Heating Element for Indirectly Heated Cathode and Method for the Manufacture of Such an Element

The present invention relates to a heating element for an indirectly heated cathode. It also relates to the method for the manufacture of such an element.

Indirectly heated cathodes which are used in electron tubes are well known from the prior art. They generally comprise an emissive disk brazed to one of the ends of a cylinder made from a non-emissive material which serves as a box or casing. A filament for indirectly heating the cathode is placed within the cylinder.

A distinction is made between two types of indirectly heated cathodes. In the first type, heating is by a "free" filament which heats the cathode by radiation. The second type involves heating by a "potted" filament. The space within the cylinder, not occupied by the filament is filled with a material which is (i) a good heat conductor, (ii) electrically insulating at the operating temperature, (iii) whose melting point is high and (iv) which does not react with the filament and the cylinder at the operating temperature.

Cathodes indirectly heated by a "potted" filament, are less vulnerable to shocks and mechanical vibrations than in the case of cathodes heated indirectly by a free filament.

The present invention relates to cathodes indirectly heated by a "potted" filament.

In the prior art, the "potting" or member which locks the filament in the cylinder is formed by alumina powder fritted at about 2000.degree. C. or by a mixture of alumina powder and calcium oxide powder fritted at between 1750.degree. and 1800.degree. C.

According to the present invention, the "potting" is formed by a mixture of alumina and at least 10% by weight of an oxide of one of the elements of column IIIB of the periodic table of elements, said mixture being fritted at between 1700.degree. and 1800.degree. C.

According to a preferred embodiment of the invention, the mixture is formed by yttrium oxide and alumina of chemical composition 3Y.sub.2 O.sub.3.5Al.sub.2 O.sub.3, plus .alpha.-phase alumina.

The "potting" according to the invention has the following advantages. Fritting is carried out at between 1700.degree. and 1800.degree. C. and this temperature does not embrittle the tungsten or rhenium tungsten filament as is the case when heating to 2000.degree. C. for fritting pure alumina powder. The "potting" is firm and compact, ensuring a good long-term, thermal contact between the filament and the cylinder. It also leads to an electrical insulation equal to that obtained with "potting" based on alumina alone. The yttrium oxide which can be used is stable and very pure. Its coefficient of .alpha. linear expansion, equal to 8.18.10.sup.-6 is very close to that of the generally used filaments and identical to that of alumina. It is therefore possible to obtain a potting, whose expansion coefficient does not depend on the alumina and oxide proportions. Moreover, the thermal conductivity yttrium oxide is identical to that of alumina (.lambda.=0.0017cal. S.sup.-1.cm.sup.-1..theta..sup.-1 at 1800.degree. C. and 0.013 at 1000.degree. C.). Finally, the melting point of yttrium oxide (2410.degree. C.) is below that of e.g. calcium oxide (2572.degree. C.).

The invention is described in greater detail hereinafter relative to non-limitative embodiments and with reference to the attached drawings, wherein show:

FIG. 1 a perspective view of an indirectly heated cathode with a "potted" filament.

FIG. 2 a detail of the phase diagram of the alumina-yttrium oxide mixture.

FIGS. 3a, b and c diagrams illustrating the manufacturing method according to the invention.

The same reference numerals are used for the same parts in the various drawings, but for reasons of clarity the dimensions and proportions of the various parts have not been respected.

FIG. 1 is a perspective view of an indirectly heated cathode with a potted filament. This cathode is formed by an emissive disk 1 occupying one of the ends of a cylinder 2 made from non-emissive material.

Cylinder 2 is generally made from molybdenum and serves as a casing for the cathode and is also known as a cathode "skirt".

A porous tungsten disk 1 is brazed to one of the ends of cylinder, this process being performed at about 1900.degree. C.

When brazing has taken place, a tungsten or rhenium tungsten filament 3 covered by cataphoresis with an alumina insulating layer is introduced into the cylinder. The "potting" 4 is formed and this locks the filament 3 in the cylinder.

At a temperature of about 1750.degree. C., the porous tungsten disk 1 is impregnated with calcium and barium aluminate, which makes said disk emissive.

As the porous tungsten is impregnated at about 1750.degree. C. whilst filament 3 is located in cylinder 2, no disadvantage results from the "potting" being formed at a temperature of 1700.degree. or 1800.degree. C., as is the case according to the invention.

According to the invention, the "potting" is formed by a mixture of alumina and at least 10% by weight of an oxide of one of the elements of column IIIB of the periodic table of elements, said mixture being fritted at between 1700.degree. and 1800.degree. C.

Column IIIB of the periodic table of elements contains four elements, namely scandium Sc, yttrium Y, lanthanum La and actinium Ac. Yttrium will be used as an example here.

FIG. 2 represents a detail of the phase diagram of the alumina-yttrium oxide mixture extracted from the work entitled "Phase diagrams for Ceramists-- 1969--supplement".

