Method of manufacturing a low-pressure mercury vapor discharge lamp and low-pressure mercury vapor discharge lamp manufactured by this method

Method of manufacturing a low-pressure mercury vapor discharge lamp (1) comprising a discharge envelope, the glass wall of which comprises at least an (alkaline earth) alkali constituent, a silicon dioxide-containing transparent layer (4) being produced on the inner wall of the discharge envelope. In the method, such a direct current is maintained through the glass that the (positive) (alkaline earth) alkali ions move away from the inner wall and upon heating the said silicon dioxide-containing transparent layer (4) is produced.

The invention relates to a method of manufacturing a low-pressure mercury 
vapor discharge lamp comprising a discharge envelope having a glass wall 
which comprises at least an alkali and/or an alkaline earth constituent, a 
silicon dioxide-containing transparent layer being produced on the inner 
surface of the glass wall. The invention further relates to a low-pressure 
mercury vapor discharge lamp manufactured by such a method. Such a method 
is known from U.S. Pat. No. 3,067,356. 
The said Patent Specification discloses that the inner surface of the 
discharge envelope of the low-pressure mercury vapor discharge lamp is 
provided with a thin transparent protective layer consisting, for example, 
of silicon dioxide. According to the U.S. Patent, it is then avoided that 
the glass wall (which consists of a glass which is commonly used for lamps 
of this type, such as lime glass, which comprises a comparatively high 
percentage of alkali and/or alkaline earth constituents) exhibits a dark 
discoloration after a comparatively small number of operating hours of the 
lamp. This dark discoloration naturally results in loss of light and in an 
unaesthetic appearance of the lamp, in particular due to the fact that the 
dark discoloration occurs irregularly, for example, in the form of dark 
spots and dots. According to the U.S. Patent, the said spots and dots 
consist of compounds of mercury and alkali constituents of the glass (such 
as sodium and potassium) present on or near the inner surface of the 
discharge envelope. 
The above-mentioned phenomenon occurs particularly at areas at which the 
discharge is in direct contact with the glass wall. This is the case in 
lamps in which the discharge envelope is entirely or partly free from 
luminescent material, as in irradiation lamps and copying lamps. It is 
further described that first an organic liquid, in which finely divided 
silicon dioxide is dissolved, is deposited on the inner wall of the 
discharge envelope for the formation of the said transparent layer. 
Subsequently, the whole is dried and, in order to obtain a satisfactory 
adhesion to the wall of the discharge envelope, sintered to a temperature 
which is slightly lower than the softening temperature of the glass of the 
wall. This process is time-consuming and expensive. Moreover, there is a 
risk that unevenness occur in the layer. This may result, during operation 
of the lamp, in undesired dark spots and dots visible on the wall of the 
discharge envelope. 
The invention has for its object to provide a method of manufacturing a 
low-pressure mercury vapor discharge lamp, in which the inner wall of the 
discharge envelope is provided in a simple manner with a silicon 
dioxide-containing transparent layer and attack and blackening of the 
glass wall of the discharge envelope during operation of the lamp is 
reduced. 
The method according to the invention is therefore characterized in that 
electrical conductors are arranged on both sides of the wall of the 
discharge envelope and, while heating the wall of the discharge envelope, 
a direct current is maintained in the glass between the conductors for 
such a period that a thin continuous silicon dioxide-containing layer is 
formed at the inner surface of the discharge envelope. This layer is 
substantially free from alkali and/or alkaline earth constituents. 
By means of the method according to the invention, a protective silicon 
dioxide-containing continuous layer is produced in the discharge envelope 
in a simple manner. Expensive and time-consuming steps during the 
manufacture, such as the preparation and the use of special organic 
solvents for the formation of the thin layer as well as the use of a 
special drying and sintering process, are avoided. 
The invention is based on the idea that an electric direct current is 
maintained in the glass by the disposition of the conductors on both sides 
of the glass wall of the discharge envelope, as a result of which the 
relatively mobile alkali and alkaline earth constituents in the glass 
(such as, for example, sodium, potassium and calcium ions) move towards 
the outer surface of the wall. Oxygen ions from the glass move towards the 
area at which all the (earth alkaline) alkali ions have disappeared from 
the glass (near and on the inner surface). It has been found that a rare 
gas plasma can serve as a conductor inside the discharge envelope. In the 
method according to the invention, there is formed at the inner surface of 
the glass wall a thin layer (in a practical embodiment approximately 50 to 
100 nm) which is free from the (alkaline earth) alkali constituents, 
penetration of mercury into the glass being avoided. It is essential in 
the method that during the current passage the glass is heated. The 
temperature then depends upon the glass to be used and is generally higher 
than 200.degree. C. In a practical embodiment, the glass wall is heated to 
a temperature of, for example, 450.degree. C. (just below the softening 
temperature of the glass), the glass relaxing and being converted into the 
continuous isolating quartzy layer (the silicon dioxide-containing layer). 
This layer extends over the whole inner wall of the discharge envelope. 
As a result of the method used, the smaller mobility of the alkaline earth 
ions with respect to the alkali ions causes a second thin layer to be 
produced between the quartzy surface layer and the inner part of the 
glass, which layer is enriched with alkaline earth ions (such as calcium 
ions) and is also substantially free from alkali constituents. (The second 
layer contains, for example, calcium silicate). 
In a preferred embodiment of the method according to the invention, a rare 
gas plasma, such as an argon plasma, is present in the discharge envelope 
as an electrical conductor, the glass wall acting as negative conductor 
for the plasma and the anode for the plasma being disposed at an end of 
the (preferably tubular) discharge envelope. The whole inner surface of 
the discharge envelope has then formed on it a uniform silicon 
dioxide-containing layer which is free from undesired ions originating 
from the conductor. In an embodiment two anodes are constituted, by 
electrodes which are arranged in the discharge envelope and between which 
a discharge is maintained during the (later) operation of the lamp. As a 
result of the direct current in the glass, negative ions (especially 
oxygen ions) and electrons are released from the inner wall (cathode 
action). In a practical embodiment, a negatively charged body of an 
electrically conducting material is arranged on the outer side around the 
discharge envelope, which body is in contact with the glass wall of the 
discharge envelope at at least a number of points regularly spaced apart. 
In the glass of the whole wall a substantially uniform density of the 
direct current is then obtained. In a practical embodiment, the negatively 
charged body is a sleeve of a feltlike material, such as graphitic carbon, 
which is capable of withstanding the comparatively high temperature during 
heating of the glass wall and does not oxidize. In an embodiment, for some 
time (for example, approximately 40 sec) a voltage difference of 
approximately 500 V is applied between this conductor and the conductor 
(an anode) present in the discharge envelope. Beforehand and during this 
process, the glass wall (which in said practical embodiment has a 
thickness of approximately 1 mm) is heated to a temperature of 
approximately 450.degree. C., as a result of which the continuous 
transparent silicon dioxide-containing layer having a thickness of 
approximately 50 nm is produced on the inner wall of the discharge 
envelope. 
The method according to the invention is preferably used in the manufacture 
of lamps having discharge envelope which are free at least in part from 
luminescent material, as, for example, in irradiation lamps and copying 
lamps. 
However, the method may also advantageously be used in the manufacture of 
low-pressure mercury vapor discharge lamps, in which the whole inner wall 
of the discharge envelope is provided with a luminescent material. The 
discharge envelope is then first provided in a usual manner with a 
luminescent layer, which is dried and sintered, whereupon the electrodes 
are arranged in the discharge envelope the discharge envelope is evacuated 
etc. and subsequently this envelope is subjected to a treatment in 
accordance with the method of the invention. It has been found that a lemp 
thus manufactured had a lower lumen loss with respect to time than a lamp 
which had not been subjected to such a treatment. In another embodiment, 
first the inner wall of the discharge envelope is subjected to a treatment 
according to the method, after which a luminescent layer is applied. It 
has been found that no alkali (alkaline earth) atoms were present in the 
luminescent material even after the sintering step to 600.degree. C.

