Patent Application
20090223699
The use of electrical leadthroughs in order to conduct currents, voltages or electric signals out of and into hermetically sealed tanks is known. For applications in which high temperatures can have an effect and/or for which a low leakage rate is required, glass is particularly suitable as an insulation material for the electrical conductor or conductors of the leadthrough. Crucial to the imperviousness of such a leadthrough is, among other things, the glass-to-metal transition between the electric conductor and the insulating glass material.
The difficulty with this type of leadthrough lies in, among other things, the fact that glass and metal generally have different coefficients of thermal expansion, which can lead to temperature stress and, consequently, to fissures in the glass material. The use of certain alloys, such as iron-nickel alloys in particular, which have a coefficient of temperature expansion matched to the glass, is a known measure for counteracting this problem. However, the problem then emerges that such alloys are not optimal with regard to the conductivity thereof. In order to improve conductivity, particularly in order to carry high current, an electrical leadthrough was manufactured in the past having a metal tube of such an alloy. Then a rod of a material having high conductivity, particularly copper, or brass or bronze was soldered into the tube in a second step.
However, a disadvantage of such a leadthrough is that a reheating when soldering still leads to thermal stress, which then considerably degrades the temperature resistance and long-term stability of such a leadthrough.
The underlying purpose of the invention therefore is to indicate a method that can be used to manufacture an electrical leadthrough that is improved with regard to the temperature resistance thereof.
This problem is solved in a highly surprisingly simple way by means of the object of the independent claims. Advantageous configurations and improvements are indicated in the subclaims.
Accordingly, the invention provides for a method for manufacturing an electrical leadthrough, for which at least one metal tube is fused in a glass insulator, whereby a rod of a highly conductive metal or metal alloy is hermetically joined to said metal tube prior to or during fusion of said tube in the glass insulation.
The metal rod need not necessarily be solid; it also is possible to use a hollow, tubular rod in order to, for example, accommodate an additional rod or in order to conduct a cooling liquid.
It is particularly preferred to solder the metal rod in the metal tube. In order to ensure a permanent bond even at higher temperatures, it is preferable, in this connection, to use hard solder to solder the metal rod into the metal tube.
Accordingly, it is possible with the method according to the invention to manufacture an electrical leadthrough having at least one conductor, comprising a metal tube and a metallic rod hard-soldered therein, fused in a glass insulator. Relative to known leadthroughs, a leadthrough manufactured according to the invention is distinguished by means of a higher temperature resistance and long-term stability, since a reheating in order to solder the inner metal rod inside the metal tube is eliminated. Such a reheating leads, in other respects, to stress between metal and glass, which leads to microfissures in the glass. Surprisingly, it is possible in this connection, to prolong the soldering process to the generally long duration of the vitrification and to nevertheless achieve a stabile soldering. Thus, the vitrification, or, as the case may be, the sealing of the tube in the glass can be carried out for a duration ranging from minutes to up to 36 h.
In addition, the metal tube and metal rod preferably comprise different materials. Here, in order to then avoid thermal stress, the metal rod is soldered to only one end of the tube in a preferred improvement of the invention.
For the glass-to-metal transition of the leadthrough, it is likewise advantageous to use a metal tube of a material having a coefficient of temperature expansion matched to the glass insulation. Coming into question here as a material for the metal tube are, among others, Ni—Fe alloys. Here, rather than the metal tube having to be composed exclusively of one such alloy, parts of the tube, such as the outer casing thereof, can be fabricated of a material having a matched coefficient of thermal expansion.
For an ability to conduct high current, it is furthermore advantageous if a copper rod is fixed in the metal tube. A suitable copper alloy having a high current conductivity can also be used.
According to yet another improvement of the invention, sealing is carried out—in contrast to a fusion under vacuum or low-pressure conditions—in an environment having a controlled gas atmosphere, particularly under normal pressure conditions. Such a controlled atmosphere can be an inert gas atmosphere, in particular.
