超高温陶瓷及其复合材料的稀土改性研究进展

超高温陶瓷及其复合材料因具有耐超高温、轻质和抗氧化烧蚀等优点,目前已成为航空航天领域热结构材料研究的热点和前沿。基于稀土化合物在热障涂层等领域的优异性能和成功应用,研究人员将稀土化合物引入超高温陶瓷及其复合材料中,改善氧化层的结构和性质,以期解决超高温陶瓷基复合材料氧化层增长速度偏快和宽温域高低温循环氧化层易剥落等问题。本文综述了稀土改性超高温陶瓷及其复合材料的研究现状,分析探讨了改性机理,并展望了未来的研究发展方向。     

高温耐磨抗氧化陶瓷材料的研究进展

介绍了陶瓷材料的氧化及磨损机理等方面的近期研究成果,总结了陶瓷材料高温耐磨性、抗氧化性的研究进展,最后提出了改善陶瓷材料耐磨性及抗氧化性的可能途径和方法.     

氧化结合法制备多孔碳化硅陶瓷及其特性

以不同粒径碳化硅为骨料,羧甲基纤维素钠(CMC)为粘结剂,在大气中利用碳化硅颗粒表面氧化成的SiO2粘接在一起低温合成多孔碳化硅陶瓷.分析了粒径大小、烧结温度、成型压力对氧化结合多孔碳化硅陶瓷特性的影响.用TG-DSC、XRD、SEM研究了碳化硅陶瓷的氧化性能,物相组成,微观形貌.结果表明:原始粒径越小,碳化硅陶瓷的活性越高,相应的氧化程度越高,在1μm时氧化率最高达到49.58％,但其在1000℃保温100 h质量增重也最高达7.53％;随着烧结温度升高,碳化硅氧化率增加,气孔率相应地降低;成型压力也对碳化硅陶瓷的氧化率,气孔率产生一定影响.     

Ultra-High Strain Ceramics with Multiple Field-Induced Phase Transitions

The induced polarization and longitudinal strain behavior of lanthanum- and titanium-modified antiferroelectric lead zirconate ceramics were measured as functions of electric field as high as 30 kV/mrn (300 kV/cm). Two field-induced phase transitions were observed, both in polarization and strain. Polarization and strain levels greater than 0.5 C/m² and 0.75%, respectively, were achieved in this materials family.     

超高温材料的研究进展

超高温材料对于航天飞行器具有重要作用,是飞行器在长时飞行、跨大气层或再入飞行中不可或缺的组成部分,对飞行器的热防护系统起着至关重要的作用。本文对近年来难熔金属及其合金、C/C复合材料、超高温陶瓷等超高温材料的最新研究成果进行归纳、总结,分析超高温材料的优缺点,提出存在的主要问题,探讨今后的主要研究目标和重点发展方向。     

High‐Temperature Isothermal Oxidation of Ultra‐High Temperature Ceramics Using Thermal Gravimetric Analysis

Oxidation of ZrB₂ + SiC composites is investigated using isothermal measurements to study the effects of temperature, time, and gas flow on oxidation behavior and microstructural evolution. A test method called dynamic nonequilibrium thermal gravimetric analysis (DNE‐TGA), which eliminates oxidation during the heating ramp, has been developed to monitor mass change from the onset of an isothermal hold period (15 min) as a function temperature (1000°C–1600°C) and gas flow (50 and 200 mL/min). In comparing isothermal to nonisothermal TGA measurements, the scale thicknesses from isothermal tests are up to 4 times greater, indicating that oxidation kinetics are faster for isothermal testing, where the oxide scale thickness is 110 μm after 15 min at 1600°C in air. Isothermal oxidation followed parabolic kinetics with a mass gain that is temperature dependent from 1000°C–1600°C. The mass gain increased from ~5 to 45 g/m² and parabolic rate constants increased from 0.037 to 2.2 g²/m⁴·s over this temperature range. The effect of flow velocity on oxidation is not significant under the given laminar flow environment where the gas boundary layer is calculated to be 4 mm. These values are consistent with diffusion of oxygen through the glass‐ceramic surface layer as rate limiting.     

