Shear-thinning behaviour of dense, stabilised suspensions of plate-like particles. Proposed structural model.

A structural model was developed to describe the shear-thinning behaviour of dense, stabilised suspensions of plate-like particles. The model is based on the following main assumptions: the particles are distributed in more or less compact layers, oriented parallel to the flow; the particles are assumed to behave as hard disks, disk thickness being the sum of crystal thickness plus twice the Debye length; when the shear stress increases, the orientation of the plate-like particles in the flow direction also increases, thus increasing layer compactness. In order to test the proposed model, a kaolin was selected and characterised. The kaolin was used to prepare more than 40 aqueous suspensions, modifying the solids volume fraction between ϕ = 0.20 and ϕ = 0.475 and the dispersant (sodium silicate) content between Xs = 0.075 and Xs = 0.225 mg dispersant/m2 solid. The flow curves of all suspensions were determined in the quasi-steady state. The results confirmed the validity of the model to satisfactorily describe the combined effect of ϕ and XS on the flow curves in the shear-thinning range

Mechanical properties obtained by nanoindentation of sintered zircon-glass matrix composites

This study was undertaken to determine the effect of zircon content and firing temperature on the hardness and indentation modulus of zircon–glass composites obtained by sintering. A standard non-devitrified borosilicate glass (SRM 717a) powder and an industrial micrometric zircon powder were used to prepare mixtures with a zircon volume/solids volume between 0 and 0.63, by the wet method. The values of these mechanical properties were determined by nanoindentation and related to the most important microstructural characteristics of the composites, such as porosity, zircon volume fraction, glass volume fraction, and average zircon grain size. Composite mechanical performance was interpreted and determined by statistically analysing the results of a large number of indentations using two maximum loads. An empirical model was developed that describes the effect of these microstructural characteristics on composite hardness and modulus of indentation. Composites of high hardness (11.3 ± 2.5 GPa) and low porosity (ε = 0.07) were obtained at 1100 °C from a mixture with a zircon volume/solids volume of 0.43

Non-isothermal sintering of powdered vitrified composites. A kinetic model

This report sets out the results obtained on studying the sintering process of glass–zircon composites, analysing the microstructural changes that developed on modifying zircon content. The sintering of composites with moderate zircon contents only developed via particle rearrangement by viscous flow. In contrast, at high zircon contents, the zircon solution–reprecipitation process was also required. A kinetic model was developed and validated that describes the effect of the heating rate and zircon volume fraction on the composite degree of non-isothermal sintering progress associated with particle rearrangement by viscous flow

Sinter-crystallisation kinetics of a SiO2–Al2O3–CaO–MgO–SrO glass-ceramic glaze

This paper examines the microstructural development and kinetics of the sintering and crystallisation processes of a SiO2–Al2O3–RO (R = Ca, Mg, Sr) glass-ceramic glaze. Crystallisation and sintering kinetics were studied by DTA and HSM, respectively, at different heating rates. The kinetic parameters of crystallisation were determined by the usual methods (Kissinger, Kissinger-Akahira-Sunose, Ozawa and Augis-Bennet methods), and the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, with an Avrami index of n = 3, characteristic of the surface crystallisation of very fine glass particles, was found to describe crystallisation kinetics very well. Sintering could also be described by the JMAK model, but with n < 1. Assuming the effect of temperature on the sintering rate to be the same as that of this variable on the inverse of glass matrix viscosity, a model was developed, based on the JMAK model, which only required a single fitting parameter. As the heating rate increased, the degree of overlap between the sintering and crystallisation processes was verified to decrease

Densification of irregular polydispersed glass particles described as a complex relaxation process

The sintering of compacts of irregular non-crystallising glass particles was studied by isothermal and constantrate heating experiments in a hot stage microscope. The resulting data fitted very well to kinetic equations developed in this study, in which sintering is assumed to be a complex relaxation process, described by the Kohlrausch–Williams–Watts (KWW) relaxation function. The effect of compact pressing pressure, heating rate, and particle size distribution on the sintering curve was determined. It was generally verified that the effect of temperature on the sintering rate could be described by the effect of temperature on the inverse of glass viscosity. For industrial particle size distributions, that the pre-exponential factor of the process rate constant (or inverse of relaxation time) increased with pressing pressure and decreased with the inverse of particle mean radius. For abnormally wide particle distributions a combination of KWW functions were required

Non-isothermal sinter-crystallisation of satin glazes: A kinetic model

Many materials of a glass-ceramic nature are obtained by glass particle sinter-crystallisation, whose process kinetics have been studied very little. The present study analyses the physico-chemical transformations that develop during the firing of a complex commercial satin glaze (containing more than five phases), with high frit content, particularly focusing on sinter-crystallisation kinetics. Glaze sintering and phase evolution were studied by hot stage microscopy (HSM) and X-ray diffraction (XRD). The glaze melting and crystallisation ranges and the kinetic parameters of the crystallisation process were determined by differential thermal analysis (DTA). Glaze sinter-crystallisation behaviour and the development of the crystal mass fractions, residual glass compositions, and effective viscosity, ηeff, during heating are discussed, based on Rietveld analysis of the XRD data. A new kinetic model was developed that describes the non-isothermal sinter-crystallisation of materials exhibiting three-stage sintering. A two-step kinetic model is involved: sintering with concurrent surface crystallisation (corresponding to sintering stages I and II) and sintering with simultaneous partial melting of crystalline phases (sintering stage III). The experimental data obtained by hot stage microscopy (HSM) at different heating rates were compared with those estimated by the model. The results obtained by the two methods exhibited very good agreement. The crystallisation kinetic parameters (activation energy, Ecr, and Avrami index, n, obtained by DTA, were consistent with the viscous flow activation energy, Q2, corresponding to the first sintering step (sintering stages I and II). The model was used to calculate the values of the effective sintering viscosity, ηs, of the glaze melt. These values were compared with the experimental effective viscosity, ηeff, data obtained by hot stage microscopy (HSM). Both sets of results exhibited good agreement. Effective sintering viscosity, ηs, which is readily measurable, helps better understand the role played by the different glaze constituents and by the firing conditions in sintering, enabling more rational design of these materials

