Porous SiC ceramics prepared via freeze-casting and solid state sintering

Porous SiC ceramics were prepared by freeze-casting process. In order to enhance the mechanical properties of the porous SiC, poly(vinyl alcohol) (PVA) was added as binder and pore morphology controller in this work. The results indicated that high porosity (>60%) SiC ceramics was obtained although the sintering temperature was over 2000 °C. The pore structure could be divided into two kinds: macropores generated by sublimation of large ice crystals, and micropores in the ceramic matrix caused by sublimating of small ice crystals, stacking of SiC particles, and burning out of PVA. With the increase of the sintering temperature, the specimens exhibited higher density, thus resulted in higher strength. Porous SiC ceramics sintered at 2100 °C showed a good flexural strength of 11.25 MPa with an open porosity as high as 66.46%.     

The relationship between microstructure and flexural strength of pressureless liquid phase sintered SiC ceramics oxidized at elevated temperatures

The oxidation behavior of pressureless liquid phase sintered SiC ceramics with Al₂O₃ and Y₂O₃ as sintering additives was investigated in the temperature range from 1000 °C to 1600 °C at the interval of 100 °C for 5 h. The relationship between residual flexural strength and microstructure was analyzed in detail. It was found that the SiC specimens suffered from mild oxidation below 1300 °C. The flexural strength of SiC specimens after oxidation at 1100 °C was the highest (90% of the original strength) due to the formation of dendritic grains, which filled pores and healed cracks. And the flexural strength was almost above 80% of the original flexural strength when the oxidation temperature was below 1300 °C. Meanwhile, the weight of specimens underwent steady increase. However, when the oxidation temperature was elevated to above 1400 °C, the specimens began to suffer from severe oxidation, which resulted in a lot of through pores and cracks on the surface, bringing about the sharp decrease of flexural strength to 30% of original strength when the oxidation temperature of 1600 °C was reached. And the weight of the specimens after huge increase began to show downtrend when the oxidation temperature was elevated to 1600 °C due to the spalling of oxidation products.     

Synthesis and characterization of Si/SiC ceramics prepared using cotton fabric

A Si/SiC ceramic was prepared from cotton fabric by the reactive infiltration of liquid silicon into the carbon template. A large density difference between the samples has been observed. This may be due to the variation in the pore size and its distribution within the sample. Scanning electron microscopy with energy dispersive spectroscopy shows the presence of three distinct phases, i.e., SiC, free Si and free carbon. X-ray diffraction pattern also confirms the presence of SiC and Si phases. However, there is no peak corresponding to carbon. So, it is inferred that the carbon exists in amorphous form. Micro-hardness, fracture toughness and bending strength of the ceramics were also studied. The values are lower than commercially available SiC ceramics. This may be due to the highly porous nature of cotton fabric-based SiC, as compared to commercially available SiC.     

Effects of oxidation cross-linking and sintering additives (TiN,B) on the formation and heat-resistant performance of polymer-derived SiC(Ti,B) films

Effects of oxidation cross-linking and sintering additives (TiN, B) on the microstructure formation and heat-resistant performance of freestanding SiC(Ti, B) films synthesized from Ti, B-containing polycarbosilane (TiB-PCS) precursor were investigated. TiB-PCS green films were first cross-linked for 1 h, 2 h, 3 h and 4 h, respectively, and then pre-sintered at 950 °C. Finally, they were sintered at 1800 °C to complete the conversion from organic films to inorganic SiC(Ti, B) films. The results reveal that curing time has a great impact on the uniformity and density of SiC(Ti, B) films. TiB-PCS films cured for 3 h yield the best quality SiC(Ti, B) films, which are composed of β-SiC crystals, C clusters, α-SiC nano-crystals, a small amount of TiB₂ and B₄C. TiB₂ and B₄C are both steady phases which can inhibit abnormal growth of β-SiC, effectively reduce sintering temperature and help consume excess C from decomposition of amorphous SiOxCy. After high temperature annealing at 1500 °C, 1600 °C and 1700 °C in argon, SiC(Ti, B) films still keep excellent mechanical properties, which makes them attractive candidate materials for microelectromechanical systems (MEMS) used at ultra-high temperatures (exceeding 1500 °C).     

Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives

In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al₂O₃ and CeO₂ as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.     

