He I-He II coexistence near the superfluid transition

We have made a numerical scheme of solving two fluid hydrodynamics near the point, which includes the mass density, momentum density, entropy density, and complex order parameter. In our model the normal fluid velocity vanishes at the boundaries due to finite viscosity. As preliminary examples we examine coexistence of a superfluid region and a normal fluid region in two space dimensions firstly in heat flow and secondly in gravity and heat flow. We observe formation of a vortex from a wall in the second case.     

On the microscopic derivation of the two-fluid thermohydrodynamic equations for4He II

The equivalence of the set of hydrodynamic equations for the macroscopic quantities associated with the ⁴ He II condensate and noncondensate—the natural parameters which occur in an O.D.L.R.O.-based microscopic theory—to the phenomenological equations for the super and normal fluids of the thermodynamic two-fluid model is uniquely established. This is achieved by defining the velocity of the normal fluid operationally as the velocity of entropy transport, from which follows a relationship between the condensate and superfluid densities. Consideration of boundary conditions together with this relation between the condensate and superfluid densities shows that the presence of the condensate allows the rest (majority) of the fluid to slip relative to a stationary boundary. The amount of slip is temperature dependent and, in particular, at absolute zero total slip occurs, i.e., the whole system is superfluid. Finally, the density relation is used to estimate the temperature dependence of the condensate density, and it is suggested that the dramatic change at about 0.5K, predicted on the basis of this relation, be investigated via measurement of the temperature dependence of the liquid structure factor.     

Transient heat transport in He II

Experimental and theoretical investigations of time dependent heat transport in He II have identified several new phenomena not previously reported. For heat fluxes greater than the steady state peak value, there is an observed time delay between the initiation of heating and the onset of film boiling. The time delay is seen to be nearly equal to the enthalpy rise in the helium divided by the applied heat flux. A well defined relationship is shown to exist between the time delay and the applied heat flux. Simple theoretical analysis of heat conductivity in He II is shown to predict the functional dependence and give approximately the correct proportionality factor. Experimental results are reported over a range of temperatures, 1.6 K to 2.1 K and for two external pressures, near saturated vapour pressure and at 0.125 MPa.     

Studies of heat transport to forced flow He II

Analytical and experimental studies of heat transport to forced flow He II are reported. The work is pertinent to the transfer of He II in space. An analytical model has been developed which establishes a condition for two phase flow to occur in the transfer line. This condition sets an allowable limit to the heat leak into the transfer line. Experimental measurements of pressure drop and flow meter performances indicate that turbulent He II can be analysed in terms of classical pressure drop correlations.     

Kapitza conductance of aluminium and heat transport through subcooled He II

Heat transport measurements in a large diameter tube containing He II are reported. The range of temperatures investigated are from 1.7 K to 2.1 K with applied pressures up to the critical pressure, 0.23 MPa. Temperature gradients established in the liquid are compared with previous experimental work and the theory of mutual friction. The Gorter-Mellink mutual friction constant, A, is observed to increase with applied pressure. At 2.01 K, a 0.1 MPa increase in pressure results in a 13.5 ± 4% enhancement of A. The results are used to predict the pressure and temperature dependence of the peak heat flux. Simultaneous measurements of the surface heat transfer from high purity aluminum to He II are reported. Kapitza conductance, film boiling and recovery heat flux data are analyzed and compared to other results on similar systems. A correlation is observed between the recovery heat flux and the film boiling heat transfer coefficient.     

Influence of the steady background turbulence level on second sound dynamics in He II - II

We report complementary results to our previous publication [Dalban-Canassy M, Hilton DK, Van Sciver SW. Influence of the steady background turbulence level on second sound dynamics in He II. Adv Cryo Eng 2006;51:371-8], both of which are aimed at determining the influence of background turbulence on the breakpoint energy of second sound pulses in He II. The apparatus consists of a channel 175 mm long and 242 mm² in cross section immersed in a saturated bath of He II at 1.7 K. A heater at the bottom end generates both background turbulence, through a low level steady heat flux (up to q_s = 2.6 kW/m²), and high intensity square second sound pulses (q_p = 100 or 200 kW/m²) of variable duration Δt₀ (up to 1 ms). Two superconducting filament sensors, located 25.4 mm and 127 mm above the heater, measure the temperature profiles of the traveling pulses. We present here an analysis of the measurements gathered on the top sensor, and compare them to similar results for the bottom sensor [1]. The strong dependence of the breakpoint energy on the background heat flux previously illustrated is also observed on the top sensor. The present work shows that the ratio of energy received at the top sensor to that at the bottom sensor diminishes with increasing background heat flux.     

