碳化硅陶瓷固相烧结的烧结机理及研究进展简

碳化硅陶瓷材料具有高硬度、高强度、抗氧化、耐高温、高热导率、低线胀系数等优良性能,同时具有优良的化学稳定性且能够耐大多数种类的酸碱溶液腐蚀,在石油、化工、建材、航空、机械等诸多领域得到了广泛应用。本文主要阐述了碳化硅陶瓷固相烧结的烧结机理,并对目前国内外关于碳化硅陶瓷固相烧结的研究进展进行了阐述。     

反应烧结碳化硅陶瓷的制备及烧结机理

用溶胶-凝胶法合成的无机／有机杂化材料结合SiC+C混合粉料制成了反应烧结碳化硅陶瓷素坯,并对由这种素坯制成的碳化硅陶瓷进行了物相鉴定和显微结构观察;借助Si-C相图对反应烧结碳化硅的烧结机理进行了研究,分析表明其主要烧结机理为溶解-再沉淀型。     

Direct coagulation casting of silicon nitride suspension via a dispersant reaction method

A direct coagulation casting method for silicon nitride suspension via dispersant reaction was reported. Tetramethylammonium hydroxide (TMAOH) was used as dispersant to prepare silicon nitride suspension with high solid loading and low viscosity. Influences of TMAOH and pH value on the dispersion of silicon nitride powder were investigated. Glycerol diacetate (GDA) was used to coagulate the silicon nitride suspension. Influences of the concentration of glycerol diacetate on the viscosity and pH value of the suspension were investigated. It was indicated that high viscosity sufficient to coagulate the suspension was achieved by adding 1.0–2.0 vol% glycerol diacetate at 40–70 °C. The coagulation mechanism was proposed that the silicon nitride suspension was destabilized by dispersant reacting with acetic acid which was hydrolyzed from glycerol diacetate at elevated temperature. Coagulated samples could be demolded without deformation by treating 50 vol% silicon nitride suspensions with 0.2 wt% tetramethylammonium hydroxide and 1.0–2.0 vol% glycerol diacetate at different temperatures. Dense silicon nitride ceramics with relative density above 98.8% had been prepared by this method using glycerol diacetate as coagulating agent sintered by different methods.     

Densification of surface-modified silicon carbide powder by spark-plasma-sintering

SiC was densified by spark plasma sintering (SPS) and the effects of surface modification of the powder particles on its sintering behavior were investigated. The pressure and temperature conditions were set to 50 MPa and 2200 °C, respectively. Specific SPS experiments at a lower temperature (i.e. 1600 °C) was performed to analyze the efficiency of the sintering and the early stage of the densification in softer conditions. The surface functionalization was carried out by grafting a thin molecular layer of a preceramic precursor on the grain surface of SiC particles, which acts as a sintering additive without producing contamination by heteroatoms since, in addition to hydrogen, it contained only Si and C in the same ratio 1:1 as silicon carbide. One of the advantages of this surface functionalization is that it reduces the temperature at which the sintering process begins and therefore it facilitates and increases the densification of the final SiC parts.     

Crack tolerant silicon carbide ceramics prepared by liquid-phase assisted oscillatory pressure sintering

Silicon carbide ceramics were prepared by liquid-phase assisted oscillatory pressure sintering (OPS) with graphene and in-situ synthesized SiC whisker as the reinforcements. The effects of sintering temperature on the densification, morphology and mechanical performances of the SiC_p-SiC_w-graphene ceramics were investigated. In the temperature range from 1700 to 1800 °C, the densification rate of SiC_p-SiC_w-graphene ceramics was accelerated, ascribing to the reduction in viscosity of the glassy phase. At 1800 °C, the flexural strength and fracture toughness of the OPS ceramics corresponded to 697 MPa and 5.8 MPa m^1/2, respectively, which were higher than that of the hot-pressed ceramics under the same temperature conditions. Multiphase toughening mechanisms, such as whisker bridging and pullout, graphene bridging and delamination, were considered as the primary mechanisms. This work demonstrates an effective strategy to prepare silicon carbide ceramics at low sintering temperature.     

