US 3679385 A
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Description (OCR text may contain errors)
M Q l2 25, 1972 s. P. SENFT I 9,
- MANUFACTURE OF INTERIOR COATED BULBS FOR HIGH TEMPERATURE GLASS LAMPS Filed Sept. 18. 1970 unm- ITWVEZTT't'OTI Stephen P Sewf' t b9 91W, 6 2111.
His A1."tZOT"TWG H United States Patent vs. c1. 65-60 a 6 Claims ABSTRACT OF THE DISCLOSURE An interior coated lamp envelope having a thick-walled bulbous portion is manufactured by first coating"an interior region of a length of quartz tubing with a, layer of boric oxide. This may be done by bubbling nitrogen gas through liquid trimethyl borate and feeding the resultant vapors, along with air, through the quartz tubing while rotating in a glass lathe and heating the region thereof which is to be formed into a thick-walled bulbous portion. While continuing the heating, the ends of the tubing are forced toward each other thereby forming a thick-walled bulbous portion and at the same time baking the deposited coating of boric oxide.
BACKGROUND OF THE INVENTION The invention is in the field of lamp manufacture, and is directed to the manufacture of interior coated bulbs for lamps which operate at high temperature.
US. Pat. No. 3,263,852 to Elmer G. Fridrich (assigned to the same assignee as the present invention) describes a method of forming a quartz envelope having a thickwalled bulbous portion, in which method a length of quartz tubing is chucked at both ends in a glass lathe, and, while heating the quartz tubing in its central region, the chucks are moved relatively toward one another. US. Pat. No. 3,305,289 to Elmer G. Fridrich (assigned to the same assignee as the present invention) describes a method of sealing electrodes in such a thick-walled bulbous envelope, to provide an arc lamp.
The above-described arc lamp is intended to operate at a high temperature, for example greater than 1000 C. and preferably at 1400" C. However, at such high temperatures the inner surface of the bulb tends to devitrifyi.e., it crystallizes and develops small cracks, which cause reduction of light output and also physical weakening of the bulb, thereby shortening the useful lamp life. US. Pat. No. 3,390,298 to Horst Werner (assigned to the same assignee as the present invention) describes a boric oxide treatment of the inner surface of the lamp bulb, which causes the inner bulb surface to become molten during lamp operation and prevents the occurrence of devitrification. The boric oxide may be applied to the bulb by filling it with a solution of trimethyl borate, draining, and baking at 1200 to 1300 C. for two hours. Alternatively, the borate may be applied inside the bulb by a flow of vapors of trimethyl borate entrained in an inert gas such as nitrogen, while heating to about 700 C. and supplying air to oxidize the trimethyl borate and produce the boric oxide coating. Instead of using trimethyl borate, tributyl borate or diborane may be used.
Although the above-described boric oxide treatment of the bulb is effective in preventing devitrification of the bulb during lamp operation, it requires considerable time and increases the cost of manufacturing the lamps.
SUMMARY OF THE INVENTION Objects of the invention are to provide an improved method of manufacturing lamp bulbs, and to provide an improved method of applying boric oxide coating inside bulbs for high-temperature lamps, resulting in reduced manufacturing time and lower cost.
3,679,385 Patented July 25, 1972 The method of the invention comprises, briefly and in a preferred embodiment, the steps of coating an interior region of a length of tubing offused vitreous silica, such as quartz, with a borating material, heating said region of the tubing, and forcing the ends of the tubing relatively toward each other, thereby forming a thick-walled bulbous portion and baking the borating material to produce boric oxide.
BRIEF DESCRIPTION OF THE DRAWING 1 illustrates the step of introducing a borating material into a" region of quartz tubing which is chucked in aj' 'gla ss lathe, while at thesam'e time heating said region of tubing, and... 7' I v 2 illustrates the step of forcing the ends of the quartztubing relatively toward 'each other while heating the region thereof to which the boratingmaterial has been applied.
