Impregnated Cathode

This invention relates to an impregnated cathode for use in an electron tube such as a display tube, a picture tube, a pick-up tube, a traveling wave tube (TWT), etc. as a high current density cathode, and particularly to an impregnated cathode with higher electron emission.

The impregnated cathode is a high current density cathode and is promising as a cathode for higher quality, particularly higher resolution and higher brightness of en electron tube.

The conventional impregnated cathode has such a basic structure that a refractory porous body composed of W, etc. is impregnated with an electron emissive material composed of a barium (Ba) compound, and has a high electron emission property, but its operating temperature necessary for obtaining the necessary current density of 10 A/cm.sup.2 for the higher quality is as high as 1,100.degree.-1,200.degree. C., which is by about 400.degree. C. higher than that of the spray type oxide cathode now generally used. Owing to the high operating temperature, the electrode material must be a high melting point metal when it is practically used in an electron tube, and furthermore, a large amount of Ba and BaO (barium oxide) evaporates from the cathode and deposits onto the electrode, causing a grid emission and bringing an adverse effect on the electron tube characteristics. Furthermore, it is very difficult to design and produce a reliable heater capable of heating the impregnated cathode for a long duration. Thus, it is the most important task in the research and development of an impregnated cathode to lower the operating temperature of the impregnated cathode. In order to lower the operating temperature, the electron emission must be increased, and as a result the operating temperature will be lowered. According to a procedure for lowering the operating temperature, that is, a procedure for increasing the electron emission, as disclosed in Japanese Patent Publication No. 47-21343, the work function of the cathode is lowered by coating the cathode surface with a metal having a high work function such as an osmium (Os) -ruthenium (Ru) alloy, etc., thereby enhancing the electron emission, where the operating temperature of the impregnated cathode can be lowered by about 100.degree.-150.degree. C., which is still by 250.degree. C. higher than the operating temperature of the spray-type oxide cathode. Some of the present inventors thus proposed an impregnated cathode provided with a thin film composed of a high melting point metal and at least one of Sc and an oxide of Sc on the electron emissive surface of a refractory porous body [Japanese Patent Application Kokai (Laid-open) No. 51-13526]. The operating temperature of the cathode could be made lower by 100.degree.-150.degree. C. than that of the impregnated cathode coated with the said Os-Ru alloy, but the cathode had a little shorter life.

An object of the present invention is to provide a highly reliable impregnated cathode having a higher electron emission at a low operating temperature and a longer life.

This and other objects of the present invention can be attained by an impregnated cathode which comprises a refractory porous body base metal, whose pore parts are impregnated with an electron emissive material including barium, and a thin film layer comprising tungsten and at least one member selected from the group consisting of scandium and an oxide of scandium on the surface of the base metal, characterized in that the thin film layer contains at least one oxide selected from the group consisting of an oxide of tungsten and an oxide of tungsten and scandium.

FIG. 1 is a schematical cross-sectional view of an impregnated cathode according to one embodiment of the present invention.

FIGS. 2 to 4 are diagrams showing the characteristics of the present impregnated cathode.

The base metal of the present impregnated cathode is a so far known ordinary cathode, that is, a refractory porous body made of W, Mo, Ir, Pt, Re or alloy powder containing these metal elements, whose pore parts are impregnated with an electron emissive material including Ba. The electron emissive material is generally based on a Ba.sub.3 Al.sub.2 O.sub.6 compound and further contains such oxides as CaO, SrO, MgO, ZrO.sub.2, Sc.sub.2 O.sub.3, Y.sub.2 O.sub.3, etc. to improve the electron emission property and the control of Ba evaporation. A thin film layer to be coated onto the cathode surface can be formed by sputtering evaporation, chemical vapor deposition (CVD), etc.

FIG. 1 is a schematic cross-sectional view of an impregnated cathode according to one embodiment of the present invention, where numeral 1 is a refractory porous body, 2 pores impregnated with an electron emissive material, 3 a thin film layer, 4 a barrier layer, 5 a sleeve, 6 a heater and 7 an alumina coating.

