Crystallization Of Cristobalite Research Articles

Reactions at the liquid silicon/silica glass interface

Experiments have been carried out to determine the nature and origin of the spots growing on silica glass surfaces in contact with liquid silicon during CZ–Si crystal growth. Silica glass ampoules were filled with silicon and tempered between 5 min and 40 h at a temperature (1693 K) slightly above the melting point of silicon. Cross sections of the ampoules with solidified silicon have been examined by scanning electron microscopy and optical polarization microscopy. In addition cross sections from commercial silica glass crucibles used in the Czochralski process or dipped into the silicon melt were investigated with the same methods. At the silicon/silica glass interface different reaction zone morphologies were detected. A solution-precipitation mechanism is suggested for the fast lateral growth of the reaction zone, which is proposed to consist of small cristobalite crystals embedded in a silica glass matrix.

Strength and Thermal Shock Resistance of Alumina-Strengthened Porcelain Containing Cristobalite

The purpose of this study is to elucidate the effect of cristobalite on the strength and thermal properties of alumina-strengthened porcelain. The effects of milling time and sintering condition of mixtures on strength, density and thermal expansion properties were investigated by controlling the proportions of raw materials in compositions that generate cristobalite crystals upon heat treatment. At a composition of 15mass% corundum, 8mass% feldspar and 37mass% fine-grained quartz powder, the highest bending strength of 230MPa was attained with a bulk density of 2.6×103kg·m-3 under appropriate milling conditions and with sintering above 1300°C. Although thermal expansion below 150°C was somewhat greater than that of general porcelain or alumina-strengthened porcelain, test bars of the porcelain resisted quenching with a temperature difference of 140°C, which assured its usability as porcelain for tableware.

Effects of precursor pH and sintering temperature on synthesizing and morphology of sol–gel processed mullite

Needlelike mullite, which shows high aspect ratios, has been synthesized by coprecipitation from aluminum nitrate enneahydrate and colloidal silica sols. The effects of precursor pH and sintering temperature on the synthesizing behavior and the morphology of the needlelike mullite have been investigated. The equilibrium mullite phase appeared in the samples, which had been sintered at and above 1200 °C for each precursor pH. Crystallization of cristobalite from excessive SiO 2 occurred during sintering from 1200 °C, and the amount of cristobalite increased with sintering temperature up to 1400 °C. However, resorption of cristobalite into mullite decreased the amount of the phase from 1500 °C. The mullite synthesized with precursor pH⩽2 (acidic sample) displayed high aspect ratios at high sintering temperature. However, morphology of the mullite synthesized with precursor pH⩾8 (basic sample) transformed to rodlike or granular shape with increasing sintering temperature.

Dimensional changes and creep of silica core ceramics used in investment casting of superalloys

Dimensional changes and creep deformation of a silica/zircon (74%/24%, respectively) and a high silica (93% silica and 3% zircon) ceramic were characterized and compared. All specimens were tested with a thermal profile that consisted of a 300°C/h heating rate to 1475 or 1525°C, followed by a one-hour isothermal hold (where each specimen was compressively crept under a static stress of 2.07, 4.14, or 6.21 MPa). The specimens were cooled at a rate of 900°C/h under stress. Dimensional changes were interpreted from apparent thermal expansion behavior during heating as well as before-and-after dimensional measurements. The silica/zircon ceramic generally exhibited less total contraction than the high silica ceramic for a specific test condition even though it crept faster at all stresses and temperatures during the one-hour isothermal/isostress segment. This indicates that the total contraction for both was dominated by reinitiated sintering and subsequent cristobalite formation that occurred during the heating segment. Minimum creep rate during the one-hour isothermal/isostress segment was examined as a function of stress and temperature for both ceramics using a power-law creep model. Creep-rate stress exponents (n) and activation energies (Q) were equivalent (within 95% confidence) for both ceramics showing that their different contents of zircon (3 vs. 24%) did not affect them. Lastly, n ≈ 1.3–1.4 and Q ≈ 170 kJ/mol indicate that diffusion-assisted crystallization of cristobalite, combined with power-law sintering owing to the high concentration of porosity (28–30%) was likely the rate-limiting mechanism in the creep deformation for both ceramics.

