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Fabrication of energy-saving MgO with large grain size and low thermal conductivity: Towards a new type of magnesia for high-temperature furnaces
Abstract
The development of advanced refractories with low thermal conductivity and long service life is of great significance for optimizing the energy consumption structure of high-temperature industries. For this purpose, a series of magnesia refractories with large grain size, low thermal conductivity, and high slag resistance had been successfully prepared at 1600 °C, in which Al2O3 and La2O3 were chosen as the additives. The results show that with the introduction of additives, MgAl2O4 and LaAlO3 as the second phase are formed inside and at the grain boundaries of the MgO matrix grains, and further promote the growth of grains and improved certain properties. Meanwhile, the grown grains (from 4.92 to 29.51 μm, increased ∼5 times), the increased density, and the enhanced strength can be attributed to the activated sintering triggered by Al2O3; the lower thermal conductivity (from 18.49 to 15.73 W·m⁻¹·K⁻¹ at 500 °C, decreased ∼15%) and the better slag resistance can be ascribed to the formation of LaAlO3 and MgAl2O4. However, due to the Nener effect, the maximum grain size is obtained at additives of 4 wt%, but the thermal conductivity is not limited by this.
... Especially in recent years, energy-saving and emission-reduction policies have been paid more and more attention worldwide, and the magnesia industry give corresponding measures. In fact, in addition to continuing to optimize the whole process of fused magnesia [10], further improving the performance of sintered magnesia, that is, preparing high-quality sintered magnesia, is a more effective strategy [12]. ...
... On an engineering level, engineers have minimized the adverse effects of aggregates by using two-(calcination → ball milling → sintering) and three-step (calcination → hydration → ball milling → re-calcination → re-ball milling → sintering) sintering methods [16]. On a technical level, researchers have identified and reported many effective sintering aids, such as CeO 2 [13], La 2 O 3 [12], TiO 2 [17], Y 2 O 3 [18], and ZrO 2 [19]. However, most of the sintering aids become new impurities in the magnesia, as well as being potentially harmful to its high temperature properties. ...
... where D b is the bulk density of the test sample, and D M is the theoretical density of MgO (3.58 g cm − 3 ); P a and P t are the apparent porosity and true porosity of the test sample (%) [12,22]. sintering method (pre-calcination + post-sintering) are reduced, the overall sintering degree is still insufficient (Fig. 2b); as expected, after adding the ball milling process on the basis of the two-step sintering method, almost no obvious open pores are observed on the surface of sample MB10, and its grains are well developed and tightly combined with each other (Fig. 2c). ...
Giving magnesia a denser microstructure is a goal relentlessly pursued by refractory scientists and related entrepreneurs. In this study, the effects of the key factors of ball milling, calcination temperature, and starting raw material (high-purity magnesite) size on the densification and grain growth behaviour of sintered magnesia were systematically investigated using a typical two-step sintering method. The results revealed that ball milling reduces the size of light-burned magnesia (pre-calcination product) to a certain extent, which is reflected in a higher tap density; moreover, both the density and grain size of the magnesia samples rise with increasing calcination temperature, implying that the activity of light-burned magnesia is not the only controlling factor for densification; in addition, the smaller magnesite particle size improves the density and grain size of the magnesia samples by weakening the negative effect of the pseudomorphic aggregates. More adequate ball milling, more moderate calcination temperature, and smaller size of magnesite (also for other magnesium salts), therefore, can be the preferred process for the two-step sintering preparation of sintered magnesia.
... where L a and L b are the lengths of the samples after and before sintering, P a and P t represent the apparent and true porosities of the sintered samples (%), D b and D r indicate the bulk density (g cm − 3 ) and relative density (%) of the sintered samples, D ti and R i denote the theoretical density (MgAl 2 O 4 , 3.58 g cm − 3 ; LaAlO 3 , 6.52 g cm − 3 ) of phase i and its proportion in the test samples, respectively [35]. Moreover, the sessile drop method was used to study the corrosion/ adhesion behavior between the cement clinker (CaO 66.23%, SiO 2 22.21%, Al 2 O 3 4.62%, Fe 2 O 3 3.55%, MgO 2.31%, K 2 O 0.68%, TiO 2 0.31%, K 2 O 0.09%) and the prepared samples. ...
... Fig. 3c demonstrates that the closed porosity of the samples first decreases from 2.52% (MA) to 1.86% (MA11) and then increases to 4.35% (MA15). In general, the formation of closed-pore structures and the increase of the closed porosity are due to the growth of grains, which represent the final stage of the sintering process of ceramics [35]. Moreover, the increased bulk density (from 2.51 to 3.01 g cm − 3 , Fig. 3d) and the corresponding decreased relative density (from 70.21% to 83.66%, Fig. 3e) also confirm that the applied pre-sintering treatment is beneficial to the subsequent densification of the samples. ...
In this study, effects of pre-sintering temperature and subsequent La2O3 addition on the densification of magnesium aluminate spinel (MgAl2O4) were investigated. Further, the role of La2O3 was evaluated in terms of sintering/mechanical properties and corrosion resistance. Results reveal that two-step sintering achieves higher relative density than one-step pressureless sintering (from 70.21% to 83.86%) because it avoids volume expansion effect of MgAl2O4, but high pre-sintering temperature leads to a decrease in the activity of the raw material (for post-sintering). Subsequent experimental data show that the introduction of La2O3 endows the sample with greater relative density (from 91.24% to 95.65%), which is attributed to the formation of LaAlO3 and defect activation effect caused by stoichiometric mismatch. Besides, corrosion resistance of MgAl2O4 gets improved due to the formation of 2CaO·4La2O3·6SiO2, CaO·2Al2O3, and CaO·6Al2O3.
... Magnesia-based refractories are widely used as lining of steel ladles and cement rotary kilns due to their superior slag corrosion resistance and high-temperature mechanical properties. [1][2][3][4][5][6][7] Conventional magnesia-based refractories prepared by sintered or fused dense aggregates as raw materials, have high thermal conductivity and bulk density, leading to excessive heat loss through refractory lining. To mitigate the heat loss, microporous aggregates were used to prepare the lightweight refractories for working linings, instead of dense aggregates. ...
The effect of spinel powder on the fracture behavior and mechanical properties of lightweight magnesia‐based refractories containing microporous magnesia aggregates with high apparent porosity (37.4%) were investigated by the wedge splitting test (WST) with the digital image correlation and acoustic emission. With the addition of spinel powder, lightweight magnesia spinel refractories showed a higher cold compressive strength compared with lightweight pure magnesia refractories. From the WST, the addition of spinel powder increased the specific fracture energy and characteristic length of lightweight magnesia spinel refractories, which improved the crack propagation resistance. The increased tortuosity of main crack and a higher ratio of crack propagation along the aggregates/matrix interface were main reasons for reducing the brittleness of lightweight magnesia spinel refractories. Besides, acoustic emission (AE) signal activity indicated that the propagation of pregenerated micro‐cracks by the thermal mismatch and the development of fracture progress zone were primary ways to consume energy in lightweight magnesia spinel refractories. The reduced proportion of crack propagation within aggregates was also detected by the peak frequency of AE signals in lightweight magnesia spinel refractories. For microporous magnesia aggregates with high apparent porosity (37.4%), lightweight magnesia spinel refractories also showed reduced brittleness fracture behavior than lightweight pure magnesia refractories.
