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Microstructure of alumina (a) sintered at 1000°C for 20 min with the SiC molding set and (b) sintered at 1150°C for 20 min with the graphite molding set. The heating rate is 50°C/min for both cases. Owing to the low etching temperature (900°C) for the SiC set, the grain boundaries in (a) are less clear. The inset is an appearance of the sintered alumina of 1 mm in thickness.
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Fully dense transparent alumina is obtained after spark plasma sintering at 1000°C by using the electrically conductive SiC molding set. Compared to the conventional graphite set, the new mold material lowered the sintering temperature by 150°C for full densification. The SiC set has a lower electrical conductivity than that of the graphite set, wh...
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Citations
... After the initial heating by the outer graphite mold, SiC gains adequate electrical conductivity and can function as a resistive heating element at high temperatures. Incidentally, the lower electrical conductivity of SiC may result in a stronger field effect during sintering which can enhances diffusion, as suggested by Kim et al. [44]. ...
Spark plasma sintering (SPS) is an advanced pressure-assisted sintering technology that combines the application of uniaxial pressure with rapid current-induced heating. The so-called high-pressure SPS (HPSPS) approach involves using specialized tooling made of robust materials that can withstand high pressures and temperatures simultaneously. The application of high pressure during the sintering process enhances densification and allows to produce materials with distinctive qualities at relatively low temperatures. This review focuses on the effects of the applied pressure on densification and the resulting functional, mechanical, optical, and physical properties. Exploring the capabilities of HPSPS for a wide range of materials. Including, but not limited to, thermally sensitive phases, nanocrystalline, ionic, bulk metallic glasses, magnetic, transparent ceramic, and composite materials, among others. The HPSPS approach not only offers a promising technique for densification, but also enables the study of fundamental aspects of high-pressure processing and various consequential materials properties.
... In the last years, SPS has been shown to be a reliable alternative technique for manufacturing transparent polycrystalline ceramics at relatively low temperatures in shorter times [4]. The principle of SPS sintering is based on the application of an associated uniaxial pressure with a pulsed direct electric current. ...
In this work, a transparent nanostructured ceramic magnesium aluminate spinel (MgAl2O4) was fabricated by Spark Plasma Sintering (SPS) from commercial spinel nano-powders at different temperatures (1300, 1350 and 1400 °C). The sintered samples were thoroughly examined to assess their microstructural, optical, and mechanical properties. Various techniques such as SEM, AFM, spectrophotometer with an integrating sphere, instrumented Vickers indenter, Pin-on-Disk tribometer, scratch tester, and sandblasting device were employed to characterize the sintered samples. The results indicated the significant impact of the sintering temperature on the properties of the spinel samples. Particularly, the samples sintered at T = 1350 °C exhibited the highest Real In-line Transmission (RIT = 72% at 550 nm and 80% at 1000 nm). These samples demonstrated the highest hardness value (HV = 16.7 GPa) compared to those sintered at 1300 °C (HV = 15.6 GPa) and 1400 °C (HV = 15.1 GPa). The measured fracture toughness of the sintered samples increased substantially with increasing sintering temperature. Similarly, the tribological study revealed that the friction coefficient of the sintered spinel samples increased with the sintering temperature, and the spinel sintered at 1350 °C exhibited the lowest wear rate. Additionally, sandblasting and scratch tests confirmed the significant influence of the sintering temperature on the mechanical properties of the fabricated spinels. Overall, the spinel sintered at 1350 °C presented the best compromise in terms of all the evaluated properties.
... In the last years, SPS has been shown to be a reliable alternative technique for manufacturing transparent polycrystalline ceramics at relatively low temperatures in shorter times [4]. The principle of SPS sintering is based on the application of an associated uniaxial pressure with a pulsed direct electric current. ...
