Cathode for electron tube

Abstract: An oxide-coated cathode for an electron tube comprises a layer (2) of an electron-emissive substance. This layer (2) contains: an alkaline earth metal oxide as a principal component containing at least Ba; an oxide of Sc; and at least one heat-resisting oxide selected from the group consisting of oxides of Aℓ, Si, Ta, V, Cr, Fe, Zr, Nb, Hf, Ta, Mo and W.

Description: BACKGROUND OF THE INVENTION This invention relates to cathodes for electron tubes such as cathode-ray tubes of TV sets and particularly to an improvement in electron emission characteristics of an oxide-coated cathode. Fig. 1 is a sectional view schematically showing a conventional oxide-coated cathode for use in a cathode-ray tube or an image pickup tube for a TV system. In the conventional oxide-coated cathode, an electron-emissive substance layer 2 made of alkaline earth metal oxides containing at least Ba and further containing Sr and/or Ca is formed on a cylindrical base 1 made of Ni as a major element containing a small amount of a reducing element such as Si or Mg. A heater 3 is provided inside the base 1 and the electron-emissive layer 2 is heated by the heater 3 to emit thermal electrons. At this time, main donors for the emission of thermal electrons are free Ba reduced by Si, Mg or the like. Such a conventional cathode is manufactured by a process as described below. First, a suspension of carbonates of alkaline earth metals (Ba, Sr, Ca, etc.) is applied on the base 1 and heated in vacuum by the heater 3. As a result, the alkaline earth metal carbonates are converted to oxides. Then, the alkaline earth metal oxides are partially reduced at a high temperature of 900 to 1100 o C so that they are activated to have a semiconductive property, whereby the electron-emissive layer 2 made of alkaline earth metal oxide is formed on the base 1. In the above described activation process, reducing elements such as Si and Mg contained in the base 1 diffuse to move toward the interface between the alkaline earth metal oxide layer 2 and the base 1, and then react with the alkaline earth metal oxides. For example, if the alkaline earth metal oxide is barium oxide (BaO), the reaction is expressed by the following formula (1) or (2). BaO + 1/2 Si = Ba + 1/2 SiO₂ (1) BaO + Mg = Ba + MgO (2) Thus, the alkaline earth metal oxide layer 2 formed on the base 1 is partially reduced to become a semiconductor of an oxygen deficient type. Consequently, an emission current of 0.5 to 0.8 A/cm² is obtained under the normal condition at an operation temperature of 700 to 800 o C. However, in the cathode thus formed, a current density higher than 0.5 to 0.8A/cm² can not be obtained for the following reasons. As a result of the partial reduction of the alkaline earth metal oxides, an interface layer of oxides or composite oxides such as SiO₂, MgO, and BaO.SiO₂ is formed in the interface region between the base 1 and the alkaline earth metal oxide layer 2 as is obvious from the formulas (1) and (2). Particularly, the interface layer tends to be formed at nickel crystal grain boundaries near the interface region and at a position of about 10µm from the interface into the electron-emissive layer 2. This interface layer is a layer of a high resistance which obstructs flow of current. In addition, it is believed that the interface layer prevents the reducing element in the base 1 from diffusing into the electron-emissive layer 2, and thus, prevents formation of a sufficient amount of Ba for emitting thermal electrons. EP-A-0 210 805 discloses a cathode comprising a base 1 of Ni containing a rare earth metal of 0.1 to 0.5 wt.%. In this cathode, oxidation of the base 1 is prevented when alkaline earth metal carbonates are decomposed to form the electron-emissive layer 2 or when barium oxide is reduced during operation of the cathode. In addition, an interface layer of composite oxides is prevented from being formed in a concentrated manner near the interface between the base 1 and the electron-emissive layer 2, and the composite oxides is formed in a diffused manner in the electron-emissive layer 1. Accordingly, a moderate diffusion of the reducing element such as Si or Mg is maintained. As a result, there is less deterioration of the electron emission characteristics in operation of the cathode even at a high current density of about 1 to 2 A/cm². EP-A-0 210 805 also discloses a cathode comprising an electron-emissive layer 2 containing a rare earth metal oxide of 0.1 to 20 wt.