Crystallization of gel-derived and quenched glasses in the ternary oxide Al 2O 3–ZrO 2–SiO 2 system

We studied glass formation in the SiO 2-rich region of the ternary oxide system Al 2O 3–ZrO 2–SiO 2. Gels with the composition 5–5–90 mol% have been synthesized by the sol–gel method in acid media and then treated up to 1650 °C and kept for 2 and 10 h. A glass with the same composition has been obtained by quenching in water from the melt. The structural evolution of gels annealed at different temperatures up to 1650 °C was followed by thermal analysis, X-ray diffraction, specific surface area, density measurements and IR spectroscopy. The materials consisted mainly of amorphous phase up to 1200 °C, at which partial crystallization of cristobalite was observed. The presence of Zr and Al in the silica network at high temperatures was investigated by IR spectroscopy. The crystallization of gels at high temperatures and the crystalline phases formed are related to the preparation conditions and the thermal treatment. For samples from quenched melts zircon was the only crystalline phase detected, while in the case of the samples slowly cooled down from 1650 °C, zircon, silica, zirconia and alumina or mullite were observed depending on the treatment time.

Effect of cristobalite formation on sinterability, microstructure and properties of glass/ceramic composites

Six glass/ceramic composite materials were prepared from pure alumina and borosilicate glass (BSG) with chemical composition of 99.0 Al 2O 3 and 80.5 SiO 2, 12.9 B 2O 3, 3.8 Na 2O, 2.2 Al 2O 3, 0.4 K 2O and 0.2 CaO wt.%, respectively. These batches were sintered at different firing temperatures up to 1300 °C. XRD, microstructure and properties, namely thermal expansion, dielectric constant and hardness of prepared composites were examined, respectively. The results reveal that the crystallization of borosilicate glass in glass/alumina composite can be prevented by varying the alumina content in the glass matrix, by which the diffused aluminum ion change the structure of glass. When approximately ⩾30 wt.% of alumina filler is added, the structure of glass changes enough to decrease the formation of cristobalite crystals and consequently decrease the thermal expansion coefficient and dielectric constant. In glass/alumina system, phase formation is influenced by firing temperature and alumina content as follows: (a) when alumina content and firing temperature are low, the precipitated phase is cristobalite, (b) at high temperature and high alumina content, mullite is formed in the glass phase.

Crystallization of sodium fluormica Na(Mg, Zn, Ca)2.5Si4O10F2 glasses

Abstract Na-fluorphlogopite glasses were prepared from natural and commercial raw materials. Substitutions of Ca or Zn for Mg in stoichiometric Na-fluorphlogopite decrease the transformation temperature and increase the crystallization range. In the parent glass, the fluormica grows at the early stage of crystallization which transforms into Mg-fluorrichterite at high temperature. Crystallization of the sample containing CaO/MgO in molar ratio (m.r.)=0.25 did not enhance Na-fluorphlogopite, but catalyzes fluorrichterite together with cristobalite. Devitrification of the glass takes place when CaO/MgO m.r. is >0.25. Substitution at the ZnO/MgO m.r.=0.25 in the base glass led to the crystallization of the Na-fluorphlogopite, whereas zinc silicate, Zn1.9SiO4 and Mg-fluorrichterite phases were formed at higher temperature or for long duration heat treatment. Crystallization at ZnO/MgO m.r.=0.66 begin with Na-fluorphlogopite and cristobalite, while willemite in addition to Mg-fluorrichterite were developed at higher temperatures. On increasing the ZnO/MgO m.r., devitrified opalescence glass was obtained. Improvement of the chemical durability of the investigated samples towards distilled water were achieved by crystallization of fluorrichterite and cristobalite or the development of the fine grain microstructures. The coefficients of thermal expansion (CTE) values of the crystalline glasses were increased by the substitution of MgO by CaO or ZnO.

