With the continuous development of the tungsten product industry, manufacturers have increasingly higher requirements for their quality and performance. As one of the important raw materials for tungsten products, the chemical purity, fluidity, bulk density, FSSS particle size, specific surface area and other properties of tungsten oxide will greatly affect the performance of tungsten products. Therefore, it is necessary to study the microstructure of tungsten oxides.
All tungsten oxides are made from APT (ammonium paratungstate) and calcined. Due to the different calcination conditions, the obtained tungsten oxide powder is also different, which may be yellow tungsten oxide (WO3), blue tungsten oxide (WO2.98) or purple tungsten oxide (WO2.72).
Yellow tungsten oxide, abbreviated as yellow tungsten, is a finely divided yellow crystalline powder. The structure of yellow tungsten depends on temperature: above 740°C it is tetragonal, 330-740°C is orthorhombic, 17-330°C is monoclinic, -50-17°C is triclinic crystal system.
Blue tungsten oxide, abbreviated as blue tungsten, refers to a class of dark blue compounds containing tungsten (VI) and tungsten (V) mixed valence states. With the different calcination conditions, the resulting blue tungsten phase composition is also different, but it is mainly WO2.98 or ammonium tungsten bronze (ATB) as the main phase.
Purple tungsten oxide, abbreviated as purple tungsten, is a purple finely divided crystalline powder. Because of its unique crystal structure, that is, all tungsten oxide agglomerates have abundant cracks inside, and have the characteristics of larger voids, lower loose packing density and significant surface effects.
In order to compare the microstructure of tungsten oxides, low-magnification and high-magnification scanning electron microscopes were used to observe the appearance and internal structure of different tungsten oxides.
a.BTO with W20O58 as main phase；b.BTO with ATB as main phase；c.VTO；d.YTO
Figure 1 Scanning electron micrographs of several tungsten oxide aggregates at low magnification
a.BTO with W20O58 as main phase；b.BTO with ATB as main phase；c.VTO；d.YTO
Figure 2 Scanning electron micrographs of several tungsten oxide aggregates at high magnification
It can be seen from Figure 1, Figure 2: (1) Different tungsten oxides have little difference in appearance, that is, they all maintain the appearance of ammonium paratungstate, but the particle size has undergone major changes.( 2) It can be seen from Figure 2 that the yellow tungsten and blue tungsten particles have sharp edges and corners, while the morphology of purple tungsten is obviously different from the two. Each large particle is a loose needle-like or rod-like particle clusters.
In order to study the internal structure of tungsten oxide pellets, a metallographic sample was made. Figure 3 is a scanning electron micrograph of the cross-section of several tungsten oxide aggregates.
a.BTO with W20O58 as main phase；b.BTO with ATB as main phase；c.VTO；d.YTO
Figure 3 Scanning electron micrographs of cross-sections of several tungsten oxide aggregates×1000
It can be seen from Figure 3 that the surface morphology of the tungsten oxides aggregates extends to the inside of the aggregates, and they all have abundant cracks, especially the purple tungsten oxide particles.
In order to study the morphology and size of tungsten oxides particles, they were characterized by transmission electron microscopy and electron diffraction, as shown in Figure 4 and Figure 5, respectively.
a.BTO with W20O58 as main phase；b.BTO with ATB as main phase；c.VTO；d.YTO
Figure 4 TEM of several tungsten oxide aggregates
a.BTO with W20O58 as main phase；b.BTO with ATB as main phase；c.VTO；d.YTO
Figure 5 Electron diffraction patterns of several tungsten oxides
It can be seen from Figure 4 that: (1) The morphology and size of different tungsten oxides particles are very different. The purple tungsten oxide particles are rod-shaped, and the rest are nearly spherical; (2) The rod-shaped purple tungsten oxide particles are the largest in size, and the cross-sectional size is about 100~200nm, and the size of blue tungsten particles with ATB as the main phase is the smallest, and the cross-sectional size is about 10nm; (3) The cross-sectional size of WO3 particles is similar to that of WO2.98 as the main phase. The size of the blue tungsten particles is about 40nm; (4) From the point of view of the degree of polymerization between the particles, the blue tungsten with ATB as the main phase has the smallest particle gap.
From the above introduction, we can see that compared to other tungsten oxides, purple tungsten oxide with a unique rod-like structure is more suitable for the production of tungsten powder and tungsten carbide powder.
