FIELD OF THE INVENTION
The present invention relates to a novel glass composition which is used for the formation of barrier ribs and capable of improving the strength while lowering the dielectric constant and being able to be fired at less than 600°C in plasma display panel (hereinafter referred to as PDP).
BACKGROUND OF THE INVENTION
In recent years, attention has been drawn to PDP as a large size thin flat plate color display device. PDP has a structure wherein many cells (fine discharge spaces) are formed as partitioned by barrier ribs between a pair of glass substrates, phosphors are provided on the surface of the respective cells, and a discharge gas is filled in such cells. Discharge takes place between electrodes in the cells to excite the discharge gas to let the phosphors in the ground state emit lights by ultraviolet rays thereby generated, to form pixels. Such PDP is a self emitting type flat display and has excellent characteristics such as wide horizontal and vertical view-angle and being thin and light in weight, and further, it can be made to have a large size. Thus, it is one of the most prospective display devices.
Generally, a PDP fluoresces by an ultraviolet light with a wavelength of 147 nm generated during a discharge of He+Xe or Ne+Xe gas to thereby display a picture including characters and graphics. Barrier ribs for dividing the discharge space extend perpendicularly from the rear substrate, and the address electrode is formed between the barrier ribs. Such discharge cells are arranged in a matrix pattern. One of the disadvantages with the PDP is that the power consumption is high. In order to reduce the power consumption, it is important to lower the dielectric constant of the barrier ribs. To this end, it is proposed to form barrier ribs of a porous structure or to use the filler powder having a low dielectric constant as the barrier rib material.
However, if the barrier ribs have a porous structure, an influence of the discharge gas passing through the barrier ribs may cause degradation in brightness or defective lighting. Furthermore, the strength of the barrier ribs is degraded to cause the barrier ribs to be broken off.
As the filler powder having a low dielectric constant, there is known silica-based filler such as α-quartz powder or fused silica powder. However, these materials are lower in mechanical strength than alumina. It is therefore difficult to form the barrier ribs having sufficient strength.
The importance of the strength of the barrier ribs is further described as follows. From the point of view of reducing the production-cost, screen-printing is the most preferred method. With the screen printing method, a substrate is covered by a mask, barrier rib paste is then coated thereon and a burning process is carried out, to thereby form the barrier rib. The screen printing may be performed several times in order to control the height of the barrier rib. However, screen printing often results in top-edge damage of the barrier rib instead of the preferred flat-top, during repetitive screen-printing steps.
With the sand blasting method, a material for barrier rib is uniformly coated on a substrate, and a protective layer acting as a mask is then formed on the barrier rib
material. Next, the exposed portions of the barrier rib material are removed using a sand blaster and the protective layer is afterwards removed to thus form the barrier ribs. In this method, however, as the hard small grains used by the sand blaster strongly collide with the barrier rib material in order to cut the unnecessary portions, the small grains also strongly collide against the protective layer with such a force that portions of the protective layer are displaced against the surface of the barrier rib causing damage. Thus, a uniform surface of the barrier ribs is not obtained. Thus, top-edge damage is also observed in barrier ribs formed by sand-blasting method.
Also, during the subsequent process of sealing the top plate with the bottom plate, mechanical force is exerted by the top plate on the conventional barrier ribs, which is expected to be damaged on the top during this sealing process. In addition, due to an exterior impact during a fabrication process and a transportation of the product, the conventional barrier ribs are likely to be damaged.
The barrier ribs prevent the ultraviolet rays generated by the discharge from being leaked (cross-talk) into the adjacent cell. Therefore, Top-edge damage has a problem because it results in leakage of the electric discharge to the adjacent cells and cross-talk is likely to result.
In order to increase the strength of the barrier rib, U.S. patent number 6,371,822 proposes a barrier rib material containing fibrous materials, like optical fibers, carbon fibers, etc. However, when this barrier rib material is used, the cost of the barrier rib powder becomes high. Also, because of the very high aspect ratios, the fibers and carbon whiskers are likely to be trapped in the mesh of the screen printer. The Present invention overcomes the problem faced in prior art.
