The present invention relates to a magnetron for use in microwave ovens and so on, and more specifically, it relates to a magnetron having a filament improved to implement a long lifetime.
Generally, a magnetron generates microwaves efficiently and is widely used in application apparatuses, particularly, such as microwave ovens and thawing apparatuses, strongly requiring stability, high quality, long lifetime, and high efficiency.
FIG. 6A shows a cathode assembly of a conventional magnetron which is mounted on home electronic ovens.
The cathode assembly is arranged on a central axis of an anode cylindrical body (not shown). The cathode assembly comprises a rod-shaped center lead pin 1 made of a high melting point metal, a top hat 2, made of a high melting point metal, connected to an upper end of the center lead pin 1 and an end hat 3, made of a high melting point metal, connected to a lower end of the center lead pin 1, side lead pins 4a and 4b, made of a high melting point metal, connected to the end hat 3, and a helical filament 5 which circles around the center lead pin 1 and whose one end is connected to the top hat 2 and the other end is connected to the end hat 3.
In order to stabilize the electron emission characteristic, the filament 5 has a carbonized layer 7 which covers an outer circumference of a core wire 6 such as a thorium-tungsten wire.
The carbonized layer 7 is formed by electrifying the core wire 6 which is molded in advance in a helical shape of a predetermined dimension under a rare gas atmosphere containing carbon and by increasing a temperature of the core wire 6 higher than that at the time of oscillating as the filament 5.
The carbonized layer 7 of the filament 5 is exhausted gradually as time passes, and, when the carbonized layer 7 is extinct, the electron emission characteristic is degraded so that the magnetron is not available any longer.
Therefore, in order to achieve a long life span of the magnetron, it is desirable that the carbonized layer 7 be formed thick.
However, an outer diameter of a wire material that can be used as the filament 5 is limited to a predetermined range (for example, about φ0.5 to 0.6 mm) according to spatial conditions that can be secured in the magnetron and required electrical characteristics. Hence, when the thickness of the carbonized layer 7 is increased, the diameter of the core wire 6 should be reduced accordingly. Further, as the thickness of the carbonized layer 7 is increased, the life span is elongated. However, in this case, a mechanical strength to vibration or shock while carrying is reduced due to a decrease in diameter of the core wire 6, which result in causing a disconnection of the filament or the like. Further, there is a problem in that oscillation performance may be degraded due to degradation of the electrical characteristics.
Thus, in order to secure a long life span without degrading the electrical characteristics or mechanical strength, it is important to properly determine the thickness of the carbonized layer 7 with respect to the range of the diameter of the wire material that can be used as the filament 5.
From such a background, conventionally, there is suggested a technology in which the thickness of the carbonized layer 7 is in a range of 5 to 30 μm, for example, and is limited to less than 5% of the value of the outer diameter D of the wire material including the carbonized layer 7, such that the long life span and the maintenance of the electrical characteristics or the mechanical strength may be stood together (for example, see JP-B-60-53418).
As described above, the carbonized layer 7 of the filament 5 is formed by electrifying the core wire 6 which is molded in advance in the helical shape of the predetermined dimension. The carbonized layer 7 formed with such a manufacturing method does not have a uniform thickness since the center of an outer circumferential circle of the carbonized layer 7 is in an eccentric state with respect to the center of the core wire 6 due to a temperature difference at the time of increasing the temperature, as shown in FIG. 7.
For this reason, as described in Patent Document 1, according to the method in which the thickness of the carbonized layer 7 formed around the outer circumference of the core wire 6 is regulated according to a ratio to the outer diameter D of the wire material, when a location for measuring the thickness of the carbonized layer 7 is deviated, a significant difference in the total amount of the substantially covered carbonized layer 7 occurs.
Specifically, according to the prior art, even when the equipped amount of the carbonized layer 7 is defined, there occurs a significant variation in the total amount of the substantially covered carbonized layer 7. As a result, there is a problem in that a variation in electrical characteristic or mechanical strength easily occurs at the time of serving as the filament 5. Furthermore, there is a problem in that a significant variation in life span also occurs at the time of serving as the magnetron.
In addition, when the electrical characteristics of the filament 5 are different from those of the conventional product, there is also a problem in that compatibility as the magnetron is not implemented.