JPH0250582B2 - - Google Patents

Description

Claim 1: A cathode unit for a silica tube, comprising a cathode 3 permanently attached to a wall 1 of the silica tube and surrounded by a cathode shield 6;
In a cathode unit in which the cathode shield 6 is not electrically connected to the cathode 3 and at least a part of the cathode shield 6 is formed of a conductive material, the cathode shield 6 accommodates the cathode 3. an opening 8 is formed in the bottom of the casing, and the open end of the casing is sealed with an electrically insulating material 9, the electrically insulating material 9 being ,
It has a hole 10 located approximately in the center thereof, and the hole 10
has an area as small as possible to minimize blackening within the tube wall 1, and is large enough so that the starting voltage of the silica tube does not exceed a predetermined value; A cathode unit for a silica tube, characterized in that the opening 8 has approximately the same area as the hole 10 of the electrically insulating material 9.

2. The cathode unit according to claim 1, wherein the area of the hole 10 is about 100 mm ² or less.

3. The cathode unit according to claim 1 or 2, wherein the opening 8 at the bottom of the box-shaped casing is approximately oval.

4. The cathode unit according to any one of the preceding claims, wherein the box-shaped casing has a substantially cylindrical portion extending in the axial direction of the silica tube.

5. The cathode unit according to any one of the preceding claims, wherein the electrically insulating material 9 is a disk, and the hole 10 of the disk is rounded. .

6. The cathode unit according to claim 5, wherein the electrically insulating material 9 is a disc made of mica, and the thickness thereof is about 0.1 mm to 0.15 mm. unit.

7. A cathode unit according to claim 5 or 6, wherein the diameter of the hole 10 in the disc is about 10 mm to 12 mm.

8. The cathode unit according to any one of the preceding claims, wherein the box-shaped casing is made of iron or nickel.

9. The cathode unit according to any one of the preceding claims, wherein the cathode shield 6 is attached to the silica tube by a brace.

10. In the cathode unit according to any one of the preceding claims, the silica tube has a diameter of about 38 mm.
A cathode unit having a diameter of .

Description The present invention provides a cathode unit for a fluorescent tube, comprising a cathode, the cathode being permanently mounted to the tube wall and surrounded by a cathode shield, the cathode shield being made of a conductive material. The present invention relates to a cathode unit formed in this way and not electrically connected to the cathode.

The service life measured in hours of combustion time is determined primarily by the service life of the tube cathode. If the cathode loses some part of the radioactive material consisting of alkaline earth oxides, its electron emitting capacity decreases and the tube stops starting or enters a flickering phase, which rapidly crushes the remaining radioactive material. It becomes like this.

As is well known, excess barium dissolved in mixed crystals of radioactive materials makes the alkaline earth oxide semiconductive and reduces the free work of electrons. This excess barium is formed by a chemical reaction between barium oxide and tungsten according to the following equation:

6BaO+W→Ba ₃ WO ₆ +3Ba The barium tungstate thus formed remains throughout the lifetime of the cathode as an interlayer between the tungsten and the actual radioactive material, while the barium is absorbed through the material. Continuously diffuses in the form of vapor.

Barium tungstate attenuates the reaction, ie, reduces the barium composition, according to the above equation. Therefore, the entire barium does not evaporate until an ordinary fluorescent tube has been burned continuously for approximately 30,000 hours. However, the stresses that occur on the cathode during the starting process are so great that under normal use conditions of the fluorescent tube, i.e. with an average connection time of 2-3 hours, the service life is reduced by a factor of 2-3. be.

The loss of the cathode material acting as radiant material and the consequent shortening of the service life of the fluorophore tube can in principle be caused by three different processes: (1) loss of the radiant material due to ion bombardment, which occurs in particular in conjunction with insufficient cathode temperature; (2) evaporation of the emissive material, and (3) chemical reaction of the emissive material with gaseous impurities within the tube.

When designing a fluorescent tube for a very long service life in which the tube is flashed a significant number of times, the three reasons mentioned above that reduce the service life of the tube cathode must be taken into account. .

A prerequisite for the removal of emissive material due to ion bombardment is that, in principle, each atom leaving the surface of the cathode never returns to the cathode. However, this can only occur in a vacuum. In practice, the cathode in a common fluorescent tube structure is surrounded by a noble gas having a pressure of about 2.5 10 ² Pa. The free intermediate length of motion for atoms and molecules released from the surface is therefore considerably shorter than the distance between the cathode and the tube wall. Many of the released atoms and molecules therefore reflect and fall towards the cathode surface. This reduces material losses considerably. However, such a reduction is insufficient for long-life cathodes.

The evaporation of the radioactive material is relatively constant during continuous operation, but with each start-up this evaporation occurs rapidly and, due to the rise in cathode temperature, follows a start-up within a few minutes. This means that the cathode for a tube with a long service life will reflect the evaporated atoms and molecules back to the cathode surface to a significant extent, so that the cathode temperature will remain reasonably stable over the actual start-up time.

The purpose of the invention is to solve the problems outlined above and
It is therefore an object to provide a cathode for use in fluorescent tubes, which significantly increases the service life of the tube.

According to the present invention, the object is to provide a cathode shield comprising a box-shaped casing, an opening formed at the bottom of the casing, the cathode being inserted into the box, and an end of the box. This is achieved by providing a cathode shield in which the section is sealed by a disk having a central hole and formed of an electrically insulating material. In this type of cathode unit, a large amount of atoms and molecules released from the surface of the cathode by ion bombardment and atoms and molecules evaporated from the surface of the cathode are reflected back to the surface of the cathode. .

