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.     

In Situ Raman Indentation of β-Eucryptite: Characterization of the Pressure-Induced Phase Transformation

A pressure-induced phase transformation in the lithium aluminum silicate β-eucryptite was studied with in situ Raman spectroscopy. Dense β-eucryptite composites were made via powder synthesis followed by sintering. The specimens were then subjected to diamond indentation up to applied contact stresses of about 8 GPa, while collecting Raman spectra. The appearance of a Raman peak (∼520 cm⁻¹) at a contact stress of about 3 GPa likely corresponds to the reversible phase transformation of β-eucryptite to the orthorhombic phase ɛ-eucryptite. Loading and unloading in situ Raman indentation experiments are discussed with regards to this transformation.     

Dissolution Kinetics of β-Si3N4 in an Mg-Si-O-N Glass

The rate of dissolution of β-Si₃N₄ into an Mg-Si-O-N glass was measured by working with a composition in the ternary system Si₃N₄-SiO₂-MgO such that Si₂N₂O rather than β-Si₃N₄ was the equilibrium phase. Dissolution was driven by the chemical reaction Si₃N₄(c)+SiO₂( l )→Si₂N₂O(c). Analysis of the kinetic data, in view of the morphology of the dissolving phase (Si₃N₄) and the precipitating phase (Si₂N₂O), led to the conclusion that the dissolution rate was controlled by reaction at the crystal/glass interface of the Si₃N₄, crystals. The process appears to have a fairly constant activation energy, equal to 621 ±40 kJ-mol⁻¹, at T=1573 to 1723 K. This large activation energy is believed to reflect the sum of two quantities: the heat of solution of β-Si₃N₄ hi the glass and the activation enthalpy for jumps of the slower-moving species across the crystal/glass interface. The data reported should be useful for interpreting creep and densification experiments with MgO-fluxed Si₃N₄.     

Topotactically Oriented α-Si3N4 Cores in Sintered β-Si3N4

α-Si₃N₄ core structures within β-Si₃N₄ grains have been studied by transmission electron microscopy. The grains were dispersed in an oxynitride glass which was previously melted at 1600°C. The cores were topotactically related to the as-grown β-Si₃N₄ crystallites and are related to epitactical nucleation during heat treatment as the most probable mechanism.     

Defects and Charge Transport in β-Ta2O5: I, Analysis of the Conductivity Observed in Nominally Pure β-Ta2O5

Simple ionic defect theory has not been successful in describing the observed defect and transport behavior in β-Ta₂O₅. The available data were reviewed, and a revised model is proposed which more adequately describes the defect behavior of nominally pure or lightly doped β-Ta₂O₅.     

Quantitative Microscopy of β-Si3N4 Ceramics

The three-dimensional grain size distribution in an experimental β-Si₃N₄ material has been determined using the hexagonal prism as a model of β-Si₃N₄ grain shape. Results from quantitative microscopy of polished and etched sections were compared with computer-generated two-dimensional stereological parameters of hexagonal prisms with different aspect ratios in order to determine an average grain shape (i.e., aspect ratio) in the microstructure. Section parameter distributions for the average grain shape were obtained from the computer simulations and used in a three-dimensional reconstruction of the microstructure. The results showed that this Si₃N₄ ceramic had the postulated fibrous microstructure and a broad grain size distribution.     

Low-Temperature Synthesis of β-Ca2SiO4 from Hillebrandite

Experiments on hydrothermal synthesis were conducted using quartz or silicic acid and lime as starting materials at Ca/Si = 2.0. It is possible to synthesize pure hillebrandite (Ca₂(SiO₃)(OH)₂) having the theoretical composition by heating at 200°C for 10 h or at 250°C for 5 h. The synthesized product is fibrous, open at each end, and has a length of 20 to 30 μm. Calcium silicate hydrate gels are produced at the initial stage of the reaction. These react further with the unreacted lime to give hillebrandite. However, when silicic acid is used as silica, hillebrandite with tricalcium silicate hydrate is observed at 250°C because of the high reaction rate of silica. On heating, hillebrandite starts to decompose at about 500°C and produces low-crystalline β-Ca₂SiO₄, which is stable at room temperature and has a remarkably large specific surface area of about 7 m²/g. The decomposition reaction rate in a single crystal is rapid, and the reaction is considered to proceed topotactically.     

