JPS62222534A - Impregnated type cathode structure - Google Patents

Abstract

PURPOSE:To stabilize the emission current property for a long period of time and realize a long service life, by specifying the porosity and the thickness of the covering layer of a porous W base. CONSTITUTION:A porous W base 11 is made by compressing and forming W powder of particle diameter 3-10mum into bar forms and sintered in a reducing atmosphere. The porosity of the base 11 is made at 20-40%. After it is combined with a cathode sleeve, a brazing material layer, a heater, fillers, and the like, and impregnated with an electron emission substance, the W base 11 is covered with Ir, for example, by a spattering. Then, at the final process, it is heat-treated in a high temperature vacuum to form the base metal of a W alloy layer, that is, Ir-W alloy layer 16 with the thickness of 3,000-30,000Angstrom . By increasing said porosity (p), the barium feeding amount to the surface of the cathode per unit time is made larger, and by increasing the thickness of the covering layer, the increasing speed of the W density over the surface of the cathode can be controlled. Therefore, an increase of the W density over the cathode, and a decrease of the barium feeding rate over the cathode are restricted, and the service life property can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Objective of the Invention (Industrial Application Field) This invention relates to an impregnated cathode structure for electron emission used in an electron tube or the like.

(Prior technology) Impregnated cathodes used in electron tubes, etc. are made of tungsten (
Barium oxide (Bad), calcium oxide (Cab), and aluminum oxide (A
t203) is melted and impregnated with an electron emitting material. Although this cathode has the disadvantage of a higher operating temperature than an oxide cathode, it has the advantage of being able to obtain a high current density, being resistant to gas poisoning, and having a long life. For this reason, they are widely used, for example, in traveling wave tubes mounted on satellites, high-power klystrons, and display tubes and projection tubes that require high operation speed. In these applications,
High reliability with characteristics such as high current density, long life, and stable operation is increasingly required.

Iridium (Ir), osmium (OB),
There is an impregnated cathode coated with a white metal such as ruthenium (Ru) or rhenium (Re), or an alloy thereof to reduce the work function. This means that at the same operating temperature, a current density that is 3 to 5 times higher than that without a coating layer can be obtained, and the operating temperature required to obtain the same current density is several tens to hundreds of degrees Celsius. is reduced. Therefore, the evaporation rate of the electron emitting material can be reduced, and the lifespan can be extended. In addition, it is possible to reduce the amount of electron emitting substances that adhere between the electrodes in the electron tube, thereby eliminating the factors that cause breakdown voltage defects and the like.

In the conventional product, the porous W cathode substrate has a specific gravity of about 16, and therefore a porosity of 16 to 17%.

In addition, in the conventional product with an Ir-W alloy coating layer, rr
The thickness of the coating layer is about 1000X to 4000X.

This alloy layer becomes a stable alloy layer with an atomic ratio of approximately 1:1,
The thickness of this alloy layer is approximately doubled by heat treatment, that is, 20
The thickness is 00 to 8000X. In terms of emission current characteristics, if the thickness of the coating layer in the case of a conventional product is thinner than the above value, the W concentration in the υ alloy layer will increase quickly during long-term operation.
The effect will not last long, and conversely, if the coating layer is thicker than the above, the supply of free barium generated in the base metal to the cathode surface will be extremely prevented by the alloy coating layer, resulting in deterioration of the emission current characteristics. It is considered to be.

(Problems to be Solved by the Invention) When an impregnated cathode having such an alloy coating layer is used at a current density of, for example, IA/cIrL2, the current value will be approximately equal to the initial value in operation for a certain period of time. This decreased to 90%, which was the typical lifespan. In recent years, demands for higher current density and longer life have become stronger, and it has become difficult for conventional devices to sufficiently meet these demands. The main reason for this is thought to be as follows. That is, the first
In addition, although the operating temperature is lower than that of an insulating cathode due to the alloy coating layer, it is normally used at a temperature of about 1000°C. Therefore, W in the base metal diffuses into the coating layer, and the W concentration on the cathode surface gradually increases.As the operating time increases, the effect of the coating layer becomes smaller and the work function increases. Secondly, the supply of free barium to the cathode surface is due to the difference in concentration of free barium between the generated part in the base metal and the vicinity of the cathode surface. Although the concentration is considered to be approximately constant, the center of its production moves deeper from the cathode surface as the operating time increases, and the path from the free barium generator to the surface becomes longer and the conductance decreases over time. In other words, the concentration gradient of free barium decreases with time. Since the barium supply to the cathode surface is considered to be approximately proportional to the conductance or concentration gradient, the supply rate decreases with time.

