Gas Discharge Display Panels

Abstract

Gas discharge display panels presenting improved electrical characteristics due to the presence, upon the surfaces in contact with the gas being ionised, of a metal oxide whose electronic configuration incorporates an unsaturated f electron-shell for example thorium oxide or gadolinium oxide.

Description

The present invention relates to improvements in gas-discharge display panels and relates more particularly to an improvement in the structure of panels of this kind, by which it is possible to improve the electrical performance and the stability of operation over a period of time.

The improvement in accordance with the invention applies more particularly to panels the technology of which is somewhat specialised and has been described by the present applicants in several of their patents, for example in U.S. Pat. No. 3,706,899.

These panels comprise two slabs, of glass for example, which are relatively thick and therefore rigid, between which there is enclosed the assembly of display cells that is to say the cells of ionisable gas, and the control electrodes.

In a simple embodiment of these panels, which we will describe more explicitly in the course of the invention itself, the control electrodes constituted by two networks of intercrossing conductive bands, are directly deposited upon the mutually opposite faces of the two slabs and covered with a film of transparent dielectric material, for example a vitrified mineral enamel. The assembly is closed off by a sealing ring, bonded between the two slabs and around their peripheries, this ring delimiting the sealed enclosure into which the ionisable gas is introduced. The gas cells can be physically delimited in relation to one another; in this case, an insulating matrix which is pierced by a rectangular network of holes is inserted into the enclosure between the two slabs; and the holes are of course opposite the points of intercrossing between the electrodes of the two crossed networks. In other embodiments, there is no matrix and the display cells are not physically defined; they are delimited, on application of a voltage to the control electrodes, by the electric field configuration which results.

Such panels present two main drawbacks due to the nature of the dielectric material film which covers the electrodes and is in contact with the gas to be ionised.

In other words, this dielectric material in order to have adequate melting properties and therefore be able to be deposited during the manufacture of the panel, is generally made of an enamel containing a large proportion of lead oxide, sometimes cadmium or again zinc. Moreover, in order to be capable of vitrification it contains silica and several of the constituents of glass (alumina, boric anhydride).

This composition means that it has quite a high work function. The striking voltage for providing the gas discharge in the display cells, is consequently high too, this voltage varying in the same sense as the work function of the material at the surface of the dielectric film. The result is that the control voltages applied to the electrodes must themselves be quite high and this is a first drawback of these panels.

Moreover, the stability of the oxides utilised as flux in the enamel, particularly lead oxide, is not very good. These oxides tend to dissociate under the effect of ion bombardment produced by the discharge, and this is translated into terms of a variation of the striking voltages of the discharge during operation. Thus, after some hundreds of hours of operation, these voltages may vary by around 10 percent thus making it a difficult if not impossible matter to achieve proper electrical control of the panel.

The object of the invention is to provide display panels contructed in accordance with this technology and utilising deposits upon thick external slabs, but which do not, or at any rate not to any major extent, exhibit these drawbacks.

A gas-discharge display panel in accordance with the invention comprises two insulating slabs which are sealed along their peripheries, in order to form a sealed enclosure between their opposing faces, said enclosure being filed with an ionisable gas, and two crossed networks of orthogonal electrodes arranged onto the opposing faces of said slabs, said opposing faces being covered with a layer of material containing an oxide of a metal of the lanthanide or actinide families.

This film, having an electronic configuration presenting an unsaturated f electron-shell has a work function which is relatively lower than that of an enamel and a presents higher stability in the face of ion bombardment; it may either be deposited upon the enamel covering the network of electrodes or deposited directly upon the electrode networks themselves.

Other features and results of the invention will become apparent from the following description, illustrated by the attached figures in which.

FIG. 1 is a schematic sectional view of an improved display panel in accordance with the invention, without a matrix;

FIG. 2 is a view identical to that of FIG. 1, of a panel with a matrix.

FIG. 1 illustrates, in section, part of a panel in accordance with the invention, without a matrix; FIG. 2 is equivalent view of a panel in accordance with the invention, with a matrix. In these two figures, similar references indicate similar elements.

As already mentioned, these panels comprise two thick slabs 1 and 2 of an insulating material, at least one of which is transparent (that through which the displayed data are observed). The two slabs are generally made of glass.

On each of these slabs there is deposited a network of parallel conductors 3 and 4 constituting control electrodes. These conductors are deposited in a conventional manner in order to form two crossed networks; the control voltages are applied to their exposed portions outside the enclosure containing the gas which constitutes the display surface proper.

In a preferred embodiment, that shown in the two Figures, the slabs and the electrodes are covered with a dielectric film 5, 6 formed by a film of vitrified enamel in accordance with the prior art technology.

