Sinterable β-Spodumene Glass-Ceramics

β-Spodumene glass-ceramic compositions were melted and studied. Compositional variations were made in the three major components as well as through minor additions of other oxides. Sintering characteristics and microstructures were studied and values for crystallization temperature, melting temperature, and crystallized thermal expansion coefficient were recorded. It was found that sinterable β-spodmene glass-ceramics could be made with a wide range of properties. Selection of an appropriate composition would be based on desired properties.     

Preparation and Properties of Glass-Ceramics Derived from Nitrogen-Containing β-Spodumene Glasses

Crystallization of Li-Al-Si-O-N oxynitride glasses based on the β-spodumene composition and the properties of the resultant glass-ceramics have been studied. The onset of the precipitation of metastable high-quartz solid solution and its transformation to β-spodumene shift to higher temperatures with increasing nitrogen content of the oxynitride glasses. Nitrided glass-ceramics crystallized at 1200°C have negative thermal expansion coefficients, since high-quartz structure is maintained up to 1000° and 1200°C. Knoop hardness and density of the glass-ceramics increase with increasing nitrogen content. There was evidence that part of the nitrogen atoms were incorporated into the high-quartz solid-solution structure and that a small amount of the minor phase of Si₂N₂O was precipitated in highly nitrided glass-ceramics.     

Mullite–β-Spodumene Composites from Aluminosilicates

Sintering studies were conducted using kaolin, metakaolin, zeolite 4A, and various synthetic mixtures of Al₂O₃ and SiO₂ in the presence of Li₂CO₃ and LiCl as fluxing agents. Various compositions of the above were prepared, and conventional sintering studies were conducted at temperatures of 900°–1450°C with soaking periods of 1–3 h. Kaolin, metakaolin, and amorphized kaolin in the presence of Li₂CO₃ showed nucleation centers of β-spodumene as pink specks, whereas synthetic mixtures of Al₂O₃ and SiO₂ failed to behave in the same manner. To determine whether the pink specks formed were color centers or F centers, the samples were subjected to UV, IR, and X-ray irradiation; however, the samples showed no tenebrescence properties. External addition of iron as an impurity in a nonlayered system also resulted in pink speck formation. This observation indicated that impurities present in the natural kaolin were the cause of this phenomenon. Moreover, the LiCl-based samples did not result in pink specks, even though the kaolinitic samples contained iron as an impurity. Therefore, although β-spodumene was formed in aluminosilicates in the presence of Li₂CO₃ and LiCl, the pink variety of β-spodumene (kunzite) formation occurred only in the presence of lithium-rich aluminosilicates and in the presence of iron as an impurity. The phase identification and microstructure were explained based on XRD, DTA, and SEM studies.     

Thermal Expansion of Synthetic β-Spodumene and β-Spodumene—Silica Solid Solutions

Synthetic β-Spodumene and several β-Spodumene-silica solid solutions were prepared, and their thermal expansion behavior was studied using high-temperature X-ray diffraction techniques. Data indicate that all β-Spodumenes are characterized by a pronounced anisotropy of thermal expansion much the same as that of the silica polymorph keatite. Anisotropy increases slightly and the volume expansion decreases as the compositions become richer in SiO₂. Specimens with more than 80.8 wt% SiO₂ at 130O°C and 1 atm contain P-cristobalite along with β-Spodumene. The lattice parameters of β-Spodumene compositions throughout the solid solution range were determined. The structural aspects of the anisotropy of thermal expansion are discussed.     

Preparation of Whisker β-Spodumene Glass–Ceramics

β-spodumene glass–ceramics with a whisker-like microstructure were prepared from the following materials (in wt%): 64.5 SiO₂, 18.0 Al₂O₃, 4.2 Li₂O, 4 ZrO₂, and 8 MgF₂. Scanning electron microscope (SEM) analysis showed that phase separation in the base glass leads to the formation of a primary crystal phase of MgF₂ that promotes the formation of spherical β-spodumene. Whisker spodumene crystals surrounded by spherical crystals are observed at 720°C after 1 h, and the whisker crystals grow at the cost of spherical crystals with increasing temperature and time. The flexural strengths of the glass–ceramics reach a maximum of 228 MPa after heat treatment at 850°C for 1 h.     

