Glassy Phase in Alumina–Silica Ceramic Shell Molds

Variation of the amount and the composition of the glassy phase in high-purity Al₂O₃-SiO₂ shell molds and the crystallization of the glassy phase have been studied. The results indicate that the two main modes of transformation from a glassy phase to a crystalline phase during firing are (1) formation of mullite through a chemical reaction and (2) devitrification of the glassy phase that comes from the original materials. The transformation is mainly influenced by the S/ ( A + S ) value ( A and S are the weight percentages of Al₂O₃ and SiO₂, respectively), the Na₂O content, and the firing temperature. Low S /( A + S ), low Na₂O, and high firing temperature are favorable to the transformation.     

Characterization and Sintering Behavior of Alkoxide-Derived Aluminosilicate Powders

Submicrometer SiO₂-Al₂O₃ powders with compositions of 46.5 to 76.6 wt% Al₂O₃ were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and Al-Si spinel at around 980°C. The compositions of mullite produced at 1400° and 1550°C were richer in Al₂O₃ than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO₂-Al₂O₃ system. Particle size decreased with increasing Al₂O₃ content. The sintered densities depended upon the amount of SiO₂-rich glassy phase formed during sintering and the green density expressed as a function of particle size.     

High-Temperature Strength and Cavitation Threshold of Silicon Nitride–Silica Ceramics

The role of high-purity silica in the fracture of Si₃N₄ at high temperatures has been investigated. The flexural strength at 1400°C was found to be greater than that at room temperature. Little plastic deformation was observed even when 10 wt% SiO₂ was added and the strain rate was decreased 2 orders of magnitude from that for a standard bend test. Microstructural observations revealed that the glassy phase was localized at intergranular pockets when SiO₂ additions were ≤ 10 wt%. High strength at 1400°C despite the presence of a fairly large amount of glassy phase is attributed to a high cavitation threshold in such glassy pockets consisting of high-purity SiO₂. However, the deformation behavior changed abruptly for SiO₂ additions of 10 and 20 wt%, which is explained by the morphological change of the glassy phase to thicker intergranular layers which allow macroscopic viscous flow.     

Preparation of a Calcium Titanium Phosphate Glass–Ceramic with Improved Chemical Durability

A calcium titanium phosphate glass–ceramic for use as a dental material with excellent chemical durability was derived from a mother glass with a small amount of fluorine. The laser Raman spectroscopic analysis showed that 35CaO–10CaF₂–30P₂O₅–25TiO₂ glass, as the nominal composition, consists of ortho-, pyro-, and meta-phosphate groups. On heating the glass at 865°C, orthophosphate crystals, such as fluorine-containing oxyapatite (Ca₁₀(PO₄)₆(O,F₂)) and the Nasicon-type phase (CaTi₄(PO₄)₆), were preferentially precipitated; the apatite particles of several tens of nanometers in size were embedded in the CaTi₄(PO₄)₆ phase. The pale bluish color of the glass–ceramic indicated that titanium ions were included in the residual glassy phase. When the glass–ceramic was treated with dilute hydrochloric acid, only the apatite particles at the surface were leached out, while no CaTi₄(PO₄)₆ phase was etched; the dissolution of the glass–ceramic was effectively controlled. Almost no dissolution of ions from the glass–ceramic occurred in water. It was suggested that the behavior is a result of the microstructure of the glass–ceramic, which consists of crystalline and glassy phases with excellent chemical durability.     

