JPS62287543A - Impreganated cathode structure - Google Patents

Abstract

PURPOSE:To substantially stabilize electron emission characteristic for a long period of time from the initial stage by specifying a crystal structure of an irridium and tungsten alloy of a coated layer to be formed on a porous substrate surface. CONSTITUTION:An irridium and tungsten alloy surface layer 15 is spread on a porous substrate 11 consisting of tungsten or the like made by melting and impregnating mixed oxides of alkaline earth metals such as barium oxide, calcium oxide and almunum oxide to form an impregnated cathode structure. By selecting a crystal structure of said layer so that it consists of epsilon phase to show a range of 2.76-2.78A for the lattice constant(a) and a range of 4.44-4.46A for the lattice constant C, the concentration composition of irridium and tungsten is fixed by epsilon alloy layer which is substantially stable from the initial stage, work function is maintained relatively at low value and electron emission characteristic is substantially stabilized for a long period of time from the initial stage.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Object of the Invention] (Field of Industrial Application) This invention relates to an impregnated cathode structure used in an electron tube, etc. Concerning crystal structure.

(Prior art) The impregnated cathode structure has barium oxide (Bad), calcium oxide (Cab), etc.
), and an electron-emitting material made of an alkaline earth metal oxide such as aluminum oxide (AI2203) is melt-impregnated. This cathode has a higher operating temperature than an oxide cathode, but has the characteristics of being able to obtain a high current density and being resistant to gas poisoning. For this reason, it is put into practical use, for example, in traveling wave tubes for use on planet Earth, and in high-power klystrons for heating fusion plasma. In such fields, high reliability such as stable operation and high current density are also required.

One measure to increase reliability is to add iridium (Ir), osmium (O5), and ruthenium (Ru) to the cathode surface.
It is known that a coating layer made of a platinum group metal or an alloy thereof is provided to lower the work function of the cathode surface, thereby reducing the operating temperature. By providing such a coating layer, the operating temperature can be lowered by several tens of degrees Celsius to hundreds of degrees Celsius while obtaining the same current density as without the coating layer.

(Problems to be Solved by the Invention) However, although such an impregnated cathode assembly can lower the operating temperature compared to the case where there is no coating layer on the surface, the operating temperature is approximately 900 - 900℃. 1000
℃, and W also forms a porous substrate with operation.
It is inevitable that the metal diffuses into the surface coating layer and gradually forms an alloy layer of the surface coating layer metal and W. This process of alloying of the surface coating layer changes the electron emission characteristics accordingly, and is one of the bottlenecks in improving reliability such as early stable operation and long life characteristics.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an impregnated cathode structure having stable electron emission characteristics from the beginning.

[Structure of the Invention] (Means for Solving the Problem) This invention provides that the crystal structure of an alloy layer consisting of an alloy layer of iridium and tungsten coated on the surface of a porous substrate has a lattice constant a within a range of 2.76 people to 2.
It is an impregnated porosity polar structure mainly composed of an ε phase exhibiting an hcp structure of 78 Å and a lattice constant C ranging from 4.44 to 4.46.

(Function) According to the present invention, since the crystal structure of the iridium-tungsten alloy layer formed on the surface of the porous substrate has the above-mentioned configuration, the alloy layer consisting of the ε phase is extremely It is stable. Therefore, the iridium and tungsten a degree compositions of the surface coating layer are approximately constant, thereby keeping the work function constant at a relatively low value. Thus, according to the present invention, it is possible to obtain an impregnated cathode structure having extremely stable electron emission characteristics from the initial stage for a long period of time.

(Example) An example will be described below with reference to the drawings. Identical parts are designated by the same reference numerals.

An example of the structure of the impregnated cathode structure of the present invention is shown in FIG. Porous tungsten with a diameter of 1.5 m and a thickness of 0.4 m with a porosity of 20% is melted and impregnated with a mixed oxide of barium oxide, calcium oxide, and aluminum oxide (molar ratio of approximately 4:1:1). After that, the surface was washed to remove excess 3a, and then a porous substrate 11 was prepared. Next, this porous substrate 11 was resistance welded to a tantalum cup 12 having a thickness of 25 μm via a rhenium wire 13. The tantalum cup 12 is then attached to a tantalum support sleeve 14.
One end was laser welded inside the opening. The support sleeve 14 was fixed to the outer support cylinder via three support straps made of a rhenium-molybdenum alloy (not shown). The surface of the porous substrate 11 of the cathode assembly thus produced was coated with iridium by sputtering to a thickness ranging from 50 to 10,000, for example to 3,500.

