Tungsten Blue Oxide--Chemical Composition

Chemical Composition. TBO is not a defined chemical compound but is a mixture of different constituents, such as ammonium, hydrogen and hydronium tungsten bronze phases, tungsten trioxide, tungsten-β-oxide(WO_2.9 or W₂₀O₅₈), and tungsten-γ-oxide(W_2.72 or W₁₈O₄₉). Under more reductions, even traces of WO2 andβ-tungsten can be present. The relative amounts of the various compounds in the TBO depend on the calcinations parameters.

•temperature,

•heating time,

•composition and pressure of atmosphere,

•mass of APT flow with time,

•gas flow,

•layer height in the boat (pusher furnace),

•slope and rotation rate (rotary furnace).

The oxygen index (molar ratio O/W) is commonly used to characterize the degree of reduction of TBO. However, since most TBOs also contain ammonia and water in addition to W and O, a more complete description is given by x(NH₃)•y(H₂O)•Wo_n.

A series of analyzed industrial samples gave the following ranges for the coefficients x and y and the index n:x=0.02-0.09, y=0.02-0.14, and n=2.82-2.99.

Qualitative and quantitative X-ray analyses of the same samples revealed quite a large scatter in composition: tungsten bronzes, 0-45%; WO₃, 0-45%; WO_2.9, 5-20%; WO_2.72, 0-25%, and amorphous, 30-55%.

Amorphous species form by dehydration which, on further heating, convert into crystalline binary tungsten oxide as well as tungsten bronzes. The conversion from amorphous to crystalline is a slow process. Therefore, if the heating period is short, as in rotary furnaces, the time available for overall crystallization is insufficient. This is why rotary-furnaces-derived TBO can be high in amorphous oxide when compared to TBO from pushers.

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