Spherulitic Growth from a Phase-Separated Vitreous Matrix in a Cordierite–Y-Stabilized Zirconia Glass-Ceramic

Crystallization behavior, especially spherulitic growth of a cordierite–3 mol% Y₂O₃–ZrO₂ glass composition annealed at 950° to 1370°C was studied by optical and electron microscopy. Microstructural analysis revealed that glass-inglass phase separation occurred prior to the crystallization of β-quartz solid solution ( ss ). ZrO₂ was involved in promoting the nucleation of β-quartz ss ; the crystallization evolved from a cellular to a spherulitic morphology, and β-quartz transformed to α-cordierite as the temperature was raised. ZrO₂ was eventually expelled intergranularly and intragranularly to form slightly misoriented domains upon annealing, and the dendritic or clustered tetragonal t -ZrO₂ crystals transformed to monoclinic ( m ) in symmetry. A possible dragging force exerted between α-cordierite and a glassy droplet phase is proposed to explain the corrugated cordierite/glass interface.     

Carbon Interfacial Layers Formed by Oxidation of SiC in SiC/Ba-Stuffed Cordierite Glass-Ceramic Reaction Couples

Reaction couples between α-SiC and cordierite (2MgO·2Al₂O₃·5SiO₂═ Mg₂Al₄Si₅O₁₈) were prepared by sandwiching (and enclosing) SiC single crystals between plates of Ba-stuffed magnesium aluminosilicate (Ba-MAS) glass and hot-pressing; the Ba-MAS was subsequently crystallized at 1000° to 1200°C in argon or air. No reaction occurred at the SiC/Ba-MAS interfaces during hot-pressing, but crystallization heat treatments caused formation of amorphous carbon reaction layers at the SiC/cordierite interfaces, due to concurrent oxidation via the reaction SiC + O₂→ SiO₂+ C. The thickness of the carbon of the carbon layer was variable. These results suggest that formation of C layers at SiC/silicate interfaces in other composites (containing Nicalon fibers, for example) depends more on thermochemistry and less on the details of SiC nonstoichiometry than has heretofore been supposed.     

Reactions in the System Li2O–MgO–Al2O3–SiO2: I, The Cordierite-Spodumene Join

Phase equilibria along the nonbinary join between cordierite (2MgO · 2Al₂O₃· 5SiO₂) and spodumene (Li₂O · Al₂O₃· 4SiO₂) were investigated in the temperature range 800° to 1550°C. using the quench technique on fourteen compositions. The phase diagram at high temperatures is characterized by a very small region of solid solution on the cordierite side, appreciable solid solution on the spodumene side, and regions of three and four phases toward the center of the system, including liquid, α-cordierite, mullite, spinel, corundum, and β-spodumene and its solid solutions. The liquidus has a flat minimum between 40 and 50% cordierite at 1347°, and rises on one side to the congruent melting point of β-spodumene (1421°) and on the other side to the temperature of complete melting of cordierite (1530°). The lowest temperature at which liquid appears is 1325°. At low temperatures a complete series of metastable solid solutions exists between μ-cordierite and β-spodumene. The significance of the data in the preparation of thermal-shock-resisting bodies is discussed.     

Crystallization behavior and sintering of cordierite synthesized by an aqueous sol–gel route

The purpose of the research was to investigate crystallization behavior and sintering of cordierite synthesized by a low-price aqueous sol–gel route starting from silicic acid and magnesium and aluminum salts. Viscous sintering of the gel occurred in the temperature range of 800–850 °C, followed by μ-cordierite crystallization at about 900 °C, which proves the homogeneity of the gel. Decreasing of μ-cordierite crystallinity in a wide temperature range prior to commencing of α-cordierite crystallization at about 1200 °C indicates reconstructive type of μ- → α-cordierite transformation. The transformation was fully completed at 1350 °C. The value of the Avrami parameter indicates that μ-cordierite crystallization was controlled by surface or interface nucleation, which implies that viscous sintering occurred in the primary gel particles, which leads to shrinkage, and thereafter nucleation occurred on the surface or interface of the particles. The overall activation energy of μ-cordierite crystallization was 382.0 kJ/mol. The sinterability of the powder obtained by calcination at 1300 °C, where well-crystallized α-cordierite was formed, was better than that of the powder obtained by calcination at 850 °C, where the most intensive shrinkage occurred before the onset of crystallization of μ-cordierite.     

