Patent Publication Number: US-2011068091-A1

Title: Ceramic Heater and Glow Plug

Description:
FIELD OF THE INVENTION 
     The present invention relates to a ceramic heater used in such applications as, for example, ignition or flame detection heater for onboard heating apparatus of combustion type, ignition heater for kerosene-burning fan heater and other combustion apparatuses, heater for glow plug, heater for various sensors such as oxygen sensor, and heater for measuring instrument. 
     BACKGROUND ART 
     Among ceramic heaters used in such applications as glow plug of automobile engine, there is known a ceramic heater comprising a ceramic base and a ceramic heating element that is embedded in the ceramic base and generates heat through electrical resistance when supplied with power through electrodes connected to both ends thereof. In the ceramic heater having such a constitution, the ceramic heating element comprises a U-shaped turn-over section extending from a base on one side and turns over at the distal end to toward a base on the other side, and two straight lead members extending in the same direction from the bases of the turn-over section (see, for example, Patent Documents 1 and 2). 
     However, in order to ensure the strength of the ceramic heater, the lead member of the ceramic heating element is made thinner than in the distal end portion, and an electrode lead-out member that connects the lead member and the electrode formed on the surface of the ceramic base is also made thinner because the lead member is thin. As a result, while the ceramic heater used in a glow plug, for example, is required to have more quick heating capability and durability at higher temperatures in recent years, there has been such a problem that the electrode lead-out member that connects the lead member and the electrode formed on the surface is more likely to deteriorate than the ceramic heating element, when used under such harsh conditions over a long period of time. This reason is supposedly because the electrode lead-out member is thin and therefore has higher electric resistance, while contact resistance between the electrode lead-out member and the lead member and contact resistance between the electrode lead-out member and the electrode formed on the surface become higher, thus resulting in more tendency to generate heat. 
     In order to solve the problem described above, for example, Patent Document 3 discloses a glow plug wherein the electrode lead-out member is formed in a direction perpendicular to the ceramic heating element and an area of a cross section of the electrode lead-out member is made larger than an area of a cross section of the ceramic heating element.
     Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 9-184626   Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. 9-184622   Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 2006-49279   

