NO146492B - HARDENABLE BINDING MATERIAL FOR USE IN THE PREPARATION OF A CASTABLE FUEL AND USING THE BINDING MATERIAL - Google Patents

Description

Getter device for electron tubes and other evacuated discharge containers.

The invention relates to a getter device for electron tubes and other evacuees

discharge containers, containing as active metal zirconium, hafnium, titanium, thorium

or one of their alloys. Here, getter device is to be understood as well as the activated part in a . evacuated vacuum container,

as the not yet activated part containing gas-binding metal or a hydride

of this metal in non-active form, ie

oxide layer is present on the surface.

There are already known getter devices consisting of pressed tablets or

pills or powder pressed into the container.

The starting material is obtained by granulation

of a larger pressing body containing

the hydride of the gas-binding metal in grains

with a diameter of less than 5 microns,

mixed with tungsten grains of considerable

smaller diameter to prevent sintering. Of

the granulated material is pressed into a fairly coarse filtered fraction for gas binders.

The fine powder in and of itself is not suitable

to be directly processed into a gas binder.

In many applications, the gas absorption rate of the known pressures

getter devices too small even if the absorption capacity is large enough. This circumstance leads to the application of many

pills or tablets or large containers

with pressed material, which is undesirable for various reasons.

In a getter device for electron tubes and other evacuated discharge containers, containing as active metal zirconium, hafnium, titanium, thorium or one of their alloys, in the form of fine grains mixed with grains of a metal that prevents sintering, such as tungsten or molybdenum in the same amount by weight as the active metal, according to the invention the primary grains, which consist both of grains of active metal and of grains of metal that prevent sintering, have a surface of 25-30 m.2/g and are compressed into secondary grains with a diameter of between 100 and 200 microns and that these secondary grains, without further compression, are arranged so that they are not mutually displaceable in an at least partially perforated container.

With the device according to the invention, a composition is obtained where the large specific surface of the former grains is formed by short micropores in the latter grains. The latter grains are formed in macro-pores between these grains. With this composition, at least an order of magnitude greater absorption rate is achieved compared to the known devices where the grains are compressed and where micropores are unlikely to occur.

In the case of the known compressed devices, mostly only a volume reaction is effective, which is consistent with the hitherto generally prevailing perception of gas binding. However, such a reaction takes place very slowly. With the device according to the invention, the entire surface can be reached by the gases to be absorbed and bound in a surface reaction. This surface reaction is achieved by the device according to the invention at high speed due to pore diffusion which is determined by the combination of micropores and macropores. The experimentally determined diffusion rates are in good agreement with the theory and the resulting calculations. In the case of the known devices, any micropores that may be at hand provide no opportunity for sufficient diffusion.

In the device according to the invention, in order to achieve the greatest possible gas absorption rate, it is necessary that the latter grains are not compressed, but it is permissible to give these grains a small pre-tension in the container to ensure tear-proof storage of the grains.

The data for the specific surfaces are given here for not yet activated material. Upon activation, a reduction of approx. 30. The metal grains which prevent sintering preferably have the same specific surface area and are present in approximately the same amount by weight.

If, in the device according to the invention, the latter grains have a considerably larger diameter, then the macropores also become larger, but the micropores through which a larger part of the active surface of the former grains is reached, become longer so that the gas absorption rate is reduced. The diameter of the latter grains should therefore lie between a few tens of microns and a few tenths of a mm, and the ratio between the largest and smallest diameters should preferably be less than 2. The most favorable dimensions lie between 100 and 200 microns. The latter grains can be obtained by granulating a larger pressing body which is produced with a pressing pressure of less than 5 tons/cm 2 , preferably 1 ton/cm 2 .

The device according to the invention not only provides a high gas absorption rate, but the degassing and especially the avoidance of hydrogen upon activation when a hydride is split, becomes much easier, so that even at higher operating temperatures, unlike with the known devices, one need not fear any noticeable hydrogen pressure.

