Ana M. Segadães

Combustion synthesis of calcium aluminates

Commercial calcium aluminate cements for refractory use are known to contain various phases, the hydration behavior of which is interdependent and not fully understood. Hydration studies normally require that pure oxides be synthesized. Solid-state synthesis of single aluminates requires high temperatures and full conversion is not guaranteed. Moreover, it is difficult to obtain a compositionally homogeneous product with this method. The present work discusses a straightforward combustion synthesis technique to prepare submicron calcium aluminates using the corresponding metal nitrates-urea mixtures, at low temperature and short reaction times. The effect of the ratio oxidizer/fuel in the redox mixture was investigated, namely, to find out if urea contents below stoichiometry were enough to trigger the explosive combustion of the fuel and the subsequent decomposition of the salts, and how that ratio affected the phase formation and the characteristics, e.g., morphology and grain size, of the powder produced.

Combustion synthesis of aluminium titanate

Abstract Initial interest in aluminium titanate was due to its low thermal expansion coefficient and high thermal shock resistance, but further research was soon discouraged following the discovery of the expansion anisotropy and the instability of the compound over a specific range of temperatures. The development of a suitable active precursor powder could provide a possible solution to the fabrication difficulties (microcracking and decomposition). The scarce available thermodynamic data for the formation of Al2TiO5from its constituent oxides indicate that the reaction is endothermic and only possible at high temperature because of the titanate being entropy stabilised. The present work describes a straightforward combustion synthesis technique to prepare submicron Al2TiO5 powders, using the corresponding metal precursors-urea mixtures, at low temperature and short reaction times. A thermodynamic interpretation of the reaction is provided and the characteristics of the powder produced, like morphology, specific surface area and grain size, are discussed. The thermal behaviour of the combustion powder is compared with that of Al2TiO5 produced via the conventional ceramic solid state route.

Combustion synthesized ZnO powders for varistor ceramics

Abstract Commercial ZnO varistor ceramics are multicomponent, with minor amounts of added oxides that play important roles, both in the strict electrical sense and for the control of the microstructure. The present work describes the straightforward combustion synthesis of pure and doped ZnO powders from stoichiometric mixtures of the relevant water soluble metal nitrates as cation precursors and urea as fuel. The mixtures were ignited at 500°C resulting in a dry, very fine powder. The as-prepared combustion products, characterized by XRD, SEM, TEM and BET, show high specific surface area, have very small particle sizes and are crystalline, with atomic level homogeneity. Implications on sintering and electrical behaviour are discussed.

Combustion synthesis of iron-substituted strontium titanate perovskites

Perovskites in the system SrOTiO2Fe2O3 can be good candidates for electrochemical applications, and the properties of sintered, fully dense bodies depend on the Fe to Ti stoichiometry and the prevailing temperature and oxygen partial pressure. Conductivity studies normally require that the oxides with the desired stoichiometry and structure be synthesized. Solid state synthesis of perovskites requires reasonably high temperatures and full conversion is not guaranteed. Moreover, with this method, it is difficult to obtain a compositionally homogeneous product. The present work discusses a straightforward combustion synthesis technique to prepare submicron iron-substituted strontium titanate, the Sr(Fe0.5Ti0.5)O3 perovskite, using the corresponding metal salts-urea mixtures, at low temperature and short reaction times. The effect of the ratio oxidizerfuel in the redox mixture was investigated, namely to find out how that ratio affected the phase formation and the characteristics, such as morphology and grain size, of the powder produced.

Assessment of the recycling potential of fresh concrete waste using a factorial design of experiments

Recycling of industrial wastes and by-products can help reduce the cost of waste treatment prior to disposal and eventually preserve natural resources and energy. To assess the recycling potential of a given waste, it is important to select a tool capable of giving clear indications either way, with the least time and work consumption, as is the case of modelling the system properties using the results obtained from statistical design of experiments. In this work, the aggregate reclaimed from the mud that results from washout and cleaning operations of fresh concrete mixer trucks (fresh concrete waste, FCW) was recycled into new concrete with various water/cement ratios, as replacement of natural fine aggregates. A 3(2) factorial design of experiments was used to model fresh concrete consistency index and hardened concrete water absorption and 7- and 28-day compressive strength, as functions of FCW content and water/cement ratio, and the resulting regression equations and contour plots were validated with confirmation experiments. The results showed that the fresh concrete workability worsened with the increase in FCW content but the water absorption (5-10 wt.%), 7-day compressive strength (26-36 MPa) and 28-day compressive strength (32-44 MPa) remained within the specified ranges, thus demonstrating that the aggregate reclaimed from FCW can be recycled into new concrete mixtures with lower natural aggregate content.

