HOW ARE THESE SUPERCONDUCTORS MADE?

This article uses a knowledge of superconductor chemistry to outline a process by which one can make superconductor disks similar to those in the Kits. This outline is not meant to suggest that the reader should attempt to make these superconductors without adequate supervision. There are various procedural and safety considerations which are not recorded here.

Since the new superconductors are ceramics, the technique for making them is quite similar to making other ceramics. Besides the precursor chemicals, all that is needed is a mortar and pestle, a die mold, and a well-ventilated kiln or furnace that can reach a temperature of about 1700 degrees Fahrenheit (975 degrees Celsius).

Using the fabrication of YBa2Cu3O7 as an example, the first step is to measure out the correct proportions of the precursor source chemicals. These proportions are described in the article, The Chemistry of Superconductors, where the elements yttrium, barium, and copper are shown to be in the mole ratios of 1 to 2 to 3 respectively. Oxides, nitrates, or carbonates are excellent sources of these elements. Care must be exercised in selecting laboratory grade chemicals for their purity. The chosen source chemicals are carefully weighed out so that the three elements are in the 1:2:3 mole ratios as described. The three powdered chemicals are then carefully mixed and ground together using the mortar and pestle. The well-powdered mixture that forms is a deep gray in color.

The resultant mixture of chemicals is then poured into the type of die mold which can be found in a machine shop. A pressure of about 20,000 pounds per square inch will produce a compressed disk from the powder. It is very important to design the mold carefully, otherwise when under pressure, it could fail mechanically.

Next, the pressed disk is fired in an electric furnace or kiln. The kiln should be capable of attaining a temperature of about 1700 degrees F, and must be well ventilated. In addition, the kiln should also have a clean shelf on which to place the pressed disk.

The pressed disk is baked in the kiln at 1700 degrees F overnight. Next, the kiln is gradually cooled to near room temperature over the course of a day. When the kiln and disk are about at room temperature, the disk can be withdrawn. It is important to circulate fresh air through the kiln during the heating and cooling cycles. Best results are obtained if the samples are slowly cooled surrounded by an oxygen atmosphere. Also, several heating and cooling cycles appear to improve the quality of the 1-2-3 ceramic superconductors. If the ceramic disk is ground into a fine powder and then pressed into a disk again between each cooling and heating cycle, an even better sample results.

The resulting disk should be hard and black. Any green color will indicate that the disk will be an insulator instead of a superconductor.

The bismuth-based, Bi2Sr2Ca2Cu3O10, requires a lower baking temperature, but the duration can be as long as 5 to 10 days. The superconductive properties are found to improve the longer the material is baked. Furthermore, it is believed that this superconductor has two distinct phases with different critical temperatures. To preferentially promote the phase with the higher critical temperature (110 Kelvin), a small amount of lead is introduced into the precursor mixture.

Information Courtesy of CSI Superconductors