Cristobalite Transformation Research Articles

Raman spectroscopic characterization of SiO2 phase transformation and Si substrate stress relevant to EBC performance

AbstractTo accurately model the long‐term durability of environmental barrier coatings (EBCs), a more complete understanding of the phase composition and transformations of the thermally grown oxide SiO2 (TGO) is desired. For the TGO formed during thermal cycling in steam, cristobalite formation and the subsequent β‐ to α‐cristobalite transformation has been identified as a potentially life‐limiting mechanism. In this study, Raman micro‐spectroscopy was used to quantify the cristobalite transformation on a polycrystalline Si coupon that was exposed to steam at 1350°C for 100 h. The phase transformation was mapped at 200–260°C on the TGO surface at different ramp rates using a heating stage and a micro‐positioning stage. The stress in the Si substrate was also determined using Raman spectroscopy by measuring the stress induced peak shift. The α→β phase transformation produced a 300–500 MPa tensile stress in the Si substrate, which compared well to the stress predicted from the volumetric expansion of the cristobalite. Quantifying the phase transformation and residual stress are critical tools in developing the next generation of high performance EBCs.

Microstructure and properties evolution of silicon-based ceramic cores fabricated by 3D printing with stair-stepping effect control

A ceramic core is the key component in the manufacture of the hollow turbine blades of aeroengines. Compared with the traditional injection molding method, 3D printing is more suitable for manufacturing ceramic cores with a complex geometry at high precision. However, the stair-stepping effect is inevitable in the 3D printing process and affects the surface roughness and strength of the ceramic core. In this study, to explore the influence of nano-silica content on the microstructure and properties of the ceramic core, silicon-based ceramic cores were fabricated with the addition of nano-silica powder by digital light processing and subsequent sintering at 1200 °C. The results showed that the apparent porosity and pore size of the ceramic core gradually decreased as both the nano-silica powder content and bulk density increased. Meanwhile, the printing interlayer spacing was significantly reduced, resulting in a low surface roughness, high flexural strength, and creep-resistance. To simulate the entire casting process of a superalloy blade, the thermal deformation behavior of the ceramic core was observed by heating and cooling cycles performed in a thermal dilatometer at 1540 °C. The total linear shrinkage decreased as the nano-silica powder content increased, which was mainly due to the phase transformation of cristobalite and the densification of the ceramic core sintered at 1200 °C. The low surface roughness and linear shrinkage as well as high flexural strength of the ceramic core can contribute to the excellent quality of cast superalloy blades.

Oxidation of 3D‐printed SiC in air and steam environments

AbstractThe high‐temperature oxidation of additively manufactured and chemically vapor infiltrated (3D‐printed SiC) has been compared to chemical vapor deposited (CVD) SiC. 100‐h isothermal exposures were conducted at 1425° and 1300°C at 1 atm under both dry air and steam environments. A SiC reaction tube was utilized to reduce silica volatility. After steam oxidation at 1425° and 1300°C, on the 3D‐printed SiC surface, which was intrinsically rougher than the CVD surface, scales were 70%–90% thicker at the convex regions compared to concave/flat regions. In the convex regions, large cracks perpendicular to the oxidizing interface were observed. After dry air oxidation, scale thicknesses were comparable between 3D‐printed SiC and CVD SiC, regardless of geometry. Finite element modeling, conducted to elucidate the relationship between SiC geometry and ß‐ to α‐cristobalite transformation stress, determined cristobalite transformation tensile stresses to be on the order of 103 MPa during cool down, assuming a 6 vol% reduction. Compared to flat SiC substrates, tensile transformation stresses were elevated at concave regions and relaxed at convex regions. Combined with specimen mass gain (accounting for the rougher surface) of 3D‐printed SiC being 15%–32% higher for 3D‐printed SiC after 1300°C and 1425°C steam oxidation, the work presented concludes that the increased oxidation of 3D‐printed SiC is primarily caused by tensile hoop stresses driven by oxidation volume expansion. Lastly, the efficacy of the 3D‐printing method is demonstrated through the production of tristructural isotropic imbedded 3D‐printed SiC fuel forms.

Effect of Mineralizers and Reaction Conditions on the Formation of Cristobalite

Crystalline silica experience phase transformation from quartz to cristobalite upon heat treatment. The influence of mineralizer types and reaction conditions on the crystallinity of cristobalite was investigated. The results show that the calcination temperature and the holding time have an effect on the phase transformation of cristobalite in the presence of mineralizers.

