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).

Comparison of K-doped and pure cold-rolled tungsten sheets: As-rolled condition and recrystallization behaviour after isochronal annealing at different temperatures

“…When only HABs are considered in in 12 , the difference amounts to even ~150 MPa (Figure 10a). This slight difference between Wpure and WK describes another contribution to the hardness beside the grain size effect and could be explained by three effects: (a) Precipitation hardening by aluminosilicates, which are forming during the sintering process [53,54]. Although most of Al and Si is removed from WK by diffusion, the chemical analysis (Table 4) shows that these elements are partially retained, potentially still as aluminosilicates.…”

The effect of heat treatments on pure and potassium doped drawn tungsten wires: Part I - Microstructural characterization

“…An improved potassium bubble strengthened non-sag tungsten wire was of great importance for the incandescent lamps due to its increased creep resistance at very high temperatures and excellent coiling properties at room temperature [16]. Following years were mostly dedicated to the investigation of topics regarding the potassium bubbles such as their formation and evolution during production [17][18][19], size modifications during operation [20], mechanisms of reshaping the bubbles during the deformation [21,22], just to name a few.…”

Growth of single-crystal W whiskers during humid H2/N2 reduction of Ni, Fe–Ni, and Co–Ni doped tungsten oxide

“…In this case, the catalyst will be rapidly mantled by the superabundant W atoms, and then lose its catalysis. As a result, these pre-formed particles only serve as the nucleation sites, i.e., nucleation aids, for the formation of W crystals and will not influence the subsequent growth of W crystals, just as the "dopant incorporation" process suggested by Schubert etc [1,9,22].…”

“…Therefore, when explain the facts that trace foreign elements (Co, Ni, Fe, and Cu) can accelerate the reduction rate of tungsten oxide, it is used to presume that these elements or their compound act as the nucleation aids for the formation W phase from WO 2 [1,[22][23][24]. However, the occurrence of W whiskers in our experiments suggests that the reductive decomposition of gaseous WO 2 (OH) 2 above the surfaces of the pre-formed foreign element particles (Ni, Fe-Ni, and Co-Ni) should proceed in a much higher speed than that above the surface of pure W (lateral side of W whiskers).…”