Microwave assisted chemical vapor infiltration Page: 3 of 7
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density composites should be attainable. Third, relatively short processing times should be
possible. Fourth, machining operations to reopen closed pores should not be necessary since
densification would occur from the inside-out. Success will of course require proper management
of heating and cooling rates. The electric field within the preform governs the local heating rate.
Interaction of an electromagnetic field with a porous preform is complicated. In general, the local
heating rate is proportional to the square of the electric field strength and to the effective dielectric
constant and loss factors for the growing composite. On the other hand, substrate heat losses occur
primarily by way of radiation and convection. One expects these losses to depend on geometry,
temperature, flow rates, and to a lesser extent, on weave architecture and lay-up pattern. The
present work is an experimental investigation into the feasibility of microwave assisted CVI.
Emphasis is on quantifying induced thermal gradients, combining CVI and microwave heating to
demonstrate inside-out densification, and examination of materials and processing issues.
In order to quantify the microwave induced thermal gradients, optical thermometry
experiments were conducted. Measurements were made in cylindrical SiC cloth lay-ups subject to
7(W W of 2.45 GHz microwave energy. Measurements were conducted in a commercial General
Electric multimode oven. Temperatures were measured along both the cylinder axis and mid-plane
radius using quartz fiber optic cables woven directly into the substrate. The optical cables were
connected via photodiodes to amplifier circuits from which voltages were read. Each optical
thermometer was first calibrated using stacks of SiC cloth placed in a conventional high
temperature furnace. The thermal profiling data shown in Figure 1 indicates that steep inverted
thermal gradients can be established in SiC cloth lay-ups.
0 MASUREENTS 00 ALONG
65 ALONG MID-PLANE CYLINDER AXIS
' I ' , I * I I ' I ' I '
0.6 0.8 1.0 1.2 0,10 0.20 0.30
DISTANCE FROM OUTER EDGE (cm) DISTANCE FROM TOP SURFACE (cm)
IGURE 1. Inverted thermal gradients in SiC cloth lay-ups.
Internal temperatures in excess of l(XX)C were observed in stagnant air along with gradients on
the order of several hundred "C/cm. Similar gradients are seen in atmospheres other than air, e.g.
I12, (' l, and C 1 It. The error bars shown in the figures include contributions from positioning
cables within the preform and variations in the voltage readings, as deduced from the calibration,
Figure 2 is a schematic of the gas delivery system and microwave CVI cavity. The latter
consisted of a modified 2.45 GI Ix, 7(X) W General Electric commercial multimode oven. A circular
hole was laser cut through the oven floor through which a Pyrex hell jar reactor was inserted. The
interio r !1K)r of the reaction vessel was a metallic plate with a circular hole in the center used for
reactant feed and product gas removal. A metallic sleeve was utili/rd to reseal the cavity.
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Devlin, D. J.; Currier, R. P.; Barbero, R. S.; Espinoza, B. F. & Elliott, N. Microwave assisted chemical vapor infiltration, article, December 31, 1991; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1273922/m1/3/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.