The invention shall be explained in more detail with reference to the drawing. Fig. 1, 2 and 3 show getter devices according to the invention. Fig. 4 shows the joint storage of the grains. Fig. 5 shows a diagram with different gas absorption curves.

In fig. 1 shows an axial section through a nickel cylinder 1 with a diameter of 4 mm and a length of 4 mm and which is closed at both ends with gas 2 or 3 of curved nickel steel. The gas consists of each

other crossing threads with a diameter of 30 microns and a space between the threads

of 30 microns. Inside the cylinder 1 there are grains 4 of a metal which prevents sintering with a diameter of 100 to 200 microns and which are produced in the following manner. Zirconium hydride powder with a specific surface area of 25-30 m2/g is mixed

with tungsten powder with approximately the same specific surface in a weight ratio of 60:40.

The mass is compressed under a pressure of 1 ton/cm2 and then granulated. A filtration fraction is taken from this granulation.

In fig. 2, the same grains 4 are shown caught between two wire nets 5 and 6 which are connected to each other along the edges 7, and thereby a better accessibility to the grains is achieved. In order to have an easy heating option during activation, a nickel ring 8 is arranged inside the container which is heated by means of eddy currents. Insertion of a tin plate is also possible.

In fig. 3 shows a cross-section of another device according to the invention, consisting of two gas rings 9 and 10 with a diameter of approximately 22 mm and a height of 12 mm and as above and below and on

eight places parallel to the axis are welded together, so that eight pockets are formed which are filled with grain 4.

In fig. 4, the grains 4 are shown on a larger scale than in fig. 1. The grains of active metal in the form of zirconium hydride are indicated by 12 and grains of tungsten by 13. Although the drawing does not show the correct proportions, it is easy to see that the micropores 14 are considerably smaller than the macropores 15.

The diagram in fig. 5 has a horizontal axis indicating the amount of gas occupied in Torr. litres, while the vertical axis indicates the gas absorption rate in liters per Sec. All measurements were carried out with carbon monoxide as the test gas because this gas forms the majority of the residual gas in electron tubes and is also very harmful. Curve I applies to a device according to fig. 1 containing 50 mg of grain. Curve II applies to a device according to fig. 2 where the gas package had a diameter of 14 mm. From those curves it is easy to see that with the same amount of absorbed gas, the absorption rate in curve II is approximately an order of magnitude greater over a larger area of capacity utilization, which means a better availability to the grains. Curve II applies to the same powder weight as curve I. Curve III applies to two bags according to fig. 2 each of which is filled with 50 mg of powder. Curve IV applies to the device according to fig. 3 filled with 500 mg of powder. Of the four curves, it can be seen that they more or less approach the ideal square curve, i.e. an ideal getter device behaves so that the gas absorption rate S remains approximately constant until full utilization of the absorption capacity Q. All the curves apply to starting powder with a specific surface of 25 m2/g.

For comparison, the curve V relating to a known device is drawn, where 26 mg of the same powder is used as the powder that was used to determine the above-mentioned curves, but in the form of a tablet with 3.5 mm and approx. 1 mm thickness is pressed into a metal strip. The diagram in fig. 5 clearly shows that the difference in the gas absorption rate between curves I and V is more than an order of magnitude greater than one would expect from the difference between the powder amounts. The greatest gas absorption rate that can be achieved with the device according to curve V is 3.10-2 liters per second at an occupied amount of 5.10—s Torr.liter.

Claims (1)

Getter device for electron tubes and other evacuated discharge containers, containing as active metal zirconium, hafnium, titanium, thorium or one of their alloys, in the form of fine grains mixed with fine grains of a metal that prevents sintering, such as tungsten or molybdenum, in approximately the same amount of weight; as the active metal, characterized by the fact that the primary grains, which consist of both grains of active metal and grains of metal that prevent sintering, have a surface area of 25-30 m2/g and are compressed into secondary grains with a diameter of between 100 and 200 microns and that these secondary grains, without further compression, are arranged so as not to be mutually displaceable in an at least partially perforated container.