Measurement of Thermal Expansion of Superconducting Wires at Cryogenic Temperature Based on Tensile Method

Abstract

The effects of variable cryogenic temperature on the thermal expansion are measured by tensile method for several commercial superconducting composite wires of NbTi/Cu, Nb3Sn/Cu, and high-temperature superconducting (HTS) Bi2212/Ag tapes. A variable temperature cryostat system is constructed to provide successive cooling environment from 300 to 77 K, and a cryogenic-type extensometer is utilized to measure the deformation of the superconducting wires arising from tensile and thermal loadings. In comparison with the mechanical deformation at room temperature, the corresponding thermal deformation at the variable cryogenic temperature is recorded and evaluated during stretching of the superconducting wires. The thermal expansion behavior of the composite wires and superconductor filaments is further captured. It shows that the thermal expansion of NbTi/Cu composite wire, NbTi filaments, Nb3Sn/Cu composite wires, and Nb3Sn filaments is almost linearly dependent upon the temperature, while those of HTS Bi2212/Ag tapes exhibit notable nonlinear features during cooling of the superconducting wires. In addition, based on the thermal expansion measured from room temperature to liquid nitrogen temperature, the thermal expansions at 4.2 K are extrapolated for low-temperature composite wires and their filaments, which are compared with the experiments using thermal methods in the existing literature. The results indicate that our variable temperature cryostat system and corresponding tensile method will be an easy way for measuring the thermal parameter of superconducting composite wires.