In recent years, there has been increasing interest in the idea of using a superheated superconducting colloid (SSC) as a detector for neutrinos and dark matter candidates. The primary objective of this work has been to investigate the basic properties of an SSC, consisting of 7 μm radius tin grains imbedded in epoxy, using a pumped ⁴He cryostat with a low vibrational noise RF-SＱUID readout system. The superheating-supercooling hysteresis curves of the colloid have been measured in applied magnetic fields ranging from 3.1 x 10⁻⁴T to 1.4 x 10⁻²T. The superconducting to normal phase transition in individual grains inside the colloid has been observed and the measured signal size is in reasonable agreement with the calculated values. Finally, it was demonstrated that the colloid could withstand up to 20Mrad of [omitted]-radiation without incurring a significant change in its superconducting-normal phase transition. A new type of sample, consisting of a planar array of 1 μm thick metal squares deposited on a mylar substrate, was developed. Both indium and tin were used as a. fabrication material. The characteristics of such samples were investigated, again using the pumped ⁴He cryostat. The full Meissner effect was only observed for applied magnetic fields less than 5 x 10⁻⁵T. For higher applied fields, the samples behaved like type-[omitted] superconductors in the mixed state regime, exhibiting flux penetration and trapping. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The behaviour of lead concentrate particles oxidizing in a stagnant gas reactor has been examined and a mathematical model which predicts the kinetics of galena particles developed. The effects of oxygen concentration, particle size, furnace temperature and concentrate composition were studied. The results showed that the concentrates all exhibited sharply defined ignition points. The ignition points were found to be strongly dependent on oxygen concentration. Reductions in ignition temperature of up to 100 K were observed when the concentrates were reacted in pure oxygen rather than in air. Iron composition was found to have a similar effect on ignition temperature. The modelling results predict short reaction times for ignited particles (less than 100 milliseconds). Particles which ignited in air attained predicted temperatures in excess of 2600 K. In oxygen, particle temperatures were calculated to be greater than 2800 K. Good agreement was found between experimental results and model predictions. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A physical model and a numerical solution procedure has been developed to predict heat transfer behaviour in supercritical fluids. A major area of concentration was the modelling of the turbulent components of shear stress and heat flux. Traditionally, the turbulent fluxes are modelled by algebraic expressions such as the familiar mixing length methods. However, the use of this technique has not been entirely satisfactory. Newer methods for constant-property flows which model turbulent fluxes by considering the transport of quantities such as turbulent kinetic energy and the dissipation rate of turbulence have been extended to supercritical fluids. This involves the solution of two additional partial differential equations that are solved simultaneously with the equations of continuity, energy, and momentum. The numerical scheme has been developed on a completely two-dimensional basis by extending the Pletcher-DuFort-Frankel finite difference method. Computed results for velocity and temperature profiles as well as wall temperature distributions exhibited reasonable agreement with previous experimental data and therefore indicate the viability of the present method. Computations were carried out for supercritical carbon dioxide flowing through a circular duct in the reduced pressure range 1.0037 to 1.098. A consideration of the influence of buoyancy on the mean momentum balance permitted the calculation of unusual velocity profiles in this investigation. The existance of such velocity profiles had been accepted previously but the nature of their growth along a pipe has probably not been suggested previous to this work. No attempt was made to include buoyancy generated turbulence or additional fluctuating property correlations in this work, but suggestions are made regarding possible avenues of approach. Some of the incidental outcomes of this work were a new continuous universal velocity profile implicit in cross stream distance an a new mixing length distribution for turbulent pipe flows. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

The purpose of this investigation was the determination of the thermal conduction properties of a single crystal of bismuth at liquid-helium temperatures in magnetic fields up to eighteen kilogauss.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 1978 / Vita. / Includes bibliographical references. / by Salvatore C. Malguarnera. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Mechanical Engineering

With the advent of the new" high Tc superconductors as well as the increasing use of cryo-cooled conventional electronics, liquid nitrogen will be one of the preferred cryogens used to cool these materials. Consequently, a more thorough understanding of the heat transfer characteristics of liquid nitrogen is required. In these investigations the transient heating characteristics of liquid nitrogen to states of nucleate and film boiling under different liquid flow conditions are examined. Using a metal hot wire/plate technique, it is verified that there is a premature transition to film boiling in the transient case at power levels as much as 30 percent lower than under steady state nucleate boiling conditions. It is also shown that the premature transition can be reduced or eliminated depending on the flow velocity The second part of this research analyses the nucleation (boiling) process from a dynamical systems point of view. By observing how the boiling system variables evolve and fluctuate over time, it is hoped that physical insight and predictive information can be gained. One goal is to discover some indicator or signature in the data that anticipates the transition from nucleate boiling to film/boiling. Some of the important variables that make up the boiling system are the temperature of the heater and the heat flux through the heater surface into the liquid nitrogen. Results, gained by plotting the system’s trajectory in the heat flux-temperature plane, are that on average the system follows a counterclockwise trajectory. A physical model is constructed that explains this behavior. Also, as the applied heater power approaches levels at which the transition to film is known to occur, the area per unit time swept out in the heat flux-temperature plane is seen to reach a maximum. This could be of practical interest as the threshold to film boiling can be anticipated and possibly prevented.

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