... It is worth noting, however, that for refractory materials used at high temperatures, the microstructure of highly densified with reasonably larger grains may be desirable because of its better hightemperature creep properties and slag resistance (Mohapatra and Sarkar, 2007;Ren et al., 2022). Therefore, it becomes apparent that previous sintering research has paid little attention to, on the one hand, addressing the issues associated with the sintering of largesized pellets and, on the other hand, the controllability of grain size growth. ...
Magnesia densification is essential for Mg-based refractories and materials to attain better thermal performance and longer service life. Few industrial-scale processes can produce magnesia with the required bulk density of above 3.40 g/cm3 (or relative density of over 95 %) from natural magnesites. The inability to achieve the desired densification is related to the low heat transfer between gas and solids inherent in large-sized greenbodies typically sintered in industrial shaft kilns. This study proposes sintering particles of 0–6 mm in primary sizes corresponding to those employed in most refractory end-products of sintered magnesia. Microstructures of the sintered particles are analyzed, and the underlying sintering mechanisms are discussed. The sintering of small particles can achieve over 95 % of densification at 1300– 1600 °C for less than 10 min. This study provides an alternative approach to producing high-density magnesia with the potential for substantial reductions in energy consumption and carbon emissions.
Lightweight refractories for the working lining of high-temperature furnaces play an important role in the smelting of advanced steels and superalloys. To prepare lightweight refractories for the working lining of high-temperature furnaces, the synthesis of lightweight aggregates is the basis. Recently, the research on the synthesis of lightweight aggregates with high service temperature, low thermal conductivity, high strength, and good slag resistance has received widespread attention. The available literature on the synthesis of lightweight aggregates was summarized, including corundum, mullite, mullite–corundum, spinel, corundum–spinel, cordierite, cordierite–mullite, calcium hexaluminate, corundum–calcium hexaluminate, bauxite, magnesia, magnesia-based, and forsterite-based aggregates. Finally, the future development trend of lightweight aggregates was proposed.
Strict requirements on low-carbon and environmentally friendly steelmaking process promote the application of the novel microporous magnesia with excellent thermal insulation as a promising refractory. This study performed static and dynamic slag resistance tests of the microporous magnesia in two tundish covering fluxes to explore its erosion and corrosion behaviors under different conditions. The fused magnesia castable was used as the control group. The phenomena were thoroughly analyzed according to macroscopic and microscopic morphology observations, supplemented with thermodynamic calculation and CFD simulation. The results showed that, castables were severely damaged through wear and peeling in the dynamic slag resistance tests, which process was enhanced by the fast dissolution of MgO. The microporous magnesia possessed stronger slag resistance since its occurrence time for wear was delayed over 5 minutes compared to fused magnesia castable, and the wear rate was lower. In addition, the microporous magnesia was less affected by the dissolution of MgO due to the formation of a spinel layer on the surface, which hindered the slag penetration and mass transfer. Finally, a Sherwood correlation was obtained based on the experimental data, which allowed one to calculate the wear rate of MgO castable at slag line in actual tundish.
The article is aimed at evaluating the promising approach to the management of the performance characteristics of ceramic refractories based on magnesia-aluminate spinel and periclase made from natural components extracted from the Satka deposit. The essence of the approach is the mechanical activation of initial coarse powders to decrease the mean particle size and dispersion, as well as to clean particle surfaces, and the variation of sintering parameters to improve the quality of adhesion between particles during sintering. It is shown that these changes in production technology allow for achieving a significant superiority of refractories obtained from activated powders over industrial samples made of powders not subjected to this kind of treatment. The obtained materials particularly have multiple higher strengths and fracture toughness. The thermal conductivity and thermal shock resistance of the obtained materials are not inferior to or superior to the corresponding characteristics of industrial refractories of similar phase composition. The analysis showed that the mechanical and thermophysical characteristics of ceramics synthesized from mechanically activated natural powders are not inferior to those of ceramics synthesized in laboratory conditions from pure precursors. The results of the study indicate a possibility of significant improvement of the mechanical and thermal characteristics of industrial periclase-spinel refractories by additional mechanical activation of mixtures of coarse powders obtained from mined natural components.
Refractories have a significant impact on the cleanliness of molten steel, especially when utilized as ceramic filters. To explore the feasibility of applying rare‐earth oxides in ceramic filters, the interactions between CeO 2 /CeAlO 3 functional refractories and steels with different Al contents are explored, and compared with the microporous magnesia refractory. In the test of CeO 2 refractory, the total oxygen (T.O) content significantly increases, while the dissolved Al content obviously decreases in 38CrMoAl steel (0.876 wt% [Al]). Furthermore, the T.O contents in 38CrMoAl and X70 steel (0.040 wt% [Al]) in the test of CeAlO 3 refractory are 22.9% and 70.9% of that in the test of microporous magnesia refractory, respectively. Meanwhile, the loss of dissolved Al in steel caused by the interaction between CeAlO 3 refractory and steel is relatively small. Therefore, CeAlO 3 is relatively stable after contact with molten steel and promising to be applicated in the ceramic filter for improving the cleanliness of molten steel.
Al2O3–Cr2O3 refractories have excellent slag corrosion resistance and can adapt to the oxidation/reduction atmosphere in the smelting reduction ironmaking furnace. Improving the corrosion resistance of chrome corundum refractories is of great significance for extending the life length of industrial kilns and decreasing carbon emissions. In this work, the influences of ZrO2 content on the phase evolution, microstructure, physicochemical properties, mechanical properties and slag corrosion resistance of the ZrO2 reinforced Al2O3–Cr2O3 composite refractories were investigated. The results show that the ZrO2 reinforced Al2O3–Cr2O3 composites are primarily composed of (Al, Cr)2O3 solid solution and m-ZrO2. The compressive strength, flexural strength and hardness of the composite refractories can be improved by adding 2.5 wt% ZrO2. However, adding more ZrO2 will reduce its mechanical properties. This is due to ZrO2 excessive content will augement the porosity of the composite refractories, and the phase transformation of ZrO2 will also lead to the formation of microcracks during the cooling process. It is worth noting that higher ZrO2 content improves the thermal shock resistance of the composites, which is primarily put down to the microcrack toughening mechanism of ZrO2. The corrosion tests show that the thicknesses of the corrosion and infiltration layers of composite refractories are significantly decreased accompanied by ZrO2 content increase. This is because the formation of high melting point CaZr4O9 reduces the CaO content and slag alkalinity, and inhibits the dissolution of Al2O3 into the slag to a certain extent. Therefore, the slag corrosion/penetration resistance of the of the composites are obviously improved with the increase of ZrO2 addition.
Free CaO containing magnesia has limited application due to poor resistance to hydration, slag penetration and thermal shock. This work aimed to improve the performances of such magnesia by introducing microscale ZrSiO4 powder. The results showed that the relative density of the specimens reduced from 92.1% to 90.9% when adding 1 wt% ZrSiO4 due to formation of Ca2SiO4 and CaZrO3, but then gradually increased to 96.2% with further increasing ZrSiO4 to 7 wt% owing to the enhanced mass transfer and MgO grains growth caused by generation of c-ZrO2 and CaMgSiO4. The addition of ZrSiO4 obviously improved hydration resistance of the specimens, whose weight gain rate after the hydration test reduced significantly from 0.38% to 0.01% as the ZrSiO4 increasing from 0 wt% to 7 wt%. Thermal shock resistance (TSR) of the specimens reduced slightly when adding 1 wt% ZrSiO4 owing to the presence of more irregular pores, but then improved dramatically with further increasing ZrSiO4 due to the reduction in thermal expansion coefficient and occurrence of crack deflection. Besides, slag penetration was accelerated in the specimen with 1 wt% ZrSiO4 due to its lowered densification, while further increasing ZrSiO4 effectively improved the slag penetration resistance, attributing to the increase in MgO grain size and density and formation of the CaZrO3 containing dense layer preventing further slag infiltration.
