NL9200939A - Far-emitting area emitting luminescent material for cathode ray tubes. - Google Patents

Description

Short designation: Far-red region emitting luminescent material for cathode ray tubes

The present invention relates to luminescent materials used in cathode ray tubes for liquid crystal displays, and more particularly to a new series of luminescent compounds having the yttrium aluminum garnet crystal structure. *

To address the amorphous silicon in the Hughes liquid crystal working light barrier (LCLV) of luminescent materials used, the following requirements must be met: the displayed spectrum must be adapted as closely as possible to the amorphous silicon (α-Si) response ; maximum radiant energy delivery in the spectral region of interest; an expiration time less than about 10 milliseconds, measured from 100% to 10% level1 s; an average particle size suitable for high resolution operation - less than approximately 6 μιη; and high resistance to either thermal or life-induced deterioration.

The spectral response of α-silicon used in the LCLV peaks at about 740 nm. Therefore, for optimum sensitivity, the incident light used to activate the α-silicon photosensitive layer should approach the response curve as closely as possible.

A large number of luminescent materials have been investigated, looking for a luminescent compound with the correct properties with regard to spectral emission, decay time, efficiency and small particle size. These luminescent compounds include aluminum oxide: cad, cadmium sulfide: Ag, zinc cadmium sulfide: Ag. zinc phosphate: Mi, yttrium oxysulfide: Eu, yttrium aluminum oxide: Eu, and the like.

The exhibited spectrum of an existing luminescent material for the far-red region (the "far-red region" means about 600-800 nm) used in luminescent lamps almost corresponds to the response of α-silicon. This material consists of an iron-activated lithium aluminum oxide (LiAl02: Fe). The iron concentration of the commercially available material is about 0.5%. However, this luminescent material, when used in cathode ray tubes, has an excessively long decay time, about 30 milliseconds.

Yttrim-almirdm-gi-mat-1xMinescent materials (YAG) are well known in the art. They are provided with various activating means, such as terbium, cerium, europium and the like. In YAG: Eu, the energy is distributed across the 591, 608, 630, and 710 nm lines, with smaller peaks at about 649 and 692 nm. Because of this displayed spectrum, this luminescent material cannot efficiently activate the light barrier.

No commercially available luminescent material has been found to meet the requirements for use with α-silicon in liquid crystal displays.

The present invention relates to a new series of luminescent materials with the yttrium aluminum garnet crystal structure. The general formula of the luminescent material of the invention is Y3.yGdyAl5.xGax012: A, where x is 0-5, y is 0-3, and A is selected from the series consisting of chromium, iron, vanadium, neodymium, dystrosium, cobalt, nickel, titanium or a combination of these.

The luminescent materials of the above formula exhibit predeterminable variations in their exhibited spectrum, brightness, and afterglow properties that meet the requirements for use with α-silicon light-sensitive layers in liquid crystal light barriers.

Fig. 1 is a schematic representation of a projection system equipped with a liquid crystal light barrier using a cathode ray tube; b

Fig. 2 is a graph of the spectral response of an amorphous silicon (α-Si) containing photoconductor, with the normalized brightness plotted against the wavelength in nanometers;

Fig. 3-10 are graphs of the light output of ¥ 3.yGdyAl5_xGaxOi2: Cr as a function of the composition, (a) where y = 0 and x = 0 (fig. 3), 1 (fig. 4), 2 (fig. 5), 3 (Fig. 6), 4 (Fig. 7) and 5 (Fig. 8), (b) where y = 3 and x = 5 (Fig. 9) and (c) where y = 3 and x = 0 (Fig. 10), where the normalized brightness is plotted against the wavelength in nanometers;

Fig. 11-15, are graphs of the light output of Y3.yGdyAl5.xGax012: Cr as a function of the process conditions, where y = o and x = 0 (fig; 11-13!) And where y = 3 and x = 5 ( 14 and 15), where the normalized brightness is plotted against the wavelength in nanometers;

Fig. 16 and 17 are graphs of particle size distribution for typical preparations of the luminescent materials of the invention, wherein; the percentage is plotted against the particle size in micrometers. !

