The Chemistry of Ceramic Superconductors
The new ceramic superconductors are a class of minerals collectively called perovskites. Perovskites are metal oxides which exhibit a stoichiometric ratio of 3 oxygen atoms for every 2 metal atoms. Perovskites are typically mixtures of several different metals. For example, in the YBa2Cu3O7 superconductor in all of the Kits, the metals are yttrium (Y), barium (Ba), and copper (Cu). Using the standard valence values for these metallic elements, one would expect the chemical formula YBa2Cu3O9. Surprisingly, scientists have found that this superconductor has about 2 oxygen atoms less than predicted, and instead has the approximate formula YBa2Cu3O7.
One should note that the proportions of the 3 different metals in the YBa2Cu3O7 superconductor are in the mole ratio of 1 to 2 to 3 for yttrium to barium to copper respectively. Thus, this particular superconductor is often referred to as the 1-2-3 superconductor. For the bismuth-based superconductor, the chemical formula is Bi2Sr2Ca2Cu3O10. Unlike the YBCO, This material has two separate superconducting phases above 77K, Bi2Sr2Can-1CunO2n+4, for n=2, 3. The crystal phase that corresponds to n=2, has a Tc of about 84K, and the crystal phase that corresponds to n=3, has a Tc of about 110K.
The 1-2-3 ratio in YBa2Cu3O7 is also an indication of the simple ratio required of the elements in the constituent original chemical precursors to make this superconductor. So, for example for making YBa2Cu3O7, three separate chemical compounds containing yttrium, barium, and copper respectively are mixed in proportions such that the three metals are in the ratio of 1-2-3. The resulting mixture is then heated and cooled several times in a kiln or electrical furnace, usually in the presence of oxygen. The amount of oxygen in the 1-2-3 compound can vary depending on the way it was made. If the sample is short on oxygen, it will be green, and will not be a superconductor. If on the other hand it has the right amount of oxygen, it will be black. This black material is a superconductor. Bi2Sr2Ca2Cu3O10 can be made in a very similar manner.
Scientists are discovering that many other metals may be substituted for the ones in our example. In fact in 1988, scientists in Arizona found a compound of thallium, barium, calcium, and copper that is perhaps an even better superconductor than Bi2Sr2Ca2Cu3O10. Unfortunately, the element thallium is very toxic, and cannot be supplied for classroom work. Since then, compounds substituting lead or vanadium for copper have been discovered.
The perovskites are ceramics, and thus share many properties with other ceramics. One of these properties is their brittleness. This has particularly bedeviled technologists because it makes it very difficult to make, for example, the flexible wires that are needed for many practical applications.
Information Courtesy of CSI Superconductors