Effect of titania on fired characteristics of triaxial porcelain

Titania was progressively added in the range 3–9 wt% into a triaxial porcelain body consisting of clay, quartz and feldspar. The composed bodies were heated at five different temperatures in the range 1200-1400°C and their fired properties as well as phases evolved were studied. The results revealed that beyond 1300°C, formation of more liquid phases caused bloating in samples which led to generation of pores. This effect is more pronounced in TiO₂ containing samples. In the present system, 1300°C appeared to be the optimum temperature at which porosity was almost negligible and strength was maximum (45 MPa), particularly in presence of TiO₂. From the results of XRD studies, it was revealed that quartz content primarily decreased with increase in TiO₂ content due to excess glass formation and its subsequent dissolution. Mullite content increased with increase in TiO₂ content. No significant effect was observed beyond 6 wt% addition. Microstructure primarily showed the presence of quartz grain and cluster of smaller sized primary mullite crystals in both the samples without and with TiO₂. Very few secondary mullite crystals were also observed. SEM picture of sample containing 9 wt% TiO₂ showed some grain boundary crack due to cooling stress generated in the glassy phase. The drastic reduction of residual strength after 8 cycles of heating at 800°C and cooling particularly in TiO₂ containing samples suggests controlled heat treatment of the vitrified samples necessary to promote secondary crystallization process for the enhancement of strength. Attempts have also been made to correlate the constitutional parameters with the properties.     

Effect of bauxite addition on densification and mullitization behaviour of West Bengal clay

The effect of bauxite addition on the densification and mullitization of reaction sintered bauxite-clay mixture had been studied in the temperature range 1400–1500°C. The maximum bulk density (2·89 g/cc) and minimum apparent porosity (0·58%) was achieved by addition of 50 wt% bauxite. The impurities present in bauxite and clay formed liquid phase which helped in particle diffusion to aid densification. The X-ray diffraction of sample fired at 1500°C showed cristoballite phase gradually disappearing and at the same time mullite and α-Al₂O₃ phase appearing at a higher level of bauxite addition. The in situ nascent alumina formed was reactive that facilitated the formation of secondary mullite by solution precipitation mechanism. The presence of bauxite also changed the morphology of the mullite particles. Two types of mullite were distinctly observed in the SEM photographs: elongated primary mullite and equiaxed secondary mullite.     

Effect of starting particle size on hot-pressing of magnesium oxide powder prepared by vapour-phase oxidation process

Effect of starting particle size on hot-pressing of magnesium oxide (MgO) powder was examined using seven kinds of MgO powders prepared by a vapour-phase oxidation process; the average primary particle sizes were 11, 25, 32, 44, 57, 107 and 261 nm. These compressed powders (compacts) were hot-pressed at a temperature between 900 and 1300°C. The densifications of these compacts during the hot-pressing proceeded via (i) the sintering of primary particles within secondary particles and the rearrangement of secondary particles/grains (900°C), (ii) the gradual grain growth controlled by the pore migration (900∼1100°C) and (iii) the rapid grain growth due to the active mass transfer (1300°C); the grain sizes of MgO compacts hot-pressed at and below 1100°C were <1 μm, while those at 1300°C attained 20∼30 μm. The transluscent compact with the relative density of 99.7% could be obtained when the compressed powder with the average primary particle size of 44 nm was hot-pressed at a temperature as low as 1100°C for 1 h. This revised version was published online in November 2006 with corrections to the Cover Date.     

Urea effect on the mechanism of mullite crystallization

Mullite is an excellent structural material due to its physical and mechanical properties. In this study, mullite was obtained by the sol–gel process, using silicic acid, aluminum nitrate, and urea. The urea effect was studied by evaluating samples obtained from urea/Al³⁺ ratio equal to 0, 1, and 3. The kinetic study was conducted using the isoconversional, non-isothermal, Flynn–Wall–Ozawa method. The sample prepared without urea, which is the least homogeneous one, formed spinel and α-alumina at 1150 °C, and Al-poor mullite together with α-alumina, at 1200 °C. The Al-poor mullite crystallization process from this sample showed the lowest yield. The sample prepared with urea/Al³⁺ ratio equal to 1, which has an intermediate behavior, formed spinel at 1100 °C, Al-poor mullite at 1150 °C, and α-alumina together with Al-poor mullite at 1250 °C. However, the sample prepared with urea/Al³⁺ ratio equal to 3, the most homogeneous, formed spinel and Al-rich mullite at 1100 °C. This sample formed Al-poor mullite at 1200 °C with the highest yield. Moreover, the sample synthesized without urea showed a higher porosity and a greater amount of hexacoordinated aluminum at 350 °C. All samples showed the same kinetic model, Šesták and Berggren (SB) for Al-poor mullite crystallization. The samples synthesized with urea crystallized mullite through the same kinetic parameters and constant values of the activation energy, but the sample prepared without urea followed different kinetic parameters and values of activation energy which changed over the course of the crystallization.     