Influences of Zirconia and Silicon Nucleating Agents on the Devitrification of Li2O · Al2O3· 6SiO2 Glasses

The effects of Si and ZrO₂ dopants on the crystallization and phase transformation process in Li₂O · Al₂O₃· 6SiO₂ glasses were investigated using differential thermal analysis, X-ray powder diffractometry (XRD), and high-resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase-separated regions in the as-quenched or heat-treated glass devitrified at ∼760°C, which in turn provided sites for the heterogeneous nucleation and growth of β-quartz(ss) (solid solution), which transformed to β-spodumene(ss) at higher temperature. Low-temperature surface crystallization in these glasses occurred as low as 760°C. ZrO₂ has limited solubility in this glass system. Small ZrO₂ crystallites (·5 nm) in the as-quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large (<5 μm) β-quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO₂. The transformation from β-quartz(ss) to β-spodumene(ss) was increasingly inhibited with ZrO₂ additions. The nucleating efficiency of Si was significantly greater than that of ZrO₂ in this glass system.     

Effect of Heat-Treatment Temperature on the Properties of a Lithium Aluminosilicate Glass

The effects of isochronal heat treatments on the properties of a lithium aluminosilicate glass have been measured. Heat treatment of the base glass at temperatures between 700° and 1100°C initially results in phase separation of the glass, followed by the formation of β-quartz solid solution ( ss ) at about 850°C. The β-quartz( ss ) converts to β-spodumene in the temperature range between 950°C and 1000°C. The dependence of the properties of the material on the heat-treatment temperature is controlled by phase separation below 850°C, by the rapid crystallization of β-quartz( ss ) between 850°C and 900°C, and by the lack of change in morphology with heat treatment above 900°C. The effects of heat treatment on the properties of this material are explained on the basis of the changes in the composition and morphology of the residual amorphous and crystalline phases.     

Alkoxide Sol-Gel-Processed Cordierite Fiber

An alkoxide sol-gel route was developed to prepare stoichometric cordierite fibers. The influences of the aging treatment and heating rate on the sinterability of the gel fibers were also examined. X-ray diffraction analysis revealed that the unaged and aged fibrous gels all remained amorphous <800&, but began crystallization into μ-cordierite and α-cordierite at ∼900°C and 1050°C, respectively; single-phase α-cordierite fibers were obtained at 1300°C. Heating the unaged fibers yielded denser microstructures, with fine grain sizes of ∼0.2–0.4 μm, whereas the aged fibers exhibited porous microstructures following heating at 1300°C. A higher heating rate and aging treatment resulted in a higher open porosity of the fired fiber.     

Effects of Annealing Temperature on Microstructural Development at the Interface Between Zirconia and Titanium

The interfacial reaction layers in the Ti/ZrO₂ diffusion couples, isothermally annealed in argon at temperatures ranging from 1100° to 1550°C for 6 h, were characterized using scanning electron microscopy and transmission electron microscopy, both attached with an energy-dispersive spectrometer. Very limited reaction occurred between Ti and ZrO₂ at 1100°C. A β'-Ti(Zr, O) layer and a two-phase α-Ti(O)+β'-Ti(Zr, O) layer were found in the titanium side after annealing at T ≥1300°C and T ≥1400°C, respectively. A three-phase layer, consisting of Ti₂ZrO+α-Ti(O, Zr)+β'-Ti (O, Zr), was formed after annealing at 1550°C. In the zirconia side near the original interface, β'-Ti coexisted with fine spherical c- ZrO_{2− x} , which dissolved a significant amount of Y₂O₃ in solid solution at T ≥1300°C. Further into the ceramic side, the α-Zr was formed due to the exsolution of Zr out of the metastable ZrO_{2− x} after annealing at T ≥1300°C: the α-Zr was very fine and dense at 1300°C, continuously distributed along grain boundaries at 1400°C, and became coarsened at 1550°C. Zirconia grains grew significantly at T ≥1400°C, with the lenticular t -ZrO_{2− x} being precipitated in c -ZrO_{2− x} . Finally, the microstructural development and diffusion paths in the Ti/ZrO₂ diffusion couples annealed at various temperatures were also described with the aid of the Ti–Zr–O ternary phase diagram.     

