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Temperature Evolution During Field Activated Sintering
Abstract
Experiments and finite element simulations have been conducted to characterize the temperature distribution in the specimen/die/punch setup and its evolution during field activated sintering. Significant temperature gradients were found in specimen in both radial and axial directions. The punches experience the highest temperatures, while the minimum is on the outer die surface, which is where the temperature is monitored for control purposes. The temperature difference between the specimen and die surface increases with temperature. In order to know the true specimen temperature, a calibration-based correction with respect to the typically used surface temperature is necessary.
... These temperatures are 1/2-1/3 lower temperatures than each material's melting point temperature. Thus, the existence of a local, high-temperature field is suggested [33][34][35][36][37]. It is noticeable that a design of sintering die and punch assembly made of graphite is an extremely important subject to Joule heating according to the sintering progress, interaction of powder material, system resistivity, and the function as direct heating elements in order to assume the role of maintaining the homogeneity of sintering temperature [36][37][38][39]. ...
... Thus, the existence of a local, high-temperature field is suggested [33][34][35][36][37]. It is noticeable that a design of sintering die and punch assembly made of graphite is an extremely important subject to Joule heating according to the sintering progress, interaction of powder material, system resistivity, and the function as direct heating elements in order to assume the role of maintaining the homogeneity of sintering temperature [36][37][38][39]. ...
... Ceramics 2021, 4 FOR PEER REVIEW 10 a local, high-temperature field is suggested [33][34][35][36][37]. It is noticeable that a design of sintering die and punch assembly made of graphite is an extremely important subject to Joule heating according to the sintering progress, interaction of powder material, system resistivity, and the function as direct heating elements in order to assume the role of maintaining the homogeneity of sintering temperature [36][37][38][39]. ...
The spark plasma sintering (SPS) method is of great interest to the powder and powder metallurgy industry and material researchers of academia for both product manufacturing and advanced material research and development. Today in Japan, a number of SPS products for different industries have already been realized. Today’s fifth-generation SPS systems are capable of producing parts of increasing size, offering improved functionality, reproducibility, productivity, and cost. For instance, pure nano-Tungsten Carbide WC powder (no additives) is fully densified with a nano-grain-sized structure for glass lens application in the optics industry. The SPS is now moving from scientific academia and/or R&D proto-type materials level usage to practical industry use product stage utilizing in the field of electronics, automotive, mold and die, cutting tools, fine ceramics, clean energy, biomaterials industries, and others. This paper reviews and introduces the peculiar phenomenon of SPS and the progress of SPS technology, method, development of SPS systems, and its industrial product applications.
... Diverse reports are available on imparts of alloying on the chemical, microstructural and phases characterization/behavior of different type of engineering materials. These include the super-alloys, steels and aluminum alloys [5][6][7], nickel nano-particles [8] and titanium base alloys, etc., and their corrosion behaviors [9] and control [10][11][12][13][14][15] in different media [16][17][18]. The current research development in the Ni-Fe-based austenitic matrix which falls under the category of Ni-based alloys is beginning to receive attention from researchers. ...
... Also, the die surface was enclosed in a permeable graphite felt (about 10 mm approx. thick) used as a heat resistor, thus minimizing heat loss due to temperature gradient and radiation [11,12]. Sintering operation was done in vacuum at constant pressure (50 MPa) and temperature (1000 o C) and was followed by soaking for 10 minutes at 1000 o C maximum temperature. ...
... Powder sintering is regarded as a thermal treatment usually performed below the fusion temperature of the major components of powder compact in order to enhance the mechanical strength and the thermo-chemical integrity [1][2][3][4][11][12]. After compaction, bordering metal powder particles are bonded by cold welds, which offer the compressed particles sufficient mechanical strength to be handled while diffusion processes form and grow necks at these contacting surfaces at the working sintering temperature [14]. ...
... The most common SPS devices investigated throughout the world are the Syntex (or Sumitomo, Japan) [5][6][7][8][9][10][11] and the FCT (Systeme GmBH, Germany) [12][13][14][15]. Both systems share various similar geometric, functional and control features, the most relevant being the tool working concept and tool geometry. ...
... Earlier SPS models utilized constant electric and thermal contact resistances. Recent SPS models implement more complex functions of temperature [e.g: 5,7,8,[11][12][13]20] and pressure [e.g: 8,11,20]. More robust SPS models may also include shrinkage [8] and electric power recordings [12]. ...
... More robust SPS models may also include shrinkage [8] and electric power recordings [12]. More simply Zavaliangos et al. [5] assume the same temperature dependence for both the contact resistances and the bulk resistivity of tool graphite. The temperatures are measured by thermocouples at selected points and/or by IR pyrometers at specified probe points. ...
... On S CD , S DE , and S ML surfaces, the conditions of heat exchange with plates from a press are set as (9) Equation (9) is obtained by approximating the heat flux in the plates and is analogous to the conditions of convective heat exchange with the corresponding effective value α eff of the heat transfer coefficient. On the S EFGHKL surface, the heat exchange with the external air environment is taken into account, that is, (10) where α air is convective heat transfer coefficients on this surface. ...
... When modeling electric and thermal fields, it is essential to take into account the conditions of contact on the contact surfaces between the elements of the apparatus, because these conditions affect the distribution of electric and temperature fields [10][11][12]. Figure 3 shows such contact surfaces, which are taken into account in this work; H denotes the horizontal contact surfaces, and V is for vertical ones. H 1 and H 2 are the contact surfaces of the upper punch with the graphite cylinder and the workpiece, respectively; H 3 and H 4 are the contact surfaces of the lower punch with the workpiece and with the graphite cylinder, respectively. ...
... The values of contact electrical and thermal conductivity on the contact surfaces and their dependence on the temperature and pressure were selected according to the data [10][11][12][20][21][22][23]. Contact electrical and thermal conductivity depends on many factors: the roughness of the contacting surfaces, the thickness of the contact layer, the contact area of the microjoints of the contact surfaces, pressure and temperature at the points of the contact surface, and the orientation of the contact surface relative to the compression force. ...
... Many works have used coupled electric-thermal modeling approaches to predict the temperature distributions in FAST/SPS, [42][43][44][45][46] and certain works have used additional thermal-mechanical steps to calculate the densification and grain size evolution of sintering. 29,47 Due to the interaction of different multi-physical fields during FAST/ SPS, more recent works have shown the concept of coupling both the procedures into a single step. ...
... The coupled FAST/SPS thermal-electrical model is based on the heat generation due to Joule heating and heat transfer due to the various modes (conduction, convection, and radiation). The heat transfer in a control volume, V, enclosed by a surface area S, is solved using the general energy balance equation 42,49 In the energy balance equation (1), D is the density of the density of the control volume, c p the specific heat of the material, T is the temperature, t is the time, and k is the thermal conductivity of the material. The surface heat flux q cond stands for heat conduction, q conv for heat convection, q rad for heat radiation and q intsruf for interfacial heating effects. ...
... The other individual terms are defined in literature. 42,49 In ABAQUS, a section of the tool geometry with the compact was modeled due to symmetrical boundary conditions and then meshed with Q3D8 element-type, which is an 8-node brick, trilinear displacement, electric potential, and temperature element. The initial temperature of all tools and the compact were set to 20°C. ...
This study aims to understand the effect of the electrical field on microstructure evolution during field‐assisted sintering or spark plasma sintering (FAST/SPS) of 10 mol% gadolinium‐doped ceria (GDC) with experimental and numerical methods. The novelty of this study has been the observation of enhanced grain growth in the region closer to the anode, even under FAST/SPS conditions with electrical fields less than 5 V/cm. The grain growth kinetics, including determination of activation energy and grain‐boundary mobility, were analyzed along the cross section of the samples for different temperatures and dwell periods. With an increase in distance from the anode, reduction in the activation energy for grain growth and grain‐boundary mobility was observed. These observations attributed to the attraction of oxygen ions to the anode region under an electrical field with an increase in defects along the grain boundaries. Thereby an increase in the grain‐boundary mobility and larger grains in that region were observed. A homogenous microstructure was observed in a case where the current did not flow through the sample. Furthermore, a numerical strategy has also been developed to simulate this behavior in addition to heat generation, heat transfer, and densification using Finite Element Methods (FEM) simulations. The simulation results provided an insight into the presence of a potential difference across the cross section of the samples. The simulation results were also in good agreement with the experimental observations.
... On the other hand, the temperature distribution can be quite inhomogeneous. [13,70] A classic misinterpretation is to define the temperature measured on thermal non-insulated die wall as sintering temperature of the sample. Zavaliangos [70] and other control. ...
... [13,70] A classic misinterpretation is to define the temperature measured on thermal non-insulated die wall as sintering temperature of the sample. Zavaliangos [70] and other control. Even if temperature is measured in vicinity of sample center e.g. by drilling a hole in the upper punch of the FAST/SPS tool, this temperature can by still quite different from the temperature at the sample edge. ...
... Further scaling up and industrialization of FAST/SPS technologies requires critical analysis of energy consumption and ideas to reduce temperature gradients, which is up to now rarely discussed in literature. [15,70,71,100] Complete energy balance including energy consumed by the water cooling system is usually not discussed. Therefore, a preliminary investigation of energy consumption and temperature gradients was performed at IEK-1 within a FAST/SPS setup with a die of 17 mm in diameter. ...
