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 SiAlON Ceramics Using Composite β-SiAlON–Glass Adhesives

Commercial β-SiAlON ceramics were joined using mixed Si₃N₄, Y₂O₃, Al₂O₃, and SiO₂ powders. At a joining temperature of 1600°C and a hold time in excess of 10 min, the adhesive was converted to an approximate 60:40 vol% composite of β-SiAlON–glass-ceramic. The grain size of the acicular β-SiAlON grains precipitated in the joint (submicrometer diameter, average aspect ratio of 10) was significantly smaller than those in the adherend ceramic (1–5 μm diameter). Intergrowth of β-SiAlON grains at the joint interface resulted in high bond strengths. The chemistry and microstructure of the ceramic adhesives used are described.     

Applicability of Optical Scattering Model to β-Quartz Solid Solution Glass–Ceramics with Nanoscale Crystalline Phase

The applicability of Rayleigh-Gans and quasi-continuum scattering models to β-quartz solid solution (s.s.) glass–ceramics was studied. We evaluated the validity of these models using two specimens with different microstructures. The refractive indices of the crystalline and glass phases of the specimens were calculated by a method utilizing the transmittance change of the specimens due to structural relaxation. It was found that the quasi-continuum model is applicable to the glass–ceramics for estimating scattering intensity and its microstructure dependence. We demonstrated that the optical transmittance of the glass–ceramics can be simulated as a function of the microstructure on the basis of this model. The result of this study provides a quantitative approach for controlling the optical transmittance of β-quartz s.s. glass–ceramics.     

Nucleation and Growth of β'-SiAlON

Elongated β'-SiAlON grains grown from several finegrained Y_m/3Si_12(m+n)Al_m+nO_nN_16–r compositions with α-Si₃N₄, AlN, Al₂O₃, and Y₂O₃ starting materials have been examined. These grains have large aspect ratios and are oriented along the [0001] axis. TEM structural and chemical analysis suggests that they are nucleated from various seed crystals, which can be α-Si₃N₄, β-Si₃N₄, or other β'-SiAlON. The β'-SiAlON seed and the initial precipitation on β-Si₃N₄ show a higher content of Al and O, indicating that a large transient supersaturation of Al and O in the liquid is instrumental for β'-SiAlON formation, whereas subsequent growth proceeds under a much lower driving force. The misfit between phases is accommodated by interfacial dislocations ( c -type and a -type). Fully grown β'-SiAlON grains usually contain several variants independently nucleated from the same seed. In particular, the two alternative α/β phase-matching possibilities result in two [0001] growth habits separated by a twin boundary.     

Formation and Stability of β-Quartz Solid-Solution Phase in the Li-Si-Al-O-N System

The development of crystalline phases in lithium oxynitride glass-ceramics was examined, with particular emphasis placed on the effect of the nitrogen source (AlN or Si₃N₄) on the formation and stability of a β-quartz solid-solution ( ss ) phase. Oxynitride glasses derived from the Li-Si-Al-O-N system were heat-treated at temperatures up to 1200°C to yield glass-ceramics in which β-quartz( ss ) and β-spodumene( ss ) of approximate composition Li₂OAl₂O₃4SiO₂ formed as major phases and in which X-phase (Si₃Al₆O₁₂N₂) and silicon oxynitride (Si₂N₂O) were present as minor phases. The nitrogen-containing β-quartz( ss ) phase that was prepared with AlN was stable at 1200°C; however, the use of Si₃N₄ as the nitrogen source was significantly less effective in promoting such thermal stabilization. Lattice parameter measurements revealed that AlN and Si₃N₄ had different effects on the crystalline structures, and it was proposed that the enhanced thermal stability of the β-quartz( ss ) phase that was prepared with AlN was due to both the replacement of oxygen by nitrogen and the positioning of excess Al³⁺ ions into interstitial sites within the β-quartz( ss ) crystal lattice.     