This cathode structure is then placed in a vacuum or an inert atmosphere, e.g.
The sample was placed in an evacuated Pel jar that was evacuated to below O-7 rorr, and a heater (not shown) inside the cathode assembly was energized to heat it at a predetermined temperature for a predetermined time.

An example of this heat treatment step is shown in FIG. That is, a lighting process (■, ■, ■, ILV, l) in which the cathode temperature is gradually increased with the main purpose of degassing, and an aging process (isothermal heating in which the brightness temperature is approximately 1180°C for a predetermined period of time).
■、■、■). Note that the temperature is shown as the brightness temperature of the cathode surface filtered at 650 degrees.

In this way, the range of the lattice constant a is 2.7 at the electron-emitting surface part.
6 to 2.78 Å, and the lattice constant C ranges from 4.44 to 4.
．． An iridium-tungsten alloy coating layer 15 having a crystal structure mainly composed of an ε phase exhibiting a 46-hcp structure was formed. Then, such an impregnated cathode structure was installed, for example, in a traveling wave tube for use in a satellite, and was operated.

Such an impregnated cathode structure has an extremely stable electron t'tli from the initial stage of operation to long-term operation.
It showed timing. The reason for this will be explained below.

First, in order to elucidate the alloying process on the surface, we used a vacuum high-temperature XtQ diffractometer to investigate changes in the crystal structure of the surface layer of the porous substrate of the cathode structure coated with iridium to a thickness of approximately 3,500 mm. It was measured in situ by X-ray diffraction. According to the cathode heating schedule shown in FIG. 2, changes in the X-ray diffraction pattern were observed as shown in FIG. 3. Note that the heating process is shown on the right vertical axis of FIG.

As can be understood from the figure, as changes during the writing process, a decrease in the iridium phase and the appearance of an intermetallic compound ε phase of iridium and dungesten were observed mainly after the writing process (IV). The ε phase shows an hcp structure. In the aging process, a series of diffraction peaks indicating the same crystal system appeared in pairs on the low angle side of each of the series of diffraction peaks exhibited by the ε phase. Then, as the aging process progressed, the initial series of peaks disappeared and were replaced by later diffraction patterns. The initially formed ε phase will be referred to as the ε■ phase, and the phase that appears on the low angle side of X-ray diffraction will be referred to as the ε■ phase. The discontinuous change in the diffraction pattern from the ε■ phase to the ε■ phase corresponds to a discontinuous change in the lattice constant. In other words, the lattice constants of the ε■ phase are a= 2゜7
35 to 2.745 Å, c = 4.385 to 4.39
There are 5 people. In addition, in the ε■ phase, a=2.760 to 2.
780 Å, C=ranged from 4.440 to 4.460. Incidentally, 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. 4.

The values of the lattice constants of the ε1 phase and the ε■ phase obtained in experiments by the present inventors are indicated by dotted diagonal lines in FIG. Their corresponding tungsten concentrations are approximately 20 to 25 atomic % for the ε1 phase and approximately 40 to 50 atomic % for the ε■ phase.

It can be seen that due to the transition from the ε1 phase to the ε■ phase, the surface layer composition changes extremely discontinuously.

The ε■ phase exhibited an extremely stable crystal structure, and the subsequent heat treatment caused almost no change in the lattice constant. That is, by heating for about 55 minutes in the aging step at 1180° C., the ε1 phase completely transitioned to the ε■ phase.

In addition, we attempted to analyze the composition of the surface alloy layer after the writing process and after the aging process in the depth direction from the surface using an Auger electron spectrometer using a sputtering method using argon ions. )
, the result of (b) was obtained.

The figure (a) shows the relative 'a degree distribution after the writing is completed, and the figure (b) shows the relative 'a degree distribution after 120 minutes of the aging process. Curves 51 and 53 show relative iridium concentrations, and curves 52 and 54 show relative tungsten concentrations.

