Oxygen dispenser for high pressure discharge lamps

Details Oxygen dispenser for high pressure discharge lamps Oxygen dispenser for high pressure discharge lamps

1998-06-23

2001-01-02

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Having vapor generating material

C313S553000, C313S562000, C313S563000

Utility Patent

active

06169361

ABSTRACT:

The present invention refers to an oxygen dispenser for high pressure discharge lamps. High pressure discharge lamps have a structure that comprises an outer glass envelope that may be kept evacuated or filled with an inert gas, generally nitrogen; inside the envelope is present a transparent discharge tube, that may be made of quartz or translucid ceramic, generally alumina. The outer envelope protects the discharge tube from inward diffusion of atmospheric gases that would occur in case of a non-protected tube, given the high temperatures reached by its surface during lamp working.
Discharge tube filling gases vary depending on the lamps, but these generally comprise at least one noble gas and, depending on the kind of lamp, little additions of sodium vapors, mercury vapors and metal halogenides (generally iodides). Two metallic electrodes are fitted into the ends of the discharge tube: when a potential difference is applied to the electrodes, a plasma is formed in the gaseous mixture filled in the discharge tube. The plasma emits radiations of wavelength in the visible and ultraviolet (UV) range. Some lamps also have on the inner surface of the outer envelope a thin layer of so-called phosphors, which function is to convert at least partially the UV radiation into visible light. Other lamps have a layer of ceramic powders, generally zirconium oxide (ZrO
2
), deposited over the two ends of the discharge tube, that helps keeping the working temperature inside the tube.
Lamps manufacturers have found that small amounts of oxygen present into the outer envelope may be advantageous to the lamp functioning.
U.S. Pat. No. 4,918,352 describes a lamp having in the outer envelope an oxygen gas adding or an oxygen dispenser that releases such gas upon heating when the lamp is turned on. According to said patent this expedient serves to oxidize the surface of electric leads present in the envelope, so as to prevent losses of sodium from the gas filled in the discharge tube.
It is known from U.S. Pat. No. 4,499,396 the advantage of having a slightly oxidizing atmosphere, due to the presence of traces of oxygen, in the outer envelope of the lamp; such atmosphere prevents the reduction and blackening of phosphors that would result in lowering in time of the lamp brightness. Blackening of phosphors may occur due to the hydrocarbons present in the outer envelope. Hydrocarbons in the lamp may come from various sources. Hydrocarbons may be introduced into the outer envelope as contaminants of components of the lamp, such as the current leads; they may come from the oil of the vacuum pumps used to evacuate the envelope; or, they may be a residue of organic binders employed in the pastes used to lay some coverings, such as those of ZrO
2
over the discharge tube ends or those of phosphors on the inner surfaces of the envelope. At the working temperature of the lamp, hydrocarbons decompose giving rise to carbon that deposits on the outer envelope and/or on the discharge tube in the form of a black layer. This black layer not only affects the maintenance in time of the lamp brightness, but also the discharge tube temperature, giving rise to a change in the lamp color. As these deposits are formed already during the first hours of lamp operation, it would be desirable to prevent their formation at a stage as early as possible of the lamp life.
A filling of gaseous oxygen in the outer envelope soon after lamp production does not allow however to check the hermetic seal of the envelope with the method commonly used by lamp manufacturers, consisting in generating an electrical discharge, called “glow discharge”, in the same envelope. As a consequence it would be advantageous having available an oxygen dispenser that releases this gas only after execution of the check of the hermetic seal of the envelope. Unfortunately the mentioned U.S. patents do not teach the use of any oxygen compound useful to this end.
APL Engineered Materials, Inc., Illinois, USA proposes in its technical-commercial catalogue the use in lamps of barium peroxide, BaO
2
. BaO
2
is introduced in the outer envelope of the lamp in a device made up of a stainless steel container with a small porous lid. According to APL's catalogue, this device maintains a slightly oxidizing atmosphere in the envelope. The device must be placed into the lamp in a position such that it is heated from the discharge tube; as a consequence of heating, BaO
2
releases oxygen that reacts with hydrocarbons (C
n
H
m
) according to the following reactions:
BaO
2
→ BaO+½ O
2
  (I)
C
n
H
m
+(n+¼ m)O
2
→n CO
2
+(m/2) H
2
O   (II)
The use of BaO
2
has however some drawbacks.
First, the use of BaO
2
in lamps had been initially proposed in U.S. Pat. No. 3,519,864 with the aim of sorbing hydrogen, generally present in lamps, that has the negative effect of increasing the voltage needed to initiate the discharge in the discharge tube. BaO
2
reacts with hydrogen according to the reaction:
BaO
2
+H
2
→Ba(OH)
2
  (III)
Thus formed Ba(OH)
2
may, in turn, decompose according to the reaction:
Ba(OH)
2
→BaO +H
2
O   (IV)
that is quite undesirable.
Moreover, reactions (I), (II) and (IV) may take place simultaneously, thus making difficult an exact dosing of BaO
2
. Such dosing is made even more complex by the fact that the rate of these reactions depends, in different ways, on the temperature. In order to overcome this problem, the commercial catalogue of the firm APL indicates that the positioning of the container of BaO
2
must be such that BaO
2
is maintained at a temperature comprised between about 250 and 325° C. This condition is however all but easy to realize, because the thermal profile inside lamps depends in a complex way on factors such the work positioning (horizontal, vertical or intermediate positioning) or on dimensions and materials making up the lamp housings.
Finally, the release of oxygen from BaO
2
takes place with high rate only at temperatures in excess of 500° C., and thus the maximum suggested temperature of 325° C. does not allow a fast release of oxygen at the very beginning of lamp life, as desirable.
Object of the present invention is to provide an oxygen dispenser for high pressure discharge lamps of fast oxygen release at relatively low temperatures.
This object is reached according to the present invention with an oxygen dispenser for high pressure discharge lamps comprising a metallic container capable of retaining solid materials but pervious to gas passage, inside which is filled silver oxide, Ag
2
O.
Ag
2
O releases oxygen according to the reaction:
Ag
2
O→2Ag+½O
2
  (V)
The use of Ag
2
O offers a series of advantages when compared to the use of BaO
2
. First, oxygen release starts at temperatures of about 300° C. As a consequence, it is possible to complete the production cycle of the lamp, including the hermetic seal check with the glow discharge method, without oxygen release. On the other hand Ag
2
O shows a fast oxygen release at temperatures of about 340° C., and a very fast release at temperatures of about 400° C., as described in the following. It is thus available a relatively broad temperature field at rather low temperatures, between about 340 and 400° C., in which Ag
2
O is effective for oxygen emission. This allows a rather free positioning of the dispenser inside the lamp, particularly in zones where the dispenser can receive heat from the discharge tube without however interfering with light output of same. The oxygen dispenser may be placed near an end of the discharge tube or parallel to the same, for instance mounted on a current lead. The freedom of positioning of the oxygen dispenser is furthermore increased by the fact that oxygen may be released by means of an activation operation after completion of the lamp production, but before first turning on of same. Activation may be done by heating the dispenser with an external heat source, for instance by means of radio freque

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