FR2480496A1 - CATHODE FOR ELECTRONIC TUBES COMPRISING TWO HOLLOW BODIES - Google Patents

Abstract

THE CATHODE PRESENTS TWO HOLLOW BODIES 1, 2 IN PYROLYTIC GRAPHITE, WHICH ARE ARRANGED CONCENTRICALLY WITH RESPECT TO THE OTHER. THE EXTERIOR HOLLOW BODY SERVES AS A SUPPORT FOR THE EMISSION MATERIAL. THE TWO HOLLOW BODIES ARE CONNECTED TO EACH OTHER THROUGH THEIR (UPPER) END BY GALVANIC TRACK AND THEIR (LOWER) END IS EQUIPPED WITH CONTACT CONNECTIONS 4 FOR ELECTRIC HEATING. PREFERREDLY, THE TWO HOLLOW BODIES ARE IN THE FORM OF COAXIAL HOLLOW CYLINDERS. THE CATHODE CONFORMING TO THE INVENTION EXTENDS THE RANGE OF APPLICATION OF THE ELECTRONIC TUBES WHICH ARE EQUIPPED WITH IT UP TO THE RANGE OF VERY HIGH FREQUENCIES. APPLICATION: TO ELECTRONIC TUBES.

Description

"Cathode for electronic tubes."

  The invention relates to a cathode for an electron tube

  that having two hollow bodies, which are arranged concentrically with respect to each other, and which are interconnected by one end (upper), the hollow body

  exterior serving as support for the emissive material.

  Such a cathode is known from US Pat. No. 3,195,003. In this known cathode, which is shown in US Pat.

  3 195 003 as double-walled cathode and as co-cathode

  xiale, the two hollow bodies are in the form of hollow cylinders and made of metal. In the enclosure between

  the two hollow cylinders are arranged a heating device

  which implies that this cathode is heated by

  indirectly, the inner cylinder acting as a reflector of the heat generated by the heating device, but called in the first place to bear

  insulation, an outer cylinder heated to a temperature of

  high erase. In addition, it should be noted that the structure

  tube according to the US-American patent NO

  3 195 003 which serves to amplify very high frequency signals.

  in principle, is not, in principle, appropriate to the aim pursued.

  French Patent No. 758,525 also describes a

  thode for a gas discharge tube, having a

  heating conductor and consisting of several cylinders

  hollows arranged concentrically relative to each other, which are electrically connected to each other and which are provided with contact connections for heating

  electrical and which serve as a support for the emissive material.

  In this known cathode, the heating conductor is made of metal, graphite or silicon carbide. However, the emissive material is applied to the support so that the cathode can serve as a storage device for

  space charge, in which accelerated ions are again

  calves are rendered extremely inactive by recombination

with electrons.

  The German patent application made available

  (Offenlegungsschrift) No 2 732 960-describes a

  a thode with a pyrite graphite heating conductor

  This electric cathode serves as a support for the emissive material. However, it has been found that the planar (open) cathodic forms which are represented in pyrolitic graphite

  not suitable for the very high frequency range.

  The aim of the invention is to indicate a cathode making it possible to

  expand the range of applications of electronic tubes

  equipped up to the very high frequency range.

  According to the invention, this object is achieved because in a cathode of the kind mentioned in the preamble,

  the two hollow bodies are connected to each other galvanically

  that and their opposite (lower) ends are provided with contact connections for electrical heating and

  made of pyrolitic graphite.

  Unlike the open planar cathode mentioned above, the embodiment according to the invention can be

  best qualified as closed planar cathode.

