The anisotropic thermal expansion of the hexagonal elements, thallium, yttrium, beryllium, and zinc, has been measured. The temperature range for the thallium, yttrium, and zinc measurements was 4.2−273.2°K, while for beryllium the range was 77−273.2°K. Dimensional changes were detected by an interferometric technique, and the movement of the interference fringes was detected with a photomultiplier tube. From the measured values of ΔL/L0 the coefficients of thermal expansion and the c/a ratios of the metals were calculated as a function of temperature. A review of previous measurements of the thermal expansion of these metals is also included.

Calorimetrically determined energy losses for a series of niobium and niobium-25 at.% zirconium samples carrying an af transport current agreed with those calculated from hysteresis loops determined by dc magnetization measurements. The results of this study showed that the energy losses in ultra-high-purity annealed Nb are less than those for any of the other samples studied at magnetic fields less than 1500 G. The magnetic field dependence of the energy losses in all of the samples studied is given by EL = E12hm, where EL is the energy dissipated per unit surface area per cycle and is independent of frequency, E12 is a constant which depends on the material and surface finish, and h = Hp/Hc1, where Hp is the peak ac field amplitude, and Hc1 is the lower critical magnetic field; m = n1 when h≤1, and m = n2 when h≥1. For both highly reversible and highly irreversible samples, n1 ≈ 3. For the least reversible samples, n2 ≈ 4, increasing to ∼8 for the most reversible samples studied. E12 generally increased with decreasing reversibility and was strongly dependent on the surface finish of the sample, increasing as the surface roughness increased. The power loss PL per unit surface area is given by PL = (EL)f. This linear frequency dependence is consistent with a hysteretic loss mechanism as is the agreement between the calorimetrically determined losses and those calculated from the measured hysteresis loops.

Niobium samples with residual resistivity ratios greater than 20,000 were prepared via a two-step process. First, a fused salt electroplating process was used to prepare niobium having an exceptionally high purity with respect to the substitutional transition metal impurities tantalum and tungsten. Second, this electroplated niobium was vacuum degassed to remove the interstitial impurities, oxygen, nitrogen, and hydrogen. The major contributions to the residual resistivity after vacuum degassing were from the impurities nitrogen, tantalum, and tungsten. The electrical resistivity of these ultrahigh purity niobium samples can be calculated from the relation pT ~" p(T) -{-10.0 X 10-e CN2 + 2.49 • 10-7 Cwa -}-11.0 • 10-TCwohm-cm where p(T) represents the thermal contribution to the resistivity at the temperature T and CN2, CTa, and Cw represent respectively the concentrations in atomic per cent of nitrogen, tantalum, and tungsten. The value of CNs for the vacuum degassed samples is given by CN2 ----6.2 • 10 -4 (PN2)1/2 exp (23,000/T) where PN2 represents the partial pressure (Torr) of nitrogen in the vacuum system and T represents the temperature (K) of the sample during outgassing.This study of the preparation and electrical resistivity of ultrahigh purity niobium was prompted by the observation that the a-c losses in superconducting niobium wire samples decreased as the residual resistivity of these samples decreased (1). Thus, it was hoped that if those factors most strongly affecting the residual resistivity were identified it might then be possible to prepare niobium samples with lower residual resistivities and hence lower a-c losses.In 1962 Stromberg and Swenson (2) reported the preparation of niobium with a residual resistivity ratio (p293/p4.2) of ~2,000. This value, the highest obtained prior to this work, was attained by vacuum degassing a niobium wire at a temperature close to its melting point at a pressure of the order of 10 -9 Torr. Attempts by others to achieve significantly higher residual resistivity ratios by either vacuum degassing and/or electron beam zone refining were unsuccessful. In some cases the highest residual resistivity ratios attained were as small as 100-500. Some of these low values can be explained by the low temperatures and/or poor vacuum used to degas the niobium. One significant difference between these studies, however, was the tantalum concentration in the starting material. Stromberg and Swenson had used niobium containing less than 200 ppm tantalum while many of the other studies were made with niobium containing as much as 1,000 ppm tantalum. No attempts to attain high residual resistivity ratios have been reported in which niobium containing less tantalum than that in the niobium used by Stromberg and Swenson was used as the starting material. Two conclusions were drawn from this early work. First, electron beam zone refining of niobium does not result in any significant reduction of the tantalum content. Second, in general the highest residual resistivity ratios were rep...

No abstract

No abstract