MORPHOLOGICAL DEVELOPMENT OF TUNGSTEN OXIDE HYDRATE GRAINS
Cs. Balázsi
Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences Hungary, Budapest XII., Konkoly-Thege út 29-33. Mail to: H-1525 Budapest-114, P.O.Box 49
Keywords: tungsten oxide hydrate, ripening, alkali content, pH, morphology
Tungsten oxide hydrates, (WO3.nH2O) are important basic materials for the tungsten industry. In this paper the morphological development of tungstic acid hydrate, WO3.2H2O is studied throughout the steps of precipitation of a gel product and the successive washing steps resulting in well defined granular crystalline product. The preparation route from Na2WO4.2H2O solution was proposed originally for the preparation of stable sols [1,2] and later applied for the preparation of hexagonal tungsten oxide [3]. Residual sodium is a persistent contaminant in WO3.2H2O [4] and the aim of this study is to follow morphological changes against [Na+] content and pH of the solution in contact with the solid phase during the washing steps. Ripening periods (the time of contact between WO3.2H2O and washing solution) were varied and controlled. Considerable changes in morphology of the grains have been registered from the original amorphous to the spindle shaped, polyhedral and rectangular crystallites.
Morphology changes have been found to depend on [Na+] content and pH of the solution in contact with the solid phase. Morphology investigations on long-time staying (1 month) samples revealed the existence of a pH band from 2,92 to 3,67 in which morphological changes occurred. Steps of shape conversion were found, which represent a crystal growing process [2]. The rectangular form proved to be the equilibrium shape of the tungsten oxide hydrate grains. The pH band favourable for shape conversion has a broadening tendency with passing of time. Out of this pH band, at pH < 3,67 morphological changes did not occur. The effect of sodium ion concentration in the solution shows similar features. Amorphous WO3.2H2O gels with [Na+] as high as 4000 ppm for gel and 500 ppm for solution have not shown any tendency for morphology changes, and the shape of very pure ([Na+] < 2ppm) crystalline precipitates has been found stable as well. The preparation route of WO3.2H2O was followed by IR spectroscopy and XRD measurements too. In agreement with the results of SEM investigations infrared absorption spectra and X-ray diffraction patterns reflected that the first precipitates had amorphous structure. After the first washing WO3.2H2O crystallises (JCPDS 18-420) and the crystalline structure of the consecutive formed product remains stable after the ripening process. Only small WO-OH, W-O-W bonding energy variations could be seen related to peptization of gels.