Thin Film Of Liquid Metal Research Articles

Electrohydrodynamical study of the instability of a thin liquid metal film: Application to planar liquid metal ion sources

Liquid metal ion sources are of interest in diverse areas of technology since they provide a high brightness, quasipoint source of ions and droplets for microfabrication, surface analysis, ultrathin film deposition, and other potential applications. N. M. Miskovsky, M. Chung, P. H. Cutler, T. E. Feuchtwang, and E. Kazes [J. Vac. Sci. Technol. A 6, 2992 (1988)] have developed an electrohydrodynamic capillary wave theory for ion and droplet emission in electrically stressed conducting viscous fluids based on a mathematical formalism introduced by J. R. Melcher and C. V. Smith [Phys. Fluids 12, 778 (1969)] and S. Grossmann and A. Muller [Z. Phys. B 57, 161 (1984)]. As the simplest analytical application of this theory they chose a model consisting of a planar fluid of thickness ‘‘a’’ supported on a rigid electrode. A parallel planar counter electrode is at a distance ‘‘b’’ from the unperturbed surface. The Navier–Stokes equation was solved subject to a dynamical Laplace–Young stress boundary condition (which includes the frictional tensor) to obtain the dispersion relation used to investigate the stability of the system. In earlier work the effect of viscosity and gravity on the spectrum of unstable wavelengths for the case of a thick film (i.e., λ≫a) was investigated within the planar surface capillary wave model. In this limit, it was found that viscosity plays a significant role in determining the growth rate and the dominant mode, suggesting that it should be included in studies of the instabilities in liquid metal ion sources. It was also found that the local electric field is much larger than the average field of the perturbed fluid surface, implying that nonlinear effects can be important. In the present study the same formalism is used to investigate the case of a thin (i.e., λ<a) liquid metal film. For thin films, that is, <1 μm, the effect of the boundary conditions at the supporting electrode can produce significant changes in the maximum growth rate (∼20%), the dominant mode (∼10%), and the maximum value of Re(ω2) (∼40%). For both the thick- and thin-film regimes viscous damping is found to have an important role increasing the stability of the fluid film. In addition, an increase in viscosity shifts the most unstable mode to longer wavelengths. This shift competes with the effect of decreasing the film thickness.

Light Induced Etching Of Gaas In A Zinc Atmosphere

ABSTRACTN-type GaAs substrates have been exposed to zinc atmospheres at 600 – 800 deg.C and focused light from a Krion laser at 647.1 nm. Etch pits are generated at optical densities of 0.2 – 10 KW/cm2 and exposure times of 5 – 60 minutes. They are characterized by a smooth profile with typical diameters of 50 – 80μm and depths of 0.1 – 50 μ The etch depth depends strongly on the laser power and on the zinc and arsenic vapor concentrations. The onset of etching is characterized by an increase in surface reflectivity of up to 10 per cent which is probably due to the formation of a thin liquid metal film. The growth of this film is initiated by optically generated holes at the semiconductor surface. The film is not saturated and dissolves GaAs from the substrate. In addition to etching, thermal diffusion of zinc can be observed in samples subjected to long exposure times or high temperatures.

A mathematical model to predict the growth and elimination of inclusions in liquid steel stirred by natural convection

A model is developed to predict the rate of removal and the change in the size distribution of inclusions in a melt stirred by natural convection. Difficulties in obtaining an exact solution to the problem due to lack of adequate knowledge for the velocity fields in the melts are discussed. The model is based on Smoluchowski’s Theory of Gradient Collision to obtain the probability of collision between two inclusions under an arbitrarily chosen velocity gradient. Initial size distributions obtained in experimental heats are used as the input to the model. Various conditions are proposed by which inclusions are removed from the melt. The rates of removal are compared with the experimentally obtained rate of removal of oxides. It is observed that a boundary layer effect and the presence of a thin liquid metal film prevent rapid removal of inclusions from the stirred melts. Inclusion size distribution predicted by the model agrees qualitatively with the experimentally observed size distribution. It is postulated that the surface forces play a significant role in coalescence and assimilation of inclusions. Finally, the application of similar models to understand the removal of inclusions in such processes as argon sparging, solidification, degassing and electroslag remelting are advocated.