Crystallization of SiO2 from Ethylsilicate-Based Gels

The mechanism of polymerization of ethylsilicate gels catalyzed by hydrochloric, nitric, and orthophosphoric acids is studied. The effect of the ethylsilicate : water ratio in the hydrolyzate on the phase composition and the optical properties of the heat-treated powder is investigated. The synthesis of opal is registered in heat treatment of ethylsilicate gels, and an explanation for the mechanism of its formation is proposed. It is demonstrated that the formation of the opal structure in gels delays the crystallization of cristobalite in heat treatment.

Crystallization of cristobalite and tridymite in the presence of vanadium

The formation of cristobalite and tridymite compounds is described in the system SiO 2 -V 2 O 5 -M 2 CO 3 (M = Na, K). Syntheses were performed with various vanadium concentrations at annealing temperatures between room temperature and 1000°C. Crystallization of cristobalite occurs at much lower temperatures (about 800°C) in the presence of vanadium than observed in the pure silica system, which remains amorphous in this temperature range. With increasing amounts of vanadium, the crystallization of NaVO 3 or K 3 V 5 O 14 is observed along with the formation of cristobalite and tridymite. XRD, IR, and NMR indicate that cristobalite is formed by catalytic processes in the presence of vanadium without measurable V-substitution in its structure. Variations in the thermal behavior of cristobalite are attributed to different states of crystallinity.

The sintering and crystallization of colloidal silica gel

The isothermal kinetics of crystallization of cristobalite from silica gel at different sintering temperatures were investigated using quantitative X-ray diffraction (XRD) method. The xerogel crystallized to cristobalite and started at the surface of the glass body during sintering at 1100°C in a He atmosphere. If the xerogel was treated with CCl 4, the nucleation and growth processes determined the crystallization during sintering to be above 1250°C. The hydroxyl contents of the specimens were about 50 ppm from calculations base on transmittance data. The activation energy for nucleation and growth is 570 kJ/mole and the constant of the Avrami equation is about 1.51. The sintering temperatures for obtaining a higher density were influenced by the bulk densities of the xerogel. The xerogel obtained by twice dispersion gives a bulk density of 0.7 g/cm 3 and can be densified by sintering at 1180°C for 4 h in a He/O 2 atmosphere; however, the xerogel with bulk density of 0.6 g/cm 3, obtained by adding 5 mol% N, N-dimethyl formamide as the drying control additive can be densified to glass by sintering at 1250°C for 4 h in a He/O 2 atmosphere.

Crystallization of Na 2O-doped colloidal gel-derived silica

Crystallization kinetics of the colloidal gel-derived silica has been investigated when the powder compacts are pressureless-sintered at 700–900°C in air. α-Cristobalite is the only crystalline phase in the sintered mixture as detected by X-ray diffractometry, whose quantity is used to establish the crystallization kinetic curves. The Avrami exponents of n=0.8–2.3 thus derived indicate that the crystallization has taken place by predominantly thermal nucleation of the diffusion-controlled nature. Both lamellar and spherulitic growth of the α-cristobalite crystals are observed by scanning electron microscopy in sintered samples containing Na 2O. The growth morphology alters from predominantly chunky crystals ( n=2.3) to spherulitic grains ( n=0.8) as the Na 2O-content of the initial colloidal sols is increased beyond a critical level. The activation enthalpy values of Δ H=216–288 kJ mol −1 suggest also that oxygen transport by a mechanism involving limited exchange of network and interstitials is the rate-determining mechanism for the crystallization of cristobalite.

Ammonium-bearing mica and mica/smectite of several pottery stone and pyrophyllite deposits in Japan: their mineralogical properties and utilization

