JPH0782808B2 - Impregnated cathode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to an impregnated cathode used in an electron tube.

(Prior Art) As is well known, recently, impregnated cathodes in which the surface of a porous cathode substrate for electron emission is coated with Ir (iridium), Os (osmium), or an alloy containing it have become practical. There is. One example is the cathode disclosed in JP-A-60-138822. It is a porous cathode substrate that is impregnated with an electron emitting material, in which a heat-resistant metal such as W (tungsten) and one or multiple elements selected from Os, Ir, and Ru are added in an amount of 5 to 30 atom%. Is.

(Problems to be Solved by the Invention) By the way, it has been revealed that such a cathode has the following disadvantages because the amount of Ir or the like added to the porous cathode substrate is too large. That is, when the ratio of the amount of Ir to the amount of W constituting the porous cathode substrate becomes too large, the melting point of Ir is lower, so that the sintering is performed during sintering, brazing in a later process, or impregnation with an electron emitting substance. The result is too advanced. As a result, the cathode substrate shrinks too much and the dimensions change. Further, as a result of the reduced porosity of the cathode substrate, a sufficient impregnation rate of the electron emitting substance cannot be obtained. Furthermore, when the electron emitting material is reduced to produce free Ba during the operation of the cathode, Ir has a lower reducing ability than W (almost cannot be expected).
Therefore, the inclusion of an excessive amount of Ir in this porous cathode substrate is not desirable.

The present invention eliminates the above inconveniences, does not cause extra changes such as shrinkage of the porous cathode substrate, keeps the alloy layer on the cathode surface stable for a long time, and stabilizes for a long time from the initial operation. It is an object to provide an impregnated cathode having electron emission characteristics.

[Structure of the Invention] (Means for Solving the Problems) The present invention is directed to a porous cathode substrate made of tungsten in which iridium is dissolved in the range of 0.5 to 2.0 atomic%, and the surface of the porous cathode substrate is It is an impregnated cathode in which an alloy coating layer of iridium and tungsten is formed on.

(Function) In order for the surface alloy coating layer of the seed-impregnated cathode to be stable during its operation, it is necessary to prevent the diffusion of Ir in the coating layer into the underlying W. Therefore, in the present invention, the surface alloy coating is obtained by pre-dissolving Ir in the porous W constituting the porous cathode substrate in a predetermined amount around the solid solution limit, that is, in the range of 0.5 atom% to 2.0 atom%. Ir from layer to substrate
It is applied that the diffusion outflow of is suppressed. As a result, the alloy coating layer maintains a stable state for a long time from the initial operation. Further, since an excess amount of Ir is not contained, there is no shrinkage or deformation in the manufacturing process of the porous cathode substrate, or reduction in porosity, and further, free Ba is generated during cathode operation.
It is unlikely that the generation of will be prevented. Thus, stable electron emission characteristics can be obtained for a long time from the initial stage. The amount of Ir solid-dissolved in the cathode substrate may be adjusted and set relatively within the above range according to the cathode operating temperature.

(Example) Hereinafter, the example will be described with reference to the drawings.

As shown in FIG. 1, a heater 12 for heating is embedded inside a sleeve 11 made of a refractory metal such as tantalum (Ta) or molybdenum (Mo) by an alumina ceramic insulator 13. The porous cathode substrate 14 is bonded via the brazing material layer 15. A surface coating layer 16 is formed on the surface of the porous cathode substrate 14.

The feature of the embodiment of the present invention is that the porous cathode substrate has about 2.
ΑW containing Ir as a solid solution in the range of 0 atom% or less and 0.5 atom% or more
It is composed of (alpha tungsten). That is, as will be described later, from the Ir-W phase diagram, the critical solid solution rate of Ir in W is estimated to be about 0.5 atom% to 2.0 atom% at an operating temperature of the cathode operating temperature of 900 ° C to 1100 ° C. Therefore, Ir is solid-dissolved in the W base in advance within this range.

Next, a preferable method for producing this cathode will be described. As the porous cathode substrate 14, 1 atom% (corresponding to about 1.04% by weight) of W powder having a particle size of 3 to 10 μm and I having a particle size of 0.1 to 1 μm
The r powders were mixed and subjected to compression molding under a hydrostatic pressure of 1.5 ton / cm ² . Next, this was pre-sintered in a hydrogen atmosphere at 1750 ° C. for 8 hours, and further, sintered in vacuum at 2200 ° C. for 2 hours. By the above-mentioned sintering, a porous cathode substrate in which Ir was solid-dissolved in W was obtained. The porosity was about 20%.