Highly efficient energy conversion and storage technologies such as high‐temperature solid oxide fuel and electrolysis cells, all‐solid‐state batteries, gas separation membranes, and thermal barrier coatings for advanced turbine systems depend on advanced materials. In all cases, processing of ceramics and metals starting from powders plays a key role and is often a challenging task. Depending on their composition, such powder materials often require high sintering temperatures and show an inherent risk of abnormal grain growth, evaporation, chemical reaction, or decomposition, especially in the case of long dwelling times. Electric current‐assisted sintering (ECAS) techniques are promising to overcome these restrictions, but a lot of fundamental and practical challenges must be solved properly to take full advantage of these techniques. A broad and long‐term expertise in the field of ECAS techniques and comprehensive facilities including conventional field‐assisted sintering technology/spark plasma sintering (FAST/SPS), hybrid FAST/SPS (with additional heater), sinter forging, and flash sintering (FS) devices are available at the Institute of Energy and Climate Research: Materials Synthesis and Processing (IEK‐1). Herein, main advantages and challenges of these techniques are discussed and the concept to overcome current limitations is introduced on selected examples.
... Evaluation of temperature gradients in SPS by experiments, modeling, or both has been performed by several research groups. [35][36][37][38][39] In general, the temperature experienced by the sample is higher than the temperature read in SPS by using an optical pyrometer or a thermocouple (TC). 35 Usually, a pyrometer is focused either radially on the outer matrix surface or axially on the bottom of a drilled upper punch at a distance of several millimeters from the sample. ...
... The temperature difference between the sample and the temperature measurement position (pyrometer or TC) depends on its position and increases with increasing temperature. As shown by Zavaliangos et al., 36 it can be as high as ΔT = 240 ○ C. Langer et al. 37 showed that the temperature at the bottom of the upper punch is closer to the sample temperature than the temperature measured at the outer matrix. Radajewski et al. 38 demonstrated that using the same thermocouple and, more importantly, the same inserted length can reduce the measuring error. ...
... As a consequence, huge temperature gradients occur between those two different configurations. According to Zavaliangos et al., 36 the punches experience the highest temperature and the outer matrix wall experiences the lowest temperature in a graphite matrix configuration. They showed that the amount of Joule heating generated in the punches is about 16 times higher than in the matrix and sample center, independent of the sample's conductivity. ...
A facile method is described to couple flash sintering (FS) and spark plasma sintering (SPS). Flash spark plasma sintering (FSPS) combines advantages of both techniques: the use of pellet-shaped samples under mechanical load with the controlled passage of electric current through the sample. FSPS is realized by partially replacing graphite pressing tools (two punches and one matrix) used in standard SPS. An insulating boron nitride matrix substitutes the conducting graphite matrix to force the electric current through the sample. Additionally, external heating of the boron nitride matrix is implemented. Microstructures of standard and flash-SPS are compared using aluminum doped zinc oxide as an example. Scanning electron microscopy reveals that different microstructures are generated for SPS and FSPS. The new setups provide novel processing routes for different current sintering methods of materials under mechanical load and assist in identifying the role of the electric current or field in the microstructure.
... They found that contact resistances are one of the parameters that control the results of thermoelectric behavior in the SPS device. The work of [11] presented a simulation using the FEM method to study the effect of thermal gradient during field-activated sintering technique. They found that there is an approximately linear correlation between the surface temperature of the sample and the temperature distribution within the sample. ...
... Other researchers have also claimed that contact resistances can be neglected by implementing measures such as reducing heat loss from the die through thermal insulation and using a nonconductive coating on its outer surface. They also suggest decreasing the resistance at the die-punch interface by preventing the entrapment of powder particles: by ensuring a smooth surface (where the punch and the die come into contact) or by using a highly conductive coating [11]. Moreover, it should be pointed out that the inclusion of contact resistances in the SPS requires specific measurements for each experiment and sintering machine [19]. ...
Spark plasma sintering (SPS) is a promising modern technology that sinters a powder, whether it is ceramic or metallic, transforming it into a solid. This technique applies both mechanical pressure and a pulsed direct electric current simultaneously. This study presents a three-dimensional (3D) numerical investigation of the ther-moelectric (thermal and electric current density fields) and mechanical (strain-stress and displacement fields) couplings during the SPS process of two powders: alu-mina (ceramic) and copper (metallic). The ANSYS software was employed to solve the conservation equations for energy, electric potential, and mechanical equilibrium simultaneously. Initially, the numerical findings regarding the thermoelectric and mechanical coupling phenomena observed in the alumina and copper specimens were compared with existing numerical and experimental results from the literature. Subsequently , a comprehensive analysis was conducted to examine the influence of current intensity and applied pressure on the aforementioned coupling behavior within the SPS device. The aim was to verify and clarify specific experimental values associated with these parameters, as reported in the literature, and identify the optimal values of applied pressure (5 MPa for alumina and 8.72 MPa for copper) and electric current (1000 A for alumina and 500 A for copper) to achieve a more homogeneous material. Abdelmalek KRIBA,
... Giuntini et al. [14] revealed that the temperature gradient was observed along the radius direction of graphite die during spark plasma sintering densification. Furthermore, Zavaliangos et al. [15] and Grasso et al. [16] discovered that such temperature gradient could be manipulated by controlling the uniaxial pressure or external electrical field combining experimental and numerical simulations. Taking advantage of the temperature gradient, continuous and symmetric graded Si 3 N 4 ceramics would be prepared by initial homogenous composition. ...
... It can be seen that as the sintering temperature increased, the temperature gradient between the edge and the center of the sample increased from 75 • C to 128 • C. These results were in agreement with Zavaliangos [15] et al. In field activated sintering, the temperature field had a considerable gradient in the radial direction both experimentally and numerically. ...
... Due to the presence of anisotropic electric (1.11×10 5 Ω −1 ·m −1 in the plane and 1.03×10 4 Ω −1 ·m −1 across the thickness) and thermal (140 W/(m·K) in the plane and 5 W/(m·K) across the thickness) properties of the 0.2-mm-thick graphite foil, the electrical contact resistance (ECR) is introduced between the graphite and the graphite foil [12,30]. The contact is closely related to the applied pressure and temperature [30,31]. ...
... Due to the presence of anisotropic electric (1.11×10 5 Ω −1 ·m −1 in the plane and 1.03×10 4 Ω −1 ·m −1 across the thickness) and thermal (140 W/(m·K) in the plane and 5 W/(m·K) across the thickness) properties of the 0.2-mm-thick graphite foil, the electrical contact resistance (ECR) is introduced between the graphite and the graphite foil [12,30]. The contact is closely related to the applied pressure and temperature [30,31]. In this study, the contact conditions were simplified according to different contact states in the vertical and horizontal directions. ...
Spark plasma sintering (SPS) is a highly efficient method for the preparation of α/β-SiAlON ceramics. However, the rapid preparation of large-scale α/β-SiAlON ceramic components with reliable mechanical properties is difficult via SPS due to their near-insulating properties. In this study, high-performance α/β-SiAlON ceramic end mill rods with large aspect ratios were successfully prepared via SPS. Two different types of sintering processes (namely vertical-round-rod (VRR) and horizontal-square-rod (HSR) processes) were developed, and their effects on the phase composition, microstructure, mechanical properties, and machining performance of the α/β-SiAlON ceramic end mill rods were studied. The electric and temperature field distributions during sintering were studied through an electro–thermal simulation. The simulated and experimental temperature distributions are in good agreement. In contrast to VRR samples, HSR samples with a small axial size show a uniform temperature distribution and satisfactory microstructures within a certain range of dimensions as well as the expected phase composition; furthermore, elongated β-SiAlON grains are preferentially oriented in the direction perpendicular to the sintering pressure direction. As a result, the HSR samples exhibit better mechanical properties and machining performance than the VRR samples.
... Temperature sensing is usually carried out either through a vertically drilled hole in the punch or through a horizontal drill in die body. In both cases, deviations from the actual sintering temperature by several hundred degrees will be inevitable as demonstrated by Zavaliangos et al. 44 It should also be noted that based on the electrical resistivity of the powder material, and the mold, there can be significant temperature variations within the die as well. This is especially significant for electrically insulating powder compacts where joule heating of the die is responsible for heating of the compact indirectly. ...
... Nevertheless, regardless of the graphite properties, it is clear that such die geometry will always show a considerable temperature variation when comparing the corners to center, especially for electrically insulating samples. These results are consistent with prior computational models by Zavaliangos et al. 44 and Olevsky et al., 71 who have demonstrated similar temperature variations between the center of the SPS-die and the outer peripheries. Figure 4. We observed the presence of a core−shell region within the matrix, where the grains are not of the same chemical composition entirely ( Figure 5). ...
