Formation conditions of hexagonal ammonium tungsten bronze during thermal decomposition of ammonium paratungstate

To define the optimum formation condition of ammonium tungsten bronze (ATB) which is very important intermediate for production of ultrafine tungsten, thermal decomposition of monoclinic and orthorhombic ammonium paratungstate (APT) was studied by thermogravimetry and differential thermal analysis in various atmospheres. The intermediate has been investigated by X-ray diffraction analysis and nitrogen measurement. ATB was formed in hydrogen, in a mixture of nitrogen and ammonia, in nitrogen, in argon, in air and in oxygen atmosphere respectively. The amount of ATB formed during thermal decomposition of APT depended on the degree of proceeding of thermal decomposition near by 630 K. In case of much sample amount, reaction rate was reduced due to the endothermic reaction related to thermal decomposition of ammonia and water in crystallization. A small amount of ammonia was necessary for a stable formation of ATB, and the nitrogen content was above 0.8% in single-phase ATB.