Abstract:
A dielectric ceramic composition having a high relative dielectric constant a high quality factor and a small temperature coefficient of resonance frequency, consisting essentially of calcium oxide, strontium, oxide, bismuth oxide and titanium oxide, and having a composition represented by the formula: 
     
       (CaO).sub.a.(SrO).sub.b.(Bi.sub.2 O.sub.3).sub.c.(TiO.sub.2).sub.d 
     
     wherein 0≦a&lt;30, 0&lt;b≦20, 10≦c≦50, 40≦d≦80, and 0&lt;a+b≦30 by mol %. 
     This composition may contain at least one of thallium oxide, yttrium oxide and manganese oxide and at least one of germanium oxide, zirconium oxide, tin oxide, cerium oxide and hafnium oxide.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a dielectric ceramic composition having a high relative dielectric constant (εr) and a small and widely controllable temperature coefficient of resonance frequency (τf). 
     Recently, ceramic filters have been widely used for movable radio transmitters and receivers such as automobile telephones and cordless telephones operable at microfrequency bands from MHz to GHz. This is due to the fact that the dielectric materials constituting the ceramic filters have high dielectric constants (εr) and quality factors Q under no load (Q 0 ), and that their temperature coefficients (τf) of resonance frequency are freely controllable on both the positive and the negative sides from 0 by changing the compositions of the dielectric materials. 
     Conventional dielectric materials are, for instance, MgO--CaO--TiO 2 , ZrO 2  --TiO 2  --SnO 2 , and BaO--TiO 2  -lanthanoid oxide, etc. 
     However, these dielectric materials have relative dielectric constants (εr) less than 100, and the miniaturization of resonance elements formed from them is inherently limited. 
     Therefore, there is a strong desire for dielectric ceramic compositions with high relative dielectric constants (εr). 
     SUMMARY OF THE INVENTION 
     As a result of intense research directed toward developing a dielectric ceramic composition having a relative dielectric constant (εr) of 100 or more, a temperature coefficient (τf) of ±100 ppm/°C. or less and a quality factor under no load (Q 0 ) of 100 or more, the inventors have found that the above properties can be obtained when the dielectric ceramic composition has a composition represented by the formula: 
     
         (CaO).sub.a.(SrO).sub.b.(Bi.sub.2 O.sub.3).sub.c.(TiO.sub.2).sub.d 
    
     wherein 0≦a&lt;30, 0&lt;b≦20, 10≦c≦50, 40≦d≦80, and 0&lt;a+b≦30 by mol %. 
     DETAILED DESCRIPTION OF THE INVENTION 
     According to a first embodiment of the present invention, the dielectric ceramic composition is represented by the general formula: 
     
         (CaO).sub.a.(SrO).sub.b.(Bi.sub.2 O.sub.3).sub.c.(TiO.sub.2).sub.d 
    
     wherein 0≦a&lt;30, 0&lt;b≦20, 10≦c≦50, 40≦d≦80, and 0&lt;a+b≦30 by mol %. 
     When the contents of CaO, SrO, Bi 2  O 3  and TiO 2  are not in the above composition ranges, the Q 0  becomes less than 100, so that the dielectric ceramic composition is unsuitable for practical applications. 
     The preferred contents of CaO, SrO, Bi 2  O 3  and TiO 2  are 0≦a≦10, 5≦b≦15, 25≦c≦30, 50≦d≦60 and 5≦a+b≦20 by mol %. 
     The dielectric ceramic composition according to a second embodiment of the present invention has a main composition represented by the general formula: 
     
         (CaO).sub.a.(SrO).sub.b.(Bi.sub.2 O.sub.3).sub.c.(TiO.sub.2).sub.d 
    
     wherein 0≦a&lt;30, 0&lt;b≦20, 10≦c≦50, 40≦d≦80, and 0&lt;a+b≦30 by mol %, and further contains at least one of the following components: 
     5 weight % or less of thallium oxide (Tl 2  O 3 ), 
     5 weight % or less of yttrium oxide (Y 2  O 3 ), and 
     0.6 weight % or less of manganese oxide (MnO). 
     When Tl 2  O 3  exceeds 5 weight %, the Q 0  becomes less than 100, and the τf becomes largely negative. And when Y 2  O 3  exceeds 5 weight %, or when MnO exceeds 0.6 weight %, similar problems take place, providing ceramic materials unsuitable for practical applications. 
     The preferred contents of CaO, SrO, Bi 2  O 3  and TiO 2  are 0≦a≦10, 5≦b≦15, 25≦c≦30, 50≦d≦60 and 5≦a+b≦20 by mol %, and the total amount of Tl 2  O 3 , Y 2  O 3  and MnO is preferably 2 weight % or less. 
     The dielectric ceramic composition according to a third embodiment of the present invention has a composition represented by the general formula: 
     
