Y xBayCuzO7− δ solid solutions

Y_xBa_yCu_zO_7– ceramics forming at isobaric conditions were studied by x-ray diffraction analysis, dynamic magnetic measurements and potentiometric titration. It was established that compositions: 0.8 x 1.2 y=2, z=3; x-1, 1.8 y 2.2, z=3; x=1, y=2, 2.7 z 3.4: are in the homogeneity range of 123. It was found that compositions with nonstoichiometric cations rations have minimum T_c.     

A generalized scaled equation of state for n-alkanes (methane to n-nonane) in the critical region

A generalized scaled equation of state has been developed to calculate thermodynamic properties of n-alkanes from methane (CH₄) to n-nonane (C₉H₂₀) in the critical region. The equation is valid in the reduced density range 0.7 _c1.3 at T=T _c and up to 1.2T _c at = _c.     

Spontaneous nuclear ferromagnetic ordering of in nuclei in AuIn2 Part I. Nuclear specific heat and nuclear susceptibility

We have measured the nuclear specific heat C_n and nuclear susceptibility _n of In nuclei (I=9/2, =5.5 _n) in the cubic intermetallic compound AuIn₂ (Korringa constant =0.11 Ksec) in the normal conducting state at 30K10mK and 2mTB115 mT. Our data show a positive nuclear Weiss temperature =+ 43 K and that the In nuclei undergo a nuclear ferromagnetic transition at T_c=35 K. The In nuclei experience an internal field of about 10 mT (obtained from C_n at T>T_c ). The nuclear ordering temperature T_c and the internal field increase with applied magnetic field. From the data we deduce exchange constants for the investigated system. The critical entropy reduction S(T_c)/S_max=8.6% and critical enthalpy E=0.28 RT_c are in reasonable agreement with the measured ordering temperature T_c,applying the Heisenberg model for a simple cubic I=9/2 spin system. The nuclear spin relaxation time calculated from the real and imaginary parts of _n is 10 msec at T>50 K, but drops to <1msec at T_c.This is the first observation of a spontaneous nuclear magnetic ordering transition in a not-hyperfine-enhanced metal at thermal equilibrium, i.e. at T _nuclear =T _electron .     

The preparation of high transition temperature superconducting Pb-Bi-Ge alloy filaments using the method of glass-coated melt spinning

The melt spinning of Pb-Bi-Ge alloys with Pyrex glass was investigated as a means of producing a superconducting long filament with highT _c. Continuous filaments with maximumT _c of more than 10 K of Pb_100-x-y Bi _x Ge _y (15x37 and 725) were obtained from the molten state at 1500 K with a winding speed of 0.95 m sec^–1. The Pb₄₉Bi₃₃Ge₁₈ filament, which was 34×10^–6 m diameter and a ductile material with a tensile strength of 20 MPa and elongation of 2.7%, exhibited superconductivity at the highestT _c of 14.3 K. These filaments were found to be polycrystalline with a grain size of more than 5000×10^–10 m and had a mixed structure of germanium (diamond) (h c p) and bismuth phases. The germanium element distributed homogeneously in the filament.     

Isochoric pvT measurements of SF6 in the density range 100 to 1200 kg · m−3

The pvT properties of SF₆ were determined by an isochoric method. The pressure p and the temperature T were measured in the pressure range 1.5 MPa p 9.0 MPa and in the temperature range 293 K T 340 K. The results are compared with data in the literature. The adsorption of SF₆ on glass and iron surfaces and its influence on the pvT measurements are also briefly discussed.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

A scaled equation of state for real fluids in the critical region

A scaled equation of state is proposed for real fluids in the critical region which incorporates asymmetry with respect to the critical isochore. In the range of reduced densities 0.65(/ _c)1.4 and for reduced temperatures (T/T _c)1.2, the equation represents P-V-T data for steam within the experimental accuracy.     