Kinetics of β-Si3N4 Grain Growth in Si3N4 Ceramics Sintered under High Nitrogen Pressure

The kinetics of anisotropic β-Si₃N₄ grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β-Si₃N₄ grains follows the empirical growth law Dⁿ– D₀ⁿ = kt , with the exponents equaling 3 and 5 for length [001] and width [210] directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β-Si₃N₄ growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperature and time.     

β-Si3N4Whiskers Embedded in Oxynitride Glasses: Interfacial Microstructure

Interfacial microstructures in βP-Si₃N₄( w )-Si-Al-Y-O-N-glass systems were investigated by systematically varying the nitrogen content and the Al:Y ratio of the glass matrix. High-resolution and analytical transmission electron microscopy (HREM and AEM) studies revealed that the interfacial microstructure is a function of the glass composition. No interfacial phases were formed in glasses with low Al:Y ratios and in glasses with high Al:Y ratios and low nitrogen content, whereas epitaxial growth of an interfacial layer (100–200 μm thick) on the βP-Si₃N₄( w ) occurred in a glass matrix with high Al:Y ratio and high nitrogen content. The interfacial layer was identified to be a β'-SiAION phase. Interfaces containing the SiAION layer exhibited high debonding energy compared to Si₃N₄( w )–glass interfaces. HREM studies indicated that the lattice-mismatch strain in the SiAION layer was relieved by dislocation formation at the SiAION–Si₃N₄( w ) interface. The difference in interfacial debonding energy was, hence, attributed to the local atomic structure and bonding between the glass-β-Si₃N₄ and the glass–β'-SiAION phases. This observation was clear evidence of the strong influence of glass chemistry on the interfacial debonding behavior by altering the interfacial microstructure.     

Kinetics of the β"-to-β Transformation in the System Na2O-A12O3

Mixtures of α-A1₂O₃ and NaAlO₂ corresponding to the eutectic composition 65 mot % A1₂O ₃ were heated in sealed platinum crucibles above the eutectic temperature (measured to be 1572°C in this study). Once melting was complete, the liquid was quenched and the resultant solid examined by X-ray diffraction. Sodium aluminate, beta alumina, and beta" alumina were detected in all cases. However, the relative amounts of beta and beta" present were a function of the cooling rate, with the amount of beta" increasing with a decreasing cooling rate. Subsequent annealing (in platinum envelopes) of the solidified eutectic at 1400° and 1530°C showed that the amount of beta" present decreased with increasing annealing time. Based on this observation, it is apparent that, in the temperature range studied, beta"-is metastable. Analysis of the time-transformation data shows that the reaction kinetics are consistent with the diffusion-controlled growth of the beta phase at the expense of the beta"-phase.     

Thermal Conductivity of β-Si3N4: II, Effect of Lattice Oxygen

Dense β-Si₃N₄ with various Y₂O₃/SiO₂ additive ratios were fabricated by hot pressing and subsequent annealing. The thermal conductivity of the sintered bodies increased as the Y₂O₃/SiO₂ ratio increased. The oxygen contents in the β-Si₃N₄ crystal lattice of these samples were determined using hot-gas extraction and electron spin resonance techniques. A good correlation between the lattice oxygen content and the thermal resistivity was observed. The relationship between the microstructure, grain-boundary phase, lattice oxygen content, and thermal conductivity of β-Si₃N₄ that was sintered at various Y₂O₃/SiO₂ additive ratios has been clarified.     

Calculation of Grain-Boundary Bonding in Rare-Earth-Doped β-Si3N4

First-principles molecular orbital calculations are performed by the discrete variational Xalpha method using model clusters of rare-earth-doped β-Si₃N₄ and the interface between prismatic planes of β-Si₃N₄ and intergranular glassy films. On the basis of the total overlap population of each cluster, the rare-earth ions are implied to be stable in the grain-boundary model, while they are not stable in the bulk model. These results are consistent with experimental observations showing significant segregation of Ln³⁺ ions at the grain boundary and no solubility of Ln³⁺ into bulk β-Si₃N₄. Grain-boundary bonding is weakened with an increase of the ionic radius of the rare-earth ions, which provides a reasonable explanation for the ionic size dependence of the crack propagation behaviors as well as the growth rate of the prismatic plane in the rare-earth-doped β-Si₃N₄ during liquid-phase sintering.     

