Method For Making A Tube

Abstract

A gas-filled discharge tube is fabricated by mounting a plurality of electrodes in an envelope, inserting an ampul containing mercury and fabricated of a material which strongly absorbs infrared radiation and has a low melting point into the envelope, evacuating the envelope, filling the evacuated envelope with a filler gas, forming the electrodes to obtain uniform surfaces and a low work function, vacuum filling the sealed envelope, and applying infrared radiation through the sealed envelope so that it is absorbed by the ampul and the mercury is released. The envelope of the tube is preferably made of a glass having weak infrared absorbing properties.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method for applying mercury in an electronic, gas-filled discharge tube, particularly a number indicator tube operating on the glow principle as well as a tube manufactured according to this method. In this method, a glass ampul containing the mercury is inserted into the tube and the mercury is released only after completion of the forming process and vacuum sealing of the tube by thermic deformation of the glass ampul.

To produce high life values for gas-filled discharge tubes, particularly for number indicator tubes according to the glow principle, it is known to provide the electrode which is at cathode potential with a thin layer, or coating, of mercury. Due to its high atomic weight, the mercury acts as an inhibitor for the ion dispersion. Since the mercury coating of the individual cathodes should be as uniform as possible, this covering can not be applied at arbitrary moment. Rather, the tube must first be pumped free of air, filled with a filler gas and then subjected to a forming process which is intended to obtain very uniform cathode surfaces and a work function as low as possible. The mercury coating must be done only after these process steps are completed. In order to accomplish this, it is known to initially insert the mercury into the envelope of the tube in an ampul and then, at the desired moment, to release the mercury by destroying or deforming the ampul.

Small glass ampuls are known for this purpose which are provided with an electric heating coil in addition to the mercury. The heating coil is then heated by the passage of current and the glass ampul is thus softened and bursts. This known method has the disadvantage that the heat of the heating coil releases easily vaporizable impurities which act on the cathode surface before the mercury is deposited thereon. As a consequence, there again appear zones with a higher work function on the cathode surfaces. Moreover, such ampuls require additional solder points and additional leads in the tube base.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process as described above which is improved particularly with respect to the above-mentioned disadvantages.

It is proposed according to the present invention to achieve the above object by manufacturing the ampul of a glass which strongly absorbs infrared radiation and which has a low melting point. The deformation of the glass ampul is effected by means of the energy of high-intensity infrared radiation directed through the walls of the tube.

A significant advantage of the present invention is that with the release of the mercury and change of form of the ampul, no additional metal parts need be heated inside the tube so that it is assured that no impurities are vaporized and deposited on the cathode surfaces. A further advantage of the present invention is that a tube constructed according to the present invention can be manufactured in a simpler manner than previous tubes because additional melting and welding points are eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a glow tube according to the present invention with infrared radiation being applied to an ampul.

FIG. 2 is a cross-sectional view of an ampul according to the present invention which has a tab for mounting it to an electrode or electrode lead.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 of the drawings shows a number indicator tube operating on the glow principle having a plurality of known cathodes 3 which glow upon the application of a potential and an anode 4, both disposed in an envelope 1. The envelope 1 is filled with a protective gas, for example an inert gas such as neon and/or argon, at a pressure of approximately 35 torr. The individual electrodes 3 and 4 are connected with respective electric leads 2 in the base 10 of the tube.

Methods of making cold cathode indicator tubes are well known. Examples of such methods are described in U.S. Pat. No. 2,946,642, issued July 26, 1960 to S. Kuchinsky and U.S. Pat. No. 2,991,386, issued July 4, 1961 to Szigeti et al.

According to the present invention, a closed ampul 5 in which a slight amount of mercury 6 is disposed is arranged in the envelope 1 before it is sealed. This ampul 5 is made of a glass which absorbs infrared radiation particularly strongly and which has a relatively low softening point. Such a glass is, for example, the lead-free IR melting glass, type 8512. This glass may be appropriately colored to increase the absorption capability, or absorptivity, for infrared radiation. Moreover, the vapor pressure of the glass of ampul 5 should be relatively low at its softening temperature in order to minimize the amount of vapor released in the envelope 1. Especially the oxygen partial pressure should be as low as possible. The softening point of the IR melting glass, type 8512, is about 660.degree. Celsius. This glass has at its softening point a sufficiently low oxygen partial pressure to avoid an oxidizing of electrode materials. The infrared radiation absorptivity of this glass is about 70 to 95 percent, for example about 95 percent at a wavelength of the radiation of 1.1 .mu.m.

The envelope 1 of the tube advisably is of a glass that absorbs infrared radiation only to a relatively slight degree. Any type of glass conventionally used for electron tubes can be used for this purpose, since their absorption capability, or absorptivity, for infrared radiation is not very high. The ampul 5 is advisably fastened to an electrode input lead as, e.g., by melting in. Alternatively, tabs or the like may be used to fasten the ampul 5 to an electrode or an electrode lead. FIG. 2 shows an embodiment of an ampul 5 enclosing an amount of mercury 6, for example a drop of mercury, and which is fastened to an electrode lead 11 by means of a tab 12 which may be melted into the wall of the ampul 5. The tab 12, which preferably consist of a metal wire, may be welded to the electrode lead 11.

The infrared radiation absorptivity of conventionally used glass for electron tubes is lower than 10 percent.

After the tube provided with the ampul has been filled with gas and the forming process for the electrodes has been completed, a suitable, known infrared radiator 8 directs a strongly bundled infrared radiation 7 from a suitable, known parabolic mirror 9 to the ampul 5 built into the tube. Radiator 8 may be, for example, a suitable, known lamp. This infrared radiation penetrates the wall of envelope 1, since the latter absorbs a relatively small amount of the infrared radiation, and reaches the ampul 5 and is strongly absorbed thereby. The absorbed infrared radiation leads to rapid softening (for example, approximately 20 seconds) and deformation, or melting, of the ampul 5 so that it will burst and release the mercury, which due to the slight vacuum in the envelope, is deposited in the desired manner on the surfaces of the electrodes 3. Since, in this process, no other metal portions are heated to any significant degree, the danger of impure electrode surfaces is substantially reduced.

It is to be understood, of course, that the method of the present invention may be used in any vacuum tube where appropriate coating of electrodes is desired.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Claims

We claim:

1. A method for fabricating a gas-filled discharged tube comprising the steps of:

a. mounting a plurality of electrodes in an envelope made of material which absorbs little infrared radiation;

b. inserting an ampul containing mercury, and which is fabricated of glass which has a low melting point and strongly absorbs infrared radiation to a degree which is substantially higher than the material of the envelope, into the envelope;

c. evacuating the envelope;

d. filling the evacuated envelope with a filler gas;

e. forming the electrodes to obtain substantially uniform surfaces and a low work function;

f. vacuum sealing the filled envelope;

g. directing high-intensity infrared radiation through the sealed envelope onto the ampul to burst the ampul and release the mercury.

2. A method as defined in claim 1, further including the step of selecting a glass for the envelope which absorbs little infrared radiation.

3. A method as defined in claim 2, further including the step of adding coloring agents to the glass of the ampul for increasing its infrared radiation absorption property.

4. A method as defined in claim 1 further comprising providing the ampul with an exposed outer surface free of surrounding metallic structure and members.