JPH0527940B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This invention relates to improvements in electrodes used in high-pressure sodium lamps having unsaturated vapor pressure characteristics, and more particularly, to improvements in electrodes used in high-pressure sodium lamps having unsaturated vapor pressure characteristics. This study relates to the improvement of electron-emitting materials.

[Prior art]

Generally, in a saturated vapor pressure type high-pressure sodium lamp, an amount of sodium and mercury in excess of the amount contributing to light emission is sealed in the arc tube in order to compensate for the loss of sodium during lamp operation. These excess amounts of sodium and mercury form an amalgam in the coldest part of the arc tube. Therefore, there is a problem in that the vapor pressure within the arc tube is affected by changes in the temperature of the coldest part and the amalgam composition ratio. Furthermore, such high-pressure sodium lamps of the saturated vapor pressure type have the disadvantage that the lamp voltage increases during the lamp stroke and eventually goes out.

On the other hand, by limiting the amount of encapsulated sodium and mercury,
In so-called unsaturated vapor pressure type high-pressure sodium lamps, in which all of the sodium and mercury enclosed in the lamp evaporate during operation, the vapor pressure inside the arc tube remains almost unchanged even if the temperature of the coldest part changes. It is already known that the lamp voltage is stable when the lamp is lit. However, in such an unsaturated vapor pressure type high-pressure sodium lamp, the amount of sodium sealed is very small, so even a small amount of sodium disappears has a significant effect on the lamp voltage. That is, there is a problem in that the lamp voltage gradually decreases as sodium disappears.

It has been known for some time that the main cause of this loss of sodium is the gases, especially oxygen, released from the electrodes when starting and lighting the lamp.

By the way, high-melting point metals such as tungsten have traditionally been used as electrode constituent materials for high-pressure sodium lamps, but it is difficult to provide sufficient electron emission even at high temperatures with tungsten alone, so electron emission is usually Electron emissivity is compensated for by using metal oxides with excellent properties.

Such electron emissive metal oxides usually include
Alkaline earth metal oxides such as barium and calcium, thorium oxide, and the like are used.

[Problem that the invention seeks to solve]

By the way, with the electrodes using metal oxides as mentioned above, the electrode temperature during lamp operation is approximately
It reaches 1700℃ and gradually decreases towards the end. During lighting operation, the electron emitting material (emitter), which is mainly composed of the metal oxide mentioned above, gradually evaporates and scatters from a portion near the tip and adheres to the inner surface of the arc tube. If the amount of electron emitting material attached to the inner surface of the arc tube increases, it becomes a major cause of a decrease in luminous flux.

As a countermeasure to these problems, conventionally, the tungsten coil wound around the electrode core was formed into a double coil, and an electron emitting material was densely filled between the inner coil and the electrode core. A method is known in which the surface of the coil is structured so that the electron emitting material is not exposed as much as possible to suppress the evaporation of the electron emitting material.

However, although the above-mentioned alkaline earth metal oxides have excellent electron emitting properties, their vapor pressure is relatively high at high temperatures, and it is difficult to completely suppress the evaporation of the oxides filled in the coil. . Also,
Oxides containing alkaline earth metals, or composite oxides with tungsten oxide, etc., partially decompose and evaporate at around 1300°C, resulting in liberated alkaline earth metals,
Releases alkaline earth metal oxides, oxygen, etc.

Among these decomposed vaporized components, oxygen in particular reacts with sodium to produce stable compounds with low vapor pressures.

This generation of sodium oxide is one of the main causes of sodium loss in the arc tube.
Particularly in unsaturated vapor pressure type high-pressure sodium lamps, the loss of sodium due to the formation of sodium oxides causes a drop in lamp voltage and shortens lamp life, so it is important to suppress this loss of sodium as much as possible. Required.