     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     When the electrode lead-out member is formed in a direction perpendicular to the ceramic heating element and the area of the cross section of the electrode lead-out member is made larger than the area of the cross section of the ceramic heating element as described in Patent Document 3, increasing the cross section of the electrode lead-out member leads to lower electrical resistance thereof, thereby decreasing the contact resistance between the electrode lead-out member and the lead member and the contact resistance between the electrode lead-out member and the electrode formed on the surface. However, since the electrode lead-out member having lower strength than ceramics is increased in volume, the strength of the ceramic heater decreases. Also the use of an expensive noble metal in the electrode lead-out member leads to a high production cost of the ceramic heater. 
     The present invention has been made to solve the problems described above and an object thereof is to provide a ceramic heater having higher durability at a low cost. 
     Means for Solving the Problems 
     A ceramic heater of the present invention comprises a heating resistor, a first lead member and a second lead member electrically connected to both ends of the heating resistor, respectively, a first electrode lead-out member and a second electrode lead-out member electrically connected to an end of the first lead member and an end of the second lead member, respectively, the end of the first lead member and the end of the second lead member being opposite to the respective ends thereof that are electrically connected to the heating resistor, a ceramic base in which the heating resistor, the first lead member, the second lead member, the first electrode lead-out member and the second electrode lead-out member are embedded, and a first electrode and a second electrode that are formed on the surface of the ceramic base, and are electrically connected to the first electrode lead-out member and the second electrode lead-out member, respectively, wherein an area of a connection part between the first electrode lead-out member and the first electrode is larger than an area of a connection part between the first electrode lead-out member and the first lead member. 
     In the ceramic heater of the present invention with the constitution described above, the first electrode lead-out member comprises an area increasing section in which an area of a cross section perpendicular to the direction increases from a side of the first lead member toward a side of the first electrode. 
     In the ceramic heater of the present invention with the constitution described above, the first electrode lead-out member comprises an area increasing section in which an area of a cross section perpendicular to the direction increases from a side of the first lead member toward a side of the first electrode. 
     In the ceramic heater of the present invention with the constitution described above, the first electrode lead-out member comprises an area decreasing section in which an area of a cross section perpendicular to the direction decreases from the side of the first lead member toward the side of the first electrode, or an area constant section in which an area of a cross section perpendicular to the direction remains constant in the direction. 
     EFFECTS OF THE INVENTION 
     According to the ceramic heater of the present invention, an area of a connection part between the first electrode lead-out member and the first electrode is larger than an area of a connection part between the first electrode lead-out member and the first lead member, that results in lower electrical resistance of the electrode lead-out member than in a case where an area of a cross section remains constant from the connection part thereof with the first lead member up to the connection part thereof with the first electrode, and therefore heat can be suppressed from being generated in the first electrode lead-out member and in the first electrode during operation. Increasing the area of the connection part between the first electrode lead-out member and the first electrode enables it to decrease the contact resistance between the first electrode lead-out member and the first electrode, thereby further suppressing the heat generation. As a result, durability of the first electrode lead-out member and the first electrode can be improved. 
     According to the ceramic heater of the present invention with the constitution described above, when the first electrode lead-out member comprises a cross section which is perpendicular to a direction from a side of the first lead member toward a side of the first electrode and is round or oval, since the profile of the cross section is formed from smooth curve, localized heat generation can be suppressed. 
     Furthermore, according to the ceramic heater of the present invention with the constitution described above, when the first electrode lead-out member comprises an area increasing section in which an area of a cross section perpendicular to the direction increases from the side of the first lead member toward the side of the first electrode, since abrupt change in electrical resistance does not occur in the first electrode lead-out member, the risk of abnormal heating can be decreased. Also because volume of the first electrode lead-out member increases continuously from a side of the first lead member to a side of the first electrode, cracks can be effectively suppressed from occurring even when the volume changes such as shrinkage in degreasing step or firing step during the production. As a result, reliability and durability of the ceramic heater as a final product can be improved. 
     Furthermore, according to the ceramic heater of the present invention with the constitution described above, when the first electrode lead-out member comprises an area constant section in which an area of a cross section perpendicular to the direction remains constant from the side of the first lead member toward the side of the first electrode, since the area of connection part of the first electrode lead-out member with the first electrode can be secured so as to keep the contact resistance low and volume of the first electrode lead-out member can be suppressed from increasing in the constant area section, quantity of expensive noble metal used can be decreased and the production cost can be decreased. 
