Proximity-effect-induced superconductivity at millikelvin temperatures

A systematic study is reported of the proximity effect in superconductivity of thick Cu layers (4d _n105 µm) in contact with Nb or Nb-Ti (46D _s1041 µm) from measurements of the ac susceptibility and of the thermal conductivity. The induced superconducting properties of Cu are studied as a function of temperature (7 mKT1 K), field (0.4 mGH 10 G), and electronic mean free path (0.2l14 µm). The Meissner screening length in Cu increases faster than the coherence length with decreasing temperature and eventually saturates, making the Cu fully superconducting. In this case, superconductivity can be destroyed sharply at a breakdown fieldH _b; the transition atH _b is hysteretic. The data are in agreement with numerical solutions of the de Gennes/Ginzburg-Landau theory, as demonstrated by the dependence of the Meissner screening length and of the breakdown field on temperature, field, and electronic mean free path. The data indicate that observation of a reduction in thermal conductivity requires a substantially larger induced pair potential than is necessary even for total field shielding. The concentration of Cooper pairs in Cu in contact to Nb is larger than in Cu in contact to Nb-Ti, even for Nb-Ti/Cu samples with a larger electron mean free path in the Cu part. No measurable superconducting field screening could be induced in the investigated temperature range in Pd (26d _n72 µm) in contact to Nb/Cu or to Nb (D _s5d _n). This is explained by the fact that the coherence length is at least an order of magnitude smaller in Pd than in Cu. Evaporating a layer of Fe on Cu in the Nb/Cu samples results in a strong depression of the proximity effect.     

Heat capacity of transition metals at high temperatures

In recent years better data have appeared for thermophysical properties of some transition elements at high temperatures (e.g., Ta, Nb, Mo, W, Pt) using subsecond techniques and refinements in thermometry. These data enable us to deduce values for the heat capacity at a constant volume C _v and compare them with the expected lattice vibrational contribution. We attribute the differences chiefly to electronic effects and compare these with results of low-temperature measurements. While calculating the C _p-to-C _v correction, we obtain values for the Grüneisen parameter which are useful in assessing the reliability of the measured data.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

A simple heat switch for use at millikelvin temperatures

A simple technique is described for fabricating a high conductivity silver-aluminium joint for use in the construction of an aluminium superconducting heat switch which can be operated in the millikelvin temperature range.     

Heat capacity of aqueous solutions of sulfuric acid at low temperatures

Results of an experimental determination of the specific heat of H₂O-H₂SO₄ solutions at low temperatures are presented and the heat capacity of a heterogeneous two-component system is analyzed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 27, No. 1, pp. 78–84, July, 1974.     

A pumping experiment in dilute3He-4He solutions at millikelvin temperatures

Very dilute³He-⁴He solutions provide a model system for the study of the effects of magnetic polarisation. It was proposed by the Lancaster group¹ that a pumping effect of³He should be observed under appropriate conditions in these solutions. In a strong magnetic field gradient, the³He should be pumped from the low field region towards the high field region, in which the³He is polarised. We have now observed this effect in 0.1% and 0.06% solutions, using a vibrating wire resonator in the low field region as the concentration indicator. The concentration change of a few percent under our present experimental conditions (9T maximum field, 10 mK) is of the expected magnitude so that development of the technique can now be considered.     

Properties of sintered silver powders and their application in heat exchangers at millikelvin temperatures

We present a systematic study of the properties of sintered silver powders used in heat exchangers at millikelvin temperatures. Scanning electron microscopy and surface area (BET) measurements have been performed on various powders with different degrees of sintering. Values of the Kapitza resistance with pure He³ and He³—He⁴ mixtures are given and compared with previously reported results.     

Thermal conductance at millikelvin temperatures of woven ribbon cable with phosphor-bronze clad superconducting wires

Woven Nomex® ribbon cables made up with superconducting niobium-titanium wire are used at millikelvin temperatures in many large cryogenic instruments. It is important to know how much heat in transmitted down such cables. However, the conductivity of the materials used is not well known. Another problem is that the wires are normally clad with alloys which exhibit some magnetism. This is a potential problem for instruments employing superconducting detectors. A safe non-magnetic alternative to the usual materials is phosphor-bronze clad niobium-titanium wiring. However, there is little experience with such wires. We have therefore measured the conductance of a ribbon cable made up with these wires. The measured values are in good agreement with our predictions, suggesting that the values we have used to model the cable are sufficiently accurate, and could therefore be used to predict the performance of ribbon cables using other cladding materials, so long as the conductivity of the cladding is reasonably well known. As part of our analysis, we consider the likely variation in thermal conductivity values for C51000 phosphor bronze caused by legitimate variations in composition.     