Direct laser sintering of reaction bonded silicon carbide with low residual silicon content

Additive manufacturing (AM) techniques are promising manufacturing methods for the production of complex parts in small series. In this work, laser sintering (LS) was used to fabricate reaction bonded silicon carbide (RBSC) parts. First, silicon carbide (SiC) and silicon (Si) powders were mixed in order to obtain a homogeneous powder. This powder mixture was subsequently laser sintered, where the Si melts and re-solidifies to bind the primary SiC particles. Afterwards, these SiSiC preforms were impregnated with a phenolic resin. This phenolic resin was pyrolysed yielding porous carbon, which was transformed into secondary reaction formed SiC when the preforms were infiltrated with molten silicon in the final step. This resulted in fully dense RBSC parts with up to 84?vol% SiC. The optimized SiSiC combined a Vickers hardness of 2045?HV, an electrical conductivity of 5.3?×?10³?S/m, a Young's modulus of 285?GPa and a 4-point bending strength of 162?MPa.     

Direct coagulation casting of silicon carbide suspension via polyelectrolyte dispersant crosslink reaction

A direct coagulation casting method for silicon carbide ceramic suspension using dispersant crosslink reaction is reported. Polymer electrolyte (polyethyleneimine, PEI) was used as dispersant to prepare silicon carbide suspension. Common food additives (carboxymethyl cellulose, CMC) were used to coagulate the electrosteric stabilized silicon carbide suspension. There was a well disperse silicon carbide suspension with 0.2 wt% PEI at pH = 5-6. Influence of coagulant on viscosity and zeta potential of the silicon carbide suspension was investigated. It indicates that the high solid loading silicon carbide suspension can be destabilized and coagulated at elevated temperature. It can be attribute to the gradual decrease of electrosteric force due to the crosslink reaction between PEI and CMC. Silicon carbide wet green body with compressive strength of 1.99 MPa could be demolded at 70°C which is higher than that prepared by traditional DCC and dispersant reaction method for nonoxide ceramics. Dense silicon carbide ceramics with relative density above 98.8% and 99.3% had been prepared by liquid phase pressureless and hot pressed sintering, respectively.     

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.     

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.     

Lattice-structured SiC ceramics obtained via 3D printing,gel casting,and gaseous silicon infiltration sintering

In view of technical difficulties in preparing ceramics with complex shapes, gel casting combined with 3D printing was here adopted to prepare silicon carbide ceramic green body, and gaseous silicon infiltration sintering was used to prepare 3D lattice-structured ceramics. The preparation of the slurry, gel curing, and ceramic molding was investigated. Results demonstrate that the ratio of components affects the fluidity and stability of slurry. However, when volume fraction of the solid phase of the slurry reaches 56%, the viscosity of slurry is only 300 mPa s, and drying shrinkage rate of green body is 6.6%; these characteristics make slurry suitable for 3D complex model injection molding. Furthermore, both the temperature and the initiator affect gel curing speed. As the temperature and initiator content increase, the induction and gel time are rapidly shortened. When demolding at 300 °C and when gaseous silicon infiltration sintering is carried out at 1550 °C, a 3D lattice-structured ceramic with relative density of 87% and average compressive strength of 433 MPa can be obtained.     

Effect of monomers content in enhancing solid-state densification of silicon carbide ceramics by aqueous gelcasting and pressureless sintering

The possibility of obtaining solid-state sintered silicon carbide (SiC) through aqueous gelcasting using commercial SiC powders was demonstrated. Green bodies were prepared from thixotropic SiC slurries in aqueous medium with optimized pH and solid-loading. The monomer system in gelcasting provides strength to the green bodies through formation of a gel network by polymerization and the carbon from polymeric gel enhances the densification of SiC, thereby avoiding addition of carbon externally to the gelcasting batches. Maximum bulk density of 3.16 g/cm³ (98.4% of relative density) was achieved in gelcast SiC on sintering at 2150 °C in argon atmosphere. The effect of carbon on SiC densification is evinced from the changes in microstructure of sintered SiC with increase in carbon content. The density and microstructure of gelcast and sintered SiC was comparable to that obtained from dry pressing and sintering of additive mixed SiC powders.     

