A Two‐Dimensional Model of Chemical Vapor Infiltration with Radio‐Frequency Heating: II. Strategies to Achieve Complete Densification

Abstract

A two-dimensional finite-element model is used to compare isothermal and radio-frequency-assisted chemical vapor infiltration of long cylindrical carbon preforms. Densification with radio-frequency heating at a constant input power initially occurs radially around the central zone and then axially toward the ends of the preform. This densification pattern results in significant entrapment of porosity at the center of the preform and requires a relatively long time for completion. A novel scheme for improved radio-frequency heating of long cylindrical preforms is proposed which entails insulating the axial ends of the preform, induction heating at a relatively high operating frequency, and linear ramping of input power with time. Simulations show that, under these conditions, radial "inside-out" densification can be achieved uniformly along the entire length of the preform. This scheme results in complete densification of the preform and reduces the overall processing time fivefold when compared to the conventional isothermal process. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 132.239.1.230 Downloaded on 2015-04-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 132.239.1.230 Downloaded on 2015-04-13 to IP

Spatio-temporal evolution of a pulsed chlorine discharge

“…Processing with ion-ion plasmas at the late afterglow offers unique possibilities. Preliminary calculations (which included the Poisson equation and resolved the sheaths) show that application of a radio-frequency bias results in bombardment of the walls by positive and negative ions alternately [20]. As demonstrated experimentally by Ahn et al [2], this has significant potential to reduce charging and notching effects associated with conventional CW electron-ion plasma processing.…”

“…Calculations were also performed by solving the energy and continuity equations (equations ( 1)-( 4)) along with Poisson equation ((equation ( 5)) for the electrostatic fields resolving the sharp gradients in the very thin sheath region [20]. In the afterglow, the electrostatic fields in the sheath region collapsed abruptly (several orders of magnitude within a few microseconds) after the bulk electrostatic fields collapsed due to the drop in electron temperature and the decay of electron density.…”

Bifurcation Analysis of Thermal Runaway in Microwave Heating of Ceramics

“…Consider solving N nonlinear equations in N state variables u, dependent on a set of m parameters p F(u, p) ϭ 0 u⑀R N , p⑀R m [5] Suppose that u ϭ u 0 is a solution for p ϭ p 0 . Let…”

“…Microwave heating has been applied in a variety of applications including sintering and joining of ceramics 1 and chemical vapor infiltration of fiber-reinforced ceramic composites. [2][3][4][5] One of the major hurdles currently faced in the application of microwave processing of ceramics is the occurrence of thermal runaway. 6 This phenomenon corresponds to a situation in which a small change in the design or operating variables (such as the thickness of the sample or microwave power) causes a rather large increase (of the order of several hundred degrees) in the temperature of the material being heated.…”

How is it possible to get optimal infiltration fronts during chemical vapor infiltration with thermal gradients ?

“…All of them are based on the solutions of balance equations for heat and species mass concentrations; some also incorporate electromagnetics in order to represent the heating system. These models span from simple 1D approximations [45,46,47,48,49,50,51,52,53,54,55,56,57] to complex, 2D or 3D Finite Element of Finite Volume analyzes [58,59,60,61,62,63]. Their raw results are temperature and mass deposition profiles or fields, for chosen processing conditions (reactor geometry, temperature, pressure, gas composition, porous preform characteristics).…”