Theory of a magnetic cooling system at very low temperatures

The totally anisotropic magnetic cooling compound, hydrated cerium magnesium nitrate, CMN, is treated at very low entropy values with the formalism of molecular-field theory. The magnetic phase-boundary line separating the ordered antiferromagnetic type region from the disordered paramagnetic region is well accounted for by the theory when compared with recent data of the Berkeley Low-Temperature Experimental Group. The ordered region refers to the canted antiferromagnetic or transverse magnetic field configuration of the Ce ions in CMN single crystals. One of the main experimental results refers to the temperature-scale functions obtained indirectly through largely arbitrary functions fitted to enthalpy-entropy data, treating simultaneously and indiscriminately both the ordered antiferromagnetic and disordered paramagnetic regions, with the data in the latter outweighing those in the former. The theoretical enthalpy-entropy function in the ordered range is in fair agreement with the corresponding data in absence of magnetic field with these data exceeding the calculated enthalpies. This may refer to the possible inclusion in the data of excitations in excess of the magnetic excitations alone accounted for by the theory. The entropy derivatives of the empirically fitted multi-parameter enthalpy-entropy functions define the indirectly derived temperature scales of strictly numerical significance at the very low temperatures. The molecular-field-theoretical entropy-temperature function, while approximate, is conceptually well grounded and may be preferable to, or at least competitive with, the indirectly derived Berkeley CMN temperature scales. There is fair agreement between them at the larger entropies approaching the zero magnetic-field critical transition entropy.Work performed under the auspices of the U. S. Atomic Energy Commission.     

Fibre-epoxy composites at low temperatures

The thermal and mechanical properties of carbon, glass and Kevlar fibre reinforced epoxy composites are discussed, with particular reference to the behaviour of these materials at cryogenic temperatures. The effects of production techniques and various fibre arrangements are determined.     

Ranque effect at low temperatures

Methods and results of experimental research on the Ranque effect at a gas temperature reduced to 80° K.     

Some electrical feedthroughs to be used at low temperatures

A type of low temperature feedthrough is described for use by low temperature physicists     

Deformation dynamics at low and ambient temperatures

The variation of tensile yield stress at a constant strain rate as a function of temperature for well-annealed pure metals show, with increasing temperatures, a rather sharp drop in yield stress (low temperature regime), followed by the intermediate temperature regime where yield stress decreases more slowly (and the ratio of yield stress to shear modulus remains more or less constant), which in turn is followed by the high temperature regime where the yield stress drops again rather sharply. The paper discusses the phenomenological framework for studying deformation dynamics in the low and intermediate temperature regimes. The approach adopted is the well-known state variable approach, where the evolutionary nature of deformation structure is described by one or more structure variables such that the current values of mechanical variables and structure variables together completely define the current state of deformation. A critical analysis of experimental results available suggest that at least for deformation at low strain rates, stress-rate is probably not a state variable of deformation. Thus deformation is most conveniently studied in terms of TASRA (thermally activated strain rate analysis) where the stress, plastic strain rate, temperature and structure are interrelated through a Gibb’s free energy for thermal activation by an Arrhenius equation. The stress-dependence of Gibb’s free energy and its maximum value then form the basis of identifying the rate-controlling obstacles. The need for careful experimentation and systematic analysis is illustrated by the example of low temperature deformation of hard hep metals. Modelling for the evolution of deformation structure is also touched upon.     

Oxidation of TiC at low temperatures

The isothermal oxidation of TiC powders was carried out at low temperatures of 350–500 °C at oxygen pressures of 3.9, 7.9 and 16 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electro-microbalance. The oxidation kinetics are described by the one-dimensional diffusion equation. It was found that oxidation consists of four steps, I (fast step), II (slow step), III (fast step) and IV (slow step), at all the pressures. Two activation energies were obtained in steps II–IV: 125–150 kJ mol^–1 below about 420 °C and 42–71 kJ mol^–1 above that temperature. The low- and high-temperature oxidation mechanisms are discussed in connection with the formation of oxycarbide/titanium suboxides and the crystallization of anatase, followed by the generation of cracks in the grains.     

