Frictional properties of metal insulator surfaces at cryogenic temperatures

When a superconducting material slides against an insulating material, frictional heat dissipation at the interface can be sufficient to drive the superconductor normal. An investigation of the sliding behaviour of various metal-insulator pairs has been carried out to discover which combinations will reduce or eliminate rapid slip events which lead to high local heating. The results indicate that desirable sliding behaviour can be achieved, at 4.2 K, when the hardness of the metal and insulator are well matched.To observe sliding behaviour, tests have been carried out at 4.2 K, 77 K and 295 K for a number of polymers, laminated composites, and coated pieces sliding against either copper or aluminum. To achieve a better understanding of the behaviour of each sliding combination, the friction coefficient has been measured as a function of sliding distance, sliding velocity, and temperature.     

Dielectric and piezoelectric properties of perovskite materials at cryogenic temperatures

Dielectric and piezoelectric properties of perovskite materials including La modified Pb(Zr, Ti)O3 (PZT's), (Ba, Sr)TiO3 (BST) polycrystalline ceramics and Pb(Zn1/3 Nb2/3)O3-PbTiO3 (PZN-PT) single crystals were investigated for capacitor and actuator applications at cryogenic temperatures. PZTs were compositionally engineered to have decreased Curie temperatures (Tc) by La and Sn doping in order to compensate for the loss of extrinsic contributions to piezoelectricity at cryogenic temperatures. Enhanced extrinsic contributions resulted in piezoelectric coefficients (d33) as high as 250 pC/N at 30 K, superior to that of conventional DOD Type PZT's (d33~100 pC/N). This property enhancement was associated with retuning to the MPB at cryogenic temperatures. 5/95 BST with a dielectric maximum at 57 K was investigated to obtain high electrostrictive properties or E-field induced piezoelectricity. Coupling coefficients (k31) 25% comparable to those of the cryogenic PLZT piezoelectrics were observed at d.c. bias of 1.5 kV/cm and 50 K. Though significantly lower than the room temperature values, PZN-PT rhombohedral single crystals exhibited d33> 500 pC/N at 30 K.     

Copper-TFE friction at cryogenic temperatures

Interfaces between metals and polytetrafluoroethylene (TFE) are common in cryogenic systems. In this paper we present results from measurements of the temperature dependence of the dynamic coefficient of friction between commercially pure copper and TFE. The effect of the copper surface finish was also determined. The effects of load and speed were evaluated over a small range, but nearly all data were taken at a surface speed of 5.4 cm s^?1 with a load of 1.63 N cm^?2. These values are typical of those encountered by the moving parts of some cryogenic machinery.     

Research into mechanical properties of reaction-bonded SiC composites at cryogenic temperatures

The mechanical properties of reaction-bonded silicon carbide (RBSC) composites at cryogenic temperatures have been reported for the first time. The results show that the flexural strength and fracture toughness increase from 277.93 ± 23.21 MPa to 396.74 ± 52.74 MPa and from 3.69 ± 0.45 MPa·m^1/2 to 4.98 ± 0.53 MPa·m^1/2 as the temperature decreases from 293 K to 77 K, respectively. The XRD analysis of the phase composition reveals that there is no phase transformation in the composites at cryogenic temperatures, indicating cryogenic mechanical properties are independent of phase composition. The enhancement of mechanical properties at 77 K over room temperature could be explained by the transition of fracture mode from predominant transgranular fracture to intergranular fracture and stronger resistance to crack propagation resulting from higher residual stress at 77 K. The above results demonstrate that such composites do not undergo similar deteriorations in the fracture toughness as other materials (some kinds of metals and polymers), so it is believed that such composites could be a potential material applied in cryogenic field.     

Creep of copper at cryogenic temperatures

The long-term creep behaviour of OFHC copper was investigated at 4.2 K and 77 K. At 77 K, steady-state creep was observed and the creep rates were tens of orders of magnitude higher than would be expected from extrapolations of ambient temperature creep data. The stress exponent obtained at 77 K is 2.2. The apparent activation energy at 77–90 K range is about 0.02 eV. At 4.2 K, transient creep was observed. TEM study of all crept specimens showed cell structures in support of the occurrence of substantial creep at cryogenic temperatures.     