For example, a liquid crystal light barrier (LCLV) is described in SID International Symposium, Digest of Technical Papers, "Video-Rate Liquid Crystal Light-Valve Using an Amorphous Silicon Ehotoconductor", RD Sterling et al, volume XXI, biz 327 -328 (1990) Fig. 1 of this reference is a schematic representation of a cathode ray tube (CRT) 12 containing base LCLV projector 10, the CRT 12 providing an input image which is applied to a liquid crystal light barrier 14 typically through a fiber optic face plate (not shown) A xenon arc lamp 16 provides the output light which is filtered by UV filter 18 and linearly polarized by pre-polarization filter 20 prior to reaching the LCLV 14. The image continues then through a polarizing mirror 22, a prism window 24, and then through a projection lens 26, by means of which the image is projected onto a screen (not shown).

The previous projector is an example of a device that uses a combination of the LCLV and CRT. Other combinations of LCLVs and CRTs are also known. While such combinations are known to those skilled in the art, none of the CRTs described use a luminescent material of the invention.

The LCLV employs a hydrogenated amorphous silicon photoconductor (a-Si: H) as known and shown in the aforementioned reference. The spectral response of α-Si photoconductor is shown in Fig. 2. It is this curve, which a luminescent material should meet as close as possible for efficient energy delivery.

The composition of the luminescent materials according to the invention is given by the formula Y3.yGdyAl5.xGax012: A, where x is 0-5, y is 0-3 and A is selected from the series consisting of chromium, iron, vanadium, neodynium, dysprosium, cobalt, nickel, titanium or combination thereof.

At least one from the series of chromium, neodymium and iron is preferably used. Most preferred is chromium, especially in the range of 0.33 to 2 at%. Outside this range, the light output is unacceptably reduced.

The preferred compositions depend on the particular desired properties. For example, the brightest composition (greatest relative light output) is obtained with chromium-activated yttrium-aluminum minium garnet, Y3Al5012: Cr (YAG: Cr), while the shortest decay time is obtained with chromium-activated gadolium-gallium garnet Gd3Ga5012: Cr (GGG: Cr). The best composition as a compromise for obtaining the maximum brightness and shortest decay time is Y3Al2Ga3012: Cr. The results are shown below:

Composition «Rel. release Expiry YAG: Cr !. 100 5-6 msec GGG: Cr 45 1 msec Y3A2Ga3012: Cr 76 2 msec.

In the preparation of the luminescent materials with the garnet structure according to the invention, a method customary in the art is used. The starting materials are the oxides of the individual elements. These are mixed in stoichiometric amounts and processed in a ball mill to obtain a thoroughly mixed, homogeneous mixture. After the reactants have been mixed and dried, they are baked in a one-step process, usually for about 2-8 hours at about 1500-1600 ° C and under oxidizing conditions, such as in the air.

Also, the oxides can be baked in a two-step process, usually about 1200 ° C for 1-4 hours, followed by 1500-1600 * 0 for 2-8 hours, again under oxidizing conditions, such as air.

Also, a three step process can be used, usually about 1200 ° C for 1-4 hours, about 1550 ° C for about 1-4 hours and about 1590 ° C for about 1-4 hours. These methods are well known in the art and are therefore not part of the present invention.

Since the light emission properties of the luminescent material become better with a smaller particle size, it must be possible to process and / or obtain particles with an average particle size smaller than about 6 μια during baking.

A sol-gel process can also be used to prepare the luminescent material. In this case, the starting materials are the nitrates or chlorides of Y, Al and Cr. These substances are dissolved in deionized water in stoichiometric amounts. A stoichiometric excess of an ammonium hydroxide solution is then added, the hydroxides of the metals being precipitated. After the precipitate is appropriately digested and stabilized,

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filtered, washed and dried. Subsequently, this starting material can be processed in the manner described above, whereby the desired film-densifying material is obtained. These methods are also known and therefore do not form part of the present invention.

In another process, stoichiometric amounts of yttrium, gadolinium, aluminum and gallium nitrates are used as the starting materials. These materials are dissolved in water, after which they are precipitated as hydroxides with ammonium hydroxide. After washing, the activator oxide can be mixed with the materials in the desired percentage, for example, using a ball mill. After ball milling, the mixture is dried and baked under the conditions described above. These methods are also known and therefore do not form part of the present invention.

In another known process, the nitrates of yttrium, aluminum, gadolinium and gallium and the activating agent are started from, and the hydroxides are precipitated in the above manner with ammonium hydroxide and processed in the above manner.

Described below is a representative method for preparing a composite luminescent material having the garnet structure according to the teachings of the present invention. This two-step process is not new and does not form part of the present invention.

