High-enthalpy hot-shot wind tunnel with combined heating and stabilization of parameters

In the present paper, we consider instrumentation and the experimental procedure for conducting tests in a highenthalpy short-duration wind-tunnel facility, namely, a hypersonic hot-shot wind tunnel. We consider operation of the hot-shot wind tunnel with the test gas (TG) parameters kept constant during the regime and also operation of the tunnel as a traditional shock tube with a decay of the TG parameters that occurs as the TG leaves a constant-volume settling chamber. Stabilization of the TG parameters is achieved by using a pressure multiplier installed coaxially with the settling chamber, the configuration presenting a linear arrangement of the two components. Unloading of pressure multiplier dynamic component is achieved by using an equalizer whose piston moves in the opposite direction to the multiplier piston system. Several modes of wind tunnel operation with various combinations of different TG heating methods (electric arc, chemical energy, adiabatic compression, or heating in an external with respect to the settling chamber heat source) are possible. The design of a device responsible for diaphragm breakdown delay is considered. The design and dimensions of the wind tunnel provide for its normal operation under the following conditions: range of Mach numbers M = 4–20, range of settling-chamber temperatures T _ch1 = 600–4000 K, and settling-chamber pressure p _ch1 up to 200 MPa (in operation with a double settling chamber, the stagnation pressure p _0n = p _ch2 can be varied from 1 to 200 MPa). The settling chamber volume (80–100 dm³) is sufficiently large, allowing obtaining a 1-m diameter hypersonic stream in the test section during ～ 100 ms (in combination with a second settling chamber).     

Thermal conductivity of solid cyclohexane in orientationally ordered and disordered phases

Thermal conductivity Λ _P of solid cyclohexane is measured at a pressure P = 0.1 MPa in the temperature range from 80 K to the melting point, which covers the ranges of low-temperature orientationally ordered phase II and high-temperature orientationally disordered phase I. Thermal conductivity Λ _V is measured at a constant volume in orientationally disordered phase I. The thermal conductivity measured at atmospheric pressure decreases with increasing temperature as Λ _P ∝ T ^−1.15 in phase II, whereas Λ _P ∝ T ^−0.3 in phase I. As temperature increases, isochoric thermal conductivity Λ _V in phase I increases gradually. The experimental data are described in terms of a modified Debye model of thermal conductivity with allowance for heat transfer by both phonons and “diffuse” modes.     

Critical behavior of the heat capacity of the manganite La<Subscript>0.87</Subscript>K<Subscript>0.13</Subscript>MnO<Subscript>3</Subscript>

The heat capacity of the manganite La_0.87K_0.13MnO₃ has been measured in the temperature range 80–350 K. The nature of the ferromagnetic phase transition and the critical properties of heat capacity near the Curie temperature have been studied. The regularities of variations in the universal critical parameters near the phase transition point have been established. The calculated critical exponent and amplitudes of the heat capacity with allowance for corrections on the scaling (α = −0.13 and A ⁺/A ⁻ = 1.178) correspond to the critical behavior of the 3D Heizenberg model.     

Discrete nature of superconducting nanochannels in the cuprate pseudogap regime at <Emphasis Type="Italic">T</Emphasis> > 200 K

Intensification of segregation processes with a decrease in the charge carrier concentration in cuprates should lead, according to the string model, to a decrease the discrete width w _ν of dominant bosonic stripes and an increase in their critical temperature T _c up to 1200 K. A comparison of the simulated results with the experimental data gives grounds to conclude that the anomalies of the electronic properties observed in the pseudogap regime of lightly dopes cuprates are most likely to be due to the formation of a metastable network of superconducting nanochannels with T _c > 200 K from the bosonic stripes limited by w _ν ≤ 2 nm.     

Estimation of Kauzmann temperature and isenthalpic temperature of metallic glasses

We propose expressions for the estimation of the isenthalpic temperature T ₀ (T ₀ = αT _m , α is a semi-empirical parameter and 0 ⩽ α < 1, T _m is the solidus temperature) and the Kauzmann temperature T _k (T _k = T _m exp(α−1)) for glass forming alloys. It is found that T _k estimated by T _k = T _m exp(α−1) is in agreement with that directly calculated from the heat capacity data, indicating that T _k = T _m exp(α − 1) can be used to estimate T _k of glass forming alloys. T ₀ estimated by T ₀ = αT _m , on the other hand, widely deviates from that of directly calculated from the heat capacity data. This suggests that the enthalpy difference of the under-cooled liquid and the crystal might be a nonlinear function of the temperature below T _k . Moreover, the Gibbs free energy difference ΔG is not sensitive to the deviation of α.     

