US 3912959 A
Description (OCR text may contain errors)
United States Patent 1191 Muta et al.
[ HIGH PRESSURE SODIUM VAPOR DISCHARGE LAMPS  Inventors: Akinori Muta; Yasuo Tsukuda, both of Tokyo, Japan  Assignee: Hitachi, Ltd., Japan  Filed: Mar. 16, 1973  Appl. No.: 341,849
 Foreign Application Priority Data Mar. 16, 1972 Japan 47-26807  US. Cl 313/220; 313/221  Int. Cl. H0lj 61/30  Field of Search 313/221, 220
 References Cited UNITED STATES PATENTS 3,363,134 l/l968 Johnson 313/221 1 Oct. 14, 1975 Primary ExaminerRudolph V. Rolinec Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or FirmCraig & Antonelli 57 ABSTRACT A ceramic tube consisting of sintered high transparent polycrystalline yttria is used as an envelope in a high pressure sodium vapor discharge lamp.
The discharge lamp having this envelope excels in operation for a long period of time because the transparency of this envelope is not reduced at all when it is exposed to saturated sodium vapor at elevated temperatures.
7 Claims, 4 Drawing Figures Sheet 1 of 3 3,912,959
US. Patent Oct. 14, 1975 FIG.
U.S. Patant 0a. 14, 1975 Sheet 2 of 3 3,912,959
Z 9 3 50 2 U) 2 D: II-
LLI g I o I 0.5 IO I5 GRAIN S|ZE( mm) US. Patent Oct. 14,1975 Sheet 3 of3 3,912,959
HIGH PRESSURE SODIUM vApoR DISCHARGE LAMPS BACKGROUND OFTl-IE INVENTION This invention relates to high pressure sodium vapor discharge lamps generally and more particularly to' such lamps which use a ceramic envelope consisting of a sintered highly transparent polycyrstalline yttrium oxide.
conventionally, a transparent polycrystalline alumina pipe was used as an envelope in a high pressure metal vapor discharge lamp. This material has a total transmission of approximately 90 percent; and, when, used as an envelope in a high pressure sodium vapor discharge lamp, it has a radiation efficiency of approximately 100 lm/W, which is lower than that of a low pressure sodium vapor discharge lamp. In addition, among the materials enclosed within the envelope, sodium is decreased gradually in its quantity, if the lamp is illuminated for a long time. Since the alumina of the envelope begins to evaporate and is changed into an oxide of lower valence as the temperature on the wall of the envelope exceeds l400C, the outer envelope is observed to blacken on its inner wall. The transparent polycrystalline body produced by sintering a mixture of yttria (Y O and thoria (ThO produced and marketed by General Electric Company, U.S.A., under the trade name of Yttrolox shows excellent transmissivity, but it is blackened when the body is heated in a sodium vapor atmosphere.
SUMMARY OF THE INVENTION An object of this invention is to obviate these drawbacks in high pressure sodium vapor discharge lamps and to provide a high pressure sodium vapor discharge lamp having a higher radiation efficiency and a lesser reduction in the quantity of sodium enclosed within the envelope, in addition to a reduction in blackening of the inner wall of the outer envelope and a longer service life. Y
According to the present invention, the above object is attained by the use of a transparent polycrystalline yttria ceramic tube as an envelope in a high pressure sodium vapor discharge lamp. By this means, the radiation efficiency can be increased to 116 to 124 Im/W, with suppression of' the decrease in sodium quantity within the envelope and the blackening of the inner wall of the outer envelope at higher than 1400C, while the service life of the discharge lamp can be prolonged.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plan view of the high pressure sodium vapor discharge lamp according to this invention using a transparent polycrystalline yttria ceramic tube as an envelope; and
FIG. 2 is a diagram showing the relation between the grain size and the in-line transmission of the yttria sintered body; and
FIGS. 3 and 4 are detailed cross-sectional views of alternative embodiments of the high pressure sodium vapor discharge lamp in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described below.
FIG. 1 shows, in plan view, a high pressure sodium v vapor discharge lamp according to the present invention, including a tubular elongated envelope 1 made of transparent yttria and sealed at both ends by niobium caps 4 and 5 with the use of a glass material consisting of, for example, 55 percent alumina, 39 percent calcia and 6 percent magnesia, said niobium caps carrying internal electrodes 2 and 3. The envelope 1 is filled with an amalgam of 30 percent sodium sodium vapor and Xenon at a pressure of 20 mm Hg. The envelope 1 is suspended in the conventional manner in an outer envelope 10 made of hard glass in which is provided electrical leads 6 and 7, connected in the known manner to a socket 8, as well as the standard getter rings 9. High vacuum pressure is maintained inside the outer envelope 10.
