Showing papers on "Critical radius published in 1985"

Evidence for a mechanism of swelling variation with composition in irradiated Fe-Cr-Ni alloys

Abstract: Irradiations with 4 MeV Ni ions and 200–400 keV He ions were carried out on two alloys, Fe-15Cr-15Ni and Fe-15Cr-35Ni, at 675°C and doses up to 84dpa. Both dual-ion irradiation experiments and sequenced He injection-anneal-Ni irradiations were used. The dual-ion experiment showed that the two alloys exhibited large differences in microstructural development, with the low nickel alloy having significantly greater swelling. The injection-anneal-irradiation experiment was designed to test the hypothesis, suggested by our earlier work, that the lower swelling of the high nickel alloy may result from a larger critical radius/critical number of gas atoms required to achieve bias driven swelling. This experiment provided a direct measurement of these critical quantities by the induction of a bimodal cavity size distribution. The measurement gave minimum critical radii of about 5nm for the high nickel alloy and < 0·5nm for the low nickel alloy, values consistent with the hypothesized mechanism. The basis...

The stability of the oscillation motion of charged grains in the Saturnian ring system

Abstract: A perturbation approach for the gravitoelectrodynamic forces encountered in the corotating plasma environment of Saturn is used to determine the stability of charged grains, given a random initial velocity. Attention is given to the implications of the Northrop and Hill (1982) and Mendis et al. (1982) results for the formation of the Saturnian ring system, and it is suggested that the marginal z stability radius at 1.5245 Saturn radii for Kepler-launched particles is due to an erosion process with ejecta of the order 0.05-0.5 microns, rather than that of the previously suggested plasma. The diffuseness of the Saturnian rings beyond the F ring is also explained in terms of instability, while a new critical radius for r instability is suggestd for the optical depth feature at 1.72 Saturn radii. The F ring is analyzed in detail.

On the possibility of critical radius measurements in the homogeneous crystal nucleation of glass

Abstract: The potential of determining the liquid-crystal surface tension during crystal nucleation in inorganic glasses without measuring the nucleation rate is examined analytically. Attention is focused on the feasibility of measuring the critical radius of the crystal nuclei as a function of temperature in order to obtain the surface tension values. Two approximate expressions are defined for the upper and lower bounds of the bulk free energy of crystallization per unit volume. The expressions, when used for a validated approximation for the critical radius, show that the critical radius is a function of the undercooling and the molar volume. The minimum undercooling necessary for detecting homogeneous crystal nucleation is then calculated, which yields an upper limit to the critical radius in the range 10-15 A. Since the size range is too small for detection, the critical radius is not a useful measure for crystal nucleation studies.

The Nucleation of Cavitation in Aqueous Media

Abstract: : This dissertation on cavitation nucleation is concerned with both the cavitation nucleus and the mechanisms by which the nucleus is stabilized. The three most plausible theories of the cavitation nucleus are: the crevice model, the surfactant skin model, and the ionic skin model. The previous crevice model required that, in order for a vaporous cavity to grow from a crevice nucleus, the liquid gas interface must reach the receding contact angle. This condition is necessary but not sufficient. An additional criterion is that the radius of curvature of the interface must be greater than a critical radius, beyond which the cavity is mechanically unstable. Using these nucleation criteria, the model is rederived. Both old and new crevice models predict the cavitation threshold as a function of surface tension and temperature well. However, they diverge when predictions of the threshold as a function of dissolved gas content are considered. The old crevice model predicts a linear dependence but experiment shows the threshold increases rapidly with decreasing gas content. This behavior is predicted by the revised model. In addition to vaporous cavitation, the revised model is applied to diffusion cavitation and is shown to predict features present in the data previously explained only in terms of the varying- permeability model.

Neutron-Induced Vaporization of Superheated Liquids: Theory and Experiment

Abstract: My interest in neutron-induced nucleation began with a simple and elegant demonstration in one of David Turnbull’s classes in which a drop of water was superheated to about 250°C as it rose in a column of heated oil. As David Glaser, the inventor of the bubble chamber, so ably demonstrated, such superheated liquids are radiation sensitive. Our test system is a simple one. Halocarbon and hydrQcarbon drops are introduced into an aqueous holding gel under pressure at room temperature. As the pressure is released, the drops become superheated. Neutrons of sufficient energy will trigger vaporization of these moderately superheated drop detectors (SSDs), but gammas and x-rays will not unless the homogeneous nucleation limit is approached. We have performed measurements on the neutron energy threshold to produce nucleation in a number of different superheated materials at different temperatures. We have also developed a theory which indicates that of the energy deposited in a critical radius, only about 5% is effective in producing bubble formation. Both theory and experiment are discussed.

Effect of Microstructure on the Minimum Critical Radius and Critical Number of Gas Atoms for Swelling

Abstract: It has been shown theoretically that bias-driven cavity swelling can only occur after either a critical cavity radius has been achieved or a critical number of gas atoms has been accumulated in a cavity. These possibilities merge into each other, as increasing the contained gas lowers the critical radius until at the critical number of gas atoms the minimum critical radius is achieved. With the addition of any more gas, the critical radius disappears and cavity swelling is insured. It is found that these critical quantities are highly sensitive to irradiation conditions and material parameters. Under fixed irradiation conditions, the critical quantities are remarkably strong functions of dislocation density and bias. These results are described and their implications for the design of swelling resistant materials are discussed.