Microscopic theory of a strongly correlated two-dimensional electron gas

The rearrangement of the Fermi surface in a diluted two-dimensional electron gas beyond the topological quantum critical point has been examined within an approach based on the Landau theory of Fermi liquid and a nonperturbative functional method. The possibility of a transition of the first order in the coupling constant at zero temperature between the states with a three-sheet Fermi surface and a transition of the first order in temperature between these states at a fixed coupling constant has been shown. It has also been shown that a topological crossover, which is associated with the joining of two sheets of the Fermi surface and is characterized by the maxima of the density of states N(T) and ratio C(T)/T of the specific heat to the temperature, occurs at a very low temperature T _⋄ determined by the structure of a state with the three-sheet Fermi surface. A momentum region where the distribution n(p, T) depends slightly on the temperature, which is manifested in the maximum of the specific heat C(T) near T _*, appears through a crossover at temperatures T ∼ T _* > T _⋄. It has been shown that the flattening of the single-particle spectrum of the strongly correlated two-dimensional electron gas results in the crossover from the Fermi liquid behavior to a non-Fermi liquid one with the density of states N(T) ∝ T ^−α with the exponent α }~ 2/3.     

Investigation of current-voltage characteristics of the <Emphasis Type="Italic">n</Emphasis>-CdS-<Emphasis Type="Italic">p</Emphasis>-CdTe structure with an extended layer of the intermediate solid solution

This paper reports on the results of investigations of n-CdS-p-CdTe heterostructures with an extended layer of the intermediate solid solution which satisfies the ratio w/L ≈ 10 (where w is the base length and L is the diffusion length of minority charge carriers). The current-voltage characteristics of these structures over a wide range of voltages are adequately described by power relationships of the type J ≈ AV _α, where the exponent α varies with an increase in the voltage. The results obtained are explained within the theory of drift mechanism of charge transfer with allowance made for the possibility of exchanging free charge carriers inside recombination complexes.     

On the mechanism of phase transformation in Ag2Se

A differential thermal analysis ΔT _y (T) in vacuum has been performed, and the temperature gradient ΔT _x (T) along the Ag₂Se sample during the transition α → β has been studied. It has been shown that the transitions α → α′ and β′ → β are displacive transitions and that the transition α′ → β′ is a reconstructive transition. It has been found that the temperature gradient along the sample during the transition α′ → β′ passes through a deep minimum due to a strong increase in the specific heat capacity.     

Radial alpha temperature exponents between 0.3 and 1.0 AU

Summary The dynamical characteristics of α-particles in the solar wind between 0.3 and 1.0 AU are studied. It has been found that the connections between alpha temperature and speed, normalized at 1AU, as well as radial temperature exponents significantly differ from those found for protons. Between α and proton temperature, generally, the α temperature depends stronger on flow speed. Nevertheless, the flow characteristics of alphas and protons concerning their speeds are similar. For example, the dependence ofT _α onV _α is stronger also for slow than for fast alpha flows. The alpha radial temperature decreases much slower than proton temperature away from the Sun, according to the relationT _α/T ₀=(r/r ₀)^−k withk varying from very low (0.10) to moderate values (0.60).T _α,T ₀ are the alpha temperatures at distancesr,r ₀(=1 AU), respectively. These findings strongly suggest an additional heating of alphas, for example by Alfven waves or Coulomb collisions. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Anomalies of the specific heat near the quantum critical point in Tm<Subscript>0.74</Subscript>Yb<Subscript>0.26</Subscript>B<Subscript>12</Subscript>

The behavior of the specific heat near the quantum critical point x ∼ 0.3 in the Tm_{1 − x} Yb _x B₁₂ system has been studied. Detailed measurements have been performed on high-quality single-crystalline Tm_0.74Yb_0.26B₁₂ samples within a wide temperature range of 1.9–300 K in a magnetic field up to 9 T. The temperature dependence of the magnetic contribution to the specific heat has a logarithmic divergence of the form C/T ∼ lnT at T < 4 K, which can be attributed to the quantum critical behavior regime suppressed by the external magnetic field. The Schottky anomaly of the magnetic contribution to the specific heat in Tm_0.74Yb_0.26B₁₂ has been analyzed.     

Influence of Thomson effect on the resultant local Seebeck coefficient in thermoelectric composite materials

The resultant local Seebeck coefficient α _R (=α _S−α _T) at the interface of a thermoelement has not yet been measured, although it is an important factor governing the thermoelectric efficiency, where α _S is the local Seebeck coefficient and α _T is the one caused by the Thomson effect. It is shown in this paper that α _S, α _T, and α _R of the p- and n-type Cu/Bi–Te/Cu composites are obtained analytically and experimentally on the assumption that the local temperature of the composite on which the temperature difference ΔT is imposed varies linearly with changes in position along the composite. They were indeed estimated as a function of position from the local experimental data of R,ΔI,ΔT, and V generated by applying an additional current of ±I to the composite, where R is the electrical resistance and ΔI is a current generated by the composite. As a result, it was found that the absolute values of α _S at the hot interface of the p- and n-type composites are approximately 1.5 and 1.4 times higher than their lowest values in the middle region of the composite, respectively, while those of α _T are less than 8% of α _S all over the composite and are so small that the relation α _R≈α _S can be held. We thus succeeded in measuring α _R at the interfaces of the composite.     