At first, the cathode surface was coated with a thin Sc.sub.2 O.sub.3 layer by sputtering evaporation, and the electron emission property thereof was measured. It was found that it had a higher electron emission property than that of the underlayer cathode. Then, oxides containing W and Sc, that is, Sc.sub.2 W.sub.3 O.sub.12 and Sc.sub.6 WO.sub.12, were synthesized and coated onto the cathode surface. It was found that they had a higher electron emission property than that of the thin Sc.sub.2 O.sub.3 film coated on the cathode surface, and further that Sc.sub.2 W.sub.3 O.sub.12 had a higher electron emission property than that of Sc.sub.6 WO.sub.12. Furthermore, sputter targets of various compositions were prepared from Sc.sub.2 O.sub.3 powder and W powder, and coated onto the cathode surface. As a result of measuring their electron emission properties, it was found that the equivalent electron emission properties could be obtained at a lower operating temperature by about 250.degree. C. Thus, it was found that it was an effective means or increasing the electron emission property of a cathode to provide a thin film layer composed of W and an oxide containing Sc and W on the cathode surface. Furthermore, it was found that coating with a multi-component target of W and Sc.sub.2 W.sub.3 O.sub.12 in a layer also had the similar effect. Further studies revealed that the composition of a thin layer film having higher electron emission properties than that of the impregnated cathode coated with a metal such as Os-Ru alloy, etc. was 2 to 50% by weight of Sc.sub.2 W.sub.2 O.sub.6 or Sc.sub.2 W.sub.3 .sub.12, most of the balance being W, and a remarkable effect was obtained in a film thickness range of 10 nm to 10 .mu.m, preferably 50 to 1,000 nm, where such a metal as Mo, Re, Pt, Ir, Ta, etc. or their alloys may be contained in an amount of less than 50% by weight of W, and this will be also applicable to the examples which follow.

It was also found that an impregnated cathode coated with a thin film layer composed of W, tungsten oxides such as WO.sub.2, and Sc.sub.2 O.sub.3 had substantially the same effect.

At first, a cathode surface was coated with a thin Sc.sub.2 O.sub.3 film layer by sputtering evaporation and its electron emission property was measured, and found to be increased. Then, another cathode surface was coated with a W and Sc.sub.2 O.sub.3 layer, and its electron emission property was measured, and found to be considerably increased, that is, the equivalent electron emission property could be obtained at a lower operating temperature by about 200.degree. C. Furthermore, it was found that the electron emission property could be much more increased by adding WO.sub.2 to the W and Sc.sub.2 O.sub.3 layer. That is, the equivalent electron emission property could be obtained at a much lower operating temperature by about 50.degree. to 100.degree. C., and the thin film layer composed of W, WO.sub.2 and Sc.sub.2 O.sub.3 was found to be effective for improving the electron emission property. A further detailed test revealed that the composition having such a high electron emission property was 2 to 30% by weight of Sc.sub.2 O.sub.3 and not more than 50% by weight of Sc.sub.2 O.sub.3 +WO.sub.2, the balance being W, and the remarkable effect was obtained in a film thickness range of 10 nm to 10 .mu.m, preferably 50 to 1,000 nm.

The present thin film layer is composed of W, WO.sub.2 and Sc.sub.2 O.sub.3, where no influence has been found on its characteristics even by replacing a portion of W with W.sub.3 O.

The present thin film layer can be prepared also by oxidizing W in the thin film layer composed of Sc and/or Sc.sub.2 O.sub.3 land W, for example, by introducing an oxidizing gas or vapor such as a well controlled oxygen gas, water vapor, etc. during the deposition of a thin film when the said conventional cathode is prepared. The amount of Sc and/or Sc.sub.2 O.sub.3 is preferably 1 to 30% by weight, and it is preferable to oxidize 1 to 50% by weight of total W amount, where the oxide may be in the form of oxides only of W such as WO.sub.2, WO.sub.3, etc., or in the form of oxides of W and Sc such as Sc.sub.2 W.sub.3 O.sub.12. The preferable thickness of the thin film layer is as described above.

In the present impregnated cathode of the said structure, the refractory porous body reacts with the electron emissive material in the impregnated cathode underlayer by heating the cathode by the heater to form Ba, and Ba reaches the cathode surface through the pores, whereas Sc and O (oxygen) are supplied to the cathode surface from the thin film layer, and Ba combines with Sc and O on the cathode surface to form a very thin (Ba, Sc, O) complex compound layer in the mono-layer order. By formation of the (Ba, Sc, O) complex compound in the mono-layer order on W, the work function is lowered from about 2.0 e.V to about 1.2 e.V. Thus, it seems that a surface of low work function is formed by providing a thin film layer on the surface of the conventional impregnated cathode, and a decrease in the work function contributes to an improvement of the electron emission property and further to a decrease in the operating temperature. Formation of the very thin (Ba, Sc, O) complex compound layer in the mono-layer order has been identified by Auger electron spectroscopy.

