Viscosity and diffusivity of a binary liquid mixture of critical composition: Study of the system 2-butoxyethanol/water

Results of measurements of the temperature dependence of the shear viscosity and the mutual diffusion coefficient of a 2-butoxyethanol/water mixture of critical composition are reported. The shear viscosities are measured with a capillary viscometer, and the diffusion coefficients by dynamic light scattering. The viscosity data are used to determine the regular (background) and the singular (critical) part of the viscosity and to analyze the crossover regime.Q ₀ has a value ofQ _o = (1.54 ± 0.60) × 10⁶ cm^–1, which is small for a binary mixture of components of small molar mass. The viscosity is dominated by singular contributions in a narrow temperature range (T _c – T) 1.2 K. The singular contributions can be neglected for temperatures (T_c–T) 11 K. The function =_b expz H, given in the literature, represents the viscosity data in the temperature range 3 mK (T _c – T) 25 K using the asymptotic form of the functionH for (q _d/q _c) 0 (q _d,q _c, parameter of the theory). This limit corresponds to the case in which the background diffusion coefficient Lib can be neglected. The reduced diffusion coefficient D^* calculated from the light-scattering data as a function of the scaling variable x(=q) is represented by the approximation of the dynamic scaling function proposed in the literature.     

Shear viscosity of the binary liquid mixture 3-methylpentane + nitroethane near the consolute point

Shear viscosity measurements of a mixture of 3-methylpentane and nitroethane at the critical concentration are presented in the temperature range 0.005 KT–T _c13.93 K, where T _c is the consolute or critical temperature. The data agree with earlier measurements.     

Isochoric Heat Capacity of Heavy Water at Subcritical and Supercritical Conditions

The heat capacity of heavy water was measured in the temperature range from 294 to 746 K and at densities between 52 and 1105 kg·m^–3 using a high-temperature, high-pressure adiabatic calorimeter. The measurements were performed at 14 liquid and 9 vapor densities between 52 and 1105 kg·m^–3. Uncertainties of the measurements are estimated to be within 3% for vapor isochores and 1.5% for the liquid isochores. In the region of the immediate vicinity of the critical point (0.97T/T _c1.03 and 0.75/_c1.25), the uncertainty is 4.5%. The original C _V data were corrected and converted to the new ITS-90 temperature scale. A parametric crossover equation of state was used to represent the isochoric heat capacity measurements of heavy water in the extended critical region, 0.8T/T _c1.5 and 0.35/_c1.65. The liquid and vapor one- and two-phase isochoric heat capacities, temperatures, and saturation densities were extracted from experimental data for each measured isochore. Most of the experimental data are compared with the Hill equation of state, and the overall statistics of deviations between experimental data and the equation of state are given.     

Fish-Tail Effect and Its Evolution Under Neutron Irradiation in Li-Doped YBa2Cu3O7?x

The evolution of the critical current density of Li-doped YBa₂Cu₃O_7–x polycrystalline samples submitted to neutron irradiation is investigated as function of magnetic field (0 B 6 T) temperature (5 T 85 K) and neutron fluence (0 9.98 × 10¹⁷ cm^–2). At fluences lower than 10¹⁷ cm^–2, a second peak in j _s vs. B dependence is present (fish-tail effect). Its magnitude decreases with increasing the fluence. Above 10¹⁷ cm^–2, the second peak of current density completely disappears; instead, the logarithmic susceptibility shows a second peak at a certain field B _infl. A dependence of B _infl on fluence is proposed.     