Thermal conductivity measurements of pitch-bonded graphites at millikelvin temperatures: Finding a replacement for AGOT graphite

Pitch-bonded graphites are among the best known thermal insulators at sub-kelvin temperatures, but are very good conductors at higher temperatures. This makes them ideal for mechanical supports which must provide good thermal isolation at an operating temperature below 1 K, but must have good conductance at higher temperatures to aid in initially cooling down an instrument (a “passive heat switch”). One type of graphite, AGOT, has been known as having the lowest thermal conductivity below 1 K not only among graphites, but also compared with any other material. It is, however, no longer available. We have carried out thermal conductivity measurements at temperatures between 60 mK and 4 K on a proposed replacement, POCO AXM-5Q graphite, as well as a sample of AGOT graphite. Our measurements show that both graphites have a difference of about six orders of magnitude in conductivity between room temperature and 100 mK, but that AGOT graphite is not as good an insulator as previously believed. We conclude that AXM-5Q graphite is not only a suitable replacement for AGOT, but in fact is somewhat superior.     

Thermal resistance between a paramagnetic salt (CMN) and liquid helium at millikelvin temperatures

We report new measurements of the thermal resistance between cerium magnesium nitrate (CMN) and liquid helium. AtT=100 mK, the thermal resistanceR for powdered CMN in contact with liquid³He-⁴He or liquid³He is about 2000 times the Kapitza boundary resistance. From the field and temperature dependences ofR we conclude that it is dominated by the phonon-bottlenecked spin-lattice resistance. However, if high-purity liquid³He is in contact with CMN, we observe, below 20 mK, a magnetic boundary resistance in parallel to the spin-lattice resistance. This resistance is two orders of magnitude smaller than the spin-lattice resistance atT=10 mK, and is described byR T ^1.4. The absolute values of the magnetic boundary resistance, as well as the spatially limited phonon-bottlenecked resistance, are extremely sensitive to the surface treatment of the CMN crystals.     

Plastic flow and fracture of titanium at low temperatures

The following features of the mechanical behaviour of titanium at low temperatures are discussed: operative slip systems, twinning, serrations in the stress-strain curves near 4.2 K, grain size hardening, strain hardening, interstitial solid solution strengthening, deformation kinetics, and fracture and ductility. Regarding plastic flow, interstitial solutes mainly influence the thermal component of the flow stress, the rate controlling mechanism being the thermally activated overcoming of interstitial solute obstacles by dislocations gliding on the first order prism planes. Strain hardening is given by the commonly observed linear relationship between the flow stress σ and the square root of the dislocation density ρ,

$\text{σ= σ}{}_{\mathrm{<mtext>f</mtext>}}\text{(T,}\text{ε}\text{?}\text{, C}{}_{\mathrm{<mtext>i</mtext>}}\text{)+ λEb[p(?, d)]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$

where σ_f, the stress due to all obstacles other than dislocations, increases with increase in strain rate ε and interstitial content C_i and with decrease in temperature T. The constant α is of the order of 0.7 and E is Young's modulus. Both σ_f and α are relatively independent of grain size, d, supporting the work hardening model for the effect of grain size on the flow stress.The fracture stress increases with decrease in grain size and with increase in interstitial content. As a general trend, the total elongation tends to increase as the temperature is lowered to about 77 K, below which a decrease elongation occurs. The improvement in ductility with decreasing temperature to 77 K is attributed to the occurrence of twinning. Therefore, the effects of grain size, temperature, and interstitial content on ductility at low temperatures reflects in part the influence these variables have on twinning.     

Elastic properties of two titanium alloys at low temperatures

Sound velocities and elastic constants were determined semi-continuously for two annealed polycrystalline titanium alloys between 4 and 300 K. Results are given for: longitudinal sound velocity, transverse sound velocity. Young's modulus, shear modulus, bulk modulus, Poisson's ratio, and elastic Debye temperature. A pulse-superposition technique was used.     

Thermal conductivity of Tecamax® SRP from millikelvin temperatures to room temperature

Tecamax® SRP (self-reinforced polyphenylene) is a new commercially available amorphous polymer which is suitable for use at cryogenic temperatures. It has a high tensile strength (210 MPa at room temperature), resulting from the molecular structure of the polymer rather than by the addition of reinforcing materials. We have measured the thermal conductivity between 60 mK and 280 K. We find that the conductivity below 10 K is similar to, but lower than, most amorphous materials, and the material offers a good combination of low conductivity at low temperatures and high tensile strength. Our results suggest that the material may in fact have a small crystalline component, which may be a partial explanation for the low conductivity. Above 10 K, the temperature dependence of the conductivity is different from most amorphous materials. We are unaware of previous measurements of the thermal conductivity of this material, even at room temperature.     

