Thesis (M.S.)--University of Montana, 2007. / Title from title screen. Description based on contents viewed Aug. 8, 2007. Includes bibliographical references.

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The reactions of lithium disks with dry and moist nitrogen and with water-vapour were investigated at temperatures from 22 to 70 degrees Centigrade with the use of a thermal balance. The reaction in nitrogen commenced with nucleation of lithium nitride at corners and edges of the sample and the reaction proceeded by lateral growth of these nuclei through the specimen. In moist gas, this reaction was accompanied by the simultaneous formation of lithium hydroxide at the plane surface of the specimen. Based on visual observations of the samples during the reaction, a model describing the geometry of nucleus formation was constructed and was used to calculate growth velocities from the reaction curves obtained with the thermal balance. The dependence of growth velocity on temperature, nitrogen partial pressure, and the moisture content of the reaction gas was investigated. The reaction with water-vapour was observed to proceed in three distinct stages. The results have been explained in terms of a model involving recrystallization and hydration of an initially coherent lithium hydroxide film. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The tensile strength or the negative pressure required to induce cavitation in a pure liquid has been a puzzling subject. On one hand, the classical nucleation theory has met great success in predicting the nucleation rates of superheated liquids. On the other hand, most of reported experimental values of the tensile strength for different liquids are far below the prediction from the classical nucleation theory. In this study, homogeneous nucleation in liquid nitrogen and its tensile strength have been investigated. In order to carry out the measurement of the tensile strength of liquid nitrogen, different approaches for determining the pressure amplitude were studied carefully. It is shown that Raman-Nath theory, as modified by the introduction of an effective interaction length, can be used to determine the pressure amplitude in the focal plane of a focusing ultrasonic transducer. The results obtained from different diffraction orders are consistent and in good agreement with other approaches including Debye's theory and solving the KZK (Khokhlov-Zabolotskaya-Kuznetsov) equation. The results from experiments in water demonstrated that as long as the nonlinearity is not too large, the experimentally determined pressure follows closely the calculated results using either Debye's theory or the KZK equation. In addition, the light diffraction contains enough information to calculate the second-order harmonic in the sound wave. In principle, it is possible that the contribution to the acoustic wave of the higher than the second-order harmonic can be obtained. The measurement of the tensile strength was carried out in a high pressure stainless steel dewar. A High intensity ultrasonic wave was focused into a small volume of liquid nitrogen in a short time period. A probe laser beam passes through the focal region of a concave spherical transducer with small aperture angle and the transmitted light is detected with a photodiode. When the voltage on the transducer reaches a critical point, nucleation in the focal region occurs and a characteristic signal associated with the nucleation was obtained. At this moment, the pressure amplitude at the focus is calculated based on the acoustic power radiated into the liquid. In the experiment, the electrical signal on the transducer is gated at its resonance frequency with gate widths of 20 ~s to 0.2 ms and temperature range from 77 K to near 100 K. The calculated pressure amplitude is in agreement with the prediction of classical nucleation theory for the nucleation rates from 106 to lOll (bubbles/cm3 sec). This work enhances our understanding of the nucleation process in liquids. It provides the direct experimental support that the validity of the classical nucleation theory can be extended to the region of the negative pressure up to 90 atm. This is only the second cryogenic liquid to reach the tensile strength predicted from the classical nucleation theory.

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