Anomalous pressure dependences of the superconducting transition temperature of dilute alloys of LaSn3 containing light rare earth impurities

The initial depressions of the superconducting transition temperature T _c of LaSn₃ by rare earth (RE) impurities are known to be anomalous. In an attempt to clarify this behavior, we have measured the pressure dependence of the T _c of (LaRE)Sn₃ alloys (RE = Y, Ce, Pr, Nd, Sm, Eu, Gd, and Lu) to pressures up to 25 kbar. The magnitude of the pressure dependence of the T _c of (LaRE)Sn₃ alloys containing RE impurities with partially filled 4f electron shells decreases monotonically with increasing RE atomic number. A general discussion of existing zero- and high-pressure data for various physical properties of (LaRE)Sn₃ alloys demonstrates that the association of large pressure dependences of T _c with the occurrence of unstable impurity 4f shells (the Kondo effect) should be made with caution. The experimental high-pressure results are analyzed in terms of appropriate theoretical models for magnetic and nonmagnetic localized impurity states in superconductors.Supported by the US DOE Contract E(04-3)-34PA227.National Science Foundation Postdoctoral Fellow.     

On properties of superconducting alloys containing Kondo impurities

The phase transition temperature T _c and the specific heat discontinuity C of a superconductor containing Kondo impurities are expressed self-consistently in terms of the t-matrix for Kondo scattering, which has been calculated within the Tomonaga approximation, thereby taking care of the infrared singularities. The approximation contains the Abrikosov-Gor'kov theory and the exact result for the classical spin as limiting cases. The initial slopes of T _c and C as a function of antiferromagnetic exchange coupling have a maximum, and the zero-temperature transition concentration has a minimum, showing that the impurity undergoes a magnetic-nonmagnetic transition when the coupling is increased. This agrees with previous results for the impurity spin susceptibility. The condition for a reentrant phase boundary is discussed in connection with its implications for the specific heat jump. The results are compared quantitatively with experimental data for LaAl₂ containing Ce and Gd impurities and qualitatively with recent data for the (La _1-x Th _x )Ce system.     

Superconducting state parameters of Pb-Tl-Bi alloys

An extensive theoretical study of the superconducting state parameters (SSPs), that is, electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _C , isotope effect exponent α, and effective interaction strength N ₀ V of Pb-Tl-Bi alloys, including Tl_0.90Bi_0.10, Pb_0.40Tl_0.60, Pb_0.60Tl_0.40, Pb_0.80Tl_0.20, Pb_0.60Tl_0.20Bi_0.20, Pb_0.90Bi_0.10, Pb_0.80Bi_0.20, Pb_0.70Bi_0.30, Pb_0.65Bi_0.35, and Pb_0.45Bi_0.55, has been conducted for the first time using model potential formalism. A considerable influence of various exchange and correlation functions on λ and μ* is found. The obtained values of the SSPs are in qualitative agreement with the available experimental data.     

Superconductivity in dilute Cu-Nb-Sn alloys containing Nb3Sn precipitates

The dilute Cu-Nb-Sn alloys containing small amounts of Nb and Sn less than 1 at % exhibited superconductivity after quenching from the liquid state and ageing. The best superconducting properties ( andJ _c=130 A cm^–2) in a Cu-0.30 at % Nb 0.15 at % Sn alloy were obtained when the sample was aged at 550° C for 384 h. This sample exhibited a structure of fine Nb₃Sn precipitates of 200 to 500Å diameter distributed homogeneously in the Cu matrix, and therefore it was concluded that superconductivity in these alloys resulted from the proximity effect of Nb₃Sn particles. In spite of the similar structure obtained by ageing at 800° C, the Cu-Nb-Sn alloys showed inferior superconducting properties compared to the Cu-0.4 at % Nb alloy and this would be explained qualitatively by the difference in the mean free path in the two alloys.     

Superconducting properties of Ti-Nb-Hf alloys

To achieve improvements in superconducting properties of the Ti-Nb superconductor, effects of ternary additions of Hf have been extensively studied on 42 Ti-Nb-Hf alloys with compositions of 25 - 65at%Nb, 0- 15at%Hf and the balance Ti. Critical temperatures are found to depend upon Hf addition and aging temperature. In as-rolled Ti-40at%Nb-3at%Hf alloy the critical temperature is raised by about 0.3K over Ti- 40at%Nb alloy. Aging at 800°C can raise critical temperatures of high Hf alloys by 0.6 - 1.8K. The upper critical field at 4.2K of as-rolled Ti-40at%Nb-3at%Hf reaches 11.7 tesla, a value higher by 0.4 tesla than that of Ti-40at%Nb. High field critical current densities are also improved by the 3at%Hf addition. 2 step aging treatment is found effective in enhancing critical current densities of high Hf alloys. No degradation in fabricability is caused by a few at% Hf additions.     

