Energy storage unit: Solid state demonstrators at 20 K and 6 K

Cryocoolers’ vibrations prevent them from being used in some sensitive applications. Stopping the cryocooler even for a short period gives rise to a steep drift of temperature. Such a drift can be significantly attenuated by adding an enthalpy reservoir to the cryocooler, separated from its cold finger by a heat switch. The whole assembly for the enthalpy reservoirs and switch is here called ESU, the energy storage unit. Two units have been built and tested based on solid state materials. One unit was designed to work up to 20 K, the other up to 6 K. This paper presents the experimental results obtained for both ESUs.Lead was used for the 20 K ESU while the ceramic GOS (Gd₂O₂S) was found adequate for the 6 K ESU. After stopping the cryocooler, a fairly slow temperature drift was measured at each ESU (from 11 to 20 K or from 3 to 6 K, respectively) while applying 10 mW for one hour, for instance. Otherwise, a temperature controlled platform experiment can use an ESU as a cold source allowing a constant temperature. Input power to the ESUs was monitored along with temperature and time. In the case of the 20 K ESU, calculations match the stored amount of energy as well as the temperature drift of the energy reservoir.A study for a high enthalpy intercept at the middle of the switch is also presented here. This intercept shall allow the attenuated temperature drift to hold for longer times. A cryogenic experiment can then be carried on using a cryocooler in a completely silent environment.     

Stress/strain characteristics of Cu-alloy sheath MgB2 superconducting wires

The mechanical properties of Cu and Cu-alloy (Cu-Zr, Cu-Be and Cu-Cr) sheath in situ PIT-processed MgB₂ superconducting wires were studied at room temperature (RT) and 4.2 K. The effects of stress/strain on the critical current (I_c) of the wires have also been studied at 4.2 K and in magnetic fields up to 5 T. Alloying the Cu sheath significantly increased the yield stress of the wires. The 0.5% flow stresses of the Cu-alloy sheath wires were 147-237 MPa, whereas that of Cu was 55 MPa. At RT, the serration in the stress-strain curves corresponding to the multiple cracking was observed around a strain of 0.4% and the curve almost saturated beyond that point. The strain dependence of I_c prior to the critical strain (ε_irr) was different depending on the magnetic field; being almost constant at 2 T and increased with strain at 5 T. The I_c decreased beyond ε_irr, which was much larger for Cu-alloy sheath wires as compared with Cu sheath wire. The magnitude of ε_irr is due to the difference in the thermal compressive strain in the MgB₂ core, which was relaxed by yielding in the sheath materials. The transverse compression tests revealed that the I_c of the Cu-alloy sheath wire did not degrade up to about 95 MPa, which is also higher than that of Cu sheath wire.     

Cryogenic design and operation of liquid helium in an electron bubble chamber towards low energy solar neutrino detectors

We are developing a new cryogenic neutrino detector: electron bubble chamber, using liquid helium as the detecting medium, for the detection of low energy p-p reaction neutrinos (<420 keV), from the Sun. The program focuses in particular on the interactions of neutrinos scattering off atomic electrons in the detecting medium of liquid helium, resulting in recoil electrons which can be measured. We designed and constructed a small test chamber with 1.5 L active volume to start the detector R&D, and performed experimental proofs of the operation principle. The test chamber is a stainless steel cylinder equipped with five optical windows and ten high voltage cables. To shield the liquid helium chamber against the external heat loads, the chamber is made of double-walled jacket cooled by a pumped helium bath and is built into a LN₂/LHe cryostat, equipped with 80 K and 4 K radiation shields. A needle valve for vapor helium cooling was used to provide a 1.7-4.5 K low temperature environments. The cryogenic test chamber has been successfully operated to test the performance of Gas Electron Multipliers (GEMs) in He and He + H₂ at temperatures in the range of 3-293 K. This paper will give an introduction on the cryogenic solar neutrino detector using electron bubbles in liquid helium, then present the cryogenic design and operation of liquid helium in the small test chamber. The general principles of a full-scale electron bubble detector for the detection of low energy solar neutrinos are also proposed.     

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.     

