GB2136195A - Cathode Ray Tube - Google Patents

Abstract

A cathode ray tube (1), resistant to burn degradation at high electron beam intensity, having a face-plate (5) of solid phosphor material, a material that is self-supportive, amorphous, translucent and refractory. This material may be of zinc yttrium silicate [ZnY2Si2O8]; of a mixture of zinc oxide and yttrium silicate [(ZnO)x(Y2Si2O7)1-x]; of yttrium oxide; or, of yttrium aluminate; and, may contain rare-earth dopant ions of cerium, terbium or europium. The face-plate (5) may be formed by a hot press, sintering process or by phosphor-volatile binder paste moulding. <IMAGE>

Description

SPECIFICATION Cathode Ray Tube Technical Field The present invention concerns cathode ray tubes, particularly those for high intensity applications such as those intended for aircraft head-up display and other types of projection display.

For projection display, high luminance is required. A luminance of 70,000 lumen/m2 (20,000 ft-lamberts) is a typical requirement.

Tubes for this application, therefore, must support and withstand high density energetic electron energisation and most exhibit resistance to "burn" under intense electron bombardment.

Background Art In a conventional cathode ray tube, particulate cathodoluminescent phosphor material is provided as a powder deposit on the internal faceplate surface of an evacuated lead;glass envelope. The rear surface of this deposit is coated with conductive material-for example evaporated aluminium, which latter provides the tube anode. For high intensity application, however, under intense electron bombardment, the phosphor material can become depleted, the glass can melt and reform at regions where the electron beam is most intense, and phosphor particles can become embedded in the glass.

These effects result in obscuration and nonuniformity of the display field, and in consequence the useful tube lifetime tends to be impractically short for high luminance application.

Recently, it has been disclosed that the powder phosphor material zinc yttrium silicate (Zn Y2 Si2 8) has superior properties, and in particular it exhibits a high resistance to "burn" degradation (of United Kingdom Patent Application GB 2,094,823). Notwithstanding, because of the high localised temperatures attained for electron absorption under intense bombardment, the utility of such tubes is limited by the melting and reformation of the face-plate glass.

One approach to this problem has been to replace the powder phosphor and glass face-plate by a face-plate of single crystal phosphor material. One example of this approach has been reported recently (Appl. Phys Lett Vol 37 No 5 pp 471-2 Sept. 1980). The high intensity projection television tube, described therein, comprises a face-plate of yttrium aluminium garnet (YAG) single crystal thermo-compression bonded by an aluminium sealant to a tube body of high density sintered alumina. Fabrication of this tube, however, requires complex and expensive technology.

Neither do single crystal face-plate cathode ray tubes prove wholly satisfactory for high intensity applications. Much of the light emitted within the crystal is totally internally reflected and emerges only at the extremities of the face-plate. The optical efficiency is low, since only a small proportion of the generated light actually leaves the front surface of the face-plate.

Disclosure of the Invention The invention is intended to provide a remedy; a cathode ray tube that will withstand the intense electron bombardment necessary for high luminance display.

It has been found here that the material zinc yttrium silicate can be formed into a dense, robust, translucent, refractory solid. Furthermore, plates of this solid material have been found here to exhibit a light output and an efficiency comparable to those of a powder on-glass deposit of the same material. Such material may be produced in the form of a self-supportive faceplate, and is resistant to "burn" degradation.

Alternative phosphor materials having similar properties are yttrium oxide, as also yttrium aluminate.

In accordance with this invention, therefore, there is provided a cathode ray tube having a face-plate formed of an amorphous, translucent, refractory, solid phosphor material.

The term "translucent" as used herein, is intended to include "transparent".

Preferably, the solids phosphor material is of zinc yttrium silicate (ZnY2 Si2 Os) or a zinc oxideyttrium silicate mixture (ZnO)x (Y2 Si2 7)1-xe This material may contain different dopants, or mixtures of dopants, to produce selected colour emissions-for example, cerium ions (blue emission); europium ions (red emission); and terbium ions (green emission).

Alternatively, the face-plate may be of yttrium oxide, or of yttrium aluminate and again may contain rare-earth dopant ions such as any of those mentioned above.

Brief Introduction of the Drawings In the drawings accompanying this specification; Figure 1, shows in cross-section a cathode ray tube, with a solid phosphor face-plate; and, Figure 2, is a graph showing spectrograms measured for a phosphor deposit-on-glass faceplate, and for a hot pressed solid phosphor faceplate respectively.