Microstructural evolution, phase transformation, and variations in physical properties of coal series kaolin powder compact during firing

Coal series kaolin (CSK) from coalmine in Shanxi, China was studied in the form of powder compact by using die-pressing technique. Mineralogical, morphological, and chemical characteristics of CSK were given. The microstructural evolution, phase transformation, and relevant variations in physical properties of the fired CSK with elevating temperature were investigated. Results indicated that mullite began to form at the temperature as low as 1002.8°C. Columnar mullite appeared when firing at 1500°C. Transformation of cristobalite to liquid phase favored the densification of the fired powder compacts and made them acquire the densest microstructure in 1580–1600°C temperature interval. Die-pressing technique and flake-like nature of CSK particle induced the interlocking texture on the plane perpendicular to die-pressing direction. Low content of impurities as TiO2 and Fe2O3 increased the formation temperature of the columnar mullite and endowed samples with high refractoriness (as high as 1600°C). The advantages of using CSK for industrial ceramic preparation are its high purity, low mullitization temperature, and high refractoriness.

Investigation on Nano-Fused Silica in Silica-Based Ceramic Cores for Investment Casting

In this work, nanofused silica and its effect on mechanical and chemical behavior of injection moulded silica-based ceramic cores were investigated. In order to simulate casting process condition, the sintered samples at 1180 °C were also heated up to 1500 °C. Flexural strength test was carried out on both sintered and heat treated samples. Phase evolution and microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the samples with 0.5 wt.% nanofused silica content had an optimal comprehensive properties for investment casting. The flexural strength of sintered samples with 0.5 wt.% nanofused silica at room temperature and at 1500 °C for 30 min were 30.3 MPa and 24.4 MPa, respectively, and the creep deformation at 1500 °C was 0.8 mm. The addition of nanofused silica was favorable for cristobalite transformation and the samples had a good leachability for the micro-cracks introduced by crystallization.

Effect of Cristobalite on the Strength of Sintered Fused Silica Above and Below the Cristobalite Transformation

Porous fused silica ceramics which had been partially devitrified to cristobalite were stronger at 350°C in the beta‐cristobalite stability range, but weaker at 25°C after the transformation to alpha‐cristobalite. The tensile strength distribution for different cristobalite fractions are compared for three types of specimens. These were beta‐cristobalite samples which had never been transformed to alpha‐cristobalite; room‐temperature samples with alpha‐cristobalite; and samples at 350°C which had retransformed to beta‐cristobalite after prior transformation to alpha‐cristobalite. The alpha‐cristobalite samples displayed 50%–75% the strength of samples tested on beta and the difference in strength was dependent on cristobalite content. Specimens retransformed to beta‐cristobalite had strength similar to virgin specimens. Microcracking associated with the transformation to alpha‐cristobalite contributes to the strength changes but cannot fully explain the impact of the beta–alpha transformation on strength.

Contribution to the understanding of the high temperature behavior and of the compressive creep behavior of silica refractory materials

Abstract Some silica (95% SiO2) refractory materials were selected and characterized by their chemical and crystallographic compositions, microstructure, refractoriness under load and bending strength versus temperature. Creep tests were realized at temperature higher than 1500 °C, using the differential method. This part of the work was mainly focalized on one material. Complementary tests: annealing treatment followed by crystallographic analysis, measurement of dimensions variation and microstructure observation were also realized. Results of annealing and creep tests demonstrate a competition between shrinkage and a volume expansion of the material, associated with a modification of crystallographic composition and of the microstructure. Results were discussed, compared with literature and a model was developed to explain the behavior and especially the expansion. The high temperature behavior and the compressive creep were explained by the appearance of a calcia–silica vitreous phase and microstructure change. The material expansion at high temperature is explained by calcia migration, from matrix into aggregates. Residual expansion after cooling is partially explained by the cristobalite transformation which induces stress and cracking.

Kinetics of Cristobalite Formation in Sintered Silica

The kinetics of the cristobalite transformation are reported for sintered silica glass from 1200°C to 1650°C and plotted as a time–temperature–transformation diagram. The 1200°C–1350°C transformation data were fit to the Johnson–Mehl–Avrami–Kolmogorov expression with an time exponent of 3.0 ± 0.6 and an apparent activation energy of 555 ± 24 kJ/mol for the kinetic constant. The temperature of maximum transformation rate was found to fall between 1500°C and 1600°C. Seeding amorphous silica powder with cristobalite resulted in accelerated transformation kinetics. Silica glass powder containing residual quartz had faster transformation kinetics than fully amorphous powder seeded with cristobalite.

