FIELD OF INVENTION:
The present invention relates to a phosphor blend. The present invention more particularly provides a stable green phosphor blend which can be excited by UV light, can emit green color and can replace the existing conventional green phosphor applied in plasma displays.
BACKGROUND ART:
The conventional AC (Alternating Current) driven PDP 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 PDP 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 147 & 172 nm, produced by Ne-Xe discharge inside PDP. 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 nm between the ribs after firing the plate for removing the organic volatiles in the paste. The major green phosphors normally employed in plasma displays are (Y,Gd)B03:Tb3+ (abbreviated as YBT), Zn2Si04:Mn2+ (abbreviated as ZSM) and the blend YBT:ZSM. Of all the three phosphors the green blend made of YBT and ZSM displays better performance in terms of brightness and color coordinates. The phosphor YBT is expensive due to the process of its synthesis and is also unstable when heated in air at high temperature (> 450° C). Hence, inventor finds a new green phosphor blend satisfying all the requirements of the YBT:ZSM blend and proposing a stable luminescent green phosphor blend comprising of Sr6(Y,Gd)Y(B03)6:Tb3+ and at least one selected from group consisting of Zn2Si04:Mn2+ and (Ba,Sr,Mg)0.aAl203:Mn (hereafter referred to as BALMN) which
is stable in air at high temperature as well as less-expensive when compare to YBT:ZSM blend.
OBJECT OF THE INVENTION:
The conventional green phosphors used in PDP have the problem of instability while heating in air at high temperature. In order to overcome this problem, a new green phosphor blend is proposed, which can be efficiently excited by UV light of wavelength 140-260 nm, can emit green color of wavelength 520- 550 nm.
The principal object of the present invention is to provide a stable luminescent green phosphor blend comprising of Sr6(Y,Gd)Y(B03)6:Tb3+ and at least one selected from group consisting of Zn2Si04:Mn2+ and (Ba,Sr,Mg)0.aAl203:Mn, which undergoes excitation with light of wavelength 140- 260 nm and gives green light in the region 520-550 nm thereby replacing the existing conventional green phosphors.
Another object of the present invention is to provide a novel phosphor blend which is suitable to work as a solid constituent to make a paste of viscosity 20-100 Pa.sec by mixing with commercial vehicles and thus novel phosphor blend 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 stable green phosphor blend which can be excited by UV light of wavelength 140-260 nm, can emit green color of wavelength 520 -550 nm
Yet another object of the present invention is to provide a Plasma display panel using the green phosphor blend which can replace the conventional green phosphors in Plasma display panel and can emit green color of wavelength 520-550 nm during the operation of Plasma display panel.
STATEMENT OF THE INVENTION:
Accordingly, the invention provides a stable luminescent green phosphor blend comprising of Sr6(Y,Gd)Y(B03)6:Tb3+ and at least one selected from group consisting of Zn2Si04:Mn2" and (Ba,Sr,Mg)0.aAl203:Mn, which undergoes excitation with light of wavelength 140- 260 nm and gives green light in the region 520-550 nm thereby replacing the existing conventional green phosphors . The present invention also provides a process for the preparation of the phosphor blend and a plasma display panel employing the phosphor blend
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 phosphor blend.
Fig 4. Illustrates the sectional view of back plate of a plasma display panel having a layer of phosphor blend 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 phosphor blend of present invention.
Fig 6. Illustrate the emission spectrum of a plasma display panel having a layer of phosphor blend 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 PDP for clear understanding of the present invention. Figure 1 (a) illustrates the cross-sectional view of conventional AC PDP with straight barrier ribs and figure 1(b) shows the Red (R), Green (G) and Blue (B) phosphor (pixel) arrangement.
In figure 1(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 (10a), G (10b), B (10c) 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 phosphor blend 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 blend of the present invention forms a thin layer (11) of thickness 10- 20 µm between the barrier ribs as shown in Fig. 4. The solid content i.e. amount of phosphor present in the paste is with in the range from 15% - 70% and viscosity of the paste ranges from 20-100 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. 1(a). In Fig. 5 (b), the back plate of the plasma display panel is described which is similar to Fig. 1(b) except that the luminescent blend 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 phosphor blend. The emission spectrum of the said luminescent blend has been shown in Fig. 6. The blend emits intense green emission with a peak at 520-550 nm under VUV light generated inside the plasma display panel. All the other emission lines of this blend composition are very weak thus showing an excellent green emission from this phosphor blend.
The present invention also provides a process for the preparation of the stable luminescent green phosphor blend comprising of Sr6(Y,Gd)Y(BO"3)6:Tb3+ and at least one selected from group consisting of Zn2Si04:Mn2+ and (Ba,Sr,Mg)0.aAl203:Mn, which comprises the following steps: making the green light emitting blend by taking the individual phosphors in suitable proportions and grinding the constituents thoroughly in an agate ball mill for a period of 30-90 min with acetone; preparing a paste of required viscosity by mixing said phosphor blend 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 blend of the present invention forms a thin layer of thickness 10- 20 µm between the barrier ribs when printed by screen printing techniques.
The phosphor blend thus formed has an average particle size of 0.5 - 3 µm. The formed paste has solid content i.e. amount of powder present in the paste with in the range from 15% to 70% and paste viscosity ranges from 20-100 Pa.sec (Pascal second).
