Novel synthesis of MoSi2 and MoSi2-Al203 ultrafine powders

Abstract

Molybdenum disilicide (MoSi₂) is a promising high temperature material with a high melting point (2032°C), low density (6.24 Mg m^-3 ), excellent high temperature oxidation resistance (up to about 1900°C), and metal like thermal and electrical conductivities. For structural applications, the formation of composites can be benejicial. Alumina (Al₂O₃) has a thermal expansion coefficient close to that of MoSi₂ in a wide temperature range from 200°C to 1400°C. Improvement in elevated temperature strength has been reported by the formation of a composite between MoSi₂ and A1₂0₃. However, the brittle nature of MoSi₂ based materials at medium to low temperatures remains an obstacle for their applications. One possible solution is to develop microstructures with ultrajine grains so that ductility can be improved through the grain boundary sliding mechanism. The present work gives preliminary results related to the synthesis ofultrajine MoSi₂-Al₂0₃ composite powders. The MoSi₂ powders (0.1–4 μm) were produced by an electric arc discharge process under varying voltage and capacitance. Three powder generating mechanisms have been identified. The influence of processing parameters (voltage, capacitance, and energy input) on the powder characteristics is discussed. Finally, the synthesis of MoSi₂-A1₂0₃ composite powders by coating individual MoSi₂ particles with an alumina gel through a sol-gel process is also demonstrated.     

Simultaneously MoSi2 and SiC-reinforced α-SiAlON composites

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Simultaneously MoSi2 and SiC-reinforced α-SiAlON composites

Abstracts are not published in this journal     

Characteristics of the combustion synthesis of TiC and Fe-TiC composites

The combustion synthesis of TiC, using the thermal explosion mode, was investigated by varying some of the process parameters including the reactant particle size, the pre-compaction pressure, and the heating rate. Based on these observations, a combustion model for the reaction was developed. When iron was added to titanium and carbon black powders, the ignition temperature was dictated by the eutectic temperature of the Fe-Ti system. Iron acted as a moderator for the reaction and led to a decrease in the combustion temperature.     

Elevated temperature mechanical properties of MoSi2/Si3N4, MoSi2/SiC composites produced by self-propagating high temperature synthesis

MoSi₂/Si₃N₄ and MoSi₂/SiC composite powders were produced via a novel self-propagating high temperature synthesis technique (SHS) and consolidated both by hot-pressing and plasma spraying. Mechanical testing subsequently was performed under four-point bending at a variety of elevated temperatures and strain rates. Damage accumulation in the hot-pressed materials, either by particle cracking or particle/matrix separation, resulted in composite mechanical properties that were reduced below the level of the unreinforced material. Plasma sprayed materials showed power-law hardening behaviour caused by Mo₅Si₃ precipitates and increased strain tolerance because of the fine grain size and reduced yield strength of the matrix.     

Multilayer composites in Al2O3/MoSi2 system

A new multilayer composite with a super-plastic layer, a hard layer and a weak interface was proposed. The hard layer can provide the strength of the multilayer composites at high temperature, the plastic layer can deform plastically at high temperature and disperse the applied stress and stop the advance of the crack, and the weak interface can deflect the propagating crack at room temperature. Such multilayer composites were prepared by tape casting in the Al₂O₃/TiC/MoSi₂–Mo₂B₅ system. It was found that such design is effective on the increase of fracture energy both at room temperature and at high temperature, and the strength at high temperature could be remained in a relatively high revel.     

Self-propagating combustion synthesis of intermetallic matrix composites in the ISS

Combustion Synthesis experiments have been performed on the ISS (International Space Station) during the Belgian taxi-flight mission ODISSEA in November 2002, in the framework of the ESA-coordinated project COSMIC (Combustion Synthesis under Microgravity Conditions). The main objective of the experiments was to investigate the general physico-chemical mechanisms of combustion synthesis processes and the formation of products microstructure. Within the combustion zone, a number of gravity-dependent phenomena occur, while other phenomena are masked by gravity. Under certain conditions, gravity-dependent secondary processes may also occur in the heat-affected zone after combustion. To study the influence of gravity, a specially dedicated reactor ensemble was designed and used in the Microgravity Science Glovebox (MSG) onboard the ISS. In this work, the experiment design is first discussed in terms of the experimental functionality and reactor ensemble integration in the MSG. To investigate microstructure formation, a sample constituted by a cylindrical portion followed by a conical one, the latter being inserted inside a massive copper block, is used. The experiment focused on the synthesis of intermetallic matrix composites (IMCs) based on the Al-Ti-B system. Depending on the composition, different intermetallic compounds (TiAl and TiAl₃) can be formed as matrix phase while TiB2 represents the reinforcing particulate phase. During the ISS mission, six samples with a relatively high green density of 65%TD have successfully been processed. The influence of the composition on the combustion process will be examined.     

