Patent Publication Number: US-6340809-B2

Title: Gas sensor with ceramic heater

Description:
The present application claims priority as a divisional application of U.S. Patent application Ser. No. 09/365,173, filed Aug. 2, 1999, the entirety of which is incorporated into the present application by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1 Technical Field of the Invention 
     The present invention relates generally to a gas sensor which may be employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of gas such O 2 , NOx, or CO, and more particularly to an improved structure of a ceramic heater used in gas sensors and a manufacturing method thereof. 
     2 Background Art 
     FIGS.  1 ( a ) and  1 ( b ) show one example of conventional ceramic heaters which is built in an oxygen sensor for use in air-fuel ratio control of automotive internal combustion engines. The ceramic heater  9  serves to heat a sensor element up to an elevated temperature to minimize a variation in measured value. 
     The ceramic heater  9  consists of a ceramic square rod  10  made of a laminate of heater substrates and a covering substrate and metallic terminals  3  mounted on side surfaces  15  of the rod  10 . The metallic terminals  3  connect electrically with leads of a heater-patterned layer in the rod  10  and joined to outer leads  4  through solders  5 , respectively. 
     In manufacturing the ceramic heater  9 , green sheets  101  and  102 , as shown in FIG.  2 ( a ), whose main component is alumina are first prepared. Next, a conductive paste is applied to the surface of each of the green sheets  101  to form a heater-patterned layer  2  consisting of pairs of a heater element  21  and a lead  22 . The two green sheets  101  and the covering green sheet  102  are laid to overlap each other to form a three-layer laminate. The three-layer laminate is cut into several pieces as shown in FIG.  2 ( b ). The metallic terminals  3  are formed on the side surfaces  15  of each piece which communicate electrically with the leads  22  to make an intermediate. Subsequently, the intermediate is baked, after which the outer leads  4  is, as shown in FIG.  2 ( c ), welded to the metallic terminals  3  through the solder  5 . Finally, welded portions of the outer leads  4  are, as indicated at numeral  6  in FIG.  1 ( b ), plated with Ni to make the ceramic heater  9 . 
     The above ceramic heater  9  and the manufacturing method thereof, however, have the following drawbacks. 
     The metallic terminals  3  are, as described above, mounted on the side surfaces  15  of the ceramic heater  9 . It is, thus, only possible to attach the metallic terminals  3  to the square rod  10  after the three-layer laminate is cut as shown in FIG.  2 ( b ). In other words, a large number of terminal attachment processes are required in mass-production of ceramic heaters. 
     In addition, the performance of the ceramic heater  9  is usually inspected after the outer leads  4  are mounted thereon. A large number of individual inspections are also required in the mass-production of ceramic heaters, thus resulting in an increase in manufacturing cost. 
     Another problem is also encountered in that the ceramic heater  9  is lower in durability than a round rod heater  91  as shown in FIG.  3 ( a ). The results of heat cycle tests show that portions of the ceramic heater  9  welded to the outer leads  4  and the metallic terminals  3  tend to be cracked as compared with the round rod heater  91 . This is because the angle β which each of the metallic terminals  3  of the ceramic heater  9 , as shown in FIG. 4, makes with the outer surface of the solder  5  is greater than the angle α which each of the metallic terminals  3  of the round rod heater  91 , as shown in FIG.  3 ( b ), makes with the outer surface of the solder  5 . The difference between the angles α and β depends upon the geometry of the heaters  9  and  91  and thus is difficult to eliminate. The use of solder which is soft enough to absorb internal stress ensures substantially the same durability of the portions of the rod  10  welded to the leads  4  as that of the round rod heater  91 , however, square rod heaters exhibiting higher durability even in use of harder solder is sought. 
     SUMMARY OF THE INVENTION 
     It is therefore a principal object of the present invention to avoid the disadvantages of the prior art. 
     It is another object of the present invention to provide an easy-to-manufacture ceramic heater used in gas sensors which has a high durability and a manufacturing method thereof. 
     According to one aspect of the invention, there is provided a ceramic heater which may be employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of gas such O 2 , NOx, or CO. The ceramic heater comprises: (a) a ceramic square rod formed with a laminate of a heater substrate on which a heater-patterned layer consisting of a heater element and leads connected to the heater element is formed and a covering substrate covering the heater-patterned layer of the heater substrate; (b) metallic terminals connected electrically to the leads of the heater-patterned layer of the heater substrate, respectively, the metallic terminals being mounted on surfaces of the ceramic square rod opposed to each other in a direction of lamination of the heater substrate and the covering substrate, respectively; and (c) at least one outer lead joined to one of the metallic terminals through a bonding layer. 
     In the preferred mode of the invention, a second outer lead is further joined to the other metallic terminal through a bonding layer. 