%. Also in this cathode, oxidation of the base 1 is prevented and formation of an interface layer is prevented. The electron emission characteristics of this cathode are little deteriorated in operation even at a high current density of 2A/cm² as in the above mentioned cathode. However, a further improvement is still required. More specifically, if the cathode after the normal activation process is operated at a high current density of more than 2A/cm², it happens that free Ba is considerably evaporated to deteriorate the electron emission characteristics. More precisely, EP-A-0 210 805 discloses a cathode having an electron-emissive layer comprising, as a principal component, an alkaline earth metal oxide containing at least barium, and also comprising a rare earth metal oxide, such as scandium oxide, or a rare earth metal such as scandium. The cathode base contains nickel as a major element upon which the electron-emissive layer is formed. The present invention provides an oxide-coated cathode for an electron tube, having stable emission characteristics in operation at a current density higher than 2A/cm². An oxide-coated cathode for an electron tube according to an aspect of the invention comprises: a base containing Ni as a major element; a reducing agent contained in the base; an electron-emissive substance layer formed on part of the external surface of the base, containing (a) an alkaline earth metal oxide as a principal component containing at least Ba, (b) Scandium oxide, and (c) at least one heat-resisting oxide selected from the group consisting of oxides of Aℓ, Si, Ti, V, Cr, Fe, Zr, Nb, Hf, Ta, Mo, and W; and a heater for heating the electron-emissive layer. An oxide-coated cathode for an electron tube according to another aspect of the invention comprises: a base containing Ni as a major element; a reducing agent contained in the base; a first electron-emissive layer formed on a part of the external surface of the base containing (a) an alkaline earth metal oxide as a principal component containing at least Ba, and (b) scandium oxide; a second electron-emissive layer formed on the first electron-emissive layer, containing (c) an alkaline earth metal oxide as a principal component containing at least Ba and (d) at least one heat-resisting oxide selected from the group consisting of oxides of Aℓ, Si, Ti, V, Cr, Fe, Zr, Nb, Hf, Ta, Mo and W; and a heater for heating the first and second electron-emissive layers. The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. Fig. 1 is a schematic sectional view illustrating a structure of an oxide-coated cathode for an electron tube. Fig. 2 is a graph showing relation between the life test period and the emission current in cathodes according to an embodiment of the invention. Fig. 3 is a schematic sectional view illustrating a structure of a cathode according to another embodiment of the invention. Fig. 4 is a graph showing the relation between the life test period and the emission current in cathodes having the structure of Fig. 3. Referring to Fig. 1, a cathode according to an embodiment of the invention comprises a base 1 including Ni as a major element containing a small amount of a reducing element such as Si or Mg, and a heater 3 in the same manner as in the conventional cathodes. An electron-emissive layer 2 in the cathode of this embodiment contains not only triple alkaline earth metal oxides of Ba, Sr and Ca and a scandium oxide, but also at least one heat-resisting oxide selected from the group consisting of oxides of Aℓ, Si, Ti, V, Cr, Fe, Zr, Nb, Hf, Ta, Mo and W. Those alkaline earth metal oxides are formed by decomposing carbonates as in the prior art and the oxides thus obtained are partially reduced and activated. Referring to Fig. 2 there are shown deterioration curves of electron emission characteristics of cathodes according to the embodiment. Those cathodes are incorporated in diode bulbs so as to be subjected to life tests at a high current density of 2.5 A/cm² and changes in the emission current under the normal condition after the tests were examined. The curve A represents a deterioration of the electron emission characteristics in a cathode comprising an electron-emissive layer 2 of an alkaline earth metal oxide of Ba, Sr, and Ca containing scandium oxide (Sc₂O₃) of 4 wt.% and heat-resisting titanium oxide (TiO₂) of 4 wt.%. The curve B represents a deterioration of the electron emission characteristics in a cathode containing heat-resisting chromium oxide (Cr₂O₃) of 4 wt.% in place of TiO₂. For comparison, the curve C represents a deterioration of the electron-emissive characteristics of a cathode containing Sc₂O₃ of 4 wt.