When one wants to prepare 10 g of luminescent material, starting from the oxide constituents, the desired amounts (see the compositions below) of the oxides are placed in a 2-ounce ball mill with about 30 g of sintered aluminum dioxide balls with a diameter of about 9 mm brought. 15 ml of deionized water is added, after which the mill is closed and placed on rollers. In order to use larger amounts of the luminescent material, these conditions must be scaled up accordingly, as is well known.

After 2 hours of rolling, during which time the oxides are thoroughly mixed and their particle size is slightly reduced, the mill is removed from the rollers and the contents are removed. collected in a clean receptacle. The rinse water from the mill is added to the α-charge. The powder is dried in an oven at 110 ° C and then sieved through a 70 mesh stainless steel sieve. The powder is placed in a sintered aluminum dioxide container and placed in an oven at 400 ° C. The temperature is raised to 1200 ° C for 2 hours. After this, the temperature is lowered to 400 ° C, after which the tray is removed from the oven. Baking in the oven is conveniently done in the air. The material is sieved again and the particle size is measured. The material is then returned to the oven and held at 1600 ° C for 2 hours. The oven is cooled to 400 ° C and the tray is removed. The material thus obtained is crushed and sieved in a mortar with a stepper, after which the particle size is measured and it is ready for use.

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A number of composite materials were prepared using the procedures described above. The influence of different compositions on a number of properties are described in Table I below. The relative release is determined by measuring the brightness with a spector radiometer in the wavelength range of 600-800 nm. These data are then converted into the relative output -v by dividing each value by the highest brightness value. The expiration time is measured in milliseconds. The baking temperature in all cases was 1200 ° C, followed by 1600 ° C. The distribution of the spectrum as a function of the composition is shown in the indicated figures:

The influence of the baking temperature on the properties is shown in Table II below. In all cases, the oven was baked for 2 hours at the indicated temperature. The influence of the baking temperature on the distribution of the spectrum is shown in the figures shown.

The influence of a number of further compositions on the relative release is shown in Table III below. Distribution of the spectrum of two of these composite materials is shown in Figures 16 and 17.

The particle size distribution of a luminescent material prepared in the manner described above with the preferred composition is shown in Figures 18 and 19. Figure 18 is for GGG, with 1.0% Cr activator. The mean particle size was 6.31 µl, with 10% of the particles less than 1.32 µl and 10% of the particles larger than 21.15 µl. Figure 19 is for TAG, with 0.5% Cr activator. The mean particle size of 6.54 μιη, with 10% of particles smaller than 1.76 pa and 10% of particles larger than 14.12 pa.

In the present application, a far-red region emitting luminescent material is disclosed, consisting essentially of Y3.yGdyAl5.xGax012: A, where x is 0-5, y is 0-3, and A is selected from the series which consists of chromium, iron, vanadium, neodymium, dysprosium, cobalt, nickel, titanium and combinations thereof. A large number of changes and adjustments will be apparent to those skilled in the art, and all these changes and adjustments are within the scope of the present invention as defined by the appended claims.

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Claims (9)

A luminescent material emitting in the far-red region, characterized in that it is a composite material having essentially the composition Y3.yGdyAl5.xGax012: A, wherein x is 0-5, y is 0-3 and A becomes selected from the range consisting of chromium, iron, vanadium, neodymium, dysprosium, cobalt, nickel, titanium or a combination thereof. Luminescent material according to claim 1, characterized in that A is chromium, neodymium or both. Luminescent material according to claim 2, characterized in that A is chromium. Luminescent material according to claim 1, characterized in that x = 0, y = 0, A is chrome and the composite material exhibits the highest brightness. Luminescent material according to claim 1, characterized in that x = 5, y = 3, A is chromium and the composite material has the shortest decay time. Luminescent material according to claim 1, characterized in that x = 2, y = 0, A is chromium, and the composite material exhibits the best combination of brightness and decay time. A process for preparing a far-red-region-emitting luminescent material according to claims 1-5, characterized in that a composite material is formed consisting essentially of Y3.yGdyAl5.xGax012: A, wherein x 0 -5 is, y is 0-3, and A is selected from the range consisting of chromium, iron, vanadium, neodymium, dysprosium, cobalt, nickel, titanium or a combination thereof. A method according to claim 7, characterized in that this method further comprises forming the luminescent material on the surface of a cathode ray tube. 9. Combination of a liquid crystal light barrier with a photosensitive layer consisting essentially of α-silicon and an operatively related luminescent material which emits in the far-red region, characterized in that it is a luminescent material according to any one of claims 1-6.