Abstract:
A multilayer ceramic structure is formed by building up a plurality of layers by sequentially coating a substrate with a series of suspensions comprising particles in a fluid medium. A composition of the sequential layers are varied to produce a structure with the desired properties. The thickness of the layers can be controlled by Theological properties of the suspension and/or by the utilization of a gelling or coagulating agent. An advantage of this method is that complete drying between the subsequent coatings is not required.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    None. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to methods for manufacturing ceramic heating elements. 
         [0004]    2. Related Art 
         [0005]    Glow plugs can be utilized in any application where a source of intense heat is required for combustion. As such, glow plugs are used as direct combustion initiators in space heaters and industrial furnaces and also as an aid in the initiation of combustion when diesel engines must be started cold. Glow plugs are also used as heaters to initiate reactions in fuel cells and to remove combustible components from exhaust systems. 
         [0006]    With regard to the example of diesel engine applications, during starting and particularly in cold weather conditions, fuel droplets are not atomized as finely as they would be at normal running speeds, and much of the heat generated by the combustion process is lost to the cold combustion chamber walls. Consequently, some form of additional heat is necessary to aid the initiation of combustion. A glow plug, located in either the intake manifold or in the combustion chamber, is a popular method to provide added heat energy during cold start conditions. 
         [0007]    The maximum temperature reached by a glow plug heating element is dependent on the voltage applied and the resistance properties of the components used. This is usually in the range of 1,000-1,300° C. Materials used in the construction of a glow plug are chosen to withstand the heat, to resist chemical attacks from the products of combustion and to endure the high levels of vibration and thermal cycling produced during the combustion process. 
         [0008]    To improve performance, durability and efficiency, new materials are constantly being sought for application within glow plug assemblies. For example, specialty metals and ceramic materials have been introduced into glow plug applications. While providing many benefits, these exotic materials can be difficult to manufacture in high volume production settings. Sometimes, they are not entirely compatible with other materials, resulting in delamination and other problems. Another common problem with specialty materials manifests as tolerance variations when formed in layers resulting from cumbersome and inefficient manufacturing techniques. 
         [0009]    Conventional methods for manufacturing ceramic heating elements, such as glow plugs, involve complex manufacturing techniques. For example, one method uses multiple layers of ceramic with different compositions. Each of those layers are built up by sequentially slip casting layers into a porous gypsum mold. The resulting part is removed from the mold and fired to produce a dense ceramic monolithic part. The casting equipment used in this type of manufacturing process is complicated and requires a complex system of pumps and hoses to inject the slurry into the molds. Moreover, the molds require careful preparation and have a very limited lifetime. Other problems exist with this method, including changes in the mold that occur after each use and result in inconsistent layer thicknesses and inconsistent performance in the fired part. Further, conventional methods are limited in their application and thickness of the layers. A thinner layer reduces the stresses associated with thermal expansion differences between layers that can result in delamination of layers during thermal cycling. 
         [0010]    Therefore, a need exists for an improved method for manufacturing ceramic heating elements which is less complex than conventional methods and eliminates the difficulties associated with plaster molds and the slurry injection equipment. A method is needed that can build a sequence of thinner layers without compounding variations in the thickness or composition of the layers or increasing stresses associated with thermal expansion differences between the layers. It being understood that high stresses can result in delamination of the layers during the thermal cycling. 
       SUMMARY OF THE INVENTION 
       [0011]    A multilayer ceramic structure is formed by building up a plurality of layers by sequentially coating a substrate with a series of suspensions comprising particles in a fluid medium. A composition of the sequential layers are varied to produce a structure with the desired properties. The thickness of the layers can be controlled by theological properties of the suspension and/or by the utilization of a gelling or coagulating agent. An advantage of this method is that complete drying between the subsequent coatings is not required. 