The pores of the porous sintered body thus obtained were impregnated with copper for facilitating cutting, and the diameter was 4.
It was cut into a disc with a thickness of 5 mm and a thickness of 1.3 mm. Next, copper was removed by heating at high temperature in nitric acid and hydrogen to complete the porous cathode substrate 14.

Next, the Mo sleeve 11 is brazed using a Ru-Mo brazing material, and the heater 12 is further embedded with an embedding agent 13 to form the Mo sleeve 11.
Embedded inside.

Further, an electron emitting material composed of BaO, CaO and Al ₂ O ₃ (molar ratio 4: 1: 1) was melt-impregnated into the porous substrate 14 in a high temperature hydrogen atmosphere.

Further, on the surface of this porous cathode substrate, an alloy of Ir and W ε _II
The coating layer 16 having a thickness of about 5000 Å consisting of phases was formed to complete the cathode.

Here, the ε _II phase means an ε phase exhibiting a saturated W concentration. The surface Ir—W alloy coating layer 16 can be formed by a dual sputtering method of Ir and W. After ordinary sputter coating, ε _II phase is formed by heating at high temperature in vacuum exhaust. Even when only Ir is coated and heated in vacuum at a temperature of about 1200 ° C. for several hours, the underlying W is diffused and alloyed in the Ir coating layer to form a surface coating layer composed of the same ε _II phase. It is formed. However,
In the case of this method, the film thickness after the εII phase formation is about twice the Ir coating film thickness, so Ir should be coated to a thickness of about 1/2 of the desired film thickness of the finished product. . The composition ratio of the surface alloy coating layer 16 is effective when Ir is in the range of 45 atom% to 80 atom% and W is in the range of 55 atom to 20 atom%, but more preferably Ir is 45 to 50 atom. %, W in the range of 50 to 55 atomic%.

The progress of alloying depends on the region showing the saturated W concentration of the ε phase, that is, ε
_It quickly progresses to Phase _{II and} stabilizes there. This was found from the measurement result by the X-ray diffractometer, which was measured while heating at high temperature in vacuum. The activation of the electron emission characteristic depends on this ε _II
It is known to proceed with the formation of phases. Therefore, in order to obtain good electron emission characteristics, it is presumed that the cathode substrate surface composed of the ε _II phase is necessary. And Ir and W
In the surface alloy coating layer of and, the state change after the ε _II phase formation is extremely small. This is the cathode operating temperature, and the diffusion rate of W in Ir or in the ε phase before reaching the saturated W concentration is
It is considered that this is because the diffusion rate of Ir into W is extremely low. By the way, as mentioned above, the cathode operating temperature (about 10
It is considered that Ir is dissolved in W at about 2.0 at. Therefore, as described above, the surface layer after the formation of the ε _II phase is relatively stable, but if the porous cathode substrate is only W, the Ir in the surface alloy coating layer will remain in the W of the substrate during long-term operation. Is expected to spread. That is, in this case, in the surface alloy coating layer in which the ε _II phase was almost 100% in the initial stage of the cathode operation after alloying, the αW phase was precipitated with the progress of the cathode operation, and the ratio was gradually increased. it is conceivable that.

As for the outermost surface of the cathode, what was covered with the ε _II phase at the initial stage of operation had a large exposed portion of W with the passage of time, and the effect of providing the metal coating layer was weakened. This is a phenomenon that leads to deterioration of electron emission characteristics and is extremely undesirable.

Therefore, the inventors of the present invention experimentally analyzed changes in the surface alloy coating layer of the Ir-coated impregnated cathode with long-term operation. That is, as comparative products, the porous cathode substrate consisting only of W and the embodiment of the present invention containing about 1.0 atom% of Ir, respectively, were comparatively early in the cathode operation (500
The surface of the cathode was analyzed by a scanning electron microscope, an X-ray diffractometer, and an Auger electron spectroscope for a time operation product) and a product operated for about 10,000 hours, and a comparative study was performed. As a test sample, a cathode having a relatively thin Ir coating layer of about 1500 Å was used so that changes in the alloy coating layer could be observed in the shortest possible time. Then, the device was operated under a high temperature forced condition of 1090 ° C.-Ta, which is higher than the normal operating temperature of 1000 ° C. b-Ta (brightness temperature of tantalum) to 920 ° C. b-Ta). The cathode current is 1A / c.
It was operated at m ² .

The results are as follows.

As a result of analysis by the X-ray diffractometer, the peaks of ε _II and W were detected in both the comparative product and the example of the present invention during the operation for 500 hours and 10,000 hours. And the lattice constant did not change.