In order to suppress the loss of sodium due to the formation of sodium oxides, it is necessary not only to use the unsaturated vapor pressure type high-pressure sodium lamp mentioned above, but also to use a sodium amalgam filled with sodium amalgam to the extent that not all but 80% or more of the filled amount evaporates during lighting. This is also a necessary requirement for high-pressure sodium lamps. In other words, if the lamp is filled with sodium amalgam to the extent that 80% or more of the amount evaporates during lighting, the composition ratio of the amalgam is the same as that of an unsaturated lamp.
It exhibits unsaturated vapor pressure characteristics rather than saturated vapor pressure characteristics, and it is equivalent to unsaturated vapor pressure lamps in that the loss of a small amount of sodium has a large effect on lamp characteristics, so the amount of fill Suppressing sodium loss is also an essential requirement for high-pressure sodium lamps with unsaturated vapor pressure characteristics, which contain an amount of sodium amalgam that evaporates not all but 80% or more. be.

Note that thorium oxide has a high melting point and a relatively small work function, so it is used as an electron emitting material. However, since this thorium oxide is an oxide of a radioactive isotope,
It must be handled in accordance with radioactive material handling standards, and there are many problems in terms of workability, so it is currently being used less and less as an electron emitting material.

The present invention was made in order to solve the above-mentioned problems with the electron emitting material applied to the arc tube electrode of a conventional high-pressure sodium lamp having unsaturated vapor pressure characteristics, and emits a substance that reacts with sodium. It is an object of the present invention to provide an electrode for a high-pressure sodium lamp having an unsaturated vapor pressure type characteristic and provided with an electron-emitting material, which is difficult to absorb and therefore reduces the lamp voltage drop during lamp operation and improves the lamp life. It is something.

[Means and actions for solving problems]

In the present invention, as a result of various experiments and studies to solve the above-mentioned problems, yttrium oxide (Y ₂ O ₃ ) is used as the electron emitting material of the electrode.

When selecting the above-mentioned yttrium oxide as the electron emitting material for the electrode, the present inventors conducted basic experiments to examine metal oxides for the electron emitting material that satisfied the conditions shown below.

(1) It must have a high melting point of at least 1,600℃ so that it does not decompose at the arc tube seal temperature of a high-pressure sodium lamp.

(2) It must have a low vapor pressure at the electrode operating temperature.

(3) The work function for thermionic radiation is relatively small.

(4) It is difficult to react with sodium at the temperature when the lamp is lit.

In this basic experiment, barium oxide (BaO) and calcium oxide, which are currently widely used as electrode emitter materials for high-pressure sodium lamps, were used.
CaO, etc., or complex oxides containing them (e.g. barium calcium tungstate)
Ba ₂ CaWO _6, etc.) has a low decomposition temperature and easily reacts with sodium, as mentioned above, and
In addition, thorium oxide ThO ₂ and composite oxides containing it were also excluded from the list of metal oxides, considering the fact that thorium is one of the radioactive elements as mentioned above.

As a result of the above experimental study, the electron emitting material that satisfies each of the conditions (1) to (4) above is scandium oxide.
Sc ₂ O ₃ , Yttrium oxide Y ₂ O ₃ , Lanthanum oxide
La ₂ O ₃ , praseodymium oxide Pr ₂ O ₃ , neodymium oxide
It was found that they were oxides of rare earth elements such as Nd ₂ O ₃ , samarium oxide Sm ₂ O ₃ , gadolinium oxide Gd ₂ O ₃ , dysprosium oxide Dy ₂ O ₃ , and mixtures of these oxides.

It was also found that the melting points of these oxides decrease in the following order: i.e. _CeO2
(2500℃), Y ₂ O ₃ (2420℃), Sc ₂ O ₃ (2405℃),
Gd ₂ O ₃ (2395℃), Dy ₂ O ₃ (2385℃), Sm ₂ O ₃ (2320
℃), Nd ₂ O ₃ (2315℃), La ₂ O ₃ (2300℃), Pr ₂ O ₃
(2290℃).

It was also found that the evaporability of these oxides increases in the following order. That is, Y ₂ O ₃ ,
Sc ₂ O ₃ , Gd ₂ O ₃ , Dy ₂ O ₃ , La ₂ O ₃ , Sm ₂ O ₃ ,
The order is Pr ₂ O ₃ , Nd ₂ O ₃ , and CeO ₂ .

