Abstract:
A heating element and, in particular, a ceramic heating element, such as ceramic heating elements used in high temperature glow plugs for diesel engines and gas igniters. The heating element includes an electrical insulator and an electrically conductive layer. The conductive layer is formed from a single material and single composition. The method of manufacture includes the steps of forming the insulative layer and molding a conductive layer around the insulative layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Application Ser. No. 60/785,334, filed Mar. 23, 2006 which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a heating element and, in particular, a ceramic heating element, such as ceramic heating elements used in high temperature glow plugs for diesel engines and gas igniters, and methods of manufacture therefore.  
         [0003]     Ceramic heating elements, such as the glow plug illustrated in  FIG. 1 , are well known in the industry. As illustrated in  FIG. 1 , a glow plug  2  typically includes a heating element having an electrically conducting core  8  surrounded by an electrically insulative layer  6 . The insulative layer  6  is in turn surrounded by an outer resistive layer  4  which makes contact with the conducting core  8  at the electrical connection area  9 . To manufacture the glow plug  2 , illustrated in  FIG. 1 , the layered structure is formed by sequentially slip casting layers from suspensions of different compositions in a porous mold, and then sintered to form a monolithic body. The resulting body is then electrically connected to form a ceramic heating element.  
         [0004]     One problem with sequential casting is that the geometric configuration of the heating element is generally limited to shapes that allow each progressive layer to be formed against the previous layer. In the case of slip casting, the configuration of any layer is generally limited to a thin layer of fairly uniform thickness or a core that is substantially solid, but may be partially hollow due to piping which occurs as the cast material solidifies. This sequential stacking of layers limits the geometric configuration and prevents each layer from being optimized for use in a heating element and being optimized for use in particular applications.  
         [0005]     Another drawback to the style of glow plugs  2  illustrated in  FIG. 1  is that the sequential layering creates discrete interfaces between layers and when the glow plug is cycled between cold and hot temperatures, failures may occur. To reduce the failure rate many manufacturers cycle the glow plugs at lower temperatures than is desired for efficient engine operation. More specifically, as the glow plug cycles between temperatures, it experiences internal stresses due to the differences in the thermal expansion rates between the differently composed layers. As the different layers expand and contract at different rates, stress may occur that may cause failure of the glow plug, commonly in the heating element of the glow plug.  
         [0006]     Yet another drawback to the style of glow plugs  2  illustrated in  FIG. 1  is that the electrical connection  9  between the conducting core  8  and the outer resistive layer  4  is in close proximity to the external surface of the glow plug  2  and may be subject to oxidation from the surrounding atmosphere during service. Sufficient oxidation at the electrical connection  9  can degrade the electrical connection  9  by the formation of an electrically insulating oxide layer, or the formation of a porous layer having an interfacial porosity, to the point where current can no longer pass between the conducting core  8  and the resistive layer  4 , resulting in a failure of the glow plug to heat when an electrical current is applied.  
         [0007]     Yet another drawback to the style of glow plugs  2  illustrated in  FIG. 1  is that the inconsistencies in the layer thickness and geometry created by the casting process leads to inconsistent resistance between manufacturing lots. The cast layers are formed by a gradual buildup of material against either the mold or against a previously formed cast surface. Once the desired thickness is achieved, excess liquid casting slip is removed. The thickness is controlled primarily by casting time, but is also affected other factors including the rheological properties of the casting slip, the permeability of the mold, and the permeability of any previously cast layers. In addition, when the casting slip is removed, the newly cast surface remains wet for a short period of time, and this small amount of remaining liquid slip may form drips or runs that further contribute to non-uniform layer thickness. Any of these factors may cause small variations in the layer thickness and the uniformity of the thickness of the layers which result in variations in the electrical resistance of the glow plugs and variances in the heating profile of the glow plug.  