Upper critical and irreversibility fields in selectively doped YBa2Cu3O7

The electrical resistivity of (Y_1–xPr_x)Ba₂Cu₃O_7–gd and YBa₂(Cu_1–xZn_x)₃O_7– thin films and (Y_1–x. Tb_x)Ba₂Cu₃O_7– thin films and single crystals has been measured as a function of 0xx_crit, 2KT300K and OTH20T. The samples were oriented withc-axis parallel to applied fields. Upper critical field H_c2 and irreversibility field H_irr values have been determined from these measurements. Increased Tb doping appears to shift H_irr to higher temperatures. This coupled with observed twin peaks in magnetization measurements reflects an enhancement of flux pinning. Unlike Tb which does not appear to alterT _c , Pr and Zn doping of this system tends to depress bothT _c and the slope of the mean field normal phase-mixed phase boundary line (dH _c2/dT).     

Measurements of the isochoric heatc v at the critical point of SF6 under microgravity: Results of the German Spacelab Mission D2

During the Second German Spacelab Mission D2 (April 26 to May 6, 1993) the isochoric specific heatc _v of SF₆ was measured along the critical isochore under microgravity conditions with a newly developed scanning radiation calorimeter. This calorimeter provided the possibility to perform comparable heating and cooling runs with variable ramp rates since the spherical sample cell was heated and cooled only by radiation. During the experimental time of 220 h, 11 heating and cooling runs with different ramp rates were performed in a temperature range ofT–T _c=±6 K. ApproachingT _c by cooling from the homogeneous one-phase region avoided significant temperature and density gradients in the fluid, which would have distorted the integral measurement ofc _v. The inhomogenities introduced by a finite ramp rate were greatly reduced by the fast dynamic temperature propagation (critical speeding up). Thec _v data achieved with slow cooling runs are in remarkably good agreement with the theoretical prediction more than one order of magnitude closer to the critical point than anyc _v measurements done so far. The preliminary value for the critical exponent is 0.107±0.02, and for the amplitude ratio we obtainedA ^–/A ⁺=1.94±0.07. In contrast to the cooling runs, the heating runs showed a strong hysteresis ofc _v. A comparison to 1g measurements is provided.Invited paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, USA.     

Equation of State and Thermodynamic Properties of Pure D2O and D2O + H2O Mixtures in and Beyond the Critical Region

A parametric crossover model is adapted to represent the thermodynamic properties of pure D₂O in the extended critical region. The crossover equation of state for D₂O incorporates scaling laws asymptotically close to the critical point and is transformed into a regular classical expansion far from the critical point. An isomorphic generalization of the law of corresponding states is applied to the prediction of thermodynamic properties and the phase behavior of D₂O + H₂O mixtures over a wide region around the locus of vapor-liquid critical points. A comparison is made with experimental data for pure D₂O and for the D₂O + H₂O mixture. The equation of state yields a good representation of thermodynamic property data in the range of temperatures 0.8T _c(x)T1.5T _c(x) and densities 0.35_c(x)1.65_c(x).     

IR Spectra of La1.85Sr0.15Cu1?xMxO4?δ(M = Zn, Ni, Mg)

Polycrystalline samples La_1.85Sr_0.15Cu_1–xZn_xO_4– (0 x 0.3), La_1.85Sr_0.15Cu_1–xNi_xO_4– (0 x 0.3), and La_1.85Sr_0.15Cu_1–xMg_xO_4– (0 x 0.3) were synthesized by the solid-state reaction method. The crystal structure and phonon vibration were investigated by means of X-ray diffraction (XRD) and infrared spectrum. Zn, Ni, and Mg doping results in the lattice parameter c decreasing and a (b) increasing. The change of the phonon modes around 504 cm^–1 and 681 cm^–1 can be satisfactorily interpreted in terms of the change of the crystal structure and the itinerant nature of charge carrier in CuO₂ sheet. The relation between the structure and phonon vibration is discussed.     

Crossover equation of state of normal hexane in the critical region

The coefficients of the basic crossover equation of state of n-hexane are determined in the critical region from experimental P, , T and C_p, P, T data. In the reduced density and temperature ranges 0.35_c1.65 and 0.982T/T_c1.23 the root mean square errors of the calculated pressure, isobaric heat capacity, and isochoric heat capacity were 0.115%, 4.87%, and 3.04%, respectively.Academician M. D. Millionshchikov Petroleum Institute, Grozny, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 2, pp.185–191, August, 1993.     

