J. Cors

Recent progress in Chevrel phase syntheses: A new low temperature synthesis of the superconducting lead compound

This paper reviews recent progress in the synthesis of Chevrel phases. The objective was to produce better microscopic homogeneity of the elements and/or precursors required to form the phase. Among these new processes, the one dealing with insertion of lead into the binary Mo6S8 compound is particularly attractive for the development of Chevrel phase wire since the phase formation occurs at temperature as low as 440°C. To achieve good microscopic homogeneity of PbMo6S8 (PMS), PbS is used and the extra sulfur is removed under a hydrogen gas flow. Basic superconducting properties, like a.c. susceptibility transition and specific heat anomaly at Tc, demonstrate extremely fine super-conducting particles. Further annealing at the moderate temperature of 600°C improves the homogeneity and/or the crystallinity of the powder.

Overview on the recent progress on Chevrel phases and the impact on the development of PbMo/sub 6/S/sub 8/ wires

The critical current density J/sub c/ of PbMo/sub 6/S/sub 8/ (PMS) wires has not reached a value required for industrial applications. However, large intragrain J/sub c/ has been observed, indicating that the grain connections play a dominant role in the J/sub c/ limitation. This behavior is similar to that of high-T/sub c/ oxides, where this limitation is thought to have an intrinsic origin related to their extremely short coherence lengths. But the coherence length of PMS lies between those of conventional superconductors and high-T/sub c/ materials. Therefore, it has been argued that an intrinsic granularity behavior is not expected for PMS and that the observed tendency towards granularity has another origin. Recent results on the phase stability, a possible structural transition, and the degradation of the grain boundaries of PMS point to different possible origins for the J/sub c/ limitation. A critical analysis of these investigations is presented, and their impact on the performance of wires is discussed.<<ETX>>

Tc variation in PbMo6S8: A critical analysis and a comparison with pure phases

Abstract Different methods were employed to synthesize a series of high quality PbMo 6 S 8 Chevrel-phase compounds. All the samples were characterized by means of X-ray diffraction, Auger electron spectroscopy, specific heat, acsusceptibility, resistivity, scanning electron spectroscopy and microprobe analysis. A marked decrease in the superconducting transition temperature (T c ) and the hexagonal lattice constant ratio (c/a) were noticed on the samples prepared in quartz tubes. Based on a simple correlation between T c and c/a ratio, we present some of the significant structure-property relations and phase purity of the samples. This interdependence is discussed mainly in terms of oxygen content in the phase. For instance, oxygen-free samples have T c invariably above 14.5 K. Interestingly, the optimal T c is found in samples having rather different, from each other, crystallographic parameters, but with a same c/a ratio. On the other hand, the wide variation of T c and structural parameters of PbMo 6 S 8 observed in the present investigation and those of many earlier works are explained by the presence of varying amounts of oxygen in the ternary phase. These results are checked in parallel with the oxygen-doped (PbMo 6 S 8−x O x ; x ≥ 0.2) samples.

Superconducting, microstructural, and grain boundary properties of hot‐pressed PbMo6S8

An alternative procedure is outlined for the synthesis of high quality, fine grained PbMo6S8. As the grain size plays an important role on the densification process, which in turn has an influence on the magnitude of Jc, an attempt has been made to produce dense samples from such powders by ‘‘Hot Pressing.’’ The effect of the hot‐pressing temperature on the superconducting, crystallographic, microstructural, and grain boundary characteristics of the ternary compound was evaluated. Scanning electron micrographs and ac‐susceptibility measurements indicate that hot pressing (1000–1200 °C) improves the grain connection as a consequence of better densification. However, at higher temperatures (1250–1400 °C) it also precipitates MoS2 as an additional phase. Calorimetric data indicate a continuous broadening, as a function of hot‐pressing temperature, of the specific heat jump at Tc. Preliminary investigation on the Tc distribution of the samples shows a progressive degradation, as indicated by a smearing in Tc ...

Magnetic penetration depth in the Chevrel phase superconductors SnMo6SySe8−y and PbMo6SzSe8−z

We report on transverse field (300 mT)μ+ SR measurements in the Chevrel phase superconductors SnMo6SySe8−y (y=1, 4, 7, 8) and PbMo6SzSe8−z (z=4.8) using the field cooling procedure (from room temperature down to 2.2 K). The London penetration depth λL was determined by the analysis of the fits with a single Gaussian relaxation function and by comparing the measured field distributions with calculated ones, where a broadening due to small random displacements of the vortices was taken into account.

Granular behavior of the PbMo6S8 Chevrel phase

PbMo6S8 is a potential superconducting mial for high magnetic field applications, but, up to now, the critical current density Jc, is still slightly to low to make it competitive for technical development. Analyses based on specific heat and ac susceptibility experiments m zero magnetic field show that the poor connections between the grains of this material limit already Jc near Tc.

The critical field of PbMo6S8 measured by specific heat up to 14 T

Calorimetric determination of the initial slope dH c2 /dT was carried out on a set of homogeneous high quality PbMo 6 S 8 samples. The specific heat jump at T c was measured in magnetic fields up to 14 T. The measurements unambigously confirm this compound as a high-field bulk superconductor (H c2 ∼50–60 tesla) and bring some insight into the nature of its upper critical field.

The role of Sn addition on the improvement of J/sub c/ in PbMo/sub 6/S/sub 8/

The influence of Sn on the enhancement of critical current densities, J/sub c/, in bulk samples of PbMo/sub 6/S/sub 8/ has been systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), AC susceptibility, and specific heat experiments. The results indicate that the addition of Sn, as an adjunction (dopant) or as a substitution, has a considerable effect on the structural, superconducting, microstructural and grain boundary properties. The observed increase in J/sub c/ is discussed in terms of improved interconnectivity between the grains, better homogeneity of the samples, and, more importantly, lower grain boundary contamination.<<ETX>>

Intrinsic critical current densities of PbMo6S8

The high upper critical field B c2 is now demonstrated to be an intrinsic property of the PbMo 6 S 8 (PMS) Chevrel phase compound. However lower values of B c2 determined using measurements based on macroscopic screening currents (like ac susceptibility and critical current densities), suggest that the granular nature of this superconductor may limit transport current by possible weak links between grains. We report inductive measurements of critical current densities on single-phased PMS powder and sintered bulk samples, those results confirm the existence of intrinsic high critical currents on PMS.

Specific heat of PbMo6S8 in high magnetic field

In order to investigate the volume upper critical field of the Chevrel-phase superconductor PbMo6Ss, sl~ecific-heat measurements have been performed in magnetic fields up to 24.5 T. The superconducting transition temperOture T~ of 14.3 ( + 0.1) K is suppressed by the maximum magnetic field to 10.1 ( + 0.3) K, which extrapolates to an upper critical field of 56 T (confirming measurements at fields up to 14 T). For a mixed state in the magnetic field a linear term 7roT, related to the core states, is present in the electronic contribution to the specific heat. At a magnetic field ot~ 24.5 T, 7m amounts to 50% of the normal state value 7 of 6.5 mJ/K 2 g-at. By extrapolation, an upper critical field of 5~ T can be derived.