Papers by Keyword: MoSi₂

Abstract: By summarizing the research and development status of electro-thermal materials, this thesis puts its focus on the types, preparation methods and applications of non-metallic high-temperature electrothermal materials, and the comparative analysis of the advantages and disadvantages of such electric heating elements as SiC, ZrO₂, LaCrO₃ and MoSi₂. At last, the gap and the main problems existing between researches made on high temperature electrothermic materials at home and abroad are pointed out, as well as their prospected development trend.

Abstract: In this paper, nano-Ti₃SiC₂/MoSi₂ composite, whose second phase was 20-150nm, was in situ prepared by mechanical activation (MA) and SPS process with the quaternary powers of Mo, Si, Ti, and C. The results showed that: (1) matrix MoSi₂ has strong repulsion to other elements, which leads to more second-phase particles inside the matrix rather than on the matrix surface; (2) matrix MoSi₂ has strong restriction on the growing of the second phase, which makes the particle diameter of the second phase inside the matrix only in 200 nm around, while that over the surface reaches to 800 nm around.

Abstract: Molybdenum disiliside is known as a ceramic material with attractive properties for high temperature structural applications. In this study, mechanical alloying was used to produce MoSi2 powder directly from molybdenum oxide. Mixture of MoO3 and Si powders with commercial purity were exposed to high mechanical activation in a planetary ball mill. The ball to powder mass ratio was selected to be constant at 33:1 and the rotation speed (cup speed) was 600 rpm during the milling operations. Crystallite sizes and structural evolutions during milling were investigated by Xray diffraction analysis. The morphology of the mechanically alloyed powders was evaluated with scanning electron microscope (SEM). From XRD results, it was observed that within 6 hours of milling MoO3 was reduced and fully converted to MoO2. After 17 hours of milling MoO2 also began to reduce and peaks of MoSi2 (both
and  phases) and Mo were detected. Further milling resulted in a gradual decrease in MoO2 peak intensities because of its continuous reduction. Peaks of MoO2 were also broadened due to refinement of MoO2 crystallite sizes. Scherrer and Williamson-Hall methods using XRD patterns were employed to calculate the mean crystallite size. Calculations indicated that in the sample ball milled for 50 hours, MoSi2 crystallite sizes were less than 100 nm.

Abstract: MoSi₂ and SiC ceramics were prepared by traditional powder metallurgy method. A novel ceramic thermocouple (CTC) used in the high temperature and high corrosion environment was assembled by SiC as the cathode and MoSi₂ as the anode. The thermo-emf of CTC was tested from 25 °C to 1600 °C in air. The results show that there is a simple relationship between the thermo-emf of CTC and the temperature. By holding temperature for 70 h at 1500 °C, the deviation of thermo-emf is ±0.37%, and better than 0.75% which is the criteria of industry thermocouple. And also the response characteristics of CTC were analyzed.

Abstract: Carbon nanotubes (CNTs) are good reinforcement of composite materials, through add appropriate amount of carbon nanotubes to MoSi2 can be improve the strength and toughness of MoSi2. In this paper, the material of CNTs/MoSi2 was made through vacuum hot pressing technology. And the split hopkison press bar (SHPB) technology was used for testing the dynamic mechanical properties of CNTs/MoSi2 composite material which has different proportion of CNTs. In the SHPB experiment, in order to get better waveforms, the waveform shaping technology was used for improving the waveform quality, and also the strain gauge technology was used for testing the real strain of the specimen in the dynamic loading process. Through calculating, the dynamic stress-strain curves which under different high strain rate are given. At the same time, the strength, deformation and other test results are analyzed, and also compared them with the static compression experiment results of the CNTs/MoSi2 specimen which is tested by the electronic universal testing machine, and then obtained the dynamic performance of CNTs/MoSi2 composite material.

