Radio frequency plasma synthesis of ultrafine titanium carbide powders

Abstract

The work reported in this thesis was part of a larger project to evaluate the use thermal plasma processing to synthesise ultrafine TiCx, TiNy, and TiCxNy powders from locally available TiO2, natural gas and NH3. The objectives for the work reported in this thesis were to develop and test the necessary experimental apparatus for a process for synthesis of the ultrafine ceramic powders, and then to investigate the synthesis of ultrafine TiCxOz powders from pigment-grade TiO2 and natural gas using the process that had been developed. The apparatus developed included a powder feeder capable of feeding ultrafine cohesive powders, a RF plasma torch, and powder collection equipment. Feeding cohesive powder at low flow rates with little flow rate variability is recognised as an extremely difficult task. Perhaps as expected, the vibrated elutriated fluidised bed (VEFB) powder feeder that was developed only partially met its design specification as it fed TiO2 agglomerates to the process that were too large to vaporise and react. The poor process repeatability was attributed to the VEFB powder feeder. It was recommended that a Wright system powder feeder be developed to overcome these limitations. The RF power supply required significant modifications before being used for plasma generation, including the addition of silicon-controlled rectifiers for continuously-variable power control and a complete redesign of the grid feedback circuit. The RF plasma torch development took several iterations, and the design used for all experiments performed adequately except for a low tailpiece reactant gas injection velocity which resulted in poor mixing between reactants injected through the tailpiece and the hot flow. The tailpiece injection velocity should be increased to at least 50 m s-1 for future experiments. The powder collection apparatus used for most experiments consisted of a 785 mm long section of 78 mm ID water-cooled stainless steel tube with smaller diameter tubes inserted within the larger tube. The mass collection efficiency of this device was greater than 75%, and was considered sufficient to ensure that statistically representative samples were collected. The experimental programme for the synthesis of ultrafine TiCxOz was centred on a base case set of experimental parameter values, and investigated the process repeatability, the plasma torch power input, the nominal reaction stoichiometry, the natural gas injection location, the tailpiece injection velocity, and the effect of H2 co-injection. Variation of only one parameter in each experiment led to the identification of required process modifications and reaction system parameters for future study. A complex multiphase mixture of four particle types was produced. Ultrafine spherical single-crystal particles ranging from 7 to 25 nm in diameter were almost always observed and were homogeneously nucleated TiO2 or Ti suboxide particles that then grew by liquid droplet coalescence. Ultrafine cuboid or rhomboid single-crystal particles up to 25 nm across parallel faces were almost always observed and were homogenously nucleated TiCxOz particles with x close to 0.99 and z close to 0 that grew by a gas-solid mechanism. Unvaporised single-crystal TiO2 feed particles and largely amorphous free carbon particles ranging from 8 to 40 nm were always observed. The crystallinity of the free carbon particles increased while the extractable content decreased when the natural gas was injected upstream of the plasma. This indicated that the formation mechanism changed from an acetylenic growth mechanism with upstream injection to a polycyclic aromatic hydrocarbon mechanism with downstream injection. The low mass vaporisation efficiency and mixing patterns meant that TiCxOz was formed unless the process power input was too low. TiO2 to TiCxOz, conversion was largely unaffected by either an increase in the reaction stoichiometry or reactant injection location. Little or no conversion of TiO2 to TiCxOz occurred in experiments performed below the minimum power input, which for this process appeared to be that required to bring all streams entering the process to a bulk temperature of around 4000 K. The major enthalpy demands for this process were heating the Ar flow required for torch operation and the natural gas reactant stream. Future experiments should not use upstream injection of reactant gases so that all available enthalpy transferred from the discharge to the cooler central core can be used for particle vaporisation. The TiCxOz synthesis experiments were modelled using free energy minimisation techniques. It was found that it was important to include all inert gases in the system specification and that the thermodynamic properties of graphite should adequately represent the thermodynamic properties of the free carbon, so the prediction of the onset of free carbon formation should be relatively accurate. The system TiCx-TiNy-TiOz forms a solid solution with complete miscibility at elevated temperatures. However, the phase and atom distributions, and the temperatures at which phase transitions occurred, were relatively unaffected by the inclusion of ideal solution TiCxOz(s,l) in the system specification. It was concluded that slight differences did not warrant the added complexity caused by including the solution phases in the system specification. Analyses of the individual experiments indicated that free carbon formation would be a serious problem, and that the high vapour pressure of TiN(g) around the temperatures at which TiC(s) formation was predicted to occur may have led to the formation of TiCxNyOz(s) with significant nitrogen levels.