Stability of (Cr)CaO.SnO2.SiO2 pink pigment in ceramic frits

Abstract

Incorporation of the (Cr) CaO.SnO₂.SiO₂ pigment in ceramic glazes yields colours with a red component the hue of which depends on the nature of the frits used to produce the glazes. Several samples were prepared by adding this pigment to different frits commonly used in the manufacture of glazed ceramic tiles, each sample subsequently being fired at the appropriate maturing temperature. A study of the resulting glazes shows that variations in colour arise as a result of the three possible phenomena which may develop in firing depending on the frit composition used. These three phenomena are pigment dissolution in the glassy phase, devitrification of crystalline phases whose nature differs from that of the pigment, and immiscible glassy phase separation.     

Grain growth of zircon pigment in tile glaze

Abstract

Glazes for ceramic materials are often opacified by a zircon pigment. The increase in optical density is related to the number and size of pigment particles, which are well dispersed in the glass. Mechanical dispersion by grinding of a fine zircon powder in a raw glaze mixture results in an unoptimised pigment dispersion in the melted glassy phase at high temperature. A significant reduction in the number of particles and a simultaneous large increase in their size is observed. In this case the optical glaze density does not attain the expected value and a very large quantity of zircon must be added. In the present work it is shown that a large part of the zircon reacts with the glass phase during firing. Simultaneously, the remaining zircon grains undergo a large increase in grain size by Ostwald ripening through the liquid phase. This process is characterised by a low activation energy (79 ± 1 kJ mol^-1) and a short diffusion path for Zr through the liquid phase between neighbouring grains. The path length is shorter than the size of the larger grains. In the usual firing range, both populations of smaller and larger grains vary in size, the smaller grains favouring growth of the larger grains.     

Effect of firing temperature and atmosphere on sintering of ceramics made from Bayer process bauxite residue

Bauxite residue, the principal waste from the Bayer process, was dried, pressed and studied for its thermal and sintering behaviour under different atmospheres, up to 1100 °C. For sintering in air and N₂, shrinkage begins at 800 °C and ranges from 2.6% to 13.9%, after firing at 1000–1100 °C. Bulk density varies from 1.7 to 2.3 g/cm³ whereas water absorption from 31.5% to 17.7%. The main crystalline phases identified on firing in air were hematite (Fe₂O₃), gehlenite (Ca₂Al₂SiO₇) and perovskite (CaTiO₃) whereas magnetite (Fe₃O₄) was also found on firing in N₂. Microstructures are characterised by irregularly shaped, <20 μm Feret diameter, pores in a ceramic matrix with interconnected porosity. The average pore size is greater in samples fired in N₂. On sintering in 4%H₂/Ar, shrinkage begins at 710 °C. After firing at 1100 °C, shrinkage is 20.1% and water absorption 1%. The main crystalline phases are magnetite, wustite (FeO), gehlenite and perovskite. Microstructures are characterised by a compact heterogeneous matrix, with isolated <15 μm Feret diameter, closed pores. The grains have reacted with the adjacent phase and their shape is rounded with no sharp facets. Increased sintering temperature results in improved physical properties for all atmospheres tested and in higher average pore size when sintering takes place in air and N₂. The use of magnetite-reducing sintering conditions can potentially assist in the production of a variety of ceramic compositions containing bauxite residue.     

Sol–gel synthesis,characterization and application of lanthanide-doped cobalt chromites (CoCr2–xLnxO4; Ln = Tm3+ and Yb3+)

Cobalt chromite based pigments CoCr_2–xLn_xO₄ (Ln?=?Tm³⁺ and Yb³⁺) with different substitutional level of lanthanide (x?=?0–0.5) have been synthesized using aqueous sol–gel synthetic approach. The XRD analysis revealed that single phase spinel was obtained only with low content of lanthanide ions (x?=?0.01–0.04). The sol–gel derived powders with higher concentration of lanthanide (x?=?0.05–0.2) contained minor amount of orthochromite phase. At the highest substitutional level (x?≥?0.2) the perovskite phase became the main crystalline phase. The colour of obtained pigments and corresponding ceramic glazes were analogous. Depending on the dominant phase, the colour varied from bluish-green (prevailing spinel phase) to dark brownish green (the main perovskite phase). This study proved that the replacement of chromium ions by thulium and ytterbium was successful at low content of lanthanides influencing the shade of pigment and corresponding glazes.     

