JPS5812695B2 - Manufacturing method of thermionic emission cathode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides high-performance and economical thermionic emission using a chip composed of a two-layer structure consisting of a single crystal and a polycrystal of lanthanum hexaboride (LaB6). Concerning a method for producing a cathode.

Hexaborides of rare earth elements such as LaB6 have a small work function, so they can be used in place of conventional tungsten and oxide-based thermionic emission cathode rod members or tips (hereinafter referred to simply as cathode tips). It is attracting attention as a material.

However, this material has the disadvantage that it cannot be used as a thermionic emitting cathode because it is very difficult to heat and support it because it reacts with most materials at high temperatures at which it emits thermionic electrons.

As an improvement method for this, a tungsten coil is placed around the cathode tip made of LaB6 to indirectly heat it (Japanese Patent Publication No. 45-40576), and the cathode tip made of LaB6 is replaced with pyrolytic graphite that does not react with LaB6. Methods have been proposed in which support and heating are carried out (Japanese Patent Publication No. 47-25911), but all of them have problems in practicality.

On the other hand, cathode chips made of LaB6 are available in polycrystalline and single-crystalline types.

A polycrystalline material obtained by cutting and processing a polycrystalline sintered body by thermo-pressure molding into a predetermined size has better electron beam characteristics than a tungsten hot cathode, but it is not sufficient.

That is, the maximum brightness of the tungsten cathode is 1×104A.
/cm2str, the luminance of the LaB6 sintered cathode is 2×105A/cm2str.

On the other hand, single-crystal ones, compared to polycrystal ones,
Although it has excellent performance such as high brightness, high current stability, atmospheric stability, and long life, it has the disadvantage of being expensive.

In other words, the method for manufacturing a single crystal of LaB6 is disclosed in, for example, JP-A-51
-Grow slowly using the floating zone method (FZ method) as shown in i18362.

However, this FZ device is expensive and takes time to grow, so the manufactured single crystal LaB6 becomes expensive.

Therefore, it is desired to reduce the use of single crystals of LaB6.

An object of the present invention is to provide an inexpensive and practical thermionic emission cathode without losing the excellent thermionic emission characteristics of single crystal LaB6.

That is, in the present invention, a single-crystal LaB6 disk with a thickness of 2 mm or less is placed in a hot press device, and then polycrystalline L is placed on top of it.
AB6 powder is placed and hot press molded to form a rod-shaped product with a two-layer structure.This is cut into a chip shape, and the tip of the single crystal part is processed to make a thermionic emission cathode chip. It is characterized by being assembled into a cathode by a conventional method.

The present invention will be further explained in detail below.

First, the rod-shaped molded product with a two-layer structure consisting of a single crystal and polycrystalline LaB6 used in the present invention is obtained by cutting polycrystalline LaB6 into a single-crystal rod-shaped body by the floating zone method to a thickness of 2 mm or less. This was put into a hot press molding machine, polycrystalline LaB6 powder was placed on top of the disc, and hot press molding was performed to obtain a two-layered integrally molded product.

The tip of the single crystal portion of the molded product thus obtained is processed into a chip, which is then assembled to form a thermionic emission cathode.

Generally, methods for bonding or coating polycrystalline materials on single-crystal ceramics include CVD, vacuum evaporation, sputtering, and ion plating, but LaB6 has a high melting point, making it difficult to obtain a dense bonded body. It has drawbacks such as slow growth rate and difficulty in maintaining high bonding strength when the bonding layer becomes thick.

On the other hand, with the hot press method, it is possible to bond a polycrystalline body of any thickness to a single crystalline body, and the bonded body is obtained in the form of a disk, which simplifies the subsequent process of making chips. It has the advantage of being

Hereinafter, the present invention will be explained in detail with reference to the drawings.

The drawings show embodiments of the present invention, and FIG. 1 is a perspective view showing an indirect heating junction type thermionic 5-emitting cathode of the present invention.

The tip end 1 of the chip is made of single crystal LaB6, and the lower part 2 is made of polycrystalline LaB6, indicating that the two are joined.

3 is a support holder made of copper, and the tip end portion 1
It's getting more heated.

FIG. 2 is a side view of the directly heated gripping type thermionic emission cathode of the present invention.

A cathode chip in which a tip portion 1 made of single-crystal LaB6 and a lower portion 2 made of polycrystalline LaB6 are integrated is heated by a heater made of graphite 4 with a current introduced from an electrode 5.