         [0008]     It is therefore desirable to provide a heating element for use in glow plugs that overcomes the disadvantages of the prior art and in particular a heating element for glow plugs that has low internal thermal stresses, optimized geometrical shape for heating, increased longevity and durability, and precisely controllable and reproducible heating characteristics.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention relates to heating elements and, in particular, to heating elements for glow plugs and gas igniters as well as the method of manufacturing thereof. The heating element generally includes a first layer formed from or acting as an electrically insulative material and a second layer formed out of a electrically conductive material that is molded around portions of the first layer. By varying the geometric profile of the first layer and the geometric profile of an injection die, the thickness of the conductive layer may be varied along the length as well as around the circumference of the heating element to provide a desirable heating profile for a specific application. The molded profile of the first layer and the profile of a die in which the electrically conductive layer is molded allows for these geometric profiles and variations in the heating profile that are not available with the slip casting method. Furthermore, by molding the electrically conductive layer as a single piece extending between a first electrical connection and a second electrical connection prevents many of the problems with the prior art methods by removing discrete interfaces between layers and eliminating the electrical interface.  
         [0010]     The invention includes a method of forming a heating element including the steps of forming a first layer, placing the first layer in a die, and molding an electrically conductive layer around the first electrically insulative layer.  
         [0011]     Further scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The present invention will become more fully understood from the detailed description given here below, the appended claims, and the accompanying drawings in which:  
         [0013]      FIG. 1  is a sectional view of a prior art slip casted heating element;  
         [0014]      FIG. 2  is a sectional view of the present invention having the heating portion focused at a first end;  
         [0015]      FIG. 3  is a sectional view of the present invention having an extended heating portion;  
         [0016]      FIG. 4  is a sectional view of the present invention having a heating portion primarily focused at the first end;  
         [0017]      FIG. 5  is a sectional view of the present invention having a heating portion primarily focused at the first end;  
         [0018]      FIG. 6  is a diagram showing the method steps in forming a heating element;  
         [0019]      FIG. 7  is a diagram of a first alternative method of forming a heating element  
         [0020]      FIG. 8  is a diagram of a second alternative method of forming a heating element;  
         [0021]      FIG. 9  is a cross-sectional view of the first layer along lines  9 - 9  in  FIG. 8 ;  
         [0022]      FIG. 10  is a cross-sectional view of the first layer in a die along lines  10 - 10  in  FIG. 8 ; and  
         [0023]      FIG. 11  is a cross-sectional view of the formed heating element along lines  11 - 11  in  FIG. 8 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]     The present invention, as illustrated in  FIGS. 2-5 , is directed to a heating element  10  having an electrically insulative layer  20 , formed from an electrically insulative material, and an electrically conductive layer  30 , formed from an electrically conductive material. As illustrated in  FIG. 2 , the conductive material is attached to a first electrical contact  40  and a second electrical contact  42  which allow electrical current to flow through the conductive material to generate heat that is primarily focused where the thickness of the conductive layer  30  is at its thinnest point and has the smallest cross cross-sectional area. Although only  FIG. 2  is illustrated with the electrical contacts,  40  and  42 , the heating element  10  will be generally formed with electrical contacts, which may vary in size, shape and configuration. The heating element also may include a base portion  14  formed in a variety of configurations and shapes.  
         [0025]     The insulative layer  20  further includes an outer surface  22  that creates a geometric profile that may vary in shape and diameter to create the desired heating profile. The insulative layer  20  generally includes a first end  26 , a second end  28 , and a center portion  27 . A passage  24  extends from the first end  26  to the second end  28 .  
         [0026]     The insulative layer  20  is generally formed from an insulative material and can be made from any known electrical insulator by any known method. Such methods may include extrusion, molding, powder compaction, and other methods. Ceramic powders with gelling additives as well as certain thermoplastic materials can be formed and sintered to make good insulators. For example, the insulative material may be a material such as silicon nitride, silicon carbide, aluminum oxide, aluminum nitride, or other ceramic materials. This list of potential insulative elements in no way should limit the materials that may be used to form the insulator. The insulative material may be formed of any material that has good electrical insulation properties or is commonly used in heating elements as an insulative material. The insulative material may also comprise electrically conductive particles in a matrix of electrically insulating material, such as a composite of molybdenum disilicide and silicon nitride wherein the conducting molybdenum disilicide particles are present below the percolation threshold and are thus electrically isolated from one another.  