Many pottery stone and pyrophyllite deposits have been developed in southwestern Japan. These deposits are hydrothermal in origin and have produced dioctahedral mica (sericite) minerals usually used for porcelain, tiles, sanitary ware, etc. Sericite specimens from two pottery stone deposits (Izumiyama and Tobe) and three pyrophyllite deposits (Yagi, Mitsuishi and Horo mines) are mineralogically characterized in terms of utilization. These sericite clays are chemically characterized by extensive substitution of NH 4 for K corresponding to the muscovite–tobelite series, resulting in the basal spacing being enlarged from 10.00 Å (muscovite) to 10.36 Å (tobelite), which is diagnostic of the interlayer composition. Infrared absorbance of the NH 4 band (1430 cm −1) standardized to the OH band (3620 cm −1) is also useful for determination of the NH 4 content. DTA of a NH 4-dominant specimen from the Tobe deposit reveals the liberation of NH 4 around 490°C and double steps dehydroxylation in the 600–800°C range, as well as crystallization of mullite and cristobalite at 1000°C. These thermal properties are considered to be effective in sintering and refractoriness of the sericite materials when fired. Most of the sericite specimens are interstratified mica/smectite with <20% smectite, which may be responsible for plasticity when treated in the wet refinement process. Of the sericite clays from the five deposits, the Tobe, Yagi and Mitsuishi specimens are characterized by a varied interlayer NH 4/K ratio, interstratification and lower Fe and Ti content, which favor refractoriness, plasticity and whiteness, respectively: these are excellent qualities for their use as raw materials for porcelain.

Thermodynamics of various polyvalent elements in a 15Na 2O · 85SiO 2 glass melt

Glass melts with the basic composition of 15Na 2O · 85SiO 2 doped with various polyvalent elements were studied by square-wave voltammetry. In the melts, the thermodynamics of the redox pairs Sb 5+/Sb 3+, As 5+/As 3+, Sn 2+/Sn 0, Sb 3+/Sb 0, As 3+/As 0, Cu +/Cu 0, V 5+/V 4+, Fe 3+/Fe 2+, Cr 3+/Cr 2+, V 4+/V 3+and Ti 4+/Ti 3+were studied quantitatively. The temperatures applied were 1000–1550°C. Two temperature ranges can be distinguished: one range from 1000°C to 1300°C and another one from 1300°C to 1550°C. In both ranges, the peak potentials depended linearly on temperature. However the slope was smaller in the high temperature range. As proved by XRD and DTA, this is caused by the crystallization of cristobalite at temperatures ⩽1300°C. This leads to an increase in the alkali content of the residual melt which then exhibits smaller standard potentials than the 15Na 2O · 85SiO 2 melt. This is shown by means of a direct comparison of the standard potentials with those determined in a melt with the basic composition of Na 2O · 2SiO 2.

Phase Transformation Sequence from Kaolinite to Mullite Investigated by an Energy‐Filtering Transmission Electron Microscope

The phase transformation from kaolinite to mullite was examined with new electron diffraction data obtained using an energy‐filtering transmission electron microscope. Kaolinite was finally transformed to mullite and cristobalite through several steps of endothermic and exothermic reactions, which resulted in metakaolinite, a microcrystalline spinel‐type phase and amorphous silica. Metakaolinite maintained its short‐range order even at 920°C. The spinel‐type phase resulted from a topotactic transformation of metakaolinite. Mullite first appeared at around 940°C, showing no clear crystallographic relationships to the parent metakaolinite structure. It coexisted with metakaolinite and the spinel‐type phase produced previously. The first strong exothermic peak on the DTA curve was mainly due to the extraction of amorphous silica from metakaolinite and the gradual nucleation of the mullite phase. The initially formed spinel‐type and mullite phases were suggested to be Al‐rich, but to progressively gain Si in their structures at higher temperatures. The spinel‐type phase decomposed completely through a second weak exothermic reaction, promoting the crystallization of cristobalite from amorphous silica, and the growth of mullite.

Hydrothermal reactivity of K-smectite at 300°C and 100 bar: dissolution-crystallization process and non-expandable dehydrated smectite formation

AbstractThe hydrothermal reactivity of a smectite saturated with K was studied experimentally at 300°C and 100 bar in (Na,K) chloride solutions (Na/K = 0, 50 and 100, liquid/solid ratio = 10/1). X-ray diffraction, TEM and microprobe results show: (1) a partial to total dissolution of the initial smectite layers; and (2) the crystallization of newly-formed euhedral I-S. Random I-S is formed after 7 days, but an ordered mixed-layer I-S containing &lt;30% expandable layers formed in the longest runs (112 days). The I-S is characterized by non-expandable layers of two distinct types: dehydrated smectite and illite. The Si content is lower in the I-S than in the initial smectite, thus creating a charge deficit, mostly compensated by the introduction of Na to the interlayer space, and yielding a silica release to the solution and subsequent crystallization of quartz and cristobalite.