Microstructural evolution and mechanical performances of SiC/SiC composites by polymer impregnation/microwave pyrolysis (PIMP) process

SiC/SiC composites were prepared by polymer impregnation/microwave pyrolysis (PIMP) process, and their microstructural evolution and the mechanical performances were characterized. Using non-coated Tyranno SA fiber preforms as reinforcement and impregnation with only allylperhydropolycarbosilane (AHPCS) into the preforms, Tyranno SA/SiC composite (TSA/SiC) with higher density was obtained. While using carbon-coated Tyranno SA fiber preforms, Tyranno SA/C/SiC composite (TSA/C/SiC) with lower density were also fabricated. In this composite, SiC particulate was loaded with polymer precursor (AHPCS) in the first cycle impregnation. Microstructural observation revealed that pore and crack formation was affected by processing conditions. Bending strength was also dependent on the microstructural evolution of the samples. In TSA/SiC composite, relatively strong interfaces contribute to effective load transfer so that higher bending strength could be reached. In the TSA/C/SiC composite, weak interfaces provide a relatively lower strength. Meanwhile, different microstructural evolution and interfacial properties of the composites lead to the variation of the fracture behaviors.     

Liquid phase flash sintering in magnesia silicate glass-containing alumina

Magnesia silicate glass-containing alumina was flash sintered using an E-Field in the 500–1500 V/cm range. The addition of glass allows to reduce the current needed for densification and improves the shrinkage obtained during field-assisted sintering process. This behaviour is related to the different sintering mechanisms involved in the two materials, i.e. solid state sintering for pure alumina and liquid phase sintering for glass-containing alumina.The estimated activation energy for conduction during flash sintering is compatible with ionic diffusion in silicate melt. Moreover, evidence of magnesium diffusion toward the cathode is recorded. The estimated sample temperature is in almost all cases lower than 1355 °C, which is the temperature at which the first liquid is formed in the ternary system MgO-SiO₂-Al₂O₃. Finally, it is shown that the application of an E-Field accounts for efficient liquid phase sintering at temperatures at which it cannot be reproduced conventionally.     

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 °C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al₂O₃/Y₂O₃ liquid was done at 1850 °C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS.     

Fabrication of porous SiC ceramics having pores shaped with Si grain templates

SiC porous ceramics were prepared by heating mixtures of Si powder and carbon black at 900 °C for 24 h in Na vapor. The grains of the Si powder were not only the source of Si for SiC but also served as templates for the pores in the SiC porous ceramics. Angular-shaped pores with sizes of 2-10, 10-150 and 50-150 μm were formed by angular Si grains with sizes of ≤10, ≤50 and ≤150 μm, respectively. The porosity of the SiC porous ceramics was around 55-59%. Spherical pores were also formed when spherical Si grains were used. A bending strength of 14 MPa was measured for the SiC porous ceramics prepared with the Si grains (≤50 μm).     

Low-temperature joining of SiC ceramics with Y2O3-Al2O3 interlayer by SiO2-based liquid phase extrusion strategy

In order to reduce the joining temperature of SiC ceramics by glass-ceramic joining, some oxides were usually introduced into to Y₂O₃–Al₂O₃ for reducing the eutectic temperature. However, the joints might have poor high-temperature resistance due to the low melting point of the joining layer. In the present work, based on novel SiO₂-based liquid phase extrusion strategy, joining of SiC ceramics with Y₂O₃–Al₂O₃ interlayer was carried out by using Y₂O₃–Al₂O₃–SiO₂ as the filler through spark plasma sintering (SPS). The SiO₂-free interlayer of Y₂O₃–Al₂O₃ was used for comparison. It was found that SiC joints using Y₂O₃–Al₂O₃ could be only joined at a high temperature of 1800 °C, and the thickness of the interlayer was about 20 μm. The shear strength of the joint obtained at 1800 °C was 89.62 ± 4.67 MPa and the failure located in the SiC matrix. By contrast, reliable joining of SiC ceramics could be finished at as low as 1550 °C by extrusion of SiO₂-containing liquid phase when using Y₂O₃–Al₂O₃–SiO₂ as the interlayer, alongside the interlayer thickness of only several microns. The joint strengths after joining at 1550 °C was 84.90 ± 3.48 MPa and the failure located in matrix position. The joining mechanism was discussed by combining the detailed microstructure analysis and phase diagram.     

