Equation of state of a3He-4He mixture near its liquid-vapor critical point

Measurements of the pressure coefficient (P/T)_,x are reported for a ³ He- ⁴ He mixture with a mole fractionX=0.805 of ³ He in the neighborhood of the liquid-vapor critical point. These include data on 16 isochores taken over the density interval–0.50.5 and over the temperature range–0.1 t0.1, where =(– _c )/ _c andt=(T-T _c )/T _c ,with _c andT _c ,respectively, the critical density and temperature of the mixture. From the discontinuity of (P/T)_,x at the boundary between the two-phase and the one-phase regions we determine the dew-bubble curve nearT _c with better precision than was done in recentPVT experiments. From the extrapolation of data not approachingT _c closer than1 mK, (P/T)_,x along the critical isochore appears to be discontinuous atT _c ,while for the isochore / _c 0.92, (P/T)_,x is continuous across the dew curve. It is found that this latter isochore cuts the dew curve at its highest temperature. These observations are discussed in terms of general thermodynamic arguments and theoretical predictions of the asymptotic behavior. We calculate (P/T)_,x from the scaling equation of state proposed by Leung and Griffiths for ³ He- ⁴ He mixtures, using their numerical parameters. In spite of some systematic deviations, especially in the two-phase region, there is in general good agreement with experimental results. In particular, the shape of the measured dew-bubble curve and the apparent discontinuity of (P/T)_,x along the critical isochore show excellent agreement with theory.Work supported by a grant from the National Science Foundation. A report of this work has been presented at the Washington Meeting of the APS [Bull. Am. Phys. Soc. 20, 618 (1975)].     

Carrier Mobility in GaSb–V2Ga5and GaSb–GaV3Sb5 Eutectic Alloys

Experimental evidence is presented that semiconductor–metal eutectics with a low content of the metallic phase (4 vol %) are similar in electronic structure to inhomogeneous semiconductors. The microstructure of undoped and Te-doped GaSb–V₂Ga₅ and GaSb–GaV₃Sb₅ eutectic alloys is examined, and the Hall mobility of carriers in these alloys is determined. The anomalous temperature variation of Hall mobility in GaSb–V₂Ga₅ ( T ²) and GaSb–GaV₃Sb₅ ( T ⁵) is interpreted in terms of infinite clusters ofn-type metallic inclusions embedded in a p-type semiconductor matrix and interconnected through overlapping inhomogeneous interfaces. It seems likely that the difference in conductivity type between the semiconductor matrix and the infinite clusters gives rise to a random large-scale potential relief. Te compensation of GaSb in the eutectic alloys causes the Hall mobility to vary more rapidly with temperature, T ³ to T ¹⁰, which is interpreted as due to an increase in the amplitude of the random large-scale potential relief, the formation of infinite clusters, and partial compensation of unintentional acceptor doping in the semiconductor matrix.     

An optimal control model of the life-cycle research productivity of scientists

A continuous time model using optimal control techniques is presented which implies that a scientist's productivity will eventually decline with age. This implication is at variance withCole's empirical findings¹ but is consistent withDiamond's empirical findings.²     

Susceptibility and Pressure Study of 3He nano-Clusters

Simultaneous measurements of pressure and magnetic susceptibility have been made in ³ He nano-clusters embedded in a ⁴ He matrix, following phase separation of the mixture. Susceptibility measurements extend from 0.5 mK to 10 mK for three different samples, which either undergo partial melting upon further cooling, or separate with liquid already present. The magnetic behavior of the clusters indicates solid fractions of 77, 54, and 19%, respectively, for pressures of 3.36 MPa, 3.06 MPa, and 2.88 MPa. The susceptibility of the 3.36 MPa sample follows a Curie law to the lowest temperature. For 3.06 MPa, we observe a kink in at 1.1 mK, which is approximately the ordering temperature T_N of the pure bulk ³ He if it existed at this pressure. However is almost constant down to 0.6 mK, with no drop at 1.1 mK, and no frequency shift, within our resolution of 10 Hz. Thus if there is magnetic ordering at 1.1 mK it is quite different than for bulk ³ He, not the U2D2 phase. For 2.88 MPa, follows a Curie-Weiss law with a positive Weiss =140 K, indicative of a ferromagnetic tendency, similar to that seen in 2D films.     