Superconducting alloys containing paramagnetic impurities with local states within the gap. Effect of a magnetic field

The effect of a uniform magnetic field on a superconductor containing magnetic impurities has been investigated by treating the scattering of conduction electrons by magnetic impurities exactly (within the Shiba-Rusinov model). Our generalized equations defining the single-particle Green function reduce to earlier known results in appropriate limits. As an application of our equations we have calculated the transition temperature T _c. For fixed values of the parameter ₀ (the normalized position of the bound state within the BCS gap), the parameter p (the normalized magnetic field), and the parameter (=S _z ²/S²), the detailed dependence of T _c on the normalized impurity concentration ovc has been shown. The dependence of T _c on p for fixed values of ₀, ovc, and has also been shown. Our results are significantly different from the earlier known results.Work supported in part by a grant from the Natural Sciences and Engineering Research Council of Canada.     

Superconducting properties of amorphous Zr-Ge binary alloys

A new type of refractory metal-metalloid amorphous alloys exhibiting superconductivity has been found in a binary Zr-Ge system by a modified melt-spinning technique. Specimens are in the form of continuous ribbons 1 to 2 mm wide and 0.02 to 0.03 mm thick. The germanium content in the amorphous alloys is limited to the range of 13 to 21 at%. These amorphous alloys are so ductile that no cracks are observed even after closely contacted bending test. The Vickers hardness and crystallization temperature increase from 435 to 530 DPN and from 628 to 707 K, respectively, with germanium content, and the tensile fracture strength is about 1460 MPa. Furthermore, the amorphous alloys exhibit a superconducting transition which occurs very sharply. The superconducting transition temperature (T _c) increases with decreasing germanium content and reaches a maximum value of 2.88 K for Zr₈₇Ge₁₃. The upper critical magnetic field for Zr₈₇Ge₁₃ alloy was of the order of 21.8 kOe at 2.0 K and the critical current density for Zr₈₅Ge₁₅ alloy was about 175 A cm^–2 at 1.70 K in the absence of an applied field. The upper critical field gradient atT _c and the electrical resistivity at 4.2 K increase significantly from 24.6 to 31.5 kOe K^–1 and from 235 to 310cm, respectively, with the amount of germanium. The Ginzburg-Landau (GL) parameter and the GL coherence length §_GL (0) were estimated to be 72 to 111 and about 7.9 nm, respectively, from these experimental values by using the Ginzburg-Landau-Abrikosov-Gorkov (GLAG) theory and hence it is concluded that the Zr-Ge amorphous alloys are extremely soft type-II superconductor with high degree of dirtiness which possesses theT _c values higher than zirconium metal, in addition to high strength combined with good ductility.     

Superconducting and normal state properties of ErRh4B4 and LuRh4B4

Detailed heat capacity measurements of the ternary compounds ErRh₄B₄ and LuRh₄B₄ have been performed between 0.5 and 36 K and in magnetic fields up to 4 kG, yielding new information on crystal field effects in these materials and on the influence of externally applied magnetic fields on the coupled superconducting-ferromagnetic reentrant transition in ErRh₄B₄. Static magnetic susceptibility data on LuRh₄B₄ are presented which allow qualitative conclusions to be drawn regarding the magnitude of exchange enhancement and orbital paramagnetic effects in the RERh₄B₄ compounds. The electrical resistivity of ErRh₄B₄ has also been determined between 4 K and room temperature.This research was supported by the Department of Energy under Contract Number Ey-76-S-03-0034-PA227-3 (LDW, HBM, RWM, MBM) and by the National Science Foundation under Grant Number NSF/DMR77-08469 (DCJ). One of us (RWM) thanks the National Science Foundation for a postdoctoral fellowship.     

Superconducting and electrical properties of amorphous zirconium-transition metal binary alloys

In order to clarify the compositional effect on the superconductivity of zirconium-transition metal (M) binary amorphous alloys, the superconducting properties and electrical resistance of the alloys were examined as functions of the concentration, group number and periodicity of the M elements.T _c for Zr₇₅M₂₅ alloys rises in the order Ru > Rh > Ir > Co > Os > Ni > Pt > Cu > Pd > Fe > Au, i.e. as the group number decreases when the periodicity belongs to the 5th period, and with decreasing M content for Zr_100–X M _X alloys. The highT _c attained in the present work is 4.55 K for Zr₈₀Rh₂₀, 4.38 K for Zr₇₅Rh₂₅ and 4.47 K for Zr₇₅Ru₂₅. The temperature gradient of the upper critical magnetic field (H _c2) near the transition temperature (T _C) tends to increase with increasing zirconium content, and the resistive state due to the flux flow phenomena appears in a wide sweeping field. Following the sharp and large decrease of the flux flow resistance due to a peak effect, the resistance recovers sharply nearH _c2. The peak effect was found to occur more distinctly for the alloys containing a magnetic element of iron or cobalt, probably because of the suppression of the pair-breaking effect due to magnetic scattering by the application of the high field nearH _c2. The dominating factor for the compositional effect onT _C is inferred to originate from the variation of through for Zr_100–x M _X alloys and from that of through N(E_f) for Zr₇₅M₂₅ alloys. Additionally, it has been found for the Zr-M amorphous alloys that the electrical resistivity(T) exhibits a maximum value at temperature ranging from 2T _C to 3T _C, suggesting that the hump phenomenon in (T) appeared through the generation of the superconducting fluctuations. The temperature coefficient of resistivity (tcr) defined by 1/R₂₅₀(dR/dT) shows negative values ranging from 1.05×10^–4 to 1.75×10^–4 K ^–1 andT _C was found to rise through the increase in with the increase in the negative value of the tcr.     

