Kapitza conductance and thermal conductivity of copper niobium and aluminium in the range from 1.3 to 2.1 K

The Kapitza conductance and thermal conductivity of ofhc-copper, niobium, ultra high purity aluminium, and of the aluminium alloy 6061 A1 have been measured in the temperature range from 1.3 to 2.1 K, yielding both quantities in the same steady state experiment. The temperature dependence of the Kapitza conductance, h_o, was between T^3.3 and T^4.6 for the different samples, which is higher than the most frequently observed T³ dependence. The magnitude of h_o for both ofhc-copper and aluminium agrees well at 1.9 K with an empirical prediction, but for niobium it is a factor of two to four lower than the value predicted. At 1.9 K, h_o is higher by a factor of two for an annealed and chemically polished niobium sample than for an untreated sample. The thermal conductivity measured from ofhc-copper and 6061 A1 as in good agreement with the value calculated from the resistivity of these materials and the Wiedemann-Franz law. The measured thermal conductivity obtained for an annealed niobium sample is a factor of 2.8 higher than the highest published value.     

Thermal contact conductance of pressed contacts at low temperatures

The influence of variations of interface temperature in the range 50-300 K on the thermal contact conductance between aluminium and stainless steel joints was determined. Predictions were done by modeling the deformation at the interface for different values of surface finish and contact pressure over the range of interface temperatures. Both elastic and plastic deformation was considered. Experiments were carried out in a closed loop cryostat and the results were shown to compare well with the predictions. A reduction of the interface temperature resulted in a smaller value of thermal contact conductance. Interfacial pressure variation had much lower influence at the smaller value of temperatures. The role of surface roughness at the contact was also seen to be less significant at lower interface temperatures and the zone of hysteresis was smaller. A correlation was developed for estimating thermal contact conductance at joints over this temperature range. An explicit dependence of contact conductance on temperature was not seen to be necessary as long as the changes in the hardness and thermal conductivity of the material with temperature are incorporated in the correlation.     

Determination of cohesive laws by the J integral approach

The determination of cohesive laws for describing large scale failure process zones is discussed. Firstly, a general approach for determination of cohesive laws, by the measurement of the J integral and end-opening of the cohesive zone of double cantilever beam specimens loaded with pure bending moments, is described. Next, two case stories are reviewed: failure of adhesive joints and splitting of a unidirectional carbon fibre/epoxy composite. For the adhesive joints, measured failure strengths of bonded panels having a central notch were found to in very good agreement with predictions from cohesive law parameters determined on test specimens. For the problem of splitting of unidirectional composites, micromechanisms were observed in situ during cracking. A cohesive law shape, predicted by a micromechanics model, was found to agree well with macroscopic cohesive law determined by the J integral approach. The cohesive law was used for predicting effect of specimen shape on strength; predictions were confirmed by experiments. Finally, some ideas regarding determination of mixed mode cohesive laws are discussed.     

Thermal conductance in screw-fastened joints at helium temperatures

The electrical resistance of simple screw-fastened joints between two bare copper blocks and blocks with various materials interposed were measured at 4.2 K. Based upon these measurements, ways to improve the low temperature thermal conductance of such joints are indicated.     

From a local stress approach to fracture mechanics: a comprehensive evaluation of the fatigue strength of welded joints

This paper investigates the possibility of unifying different criteria concerned with the fatigue strength of welded joints. In particular, it compares estimates based on local stress fields due to geometry (evaluated without any crack-like defect) and residual life predictions in the presence of a crack, according to LEFM. Fatigue strength results already reported in the literature for transverse non-load-carrying fillet welds are used as an experimental database. Nominal stress ranges were largely scattered, due to large variations of joint geometrical parameters. The scatter band greatly reduces as soon as a 0.3-mm virtual crack is introduced at the weld toe, and the behaviour of the joints is given in terms of Δ K _I versus total life fatigue. Such calculations, not different from residual life predictions, are easily performed by using the local stress distributions determined near the weld toes in the absence of crack-like defects. More precisely, the analytical expressions for K _I are based on a simple combination of the notch stress intensity factors K ₁^N and K ₂^N for opening and sliding modes. Then, fatigue strength predictions, as accurate as those based on fracture mechanics, are performed by the local stress analysis in a simpler way.     

