Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
1999-05-17
2001-12-25
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C428S426000, C428S428000, C501S073000, C501S074000, C345S037000, C345S041000
Reexamination Certificate
active
06333116
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to lead silicate glass frit compositions useful for forming freestanding glass structures, in particular, glass rib barrier layers such as are used in plasma flat panel display applications.
BACKGROUND OF THE INVENTION
Color plasma display panels have been considered the most suitable flat display device for large sized applications, and are believed to be adaptable to high definition television, as well as other visual display applications. Typically, such display panels are formed of two parallel substrates separated from each other to form a discharge space therebetween in which a discharge gas, such as a mixture of neon, xenon, and helium, is filled. The inner-facing surface of each of the substrates bears a pattern of spaced parallel electrodes, with the electrodes on one substrate being oriented in a direction that intersects the direction of the electrodes on the other substrate. The electrode bearing surfaces of the substrates are typically covered with a dielectric layer, and red, green and blue phosphors are located on the internal surface of the dielectric layer on one of the two substrates. The Dielectric layers are generally lead-based glass frits fired between 500 and 600° C., depending on their formulation and the level of uniformity required. The display picture is produced by plasma discharges which are induced locally in the gas by applying a suitable voltage between the electrodes of one substrate and the electrodes of the other substrate. Ultraviolet light emitted locally by the gas discharge induces luminescence of the neighboring phosphors.
In order to prevent luminous cross-talk between neighboring pixels in such displays, barrier ribs are typically disposed vertically or as closed cells, on at least one of the substrates (typically the rear one) in order to optically insulate each discharge cell. These ribs are typically formed either by sandblasting or screen printing glass frits containing low melting temperature glasses such as lead silicates and zinc, lead, or phosphate glasses. The barrier rib structure is typically periodic with a pitch of from 200 &mgr;m to 400 &mgr;m, depending on the panel resolution. These ribs are about 30-80 &mgr;m wide and 100-200 &mgr;m thick. Alternatively, a closed cell design has been employed having square cells which are about 200-400 &mgr;m on each side. The “ribs” which form these square cells are about 30 &mgr;m to 70 &mgr;m wide and about 30 to 200 &mgr;m high.
The frit containing materials used in the preparation of the dielectric layers and barrier ribs are often provided with a quantity of crystalline filler, e.g. a crystalline material selected from the group consisting of mineral, ceramic, or glass ceramic materials. Typically, such crystalline materials have a coefficient of thermal expansion and are employed in an amount so that, over the temperature range of 0 to 300° C., the average or resultant coefficient of thermal expansion of the frit containing material is between about 77 to 90×10
−7
/°C. Such fillers assist in preventing misalignment of the various layers of the plasma structures during their consolidation. In particular, the fillers promote surface nucleated crystallization of the frit that improves the mechanical strength and enhances the rigidity of the frit. This technique is very useful to prevent excessive flow and is therefore used in dielectric layers as well as barrier ribs to maintain structural geometry after firing.
The frits used in the barrier ribs and the dielectric layers are typically of different formulations. This is generally necessary if sand blasting processes are used to form the barrier ribs on the surface of the dielectric layers to prevent removal of the dielectric layer between the ribs. Recently, however, a process has been developed that allows the simultaneous formation of the dielectric layer and barrier ribs in a single step. According to this process, the barrier ribs and dielectric layer can be formed on the substrate by an embossing or intaglio printing process. The method and apparatus for accomplishing this result are described in detail in U.S. patent application Ser. No. 08/820,206 filed Mar. 18, 1997, the disclosure of which is expressly incorporated herein by reference.
Accordingly, to effectuate the advantages inherent in such a process, glass frit compositions suitable for both the dielectric layer and barrier ribs are necessary. Additionally, it would be advantageous for such frits to devitrify without the addition of fillers such as zircon, alumina or glass ceramic fillers commonly added to promote surface nucleated crystallization. These needs are fulfilled by the glass frit compositions described herein.
SUMMARY OF THE INVENTION
The present invention relates to glass frit compositions that are particularly well-suited for use in the formation of dielectric layers and barrier ribs utilized in plasma display panels. The compositions permit the simultaneous formation of the dielectric layers and the barrier ribs, which significantly simplifies their manufacture. Further, the compositions are initially devitrified during the melt through internal crystallization without the need for the addition of crystalline fillers, and provide, when consolidated under normal processing conditions at temperatures of from about 520 to about 580° C., desired shape retention. As a consequence, the glass frit compositions of the invention typically exhibit great compaction after firing, resulting in a low porosity and surface roughness. These attributes lead to better control of the thickness and homogeneity of the dielectric layers, which are important in obtaining uniform electrical behavior over their surface.
In one embodiment of the invention, the glass frits comprise by weight, 18 to 24 percent silica (SiO
2
), 69 to 80 percent PbO, and 1 to 3 percent titania (TiO
2
). Optionally, the composition may contain up to 15 percent zirconia (ZrO
2
), as well as small quantities of ceramic dielectric materials such as BaTiO
3
.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
The lead glass frit compositions of the present invention provide crystallized glass formulations that can be used alone to form structures having a glossy aspect, or made milky reflectant by the addition of pigments. Additionally, the dielectric properties of the resulting structures can be adapted by the addition of small quantities of specific ceramic dielectric materials such as BaTiO
3
.
Unlike the prior art, wherein various glass frits are used to provide the dielectric layers and the barrier rib structure, the present composition provides a single material formulation that can be used in both of these layers. The composition of this invention exhibits minimal variations in glass substrate dimensions when the substrate is heated up to its annealing range, and allows the formation of structures with very good dimensional shape control when consolidated in a temperature range of about 520 to about 580° C., preferably at about 550° C. In particular, due to its strong devitrifying character, which may be augmented by the addition of ZrO
2
to the glass frit composition, combined with a softening point in the range of from about 450 to about 550° C., the level of compaction that occurs during firing is high, and the resulting glass substrate has low porosity and surface roughness. As a result, it is possible to control dimensional shapes precisely at a given temperature (preferably at about 550° C.) and achieve a glass substrate exhibiting improved mechanical strength and uniformity. Furthermore, the thermal expansion coefficient is maintained in the range of from about 70 to about 90 * 10
−7
/° C. to prevent stresses and cracks induced by potential excessive expansion mismatch between the substrate (generally a sodalime glass) and the dielectric layers.
The crystallizing glasses of this invention are lead silicate materials containing TiO
2
. The glass frit compositions of this invention contain, on a weight percent basi
Corning Incorporated
Jones Deborah
Klee Maurice M
McNeil Jennifer
Nwaneri Angela N.
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