Superconducting properties and microstructure of crystallized Hf-Nb-Si and Hf-V-Si amorphous alloys

Amorphous Hf-Nb-Si and Hf-V-Si alloys have been produced by rapidly quenching the melts using a melt-spinning technique. The silicon content in the amorphous alloys was limited to 14 to 20 at% and the niobium or vanadium content was limited to 0 to 45 at% and 0 to 35 at%, respectively. These amorphous alloys did not show any superconducting transition down to liquid helium temperature (4.2 K). However, a transition was detected above 4.2 K after inducing crystallization in these alloys by annealing at appropriate temperatures. The highest superconducting transition temperatures, T _c, attained were 8.9 K for the Hf₄₅Nb₄₀Si₁₅ alloy annealed for 1 h at 1273 K and 6.7 K for the Hf₅₀V₃₅Si₁₅ alloy annealed for 1 h at 1173 K. The upper critical magnetic field, H _c2, at 4.2 K and the critical current density, J _c, at zero applied field and 4.2 K were about 5.1×10⁶A m^–1 (6.4 T) and more than 1×10⁴ A cm^–2 for the Hf₄₅Nb₄₀Si₁₅ alloy and more than 8.0×10⁶ A m^–1 (10 T) and 5×10³ A cm^–2 for the Hf₅₀V₃₅Si₁₅ alloy. Detailed transmission electron microscopic studies of the annealed structure of these amorphous alloys established that, after crystallization, these alloys contain a body-centred cubic -Hf(Nb) solid solution and body-centred tetragonal Nb₃Si phases in the Hf₄₅Nb₄₀Si₁₅ alloy and hexagonal Hf₅Si₃, face-centred cubic HfV₂ and cubic V₃Si phases in the Hf₅₀V₃₅Si₁₅ alloy. Since Nb₃Si and Hf₅Si₃ are not superconducting above 4.2 K, it has been concluded that superconductivity in these crystallized alloys is due to the precipitation of -Hf(Nb) solid solution in the Hf-Nb-Si alloys and to the precipitation of HfV₂ and V₃Si compounds in the Hf-V-Si alloys.     

Critical fields and generalized Ginzburg-Landau parameters for Mo-Re alloys containing paramagnetic impurities

We have investigated the temperature dependence of the upper critical fieldH _c2 and the thermodynamical fieldH _c for superconducting Mo-Re alloys containing magnetic impurities. In addition, the temperature dependence of the Ginzburg-Landau parameters ₁(T) and ₂(T) has been determined by measuring the magnetization and the specific heat. Our findings are compared with the theory of Fulde and Maki. In agreement with theory, our measurements show that the two pair-breaking mechanisms involved (magnetic field and magnetic impurities) are nonadditive if one departs from the linearized Ginzburg-Landau equation.     

形变热处理对低浓度CuCr合金性能的影响

用两种形变热处理工艺制备了Cu-0.4Cr合金,并通过测定不同工艺合金的硬度和电导率,研究了形变热处理工艺对合金性能的影响,以得到硬度和电导率综合性能优异的且适宜工业化批量生产的工艺.结果表明,CuCr合金在线热轧淬火后经60%冷轧并在450℃时效30min后其硬度和电导率分别为156Hv,86.4%IACS,该工艺适合工业化批量生产;经固溶一冷轧80%-450℃时效30min后,合金硬度和电导率可达176Hv,80.2%IACS,虽然该工艺所制备合金性能优异,但难以实现工业化批量生产.对所得结果分析表明,所制备CuCr合金极易过时效,且冷轧变形量越大,时效温度越高,合金过时效越明显,生产中难以控制.微合金化以延缓时效是该合金值得研究的方向.     

