Low-pressure mercury vapor discharge lamp

A low-pressure mercury vapor discharge lamp according to the invention is provided with a discharge vessel having a tubular portion and a first and a second end portion. The discharge vessel encloses a discharge space provided with a filling of mercury and a rare gas in a gastight manner. Each end portion supports an electrode which is arranged in the discharge space. Current supply conductors extend from the electrodes through the end portions to outside the discharge vessel. The tubular portion of the discharge vessel is provided with a metal oxide layer on a surface which faces the discharge space. The first and the second end portion are also provided with metal oxide layers at surfaces which face the discharge space. The lamp according to the invention has a comparatively low mercury consumption.

BACKGROUND OF THE INVENTION 
The invention relates to a low-pressure mercury vapour discharge lamp 
provided with a discharge vessel having a tubular portion which transmits 
radiation generated in the discharge vessel and having a first and a 
second end portion, which discharge vessel encloses a discharge space 
provided with a filling of mercury and a rare gas in a gastight manner, 
while the end portions each support an electrode arranged in the discharge 
space and current supply conductors issue from the electrodes through the 
end portions to outside the discharge vessel, the tubular portion of the 
discharge vessel being provided with a metal oxide layer on a surface 
which faces the discharge space. 
Such a lamp is known from U.S. Pat. No. 4,544,997. The tubular portion of 
the discharge vessel of the known lamp has a layer of at least one oxide 
of at least one element from the group formed by scandium, yttrium, 
lanthanum, gadolinium, ytterbium, and lutetium. The metal oxide layer 
counteracts attacks on the wall of the tubular portion of the discharge 
vessel owing to interactions with mercury and thus has a favourable 
influence on the maintenance of the radiation output of the lamp. The 
metal oxide layer was obtained in that a solution of an organometallic 
compound was flushed over the surface of the discharge vessel facing the 
discharge space, and the film remaining on the surface facing the 
discharge space was subsequently dried and sintered. 
The metal oxide layer results in that the mercury consumption of the lamp, 
i.e. the quantity of mercury bound to lamp components during lamp 
operation and thus no longer available for lamp operation, is 
comparatively low as compared with that in lamps not having a metal oxide 
layer. Nevertheless, a comparatively high mercury dose is necessary also 
for the known lamp in order to realise a sufficiently long life. This is 
detrimental to the environment in the case of inexpert disposal at the end 
of lamp life. A high mercury dose in addition prevents an economically 
feasible use of mercury enriched with .sup.196 Hg. It is known from U.S. 
Pat. No. 4,379,252 that a lamp whose mercury filling is enriched with this 
isotope has a comparatively high efficacy. This isotope, however, is 
comparatively expensive, so the advantage is wiped out in the case of a 
high mercury dose by the cost price of the required quantity of the 
isotope accompanying this dose. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a lamp of the kind described in 
the opening paragraph which consumes comparatively little mercury. 
According to the invention, the lamp is for this purpose characterized in 
that the first and the second end portion are also each provided with a 
metal oxide layer on surfaces facing the discharge space. 
The overall surface area of the end portions facing the discharge space is 
small compared with that of the tubular portion. The inventors have found, 
however, that nevertheless substantially the same quantity of mercury is 
bound to the end portions in the known lamp as to the tubular portion. It 
was surprisingly found, moreover, that the presence of a metal oxide layer 
on the end portions not only strongly reduces the binding of mercury to 
the end portions, but also causes the quantity of mercury bound to the 
tubular portion to decrease considerably. It is assumed that the end 
portions of a lamp not according to the invention desorb impurities such 
as CO.sub.2 and H.sub.2 O when a metal oxide layer on the end portions is 
absent. The end portions assume a comparatively high temperature during 
operation, which accelerates the desorption of said impurities. Said 
impurities can be transported to other lamp components through the 
discharge space and react with mercury from the filling there, so that 
this mercury is lost to lamp operation. It is assumed that the metal oxide 
layer on the end portions reduces the desorption of impurities. 
