GB2032909A

GB2032909A - Sealing Glass Compositions

Info

Publication number: GB2032909A
Application number: GB7922381A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1978-08-09
Filing date: 1979-06-27
Publication date: 1980-05-14
Also published as: DE2930249C2; FR2433001A1; GB2032909B; JPS5527896A; JPS5647142B2; FR2433001B1; DE2930249A1

Abstract

Glasses having high softening points as well as exceptionally low liquidus temperatures are within the following compositional limits and percent by weight: <IMAGE> except for incidental impurities, residual fluxes, and refining agents and have a ratio BaO/CaO of 2.3 to 3.5. These glasses have improved reboil resistance and can be sealed directly to tungsten or molybdenum metals to provide a direct hermetic seal, and are thus especially useful in high temperature lamp envelopes. <IMAGE>

Description

SPECIFICATION High Temperature Seal Glass Composition This invention relates to improvements made upon high temperature aluminosilicate seal glasses containing CaO and BaO which are described and claimed in U.S. Pat. No. 4,060,423 and assigned to the assignee of the present invention. The present glasses maintain the metal oxide combinations within specified weight percent ranges as follows: SiO+A1203 73-83, BaO+CaO 1 7-30 and with further ratios between BaO and CaO being maintained within the range 2.3 to 3.5, for greater reboil resistance. The term "reboil" as used herein is a tendency of the glass upon flame reworking or reheating to form many small bubbles of entrapped gases in the glass which produce poor light transmission as well as weaken the reheated areas.While small amounts of alkali metal oxides have been used in the past to reduce reboil, the tungsten-halogen cycle lamp cannot tolerate more than 300 ppm of an alkali metal oxide in the lamp glass envelope without experiencing operating difficulties, On the other hand, it is within contemplation of this invention to have 0--3 weight percent of an alkali metal ion in the present glasses for use in other high temperature lamp types requiring direct hermetic seals to tungsten and molybdenum components such as mercury, multivapor, or other vapor discharge lamps.

It has been discovered by the present applicant that the ratio of BaO to CaO in high temperature aluminosilicate glasses is critical for controlling reboil of the glass. Glasses having a ratio less than about 2.3 will tend to have some reboil on reheating and flame working of the glass, with the reboil becoming more severe as this ratio decreases. While increasing the SiO2 content in the glass can reduce reboil, such increased SiO2 content lowers the thermal expansion of the glass and for glasses to be useful in direct hermetic sealing to tungsten as well as molybdenum metals, an average thermal coefficient of expansion in the O-30O0C temperature range between 37x 10-7 cm/cm/ C to about 50x 10-7 cm/cm/0C is required. The prior art glasses disclosed in the aforementioned issued U.S.

patent have a thermal expansion coefficient suitable for hermetic seals only to metals with an expansion coefficient matching molybdenum. It is now possible by maintaining the BaO content at 10 percent by weight or greater and controlling the remaining metal oxides within critical proportions to achieve a proper match with metals having a lower expansion coefficient such as tungsten It is an object of the present invention, therefore, to provide an improved aluminosilicate glass which is useful as the envelope material in high temperature lamps utilizing either tungsten or moiybdenum components to which the glass is directly sealed. It is a further object to provide an improved aluminosilicate glass for direct hermetic sealing to ether molybdenum or tungsten in leads and which is essentially free of reboil tendency during the sealing operation.A still further important object of the present invention is to provide a glass composition having SiO2,Al2O3, CaO and BaO in critical proportions as the essential metal oxides but which can further contain minor amounts of alkali metal oxides without producing detrimental effects upon the desired sealing characteristics.