For avoiding the top-edge damage of the barrier rib, filler to glass ratio may also be increased for improving the rigidity of the rib. However, there is a problem that the firing temperature of the barrier rib is likely to increase if the filler to glass ratio is increased. The firing temperature must be less than 600°C, so that the substrate glass is not deformed during firing
The materials for forming such barrier ribs are desired to be one which can be fired at a low temperature such as at most 600°C, to prevent deformation of the glass substrates.
OBJECT OF THE INVENTION
It has already been proposed in the PDP manufacturing process that top and bottom plate (comprising barrier ribs) have to be joined. During the joining process, barrier ribs are subjected to mechanical abuse. Also, the barrier ribs prevent the ultraviolet rays generated by the discharge from being leaked (cross-talk) into the adjacent cell. Therefore, Top-edge damage results in leakage of the electric discharge to the adjacent cells and cross-talk is likely to result, so in order to overcome these problems the strength of the barrier ribs must be increased. Further in the said PDP barrier ribs of the prior art have a high dielectric constant which results in increased power consumption of the PDF. In order to overcome this problem the dielectric constant must be reduced.
The principal object of the present invention is to provide a novel glass composition for making a glass powder which is used for making barrier ribs for a plasma display panel having synergistically sufficient strength and lower dielectric constant.
Yet another object of this invention is to provide a glass composition for making a glass powder which is fired below 600°C. Also, if the firing temperature of the barrier rib is
more than 600°C, the substrate glass panel will be deformed. Thus to form the barrier ribs the firing temperature must be less than 600°C.
Yet another object of the present invention is to provide a process for the preparation of a glass composition to be used for manufacturing the barrier ribs wherein the said composition comprising essentially by weight percent, from 30 to 60% of PbO, from 20 to 60% of B203, from 0.01 to 10% of Si02, less than 2.0% of TI02, which comprises melting of the said glass composition at 1250°C in a platinum lined furnace for 2 hours; obtaining a glass body from the said glass melt by pouring; grinding the said glass body in an alumina ball mill to obtain a glass powder; classification of the said glass powder using a screen with an opening of 53 µm; mixing a ceramic filler such as herein described to said glass powder whereby a glass ceramic powder is obtained; kneading the said glass ceramic powder after adding the binder and the solvent to obtain a glass paste; the said glass paste is screen printed in a predetermined manner , followed by drying to obtain barrier ribs, such ribs having the dielectric constant between 5 - 8.4, the thermal expansion coefficient between 60-65 (x 10"7 /°C), firing temperature less than 600°C and preferably flat shape.
STATEMENT OF THE INVENTION
Accordingly the invention provides a glass wherein the glass composition comprises essentially by weight percent, from 30 to 60% of PbO, from 20 to 60% of B2O3, from 0,01 to 10% of Si02, less than 2.0% of Ti02 for forming barrier ribs in a PDF.
DETAIL DESCRIPTION OF THE INVENTION
The present invention describes a novel glass composition which comprises 30 to 60% of PbO, from 20 to 60% of B203, from 0.01 to 10% of Si02, less than 2.0% of TiO2. The said glass composition is used for preparing a glass powder. The said glass powder is mixed with a ceramic filler to obtain a glass-ceramic powder. The glass ceramic powder essentially comprises by weight percent, 85-95% the said glass powder and 5 to 15% of ceramic filler, AI203 and/or Si02.
In PDP of the present invention, to form barrier ribs on a substrate for PDP, the above mentioned glass-ceramic powder is formed into a paste, which is then screen-printed in a predetermined pattern, followed by firing at a temperature ranging from 500° to 600°C for 5 to 20 minutes to form the barrier ribs, Otherwise the above mentioned paste may be printed over the entire surface of the substrate, and then processed to form a pattern of barrier ribs. For example, it is possible to employ a method wherein a photosensitive resin is mixed in the paste, so that the pattern of barrier ribs can be formed by photolithography, or a method wherein the pattern of barrier rib is formed by sandblasting.