% but not containing TiO₂ nor Cr₂O₃ and the curve D represents a deterioration of the electron emission characteristics of a cathode not containing any of Sc₂O₃, TiO₂ and Cr₂O₃. As is evident from those curves, it is understood that the cathodes containing the heat-resisting oxide Ti₂O₃ or Cr₂O₃ in addition to Sc₂O₃ exhibit less deterioration in the electron emission characteristics during operation at a high current density, compared with the cathodes of the prior art. It is believed that this improvement is obtained because added TiO₂ or Cr₂O₃ prevents evaporation of free Ba as donor for thermionic emission. As a result of observation of the surface of the electron-emissive layer 2 containing TiO₂ or Cr₂O₃ by using the Auger spectral analysis method, it was found that a sufficient amount of Ba exists on particles of TiO₂ or Cr₂O₃. Generally, if high current flows in the electron-emissive layer 2, temperature rises due to Joule heat and an evaporated amount of Ba increases. Accordingly, the increase of the evaporated Ba results in a short life of the cathode. In other words, it is believed that the oxide TiO₃ or Cr₂O₃ absorbs Ba and prevents evaporation thereof, thus, prolonging the life of the cathode even after operation at a high current density. As a result of conducting experiments as to amounts of addition of Sc₂O₃, TiO₂ and Cr₂O₃, it was found that the addition amounts are preferably 0.1 to 20 wt.% for Sc₂O₃ and 0.5 to 10 wt.% for TiO₂ and/or Cr₂O₃. More specifically, if the amount of Sc₂O₃ exceeds 20 wt.%, the initial emission current is lowered and if it is less than 0.1 wt.%, an interface layer can not be effectively prevented from being formed. If TiO₂ or Cr₂O₃ exceeds 10 wt.%, the initial emission current is also lowered and if it is less than 0.5 wt.% conversely, evaporation of Ba can not be effectively prevented. Aℓ₂O₃, SiO₂, V₂O₅, Fe₂O₃, ZrO₂, Nb₂O₅, HfO₂, Ta₂O₅, MoO₃ or WO₃ for example may be used in place of TiO₂ and/or Cr₂O₃. Referring to Fig. 3, there is shown a structure of a cathode according to another embodiment of the invention. The cathode of Fig. 3 is similar to that of Fig. 1, except that the electron-emissive layer 2 in Fig. 3 includes a first sub layer 2a and a second sub layer 2b. Those sub layers can be manufactured by the below described process. First, in order to form the first sub layer 2a, a first suspension is prepared by adding and mixing scandium oxide of 50 wt.% (wt.% after barium carbonate has been converted to an oxide) into a carbonate of Ba. This suspension is applied on the base 1 to a thickness of about 10µm by using a spray. Then, in order to form the second sub layer 2b, a second suspension is prepared by mixing TiO₂ or Cr₂O₃ of 4 wt.% into carbonates of Ba, Sr and Ca. This second suspension is applied on the first suspension layer to a thickness of about 90µm. After that, the carbonates are decomposed in vacuum and an activation process is applied, whereby the cathode of Fig. 3 is completed. Fig. 4 shows the results of life test at a high current density of 2.5 A/cm² for cathodes thus manufactured. The curve E represents a deterioration of the electron emission characteristics in the cathode including the first sub layer of BaO-50wt.%Sc₂O₃ and the second sub layer of (Ba·Sr·Ca)O-4wt.%TiO₂. The curve F represents a deterioration of the electron emission characteristics in the cathode including the second sub layer of (Ba·Sr·Ca)O-4wt.%Cr₂O₃ in place of (Ba·Sr·Ca)O-4wt.%TiO₂. The curves C and D in Fig. 4 are the same as in Fig. 2. As is clear from Fig. 4, it is understood that the cathodes as shown in Fig. 3 exhibit less deterioration in the electron emission characteristics during operation at a high current density compared with the conventional cathodes. The first sub layer may contain an alkaline earth metal oxide containing at least Ba, and Sc₂O₃ and accordingly it may further contain an oxide of Sr or Ca. The thickness of the first sub layer is preferably less than 50µm and more preferably 10 to 20µm. This is because if the first sub layer 2a has a large thickness, the distance for the reducing agents Si and/or Mg in the base 1 to migrate to the second sub layer becomes long. In addition, since the first sub layer is sufficiently thin and a sufficient amount of free Ba is formed in the second sub layer, the initial emission current is not lowered even if Sc₂O₃ of more than 20 wt.% is contained in the first sub layer. On the other hand, the heat-resisting oxide in the second sub layer is contained preferably in the range from 0.05 to 10 wt.% in order to avoid lowering of the initial emission current. In the above described embodiments, a small amount of metal powder of Ni, Co, Fe, Aℓ, Ti, Zr, Hf, Nb, Ta, Mo, W, Mg, Re, Os, Ir, Pt, Pd, Rh, Au, V, Cr, Mn, Cu, Zn, Bi and the like may be added into the electron-emissive layers 2a and 2b and then conductivity of the electron-emissive layers can be improved.