While the cathode is working, supply and evaporation of these elements are balanced and brought into a steady state, where O is supplemented by decomposition of oxides of W or oxides of W and Sc in the thin film.

The present invention will be described in detail below, referring to Examples and the accompanying drawings.

In FIG. 1, the present impregnated cathode is schematically shown in cross-section, where numeral 8 is a pellet, 1.4 mm in diameter, of cathode material, composed of a porous W body 1 having a porosity of 20 to 25% and pores 2. The pores 2 are impregnated with a mixture of BaCO.sub.3, CaCO.sub.3 and Al.sub.2 O.sub.3 in a molar ratio of 4:1:1 as electron emissive materials. Electron emissive materials in different molar ratios or containing different kinds of materials may be used. The pellet 8 is placed in a Ta cap 4, which is then laser welded to a Ta sleeve 5. A soldering material may be used in place of the laser welding. A heater comprising a W core wire 6 coated with alumina 7 is used for heating the cathode. The foregoing is a Ba supply source. The rate of Ba to be supplied depends on a heating temperature, but can be adjusted by changing the molar ratio of the electron emissive material or adding such an activator as Zr, Hf, Ti, Cr, Mn, Si, Al, etc. to the base metal material. As a Sc.sub.2 O.sub.3 supply source, a thin film 3 having a thickness of 10 nm to 10 .mu.m, composed of W and Sc.sub.2 O.sub.3, is deposited onto the pellet 8 by vacuum sputtering.

Before the vacuum sputtering, the oxygen partial pressure in a sputtering chamber is adjusted to 1.times.10.sup.-5 to 1.times.10.sup.-4 Torr by introducing an oxygen gas of high purity (99.9%) thereto through a gas regulator, while measuring the oxygen partial pressure by a small mass spectrometer provided at the sputtering vessel.

By this operation, W in the thin film 3 can be oxidized. It is also possible to oxidize only a portion of the thin film 3 by introducing an oxygen gas under the premeasured partial pressure in the course of sputtering. Other oxidizing gases than the oxygen gas can be introduced in place of the oxygen gas. The degree of W oxidization can be determined by measuring the electrical resistivity of a thin film sample deposited on a glass plate in advance or by X-ray photo-electron spectroscopy.

With this cathode, a saturation current density is measured by applying high pulse repetitions of 100 Hz with a width of 5 .mu.S to the anode according to a cathode-anode diode configuration. The results as shown in FIG. 2 are obtained, where line 9 shows the characteristics of the cathode coated with a thin film composed of W and Sc.sub.2 O.sub.3 according to the present invention, line 10 shows characteristics of the cathode coated with a thin film without oxidation treatment, and line 11 shows the characteristics of the cathode without the thin film. The present cathode has a life of more than 20,000 hours at 900.degree. C.

An impregnated cathode underlayer is prepared from a porous W body 1 having a porosity of 23%, prepared by press molding W powder having particle sizes of 5 .mu.m, and subjecting the molding to presintering in hydrogen and then to sintering in vacuum. Then, an electron emissive material having a composition of 4BaO.CaO.Al.sub.2 O.sub.3 is melted by heating in a hydrogen atmosphere, and the porous W body is impregnated with the molten electron emissive material to prepare the impregnated cathode underlayer.

A thin film layer 3 for the impregnated cathode according to the present invention is formed in an R.F. sputtering chamber. The composition of the thin film layer 3 is determined by inductively coupled plasma spectroscopy (ICPS method) and by fluorescence X-ray analysis (FLX method), and W and oxides containing W and Sc (Sc.sub.2 W.sub.3 O.sub.12 and Sc.sub.6 WO.sub.12) are confirmed by X-ray diffraction. Sputtering targets are prepared by mixing W powder and Sc.sub.2 W.sub.3 O.sub.12 or Sc.sub.6 WO.sub.12 powder synthesized in advance in various mixing ratios and press molding the resulting mixtures. Then, the impregnated cathode underlayer and the target composed of W and Sc.sub.2 W.sub.3 O.sub.12 or Sc.sub.6 WO.sub.12 are placed in the sputtering chamber, and, after the chamber has been evacuated to the order of 10.sup.-7 Torr, the thin film layer 3 composed of W and Sc.sub.2 W.sub.3 O.sub.12 or Sc.sub.6 WO.sub.12 is formed on the surface of the impregnated cathode underlayer in an Ar gas atmosphere in the order of 10.sup.-2 Torr by introducing an Ar gas into the chamber. The thin film layer 3 is formed from the targets of various compositions, and the thickness of the thin film layer 3 is changed by adjusting the sputtering time.