The cathode is generally used in a space charge limited region, but there are also parts of the cathode surface that are saturated, such as parts far from the pores. When operating at a high current density, the proportion of the saturation region naturally increases. Due to the above-mentioned factors, the proportion of the saturated region on the cathode surface increases with operating time, and it is therefore inevitable that the emission current value under a constant applied voltage will decrease.

The object of the present invention is to obtain an impregnated cathode whose emission current characteristics are more stable over a long period of time and which have long life characteristics.

[Structure of the invention]

(Means for Solving the Problems) This invention provides rr
* In an impregnated cathode formed with an alloy coating layer consisting of at least one metal selected from Os + Ru r Re and W, the porosity of the porous W substrate is 2.
The range is 0% to 40%, particularly preferably 25 to 35%, and the thickness of the coating layer is in the range of 3,000X to 30,0001, particularly preferably 10,000X to 25,000.
This is an impregnated cathode structure set within the range of X.

(Function) According to the present invention, both of the main causes of attenuation of electron emission characteristics during long-term operation of this type of impregnated cathode are an increase in the W concentration on the cathode surface and a decrease in the barium supply rate to the cathode surface. This makes it possible to significantly improve life characteristics.

(Example) Examples thereof will be described below with reference to the drawings.

Identical parts are designated by the same reference numerals.

As shown in FIG. 1, a porous W substrate II is joined to the opening of the cathode sleeve 12, and a heater 15 for heating is provided in the sleeve separated by a brazing material layer 13 with an insulating filling 14.
is embedded. Inside the porous W substrate 11, Ba
It is impregnated with an electron-emitting material such as O+ CaO+ At203. The surface of this porous W substrate 11 is coated with at least one metal layer (Z6) selected from Ir r Os +Ru*Re.

The porous W substrate 11 is made by compression molding W powder having a particle size of 3 to 10 mm into a rod shape, and sintering this in a reducing atmosphere. The porosity of conventional substrates is about 16 to 17 inches as mentioned above, but in the present invention it is in the range of 20 to 40%, for example about 32%.

The pores of the sintered body thus obtained are impregnated with steel or the like to facilitate cutting. This is cut into a disk shape having, for example, an outer diameter of 4.5-1 and a thickness of 1.5+a. Next, copper is removed using nitric acid or heating in a hydrogen furnace. Then, it is combined with a cathode sleeve, a brazing material layer, a heater, a filler, etc., and then impregnated with the electron emitting material as described above.

After impregnating the electron emitting material, the cathode surface is cleaned and, for example, Ir is applied onto the W substrate 10 by sputtering.
,000 coated. In the final step, this is heat-treated in a vacuum at a high temperature of 1000 DEG C. or higher to form an alloy layer with W as the base metal, that is, an Ir-W alloy layer I6 with a thickness of approximately 20,000X, on the surface portion.

Next, the functions and effects of the present invention will be briefly explained using a simple operational model. FIGS. 2 and 3 schematically show how free barium is supplied to the cathode surface at the surface of the porous cathode substrate of this impregnated cathode structure after a certain long-time operation. The code 11h in the figure is a W particle, Ilb is a hole, 1
6 is an alloy coating layer, 17 is a consumption area of electron emitting material, 18
is raining down areas where electron emitting materials remain. The main causes of the deterioration of the electron emission characteristics due to long-term operation of this cathode are, as mentioned above, the increase in the W concentration on the alloy coating surface and the decrease in the barium supply rate to the cathode surface due to the retreat of free barium production centers. This is considered to be attenuation. The feature of this invention is that it suppresses the rate of increase in W concentration on the cathode surface and the rate of retreat of free barium generation centers at the same temperature compared to conventional products, and therefore reduces the rate of deterioration of electron emission characteristics over operating time. It is about to be significantly reduced.

