Phase relationships in the zirconia-yttria system

Metastable and equilibrium phase relationships in the system ZrO₂:YO_1.5 have been studied by X-ray diffraction. The conditions for the retention of a zirconia-rich tetragonal phase at ambient temperature are established. The existence of a miscibility gap, closed below the solidus temperature, in the yttria-rich solid solution region is proposed. Some evidence for partially ordered phases is presented.     

Phase relationships in the system Si3N4-SiO2-La2O3

Phase relationships in the system Si₃N₄-SiO₂-La₂O₃ have been investigated after cooling from 1700° C. Two phases, 2Si₃N₄·La₂O₃ (monoclinic) and La₅ (SiO₄)₃N (hexagonal), were identified; the other two phases in the system, LaSiO₂N (monoclinic) and La₄Si₂O₇N₂ (monoclinic), were found to dissociate to La₅ (SiO₄)₃N and a glass after cooling from temperatures above 1650° C. The unit cells of 2Si₃N₄·La₂O₃, LaSiO₂N and La₄Si₂O₇N₂ have been determined and compared with those of preceding works. The results are discussed in relation to the intergranular phases observed when Si₃N₄ is sintered with La₂O₃ additions.     

Phase relationships in the system Si3N4-Y2O3-SiO2

Examination of compositions in the system Si₃N₄-Y₂O₃-SiO₂ using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at 1600° C at high SiO₂ concentrations (13 mol% Si₃N₄ + 19 mol% Y₂O₃ + 68 mol% SiO₂) and at 1650° C at high Y₂O₃ concentrations (25 mol % Si₃N₄ + 75 mol % Y₂O₃). Two ternary phases 4Y₂O₃ ·SiO₂ ·Si₃N₄ and 10Y₂O₃ ·9SiO₂ ·Si₃N₄ and one binary phase Si₃N₄ ·Y₂O₃ were observed. The 4Y₂O₃ ·SiO₂ ·Si₃N₄ phase has a monoclinic structure (a= 11.038 Å, b=10.076 Å, c=7.552 Å, =108° 40) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y₂O₃ ·9SiO₂ ·Si₃N₄ phase has a hexagonal unit cell (a=7.598 Å c=4.908 Å). Features of the Si₃N₄-Y₂O₃-SiO₂ systems are discussed in terms of the role of Y₂O₃ in the hot-pressing of Si₃N₄, and it is suggested that Y₂O₃ promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si₃N₄ at the solid-liquid interface.Visiting Research Associate at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433, under Contract No. F33615-73-C-4155 when this work was carried out.     

Phase relationships in the yttria-rich part of the yttria-zirconia system

Phase equilibria in yttria-zirconia mixtures containing not less than 50 mol % YO_1.5 have been studied between 1200 and 2200° C. The solubility of zirconia in yttria is shown to decrease with increasing temperature: the miscibility gap between the zirconia (fluorite) and yttria (type C rare earth) solid solutions extends from 64 to 75 mol % YO_1.5 at 1400° C, from 66 to 79% at 1700° C and from 73 to 85% at 2000° C. An ordered fluorite-related phase with the ideal formula Zr₃Y₄O₁₂ and a narrow homogeneity range, which disorders at about 1370° C, is reported.     

Phase relationships and properties in the VTe system

The V-Te phase diagram was investigated in the composition range V_0.5Te-V_1.25Te. The V₃Te₄ phase with a Cr₃S₄-type structure has a wide homogeneity range V_0.64Te-V_0.825Te at 773 K, showing the reversible transition to a CdI₂-type structure at temperatures above about 1000 K. The V₅Te₈ phase with a V₅S₈-type structure is stable below 720 K in the narrow range V_0.63Te-V_0.67Te. The tellurium-rich phase is separated into three structurally-related phases: the α-V_1+xTe₂ phase with a monoclinic CdI₂-like structure (low temperature phase), the β-V_1+xTe₂ phase with an unknown CdI₂-like structure and the γ-V_1+xTe₂ phase with a CdI₂-type structure (high temperature phase). The γ-V_1+xTe₂ phase spreads over to the high-temperature region of the V₃Te₄ phase. The magnetic and electrical properties of these compounds are also reported.     

