Temperature compensating titanate ceramic capacitor with nickel or copper electroless metal electrodes
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ceramic dielectric composition for temperature for compensation. 
     2. Description of the Prior Art 
     Japanese Patent Publication No. 17771/1981 of the Japanese patent application filed in the name of TDK Electronics Company Inc., Tokyo, Japan, discloses a composition of SrTiO 3  --CaTiO 3  --Nb 2  O 5  system by way of an example of a ceramic dielectric composition for temperature compensation exhibiting electrical characteristics of a dielectric constant of larger than 200, the temperature coefficient of a dielectric constant of up to -1000×10 -6  /°C. and quality factor (Q) of larger than 1000. 
     It has already been reported that such system exhibits electrical characteristics of the dielectric constant being 200 to 320 and of the quality factor being in the range of 1500 to 5000, in the range of the temperature coefficient of a dielectric constant of -1000×10 -6  /°C. to -3000×10 -6  /°C. 
     In addition, it has been confirmed that such system exhibits characteristics in which the dielectric constant is decreased in the ranges of 50 to 80% by weight of SrTiO 3  and of 20 to 50% by weight of CaTiO 3  in terms of proportion of SrTiO 3  --CaTiO 3  which is a principal constituent of the system, while the quality factor is increased as the temperature coefficient of a dielectric constant is decreased. 
     Generally, the electric characteristic of a ceramic capacitor is shown by the values by measurement of a sample of a ceramic element having electrodes of fired silver. The above described system is not an exception to the same and measurement has been made upon formation of electrodes of fired silver. 
     However, in view of the fact that the recent rise of the cost of silver per se has increased the cost of capacitors, ceramic capacitors using a base metal, such as nickel, copper or the like, cheaper than silver have been put into practical use. Usually, electrodes of a base metal are formed by an electroless plating method. 
     Accordingly, formation of electrodes by an electroless plating method on a ceramic element of the above described SrTiO 3  --CaTiO 3  --Nb 2  O 5  system can be readily considered. However, formation of electrodes of nickel or copper by an electroless plating method on a ceramic capacitor element made of such system has revealed a large difference, as compared with a sample having electrodes of fired silver. More specifically, although the quality factor has been in the range of 1500 to 5000 according to a sample having silver electrodes, the quality factor of a sample having nickel or copper electrodes remains as low as 200 at the most, revealing that an element which has electrodes formed by an electroless plating method can be hardly put into practical use. Therefore, in formation of electrodes a combination of a ceramic element with electrodes need be taken into consideration and, failure in formation of inexpensive electrodes by an electroless plating method is incompatible with cost reduction. 
     In the light of the foregoing, even when a ceramic element for capacitors having excellent electric characteristics can be obtained, it is undesirable that the ceramic element brings about degradation of electric characteristics in the case of electrodes of electroless plating. Therefore, it is desired that a ceramic element brings about no degradation of electric characteristics irrespective of whether electrodes are of fired silver or of an electroless plating method. 
     SUMMARY OF THE INVENTION 
     Accordingly, a principal object of the present invention is to provide a ceramic dielectric composition for temperature compensation comprising a composition system satisfying the above described requirement. 
     The present invention comprises a composition system including SrTiO 3  --CaTiO 3  --Bi 2  O 3  or --Bi 2  O 3 .nTiO 2  (where n=1 to 5), to which MgTiO 3  is added as necessary. 
     More specifically, the present invention comprises 64 to 70.5% by weight of SrTiO 3 , 28 to 34% by weight of CaTiO 3 , and 1.5 to 4.5% by weight of Bi 2  O 3  or Bi 2  O 3 .nTiO 2 . Not more than 10% by weight of MgTiO 3  may be further added, as necessary. 
     The reason for the above described limitation to the ranges of composition will be described in the following. If and when SrTiO 3  is less than 64% by weight and CaTiO 3  is more than 34% by weight, the temperature coefficient of a dielectric constant goes too much in the minus direction. On the other hand, if and when SrTiO 3  is more than 70.5% by weight and CaTiO 3  is less than 28% by weight, the temperature coefficient of a dielectric constant also becomes large in the minus direction. If and when Bi 2  O 3  or Bi 2  O 3 .nTiO 2  is less than 1.5% by weight, sintering is difficult and both the dielectric constant and the quality factor become small and the temperature coefficient of a dielectric constant becomes large in the minus direction, whereas if and when the same is larger than 4.5% by weight, a preferred temperature coefficient of a dielectric constant is exhibited but the quality factor is decreased and a needle-like crystal appears on the surface of a ceramic element, with the result that the adhesive strength between the surface of ceramic and electroless plating layers as electrodes is decreased. If and when n becomes larger than 5, production of a needle-like crystal is suppressed but an increasing effect of a dielectric constant becomes lost. If and when MgTiO 3  is added, the temperature coefficient of a dielectric constant becomes small and sintering becomes easy; however, if MgTiO 3  becomes more than 10% by weight, the dielectric constant becomes low. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a triangular diagram showing a dielectric constant (ε) at the respective composition points in a composition including SrTiO 3  --CaTiO 3  --Bi 2  O 3 .3TiO 2  ; 
     FIG. 2 is a triangular diagram showing a quality factor (Q) at the respective composition points in a composition including SrTiO 3  --CaTiO 3  --Bi 2  O 3 .3TiO 2  ; 
     FIG. 3 is a triangular diagram showing a temperature coefficient of a dielectric constant (TC) at the respective composition points in a composition including SrTiO 3  --CaTiO 3  --Bi 2  O 3 .3TiO 2  ; 
     FIG. 4 is a graph showing a relation between the dielectric constant (ε) and the temperature coefficient of a dielectric constant (TC) if and when MgTiO 3  is added to SrTiO 3  --CaTiO 3  --Bi 2  O 3 .3TiO 2  ; and 
     FIG. 5 is a graph showing a relation between the dielectric constant (ε) and the length (μm) of a needle-like crystal when n of Bi 2  O 3 .nTiO 2  is changed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now the present invention will be described further in detail in the following by referring to the embodiments. 
