BULB HAVING INTERIOR SURFACE COATED WITH RARE EARTH OXIDE

Abstract: A lamp bulb (1) includes a light transmissive envelope (2), a fill material disposed inside the envelope including a rare earth metal which forms a plasma discharge when excited, and a coating (3) disposed on an interior surface of the envelope which inhibits reaction between the plasma discharge and the envelope (2), wherein the coating (3) comprises an oxide of the same rare earth metal of the fill material. A process for coating the interior of a lamp bulb envelope (2) includes preparing a sol gel solution having a precursor of the desired coating, introducing the sol gel solution into a lamp preform, distributing the sol gel along an interior surface of the bulb, and drying and firing the sol gel to obtain the coated bulb.

Description: BULB HAVING INTERIOR SURFACE COATED WITH RARE EARTH OXIDE CROSS REFERENCE TO RELATED APPLICATIONS The present application is based on and claims the benefit of priority of U.S. provisional patent application nos. 60/108,440, filed November 13, 1998, and 60/130,979, filed April 26, 1999, each of which is herein incorporated by reference in its entirety. BACKGROUND 1. Field of the invention The invention relates to discharge lamps and more specifically to lamp bulbs for discharge lamps which bear a coating on an interior surface thereof to reduce the reaction between the bulb wall and the fill material. 2. Related art Various rare earth metal halides are known to be used as lamp fills because of desired light characteristics. For example, U.S. Patent Nos. 5,363,015 and 5,479,072 disclose praseodymium and neodymium as suitable mercury-free lamp fills. However, such substances are reactive to various degrees with quartz at typical lamp operating temperatures and the color characteristics of the lamp degrade over a relatively short period of time. It is well known to utilize a coating on the interior of a bulb to reduce the reaction between the lamp fill material and the bulb wall. However, prior art lamps with such coatings may still have a limited useful life and are not necessarily suitable for use with a rare earth metal halide fill. Another problem with prior art coatings is that a coating which is thick enough to prevent the undesired reaction may be too thick to adhere well to the bulb wall because of, for example, different thermal expansion coefficients and a coating which is thin enough to avoid the expansion problem may be too thin to prevent the undesired reaction. SUMMARY It is an object of the invention to provide a discharge lamp bulb with a longer useful life when used in conjunction with a lamp fill which includes a rare earth metal. According to the invention, a discharge lamp bulb is coated on an interior surface thereof with a rare earth oxide which inhibits interaction between a fill material and the bulb material, where the fill material includes the same rare earth metal. For example, for a fill which includes a rare earth metal, a coating of rare earth oxide is applied to the interior of a bulb envelope. The coating may be applied by means of a sol gel solution which is formulated to yield the desired rare earth oxide coating after evaporation of the sol gel and higher temperature firing of the coated bulb envelope. An advantage of the sol gel coating process is that the final coating is relatively thin (e.g. between about 50 and 1000 nm), but is still thick enough to inhibit the undesired reaction. In one example, an oxide of praseodymium is applied to the interior of the bulb. The praseodymium oxide coating facilitates the use of a praseodymium fill with significant inhibition of devitrification of the fused quartz bulb because there is no thermally dependent chemical potential driving devitrification. An exemplary process according to the invention for applying the rare oxide coating is as follows. A rare earth oxide precursor is used to prepare a sol gel solution. The sol gel solution is poured into a lamp preform and then poured out in a controlled manner to leave a relatively uniform thickness of coating behind. Alternatively, the sol gel is spin coated onto the interior surface of the bulb. The coating is then dried and fired. Several layers may be applied in this manner. Examples of the rare earth oxide precursor include praseodymium iso-propoxide [Pr(OC 3 H 7 ) 3 ] and praseodymium methoxyethoxide. The foregoing and other aspects of the invention are achieved individually and in combination. The invention should not be construed as requiring two or more such aspects unless expressly required by the claims. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings, in which reference characters generally refer to the same parts throughout the various views. The drawings are not necessarily to scale, the emphasis instead being placed upon illustrating the principles of the invention. Fig. 1 is a schematic, cross-sectional view of a bulb coated in accordance with a first aspect of the invention. Fig. 2 is a schematic, cross-sectional view of a bulb coated in accordance with a second aspect of the invention. Fig. 3 is a flow diagram of a first method of coating a bulb interior in accordance with an aspect of the invention. Fig. 4 is a flow diagram of a second method of coating a bulb interior in accordance with an aspect of the invention. Fig. 5 is a graph of spectral distribution for a praseodymium oxide coated bulb with a praseodymium fill. DESCRIPTION In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular structures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art having the benefit of the present disclosure that the invention may be practiced in other examples that depart from these specific details. In certain instances, descriptions of well known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. In Fig. 1 , a discharge lamp bulb 1 comprising a fused quartz envelope 2 has a coating 3 on an interior surface thereof. In Fig. 2, the bulb 1 further includes a diffusion barrier 4 between the fused quartz envelope 2 and the coating 3. The bulb 1 may be used in a microwave excited electrodeless discharge lamp, for example, as described in U.S. Patent 5,404,076. Bulb 1 may also be used in an inductively coupled electrodeless discharge lamp, for example, as described in PCT Publication No. WO 99/36940. The bulb 1 may also be used in a capacitively coupled electrodeless discharge lamp, for example, as described in U.S. Patent No. 5,825,132, or a travelling wave electrodeless discharge lamp. According to the invention, the coating 3 comprises a rare earth oxide which corresponds to a rare earth metal fill material. A method according to the invention for applying the coating 3 is by means of a sol gel coating process, as hereinafter described in detail. The rare earth oxide coating mitigates devitrification of the quartz caused by like rare earth metal halides in the lamp fill which is driven by temperature sensitivity of the reaction (not balanced): 3MI 3 + 5Si0 2 ^ M 2 Si0 7 + MOI(g) + SiO(g) + SiOl n (g) + Sil 4 (g) (Eq. 1 ) where: M is a rare earth metal; I is a halogen, non balanced form; g denotes gas phase material; and n is an integer. The equilibrium partial pressures of the volatile products decrease with decreasing temperature, which drives a mechanism for the transport of vitreous silica from a hotter region of the lamp to deposit amorphous or crystalline silica in a colder region of the lamp. According to the invention, the reaction is inhibited by coating the interior of the bulb with a rare earth oxide which corresponds to the rare earth element in the fill. For example, if praseodymium tri-bromide is the fill element and an oxide of praseodymium coats the interior of the bulb the reaction between the fill and the coating may be described as follows (not balanced): PrBr 3 + Pr 6 On ^ PrBr 3 + Pr 6 On + Pr 2 0 3 + PrO x Br y (Eq. 2) This reaction is further stabilized by the extremely high melting point of various forms of praseodymium oxide (e.g. Pr 6 On, Pr 2 O 3 ). The exact ratio of Pr to O which will be sustained in the coating will be determined by the operating temperature of the lamp and the processing temperature of the deposited coating. Fig. 3 is a flow diagram of a first method according to the invention for applying the coating to the bulb interior. Advantageously, the sol gel process according to the invention is inexpensive and simple as compared to other processes for coating an interior of a bulb. First, a sol gel solution is prepared or provided which includes a precursor of rare earth oxide (step 11). Next, the sol gel solution is poured into a lamp preform, such as a bulb blank (step 13). Then, the solution is poured out in a controlled manner to leave a relatively uniform thickness of coating behind (step 15). The coating is then dried and fired (step 17). Several layers may be applied in this manner. The bulb is then dosed with the fill material and a starting gas (if any) and sealed off. One precursor for the solution is praseodymium iso-propoxide [Pr(OC 3 H ) 3 ], which is a moisture sensitive solid in powder form which is stored in a glove box under dry atmosphere. Praseodymium iso-propoxide is commercially available from Strem Chemicals. Other precursors (e.g. praseodymium methoxyethoxide) may alternatively be used. Suitable precautions are taken with respect to handling the chemicals. An exemplary process for preparing the sol gel solution is as follows. 1 ) A magnetic stirrer is placed in a 25 ml, 3-neck glass flask. One neck is closed with a suba seal. A 90° bend adapter with a teflon stopcock plug is placed on a second neck. The precursor is subsequently loaded through the third neck. 2) The flask and a stopper are placed in the glove box with the precursor. About 0.32 grams (1 millimole) is deposited in the flask through the third, open neck. Loss of a small amount of powder is not critical. 3) The third neck is sealed with the stopper and wrapped with parafilm. The flask is then removed from the glove box. 4) The flask is secured above a stir plate under a fume hood and dry nitrogen is applied (flowing gently) through the 90° bend adapter. Care is taken not to expose the precursor solution to the atmosphere. 5) A syringe is prepared with 2 ml of 2-methoxy ethanol [CH3OCH2CH20H] (99.9+%, HPLC grade). This is added to the precursor through the suba seal. This solution is stirred for about 1 hour under N 2 atmosphere. Afterwards, the iso- propoxide is completely dissolved. 