         [0027]     By molding or by the other listed methods, the insulative layer  20  can be formed in a variety of shapes such as those in  FIGS. 3 and 4 , which were not previously possible using the slip casting method. It is preferable for the insulative layer  20  to be formed from a material that may be reliably molded into various shapes.  FIGS. 4 and 5  illustrate profiles that have an outer diameter at a first end  26  that is greater than the outer diameter at a center portion  27 . The second end  28  may also have a diameter that is greater than the center portion  27  and sometimes a diameter that is greater than the first end  26 . As may be seen, the insulative layer  20  may be highly customized to provide specific heating profiles when combined with the conductive layer  30 .  
         [0028]     The conductive layer  30  is generally formed from a conductive material that allows electrical current to flow between a first electrical contact  40  and a second electrical contact  42 . The conductive layer  30  generally forms an outer surface  12  of the heating element  10 . By varying the thickness between the insulative outer surface  22  and the conductive outer surface  32 , the heating profile may be adjusted. For example, as illustrated in  FIG. 3 , the center passage  24  is filled with conductive material of the conductive layer  30  and has a relatively large thickness which allows for less resistance and easier electrical current flow. However, the thickness of the conductive layer  30  in the heating portion  16  of the heating element  10  is much thinner which creates a greater resistance and increases the amount of heat output near the heating portion  16 . Therefore, as current passes between the first and second electrical contacts  40  and  42 , in the thinner areas of the conductive layer  30 , the heating output will be the greatest. As illustrated in  FIG. 2 , the thin area is limited to only a portion of the tip of the heating element  10  thereby creating a heating profile that is primarily focused in the vicinity of the first end  26 . The heating profile may be varied by changing the profile of either the insulative layer  20  or the conductive layer  30 .  
         [0029]     As illustrated in  FIG. 3 , the conductive layer  30  extends from an area proximate to the first end  26  of the insulative layer  20  towards the second end  28  along the center portion  27 . This creates a heating profile that extends further along with a greater heating capacity than the heating element illustrated in  FIG. 2 .  
         [0030]     The heating element  10  illustrated in  FIG. 4  includes a heating portion that is primarily focused near the first end  26  of the insulative layer  20  where the thickness is much thinner than the thickness near the center portion  27  of the insulative layer. Therefore, the heating profile of the heating element  10  is primarily focused near the first end  26  of the insulator, however, the heating element does provide some heat along the center portion  27  of the insulator.  FIG. 5  is a further variation of the heating element in  FIG. 4  with the conductive layer  30  extending further along the center portion  27  of the insulator toward the second end  28 .  
         [0031]     As illustrated in step  301  of  FIG. 8  and  FIGS. 9-11 , the heating element may include projections along the outer surface  22  that allows centering of the heating element in the die that receives the insulative layer  20  for overmolding with the conductive layer. These projections formed from the insulative layer  20  may also modify the heating profile by creating areas on the outer surface  12  of the heating element  10  that do not generate heat. Typically, at least three of these projections would be used to center the insulative portion within the die; however more or less may be used depending upon the geometric shape and the die. The conductive layer  30  may be formed from a variety of known conductive materials such as conductive materials formed from ceramic matter that are typically used in glow plugs today including molybdenum disilicide, titanium nitride, zirconium nitride and titanium boride. The conductive material may also comprise electrically insulating particles in a matrix of electrically conducting material, such as a composite of molybdenum disilicide and silicon nitride wherein the conducting molybdenum disilicide grains are present above the percolation threshold and thus form a continuous electrically conductive path through the material. The conductive layer may also comprise metals such as platinum, iridium, rhenium, palladium, rhodium, gold, copper, silver, tungsten and alloys of these to name a few. Generally the conductive layer  30  needs to be formed of a conductive material that allows for easy molding in a die. Any conductive material or resistive heating material currently in use with heating elements may be used.  