Thermal design of the CFRP support struts for the spatial framework of the Herschel Space Observatory

Thermal finite element (FE) models, of low thermal conductance struts which are required to provide support for the low temperature components of the Herschel Space Observatory, have been validated by measurements at temperatures below 20 K. The Herschel Space Observatory structure is introduced. FE modelling of two designs of support strut is briefly discussed and the final designs presented. Validation of the design models was made in two experiments. The first of these provided specific thermal conductivity data for component CFRP materials, whose composition was initially designed on the basis of data available in the literature. The second experiment was performed to confirm the thermal conductance (Q′/ΔT), of the completed struts. The validation test rigs are described together with details of the experimental methods employed. Values of conductance were at the level of 5 × 10⁻⁵ W/K at a mean temperature of 6 K. The measured data are presented and discussed with reference to the thermal models. Sources of measurement inaccuracy, are also discussed.     

Some experiments on the influence of surface treatment on the Kapitza conductance between copper and He4 at temperatures from 1.2 to 2.0 k

This paper reports on some experiments on the influence of a number of methods of surface treatment on the Kapitza conductance between copper and He⁴ at not very low temperatures. Attention is paid to the magnitude, the temperature dependence, and the reproducibility between different samples. The results are compared with some theoretical predictions.     

Strömungsleitwert der dünnen Blende

The conductance of a thin circular orifice with 1.2 mm nominal diameter was measured for 8 gas species at inlet pressures from 10^?3 to 10³ mbar and negligibly small outlet pressures. The experimental data cover the various regimes of gas flow ranging from molecular to viscous. In order to compare data of different gases, these have been scaled: the pressure by the inverse Knudsen number and the conductance by the conductance in the molecualr regime. As is found, the transition from molecular to viscous flow for different gases scales only approximately by the inverse Knudsen number. The data in the viscous regime correspond to the predictions for gasdynamical flow, if the flow contraction is taken into account. At Knudsen numbers Kn ≈ 1/100 a maximum of the conductance occurs whose shape depends on the gas species.     

Kapitza conductance of molybdenum and beryllium between 1 and 2 K

Using the same experimental methods as in earlier work, studies have been made of the Kapitza conductance between liquid He⁴ and clean surfaces of annealed molybdenum and beryllium samples at temperatures between 1 and 2 K. The conductance of molybdenum at 1.5 K is 75 W m^?2 K^?1 and for beryllium 24 W m^?2 K^?1. The respective temperature dependences can be described fairly well by T^1.1 and T^1.2. The results for different samples reproduce reasonably well and the general behaviour is much the same as for the copper and silver samples studied earlier, though some of the effects are rather smaller. On one of the beryllium samples the Kapitza conductance was measured as a function of the liquid pressure up to 2.75 atm. No significant dependence was found. To our knowledge these materials have not been studied before.     

Strain rate-dependant mechanical properties of OFHC copper

The mechanical properties of high purity copper have been extensively studied in the literature, with yield and flow stresses measured as a function of strain rate, grain size, and temperature. This paper presents a comprehensive study of the strain rate and grain size dependence of the mechanical properties of OFHC copper, including an investigation of the previously observed upturn in rate dependence of flow stress at high rates of strain (≥500 s^?1). As well as a comprehensive review of the literature, an experimental study is presented investigating the mechanical properties of OFHC copper across a range of strain rates from 10^?3 to 10⁵ s^?1, in which the copper samples were designed to minimize the effects of inertia in the testing. The experimental data from this study are compared with multiple sources from the literature varying strain rate and grain size to understand the differences between experimental results on nominally the same material. It is observed that the OFHC copper in this study showed a similar increase in flow stress with strain rate seen by other researchers at high strain rates. The major contribution to the variation between experimental results from different studies is most likely the starting internal structure for the materials, which is dependent on cold working, annealing temperature, and annealing time. In addition, the experimental variation within a particular study at a given strain rate may be due to small variations in the internal structure and the strain rate history.     