     Moreover, when the first electrode lead-out member comprises an area decreasing section in which an area of a cross section perpendicular to the direction decreases from the side of the first lead member toward the side of the first electrode, the area of the connection part with the first electrode of the first electrode lead-out member can be secured to keep the contact resistance low and the area of the connection part with the first lead member can be secured to keep the contact resistance low, thus making it possible to suppress heat generation in the first electrode lead-out member. Furthermore, since volume can be suppressed from increasing in the middle portion of the first electrode lead-out member, quantity of expensive noble metal used can be decreased and the production cost can be decreased. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The ceramic heater according to one embodiment of the present invention will be described in detail below with reference to the accompanying drawings.  FIG. 1  is a longitudinal sectional view showing a ceramic heater according to one embodiment of the present invention, and  FIG. 2  is an enlarged plan view of the vicinity of the first electrode of the ceramic heater shown in  FIG. 1  as viewed in the direction of an arrow V. In the following drawings including these drawings, hatchings to be depicted in cross sections of the ceramic base will be omitted. As shown in  FIG. 1 , the ceramic heater  11  has a heating resistor  13 , a first lead member  15  and a second lead member  17  electrically connected to both ends of the heating resistor  13 , a first electrode lead-out member  19  and a second electrode lead-out member  21  electrically connected, respectively, to the ends of the first lead member  15  and the second lead member  17  opposite to the ends thereof that are electrically connected to the heating resistor  13 , and a bar-shaped ceramic base  23  in which the heating resistor  13 , the first lead member  15 , the second lead member  17 , the first electrode lead-out member  19  and the second electrode lead-out member  21  are embedded. The heating resistor  13  is embedded on the first end  12  side of the ceramic base  23 . 
     The ceramic base  23  comprises a first electrode  25  and a second electrode  27  electrically connected, respectively, to the first electrode lead-out member  19  and the second electrode lead-out member  21 , formed on the surface thereof. The first electrode  25  is formed on a side face of the ceramic base  23 . 
     As shown in  FIG. 3  which is an enlarged sectional view of the vicinity of the first electrode lead-out member  19  shown in  FIG. 1 ,  FIG. 4  which is an enlarged sectional view of another embodiment and  FIG. 5  which is an enlarged sectional view of still another embodiment, the first electrode lead-out members  19 ,  31 ,  32  have area S 1  of the connection part with the first electrode  25  larger than the area S 2  of the connection part with the first lead member  15 . This is an important feature of the present invention. 
     According to the present invention, since the area S 1  of the connection part between the first electrode lead-out member  19  and the first electrode  25  is larger than the area S 2  of the connection part between the first electrode lead-out member  19  and the first lead member  15 , electrical resistance of the electrode lead-out member can be made lower than that of a case where an area of a cross section remains constant from the connection part thereof with the first lead member  15  up to the connection part with the first electrode  25 , and therefore heat can be suppressed from being generated in the first electrode lead-out member  19  and the first electrode  25  during operation. Increasing the area of the connection part between the first electrode lead-out member  19  and the first electrode  25  enables it to decrease the contact resistance between the first electrode lead-out member  25  and the first electrode  25 , thus further suppressing the heat generation. As a result, durability of the first electrode lead-out member  19  and the first electrode  25  can be improved. 
     Particularly, as the area S 1  of a portion of the first electrode lead-out member  19  near the surface of the ceramic base  23  is increased, this improves heat dissipation from the first electrode lead-out member  19  through the first electrode  25  and suppresses the temperature from rising in the portion near the surface of the ceramic base  23 . As a result, the first electrode lead-out member  19  can be suppressed from deteriorating, and cracks can be suppressed from occurring in the ceramic base  23  due to heat generated in the first electrode lead-out member  19 . Particularly it is made possible to effectively suppress cracks from occurring in the surface of the ceramic base  23 . 
     It is preferable that a ratio S 1 /S 2  of the area S 1  of the connection part with the first electrode  25  to the area S 2  of the connection part with the first lead member  15  in the first electrode lead-out member  19  is 1.1 or more, more preferably 1.2 or more, and still more preferably 1.5 or more, in order to make the electrical resistance of the first electrode lead-out member  19  lower than that of a case in which an area of a cross section remains constant from the connection part thereof with the first lead member  15  up to the connection part thereof with the first electrode  25 . There is no particular limitation on the upper limit to the ratio S 1 /S 2 , which may be appropriately determined with consideration given to such factors as dimensions and arrangement of the ceramic base  23  and other members. 
     It is preferable that the first electrode lead-out member  19  comprises a cross section which is perpendicular to a direction from a side of the first lead member  15  toward a side of the first electrode  25  and is round or oval. The cross section having a round or oval shape results in the profile of the cross section having a smooth curve, that enables it to suppress localized heat generation. 
     The first electrode lead-out member  19  described above is preferably formed, for example, by the injection molding method as shown in a production method described hereinafter. When the first electrode lead-out member  19  is formed by the injection molding method, the first electrode lead-out member  19  can be formed with round or oval cross section more easily than in the case of using the printing method. When the first electrode lead-out member  19  is formed by printing method, it is necessary to carry out the printing operation a plurality of times since it is difficult to ensure sufficient thickness by a single printing operation. This takes time since it is necessary to correctly align the position every time the printing operation is carried out while positional displacement is likely to occur between the printing operations, thus making it difficult to form smooth round or oval cross section. To the contrary, when the first electrode lead-out member  19  is formed by the injection molding method, the forming method is completed by a single molding operation using a die, so that the first electrode lead-out member  19  can be formed with round or oval cross section easily with high accuracy. 
     In the example shown in  FIG. 3 , the first electrode lead-out member  19  comprises an area increasing section in which an area of a cross section perpendicular to the direction increases from the side of the first lead member  15  toward the side of the first electrode  25 . Thus the first electrode lead-out member  19  in this example has a conical shape truncated at the top. Such a structure enables it to make the electrical resistance of the first electrode lead-out member  19  lower than that of a case in which an area of a cross section remains constant from the connection part thereof with the first lead member  15  up to the connection part thereof with the first electrode  25 , thereby to suppress heat generation in the first electrode lead-out member  19  and in the first electrode  25  during operation. Also increasing the area of the connection part of the first electrode lead-out member  19  with the first lead member  15  decreases the contact resistance between the first electrode lead-out member  19  and the first lead member  15 , so as to further suppress the heat generation. As a result, durability of the first electrode lead-out member  19  and the first electrode  25  can be improved. 
