Alloys superconducting properties

The metal has unusual superconductive properties. As little as 1 percent gadolinium improves the workability and resistance of iron, chromium, and related alloys to high temperatures and oxidation. 

Several of the low-temperature superconducting metals, such as lead, brass, and some solders (particularly lead-tin alloys), experience property changes when they become superconducting. Such changes can include specific heat, thermal conductivity, electrical resistance, magnetic permeability, and thermoelectric resistance. Consequently, the use of these superconducting metals in the construction of equipment for low-temperature operation must be evaluated carefully. 

Since proximity to the MIT controls the normal state transport properties it is natural to assume that it might also influence the superconductive properties. To include these effects in a model, we modified the the Morel-Anderson model for Tc, to span the entire metallic range of an alloy ... 

Titanium-aluminum-niobium alloys have been developed for biocompatible, high-strength surgical implants (Semlitsch et al. 1985), while metal-resin composites containing niobium as filler have potential use as restorative materials in dentistry (Misra and Bowen 1977). The metal possesses superior superconductive properties in strong magnetic fields, which may be... 

The following tables include superconductive properties of selected elements, compounds, and alloys. Individual tables are given for thin fUms, elements at high pressures, superconductors with high critical magnetic fields, and high critical temperature superconductors. 

The Interstitial-Electron Theory has been applied to the structure of metals, alloys and interstitial compounds, to their magnetic and superconducting properties, as well as to a range of surface phenomena. This work has seemingly not come to the attention of the wider scientific community perhaps because it was published only in Japanese journals. It merits wider recognition and a critical evaluation. 

The selection of the critical temperature lixnn a transition in the effective permeability, or the change in resistance, or possibly the incremental changes in frequency observed by certain techniques is not often obvious liomthe literature. Most authors choose the mid-point of such curves as the probable critical temperature of the idealized material, while others will choose the highest temperature at which a deviation from the normal state property is observed. In view of the previous discussion concerning the variability of the superconductive properties as a function of purity and other metallurgical aspects, it is recommended that appropriate literature be checked to determine the most probable critical temperature or critical field of a given alloy. 

The nature and origin of superconductivity was described in 1957 by John Bardeen, Leon Neil Cooper, and John Robert Schrieffer. Together they created the Bardeen Cooper Schrieffer (BCS) model. It occurs for many metals, alloys, intermetallic compounds, and doped semiconductors. The transition temperatures range from 92.5 K for Ybc CUjOg j, down to 0.001 K for the element Rh. And there are some materials that become superconducting only under high-pressure conditions. These materials all have to be extremely pure, even just one impurity in 10,000 atoms can severely affect the superconducting property. 

Manning and Briscoe offer two possible explanations for the difference in superconducting properties between vapour-quenched and liquid-quenched alloys. The first explanation is based on the assumption that the liquid-quenched alloys are microcrystalUne rather than amorphous. The observation of a strongly exothermic heat effect (T ) observed by Buschow in the differential scanning calorimetry experiments does not corroborate this view. The second explanation is based on the assumption of an atomic arrangement in the vapour-quenched alloy, which is much more uniformly disordered than in the liquid-quenched alloy. The rather low transition widths A 7 seem to be in favour of this explanation, although even lower ATq values were found in liquid-quenched La .AUj alloys by Johnson and Tsuei (1976). 

Apart from the amorphous rare earth base materials mentioned above, numerous other amorphous metals and alloys have been studied. Collver and Hammond (1973) made a systematic study of amorphous transition metal films obtained by vapour deposition on cryogenic substrates. The superconducting properties of these amorphous alloys show clear differences with those of crystalline alloys. The 7J, values of the latter vary continuously with valency and give rise to two sharp peaks for electron-to-atom ratios equal to 4.5 and 6.5, respectively (Matthias, 1957). In contrast, the Tq values of the amorphous alloys show a more moderate variation with... 

Superconductivity is no stranger to the rare earths, in particular La and several of its compounds. Under pressure, Ce also becomes a superconductor as do Y, Lu (see Probst and Wittig 1978) and certain alloys however, for those lanthanides in which well-defined local 4f moments are present, superconductivity is inhibited by the pair breaking associated with the spins. And yes,- rare earths play a role, albeit not central, in what has been described as the condensed matter event of the 1980s -the remarkable discovery of superconductivity above 90 K in YBaCuO (Wu et al. 1987, Cava et al. 1987). In this case the rare earth is electronically isolated from the superconducting electrons, and as a result the substitution of heavy lanthanides with large 4f spins for the yttrium (Fisk et al. 1987) has virtually no dfect on the superconducting properties. 

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