Magnetic Ordering of Dy3+ Ions in RbDy(WO4)2 Single Crystal

The specific heat C(T) of the monoclinic RbDy(WO ₄ ) ₂ crystal has been studied at very low temperatures 0.2T1.9 K and in magnetic fields 0H0.38 T. The Neel temperature was shown to be equal to T_N = 0.818 ± 0.005 K. The experimental value of the effective exchange parameter was obtained to be equal to J/k = – 0.798 K. The C T) dependence below Neel temperature 0.5T_N0.99T_N ) is well described by 2D Ising model, whereas in the temperature region close above T_N (1.01 T_N2T_N) it can be described by neither 2D, nor 3D Ising model. The experimental and theoretical H-T_N diagrams for field H a are in a reasonable agreement for simple quadratic lattice.     

Fluctuations of the Order Parameter''s Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c T _c T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

Valence Charge Fluctuations and Tc Retention in Tl1?xPbxSr1+xLa1?xCuO5?y (0.0 ≤ x ≤0.5) System

Structural and x-ray photoelectron spectroscopy (XPS) studies of the Tl_1–xPb_xSr_1+xLa_1–xCuO_5–y (0.0x0.5) system have been carried out. The unit cell parameters increase with x. Irrespective of x the T_c is retained in this series and the optimum hole concentration (n_h) is maintained for x 0.5. An increase in Tl 4f and O ls binding energy with x suggests a reduction in their oxidation state. Origin of holes is discussed in terms of charge transfer between Tl, Pb and CuO₂ layers.     

The Nearly Classical Behavior of a Pure Fluid on the Critical Isochore Very Near the Critical Point Under the Influence of Gravity

Comprehensive measurements of the isothermal compressibility along the critical isochore in the critical region of pure sulfur hexafluoride (SF₆) and pure carbon dioxide (CO₂) were carried out. All measurements were performed with a multicell apparatus, especially designed for pT measurements in the critical region. The height of the measuring cells was either 30 or 11 mm. Independent of the cell height, we found for both fluids nearly classical values for the critical exponent in the limiting approach to the critical point. However, at a certain distance from the critical point {(T – T _c) 90 mK or 2.82 × 10^–4 for SF₆ and (T – T _c) 55 mK or 1.81 × 10^–4 for CO₂}, we observed a transition to values of the critical exponents which nearly meet the predictions of the renormalization-group theory. Since in the fitting range (T – T _c 1 mK) the correlation length ( 0.5 m) is very much smaller than the geometrical dimensions of the measuring cells, we conclude that the reason for the different behavior is an explicit gravity effect governing the inner critical region. The two different loci of the transition points for sulfur hexafluoride and carbon dioxide can be attributed to the different gravity impact on the fluid corresponding to the different critical densities of the two substances.     

Simultaneous Measurement of the Density and Viscosity of Compressed Liquid Toluene

A vibrating-wire densimeter described previously has been used to perform simultaneous measurements of the density and viscosity of toluene at temperatures from 222 to 348 K and pressures up to 80 MPa. The density measurements are essentially based on the hydrostatic weighing principle, using a vibrating-wire device operated in forced mode of oscillation, as a sensor of the apparent weight of a cylindrical sinker immersed in the test fluid. The resonance characteristics for the transverse oscillations of the wire, which is also immersed in the fluid, are described by a rigorous theoretical model, which includes both the buoyancy and the hydrodynamic effects, owing to the presence of the fluid, on the wire motion. It is thus possible, from the working equations, to determine simultaneously, both the density and the viscosity of the fluid from the analysis of the resonance curve of the wire oscillation, the density being related essentially to the position of the maximum and the viscosity to its width. New results of measurements of the density and viscosity of toluene in the compressed liquid region are presented, and compared with literature data. The density results extend over a temperature range 222 KT348 K, and pressures up to 80 MPa. The viscosity results cover a temperature range of 248 KT348 K and pressures up to 80 MPa. The uncertainty of the present density data is estimated to be within ±0.1% at temperatures 298 KT350 K, and ±0.15% at 222 KT273 K. The corresponding overall uncertainty of the viscosity measurements is estimated to be ±2% for temperatures 298 KT350 K, and ±3% for 248 KT273 K.     

