Abstract:
A ceramic substrate for a ceramic heater includes aluminum nitride, silicon nitride or silicon carbide as the main component for increasing mechanical strength and improving thermal shock resistance, and a proper additive for controlling thermal conductivity. A temperature gradient from a heating element to a power feeding electrode is reduced by providing a dimensional ratio of the substrate effective for preventing oxidation of a power feeding contact that contacts the electrode of the heating element formed on the surface of the ceramic substrate. The dimensional ratio A/B≧20 is satisfied, wherein A represents the distance from the contact between a circuit of the heating element and the electrode to an end of the ceramic substrate closer to the electrode, and B represents the thickness of the ceramic substrate. The thermal conductivity of the ceramic substrate is adjusted to 30 to 80 W/m·K.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ceramic heater having a heating element formed on a ceramic substrate (hereinafter simply referred to as a substrate), and more particularly, it relates to a ceramic heater usefully applied to an electric or electronic apparatus. 
     2. Description of the Prior Art 
     In general, ceramics having an excellent insulation property and a high degree of freedom in design of a heater circuit is applied to various types of heater substrates. In particular, an alumina sintered body, having high mechanical strength among ceramic materials with thermal conductivity reaching 30 W/m·K, relatively excellent in thermal conductivity and thermal shock resistance and obtained at a low cost, is widely employed. When the alumina sintered body is applied to a A substrate, however, the substrate cannot follow abrupt temperature change of a heating element and may be broken due to a thermal shock. 
     Japanese Patent Laying-Open No. 4-324276 (1992) discloses a ceramic heater employing aluminum nitride having thermal conductivity of at least 160 W/m·K. A substrate having such a degree of thermal conductivity is not broken by abrupt temperature change dissimilarly to the substrate of alumina. This gazette describes that the uniform heating property of the overall heater can be secured by stacking about four layers of aluminum nitride and forming heating elements having different shapes on the respective layers while locating an electrode substantially at the center of the substrate for uniformizing temperature distribution in the ceramic heater. 
     Japanese Patent Laying-Open No. 9-197861 (1997) discloses employment of aluminum nitride for a substrate of a heater for a fixing device. According to this prior art, a substrate having thermal conductivity of at least 50 W/m·K, preferably at least 200 W/m·K can be obtained by setting the mean particle diameter of aluminum nitride particles to not more than 6.0 μm, optimizing combination of sintering agents and performing sintering at a temperature of not more than 1800° C., preferably not more than 1700° C. This gazette describes that the substrate having excellent thermal conductivity is employed for the heater for a fixing device thereby efficiently transferring heat of a heating element to paper or toner and improving a fixing rate. 
     In addition, Japanese Patent Laying-Open No. 11-95583 (1999) discloses the use of silicon nitride for a substrate of a heater for a fixing device. This prior art reduces the thickness of the substrate itself by employing silicon nitride having a relatively high strength with a flexural strength of 490 to 980 N/mm 2  and a thermal conductivity of at least 40 W/m·K, preferably at least 80 W/m·K, and reducing the heat capacity thereof, thereby reducing the power consumption. This gazette describes that silicon nitride has a lower thermal conductivity than aluminum nitride and hence the heat of a heating element is not readily transmitted to a connector of a current feeding part and oxidation of an electrode of the heating element can be prevented for avoiding a contact failure. 
     When thermal conductivity of a substrate is increased, the quantity of diffusion to parts other than a heating part is also increased although heat propagation efficiency from a heating element is improved, to consequently increase power consumption. In order to prevent oxidation of a contact between an electrode of the heating element and a connector of a feeding part, therefore, it is effective that a uniform heating property around the substrate is excellent and a temperature around the electrode of the heating element is lower by at least several % than that of the heating element region. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a ceramic heater having an increased mechanical strength of a substrate and an improved thermal shock resistance. 
     Another object of the present invention is to provide a ceramic heater capable of controlling the thermal conductivity of a substrate and reducing the steepness of a temperature gradient from a heating element to an electrode thereby preventing oxidation of a contact between the electrode of the heating element and a connector of a current feeding part. 
     In a ceramic heater according to the present invention, a ceramic substrate provided with an electrode and a heating element on its surface is formed in a shape satisfying A/B≧20 assuming that A represents the distance from a contact between the heating element and the electrode to an end of the substrate closer to the electrode and B represents the thickness of the substrate, and the thermal conductivity of the substrate is adjusted to 30 to 80 W/m·K. 
     The main component forming the substrate is aluminum nitride, silicon nitride or silicon carbide, and a subsidiary component having thermal conductivity of not more than 50 W/m·K is added thereto. 
     If the main component of the ceramic is aluminum nitride, 5 to 100 parts by weight of aluminum oxide, 1 to 20 parts by weight of silicon and/or a silicon compound in terms of silicon dioxide or 5 to 100 parts by weight of zirconium and/or a zirconium compound in terms of zirconium oxide is added to 100 parts by weight of aluminum nitride, in order to adjust thermal conductivity thereof. 