Flux line lattice states and pinning in niobium wire networks in high magnetic fields

The properties of the flux line lattice in the thin strands of superconducting niobium wire networks with thickness and width comparable to the penetration depth are investigated in the limit of very few flux lines. An angle dependent upper critical field is observed that is consistent with the existence of surface superconductivity. Our results indicate the existence of a flux line lattice in the complete field range B_c1 B B_c 1.4 B_c2. The flux line lattice undergoes a dimensional crossover when the transversal correlation length R_c is of the order of the strand width. The field dependence of the Labusch parameter due to surface pinning is determined and compared to a model for surface pinning by Bean-Livingston barriers.     

Heat capacity of titanium hydride at millikelvin temperatures

The heat capacity C of a TiH_1.98 bulk sample in zero and 72 mT magnetic fields has been measured at 8 mKT 600 mK. In zero magnetic field the sample shows metallic behavior. In a magnetic field the protons give an additional nuclear heat capacity contribution, whose value is in good agreement with the calculated one. From our data we also obtain an upper limit for the nuclear spin-lattice relaxation time ₁ and for the molecular hydrogen impurity concentration in the sample. ForT 10 mK, no superconducting or any other phase transition has been observed.     

Heat capacity and magnetic properties in the (PdCO y )H x -system at low temperatures

Results of heat capacity measurements on (PdCo _y )H _x (y 2.5 at %; 0 x 0.8) in the temperature range from 2 to 12 K are reported. The dependence of the coefficient of the electronic heat capacity on the Co content and on the x value is considered. For the binary alloy PdCo _y a decreasing spin wave contribution to the heat capacity (T ^3/2) is found, which agrees well with the known ferromagnetic properties of the alloy. In contrast, for the phase of (PdCo _y )H _x a contribution to the heat capacity can be separated, probably caused by the thermal excitation of magnetic clusters. The temperature dependence of this cluster contribution can be well explained by means of an Einstein function. Conclusions are drawn with regard to the intercluster interaction and to the number of atoms per cluster.     

Mean polarizabilities and second and third virial coefficients of the gases C2H4, C2H6, and SF6

Mean polarizabilities ₀(₀, T) as well as second and third virial coefficients B(T) and C(T) of the equation of state of the gases C₂H₄, C₂H₆, and SF₆ have been determined from refractive index measurements with a Michelson interferometer for wavelength ₀=632.99 nm in the overall ranges 250 KT 340 K and 0p3 MPa of temperature T and pressure p. Some negative C(T) values at low temperatures have been obtained. The C(T) data could be fitted satisfactorily to the simple three-parameter function C(T)= a(T–T ₀) exp[ –b(T–T ₀)], with the maximum near the critical temperature T _c. A possible correlation between C(T) and the vapor pressure p _v(T) is discussed.     

Nuclear ferromagnetic ordering of141Pr in the diluted Van Vleck paramagnets Pr1−x Y x Ni5

We report on the first investigation of the effect of magnetic dilution on nuclear magnetic spin interactions in metals; we studied this effect in the diluted Van Vleck paramagnets Pr_1–xY_xNi₅. In addition, we investigated the electronic magnetic properties of these intermetallic compounds. For this purpose, we have measured the nuclear and electronic susceptibilities of Pr_1–xY_xNi5 with x = 0 to 0.2 at 50K T 8 mK and at 2.2 K T 300 K. We observe a linear decrease of the electronic Van Vleck susceptibility and of the hyperfine enhancement factor by 13% (per mole Pr³⁺). The hyperfine-enhanced nuclear susceptibility decreases by about a factor of three when going fromx = 0 tox = 0.2. The samples show nuclear ferromagnetic transitions of¹⁴¹Pr with nuclear Curie temperatures T_c decreasing from 370 K to 100 K and Weiss temperatures decreasing from 218 K to 13 K in this concentration range. These data are compared with the results of mean-field calculations. In addition, we report on measurements of the nuclear spin relaxation time of these compounds, for which we find values of a few msec at millikelvin temperatures and a critical speeding-up at T_c. Our data give hints that for x > 0.2 the character of the nuclear magnetic transition may change, possibly to a nuclear spin glass freezing.     

