Abstract:
A ceramic heater element and a glow plug incorporating the novel heater element. The heater element has a base portion and a heater portion. Conductive, insulative and resistive layers extend through both the base and heater portions. An outer conductive layer is applied to the outside of the base portion to provide a highly conductive return path. This lends to limit the heating of the resistive layer in the base portion and results in better and more reliable heat concentration in the heater portion. The heater element is further provided with a waterproof non-electrically conductive outer layer. The heater element can be assembled to form a glow plug for a diesel engine.

Description:
FIELD OF THE INVENTION 
   This invention relates to ceramic heater elements. In particular, this invention relates to ceramic heater elements, and methods of manufacture therefor, such as ceramic heaters used in high-temperature a glow plugs for diesel engines. 
   BACKGROUND OF THE INVENTION 
   It is well known to manufacture ceramic glow plugs having a multi-layered construction. Examples of such conventional glow plugs are described in U.S. Pat. Nos. 4,742,209, 5,304,778 and 5,519,187. In general, these glow plugs have a ceramic heater with a conductive core enclosed by insulative and resistive ceramic layers, respectively. The layers are separately cast and fitted together. The resulting green body is then sintered to form a ceramic heater. Such ceramic heaters suffer several drawbacks. Used in a glow plug, they experience cyclic heating and cooling, which results in high internal stresses at the interfacial junction between the ceramic layers, promoting eventual failure of the glow plugs. To reduce this failure rate, such ceramic heaters tend to be cycled at lower temperatures than would be optimal in a diesel engine. 
   The internal stresses of a layered glow plug are mainly the result of differences in the coefficients of thermal expansion between the differently composed layers. The different layers of the glow plug expand and contract at different rates. Further, residual stresses are the result of manufacture, particularly from uneven contraction in the cooling period which occurs below the plastic deformation state of the ceramic composition, and from non-uniform attachment between the layers. 
   A ceramic heater that has reduced internal stress is described in U.S. patent application Ser. No. 08/882,306, filed Jun. 25, 1997. This application discloses a ceramic heater that is slip cast as a unitary body with a graduated composition in the interfacial boundary zones. While the ceramic heater described in this application has reduced internal stresses, it has been found to be difficult to manufacture to the stringent standards required of such heaters. In particular, the layer thicknesses are difficult to control precisely, and even minor discrepancies can lead to widely varying heat output in the final heater. Precise control of heating characteristics, and limiting heating losses in the base portion of the heater element, is important if the ceramic heaters are to be mass produced for vehicle and engine manufacturers. 
   Additionally, it has been found that the performance of prior ceramic heater elements can be affected by moisture. 
   It is, therefore, desirable to provide a ceramic heater element that overcomes the disadvantages of the prior art. In particular, it is desirable to provide a ceramic heater element that has low internal thermal stresses, precisely controllable and reproducible heating characteristics that are focussed mainly to the heating tip of the element and that is more resistant to the effects of moisture. 
   SUMMARY OF INVENTION 
   Generally, the present invention provides a ceramic heater element and a glow plug incorporating a novel heater element. The heater element has a base portion and a heater portion. Conductive, insulative and resistive layers extend through both the base and heater portions. An outer conductive layer is applied to the outside of the base portion to provide a highly conductive return path. This tends to limit the heating of the resistive layer in the base portion and results in better and more reliable heat concentration in the heater portion. A waterproof non-conductive outer layer is provided over the outer surface of the heater element. The heater element can be assembled to form a glow plug for a diesel engine. 
   In a preferred embodiment of the present invention, the ceramic heater includes a base portion with a heater portion formed at one end. The heater portion has a lesser diameter than the base portion. The base portion and heater portion each having a conductive ceramic layer and a resistive ceramic layer, which are separated by an insulative ceramic layer except at a tip of the heater portion where they are electrically connected. The base portion further has an outer conductive ceramic layer in electrical contact with the resistive ceramic layer. A waterproof outer layer of non-electrically conductive ceramic extends over the base and the heater portions. An optional central conductive core can be included in this heater, which extends substantially the length of the base portion. 
   In a further embodiment of the present invention, there is provided a glow plug for a diesel engine, employing the above-described heater element. The glow plug has a metallic housing, including a barrel and a tapered sleeve. A ceramic heater element, having a base portion tapered to wedgingly fit within the sleeve, is mounted within the housing. The heater element has a heater portion formed at an end of the base portion. The heater portion has a lesser diameter than the base portion, and generally extends beyond the housing. The base portion and heater portion each having a conductive ceramic layer and a resistive ceramic layer, which are separated by an insulative ceramic layer except at a tip of the heater portion where they are electrically connected. The base portion further has an outer conductive ceramic layer in electrical contact with the resistive ceramic layer. A waterproof non-electrically conductive outer layer extends over the base and the heater portions. An optional central conductive core can be included in this heater, which extends substantially the length of the base portion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described, by way of example only, with reference to the attached Figures, in which: 
       FIG. 1  is a schematic cross sectional view of a ceramic heater element according to an embodiment of the present invention, sectioned along its longitudinal axis; 
       FIG. 2  is a schematic cross sectional view of the ceramic heater element according of  FIG. 1 , along the line A—A; 
       FIG. 3  is a schematic cross sectional view of the ceramic heater element according to  FIG. 1 , along the line B—B; 
       FIG. 4  is a cross section of a glow plug according to the present invention; and 
       FIG. 5  is a cross section of a further embodiment of the heater element of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will be now be described with reference to  FIGS. 1 and 2 . A schematic view of a ceramic heater element according to a first embodiment of the present invention is shown in cross-section along its longitudinal axis in  FIG. 1 , and in cross-section along line A—A in FIG.  2 . The heater element is not shown to scale and is generally designated at reference numeral  10 . 