PbO is a component to lower the firing temperature and is essential. If it is less than 30%, firing temperature will be high. It is preferably at least 37%. If it exceeds 60%, coefficient of thermal expansion will increase and the strength of the barrier rib will be poor. It is preferably at most 53%.
B2O3 is a glass forming element for widening the vitrification range and lowering the dieiectric constant. The content should be 20-60% by weight percent, preferably 29-52%. B203 contents less than 20% makes dielectric constant high. However, when it is preferably incorporated in an amount of at least 29 wt. %, it provides an effect of lowering the dielectric constant. On the other hand, if it is too much, the chemical
durability tends to be poor. Therefore, it is preferably at most 60 wt. %, more preferably at most 52 wt. %.
Si02 is also a glass forming element and should be selected, in content, 0.01-10% by weight percent, preferably 0.04-8.5%. Use of SiO2 more than 10% results in a softening point higher than 600°C, and cannot therefore produce a dense body of the rib by firing at a temperature of 600°C or lower. Preferably, it is at most 8.5%. Si02 is a component to also stabilize the glass. If it is less than 0.01%, the glass tends to be unstable. Preferably, it is at least 0.04%. The total content of B2O3 and SiO2 is from 20.01 to 70%, preferably from 30 to 60%.
Ti02, ZrO2, La203 and Ta205 are components to stabilize the dielectric constant, and at least one of them is required to be contained. If there is no TiO2, ZrO2, La203 or Ta205, dielectric constant tends to be too small. Preferably, the total content is at least 0.01%. If it exceeds 2%, dielectric constant will increase and the power consumption and crosstalk of the PDP will increase. Preferably, it is at most 1%.
Such a glass-ceramic powder has a dielectric constant which varies between 5-8.4. Besides, it has adequate fluidity at a temperature ranging from 500° to 600°C, and the glass powder and the ceramic filler are sufficiently wetted to form a thick film of the glass-ceramic.
If the content of such a glass powder is small, the ceramic filler can not adequately be wetted, whereby dense sintering tends to be difficult, and the strength of barrier ribs tends to be low. On the other hand, if the content of such a glass powder is large, the shrinkage upon firing tends to be too large. Therefore, in the present invention, the content of the glass powder is preferably from 85 to 95 wt% and 5-15 wt% is the ceramic filler powder of the entire glass-ceramic powder.
In order to prepare the paste from the glass-ceramic powder, the glass-ceramic powder described above is mixed with a binder and a solvent.
The paste comprises, by weight percent, the glass-ceramic powder of 70-90%, the binder of 1-5% and the solvent of 9-25%.
The barrier ribs of the present PDP in the present invention can be prepared, for example, in the following manner: An organic vehicle comprising an organic resin binder and a solvent is added to the above-described glass-ceramic powder, followed by kneading to obtain a paste. As the organic resin binder and the solvent, those commonly used in the field of preparing glass pastes may be employed. For example, as the organic resin binder at least one is selected from a group of poly butyl methacrylate, polyvinyl butyral, poly methyl methacrylate, poly ethyl methacrylate, and ethyl cellulose. The solvent is at least one selected from a group of terpineol, butyl carbitol, butyl carbitol acetate, diethylene glycol monobutyl ether acetate, and 2,2,4-trymethyl-l,3-pentanediolmonoisobutylate. To the paste, a surfactant may be incorporated as a dispersant. Further, in a case where a pattern of barrier ribs is formed by photolithography, a photosensitive resin is incorporated to the paste.
If the particle size of the glass-ceramic powder is too small, it is likely to be difficult to form a paste. On the other hand, if the particle size of the glass-ceramic powder is too large, no adequately dense sintered layer can be formed at the time of firing, and many voids are likely to form. Therefore, in the present invention, the average particle size of the glass powder is preferably within a range of from 1 to 10 jam, and the average particle size of the glass-ceramic powder is preferably within a range of from 0.1 to 10 µm.
