JPS6238819B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cathode assembly for an electron tube, and particularly to a cathode assembly suitable for a magnetron used in a microwave oven or the like.

Among electron tubes, magnetrons are incorporated into microwave ovens and used for thawing and cooking frozen foods because they efficiently generate microwaves.

BACKGROUND ART Conventionally, for a cathode structure which is a microwave generation source of a magnetron, a heating wire made of a tungsten alloy containing thorium is formed into a coiled wire, and both ends are supported by supporting members. This cathode structure is required to have various characteristics desirable as a magnetron cathode, such as high electron emission density and instantaneous operation. For this reason, the surface of the heating filament is usually carbonized to stabilize the electron emission characteristics.

However, in the case of usage in which the oscillation output is adjusted by flashing a large amount of current at short intervals,
With conventional cathode structures, it has been difficult to maintain stable electron emission for a long period of time.

The inventors conducted various studies regarding the cause of this and found the following. That is, it is thought that the biggest cause of the decrease in life is that the shape of the surface carbonized layer gradually deforms during use, the surface area increases, the temperature decreases, and electron emission deteriorates. The reason why the shape of the surface carbonized layer is distorted in this way is that the sliding of crystals within the carbonized layer is promoted by the pores present in the carbonized layer. Since heated filaments made of tungsten alloy are obtained by sintering powder, pores are trapped inside, but pores are concentrated especially in the carbonized layer on the surface, and after this carbonization treatment. It is thought that the vacancies present in the initial state serve as nuclei and grow during use, promoting surface deterioration.

In view of this, the present invention has discovered that the size and pore density of the pores that initially exist in the carbonized layer after carbonization treatment greatly affect the deformation of the surface carbonized layer during use, and based on this knowledge, The purpose of the present invention is to provide a cathode structure for an electron tube that has a long service life by defining the size of pores initially present in the carbonized layer and their density based on the following.

That is, in the present invention, a cathode assembly for an electron tube is formed of a tungsten alloy containing 0.5 to 4.0% by weight of thorium oxide, and a carbonized layer having a layered structure of W ₂ C and W is formed on the surface of the cathode assembly. It is characterized in that the size of the pores existing therein is 5 μm or less, and the pore density is 100 pores/mm ² or less with a size of 3 μm or more.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an embodiment of the cathode assembly of the present invention, and this cathode assembly has a structure in which supporting members 2 and 3 support and fix both ends of a heating wire 1 wound into a coil.

The heating filament contains 0.5 to 4.0% by weight of thorium oxide.
The base material is a tungsten alloy containing tungsten carbide, and a layered structure of tungsten carbide (W ₂ C) and W is formed on its surface. In the present invention, thorium oxide added to tungsten has the effect of increasing the electron emissivity of the heated wire, and the reason why the amount added is limited to the above range is that if it is less than 0.5% by weight, a sufficient amount of electron emission cannot be obtained. First, if the content exceeds 4% by weight, processability deteriorates. Further, in addition to thorium oxide, rhenium, iron, nickel, cobalt, chromium, molybdenum, etc. may be added alone or in combination, if necessary.

In addition, tungsten carbide, which forms a carbonized layer on the surface, contributes to electron emission, and its thickness is 10
A preferable thickness is about 60 μm.

The average size of pores existing in this carbonized layer in the initial state after carbonization is preferably 5 μm or less. The relationship between the size of the pores present in this carbonized layer and the life and strength of the heating wire is shown in the diagram of FIG. When the pore size was changed from 2 μm to 8 μm, it was observed that the life of the heated filament was improved by about twice that of the conventional level at 5 μm or less, and the strength was also improved by more than 1.5 times. It is recognized that there are

The lifespan was determined by repeating blinking at 5-second intervals with the heating filament assembled as a cathode structure as shown in Figure 1, and calculating the cumulative lighting time until the amount of electron radiation fell below the standard value. .

Also, looking at the relationship with the pore density, as is clear from the diagram in Figure 3, pores with a size of 3 μm
At less than 100 pieces/mm ² , it is recognized that the life of the heated filaments is approximately twice as high as the conventional level, and the strength is also approximately 1.7 times or more improved as compared to the conventional level.

The pore density was measured by cutting the carbonized layer and enlarging the cut surface using an electron microscope photograph.
This is a count of pores larger than μm.

Further, as a method of suppressing the size and density of the pores existing in the carbonized layer in the initial state within the above range, the following method can be used, for example.

First, tungsten powder, thorium oxide powder, and other additive element powders as needed are mixed and formed into a rod shape, and then sintered by heating with electricity in a non-oxidizing atmosphere. Next, after swaging this, it is drawn with a die and wire drawn to obtain a desired wire diameter. Next, this linear body is wound into a coil, and then electrically heated in a hydrogen gas atmosphere containing a hydrocarbon gas such as benzene to carburize the surface of the linear body to form a layer mainly composed of WC. In this case, the concentration of hydrocarbon gas is 0.2 to 0.2 to the hydrogen gas flow rate.
1.2% by volume is desirable, and heating is at 1800-2200℃
It is desirable to do this for 10 to 120 seconds. then on the surface
The linear body on which the WC layer has been formed is heated to 1800 to 2200°C for 10 to 120 seconds in a hydrogen gas atmosphere to convert the surface carbonized layer from a WC layer to a layered structure layer of W ₂ C and W. It is possible to obtain a heated filament with excellent surface carbonization and a carbonized layer with few pores.

The present invention is not limited to cathode assemblies for magnetrons, but can be applied to all cathode assemblies that operate intermittently, such as those for power tubes.

As explained above, according to the cathode structure for an electron tube according to the present invention, by specifying both the size and the density of the pores existing in the initial state in the heated linear surface carbonized layer, the intermittent operation is possible. Even if this is repeated, the growth and increase of vacancies is suppressed. Therefore, the vacancies existing in the initial state of the carbonized surface layer become nuclei and the surface carbonized layer is less likely to collapse, making it possible to maintain good electron emission characteristics over a long period of time and also having excellent strength. , the life of the cathode structure can be extended.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the general shape of the cathode structure, Figure 2 is a line diagram showing changes in life and strength as a function of hole size, and Figure 3 is a line diagram showing changes in life and strength as a function of hole density. It is a diagram. 1... heating filament, 2, 3... support member.

Claims (1)

[Claims] 1 A cathode structure in which a coiled heating wire is supported between a pair of electrodes, the heating wire is made of a tungsten alloy containing 0.5 to 4.0% by weight of thorium oxide, and the surface of the heating wire is A carbonized layer with a layered structure of W ₂ C and W is formed, and the average size of pores in this carbonized layer is 5 μm or less, and the pore density is 100 pores/mm ² or less with a size of 3 μm or more. A cathode structure for an electron tube characterized by the following.