Low-temperature electronic transport in single K(0.27)MnO(2)·0.5H(2)O nanowires: enhanced electron-electron interaction

The current-voltage (I-V) characteristics and electrical resistivity of isolated potassium manganese oxide (K(0.27)MnO(2)·0.5H(2)O) nanowires prepared by a simple hydrothermal method were investigated over a wide temperature range from 300 to 4?K. With lowering temperature, a transition from linear to nonlinear I-V curves was observed around 50?K, and a clear zero bias anomaly (i.e., Coulomb gap-like structure) appeared on the differential conductance (dI/dV) curves, possibly due to enhanced electron-electron interaction at low temperatures. The temperature dependence of resistivity, [Formula: see text], follows the Efros-Shklovskii (ES) law, as expected in the presence of a Coulomb gap. Here we note that both the ES law and Coulomb blockade can in principle lead to a reduced zero bias conductance at low temperatures; in this study we cannot exclude the possibility of Coulomb-blockade transport in the measured nanowires, especially in the low-temperature range. It is still an open question how to pin down the origin of the observed reduction to a Coulomb gap (ES law) or Coulomb blockade.     

Thermal conductance characterization of a pressed copper rope strap between 0.13 K and 10 K

Mechanically pressing the ends of a copper braid in solid copper is an effective way of constructing solderless conductive straps for cryogenic applications. In this paper we present thermal conductance data of such a copper strap measured using the two-heater one-thermometer method. The measurements span a wide temperature range of 0.13–10 K applicable to a variety of cryogenic systems employing liquid helium, pulse tube coolers, adiabatic demagnetization refrigerators, and others. Above ≈1.5 K, the braid thermal conductivity dominates the strap conductance resulting in a near-linear dependence with temperature. The variation with temperature below ≈1.5 K is near-quadratic indicating dominance of the pressed contact conductance at the strap ends. Electron-beam welding the braid to the strap ends is shown to be a promising solution for improving sub-Kelvin thermal conductance of the strap.     

Model calculations on heat flow in inhomogeneous thermal systems 2-Sintered copper heat exchanger and Kapitza conductance

Using the methods applied in a previous paper¹ to the analysis of a thermal conductivity cell, the thermal conductance of copper sinter in contact with liquid He³ is studied as a function of the number of breaks and blockages in the sinter. The reduction in sinter conductance is obtained as well as the change in electrical resistance of the sinter. The results are discussed in terms of the filling factor. Also the anomalous Kapitza conductance which is sometimes found in sinters is discussed.     

On the experimental reproducibility in measurements of the Kapitza conductance of copper between 1 and 2 K

The reproducibility of measurements of the Kapitza conductance between electropolished copper samples and He⁴ has been investigated at temperatures between 1.2 and 2.0 K. It is shown that the results are sensitive to the vacuum treatment to which the sample is subjected. It appears that after leaving samples for a long time under a vacuum of 10⁻⁷ mm Hg, obtained with a getter-ion pump, fairly reproducible data may result.     

