R 1025
Philips J. Res. 35, 326-336,1980
MORPHOLOGICAL EFFECTS OF TUNGSTEN
FILAMENTS IN HALOGEN LAMPS: THEIR INFLUENCE
ON THE TEMPERATURE DISTRIBUTION AND
EVIDENCE FOR HOT SPOT GROWTH DUE TO
DIFFERENT FACETING*)
by S. GARBE
Abstract
The recrystallization of the tungsten filament and the formation of
equilibrium surfaces with lower surface energy - a process which is known
as faceting - changes the original temperature distribution of tungsten
coils in certain halogen lamps. At constant voltage a temperature decrease
of 100 to 150 K is observed at initial temperatures around 3000 K, due to
the increase of the effective emissivity of the faceted surface. Growth of
low-index facets (e.g. (110) with (211) edges) proceeds with a ti time
dependence, the activation energy for the growtli of the facet width is
between 80 and 120 kJoule. Dissimilar facet fields of neighbouring turns of
a coil form local differences of the effective emittance. The emissivity
profile of a coil can be measured by simultaneous brightness and twocolour pyrometry. Temperature differences as large as 50 K develop due to
effective emittance gradients. Such temperature gradients between adjacent
turns of a coil increase the axial transport of tungsten and may lead to hot
spot formation. Together with grain boundary effects and the formation of
large potassium bubbles, faceting and notching may lead to fracture of the
wire and premature lamp breakdown.
1. Introduetion
Morphological effects of tungsten filaments in halogen lamps have in the
past mainly been discussed in order to demonstrate the direction of transport
or some unwanted nonideal behaviour. At the very high burning temperatures
of halogen lamps, the recrystallization of the filament takes place very early in
life and the rearrangement of the surface microstructure, which occurs with a
gain in free energy, may cause deviations from the ideal behaviour of the
regenerative cycle, especially in long-life halogen lamps.
2. Faceting of tungsten filaments in halogen lamps
Facet growth is a process by which the surface rearranges itself and a hill
and valley structure instead of a smooth surface is formed. Two different conditions have been discussed which favour the growth of facets:
*) Paper No. 28 at the 2nd Int. Symp. on Incoherent Light Sources, Enschede, The Netherlands,
April 9-12, 1979'.
326
.
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1980
Morphological effects of tungsten filaments in halogen lamps
(i) A crystalline material, which is in equilibrium with its vapour may
reduce the free energy of its surface by rearranging the atoms into hills and
valleys as a result of the different surface energies y(ii) of single crystal surfaces A 1). The equilibrium shape of a crystal would be the shape for which the
surface integral f y(ii) dA is a minimum (fig. I), y(ii) possesses the symmetry
Fig. 1. Facet growth. Breakup of surface orientation
(from N. Cabrera and R. V. Coleman, ref. 3).
n into
new surfaces of orientation
n) and n
2
elements of the crystal and the equilibrium shape can be deduced from a
Wulff-plot 2), wliich defines the stability of a surface against faceting. If the
macroscopie surface of a crystal does not coincide with some portion of the
equilibrium shape determined by the Wulff-plot, there will exist some hill and
valley structure, which has a lower free surface energy than the original
surface despite an increase of the surface area. The form of the y-plot also
defines areas of such orientations which are stable against faceting. Cabrera
and Coleman 3) have shown, that cusps in the y-plot define orientations with
smooth areas, which may form curved surfaces with microsteps instead of
faceting.
