Blue tungsten oxide (BTO) is a kind of transparent thermal insulation nanopowder. “Transparent” is used because it is transparent when it is applied in the thermal insulation films. The thermal insulation characteristic of blue tungsten oxide is determined by its special structure, which makes it has photochromic properties.
When blue tungsten oxide is prepared by reducing ammonium paratungstate (APT), the phase transformation of BTO is as follows:
APT→WO3+ammonium tungsten bronze→WO2.90→WO2.72
The products in this phase transformation process are all blue in color. In other words, they are all blue tungsten oxide. But, the phase composition of these blue tungsten oxides varies with the change of reduction temperature. It can be seen that blue tungsten oxide is a mixture of intermediate oxides of various valence states of tungsten with variable composition, and blue tungsten oxide with different phase compositions may be prepared by controlling the reduction process conditions.
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