6) A syringe is prepared with 0.2 ml of acetic acid (99.7% A.C.S. Reagent). The acetic acid is added to the solution through the suba seal. The solution is stirred for about 10 minutes (a precipitate begins to form after about 90 minutes). The acetic acid is a chelating agent which slows hydrolysis and condensation of the alkoxide. The resulting solution is clear. In the case of praseodymium methoxyethoxide, a pre-prepared sol gel solution of praseodymium (2- methoxyethoxide) 2 in 2-methoxyethanol is commercially available from Chemat Technologies, Inc. The moisture sensitivity of the alkoxide solution precursors may be reduced by using more complex alkoxy groups or by adding a chelating agent (e.g. acetic acid). An optional further steps includes hydrolizing the precursor before coating. Hydrolysis helps to lower the amount of residual organic content in the coating and reduces sensitivity to atmospheric moisture. The precursor is hydrolized by adding 2 moles of H 2 O per mole of alkoxide, which leads to substitution by a hydroxy group. When water is added to the precursor solution in the absence of the chelating agent, the solution turns turbid instantaneously. In the presence of the chelating agent, the solution turns turbid after a period of about one hour. Test results Tests were performed on three 12.5 mm by 12.5 mm (1/2 inch by 1/2 inch) fused quartz substrates to determine the phase of the oxide and the reaction of the coating with quartz. The substrates are coated as follows. The solution is spin coated onto the substrate at about 3000 r.p.m. for about 60 seconds. The substrate is then placed on a hot plate at about 175°C for 4 minutes to evaporate the solvent. The spin coating and heating is performed 3 times to produce a 3 layer coating. The deposited coating is then sintered. The heating and cooling rate is about 5° C per minute. Each of the three tested samples were first sintered to about 900° C for about 30 minutes, after which a yellowish appearance is observed. The three samples were then sintered to about 1100° C for 1 hour (sample #1), 2 hours (sample #2), and 5 hours (sample #3), respectively. The films became transparent after the second sintering step. The thickness of the three samples, as measured by a tencor profilometer, is 183 nm (sample #1), 203 nm (sample #2), and 170 nm (sample #3). An alternative sintering process is to first heat the sample to 500° C and then raise the temperature to 1100° C at a rate of about 5° C per minute. The sample is held at 1100° C for about 30 minutes. The resulting film is transparent. Specifically, an SEM micrograph of the microstructure at 1000° C shows close packed nano- scale grains of praseodymium oxide which is a desirable microstructure for optical transparency. The absorbance of the coating as a function of wavelength was determined using a UV-VIS spectrometer. In general, the absorbance was found to decrease with increasing wavelength (for the range 190nm to 820 nm). X-ray diffraction tests on the spin coated samples showed the presence of crystalline phases, including the Pr 6 On phase which may be reduced to Pr 2 O 3 at around 1000° C. Differential thermal analysis (DTA) shows that Pr 6 On crystallizes at about 650° C by the methoxyethoxide route as compared to about 500° C by the iso- propoxide route. Analysis of the crystalline phase development on thin films by the iso-propoxide route indicates that Pr 6 On is the main phase, but at 1000° C, an unidentified phase forms. Also, further unidentified phases appear after heating to higher temperatures, possibly due to inter-diffusion. For the iso-propoxide route, cristoballite is apparent after heating to 1000° C, indicating devitrification of the fused quartz. Analysis of the crystalline phase development on thin films by the methoxyethoxide route indicates that Pr 6 On crystallizes at higher temperatures than the iso-propoxide solution and no unidentified phases appear. Formation of Pr 2 O 3 from Pr 6 On occurs at about 900°C. With the methoxyethoxide route, no cristoballite is observed. Depth profiling using Auger Electron Spectroscopy shows no Si at the surface of the tested substrates. Further analysis shows Si present after sputtering off about 108 nm from the film using Ar. A Rutherford back scattering spectrum shows that interdiffusion of Pr and Si occurs for a sample heated to 1100° C for 1 hour (iso-propoxide route). Depending on the temperature of the application, one route may be preferred over the other. As noted above, the iso-propoxide route crystallizes at lower temperatures. However, the methoxyethoxide solution coated on fused quartz generates only the Pr 6 On phase. For example, this stable, polycrystalline praseodymium oxide phase by the methoxyethoxide route may be important to the long term stability of an operating discharge lamp for lumen and / or color maintenance. High temperature tests show that interdiffusion between the praseodymium oxide and the quartz may occur at temperatures above 1000°C. Accordingly, in an operating discharge lamp, interdiffusion may be inhibited by keeping the bulb temperature less than 1000°C, and