         [0032]     The heating element  10  is generally formed by a method of first forming an insulative layer  20  illustrated as steps  101  in  FIG. 6, 201  in  FIG. 7 , and  301  in  FIG. 8 . In a second step, an injection molding die  50  is provided having a geometric profile that will form the outer surface  12  of the heating element  10  and is illustrated as step  102  in  FIG. 6 , step  202  in  FIG. 7 , and step  302  in  FIG. 8 . Once the insulative layer  20  is formed to the desired geometric shape out of an insulative material such as by molding powder formation or other methods, the insulative layer  20  is inserted in an injection molded die  50  as shown in steps  103  in  FIG. 6, 203  in  FIG. 7 , and  303  in  FIG. 8 . After the insulative layer  20  is placed into the die  50 , the molten conductive material is forced into the die as illustrated in steps  104  in  FIG. 6, 204  in  FIG. 7 , and  304  in  FIG. 8 . With the molten conductive material in the die and substantially filling the voids, the material is allowed to cool and harden as illustrated in steps  105  of  FIG. 6  and  305  of  FIG. 8 . The formed heating element  10  is then removed from the die  50  as illustrated in step  106  of  FIG. 6, 205  of  FIG. 7 , and  306  of  FIG. 8 . The heating element  10  is then sintered to form a monolithic material (not shown). In the method illustrated in  FIG. 7 , the excess material is removed in step  206 .  
         [0033]     It will be understood by one that is skilled in the art that ceramic materials are commonly formed by first forming an assembly of finely divided particles and subsequently firing the assembly to sinter the particles in to a monolithic article. Ceramic materials are commonly injection molded by mixing the particles with a thermoplastic medium or binder such as, but not limited to, wax or polyethylene or a blend of the two, and heating the resulting mixture so that the molten mixture is sufficiently fluid to fill a die cavity, and subsequently cooling the molded article to form a rigid part that can be removed from the die. Alternatively, non-thermoplastic binder medium such as agar/water may also be employed. The binder medium is then removed by a process commonly known as debinding, which may include solvent extraction and thermal debinding steps. The part is then fired under suitable conditions to sinter the particles together and form the final monolithic article.  
         [0034]     A first layer can be formed from a material that is insulative or even non-insulative in some embodiments. By forming the first layer from a material that is later removable from the final cast part allows a method forming a heating element substantially following the above steps however, it would have an additional step (not shown) of removing the first layer material from within the conductive layer  30 . Removal of the first layer would create a substantially air core to the conductor which would act as an insulator. By having a hollow core all differences in thermal expansion are eliminated and may provide longer life to the heating element. Therefore the first layer may use any material that is known in the casting or molding process to be able to be later removed or destroyed during the casting process. To provide rigidity to the conductive layer, after the first layer is removed, an insulative or rigid layer may be added to fill the pockets within the conductive layer, such as an insulator that is not conducive to the overmolding process.  
         [0035]     As is illustrated in  FIGS. 6 and 7 , the second end  28  of the insulative material  20  may engage the inner surface of the die to hold the insulative layer  20  in place within the cavity of the die  50 . This ensures proper placement within the die  50  so that as the molten conductive layer  30  flows and is forced into the die the desired profile is created and the insulative layer  20  does not move. However, in some embodiments it may be desirable to have projections, as illustrated in  FIG. 9 , on the insulative layer  20  so that the projections engage the die as shown in step  303  in  FIG. 8  and  FIG. 10 . These projections ensure that the insulative layer  20  stays in place during the molding process by providing two areas of contact with the die that are removed from each other.  
         [0036]     By forming a first layer through extrusion molding or powder compaction methods similar to that currently used to form spark plug insulators, the first layer can be created with a specific geometric profile. The first layer can be formed from an insulative material for use in conjunction with the conductive layer, or from a material that is easily removable once the conductive layer is overmolded on the first layer. When this geometric profile the first layer is combined with the geometric profile of the conductive layer  30 , a heating profile may be created for the heating element  10  that allows hot and cold spots and even areas that gradually change on the heating element, both around the circumference as well as along the length. Therefore, if needed, a heating profile can be created that has, for example, a hot spot on half of the circumference of the heating element  10  and removed from the first end  26  of the insulative layer  20  so that on the heating element  10  the tip of the heating element as well as at least half of the circumference and the portion toward the second end may be cooler than the desired hot spot. These types of heating profiles were previously unobtainable by the prior art methods of creating heating elements.  
         [0037]     The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.