以水玻璃为硅源喷雾干燥制备SiC前驱体

以廉价的水玻璃和炭黑为原料,通过制备均匀混合的前驱体,利用碳热还原反应合成出超细SiC粉体。比较研究了前驱体喷雾干燥与搅拌干燥两种制备方法对体系碳热还原反应的影响,重点考察了喷雾干燥制备过程中主要工艺参数对前驱粉体密度及收率的影响。结果表明:喷雾制备的前驱体具有更高的反应活性,在1550℃下反应2h就可使SiO2转化率达到89.4%,搅拌干燥制备的前驱体转化率只有65.2%。喷雾干燥过程中,适当提高进口温度、喷雾头转速并降低料液的固体含量,可获得高收率、高产率的前驱粉体,利于整个制备工艺生产效率及产率的提高。     

Production of porous SiC by liquid phase sintering using graphite as sacrificial phase: Influence of SiO2 and graphite on the sintering mechanisms

Porous silicon carbide (SiC) is a promising ceramic for high-temperature applications due to its unique combination of properties. In the present work, a fabrication route for porous SiC is presented using graphite spherical powder as sacrificial phase to introduce porosity. By varying the initial amount of sacrificial phase, high-performance SiC materials with porosities in the range 30–50% were manufactured and characterized in terms of microstructure, density, thermal conductivity and flexural strength. The materials were fabricated by liquid phase sintering in presence of 2.5 wt.% Al₂O₃ and Y₂O₃ as sintering additives. The results indicate that the SiO₂ present in the starting SiC powders interacts with the sintering additives forming liquid phases that promote densification and weight loss. Besides, an Al-Si liquid phase is formed at higher sintering temperatures, whose contribution to densification is inhibited in presence of graphite due to the formation of Al-rich carbides.     

Preparation of TaB2-SiC oxidation protective coating for carbon materials by liquid phase sintering

To control the microstructure and amounts of TaB₂ phase in the TaB₂-SiC coating, a novel liquid phase sintering method was developed on the basis of in-situ reaction method to prepare the TaB₂-SiC coating, which includes synthesis of TaB₂ powders and further preparation of TaB₂-SiC coating. With Ta₂O₅, B₂O₃ and C employed as raw materials, hexagonal TaB₂ powders were prepared by carbothermal reduction method at 1500?°C, whose mean particle size is 491?nm. The TaB₂, SiC, C powders, and the low melting point phases Si and silica sol were used to prepare the TaB₂-SiC coating by liquid phase sintering at 2373?K. The thickness of the coating is about 350?µm. Compared with the SiC coating, the weight loss of the samples modified by TaB₂ decreased from 17.7% to 11.8%, and the average weight loss rate of the fastest weightloss zone reduced from ?6?×?10^?3 mg?cm^?2 s^?1 to ?5?×?10^?3 mg?cm^?2 s^?1. During oxidation, the Ta-oxides would gradually dissolve in the silicate glass to form Ta-Si-O glass ceramics with dendritic structure, which significantly improved the toughness and stability of the glass layer. The Ta-Si-O glass ceramics possesses the ability of sealing and arresting the microcracks, which can enhance the oxidation protective ability of the coating.     

Low-temperature sintering of silicon carbide membrane supports from disks to single- and 19-channel tubes

As the configuration of asymmetric ceramic membranes is determined by the membrane supports, a low temperature sintering process was developed to prepare ceramic membrane supports from disks to single- and 19-channel tubes. The residual of NaA zeolite was introduced into the silicon carbide (SiC) matrix, and porous SiC membrane supports were successfully prepared at 1100 °C. Compared with the disk-like support, the open porosity, average pore size and pure water permeance of the single-channel supports obviously increased, and their bending strength decreased accordingly. These differences were mainly attributed to the incorporation of organic plasticizer and the change in molding process. Notably, the pure water permeance of disks, single- and 19-channel tubes was 70 ± 3, 74 ± 4 and 22 ± 1 m³·m^?2·h^?1·bar^?1, respectively, which was much higher than the previously reported values. Therefore, this work provides important guides to develop of new generation ceramic membranes for practical application.     