Heat conductivity of a nonlinear β-FPU lattice

In this work, we propose a new approach to the computation of heat conductivity in nonlinear systems. The total heat conductivity process is decomposed into two parts: one part is an equilibrium process at the same temperature T of either end of the lattice, which does not transfer energy and the other is a nonequilibrium process at temperature ΔT of one end and a zero temperature of the opposite end of the lattice. This approach makes it possible to somewhat reduce the time of computation of heat conductivity at ΔT → 0. The threshold temperature T _thr is found to behave as T _thr ∼ N ⁻³, where N is the lattice length. The threshold temperature conventionally separates two mechanisms of heat conductivity: at T < T _thr, phonon heat conductivity is dominant; at T > T _thr, the contribution of soliton heat conductivity increases with increasing temperature. The problem of the computation of heat conductivity in the limit ΔT → 0 reduces to the heat conductivity of a harmonic lattice with time-dependent bond rigidities determined by an equilibrium process at temperature T. An exact expression for the temperature dependence of sound velocity in a lattice with a β-FPU potential at T < 10 is derived. A numerical experiment confirmed the existence of solitons and breathers that correspond to a modified Korteweg-de Vries (KdV) equation. The problem of the quantitative contribution of solitons and breathers to heat conductivity requires further study.     

Phenomenological theory of the magnetoelectric effect in some boracites

An explanation is given for the narrow peak in the temperature dependence of the component α ₃₂ of the magnetoelectric effect tensor near the transition at T=T _c to the m′m2′ phase observed in the boracites. A phenomenological approach is used which is based on the symmetry of the cubic phase of the crystals. The change in the sign of α ₂₃ and α ₃₂ observed in some of the boracites as the temperature is lowered is attributed to the low value of T _c. Zh. éksp. Teor. Fiz. 111, 536–546 (February 1997)     

Third-order elastic moduli of single-layer graphene

In the framework of the Keating model with allowance made for the anharmonic constant of the central interaction between the nearest neighbors μ, analytical expressions have been obtained for three third-order independent elastic constants c _ijk (μ, ζ) of single-layer graphene, where ζ = (2α − β)/(4α + β) is the Kleinman internal displacement parameter and α and β are the harmonic constants of the central interaction between the nearest neighbors and the noncentral interaction between the next-nearest neighbors, respectively. The dependences of the second-order elastic constants on the pressure p have been determined. It has been shown that the moduli c ₁₁ and c ₂₂ differently respond to the pressure. Therefore, graphene is isotropic in the harmonic approximation, whereas the inclusion of anharmonicity leads to the appearance of the anisotropy.     

Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

Thermal conductivity of high temperature gas mixture for lighting engineering

Paper presents experimental values of the coefficient of thermal conductivity of a ternary gaseous mixture (0·15 N₂-0·1 Ar-0·75 Kr) in the temperature range between approx. 1500 and 4000 K at about a mospheric pressure. The unsteady method based on the analysis of the heat flux from gas heated by the reflected shock wave to the end wall of a shock tube was used. Presented values agree well with rigorous kinetic theory calculations and with a combined Dulnev's model.Notation (in SI units) a thermal diffusivity - c _p isobaric heat capacity - E voltage - p pressure - T thermodynamic temperature - t time - x coordinate - temperature coefficient of resistance - mass density - coefficient of thermal conductivity Indexes 1 gas state before the experiment - 2 gas state behind the primary shock wave - 5 gas state behind the reflected shock wave - f relates to the solid wall - F relates to the solid wall on the solid-gas phase boundary - g relates to the test gas - G relates to the gas on the gas-solid phase boundary     

Pressure and temperature variation of the depolarized light scattering spectra of the fragile glass-former liquid salol

The pressure and temperature dependences of the depolarized light scattering spectra of salol have been measured at isobaric condition P = 1 bar for T between 328 and 393 K, and at isothermal condition T = 343 K for P between 1 and 625 bar, i.e. outside the metastable liquid region. The experimental results for both the α- and β-relaxations can be well described by the Mode Coupling Theory. The independence of the power-law parameters a, and consequently b and γ, from the thermodynamic variables T and P is demonstrated.The critical temperature T _c at 1 bar and the critical pressure P _c at T = 343 K have been determined from the normal liquid state. Received 2 May 2000     

Rapid solidification of Al-Cu-Ag ternary alloy under the free fall condition

The rapid solidification of Al-30%Cu-18%Ag ternary alloy is investigated by using the free fall method. Its solidified microstructure is composed of θ(Al₂Cu), α(Al) and ξ(Ag₂Al) phases. The liquidus temperature and solidus temperature are determined as 778 and 827 K, respectively. The alloy melt undercooled amounts up to ΔT _Max=171 K (0.20T _L). Its microstructural evolution is investigated based on the theoretical analysis of undercooling behavior and nucleation mechanics. It is found that the undercooling increases with the decrease of the diameter of the alloy droplet. When ΔT<78 K, the primary θ(Al₂Cu) phase of the alloy grows into coarse dendrite. When 78 K⩽ΔT⩽171 K, its refined θ(Al₂Cu) phase grows alternatively with α(Al) phase. Once ΔT⩾171 K, its microstructure is characterized by the anomalous (θ+α+ξ) ternary eutectic. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105)