Therefore, we decided to reduce the barium concentration near the free barium generation surface to N
o (constant depending on temperature), N is the barium concentration near the cathode surface, and C is the total conductance of the base metal and the coating layer with respect to barium supply from the barium production center to the cathode surface. In addition, the amount of free barium generated per unit volume of porous W is m, and the cathode surface area (cross-sectional area) is 81 barium consumption depth of porous W (including the depth of the initial depletion layer).
Let be tp. Assuming that the electron emitting material is consumed sequentially from the cathode surface side, the following equation holds using these physical quantities and the cathode operating time t.

That is, in a state of thermal equilibrium, the amount of free barium produced per unit time (the left side of the equation) is equal to the amount of free barium per unit time flowing through the pores to the cathode surface (the right side of the equation 0). In the case of an actual cathode as well, barium production is thought to progress gradually from near the surface to the depths.

Here, the total conductance C can be expressed by the conductance Op of the porous W portion and the conductance (4) of the alloy coating layer as follows.

CCp Cf Further, if the porosity of the porous W is P, and the pore area in the cross section of the porous W is Sp, the following equation holds true.

Co-8aP Cp = □ ...■ p Here, Co is a constant proportional coefficient independent of porosity, and is an amount corresponding to conductivity. Then, using the 0 formula and the ■ formula, it becomes as follows.

In this equation, the denominator (Go-8-P/C7) is a converted depth obtained by converting the conductance of the coating layer portion by the conductance of the porous W portion, which corresponds to the barium consumption depth. In the case of the coating layer after alloying is completed, the change in the layer structure is extremely small, and therefore it is considered to be a constant that is almost independent of the operating time.

Also, if the entire denominator of equation (2) is t, and the amount of free barium generated from a unit volume of electron emitting material is M, then m in equation 6 uses the porosity p, and m = Mp. . Therefore, if we rewrite equation 0 using t(!:M), we get
It will look like this:

dt CoCo-8 MP S −= (No −N) ・・
...■dt t Now, consider long-term cathode operation at the same temperature.

The barium concentration No. at the center of free barium production is considered to be constant regardless of time, and the function of the operating time t in the formula is t
and N, and therefore it can occur as follows.

dl　　　C.

t=(No-N) ・・・・・・■dt
M Integrate this 0 equation over time t. Here, the value obtained by integrating the barium concentration N near the cathode surface over the operating time is the product of the amount of barium consumed in the porous W and the average life time τ for barium evaporation on the cathode surface, and this is the value of the barium concentration N near the cathode surface.
(L-t(o))τ. Note that L (o) is t(
1=0).

Using these, the formula 0 becomes as follows under the conditions of 1=0 and t=t(o).

Therefore, using equation 0, we get the following from equation (2).

The barium supply rate to the cathode surface is expressed as follows.

As can be seen from ■ and Equation 0, when comparing the same temperature and the same time, as the porosity p increases, the amount of barium supplied to the cathode surface per unit time increases, and the surface barium concentration also increases proportionally. Become.

However, the amount of barium evaporated from the cathode surface is equal to the amount supplied to the cathode surface. Therefore, if the porosity of conventional products is simply large, excessive barium consumption and poor withstand voltage due to barium evaporation may occur. This is easy and inconvenient.

Therefore, according to the present invention, the porosity p is increased, and the converted depth t(0) including the coating layer portion in the initial state is increased.
), the initial barium supply rate can be made equal to that of conventional products, and the change in supply rate with respect to operating time can be reduced.

For example, focusing on P and t (o) in the denominator of the right side of equation O,
Considering the case where both P and t(o) are doubled,
The operating time for which the initial barium supply amount does not change and the barium supply rate remains the same increases by four times.