To improve the service life of periclase-forsterite refractories, it is important to develop aggregates with high thermal shock resistance. In this study, periclase-forsterite aggregates with good resistance to thermal shock and micro-nanopores were prepared using high-silicon magnesite, silica, and silica sol. Microcracks were generated in the multiphase aggregates, which inhibited the continuous propagation of cracks during thermal shock through mismatched thermal expansion coefficients. Based on Hasselman's thermal shock stability factor, the reduction in the average thermal expansion coefficient and improved mechanical characteristics were critical factors in improving the thermal shock resistance of the multiphase aggregates. As a binder, silica sol provided nano-SiO2 and superplasticity, which facilitated the formation of micro-nanopores and strengthened the combination of the various phases in the aggregates.
Resource utilization of industrial solid waste can be a preferred solution to waste management. This study proposes a new strategy to prepare high closed porosity foamed ceramic with coal gangue waste as the major material. The chemical composition and crystalline phase of the starting material, formation mechanism and the effects of sintering process parameters on the pore structure and properties of the samples are discussed. The results show that the density, compressive strength, and true porosity of the foamed ceramic are 0.22 g/cm³, 1.51 MPa, 91.38%, respectively, at the optimum sintering temperature and time of 1200 °C and 30 min. The high closed porosity of 83.95% compared to conventional foamed ceramics is a very favorable factor for the reduction of thermal conductivity and water absorption, resulting in a low of 0.11 W/(m·K) and 0.68%, respectively, which can be used as a cost-effective inorganic thermal insulation material in the building industry.
Owing to the periodic mechanical stresses frequently suffered by refractories applied in cement rotary kilns, the study of fracture behavior is particularly significant for long life, safety, and reliable operation. In this work, bauxite aggregates and lightweight mullite aggregates with and without glass ceramic coatings were utilized in mullite-SiC refractories, and the fracture properties of the refractories were investigated and compared using the Brazilian test, digital image correlation technology, crack propagation path analysis, etc. The results show that the application of a glass ceramic coating on the surface of lightweight aggregates strengthens the interface between the porous mullite aggregates and the matrix, not only improving the compressive and tensile strength of the material but also increasing the number of crack branches and further improving fracture toughness. The compressive strength, tensile strength, and tensile displacement of refractories with 50 vol% coated lightweight aggregate increased by 53.1%, 67.6%, and 42.3%, respectively, compared to traditional mullite-SiC refractories.
In this study, lightweight thermal insulation materials were successfully prepared in this study using a high-temperature micro-foaming method at 1200°C, where granite waste was the main raw material and SiC was the foaming agent. Further, the thermal insulation material’s macrostructure, microstructure, room temperature, and high-temperature performance have been investigated. This material has a small closed-cell porosity of up to 73.8%, high compressive strength of 18.2 MPa, low thermal conductivity of 0.22 W/(m•K) at room temperature and 0.17 W/(m•K) at 350°C, positive reheating linear change, and the refractoriness under load of 812°C, confirming that these materials can be used as good thermal insulation materials up to ~800°C.
Partial/full replacement of traditional dense refractory aggregates (for furnace lining) with lightweight aggregates is considered to be an effective and promising strategy for energy saving and emission reduction. In this work, lightweight magnesia refractory ceramics with tailored closed porosity were prepared by one-step sintering at 1600 °C from high-purity magnesite added with silicon kerf waste in different ratios, with the emphasis on the evolution of their phase compositions, micromorphologies, and various properties. With the addition of additives, Mg2–xFexSiO4 solid solution was formed at the grain boundaries of the MgO matrix, and then the volumetric expansion effect (interfacial reaction) and activated sintering effect (vacancy defect) promote the decrease of apparent porosity and the increase of closed porosity, respectively. Consequently, MgO ceramics with apparent/closed porosity of 0.6%/6.5%, bulk density of 3.25 g cm⁻³, and lightweight index of 7.8% were successfully prepared, suitable for the working lining of metallurgical furnaces.
This work contains the results of investigations into the influence of titanium oxide (TiO2) addition on the properties of refractory magnesia ceramics. The presented research involved adding titanium oxide in a classic way, i.e. directly to the ceramic mix. The conducted laboratory tests revealed a significant impact of this oxide on the properties of refractory materials. Addition of a small amount of TiO2 favoured the ceramic mix sintering whereas adding a bigger amount—more than 10 wt% resulted in the formation of refractories characterised by considerable porosity and low mechanical strength. Addition of this oxide also slightly improves the corrosive resistance of refractories.
Sintered MgO was prepared by using MgO powder with different particle sizes by calcination of by crystal magnesite as the raw material and CeO2 powder as an additive. In addition, the effect of the CeO2 addition on the performance of the sintered MgO was investigated. The results showed that (i) the densification of the sintered MgO with a small amount of CeO2 addition appreciably promoted, (ii) the maximum bulk density reached 3.48 g cm⁻³, and (iii) the apparent porosity was 0.35%. In the sintering process, the Ca²⁺ could approach into CeO2 lattices, forming O²⁻ vacancy at CeO2 grains, which resulted in promoting the growth of CeO2 grains filling the pores between the MgO grains, and enhanced the densification of sintered MgO. In addition, the solid solution reaction limited the formation of silicate phase in MgO, improving the bonding degree between the MgO grains.
In this study, lightweight alumina containing nanoscale intracrystalline pores was fabricated by introducing zirconia sol and alumina sol into α‐Al2O3 micropowder. The effects of zirconia sol on the sintering behavior of lightweight alumina were also investigated. With the introduction of zirconia sol, the grain growth and phase transformation of nano‐alumina are delayed. Therefore, the structure of the nanoscale pores between nanoparticles can be stabilized during heat treatment, and numerous nanoscale intracrystalline pores with diameters in the range of 50‐300 nm are trapped in the alumina grains. Because of the existence of these nanoscale intracrystalline pores, the thermal conductivity of lightweight alumina decreases significantly. Lightweight alumina with the addition of 0.75 wt% zirconia sol exhibits 31% lower thermal conductivity compared to alumina without zirconia sol.
To efficiently reduce heat loss in high-temperature furnaces, the use of a working lining with low thermal conductivity, in lightweight refractories is a significant development. Conventional lightweight refractories focus on the fabrication of Al2O3-based, spinel-based, or Al2O3-spinel based refractories with micro-sized closed pores. In this study, lightweight magnesia-based aggregates with smaller nano-sized pores were fabricated by the decomposition of magnesite by using nano-sized Al2O3 and ZrO2 as additives. The lightweight magnesia containing nano-sized intracrystalline pores (100–300 nm) had a relatively low thermal conductivity of 4.539 W⋅m−1K−1 at 500 °C with a bulk density of 3.37 g/cm3 and a closed porosity of 4.3%. Moreover, the formation mechanism of nano-sized intracrystalline pores was proposed, and the effect of nano-sized additives on the sintering properties was discussed. We concluded that nano-sized Al2O3 and ZrO2 raise the number of nano-sized intracrystalline pores by increasing their migration distance required to separate from the magnesia grains. With the joint addition of nano-sized Al2O3 and ZrO2, the lightweight magnesia possessed the lowest thermal conductivity, as well as excellent strength, owing to the generation of intergranular MgAl2O4 spinel. Furthermore, the nano-sized Al2O3 and ZrO2 also promoted the sintering of magnesia resulting in the formation of cation vacancies (VMg⁎⁎). Keywords: Lightweight refractories, Magnesia, Nano-sized intracrystalline pores, Sintering
Magnesium aluminate (MgAl2O4) spinel (MAS) is a synthetic material with cubic crystal structure and excellent chemical, thermal, dielectric, mechanical and optical properties. These properties made MAS an indispensable material for optically transparent windows, domes and armours, and for certain refractory applications. High processing cost of dense MAS ceramics is in the main responsible for its limited usage in certain important applications despite its excellent performance in them. The volume expansion (∼8%) associated with MAS phase formation from alumina and magnesia does not allow obtaining dense MAS bodies in a single-stage reaction sintering process. Therefore, dense MAS bodies are made by following a double stage firing process, which is expensive. The existing literature suggests that the processing cost of dense MAS ceramics could be reduced to a great extent by decisive selection of starting raw materials, powder processing and densification conditions, and by understanding the underlying mechanisms of MAS formation and densification. Since there is no review article covering the complete comprehensive information of MAS, an attempt is made to write this review article with a main perspective of synthesis, processing and important applications of MAS and its utility for certain future emerging novel and innovative applications.