(ii) In the case of a net condensation or evaporation the shape of the
reacting surface will be affected by the kinetics of the mass transfer 4) and the
effective surface free-energy distribution may also depend on the adsorption
kinetics of a reacting gas. The equilibrium shape under etching conditions is
given by a similar condition to the Wulff-plot minimizing the orientation
dependent etch rate 5). Of course also the approximation of equilibrium conditions needs some kinetic phenomena to change the surface into an equilibrium structure. Under the conditions of a small supersaturation which holds
for most part of the filament of a non-blackening halogen lamp a faceting
structure develops while in the case of a net transport from part of the filament to other places - the hot spot development - the atoms from the corners of the facets will have a greater probability for evaporation and a smooth
equilibrated surface structure results. On "amicroscopie scale the processes of
evaporation of tungsten, re-adsorption and surface diffusion take care of predominant lateral growth of densely packed low-index planes, while the more
"PhilipsJourooi of Research
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1980
327
s. Garbe
open structures of kink planes - the higher index surfaces - have no
preferentlal adsorption sites and therefore these planes grow perpendicular to
their surfaces. If orientation-dependent surface diffusion limits the time law
of facet growth, the facet width increases with s ~ (Bt)1, where B is proportional to the surface diffusion coefficient 6). Because the recrystallized wire of
single coil halogen lamps often forms single crystals with about one grain
boundary per turn, a highly periodic facet structure can be observed in early
life (fig. 2). Grain boundaries cause preferential evaporation under certain
conditions of the orientations of the grains. Smooth areas of low-index planes
will be formed and these certainly differ in their radiation properties from
faceted areas (fig. 3). Very large facets may grow in long life lamps. Figure 4
shows the well-developed (lOO)-facets after 5500 hours.
Channeling diagrams, which were obtained in the Scanning Microscope,
were used to determine the orientations of facets. Figure 5 shows such a
diagram of the (100) surface. After rotation by 45 the channeling diagram of
the (110) orientation was obtained and tilting by 35 resulted in the channeling
diagram of the (111) direction. This sequence of the channeling patterns
0
0
Fig. 2. Periodic
328
faceting
structure
of H 3 coil after 123 hours
Philips Journalof
at 3350 K.
Research
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Nos.4/5
1980
Morphological effects oftungstenfilaments
Fig. 3. Faceting structure of H 3 single coil after 3096 hours
ness causes different radiation emission of individual turns.
Fig. 4. (lOO)/(lOO)-surface
steps,
(To = 3000 K, t = 5500 h).
Philips Journalof Research
which
Vol. 35 Nos.4/5
1980
indicate
in halogen lamps
at 3000 K. Different
predominating
radial
transport
surface rough-
of H 3 lamps
329
S. Garbe
Fig. 5. (IOO)-channeling
obtained
crystal.
diagram,
showed that the investigated
obtained
surfaces
from H 3 single coil.
belonged
to the same single
3. Facet growth
The facet growth occurs by surface-diffusion-controlled
growth selection
of a train of migrating steps. The observations on a large number of coils
for (110) steps with mainly (211) edges enabled the step distance to be fitted
,-
,-
-,
-,102
r
103
10
-Uh}
Fig. 6. Time Dependence
330
of IIIO}-facet
width at To = 3000 K and 3300 K.
Philips Journalof
Research
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Nos.4/5
1980
Morphological effects of tungstenfilaments
in halogen lamps
,--------------------------------,10
Qo= 101 kJoule
5
3.0
3.1
32
33
3.4
3.5
3.6
--Fig. 7. Temperature
dependence
3.7
(a.u.)
38
10"/r (K-I)
of [Ll Oj-facet step width.
on a log-log-plot
to a 4th root time dependence (fig. 6). The temperature
dependence of the facet widths is given by the corresponding surface diffusion
activation energy (fig. 7). For (IlO) surfaces a value of 101 kJoule was fitted
to the measured points.
Fig. 8. Surface
t = 3570 h).
Philips Journalof
structure
Research
of faceted
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Nos.4/5
wire, showing V-shaped
1980
grooves
and holes (To = 3000 K,
331
S. Garbe
4. Influence of faceting on emissivity and temperature distribution
The structure of faceted areas leads to an increase of the emitting surface
area and multiple reflection and hence and increase of the effective emissivity.
Some areas show V-shaped grooves and even holes (fig. 8). The radiation
from such bodies shows a selective increase of the emittance. Calculations of
the change of the directional emissivity of V-shaped grooves with apex angles
of 60 90 and 120 respectively 7), and for semi-spherical cavities 8) have
been made in the past and their result is given in fig. 9.
The change of the effective emissivity of halogen lamp filaments (H 3) was
measured during life by simultaneous brightness and two-colour pyrometry 9).
To a first approximation the change of the radiating area does not influence
the two-colour pyrometry and the effective emittance can be calculated from
0,
0
0
,
0.7
feff
t
0.6
///
//
/
/
/
/
/
0.5
/
/
/
/
0.4
1L-----I--...----+-----I-0.3
0.5
0.4
0.3
0.6
-ê
Fig. 9. Effective emittance ë.rr as function of the emission coefficient e for V-shaped grooves with
apex angle 8 or spherical holes with opening to diameter ratio do/D.