     SrCO 3 , CaCO 3 , TiO 2 , Bi 2  O 3 , MgCO 3  were used as raw materials. Then SrTiO 3 , CaTiO 3 , MgTiO 3  were in advance prepared and these were mixed with Bi 2  O 3 , TiO 2  to provide ceramics of compositions shown in Table 1, whereupon the mixed raw materials were milled by a wet process together with a binder and the mixture was dehydrated and dried. The powders thus obtained were formed into discs of 10 mm in diameter and 0.5 mm in thickness under pressure of 500 kg/cm 2 . Then the compact thus obtained was fired for one hour in a natural atmosphere at the temperature of 1250° to 1400° C. 
     The ceramic thus obtained was rinsed and etched and sensitized and then activated, whereupon the same was dipped in a nickel plating bath, whereby electroless nickel plating electrodes were formed on the surface. Since an unnecessary plating layer has been formed on the outer peripheral end surface of the ceramic disc, the layer on such surface was removed by grinding. 
     With respect to the samples thus obtained, the respective electric characteristics of the dielectric constant (ε), the quality factor (Q) and the temperature coefficient of the dielectric constant (TC) were measured at the temperature of 20° C. The result of the measurement is shown in Table 2. 
     Referring to Tables 1 and 2, the samples marked with an asterisk (*) are those obtained not in accordance with the present invention and those other than the samples marked with an asterisk are those obtained in accordance with the present invention. 
     
                       TABLE 1                                                     
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Sample                                                                    
      SrTiO.sub.3                                                         
              CaTiO.sub.3                                                 
                        Bi.sub.2 O.sub.3 or                               
                                    MgTiO.sub.3                           
No.   (wt %)  (wt %)    Bi.sub.2 O.sub.3.nTiO.sub.2 (wt                   
                                    (wt %)                                
______________________________________                                    
 1*   63        34.5    Bi.sub.2 O.sub.3.2TiO.sub.2 :2.5                  
 2    64.5    33        Bi.sub.2 O.sub.3 :2.5                             
                                    --                                    
 3    &#34;       &#34;         Bi.sub.2 O.sub.3 TiO.sub.2 :2.5                   
                                    --                                    
 4    &#34;       &#34;         Bi.sub.2 O.sub.3.2TiO.sub.2 :2.5                  
                                    --                                    
 5    &#34;       &#34;         Bi.sub.2 O.sub.3.2TiO.sub.2 :2.5                  
                                    5                                     
 6*   &#34;       &#34;         Bi.sub.2 O.sub.3.2TiO.sub.2 :2.5                  
                                    13                                    
 7    &#34;       &#34;         Bi.sub.2 O.sub.3.4TiO.sub.2 :2.5                  
                                    --                                    
 8    &#34;       &#34;         Bi.sub.2 O.sub.3.5TiO.sub.2 :2.5                  
                                    --                                    
 9*   &#34;       &#34;         Bi.sub.2 O.sub.3.6TiO.sub.2 :2.5                  
                                    --                                    
10    66.5    29        Bi.sub.2 O.sub.3.4TiO.sub.2 :4.5                  
                                    --                                    
 11*  66.5    28        Bi.sub.2 O.sub.3.4TiO.sub.2 :5.5                  
                                    --                                    
12    66.5    31        Bi.sub.2 O.sub.3.2TiO.sub.2 :2.5                  
                                    --                                    
13    &#34;       &#34;         Bi.sub.2 O.sub.3.4TiO.sub.2 :2.5                  
                                    --                                    
14    67.5    29        Bi.sub.2 O.sub.3 :3.5                             
                                    --                                    
15    &#34;       &#34;         Bi.sub.2 O.sub.3.2TiO.sub.2 :3.5                  
                                    --                                    
16    68.5    29        Bi.sub.2 O.sub.3.TiO.sub.TiO.sub.2 :3.5           
                                    --                                    
17    &#34;       &#34;         Bi.sub.2 O.sub.3.2TiO.sub.2 :2.5                  
                                    --                                    
18    &#34;       &#34;         Bi.sub.2 O.sub.3.2TiO.sub.2 :2.5                  
                                    8                                     
19    69.5    29        Bi.sub.2 O.sub.3.4TiO.sub.2 :1.5                  
                                    --                                    
 20*  70.5      29.5      0         --                                    
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                       TABLE 2                                                     
______________________________________                                    
Sample                      TC                                            
No.      ε   Q      (× 10.sup.-6 /°C.)               
