An Improved Thin-film Multijunction Thermocouple for the Application in a Calorimetric Thermal Voltage Converter

Quantum AC voltage standards, although significantly advanced, remain constrained by frequency range, amplitude, and high costs. Traditional thermal voltage converters (TVCs), particularly calorimetric thermal voltage converters (CTVCs), offer superior accuracy above 100 kHz. CTVCs utilize a coaxial waveguide terminated with a microwave rod resistor and a multijunction thermocouple to measure temperature rise. However, the original CTVC design was limited by a handmade thermocouple with restricted sensitivity and reproducibility. This research proposes a novel multijunction thermal sensor design based on a prefabricated polyimide foil, with a copper-clad covering of bismuth and antimony, which demonstrates improved sensitivity and reduced internal resistance compared to other designs. To optimize the sensor's performance, a comprehensive multiphysics simulation was conducted using finite element analysis. The simulation meticulously considered various factors, including geometry, material properties, and thermal effects.