Enhanced Surface Crystallization of β-Barium Borate on Glass Due to Ultrasonic Treatment

β-Barium borate (BBO) surface crystallized glass composites were prepared for a glass of composition 40BaO·15Tio₂–45B₂O₃ (in mol%). Enhanced nucleation was attained by ultrasonic surface modification with an aqueous suspension of BBO particles, resulting in dense BBO thin films on the glass surfaces. Second harmonic generation signals were observed for samples subjected to ultrasonic treatment.     

High-Temperature Oxidation Behavior of High-Purity α-, β-, and Mixed Silicon Nitride Ceramics

High-temperature oxidation behavior, microstructural evolution, and oxidation kinetics of additive-free α-, β-, and mixed silicon nitride ceramics is investigated. The oxidation rate of the ceramics depends on the allotropic ratio; best oxidation resistance is achieved for ceramics rich in α-phase. Variations in the oxidation kinetics are directly related to average grain size and glass distribution in the oxidation scale. The oxygen contents incorporated into the Si₃N₄ phase before its dissolution at the oxidation front affects the local glass composition and thereby yields nucleation and growth rates of SiO₂ crystallites within the glass phase and a final oxidation scale microstructure, which depend on the incorporated oxygen contents. For the α-polymorph, the dynamic oxygen solubility is found to remain negligible; therefore, a nitrogen-rich glass forms at the oxidation front, which promotes devitrification and yields a scale with small grain size and thin intergranular glass films. β-Si₃N₄ is observed to form oxygen-rich solid solutions on oxidation, which are in contact with silicon oxynitride or oxygen-rich glass. Nucleation of cristobalite in the latter is sluggish, yielding coarse-grained oxidation scales with thick intergranular glass film.     

The Mechanical Properties of β-SiAlON Ceramics Joined Using Composite β-SiAlON-Glass Adhesives

The mechanical properties of β-SiAlON ceramics joined using β-SiAlON-glass-forming adhesives consisting of mixed Si³N⁴, Y²O³, Al²O³, and SiO² powders are described. Use of adhesives with a β-SiAlON:glass ratio of 60:40 gave an optimum joint strength of 650 MPa in four-point bending mode, i.e., 85% of that of unbonded material, when joining was carried out at 1600°C for 10 min, under an applied uniaxial pressure of 2 MPa. Bonding pressures in excess of 2 MPa caused excessive compressive creep distortion during the joining operation. The strengths of postjoined HIPed material and HIPed, unbonded material, differed by only 4%, i.e., 975 and 1010 MPa, respectively, which indicates that HIPing reduces the size of critical defects in the joint. Fracture toughness of the joint also improved upon HIPing.     

转炉渣微晶玻璃热处理制度研究

本文以60%转炉渣和40%硅石为原料,采用熔融法制备了微晶玻璃.借助XRD,SEM和DTA等分析手段研究了晶化和核化时间对微晶玻璃析晶行为和性能的影响.在不同的晶化和核化时间下,微晶玻璃的晶相均为钙铁辉石和铁透辉石,晶体均为球形.随着晶化时间的延长,晶体析出量增加,微晶玻璃的平均粒径和显微硬度均呈先增大后减小的趋势,当晶化时间为75 min时达到最大值,分别为0.44 μm和733 Hv;随着核化时间的延长,晶体尺寸逐渐减小,晶核数量逐渐增加,显微硬度变化平缓.     

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.     

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.     

Nucleation and Crystallization of a Lithium Aluminosilicate Glass

An aluminosilicate glass of composition 61SiO₂6Al₂O₃10MgO6ZnO·12Li₂O·5TiO₂ (mol%) has been prepared by a melting process and investigated as far as crystallization is concerned. Glass-ceramic is easily obtained because glass shows a high tendency to crystallize starting from 700°C. The crystalline phases evolve with temperature, showing the aluminosilicates to be the main phase up to 1050°C, followed by metasilicates and silicates, some of which have lower melting points. The titanates of Mg and Zn develop from the phase-separated glass, soon after T _g, and grow to form nucleation centers for the other crystalline phases. The evolution from phase-separated glass to glass-ceramic has been followed by many thermal, diffractometric, spectroscopic, and microscopic techniques.     

