Programmable multizone furnaces with enhanced designs have been proposed for use in processing materials by directional solidification or, optionally, by related techniques of float-zone or isothermal solidification. Like some other directional-solidification furnaces reported previously in NASA Tech Briefs and elsewhere, these furnaces would incorporate multiple, individually controllable electric heaters stacked at a succession of axial locations in a stationary cylindrical main canister for generating, shaping, and translating specified axial temperature profiles (specified temperatures and/or temperature gradients) under electrical control. These furnaces would also incorporate cold-end modules that could be translated axially and telescoped into the main canisters to provide additional versatility for control of cold-end temperature profiles.

Two BOSS Furnace Assemblies are shown in simplified form to illustrate the arrangement of major components. For the sake of clarity, outer containers and drive mechanisms for translating the cold-end modules are omitted from these drawings.

Because it would afford options for a combination of mechanical (translational) and electrical control of temperature profiles, a furnace of this type would be called a "bi-operational solidification system" (BOSS) to distinguish it from older multizone furnaces that feature electrical or translational (but not both) modes of temperature-profile control. The BOSS concept offers opportunities for achieving (1) compact furnace design, (2) increased versatility for programming a variety of stationary and moving temperature profiles, and (3) high thermal efficiencies through reduced (relative to other directional-solidification furnaces) heat losses, with total power-consumption levels comparable to those of translation (only)-type directional-solidification furnaces.

A typical BOSS (see figure) would include 12 zone heaters separated by spacers that would reduce heat conduction between zones and thereby also enhance thermal control of each zone. The axial lengths of the end heaters would be made greater than those of the other heaters to provide additional heat capacity to overcome heat losses from the ends. Each heater would comprise an electrically resistive wire wound in a groove on the radially outermost edge of a washerlike disk of thermally insulating material. (Consequently, each heater would act as an insulator when not powered.) Thermocouples for measuring temperatures in the heater zones would be included. Power and thermocouple leads could be routed axially or radially through a layer of insulating material that would surround the heaters.

The cold-end module could be of either of two types: (1) a quench module (QM) or (2) a crystal-growth module (CGM). A QM would include a water-cooled cold block (or quench block), a quench-spray port or nozzle at the entrance to the cold block, and a gradient-control heater separated from the cold block by an adiabatic barrier. In conjunction with the zone heaters in the stationary main canister, a QM would provide a controllable, translatable temperature gradient.

A CGM would also include a gradient-control heater and an adiabatic barrier, but would differ from a QM in that the quench capability would be replaced by a cold-end temperature-control capability. Instead of a single cold block, there would be two cold blocks, which could be actively cooled by water or operate in a passive (uncooled) mode. Two cold-end heaters mounted on a common mandrel would provide a capability for finer control of the cold-end temperature profile.

This work was done by Jack E. Robertson of AI Signal Research, Inc., for Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category. MFS-26524


NASA Tech Briefs Magazine

This article first appeared in the April, 1999 issue of NASA Tech Briefs Magazine.

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