     In order to obtain a ceramic sintered body having high mechanical strength, 1 to 10 parts by weight of an alkaline earth element and/or a rare earth element of the periodic table is introduced as a sintering agent with respect to 100 parts by weight of aluminum nitride. Calcium (Ca) is preferably selected as the alkaline earth element of the periodic table, while neodymium (Nd) or ytterbium (Yb) are preferably selected as the rare earth element of the periodic table. 
     The material for the substrate of the ceramic heater according to the present invention is preferably mainly composed of aluminum nitride (AlN), silicon nitride (Si 3 N 4 ) or silicon carbide (SiC). While a substrate having thermal conductivity exceeding 100 W/m·K can be obtained by sintering material powder of such ceramic with addition of not more than several % of a proper sintering agent, the thermal conductivity of the substrate can be reduced to 30 to 80 W/m·K by adding a subsidiary component having thermal conductivity of not more than 50 W/m·K to the material powder. 
     If the thermal conductivity of the substrate is less than 30 W/m·K, there is a high possibility that the substrate itself is unpreferably broken by a thermal shock due to abrupt temperature increase of the heating element as energized. If the thermal conductivity of the substrate exceeds 80 W/m·K, the heat of the heating element is propagated to the overall substrate to unpreferably increase the quantity of diffusion to parts other than a heating part while also increasing power consumption, although a uniform heating property is excellent. 
     When adding aluminum oxide (Al 2 O 3 ) to aluminum nitride (AlN), it is preferable to add 5 to 100 parts by weight of the former with respect to 100 parts by weight of the latter. The added aluminum oxide solidly dissolves oxygen in aluminum nitride in the sintered body thereby reducing the thermal conductivity while aluminum oxide having thermal conductivity of about 20 W/m·K itself is present in a grain boundary phase of aluminum nitride to effectively reduce the thermal conductivity of the ceramic sintered body. If the content of aluminum oxide is less than 5 parts by weight, the thermal conductivity may exceed 80 W/m·K. If the content of aluminum oxide exceeds 100 parts by weight, aluminum nitride reacts with aluminum oxide to form aluminum oxynitride. This substance has extremely low thermal conductivity, and hence the thermal conductivity of the overall substrate may be less than 30 W/m·K in this case. 
     Silicon and/or a silicon compound can be added to aluminum nitride (AlN) for adjusting the thermal conductivity. Silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ) or silicon carbide (SiC) may be employed as the added silicon compound. Such a substance is present in a grain boundary phase in the sintered body, and serves as a thermal barrier phase inhibiting thermal conduction between aluminum nitride particles. Such silicon and/or a silicon compound is preferably added by 1 to 20 parts by weight in terms of silicon dioxide (SiO 2 ) with respect to 100 parts by weight of aluminum nitride. If the content of silicon and/or a silicon compound is less than 1 part by weight, the thermal barrier effect of silicon tends to be insufficient and hence the thermal conductivity may exceed 80 W/m·K. If the content of silicon and/or a silicon compound exceeds 20 parts by weight, the thermal conductivity tends to be less than 30 W/m·K. 
     Zirconium and/or a zirconium compound can be added to aluminum nitride (AlN) for adjusting the thermal conductivity. A typical example is zirconium oxide (ZrO 2 ). This substance is present in a grain boundary phase in the sintered body and serves as a thermal barrier phase inhibiting thermal conduction between aluminum nitride particles. 5 to 100 parts by weight of zirconium oxide is preferably added with respect to 100 parts by weight of aluminum nitride. If the content of zirconium oxide is less than 5 parts by weight, the thermal barrier effect of zirconium tends to be insufficient and hence the thermal conductivity may exceed 80 W/m·K. If the content of zirconium exceeds 100 parts by weight, the thermal conductivity tends to be less than 30 W/m·K. 
     Titanium oxide, vanadium oxide, manganese oxide or magnesium oxide can also be added as another subsidiary component, in order to reduce the thermal conductivity of aluminum nitride. 15 to 30 parts by weight of titanium oxide, 5 to 20 parts by weight of vanadium oxide, 5 to 10 parts by weight of manganese oxide or 5 to 15 parts by weight of magnesium oxide is preferably added with respect to 100 parts by weight of aluminum nitride. 
     Also when the ceramic is mainly composed of silicon nitride (Si 3 N 4 ), aluminum oxide, zirconium oxide, titanium oxide, vanadium oxide, manganese oxide or magnesium oxide can be added for adjusting thermal conductivity. 2 to 20 parts by weight of aluminum oxide, 5 to 20 parts by weight of zirconium oxide, 10 to 30 parts by weight of titanium oxide, 5 to 20 parts by weight of vanadium oxide, 5 to 10 parts by weight of manganese oxide or 10 to 20 parts of magnesium oxide is preferably added with respect to 100 parts by weight of silicon nitride. 
     When the ceramic is mainly composed of silicon carbide (SiC), aluminum oxide, zirconium oxide, titanium oxide, vanadium oxide, manganese oxide or magnesium oxide can be added for adjusting thermal conductivity. 10 to 40 parts by weight of aluminum oxide, 5 to 20 parts by weight of zirconium oxide, 15 to 30 parts by weight of titanium oxide, 10 to 25 parts by weight of vanadium oxide, 2 to 10 parts by weight of manganese oxide or 5 to 15 parts of magnesium oxide is preferably added with respect to 100 parts by weight of silicon carbide. 
     When the main component is prepared from aluminum nitride (AlN) in the present invention, at least 1 part by weight of an alkaline earth element and/or a rare earth element of the periodic table is preferably introduced as a sintering agent with respect to 100 parts by weight of material powder of the main component, in order to obtain a dense sintered body. The alkaline earth element of the periodic table is preferably calcium (Ca), while the rare earth element of the periodic table is preferably neodymium (Nd) or ytterbium (Yb). Sintering can be performed at a relatively low temperature by adding such element(s), for reducing the sintering cost. 
