Production of opaque frits with low ZrO<Subscript>2</Subscript> and ZnO contents and their industrial uses for fast single-fired wall tile glazes

The amounts of zirconium and zinc oxides, which raise the production costs of ceramic glazes, were decreased in fast single-fired wall tile frit compositions and an industrial frit production was conducted. Opacity of the fired frit-based glazes was accomplished by compositional modifications of frits with no other nucleating agent. It was determined that the ratios of Al₂O₃/ΣR₂O, Al₂O₃/ΣRO, and Al₂O₃/B₂O₃ have significant effects on decreasing ZrO₂ and ZnO levels in the frit composition. A reduction of 25% in both zirconia and zinc oxide contents of frit batches, with respect to the reference frit (R) containing 6–10% ZrO₂ and 6–10% ZnO for a glossy white opaque wall tile glaze, was achieved in the ZD glaze consisting of 4.5–7.5% zirconia and 4.5–7.5% ZnO in its frit composition. It was concluded that zircon was the main crystalline phase of the glaze contributing the opacity. The ZD frit-based glaze has a thermal expansion coefficient value of 61.13 ± 0.32 × 10⁻⁷ °C⁻¹ at 400 °C which well matches to that of the wall tile body. TS EN ISO 10545 standard tests were also applied to the final ZD glaze. It is confirmed that the production cost of a fast single-fired wall tile glaze can be decreased by 15–20% with the successful new frit developed.     

Ceramic wastes as alternative raw materials for Portland cement clinker production

The cement industry has for some time been seeking procedures that would effectively reduce the high energy and environmental costs of cement manufacture. One such procedure is the use of alternative materials as partial replacements for fuel, raw materials or even clinker. The present study explores the reactivity and burnability of cement raw mixes containing fired red or white ceramic wall tile wastes and combinations of the two as alternative raw materials.The results showed that the new raw mixes containing this kind of waste to be technically viable, and to have higher reactivity and burnability than a conventional mix, providing that the particle size of the waste used is lower than 90 μm. The mineralogical composition and distribution in the experimental clinker prepared were comparable to the properties of the clinker manufactured with conventional raw materials. Due to the presence of oxides such as ZnO, ZrO₂ and B₂O₃ in tile glazing, the content of these oxides was higher in clinker made with such waste. The mix of red and white ceramic wall tile waste was found to perform equally or better than each type of waste separately, a promising indication that separation of the two would be unnecessary for the purpose described above.     

Production of stabilized and non-stabilized ZrO2 by carbothermic reduction of ZrSiO4

The kinetics of reduction of zircon by carbon have been investigated in the temperature range 1400–1650 °C. Volatilization of SiO permits production of zirconia of about 96% purity direct from zircon with 100% removal of the silicon content being achieved in less than 2 h at 1650 °C. Addition of MgO, Y₂O₃ or CaO stabilizers to zircon prior to the reduction permits direct production of partially stabilized zirconia from zircon by means of carbothermic reduction. The presence of such stabilizers has been found to produce a slight increase in the reduction of kinetics of zircon.     

Microwave induced plasma processing of nuclear waste calcines

Microwave induced plasma processing was used to sinter synthetic Idaho Chemical Processing Plant (ICPP) alumina and zirconia based high level nuclear waste calcines in a nitrogen atmosphere. The microwave densification behaviour of these nuclear waste calcines was observed parallel with identification of the phases formed after sintering. Sintered densities of > 3.20 g cm^–3 were obtained within 10 min of microwave sintering of pure calcines. Glass frit containing calcines showed lower sintering densities (< 2.0 g cm^–3) due to reactions between the frit and volatile substances in both zirconia based and alumina based calcines; prior removal of frit volatiles increased the sintered density. Phases formed in the microwave sintered calcines were identified by X-ray diffraction.     

Fractographic study of the alumina and zirconia particles embedded in mullite prepared by reaction sintering

Four different mullite-alumina-zirconia composites have been prepared by reaction sintering between alumina and zircon powders using magnesia or spinel (MgAl₂O₄) to increase the sintering and reaction rates. The microstructure of these materials can be described as composed of two parts: the first one is the mullite matrix containing various kinds of zirconia and alumina particles, whereas the second part is an amorphous phase in which alumina submatrices, zirconia and spinel particles are embedded. Examination of fracture surfaces allows one to identify the crack paths and shows that the main differences are related to zirconia inclusions. Analysis of mechanical properties and fracture features leads to the conclusion that crack deflection and microcracking are operative toughening mechanisms for the various materials. Moreover, a crack bowing mechanism is proposed to explain the higher modulus of rupture found for the series of materials prepared with magnesia as a reaction sintering aid. On leave from Instituto de Ceramica Y Vidrio, CSIC, Arganda del Rey, Madrid, Spain.     

