MoSi2基复相材料表面特性的研究

对MoSi2及其复相材料在高温下长时间老化后表面玻璃膜结构进行了研究,并用电学测试手段对表面玻璃膜的生成过程进行跟踪测试.研究了M0Si2及MoSi2-SiC的氧化机理,发现其氧化速度顺序为MoSi2＞MoSi2-SiC.对添加氧化物的复相材料来讲,其玻璃膜的生成速度顺序为MoSi2＞MoSi2-莫来石＞MoSi2-A12O3.     

MoSi2复相材料表面特性的研究

对MoSi2及其复相材料在高温、长时间老化后表面玻璃膜结构进行了研究.并用电学测试手段对表面玻璃膜的生成过程进行跟踪测试,以研究各复相材料表面玻璃膜的生成速度及机理.实验工作研究了MoSi2及MoSi2/SiC的氧化机理.氧化速度顺序为MoSi2＞MoSi2/SiC.对于复相材料来讲,其玻璃膜的生成速度顺序为MS＞MSM＞MSA.     

碳/碳复合材料表面MoSi2-SiC复相陶瓷涂层及其抗氧化机制

对MoSi2-SiC复相陶瓷涂层的显微形貌、相组成及成分进行了观察与分析,考察并研究了有涂层的碳／碳复合材料在1650℃以下温度的等温氧化性能,以及涂层的结构与组成对抗氧化性能的影响,阐明了涂层的抗氧化过程及机理,并进一步提出了合理的涂层结构。结果表明,碳／碳复合材料表面MoSi2-SiC复相陶瓷涂层的抗氧化性能取决于氧在涂层中的扩散过程。     

反应熔渗烧结法制备MoSi2/SiC复合材料

采用反应熔渗烧结方法制备了弯曲强度达262 MPa的MoSi2／SiC复相材料，并研究了其烧结过程和烧结机理。结果表明：MoSi2／C坯体中熔渗硅的方法制备MoSi2／SiC复相材料，液态硅自然浸渗过程在1450℃即可完成，而且在该温度下液态硅的浸渗速率大于或等于反应速率。在1750℃时虽然C Si→β-SiC反应全部完成，但生成的β-SiC发生再结晶，使复相材料的强度降低。成型压力对反应浸渗材料的强度影响不大。     

水热温度对SiC-C/C复合材料表面水热电泳沉积SiCn-MoSi2复合抗氧化涂层的影响

采用水热电泳沉积法在SiC–C/C复合材料表面制备了纳米碳化硅和二硅化钼的复相(SiCn–MoSi2)抗氧化涂层。采用X射线衍射和扫描电子显微镜等对制备涂层的晶相组成、表面及断面微观结构进行了表征。研究了水热温度对制备涂层的结构及高温抗氧化性能的影响,分析了涂层在1 600℃静态氧化行为及失效机理。结果表明:外涂层主要由MoSi2和β-SiC晶相组成。复相外涂层的致密程度、厚度及抗氧化性能随着水热温度的升高而提高。SiCn–MoSi2/SiC复合涂层具有较好的抗氧化和抗热震能力,在1 600℃氧化80 h后氧化质量损失为3.6×10–3 g/cm2。复合涂层在1 600℃的氧化失效主要是由于经过长时间氧化后SiO2玻璃膜层不能及时有效填补涂层中的缺陷,涂层中出现贯穿性的裂纹和孔洞导致的。     

水热温度对SiC–C/C复合材料表面水热电泳沉积SiCn–MoSi2复合抗氧化涂层的影响

采用水热电泳沉积法在SiC–C/C复合材料表面制备了纳米碳化硅和二硅化钼的复相(SiCn–MoSi2)抗氧化涂层。采用X射线衍射和扫描电子显微镜等对制备涂层的晶相组成、表面及断面微观结构进行了表征。研究了水热温度对制备涂层的结构及高温抗氧化性能的影响,分析了涂层在1 600℃静态氧化行为及失效机理。结果表明:外涂层主要由MoSi2和β-SiC晶相组成。复相外涂层的致密程度、厚度及抗氧化性能随着水热温度的升高而提高。SiCn–MoSi2/SiC复合涂层具有较好的抗氧化和抗热震能力,在1 600℃氧化80 h后氧化质量损失为3.6×10–3 g/cm2。复合涂层在1 600℃的氧化失效主要是由于经过长时间氧化后SiO2玻璃膜层不能及时有效填补涂层中的缺陷,涂层中出现贯穿性的裂纹和孔洞导致的。     

