Patent Application
20060138962
This invention relates to ceramic discharge vessels and in particular to discharge vessels for high-intensity discharge applications that include a sapphire tube body.
Ceramic discharge vessels are generally used for high-intensity discharge (HID) lamps such as high-pressure sodium (HPS), high-pressure mercury, and metal halide lamps. Typically, these discharge vessels are formed from multiple ceramic components that are co-sintered to form hermetic seals between the parts without the use of a frit material. This technique relies on a differential shrinkage of the ceramic components to create an interference fit between the parts. The preferred ceramic for HID lamp applications is polycrystalline alumina (PCA), although other ceramics such as sapphire, yttrium aluminum garnet, aluminum nitride and aluminum oxynitride may also be used.
Sapphire is an excellent transparent ceramic material, except that it is limited to straight shapes defined by crystal growth techniques. Single-crystal sapphire tubes typically grown by the EFG (edge-defined, film-feed growth) method are useful for ceramic metal halide lamps. However, while PCA to PCA seals rely on the differential shrinkage of the PCA parts, this technique is normally not applicable to sapphire to PCA seals because the sapphire tube does not shrink during the sintering of PCA parts. As a solution, some recent designs of ceramic discharge vessels for low wattage, automotive applications use polycrystalline alumina hats that fit over the ends of the sapphire tube. See, e.g., International Patent Application No. WO 99/41761. The PCA hat shrinks around the end of sapphire tube during sintering to create a seal with the exterior surface of the tube. However, the high thermal mass of the PCA hat causes high heat losses via radiation from the hat's surface and induces a severe thermal stress that can lead to a high incidence of cracking. In addition, the cost of this sapphire-PCA construction is relatively high compared to competing technologies. Unfortunately, sapphire tubes with frit-sealed PCA plugs are not a suitable alternative since the frit, in this type of construction, is too close to the higher temperature region and is consequently attacked more rapidly by the corrosive metal halide fill leading to early failure of the lamp.
Reaction-bonded aluminum oxide (RBAO) is a relatively new class of ceramic material with low (<1%) dimensional shrinkage. RBAO is new relative to conventional sintering of alumina in that both reactions and sintering take place in the compacted body simultaneously during heating. The method of producing strong RBAO bodies starts with compacts of milled mixtures of aluminum metal and aluminum oxide powders that are heat treated at about 1200 to about 1550° C. Typically, it is desired that the expansion due to the Al metal to Al2O3 reaction and the shrinkage on sintering of the Al2O3 be nearly balanced.
However, unlike prior applications for RBAO, the present invention involves coaxing the RBAO into a range where densification is accompanied by a small expansion during heating. The expansion of the RBAO component is used to create the hermetic seal between ceramic components. In effect, the RBAO part swells and seals against a constricting surface. This is can be thought of as the opposite of the differential shrinkage method used for PCA.
Since the RBAO expands during sintering, it is possible to use an inserted RBAO plug in a sapphire tube to seal the ends of the tube and thereby eliminate the use of an external PCA hat. This construction should result in a better thermal profile, less stresses, and higher survivability. While an internally-sealed plug construction is preferred, the use of expanded RBAO is not limited to forming internal seals within the arc tube. For example, it is also possible to use the expanded RBAO in the hat configuration whereby the expansion of the RBAO during sintering causes a constriction of the hat around the outer surface of the tube to form an external seal. Thus, the use of expanded RBAO for creating hermetic seals allows more flexibility in the manufacturing of ceramic discharge vessels.
Therefore, in accordance with an aspect of the invention, there is provided a ceramic discharge vessel comprising a ceramic body and at least one expanded reaction-bonded aluminum oxide member hermetically sealed to the ceramic body.
In accordance with another aspect of the invention, there is provided a method of forming a hermetic seal in a ceramic discharge vessel, the method comprising: (a) forming a ceramic body; (b) forming a reaction-bonded aluminum oxide member in a green state by compacting a mixture of aluminum metal and aluminum oxide powders; (c) assembling the ceramic body and the reaction-bonded aluminum oxide member in the green state to form an assembly; and (d) reaction sintering the assembly to cause the reaction-bonded aluminum oxide member to expand and form a hermetic seal with the ceramic body.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.
The general equation for the total dimensional change, S, after a complete reaction-bonding cycle for an RBAO ceramic is given by Equation (1) below:
S=[(1+ΣνiVi)(ρo/ρ)/(1+0.28fVAl)]1/3−1 (1)
where νi is the volume expansion associated with the respective oxidation (e.g., νAl=0.28, νZr=0.49, νTi=0.76, νCr=1.02), Vi is the volume fraction of metal (including VAl) or ceramic phases added in the original powder mixture, f is the Al fraction oxidized during milling, and ρo and ρ are the green and final densities, respectively. Equation (1) indicates that a higher volume fraction of Al and a high green density can yield a final expansion (rather than shrinkage) during sintering of the Al/Al2O3 compacts. Typically a volume expansion of about 1-4% occurs at ˜700° C. because of the melting of the Al phase. When this expansion occurs, it is theorized that the molten Al metal will spread from the Al/Al2O3 part to fill the gap between the parts so as to form a hermetic seal upon further reaction sintering.
