Industrial grade yellow tungsten oxide is a yellow powder with purity higher than 99.95% and an average particle size of less than 100nm, which can be prepared by ammonium paratungstate calcination method. The purity of yellow tungsten oxide depends on the purity of ammonium paratungstate (APT). The crystal form of yellow tungsten oxide inherits the crystal form of APT. Therefore, by controlling the crystal form and purity of APT, yellow tungsten oxide with desired crystal form and purity can be obtained.
APT Calcination Method to Produce Yellow Tungsten Oxide
Calcinate APT in the air, after it reaches to certain temperature, the decomposed material is yellow tungsten oxide. When calcinating APT in the air, it will have different phases under different temperature. When it reaches 500°C, it converts to yellow tungsten oxide. 
Adopt differential thermal analysis (DSC) and X-ray diffraction (XRD) to analyze the process of APT calcination. It was found that the phase change of APT in air is:
1. APT converts to AMT if temperature is lower than 240°C.
2. It converts to amorphous substance (ammonium tungsten bronze) if it reaches 300~350°C.
3. Hexagonal structure of WO3 is obtained if it goes up to 400°C.
4. Triclinic crystalline model of yellow tungsten oxide is obtained if it heats up to 500°C.
More details, please visit:
http://www.yellow-tungsten-oxide.com/index.html