Spark plasma sintering of graphene reinforced zirconium diboride ultra-high temperature ceramic composites

Spark plasma sintering (SPS) of monolithic ZrB₂ ultra-high temperature ceramic and 2–6 vol% graphene nanoplates (GNPs) reinforced ZrB₂ matrix composites is reported. The SPS at 1900 °C with a uni-axial pressure of 70 MPa and soaking time of 15 min resulted in near-full densification in ZrB₂–GNP composites. Systematic investigations on the effect of GNP reinforcement on densification behavior, microstructure, and mechanical properties (microhardness, biaxial flexural strength, and indentation fracture toughness) of the composites are presented. Densification mechanisms, initiated by interfacial reactions, are also proposed based on detailed thermodynamic analysis of possible reactions at the sintering temperature and the analysis of in-process punch displacement profiles. The results show that GNPs can be retained in the ZrB₂ matrix composites even with high SPS temperature of 1900 °C and cause toughening of the composites through a range of toughening mechanisms, including GNP pull-out, crack deflection, and crack bridging.     

Nucleation and growth of SiC at the interface between molten Si and graphite

In the present work, the evolution of the SiC layer formed at the interface between liquid silicon and solid carbon is studied using a diffusion couple configuration. Reaction conditions were isothermal, with a temperature of 1450 °C maintained from 2 min to 4 h. The rapid heating and cooling of the Si–C diffusion couple specimens were achieved using a Pulse-Electric Current Sintering system. Crystallographic, compositional, and phase distribution data obtained after different reaction times were used to develop a two-stage model for SiC growth at the interface between molten Si and C. Initially, the formation of SiC at the interface is governed by diffusion of C into the molten Si and dissolution/reprecipitation of formed SiC nuclei. These nuclei further grow into larger SiC grains at the Si–C interface and this initial stage is successfully modeled using the Johnson-Mehl-Avrami-Kolmogorov model. Once a continuous SiC layer forms at the Si–C interface, the growth of SiC is controlled by the diffusion of C through the SiC layer, which can be modeled using a power rate law. However, the nature of this diffusion is difficult to determine with certainty since the rate laws for both grain boundary and bulk diffusion fit experimental data equally well.     

Kinetic field approach to study liquid phase sintering of ZnO based ceramics

Liquid phase sintering kinetics in the system ZnO–Bi₂O₃–Sb₂O₃ was studied using closed crucibles and an optical dilatometer. A modified kinetic field technique was applied for the first time to investigate the densification rates. The values obtained were assessed with existing liquid phase sintering models. Grain growth data were derived from the kinetic field diagram and compared to those obtained from microstructure analysis of quenched samples. Good agreement was obtained between both techniques. Values for both the activation energies (activation energies for grain growth and densification) were also reported for the ZnO–Bi₂O₃–Sb₂O₃ system for the first time. In the initial sintering stage mechanisms were identified which retard densification and are essentially unaffected by temperature. It was shown how the position and slope of the iso-strain lines in the modified kinetic field diagram can be used for a qualitative understanding of the interaction of coarsening, liquid redistribution and densification during sintering.     

Liquid phase assisted high pressure sintering of dense TiC nanoceramics

To address the difficulty of achieving high density while effectively suppressing grain growth in the fabricating of nanoceramics, this paper demonstrates a novel integrated approach, consisting of a solid-liquid reaction, high pressure and low temperature sintering, to prepare dense nanocrystalline TiC ceramics. Using Si as the additive, a low-viscosity Si liquid phase is formed under the sintering condition of 1200?°C/4.5?GPa. It is shown that both of the sintering aid and high pressure are crucial in achieving high density nanostructured TiC ceramics and controlling their microstructure and thus their mechanical properties. The pure TiC can be sintered to reach 95.3% of its theory density and, with the assistance of liquid Si additive, it can be sintered to full densification without grain growth by high pressure technique.     

Phase formation during liquid phase sintering of ZnO ceramics

ZnO doped with Bi₂O₃ and Sb₂O₃ (ZBS), is the basic system for ceramic varistors. Phase formation during sintering of ZBS was measured in situ, using 1 mm thick samples and synchrotron X-rays. Sintering shrinkage was measured in different atmospheres by an optical method. Thermodynamic calculations were performed to explain phase formation, composition, stability of additive oxides and influence of the oxygen fugacity on sintering. Sb₂O₄, pyrochlore, trirutile and spinel were formed at temperatures of 500–800 °C. The oxidation of antimony was controlled by the oxygen partial pressure and affected both, phase formation and sintering kinetics, in the ZBS system.     

