Kinetics of the α-to-α'H Polymorphic Phase Transition of Ca2SiO4 Solid Solutions

The α-to-α'_H transition of Ca₂SiO₄ solid solutions (C₂S(ss)) is a nucleation and growth process. This process was shown on time–temperature–transformation (TTT) diagrams for C₂S(ss) with different concentrations of foreign oxides (Na₂O, Al₂O₃, and Fe₂O₃). The kinetic cutoff temperature and the activation energy for growth of the α'_H phase increase steadily with increasing concentration of impurities. The activation energy for nucleation also increases above 950°C. The α'_H phase, which exists in equilibrium with the α phase at 1280°C, is formed at a maximum rate at around 1100°C regardless of the chemical composition. The TTT diagrams enable us to predict, as a function of cooling rate, the phase constitution of C₂S(ss) at ambient temperature.     

Phases in the System Ba2SiO4-Ca2SiO4

The system Ba₂SiO₄-Ca₂SiO₄ was studied by heating mixtures of Ba₂SiO₄ and Ca₂SiO₄ at 1723 K. Six distinct phases resulted; they were examined by both X-ray diffraction and differential thermal analysis. The phases β -(Ba_0.05Ca_1.95)SiO₄ and α-(Ba_0.15Ca_1.85)SiO₄ are isostructural with corresponding high-temperature modifications of Ca₂SiO₄. The X phase (Ba_0.48Ca_1.52SiO₄) is orthorhombic, is a pure phase rather than a solid solution, and is defined for the first time in the present work. The T phase (Ba_1.31Ca_0.69SiO₄) is hexagonal and interpreted in terms of a unit cell with a doubled c parameter, in contrast with literature data.     

Further Consideration of Phases in the System Ba2SiO4–Ca2SiO4

The results of an investigation of the system Ba₂SiO₄–Ca₂SiO₄ by powder X-ray and electron diffraction suggest a greater complexity than supposed hitherto. The previously recognized phases α, α' h α' l , X, T, and the newly reported Y have room-temperature structures that are modulated distortions of hexagonal (or pseudohexagonal) parent structures. Each displays characteristic and distinctive modulations. The phases are more readily distinguished in this way than by their unit-cell dimensions and compositions which, for a given phase, can vary with bulk starting composition and thermal history.     

Preparation of Porous Ca10(PO4)6(OH)2 and β-Ca3(PO4)2 Bioceramics

Submicrometer-sized, pure calcium hydroxyapatite (HA, (Ca₁₀(PO₄)₆(OH)₂)) and β-tricalcium phosphate (β-TCP, Ca₃(PO₄)₂) bioceramic powders, that have been synthesized via chemical precipitation techniques, were used in the preparation of aqueous slurries that contained methyl cellulose to manufacture porous (70%–95% porosity) HA or β-TCP ceramics. The pore sizes in HA bioceramics of this study were 200–400 μm, whereas those of β-TCP bioceramics were 100–300 μm. The pore morphology and total porosity of the HA and β-TCP samples were investigated via scanning electron microscopy, water absorption, and computerized tomography.     

Crystal Field Studies of Co2+ in α-Zn3(VO4)2

Complete solid solubility was found in the system Cog-(VO₄)₂−α-Zn₃(VO₄)₂. Optical spectra of Co²⁺-containing α-Zn₃(VO₄)₂ samples are discussed in the light of crystal field theory. The calculated and theoretical frequencies were in good agreement for octahedral symmetry in the zinc sites. The crystal field parameter Dq was consistent with the ionic approximation rule. The nephelauxetic ratio did not show any relation to cation-anion distance. Most probably the expanding d-d electron clouds of Co²⁺ interacted.     