Anomalous specific heat of the La-Ge system

The specific heat of the orthorhombically distorted -ThSi₂-type La-Ge intermetallic compound system has been measured in the temperature range 0.07 T 10 K. An anomalous behavior of the lattice part of the specific heat has been discovered. The specific heat data are analyzed and discussions are given, with particular attention to the superlattice structure observed in these compounds.     

Crystal structure and hydrogen occupation in H x V2O5(x=0.0–3.9)

Changes in the crystal structure and the lattice constants of powdered crystalline V₂O₅ after absorption of hydrogen were measured, as a function of the concentration, by X-ray diffractometry. The diffraction spectra obtained showed that three kinds of phases, (x<0.4 in H _x V₂O₅), (co-existing with either terminal phase in the range of 0.4x<3.2) and (3.2x3.9), appear up tox=3.9, which was found to be the upper limit of the absorption at 70C under a hydrogen pressure of 5 MPa. The crystal system of the three phases was determined to be orthorhombic, although the lattice constants differed. The diffraction lines were broadened, and the background of the X-ray spectra grew with hydrogenation, especially in the -region, which indicates the growth of the lattice distortion and a quasi-amorphous state. The occupation of hydrogen atoms in each phase is discussed on the basis of the experimental results.     

A four-parameter corresponding-states method for prediction of Newtonian,pure-component viscosity

The extended Lee-Kesler (ELK) method, introduced for calculating thermodynamic properties of polar as well as nonpolar fluids and their mixtures, has been adapted to the calculation of Newtonian, pure-fluid viscosity. The method is a four-parameter, corresponding-states technique requiring as input the critical temperature, critical pressure, a size/shape parameter , and a polar interaction parameter . Because and have been previously tabulated for many fluids (for calculation of thermodynamic properties) and may also be obtained directly from the radius of gyration and a single liquid density, respectively, the method contains no adjustable parameters and is predictive in nature. ELK viscosity predictions were compared to experimental data for nonpolar and polar fluids. For 36 different nonpolar fluids and a total of 5748 different points, the comparison yielded an absolute average deviation (AAD) of 7.88% with a bias of –4.45%. Similarly, the AAD was 10.62% with a bias of –5.34% for a comparison of 15 different polar fluids involving 1500 different points. With this method, viscosities can be calculated within the range 0.55 T _r2.00 and 0<P _r10.     

Temperature effects on the properties of Ge thin films

The effects of substrate temperature (T_s) on the properties of vacuum evaporated p-type Ge thin films have been investigated for 25s<400°C. Increase in the substrate temperature improves the crystallinity and increases the grain size resulting a gradual change from amorphous to polycrystalline structure which was attained above a substrate temperature of 225°C. Low resistive (1×10^–2 ohm-cm) and high mobility (280 cm²/V·s) films were obtained at T_s=400°C. It has been observed that the conduction mechanism in polycrystalline films was dominated successively by hopping, tunneling and thermionic emission as the sample temperature was increased from 40 to 400 K. In amorphous samples, conduction was described in terms of different hopping mechanisms.     

Non fermi liquid ground states in strongly correlated f-electron materials

Experimental efforts to characterize and develop an understanding of non Fermi liquid (NFL) behavior at low temperature in f-electron materials are reviewed for three f-electron systems: M_1–xU_xPd₃ (M = Sc, Y), U_1–xTh_xPd₂Al₃, and UCu_5–xPd_x. The emerging systematics of NFL behavior in f-electron systems, based on the present sample of nearly ten f-electron systems, is updated. Many of the f-electron systems exhibit the following temperature dependences of the electrical resistivity p, specific heat C, and magnetic susceptibility for T T₀, where T_o is a characteristic temperature: P(T) 1 –aT/T ₀, where a < 0 or > 0, C(T)/T (-1/T _o) In (T/bT ₀), and (T) 1 –c(T/T_o)^1/2. In several of the f-electron systems, the characteristic temperature T_o can be identified with the Kondo temperature T_k.     

