JPH0795422B2 - Method for manufacturing impregnated cathode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention relates to a method for producing an impregnated cathode used in an electron tube or the like, and particularly to an alloy having a stable crystal structure on its electron emission surface. A manufacturing method for forming a layer.

(Prior Art) As is well known, the impregnated cathode assembly has barium oxide (BaO) and oxide in the pores of a porous substrate formed by sintering a powder of a refractory metal such as tungsten (W). Calcium (C
aO) and an electron-emitting substance composed of an alkaline earth metal oxide such as aluminum oxide (Al ₂ O ₃ ) are melt-impregnated. Although this cathode has a higher operating temperature than an oxide cathode, it has the characteristics that a high current density can be obtained and that it is resistant to gas poisoning. Therefore, for example, it is practically used in a traveling wave tube for mounting on a satellite, a high power klystron for heating fusion plasma, and the like. In such a field, high reliability such as long life and stable operation and high current density are required.

As one measure to improve reliability, a coating layer made of platinum group metal such as iridium (Ir), osmium (Os), ruthenium (Ru) or its alloy is provided on the cathode surface to improve the work function of the cathode surface. It is known to lower the operating temperature by lowering it. By providing such a coating layer, the operating temperature can be lowered by several tens to several hundreds of degrees Celsius in order to obtain the same current density as in the case where the coating layer is not provided.

(Problems to be Solved by the Invention) However, although such an impregnated-type cathode assembly can lower the operating temperature as compared with the case where there is no coating layer on the surface, the operating temperature is about 900- It is 1000 ° C., and it is unavoidable that W, which also forms a porous substrate along with the operation, diffuses into the surface coating layer and gradually forms an alloy layer of the surface coating layer metal and W. As the alloying process of the surface coating layer changes, the electron emission characteristics are changed accordingly, and it is possible to improve reliability such as early stable operation and long life characteristics.
It is one bottleneck. And it is effective to coat the surface of the porous substrate with an alloy layer of the above metal and tungsten, which is the substrate metal, which has been performed conventionally, but the crystal structure does not change from the beginning of the operation and the electron emission characteristics are not changed. Obtaining a stable alloy layer has been difficult in practice.

The present invention has been made in view of the above circumstances, and provides a method for manufacturing an impregnated cathode in which a surface alloy layer having stable electron emission characteristics from the initial stage of operation can be easily obtained with good reproducibility. To aim.

[Structure of the Invention] (Means for Solving the Problem) The present invention applies a layer of iridium or an alloy of iridium and tungsten to the surface of a porous substrate impregnated with an alkaline earth metal oxide. Then, in a vacuum or in an inert atmosphere, this porous substrate is then heated at a temperature in the range of 1100 ° C to 1260 ° C for a predetermined time so as to form an alloy layer composed of the ε _II phase of the metal compounds of iridium and tungsten on the surface. It is a method of manufacturing an impregnated cathode which is heat-treated.

(Operation) According to the present invention, the crystal structure of the iridium-tungsten alloy layer formed on the surface of the porous substrate is extremely stable from the initial stage of operation. Therefore,
The iridium and tungsten concentration compositions of the surface coating layer are almost constant, whereby the work function is kept constant at a relatively low value. Thus, according to the present invention, an impregnated cathode having extremely stable electron emission characteristics over a long period from the initial operation can be obtained with good reproducibility.

(Example) Hereinafter, an example will be described with reference to the drawings. The same parts are represented by the same symbols.

FIG. 1 shows an example of the structure of the impregnated cathode assembly obtained by the present invention. Porous tungsten with a diameter of 1.5 mm and a thickness of 0.4 mm and a porosity of about 20% was melt impregnated with a mixed oxide of barium oxide, calcium oxide and aluminum oxide (molar ratio of about 4: 1: 1). After cleaning the surface and removing excess Ba, a porous substrate 11 was produced. Then this porous substrate
11 is a 25 μm thick tantalum cup 12 with rhenium wire
Resistance welded through 13. Then tantalum cup
12 was laser-welded to the inside of the opening at one end of a support sleeve 14 made of tantalum. The support sleeve 14 was fixed to the outer support cylinder via three support straps (not shown) made of rhenium-molybdenum alloy. The surface of the porous substrate 11 of the cathode assembly thus manufactured was coated with iridium to a thickness in the range of 50Å to 10,000Å, for example, 3,500Å by sputtering.