  The two hollow bodies are in the form of two bodies

  concentric coaxials, for example spheres, half

  spheres, cones or truncated cones. The cylindrical shape

  that is a preferential embodiment, because of the

  that it is of frequent use in the technique of

  truction of the tubes. The application of pyrolytic graphite.

  anisotropic material for the heating conductor and the transmitter support for incandescent cathodes

  in electronic tubes is particularly advantageous.

  compared to the application of metals, thanks to the following properties: 1) Resistance to high temperatures up to 25000K (vapor pressure at this temperature: 10 bar), 2) no melting but a beginning of sublimation (in general no

  harmful) of carbon at temperatures equal to or higher than

  res at 25001K: 3) mechanical resistance that increases with temperatures; 4) low specific gravity (2.0 to 2.2 g cm) and, as a result, low inertia forces in the case of mechanical vibrations; 5) fairly high resistivity (depending on the structure) = 1.5 to 4.8. 10 4 Q.cm, which means ten to a hundred times greater than that of metals; therefore the material is particularly advantageous, also for thin planar conductors

  (e 1 means that the value indicated concerns the resistance

  parallel to the preferred orientation of base faces in anisotropic pytolitic graphite; 6) very good thermal conductivity parallel to the layers; A Ill = 2.1 to 4.2 J K 1 cm s 1, therefore of the same order of magnitude as the conductivity of copper; thereby,

  a very homogeneous temperature distribution, which

  bl very quickly in the case of switching on

of the heating system.

  The thermal conduction is also dependent on the structure and increases with the crystalline perfection of pyrolitic graphite up to five to ten times the value.

  indicated above of 2, 1 J K_1 cm1l s-1).

  In the cathode according to the invention, the

  design of the fast heating cathode or the

  "fast" heating unit for internally heated cathodes

  direct. In addition, the cathode according to the invention can be made in a very advantageous way both from the point of view

electric than thermal.

  The following description with reference to the

  xé, all given as a non-limitative example, will

  understand how the invention can be realized.

  Figure 1 shows a heating conductor.

  FIG. 2 represents a cathode whose heating conductor has a modified profile compared to that

according to Figure 1.

  Figure 3 shows a substrate for separation

of pyrolitic graphite.

  Figure 4 shows a layout for the composition

heating conductors.

  FIGS. 5a to 5d show several embodiments of the junction between two hollow cylinders and

  FIG. 6 shows a cathode presenting another

  modified wire of the heating conductor.

  FIG. 1 shows two thin-walled hollow cylinders 1.2 which are arranged concentrically with respect to each other and which are generally constituted by pyrolitic graphite and whose upper ends 3 are galvanically connected. between them, while the opposite ends (lower) are provided with contact connections 4 ensuring the supply of an electric current for

electric heating.

  Such a device offers the following advantages: a) In the case of a construction height determined for

  the cathode in an electron tube, the realization to -

  double wall makes it possible to obtain in practice more than twice the electrical resistance and, therefore, a ratio

  current-voltage more favorable for heating energy

  b) the heating energy supplied is better used because the heat radiated from the lower part is largely absorbed in the outer cylinder 2, which

  simultaneously serves as a support for the emissive material 5 (FIG.

  re 2) and it also contributes to heating (more

  pide) of the cathode c) the arrangement of the double cylinders rigidly connected to each other provides a fairly high mechanical strength, which can be increased further by the application, with a certain clearance, of at least one spacer 6 in the enclosure between the two cylinders, which parts are made of thermosolant ceramic material and

  electro-insulator for example (see Figure 2). These dis-

  positions allow the cathodes to be

  be self-supporting without resorting to ad-

  the structure and this in the case of very small wall thicknesses of 10 / one

  for example, which leads to the benefits that are inherent

  fast heating for relatively low heating currents. Pyrolytic graphite double-wall cylinders are produced by methods known per se for separating pyrolytic graphite suitably oriented from a carbon-containing gas phase. For this purpose, it is preferable to use so-called wall pyrolysis.

  hot-wall) because, in this way, heating

  ge required for uniform separation of more or less complicated substrate shapes and maximum ssudefarn. Hot wall pyrolysis is described in "Carbon" 5 (1967)

  pages 205 to 217. As a material for substrates, it is

  advantageously reads electrographite. However, it has been found that the production of the products according to the invention for

  hollow cylinders rigidly connected to one another by their

  (see Figure 1) in a separation step, in addition to

  very "one-piece" terms pose problems. This is due to the known fact that materials coming online from

  account for the substrate, here especially the electrographite, pre-

  have a higher coefficient of thermal expansion than the separate layers of pyrolytic graphite. It

  as a result of the lowering of the

  paration (about 2000C) at the ambient temperature after coating, the substrate contracts more strongly than the layer. (In the case of the realization of grid electrodes known from the German patent (Auslegeschrift) NO