Processing Aluminum Nitride-Silicon Carbide Composites via Polymer Infiltration and Pyrolysis of Polymethylsilane, a Precursor to Stoichiometric Silicon Carbide

AlN-SiC-particle-reinforced composites have been prepared at lessthan equal to1400°C using submicrometer AlN, -325 mesh alpha-SiC particles, and polymethylsilane (PMS; -(CH₃SiH) _n -) via a polymer infiltration and pyrolysis (PIP) process. PMS is an organometallic SiC polymer precursor that can be modified with 16 wt% cross-linking aid to provide mPMS. mPMS converts to nanocrystalline β-SiC with >80% ceramic yield (1000°C in argon) with some excess (<5 wt%) graphitic carbon. mPMS has been used successfully as a nonfugitive binder for AlN-SiC compacts. Densities of 2.5 versus 2.1 g/cm³ have been obtained after nine PIP cycles for disk-shaped compacts formulated with and without mPMS binder, respectively. alpha-SiC seeds crystallization of β-SiC derived from mPMS at temperatures as low as 1000°C. Some evidence suggests that AlN-SiC solid solutions form at particle/matrix interfaces.     

Microstructural Control of a 70% Silicon Nitride– 30% Barium Aluminum Silicate Self-Reinforced Composite

The processing response of a 70% silicon nitride–30% barium aluminum silicate (70%-Si₃N₄–30%-BAS) ceramic-matrix composite was studied using pressureless sintering, at temperatures ranging from 1740°C, which is below the melting point of BAS, to 1950°C. The relationship between the processing parameters and the microstructural constituents, such as morphology of the β-Si₃N₄ whisker and crystallization of the BAS matrix, was evaluated. The mechanical response of this array of microstructures was characterized for flexural strength, as well as fracture behavior, at test temperatures up to 1300°C. The indentation method was used to estimate the fracture resistance, and R -curves were obtained from modified compact-tension samples of selected microstructures at room temperature.     

Classical and Differential Thermal Analysis Studies of the Glass-Ceramic Transformation in a YSiAlON Glass

Nucleation and crystallization studies were conducted on a YSiAlON glass that contained 17 equiv.% nitrogen (7.5 at.%) by using a two-stage nucleation-and-growth treatment. Classical and differential thermal analysis (DTA) techniques were both used to study the crystallization process, to ensure that the optimum heat-treatment schedule that yielded a fine microstructure and minimum residual glass was applied. The optimum nucleation and crystallization temperatures were determined from DTA traces that were recorded from isothermal heat treatments at different nucleating temperatures, ranging between T _g - 40°C and T _g+ 100°C for 1 h and crystal-growth temperatures in the range of 1170°-1310°C for 0.5 h, respectively. The activation energy for the crystallization process was determined, based on the analysis of the variation of peak temperature at five different heating rates. Specimens heat treated in a tube furnace under nitrogen gas were subjected to microscopical investigation, and results showed variations in the volume fraction of crystalline phases and crystal size with nucleation temperature. The nucleation temperature, T _g+ 40°C (1025°C), which corresponded to the maximum volume fraction of crystalline phases and minimum crystal size, was consistent with the optimum nucleation temperature, T _g+ 35°C (1020°C), as determined from DTA. The time and temperature of nucleation and crystal growth dictated the nature and size of the crystalline phases. Properties such as hardness and density were assessed and correlated to the nucleation temperature. The influence of sample specific surface on the devitrification mechanisms was estimated, and bulk nucleation was observed to be the dominant nucleation mechanism.     