Mechanical Properties of β-Si3N4-Whisker/Si3N4-Matrix Composites

Composites containing 30 vol%β-Si₃N₄ whiskers in a Si₃N₄ matrix were fabricated by hot-pressing. The composites exhibited fracture toughness values between 7.6 and 8.6 MPa · m^1/2, compared to 4.0 MPa · m^1/2 for unreinforced polycrystalline Si₃N₄. The improvements in fracture toughness were attributed to crack wake effects, i.e., whisker bridging and pullout mechanisms.     

Synthesis of β-Si3N4 by Chemical Vapor Deposition

Beta-type CVD-Si₃N₄ plates (up to 1.1 mm thick) have been prepared by adding TiCl₄ vapor to the system SiCl₄-NH₃-H₂ at deposition temperatures of 1350° to 1450°C, while α-type or amorphous CVD-Si₃N₄ was obtained without TiCl₄ vapor at the same deposition temperature. Three to four wt % 777V was included in the β-type CVD-Si₃N₄ matrix. The density, preferred orientation, and lattice parameters of β-type CVD-Si₃N₄ were examined.     

Anisotropic Thermal Expansion of β-Ca2SiO4 Monoclinic Crystal

Crystals of β-Ca₂SiO₄ (space group P 12₁/ n 1) were examined by high-temperature powder X-ray diffractometry to determine the change in unit-cell dimensions with temperature up to 645°C. The temperature dependence of the principal expansion coefficients (α_i) found from the matrix algebra analysis was as follows: α₁= 20.492 × 10⁻⁶+ 16.490 × 10⁻⁹ ( T - 25)°C⁻¹, α₂= 7.494 × 10⁻⁶+ 5.168 × 10⁻⁹( T - 25)°C⁻¹, α₃=−0.842 × 10⁻⁶− 1.497 × 10⁻⁹( T - 25)°C⁻¹. The expansion coefficient α₁, nearly along [302] was approximately 3 times α₂ along the b -axis. Very small contraction (α₃) occurred nearly along [     01]. The volume changes upon martensitic transformations of β↔α_L' were very small, and the strain accommodation would be almost complete. This is consistent with the thermoelasticity.     

Defects and Charge Transport in β-Ta2O5: II, Analysis of the Conductivity of Nb2O5-doped β-Ta2O5

The model proposed to explain the defect behavior of nominally pure β-Ta₂O₅ was extended to the case of defect equilibria and charge transport in β-Ta₂O₅ doped with a multivalent cation, i.e., Nb₂O₅-doped P-Ta₂O₅. This case is of practical interest in the electronics industry, where Nb₂O₅, is a dominant impurity in capacitor-grade β-Ta₂O₅.     

High-Resolution Electron Microscopy of Chemically Vapor-Deposited β-Si3N4-TiN Composites

Microstructures of Si₃N₄-TiN composites prepared by chemical vapor deposition (CVD) were investigated by the multibeam imaging technique using a 1 MV electron microscope. High-resolution images showed a number of fibrous TIN crystallites dispersed in the matrix of CVD β-Si₃N₄. Crystallographic orientation relations between β-Si₃N₄ and TiN were determined directly from the observed images in the subcell scale. The fibrous axis of TiN is parallel to the (110) direction of the NaCl structure and lies along the c axis of the hexagonal β-Si₃N₄ crystal. Domain boundaries, planar faults, nonplanar faults, and dislocations were found in the CVD β-Si₃N₄ matrix near the TiN crystallites. The origin of the structure defects is briefly discussed in connection with the formation of TiN crystallites in the matrix.     

Formation of β-Si3N4 Coatings by Chemical Vapor Deposition

BetaSi₃N₄ coatings were obtained by chemical vapor deposition in a fused-silica reaction tube by outside heating of the system SiCl₄-NH₃-N₂ at a deposition temperature (reaction tube temperature) of 1300°C, whereas α- and α+β-phase coatings were obtained at depositon temperatures of 1150° and 1250°C, respecively. Formation of β-phase coatings at relatively low temperatures is explained in terms of the effect of a catalytic impurity, SiO vapor from the reaction tube. The X-ray diffraction patterns and sulface morphologies of the coatings were studied.