Abstract:
An electrically heatable glow plug and a method for manufacturing an electrically heatable glow plug are proposed that enable a protection of a heating coil of the glow plug against nitridation and evaporation of the aluminum from the heating conductor alloy. The glow plug includes a glow tube that is closed at the end, into which the electrically conductive heating coil is inserted, the heating coil being formed at least partially of aluminum, in particular of an aluminun-iron-chromium alloy. In the glow tube, oxygen donors are provided in order to form an aluminum oxide layer on the surface of the heating coil before or during the heating of the heating coil.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an electrically heatable glow plug and a method for manufacturing an electrically heatable glow plug. 
   BACKGROUND INFORMATION 
   German Patent No. 19928037 describes an electrically heatable glow plug for internal-combustion engines that includes a glow tube that is closed at its end and is corrosion-resistant, and that accommodates a filling of a compressed, electrically nonconductive powder in which there is embedded an electrically conductive filament. The filament includes a heating coil. This heating coil is formed from an iron-chromium-aluminum alloy. In the area of the heating coil, the electrically conductive filament is hardened on its surface. In this way, the filament can withstand the mechanical stress during the compression process without damage. 
   German Patent No. 19756988 describes an electrically heatable glow plug for internal-combustion engines that has a glow element made of a corrosion-resistant metal jacket. In the glow element there is contained a compressed powder filling. An electrically conductive filament is embedded in the filling. In order to increase the life span of the filament, a getter material is provided in the glow element for the binding of the oxygen contained in the compressed powder filling. The getter material can be distributed in the compressed powder filling in the form of electrically non-conductive particles. These particles can be made of silicon or metal oxides of metals that oxidize in several oxidation stages and that have a higher affinity to oxygen than does the filament material; in the initial state, the getter material can contain the metal oxides in their first oxidation stage. 
   European Published Patent Application No. 0079385 describes a heating element in which a filament is situated in a sheath and is embedded in an electrically insulating powder. The powder has 0.1 to 10 weight percent of an oxide, and in this way prevents the oxidation of the metallic portion of the filament. 
   SUMMARY OF THE INVENTION 
   In contrast, the electrically heatable glow plug and the method for manufacturing an electrically heatable glow plug have the advantage that in the glow tube oxygen donors are provided, in order to form a layer of aluminum oxide on the surface of the heating coil before or during the heating of the heating coil. In this way, in the case of a penetration of air into the glow tube, the formation of nitrides in the edge layers of the heating coil, and thus a local increase of the electrical resistance and a premature failure of the heating coil, are prevented. 
   A further advantage is that an evaporation of aluminum from the alloy can largely be suppressed. 
   An economical realization of the supply of oxygen donors results when the heating coil in the glow tube is embedded in a first insulating powder, the first insulating powder including a material that acts as an oxygen donor. 
   It is particularly advantageous if the oxygen donor is formed as a metal oxide that can oxidize in several oxidation stages and that is present in its highest oxidation stage. In this way, the oxygen release of the metal oxide is promoted considerably. 
   The same holds correspondingly if the oxidic ceramic powder includes a metal oxide that, under reducing conditions, can release oxygen through defect formation. 
   It is also advantageous if the oxygen donors are brought into the glow tube in the form of oxygen molecules under pressure. In this way, through the pressure the concentration of oxygen in the glow tube can be increased, and through the oxygen molecules an oxidation can be realized on the heating coil surface for the formation of aluminum oxide, without requiring a heating of the heating coil by a heating current for this purpose. In this way, the heating coil can be protected from nitridation by an oxide layer already before the first operation, i.e., before the first heating by a heating current. 
   A further advantage is that a control coil, connected to the heating coil, is embedded in a second insulating powder that is as free as possible of oxygen donors and/or includes getter material for the binding of oxygen. In this way, a material can be used for the control coil that does not form a protective oxide layer under the influence of oxygen donors, as is the case for example for cobalt-iron alloys. A corrosion of the control coil can thus be prevented, or at least considerably delayed, through the use of the second insulating powder that is as free as possible of oxygen donors. 