The cathode shield should preferably be made of iron or nickel. Preferably, the disk is made of mica, and is made of a material that does not shatter when ion bombardment is performed. The holes in the disk should be as small in diameter as possible to reduce darkening on the inside of the tube wall. However, if the diameter of the hole is too small, the starting voltage of the fluorescent tube will increase significantly. Therefore, the diameter of the hole in the disk should be as small as possible, and the starting voltage of the tube should not exceed a certain value. It has been found that the most suitable hole diameter is 10-12 mm for a common fluorescent tube where the tube diameter is 38 mm.

When manufacturing tubes, an effective pumping process removes all traces of the various gases, since undesirable chemical reactions between the radiant material and gaseous impurities in the tube can be a major hindrance to the tube's service life. It is important to remove Experiments have shown that the most effective pumping process is an automatic pumping unit, which combines a high-temperature vacuum pumping action with an "internal pumping action" obtained by dropping mercury into a hot fluorescent tube. I found out that it will be done. When a drop of mercury hits a fluorescent tube, the mercury evaporates explosively, creating a diffusion pump effect within the fluorescent tube. This is very effective for removing impurities. However, in order to obtain a sufficient effect of this kind, it is important that the cathode does not have an excessively limiting effect on the efficiency of the process. Therefore, it is desirable that the bottom opening of the cathode shield has an area at least equal to the area of the hole in the disk.

The invention will now be described in detail in the form of an exemplary design and with reference to the accompanying drawings, in which: FIG. FIG. 1 shows one end of a fluorescent tube having a cathode unit designed and constructed in accordance with the present invention. Figures 2a and 2b are a vertical cross-sectional view and a bottom view, respectively, of the cathode shield used for the cathode unit. FIG. 3 is a plan view of a mica disk for covering the open end of the cathode shield shown in FIGS. 2a and 2b. FIG. 4 is a chart showing the relationship between the starting voltage, the degree of blackening, and the pore diameter of the mica disk.

FIG. 1 is a cross-sectional view of one end of a fluorescent tube designed and constructed in accordance with the present invention. The glass wall 1 of the tube is sealed at one end by a foot 2 in the usual manner. This foot is also the cathode support 4
The support supports the tube cathode 3. The cathode support, which is electrically conductive, is connected by a supply line 5 fused into the foot 2, so that a current passes through the cathode 3 and heats it. The cathode 3 is surrounded by a cathode shield 6, preferably made of iron or nickel. Said shield 6 is supported by a brace 7 fused into the leg 2, which brace is electrically insulated from the cathode 3.

2a and 2b, the cathode shield 6 has a box-like shape, at the bottom of which an oval hole 8 is formed, and the cathode 3 and the cathode support 4 are It is now possible to insert parts. The open end of the cathode shield 6 is sealed by a mica disk 9, the thickness of which is preferably 0.10-0.15 mm. As can be seen in FIG. 3, said mica disk 9 has a central hole 10, preferably circular. The bore 10 has a diameter of 10-12 mm for a typical fluorescent tube having a tube diameter of 38 mm. Smaller diameters generally reduce blackening inside the tube wall, but at the same time increase the starting voltage to unacceptable values, as shown in FIG. This figure shows the starting voltage (U) in volts and the relative degree of blackening as a function of the diameter (D ₁₀ ) of the hole 10 in millimeters. Larger hole diameters reduce the starting voltage only slightly, but the blackening of the tube wall increases considerably.

It is important that the disk 9 is made of mica or other non-conductive material that does not generate gas.
The reason for this is that if this disk were made of iron, for example, it would generate more powdery material and thus increase the blackening of the tube wall.

A design as described above provides other advantages.
That is, when half of the cycle is completed, spiral 3
acts as an anode. Since the discharge has to pass through the perforated mica disk 9, the electron density adjacent to the spiral 3, which acts as an anode, increases significantly. The anodic drop is therefore reduced. This reduces the cathode temperature and therefore the rate of evaporation.

As mentioned above, the tube is preferably evacuated by a pumping process in which the vacuum pumping action is combined with an "internal pumping action" obtained by impinging a drop of mercury on the hot tube. Ru. This type of drop is diagrammatically indicated by 11 in FIG. Fluorescent tube (wall 1 and or foot 2) in which the drop was heated
When hit, the mercury evaporates explosively and the mercury vapor thus formed quickly escapes. Arrows 12, 13 diagrammatically indicate the most important flow paths for steam. The mercury vapor passing through the path represented by the arrow 13 should be removed effectively if the carbon dioxide located in the emissive layer and formed by the conversion of carbonates to oxides is to be removed, and if the internal When the pumping action must be effective, the cathode shield 6 and the mica disk 9
must not be disturbed by the structure formed by the For these reasons, the diameter of the hole in the mica disk 9 should be at least 10 mm (for a fluorescent tube with a tube diameter of 38 mm), and the bottom hole 8 in the cathode shield 6 should be at least 10 mm in diameter (for a fluorescent tube with a tube diameter of 38 mm). It should have the same area as the hole in the mica disk, preferably a larger area.

The cathode design as described above provides a service life 3-4 times longer than conventional fluorescent tubes while maintaining a normal burn time of 3 hours per connection.