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
<5
0.5
Al
[ppm]
9
16
<5
0.4
Si
[ppm]
6
8
<5
0.6
O
[ppm]
13
24
5
<5
݊
Deformation
Characterization
1.0/2.4/3.6
1.0/2.4
1.0/2.4/3.6
1.3/2.7/3.9
rolling
swaging
rolling
rolling
Porosity, Rxx
Rxx
Porosity, Rxx
Porosity, Rxx
To simulate the behavior of the different tungsten grades as electrode material, ground samples with
the dimension of 8 x 8 x 7 mm were prepared from the center area of radially swaged rods (Table II).
The center area is usually the functional area of the anode where the electrical arc attaches on the
anode surface. These samples were subjected to a defined thermal load in axial direction in an electron
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beam chamber. The thermal load was applied by an electron beam on one of the 8 x 8 mm surfaces of
the sample. The electron beam was defocused to a diameter of 3.5 mm to apply the thermal load to a
sufficiently large area of the sample. Before all samples were recrystallized by an 1800°C/ H2/ 1h
annealing. The applied temperature during the electron beam treatment was estimated to ~3200°C
from the applied power which was required to melt tungsten samples. The samples were heated up in
60s and held at maximum power input for 300s. After this treatment the treated 8 x 8 mm surface was
investigated by SEM and the local deformation of the electron beam treated area was measured by
white-light interferometry (FRT CWLF, detector head 186-07).
Table II: Chemical composition in wt.-ppm, logarithmic degree of deformation, and type of
deformation of the electron beam treated material; * indicates GDMS measurement
Grade
K- doped 30ppm
K- doped 19ppm
Pure W
W-UHP
K
[ppm]
30
19
<5
0.1*
Al
[ppm]
38
26
<5
0.05*
Si
[ppm]
30
15
<5
1*
O
[ppm]
34
<5
<5
<5
݊
Deformation
Characterization
1.1
1.1
1.45
1.6
swaging
swaging
swaging
swaging
shape stability
shape stability
shape stability
shape stability
RESULTS
Evolution of the porosity from powder to the deformed rod
Differences in the porosity can be observed right away from the powder (Figure 4, Figure 6 and Figure
8). Very typical is the observed porosity on the tungsten particles of the K doped powder, while for the
pure tungsten as well as for the tungsten UHP powder such porosity cannot be detected. The pore size
is typically between 100nm and 200nm and is a result from the potassium doping.
After the sintering process the overall picture does not change significantly (Figure 5, Figure 7, Figure
9). Two types of porosity can be distinguished in the sintered ingots: macroporosity with more or less
irregularly shaped pores as a result from residual pores which remains after the sintering process
between the particle necks (1-10µm) and roundly shaped microporosity with pore sizes <200 nm.
Macropores can be detected for all three investigated tungsten grades. They are similar in size and
distribution. Significant differences were observed for the microporosity. The amount of micro
porosity is significantly reduced in pure tungsten as compared to potassium doped tungsten while
tungsten UHP is almost free of micropores. The micropores show a pore size of 50-200 nm in the
sintered ingot. This is in good agreement with the experimentally determined values reported in the
literature as well as with values calculated from equilibrium pressure between the Laplace pressure
and the pressure for an ideal gas filled bubble [6, 11].
From literature it is known that initially the potassium is located in the microporosity right away from
the powder and remains there up to sintering temperatures below 2300°C. At 2300°C potassium starts
to evaporate from the micropores so that it can be located in the macropores as well [6]. From this
statement it cannot be excluded that potassium is located in the macropores in the investigated
potassium doped tungsten.
Interesting is the fact that sintered pure tungsten shows some microporosity although it is not
potassium doped and tungsten UHP is free of microporosity. The small pore size of 100 nm in the pure
tungsten will result in a relatively high value for the Laplace pressure which itself would result in
collapsing of the pore at sintering temperature. One possible explanation for the existence of such
small pores even after sintering is that these small bubbles are filled with insoluble accompanying
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substances. Such substances can be insoluble pure elements like K as well as insoluble compounds
like oxides. The lack of micropores in the sintered tungsten UHP in combination with the high purity
confirms this idea.