Briefly stated, the present glass composition consists essentially of metal oxides as calculated from the starting batch of materials in approximate percents by weight 54-71 SiO,, 12--18 As203, 10-23 BaO, 4-8 CaO, and 0--3 R2O, wherein R is an alkali metal ion, along with minor amounts of incidental impurities, residual fluxes and refining agents such that the aggregate SiO2 and Awl203 content lies within the approximate range 73-83 weight percent, the aggregate BaO and CaO content lies within the approximate range 1 7-30 weight percent, and the weight ratio between BaO and CaO lies within the approximate range 2.3 to 3.5 so as to provide improved reboil resistance. A preferred glass composition for sealing directly to tungsten metals contains an aggregate BaO and CaO content lying in the approximate range 17-21 weight percent and exhibits an average coefficient of linear thermal expansion in the 0--3000C temperature range between about 37x 10-? cm/cm/ C to about 41 x10-7 cm/cm/ C. A different preferred glass for sealing directly to molybdenum metals contains an aggregate BaO and CaO content lying in the approximate range 21-30 weight percent and exhibits an average coefficient of linear thermal expansion in the O3aOoc temperature range between about 41 x 10-' cm/cm/ C and 50x 10-' cm/cm/ C. Both preferred glasses further desirably exhibit a strain point of at least about 7300C and a softening point in the 1,000--1,1400C range while retaining a relatively low liquidus temperature not exceeding 1 ,3500C.

The present invention will be further described, by way of example only, with reference to the accompanying drawing which is a side elevation view of one type regenerative cycle incandescent lamp employing improved glass-to-metal seals according to the present invention.

Referring to the accompanying drawing, a double-ended type lamp 10 is shown which includes a transparent envelope 12 of the present glass compositions, a tungsten filament 14, pinch seal portions 16, lead-in conductors 18, and an inert gas filling (not shown). Said inert gas filling can comprise an inert gas such as argon which further includes a halogen such as iodine to produce the well-known tungsten-halogen cycle of lamp operation. The lamp gas filling is introduced through an exhaust tube (not shown) with the tipped-off residue being shown at 20. The lead-in conductors 1 8 each comprise an outer lead wire 22, foil element 24, and inner-lead 26 which are now commonly formed of molybdenum metal.Said lead-in conductor assembly is hermetically sealed in both ends of the lamp envelope 12 at the pinch seal portion 1 6. As can be further noted from said drawing, the particular glass-to-metal seal being depicted is provided by a direct vacuum-tight seal of the glass around the individual refractory metal foil and wire elements of the lead-in conductor assembly. In forming the desired hemetic seal, it has not been necessary to specially pretreat the metal surfaces since the molten glass composition sufficiently bonds to a relatively clean metal surface at the seal-forming temperature. The pinching step employed during lamp manufacture is also sufficient to produce a relatively strain-free seal without need for annealing post-treatment.

As previously indicated, the improved hermetic seals obtained m accordance with the present invention are more free from reboil effect than is obtained with the glass composition disclosed in the aforementioned issued U.S. Pat. No. 4,060,423. It is also further possible with the present glass composition to obtain such improved seal in other high temperature lamps than the above illustrated tungsten-halogen cycle lamps as well as permit further flexibility in construction of all said lamps by substitution of tungsten components for molybdenum components whenever otherwise suitable for the iead-in conductor assembly of the particular lamp.The present glass compositions are still further distinctive from the glass compositions disclosed in the aforementioned issued U.S. patent with respect to certain other physical properties useful in construction and operation of these type lamps.

Specifically, present glass compositions exhibit a softening point in the approximate range 1,000-- 1 ,1400C along with a strain point in the approximate range 730--8200C which raises the high temperature resistance to deforming of the glass when being used in a high temperature environment.

On the other hand, the present glasses can still be drawn into tubing by conventional methods in high temperature glass melting furnaces and glass drawing equipment since the liquidus temperatures during glass melting and forming still lie in the approximate temperature range 1200--13250C.

Preferred glasses according to the present invention are reported in Table I below. As is common in glass technology, the glass compositions are reported in terms of oxides as calculated from the batch starting materials. Although there may be minor differences between the glass composition as calculated in the conventional manner from the batch constituents and the actual glass composition obtained therefrom, both compositions will be essentially the same. There is only slight volatilization of the batch constituents in the present glass composition during melting and which can be accompanied by pickup of ZrO2 and At203 and other constituents at the trace level from any refractories employed to melt the glass.Consequently, the present invention contemplates a glass composition having the same composition ranges above set forth as calculated in the conventional manner from the starting batch formulations.