Chemical vapor deposition of titanium nitride at low temperatures

A new method is proposed for making titanium nitride (TiN) films at substrate temperatures between about 400 and 700°C. The films are formed from TiCl₄ and NH₃ by chemical vapor deposition. The method is versatile with growth rates of up to 0.1 μm s⁻¹ possible. The optical properties of the films are measured and fitted theoretically using the Drude theory to obtain the plasma frequency of the films. A modification of the Drude theory according to Maxwell Garnett explains a decreased reflectance in the IR region for the thinner films. Use of the films as heat mirrors and other applications are discussed.     

Heat capacity of the water-lithium bromide system and the water-lithium bromide-zinc bromide-lithium chloride system at high temperatures

The heat capacities of the water-lithium bromide system were measured in the temperature range 313.15–433.15 K. Those of the water-lithium bromide-zinc bromide-lithium chloride system wer also measured in the temperature range 373.15–433.15 K. On the basis of the experimental data, an empirical formula for the water-lithium bromide system was obtained by the least-squares method. The calculated values from the empirical formula were in good agreement with literature values obtained in the range of low temperatures. In addition, the heat capacity data obtained for the water-lithium bromide-zinc bromide-lithium chloride system were compared with the calculated values from Ueda's equation which is applicable at low temperatures, and they agreed well.     

Surface characterization of titanium hydride powder

Titanium hydride is well known for its stability in air because of formation of thin TiO₂ oxide layer on top of TiH₂ powder particles. Formation of the oxide layer is directly related to high affinity of Ti to oxygen and it can be formed in few seconds also on top of TiH₂ particles in low vacuum. On the other hand, thickness of TiO₂ layer on the formed TiH₂ powder is in the range of 130 nm. Existence of oxide layer on top of powder particles was confirmed with AES, XPS and EDXS cross section line scans. Formation of thin oxide was confirmed also with calculations based on chemical and TG analyses.     

Sintering densification of titanium hydride powders

The sintering densification behaviors of titanium hydride are investigated at different compaction pressures, compared with pure titanium. The results show that the shrinkage and densification of the TiH₂ specimens after sintering are obviously higher than those of pure Ti. It is also found that the densification mechanism of the TiH₂ is mainly attributed to the volume change in the TiH₂ sintering process, and the surface cleaning effect of the released H atoms. In addition, it is also validated experimentally that the released H atoms are accompanied by the formation of H₂O, which reduces the oxide film on Ti surface, increases the chemical activity of the Ti surface, and promotes the sintering densification.     

Boriding titanium alloys at lower temperatures using electrochemical methods

In this study the boriding of titanium alloys was attempted with the use of electrochemical methods at room temperature followed by a tempering process at much lower temperatures than those reported in literature. The electrochemical boriding process with tempering at 500 °C resulted in the formation of compact TiB and TiB₂ layers.     

Electrical conductivity in non-stoichiometric titanium dioxide at elevated temperatures

The electrical conductivity of polycrystalline titanium dioxide prepared by a liquid mix technique was measured for the oxygen partial pressure range of 10° to 10^–19 atm and temperature range of 850 to 1050° C. The data were found to be proportional to the –1/6 power of oxygen partial pressure for the oxygen pressure range 10^–19 to 10^–15 atm, and proportional to for the oxygen pressure range >10^–15 atm. The region of linearity where the electrical conductivity varied as the –1/4 power of increased as the temperature was decreased. There was evidence of p-type behaviour for 10^{ - 2}$$ " align="middle" border="0"> atm in the temperature range 950 to 850° C, although the measured data were insufficient to assign a pressure dependence. Electrical conductivity minima in the log against log plot moved to lower as the temperature was decreased in the range 950 to 850° C. The measured oxygen pressure dependence of electrical conductivity in the lowest region supports the oxygen vacancy defect model. The observed data are consistent with the presence of very small amounts of acceptor impurities. A binding energy of 0.67 eV between the acceptor impurity and its compensating oxygen vacancy was also determined.     

Heat capacity of natural fruit juices and of their concentrates at temperatures from 10 to 120°C

The heat capacity of natural apple, cherry, and raspberry juices and of their concentrates has been studied at temperatures from 10 to 120°C and pressures of 0.1 and 2 MPa. A setup based on the method of adiabatic calorimeter was assembled to carry out investigations. An estimate of the error of setup operation as well as control measurements on water prove the accuracy of experimental data to be within ±0.8% at the indicated parameters of state. For the heat capacity of juices 175 values have been obtained. Equations that describe experimental data as functions of temperature and content of dry matter have been constructed. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 5, pp. 192–200, September–October, 2007.