The barrier ribs are formed in the following manner. Firstly, the paste prepared as described above, is screen-printed in a predetermined pattern, followed by drying. Then, this printing and drying operation is repeated until the film thickness after drying the paste will be about 150 to 200 µm. Then, the paste is fired at a temperature of from 0° to 600°C from 5 to 20 minutes to obtain barrier ribs,
In the process for the preparation of a glass composition to be used for manufacturing the barrier ribs, the glass-powder is having average particle size of from 1 to 10 µm, and the average particle size of the glass-ceramic powder is preferably within a range of from 0.1 to 10 µm. The glass composition is used ranging from 85% to 95% being added with ceramic filler ranging from 5 to 15 in weight % to obtain a glass-ceramic powder in the process for the preparation of a glass composition to be used for manufacturing the barrier ribs. A glass paste is obtained in the process for the preparation of a glass composition to be used for manufacturing the barrier ribs as glass-ceramic powder ranging from 70 to 90 weight % being added with a binder ranging from 1 to 5 weight % and a solvent ranging from 9 to 25 weight %,
The glass ceramics filler powder is at least one selected from alumina powder, siilca powder, corderite powder, cristobalite and α-quartz powder, the binder is at least one selected from a group of poly butyl methacryiate, polyvinyl butyral, poly methyl methacrylate, poly ethyl methacryiate, and ethyl cellulose and solvent is at least one selected from a group of terpineol, butyl carbitol, butyl carbitol acetate, diethyiene glycol monobutyl ether acetate, and 2,2,4-trymethyl-l,3-pentanediolmonoisobutylate in a process for the preparation of a glass composition to be used for manufacturing the barrier ribs.
In the process for the preparation of a glass composition to be used for manufacturing the barrier ribs the thermal expansion coefficient of the glass-ceramic powder is between 60-65 (x 10"7 / °C) and dielectric constant of the glass-ceramic powder is between 5 - 8.4 further glass paste has a firing temperature less than 600°C.
The strength of the barrier rib is determined by the following equation:
α= E •Δa•ΔT (1)
In equation (1) a is the strength of the barrier rib, E is its Young's modulus, n is its Poisson's ration, Aa is the difference in the coefficient of thermal expansion of barrier rib and underlying dielectric. AT is the difference in the firing temperature and room temperature. In equation (1) all the parameters on the right hand side are almost the same for different barrier ribs except Aa. Reducing the thermal expansion coefficient of the barrier rib, can increase Δα thus increase the strength of the barrier rib. Barrier rib is typically screen printed in 8 steps, the thermal expansion of the barrier rib is disclosed to be reduced in steps to form a thermal expansion coefficient gradient in the barrier rib, Thermal expansion coefficient of the glass-ceramic powder for forming the barrier ribs should be between 60 to 65 x 10"7 / °C from room temperature to 300 °C for improving the strength of the barrier rib.
The paste for barrier ribs may be printed in any pattern of cell-form, stripe-form or solid printing. However, when solid printing is applied, post-processing is required to form a predetermined pattern. For example, cell-form or stripe-form patterning may be carried out by, e.g., photolithography, etching or sand blasting,
Now, the present invention will be described in further detail with reference to Examples (No. 1-8) and comparative example (No.9). However, it should be understood that the present invention is by no means restricted to such specific examples. These examples do not limit the scope of the present invention but only describe the working of the present invention.
(Table Removed)

Each of the samples was prepared by the following steps. A charge of raw materials for the glass composition was blended for each of samples shown in Table 1 and was melted in a platinum crucible at 1,250°C for two hours. Then, a glass body was obtained from the molten glass, crushed and ground in an alumina ball mill, and classified by a screen having openings of 53 µm to obtain glass powder,
The glass powder of each glass composition of the samples was mixed with ceramic filler as shown in Table I to obtain a glass-ceramic powder thereof, which was examined in firing quality. Samples Nos. 1-8 were confirmed good in the firing quality and the barrier rib top was flat, but sample No. 9 could not provide a flat barrier rib top.