The Influence of Acoustic Emission on the Onset of Turbulent Flow in He II

An experimental study is carried out on the influence of acoustic emission on the development of turbulent flow in He II at temperature 370 mK and at various pressures, from the saturated vapor pressure to that of ⁴He crystallization. The experimental technique of oscillating quartz tuning fork was applied for simultaneously exciting both superfluid flow, near the oscillating tuning fork prongs, and the acoustic wave emission. The flow rate of He II and the amplitude of the radiated acoustic waves were driven by the fork prongs, excited by voltage oscillations. Furthermore, the power of an acoustic wave depends on the density and sound velocity in He II as ρ/c ⁵, which in turn depends on the pressure. This allowed to measure the influence of the amplitude of acoustic emission on the flow in He II. It has been shown that the occurrence of acoustic radiation by tuning fork leads to an increased flow velocity for the beginning of nonlinear flow regime.     

Heat transport in dilute mixtures of3He in superfluid4He

We report new measurements of the effective thermal conductivity K_eff and relaxation time τ in dilute mixtures of³He in superfluid⁴He, with molar concentrationsX≤10⁻³. The temperature range extended fromT≈1.4 K toT _λ. Both K_cff and τ are found to agree with theoretical predictions, in contrast to previous experiments where significant differences were observed. A new thermal conductivity cell design was used which almost completely eliminates extraneous volumes and surfaces, and the earlier results are explained in relation to these design changes.     

Comparison of cavitation flows in He I and He II

This paper describes some of the results of an experimental study on the cavitation phenomena in He II and He I by the visualization and the measurements of the pressure and the temperature. The cavitation flow is generated in liquid helium driven by a bellows pump in the downstream region of the throat in a Venturi channel with a rectangular cross section. It is found that there are some definite differences in the appearance of cavitation between He I and He II flows, and that the λ-phase transition from He I to He II is sometimes induced because of the temperature drop in cavitating He I flow. In addition, the expression for the relation between the amount of the temperature drop induced by cavitation and the void fraction was described, which was found to be qualitatively in good agreement with the experimental result.     

Influence of4He impurities on nuclear spin relaxation in solid3He

The NMR properties of solid³He, mainly the ratio of the heat capacities of exchange and Zeeman energies and the exchange-lattice relaxation times are very sensitive to the presence of⁴He impurities, while the transverse relaxation timeT ₂ does not depend on the impurity concentration when the latter remains small. These properties can be explained in two different ways. (1) We assume an enhancement of exchange interactions around⁴He impurities. We derive the consequences of such an assumption and compare them with experimental results. For two molar volumes in the bcc phase, the locally enhanced exchange is equal to approximately7J, withJ being the exchange in pure³He. (2) Guyer and Zane introduce a mass fluctuation wave to explain the excess of heat capacity and the dependence of the longitudinal relaxation time with concentration. Both these models give rise to a four-energy bath system. As in the bcc phase, the exchange-lattice relaxation time in the hcp phase decreases when × increases at low enough⁴He concentrations. ForV=18.48 cm³ we deduce the coefficient for translational diffusion from high-temperature experiments with the help of Torrey's theory for spin-lattice relaxation.This paper is based on a thesis to be submitted in partial fulfillment of the requirements for the degree ofDocteur ès Sciences in Physics at theFaculté des Sciences d'Orsay in 1970. This thesis will be registered at the CNRS under the number AO 3704.     