     When the first electrode lead-out member  19  comprises an area increasing section in which an area of a cross section perpendicular to the direction increases from the side of the first lead member toward the side of the first electrode as shown in  FIG. 3 , since abrupt change in electrical resistance does not occur in the first electrode lead-out member  19 , the risk of abnormal heating can be decreased. Also because volume of the first electrode lead-out member  19  increases or decreases continuously from a side of the first lead member  15  to a side of the first electrode  25  in the area increasing section, cracks can be effectively suppressed from occurring even when shrinkage occurs in degreasing step or firing step during the production. As a result, reliability and durability of the ceramic heater as a final product can be improved. Furthermore, since defects such as crack can be suppressed from occurring in the green compact, yield of production can be improved. 
     In the example shown in  FIG. 4 , the first electrode lead-out member  31  comprises the constant area section  31   a  in which an area of a cross section remains constant in the direction of arrow D 1  from a side of the first lead member  15  toward a side of the first electrode  25  and the area increasing section  31   b  in which an area of a cross section increases in the direction of arrow D 1 . 
     When the first electrode lead-out member  31  comprises an area constant section  31   a  in which an area of a cross section perpendicular to the direction remains constant from the side of the first lead member  15  toward the side of the first electrode  25  as described above, since the area of the connection part of the first electrode lead-out member  31  with the first electrode  25  can be made larger to keep the contact resistance low and volume of the first electrode lead-out member  31  can be suppressed from increasing in the constant area section  31   a , quantity of expensive noble metal used in the first electrode lead-out member  31  can be decreased and the production cost can be decreased. 
     When the area increasing section  31   b  and the constant area section  31   a  are combined as described above, there is a portion where the direction in which the side face of the first electrode lead-out member  31  inclines changes in the border between these sections. As a result, when the ceramic heater  11  is molded and fired, or when an external stress is applied, the portion where the direction in which the side face of the first electrode lead-out member  31  inclines changes in the ceramic base  23  serves as a hook, to prevent the first electrode lead-out member  31  from moving and position shifting in the ceramic base  23 . 
     In the example shown in  FIG. 5 , the first electrode lead-out member  32  has the area decreasing section  32   a  in which an area of a cross section perpendicular to the direction of arrow D 1  decreases as it goes toward the direction of arrow D 1 , the constant area section  32   b  in which an area of a cross section remains constant in the direction of arrow D 1  and the area increasing section  32   c  in which an area of a cross section increases as it goes toward the direction of arrow D 1 . When the area decreasing section  32   a , the constant area section  32   b  and the area increasing section  32   c  are combined, or the area decreasing section  32   a  and the area increasing section  32   c  are combined, in any case, there are one or more portions where the direction in which the side face of the conductor inclines changes in the border of the conductor. As a result, when the ceramic heater  11  is molded and fired, or when an external stress is applied to the ceramic heater  11 , the portion where the direction in which the side face of the first electrode lead-out member  32  inclines changes in the ceramic base  23  serves as a hook, to prevent the first electrode lead-out member  32  from moving and position shifting in the ceramic base  23 . 
     Such a constitution makes it possible to ensure the area of the connection part of the first electrode lead-out member  32  with the first electrode  25  and the area of the connection part between the first electrode lead-out member  32  and the first lead member  15  can be respectively maintained in the area increasing section  32   c  and in the area decreasing section  32   a , so as to keep the contact resistance in the connection part low and volume of the first electrode lead-out member  32  can be suppressed from increasing in the constant area section  32   b  in which an area of a cross section does not change, and therefore quantity of expensive noble metal used in the first electrode lead-out member  32  can be decreased and the production cost can be decreased. 
     As shown in  FIG. 1 , the second electrode  27  is formed to cover an end face  14   a  and a lateral face  14   b  of a second end portion  14  of the ceramic base  23 . As shown in  FIG. 1 ,  FIG. 6  that is an enlarged sectional view of the vicinity of the second electrode lead-out member  27  of the ceramic heater shown in  FIG. 1 ,  FIG. 7  that is a front view of the ceramic heater shown in  FIG. 1  as viewed in the direction H indicated with arrow in  FIG. 1 , and  FIG. 8  that is a sectional view taken along lines A-A in  FIG. 1 , the second electrode lead-out member  21  has an area of a connection part with the second electrode  27  larger than an area of a connection part with the second lead member  17 , and therefore enables it to make the electrical resistance of the second electrode lead-out member  21  lower than that of a case in which an area of a cross section remains constant from the connection part thereof with the second lead member  17  up to the connection part thereof with the second electrode  27 , thereby to suppress heat generation in the second electrode lead-out member  21  during operation, so that the second electrode lead-out member  21  can be suppressed from deteriorating. 
     It is preferable that a ratio S 3 /S 4  of the area S 3  of the connection part with the second electrode  27  to the area S 4  of the connection part with the second lead member  17  in the second electrode lead-out member  21  is 1.3 or more, and more preferably 3.7 or more, in order to make the electrical resistance of the second electrode lead-out member  21  lower than that of a case in which the area remains constant from the connection part thereof with the second lead member  17  up to the connection part thereof with the second electrode  27 . There is no particular limitation on the upper limit to the ratio S 3 /S 4 , which may be appropriately determined with consideration given to such factors as dimensions and arrangement of the ceramic base  23  and other members. 