A metal oxide layer may be provided on the end portions in that these 
portions are dipped in a suspension of metal oxide particles and the layer 
remaining on them is subsequently dried and sintered, i.e. heated so as to 
drive out auxiliary substances such as binders from the layer. 
Alternatively, such a layer may be provided, for example, in that the end 
portions are dipped in a solution of an organometallic compound and the 
layer is subsequently dried and heated. 
A favourable embodiment of the lamp according to the invention is 
characterized in that the metal oxide layers on the surfaces of the end 
portions facing the discharge space comprise aluminum oxide and/or yttrium 
oxide. A comparatively strong decrease in the mercury consumption was 
found with a layer of aluminum oxide and/or yttrium oxide. 
An attractive embodiment of the lamp according to the invention is 
characterized in that the metal oxide layer on the surface of the tubular 
portion facing the discharge space comprises at least one oxide of at 
least one element from the group formed by magnesium, aluminum, titanium, 
zirconium, and the rare earths. The term "rare earths" in the present 
description and claims is understood to mean scandium, yttrium, lanthanum, 
and the lanthanides. Such a layer is highly inert so that the mercury 
consumption caused by reactions between mercury from the filling and the 
metal oxide layer is small also in the long term. 
Favorable results were obtained with an embodiment of the lamp according to 
the invention which is characterized in that the metal oxide layer of the 
tubular portion comprises aluminum oxide and/or yttrium oxide. Such a 
layer may be provided, for example, in the form of a suspension of 
aluminum oxide/yttrium oxide particles, for example through atomizing of 
the suspension or by having this suspension flow over the inner surface of 
the discharge vessel. 
An advantageous embodiment is characterized in that the tubular portion of 
the discharge vessel carries a further metal oxide layer, which acts as a 
layer repelling alkali metals, between the surface facing the discharge 
space and the metal oxide layer (called protective layer hereinafter). A 
layer repelling alkali metals hampers the transport of alkali metal ions, 
such as sodium and potassium ions, from the discharge vessel wall to the 
discharge space. Mercury consumption caused by amalgam formation with 
alkali metals is counteracted thereby. 
A favorable embodiment of the lamp according to the invention is 
characterized in that the further metal oxide layer comprises silicon 
oxide. Silicon oxide forms a very good barrier against alkali metal ions. 
Such a layer is readily provided. It suffices to flush a solution of 
hydrolyzed tetraethyl orthosilicate over the discharge vessel surface 
which faces the discharge space. After the silicon oxide layer thus 
provided on the surface has been dried, the metal oxide layer may be 
directly provided. A heat treatment is favourable for increasing the 
density of the layer. The heat treatment coincides, for example, with a 
heat treatment for the protective layer. If a separate heat treatment is 
unnecessary also for the protective layer, it is possible to have the heat 
treatment coincide with a heat treatment for driving auxiliary substances, 
such as binders, from a suspension of luminescent material in the case in 
which a luminescent layer is provided on the lamp in the form of a 
suspension. 
The discharge vessel supports, for example, a luminescent layer composed of 
blue-luminescing barium-magnesium aluminate activated by bivalent europium 
(BAM), green-luminescing cerium-gadolinium-terbium pentaborate in which 
terbium acts as an activator (CBT), and red-luminescing yttrium oxide 
activated by trivalent europium (YOX). This embodiment of the lamp is 
suitable for lighting purposes. A luminescent layer is absent in another 
embodiment of the lamp according to the invention. This embodiment of the 
lamp is suitable, for example, as a UV radiator for disinfection purposes.