     The metallic terminals are electrically connected to the leads through holes formed in at least one of the covering substrate and the heater substrate. 
     Each of the metallic terminals is mounted on an area inside edges of the surface of the ceramic square rod. 
     The bonding layer occupies an area of a surface of the metallic terminal inside edges of the metallic terminal. 
     The one of the metallic terminals contains 70 Wt % of W or more. The bonding layer contains 40 to 98 Wt % of Cu and 2 to 20 Wt % of Ni. 
     The bonding layer may contain 60 Wt % of Au or less. 
     An Ni-plated layer may be formed on the one of the metallic terminals, having a thickness of 3 μm or less. The outer lead is joined to the Ni-plated layer through the bonding layer. 
     According to the second aspect of the invention, there is provided a ceramic heater. The ceramic heater comprises: (a) a ceramic square rod formed with a laminate of heater substrates each having formed thereon a heater-patterned layer consisting a heater element and first and second leads connected to the heater element and a covering substrate interposed between the heater substrates; (b) first and second metallic terminals connected electrically to the first and second leads of the heater-patterned layers of the heater substrates, respectively, the metallic terminals being mounted on surfaces of the ceramic square rod opposed to each other in a direction of lamination of the heater substrates and the covering substrate; and (c) outer leads joined to the first and second metallic terminals through bonding layers, respectively. 
     In the preferred mode of the invention, the first metallic terminal is connected to the first leads of the heater substrates through conductive material-coated holes formed in the covering substrate and one of the heater substrates. The second metallic terminal is connected to the second leads of the heater substrates through conductive material-coated holes formed in the covering substrate and the other heater substrate. 
     Each of the bonding layers occupies an area of a surface of one of the metallic terminals inside edges of the metallic terminal. 
     Each of the metallic terminals contains 70 Wt % of W or more. Each of the bonding layers contains 40 to 98 Wt % of Cu and 2 to 20 Wt % of Ni. 
     Each of the bonding layers contains 60 Wt % of Au or less. 
     An Ni-plated layer formed on each of the metallic terminals, having a thickness of 3 μm or less. The outer leads are joined to the Ni-plated layers through the bonding layers. 
     According to the third aspect of the invention, there is provided a method of manufacturing ceramic heaters which comprises the steps of: (a) preparing a first green sheet; (b) preparing a second green sheet; (c) printing a first surface of the second green sheet an array of heater-patterned layers each consisting of a heater element and leads connected to the heater element; (d) printing a second surface of the second green sheet opposite the first surface with an array of metallic terminals; (e) attaching the first green sheet to the second green sheet so as to cover the first surface of the second green sheet to form a laminate; (f) baking the laminate to form a ceramic board; (g) joining outer leads to the metallic terminals through bonding layers, respectively; and (h) cutting the ceramic board into a plurality of square rods constituting units of the ceramic heaters. 
     In the preferred mode of the invention, a step is further provided which forms through holes in the first green sheet for electrical connections of the leads of the heater-patterned layers and the metallic terminals. 
     A step is further provided which forms grooves in a surface of the ceramic board between adjacent two of the units of the ceramic heaters to be cut by the cutting step. 
     According to the fourth aspect of the invention, there is provided a method of manufacturing ceramic heaters which comprises the steps of: (a) preparing a first green sheet; (b) preparing second green sheets; (c) printing a first surface of each of the second green sheets an array of heater-patterned layers each consisting of a heater element and leads connected to the heater element; (d) printing a second surface of each of the second green sheets opposite the first surface with an array of metallic terminals; (e) interposing the first green sheet between the second green sheets so as to cover the first surfaces of the second green sheets to form a laminate; (f) baking the laminate to form a ceramic board; (g) joining outer leads to the metallic terminals formed on at least one of the second green sheets through bonding layers, respectively; and (h) cutting the ceramic board into a plurality of square rods constituting units of the ceramic heaters. 
     According to the fifth aspect of the invention, there is provided a gas sensor which comprises: (a) a gas sensing element having a gas-exposed portion, the gas sensing element having formed therein a chamber; (b) a ceramic heater disposed within the chamber of the gas sensing element to heat the gas sensing element; (c) a first cylindrical holder fitted in the chamber of the gas sensing element, the first holder including a heater holding portion for holding the ceramic heater and a sensor contact in contact with an inner wall of the gas sensing element, the sensor contact having a sensor signal output terminal; (d) a second cylindrical holder mounted on an outer wall of the gas sensing element, having a sensor signal output terminal; and (e) a slit formed in the first holder to define a C-shaped cross section, the slit being located 90°±20° apart from the sensor signal output terminal of the first cylindrical holder. The ceramic heater includes, (a) a ceramic square rod formed with a laminate of a heater substrate on which a heater-patterned layer consisting of a heater element and leads connected to the heater element is formed and a covering substrate covering the heater-patterned layer of the heater substrate, (b) metallic terminals connected electrically to the leads of the heater-patterned layer of the heater substrate, respectively, the metallic terminals being mounted on surfaces of the ceramic square rod opposed to each other in a direction of lamination of the heater substrate and the covering substrate, respectively, and (c) at least one outer lead joined to one of the metallic terminals through a bonding layer. 