Abstract: SiC reinforced MoSi2 composites have been successfully prepared by pressureless sintering from mechanical-assistant combustion synthesized powders. The sintering temperatures and holding time were 1500°C~1650°C at a heating rate of 10K/min and 1 hour, respectively. The microstructure and mechanical properties of the as-sintered composites were investigated. SEM micrographs of SiC/MoSi2 composites showed that SiC particles were homogeneously distributed in MoSi2 matrix. The Vickers hardness, flexural strength and fracture toughness of the SiC/MoSi2 composites were up to 15.50GPa, 468.7MPa and 9.35MPa•m1/2, respectively. The morphologies of fractured surface of the composites revealed the mechanism to improve mechanical properties of MoSi2 matrix. At last, the cyclic oxidation behavior of the composites was discussed. The results of this work showed that in situ SiC/MoSi2 composite powder prepared by MASHS technique could be successfully sintered via pressureless sintering process and significant improvement of room temperature mechanical and anti-oxidation properties could be achieved.

Abstract: The MoSi2 nanocrystal was prepared by mechanical alloying (MA) large particle-sized starting powders, in which the milling time is much longer than usual MA time. It was found that the Mo-Si powder mixture mixed at stoichiometry proportion forms α-MoSi2 and β-MoSi2 in the MDR mode rather than pure α-MoSi2 in the SHS mode. The grain size of MoSi2, calculated using Scherrer′s formula, is 18nm when milled for 96h, and decreases to 12nm when further milling to 144 h. This is because that the milling balls provide enough energy to refine most of the rough crystal grain. The average grain size increased to 15nm when milled for 192 h, which indicates that further expand time could not refine the crystal grain while cause the growth of a part of the crystal grain. The particle size of MoSi2 is about 0.5μm when milled for 96 h and the agglomerating phenomenon is severe. The particle size of MoSi2 decreases to 0.4μm and releases the agglomerating phenomenon with the milling time for 144 h.

Abstract: Si3N4 particle reinforced MoSi2 composite powder has been successfully synthesized combustion synthesis method. XRD and SEM results showed that the combustion product was mainly composed by MoSi2 and Si3N4. The as-prepared Si3N4/MoSi2 composite powder has been pressureless sintered at 1600°C for 1h. The microstructure and mechanical properties of the composite were investigated. Relative densities of the monolithic material and composite were 92.2% and 89.6%, respectively. The composite has higher Vicker’s hardness and flexural strength than monolithic MoSi2. Especially the room-temperature fracture toughness of the composite is from 4.21MPa•m1/2 for MoSi2 to 7.25MPa•m1/2 for composite, increased by 72.2%, respectively. The morphology of fractured surface of composite revealed the mechanism of improving mechanical properties of MoSi2 matrix. The results of this work showed that in situ Si3N4/MoSi2 composite powder prepared by combustion synthesis could be successfully pressureless sintered and significant improvement of mechanical properties could be achieved.

Abstract: Thin films of Si-added-MoSi2 were deposited on Si3N4 or alumina substrate using an RF magnetron sputtering with a target made of MoSi2 and Si powders. The thin film consisted of a mixture of Si and MoSi2. The resistance of MoSi2-thin-film heaters fabricated using Si3N4 substrate heated for a long time in air increased with increasing heating-time at temperatures near 420°C due to generation of MoO3, which is an insulator, in thin film due to the oxidation of Mo in MoSi2. On the other hand, oxidation-resistant capability of thin-film heater fabricated by adding Si to MoSi2 was drastically improved compared with MoSi2 thin-film heater, because Si particles or the Si layer on a MoSi2 particle prevented the oxidation of MoSi2. The oxidation-resistant capability was excellent in air at high temperatures near 480°C for a long heating-time exceeding 950h.

Abstract: Starting from some fundamentals of solid-state diffusion, we remind the reader to the major techniques for lattice diffusion measurements. Self-diffusion is the most basic diffusion phenomenon in any solid. The paper covers main features of self-diffusion in pure fcc and bcc metals and some important facts about diffusion of substitutional solutes in metals. Binary intermetallics are compounds of two metals or of a metal and a semimetal. Their structures are different from those of the constituents. Some intermetallics are interesting functional materials others have attracted attention as high-temperature structural materials. The paper reviews some results mainly from our laboratory on diffusion in binary intermetallics from the systems Cu-Zn, Ni-Al, Fe-Al, Ni-Ge, Ni-Ga, Fe-Si, Ti-Al, Ni-Mn, Mo-Si and Co-Nb, which have been published in detail elsewhere. Some results for the ternary system Ni-Fe-Al are also mentioned.