Abstract
This study aims to explore superconducting HEAs and to find their critical parameters, Tc, Jc, & Hc, and characteristic parameters, ξ, λL, & κ, by experimental analyses and accompanied theoretical calculations, respectively. The materials selected were alloys that were added by Hf, V, Mo, or Fe to pseudo-unitary BCC quaternary NbTaTiZr-based HEAs, and were arc-melted in as-cast equi-molar specimens containing numbers of alloying elements from four to seven. Samples were homogenized at 1100oC for 24 h in a quartz tube filled with argon gas and then quenched in water. Both as-cast and homogenized samples were characterized by XRD, SEM-BEI, EDS, four-point probe resistivity (4.2 K ~ 300 K), magnetization vs. temperature curve (2 K ~ 300 K), magnetic hysteresis loop (2 K, 5 K and 300 K) and Hall (300 K) measurements. NbTaTiZr, NbTaTiZrHf, & NbTaTiZrHfV are single-pseudo-unitary BCC solid solutions in both their as-cast and homogenized states. As-cast and homogenized NbTaTiZrMo alloys are composed of one BCC and one HCP. As-cast NbTaTiZrHfVMo has two BCCs, while the HCP phase will appear after homogenization. One Laves phase, which is composed of small Fe atoms and large Nb, Ta, Ti, and Zr atoms, as well as one BCC phase, occurs in NbTaTiZrFe, because the negative mixing enthalpy of Fe with Nb, Ta, Ti, and Zr are large. Most HEAs in this study show superconductivity in resistivity vs. temperature measurements. Tc falls as number of alloying elements in alloys increases. Superconducting transitions are in temperature intervals, ΔTc, and ΔTc < 1 K. Values of residual resistivity ratio (RRR) range from 1.06 to 1.15, that is to say, the effect of “lattice defects” to electrical conducting is larger than does that of “temperature” in HEAs. For as-cast HEAs, except NbTaTiZrMo, magnetization at 100 Oe vs. temperature curves show diamagnetism. Values of critical temperatures, Tc, range from 3.49 K to 7.97 K. Except that in NbTaTiZrMo and NbTaTiZrHfVMo, Tc of as-cast state in other alloys is higher than their corresponding Tc of homogenization. Some factors that affect the values of Tc in superconducting HEAs are demonstrated. According to their contribution order, they are, successively, (1) Nb + Ta contents in superconducting phase, (2) e/a ratio, (3) lattice distortion, (4) electron density of state, D(εF), and (5) Debye temperature, θD. Because susceptibility of the as-cast superconducting HEAs does not change with temperature, and their magnetic hysteresis loops at 300 K show paramagnetism, accordingly, these HEAs are Pauli paramagnetism at their normal state. From the results of magnetic hysteresis loops at 2 K and 5 K, both as-cast and homogenized alloys are type II superconductors. Values of upper critical fields, Hc2, range from 0.50 T to 2.01 T. For as-cast alloys, Hc2 increases as number of alloying elements increases. Hc2 decreases after homogenization. Values of critical current densities, Jc, range from 0.6 x 104 A/cm2 to 1.5 x 105 A/cm2. As-cast NbTaTiZrHfVMo has the largest Jc, and Jc for alloys decreases after homogenization. The results of Hall measurement at 300 K show carriers of as-cast alloys are “hole-like”. Carrier concentrations are in order of magnitude of 1022 cm-3, which are similar to that of non-superconductig HEAs ever investigated. In this study, characteristic parameters, ξ, λL, & κ, are calculated from the above-mentioned carrier concentrations by relevant theories. Values of κ range from 2.3 to 12.5, which also show that the superconducting HEAs are type II superconductors.