   With the use of getter material in the second insulating powder, disturbing oxygen molecules in the area of the control coil can be bound. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a first exemplary embodiment of an electrically heatable glow plug according to the present invention. 
       FIG. 2  shows a second exemplary embodiment of an electrically heatable glow plug according to the present invention. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1 , reference character  1  designates a glow plug, formed as a sheathed-element glow plug, for an internal-combustion engine. Sheathed-element glow plug  1  includes a plug housing  40  having a threading  45  for screwing into a cylinder head of the internal-combustion engine. Plug housing  40  further includes a hexagon  50 , via which the sheathed-element glow plug or plug housing  40  can be screwed into or out of the cylinder head using a twisting tool, for example a wrench for hexagon nuts. A glow tube  5  is pressed into plug housing  40 , which is formed in the shape of a tube, and this glow tube protrudes from plug housing  40  at the side of the combustion chamber, i.e., at the end of plug housing  40  situated opposite hexagon  50 . At the side of the combustion chamber, glow plug  5  is closed at its end. In an area  20  at the combustion-chamber-side tip  55 , formed in this way, of glow plug  5 , the cross-section of glow plug  5  can be reduced, as is the case in this example. However, a reduction of this cross-section is not absolutely necessary. Only area  20 , having reduced cross-section, of sheathed-element glow plug  1  protrudes into the combustion chamber. In area  20  having reduced cross-section, glow plug  5  has a heating coil  10  that is welded to combustion-chamber-side tip  55  of glow tube  5 . Adjoining heating coil  10  is a control coil  60 , situated in the area of glow tube  5 , whose cross-section is not reduced. At the end of glow tube  5  situated away from the combustion chamber, control coil  60  contacts a connecting bolt  65  that can be connected with the positive pole of a vehicle battery. In the direction towards the opening of plug housing  40  situated away from the combustion chamber, glow tube  5  is sealed, still inside plug housing  40 , against environmental influences by a Viton ring  70 . A further sealing ring  75  seals connecting bolt  65 , which protrudes from plug housing  40  away from the combustion chamber, against plug housing  40 . An insulating disk  80 , connected to sealing ring  75  away from the combustion chamber, is used to electrically insulate connecting bolt  65  from plug housing  40 , and thus electrically insulates connecting bolt  65  from plug housing  40 , whose electrical potential is at vehicle ground. A ring nut  85  presses insulating disk  80  onto plug housing  40 , and presses sealing ring  75  into plug housing  40 . 
   Glow tube  5  is of metallic construction, and, due to being pressed into plug housing  40 , its electrical potential is likewise at vehicle ground. Heating coil  10  is welded, with control coil  60 , to a connection point  90 . 
   The function of Viton ring  70  is of considerable importance, because it is made of a soft, insulating material, and thus not only seals connecting bolt  65  in electrically insulating fashion against plug housing  40  at its end protruding into glow tube  5  for the contacting of control coil  60 , but also prevents the penetration of air into glow tube  5 , which is open at its end away from the combustion chamber. This sealing should be as reliable as possible. 
   Heating coil  10  is made for example of a ferritic steel having an aluminum portion, for example of an iron-chromium-aluminum alloy. The control coil can for example be made of pure nickel or of a cobalt-iron alloy, having a portion of 6-18 weight percent cobalt, and has the function of a control resistance having a positive temperature coefficient. 
   In addition, in glow tube  5  an electrically insulating powder filling  25 ,  30 , which is compressed after the hammering of glow tube  5 , is provided, which ensures that heating coil  10  and control coil  60  in the interior of glow tube  5  are housed and fixed in stationary fashion, as well as being electrically insulated against glow tube  5 , apart from tip  55  of glow tube  5 . As a powder filling, in general magnesium oxide is used. Moreover, the powder filling provides a thermal connection between glow tube  5  and heating coil  10 , or control coil  60 . 
