Papers by Keyword: Cristobalite

Abstract: This study focused on the production of bioactive pure cordierite ceramic and the evaluation of its bioactivity by immersing it in simulated bodily fluid (SBF) and coating it on CP-Ti utilizing the dip coating Procedure. Cordierite is created by chemical coprecipitation, which involves combining Cordierite gel with a water-based sol-gel composed of Alumina Al2O3, magnesium oxide MgO, and silicon oxide SiO2. They used a hydraulic press to form cordierite into a cylindrical shape with a pressure of 4 bar and a holding time of 30 minutes at this pressure. Firing presses reach temperatures of 1250 C^o and have a holding time of 2 hours. A Field Emission Scanning Electron Microscope (FESEM) was used to examine the microstructure, which revealed a homogenous distribution of the compounds with negligible porosities. The phase transitions that cur during the sintering Process is studied using X-ray diffraction (XRD). The structural investigation of specimens revealed the formation of the cordierite phase at 1250 C^o, as well as the development of cristobalite, spinel, protoestatite, and corundum with cordierite phase. The dip coating Procedure was utilized to cover commercial pure titanium (CP-Ti) samples with cordierite powder. CP and Tafel exploration tests show that coated CP-Ti has superior passivation than uncoated Substrate. By comparing the findings of uncoated and coated CP-Ti, this paper concludes that the cordierite system acts as a good passivation layer.

Abstract: Boron Oxide (B₂O₃) was doped in fused silica using slip casting technique. Effects of addition of B₂O₃ on mechanical as well as dielectric properties along with microstructure were studied. It was observed that B₂O₃ acts as devitrification inhibitor by reducing the formation of cristobalite content during sintering of fused silica. Mechanical properties were improved by addition of B₂O₃. Density and flexural strength with (3% by weight) addition of B₂O₃ was 1.89 g/cc and 52MPa respectively.

Abstract: Due to a wide range of studies, it has been established that siliceous clays can be an alternative source of raw materials for the production of various types of structural ceramics. Their wide occurrence contributes to their usage in industrial production. Siliceous clays have a specific mineralogical composition and were previously considered only as an additive to increase the plasticity of the main raw material. The chemical composition, properties of green and fired siliceous clays of the Malchevsky deposit has been studied in detail. They consist of 67-70% silica, 13-15% alumina, and 1-3% alkaline earth oxides. The main minerals are opal silica and beidellite. These thermographs confirm the presence of montmorillonite, zeolites, mica and hydromica, opal silica and quartz. It is established that siliceous clays belong to the groups of medium dispersed and highly plastic raw materials. Despite the increased molding moisture and great shrinkage, they are moderately sensitive to drying and allow you to get molding masses with high binding property. At the firing temperature of 1000-1020 ° C, the brick is free from defects, the strength grade is M200-250, and the frost resistance is F75. There are X-ray data of siliceous clays burned at temperatures of 900, 950, 1000, 1050, 1100° C. Significant phase transformations become noticeable at a temperature of 1000° C, when the process of transition of amorphous opal silica to cristobalite begins. At 1050° C, the formation of a glass phase begins. The results obtained suggest that the main phases of the fired material based on siliceous clays are quartz and cristobalite with a low degree of structural perfection and the formation of the microstructure occurs in the interval 1000-1100° C. So, siliceous clays can be considered as the main and additional material for the production of various types of structural ceramics.

Abstract: Fused silica ceramics, named fused silica articles by means of ceramics fabrication processes, had been widely used in metallurgy, refractory, aviation and many other areas. How to get fused silica ceramics with high density and less cristobalite is on the focus. In this paper, effects of sintering temperature (1150°C, 1200°C, 1250°C, 1300°C, 1350°C) on phases, microstructures and properties of fused silica ceramics were investigated. The results showed that the bulk density of samples increased and the apparent porosity of samples minished gradually, with the increase of sintering temperature. However, the bending strength increased to maximum firstly and then decreased because the emergency of cristobalite at 1300°C. When the sintering temperature was 1250°C, the bulk density, apparent porosity and bending strength was 1.71g·cm^-3, 17.6% and 38.87MPa, respectively.

Abstract: In this work, chamotte with dispersions of up to 30wt% zircon is sintered at 1500 °C to investigate the effect of zircon content on the microstructure and physical properties of chamotte refractories. The microstructure and physical properties of samples are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending and dilatometer. The result shows that the main phases of chamotte with and without sintering at 1500 °C are mullite and cristobalite, the addition of zircon has no impact on the main phase compositions of chamotte matrix. For the sintered samples, zircon grains are wrapped with glass and no obvious bonding are detected between the interface of zircon and mullite grains. Increasing zircon content leads to the increase in bulk density and has a negative effect on the flexural strength, besides, linear thermal expansion ratio decreases as the zircon content increases.