Basically, the transparent yttria envelope 1 according to this invention is produced by the process comprising the steps of 1. Providing finely powdered yttrium oxide of high purity;
2. Pressure molding said powders into a tubular body and, if desired, subjecting the latter to a vacuum degasing operation; and
3. Heating the molded body for more than 10 minutes at a temperature of l950 to 2400C under an atmosphere of hydrogen or oxygen, or in a vacuum.
The purity of the powdered material can be varied, depending upon the usage of the powdered material or the property of the impurities mixed into the materialv For instance, in cases where the sintered body is used as an envelope, a window, or a lens in an optical system required to be heat resistant and have a high transparency, it is necessary to maintain the purity approximately higher than 99.99 percent and,-"above all, to avoid the mixture of terbium, molybdenum and other coloring impurities into the sinteredbody as much as possible.
The powdered material withgrain sizes less than 30p. may safely be employed, but a powdered material having grain sizes as small as possible, especially grain sizes in the range of 0.2 to 1.011.,IS preferred from the practical point of view.
A hydraulic press, for instance, can be used for pressure molding the powdered material. While the molding pressure is not so critical, Sinterability tends to abruptly decrease with molding pressures less than 0.2 ton/cm Therefore, a molding pressure higher than 0.2 ton/cm is preferred. Sinterability can be increased by using a higher molding pressure, however, when the pressure is increased excessively, the density of the sintered body may be ocasionally lowered, due to the expansion of the pores of the sintered body occurring at the time of sintering. Therefore, when the molding operation is carried out under a higher molding pressure, it is necessary to use a lower press speed or an atmosphere of reduced pressure, orjt is necessary to subject the molded body to degassing by heating the latter in an atmosphere of reduced pressure so that the gas included in the molded body can be reduced as much as possible. Expansion of the sintered body can also be prevented to a limited extent by using a reduced heating speed in the sintering process.
With a molding pressure higher than 8 to 9 tons/cm it is necessary to carry out a degasing operation as de-.
scribed above, except where an extremely low press speed is employed. With a molding pressure higher than 25 to 30 tonslcm increased molding pressures can hardly be expected to bring about an increase in sinterability and, in addition, the above-mentioned degasing operation can be effected only with considerable 4 A similar high pressure sodium vapor discharge lamp, such as described in conjunction with FIG. 1, was produced with the use of a transparent alumina tube as the 1 envelope 1, and a comparative test was carried out difficulty. Therefore, it is usually preferre to use a 5 under lighted conditions with this discharge lamp and molding pressure less than 30 ton/cm? For instance, the product of the present invention. when the pressure of 7 to 10 tons/Cm is used f r a It was observed that,with the use of the conventional molding operation continuing for 10 minutes, a egasalumina tube as the envelope, the inner wall of the ing operation is preferably carried out for about on outer envelope l0 began to blacken as the temperature hour under a vacuum pressure of 10 mm Hg while 10 of the envelope exceeded 1400" C, but the envelope heating to 100 to 200 C. When the molding pressure made of transparent yttria was not subjected to blackof 20 to 3 tons/cm is used, a degasing operation is ening and could be safely used for a long time at a tempreferably carried out for more than 1 hour under a ture of 1400 C. vacuum pressure of IO" mm Hg while heating to Table 2 shows the radiation efficiency of the high hig er than C- pressure sodium vapor discharge lamp using the above- The heating atmosphere used in the sintering process mentioned transparent yttria envelope and that using is not strictly limited and may be Sele t d epending On the transparent alumina envelope. It is seen from this the type of heater used in the heating furnace. Thus, Table that the discharge lamp made of transparent with the use of an x de r highelting m l h at yttria has a radiation efficiency of about 6 to 14 lm/W an oxygen atmosphere of a hydrogen or vacuum atmohigher than that of the comparative lamp. sphere may be used, respectively.