     According to the present invention, the sintering body may be prepared by a well-known method. For example, an organic solvent, a binder etc. may be added to a prescribed quantity of material powder for preparing a slurry through a mixing step in a ball mill, forming the slurry into a sheet of a prescribed thickness by the doctor blade method, cutting the sheet into a prescribed size/shape, degreasing the cut sheet in the atmosphere or in nitrogen, and thereafter sintering the sheet in a non-oxidizing atmosphere. 
     The slurry can be formed through general means such as pressing or extrusion molding. In order to prepare the heater, the heating element can be formed in a prescribed pattern by sintering a layer of a high melting point metal consisting of tungsten or molybdenum on the sintered body by a technique such as screen printing in a non-oxidizing atmosphere. The electrode serving as a feeding part for the heating element can also be simultaneously formed by screen-printing the same on the sintered body. In this case, however, degreasing must be performed in a non-oxidizing atmosphere of nitrogen or the like in order to prevent oxidation of a metallized layer. Further, Ag or Ag—Pd can be employed as the heating element. While Examples of the present invention are described with reference to ceramic heaters for soldering irons, the present invention is not restricted to this application. 
     In the ceramic heater according to the present invention, the thermal conductivity of the substrate is adjusted to 30 to 80 W/m·K and the relation between the distance A from the contact between the heating element and the electrode on the substrate to the end of the substrate closer to the electrode and the thickness B of the substrate is set to satisfy A/B≧20, thereby increasing mechanical strength of the substrate, improving thermal shock resistance, relaxing or reducing a temperature gradient from the heating element to the electrode, inhibiting oxidation of the contact of the electrode part and preventing a contact failure. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a ceramic heater according to the present invention; 
     FIG. 2 is a sectional view of the ceramic heater taken along the line II—II in FIG. 1; and 
     FIG. 3 is a sectional view of a heater for a soldering iron according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     In each sample, the quantity of aluminum oxide (Al 2 O 3 ) added to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic was selected as shown in Table 1, while 2 parts by weight of Yb 2 O 3 , 2 parts by weight of Nd 2 O 3  and 0.3 parts by weight of CaO were added as sintering agents with addition of an organic solvent and a binder, and these materials were mixed in a ball mill for 24 hours. A slurry obtained in this manner was formed into a sheet by the doctor blade method so that the thickness after sintering was 0.7 mm. 
     The sheet was cut so that the dimensions of both substrates  1   a  and  1   b  shown in a plan view of a ceramic heater in FIG. 1 were 50 mm by 5 mm after sintering, and degreased in the atmosphere at 500° C. Then, the degreased body was sintered in a nitrogen atmosphere at 1800° C., and thereafter polished into a thickness (B) of 0.5 mm. Further, a heating element  2  and an electrode  3  were screen-printed on the substrate  1   a  with Ag—Pd paste and Ag paste respectively, and sintered in the atmosphere at 880° C. As to the size/shape of the ceramic heater, the longitudinal length of the circuit of the heating element  2  was set to 40 mm for satisfying the condition A/B≧20 assuming that A represents the distance from the contact between the heating element  2  and the electrode  3  to an end of the substrate  1   a  closer to the electrode  3  and B represents the thickness of the substrate  1   a.    
     Further, pasty sealing glass  4  was applied in order to protect the heating element  2  as shown in FIG. 2, the substrate  1   b  of 45 mm by 5 mm was placed thereon and sintered in the atmosphere at 880° C. for bonding the substrates  1   a  and  1   b  to each other, thereby preparing a heater for a soldering iron  10  shown in a sectional view of FIG.  3 . The substrates  1   a  and  1   b,  made of ceramic, are identical in size and material to each other except slight difference between the total lengths thereof. Table 1 shows values of thermal conductivity in Example 1 measured by applying a laser flash method to the substrate  1   a.    
     On the forward end of the soldering iron  10 , a frame  12  of a metal thin plate holds a tip  11  consisting of the substrates  1   a  and  1   b.  A heat insulator  13  consisting of mica or asbestos is interposed between the frame  12  and the tip  11 , while a wooden handle  14  is engaged with the outer periphery of the frame  12 . In order to connect the electrode  3  with a lead wire  15 , a contact  16  on the side of the lead wire  15  is brought into pressure contact with the electrode  3  by a spring seat  17  and a clamp bolt  18  for attaining mechanical contact bonding since a deposited metal such as solder is readily thermally deteriorated. If the temperature is repeatedly increased beyond 300° C. in the atmosphere, the contact  16  is oxidized to readily cause a contact failure. Numeral  19  denotes a window for observing the temperature of the part of the electrode  3 . 
     While the material for the tip  11  of the soldering iron  10  is generally prepared from copper due to excellent affinity with solder and high thermal conductivity, adhesion of solder is readily caused due to the excellent affinity with solder. When the tip  11  must not be covered with solder in a specific application, therefore, the material therefor is prepared from ceramic. The solder, which is prepared from an alloy of tin and lead while the melting point thereof is reduced as the content of tin is increased, is generally welded at a temperature of about 230 to 280° C. A toner fixing temperature of a heater for a fixing device is 200 to 250° C. 
     The quantity of current was adjusted with a sliding voltage regulator so that the temperature of a portion of the soldering iron  10  where the tip  11  was exposed was stabilized at 300° C., for measuring power consumption. At the same time, the current temperature of the part of the electrode  3  was measured with an infrared radiation thermometer through the window  19  for temperature observation. Table 1 also shows the results. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Content of 
                   