Description of the Embodiments Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings.

There is shown in Figure 1 a cathode ray tube 1 comprised of a tubular lead-glass body 3 with a solid phosphor face-plate 5 sealed onto this body 3 at one end. This seal comprises an annular housing 7 of machined glass ceramic which has been glass frit bonded onto the tubular body 3.

The face-plate 5 fits into a recess at the front of the annular housing 7 and has been sealed to this by a malleable silver chloride sealant 9. This sealant has been introduced in the form of a ring cut from rolled sheet material and has been melted in vacuum, with the housing and face plate in forced abutment, until it has flowed across the entire interface between the abutting surfaces of the tube 3 and the face-plate 5. The seal is of sufficient malleability to accommodate any differential expansion of the adjoined parts.

An anode contact 11 is sealed into the side of the tube 3; and, this contact 11 and the rear surface of the face-plate 5 have been covered by a layer 13 of evaporated aluminium material, which later serves as the cathode ray tube anode. A cathode gun and focusing optics (not shown) are assembled in the tube 3 at its other end and the tube is evacuated, heat treated to remove all residual gas, and sealed.

The solids phosphor face-plate 5 is formed of a translucent refractory amorphous material, the material zinc yttrium silicate (Zn Y2 Si2 0s), and is self-supporting, of about 2.5 cm (1") diameter and 2 mm thickness.

This face-plate 5 may be formed by either one of the two methods described below: Method 1 The powdered phosphor is formed into a plate of the required diameter and thickness by making a thick paste of the powder with a volatile liquid binder. A solution of ammonium fluoride, or ammonium chloride in water, or various organic liquids such as alcohol prove suitable binders for zinc yttrium silicate. This thick paste is placed into a mould, and is dried out and heated to at least 1 35O0C for a period of at least four hours. The hard plate resulting is then ground and polished prior to tube assembly.

Method 2 Alternatively the face-plate can be produced by a hot press sintering process; a fast and efficient process. In this case the powdered phosphor is loaded into a press and moulded into the required shape. It is heated to a temperature of at least 13500C in an atmosphere of air whilst at the same time a mechanical pressure of approximately 1 5 hectobar (10 ton f/in2) is applied. The plate thus formed is hard and of high density (99.9% of the theoretical density value), and is translucent. Again this plate is polished prior to tube assembly.

Plates, formed as above, have been mounted in an electron microscope and the light output measured for comparative test. Such tests show that the light output and efficiency of these refractory plates are indeed comparable to, and marginally better than, those that obtain in the case of an equal thickness powder deposit-onglass. This can be seen from an inspection of the spectrograms shown in Figure 2.

An alternative to zinc yttrium silicate, a nonstoichiometric compound of zinc oxide-yttrium silicate ((no), (Y2 Si 07)1x) may be substituted.

Alternatively, plates of yttrium oxide or yttrium aluminate may be produced and used in manner similar to that already described.

Claims (9)

1. A cathode ray tube having a face-plate formed of amorphous, translucent, refractory, solid phosphor material.

2. A cathode ray tube as claimed in claim 1, wherein the face-plate material is of zinc yttrium silicate [Zn Y2 Si2 Oa] or of a non-stoichiometric zinc oxide-yttrium silicate mixture [(ZnO), (Y2 Si 0,)ix].

3. A cathode ray tube as claimed in claim 1, wherein the face-plate material is of yttrium oxide.

4. A cathode ray tube as claimed in claim 1, wherein the face-plate material is of yttrium aluminate.

5. A cathode ray tube as claimed in any one of the preceding claims 2 to 4, wherein the faceplate material contains dopant ions of any one or more of the rare-earth elements:-- cerium; terbium; or, europium.

6. A cathode ray tube constructed, adapted, and arranged to operate, substantially as described hereinbefore with reference to and as shown in the accompanying drawings.

7. A face-plate, for a cathode ray tube, formed of amorphous, translucent, refractory solid phosphor material.

8. A face-plate as claimed in claim 7 above of zinc yttrium silicate; of a mixture of zinc oxide and yttrium silicate; of yttrium oxide; or, of yttrium aluminate.

9. A face-plate is claimed in claim 8 formed by a process wherein the phosphor powder is loaded into a press, pressed into shape and heated to sinter temperature, to form a solid plate.