Method of depositing a material on an electrode for an arc tube

Abstract: An electron emitting, barium hafnate material for arc discharge lamp electrodes is manufactured by a method comprising the steps of: forming a mixture of barium carbonate and hafnium oxide in a 48 to 52 mole ratio; adding the mixture to a suitable carrier to form a slurry; ball milling the slurry for about 2 hours to form a powder mix; drying the powder mix; firing the dried powder mix in air at about 1500°C for about 22 hours to produce a product; and vibration milling the product in methanol with zirconia media to produce the barium hafnate. The latter material is then preferably vacuum impregnated into a tungsten electrode (7,8,9) and hydrogen fired to produce a single phase barium oxide/hafnium oxide compound (10).

Description: This invention relates to high pressure arc discharge lamps such as, for example, mercury lamps and high pressure sodium lamps and particularly to a new electrode coating for such lamps. The efficacy of hafnium oxide as an electron emitting material is known from US-A-2,843,801. US-A-4,044,276 discloses emitter materials composed of barium oxide, calcium oxide and hafnium oxide. Attempts to employ the latter material, while successful, have problems due to moisture reaction. This material was coated on electrodes as the carbonates and oxides and subsequently fired in hydrogen or vacuum to decompose the carbonates to the oxides. However, after firing, it has been discovered that the materials can deteriorate rapidly when exposed to ambient atmosphere. Calcium oxide can react vigorously and exothermally with water vapour while the calcining of barium carbonate is an equilibrium reaction, thus: BaCO 3 ⇄ BaO + CO 2 and, while this reaction can be driven to the right, exposure to CO 2 after firing can result in the reformation of BaCO 3 . It is known from US-A-5111108 (over which claim 1 has been characterised) to provide an emitter material for an electrode comprising Ba x Sr 1-x HfO 3 , where 1 ≥ x ≥ 0. According to the present invention there is provided a method of depositing a material on an electrode for an arc tube, the material comprising an oxide for enhancing the electron emission of the electrode, characterised in that the electron emission enhancing material is deposited as single phase barium hafnate with an excess of hafnia. The excess of hafnia is to substantially prevent the oxide from undergoing a reverse reaction. The electron emission enhancing material is preferably deposited as a slurry of powdered material suspended in a carrier. Preferably, the slurry is made by performing the steps of forming a mixture of a barium compound and hafnium oxide, adding a carrier to form an initial slurry, milling the mixture, drying the mixture, firing the mixture to form a product of single phase barium hafnate with an excess of hafnia, milling the product and adding a further carrier to form said slurry. Preferably the mole ratio of the hafnium oxide to the barium oxide is between 50 to 55. Most preferably, the mole ratio of the barium compound and the hafnium oxide is 48 to 52. According to one preferred embodiment, there is provided a method of making an electron emitting, barium hafnate material for arc discharge lamp electrodes comprising the steps of: forming a mixture of barium carbonate and hafnium oxide in a 48 to 52 mole ratio; adding said mixture to a suitable carrier to form a slurry; ball milling said slurry for about 2 hours to form a powder mix; drying said powder mix; firing said dried powder mix in air at about 1500°C for about 22 hours to produce a product; and vibration milling said product in methanol with zirconia media to produce single phase barium hafnate with an excess of hafnia. Applying the barium hafnate material to a suitable electrode is accomplished by forming an emission mix slurry as is known in the art and impregnating the electrodes therewith. Vacuum impregnation of the electrodes from a slurry of methanol is preferred. After coating, the electrode is allowed to dry and any excess oxide material is cleaned off. The coated electrodes are then fired at 1600°C to form the oxides. This procedure insures complete reaction of the oxides and prevents the reformation of the carbonates. Some preferred embodiments will now be described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 is an elevational view, partially in section, of a preferred arc tube of the present invention; Fig. 2 is an expanded view of a preferred electrode; Fig. 3 is a graph of the moisture reaction of barium hafnate compared to the prior art emitter; and Fig. 4 is an elevational view, in section, of a further preferred arc tube. In Fig. 1, there is shown an arc tube 1 made of a high silica glass such as quartz, having seals 2, preferably press seals as is shown in Fig. 1, at each end thereof. At each end of arc tube 1 is an electrode 3 which is connected to a molybdenum ribbon 4, which in turn is connected to an external lead wire 5. The arc tube has a starting electrode 6 as is known in the art. An electron emitting composition, as will be described later, is disposed on each electrode 3. In one embodiment of electrode 3, as shown in Fig. 2, the electrode comprises a tungsten rod 7 having inner tungsten coil 8 thereon encircling a portion of rod 7 and secured thereto. Outer tungsten coil 9 is threaded on coil 8. The emitting material 10 is disposed in the recesses between coils 8 and 9. Coil 8 may have some open turns, as shown in Fig. 2, to accommodate extra emitter material. The emitter material is preferably prepared by a method comprising the steps of: forming a mixture of barium carbonate and hafnium oxide in a 48 to 52 mole ratio; and adding the mixture to a suitable carrier to form a slurry. In one preferred embodiment the slurry material is methanol; however, other volatile carriers, such as water, ethanol or butyl acetate can be used. The slurry is then ball milled for about 2 hours using zirconia media, to form a powder mix. This powder mix is then dried and fired in air at about 1500°C for about 22 hours to produce a single phase barium hafnate with a slight excess of hafnia. This product is then mixed in methanol, or other suitable carrier, and vibration milled with zirconia media to produce a slurry with a mean particle diameter of 3.5 micrometers The barium hafnate is then prepared with an emission slurry, as is known in the art, and vacuum impregnated into the electrodes 3. The now coated coils are allowed to dry and any excess oxides which may be present are cleaned from the coils. The coils are then fired in hydrogen at about 1400 to 1700°C to sinter the electrodes. The preferred temperature is about 1600°C. Employing this procedure provides for the creation of single phase barium hafnate with a slight excess of hafnia which effectively prevents the reformation of the carbonates, or reaction with atmospheric moisture. Measurements have been made of the moisture reaction of the prior art (as epitomized in US-A-4,044,276) and the composition described herein. Coated coils were prepared in accordance with the procedures described above and exposed to air with a fixed relative humidity of 57% at room temperature. The results, as shown in Fig. 3, reveal that the new composition reacts with the atmosphere 400 times slower than the prior art. The advantages of the present invention can also be obtained by using other precursor materials such as the hydroxides, nitrates, oxalates or other materials which react in oxygen and heat to form oxides. Referring now to Fig. 4, there is shown a preferred sodium lamp arc tube 11 which comprises a ceramic cylindrical body 12 having an outside diameter 14 and an inside diameter 16. Adjacent the ends 18 and 20 the inside diameter widens to form an intermediate diameter 22 which is greater than diameter 16 but less than diameter 14, thus forming a chamfer 24. A sealing disk 26, which includes a frusto-conical portion 28 formed to mate with chamfer 24 is sealed into each end of body 12 by means of a sealing material 30 which forms a sealing annulus. An electrode 32, which can be a conventional electrode for a high pressure sodium lamp including a preferred electron emissive material of the present invention, is sealed into a centrally located aperture in disk 26. A wire stop 34 can hold electrode 32 in position.