Hysteresis upon Repeated Cycling through the Beta-Alpha Cristobalite Transformation

Beta-to-alpha transformation in cristobalite was examined by means of differential scanning calorimetry (DSC) to address how enthalpy, transformation hysteresis, and transformation onset temperature change upon repeated thermal cycling. Cristobalite powder was repeatedly cycled from 190 – 280 °C and back, cycling through the firstorder displacive transformation between high-temperature, cubic beta-cristobalite and low-temperature, tetragonal alpha-cristobalite. The enthalpy of the transformation did not changewith cycling, but the exothermic beta-to-alpha enthalpy at 1560 ± 27 J/mol was larger than endothermic alpha-to-beta enthalpy at 1260 ± 8 J/mol. Transformation onset temperatures and hysteresis varied systematically with repeated cycling of the transformation. The onset temperature of the beta-to-alpha transformation increased logarithmically with cycling, resulting in reduced undercooling and hysteresis. The onset temperature of the alpha-to-beta transformation decreased logarithmically with cycling, resulting in reduced super-heating and hysteresis. The reduced hysteresis indicates a lowered barrier to transformation. We propose this reduced hysteresis indicates particle refinement through microfracture caused by the -4.9% volume change on the beta-to-alpha transformation. This is supported by the observation of powder size dependence. Powder with particles finer than 38 μmhad no change on cycling, suggesting 38 μm is below the critical minimum size.

A kinetic study of the quartz–cristobalite phase transition

Abstract Cristobalite is a common silica polymorph in ceramics, as it can crystallize in SiO 2 -rich systems during high temperature processes. Its occurrence in final traditional ceramic bodies remarkably affects their thermal expansion, thus playing an important role in the shrinkage upon cooling. The quartz–cristobalite transformation kinetics is investigated by in-situ isothermal X-ray powder diffraction experiments and then correlated to the average particle size (〈 d 〉) of the starting quartz using a model here developed. An Avrami-like rate equation, i.e. α ( t ) = 1 − exp(− k × t ) n , in which the n -term is assumed to account for the dependence on the average particle size, has provided the best fitting of theoretical to experimental data, yielding activation energy values that range from 181 to 234 kJ mol −1 , and exponential n -coefficients from 0.9 to 1.5. Ex-situ observations have demonstrated that the formation of cristobalite from quartz after 50 min, 2, 4 and 6 h at 1200 and 1300 °C, exhibits a remarkable dependence on 〈 d 〉 of quartz, showing comparable behaviours in the case of 〈 d 〉 equal to 15.8 and 28.4 μm, but significant differences for 〈 d 〉 of 4.1 μm. The formation of cristobalite is boosted remarkably at temperature higher than 1200 °C, with an increase by weight even of 500%, with respect to its content at lower temperature. The method of sample preparation (dry powder, wet powder and tablet of compressed dry powder) seems to influence the results only at temperature > 1200 °C and in the case of fine powder.

Occurrence of silica polymorphs nanocrystals in tuffaceous rocks, Province of the Mesa Central, Mexico, and their formation from subcritical Si-rich fluids

Cristobalite-tridymite blade nanocrystals cemented by SiO2-glass and tridymite nanocrystals fill separately SiO2-glass spherules released by explosive volcanism, in rhyolitic tuffs from the Province of the Mesa Central, Mexico. This paper presents the mineralogy of silica polymorphs, occurrence, and process of formation in the Province of the Mesa Central. The understanding of the origin of these pure SiO2-polymorphs, their association with high-temperature minerals, fractionation of magmas, and role of volatiles contribute to our knowledge on silica minerals, their technological implications and damaging health effects. Results indicate that a precursor magma, from which kyanite crystallized, partitioned into an immiscible Fe-rich magmatic liquid that crystallized Fe-cordierite, Fe-amphiboles, and fayalite and into a siliceous melt that led to low-temperature glasses of 78.22–80.01 wt% SiO2 and Si/Al ratio 4.07–5.65. Presence of amphiboles, sulfur in cristobalite-tridymite crystals and alunite suggest association of volatiles. The crystallization of silica polymorphs is associated with the dissolution of water vapor and volatiles in the precursor magma, establishing a silicate melt–water system of two critical points, one of them at pressure and temperature near the critical point of water and another close to the critical point of SiO2, and defining a critical curve between them and supercriticality at pressures and temperatures lower than the critical point of SiO2. Decreasing the ambient conditions from supercritical to subcritical would have allowed the separation of liquid and gases and the crystallization of cristobalite-tridymite and tridymite nanocrystals from the vapor phase. Cristobalite single crystals were not formed and transformation of cristobalite and trydimite to quartz did not occur. We conclude that glasses did not crystallize cristobalite or tridymite. Components in excess of the pure phases precipitated as nanoparticles of siliceous glass forming agglomerates, some containing iron hydroxides and alunogen.