preferably 950°C or less. Lamp bulb data Fig. 4 is a flow diagram of a second method according to the invention for coating an interior surface of a lamp bulb. The sol gel solution may be prepared or provided as described above (step 21 ). The sol gel is introduced into a lamp preform (step 23). The preform is spun or rotated at a sufficient speed to distribute the sol gel solution along the interior surface of the preform (step 25). Any excess sol gel solution is removed from the preform (step 27, e.g. by pouring the excess out). The coated preform is then dried and fired as described above (step 29). As applied to an exemplary lamp bulb having a 9 mm outer diameter and an 8 mm inner diameter, the process is as follows. The bulb is formed on the end of a quartz rod with an opening in a pinch-off region which is not yet sealed. A drop of sol gel solution prepared in accordance with the iso-propoxide route is deposited into the bulb interior. The bulb is then rotated at a speed of 3000 RPM and heated at a temperature of 90° C by a cylindrical heating mat. The bulb is rotated for about two minutes and then the axis direction of rotation is reversed by 180° and the bulb is spun for another two minutes. This process is repeated three times for a total rotation time of about 8 minutes. Other schemes for uniformly distributing the sol gel may alternatively be employed (e.g. shaking or agitating the preform). Any residual sol gel liquid is then extracted from the bulb. The bulb is then dried at about 200° C for 15 minutes. Several layers may be applied in this manner. Thereafter, the bulb is fired in a furnace at 1000° C for 30 minutes at a heating / cooling rate of 5° C per minute. The bulb is then dosed with the fill material, which in this example is 2 mg of praseodymium tri-chloride (PrCI 3 ), and a suitable starting gas (e.g. 50 Torr Krypton). Fig. 5 is a graph of spectral power distribution for the coated bulb with this fill. The bulb is encased in a reflecting ceramic jacket with a 5 mm diameter round aperture. Approximately 150 watts of RF power are applied to the bulb with an inductively coupled lamp circuit. As can be seen from Fig. 5, the fill provides light output in a broad, continuous spectrum throughout the visible range. Advantageously, the coated lamp bulb according to the invention maintains good light output many times longer than uncoated bulbs with identical fills. Preliminary results suggest that the bulbs employing the coating of the present invention have a useful life which is at least an order of magnitude longer than uncoated bulbs, and potentially several orders of magnitude longer useful life. Diffusion barrier As noted above, the potential exists for the diffusion of praseodymium into the silica substrate at high bulb temperatures. For the extremely long time scales at high temperatures required by lamp envelopes, it may be advantageous to first deposit a substantially transparent diffusion barrier layer on the silica, as shown in Fig. 2, prior to depositing the praseodymium oxide. Suitable barrier layers include thin films of alumina, aluminum nitride, silicon nitride, praseodymium nitride, and titanium oxide. Titanium oxide (TiO 2 ) is one example of a suitable diffusion barrier layer according to the invention. Ti0 does not react with SiO 2 until temperatures of up to about 1400° C. According to a present aspect of the invention, a diffusion barrier coating is applied by sol gel processing as follows: 1 ) A precursor of Ti(IV) iso-propoxide is mixed with a solvent of isopropanol. 2) A chelating agent is added such that the precursor : chelating agent ratio is maintained at 1 :2 moles. 3) H 2 0 (10 moles per mole of the precursor) is introduced into the system drop wise. 4) After a few minutes, a drop of HCI is added as a peptizing agent. The titania precursor solution is spin coated onto fused quartz substrates at 3000 RMP for 60 seconds. After spin coating, the coated substrate is heated to 105° C for four minutes to evaporate the solvent. Four layers are deposited in this manner. The coated substrate is then fired to 700° C for 30 minutes at 5° C per minute. The coating thickness is about 300 nm. The coating forms the anatase phase at 700° C and the rutile phase on heating to 1000° C. Thereafter, the substrate is coated with rare earth oxide as described above. At about 700° C, a Pr 6 On phase forms on the anatase phase. On heating to 1000° C for 30 minutes, the rare earth oxide phase interacts with the underlying anatase phase forming a Pr-O-Ti phase. Micro-diffraction tests show that anatase phase remains at 1000° C with no interaction between the anatase and SiO 2 . Also, no cristoballite formation is observed. While the invention has been described in connection with preferred examples, variations will occur to those skilled in the art. For example, the coating may be used by lamps having internal electrodes and also with other rare earth metal fills. Also, the coating may be utilized with bulbs having any of a variety of shapes. Accordingly, the foregoing description should be considered as illustrative and not limiting, with the scope and spirit of the invention instead being defined by the appended claims.

