The high-temperature superconductors known as Perovskites are a mixture
of metal oxides which display the mechanical and physical properties of
ceramics. YBaCu
Ox, (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 YBa
Cu
O
.
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
YBaCu
O
to superconductors
at YBa
Cu
O
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
YBa
Cu
O
. However, it has been found that
these superconductors usually
have more oxygen atoms than predicted. According to the formula,
YBa
Cu
O
, the metals are in a mole ratio of 1-2-3.
YBaCu
O
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 deficient YBaCu
O
,
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 T
as a
function of x are shown in Figure
(16), a graph of T
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.
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