Howard E. Flotow

Calibration and Use of Germanium Resistance Thermometers for Precise Heat Capacity Measurements from 1 to 25°K. High Purity Copper for Interlaboratory Heat Capacity Comparisons

The cryogenic apparatus and techniques used to calibrate 3 platinum‐encapsulated 4‐lead germanium resistance thermometers for use in precision calorimetry from 1 to 25°K are described. The calibration data for each thermometer were fitted with the aid of a digital computer by a polynomial expressing logR in terms of powers of logT. The sensitivity and temperature‐cycling stability of these thermometers were adequate to recommend this type of temperature sensor for accurate low temperature calorimetry. A test of the temperature scale and of an isothermal type calorimeter was made by measuring the heat capacity of a sample of 99.99+% copper from the 1965 Calorimetry Conference Copper Standard and comparing the results with those obtained by other investigators. An equation to represent the heat capacity of pure copper from 1 to 25°K, based upon appropriate literature results and the present data, is offered as a convenient copper reference equation for intercomparison of heat capacity results from various l...

Vibration Spectra of Vanadium Hydride in Three Crystal Phases by Inelastic Neutron Scattering

The vibration spectra of vanadium hydride in three crystal phases have been investigated by the energy‐gain scattering of cold neutrons. The measured spectra are generally split into two broad bands above and below about 300 cm− 1, which are primarily associated with optical hydrogen vibrations and metal‐atom vibrations, respectively. Pseudofrequency distributions for the hydrogen vibrations were derived from the measured neutron spectra. These indicate broad optical vibration bands peaked at 970 ± 50 and ∼1400 cm− 1 in the α (bcc) phase, 440 ± 20, 970 and ∼1400 cm− 1 in the β (bct) phase, and ∼1300 cm− 1 in the γ (fcc) phase. The width of the 440‐cm− 1 band increases with hydrogen concentration, possibly due to hydrogen–hydrogen interactions. The “metal‐atom” vibration spectra also show changes in peak positions and widths as the distribution of phases is changed. The changes in the neutron spectra with temperature and as a function of composition between VH0.20 and VH1.71 correlate quite well with the p...

Deuterium site occupation in the oxygen‐stabilized η‐carbides Zr3V3ODx. I. Preparation and neutron powder diffraction

The structures of the single‐phase, well‐crystallized family of hydrides Zr3V3ODx for x=0, 1.86, 2.85, and 4.93 have been refined by the Rietveld method from neutron powder diffraction data taken at 298 K. The alloy and the three deuterides studied all possess the η‐carbide structure (cubic Fd3m; origin at 43m). Deuterium is found to occupy four different tetrahedral interstices preferentially. Sites with the largest number of Zr near neighbors appear to fill first unless occupation is hindered by nearby oxygen or simultaneously occupied deuterium sites. The volume expansion per number of deuterium atoms is nearly linear, even though the lowest stable hydride concentration at 298 K is near Zr3V3OD1.5 and markedly nonlinear distortions occur within the unit cell as different sites fill.

Uranium Monosulfide. The Ferromagnetic Transition. The Heat Capacity and Thermodynamic Properties from 1.5° to 350°K

The heat capacity of uranium monosulfide was measured from 1.5° to 22°K by an isothermal (isoperibol) method and from 6° to 350°K by an adiabatic technique. The ferromagnetic transition at 180.1°K has a characteristic lambda shape and associated magnetic ordering entropy and enthalpy increments of 1.62 ± 0.2 cal °K−1mole−1 and 231 ± 20 cal mole−1, respectively, over the temperature range 0° to 230°K. The correlation of the thermal data with magnetic studies is discussed. The heat capacity below 9°K is represented by Cp = 5.588 × 10−3T + 2.627 × 10−4T3 / 2 + 6.752 × 10−5T3cal °K−1mole−1, in which the successive terms represent conduction electronic, magnetic, and lattice contributions. Values of the entropy [S°], enthaply function [(H° − H°0) / T], and Gibbs‐energy function [(G° − H°0) / T] are 18.64 ± 0.005, 8.94 ± 0.002, and − 9.70 ± 0.02 cal °K−1 mole−1, respectively, at 298.15°K. The Gibbs energy of formation at 298.15°K is − 72.9 ± 3.5 kcal mole−1.

Heat Capacities and Thermodynamic Functions of ZrH2 and ZrD2 from 5 to 350°K and the Hydrogen Vibration Frequency in ZrH2

The heat capacities of ZrH2 and ZrD2 were measured in an adiabatic type calorimeter from 5 to 350°K. X‐ray analyses showed the hydrides consist of a single face‐centered tetragonal phase. The data for both compounds below 11°K were found to fit the equation Cv=9.8×10—4T+464.6(T/θ)3 cal deg—1 mole—1, where θ=311.4, and this equation was used to extrapolate the heat capacities below 6°K. Between 20 and 100°K the Cp of ZrD2 averages 1% lower than that of ZrH2 but above 110°K the Cp of ZrD2 becomes increasingly greater than that of ZrH2. At 298.15°K the heat capacities and thermodynamic functions calculated from the data are Cp=7.396±0.015 cal deg—1 mole—1, S°=8.374±0.02 cal deg—1 mole—1, H°—H0°=1284.1±2 cal mole—1, and (F°—H0°)/T=—4.067±0.01 cal deg—1 mole—1 for ZrH2, and 9.631±0.019, 9.168±0.02, 1474.4±3, and —4.223±0.01, respectively, for ZrD2. The free energy of formation of ZrH2 at 298.15°K is —30.9±2 kcal mole—1 and that of ZrD2 is —31.2±2. On the assumption that the difference in the heat capacity betw...