The thick line curve indicates the melting temperature of the mixture of alumina Al.sub.2 O.sub.3 and yttrium oxide Y.sub.2 O.sub.3 as a function of the percentages by weight of the alumina and the yttrium oxide. The curve is discontinuous. For certain alumina and yttrium oxide percentages, melting takes place at a lower temperature than for other percentages, these being eutectic compositions.

In FIG. 2, it can be seen that for point A which essentially corresponds to 60% alumina and to 40% yttrium oxide, the melting point is 1760.degree. C. Around point A, the melting point exceeds 1760.degree. C. In the same way, it is remarkable that on the basis of more than 43% alumina, the mixture obtained in the solid state has the same chemical constitution, namely 3Y.sub.2 O.sub.3.5Al.sub.2 O.sub.3, plus .alpha.-phase alumina.

It is of interest that the alumina-yttrium oxide mixture is an eutectic because, compared with pure alumina, the fritting temperature can be reduced. It is also of interest that the chemical composition of the solid product obtained is the same within a wide range of respective alumina and yttrium oxide percentages. Thus, it is difficult when forming a mixture of powders (in the present case alumina powder and yttrium oxide powder) to ensure that the mixture is completely satisfactorily formed and that the percentage of the substances present is constant. It is therefore of importance to obtain the same chemical compound, even if the mixture is not completely homogeneous.

Thus, it is desirable to utilize the advantages of the alumino-yttrium oxide mixture compared with pure alumina, whilst attempting to obtain a mixture containing the maximum quantity of alumina. If the mixture contains too much yttrium oxide, there is a risk of the potting being disengaged from the cylinder acting as a casing, as well as the detachment of the filament from its "potting".

It has experimentally been found that the potting leading to the maximum number of advantages is obtained by fritting at between 1700.degree. and 1800.degree. C. whilst limiting the yttrium oxide percentage to about 10% by weight.

However, a good potting is obtained by fritting a mixture containing approximately 50 to 99% alumina and consequently approximately 1 to 50% yttrium oxide at between 1700.degree. and 1800.degree. C. In all cases, an yttrium oxide-alumina mixture is obtained of chemical composition 3Y.sub.2 O.sub.3.5Al.sub.2 O.sub.3, plus .alpha.-phase alumina.

FIG. 2 only shows the interesting part of the alumina-yttrium oxide phase diagram. In the rest of the diagram, the alumina percentage is low (below 40%) and the melting point and consequently fritting temperature are too high.

The alumina used in the potting composition can be constituted by several alumina varieties of different grain size distribution. Thus, for example, it is possible to use grains with a diameter of less than 10 .mu.m, as well as those with a diameter of 10 to 50 .mu.m.

The mixture of several varieties of alumina makes it possible to make a compromise between the defects and the advantageous qualities inherent in each variety. Thus, fine-grained alumina easily solidifies, but suffers from significant contraction, whilst large-grained alumina solidifies more difficultly, but forms a porous mass without contraction.

In the same way as described in detail for yttrium, the "potting" can be formed by fritting at between 1700.degree. and 1800.degree. C. a mixture of alumina and less than 10% of scandium, lanthanum or actinium oxide. The chemical composition of the bodies obtained will, in the case of lanthanum oxide, be La.sub.2 O.sub.3.11Al.sub.2 O.sub.3, plus .alpha.-phase alumina and, in the case of scandium oxide Sc.sub.2 O.sub.3.Al.sub.2 O.sub.3, plus .alpha.-phase alumina.

It is pointed out that the insulating layer deposited by cataphoresis on filament 3 can be an alumina layer, as stated in connection with FIG. 1. However, this insulating layer can also have the same composition as the mixture used for the potting, e.g. alumina and yttrium oxide.

FIGS. 3a, b and c illustrate a method for the manufacture of a heating element according to the invention.

According to this method, the powder of an oxide of one of the elements of column IIIB of the periodic table of elements and one or more powders of alumina of different grain sizes are intimately mixed, whilst stirring for at least 24 hours. The alumina powder must not exceed 10% by weight of the mixture. A solvent is then added to the mixture so as to obtain a paste.

The surface of the emissive disk 1 directed towards the inside of cylinder 2 is then coated with this paste 5. This stage is shown in FIG. 3a. The solvent is then slowly evaporated by using an e.g. 100 W electric lamp or by allowing to dry naturally. Filament 3 is then introduced into cylinder 2, this stage being shown in FIG. 3b.

The cylinder is then filled a number of times with the paste, whose consistency can be modified by adding the solvent. On each occasion, when paste has been added to the cylinder, the solvent is evaporated by using the electric lamp. The solvent can be acetone.

Finally, fritting takes place under hydrogen, i.e., in a hydrogen atmosphere at e.g. atmospheric pressure at between 1700.degree. and 1800.degree. C. for approximately 30 minutes so as to obtain potting 4.

By slowly evaporating the solvent in proportion to the addition of the paste layer to the cylinder, the formation of bubbles due to a rapid evaporation of the solvent from all the paste filling the cylinder is avoided.