CN1325538A - Light bulbs coated with rare earth oxides on the inner surface - Google Patents

Abstract

A light bulb (1) comprises a light-transmitting shell (2), a filling material placed in the shell, and a coating (3) deposited on the inner surface of the shell. The filling material forms a plasma discharge when excited, and the coating inhibits the reaction between the plasma discharge and the shell (2). The coating (3) comprises an oxide of the same rare earth metal as the filling material. A method for coating the inner wall of the light bulb shell (2) comprises the following steps: preparing a sol-gel solution containing a desired coating precursor, distributing the sol-gel on the inner surface of the light bulb, and drying and firing the solution to obtain a coated light bulb.

Description

Light bulbs coated with rare earth oxides on the inner surface

related application

This application is based on and claims priority to U.S. Patent Application 60/108,440, filed November 13, 1998, and U.S. Patent Application 60/130,979, filed April 26, 1999, both of which are hereby incorporated by reference in their entirety .

Background of the invention

1. field of invention

The present invention relates to discharge lamps and in particular to bulbs for discharge lamps which have a coating on their inner surfaces to reduce the reaction of the bulb wall and the filling material.

2. Related areas

Due to the required optical properties, various rare earth metal halides are known for use as filling materials in bulbs. For example, US Patents 5,363,015 and 5,479,072 disclose praseodymium and neodymium as suitable fill materials for mercury-free lamps.

However, these substances react to varying degrees with the bulb material fused quartz at the normal operating temperature of the lamp, and the color characteristics of the lamp deteriorate over a short period of time.

It is known to use coatings on the inside of the bulb to reduce the reaction of the lamp fill material and the bulb wall. However, prior art lamps with these coatings still have a limited lifetime and are not necessarily suitable for use with fill materials such as rare earth metal halides. Another problem with prior art coatings is that while the coating is thick enough to prevent undesired reactions, it is too thick to adhere to the bulb wall due to, for example, different coefficients of thermal expansion; Too thin to prevent undesired reactions while avoiding swelling problems.

Summary of the invention

It is an object of the present invention to provide a discharge bulb which has a longer service life when a bulb filling material containing a rare earth metal is used.

The discharge bulb of the present invention is coated on its inner surface with a rare earth oxide coating which inhibits the reaction between the filling material containing the same rare earth metal and the bulb wall material. For example, for a fill material containing a rare earth metal, a coating of rare earth oxide is applied to the inside of the bulb. The coating can be applied by means of a sol-gel solution formulated to allow the desired rare earth oxide coating to form after the coated bulb envelope has been dried and fired at a relatively high temperature.

The advantage of the sol-gel coating method is that the final coating is relatively thin (eg, about 50-1000 nm), but thick enough to suppress undesirable reactions.

In one embodiment, praseodymium oxide is applied to the inner wall of the bulb. The praseodymium oxide coating facilitates the use of praseodymium filling materials and significantly inhibits devitrification of fused silica bulbs, since there is no temperature-dependent chemical tendency to cause devitrification.

The following is an exemplary method of applying the rare earth oxide coating according to the present invention. A sol-gel solution was prepared using a rare earth oxide precursor. This sol-gel solution is poured into the bulb preform, which is then poured out in a controlled manner, leaving a coating of uniform thickness. It is also possible to spin coat the sol-gel solution on the inner surface of the bulb, then dry and fire the coating. Several layers can be applied in this way. Examples of rare earth oxide precursors include praseodymium isopropoxide [Pr(OC ₃ H ₇ ) ₃ ] and praseodymium methoxyethoxide.

The foregoing and other aspects of the invention may be performed individually or simultaneously. The invention should not be limited to these two or several aspects unless explicitly required by the claims.

Brief description of the drawings

The foregoing and other objects, features and advantages of the present invention will be better understood from the following detailed description of the embodiments illustrated in the accompanying drawings. Like numerals in the drawings generally refer to like parts in the various drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Figure 1 is a cross-sectional view of a bulb coated according to the first aspect of the invention.

Figure 2 is a cross-sectional view of a bulb coated according to a second aspect of the invention.

Figure 3 is a flow chart of a first method of coating the interior of a light bulb in accordance with one aspect of the present invention.