     In the preferred mode of the invention, the sensor signal output terminal of the first cylindrical holder is located 180° apart from the sensor signal output terminal of the second cylindrical holder. 
     The slit is located 90° apart from the sensor signal output terminal of the first cylindrical holder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
     FIG.  1 ( a ) is a perspective view which shows a conventional ceramic heater; 
     FIG.  1 ( b ) is a cross sectional view taken along the line A—A in FIG.  1 ( a ); 
     FIGS.  2 ( a ),  2 ( b ), and  2 ( c ) are perspective views which show a sequence of manufacturing processes of a conventional ceramic heater; 
     FIG.  3 ( a ) is a perspective view which shows a conventional ceramic heater made of a round bar; 
     FIG.  3 ( b ) is a cross sectional view taken along the line B—B in FIG.  3 ( a ); 
     FIG. 4 is a sectional view which shows a welded angle of an outer surface of an end of a bonding layer with a metallic terminal; 
     FIG.  5 ( a ) is a perspective view which shows a ceramic heater according to the invention; 
     FIG.  5 ( b ) is a sectional view taken along the line C—C in FIG.  5 ( a ); 
     FIG. 6 is an exploded view which shows the ceramic heater in FIG.  5 ( a ); 
     FIGS.  7 ( a ),  7 ( b ), and  7 ( c ) are perspective views which show a sequence of manufacturing processes of a ceramic heater; 
     FIGS.  8 ( a ) and  8 ( b ) show modifications of an outer lead connected to a ceramic heater; 
     FIG. 9 is a graph which shows the relation between the hardness of solder and a component ratio of Au to Cu of the solder; 
     FIG. 10 shows the second embodiment of the manufacturing processes of the ceramic heater  1 . 
     FIGS.  11 ( a ) and  11 ( b ) show manners to measure the surface roughness of a metallic terminal of the invention and a conventional metallic terminal; 
     FIG. 12 is a vertical sectional view which shows an oxygen sensor in which the ceramic heater shown in FIGS.  5 ( a ) and  5 ( b ) is built; 
     FIG.  13 ( a ) is a perspective view which shows a minus holder for holding a gas sensing element; 
     FIG.  13 ( b ) is a perspective view which shows a plus holder for holding a ceramic heater; 
     FIG.  14 ( a ) is a plan view of a plus holder; 
     FIGS.  14 ( b ) and  14 ( c ) are side views of the plus holder in FIG.  14 ( a ); 
     FIGS.  15 ( a ) and  15 ( b ) are side views of a plus holder in which a ceramic heater is fitted; 
     FIG. 16 is a plan view which shows a plus holder in which a ceramic heater is fitted; 
     FIG. 17 is a plan view which shows a plus holder holding therein a ceramic holder fitted in a gas sensing element and a minus holder; 
     FIGS.  18 ( a ) and  18 ( b ) are plan views which a comparative example; and 
     FIG. 19 is a sectional view which shows a welded angle of an outer surface of an end of a bonding layer with a metallic terminal. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like numbers refer to like parts in several views, particularly to FIG.  5 ( a ) and  5 ( b ), there is shown a ceramic heater  1  of an oxygen sensor according to the invention which is employed in automotive air-fuel ratio control systems to measure an oxygen content in exhaust gasses of an internal combustion engine. Note that the present invention is not limited to an oxygen sensor and may alternatively be used with a variety of gas sensors such as HC, CO, and NOx sensors. 
     The ceramic heater  1  includes a ceramic square rod  10  which is, as clearly shown in FIG. 6, made of a laminate of two heater substrates  11  and a covering substrate  12 . Each of the heater substrate  11  has formed thereon a heater-patterned layer  2  consisting of a heater element  21  and leads  22  connected to the heater element  21 . The covering substrate  12  is interposed between the heater substrates  11  to cover the heater-patterned layers  2 . 
     The ceramic heater  1  also includes a pair of metallic terminals  3  which are attached to upper and lower surfaces  17  and  18 , as viewed in FIGS.  5 ( a ) and  5 ( b ), of the heater substrates  11  and which are electrically connected to the leads  22 . Outer leads  4  are welded to the terminals  3  through bonding layers  5 , respectively. 
     The covering substrate  12  and the heater substrates  11 , as clearly shown in FIGS.  5 ( b ) and  6 , have conductive material-coated through holes  71 ,  72 ,  73 , and  74 , respectively, to establish electrical communication between the heater-patterned layers  2  of the heater substrates  11  and the metallic terminals  3 . 
     The metallic terminals  3  are, as clearly shown in FIG.  5 ( b ), disposed on flat portions of the surfaces  17  and  18  of the heater substrates  11  so that side ends  31  thereof may be located inside side edges  171  and  181  of the heater substrates  11 , respectively. 