Electrical and Pyroelectric Properties of Highly (001)-Oriented (Pb0.76Ca0.24)TiO3 Thin Films Grown by a Sol–Gel Process

Highly (001)-oriented (Pb_0.76Ca_0.24)TiO₃ (PCT) thin films were grown on Pt/Ti/SiO₂/Si substrates using a sol–gel process. The PCT film with a highly (001) orientation showed well-saturated hysteresis loops at an applied field of 800 kV/cm, with remanent polarization ( P _r) and coercive electric field ( E _c) values of 23.6 μC/cm² and 225 kV/cm, respectively. At 100 kHz, the dielectric constant and dielectric loss values of these films were 117 and 0.010, respectively. The leakage-current density of the PCT film was 6.15 × 10⁻⁸A/cm² at 5 V. The pyroelectric coefficient ( p ) of the PCT film was measured using a dynamic technique. At room temperature, the p values and figures-of-merit ( F _D) of the PCT film were 185 μC/m²K and 1.79 × 10⁻⁵ Pa^−0.5, respectively.     

Structure and Microtexture Changes in Phosphorous-Bearing Ca2SiO4 Solid Solutions

The crystal structure and microtexture of P-bearing Ca₂SiO₄solid solutions (C₂S( ss )) were studied as a function of x = P/(Si + P) ranging from 0.085 to 0.398. All the samples were prepared at the stable-temperature region of the α' _l -phase and quenched in air. The structures were described in terms of the orthohexagonal or hexagonal cell of the former α-phase. The crystal with x = 0.085 was composed entirely of the orthorhombic α' _l -phase, the modulation wavelength of which was N = 3 along the c -axis. With x = 0.118 and 0.156, the crystal grains were made up of both α' _l and incommensurate orthorhombic phases. The volume fraction of the α' _l -phase decreased with increasing x . With x = 0.197, the crystal was made up exclusively of the incommensurate phase, with the modulation wave vector k given by (1/ N ) a^*+ c^*. A good correlation N = 4.370 – 2.50 x was observed between N (3.75 ≤ N ≤ 4.09) and x (0.118 ≤ x ≤ 0.250). The crystal with x = 0.3.98 consisted of a single hexagonal phase. The modulation wavelength was N = 2 along the a-axis and N = 3 along the c -axis.     

Hydration in the System Ca2SiO4–Ca3(PO4)2 at 90°C

Compositions along the Ca₂SiO₄–Ca₃(PO₄)₂ join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca₃(PO₄)₂ were fired at 1500°C, forming nagelschmidtite + a ¹-CaSiO₄, A -phase and silicocarnotite and a -Ca₃(PO₄)₂, respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca₂SiO₄≠ 0 and portlandite when Ca₂SiO₄ is >50%. Relative hydration reactivities are a -Ca₃(PO₄)₂ > nagelschmidtite > α ¹-Ca₂SiO₄ > A -phase > silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14-A tobermorite rather than calcium silicate hydrate gel.     

Formation Mechanism and Synthesis of Apatite-Type Structure Ba2+xLa8−x(SiO4)6O2−δ

The formation process of Ba₂La₈(SiO₄)₆O₂ was clarified using thermogravimetry–differential thermal analysis (TG-DTA) and a high-temperature powder X-ray diffraction (HT-XRD) method. Phase changes identified from the HT-XRD data surprisingly corresponded to the weight loss and/or endothermic peaks observed in the TG-DTA curves. Raw material with the composition Ba₂La₈(SiO₄)₆O₂ was completely reacted at 1400°C and produced only an apatite-type compound without a secondary phase. Moreover, the synthesis of Ba_{2+ x} La_{8− x} (SiO₄)₆O_2−δ crystals with x = 0–2 was attempted using a solid-state reaction.     

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.     

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.     

Determination of Precise Unit-Cell Parameters of the α-Tricalcium Phosphate Ca3(PO4)2 Through High-Resolution Synchrotron Powder Diffraction

Unit-cell parameters of the α-tricalcium phosphate [TCP; Ca₃(PO₄)₂] were investigated using high-resolution synchrotron powder diffraction and the Rietveld method. The diffraction experiment was conducted at 29°C at the BL-15XU experimental station of SPring-8, Japan. Precise unit-cell parameters of the α-TCP were obtained; a =12.87271 (9), b =27.28034(8), c =15.21275(12) Å, α=γ=90°, and β=126.2078(4)°. The calculated density of α-TCP (2.8677 g/cm³) is smaller than that of β-TCP, indicating the "looser" structure of α-TCP.     

