DE2122949A1 - Process for the production of an anticathode for an X-ray tube - Google Patents

Description

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2122349

PHB- 32 059 PHB. 32059 .

May 7, 1971

Va / WJM.

Process for the production of an anticathode for an X-ray tube.

The invention relates to a method for producing an anticathode for an X-ray tube, which Electrode consists of a tungsten layer, which is on a

refractory substrate is deposited. ™

As is well known, X-ray tubes contain an anticathode and an electron source for generating electrons, which are drawn to the anticathode during operation, so that they hit this cathode at a very high speed on hitting, which entails. that the anticathode x-rays generated. The bombardment of the anticathode with the electrons also generates a large amount of heat, whereby the Anti-cathode, although it can stand still, in high-power

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operation is normally rotated, so that with The area bombarded by the electrons is always changed, with the result that each part of the anticathode only occurs during one electron bombardment is subjected to a small fraction of the total operating time. The surface of the anticathode Usually consists of tungsten, although - despite the very high melting point of this material - this tungsten surface becomes rough or cracked with continued operation, even if the anticathode is of the rotatable type. This again causes absorption and scattering of the emitted X-rays, as a result of which the efficiency of the anticathode is impaired more and more, so that it finally must be removed.

It is known that by adding an amount (~ 5 - 10 $) of rhenium to the tungsten of the anticathode this Roughening of the surface is avoided, so that the service life of the anticathode can be extended considerably. rhenium however, is a particularly rare and expensive material, so adding this material to the material the anticathode the cost price of the anticathode is significantly increased.

Tungsten itself is also quite expensive and is a dense material. Made from solid tungsten Anticathodes are therefore expensive per se and also have considerable mechanical inertia. The latter is disadvantageous if the anticathode is of the rotatable ffyp, because either reaching the operating speed

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this cathode then takes a lot of time, or a very powerful motor has to be used to accelerate the cathode. It has therefore been proposed to manufacture the base body of the anticathode from a different refractory material than tungsten and to apply a surface layer of tungsten or tungsten-rhenium to this body. For example, molybdenum has been used for this purpose, which is advantageous both in terms of cost and density. A rotatable anticathode of this type is, for example, in the M

French patent 1,470,382.

Because of the chemical properties of tungsten a sheet of tungsten is difficult to apply to any other refractory material. According to a standard procedure two separate layers of the two metals are sintered together or melted. Methods for depositing tungsten are also known; after the most famous of these Process is gaseous tungsten hexafluoride (WF ^) through Hydrogen is reduced to metallic tungsten, with a reaction of the general form: ™

WFz- 3H + _O 4 ± W + 6HF

O dd

is carried out.

A layer of tungsten deposited in this way on a molybdenum substrate was found to contribute Application as an anticathode for an X-ray tube has almost the same properties as solid tungsten, provided that the layer is of sufficient thickness (about 100 µm or more).

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British patent 784. kok describes a method of depositing a layer consisting of tungsten, molybdenum or an alloy of tungsten and molybdenum, according to which the material of the layer to be formed is applied in the vicinity of the object on which the layer is to be deposited must, while the material is ^{heated to a temperature above 1900 0} C but below its melting point, the object itself having a temperature between 500 ⁰ and 1400 ⁰ C, and the space between the mentioned material and the mentioned object with a Hydrogen and water vapor and / or oxygen-containing atmosphere is filled.

The invention has the purpose of creating a method for producing an anticathode for an X-ray tube, which cathode consists of a tungsten layer which is deposited on a refractory substrate, characterized in that tungsten is applied in the vicinity of the substrate and at a temperature above, is 19OO ⁰ C but heated below its melting point, where the substrate itself has a temperature between 500 ° C and 14OO ° C, and wherein the space between the tungsten and the substrate with a hydrogen and water vapor and / or oxygen-containing atmosphere is filled out.