From this result, at the end of the writing process,
There is little gradient in the tungsten concentration on the surface, suggesting rapid diffusion of tungsten into iridium. In addition, it was found that the tungsten concentration on the surface and in the alloy layer was 40 to 50 atomic % in the aging process (I[I) completed stage. These facts are consistent with the compositional variation of the surface coating layer inferred from the change in lattice constant shown in FIG.

As described above, by coating iridium on the surface of a porous substrate and heat-treating it at a predetermined temperature for a predetermined time,
An iridium-tungsten alloy coating layer consisting of a stable ε■ phase can be formed on the surface portion.

Next, the relationship between the thickness of the iridium layer coated on the surface of the porous substrate and the heat treatment, ie, aging conditions, was investigated. The iridium coating layer has a diameter of 1000 Å and approximately 200 Å.
Thicknesses of 0 Å, about 3500 Å, and about 5000 Å were prepared, and all were heated at 1180° C. for a predetermined time. The result was XFJ! The diffraction results are shown in FIG. In the figure, the a-axis X-ray diffraction intensity ratio is the ratio of the diffraction peak intensity of the ε■ phase to the sum of the main diffraction peak intensities of the ε■ phase, the ε■ phase, and iridium. From this result, the heat treatment time required to transition from the ε■ phase to the ε■ phase depends on the thickness of the iridium coating layer, and the thicker the iridium layer, the longer the heat treatment time required to form the ε■ phase. It turns out that it requires In addition, curve 6 is 43 in the same figure.
1 is when the iridium coating layer thickness is about 1000 people,
62 is about 2000 Å, 63 is about 3500 Å, 64
The cases of approximately 5,000 people are summarized in each case. Furthermore, when the aging time was kept constant, the thicker the iridium coating layer, the higher the need for heat treatment at a higher temperature to completely generate the ε■ phase up to the surface.

Further, FIG. 7 shows the change in the maximum emission value, ie, fVLIsc, 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 of f3 in this space charge limited region is expressed as the value 1 second after the start of application when anode voltage is applied for 2 seconds, and this value is based on the pulse type that is normally used as the anode voltage application method. The characteristics are approximately intermediate between the DC case and the DC case. In the same figure, the curve 71 indicates that the iridium coating layer thickness is approximately 1,000 mm, and the curve 72 is approximately 20 mm.
00 Å, 73 is about 3,500 Å, and 74 is about 5,000 people.

From these results, the thicker the iridium coating layer, the more M
It was confirmed that the increasing trend of the ISC value was gradual and that emission activation also required a long time. In this way, the electron emission characteristics have a strong correlation with the formation rate of the ε■ phase, and the
It has become clear that when the phase is completely generated up to the substrate surface, the largest electron number OA current is 19, and it is stable. It was determined from X-ray diffraction that the surface alloy layer of a sufficiently activated cathode substrate consists of almost only the ε■ phase.

In addition, the cross section of the cathode after alloying was observed using a scanning electron microscope to examine the relationship between the thickness of the formed alloy layer and the thickness of the iridium coating layer. The results are shown in FIG. As a result, it was confirmed that the thickness of the formed alloy layer was approximately twice the thickness of the initially deposited iridium layer.

Considering the results described above, we next considered the relationship with electron emission characteristics. The outline is described below.

A sample was prepared in which the surface of a porous substrate was coated with iridium by sputtering to an arbitrary thickness ranging from about 508 to about io, ooo, and was subjected to a predetermined heat treatment. This surface alloying treatment by heat treatment is carried out by in-tube heating, which is a method in which alloying is performed by energizing a heater inside the cathode assembly while it is incorporated in the electron tube, and by single-unit heating, which is a glass Pelger that is evacuated. The method was carried out by placing a cathode structure inside and heating it. Each of these methods takes into consideration that the former, ie, tube heating, is suitable for relatively low-voltage electron tubes, and the latter, ie, the unit heating method, is suitable for large-sized or high-voltage operation electron tubes. The electron emission characteristics were evaluated by fabricating a parallel plate diode glass dummy tube.