Kinetics of Dissolution and Crystallization in a β-Spodumene Glass-Ceramic

The kinetics of partial melting in a β-spodumene glass-ceramic was measured by holding fully crystallized material above the solidus but below the liquidus temperature. The kinetics was apparently limited by the rate of diffusion of silica in the crystalline phase. The measured lattice diffusivity of silica was 8×10⁻¹⁰ exp(-251 kJ mol⁻¹ /RT ) m² s⁻¹ with a possible error of ±60 kJ mol⁻¹ in the activation energy. The partially molten samples were heat-treated below the solidus to obtain the kinetics of crystallization, which was found to be interface-reaction-controlled.     

Crystallization and Chemical Strengthening of Stuffed β-Quartz Glass-Ceramics

Metastable solid solutions with the β-quartz structure can be crystallized from most glasses in the system SiO₂-Mg(AlO₂)₂-LiAlO₂ as well as from many containing the additional components Zn(AlO₂)₂ Al(AlO₂)₃, Li₂ZnO₂, and Li₂BeO₂. Internal nucleation is afforded by additions of ZrO₂ or TiO₂. Either transparent or opaque crystalline materials can be formed from glasses containing about 70% SiO₂. The transparency is due to a combination of low birefringence in the major stuffed β-quartz phase and minute crystal size. Thermal expansions vary from -20 to +50 × 10⁻⁷/°C. Thermal stability is highly variable. Breakdown products include spinel, cordierite, β-spodumene, willemite, mullite, and cristobalite. Magnesian compositions can be strengthened by a 2Li⁺⇌ Mg²⁺ ionexchange reaction. Abraded flexural strengths range from 30,000 to 160,000 psi.     

Joining of Calcium Phosphate Invert Glass-Ceramics on a β-Type Titanium Alloy

A bioactive calcium phosphate invert glass-ceramic containing β-Ca₃(PO₄)₂ crystals could be joined strongly with a Ti–29Nb–13Ta–4.6Zr alloy consisting of a β-titanium phase by heating the metal on which the mother glass powders with a composition 60CaO·30P₂O₅·7Na₂O·3TiO₂ (mol%) were placed, at 800°C for 1 h in air; the tensile joining strength was estimated to be ∼26 MPa on average. A compositionally gradient layer was developed on the metallic substrate during the heating. When the metal with glass powders on it was heated at 850°C in air, the phosphate glassy phase flowed viscously, permeating the oxide layer formed around the surface of the metal, which was thicker than that formed by heating at 800°C; a compositionally gradient layer was not developed, and a strong joining was not realized. The joining between the glass-ceramic and the metal is suggested to be controlled by viscous flow of the glassy phase in the glass-ceramic and by reaction of the glassy phase with the oxide phase formed around the surface layer of the metal.     

Superplastic Behavior of Fine-Grained β-Silicon Nitride Material under Compression

The deformation behavior of a hot-pressed, fine-grained β-Si₃N₄ ceramic was investigated in the temperature range 1450°—1650°C, under compression, and the results for strain rate and temperature dependence of the flow stress are presented here. The present results show that the material is capable of high rates of deformation (∼10⁻⁴—10⁻³ s⁻¹) within a wide range of deformation temperatures and under a pressure of 5—100 MPa; no strain hardening occurs in the material, even at slow deformation rates, because of its stable microstructure; Newtonian flow occurs, with a stress exponent of approximately unity; and the material has activation energy values for flow in the range 344—410 kJ·mol⁻¹. Grain-boundary sliding and grain rotation, accommodated by viscous flow, might be the mechanisms of superplasticity for the present material.     

Diffusion Bonding of Silicon Nitride Using a Superplastic β-SiAlON Interlayer

A superplastic β-SiAlON was used as an interlayer to diffusionally bond a hot-pressed silicon nitride to itself. The bonding was conducted in a graphite furnace under a constant uniaxial load of 5 MPa at temperatures varying from 1500° to 1650°C for 2 h, followed by annealing at temperatures in the range of 1600° to 1750^oC for 2 h. The bonds were evaluated using the four-point-bend method at both room temperature and high temperatures. The results indicate that strong, void-free joints can be produced with the superplastic β-SiAlON interlayer, with bond strengths ranging from 438 to 682 MPa, and that the Si₃N₄ joints are heat resistant, being able to retain their strength up to 1000°C (635 MPa), and therefore have potential for high-temperature applications.     

