The temperature dependence of the dielectric constant in solid H2, D2,4He,and Ne

The dielectric constant ε of solid hexagonal close-packed H₂ (X?0), H₂(X?0.7), D₂(X=0.33), and He ⁴ at constant volume and of H₂(X=0.75) and Ne at saturated vapor pressure is studied as a function of temperature between 1.17 and 10 K. HereX is the fraction of molecules with angular rotational momentumJ=1, the fraction 1?X havingJ=0. These are the first reported measurements on van der Waals solids at constant volume. The experiments were carried out using a thick-walled capacitance cell, the measurement frequency being 14 MHz. The observed changes of ε withT are expressed in terms of the effective polarizability α_CM defined by the Clausius-Mosotti relation. A reduced polarizability α*=α_CM (T)/α_CM (T=1.17 K) is presented as a function of temperature. The initial purpose of the measurements was to observe the effect of rotational ordering of the(J=1) molecules in H ₂ , to be detected by a change in polarizability, as suggested by A. B. Harris. With increasing temperature it was found that α_CM decreased of the order of 0.1% for H ₂ and D ₂ . Also, differences were found between H₂(X=0) and H₂(X=0.75), which are briefly discussed. For He ₄ , α_CM decreased by about 3×10 ^?5 between 1.17 K and the melting point, 3.8 K. An analysis of the results showed that the temperature variation of α_CM could not be caused by the effect of lattice vibrations. Also, a simple calculation shows that the rearrangement of molecular positions in H ₂ and D ₂ at constant volume would have to be quite substantial to account for the observed temperature change in α_CM, which change is therefore not understood. The dielectric constant of Ne was measured at saturated vapor pressure and corrections were made to obtain the change of α_CM at constant volume. Rather different results were obtained for Ne than for the other solids: α_CM rose sharply with temperature, and this behavior also is not understood. Measurements of the diectric constant of H₂(X=0) and H₂(X=0.73) near the melting curve showed that the polarizability α_CM is the same within 0.1% in both the liquid and the solid phases. In the solid and liquid phases the density differences between H₂(X=0) and H₂(X=0.75) and also the differences in the respective melting pressures are in good agreement with the previous published work.     

Self-consistent measurement of the adsorption of He on Cu,CO2, Ar,and Ne

The chemical potentials as a function of He coverage have been determined from He adsorption isotherms measured at 1.72 K for the following thick substrates: Ne, Ar, and CO₂-plated Cu, and bare Cu. The adsorption isotherm data were analyzed self-consistently using the modified Brunauer-Emmett-Teller method for low coverages and the Frenkel-Halsey-Hill vapor pressure relation at high coverages. The analysis establishes the surface area, chemical potential constant, =–d ³, and capacity increases and heats of adsorption of the first and second monolayers of He for each adsorbent. The measured chemical potentials follow the expected inverse-cube law for coverages greater than 2.5 layers, but differ substantially for smaller coverages. We use results of the isotherm analysis to interpret measurements of third sound velocities, and to make precision measurements of the He inert (nonsuperfluid) layer coverages on these substrates     

On the conditions for contraction of a multielectrode corona discharge in He/Ar, Kr, Xe mixtures

The results of an investigation of the contraction of a corona discharge in “needle-grid” and “needle-plane” electrode systems maintained by a constant negative voltage are presented. The discharge was ignited in working media of infrared lasers on p-d transitions of atoms of heavy inert gases (He/Ar, Kr, Xe mixtures). Investigation of the current-voltage characteristics showed that in the mixtures He/Ar/Kr there is no hysteresis on the rising section of the current-voltage characteristic, while hysteresis does appear for a corona discharge in the mixture He/Xe. For voltages ≥5.5 kV on the needles and ballast resistance R_b≥0.5 MΩ dynamical contraction of a corona discharge together with current pulses with f=3–5 kHz and hysteresis loops were found on the descending section of the curve I=f(U), where I and U are the average current and voltage on the discharge gap. The maximum content of atoms of heavy inert gases for [He]=200–300 kPa is [Ar]≤12, [Kr]≤8, and [Xe]≤4 kPa. Pis’ma Zh. Tekh. Fiz. 25, 90–94 (September 12, 1999)     

Experimental investigation of thermal conductivity and the temperature jump in nonreactive gases at low pressures

The thermal conductivities of inert gases, air, and sulfur hexafluoride in the low-pressure region are investigated with measurement cells having different geometries.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 3, pp. 376–383, September, 1984.     

Experimental investigation and modeling of effective thermal conductivity and its temperature dependence in a carbon-based foam

The effects of test temperature and a graphitization heat treatment on thermal and thermo-mechanical properties of a carbon-based foam material called CFOAM^® are investigated experimentally. Thermal diffusivity is determined using a laser flash method, heat capacity via the use of differential scanning calorimetry, while (linear) thermal expansion is measured using a dilatometric technique. Experimental results are next used to compute the effective thermal conductivity and the coefficient of thermal expansion as a function of test temperature. The computed thermal conductivity results are then compared with their counterparts obtained using our recent model. The agreement between the experiment-based and the model-based results is found to be fairly good only in the case when the graphitization temperature is high relative to the maximum test temperature and, hence, CFOAM^® does not undergo a significant additional graphitization during testing. A potential use of CFOAM^® as an insulation material in thermal protection systems for the space vehicles is discussed.     

