Superconducting copper oxides Oxide

La2Cu04, Sr2Cu04. As we show in chapter 6, when a perovskite forms a composite or intergrowth with other structures, new compounds of interest in catalysis can be formed (such as in high-temperature superconducting copper oxides) and EM is used to determine the structures and properties of these complex compounds. The merits of using perovskites in steam reforming, membrane catalysis and fuel cells are discussed in chapter 6. 

The discovery of high Tq superconductivity in La2-xMxCu04 (M = Ba, Sr) (Bednorz and Muller 1987) based on perovskite and rock-salt structures has led to an international effort in superconductivity research over the last decade. The principles that govern superconducting copper-oxide-based materials have enormous significance in the application of these oxides as potential catalysts... 

Torardi et al, 1987 David et al, 1987). The structure is closely related to that of the superconducting copper oxide YBa2Cu307, consisting of square-pyramidally coordinated copper (Cu-O sheets) and square-planar copper (Cu-O chains). Excess oxygen in the lanthanum compound is located interstitially converting partially the chain copper to square-pyramidal and/or octahedral coordination. 

Raveau, B., Michel, C., Hervieu, M. Groult, D. (1991) Crystal Chemistry of High T Superconducting Copper Oxides, Springer-Verlag. 

One of the simple models of high-Tc superconducting copper oxides is a two-band Hubbard Hamiltonian (so called Emery model) [46]... 

B. Raveau, C. Michel, M. Hervieu and D. Groult, Crystal Chemistry of High Tc Superconducting Copper Oxides, Springer, Berlin, 1991. 

There is little doubt that many materials that at present are described as containing ordered arrays of point or extended defects will be successfully described as notionally defect-free modulated structures. For example, the intergrowth Aurivillius phases, described as containing extended planar defects, have recently been described compactly as modulated structures. " The same formalism has been applied to hexagonal perovskite structures and superconducting copper oxides. Others will certainly follow. 

Owing to the large volume of information on low temperature superconductivity already available in the books and excellent reviews, and the high cost of refrigeration required for using the low temperature superconductors, we describe briefly the preparation of a few low temperature superconductors and then focus on the preparation of high temperature superconducting copper oxides. 

C. N. R. Rao, The Chemistry of the High Temperature Superconducting Copper Oxides, Springer, Heidelberg, 1991. 

Structure—Property Correlations in Superconducting Copper Oxides... 

All the stmctures of the superconducting copper oxides isolated up to now are based on these principles. 

Y. Tokura, H. Takagi, and S. Uchida, Nature, 337, 345 (1989). A Superconducting Copper Oxide Compound with Electrons as the Charge Carriers. 

Horn et al. (1988) have compared the electronic structure of several superconducting copper oxide perovskites with closely related non-superconductors using several spectroscopic techniques including APS. Using XPS, a number of workers have... 

Figure 44 also shows that the character of the thermoelectric power a(T) changes dramatically between the underdoped composition x = 0.10 and the bulk superconductor x = 0.15. Figure 46 shows that a(T) for x = 0.15 is nearly temperature-independent above a critical temperature Tj. At low temperatures it exhibits an unusual enhancement with a maximum value near 140 K a phonon-drag enhancement would have its maximum near 70 K. We have shown that this unusual enhancement is a characteristic and unique feature of all the superconductive copper oxides [284-286]. 

The mechanism responsible for the formation of Cooper pairs in the superconductive state remains unsolved. Extensive spin-polarized inelastic neutron-scattering experiments have revealed a 41 meV resonance in the spin-excitation spectrum of the superconductive copper oxides that has caught theoretical attention [317]. Carbotte et al. [318] have noted that if these spin excitations are strongly coupled to the charge carriers, they should also be seen as a peak in the optical conductivity. They therefore calculated a((o) for a d-wave superconductor with inelastic scattering from the neutron data. Comparison with a-axis optical-conductivity data [319] showed that the... 

Over the past few years there has been a renewed experimental search for high-temperature superconductors. This effort has been augmented by the unexpected discovery of a superconducting copper-oxide based ceramic by Bednorz and Muller. This flurry of activity as shown in Fig. 3.26 has culminated in the development of a completely new class of mixed-oxide materials that is superconducting to temperatures above the temperature of... 

Yttrium—barium—copper oxide, YBa2Cu202 is a newly developed high T material which has been found to be fully superconductive at temperatures above 90 K, a temperature that can be maintained during practical operation. The foremost challenge is to be able to fabricate these materials into a flexible form to prepare wines, fibers, and bulk shapes. Ultrapure powders of yttrium—barium—copper oxide that are sinterable into single-phase superconducting... 

Bismuth trioxide forms numerous, complex, mixed oxides of varying composition when fused with CaO, SrO, BaO, and PbO. If high purity bismuth, lead, and copper oxides and strontium and calcium carbonates are mixed together with metal ratios Bi Pb Sn Ca Cu = 1.9 0.4 2 2 3 or 1.95 0.6 2 2 3 and calcined at 800—835°C, the resulting materials have the nominal composition Bi PbQ4Sr2Ca2Cu20 and Bi 25PbQgSr2Ca2Cu20 and become superconducting at about 110 K (25). 

Tertiary bismuthines appear to have a number of uses in synthetic organic chemistry (32), eg, they promote the formation of 1,1,2-trisubstituted cyclopropanes by the iateraction of electron-deficient olefins and dialkyl dibromomalonates (100). They have also been employed for the preparation of thin films (qv) of superconducting bismuth strontium calcium copper oxide (101), as cocatalysts for the polymerization of alkynes (102), as inhibitors of the flammabihty of epoxy resins (103), and for a number of other industrial purposes. 

In addition, the copper industry s market development activities have resulted in appHcations such as clad ship hulls, sheathing for offshore platforms, automotive electrical systems including electric vehicles, improved automobde radiators, solar energy, fire sprinkler systems, parts for fusion reactors, semiconductor lead frames, shape memory alloys, and superconducting ceramics (qv) containing copper oxides. 

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. 

Ba 4d spectrum also changes by increasing in intensity and conforming mostly to that expected of a barium silicate. As a result of the latter changes the superconducting properties of the film were destroyed. The Y 3d and Cu 2p spectra establish that yttrium and copper oxides are also formed. 

The discovery of high-temperature superconductivity in mixed oxides, such as the lanthanum-barium-copper oxide complexes, has created a great deal of interest in these materials. Superconductivity, that is, the absence of any resistance to the flow of electric current, is now possible at temperatures above the temperature of liquid nitrogen (77K). Many problems remain in the development of practical processes for these materials and commercialization is not likely to occur until these problems are solved. Among the several processing techniques now used, CVD appears one of the most successful. 

About 20 HTe superconducting compounds and copper oxidic systems Correlations of the Cu NQR/NMR data with the Cu( Zn) emission Mossbauer data for HTSC lattices as a tool for the determination of atomic charges... 

KimuraY., Ito, T., Yoshikawa, H., Hiraki, A. Superconducting Yttrium Barium, Copper Oxide (YBa2Cu307.delta) particles prepared from freeze-dried nitrates. Jpn. J. Appl. Phy s., Part 2, 30 (5A), L 798 to L 801, 1991... 

---