Crystalline phases and physical properties of modified stoneware body with glaze sludge

Ceramic stoneware body has been modified with ceramic and glass industrial wastes by replacing 25–100% as flux in the formula. The effects of solid wastes added to the bodies were studied after firing in the temperature range 950–1280 °C. The physical properties of linear shrinkage, bulk density, apparent porosity, water absorption and 3-point bending strength were determined. A composition which related to the general stoneware properties was found when using soda-lime cullet and glaze sludge. It had a firing range lowered to 1050–1100 °C. SEM results demonstrated the sintered microstructure increased in density with increase in solid waste in the modified body. XRD results after firing showed the crystalline phases comprised of mullite, albite calcian and quartz. Thermal expansion was measured in the range 6.53–6.67 × 10^?6 K^?1 at 30–500 °C. The modified bodies were capable of forming prototype products by slip casting and jiggering.     

Optimization of the technological properties of porcelain tile bodies containing rice straw ash using the design of experiments methodology

This study examines the effects of replacing fluxing and filler materials with rice straw ash (RSA) in manufacturing porcelain stoneware tile, using the design of experiments (DOE) methodology. The results of the characterization were used to obtain statistically significant, valid regression equations, relating the technological properties of the dried and fired test pieces to the raw materials content in the unfired mixtures. The regression models were analysed in relation to the X-ray diffraction and scanning electron microscopy results and used to determine the most appropriate combinations of traditional raw materials and RSA to produce porcelain stoneware tiles with specific technological properties. The studied range of tile body compositions: clay (40 wt%), feldspar (20–50 wt%), feldspathic sand (5–20 wt%), and RSA (0–25 wt%) was shown to be appropriate for porcelain stoneware tile manufacture.     

Influence of composition on some industrially relevant properties of traditional sanitary-ware glaze

Two series of glazes have been produced from different combinations of the same raw materials in the range of interest for sanitary-ware applications: they are designed to allow one to get insight into network-forming and network-modifying species. Fusibility tests and hot stage microscope observations show the influence of even low differences in the starting chemical compositions on the transformation temperatures. X-ray powder diffraction, wavelength dispersion spectrometry and scanning electron microscopy prove that: (i) zircon, the most abundant crystalline phase, is homogeneously distributed and decreases by a 3% from its starting value; (ii) the glass-phase of glaze has a quasi-uniform composition. X-ray synchrotron radiation micro-tomography shows that glaze porosity is 15% by volume, and voids are prevalently not interconnected and with size up to 50 μm. The linear thermal expansion of the glass phase of glaze ranges between 6 and 7 × 10^?6 °C^?1, without apparent correlation with composition.     

Glass-ceramic glazes for future generation floor tiles

Glaze in the CaO–MgO–Al₂O₃–SiO₂ system was heated at 950–1190 °C for 2 h and characterized. X-ray diffraction showed that only trace amount of mullite was formed in the glass-ceramic glaze heated at 950 °C. Both mullite and α-cordierite were formed in the glass-ceramic glaze heated at 1050 °C as primary and secondary phases. Glass-ceramic glazes heated at 1120 °C and 1190 °C contained α-cordierite and mullite as major and minor phases. Rietveld analysis revealed that the amount of α-cordierite increased and mullite decreased with increasing heating temperature. Field emission scanning electron microscopy showed presence of mullite crystals dispersed within residual glassy phase in the glass-ceramic glazes heated at 950 °C and 1050 °C. In the microstructures of glass-ceramic glazes heated at 1120 °C and 1190 °C α-cordierite crystals were mainly appeared. Energy Dispersive X-ray analysis corroborated X-ray diffraction results. Vickers microhardness measurement demonstrated highest hardness (8.38 ± 0.07 GPa) of the glass-ceramic glaze heated at 1190 °C.     

Development of gold-bronze metallic glazes in a clay-based system for stoneware bodies

In this study, a triaxial glaze system consisting of red clay, kaoline, quartz, MnO, CuO and CoO is systematically developed to produce gold-bronze raw metallic glazes for stoneware bodies. At first, all of the glazed samples in the developed system were fired in an electrically-heated kiln at 1160 °C. Then, the selected successful gold-bronze metallic glazes were applied onto 3-D forms of stoneware bodies and fired at the same conditions. Microstructural characterizations of the glazes are done with scanning electron microscopy (SEM) and energy dispersive x-ray analyses (EDS). This study revealed that triaxial blending of the ceramic raw materials is a beneficial method for glaze production and gold-bronze surfaces are obtained in glazes G 9, G 26, and G 34. It is observed that chemical composition of the glazes directly influence the color and the amount of CuO is more significative than MnO for achieving gold-bronze effect.     