The dimensions of the chip according to the present invention are usually about 1 mm in diameter and 15 mm in length in the case of the cathode shown in FIG. 1, and about 2 mm in length with each side of 0.5 mm in the case of FIG. 2. A small one is used.

In any chip, the part that has the role of emitting thermionic electrons is only a very small part of the pointed tip, and the other part has no relation to thermionic emission, so it has a diameter of about 0.5 mm, for example. The part other than the tip is made of expensive single crystal LaB.
There is no need to set it to 6.

In addition, in the present invention, the tip and lower part of the chip are made of the same type of LaB6 material, although the type of crystal is different, because the material must be chemically inert and when used repeatedly, This is to prevent cracking and peeling from the joint due to differences in expansion coefficients.If this is made of different materials with different coefficients of thermal expansion, it will easily peel off from the joint due to repeated use at high temperature and room temperature. This is because it becomes impossible to make a durable chip.

In order to integrate the tip and the lower part, it is preferable to use a hot press method as shown in the examples.

As described above, the present invention provides a hot cathode using a cathode chip in which only the tip that plays the role of thermionic emission is made of single crystal LaB6, and the part that does not have the role of thermionic emission is made of polycrystalline LaB6. This is the manufacturing method for La that makes up the chip.
This has the effect of providing a high-performance hot cathode at a low cost in which the B6 single crystal and polycrystal do not separate.

Hereinafter, the present invention will be explained in more detail with reference to examples.

Example A rod-shaped molded product having a diameter of 20 mm and a length of 200 mm was prepared from LaB6 powder using a rubber press, and the rod-shaped molded product was further heated in an inert atmosphere to obtain a sintered LaB6 rod.

After machining this into a size of 15 mm in diameter and 180 mm in length, it was placed in an FZ device, and a part of the sintered rod was partially heated by high frequency heating under 30 atmospheres of argon gas to create a melt zone. A high purity LaB6 single crystal rod was obtained by moving up and down several times.

From this single crystal LaB6 rod, the diameter is 10 mm and the thickness is 2 mm.
A disk was created by mechanical processing.

This disc has an inner diameter of 10 mm, an outer diameter of 70 mm, and a length of 100 m.
The sintered body was placed in a graphite die of 1.5 m in diameter, filled with polycrystalline LaB6 powder so that the thickness of the sintered body after molding was 18 mm, and hot press sintered at 2050°C and 350 kg/cm2 for 60 minutes. It was joined.

After hot pressing, the molded product taken out had single crystal LaB6 on one end for 2 mm, had a diameter of 10 mm, and a length of 20 mm.
This is a LaB6 bar.

This hot pressing operation does not convert the single crystal LaB6 into a polycrystal.

This is because single crystals are more energetically stable.

The hot cathode chip of the present invention was obtained by mechanically processing the rod in which the single crystal and polycrystal were joined so that the tip from which thermionic electrons were radiated became a single crystal.

A hot cathode was assembled as shown in Figure 2, and the temperature was raised to 1550°C.
Thermionic emission tests were conducted at

As a result, there was no peeling or cracking from the joints even after 500 hours of use.

In addition, in the case of a chip composed only of polycrystalline LaB6,
It became unusable after about 300 hours.

In addition, the measured value of brightness is 7 × 105 A/cm2,
This value was almost the same as the value when using a chip made only of single-crystal LaB6.

Furthermore, the stability of the electron beam was 1% or less per hour, which was very good.

Comparative Example Zirconium boride (Zr) was used instead of polycrystalline LaB6 powder.
A chip was manufactured in the same manner as in Example 1 except that B2) or titanium boride (TiB2) was used, and this chip was used as a hot cathode.

However, all of these could not be used as commercial products because they cracked and had large variations between chips, causing chips to fall off when used as hot cathodes.

[Brief explanation of the drawing]

The drawings show an embodiment of the present invention, and FIG. 1 is a perspective view of an indirectly heated bonding type LaB6 hot cathode, and FIG. 2 is a side view of a directly heated and gripped type LaB6 hot cathode. 1... Single crystal LaB6, 2... Polycrystalline LaB6, 3... Copper support holder, 4...
...Pyrolytic graphite heater, 5... Electrode.

Claims (1)

[Claims] 1. A single-crystalline LaB6 disk with a thickness of 2 mm or less is placed in a hot press machine, and then polycrystalline LaB6 powder is placed on top of it and hot-press molded to form a two-layered integrally molded product, which is then shaped into a chip. 1. A method for manufacturing a thermionic emission cathode, which comprises cutting out the single crystal, processing the tip of the single crystal part to make a thermionic emission cathode chip, and assembling this chip into a cathode by a conventional method.