It was also found that the work functions of these oxides increase in the following order: That is,
Y ₂ O ₃ (2.0eV), Gd ₂ O ₃ (2.1eV), Dy ₂ O ₃ (2.1eV),
Nd ₂ O ₃ (2.3eV), La ₂ O ₃ (2.8eV), Pr ₂ O ₃ (2.8eV),
Sm ₂ O ₃ (2.8eV), CeO ₂ (2.9eV), Sc ₂ O ₃ (3.2eV)
The order is

Among these rare earth element oxides, when yttrium oxide Y ₂ O ₃ was used as the electron emitting material, the drop in lamp voltage was significantly suppressed and the starting characteristics were also relatively good.

Therefore, the present invention uses yttrium oxide Y ₂ O ₃ as described above as an electron emitting material for an electrode for a high-pressure sodium lamp having unsaturated vapor pressure characteristics. It is not clear why the use of this yttrium oxide Y ₂ O ₃ significantly suppresses the drop in lamp voltage and provides relatively good startability, but as mentioned above, the melting point, evaporability, and work function Superior to other oxides in all aspects,
This is thought to be because it is the substance that is least likely to release free oxygen and therefore has low reactivity with sodium, making the effect of suppressing the disappearance of sodium significant.

As described above, by using yttrium oxide Y ₂ O ₃ as the electron-emitting material for the electrodes of high-pressure sodium lamps, which have unsaturated vapor pressure characteristics, the drop in lamp voltage is suppressed, the lamp life is extended, and the lamp life is improved. It becomes possible to achieve the objectives of

〔Example〕

Examples of the present invention will be described below. First, as the electrode constituent members of the electrode according to the present invention, the first
As shown in the figure, a double coil 2 is disposed in contact with the outer periphery of the electrode core 1, and a single coil 3 is further disposed outside the double coil 2.
Use electrode components for a very common 360 watt high pressure sodium lamp equipped with
In order to reduce the heat capacity of the electrode, the diameter of the electrode core 1 was set to 0.8 mm).

On the other hand, the electron emitting material is made into a suspension by adding butyl alcohol and nitrocellulose lacquer as a binder to commercially available 99.99% pure yttrium oxide powder and stirring well.

Next, the electrode constituent member is immersed in this suspension, the electron emitting material 4 is applied thereto, and after drying, the electron emitting material 4 on the surface of the outer coil 3 is removed.
Afterwards, it was fired at a temperature of approximately 2200°C in a hydrogen atmosphere to create an electrode. In this firing process,
Since the melting point of yttrium oxide Y ₂ O ₃ is as high as 2420°C, it is desirable that the firing temperature be at least 2000°C or higher in order to improve the bonding properties with the metal of the electrode component. Note that discharge firing can also be used in this firing step.

FIG. 2 shows an arc tube for a 360 watt high pressure sodium lamp incorporating electrodes made as described above. In the figure, 5 has an inner diameter of 8.0mm.
The arc tube body 5 is made of polycrystalline alumina, and conductive heat-resistant metal tubes 7a and 7b are provided at both ends of the arc tube body 5 with main electrodes 6a and 6b.
The distance between the heat-resistant metal tubes 7a and 6b is 84 mm, and the gap between the heat-resistant metal tubes 7a and 7b and the arc tube body 5 is hermetically sealed with a sealing frit.

The arc tube with such a structure contains 0.06 mg of sodium and 5.5 mg of mercury, enough to evaporate all the sodium and mercury sealed during lamp operation.
150 Torr of xenon is sealed as an auxiliary gas for starting, and is in a so-called unsaturated vapor pressure state.

The arc tube configured in this way is attached to an outer sphere (not shown).
In combination with a ballast for a 400W high-pressure mercury lamp, a lamp life test was conducted at an input power of 360W. As a result, in a lamp equipped with an electrode coated and fired with Y ₂ O ₃ as the electron emitting material, the initial lamp voltage was 135 V on average, and 3000 V.
The average lamp voltage after lighting for a certain period of time was 130V, and the lamp voltage drop width from the initial point was 5V.

