Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
1997-03-25
2001-01-09
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S493000, C313S422000
Reexamination Certificate
active
06172457
ABSTRACT:
BACKGROUND—FIELD OF THE INVENTION
The present invention relates generally to ceramic compositions and more specifically to ceramic components used in flat panel display devices, wherein high thermal resistance, high compressive and bending strength, and a coefficient of thermal expansion similar to that of soda lime silica glass are desirable.
BACKGROUND—DESCRIPTION OF PRIOR ART
Construction of flat panel display devices presents unique challenges to selection of materials. The display devices are often comprised of a proportionally large frontal area of soda lime silica glass (SLG) patterned with stripes of Indium Tin Oxide (ITO) and phosphors, (also referred herein as the Display Panel or display glass), physically bonded to a similar areas of metal, ITO coated glass, or ceramic (collectively referred herein as the Backplane or backpanel) by means of a series of SLG spacers. The envelope formed by the combination of the Display Panel, the SLG spacers, and the Backplane contains the interior components for electron generation (cathode) and electron targeting systems (grids or gates). To operate, the entire assembly must be sealed in such manner to maintain a high vacuum within the envelope.
Typically, the assembly components are bonded by means of heat sealing; that is devitrifiable solder glass or vitreous solder glass (collectively referred herein as “frit”) is applied to the edge surfaces (interface) where the Display Panel and Backplane come into contact with the glass spacers. The device is placed in a sealing oven which raises the temperature of the assembly to or above the temperature at which frit melts. At that elevated temperature, the glass spacers become bonded to the Display Panel and Backplane. As the frit crystallizes, any differential between the net CTC (coefficient of thermal contraction) of the glass spacers and either the Display Panel or the Backplane will create strains in the assembly at the interface. As SLG is strong in compression but weak in tension, there is always the concern that any thermal coefficient mismatch between the glass spacers and either the Display Panel or the Backplane will create a fracture surface or failure at the interface area. Due to the extremely low fracture toughness of the glassy material in this region, initiation and propagation of cracks and subsequent inability of the assembly to hold a sufficiently high vacumn effectively prevents operation of the interior components.
Additionally, due to mismatches between the interior cathode and grid structures both of which extend their electrical contacts through the glass spacers and rely upon these same spacers for support by means of frit sealing, fractures occur in the sub-structures glass surface when the assembly is subjected to thermal changes. Thermal shocking of the interior cathode and grid support structures can occur when the assembly is heated during evacuation, and again during operation of the display.
Thermal shocking has been found to produce microfissures in the frit at the point where the interior cathode and grid structures are sealed to the glass spacers. Such microfissures in the frit material result in contamination of the vacuum environment within the assembly after pump-down and seal-off due to outgassing through the microfractured frit.
Ceramic materials have long been used in connection with shadow mask supporting structures in Cathode Ray Tubes (CRT) where similar problems involving the CTC of the support structure and the glass face plate exist. In accordance with the teachings of this invention, as described in detail hereafter, the SLG spacers are replaced by a ceramic material of a composition between fosterite (2MgO·SiO
2
) and steatites (MgO·SiO
2
) extruded into the shape of a collar. By varying the amount of SiO
2
and MgO to control the CTC in the ceramic material, the collar places the Display Panel and Backplane in compression after sealing while withstanding tensile forces. Ceramic collars of the composition hereinafter described are especially useful in flat panel display devices due to their relatively low weight, high tensile strength, and resistance to damage from vibration, impact and thermal cycling.
In U.S. Pat. No. 4,745,330—Capek et al teaches the use of a three layer ceramic system in a CRT mask supporting structure to vary the CTC in steps of 2×10
−7
in./in./degree Celsius thus “stepping down” or buffering the tensile forces present at the interface during thermal cycling.
Using a similar ceramic compositions described in U.S. Pat. 5,086,251—Capek et al improved upon the '330 patent by showing, also in connection with a CRT mask supporting structure, that the use of a ceramic element having varying the amount of MgO content in a fosterite composition coupled with the use of a cement having a CTC intermediate between the faceplate and the mask supporting structure and a grooved holder to further reduced tensile forces.
In accordance with an aspect of this invention both SiO
2
and MgO are varied to control the CTC with the result of extremely precise strain control in the seal provided at the interface area.
Prior Art
U.S. Pat. No. 4,745,330—Capek et al; U.S. Pat. No. 5,086,251—Capek et al; U.S. Pat. No. 5,369,062 —Chiang et al.
Objects of the Invention
It is a general object of the invention to provide a flat panel display assembly having an improved spacing component between the Display Panel and the Backplane.
It is another object of the invention to provide a flat panel display in which the assembly of the Display Panel, spacing component, and the Backplane are so constructed as to overcome the aforementioned problem of glass failures due to cracks propagating from the interface between the spacing component and the Display Panel and/or spacing component and the Backplane.
It is another object of the invention to provide a flat panel display having such an improved spacing component that yield losses due to outgassing and spallation at the places where the supported internal structures are bonded to the spacing component are reduced.
It is another object of the invention to provide a flat panel display having an improved assembly that can readily be adjusted to accommodate Display Panel glass of differing compositions and differing flat panel display designs and/or production objectives such as maximized throughput, maximized yield, varying display sizes, etc.
It is another object of the invention to provide a flat panel display having an improved assembly that permits the use of higher thermal gradients during envelope evacuation, with resulting greater yields and/or throughput and consequent reduced manufacturing costs.
It is another object of the invention to provide a flat panel display by which the desired tensile strain at the interface of the spacing component and the Display Panel and/or spacing component and the Backplane can be accurately controlled in a mass production environment.
It is another object of the invention to provide a flat panel display by which the materials used in the Display Panel and the Backplane can be thinner and lighter in weight.
These and other objects and advantages will become more apparent after consideration of the ensuing description and the accompanying drawings.
SUMMARY OF INVENTION
The invention consists of a novel use of a ceramic composition as a spacing component for flat panel displays. By careful selection of the composition of the ceramic material, precise control over the stresses imposed upon the bonded components may be achieved. By selecting material which places the Display Panel and Backplane in compression, the sealed envelope so formed will withstand the stress of repeated heatings and pump-downs. Upon sealing a very high interior vacumn may be supported. The ability of ceramic material to be extruded in unique shapes with uniform strength permits the design of alternative styles of spacing components. which adds to the structural strength of the apparatus while permitting the use of thinner, lighter weight Display Panel.
REFERENCES:
patent: 4745330
Brown Christina M
Capek Raymond G
Day Michael H.
Kolakowski Victoria S.
Telegen Corporation
Weiland Leigh Ann
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