A planar Bi-Sb multijunction thermal converter with small ac-dc transfer differences

A planar Bi–Sb multijunction thermal converter was fabricated on the LPCVD Si3N4/SiO2/Si3N4-diaphragm, prepared by silicon bulk micromachining, which thermally isolated a bifilar Ptor NiCr-heater and the hot junctions of a Bi–Sb thermopile from the silicon substrate. The voltage responsivity, the ac–dc transfer difference, and the fluctuations of the output thermoelectric voltage and heater resistance were discussed to investigate the design factors of a thermal converter. The respective voltage responsivities in air and in a vacuum of the thermal converter with a built-in NiCr-heater were about 14.0 mV/mW and 54.0 mV/mW. The ac–dc voltage and the current transfer differences in air were about 0.60 ppm and 0.11 ppm in the dc reversing frequency range from 10 Hz to 10 kHz. They are sufficiently small to be used as practical ac standards. Compared to the thermal converter with a built-in Pt-heater, the thermal converter with a built-in NiCr-heater demonstrated a higher voltage responsivity and smaller ac–dc transfer differences, while exhibiting slightly larger fluctuations in output thermoelectric voltage and in heater resistance.