An automated heating/cooling laboratory apparatus enables the accelerated thermal-cycle life testing of thin-film samples and other small material samples. In the original application for which the apparatus was constructed, the thin-film samples are candidates for use as components of thermal-insulation blankets, and there is a requirement to subject the samples to at least 20,000 thermal cycles between the temperatures of +50 and - 100 °C. Without the pres-ent apparatus, it would be necessary to test the samples in an environmental chamber that takes about 30 minutes to complete a thermal cycle and would therefore take more than a year to complete 20,000 cycles. In contrast, the present apparatus operates with a thermal-cycle time of 15 to 20 seconds, making it possible to complete the 20,000 cycles in less than five days.

Samples Are Alternately Cooled and Heated by liquid nitrogen and electrically heated nitrogen, respectively, under automatic timing control.

The apparatus includes a hot-air gun and a liquid-nitrogen inlet mounted above and aimed at the samples. All of the aforementioned components are located in a chamber purged with nitrogen (see figure). A solid-state relay is used to control the opening and closing of an electrically actuated valve that controls the flow of liquid nitrogen through the inlet. Another solid-state relay is used to turn the hot-air gun on and off. (Because the gun is located in the nitrogen-purged chamber, it actually blows hot nitrogen, not hot air, when it is turned on.) The samples are taped and clamped to a fixture. Thermocouples are attached to the fixture and to one of the samples for monitoring test temperatures.

The solid-state relays are toggled by an amplified 1-volt rectangular-wave signal that establishes the timing of the cooling and heating periods of the thermal cycle. When the signal is 1 V, the liquid-nitrogen valve is opened and the hot-air gun is turned off. When the signal is 0 V, the liquid-nitrogen valve is closed and the hot-air gun is turned on. The duty cycle of the rectangular wave is adjusted to obtain the required thermal cycle; for example, in initial experiments, the liquid-nitrogen valve was opened during about 38 percent of the thermal cycle and the hot-air gun was on during the remaining 62 percent of the cycle.

This work was done by Charles Powers and Bruno Munoz of Goddard Space Flight Center. GSC-13974

NASA Tech Briefs Magazine

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

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