Tungsten oxide (WO₃) thin film is a kind of important functional material due to the advantages of WO₃ thin films have higher transmittance change, good reversibility, relatively low price, long service life and low toxicity. It is the earliest and the most widely used electrochromic materials. Tungsten oxide (WO₃) thin film can employ in buildings and smart window.

The preparation methods of tungsten trioxide thin film include sol gel method, sputtering method and chemical vapor phase method. It makes special introduction of sol gel method and sputtering method.

1.    Sputtering method

Sputtering method is an advanced method to prepare thin film which is characterized by high speed and low temperature, and the high density of the thin film.

It mainly refers to the ion source to generate ion. The ion increases and accumulates into high speed integrated ion in vacuum to bombard target surface. Then the ion and atoms of target surface undergoes kinetic exchange to make the atom of target surface leaving substrate surface, and finally forms into the film. Sputtering method is divided into direct current sputtering, radio frequency sputtering, magnetron sputtering, reactive sputtering, medium frequency sputtering, pulsed sputtering, bias sputtering and ion beam sputtering etc..

Direct current magnetron sputtering is prepared on silicon wafer with the thickness of tungsten oxide thin films 300nm. Specific methods: use 99.99% purity and the diameter is 10mm annular metal tungsten as target. The volume of argon and oxygen is based on ratio of 1:1 mixed gas as gas discharge. The pressure of discharge gas is 0.4~12Pa; the current is 50mA; the voltage changes with the pressure of the discharge gas; and the varying range is controlled at 540~420V. First close the sputtering chamber for five minutes before sputtering.

2.    Sol gel method

The principle of sol gel method is that the precursor（inorganic salt or metal alcohol salt）is dissolved in the solvent to prepare solution by mixing the solution. The hydrolysis product is formed by the condensation reaction to form nano particles and a sol. On the basis of sol, you can take a dip coating, spin coating, spray pyrolysis, screen printing and so on to produce tungsten trioxide thin films.

Mix tungstate powder and 30% H₂O₂ of WO₃ solution and stir continuously 2-4h to get  peroxotungstic acid (PTA). PTA solution heats and losses water into the precursor powder, and then calcinates four hour in the air at 600℃ to obtain WO₃ powder. The WO₃ powder is dissolved in the organic solvent made of printing paste. Finally use screen printing method in alumina ceramic substrate to prepare tungsten oxide thin films.

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