The composition of the controlled atmosphere can be determined, in particular, by means of the glass type of the leadthrough and the metals used. Certain glass types and metals can be better processed in reduced or neutral environments. However, a controlled atmosphere can also have an oxidizing effect, in particular. This is of advantage in order to achieve, among other things, a particularly good glass-to-metal transition. Thus, the exterior surface of the metal tube can be oxidized prior to or during the sealing by means of, among other things, a suitable atmosphere. In an oxidizing atmosphere, an oxide layer, which can bond to the glass, forms on the metal tube. A targeted oxidizing of the exterior surface can however also occur by means of other alternative or additional measures. In addition, an oxidizing environment also suppresses or retards a conversion of oxidic components of the glass or metals.
In general, oxidizing gases can be released—even in a neutral or reducing atmosphere—as the glass and/or metals of the leadthrough conductors are heated. However, such elements are disadvantageous, particularly for fixing the metal rod in the metal tube by means of soldering. This applies particularly if the metal rod is soldered in without the use of flux, according to a preferred configuration of the invention. An oxidizing atmosphere worsens the wetting of the solder with the components to be joined; moreover, the solder can oxidize in the course of the generally quite long vitrification process and, consequently, can have no wetting or a sharply reduced wetting.
In order to nevertheless enable a soldering-in during the vitrification of the leadthrough, it is provided according to an improvement of the invention that a cap or sleeve that protects the soldering location during the sealing is placed over the metal tube. In particular, the cap or sleeve that protects the soldering location during the sealing can enclose the soldering location during the sealing. With such a cap, oxidizing gases can at least be partially kept away from the mounting location of the tube to the rod during the sealing. In order to further improve the effect of the cap, the cap can also be configured such that the cap absorbs or transforms oxidizing gases during the sealing. Such an effect can be achieved in a surprisingly simple way if the soldering location is protected, in particular, enclosed by a graphite cap or graphite-containing cap.
In order to achieve a protective effect for the soldering location, the cap or sleeve need not be composed exclusively of graphite, even if this embodiment is particularly preferred. It is also conceivable to use, for example, a cap of a fireproof carrier, which is lined or provided with graphite. Thus a metallic or ceramic cap, for instance, can be used which has been coated with graphite. In addition, the material of the cap can also have, in general, a high reaction bonding effect for oxidizing gases, at least in a hot state, i.e., a getter effect.
If a leadthrough is manufactured having several conductors, thus, accordingly, several metal tubes, the several metal tubes can also be covered with a common cap or sleeve. In particular, several metal tubes can also be fused in a common glass insulator.
In a preferred improvement of the invention, a sintered glass body is used, in which the metal tube or tubes are inserted. A sintered body assembled in this way is then melted in order to manufacture an impervious glass-to-metal bond to the metal tube.
In addition, the glass can also be melted in a metal body of the leadthrough, e.g., of a metal sleeve or of a flange, in order to manufacture an impervious bond of the glass with the metal part as a component of the leadthrough. If the glass is melted in the metal body, then an impervious bond results with the glass melted on the metal body. In this connection, alignment elements preferably are used to fix the metal tube in alignment to the metal body during the sealing, in order to obtain a precise alignment of the conductor or conductors to the metal body of the completed electrical leadthrough.
The method according to the invention can be used to solder the metal tube and rod to each other in such a way that the soldering location with which the metal tube and rod are bonded reaches very close to the surface of the glass insulation. Thus, yet another improvement of the invention provides for the soldering location to be arranged even at a distance ranging from 2-20 millimeters away from the glass surface.
A permanent bond is achieved with soldering if the metal tube and rod are bonded with a fillet weld or capillary weld. The fillet weld can preferably also reach in the tube, or, at the end of the tube, bond the inside thereof to the rod.
Among other things, the invention is particularly suitable for the manufacture of an electrical leadthrough for a safety tank. A preferred application is the manufacture of an electrical leadthrough for the reactor safety tank of a nuclear power plant. The invention is also outstandingly suited for manufacturing electrical leadthroughs for pressure or vacuum tanks.
In the following, the invention will be explained in detail with the aid of embodiments and with reference to the appended drawings. In this connection, identical reference numbers denote identical or similar parts.
Shown are:
Illustrated in
The leadthrough 1 comprises a metal body configured as a flange 3 having three individual leadthroughs 5, 6, 7. Screw holes 30 in the flange serve to fasten the leadthrough, e.g., to an opening of a safety tank, or of a pressure tank. Such a safety tank can be a reactor safety tank, in particular, of a nuclear power plant.