                   
                   
               
               
                   
                 Al 2 O 3   
                 Thermal 
                 Temperature of 
                 Power 
               
               
                 Sample 
                 (parts by 
                 Conductivity 
                 Electrode Part 
                 Consumption at 
               
               
                 No. 
                 weight) 
                 (W/m · K) 
                 (° C.) 
                 300° C. (W) 
               
               
                   
               
             
             
               
                 ⋆1 
                  0 
                 148  
                 232 
                 120  
               
               
                 ⋆2 
                  4 
                 99 
                 241 
                 105  
               
               
                    3 
                  5 
                 80 
                 273 
                 80 
               
               
                    4 
                 10 
                 72 
                 277 
                 75 
               
               
                    5 
                 25 
                 50 
                 281 
                 73 
               
               
                    6 
                 70 
                 37 
                 283 
                 70 
               
               
                    7 
                 100  
                 30 
                 285 
                 68 
               
               
                 ⋆8 
                 120  
                 20 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                 upon energization 
               
               
                   
               
               
                 Marks ⋆ denote comparative examples.  
               
             
          
         
       
     
     Referring to Table 1, power consumption increased in samples Nos. 1 and 2 having thermal conductivity exceeding the upper limit of the present invention, while a crack similar to a quenching crack frequently observed in earthenware was caused in the substrate  1   a  of a sample No. 8 having thermal conductivity less than the lower limit due by to a thermal shock. The temperature gradient of the part of the electrode  3  with respect to the heating element  2  was not severe within the range of thermal conductivity recommended in the present invention, to indicate that the uniform heating property of the substrate  1   a  is excellent. 
     EXAMPLE 2 
     In each sample, the quantities of silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ) and silicon carbide (SiC) added to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic were selected as shown in Table 2, while 2 parts by weight of Yb 2 O 3 , 2 parts by weight of Nd 2 O 3  and 0.3 parts by weight of CaO were added as sintering agents for preparing a substrate by a method similar to that in Example 1. The substrate was assembled into the soldering iron  10  shown in FIG. 3, and the characteristics of the substrate serving as a ceramic heater were evaluated through a procedure similar to that in Example 1. Table 2 also shows the results. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Content in 
                 Thermal 
                 Temperature 
                 Power 
               