Xenon Tetrafluoride: Heat Capacity and Thermodynamic Functions from 5 to 350°K. Reconciliation of the Entropies from Molecular and Thermal Data

The heat capacity of a very pure sample of xenon tetrafluoride, XeF4, was determined by adiabatic calorimetry between 5 and 350°K. No irregular thermal behavior was observed, except in the region near 210°K, where slightly enhanced heat capacity values, ascribed to a small amount of impurity, were observed. The values of the thermodynamic quantities Cp°, S°, (H° − H °0)/T, and (G° − H °0)/T at 298.15°K are 118.39 ± 0.12, 167.00 ± 0.17, 77.24 ± 0.08, and −89.76± 0.09 J °K−1 · mole−1, respectively. The standard entropy of the gas at 298.15°K was calculated from the thermal data to be 323.2± 2.0 J °K−1· mole−1, in excellent agreement with the value 323.98 ± 0.4 J °K−1· mole−1 calculated from electron‐diffraction data and the frequency assignment of Tsao, Cobb, and Claassen. The standard entropy of formation of solid XeF4 at 298.15°K was found to be −407.95 ± 0.17 J °K−1 · mole−1, and the standard Gibbs energy of formation at 298.15°K is −145.48 ± 0.88 kJ mole−1. The corresponding quantities for gaseous XeF4 ...

Vibration Spectra of Yttrium and Uranium Hydrides by the Inelastic Scattering of Cold Neutrons

The vibration spectra of the hydrides and deuterides of yttrium and uranium have been investigated by the energy‐gain scattering of cold neutrons. The measured spectra are all split into two bands, one at higher energies, due to the optical hydrogen vibrations, and another at lower energies due to metal—atom vibrations. Values have been obtained for the peaks and widths of the single optical hydrogen bands in YH2, YD2, UH3, and UD3 by the calculation of approximate frequency distributions from the observed neutron time‐of‐flight spectra. The derived peak frequencies in cm−1 are: YH2, 1025±60; YD2, 725±40; UH3, 970±60; UD3, 710±40. The relative width at half‐maximum for the hydrogen band in UH3 is about double that for YH2. The distribution of hydrogen vibrations in hexagonal YH3 and YD3 is considerably broader, and exhibits several maxima, due to hydrogens at different sites in the lattice. The hydrogen modes in all the compounds can be correlated reasonably well with the known crystal structures. The res...

Quasielastic Thermal Neutron Scattering by Hydrogen in α‐Vanadium Hydride

The diffusion process of hydrogen in vanadium has been studied by quasielastic thermal neutron scattering. Neutron linewidths associated with diffusion broadening have been determined at 485°K for VH0.198 and VH0.570. These measurements cover a range of momentum transfers (Q) for elastic scattering up to 4.1 A−1. This is much larger than the momentum transfers obtained in earlier cold‐neutron experiments, where no conclusive comparison was possible between data and proposed models involving diffusive jumps between octahedral and tetrahedral sites. The linewidth results for α‐VH0.57 are compared with these models and with an extension developed in the present paper. Although no complete agreement was obtained, this comparison leads to the conclusion that jumps between tetrahedral sites are predominant in the diffusion process. Differences in the linewidth behavior for VH0.198 and VH0.570, and the fact that the measured widths at large Q increase rapidly with Q rather than approaching an asymptotic value as...

Thermodynamics of the lanthanide trifluorides. I. The heat capacity of lanthanum trifluoride, LaF3 from 5 to 350°K and enthalpies from 298 to 1477°K

The heat capacity of a sample of LaF3 was determined in the temperature range 5–350°K by aneroid adiabatic calorimetry and the enthalpy from 298.15 to 1477°K by drop calorimetry. The heat capacity at constant pressure C°p(298.15°K), the entropy S° (298.15°K), the enthalpy [H° (298.15°K)−H° (0)] and the Planck function −[G° (298.15°K)−H° (0)]/298.15°K; were found to be (90.29±0.09) J °K−1⋅mole−1, (106.98±0.11) J °K−1⋅mole−1, (16717±17) J mole−1, and (50.91±0.05) J °K−1⋅mole−1. The thermal functions from the present research were extended up to the melting temperature (1766°K) by combination with previously published results. The anomalously high heat capacity from about 1100 to 1766°K is discussed.

Thermodynamics of the lanthanide trifluorides. IV. The heat capacities of gadolinium trifluoride GdF3, lutetium trifluoride LuF3, and yttrium trifluoride YF3 from 5 to 350 °K

The heat capacities of three isostructural trifluorides GdF3, LuF3, and YF3 were determined from 5 to 350 °K by adiabatic calorimetry. Below 15 °K GdF3 contained an excess heat capacity contribution over the usual lattice heat capacity; LuF3 and YF3 showed no unusual heat capacities over the entire temperature range. Results in tabular form which list the heat capacity C°p, the entropy S°, the enthalpy [H°(T)−H°(0)], and the Planck function −[G°(T)−H°(T)]/T are given for the three trifluorides. Also presented are recommended thermochemical functions at 298.15 °K and tables of recommended high‐temperature thermodynamic functions from 400 °K to the melting temperatures. The excess entropy associated with GdF3 is discussed and values of the lattice heat capacity of GdF3 below 15 °K are estimated.