     The bonding layers  5  are formed with solder made of, for example, Cu/Si, Cu/Au, or Cu/Ni material and, as can be seen in FIG.  5 ( b ), formed on flat surfaces of the terminals  3  so that side edges  51  thereof may be located inside the side ends  31  of the terminals  3 . 
     The ceramic square rod  10  has, as shown in FIG. 6, an overall length L of 54 mm, an overall width W of 2.9 mm, and a thickness T of 1.6 mm (see FIG.  5 ( a )). The length C of the heater element  21  of each of the heater-patterned layers  2  is 9 mm. The length D of each of the leads  22  is 42 mm. 
     The leads  22  formed on each of the heater substrates  11  extend in parallel at an interval F 2  of 0.228 mm away from each other. Each of the leads  22  is disposed at an interval F 1  of 0.25 mm away from the side of the heater substrate  11  and at an interval F 3  of 1 mm away from a rear end of the heater substrate  11 . 
     The through holes  71  to  74  are arrayed with a pitch P 1  of 3.6 mm in a lengthwise direction of the heater substrate  11  and a pitch P 2  of 1.4 mm in a widthwise direction of the heater substrate  11  and have a diameter of 3 mm. The metallic terminals  3  each have a length E 1  of 5.5 mm and a width E 2  of 2.3 mm. 
     A sequence of manufacturing processes of the ceramic heater  1  will be discussed below with reference to FIGS.  7 ( a ),  7 ( b ), and  7 ( c ). 
     A powdered raw material containing about 92 Wt % of Al 2 O 3  and a total of about 8 Wt % of SiO 2 , CaO, and MgO is first prepared to make slurry. 
     Next, a green sheet is formed with the slurry using the doctor blade and then punched by a punch press to form green sheets  101  measuring 120 mm×120 mm for making the heater substrates  11  and a green sheet  102  measuring 120 mm×120 mm for making the covering substrate  12 . The through holes  71  to  74  are formed in the green sheets  101  and  102 . 
     The making of the green sheets  101  and  102  may alternatively be achieved with the extrusion molding. 
     A conductive paste whose main constituent is metal such as W or Mo is prepared and coated on surfaces of the green sheets  101  to form heater-patterned layers  2 , as shown in FIG.  7 ( a ), and inner walls of the through holes  71  to  74  using printing techniques. The heater-patterned layers extend parallel to each other. 
     On a surface of each of the green sheets  101  opposite to the heater-patterned layers  2 , a conductive paste is coated to form the metallic terminals  3  in line using printing techniques. The conductive paste is made of a main constituent of metal containing 70 Wt % or more of W and a remaining content of Mo, but may be identical with that used in forming the heater-patterned layers  2 . 
     The two green sheets  101  are arranged so that the heater-patterned layers  2  may face each other. Subsequently, the green sheet  102  is interposed between the green sheets  101  to form a three-layer laminate. The three-layer laminate is baked at 1400 to 1600° C. in a reducing atmosphere of N 2  and H 2  gasses to make an intermediate. 
     The outer leads  4  are, as shown in FIG.  7 ( b ), soldered to the metallic terminals  3 , respectively. The soldering is achieved by placing solder and the outer leads  4  on the metallic terminals  3  and heating them at 1000 to 1200° C. to form the bonding layers  5 . 
     Each of the outer leads  4  may be made either of a round bar, as shown in FIG.  7 ( b ), or of a square bar, as shown in FIGS.  8 ( b ) and  8 ( b ). 
     The overall surface of each of the bonding layers  5  is, as clearly shown in FIG.  5 ( b ), covered with an Ni-plated layer  6 . 
     The intermediate is, as shown in FIG.  7 ( c ), cut into several pieces, i.e., units of the ceramic heaters  1 . 
     Finally, an end of each of the ceramic heaters  1  opposite to the outer leads  4  is rounded using a grinding machine. 
     Note that after the three-layer laminate is braked, the intermediate is tested for heater performance. 
     Each of the bonding layers  5  may contain 40 to 98 Wt % of Cu and 2 to 20 Wt % of Ni. The metallic terminals  3 , as described above, contains W, thus resulting in improved wettability between the bonding layers  5  and the metallic terminals  3 , which eliminates the need for the metallic terminals  3  to be plated with Ni in conventional manufacturing processes. 
     When the content of Cu in the bonding layers  5  is small, less than 40 Wt % and when the leads  4  do not contain Ni, it will cause no Ni to be diffused from the leads  4  to the bonding layers  5 , so that the content of Ni in the bonding layers  5  will be smaller than that when the content of Cu is more than 40 Wt %, which results in lowered wettability of the bonding layers  5  to the metallic terminals  3  and a decrease in strength of joints of the bonding layers  5  and the metallic terminals  3 . 
     When the content of Cu in the bonding layers  5  is greater than 98 Wt %, the content of Ni in the bonding layers  5  will be smaller than that in the metallic terminals  3 , thereby causing the wettability of the bonding layers  5  to the metallic terminals  3  to be lowered, which results in a decrease in strength of the joints of the bonding layers  5  and the metallic terminals  3 . 
     When the content of Ni in the bonding layers  5  is less than 2 Wt %, it will cause the wettability of the bonding layers  5  to the metallic terminals  3  to be lowered, resulting in a decrease in strength of the joints of the bonding layers  5  and the metallic terminals  3 . Alternatively, when the content of Ni in the bonding layers  5  greater than 20 Wt %, it will cause a W—Ni intermetallic compound to be precipitated during manufacture, resulting in a decrease in strength of joints of the bonding layers  5  and the metallic terminals  3 . 