Characterization of transparent glaze for single-crystalline anorthite porcelain

The objective of this work was to design a transparent glaze for matching single-crystalline anorthite porcelain. Excessive amounts of quartz were used in glaze to improve surface hardness. Technological properties including hardness and thermal shock resistance were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies were also carried out to analyze the microstructure. The phases found in glaze were aluminosilicate glass, quartz and cristobalite crystals. The Vickers hardness of the transparent glaze was about 2.48 GPa, which was much higher than that of commercial soft glaze and was close to hard porcelain glaze due to forming dispersed crystal particles (quartz and cristobalite) in the glass matrix. Moreover, the thermal expansion coefficient of the glaze was slightly lower than that of porcelain body which was easy to produce compressive stress in glaze surface to increase the strength of porcelain. And no cracks were observed on glaze surface after heat exchange three times from 220 °C to 25 °C, presenting excellent thermal shock resistance.     

The influence of particle size distribution on the surface appearance of glazed tiles

Matte glaze was milled in for different times (15–50 min) and the glazes obtained were applied over ceramic bodies which were then fired using an industrial cycle and the final surfaces were analyzed using a glossmeter. The same glazes were also uniaxially pressed, forming cylindrical samples; the specimens were placed on engobe surfaces and fired at 1100 °C in a laboratory oven. The contact angles between the melted specimens and the ceramic surfaces were measured to determine their wettability. The results showed a correlation between the glaze particle size distribution and the surface aspect of the final product.     

Synthesis of glass-ceramic glazes in the ZnO–Al2O3–SiO2–ZrO2 system

Glazes in the ZnO–Al₂O₃–SiO₂–ZrO₂ system with crystallization ability of gahnite (ZnO·Al₂O₃) and β-quartz solid solution (βqss) were synthesized. The compositions were designed based on calcium and magnesium oxide replacement (from a CaO–MgO–Al₂O₃–SiO₂ glass-ceramic glaze system) with zinc oxide and simultaneous increasing aluminum oxide. By this replacement, diopside eliminated and co-precipitation of gahnite and zirconium silicate observed. However, a little addition of Li₂O changes the crystallization path by precipitation of βq_ss and willemite (2ZnO·SiO₂) at low temperatures (800–900 °C) which dissolved into glaze by development of firing temperature. The experiments showed that while the micro-hardness of gahnite-based glass-ceramic glazes is almost equal with the diopside based one, it is more than the traditional floor tile glazes.     

Metallurgy dusts as a pigment for glazes and engobes

This study is focused on using the dust from metallurgy as a pigment. The agglomerating dust is formed during metallurgical processes. This waste product is interesting for recycling process. The main mineralogical phase of dust is hematite α-Fe₂O₃. Both synthetic and natural iron oxides are commonly used as pigments in ceramic industry. In this experiment the metallurgy dusts were used as a pigment for preparation of glazes and engobes. Agglomerating dusts were used both precalcined thermally at 700 °C and 900 °C and in an original state. The prepared glazes were composed of a transparent glaze base with 10 wt% agglomerating dusts as pigment. The glazes calcined at 1060 °C were finally yellow colored and glazes calcined at 900 °C were brown colored. Engobes contained a ceramic clay base with 1, 5, 10 and 50 wt% of dust as pigment. Engobes calcined at 900 °C were red and grey colored. The pigments were characterized by X-ray diffraction (XRD), chemical (XRFS) analysis, granulometry (PSD), thermogravimetric (TG) and differential thermal (DTA) analysis, scanning electron microscopy (SEM) and CIELab values.     

Fabrication and characterization of anorthite-based ceramic using mineral raw materials

The purpose of this study is to design a novel single crystalline phase ceramic based on anorthite whose properties fulfill the tableware market requirements such as high appearance quality, strength and thermal shock resistance. To obtain the single phase anorthite ceramic, ball clay, quartz, calcite, feldspar and alumina were used as raw materials. The single phase anorthite ceramic was fabricated by slip casting and sintering at 1230 °C for 1 h. It has a high flexural strength of 103 MPa, which is higher than that of the conventional porcelain. The single phase anorthite ceramic had relatively low (4.9 × 10^?6 K^?1) thermal expansion coefficient which can be matched with applicable glaze easily. Furthermore, the single phase anorthite ceramic had high degree of whiteness (L* = 94) and excellent translucency behavior which could achieve a high-quality decorative effect.     

Mullite development on firing in porcelain stoneware bodies

Microstructural evolution on heating was investigated in a reference industrial composition (50% kaolinitic clay, 40% feldspar and 10% quartz) of porcelain stoneware, fast fired at different temperatures (500–1400 °C). The evolution of mullite crystals, regarding shape and size progress, was examined by scanning electron microscopy (SEM). The proportion of Type I mullite crystals decreases with firing temperature and simultaneously, the size of crystals increases, reaching the maximum value of aspect ratio (3:1) at 1400 °C. Type II and Type III secondary mullite needles increase with temperature in both number and length, which leads to an increase in the aspect ratio from 5:1 to ～20:1 in Type II crystals and from ～33:1 to 50:1 in Type III mullite needles. Finally, clusters of Type III mullite fibres are observed in porcelain stoneware samples fast fired in the 1250–1280 °C interval.     