Phase diagram and structure-property relationships in the lead-free piezoelectric system: Na0.5K0.5NbO3-LiTaO3

A phase-diagram for the Na(0.5)K(0.5)NbO(3)-LiTaO(3) solid solution series (NKN-LT) is presented for compositions ≤ 10 mol% LT, based on the combined results of temperaturevariable X-ray powder diffraction and dielectric measurements. In addition to the reported orthorhombic and tetragonal polymorphs of NKN-LT, a monoclinic phase is revealed. Changes to electrical properties as a function of LT substitution are correlated to phase content. Increasing the LT content from 5 to 7 mol% LT led to improved temperature stability of piezoelectric properties because of the avoidance of the monoclinic-tetragonal polymorphic phase transition during thermal cycling (at >25°C). For 7 mol% LT samples: d(33) = 200 pC/N; T(c) = 440°C; ε(r) = 550 and tan δ = 0.02 (at 20°C). Modification of this composition by solid solution with BiScO(3) led to a decrease in d(33) values. Transmission electron microscopy of a sample of 0.95[0.93 NKN-0.07LT]-0.05BiScO(3) indicated a core-shell grain structure which led to temperature-stable dielectric properties.     

Phase equilibria in the system BaTiO3-BaGeO3

The system BaTiO₃-BaGeO₃ has been investigated by DTA, metallographic and X-ray diffraction methods. A number of selected samples were also analysed on the electron probe micro-analyser. The system was found to be of the simple binary-eutectic type with partial solid solubility at both ends. The eutectic composition was established as 68±0.5 mol% BaGeO₃ and its melting temperature as 1120±5° C. The solid solubility of BaGeO₃ in BaTiO₃ at 1120°C was found to be 1.8 mol % and that of BaTiO₃ in BaGeO₃ 2.2 mol %. Additions of BaGeO₃ to BaTiO₃ did not appear to cause any change in the temperature of the cubic to hexagonal inversion of BaTiO₃ but the temperature of the tetragonal to cubic inversion was raised slightly with such additions. The transformation temperature of 1180° C for the low- to high-temperature form of BaGe0₃ was not affected by additions of BaTiO₃     

Phase relationships in the system Ni-W-O and thermodynamic properties of NiWO4

The phase diagram of the Ni-W-O system at 1200 K was established by metallographic and X-ray identification of the phases present after equilibration at controlled oxygen potentials. The oxygen partial pressures over the samples were fixed by metered streams of CO+CO₂ gas mixtures. There was only one ternary oxide, nickel tungstate (NiWO₄), in the Ni-W-O system at a total pressure of 1 atm, and this compound decomposed to a mixture of Ni+WO_2.72 on lowering the oxygen potential. The Gibbs' free energy of formation of NiWO₄ was determined from the measurement of the e.m.f. of the solid oxide galvanic cell, Pt, Ni+NiWO₄+WO_2.72/CaO-ZrO₂/Ni+NiO, Pt and thermodynamic properties of tungsten and nickel oxides available in the literature. For the reaction, NiO(s)+WO₃(s)NiWO₄(s) G°=–10500–0.708 T (±250) cal mol^–1.     

Phase relationships in the system HxMoO3 (0<x⩽2.0)

Four solid phases of H_xMoO₃ in the range 0<x?2.0 have been synthesised and characterised following Glemser and co-workers (1, 2, 3). Unit-cell dimensions for all phases have been determined from Guinier powder X-ray analysis. Ranges of homogeneity exist for three of the phases and approximate limits have been determined as follows: blue orthorhombic 0.23<x<0.4, blue monoclinic 0.85<x<1.04, red monoclinic 1.55<x<1.72.     

Phase relations in the MoO3-MgMoO4 system

Journal of Materials Science Letters -     

Phase relations in the BiI3-ZnI2 system

The liquidus diagram of the BiI₃-ZnI₂ system has been studied for the first time by differential thermal analysis, x-ray diffraction, and optical microscopy. The crystallization temperatures and melt compositions corresponding to eutectic and transition points have been determined, and a compound of composition ZnBiI₅, melting congruently at 400°C, has been identified. Some of its physicochemical properties have been studied.