In order to compare this, we performed a life test on an unsaturated high-pressure sodium lamp of the same structure, equipped with an electrode using a conventional electron-emitting material mainly composed of barium calcium tungstate Ba ₂ CaWO ₆ . The initial lamp voltage is an average of 125V, and the lamp voltage after 3000 hours of operation is an average of 125V.
At 105V, the lamp voltage drop width from the initial point is
It was 20V. After being lit for 3,000 hours, this lamp had almost no sodium emission color and only mercury emission, making it a mercury lamp.

Furthermore, yttrium oxide is used as an electron emitting material.
For the purpose of comparing cases where rare earth oxides other than Y ₂ O ₃ are used, cerium oxide CeO ₂ , neodymium oxide
When a similar life test was conducted on lamps using Nd ₂ O ₃ and samarium oxide Sm ₂ O ₃ as electron emitting materials, the lamp voltage of all lamps was 20 to 30 V within 500 hours of lamp operation. It decreased significantly, and the sodium emission color disappeared within 1000 hours.

Although the reason for this is not clear, CeO ₂ has a higher melting point but also higher evaporability and work function than yttrium oxide Y ₂ O ₃ used in the present invention.
Also, Nd ₂ O ₃ and Sm ₂ O ₃ are yttrium oxide Y ₂ O ₃
This is probably due to its relatively high reactivity with sodium, as it has a lower melting point, higher evaporability, and higher work function.

In the above embodiment, the electrodes are applied to an unsaturated vapor pressure type lamp in which the sodium and mercury sealed in the arc tube completely evaporate during lighting operation. Even in high-pressure sodium lamps with unsaturated vapor pressure characteristics, in which an amount of sodium amalgam is sealed in such an amount that more than 80% of the sodium amalgam evaporates during lighting operation, the loss of sodium due to free oxygen released from the electrodes has a large effect on the lamp characteristics. Because it affects
The present invention can also be applied to electrodes of high-pressure sodium lamps having such unsaturated vapor pressure type characteristics, and similar effects can be obtained.

In the above example, only yttrium oxide Y ₂ O ₃ was used as the electron emitting material of the electrode. For the purpose of further improvement, it is also possible to add a trace amount of thorium oxide ThO ₂ , barium oxide BaO, calcium oxide CaO, strontium oxide SrO, etc. to yttrium oxide Y ₂ O ₃ .

〔Effect of the invention〕

As explained above in detail based on the examples,
The present invention is a high-pressure sodium lamp with unsaturated vapor pressure characteristics in which a trace amount of sodium is sealed together with xenon gas and mercury inside the arc tube, and the electrodes are constructed by coating an electron-emitting substance made of yttrium oxide. Without making it difficult to start the discharge, the lamp voltage drop due to sodium disappearance can be reduced, and the lamp life can be improved.

Furthermore, since the yttrium oxide used as the electron emitting material is harmless to the human body, it is easy to handle, which greatly simplifies the process of manufacturing the electron emitting material, resulting in cost reduction. .

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of an electrode for a high-pressure sodium lamp, and FIG. 2 is a plan view of an arc tube of the high-pressure sodium lamp. In the figure, 1 is an electrode core, 2 is an inner double coil, 3 is an outer coil, 4 is an electron emitting material, 5 is an arc tube, 6a and 6b are electrodes, and 7a and 7b are conductive heat-resistant metal tubes.

Claims (1)

[Claims] 1. An electrode for a high-pressure sodium lamp having unsaturated vapor pressure characteristics, characterized in that yttrium oxide is used as an electron emitting material. 2. The high-pressure sodium lamp having unsaturated vapor pressure characteristics is characterized in that sodium malgam is sealed in the arc tube in an amount that completely evaporates during lighting operation. Electrodes for high pressure sodium lamps. 3. The above-mentioned high-pressure sodium lamp with unsaturated vapor pressure type characteristics is characterized in that the arc tube is filled with sodium amalgam in an amount that does not completely evaporate during lighting operation, but evaporates more than 80% of the filled amount. An electrode for a high-pressure sodium lamp according to claim 1.