The cathode assembly is then placed in a vacuum or inert atmosphere, such as 10
^Put it in the vacuum exhaust bell jar exhausted below ^-7 Torr,
A heater (not shown) inside the cathode assembly was energized to perform heat treatment at a predetermined temperature for a predetermined time. An example of this heat treatment process is shown in FIG. That is, a writing process (I, II, III, I) for gradually increasing the cathode temperature mainly for degassing
V, V, VI) and an aging step (I, II, III) of isothermal heating at a brightness temperature of about 1180 ° C. for a predetermined time. The temperature is shown as a brightness temperature obtained by filtering the cathode surface by 650 nm.

Thus, the range of the lattice constant a on the electron emission surface is 2.76.
Å to 2.78Å, the lattice constant c range is 4.44Å to 4.46Å
An iridium-tungsten surface alloy layer 15 having a crystal structure mainly composed of an ε phase showing an hcp structure was formed. Then, such an impregnated cathode structure was incorporated into a traveling wave tube for mounting on a satellite and operated.

Such an impregnated-type cathode assembly showed an electron emission characteristic with excellent stability in the operation for a long time from the initial operation. The reason will be described below.

First, in order to elucidate the alloying process of the surface part, a vacuum high temperature X-ray diffractometer was used to measure the change in crystal structure of the surface layer of the porous substrate of the cathode assembly coated with iridium to a thickness of about 3,500Å. Field observation was performed by X-ray diffraction. Along the cathode heating schedule shown in FIG. 2, it was confirmed that the X-ray diffraction pattern changed as shown in FIG. The heating process is shown on the right vertical axis in FIG.

As can be seen from the figure, as changes during the writing process, a decrease in the iridium phase and the appearance of the intermetallic compound ε phase of iridium and dangsten were observed mainly after the writing process (IV). The ε phase exhibits a hcp structure. In the aging step, a series of diffraction peaks showing the same crystal system as a pair appeared on the low angle side of the series of diffraction peaks shown by the ε phase. Then, as the aging process proceeded, the first series of peaks disappeared and were replaced by later diffraction patterns. The ε phase formed at the beginning is ε _I
The phase, and the phase that appears later on the lower angle side of X-ray diffraction will be referred to as the ε _II phase. The discontinuous change in diffraction pattern from the ε _I phase to the ε _II phase corresponds to the discontinuous change in the lattice constant. That is, the lattice constants of the ε _I phase are a = 2.735 to 2.7, respectively.
45Å, c = 4.385 to 4.395Å. In the ε _II phase,
The ranges of a = 2.760 to 2.780Å and c = 4.440 to 4.460Å are shown. The relationship between the values of these lattice constants and the tungsten concentration in the iridium-tungsten alloy has already been reported, and is the relationship shown by the solid line in FIG.
The values of the lattice constants of the ε _I phase and the ε _II phase obtained by the experiments of the present inventors are shown by the dotted diagonal lines in FIG. These corresponding tungsten concentrations range from about 20 to about ε _I phase.
25 atom%, about 40 to 50 atom% in the ε _II phase. It can be seen that due to the transition from the ε _I phase to the ε _II phase, the surface layer composition also changes extremely discontinuously. The ε _II phase showed an extremely stable crystal structure, and the subsequent heat treatment hardly changed the lattice constant. Ie 11
About 55 minutes of heating in the aging step at 80 ° C. caused a complete transition from the ε _I phase to the ε _II phase.

In addition, regarding the surface alloy layer after the writing process and after the aging process, the composition analysis in the depth direction from the surface is performed by the Auger electron spectroscopy apparatus using the sputtering method using argon ions, as shown in FIG. ) And (b) were obtained. The figure (a) is a relative density distribution after 120 minutes of the aging process after the end of writing, and the figure (b) is. Curves 51 and 53 show relative iridium concentration, and curves 52 and 54 show relative tungsten concentration. From this result, it is inferred that the tungsten concentration in the surface at the end of the writing step has a small gradient, and that tungsten is rapidly diffused into iridium. It was also found that the tungsten concentration in the surface and in the alloy layer was 40 to 50 atom% at the end of the aging step (III). These facts are in agreement with the compositional variation of the surface coating layer, which is inferred from the variation of the lattice constant shown in FIG.

