Rasit Koc

Carbothermal synthesis of titanium carbide using ultrafine titania powders

The synthesis of titanium carbide (TiC) by the carbothermal reduction of carbon coated titanium dioxide (TiO2), a novel synthesis process, and titanium dioxide (TiO2) mixed with carbon black was investigated. A high surface area (64 m2g-1) TiO2 powder consisting of anatase and rutile phases was used for starting powders. The carbon coated method is a two-step process that utilizes a precursor derived from decomposing propylene (C3H6) and depositing carbon on the TiO2 particles. TiO2 powders were also mechanically mixed with carbon black for comparison. Both starting precursors and mixtures were reacted in a tube furnace for 2 and 4 h at temperatures of 1100°C to 1550°C under 1 l min-1 flowing argon. The TiC powders were characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analyser, chemical analysis (oxygen and carbon) and transmission electron microscopy (TEM). The carbon coating process provides high contact area between the reactants which results in a TiC product with lower oxygen content (0.6 wt%), finer particle size (0.1 μm), and uniform shape when synthesized at 1550°C for 4 h.

Kinetics and phase evolution during carbothermal synthesis of titanium carbide from ultrafine titania/carbon mixture

The kinetics and phase evolution of the TiC formation process by carbothermal reduction of ultrafine titania/carbon mixture were investigated using thermogravimetric analysis (TGA), X-ray diffraction (XRD) and oxygen analysis. Titania (TiO2) first lowered its oxidation state to Ti3O5 via an unidentified phase (possibly one of the Magneli phases). Then Ti3O5 was further reduced to Ti2O3, followed by the formation of titanium oxycarbide (TiCxOy) phase and its purification toward high purity TiC thereafter. Ti2O3 was the oxide phase with the lowest oxidation state before forming TiCxOy phase. In the isothermal TGA trace, the formation of Ti3O5 showed a diffusion-controlled process; possibly carbon diffusion limited the solid state reaction. The formation of Ti2O3 and TiCxOy was interpreted to be associated with CO gas-assisted reduction reaction, based on constant reaction rate for each process. The activation energy for the formation of Ti2O3 (from Ti3O5) and TiCxOy (from Ti2O3) phase were calculated to be 415.6 and 264.3 kJ mol-1, respectively. The TiC powder synthesized at 1550 °C for 4 h in flowing argon atmosphere showed fine particle size (0.3–0.6 μm) with oxygen content of 0.7 wt % and lattice parameter of 0.4328 nm while interparticle agglomeration was moderate.

Formation studies of TiC from carbon coated TiO2

This paper deals with the formation of titanium carbide from carbon coated titanium dioxide precursors. This study makes use of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and both scanning and transmission electron microscopy (SEM and TEM). DSC curves of both coated and mixed 33.2 wt % carbon containing titania demonstrates the superiority of the coated precursor by exhibiting both more reactions and reactions at lower temperatures than the mixed powder. Weight loss as powders were reacted in argon at varying temperatures was measured using TGA, while heat flow vs. temperature was measured by DSC. The weight loss allowed for calculation of the activation energy of TiC via the formation of various lower oxides of titanium. The activation energy was calculated as 731.6 ± 24.2 kJ/mol. XRD was used to characterize the products resulting from the reaction of the carbon coated precursor at isotherms at each 100 °C interval from 1100 to 1500 °C, inclusive. These diffraction patterns support the hypothesis that the TiC formation proceeds through the formation of lower oxidation states of titanium.

Sintering properties of submicron TiC powders from carbon coated titania precursor

The sintering behavior of submicron titanium carbide (TiC) synthesized from carbon coated titania (TiO2) precursor was investigated in TiC-Ni system. The densification was examined as functions of initial carbon content (30.95–34 wt.%) and Ni content (3–20 wt.%). The sintered density of TiC-Ni was markedly decreased with increased carbon content in the precursor. The amount of Ni had a relatively small influence on the densification of submicron TiC-Ni cermet compared with TiC (commercially available HCS)-Ni cermets. The results show that submicron TiC with only 3 wt.% Ni can be sintered to densities above 95% TD in flowing Ar+10H2 at 1500°C and below. The improvements in densification result from the capillary force increase since it is inversely dependent on the particle size. With decreased Ni content, the Vickers hardness increased and the fracture toughness decreased, as expected. However, the sufficient densification cannot be achieved for commercial HCS TiC powder sintered with Ni (<10 wt.%) under the same conditions. Therefore, both the Vickers hardness and fracture toughness decreased as the Ni content decreased. This was due to the increase of porosity in the sintered samples containing commercial TiC powder.