Oxidation of Polymer-Derived SiAlCN Ceramics

The oxidation behavior of polymer-derived amorphous SiAlCNs was studied in the temperature range of 900°–1200°C. The results revealed that while at 900°C the oxidation of the SiAlCNs follows typical parabolic kinetics, at higher temperatures the oxidation rates of the materials decrease with annealing time. Long-term oxidation rate of the SiAlCNs is much lower than the lowest values reported for chemical vapor deposition of SiC and Si₃N₄. Structures of the oxide scales were studied using solid-state nuclear magnetic resonance. We proposed that oxide scales formed for the SiAlCNs possess a unique network structure of the oxide scale in which aluminum atoms block the path of oxygen diffusion, thus lowering the oxidation rates. Such a unique structure was likely formed gradually with annealing time, leading to a continuous decrease in oxidation rate.     

AlN陶瓷制备过程中氧化行为研究

本论文以氮化铝陶瓷的制备为研究对象,围绕其在烧结过程中的氧化进行了研究,分析了氮化铝陶瓷在烧结过程中氧化的机理,提出了防止AlN陶瓷制备过程氧化的措施,确定了合适的埋粉碳粉和AlN粉体,并确定了合适的配比点。     

Stress-Enhanced Oxidation of Silicon Nitride Ceramics

Silicon nitride ceramics show an accelerated oxidation rate under load in air. This phenomenon was observed for porous and dense ceramics with and without additives in a wide temperature range (700°–1450°C) and can be interpreted as stress corrosion in oxygen-containing environments. Stresses cause an alteration of the amount and composition of oxidation products, formation of pits and cracks on stressed parts of specimens, and changes of the surface coloration and oxide scale morphology. Both tensile and compressive stresses can affect the oxidation process. An exponential dependence of mass gain on stress was found. On the other hand, oxidation of silicon nitride-based ceramics can affect the material response to mechanical stresses as, for example, deformation, cavitation and cracking. Stress-assisted chemical reaction at lower temperatures and stress-affected diffusion at higher temperatures seem to be the main reasons for the susceptibility of Si₃N₄ ceramics to stress corrosion. The effect of stress corrosion on mechanical properties is discussed.     

Precursor-Derived Si-B-C-N Ceramics: Oxidation Kinetics

The oxidation behavior of three precursor-derived ceramics—Si_4.46BC_7.32N_4.40 (AMF2p), Si_2.72BC_4.51N_2.69 (AMF3p), and Si_3.08BC_4.39N_2.28 (T2/1p)—was investigated at 1300° and 1500°C. Scale growth at 1500°C in air can be approximated by a parabolic rate law with rate constants of 0.0599 and 0.0593 μm²/h for AMF3p and T2/1p, respectively. The third material does not oxidize according to a parabolic rate law, but has a similar scale thickness after 100 h. The results show that at least within the experimental times these ceramics develop extremely thin scales, thinner than pure SiC or Si₃N₄.     

低温共烧陶瓷

所谓低温共烧陶瓷（Low－Temperature Cofired Ce－ramics，LTCC）技术，就是将低温烧结陶瓷粉制成厚度精确且致密的生瓷带作为电路基板材料．在生瓷带上利用激光打孔、微孔注浆、精密导体浆料印刷等工艺制出所需要的电路图形，并将多个无源元件埋入其中．然后叠压在一起，在900℃下烧结，制成三维电路网络的无源集成组件；也可制成内置无源元件的三维电路基板，     

Strength Distributions of SiC Ceramics After Oxidation and Oxidation Under Load

The room-temperature strength distributions of a sintered and a hot-pressed SiC were examined as-machined, after oxidation at 1370°C, and after oxidation under load at 1370°C. The strengths were observed to be dependent on both the duration of oxidation and the magnitude of the applied load. Processes resulting in both strengthening and weakening behavior were observed to occur, at times simultaneously within the same strength distribution. This dynamic situation indicates that the strength-controlling flaw populations are highly transient in nature.     

Preparation and Characterization of UltraHigh‐Temperature Ternary Ceramics Ta4HfC5

Tantalum hafnium carbide (Ta₄HfC₅) powders were synthesized by solvothermal treatment and carbothermal reduction reactions from an inorganic hybrid. Tantalum pentachloride, hafnium chloride, and phenolic resin were used as the sources of tantalum, hafnium, and carbon, respectively. Pyrolysis of the complexes at 1000°C/1 h initiated the carbothermal reduction to result in multiplex phases including tantalum carbide and hafnium oxide which after heat treatment at 1400°C–1600°C transformed to single‐phase solid solution Ta₄HfC₅ by solid solution reaction. The mean crystallite size of Ta₄HfC₅ particles was less than 80 nm, and the composition of Ta, Hf, and C elements was near stoichiometric and homogeneously distributed in the powder samples. XRD pattern for Ta₄HfC₅ powders was analyzed.     