Experimental determination of the accommodation coefficients of argon and xenon on nickel at high temperatures

The accommodation coefficients of argon and xenon on nickel are obtained in the temperature range from 940 to 1150°K. The data are obtained for measurements of the temperature change determining the thermal conductivity of gases by the coaxial cylinders method.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 25, No. 5, pp. 847–850, November, 1973.     

Experimental investigation of the length of a free diffusion jet of fuel gases diluted with inert gases

Experimental investigation of the length of single burning jets of methane and hydrogen previously diluted with an inert gas (nitrogen or helium) was carried out. Efflux of fuel gases into the atmosphere occurred through cylindrical extension pieces 4 and 8 mm in diameter. The Reynolds numbers at the cut of a piece varied in the range from 400 to 12,000. A clearly defined dependence of the jet length on the quality of the added inert gas is obtained. The correlation of experimental data made it possible to recommend formulas for engineering calculations of free laminar and turbulent jets.     

行走激励的傅里叶级数模型及其参数的实验研究

利用三维动作捕捉技术结合三向测力板,开展了7种不同步行频率下行走激励的动力特性实验,得到73人次5 004条有效单足落步荷载曲线。利用实测的双足重合的准确时间点将单足落步荷载曲线拓展成连续时程曲线,进而得到行走激励竖向及横向分量的傅里叶级数模型的前5阶系数及相位角值。与其它国家现有模型的比较反映出了不同人群特性的差异,所建议模型可用于大跨工程结构考虑行走作用的振动舒适度问题的设计与分析。     

Experimental investigation of the surface temperature and water retention effects on the frosting performance of a compact microchannel heat exchanger for heat pump systems

Frost formation on a louvered fin microchannel heat exchanger was experimentally investigated in this paper with the aim of determining the dominant factors affecting the time of frosting and frost growth rate. A novel methodology was developed to measure frost thickness and frost weight at intervals during the frosting period. Frost mass and thickness growth rates, corresponding coil heat transfer, capacity degradations and air pressure drop are measured and discussed. The experimental data showed that at a given air dry bulb temperature, the fin surface temperature and air humidity are the primary parameters that influence the frost growth rates. Water retention and air velocity had a secondary impact on the frosting performance. From digital images of the frost growth it was observed that frost does not nucleate from the water droplets retained in between fins but it developed from the leading edges of the fins.     

Studies of the dielectric constant in solid H2,4He,and Ne

Measurements of the static dielectric constant and of the resulting polarizability _CM defined by the Clausius-Mosotti relation are reported for several molecular solids under conditions of quasiconstant density. The range of measurements was between 0.4 and about 15 K, and pressures up to about 1 kbar were used. In solid H₂, the changes of as a function of ortho concentration are reported at 4.2 and 12 K, from which it is concluded that [(ortho)–(para)]/0.005, which is approximately 2 1/2 times larger than in the liquid phase. Furthermore the temperature change of _CM is studied for various densities at comparable ortho concentrationsX. Particular attention is paid to the orientational order-disorder transition in solid H₂ withX0.65, where _CM shows an abrupt change that appears to be a function ofX, density, and thermal history. In general _CM is lower in the ordered (cubic) phase than in the disordered (hexagonal) phase. The temperature change of _CM in almost pure para-H₂, measured between 4 and 0.4 K, is presented and is compared with other recently reported experiments. It is concluded that the mechanism that produces the change of _CM withT and at the order-disorder phase transition is not understood. In solid Ne, measurements of _CM are reported for one sample assumed to be at zero pressure and one where the density is quasiconstant. There is a strong temperature dependence in _CM for the former above 5 K (in agreement with previous findings), while for the latter _CM appears to be constant within the uncertainties introduced by strain-induced hysteresis effects. The sample of solid⁴He, investigated between its melting point at 4.3 and 0.8 K at constant density, shows a very weak temperature dependence of _CM, which is also consistent with previously reported measurements.Research supported by a grant of the U.S. Army Research Office (AROD) and from a contract with the Office of Naval Research. A preliminary account of this research was given inBull. Am. Phys. Soc. 20, 672 (1975).     

Stability,orientational effects,and temperature dependence of nonunitary vortex structures in superfluid3He

First we set up the nine Ginzburg-Landau (GL) equations for the ninecomplex radial functions of an axisymmetric vortex line in superfluid³He. These GL equations necessarily yield a nine-componentnonunitary state for the A-phase vortex that corresponds to the Mermin-Ho trial state. We then solve the GL equations and calculate the energies for the five B-phase vortices of different symmetries that have been discussed by Salomaa and Volovik. We find that the only stable vortex is theirv-vortex, which has a superfluid and ferromagnetic core. Theo-vortex, which has a normal core, turns out to bemetastable (it is a saddle point on the energy surface describing theuvw-vortices). Theu-,w-, anduvw-vortices are found to beunstable. In the second part of the paper we calculate, with the help of the generalized GL theory, the temperature corrections to the GL structures of theo- andv-vortices. It turns out that the corresponding temperature variations of the two orientational energies for the -vector that are linear and quadratic in the field disagree with the measured temperature variations of the parameters and (see Hakonenet al.). However, a novel new orientational energy due to the interaction of the field and superflow around the vortex line increases linearly asT is decreased, in agreement with the measured behavior of .     