Deformation rate dependence of mechanical properties of epoxy resin at cryogenic temperatures

Effects of deformation rate on the mechanical properties of epoxy resin used in superconducting magnets have been studied at cryogenic temperatures. Compressive and flexural tests were made to reveal the mechanical behaviour. In the case of compressive tests, the increase in the deformation rate caused on increase of elastic modulus and a decrease of breaking stress and strain. In the case of flexural tests, different results in breaking stress were obtained. The results indicate that the following two problems must be elucidated for practical use of polymers in magnets, ie (i) impact strength of the polymers, (ii) the stress condition applied to the polymers in the magnet.     

Fracture and strength properties of selected austenitic stainless steels at cryogenic temperatures

Austenitic stainless steels have an excellent combination of mechanical and physical properties for load-bearing structures of large superconducting magnets for plasma containment in magnetic fusion experiments. To assess their relative suitability fracture toughness, fatigue crack growth, and tensile properties data for five austenitic steels at 295, 76, and 4 K have been obtained. The steels were AISI 304, 316, 304LN, and 316LN, and an Fe-21cr-12Ni-5Mn alloy with a higher nitrogen content than the other four grades. The two principal findings were the systematic variation of yield strength with nitrogen content and a systematic inverse correlation between fracture toughness and yield strength. Data from previous studies are reviewed which confirm the trends of the present data.     

Eddy current losses at cryogenic temperatures

The effect of thermal processes on eddy-current losses in the structural elements of cryogenic and superconducting devices are analyzed. Maxwell's equations coupled with the heat-conduction equation are solved, taking into account the dependence of resistivity, heat capacity, and heat-transfer coefficient on temperature. Losses are analyzed as a function of magnetic field, frequency, and geometry for a thin strip in a uniform magnetic field. It is shown that losses calculated taking the thermal processes into account may differ from those obtained at constant temperature     

Behaviour of power MOSFETs at cryogenic temperatures

In this paper an investigation is reported on Siemens-power-metal-oxide-semiconductor (SIPMOS) transistors of both p and n channel types, for their suitability for cryogenic applications. The drain characteristics, temperature dependence of R_ds(on) and switching behaviour have been studied in the temperature range 4.2 – 300 K in BSS91 and BSS92 MOSFETs. The experiments reveal that these types of power transistors are well suited for operations down to ≈ 30 K. However, below 30 K the operating characteristics make them unsuitable for application. This arises because of carrier freeze-out in the n⁻ region on the substrate, which forms a drain.     

Interlaminar shear properties of composite insulation systems for fusion magnets at cryogenic temperatures

This paper reports the cryogenic interlaminar shear properties of composite insulation systems for the superconducting magnet coils in the International Thermonuclear Experimental Reactor (ITER). Short beam shear tests were performed at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) on three insulation systems consisting of woven glass fiber reinforced plastic (GFRP) composites with different polymer resins and polyimide films, and the dependence of their apparent interlaminar shear strength on the temperature and the polymer resins was discussed. A detailed observation of failed specimens was made to verify the failure mechanisms. Insulation systems subjected to gamma irradiation at room temperature were also considered, and the effect of irradiation on the cryogenic interlaminar shear properties was examined.     

Frictional properties of structural steel JN2 at cryogenic temperatures in a vacuum

To assess the frictional properties at the wedge of a toroidal field coil of a tokamak fusion reactor, friction tests were conducted using structural steel, JN2, against various materials in a vacuum at low temperatures under a normal load of 10 N and a peak-to-peak sliding amplitude of 100 μm. The temperature was mainly ≈ 5 K and the ambient pressure ≈ 10⁻³ Pa. The relationship between the frictional characteristics and the number of cycles depends on both the material combination and temperature. Two typical patterns of behaviour were observed at 5 K depending on the combination; fairly constant friction and very high and fluctuating friction. The high friction was caused by severe adhesion between surfaces. Temperature dependence of friction was also observed for JN2-copper.     