Square-shaped boron carbide ceramic composites have been produced by spark plasma sintering with the addition of 5 to 20 vol % titanium metal powder in the B4C matrix in order to initiate an in situ self-propagating high-temperature synthesis (SHS) of TiB2. The SHS reaction not only enhances many of the physical and mechanical properties of B4C, but also reduces the required sintering temperature and pressure because of the enthalpy of reaction between metallic Ti and B4C. Sintering has been carried out in the SPS-temperature range of 1450 to 1550 °C with a uniaxial pressure of 40 MPa and a dwell time of 4 min under a 1 atm argon atmosphere. The effects of various amounts of Ti additions and sintering temperature on the phase composition, density, hardness, fracture toughness, and microstructure are examined. X-ray diffraction and transmission electron microscopy evaluations have shown that added Ti completely transforms into TiB2, resulting in a core–shell microstructure with a carbon core, surrounded by a TiB2 shell in the B4C matrix. Moreover, by carrying out a control experiment where TiB2 was added instead of Ti, and performing a molecular dynamics simulation of the B4C-Ti interface, the significance of the in situ SHS process has been validated.
... Modeling the SPS process is a complex task that requires accounting for several physics at once. The case of conducting metallic materials has been considered in the following references: [64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79]. As depicted by Achenani et al. [78], the ongoing modeling effort can be divided in two categories: thermoelectric coupling to simulate temperature gradients in fully-dense, static conditions [64][65][66][67]70,74,76,78,79], and stress distribution calculations to model the densification kinetics [28,68,69,[71][72][73]75,77]. ...
... The case of conducting metallic materials has been considered in the following references: [64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79]. As depicted by Achenani et al. [78], the ongoing modeling effort can be divided in two categories: thermoelectric coupling to simulate temperature gradients in fully-dense, static conditions [64][65][66][67]70,74,76,78,79], and stress distribution calculations to model the densification kinetics [28,68,69,[71][72][73]75,77]. ...
After a few decades of increasing interest, spark plasma sintering (SPS) has now become a mature powder metallurgy technique, which allows assessing its performances toward fabricating enhanced materials. Here, the case of metals and alloys will be presented. The main advantage of SPS lies in its rapid heating capability enabled by the application of high intensity electric currents to a metallic powder. This presents numerous advantages balanced by some limitations that will be addressed in this review. The first section will be devoted to sintering issues, with an emphasis on the effect of the electric current on the densification mechanisms. Then, typical as-SPS microstructures and properties will be presented. In some cases, they will be compared with that of materials processed by conventional techniques. As such, examples of nanostructured materials, intermetallics, metallic glasses, and high entropy alloys, will be presented. Finally, the implementation of SPS as a technique to manufacture complex, near-net shape industrial parts will be discussed.
... Several authors have identified the electrical and thermal contact resistances (ECR and TCR respectively) as crucial parameters to obtain a good thermo-electrical model of the process [8]- [13]. Those resistances can be found at each interface between each part of the SPS apparatus (tooling parts and sample). ...
... However, vertical contact resistances in the SPS have been studied in the literature. First, they were considered as constants with temperature and pressure [8,9]. However, the contact pressure varies over the SPS trial as the temperature increases and thus as the materials creep. ...
The electrical and thermal contact resistances are key parameters for obtaining an accurate electro-thermal model of the spark plasma sintering (SPS) process. However, due to the lack of a general expression, these parameters are usually determined empirically. Thus, they are only valid for a specific material and SPS configuration. A simple method based on a limited amount of experiments as well as a new formulation of the electrical and thermal contact resistances are developed. First, the evolution of those resistances is optimized on simple shapes (pellets) experiments. They are then transferred into the electro-thermal simulation of complex shapes configurations, which showed a good agreement between the experimental and computed data.
... Furthermore, In SPS, the temperature measured by the pyrometer does not represent the actual annealing temperature. Various investigations have shown that the actual sintering temperature could be higher than the pyro temperature by about 250°C [6, [19][20][21][22][23]. Additionally, application of pressure during SPS annealing is also known to compact the various layers of the foil causing a reduction in thickness which in turn leads to a decrease in electrical resistance of the graphite foils [24,25]. ...
The progression of the electrical resistance of graphite foils during spark plasma sintering process (SPS) was investigated at constant temperature and pressure. The study applied various set-ups of the SPS device, and the electrical data used for the evaluation of electrical resistance (heating power and current) was obtained from the SPS apparatus in real-time. The contact resistance and resistance due to graphite foil/s was evaluated by subtracting the resistance of the single punch set-up from the set-up of two punches in direct contact and the set-ups with various graphite foils . The results showed that during the initial stages of sintering, set-up resistance increases with time and that, overall, set-up resistance increases with number of graphite foils. Both contact resistance and resistance due to graphite foils was found to decrease with sintering time. In contrast to previous conceptions, the electrical resistance of graphite foils changes in response to sintering conditions during the SPS process.
... The temperature distribution in SPS machines has been thoroughly studied by finite element (FE) modelling to understand the heating of the tool setup, and the heating and sintering of the powder compact, especially to evaluate differential sintering due to temperature inhomogeneities [13][14][15][16][17][18]. Later, strategies to achieve a homogeneous temperature distribution in the specimen have been proposed based on these simulations [19], on the one hand because such inhomogeneities have to be avoided to achieve homogeneous part properties, on the other hand because the study of fundamental aspects of the SPS process by model experiments would be impaired by such phenomena. ...
Spark Plasma Sintering (SPS) is an innovative sintering technique, whereby many of the beneficial effects of this process on sintering are still elusive. To allow for the detailed investigations of the SPS process, a custom experimental set-up and a corresponding finite element (FE) model was developed. The miniaturised setup allows for very high current intensities, custom pulse patterns, a wide pressure range and dilatometric measurements. The FE model was employed to calculate the temperature field in the set-up and the sintering specimen itself. A very good correlation of the temperature, current and voltage over the entire process was observed. Our investigations show that the contact conductivities have a significant impact on the process temperature. Also, the imperfect contacts at the interfaces between the graphite foil and the real specimen may lead to a significant variance of the currents necessary to obtain the desired sintering temperature.
... SPS is a process where three main physical phenomena are involved and interconnected: densification, thermal distribution and electrical behavior of the specimens. Powder densification can be modeled based on studies presented in literature [11,[37][38][39][40]. SPS involves Joule heating [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57], densification and field phenomena [1,11,37,39,[58][59][60]. To simulate the thermal and electrical current distribution and the densification during SPS, Finite Element Method (FEM) is largely used [9,[61][62][63][64]. ...
A novel method of producing complex ceramic and metallic parts with designed internal channels is developed. The method utilizes a combination of the additive manufacturing technique of solvent jetting and spark plasma sintering (SPS.) The developed manufacturing approach brings benefits in producing complex shapes with internal channels. Along with geometric customization of the 3D printed mold, a major advantage of this method is the removal of the need for a long debinding process, usually necessary with other 3D printing methods, by using the SPS. High density ceramic and metallic complex parts with internal channels were successfully produced with close to theoretical densities. The conducted studies include the development of a model that can predict the evolution and/or distortions of the complex-shaped powder assembly during the sintering process. The model is based on the continuum theory of sintering formulations embedded in a finite element code.
... Graphite foil with a thickness of 0.2 mm was placed between the die and powder for easy removal and to ensure that the cooling of the sintered ZrC samples was homogeneous. The outside of the graphite die was covered with an insulating felt to reduce radiation loss [15]. After the powder was placed in the graphite die, it was first cold pressed using a hydraulic press to consolidate the powder together and then spark plasma sintered (SPS). ...
Zirconium carbide (ZrC) samples were prepared by spark plasma sintering (SPS), at temperatures of 1700 °C, 1900 °C and 2100 °C, all at pressure of 50 megapascal (MPa). The density of ZrC ceramic pellets was measured using a Micromeritics AccuPyc II 1340 Helium Pycnometer. The density of ZrC ceramic pellets was found to increase from (6.51 ± 0.032) g/cm 3 to (6.66 ± 0.039) g/cm 3 and (6.70 ± 0.017) g/cm 3 when the temperature of the SPS was increased from 1700 o C to 1900 o C and 2100 o C respectively. Moreover, the hardness of ZrC ceramic pellets were measured using Rockwell hardness test. The hardness of ZrC ceramic pellets increased from (7.4 ± 0.83) to (17.0 ± 0.073) and (18.4± 0.05) gigapascals (GPa) at temperatures of 1700 o C, 1900 o C and 2100 o C respectively. X-ray diffraction shows the absence of spurious phases or impurity. XRD results showed that, all prepared ZrC samples has the same preferred orientation of the planes (i.e., 200). Furthermore, the average grain size of ZrC was calculated using Sherrers's equation. The average grain size of the pure ZrC powder increased from 67.46 nm to 72 nm, 79 nm and 83 nm when the ZrC powder was sinteried at temperatures of 1700 o C, 1900 o C and 2100 o C respectively. The differences in the average grain size between the prepared samples leads to show different surface morphologies that monitored by scanning electron microscopy (SEM)., et al. Microstructure of zirconium carbide ceramics synthesized by spark plasma sintering
... They also reported that the temperature distribution affected the mechanical stresses and that the applied external pressure played an important role in the densification. Zavaliangos et al. [31] used both experimental and numerical simulations to obtain the temperature distribution in the specimen/die/punch setup during field-activated sintering. They found a considerable radial and axial temperature gradients in the specimen. ...
