The preliminary exploration on change mechanism of Seebeck coefficient for NiCr/NiSi thin film thermocouple with different thickness

Abstract

By controlling the thickness of NiCr and NiSi films, a series of NiCr/NiSi thin film thermocouples (TFTCs) were prepared by the sputtering technique. With the standard static calibration procedure for wire thermocouple, the temperature difference mainly happened between NiCr/NiSi compensation wires, but not between pad and hot junction of NiCr/NiSi TFTCs. The incorrect temperature difference resulted in the distorted Seebeck coefficient of NiCr/NiSi TFTCs, which was close to the standard NiCr/NiSi wire thermocouple. Only considering the temperature difference between pad and hot junction of NiCr/NiSi TFTCs, Seebeck coefficient of NiCr/NiSi TFTCs were accurately obtained. Seebeck coefficient of NiCr/NiSi TFTCs was about ∼10 μV/°C, which was obviously lower than that of standard K type NiCr/NiSi wire thermocouple (∼40μV/°C). Besides, Seebeck coefficient of NiCr/NiSi TFTCs gradually increased from 7.32 to 13.36 μV/°C with the increase of NiCr/NiSi film thickness from 400 to 2000 nm. NiCr and NiSi films with different thickness displayed the similar crystal structure, grain size, metal bonds structure, Ni/Cr and Ni/Si atom ratio, as well as similar surface morphology and roughness, which have little influence on Seebeck coefficient of NiCr/NiSi TFTCs. With the increase of NiCr and NiSi film thickness, the sheet concentration of free electron increased gradually, which resulted in the increase of Seebeck coefficient of NiCr/NiSi TFTCs. Because of the limited thickness of NiCr and NiSi film, all Seebeck coefficient values of NiCr/NiSi TFTCs were lower than that of standard K type thermocouple. NiCr/NiSi TFTCs with small thickness NiSi film and large thickness NiCr film could display the relative higher Seebeck coefficient value. And, NiCr/NiSi TFTCs with 800 nm NiSi and 2000 nm NiCr films displayed Seebeck coefficient value of 21.30 ± 0.69 μV/°C. o