DETAILED DESCRIPTION OF THE DRAWING 
FIG. 1 shows a low-pressure mercury vapour discharge lamp provided with a 
glass discharge vessel 10 having a tubular portion 11 which transmits 
radiation generated in the discharge vessel 10 and having a first and a 
second end portion 12a, 12b. The tubular portion 11 has a length of 120 cm 
and an internal diameter of 2.5 cm. The discharge vessel 10 encloses a 
discharge space 13 provided with a filling of 1 mg mercury and a rare gas, 
here argon, in a gastight manner. The end portions 12a, 12b each support 
an electrode 20b (the electrode at the first end portion 12a is not shown) 
arranged in the discharge space 13. Current supply conductors 30a, 30a'; 
30b, 30b' extend from the electrodes 20b through the end portions 12a, 12b 
to outside the discharge vessel 10. The current supply conductors 30a, 
30a'; 30b, 30b' are connected to contact pins 31a, 31a'; 31b, 31b' which 
are fastened to lamp caps 32a, 32b. An electrode ring 21a is positioned 
around each electrode 20b (the electrode ring at the second end portion 
12b is not shown). A glass capsule 22, with which mercury was dosed, is 
clamped on the electrode ring 21a. A metal wire 23 tensioned over the 
glass capsule 22 was inductively heated in a high-frequency 
electromagnetic field, whereby the capsule 22 was cut open and the mercury 
to be dispensed was released from the capsule 22 into the discharge space 
13. 
The tubular portion 11 of the discharge vessel 10 is provided with a metal 
oxide layer 15 (see FIG. 2) at a surface 14 which faces the discharge 
space. 
The first and the second end portion 12a, 12b are also provided with metal 
oxide layers 15a, 15b at surfaces 14a, 14b which face the discharge space. 
The metal oxide layers 15a, 15b on the surfaces 14a, 14b of the end 
portions 12a, 12b facing the discharge space comprise yttrium oxide in 
this case. The yttrium oxide layers 15a, 15b, which have a coating weight 
of 15 to 30 .mu.g/cm.sup.2, were provided in that the end portions 12a, 
12b were immersed in a solution of yttrium acetate, whereupon the layer 
remaining on the end portions 12a, 12b was dried and sintered. In the 
embodiment shown, the yttrium oxide layers 15a, 15b extend to 2 to 3 mm 
away from the seam with the tubular portion 11. This facilitates the 
fusion of the end portions 12a, 12b to the tubular portion 11. 
The metal oxide layer 15 on the surface 14 of the tubular portion 11 which 
faces the discharge space comprises an oxide of at least one element from 
the group formed by magnesium, aluminium, titanium, zirconium, and the 
rare earths. The metal oxide layer 15 in this case is an yttrium oxide 
layer with a coating weight of 30 .mu.g/cm.sup.2. 
The tubular portion 11 of the discharge vessel 10 supports a further metal 
oxide layer 16, which acts as an alkali metal repelling layer, between the 
surface 14 facing the discharge space and the yttrium oxide layer 15. In 
the embodiment shown, the further metal oxide layer 16 is made of silicon 
oxide and has a coating weight of 12 .mu.g/cm.sup.2. The yttrium oxide 
layer 15 supports a luminescent layer 17 with a coating weight of 1.8 
mg/cm.sup.2 comprising the luminescent materials BAM, CBT and YOX. 
A lamp not according to the invention was manufactured for comparison 
purposes, lacking a metal oxide layer on the end portions but 
corresponding to the lamp according to the invention in all other 
respects. 
The lamps were subjected to an endurance test of 5000 hours. After the 
endurance test, the quantity of mercury bound to the end portions (A) and 
to the tubular portion (B) was ascertained by means of a wet-chemical 
analysis. The results (in .mu.g) are shown in Table 1 for the lamp 
according to the invention (I) and the lamp not according to the invention 
(II). 
TABLE 1 
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Quantity of bound mercury in .mu.g in the lamp 
according to the invention I and not according to the 
invention II. 
Lamp component 
I II 
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A 10 61 
B 24 73 
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The measure according to the invention leads to a strong reduction in the 
quantity of mercury bound at the end portions (A), but the quantity of 
bound mercury at the tubular portion (B) is also considerably smaller. 
