Hypersonic flight at a speed greater than Mach 5 (1715 ms-1) requires materials that can withstand temperatures up to 3000°C, high heat flux, rapid heating and disassociated reactive oxygen in the extreme environment of space and during re-entry. A number of advanced ceramic materials have melting points over 3000°C, of which the refractory metal carbides and borides are of main interest due to their excellent thermal conductivity from room temperature to over 2500°C, good chemical stability and ablation resistance at high temperatures. These materials are classified as ultra-high-temperature ceramics (UHTCs). Among the family of UHTCs, ZrB2 and HfB2 are reported as the most promising candidates to be used as thermal protection systems (TPS) for the nose tip and sharp leading edges. However, the issue of using monolithic ZrB2 and HfB2 is the phase transformation of ZrO2 and HfO2 oxide by-products at elevated temperature, leading to a volume change that results in cracking of the formed oxide scale. Hence, it is necessary to use dopants to stabilize the oxidation products of ZrB2 and HfB2 in-situ and to minimise the transformation induced cracking and thus improving the oxidation resistance. This research is focused on introducing dopants, such as Y and Ta into HfB2 and to understand its effect on the oxidation behaviour of HfB2 based UHT ceramics. The primary objectives were to: (a) Synthesize sub-micron pure and doped HfB2 powders; (b) Sinter the HfB2 based ceramics to achieve relative density > 95% (i.e. with close porosity); (c) Assess the effect of dopants on the oxidation resistance of HfB2 ceramics at high temperatures. Sub-micron pure HfB2 powder of ~200 nm was synthesized by a modified sol-gel approach combined with subsequent carbothermal reduction process using hafnium tetrachloride, boric acid, and phenolic resin as the starting materials. HfC and residual carbon were found to be the main impurity phase, owing to the lack of removal of carbon-containing species in the argon atmosphere during the heat treatment. Therefore, a precipitation approach was developed to transfer hafnium tetrachloride into hafnium hydroxide during the mixing stage to get rid of the Cl- and carbon-containing functional groups. Based on the detailed study of the formation mechanism of HfB2, it was found that the particle size of the HfB2 powders was decided by the particle size of the starting Hf source. Although the powders were slightly coarser (~400-800 nm) from the precipitation approach, importantly phase-pure HfB2 was formed at the same furnace heating conditions (1600°C/2 hrs). The precipitation method was also used to prepare doped HfB2 powders as the homogeneity of the dopants (TaB2, Y2O3) could be improved by controlling the pH values at ~8.5 to achieve the simultaneous precipitation of the dopants and HfB2 precursors. As a result, (Hf,Ta)B2 solid solution was prepared successfully at the temperature of 1600°C. Spark plasma sintering (SPS) was used to densify the pure and doped HfB2 powders. The optimized density achieved was around 97% at 2150°C without the use of any sintering aids and the addition of TaB2 slightly improved the sinterability of the HfB2 based powders due to the formation of the (Hf,Ta)B2 solid solution. The sintered density of commercial micron HfB2 powders (Treibacher) was only 94% in the same condition, and the resultant grain size (5-10 μm) is also significantly larger than that from synthesized HfB2-based ceramics (2-6 μm). The oxide impurities, such as HfO2 and B2O3, on the surface of the fine HfB2 based powders were attributed as the main reason for inhibiting further densification. The oxidation behaviours of the HfB2 based ceramics were investigated via both static oven oxidation and oxyacetylene torch testing. In low and intermediate temperature regime ( < 1600°C), it was indicated that the addition of dopants didn't significantly improve the oxidation resistance as the glassy B2O3 was the critical factor controlling the oxygen permeation rate. However, in the high-temperature regime ( > 1600°C), it was found the oxidation product was mainly tetragonal HfO2, which was stabilized by the Ta-dopants at temperatures well below the HfO2 phase transformation temperature. Therefore, the cracking and volume change due to phase transformation can be avoided and in return, oxidation resistance was improved at high temperature, which should be beneficial for the application of these materials in hypersonic aviation.

