Estudio del carbonato de litio como ayuda de sinterización para óxido de estaño mediante diseños de experimentos: variables principales y cambios microestructurales

Tin oxide is one of the most extensively studied semiconductor materials due to its broad field of applications. On the one hand, its high conductivity and its corrosion resistance are the most remarkable properties. Therefore, one of the most developed uses in the recent decades has been as ceramic electrode for electrooxidation process. On the other hand, its poor sinterability hinders a broader use. As a result, the use of advanced techniques or sin- tering aids for obtaining low-porosity specimens is necessary. So far, many additives have been studied, CaCO3, Co3O4, Nb2O5 or MnO2, among others. In the present work, the sinter- ing behaviour of SnO2-based powder, containing Li2CO3 as a sintering aid, which generates a liquid phase, has been analysed, since it is one of the additives that has been studied to a lesser extent. The effect of the amount of sintering aid just like the thermal treatment parameters (maximum temperature, heating rate and soaking time) on volumetric contrac- tion’s evolution has been studied through a factorial experiment designs 2n. The results show that an amount of lithium carbonate greater than 1 mol.% is unfavourable to densification. With regards to the thermal cycle’s parameters, it is advisable to have thermal treatments at high temperatures (1300 ◦C) with moderate soaking times (1 h), as maximum temperatures have the biggest influence on the densification followed by soaking time while the heating rate has a lesser influence. Under these conditions, a microstructure of closed and rounded pores is obtained, in which a residual phase is enclosed, but the small proportion of which prevents its characterisation.El óxido de esta ̃no es uno de los materiales más estudiados, dado su extenso campo de aplicación. De entre sus propiedades, cabe destacar su alta conductividad eléctrica y su resistencia a la corrosión, de ahí que entre sus aplicaciones se encuentre la de electrodo cerámico. Sin embargo, su baja capacidad de densificación dificulta su uso. Como con- secuencia, se requieren técnicas avanzadas o aditivos de sinterización que faciliten la obtención de piezas con baja porosidad. Hasta el momento han sido muchos los aditivos estudiados, CaCO3, Co3O4, Nb2O5 o MnO2, entre otros. En este trabajo se estudia el proceso de sinterización de piezas de SnO2 a las que se ha incorporado Li2CO3 como aditivo de sinterización, el cual genera una fase líquida, por ser uno de los aditivos en cuyo estudio se ha profundizado en menor medida. A través de dise ̃nos factoriales de experimentos 2n, se han determinado los efectos de los diferentes parámetros del ciclo de cocción (temper- atura máxima, velocidad de calentamiento y tiempo de permanencia), así como la cantidad de aditivo de sinterización, sobre la evolución de la contracción volumétrica. Los resulta- dos muestran que la incorporación del carbonato de litio, en porcentajes superiores al 1% molar, no es favorable para la densificación. Respecto a los parámetros de cocción, resul- tan recomendables tratamientos térmicos a elevada temperatura (1.300 ◦C) con tiempos de permanencia moderados (1 hora), mientras que la velocidad de calentamiento ejerce una menor influencia. En estas condiciones se obtiene una microestructura de poros cerrados y redondeados, en la que queda encerrada una fase residual cuya reducida proporción impide su caracterización

Solution combustion synthesis of (Ni,Fe)Cr2O4 pigments: Effect of post-synthesis thermal treatments

Mixed spinel pigments Ni1− ΨFeΨCr2O4 (0 ≤ Ψ ≤ 1) were synthesized by means of a method known as Solution Combustion Synthesis, using urea as fuel. Effects on mineralogy, microstructure and colouring power of composition and parameters of post-synthesis thermal treatments (temperature and kiln atmosphere) were studied. The as-synthesized powders were calcined at two different temperatures (800 and 1000 °C) and atmospheres (oxidizing and inert). Powders were characterized by X-ray diffraction (XRD), thermogravimetry (TG), scanning electron microscopy (FEG-SEM) and colour development in glaze. Mineralogy was highly affected by post-synthesis parameters. A progressive evolution from a Fd-3m face-centred spinel to a rhombohedral (Fe,Cr)2O3 structure was identified as the calcination temperature increased and composition enriched in iron. Inert atmosphere slowed down spinel→(Fe,Cr)2O3 transformation. Moreover, crystallite size of both phases showed characteristic critical points with composition and calcination conditions. On the other hand, microstructure analysis showed an important grain growth as the calcination temperature increased using the oxidizing atmosphere, but it was considerably altered with inert atmosphere. Regardless of post-synthesis thermal treatment parameters, all pigments showed high colouring power in a glaze. As Ψ increased, generated colours evolved from yellow-greenish to brown. Only luminosity coordinate L* of glazes subtly decreased as calcination temperature of the pigment increased, while a* and b* were nearly constant. This behaviour is related to the evolution of the pigment during the heating interval of the firing cycle of the glazed samples