On these dielectric films 5 and 6 there are respectively deposited thin films 7 and 8 of a metal oxide, the electronic configuration of which comprises an unsaturated f electron-shell, that is to say a metal belonging to the lanthanide or actinide. Amongst these oxides, advantageously gadolinium oxide or thorium oxide will be chosen, these having very good thermal stability and having work functions in the order of 2 to 3 electron-volts that is to say much lower than the work functions of the enamel which is between 4 and 5 electron-volts.

A sealing ring 9 hermetically seals the two slabs at their periphery and thus closes off the enclosure 10 which is filled with an ionisable gas. The seal is produced by means of an enamel, a cement, or any other suitable material. The ionisable gas will for example be introduced into the enclosure 10, after sealing, through a hole (not shown) formed in one of the slabs and communicating with a pip (not shown) which enables pumping to be carried out, filling with gas and then sealing.

In the example shown in FIG. 1, the panel has no matrix; the ionisation of the gas is limited substantially to a small cylinder (as indicated at 11 and 12 for two cells in FIG. 1), by the configuration of the electric fields created by the control voltages applied to the electrodes.

In the example of FIG. 2, the matrix 13 of insulating material, containing a rectangular network of cylindrical holes 14, defines said cells.

It should be noted that the metal oxide constituted in films 7 and 8 (thorium oxide will be referred to in the following although the invention is in no way limited to this oxide alone, as mentioned hereinbefore) is deposited solely upon the display surface proprer where the gas discharges are produced; the presence of this oxide on the part serving to bond the two slabs together, could introduce unwanted mechanical stresses.

Thorium oxide is deposited for example by the conventional process of vaporisation under vacuum, using electron bombardment. The film thickness should be sufficient to resist ion bombardment throughout the service life of the panel. It is therefore made more than a few hundred A units in thickness.

In the embodiment shown in FIG. 2, the thorium oxide film can, of course, be deposited solely on the surfaces corresponding to the discharge zones, that is to say at the locations corresponding to the holes 14 in the matrix.

In one possible variant embodiment, the thorium oxide film is deposited directly upon the slabs and itself covers the network of electrodes. Thus it replaces the enamel films 5 and 6. The drawback of this embodiment, is that the thorium oxide film then has to be thicker and since its deposition by vaporisation takes longer, the solution is a more expensive one than the others mentioned before. However, it is one which can be considered.

Although one possible method of depositing thorium, as mentioned before, is vaporisation under vacuum by electron bombardment, a conventional method of depositing thin transparent films of high uniformity and high quality, other methods are possible such for example as cathode-sputtering.

Yet another method can be used to obtain the thorium oxide films at 7 and 8; however, compared with the aforesaid methods it has the drawback that the thorium oxide is no longer deposited solely upon the surfaces which are in contact with the gas being ionised, and this to some extent reduces the advantages of the invention. However, this solution is still preferable to the prior art ones. It consists in mixing with the enamel constituting the films 5 and 6, a small quantity of thorium oxide, either by mixing thorium oxide powder with the enamel powder, prior to the melting of the enamel, or by introducing a small quantity of thorium oxide into the composition of the glass forming the enamel.

It should also be noted that the thorium oxide films can, without any drawback, contain quite a high proportion of metallic thorium or one of the other known refractory oxides.

It goes without saying that everything that has been said with regard to thorium oxide, applies to the other oxides mentioned, and in particular to gadolinium oxide.

The significance of the invention will be better understood from a consideration of an example of the characteristics obtained in true panels filled with one and the same gas mixture and having identical structures with the exception of the fact that the second panel has a thorium oxide film 2,500 A units in thickness, deposited upon each slab on top of an enamel film 25 microns in thickness, whilst the first panel simply has said enamel film alone.

The mean voltages required to maintain discharges are as follows:

1st panel : 180 V peak

2nd panel : 120 V peak

The variations in maintaining voltage for 1000 hours of operation are as follows:

1st panel : 24 V

2nd panel : 9 V

The other characteristics are very similar in both panels.

Claims

What is claimed, is:

1. A gas discharge display panel comprising two insulating slabs which are sealed along their peripheries, in order to form a sealed enclosure between their opposing faces, said enclosure being filled with ionisable gas, and two crossed networks of orthogonal electrodes arranged onto the opposing faces of said slabs, said opposing faces being covered with a layer of material containing an oxide of a metal of the lanthanide or actinide families.

2. A display panel as claimed in claim 1, wherein said film is a film of thorium oxide or gadolinium oxide.

3. A display panel as claimed in claim 2, wherein said oxide film is a thin film deposited upon a film of a dielectric material such as an enamel, itself deposited upon said opposite faces of said slabs.

4. A display panel as claimed in claim 1, wherein said film is a film of a dielectric material comprising a small quantity of thorium oxide or gadolinium oxide.