The present work aims to investigate the geometrical parameters of the graphite die on energy consumption needed for sintering of a ZrB2 sample. The Maxwell and electrical charge conservation equations are solved to obtain the electrical potential and current of the system. The governing equations are discretized by the Galerkin method and solved using the finite element method. The electric current distribution is obtained at each geometry and the temperature contours are obtained. The results showed that the height of die has a direct effect on power consumption. This can be attributed to the increased electric resistance and consequent increased Joule heating. On the other hand, increasing the die height resulted in more uniform temperature distribution through the sintered sample.
... The finite-element method is usually used for the simulation at the component level for a part of or for the whole sintering machine. Typically, such modeling is limited to a thermal or thermoelectric analysis [44][45][46][47][48][49][50][51][52], although, multi-physics simulations of powder systems are getting more widespread for various technological processes involving powders [53]. The finite-element simulation of the three-way coupling of thermal, electrical and mechanical behavior with spatial distribution of the stress and strain fields was analyzed only in a few works [54][55][56][57][58]. ...
In the present research, a numerical modeling approach of the initial stage of consolidation during spark plasma sintering on the microscopic scale is presented. The solution of a fully coupled thermo-electro-mechanical problem also accounting for grain boundary and surface diffusion is found by using a staggered way. The finite-element method is applied for solving the thermo-electro-mechanical problem while the finite-difference method is applied for the diffusion problem. A Lagrange-based non-linear formulation is used to deal with the detailed description of plastic and creep strain accumulation. The numerical model is developed for simulating the structural evolution of the involved particles during sintering of powder compacts taking into account both the free surface diffusion of the particles and the grain boundary diffusion at interparticle contact areas. The numerical results obtained by using the two-particle model—as a representative volume element of the powder—are compared with experimental results for the densification of a copper powder compact. The numerical and experimental results are in excellent agreement.
... A constant cooling water temperature of Tw = 25.5 • C was used. According to recommendations published by Zavaliangos et al., an emissivity coefficient of 0.8 was taken for graphite (the dummy and the spacers) [18]. An emissivity of 0.2 was applied on external surfaces of electrodes. ...
Spark Plasma Sintering (SPS) is a technology used for fast consolidation of metallic, ceramic, and composite powders. The upscaling of this technology requires a reduction in energy consumption and homogenization of temperature in compacts. The application of Carbon Fiber-Reinforced Carbon (CFRC) insulating plates between the sintering setup and the electrodes is frequently considered as a measure to attain these goals. However, the efficiency of such a practice remains largely unexplored so far. In the present paper, the impact of CFRC plates on required power, total sintering energy, and temperature distribution was investigated by experiments and by Finite Element Modeling (FEM). The study was performed at a temperature of 1000 °C with a graphite dummy mimicking an SPS setup. A rather moderate influence of CFRC plates on power and energy demand was found. Furthermore, the cooling stage becomes considerably longer. However, the application of CFRC plates leads to a significant reduction in the axial temperature gradient. The comparative analysis of experimental and modeling results showed the good capability of the FEM method for prediction of temperature distribution and required electric current. However, a discrepancy between measured and calculated voltage and power was found. This issue must be further investigated, considering the influence of AC harmonics in the DC field.
... It has been recognized that the contact conductance is a function of several parameters, such as contacting interface geometry, surface roughness, temperature, interfacial pressure, etc. [35][36][37][38][39]. In the multivariate inverse method, as another two outputs from the simulation, thermal contact conductance between the powder and solid shell on the top and at the bottom are principal parameters interfering with heat transfer in the powder-enclosed specimens. ...
This study investigates the thermal conductivity of 17-4PH stainless steel powder that was encapsulated within specimens with different internal geometries in laser powder bed fusion (L-PBF) additive manufacturing (AM). The objective is to evaluate the effect of the internal geometry of the specimens on the measurement of the powder thermal conductivity and to compare the thermal properties amongst the 17-4PH and two additional powder materials used in L-PBF. Continued from the previous work [1], three new cone configurations in the hollow specimens were designed and fabricated in an L-PBF system. The thermal conductivity of the internal powder was indirectly measured using an experimental-numerical approach, combined with laser-flash testing, finite element (FE) heat transfer modeling and multivariate inverse method. The results reveal that the thermal conductivity of 17-4PH powder ranges from 0.67 W/(m·K) to 1.34 W/(m·K) at 100 °C to 500 °C, and varies with the internal geometry of the specimens. In addition, the measurement of the hollow specimen with a convex cone seems to be a more reliable evaluation. Further, the thermal conductivity ratio of the powder to the solid counterpart of 17-4PH approximately ranges from 3.9 % to 5.5 % at tested temperatures, which is similar to the results obtained from the nickel-based super alloy 625 (IN625) and Ti-6Al-4V (Ti64) powders measured in a previous study.
... The reduction of the temperature gradients inside the SPS tooling [21][22][23] and inside the processed samples requires an adjustment of the tooling and of the electric current path 24 , which can be accomplished with the help of finite element simulations. A special attention should be paid to the predominant role of the electric and thermal contacts at all the tooling interfaces in the formation of the overall temperature field [25][26][27][28][29][30][31][32] . On the other hand, the productivity and the scale up of the SPS process represent a similar challenge because increasing the productivity requires the simultaneous processing of multiple samples 5 which, in turn, introduce large processed volume dimensions. ...
An energy efficient spark plasma sintering method enabling the densification of large size samples assisted by very low electric current levels is developed. In this method, the electric current is concentrated in the graphite foils around the sample. High energy dissipation is then achieved in this area enabling the heating and full densification of large (alumina) parts ({{\O}} 40 mm) at relatively low currents (800 A). The electrothermal mechanical simulation reveals that the electric current needed to heat the large samples is 70 % lower in the energy efficient configuration compared to the traditional configuration. The presence of thermal and densification gradients is also revealed for the larger size samples. Potential solutions for this problem are discussed. The experiments confirm the possibility of full densification (96-99 %) of large alumina samples. This approach allows using small (and low cost) SPS devices (generally limited to 10-15 mm samples) for large size samples (40-50 mm). The developed technique enables also an optimized energy consumption by large scale SPS systems.
... The electric and thermal contact resistances (ECR and TCR) on each interface of the NSFSPS setup are very important parameters influencing the overall temperature distribution [60][61][62][63][64] . ...
A new flash (ultra-rapid) spark plasma sintering method applicable to various materials systems, regardless of their electrical resistivity, is developed. A number of powders ranging from metals to electrically insulative ceramics have been successfully densified resulting in homogeneous microstructures within sintering times of 8-35 s. A finite element simulation reveals that the developed method, providing an extraordinary fast and homogeneous heating concentrated in the sample's volume and punches, is applicable to all the different samples tested. The utilized uniquely controllable flash phenomenon is enabled by the combination of the electric current concentration around the sample and the confinement of the heat generated in this area by the lateral thermal contact resistance. The presented new method allows: extending flash sintering to nearly all materials, controlling sample shape by an added graphite die, and an energy efficient mass production of small and intermediate size objects. This approach represents also a potential venue for future investigations of flash sintering of complex shapes.
... The finite element method (FEM) is often used to assess the expected thermal gradients and densification in-homogeneities [18][19] and to find optimized process conditions. Numerous electro-thermal models highlight the prominent role of the electric and thermal contacts in the temperature generation and distribution during SPS [20][21][22][23][24][25][26]. ...
The stability ofthe proportional--integral--derivative (PID)controlof temperature in the spark plasma sintering (SPS) process is investigated.ThePID regulationsof this process are tested fordifferent SPS toolingdimensions, physical parameters conditions,andareas of temperature control. It isshown thatthe PID regulation quality strongly depends on the heating time lag between the area of heat generation and the area of the temperature control. Tooling temperature rate maps arestudied to revealpotential areas forhighlyefficientPID control.The convergence of the model and experiment indicatesthat even with non-optimal initial PIDcoefficients, it is possible to reduce the temperature regulation inaccuracy to less than 4K by positioning the temperature control location in highlyresponsiveareas revealed by the finite-element calculationsof the temperature spatial distribution.
... As described by numerous authors [5,[14][15][16][17], the main challenge related to the Joule heating modeling is to identify the non-ideal electric and thermal contacts in the SPS toolingspecimen setup as the dominant parameters controlling the temperature field distribution. ...
The modeling of powder compaction process, such as spark plasma sintering (SPS), requires the determination of the visco-plastic deformation behavior of the particle material including the viscosity moduli. The establishment of these parameters usually entails a long and difficult experimental campaign which in particular involves several hot isostatic pressing tests. A more straightforward method based on the coupled sinter-forging and die compaction tests, which can be easily carried out in a regular SPS device, is presented. Compared to classical creep mechanism studies, this comprehensive experimental approach can reveal the in situ porous structure morphology influence on the sintering process.
... In addition to the fast densification rates observed in SPS, the pressure and temperature profiles as well as the die geometry effects on the electrical-thermal-mechanical system involved in SPS are of prime relevance [19][20][21]. Indeed, on the way towards industrial relevance of SPS processes, advanced understanding of such physical and geometrical factors is needed to produce globally homogeneous ceramic parts. ...