In situ synthesis of orange rutile ceramic pigments by non-conventional methods

Different precursor-mixtures of orange Cr,Sb-TiO₂ ceramic pigment have been obtained by non-conventional methods (heterogeneous ammonia coprecipitation, urea homogeneous coprecipitation, PECHINI polyester method and an original aqueous–organic coprecipitation method in water–diethylenglycol medium) in order to produce in situ the pigment through the ceramic body firing. The pigmenting performances of powders were appraised in two cases: (a) as ceramic pigment for glazed porcelain stoneware and (b) as ceramic inks for screen printing of porcelain stoneware. Samples were characterised by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), UV–vis-NIR spectroscopy by diffuse reflectance method, CIE-L*a*b* colour parameters, BET specific surface area and crystallite size measured by the Scherrer method. The colouring performance of raw powders obtained by non-conventional methods in glazed porcelain stoneware improves that of the ceramic samples fired at 1100 °C used as reference. TEM observations indicate nanostructured powders with pigmenting performance depending on factors such as their specific surface area (BET), the crystalline phases detected by XRD (e.g. anatase–rutile presence) and their crystallite size (Scherrer measurements). Ammonia coprecipitated samples, both in water and in water–diethylenglycol medium without surfactant addition, or modified by the addition of sodium dodecyl sulphate as surfactant, stand out by their colouring performance.     

Development of alumina primary coat for single crystal investment casting ceramic mould

Abstract

The use of an alumina primary coat to eliminate the interaction between aluminium in NiAl single crystal alloy and silica in the ceramic investment casting mould has been examined in this study. The properties of the alumina slurry were characterised and the main factors controlling stability identified. It was found that the pH of the slurry must be carefully controlled to avoid polycondensation reactions. The most stable slurry was produced using filler with lower mean particle size and alkalinity. Alumina primary coat produced with low filler loading does not survive completely during wax removal. The surface condition can be improved by increasing the plate weight, which corresponds to a higher filler loading in the slurry. Moulds with a consistent alumina primary coat exhibit slightly higher green and sintered strength than moulds with the standard zircon-silica primary coat.     

Spodumene-bearing rock as flux for triaxial ceramic bodies

Abstract

Abstract

In this work, spodumene-bearing rock, a waste from mineral processing, was used as a flux in ceramic bodies, and its reactivity to other raw materials such as quartz and kaolin was analysed. The petrology properties of the pegmatite (parent rock) from which the flux is extracted were characterised. The grindability, fluxing strength and pyroplastic deformation of the spodumene-bearing rock were investigated. In addition, the influence of spodumene-bearing rock on technological properties of the final pieces, mainly in relation to the firing temperature, phases formation and microstructure was also analysed. Then, the firing behaviour to other commercial fluxes such as albite and feldspar was compared. The spodumene-bearing rock was considered an effective flux, capable of reducing the firing temperatures of the tested bodies by viscous flow.     

Behaviour of different industrial ceramic pastes in extrusion process

Abstract

It is well known that the extrudability of a ceramic paste is strongly dependent on materials composition and formulation, basically because it controls the resulting plasticity. Accordingly, the characterisation of the plastic behaviour is essential to adjust the fluxing properties of the material. In the current work, three industrially prepared ceramic pastes (porcelain, earthenware and terracotta) were tested, using stress–deformation curves obtained by compression of cylindrical probe tests. To achieve good extrusion performance, the design and operating conditions also need to be considered in detail.

The ram extrusion of adjusted pastes was followed by the Benbow–Bridgwater model. Differences in the flow behaviour were related with stress–deformation curves and with the surface quality of the extrudate (by using the static friction coefficient).     

Sodium-bismuth titanate based lead-free ferroelectric materials

In search of lead-free ferroelectric ceramics with improved properties, an investigation was carried out on barium and strontium-modified Na_0.5Bi_0.5TiO₃ (NBT) ceramics. Many compositions among the Ba and Sr-modified ceramic materials exhibit diffuse phase transition and are characterised by a strong temperature and frequency dispersion of the permittivity which would be connected with the cation disorder in the A site of the perovskite unit cell.     

Influence of firing temperature on the color developed by a (Zr,V)SiO4 pigmented opaque ceramic glaze

The analysis of the physical interactions between pigments, opacifiers and glazes is fundamental to understand the optical behavior of ceramic glazes. Furthermore is important to verify if the glaze devitrifies because the crystallized phases can contribute to the optical properties of the system. The size and the quantity of the formed crystals can change significantly the glaze color. The mean goal of this study was to evaluate the influence of firing temperature on the color stability of an opaque ceramic glaze colored by a blue vanadium–zircon pigment taking into account all the optical components. Quantitative X-ray diffraction analysis was conducted in order to evaluate the pigment dissolution at the three studied temperature and the quantity of the in situ formed zircon crystals from the used frit. The reported study demonstrated the importance to consider all the components in a multicomponent optical system as a ceramic glaze.     