In a preferred method of manufacture, the sealing members 7 in their green state would be inserted into the ends of a PCA or sapphire tube and expanded by reaction sintering. As the diameter of the RBAO plugs expands during reaction sintering, an interference fit is created with the constricting inner surface 5 of the tubular body 3 and a hermetic seal is formed between the tubular body 3 and the sealing members 7. In the case of PCA, the PCA tube may be fully sintered to prior to combining it with the RBAO plug in which case only minimal shrinkage of the PCA tube may occur during the reaction sintering of the RBAO parts, or the PCA tube may be only prefired in which case the PCA tube will shrink as the RBAO parts are expanded during the reaction sintering step. In the latter case, the alumina tube is prefired at 850° C. to 1350° C. before being combined with the green RBAO part. The assembled parts are then reaction sintered at a temperature less than 1350° C. to at least partially bond the parts and then sintered at a higher temperature to about 1850° C. in hydrogen, an N2-H2 mixture, or vacuum, to increase transmittance and finish the seal. A sinter-HIP (hot isostatic pressing) process which sinters the assembly to a closed-pore stage followed by HIP may also be used to bring about high transmittance.
With respect to the discharge vessels shown in
In order to demonstrate the capability of expanded RBAO to form hermetic seals, solid RBAO plugs were made and sealed into the ends of sintered PCA tubes. In particular, aluminum metal powder having an average 20 μm particle size (Johnson-Matthey) was admixed with alumina powder (CR6 or CR1 from Baikowski) in amounts of 30, 40, 50, and 60 volume percent (vol %). CR6 alumina powder which has a surface area of 6 m2/g was preferred because of its sinterability. Finer aluminum powders are available, but submicron aluminum powders would require special precautions as spontaneous combustion could occur. For aluminum powders greater than 1 μm, handling in air at ambient temperature is acceptable. Aluminum metal volume content may be in a range from 10 to 70 volume percent, and preferably from 50 to 60 vol %. When the aluminum metal content is high (>60 vol %), the pressed parts tend to be soft and frail making handling more difficult. The Al/Al2O3 mixtures were ball-milled for 2 hours in methanol using 5 mm ZrO2 balls and high-density polyethylene bottles. Methanol was used for ball milling since aluminum metal powder reacts with water. Ball milling was limited to 2 hours to prevent excessive pick up of ZrO2 from the media. After pan drying, the powder was broken up using mortar/pestle. The powders were uniaxially pressed or isopressed at 35 ksi or higher. Because aluminum metal deforms under pressure, the Al/Al2O3 compacts could achieve a high green density of 60-80% of theoretical density. If needed, the green plugs could be machined to a predetermined size. The green RBAO plugs were 4.90 mm in diameter by 2 mm thick, and the PCA outer tubes had a 4.95 mm ID. After assembly with the outer tube, the entire samples were reaction-sintered under flowing air or oxygen using the following temperature cycle: (1) heating at 1° C./min to 700° C. with a hold at 700° C. for 24 h; (2) continue heating at 1° C./min to 1100° C. with a hold at 1100° C. for 24 h; (3) continue heating at 1° C./min to 1550° C. with a hold at 1550° C. for 24 h; and finally cooling at 30° C./min to room temperature. The final hold temperature could be higher than 1550° C., e.g., 1600-1900° C., in order to promote full densification. This depends on the starting green density and particles sizes of the Al and Al2O3 phases. A pure oxygen atmosphere is preferred, because it results in a faster oxidation of the Al metal particles to Al2O3. A slow ramping of the temperature limited cracking of the tube. Higher ramp rates such as 2° C./min to 5° C./min resulted in cracking of the outer tubes, probably because of a lack of stress relaxation. A temperature of 1550° C. was sufficient to form hermetic body and direct bonds of the expanded RBAO plugs to the outer alumina tubes.
During reaction sintering, the RBAO plugs had a final expansion of 0.35 mm as the diameter increased from 4.90 mm to 5.25 mm resulting in a net interference of about 6%. Longitudinal cracks in the outer PCA tubes appeared after the reaction sintering cycle when high temperature ramp rates (5° C./min) were used. The length of the bond between the expanded RBAO plug and outer alumina tube was 2 mm. Successfully bonded tubes were leak tight to <10−9 scc/sec.
While there has been shown and described what are at the present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.