Infiltration of Reaction-Bonded Silicon Nitride with Equilibrium Y-Si-O-N Melt

An equilibrium Y-Si-O-N melt was infiltrated to eliminate the open porosity of reaction-bonded silicon nitride at 1600–1800°C. This oxynitride melt contained two equilibrium phases, a β-Si₃N₄ solid phase and a liquid phase at high temperatures. Before infiltration, porous reaction-bonded silicon nitride compacts were heat-treated to completely transform to the β-Si₃N₄ phase. After infiltration, the flexural strength of the reaction-bonded silicon nitride material increased from 200 to 600 MPa at 25°C, from 200 to 300 MPa at 1400°C in air.     

Preparation of Synthetic Cordierite by Solid-State Reaction via Bismuth Oxide Flux

A comparative study of the solid-state reaction synthesis of cordierite with and without the use of a flux was performed, varying the sintering temperatures between 900° and 1400°C. Bi₂O₃ proved to be a useful additive for lowering the temperature needed for the reaction to take place. The deviation from the ordered β-cordierite phase was analyzed using the distortion index Δ as a factor of merit. The values obtained for Δ in all compositions showed that cordierite tended to a much-ordered structure with increased sintering temperatures.     

Fabrication and properties of cordierite based glass/AlN composites by sol–gel and pressureless sintering

In this study, the effect of AlN content on the crystallization behavior of cordierite based glass, was firstly investigated. Results show that μ-cordierite appeared in the composites with high AlN content even at high temperatures, which implied that the AlN may broad the crystallization temperature range of μ-cordierite and depress the transformation of μ→α-cordierite. The crystallization temperature of α-cordierite was about 980 °C for the pure glass and the temperature increased with AlN content for composites, but the crystallization temperature of μ-cordierite had reverse trend. The composites owned excellent bending strength when the AlN content was 20 wt%. With increasing of AlN content, the dielectric loss was increased which was caused mainly by the structural loss and the appearance of μ-cordierite, but the dielectric constant had crosscurrent. It was observed that the composites were beneficial in producing LTCC material which can be highlighted with high strength, low shrinkage and good dielectric properties at 1 MHz.     

Synthesis of Yttrium Aluminum Garnet from a Mixed-Metal Citrate Precursor

Yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized using a polymeric precursor derived from a mixed-metal citric acid/ethylene glycol/ethanol solution. YAG was found to crystallize directly from an amorphous precursor beginning at temperatures as low as 600°C within 1 h in air. The polymer resin concentration was found to have an effect on the temperature of crystallization initiation. However, all precursors produced a well-crystallized YAG powder within 1 h at 800°C in air. Formation of phase-pure YAG in an argon atmosphere did not occur until heating for 1 h at 1000°C. An optimum cation-to-resin ratio to maximize reactivity provides a polymeric network to ensure a homogeneous dispersion of cations, yet minimizes cation diffusion distances within the char by limiting excess free carbon after polymer pyrolysis.     

Silica Transformations in the System PbO-SiO2

Mixtures of PbO and silicic acid were fired at 710°, 820°, and 910°C for periods up to 1000 hr in air. At each temperature cristobalite and tridymite crystallized rapidly, with the greatest development in samples with compositions of 1 mole PbO to 50 and 60 moles SO₂. At 820° and 91O°C tridymite continued to crystallize at the expense of cristobalite. At 710°C quartz formed, after a nucleation time of approximately 200 hr, at the expense of cristobalite and tridymite. The equilibrium assemblages were cristobalite and tridymite at 820° and 910°C and quartz and PbSiO₂ at 710°C. The results are interpreted in terms of the tentative phase relations outlined by Holmquist for the high-silica regions of binary silicate systems. Phase development curves and lattice parameter measurements characterize the initially formed cristobalite as a metastable solid solution containing approximately 2 mole % PbO. Equilibrium was approached by exsolution of PbO from the cristobalite structure.     

