Abstract:
A gas discharge display panel adapted for effecting display by a plurality of gas discharge elements arranged in a matrix has a cathode formed by coating the substrate surface with a (100) preferred film of a rare earth hexaboride such as LaB 6 . The gas discharge display panel can be driven at relatively low voltage.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a gas discharge display panel adapted for effecting display by means of a plurality of gas discharge elements arranged in a matrix, and also pertains to a cathode for use in such gas discharge display panel. More particularly, the present invention is concerned with a gas discharge display panel that employs as a cathode material rare earth hexaborides, known as substances which exhibit excellent electron emission characteristics, and also pertains to a cathode for use in such display panel. 
     This type of gas discharge display panel is disclosed in, e.g., U.S. Pat. No. 4,206,386. One example of conventional gas discharge display panels is shown in FIG. 1. As illustrated, this device has a two-electrode group structure in which a Y-direction electrode group K R , K 1 , K 2 , K 3 , K 4 , and K 5  adapted to operate as cathode elements and an X-direction electrode group A 1 , A 2 , A 3  and A 4  adapted to operate as anode elements are provided between glass plates 1, 2 so that the two electrode groups extend orthogonally to each other. Relatively long and narrow strip-like dielectrics 16 are provided so as to extend parallel to respective anode elements, thereby partitioning the discharge space into individual discharge areas which are vacuum-sealed in a gas that emits light when an electric discharge occurs therein. Discharge dots 15 are respectively formed at intersections between the X-Y electrode elements. 
     It is strongly demanded to lower the driving voltage in such gas discharge display panel with the view to simplifying the driving circuit, reducing the costs and extending the lifetime of the panel itself. 
     The application of rare earth hexaborides, represented by LaB 6 , to cathode elements of a gas discharge display panel has already been practiced in DC discharge display panels, and it has been confirmed that rare earth hexaborides are effective in lowering the discharge maintenance voltage (see the Television Society Technical Report, ED-572 , 1981, or the specification of Japanese Patent Laid-Open No. 62647/1980). However, in these examples, LaB 6  film is formed by plasma spray coating or thick film printing, and in these methods a polycrystalline film having irregular crystalline orientation is employed. In addition, LaB 6  film may also be formed by electron beam heating (see the specification of Japanese Patent Publication No. 17780/1981). In this case, however, no consideration has heretofore been taken for the preference of a crystal plane of a film deposited by evaporation. 
     The discharge maintenance voltage in a gas discharge display panel can be made lower as the work function of the cathode thereof is decreased. However, the work function of rare earth hexaborides such as LaB 6  greatly differs in accordance with the kind of crystal plane. Accordingly, conventionally employed LaB 6  films, which are polycrystalline films or in which no consideration is taken for the preference of the crystal plane, have variations in discharge maintenance voltage and have not satisfactorily lowered the discharge maintenance voltage. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a gas discharge display panel which employs a rare earth hexaboride and which is very effective in lowering the discharge maintenance voltage, together with a cathode for use in said display panel. 
     To this end, the present invention provides a gas discharge display panel adapted for effecting display by means of a plurality of gas discharge elements arranged in a matrix, the display panel comprising a cathode formed by coating the substrate surface with a (100) preferred film of a rare earth hexaboride. The present invention also provides a cathode for use in the abovedescribed display panel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary perspective view of a gas discharge display panel; 
     FIG. 2 is a graph showing the relationship between the film thickness of an LaB 6  film and the FWHM (Full Width Half Maximum) of (100) diffraction rocking curve; 
     FIG. 3 shows X-ray diffraction patterns of LaB 6  thin film, in which: 
     FIG. 3(a) shows an X-ray diffraction pattern of LaB 6  finely-ground powder; 
     FIG. 3(b) shows an X-ray diffraction pattern of an LaB 6  thin film having a film thickness of 3,500 Å; 
     FIG. 3(c) shows an X-ray diffraction pattern of an LaB 6  thin film having a film thickness of 8,000 Å; and 
     FIG. 3(d) shows an X-ray diffraction pattern of an LaB 6  thin film having a film thickness of 20,000 Å. 