Surprisingly, it has been found that a layer of tungsten deposited on a substrate existing and through this procedure (and through the in the

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patent application filed at the same time ... (PHB ...) described method) produced anticathodes for X-ray tubes can have a much longer useful life than anticathodes of similar structure with a layer deposited by the tungsten hexafluoride process and also than solid tungsten anticathodes. Their lifespan can be the same or even longer than that of anticathodes that are attached to a substrate. μ applied tungsten-rhenium layer. In addition, with an anti-cathode produced by a method according to the invention, the same service life as with a conventional anti-cathode can be obtained if the anti-cathode produced in this way has a smaller electron impact spot than the conventional anti-cathode. In this way, an increased resolving power of the X-ray radiation can be obtained without the service life of the anti-cathode falling below the service life normally obtained with conventional anti-cathodes. Attempts have shown "

that the beneficial effect occurs even when the layer material and the substrate material are both tungsten. These The appearance cannot be fully explained, but tests have shown that with a by an inventive The anticathode method produced the electrode surface to a much lesser extent by the incident electrons is damaged than with the known anticathodes made of the same materials. It was found that conventional ones made of solid tungsten or made of by the tungsten hexa-

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fluoride process on molybdenum-deposited tungsten Existing anticathodes have the disadvantage that their surface becomes rough, which is caused by thorough recrystallization and granule separation is brought about or is associated with these phenomena. Probably this is due to a low intergranular strength and the granule boundaries high energy of the material on the surface of these known anticathodes, one according to the invention The anticathode produced by the method has been found to have a considerable amount in this regard Improvement on.

Since the incident electrons penetrate into the anticathode layer during operation, this layer preferably has a thickness of at least 100 µm. With thinner layers, a significant number of the noticeable Electrons reach the substrate, especially as surface damage progresses.

The substrate is preferably made of tungsten-molybdenum or an alloy of tungsten and molybdenum or from alloys based on these metals.

In a method for producing an anti-cathode for an X-ray tube according to the invention, the tungsten is preferably heated to a temperature between 2200 ° C and 3000 ° C, while the substrate is heated itself to a temperature between 65O ⁰ C and I3OO ⁰ C.

In practice, the precipitation method has the offset effect when carried out in an atmosphere

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Hydrogen and water vapor as well as in an atmosphere Hydrogen with an admixture of oxygen, the same relative amounts of hydrogen and oxygen being obtained because in the latter case the oxygen together with some of the hydrogen on contact with the hot material to be precipitated will react as water. The precipitation rate is determined by the total gas flow rate and affects the relative proportions of hydrogen and water (or oxygen). If desired the atmosphere can also contain one or more inert gases such as nitrogen or the noble gases. The precipitation process is advantageously carried out at atmospheric pressure.

The invention is explained in more detail below, for example, with reference to the accompanying drawing. Show it:

Fig. 1 by the inventive method manufactured anticathode for a rotatable type X-ray tube, and

2 shows the results of an accelerated life test on the anticathode according to FIG. 1 (curve 1), as well as corresponding results from life tests on similar anticathodes made of solid tungsten, (curve 2), from a tungsten layer produced by the usual method, in which method tungsten hexafluoride is deposited (curve 3)> or from an alloy of tungsten and 10 $ rhenium, the electrode being manufactured again by the usual method (curve k) .

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In the manufacture of the anticathode according to Fig. 1, a preformed molybdenum substrate 1 becomes airtight in an air-tight manner closed space attached and by induction to about Heated to 1200 ° C. Hydrogen gas is initially supplied by a Humidification system, which is formed e.g. by a vessel, the water or an aqueous solution on a contains such a temperature that a vapor pressure is obtained which causes the humidification system to produce a starting mixture of hydrogen and water vapor in a ratio of 1: 1. The starting mixture of the humidification system is then passed through a tungsten tube, the outlet opening of which is brought to a temperature above by induction 2400 ° C is heated. The water vapor in the mixture of hydrogen and water vapor reacts with the tungsten of the Rohres, such that a gaseous tungsten hydroxyl plus hydrogen is obtained.

The reaction products emerge from the outlet opening and are then passed over the heated substrate, on the surface of which the reverse reaction takes place, with metallic tungsten on the molybdenum substrate being knocked down. The process continues until the layer 2 of tungsten is deposited on the substrate has a thickness of 100 µm or more. The substrate is rotated slowly during the process so that a deposit that is as even as possible is obtained. Preferably only those in the finished X-ray tube are to be bombarded with electrons

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Parts of the substrate coated with tungsten. This can be achieved in that the substrate is masked in a suitable manner and / or with respect to the outlet opening of the tungsten tube attached and this outlet opening is formed in such a way that the gases almost uniquely to those Get parts of the substrate that are to be coated.