               
                 Sample 
                   
                 Terms of SiO 2   
                 Conductivity 
                 of Electrode 
                 Consumption at 
               
               
                 No. 
                 Additive 
                 (parts by weight) 
                 (W/m · K) 
                 Part (° C.) 
                 300° C. (W) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 ⋆9 
                 SiO 2   
                 0.5 
                 120 
                 237 
                 111 
               
               
                 ⋆10 
                 Si 3 N 4   
                 0.5 
                 131 
                 235 
                 115 
               
               
                 ⋆11 
                 SiC 
                 0.5 
                 118 
                 238 
                 108 
               
               
                 12 
                 SiO 2   
                 1.0 
                 75 
                 276 
                 72 
               
               
                 13 
                 Si 3 N 4   
                 1.0 
                 79 
                 275 
                 75 
               
               
                 14 
                 SiC 
                 1.0 
                 74 
                 277 
                 72 
               
               
                 15 
                 SiO 2   
                 5.0 
                 63 
                 279 
                 70 
               
               
                 16 
                 Si 3 N 4   
                 10.0 
                 58 
                 280 
                 68 
               
               
                 17 
                 SiO 2   
                 15.0 
                 41 
                 281 
                 65 
               
               
                 18 
                 SiC 
                 20.0 
                 32 
                 285 
                 63 
               
               
                 19 
                 SiO 2   
                 20.0 
                 33 
                 284 
                 63 
               
               
                 ⋆20 
                 SiO 2   
                 25.0 
                 24 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 ⋆21 
                 Si 3 N 4   
                 25.0 
                 27 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                   
               
               
                 Marks ⋆ denote comparative examples.  
               
             
          
         
       
     
     Referring to Table 2, the thermal conductivity was adjusted in the proper range and the power consumption was suppressed in samples Nos. 12 to 19 having contents of additives in terms of SiO 2  within the range recommended in the present invention. The temperature gradient of the part of the electrode  3  with respect to the heating element  2  also exhibited a stable uniform heating property. 
     EXAMPLE 3 
     In each sample, the quantity of zirconium dioxide (ZrO 2 ) added to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic was selected as shown in Table 3, while 2 parts by weight of Yb 2 O 3 , 2 parts by weight of Nd 2 O 3  and 0.3 parts by weight of CaO were added as sintering agents for preparing a substrate by a method similar to that in Example 1. Table 3 shows results of characteristics of the substrate serving as a ceramic heater for the soldering iron  10  shown in FIG. 3 evaluated through a procedure similar to that in Example 1. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Content of 
                   
                   
                   
               
               
                   
                 ZrO 2   
                 Thermal 
                 Temperature of 
                 Power  
               
               
                 Sample 
                 (parts by 
                 Conductivity 
                 Electrode Part 
                 Consumption at 
               
               
                 No. 
                 weight) 
                 (W/m · K) 
                 (° C.) 
                 300° C. (W) 
               
               
                   
               
             
             
               
                 ⋆22 
                  4 
                 104  
                 238 
                 113  
               
               
                    23 
                  5 
                 77 
                 275 
                 78 
               
               
                    24 
                 10 
                 70 
                 278 
                 72 
               
               
                    25 
                 25 
                 65 
                 280 
                 71 
               
               
                    26 
                 70 
                 45 
                 282 
                 69 
               
               
                    27 
                 100  
                 32 
                 284 
                 68 
               
               
                 ⋆28 
                 120  
                 19 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                 upon energization 
               
               
                   
               
               
                 Marks ⋆ denote comparative examples.  
               