In this work, a transparent nanostructured ceramic magnesium aluminate spinel (MgAl2O4) was fabricated by Spark Plasma Sintering (SPS) from commercial spinel nano-powders at different temperatures (1300, 1350 and 1400 ◦C). The sintered samples were thoroughly examined to assess their microstructural, optical, and mechanical properties. Various techniques such as SEM, AFM, spectrophotometer with an integrating sphere, instrumented Vickers indenter, Pin-on-Disk tribome ter, scratch tester, and sandblasting device were employed to characterize the sintered samples. The results indicated the significant impact of the sintering temperature on the properties of the spinel samples. Particularly, the samples sintered at T = 1350 ◦C exhibited the highest Real In-line Transmission (RIT = 72% at 550 nm and 80% at 1000 nm). These samples demonstrated the highest hardness value (HV = 16.7 GPa) compared to those sintered at 1300 ◦C (HV = 15.6 GPa) and 1400 ◦C (HV = 15.1 GPa). The measured fracture toughness of the sintered samples increased substantially with increasing sintering temperature. Similarly, the tribological study revealed that the friction coefficient of the sintered spinel samples increased with the sintering temperature, and the spinel sintered at 1350 ◦C exhibited the lowest wear rate. Additionally, sandblasting and scratch tests confirmed the significant influence of the sintering temperature on the mechanical properties of the fabricated spinels. Overall, the spinel sintered at 1350 ◦C presented the best compromise in terms of all the evaluated properties
... Carbon contamination can also be avoided by performing two-stage SPS [19]. SPS at a slow heating rate produces isolated pores below a temperature called "critical temperature", where carbon deposition in pores is not thermodynamically favored; consequently, carbon contamination can be minimized [13,28]. However, the two-stage SPS is time-consuming. ...
The effects of LiOH doping of magnesium aluminate spinel powders and various Spark Plasma Sintering (SPS) schedules on densification behavior and final transparency of polycrystalline magnesium aluminate spinel were studied. Two commercial magnesium aluminate spinel powders, with different specific surface areas, were doped with up to 0.6 wt.% of LiOH and consolidated using SPS with slow (2.75 °C/min) and fast (100 °C/min) heating rates. The slow heating rate was optimal for undoped magnesium aluminate spinel (LiOH-free) with the best real in-line transmittance (RIT) of 84.8% (measured at 633 nm on a disc 0.8 mm thick). For the magnesium aluminate spinel doped with 0.3 wt.% of LiOH, the fast heating rate was beneficial, and an RIT of 76.5% was achieved. μ-Raman analysis confirmed that the addition of LiOH suppressed carbon contamination.
... In comparison to the results of other works in literature, the 3D printed samples in this report were able achieve similar results. Kim et al. reported on SPS processed alumina ceramics of 1 mm thickness which achieved a maximum total transmittance of approximately 70 % at 800 nm and greater than 80 % at 1000 nm and above [38]. Yang et al. also reported on 1 mm thick transparent alumina ceramics fabricated by isostatic pressing and sintering in H 2 atmosphere which achieved a total transmittance as high as 86 % from 300 to 800 nm [39]. ...
Transparent alumina ceramics were fabricated using an extrusion-based 3D printer and post-processing steps including debinding, vacuum sintering, and polishing. Printable slurry recipes and 3D printing parameters were optimized to fabricate quality green bodies of varying shapes and sizes. Two-step vacuum sintering profiles were found to increase density while reducing grain size and thus improving the transparency of the sintered alumina ceramics over single-step sintering profiles. The 3D printed and two-step vacuum sintered alumina ceramics achieved greater than 99% relative density and total transmittance values of about 70% at 800 nm and above, which was comparable to that of conventional CIP processed alumina ceramics. This demonstrates the capability of 3D printing to compete with conventional transparent ceramic forming methods along with the additional benefit of freedom of design and production of complex shapes.
... HPSPS setups with high-pressure tooling made of tungsten carbide (WC) [5] or silicon carbide (SiC) [16] provide an effective single-stage sintering process that allows for the fabrication of transparent alumina from untreated commercial α-alumina powders [5,17,42]. To date, alumina exhibiting an in-line transmittance of about 65-69% at a wavelength of 640 nm was fabricated by HPSPS under pressures up to 500 MPa at relatively low sintering temperatures of 1000-1200 °C. ...