Effect of Al<sup>3+</sup> Additions on the Properties of Quartz Ceramics

Quartz ceramic has a great potential use in engineering ceramic materials. However, due to the cristobalite transformation in sintering process, the mechanical properties are weakened. The influence of Al3+ additions on bending strength and open porosity of quartz ceramics was determined by adding Al2O3 (sample 1) and aluminum sulfate solution (sample 2). The content of Al3+ ranged from 1% to 7%. Both samples were sintered at 1200°C for 2h. The bulk density of samples was measured by Archimede’s displacement method and the flexural strength of the specimens was tested using a conventional three point flexural method. The addition of 3% Al3+ of aluminum sulfate leads to better properties (bending strength at 14Mpa and open porosity at 23%). Glass theory was applied to explain the mechanisms of inhibiting crystallization of Al3+ additions.

Microstructure and Thermal Shock Resistance of Molten Glass‐Coated Carbon Materials Fabricated by Interfacial Control

Carbon substrates were coated completely with a molten silicate glass, where the wettability of carbon to glass was improved by infiltration and pyrolysis of perhydropolysilazane. Microstructures of the carbon–glass interface were dependent on Pn2 during coating. Coating at lower Pn2 induced the formation of cristobalite at the carbon–glass interface. When the coating was performed at higher Pn2, the glass and carbon were strongly adhered, without the formation of cristobalite. Coating at higher Pn2 improved the thermal shock resistance of the glass layer, because crack initiation was not induced by the phase transformation of cristobalite during the cooling process. In the case of coating at higher Pn2, an oxynitride glass layer was formed at the glass subsurface by dissolution of N2. A porous glass subsurface layer with uniform spherical micro‐pores could be produced by soaking near the glass transition temperature in a steam environment. The porous layer with fine and homogeneous microstructure acts as a thermal shock absorbing layer, so that glass‐coated carbon with a porous glass layer has excellent thermal shock resistance in addition to steam oxidation resistance.

Cristobalite transformation in two refractory materials with 45 and 42 % alumina

A natural kaolin material, which contains 45 wt % of alumina and less than 1 wt % of metal oxide impurities, has been submitted to thermal treatments in the temperature range 1 000-1 450 °C. The cristobalite development is studied by X-ray diffraction, using the broadening of line {101}. The apparent grain sizes of cristobalite, measured by the integral width of the line, change as a function of thermal treatment. Nevertheless, the apparent total content of the cristobalite remains constant. In the case of a chamotte material, which contains 42 wt % alumina and 3 to 4 wt % of metal oxide impurities, the apparent cristobalite grain sizes are of order of magnitude of the nanometer and the total apparent content is slightly lower. Finally, the thermal expansion coefficient, α (T), of the first material is higher, during the cristobalite transformation α → β , than that of the chamotte material.

Ab initiomolecular dynamics study of the pressure-induced phase transformations in cristobalite

The mechanism for the pressure-induced transformation of cristobalite to stishovite and post-stishovite phases has been obtained from constant pressure ab initio molecular dynamics simulations. The cristobalite to stishovite transformation is found to be a two step process where ${\mathrm{SiO}}_{4}$ tetrahedra first rotate followed by a lattice distortion to yield the six-coordinated stishovite structure. Further compression of stishovite yields the ${\mathrm{CaCl}}_{2}$ structure and is followed by another six-coordinated structure with symmetry ${P2}_{1}/n$ (at 11 Mbars) which remains stable to a pressure of about 14 Mbars and then transforms into a nine-coordinated ${P2}_{1}/m$ structure.

