Development of Thin-Film Multijunction Thermal Current Converters With Increased Rated Current

Thin-film multijunction thermal converters (MJTCs) with a heater fabricated on an AlN chip have been developed for ac-dc current transfer standards. The rated current of the MJTC can be increased due to the good thermal conductivity of the AlN substrate, which acted as a heat sink drawing away heat from the heater. The AlN chip was mounted on a polyimide foil with thermocouples of the MJTC produced separately. In the case of an MJTC with a heater resistance of 5 Ω, which has a bifilar heater pattern to reduce the thermoelectric effects, the nominal current is 200 mA. The observed current level dependence of the ac-dc current transfer differences from 50 to 200 mA for the 5-Ω MJTC was within 1 μV/V at frequency from 1 to 100 kHz. Compared with a conventional thermal converter, the wide range of available input current and small current level dependence of the new MJTCs may reduce uncertainties of ac-dc current transfer standards.