CMOS operational amplifier performance at cryogenic temperatures

The performance of three different two-stage CMOS operational amplifiers (op-amps) at temperatures below 100 K was investigated. The open-loop gain and unity-gain frequency increased by at least a factor of 2 and 1.3, respectively, when cooled from room temperature to 50 K. However, because of current kinks in the n-channel output transistors, all of the op-amps developed severe non-linearities in their output characteristics at temperatures below 50 K with ±5 V supplies. Reducing the supply voltages extended the op-amps useful cold temperature operating range. One op-amp was found to operate quite well down to 30 K with the supplies reduced to ±2.5 V. Methods of improving the cold temperature operation of CMOS op-amps are discussed.     

Thermal expansion at low temperatures of glass-ceramics and glasses

The linear thermal expansion coefficient, α, has been measured from 2 to 32 K and from 55 to 90 K for a machineable glass-ceramic, an ‘ultra-low expansion’ titanium silicate glass (Corning ULEE), and ceramic glasses (Cer-Vit and Zerodur), and for glassy carbon. α is negative for the ultra-low expansion materials below 100 K, as for pure vitreous silica. Comparative data are reported for α-quartz, α-cristobalite, common opal, and vitreous silica.     

Quartz crystal unit modeling at cryogenic temperatures

The aim of this paper was to develop the new quartz crystal electrical model including its temperature properties in the temperature range from 83.15 K (−190 °C) to 303.15 K (+30 °C) through experimental set-up and simulation analysis. Both the methodology of the quartz resonator measurements, the instrument setup, and the measurement methods needed to collect the necessary data as well as polynomial approximation of temperature dependence were described. The electrical model of AT-cut type quartz crystal for cryogenic temperatures was developed, in which its elements were expressed as functions of temperature. Using these polynomials, the behavioral model for PSPICE computer program has been worked out.     

Austenitic stainless steels at cryogenic temperatures 1—Structural stability and magnetic properties

The structure and magnetic properties of some 15 austenitic stainless steels were examined after cyclic cooling treatments and low temperature deformation. Magnetic measurements at room temperature, 77 K, and 4.2 K and subsequent metallographic examination suggest that many of the AISI 300 stainless steels such as 301, 302, 303, 304, 304L, 305, 316L, 321, and 347 must be considered potentially unstable with respect to the formation of the ferromagnetic α′ martensite phase on repeated cooling to low temperatures. This structural instability was increased significantly after a sensitizing treatment in the weldable steels 304L, 321, and 347 leading to the formation of up to 11.2% a′ martensite, part of which formed isothermally. Low temperature deformation is even more potent in promoting the transformation, at least 50% α′ martensite being induced by deformation at 4.2 K in the otherwise stable alloys such as 309 and the 0.2% N versions of 304L and 316L. The high alloy steels 310 and Kromarc 55 remain fully a austenitic even after deformation to rupture at 4.2 K. The temperature dependence of the magnetic susceptibility of the latter alloys and Incoloy 800 indicates that their low temperature structural stability is associated with magnetic transitions which occur within the austenite phase.     

Transmissivity testing of multilayer insulation at cryogenic temperatures

The problem of degraded emissivity of thin films at low temperatures has been a long observed phenomena. Previous efforts at measuring properties have suggested that transmission of energy through the films may play a key role in the thermal performance of multilayer insulation systems at low temperatures. Similarly, recent testing on tank applied systems has suggested a radiative degradation at low temperatures. Two different approaches were used to attempt to measure the transmission of energy through MLI at low temperatures. A laser based measurement system was set up to directly measure transmittance and a calorimetric based measurement system was used to measure relative emittance of a single layer between aluminum foil and double aluminized Mylar. Minimal transmission at long wavelengths were observed through standard MLI blanket materials at deposition thicknesses of even 35 nm. Where transmission was measured, it was too low to effect the performance of a multilayers system. Similarly, the calorimeter showed similar increases of emissivity for both standard blanket materials and aluminum foils. Multiple different methodologies of measurement have all yielded the same result: that there is no transmission through standard MLI blanket materials at wavelengths associated with temperatures as low as 2 K.