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Titanium and boron nitride and carbide, titanium diboride were synthesized by carbothermic reduction as single phase as well as mixtures intended to form composite materials. The aim of the project is to study the physical chemistry of carbothermic reduction for the production of pure nonoxide ceramic powders and also for the in-situ formation of ceramic/ceramic partially-densified composites. The thermodynamic and kinetic factors that govern the phase constituents are discussed and the effect of processing parameters on the morphology and extent of reduction are also established. The first part of the present investigation is aimed at the production of titanium nitride, carbonitride and carbide powders and the in-situ formation of TiN/TiC partially-densified composites by the carbothermic reduction of titania in suitable nitriding atmospheres. The investigation includes the aspects of the thermodynamics and kinetics of the nitriding reaction and points out the reaction mechanism by identifying the phase formed after the nitridation process. The microstructures produced after the reduction-nitridation process have been correlated with the thermodynamic and kinetic parameters. The synthesized titanium nitride powder was identified as the carbonitride phase, Ti(CxN1_x), having a range of composition. The rate of reduction of TbO2 was found to be determined by the rate of oxygen diffusion in the sub-oxide lattice and the derived value of activation energy in the temperature range 1473K to 1773K from the Arrhenius plot is 120 kJ-mole-1 of T102. TI305 was found as a high temperature precursor phase for the formation of titanium nitride. The use of iron chloride as catalyst and activated charcoal in the mixtures of oxide increased the yield of titanium nitride phase by enhancing the rate of reduction of titanium oxides. The morphology of titanium carbonitride particles was dependent upon the reactivity of carbon and the temperature. The calculated equilibrium phase fields were found to be in agreement with the experimental data and provide a means to select the variables for the reduction condition for designing a required ceramic microstructure. The microstructure of boron nitrides is closely related to the structural chemistry of carbon and nitriding agent. The main aim of the second part of the project was to synthesize boron nitride and carbide powders and whiskers by carbothermic reduction of boric anhydride (6203) in nitrogen atmosphere and also to understand a relation between the processing parameters and the phases produced. The effect of processing conditions such as the gas composition, reactivity of carbon, reaction temperature and time as well as the composition of starting materials on the synthesis of boron nitride and carbide phases were studied. The reactivity of carbon, B/C ratio and gas composition were the most important variables that determined the formation, structure and morphology of the nitride. During the nitridation process, boron carbide phase also formed and played a significant role. The investigation also reports the evidence for the formation of metastable forms of BN i. e wurtzite and cubic BN. We also report the results of the solubility of nitrogen in C-saturated B4C structure. The third part of the present work is aimed at the production of TiB2 powders. Aspects of the formation of two or three ceramic phase mixtures were also examined together with the relative stability of the single phase mixed diborides with respect to pure diboride phase. The central aim of this part is to establish the mechanism of the synthesis reaction leading to the formation of uniform size of titanium diboride crystals. Titanium boride (TiB2) powder was produced in the powder form by the reduction of ingredient oxides with carbon via a gas-solid phase reaction. For the production of the composite microstructure, the nitrogen partial pressure was found to be the most critical factor. In the composite microstructure, the titanium nitride particles have a submicrometer size whereas the boride particle size is only a few micrometers with predominantly hexagonal morphology. Some calculated equilibrium phase fields have been experimentally verified. The empirical verification is a useful tool to establish the correctness of the calculated phase diagram. The theoretical approach therefore enables to identify the condition for the formation of phase mixtures. The constituent phases depend on the reduction conditions. For example, nitrides in equilibrium with Ti62 can only form above a critical nitrogen partial pressure whereas TiC or B4C form in the inert atmospheres. This result is applicable to all other ceramics. The investigation also shows the viability of production of the composite powder mixture via the oxide co-reduction technique. The synthesis of TIB2/TiN, TiB2ýC, TB2/TN/BN and mixed diboride composites is possible by employing the reduction route.