In other words, by increasing the thickness of the coating layer, L
(o) can be increased.

Increasing the thickness of the coating layer also has the effect of suppressing the rate of increase in the W concentration on the cathode surface. Incidentally, in the initial state of operation after completion of alloying, the thickness of the alloy layer is d, the concentration of W is no, and the W diffusion coefficient in the coating layer at the operating temperature is D. Solving the diffusion problem and approximating the W concentration n (t) at the outermost surface of the cathode using the solution results in the following equation.

n(t) = 1-(1-no) er f (= wealth)
=@4Dt Therefore, the thicker the layer, the slower the increase in W concentration on the surface can be made.

Next, the effects of this invention will be described. In the embodiment of the present invention, as mentioned above, the porosity of the W body is about 32, and the I
This is the case when the thickness of the r-W alloy coating layer is approximately 20,000X. On the other hand, the conventional product compared with this has a porosity of 16 cm.
The alloy coating layer has a thickness of about 4,000×, and an enlarged schematic diagram of its surface is shown in FIG.

Figure 5 shows the change in the electron emission characteristics of both devices depending on the operating time.
The life test was started after sufficient activation.

The initial emission current is IA/crn2. For example, when comparing the two after 20,000 hours, the decay rate of the emission current is about 6.3 inches for the conventional product, while it is about 3.2 inches for the embodiment of the present invention, which is about 14 degrees. It's staying. The current attenuation rate is approximately proportional to the A power of the operating time t from approximately 1,000 hours for the conventional product and approximately 4,000 hours for the inventive product. It can be said that the amount is about four times that of the conventional one. Incidentally, the current decay rate is 1
The expected operating time for the conventional product to reach 0% is approximately 50,000 hours, and that of the present invention is approximately 200,000 hours. As described above, according to the present invention, the life characteristics are significantly improved.

As explained above, this invention is characterized by increasing the porosity of the base metal and the thickness of the surface coating layer within predetermined ranges. Regarding the effectiveness of this numerical range, the porosity is preferably in the range of approximately 20q6 to 40%, as described above. The thickness of the coating layer is determined by the operating temperature of approximately 1.5 mm. O
At OO'C, the barium thermal evaporation amount averages from about 1 to 3/15 I/(cr
IL2h) may be selected. When the porosity is 20%, the alloy coating layer is approximately 3,00%
When the porosity is about 0.0X and the porosity is 40%, the thickness is about 30.
A relationship of about 0001 is preferable. By increasing the porosity and layer thickness in this manner, the increasing amount of barium supplied can be moderately suppressed, and long-life characteristics can be obtained.

〔Effect of the invention〕

As described above, according to the present invention, both the increase in the W concentration on the cathode surface and the decrease in the barium supply efficiency to the cathode surface, which are the main causes of attenuation of electron emission characteristics during long-term operation of this type of impregnated cathode, can be reduced. It is possible to suppress the factors and improve the life characteristics.

[Brief explanation of drawings]

Fig. 1 is a schematic vertical sectional view showing an embodiment of the present invention, Fig. 2 is a schematic diagram showing an enlarged surface portion thereof, Fig. 3 is an enlarged schematic diagram of the main part thereof, and Fig. 4 is a schematic diagram of the conventional configuration. FIG. 5 is an enlarged schematic diagram of the main part of the product and a comparative characteristic diagram. 11... Porous cathode substrate, I6... Alloy coating layer. Applicant's representative Patent attorney Takehiko Suzue Figure 1 Figure 2

Claims (1)

[Claims] A cathode substrate made of a high melting point metal is impregnated with an alkaline earth metal oxide, and the electron emitting surface of the cathode substrate is coated with at least one selected from iridium, osmium, ruthenium, and rhenium. In an impregnated cathode structure in which an alloy coating layer of a metal and the metal of the porous cathode substrate is formed, the porosity of the porous cathode substrate is in the range of 20% to 40%, and the alloy coating layer is Thickness of 3,000 Å or more, 3
An impregnated cathode structure characterized in that the thickness is in the range of 0,000 Å or less.