The cubic arrangement described above with an atom in the center of a cube is typical for perovskites, a group of ceramic materials with a variety of interesting electrical properties. The high--temperature superconductors are among these. In the cubic phase, tungsten bronzes are metallic and conduct electricity. However,in the hexagonal phase, they become superconductors. William Moulton, at Florida State University in Tallahassee, has done a lot of work with potassium, rubidium and cesium tungsten bronze superconductors. Dr. Moulton points out that these differences in properties depending on the direction of measurement in the crystal. The temperature at which a material becomes superconducting, of about 6K.
Iowa State University in Ames was another center for tungsten bronze research. There, Douglas Finnemore studied the effects of pressure on the transition temperature of potassium tungsten bronze. The object was to enhance the interaction between electrons and the lattice vibrations, or phonons. However, these tungsten bronzes were still superconductive at only 4K.
Howard Shanks, also at Iowa State, was able to produce sodium tungsten bronze compounds that were superconductors at as high as 10K. Part of his success was due to techniques he developed tp grow large crystals of this material, some as large as 3 inches. Dr. Shanks finds it ironic, in light of today’s superconductor research, that one of the reasons why work on tungsten bronze was dropped was because so many saw no future in oxide superconductors.
Other work at Iowa State has included using sodium tungsten bronze as a coating for one of the electrodes in a fuel cell that used hydrogen and oxygen as fuel to produce electricity. The test cell that was built ran for about a year. Another application that was investigated was using tungsten compounds for hydrogen storage. It was found that for HxW03 with x<0.5 hydrogen could move in and out of the material with ease. Some of this work was also done in Germany.
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