Figure 4. SEM image of the K doped tungsten Figure 5. Macroporosity preferably located at
powder
triple junction as well as microporosity on the
grain boundary for K doped tungsten (50ppm K)
Figure 6. SEM image of pure tungsten powder
Figure 7. Pure tungsten after sintering
showing very few micropores at the grain
boundary
Figure 8. SEM image of tungsten UHP powder
Figure 9. Tungsten UHP after sintering which is
virtually free of microporosity
The observation of the two types of porosity does not even change after applying moderate degrees of
deformation (ࡏ1.0-1.3) and subsequent recrystallization at 1600°C/1h. In potassium doped tungsten
still macro- and microporosity can be detected. In Figure 10 one elongated pore can be seen in the left
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side of the image which has been a macropore in the sintered ingot. Elongated macropores can be
detected for tungsten UHP as well (Figure 11).
On the upper right sight in Figure 10 typical micropores for potassium doped tungsten are shown.
These micropores show almost no elongation and a pore size comparable to the sintered ingot. The
reason for the apparent lack of deformation of the microporosity in the potassium doped tungsten is
the 1600°C/1h annealing prior to the investigation of the fractured surfaces. Bewlay [6] demonstrated
that a 1600°C/1h annealing is sufficient to change the shape of the elongated pore with a radius of 100
nm by surface diffusion. Nevertheless the total degree of deformation of ݊ = 1.0 is not high enough to
produce the well-known potassium rows. Therefore the micropores reshaped from their elongated
shape after deformation to their initial shape. Due the large size of the macropores the 1600°C/1h
annealing was not sufficient to reshape the macropores.
A reshaping of the macroporosity into rows of pores can be observed by increasing the logarithmic
degree of deformation to values > 1.3 and subsequent annealing at 1600°C/1h. Such rows are well
known for potassium doped qualities. It is surprising that macropores in pure tungsten as well as
tungsten UHP can form rows of pores, although they are obviously not filled with potassium which
arises from a doping process. Figure 13 shows such a row of pores in W-UHP and Figure 3 shows
such rows of pores for pure tungsten with a logarithmic degree of deformation of 1.5.
By further increasing the deformation to values >3.5 no more elongated macropores can be observed
after applying a 1600°C/1h annealing. In the potassium doped tungsten a high amount of micropores is
still present. The size of these micropores is still in the same order of magnitude as they were already
observed in the sintered ingot (Figure 5). All pores have a round shape. Interesting is the fact that for
tungsten UHP with ݊ of 3.9 microporosity can be detected (Figure 15). Remember the fact that in the
sintered ingot as well as in the samples with a low degree of deformation no microporosity was
present in the material. The reason for this emerging microporosity in tungsten UHP is not fully clear
and needs further research.
One may speculate that some insoluble substances were entrapped in the macropores of the sintered
tungsten UHP. By deforming the tungsten UHP with a high degree of deformation the macropores
were deformed and pore multiplication and pore refinement took place. The observed pores in
tungsten UHP are significantly smaller as compared to the observed microporosity in the potassium
doped tungsten. As previously discussed these pores have to be filled with insoluble substances.
Without any substances which may produce additional pressure in the pore, the micropores would
collapse immediately as a result of the high Laplace pressure during the 1600°C/1h annealing. The
thesis of impurities seems to be in contrast with the fact that the tungsten is ultra-high purity tungsten.
The K concentration in the tungsten UHP was 2 orders of magnitude lower (0.4ppm) as compared to
the potassium doped tungsten with 50 ppm. Nevertheless one has to consider the equilibrium radius of
a gas filled pore. The equilibrium radius for the potassium bubbles in both materials can be calculated
according to the following equation:
‫ ݎ‬ൌ ሼሺ͵ܴ݊ܶሻȀሺͺߨߛሻሽଵΤଶ
[6]
Here n is the molar amount of the gaseous species, R is the ideal gas constant and
ஔ is the surface
energy. Assuming the same potassium bubble density in both materials, the calculated equilibrium
radius between the two materials differ only by one order of magnitude even if the total potassium
concentration differs by 2 orders of magnitude. This effect may explain that even very low
concentrations of insoluble substances like potassium may be visible as microporosity.
