Superconductor YBCO

Yttrium barium copper oxide, often abbreviated YBCO, is a family of crystalline chemical compounds, famous for displaying "High-temperature superconductivity". It includes the first material ever discovered to become superconducting above the boiling point of liquid nitrogen (77 K) at about 90 K. Many YBCO compounds have the general formula YBa₂Cu₃O_7x (also known as Y123), although materials with other Y:Ba:Cu ratios exist, such as YBa₂Cu₄O_y (Y124) or Y₂Ba₄Cu₇O_y (Y247).

All materials developed before 1986 became superconducting only at temperatures near the boiling points of liquid helium (T_b = 4.2 K) or liquid hydrogen (T_b = 20.28 K) — the highest being Nb₃Ge at 23 K. The significance of the discovery of YBCO is the much lower cost of the refrigerant used to cool the material to below the critical temperature.

…. the properties of YBCO are influenced by the crystallization methods used… However, new possibilities have been opened since the discovery that trifluoroacetic acid (TFA)), a source of fluorine, prevents the formation of the undesired barium carbonate (BaCO₃). Routes such as CSD (chemical solution deposition) have opened a wide range of possibilities, particularly in the preparation of long length YBCO tapes. This route lowers the temperature necessary to get the correct phase to around 700 °C. This, and the lack of dependence on vacuum, makes this method a very promising way to get scalable YBCO tapes.

Although YBa₂Cu₃O₇ is a well-defined chemical compound with a specific structure and stoichiometry, materials with fewer than seven oxygen atoms per formula unit are non-stoichiometric compounds. The structure of these materials depends on the oxygen content. This non-stoichiometry is denoted by the YBa₂Cu₃O_7-x in the chemical formula. When x = 1, the O(1) sites in the Cu(1) layer are vacant and the structure is tetragonal. The tetragonal form of YBCO is insulating and does not superconduct. Increasing the oxygen content slightly causes more of the O(1) sites to become occupied.

Applications

Many possible applications of this and related high temperature superconducting materials have been discussed. For example, superconducting materials are finding use as magnets in magnetic resonance imaging, magnetic levitation, and Josephson junctions. (The most used material for power cables and magnets is BSCCO.)

YBCO has yet to be used in many applications involving superconductors for two primary reasons:

• First, although single crystals of YBCO have a very high critical current density, polycrystals have a very low critical current density: only a small current can be passed while maintaining superconductivity. This problem is due to crystal grain boundaries in the material. When the grain boundary angle is greater than about 5°, the supercurrent cannot cross the boundary. The grain boundary problem can be controlled to some extent by preparing thin films via CVD or by texturing the material to align the grain boundaries.
• A second problem limiting the use of this material in technological applications is associated with processing of the material. Oxide materials such as this are brittle, and forming them into wires by any conventional process does not produce a useful superconductor. (Unlike BSCCO, the powder-in-tube process does not give good results with YBCO.)

It should be noted that cooling materials to liquid nitrogen temperature (77 K) is often not practical on a large scale, although many commercial magnets are routinely cooled to liquid helium temperatures (4.2 K).

 

http://en.wikipedia.org/wiki/Yttrium_barium_copper_oxide