FIELD OF INVENTION:
The present invention relates to a glass composition. The present invention more particularly provides a glass composition which can be excited by UV light, can emit green color and replacing the existing conventional green phosphor applied in plasma displays.
BACKGROUND ART:
The conventional AC (Alternating Current) driven PDF possesses three-electrode structure on two glass plates forming front plate and back plate. The front plate has formed therein a plurality of pairs of display electrodes known as sustain and scan electrodes. These electrodes are formed of ITO (Indium-Tin-Oxide) material. The ITO electrode sheet resistance is decreased with the introduction of metal bus lines of electrically conducting material over the ITO electrodes. The display electrodes are covered with a transparent dielectric layer to limit the discharge current. A thin electron emissive layer is formed over the transparent dielectric layer to emit secondary electrons and to protect the transparent dielectric layer from sputtering by ion bombardment. The back plate has formed therein a plurality of address electrodes that are orthogonal to the display electrodes. A pair of sustain and scan electrodes along with an address electrode form a sub-pixel. A sub-pixel comprises of Red, Green or Blue color phosphor that make Red, Green or Blue sub-pixel respectively. A combination of Red, Green and Blue sub-pixels forms a pixel. The straight channel barrier ribs are formed on the back plate to create the discharge volume and also to separate different sub-pixels. The main co-planar discharge is created with the display electrodes by square pulse voltage. The VUV radiation produced in the discharge phenomenon excites the phosphor to emit visible light. This type of conventional AC PDF is described in US Patent no. 5661500.
The convention plasma display panels employ RGB phosphors as light emitters in three different regions viz., red (595-620 nm), green (525-540 nm) and blue (450 nm) when excited by VUV light of wavelength 172 nm, produced by Ne-Xe discharge inside PDF. The phosphors are applied on the back plate, between the barrier ribs, conventionally by screen printing techniques after converting the phosphors into suitable pastes. The phosphor powders form a thin layer of thickness 10-20 between the ribs after firing the plate for removing the organic volatiles in the paste. These phosphors have the problem of non-uniformity of coating, adherence with the rib and bottom materials of the glass and hence peel off from the surface. In addition the porosity of the layers is high. The phosphors also suffer degradation by continuous bombardment of plasma particles inside the panel on prolonged operation of the panel.
So far no glasses have been applied in PDPs which can be excited by VUV. Hence it is thought of applying a glass composition which can efficiently replace at least one of the phosphors inside PDPs and which can also alleviate the above mentioned drawbacks of phosphors.
OBJECT OF THE INVENTION:
The conventional phosphors used in PDF have the problem of non-uniformity of coating, adherence with the rib and bottom materials of the glass and hence peel off from the surface. In addition the porosity of the layers is high. The phosphors also suffer degradation by continuous bombardment of plasma particles inside the panel on prolonged operation of the panel. Different types of Phosphor compositions have been proposed over a long period of time having reduced porosity and high stability but none of them are found to be suitable for application in PDF. In order to overcome all these problems a luminescent glass composition is proposed, which can be excited by UV light of wavelength 160-260 nm, can emit green color of wavelength 540- 550 nm.
The principal object of the present invention is to provide a Tb3+ activated borosilicate glass having a composition B2C>3 (70-45%), SiO2 ( 20-45%), CaCOj (0-10%), Tb4O7 (1-20%), CeO2 ( 3-30%), Na2CO3 ( 1-20%), MnCO3 (0-5%) and A1(OH)3 (5-25%) which undergoes excitation with light of wavelength 160- 260 nm and gives green light in the region 540-550 nm thereby replacing the existing conventional green phosphor applied in PDPs.
Another object of the present invention is to provide a novel glass composition for making a luminescent glass powder which is suitable to work as a solid constituent to make a paste of viscosity 30-100 Pa.sec by mixing with commercial vehicles and thus novel glass composition can act as a substitute for conventional green phosphors.
Yet another object of the present invention is to provide a process for the preparation of a glass composition which can be excited by UV light of wavelength 160-260 nm, can emit green color of wavelength 540 -550 nm
Yet another object of the present invention is to provide a Plasma display panel using the glass composition which can replace the conventional green phosphors in Plasma display panel and can emit green color of wavelength 540- 550 nm during the operation of Plasma display panel.