The thermal expansion coefficient was measured at a temperature range of 30-300°C of a sample piece which was formed by the following steps. Each of the sample powders were press-formed, fired, and ground to form the sample piece of a cylindrical rod having a diameter of 4 mm and a length of 40 mm.
For dielectric constant measurement, glass powder was re-melted, molded into a plate shape and then processed into 50 mm x 50 mm x 3 mm, whereupon aluminium electrodes were formed on both sides by a vapor deposition method to obtain a sample. The relative dielectric constant at 1 MHz at 20°C of this sample was measured by a LCR
meter.
The thermal expansion coefficient and the dielectric constant of the glass-ceramic powder was measured and recorded. As seen from Table I, samples 1-8 have the
thermal expansion coefficient of 60-65 (x 10-7 / °C) and the dielectric constant varies between 5.5 - 8.0. Comparing to this, sample 9 has a high thermal expansion coefficient of 73.9 x 10"7 /°C and the dielectric constant is 12.9.
The firing quality was examined by the following method. Each of samples was mixed with 2% Ethyl Cellulose, 2% Butyl Cellulose and 16% Butyl Cellulose Acetate solution and kneaded to form a paste. The paste was coated onto a glass plate having a thermal expansion coefficient of 85 x 10"7 /°C. by the screen printing process to form a coating layer having a thickness of 200 µm. Then, the coating layer was fired at 550 °C for 30 minutes in an electric furnace to form a thin glass film. Then the cross-section of the barrier rib was checked under a microscope to examine the flatness of the barrier rib top.
As described above, the composition of this invention can provide a dense body by firing at a temperature of 600°C or less and is, therefore, useful for forming ribs on a rear glass ptate of PDF.
The Barrier ribs made up of the said composition are formed by any of the following processes, like screen printing, sand blasting, photolithography and embossing/lamination.

WE CLAIM:
1. A glass composition comprising essentially by weight percent, from 30 to 60% of
PbO, from 20 to 60% of B2O3, from 0.01 to 10% of SiO2, less than 2.0% of TiO2.
2. The glass composition as claimed in claim 1, wherein the composition comprises
preferably by weight percent, 37 to 53% of PbO, from 29 to 52% of B2O3, from 0.04 to
8.5% of Si02, from 0.01 to 2.0% of Ti02.
3. The glass composition as claimed in any one of the above claims wherein the said
glass composition is used for preparing a glass-powder, the said glass-powder has
average particle size of from 1 to 10 µm.

4. The glass powder as claimed in claim 3, wherein by weight % the said glass powder is
used is ranging from 85% to 95% being added with ceramic filler such as herein
described ranging from 5 to 15 in weight % to obtain a glass-ceramic powder.
5. The glass-ceramic powder as claimed in claim 4, wherein the said ceramic filler
powder is at least one selected from alumina powder, silica powder, corderite powder,
cristobalite and a-quartz powder.
6. The glass-ceramic powder as claimed in claim 4, wherein the thermal expansion
coefficient of the said glass-ceramic powder is between 60-65 (x 10~7 /°C).
7. The glass-ceramic powder as claimed in claim 4, wherein the dielectric constant of the
said glass-ceramic powder is between 5 - 8.4.
8. The glass-ceramic powder as claimed in claim 4, wherein the average particle size of
the glass-ceramic powder is preferably within a range of from 0.1 to 10 µm.
9. The glass-ceramic powder as claimed in claim 4, wherein in by weight % the said
glass -ceramic powder used is ranging from 70 to 90 in weight % being added with a
binder such as here in described ranging from 1 to 5 in weight % and a solvent such as
here in described ranging from 9 to 25 in weight % to obtain a glass paste.