Recommended values for the thermal conductivity of aluminium of different purities in the cryogenic to room temperature range, and a comparison with copper

The thermal conductivity of pure aluminium at cryogenic temperatures varies by many orders of magnitude depending on purity and treatment, and there is little information in the literature on the likely values to be obtained for samples of a given purity. A compilation of measurements from the literature has been assembled and used to provide recommended ranges of values for aluminium of different purities (4N, 5N and 6N) in the normal (non superconducting) state. The number of direct thermal conductivity measurements is too limited to be used alone. Electrical resistivity measurements have thus also been used by converting to thermal conductivity using the Wiedemann-Franz law, which is shown to be valid. Since low temperature measurements can easily be extrapolated to higher temperatures, the results cover the range from 1.2 K (the superconducting transition temperature) to room temperature. Values for 5N purity copper have also been examined in a similar manner, to allow a comparison between the two materials. The main application of these results is in the design of cryogenic thermal links; a discussion of the advantages and disadvantages of both materials for this use is given. The use of silver is also investigated briefly. Trends in the behaviour of the conductivity of aluminium in the superconducting state (to temperatures as low as 50 mK) are also discussed.     

Predicting the thermal conductivity of aluminium alloys in the cryogenic to room temperature range

Aluminium alloys are being used increasingly in cryogenic systems. However, cryogenic thermal conductivity measurements have been made on only a few of the many types in general use. This paper describes a method of predicting the thermal conductivity of any aluminium alloy between the superconducting transition temperature (approximately 1 K) and room temperature, based on a measurement of the thermal conductivity or electrical resistivity at a single temperature. Where predictions are based on low temperature measurements (approximately 4 K and below), the accuracy is generally better than 10%. Useful predictions can also be made from room temperature measurements for most alloys, but with reduced accuracy. This method permits aluminium alloys to be used in situations where the thermal conductivity is important without having to make (or find) direct measurements over the entire temperature range of interest. There is therefore greater scope to choose alloys based on mechanical properties and availability, rather than on whether cryogenic thermal conductivity measurements have been made. Recommended thermal conductivity values are presented for aluminium 6082 (based on a new measurement), and for 1000 series, and types 2014, 2024, 2219, 3003, 5052, 5083, 5086, 5154, 6061, 6063, 6082, 7039 and 7075 (based on low temperature measurements in the literature).     

Analytical and finite element investigations of shear/compression test fixtures

Insulation systems are critical components of the international thermonuclear experimental reactor (ITER). They must meet the super conducting magnets design requirements, including mechanical strength under combined shear and compressive stresses at cryogenic temperatures. Past cryogenic magnet systems often relied on woven glass/epoxy materials for insulation. An important point is to find a reliable shear/compression test method for these materials. The present work investigates a commonly used shear/compression setup and aims at measuring the reliability of the obtained test results. Therefore, the stress and failure analysis is performed analytically and numerically using the finite element method. The model is based on woven glass fiber reinforced materials which are subjected to combined shear and compressive stresses as well as to thermal loading, that results from cooling from 293 K to the test temperature of 77 K. A short analytical section shows the problems of common failure criteria which are used to describe the interaction of the shear and compression stresses. The numerical—finite element—section is based on three-dimensional linear elastic finite element models under thermo-mechanical loading. The locations of high stress gradients are investigated using an average stress criterion. Three different model geometries (15°, 45°, and 70°) are analyzed and finally compared with respect to their reliability.     

A new apparatus for mechanical Q-factor measurements between 5 and 300 K

Today’s laser interferometric gravitational wave detectors are sensitivity limited by thermal noise of the optical components within the detection band of about 0.1-1 kHz. Cooling down these parts to cryogenic temperatures is a promising technique to increase the sensitivity of the gravitational wave detectors by at least one to two orders of magnitudes. Cooling substantially increases the material Q-factor contributing to reduced thermal noise. This article describes a new cryogenic apparatus which allows the measurement of the mechanical Q-factor - as a measure of internal losses - in a temperature range from 5 K up to 300 K. The requirements for cryogenic Q-factor measurements and their realization are shown. The measuring technique as well as the key parameters are discussed. Exemplary, measurements on crystalline quartz and silicon (1 0 0) are given to characterize the setup.     

Influence of different cryotreatments on tribological behavior of 80CrMo12 5 cold work tool steel

This experimental study investigated the effect of cryogenic treatments on the wear behavior of 80CrMo12 5 tool steel. For this purpose, two different cryogenic temperatures were used: −80 °C as the shallow cryogenic temperature and −196 °C as the deep cryogenic temperature. The results showed that the cryogenic treatments decrease retained austenite, which is more effective in the case of the deep cryogenic treatment (DCT). As a result, a remarkable improvement in the wear resistance of the cryogenically treated specimens was observed. In addition, DCT increases the percentage of carbides and their homogeneity in distribution. An optimum holding time was found in the deep cryogenic temperature, in which the hardness and wear resistance show maximum values. Moreover, the wear debris and worn surfaces showed that the dominant mechanism in the wear test is adhesive.     