Summarizing the work of nearly a decade of research on spark plasma sintering (SPS), a review is given on the specificities and key factors to be considered in SPS of ceramic materials, based on the authors’ own research. Alumina is used primarily as a model material throughout the review. Intrinsic inhomogeneities linked to SPS and operational parameters, which depend on the generation of atomistic scale defects, are discussed in detail to explain regularly observed inhomogeneities reported in literature. Adopting an engineering approach to overcome these inherent issues, a successful processing path is laid out towards the mastering of SPS in a wide range of research and industrial settings.
... The character of the curve is the same as that in the estimated dependences. After 90 s, the final pressure was applied over 20 s and the temperature fell to 635 °С [25]. ...
The reported estimation calculations have determined the kinetics of temperature changes in the model clamp‒dielectric matrix‒upper electrode‒punch‒sintered ring product‒lower electrode‒punch‒stand. This has made it possible to determine the temperature of the control volumes of the product and tooling in a random period. The experiments have confirmed that the estimated values of temperature in the upper and lower layers of the sintered product are rather similar and differ little from actual ones. Specifically, it has been established that the sintered product concentrates 24 % of the thermal energy, which is released throughout the entire unit (press-tool‒product). Under a conductive heating method, it is very important that the maximum temperature in the zone of the sintered product should be reached in the shortest period.
It has been shown that during exploitation the material of a mold under conditions of electric sintering is exposed to the thermocyclic and thermomechanical influence. The tooling components have different resistance to wear: a resource of the isolated insert is 20 cycles of sintering, of electrodes–punches ‒ 50 cycles. This allows us to argue that it is necessary to choose a material for the components of a mold, which could meet the following requirements:
– the minimal heating of the tooling elements, thereby ensuring its structural reliability and operational adaptability (a holder, a matrix, a stand);
– the maximum heating of the chain: an upper electrode‒punch‒needle‒the sintered product‒a lower ring electrode-punch, thereby contributing to the accumulation of heat in the contact area between the components of the mold and a would-be product.
The above measures lead to the increased structural strength of sintered products, as well as to the optimization of control over would-be technological operations and prompt registration of important experimental data.
Thus, there are reasons to assert the possibility of the targeted adjustment of temperature fields through the preliminary calculation and selection of the tooling materials based on the thermal-physical characteristics. The application of a given mathematical model is an effective way to resolve the above issue
... Les rampes de chauffage rapides, les propriétés thermo-électriques du matériau pulvérulent ou encore la température de frittage visée sont des paramètres qui influencent la distribution de la température au sein de l'échantillon. [113]. [114]. ...
Cette étude a pour objectif d’élaborer de nouveaux matériaux de carbure de bore utilisés en tant qu’absorbants neutroniques en réacteurs à neutrons rapides. La stratégie adoptée vise l’affinement de la microstructure des matériaux afin de limiter le phénomène de déformation anisotropique des grains sous irradiation qui est responsable de la dégradation des pastilles en fonctionnement. Deux nuances de matériaux ont été élaborées par le procédé SPS avec des microstructures submicroniques et nanométriques, permettant une diminution des tailles de grains par rapport au matériau de référence historiquement utilisé par le CEA. Les matériaux SPS ainsi que le matériau de référence ont été caractérisés et comparés du point de vue chimique, mécanique et thermique. Ce second volet de l’étude a permis de sélectionner le matériau SPS submicronique et d’approfondir les caractérisations en matière de résistance à la rupture et de tenue aux chocs thermiques. Il a ainsi été montré un gain de performance par rapport au matériau de référence. D’autre part, le comportement au fluage à haute température du matériau SPS a été évalué et les mécanismes de déformation associés identifiés. Par ailleurs, la fabrication des pastilles d’absorbant nécessitant un accroissement du rapport hauteur sur diamètre par rapport aux pastilles SPS classiques, un modèle numérique a été développé. L’acquisition des différentes données du procédé nécessaire à cette modélisation a reposé sur une instrumentation spécifique aux mesures thermiques et électriques. D’autre part, les paramètres de densification du matériau SPS ont été déterminés à partir d’un modèle d’écoulement visqueux non linéaire. Les phénomènes thermiques, électriques et mécaniques décrits par le modèle ont alors été validés par la confrontation au suivi expérimental du retrait d’un échantillon de carbure de bore.
... In addition, as two material systems are simulated in this study, the contact resistances between the sample and the die and the sample and the punches were taken into account through the current and heat fluxes across the interfaces. The equations governing these fluxes and the corresponding coefficients are reported in [18,25,37]. The ...
Functionally graded materials (FGMs) exhibit good performance owing to their gradual and directional property and compositional changes occurring in the material. Field-assisted sintering is one method to fabricate FGMs by utilizing a heating geometry that provides a temperature gradient within the sample. Here, a heating geometry that provides ultra-large temperature gradients is proposed. Using finite element analyses, the geometrical parameters of the proposed design were optimized through a systematic parametric investigation. The resulting temperature gradients were evaluated for electrically conductive stainless steel (SUS) 304L and insulating 8 mol% yttria-stabilized zirconia (8YSZ). The temperature gradients within the samples were 80 and 122 °C/mm for SUS 304L and 8YSZ, respectively. The heating geometry was used to fabricate functionally graded versions of the two materials, which showed gradual changes in the porosities, grain sizes, and hardness. Lastly, the temperature gradients were then quantitatively validated through temperature measurements and single-temperature sintering experiments.
... They also reported that the maximum temperature occurs at the lateral surface of the sample in the graphite foil, and it has an essential effect on the grain growth and densification process. Zavaliangos et al. [102] analyzed the effect of thermal gradient during the field activated sintering using experimental measurements and performed numerical simulations by the finite element method and observed the significant temperature gradients in the specimen in both radial and axial directions. They also realized that an approximately linear correlation exists between the die surface temperature and the inside temperatures. ...
Spark plasma sintering is a novel sintering technique in manufacturing of ultra high temperature ceramics. Temperature distribution during sintering process controls microstructure and consequent thermomechanical properties of manufactured samples.
Therefore, temperature distribution in the spark plasma sintering of TiB2 is investigated numerically in this work. A pulsed direct electrical current was applied during the sintering process and current density distribution as well as generated heat, as a result of Joule heating effect, were obtained at each point. The thermoelectrical governing equations were solved by finite element method. The obtained temperature contours showed that the heat generation rate is higher at the sample camped to the die, so the heat flow is from the sample center toward the die and finally the surroundings. The obtained results showed a uniform temperature distribution in the sample, so that a maximum temperate
difference with respect to the sample center was 75 ºC at the sample/die interface at the sintering temperature of 2200 ºC. This uniformity is one of the advantages of spark plasma sintering which results in a uniform microstructure in the as-sintered sample.
Spark plasma sintering as the prominent field‐assisted sintering technique (FAST/SPS) is a novel technology for the rapid, pressure‐assisted consolidation of powder materials. The main feature of FAST/SPS is the direct Joule heating of the applied tooling. Tooling is a challenging part of the FAST/SPS setup, which must withstand high pressure at elevated temperatures and ensure a uniform temperature distribution in the sintered part. This review looks at the standard FAST/SPS tooling, the specific tooling for sintering complex‐shaped parts, and for pressureless sintering. A particular focus lies on graphite, the commonly used tooling material, and on alternative materials such as steel, alloys, ceramics, and composites. The review also considers the add‐on tooling elements, such as spacers, foils, and thermal insulation. Furthermore, the article discusses the basics of FAST/SPS modeling, and the computer‐based optimization of FAST/SPS tooling, the procedure used, and the modeling accuracy. The review briefly describes the tooling and equipment for manufacturing upsized parts and large‐scale production. In addition, the article considers the tooling for FAST/SPS sintering under high pressure (up to 1 GPa) and ultra‐high pressure (over 1 GPa). The article concludes with an analysis of the challenges and prospectives for the smart design of FAST/SPS tooling.
Multivariant experimental investigations and multiphysics microstructural modeling of the spark plasma sintering process of metallic powders have been performed up to a relative density of approximately 80%. In comparison, the effect of sintering temperature, pressure, and particle size on the interparticle contact area growth and axial shrinkage of cylindrical specimens of copper and nickel particles is measured in laboratory scaled tests. Herein, for the first time all relevant for sintering phenomena are considered simultaneously: the fully coupled thermo‐electro‐mechanical modeling of the spark plasma sintering processes, additionally taking into account for lattice, grain boundary, surface diffusion, electromigration, and thermomigration, has been carried out. The computational analysis of various physical phenomena allows to identify dominant and insignificant mechanisms. The two‐level numerical simulation includes the modeling of the sintering setup at the macroscopic level and the neck formation process in particle chain systems at the microscopic level. The results of the numerical simulations show a very good agreement with the experimental data. Therefore, the impact of electrical and mechanical loads as well as of particle size on microscopic distribution of temperature, inelastic strain, and on densification has been studied by the finite element simulations.