Densification of zirconia-containing sialon composites by Sm2O3

Sm₂O₃ was used as an additive for ZrO₂/O-sialon composites and showed remarkably good densification behaviour on sintering. Sm₂O₃ appeared to behave as a densification rather than a stabilization additive and the low eutectic temperature in this system facilitated densification before zircon dissociation. An important advantage is that the final sintering temperature is then determined by the minimum temperature at which full conversion from -Si₃N₄ to O-sialon can be achieved. At this temperature (1500–1550 °C), grain growth of zirconia is minimal, and this allows control of small grain size for the zirconia particles.     

Effects of nucleating agents on diopside crystallization in new glass-ceramics for tile-glaze application

The effect of crystallization produced by addition of TiO₂, ZrO₂ and P₂O₅ oxides to glass-ceramic of the system CaO-MgO-SiO₂ was studied using structural and thermal techniques. The devitrification process was independent of thermal treatment. X-ray diffraction studies performed on the glass-ceramic system indicated that diopside crystalline phase was more thermodynamically favourable than other phases. The effect of the nucleating agent depends on its nature: TiO₂ decreased the activation energy while P₂O₅ and ZrO₂ did not. SEM analysis of the doped glass-ceramics showed randomly distributed crystals with significant dimensional variations from those of the undoped system. All these formulations, showing a high crystallization rate, and a fast heating rate, can be used as tile glazes and/or tile-glaze components.     

Effect of controlled crystallization on the chemical durability of a lead-containing waste glass

A favourable microstructure was tailored by controlled crystallization of a quenched glass made of a combination of metal industry slags and a lead bisilicate frit. The frit was used to simulate a lead-containing waste. In the devitrified glass microstructure, lead-rich glassy pockets were contained in an akermanite (Ca₂MgSi₂O₇) crystalline matrix. Leaching of lead was reduced by a factor of two as a result of devitrification processing; the low-lead crystalline matrix protected the lead-rich pockets from chemical attack.     

Control of the tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia in hot water

The tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia caused by annealing in hot water was investigated in the temperature range 80 to 200° C using sintered bodies in zirconia containing 2, 3 and 4 mol % Y₂O₃. Three approaches, alloying ZrO₂(Y₂O₃) with 0 to 20wt% CeO₂, dispersing 0 to 40 wt % Al₂O₃ into ZrO₂(Y₂O₃) ceramics and decreasing the grain size of zirconia, were examined to inhibit the tetragonal-to-monoclinic phase transformation. The amount of monoclinic phase formed decreased with increasing concentrations of CeO₂ alloyed and Al₂O₃ dispersed, and with decreasing grain size of zirconia.     

An X-ray diffraction and NMR study into the mechanism of zircon formation from aqueous sols

Solid solutions of the formula Zr₁–xSi _x O₂ (x=0–0.5) were produced by an aqueous inorganic sol-gel route. X-ray diffraction and solid state magic-angle spinning-nuclear magnetic resonance (MAS-NMR) were used to characterize the products at various calcination temperatures. The solubility limit of silica in zirconia at <950°C was found to be x=0.15 and that this amount stabilized the tetragonal zirconia phase to higher temperatures and inhibited the formation of zircon. Withx=0.5, zircon formation was 70% at only 1100°C.     

A structural study of some SnO2-Sb2O5 semiconducting glazes

The use of resistive glazes on high tension insulators to control flashover caused by pollution has proved only partially successful as a result of deterioration of the glazes in service. The most promising and now most widely used glaze relies upon the semiconducting properties of SnO₂ doped with Sb₂O₅, incorporated in an alumino-silicate base glaze. The structure of glazes containing various amounts of SnO₂ has been studied by a variety of techniques and particularly by scanning microscopy and electron probe microanalysis. These techniques have revealed both the existence and composition of relatively conducting and relatively insulating areas in the glaze, and a detailed analysis of the distribution of all the elements across the glaze surface and through the glaze thickness has been carried out. The results of this structural survey suggest that activated conduction proceeds through overlapping solubility rims around SnO₂ particles in the glaze.     

