Characterization and control of the fiber-matrix interface in ceramic ...

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was not deoxygenated. Other gases such as silane, ammonia, and diborane
were not treated but were used as received (at least 99.99% purity).
Methyltrichlorosilane (MTS), which is a liquid at room temperature,
was carried to the reactor by a flow of hydrogen through an evaporator,
and flow was metered using a vapor source controller.* On bubbling the
hydrogen carrier gas through the reservoir containing the liquid
reactant, the gas stream becomes saturated with vapor. The vapor-source
controller monitors the heat capacity of the incoming gas and the exit
gas mixture and, by subtracting the incoming carrier flow signal, can
measure the vapor concentration in the outgoing mixture. The controller
varies the flow of hydrogen to the reservoir to maintain the given vapor
concentration. The gas-vapor mixture is then diluted to the proper ratio
using a mass flow controller that is placed in series with the vapor-
source controller.
The system is designed to allow for complete control
of not only vapor flow but gas-to-vapor ratios. Although the source
controller maintains the flow of reactant independent of pressure or
temperature, the reservoir temperature was held at 297 K using a constant
temperature bath to minimize cooling effects caused by the evaporation of
the MTS.
On completion of densification, a programmed cool-down sequence was
used to minimize thermal shock effects. Temperature control was switched
from the optical pyrometer to a thermocouple controlled programmer. **
*Research, Inc . , Minneapolis, Minnesota.
""Tylan, Inc., Torrance, California.