NMR of 89Y in the Copper-Oxide Spin-Chain Compound Ca2+x Y2−x Cu5O10

We report NMR lineshape, spin-lattice relaxation time T ₁, and spin-spin relaxation time T ₂ data at 17 MHz (8.07 T) for ⁸⁹Y in the copper-oxide spin-chain compound Ca_2+x Y_2–x Cu₅O₁₀. For x=0, a broad, asymmetric line with width 90 kHz is observed for T=250–300 K. The spectra exhibit an appreciable average shift (H/H+0.7%) and sharpen at lower temperature, possibly due to increasing intrachain ferromagnetic correlations. T ₁ and T ₂ decrease with decreasing temperature. The Tl data imply a short correlation-time limit, with _e=3–5×10^–11 s. The T ₂ data apparently include a contribution from dipolar interactions with copper nuclei. Relaxation time data for a doped (x=0.5) compound surprisingly show more rapid relaxation.     

On the solution of nonstationary heat-conduction problems with variable heat-transfer coefficient

The article describes an exact method for calculating the temperature field in solids when they are heated in a medium with a variable heat-transfer coefficient and a nonuniform initial temperature distribution.Notation temperature - L thickness of plate - x space coordinate - a thermal diffusivity - thermal conductivity - heat-transfer coefficient - t time - X=x/L dimensionless coordinate - Fo=at/L² Fourier number - Bi(Fo)=(Fo)L/ Biot number Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 5, pp. 921–924, May, 1971.     

High-temperature deformation of solution-treated Al-0.6% Si-1% Mn-0.7 Fe alloy

The tensile properties of Al-0.6% Si-1% Mn-0.7% Fe alloy were investigated in the temperature range 295–773 K, to assess the effect of the precipitation of Si, Mn and Fe during ageing and deformation on the mechanical properties. The alloy showed a pronounced drop in ductility at elevated temperatures. Elongation-to-fracture versus temperature-of-deformation curves are evaluated as a function of the strain rate. The elevated-temperature yield and ultimate tensile strength (UTS), ductility, strain-rate sensitivity and strain-hardening exponent have a strain-rate dependence. The minimum in the strain-rate sensitivity versus temperature curve is coincident with the elongation minimum temperature. At low and high temperature ranges the flow could be represented by the constitutive equations = K ₂ ⁿ and = K ₃ ^m , respectively. There is also a discussion of the activation energy for deformation in the vicinity of the ductility minima and from plotting the logarithm of the strain rate versus the reciprocal absolute temperature at a constant yield strength (18 MN m^–2). A tentative model based on the diffusion of Si, Fe and Mn in Al and the subsequent precipitation of Si, FeAl₃, MnAl₆ and -Al₁₂Mn₃Si is postulated to explain the loss in ductility at high temperatures and the corresponding change in strength. An attempt is made to correlate strength, ductility and structural changes at elevated temperatures.     

Isochoric (p-π-T) measurements on liquid and gaseous air from 67 to 400 K at pressures to 35 MPa

Comprehensive isochoricp--T measurements have been carried out on liquid and gaseous air along 16 isochores at densities ranging from 2 to 32 mol · dm^–3. The air mixture has a nominal composition of 0.7813 N₂ + 0.2096 O₂ + 0.0092 Ar. Thep--T data cover a temperature range from 67 to 400 K at pressures up to 35 MPa. Comparisons with experimental results from independent sources are presented using a fundamental equation of state based. in part, on thep--T data from this study.     

Phase diagram for the helical texture in 3He-A

The stability region of the helical textures in³He-A in thev _s-H phase diagram is theoretically determined. Herev _sis the superflow velocity andH (v _s) is the external magnetic field. The pitch of the helixk ^–1 that minimizes the Gibbs free energy and the corresponding inclination angles ofl andd are determined numerically as functions ofv _sandH. The NMR frequencies (two longitudinal and four transverse frequencies) associated with the helical texture are obtained.Work supported by the National Science Foundation under Grant No. DMR 76-21032.     