         (CaO).sub.a.(SrO).sub.b.(Bi.sub.2 O.sub.3).sub.c.(TiO.sub.2).sub.d.(RO.sub.2).sub.e 
    
     wherein R represents at least one of Ge, Zr, Sn, Ce and Hf, and a, b, c, d and e satisfy 0≦a&lt;30, 0&lt;b≦20, 10≦c≦50, 40≦d≦80, 0&lt;e&lt;5, and 0&lt;a+b≦30 by mol %. 
     When GeO 2 , ZrO 2 , SnO 2 , CeO 2  and HfO 2  reach 5 mol %, the Q 0  becomes less than 100, and the τf cannot be detected at high temperatures because measured peaks are concealed by noises or become increasingly negative, making the dielectric ceramic composition unsuitable for practical applications. 
     The preferred contents of CaO, SrO, Bi 2  O 3  and TiO 2  are 0≦a≦10, 5≦b≦15, 25≦c≦30, 50≦d≦60 and 5≦a+b≦20 by mol %, and RO 2  is preferably 2 mol % or less. 
     The dielectric ceramic composition of the present invention can be prepared by mixing and sintering starting material powders in the predetermined proportions. The starting materials may be carbonates, nitrates, organic acid salts, etc. because they are thermally decomposed to form the corresponding oxides. 
    
    
     EXAMPLE 1 
     CaCO 3  powder, SrCO 3  powder, Bi 2  O 3  powder and TiO 2  powder were introduced into a polyethylene pot in the proportions as shown in Table 1 together with agate balls and acetone, and mixed for 16 hours. 
     The resulting slurry was dried by heating and classified with a 5-mesh sieve, and then burned at 1000° C. for 2 hours in the air. The burned product was again introduced into the polyethylene pot containing agate balls together with acetone and pulverized for 16 hours. 
     The resulting slurry was dried by heating, mixed with a polyvinyl alcohol aqueous solution, and then granulated with a 32-mesh sieve. 
     The granulated powder was pressed at 1 ton/cm 2 , and sintered in the atmosphere at 1200°-1400° C. for 4 hours. The resulting sintered product was worked to have a diameter of about 30 mm and a height of about 15 mm. This sample was measured at about 1 GHz to obtain a peak in a TE 011  mode, and the sample&#39;s εr and Q 0  were calculated from the above peak. Next, the τf was determined from the variation or resonance frequency between -20° C. and +60° C. The results are shown in Table 1. 
     
                       TABLE 1______________________________________Sample Composition (mol %)         τfNo.    CaO    SrO    Bi.sub.2 O.sub.3                      TiO.sub.2                           εr                                Q.sub.0                                      (ppm/°C.)______________________________________1      0      14.29  28.57 57.14                           198  168   -302      1      13.29  28.57 57.14                           196  182   -103      2      12.29  28.57 57.14                           192  188   +14      2.86   11.43  28.57 57.14                           188  196   +185      4      10.29  28.57 57.14                           184  200   +406      5      9.29   28.57 57.14                           180  210   +627      5.71   8.57   28.57 57.14                           176  230   +758      6      8.29   28.57 57.14                           177  220   +809      6.5    7.79   28.57 57.14                           176  220   +8410     20     10     40    30   169  53    +3011     5      25     20    50   150  90    +4212     10     10     5     75   120  95    +320______________________________________ Note: Sample Nos. 1-9: Examples of the present invention Sample Nos. 10-12: Comparative Examples 
    
     EXAMPLE 2 
     CaCO 3  powder, SrCO 3  powder, Bi 2  O 3  powder, TiO 2  powder, Tl 2  O 3  powder, Y 2  O 3  powder and MnCO 3  powder were introduced into a polyethylene pot in the proportions as shown in Table 2 together with agate balls and acetone, and mixed for 16 hours. 
     The resulting slurry was dried by heating and classified with a 5-mesh sieve, and then burned at 1000° C. for 2 hours in the air. The burned product was again introduced into the polyethylene pot containing agate balls together with acetone and pulverized for 16 hours. 
     The resulting slurry was dried by heating, mixed with a polyvinyl alcohol aqueous solution, and then granulated with a 32-mesh sieve. 
     The granulated powder was pressed at 1 ton/cm 2 , and sintered in the atmosphere at 1200°-1400° C. for 4 hours. The resulting sintered product was worked to have a diameter of about 30 mm and a height of about 15 mm. This sample was measured at about 1 GHz to obtain a peak in a TE 011  mode, and the sample&#39;s εr and Q 0  were calculated from the above peak. Next, the τf was determined from the variation of resonance frequency between -20° C. and +60° C. The results are shown in Table 2. 
     