MgO-in situ spinel substrate was prepared at 1773 K (1500 °C) using colloidal alumina suspension and coarse MgO as raw materials. While the addition of 10 mass pct colloidal alumina had limited effect, addition of 20 mass pct colloidal alumina into the MgO matrix improved greatly the resistance of the substrate against the slag penetration at 1873 K (1600 °C). The improvement was found to be mainly related to the formation of solid phases, CaO·Al2O3 and CaO·MgO·Al2O3 at the grain boundaries due to slag-spinel reaction. Putting the reacted substrate into contact again with new slag revealed no appreciable new slag penetration. The results showed a potential solution to improve the resistance of MgO-based refractory to slag penetration and to improve steel cleanness.
In order to achieve high-quality and stable production of special steel, the performance of low-carbon MgO-C refractories needs to be further optimized. For this purpose, low-carbon MgO–Al2O3–La2O3–C refractories with enhanced thermal shock resistance and slag resistance were designed and successfully prepared by introducing Al2O3 as a reinforcer and La2O3 as a modifier. The results showed that the refractory samples with additives show better overall performance than those without additives. When 10 wt.% of Al2O3 and La2O3 were added, the oxidation resistance, thermal shock resistance and slag resistance of the refractory samples coked at 1400 °C are increased by 13.57%, 17.75% and 43.09%, respectively. The analysis found that this can be mainly attributed to the formation of MgAl2O4, Mg2SiO4, and 2CaO·4La2O3·6SiO2 and the consequent volume expansion effect and intergranular phase enhancement effect. Therefore, a low-cost and enforceable reinforcement strategy for low-carbon MgO-C refractories is proposed, which is expected to be applied in steelmaking.
In this study, high-temperature foaming was used to successfully prepare foamed ceramics with high closed-pore porosity and high strength using granite waste as the main raw material, SiC as the foaming agent, and B4C waste as the flux. The effects of the amount of B4C waste on the microstructure and physical properties of foamed ceramics were thoroughly investigated. Experimental results showed that under optimal preparation parameters, foamed ceramic has a low bulk density of 0.48 g/cm³, available compressive strength of 4.97 MPa, the thermal conductivity of 0.18 W/(m·K), and high closed-pore porosity of 74.2%. The addition of a trace amount of B4C waste can significantly reduce firing temperature and energy consumption. Overall, these results indicated that the preparation of foamed ceramics is a promising strategy for recycling industrial waste into new types of insulation materials.
Two types of MgO-C refractories with tight particle grading and non-tight particle grading were Two types of MgO-C refractories with tight particle grading and non-tight particle grading were prepared according to Andreasen's continuous packing theory. Fracture behaviors were investigated using wedge splitting tests combined with digital image correlation method and acoustic emission techniques. The results indicated that MgO-C refractory with non-tight particle grading treated at 1400 ℃ had more in situ phases (e.g., AlN and MgAl2O4) and exhibited less brittleness than specimens with tight particle grading even though they had similar nominal tensile strengths. In contrast, specimens with non-tight particle grading had greater horizontal strain under various loading stages, reflecting their better ability to resist rupture deformation. In addition, more microcracks were initiated earlier in the pre-peak region, and more energy was consumed. The decrease in coarse particles and corresponding increase in fine powder content increased the interface between particles, benefiting for reducing the local stress concentration and improving the thermal shock resistance of refractories.
Three composite materials with different CaZrO3/MgO fractions (2/3, 1/2, 1/3) and two single-phase materials (CaZrO3 and MgO) were fabricated and their thermal conductivity was investigated.
Complete thermal and mechanical characterizations (thermal expansion coefficient, thermal diffusivity, specific heat, hardness and toughness) of the materials were performed. Values of the thermal conductivity up to 480 °C of the composites were compared with those calculated with the main analytical models. From the real microstructures of the three composites, representative volume elements (RVE) were built and used for finite element modelling (FEM) of thermal conductivity using conductivities of the single-phase materials as inputs.
The FEM results showed no differences for the 3 spatial directions of the RVE, nor for the different edge lengths (11, 14 and 17 µm).
Results of all analytical models are statistically different from the experimental ones, being those from the Bruggeman model the closest. Results of the proposed FEM are statistically coincident with the experimental ones, showing sensitivity to temperature variation.
MgO-based refractories have been regarded as ideal linings for various furnaces owing to high refractoriness and excellent corrosion resistance to basic slag. However, thermal shock damage resulting from poor thermal shock resistance (TSR) of magnesia is the common failing. The present work aimed at improving TSR of magnesia aggregates by introducing microscale monoclinic ZrO2. The results showed that the ZrO2 added could increase cation vacancy concentration and inhibit abnormal growth of MgO grains, which therefore enhanced densification of the specimens. However, when the ZrO2 amount exceeded 15wt%, densification decreased slightly due to agglomeration of ZrO2 and formation of more microcracks at the MgO grain boundaries. With increasing ZrO2 content, although the flexural strength degraded, the fracture toughness increased constantly because of the toughening effects of crack deflection and crack branching. Besides, although the addition of ZrO2 decreased the thermal conductivity, TSR of the specimens was significantly improved with increasing ZrO2 content due to the increase in toughness and decrease in strength, thermal expansion coefficient as well as Young’s modulus. After thermal shock tests, 15wt% ZrO2 containing specimen exhibited the highest TSR, whose residual strength ratio was about triple of that of the pure magnesia specimen. However, further increasing ZrO2 content to 20wt% reduced the TSR attributing to the spalling of intergranular ZrO2 agglomerations.
This work reported the feasibility of using silicon kerf waste as raw material for the preparation of porous SiC ceramics, as well as effects of sintering atmospheres and additives on the properties of as-prepared ceramics. Samples sintered in Ar, N2, and carbon embedded atmospheres exhibited entirely different properties because of the formed bonding phases of SiCw, Si3N4/Si2N2O, and Si2N2O. Moreover, the apparent porosity and closed porosity of the samples were increased with the additions of feldspar and starch. Preferably, a series of porous SiC ceramics with apparent/closed porosity of 24.15%–59.59%/5.83%–14.77%, bulk density of 1.13–1.91 g·cm⁻³, cold compressive strength of 34.73–249.89 MPa, and thermal shock resistance of 110–118 cycles was synthesized at 1500 °C in carbon embedded atmosphere. Therefore, a low-cost and facile synthesis strategy for preparing high-quality porous SiC ceramics from solid waste is proposed in present study.