Synthesis of hydroxyapatite (Ca10(PO4)6(OH)2, HAp) from commercial grade plaster of paris (CaSO4&bull / 0.5H2O, PoP) and gypsum (CaSO4&bull / 2H2O) has been performed. HAp synthesis was achieved by reacting 1 M of (NH4)2HPO4 (or 0.5 M of (NH4)2HPO4) solutions with solid calcium sulfate precursors under ambient pressure (1 atm) and hydrothermal-like (2 &plusmn / 0.2 atm, 120 &deg / C) conditions. Under ambient conditions, HAp formation kinetics was investigated at 25 &deg / C, 50 &deg / C and 90 &deg / C using 1 M of (NH4)2HPO4 solution. Conversion to HAp at such low temperature takes more than 21 days and it also promotes formation of additional calcium phosphate with HAp. At 25 &deg / C, HAp formation started after 7 days accompanied with formation of brushite (CaHPO4&bull / 2H2O). At 50 &deg / C no significant conversion was observed after 6 h. However, at 90 &deg / C, phase pure HAp was formed after 2 h. On the other hand, under hydrothermal-like conditions, the HAp formation proceed much faster and it was also shown that HAp could be also synthesized from gypsum powders and bulk gypsum pellets. Using 1 M of (NH4)2HPO4 solution, HAp formation from PoP started 15 min and completed almost in 30 min, whereas 0.5 M of (NH4)2HPO4 reactant solution slowed down the conversion. The exact chemical identity of the HAp product of hydrothermal-like reaction was evaluated by post-synthesis calcinations and the thermal phase stability was related with the stoichiometry (Ca/P at ratio) of the HAp. The HAp phase was stable up to 600 &deg / C and above 600 &deg / C, &beta / -tricalcium phosphate (&beta / -Ca3(PO4)2, &beta / -TCP) was formed, suggesting that the resultant HAp was calcium-deficient. Mechanical testing by diametrical compression was performed to the HAp samples produced from bulk gypsum pellets. The strength was measured 1.2 MPa with highest solid to liquid (s:l) ratio 3.33 and decreased with s:l ratio. This change was found to be related with the porosity differences due to differences in s:l ratio. Additional mechanical tests were applied to the polycaprolactone (PCL) coated bulk HAp pellets for which the tensile strength was doubled. This study presents an easy and feasible method for production of HAp from a cheap and abundant calcium source &ndash / PoP. In addition, the findings provide a potential processing route for developing irregularly shaped bulk porous HAp structures.

O uso do hidrogênio, como vetor energético, representa uma opção promissora a fim de se reduzir a dependência dos combustíveis fósseis e controlar a emissão de poluentes na atmosfera. Atualmente, uma das rotas mais propícias para produção de hidrogênio envolve a reação de reforma a vapor de álcoois utilizando-se catalisadores de níquel suportados em alumina. O níquel é amplamente utilizado em catálise devido ao baixo custo e à elevada atividade para ruptura da ligação C-C. A alumina, por sua vez, promove maior dispersão do metal ativo devido aos valores elevados de área superficial das estruturas cristalinas de transição, sobretudo da fase gama, característica esta diretamente relacionada às condições de síntese. A incorporação de lantânia, como aditivo, tem sido considerada por moderar a acidez da superfície do suporte e minimizar a deposição de carbono no catalisador durante a reação de reforma. Tendo em vista que a atividade dos catalisadores na reação de reforma é função das características físicas e químicas desses materiais, avaliou-se, no presente trabalho, a rota de síntese por coprecipitação de hidróxidos em associação ao uso de surfactante e tratamento solvotérmico. A rota de mistura de pós de óxido de níquel com alumina, ambos preparados por precipitação individual, foi adotada para comparação dos resultados obtidos por coprecipitação. Nas condições otimizadas do processo, estudou-se o efeito da adição de óxido de lantânio nas propriedades dos materiais sintetizados. Verificou-se que o método de coprecipitação permitiu a obtenção de óxidos mistos com elevada área superficial (na faixa de 170 a 260 m2g-1), ao passo que a mistura de óxidos conduziu à formação de pós constituídos por fases distintas de alumina e óxido de níquel, com menor área superficial (na faixa de 60 a 180 m2.g-1). Após avaliação do comportamento de redução do óxido de níquel contido nesses materiais, as propriedades dos pós obtidos foram correlacionadas com o desempenho na produção de hidrogênio pela reforma do etanol. De acordo com os resultados, o catalisador preparado por coprecipitação apresentou menor atividade na produção de hidrogênio, comparativamente aos materiais obtidos por coprecipitação em associação com tratamento solvotérmico e mistura de óxidos. Em contraste, verificou-se uma menor quantidade de carbono acumulado sobre o catalisador preparado por coprecipitação, indicativo da elevada estabilidade catalítica deste material durante a reação de reforma. / Hydrogen use as an energy vector represents a promising option in order to reduce the dependence on fossil fuels and to control the emission of pollutants into the atmosphere. Nowadays, one of the most important routes for hydrogen production includes steam reforming reactions of alcohols over alumina-supported nickel catalysts. Nickel is largely applied in catalysis because of its low cost and high activity for C-C bond rupture. Alumina, in turn, promotes appropriate dispersion of the active metal due to the high surface area values of its transition crystalline structures, especially gamma phase. These characteristics are related to synthesis conditions. Incorporation of lanthana as an additive has been considered to control alumina surface acidity and to inhibit catalyst deactivation by carbon deposition during reforming reaction. Considering that catalyst activity is a function of materials physical and chemical properties, it was evaluated in this work powder synthesis route by coprecipitation in association with surfactant templating method and solvothermal treatment. Mechanical milling of nickel oxide and alumina powders, which were individually prepared by chemical precipitation, was adopted for comparison purposes. Under optimized preparation conditions, the effect of lanthanum oxide addition on the materials properties was studied. It was verified that coprecipitation allowed the production of high surface area mixed oxides (170-260 m2g-1). Mechanical mixture led to the formation of materials constituted by alumina and nickel oxide phases, with low surface area (60-180 m2.g-1). After evaluation of nickel oxide reduction behavior, in hydrogen atmosphere, a correlation between properties and performance in hydrogen production by ethanol steam reforming was established. According to the results, the catalyst prepared by coprecipitation was less active for hydrogen generation compared to the ones obtained by coprecipitation followed by solvothermal treatment and mechanical milling methods. In contrast, the lowest amount of carbon deposits was found on the catalyst prepared by coprecipitation, which is an indicative of the high catalytic stability during reforming reaction.