自蔓延高温燃烧合成MoSi2

以Mo粉和Si粉为原料,通过自蔓延高温燃烧合成(SHS)的方法成功地制备了MoSi2材料.研究了反应物原料粒度、反应物料坯相对密度、反应物预热温度、稀释剂加入量以及反应气氛对MoSi2燃烧合成的影响.并通过XRD和SEM对燃烧合成产物的物相组成和形貌进行了分析.     

In situ fracture monitoring of plasma-sprayed MoSi2-Ta composites

Initial work has shown that the ambient toughness of brittle silicide intermetallics such as MoSi₂ have been toughened by incorporating irregular pancake-shaped Ta phases via deposition of powders by plasma-spray processing techniques. The mechanisms of toughness enhancement were observed in situ by conducting fracture toughness tests in an instrumented loading stage situated in a scanning electron microscope. It was observed that increases in toughness were obtained via ductile-phase toughening and the development of bridgedzones. The initiation and peak toughness of the composites varied with testing orientation, with the highest values measured when the crack plane intersected the Ta particles perpendicular to their edge. The shape of the resistance curve, however, was independent of Ta orientation.     

在电场中燃烧合成Al2O3-TiC-Al复合材料的结构形成机理

以3TiO2 3C (4 x)Al为反应体系,用电场激发燃烧合成技术并使用合成中形成的液态Al对产物的渗透作用,制备出致密度为92.5%的Al2O3-TiC-Al复合材料,采用燃烧波峰淬熄法研究了原位合成Al2O3-TiC-Al复合材料的结构形成机理.结果表明:电场提供的焦耳效应可提高体系的绝热燃烧温度,从而可突破该体系只能在x＜10 mol下发生SHS反应的热力学限制;在Al2O3-TiC-Al复合材料动力学过程中,首先Al粉熔融,进而加速与TiO2的反应生成Al2O3;然后Al与TiO2反应还原出Ti并与C反应生成TiC;液态Al的渗透将Al2O3和TiC颗粒粘结起来,形成致密的复合材料组织.     

Oxidation of MoSi2 and MoSi2-based materials

Oxidation experiments, at 500°C, of MoSi₂ and MoSi₂-based compounds such as Mo(Al,Si)₂ and MoSi₂+1 wt% C compacts have been carried out. These compacts were prepared byin situ synthesis and a compaction method, starting from the elemental powders. For comparison, commercial MoSi₂ and Mo(Al,Si)₂ infiltrated into SiC preform were also studied under similar conditions. It was found that the synthesized high density MoSi₂ and Mo(Al,Si)₂ infiltrated into SiC preform did not show any oxidation even after 100 h of heating in air. The colour of the polished surfaces of commercial MoSi₂, Mo(Al,Si)₂ and MoSi₂+1 wt% C had changed. The SEM of Mo(Al,Si)₂ showed open blisters with rods of MoO₃ in them whereas MoSi₂+1 wt% C surface had MoO₃ rods but no blisters and the oxidation was superficial with no penetration into the compact. It is suggested that in compounds, the presence of small amounts of impurities is not as detrimental to pesting as presence of defects like open pores or cracks. Hence, high density of the compact is essential for the prevention of complete disintegration of the compact.     

稀土/MoSi2复合材料的磨损性能及机理研究

采用磨损试验机、压痕法、X射线衍射仪（XRD）、扫描电子显微镜（SEM）及微探针等研究了稀土／MoSi2复合材料的力学性能、磨损性能及机理。结果表明，随着磨损载荷的增加，RE／MoSi2复合材料的摩擦系数逐渐下降，当负荷〉90N时，其摩擦系数与纯MoSi2样品的摩擦系数相当。当磨损载荷为50N时，RE／Mo—Si2复合材料的磨损率最大（28．95mg／km），当负荷为80～120N时，其磨损率比纯MoSi2材料下降了至少65％。该复合材料的磨损机制主要是粘着磨损和脆性断裂。     