Blue cobalt doped-hibonite pigments prepared from industrial sludges: Formulation and characterization

The structural stability of the pigment was investigated using X-ray diffraction coupled with SEM and UV–vis–IR analysis, as a function of the relative Co content and calcination temperature. A standard formulation prepared from commercial reagents was also prepared and characterized for the sake of comparison. The pigment was added to a transparent glaze and to a porcelain stoneware body. Since the relative amount of cobalt was low and the sintering temperature of the pigment was not too high (1350–1400 °C), the new pigment system offers an interesting alternative to spinel, olivine and willemite commercial blue pigments. Moreover, the use of several wastes in the pigment formulation does not diminish its colouring performance.     

Sintered esseneite–wollastonite–plagioclase glass–ceramics from vitrified waste

Dense sintered esseneite–wollastonite–plagioclase glass–ceramics have been successfully prepared from a vitrified mixture of important inorganic waste (Bayer process red mud, fly ash from lignite combustion and residues from the polishing of porcelain stoneware tiles). The enhanced nucleation activity of fine glass powders, favoured by particular oxidation conditions, caused a substantial crystallisation, even in the case of very rapid thermal treatments at 900 °C, which led to remarkable mechanical properties (bending strength and Vickers micro-hardness exceeding 130 MPa and 7 GPa, respectively) and a promising chemical durability.     

Environmental-friendly yellow pigment based on Tb and M (M = Ca or Ba) co-doped Y2O3

A yellow inorganic ceramic pigment with general formula Y_1.86?xM_xTb_0.14O_3?x/2 (M = Ca and/or Zn) with x = 0.06, 0.32 and 0.64 were synthesized by a modified Pechini method. XRD, SEM and HRTEM/EDX analysis showed the formation of solid solution at 1300 °C when x = 0.06 and 0.32. The best b* yellow coordinates were obtained for Ca and Zn co-doped Y_1.86Tb_0.14O₃ samples. The intensity of the yellow colour in the samples is related to the presence of Tb⁴⁺ ions. Samples with higher concentration of Tb⁴⁺ ions lead to a better yellow colour. The chemical stability of these pigments was determinate in an industrial glaze. The glazing tests indicated that the powder samples with x = 0.06 and 0.32 fired at 1300 °C were stable in the glaze. These results make it a potential candidate for environmental friendly yellow ceramic pigment to be used in applications such as pigment for glazes or inkjet printers.     

Temperature dependence of electrical conductivity of a green porcelain mixture

AC conductivity of a green porcelain body was investigated using impedance spectroscopy over a temperature range of 100–950 °C. The results showed that during the heating, conductivity at 100–200 °C increased mainly arising from H⁺ and OH⁻ ions generated from adsorbing physical water. The activation energy increased below the dehydroxylation of clay resulting from movement of monovalent ions. At the dehydroxylation of clay, a combination of H⁺, OH⁻ and monovalent ions dominated the conductivity. The activation energy rose to 1.14 eV (600–950 °C) controlled by diffusion of Na⁺, and K⁺ ions. During the cooling, conductivity showed single activation energy with 0.86 eV resulting from denser microstructure and change in mineralogical constituents and the heat treated porcelain sample showed higher electrical conductivity at the same temperature. Understanding conduction behaviour of the green porcelain enabled more accurate control of furnace temperature in flash sintering, a process which relies on electrical conductivity at high temperatures.     

Refractory glass–ceramics based on alkaline earth aluminosilicates

Glass–ceramics that can be used at temperatures of 1200–1500 °C are found in the alkaline earth aluminosilicate field, and are generally nucleated internally with titania. These glass–ceramics have good strength (>100 MPa, abraded), can be tailored to produce high fracture toughness (2–5 MPa m^1/2), and have good dielectric properties. Coefficients of thermal expansion (CTEs) are low to moderate ((25–45) × 10^?7 °C^?1, from 25 to 1000 °C).The major crystalline phase in the glass–ceramics exhibiting the lowest CTEs is hexagonal cordierite (indialite), while important toughening accessory phases are enstatite and acicular magnesium dititanate.The most refractory glass–ceramics that are easily melted at 1650 °C, yet when crystallized do not deform at 1450 °C, are based on strontium and barium monoclinic feldspars of the celsian type. CTEs range from 35 to 45 × 10^?7 °C^?1. Acicular mullite is an important accessory phase aiding fracture toughness in these materials.Mullite glass–ceramics which contain considerable siliceous residual glass are probably the most refractory of these glass–ceramics, but they require melting above 1700 °C. Nevertheless, they can be used at temperatures near 1600 °C.Potential applications for refractory glass–ceramics include improved radomes, engine components, substrates for semiconductors and precision metallurgical molds.     