Influence of fluorine source and TiO2 on glass ceramics based on red clay and magnesite

Abstract

Within the Fe₂O₃–BaO–Al₂O₃–SiO₂ glass system, the influence of different fluorine source and TiO₂ on the crystallisation behaviour and thermal expansion characteristics has been investigated. These aluminosilicate glasses, with different nucleation catalysts (TiO₂, AlF₃ and NH₄F), were successfully prepared from Saudi Arabia raw materials (red clay and magnesite). Thermal behaviour, crystalline phases, microstructures, and thermal expansion coefficient were studied by DTA, XRD, SEM techniques, as well as a dilatometer. Spinel, hexacelsian, monocelsian, kinoshitalite, and Ba-osumilite were developed in glass ceramic samples. Spinel was the early phase formed in all glasses; however, with increasing temperature, hexacelsian and kinoshitalite were developed in fluorine free glasses and fluorine containing glasses respectively. Ba-osumilite was developed and hexacelsian was converted into monocelsian during lengthy heat treatment at 1150°C. A bulk crystalline microstructure was obtained; however, a microscale structure was observed in fluorine free samples and a homogeneous nanoscale microstructure was developed in fluorine containing samples. The coefficient of thermal expansion (CTE) varied according to the phases and the heat treatment parameters. Development of spinel, hexacelsian and kinoshitalite in glass ceramic samples gave CTE values between 86·44 × 10^?7 and 52·53 × 10^?7°C^?1, whereas crystallisation of Ba-osumilite and monocelsian gave CTE values between 51·29 × 10^?7 and ?7·35 × 10^?7°C^?1.     

Low cost ceramic moulding composites: materials and manufacturing technology

Abstract

Low cost ceramic moulding compounds/composites are composed of inorganic metal silicates, fillers and different types of chopped fibre reinforcements, such as glass fibres. This research investigated manufacturing process of a series of ceramic moulding compounds that could be moulded at modest temperature (～150°C) and pressure (0–8 MPa). This manufacturing process replicated a process of the fibre reinforced polymeric moulding composites or dough moulding compounds. The conventional polymeric matrix of the dough moulding compounds has been replaced by a soluble metal silicates based on an inorganic system which can be compounded with fibres, minerals or synthetic fillers and hardening additives, to produce a thick paste or a doughy substance, so called ceramic dough moulding compounds. The ceramic dough moulding compounds demonstrated a good handle ability and can be adjusted by the viscosity of the matrix through the use of various amount of fillers and additives. The mechanical properties of the compounds have been tested and additional formulation changes have been introduced to maintain desirable processing characteristics.     

Investigation of properties of electric arc ion deposited TiN coating on Al2O3-based ceramic composite

Electric arc ion deposition technique was adopted to deposit TiN coating on Al2O3-based ceramic composite. Scanning electron microscopy and secondary ion mass spectrometry were used to analyze the microstructure, phase constitution, and quality of the TiN coating and the interface. Surface roughness and micro-hardness of the TiN coating were measured to evaluate its quality. Flexural strength of ceramic materials is dependent on both their inherent resistance to fracture and the presence of defects, thus it was used to investigate the effect of electric arc ion deposition technique on the surface modification of Al₂O₃-based ceramic composite. Experimental results show that the higher the deposition bias voltage, the better the coating quality. The TiN coating is homogeneous, with a uniform surface, and free of defects when the deposition bias voltage is 300 V. The TiN coating strongly adheres to the Al₂O₃-based ceramic composite, and the observed elemental interface diffusion strengthens the interface bonding.     

Gelcasting of alumina based ceramic cores containing yttria for single crystal and directional solidification blades

Abstract

Ceramic cores play an essential role in investment casting. In this paper, gelcasting process has been successfully employed to fabricate alumina based ceramic cores containing yttria for single crystal and directional solidification blades. Based on an investigation of the formability of different ceramic slurries, material compositions of ceramic cores are determined by experiments. A proper sintering process is developed to get low sintered shrinkage, high apparent porosity and high room temperature flexural strength. The high temperature properties of ceramic cores are improved by dipping in water based yttria sol and resintering. The test results show that comprehensive properties of alumina based ceramic cores containing yttria fabricated by gelcasting are better than those of AC-1 ceramic cores made by the Beijing Institute of Aeronautical Materials, China, and that the ceramic cores can be applied to single crystal and directional solidification blade casting.     

