Abstract
Ammonium paratungstate tetrahydrate (NH 4) 10[H 2W 12O 42]·4H 2O (APT), a starting material in powder metallurgy of tungsten, has been subjected to a complex thermoanalytical study. The weight loss stages and the various heat effects have been followed by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) in flowing 10% H 2/Ar and 10% H 2/He atmospheres up to 600 °C. Meanwhile evolution of gaseous products has also been analyzed and monitored by both on-line coupled mass spectrometer (TG/DTA-MS) and infrared gas cell (TG-FTIR). In atmospheres containing 10% H 2, the evolution courses of H 2O and NH 3 traced by these EGA-methods and the TG/DTA curves up to 380 °C have been found quite similar to those of measured in air or in pure He. Effect of the reductive medium occurred above 380 °C, where solid products of intermediate temperatures have been structurally evaluated by both FTIR spectroscopy and powder X-ray diffraction (XRD). Based on significant changes in XRD patterns, the only small exothermic heat effect observed at 420–440 °C is assigned to the crystallization heat of a mixture of partially reduced tungsten oxide bronzes.
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