As described above, by coating the surface of the porous substrate with iridium and heat-treating it at a predetermined temperature for a predetermined time, it is possible to reliably form a stable iridium-tungsten alloy layer composed of the ε _II phase on the surface portion. You can

Next, the relationship between the thickness of the iridium layer coated on the surface of the porous substrate and the heat treatment, that is, the aging condition was examined. Iridium coating layer is approx. 1000Å, 2000Å, approx.
Available in thicknesses of 3500Å and about 5000Å, all 1180
Heated at ℃ for a predetermined time. The result of the X-ray diffraction obtained as a result is shown in FIG. In the figure, the X-ray diffraction intensity ratio on the vertical axis represents the ratio of the diffraction peak intensity of the ε _II phase to the sum of the main diffraction peak intensities of the ε _I phase, the ε _II phase, and the iridium. From this result, from the ε _I phase to ε _II
It was found that the heat treatment required for transition to the phase depends on the thickness of the iridium coating layer, and that the thicker the iridium layer, the longer the heat treatment time is required to form the ε _II phase. In the figure, the curve 61 shows the case where the iridium coating layer has a thickness of about 1000Å, the curve 62 has about 2000Å, the curve 63 has about 3500Å, and the curve 64 has about 5000Å. Moreover, when the aging time was kept constant, the thicker the iridium coating layer was, the more the heat treatment at a higher temperature was required for the complete formation of the ε _II phase up to the surface.

Further, FIG. 7 shows changes in the maximum emission value, that is, MISC, in the space charge limited region with respect to the iridium coating layer thickness and the aging heat treatment time. The maximum emission value MISC in this space charge limited region is expressed as a value one second after the start of application when the anode voltage is applied for two seconds.
This value shows an almost intermediate characteristic between a pulse type which is usually used as an anode voltage applying method and a DC type. In the figure, curve 71 indicates that the iridium coating layer thickness is about 1000.
In the case of Å, the 72 is about 2000 Å, the 73 is about 3500 Å,
74 indicates the case of about 5000 Å.
From these results, the thicker the iridium coating layer is, the MI
It was confirmed that the increasing trend of the SC value is gradual and that it takes a long time to activate the emission. Thus electron emission characteristic has a strong correlation with the rate of formation of epsilon _II phase, if epsilon _II phase is completely generated until the base surface can be obtained the largest radiation current, clear that and is stable Became. Then, it was associated that the surface alloy layer of the sufficiently activated cathode substrate was composed of only the ε _II phase in X-ray diffraction.

Furthermore, the cross section of the cathode after alloying was observed by a scanning electron microscope, and the relationship between the thickness of the produced alloy layer and the thickness of the iridium coating layer was observed. The results are shown in FIG. As a result, it was confirmed that the thickness of the alloy layer to be formed was about twice the thickness of the iridium layer to be deposited first.

Considering the above-mentioned results, the relation with the electron emission characteristics was examined next. The outline is described below. A sample was prepared by coating the surface of a porous substrate with iridium by sputtering to an arbitrary suitable thickness in the range of about 50Å to about 10,000Å, and subjected to a predetermined heat treatment. The surface alloying treatment by this heat treatment is carried out by heating the inside of the tube, that is, by assembling it in the electric tube to energize it by heating the heater in the cathode assembly to heat it, and by heating it alone, that is, inside the glass bell jar that is evacuated. The method was carried out by arranging the cathode assembly on the substrate and heating. Each of these methods takes into consideration that the former, that is, heating in a tube is suitable for a relatively low-voltage electron tube, and the latter, that is, heating method for a single body is suitable for an electron tube for large-sized or high-voltage operation. Then, the electron emission characteristics were evaluated by producing a glass dummy tube of a parallel plate type bipolar tube. In the case of the in-tube heating method, the writing step was carried out during the exhaust of the glass dummy tube, after which the exhaust tube was sealed and cut off, and then subjected to aging heat treatment at a predetermined equal temperature. Changes in electron emission characteristics during the aging process were measured. Further, in the single heating method, a predetermined lighting and aging process is performed with a glass bell jar, and then the obtained cathode structure is incorporated into a glass dummy tube of a parallel plate type bipolar tube,
The electron emission characteristics were measured by the same method as in the tube heating method. In addition, in order to prevent the aging effect during the measurement, the electron emission characteristics were measured by lowering the cathode temperature to 1000 ° C.

The comparison results are shown in Table 1. In the table, in the examples and the subsequent analysis, ε _II was formed on almost the entire surface alloy layer.
The formation of phases was confirmed in the examples of the present invention, and the comparative example is the case where both ε _I phase and ε _II phase were observed in the surface alloy layer.

From the results shown in Table 1, by performing a predetermined heat treatment according to the thickness of the first coated iridium layer to form an alloyed layer consisting of almost ε _II phase, stable operation is possible even in the initial and long-term operation. It was confirmed that various electron emission characteristics can be obtained. By setting appropriate heat treatment conditions according to the thickness of the iridium layer as described above, the iridium-tungsten alloyed layer composed of the ε _II phase can be reliably and reliably obtained.