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O presente trabalho apresenta um equipamento para o processamento de reduções carbotérmicas sob campo de micro-ondas, permitindo pleno controle da potência fornecida e da medida da energia de micro-ondas efetivamente aplicada à carga em processo de redução podendo assim realizar balanços de energia e determinação de taxa de reação com altos níveis de confiança. O equipamento permite, sobretudo, a medida simultânea de temperatura e de massa do material sob redução, e a fácil reprodutibilidade das condições de ensaio. O protótipo opera com um gerador de micro-ondas de 2.45 GHz e potência variável até 3000 W. A amostra é mantida sob atmosfera de argônio. Nos testes operacionais do protótipo procedeu-se à redução carbotérmica de minério de ferro de Carajás, utilizando coque de petróleo como redutor através de pelotas esféricas com massa de 3,5 g e diâmetro de 15 mm. Obtiveram-se curvas cinéticas de redução de minério de ferro e de consumo de energia durante o processo. Os dados colhidos com este protótipo permitiram realizar uma análise da energia de micro-ondas efetivamente consumida na redução do minério. / This work presents of a device for microwave enhanced carbothermic reductions which permits the full control of microwave power irradiated, the measurement of effectively applied microwave power to the charge in process thus performing a high confidence determination of rate of reaction and energy balance. In particular, the simultaneous records of its temperature and mass loss, and good reproducibility of operation parameters, are obtainable. This prototype works with a 2.45 GHz microwave generator with variable power up to 3000 W. Samples are processes under argon gas atmosphere. Operational tests for Carajás iron ore reduction with petroleum coke were performed using spherical pellets with weight of 3,5 g and diameter of 15 mm. Kinetic curves of iron ore reduction and effective consumption of microwave power curves were determined. These results made possible to perform an analysis of actual energy consumption for enhanced microwave iron ore carbothermic reduction.

No / To interpret nitrogen sintering of chromium and manganese steels without the formation of deleterious oxides, but with manganese and carbon the formation of deleterious oxides, but with manganese and carbon modifying the local microclimate, the role of the volatile Mn and modifying the local microclimate, the role of the volatile and Mn carbothermic reactions were considered. carbothermic reactions were considered. Reduction of Cr Reduction of Cr 2 2 O About 3 3 by Mn vapour by Mn vapor is always favourable. is always favorable. CO is an effective reducing agent, however, whereas CO is an effective reducing agent, however, whereas at atmospheric pressure it will reduce FeO at730°C, temperatures some at atmospheric pressure it will reduce FeO at730 ° C, temperatures some 500 and 700°C higher, ie above those for conventional sintering, are 500 and 700 ° C higher, ie above those for conventional sintering, are necessary for reducing Cr necessary for reducing Cr 2 2 O About 3 3 and MnO, respectively. and MNO, respectively. Accordingly partial Accordingly partial pressures must be considered and the sintering process is modelled at a pressures must be considered and the sintering process is modeled at a conglomerate of several surface oxidised alloy particles surrounding a pore conglomerate of several surface oxidised alloy particles surrounding a pore with graphite present and a tortuous access to the nitrogen-rich atmosphere with graphite and present a tortuous access to the nitrogen-rich atmosphere containing some water vapour and oxygen. containing some water vapor and oxygen. The relevant partial pressures The relevant partial pressures were calculated and reduction reactions become thermodynamically were calculated and reduction reactions become thermodynamically favourable from 200°C. favorable from 200 ° C. Kinetics, however, dictates availability of CO and Kinetics, however, dictates availability of CO and the relevant reactions are the water-gas, C + H the relevant reactions are the water-gas, C + H 2 2 O = CO + H O = CO + H 2 2 from 500°C from 500 ° C and the Boudouard, C + CO and the Boudouard C + CO 2 2 = 2CO, from 700°C. = 2CO, from 700 ° C. Discussion of sintering Discussion of sintering mechanisms is extended to processing in semi-closed containers, also mechanisms is extended to processing in a semi-closed containers, also possessing specific microclimates.