   Element  10  consists of a base portion  20  and a heater portion  22 . Base portion  20  and heater tip portion  22  form a generally cylindrical heater element that is thicker in diameter through base portion  20  and tapers to a thinner diameter heater portion  22 . As is well known to those of skill in the art, base portion  20  is typically sized to be received in a metal housing, including appropriate electrical contacts, to form a glow plug for a diesel engine. As described in U.S. Pat. No. 5,880,432, entitled “Electric heating device with ceramic heater wedgingly received within a metallic body”, the contents of which are incorporated herein by reference, one means of forming base portion  20  is to taper base portion  20  to permit it to be wedged into a suitable metal housing. It is fully within the contemplation of the present inventor that base portion  20  of heater element  10  can be so formed, but the present invention can be employed advantageously with any ceramic heater element, regardless of its particular shape and dimensions. 
   As is well know in to those of skill in the art, heater portion  22  has a lesser diameter than base portion  20 . This results in a higher resistance in heater portion  22 , and, consequently, a higher heat output. Thus, heating of element  10  is ideally concentrated in heater portion  22 . 
   Referring to the preferred embodiment shown in  FIGS. 1 and 2 , base portion  20  is formed of six layers of ceramic material. As is well known, the composition of the layers differs, particularly in the amount of conductive ceramic component such as MoSi 2 , such that the electrical conductivity of the different layers can be controlled. Beginning at the centre, base portion  20  consists of an inner electrically conductive core  24 , an electrically conductive layer  26 , an electrically insulative layer  28 , an electrically resistive layer  30 , an outer electrically conductive layer  32  and an outer insulative waterproof layer  38 . Generally, base portion  20  also includes hole  4  that permits connection to an electrical lead (not shown) when element  10  is assembled as a glow plug. For the purposes of description, conductive layer  26  and resistive layer  30  are differentiated. However, as will be further described below, these two layers have similar characteristics, and any heating ascribed to resistive layer  30  can be equally well accomplished in conductive layer  26 . 
   Referring to  FIGS. 1 and 3 , heater portion  22  is formed of four layers of ceramic material. Beginning again at the innermost layer, heater portion  22  consists of conductive layer  26 , insulative layer  28 , resistive layer  30  and an outer insulative waterproof layer  38 . The distal end of heater portion  22  is formed into a tip  36  that forms an electrical connection between conductive layer  26  and resistive layer  30 . 
   Generally, the ceramic material forming the various layers is selected from the group comprising Si 3 N 4 , Y 2 O 3 , silicon carbide, aluminum nitride, alumina, silica and zirconia. These non-conductive ceramic materials are then doped with one or more conductive components selected from the group comprising MoSi 2 , TiN, ZrN, TiCN and TiB 2 . The percent concentration of the conductive component, in conjunction with the layer thickness, determines the resulting conductivity of the ceramic material. A sintering additive from about 10 to about 0 percent by volume can also be included. The sintering additive includes yttrium, magnesia, calcium, hafnia and others of the Lanthanide group of elements. The conductive and non-conductive components are supplied as finely ground particles. Optimally, the particles can range in size from about 0.2 to about 0.8 microns. The finely ground components are mixed and suspended in a solvent, such as water, to form a slurry. A suitable deflocculant, such as ammonium polyacrylate, known commercially as DARVAN C™ can also be added. 
   In a preferred embodiment, the non-conductive ceramic material is Si 3 N 4  and the conductive component is MoSi 2 . Inner core  24  can have 41-80 vol. % MoSi 2 , conductive layer  26  can have 30-45 vol. % MoSi 2 , insulative layer  28  and waterproof outer layer  38  can have 0-28 vol. % MoSi 2 , resistive layer  30  can have 30-45 vol. % MoSi 2 , and outer layer  32  can have 41-80 vol. % MoSi 2 . 
   While this preferred embodiment has been described as having an inner conductive core  24 , it is contemplated by the present inventor that heater element  10  can be formed of five layers, without a core. In this case, conductive layer  26  also occupies the volume of conductive core  24 . The advantage that core  24  is presently believed to provide to heater element  10  is an improved conduction of electricity through base portion  20  to concentrate heat development in heater portion  22 . It is also contemplated that heater element  10  can include a core that extends beyond the length of base portion  20 . For example, for certain applications it can be desirable to have core  24  extend nearly to tip  36 . 