Inkjet technology for ceramic products. Influence of some process variables on ink penetration

Inkjet technology has become very popular in recent years and is nowadays widely used for decorating ceramic products. This has led to in-depth study of the operating principles and process variables involved. These notably influence the properties of the deposited ink layers, as do the physicochemical properties of the applied inks and of the glazes on which the inks are deposited. The problems associated with these factors are further heightened when inks with complex formulations designed to obtain special effects and specific surface properties are used. This study examines the characteristics of the interface that developed on applying an ink on a certain type of glaze, as well as the surface properties of the final product. The relationship of the amount of ink applied per unit area and the particle size distribution of the glaze on which it was deposited to ink penetration into the glaze layer was also investigated. The resulting microstructures were observed by SEM, while the variation of glaze layer composition with distance to the surface was determined by SEM-EDS. The results obtained were successfully correlated with the porous texture of the unfired glaze (determined by SEM and mercury porosimetry) and with unfired glaze particle size distribution

Determinación de sales solubles y eflorescencias en tejas cerámicas

The appearance of efflorescences constitutes a serious problem for ceramic roofing tile quality, since efflorescences affect roofing tile technical and aesthetic qualities. This paper describes a method of determining the soluble salts present in the raw materials used in ceramic roofing tile manufacture and in the resulting roofing tiles, as well as an analysis of the phases in roofing tile efflorescences. The study was divided into three parts. First, a method of determining the soluble salts was developed, optimising the extraction method and fine-tuning an anion measurement method by ionic chromatography. Secondly, the changes in solubility, as well as the chemical and structural changes that occurred when the raw materials were subjected to firing, were studied. Finally, the efflorescences of a group of unfired and fired ceramic roofing tiles were characterised, in order to relate them to the quantity and type of soluble salts contained. The study concludes that it is necessary to conduct solubility tests on fired samples under real conditions, since important structural changes occur in the firing process, which lead to considerable variations in the soluble salts. Furthermore, in addition to analysing Na(I), K(I), and Mg(II) as indicated in the standard, it is advisable to determine Ca(II) and SO42-, since these are the major components in the efflorescences.La aparición de eflorescencias constituye un serio problema para la calidad de las tejas cerámicas ya que afectan a las características técnicas y estéticas de éstas. En este trabajo se ha realizado un estudio del método de determinación de las sales solubles presentes en materias primas y en tejas cerámicas, así como un análisis de fases de las eflorescencias halladas. El trabajo se ha dividido en tres fases. Primero, se ha desarrollado un método de determinación de las sales solubles, optimizando el método de extracción y poniendo a punto un método de medida de los aniones mediante cromatografía iónica. Seguidamente, se han estudiado los cambios de solubilidad, químicos y estructurales que ocurren al someter los materiales al proceso de cocción. Finalmente, se han caracterizado las eflorescencias de un conjunto de tejas crudas y cocidas para relacionarlas con el contenido y tipo de sales solubles presentes. Del estudio se concluye que es necesario realizar los ensayos de solubilidad sobre las muestras cocidas en condiciones reales ya que en el proceso de cocción ocurren importantes cambios estructurales que conllevan una considerable variación de las sales solubles; y que además de analizar Na+, K+ y Mg2+, como se indica en la norma, se aconseja determinar Ca2+ y SO 2-, por ser los componentes mayoritarios de las eflorescencias

Development of a yellow ceramic pigment based on silver nanoparticles

In this study a yellow pigment was obtained for third-fire ceramic decorations, based on silver nanoparticles synthesised by the method of chemical reduction in aqueous phase, using silver nitrate and polyvinylpyrrolidone as raw materials. Monitoring of the nanoparticle synthesis reaction by UV–vis spectroscopy allowed optimum operating conditions to be defined in preparing these particles for use as chromophores. Under these conditions, a stable suspension of Ag nanoparticles, which were well dispersed and had an average diameter of 20–30 nm, was obtained. Polyvinyl alcohol and tetraethyl orthosilicate were then added to the nanoparticle suspension to obtain the pigment precursor. The pigment precursor was directly applied on to fired glazed ceramic tile. Subsequent thermal treatment at moderate temperature (700 °C) yielded a layer less than one micron thick, which generated an intense yellow colour

Densification of monomodal quartz particle beds by tapping

A study has been undertaken to determine how the mean particle size of monomodal quartz particle beds and the bed preparation method affect initial and final bed apparent density and the kinetics of densification by tapping. The study was conducted on eight quartz particle size fractions obtained from a commercial quartz powder by sieving, each having a different mean particle size and a very narrow (monomodal) particle size distribution. The amplitude of the distribution and the particle shape were practically the same for all fractions. Densification experiments were conducted on the beds obtained with each fraction, using an assembly designed for this purpose. A kinetic model representing the densification process of monomodal powder beds by tapping is proposed, in which good relations are obtained between the parameters of the model and the foregoing variables. © 2008 Elsevier Ltd. All rights reserved