Mayr, Walter c/o Philips Intellectual Property & Standards GmbH (Weisshausstr. 2, Aachen, 52066, DE)
Schmidt, Peter J. c/o Philips Intellectual Property & Standards GmbH (Weisshausstr. 2, Aachen, 52066, DE)
KONINKLIJKE PHILIPS ELECTRONICS N.V. (Groenewoudseweg 1, BA Eindhoven, NL-5621, NL)
Jüstel, Thomas c/o Philips Intellectual Property & Standards GmbH (Weisshausstr. 2, Aachen, 52066, DE)
Mayr, Walter c/o Philips Intellectual Property & Standards GmbH (Weisshausstr. 2, Aachen, 52066, DE)
Schmidt, Peter J. c/o Philips Intellectual Property & Standards GmbH (Weisshausstr. 2, Aachen, 52066, DE)
| 1. | A plasma picture screen provided with a phosphor layer which comprises a phosphor having a host lattice and Pr3+ as an activator, characterized in that said host lattice of the phosphor is chosen from the group of oxides, silicates, niobates, tantalates, and niobatetantalate mixed crystals. |
| 2. | A plasma picture screen as claimed in claim 1, characterized in that the phosphor is chosen from the group of (YlxGdx) 203: Pr with 0 # x # 1, (Ca1xSrx)2SiO4 : Pr with 0 < x < 1, and Y (Nb1xTax)O4 : Pr with 0 # x # 1. |
| 3. | A plasma picture screen as claimed in claim 1, characterized in that the phosphor layer additionally comprises a further redemitting phosphor chosen from the group of (YlxGdx) B03 : Eu with 0 # x # 1, Y (V1xPx)O4 : Eu with 0 < x 1, and (Yl xGdx) 203: Eu with 0 # x # 1. |
The invention relates to a plasma picture screen provided with a phosphor layer which comprises a red-emitting phosphor.
Plasma picture screens render possible color pictures with high resolution and a large picture screen diameter and are of compact construction. A plasma picture screen comprises a hermetically closed space which is filled with a gas, with electrodes arranged in a grid. Individually controllable plasma cells are created by means of separating ribs, in which cells a gas discharge is generated by the application of a voltage, thus generating light in the ultraviolet range. This light can be converted into visible light by phosphors and can be emitted through the front plate of the cell to the viewer.
The red phosphor used in many plasma picture screens is (Y, Gd) B03 : Eu, because this phosphor has a higher luminous efficacy when excited by VUV radiation than other red-emitting phosphors. Considerable disadvantages of this phosphor are on the one hand the color point, which is too orange for video applications with x = 0.643 and y = 0.357, and on the other hand the comparatively long decay time of ilo = 9 ms. US 5,136, 207 describes, for example, a plasma picture screen with (Y, Gd) B03 : Eu as the red-emitting phosphor.
The orange color point of (Y, Gd) B03 : Eu leads to a reduced color space in plasma picture screens in comparison with cathode ray tubes, in which Y202S: Eu is used as the red phosphor. The latter has a color point of x = 0.659 and y = 0.332.
It is accordingly an object of the invention to provide an improved plasma picture screen.
This object is achieved by means of a plasma picture screen provided with a phosphor layer which comprises a phosphor having a host lattice and Pr3+ as an activator, said host lattice of the phosphor being chosen from the group of oxides, silicates, niobates, tantalates, and niobate-tantalate mixed crystals.
According to quantum theory models, the lowest 5d energy level of the Pr3+ cation lies far below the'So energy level of the Pr3+ cation.
In Pr3+-activated phosphors, in which the lowest 5d energy level of the Pr3+ cation lies below the'So energy level of the Pr3+ cation according to qualitative quantum theory models, no transitions from the'So energy level to the ground state take place in which light is emitted in the UV-C range or the blue spectral range. Instead, radiationless relaxations take place via 5d energy levels lying below the'So energy level towards lower energy levels. The latter are, for example, the 3PJ energy level or the ID2 energy level. The electrons return from the 3PJ energy level or the 1Da energy level to the ground state under emission of red light.