Electric current‐assisted sintering (ECAS) is a promising powder consolidation technique that can achieve short‐term sintering with high heating rates. Currently, main methods of performing ECAS are indirect heating of the powder compact in a conductive tool or direct heating with current flowing through the powder compact. Various influencing factors have been identified to explain the rapid densification during ECAS, such as ultrahigh heating rates, extra‐high temperatures, and electric field. However, the key consolidation‐enhancing factor is still under debate. This study aims at understanding the role of heating rate on the enhanced densification during ECAS of 8 mol% Y2O3‐stabilized ZrO2 (8YSZ) by experimental and numerical methods. Two different heating modes, ultrafast high‐temperature sintering (UHS, indirect heating) and flash sintering (FS, direct heating), are studied. The novel UHS technique is successfully applied to consolidate the 8YSZ samples. Additionally, finite element methods (FEM) combined with a constitutive model is adopted to predict the densification and grain growth. Furthermore, a comparison of UHS and FS is performed to investigate the thermal effect (heating rate) and athermal effect (electric field) individually. The results indicate that the high heating rate is the key factor of the rapid densification during UHS and FS of 8YSZ.
As a promising sintering technique, flash sintering utilizes high electric fields to achieve rapid densification at low furnace temperatures. Various factors can influence the densification rate during flash sintering, such as ultrahigh heating rates, extra‐high sample temperatures, and electric field. However, the determining factor of the densification rate and the key mechanism during densification are still under debate. Herein, the densification and grain growth kinetic during flash sintering of 8 mol% Y2O3‐stabilized ZrO2 (8YSZ) is studied experimentally and numerically using finite element method (FEM). The roles of Joule heating and heating rate on the densification are investigated by comparing flash sintering with conventional sintering. An apparently smaller activation energy for the material transport resulting in densification is obtained by flash sintering (Qd =424 kJ mol⁻¹) compared to the conventional sintering (Qd = 691 kJ mol⁻¹). In addition, a constitutive model is implemented to study both the densification and the grain growth during flash and conventional sintering. Furthermore, the effect of electrical polarity on the density and the grain size evolution during flash sintering of 8YSZ is also investigated. The simulation results of average density and grain size inhomogeneity agree well with the experimental data.
Le procédé Spark Plasma Sintering (SPS) est une méthode de frittage non conventionnelle et prometteuse car permettant d’élaborer des matériaux denses tout en limitant le phénomène de croissance granulaire. Cette technique repose sur l'application simultanée d'une contrainte uniaxiale et d’une vitesse de chauffage élevée induite par le passage d'un courant électrique pulsé au sein de l’outillage en graphite, voire dans le milieu granulaire (si celui-ci est conducteur). Cependant, les phénomènes physiques générés par le passage du courant pulsé demeurent mal compris et sujets à controverse. Ainsi, le rôle du courant pulsé et/ou du champ électromagnétique induit par le traitement SPS sur les mécanismes de densification et de consolidation ont fait l’objet d’une investigation particulière. Dans le but de dissocier les différents champs physiques intervenant lors du procédé SPS, trois enceintes originales ont été mises en place et utilisées : i) le générateur de courant continu couplé à un chargement mécanique ; ii) le Cohéreur de Branly ; iii) le démonstrateur équipé d’un générateur de courant électrique pulsé. Quelque soit le dispositif utilisé, la réponse électrique d’un empilement granulaire « modèle » (i.e. constitué de particules de cuivre sphériques (87 μm) et pré-oxydées), sollicité soit par l'application d'un courant continu/pulsé soit par la génération d’ondes électromagnétiques à leur proximité, est finement caractérisée. Le comportement électrique du milieu granulaire est marqué par une transition d'un état isolant à un état conducteur. Cette transition, connue sous le nom d'effet Branly, est marquée par la chute apparente de la résistance électrique de l’empilement granulaire de plusieurs ordres de grandeur. Ces investigations expérimentales ont permis d'identifier des mécanismes spécifiques au procédé « SPS » tels que l’effet Branly et la surchauffe locale. Ces phénomènes se produisent dans les premiers stades du traitement, soit avant le début de la densification. Ils s’accompagnent de la destruction de la couche d'oxyde isolante présente au niveau des microcontacts par un phénomène de claquage diélectrique. Des micro-soudures se forment entre les particules, créant ainsi des chemins privilégiés pour le passage du courant pulsé.
The competition between sintering and coarsening is cited by numerous authors as one of the potential factors for explaining the ultra-rapid sintering kinetics of flash sintering. In particular, surface diffusion is a mechanism decreasing the driving force of sintering by changing the initial highly reactive microstructures (particle contact) into poorly reactive porous skeleton structures (spherical porosity). We show by finite element simulations that flash SPS experiments high specimen temperatures close to 2000°C. These high temperatures are not sufficient to explain the ultra-rapid sintering kinetics if typical spherical pore theoretical moduli are employed. On the contrary, reactive experimentally determined moduli succeed in explaining the ultra-rapid sintering kinetics. Mesoscale simulations evidenced that the origin of such reactive experimental moduli is a porous skeleton geometry with a significant delay in surface diffusion and particle rearrangement. This highlights the important role of the surface diffusion negation (favoring higher stress intensification factor) in flash sintering.
The effects of representative sintering techniques on the residual thermal stress (RTS) in the as-prepared cermets were investigated using the WC-Co composites as an example. The real processing parameters of the spark plasma sintering (SPS) and sinter-hot isostatic pressing (Sinter-HIP) techniques were introduced to quantify RTS and its mechanisms. The real microstructures of cermets were used in the models for evaluation of RTS distributions in different phases. The macro-mesoscale coupled calculations indicated that during the cooling of both the SPS and Sinter-HIP processes, the WC phase of the sintered cermets has a state of compressive stress, while Co phase has a tensile stress state. The internal stress magnitude of both phases increases linearly with the decrease of temperature. There are larger temperature and displacement fields in the SPSed cermets compared to those prepared by Sinter-HIP. The stress accumulation in the SPS process is faster than that in the Sinter-HIP by 30%. The RTS in the SPSed cermets is highly concentrated at the acute dihedral angles of WC/Co interfaces, which tends to promote the face-centred cubic (fcc) to hexagonal close-packed (hcp) martensitic transformation of the Co phase.
This work investigates amorphous powders’ temperature distribution and densification mechanism at macroscopic and particle scales during spark plasma sintering. The evolution of contact necks between powders is studied by quasi‐in situ microstructure examination. Amorphous powders’ contact mode changes from point contact to surface contact as sintering proceeds. The high current density concentration leads to local overheating at the particle necks. As the contact radius ratio increases, the temperature gap between the necks and the interior of the particles becomes less noticeable. In the early stage, localized high temperature and high stress around the necks significantly reduce the densification onset temperature of amorphous powders. As the relative density increases, the uneven temperature field at the particle scale disappears, while the macroscopic temperature gradient increases. The dominant densification mechanism of amorphous powders has transitioned from particle‐scale to macroscopic temperature. The sample temperature is significantly higher than the nominal sintering temperature, promoting the contribution of viscous flow to porosity elimination. This work can provide new insights into the evolution of multi‐scale temperature distribution during spark plasma sintering and its impact on the densification behavior of amorphous powders.
Herein, titanium (Ti) and zirconium (Zr) metal powders are used as starting powders and GH4169 superalloy powder is used as a sintering aid. Spark plasma sintering process is used to prepare Ti–Zr–nickel (Ni) alloy with the fine and uniform grain size of α and β phases mixed under low‐temperature conditions of 1273, 1373, and 1573 K. The thermal–mechanical–electric three‐field coupling simulation of the sintering process of alloy powder is conducted on the basis of the voltage and current data of the discharge sintering test. Results show that experimental and simulation temperatures are consistent and extension of holding time helps improve the uniformity of temperature and relative density. In the axial direction, the temperature and relative density gradually decrease from the upper surface to the lower surface of the sample. In the radial direction, the temperature gradually decreases from the center to the edge of the sample, and the relative density of the upper surface of the sample gradually decreases from the center to the edge, with an opposite change in the lower surface. The relative density of the central layer is stable and uniformity is optimal.
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The need for fully dense material with well-engineered microstructures has led to the promising emergence of innovative sintering technologies among which the Spark Plasma Sintering (SPS) is one of the most favorite. Unlike the conventional sintering processes, SPS takes advantage of a current flow passing through the sintering die and metallic powders by which fast densification with minimal grain growth and enhanced physicomechanical properties can be obtained. Albeit there is a growing interest in the exploitation of SPS in producing sufficiently consolidated metallic parts, no analytical review has been released over the effects of SPS parameters on the densification behavior, microstructure evolution, and resultant physicomechanical properties of metallic parts and their alloys. In the present review, recent developments and ongoing challenges in modeling the SPS of metallic systems are thoroughly explored. Then, the effects of main SPS parameters including sintering temperature, dwell time, heating rate, and pressure on the microstructure and physicomechanical properties of metals and alloys are comprehensively investigated. These properties are categorized into two groups: (i) physical properties including relative density, electrical and thermal conductivities; (ii) mechanical properties with a systematic focus on hardness, elastic modulus, and tensile, compressive, and bending strengths. In each section, the general trends along which SPS parameters grow to affect each corresponding property are comprehensively discussed. Additionally, various microstructural phenomena being more likely to occur at the given metallic systems are fully addressed. The present work seeks to elaborate on the aforementioned issues and provide an overview of the unresolved challenges and proposed solutions to them.
