Formation and incorporation of dopant phases during technical reduction of NS-doped tungsten blue oxide

Abstract

This paper describes the formation and incorporation of dopant phases during technical hydrogen reduction of NS-doped tungsten blue oxide, as required for the development of non-sag properties in the final tungsten wire. Proper dopant incorporation during reduction is essential for assuring that the potassium remains in the sintered tungsten ingot. It is this potassium that enables the formation of rows of potassium bubbles during wire fabrication and consequently the formation of a long-grained interlocking microstructure during subsequent filament operation, which is the key to the non-sag characteristics. In the first part of this paper the industrial production of tungsten metal powder is discussed. Although in principle the process is the same for undoped as for K-, Al- and Si-doped tungsten powders, there are some specific differences which are presented. Both thermodynamic and kinetic aspects are considered. In the second part the chemical reactions of the dopants with the WO matrix, which finally lead to a partial dopant incorporation into the tungsten grains, are examined. A model for the dopant incorporation is proposed based on the authors' own experiences. It agrees with experimental observations and explains the individual actions of the doping elements K, Al and Si. In the final part, the incorporation of dopant phases is discussed on a more general basis considering further third-element interactions. It is shown that incorporation is closely linked with the intermediate appearance of dopant-WO phases (tungsten bronzes, tungstates).