     It is preferable that the second electrode lead-out member  21  has round or oval area of a cross section perpendicular to the direction from the second lead member  17  toward the second electrode  27 . The cross section having round or oval shape enables it to suppress localized heat generation. The cross section having round or oval shape enables it to suppress heat from being generated locally. The cross section having round or oval shape also decreases heat generation in the connection part thereof with the second electrode  27  and in the connection part thereof with the second lead  17 . 
     As shown in  FIG. 6 , the second electrode lead-out member  21  has the area increasing section  21   a  in which an area of a cross section perpendicular to the direction of arrow D 2  increases in the direction of arrow D 2  from the second lead member  17  toward the second electrode  27 . Therefore, since abrupt change in electrical resistance does not occur in the second electrode lead-out member  21 , heat generation by the second electrode lead-out member  21  can be further suppressed. Also because volume of the second electrode lead-out member  21  increases or decreases continuously between the second lead member  17  and the second electrode  27 , cracks can be effectively suppressed from occurring in the ceramic base  23  even when shrinkage occurs in degreasing step or firing step during the production of the ceramic heater. As a result, reliability and durability of the ceramic heater as a final product can be improved. Furthermore, since defects such as crack can be suppressed from occurring in the green compact of the ceramic base  23 , yield of production can be improved. 
     In the example shown in  FIG. 6 , the second electrode lead-out member  21  has the area decreasing section  21   b , in which an area of a cross section decreases as it goes toward the direction of arrow D 2 , provided at the position located in the direction of arrow D 2  from the area increasing section  21   a . Regarding the second end portion  14 , as it goes toward the end face  14   a  of the second end portion  14 , the diameter thereof becomes smaller (hereafter referred to as a small-diameter section  14 ). The area increasing section  21   a  and the area decreasing section  21   b  of the second electrode lead-out member  21  are embedded in the small-diameter section  14 , and the area decreasing section  21   b  is disposed along the small-diameter section  14 . The second electrode lead-out member  21  is constituted by disposing the area increasing section  21   a  and the area decreasing section  21   b  in this order from the second lead member  17  side toward the second electrode  27 . When the area increasing section  21   a  in which an area of cross section increases as it goes toward the direction of arrow D 2  and the area decreasing section  21   b  in which the area of the cross section decreases are provided in this way, strength of the product can be enhanced in the vicinity of the second electrode lead-out member  21  by decreasing the volume of the electrode lead-out material that has low hardness while maintaining an area of a cross section enough to flow electric current, thus enabling it to provide a product of higher reliability. 
     As shown in  FIG. 9  which is an enlarged sectional view of the vicinity of the second electrode lead-out member  33  of the ceramic heater  11  of another embodiment, the second electrode lead-out member  33  may also be constituted from the area increasing section  33   a  in which a cross sectional perpendicular to this direction area increases as it goes from the second lead member  17  toward the second end portion  14 , the constant area section  33   b  of which an area of a cross section remains constant and the area decreasing section  33   c  of which an area of a cross section decreases. Such a constitution enables it to decrease the volume of the electrode lead-out material that has low hardness, thereby further increasing the strength of the product in the vicinity of the second electrode lead-out member  21 . 
     The second electrode  27  is formed on the end face  14   a  of the second end portion  14  and a lateral face  14   b  of the second end portion  14  connected to the end face  14   a . As shown in  FIG. 10  which is a side view depicting a state of a metal fitting section  35  fitted onto the second end portion  14  of the ceramic heater  11  shown in  FIG. 1 , the metal fitting section  35  having a recess is fitted onto the small-diameter section (the second end portion)  14  so as to cover the second electrode  27 . This configuration enables it to suppress the second electrode  27  from being oxidized. Particularly as shown in  FIG. 11  which is a side view depicting another embodiment of the connection part structure between the second end portion  14  and the metal fitting section  35 , it is preferable that the metal fitting section  35  covers the entire surface of the second electrode  27 . This enables it to further improve the effect of suppressing the second electrode  27  from being oxidized, and also increase the contact area between the metal fitting section  35  and the second electrode  27 , thereby decreasing the electrical resistance of this portion and further suppressing heat generation. 
     It is possible to use, as a heating resistor  13 , materials containing carbides, nitrides and silicades of W, Mo and Ti as main component. Of these materials, WC is excellent as the material of the heating resistor  13  in view of thermal expansion coefficient, heat resistance and resistivity. The heating resistor  13  contains an inorganic electric conductor WC as the main component and, for example, when the ceramic base  23  is produced using silicon nitride ceramics as described hereinafter, it is preferred to adjust the proportion of silicon nitride to be added in the heating resistor  13  to 20% by mass or more. Among silicon nitride ceramics, since a conductor component, that would be turned into the heating resistor  13 , has a larger thermal expansion coefficient than that of silicon nitride, it is in a state where tensile stress is applied. To the contrary, the addition of silicon nitride itself, as a common material, to the heating resistor  13  brings the thermal expansion coefficient close to that of silicon nitride as the base material, thus making it possible to release stress due to difference in thermal expansion of the ceramic heater  11  upon heating and cooling. 
     When the additive amount of silicon nitride is 40% by mass or less, it is possible to satisfactorily stabilize electrical resistance. The additive amount of silicon nitride is preferably adjusted within a range from 25 to 35% by mass. It is also possible to add, as an additive to the heating resistor  13 , 4 to 12% by mass of boron nitride instead of silicon nitride. 
     It is possible to use, as the materials of the first lead member  15  and the second lead member  17 , same materials as those of the heating resistor  13 . Of these materials, WC is excellent as the material of lead members  15 ,  17  in view of thermal expansion coefficient, heat resistance and resistivity. The first lead member  15  and the second lead member  17  contain an inorganic electric conductor WC as the main component. Similarly to the heating resistor  13  described above, when a ceramic base  23  is produced using silicon nitride ceramics, it is preferred to adjust the proportion of silicon nitride to be added in the first lead member  15  and the second lead member  17  to 15% by mass or more. As the additive amount of silicon nitride increases, it is possible to bring the thermal expansion coefficient of the first lead member  15  and the second lead member  17  close to that of silicon nitride as the base material. When the additive amount of silicon nitride is 40% by mass or less, since electrical resistance becomes stable, the additive amount of silicon nitride is preferably adjusted to 40% by mass or less. More preferably, the additive amount of the silicon nitride is adjusted within a range from 20 to 35% by mass. 