             
          
         
       
     
     Referring to Table 3, the thermal conductivity was adjusted in the proper range and the power consumption was suppressed in samples Nos. 23 to 27 having contents of zirconium oxide (ZrO 2 ) within the range recommended in the present invention. The temperature gradient of the part of the electrode  3  with respect to the heating element  2  also exhibited a stable uniform heating property. 
     EXAMPLE 4 
     In each sample, the quantities of aluminum oxide (Al 2 O 3 ), zirconium oxide (ZrO 2 ), titanium dioxide (TiO 2 ), vanadium oxide (V 2 O 5 ), manganese dioxide (MnO 2 ) and magnesium oxide (MgO) added to 100 parts by weight of silicon nitride (Si 3 N 4 ) forming the main component of ceramic were selected as shown in Table 4, while 10 parts by weight of yttrium oxide was added as a sintering agent for forming a sheet by a method similar to that in Example 1. Thereafter the sheet was degreased in a nitrogen atmosphere at 850° C., and sintered in a nitrogen atmosphere of 1850° C. for three hours thereby preparing each substrate shown in Table 4. Table 4 also shows results of characteristics of the substrate serving as a ceramic heater for the soldering iron  10  shown in FIG. 3 evaluated through a procedure similar to that in Example 1. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                 Thermal 
                 Temperature 
                 Power 
               
               
                 Sample 
                   
                 Content 
                 Conductivity 
                 of Electrode 
                 Consumption at 
               
               
                 No. 
                 Additive 
                 (parts by weight) 
                 (W/m · K) 
                 Part (° C.) 
                 300° C. (W) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 ⋆29 
                 — 
                 — 
                 100 
                 239 
                 111  
               
               
                 30 
                 Al 2 O 3   
                 2 
                 79 
                 273 
                 80 
               
               
                 31 
                 Al 2 O 3   
                 5 
                 52 
                 280 
                 73 
               
               
                 32 
                 Al 2 O 3   
                 10.0 
                 41 
                 283 
                 71 
               
               
                 33 
                 Al 2 O 3   
                 20.0 
                 31 
                 284 
                 69 
               
               
                 ⋆34 
                 Al 2 O 3   
                 30.0 
                 15 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 35 
                 ZrO 2   
                 5.0 
                 75 
                 274 
                 80 
               
               
                 36 
                 ZrO 2   
                 10.0 
                 51 
                 281 
                 74 
               
               
                 37 
                 ZrO 2   
                 20.0 
                 35 
                 284 
                 72 
               
               
                 ⋆38 
                 ZrO 2   
                 30.0 
                 19 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 39 
                 TiO 2   
                 10.0 
                 74 
                 275 
                 78 
               
               
                 40 
                 TiO 2   
                 30.0 
                 45 
                 282 
                 72 
               
               
                 ⋆41 
                 TiO 2   
                 50.0 
                 26 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 42 
                 V 2 O 5   
                 10.0 
                 72 
                 275 
                 80 
               
               
                 43 
                 V 2 O 5   
                 20.0 
                 43 
                 285 
                 72 
               
               
                 ⋆44 
                 V 2 O 5   
                 30.0 
                 unsinterable 
                 — 
                 — 
               
               
                 45 
                 MnO 2   
                 5.0 
                 69 
                 277 
                 77 
               
               
                 46 
                 MnO 2   
                 10.0 
                 35 
                 285 
                 71 
               
               
                 ⋆47 
                 MnO 2   
                 20.0 
                 23 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 48 
                 MgO 
                 10.0 
                 74 
                 274 
                 80 
               
               
                 49 
                 MgO 
                 20.0 
                 53 
                 279 
                 75 
               
               
                 ⋆50 
                 MgO 
                 30.0 
                 23 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                   
               
               
                 Marks ⋆ denote comparative examples.  
               
             
          
         
       
     
     Referring to Table 4, the thermal conductivity was adjusted in the proper range and the power consumption was suppressed in samples Nos. 30 to 33, 35 to 37, 39 and 40, 42 and 43, 45 and 46 and 48 and 49 having contents of the additives within the range recommended in the present invention. The temperature gradient of the part of the electrode  3  with respect to the heating element  2  also exhibited a stable uniform heating property. 
     EXAMPLE 5 
     In each sample, the quantities of aluminum oxide (Al 2 O 3 ), zirconium oxide (ZrO 2 ), titanium dioxide (TiO 2 ), vanadium oxide (V 2 O 5 ), manganese dioxide (MnO 2 ) and magnesium oxide (MgO) added to 100 parts by weight of silicon carbide (SiC) forming the main component of ceramic were selected as shown in Table 5, while 1.0 part by weight of boron carbide (B 4 C) was added as a sintering agent for forming a sheet by a method similar to that in Example 1. Thereafter the sheet was degreased in a nitrogen atmosphere at 850° C., and sintered in an argon atmosphere of 2000° C. for three hours thereby preparing each substrate shown in Table 5. Table 5 also shows results of characteristics of the substrate serving as a ceramic heater for the soldering iron  10  shown in FIG. 3 evaluated through a procedure similar to that in Example 1. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                 Thermal 
                 Temperature 
                 Power 
               