Synthesis of iron zircon coral by coprecipitation routes

An iron-doped (15 mol-% of Fe₂O₃) zircon ceramic pigment has been prepared using binary (ZrO₂-Fe₂O₃ and SiO₂-Fe₂O₃) and ternary (ZrO₂-Fe₂O₃-SiO₂) colloidal gels or coprecipitates as precursors. The obtained raw powders, precalcines, and fired pigments have been characterized by XRD, thermal analysis (TGA/DTA) and SEM/EDX to analyze the effect of binary interactions (occlusion and adsorption phenomena) on the synthesis of the iron-zircon coral as well as on the coloring yield. The use of a binary raw coprecipitate as precursor prepared with colloidal silica and ferrous sulphate leads to a higher efficiency in the hematite occlusion, since a more intense coral hue (L ^* = 59.6, a ^* = 29.3 and b ^* = 25.8 at 950°C) is obtained, similar to an optimal ceramic reference. The protection or occlusion of -Fe₂O₃ in amorphous silica agglomerates of high specific surface appears to be a more effective reaction intermediate than the observed adsorption of micronic -Fe₂O₃ particles on tetragonal zirconia monoliths. The necessary transformation of tetragonal zirconia into its monoclinic form prior to zircon formation, and the higher -Fe₂O₃ segregation from the coprecipitate obtained with Zr and Fe precursors, seem to lower the efficiency of the hematite coarsening-occlusion process involved in the coral pigment formation.     

Microstructure and mechanical properties of zirconia-toughened lithium disilicate glass–ceramic composites

The lithium disilicate glass–ceramics composites reinforced and toughened by tetragonal zirconia (3Y-TZP) were prepared by hot-pressing at 800 °C with varying zirconia content from 0 to 30 wt.%. In the case of the composites of small zirconia content (below 10 wt.%), zirconia acted as nucleation agent primarily, and the microstructure was refined continuously. The morphology of Li₂Si₂O₅ crystals transformed from rod-shaped to spherical structure, and the mechanical properties decreased inevitably. For the composites of large zirconia content (from 15 wt.% to 30 wt.%), however, zirconia restrained the phase separation of glass. The morphology of Li₂Si₂O₅ crystals transformed to rod-shaped structure again. The mechanical properties of the composite at zirconia content of 15 wt.% increased up to 340 MPa and 3.5 MPa m^1/2 which were much higher than those of zirconia-free glass–ceramics. The improved properties were attributed mainly to compressive stress reinforcement, phase transformation and bridging toughening mechanisms.     

Preparation and characterization of precursor powders for yttria-doped tetragonal zirconia polycrystals (Y-TZP) and Y-TZP-Al2O3 composites

Precursor powders for the preparation of tetragonal 2.5 mol% Y₂O₃-ZrO₂ containing 0 to 30 wt% Al₂O₃ were made by coprecipitation. The behaviour of this powder during calcination from room temperature to 1200° C was studied using differential thermal analysis. X-ray diffraction and transmission electron microscopy methods, and measurements of surface area. The uncalcined powder was essentially amorphous. On heating alumina-free powder, zirconia crystallized at 485° C: for increasing alumina content, zirconia crystallized from an amorphous aluminous matrix at increasing temperatures (850° C for 20 wt% Al₂O₃), while the crystallite size decreased and the surface area of the powder increased. The zirconia first crystallized as cubic, but transformed to the tetragonal form near 1100° C. The alumina crystallized as corundum at 1200° C. No monoclinic zirconia could be detected when calcined aluminous material was cooled to room temperature. The sintering behaviour of the calcined powder is discussed.     

Characterization and densification of lanthana-zirconia powders prepared by high temperature hydrolysis

Zirconia-lanthana powders containing 4.5, 7, 10, 15 and 20 mol % La₂O₃ were prepared by hydrolysis. The hydrolysis process was carried out in a laboratory stainless steel autoclave for their equivalent hydroxides for 2 h at 200‡ C. The powders were investigated using X-ray diffraction, infrared spectrometry, and transmission electron microscopy techniques. No other phases except the cubic phase zirconia of fluorite-type structure were detected. The prepared materials were examined for their thermal stability and phase constitution, by X-ray and infrared analyses, on heating up to 1400‡ C. The cubic phase zirconia remains stable up to 1000‡ C at which it starts to decompose yielding monoclinic zirconia and lanthanum zirconate. At 1200‡ C, the cubic phase nearly disappears in the sample containing 4.5 mol % La₂O₃. Increasing La₂O₃ content up to 20 mol % retards its destabilization, reduces the yielded monoclinic phase, and in the same time increases the formed lanthanum zirconate phase. At 1500‡ C only monoclinic ZrO₂ and La₂Zr₂O₇ are present. The La₂Zr₂O₇/monoclinic ZrO₂ ratio increases with increasing La₂O₃ content. Pressed specimens of the prepared materials were fired for 2 h at 800 to 1400‡ C. The sintering activity of the prepared powders resulted in a 92% theoretical density body for the 4.5 mol % ZrO₂ material. The densification properties in relation to changes in the phase constitution are discussed.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> stabilizer content and annealing on the structural transformations of ZrO<Subscript>2</Subscript>

The structure of partially stabilized zirconia crystals has been studied by transmission electron microscopy before and after annealing. Structural characterization of Y₂O₃-doped (2.8 to 4 mol %) zirconia before annealing showed that all of the samples consisted of twin domains whose size was dependent on the stabilizer content. Annealing at 2100°C increased the domain size in the composition range 2.8–3.7 mol % Y₂O₃ and reduced it at 4 mol % Y₂O₃. These structural changes allowed us to determine the position of the representative point relative to the phase boundary in the equilibrium phase diagram of the system.     