   Given the presence of sufficient oxygen, the alloy of heating coil  10  normally protects itself in a short time against further corrosion through the formation of a thin Al 2 O 3  layer. However, this precondition is not met in sheathed-element glow plug  1 , due to an initial lack of oxygen that is as a rule initially present. During the cyclical thermal loading of the sheathed-element glow plug in its use in the cylinder head, air can penetrate into glow tube  5  despite sealing ring  75  and Viton ring  70 . This leads to a simultaneous reaction of the material of heating coil  10  with oxygen and nitrogen. In contrast to oxygen, which forms a protective aluminum oxide layer in the surface of heating coil  10 , nitrogen causes an interior nitridation, i.e., formation of aluminum nitride in the material of heating coil  10 . The consequence is a local increase of the electrical resistance of heating coil  10 , resulting in a higher voltage drop, and thus a greater heating at heating coil  10 ; this can cause a premature failure of heating coil  10 . 
   For this reason, a material that acts as an oxygen donor is added to the insulating powder filling, said material releasing oxygen at high temperatures and thus promoting the formation of a protective aluminum oxide layer on heating coil  10 . In this way, in the case of a penetration of air into glow tube  5 , the formation of nitrides in the edge layers of heating coil  10  is prevented. The aluminum oxide layer is here at least partially realized by a heating current already during the first heating of heating coil  10 , in which temperatures of greater than 1000 degrees Celsius are reached. 
   If the material of control coil  60  has no aluminum portion and also no silicon portion, as in the example described here, then it does not form a protective oxide layer with the oxygen released by the oxygen donors, but rather corrodes. This should be prevented. For this reason, in this case the material of the insulating powder filling acting as an oxygen donor is added only in area  20  at tip  55  of glow tube  5 , in which heating coil  10  is located. The material acting as an oxygen donor should thus be present only in the area of heating coil  10 , and not in the area of control coil  60 . For this purpose, in the assembly of sheathed-element glow plug  1 , first glow tube  5  is filled with the insulating powder having the material acting as an oxygen donor until heating coil  10  is embedded therein as completely as possible, and control coil  60  does not come into contact with the material acting as an oxygen donor even after a hammering of glow tube  5 . The insulating powder filling enriched with the material acting as an oxygen donor is designated with reference character  25  in  FIG. 1 , and is referred to in the following as the first insulating powder. The insulating powder with which glow tube  5  is subsequently filled, and in which control coil  60  is embedded, should in this example contain no material acting as an oxygen donor, and should for example be formed from pure magnesium oxide. In this way, the oxidation is supported only in the area of heating coil  10 , so that both a nitridation of heating coil  10  and a corrosion of control coil  60  can be prevented. The insulating powder, which is free of materials acting as oxygen donors, is designated in  FIG. 1  with reference character  30 , and represents a second insulating powder. Alternatively, or in addition, second insulating powder  30  can include a getter material for the binding of oxygen, such as for example Si, Ti, Al, or reduced metal oxides, such as for example FeO, Ti 2 O 3 . Given an electrically conductive getter material, such as for example Si, Ti, Al, second insulating powder  30  contains electrically insulating material, such as for example MgO, in a significantly greater concentration than the getter material. 
   The material acting as an oxygen donor can for example be formed as an oxidic ceramic powder. Here, the ceramic powder can be a metal oxide of a metal that can oxidize in several oxidation stages. In order to promote the releasing of oxygen, in an initial state this metal oxide can be present in its highest oxidation stage. Here, for example TiO 2  can be used as an oxygen donor. 
   A further possibility is to use as an oxygen donor an oxidic ceramic powder or metal oxide that releases oxygen under reducing conditions, such as those present in area  20  at tip  55  of glow tube  5  due to the aluminum portion of heating coil  10 , so that a defect results in the crystal grid of the relevant metal oxide due to missing oxygen atoms. ZrO 2  can for example be selected as such an oxygen donor. 