Spark plasma reaction sintering of ZrO2–mullite composites from plasma spheroidized zircon/alumina powders

Mullite–zirconia ceramic composites are prepared by reaction sintering of plasma spheroidized (PS) zircon–alumina powders in a spark plasma sintering (SPS) system at 1000, 1100, 1200 and 1300 °C with duration of 10 and 30 min. At SPS temperature of 1000 °C, evidence of zircon decomposition is detected, while at 1200 °C, mullite formation dominates the process, resulting in significant increases in microhardness, Young's modulus and fracture toughness values. At SPS temperature of 1300 °C, due to re-crystallization, rapid grain growth, and intergranular micro cracking, there is a slight decrease of microhardness and Young's modulus values. Yet, fracture toughness as high as 11.2±1.1 MPa m^1/2 is obtained by the indentation technique. The results indicate that with optimized sintering parameters, a combination of PS and SPS is effective in preparing high performance mullite/ZrO₂ composites from zircon/alumina mixtures at a relatively low reaction sintering temperature.     

Preparation and characterization of ceramic porous sheet composed of platelet (Cr, Al)2O3 crystals

A compact powder mixture consisting of 3Al2O3·2SiO2 (mullite) and Cr2O3, which was heated in a carbon powder bed, was cut into a 1 mm thickness sheet. The ceramic porous sheet composed of platelet (Cr, Al)2O3 crystals was prepared by leaching SiO2 contained in the sheet by hydrofluoric acid. The porous sheet produced from the 50 mass % mullite/50 mass % Cr2O3 compact, heated at 1500°C for 2 h, was composed of platelet crystals with 30 μm average length and 4.6 μm average thickness. Its relative density and specific surface area were 73 % and 4.4 m2 g-1 respectively. This revised version was published online in November 2006 with corrections to the Cover Date.     

Paper pulp waste— a new source of raw material for the synthesis of a porous ceramic composite

A synthetic porous ceramic composite material consisting of the mullite, cordierite and cristobalite phases is produced from a mixture of paper pulp waste and clay by reaction sintering at 1400°C. Physicomechanical properties such as bulk density, porosity, cold crushing strength and cold modulus of rupture have been studied. The presence of mullite, cordierite, cristobalite and quartz as major phases and montellecite, tatanite, forsterite and anorthite as minor phases have been confirmed by X-ray diffraction pattern. SEM studies revealed the presence of well developed needle shaped mullite and quartz crystals. The paper also discusses the possible uses of this type of porous composite material.     

Preparation of mullite-based iron magnetic nanocomposite powders by reduction of solid solution

In this article, the preparation of mullite-based iron magnetic nanocomposite powders by hydrogen reduction of Fe-doped mullite solid solution with a nominal composition of Al_5.4Fe_0.6Si₂O₁₃ is reported. The formation process of Al_5.4Fe_0.6Si₂O₁₃ solid solution was analyzed using X-ray diffraction analysis (XRD), Fourier Transform Infrared Spectrum (FT-IR), thermogravimetric, and differential thermal analysis (TG-DTA). It is found that doping with Fe³⁺ cation affects the crystallization temperature of mullite. During the hydrogen reduction process, more than 89% Fe³⁺ cation in solid solution were transformed into α-Fe phase when reduction temperature reached 1200 °C. Microstructure characterization of nanocomposite powders reduced at 1300 °C reveals that there are two types of α-Fe particles in mullite matrix. Fe nanoparticles with a size of approximately 10 nm were precipitated within the mullite grains, while Fe particles larger than hundreds of nanometers were located at the surfaces of the mullite grains. The measurement of the magnetic properties of nanocomposite powders indicates that large particles and nanoparticles of α-iron have the ferromagnetic and superparamagnetic behavior at room temperature, respectively.     

Effect of microstructure on the high temperature strength of nitride bonded silicon carbide composite

Four compositions of nitride bonded SiC were fabricated with varying particle size of SiC of ∼ 9.67, ∼ 13.79, ∼ 60 μ and their mixture with Si of ∼ 4.83 μ particle size. The green density and hence the open porosity of the shapes were varied between 1.83 to 2.09 g/cc and 33.3 to 26.8 vol.%, respectively. The effect of these parameters on room temperature and high temperature strength of the composite up to 1300°C in ambient condition were studied. The high temperature flexural strength of the composite of all compositions increased at 1200 and 1300°C because of oxidation of Si₃N₄ phase and blunting crack front. Formation of Si₃N₄ whisker was also observed. The strength of the mixture composition was maximum.     