Resistive SQUID calorimetry at low temperatures

A resistive SQUID may be used to measure heat current. We call an RSQUID used in such a way an RSqUID: It acts as a heat-current to frequency (q–f) converter of sensitivity =f/q. Two methods of heat capacity measurement by the use of an RSqUID are described. In the direct method the number of cycles of oscillation when the temperature of the RSqUID and specimen is changed by T is counted. The measured heat capacity C _m (to be corrected for the addenda) is given by C_m=n/(T). In the step method the specimen and heater together are separated from the RSqUID by a thermal resistance R. The RSqUID is biased to a frequency f ₁. The heater is switched on and a number of cycles is obtained which represents the amount of heat required to bring the specimen to equilibrium in the new steady state, in which f=f₂+f. By this method the quantity C_m=n/(Rf) is obtained. In both methods the heat current is integrated by counting to obtain an amount of heat Q=n/, which may thus be measured very precisely if the number of cycles is sufficiently large [and (T) calibrated accurately]. The first RSqUID constructed for heat capacity measurement is described. In this RSqUID the high thermal resistance of press contacts has been avoided and soft solder as well as niobium used for superconducting material. The sensitivity varied from 0.8 to 0.4 Hz nW_–1 between 2 and 7 K. The results of measurements made between 2 and 7K by the step method with this RSqUID in a rather unsophisticated cryostat achieved a relative accuracy of ~0.1% with samples of mass about 1 g. Results of measurements made on samples of pure copper and indium are described.     

Vapour-phase graphene epitaxy at low temperatures

We report an epitaxial growth of graphene, including homo- and hetero-epitaxy on graphite and SiC substrates, at a temperature as low as ∼540 °C. This vapour-phase epitaxial growth, carried out in a remote plasma-enhanced chemical vapor deposition (RPECVD) system using methane as the carbon source, can yield large-area high-quality graphene with the desired number of layers over the entire substrate surfaces following an AB-stacking layer-by-layer growth model. We also developed a facile transfer method to transfer a typical continuous one layer epitaxial graphene with second layer graphene islands on top of the first layer with the coverage of the second layer graphene islands being 20% (1.2 layer epitaxial graphene) from a SiC substrate onto SiO₂ and measured the resistivity, carrier density and mobility. Our work provides a new strategy toward the growth of graphene and broadens its prospects of application in future electronics.      

Deformation of silicon at low temperatures

The dislocation arrangements produced around microhardness indentations made in silicon at room temperature have been studied by transmission electron microscopy. Loops consisting of 30°- and 60°-dislocations are produced and move on the {111} planes. It is suggested that, during indentation, the theoretical shear strength is exceeded locally and that the observed dislocations arise as a result of the accommodation of the displacements due to block slip. On annealing up to 1030° C the loops do not appear to be mobile, rather new loops consisting of edge and screw components are formed which can move large distances.     

Strength of adhesives at low temperatures

The paper reports tests on the shearing strength of a range of adhesives at 77K.     

Fabry-Perot optical resonator at low temperatures

Results of an experimental study of an optical Fabry-Perot resonant cavity operated between temperatures of 77 and 300 K are presented. In particular, measurements of the cavity finesse and thermal expansion are discussed in terms of new and current applications for such a device. These include energy sensors for Weber-bar type gravitational wave antennas, high-stability laser oscillators, and high-sensitivity thermal-expansion studies of condensed matter.     

Thermoelectric refrigeration at very low temperatures

It is shown that thermoelectric refrigeration might become an effective technique at or below a temperature of several mK. This is because the heat conduction due to the lattice vibrations becomes very small at these low temperatures. The thermoelectric material should have a high Debye temperature and a low carrier concentration, controlled so as to correspond to a Seebeck coefficient of a few hundred μV K^?1. One of the major difficulties in the achievement of low temperature thermoelectric refrigeration would, in fact, be the establishment of this optimum carrier concentration; it is suggested that it might be established through either an injection process or the application of a high magnetic field.     

Liquid density measurement at low temperatures

Liquid density measurement at low temperatures is discussed. Problems encountered and their possible solutions are considered, such as safety aspects, calibration, and installation. Use of the vibrating element density meter fitted to a mechanical tank gauge has given a direct accurate tool for determining the density and density profiles that exist in cryogenic tanks.     

Optical interferometry at ultra low temperatures

We have developed optical, interferometric methods for investigations of interfaces at ultra low temperatures. In our scheme conventional optical windows are avoided: laser illumination (He-Ne) is guided into the cryostat via a single-mode optical fiber and images are taken using a CCD sensor mounted inside the 4-K vacuum can. A real-time video camera has been successfully used in investigations of superfluid³He down to 0.6 mK whereas a slow-scan camera has been employed for optimal contrast in low-intensity imaging of liquid/solid interfaces (reflection coefficient 10^–6). The investigated topics include (1) superfluid³He surface in rotation and during rapid deceleration, (2) hydrodynamics of thin superfluid³He layers, (3) superfluid/solid interface in⁴He, and (4) wetting of superfluid⁴He by normal³He in phase separated mixtures. A vertical resolution of 10 nm and even below has been achieved in these studies.     

Thermal conductivity measurements at low temperatures

We describe briefly the experimental facilities developed for the measurement of thermal conductivity of solids in the temperature range 10K–300K. Different techniques have been used for the determination of thermal conductivity, depending on the relaxation time of the system under investigation. Measurements on stainless steel 304, using steady state and non-steady state methods are presented. Values of thermal conductivity obtained by both these methods agree to each other and are consistent with those reported earlier. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995