Maximum Voltage and Possible Over Voltage Failure Mechanism of Multijunction Thermal Converters

Abstract

Multijunction Thermal Converters (MJTCs) with heater resistances between 200 Ω and 250 Ωwere tested to determine the maximum voltage prior to failure. The MJTC chips were mounted on alumina substrates and their temperature monitored with 100 Ω (resistance temperature detectors) RTDs. Thermal losses were considered to be minimal over the few millimeters from the MJTC chip to the RTD on the substrate. Thermal imaging was used to map and validate the temperature distribution across the MJTC chip. Voltage was applied to the MJTC in steps taking several minutes each, allowing the MJTC output voltage and substrate temperature to equilibrate. With 20 V applied to an MJTC for over 20 minutes the MJTC output was over 2.1 V, and the substrate temperature increased to 341 K prior to device failure. Based on these measured quantities, the temperature of the resistive element was estimated to have reached approximately 640 K. A Multiphysics model was developed to explore the experiment and confirmed the resistive element of the MJTC design could reach a temperature of approximately 700 K with 20 V applied. Further analysis of the heating of the resistive element, a 70 nm thick, Ni75Cr20Al2.5Cu2.5 film, revealed that at these high temperatures, the major constituents of the alloy could evaporate at a significant enough rate to remove the film within several minutes. Postmortem examination of the MJTCs revealed a pattern indicative of evaporation occurring with a hot spot in the center of the resistive element. With a better understanding of the MJTC failure mechanisms and operating parameter space it is possible to explore new design techniques to further expand the usable voltage range for multijunction thermal converters.