Síntesis del pigmento de hierro-circón: Propuesta de un índice de mezclado para mezclas de materias primas

Iron zircon coral pigments are very interesting from an industrial point of view because of their high colouring power and their stability at high temperatures. However, the pigment's synthesis is particularly troublesome due to its specific reaction mechanism. As an encapsulated pigment it becomes very important how the raw materials are distributed in the reaction mixture. To evaluate the effectiveness of the mixing process, it would be convenient to define a parameter, that is the mixing index, to estimate the degree of homogeneity of the system. In the current investigation, a mixing index is proposed derived from the power spectrum of Fourier transform of scanning electron microscope (SEM) images of the raw material mixture. Concretely, the number of pixels in a certain range of values in the image of the power spectrum, seems to behave relatively well as mixing index. This index allows us to distinguish between samples with different zirconia and iron oxide used as precursors. The proposed mixing index seems to be related to the colouring power of the final pigment when the synthesis generates enough zircon to encapsulate hematite particles.Los pigmentos coral de hierro-circón son muy interesantes desde el punto de vista industrial ya que n un alto poder colorante y estabilidad a altas temperaturas. Sin embargo, la síntesis del pigmento es particularmente problemática debido a su mecanismo de reacción. Al tratarse de un pigmento encapsulado, resulta fundamental cómo se encuentran distribuidas las materias primas en la mezcla de reacción. Para evaluar la efectividad del proceso de mezclado, es conveniente definir un parámetro, que es el índice de mezclado, para estimar el grado de homogeneidad del sistema. En el presente trabajo de investigación se propone un índice de mezclado basado en el espectro de potencia de la transformada de Fourier de imágenes obtenidas con el microscopio electrónico de barrido de las mezclas de materias primas. Concretamente, el número de píxeles en un cierto rango de valores en la imagen del espectro de potencia parece funcionar relativamente bien como índice de mezcla. Este índice permite distinguir entre muestras con diferentes circonas y óxidos de hierro uitilizados como precursores. El índice de mezcla propuesto se relaciona con el poder colorante del pigmento final cuando durante la síntesis se genera suficiente cantidad de circón para encapsular las partículas de hematites

Microstructure assessment of suspension plasma spraying coatings from multicomponent submicronic Y-TZP/Al2O3/SiC particles

In this research, Suspension Plasma Spraying (SPS) technique was used for the thermal deposition of a multicomponent mixture made up of an Y-TZP/Al2O3 matrix with SiC particles. Two suspensions of Y-TZP and Al2O3 with different SiC particles content (6 wt% and 12 wt%) were tested as feedstocks in the SPS process. Three stand-off distances were varied in order to assess coating microstructure and evaluate the presence of SiC in the final coatings. Coatings were characterised in terms of porosity, microstructure and phase distribution. The estimate of the amount of SiC in the coating was carried out by XRD technique. Findings showed typical cauliflower-like SPS microstructure which intensifies with stand-off distance. Coatings porosity varied significantly between 8% and 25% whereas minimum porosity was found for the intermedium stand-off distance of 40 mm. Microstructure analysis also revealed the presence of SiC particles in the coatings which was confirmed by EDX analysis, overall XRD tests as well as TG analysis. Finally, evaluation of SiC content in the final coatings by means of XRD analysis showed that most of SiC particles (c.a 80%) of the feedstocks were preserved in the final coatings

Desenvolvimento de vidrados cerâmicos com propriedades bactericidas e fungicidas

Nos últimos anos foram desenvolvidas técnicas que permitem a síntese de materiais que, ao menos em uma de suas dimensões, são de tamanho nanométrico (1 a 100 nm), apresentando assim propriedades e funções significativamente diferentes das observadas nos materiais tradicionais de tamanho micrométrico. Os nanomateriais têm uma ampla faixa de aplicações especiais, especialmente em eletrônica, ciência dos materiais, comunicações e sistemas biológicos. As análises econômicas indicam que o mercado de nanotecnologia movimentará entre 750 milhões e 2 bilhões de euros a partir de 2015. No entanto, no setor cerâmico ainda não se começou a trabalhar com nanomateriais, pois as potenciais vantagens destes materiais ainda não foram valoradas convenientemente. É o que ocorre com as propriedades antimicrobianas que determinados nanomateriais podem aportar aos revestimentos cerâmicos. A aparição desta nova geração de materiais pode permitir a obtenção de produtos cerâmicos com propriedades bactericidas e fungicidas, cujas superfícies sejam capazes de impedir e eliminar o crescimento de organismos patogênicos, mantendo deste modo as melhores condições de segurança e higiene ambiental. Com isto, o setor cerâmico pode desenvolver produtos inovadores, de maior qualidade e alto valor agregado, com objetivo de aumentar sua competitividade.Este trabalho foi realizado graças ao apoio do Conselho da Indústria, Comércio e Navegação da Generalitat Valenciana, através do IMPIVA (n° de expediente IMIDIC/2009/10) e do Fundo Europeu de Desenvolvimento Regional

