Nuclear magnetic relaxation of3He gas. II.3He-4He mixtures

Longitudinal relaxation timesT ₁ have been measured in³He-⁴He gas mixtures, using pulsed NMR, in the temperature range 0.6–15 K. Helium-3 number densities of the order of 10²⁴ atoms m^–3 were used. Relaxation takes place on or near the walls of the Pyrex sample cells and measurements ofT ₁ give information about the surface phases. A cryogenic precoating of solid molecular hydrogen was used to reduce the helium-substrate binding energy from 100 K on Pyrex to 13 K for³He and 15 K for⁴He. TheT ₁ data at high temperatures were similar to those observed previously in the pure³He-H₂ system. The presence of⁴He generally causedT ₁ to rise on cooling below 2 K due to the preferential adsorption of⁴He over³He at the surface. However,³He atoms that go into quasiparticle states in the superfluid helium film can be an extra source of relaxation. In uncleaned cells, relaxation probably takes place in quasiparticle states at the free surface of the superfluid film, which are bound with an energy of 5.1±0.3 K. Baking the Pyrex cells under vacuum and rf discharge cleaning the walls before sealing in the sample gas were found to increase the bulk gasT ₁ by two or three orders of magnitude. In a cleaned, sealed cell aT ₁ of 8 h was measured at 7.7 MHz and 0.8 K. In this case relaxation is probably occurring two or three helium layers away from the helium-hydrogen interface. It may be possible to observe a predicted minimum in the intrinsic dipolarT ₁ of the bulk gas by using a⁴He wall coating to suppress wall relaxation effects (which usually dominate the nuclear relaxation of the bulk gas).     

Nuclear magnetic relaxation of3He gas. I. Pure3He

Longitudinal relaxation timesT ₁ have been measured in³He gas, using pulsed NMR, for number densities between 3 × 10²³ and 6 × 10²⁵ spins m^–3 and temperatures between 0.6 and 15 K. Relaxation takes place on or near the walls of the Pyrex sample cells and measurements ofT ₁ give information about the surface phases. A cryogenic wall coating of solid molecular hydrogen was found to delay the formation of a³He monolayer on cooling, andT ₁ measurements were consistent with a binding energy of 13 K for a³He atom to a hydrogen surface. At temperatures below 2 K a completed³He monolayer forms on the H₂ coating. No variation of the areal density of monolayer completion with bulk number density at fixed temperature could be observed and the completed³He monolayer is thought to be a dense fluid. Baking the Pyrex sample cells under vacuum and using an rf discharge in³He gas to clean the walls before sealing in the sample gas were found to increase the observed T₁'s by up to three orders of magnitude. Once a³He monolayer has formed on the H₂ surface in these cleaned, sealed cells, the dipolar interaction between adsorbed spins is thought to be the dominant source of longitudinal relaxation. The data are consistent with a dipolar relaxation model with a correlation time of 2 × 10^–9 sec. This time is long compared to the value of 10^–11 or 10^–12 sec in the 3D fluid. This suggests that if the surface phase is a 2D fluid and the dipolar mechanism is indeed the dominant one, then the atoms in the 2D fluid are less mobile than in three dimensions. This is consistent with recent susceptibility measurements.     

X-ray Diffraction Study of Y2W3O12· 3H2O

Under ordinary conditions, yttrium tungstate exists in the form of the trihydrate Y₂W₃O₁₂· 3H₂O and has a monoclinic structure (sp. gr. P2/m; a= 14.308 Å, b= 4.354 Å, c= 12.430 Å, = 119.78°). The dehydration Y₂W₃O₁₂· 3H₂O Y₂W₃O₁₂occurs at 142°C.     

Gibbs energy of formation of Cu2 Ln 2O5 (Ln = Yb,Tm, Er,Ho, Dy) and CuGd2O4 compounds by the e.m.f. method

Employing electrochemical cells with the solid zirconia electrolyte Cu₂O, Cu₂ Ln ₂0₅, Ln ₂O₃O^2– air and Cu₂O, CuGd₂O₄, Gd₂O₃O^2– air the Gibbs free energy of formation of the solid Cu₂ Ln ₂O₅ (Ln=Yb,Tm, Er, Ho, Dy) and CuGd₂O₄ phases was determined. The results obtained were used to derive Gibbs free energy change of reactions of formation of these compounds from the respective oxides in the following form G _oxides ⁰ (Cu₂Yb₂O₅)=19 429-22.02TJmol^–1 G _oxides ⁰ (Cu₂Tm₂O₅)=35 275-34.09TJmol^–1 G _oxides ⁰ (Cu₂Er₂O₅)=17 427-19.61TJmol^–1 G _oxides ⁰ (Cu₂Ho₂0₅)=18 165-19.49TJmol^–1 G _oxides ⁰ (Cu₂Dy₂O₅)=16 648-17.58TJmol^–1 G _oxides ⁰ (CuGd₂O₄)=9562-12.29TJmol^–1     