Abstract: In this study, cristobalite was used as the addition to prepare silica based ceramic cores, and then the effect of cristobalite content on the mechanical and chemical properties was investigated. The increasing of pre-added cristobalite content resulted in more crystallized cristobalite in the sintered samples and the variation of the linear shrinkage and mechanical properties. When pre-added cristobalite content was 6.0 wt%, the shrinkage of ceramic core was 0.42%, the creep at 1540°C for 30 min was 0.10 mm, and the flexural strength at room temperature and at 1550°C were 25.2 MPa and 28.7 MPa, respectively, which meets the need of ceramic cores of hollow turbine blades.

Abstract: It is found out that activation of the sintering process for refractory clay-containing raw materials (rusk kaolin and wocheinite) is provided using additives of 3d-transition elements oxides (Fe₂O₃ and MnO₂) in the amount of 2 - 5 wt. % due to formation of defective solid solutions of mullite by isovalent substitution mechanism as well as fluxing additives of alkaline and alkaline-earth oxides (Na₂O, MgO) in the amount of 2 wt. % by regulating rheological properties of silicate melts.

Abstract: Youngs moduli of talc-based ceramics from the system MgO-Al₂O₃-SiO₂ are measured for temperatures up to 1000 °C via impulse excitation. It is shown that, after pressing at 50 MPa and firing at 1280 °C, MgO-rich compositions exhibit higher porosity and lower Youngs moduli (approximately 2030 % lower than predicted via micromechanical relations). The Young moduli of materials with less MgO decrease with temperature, but those of MgO-rich ceramics increase with temperature and exhibit a large hysteresis between heating and cooling. Lower absolute values are mainly due to increased porosity, but the reason for the modulus increase with temperature and the hysteresis is the higher enstatite content in the MgO-rich compositions. For a special composition the Youngs moduli are more or less temperature-independent and without significant hysteresis effects, probably due to the low content of enstatite and the high content of sapphirine.

Abstract: In the present investigation, nanocrystalline silica xerogel (NSX) powders were produced from an amorphous silica xerogel (ASX) extracted from sago waste ash. The NSX powders have been calcined at 1200oC, milled and then annealed at temperatures ranging from a room temperature to 1200oC. Their properties (and most notably the size of the particles) have been characterized on the basis of the experimental data obtained using thermal analysis (DSC/TGA), X-ray diffraction (XRD), Infrared and Raman spectroscopy. For the crystalline silica xerogel powders the results show a narrow distribution of the particle sizes centered around an average value of 636  67 nm. The DSC analysis of NSX indicates that in the temperature range from a room temperature to 300oC five distinct stages of the crystallization process take place, which are delimited by the transition temperature of 38oC, 92oC, 129oC, 168oC, and 246oC, respectively. Above 300oC, the crystalline phase is similar to an amorphous silica xerogel (ASX), i.e. cristoballite-like and tridymite-like crystalline silica phases confirmed by the XRD analysis. It has been observed that the characteristic band of cristoballite is strongly dependent on the thermal history and the NSX transforms into a stable form at a temperature of 1200oC. Both the Raman and the FTIR spectra elucidate the bonding system of the constituent atoms and groups (such as Si, O and OH) and throw light on their underlying structure. The obtained results are important for optimization of the parameters of the technological processes for production of nanocrystalline silica glass ceramics used as a host matrix for luminescence materials, each of which requires a specific porosity and structure.

Abstract: Sintered fused silica is often used for making sacrificial cores in investment castings of Ni superalloys. Their usage is fundamental in the manufacture of precise superalloy gas turbine components with complex internal cooling passages. In this study SiO2/ZrSiO4/TiO2 cores were prepared from fused silica powders with different grain size and zircon and TiO2 content by slip casting method. Green samples were sintered at 1230°C at various soaking time: from 0,5 to 10 hours. Thermomechanical and microstructural properties of optimized silica obtained by add of 1,5%wt of TiO2 to SiO2/ZrSiO4 composition have been investigated by three point bending tests, XRD and Hg porosimetric analysis. The influence of cristobalite content on thermal stability at high temperature was studied by an optical dilatometer. At temperature below 1200°C TiO2 appears to act as a phase transformation inhibitor reducing the transformation rate of fused silica to cristobalite at high temperatures. At higher temperature it speeds up the formation of cristobalite. A comparison with commercial silica cores made by injection moulding has been performed. A prototype core was obtained and an investment casting was performed on that.