Ammonium Metatungstate Prepares Cesium Tungsten Bronze Ultrafine Powder
Tungsten(VI) oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygenand the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce WO3. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal. Tungsten(VI) oxide occurs naturally in the form of hydrates, which include minerals: tungstite WO3•H2O, meymacite WO3•2H2O andhydrotungstite. These minerals are rare to very rare secondary tungsten minerals. Tungsten has a rich history dating back to its discovery during the 18th century.
WO3 has excellent performance in electrochromic, catalysis, gas sensing and other aspects, the composite oxides – tungsten bronze is a kind of metal oxides generally with deep metallic luster color, and usually a metal conductor or a metal semiconductor, the formula of tungsten bronze is AxWO3, but with the change of cation kind and the different number of moles （different value of a and x）, the structure and color depth of the compounds may vary. Currently, the researches are more about potassium cesium bronze tungsten and cesium tungsten bronze tungsten, because of the low resistance and excellent transmittance of visible and near-infrared shielding properties of cesium tungsten bronze tungsten, it’s widely used in the preparation of the conductive thin film used in a transparent glass partition thermal insulation coatings as agent.
The tungsten bronzes are a very interesting, but little appreciated, family of materials. They are not related to bronze, an alloy of copper and tin, except coloration. However, the structure of tungsten bronzes are similar to the high-temperature copper oxide superconductors. In fact, the tungsten bronzes were the first oxide superconductors and were the focus of extensive research 10-15 years ago. But by the early 1980s, most of this work had been set aside in favor of other pursuits.
The tungsten bronzes are a group of compounds made up of tungsten trioxide, WO3, and an alkali metal, such as sodium (Na) , potassium (K), rubidium (Rb) , or cesium (Cs). The general chemical form is MxW03, where M=Na, K, Rb, or Cs, and O<x<l. The color of these compounds varies with composition, at x=0.93 the color is a bronzelike golden-yellow, hence the name; at x =0.32 the color is a blue-violet. For this reason tungsten bronzes are use as pigments in dyes allld paints.
The variation in composition also affects the structure of the compound. Imagine a ¢ube with a tungsten atom at each comer, an oxygen atom in the middle of each edge and an atom of an alkali metal in the center of the cube. However, in a tungsten bronze there is not an atom at the center of every cube. When x< 1, only a certain fraction of the cubes will contain an alkali atom. If x is large, close to 1, the structure of the crystal lattice will be cubic. As x decreases, and fewer of the cubes are filled, the structure changes. At about x<0.3, or with less than 30% of the cubes full, the structure becomes hexagonal, with atoms arranged in hexagonal plates.
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.
There was many methods to prepare cesium tungsten bronze, for example, a cesium tungsten bronze powder preparation method, the use of tungstate tungsten source, Cs / W molar ratio of 1: (2.857 100), at 180 ~ 200 ° C reaction conditions 1 ~ 3 days. The method for preparing a long reaction period, low efficiency, is not conducive to industrial production.
The present invention is directed to the above-mentioned deficiencies in the prior art, there is provided a method for preparing save raw materials, short reaction period cesium tungsten bronze ultrafine powders.
A method for preparing a cesium tungsten bronze ultrafine powder, comprising the steps of: (1) in accordance with Cs / W molar ratio of 1: (1.5 to 2.8) was weighed cesium salt, a tungsten salt, a chelating agent and an alcohol reagent, under conditions of 170 ° C reaction 3h; mixture (2) in the step (1) was charged into a pressure vessel bomb reacted at 260 ~ 270 ° C reaction conditions 5 ~ 8h; (3) the step (2) a reaction product obtained by ethanol washing, centrifugation, at 80 ° C and dried in vacuo to give crystals of the condition of complete crane cesium powder.
On the basis of the above method, the present invention also can do the following improvements.
Further, the tungsten salt of the step (1) is ammonium tungstate, ammonium paratungstate or ammonium metatungstate; the cesium salt is cesium nitrate or cesium hydroxide;
With the above further aspect, the beneficial effect is that the tungsten salts used contain NH4 +, not only to provide a tungsten source, and ammonia at high temperatures generated by the reaction can be used as a reducing atmosphere to protect its color stability, the resulting cesium tungsten bronze powder is obtained.
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