     It is possible to use, as the material of the ceramic base  23 , ceramics having insulating properties, such as oxide ceramics, nitrides ceramics or carbides ceramics. It is particularly preferred to use silicon nitride ceramics. The reason why silicon nitride ceramics are particularly preferred is that silicon nitride as the main component is excellent in view of high strength, high toughness, high insulating properties and heat resistance. The silicon nitride ceramics can be obtained, for example, by mixing silicon nitride as the main component with sintering aids, for example, 3 to 12% by mass of rare earth element oxides such as Y 2 O 3 , Yb 2 O 3  and Er 2 O 3 , 0.5 to 3% by mass of Al 2 O 3 , and 1.5 to 5% by mass of SiO 2  in terms of SiO 2  contained in the resultant sintered body, and forming the mixture into s predetermined shape, followed by firing through hot pressing at 1,650 to 1,780° C. 
     When silicon nitride is used as the material of the ceramic base  23 , it is preferred that MoSiO 2  or WSi 2  are dispersed. The reason is that durability of the ceramic heater  11  can be improved by bringing the thermal expansion coefficient of the base material closer to that of the heating resistor  13 . 
     A method of producing the ceramic heater  11  of the above-mentioned embodiment will be described below. The ceramic heater  11  of the present embodiment can be molded by an injection molding method using a die fabricated so as to form the first electrode lead-out member  19  having the area of the connection part with the first electrode  25  larger than the area of the connection part with the first lead member  15 . 
     First, a mixed material for conductor containing an electrically conductive ceramic powder and a binder and a mixed material for a substrate containing an insulating ceramics and a binder are prepared. The mixed material for conductor is used to form a green compact for heating resistor by an injection molding method. While holding the green compact for heating resistor thus obtained in an injection molding die, the die is filled with the mixed material for conductor, thereby to mold the green compact for lead member. This results in a green compact for conductor comprising the green compact for heating resistor and the green compact for lead member held within the die. 
     Using the green compact for conductor held in the die, a part of the die is replaced with a component used to form the ceramic base, and the die is filled with the mixed material for substrate. This results in a green compact of element comprising the green compact for conductor covered by the green compact for ceramic base. The green compact of element is then fired so as to make the ceramic heater. The firing operation is preferably carried out in a non-oxidizing atmosphere. 
     &lt;Glow Plug&gt; 
     The glow plug according to one embodiment of the present invention will be described below. As shown in  FIG. 12  which is a sectional view of a glow plug according to one embodiment of the present invention, the glow plug  51  comprises the ceramic heater  11  inserted into a tubular metal fitting  53 . The tubular metal fitting  53  is used as a cathode, and is electrically connected to the first electrode  25  that is exposed on the side face of the ceramic heater  11 . Disposed in the tubular metal fitting  53  is an anode metal fitting  55  that is electrically connected to the second electrode  27 . When electric current is supplied to flow through the tubular metal fitting  53  and the anode metal fitting  55 , the glow plug of the present embodiment functions as a heat source, for example, to start an engine. 
     EXAMPLES 
     The ceramic heater according to one embodiment of the present invention was made as follows. First, a material consisting of WC and silicon nitride as the main components was injected into a die thereby to mold the green compact for heating resistor. While holding the green compact for a heating resistor thus obtained in an injection molding die, the die was filled with the green compact for a lead member, thereby to integrate the green compact for a heating resistor and the green compact for a lead member within the die and obtain the green compact for conductor. Specimens Nos. 1 through 16 shown in Table 1 and Table 2 are samples that were molded by using dies having electrode lead-out members of various shapes. The electrode lead-out member of each specimen was formed so that the cross section perpendicular to the direction from the lead member to the electrode would have oval shape. The yield of molding for each specimen was evaluated and the shapes thereof were compared. 
     Using the green compact for conductor held in the injection molding die, a ceramic material, prepared by adding a sintering aid composed of an oxide of ytterbium (Yb) and MoSi 2  used to control the thermal expansion coefficient to a value near that of the heating resistor and the lead member to silicon nitride (Si 3 N 4 ), was molded by an injection molding method. Thus, a structure comprising the green compact for conductor embedded in the green compact for a ceramic base was obtained. 
     The green compact thus obtained was put into a tubular carbon die and was fired by a hot press method at a temperature in a range from 1,650° C. to 1,780° C. under a pressure from 10 to 50 MPa in a reducing atmosphere. Metal fittings were brazed onto the first electrode lead-out member and the second electrode lead-out member exposed on the surface of the sintered body thus obtained, thereby making the ceramic heater. Using a K thermocouple attached to these metal fittings, temperature of the electrode lead-out member was measured in the state of saturated energization. Design temperature of the electrode is usually considered to be desirably 300° C. or lower, and therefore the temperature not higher than this level is thought to be advantageous in terms of durability of the electrode. 
     The ceramic heaters made as described above were subjected to a thermal cycle test. One cycle was set to consist of 5 minutes of supplying current to the ceramic heater with voltage applied so that the electrode would be heated to 400° C. and 2 minutes of shutting off the current, and ten thousand thermal cycles were repeated. Electrical resistance of the ceramic heater was measured before and after energization, and specimen that showed 5% or more change in electrical resistance was evaluated as NG. Cracks generated in the electrode or the electrode lead-out member were observed in the specimens evaluated as NG. The results are shown in Table 1 and Table 2. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 First electrode lead-out member 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Area 
                 Area 
                 Constant 
                   
