JP2005259606A - Thermionic emission filament - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deformation and disconnection due to filament deterioration and to extend the life, maintain stable measurement sensitivity even after long use, and to be used for a measuring instrument typified by an ionization vacuum gauge In this case, it is an object of the present invention to provide a thermionic emission filament capable of reducing the overall energy by reducing the wire diameter to a high resistance and reducing a voltage drop in the cable.
SOLUTION: The surface of tungsten as a first metal for heat generation to be a bus bar of a filament wire is coated with iridium as a second metal as a protective film for preventing oxidation, and the second metal film The surface is coated with thorium oxide or yttrium metal oxide for thermionic emission.
[Selection] Figure 1

Description

  The present invention relates to a thermionic emission filament that can be used in a measuring instrument or the like that measures a pressure in a vacuum or a partial pressure of a gas species by exciting or ionizing gas molecules in a vacuum.

  Conventionally, a filament is placed in a vacuum, heated by applying a filament current to the filament, and then heated to a high temperature to emit thermoelectrons. One such application is using the filament. An ionization vacuum gauge is a measuring instrument that measures the pressure in a vacuum or the partial pressure of a gas species by emitting thermal electrons to excite or ionize gas molecules in the vacuum.

  FIG. 2 is a block diagram of an ionization vacuum gauge using a thermionic emission filament. As shown in FIG. 2, the ionization vacuum gauge using the thermoelectron emission filament 20 is composed of a thermoelectron emission filament 20, a grid 3 and an ion collector 5, a filament energization circuit 10, and an emission detection control circuit 11. And a high voltage application circuit 12 and an ion current detection circuit 13.

  The filament energization circuit 10 energizes the filament wire of the thermoelectron emission filament 20 via the cable 14 to heat the thermoelectron emission filament 20. The emission detection control circuit 11 detects an electron current emitted from the thermionic emission filament 20, and from the filament energization circuit 10 that energizes and heats the thermionic emission filament 20 so that the electron current always becomes a constant value. The output energizing current voltage is controlled.

  The grid 3 formed in a coil shape gives a positive potential to the thermoelectron emission filament 20 and gives sufficient energy to ionize the gas to the thermoelectrons emitted from the thermoelectron emission filament 20. Is for.

  The ion collector 5 having a needle shape is normally at a ground potential, that is, 0V. In such a potential arrangement, when the thermoelectron emission filament 20 rises to an appropriate temperature, the energy of the electrons exceeds the value of the work function required to jump out into the vacuum, and the thermoelectrons of the thermoelectron emission filament 20 Released from the surface. Most of the emitted thermoelectrons are grid-like in the grid 3 having a high potential, so that most of them do not directly jump into the grid 3, and after drawing various trajectories, the grid-like high-potential is finally formed. It flows into the grid 3. If collision with gas molecules in the vacuum occurs before electrons flow into the grid 3, the gas molecules are ionized, the ionized gas molecules flow into the ion collector 5, and the ion current detection circuit 13 detects the collector current. And measure the degree of vacuum from the collector current.

  JP-A-7-151816 (Patent Document 1) discloses an example thereof.

JP 7-151816 A

  As the thermoelectron emission filament 20, a filament wire material that is a bus bar made of iridium is now well known. Since iridium is a metal having a very low chemical reactivity and a stable property, the filament 20 in which the filament wire, which is the bus bar, is made of iridium is difficult to react with oxygen remaining in the vacuum, and disconnection due to alteration is prevented. This is because there is an advantage that the filament life is long.

  However, when the filament wire that is the bus bar is made of iridium, iridium has a low chemical reactivity and a stable property, but its melting point is 2727K, and the thermoelectron that the filament wire that is the bus bar is made of iridium. When the emission filament 20 is operated at a temperature of about 1500 to 1600 K for a long time, the thermoelectron emission filament 20 is deformed. When such deformation occurs, for example, in an ionization vacuum gauge, the electron trajectory until electrons emitted from the thermionic emission filament 20 reach the grid 3 changes, so that the sensitivity of measurement important as a vacuum gauge changes. There was a problem to do.

  Therefore, there is a demand for a thermoelectron emission filament that is less deformed even when the thermoelectron emission filament 20 is operated for a long time.