Measurement of linear thermal expansion coefficient of alkali-activated aluminosilicate composites up to 1000 °C

The effect of high temperatures up to 1000 °C on the length changes of two alkali-activated aluminosilicate composites, one of them with quartz sand aggregates, the second with electrical porcelain, is analyzed in the paper. The thermal strain vs. temperature functions of both materials are found to increase monotonically in the whole temperature range studied so that the thermal expansion mismatch (the gel undergoes thermal shrinkage, the aggregates expand with increasing temperature) results in positive values of the apparent linear thermal expansion coefficient. The composite material with electrical porcelain aggregates exhibits a more desired thermomechanical behavior which is a consequence of the better high-temperature thermal stability of electrical porcelain as compared to quartz. In a comparison with Portland-cement based composites, the linear thermal expansion coefficient of both studied aluminosilicates is substantially lower in the whole temperature range of 20–1000 °C.     

Reaction between ∼0.7- to 1-μm α-Al2O3 particles and SiO2 during high-temperature sintering

Al₂O₃:Cr³⁺ and Al₂O₃:Ti³⁺ particles with an average size of ∼1 and 0.7 μm, respectively, have been prepared through mechanical grinding of bulk crystals. Using purpose-designed accessories made from chromium and titanium, we were able to prevent Fe contamination. We studied the reaction between α-Al₂O₃ and SiO₂ particles during high-temperature sintering. The results demonstrate that, in this system at temperatures above 1300°C, corundum particles dissolve in silica to form mullite. The reaction temperature and rate depend on the particle size composition of the Al₂O₃. The heating rate is shown to influence the dissolution rate of corundum particles. Increasing the heating rate from 7 to 15°C/min shifts the dissolution range of alumina particles from 1500 to 1630°C.     

Phase formation and texture development in mullite/zirconia composites fabricated by templated grain growth

Mullite is a promising candidate for advanced ceramic applications but its low fracture toughness and difficulties in sintering are the main limitations for more widespread industrial applications. Therefore, mullite/zirconia composites were prepared from a reactive mixture of alumina and zircon powders. Additives, TiO₂ and MgO, were used to modify aluminosilicate glass to increase densification and <001> aluminum borate templates were incorporated to texture mullite in [001] by templated grain growth. Mullite/zirconia phase formation was complete at 1450°C in the presence of both templates and additives, as compared to 1500°C for the samples with only additives and to 1600°C for the samples with only templates. Dense mullite/zirconia composites with highly <001>-textured mullite grains (Lotgering factor ∼1) and a retention of ∼13% tetragonal ZrO₂ were fabricated after sintering at 1450°C for 2 h. A high quality of mullite texture with a degree of orientation parameter of 0.22 and a narrow distribution of elongated mullite grains within 8.8° around [001] were successfully obtained in the composites.     

The preparation and characterization of SiC-AlN-Fe2Si composites with high porosity from allophane

The reaction products of an allophane heated with carbon at 850–1600 °C in the stream of nitrogen for a given time were characterized by X-ray diffractometry. As a result, it was found that cristobalite and mullite were stable phases at 850–1300 °C, β-Si₃N₄ and α-Al₂O₃ at 1300–1500 °C, and SiC-AlN-Fe₂Si at temperatures higher than 1500 °C. SiC-AlN-Fe₂Si composites with high porosity of about 50% were easily prepared by a heat treatment at a temperature higher than 1500 °C with carbon in a stream of nitrogen. The formation mechanism of the composites is kinetically discussed from a viewpoint of small-pore shrinkage and large-pore expansion by volume diffusion during heating. The resultant microstructure of the composites obtained is also discussed.     

Hot-pressed hydroxylapatite/monoclinic zirconia composites with improved mechanical properties

Composites of hydroxylapatite (HA) and monoclinic zirconia were hot-pressed at 1100 °C and 1200 °C under vacuum to study the phase transformations and the mechanical properties. X-ray diffraction results showed a higher phase transformation from monoclinic-ZrO₂ to tetragonal-ZrO₂ when the sintering temperature increased from 1100 °C to 1200 °C. HA decomposed faster when the amount of ZrO₂ in the composites increased. Moreover, small amount of α-TCP and CaZrO₃ was observed in the composites hot-pressed at 1200 °C. Hot-pressing at 1100 °C resulted in better mechanical properties than the hot-pressing at 1200 °C because of less reaction between HA and zirconia at 1100 °C. 40 wt% monoclinic zirconia and HA composite hot-pressed at 1100 °C resulted in promising mechanical properties which are 6.5 GPa of Vickers μ-hardness, 2.23 MPa√m of fracture toughness, and 66 MPa of diametral strength.     