Preparation, Structure, and Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + δ Ceramics with Si3N4 Additions

The microstructure, phase composition, texture, and superconducting properties (T _c, T _c, j _c(T), and R(T) at H= 0 and 5 mT) of (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +} ceramics (sintering at 840°C for 36 h) with ultrafine Si₃N₄ additions (0.05–0.2 wt %) are studied. The introduction of 0.05–0.1 wt % Si₃N₄ is shown to reduce the width of the superconducting transition by 2–3 K and to raise the critical current density at temperatures below 95 K.     

Thermodynamic stability of Sb2O4 by a solid oxide electrolyte e.m.f. method

The standard Gibbs energy change for the reaction Sb₂O₃ (orthorhombic) (orthorhombic) was measured using the following galvanic cell: Pt, Sb₂O₃(ortho), Sb₂O₄ (ortho), platinum black |15 Wt % Y₂O₃-stabilized ZrO₂|O₂(air,P _{o 2} = 0.21 atm), Pt over the range 585 to 828K. Reproducible e.m.f. values were obtained only when the otherwise poorly electronic conducting oxide mixture was blended with platinum black. The standard Gibbs energy change forthis reaction was found to be (G_R ^o ± 0.81) (kJ) = –16833 + 0.0S019T/K. This yielded thestandard Gibbsenergy offormation G_R ^o of Sb₂O₄ (ortho) to be as follows: [\gDG₁ ^o(Sb₂O₄, ortho) \+- 1.61] (kJ) \s= \t-854.89 \s+ 0.33614T/K. A third-law treatment yielded a value of \t-880.55 (\+-1.21) kJmol^\t-1 for \gDH _1,298 ^o of Sb₂O₄ (ortho).     

Multilayer adsorption of3He and4He on zeolite from 4 K to 80 K

The adsorption isotherms of⁴He, N₂, and argon have been measured on synthetic zeolite (Linde Molecular Sieve 13X) at 78 K, and of³He and⁴He, also on zeolite 13X, in the temperature range 4 K to 20 K. The results are presented in tabular and graphical form. The N₂ isotherms, which showed characteristic step-like behavior, served to assess the specific surface area, which was 527 m² g^–1 based on a standard N₂ molecular area of 16.2 Ų. It also provided a value ofE ₁ equal to 2530 cal mole^–1. The argon isotherm at 78 K yielded a specific surface area for the zeolite 13X in fair agreement with that from the N₂ data. Nine isotherms were taken for⁴He between 4 K and 20 K and four for³He in the same temperature range. These isotherms permitted good evaluations of the isosteric heats of adsorption to be made and plotted as a function of coverage, yielding, for⁴He,Q _st =1580 j mole^–1 at zero coverage,Q _st =1030 j mole^–1 at monolayer coverage andE ₂=480 j mole^–1 at two-layer coverage. For³He, which showed everywhere smaller Q _st values. Q _st =1420 at zero coverage. By use of the Steele equation applied to⁴He, we found that the monolayer coverageV _m1 0.29 cm³ (STP) m^–2, and the second-layer coverage,V _m2 0.10 cm³ (STP) m^–2.Supported in part by a grant from the National Science Foundation and by contracts with ONR and the Department of Defense (Themis).     

Microstructure and properties of ceramic coatings produced on 2024 aluminum alloy by microarc oxidation

The microstructures of the microarc oxidation coatings and 2024 aluminum alloy substrate were observed using the scanning electron microscope (SEM) and the phase composition of the coatings was analyzed by X-ray diffraction (XRD). Furthermore, the profiles of the nanohardness, H, and elastic modulus, E, along the coating depth were first determined using the mechanical properties microprobe. The microarc oxidation coatings consist of two layers—a loose layer and a compact layer. The H and E in the compact layer are about 18–32 GPa, 280–390 GPa, respectively. The H and E profiles are similar, and both of them exhibit a maximum value at a same depth of the coatings. The distribution of -Al₂O₃ phase content determines the H and E profiles in the coatings. The changes of -Al₂O₃ and -Al₂O₃ contents result from the different cooling rates of the molten alumina in the microarc discharge channel at the different depths of the coatings. After the microarc oxidation treatment, the microstructure of the alloy substrate, even near the Al/Al₂O₃ interface, has not been changed.     