Noncontact Ultrasonic Characterization of Ceramics

A noncontact ultrasonic technique for the characterization of piezoelectric ceramics is described. The velocity of a laser-generated surface-acoustic-wave pulse propagating at the surface of the ceramic material is measured by an optical probe. The laser beam has an annular section in order to produce a conveniently strong ultrasonic pulse without overheating the surface. Applications to the characterization of PZT samples are described.     

Mechanism and Kinetics of Oxidation of ZrN Ceramics

Oxidation of ZrN ceramics from 973–1373 K under static conditions reveals parabolic rate behavior, indicative of a diffusion‐controlled process. In‐situ high temperature powder XRD found the oxidation mechanism begins with destabilization of ZrN through formation of a ZrN_1?x phase with oxide peaks initially detected at around 773 K. The zirconium oxide layer was found to be monoclinic by in‐situ XRD with no evidence of tetragonal or cubic polymorphs present to 1023 K. Bulk ceramic samples oxidized at 1173 and 1273 K underwent slower oxidation than those oxidized at 973 and 1073 K. This change in oxidation rate and hence mechanism was due to formation of a denser c‐ZrO₂ polymorph stabilized by nitrogen defects. This N‐doped dense ZrO₂ layer acts as a diffusion barrier to oxygen diffusion. However, at an oxidation temperature of 1373 K this layer is no longer protective due to increased diffusion through it resulting in grain boundary oxidation.     

ZrB2–SiC多层陶瓷的抗氧化性

用传统陶瓷的流延工艺制备ZrB2–SiC多层陶瓷。用Archimedes法测定ZrB2–SiC多层陶瓷的相对密度。用扫描电子显微镜观察其显微结构,并进行循环抗氧化性能评价。结果表明：ZrB2–SiC多层陶瓷在1 950℃烧结的致密度达到99.7%,材料的抗氧化过程主要可分为两个阶段：第一阶段低熔点相的挥发,出现质量损失;第二阶段氧化层的形成,降低进一步氧化速率。抗氧化性能较ZrB2–SiC复相陶瓷有很大提高。     

Oxidation Behavior of NBD 200 Silicon Nitride Ceramics

The oxidation behavior of NBD 200 Si₃N₄ containing 1 wt% MgO sintering aid was investigated in oxygen at 900°-1300°C. The oxide growth followed a parabolic rate law with an apparent activation energy of 260 kJ/mol. The oxide layers were enriched with sodium and magnesium because of outward diffusion of intergranular Na⁺ and Mg²⁺ cations in the ceramics. The 2-4 orders of magnitude higher oxidation rate for NBD 200 Si₃N₄ than for other Si₃N₄ ceramics with a similar amount of MgO could be attributed to the presence of sodium. The oxidation process was most likely rate limited by grain-boundary diffusion of Mg²⁺.     

相变陶瓷板调温性能的研究

本文以石蜡为芯材,水性环氧树脂为壁材,通过化学聚合法制备含有相变材料的胶囊。将制成的相变胶囊填入空心陶瓷板内的夹层。分别测试掺入相变胶囊的的陶瓷板和未掺相变胶囊的陶瓷板在升温和降温过程中的温度变化情况。实验表明,在陶瓷板中掺入相变胶囊可以起到调节温度的作用。在相变胶囊吸热过程中,调温效果最大可达3.7℃;放热过程中,调温效果最大可达3℃。在建筑材料中加入相变材料可以起到减少制冷或采暖设备的运行时间的作用,达到节约能耗的目的。     

Rapid Rate Sintering of Yttria Transparent Ceramics

This paper reports on the influence of rapid rate sintering (RRS) on densification and microstructure evolution of yttria transparent ceramics by using vacuum sintering. The presence of temperature gradient has been confirmed during the RRS process. The higher the heating rate (HR), the larger the temperature gradient in the samples would be. By using RRS, e.g., HR = 40°C/min, the samples could be densified very fast to a relative density of 99.6%. However, these samples could not be further densified, due to the presence of difference in densification caused by a heating rate‐induced temperature gradient. By using a two‐step RRS with an intermediate‐temperature thermal treatment, this problem has been successfully addressed. The intermediate‐temperature treatment allowed for the particle neck growth, so that effective thermal conductivity of the compacts was increased greatly. Therefore, the temperature gradient and differentiate densification were effectively prevented. Samples sintered using the two‐step RRS process could be fully densified and excellent in‐line optical transmittance was achieved. It is believed this strategy is applicable to other transparent ceramics, as well as other engineering ceramics.