As we all know, annealing, or thermal, treatment is one of the most effective ways to influence the structure and the properties of many films. To investigate the effects of the annealing temperatures on the structure and optical property of tungsten oxide thin films that were deposited by magnetron sputtering of WO3 bulk in a vacuum, the deposited films were annealed at 200℃ and 300℃ for 60 min and at 400℃ for 60 min and 180 min in air, respectively. We find that the annealing temperature of 400℃ can effectively influence the structure and optical property of the deposited films. The films annealed at 200℃ and 300℃ display violetred under the sunlight as well as the as-deposited ones. Nevertheless, the film annealed at 400℃ exhibits transparency and appeared to be blue colored.

 

 

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Several preparation techniques have been used to deposit tungsten oxide thin films, including sol-gel, hydrothermal technologies, chemical vapor deposition, thermal evaporation, pulsed laser deposition, and magnetron sputtering. In fact, reactive magnetron sputtering is one of the most versatile techniques to deposit oxide films, allowing the control of many properties of the films by changing the partial pressure of the reactive gas, the components with the different targets and even sputtering power. Many reports have been published explaining the mechanisms involved in thin film growth using magnetron sputtering, but very often these refer to tungsten oxide thin films obtained from a metallic tungsten target. In contrast, the reports on tungsten oxide thin films obtained from a WO3 target are seldom reported. In 2009, Acosta et al. reported the studies on the optical properties of tungsten oxide thin films by non-reactive sputtering of WO3 target and found that the argon pressure had a strong influence on the optical properties of the film. It has been predicted that a large amount of oxygen vacancies, defects and disordered structures in the deposited films, which are very critical in determining the crystalline, optical, photochemical and electric properties of tungsten oxide thin film, would be produced when the sputtering plasma interacts with WO3 bulk.

 

 

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In the past few years, an increasing interest has been put on the tungsten oxide films due to their potential applicanons in smart windows, gas sensors, photocatalytic reactions, and optoelectronic devices, etc. The unique properties of the tungsten oxide films were usually determined by the oxygen defects and the valence states of tungsten ions. Besides, it is very important to improve the properties of tungsten oxide films and other oxide films by controlling their morphologies and crystalline phases, which mainly depend on the preparation technologies and thermal treating temperatures and atmospheres.

 

 

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  (1)The nanoscale tungsten powder with an average particle size of 19 nm was obtained by gradually reducing WO3, which was prepared by ultrasonic spray and thermochemical conversion process, into WO2.90 and W.

 

  (2)Compacting with a rubber die is a preferred method for making a tungsten green compact with the nanoscale powder, whereby a relative density of 46.2% was achieved.

 

  (3)The compact made from the nanoscale tungsten powder starts shrinkage at 1050℃, and the shrinkage rate is maximum at 1210℃. The shrinkage almost ends at 1500℃. In the case of the traditional tungsten powders, however, the compacts barely shrink.

 

  (4)After sintered in hydrogen at 1500℃, tungsten samples with a relative density of 96.4% and a particle size of 5.8 pm were prepared. As for the traditional tungsten powder, the relative density is only 68.2% after sintering.

 

 

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Nanoscale tungsten powders were compacted with a steel die and a rubber die respectively. In the case of compaction with a steel die, the relative density of the resulted compact is 28.3% when the unit compacting pressure is 200 MPa. When the compacting pressure was higher than 200 MPa, some defects such as delamination and fracture were observed in the compacts.

 

The compacting pressure for a rubber die can be up to 300 MPa, resulting in a compact free of detects with a relative density of 46.2%. Even with a compacting pressure of 200 MPa, the relative density of the compact can also be up to 44.0%, which is remarkably more than that obtained by compacting with a steel die. It shows that the green density of nanoscale tungsten powder can be greatly improved by compacting with a rubber die.

 

 

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Blue tungsten oxide WO2.90 was prepared by reduclion of nanosclae WO3 at 500℃ for 40 min in hydrogen. Subsequently, the resulted blue tungsten oxide was reduced at a series of temperatures for 60 min to get tungsten powders.