Influence of Microstructure and Temperature on the Interfacial Fracture Energy of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

The microstructure and interfacial fracture energy of silicon nitride/boron nitride fibrous monoliths, Gamma_BN, were determined as a function of starting silicon nitride composition and temperature using the method described by Charalambides. The glassy phase created by the sintering aids added to the silicon nitride cells was shown to migrate into the boron nitride cell boundaries during hot-pressing. The amount of glassy phase in the boron nitride cell boundaries was shown to strongly influence Gamma_BN at room temperature, increasing the fracture energy with increasing amounts of glass. Similar trends in the interfacial fracture energy as a function of temperature were demonstrated by both compositions of fibrous monoliths, with a large peak in Gamma_BN observed over a narrow temperature range. For silicon nitride cells densified with 6 wt% yttria and 2 wt% alumina, the room-temperature interfacial fracture energy was 37 J/m², remaining constant through 950°C. A sharp increase in Gamma_BN, to 60 J/m², was observed between 1000° and 1050°C. This increase was attributed to interactions of the crack tip with the glassy phase in the boron nitride cell boundary. Measurements at 1075°C indicated a marked decrease in Gamma_BN to 39 J/m². The interfacial fracture energy decreased with increasing temperature in the 1200° to 1300°C regime, plateauing between 17 to 20 J/m². A crack propagation model based on linkup of existing microcracks and peeling/cleaving boron nitride has been proposed.     

Silica Glass Segregation in 3 wt% LiF-Doped Hot-Pressed Y2Si2O7

Hot-pressed yttrium disilicate ceramics have been characterized using analytical transmission electron microscopy (TEM). The microstructure consists of large grains of the γ phase of stoichiometric γ-Y₂Si₂O₇ containing rounded glassy Y-doped SiO₂ inclusions; excess glassy SiO₂-rich material is also found at the grain boundaries. Two main reasons are found for the inhomogeneity: a slight SiO₂ excess is inferred from the composition measurements, and the LiF flux used in hot pressing would also promote glass formation. Improved high-temperature mechanical properties would only be possible if residual glass formation was minimized, strategies for doing so are discussed, and the importance of analytical TEM for studying such submicron scale inhomogeneity is underlined.     

Magnetic Behavior and Microstructure of Vanadium Phosphate Glasses

The magnetic properties and microstructures of the vanadium phosphate glass system over the composition range 60 to 90 mol% V₂O₅ were investigated to study magnetic ordering in the glass and the effect of microstructure on its magnetic properties. Direct antiferromagnetic coupling between V⁴⁺ ions in the glassy matrix exists, and a transition temperature near - 70°C was observed. As-cast glasses with high V₂O₅ concentrations separated into two glassy phases; this separation increased the ESR line width as a result of inhomogeneity broadening. The separation, which concentrated the vanadium ions in a vanadium-rich phase, caused a hysteresis in the plot of ESR line intensity vs temperature at the transition temperature. Reduction of the vanadium ions by dextrose added to the melt enhanced phase separation and resulted in weak antiferromagnetic transitions at +70° and -120°C, the Neel temperatures of VO₂ and V₂O₃, respectively.     

Heat-Resistant Silicon Carbide with Aluminum Nitride and Erbium Oxide

Fully dense SiC ceramics with high strength at high temperature were obtained by hot-pressing and subsequent annealing under pressure, with AlN and Er₂O₃ as sintering additives. The ceramics had a self-reinforced microstructure consisting of elongated SiC grains and a grain-boundary glassy phase. The strength of these ceramics was ∼550 MPa at 1600°C, and the fracture toughness was ∼6 MPa·m^1/2 at room temperature. The beneficial effect of the new additive composition on high-temperature strength might be attributable to the introduction of aluminum from the liquid composition into the SiC lattice, resulting in a refractive grain-boundary glassy phase.     

Ionic Conductivity of the System Si-Al-O-N from 850° to 1400°C

Electrical conductivity was measured from 850° to 1400°C for β-sialon and pure X phase as well as for the sintered system Si₃N₄-Al₂O₃, containing β-sialon, X phase, β-Si₃N₄, and glassy phase. Ionic conductivity was measured at >1000°C. The charge carriers were identified by electrolysis. The results showed that pure β-sialon is ionically conducting because of Si⁴⁺ migration for the temperature range studied. Pure X phase shows ionic conduction by Si⁴⁺ above 1000°; below 1000°C, it shows electronic conduction because of impurities. The conductivity of the sintered system Si₃N₄-Al₂O₃ containing β-sialon, β-Si₃N₄ X phase, and glassy phase changes as the relative quantities of β -sialon and X phase change. The apparent activation energies for the ionic and electronic conductivities are 45 and 20 kcal/mol, respectively.     

