Synthesis

WO3 can be synthesized into nanoparticles or thin films and then used in various applications. There's several different ways to synthesize WO3. WO3 nanoparticles can by synthesized by a 2-stage method^[5]. Firstly, ammonium tungstate pentahydrate was ball-milled and then it when through calcination process ^[5]. The ball-milling process took 8 hours and the calcination process was held in the electrical furnace where temperature was increased to 500 °C and held there for 30 minutes before cooling back to room-temperature ^[5].

In one study, WO3 thin films can be obtained for example by hydrothermal reaction ^[6]. Tungsten-source was an aqueous Na2WO4 and in acidic conditions, WO3 was synthesized by hydrothermal reaction at 180 °C ^[6]. Stainless steel was used as a substrate and it was found that when the Na2WO4 solution had pH value of 2.5, there was a hexagonal WO3 thin film formed while the sample was dried in an autoclave ^[6]. In other paper, a carbonized WO3 thin films were obtained by a 3-stage synthesis ^[7]. The 3-stage synthesis consisted of radio-frequency sputtering system, annealing and the carbonization process at high temperature

^[7].

Structure

WO3 owns a triclinic structure and its space group and structure type differ depending on the surrounding temperature^{[8, p.1105-1106]}

Table 2. Different structures of tungsten trioxide depending on temperature.

In Figure 1. is presented the structure of WO3 in under 17 °C where its crystal system is triclinic. 

Figure 1. The structures of WO3(aP32) (Figure: Eino Kilpeläinen)

In Figure 2. is shown the most common structure of WO3 because monoclinic crystal system appears in room temperature.

Figure 2. The structures of WO3(mP32) (Figure: Eino Kilpeläinen)

When temperature is between 330 and 740 °C, WO3 have an orthorhombic crystal system as seen in Figure 3. 

Figure 3. The structures of orthorhombic WO3. (Figure: Eino Kilpeläinen)

When temperature is increased to 740-900 °C, the crystal system changes quite radically into tetragonal and its number of formula units drops form 8 to 2 as shown in Figure 4.

Figure 4. The structures of WO3(tP8). (Figure: Eino Kilpeläinen)

Applications

As seen in Table 1. WO3 appears as a heavy yellow powder and it is also insoluble in water ^[1]. These properties makes WO3 as a great pigment used in ceramics ^[1]. WO3 has arose interest in various of fields due to its fascinating physical and chemical characteristics ^[5]. In Table 3. is shown some of the physical and chemical characteristics of WO3. WO3 is used as a ingredient to produce tungstates for x-ray screens ^[1]. WO3 owns suitable properties for different high-tech applications ^[5]. In one study about thin films, was described that tungsten oxides suit well as rechargeable lithium batteries in electrochemical devices ^[6]. With so fascinating properties, WO3 has been presented to fit for microelectronics, selective catalysis and environmental engineering ^[5]. 

Table 3. WO3 has a lot of interesting properties ^[5].

Metal-ceramics are used for example in dentistry because they meet the esthetic criteria and suit well under existing partial removable dental prosthesis ^[3] Metal-ceramics are so common in dentistry that they have 11 evaluation criteria to measure and compare their fit^{[9, p.168-170]}

^[10].

Characterization

In one study, SEM, X-ray diffraction and Fourier transform infrared were used to support the analysis of adsorption isotherm of densed monoclinic tungsten trioxide nanoparticles ^[5]. With the results of these characterization methods, the researchers could study how chemical and physical interactions occur during adsorption process ^[5]. In other research, a WO3 thin film and powder were characterized by SEM, XRD and TG measurements ^[6]. For example, with SEM the surface of WO3 thin film could be studied ^[6]. 

References