               
                 Sample 
                   
                 Content 
                 Conductivity 
                 of Electrode 
                 Consumption at 
               
               
                 No. 
                 Additive 
                 (parts by weight) 
                 (W/m · K) 
                 Part (° C.) 
                 300° C. (W) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 ⋆51 
                 — 
                 — 
                 162 
                 221 
                 132  
               
               
                 52 
                 Al 2 O 3   
                 10.0 
                 79 
                 269 
                 82 
               
               
                 53 
                 Al 2 O 3   
                 20.0 
                 61 
                 275 
                 77 
               
               
                 54 
                 Al 2 O 3   
                 30.0 
                 46 
                 280 
                 72 
               
               
                 55 
                 Al 2 O 3   
                 40.0 
                 32 
                 285 
                 69 
               
               
                 ⋆56 
                 Al 2 O 3   
                 50.0 
                 16 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 57 
                 ZrO 2   
                 5.0 
                 74 
                 271 
                 83 
               
               
                 58 
                 ZrO 2   
                 10.0 
                 49 
                 279 
                 76 
               
               
                 59 
                 ZrO 2   
                 20.0 
                 33 
                 285 
                 73 
               
               
                 ⋆60 
                 ZrO 2   
                 30.0 
                 17 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 61 
                 TiO 2   
                 15.0 
                 78 
                 269 
                 82 
               
               
                 62 
                 TiO 2   
                 30.0 
                 48 
                 280 
                 76 
               
               
                 ⋆63 
                 TiO 2   
                 50.0 
                 26 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 64 
                 V 2 O 5   
                 10.0 
                 69 
                 272 
                 79 
               
               
                 65 
                 V 2 O 5   
                 25.0 
                 39 
                 283 
                 71 
               
               
                 ⋆66 
                 V 2 O 5   
                 40.0 
                 18 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 67 
                 MnO 2   
                 2.0 
                 77 
                 270 
                 83 
               
               
                 68 
                 MnO 2   
                 10.0 
                 42 
                 282 
                 71 
               
               
                 ⋆69 
                 MnO 2   
                 20.0 
                 21 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 70 
                 MgO 
                 5.0 
                 70 
                 270 
                 82 
               
               
                 71 
                 MgO 
                 15.0 
                 51 
                 278 
                 77 
               
               
                 ⋆72 
                 MgO 
                 30.0 
                 24 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                   
               
               
                 Marks ⋆ denote comparative examples.  
               
             
          
         
       
     
     Referring to Table 5, the thermal conductivity was adjusted in the proper range and the power consumption was suppressed in samples Nos. 52 to 55, 57 to 59, 61 and 62, 64 and 65, 67 and 68 and 70 and 71 having contents of the additives within the range recommended in the present invention. The temperature gradient of the part of the electrode  3  with respect to the heating element  2  also exhibited a stable uniform heating property. 
     EXAMPLE 6 
     In each sample, the quantities of titanium dioxide (TiO 2 ), vanadium oxide (V 2 O 5 ), manganese dioxide (MnO 2 ) and magnesium oxide (MgO) added to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic were selected as shown in Table 6, while 2 parts by weight of Yb 2 O 3 , 2 parts by weight of Nd 2 O 3  and 0.3 parts by weight of CaO were added as sintering agents for preparing a substrate by a method similar to that in Example 1. Table 6 also shows results of characteristics of the substrate serving as a ceramic heater for the soldering iron  10  shown in FIG. 3 evaluated through a procedure similar to that in Example 1. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                   
                 Thermal 
                 Temperature 
                 Power 
               
               
                 Sample 
                   
                 Content 
                 Conductivity 
                 of Electrode 
                 Consumption at 
               
               
                 No. 
                 Additive 
                 (parts by weight) 
                 (W/m · K) 
                 Part (° C.) 
                 300° C. (W) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 ⋆73 
                 TiO 2   
                 5.0 
                 123 
                 235 
                 112  
               
               
                 74 
                 TiO 2   
                 15.0 
                 74 
                 275 
                 77 
               
               
                 75 
                 TiO 2   
                 30.0 
                 40 
                 282 
                 73 
               
               
                 ⋆76 
                 TiO 2   
                 50.0 
                 23 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 77 
                 V 2 O 5   
                 5.0 
                 70 
                 278 
                 74 
               
               
                 78 
                 V 2 O 5   
                 20.0 
                 36 
                 283 
                 70 
               
               
                 ⋆79 
                 V 2 O 5   
                 40.0 
                 17 
                 271 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 80 
                 MnO 2   
                 5.0 
                 71 
                 277 
                 74 
               