     The metallic terminals  3  contain, as described above, 70 Wt % of W or more (including 100 Wt % of W) and thus have good conformability to a ceramic particularly containing alumina (i.e., the square rod  10  of the ceramic heater  1 ) and good heat resistance. When the content of W is less than 70 Wt %, it may result in decreases in strength of a joint of the metallic terminals  3  and the square rod  10  and heat resistance. 
     The bonding layers  5  may contain 60 Wt % of Au or less for avoiding precipitation of a W—Ni intermetallic compound to increase the strength to join the leads  4  to the metallic terminals  3 . When the content of Au in the bonding layers  5  is more than 60 Wt %, the content of Cu will be decreased. Thus, when the leads  4  do not contain Ni, it will cause no Ni to be diffused from the leads  4  to the bonding layers  5 , so that the content of Ni in the bonding layers  5  will be smaller than that when the content of Au is less than 60 Wt %, which results in lowered wettability of the bonding layers  5  to the metallic terminals  3  and a decrease in strength of joints of the bonding layers  5  to the metallic terminals  3 . Specifically, when the content of Au is, as shown in FIG. 9, 60 to 90 Wt %, the hardness of the solder forming the bonding layers  5  becomes too high, thus resulting in a decrease in durability against cyclic changes in ambient temperature. When the content of Au is greater than 90 Wt %, the hardness of the solder is lower, but manufacturing costs will increase. 
     A major surface of each of the metallic terminals  3  to which the leads  4  are to be joined through the bonding layer  5  may be plated with Ni. The thickness of the Ni-plated layer is 3 or less μm. The formation of the Ni-plated layer improves the wettability of the bonding layer  5 , thereby decreasing the welded angle which the outer surface of each side end of the bonding layer  5  makes with the metallic terminal  3 , resulting in a decrease in thermal stress contributing to cracks. When the thickness of the Ni-plated layer is more than 3 μm, a metallic alloy will be produced between the Ni-plated layer and the metallic terminal  3  which decreases the strength to join the bonding layer  5  and the metallic terminal  3 . 
     The laminate produced in the process shown in FIG.  7 ( a ) may consist only of the single green sheet  101  and the green sheet  102 . In this case, the metallic terminals  3  are also formed on a surface of the green sheet  102  opposite to a surface covering the heater-patterned layers  2  of the green sheet  101 . 
     As can be seen from the above discussion, the metallic terminals  3  and the outer leads  4  are disposed on the surfaces  17  and  18  of the square rod  10  opposed in a direction of lamination of the substrates  11  and  12 , thereby allowing the joining process wherein the outer leads  4  are joined to the metallic terminals  3 , respectively, to be performed before the intermediate is cut into units of the ceramic heaters  1  in the course of manufacture. This will result in great rationalization of the manufacturing processes. 
     In addition, the performance test may be, as described above, performed before the intermediate is cut into unit of the ceramic heaters  1 , thus resulting in rationalization of procedure of the test. 
     The metallic terminals  3  and the bonding layers  5  are, as described above, arranged on the surfaces  17  and  18  of the square rod  10  out of alignment of side ends with each other, thus avoiding concentration of stress on the side edges  171 ,  181 ,  31 , and  51 , which will result in improved durability of the ceramic heater  1 . 
     One of the metallic terminals  3  of the ceramic heater  1  may be connected directly to a connector leading to, for example, a ground terminal without use of the outer lead  4 . In this case, the single outer lead  4  may be joined to either of the metallic terminals  3 . 
     FIG. 10 shows the second embodiment of the manufacturing processes of the ceramic heater  1 . 
     Before the three-layer laminate of the green sheets  101  and  102  is braked, cutting notches or grooves  7  are machined in upper and lower surfaces of the three-layer laminate which extend parallel between adjacent two of the metallic terminals  3  for facilitating ease of cutting the three-layer laminate into units of the ceramic heaters  1  after being baked. 
     The formation of the cutting grooves  7  is achieved by grooving the upper and lower surfaces of the three-layer laminate to a depth less than half a thickness of the laminate using a cutting machine. 
     Other manufacturing processes are identical with those of the first embodiment, and explanation thereof in detail will be omitted here. 
     Ten samples of the ceramic heater  1  made in the manufacturing processes of the first embodiment were tested for the surface roughness of the metallic terminals  3  which may be thought of as one of factors of improvement of durability. The measurement of the surface roughness was accomplished, as shown in FIG.  11 ( a ), by scanning the surface of the metallic terminal  3  of each sample over 0.8 mm in a direction, as indicated by S in FIG.  11 ( a ). For comparison, the same tests were performed, as shown in FIG.  11 ( b ), for ten conventional ceramic heaters identical with the one shown in FIGS.  1 ( a ) and  1 ( b ). The results of the tests are shown in table 1 below. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Sample 
                 Prior art 
                 Invention 
               