O uso do hidrogênio, como vetor energético, representa uma opção promissora a fim de se reduzir a dependência dos combustíveis fósseis e controlar a emissão de poluentes na atmosfera. Atualmente, uma das rotas mais propícias para produção de hidrogênio envolve a reação de reforma a vapor de álcoois utilizando-se catalisadores de níquel suportados em alumina. O níquel é amplamente utilizado em catálise devido ao baixo custo e à elevada atividade para ruptura da ligação C-C. A alumina, por sua vez, promove maior dispersão do metal ativo devido aos valores elevados de área superficial das estruturas cristalinas de transição, sobretudo da fase gama, característica esta diretamente relacionada às condições de síntese. A incorporação de lantânia, como aditivo, tem sido considerada por moderar a acidez da superfície do suporte e minimizar a deposição de carbono no catalisador durante a reação de reforma. Tendo em vista que a atividade dos catalisadores na reação de reforma é função das características físicas e químicas desses materiais, avaliou-se, no presente trabalho, a rota de síntese por coprecipitação de hidróxidos em associação ao uso de surfactante e tratamento solvotérmico. A rota de mistura de pós de óxido de níquel com alumina, ambos preparados por precipitação individual, foi adotada para comparação dos resultados obtidos por coprecipitação. Nas condições otimizadas do processo, estudou-se o efeito da adição de óxido de lantânio nas propriedades dos materiais sintetizados. Verificou-se que o método de coprecipitação permitiu a obtenção de óxidos mistos com elevada área superficial (na faixa de 170 a 260 m2g-1), ao passo que a mistura de óxidos conduziu à formação de pós constituídos por fases distintas de alumina e óxido de níquel, com menor área superficial (na faixa de 60 a 180 m2.g-1). Após avaliação do comportamento de redução do óxido de níquel contido nesses materiais, as propriedades dos pós obtidos foram correlacionadas com o desempenho na produção de hidrogênio pela reforma do etanol. De acordo com os resultados, o catalisador preparado por coprecipitação apresentou menor atividade na produção de hidrogênio, comparativamente aos materiais obtidos por coprecipitação em associação com tratamento solvotérmico e mistura de óxidos. Em contraste, verificou-se uma menor quantidade de carbono acumulado sobre o catalisador preparado por coprecipitação, indicativo da elevada estabilidade catalítica deste material durante a reação de reforma. / Hydrogen use as an energy vector represents a promising option in order to reduce the dependence on fossil fuels and to control the emission of pollutants into the atmosphere. Nowadays, one of the most important routes for hydrogen production includes steam reforming reactions of alcohols over alumina-supported nickel catalysts. Nickel is largely applied in catalysis because of its low cost and high activity for C-C bond rupture. Alumina, in turn, promotes appropriate dispersion of the active metal due to the high surface area values of its transition crystalline structures, especially gamma phase. These characteristics are related to synthesis conditions. Incorporation of lanthana as an additive has been considered to control alumina surface acidity and to inhibit catalyst deactivation by carbon deposition during reforming reaction. Considering that catalyst activity is a function of materials physical and chemical properties, it was evaluated in this work powder synthesis route by coprecipitation in association with surfactant templating method and solvothermal treatment. Mechanical milling of nickel oxide and alumina powders, which were individually prepared by chemical precipitation, was adopted for comparison purposes. Under optimized preparation conditions, the effect of lanthanum oxide addition on the materials properties was studied. It was verified that coprecipitation allowed the production of high surface area mixed oxides (170-260 m2g-1). Mechanical mixture led to the formation of materials constituted by alumina and nickel oxide phases, with low surface area (60-180 m2.g-1). After evaluation of nickel oxide reduction behavior, in hydrogen atmosphere, a correlation between properties and performance in hydrogen production by ethanol steam reforming was established. According to the results, the catalyst prepared by coprecipitation was less active for hydrogen generation compared to the ones obtained by coprecipitation followed by solvothermal treatment and mechanical milling methods. In contrast, the lowest amount of carbon deposits was found on the catalyst prepared by coprecipitation, which is an indicative of the high catalytic stability during reforming reaction.