Here again make some additional explanation:

(1) In fact, tungsten trioxide tungsten powder is not reduced to one step, took a three-tungsten oxide crystal, the reduction when it was first reduced to WO2, then surface WO2.9 is further reduced to WO2.72 In this case the crystal or WO2.9, while continuing to restore, and then into WO2, finally being completely reduced to tungsten powder. So, when the tungsten trioxide reduction, the surface and the interior of the restore process is not synchronized. The crystal tungsten trioxide necessary reduction resulting H2O spread to the outside through the outer layer of crystalline tungsten restored, in this slow diffusion process can easily produce the crystal fracture, in times of higher electron microscope you can see these photos crack.

 

 

(2) After reduction of tungsten powder to be oxidized again, they must first adsorption must then react with oxygen. After imagine, the crystal surface tungsten powder is very easy to absorb oxygen, to be oxidized very simple, but the tungsten oxide crystal powder inside, you have to wait for the oxygen atoms diffuse into the crystal after, or through cracks in the surface of the tungsten powder absorb large amount of oxygen atoms to react. This process is very slow. When the internal crystal tungsten powder is oxidized tungsten trioxide, its inevitable than the original volume tungsten powder should be large, so that it is possible to hold a larger volume of crystals cracking, whole grain to grain is smaller than the original.

 

(3) With regard to grain reunion. The higher activity of smaller crystal particles, the closer proximity of the crystal more easily attracted to each other, easier to form aggregates. Tungsten trioxide particles obtained in the last step has been very small, it is possible to obtain sub-micron scale tungsten trioxide powder, to achieve the desired effect experiments.