Synthesis of mullite precursors in molten salts. Influence of the molten alkali nitrate and additives

Silica gel and aluminium salt, in the stoichiometric ratio for mullite (Al/Si=3), have been used to prepare the mullite precursor by reaction in molten alkali nitrates at 450°C. The influence of the nitrate (NaNO 3, KNO 3 or mixture NaNO 3–KNO 3) and the additives (Ca 2+, Ba 2+, Bi 3+) on the phase transformations has been followed between 1000 and 1400°C. It appears that molten NaNO 3 favours the crystallization of cristobalite and α-Al 2O 3 whereas KNO 3 inhibits the formation of silica and mullite. The mixture NaNO 3–KNO 3 seems to be the best choice. Addition of foreign cations improves the mullite formation and favours the synthesis of θ-Al 2O 3, especially in the case of Bi 3+. But the crystallization temperature of mullite is not changed.

Crystallization of Cristobalite from Glass Phase in Mullite Ceramics with Excess SiO2 Compositions.

Crystallization of glass phase to cristobalite was investigated in mullite-glass ceramics with excess SiO2 compositions (5.3-16.4mass%), namely, higher than stoichiometric mullite. Starting powders were prepared by alkoxide hydrolysis method. Compact specimens were sintered at 1600°C above the eutectic temperature (1587°C) for 2h. Crystallization treatment was carried out at 1500°C, below the eutectic temperature, for times from 4h to 96h. Crystallization of glass phase proceeded from the surface of the specimen toward its inner part, in linear dependence on the annealing time. At the polished surface of the specimen, crystallization started in large glass pockets and the crystallized area extended spherically toward the inner part. This phenomenon suggests that nucleation occurs at the minimum parts in the elastic energy generated by the volume change involved in the crystallization of glass phase to cristobalite.

仙台市太白山産クリストバライト

Minute hexagonal platy crystals in druse of pyroxene andesite from Taihakusan, Sendai were examined by polarized optical microscope, SEM, EPMA and X ray studies. Although the mineral looks like trydimite, X ray analyses (microdiffraction and precession camera) demonstrate it to be cristobalite. It is assumed that the cristobalite crystals with fine stacking faults and polysynthetic twins were produced in the druse from a gas phase after andesite was solidified.

An Occurrence of Metastable Cristobalite in High-Pressure Garnet Granulite

High-pressure (0.8 gigapascals) granulite facies garnet from Gore Mountain, New York, hosts multiple solid inclusions containing the low-pressure silica polymorph cristobalite along with albite and minor ilmenite. Identification of cristobalite is based on Raman spectra, electron microprobe analysis, and microthermometric measurements on the alpha/beta phase transformation. The cristobalite plus albite inclusions may have originated as small, trapped samples of hydrous sodium-aluminum-siliceous melt. Diffusive loss of water from these inclusions under isothermal, isochoric conditions may have resulted in a large enough internal pressure decrease to promote the metastable crystallization of cristobalite.

Characterization of hydrated silicate glass microballoons

Glass microballoons (GB) of about 1 μm in diameter were prepared by ultrasonic spray pyrolysis from sodium silicate solution. A silica-rich type of glass microballoons (SB) was prepared by acid treatment of GB. The structural changes of both microballoons with thermal treatment up to 973 K were examined. Both GB and SB showed properties similar to hydrated sodium silicate glass, to some extent. SB was more thermally stable than GB, but the spherical structures of both microballoons were collapsed by heating at 973 K; cristobalite was observed in samples heated at 973 K. The loosely and tightly incorporated water molecules evolved up to 573 K and near 850 K, respectively. The crystallization of cristobalite caused tightly incorporated water molecules to develop. The ultramicropores accessible only to H2O molecules in SB gradually decreased by heating and disappeared by heating at 773 K.