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Figure 10. K- doped tungsten (50ppm), ݊1.0, Figure 11. W-UHP, ݊1.3, 1600°C/1h annealed
1600°C/1h annealed
Figure 12. K- doped tungsten (50ppm), ݊2.4, Figure 13. W-UHP, ݊2.7, 1600°C/1h annealed
1600°C/1h annealed
Figure 14. K- doped tungsten (50ppm), ݊3.6, Figure 15. W-UHP, ݊3.9, 1600°C/1h annealed
1600°C/1h annealed
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Recrystallization behavior and grain aspect ratio
The observed increase in recrystallization temperature was rather low for both K- doped qualities. For
pure tungsten as well as tungsten UHP the recrystallization temperatures were between 1250 - 1300°C.
No increase in recrystallization temperature with increasing degree of deformation was observed for
these materials. Figure 16 shows the decrease in Vickers hardness with increasing annealing
temperature for the potassium doped tungsten with 50 ppm potassium. With increasing degree of
deformation the temperatures for 95% recrystallization increases slightly from 1350°C for ݊ =1 to
1480°C for ݊ =3.6. Figure 17 shows the 95% recrystallization temperature of the 4 investigated
materials. Additionally available data for a tungsten UHP wire and a potassium doped tungsten wire
were added to Figure 17. From this data it clearly comes out that with increasing the degree of
deformation ݊ > 6 the temperatures for 95% recrystallization increases up to 2100°C for the potassium
doped quality. This is best explained be the continuous refinement of the potassium bubble
distribution. For qualities like the tungsten UHP the recrystallization temperature drops slightly
because with increasing degree of deformation the driving forces for recrystallization increase and
there are no alloying elements which impede the recrystallization.
Figure 16. Change of Vickers hardness HV30 depending on annealing temperature and total degree of
deformation for potassium doped tungsten (50ppm)
8
Figure 17. Temperatures for 95% recrystallization determined from Vickers hardness measurements
depending on the total degree of deformation for different tungsten grades
The observed changes in grain aspect ratios (GAR) were moderate for the 4 different investigated
materials. The maximum value of 2.6 was observed for the potassium doped tungsten with a degree of
deformation of 3.6. This is far beyond the values reported for thin wires with GAR >10-15 [8].
Nevertheless a slight increase in the grain aspect ratio can be observed for both potassium doped
qualities. The maximum GAR value for the pure tungsten qualities was 1.6. for pure tungsten with ݊ =
1.0. For the tungsten UHP the value stays at 1.0 with increasing the degree of deformation to 3.9.
9
3
2,5
K-doped (50 ppm)
K-doped (22 ppm)
grain aspect ratio
2
pure W
UHP-W
1,5
1
0,5
0
0,00
1,00
2,00
3,00
4,00
5,00
logarithmic degree of deformation ࡏ
Figure 18. Grain aspect ratio in longitudinal direction after recrystallization at 1800°C/1h
Shape Stability of the Electrode Materials
Due to the thermal load which was applied during the electron beam treatment the grain size changed
significantly depending on the investigated grade (Table III). Potassium doping of 19 ppm as well as
30 ppm were very effective in impeding the grain growth during electron beam treatment while
tungsten UHP showed excessive grain growth.
Table III: Comparison of the grain size in µm in the recrystallized condition (1800°C/1h) and after
electron beam treatment at an estimated temperature of 3200°C/5min
Grade
W-UHP
Pure Tungsten
K-doped (19ppm)
K-doped (30ppm)
1800°C/1h
40
30
39
35
Treated Surface
2500
367
56
50
Beside the different grain growth behavior significant differences in the surface appearance after the
electron beam treatment were observed. Figure 19 shows the appearance of the electron beam treated
surface of tungsten doped with 19 ppm K. Figure 20 shows the evolution of the surface undulation
measured by white-light interferometry after electron beam treatment for tungsten UHP and potassium
doped tungsten (19ppm K). Significant deformation of the surface can be observed for tungsten UHP
while the potassium doped tungsten shows very good shape stability. By comparing the measured
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surface undulation in all 4 investigated materials in Figure 21 it clearly comes out that with increasing
amount of potassium the shape stability increases significantly.
