Low-pressure sodium discharge lamp with specific current supply coatings

The low pressure sodium discharge lamp has a discharge vessel (1) having pinch seals (2) through which current conductors (4) extend towards electrodes (3) inside the discharge vessel (1). The current conductors (4) each have a first glass coating (5), which extends from inside a pinch seal (2) into the discharge vessel (1) and a second lime glass coating (6) abutting the first (5) and extending to outside the discharge vessel (1). A new first sodium resistant glass coating (5), which is substantially devoid of borate, is defined.

The invention relates to a low-pressure sodium discharge lamp comprising: 
a discharge vessel with pinch seals which is closed in a vacuumtight manner 
and which has a filling comprising sodium and rare gas; 
electrodes arranged in the discharge vessel and each connected to at least 
one corresponding current conductor which issues through a pinch seal to 
the exterior, 
which current conductors each have a first, comparatively thin glass 
coating which extends from the relevant pinch seal to inside the discharge 
vessel, and a second, comparatively thick coating of lime-barium glass 
abutting the first coating and extending from said pinch seal to outside 
the discharge vessel; and 
an evacuated outer bulb surrounding the discharge vessel and provided with 
an IR reflection filter. 
Such a low-pressure sodium discharge lamp is known from U.S. Pat. No. 
4,783,612. 
The first, comparatively thin, for example approximately 0.3 mm thick and 
mechanically comparatively weak coating protects the current conductors 
against electrical contact with solid or liquid sodium which is deposited 
against the pinch seal. It is prevented thereby that the discharge arc 
applies itself to the sodium during lamp operation and leads to violent 
reactions which may result in damage to the current conductor and to the 
pinch seal. The glass of the first coating comprises some tens of percents 
by weight of boron oxide and in addition more than 50% by weight of barium 
oxide, but only a few percents of silicon oxide. The glass is resistant to 
sodium in the case of operation at mains frequency. 
The second, comparatively thick, for example approximately 0.7 mm thick 
coating serves to provide the current conductor with a mechanically strong 
envelope, absorbing the difference in coefficient of thermal expansion 
between the current conductor and the discharge vessel and providing a 
vacuumtight seal. 
It was found that the first coating is attacked by sodium in the case of 
high-frequency operation of the lamp, for example at several kHz, for 
example 45 kHz, as a result of which the discharge vessel exhibits cracks 
in its pinch seals after a comparatively short operating period of two to 
three thousand hours already and becomes leaky. The low viscosity of the 
glass of the first coating during lamp manufacture, leading to an 
irregular thickness of the coating, and electrolytic processes taking 
place at high-frequency operation as a result are to blame for this. 
A low-pressure mercury discharge lamp is known from U.S. Pat. No. 5,498,927 
where a first coating of the current conductor made from sodium-resistant 
glass of low viscosity at the pinching temperature extends from inside the 
discharge vessel through the pinch seal to outside the discharge vessel. 
The first coating is surrounded by a second coating of the more viscous 
glass from which the discharge vessel is made from inside the pinch seal 
to outside the discharge vessel. The second coating protects the first 
coating while the pinch seal is being made, so that the first coating has 
a more homogeneous thickness and is better resistant to high-frequency 
operation. It was found, however, that it is difficult to manufacture the 
lamp according to this Patent in an industrial process. 
A low-pressure sodium discharge lamp is known from JP-A-491974!-33870, 
where the current conductors each have only one coating of borate glass 
which extends from inside the discharge vessel through the pinch seal to 
outside the discharge vessel. The lamp known from this document was found 
to become leaky prematurely in the case of high-frequency operation. 
It is an object of the invention to provide a low-pressure sodium discharge 
lamp of the kind described in the opening paragraph which is of a simple, 
mechanically strong construction which can be readily manufactured on an 
industrial scale and which is resistant to sodium also in high-frequency 
operation. 
According to the invention, this object is achieved in that the first glass 
coating is made from a glass comprising the following constituents in % by 
weight: SiO.sub.2 30-50; Al.sub.2 O.sub.3 5-10; ZrO.sub.2 2-6, with the 
total quantity of Al.sub.2 O.sub.3 and ZrO.sub.2 being 7-15; Li.sub.2 O 
1-4; Na.sub.2 O 4-7; K.sub.2 O 0-0.5; MgO plus CaO in a total quantity of 
8-12; SrO 3-9; BaO 20-32; rest &lt;1. 
The low-pressure sodium discharge lamp according to the invention is of a 
mechanically strong construction which is readily obtained industrially. 
The lamp is resistant to sodium also in the case of high-frequency 
operation. 
Each of the constituents of the glass of the first coating with its 
quantitative limits is of essential importance for the properties of the 
lamp. A too low SiO.sub.2 content, i.e. a content lower than the bottom 
value indicated, involves the risk of the glass crystallizing, which 
renders it difficult or impossible to process. Stresses also arise in the 
glass then, involving the risk of cracks, owing to a too high coefficient 
of thermal expansion. If the SiO.sub.2 content is too high, i.e. higher 
than the upper value indicated, the glass will have a bad sodium 
resistance. An SiO.sub.2 content of 30 to 40% by weight is favorable for a 
high sodium resistance. If the quantity of Al.sub.2 O.sub.3 plus ZrO.sub.2 
were to pass below the bottom limit indicated or above the upper limit 
indicated, thus increasing or decreasing the SiO.sub.2 content, the glass 
would have a bad sodium resistance or crystallize, respectively. Too much 
of these oxides would make the melting point too high, so that the glass 
would be difficult to process. It is favorable for the sodium resistance 
when the sum of these oxides is 10-15% by weight. The values of Li.sub.2 O 
are important on account of a too low coefficient of expansion and a too 
high melting point, respectively a too high coefficient of expansion, 
respectively crystallization. Relevant for Na.sub.2 O are a too low 
coefficient of expansion and a too high electrical conductivity and 
accordingly a bad sodium resistance; for MgO, CaO, and SrO a bad sodium 
resistance, a too high melting point, and the risk of crystallization 
versus a too low melting point and crystallization; for BaO a bad sodium 
resistance and a too high melting point versus crystallization. If there 
is too much K.sub.2 O, the sodium resistance will be bad and potassium 
will be exchanged with sodium in the glass, so that potassium having a low 
efficacy will take part in the discharge. The residual content of the 
glass may be SO.sub.3 and/or Sb.sub.2 O.sub.5 originating from a purifying 
agent, and impurities such as Fe.sub.2 O.sub.3, chlorides, fluorides, and 
the like. 
It is favorable when the glass of the first glass coating has a composition 
of SiO.sub.2 37.1.+-.3.0; Al.sub.2 O.sub.3 8.1.+-.1.0; ZrO.sub.2 
4.0.+-.0.5; Li.sub.2 O 2.3.+-.0.2; Na.sub.2 O 6.2.+-.0.6; K.sub.2 O 
0.06.+-.0.05; MgO 4.1.+-.0.5; CaO 5.9.+-.0.5; SrO 6.0.+-.0.5; BaO 
26.0.+-.2; rest 0.24.+-.0.1% by weight.