Low temperature radiative properties of materials used in cryogenics

Radiative heat transfer between two parallel surfaces, a sample surface and a black surface, was measured. One of the surfaces was cooled with liquid helium to about 5 K and the other one was step by step heated to temperatures ranging between 30 and 140 K. As a result, the total hemispherical absorptivity and emissivity of the sample surface were determined in dependence on the temperature of the heat radiation. Aluminium samples were made of Al sheet, Al foil and aluminized mylar. Further measurements were performed on sheets of aluminium alloy, Cu, zinc brass and stainless steel. The influence of different types of sample treatment such as chemical and mechanical surface finishing and material annealing on the radiative properties is presented.     

Magnetoresistance up to 50 T of highly strengthened Cu-Ag conductors for pulsed high field magnets

The resistance of cold worked Cu-x wt.%Ag alloys (x = 7 and x = 24) is measured in dependence of magnetic field and temperature. The magnetoresistance (MR) in the field range 0 T ? B ? 50 T is positive and increases with magnetic field. If the magnetic field B is applied perpendicular to the transport current I, the magnetoresistance increases in the temperature range from 77 K to 300 K. The highest value of 14% for the MR is measured at 77 K and 50 T. For I‖B it was found that the MR is independent of the temperature in the investigated range from 77 K to 199 K. The MR is attributed to the microstructure of the alloys and appears to be independent of the Ag content of the alloys under investigation.     

Performance evaluation of the ITER Toroidal Field Model Coil phase I: Part 2: M&M analysis and interpretation

The ITER Toroidal Field Model Coil (TFMC), a large (2.7 m × 3.8 m × 0.8 m) superconducting (Nb₃Sn) DC coil designed and constructed in collaboration between EU industries and laboratories coordinated by EFDA, has been tested during 2001 in the TOSKA cryogenic facility at Forschungszentrum Karlsruhe, Germany, achieving the nominal 80 kA at 7.8 T peak field and 86 MJ stored energy as a standalone coil (Phase I). The results of the current sharing temperature (T_CS) measurements at I=80, 69 and 57 kA, presented in a companion paper (Part 1), are evaluated here using the M&M code. The critical properties best fitting the experimental voltage-inlet temperature characteristic of the P1.2 pancake are deduced from the TFMC data under the assumption of an ideal collective behaviour of the strands. The TFMC results are compared first with the expected conductor performance, showing that at the maximal current the performance was borderline with what was expected, while at the minimal current tested it was better than expected. Second, they are compared with the performance of the single strand as measured in the lab, showing that, in order to reproduce the TFMC data, one has to invoke that some degradation, larger at higher current, occurred when going from the strand to the cable.     

Cryogenic delamination growth in woven glass/epoxy composite laminates under mixed-mode I/II fatigue loading

This paper investigates the fatigue delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode I/II conditions at cryogenic temperatures. Fatigue delamination tests were performed with the mixed-mode bending (MMB) test apparatus at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), in order to obtain the delamination growth rate as a function of the range of the energy release rate, and the dependence of the delamination growth behavior on the temperature and the mixed-mode ratio of mode I and mode II was examined. The energy release rate was evaluated using three-dimensional finite element analysis. The fractographic examinations by scanning electron microscopy (SEM) were also carried out to assess the mixed-mode fatigue delamination growth mechanisms in the woven GFRP laminates at cryogenic temperatures.     

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.     

Mechanical properties of pure Ni and Ni-alloy substrate materials for Y-Ba-Cu-O coated superconductors

Mechanical properties of rolling-assisted, biaxially-textured substrates (RABiTS) and substrates for ion-beam assisted deposition (IBAD) coated superconductors are measured at room temperature, 76, and 4 K. Yield strength, Young’s modulus, and the proportional limit of elasticity are determined, tabulated and compared. Results obtained are intended to serve as a database of mechanical properties of substrates having the same anneal state and texture as those incorporated in the general class of RE-Ba-Cu-O coated conductor composites (RE = rare earth). The RABiTS materials measured are pure Ni, Ni-13at.%Cr, Ni-3at.%W-2at.%Fe, Ni-10at.%Cr-2at.%W, and Ni-5at.%W. The IBAD substrate materials included Inconel 625 and Hastelloy C-276. The Ni alloys are substantially stronger and show higher strains at the proportional limit than those of pure Ni. Substrates fully coated with buffer layers, ≈1 μm of Y-Ba-Cu-O, and 3-5 μm of Ag have similar mechanical properties (at 76 K) as the substrate alone. Somewhat surprisingly, plating an additional 30-40 μm of Cu stabilizer onto high-yield-strength (690 MPa) Hastelloy coated conductors ∼100 μm thick, reduces the overall yield strength of the composite structure by only about 10-12% at 76 K and 12-14% at room temperature; this indicates that the Cu layer, despite its relatively soft nature, contributes significantly to the overall strength of even high-strength coated conductors.     