Figure 19. Image of the electron beam treated surface of potassium doped W (19ppm)
Figure 20. Color coded images of the surface undulation of the electron beam treated surfaces, left:
W-UHP; right: potassium doped W (19ppm K) with almost negligible undulation of the surface
11
Figure 21. Measured surface undulation after electron beam treatment depending on the potassium
content in the material
To understand the effect of potassium on the shape stability, the SEM images of the electron beam
treated surfaces have to be considered. A detailed view of the electron beam treated surface of
tungsten UHP is shown in Figure 22. Microscopic roughness showing regular patterns within the
individual grains can be detected. The observed patterns differ significantly from patterns which
typically appear by evaporation and condensation on tungsten electrodes in short arc lamps (Figure
23). Moreover, the pattern shows similarity to slip lines as they occur during plastic deformation. So
obviously plastic deformation by dislocation glide took place during the electron beam treatment
which thereby causes the undulation of the electron beam treated surface. This observation is in
agreement with the observed undulation of the tungsten anode as shown in Figure 1. Additionally, sub
grain formation in the tungsten single crystal can be detected on this anode. The only reasonable
explanation for the formation of sub grains within the single crystal is dislocation activity caused by
stresses. From previous calculations it is known that thermally induced stress by temperature gradients
can easily exceed the stress level required for dislocation creep or even the 0.2% yield point [12].
Potassium may now contribute to the observed differences in shape stability directly by interactions
between the potassium bubbles and the dislocation as well as indirectly by influencing the grain size as
reported in Table III. Wherease the grain size stabilizing effect of potassium during electron beam
treatment is very obvious, the influence on dislocation movement is rather small. For dislocation
creep at temperatures of T > 0.8 of the homologous temperature and a logarithmic normalized shear
stress of > -4.5 the deformation rate of W is independent of the grain size [13]. So the observed
differences in grain size will not explain the differences in shape stability.
The high shape stability of the potassium doped tungsten is most probably caused by the attractive
interaction with potassium bubbles and the dislocations which are induced by the thermal stresses. The
effect of K is somewhat surprising because the total degree of deformation of these samples was rather
low (ࡏ = 1.1- 1.6) compared to the reported refinement of K in tungsten wires with ࡏ > 7. The
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macropores observed for all tungsten grades are too big to produce sufficient attractive interactions
with dislocations. Obviously the potassium filled micropores in potassium doped qualities contribute
sufficiently to the increase in shape stability.
Figure 22.SEM surface image of the electron beam treated surface left: 50x magnification; right: 300x
magnification
Figure 23.SEM surface of tungsten facets caused by condensation and evaporation of a tungsten
cathode
SUMMARY
Besides the well-known incandescent lamp wire potassium doped tungsten has been established as
anode material in short arc lamps. The total degree of deformation for potassium doped tungsten
electrodes used in those lamps is limited due to the ingot size and the final diameter which is necessary
for high wattage short arc lamps. Due to the low degree of deformation (ࡏ ” no significant
refinement of the bubble distribution was observed on deformed samples and thereby the increase in
recrystallization temperature and GAR value are rather moderate. Nevertheless potassium turned out
to be very effective in preventing deformation at operational temperatures of 3200°C. The reported
stresses on anodes indicate that dislocation creep or even plastic deformation can occur under
operating conditions. This is in contrast to incandescent wire where mainly diffusional creep under the
dead load of the wire is active.
13
Microstructural investigation showed that potassium in potassium doped tungsten can be easily traced
as microporosity from the powder to the deformed product. The reported mechanism of elongation of
potassium filled pores during deformation and the subsequent formation of bubble rows after applying
a heat treatment was also observed for macropores in not potassium doped tungsten qualities.
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