Abstracts:Properties of cement-based materials at cryogenic temperatures are quite different from those at room temperatures. The strength of mortars at cryogenic temperatures was experimentally studied and an empirical model was established. The freezing thermodynamic process of pore water and pore size distribution in mortars were characterized by differential scanning calorimeter (DSC) and thermoporometry (TPM), respectively. The relationship between the increased cryogenic strength and pore ice formation was discussed. The results showed that flexural strength of mortars increased at a higher rate than compressive strength. Water content and initial strength at room temperatures were the main factors influencing the cryogenic strength. Higher water content and higher initial strength resulted in higher cryogenic strength. Ice formation in pores is one of the main reasons for the mortar’s cryogenic strength increase. Nearly half of the water remained unfrozen in pores with radius less than 40 nm at −40 °C. Both ice formed in capillary pores and gel pores contributes to the strength increase observed at cryogenic temperatures.

Abstracts:In practical applications, such as superconducting magnetic energy storage (SMES) magnets or alternate current applications of superconducting power cables, high temperature superconducting (HTS) tapes are subject to cyclical loads caused by repeated thermal cycles and periodic electromagnetic forces. These repeated loads will affect electromechanical properties of the tapes. Previous electromechanical properties studies have provided useful information on the performance of multifilament Bi-2223 tapes under different forms of fatigue loading. The impact of axial tensile fatigue loading on the critical current (I c) under external magnetic field, however, has not been reported. In this work, we studied the variation of I c of the Bi-2223 tapes with the background magnetic field after different cycles of fatigue at 77 K. It was found that the effect of stress amplitude on the degradation of I c for Bi-2223 tapes was more significant than the maximum stress. When the maximum fatigue stress approached the irreversible stress limit, the copper alloy reinforced Bi-2223 tapes still had good current-carrying properties after 10⁵ fatigue loadings. I c became less sensitive to the external magnetic field with the increase of the number of fatigue cycle.