For further investigation, two groups of 10 lamps according to the 
invention and one group of 10 lamps not according to the invention were 
manufactured. The tubular portion of the discharge vessel was provided 
with a luminescent layer in all cases, while an aluminium oxide layer was 
present between the surface of the tubular portion facing the discharge 
space and the luminescent layer. The aluminum oxide layer was obtained 
from a suspension of aluminium oxide particles, here of the Degussa Alon-C 
type. Five out of each group of ten lamps was in addition provided with an 
alkali metal repelling layer of silicon oxide between the surface of the 
tubular portion facing the discharge space and the aluminum oxide layer. 
The silicon oxide layer was provided in that a solution of hydrolyzed 
tetraethyl orthosilicate was flushed over the surface of the discharge 
vessel which faces the discharge space. The silicon oxide layer and the 
aluminum oxide layer have respective coating weights of 12 and 55 
.mu.g/cm.sup.2. The coating weight of the luminescent layer is 1.8 
mg/cm.sup.2. The end portions of the lamps according to the invention of 
the first group are provided with an yttrium oxide layer with a coating 
weight of approximately 30 .mu.g/cm.sup.2. In the second group of lamps 
according to the invention, the end portions are provided with an aluminum 
oxide layer with a coating weight of approximately 250 .mu.g/cm.sup.2, 
while the group of lamps not according to the invention has no metal oxide 
layer on the end portions. 
The lamps were provided with a filling of 0.4 mg mercury and argon. The 
total quantity of bound mercury was measured after 1000 hours of 
operation. The measuring method used is based on the phenomenon that free 
mercury moves to the negative electrode in a DC-operated lamp. The 
displacement of mercury is visible in the form of a decrease in intensity 
of the light radiated by the lamp near the end of the positive electrode. 
In the embodiment of the measuring method used during the test, the 
polarity of the DC voltage was reversed the moment the luminous intensity 
near the end of the positive pole had dropped to 60% of the rated value. 
The time which elapses between this moment and the moment at which the 
luminous intensity near the opposite end has dropped to 60% of the rated 
value is a measure for the quantity of free mercury still available, and 
thus for the mercury consumption. The measuring method was calibrated by 
means of the results obtained by a wet-chemical analysis. 
The mercury consumption (in .mu.g) in the period up to 1000 hours is shown 
in Table II. The Table also indicates between parentheses the mercury 
consumption in the period from 1 to 1000 hours. 
TABLE 2 
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Mercury consumption in .mu.g in the operating period up to 1000 hours, 
and mercury consumption in the operating period from 1 to 1000 hours 
(between parentheses) for lamps according to the invention and 
lamps not according to the invention. 
Coating of Coating of end portions 
tubular portion 
Y.sub.2 O.sub.3 
Al.sub.2 O.sub.3 
-- 
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Al.sub.2 O.sub.3 
225 (144) 220 (114) 
291 (196) 
SiO.sub.2 /Al.sub.2 O.sub.3 
168 (83) 147 (69) 200 (112) 
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It is again apparent from the measurements that the measure according to 
the invention results in a significant decrease in the mercury 
consumption. The mercury consumption during the first hour of operation 
(80 to 110 .mu.g) is substantially independent of the coating of the 
tubular portion and the coating or absence thereof on the end portions. In 
the period after the first hour of operation, the measure according to the 
invention results in a comparatively strong reduction in the mercury 
consumption. The reduction ranges from 26% with the use of an yttrium 
oxide layer as the metal oxide layer on the end portions of lamps having a 
metal oxide layer of yttrium oxide combined with an alkali metal repelling 
layer of silicon oxide on the tubular portion, to 42% with the use of an 
aluminum oxide layer on the end portions of lamps having besides a 
luminescent layer exclusively an aluminum oxide layer on the tubular 
portion. The lowest mercury consumption was found in lamps whose tubular 
portions of the discharge vessels were provided with an aluminium oxide 
layer supported by a silicon oxide layer, while the end portions were 
coated with aluminium oxide.