Phase Behavior of N2O and CO2 in Room-Temperature Ionic Liquids [bmim][Tf2N], [bmim][BF4], [bmim][N(CN)2], [bmim][Ac], [eam][NO3], and [bmim][SCN]

The gas solubility of nitrous oxide (N₂O) in room-temperature ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium thiocyanate, and ethylammonium nitrate has been measured at isothermal conditions from about (283 to 348)K using a gravimetric microbalance. The observed pressure–temperature composition (PTx) data have been analyzed by use of a generic Redlich-Kwong equation-of-state (EOS) model, which has been successfully applied in our previous works. The interaction parameters have been determined using our measured vapor–liquid equilibrium data. Vapor–liquid–liquid equilibrium measurements have been made and validate EOS model predictions which suggest that these systems demonstrate Type III and Type V phase behavior, according to the classification of van Konynenburg and Scott. The global phase behavior of N₂O has also been compared with both the measured data from this study and literature data for carbon dioxide (CO₂) in each ionic liquid and Henry’s law constants are compared at room temperature (298.15 K).     

Reactively evaporated films of copper molybdenum sulfide

Films of superconducting Chevrel-phase copper molybdenum sulfide Cu_xMo₆S₈ were deposited on sapphire substrates by reactive evaporation using H₂S as the reacting gas. Two superconducting temperatures (10.0 K and 5.0 K) of the films were found, corresponding to two different phases with different copper concentrations. All films were superconducting above 4.2 K and contained Chevrel-phase compound as well as free molybdenum. The critical current was measured as a function of applied field. One sample was found to deviate from the scaling law found for co-evaporated or sputtered samples, which possibly indicates a different pinning mechanism or inhomogeneity of the sample.     

Thermal resistance of indium coated sapphire–copper contacts below 0.1 K

High thermal resistances exist at ultra-low temperatures for solid–solid interfaces. This is especially true for pressed metal–sapphire joints, where the heat is transferred by phonons only. For such pressed joints it is difficult to achieve good physical, i.e. thermal contacts due to surface irregularities in the microscopic or larger scale. Applying ductile indium as an intermediate layer reduces the thermal resistance of such contacts. This could be proven by measurements of several researchers. However, the majority of the measurements were performed at temperatures higher than 1 K. Consequently, it is difficult to predict the thermal resistance of pressed metal–sapphire joints at temperatures below 1 K.In this paper the thermal resistances across four different copper–sapphire–copper sandwiches are presented in a temperature range between 30 mK and 100 mK. The investigated sandwiches feature either rough or polished sapphire discs (Ø 20 mm × 1.5 mm) to investigate the phonon scattering at the boundaries. All sandwiches apply indium foils as intermediate layers on both sides of the sapphire. Additionally to the indium foils, thin indium films are vapour deposited onto both sides of one rough and one polished sapphire in order to improve the contact to the sapphire.Significantly different thermal resistances have been found amongst the investigated sandwiches. The lowest total thermal resistivity (roughly 26 cm² K⁴/W at 30 mK helium temperature) is achieved across a sandwich consisting of a polished sapphire with indium vapour deposition. The thermal boundary resistance between indium and sapphire is estimated from the total thermal resistivity by assuming the scattering at only one boundary, which is the warm sapphire boundary where phonons impinge, and taking the scattering in the sapphire bulk into account. The so derived thermal boundary resistance agrees at low temperatures very well with the acoustic mismatch theory.     

He gas gap heat switch

Thermal control at 1 K is still demanding for heat switches development.A gas gap heat switch using ³He gas as the heat-transfer fluid was tested and characterized. The switch is actuated by a sorption pump, whose triggering temperatures were also characterized. Switching times were recorded for different thermalizations of the sorption pump.This paper presents the conductance results of such switch. The temperature scanning of the actuator is also presented. The effect of filling pressure is discussed as well as the thermalization of the sorption pump.About 60 μW/K OFF-state conductance and 100 mW/K ON-state conductance were obtained at 1.7 K. The actuation temperature is slightly adjustable upon the charging pressure of the working gas. Thermalization of the sorption pump at about 8–10 K is enough for producing an OFF state – it can be comfortably linked to a 4 K stage. Temperatures of 15–20 K at the sorption pump are required for reaching the viscous range for maximum ON conduction. Switching time dependence on the thermalization of the sorption pump is discarded.