332
PhilipsJournnI of Research
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1980
Morphological effects of tungsten filaments in halogen lamps
0.70
•
.1
........
0.60
+
+
0.50
0.45
./+
v-
V
-
+
l----.
v· ..
+
+
3000K
.-
•
+
i
1000
2000
3000
Fig. 10. Change of the mean directional spectral emittance Ëcff at
To = 3000 K.
4000
-t(h)
À. =
5000
0.65 urn with time at
the results of the two measurements. The increases of the effective emissivity
(fig. 10) has two consequences:
(1) The mean temperature of the coil drops, which increases life. The temperature drop is about 150 K at an initial temperature of 3000 K.
(2) Local variations of the facet structure (fig. 11) may cause temperature differences between neighbouring turns (fig. 12). Temperature differences of
20.40 K can be introduced and they may lead to premature lamp failure.
The increase ofaxial transport due to differences of the local faceting pattern was analysed in a number of experiments. Cross-sectioning of the coils at
such defects supports the view that surface-structure-induced local emissivity
variations lead to temperature gradients and hot spot development.
5. Grain boundary effects and bubble growth
They will only lead to hot spots if large potassium bubbles have
accumulated at the "grain boundary (fig. 13). More often they may cause a
deformation of the coil owing to grain boundary sliding. The formation of
large grooves influences mainly the radiation properties of the coil and hence
the temperature distribution.
Philips Journal of Research
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Nos.4/5
1980
333
s. Garbe
front
back
Fig. 11. Surface structure of H 3 single coil after 3096 hours at 3000 K. The 10th turn exhibits
large smooth areas of low emittanee at the front and back side.
334
Philips Journalof Research Vol. 35
Nos.4/5
1980
Morphological effects of tungsten filaments in halogen lamps
3150
H3lamp
lifetime 3096 hours
3100
TS/R' TR {KJ
I
3050
3000
1.0
2950
0.9
6eff.O.65
2900
0.8
0.7
2850
Î
0.6
2800
0.5
2750
0.4
2
4.
6
8
12
10
-
16
14
winding no.
Fig. 12. Emissivity profile eer! of a H 3 single coil after 3096 hours at 3000 K. The resulting
brightness temperature profile TR is mainly given by local emittance differences. The temperature
spot TS/R which is measured by two-wavelengths pyrometry at the 10th turn is given by the low
emittance at that turn.
6. Conclusion
The morphological effects of tungsten wires - faceting and grain boundary
effects - change the temperature and the temperature profile. At moderate
temperature - about 3000 K - they introduce increased axial transport via
discontinuities of the local emission coefficient.
PhIlIpsJoumal of Research
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1980
335
S. Garbe
Fig. 13. lntergranular
fracture
due to bubble
growth
after 3570 hours at 3000 K.
Aachen, May 1980
Philips GmbH Forschungslaboratorium
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,)
2)
3)
4)
5)
6)
7)
8)
9)
H. J. Leamy,
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1975, p. 121.
C. Herring,
Some theorems on the free energy of crystal surfaces, Phys. Rev. 82, 87, 1951
N. Cabrera
and R. V. Co le m a n, in J. Gilman
(ed.), The art and science of growing
crystals, Wiley, N.Y., 1963, p. 3.
H: My k u r a, The equilibrium
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R. D.
Gretz and R. I. Jaffee
(eds), Molecular Processes on solid surfaces, McGraw-Hill
Book
Comp., N.Y., 1968, p. 129.
R. J. Jacodine,
Use of modified free energy theorems to predict equilibrium growing and
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W. W. Mullins,
Theory of linear facet growth during thermal etching, Phil. Mag. 6,1313,
1961
E. M. Sparrow
and S. H. Lin, Absorption of thermal radiation in a V-groove cavity, Int. J.
Heat Transfer 5, 1111, 1962.
M. A. Bramson,
Infrared radiation, Plenum Press, N.Y. 1968, p. 242.
W. Lechner
and O. Schob,
Temperature measurement of filaments above 2500 K applying
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pyrornetry,
lnst. Phys. Conf. Ser. No. 26, 297, 1975.
336
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1980