Prediction of thermodynamic properties for fluid nitrogen with molecular dynamics simulations

Molecular dynamics simulation results in the isochoric isothermal ensemble are reported for a two-center Lennard Jones model of fluid nitrogen characterized by the fixed molecular elongationL = 1 = 0.3292, New values of and were determined by fitting the vapor pressure and the saturated liquid density of the model to experimental data at 94,67 K. The required vapor liquid equilibrium data of the model were taken from a study using the NpT + test particle method. The resulting values are k = 36.32013 K (36.673 K) and = 0.32973 nm (0.33073 nm), with values in parentheses being those obtained previously from a Weeks Chandler Andersen-type perturbation theory. Then pressures and internal energies were calculated by molecular simulations for 110 state points in the temperature range from 72 to 330 K and for densities up to 35 mol · L¹. Comparison of the predictions based on the new parameters with the empirical equation of state of Jacobsen et al. shows good to excellent agreement except in the near-critical region. Moreover. for almost all state points the new parameters yield an improvement over old ones from perturbation theory.     

Observation ofd-wave interference pattern:Short junction is crucial

The properties of a one-dimensional 0--Josephson junction, consiting of ans-d junction or selected boundaries between three suitably orientedd-wave superconductors, are studied. It is shown that ad-wave type interference pattern can only be observed in the field-modulated critical current if the junction is short compared to the Josephson penetration depth. However, the behavior of a 0- junction is found to be indistinguishable from that of a 0-junction betweens-wave superconductors in the long junction limit, due to the formation of a -vortex.     

Temperature variation of the Lorenz ratio for high-purity gallium between 1.4 and 4.2 K

The electrical and thermal resistivity ( andW, respectively) of a large (1 cm diam) single crystal of high-purity gallium, oriented along theC axis, have been measured as a function of temperature over the range of 1.4–4.2K. Both andWT can be fitted to the same power law with an exponent close to three. As a consequence, the temperature-dependent part of the Lorenz ratio has a relatively constant value of about 0.4×10^–8V²·K^–2 for the entire range. Qualitative explanation for this is attempted in terms of the possible scattering mechanisms involved.     

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*.     

Saturated liquid viscosity correlations for alternative refrigerants

This article presents dimensionless equations for the temperature dependence of the saturated liquid viscosity of R32, R123 R124, R125, R134a. R141b. and R152a valid over a temperature range of engineering interest. The correlation has the form _n ^D =A +BT, where _D is the dimensionless fluidity (l _D) andT _D is a dimensionless temperature.n. A. andB are evaluated for each of the above refrigerants based on a least-squares fit to experimental data. This equation is found to provide an improved fit over those existing in the literature up toT _D=0.8.     

Consistency conditions for the integral equations of liquid structures

A thermodynamic consistency principle is established for the closure relations in integral equations that can yield accurate correlation functions as well as accurate thermodynamic properties, A brief lour d'horizon is given for existing consistency approaches. In addition to the common pressure consistency and the pressure energy consistency, we introduce a third requirement based on the Gibbs-Duhem relation. We found that Gibbs Duhem relation, mediated through the chemical potential, is instrumental in procuring accurate behavior of the bridge function and cavity Junction in the overlapping region (0 <r < ). We test the Lennard Jones fluid over wide ranges ofT ^* andp ^* (T ^* as low as 0.72 andp ^* up to 11,90(, For more than IS state points we obtain excellent agreement in internal energy, pressure, and chemical potential. Comparison with Monte Carlo data on the bridge Junction and the radial distribution function also shows that the present approach is highly accurate.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Parallel-plate resonator of variable spacer thickness for accurate measurements of surface impedance of high-Tc superconductive films

Measurements of microwave surface impedance of high-T _c films at gigahertz frequencies and nitrogen temperature are performed. A simple technique employing a parallel-plate resonator with liquid nitrogen as a dielectric spaces is suggested. The use of a precise mechanical device provides smooth changing of distance between films from 200m down to zero. Coupling to the resonator is accomplished by means of two small antennas-half-wave vibrators for frequency 10 GHz. The method for determining resistivity and magnetic field penetration depth was based on the analysis of spacer thickness dependences of the resonator quality factor and frequency. YBa₂Cu₃O₇ films produced by a laser deposition technique on CaNdAlO₄ substrates withT _c =91 K andj _c =10⁷ A/cm² and on NdGaO₃ substrates withT _c =91 K andj _c =10⁶ A/cm² are examined, and the valuesR _s =0.6 m,=348 nm atf=8.97 GHz andR _s =0.5 m,=250 nm atf=10.12 GHz, respectively, are obtained at 77 K.     