     FIG. 4 is a discharge characteristic chart showing the relationship between the discharge maintenance voltage and the gas pressure. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The work function of rare earth hexaborides greatly differs in accordance with the kind of crystal plane. In the case of, for example, LaB 6 , its work function is 2.5 eV, 2.7 eV, 2.9 eV and 2.8 eV for (100), (110), (111) and (321) planes, respectively. Thus, the work function of LaB 6  is lowest at the (100) plane. The discharge maintenance voltage of a gas discharge display panel can be lowered as the work function of its cathode is lowered. Accordingly, if the cathode surface is coated with a rare earth hexaboride having a (100) plane, it is possible to drive the gas discharge display panel at the lowest driving voltage. 
     According to the present invention, among the crystal planes of a rare earth hexaboride, the (100) plane, which has the lowest work function, is selected and coated on the cathode substrate surface so as to be exposed, whereby the discharge maintenance voltage of the gas discharge display panel is lowered most efficiently. 
     Methods of obtaining the above-described (100) preferred film of a rare earth hexaboride are not particularly limited, but the electron beam evaporation method is generally preferable. When a rare earth hexaboride thin film is formed by this method, the thin film easily has an amorphous structure when the film is extremely thin. However, when the film thickness is increased to a certain extent, a (100) preferred film can be obtained. It is only necessary for the film thickness at this time to be more than about 5,000 Å, although it depends on various conditions in which the thin film is formed. FIG. 2 shows the effect of the film thickness on FWHM (Full Width Half Maximum) of the (100) diffraction rocking curve of lanthanum hexaboride (LaB 6 ) film. When the film thickness exceeds 5,000  Å, the FWHM decreases, and the preference of the crystal film is improved. Therefore, the film thickness of a rare earth hexaboride film in accordance with the present invention is preferably 5,000 Å or more. Although the upper limit of the film thickness is not particularly restricted, the preferable upper limit is 10 μm from the economical point of view. Accordingly, a preferable film thickness range is from 5,000 521  to 10 μm. Although it has already been mentioned that the rare earth hexaboride film in acocrdance with the present invention is excellent in preference, it is preferable to employ a film whose FWHM of the (100) diffraction rocking curve is 8° or less as shown in FIG. 2. 
     Examples of the present invention and advantages offered thereby will be described hereinunder in detail. 
     EXAMPLE 1 
     Among the rare earth hexaborides, LaB 6  was selected, and an LaB 6  thin film was prepared by the electron beam evaporation method. As an evaporation material, a single crystal material grown by an infrared ray heating-floating zone method from a polycrystalline material was employed. As an evaporation substrate, soda glass was employed and heated at 350°. The degree of vacuum at the time of evaporation was 5×10 -6  Torr. FIG. 3 shows diffraction patterns obtained by subjecting LaB 6  thin films having various thicknesses to the X-ray diffraction by characteristic X-rays of CuKα. Diffraction conditions were 40 kV, 30 mA and 1×10 3  cps on full scale. FIG. 3(a) shows the results of the X-ray diffraction in the case where LaB 6  finely-ground powder was employed as a standard sample. Although the strongest peak appeared at (110) plane, the pattern represented well a simple cubic structure of LaB 6 . FIG. 3(b) shows a diffraction pattern in the case of an LaB 6  film having a film thickness of 3,500 Å. Although a slight diffraction peak was recognized at the (100) plane, no peak was present at the other planes, and this film generally had an amorphous structure. FIG. 3(c) shows a diffraction pattern of an LaB 6  film having a film thickness of 8,000 Å. In this case, the diffraction peaks of the (100), (110) and (200) planes were clearly observed. However, the diffraction peak of the (100) plane was stronger than that of the (110) plane, and preference was recognized in which the respective (100) planes of crystallites within the evaporated film extended parallel to the substrate. FIG. 3(d) shows a diffraction pattern of an LaB 6  film having a film thickness of 20,000 Å. In this case, strong diffraction peaks of (100) and (200) planes were observed, and it has been found that the (100) plane is strongly preferred with respect to the substrate and the crystallizability is also improved. It may be concluded from the above that, when an LaB 6  thin film is formed by the electron beam evaporation method, a portion of the film which is deposited in an early stage of evaporation to a thickness of 5,000 Å is easily affected by the glass substrate so as to have an amorphous structure, but as the thickness of the evaporated film increases, it becomes easy for a (100) preferred film of LaB 6  to be grown. 
