Impregnated Cathode

The present invention relates to an impregnated cathode and an electron tube, especially a display tube and a pick up tube, using said impregnated cathode, and further to a cathode having a low work function mono-layer required for a particular low temperature operation on a cathode surface and an electron tube using said cathode.

The impregnated cathode formerly used for a low temperature operation is, as indicated in the Japanese Patent Application Laid Open No. 154131/1983, characterized in that it has a constitution of a porous base body consisting of W and Sc.sub.2 O.sub.3 impregnated with electron emissive materials; also it has a mono-layer consisting of Ba, Sc, and O on the cathode surface; and it forms a low work function surface. However, this mono-layer is defective in the following respects that: it is unstable against thermal shock or ion bombardment; it is short in life because of its non-uniform distribution, and also its electron emissive property deteriorates at a low electric field.

An object of the present invention is to provide an impregnated cathode which has a mono-layer of the low work function maintained stably for a longer period and which has uniform work function on the cathode surface. Another object of the present invention is to provide an electron tube using this cathode.

These objects are achieved by an impregnated cathode and an electron tube using this cathode, wherein this cathode is featured in that it is structured by attaching at least two layered thin films, i.e., an under layer consisting of a high melting point metal thin film and an over layer consisting of a high melting point metal layer which contains Sc.sub.2 O.sub.3 and is placed over said under layer, on the surface of the impregnated cathode pellet prepared by impregnating a refractory porous base body with electron emissive materials.

FIG. 1 shows a schematically illustrated cross section of the impregnated cathode in an embodiment of this invention.

FIG. 2 is a graph comparing the electron emissive properties between the cathode in the present invention and the conventional low temperature operation impregnated chathode.

The present invention has proposed a cathode newly structured so as to form a mono-layer which is stable against thermal shock or ion bombardment and is uniform, by using the conventional standard type impregnated cathode (which is made up by impregnating a refractory porous base body with electron emissive materials) for a Ba supply source, by attaching the high melting point metal thin film on the cathode surface so as to smoothen the surface, and by attaching the high melting point metal thin film containing Sc.sub.2 O.sub.3 even over the surface as a Sc and O supply source.

The mono-layer consisting of Ba, Sc and O on the conventional cathode surface is formed by combining Sc.sub.2 O.sub.3, showing no reaction against the electron emissive materials at the time of impregnation, with Ba diffusing out of pores in a porous base body, thereby showing no existence once a supply of Sc.sub.2 O.sub.3 discontinues. In addition, non-reactive Sc.sub.2 O.sub.3 remains a very small quantity, thus being difficult to control.

In the present invention, a high melting point metal thin film containing Sc.sub.2 O.sub.3 such as a thin film of at least one metal selected from the group consisting of W, Mo, Ta, Ir. Os, Re, Ru, Rh, Pd, and Pt is employed as a supply source of Sc.sub.2 O.sub.3 and the film is desirable to have a thickness of 10 nm to 1 .mu.m.

The above-said standard type impregnated cathode surface to work as under layer has pores 5 .mu.m diameter on average. If the above-stated metal thin film is directly formed on this surface, there will be caused some inconveniences, for example, a supply of Ba tends to concentrate on the pores directly under the thin film and the thin film is not likely to be evenly formed. This invention provides an under thin film layer under said metal thin film so as to prevent such inconveniences. A high melting point metal thin film is enough for this under layer, however it is desirable to adopt at least one metal which is selected from the high melting point noble metals such as Os, Re, Pt, Ru, etc. showing low reactivity against the electron emissive materials.

It is also proper to provide micro pores or cracks artificially controlled in this under layer thin film, allowing a structure to easily diffuse Ba onto the over layer thin film. It is recommendable to provide 10 nm to 2 .mu.m for the small pore diameter or for the crack width, also ranging preferably from 10 nm to 1 .mu.m.

For the Ba source, materials capable of supplying Ba like a pressed cathode can also be used in addition to the above-mentioned standard type impregnated cathode.

