| The formation of tungstic acid a nd the growth of tungsten oxides (mainly trioxide) by firing were studied with electronmicroscope, X-ray powder method and B. E. T. method. A white precipitate of tungstic acid was observed from an ammonium tungstate solution when dilute hydrochloric acid was used, on the other hand a yellow precipitate tungstic acid was obtained wnen concentrated hydrochloric acid was used. X-ray analysis showed that the crystal structure of the white tungstic acid was different from that of the yellow tungstic acid. The particles of white tungstic acid were smaller than those of yellow tungstic acid. Yellow tungstic acid obtained from calcium tungstate had the largest particle size, and was in a state of a spherical aggregate composed of needle crystals resembling to the calcium tungstate particles. These precipitates and ammonium paratungstate crystals were fired at various temperatures, 300, 400, 500, 600, 800, 1000 and 1230℃ for 2 hours. In electronmicroscopic method and B. E. T. method it was found that the sizes of particles were affected by the sizes and crystal structures of the tungstic up to 400℃. At 800℃ the particle size observed in the electronmicroscopic method was slightly affected by the particle size of the tungstic acid, whlie the specific surface measured by B. E. T. method had the same values. In the case of precipitate containing a trace of potassium, the growth of the praticles was promoted at all temperatures. The particles obtained from white tangstic acid, calcium tungstate and ammonium paratungstate were uniform in their size, but the particles from yellow tungstic acid were not uniform. When ammonium paratungstate was decomposed in vacuum, tungsten oxide was produced, the growth of the particles was quite different from that of tungsten trioxide particles. |
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