Effect of Sintering Additives on Superplastic Deformation of Nano-Sized β-Silicon Nitride Ceramics

The effect of sintering additives on superplastic deformation of nano-sized β-Si₃N₄ ceramics has been studied by compression tests at 1500°C. The sintering additives were (i) Y₂O₃+Al₂O₃; (ii) Y₂O₃+MgO; and (iii) Y₂O₃. Nano-sized Si₃N₄ ceramics with different sintering additives had similar microstructures. For the first two sintering additives, the stress exponents were determined to be ∼2 at a lower stress region and ∼1 at a higher stress region, where the strain rate was dependent on sintering additives only at the higher stress region, and was independent at the lower stress region. Nano-ceramics with Y₂O₃ additives had only one region, which had a stress exponent of ∼1 within the stress range that we studied. The results could be explained by the different deformation mechanisms at the higher and lower stress regions and the influence of viscosity of liquid phase on the transition stress.     

Effects of Microstructure on Superplastic Behavior and Deformation Mechanisms in β-Silicon Nitride Ceramics

The influence of grain shape and size on superplastic behavior and deformation mechanisms was investigated in annealed β-silicon nitride materials and compared with the results for hot-pressed material. The microstructure of the annealed materials consisted of fine equiaxed β-grains together with some elongated ones. Similar to the deformation behavior in the hot-pressed material, strain hardening did not occur in these annealed materials. Moreover, in contrast to the deformation behavior under tension, grain alignment under compression resulting from the development of a mild texture did not give rise to strain hardening. An annealed material with small elongated grains had a flow-stress dependency of n = 1, whereas other annealed materials with large elongated grains exhibited a flow-stress dependency of n = 1.6. In terms of texture development and the effect of grain shape on the creep rate when diffusion was the rate-controlling mechanism, a single curve with a stress exponent of ∼1 and a grain-size exponent of 3 were obtained for all materials. This suggests that the deformation mechanism in these annealed materials was the same as that of fine equiaxed β-silicon nitride.     

Hydration of Sodium β- and β'-Aluminas

The enthalpy and entropy for the hydration of sodium β - and β '-aluminas have been determined by measuring the infrared absorbance of single-crystal samples following equilibration at elevated temperatures in water vapor pressures ranging from 3 to 70 kPa. The equilibrium water concentrations can be varied continuously and reversibly up to saturation concentrations of about 0.9H₂O-Na_1.2AI₁₁O_17.1 for the β-phase and 0.4H₂O.Na_1.67Mg_.67Al_10.33O₁₇ for the β'-phase. The hydration reactions are exothermic; Δ H =−56 ± 2 for sodium β '-aIumina and ΔH =−67 ± 2 kJ/mol for sodium β-alumina. The entropies of hydration are about −143 ± 6 J/(mol-K) for both sodium β - and β '-aIuminas.     

Edge-Defined Film-Fed Growth of β-Alumina and Mg-Substituted β-Alumina

High Na vapor pressure, peritectic decomposition, and high reactivity of the melt complicate the growth of α-alumina crystals. These difficulties were overcome by using a high-pressure (300 psig) growth chamber, Na₂O-rich melts, and Ir for all surfaces in contact with the melt. These procedures were combined with the edge-defined film-fed growth technique to produce single-crystal β alumina tubes and ribbons.     

Phase Equilibrium Investigations on Na–K–(β+β")-Alumina

The activities of Na₂O and K₂O dissolved in mixed-alkali Na–K–(β+β")-Al₂O₃ (NKBA) have been determined by using yttria stabilized zirconia (YSZ) as a solid electrolyte in the following galvanic cells: The approach enables to verify in situ the establishment and maintenance of the β/β"-equilibrium, and to characterize it as a function of the phase composition of NKBA. The results can be expressed as follows:      

Topotactic Growth of β-Alumina on α-Alumina

Thin foils of polycrystalline α-alumina were reacted with a potassium-rich vapor at ≤900°C. Potassium β-alumina formed along α-alumina grain boundaries and protruded from holes in the foils. Conventional transmission electron microscopy was used to analyze the α-alumina/β-alumina phase boundary for possible orientation relations.     

Effect of α to β (β') Phase Transition on the Sintering of Silicon Nitride Ceramics

By using α-Si₃N₄ and β-Si₃N₄ starting powders with similar particle size and distribution, the effect of α-β (β') phase transition on densification and microstructure is investigated during the liquid-phase sintering of 82Si₃N₄·9Al₂O₃·9Y₂O₃ (wt%) and 80Si₃N₄·13Al₂O₃·5AIN·5AIN·2Y₂O₃. When α-Si₃N₄ powder is used, the grains become elongated, apparently hindering the densification process. Hence, the phase transition does not enhance the densification.