APT Thermal Decomposition

Thermal decomposition. On heating APT to 300-800℃, ammonia and water are evolved. The decomposition becomes more complete as temperature and duration of heating are increased. The final decomposition product is determined by the decomposition the retention time in the furnace, and the reduction potential of the decomposition atmosphere.

If heating takes place with exclusion of air or in an inert gas atmosphere (e.g., nitrogen), part of the ammonia evolved is cracked and the hydrogen formed can cause a slight reduction of the hexavalent tungsten matrix(“autoreduction capability of APT”; partial formation of pentavalent tungsten). The degree of reduction and the formation of compounds is determined by the decomposition conditions. The product of this type of calcination is greenish blue to flashing dark blue in color and is called tungsten blue oxide (TBO).

If the decomposition is carried out under oxidizing conditions, a slight reduction can occur intermediately at low decomposition temperatures, but the final product is always tungsten trioxide (WO₃).

Besides temperature, time, and decomposition atmosphere, the amount of APT plays an important role since the mass of APT itself is responsible for producing a certain amount of ammonia and water as well as hydrogen and nitrogen. The powder layer, depending on its thickness and porosity (which increases during decomposition due to an increase in density), retains the gases released for some time. This fact explains the occasional contradictory literature on the decomposition of APT. For example, it is easy to understand that in a boat of comparable size 10g or 1000g will produce different atmospheres, especially inside the powder layers whose heights are also quite different.

 Under oxidizing conditions, the APT decomposition path is as follows:

 Between 10 and 100℃ only dehydration occurs and the product is crystallized, dehydrated APT:

(NH₄)₁₀[H₂W₁₂O₄₂] •4H₂O→(NH₄)₁₀[H₂W₁₂O₄₂]+4H₂O

In the temperature range 180-225℃, ammonia is released and the APT converts to amorphous ammonium metatungstate (AMT):
(NH₄)₁₀[H₂W₁₂O₄₂]→(NH₄)₆[H₂W₁₂O₄₀] •2H₂O+4NH₃

Between 230 and 325℃, ammonia as well as water vapor are evolved. The product is also amorphous:

(NH₄)₁₀[H₂W₁₂O₄₀] •2H₂O→(NH₄)₂[W₁₂O₃₇]+4NH₃+5H₂O

By increasing the temperature to 400-500℃, all residual ammonia and water is released, and the reaction product is tungsten trioxide:

(NH₄)₂[W₁₂O₃₇]→12WO₃+2NH₃+H₂O

Under a slightly reducing atmosphere between 220 and 325℃ amorphous, and above 325℃, crystallized, ammonium tungsten bronzes form: (NH₄)_χWO₃. Under stronger reducing conditions, conversion to lower tungsten oxides takes place.

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