Bioactive glass coatings by suspension plasma spraying from glycoletherbased solvent feedstock

Bioactive glasses are emerging as a substitute of hydroxyapatite in the development of bioactive coatings for biomedical applications. The deposition of these coatings is carried out by a wide range of methods, being atmospheric plasma spraying the most employed technique. However, the research on the deposition of these coatings from suspension feedstocks by thermal spraying is still incipient, therefore more research about this topic is needed. Thus, a bioactive glass suspension, composed of fine glass particles, was prepared and stabilised through rheological and sedimentation tests to be used as a feedstock in plasma spraying. The solvent used in the suspension preparation was dipropylene glycol methyl ether in order to develop a new type of bioactive suspension. Consequently, as a new type of solvent was used, its effect on the plasma torch properties was determined. On other hand, the rheological behaviour of the suspension feedstock was assessed by means of a simple viscosity model. This suspension was deposited onto metallic substrates by plasma spraying, employing several spraying distances. All coatings displayed a suitable adherence and similar thickness. However, the microstructure of the obtained coatings is highly affected by the spraying distance as it can be seen in coatings surface and cross-section field emission gun environmental scanning electron microscopy examination. Thus, a relation between the spraying distance and coatings microstructure was found. On the other hand, X-ray diffraction confirmed the amorphous nature of the obtained coatings

Preparation of chamott es as raw material for low - cost ceramic membranes

Low cost ceramic membranes are usually prepared fro m a mixture of natural raw materials and some organic porogen agent, as starch . The fact that the porogen must be completely eliminated during firing, leaving an int erconnected porous structure, impose large firing times, increasing the final price. A s tudy about the synthesis of porous chamottes as an alternative to organic pore formers was conducted to reduce firing costs. Chamottes were obtained from mixtures of a c lay and starch. Different starches were used and the influence of the composition and processing variables were studied. The viability of the porous chamottes was demonstra ted.Supported by the Spanish Ministerio de Economía y C ompetitividad (Plan Nacional de I+D, ref. CTQ2012 37450 C02 02)

Microstructural and in vitro characterisation of 45S5 bioactive glass coatings deposited by Solution Precursor Plasma Spraying (SPPS)

The present work focused on the development of bioactive glass coatings employing Solution Precursor Plasma Spraying. Precursors of SiO2, CaO, Na2O and P2O5 were mixed in distilled water to prepare concentrated solutions with a composition close to the 45S5 bioactive glass. Solutions were rheologically characterised to assess their stability with time and deposited onto AISI type 304 stainless steel to develop coatings under different parameters related to the thermal spraying technique. The effect of these parameters on coatings microstructure was studied by scanning electron microscopy. Coatings were also analysed by X–ray diffraction and scratch test to complete the microstructural characterization. Moreover, coatings bioactivity was evaluated by immersing them in Simulated Body Fluid. The study showed that using short spraying distances and low argon flow rates, gave rise to the typical microstructure derived from liquid feedstocks whereas some crystallization associated to the long spray distance used occurred. Scratch test revealed that the resulting coating possessed good mechanical properties when compared with similar coatings obtained by other plasma spraying techniques. Moreover, the obtained coating could develop an hydroxycarbonate apatite layer when in contact with Simulated Body Fluid as demonstrated by scanning electron microscopy, X–ray diffraction and Fourier transform infrared spectroscopy

Microfluidic rheology: A new approach to measure viscosity of ceramic suspensions at extremely high shear rates

This paper describes the operation principles of a micro-scale rheometry equipment based on the technology of microchannels on a chip and compares it with a conventional macro-scale rotational rheometer. Both techniques are used for the study of different solutions and suspensions of ceramic particles with different saccharides, which are subjected to a wide variety of shear rates from their preparation to their processing and injection in plasma spraying processes. The results have shown a clear difference between the rheological values obtained between both equipment, clearly influenced by the different measurement method. However, the microfluidic technique has higher accuracy to characterise fluids at high shear rates and low viscosities, and the apparent shear thickening observed in rotational rheometry for low viscosity samples due to wall slippage is not observed. In contrast, the fine microchannels of are easily clogged in concentrated suspensions