FR2481849A1 - VOLTAGE LIMITER WITH GAS-TIGHT ENCLOSURE IN INSULATING MATERIAL - Google Patents

Abstract

SUBSTANCE 4.5 WITH A HIGH ELECTRONIC EMISSIONS IS AN ALLOY BASED ON BARIUM AND NICKEL CONTAINING 60 TO 99.8 BY WEIGHT OF NICKEL AND AT THE SAME TIME 0.2 TO 40 BY WEIGHT OF BARIUM.

Description

248 1849

  The present invention relates to a

  voltage comprising a gas-tight enclosure which is reacted

  made of an insulating material and inside which are arranged at a certain distance one from the other electrodes, at least one of which carries a substance containing barium and having a high emissivity of electrons. In voltage limiters of this type, the substance with high emissivity of electrons aims to reduce the

  ignition voltage at a low stable value desired by - -

  example when the voltage limiter is used in low voltage systems and in particular in installations

telephone calls. Substances containing alkaline metals

earthy or alkaline, including the suitable barium

especially well can be used for this purpose. However

the use of barium in its pure state creates great diff

  difficulties. Known from patent application DE-OS l 950 090, a voltage limiter whose electrodes are coated with a

  activation layer made of an alloy of barium and aluminum.

The disadvantage of this known embodiment is

  in that this layer is easily reduced to push

sier and creates a layer on the inner wall of the envelope

eg in insulating material during operation and in particular during high electrical stress, which causes

  in the long run a degradation of the insulation.

  In order to improve the characteristics of these voltage limiters, provision has been made, for example according to the requests for

  DE-OS patent 2,537,964 and 2,619,866, to add bro-

  potassium mure or titanium hydride with barium-aluminum alloy. This method has the disadvantage that the mixture of these different compounds must be applied in the form of a paste and by using a binder on the electrodes the binder must then be separated and eliminated by a particular method. The production of these voltage limiters is consequently complicated, expensive and not suitable for a

  rational mass production.

  The object of the present invention is to create a limit

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voltage tester operating under low voltage and mistletoe

  has a long service life without the characteristics

  electrical characteristics deteriorate during operation

  or during strong electrical stresses as well as a pro-

sold for rational mass production of limi-

voltage sensors of this type.

These problems are solved in accordance with the invention by a voltage limiter which is characterized in that the substance with high emissivity of electrons is composed of an alloy of barium and nickel. A voltage limiter of this type can be produced in accordance with the invention by

  forcing on the surface of at least one of the electro

trodes a mixture of nickel powder and

  barium-containing substance before assembling the elect

  trodes with the envelope of insulating material.

  Various other features of the invention res-

  emerge from the detailed description which follows.

  Two embodiments of the subject of the invention

are shown, by way of nonlimiting examples, above

sin annexed.

  Fig. l shows in section a voltage limiter

according to a first embodiment.

Fig. 2 is a section of a following voltage limiter

  a second embodiment.

  The voltage limiter shown in fig. l-

carries a cylindrical casing l of insulating material, for example a ceramic tube of aluminum oxide. Two electrodes 2 and 3, for example of a fusible Fe-Ni or Fe-Ni-Co alloy, are introduced into the open ends

  of the tube 1 and connected by fusion to the latter in a watertight manner

gas and so that the middle parts of the elect

  trodes 2, 3 are at a certain distance one in

opposite each other.

  In voltage limiters used in the

  telecommunications technology, particularly in ins-

  telephone installations, the envelope of insulating material may have a diameter of between 5 and 8 mm and it

  it is advantageous to use a ceramic tube whose thickness

wall thickness is between 1 and 1.5 mm, the thickness

electrodes being between 0.5 and 1.0 mma.

On their front faces located at a certain distance opposite one another, the electrodes 2, 3 carry a layer 4 and 5 of a substance with a high emissivity of electrons. This substance is preferably placed in

  hollow parts 6 and 7 of the front surfaces of the electrodes.

  In order to increase the adhesive property, the median surfaces of the front faces can have an embossed design 8, 9. The substances 4, 5 consist of a barium alloy

and nickel in which the proportion of nickel is advanced

  markedly between 60 and 99.8% by weight and that of

  barium between 0.2 and 40% by weight. A composition comprising

both 94% by weight of nickel and 5% by weight of barium

is particularly appropriate.

The gas-tight interior space 10 is filled with a suitable inert gas, for example argon, to which hydrogen may optionally be added. To avoid

  unwanted ignition delays, it is possible to ad--.

  add a small amount of tritium. It is also possible to provide other known ignition auxiliaries, for example in the form of conductive lines, etc. Fig. 2 shows an example of a voltage limiter which differs from that according to FIG. 1 due to the fact that the high electron emissivity layer is applied

on the electrodes. Unlike in fig. 1 the elect

  trodes of fig. 2 have median surfaces 10, it

  on which the layer 12 with high emissivity of elect

  trons is added so that its edges 13 match the

angles of the middle parts of the electrodes.

