Medium expansion leaded copper boroaluminosilicate glasses

Details Medium expansion leaded copper boroaluminosilicate glasses Medium expansion leaded copper boroaluminosilicate glasses

2000-06-01

2002-08-13

Dunn, Tom (Department: 1725)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S059000, C501S061000, C501S074000, C501S017000

Reexamination Certificate

active

06432853

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to copper aluminosilicate glasses, and in particular to such new glass compositions of leaded copper boroaluminosilicate suitable for sealing borosilicate glasses or glass-ceramic materials having a high strain point.
2. Description of the Related Art
The joining of component parts together by means of a fused glass seal to form a composite article is a well-cultivated art. In particular, numerous special sealing glasses have been developed for use in joining glass parts with each other, or with metals, alloys, or ceramics. In making a fusion-type seal, the sealing material must be heated to a temperature where it becomes soft enough to wet the sealing surface and form an adhesive, hermetic bond.
The type of glass used in forming a fusion-type seal varies according to the desired properties of the article being sealed. For many purposes, manufacturers want to keep the softening temperature (softening point) as low as possible, while also maintaining a low to medium coefficient of thermal expansion (CTE). This is particularly true for lamp work having electrical and electronic articles where thermally sensitive parts or coatings are commonly used.
Typically, electrical lamps employ borosilicate glass envelopes or bulbs and require joining or sealing of these envelopes to either another piece of glass or metal (usually electrodes) to achieve hermeticity within the glass vessel. Borosilicate glass has a coefficient of thermal expansion (CTE) of 30-40×10
−7
/°C. The normal CTE for borosilicate lamp glasses, such as Pyrex® (Glass Code #7251) by Corning, is 38×10
−7
/°C. The elements enclosed in the glass vessel may include phosphor coatings (fluorescent lamps), metal electrodes, or metallic reflective coatings. Coatings and electrodes require a hermetic environment, otherwise their properties diminish, and as a result the lifetime of the device is dramatically shortened. The coatings elements, however, are typically sensitive to relatively high temperatures, especially greater than 600° C., and thus sealing temperatures must be maintained low, and processing times must be maintained short to avoid decomposition or degradation of these components.
The current workable sealing glasses used in lamps of the type described above are phosphate based frits. Phosphate frits have the advantage of low softening points and, hence, are considered low temperature sealing glasses. Solder glasses currently available can be classified in two categories: 1) leaded phosphate sealing glasses and 2) non-leaded phosphate sealing glasses. Both categories contain compositions that can be classified as vitreous or crystallized, in which crystallized sealing glasses are resistant to deformation under the conditions of reheating in vacuum—an advantage, for example, in television manufacture applications. In the prior art, medium expansion of sealing glasses is established typically by adding low expansion fillers (usually cordierite, lithium aluminosilicate glass-ceramics, or crystalline cobalt pyrophosphate, or magnesium pyrophosphate) to relatively high expansion base glass compositions (60-120×10
−7
/°C.) that have a relatively low softening points (350-400° C.).
The two categories of phosphate sealing glasses mentioned above, however, suffer from a number of disadvantages. First, as stated before, to bring down the CTE, phosphate glasses require adding substantial amounts of fillers. The fillers can add significantly to the overall cost of the glass frit. Second, the addition of filler mandates that the sealing glass be used in a powdered form, which is less desirable for tubular geometries. Third, these phosphate frits tend to create seals that are not consistently hermetic when used to seal borosilicate glass, particularly when a desired application calls for the use of a tube geometry. Phosphate glass frits by their inherent nature need to be used in either a dry powder or paste form. Air permeates the seal because the frit powder or paste does not densify completely. Gaps and pores often will appear during the sintering process as the organic binders in the paste de-gas when burned-out under high temperatures. Additionally, as the phosphate frit melts, the glass tends to flow under gravity towards the bottom. Even though, phosphate frits can seal flat glass applications without much problems, some gaps or pores, however small, will always exist between the frit particles in complex geometric applications aside from flat glasses. Moreover, it is difficult to cover or seal certain geometric configurations, such as round or cylindrical forms with loose powder or even paste. Since sealing glasses used in the powdered form are susceptible to porosity within the seal, hermeticity is harder to achieve in the seal.
At the present time, a need for improvement continues to be unsatisfied in the medium-expansion sealing/solder glass industry, especially in seals for electrical lamp constructions such as photoflash lamps, vehicle headlamps and lamps for fluorescent lighting. If one has to use a solder glass to join together various glass articles or pieces that are characterized as having medium expansion, one has limited options.
It is known that all of the various monovalent and divalent oxides are effective in reducing the melting temperature of glass frit. The fluxing power of these oxides is as follows, in decreasing order: Li
2
O>PbO>Na
2
O>K
2
O>BaO>CaO>SrO>MgO>ZnO. Replacement of silica by B
2
O
3
decreases the melting temperature, as does a decrease in the silica, zirconia, and alumina content. Moreover, use of several fluxing ingredients in proper proportion is more effective in decreasing the melting temperature than use of any single oxide. It is also known that in general, those oxides that have the greatest fluxing power give the highest expansion.
As alternatives to phosphate glasses, lead glasses can be used as seals for lamp envelopes. In the past, the glass industry has developed lead sealing glasses for use at relatively low temperatures, in such applications as sealing color television bulbs. For example, stable lead sealing glasses that have softening points in the 430-500° C. range, and coefficients of thermal expansion in the range of 70-90×10
−7
/°C. are disclosed in U.S. Pat. No. 2,6462,633 (Dalton). Nevertheless, these glasses, in general, still had a relatively high coefficient of thermal expansion, which made them unsuitable for use with borosilicate glasses. Lead-containing frits, likewise, have been deemed unsatisfactory for the same reasons. See, John S. Nordyke, ed., Lead in the World of Ceramics, “Lead Frits,” pp. 99-105, The American Ceramic Society (1984). While some of the leaded frit compositions that are disclosed, melt in the relatively low temperature range of 500-675° C., these glasses still exhibit coefficients of thermal expansion that are too high to be compatible with medium expansion glasses, such as borosilicate articles. On the other hand, those leaded frits that exhibit lower coefficients of thermal expansion, melt in the relatively high temperature range of 980-1120° C.
In contrast to the more dense lead glasses, sealing glasses composed essentially of copper aluminum and silicon oxides are known in the art to have low to medium coefficients of thermal expansion that typically do not exceed 20×10
−7
/°C., and are often lower than 10×10
−7
/°C., over a broad temperature range. Briefly studied in the 1960s and early 1970s, this property made copper aluminosilicates a favored type of sealing glass for joining fused silica, fused quartz, and other low-expansion glass and glass-ceramics materials.
To illustrate, the following patents describe some of the qualities and applications of copper sealing glasses that are known. U.S. Pat. No. 3,414,465 (Baak et al.) discloses a copper sealing glass used for forming fused quartz to fused quartz seals and fused silica-to fused silica seals. The gl

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