Nucleation and Growth of α'-SiAlON on α-Si3N4

Morphology, composition, and growth defects of α'-SiAION have been studied in a fine-grained material with an overall composition Y_0.33Si₁₀Al₂O₁N₁₅ prepared from α-Si₃N₄, AlN, Al₂O₃, and Y₂O₃ powders. TEM analysis has shown that fully grown α'-SiAloN grains always contain an α-Si₃N₄ core, implicating heterogeneous nucleation operating in the present system. The growth mode is epitaxial, despite the composition and lattice parameter difference between α-Si₃N₄ and α'-SiAlON. The inversion boundary that separates two domains in the seed crystal is seen to continue in the grown α'-SiAION. Lacking a special growth habit, the growth typically proceeds from more than one site on the seed crystal, and the different growth fronts impinge on each other to give an equiaxed appearance of α'-SiAlON. Misfit dislocations on the α/α'interface are identified as [0001] type ( b = 5.62 Å) and 1/3 [1 2 10] type ( b = 7.75 Å). These nucleation and growth characteristics dictate that microstructural control of α'-SiAlON must rest on the size distribution of the starting α-Si₃N₄ powder.     

Structural and Thermophysical Properties of (PbO)0.5(SiO2)0.5 Glasses: Effect of SnO2 Incorporation

¹¹⁹Sn and ²⁹Si solid-state nuclear magnetic resonance studies on lead silicate glasses containing different amounts of SnO₂ confirmed that tin exists in the glass as distorted SnO₆ polyhedra and there is no direct interaction between tin and silicon structural units. Transmission electron microscopic studies have established that tin structural units are uniformly distributed in the glass. Significant changes in the values of glass transition temperature, microhardness, and thermal expansion coefficient with SnO₂ incorporation into the glass have been attributed to the increased rigidity of the glass network brought about by the replacement of weaker Pb–O linkages with stronger Sn–O linkages.     

High-Temperature Mechanical Properties of Ceramic Materials: V, (Zn0.65, Mg0.35)2SiO4

The composition (0.65Zn,0.35Mg)₂ SiO₁ was investigated. Its thermal expansion was 32 × 10^-7/°C from room temperature to 1000°C. Modulus of rupture was approximately 7000 psi between room temperature and 800°C, whereas Young's modulus held at approximately 11 × 10° psi over the same range. The substitution of 0.35 m oles Mg⁺⁺ for Zn⁺⁺ in Zn₂Si0₄ causes little change in many of the physical properties, but the solid solution sinters much more readily than pure Zn₂Sio₄. The willemite solid solution studied has very good thermal shock resistance between room temperature and 1000°C.     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

Electrical Conductivity of Polycrystalline Li2SiO3 and γ-LiAlO2

Conductivities of polycrystalline Li₂SiO₃ and γ-LiAlO₂ were measured by the two-terminal ac method at T =450° to 1000°C. Intrinsic and extrinsic regions were observed. The activation energies of conductivities in the extrinsic region for both samples agreed well with those of lithium diffusion obtained by T ₁ analysis of nuclear magnetic relaxation.     

Electrical Conduction in Sintered α-Sb2O4

Electrical conductivity and thermoelectric power were measured on sintered α-Sb₂O₄ at 250° to 780°C. Oxygen partial pressure dependence of the conductivity and sign of the Seebeck coefficient showed α-Sb₂O₄ to be a p -type semiconductor above 600°C in the oxygen pressure range of lo⁵ to 10² Pa. A hopping conduction was proposed from very small hole mobility with an activation energy of 18 kJ/mol.