  2,450,261, the required simple separation of the

  pyrolytic graphite) and the massive substrate is

  on this phenomenon). Or, as represented by the finre when the siustr

  it has the shape of a Paxoc pino cylinder or a thick neor S E Jecto -

  graphite, the cylinder of the substrate shrinks during

  cooling away from the cylindrical layer.

  external drique 2, while layer 1 deposited in the

  The interior is simply clamped. Shrinkage

2480496 =

  is indicated by the arrow 3. In general, a sepa-

  perfect ration of the substrate S and layers 1, 2 is not possible. Obviously, the substrate can in principle be removed from the envelope mechanically, for example by turning or grinding, but this method can only be used in

  certain special cases, for example in the case of large

  between the outer and inner cylinders, and

  at the same time, fairly large wall thicknesses of the layers.

  In general, to obviate said disadvantages,

  the production of double wall cylinders takes place in.

  several stages of manufacture, which will be explained below.

  after in detail with examples.

  For the inner cylinder as well as the outer cylinder are formed massive substrates of shape and dimensions required. These substrates are coated in a known manner (for example at 200 ° C.) under a pressure of 2 to 3 mbar of propane or similar hydrocarbons for a period of 3 to 5 hours. Under these conditions, an enveloping coating with a thickness of 200 to 300 μm is obtained, which

  can easily be removed from the substrate after removal

  the bottom part, for example by grinding. (The same technique is used for the realization of "semi-worked"

for grid electrodes).

  Or the two partial cylinders thus obtained 1 and 2 are compounded in pairs, possibly after treatment,

  in order to obtain a certain adaptation to the places 3

  for the junction. To ensure a rigorous fixation

  and concentric, we must generally use what we call

  the template. A device of the type described is illustrated schematically in Figure 4 where the template is designated by L. For the joining of the points to 3 can be used several techniques, such as welding, clamping, screwing etc.,

  but especially the methods of depositing from chemical vapor

  nomic. For the tests, preference was given to pyrolytic carbon separation and welding (for example

with Zr / Ni) for the junction.

  The partial bodies to be connected can present very diverse forms. Figures 5a to 5d illustrate (each

  only one section of the junction area is

  felt on the drawing; the junction place, even is

  re a circle). FIG. 5d symbolizes a junction area with the application of a centering element

  nel (hatched) of metal or carbon).

  Not only the shape of the junction place as well as the joining technique, but also the shape of the

  two partial bodies of the cathode can be widely

  That is why instead of the preferential cylinders

  used, it is possible to choose hollow bodies having cross sections or non-circular profiles, in which case a better fixation of the emitting material, for example the

form shown in Figure 6.

  It is not possible to prescribe energies of

  compulsory heating to obtain certain temperatures

  superficial erosion, since this is especially

  undermined by the kind and size of each coaxial cathode

  chosen, as well as by radiation and over-

  all also by the heat evacuation of the contacts. always

  However, it is apparent from the measurement that after commissioning

  heating energy, systems of the kind

  almost spontaneously, that is to say in less than one se-

  conde their final temperature and that equilibrium states

  of temperature settle quickly.

Claims (3)

  1. Cathode for an electron tube comprising two hollow bodies, which are arranged concentrically with respect to each other and which are interconnected by one end (upper), the outer hollow body serving as a support to the emissive material, characterized in that   two hollow bodies (1, 2) are connected to each other by way of   vanic and their opposite (lower) ends are   provided with contact connections (4) for electrical heating   and consist of pyrolytic graphite.   2. Cathode according to claim 1, characterized in that the two hollow bodies (1, 2) have the shape of coaxial hollow cylinders.   3.- Electronic tube preferably for very high frequencies, characterized in that it comprises a cathode   according to one of claims 1 and 2.