POTASSIUM DOPED TUNGSTEN BEYOND INCANDESCENT LAMP WIRES

Andreas Hoffmann1, Ingmar Wesemann1 1

Plansee SE, A-6600 Reutte

K doped tungsten has been used as creep resistant wire material for incandescent lamps and is well investigated for decades. To achieve excellent high temperature creep properties a fine potassium distribution in combination with a rather high degree of deformation is required. Beside this conventional application potassium doped tungsten gained significant importance as electrode material in discharge lamps. The total degree of deformation for such electrode materials is significantly lower and the effect of potassium is less intensely investigated. Therefore the evolution of microstructure of different powder metallurgically produced K doped tungsten grades as well as pure tungsten grades were investigated from powder to final product. Furthermore these materials were subjected to very high temperature (0.9 T m) and characterized regarding the microstructural stability as well as shape stability towards thermally induced stresses which occur during application.

INTRODUCTION Bubble strengthened non sag tungsten is used as creep resistant wire material in incandescent lamps with excellent coiling properties at room temperature for decades. The outstanding creep properties are achieved by additions of aluminum, silicon and potassium (AKS-Tungsten). Aluminum and silicon are added in form of an aqueous solutions of potassium disilicate, aluminum nitrate or aluminum chloride to the tungsten blue oxide (TBO) followed by a reduction of the doped blue oxide [1, 2, 3, 5]. During direct sintering the majority of the additives are removed by evaporation. Despite the very high vapor pressure of K during sintering it is possible to retain 60-120 ppm metallic K in form of partly filled bubbles. This is achieved by the addition of Al and Si as described above [4, 6]. Excellent creep properties in the final wire are achieved by the formation of an interlocking grain structure with grain aspect ratios (GAR) > 10-15 after secondary recrystallization [8]. Typical recrystallization temperatures range from 1900- 2300°C depending on the total degree of deformation for wires below 500µm [7]. The interlocking grain structure impedes diffusional creep along unfavorably orientated grain boundaries in transversal direction of the wire which can occur under the dead load at typical operation temperatures of 2200- 2900°C. It is widely accepted that the interlocking grain structure can only be achieved by the potassium doping in combination with a sufficiently high degree of deformation ݊ > 8.0. During the deformation step the potassium filled bubbles in the sintered ingot elongate along the wire axis. After a sufficiently high deformation and intermediate annealing these rows of elongated potassium bubbles start to split up into rows of smaller bubbles and a refinement of the inital potassium distribution can be achieved. The total number of bubbles is determined by the total degree of deformation, deformation temperature as well as intermediate annealing steps [10]. A lot of work was done to investigate the potassium bubble distribution in the pores of the sinter ingot, the deformability of the pores as well as mechanism of

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reshaping of the bubbles during deformation and external influences which lead to undesired bubble growth [6, 9, 10, 11]. Final wire diameters of potassium doped tungsten typically range from 10 µm for multi coiled incandescent lamp wires and 400µm for single coiled halogen lamp wires. Beside these traditional applications potassium doped tungsten is also used as electrode material in discharge lamps. Typical diameters range from 250µm for electrodes used in metal halide lamps and up to 15- 40 mm for anodes used in short arc lamps. Large diameter anodes are mainly used in xenon short arc lamps for cinema projection as well as for mercury containing short arc lamps used for photolithography processes. For these anode materials the typical K content is rather low with 15 - 40 ppm and the achievable degree of deformation is limited to the ingot size and the required dimension of the final anode. Despite the significantly lower K- concentration and the low degree of deformation (݊ < 3) potassium doped tungsten is reported to show significant advantages regarding the shape stability of the electrode compared to pure tungsten [14]. Shape stability describes the resistance towards deformation of the anode plateau which would lead to undesired changes in the intensity distribution of the electric arc as well as local arc attachment followed by excessive evaporation of tungsten. An example for the deformation of an initially flat anode plateau is given in Figure 1. Stresses in anodes of short arc lamps are predominantly induced by thermal gradients caused by the electric arc which attaches on the electrode [12]. Stress under the dead load has only a minor impact which is a significant difference compared to wires materials used in incandescent lamps. Operation temperatures of such discharge lamps can range from 2000°C up to temperatures above 3000°C. Nevertheless the mechanism leading to the increased shape stability for these potassium doped electrode materials is not fully clarified.

Figure 1. SEM picture of a pure W single crystal anode showing subgrain formation and irregular deformation on the initially flat anode plateau , Hg-Xe short arc lamp, anode diameter= 15 mm, anode length= 30 mm, testing time~ 200h [12]

EXPERIMENTAL To get a better understanding about the role of the potassium-doping in electrode materials, different tungsten grades with K contents between 19 - 50 ppm and with a moderate degree of deformation (݊ = 0.75 - 4.0) were investigated and compared to a standard pure tungsten with a metallic purity of > 3.7N and an ultra-high purity (UHP) tungsten with a metallic purity of 4.8N. All materials were produced by powder metallurgical production route at PLANSEE SE. Samples were indirectly

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sintered at temperatures between 2300 - 2400°C which is in contrast to the reported sintering temperatures of T > 2800°C for directly sintered potassium doped wire material. Afterwards the different tungsten grades were deformed by swaging or rolling. The evolution of the porosity from the powder to the sintered ingot and the deformed product was investigated on samples as listed in Table I. Porosity was investigated by SEM with 10.000 times magnification. The deformed samples were recrystallized prior the investigation. In the as deformed condition the microstructure showed a high density of very small subgrains which make a clear identification of pores difficult (Figure 2). After full recrystallization the porosity becomes significantly better visible (Figure 3).

Figure 2. Fractured surface of pure W, ݊ 1.5, Figure 3. Fractured surface of pure W, ݊ 1.5, as deformed 1800°C/1h annealed The chemical composition of the sintered ingot was characterized regarding aluminum by Inductive Coupled Plasma Optical Emission Spectroscopy (ICP-OES), silicon by Graphite Atomic Tube Absorption Spectroscopy (GFAAS), potassium by Flame Atom Absorption Spectroscopy (FAAS) and the oxygen content by Carrier Gas Hot Extraction (CGHE). Potassium, aluminum and silicon content for the tungsten UHP were determined by Glow Discharge Mass Spectroscopy (GDMS). From the same materials the temperatures for 95% recrystallization were determined from hardness measurements on metallographic cuts after applying H2 annealing between 1000 -1800°C for 1 hour. The temperature for 95% recrystallization was defined by the annealing temperature at which the hardness was 5% above the hardness value of that of the fully recrystallized condition. The fully recrystallized condition was defined by the temperature range where no further drop in Vickers hardness was observed. Furthermore the grain aspect ratio in the recrystallized condition (1800°C/ 1h) was determined from metallographic cuts in longitudinal direction. Table I: Chemical composition in wt.-ppm, logarithmic degree of deformation, type of deformation and performed characterization of the investigated material Grade K- doped 50ppm K- doped 22ppm Pure W W- UHP

K [ppm] 50 22