Vibrational Spectra of Ca3SiO5: Ultraviolet Laser Raman Spectroscopy at High Temperatures

The Raman spectra of Ca₃SiO₅ have been successfully obtained at temperatures up to 1353 K by a continuous-wave ultraviolet Raman spectroscopic system. The Raman spectra do not change significantly with an increase in temperature, despite triclinic ( T )→monoclinic ( M )→rhombohedral ( R ) transitions. The present result is described in terms of the parameter related to temperature variation at constant pressure, defined in a manner similar to the Gruneisen parameter. Raman bands located above 350 cm⁻¹ show smaller values of the parameter than lower frequency modes, suggesting that these higher frequency modes are assigned to SiO₄ internal modes.     

Phase Equilibrium in the System MgO-H2O at High Temperatures and Very High Pressures

Phase equilibrium in the system MgO-H₂O was investigated from 500° to 1450° C at 5 to 40 kbars by a quenching technique using a piston-cylinder apparatus. Brucite decomposed to periclase plus vapor at 810° and 10 kbars, 945° and 20 kbars, 1000° and 32 kbars, and 975°C and 40 kbars. The slope of the decomposition P-T curve became steeper with increasing pressure, infinite at 32^- kbars and 1000C, and then negative above 32 kbars. The extrapolation of this curve to lower pressures agrees well with data of other workers who used hydrothermal techniques.     

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.     

Deformation of UO2 at High Temperatures

The effects of temperature, strain rate, and grain size on the mechanical properties of UO₂ were investigated using the four-point bending technique. Strain rates were varied by two orders of magnitude, and test temperatures up to 1800°C were used. Data are presented on the ultimate tensile stress, yield stress, and plastic strain-to-fracture. Below the brittle-to-ductile transition temperature, T_c , the material fractured in a brittle manner, with no macroscopic plastic deformation. Between T_c and a second transition at a higher temperature, T_t , a small amount of plastic deformation was measured before fracture. Beyond T_t , the strength of UO₂ decreased continuously, and extensive plasticity was observed. This high-temperature plasticity was characterized by a thermally activated rate-controlling process; this behavior is consistent with observations of creep behavior under high stresses. The following phenomenological equations for the strain rate fit the data for the material with 8-μm grain size above T_t :
and
where σ_p and σ_88f are the proportional limit and steady-state flow stress, respectively, and temperature T is in °K.     

Phase Equilibria in the System Na2SiO3-Li2SiO3-SiO2

Ternary phase relations have been studied and several modifications made to Kracek's phase equilibrium diagram. These include location of the primary phase fields of Na₆Si₈O₁₉ and Li₂Si₂O₅. Metastable phases and polymorphs were often encountered, notably a primary phase, δ-Na₂Si₂O₅, and a new polymorph of Li₂Si₂O₅.     

Thermal Expansion of Y2SiO5 Single Crystals

The thermal expansion of Y₂SiO₅ crystals has been measured for the principal crystallographic directions and two orthogonal directions in the (010) plane in the temperature range 25° to 200°C. This monoclinic crystal has strongly anisotropic expansions with coefficients which range from 0.6 × 10⁻⁶/°C for [100] to 11.4 × 10⁻⁶/°C for [001]. Third-order polynomials have been calculated from the expansion curves. Data for the β angle and cell volume as a function of temperature are also given. The thermal expansion of Y₂SiO₅ crystals is not affected by doping with 5% Tb.     

Sublimation of Cr2O3 at High Temperatures

The sublimation of chromic oxide, Cr₂O₃, has been observed in vacuum by the Langmuir technique using induction and solar heating. Extensive sublimation did not yield any new phases on the basis of X-ray powder studies, and condensates of Cr₂O₃ were always obtained. Flash vaporization and flow experiments in CO or O₂ atmospheres and in vacuum indicated no appreciable differences in rates of sublimation. Weight-loss experiments showed that the rate of sublimation was slightly higher than predicted for decomposition to the elements and suggested that small amounts of complex molecules, e.g. CrO and CrO₂, were also present in the equilibrium vapor.     

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.     

Activity-Composition Relations in Co2SiO4-Fe2SiO4 Solid Solutions at 1180°C

Activity-composition relations of cobalt orthosilicate in cobalt-iron-orthosilicate solid solutions were determined at 1180°C by studying the equilibrium between these solid solutions, silica, metallic cobalt, and a gas phase of known oxygen pressures. The solution shows a slight positive deviation from ideality.     

Phase Relationships in the System CaO—AI2O3—P2O5

Primary fields of crystallization in the system CaO-Al₂O₃-P₂O₅ at temperatures from 900° to 1600°C. were determined by the method of quenching. Three ternary eutectics were established: CaO·P₂O₅-Al₂O₃·3P₂O₅-Al₂O₃·P₂O₅, 2CaO·P₂O₆-CaO·P₂O₅-Al₂O₃·P₂O₅, and 3CaO·P₂O₆-2CaO·P₂O₅-Al₂O₃·P₂O₅. The rate of decomposition of Al₂O₃·3P₂O₅ was determined at several temperatures. The boundary was established between the field A1₂O₃·P₂O₅, which covers about 35% of the ternary diagram, and the fields Al₂O₃·3P₂O₅, 2CaO·P₂O₅, and CaO·P₂O₅. A portion of the Al₂O₃·P₂O₅-3CaO·P₂O₅ boundary also was established. A compound with the composition 2Al₂O₃·3P₂O₅ did not appear in the system. No calcium aluminum phosphates were found.     