Reaction Hot Pressing of α'- and β'- SiAlON Ceramics

Hot pressing kinetics of α-Si₃N₄, AIN, Al₂O₃, and Y₂O₃ powder mixtures forming α'- and β'-SiAlONs have been studied. Densification proceeds in two steps, first by a small shrinkage upon ternary eutectic oxide melting (SiO₂–Al₂O₃–Y₂O₃) at 1340°C, followed by a massive particle rearrangement and further shrinkage at higher temperature when nitride dissolution begins. With better wettability, AIN initially traps the oxide melt and delays densification. In addition, the preferential dissolution of AIN at 1450°C enriches the melt composition in AI, triggering transient precipitation of supersaturated β'-SiAlON. Full densification is readily achieved at 1550°C without complete α-Si₃N₄ conversion.     

Duplex α,β-Sialon Ceramics Stabilized by Dysprosium and Samarium

Duplex αβ,-sialon ceramics with a minimum volume fraction of residual intergranular glass have been prepared using Dy or Sm as the α-sialon stabilizing element. These microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix. A number of the larger α-sialon grains contained dislocations and showed a core/shell structure. Dy gave an α-sialon which was stable over a wide temperature range (1350–1800°C) for long holding times, while the use of Sm resulted in less stable α-sialon structures at medium temperatures (1450°C) and the formation of melilite, R₂Si_3−xAl_xO_3+xN_4−x, β-sialon, and the 21R sialon polytype during prolonged heating. High α-phase contents gave a very high hardness ( H _V10 is approximately 22 GPa) but a comparatively low indentation fracture toughness (around 4.4 MPam^1/2). Duplex sialons fabricated from powder mixtures corresponding to an α-to-β sialon ratio of around 50:50 resulted in a sialon material with a favorable combination of high hardness (around 22 GPa) and increased toughness (to around 5.5 MPam^1/2).     

Nonuniformity of Potassium Ions in β-Alumina Ceramics

A nonuniform distribution of K was found in β -alumina that had been in contact with sodium polysulfide or sodium nitrate melts containing small amounts of K. Large β -alumina grains in contact with the melt had approximately the equilibrium concentration of K expected from studies on single crystals of β -alumina, but surrounding grains contained little or no K. Thus the mobility of K ions in grain boundaries in β -alumina appears to be very low. Stresses from K concentration at the ceramic surface could enhance other incipient sources of failure such as seal stresses, cracks, and other defects.     

Fine-Grained Silicon Nitride Ceramics Prepared from β-Powder

Fine β-powder with an average particle size of 0.28 μm was prepared by grinding and centrifugal sedimentation of sub-micrometer β-powder. Fine- and uniform-grained ceramics were fabricated from the powder by hot pressing. The average grain size of the ceramic was 0.21 μm. It was shown that this kind of microstructure was desirable for the matrix of in situ composite. It was also shown that the ceramics could be superplastically deformed at a temperature as low as 1500°C.     

Synthesis and Properties of Porous Single-Phase β'-SiAlON Ceramics

Single-phase β'-SiAlON (Si_{6− z} Al _z O _z N_{8− z} , z = 0–4.2) ceramics with porous structure have been prepared by pressureless sintering of powder mixtures of á-Si₃N₄, AlN, and Al₂O₃ of the SiAlON compositions. A solution of AlN and Al₂O₃ into Si₃N₄ resulted in the β'-SiAlON, and full densification was prohibited because no other sintering additives were used. Relative densities ranging from 50%–90% were adjusted with the z -value and sintering temperature. The results of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses indicated that single-phase β'-SiAlON free from a grain boundary glassy phase could be obtained. Both grain and pore sizes increased with increasing z -value. Low z -value resulted in a relatively high flexural strength.     

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.     

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.