               
                 No. 
                 (μm) 
                 (μm) 
               
               
                   
               
             
            
               
                 1 
                 3.642 
                 1.481 
               
               
                 2 
                 3.932 
                 1.098 
               
               
                 3 
                 2.47 
                 1.018 
               
               
                 4 
                 3.782 
                 0.978 
               
               
                 5 
                 3.146 
                 1.294 
               
               
                 6 
                 2.858 
                 1.893 
               
               
                 7 
                 3.431 
                 1.149 
               
               
                 8 
                 3.278 
                 1.19  
               
               
                 9 
                 2.685 
                 1.435 
               
               
                 10  
                 2.891 
                 1.215 
               
               
                 Average 
                 3.212 
                 1.275 
               
               
                   
               
            
           
         
       
     
     The table 1 shows that the surface roughness (Rz) of the metallic terminals  3  of the ceramic heater  1  is greatly improved as compared with the conventional ceramic heaters. The improvement of the surface roughness of the metallic terminals will facilitate flow of solder on the surfaces of the metallic terminals  3  when the outer leads  4  are joined to the metallic terminals  3 , thereby increasing an area of the bonding layers  5 , which results in improvement of initial strength to join the outer leads  4  to the metallic terminals  3  and a decrease in thermal stress acting on the joints produced by cyclic temperature changes, thus improving the durability of the ceramic heater  1 . 
     FIG. 12 shows an oxygen sensor  8  in which the ceramic heater  1  is built. 
     The oxygen sensor  8  is used in an automotive internal combustion engine control system and includes a gas sensing element  81  with a gas-exposed portion  811  exposed to the gas to be measured. 
     The gas sensing element  81  is of a cup-shape having formed therein an inner chamber  810 . Within the inner chamber  180 , the ceramic heater  1  is disposed for heating the gas sensing element  81 . 
     On outer and inner surfaces of the gas sensing element  81 , minus and plus holders  86  and  87  are installed which have sensor signal output terminals  869  and  879 , respectively. The pulse holder  87  includes, as shown in FIGS.  13 ( b ) and  14 ( a ) to  14 ( c ), a heater holding portion  871  for holding the ceramic heater  1  and a sensor contact  873  for making contact with the inner surface of the gas sensing element  81 . The sensor signal output terminal  879  extends from an end of the sensor contact  873 . The heater holding portion  871  and the sensor contact  873  have formed therein slits  877  and  878  to define C-shape in section so that they may be elastically deformable to have spring properties. The slits  877  and  878  extend in a lengthwise direction of the pulse holder  87  and are shifted approximately 90° away from each other. 
     The heater holding portion  871  and the sensor contact  873  are joined through a frusto-conical connector  872 . The connector  872  has formed therein an L-shaped slit which connects the slits  877  and  878 . The heater holding portion  871  and the sensor contact  873  are eccentric so that the ceramic heater  1  may be coaxial with the gas sensing element  81  when the plus holder  87  is fitted in the gas sensing element  81 . 
     The slit  878  formed in the sensor contact  873  is, as can be seen in FIG.  13 ( b ), diametrically opposed to the sensor signal output terminal  879  and thus is located at an angular interval of 90° away from the slit  877  formed in the heater holding portion  871 . 
     The sensor contact  873  has formed on the end thereof a plurality of claws  874  which engage an upper end of the gas sensing element  81  for orientation to the gas sensing element  81 . 
     The sensor contact  873  has an outer diameter slightly greater than an inner diameter of the gas sensing element  81  so that the sensor contact  873  may be installed elastically within the gas sensing element  81  by a press fit. The heater holding portion  871  has an inner diameter slightly smaller than a maximum outer diameter of the ceramic heater  1  for establishing tight engagement with the ceramic heater  1  when fitted in the heater holding portion  871 . 
     The minus holder  86 , as clearly shown in FIG.  13 ( a ), has formed therein a slit to have spring properties like the plus holder  87 . In order to enhance the spring properties, the plus holder  87  and the minus holder  86  are both made of a heat resisting spring steel. 
     FIGS.  15 ( a ),  15 ( b ), and  16  show the plus holder  87  in which the ceramic heater  1  is fitted. 