The layer to be deposited on the surface of the porous substrate first is not limited to iridium, but may be coated with an alloy mainly composed of iridium and tungsten. Then, as described above, in a vacuum or in an inert atmosphere, a predetermined temperature is set in a range of about 1100 ° C to 1260 ° C so as to form an alloy layer composed of the ε _II phase of the metal compound of iridium and tungsten on the surface. Heat treatment may be performed for an hour.

Thus, in the method for producing an impregnated cathode in which an alloy layer containing iridium is formed on the surface, iridium, or iridium-tungsten alloy is deposited on the surface of the porous substrate, and the surface of the iridium is deposited in vacuum or in an inert atmosphere. The heat treatment at a temperature in the range of about 1100 ° C to 1260 ° C for a certain period of time to form an alloy layer consisting of the ε _II phase of the metal compound of iridium and tungsten in the part is extremely stable and good Impregnated cathodes having excellent electron emission characteristics can be manufactured with good reproducibility. That is, the alloy layer made of the ε _II phase formed on the surface has a characteristic that it is extremely stable and its change is extremely small even after long-term operation. The obtained ε _II phase has a lattice constant a
Range is 2.76 to 2.78Å, lattice constant c is 4.44 to 4.46Å
In other words, the hcp structure in the range is shown, which is characterized in that it is composed of the ε phase of an iridium-tungsten alloy in which tungsten is a saturated solid solution. In this way, the iridium and tungsten concentration compositions on the surface are made constant, whereby the work function is kept constant at a relatively low value. Therefore, extremely stable electron emission characteristics can be obtained from the initial operation to the lifetime.

The thickness of the iridium coating layer on the surface of the porous substrate is 50Å to 10,0 from the viewpoint of easy practical control of heat treatment time.
A range of 00Å is suitable. As a result, the thickness of the alloy coating layer consisting of ε _II phase is approximately doubled from 100 Å to 20,000 Å
The range is the preferred thickness. The heat treatment conditions in that case are approximately 1100 ° C. to 1260 ° C., and the heat treatment time is approximately 1 minute to 360 minutes, and appropriate treatment may be performed according to the iridium thickness as described above. Good.

If the heat treatment temperature is higher than 1260 ° C., the amount of barium evaporated in the substrate will be too large, and the desired electron emission characteristics may be impaired. At 1100 ° C or lower, alloying of the ε _II phase requires an extremely long time, which is not practical. Further, if the thickness of the ε _II phase alloy layer is less than 100Å, the life becomes short, while if it exceeds 20,000Å, disadvantages such as high operating temperature are required.

[Effects of the Invention] As described above, according to the manufacturing method of the present invention, an iridium-tungsten alloy layer composed of an ε _II phase having an extremely stable crystal structure is reliably and reliably formed on the surface of a porous substrate. it can. Therefore, it is possible to reliably and easily obtain an impregnated cathode having excellent electron emission characteristics that is extremely stable from the initial operation to long-term operation.

[Brief description of drawings]

FIG. 1 is a perspective view showing a partial cross section of an impregnated cathode obtained by the present invention, FIG. 2 is a view showing a cathode heat treatment step in an embodiment of the present invention, and FIG. 3 is a surface portion in the heat treatment step. X-ray diffraction pattern diagram, FIG. 4, FIG. 5 (a), (b), FIG. 6, FIG. 7, and FIG. 8 are characteristic diagrams in each step. 11 ... Porous substrate, 15 ... Surface alloy layer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsumi Yanagibashi 72 Horikawa-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Higashi Shiba-Horikawa-cho, Ltd. (72) Inventor Yoshiaki Ouchi 72 Horikawa-cho, Sai-ku, Kawasaki, Kanagawa Higashi Shiba Horikawa-cho Factory (56) Reference JP-A-57-210538 (JP, A) JP-A-54-133871 (JP, A)

Claims (1)

[Claims] 1. A method for producing an impregnated cathode, comprising the step of impregnating a porous substrate with an alkaline earth metal oxide and forming an alloy layer of iridium and tungsten on the surface of the porous substrate. A layer of iridium or an iridium and tungsten alloy is deposited on the surface of the substrate, and the porous substrate is then subjected to ε _{II of} a metal compound of iridium and tungsten on the surface in vacuum or in an inert atmosphere.
1. A method for producing an impregnated cathode, which comprises heat-treating at a temperature in the range of 1100 ° C. to 1260 ° C. for a predetermined time so as to form an alloy layer of phases.