Effect of SiC ceramics on thermoelectric properties of SiC/SnSe composites for solid-state thermoelectric applications

Tin selenide (SnSe) based thermoelectric materials with varying amounts of embedded silicon carbide (SiC) particles were fabricated, and their thermoelectric properties were investigated. The SiC particles were evenly distributed in the SnSe matrix, thereby leading to the formation of the SiC/SnSe composite samples. The introduction of SiC into the SnSe matrix improved the power factors, owing mainly to an increase in the Seebeck coefficient, and a decrease in the thermal conductivity arising from the formation of phonon-scattering centers. Consequently, a ZT of 0.125 (at 300 K) was obtained for the SiC/SnSe composite with a SiC content of 1 wt%; this value was larger than that of the pristine SnSe. The results of this study indicate that the introduction of SiC particles into the SnSe matrix constitutes an efficient strategy for achieving thermoelectric enhancement for solid-state applications.     

Tin oxide-based ceramics of high density obtained by pressureless sintering

Dense tin oxide-based ceramic semiconductors have a high potential as electrodes for aluminum production, glass industry, sputtering targets for transparent conducting thin films, varistors, and thermoelectrics due to their specific electrical properties, high corrosion resistance and ability to withstand high temperatures. The application of these ceramics is still limited because of the necessity to reach density of 99% of TD or greater and high electrical conductivity, and because of the manufacturing difficulties to produce components of different shapes and sizes with low cost. The paper reviews the results of the works conducted towards obtaining tin oxide-based ceramics with density up to 99.5+% of TD through low-temperature pressureless sintering. The selected sintering aids/dopants and firing conditions promoted both liquid phase sintering and electrical conductivity. The uniform microcrystalline structure is obtained.     

Enhancing toughness and strength of SiC ceramics with reduced graphene oxide by HP sintering

In this paper, the silicon carbide-reduced graphene oxide (SiC/rGO) composites with different content of rGO are investigated. The hot pressing (HP) at 2100?°C for 60?min under a uniaxial pressure of 40?M?Pa resulted in a near fully-dense SiC/rGO composite. In addition, the influence of graphene reinforcement on the sintering process, microstructure, and mechanical properties (fracture toughness, bending strength, and Vickers hardness) of SiC/rGO composites is discussed. The fracture toughness of SiC/rGO composites (7.9MPam^1/2) was strongly enhanced by incorporating rGO into the SiC matrix, which was 97% higher than the solid-state sintering SiC ceramics (SSiC) by HP. Meanwhile, the bending strength of the composites reached 625?M?Pa, which was 17.3% higher than the reference materials (SSiC). The microstructure of the composites revealed that SiC grains were isolated by rGO platelets, which lead to the toughening of the composite through rGO pull out/debonding and crack bridging mechanisms.     

Properties and mechanism of mullite whisker toughened ceramics

High-strength ceramics were prepared from high alumina fly ash (HAFA) and activated alumina as raw materials with magnesia as a sintering additive. The growth kinetics and influence mechanism of secondary mullite whiskers were investigated. Meanwhile, the effects of the Al₂O₃/SiO₂ mass ratio (A/S) and the amount of magnesia on the content and morphology of mullite in the green body were investigated, so as to emphasize the effect of the liquid phase in the sintering process on the growth of secondary mullite whiskers. The results showed that the aspect ratio of secondary mullite whiskers increased significantly after adding activated alumina to increase the A/S ratio of raw materials. When 30 wt% activated alumina was added, the mullite content increased by 5.39%, and the whisker length increased from 1.36 μm to 4.18 μm. The addition of magnesia improved the liquid phase formed during the sintering process and the K value method was used to determine the sintering liquid phase content under various conditions. It was observed that increasing the magnesia level by 1 wt% could raise the liquid phase content by 5–7%. When the total liquid content of the system was 30–40%, the growth activation energy in the diameter direction of the whisker reduced significantly, promoting the growth of secondary mullite whiskers along the C axis. The morphology of mullite gradually developed from fibrous to long columnar crystal, making it combine more densely with the green body matrix. Furthermore, the staggered long columnar mullite crystal structure changes the fracture mode of ceramics from intergranular to transgranular fracture, which fully uses the high mechanical strength of mullite. As a result, the fracture energy and strength of ceramics are significantly improved.     

Gas pressure sintering of Al2O3/TiCN composite

Al₂O₃/TiCN composites have been fabricated by gas pressure sintering, which overcomes the limitations of hot pressing. The densification behavior and mechanical properties of the Al₂O₃ gas pressure sintered with 30 wt.% TiCN at different temperatures have been investigated. The gas pressure sintered Al₂O₃–30 wt.%TiCN composite achieved a relative density of 99.5%, a bending strength of 772 MPa, a hardness of 19.6 GPa, and a fracture toughness of 5.82 MPa·M^1/2.