Pressure-volume-temperature relations in liquid and solid3He

Liquid and solid³He P-V-T relations were measured over 0.26–1.8 K with concentration on regions near the melting curve. The method used a cell whose volume was varied with diaphragms, the positions of which determined the volume and pressure. The molar volumes of liquidV _lm and solidV _sm along the melting curve were consistent with the directly measured volume change on melting V _m . Below 1 K,V _lm , V_sm, and V _m were greater than previous results. Along the melting curve, the compressibility of the solid became greater than the compressibility of the liquid atT<1.2 K, the difference rising to 12% at 0.32 K. The thermal expansion of the solid became negative below 0.30–0.35 K. The melting curve minimum was measured at 28.932±0.003 atm and 0.319±0.003 K. Starting with the minimum, the melting curve was calculated to 0.02 K, where it should be useful in thermometry. A set of self-consistent data is presented.Work performed under the auspices of the U.S. Atomic Energy Commission.     

Standard Gibbs' energies of formation of BaCuO2, Y2Cu2O5 and Y2BaCuO5

The Gibbs' energies of formation of BaCuO₂, Y₂Cu₂O₅ and Y₂BaCuO₅ from component oxides have been measured using solid state galvanic cells incorporating CaF₂ as the solid electrolyte under pure oxygen at a pressure of 1.01×10⁵ Pa BaO + CuO BaCuO₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=–63.4–0.0525T(K) Y₂O₃ + 2CuO Y₂Cu₂O₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=18.47–0.0219T(K) Y₂O₃ + BaO + CuO Y₂BaCuO₅ G _f,ox ^o (± 0.7) (kJ mol^–1)=–72.5–0.0793T(K) Because the superconducting compound YBa₂Cu₃O_7– coexists with any two of the phases CuO, BaCuO₂ and Y₂BaCuO₅, the data on BaCuO₂ and Y₂BaCuO₅ obtained in this study provide the basis for the evaluation of the Gibbs' energy of formation of the 1-2-3 compound at high temperatures.     

Magnetic properties of solid 3He down to 0.3 mK

Nuclear magnetization of solid ³He has been studied by static magnetization measurements from 10 mK down to 0.3 mK for molar volumes ranging from V = 24.14 to 21.02 cm³/mole in the bcc phase and from V= 19.83 to 19.26 cm³/mole in the hcp phase. In the bcc phase, both the antiferromagnetic transition temperature T _N and the reciprocal of the maximum magnetization V _max ^–1 at T _N vary in proportion to V ^16.5±1, and the magnetization below T _N is constant. The magnetization reduced by M _max is found to be represented by a universal function of the reduced temperature T/T _N. In the hcp phase, the magnetization can approximately be represented by Curie's law, and the estimated Weiss temperatures are below 50 K. We also observed that the boundary magnetism of liquid ³He depends considerably on pressure. The transition temperature of solid ³He to the antiferromagnetic phase coexisting with liquid in a restricted geometry is 15% higher than that of the bulk solid on the melting curve.     

Electrical Conductivity of H2V12 – y W y O31 + δ · nH2O and H x V x W1 – x O3 · nH2O

The electrical conductivity of H₂V_{12 – y} W _y O_{31 +} · nH₂O (0 y 3) and H _x V _x W_{1 – x} O₃ · nH₂O (x = 0, 0.08, 0.17, 0.25, 0.33) prepared by the sol–gel method was measured between 293 and 473 K at a relative humidity of 12%. Partial substitution of tungsten for vanadium or V⁵⁺ and H⁺ for tungsten was found to reduce the conductivity of the phases studied. The only exception is the x = 0.33 phase. Below 373 K, the activation energy of conduction is 0.22–0.29 eV in the amorphous phases (0 y 3; x= 0.25, 0.33) and 0.06–0.11 eV in the crystalline phases (x = 0, 0.08, 0.17). The possible mechanisms of electrical transport are discussed.     