Figure 4 is a flow diagram of a second method of coating the interior of a light bulb in accordance with an aspect of the present invention.

Fig. 5 is a spectrum distribution diagram of a bulb coated with praseodymium oxide using a filling material containing praseodymium in the bulb.

Description of the invention

In the following description, for purposes of illustration and not limitation, some specific details are set forth, such as specific structures, interfaces, methods, etc., in order to provide a complete understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced to advantage in other ways that depart from these specific details set forth. In some instances, descriptions of known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

In Fig. 1, the discharge bulb 1 is a fused quartz envelope 2 with a coating 3 on its inner surface. In FIG. 2 , the bulb 1 also comprises a diffusion membrane 4 between the fused quartz envelope 2 and the coating 3 . Bulb 1 may be used in a microwave activated electrodeless discharge lamp such as described in 5,404,076. The bulb 1 can also be used in an inductively coupled electrodeless discharge lamp, as described, for example, in PCT publication WO 99/36940. Bulb 1 can also be used in capacitively coupled electrodeless discharge lamps, as described in US Patent No. 5,825,132, or traveling wave electrodeless discharge lamps.

According to the invention, coating 3 contains rare earth oxides corresponding to rare earth metal filling materials. The method of applying the coating layer 3 in the present invention is a sol-gel coating method, which will be described in detail below. Rare earth oxide coatings mitigate blister quartz devitrification caused by the same rare earth metal halides in the bulb fill material, which is driven by the temperature sensitivity of the following reactions (imbalances):

3Ml ₃ +5SiO ₂ =M ₂ SiO ₇ +MOl(g)+SiO(g)+SiOl _n (g)+Sil ₄ (g) (Formula 1) where,

M is a rare earth metal; l is a halogen, a non-equilibrium form; g refers to a gas phase substance; n is an integer.

The equilibrium partial pressure of volatile products decreases as the temperature decreases, and the volatility of the products drives the glassy quartz to migrate from the hotter region of the bulb through the gas phase to the cooler region of the bulb, depositing amorphous or crystalline quartz on it.

According to the present invention, the above-mentioned reaction can be suppressed by coating the rare earth oxide corresponding to the rare earth element in the filling material on the inner wall of the bulb. For example, if praseodymium tribromide is the filling material and a praseodymium oxide is coated on the inner wall of the bulb, the reaction between the filling material and the coating occurs as follows (unbalanced):

PrBr ₃ +Pr ₆ O ₁₁ =PrBr ₃ +Pr ₆ O ₁₁ +Pr ₂ O ₃ +PrO _x Br _y (Formula 2)

Various forms of praseodymium oxides with very high melting points (such as Pr ₆ O ₁₁ , Pr ₂ O ₃ ) can further stabilize the above reaction. The exact ratio of Pr to O in the coating is determined by the operating temperature of the lamp and the process temperature of the deposited coating.

Fig. 3 is a flowchart of the first method of applying paint to the inner wall of the bulb according to the present invention. The sol-gel method of the present invention has the advantage of being cheap and simple compared to other methods of coating the inside walls of bulbs. First, a sol-gel solution containing a rare earth oxide precursor is prepared or provided (step 11). Thereafter, the sol-gel solution is poured into the bulb preform as the bulb blank (step 13). The solution is then decanted in a controlled manner, leaving a coating of uniform thickness (step 15). The coating is then dried and fired (step 17). Several layers can be applied in this way. The bulb is then filled with filling material and starting gas (if any) and sealed.

One precursor in this solution is praseodymium isopropoxide [Pr(OC ₃ H ₇ ) ₃ ], a powdered moisture-sensitive solid stored in a dry atmosphere glove box. Praseodymium isopropoxide is commercially available from Strem Chemicals. Other precursors (such as praseodymium methoxyethoxide) may also be used. Exercise due care when working with these chemicals. An exemplary method of preparing a sol-gel solution is as follows:

1) Put a magnetic stirrer in a 25 ml three-necked flask. Close one neck with suba sealant. A 90° elbow with a tetrafluoroethylene plunger is placed in the second neck. The precursor is then added through the third neck.

2) There is a flask containing the precursor and a stopper placed in the glove box. About 0.32 g (1 mmol) was deposited through the third open neck. A small amount of powder loss is not critical.

