Transparent furnaces are being developed for use in research on properties and processing of materials at high temperatures. Full optical access to the interiors of furnaces is intended to provide the capability for nonintrusive diagnosis and feedback control of the subtleties of high-temperature processes. Small furnace windows are now included in some otherwise opaque furnaces to provide visibility for assessing crystal quality, but only small portions of the objects of interest can be viewed, and these windows cause thermal disturbances that affect crystal-growth processes.
A standard transparent furnace of the type developed thus far includes a quartz tube that is coated with a thin layer of gold and that surrounds the hot zone. The gold layer is about 80 percent transparent to visible light and about 95 percent reflective to infrared radiation; thus, it enables visual observation of the interior of the furnace while acting as a radiant heat insulator to impede leakage of heat from the furnace.
The furnace is heated by one or more helically wound resistance heating coils. The pitch of the coils is made large enough to make it possible to look between the turns of the coil(s) and see the interior region of interest. A quartz shield tube is located between the heater and the gold-coated mirror surface to prevent the outgassing heater material from coating the gold and thereby reducing its infrared reflectivity. A quartz "muffle" tube is mounted as an impurity barrier between the heater and the material sample to be heated and observed. Typically, the hot zone is 6.5 cm in diameter and 13 cm long, and a controlled atmosphere is maintained in the hot zone.
Standard transparent furnaces developed thus far have been limited to operating temperatures below 1,000 °C. Transparent furnaces at the leading edge of development are being modified for operation at higher temperatures; the modifications include improvements in containment of thermal radiation, reduction in convective transfer of heat, the use of materials that are transparent at high temperatures, and improved design of transparent-heater components. In a theoretical analysis that coupled energy-balance analysis and heat-shield design with modifications of a commercial transparent furnace, it was shown to be feasible to raise the maximum operating temperature to 1,200 °C.
This work was done by David W. Yoel of Centorr/Vacuum Industries, Inc., for Lewis Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7-3
21000 Brookpark Road
Refer to LEW-16064.