High-temperature superconductors,

Superconductors, high temperature general properties, 12-73 to 74 phase diagram, 12-182 to 199 Supercooled water, 6-8... 

Abstract An Eddy current method applying a High Temperature Superconductor ( HTS ) DC SQUID sensor operating at Uquid nitrogen temperature (77K) is presented. The method is developed for the detection of surface or surface near defects. We compare the performance of the SQUID system with the performance gained from a commercial Eddy current system, while using identical probes. The experimental data are obtained on defects in gas turbine blades. The advantage of planar conformable probes for the use with the SQUID is discussed. 

The development of neutron diffraction by C G Shull and coworkers [30] led to the detennination of the existence, previously only a hypothesis, of antiferromagnetism and ferrimagnetism. More recently neutron diffraction, because of its sensitivity to light elements in the presence of heavy ones, played a cmcial role in demonstrating the importance of oxygen content m high-temperature superconductors. 

Niobium is important as an alloy addition in steels (see Steel). This use consumes over 90% of the niobium produced. Niobium is also vital as an alloying element in superalloys for aircraft turbine engines. Other uses, mainly in aerospace appHcations, take advantage of its heat resistance when alloyed singly or with groups of elements such as titanium, tirconium, hafnium, or tungsten. Niobium alloyed with titanium or with tin is also important in the superconductor industry (see High temperature alloys Refractories). 

CUO2 layers appear in all cuprate superconductors and appear to be a necessary but not sufficient condition for high temperature superconduction. The La2SrCu20g 2 compound has CUO2 layers but does not superconduct. Experiments also indicate that T is proportional to the carrier density in the CUO2 layer but not to the volume carrier density, which is further evidence that the YBa2Cu202 is a two-dimensional superconductor. 

Superconductivity The physical state in which all resistance to the flow of direct-current electricity disappears is defined as superconductivity. The Bardeen-Cooper-Schriefer (BCS) theoiy has been reasonably successful in accounting for most of the basic features observed of the superconducting state for low-temperature superconductors (LTS) operating below 23 K. The advent of the ceramic high-temperature superconductors (HTS) by Bednorz and Miller (Z. Phys. B64, 189, 1989) has called for modifications to existing theories which have not been finahzed to date. The massive interest in the new superconductors that can be cooled with liquid nitrogen is just now beginning to make its way into new applications. 

There are presently four famihes of high-temperature superconductors under investigation for practical magnet appheations. Table 11-25 shows that all HTS are copper oxide ceramics even though the oxygen content may vary. However, this variation generally has little effect on the phvsical properties of importance to superconductivity. 

M. S. Dresselhaus, G. Dresselhaus, and R. Saito. In Physical Properties of High Temperature Superconductors IV, edited by D. M. Ginsberg, World Scientific Publishing Co., Singapore, 1994. Vol. IV, Chapter 7. 

The electronic theory of metallic superconduction was established by Bardeen, Cooper and Schrieffer in 1957, but the basis of superconduction in the oxides remains a battleground for rival interpretations. The technology of the oxide ( high-temperature ) superconductors is currently receiving a great deal of attention the central problem is to make windable wires or tapes from an intensely brittle material. It is in no way a negative judgment on the importance and interest of these materials that they do not receive a detailed discussion here it is simply that they do not lend themselves to a superficial account, and there is no space here for a discussion in the detail that they intrinsically deserve. 

The alkoxides and aryloxides, particularly of yttrium have excited recent interest. This is because of their potential use in the production of electronic and ceramic materials,in particular high temperature superconductors, by the deposition of pure oxides (metallo-organic chemical vapour deposition, MOCVD). They are moisture sensitive but mostly polymeric and involatile and so attempts have been made to inhibit polymerization and produce the required volatility by using bulky alkoxide ligands. M(OR)3, R = 2,6-di-terr-butyl-4-methylphenoxide, are indeed 3-coordinate (pyramidal) monomers but still not sufficiently volatile. More success has been achieved with fluorinated alkoxides, prepared by reacting the parent alcohols with the metal tris-(bis-trimethylsilylamides) ... 

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