3) Seal the third neck with a stopper and wrap it with parafilm. The flask was then removed from the glove box.

4) The flask is fixed on the stirring plate under the fume hood, and dry nitrogen gas (moderate airflow) is introduced through a 90° elbow, being careful not to expose the precursor solution to the atmosphere.

5) Prepare an infusion solution with 2 mL of 2-methoxyethanol [CH ₃ OCH ₂ CH ₂ OH] (99.9+%, HPLC grade). It is added to the precursor through the suba seal. The solution was stirred for about 1 h under _N2 atmosphere. Afterwards, the isopropoxide was completely dissolved.

6) Prepare an infusion with 0.2 ml of acetic acid (99.7% A.C.S. Reagent) and add the acetic acid infusion in solution through the suba seal. The solution was stirred for about 10 minutes (a precipitate started to form after about 90 minutes). Acetic acid is a chelating agent that slows down the hydrolysis and condensation of alcohol salts.

The resulting solution is transparent. Prefabricated sol-gel solutions of (2-methoxyethanol) ² praseodymium in 2-methoxyethanol are available from Chemat Technologies, Inc. if using praseodymium methoxyethanol as a precursor. purchased. The moisture sensitivity of the alkoxide solution precursors can be reduced by using more complex alkoxy groups or adding chelating agents such as acetic acid.

Still other steps that can be taken include hydrolyzing the precursors prior to coating. Hydrolysis helps reduce residual organic content in the coating and reduces sensitivity to ambient humidity. Addition of 2 moles of _H2O per mole of alkoxide hydrolyzes the precursor, resulting in substitution by hydroxyl groups. In the absence of chelating agent, water was added to the precursor solution, and the solution immediately became cloudy. If a chelating agent is present, the solution becomes cloudy after about 1 hour.

test results

Tests were performed on three 12.5 x 12.5 mm (1/2 x 1/2 in) fused silica substrates to determine the oxide phase and the reaction between the coating and the quartz. Coat the substrate as described below. The solution was spin coated on the substrate at about 3000 rpm for about 60 seconds. The substrate was then placed on a hot plate at about 175°C for 4 minutes to evaporate the solvent. Spin coating and heating were performed three times to form a three-layer coating.

The deposited coating is then fired. The heating and cooling rate is about 5°C/min. All three test samples were first fired at about 900° C. for about 30 minutes, after which a light yellow appearance was observed. These three samples were then fired at about 1100°C for about 1 hour (sample #1), 2 hours (sample #2) and 5 hours (sample #3), respectively. The film became transparent after the second firing step. The coating thicknesses of the three samples were measured with a tencor profilometer and were 183 nm (sample #1), 203 nm (sample #2) and 170 nm (sample #3).

Another method of firing is to first heat the sample to 500 °C, and then increase the temperature to 1100 °C at a rate of about 5 °C/min. The samples were incubated at 1100°C for about 30 minutes. The resulting film was transparent. Specifically, SEM micrographs at 1000 °C revealed densely packed nanoscale grains of praseodymium oxide, a microstructure required for optical transparency.

UV-VIS spectrometer was used to measure the relationship between the absorbance of coating and the change of wavelength. It was found that the absorbance generally decreased with increasing wavelength (in the range of 190-820 nm). X-ray diffraction experiments on spin-coated samples showed the presence of crystalline phases, including a Pr ₆ O ₁₁ phase that reduces to Pr ₂ O ₃ at about 1000 °C.

Differential thermal analysis (DTA) showed that Pr ₆ O ₁₁ crystallized at about 650 °C with methoxyethanolate as precursor and at about 500 °C with isopropoxide as precursor. Analysis of crystallographic phase growth on films formed with isopropoxide as precursor showed that Pr ₆ O ₁₁ was the main phase, but an unidentified phase formed at 1000°C. Upon heating to higher temperatures, some unidentified phases also appear, probably due to internal diffusion. When isopropoxide is used as the precursor, cristobalite appears after heating to 1000°C, indicating the crystallization of fused quartz.