The individual leadthroughs 5, 6, 7 are arranged, in each case, in boreholes 10 in the flange 3 and comprise, in this embodiment, in each case, one conductor 9, which, with glass insulation 12, is insulated relative to the inner wall of the borehole 10. The conductors 9 comprise, in each case, a metal tube 14 in which a metal rod 16 is inserted and soldered in with hard solder without flux.
In this connection, soldering-in takes place prior to, or preferably during, sealing of the tube 14 in the glass insulation 12. A sealing of the conductors 9 in the glass insulation in the flange is also performed. For this reason, the glass of the insulation is melted to the metal body and a hermetic seal is also created on the inner wall of the borehole 10.
The metal tube 14 is fabricated from a different material than the copper rod 16. In order to improve the temperature resistance and resistance to thermal shock of the electrical leadthrough 1, it is preferable to use a material for the metal tube 14 having a coefficient of temperature expansion matched as closely as possible to the glass insulation 12. A preferred material for this is a nickel-iron alloy.
For this embodiment, the soldering location 20 is designed as a fillet weld, which is formed in the fillet of the exterior surface of the copper rod 16 projecting from the metal tube 14 and the end surface of the metal tube 14. The manufacturing method according to the invention allows the soldering location 20 to be arranged very close to the surface of the glass insulation 12. The distance here lies in a range from 2-20 millimeters.
In order to prevent temperature stress between the parts 14, 16 bonded to each other, a soldering location 20 is provided at only one end of the metal tube 14. The rod 16 can then move at the other end of the tube 14 longitudinally relative to the tube, owing to differing thermal expansion.
In order to connect cables to the conductors 5, 6, 7, the copper rods 16 each have a truncated end 17 having a through-hole 18. In this connection, cables can be fastened to the conductors 5, 6, 7 with a screw connection through the through-holes 18; however, other connecting techniques are also possible.
In addition, the alignment element 32 has a flat inner, cylindrical section 34 and a rim 36. The alignment element 32 is placed on the metal tube with the inner section 34 facing toward the opening 10 in the flange. The inner section here is shaped to correspond to the shape of the opening 10, such that the exterior surface 35 of section 24 can be pushed into the opening 10, until the rim 36 is supported against the outside of the flange 3. The borehole 33 and also the metal tube 14 inserted through it are therewith centered relative to the opening 10 of the flange 3. Graphite also is preferably used for the alignment element, since graphite does not adhere to molten glass.
Illustrated in
For centering, an alignment element 32 as illustrated in
In addition a graphite sleeve, as represented in
A flange thus equipped is then heated in a controlled-atmosphere furnace under normal pressure conditions, preferably under a slight overpressure. The composition of the atmosphere is selected, preferably, among other things, on the basis of the flange material and glass used. Melting of the sintered body 13 and sealing of the metal tube takes place over a period of time ranging from minutes to up to 36 h. During the often comparatively long period of time, soldering can be supported by means of the melting solder 21, rather than with flux, such that soldering occurs in a flux-free manner.
In order to improve the glass-to-metal bond, it can be of additional advantage to oxidize the exterior surface of the metal tube or tubes 14 prior to or during sealing. The oxide layer thus formed then bonds outstandingly with the glass.
Oxidizing gases, which otherwise are kept from the soldering location by the flux, can, however, oxidize the solder and/or the surfaces to be joined and, furthermore, can degrade the wettability thereof. Soldering can be achieved in a surprisingly simple way by means of protection with a graphite sleeve. However, the graphite sleeve absorbs oxidizing gases of the environment and transforms these in the case of carbon dioxide or oxygen and thus provides for a reducing or at least neutral atmosphere in its interior. The sleeve 25 can at least partially keep away oxidizing gases, in particular, for the entire duration of sealing such that a stabile, impervious soldering is achieved after the hard solder has set during cooling of the leadthrough in the furnace.
It is obvious to those in the art that the invention, rather than being limited to the aforementioned embodiments, can be varied in multifaceted ways. In particular, the features of the individual example embodiments can also be combined with each other.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 061 175.9 | Dec 2007 | DE | national |