DESCRIPTION OF Til-IE PREFERRED EMBODIMENT As shown in FIG. 1, a length of tubing 11 of fused vitreous silica, such as quartz, is held in and rotated by a chuck 12 of a glass lathe. A flame burner 13 is positioned to heat a region 14 of the tubing 11 to a temperature of at least 700 C. A vessel 16 contains a solution of borating material, such as trimethyl borate, tributyl borate, or diborane. A nozzle or tubing 18 extends into the borating liquid 17, and an inert gas such as nitrogen is flowed into the tubing 18 and bubbles through the borating liquid material 17. A nozzle 21 extends from the vessel 16 and partly into the quartz tubing 11, so that the inert gas, carrying vapors of the borating material 17, enters the quartz tubing 11 at the region 14, whereby the borating material deposits on the inner surface of the tubing 11 as the heated region 14. An air inlet 22 is provided, for combining air with the vapor carrying nitrogen, so that the borating material will oxidize where deposited at the heated region 14 inside the tubing 11 and form boric oxide.
The borating nozzle 21 is removed from the quartz tubing 11, and the free end of the tubing 11 is chucked in a second chuck 23, as shown in FIG. 2., and, while continuing to heat the region 14 of the tubing by means of flame burner 13, the chucks 23 and 12 are forced relatively toward each other, as indicated by the arrow 24, thereby forming a thick-walled bulbous portion 26 in the manner described in the above-referenced Pat. 3,263,852, while at the same time baking the deposited borating material on the inner wall of the region 14. Thus, in a continuous operation, the boric oxide layer is formed within the bulbous portion 26, and baked, and the bulbous portion 26 is formed. This simple continuous operation saves considerable time in manufacturing the coated bulb, and reduces the manufacturing cost, because it eliminates the steps, previously deemed necessary, of cooling the formed bulb after it is made, storing it, and then chuck ing it again in a glass lathe and heating up the bulbous portion while introducing the borating material.
Instead of removing the nozzle 21 before chucking the free end of the tubing '11 in the chuck 23, as described above, both ends of the quartz tubing may initially be chucked as shown in FIG. 2, and a nozdle 21 may be inserted through a hollow portion of one of the chucks 23 and into an end of the quartz tubing 11, in order to introduce the borating material therein. If desired, the borating material may thus be introduced into the tubing 11 at the same time as the chucks 23 and 12 are being moved relatively toward each other for forming the bulbous portion 26. In a modification of the method above-described, air may continue to be flowed through the intake 22 and out of the nozzle 21, after the flow of nitrogen has been terminated in the tubing 18, so as to facilitate oxidizing the borate material while it is being bakedduring the formation of the bulb portion 26. During the latter stages of thelafore-described continuous method, it is desirable to increase the; temperature of the heated region. Also, if desired, the borating vapors may be fed into the region 14 before, during, and/ or after formation of the' bulb portion 26, taking advantage of the bulbgf orrr'iing heat for depositing and baking the layer of borating material within the bulb portion 26.
' In ajpr'eferred embodiment of the invention, the flow "rate of the inert gas in tubing 18 isapproximately 0.1 to 0.03 c.f.h., the flow of air into tubing 22 is'at a rate of about:0.5=to 1.0 c.f.h., the time duration of flowing the ,vaporsinto the bulb region 14 is approximately 5 to 40 seconds, and the amount of boric oxide formed in a typical lamp envelope, having a bulbous volume of less than 1 cubic centimeter, is about 0.1 to 6.0 milligrams. While a preferred embodimentpand modification of the invention have been shown and described, various other embodiments and, modifications'will become. apparent to persons skilled in the art, and'will fall within the scope of invention as defined in the following claims.
What I claim asnew and desire to-secure by Letters Patent of the United'States is: v
1. A method of making a lamp envelope having a bulbous portion interiorly coated with boric oxide, cornprising the steps of introducing a borating material into a' length of tubing of fused vitreous" silica at a region where said bulbous portion is desired,heating said region,
and forcing the ends of the tubing relativelytoward each other thereby forming said bulbous portion at said heated region and baking said borating material to form said coating of boric oxide, said steps being performed in'a continuous operation.
2. A method as claimed in claim 1, in which said borating material is introduced into said tubing prior to said forming of the bulbous portion, whereby the borating material is baked during the entire time said regionof the tubing is heated for forming said bulbous portion.
3. A method as claimed in claim 1, in-which said borating material is trimethyl borate.
4. A method as claimed in claim 1, in which said borating material is .tributyl borate. w
5. A method as claimed in claim 1, in which said borating material is diborane. I 6. A method as claimedin claim 1, in which air is flowed in said'tubing during said heating of said region thereby'facilitating the formation of said boric oxide.
7 References Cited 6 UNITED STATES PATENTS 1,948,560