Magnetic field properties of 1 at.% transition metal substituted Nb3Ge

Measurements of the critical magnetic field slope near the superconducting transition temperature T _care reported for samples of Nb_1–x Ge _x , (Nb_0.99Ti_0.01)_1–x Ge _x , and (Nb_0.99Zr_0.01)_1–x Ge _x for 0.15x0.25. The samples investigated were produced by trisputtering onto heated polycrystalline alumina substrates. The critical magnetic field slope near T _cfor substituted samples is comparable to that of Nb₃Ge alone. The highest T _cvalues, the highest resistance ratios, and the narrowest transitions, however, occur for samples with excess transition metal component.Supported by PSC-BHE Research Award Program.     

Critical light scattering in4He near the gas-liquid critical point

Scattered intensities of light were measured near the gas-liquid critical point of ⁴ He at scattering angles of 30, 60, and 90° as functions of the reduced temperature =|T–T _c |/T _c along the critical isochore (T>T _c ) and the coexistence curve (T>T _c ). The temperature range was 3×10 ^–5 <<1.5×10 ^–2 . Critical exponents and coefficients describing divergence of the generalized susceptibility and the correlation length are obtained as (T>T _c )=1.31±0.02, v(T>T _c )=0.66±0.02, ₀ (T>T _c )=4.2±0.6 Å, (T>T _c )=1.32±0.02, v(T>T _c )=0.68±0.02, _± (T>T _c )=2.6±0.7 Å, =0.06±0.06(T>T _c ), 0.05±0.05(T>T _c ), and ₀(T>T _c )/x_± (T>T _c )=3.6±0.4. It is pointed out that the quantal nature of ⁴ He has remarkable influence on the critical behavior of ⁴ He in the above-mentioned temperature region.     

Influence of proton irradiation on the critical current densityj c(B,T) of NbTi

The critical current densityj _c of a copper-coated single-core wire of Nb-50 wt.% Ti has been measured as a function of the magnetic flux density (1B8T) and the temperature (2.5 KTT _c). Irradiation with 3.1-MeV protons at 25 K up to a flux of1·10 ¹⁷ cm ^–2 led to aT _c drop of 0.17 K and an almost uniform degradation of 19% over the completej _c(B, T) surface. After annealing for 1 h at 285 K only 40% of thej _c degradation remained. Thej _c(B, T) surface can be described, independent of the irradiation dose, approximately byj _c(B, T) B _c2(T) – B, giving a pinning force density ofF _p=j _cB4F _p,max b(1–b), whereb=B/B _c2 andF _p,max B _c2 ² . The defect structure responsible for the vortex pinning is not affected by irradiation. Thej _c decrease on irradiation results from approximately 70% from a decrease ofF _p,max and 30% from theT _c decrease.This work has been supported by the technological program of the Federal Department of Research and Technology of the FRG. The author alone is responsible for the contents.     