                   
                   
               
               
                 Specimen 
                   
                 increasing 
                 decreasing 
                 area 
                 Yield of 
                 Electrode 
               
               
                 No. 
                 S1/S2 
                 section 
                 section 
                 section 
                 molding 
                 temperature 
                 Durability 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                 1.5 
                 Provided 
                 None 
                 None 
                 100% 
                 230° C. 
                 OK 
               
               
                 2 
                 1.2 
                 Provided 
                 None 
                 None 
                 100% 
                 240° C. 
                 OK 
               
               
                 3 
                 1.1 
                 Provided 
                 None 
                 None 
                 100% 
                 265° C. 
                 OK 
               
               
                 4 
                 1.2 
                 Provided 
                 Provided 
                 None 
                 100% 
                 260° C. 
                 OK 
               
               
                 5 
                 1.2 
                 Provided 
                 None 
                 Provided 
                 100% 
                 245° C. 
                 OK 
               
               
                 6 
                 1.2 
                 Provided 
                 Provided 
                 Provided 
                 100% 
                 250° C. 
                 OK 
               
               
                 7 
                 1.0 
                 None 
                 None 
                 Provided 
                 65% 
                 360° C. 
                 NG 
               
               
                 8 
                 0.8 
                 None 
                 Provided 
                 None 
                 40% 
                 430° C. 
                 NG 
               
               
                   
               
               
                 S1 is the area of the connection part of the first electrode lead-out member with the first electrode. 
               