STATEMENT OF THE INVENTION:
Accordingly the invention provides a glass composition comprising B2O3 (70-45%), Si02 ( 20-45%), CaCO3 (0-10%), Tb4O7 (1-20%), CeO2 ( 3-30%), Na2CO3 ( 1-20%), MnCO3(0-5%) and A1(OH)3 (5-25%) gives green light in the regions 540-550 nm of the visible spectrum under excitation with light of wavelength 160 - 260 nm, thereby replacing the existing conventional green phosphors. The present invention also provides a process for the preparation of the glass composition and a plasma display panel employing the glass composition.
BRIEF DESCRIPTION OF DRAWINGS:
Fig 1 illustrates the structure of front glass substrate and back glass substrate of a conventional plasma display panel
Fig 2 illustrates sectional view of back plate of the conventional plasma display panel.
Fig 3 illustrates the sectional view of back plate of a plasma display panel having layer of paste of the present glass composition
Fig 4. Illustrates the sectional view of back plate of a plasma display panel having a layer of glass composition of present invention.
Fig 5. Illustrates the structure of front glass substrate and back glass substrate of a plasma display panel having a layer of glass composition of present invention.
Fig 6. Illustrate the emission spectrum of a plasma display panel having a layer of glass composition of present invention.
Detailed description of the invention with reference to drawings and Examples:
Before starting the detailed description of the present invention, it is necessary to discuss the conventional AC PDF for clear understanding of the present invention. Figure l(a) illustrates the cross-sectional view of conventional AC PDP with straight barrier ribs and figure l(b) shows the Red (R), Green (G) and Blue (B) phosphor (pixel) arrangement.
In figure l(a), the front glass substrate (1) and back glass substrate (2) are shown. In the front glass substrate (1), display electrodes are made of transparent ITO sheet (3). To reduce the resistance of the display electrode, opaque electrically conducting bus electrodes (4) are made over the ITO electrodes. The display electrode is covered with a transparent dielectric layer (5) to limit the discharge current. Then the electron emissive layer (6) is deposited over the transparent dielectric layer (5). On the back glass substrate (2), a plurality of address electrodes (7) are formed with one address electrode (7) is formed in each sub-pixel. The address electrodes (7) are covered with a dielectric layer
(8) to limit the discharge current and for light reflection. The straight channel barrier ribs
(9) are formed over the dielectric layer (8). The R (lOa), G (lOb), B (lOc) phosphor
layers are formed in the barrier rib (9) channel spaces. One PDP cell or sub-pixel
comprises of one pair of display electrodes (3), (4) and an address electrode (7).
Fig.2 describes the back plate of the plasma display panel. In Fig.2 it is shown the back plate surface (2), the address electrode (7), the white reflecting layer (8) and the barrier rib (9). The barrier ribs are shown without any light emitting material in between. Fig. 3 clearly shows the formation of layer of paste of the glass composition of the present invention, printed between the barrier ribs. The paste occupies the region between the ribs in full (11), as shown in Fig.3. The plate thus fabricated is then dried at 120° C
for 15-40 min and fired at 400- 500° C, to remove the organic volatiles contained in the paste, As a result, the luminescent glass of the present invention forms a thin layer (11) of thickness 10- 20 urn between the barrier ribs as shown in Fig. 4. The solid content i.e. amount of glass present in the paste is with in the range from 15% - 70% and viscosity of the paste ranges from 30-100 Pa.s (Pascal second).
In figure (5a), the top plate of the plasma display panel is described, which is similar to the one described in Fig. l(a). In Fig. 5 (b), the back plate of the plasma display panel is described which is similar to Fig. l(b) except that the luminescent glass is coated on the back plate of the panel (11) keeping all the other parts the same.
The above fabricated plasma display panel as shown in Fig.5 is characterized for the performance of the luminescent glass. The emission spectrum of the said luminescent glass has been shown in Fig. 6. The glass emits intense green emission with a peak at 540 nm due to SD4 - 7F? transition of Tb3+ ion under VUV light generated inside the plasma display panel. All the other emission lines of Tb3+ in this glass composition are very weak thus showing an excellent green emission from this glass composition.