Sound Propagation in 3He and 4He Above the Liquid-Vapor Critical Point

A scaled plot is presented of published sound attenuation and dispersion data of ³ He and ⁴ He along the critical isochore above T_c as a function of the reduced frequency ^* . Here ^* / with the frequency and the order parameter relaxation rate, where the latter is determined from experimental transport and thermodynamic data. For a given isotope, the scaled data obtained at different frequencies lie on a single curve. These curves are different, however, for ³ He and ⁴ He. The resulting scaled plot is compared with recent predictions by Onuki, and by Folk and Moser. The crossover-term contribution in from the background thermal conductivity data is compared with the form derived by Bhattacharjee and Ferrell. In the Appendix, the calculation of for both ³ He and ⁴ He from experiments is described and discussed.     

A new formulation of Gor'kov's theory of clean type II superconductors

From Gor'kov's theory a functional for the free energy of the flux line lattice is derived which depends explicitly on the potentials andA. It may be interpreted as an extension of the Ginzburg-Landau functional to arbitrary temperatures. For periodic || ² andH=rotA, the functional is a rapidly converging series, of which often the main term (corresponding to the Brandt-Pesch-Tewordt approximation) plus a correction term depending on the lattice structure suffices. From the cutoff series all previously known results for the special casesT T _c ,B H _c2 ,orB 0 are reproduced by minimization with respect to || ² andH.     

Assessing assessments of British science. Some facts and figures to accept or decline

Conclusions After a rather comprehensive analysis of severalSCI based publication productivity indicators, it can be concluded that neither the view of a continuing decline⁵ nor that of a remarkable increase⁶ of British science in the first half of the eighties can be supported by valid bibliometric/scientometric arguments. The annual changes of any of the indicators considered had no statistical significance, and no trend distinguishable from the effect of random fluctuations could be observed. Some of the indicators showed a local minimum in 1982, the significance of which can be clarified only in the frame of a longer range study.All the abovementioned refers to British science in general. As a previous paper⁵ reported also a decline of British publication performance in some science fields and subfields these would have to be investigated one by one with the above described methodology. The present authors did investigate the research performance of British analytical chemistry which has been mentioned as a declining subfield.⁵ Our results however showed¹² that analytical chemistry is a flourishing subfield in Britain.Finaly we mention thatNederhof in a recent study on the performance of six industrial nations in the field of biotechnology insisted that his findings contradict earlier reports⁴ showing UK losses in article productivity in basic research.¹³     

Signal and noise theory for a dc SQUID amplifier

A theory is presented for the gain and noise in an amplifier based on a dc SQUID. In the lumped circuit approximation, the total inductance of the input circuitL _T is coupled to the SQUID inductanceL via a mutual inductanceM _ii = _e (LL _T )^1/2 and is in series with a voltage source with a resistanceR _i and a capacitanceC _i . The results are expressed in terms of parameters for a SQUID with reduced inductance (1– _e ² )L. The voltage gain of the amplifier at frequency /2 isM _i V ^r /Z _T ^* (), while the total voltage noise at the output of the SQUID isV _N ^r ()+M _i ² V ^r J _N ^r () (R _i + 1/jC _i )/L _TZ _T ^* (). Here,Z _T ^* ()=Z _T ()–J ^r M _i ² (R _i + 1/jC _i )/L _i , whereZ _T () is the total impedance of the unloaded input circuit,V ^r andJ ^r are the flux-to-voltage and flux-to-circulating current transfer functions of the reduced SQUID, andV _N ^r () andJ _N ^r () are the noise voltage and noise current of the reduced SQUID.