Yes / To interpret nitrogen sintering of chromium and manganese steels without the formation of deleterious oxides, but with manganese and carbon modifying the local microclimate, the role of the volatile Mn and carbothermic reactions were considered. Reduction of Cr2O3 by Mn vapour is always favourable. CO is an effective reducing agent, however, whereas at atmospheric pressure it will reduce FeO at ~730°C, temperatures some 500 and 700°C higher, i.e. above those for conventional sintering, are necessary for reducing Cr2O3 and MnO, respectively. Accordingly partial pressures must be considered and the sintering process is modelled at a conglomerate of several surface oxidised alloy particles surrounding a pore with graphite present and a tortuous access to the nitrogen-rich atmosphere containing some water vapour and oxygen. The relevant partial pressures were calculated and reduction reactions become thermodynamically favourable from ~200°C. Kinetics, however, dictates availability of CO and the relevant reactions are the water-gas, C + H2O = CO + H2 from ~500°C and the Boudouard, C + CO2 = 2CO, from ~700°C. Discussion of sintering mechanisms is extended to processing in semi-closed containers, also possessing specific microclimates.

A redução carbotérmica do minério de ferro é a mais importante reação na fabricação do ferro, e tem sido obtida principalmente em Alto-Forno. Nos últimos anos, muitos processos novos foram propostos como alternativas e vários tipos de reatores foram testados. O processo mais promissor foi aquele em que a mistura minério de ferro e material carbonáceo eram aquecidos a altas temperaturas, promovendo a reação com formação do ferro metálico. Tornou-se claro que um dos principais obstáculos para a rápida reação é a transferência de calor da região externa para o centro da mistura. Por outro lado, muitos estudos têm mostrado que o aquecimento por microondas é muito efetivo em processos industriais, como secagem e sinterização de cerâmicos. No aquecimento por microondas, o material é aquecido a partir de seu interior, logo evita a obrigação de um aquecimento de fora para dentro do material. Neste trabalho, o aquecimento por microondas foi aplicado para a redução carbotérmica da hematita. Os resultados obtidos mostraram que é possível aquecer a mistura minério de ferro - carbono acima da temperatura de reação e que a taxa de reação é comparável à obtida por aquecimento convencional utilizando-se a mesma mistura. / The carbothermic reduction of iron ores is the most important reaction in ironmaking, and has been performed mainly in the Blast Furnace. In the last years, several new processes have been proposed as alternatives, and many types of reactors have been tested. The most promising processes are those in which a mixture of iron ore and carbonaceous material is heated at high temperatures, promoting the reaction with formation of metallic iron. It became clear that one of the main obstacles to a fast reaction is heat transfer from the surroundings to the core of the mixture. On the other hand, several studies have shown that microwave heating is very effective in some industrial processes, like drying and sintering of ceramics. In the microwave heating, the material is heated from the inside, thus avoiding the constraints of heat transfer from the surroundings to the inner part of the material. In this work, microwave heating has been applied to the carbothermic reduction of hematite. The obtained results have show that it is possible to heat iron-carbon mixture above the reduction temperature, and the reaction rates have been compared to those obtained employing conventional heating with the same mixtures .

A mathematical model to simulate the reaction kinetics of dust–carbon composite agglomerates in an RHF was developed. Major chemical, thermal and physical phenomena taking place during RHF treatment were formulated and the corresponding equations were solved to yield the trend of solid composition, temperature and gas composition of the agglomerates. The model calculation results indicate that the pellet reduction is accelerated by the reducing gas from high–volatile reductants if the gas is released after the pellet temperature is sufficiently high for reduction. The reduction of zinc oxide can also be represented using the model by implementing its small particle size caused by the inherent vaporization/re–oxidization process that zinc comes through in a melter. It was demonstrated that the proposed model can be used as an engineering tool to determine the optimum operating conditions for the RHF process to promote recycling a wide range of waste materials.