Nanocrystalline WC–Co materials have been the subject of interests and focus of research programs around the world for the past two decades owing to the expectations that the mechanical behavior of the material may improve significantly when grain sizes reduce to nanometer scale. However, although numerous technologies are available for making nanosized tungsten carbide powders, obtaining true nanocrystalline WC–Co (average WC grain size

Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide - A review

Spectroscopic ellipsometry (SE) was employed to get insights on the optical, electronic, and transport properties of nanocrystalline titanium nitride (TiNx) films with respect to their microstructure and stoichiometry. The films’ properties can be tailored by varying the energy of bombarding ions during sputter deposition and the substrate temperature (Td). The best metallic behavior of TiNx (resistivity 40 μΩ cm and conduction density 5.5×1022 electrons/cm3) has been observed in films developed with energy above 100 eV and Td⩾400 °C. A redshift of the optical gaps has been observed for overstoichiometric films, suggesting it as a sensitive probe to investigate the TiNx stoichiometry. The energy, strength, and broadening of the interband transitions were studied with respect to the energy of ions and Td and they were explicitly correlated with the TiNx crystal cell size and grain orientation. On the other hand, the study of intraband absorption has provided the conduction electron density with respect to ...

Optical, electronic, and transport properties of nanocrystalline titanium nitride thin films

Progress in developing a new class of support materials based on silicon carbide (SiC)is reviewed. Silicon carbide has superior mechanical and thermal properties which, coupled to chemical inertness,avoids several of the problems inherent in the use of commercial oxide and carbon based supports and catalysts. High surface area SiC can now be prepared easily in a commercially viable shape,with good mechanical properties,and at reasonable cost.I t can be shaped directly into monolith or honeycomb forms including some catalytically active material, rendering fabrication simple and cost effective. Furthermore, it can be modified for specific catalytic applications through the addition of metals. In many respects, it combines the best properties of oxide and carbon based supports without suffering many of their disadvantages.

Silicon Carbide: A Novel Catalyst Support for Heterogeneous Catalysis

New synthesis and uses of high-specific-surface SiC as a catalytic support that is chemically inert and has high thermal resistance

The N-Ti (Nitrogen-Titanium) System

Black powders of tungsten carbides (WC, W2C), including tungsten metal, were produced by the vapor phase reaction of the WCl6-CH4-H2 system at 1000–1400 °C. The C/W ratio of the product increased sharply with increasing reaction temperature and increasing methane concentration. At 1400 °C the product consisted of almost pure WC. The particle size of the carbide powders, which was significantly influenced by the mixing temperature of WCl6 and H2, was 0.04–0.05 μm for low mixing temperatures and 0.08–0.11 μm for high mixing temperatures. The formation of tungsten carbide powders in this reaction system was found to occur in two steps, namely the initial formation of W particles and their subsequent carburization. This was confirmed by separate examinations of the hydrogen reduction of WCl6 and the carburization of W powders with methane.

Tungsten carbide powders produced by the vapor phase reaction of the WCl6-CH4-H2 system

The formation of silicon carbide powders by the vapor phase reaction of SiCl4 + CH4 (1400–1500°C), CH3SiCl3 (800–1400°C) and (CH3)4Si (800–1400°C) was studied. The effectiveness of the silicon sources was in the sequence (CH3)4Si > CH3SiCl3 > SiCl4, in accordance with the thermodynamic parameters. Fine β-SiC powders were produced by the pyrolysis of (CH3)4Si in hydrogen above 900°C. The crystallinity of the products increased with increasing reaction temperature. The particles were spherical and had average sizes of 0.02–0.12μm which decreased with increasing reaction temperature and with decreasing (CH3)4Si concentration. A reaction process consisting of the formation of particles of organosilicon polymers and their subsequent decomposition was proposed for the formation of SiC particles by the pyrolysis of (CH3)4Si.

Formation of fine silicon carbide powders by a vapor phase method

Nitride powders of titanium and vanadium prepared by vapour phase reactions have been investigated with regard to their defect structure, thermal stability, sinterability and electrical conductivity. Ti nitride and V nitride containing excess of the non-metallic element and with large numbers of vacancies on the metal and non-metal sublattices liberated nitrogen, with the disappearance of vacancies, when held above 600°C under vacuum. The fractional occupancies of both sublattices were near unity at an atomic ratio (N + O) M of 1.00. However, they decreased again when the (N + O) M ratio fell below 1.00. The dissociation pressures of Ti and V nitrides at 1100°C increased sharply when the (N + O) M ratio was close to or above 1.00. The fine powders of Ti and V nitrides (0.03–0.05μm particle size) showed very high sinterabilities. The sintering proceeded rapidly at temperatures of 1000–1300 °C for titanium nitride and 900–1100°C for vanadium nitride without noticeable grain growth. The resistivities of fine powder compacts of Ti and V nitrides were 10−2–10−1 Ω cm and about 10−3 Ω cm, respectively, in the range -173 –400 °C and the temperature dependences were slightly negative.

Defect structure, thermal and electrical properties of Ti nitride and V nitride powders

Formation of SiC particles from Si(CH3)Cl3 by a plasma gas phase reaction

The formation of fine molybdenum carbide powder by the vapor phase reaction of the MoCl 4 -CH 4 -H 2 system was studied at 800–1400 °C. The mixing temperature of MoCl 4 and H 2 had a significant effect on the properties of the powders. After low temperature (600 °C) mixing (method A), Mo 2 C powders free from Mo with a particle size of about 0.08 μm were produced by the reaction at 1200 °C and [CH 4 ] ≧ 0.5%. After mixing at the reaction temperature (method B), pure Mo 2 C powders with a particle size of 0.15–0.2 μm were produced at 1000 – 1200 °C and [CH 4 ] = 1 – 2%, but MoC was formed as the major carbide phase at [CH 4 ] ≧ 2.5%. The Mo 2 C particles produced by method A were polycrystalline and those produced by method B were single crystalline. It was concluded that the formation of molybdenum carbide powders in the present reaction system occurs as follows: formation of Mo particles and their subsequent carburization in method A, and growth of carbide particles by the deposition of Mo and its immediate carburization in method B.

J. Hojo

Papers

Tungsten carbide powders produced by the vapor phase reaction of the WCl6-CH4-H2 system

Formation of fine silicon carbide powders by a vapor phase method

Defect structure, thermal and electrical properties of Ti nitride and V nitride powders

Formation of SiC particles from Si(CH3)Cl3 by a plasma gas phase reaction

Molybdenum carbide powders obtained from the vapor phase reaction of the MoCl4-CH4-H2 system