               
                 81 
                 MnO 2   
                 10.0 
                 47 
                 285 
                 73 
               
               
                 ⋆82 
                 MnO 2   
                 20.0 
                 22 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                 83 
                 MgO 
                 5.0 
                 67 
                 279 
                 73 
               
               
                 84 
                 MgO 
                 15.0 
                 49 
                 281 
                 72 
               
               
                 ⋆85 
                 MgO 
                 30.0 
                 18 
                 — 
                 substrate cracked 
               
               
                   
                   
                   
                   
                   
                 upon energization 
               
               
                   
               
               
                 Marks ⋆ denote comparative examples  
               
             
          
         
       
     
     Referring to Table 6, the thermal conductivity was adjusted in the proper range and the power consumption was suppressed in samples Nos. 74 and 75, 77 and 78, 80 and 81 and 83 and 84 having contents of the additives within the range recommended in the present invention. The temperature gradient of the part of the electrode  3  with respect to the heating element  2  also exhibited a stable uniform heating property. 
     EXAMPLE 7 
     Substrates similar to that shown in FIG. 1 were formed by samples Nos. 2a, 2b and 2c prepared by adding 4 parts by weight of aluminum oxide (Al 2 O 3 ) to 100 parts by weight of aluminum nitride (AlN) forming the main component of ceramic, samples Nos. 5a, 5b and 5c prepared by adding 25 parts by weight of aluminum oxide (Al 2 O 3 ) to 100 parts by weight of aluminum nitride, samples Nos. 15a, 15b and 15c prepared by adding 5 parts by weight of silicon dioxide (SiO 2 ) to 100 parts by weight of aluminum nitride and samples Nos. 25a, 25b and 25c prepared by adding 25 parts by weight of zirconium oxide (ZrO 2 ) to 100 parts by weight of aluminum nitride while setting distances A from starting points of circuits of heating elements  2  to ends of substrates  1   a  closer to electrodes  3  to 5 mm, 10 mm 10 and 20 mm respectively. Each substrate was assembled into the soldering iron  10  shown in FIG. 3, and the characteristics of the substrate serving as a ceramic heater were evaluated through a procedure similar to that in Example 1. Table 7 also shows the results. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                   
                   
                 Distance A 
                   
                   
                 Power 
               
               
                   
                 Thermal 
                 to End of 
                   
                 Temperature 
                 Consumption 
               
               
                 Sample 
                 Conductivity 
                 Substrate 
                   
                 of Electrode 
                 at 300° C. 
               
               
                 No. 
                 (W/m · K) 
                 (mm) 
                 A/B 
                 Part (° C.) 
                 (W) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 2a 
                 ⋆99 
                 ⋆5 
                 10 
                 272 
                 113 
               
               
                 2b 
                 ⋆99 
                 10 
                 20 
                 241 
                 105 
               
               
                 2c 
                 ⋆99 
                 20 
                 40 
                 182 
                 97 
               
               
                 5a 
                 50 
                 ⋆5 
                 10 
                 290 
                 104 
               
               
                 5b 
                 50 
                 10 
                 20 
                 281 
                 73 
               
               
                 5c 
                 50 
                 20 
                 40 
                 262 
                 52 
               
               
                 15a 
                 63 
                 ⋆5 
                 10 
                 280 
                 101 
               
               
                 15b 
                 63 
                 10 
                 20 
                 279 
                 70 
               
               
                 15c 
                 63 
                 20 
                 40 
                 258 
                 49 
               
               
                 25a 
                 65 
                 ⋆5 
                 10 
                 290 
                 102 
               
               
                 25b 
                 65 
                 10 
                 20 
                 280 
                 71 
               
               
                 25c 
                 65 
                 20 
                 40 
                 270 
                 50 
               
               
                   
               
               
                 Marks ⋆ denote comparative examples  
               
             
          
         
       
     
     When gradually increasing the distance A from the starting point of the circuit of the heating element to the end of the substrate closer to the electrode while keeping the length of the substrate constant, the circuit of the heating element is shortened and hence power consumption is reduced as a matter of course. Referring to Table 7, power consumption is excessive in the samples 2a, 2b and 2c having thermal conductivity exceeding the upper limit of the range recommended in the present invention although the temperature of the electrode part does not reach a temperature region facilitating oxidation of the part of the electrode. Similarly, power consumption is excessive in the samples 5a, 15a and 25a not satisfying the relation A/B≧20 between the distance A to the end of the substrate and the thickness B of the substrate. As to the remaining samples, the temperature gradient from the heating element to the part of the electrode is low and power consumption is suppressed. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.