In FIG. 1, the low-pressure sodium discharge lamp has a discharge vessel 1 
which is closed in a vacuumtight manner and which has pinch seals 2, see 
FIG. 2, and a filling comprising sodium and rare gas. Electrodes 3, made 
of tungsten in the Figures, are arranged in the discharge vessel 1 and are 
each connected to at least one corresponding current conductor 4 which 
issues through a pinch seal 2 to the exterior. Each electrode 3 in FIG. 2 
is connected to two current conductors made of FeNiCr. The current 
conductors 4 each have a first, comparatively thin glass coating 5, having 
a wall thickness of approximately 0.3 mm in FIG. 2, which extends from the 
relevant pinch seal 2 to inside the discharge vessel 1, and a second, 
comparatively thick lime-barium glass coating 6, having a wall thickness 
of approximately 0.7 mm in the Figure. The second coating 6 has a butt 
joint against the first coating 5 and extends from the relevant pinch seal 
2 to outside the discharge vessel 1. The discharge vessel 1, see FIG. 2, 
internally has a sodium-resistant coating 1'. An evacuated outer bulb 8 
provided with an IR reflection filter 7, for example made of tin-doped 
indium oxide, surrounds the discharge vessel 1. 
The first coating 5 is made from glass having a composition shown in column 
"5" in Table 1. In the embodiment shown, the glass has the composition of 
"5" example 1 ("5" ex 1) from the Table. An alternative is shown in column 
"5" example 2 ("5" ex 2). The Table further shows a composition "6" of the 
lime-barium glass of the second coating 6, the composition of the glass 
"1" of the discharge vessel 1, and the composition of the borate glass 
coating 1' on the inside of the discharge vessel 1. 
A striking feature in the coatings 5 is the high content of SiO.sub.2 plus 
Al.sub.2 O.sub.3 plus ZrO.sub.2 compared with the sodium-resistant glass 
1' on the inside of the discharge vessel 1, and the substantial lack of 
B.sub.2 O.sub.3. 
Lamps as shown in the drawing and each provided with a first glass coating 
5 having the composition of the examples of Table 1 were operated at high 
frequency. The lamps were still fully intact after 7000 hours of operation 
and showed no trace of electrolysis. 
TABLE 1 
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component.backslash. 
glass "5" "5" ex 1 "5" ex 2 
"6" "1" "1'" 
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SiO.sub.2 
30-50 37.1 43.7 66.3 63.3 5.7 
Al.sub.2 O.sub.3 
5-10 8.1 5.0 2.4 4.7 9.2 
ZrO.sub.2 
2-6 4.0 3.0 
Al.sub.2 O.sub.3 + 
7-15 12.1 8.0 
ZrO.sub.2 
Li.sub.2 O 
1-4 2.3 3.0 
Na.sub.2 O 
4-7 6.2 5.0 7.0 17.1 
K.sub.2 O 
0-0.5 0.06 0.05 8.8 0.7 
MgO + CaO 
8-12 10.0 10.0 
MgO 4.1 4.1 3.1 5.0 
CaO 5.9 5.9 4.7 10.0 
SrO 3-9 6.0 4.0 0.25 1.1 
BaO 20-32 26.0 26.0 13.6 5.2 50.4 
B.sub.2 O.sub.3 1.5 0.8 18.5 
rest &lt;1 SO.sub.3 SO.sub.3 
F 0.1 
SO.sub.3 
0.16 0.16 0.07 
Fe.sub.2 O.sub.3 
Fe.sub.2 O.sub.3 
0.05 
0.33 0.1 
0.08 0.07 
0.02 
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