The high-temperature superconductors known as Perovskites are a mixture
of metal oxides which display the mechanical and physical properties of
ceramics. YBa2Cu3Ox, (YBCO) is
a very common Type II superconductor. A
key element to the behavior of these materials is the presence of planes
containing copper and oxygen atoms chemically bonded to each other. The
special nature of the copper-oxygen chemical bond permits materials to conduct
electricity very well in some directions. See Figure (15), a drawing of the molecular cell structure in
YBa2Cu3O7.
Most ceramic materials are considered good electrical insulators. YBCO compounds, also known as 1-2-3 compounds, are very sensitive to oxygen content. They change from semiconductors at YBa2Cu3O6.5 to superconductors at YBa2Cu3O7 without losing their crystalline structure. The high sensitivity of superconductors to oxygen content is due to the apparent ease to which oxygen can move in and out of the molecular lattice. Using the standard valance charges for the metallic elements, one would expect a formula of YBa2Cu3O6.5. However, it has been found that these superconductors usually have more oxygen atoms than predicted. According to the formula, YBa2Cu3O7, the metals are in a mole ratio of 1-2-3.
YBa2Cu3O7 was the first material found to be superconducting above liquid nitrogen temperature. It exhibits a very interesting and complex relationship between its chemistry, crystal structure and physical properties. A very subtle electronic charge balance exists between the one dimensional copper-oxygen chains, which have variable oxygen content, and the two dimensional copper-oxygen pyramidal planes, where superconductivity originates.
In oxygen deficientYBa2Cu3Ox, oxygen is removed from the CuO chains. A 90 K superconductor is obtained for 0<× <0.2, a 60 K superconductor for 0.3<× <0.55, and an antiferromagnetic semiconductor for 0.55<× <1.0. These changes in Tc as a function of x are shown in Figure (16), a graph of Tc versus oxygen content.
Since perovskites are ceramics, the procedure for making them is very similar to making other ceramics. All that is needed is a mortal and pestle, a die cast mold,a well-ventilated kiln or furnace and the necessary chemicals. Oxides, carbonates, and nitrates are good sources for the metals needed to make YBCO. The following recipe for making YBCO superconductors allows ambitious and outstandingly competant readers to make their own superconductors. Excellent quality commercially produced superconductors may be purchased at very reasonable cost through various vendors.