The present invention also provides a process for the preparation of a glass composition capable of emitting green color , the composition comprising BiCh (70-45%), SiO2 ( 20-45%), CaCO3 (0-10%), Tb4O7(l-20%), CeO2(3-30%), Na2C03 ( 1-20%), MnCO3 (0-5%) and A1(OH)3 (5-25%), which comprises the following steps: synthesizing the green light emitting glass composition by taking the constituents in suitable proportions and grinding the constituents thoroughly in an agate ball mill for a period of 3 hrs with acetone ; transferring the ground constituents in to an alumina crucible of one liter capacity; heating the constituents at temperatures in the range 900-1100" C under mild reducing atmosphere created by ammonia decomposition in a muffle furnace thereby converting it into a melt; Quenching the melt in air to condense; grinding the condensed melt in a ball mill to form the glass powder; preparing a paste of required viscosity by mixing said glass powder with suitable commercial vehicles and Applying the said paste in lieu of Green Phosphor .
The paste is applied between barrier ribs of back plate of plasma display panel, dried at 120° C for 15-40 min and fired at 400- 500° C, to remove the organic volatiles contained in the paste. As a result, the luminescent glass of the present invention forms a thin layer of thickness 10- 20 u,m between the barrier ribs when printed by screen printing techniques.
The glass composition thus formed has an average particle size of 0.5 - 2 j.im. The formed paste has solid content i.e. amount of glass present in the paste with in the range from 15% to 70% and paste viscosity ranges from 30 -100 Pa.sec (Pascal second).
In present invention, a plasma display panel has been fabricated with the aforesaid glass composition as green emitting component instead the conventional green phosphor. The glass composition has been made into a paste, printed in the panel by means of screen printing techniques, dried and fired to form a thin layer of glass. This glass
composition emits green light with a wavelength of 540 nm when excited by VUV light of wavelength 172 nm inside the said PDF. The red emitting component is the conventional red phosphor (Y,Gd)BO3:Eu3+, and the blue emitting component is the conventional BAM phosphor.
EXAMPLE:
In an actual experiment, 300 gm of the said luminescent glass has been prepared with the raw materials taken in the following proportions. 6263 (135 gm), SiO2 (120 gm), Tb4O7(6 gm), CeO2(15gm), A1(OH)3 (15 gm) and Na2CO3 (9 gm).
The constituents are first ground thoroughly in an agate ball mill for a period of 3 hrs with acetone and then transferred in to an alumina crucible of one liter capacity. The crucible was then placed inside a muffle furnace having a provision to pass gas from ammonia crackle se-up kept outside the furnace. The forming gas composition was in the ratio N::3Hi. The presence of H? was to reduce the Tb4+ ions to Tb~ ^ during the course of the reaction. The furnace was heated up gradually to a temperature of 900° C and kept at this temperature for 40 min. It was then quenched suddenly to room temperature by furnace shut-off and by stopping the gas passage. The melt was then poured on to a brass dish. The condensed melt in the form of a solid glass was then ground thoroughly in an agate mortar for 30 min when the entire solid became a glass powder.
The glass composition thus formed was found to have an average particle size of 0.5 - 2 |.im. The glass composition in the form of powder has been made in to a paste with commercial vehicles. The formed paste has a solid content of 40% and remaining 60% is vehicle having viscosity of 40 Pa.s. It has been printed on the back plate of the plasma display panel between the barrier ribs by screen printing techniques. The back plate was then dried in air at 120°C for 20 min and fired at 500° C in air for 30 min with the back being inserted in to the furnace at the start and taken after out of the furnace after cooling it to room temperature, to form a thin layer of glass between the ribs of uniform thickness of 12- 14
The Plasma display panel formed with the said back plate was characterized for green emission. The spectra showed intense green emission with a peak at 540 nm as that of the conventional (Y,Gd)BO3:Tb" + green phosphor.
ADVANTAGES;
The application of the above mentioned glass as a green component instead of the conventional green phosphor has the following advantages:
1. The glass composition can be excited efficiently by light of wavelength 160-260
nm.
2. No luminescence degradation observed due to the stable valence state of Tb3+ ion
in this glass.