Si3N4 rodlike crystal-reinforced MoSi2 matrix composites

A MoSi₂-based composite reinforced with 20 vol.% Si₃N₄ rodlike crystals was fabricated by Spark Plasma Sintering (SPS) process. The microstructure and mechanical properties of the composite were investigated. Relative densities of the monolithic material and composite were above 95%. No reactions between Si₃N₄ and MoSi₂ were observed. The composite containing Si₃N₄ rodlike crystal had higher hardness than monolithic MoSi₂. The room-temperature flexural strength increased 60% compared to that of pure MoSi₂. Especially the room-temperature fracture toughness of the composites was higher than that of MoSi₂, from 3.6 MPa m^1/2 for MoSi₂ to 5.1 MPa m^1/2 for composites with 20 vol.% Si₃N₄, respectively. Besides, the yield strength at elevated temperature and the low-temperature oxidation resistance for 20 vol.% Si₃N₄–MoSi₂ composite exhibited considerable improvement over that of monolithic MoSi₂. These results showed that Si₃N₄ rodlike crystal is an effective reinforcement for MoSi₂.     

Preparation of calcium aluminate matrix composites by combustion synthesis

CaO–Al₂O₃–TiB₂ composites have been produced by the Combustion Synthesis technique. These materials have matrices based on binary calcium-aluminate compounds, i.e., Ca₃Al₂O₆ (C₃A), Ca₁₂Al₁₄O₃₃ (C₁₂A₇), CaAl₂O₄ (CA), CaAl₄O₇ (CA₂) and CaAl₁₂O₁₉ (CA₆). Except for samples with the matrix composition of C₃A, the combustion synthesis reactions can be characterized as stable self-propagating waves with combustion temperatures ranging from 2125 K to 2717 K and combustion wave velocity from 4.0 mm/s to 10.6 mm/s. For samples with a matrix composition of C₁₂A₇, CA, and CA₂, predominantly equilibrium compound phase was formed, while for samples with a matrix composition of C₃A, non-equilibrium phases were also present. There was no evidence of CA₆ formation for samples with a matrix composition corresponding to CA₆.     

MoSi2及MoSi2基复合材料制备技术的新进展

MoSi2是极具应用前景的新型高温结构材料.综述了利用放电等离子烧结、微波烧结、先驱体转化法、自蔓延高温合成和反应熔渗法制备MoSi2及其复合材料的最新研究进展,并提出合金化和纳米颗粒复合化是今后MoSi2基复合材料的研究发展方向.     

铟-锡复合氧化物粉末的制备及甲烷催化燃烧性能研究

采用共沉淀法制备In2O3-SnO2复合粉末,以TEM、XRD、BET等实验手段对催化剂粉体的形貌和结构进行了表征.结果表明制备的催化剂样品颗粒均匀,粒径范围在10～40nm.该复合氧化物对甲烷的催化燃烧表现出了良好的催化活性.最佳制备条件为铟离子摩尔分数为40%,焙烧温度为600℃.     

A study of combustion synthesis of Ti-Al intermetallic compounds

The mechanisms involved in the combustion synthesis of Ti-Al intermetallics have been studied by quenching partially transformed pellets to suppress the reaction prior to its completion. The reaction steps leading to the formation of TiAl₃, TiAl or Ti₃Al were found to be similar in all three compositions used for the study. In each case the initial product detected by X-ray diffraction was TiAl₃, while substantial dissolution of Ti into molten Al was found. At the same time the molten Al-Ti liquid adsorbed oxygen, and on quenching the partially transformed pellet, a glass was formed from the liquid. If the combustion synthesis process was completed, the oxygen-rich liquid decomposed, yielding a mixture of oxides and intermetallic compounds.     

Thermal properties of MoSi2 and SiC whisker-reinforced MoSi2

The heat capacity, thermal conductivity and coefficient of thermal expansion of MoSi₂ and 18 vol % SiC whisker-reinforced MoSi₂ were investigated as a function of temperature. The materials were prepared by hot isostatic pressing between 1650 and 1700 °C, the hold time at temperature being 4 h. The heat capacity of MoSi₂ showed an increase from about 0.44 Wsg^–11K^–1 at room temperature to 0.53 at 700 °C. Whisker reinforcement increased heat capacity by about 10%. Thermal conductivity exhibited a decreasing trend from 0.63 Wcm^–1 K^–1 at room temperature to 0.28 Wem^–1 K^–1 at 1400°C. Whiskers reduced conductivity by about 10%. The thermal expansion coefficient increased from 7.42 °C^–1 between room temperature and 200 °C to 9.13 °C^–1 between room temperature and 1200 °C. There was a 10% decrease resulting from the whiskers. The measured data are compared with literature values. The trends in the data and their potential implications for high-temperature aerospace applications of MoSi₂ are discussed.