A voltage limiter of this type can be produced,

for example, as follows.

  A mixture of a substance containing barium and a

  nickel powder is introduced under pressure and without using

  sation of a binder in the central hollow parts 6, 7 of the electrodes 2, 3 before assembly by fusion of the latter with the casing 1 of insulating material. The barium contained in the substance can be in the elementary state or it is possible to use thermodegradable barium salt, for example BaH2, BaS, BaS4, BaBr and BaN6. Thanks to the drawing

  embossed with surfaces 8, 9, one obtains a property of adhesion

  sufficient. The electrodes thus prepared are connected to the envelope 1 of insulating material in a known manner, for example by patent CH 378,765, by fusion,

gas-tight by possibly inserting

  appropriate intermediary ches. This assembly is carried out by bringing the parts, optionally coated with suitable intermediate layers, into contact and by heating the parts firstly to around 450 ° C. under vacuum and then to around 950 ° C. under an inert atmosphere corresponding to the filling gas provided. During heat treatment

  the barium composition breaks down to form a subs-

  solid tance which contains barium and nickel. It is assumed that this substance then presents itself, preferably, under

  the form of an alloy of barium and nickel.

  The voltage limiters whose electrodes are coated with an activating substance of this type surprisingly have significant advantages compared to known voltage limiters comprising as

  activating substance of other barium alloys and in particular

alloys of barium and aluminum. It has been found that the reduction in dust of the activating substance during operation, and in particular during a strong

  electrical stress, is significantly lower when

that an activating substance containing barium and nickel is used at the same time as when using the known activating layers based on barium. As a result, voltage limiters of this type allow better

  preservation of the insulation of the interior wall of the

  even for extended periods of service.

  Consequently, these voltage limiters have a large

  longevity which is unexpected for those skilled in the art knowing the characteristics of alloys based on

  barium and nickel made this way.

  The manufacture of the described voltage limiters is furthermore very simple because the activating substance

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can be applied (without using a binder and therefore the pumping and gas filling operations are significantly simplified. In addition, no residue remains from the presence of a binder. The creation of the substance activation can be carried out at the same time as the assembly by fusion of the electrodes with the ceramic body by a single heat treatment. Consequently the process according to the invention allows a

  non-polluting, rational and mass production.

It should be noted that the method according to the invention is also suitable for voltage limiters of different shapes, in particular those which have more than two electrodes, for example double path voltage limiters. Instead of a ceramic envelope, an envelope made of another insulating material, for example glass, can be used. It is also possible to carry out the assembly using another fusion process. The nickel powder can also be doped with radioactive nickel 63, as is known from patent application DE-OS 2 445 063, which makes it possible to dispense with the addition of tritium to the filling gas.

In the case of installations operating under voltage

continuous ions, it may be sufficient not to coat with a layer or substance that reduces electron emission

  that the electrode which serves as cathode although it is general-

It is preferable, especially in the case of AC voltage limiters, to coat the two electrodes

an activation layer.

  The voltage limiter described and produced according to the method of the present invention is particularly suitable for protection against overvoltage of devices or systems operating at low voltage, for example telephone installations requiring a low and stable ignition voltage which must be kept for long periods of service, even for use

high electrical loads.

o

Claims (6)

1 - Voltage limiter comprising a gas-tight envelope (1) which is made of an insulating material and inside which are arranged at a certain distance opposite one another electrodes (2, 3) of which at least one carries a substance (4, 5) containing barium and having a high emissivity of electrons, characterized in that the substance (4, 5) with high emissivity of electrons is a barium-based alloy and of nickel.   2 - Voltage limiter according to claim 1, characterized in that the substance (4, 5) contains 60 to 99.8% by weight of nickel and at the same time 0.2 to 40% in barium weight.   3 - Voltage limiter according to claim 2, characterized in that the substance (4, 5) contains a proportion by weight of nickel of an order of magnitude of 95% and a proportion by weight of barium of an order of magnitude 5%.   4 - Voltage limiter according to one of the claims   cations 1 to 3, characterized in that the nickel present   in the substance (4, 5) is doped with radioactive nickel 63   active. - Method for producing a voltage limiter   according to one of claims 1 to 4, characterized in   that a mixture of nickel powder and a barium-containing substance is forcibly applied to the   surface of at least one of the electrodes (2, 3) before the as- appearance of the electrodes (2, 3) with the casing (1) in insulating material.   6 - Process according to claim 5, characterized in that the substance containing barium is elementary barium. 7 - Process according to claim 5, characterized in that the substance containing barium is a barium salt. 8 - Process according to claim 5, characterized in that the substance containing barium is a compound   from the group BaH2, BaS, BaS4, BaBr or BaN6.   9 - Method according to one of claims 5 to 8,   characterized in that the force applied mixture is heated   at a temperature that is higher than the decomposition temperature of the barium-containing substance during assembly by fusion of the electrodes (2, 3) with the envelope pe (1) in insulating material.