Thermodynamic Properties of Lanthanum Chlorides

The enthalpies of atomization of LaCl, LaCl₂, and LaCl₃ are determined by measuring the vapor pressure over liquid and solid LaCl₃ and assessing the equilibrium constants of the gas-phase reactions BaCl + La = Ba + LaCl and 2BaCl + La = 2Ba + LaCl₂, using the appearance potentials of ions and the ionization potentials of the lanthanum chlorides: _at H ⁰(LaCl, 298 K) = 502.4 ± 0.5 kJ/mol, _at H ⁰(LaCl₂, 298 K) = 999.2 ± 0.8 kJ/mol, and _at H ⁰(LaCl₃, 298 K) = 1524.6 ± 2.0 kJ/mol. In addition, the enthalpies of formation of gaseous lanthanum chlorides are determined using new data on the thermodynamic functions of condensed LaCl₃ and gaseous LaCl, LaCl₂, and LaCl₃: _f H ⁰(LaCl, g, 298 K) = 48.9 ± 1.0 kJ/mol, _f H ⁰(LaCl₂, g, 298 K) = –326.6 ± 1.2 kJ/mol, and _f H ⁰(LaCl₃, g, 298 K) = –730.7 ± 2.0 kJ/mol. The ionization and appearance potentials of ions resulting from electron impact ionization of LaCl₃ are determined.     

Specific heats of solid hydrogen,solid deuterium,and solid mixtures of hydrogen and deuterium

Measurements have been made in the temperature range 0.5–5 K of the specific heats of pure solid H₂, pure solid D₂, and four solid mixtures of H₂ and D₂ with D₂ fractions between 15 and 95%, all of which systems had low concentrationc of molecules in the rotational stateJ=1. For the H₂,c=0.0023, and for the D₂,c=0.030. It was found that the observed specific heats could be described as the sum of three terms as follows. (1) A lattice specific heat. This was found to follow aT ³ function with _D =122 K for solid H₂ and _D=113.8 K for solid D₂. For the solid mixtures the lattice specifi heats have in first approximation values given by a weighted mean of the lattice specific heats of the pure components. (2) A Schottky term with a maximum at about 1.3 K for solid H₂ and 1.4 K for solid D₂, which can quantitatively be described in terms of electric quadrupole-quadrupole (EQQ) interactions in nearest-neighbor pairs and triples of molecules in the rotational stateJ=1. This term led to evaluation of , the EQQ interaction energy, giving /k=0.9 ± 0.1 K for solid H₂ and 0.93 ± 0.05 K for solid D₂. It was found, moreover, that in solid H₂ there was an appreciable enhancement with time of the number of clusters of molecules in the rotational stateJ=1 at the expense of the number of isolated singles of similar molecules, due to rotation diffusion. On the other hand no rotation diffusion was observed in solid D₂ or in the solid mixtures of H₂ and D₂, except possibly in those mixtures containing 85% H₂. (3) A second anomalous term occurring at temperatures below about 0.8 K and, by extrapolation, having its maximum below 0.5 K (the lowest temperature of our measurements). This anomaly was observed only in those samples containing D₂ and is, as yet, unexplained in detail. A further major conclusion from our results in the temperature range 0.5–5 K is that there was no evidence in the form of a sharp anomalous jump in the specific heat of any of the mixtures which could be interpreted as indicating the occurrence of isotopic phase separation. This absence of phase separation is noted, in spite of the fact that thermodynamically it is energetically favorable in the temperature range of observation.Work partially supported by a grant from the National Science Foundation and by contracts with the Office of Naval Research and DOD (Themis Program).     

RF magnetron sputtered aluminium oxide coatings on iridium

The effects of process parameters on the microstructural morphology of aluminium oxide (Al₂O₃) coatings on Ir have been studied. Al₂O₃ coatings were deposited on Ir-coated isotropic graphite (IG) substrates at substrate temperatures of room temperature (RT)-1073 K, RF power of 200–600 W in an Ar, or Ar+1–10% O₂, sputtering gas atmosphere by RF magnetron sputtering. Al₂O₃ coatings which were deposited at high substrate temperatures and high RF powers in an Ar, or an Ar+O₂, sputtering gas atmosphere were found to contain a dense, fine columnar structure with a -Al₂O₃ phase, low Ar content and a relatively high hardness value of ca. 1050 H _v. Furthermore, high resolution transmission electron microscopy (HRTEM) results revealed the epitaxial growth of Al₂O₃ coatings on Ir-coated IG substrate. It was found that the interface between Al₂O₃ and Ir coatings was sharp and Al₂O₃ coatings remained intact with the Ir-coated IG substrate.     