Claims: An oxide-coated cathode for an electron tube, comprising: a base (1) containing Ni as a major element, said base having an external surface; a reducing agent contained in said base (1); a layer (2) of an electron-emissive substance formed on a part of said external surface and containing (a) an alkaline earth metal oxide as a principal component containing at least Ba, and (b) scandium oxide; and a heater (3) for heating said layer (2) of the electron-emissive substance, characterised in that said electron-emissive layer (2) comprises: at least one heat-resisting oxide selected from the group consisting of oxides of Al, Si, Ti, V, Cr, Fe, Zr, Nb, Hf, Ta, Mo and W. The cathode of claim 1, wherein said scandium oxide is Sc₂O₃ in the range from 0.1 to 20 wt.%. The cathode of claim 1, wherein said heat-resisting oxide is contained in the range from 0.05 to 10 wt.%. The cathode of claim 1, wherein said layer of the electron-emissive substance contains a small amount of metal powder for improving conductivity. An oxide-coated cathode for an electron tube, comprising: a base (1) containing Ni as a major element, said base having an extenal surface; a reducing agent contained in said base (1); a layer (2a,2b) of an electron-emissive substance formed on a part of said external surface; and a heater (3) for heating said layer (2a,2b) of the electron emissive substance; said electron-emissive layer comprising: a first electron-emissive sub-layer (2a) formed on a part of said external surface and containing; (a) an alkaline earth metal oxide containing at least Ba; and (b) scandium oxide; a second electron-emissive sub-layer (2b) formed on said first electron-emissive sub-layer (2a) and containing; (c) an alkaline earth metal oxide as a principal component containing at least Ba; characterised in that said second electron-emissive sub-layer (2b) comprises: (d) at least one heat-resisting oxide selected from the group consisting of oxides of Al, Si, Ti, V, Cr, Fe, Zr, Nb, Hf, Ta, Mo and W. The cathode of claim 5, wherein said first electron-emissive layer has a thickness of less than 50 µm. The cathode of claim 6, wherein said first electron-emissive layer has a thickness in the range from 10 to 20 µm. The cathode of claim 5, wherein said heat-resisting oxide is contained in said second electron-emissive layer in the range from 0.05 to 10 wt.% The cathode of claim 5, wherein at least either said first electron-emissive layer or said second electron-emissive layer contains a small amount of metal powder to improve conductivity. An electron tube comprising an oxide coated cathode, which cathode is as claimed in any one of the preceding claims.

Inventors: FUKUYAMA KEIJI MITSUBISHI DENK, MASAKO ISHIDA MITSUBISHI DENKI, SAITO MASATO MITSUBISHI DENKI, SUZUKI RYO MITSUBISHI DENKI K, WATANABE KEIJI MITSUBISHI DENK, FUKUYAMA, KEIJI MITSUBISHI DENKI K.K., MASAKO, ISHIDA MITSUBISHI DENKI K.K., SAITO, MASATO MITSUBISHI DENKI K.K., SUZUKI, RYO MITSUBISHI DENKI K.K., WATANABE, KEIJI MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: H01J1
IPC codes: H01J1