It is obvious that the temperature of the tungsten tube, the substrate temperature and the ratio between water vapor and hydrogen in the mixture passed through the tungsten “tube can vary within wide limits All through W + H ₂ O £? W (OH) _x + H ₂

can be expressed in the clockwise direction at high temperatures and in the opposite direction at low temperatures. The state of equilibrium is thus regulated by the temperature and the water vapor / hydrogen ratio. »

When this ratio is nearly equal to 1, there is a rapid clockwise response at a temperature above about 2000 ° C and in the opposite direction to this at a temperature below about 1200 ° C. When using a higher substrate temperature higher water vapor / hydrogen ratios can be obtained within certain limits; This is advantageous and results in an improvement in the tungsten transport speed without reducing the total volume of the

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deten gas mixture increases. It should, however, be secured that a higher substrate temperature does not have any adverse changes the physical or chemical properties of the Substrate. An increase in the substrate temperature can, for example, result in the substrate being different from the Gas mixture is oxidized.

Considering these factors, the tungsten pipe is preferably heated to a temperature above 1900 ⁰ C and the substrate to a temperature between 500 ° C and 14OO ° C.

The water vapor / hydrogen ratio in the

The gas mixture used can be between 0.1 and above 1; the temperatures of the tungsten and the Substrate chosen so that they are adapted to the ratio actually used. If desired, the Hydrogen can be replaced by a mixture of hydrogen and an inert gas such as nitrogen.

Although molybdenum is the preferred material for the substrate because of its price and density, it should it is obvious that other materials, such as tungsten, a tungsten-molybdenum alloy or other alloys based on molybdenum can be used. However, it should be taken into account that certain materials are less suitable because they are subject to the electroplating conditions oxidize, in which case it is difficult to obtain a coherent wolf fall. Hesse.

For example, carbon can be used as the refractory material

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can be used, but in this case it would probably be necessary to combine this material with e.g. tungsten or tantalum would be coated before the inventive Procedure would be used.

Although the manufacture of a rotatable type X-ray tube anticathode has been described, it is It is obvious that the process can also be used to manufacture an anti-cathode of the stationary type *

2 shows curves of the relative X-ray radiation capability E as a function of time t in minutes for an anticathode produced by the method described from a 250 μm thick tungsten layer (curve 1), made of solid tungsten (curve 2), for a similar by the usual one Electrode produced by the tungsten hexafluoride method (curve 3) and for a similar electrode with a layer of tungsten with 10 $ rhenium deposited on a molybdenum substrate by the usual method (curve k) . It turns out that the useful life of the electrode produced by the method according to the invention is considerably longer than that of the electrode produced by the usual method and is equal to the life of the electrode provided with a tungsten-rhenium layer.

It is advantageous to sand the surfaces of the deposited layers smooth; thereby the X-ray yield increased.

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Claims (7)

PHB. 32059 - 12 PATENT CLAIMS. '1 J A process for preparing an anti-cathode for an X-ray tube which electrode consists of a deposited on a refractory substrate tungsten layer, characterized in that the tungsten attached in the vicinity of the substrate and to a temperature above I9OO ⁰ C, but below its melting point , is heated, wherein the substrate itself has a temperature between 500 ° C and 14OO ° C, and wherein the space between the tungsten and the substrate is filled with an atmosphere containing hydrogen and water vapor and / or oxygen. 2. The method according to claim 1, characterized in that that the tungsten to a temperature between 2200 ° C and 3000 ° C and the substrate itself to a temperature between 650 ° C and 1300 ° C is heated. 3. The method according to claim 1 or 2, characterized in that the process is continued until a tungsten layer is obtained with a thickness of at least 100 / µm on the substrate. 4. The method according to one or more of the preceding claims, characterized in that the substrate consists of tungsten, molybdenum or an alloy of tungsten and molybdenum. 5. The method according to one or more of the preceding claims, characterized in that the tungsten in Shape of a tube is designed through which the mentioned 10 ^ 830/1 108 2122049 PHB. 32059. Atmosphere is passed through. 6. The method according to one or more of the preceding claims, characterized in that the ratio between water vapor and hydrogen in the atmosphere mentioned is between 0.1 and 1. 7. Anticathode for an X-ray tube produced by the method according to one or more of the preceding Claims is made. 10a8oO / 1 108 Le e rs e ι te