Claims: A method of depositing a material on an electrode (3;32) for an arc tube (1;11), the material comprising an oxide (10) for enhancing the electron emission of the electrode, characterised in that the electron emission enhancing material (10) is deposited as single phase barium hafnate with an excess of hafnia. A method as claimed in claim 1, characterised in that the electrode is fired in hydrogen at a temperature of between 1400 and 1700°C to sinter the electron emission enhancing material (10). A method as claimed in claim 1 or 2, characterised in that the emission enhancing material (10) is deposited on the electrode as a slurry comprising powdered material suspended in a carrier. A method as claimed in claim 3, characterised in that the slurry is vacuum impregnated into the electrode. A method as claimed in claim 3 or 4, characterised in that the mean particle diameter of the slurry is about 3.5µm. A method of making a slurry for use in the method of claims 3 to 5, comprising the steps of forming a mixture of a barium compound and hafnium oxide, adding a carrier to form an initial slurry, milling the mixture, drying the mixture, firing the mixture to form a product of single phase barium hafnate with an excess of hafnia, milling the product and adding a further carrier to form said slurry. A method as claimed in claim 6, characterised in that in the mixture, the mole ratio of the barium compound and the hafnium oxide is 48 to 52. A method as claimed in claim 6 or 7, characterised in that the mixture is fired at 1500°C for 22 hours to form the product of single phase barium hafnate with an excess of hafnia.

Inventors: DUGGAN GEORGE L, GOODMAN DAVID A, DUGGAN, GEORGE L., GOODMAN, DAVID A.
Applicants: FLOWIL INT LIGHTING, FLOWIL INTERNATIONAL LIGHTING (HOLDING) B.V.
Application year: 1996
Main classification: H01J9
IPC codes: H01J9, H01J61