Acoustic quality factor of niobium and vanadium at low temperatures

Abstract

The amplitude-independent acoustical mechanical quality factors Q of polycrystalline commercially prepared samples of 99.8% pure niobium and 99.75% pure vanadium have been measured as a function of temperature and heat treatment from 50 mK to 300 K. Data were obtained from 1 kHz torsional modes of resonators designed to minimize extraneous loss effects. Thermal annealing at temperatures above the recrystallization temperatures yielded strongly enhanced Q. Maximum Q values of 4.24×107 in Nb and 3.91×10 7 in V were observed in annealed material at the lowest measurement temperatures. Internal friction peaks are interpreted in terms of mechanisms identified from previous studies of predeformed and hydrogen-charged materials. Intergrain thermal diffusion is shown to account for a significant portion of the background acoustic loss from 60 to 300 K. Relative shear moduli, obtained from the square of the resonator frequency, are given from 50 mK to 300 K. High-resolution frequency measurements at the lowest temperatures allow evaluation of the superconductivity component of the shear modulus.