When using methoxyethanolate as precursor, the analysis of crystalline phase growth on the film shows that Pr ₆ O ₁₁ crystallizes at a higher crystallization temperature than isopropoxide as precursor, and no unidentified phase appears . Pr ₆ O ₁₁ transforms into Pr ₂ O ₃ at about 900°C. With methoxyethanolate as precursor, no cristobalite was observed.

Depth analysis using Auger electron spectroscopy showed no Si on the surface of the tested substrates. Further analysis showed the presence of Si after about 108 nm sputtered from the film using Ar.

The Rutherford backscattering spectrum shows that the interdiffusion of Pr and Si occurs on the sample heated to 1100°C for about 1 hour (isopropoxide as the precursor).

Depending on the application temperature, one route may be more suitable than another (meaning using another precursor). As noted previously, the isopropoxide route produces crystallization at lower temperatures. However, the methoxyethanolate solution coated on fused silica produced only _{the Pr6O11} phase. For example, the stable polycrystalline praseodymium oxide phase produced by this methoxyethanolate route is important for the stability of the luminosity and/or the color of long-term discharge lamps.

High-temperature tests have shown that interdiffusion between praseodymium oxide and quartz occurs at about 1000°C. Therefore, in a discharge lamp, keeping the bulb operating temperature below 1000°C, preferably 950°C or lower, suppresses this interdiffusion.

bulb data

Figure 4 is a flow chart of a second method of coating the interior surface of a light bulb according to the present invention. A sol-gel solution is prepared or provided as described above (step 21). The sol-gel solution is passed into the bulb preform (step 23). The preform is rotated at a sufficient speed to distribute the sol-gel solution over the inner surface of the preform (step 25). Excess sol-gel solution is removed from the preform (step 27, eg pouring off excess solution). The coated preform is dried and fired (step 29) as described above.

Apply the above procedure to a light bulb with an outer diameter of 9 mm and an inner diameter of 8 mm as follows. A bulb is formed on one end of the quartz rod, with a pinch-off opening that remains unsealed. Add a drop of the sol-gel solution prepared according to the isopropoxide route to the inside of the bulb. The bulb was then rotated at 3000 rpm and heated at 90°C with a cylindrical heating pad. The bulb rotates for about 2 minutes, then the axis of rotation is reversed 180°, and the bulb rotates for another 2 minutes. The process was repeated 3 times for a total spin time of approximately 8 minutes. Other methods of uniformly distributing the sol-gel (such as vibrating or rocking the bulb preform) may also be used. The residual sol-gel liquid was then aspirated from the bulb. The bulbs were dried at about 200°C for 15 minutes. Several layers are applied in this way. Thereafter, the bulb was fired in a furnace at 1000°C for 30 minutes at a heating/cooling rate of 5°C/min.

Then a filling material and a suitable starting gas (such as 50 Torr Krypton) are added to the bulb. In this embodiment, the filling material is 2 mg of praseodymium trichloride (PrCl ₃ ). Figure 5 is a graph of the spectral power distribution of a coated bulb containing this fill material. It is measured by fitting the bulb in a reflective ceramic jacket having a circular hole with a diameter of 5 mm. Approximately 150 watts of RF power was applied to the bulb using inductively coupled lamp wiring. As can be seen from FIG. 5, the filling material provides light output over a wide continuous visible spectral range.

An advantage of coated bulbs according to the invention is that they maintain good light output several times longer than uncoated bulbs containing the same fill material. Preliminary results show that bulbs using the coatings of the present invention last at least an order of magnitude longer than uncoated bulbs, and likely provide several orders of magnitude longer lifetimes.

barrier film

As mentioned above, at the high temperature of the bulb, there is a possibility of diffusion of praseodymium into the quartz substrate. For lamp envelopes requiring a long service life at high temperatures, as shown in Figure 2, it is advantageous to first deposit a clearly transparent diffusion barrier film on the fused silica bulb envelope before depositing the praseodymium oxide. Suitable barrier films include thin films of aluminum oxide, aluminum nitride, silicon nitride, praseodymium nitride, and titanium dioxide.

Titanium dioxide ( _TiO2 ) is an example of a suitable diffusion barrier film of the invention. _TiO2 does not react with _SiO2 below about 1400°C. According to this aspect of the invention, the diffusion barrier film coating is applied by the sol-gel method as follows:

1) Mix Ti(IV) isopropanol as a precursor with isopropanol solvent.

