Pulsed-power volume-heating chemical vapor infiltration

Abstract

The dynamic behavior of a novel chemical vapor infiltration (CVI) technique called pulsed-power volume-heating CVI is investigated using a diffusion-reaction model. In this technique, a volume-heating source (e.g., RF or microwave) is used to heat the preform. The source power is modulated in time (e.g., square-wave modulation) with a specific period and duty cycle. During the low-power part of the cycle, the temperature of the composite drops, reducing the reaction rate and thus allowing the precursor gas to diffuse into the composite, essentially "refilling" the composite. This alleviates reactant concentration gradients within the composite minimizing density nonuniformities. The high-power part of the cycle is used to achieve rapid reaction rates, thereby minimizing processing time. CVI of a carbon fiber preform with carbon resulting from methane decomposition is taken as an example to illustrate the technique. The results reveal the dependence of density uniformity and processing time on relevant variables such as pulse period, duty cycle, power density level, and methane mole fraction. It is shown that pulsed-power volume-heating CVI can provide a window of operating conditions leading to rapid and complete densiflcation.

Microwave Heated Chemical Vapor Infiltration: Densification Mechanism of SiCf/SiC Composites

“…4,15,20 In an attempt to overcome this problem, an approach based on the forced flow of gases, combined with the deliberate creation of a temperature gradient across the preform (FCVI), has been developed. 30,32 Experimental work has yielded this inside-out densification pattern in a number of different ceramic systems, including SiC, Si 3 N 4 , Al 2 O 3 , and ZrO 2 . The use of a pulsed-pressure CVI process (PCVI) in which the pressure is modulated 24,25 has also yielded faster infiltration rates, improved levels of densification, and greater infiltration uniformity as a result of increased mass transport compared with ICVI.…”

“…27,30,31 Premature pore closure due to crusting is completely avoided, potentially allowing densification to occur an order of magnitude faster than with ICVI. 30,32 Experimental work has yielded this inside-out densification pattern in a number of different ceramic systems, including SiC, Si 3 N 4 , Al 2 O 3 , and ZrO 2 . 28,31,33 The work presented here investigates the densification mechanisms for SiC fiber-reinforced SiC matrix composites (SiC f /SiC) processed by MCVI using principally scanning electron microscopy (SEM) and X-ray absorptiometry (XRA).…”

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

“…In this paper we use a randomly overlapping uniform radius fiber model2' (as opposed to a pore model used in ..E. [7] B=[) 8 where r, is the fiber radius and is the pore radius defined than 0.5. The GEMT model was used in the present simula- Figure 3 compares the effective binary and Knudsen diffutions to calculate the effective electrical and thermal consivities for the fiber and capillary models.…”

Chemical Vapor Infiltration of Non-Oxide Ceramic Matrix Composites

“…This is still a slow process due to the dependency on diffusion. In a new development for this technique, there have been some exploratory efforts in using microwave heating to produce the thermal gradient [6][7][8][9][10]. The outer regions of a relatively low thermal conductivity preformwill be radiatively cooled, while the central volume rises to temperatures up to several hundred degrees centigrade higher.…”