The high-temperature superconductors known as Perovskites are a mixture of
metal oxides which display the mechanical and physical properties of
ceramics. YBa Cu 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 YBaCuO .
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 YBaCuO  
to superconductors at YBaCuO
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
YBaCuO.
However, it has been found that these superconductors usually have more
oxygen atoms than predicted. According to the formula, YBaCuO,
the metals are in a mole ratio of 1-2-3.
YBaCuO
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 YBaCuO  ,
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. |