Available publications on crystalline materials used to immobilize HLW and actinides have been analyzed. Select compounds and solid solutions covered in this paper are phosphates of the following structural families: monazite, xenotime, kosnarite (NZP),apatite and britholite, langbeinite, whitlockite and thorium phosphate diphosphate (TPD). These materials have been shown to be fit for “nature-like” disposal (except TPD), they have a wide spectrum of isomorphic substitutions of cations and/or anions, and high heat, chemical, and irradiation stability. They can be also used in geoenvironmental engineering. The studies taken into consideration follow the materials science approach that rests on the “composition – structure – synthesis – property” basis. The analyzed papers describe materials synthesized using Spark Plasma Sintering, a process that results in rapid formation of virtually poreless ceramics and improves environmental safety both at stage of HLW immobilization and during long-term storage.
Flash sintering is a very promising method with a great potential for the ultra-rapid production of objects with firing processes of mere seconds. However, the transition from few millimeters samples to larger scales is a key issue. In this study, we explore the spark plasma sintering approach allowing a stable hybrid heating in flash condition and producing near net shape sintered specimens. Two electric current configurations are employed for diameters of 15 and 30 mm to determine the effect of scale change and of electric current concentration. These experiments coupled with a Multiphysics simulation indicate that stable flash conditions can be reached for 30 mm specimens. However, even if the electrical current concentration is very effective at small sizes, it generates peripheral hot spots for large specimen dimensions. The blackening effect on zirconia flash specimens acts then as an indicator of the specimen thermal history.
The coupled electrical-thermal-mechanical finite element method in the continuum scale has been widely used to investigate the spark plasma sintering process. An accurate constitutive model of powder material is pivotal for precise continuum finite element simulation. In this study, the Drucker-Prager-Cap model, which is highly accurate in describing the densification behaviour of powder material, was adopted to numerically analyse the spark plasma sintering process of boron carbide powder. First, the parameters of the model were defined to be dependent on temperature and density for higher accuracy; they were determined by minimising the discrepancy between the simulated and experimental results. Based on a spark plasma sintering experiment with a cylindrical sample, the parameters of the Drucker-Prager-Cap model were identified at 1500 °C, 1600 °C, 1700 °C, 1800 °C, and 1900 °C. A coupled electrical-thermal-mechanical finite element simulation with the model was performed for spark plasma sintering of boron carbide powder at 1750 °C and 1850 °C. The temperature, stress, and relative density were investigated numerically. By comparing the model results with the temperature and relative density measured in the experiment, the continuum finite element method with the Drucker-Prager-Cap model was validated.
Ultra‐high temperature ceramics (UHTCs) are a group of advanced ceramic materials that possess excellent high temperature capabilities, which make them especially suitable for extreme environment engineering applications. As an effective assembling method, joining is frequently required for fabricating sophisticated structures for such applications due to the excessive challenges and costs in producing near‐net shapes. Here, we introduce a promising new joining technique to effectively join UHTCs called Instantaneous Nanowelding, which uses direct electric current assisted rapid Joule heating to instantaneously bond hafnium diboride (HfB2) to zirconium diboride (ZrB2) in 1 s down to atomic scale. Our approach is analogous to high temperature spot welding, and the entire process is complete in 10 min, and the instant diffusion occurs in 1 s. Seamless HfB2/ZrB2 interfaces are formed at 1750 for a duration of 1 s. A series of characterizations are done at the interfaces using techniques including SEM, WDS, EBSD, and S/TEM to observe ZrxHf1−xB2 solid solution formation. Highly coherent transition with perfect lattice alignment at atomic scale from ZrB2 to HfB2 is observed using S/TEM, meaning that the two materials are brought to atomic contact.
Spark plasma joining of nanostructured ferritic 14YWT alloys has been carried out at 800°C, 1020 °C and 1030 °C in vacuum. The characteristics of joining as a function of temperatures were investigated using bulk density, nanoindentation and electron backscattered diffraction techniques. Joining performed at 1020 °C and 1030 °C shows a complete, uniform bonding of the samples, whereas the samples joined at 800 °C showed a visible crack that runs along the entire sample length. Bulk density measurements indicate that the reduction in density of samples joined at 800 °C is due to the presence of voids at the interface. The presence of columnar grains only close to the interface at 800 °C suggests the possibility of a steep temperature rise locally at the interface that is attributed to high electrical contact resistance at the interface.
In this work, the synthesis, characterization and mechanical properties of sintered In situ Ni-based quaternary superalloy (Ni–9Fe–22Cr–10Co) were studied. All the Ni–9Fe–22Cr–10Co quaternary alloy specimens were consolidated via spark plasma sintering technique at temperature range and sintering pressure of 850–1100 °C and 50 MPa, respectively. The densification of sintered alloys was obtained through Archimedes technique while the micro-indentation hardness of the alloys was conducted using Vickers microhardness tester. The specimens were characterized using the optical microscope and scanning electron microscope equipped with energy-dispersive spectroscopy while the phase identification was carried using X-ray diffractometer. The microstructural characterization results revealed that the relative density, surface morphology as well as the micro-indentation hardness properties depend on the sintering temperature. In addition, it was observed that high sintering temperature aided grain refinement and bulk compaction of the material leading to high relative density. XRD analysis shows the formation of Ni–Fe and Co–Fe as the major phases. Generally, the densification and grain size of the alloys increased with increasing sintering temperature with optimum results obtained at a temperature of 1100 °C and sintered density of 98% while the maximum Vickers hardness value of 439.17 HV1 was recorded.
Ultrafine grain tungsten heavy alloys (WHAs) were successfully produced from the nanocrystalline powders using spark plasma sintering. The present study mainly discussed the effects of sintering temperature on the density, microstructure and mechanical properties of the alloys. The relative density of 98.12% was obtained at 1 050 °C, and the tungsten grain size is about 871 nm. At 1 000 °C-1 200 °C, the mechanical properties of the alloys tend to first rise and then goes down. After SPS, the alloy exhibits improved hardness (84.3 HRA at 1 050 °C) and bending strength (987.16 MPa at 1 100 °C), due to the ultrafine-grained microstructure. The fracture mode after bending tests is mainly characterized as intergranular or intragranular fracture of W grains, interfacial debonding of W grains-binding phase and ductile tearing of binding phase. The EDS analysis reveals a certain proportion of solid solution between W and Ni-Fe binding phase. The good mechanical properties of the alloys can be attributed to grain refinement and solid solution strengthening.
Sample temperature and neck growth were investigated on pulse-current pressure sintering of coarse cast-iron powder (diameter: 200 μm) produced by a gas atomizing technique. At 873 K of die temperature, the temperatures of sample inside and its surface were 950 K and 910 K, respectively. Microstructure observation of the sintered cast-iron revealed that there were no melting and remarkable deformation around the grain interface. Neck growth was governed by plastic deformation during the initial stage of the process. A model calculation of neck growth by assuming the plastic deformation of particles is in good agreement with the experimental results. The results indicate that the initial stage on pulse current pressure sintering is similar to conventional hot-press sintering for coarse cast-iron powder.
The paper presented a brief theoretical temperature distribution in the sample and the die in spark plasma sintering (SPS). It aimed to point out that, for high conductance and low heat conductivity specimen, temperature-difference in sample of SPS system is inevitable; for good conductivity specimen the temperature-difference will be small. Experiments proved the existence of such a temperature difference and showed some interesting results about the electric insulated sample.
The pulse current pressure sintering and hot press process were applied for sintering of aluminum powder produced by water and gas atomizing methods. The pulse current pressure sintering process could densify the water atomized powder in a short time compared with the hot pressing because it could remove the H-2 gas at lower temperature. The gas prevents sintering of aluminium powder. The tensile strength of the pulse current pressure sintered gas atomized powder which contains low H-2 was higher than that of hot pressed specimen. The electric resistivity of sintered specimens obtained by pulse current pressure sintering process was lower than that of hot pressed specimen. The oxide layer destruction was confirmed by this measurement of electric resistivity. The oxide layers were supposed to be fractured by the high temperature region which is attributed to Joule heat by contact resistance.
A comprehensive discussion of heat transfer by thermal radiation is presented, including the radiative behavior of materials, radiation between surfaces, and gas radiation. Among the topics considered are property prediction by electromagnetic theory, the observed properties of solid materials, radiation in the presence of other modes of energy transfer, the equations of transfer for an absorbing-emitting gas, and radiative transfer in scattering and absorbing media. Also considered are radiation exchange between black isothermal surfaces, radiation exchange in enclosures composed of diffuse gray surfaces and in enclosures having some specularly reflecting surfaces, and radiation exchange between nondiffuse nongray surfaces. The use of the Monte Carlo technique in solving radiant-exchange problems and problems of radiative transfer through absorbing-emitting media is explained.
Thermal management systems consist of external cooling mechanisms,
heat dissipaters, and thermal interfaces. The primary function of heat
dissipaters, e.g. heat sinks, is to create the maximum effective surface
area where heat is transferred into and removed by the external cooling
medium. Heat dissipater performance is characterized by its intrinsic
thermal conductivity, physical surface area, and pressure drop (or drag)
coefficient (Kraus and Bar-Cohen, 1995). Another variable, the heat
spreading coefficient, introduced by Tzeng et al (PCIM, 2000), must be
considered when the heat dissipater is a thermally anisotropic material.