密封器件压氦和预充氦细检漏过程中环境氦分压的影响

分别推导和分析了环境大气氦分压对压氦法的影响,地球干洁大气氦分压对预充氦法和预充氦密封器件压氦法复检的影响.证明对于压氦法,不需要考虑地球干洁大气氦分压的影响.但是如果候检室环境大气氦分压显著升高,对于内腔有效容积大,且等效标准漏率小的密封器件,会加大测量漏率值,所以压氦后,被检器件应尽快离开压氦设备所在的房间.对于预充氦法,地球干洁大气氦分压会使测量漏率通过极大值后出现极小值,且当候检时间与内腔有效容积之比大于100 h/cm3时,极小值点的气流仍处于分子流状态,不能靠粗检鉴别,所以需压氦法复检加粗检,才能防止漏检.另外,地球干洁大气氦分压会使预充氦法候检时间的第一特征点变大,从而扩大了需压氦法复检加粗检的范围,但第二特征点不变,也不影响压氦法复检加粗检的结果.     

Influences of interfacial resistances on gas transport through carbon nanotube membranes

Carbon nanotubes have significant promise as gas separation membranes. Gas permeation through nanopores involves mass transfer resistances from molecules entering and leaving pores (so-called surface resistances) and diffusion within the pores. We use molecular simulations to give the first estimates of surface resistances for gas transport through nanotubes. For CH4 transport through (20,0) carbon nanotubes at 300 K, surface resistances are small for nanotubes 5-10 mum in length but can be significant for shorter nanotubes.     

Coalescence of Levitated He II Drops

We describe experiments to study the coalescence of He II drops levitated in a magnetic trap. Using a high speed CCD camera, we have produced movies of drops coalescing at temperatures as low as 0.7 K. We examine some interesting features of the motion during and following coalescence.     

Influence of ion-induced interfacial chemical reactivity on contact resistance

Contact resistance measurements of chromium contacts deposited by partially ionized beam deposition on transparent conducting indium tin oxide (ITO) were performed. These provide a direct experimental evidence of the influence of interfacial chemical interaction on the contact resistance. The interfacial reactivity is controlled by modifying the energy and flux of ionized chromium atoms deposited on ITO employing a specially designed partially ionized deposition system with very high ionization efficiency. The true contact resistivityρ _c is obtained by iteratively correcting the experimentally measured values for the finite sheet resistance of the ITO layer.ρ _c decreases linearly with the energy of the ionized chromium. Auger sputter profiling shows no structural modifications at the interface due to a change in the energy of the chromium atoms, confirming that the observed change in the contact resistivity is directly related to interfacial chemical bonding of the atoms with the oxygen atoms in the ITO leading to a local increase of carrier concentration and lower interfacial resistance.     

The dispersion of3He quasiparticles in He II from neutron scattering

In an inelastic neutron scattering (INS) experiment on³He-⁴He mixtures one observes, besides the photon-roton mode which is barely modified by the admixture of³He, an additional excitation at lower energies which is interpreted as quasi-particle-hole excitations of a nearly free Fermi gas. We reanalyse INS data ofx ₃=1% and 4.5% mixtures at various pressures to extract the mean energy of the fermions. In the momentum range 9<q<17 nm^–1 (above 2k _F ) follows very closely the relation =A ₂ q ²+A ₄ q ⁴ at all concentrations, pressures and temperatures observed. In a 4.5% mixture (T _F 0.3 K), measurements were performed for temperatures in the range 0.07<T<0.9 K. We find bothA ₂ andA ₄ to be strongly temperature dependent. For the interpretation of thermodynamical properties, the single particle energy _k is parametrized as _k =_o+1/(2ms*) ·k ₂ · (1+k ²). Neglecting interactions between fermions, we calculate from the free-particle _k the scattering functionS(q, ) and the mean value of the fermion peak energy _q = S ₃(q, )d/S ₃(q, )d. We find that follows closely _q , deviating at most by 10%. A comparison to the measuredA ₂ andA ₄ directly yieldsms* (x ₃,p, T) and (x ₃,p, T). In the limitx ₃=0,p=0 andT=0, the density and concentration dependence of the inertial mass is in excellent agreement with values found by Sherlock and Edwards. The temperature dependence of the specific heat data from Greywall and Owers-Bradleyet al. are well represented by our model atT<0,5 K.     