         [0012]    The method provides the manufacture of multilayer ceramic heating elements such as those used for glow plugs to be automated and eliminates difficulties associated with plaster molds and the slurry injection equipment. Further, the sequential building up of thin layers produces a product that has smaller variations in thickness or composition than are possible with slip casting, injection molding or extrusion. The reduced stresses associated with thermal expansion differences between layers resists delamination of the layers during thermal cycling. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention will become more fully understood from the detailed description given here below, the appended claims, and the accompanying drawings in which: 
           [0014]      FIG. 1  is a simplified cross-sectional view of an exemplary glow plug installation in the pre-combustion chamber of a diesel engine; 
           [0015]      FIG. 2  is a cross-sectional view of a glow plug assembly in accordance with an embodiment of the invention; 
           [0016]      FIG. 3  is a fragmentary, cross-sectional view of the high temperature tip region of a glow plug according to one embodiment of the invention; and 
           [0017]      FIG. 4  is a flowchart illustrating the method for manufacturing the heating device, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, a diesel engine is generally shown at  10  in  FIG. 1 . The engine  10  includes a piston  12  reciprocating in a cylinder. The cylinder is formed in a block  14 . A cylinder head  16  covers the block  14  to enclose a combustion chamber. An intake manifold routes through the cylinder head  16  and includes a fuel injector  18  which, at timed intervals, delivers a charge of atomized fuel into the combustion chamber. A glow plug, generally indicated at  20 , includes a high temperature tip  22  positioned, in this example, within a pre-combustion chamber  24 . The arrangement of components as illustrated in  FIG. 1  is typical of one configuration style for a diesel engine. However, there are many other diesel engine types for which a glow plug  20  according to the invention is equally applicable. Furthermore, many other types of devices can utilize the subject glow plug  20 , such as space heaters, industrial furnaces, fuel cells, exhaust systems, and the like. Accordingly, the subject glow plug  20  is not limited to use in diesel engine applications. 
         [0019]    Referring now to  FIG. 2 , a cross-sectional view of the glow plug  20  is depicted. Here, the high-temperature tip  22  is shown forming the distal end of a heating element, generally indicated at  26 . The heating element  26  is a composite structure which protrudes from the end of a hollow shell  28 , such as by a copper ring  30  and a brazed joint  32 . By these means, the heating element  26  is both securely fixed in position relative to the shell  28  and held in electrically conductive relationship therewith. A proximal end of the heating element  26  is affixed to a conductive center wire  34 , such as via a tapered and brazed joint. The proximal end of the center wire  34  holds a terminal  36  used to join an electrical lead (not shown) from the ignition system. The center wire  34  and terminal  36  are held in electrical isolation from the conductive shell  28  by way of an insulating layer of alumina powder  38 , epoxide resin  40  and plastic gasket  42 . Of course, alternative materials may be suitable to hold the center wire  34  and terminal  36  in position and in electrical isolation from the shell  28 . The exterior of the shell  28  is provided with a tool fitting  44  and threads  46 . Of course, the glow plug  20  can take numerous other forms and constructions, depending upon the materials used and its intended application. 
         [0020]    Generally stated, the heating element  26  operates by passing an electrical current through a resistive material. The current is introduced to the heating element  26  through the center wire  34 . Current flows through the heating element  26  and into the shell  28  which is typically metallic and grounded through the cylinder head  16  or other component of the device. 
         [0021]    A fragmentary, cross-sectional view taken through the lower end of the heating element  26  is depicted in  FIG. 3 . Here, the heating element  26  is shown including a starter substrate  48 . Starter substrate  48  is used as a foundation for forming a layered structure. Substrate  48  may be a fired or unfired ceramic, ceramic composite or metal form that will become a part of the final structure. The present invention also contemplates that substrate  48  may be a form that can be removed from the multilayer structure before it is fired. For example, substrate  48  may be a metal mandrel. 
         [0022]    Alternatively, substrate  48  may be a pre-form that is configured to be removable by pyrolosis during heat treatment of the layered resistive core. In an embodiment of the invention, the substrate  48  has a surface treatment or a configuration that promotes the adhesion of subsequent layers as described here below. 