Sintered and fused magnesia (FM) produced from the macrocrystalline magnesite in China have attracted attention worldwide for the production of various refractories. Herein, dead burnt magnesia (DBM) with varying compositions was investigated. The results revealed that the periclase crystals of the DBM92 sample were subrounded to rounded euhedral, whereas the periclase crystals of the DBM95 sample were subhedral and idiomorphic. In addition, a significant amount of periclase–periclase bonding with straight boundaries was observed in the DBM97 sample. The periclase crystals of the DBM98 sample with no clear boundaries exhibited the densest packing among the samples. The silicate matrix around the periclase grains of the DBM92 sample contained forsterite and monticellite, whereas that around the periclase grains of the DBM95 sample was mainly composed of monticellite and merwinite. Dicalcium silicate and merwinite were observed in small amounts as interstitial phases in the DBM97 sample. In contrast, only dicalcium silicate, which was concentrated in small triangular pockets, was observed in the densely packed periclase grains of the DBM98 sample. The hot modulus of rupture tests revealed that an alkali-resistant DBM95-based brick can be prepared at a lime–silica ratio of <0.5. The best-quality DBM97-based brick can be prepared at a lime–silica ratio of approximately 2.2, which ensures that dicalcium silicate is the only interstitial phase. The periclase crystals of the two-step FM were significantly larger than those of the one-step FM and exhibited remarkably fewer silicate boundaries. The superior structure and large crystal size of the two-step FM can endow magnesia–carbon bricks with slag corrosion resistance.
A new strategy to improve the mechanical strength and the cement clinker corrosion resistance of refractories for cement kiln was proposed in this work. For this purpose, advanced lightweight periclase-magnesium aluminate spinel refractories (LPSR) with high mechanical properties and excellent cement clinker resistance were fabricated using microporous magnesia aggregates. Their microstructures and properties were then compared to conventional dense periclase-magnesium aluminate spinel refractories (DPSR) containing sintered magnesia aggregates. The apparent porosities of the microporous and dense magnesia aggregates were 43.1% and 5%, respectively. Scanning electron microscopy observation results showed that the interface between the microporous aggregates and the matrix was better, which significantly improved the strength and thermal shock resistance of the LPSR. Additionally, the microporous magnesia aggregates absorbed some of the penetrating slag from the matrix, which prevented further infiltration. Thus, after substituting the microporous magnesia aggregates for dense magnesia aggregates to fabricate LPSR, the practically same cement clinker resistance was obtained for the LPSR compared to DPSR. Meanwhile, the bulk density was decreased from 2.87 g/cm³ to 2.61 g/cm³, while the compressive strength and flexural strength increased from 48.7 MPa to 73.1 MPa and 6.0 MPa to 10.1 MPa, respectively. Meanwhile, practically the same cement clinker resistance was obtained for the LPSR compared to DPSR.
The recycling of solid waste is win-win solution for humans and nature. For this purpose, magnesite tailings and silicon kerf waste were employed to prepare MgO–Mg2SiO4 composite ceramics by solid-state reaction synthesis in present work. Then, effects of sintering temperature and raw material ratio on as-prepared ceramics were systematically studied. As-prepared ceramics showed improvement in their relative density (from 47.55%–68.12% to 90.96%–95.25%) and cold compressive strength (from 7.34–118.66 MPa to 303.39–546.65 MPa) with the increase in sintering temperature from 1300 to 1600 °C. In addition, it was found that Si promoted synthesis process of Mg2SiO4 phase through transient liquid phase sintering and Fe2O3 accelerated sintering process through activation sintering. Consequently, the presence of Mg2SiO4 phase effectively improved the density and strength of MgO–Mg2SiO4 composite ceramic, while reducing its thermal conductivity. This work provides potential reutilization strategy for magnesite tailings, and as-prepared products are expected to be applied in fields of construction, metallurgy, and chemical industry.
In this study, the degradation mechanisms of dense periclase-magnesium aluminate spinel refractory bricks used in the upper transition zone of cement rotary kilns were investigated by XRD, SEM and EDS. And the results indicated that the damage of the used brick was main due to the structural spalling as well as the formation and extension of cracks, caused by the corrosion of cement clinker, the deposition of alkali salts and thermal stress resulted from changing temperature field. The bricks reacted with the cement clinker forming a liquid phase at the reacting interface, and then the penetration of this liquid phase by reaction sintering altered the microstructure and compositions of bricks. Additionally, the constantly changing temperature field and the precipitation of alkali salts with the high thermal expansion coefficient in colder zones led to tremendous thermomechanical stress. Thus, joint effects of these factors caused the structural spalling and deterioration of refractory linings.
The rich mineral resources of magnesite have been decreasing rapidly, while the low-grade magnesite is abandoned, and the requirement on utilization of low-grade magnesite is causing much attention. La2O3 has been introduced as an additive in this study, considering its positive role in the modification of the magnesia matrix. Low-grade magnesite containing 0–3 wt% La2O3 was fired at 1550 °C for 3 h. The effect of La2O3 on the microstructural evolution and thermomechanical property of sintered low-grade magnesite was investigated. The results showed that La2O3 markedly improve the microstructure of sintered low-grade magnesite. The distribution state of the low melting monticellite phases among the periclase grains evolved from distribute continuously to concentrate in isolation at the triple-point junction of the periclase grains, and the generated CaO·3SiO2·2La2O3 based dendrites phases interlaced in the magnesia matrix. The hot modulus of rupture (HMOR) was significantly enhanced with increasing addition of La2O3 due to the low-melting monticellite phases were transformed to CaO·3SiO2·2La2O3 phases based dendrites with large aspect ratio, effectively reducing the availability of monticellite. Besides, the bridging phenomenon resulting due to the dendritic phase exhibited an optimal toughening effect, attributing to the increase in crack deflection and resistance to crack propagation.
Sintered magnesia refractories combined by in situ intergranular CaZrO3 phases were synthesized using natural MgO-containing natural minerals with CaO and SiO2 impurities and nano-sized ZrO2 additive. A homogenous distribution of intergranular CaZrO3, independent from the intergranular CaO-MgO-SiO2 phases, was formed in situ within the sintered magnesia aggregates by introducing 0.75 wt pct nano-sized ZrO2 into the magnesite. The formation mechanism of the in situ intergranular CaZrO3 phases was determined. The nano-sized ZrO2 was introduced and uniformly distributed at grain boundaries of the magnesia due to the micron-nano-sized particles composite system and wetting grinding process. Then the CaO in impurities were prior to SiO2 to react with the ZrO2 for generating CaZrO3 at the grain boundaries by increasing sintering temperature. Nevertheless, the nano-sized ZrO2 particles were encapsulated in the MgO crystallites with similar particle size decomposed from brucite and prevented from reacting with CaO impurities in magnesite. The mixing homogeneity of magnesite particles and ZrO2 particles and the direct contact between ZrO2 and CaO impurities in magnesite has a crucial effect on the formation of intergranular CaZrO3 phases. Furthermore, the intergranular CaZrO3 phases could enhance the bonding of magnesia grains and have great potential for improving the service performance of magnesia.
Magnesium aluminate (MgAl2O4) spinel ceramics have gained widely using in some crucial industries by right of superior properties. However, their further applications have been restricted by the complicated preparing technology and high processing cost. In the present work, a series of MgAl2O4 spinel ceramics with high-density have been fabricated by solid-state reaction sintering. Role of different composite oxide additives including Sm2O3–Y2O3, Sm2O3–Nb2O5, and Sm2O3–La2O3 respectively, on phase compositions, morphologies, sintering characteristics, mechanical properties, and thermal stability of as-prepared MgAl2O4 spinel ceramics was investigated. Some secondary phases such as Y3Al5O12, SmAlO3, and MgAl11LaO19 were formed. Due to the synergistic effect of the secondary phases and consequent solid solutions, the densification and properties of MgAl2O4 spinel ceramics were obviously promoted. Simultaneously, the positive effects were also intuitively manifested in the decrease of apparent porosity and increase of the cold compressive strength and thermal shock resistance.