Sintering Additives to Eliminate Interphases in Cordierite Ceramics

Effects of the sintering additives CeO₂ and K₂O on eliminating interphases of cristobalite and spinel were studied by means of XRD, SEM/energy-dispersive spectrometer, and TMA techniques to obtain high cordierite contents. Adding 4 wt% CeO₂ significantly stimulates the conversion of cristobalite to cordierite, depresses spinel formation and yields less glassy phase. It is helpful to prepare a ceramic with a high content of cordierite and a low thermal expansion coefficient when sintered at 1400°C for 3 h. Addition of 1–2 wt% K₂O results in a large amount of glassy phase and is less effective in eliminating the interphases.     

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.     

Modification of the Hydrofluoric Acid Leaching Technique: I, Corundum–Mullite–Glassy Phase Materials

A method to modify the HF leaching technique is proposed. By the use of this method, the error caused by the dissolution of mullite can be measured and removed from the analysis results, and satisfying accuracy of the results can be reached. Experimental comparison shows that the accuracy of the modified HF leaching technique is much higher than that of the unmodified HF leaching technique and the X-ray diffraction technique when these techniques are used to determine the amount of glassy phase in Al₂O₃–SiO₂ ceramics. An example is given to show how to use the modified HF leaching technique.     

Stability of Mullite as Derived from Equilibria in the System CoO-Al2O3-SiO2

The free energy of reaction for the formation of mullite from its oxide components was derived from equilibrium studies in the system CoO-Al₂O₃-SiO₂. Within this system there appears, at solidus temperature in a certain composition area, the phase assemblage mullite + silica + spinel (= cobalt aluminate) + liquid. Determination of the oxygen pressure of a gas phase at which metallic cobalt precipitates from this phase assemblage and from the phase assemblage spinel (= cobalt aluminate) + corundum in the system CoO-Al₂O₃ permits calculation of ΔG° for the reaction 3Al₂O₃+ 2SiO₂= Al₆Si₂O₁₃. The value obtained at 1422°C is -5.8 kcal.     

Sintering of Industrial Mullites in the Presence of Magnesia as a Sintering Aid

The sinterability of two industrial mullite powders, in the presence of MgO as a sintering aid, was investigated. A glassy phase, which was generated during preparation, was present in both powders; this glassy phase had a strong influence on sintering, depending on its content, composition, and spatial distribution. MgO promoted sintering in the presence of a liquid phase, both in the as-received materials and in samples washed with HF, in which most of the pre-existing glassy phase was eliminated. Investigations using transmission electron microscopy, coupled with energy-dispersive spectroscopy, as well as dilatometric measurements and X-ray diffraction data, on washed and unwashed materials and on quenched and slow-cooled samples allowed a better understanding of the influence of MgO and the glassy phase on the sintering behavior and the formation of new phases. Most of the phases, in fact, can be explained by using the MgO–Al₂O₃–SiO₂ phase diagram, even in such complex systems.     

Evaluation of Glassy Phase in Sintered Silicon Nitride by Low-Temperature Specific Heat Measurements

The specific heat of HIP sintered Si₃N₄ with 3 mol% Y₂O₃ and 3 mol% Al₂O₃ additives was measured at different temperatures ranging from 2 to 10 K, in order to confirm the presence of a glassy phase in the sintered body. The grainboundary glassy phase in the sintered Si₃N₄ was evaluated by specific heat measurements. The difference between the experimental value and the lattice specific heat calculated from the Debye theory confirmed the existence of a glassy phase in sintered Si₃N₄.     