         [0023]    With reference to both  FIGS. 3 and 4 , a method  90  for forming the multi-layered structure will now be described, in accordance with an embodiment of the present invention. In an initial step, as represented by block  100 , a starting substrate or pre-form  48  is provided upon which the multi-layered structure will be built. At step  102 , substrate  48  is immersed in a suspension of particles in a fluid medium to produce a first coating  50  (shown in  FIG. 3 ) on substrate  48 . First coating  50  is caused to set into a non-fluid layer, as represented by block  104 . First coating  50  is transformed into a non-fluid layer by chemical or physical means. At block  106 , second coating  52  is applied over first coating  50  in a similar manner. The present invention contemplates that second coating  52  has the same composition or a different composition relative to first coating  50 . Additional coatings such as third coating  54  are sequentially applied until the desired multi-layer structure is completed, as represented by block  108 . There may be additional coatings or layers over the third coating  54 . In some applications, it may be desirable to modify one or more of the coating layers  50 ,  52 ,  54  to provide for an electrical interconnect. For example, as illustrated in  FIG. 3 , the tip of the second coating  52  may be ground flat so that the first  50  and third  54  coating layers can establish an electrical connection therebetween. Once any such optional modifications have been made, and all desired layers built, the assembly is fired to consolidate the multilayered structure, as represented by block  110 . The multi-layered structure may be further treated before or after firing to provide electrical contacts with one or more of the various layers, as represented by block  112 . As shown in  FIG. 3 , this electrical contact may be established between the first  50  and third  54  coatings. The fired structure may be further combined with other components to form a device such as a glow plug  20  to be used in a diesel engine  10 , as represented by block  114 . 
         [0024]    In an embodiment of the present invention, first coating  50  is a suspension of ceramic particles in a water that also contains a gelling binder such as alginate. After the layer is formed, the alginate-containing suspension can be caused to set by immersing the coated pre-form in a solution containing dissolved calcium ions. The calcium ions chemically interact with the alginate causing the suspension to gel. Once the coating has gelled it may be desirable to wash the surface to remove excess gelling agent before forming the next layer. Alternatively, the substrate might be first coated with a calcium-containing solution and then subsequently dipped into alginate-containing slurry to form a gelled layer. The thickness of the layer is controlled by the amount of calcium in the calcium-containing solution. 
         [0025]    In yet another embodiment of the present invention, other gellation reactions as an alternative to alginate and calcium may be used. For example, a slurry containing polyacrylic acid can be gelled by changing the pH or the temperature of the slurry. In operation, the substrate  48  is coated by dipping the substrate  48  into a slurry of particles that contain polyacrylic acid. The coating is then gelled either by dipping the coated substrate  48  into an acidic or basic solution depending on the type of polyacrylic acid used or by dipping it into a bath containing an immiscible liquid. The immiscible liquid is held at an elevated temperature, which causes gellation. 
         [0026]    Alternatively, an organic monomer may be used as a gelling agent in a suspension of ceramic particles. The organic monomer is coated on substrate  48  and gelled by polymerization initiated by a chemical initiator. Other types of binders could be gelled by ultraviolet radiation. A large number of gellation binder systems are known in the ceramic art and any of these could be used in this method. 
         [0027]    Any one of the layers might also be modified in such a way as to form interconnects between layers. For example, in a three layer structure a first conductive layer might be formed followed by an insulating layer and finally a resistive layer. After the insulating layer is formed, a portion of the insulating layer is removed exposing the conductive layer and forming an electrical contact between the conductive layer and the resistive layer during a final coating operation. 
         [0028]    In a manufacturing setting the method of the present invention is performed, for example, by setting up a series of slurry tanks and solution tanks in a line with the substrates suspended above the tanks on a moving conveyor. Alternately, the substrates may be dipped and then set or hung on draining racks to drain and then moved to the next tank to be dipped and drained. This process is repeated until the desired coatings have been built up on the substrate  48 . 
         [0029]    In yet another embodiment of the invention, a method is provided whereby the substrate  48  is sprayed to create the coating layers prior to the gellation step. The gellation of these coating layers may also be accomplished by spraying any of the gelling solutions described above instead of dipping the substrate. The addition of subsequent coatings allows individual conductors, resistors and insulators to be merged into one another gradually to reduce thermal shock and delamination. More specifically, the layers may be designed by slurry rheology to produce thicknesses of 0.001 inch (i.e., about 25 microns) after dipping. Thus, the difficulty of injection molding plaster casting (and other methods) is eliminated and makes the process easy to semi-automate into high volume production. 
         [0030]    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.