Lightweight microporous magnesia aggregates with closed porosity of 6.2%, apparent porosity of 2.1% and median pore size of 2.36 μm were fabricated by introducing nano-sized ZrO2 combined with magnesite decomposition and sintered at 1780 °C. The sintering properties and microstructure revealed that nano-sized ZrO2 promoted the sintering and formation of closed intragranular pores by increasing abundant crystal defects and producing cation vacancies (VMg'') raising the migration rate of grain boundaries, which induced a change in porosity type from large open pores to small closed pores. The nano-sized ZrO2 changed the bonding of magnesia grains from single CaO–SiO2–MgO phases bonding to composited CaO–SiO2–MgO phases-CaZrO3 phases bonding, significantly enhancing the slag penetration resistance. The closed porosity played a more important role in reducing the thermal conductivity compared with total porosities. The microporous magnesia aggregates and thermal conductivity of 8.994 W (m K)⁻¹ at 800 °C exhibit potential for application in wear lining refractories.
Foamed mullite-SiC ceramics were prepared by using white clay, industrial alumina and silicon powder as raw materials. The effects of the clay content on the phase composition, microstructure and thermal properties of foamed ceramics were investigated by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The fired foamed mullite-SiC ceramics contained mullite, corundum and β-SiC. With an increase of clay content, the apparent porosity decreased, while the compressive strength and thermal conductivity increased. The optimized composition had a clay content of 44 wt% clay, which yield a high porosity of 72.9%, an excellent compressive strength of 5.6 MPa and a low thermal conductivity of only 0.267 W/(m·K). An adjusted GE thermal conductivity model assuming an appropriate emissivity of 0.35 was highly capable to predict the effective thermal conductivities of the foamed mullite-SiC ceramics with different pore characteristics at 800 °C.
Mg-α-SiAlON bonded periclase composites were prepared at 1550 °C in a nitrogen atmosphere using sintered magnesia particles, Al metal powder, Si powder, magnesia, and α-Al2O3 powder as raw materials and were bonded with a phenolic resin. The modification mechanism was investigated. Tabular-shaped Mg-α-SiAlON can be obtained through the diffusion-reaction-dissolution-precipitation mass transfer mechanism. It was found that modification of the matrix for the magnesia material led to a direct bonding mode and a card structure. The optimal percentage of powder material was determined to be 40 wt%, by which a well-developed card structure was obtained. The sintered Mg-α-SiAlON bonded periclase composites exhibited excellent cold modulus of rupture (12.5 MPa), and the residual strength retention rate was 3 times higher than that of a magnesia specimen after the third thermal shock. The enhanced thermal shock resistance was attributed to a lower thermal expansion coefficient and the “crack deflection” mechanism of tabular-shaped Mg-α-SiAlON.
The blank of phase equilibrium relations, liquidus, and other information in the CaO-SiO2-La2O3 basic slag system phase diagram limits the application of rare earth elements in the smelting, processing, and high temperature materials. In the current work, phase equilibrium relations in the CaO-SiO2-La2O3 system at 1373K-1873K were studied experimentally by using thermodynamic equilibrium experiment followed by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). According to the experimental data, primary crystal field of SiO2, La2O3·2SiO2, CaO·3SiO2·2La2O3, and 2CaO·SiO2 were identified respectively. The 1473K-1873K liquidus lines in SiO2 primary crystal field and 1373K-1873K liquidus lines in CaO·3SiO2·2La2O3 primary crystal field were also determined. Sub-solidus phase fields in the ternary system were identified, and then the reaction type and temperature range of relevant invariant points were determined. Finally, the CaO-SiO2-La2O3 ternary system phase diagram within specific region was obtained.
Small amount of Y2O3 (0-3 wt%) was added into off-grade natural magnesite and its effects on the phase assemblage, microstructural evolution in correlation with mechanical and thermomechanical properties were investigated. Presence of Y2O3 facilitated the formation of calcium yttrium silicate and yttrialite phases, which prohibited the formation of detrimental-phase monticellite, which was gradually reduced with the increase in Y2O3 content. Y2O3 promoted periclase grain growth and segregated the secondary phases at triple-point junction of periclase grains. Also, high-temperature (at 1200°C) flexural strength of the samples increased from 77.2 MPa (without Y2O3) to 137.45 MPa with the addition of 2 wt% Y2O3. Greater degree of direct bonding among periclase grains, compact microstructure, and uniform grain size distribution in addition to reduced amount of monticellite were responsible for the improvement in mechanical and thermomechanical properties.
In the Blast Furnace (BF) process, the viscous behaviors of molten slags are essential to the efficiency and the productivity of operations. In this work, the effects of CaO/SiO2 and Al2O3 on the viscosity, activation energy for viscous flow, and break point temperature of titanium-bearing BF slags are analyzed. To connect the viscosity and the slag structure, Fourier transform infrared spectroscopy (FTIR) is performed on the as-quenched slags. It is found that the viscosity and the activation energy for viscous flow of slags decrease as the CaO/SiO2 content increases from 1.00 to 1.20 and increase with increasing the Al2O3 content from 11.00 to 15.00 wt%. As for the break point temperature, it raises when the CaO/SiO2 and the Al2O3 content increases. FTIR results reveal that the polymerization degree of complex viscous units in the slag decreases as the CaO/SiO2 increases. However, it is opposite when the Al2O3 content increases. The variation of viscosity and activation energy for viscous flow for the different CaO/SiO2 and Al2O3 content slags are explained.
MgO-2CaO∙SiO2-3CaO∙SiO2 refractory compositions incorporated with zircon (ZrO2∙SiO2) were obtained by solid state sintering at 1550 °C for 3 h. The effect of different ZrO2 contents (0, 0.5, 1.0, 1.5, 2.0 and 2.5%) on the physical properties and adherence ability to cement clinker were investigated. Specimens were characterized by bulk density, apparent porosity, cold crushing strength, cold and hot modulus of rupture and coatability adherence with the combination of crystalline phase formation (XRD) and microstructural analysis (SEM). The results showed that CaZrO3 was generated in the matrix grain boundaries and triple points, forming direct bonding between MgO and calcium silicate. Densification of the composites was promoted, and the apparent porosity decreased to 8.9% when the content of ZrO2 was 1.5%. The cold crushing strength increased steadily from 65 MPa to 124.2 MPa. The cold modulus of rupture slightly decreased in the ZrO2 content range of 0.5–1.5% and then reached 43.2 MPa, whereas the hot modulus of rupture reached its highest value of 4.4 MPa with 1.5% ZrO2 addition. The highest adherence strength (7.1 MPa) was obtained for 0.5% ZrO2 addition because the dissolution of CaZrO3 into the cement clinker increased the viscosity of the clinker and the bonding with the specimen. At high ZrO2 concentrations (1.0–2.5%), penetration of the clinker into the matrix was hindered by CaZrO3, thus resulting in lower adherence strength.