Effect of size of fly ash particle on enhancement of mullite content and glass formation

Quartz is widely replaced by fly ash in traditional porcelain composite. Increased strength and stability of the fly ash-mixed composite depends on the quantity and crystallinity of the mullite phase in the fly ash. Our aim in this investigation is to increase the formation of mullite in nanocrystalline form and study the effect of temperature. Quantitative estimation of mullite and residual quartz content were done by X-ray diffraction (XRD) and nanostructure and crystallization were studied using differential thermal analysis (DTA), field effect scanning electron microscopy (FESEM), XRD and Fourier transform infrared (FTIR) spectroscopy. The results show that fly ash sieved through 250 holes/cm² mesh contain more mullite initially and growth of mullite as well as glass formation was faster in this sample compared to coarse fly ash. The maximum mullite in these samples was formed at 1600°C. Transformation of quartz and cristobalite phases into glassy phase was also faster for smaller particle sizes of fly ash.     

Templated grain growth of textured mullite/zirconia composites

Mullite is an attractive material for advanced ceramic applications, but its low fracture toughness prevents it from widespread industrial applications. Therefore, mullite/zirconia composites were prepared from a reactive mixture of alumina and zircon with additives of TiO₂ and MgO to increase mechanical properties and densification. <001> aluminum borate templates were used to nucleate, and texture mullite in [001]. Mullite/zirconia formation started at 1350 °C and was complete at 1450 °C. Dense mullite/zirconia composites with highly textured mullite were produced after sintering at 1450 °C. A relatively constant tetragonal ZrO₂ content of 11±2 wt.% was retained at room temperature after sintering between 1350 and 1550 °C. A high quality of texture with an orientation parameter of 0.22 and a very narrow distribution of elongated mullite grains within 8.8° around [001] were successfully produced.     

Effect of chemical composition and alumina content on structure and properties of ceramic insulators

In the present work, six electrical porcelain compositions with different amount of alumina and silica have been prepared and fired in an industrial furnace at 1300 °C. Density, porosity, bending strength and electrical strength were measured in the samples. In order to find a relationship between properties and sample microstructures, samples were analyzed by scanning electron microscope (SEM) and x-ray diffraction (XRD) techniques. The results showed that, with chemical composition of 53·5 wt.% SiO₂ and 37·5 wt.% alumina, highest electrical strength of 21·97 kV/mm was achieved in fabricated electrical porcelains. Increasing amount of alumina up to 30 wt.% decreases quartz and cristobalite phases, but increases corundum phase 3 to 5 times. SEM observation revealed that dense particles and uniform distribution of long and thin needle shaped mullite are predominant in sample microstructures with highest electrical strength.     

Preparation of mullite bonded porous SiC ceramics by an infiltration method

A powder compact of α-SiC and α-Al₂O₃ was infiltrated with a liquid precursor of SiO₂, which on subsequent heat treatment at 1500 °C produced a mullite bonded porous SiC ceramics. Results showed that infiltration rate could be estimated by using weight gain measurements and theoretical analysis. The bond phase was composed of needle-shaped mullite which was observed to be grown from a siliceous melt formed during the process of oxide bonding. The porous SiC ceramics exhibited a density and porosity of 2 g cm⁻³ and 30 vol%, respectively, and also a pore size distribution in a range of 2–15 μm with an average pore size of 5 μm. No appreciable degradation of room temperature flexural strength (51 MPa) was observed at high temperatures (1100 °C).     

Crystal growth of tetragonal ZrO2 in the glass system ZrO2-SiO2 prepared by the sol-gel process from metal alkoxides

Glasses in the system ZrO₂-SiO₂ containing 40 to 60 mol % ZrO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by heat treatment at 800 and 1200° C, and its crystal growth was measured by differential thermal and X-ray diffraction analyses. At 800 to 900° C, tetragonal ZrO₂ crystals grew three-dimensionally and the activation energy for growth was calculated as about 680 kJ mol⁻¹. On the other hand, the secondary growth of tetragonal ZrO₂ at 1000 to 1200° C followed the cube-root-of-time law. The activation energy for secondary growth was about 380 kJ mol⁻¹. It is suggested that the diffusion of Zr⁴⁺ ions is the rate-limiting process for the secondary crystal growth of tetragonal ZrO₂.     

Effects of aerosol heating rate on the properties of aggregates of lead zirconate titanate nanoparticles produced by spray pyrolysis

The dependence of the shape and size distribution of aggregates of lead zirconate titanate nanoparticles prepared by spray pyrolysis of a sol–gel precursor solution is reported. Decreasing the average heating rate from 300 to 160 °C s⁻¹ in the sub-200 °C section of the reactor decreased the proportion of non-spherical particle aggregates and decreased the maximum size from ~10 to ~5 μm. Microtome sectioning revealed an internal structure composed of <100 nm primary particles. Both solid and hollow particle aggregates were present.