In present invention, a plasma display panel has been fabricated with the aforesaid phosphor blend as green emitting component instead the conventional green phosphor. The phosphor blend 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 powder. This powder composition emits green light with a wavelength of 520-550 nm when excited by VUV light of wavelength 147 & 172 nm inside the said PDP. The red emitting component is the conventional red phosphor (Y,Gd)BO"3:Eu3+, and the blue emitting component is the conventional BAM phosphor.
EXAMPLE 1:
In an actual experiment, 300 gm of the said luminescent phosphor blend has been prepared with the raw materials taken in the following proportions: SRYBT (150 gm) and ZSM(150gm).
The constituents are first ground thoroughly in an agate ball mill for a period of 45 min with acetone. The phosphor blend thus formed was found to have an average particle size of 0.5 •• 2 µm. The phosphor blend in the form of powder has been made in to a paste with commercial vehicles. The formed paste has a solid content of 30% and remaining 70% 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 powder between the ribs of uniform thickness of 12- 15 urn
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 530-545 nm as compare to conventional YBT:ZSM green phosphor blend.
EXAMPLE 2:
In an actual experiment, 300 gm of the said luminescent phosphor blend has been prepared with the raw materials taken in the following proportions: SRYBT (150 gm), ZSM (75 gm), BALMN (75 gm).
The constituents are first ground thoroughly in an agate ball mill for a period of 60 min with acetone. The phosphor blend thus formed was found to have an average particle size of 0.5 - 3 urn. The phosphor blend in the form of powder has been made in to a paste with commercial vehicles. The formed paste has a solid content of 25% and remaining 70% is vehicle having viscosity of 45 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 powder between the ribs of uniform thickness of 12- 15 µm.
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 520-545 nm as compared to conventional YBT: ZSM green phosphor blend.
ADVANTAGES:
The application of the above mentioned phosphor blend as a green component instead of the conventional green phosphor has the following advantages:
1. The phosphor blend can be excited efficiently by light of wavelength 140 - 260 nm.
2. No luminescence degradation observed due to the stable valence state of Tb3+ ion in this blend.
3. The blend is thermally stable in air at high temperature (up to 700°C), stable to dispersion in aqueous and organic solvents.
4. The green phosphor blend 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.
5. The phosphors in the blend contain less expensive ions and hence cost reduction is feasible.
The term "comprising" as used herein is synonymous with "including," "containing," or "characterized by," and is inclusive or open-ended and does not exclude additional, un recited elements or method steps.
All numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of any claims in any application claiming priority to the present application, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches.
The above description discloses several methods and materials of the present invention. This invention is susceptible to modifications in the methods and materials, as well as alterations in the fabrication methods and equipment. Such modifications will become apparent to those skilled in the art from a consideration of this disclosure or practice of the invention disclosed herein. Consequently, it is not intended that this invention be limited to the specific embodiments disclosed herein, but that it cover all modifications and alternatives coming within the true scope and spirit 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 stable luminescent green phosphor blend comprising of Sr6(Y,Gd)Y(B03)6:Tb3+and at least one selected from group consisting of Zn2Si04:Mn2+ and (Ba,Sr,Mg)0.aAl203:Mn, which undergoes excitation with light of wavelength 140- 260 nm and gives green light in the region 520-550 nm thereby replacing the existing conventional green phosphors .
2. The phosphor blend as claimed in claim 1, wherein, the phosphor blend can be excited directly by light of wavelength 147 as well as 172 nm and emits green light of wavelength 520 - 550 nm.
3. The phosphor blend as claimed in claim 1, wherein, the phosphor blend has particle size in the range from 0.5 - 3 µm.
4. The phosphor blend as claimed in claim 1, wherein, the phosphor blend is stable in air at high temperature and at dispersion in water or any organic solvents.
5. The phosphor blend as claimed in claim 1, wherein, a paste made of said phosphor blend forms a uniform layer of thickness 10-20 jam between barrier ribs inside a Plasma Display Panel.
6. The phosphor blend 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 the stable luminescent green phosphor blend, wherein the blend comprising of Sr6(Y,Gd)Y(B03)6:Tb3+ and at least one selected from group consisting of Zn2Si04:Mn"+ and (Ba,Sr,Mg)0.aAl203:Mn, which comprises :
a) Taking the individual phosphors in suitable proportions and grinding
the constituents thoroughly in an agate ball mill for a period with in
range of 30-90 min with acetone;
b) Preparing a paste of required viscosity by mixing said phosphor blend Powder with suitable commercial vehicles and
c) Applying the said paste in lieu of conventional Green Phosphor.
8. The Process as claimed in claim 7, wherein, the phosphor blend has
particle size in the range from 0.5 - 3 µm.
9. The Process as claimed in claim 7, wherein, the phosphor blend 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 20 -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 µm between barrier ribs inside a Plasma Display Panel.
12. A plasma display panel is fabricated by using the phosphor blend as claimed in claims 1 to 11, wherein, the phosphor blend 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 powder which emits green light in the region 520-550 nm, replaces the conventional green phosphor.
13. The phosphor blend as claimed in claim 1, wherein, the phosphor blend 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 stable luminescent green phosphor blend, a process for the preparation of blend and a plasma display panel substantially herein described with reference to the accompanying drawing and examples.