V. S. Stubican

Phase Equilibria and Ordering in the System ZrO2‐CaO

The phase diagram of the system ZrO2-CaO for the region ZrO2-CaZrO3 was redetermined. The extent of the cubic solid solution field was ascertained by using a high-temperature X-ray furnace, precise lattice parameter measurements, and a hydrothermal technique. At 850 ± 150°C, the cubic solid solutions decompose by a eutectoid reaction. A reasonable agreement for the position of the eutectoid in this system was obtained by comparing the extrapolated data from high temperatures with the data obtained by the hydrothermal technique. Only one ordered compound, CaZr4O9, with monoclinic symmetry, was found; this compound is unstable above 1310 ± 40°C.

A new approach to assignment of infra-red absorption bands in layer-structure silicates

The infra-red spectra of layer s tructures synthesized under controlled hightemperature , high-pressure conditions in sealed noble-metal systems have been examined in the region 400—5000 c m 1 (25—2 μ). B y complete replacement of an ion, e.g., Mg+ by Ni 2 + or Fe+, Al+ by Ga+ or Fe+, and Si by Ge it was possible t o assign the main absorption bands of layer-structure silicates. Bands involving various OH-vibrations were located by complete replacement of OH~ by O D The results obtained in this s tudy suggest certain other relations between the bonds involved and IB. spectra of t he complex crystalline phases.

Interdiffusion studies in the system MgOAl2O3

Abstract The rate of formation of single crystal MgCr 2 O 4 from MgO single crystal and polycrystalline Cr 2 O 3 in air can be described by the expression k = 16.3 exp(80,000±9,000/ RT ) cm 2 sec −1 . The single crystal of MgCr 2 O 4 grew by the advancement of(110) planes at the MgCr 2 O 4 /Cr 2 O 3 interface. Two different methods were used for the calculation of interdiffusion coefficients of Cr 3+ in MgO in the temperature range 1340–1640°C. Wagners recent analysis was applied for the calculation of interdiffusion coefficients in the MgO-Cr 2 O 3 system where an intermediate spinel phase exists. A different mathematical treatment was derived for the calculation of interdiffusion coefficients at the saturation concentration of Cr 2 O 3 , in MgO single crystal. The interdiffusion coefficients increased linearly with increasing mole fraction of Cr 3+ cations and with increasing mole fraction of cation vacancies. For a concentration of one mol.% Cr 2 O 3 MgO, the interdiffusion coefficient can be expressed as D = 5·50 × 10 −3 exp(67,000 ±2,000/ RT ) cm 2 sec −1 . The enthalpy of activation was independent of cation vacancy concentration while the entropy of activation increased with increasing vacancy concentration. The measured interdiffusion coefficients were deduced to be the intrinsic diffusion coefficients for Cr 3+ cations in MgO. Interdiffusion coefficients were in the range of 10 −12 −10 −9 cm 2 sec −1 for the temperatures and concentrations studied.

Divalent chromium in magnesium-chromium spinels

Abstract By heating mixtures of MgO and 50–66 mol.% Cr2O3 in a slightly reducing atmosphere at 1800–2000°C a new spinel-type crystalline solution with the unit cell formula (Mg8−xCrx2+)IVCr163+VIO32 was obtained. Data concerning synthesis, structure, electronic spectra, magnetic susceptibility and electrical conductivity for these crystalline solutions are presented. Strong tetragonal (Jahn-Teller) distortion with c a was observed when a critical value of approx. one Cr2+(3d4) ion per unit cell formula in the tetrahedral sites was reached. A strong absorption band in the near infra-red region with Dq = 667 cm−1 could be assigned to the5T2g → 5Eg transition for Cr2+ in the tetrahedral environment. Both the ground state (5T2g) and the excited state (5Eg) split in the tetragonal symmetry and a strong displacement of the absorption maximum towards higher energies is observed. The spinel crystalline solution with x = 3.04 is anti-ferromagnetic with a Neel temperature at 10°K. Magnesium-chromium spinels containing Cr2+ in the tetrahedral sites display high spin magnetic susceptibilities. A relatively high electrical resistivity of 1.2 × 105 Ω-cm at 400°C for the crystalline solution with x = 2.48, indicates that Cr2+ and Cr3+ ions are not in the same coordination sites in the structure. From the electrical resistivity measurements between 200–450°C an activation energy of 0.85 eV was obtained.

Mechanism of the precipitation of the spinel from MgO-Al2O3 solid solutions

Abstract The recently reported crystalline solubility of Al2O3 in MgO was confirmed. Unit cell dimensions and densities were measured independently and the existence of a predominant cationvacancy model established for these crystalline solutions. The precipitation of the spinel was effected in the air at 1350° and 1100°C and studied by quantitative X-ray diffraction. The precipitation is a diffusion controlled process and the equation y = 1− exp [− ( t τ ) n ] fits the experimental results for y = 0.05 − 0.95. The exponent n was found to be very close to 3 2 , suggesting that under the experimental conditions the growing particles were approximately spheroidal. Electron probe investigations showed that the preferred sites for the nucleation and the precipitation of spinel were the grain boundaries and intergranular voids.