Indian magnesite mineral has substantial amount of impurities like CaO, SiO2 & Fe2O3. During sintering at elevated temperature these impurities react to form low melting phases like monticellite (CMS), which can degrade the high temperature properties of magnesite. In the present study, ZrO2 was added to reduce the formation of low melting phase in order to improve the hot strength. Amount of additive was varied between 2 and 6 wt.% with respect to raw magnesite. It was observed that addition of ZrO2 reduces the formation of low melting CMS at higher temperature and improves the flexural strength at 1200 °C. Periclase grain shape also changed from rounded to subrounded in the presence of zirconia.
Ca2+/Cr3+ doped LaAlO3 microspheres were prepared by using a facile flame-spraying synthesis (FSS) technique. Phase composition and microstructure of the samples were characterized by XRD, EDS and SEM. The particle size of pure and doped LaAlO3 microspheres was 5–20 μm with a narrow size distribution. The effects of Ca2+/Cr3+ doping concentration and annealing temperature on infrared emissivity of doped LaAlO3 microspheres were investigated. It revealed that the infrared emissivity of LaAlO3 could be significantly enhanced by Ca2+–Cr3+ co-doping and it increased mainly with the content of Cr3+, reaching as high as 0.93. After heat-treatment at 900–1500 °C, the infrared emissivity of doped LaAlO3 microspheres was further improved. It indicates as-synthesized Ca2+–Cr3+ co-doped LaAlO3 microspheres are expected to be the next generation of infrared radiation agent for energy-saving applications, due to their high emissivity and melting point, and excellent chemical stability at high temperature.
Up to 5 wt% of nano-alumina or nano-iron oxide was added to magnesia refractory matrix. The crystalline phases and microstructure characteristics of specimens sintered at 1600 °C for 4 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of density and porosity. The mechanical behavior was studied by a cold crushing strength (CCS) test. In addition, the chemical behavior with regard to slag attack was studied by the SEM technique. As a result, it was found that the presence of nano-iron oxide in the magnesia matrix induced magnesioferrite spinel formation, which improved the sintering process. Nano-iron oxide also influenced the bonding structure through a direct bonding enhancement. On the other hand, the presence of nano-alumina in the magnesia matrix induced magnesium-aluminate spinel formation, resulting in lower properties in comparison with those obtained by nano-iron oxide addition.
A direct observation of the grain growth and densification of BaTiO3 ceramics were made in a TEM equipped with an in-situ installed heating platform. Both real two-sphere and three-sphere models for BaTiO3 ceramics were found and parallel processes of the shrinkage between particles and the grain growth (coarsening) were observed during the heating process. Attempts of relating the grain growth and densification was made which reveals, though roughly, the close relation between the two competing process: they took place simultaneously and in parallel, therefore it is believed that the process took place via the same mass transport mechanism. Surface diffusion on the TiO2 particle surface was observed directly, which takes place with the co-motion of several lattice layers from a smaller grain onto the surface of an adjacent larger one. The surface diffusion was accompanied by the diminishing of a small grain, i.e., overall grain growth.
The effect of Cr2O3 particle size on the densification of magnesia refractories was investigated. Magnesia grains (<45 μm) were mixed with 2 wt% of micro-Cr2O3 (2 μm) and nano-Cr2O3 particles (10–20 nm) and sintered at 850–1450 °C, for 5 h in air. The progress of the densification and phase evolution of samples was studied with the support of X-ray diffraction phase analysis (XRD), Fourier transformer infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). It was shown that the densification of magnesia was enhanced by reducing the particle size of the added chromia to the range of 20 nm. According to the phase analysis results, the higher dissolution rate of Cr2O3 in MgO in the MgO–Cr2O3 system was responsible for the faster densification of nano-Cr2O3 containing mixes.
The process of isothermal sintering of magnesium oxide obtained from sea water (by precipitation with 80% and 120% of the stoichiometric quantity of the calcined dolomite and magnesium oxide p.a. (pro analysi purity grade) ) was examined with the addition of SiO2, Al2O3 and TiO2, at temperatures in the range 1300–1800 °C. The process was followed by determining the product density, as well as densification of the compacts. With compacts containing SiO2 or Al2O3 densification was examined in relation to the sintering temperature and quantity of the sintering aid; with compacts containing TiO2 densification was examined in relation to the sintering temperature and pressing pressure. Sample densification and pore removal during isothermal sintering have been shown to be a function of the exponent α in the expression (Δ V/V0) − m = τα. The specific surfaces, chemical compositions and particle size distribution were determined for the magnesium oxide samples examined.
The behavior of slag penetration into MgO refractory was investigated by combining in-situ X-ray observation with microstructural analysis of the samples after penetration experiments. The following results are obtained. The slag penetrates rapidly into the refractory, reacting with MgO particles in the refractory. The slag penetrates unevenly into the refractory with uneven pore size in a route like a tree, firstly along the main route (the surface of large MgO particles), and then extending to the branch route, while evenly into the refractory with even pore size. The rate of slag penetration increases with increasing pore radius and apparent porosity of the refractory, T · Fe concentration in the slag and temperature, and with decreasing the slag basicity (C/S ratio). In the case of the Al2O3-bearing slag, the rate of slag penetration is less than that of Al2O3-free slag in the initial stage. The penetration also stops much earlier than that of the Al2O3-free slag, and then the penetration height remains almost constant. In the initial stage of penetration, the penetration height is proportional to the square root of penetration time. Slag penetration is deduced as stopping due to the following: (1) the melting point and viscosity of the penetrated slag increase, and the surface tension of the penetrated slag decreases with decreasing the FetO concentration in the penetrated slag consumed by the reaction between FetO and MgO particles; (2) in the 10mass% Al2O3-bearing slag, the pore size in the refractory is reduced by the spinel formed on the pore surface by the reaction between Al2O3 in the penetrated slag and MgO particles in the refractory.
It is shown that Ryshkewitch's exponential and Schiller's logarithmic formulae for the strength of porous materials are numerically indistinguishable except in the neighbourhood of the extremes of 0% and 100% porosity.ZusammenfassungEs wird gezeigt, dass Ryshkewitch's exponentielle und Schiller's logarithmische Formel fuer die Festigkeit poroeser Stoffe numerisch ununterscheidbar sind, ausser in der Nachbarschaft der Extreme von 0% und 100% Porositaet.
Solid-state reaction was adopted to prepare MgO-rich MgAl2O4 spinel from commercially available sintered seawater magnesia and α-alumina. The starting materials were mixed in weight ratio (Al2O3:MgO) of 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4. Calcination led to the development of MgAl2O4 spinel crystal seed, which was varied (5–50%) with respect to MgO content and temperature. Spinellisation of 70vol.% was observed for equal weight proportionate calcined powder, when calcined at 1000°C for 2h and subsequent sintered at 1600°C for 4h. The initial calcination temperature and hence the primary spinel seed content was found to influence the densification and microstructure of sintered specimen. Finally attempts were made to correlate the effect of spinel seed content on the crystallization behavior and microstructure.
The sintering and microstructure development of magnesia containing 0–10 wt% TiO2 at temperatures in the range 1300–1600°C have been investigated. The addition of TiO2 markedly promoted densification at relatively low temperature, and grain growth. Excess TiO2 over the solid solubility limit of TiO2 (0.3 wt%) reacted with magnesia to form inter- and intra-granular magnesium titanate (Mg2TiO4) above 1300°C. The grain size of MgO increased with increasing TiO2 content, and densification was mainly governed by MgO grain growth. © 1998 Kluwer Academic Publishers
Sintering occurs when packed particles are heated to a temperature where there is sufficient atomic motion to grow bonds between the particles. The conditions that induce sintering depend on the material, its melting temperature, particle size, and a host of processing variables. It is common for sintering to produce a dimensional change, typically shrinkage, where the powder compact densifies, leading to significant strengthening. Microstructure coarsening is inherent to sintering, most evident as grain growth, but it is common for pore growth to occur as density increases. During coarsening, the grain structure converges to a self-similar character seen in both the grain shape distribution and grain size distribution. Coarsening behavior during sintering conforms to classic grain growth kinetics, modified to reflect the evolving microstructure. These modifications involve the grain boundary coverage due to pores, liquid films, or second phases and the altered grain boundary mobility due to these phases. The mass transport rates associated with each of these interfaces are different, with different temperature and composition dependencies. Hence, the coarsening rate during sintering is not constant, but changes with the evolving microstructure. Core aspects treated in this review include models for coarsening, grain shape, grain size distribution, and how pores, liquids, dispersoids, and other phases determine microstructure coarsening during sintering.