Influence of porosity on the high temperature oxidation of a SiC-reinforced Si3N4 ceramic composite

In this work the oxidation kinetics of a porous (21.5% porosity) nitride bonded silicon carbide containing 20% SiC was investigated in the temperature range of 1100–1400 °C. Two oxidation stages were found which corresponded to (i) a rapid parabolic oxidation rate during short term oxidation exposure (0–10 hours), and (ii) a parabolic scale growth after some exposure time. During short term oxidation a continuous oxide film developed, but it was unable to block internal oxidation at the pore channels. An experimental activation energy of 55 kJ/mol was obtained which was attributed to intially rapid external and internal oxidation through the open pore network. After 1–10 hours depending on the actual temperature, closure of the surface pores through oxidation lead to a transition where a continuous SiO2 scale grew on the outer sample surface through diffusion. An activation energy of 132 kJ/mol was associated with this parabolic growth suggesting that inward oxygen diffusion through the SiO2 scale was rate limiting. Metallographic observations indicated severe cracking of the scale developed. This was attributed to the relatively large shrinkage (≈1%) associated with the β–α cristobalite transformation occurring at temperatures below 250 °C. Moreover, X-ray diffraction indicated the presence of cristobalite and tridymite, but it was unable to identify a discontinuous phase developed beneath the SiO2 scale during oxidation.

High temperature oxidation of nitride bonded SiC-ceramics

The oxidation kinetics of a porous (21.5% porosity) nitride bonded silicon carbide (NBSC) ceramics containing 20% SiC was investigated in the temperature range of 1100–1400°C. A rapid linear oxidation rate during the initial exposure to air (0–10 h), was found to be followed by parabolic scale growth. During parabolic oxidation a continuous SiO2 scale developed whose growth had an activation energy of 132 kJ/mol. Apparently, inward oxygen diffusion through the SiO2 scale was rate limiting. In addition, severe scale cracking was found probably as a result of the shrinkage (≈1%) associated with the β–α cristobalite transformation below 250°C.

Correlation between the oxidation behavior and the microstructure of SiC coatings deposited on graphite substrates via chemical vapor deposition

The dependence of high temperature oxidation behavior of β-SiC coatings on their microstructures were investigated. Sample coatings with various surface morphologies and microstructures were synthesized on graphite by chemical vapor deposition, and were exposed to air or methane combustion exhaust gas at a pressure of 100 kPa and temperatures ranging from 1500 to 1700°C. It was found that the formation of cristobalite decreased with decreasing content of 〈111〉-oriented grains at the specimen surfaces both in air and methane combustion exhaust gas. Because the formation of cristobalite is known to reduce the protectability against oxidation of SiC coatings because of the cracking and peeling of the protective oxide overlayer by thermal expansion and the fast transformation of cristobalite during cooling to room temperature, the present result indicates that oxidation protectability increases with decreasing 〈111〉-oriented grains in the SiC coatings.

The Oxidation Behavior of a Si-Ti-C-O Fiber with a Low Oxygen Content

The Si-Ti-C-O fiber (STC (6)) with a low oxygen content of 6mass% was oxidized at temperatures in the range of 1273 to 1773K in order to measure its oxidation rate and thereby to elucidate its oxidation behavior. These results were compared to those of the commercial fibers having 18 and 13mass% O (STC (18) and STC (13)). The kinetic data of STC (6) were expressed by the contracting-cylinder formula. The activation energies were calculated to be 72 and 430kJ/mol at the low-temperature (T 1473K) regions, respectively. On the other hand, STC (18) and STC (13) had activation energies of oxidation of E=72kJ/mol and E=70kJ/mol within the whole temperature range, respectively. Oxidation of STC (6) with a low oxygen content is considered to be similar in kinetic behavior to that of pure SiC.The oxide film consisted of SiO2 and TiO2; and the former was amorphous during the low-temperature oxidation process tending to be crystallized into cristobalite at higher temperatures. The average size of crystallized β-SiC particles of the interface between the oxide film and unoxidized core fiber was larger than that of core of the fiber. This is because the progress of the crystallization in O2 stream is considered to have been suppressed when the fiber was completely covered with the oxide film. When the fiber was oxidized at 1773K, the interface between the oxide film and the unoxidized core fiber contained many microcracks; moreover, the core fiber lost 75% of its original strength at room temperature. It is quite possible that these defects were produced by a contraction due to transformation of cristobalite (α-β) during cooling from the oxidation temperatures.