Transparent alumina was fabricated from untreated commercial powder by high-pressure spark plasma sintering (HPSPS) at temperatures of 1000, 1050 and 1100 °C under pressures of 250-800 MPa. It was established that transparency strongly depends on the HPSPS parameters. At all temperatures, there was a certain point when increasing the pressure led to decreasing transparency. At 1100 °C, relatively high pressure led to excessive grain growth, as well as the formation of creep-induced porosity at the center of the samples. Hardness values decreased with pressure due to grain growth, correlated with the Hall-Petch relationship. The optimal combination of optical and mechanical properties (68% in-line transmittance at a wavelength of 640 nm and a hardness value of about 2300 HV2) was achieved after sintering at 1050 °C under 600 MPa.
... They used combination of SiC and graphite as mold and performed sintering process at 300 MPa and 1100°C. In other similar work, KIM et al. [22] used SiC to fabricate transparent alumina at 1000°C with high pressure SPS process. Fu-Chi Wang et al. [13] prepared Ti-MWCNT composite at 550°C and final pressure of 300 MPa by using a refractory steel mold. ...
The vanadium carbide reinforced aluminium matrix composite has been fabricated successfully by using high-pressure spark plasma sintering at very low temperature (220 °C) for the first time. Applying 30, 150 and 300 MPa pressure have prepared three different samples. The microstructure investigation revealed layer-by-layer formation of the composite for all samples. It is observed that layer to layer interfacial bonding was perfect by applying 300 MPa during sintering process. The proposed mechanism for sintering behavior at ultra-low temperature was introduced cold welding; plastic deformation and micro spark between particles. The optimum physical and mechanical properties obtained from composites prepared at maximum applied pressure (300 MPa) showing 99% of theoretical density, 332 MPa bending strength and 264 hardness (Vickers).
Silicon carbide (SiC) is an electrical semiconductor with a wide bandgap. Recently, highly conductive liquid-phase sintered SiC (LPS-SiC) ceramics have been developed by the successful doping of N atoms into a SiC lattice. Fully dense N-doped SiC ceramics with electrical conductivity as high as 300 S·cm⁻¹ at 25 °C have been obtained. These ceramics can be formed into complex shapes by electrical discharge machining. This article reviews the influence of critical factors on the electrical conductivity of LPS-SiC ceramics, including SiC polytype, grain boundary structure, soluble atoms, porosity, second-phase addition (nitride, carbide, boride, graphene), grain size, and neutron irradiation. We also discuss the relationship between the solubility of nitrogen in the liquid phase and the electrical conductivity of the resulting ceramics, along with the mechanism for N doping in the SiC lattice and the possibility of controlling the electrical conductivity of the LPS-SiC ceramics. The N doping method used for LPS-SiC ceramics may also be applied for doping other soluble atoms in liquid-phase sintered ceramics.
In order to clarify the symmetry of sintering parameters under compressive and tensile stresses, uniaxial viscosity measurements were performed for 0.1 mol% yttrium doped ceria under tension and compression, and without and with AC electric field. Major findings are: (i) the strain rate was confirmed to be linear under compressive/tensile loading and electrical fields; (ii) as a result, for a given relative density, the uniaxial viscosity is the same under tension and compression; (iii) the uniaxial viscosity decreases under moderate electrical fields, which implies the effect of electrical fields is not restricted to a specific doping level.
During spark plasma sintering of Y2O3 ceramics, the evolution of non-uniform microstructure was observed, depending on the heating rate. At low heating rates, the appearance of the sintered bodies is relatively uniform, and the grain size is slightly larger in the center than in the periphery. At high heating rates, however, the sintered bodies are apparently non-uniform to reveal opaque center and translucent periphery, and a difference in the grain size between the center and the periphery increases remarkably. The porosity and the pore size in the center also increase with holding time. The evolution mechanism of the non-uniform microstructure is explained by using a concept of dynamic grain growth and by assuming defect diffusion from the periphery to the center under complicated electric and magnetic fields during spark plasma sintering.