High-T c superconductivity in theRBa2−xSrxCu3O7 system forR ≡ Y,Gd, Sm and Nd

RBa_2–x Sr_xCu₃O₇ samples whereRY, Gd, Sm and Nd were synthesized in oxygen between 930 and 1070 °C. The effect of the Ba substitution by Sr on the sample optimum synthesis temperature, microstructure and electrical properties was investigated for 0x1.0. The magnitude of the resistivity increase and of theT _c decrease due to the Ba substitution by Sr is found to depend on theR ionic radius. Moreover, the Sr addition has a decreasing effect onJ _c for allR elements.     

Critical review of the thermodynamics of the Na-Al-O system

The free energies of formation of various phases in the Na-Al-O systems are critically reviewed for the composition range between pure alumina and sodium-aluminate. The results clearly indicate that in the temperature range between 600 and 1100 K neither-alumina nor'-alumina (Na₂O.5Al₂O₃) are stable with respect to sodium; the most stable phase in that range being sodium aluminate. Least square fitting and statistical analysis of all the available data on the free energy of formation of-alumina deemed accurate yields G _form ⁰ (from oxides) = –8772+1.53T (J/mole of O) (600 T 1200K) If the composition of-alumina is assumed to be Na₂0.11Al₂O₃ then G _form ⁰ (Na_1/17Al_11/17O, from elements) = –565100+109.6T (J mole of O) (600 T 1200 K) If the composition is assumed to be 19 then: G _form ⁰ (Na_1/14Al_9/14O, from elements) = –564000+109.6T (J/mole of O) (600 T 1200K) Based on the only study to date, the best estimate for the free energy of formation of-alumina is G _form ⁰ (Na_1/8Al_5/8O, from the elements) = –567860+114T (J/mole of O) (600 T 1200 K)The uncertainty (95% confidence band) is estimated to be about ±200 J/mole of O. Based on these results-alumina should be thermodynamically stable at lower temperatures, but become unstable at temperatures >900 K and should, kinetics permitting, dissociate into-alumina and sodium aluminate. Experimental evidence, however exists that conclusively shows that-alumina is thermodynamically stable at much higher temperatures than 900 K which leads to the conclusion that G °_form for-alumina reported to date is too high and should be more negative.     

Experimental confirmation of the isotopic volume effect in superconducting molybdenum by means of energy-dispersive x-ray diffration at low temperatures

Differences in the lattice constants of Mo-100 and Mo-92 have been measured by x-ray diffraction, in order to search directly for the volume effect of isotopes in a superconductor. No significant difference in the lattice constanta(Mo-100)–a(Mo-92) could be detected at 290 K, while the differences –0.0014±0.0008 and –0.0029±0.0009 Å were detected at 85.3 and 4.31 K, respectively. These values, and their temperature dependence, are considered to be theoretically reasonable. The exponent in the isotope effect defined by T_cM^- is represented thermodynamically by =–( lnT _c / lnM)–( lnT _c / lnV)(d lnV/d lnM). From the results, d lnV/d lnM is found to be –0.033±0.009 at 4.31 K. Then, the second term representing the isotopic volume effect is estimated to be about 0.09, with lnT _c / lnV2.81. The observed value of is 0.33, so that the contribution of the second term, 0.09, is 27% of the value of . It becomes quite clear that the isotopic volume effect in superconducting Mo should not be neglected.     

On the calculation of second virial coefficients for nonspherical molecules

A self-consistent scheme is given for the calculation of the contributions of nonspherical molecular interactions to second virial coefficients. The usual longrange nonspherical interactions, which are asymptotically valid only at large separations, are modified by damping functions that render them valid at small separations as well. Numerical tables of new auxiliary functions J _n ^D (T^*) are given for 6n30 and 0.5T ^*10. These are designed to be used in the identical statistical-mechanical formalism for the second virial coefficient that had been developed for the undamped long-range nonspherical interactions.