SrCo0.9Sc0.1O3−δ perovskite hollow fibre membranes for air separation at intermediate temperatures

SrCo_0.9Sc_0.1O₃ (SCSc) perovskite powders with sub-micron particle size were synthesized by a modified Pechini method combined with a post-treatment of sintering and ball-milling. From the prepared powders, the SCSc hollow fibre membranes with asymmetric structure and gas-tight property were fabricated by spinning a polymer solution containing 58.4 wt% SCSc followed by sintering at 1200 °C for 5 h. The oxygen permeation properties of the obtained SCSc fibres were measured under air/He gradients at 500–800 °C. This showed the oxygen flux of 1 mL cm^?2 min^?1 at 750 °C and 4.41 mL cm^?2 min^?1 at 900 °C. Modeling analysis reveals that the oxygen permeation process is predominated by oxygen surface exchange kinetics with an activation energy of 95.0 kJ mol^?1. The SCSc membranes showed excellent oxygen permeation performance while exhibiting high structural and permeating stability at intermediate temperatures (500–800 °C).     

The use of pumice (pumicite) in transparent roof tile glaze composition

In this study, the use of pumice as an alternative fluxing agent instead of Na-Feldspar in the transparent roof tile glaze composition was investigated. The experiments were designed as a mixture experiment. According to heating microscope results, pumice was observed deforming at 850 °C and flowing at 1270 °C. Considering the properties of the pumice it is possible to use it in raw glaze (without using frit) applications instead of feldspar. Colour, brightness and hardness values of fired samples were measured. These data were analysed with MINITAB 13.0 statistical software programme. Microstructure of glaze/tile interfaces of single fired samples was studied by using a scanning electron microscope (SEM). The use of pumice in the glaze composition showed no negative effect on the colour, brightness and hardness values and decreased the flowing temperature of the glaze. It was concluded that pumice could easily replace Na-Feldspar as an alternative fluxing agent in appropriate proportions in such kind of a roof tile glaze composition without causing undesired consequences.     

Preparation of zinc oxide ceramics with a sustainable antibacterial activity under dark conditions

Fabrication of ZnO ceramics with a sustainable antibacterial activity even in the dark has been conducted. Fine ZnO powders were hydrothermally treated in 0.5–3 mol ml^?1 Zn(NO₃)₂ aqueous solutions at 110–180 °C for 3–20 h. After an uniaxial pressing of the ZnO powders thus prepared, they were sintered at 400–600 °C for 1 h in air. Sustainability in antibacterial activity was evaluated using a colony count method with Escherichia coli bacteria on nutrient agar medium (36 °C/24 h) in a Na–P-buffer solution. The best data was attained for the ZnO ceramics prepared from the following conditions: a 3 mol ml^?1 zinc nitrate solution for the hydrothermal treatment at 120 °C for 7 h and sintering in air with a step-by-step pattern (470 °C/1 h–485 °C/1 h–500 °C/1 h). ESR and chemical photoluminescence analyses have cleared that radical oxygen of super-oxide (O₂^?) originated from the surface of ZnO might exhibit an antibacterial activity even under the dark condition.     

Ce0.9Gd0.1O1.95 supported La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes for solid oxide fuel cells

Lanthanum-based iron- and cobalt-containing perovskite has a high potential as a cathode material because of its high electro-catalytic activity at a relatively low operating temperature in solid oxide fuel cells (SOFCs) (600–800). To enhance the electro-catalytic reduction of oxidants on La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF), Ga doped ceria (Ce_0.9Gd_0.1O_1.95, GDC) supported LSCF (15LSCF/GDC) is successfully fabricated using an impregnation method with a ratio of 15 wt% LSCF and 85 wt% GDC. The cathodic polarization resistances of 15LSCF/GDC are 0.015 Ω cm², 0.03 Ω cm², 0.11 Ω cm², and 0.37 Ω cm² at 800 °C, 750 °C, 700 °C, and 650 °C, respectively. The simply mixed composite cathode with LSCF and GDC of the same compositions shows 0.05 Ω cm², 0.2 Ω cm², 0.56 Ω cm², and 1.20 Ω cm² at 800 °C, 750 °C, 700 °C, and 650 °C, respectively. The fuel cell performance of the SOFC with 15LSCF/GDC shows maximum power densities of 1.45 W cm^?2, 1.2 W cm^?2, and 0.8 W cm^?2 at 780 °C, 730 °C, and 680 °C, respectively. GDC supported LSCF (15LSCF/GDC) shows a higher fuel cell performance with small compositions of LSCF due to the extension of triple phase boundaries and effective building of an electronic path.