Cf/SiC 复合材料的 ZrB2-SiC/SiC 超高温陶瓷涂层研究

采用两步包埋法在Cf/SiC复合材料表面制备了Zr B_2-SiC/SiC超高温陶瓷涂层。借助SEM、XRD对涂层的微观结构及物相组成进行了分析研究,并进行了高温静态氧化和热震测试。研究表明,1500°C氧化5 h后,涂层表面覆盖有平整的玻璃相氧化层,氧化失重率为6.4%;热震测试10次后涂层的氧化失重率为14%。Zr B_2-SiC/SiC涂层能有效提高Cf/SiC复合材料的高温抗氧化性能。     

莫来石结合Al2O3-SiC浇注料的显微结构特征

借助SEM和EDAX研究了莫来石结合Al2O3 -SiC浇注料的显微结构特征。结果表明 :在莫来石结合Al2 O3 -SiC浇注料中 ,莫来石相与填充在其间隙的玻璃相形成连续基质 ;刚玉颗粒与基质中SiO2 反应生成的二次莫来石与基质中的原生莫来石交错存在 ,构成网状结构 ;SiC颗粒表面高温氧化生成的SiO2 膜 ,改善了SiC颗粒与基质的润湿性 ,是其与基质直接结合的媒介。     

原位合成TiB2-SiC基复相陶瓷及其高温摩擦学性能的研究

本研究以SiC为基体，用TiC和B4C为原料，采用新的反应原理生成TiB2，原位合成了TiB2-SiC基复相陶瓷，提高了SiC陶瓷的物理性能和高温摩擦学性能：随着材料中TiB2物相重量百分比的增加，材料的高温摩擦学性能提高。在以下摩擦环境参数下TiB2（wt25％）SiC基复相陶瓷自对偶在空气中高温摩擦磨损性能较好，呈现良好的高温自润滑性能：在升温状态下、空气中、环境温度为200℃-1000℃、外加载荷为0．2MPa、摩擦速度为0．3m／s，温度和外加载荷对TiB2-SiC基复相陶瓷自对偶比磨损率的影响具有依存性。高温摩擦氧化是TiB2-SiC基复相陶瓷自对偶高温磨损主要机理，磨损试样磨损断面包含摩擦氧化层、过渡层和基体亚表面三层。氧化层和过渡层接触紧密；磨屑具有典型包裹结构。     

一种MoSi2发热元件的组成和结构分析

采用X射线衍射、扫描电镜和能谱分析技术研究了国外某MoSi2发热元件的组成和结构.结果表明:该MoSi2发热元件的主要组成为MoSi2、Mo5Si3以及以Al2O3和SiO2为主的玻璃相;黑色相和亮色相均匀分布在灰色基体中,亮色相主要伴随黑色相形成;断口以解理穿晶特征为主.MoSi2发热元件表面有一层20μm厚的以SiO2和Al2O3为主的玻璃保护膜,在保护膜表面有大量针状凸起生成.保护膜外表面形貌以及发热元件中黑色相和保护膜表面针状凸起中Al含量的增加是该MoSi2发热元件区别国内外其它MoSi2发热元件的主要特征.     

MoSi2–再结晶SiC复合材料的高温抗氧化性能及氧化机理

在SiC粉中添加MoSi2粉,采用模压成型、无压烧成方法制备MoSi2–再结晶SiC(RSiC)复合材料。利用扫描电子显微镜、X射线衍射和等温氧化法研究复合材料的高温抗氧化性能及氧化机理。结果表明,所得复合材料中SiC为6H型,部分MoSi2转变为六方结构Mo4.8Si3C0.6,添加MoSi2前后样品的氧化产物均为方石英,样品表面生成的氧化膜形貌相似。氧化过程中样品质量变化与时间关系遵循抛物线规律,随MoSi2添加量增加,复合材料的抗氧化性能显著提高,其中,添加20%(质量分数)MoSi2所得复合材料在1500℃循环氧化100h后质量增加量仅为未添加MoSi2样品的37%。当MoSi2添加量为10%时,复合材料的抗氧化性能随样品烧成温度的升高先提高后降低,2 300℃烧成所得材料有较好的高温抗氧化性能,其氧化速率常数为0.99mg2/(cm4.h)。在氧化初始阶段,Mo4.8Si3C0.6和MoSi2首先发生氧化反应,随氧化时间增加,Mo4.8Si3C0.6和MoSi2消耗殆尽,此后的氧化则主要为Mo5Si3和SiC的氧化。SiO2膜的致密性和膜厚度与膜中Mo5Si3的含量有关。     