反应烧结碳化硅高温性能的研究进展

反应烧结碳化硅具有优良的力学性能、抗侵蚀性能和抗氧化性能等优点,是一种高致密度、低成本和净尺寸成型的材料.但由于反应烧结法的特殊工艺,反应烧结碳化硅中常含有较多游离硅,严重损害了材料的高温性能.主要阐述了反应烧结碳化硅高温力学性能、抗氧化性能、导热性能和抗热震性能的研究现状,并总结了近年来降低游离硅含量、提高反应烧结碳...     

Liquid phase sintering and characterization of SiC ceramics

The present study focuses on the sintering of silicon carbide-based ceramics (SiC) by liquid phase sintering (LPS) followed by characterization of the produced ceramics. AlN/Re₂O₃ mixtures were used as additives in the LPS process. In the first step, the LPS-SiC materials were produced in a graphite resistance furnace in the form of discs at different temperatures. The conditions with the best results regarding real density and relative density were taken as reference for sintering in the form of prismatic bars. In the second step, these samples were evaluated regarding fracture toughness (K_IC), by the Single Edge V Notch Beam – SEVNB – method, and flexural strength. K_IC behavior was evaluated according to the depth and curvature radius of the notches. Reliable K_IC values were presented when the ceramic displayed a small curvature radius at the notch tip. When the radius was large, it did not maintain the square root singularity of the notch tip. Tests were carried out to determine K_IC values in atmospheric air and water. K_IC results were lower in water than air, with a decrease ranging between 2.56% and 11.26%. The observations indicated a direct grain size correlation between K_IC values and fracture strength of the SiC ceramics.     

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.     

溶胶-凝胶法引入烧结助剂制备SiC-Y3Al5O12复相陶瓷

以碳化硅、六水硝酸钇、九水硝酸铝和六次甲基四胺为主要原料,通过溶胶-凝胶法引入Al2O3和Y2O3复合烧结助剂,液相烧结制备得到SiC-Y3Al5O12(Y3Al5O12简称YAG)复相陶瓷.采用DTA、TEM、XRD等分析测试技术研究了溶胶-凝胶法引入复合烧结助剂过程及复合烧结助剂对SiC-YAG陶瓷的烧结性能、力学性能、物相组成与显微结构的影响.结果表明干凝胶在920℃左右已完全转变成YAG相,最终获得的YAG粒径小,并均匀分散在SiC表面的SiC-YAG复合粉体;复合粉体先干压、再等静压成型后,在1860℃下烧结45 min,所制得复相陶瓷的相对密度达到了96.5%,抗弯强度达到486 MPa,断裂韧性达到5.7 MPa·m1/2.     

Concurrent reaction-bonded joining and densification of additively manufactured silicon carbide by liquid silicon infiltration

In this study, additive-manufactured silicon carbide preforms were joined and densified by reaction bonding via liquid silicon infiltration. The silicon carbide preforms were first printed by binder jetting additive manufacturing. To demonstrate concurrent joining and densification, two preforms with carbon or parchment papers at the interface were concurrently joined and infiltrated by liquid silicon. Results showed a robust interface with thicknesses ranging from 150 to 500 µm, depending on the paper type and the number of paper layers. High-energy synchrotron X-ray revealed that β-phase silicon carbide was formed inside the interface. Finally, two additively manufactured samples with complicated channel geometry were successfully joined. Energy dispersive spectroscopy of the interface of the channeled samples showed a consistent and robust joining. This concurrent approach of joining and densification enables efficiency improvement of fabricating silicon carbide parts with complicated geometries and widens geometry freedom for additive manufacturing of silicon carbide.