High-temperature Solid Phases Research Articles

Effect of Molecular Structure on Mesomorphism: 5. Comparison of Methyl and Trifluoromethyl as Lateral Substituents in the Solid and Nematic Phases.1

Abstract Two laterally-substituted nematogenic liquid crystals, methyl-p-phenylene-di-p-butoxybenzoate and trifluoromethyl-p-phenylene-di-p-butoxybenzoate, have been studied by differential scanning calorimetry, powder x-ray diffraction, and polarized light microscopy (hot stage microscopy and hot-wire stage microscopy). Comparison of thermodynamic data for the nematicisotropic transitions of these two compounds suggests a strongly repulsive electronic interaction between molecules in the nematic phase of the trifluoromethyl compound. Detailed molecular structural augments are presented to rationalize the thermodynamic data. The room temperature solid phases of the two compounds were found to have different crystal structures. However, the high temperature solid phases were found, by a novel application of hot-wire stage microscopy, to be isomorphous.

Vibrational spectra of solid and molten phases of the alkali metal tetrafluoroborates

Raman and i.r. spectra of the low temperature (orthorhombic) and high temperature (cubic) solid phases of KBF 4, RbBF 4 and CsBF 4 have been measured, and Raman spectra of the molten salts (including LiBF 4) have been observed. Studies of the low temperature phase include a determination of the longitudinal optical mode frequencies of the ν 3 mode by polarized specular reflectance i.r. spectroscopy. The frequency shift observed for the Raman active component of ν 1 in crystalline lithium, sodium, potassium, rubidium, and cesium fluoroborates is interpreted in terms of a model based on an isolated BF 4 − ion contained in a static field.

Thermal diffusitivities of lanthanum, cerium and plutonium at high temperatures

The thermal diffusivities of La, Ce and Pu were measured in the high temperature regions which spanned their respective fcc, bcc and liquid phases. The transient heating technique was evaluated by successful measurements of the thermal diffusivity of liquid lead. The calculated thermal conductivity values for liquid La and Ce were the same as or less than their respective high temperature solid phases, while liquid Pu was a better conductor than any of its solid phases. The Lorenz numbers for liquid La and Ce were 10 and 30%, respectively, less than the free-electron value.

Preparation of sound high purity plutonium rods: Part 2. Observed phase transformations during quenching from elevated temperatures

Cooling curves of high purity plutonium (1) chill cast from the liquid and (2) quenched from high temperature solid phases have been subjected to heat flow analysis to yield information concerning the transformation behavior of the rapidly cooled material. The most complex transformation behavior was found in rods formed when molten plutonium was chill cast into molds at ambient temperature. The plutonium solidified as ε phase and the bulk of the material subsequently transformed to γ, then to β and finally to α phase during the cooling process. When the mold temperature was −72 °C prior to the pour, the ε phase material transformed to α phase at a temperature of 80 °C. High density α phase rods were heated into and then quenched from β, γ and δ phase fields. The high temperature phases which were formed then transformed to α phase below 90 °C. If the formation of the brittle α phase proceeded from the circumference of the rod toward the center, a relatively crack-free rod was produced. However, if the formation of α phase occurred randomly across the cross section, a badly microcracked rod resulted.

Phase Changes and Electrical Conductivity of Concentrated Lithium—Ammonia Solutions and the Solid Eutectic

The electrical conductivities of concentrated solutions of lithium in ammonia have been measured over the temperature range 65° to 300°K by both conventional dc techniques and electrodeless techniques. In the liquid phase, conductivities ranged from 400 to 15 000 Ω−1·cm−1 at 240°K; the temperature coefficient of conductivity varied from +1.0% deg−1 to zero. Phase changes were indicated by discontinuities in either the conductivity or the temperature derivative of conductivity. No sign of a maximum in the melting-point-vs-concentration curve near the tetra-amine lithium [Li(NH3)4] concentration was found. The eutectic was found to be 89.6°±0.3°K. The conductivity of the solid was 6.7 times as large as that of the liquid at the melting point. In addition to phase changes in the liquid, two solid-phase changes were observed; one at 82°K and the other at 69°K. Conductivities in the solid were decreasing functions of the temperature in each solid phase, yet were higher in the high-temperature solid phases. The highest conductivity observed was 90 000 Ω−1·cm−1 (at 83°K). The solid does not behave as do the eutectics of sodium—ammonia or potassium—ammonia solutions. A comparison with other metal—ammonia solutions is made.