Elastic Effect on Domain Morphology and Kinetics of Spinodal Decomposition in the Tetragonal System

A method was proposed for calculation of the effect of elastic strain on spinodal decomposition in the tetragonal system. An effective free energy for the composition field was derived by eliminating elastic fields which are coherently induced by composition inhomogeneities It was shown that anisotropic long-range interactions between composition fields play a major role in determining both the domainmorphology and domain growth law of spinodal decomposition with the coherence of the lattice. Computers simulations were performed on the basis of a two-dimensional model by taking into account those long-range interactions. The results demonstrated the appearance of lamella structure and its coarsening in the late stage of the phase separation The calculation for the TiO₂-SnO₂ system showed slow coarsening due to the anisotropic elastic long -range interactions The asymptotic growth of the lamella size was described by λα tⁿ, where n is 0.18.     

Microstructural Evolution in Fast-Heated Cordierite-Based Glass-Ceramic Glazes for Ceramic Tile

The crystallization mechanism of α-cordierite from a B₂O₃- and TiO₂-containing glass submitted to fast heating in the cordierite primary phase field of the CaO–MgO–Al₂O₃–SiO₂ quaternary system was investigated. Addition of B₂O₃ to a SiO₂-rich glass suppressed the formation of μ-cordierite. This suppression facilitated densification by viscous flow before crystallization. Powder X-ray diffractometry, field-emission electron scanning microscopy, and energy-dispersive X-ray analysis revealed that α-cordierite nucleated directly from glass on the boundaries of original particles and was probably favored by TiO₂, which acted as a nucleant. The growth kinetics of α-cordierite crystals was fast, and the crystals seemed to grow by consuming glass directly from the growing interphase. The estimated amount of α-cordierite in the glass heated at 1160°C was 69.5 wt%, as determined using the Alegre method. The final microstructure consisted of an arrangement of well-shaped hexagonal prisms, with sizes <3 μm, immersed in a residual glassy phase. The feasibility to develop new glass-ceramic glazes using the present and previous works is considered.     

Phase Equilibria and Superconducting Properties of BiSr2YCu2O7 (1212 Phase)

Phase equilibria of the quasi-quaternary system BiO_1.5–SrO-YO_1.5–CuO have been studied at a temperature of 950°C in air, with special regard to the 1212 phase. The 1212 phase reveals only very small changes in the cation ratio. Single-phase samples exist for (Bi_0.24–0.36Cu_0.42-0.55)–Sr₂Y_1.27Cu₂O _y compositions. The bismuth-rich composition of the 1212 phase is in thermodynamic equilibrium with a liquid and the 2212 phase, whereas the copper-rich composition is in equilibrium with five other phases. The influence of combined calcium and lead doping also has been studied. Exceeding the calcium saturation of the 1212 phase increases the amount of 2212 as a secondary phase. Single-phase 1212 samples do not show any superconductivity in either the as-prepared or the post-annealed state. The only compositions with bulk superconductivity are those with calcium and lead doping after annealing at a temperature of 980°C. The superconductivity is attributed to the 2212 phase crystallizing from the melt during slow cooling.     

Microstructure of 96% Alumina Ceramics: I, Characterization of the As-Sintered Materials

Characterization of the microstructure and microchemistry of a group of commercial 96% Al₂O₃ ceramics, using analytical and conventional electron microscopy techniques, was conducted. A continuous glassy grain-boundary phase was found in all samples, in addition to a number of intragranular and intergranular crystalline second phases; the phases present depended on the original boundary-glass composition. The faceted nature of many of the Al₂O₃-glass interfaces was studied and is thought to be an equilibrium structure.     

Glass-Reduced SiAlONs with Improved Creep and Oxidation Resistance

Ceramics containing α-SiAlON with improved high-temperature properties such as thermal stability and creep and oxidation resistance were synthesized. The influence of starting composition on the amount of residual grain-boundary phase has been explored. Results have been interpreted in terms of compatibility phase relationships in the Si₃N₄–AlN–Al₂O₃–Re₂O₃ system and transient evolutions.