Research on microstructure and oxidation resistant property of ZrSi2-SiC/SiC coating on HTR graphite spheres

ZrSi₂-SiC/SiC coating was prepared on the surface of high temperature gas-cooled reactor (HTR) matrix graphite spheres by two-step pack cementation and sintering process. The microstructure, oxidation resistance and thermal shock resistance properties of the as-prepared coatings with different original powder mixtures were investigated. Results show that dense microstructure of the ZrSi₂-SiC/SiC coating and continuous ZrSiO₄-SiO₂-ZrO₂ glass phase generated during the oxidation process were the key factors for the outstanding thermal properties. When the mole ratio of Zr:Si:C reaches 1:7:3 in the second pack cementation powders, the coated graphite spheres have optimum oxidation resistant ability. The weight gain is only 0.6 wt% after 15 times thermal shock tests and 0.12 wt% after isothermal oxidation test at 1500 °C for 20 h in air. The oxidation resistant mechanism of the coating was also discussed. The dense inner SiC layer and the outer glass layer generated during the oxidation process could protect the ZrSi₂-SiC/SiC coating from further oxidation.     

Model of morphology evolution in the growth of polycrystalline β-SiC films

The growth of β-SiC films via chemical vapor deposition (CVD) has been under intensive investigation because this is viewed to be an enabling material for a variety of new semiconductor devices in areas where silicon cannot effectively compete. However, the difficulty in achieving single-crystal or highly textured surface morphology in films with low bulk defect density has limited the use of β-SiC films in electronic devices. Although several researchers have reported results relating the morphology of β-SiC films to deposition parameters, including substrate temperature and gas composition, detailed knowledge of the effects of deposition parameters on film morphology and crystallographic texture is still lacking. If these relationships between deposition parameters and film morphology can be quantified, then it may be possible to obtain optimal β-SiC film morphologies via CVD for specific applications such as high-power electronic devices.The purpose of this study is to predict the dependence of the surface morphology of β-SiC films grown by CVD on substrate temperature and inlet atom ratio of Si:C, and to model the morphological evolution of the growing polycrystalline film. The Si:C ratio is determined by the composition of the reactant gases, propane (C₃H₈) and silane (SiH₄). A two-dimensional numerical model based on growth rate parameters has been developed to predict the evolution of the surface morphology. The model calculates the texture, surface roughness, and grain size of continuous polycrystalline β-SiC films resulting from growth competition between nucleated seed crystals of known orientation. Crystals with the fastest growth direction perpendicular to the substrate surface are allowed to overgrow all other crystal orientations. When a continuous polycrystalline film is formed, the facet orientations of crystals are represented on the surface. In the model, the growth parameter α_2D, the ratio of the growth rates of the {10} and {11} faces, determines the crystal shapes and, thus, the facet orientations of crystals. The growth rate parameter α_2D used in the model has been derived empirically from the textures of continuous β-SiC films reported in the literature.     

Deposition of diamond/β-SiC composite gradient films by HFCVD: A competitive growth process

Surfaces featuring gradients of chemical composition and/or morphology allow high-throughput investigations and systematic studies in disciplines such as physics, chemistry, materials science, and biology. In this work, novel diamond/β-SiC composite films exhibiting a gradient composition were synthesized by a hot filament chemical vapor deposition (HFCVD) technique utilizing H₂, CH₄, and tetramethylsilane (TMS) as reaction gases. A specific filament-sample arrangement in the HFCVD chamber induced a gradation in chemical composition of the gas phase above the substrate surface, which, in turn, leads to a gradual change in the composition of the deposited films potentially ranging from pure diamond to pure β-SiC. It was possible to control the actual details of the diamond/β-SiC ratio in the gradient films by adjusting deposition pressure and TMS concentration. Aside from film characterization by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy were employed to determine the presence and quality of both diamond and β-SiC phases, respectively. The wealth of information provided by such diamond/β-SiC composite films allowed for a systematic investigation of the mechanism governing their growth. It turned out, that the growth process features nonequilibrium characteristics. It is dominated by a competition between a kinetic product (diamond) and kind of a thermodynamic product (β-SiC) to occupy any available positions on the substrate and the growing surface, respectively. With higher hydrogen radical concentration [H] and substrate temperature, the deposition is kinetically controlled, leading to diamond dominated films. On the other hand, a lower [H] and substrate temperature, consequently resulted in a predominantly thermodynamically controlled deposition, featuring a higher β-SiC content in the film.     