     No conventional method has succeeded in producing a strong (100) preferred film such as that shown in this example by evaporation (see the specification of Japanese Patent Publication No. 17780/1981). It may be considered that, since the prior art employs an LaB 6  sintered material as an evaporation material, the degree of vacuum at the time of evaporation may be unfavorably low, and a relatively large amount of impurities may be included; therefore, conditions are not suitable for producing an LaB 6  film of high quality. 
     EXAMPLE 2 
     The effectiveness of employing a (100) preferred film of LaB 6  as a cathode material has been confirmed by an actual gas discharge display panel. 
     A multiplicity of Ni cathode lines were provided on a soda glass substrate at a pitch of 0.2 mm and a line width of 0.1 mm by thick film printing. An LaB 6  film having a thickness of about 10,000 Å was deposited thereon by the electron beam evaporation method under the conditions mentioned in Example 1. The LaB 6  film was confirmed that it was a (100) preferred film by means of X-rays as shown in FIG. 3. A multiplicity of Ni anode lines were formed on a transparent soda glass at a pitch of 0.2 mm and a line width of 0.03 mm by thick film printing. Then, the two soda glasses were laid one upon the other in such a manner a vacuum space is formed therebetween in which the cathode and anode lines face and extend orthogonally to each other. Then, Ne-4% Ar gas was introduced into the vacuum space at various gas pressures, and a predetermined voltage was applied to the intersections between the cathode and anode lines to produce a plasma, thereby examining effects of the gas pressure on the discharge maintenance voltage. The results of the experiment is shown as a discharge characteristic curve 3 in the graph of FIG. 4. FIG. 4 also shows the discharge characteristic curve 1 of a conventional Ni cathode - Ni anode structure and the discharge characteristic curve 2 of a polycrystalline LaB 6  cathode - Ni anode structure for comparative purposes. The polycrystalline LaB 6  cathode lines were formed from LaB 6  powder by thick film printing. It may be understood from FIG. 4 that the gas discharge display panel according to the present invention which includes cathode lines coated with a (100) preferred film of LaB 6  and Ni anode lines has a discharge maintenance voltage lower than those of the display panels respectively including Ni cathode lines and polycrystalline LaB 6  cathode lines. This shows that the (100) plane which has a relatively low work function in the LaB 6  crystal effectively acts to lower the discharge maintenance voltage of the gas discharge display panel. 
     EXAMPLE 3 
     Other rare earth hexaborides than LaB 6 , i.e., CeB 6 , PrB 6 , NdB 6 , SmB 6  and GdB 6  were employed to form evaporated films similar to that described in Example 1. It has been confirmed from experiments that the (100) plane which has a relatively low work function is effective in lowering the discharge maintenance voltage in the gas discharge display panel to substantially the same extent as that of the example shown in FIG. 4. 
     As will be clear from the above-described examples, it is possible, according to the present invention, to greatly lower the discharge maintenance voltage of a gas discharge display panel by coating the substrate surface with a (100) preferred film of a rare earth hexaboride. Therefore, the present invention can contribute to simplifying the driving circuit, reducing the costs and extending the lifetime of the panel itself. Thus, the present invention is of great industrial value.