Effect of the milling process on the properties of CoFe2O4 pigment

In this study, CoFe₂O₄ pigments were synthesised using both co-precipitation and conventional ceramic methods. Pigment particles prepared using the conventional ceramic method were subsequently milled to submicron size. The effects of the solvent, dispersant and milling type in the milling process were investigated. This study showed that planetary milling in a diethylene glycol (DEG) medium with sodium tripolyphosphate (STPP) was an effective method for producing submicron-sized pigment powders from pigments synthesised using the conventional method. With this method, submicron-sized pigment particles (approximately 190 nm) were obtained after milling for 4 h. Planetary milling was more efficient in reducing particle size compared to attrition milling. Co-precipitated pigment had a more intense black colour, due to the nanoscale particle size (<100 nm). However, conventional ceramic pigments also had an adequately intense black colour that increased after milling compared to unmilled conventional pigments. When considering production of industrial scale submicron-sized pigments, the milling of these pigments to submicron size can be a good alternative method for the production of ink colourants.     

Synthesis of new vanadium cordierite pigment by sol–gel processing

Abstract

A new ceramic pigment with the cordierite high temperature structure was synthesised by sol–gel processing. A small region of a solid solution phase was detected containing <5 wt-%V₂O₅ . The crystallisation behaviour and thermal stability of the pigment was studied. Unit cell parameters, crystallite size, and the phase constitution of calcined powders were determined by X-ray diffraction. The xerogel powders were studied by thermal analysis and infrared spectroscopy. The compositions of fired samples were obtained by inductively coupled plasma analysis and the L ^*a^*b^*colour parameters were measured.     

Change of phase composition and morphology of sonochemically synthesised hydroxyapatite nanoparticles with glycosaminoglycans during thermal treatment

Abstract

Based on the preparation of glycosaminoglycans (GAGs) stabilised hydroxyapatite (HAP) nanoparticle suspension via sonochemical synthesis, the change of phase composition and morphology of freeze dried HAP nanoparticles with GAGs was investigated from 500 to 1200°C by TEM, X-ray diffraction, Fourier transform infrared spectroscopy and SEM. Results show that thermal treatment brought the phase transformation and the morphology change of HAP nanoparticles. In the low temperature stage (～650°C), the samples were mainly composed of HAP as main crystalline phase and β-NaCaPO₄ as minor crystalline phase. This phase transformation was mainly attributed to the residues containing sodium derived from combustion of GAGs. The particles were near spherical, and the nanocrystalline nature was retained. In the high temperature stage (650–1200°C), the samples were glass ceramic powders composed of HAP, β-tricalcium phosphate, Na₃Ca₆(PO₄)₅, β-NaCaPO₄ and Na–Ca–P–O glass phase. The grains rapidly grew into larger particles with morphology transformation from rodlike shape to irregular shape and the size increase from (0·1–0·15) × (0·3–0·5) μm to 1·5–10 μm.     

Complex permittivity andpermeability of Zn–Co substituted Z type hexaferrite prepared by citrate sol–gel process

Abstract

A series of Z type hexaferrite samples with composition Ba₃Co_xZn_{2 -x}Fe₂₄O₄₁ (x varying from 0·0 to 2·0 in steps of 0·4) were prepared by a citrate sol-gel process. Samples were characterised by TG-DSC, X-ray diffraction, and scanning electron microscopy. Complex permittivity and permeability, and dielectric and magnetic losses were studied as a function of measurement frequency, composition, and annealing temperature. The dependence of natural resonant frequency on annealing temperature as well as on cobalt content was investigated in the range 100 MHz to 6 GHz. Reflection loss has been calculated as a function of frequency according to transmission line theory.     

Bone china formulated with waste glass

Abstract

Abstract

Bone china is known for its technical and aesthetical quality with distinguishable whiteness, brightness and high mechanical strength. In this work, the use of waste glass was investigated in a bone china composition. This makes the production of this porcelain even more interesting from the environmental point of view, since most of its raw materials are from recyclable sources. Potash feldspar was partially replaced by waste glass, and the technical and slip rheological properties, phase development and microstructure after firing of the ceramic bodies were evaluated. The results revealed that waste glass provides a reduction in firing temperature and large plateau for firing while maintaining the quality of the final porcelain, including thermal expansion and mechanical strength.     

Preparation and characterisation of self-flowing refractory material containing 971U type microsilica

Abstract

A newer composition of self-flowing low cement brown fused alumina castable containing 971U type microsilica was developed. Optimum flow characteristics were achieved using water addition of 4·6?wt-%. This batch was sintered at different firing temperatures up to 1500°C. To understand the effect of both firing temperatures and corresponding phases, the present castable was characterised in terms of X-ray diffraction, scanning electron microscopy, bulk density (BD), apparent porosity (AP), water absorption (WA), cold crushing strength (CCS) and self-leveling flowability. The results revealed that gradual increase in firing temperature from 1100 through 1500°C caused low AP and WA, and high CCS properties due to the densification of the castable.