A high degree of thermal anisotropy reduces the temperature gradient in
the component plane and increases effective heat transfer area,
characteristics that are most desirable for electronics with high
heat-intensity components. The ability to direct heat in a preferred
direction is a further advantage of anisotropic heat-spreader materials.
Carbon and graphite-based materials are attracting interest as
anisotropic heat-spreaders, with another advantage being their low
density. Most carbon and graphite-based materials used to date are based
around carbon fibers. These are high cost due to the need for high
temperature graphitization processes to develop the required fiber
thermal properties. A new form of graphite heat-spreader material is
described in this paper, based around naturally occurring graphite.
Since this material has been graphitized by nature, anisotropic
heat-spreaders with high thermal conductivity can be manufactured
without carbon fiber-based additives
This book is a completely revised and rewritten edition of "Electric Contacts Handbook" published in 1958. A large number of new in vestigations are considered, and many of the basic theories are revised in detail and even in general. The body of information had to be limited as it was not advisable to increase the volume of the book. In particular, no attempt was made to cover all of the practical applications. They appear as examples following concentrated explanations of basic phenomena. As in several branches of technology, the solutions of problems ari sing in the field of electric contacts involve insight into various disci plines of physics. It is feit that reviews of some of those topics, especi ally adapted to electric contact phenomena, are welcome to many readers. For example, chapters have been devoted to the structure of carbon, the band theory of electric conduction in solids, certain pro blems in statistics, and the theory of the electric arc. As regards arc problems, new ideas have been introduced. In order to make the main text less cumbersome, such reviews are presented as appendices. Throughout this edition, the mksa-unit system is used in accord with the latest recommendation for standardization of units in scientific and technical writings. The chapter "History of Early Investigations on Contacts" forming Part IV in the preceding edition of 1958 has not been repeated in this book.
Instrumented pulse electro-discharge consolidation of mechanically alloyed amorphous TiAl powder is proposed as a method for synthesizing a high-strength titanium aluminide compact composed of fully densified nano-scaled structure by controlling the process variables of temperature, stress and pulse electric current.
We obtained a fully densified compact consisting of titanium aluminide{γ(TiAl)+α2(Ti3Al)} with the dispersion of an island-shaped α (Ti) phase and Al3Ti particles using a relatively low stress of 29 MPa with an electro-discharge consolidating time less than 600 s. The consolidating temperature necessary to obtain a fully densified TiAl compact decreases from 1312 to 1051 K with increasing applied stress from 29 to 147 MPa. Such a decrease is conducive to the great decreases in size of titanium aluminide grains and Al3Ti second phase particles. Moreover, we found that the fully densified compact of titanium aluminide has an increasing Vickers hardness up to 1050 DPN at 1100 K, following a decrease with decreasing consolidating temperature.
The process viscosity(η) for pulse electro-discharge consolidation of amorphous TiAl is fairly well expressed by an Arrhenius-type equation of η=ηoexp(HV⁄kT) with 87-115 kJ mol⁻¹ for activation energy(HV), which is smaller than 339 kJ mol⁻¹ for specimen viscosity, and a significantly decreasing ηo with increasing applied current. These results indicate that pulse electro-discharging strongly accelerates the rate of the densification via viscous flow in an amorphous TiAl powder compact.
December 18, 1973 Master and Servant — Redundancy — Dismissal for redundancy — Employee contracted to work at any of employers' establishments — Employee's refusal to move place of work — Dismissal — Whether dismissal by reason of redundancy — “Where he was so employed” — “Place in which he would be employed” — Redundancy Payments Act, 1965 (c.62), ss. 1 (2) (b), 2 (3).
Silicon carbide ceramics which 5 mass% Al2O3 and 2 mass% Y2O3 were added to were prepared by the spark plasma sintering (SPS) method under the conditions of 30 MPa and 5 min. Mechanical properties at room temperature were eximined. The SPS brought dense silicon carbide ceramics at a sintering temperature of 1800°C, which was about 200°C lower than that of the hot-pressing process. The silicon carbide obtained by SPS had higher strength and fracture toughness than those obtained by hot-pressing. The results suggest that the inside temperature of the sintered bodies during spark plasma sintering was higher than the measured temperature.
Alumina ceramics were fabricated by spark-plasma-sintering (SPS) and hot-pressing at 1100 to 1600°C under 30 MPa for 5 min. The room temperature mechanical properties were measured. The SPS brought about dense alumina ceramics at a sintering temperature lower than that in the hot-pressing. The alumina ceramics obtained by SPS had strength, hardness and fracture toughness higher than those by hot-pressing. The alumina grains in the sintered bodies prepared by SPS grew larger than those by hot-pressing.
Specimen temperature and sintering behavior of Ni powder by spark plasma sintering (SPS) were investigated. Microstructure observation of the sintered compact revealed that the fast densification of the powder materials was due to the promotion of neck growth and plastic deformation of powder particles in the initial stage of SPS. With die temperature at 1073 K, the temperature of the specimen center was 1203 K. The temperature difference between specimen and graphite die exponentially increased with the increase of the temperature of the graphite die. When the graphite die was kept warm by a thermal insulation jacket, the temperature difference between specimen and graphite die was smaller than when using a non-insulated die, and the temperature difference lineally increased with the increase of the temperature of the graphite die.
Discharge and sintering phenomena are investigated by measuring current and voltage in pulsed electric current sintering. Fine Al powders under 10 μm in particle size and coarse Al powders over 100 μm are used. A BN die and a graphite die are used and a graphite punch is used for each die. With BN die and fine Al powders, no current flows until voltage attains 24 V. At this voltage a large current over 600 A suddenly flows. Some large pores produced by melting and evaporating are observed in the specimen. Discharge occurs when a BN die is used. This discharge can occur in the range of resistivity 10 kΩ-1 MΩ. The coarse powders can be sintered without discharge. When fine Al powders are sintered using a graphite die, the current and voltage increase simultaneously at the starting stage of sintering. The current depends on the resistivity of powders. The powder compact is sintered from the surface, and then the center region is sintered later. A current almost flows through the graphite die except for fine Al powders. The coarse Al powders can be sintered homogeneously because of current flows through the powders. No discharge occurs with graphite die, and the temperature rises by joule heating.
In spark sintering copper powder and alumina one with a graphite and insulated dies under a constant pressure and a constant electric current, electric voltage between electrodes and temperature distributions of die, punch and specimen were measured. And, we considered effect of the electric current paths on the spark sintering. In the first case of sintering the copper powder with the graphite die, during initial electricity, almost all the electric current went through the specimen. During sequential electricity, the electric current went through both the graphite die and the specimen. Therefore, fluctuation of temperature distribution in the specimen was decreased as the discharged time was spent. In the second case of sintering the copper powder with the insulated die, the electric current went through the graphite punch and the specimen. The fluctuation of temperature distribution in the specimen was larger than that of the case which the graphite die was used, for the specimen was not heated by the graphite die. In the third case of sintering of alumina powder with the graphite die, the electric current went through the graphite punch and the graphite die. Therefore, the temperature distribution of both the graphite mold and the specimen was considerably different from that of the case of the copper powder with the graphite die.
Al2O3 ceramics were superfast densified using spark plasma sintering (SPS) by heating to a sintering temperature between 1350 and 1700°C at a heating rate of 600°C/min, without holding time, and then fast cooling to 600°C within 3 min. High-density Al2O3 ceramics could be achieved at lower sintering temperatures by SPS, as compared with that by conventional pressureless sintering (PLS). The bending strength of Al2O3 superfast densified by SPS in the range of sintering temperature between 1400 and 1550°C reached values as high as 800 MPa, almost twice that obtained by the PLS. SEM observations indicated that intragranular fracture was the preponderant fracture mode in these samples, resulting in these excellent bending strength values.
A Si3N4/SiC composite with additions of La2O3, Y2O3 and SiO2 was fabricated by spark plasma sintering (SPS). The porosity and phase formation resulting by this method are compared with those obtained by sinter/hot isostatic pressing (sinter/HIPing). The open porosity was completely eliminated by SPS in 10min of heating to 1900°C and 5min dwell time. The complete α to β transformation was achieved with the formation of elongated β Si3N4 grains. The same result was achieved by sinter/HIPing at 1750°C with a dwell time of 160min. There were differences in the % β transformation from α, open porosity and the presence of minor phases between the two processes. The comparison of SPS with sinter/HIP showed the former to be a rapid fabrication process for Si3N4 which warrants further investigation to optimise the microstructure.
Superfast densification of ceramic composites in the pseudobinary system Al2O3-Y3Al5O12 was carried out by using a new technique called spark plasma sintering (SPS). Dense ceramic composites were obtained by heating appropriate powder mixtures to 1573 K in an SPS unit at a rate of 600 K/min. No holding time at 1573 K was applied. Scanning electron microscopy studies showed the compacted materials to contain submicrometer-sized grains of the same sizes as the precursor powder mixtures; i.e., no significant grain growth had occurred.
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