 

The average particle size of tungsten powder reduced at 720℃ is 19 nm, which is much less than that of conventional tungsten powder. The particle size of tungsten powder increases with increasing the reduction temperature. In addition, the tungsten powder reduced at 680℃ is so fine that it self-ignited after taken out from the reduction furnace. Nanoscale tungsten powders reduced at 720℃ and 780℃ keep stable in the air. Therefore, 680℃ can not be applied as a reduction temperature. The nanoscale tungsten powder should be stored in vacuum bags or bags filled with inert gas.

 

 

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The melting temperature of tungsten is up to 3410℃, which is the highest one in metals. Furthermore, tungsten has an excellent strength at high temperature. With these characteristics, tungsten is very useful for applications at high temperature. With the development of tungsten and tungsten-based alloys (e.g., W-Cu, W-Ni/Fe), there exit requirements for uniform mcrostructure, superfine grains and good plastics. However, conventional techniques can not meet these requirements.

 

The nano-related technology can provide a new process for producing ultrafine grain materials. In recent years, ultrafine grain cemented carbides with high performance can be prepared using nanoscale WC powder, and nanoscale WC powder was prepared by reduction carbonization of the nanoscale WO3 powder.

 

 

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At present, micrometer tungsten powder was used for the production and research of tungsten materials. This type of tungsten powder is sintered at a temperature higher than 2000℃ in order to be densified. Thereafter, it can be further processed. However, tungsten grain size increases quickly during the sintering, and may be up to several hundreds micrometers. These coarse tungsten grains lower the mechanical and physical properties as well as rolling characteristic. Nanoscale powder has more surface energy and can be densified more easily than micrometer powder. Nanoscale WO3 powder was prepared from the APT (ammonium paratungstate) solution by the ultrasonic spray thermo-conversion process, and was then reduced in hydrogen. The reduction was canned out in two steps. WO3 was first reduced to the blue tungsten oxide WO2.90, and then WO2.90 was reduced to tungsten. In the above process, nanoscale WO3, WO2.90 and tungsten powders were milled by the high energy shear miller. The specific surface area of these nanoscale powders was measured by an ST-03 specific surface area tester, by which an average particle size was calculated. The powder shape was observed with an H-800 TEM (transmission electron microscopy). Nanoscale tungsten powder was formed with a rubber die and a steel die respectively. The compact density was measured based on the Archimedes method. Nanoscale tungsten compact was sintered in hydrogen at a temperature elevating rate of 10℃/min with a high temperature dilatometer. Furthermore, conventional tungsten powder with an average particle size of 2.5 um was compacted and sintered to compare with the nanoscale tungsten powder.

 

 

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Chemical Formula：WO3
Molecular weight: 231.85

Density: 7.16 g/cm3

Tungsten outer electron configurations: sd46s

melting point: 1473 ℃
Solubility: insoluble in water, insoluble in acid, slightly soluble in HF, soluble in hot alkali, ammonia

Hazardous characteristics: violently reacts with halogen compounds such as bromine pentafluoride, chlorine trifluoride
Hazardous combustion products: harmful toxic fumes

 

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Tungsten trioxide films have been shown to have applications as the sensing element in gas sensors. One of the most important part of the gas sensor is gas sensitive element. The sensor's sensitivity, selectivity, to a large extent dependent, depends on the performance of gas sensitive element. The general gas sensor is thin film material. Study on tungsten trioxide films began 20 years ago, most is on the electrically induced discoloration, photochromic, electrochemistry and gas sensor performance. For hydrogen sensor, tungsten trioxide films have enormous potential. When tungsten trioxide contacts with hydrogen, the most change is the film colour from transparent to blue. At the same time, changes have taken place in film measured by optical instruments. So tungsten trioxide films are good hydrogen sensitive materials. After doping of rare metal, such as platinum and palladium, hydrogen sensitive properties of tungsten trioxide films can greatly improve. Tungsten trioxide films not only can be used for hydrogen sensor, after doping different elements in different conditions, can also be used for other gases sensitive materials, such as NH4, NO2, H2S, O3 and O2.

Tungsten trioxide films material is a kind of wide application of functional materials, has the very broad application prospects. This material can be used in electrically induced discoloration electrochromics, smart Windows, etc. In different field of application of the film materials have different requirements.

 

 

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