Incorporação de lama de mármore e granito em massas argilosas

A utilizacao de residuos industriais como aditivos na fabricacao de produtos cerâmicos vem despertando um crescente interesse dos pesquisadores nos ultimos anos e esta se tornando pratica comum. Este trabalho descreve a variacao do comportamento de uma argila utilizada numa industria de cerâmica vermelha, resultante de adicoes de uma lama de marmore e granito, tal como e produzida em uma industria de beneficiamento de pedras ornamentais do estado do Rio Grande do Norte. Misturas de argila e rejeito (10 - 50% em peso) foram compactadas uniaxialmente e sinterizadas a temperaturas entre 950 e 1150 °C. Os resultados obtidos por analise quimica e mineralogica (DRX e FRX), analises termicas (DTA, TG e dilatometria), medidas de distribuicao granulometrica, porosidade, absorcao de agua e tensao de ruptura a flexao, revelam que a lama de marmore e granito pode ser incorporada na massa de argila sem perda ou comprometimento das propriedades dos corpos sinterizados de cerâmica vermelha.

Effect of quartz sand replacement by agate rejects in triaxial porcelain

The ceramics industry, given the high volume of materials processed, stands as one of the largest consumers of natural raw materials but has also the capacity and potential to make significant contributions to solving environmental problems associated with other industries rejects. This work investigates the effects of quartz sand replacement by agate rejects (scrap) in a traditional triaxial porcelain composition. The study was carried out using the design of experiments (DoE) method. Characterization results were used to calculate statistically significant and valid regression equations, relating dried and fired body properties with clay, feldspar and agate scrap contents in the unfired mixture. The regression models were then discussed against X-ray diffraction and scanning electron microscopy results and used simultaneously to delimit the combinations of those three raw materials most adequate to produce a porcelainized stoneware floor tile with specified properties. Thus, an alternative use of an otherwise waste material is proposed, which can be translated into economic benefits and an important and welcome relief on environmental and waste disposal concerns.

Phase equilibrium relationships in the system Al2O3-TiO2-MnO, relevant to the low-temperature sintering of alumina

Abstract A great deal of research work has been devoted to lowering the sintering temperature of ceramic powders of varied nature, to fulfil a variety of purposes. Both experimentation and theory show that the sintering temperature of alumina can be lowered to 1400 °C and below by using small particle sizes and certain additives like TiO2 and/or MnO. The general idea is that sintering is aided by the development of a liquid phase at this low temperature, due to the presence of the additives. However, there is no phase diagram available to throw light on this matter. For this reason, the present work was aimed at investigating the phase equilibrium relationships in the ternary, non-condensed system Al2O3-TiO2-MnO, in air. Selected compositions in this system were prepared from reagent-grade oxides, uniaxially pressed into 6 mm cylindrical pellets, fired at temperatures between 1000 and 1650 °C for 2 to 22 h, water-quenched, and observed by X-ray diffraction and scanning electron microscopy, the composition of some of the phases identified being evaluated by energy-dispersive spectroscopy. These experiments led to the definition of the compatibility triangles and a tentative location of the boundary curves between primary phase fields is presented.

Evaluation of Pd/alumina catalysts, produced by combustion synthesis, in the ethanol oxidation reaction to acetic acid

Abstract Combustion synthesis is a quick, straightforward preparation process to produce homogeneous, very fine, crystalline and unagglomerated multicomponent oxide ceramic powders, without the intermediate decomposition and/or calcining steps, and was used to prepare a palladium/alumina catalyst oxide precursor. The present work describes the synthesis of the catalyst and compares its performance, in the acetic acid formation from ethanol oxidation reaction, with that of the equivalent catalyst synthesized via the usual technique of an independent support impregnation step. The phase formation and characteristics of the powder produced (morphology and grain size) were investigated and are also discussed. Results show that the combustion synthesis produces solids with specific surface areas 60% higher than those obtained by the conventional technique. The catalytic test results show that the combustion synthesis catalysts have higher activity and acetic acid selectivity, in the ethanol oxidation reaction.

Refractory Formulations Made of Different Wastes and Natural Sub-Products

This work describes studies on the preliminary processing of calcium aluminate mortars based on several industrial wastes and natural sub-products. The starting materials are sludges (generated in Al-anodising and surface coating industrial processes, potable water filtration/cleaning operations and cutting process of natural marble rocks), and foundry sands. These materials are all classified as non-hazardous but high daily-produced amounts involve high transportation costs for disposal. Reusing alternatives should be sought. The characterization of the waste materials is reported, including physical and chemical parameters and thermal behaviour. Several powder mixtures were prepared and fired at different temperatures. Compositional changes were assessed by XRD and powder density, average particle size, and specific surface area, were determined. The compression and flexural strengths of the corresponding pressed and sintered samples are also given.