                                           TABLE 2__________________________________________________________________________SampleComposition (mol %)             Additives    τfNo.  CaO   SrO      Bi.sub.2 O.sub.3          TiO.sub.2              (weight %)                    εr                       Q.sub.0                          (ppm/°C.)__________________________________________________________________________1    2.86   11.43      28.57          57.14             Tl.sub.2 O.sub.3                 0.1                    187                       174                          -62    2.86   11.43      28.57          57.14             Tl.sub.52 O.sub.3                 1.0                    188                       179                          -93    4.0   10.29      28.57          57.14             Tl.sub.2 O.sub.3                 3.0                    184                       200                          +54    2.86   11.43      28.57          57.14             Y.sub.2 O.sub.3                 0.5                    179                       163                          +75    2.86   11.43      28.57          57.14             MnO 0.006                    188                       203                          +16    4.0   10.29      28.57          57.14             MnO 0.003                    184                       220                          +57    2.86   11.43      28.57          57.14             Y.sub.2 O.sub.3                 3.0                    169                       138                          +28    4.0   10.29      28.57          57.14             Tl.sub.2 O.sub.3                 1.0                    185                       170                          +3             MnO 0.0069    2.86   11.43      28.57          57.14             Y.sub.2 O.sub.3                 0.5                    180                       199                          +2             MnO 0.00610   2.86   11.43      28.57          57.14             Tl.sub.2 O.sub.3                 1.0                    182                       179                          -1             Y.sub.2 O.sub.3                 0.111   2.86   11.43      28.57          57.14             Tl.sub.2 O.sub.3                 0.05                    185                       186                          -2             Y.sub.2 O.sub.3                 0.112   2.86   11.43      28.57          57.14             Tl.sub.2 O.sub.3                 6.0                    179                       90 -2213   2.86   11.43      28.57          57.14             Y.sub.2 O.sub.3                 6.0                    142                       89 -4814   2.86   11.43      28.57          57.14             MnO 0.9                    186                       85 -52__________________________________________________________________________ Note: Sample Nos. 1-11: Examples of the present invention Sample Nos. 12-14: Comparative Examples 
    
     EXAMPLE 3 
     CaCO 3  powder, SrCO 3  powder, Bi 2  O 3  powder, TiO 2  powder, GeO 2  powder, ZrO 2  powder, SnO 2  powder, CeO 2  powder and HfO 2  powder were introduced into a polyethylene pot in the proportions as shown in Table 3 together with agate balls and acetone, and mixed for 16 hours. 
     The resulting slurry was dried by heating and classified with a 5-mesh sieve, and then burned at 1000° C. for 2 hours in the air. The burned product was again introduced into the polyethylene pot containing agate balls together with acetone and pulverized for 16 hours. 
     The resulting slurry was dried by heating, mixed with a polyvinyl alcohol aqueous solution, and then granulated with a 32-mesh sieve. 
     The granulated powder was pressed at 1 ton/cm 2 , and sintered in the atmosphere at 1200°-1400° C. for 4 hours. The resulting sintered product was worked to have a diameter of about 30 mm and a height of about 15 mm. This sample was measured at about 1 GHz to obtain a peak in a TE 011  mode, and the sample&#39;s εr and Q 0  were calculated from the above peak. Next, the τf was determined from the variation of resonance frequency between -20° C. and +60° C. The results are shown in Table 3. 
     
                                           TABLE 3__________________________________________________________________________Sample    Composition (mol %)          τfNo. CaO  SrO Bi.sub.2 O.sub.3          TiO.sub.2               RO.sub.2                     εr                        Q.sub.0                            (ppm/°C.)__________________________________________________________________________1   2.86  11.43      28.57          57.04              GeO.sub.2                  0.1                     186                        179 -52   2.86  11.43      28.57          56.14              GeO.sub.2                  1.0                     181                        166 -153   2.86  11.43      28.57          56.64              ZrO.sub.2                  0.5                     185                        170 +24   2.86  11.43      28.57          56.14              ZrO.sub.2                  1.0                     181                        140 -125   2.86  11.43      28.57          57.06              SnO.sub.2                  0.08                     188                        175 +86   2.86  11.43      28.57          56.14              SnO.sub.2                  1.0                     175                        133 -607   2.86  11.43      28.57          56.54              CeO.sub.2                  0.6                     186                        192 -18   2.86  11.43      28.57          56.14              CeO.sub.2                  1.0                     182                        191 -169   2.86  11.43      28.57          56.64              HfO.sub.2                  0.5                     182                        165 +210  2.86  11.43      28.57          56.14              HfO.sub.2                  1.0                     178                        138 -5411  5.0  9.29      28.57          56.14              GeO.sub.2                  1.0                     179                        205 +412  2.86  11.43      28.57          52.14              GeO.sub.2                  5  159                        90  -8913  2.86  11.43      28.57          52.14              ZrO.sub.2                  5  171                        50  --*14  2.86  11.43      28.57          52.14              SnO.sub.2                  5  170                        75  --*15  2.86  11.43      28.57          52.14              CeO.sub.2                  5  148                        58  -19016  2.86  11.43      28.57          52.14              HfO.sub.2                  5  172                        26  --*__________________________________________________________________________ Note: Sample Nos. 1-11: Examples of the present invention Sample Nos. 12-16: Comparative Examples *Unmeasurable 
    
     As described above in detail, the dielectric ceramic composition of the present invention shows high εr and Q 0 , and its τf can be widely controlled by adjusting the ratio of CaO/SrO, the amounts of Tl 2  O 3 , Y 2  O 3  and MnO, and the amounts of GeO 2 , ZrO 2 , SnO 2 , CeO 2  and HfO 2 . It is highly suitable for microwave dielectric elements and temperature compensating capacitors, etc.