Claims: CLAIMS What is claimed is: 1. A lamp bulb, comprising: a light transmissive fused quartz envelope; a plasma forming fill material disposed in the quartz envelope, the fill comprising a halide of praseodymium and a rare gas; and a coating disposed on substantially an entire interior surface of the quartz envelope, wherein the coating comprises an oxide of praseodymium. 2. The lamp bulb as recited in claim 1 , wherein the fill material comprises one of PrBr 3 and PrCI 3 and the coating comprises at least one of Pr 6 On and Pr 2 O 3 . 3. The lamp bulb as recited in claim 1 , wherein the fill consists essentially of only a halide of praseodymium and a rare gas, and wherein the coating consists essentially of only an oxide of praseodymium. 4. The lamp bulb as recited in claim 1 , further comprising a diffusion barrier between the coating and the envelope. 5. The lamp bulb as recited in claim 4, wherein the diffusion barrier comprises one of silicon nitride and titanium oxide. 6. A lamp bulb, comprising: a light transmissive envelope; a fill material disposed inside the envelope including a rare earth metal which forms a plasma discharge when excited; and a coating disposed on an interior surface of the envelope which inhibits reaction between the plasma discharge and the envelope, wherein the coating comprises an oxide of the same rare earth metal of the fill material. 7. The lamp bulb as recited in claim 1 , wherein the fill consists essentially of only a halide of the rare earth metal and a rare gas, and wherein the coating consists essentially of only an oxide of the rare earth metal. 8. The lamp bulb as recited in claim 6, further comprising a diffusion barrier between the coating and the envelope. 9. The lamp bulb as recited in claim 8, wherein the diffusion barrier comprises one of alumina, aluminum nitride, silicon nitride, rare earth nitride, and titanium oxide. 10. A method of coating a lamp bulb, comprising: i) introducing a sol gel solution into a preform of the lamp bulb; ii) distributing the sol gel solution along an interior surface of the preform; iii) extracting excess sol gel, if any, from the preform; and iv) processing the sol gel solution which remains in the preform to obtain a desired coating. 11. The method as recited in claim 10, wherein step (iv) comprises drying and firing the coating. 12. The method as recited in claim 10, further comprising: v) dosing the preform with fill material; and vi) sealing the preform to form the lamp bulb. 13. The method as recited in claim 10, wherein the sol gel solution as introduced in step (i) comprises a precursor of the desired coating. 14. The method as recited in claim 13, wherein the precursor comprises an oxide of a rare earth metal and a desired fill material comprises a halide of the same rare earth metal. 15. The method as recited in claim 10, wherein steps (i) to (iii) are repeated one or more times prior to step (iv). 16. The method as recited in claim 10, wherein step (ii) comprises spinning the preform.

Inventors: FRANCIS LORRAINE F, KIRKPATRICK DOUGLAS A, FRANCIS, LORRAINE, F., KIRKPATRICK, DOUGLAS, A.
Applicants: FRANCIS LORRAINE F, FUSION LIGHTING INC, KIRKPATRICK DOUGLAS A, FRANCIS, LORRAINE, F., FUSION LIGHTING, INC., KIRKPATRICK, DOUGLAS, A.
Application year: 1999
Main classification: H01J17
IPC codes: H01J9, H01J61, H01J65