______________________________________                                    
 1*      228         1800   -1160                                         
 2       284         4300   -970                                          
 3       273         4900   -910                                          
 4       269         5100   -850                                          
 5       265         5300   -800                                          
 6*      212         6500   -760                                          
 7       253         5600   -870                                          
 8       241         6100   -840                                          
 9*      232         5900   -920                                          
10       267         2600   -630                                          
11*      315         1800   -710                                          
12       294         5400   -590                                          
13       272         6300   -650                                          
14       305         2600   -580                                          
15       295         3200   -690                                          
16       293         4700   -780                                          
17       287         5100   -830                                          
18       268         5500   -760                                          
19       238         4700   -920                                          
20*      230          250   -1640                                         
______________________________________                                    
 
    
     FIGS. 1, 2 and 3 are triangular diagrams showing the dielectric constant (ε), the quality factor (Q) and the temperature coefficient of a dielectric constant (TC), respectively, at the respective composition points of a composition including SrTiO 3  --CaTiO 3  --Bi 2  O 3 .3TiO 2 . Referring to the figures, those shown in parentheses among the respective measured values are those obtained by measurement using electrodes of fired silver. 
     FIG. 4 is a graph showing a change of the dielectric constant (ε) and the temperature coefficient of a dielectric constant (TC) when MgTiO 3  is added to a composition comprising 66.5% by weight of SrTiO 3 , 31% by weight of CaTiO 3  and 2.5% by weight of Bi 2  O 3 .3TiO 2 , in which the solid line denotes the dielectric constant (ε) and the dotted line denotes the temperature coefficient of a dielectric constant (TC). 
     FIG. 5 is a graph showing an influence upon the dielectric constant (ε) caused by a change of n of Bi 2  O 3 .nTiO 2  in a composition comprising 66.5% by weight of SrTiO 3 , 31% by weight of CaTiO 3  and 2.5% by weight of Bi 2  O 3 .nTiO 2 . The length (μm) of a needle-like crystal formed on the ceramic surface was simultaneously measured. 
     As seen from Tables 1 and 2 and FIGS. 1 to 3, by selecting the values in the ranges in accordance with the present invention, that which has an excellent characteristic of the dielectric constant (ε) being at least 235, of the quality factor (Q) being at least 2000 and of the temperature coefficient of a dielectric constant (TC) being up to -1000×10 -6  /°C. was obtained. Furthermore, by selecting the values in the ranges in accordance with the present invention, a composition of an electric characteristic of little diversification is provided even in case of electrodes of fired silver or electrodes of electroless plating. Accordingly, inexpensive electroless plating electrodes can be formed and the cost can be advantageously reduced. 
     As is further seen in FIG. 5, by inclusion of Bi 2  O 3 .nTiO 2  in SrTiO 3  --CaTiO 3 , and increasing the amount of TiO 2  in such case, production a needle-like crystal on the surface of ceramic can be suppressed and the adhesiveness of electroless plating electrodes to the surface of ceramic is improved. 
     Furthermore, by adding a mineralizer of such as SiO 2 , Al 2  O 3 , MnO 2 , Fe 2  O 3  or the like to the inventive composition, the ease of sintering can be further enhanced. 
     Although SrTiO 3 , CaTiO 3 , MgTiO 3  were in advance prepared in the above described examples, alternatively raw materials of SrCO 3 , CaCO 3 , MgCO 3  may be used and the same may be admixed with Bi 2  O 3 , TiO 2  and then may be subjected to calcination. 
     Japanese Patent Publication No. 17771/1981 described above discloses a composition including Bi 2  O 3 .3TiO 2  included as a secondary constituent in SrTiO 3  --CaTiO 3  and the same points out that whereas the dielectric constant (ε) is increased, the quality factor (Q) and the temperature coefficient of a dielectric constant (TC) are degraded. However, as a result of research made in conjunction with the present invention, not only the electric characteristic of the same level as compared with the SrTiO 3  --CaTiO 3  --Nb 2  O 5  system can be attained in the specified range of composition, but also the above described characteristics can be secured by electroless plating electrodes as well as by electrodes of fired silver, with the result that the present invention is extremely effective in industrial production of the composition.