2) Add a chelating agent, precursor; the ratio of chelating agent is kept at 1:2 moles.

3) Add H ₂ O (10 moles of water per mole of precursor) dropwise into the system.

4) After a few minutes, add a drop of HCl as a peptizer.

The above titania precursor solution was spin-coated on a fused silica substrate at a speed of 3000 rpm for 60 seconds. After spin coating, the coated substrate was heated at 105°C for 4 minutes to evaporate the solvent. Four layers were deposited in this manner. The coated substrate was then fired at 700°C for 30 minutes at 5°C/min. The resulting coating thickness was about 300 nm. The coating forms an anatase phase at 700°C and turns into a rutile phase when heated to 1000°C.

The substrate is then coated with a rare earth oxide as described above. At about 700 °C, _{a Pr6O11} phase forms on the diffusion barrier layer of the anatase phase. Heating at 1000 °C for 30 minutes, the rare earth oxide phase interacts with the underlying rutile phase to form a Pr-O-Ti phase. X-ray diffraction experiments show that at 1000°C, the rutile phase remains and no reaction with _SiO2 occurs. Formation of cristobalite was also not observed.

The invention has been described with reference to preferred embodiments, and various changes will occur to those skilled in the art. For example, the coatings of the invention can be used in bulbs having internal electrodes and also other rare earth metal filling materials. Bulbs of all shapes can use this coating. Accordingly, the foregoing description should be considered as illustrative only and not restrictive, with the spirit and scope of the invention being defined by the appended claims.

Claims (16)

1. A light bulb comprising: Light-transmitting fused quartz casing; A plasma-forming filling material and a rare gas filled in the quartz envelope, said filling material comprising a halide of praseodymium and a rare gas; A coating that is deposited over substantially the entire inner surface of a quartz envelope, the coating comprising oxides of praseodymium. 2. The light bulb of claim 1, wherein said filling material comprises one of PrBr ₃ and PrCl ₃ , and said coating comprises at least one of Pr ₆ O ₁₁ and Pr ₂ O ₃ . 3. 2. The lamp of claim 1, wherein said filling material consists essentially only of praseodymium halides and noble gases, and said coating consists essentially only of praseodymium oxides. 4. The light bulb of claim 1 further comprising a diffusion barrier film between said coating and said envelope. 5. The light bulb of claim 4, wherein said diffusion barrier film comprises one of silicon nitride and titanium dioxide. 6. A light bulb comprising: Light-transmitting shell; a fill material containing a rare earth metal contained within the enclosure, said fill material capable of producing a plasma discharge when energized; A coating deposited on the inner surface of the enclosure, said coating inhibiting the reaction between the plasma discharge and the enclosure, comprising oxides of the same rare earth metals as the filling material. 7. The light bulb of claim 1, wherein said filling material consists essentially only of rare earth metal halides and rare gases, and said coating consists essentially only of rare earth metal oxides. 8. The light bulb of claim 6 further comprising a diffusion barrier film between said coating and said envelope. 9. The light bulb of claim 8, wherein said diffusion barrier film comprises one of aluminum oxide, aluminum nitride, silicon nitride, rare earth nitride, and titanium dioxide. 10. A method of coating a light bulb, the method comprising the steps of: i) filling a sol-gel solution into the bulb preform; ii) distributing said sol-gel solution on the inner surface of said preform; iii) removing possible excess sol-gel from said preform; iv) Treating the sol-gel solution remaining in the preform to obtain the desired coating. 11. The method of claim 10, wherein step (iv) includes drying and firing said coating. 12. The method of claim 10, further comprising: v) adding filler material to said preform; ix) Sealing the preform to form a bulb. 13. The method of claim 10, wherein said sol-gel solution added in said step (i) contains a precursor of a desired coating. 14. 13. The method of claim 13, wherein said precursor comprises a rare earth metal oxide and the desired filler material comprises a halide of the same rare earth metal. 15. The method according to claim 10, characterized in that before said step (iv), said steps (i)-(iii) are repeated one or more times. 16. The method of claim 10 wherein said step (ii) includes rotating said preform.

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