The following is a description of one of the typical examples of the present invention as shown in FIG. 1.

FIG. 1 is a cross section which schematically illustrates the impregnated cathode in this invention. In this illustration, numeral 1 denotes a pellet of 1.4 mm diameter of the cathode base body materials and is structured of porous tungsten base body 2 prepared by impregnating pore 3 having porosity of from 20 to 25% with the electron emissive materials. The electron emissive materials mixed with BaCO.sub.3, CaCO.sub.3, and Al.sub.2 O.sub.3 at a mole ratio of 4:1:1 are used. It is also proper to use materials mixed at a different mole ratio or to which different substances are added. It is also suitable to use the porous base body of Mo, Ta, Re, Ru, Rh, Pd, Os, Ir, Pt besides W, or an alloy of such substances.

First, a Ta cup 4 is plugged with the pellet 1 and then the Ta cup 4 is laser welded on the upper part inside Ta sleeve 5. Instead of laser welding, soldering is also recommended. Cathode pellet 1 is heated by heater 7 which is core wire 6 coated with alumina and provided in the lower part inside Ta sleeve 5. Such is the standard type impregnated cathode, serving as the Ba source. The Ba supply amount depends on the temperature at which cathode pellet 1 is heated, however it can also be adjusted by altering mole ratios in the composition of the above electron emissive materials or by containing such activators as Zr, Hf, Ti, Cr, Mn, Si, and Al in said base body material.

An Os layer approx. 500 nm in thickness is attached, adopting electron beam bombardment heating, as high melting point metal thin film 8 which is provided on the pellet 1 surface. For the materials to form this film, noble metals like Ru, Rh, Pd, Ir, Pt, Re in addition to Os; high melting point metals like Mo, W, and Ta; and an alloy of these substances can also be used. A thickness of 10 nm to 1 .mu.m is appropriate for the film. The Sc.sub.2 O.sub.3 source is made up by attaching thin film 9 consisting of W and Sc.sub.2 O.sub.3 and having 10 nm to 1 .mu.m, thick, employing the vacuum sputtering method. Instead of W, it is also appropriate to use Mo, Re, Ru, Rh, Pd, Os, Ir, Pt, and Ta or an alloy of these substances. 10 weight % is opted for the content of Sc.sub.2 O.sub.3 in W in this case, and it is recommended to select a range from 1 to 50 weight %.

By using the cathode like this, saturation current density is measured by applying high voltage pulse 5 .mu.s wide and at a repetition cycle of 100 Hz to an anode with a cathode/anode diode configuration. The result of this is shown in FIG. 2. Numeral 10 in the figure denotes the emission characteristics of the cathode which contains 10 weight % of Sc.sub.2 O.sub.3 and which has W thin film 9 about 100 nm thick and Os thin film 8 approx. 500 nm thick. The conventional cathode having no thin films 8 and 9 mentioned above is identical to characteristic 10, however, the characteristic degrades as illustrated by numeral 11 after removal of mono-layer consisting of Ba, Sc and O due to sputtering at Ar atmosphere of 5.times.10.sup.-5 Torr. Whereas the cathode in this invention produces nearly no electron emissive deterioration subsequent to the mono-layer removal and restores characteristic 10 when heated at 1150.degree. C. for a period of 15 min in case the cathode shows any deterioration. The cathode, in addition, has remarkably improved electron emission characteristics under low electric field as compared with a cathode having no smoothing treatment film.

It is obvious from what has been described that the present invention has effects that if the mono-layer consisting of Ba, Sc and O which are all essential to maintain a low work functional condition is destroyed, there can be observed no deterioration in the electron emission characteristic because a mono-layer is newly supplied. And if there should be deterioration in the characteristic, the cathode forms a complete mono-layer if only heated at about 1150.degree. C. for roughly 15 to 30 min, thus maintaining its characteristics of a long service life and a low temperature operation.

Further, since smoothing treatment by the high melting point metal thin film improves deterioration in the electron emission under the low electric field, application to various electron tubes like normal display tubes has created low temperature operational effects.