Electromagnetic processing: Application in ceramic composites and metal casting.

摘要

This two part dissertation is an investigation of electromagnetic processing of ceramic composites and metal casting. The first part introduces a novel idea of employing microwave heating and computationally demonstrates the applicability of the technology to produce better ceramic fiber-matrix composites. The second part is a theoretical examination of a magnetic field calculation and instability at the liquid surface in electromagnetic casting of aluminum.; A ceramic body exposed to microwaves exhibits unusual temperature gradients in that the interior of the material is hotter than the surface. The calculations entail determination of fields and subsequent solution of the heat conduction equation for temperatures in the ceramic piece. In a one-dimensional model, these calculations are analytical while in the two-dimensional model, an integral formulation is used to determine the electromagnetic fields and the calculations are numerical. The results are computed with estimates for properties of SiC, including a case where the complex dielectric constant is varied with temperature.; The computed results demonstrate that SiC can be heated to high temperatures (1000-1500K) and that both the temperature and the temperature gradient can be controlled by varying the power density of the microwaves and external cooling. The results also exhibit a high sensitivity of temperatures to the dimensions of the material, its loss factor and the orientation in which the microwaves impinge on the ceramic body.; The temperature profiles computed for preforms subjected to microwaves are input to mass transport models describing chemical vapor infiltration (CVI) of the preform. A one-dimensional model is based on Knudsen flow, while the mass flux in a two-dimensional model is expressed according to the "dusty gas" model. The models are solved numerically for parameters corresponding to the infiltration of SiC preform by pyrolysis of trichloromethylsilane. The computed results clearly demonstrate an advantage in employing microwave heating in CVI. Also, greater infiltration is achieved in a preform of greater pore size and at lower temperature. However, for successful infiltration, diffusion/chemical-reaction-controlled CVI is shown to impose limits on the preform size.; A previously existing model for describing electromagnetic casting of aluminum used differentiation of known current densities to estimate the magnetic field. In this dissertation, as an improvement, an alternative approach based on integration of the current densities is suggested. The integral method is expected to yield more accurate results than the differential method.; Also, the stability of liquid aluminum/air interface in electromagnetic casting of aluminum is examined as an extension of Garnier and Moreau{dollar}sp1{dollar} work on stability of an interface between inviscid fluids subjected to a high frequency magnetic field. This dissertation extends that study to include the viscosity of both fluids in order to estimate the damping rates of the perturbations. Furthermore, Garnier and Moreau made an assumption that the frequency of oscillation of the interface could be neglected compared to the frequency of the applied field; they concluded that the applied field was not destabilizing. That (apparently reasonable) assumption has been lifted in the present work and the consequence is that the magnetic field is seen to lead to instability over a significant range of wavelengths. Application of this analysis to the electromagnetic casting of aluminum is discussed. ftn{dollar}sp1{dollar}Garnier, M. and Moreau, R., "Effect of finite conductivity on the inviscid stability of an interface submitted to a high frequency magnetic field," J. Fluid Mech., Vol. 127, 1983, pp. 365-377.