The Join Ca2SiO4-CaMgSiO4

New data obtained on the join Ca₂SiO₄-CaMgSiO₄ established a limit of crystalline solubility of Mg in α-Ca₂SiO₄ corresponding to the composition Ca_1.90Mg_0.10SiO₄ at 1575°C. The α-α'Ca₂SiO₄ inversion temperature is lowered from 1447° to 1400°C by Mg substitution in the lattice. α'-Ca₂SiO₄ takes Mg into its lattice up to the composition Ca_1.94Mg_0.06SiO₄ at 1400°C and to Ca_1.96Mg_0.04SiO₄ at 900°C. A new phase (T) reported previously by Gutt, with the approximate composition Ca_1.70Mg_0.30SiO₄, was stable between 979° and 1381°C, and should be stable at liquidus temperatures in multicomponent systems involving CaO–MgO–SiO₂.     

Determination of the Pcmn/Ibm2 Phase Boundary at High Temperatures in the System Ca2Fe2O5-Ca2Al2O5

The crystals of Ca₂(Al _x Fe_{1− x} )₂O₅ with 0 ≤ x ≤ 0.4 were examined using powder XRD at temperatures between 25° and 1000°C. The Pcmn to Ibm 2 phase transition was readily detected by the disappearance of the 131 and 151 reflections ( h + k + l odd). Thus, the phase relationship has been determined as combined functions of the temperature and x -value. At 25°C, the space group changed from Pcmn (0 ≤ x ≤ 0.23) to Ibm 2 (0.24 ≤ x ≤ 0.40) across x = 0.235. With increased x -value, the cell dimensions of both phases steadily decreased, showing the nearly complete continuity at that phase boundary. For the crystal with x = 0 (Ca₂Fe₂O₅), the cell dimensions steadily increased during heating to 685°C, at which temperature the phase transition occurred. There was a slight discontinuous decrease in cell dimensions just after the transition. Subsequent heating to 1000°C of the Ibm 2 phase led to a steady thermal expansion along the b -axis and c -axis, while a slight contraction occurred along the a -axis up to 800°C.     

CrO2-Cr2O3 Phase Boundary Under High O2 Pressures

The phase boundary between CrO₂ and Cr₂O₃ was reinvestigated under high O₂ pressures by using a new type of gas compressor. The boundary curve can be represented as log Po₂= 7.16-(3579/ T ). Using the observed data, Δ G °, Δ H °, and Δ S ° for the reaction 2CrO₂⇋Cr₂O₃+½O₂ were calculated to be: Δ G °= -(1.55/100) T +7.60 kcal/mol, Δ H °= -8.19 kcal/mol, and Δ S °= (-15.8/ T )+0.0155 kcal/mol.     

Activities and Structure of Some Melts in the System Na2SiO3-Na2Si2O5

Different structural models for melts between Na₂SiO₃ and Na₂Si₂O₅ were tested by comparison of activities computed from the models with activities determined from the calorimetric heat of fusion. Data were used from the liquidus curve of the phase diagram for the system Na₂SiO₃-Na₂Si₂O₅ (Kracek and Morey and Bowen). A group model involving (SiO₃)_x^2x- rings or chains, with a random distribution of (SiO_2.5)₂ pairs which bridge between rings or chains, gives activities in good agreement with those determined from the calorimetric heat of fusion.     

Spheroidization of Tubular Voids in Al2 O3 Crystals at High Temperatures

Cracks introduced into single-crystal sapphire broke up after annealing, first into channels of cylindrical voids and ultimately into rows of spherical pores, with the thicker gap spacings in the original crack remaining open. Breakup of the cylindrical voids on subsequent annealing conformed to the models for surface-diffusion-controlled material transport. At the temperatures of measurement, the magnitudes of the calculated surface diffusivities agree well with values reported previously.     

Stability of SrFeO3-Based Materials in H2O/CO2-Containing Atmospheres at High Temperatures and Pressures

The chemical stability of SrFeO₃-based perovskites in H₂O- and CO₂-containing atmospheres at high temperatures and pressures has been examined. The extent of reaction as a function of p CO₂, p H₂O, temperature, and time has been determined. Either strontium carbonate or Sr(OH)₂·H₂O was observed on sample surfaces after exposure. Observation of two different reaction-rate behaviors could be explained by the formation of different products. The stability of the perovskite has been found to increase when the activity of Sr is decreased. Chemical stability in H₂O/CO₂ is important to understand in order to use these membrane materials for syngas production.     

Deviation from Stoichiometry in Cr2O3 at High Oxygen Partial Pressures

The deviation from stoichiometry, δ, in Cr₂−δO₃ was measured by a tensivolumetric method in the high pO₂ range of ≊10⁴ to 10⁴ Pa at 1100°C. The value of δ, or chromium vacancy concentration, was≊9×10⁻⁵ mol/mol Cr₂O₃ in air for Cr₂O₃ with 99.999% purity. The chemical diffusion coefficient, DT, determined from equilibration data was ≊4.6× cm²·s⁻¹ at 1100°C for pO₂= 2.2 ×10¹ Pa. The self-diffusion coefficient of Cr ions was calculated from and δ and found to be≊1.6×10-17 cm²-s⁻¹, in good agreement with recently measured values.     

Chemical Durability of Sodium Silicate Glasses Containing AI2O3 and ZrO2

A sodium silicate glass and ternary glasses derived from it by substituting AI₂O₃ and ZrO₂ for SiO₂ were exposed to water and aqueous solutions of pH 1.4 to 12.7; the kinetics of the reactions were studied. Diffusion of alkali ions and leaching of alkali and SiO₂ from the glasses were influenced by the occupancy of surface sites by alkali ions above a critical pH; however, the activation energies of the processes varied linearly with the logarithm of mole fraction of surface sites occupied by H^plus;. Identical slopes were obtained for all glasses for a given process. The results are explained on the basis that transport of alkali ions is retarded as a result of increased boundary concentration and that suitable sites for reaction are lacking.