Barium oxide, superconductors

Both industrial processes have long been superseded. There has been a recent renewal of interest in the solid state chemistry of barium oxides as reactants in the preparation of high-temperature superconductors. 

In addition to the above, preparation in w/o microemulsions of nanoparticles of various other types of compounds, viz. silica-coated iron oxide, Fe203-Ag nanocomposite, oxides of ytrium, erbium, neodymium, vanadium and cobalt, titanates of barium and lead, ferrites of barium, strontium, manganese, cobalt and zinc, oxide superconductors, aluminates, zirconium silicate, barium tungstate, phosphates of calcium, aluminium and zinc, carbonates of calcium and barium, sulphides of molybdenum and sodium, selenides of cadmium and silver etc. have been reported. Preparative sources and related elaboration can be found in [24]. 

Fransaer J., Roos I.R., Delaey L., Van der Biest O., Arkens O., Cells J.P. Sol-gel preparation ofhigh-Tc bismuth calcium strontium coppo oxide and yttrium barium copper oxide superconductors. J. Appl. Phys. 1989 65 3277-3279... 

Electrical and Electronic Applications. Silver neodecanoate [62804-19-7] has been used in the preparation of a capacitor-end termination composition (110), lead and stannous neodecanoate have been used in circuit-board fabrication (111), and stannous neodecanoate has been used to form patterned semiconductive tin oxide films (112). The silver salt has also been used in the preparation of ceramic superconductors (113). Neodecanoate salts of barium, copper, yttrium, and europium have been used to prepare superconducting films and patterned thin-fHm superconductors. To prepare these materials, the metal salts are deposited on a substrate, then decomposed by heat to give the thin film (114—116) or by a focused beam (electron, ion, or laser) to give the patterned thin film (117,118). The resulting films exhibit superconductivity above Hquid nitrogen temperatures. 

The acetylacetonates are stable in air and readily soluble in organic solvents. From this standpoint, they have the advantage over the alkyls and other alkoxides, which, with the exception of the iron alkoxides, are not as easily soluble. They can be readily synthesized in the laboratory. Many are used extensively as catalysts and are readily available. They are also used in CVD in the deposition of metals such as iridium, scandium and rhenium and of compounds, such as the yttrium-barium-copper oxide complexes, used as superconductors. 1 1 PI Commercially available acetyl-acetonates are shown in Table 4.2. 

The deposition of thin films of the high-temperature superconductor yttrium-barium-copper oxide, YBa2Cu307, is obtained from the mixed halides, typically YCI3, Bal2, and CUCI2, with O2 and H2O as oxygen sources. Deposition temperatures are 870-910°C.f ]... 

One of the most exciting developments in materials science in recent years involves mixed oxides containing rare earth metals. Some of these compounds are superconductors, as described in our Chemistry and Technology Box. Below a certain temperature, a superconductor can carry an immense electrical current without losses from resistance. Before 1986, it was thought that this property was limited to a few metals at temperatures below 25 K. Then it was found that a mixed oxide of lanthanum, barium, and copper showed superconductivity at around 30 K, and since then the temperature threshold for superconductivity has been advanced to 135 K. 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

The two copper oxide layers can be considered as polymeric since the covalent character is in the same range as for the carbon fluoride bond in Teflon. Thus, the 123-superconductors consist of two types of polymeric copper oxide layers held together by ionic bonding metals such as barium and yttrium. This theme of polymeric layers held together by ionic bonding to metals is common in the silicates and other minerals. 

The discovery of a barium-doped lanthanum copper oxide which became superconducting at 35 K led to a flood of new high temperature superconductors some of which were superconducting above the boiling temperature of nitrogen, 77 K. Over 50 high temperature superconductors, almost all containing copper oxide layers, are now known. 

A common feature of all the new ceramic superconductors is that they are cuprates, that is, they are complex copper oxides. The structure of YBCO is given in Fig. 19.3, which also shows that it is related to the perovskite structure (Fig. 4.17). Synthesis of YBCO is remarkably easy appropriate amounts of dry yttrium oxide (Y203), copper oxide (CuO), and barium carbonate (BaC03) are ground together into a fine, well-mixed... 

A prototypical high-temperature superconductor is yttrium barium copper oxide, YBa2Cu307, in which two-thirds of the copper is in the +2 oxidation state and one-third is in the unusual +3 state. Another example is BijSrjlCaQgYoyjCujOgjys, in which the average oxidation state of copper is +2.105 and the average oxidation state of bismuth is +3.090 (which is formally a mixture of Bi3+ and Bi5+). The most reliable means to unravel these complex formulas is through wet oxidation-reduction titrations, described in this chapter. 

A breakthrough in superconductor technology came with the discovery24 of yttrium barium copper oxide, YBa2Cu307, whose crystal structure is shown here. When heated, the material readily loses oxygen atoms from the Cu-O chains, and any composition between YBa2Cu307 and YBa2Cu3Ob is observable. 

When high-temperature superconductors were discovered, the oxygen content in the formula YBa2Cu3Or was unknown. YBa2Cu307 represents an unusual set of oxidation states, because the common states of yttrium and barium are Y3+ and Ba2, and... 

Metal alkoxides are of interest as precursors to oxide materials with ceramic or electronic applications.1 Barium incorporation in such a precursor is particularly interesting because of the known high-Tc superconductor, YBa2Cu307. The title compound has vast possibilities for reactions of the type... 

Since 1911, scientists have been searching for materials that superconduct at higher temperatures, and more than 6000 superconductors are now known. Until 1986, however, the record value of Tc was only 23.2 K (for the compound Nb3Ge). The situation changed dramatically in 1986 when K. Alex Muller and J. Georg Bednorz of the IBM Zurich Research Laboratory reported a Tc of 35 K for the non-stoichiometric barium lanthanum copper oxide BavLa2-.vCu04, where x has a... 

The 1-2-3 ceramic superconductor YBa2Cu307 has been synthesized by the sol-gel method from a stoichiometric mixture of yttrium ethoxide, barium ethoxide, and cop-per(II) ethoxide in an appropriate organic solvent. The oxide product, before being heated in oxygen, has the formula YBa2Cu3065. Write a balanced equation for the hydrolysis of the stoichiometric mixture of metal ethoxides. 

Something like that can go on in the new ceramic superconductors. In the yttrium-barium-copper oxide material, for example, if annealing—the heat treatment used to soften a material and make it more workable, and to relieve internal stresses and instabilities—goes on for too long, the ceramic begins to decompose if the annealing step is too short, it doesn t superconduct. 

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