The host lattice is responsible for the degree of resolution of the 5d states of the Pr3+ cation. The advantageously chosen host lattices comprise co-ordinating anions with a high charge density and thus achieve a major resolution of the 5d states of the Pr3+ cation. In the Pr3+-activated phosphors with these host lattices, the lowest 5d energy level lies below the 'So energy level of the Pr3+ cation, and a red emission is obtained after excitation with (V) UV radiation.
The advantageously chosen Pr3+-activated phosphors of claim 2 have a lower y-value of the color point than (Y, Gd) B03 : Eu.
The advantageous embodiment of claim 3 renders it possible to vary the color point of a red-emitting phosphor layer.
The invention will be explained in more detail below with reference to four Figures, in which Fig. 1 shows the construction and operating principle of a single plasma cell in an AC plasma picture screen, Fig. 2 shows the energy level diagram of Pr3+, Fig. 3 shows the excitation and emission spectrum of Ca2Si04 : 1 % Pr, and Fig. 4 shows the excitation and emission spectrum of Y203 : 0.5% Pr.
In Fig. 1, a plasma cell of an AC plasma picture screen with a coplanar arrangement of the electrodes comprises a front plate 1 and a carrier plate 2. The front plate 1 comprises a transparent plate 3, for example made of glass, on which a dielectric layer 4,
preferably comprising low-melting glass, and thereon a protective layer 5, preferably comprising MgO, are present. Parallel, strip-shaped discharge electrodes 6,7 covered by the dielectric layer 4 are provided on the transparent plate 3. The discharge electrodes 6,7 are made, for example, from metal, ITO, or a combination of a metal and ITO. Preferably, the discharge electrodes 6,7 each comprise a strip of ITO on which a narrower layer of Al or Ag is provided each time as a bus electrode. The carrier plate 2 is preferably made of glass, and parallel, strip-shaped address electrodes 10, for example of Ag, are preferably provided on the carrier plate 2 so as to run perpendicularly to the discharge electrodes 6,7. Said address electrodes 10 are covered with a phosphor layer 9 which emits light 13 in one of the three basic colors red, green, and blue. The phosphor layer 9 is subdivided into several color segments for this purpose. Individually controllable plasma cells, in which corona discharges take place, are formed by a ribbed structure 12 with separating ribs which are preferably made of a dielectric material.
A gas is present in the plasma cell, and also between the discharge electrodes 6,7, of which one is the cathode and the other the anode, in alternation. The gas may be, for example, a rare gas, a mixture of rare gases with Xe as the UV-emitting component, nitrogen, or a mixture of nitrogen and at least one rare gas such as, for example, He, Ne, Kr, or Xe.
After the surface discharged has ignited, such that charges can flow along a discharge path situated between the discharge electrodes 6,7 in the plasma range 8, a plasma is formed in the plasma range 8 by means of which radiation 11 in the UV range is generated in dependence on the composition of the gas. The radiation 11 excites the phosphor layer 9 into luminance so as to emit visible light 13 which issues through the front plate 1 to the exterior and thus forms a luminous dot on the picture screen. The phosphor layer 9 is subdivided into a plurality of color segments. Usually, the red-, green-, and blue-emitting color segments of the phosphor layer 9 are provided in the form of perpendicular strip triplets. A plasma cell with one color segment forms a so-termed sub-pixel. Three adjoining plasma cells with a red- , green-, and blue-emitting color segment, respectively, together form one pixel or picture element.
A green-emitting color segment may comprise, for example, Zn2Si04 : Mn as its phosphor, and a blue-emitting color segment may comprise, for example, BaMgAlloOl7 : Eu as its phosphor.
A red-emitting color segment comprises a red-emitting Pr3+-activated phosphor. The host lattice of the Pr3+-activated phosphor is preferably an oxide, a silicate, a niobate, a tantalate, or a niobate-tantalate mixed crystal. The Pr3+-activated phosphor used
may be, for example, (Y1-xGdx) 203: Pr with 0 < x < 1, (Cal xSrx) 2SiO4 : Pr with 0 < x < 1, or Y (Nbl xTax) o4 : Pr with 0 # x # 1 in a red-emitting color segment of the phosphor layer 9.