Rotational and translational accommodation coefficients of nitrogen on nickel,silver and gold

An electron beam excited fluorescence technique was used to make simultaneous measurements of rotational and translational accommodation of nitrogen on nickel, silver, gold and stainless steel solid surfaces. In addition to the advantage of being able to make independent measurements of rotational accommodation, this apparatus allows a wide range of solid surface temperatures to be used and operates at lower gas density than most previous experiments.Rotational accommodation coefficients for nickel and gold varied from α_R = 0.12 at 400°K to α_R = 0.18 at 850°K while the value of α_R for silver increased more quickly from α_R = 0.03 at 430°K to α_R = 0.2 at 700°K.A linear variation of translational accommodation coefficient, α_T, with surface temperature was observed. An empirical relationship based on molecular weights was able to account for variation in dα_T/dT_s observed. The metal surfaces used were thought to be free of adsorbed gas and oxide layers.     

Experimental investigation of thermal conductivity and the temperature jump in dissociating gases. Vapors of carbonaceous acids. I

We investigated the thermal conductivity of propionic acid vapor in the lowpressure range by means of measuring cells with different geometries.     

Experimental investigation of the sputter-topographic evaluation of a cylindrical surface

An experimental method is used to develop the evolutionary contours of circular surface erosion produced by ion bombardment. Silica glass fibres are bombarded from one direction, normal to the fibre axis, with Ar⁺ ions at energies from 2 to 16 keV, and the fibre cross-section profile variations are observed as a function of ion fluence. The results indicate that two equilibrium optimum sputtering rate angles obtain, one on each side of the convex hemispherical fibre surface, and the slopes defined by these angles move parallel to themselves into the surface and eventually intersect to form an isosceles triangle with rounded base. Sequential profile variations show close agreement with theoretical predictions, and it is evident that the apex angle varies as a function of energy for the range investigated.     

Influence of the specific input energy on the characteristics of a nuclear-pumped Ar/Xe laser

Results are presented of experiments and model calculations on the pumping of a 1.73 μm Ar/Xe laser with 235U fission fragments over a wide range of specific powers. An analysis is made of the mechanism for quenching of the lasing at high specific input energies. Pis’ma Zh. Tekh. Fiz. 24, 16–20 (July 26, 1998)     

Effect of He addition on the heating characteristics of substrate surface irradiated by Ar thermal plasma jet

Effect of He addition to Ar as the discharge gas on the substrate surface temperature during thermal plasma jet (TPJ) annealing is investigated. The maximum surface temperature (T_max) increases from 1370 to 1470 K with increasing He fraction (R_He) from 0 to 28%, then decreases to 1040 K with increasing R_He to 86%. It has been confirmed that the TPJ is efficiently concentrated when He addition is around 30%, which suggests that the increase in T_max is due to the increase in the power density induced by thermal pinch effect. By adding 30% He and increasing the total gas flow rate to 15.4 L/min, T_max is increased to 1580 K.     

Density matrix of superfluid4He and temperature dependence of the static structure factor

We have computed properties of superfluid⁴He on the basis of a new density matrix which is based on a shadow wave function for the roton excitation. We deduce the separate contribution of kinetic and potential energy to a roton and find that a roton lowers the potential energy of the system. We find that S (k; T) and g(r; T) have an anomalous T dependence as found experimentally and the extra correlations present at finite T are induced by rotons. By computing the condensate fraction n₀(T) we find that the Cummings et al relation between g(r; T) and n₀(T) is violated and that it leads to a value of n₀(T) larger of the true one.     

Deformation of carbon nanotubes colliding with a silicon surface and its dependence on temperature

Using molecular dynamics simulation, we investigated the carbon nanotubes (CNTs) colliding with a silicon surface at a speed of 600 m/s, mimicking cold spray experiments of CNTs. Depending on temperature (300-900 K), the CNT is deposited on or bounces off the surface after impact on the surface. The CNT was more deformed as its temperature rose. The deformation of CNT was maximal for the collision geometry where the long axis of CNT lies parallel to the surface plane. However, its vibrational energy was maximal when the CNT collided with its long axis perpendicular to the surface.     

Roton backflow and the temperature dependence of the static structure factor of superfluid4He

The temperature dependence of the static structure factor of superfluid⁴He is given an interpretation in terms of the Feynman-Cohen picture of the elementary excitations by constructing the corresponding microscopic density matrix and including the effect of three-body terms. The required information on the ground-state wave function is taken over from configurations drawn from a Green's Function Monte Carlo computation. Correlations become stronger at finite temperatures in agreement with experiment. Backflow has only a modest effect onS(k, T).