In the case of the in-tube heating method, the lighting process was carried out while the glass dummy tube was being evacuated, and after the exhaust pipe was sealed and cut, aging heat treatment was performed at a predetermined constant temperature. Changes in electron emission properties during the aging process were measured. In addition, with the single heating method, the specified lighting and
The aging process was carried out, and then the obtained cathode structure was assembled into a glass dummy tube of a parallel plate type diode, and the electron emission characteristics were measured in the same manner as in the case of the in-tube heating method.

In order to prevent aging effects during measurements, all electron emission characteristics were measured with the cathode temperature lowered to 100'C.

Table 1 shows the comparison results. In the same table, examples indicate that almost the entire surface gold layer was ε■■ as determined by subsequent analysis.
Examples of the present invention in which phase formation was confirmed, and comparative examples include cases in which both ε■ phase and ε■ phase were observed in the surface alloy layer.

(Margins below) Table 1 (Continued) *marks indicate (℃ 2 minutes) From the results in Table 1, it is possible to create an alloy consisting almost only of the ε■ phase by applying a predetermined heat treatment depending on the thickness of the iridium layer initially coated. It was confirmed that by forming a layer, it was possible to obtain stable and good electron emission characteristics even at the initial stage and during long-term operation. As mentioned above, depending on the thickness of the iridium layer, the iridium
The predetermined heat treatment process for obtaining the tungsten alloyed layer has good reproducibility and is extremely practical.

In this way, in an impregnated cathode whose surface is coated with an alloy layer containing iridium, by making the alloy layer ε■ phase, the impregnated cathode is stable from the initial stage of operation and has good electron emission characteristics. can be realized. This ε
(2) The phase is extremely stable, and changes are extremely small even during long-term operation. In addition, this ε■ phase has a lattice constant a in the range of 2.76 to 2.78 Å, and a lattice constant C
ranges from 4.44 to 4.46 indicating the human hcp structure,
In other words, it is composed of the ε phase of an iridium-tungsten alloy in which tungsten is a saturated solid solution: the ri sign. In this way, the iridium and tungsten concentration composition of the surface portion is constant, thereby keeping the work function constant at a relatively low value.

Therefore, extremely stable electron emission characteristics can be obtained from the initial stage of operation to the entire lifetime.

In addition, for ease of practical management of heat treatment time, the thickness of the iridium coating layer on the surface of the porous substrate is set at 50 to
A range of o, ooo people is appropriate. Therefore, the thickness of the alloy coating layer consisting of the ε■ phase is preferably in the range of about 100 to 20,000, which is about twice the thickness. In that case, the heat treatment conditions are approximately 1100°C to 1260'C, the heat treatment time is approximately 1 minute to 360 minutes, and appropriate treatment is performed depending on the iridium thickness as described above. All you have to do is implement it.

Note that if the heat treatment temperature is higher than 1260° C., the amount of barium evaporated in the substrate becomes too large, which may actually impair the desired electron emission characteristics. Further, at temperatures below 1100°C, it takes a very long time to alloy the ε■ phase, which is impractical. Furthermore, if the thickness of the ε■ phase alloy layer is less than 100,000 people, the service life will be shortened, while if it exceeds 20,000 people, disadvantages such as a high operating temperature will become noticeable.

[Effects of the Invention] As explained above, according to the present invention, the iridium-tungsten alloy coating layer consisting of the ε phase formed on the surface of the porous substrate provides extremely stable performance from the initial stage of operation to long-term operation. Electron emission characteristics can be obtained.

[Brief explanation of the drawing]

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

Claims (2)

[Claims] (1) In an impregnated cathode structure in which a porous substrate is impregnated with an alkaline earth metal oxide and an alloy layer of iridium and tungsten is formed on the surface of the porous substrate, the crystal structure of the alloy layer is as follows: The range of lattice constant a is 2.76 Å to 2.78 Å, and the range of lattice constant C is 4
．． An impregnated cathode structure comprising an ε phase exhibiting an hcp structure of 44 Å to 4.46 Å. (2) The impregnated cathode structure according to claim 1, wherein the thickness of the alloy layer is in the range of 100 Å to 20,000 Å.