     As clearly shown in FIG. 16, the ceramic heater  1  is disposed in the plus holder  87  with one of the surfaces on which the outer leads  4  are installed facing the slit  877  so that the outer leads  4  may be both located 90° apart from the sensor signal output terminal  879 . 
     FIG. 17 shows the plus holder  87  holding therein the ceramic holder  1  fitted in the gas sensing element  81  and the minus holder  86  installed on the outer surface of the gas sensing element  81 . The sensor signal output terminal  869  of the minus holder  86  is located approximately 180° away from the sensor signal output terminal  879  of the plus holder  87 . The sensor signal output terminals  869  and  879  are, therefore, arranged at angular intervals 90° away from the outer leads  4 , respectively. 
     The gas sensing element  81  has, as shown in FIG. 12, a reference gas chamber  812  formed in the inner chamber  810  and defines a gas chamber  813  between itself and a protective cover assembly  82 . An outer electrode  815  and an inner electrode  814  both made of platinum are installed on the gas-exposed portion  811  and the inner surface of the gas sensing element  81  in connection with the minus holder  815  and the plus holder  87 , respectively. 
     The sensor signal output terminals  869  and  879  of the holders  86  and  87  and the leads  4  of the ceramic heater  1  are electrically connected to four leads  891  to  893 , respectively, through connectors  995  and  896 . The connectors  995  and  895  are disposed in an insulator  85  at regular intervals of 90° for avoiding interference with each other. 
     The gas sensing element  81  is installed in a sensor mount  84  which is used in mounting the oxygen sensor  8  in an exhaust pipe of an automotive engine. The protective cover assembly  82  is mounted on an end of the sensor mount  84  to cover the gas sensing element  81 . A dust cover  83  is mounted on the sensor mount  84 . 
     The sensor mount  84  has a cylindrical wall which extends upward from the flange thereof and in which an insulator  881 , a talc  882 , and a ring spacer  883  are disposed to retain the gas sensing element in the sensor mount  84 . An end  841  of the cylindrical wall of the sensor mount  84  is crimped inward to elastically press the ring spacer  883  downward, as viewed in FIG. 12. A float packing  884  is interposed between an inner wall of the sensor mount  84  and an outer wall of the gas sensing element  81  to seal the gas chamber  813  hermetically. 
     The sensor mount  84  has formed in the end  842  thereof an annular groove  843  to form an outer skirt  844  and an inner skirt  845 . The protective cover assembly  82  consists of an outer cover  821  and an inner cover  822  both made of a cup-shaped member. The outer and inner covers  821  and  822  have flanges  828  and  829  which are retained in the groove  843  of the sensor mount  84  by crimping the outer skirt  844  inward. The outer and inner covers  821  and  822  have formed in side walls thereof a plurality of holes through which a gas to be measured passes to enter the gas chamber  813 . 
     The dust cover  83 , as shown in FIG. 12, consists of a small-diameter cylinder  831 , a large-diameter cylinder  832 , and a shoulder portion  833  connecting the cylinders  831  and  832 . The dust cover  83  is, as described above, welded at a circumferential portion  834  thereof to a boss of the sensor mount  84  and retains therein the insulator  85 . 
     A cylindrical cover  839  is mounted on the periphery of the small-diameter cylinder  831  of the dust cover  83  by crimping. A water-repellent filter  857  is installed between the cylindrical cover  839  and the small-diameter cylinder  831 . The cover  839  and the dust cover  83  have formed therein first air vents  858  and second air vents  859 , respectively, which communicate with the reference gas chamber  812  formed in the gas sensing element  81  to fill the reference gas chamber  812  with air. 
     A heat-resisting rubber bush  895  is mounted in the end of the small-diameter cylinder  831  of the dust cover  83  to retain the leads  891  to  893  at angular intervals of 90°. 
     The insulator  85  consists of a sleeve  851  in which the leads  891  to  893  are disposed and a flange  852  greater in diameter than the sleeve  851 . The small-diameter cylinder  831  of the dust cover  83  has the inner diameter greater than the outer diameter of the sleeve  851  of the insulator  85  and smaller than the outer diameter of the flange  852 . The large-diameter cylinder  832  of the dust cover  83  has the inner diameter greater than the outer diameter of the flange  852  of the insulator  85 . 
     The insulator  85  is retained in the large-diameter cylinder  832  of the dust cover  83  in engagement of an upper end of the flange  852  with the shoulder portion  833  of the dust cover  83  by a stop ring  899  press-fitted in the large-diameter cylinder  832 . 
     The gas sensing element  81  produces the electromotive force as a function of a difference in oxygen concentration between the air in the reference gas chamber  812  and the gas in the gas chamber  813  and outputs a signal indicative thereof through the leads  891  and  892 . The operation of the oxygen sensor  8  is well known in the art, and explanation thereof in detail will be omitted here. 