   A content of the material acting as an oxygen donor in first insulating powder  25  in a range from as low as approximately 0.1 weight percent up to approximately 20 weight percent has proven sufficient for the introduction of the oxidation on heating coil  10  upon heating; the remaining portion of first insulating powder  25  can for example be formed by magnesium oxide. 
     FIG. 2  shows a second exemplary embodiment of a glow plug according to the present invention, in which identical reference characters designate the same elements as in FIG.  1 . In contrast to the first specific embodiment according to  FIG. 1 , in the second specific embodiment according to  FIG. 2  glow tube  5  does not have a control coil, but rather has an electronic control element  95  that is protected against oxidation, which can for example include a temperature sensor and a keying, dependent on the determined temperature, of the current supplied to heating coil  10 , and which is not described here in more detail. A control coil or a control element can also be omitted entirely. Moreover, instead of first insulating powder  25  and second insulating powder  30 , a third insulating powder  15  is provided in the entire area of glow tube  5 , this third powder being made of an electrically insulating material, for example magnesium oxide, and being free of oxygen donors. Heating coil  10  is connected with connecting bolt  65  via control element  95 ; here control element  95  can also be situated as far from the combustion chamber as possible, so that it will not be heated too strongly. It can now be provided that before the first operation of sheathed-element glow plug  1 , an opening  35  is bored into glow tube  5 ; here opening  35  should be situated outside area  20  at tip  55  of glow tube  5  having heating coil  10 , because this area could be too sensitive for a boring due to its reduced cross-section. If, however, there are no stability problems in area  20  at tip  55  of glow tube  5 , it is also conceivable to make bored opening  35  there; i.e., directly in the area of heating coil  10 . Here, opening  35  is made only after heating coil  10  and, if necessary, control element  95  have been brought into area  20  at tip  55  of glow tube  5 , and glow tube  5  has been filled with third insulating powder  15 . Only then is opening  35  bored into glow tube  5 . Through opening  35 , oxygen molecules are then brought into glow tube  5  under a gas atmosphere with controlled partial pressure. This process can for example last between approximately one hour and approximately 20 hours; the limits of this time span can also be adjusted upward or downwards. Subsequently, opening  35  formed by the boring is again closed. The closing can for example take place through welding. Through the controlled partial pressure, the concentration of oxygen in glow tube  5  is increased. The higher the partial pressure is, the higher the concentration of the oxygen in glow tube  5  becomes. Due to the high concentration of oxygen, and above all due to the presence of pure oxygen molecules, an oxidation on the surface of heating coil  10  can be accelerated, so that a passivation of heating coil  10  through the formation of a thin Al 2 O 3  layer on the surface of heating coil  10  can be realized in a short time, already before or during the first operation of sheathed-element glow plug  1  in the internal-combustion engine, the Al 2 O 3  layer here exercising a protective function and, in the case of a penetration of small quantities of air during the operation of the sheathed-element glow plug, preventing the formation of nitrides on heating coil  10 . In this way, the life span of sheathed-element glow plug  1  can be increased. In this case, this takes place through pre-oxidation of heating coil  10  before the first setting into operation of sheathed-element glow plug  1 . Through corresponding predetermination of the partial pressure for the bringing of oxygen into glow tube  5 , and given corresponding predetermination of the time in which the oxygen is brought into glow tube  5 , a protective layer can be produced on heating coil  10  that is defined in its composition; in this example it is formed as an aluminum oxide layer. 
   If the oxygen brought into glow tube  5  in this way is also distributed outside the area having heating coil  10  in glow tube  5 , the use of a control coil susceptible to oxidation and corrosion is not recommended in the second exemplary embodiment, and the use of a control element that is resistant to oxidation and to corrosion, as described for example on the basis of control element  95 , or the omission of a control coil or control element, is to be preferred.