  In order to reduce the power of the measuring instrument, it is effective to increase the resistance value of the thermoelectron emission filament 20 to reduce the power consumed by the thermoelectron emission filament 20 and the cable. However, if the resistance value in the thermionic emission filament 20 is increased, the current value is decreased, and the power consumed by the cable, that is, the voltage drop is decreased.

  However, as described above, since the thermoelectron emission filament 20 in which the bus of the filament wire is made of iridium has a problem of deformation, the thinner the wire diameter, the easier the deformation, and the stability of measurement sensitivity. There is a problem that it becomes impossible to maintain.

  Therefore, the present invention has been made in view of the drawbacks of conventional thermionic emission filaments, that is, by taking advantage of the low chemical reactivity of iridium and excellent stability, the life of the filament is extended. At the same time, it is possible to prevent the filament from being deformed and maintain a stable measurement sensitivity for a long time. Furthermore, it is intended to provide a thermionic emission filament that can be used for an ionization vacuum gauge and the like that can reduce the overall energy by reducing the voltage drop in the cable by reducing the wire diameter of the wire and increasing the resistance. To do.

  The filament for thermoelectron emission according to the present invention is a filament for thermoelectron emission for heating a filament to emit thermoelectrons, and coating the second metal on the surface of the first metal that becomes the bus bar of the filament wire. The surface of the second metal film is coated with a metal oxide. Here, the bus | bath of a filament wire is a part used as the core which supports the structure as a filament wire. The first metal of the thermionic emission filament according to the present invention is made of tungsten, tantalum or molybdenum. The second metal of the thermoelectron emitting filament according to the present invention is made of iridium. The metal oxide of the thermoelectron emission filament according to the present invention is made of thorium oxide or yttrium oxide. In addition, the first metal of the thermoelectron emission filament according to the present invention has a melting point higher than that of the second metal. Further, the metal oxide of the thermoelectron emitting filament according to the present invention is obtained by coating the surface of the second metal only in a region where thermoelectrons are generated.

  According to the thermionic emission filament of the present invention, for example, tungsten, which is a metal having excellent heat resistance, is used as a bus bar of the filament wire, and the surface of tungsten is coated with iridium, which is a metal having low chemical reactivity, to provide a protective film. Since the structure is employed, even if the thermoelectron emission filament is heated, tungsten is not in direct contact with oxygen gas or the like, so that disconnection due to deterioration can be prevented and a long life can be achieved.

  Further, according to the thermionic emission filament according to the present invention, since it is difficult to be deformed even by long-time heat generation as described above and disconnection can be prevented, the wire diameter is used when used in a measuring instrument typified by an ionization vacuum gauge. It is possible to reduce the overall energy by reducing the voltage to a high resistance and reducing the voltage drop in the cable.

  Hereinafter, an embodiment of a thermionic emission filament according to the present invention will be described with reference to FIG. FIG. 1A is a cross-sectional view along the central axis of a thermionic emission filament according to the present invention, and FIG. 1B is a cross-sectional view in the radial direction of the filament.

  As shown in FIG. 1, the thermoelectron emission filament 1 includes a first metal (filament wire) 1a that is a bus, a second metal 1b that forms a metal film on the surface of the filament wire 1a, and a second metal. The surface of 1b is comprised with the metal oxide 1c for thermionic emission.

  The filament wire 1a serving as a bus bar is for heating the metal oxide 1c for thermionic emission that emits thermionic electrons. Moreover, it is also a part which supports the structure of the thermoelectron emission filament 1, and should not be deformed by heating due to long-time use. For the filament wire 1a, tungsten, tantalum or molybdenum having high heat resistance is used to support the structure of the filament wire 1a.

  Tungsten as the filament wire 1a has a melting point of 3655K, tantalum has a melting point of 3219K, and molybdenum has a melting point of 2883K, both of which are refractory metals.

  Tungsten, tantalum, and molybdenum are all metals with high heat resistance, but easily bond with oxygen gas or the like under high temperature conditions used as filaments, so that they are released as oxides and deteriorate, leading to disconnection. In order to prevent this disconnection, an iridium film is formed on the surface by sputtering or the like. As described above, iridium has a very low chemical reactivity and hardly reacts with oxygen gas or the like. Therefore, iridium is used as a protective film for preventing oxidation. The melting point of iridium is 2727K, which is lower than the melting points of tungsten, tantalum and molybdenum as the filament wire 1a serving as the bus bar. It is preferable that the iridium coating on the surface of the filament wire 1a serving as a bus has a thickness of about 1 μm, but the protective film 1b is for preventing tungsten or the like from coming into contact with the outside air. The film thickness of iridium may be 1 μm or less.