               
                 S2 is the area of the connection part of the first electrode lead-out member with the first lead member. 
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Second electrode lead-out member 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Area 
                 Area 
                   
                   
                   
                   
                   
                   
               
               
                 Specimen 
                   
                 increasing 
                 decreasing 
                 Constant 
                 Small-diameter 
                 Metal fitting 
                 Yield of 
                 Electrode 
               
               
                 No. 
                 S3/S4 
                 section 
                 section 
                 area section 
                 section 
                 section 
                 molding 
                 temperature 
                 Durability 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 9 
                 5.8 
                 Provided 
                 Provided 
                 None 
                 Provided 
                 Entire electrode 
                 100% 
                 180° C. 
                 OK 
               
               
                   
                   
                   
                   
                   
                   
                 surface 
               
               
                 10 
                 4.9 
                 Provided 
                 Provided 
                 None 
                 Provided 
                 Entire electrode 
                 100% 
                 190° C. 
                 OK 
               
               
                   
                   
                   
                   
                   
                   
                 surface 
               
               
                 11 
                 3.7 
                 Provided 
                 Provided 
                 None 
                 Provided 
                 Entire electrode 
                 100% 
                 205° C. 
                 OK 
               
               
                   
                   
                   
                   
                   
                   
                 surface 
               
               
                 12 
                 4.9 
                 Provided 
                 Provided 
                 None 
                 Provided 
                 Part of electrode 
                 100% 
                 200° C. 
                 OK 
               
               
                   
                   
                   
                   
                   
                   
                 surface 
               
               
                 13 
                 1.3 
                 Provided 
                 None 
                 None 
                 None 
                 Entire electrode 
                 100% 
                 250° C. 
                 OK 
               
               
                   
                   
                   
                   
                   
                   
                 surface 
               
               
                 14 
                 1.0 
                 None 
                 Provided 
                 Provided 
                 Provided 
                 Entire electrode 
                 70% 
                 310° C. 
                 NG 
               
               
                   
                   
                   
                   
                   
                   
                 surface 
               
               
                 15 
                 0.9 
                 None 
                 Provided 
                 None 
                 Provided 
                 Entire electrode 
                 50% 
                 370° C. 
                 NG 
               
               
                   
                   
                   
                   
                   
                   
                 surface 
               
               
                 16 
                 1.0 
                 None 
                 None 
                 provided 
                 None 
                 Entire electrode 
                 70% 
                 350° C. 
                 NG 
               
               
                   
                   
                   
                   
                   
                   
                 surface 
               
               
                   
               
               
                 S3 is the area of the connection part of the second electrode lead-out member with the second electrode. 
               
               
                 S2 is the area of the connection part of the second electrode lead-out member with the second lead member. 
               
            
           
         
       
     
     As is apparent from Table 1 and Table 2, specimens Nos. 7, 8 and 14 to 16 with no area increasing section showed low yield of molding in a range from 40% to 70%. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal sectional view showing a ceramic heater according to one embodiment of the present invention. 
       FIG. 2  is an enlarged plan view of the vicinity of the first electrode of the ceramic heater shown in  FIG. 1  as viewed in the direction of a dash line V. 
       FIG. 3  is an enlarged sectional view of the vicinity of the first electrode lead-out member shown in  FIG. 1 . 
       FIG. 4  is an enlarged sectional view showing another embodiment of the vicinity of the first electrode of the ceramic heater. 
       FIG. 5  is an enlarged sectional view showing still another embodiment of the vicinity of the first electrode of the ceramic heater. 
       FIG. 6  is an enlarged sectional view of the vicinity of the second electrode lead-out member of the ceramic heater shown in  FIG. 1 . 
       FIG. 7  is a front view of the ceramic heater shown in  FIG. 1  as viewed in the direction H indicated with arrow in  FIG. 1 . 
       FIG. 8  is a sectional view taken along lines A-A in  FIG. 1 . 
       FIG. 9  which is an enlarged sectional view of the vicinity of the second electrode lead-out member of the ceramic heater of another embodiment. 
       FIG. 10  is a side view depicting a state of a metal fitting section fitted onto the second end portion of the ceramic heater shown in  FIG. 1 . 
       FIG. 11  which is a side view depicting another embodiment of the connection part structure between the second end portion and the metal fitting section. 
       FIG. 12  is a sectional view of a glow plug according to one embodiment of the present invention 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           11 : Ceramic heater 
           12 : First end 
           13 : Heating resistor 
           14 : Second end portion (Small-diameter section) 
           14   a : End face 
           14   b : Lateral face 
           15 : First lead member 
           17 : Second lead member 
           19 ,  31 : First electrode lead-out member 
           21 ,  33 : Second electrode lead-out member 
           21   a ,  31   b ,  32   c ,  33   a : Area increasing section 
           21   b ,  32   a ,  33   c : Area decreasing section 
           23 : Ceramic substrate 
           25 : First electrode 
           27 ,  33 : Second electrode 
           31   a ,  32   b ,  33   b : Constant area section 
           35 ,  37 : Metal fitting section 
           51 : Glow plug