     The operation and effects of this embodiment will be described below. 
     The four connectors  896  and  995  are disposed in an insulator  85  at regular intervals of 90° for avoiding interference with each other. The sensor signal output terminals  879  and  869  of the holders  86  and  87  and the leads  4  of the ceramic heater  1  are, therefore, located at regular intervals of 90° away from each other. 
     The sensor signal output terminal  879  installed on the sensor contact  873  of the plus holder  87  is, as described above, located approximately 90° away from the slit  877  formed in the heater holding portion  871 , thereby allowing the ceramic heater  1  to be, as shown in FIGS. 16 and 17, fitted firmly in the heater holding portion  871  of the plus holder  87  so that the leads  4  of the ceramic heater  1  may be located at angular intervals of 90° away from the sensor signal output terminal  879 . 
     For comparison with this embodiment, a plus holder  97  used in conventional oxygen sensors is shown in FIGS.  18 ( a ) and  18 ( b ). The plus holder  97  has a slit  977  formed in a heater holding portion  971  at an angular interval of 180° away from a sensor signal output terminal  979 . The slit  977  is located at the same angular position as that of a slit  978  formed in a sensor contact  973  of the plus holder  97 . Arranging the leads  4  of the ceramic heater  1  90° apart from the sensor signal output terminal  979  requires, as shown in FIG.  18 ( a ), retaining side walls of the ceramic heater  1  between vertical edges  999  and an opposite inner wall of the heater holding portion  971  defining the slit  977 , thus resulting in instability of installation of the ceramic heater  1 . 
     The stable installation of the ceramic heater  1  in the plus holder  97  requires, as shown in FIG.  18 ( b ), retaining the side walls of the ceramic heater  1  between opposite portions of the inner wall of the plus holder  97  located 90° apart from the slit  977 . In this case, the leads  4  are oriented in alignment with the sensor signal output terminal  979 , so that they are twisted undesirably when connected to the connectors  896  and  995 . 
     The structure of this embodiment allows, as described above, the leads  4  of the ceramic heater  1  to be located 90° apart from the sensor signal output terminal  879  without compromising the installation of the ceramic heater  1  in the plus holder  87 . 
     The positional relation between the sensor signal output terminal  879  of the sensor contact  973  and the slit  877  of the heater holding portion of the plus holder  87  is not limited to 90°, but may be within an angular range of 90°±20°. This also achieves firm installation of the ceramic heater  1  in the plus holder  87  without interfering the connectors  896  and  995  with each other. 
     The inventors of this application analyzed the relation between the durability of the ceramic heater  1  and a welded angle which the outer surface of each side end of the bonding layer  5  makes with the metallic plate  3 . The analysis was made by preparing samples whose welded angles γ, as shown in FIG. 19, are 25° to 60° and performing a temperature cycle test a hundred times in which each sample was subjected to intense heat at 450° C. for four minutes and then left at room temperature for four minutes. After the hundred temperature cycle tests, each metallic terminals  3  was checked for cracks, and the strength of a joint of the bonding layer  5  and the metallic terminal  3  was measured. The measurement of the strength was performed in tensile tests. The results of the tests are shown in table 2 below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Welded 
                   
                 Joint 
                   
               
               
                   
                 Angle 
                   
                 Strength 
                   
               
               
                   
                 γ 
                 Cracks 
                 (kgf) 
                 Evaluation 
               
               
                   
                   
               
             
            
               
                   
                 60° 
                 many 
                 1 or less 
                 X 
               
               
                   
                 50° 
                 many 
                 1 or less 
                 X 
               
               
                   
                 40° 
                 few 
                 3 
                 Δ 
               
               
                   
                 30° 
                 few 
                 4 
                 ∘ 
               
               
                   
                 20° 
                 few 
                   4.5 
                 ∘ 
               
               
                   
                   
               
            
           
         
       
     
     where ∘ indicates excellent durability, Δ indicates allowable durability, and X indicates lack of durability. 
     The table 2 shows that the ceramic heater  1  has high durability when the welded angle γ is 40° or less. 
     While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.