In the analysis using a scanning electron microscope, the exposed part of the base material was observed in the comparative product, and the alloyed Fukui layer after 500 hours of operation was observed in a roughly lace-like structure. It can be seen, but the boundary with the base is somewhat unclear. In contrast, the embodiment of the present invention is 500
In both the time-actuated product and the 10,000-hour actuated product, the W exposed portion of the base is hardly recognized.

In the depth profile analyzed by Auger electron spectroscopy using an argon ion gun, the average W concentration near the outermost surface of the comparative product is about 10 in the 10,000-hour operation product compared to the 500-hour operation product. % To about 65%. This is considered to correspond to the analysis result by the above-mentioned scanning electron microscope.

On the other hand, it was revealed that in the example of the present invention, about 52%, there was almost no change from the initial stage of operation.

FIG. 2 shows the results of comparison of changes in electron emission characteristics between the cathode of the embodiment of the present invention and the comparative product due to long-term operation. These evaluations were carried out at 1100 ° C b-Mo high temperature forced operation, and a constant anode voltage was applied so as to obtain electron emission with a cathode current equivalent to 2 A / cm ² as an initial value, and the change in cathode current was traced. It was done. A curve A in the figure shows the electron emission characteristic of the embodiment of the present invention, and a curve B shows the characteristic of the comparative product. Life test 1
The decay rate of the cathode current at ten thousand hours was about 10% for the comparative product, while it was only about 4% for the cathode of the present invention, clearly indicating that the inventive example showed better characteristics. Therefore, for example, if the time when the cathode current decays by 10% is regarded as the life, the cathode of this invention example is expected to be four times as long as 74,000 hours or more, compared to about 10,000 hours of the comparative product.

In this way, it was confirmed that the cathode surface deterioration of the comparative product progressed with the operation of the cathode. That is, after the surface alloy layer composed of the Ir-W εII phase showing the saturated W concentration on the surface is formed, Ir gradually diffuses from this surface alloy layer into W of the base metal. Along with this, the α-W phase is precipitated in the surface layer, and the proportion thereof increases with the lapse of operating time. Therefore, the α-W phase is exposed on the outermost surface, and the cathode surface coating layer is deteriorated by long-term operation and its surface effect is reduced. Therefore, the electron emission characteristics are greatly deteriorated.

The cathode disclosed in JP-A-60-138822 is a cathode in which a large amount of Ir is dissolved in the W cathode substrate such as 5 to 30 atom%, and the cathode substrate is sintered as described above. At the time of brazing in a later step, or when impregnated with an electron emitting substance, sintering proceeds excessively, and the cathode substrate shrinks too much to change the dimensions. Further, since the porosity of the cathode substrate becomes small, the impregnation rate of the electron emitting substance cannot be sufficiently obtained. In addition, since the reducing ability by Ir can hardly be expected even when reducing the electron emitting substance to generate free Ba during the cathode operation, the electron emission characteristics are generally unstable from the initial operation to the long-term operation and the comparison is made. It is considered that the tendency tends to be low.

[Effects of the Invention] As described above, according to the present invention, by preliminarily forming a solid solution of Ir having a saturation concentration in W of the base metal, diffusion outflow of Ir from the surface alloy layer can be suppressed. Therefore, stable electron emission characteristics can be obtained for a long time from the initial operation. Further, since an excessive amount of Ir does not form a solid solution in the porous cathode substrate, sintering and shrinkage of this substrate do not proceed excessively. Therefore, there are many manufacturing advantages such as dimensional change of the cathode substrate and little decrease in porosity.

The present invention is particularly effective when the surface coating layer is thin. That is, the function of the surface alloy coating layer is stably exerted for a long time even if the film thickness is considerably reduced. A suitable film thickness is 50Å to 10,000Å.

[Brief description of drawings]

FIG. 1 is a sectional view of an impregnated cathode according to an embodiment of the present invention, and FIG. 2 is an electron emission characteristic diagram. 14 …… Porous cathode substrate 16 …… Surface alloy coating layer

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Katsu Nikaido 72 Horikawa-cho, Sachi-ku, Kawasaki-shi, Kanagawa Higashi-shiba Horikawa-cho, Higashi Co., Ltd. (56) References JP-A-60-138822 (JP, A)

Claims (2)

[Claims] 1. An impregnated cathode comprising a porous cathode substrate obtained by sintering a refractory metal powder and an electron-emitting substance impregnated in the porous cathode substrate, wherein An impregnated cathode in which the cathode substrate is made of tungsten in which iridium is solid-soluted in the range of 0.5 at% to 2.0 at%, and an alloy of iridium and tungsten is formed on the surface of the porous cathode substrate. 2. The impregnated cathode according to claim 1, wherein the film thickness of the alloy layer is in the range of 50Å to 10,000Å.