RELATION OF EQUILIBRIUM PHASE TRANSITION PRESSURE TO IONIC RADII

P-t equilibria for a series of some 20 rare earth arsenates (LnAsO/sub 4/ ) and vanadates (LnVO/sub 4/) which undergo reconstructive transitions (xenotime in equilibrium scheelite) in the region 10,000 to 60,000 atm were obtained. From these curves, a set of data was extracted representing (under a fair approximation to corresponding states) the change of equilibrium pressure for a transition as a function of the radius of the Ln/sup 3+/ ion. The result is a V shaped curve in both the arsenates and the vanadates showing a minimum transformation pressure near Dy/sup 3+/ or Ho/sup 3+/. An even-odd effect is also noted. (auth)

Measurement of spectral, directional reflectivities of solids at high temperatures between 9 and 11 μm

A new apparatus to measure spectral,<directional reflectivities of solids for temperatures up to 1100 °C is developed. Reflectivities of two ceramic materials, silicon nitride (Si(3)N(4)) and silicon carbide α-SiC, are measured at wavelengths between 9 and 11 µm (the operating range of tunable CO(2) lasers) for various temperatures, angles of incidence, and for two types of polarization, the electric vector perpendicular and parallel to the plane of incidence. Reflectivities are measured by comparing the power of the beam reflected from the sample (heated in the furnace) with that of the incident beam. This experimental setup is limited to relatively specular surfaces (with a collection half-angle of 15°). The measurements show that the reflectivity of α-SiC at room temperature rises sharply near ~10.2 µm because of the presence of a 12.6-µm reflection band (restrahlen band), and the occurrence of this phenomenon gradually shifts to longer wavelengths as the temperature is raised to 1000 °C. At 10.6 µm, where most CO(3) lasers operate, the reflectivity of SiC diminishes rapidly as the temperature is raised. Si(3)N(4) has two restrahlen bands on both sides of 9.9 µm at room temperature that gradually shift to longer wavelengths with temperature. However, the decrease in reflectivity of Si(3)N(4) with temperature at 10.6 µm is very small.

GRAIN BOUNDARY PRECIPITATION IN OXIDE SYSTEMS

The spatial distribution of precipitate in several polycrystalline oxide systems after annealing of the crystalline solutions was investigated by using an electron probe microanalyzer. Preferential grain boundary and intergranular voids precipitation of the minor component was found in the majority of systems with limited crystalline solubility, e.g., MgO–Al2O3, MgO–CaO, UO2–CaO, CaO–Yb2O3, as well as with the ZrO2–MgO system which has an eutectoid. The formation of an intermediate phase during the precipitation may have a varied effect on the contribution of the surface free energy to the activation energy for nucleation. The intermediate phase appears in both MgAl2O4–Al2O3 and CaO–Yb2O3 systems, although complete phase separation occurs in the former and grain boundary precipitation in the latter.

Diffusion of 51Cr in Cr-Doped MgO

Abstract An arc fusion technique was used to grow single crystals of MgO and Cr-doped MgO. Diffusion coefficients for 51 Cr in Cr-doped single crystals were measured at three temperatures 1383, 1444 and 1495°C using a high specific activity isotope 51 Cr. An approximately linear relationship between the concentration of Cr-ions in MgO and diffusion coefficients of 51 Cr was obtained. It is shown that the activation energy of 19.6 kcal mole obtained for the doped crystals is the difference between the energy for motion and the energy for association of the Cr-vacancy complexes. Using a previously determined value of 39.9 kcal / mol for the energy of motion, the energy of association for the Cr-vacancy complex is calculated to 20.3 ± 3 kcal / mol or 0.88 ± 0.13 eV.

The relationship between chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice

Abstract The theoretical model developed by Lidiard was extended to describe the relationship between the chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice. It is shown that the relationship between the chemical diffusion coefficient, D , and the tracer diffusion coefficient, D ∗ , is D = 2 D ∗ if the migration of dimers is the principal mechanism of transport and for the migration of trimers D = 3 D ∗ if the concentration of impurity ion is relatively small. These relationships are valid regardless of the charge of the aliovalent or lattice ions. The chemical diffusion coefficients of Cr 3+ in Cr-doped MgO were determined for three different temperatures, 1656, 1717 and 1768K, and for the concentration region 2.5×10 −2 −2.8×10 −1 mole% Cr 2 O 3 . Using previously determined values for the tracer diffusion coefficient of 51 Cr in Cr-doped MgO it was found that for the temperature and concentration region investigated D = (2.00±0.17) D ∗ which indicates that diffusion proceeds primarily by the migration of dimers.