含钪Al-Cu-Li-Zr合金的组织与性能

为了研究微量Sc对Al-3.5Cu-1.5Li-0.12Zr合金组织与性能的影响,采用铸锭冶金法,制备了4种不同Sc含量的Al-3.5Cu-1.5Li-0.12Zr合金.采用室温拉伸力学性能实验、金相显微镜和透射电镜研究了微量Sc对Al-3.5Cu-1.5Li-0.12Zr合金微观组织和拉伸性能的影响.结果表明:添加0.1%Sc能消除铸态合金的枝晶组织,有效地抑制了再结晶的发生,具有一定的强化作用和明显的增塑效应;添加O.15%Sc和0.25%Sc能显著细化合金铸态的晶粒组织,但添加0.15%Sc不能抑制合金固溶过程中再结晶;添加0.25%Sc会促进合金固溶过程中的再结晶,从而降低合金的强度.合金中较适宜的Sc加入量为0.10%～0.15%,此时合金既具有较高的强度,又兼具较好的塑性.     

Thermal transport properties in the normal phase of dilute3He-4He mixtures

We present steady-state measurements of the thermal diffusion ratiok _T and of the heat conductivity for three dilute mixtures of³He in⁴He with concentrations 9×10^–3X(³He)5×10^–2 at saturated vapor pressure in the normal phase close to the superfluid transition. The data are compared with predictions by Dohm and Folk from the renormalization group (RG) theory. From auxiliary determinations of thermodynamic derivatives for these mixtures, we obtain the separation factor =–(k _T /T)×(/X) _T,P /(/T) _X,P above T over the range wherek _T is positive. Here is the mass density. From the transients of X(t) as a function of time, we obtain an estimation for the mass diffusion coefficientD and compare the results with predictions by Dohm and Folk and with results from other experiments.     

Determination of the bulk cobalt valence state of co-perovskites containing surface-adsorbed impurities

We used thermogravimetric hydrogen reduction and iodometric titration to determine the bulk valence state of cobalt in Co-perovskites containing surface carbonate hydroxide or hydroxyl groups. It could be shown that thermogravimetric hydrogen reduction experiments are very sensitive to volatile surface groups, but due to their volatility, they can be specified and the bulk valence state of cobalt can still be deduced from these experiments. The iodometric titration is less sensitive to small volatile surface impurities, but precaution has to be taken that oxygen or iodine does not escape from the solution during dissolution of the sample. Best results were obtained if the sample was titrated during dissolution in a closed argon floated titration apparatus. We tested the two methods using LaCoO3 perovskite as a sample with a known valence state. Both methods delivered satisfactory results, and the valence state could be determined with an accuracy of better than 1%.     

Magnetic and nonmagnetic impurities in some A-15 structure systems. I. Superconducting properties

We present measurements of the critical temperature T _c and the critical field H _c2 of the A-15 structure systems Nb _y ,Pt_100–y–x M _x where M = Fe and Ru (y = 73–78). T _c is influenced by two effects: one due to the magnetic Fe atoms, which causes a strong decrease, and the other due to the nonmagnetic effect of the Fe atoms, which causes a smaller but non-negligible effect. For the study of the nonmagnetic effect we introduced Ru impurities instead of Fe. Anomalous high critical field values are found as T 0, and the curves H _c2 versus T do not follow the WHH theory generalized to include magnetic impurities. The introduction of an empirical parameter permits us to fit our experimental results. This abnormal behavior of H _c2 is discussed in terms of strong-coupling and anisotropy effects.     

Solid state phase transformations in aluminium alloys containing lithium

Abstract

The microstructure and solid state phase transformations which take place in aluminium alloys containing lithium have been assessed. In addition to the binary Al–Li system, the Al–Li–Cu, Al–Li–Mg, and Al–Li–Cu–Mg systems are discussed in detail together with the role of dispersoid forming elements. The physical metallurgical basis of current alloy development is thereby established.

MST/595     

Creep of dilute zinc-copper alloys

A series of zinc-copper alloys containing up to 2.0 wt % Cu have been prepared and tested in creep under constant load at temperatures up to 200° C. A metallographic study has also been made of the crept specimens. The creep resistance of Zn is shown to increase as the Cu content is raised, although the creep strength increment is small above 1 wt % Cu. Ageing the alloys also improves the creep strength, but precipitation during creep can generate voids which may lead to premature failure. The effect of increasing the Cu content is to make slip and grain-boundary sliding progressively more difficult, and to raiseΔH _c, the apparent activation energy for creep. A higher copper content also enables a low value of the stress exponent to persist to higher creep stresses.     

Magnetization of dilute Al-Mn and Al-Fe alloys

The magnetization of very dilute Al-Mn and Al-Fe alloys was measured from 2 to 0.01 K in order to obtain a limit on how nonmagnetic such systems are. Results indicate that the impurity contribution to the magnetization is almost temperature independent and that the moment per impurity is less than 1.7 × 10^–2. The Al nuclear paramagnetism is mainly responsible for the uncertainty in this value.Supported by NSF grant DMR 76 21702.