Especially, if desired to readily and completely remove the pores from crystal grains of yttrium oxides, TABLE 2 dry hydrogen having a dew point approximately below 20 C may be preferably employed as an atmosphere Envelope Lighting Conditions Efficiency to introduce oxygen defects into the yttrium oxide L mp Lamp Total light Efficiency voltage current flux (lm) (lmlw) grams. It 18 to be noted that, when the heating is carried v (A) out under such an atmosphere for a long time, the sintered body is colored black partially or in its entirety. xfis 91.0 505 44.610 H63 in this case, a colorless product can be obtained if the Transparent sintered body is re-heated in a humid or wet hydrogen figi 910 46-800 120-0 atmosphere having a dew point in the range of 0 C to m; 47,750 24.0 room temperature. Tranparem While the sintering temperature and time vary alumna 10o about no slightly depending on the'sintering atmosphere, for example, it is necessary to heat the mixture for more than l0 minutes at a temperature higher than 1950 C. FIG. 2 shows the relation between the grain size and While the upper limit of the sintering temperature is inline transmission of the transparent yttria sintered 2400" C, which is slightly below the melting point of body produced by the above process. lt is seen from yttria, the mixture is preferably heated for more than 40 this Figure that, with the grain size more than 0.1 mm, 45.minutes at a temperature of 2000 to 2300 C, if dethe in-line transmission exceeds 40 percent, which sired to obtain a sintered body having high density and means that the transparency of the sintered body is satgood transparency as well as uniform structure utilizisfactory for practical applications. However, with the able with the present invention. grain size more than 5 mm, the sintered body is subject Table Lshows the result ofsodium resistance tests on to cracks and a course grain boundary, although the the transparent yttria prepared by the above process, as data are not shown in FIG. 2. Therefore, the grain size well as transparent alumina prepared by the convenless than approximately 5 mm is usually preferred. tional process and commercially available Yttrolox. It will be apparent from the above description that a These sodium resistance tests were carried out in a way high pressure sodium vapor discharge lamp using a such that each sample was enclosed in a sodium-sealed transparent yttria envelope in accordance with the niobium tube in which saturated sodium vapor is intropresent invention has excellent sodium resistance and duced and the tube was then heated under the condiradiation efficiency as compared to the conventional tions shown in the Table. sodium vapor discharge tube using a transparent alu- TABLE 1 Samples Treatment in Sodium-saturated Appearance X-ray Vapor After Analysis Treatment Temp. Time (C) (hours) (This invention: pure 1200 3 Unchanged Yttria yttria) (Comparative: Large transparent 1200 3 cracks, y-alumina alumina) opaque (Comparative: yttria con- 1200 3 Dcvitritaining thoriafication Yttrolox) mina or Yttrolox envelope. Moreover, it has a lesser blackening and a longer service life than the abovementioned conventional discharge tube and has a great practical utility.
It is usually preferred to have the thickness of both ends 11 of the envelope 1 shown in FIG. 1 larger than that of the outer region thereof as seen in FIG. 3. With the envelope of the uniform thickness, the outer surface of both ends of the envelope is preferably coated (see FIG. 4) with powdered heat-resistant material 12, such as alumina or yttria, so that the envelope may have a lower heat conductivity in the end portions than in the central zone. By this measure, stabilized discharge characteristics can be obtained as a result of obviating a temperature decrease at the end portions of the envelope. As the powdered heat-resistant material to be coated on the outer surface of the end portions of the envelope, it is preferred to use oxides which have a thermal conductivity less than yttria and are chemically stable at operating temperatures. In addition, a powdered heat-resistant material is preferably coated around outside of the opposite ends of the envelope up to the height of the electrodes.
What we claim is:
1. A high pressure sodium vapor discharge lamp comprising:
a tubular elongated envelope of sintered polycrystalline yttria consisting of corrosion resistant yttrium oxide and having an average grain size of 0.1 to 5 mm, said envelope having a pair of electrodes sealed into opposite ends, and
an ionizable medium including sodium vapor sealed within said envelope.
2. The high pressure sodium vapor discharge lamp according to claim 1, in which said envelope has a heat resistant oxide layer coated on the outer sides of opposite ends thereof.
3. The high pressure sodium vapor discharge lamp according to claim 2, in which said heat resistant oxide layer is selected from the group consisting of alumina and yttria.
4. The high pressure sodium vapor discharge lamp according to claim 1 wherein the thickness of the ends of said envelope in which said electrodes are sealed is greater than the thickness of the side of said envelope.
5. The high pressure sodium vapor discharge lamp according to claim 1 wherein a powdered heat-resistant material is coated on the outside of the opposite ends of said envelope up to the height of the electrodes sealed therein.
6. The high pressure sodium vapor discharge lamp according to claim 1, in which the corrosion resistant yttrium oxide of said envelope has an average grain size of 0.2 mm to 5 mm,
7. The high pressure sodium vapor discharge lamp according to claim 1, in which the corrosion resistant yttrium oxide of said envelope has an average grain size of 0.3 mm to 5 mm.