Fig. 2 shows the energy level diagram of Pr3+ and the positions of the 5d states in the phosphors according to the invention. After excitation of an electron into a 5d state of the Pr3+ cation, a radiationless relaxation takes place to the'So energy level. Further radiationless relaxations take place from there, first to a further, lower 5d state of the Pr3+ cation, and subsequently to further energy levels, for example 3PJ, iI6, or ID2. The electrons return from these levels to their ground state with radiation. The transitions 3PJ X 3H6 and ID2 o 3H4 are responsible for the red emission of these phosphors. Since the lowest 5d state of the Pr3+ cation lies below the'So energy level of the Pr3+ cation, no radiation-emitting transitions take place from this'So energy level to the ground state under emission of short- wave radiation, i. e. for example in the UV-C or blue spectral range, as are lcnown from other Pr3+-activated phosphors such as YP04 : Pr or YBo3 : Pr.
It may be advantageous that the red-emitting color segments of the phosphor layer 9 comprise besides the Pr3+-activated phosphor a further red-emitting phosphor such as, for example, (Y1-xGdx)BO3 : Eu with 0 # x # 1, Y (v1-xPx)O4 : Eu with 0 < x # 1, or (Y1-xGdx) 203: Eu with 0 # x < 1.
Embodiment 1 To prepare Ca2Si04 : 1% Pr, 20.00 g CaC03 (199.8 mmole Ca), 6.215 g Si02 (103.4 mmole Si), and 437.0 mg Pr (N03) 3 (H20) 6 (1.005 mmole Pr) were dispersed or dissolved, as applicable, in 200 ml water and subsequently placed in an ultrasonic bath for 10 minutes. The water was removed by distillation, and the residue was dried at 100 °C. Then 1.6 g NH4C1 was added, and the resulting mixture was pounded. Finally, the mixture was calcinated for 12 hours at 850 °C in a CO-containing atmosphere (heating-up rate: 200 °C/h).
After cooling down, the powder was milled and passed through a 36 llm sieve.
Fig. 3 shows the excitation and emission spectrum of Ca2Si04 : 1% Pr. The color point (x, y) of the resulting phosphor lies at x = 0.605, y = 0. 350.
Ca2Si04 : 1% Pr was used as a red-emitting phosphor in a plasma picture screen, which exhibited an improved color point and short decay times after excitation of the red- emitting sub-pixels or color segments.
Embodiment 2 Ca2Si04 : 1 % Pr was prepared as in embodiment 1 and was used together with YP04 : Eu in a plasma picture screen, which exhibited an improved color point and short decay times after excitation of the red-emitting sub-pixels or color segments.
Embodiment 3 To prepare Y203: 0.5% Pr, 30.00 g Y (N03) 3 (H20) 6 (78.36 mmole Y) and 167.0 mg g Pr (N03) 3 (H20) 6 (396.0 llmole Pr) were dissolved in 200 ml water. The pH value was adjusted to a value of 5.0 by means of 1 M NH3. The resulting solution was heated to 60 °C.
74.43 g oxalic acid dihydrate (590.4 mmole oxalic acid) was dissolved in 500 ml water and was dripped into the nitrate solution within a period of 45 minutes. Stirring then took place for a further 15 minutes at 60 °C. The resulting precipitate was sucked off, washed twice with water, and then dried at 120 °C. The solid substance was subsequently calcinated for three times for 1 h at 900 °C in air (heating-up rate: 250 °C/h), for 4 h at 1400 °C in a CO- containing atmosphere (heating-up rate: 200 °C/h), and for 2 h at 1100 °C in a CO-containing atmosphere (heating-up rate: 300 °C/h).
Fig. 4 shows the excitation and emission spectrum of Y203: 0.5% Pr. The color point (x, y) of the resulting phosphor lies at x = 0.671, y = 0.304.
Y203: 0.5% Pr was used as a red-emitting phosphor in a plasma picture screen which exhibited an improved color point and short decay times upon excitation of the red- emitting sub-pixels or color segments.