Nucleation and Growth of Sodium Disilicate Crystals in Sodium Disilicate Glass

A high-temperature microscope for use with transmitted or reflected light up to × 400 magnification and up to 1400°C. was constructed. Nucleation and crystal growth in sodium disilicate glass was studied with this microscope. Heterogeneous nucleation occurred at the interface between the glass and the platinum heating element from 450° to 870° C. with a maximum at 600° C. Heterogeneous nucleation also occurred at the glass-atmosphere interface from 25° to 375° C. as a result of surface reactions with water vapor. No homogeneous nucleation was observed. Linear growth rates of α and β sodium disilicate crystals were measured from 600° to 870° C. in two sodium disilicate glasses. Metastable growth of both α and β was observed, and growth rates of α and β were the same at a given temperature for a given glass. An equation relating fluidity and undercooling to growth rate was applied.     

Highly Reactive β-Dicalcium Silicate

Calcium silicate hydrate was prepared by hydrothermal reaction between calcium oxide and silica (C/S = 2.0) at a temperature of 205°–215°C and a pressure of 17–19 bar. This reaction with decomposition at 900°C produced highly reactive β-dicalcium silicate (specific surface area 4.55 m²/g) contaminated with small amounts of wollastonite as an impurity. Infrared spectral studies have shown that β-dicalcium silicate prepared at 900°C is less symmetric compared with the control prepared at 1450°C using boron trioxide as a stabilizer. The specific surface area of β-dicalcium silicate decreased with temperature. The hydration studies were done by determining the nonevaporable water, calcium hydroxide (CH) contents, and specific surface area of the hydrated samples. X-ray diffraction studies were also done. The results showed that prepared β-dicalcium silicate is highly reactive. Calcium chloride (1.0 wt%) and gypsum retard the hydration. Possible causes of high reactivity have been discussed.     

Synthesis and Structural Characterization of a Metastable Mullite-Like Alumina Phase

A solution combustion technique for the synthesis of different β-alumina compositions in the Na₂O· x Al₂O₃ system (where x =5, 6, or 7) is described along with the structural characterization of the materials prepared. The amorphous powder obtained after a combustion reaction between the nitrate salts of the cations and aminoacetic acid was calcined in air at different temperatures in the range from 600°C up to 1300°C. The phases were investigated by powder X-ray diffraction (XRD) and infrared spectroscopic measurements. A metastable mullite-like alumina phase was found to form as an intermediate at a minimum calcination temperature of 750°C and stable up to 1000°C, which transformed completely into β/β"-alumina phases beyond a temperature of 1100°C. The crystal structure of the mullite-like alumina phase was deduced by rietveld refinement of slow scan powder XRD data. A better understanding of the crystal structure of the mullite-like alumina was possible using other supplementary experimental evidences.     

Effect of P2O5 on the Crystallization of Lead Silicate Glasses

The effect of P₂O_S on the devitrification of binary lead silicate glasses containing 64 and 59 mol% PbO was studied. Glasses underwent isothermal crystallization treatments at 400°, 450°, 500°, and 550°C. A polymorph of 3PbO₂SiO₂ was the major product of crystallization for all compositions of glasses. Secondary products of crystallization were found to be a polymorph of PbOSiO₂ in the 59 mol% and a low-temperature polymorph of 2PbOSiO₂ in the 64 mol% PbO glasses. Dominant mode of crystallization in both binary glasses was surface devitrification at all temperatures studied. Addition of P₂O₅ promoted internal crystallization in the form of spherulites. 400°C was found to be the most effective temperature for nucleating spherulitic growth. Crystallization at 400°C led to high concentrations of spherulites in all glasses containing at least 0.5 mol% P₂O₅. Concentrations of 0.5 mol% P₂O₅ were needed to produce detectable levels of spherulitic nucleation at r<400°C.