To make an yttrium-barium-copper-oxide superconductor, you will need:
* Yttrium Oxide
* Barium Carbonate (TOXIC)
* Cupric Oxide
* A Laboratory Furnace or a converted pottery kiln.
* Labware made of alumina.
* An Oxygen Source
* Liquid Nitrogen and a rare-earth magnet for testing and demonstrating the superconductors
There are a number of methods of producing ceramic superconductors like this, but the simplest is the so-called “shake and bake” method, which involves a four step process:
1. Mixing the chemicals;
2. Calcination(the initial firing);
3. The intermediate firing(s) (oxygen annealings);
4. The final oxygen annealing.
The number of intermediate firings and the length of the firings are largely up to the user. In general, the more intermediate firings, and the longer the duration of the firings under oxygen flow, the better the superconductor. But definite signs of superconductivity can usually be obtained without any intermediate firing at all. In fact, if the initial mixing of the chemicals is sufficiently thorough, the intermediate firing is not necessary at all.
1. Mixing the chemicals
The starting mix is a grey powder made by thoroughly mixing yttrium oxide, barium carbonate and cuprix oxide in the ratios 1:2:3 (This superconductor is often referred to as “1-2-3″ as a result) –
* Yttrium Oxide, Y2O3 – 11.29 grams
* Barium Carbonate, BaCO3 – 39.47 grams
* Cupric Oxide, CuO – 23.86 grams
For the initial heat treatment, called calcination, the mix is heated at 925-950 degrees Celsius for about 18-24 hours. This first treatment may be done in a crucible or evaporating dish made of alumina or of a good grade of laboratory porcelain. This forms the basic crystal structure of YBa2Cu3O6.5, and gets rid of the carbon dioxide from the barium carbonate. (Barium carbonate is used instead of barium oxide because barium oxide of any reasonable purity is difficult to obtain. Also, exposing barium oxide to air tends to quickly convert much of it to barium carbonate and barium hydroxide.) The result of this first firing is a porous black or very dark gray clump. The coloration should be fairly even.
An uneven green coloration is an indication that the powders are not as thoroughly mixed as they should have been, and that extra time and care should be taken to insure thorough grinding and mixing on subsequent steps. The material will seem to shrink rather dramatically during the initial firing as it loses its carbon dioxide and becomes much denser than the original powder mix.
3. Intermediate firing(s)
The porous black clump is ground into a fine powder and placed in the furnace in an alumina dish. After the furnace temperature reaches about 500 degrees Celsius, begin a slow flow of oxygen into the furnace. This heat treatment under oxygen flow is called oxygen annealing. A final furnace temperature of 925 to 975 degrees Celsius is recommended for the intermediate firings. A temperature much higher than this will result in a material that is much harder to re-grind. Temperatures above 1030 degrees Celsius may destroy the crystal structure.After the mix has heated in the furnace for at least 18 hours at 925-975 degrees Celsius, reduce the temperature slowly. If you plan to test the sample for superconductivity after this firing, the cooling rate must be no more than 100 degrees per hour until 400 degrees Celsius is reached. The rate of cooling from 400 degrees down to room temperature can be increased to about 200 degrees per hour. If you do not plan to test for superconductivity after this firing, a cooling rate in excess of 100 degrees per hour may be used; however a cooling rate in excess of 250 degrees per hour is not recommended. Do not remove the oxygen flow until the indicated furnace temperature has fallen below 400 degrees Celsius.
The material should be thoroughly re-ground in a mortar and pestle (or similar device) between each firing. (If, after an intermediate firing, there is some green coloration in the resultant disk, it is important to take extra time and care in re-grinding and mixing the material before the next firing.) Problems that occur in the mixing and grinding process in any of these steps are often due to hard, coarse particles being mixed in with the finely powder material. An ordinary kitchen tea strainer can come in handy at this point to separate the coarser particles or lumps so they may be ground separately. IMPORTANT: If you an ordinary tea strainer, make sure it is made of a non-magnetic material, or make sure you are satisfied that none of the material in the sifter or strainer will contaminate the chemicals.
Even very small quantities of magnetic materials in the chemical mix can diminish or destroy the potential superconductivity. (It is also for this reason that “ceramic grade” chemicals, which tend to have iron impurities, are not often usable for making superconductors.) Shortcuts in grinding the materials, such as using an electric coffee grinder, often contaminate the compound with elements that destroy the superconducting properties. Some contaminates will destroy superconductivity in very tiny amounts. To keep your chances of success high, grinding with a good-quality mortar and pestle is the best method. This manual grinding can be an arduous process, but the results are worth the trouble.
4. The final oxygen annealing
The sample should be thoroughly reground, and the resultant black powder placed back in the alumina dish. The thickness of the layer of loose powder in the dish should match the desired thickness of the final superconducting disk. For this final firing, the powder should be as finely-ground and as densely-packed as possible. Do NOT pack the powder into the dish by pressing on it from the top (as this can makes the superconductor tend to stick to the alumina dish). Better results can usually be obtained by tapping the alumina dish with a pestle or a similar object so that the particles of the mix settle together in an evenly packed disk.
For this final heat treatment, heat the sample to between 950 degrees and 1000 degrees Celsius for about 18 hours. The higher temperature is better, but be sure of the accuracy of your temperature indicator before getting too close to 1000 degrees. Temperatures above 1020 degrees risk decomposition of the crystal structure and the possiblity of the material sticking to the alumina dish. On the other hand, a final oxygen annealing at only 950 degrees Celsius will yield a superconductor that will crack easily, but will otherwise be satisfactory.
It is absolutely necessary that the cool-down take place very slowly and under adequate oxygen flow. The rate of cooling must be no more than 100 degrees Celsius per hour, especially during the critical temperature region between 750 and 400 degrees Celsius. Take special care to insure that the sample has access to plenty of oxygen, especially in during the cool-down from 900 to 300 degrees. Brief interruptions in oxygen flow when the material is above 900 are unimportant, but continuous flow must be maintained during cool-down. If the atmosphere in the furnace is not oxygen-rich while the sample is still above about 400 degrees, the material can lose vital oxygen from its crystal structure. After the furnace temperature reaches about 500 degrees, the rate of cooling can be increased.
During this final heat treatment, a superconductor that is more resistant to cracking during thermal stress can be produced by subjecting the sample to high-temperature thermal cycling. To do this, vary the temperature between 750 and 1000 degrees at rates of change of about 200 per hour. Then raise the temperature to about 1000 degrees for an hour or more before beginning the final slow cool-down. This thermal cycling is not a necessity at all, but it will add significantly to the mechanical strength of the sample.
Testing the superconductor
The most foolproof test for superconductivity is the simplest. This is the test for diamagnetism using small rare earth magnets made of samarium-cobalt or neodymium-iron-boron.
Use a very small rare-earth magnet at first. Start with a rare-earth disk magnet about 6 mm. in diameter. If you have made a good-quality superconductor, the magnet will levitiate at least 3 mm. above the surface of the superconducting disk. A superconductor with poor levitation can usually be improved by re-grinding it and giving it an additional oxygen annealing.
When a superconductor levitates a magnet, a magnetic mirror image is formed in the superconductor of the levitating magnet due to the exclusion of the magnetic field (the Meissner effect). The magnetic mirror image insures that there is always a north pole induced in the superconductor directly below the north pole of the levitating magnet.
There is a south pole induced in the superconductor directly below the south pole of the levitating magnet. This mirror image moves with the magnet as the magnet is moves, so that the disk magnet can be given a rapid spin without affecting its levitation. In fact the magnet may continue to spin for quite a long time because its spinning encounters no friction other than the friction of air resistance.