   Ceramic heater element  10  is preferably manufactured by slip casting, such as is described in U.S. patent application Ser. No. 08/882,306, the contents of which are incorporated herein by reference. The method described therein is modified somewhat to incorporate the additional layers: inner core  24  and outer layer  32 . An absorbent, tubular mold, open at both ends, is provided. The mold can be fabricated from plaster of Paris or any other suitable absorbent material. In a preferred embodiment the mold is provided with a smaller inner diameter step to produce element  10  having a relatively small diameter at heater portion  22 . 
   Generally, successive layers of element  10  are added to the mold from the tip  36  end. The method commences by laying down outer waterproof insulative layer  38 , then outer electrically conductive layer  32 , and then forming resistive layer  30 . Next, insulative layer  28  is formed in the mold. It has been found, in a standard sized heater element, that insulative layer  28  needs to be at least 0.3 mm to provide an effective electrically insulative barrier between resistive layer  30  and conductive layer  26 . And finally, conductive layer  26  is formed in a well known manner. Inner core  24  is then injected into the mold from the opposite end of the mold such that it extends substantially the length of base portion  20 . Connecting hole  34  can be formed in inner core  24  at this time. To form an integral electrical connection between conductive layer  26  and resistive layer  30 , tip  36  of the green body is reformed by, for example, applying low intensity vibrations from an ultrasonic wand to tip  36  before the green body is removed from the mold. The low intensity vibrations cause the particles at the tip to be blended into an electrically conductive tip joining the inner and outer volumes. Once the liquid phase has been substantially absorbed through the walls of the mold, the green body with a reformed tip is removed from the mold and allowed to air dry. 
   Alternatively, the ceramic heater element  10  can be formed by commencing with resistive layer  30 , and continuing as described above. Then prior to sintering the green body, it is dipped into a conductive ceramic slurry to form outer layer  32 . This results in very thin coating of conductive material that covers base portion  20 . Next, the green body is dipped into an insulative ceramic body to form outer waterproof insulative layer. As is well known, the green ceramic body is then sintered and polished to produce element  10 . Casting outer layer  32  is presently preferred, as greater control of the layer  32  thickness is achieved. 
   Referring to  FIG. 4 , element  10  can then be assembled to form a glow plug assembly  40 , as described in the aforementioned U.S. Pat. No. 5,880,432. Element  10  is inserted into a metallic housing  42  consisting of a barrel  44  and a sleeve  46 . Sleeve  46  is tapered to match the outer taper of base portion  20  such that element  10  is wedgingly held in place within housing  42 . A conductive wire  48  is inserted into hole  34  of element  10 , and element  10  and wire  48  are secured in place by filling barrel  44  with an epoxy, or other fixant suitable for operation in a corrosive, high temperature atmosphere. Barrel  44  is then sealed with connector cap  50 . 
   As can be seen from  FIG. 4 , sleeve  46 , and hence housing  42 , is in electrical contact with outer layer  32 , while wire  48  is in electrical contact with inner core  24 . In operation, an electrical potential is applied across housing  42  and conductive wire  48 . This causes an electrical current to flow from conductive wire  48  through conductive inner core  24  to conductive layer  26 . The current then flows through resistive layer  30  at the exterior of heater portion  22 , and returns along outer layer  32  to housing  42 . As the current flows through resistive layer  30  in the region of heater portion  22 , it heats heater portion  22  to a temperature sufficient for diesel fuel ignition. Experimental testing of element  10  has resulted in repeated cycling to heater temperatures in the range of 1500° C. without failure of the element  24 . As will be understood by those of skill in the art, the high conductivity of outer layer  32  results in little current flow through the resistive layer  30  in the base portion  20 , thus limiting the heating of the base portion, and improving the concentration of heat in the resistive layer  30  of heater portion  22 . 
   Referring to  FIG. 5 , a further embodiment a ceramic heater element of the present invention is shown and generally designated at reference numeral  60 . This embodiment differs from the first embodiment in that it has no inner core. Instead conductive layer  26  fills the interior volume of element  10  and forms the inner core. Generally, this four layer ceramic heater element  60  relies on the conductive layer  26  to carry the electrical current to heater portion  22 . The slightly less efficient resistivity of layer  26  results in slightly lower operating temperatures, typically in the range of 1300° C., but has the benefit of lowering the production costs of the ceramic heater elements. 
   As will be appreciated by those of skill in the alt, the ceramic heater element of the present invention has a number of advantages over the prior art. The four or five layer structure and and outer layer  32  result in a more efficient concentration of heat at heater portion  22 , and enhances the stability and uniformity of the ceramic heater elements. The non-conductive waterproof outer layer  38  further enhances performance by protecting the heater element and minimizing the effects of atmospheric moisture on the electrical properties of the heater element. Consequently, this results in the manufacture of fewer rejected pieces, thereby lowering production costs and increasing profit. The concentration of heat also results in a heater element that can be repeatedly cycled to approximately 1300-1500° C., which is a significant improvement over prior art ceramic heater elements which typically operate at 900-1100° C. 
   Although the disclosure describes and illustrates the preferred embodiments of the invention, it is understood that the invention is not limited to these particular embodiments. Many variations and modifications will now occur to those skilled in the art. For definition of the invention, reference is made to the appended claims.