Nanocrystalline zirconia-yttria system–a Raman study

Nano sized zirconia (ZrO₂) powders doped with different amount of yttria (Y₂O₃) (3, 5 and 8 mol%) were prepared through coprecipitation method. The crystallite size estimated from the X-ray peak broadening is around 10 nm. Phase identification was carried out using XRD and Raman spectroscopy. Raman spectroscopic study of the synthesized materials show clear evidence of the presence of single phase cubic structure in the case of 8 mol% Y₂O₃ doped fully stabilized zirconia (8Y-FSZ); tetragonal phase in the case of zirconia doped with 3 mol% Y₂O₃ (3Y-TZP-tetragonal zirconia polycrystal) and a mixture of cubic and tetragonal phases for 5 mol% Y₂O₃ doped partially stabilized zirconia (5Y-PSZ). Raman technique is therefore an effective tool to distinguish the phases present in the calcined nano sized powders of zirconia.     

Glass-ceramic bonding in alumina/CBN abrasive systems

A glass-ceramic bond which can be applied in alumina/CBN abrasive systems was developed by the method of liquid-phase sintering of a homogenized mixture of alumina abrasives and bonding medium containing various amounts of B₂O₃. Microstructural and mechanical examinations have shown that the amount of B₂O₃ present in the starting glass frit determines the ultimate properties of the glass-ceramic bond which can be successfully used in the Al₂O₂/CBN abrasive systems.     

Sintering and crystallisation of a P2O5-added Li2O-ZrO2-SiO2 glass powder system

Sintering and crystallisation of a 11.5 wt % Li₂O, 22.8 wt % ZrO₂, 65.7 wt % SiO₂ glass powder with P₂O₅ added were investigated. By means of thermal shrinkage measurements, sintering was found to start at about 650°C and was completed in a very short temperature interval (T 100°C) in less than 30 min. Crystallisation took place just after completion of densification and was almost completed at about 900°C in 20 min. Secondary porosity prevailed over the primary porosity during the crystallisation stage. The glass powder compacts first crystallised into lithium metasilicate (Li₂SiO₃) and/or zircon (ZrSiO₄) and tridymite (SiO₂) which transformed and/or grew into lithium disilicate (Li₂Si₂O₅), zircon and tridymite after the crystallisation process was essentially complete, so that, a crystallinity degree between 52.4 ± 2.0 and 68.5 ± 3.2 wt % was obtained. P₂O₅ doping little affected the densification. However, adding P₂O₅ remarkably enhanced the zircon and tridymite crystallisation while delaying the Li₂SiO₃ to Li₂Si₂O₅ transformation. The microstructure is characterised by fine crystals uniformly distributed arbitrarily oriented throughout the residual glass phase.     

Low temperature phase equilibria and ordering in the ZrO2-rich region of the system ZrO2-CaO

From co-precipitated powder samples, the solid state reactions occurring between room temperature and 1500° C in the ZrO₂-CaO system have been studied. At low temperatures, compositions containing < 25 mol% CaO show a complex picture of phase transformation and ordering in the system. From the obtained results the following singular reactions have been established. (i) Tetragonal zirconia solid solution decomposes eutectoidally at 7 mol% CaO and 1048 ± 4° C into monoclinic zirconia solid solution and calcium zirconate (CZ). (ii) Cubic zirconia solid solution undergoes a eutectoidal decomposition at 17.5 mol% Cao and 1080 ±20° C into tetragonal solid solution + calcium zirconate. (iii) The monoclinic ordered phase, CaZr₄O₉ (₁), ), undergoes an order-disorder transformation into cubic zirconia solid solution at 1232 ± 5° C. (iv) Cubic zirconia solid solution undergoes a eutectoidal decomposition into two ordered phases, ₁ + ₂ at 21 mol% CaO and 1200 ± 10°C. (v) Hexagonal ordered phase Ca₆Zr₁₉O₄₄ (₂) decomposes peritectoidally into cubic zirconia solid solution + calcium zirconate at 1360 ± 10° C. The two ordered phases ₁ and ₂ seem to be unstable below 1100° C. By using DTA, X-ray diffraction and SEM techniques, the extent of the tetragonal and cubic zirconia solid solution fields have been established. From the above experimental results a new tentative phase diagram is given for the ZrO₂-rich region of the system, ZrO₂-CaO.