Influence of interfacial microcracks on the elastic properties of composites

A model is established to quantify the influence of interfacial microcracks on the elastic properties of a particulate composite using a combination of theoretical and finite element analysis. A unique way to construct physical models which could accommodate both crack size and crack density is proposed. Based on energy principles, the influence of a dilute concentration of interfacial microcracks is first studied. The case of a finite concentration of microcracks is solved subsequently by combining the dilute concentration solutions and the differential scheme. Both cases agreed well with existing composite theories for the limiting condition of complete decohesion. The final model predicts the effective elastic properties as functions of both crack size and microcrack density.     

Influence of 4He Preplating on 3He Fluid Formed in One-Dimensional 28 Å-Pores

No Heading A degenerate Fermi fluid has been realized recently for ³He films adsorbed in one-dimensional (1D) pores 28 Å in diameter, preplated with ⁴He layers. In order to study the influence of the ⁴He preplating on the ³He Fermi fluids, heat capacity was measured down to 4 mK for two preplated ⁴He films of different thickness. At low temperatures, no substantial difference was found between the two cases. On the other hand, a large difference appeared above about 150 mK. The large ³He heat capacity for the thick ⁴He preplating suggests thermal excitation of the ³He atoms to the excited energy levels of the Andreev states in the ⁴He fluid layers.PACS numbers: 67.70 +n. 67.55 Cx.     

The transverse acoustic impedance of He II

The complex shear acoustic impedance of liquid He II has been measured at frequenciesf(=/2) of 20.5, 34.1, and 47.8 MHz from 30 mK to the -point T (2.176 K). The impedanceZ was found from the temperature dependence of the quality factor and the resonant frequency of a thickness shear mode quartz crystal resonator immersed in the liquid. The relationship for a hydrodynamic viscous liquidZ(T)=(1–i)(f _n )^1/2 was used to measure the temperature dependence of the viscosity (T) using tabulated values of the normal fluid density _n (T). Deviations from hydrodynamic behavior occurred when the viscous penetration depth was less than the superfluid healing length, the phonon mean free path, and the roton mean free path. Near the -point,Z(T)/Z(T) was frequency dependent and a value for the superfluid healing lengtha=(0.10±0.01)^–2/3 nm was found, where =(T–T)/T. The effects of van der Waals forces near the crystal surface were also observed and a layer model was used to interpret the measurements. Below 1.8 K only rotons contribute significantly toZ and we determined the roton relaxation time as _r =8.5×10^–14 T ^–1/3 exp (8.65/T) sec. Below 1.2 K, _r >1 and we investigated the breakdown of hydrodynamics in this region. ForT<0.6 K the resonant frequency of the crystals decreased by f/f=2×10^–7, but the origin of this effect is not yet known.Financial support provided by the SERC, Bedford College, and the Central Research Fund, University of London.     

Calculation of He II flow in tubes

The flow of superfluid helium through a tube with different temperatures at the ends differs considerably from that of a Newtonian fluid. The strong dependence of the thermodynamic properties on temperature, the internal convection mechanism and the structure of superfluid turbulence causes unusual flows. The equations for the flow of He II are integrated using a new one-dimensional, steady state model to study the flow in a tube. A wide range of driving conditions is studied. The temperature and pressure profiles along the tube fall into four classes. A dimensionless parameter called is defined which determines the progression through the four classes of behavior. The deviation of the flow from Newtonian is measured by . Significant maxima of the temperature and pressure can occur between the ends of the tube for large values of . The shapes of the profiles and the mass flux depend primarily on , the geometry and the boundary conditions. Formulas are presented which relate the variables of interest to the boundary conditions. These formulas result from averaging the equations of motion along the tube. A general and unified approach, based on , is presented for analyzing experimental data and designing new experiments. It is shown that the common practice of neglecting the pressure term in the energy equation results in poor prediction for many situations. The occurrence of large maxima of pressure and temperature imply that the interpretation of some of the experimental data of the literature should be reconsidered.