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.
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