Acoustic emission and fracture behavior of GFRP woven laminates at cryogenic temperatures

This paper describes an experimental and analytical study on fracture and damage behavior of GFRP woven laminates at cryogenic temperatures. CT (compact tension) tests were carried out at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) to evaluate the critical values of the fracture mechanics parameters. During the CT tests, AE (acoustic emission) method was implemented. AE signals can identify the critical load at which gross failure occurs. A FEA (finite element analysis) was also applied to calculate the fracture mechanics parameters. The failure criteria (Hoffman criterion and maximum strain criterion) or the damage variable based on the continuum damage mechanics was incorporated into the model to interpret the experimental measurements and to study the damage distributions within the specimen. Several methods of calculating J-integral are discussed.     

Cryogenic fatigue behavior of plain weave glass/epoxy composite laminates under tension-tension cycling

This paper focuses on understanding the tension-tension fatigue behavior of woven glass fiber reinforced polymer laminates at cryogenic temperatures. Tension-tension fatigue tests at frequencies of 4 and 10 Hz with a stress ratio of 0.1 were conducted at room temperature, 77 and 4 K. The fatigue stress versus cycles to failure (S-N) relationships and fatigue limits for 10⁶ cycles were obtained. Fractured specimens tested under fatigue tests were also examined with optical microscope.     

Very-low temperature specific heat of Torlon

The specific heat of Torlon has been measured in the 0.15-4.2 K temperature range. Data below 1 K can be represented by c(T) = P₁T^1+δ + P₂T³, with P₁ = (5.41 ± 0.08)·10⁻⁶J K^−(2+δ) g⁻¹, P₂ = (2.82 ± 0.03) ·10⁻⁵JK⁻⁴g⁻¹ and δ = 0.28 ± 0.01, as predicted by the tunnelling theory. Above 1 K, the behaviour of c(T) is similar to that of other amorphous materials and can be expressed as: c(T) = P · T^Ω with P = (2.68 ± 0.07)·10⁻⁵JK^Ω+1g⁻¹ and Ω = 3.32 ± 0.02.     

The reinforcing effect of graphene nanosheets on the cryogenic mechanical properties of epoxy resins

The reinforcing effect of graphene in enhancing the cryogenic tensile and impact properties of epoxy composites is examined at a weight fraction of 0.05–0.50%. The micro-structure and cryogenic mechanical properties of the graphene/epoxy composites are investigated using scanning electron microscopy, transmission electron microscopy, small-angle X-ray scattering and mechanical testing techniques. The results show that the graphene dispersion in the epoxy matrix is good at low contents while its aggregation takes place and becomes severer as its content increases. And the cryogenic tensile and impact strength at liquid nitrogen temperature (77 K) of the composites are effectively improved by the graphene addition at proper contents. The cryogenic Young’s modulus increases almost linearly with increasing the graphene content. Moreover, the results for the mechanical properties at room temperature (298 K) of the graphene/epoxy composites are also presented for the purpose of comparison.     

Regenerator performance improvement of a single-stage pulse tube cooler reached 11.1 K

In order to improve the cooling performance of pulse tube cooler (PTC) at 20-40 K, hybrid regenerators are often employed. In this paper a three-layer regenerator, which consists of woven wire screen, lead sphere and Er₃Ni is optimized to enhance the cooling performance and explore the lowest attainable refrigeration temperature for a single-stage PTC. The efforts focus on the temperature range of 80-300 K, where woven wire screens are used. Theoretical and experimental studies are carried out to study the metal material and the mesh size effect of woven wire screens on the performance of the single-stage G-M type PTC. A lowest no-load refrigeration temperature of 11.1 K was obtained with an input power of 6 kW. The PTC can supply 17.8 W at 20 K and 39.4 W at 30 K, respectively.