The critical constants of long-chain normal paraffins

Because of the recent availability of the critical constants of normal alkanes up to octadecane, some modifications in the estimation procedures for the critical constants have become necessary. It has been shown that the equation of Ambrose for the critical temperature of normal alkanes leads to the result that as n , the limiting value for the critical temperature is equal to the limiting value for the normal boiling point and the limiting value for the critical pressure is 1 atm. Currently, the CH₂ increment for the critical volume is considered constant. The recent data of Teja have shown that the CH₂ increment increases indefinitely in a homologous series until the critical volume reaches its limiting value. This has made the current procedure for estimating the critical volume obsolete. Taking into account the new measurements of Teja, we have now developed new equations for estimating the critical constants. The limiting values for an infinitely long alkyl chain for T _b, T _c, P _c, and V _c have been found to be 1021 K, 1021 K, 1.01325 bar, and 18618 cm³ · mol^–1, respectively. These new concepts have been applied to the estimation of various properties other than the critical constants.Nomenclature M Molar mass, kg·mol ^–1 - V _c Critical volume, cm³·mol^–1 - V ₁ Saturated liquid volume, cm³·mol^–1 - P _c Critical Pressure, bar - T _c Critical temperature, K - T _b Normal boiling point, K - T _B Boyle temperature, K - T _A Temperature at which the third virial coefficient is zero, K - V _c Limiting value of critical volume = 18,618 cm³ · mol^–1 - P _c Limiting value of critical pressure=1.01325 bar - T _c Limiting value of critical temperature = 1021 K - T _b Limiting value of normal boiling point = 1021 K - P _b Pressure at the normal boiling point, 1 atm - Z _c Critical compressibility factor - Z _c Limiting value for the critical compressibility factor = 0.22222 - R Gas constant, 83.1448×10^–6m³ · bar · K^–1 · mol^–1 - Acentric factor - X (T _c–T _b)/T _c - X ₁ (T _c–T)/T _c - X ₂ 1–(T _B/T)^5/4 - X ₃ 1–(T _A/T)^5/2 - Y P _c/RT _c - Surface tension, mN · m^–1 - B Second virial coefficient, cm³ · mol^–1 - B Limiting value for the second virial coefficient = –30,463 cm³ · mol^–1 - C Third virial coefficient, cm⁶ · mol^–2 - C _b Third virial coefficient at the normal boiling point, cm⁶ · mol^–2 - C _c Third virial coefficient at the critical temperature, cm⁶ · mol^–2 - C _B Third virial coefficient at the Boyle temperature, cm⁶ · mol^–2 - H _vb Enthalpy of vaporization at the normal boiling point, kJ · mol^–1 - n Number of carbon atoms in a homologous series - p Platt number, number of C-C-C-C structural elements - a, b, c, d, e, etc Constants associated with the specific equation - T _c ^* , T _b ^* , P _c ^* , V _c ^* , etc. Dimensionless variables     

Spin Gap Effects on Bulk R1+xba2?x Cu3O7?δ (R=Eu or Nd and 0≤x≤0.4)

Spin gap effects on the underdoping states of the bulk system of R_1+x Ba_2–x Cu₃O_7– (R = Eu or Nd and 0 x 0.4) were investigated through transport property measurements. The underdoping states were achieved by, alternatively substituting R³⁺ for Ba²⁺ ions in the system rather than adjusting the oxygen deficiency. The excess R³⁺ ions were to occupy the Ba sites of the crystalline lattice as revealed from Rietveld analysis for powder X-ray diffraction. The underdoped materials were observed to first undergo spin pairing transition in the temperature range well above T _c, and come across with superconducting transition at T _c. The increasing feature observed for spin gap temperature and the decreasing one for T _c, as the concentration of holes decreases, are in qualitatively good agreement with theoretical predictions from the mean-field RVB model.     

Superconductivity in bulk A15 Nb3Si under pressure

We have determined the effect of hydrostatic pressureP on the superconducting transition temperatureT _c of bulk, A15 Nb₃Si. For 0P20 kbar (2 GPa),T _c decrease linearly with increasing pressure at a rate T _c/P=–2.67×10^–5 K/bar. From an estimate of T _c/P obtained using recent band structure calculations for the density-of-electronic-states change as a function of lattice parameter in Nb₃Si, we conclude that the pressure dependence of the electron-phonon interaction primarily determines T _c/P.Work performed under the auspices of the U.S. DOE.