Improved electrical connections have been developed for thin-film thermocouples used to measure temperatures on the surfaces of ceramic-matrix composite-material specimens during high-temperature tests. In a test, specimens are arranged in a four-sided configuration in a test rig, where they are heated from inside the rig by a flame. Composite specimens have endured up to 3,200 °F (1,760 °C) on the hot sides, while temperatures at electrical connections for thermocouples have reached 1,200 °F (650 °C). Previously, the electrical connections on the thermocouples could not withstand the test temperatures; they failed so frequently that no useful data could be taken. In addition, the design of the electrical connections made it difficult to change specimens in the test rig. The improved electrical connections withstand the test temperatures and make it possible to change specimens easily.

Thin-film thermocouples were installed on specimens by sputtering. Wires with a diameter of 5 mils (0.13 mm) were attached to the thermocouples by parallel-gap welding. Each specimen was then mounted on a spring plate by use of straps made of a nickel alloy.

Electrical Connections for Thermocouples on a test rig withstand high temperatures. Thermocouples can readily be connected or disconnected at the transition fixture.

The figure illustrates key features of the improved electrical connections. Small ceramic tubes were strapped to the inside perimeter of the string plate. Wires with a diameter of 10 mils (0.25 mm) were run through the ceramic tubes. Starting from the outer ends of the ceramic tubes, the 10-mil wires were covered with fiberglass sheaths and run from the hot interior of the test chamber to a transition fixture (described in the next paragraph) located near the specimen, but not so close as to be subjected to the maximum test temperature. At the inner ends of the ceramic tubes, the 10-mil wires were threaded through and bent around ceramic beads and attached to the 5-mil wires. The specimen as thus prepared was ready for installation in the test chamber.

The key to interchangeability of specimens was the transition fixture, which was made from a block of machinable ceramic. Small wishbone-shaped slots were milled into the block. A metal-sheathed thermocouple extension wire was stripped back and laid into the single-slot end of each wishbone, with its leads in the branch slots at the other end of the wishbone. In each branch slot, the thermocouple extension lead was placed in contact with the outer end of the corresponding 10-mil lead from the specimen. The metal-sheathed extension wires were held in place by washers and stainless-steel screws that engaged threads tapped into the block. Electrical contact between the thermocouple leads and the 10-mil specimen leads could easily be made and broken at the transition fixture.

This work was done by Chip Redding of Lewis Research Center. No further documentation is available. 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, Cleveland, Ohio 44135.

Refer to LEW-16688.


Electronics Tech Briefs Magazine

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

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