In FIG. 1, reference numeral 1 designates a tubular discharge envelope of a 
low-pressure mercury vapor discharge lamp. Two electrodes 2 and 3 are 
arranged at the ends of the discharge envelope. During operation of the 
lamp, a discharge is maintained between these electrodes. The inner wall 
of the discharge envelope is completely free from luminescent material. 
The glass of the wall of the discharge envelope has the following 
composition (% by weight): 72.3% of SiO.sub.2 ; 16.9% of Na.sub.2 O; 0.8% 
of K.sub.2 O; 5.3% of CaO; 2.6% of MgO; 1.7% of Al.sub.2 O.sub.3 ; 0.33% 
of Sb.sub.2 O.sub.3 ; 0.12% of Fe.sub.2 O.sub.3. 
A continuous transparent protective layer 4 containing silicon dioxide is 
present on the whole inner side of the wall. This layer has been produced 
with the aid of the method according to the invention, which is explained 
with reference to FIG. 2. 
In this method, the discharge envelope is first closed and provided with 
argon at a pressure of 400 Pa. In the discharge envelope, there are 
arranged in the proximity of the electrodes 2, 3 respective titanium 
plates 5,6, which are each connected through a respective wire 7 and 8, to 
a respective one of the supply wires 9, 10 of the electrodes. A positive 
voltage is applied to the connection wires 11, 12 connected to these 
supply wires. Each titanium plate 5,6 is then charged positively and acts 
as an anode for the argon plasma in the discharge envelope. A negatively 
charged electrical conductor is arranged around the outer wall of the 
discharge envelope. This conductor consists of an elastic felt-like 
blanket of graphite carbon in the form of a sleeve 13 (thickness 10 mm). 
The sleeve bears on the outer wall of the discharge envelope 1. A voltage 
difference of approximately 500 V is applied between the sleeve 13 and the 
titanium plates 5,6 for approximately 40 sec. A direct current is then 
produced in the glass between plates 5,6 and sleeve 13, as a result of 
which the relatively mobile alkali and alkaline earth constituents in the 
glass move towards the outer wall of the discharge envelope. The current 
density in the glass then decreases from 750 mA/cm.sup.2 to 100 
mA/cm.sup.2. A part of the oxygen ions present in the glass moves towards 
the inner wall and even moves to the plasma present in the discharge 
envelope. The oxygen ions are conducted away with the aid of a gas current 
(argon) through the exhaust tubes 14, 15 connected to a pump. Before and 
during the maintenance of the direct current in the glass, the discharge 
envelope is heated to a temperature of approximately 450.degree. C. The 
body 13 is not attacked by oxidation. Due to this heating, the glass 
relaxes at the inner surface and is converted into a thin continuous 
isolating quartzy layer of silicon dioxide, which is free from alkali 
and/or alkaline earth constituents. After the glass wall has been exposed 
to the direct current during the period of time, the connection wires 7 
and 8 are melted by high energy current pulses. Finally, the mercury is 
dosed in the discharge envelope (cf., for example, GB-PS 1,475,458), 
whereupon the lamp (20 W, length 40 cm, diameter 2.5 cm) is ready for use. 
It has been found that the decrease of the radiation output of this lamp 
amounted to only 2% after 8000 operating hours.