Stability of Si3N4-Al2O3-ZrO2 composites in oxygen environments

Oxidation of dense Si₃N₄-Al₂O₃-ZrO₂ and Si₃N₄-Al₂O₃ compacts, at 873–1773 K and 98 KPa air atmosphere, results in two different parabolic oxidation regimes. Oxygen diffusion is likely to be the governing step at low temperature (T<1623 K (H= 100kJ mol^–1)), whereas at T> 1623 K (H = 800kJ mol^–1) metal cation diffusion through the grain boundary phase appears limiting. The excellent stability in oxygen environments of the Si₃N₄-Al₂O₃-ZrO₂ composites compared to other ZrO₂-Si₃N₄ materials derives from (i) absence of easy-to-oxidize Zr-O-N phases; (ii) reduced amount of grain boundary phase, and possibly (iii) decreased solubilization rate of the nitride phases in the high viscous oxide film.     

Nucleation in Dilute 3He-4He Liquid Mixtures at Low Temperatures

We present a study of phase separation from supersaturated ³ He- ⁴ He liquid mixtures at low temperatures addressing both the degree of critical supersaturation x_cr and the thermal-to-quantum crossover temperature T* for the nucleation process. Two different nucleation seeds are investigated, namely ³ He droplets and ⁴ He vortex lines with cores filled with ³ He. We have found that the experimental T* is reproduced when we consider that nucleation proceeds from ³ He droplets, whereas x_cr is reproduced when we consider ⁴ He vortex lines filled with ³ He. However, neither nucleation configuration is able to simultaneously reproduce the current experimental information on x_cr and T*.     

Interaction between a dislocation and various divalent impurity in KCl single crystals for the Fleischer's model taking account of the Friedel relation

The interaction between a dislocation and the impurity in KCl : Mg²⁺ (0.035 mol% in the melt), KCl : Ca²⁺ (0.035 and 0.065 mol% in the melt) and KCl : Ba²⁺ (0.050 and 0.065 mol% in the melt) was investigated from the strain-rate cycling test during the Blaha effect measurement. This was carried out at 77–254 K. As a result, it was found that the Fleischer's model taking account of the Friedel relation seems to be suitable for KCl : Ca²⁺ and KCl : Ba²⁺. However, it was not appropriate for KCl : Mg²⁺. Furthermore, the values of T _c, H(T _c) and G ₀ were obtained for the specimens. T _c is the critical temperature at which effective stress is zero. H(T _c) and G ₀ are the enthalpy and the Gibbs free energy of activation for the breakaway of the dislocation from the impurity, respectively. H(T _c) was almost the same for the specimens except KCl : Mg²⁺. G ₀ increased with increasing the divalent cation size. In addition, the tetragonality around the divalent ion-positive ion vacancy pair was estimated on the basis of G ₀ for the each specimen.     

X-ray Diffraction and Thermodynamic Studies of GdLiCr2O5

GdLiCr₂O₅ is synthesized from Gd₂O₃, Cr₂O₃, and Li₂CO₃ by solid-state reaction. This compound has an orthorhombically distorted perovskite structure with lattice parametersa= 10.78 Å, b= 10.63 Å, and c= 11.04 Å (V = 1263.9 ų, V _subcell = 158.0 ų, _x = 3.66 g/cm³, _meas = 3.57 ± 0.09 g/cm³). The heat capacity of GdLiCr₂O₅ is measured between 298.15 and 673 K by dynamic calorimetry. At 423 K, C _p ⁰ exhibits a lambda-type anomaly due to a second-order phase transition. The best fit equations for C _p ⁰(T) are derived, and the thermodynamic functions S ⁰(T), H ⁰() – H ⁰(298.15 K), and "(T) are calculated.     

Measurement of Gibbs energies of formation of CoF2 and MnF2 using a new composite dispersed solid electrolyte

Gibbs energies of formation of CoF₂ and MnF₂ have been measured in the temperature range from 700 to 1100 K using Al₂O₃-dispersed CaF₂ solid electrolyte and Ni+NiF₂ as the reference electrode. The dispersed solid electrolyte has higher conductivity than pure CaF₂ thus permitting accurate measurements at lower temperatures. However, to prevent reaction between Al₂O₃ in the solid electrolyte and NiF₂ (or CoF₂) at the electrode, the dispersed solid electrolyte was coated with pure CaF₂, thus creating a composite structure. The free energies of formation of CoF₂ and MnF₂ are (± 1700) J mol⁻¹; {fx37-1} The third law analysis gives the enthalpy of formation of solid CoF₂ as ΔH° (298·15 K) = −672·69 (± 0·1) kJ mol⁻¹, which compares with a value of −671·5 (± 4) kJ mol⁻¹ given in Janaf tables. For solid MnF₂, ΔH°(298·15 K) = − 854·97 (± 0·13) kJ mol⁻¹, which is significantly different from a value of −803·3 kJ mol⁻¹ given in the compilation by Barinet al.