Structure, Electrical Resistivity and Superconductivity of Low-alloyed γ-U Phase Retained to Low Temperatures by Means of Rapid Cooling

doi:10.1007/s40195-016-0400-7

Abstract

Abstract:

The γ-U phase alloys can be retained down to low temperatures with less required alloying concentration by using the splat-cooling technique with a cooling rate better than 10⁶ K/s. Doping with 15 at.% Mo, Pt, Pd, Nb leads to a stabilization of the cubic γ-U phase, while it requires much higher Zr concentrations (≥30 at.% Zr). All U-T splats become superconducting with T _c in the range of 0.61-2.11 K. A good agreement of the experimentally observed specific-heat jump at T _c with that from BCS theory prediction was obtained for U-15 at.% Mo consisting of the γ-U phase with an ideal bcc A2 structure.

Key words: Crystal, structure, Electrical, resistivity, Metallic, system, Superconductivity

M. Krupska, N.-T. H. Kim-Ngan, S. Sowa, M. Paukov, I. Tkach, D. Drozdenko, L. Havela, Z. Tarnawski. Structure, Electrical Resistivity and Superconductivity of Low-alloyed γ-U Phase Retained to Low Temperatures by Means of Rapid Cooling[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(4): 388-398.

Figures/Tables 9

Fig. 1 Photograph of the bulk ingot with a mass of ≈300 mg (left) prepared by the arc furnace and the resulting splat-cooled sample disk (right) produced by HV splat cooler

Fig. 2 EBSD crystallographic orientation map a and the Kikuchi pattern b highlighting the surface microstructure of the splat-cooled U-12 at.% Mo alloy which identified only γ-U phase and UC (small black areas) with no evidence for α-U or α-U-related phases

Fig. 3 XRD patterns of the as-formed splat-cooled U-Pt alloys a and U-Nb alloys b. Each curve was normalized to the maximal intensity of the most intense peak at 2θ = 36°-37° and then shifted upwards with respect to that of pure U splat. The color vertical ticks indicate the main XRD patterns of orthorhombic (blue) and cubic (red) structures and of the surface impurities (black). The four main γ-reflections are also indicated

Fig. 4 XRD patterns of the as-formed splat-cooled U-Zr alloys shown as normalized ones a. The same notations of the color vertical ticks are used as those in Fig. 3. The enlarged low-angle XRD patterns b. For U-Zr splats, apart from surface impurities UO2 and UC, the peaks of spurious ZrC are observed

Fig. 5 Comparison of the most intense reflection in XRD patterns of U-T splats with 15 at.% T alloying [T = Mo, (Mo + Zr), Pt, Nb] in the as-formed state. Sharp γ-phase peaks and no trace of α-phase peaks were observed for only U-15 at.% Mo splat

Table 1 Summary of low-temperature properties of U-T splat alloys: resistivity values at 300 K and at 4 K (ρ 300 K, ρ4 K), superconducting transition temperatures (T c) determined from the ρ(T) jump and/or from the specific heat C(T), the width of the superconducting transition in the resistivity (ΔT ρ ), the Sommerfeld coefficient of electronic specific heat (γ e) and Debye temperature (Θ D). The structure and lattice parameters (a, c) are also given

Tcontent (at.%)	Type	a, c(Å)	ρ _{300 K}(µΩ cm)	ρ _{4 K}(µΩ cm)	T _c (K) [ρ(T)]	ΔT _ρ (K)	T _c(K) [C(T)]	γ _e[mJ/(K²mol)]	Θ_D(K)
Pure U	α		53	14	1.24	0.20	0.65	11.0	179
15% Mo	γ	3.441	89	95	2.11	0.02	2.11	16.0	139
15% Pt	γ	3.469	164	166	0.95/0.61	0.08/0.04	0.75	19.5	145
15% Nb	γ ⁰	3.435 (a) 3.565(c)	83	86	1.90	0.15	1.90	13.7	153
15% (Mo + Zr)	γ ⁰	3.431 (a) 3.482 (c)
30% Zr	γ	3.543	75	73	0.81	0.08	0.60	11.8	165

Fig. 6 Temperature dependence of electrical resistivity (in zero field) of U-Mo splats in the normal state a and around the superconducting transition temperature b. For an easier comparison, the curves were normalized to respective resistivity values at T = 300 K and at T = 4 K. All U-Mo splats with the γ-U phase (≥11 at.% Mo doping) have a negative temperature coefficient (dρ/dT < 0). The pure single γ-U phase U-15% Mo splat has a highest critical temperature (T c = 2.11 K) and sharpest resistivity drop

Fig. 7 Temperature dependence of the normalized electrical resistivity in zero field of splat-cooled U-T splats having the γ-U phase compared with that of pure uranium splat a. All the alloys with 15 at.% T doping (T = Mo, Nb, Pt) have a negative dρ/dT, except for U-30 at.% Zr having a positive one. Superconducting phase transitions indicate by the abrupt resistivity drops in the range of 0.61-2.11 K b

Fig. 8 Specific-heat anomalies related the superconducting phase transition for selected U-T splats. A pronouncedλ-type specific-heat anomaly was observed only for U-15 at.% Mo splat consisting of single γ-U phase with ideal bcc A2 structure

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