3. The non-porous nature of this luminescent glass reduces the loss of VUV inside
which normally occurs in phosphors due to scattering of the phosphors at the
voids and absorption by the bottom surface of the plate.
4. This glass enhances the formation of a compact and uniform layer between barrier
ribs in PDPs,
5. Better adherence of this glass with the bottom layer of the back plate of POP and
hence no peel-off of this glass inside PDF. This helps to reduce the pixel defects.
6. The glass is thermally stable in air, stable to dispersion in aqueous and organic
solvents.
7. The glass can be used as green emitter in various devices such as low pressure
mercury vapor lamps, dielectric barrier discharge flat lamps and in plasma display
panels
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined.

We Claim:
1. A glass composition comprising B203 (70-45%), SiO2 ( 20-45%),
CaCO3 (0-10%), Tb4O7 (1-20%), CeO2 ( 3-30%), Na2CO3 ( 1-20%),
MnCO3 (0-5%) and A1(OH)3 (5-25%) gives green light in the regions
540-550 nm of the visible spectrum under excitation with light of
wavelength 160 - 260 nm, thereby replacing the existing conventional
green phosphors.
2. The glass composition as claimed in claim 1, wherein, the glass
composition can be excited directly by light of wavelength 172 nm and
emits green light of wavelength 540 nm.
3. The glass composition as claimed in claim 1, wherein, the glass
composition has particle size in the range from 0.5 - 3 (im.
4. The glass composition as claimed in claim 1, wherein, the glass
composition is stable in air and at dispersion in water or any organic
solvents.
5. The glass composition as claimed in claim 1, wherein, a paste made of
said glass composition forms a uniform layer of thickness 10 - 20 u
between barrier ribs inside a Plasma Display Panel.
6. The glass composition as claimed in claim 5, wherein, the paste
viscosity ranges from 30-100 Pa.s (Pascal second) and solid content of
the paste ranges from 15% to 70%.
7. A process for the preparation of a glass composition, wherein the
composition comprising B2O3 (70-45%), SiO2 ( 20-45%), CaC03 (0-
10%), Tb4O7 (1-20%), CeO2 ( 3-30%), Na2CO3 ( 1-20%), MnCO3 (0-
5%) and A1(OH)3 (5-25%), which comprises :

a) synthesizing the green light emitting glass composition by taking the
constituents in suitable proportions and grinding the constituents
thoroughly in an agate ball mill for a period of 3 hrs with acetone ;
b) Transferring the ground constituents in to an alumina crucible of one
Liter capacity;
c) Heating the constituents at temperatures in the range 900-1100" C
Under mild reducing atmosphere created by ammonia decomposition
in a muffle furnace thereby converting it into a melt;
d) Quenching the melt in air to form a condensed melt;
e) Grinding the condensed melt to form a glass powder in a ball mill;

f) Preparing a paste of required viscosity by mixing said glass powder
With suitable commercial vehicles and
g) Applying the said paste in lieu of Green Phosphor.
8. The Process as claimed in claim 7, wherein, the glass composition has
particle size in the range from 0.5 - 3 jam.
9. The Process as claimed in claim 7, wherein, the glass composition is
stable in air and at dispersion in water or any organic solvents.
10. The Process as claimed in claim 7, wherein, the paste viscosity ranges
from 30 -100 Pa.sec (Pascal second) and solid content of the paste
ranges from 15% to 70%.
11. The Process as claimed in claim 7, wherein, the paste forms a uniform
layer of thickness 10 - 20 |j, between barrier ribs inside a Plasma
Display Panel.
12. A plasma display panel is fabricated by using the glass composition as
claimed in claims 1 to 11, wherein, the glass composition is printed
between barrier ribs of the plasma display panel in the form of paste
which is subsequently dried and fired to form a thin layer of glass
which emits green light in the region 540-550 nm, replaces the
conventional green phosphor.
13. The glass composition as claimed in claim 1, wherein, the glass
composition can be used as green light emitter in various
display devices such as low pressure mercury vapor lamps,
dielectric barrier discharge flat lamps and in plasma
display panels.
14. A glass composition, a process for the preparation of glass composition
and a plasma display panel substantially herein described with reference
to the accompanying drawing and examples.