  As described above, iridium (second metal 1b) is coated on the surface of tungsten, tantalum or molybdenum of the filament wire 1a, which is the bus bar, by a sputtering method or the like, thereby preventing deformation due to deterioration caused by long-time heat generation. In addition, since tungsten, tantalum, or molybdenum of the filament wire 1a does not cause a chemical reaction by being combined with oxygen gas or the like, it can be used for a long time. Further, since tungsten, tantalum or molybdenum as the filament wire 1a has sufficient heat resistance, it is possible to form a high-resistance thermionic emission filament by reducing the wire diameter of the filament wire 1a. The current value flowing through the cable 14 (shown in FIG. 2) can be kept low. Therefore, since the voltage drop in the cable portion is reduced, the energy can be reduced when used in a measuring instrument represented by an ionization vacuum gauge. In addition, although it is suitable for the diameter of the filament wire 1a to have a diameter of 0.1 mm to 0.2 mm, it is not limited to this range.

  Further, the surface of iridium (second metal 1b) of the thermoelectron emission filament 1 according to the present invention is coated with a metal oxide 1c that emits thermoelectrons. Thorium oxide or yttrium oxide is used as the metal oxide 1c for thermionic emission. Thorium oxide or yttrium oxide is a material capable of emitting required thermoelectrons at a temperature of about 1500K to 1600K.

  Moreover, since the thermoelectron emission filament 1 according to the present invention is coated with the metal oxide 1c only in a range where the thermoelectron emission is performed on the filament, no thermoelectrons are emitted from a portion where the metal oxide 1c is not coated. There is no need to coat the filament for thermionic shielding.

  Although the shape of the thermoelectron emission filament 1 described above is a U-shape, the overall shape is not an essential problem. Even if the shape of the thermoelectron emission filament 1 is linear, it is coiled. It may be.

  As described above, according to the thermoelectron emission filament 1 of the present invention, tungsten having excellent heat resistance is used as the bus bar of the filament wire material. Even if it is used, there is no great difference in the trajectory until the thermoelectrons reach the grid 3, and stable measurement sensitivity can be maintained.

  Further, according to the thermoelectron emission filament 1 of the present invention, the voltage drop in the cable 14 can be reduced by reducing the wire diameter and increasing the resistance, so that the energy can be reduced as a whole.

It is sectional drawing which shows the structure of the metal material currently used for the filament for thermal electron emission by this invention, (a) is sectional drawing along the central axis of the filament for thermal electron emission, (b) is thermoelectron It is sectional drawing of the radial direction of the filament for discharge | release. The block diagram of the ionization vacuum gauge using the filament for conventional thermoelectron emission is shown.

Explanation of symbols

1 Thermionic emission filament 1a of the present invention The first metal (filament wire)
1b Second metal (protective film)
1c Metal oxide (coating oxide for electron emission)
3 Grid 5 Ion collector 10 Fragment energization circuit 11 Emission detection control circuit 12 High voltage application circuit 13 Ion current detection circuit 14 Cable 20 Conventional filament for thermoelectron emission

Claims (6)

  A thermoelectron emitting filament that emits thermoelectrons by heating a filament, wherein a second metal is coated on a surface of a first metal that serves as a bus bar of the filament wire, and a surface of the second metal film A thermionic emission filament characterized by being coated with a metal oxide.   2. The thermionic emission filament according to claim 1, wherein the first metal is tungsten, tantalum or molybdenum.   The thermoelectron emission filament according to claim 1 or 2, wherein the second metal is iridium.   4. The thermoelectron emission filament according to claim 1, wherein the metal oxide is thorium oxide or yttrium oxide.   The filament for thermoelectron emission according to any one of claims 1 to 4, wherein the first metal has a melting point higher than that of the second metal.   6. The thermoelectron emission filament according to claim 1, wherein the metal oxide is coated only on a region where thermoelectrons are generated on the surface of the second metal. 7.

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