Table I Weight percent 1 2 3 4 5 6 7 SiO2 61.1 62.3 59.8 58.0 64.7 63.6 55.8 Al203 14.2 12.8 15.5 16.2 17.5 17.3 15.0 BaO 18.1 18.2 18.0 19.3 13.0 13.0 22.5 CaO 6.6 6.7 6.6 6.5 4.8 4.8 6.7 K20 0 0 0 0 0 1.3 0 Softening Pt., OC 1014 1006 1030 1029 1120 1084 984 Strain Pt., OC 750 - 748 747 801 784 Coef. Exp. (0--3000C)/OC 45.2 45.7 45.0 46.0 38.8 40.5 50.0 Ratio BaO/CaO 2.7 2.7 2.7 3.0 2.7 2.7 3.4 % SiO2+AIz03 75.3 75.1 75.3 74.2 82.2 80.9 70.8 % BaO+CaO 24.7 24.9 24.6 25.8 17.8 17.8 29.2 As can be noted from the foregoing Table, the glass composition in Example 6 further includes an alkali metal oxide in modest amounts while still maintaining all other metal oxide constituents within the critical proportions required to provide a seal glass composition exhibiting the desired improvements.

The present glass composition is further amendable to incorporation of even lesser amounts of other oxides known to produce desired further effects such as absorption of ultraviolet radiation within the compositional ranges specified for the critical metal oxides and without detrimental effect upon the desired physical properties.

It will be apparent from the foregoing description, therefore, that a novel aluminosilicate glass composition is provided affording significant advantages as a general purpose glass for direct hermetic sealing to both tungsten and molybdenum type metals. It will also be apparent that minor variations in said glass compositions other than above specifically disclosed are included within the present glass compositions. It is intended to limit the present invention, therefore, only by the scope of the following

Claims

claims. Claims

1. A glass-to-metal seal glass comosition which consists of oxides in approximate percent by weight 54--71 SiO2, 12-18A1203, 4-8 CaO, 10--23 BaO, and 0--3 R20 wherein R is an alkali metal ion, along with minor amounts of incidental impurities, residual fluxes, and refining agents, such that the aggregate SiO2 and Awl203 content lies within the approximate range 73-83 weight percent, the aggregate BaO and CaO content lies within the approximate range 1 7-30 weight per cent; and the ratio between BaO weight percent and CaO weight percent lies within the approximate range 2.3to 3.5 so as to provide reboil resistance.

2. A seal glass composition as claimed in claim 1 having a strain point of at least about 7300 C.

3. A seal glass composition as claimed in claim 1 or claim 2 wherein the aggregate BaO and CaO content lies in the approximate range 1 7-21 weight percent and the average coefficient of linear thermal expansion in the 0--3000C temperature range is between about 37xl0-7 cm/cm/ C to about 41 x10-7 cm/cm/ C.

4. A seal glass composition as claimed in claim 1 or claim 2 wherein the aggregate BaO and CaO content lies in the approximate range 21-30 weight percent and the average coefficient of linear thermal expansion in the O-3000C temperature range is between about 41 x10-7 cm/cm/ C and 50x 10-7 cm/cm/ C.

5. A glass-to-metal seal composition as claimed in any one of the preceding claims wherein said metal is molybdenum or tungsten.

6. A glass-to-metal seal composition as claimed in claim 5 wherein said metal is a molybdenum lead-in conductor in a regenerative cycle halogen-containing incandescent lamp.

7. A glass-to-metal seal composition as claimed in claim 1 substantially as hereinbefore described in the accompanying drawing.

8. A glass-to-metal seal composition as claimed in claim 1 substantially as hereinbefore described in any one of the Examples.