MoSi2和(Mo,W)Si2涂层的宽温域氧化过程

采用包渗法在Mo及Mo?W基体上分别制备MoSi2及(Mo,W)Si2涂层,研究了W掺杂对MoSi2涂层抗氧化性能的影响规律和作用机理。结果表明,W元素固溶到MoSi2涂层中,形成(Mo,W)Si2固溶体,涂层微观结构更加致密化。在1600℃高温下静态氧化,(Mo,W)Si2涂层抗氧化失效时间长达70 h,1200℃下氧化1000 h仍具有良好的防护性能,抗氧化性能大幅提升。加入W元素阻碍了Si元素与基体间的扩散反应,降低了涂层中Si元素的消耗速率,显著增强了(Mo,W)Si2涂层抗高温氧化性能。在500℃低温下静态氧化50 h,与MoSi2涂层相比,(Mo,W)Si2涂层氧化产生明显的“Pest”现象,涂层严重粉化失效。加入W元素降低了涂层中Si元素的扩散速率,导致低温下涂层表面无法形成致密氧化层,加剧涂层的快速氧化。     

Oxidation protection of B4C modified HfB2-SiC coating for C/C composites at 1073–1473 K

B₄C modified HfB₂-SiC coating for C/C substrate was designed to expand the application of HfB₂-SiC based coating in low-medium temperature environment. The oxidation protection behavior of HfB₂-SiC based ceramic coatings with and without B₄C at 1073, 1273 and 1473 K was tested and analyzed. The experimental results reveal that the oxidative damage of HfB₂-SiC coated C/C reduces by over 20% after introducing B₄C, which may be due to the protection of borosilicate glass with more suitable viscosity during oxidation. Meanwhile, B₄C can improve the oxidation protection ability of HfB₂-SiC coating best at 1473 K. And the introduction of B₄C can reduce the mass loss of HfB₂-SiC coated C/C sample by 77.6% after oxidation for 58 h at 1473 K. The fluidity of glass film becoming better with temperature-rising, and the fluid borosilicate glass layer makes the coated samples have the best anti-oxidation properties at 1473 K among these three temperatures.     

Low temperature synthesis of pure phase TaB2 powders and its oxidation protection modification behaviors for Si-based ceramic coating in dynamic oxidation environments

To reveal the generation mechanisms of the Ta-Si-O glass ceramics layer in dynamic oxidation environments, a 40?wt% TaB₂-SiC coating was prepared by liquid phase sintering method. To obtain pure phase TaB₂ powders at lower temperature (1500?°C), excessive B₂O₃ powders were added in raw materials to eliminate the TaC byproduct phase. The hexagonal pure phase TaB₂ powders own average particle size of about 386?nm. During the TGA dynamic oxidation tests, after the modification of 40?wt% TaB₂, the initial weight loss temperature of the sample delayed by about 48%, while the weight loss percentage and rate in fastest weight loss zone decreased by about 61% and 53%, respectively. During oxidation, the generated Ta-oxides were peeled and carried away by the formed fluid SiO₂ glass layer to form “Ta-oxides halation” at first, which results the dissolution of Ta-oxides in the SiO₂ glass, thus forming the Ta-Si-O glass ceramics with dendritic structure. With the spread of the SiO₂ glass layer and growth of the Ta-Si-O dendrite, the Ta-Si-O glass ceramics gradually cover on the surface of the SiO₂ glass layer, forming the structure of Ta-Si-O/SiO₂ double glass layer that is capable of sealing and arresting of microcracks.     

ZnFe_2O_4和TiO_2纳米复合薄膜光电催化性能的研究

采用溶胶凝胶法,在导电玻璃上制备了纳米ZnFe2O4和TiO2的复合薄膜,利用X射线衍射仪及扫描电镜对其进行了表征,通过复合薄膜对甲基橙的降解试验研究了其光电催化性能及催化机理。并将复合薄膜、ZnFe2O4薄膜和TiO2薄膜在不同电压下的光电催化效果进行了对比,结果表明:复合薄膜的光电催化降解效率有了明显提高,三层复合薄膜ZnFe2O4+TiO2+ZnFe2O4的光催化效果最好,在外加偏压0.2-6V范围内随电压增大不同薄膜对甲基橙的降解率均呈现波动性增长。