Cristobalite Transformation Research Articles

Young's modulus and mechanical strength of porcelain at the firing in the cooling stage

Young's modulus and mechanical strength of alumina-enriched porcelain (30% kaolin and clay, 25% Al 2O 3, 30% feldspar, 15% chamotte; 19% and 0·8% total amount of quartz and cristobalite impurities respectively) were measured in the cooling stage of firing (5°C/min). It was confirmed that β → α transformation of quartz is a source of the microcracks and the same influence on the Young's modulus and strength had the β → α transformation of cristobalite: the sharp minimums of Young's modulus at 570 and 280°C were discovered. Mechanical strength increased transienlty after β → α changes.

Thermal properties of gel-derived materials of Al 2O 3SiO 2 system

Gels of Al 2O 32SiO 2 composition were prepared from various raw materials, and the crystallization tendency and thermal expansion coefficient of gel-derived materials were compared. A monolithic gel prepared from a colloidal suspension of alumina in TEOS transformed into a pore-free material consistend of cristobalite and θ- and δ-alumina by the heat treatment at 1230°C for 1 h. Its thermal expansion coefficient was about 43 × 10 −7/°C in the temperature range between 250–800°C. In the range from room temperature to 250°C, however, the expansion due to transformation of cristobalite between high- and low-forms was observed. A monolithic gel from the colloidal suspension of Aerosil in the mixture of boehmite sol and TEOS became pore-free by heat treatment at 1150°C for 1 h without precipitation of cristobalite. The material was thermally stable and its thermal expansion coefficient remained unchanged at 38 × 10 −7/°C even after treatment at 1200°C for 1 h. A gel from silicon-aluminum ester became pore free material by heat treatment at 950°C for 6 h, although it fractured.

Adherence of oxide scale formed on Ni20Cr1Si alloys with small additions of rare earth elements

The isothermal and cyclic oxidation behaviour of Ni20Cr1Si (where the composition is in approximate mass per cent) alloys with small additions of rare earth elements has been studied in air over the temperature range 1273–1523 K. The rate of oxidation was reduced by the addition of cerium. In most alloys except the alloy with the addition of 0.34 mass% Ce, the oxide scales began to spall off at about 1000 K during cooling; this was caused by the difference between the thermal contraction of the alloy and that of the oxides. The scales severely spalled again at around 500 K in the alloys oxidized above 1473 K. In these alloys a continuous layer of cristobalite was formed beneath the oxide scales. The spalling is probably due to the stress caused by the displacive transformation of cristobalite which involves a substantial volume change. The best improvement in the scale adherence was attained by the addition of 0.34 mass% Ce even in the cyclic oxidation. In this alloy, SiO 2 is formed as cristobalite in a spherical shape beneath the protective Cr 2O 3 layer. The stress generated by the transpormation of cristobalite is relaxed by the deformation of the alloy adjacent to the spherical SiO 2. The scale adherence was also improved by the superficial application of CeO 2 powder. The suppression of spalling was also observed in the alloy with the addition of yttrium, lanthanum or erbium. The good adherence of the scales is due to the prevention of void formation at the alloy-oxide interface and to the keying-on structure of scales.

Sintering of Mullite-Silica Ceramics and Some Properties for Insulating Substrate Material

The sinterability of SiO2-Al2O3 powders containing 20, 30, 40wt% Al2O3, synthesized by hydrolysis of metal alkoxide, was examined. The relationship between the chemical composition and the properties, such as thermal, electrical and mechanical properties, of the sintered bodies has been studied. These powders densified to almost full density at 1300°-1350°C for 4h without forming cristobalite. When heated above 1350°C, however, crystallization of cristobalite was recognized which resulted in the increase in thermal expansion of the sintered body due to α-β phase transformation of cristobalite. The densification rate increased with increasing SiO2 content in the powder. The sintered bodies consisted of very fine acicular mullite particles and glassy matrix. The aspect ratio of the mullite particle increased with the decrease in Al2O3 content. The dielectric constants, thermal expansion coefficients and flexural strength of the sintered bodies containing no cristobalite phase were 4.3-5.3 (at 1MHz), 1.8-2.9×10-6/C (30°-400°C) and 124-201MPa, respectively. These values seem to be more advantageous in application for the insulating substrate material than those of conventional alumina ceramics. The above mentioned values increased with the increase in Al2O3 content which determines the amount of mullite phase in the sintered body. Consequently, the optimum property values can be designed by adjusting Al2O3/SiO2 ratio in the starting powder.

Effects of phase transformations of silicas and calcium sulfates on the compressive strength of gypsum-bonded investments at high temperatures.

The effects of transformations of silicas and calcium sulfates on high temperature compressive strength were investigated in commercial and experimental investment materials containing fused quartz as silica. The strength is only slightly affected by the alpha leads to beta transformation of cristobalite and not by the quartz alpha leads to beta transformation. The state of the calcium sulfates is the main factor influencing variations in the strength at a particular temperature.

About the kinetic equation in the DTA study of the silica gel to cristobalite transformation

The method suggested by Marotta and Buri for performing the kinetic analysis of the silica gel to cristobalite transformation from the DTA trace is criticized. It is shown that this method does not allow determination of the value of the exponentn in the Avrami-Erofeev law. This is in contradiction with the theoretical deduction made by the above authors.

The kinetic equation in the DTA study of the silica gel to cristobalite transformation

The importance of the choice of a suitable kinetic equation in the DTA study of the silica gel to cristobalite transformation is pointed out. It is shown that isothermal and non-isothermal data reported in the literature are better fitted by a Johnson-Mehl-Avrami equation withn=3, rather than a “first-order” one.

The formation of cristobalite in evacuated silica ampoules and the transformation of cristobalite to amorphous silica

Well crystalline specimens of cristobalite have been observed to form by a gas transport reaction in sealed evacuated ampoules of silica glass containing tungsten oxides plus other oxides at temperatures well below the lower stability limit of this modification of SiO 2. These crystals were found to be physically intergrown in the tungsten oxide mixture. In addition, the inner walls of the silica tubes were often recrystallised into the cristobalite form. The transported crystals were studied by powder X-ray diffraction, and optical and electron microscopy. Powder X-ray diffraction studies gave a value of a = 0.4975 nm and c = 0.6942 nm and c = 0.6942 nm for the lattice parameters of cristobalite. Examination by high-resolution electron microscopy allowed the transformation of the cristobalite to amorphous silica to be observed directly. Possible reasons for the formation of the cristobalite form of silica in the systems studied are discussed.