The electron emission property of the present impregnated cathode 8 provided with the thin film layer 3 thus formed is determined by applying a positive pulse voltage to the anode according to a cathode-anode diode configuration in a vacuum chamber in the order of 10.sup.-9 Torr. Typical results are shown in FIG. 3, where line 11 shows the electron emission characteristics of the conventional impregnated cathode underlayer, line 10 those of the metal film-coated, impregnated cathode, as coated with Os-Ru alloy to a layer thickness of 500 nm, and line 12 those of the present impregnated cathode provided with the thin film layer 3. The composition and the thickness of the thin layer film 3 shown in FIG. 3 are 93 wt.% W--7 wt.% Sc.sub.2 W.sub.3 O.sub.12, as calculated from the analytical results and 210 nm, respectively.

Decrease in the operating temperature is determined from the characteristics 12 of the impregnated cathode 8 obtained according to the present invention. The present impregnated cathode can be operated at a lower temperature at least by 250.degree. C. than that of the conventional impregnated cathode underlayer (characteristics 11) and at least by 100.degree. C. than that of the conventional Os-Ru-coated, impregnated cathode (characteristics 10). Furthermore, the amount of evaporated barium and barium oxide is measured by mass spectrometry, and has been found to decrease proportionately to lowered operating temperature. Specifically, it has been found to decrease by the order of 1-1.5, as compared with that of the conventional impregnated cathode underlayer. By lowering the operating temperature at least by 100.degree.-250.degree. C., the power consumption decreases without changing the electrode material of a bulb, and furthermore the heater can have a life of a few ten thousand hours, which is substantially equivalent to that of the spray-type oxide cathode as heated. Thus, a highly reliable impregnated cathode can be obtained in the present invention.

A conventional impregnated cathode underlayer is prepared from a porous W body 1 having a porosity of 23%, prepared by press molding W powder having particle sizes of 5 .mu.m, subjecting the molding to presintering in hydrogen and then to sintering in vacuum, and impregnating the sintered molding with a molten electron emissive material having a composition of 4BaO Al.sub.2 O.sub.3.CaO in a hydrogen atmosphere. The present thin film layer of the impregnated cathode is formed in a sputtering chamber, and its composition is determined by inductive coupled plasma spectroscopy (ICPS method) and by fluorescence X-ray analysis (FLX method). Sputtering targets are prepared by mixing W, WO.sub.2 and Sc.sub.2 O.sub.3 powder in various mixing ratios and press molding the resulting mixtures. Then, the impregnated cathode underlayer and the target composed of W, WO.sub.2 and Sc.sub.2 O.sub.3 are placed in the sputtering chamber, and, after the chamber has been evacuated to the order of 10.sup.-7 Torr, the thin layer 3 composed of W, WO.sub.2 and Sc.sub.2 O.sub.3 is formed on the surface of the impregnated cathode underlayer in an Ar gas atmosphere in the order of 10.sup.-2 Torr by introducing an Ar gas into the chamber. The thin film layer 3 is formed from the targets of various compositions, and the thickness of the thin film layer 3 is changed by adjusting the sputtering time.

The electron emission property of the present impregnated cathode 8 provided with the thin film layer 3 thus formed is determined by applying a pulse voltage to the anode according to a cathode-anode diode parallel plate configuration in a vacuum chamber in the order of 10.sup.-9 Torr. Results are shown in FIG. 4, where line 11 shows the electron emission characteristics of the conventional impregnated cathode underlayer, line 10 those of the metal-coated, impregnated cathode, as coated with Os-Ru alloy to a layer thickness of 500 nm, and line 13 those of the present impregnated cathode coated with the thin film layer. The composition of the thin film layer shown in FIG. 4 is 78 wt.% W--17 wt.% WO.sub.2 --5 wt.% Sc.sub.2 O.sub.3.

The present impregnated cathode shown by line 13 can be operated at a lower temperature by about 300.degree. C. than that of the conventional impregnated cathode underlayer shown by line 10 and by about 150.degree. C. than that of the conventional Os-Ru-coated, impregnated cathode shown by line 10. Furthermore, the amount of evaporated barium and barium oxide is measured by mass spectroscopy, and has been found to decrease by the order of 1.5-3, as compared with that of the conventional impregnated cathode under layer. By lowering the operating temperature by 150.degree.-300.degree. C., the power consumption decreases and furthermore the heater can have a life of a few ten thousand hours, which is substantially equivalent to that of the spray-type oxide cathode as heated. Thus, a highly reliable impregnated cathode can be obtained in the present invention.