Pressureless sintering studies have been conducted for excess Al2O3, stoichiometric, and excess MgO compositions of MgAl2O4 at 1500-1625°C. Initial powders of various compositions are prepared by solid-state reaction of MgO and Al2O3. A Brouwer defect equilibrium diagram is constructed that assumes intrinsic defects of the Schottky type. The densification rate derived from sintering kinetics is compared with the compositions investigated when the concentration is converted to the activity of the two oxide components in MgAl2O4. The grain-size exponent of p similar/congruent 3 suggests that densification takes place by a lattice-diffusion mechanism in the solid state. Determined activation enthalpies of 489-505 kJmol-1 are close to those obtained from oxygen self-diffusion derived in previous sintering studies. It is, therefore, proposed that oxygen lattice diffusion through vacancies is the rate-controlling mechanism for the sintering of nonstoichiometric MgAl2O4 compositions. The discrepancy between densification-rate ratios in experimental results and oxygen vacancy concentration in the Brouwer diagram is accounted for by the defect associates formed in the nonstoichiometric compositions.
The influence of controlled porosity on the compression strength of sintered pure alumina and of partly magnesia-stabilized zirconia was investigated. Bodies with porosities ranging from approximately 3 to 60% by volume were prepared utilizing hydrogen peroxide to induce pore formation. Cubes of approximately 1.2-cm, unit length were used in testing for compression strength at room temperature. The spatial arrangement of pores in sintered alumina was found to exert an influence, inasmuch as bodies with pores lined parallel to the pressure direction revealed a higher strength than bodies of the same porosity but with pores lined mainly perpendicular to this direction. It was found that an increase of porosity by 10 volume % decreased the strength of both sintered alumina and sintered zirconia by half of their initial respective values.
A review of the densification mechanisms and the microstructural development for transparent spinel made by free sintering and by hot pressing is given. The paper is divided into two main parts. The first part considers spinel without any sintering additives because there still is some controversy concerning the role of cation stoichiometry on sintering and grain growth. The second part discusses the role of the classic sintering aid, LiF, in processing transparent spinel. LiF is shown to have multiple behaviors: (1) it initially wets spinel and forms a liquid phase at relatively low temperatures, which affects early-stage densification and also grain growth; (2) upon cooling from intermediate temperatures, or even from higher temperatures if microstructure evolution (e.g., formation of closed porosity) prevents volatization, the LiF-containing liquid dewets and resides in isolated pockets; (3) LiF alters the cation stoichiometry, thereby enhancing diffusion via an increase in the concentration of oxygen vacancies; this affects both the densification rate and grain growth; and (4) it reacts with impurities in the system, thereby acting as a cleanser. For the production of transparent spinel, it is critical that LiF or associated reaction products not be retained as a secondary phase.
A sintering model has been developed to predict the consequences of independently varying the grain growth rate in alumina during final-stage sintering of a microstructure containing both small (first-generation) and large (inter-agglomerate second-generation) pores. The model shows that although it may be thermodynamically favorable to increase the grain growth rate, the kinetics of densification are such that it almost always pays to inhibit grain growth. This conclusion was verified by experiments on undoped, MgO-doped, and ZrO2-doped alumina impregnated with model spherical large pores produced by the burnt-out latex sphere method. A new type of ceramic processing map has also been developed to aid in the selection of the optimum processing conditions for the sintering of ceramics containing large pores.
A growth equation for individual grains in single-phase materials is suggested. It is used to calculate a rate equation for normal grain growth and the size distribution in the material. It predicts a maximum size of twice the average size. The theory is modified to take into account the effect of second-phase particles. In an alternative treatment the array of grains is described in terms of a kind of defects introduced into a perfect array. The defects move through the array during grain growth. The rate of grain growth is calculated from the number of defects and their mobility. The defect concentration is predicted by comparing the two treatments. The defect-model predicts two grain size limits due to second-phase particles. Normal grain growth takes place below the lower limit. Abnormal grain growth can take place between the two limits if the material contains at least one very large grain. No grain growth can take place above the higher limit. Several possible mechanisms for the development of abnormal grain growth are examined. An explanation is offered for the observation that most of the well-known cases occur as the second-phase is dissolving.
The stability of closed pores in two and three dimensions has been discussed and it is found that the stability of pores in two dimension can be determined mathematically from their particle coordination number and dihedral angle; while those in three dimension can be approximately determined by a spherical pore model. This model is set up by first excluding the effect of interface tension, so the pore was supposed to be spherical, and then the tensile stress arising from the interface tension was allowed to act on this hypothesized spherical pore. On the basis of the spherical pore model, pore microstructure models for real powder compacts were set up and the densification equations for the intermediate and final stages of sintering were derived. The criterion for pore shrinkage, and the effect of pore size distribution and green density were discussed according to the derived equations. The densification equations for pressureless solid state sintering can be easily extended to describe the densification behaviour during hot-pressing or hot-isostatic-pressing. Densification characteristics in liquid state sintering were also considered from the result of solid state sintering.
This paper reviews the influence of particle size distribution, agglomerates, rearrangement, sintering atmospheres and impurities on the pore evolution of some commonly studied oxides. These factors largely affect sintering mechanisms due to modifications of diffusion coefficients or evaporation-condensation. Very broad particle size distribution leads to grain growth and agglomerates densify first. Rearrangement of particles due to neck asymmetry mainly in the early stage of sintering is responsible for a high rate of densification in the first minutes of sintering by collapse of large pores. Sintering atmospheres play an important role in both densification and pore evolution. The chemical interaction of water molecules with several oxides like MgO, ZnO and SnO2 largely affects surface diffusion. As a consequence, there is an increase in the rates of pore growth and densification for MgO and ZnO and in the rate of pore growth for SnO2. Carbon dioxide does not affect the rate of sintering of MgO but greatly affects both rates of pore growth and densification of ZnO. Oxygen concentration in the atmosphere can especially affect semiconductor oxides but significantly affects the rate of pore growth of SnO2. Impurities like chlorine ions increase the rate of pore growth in MgO due to evaporation of HCl and Mg(OH)Cl, increasing the rate of densification and particle cuboidization. CuO promotes densification in SnO2, and is more effective in dry air. The rate of densification decrease and pore widening are promoted in argon. An inert atmosphere favors SnO2 evaporation due to reduction of CuO.
An inverse Hall–Petch effect has been observed for nanocrystalline materials by a large number of researchers. This effect implies that nanocrystalline materials get softer as grain size is reduced below a critical value. Postulated explanations for this behavior include dislocation-based models, diffusion-based models, grain-boundary-shearing models and two-phase-based models. In this paper, we report an explanation for the inverse Hall–Petch effect based on the statistical absorption of dislocations by grain boundaries, showing that the yield strength is dependent on strain rate and temperature and deviates from the Hall–Petch relationship below a critical grain size.