Synthesis of beta silicon carbide powders using carbon coated fumed silica

The synthesis of beta silicon carbide (β-SiC) powders by carbothermic reduction of carbon coated silica and silica mixed with carbon black was investigated. The production of β-SiC powders by using carbon coated silica consists of two steps. The first step is to prepare the carbon coated silica precursor by coating fumed silica particles with carbon by pyrolytic cracking of a hydrocarbon gas (C3H6). This provides intimate contact between the reactants and yields a better distribution of carbon within the fumed silica. Fumed silica was also mechanically mixed with carbon black for comparison. Both starting mixtures were reacted in a tube furnace for 2 h at temperatures of 1300°C to 1600°C in 1 l min-1 flowing argon. The reaction products were characterized using weight loss data, X-ray diffraction (XRD), a BET surface area analyser, oxygen and free carbon analysis and transmission electron microscopy (TEM). The carbon coating process resulted in a more complete reaction, purer product and high yield SiC powders with very little agglomeration at temperatures of 1500°C and 1600°C. The β-SiC powders produced at 1600°C for 2 h in argon gas flow have oxygen content of 0.3 wt%, a very fine particle size 0.1–0.3 μm and uniform shape.

Kinetics and phase evolution during carbothermal synthesis of titanium carbide from carbon-coated titania powder

Abstract Kinetics and phase evolution of the TiC formation process by carbothermal reduction of carbon-coated titania (TiO 2 ) powder were investigated using TGA, XRD, and chemical analysis. The TiC synthesis in the present work proceeded via forming titanium oxycarbide (TiC x O y ) followed by its purification toward titanium carbide (TiC). The formation of TiC x O y was achieved via two routes. The uniformly coated pyrolytic carbon on fine titania particles formed the first TiC x O y phase at temperature of 1000 °C. The additional TiC x O y formed from Ti 3 O 5 as temperature increased. The activation energy of the additional TiC x O y formation process was calculated to be 278.1 kJ mole . The resultant TiC powders prepared at 1550 °C for 4 h in flowing argon showed fine particle size (0.1–0.3 μm), oxygen content of 0.57 wt%, lattice parameter of 4.331 A, uniform particle shape, and loose agglomeration between particles.

Electrical and thermal transport properties of (La,Ca)(Cr,Co)O3

Abstract DC electrical conductivity and Seebeck data as functions of temperature and oxygen activity were used to explain the electrical and thermal transport properties of (La,Ca) (Cr,Co)O3. The electrical conductivity data of La(Cr,Co)O3 suggested that it depends upon Co content. At Co concentrations less than 20 mol%, Co acted as traps for the carriers and decreased the electrical conductivity of the compositions at temperatures less than 900 °C. As the Co concentration increased to above 20 mol%, electrical conductivity increased significantly due to a connecting path of available Co sites. Additional Ca substitution for La increased the electrical conductivity in accordance with Verways principle. Defect models derived from the electrical conductivity data were found to relate the carrier concentrations to the Ca, Co and oxygen vacancy concentrations. The measured Seebeck coefficients were found to be positive even for the most reducing conditions, indicating that (La,Ca)(Cr,Co)O3 compositions with Co ≤ 0.3 and Ca ≤ 0.3 were as stable as LaCrO3. The Heikes formula was adopted to interpret the Seebeck coefficient results. These results indicated that electrical conduction in La(Cr,Co)O3 and (La,Ca)(Cr,Co)O3 occurs via the non-adiabatic and adiabatic small polaron mechanisms, respectively.

β-SiC production by reacting silica gel with hydrocarbon gas

A novel synthesis process, developed for producing high purity, submicron, non-agglomerated and low cost β-SiC powders. The process is based on carbothermal reduction reaction of a novel coated precursor. The precursor is derived from a silica gel and a hydrocarbon gas and provides high contact area between reactants. This yields a better distribution of carbon within the silica gel and results in a more complete reaction and a purer product. The powders produced in this process have a low oxygen content (less than 0.8 wt.%), very fine particle size (0.1–0.3 μm), narrow particle size distribution, non-agglomerated and are low cost. The sintering tests demonstrated that these powders can be pressureless-sintered to near theoretical density at about 2100°C in an inert atmosphere. No decarburization and no acid purification process was required before sintering.

Synthesis of α-Si3N4 from carbon coated silica by carbothermal reduction and nitridation

Abstract The production of α-Si 3 N 4 powder was performed by carbothermal reduction and nitridation of carbon coated fumed silica precursor. The precursor is derived from a fumed silica and a hydrocarbon gas (C 3 H 6 ) and provides intimate contact between the reactants. This yields a better distribution of carbon within the silica and inhibits the agglomeration among the oxides of silicon during the reactions, resulting in a more complete reaction and purer product at a comparatively low temperature. The Si 3 N 4 powders produced at 1500 °C for 5 h in flowing nitrogen gas have low oxygen content (1.2 wt%), submicrometer particles (0.3–0.7 μm), high α phase content and no agglomeration.

Formation of WC powders using carbon coated precursors

This paper deals with the formation of tungsten carbide from carbon coated tungstic oxide precursors. This study makes use of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). DSC and TGA data for both coated and mixed 17.2 wt% carbon containing tungstic oxide demonstrate the superiority of the coated precursor in the formation of tungsten carbide, as conversion is initiated at lower temperature. XRD patterns of products from each 100°C isotherm from 900–1400°C, inclusive, illustrate the formation as it proceeds through lower oxides into tungsten then carburizes into tungsten carbide for both the mixed and coated samples.