Morphology of W18O49 Crystals by Microwave Decomposition of Ammonium Paratungstate (APT)

Ammonium paratungstate (APT) as an important precursor in tungsten industry. It is the raw material for the majority of tungsten products. For example, tungsten trioxide (WO₃), tungsten blue oxide, tungstic acid and ammonium metatungstate, etc. These are all intermediates by subsequent calcination and/or reduction of APT. Another significant middle product is monoclinic W₁₈O₄₉, (WO_2.72), one of the tungsten suboxides, is an important intermediate product of WO₃, influencing significantly the morphology of the tungsten powder end-product. W₁₈O₄₉ crystallises in needles and agglomerates of needle-like crystallites. This is true for the formation by vapour transport, in oxidative, and reductive atmospheres and for the transformation in the solid phase.

However, the main interest has been focused on its formation by gas phase reduction of higher oxides: WO 3 or WO_3-x using H₂, or CO, as reductive agents.

Microwave decomposition of APT has several significance for the morphological features and the growth mechanism of W₁₈0₄₉. Since conditions during microwave decomposition of APT have been found favourable to the growth of W18049 whiskers. By understanding the morphology of W₁₈O₄₉ needle crystals, we can illuminate the function of whisker growth of tungsten oxide compounds. Scientists have conducted an experiment to clarify the morphology of W₁₈O₄₉ needle crystals prepared by microwave decomposition of APT.

The experiment procedures are as follows:

Samples were prepared by decomposition of APT under a neutral gas ambient in a 2.45 GHz microwave reactor. The microwave irradiation began with a slow increasing temperature of APT in the microwave reactor, later with a very quick increase temperature, then a rapid decomposition of APT occured. NH₃ released from APT started to play the role of reductive agent and the reduction of WO₃ came to an end after the escape of NH₃. Released water of hydration causes large H₂0 pressure peaks in the gas phase confined in the pores of the tungsten oxide powder.

Quantitative X-ray powder diffraction was used in an attempt to By information on the amorphous fraction and on the crystalline phases. The morphology was studied by scanning electron microscopy as well as by transmission electron microscopy (TEM). For investigation by TEM the material was ground in an agate mortar, and a large number of small fragments were dispersed on carbon-coated specimen grids• Crystal structure of individual needles was established by electron diffraction. Attempts were made to reveal growth features and crystal defects by chemical etching of the samples using HF, ammonia water or K3FeCN 6 based etchants. Growth features were examined before and after chemical etching.

Conclusion

The results presented in this article demonstrate that the reductive formation of the needle crystals of W18049 occurs as a crystallisation process with the following main steps. (1) Root growth of a leader from a limited volume high mobility layer on the surface of the parent particle. (2) Growth in thickness with root growth of secondary needles parallel to the surface of the primary whisker combined with vapour deposition filling gaps. The model is suggested to explain morphologies of W18049 produced not only by microwave decom- position of APT but by reductions under wet conditions reported in the literature.

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