10. The glass paste as claimed in claim 9, wherein the said binder is at least one
selected from a group of poly butyl methacrylate, polyvinyl butyral, poly methyl
methacrylate, poly ethyl methacrylate, and ethyl cellulose.
11. The glass paste as claimed in claim 9 wherein the said solvent is at least one
selected from a group of terpineol, butyl carbitol, butyl carbitol acetate, diethyiene
glycol monobutyl ether acetate, and 2,2,4-trymethyl-l,3-pentanediolmonoisobutylate.
12. The glass paste claimed in claim 9, wherein the said glass paste has a firing
temperature less than 600°C.
13. A process for the preparation of a glass composition to be used for manufacturing
the barrier ribs wherein the said composition comprising essentially by weight percent,
from 30 to 60% of PbO, from 20 to 60% of B203, from O.O1 to 10% of Si02, less than
2.0% of TiO2, which comprises:
melting of the said glass composition at 1250°C in a platinum lined furnace for 2 hours; obtaining a glass body from the said glass melt by pouring; grinding the said glass body in an alumina ball mill to obtain a glass powder; classification of the said glass powder using a screen with an opening of 53 µm;
mixing a ceramic filler such as herein described to said glass powder whereby a glass ceramic powder is obtained;
kneading the said glass ceramic powder after adding the binder and the solvent to obtain a glass paste;
the said glass paste is screen printed in a predetermined manner , followed by drying to obtain barrier ribs.
14. The process as claimed in claim 13, wherein the said composition comprises
preferably by weight percent, 37 to 53% of PbO, from 29 to 52% of B203, from 0.04 to
8.5% of Si02, from 0,01 to 2,0% of Ti02.
15. The process as claimed in claim 13, wherein said glass-powder is having average
particle size of from 1 to 10 µm.
16. The process as claimed in claim 13, wherein by weight % the said glass composition
is used ranging from 85% to 95% being added with ceramic filler such as herein
described ranging from 5 to 15 in weight % to obtain a glass-ceramic powder.
17. The process as claimed in claim 13, wherein the said glass ceramics filler powder is
at least one selected from alumina powder, siilca powder, corderite powder, cristobalite
and α-quartz powder.
18. The process as claimed in claim 13, wherein the thermal expansion coefficient of the
said glass-ceramic powder is between 60-65 (x 10"7 /°C),
19. The process as claimed in claim 13, wherein the dielectric constant of the said glass-
ceramic powder is between 5 - 8,4.
20. The process as claimed in claim 13, wherein the average particle size of the glass-
ceramic powder is preferably within a range of from 0.1 to 10 µm.
21. The process as claimed in claim 13, wherein in by weight % the said glass
ceramic powder used is ranging 70 to 90 weight % being added with a binder such as
here in described ranging from 1 to 5 weight % and a solvent such as here in described
ranging from 9 to 25 weight % to obtain a glass paste.
22 The process as claimed in claim 13, wherein the said binder is at least one selected from a group of poly butyl methacrylate, polyvinyl butyral, poly methyl methacrylate, poly ethyl methacrylate, and ethyl cellulose.
23. The process as claimed in claim 13, wherein the said solvent is at least one selected from a group of terpineol, butyl carbitol, butyl carbitol acetate, diethylene glycol monobutyl ether acetate, and 2,2,4-trymethyi-l,3-pentanediolmonoisobutylate.
24. The process as claimed in claim 13, wherein the said glass paste has a firing
temperature less than 600°C.
25. A Barrier rib made up of the composition as claimed in claims 1 to 24 , such rib
having the dielectric constant between 5 - 8.4, the thermal expansion coefficient
between 60-65 (x 10-7 /°C), firing temperature less than 600°C and preferably flat shape.
26. A Barrier rib made up of the composition as claimed in claims 1 to 24 , wherein such
barrier ribs are formed by any of the following processes, like screen printing, sand
blasting, photolithography and embossing/lamination.
27. A glass composition and a process for the preparation of glass composition for
manufacturing of barrier ribs substantially herein described with reference to the
accompanying examples.