Abstract:
A dielectric ceramic composition consisting essentially of: 
     
       Pb(M.sub.1/3 Nb.sub.2/3).sub.x (M&#39;.sub.a Nb.sub.1-a).sub.y (Fe.sub.2/3 
     
      W 1/3 ) z  O 3 , 
     wherein M represents Ni or Mg and x+y+z=1, and 
     when M is Ni, M&#39;=Fe, a=1/2, 0.01≦x≦0.40, 0.45≦y≦0.80 and 0.05≦z≦0.50; and 
     when M is Mg, M&#39;=Zn, a=1/3, 0.01≦x≦0.70, 0.15≦y≦0.45 and 0.05≦z≦0.60. 
     The ceramic composition allows low-temperature sintering and exhibits high dielectric constant.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     The present invention relates to dielectric ceramic compositions allowing low-temperature sintering, and exhibiting high dielectric constant, and being suitable for use in multilayer ceramic capacitors. 
     2. Description of the Prior Art: 
     As promising ceramic compositions for this application, ceramic compositions essentially consisting of BaTiO 3  have been widely used as a materials of high dielectric constant. However, the BaTiO 3  system ceramics must be sintered at a very high temperature in the range of 1300° C. to 1400° C., so that where they are used as a dielectric material of multilayer ceramic capacitors, expensive metals such as platinum or palladium which can stand such high sintering temperature should be used as internal electrodes of the capacitors. Therefore there has still been a demand for dielectric ceramic materials which can be sintered at a temperature as low as below 1000° C. for enabling the use of relatively cheap metal such as silver for the internal electrodes. 
     U.S. Pat. No. 4,078,938 has described binary system ceramic compositions of PbFe 2/3  W 1/3  O 3  --PbFe 178  Nb 1/2  O 3   which can be sintered at a relatively low temperature, and which exhibit high dielectric constant. However, the ceramics have low specific resistance. 
     U.S. Pat. No. 4,236,928 has described binary system ceramic compositions of Pb(Fe 2/3  W 1/3 )O 3  --Pb(Zn 1/3  Nb 2/3 )O 3  which can be sintered at a temperature below 1000° C. However, the ceramics have relatively low dielectric constant. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide excellent dielectric ceramic compositions which have low sintering temperature, high dielectric constant and high specific resistivity. 
     Another object of the present invention is to provide dielectric ceramic compositions which have low sintering temperature along with high dielectric constant and low temperature coefficient of the dielectric constant. 
     In accomplishing these objects, a ceramic composition of the present invention has a dielectric ceramic composition consisting essentially of a material represented by the formula 
     
         Pb(M.sub.1/3 Nb.sub.2/3).sub.x (M&#39;.sub.a Nb.sub.1-a).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z O.sub.3, 
    
     wherein 
     M represents one element selected from the group consisting of Ni and Mg, M&#39;=Fe, a=1/2, 0.01≦x≦0.40, 0.45≦y≦0.80 and 0.05≦z≦0.50 when Ni is selected for M, M&#39;=Zn, a=1/3, 0.01≦x≦0.70, 0.15≦y≦0.45 and 0.05≦z≦0.60 when Mg is selected for M, and x+y+z=1. 
     These novel compositions of the present invention can be sintered at a temperature as low as 1000° C. or less, and have a high dielectric constant, a high specific resistivity and a low temperature coefficient of the dielectric constant. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is based on the discovery that within certain particular compositional ranges of these systems Pb(Ni 1/3  Nb 2/3 )O 3  --Pb(Fe 1/2  Nb 1/2 )O 3  --Pb(Fe 2/3  W 1/3 )O 3  and Pb(Mg 1/3  Nb 2/3 )O 3  --Pb(Zn 1/3  Nb 2/3 )O 3  --Pb(Fe 2/3  W 1/3 )O 3  the specimens exhibit very high dielectric constant along with low sintering temperature. 
    
    
     The compositions described herein may be prepared in accordance with various well-known ceramic procedures. 
     EXAMPLE 1 
     The starting materials, viz., lead oxide (PbO), nickel oxide (NiO), ferric oxide (Fe 2  O 3 ), niobium oxide (Nb 2  O 5 ) and tungsten oxide (WO 3 ), all of relatively pure grade, were intimately mixed in a ball mill with distilled water. Thereafter the mixtures were dried and then calcined at 750° C. for 2 hours. Thus obtained materials were wet-ground in a ball mill, dried, mixed with polyvinyl alcohol as a binder solution, and then pressed into columns of about 13 mm in diameter and about 8 mm in length at a pressure of 700 kg/cm 2 . After burning out binder at about 700° C., the pressed columns which were put into magnesia crucible were sintered at a temperature of 880° C. to 1040° C. for 2 hours. The sintered bodies were cut into discs of about 1 mm in thickness, and then were attached Cr--Au electrodes on the both surfaces of the discs by a method of vacuum evaporation. Various properties of the ceramic discs thus obtained are shown in Table 1. The dielectric constant (ε r ) and the dielectric loss (D) were measured at a frequency of 1 kHz and a voltage of 1 V under room temperature. The temperature coefficient of a dielectric constant was obtained by measuring a dielectric constant at a temperature range of -25° C. to 85° C. and calculated with reference to the dielectric constant at 20° C. The specific electrical resistivity was measured at room temperature by applying a D.C. voltage of 1 kV. 
     
                                           TABLE 1__________________________________________________________________________       Sintering       temper-       Change of                              SpecificCompositions       ature    D    ε.sub.r (%)                              resistivityNo.   x  y  z  (°C.)            ε.sub.r                (× 10.sup.-4)                     -25° C.                          85° C.cm)                                (Ω__________________________________________________________________________ 1*   0  0.50    0.50       940  8740                 41   72  -75 1.5 × 10.sup.72  0.01 0.59    0.40       920  18690                245  -74  -69 6.1 × 10.sup.8 3*   0.05 0.85    0.10       920  6130                576  -62   44 1.6 × 10.sup.94  0.05 0.45    0.50       900  8570                 80   36  -59 2.3 × 10.sup.9 5*   0.05 0.40    0.55       880  5710                 72   55  -53 7.5 × 10.sup.86  0.10 0.80    0.10       900  8260                483  -58   27 3.2 × 10.sup.117  0.10 0.60    0.30       880  22620                260  -68  -76 7.1 × 10.sup.118  0.10 0.50    0.40       900  13450                 67  -15  -71 6.5 × 10.sup.119  0.20 0.60    0.20       940  23410                136  -71  -75 5.0 × 10.sup.1110 0.30 0.60    0.10       960  15950                293  -43  -60 4.3 × 10.sup.1111 0.30 0.50    0.20       960  11290                442  -27  -53 8.9 × 10.sup.1012 0.40 0.55    0.05       980  8230                124   16  -47 1.7 × 10.sup.1113*   0.45 0.55    0  1020 7400                526  -56  -29 5.2 × 10.sup.1014*   0.50 0.40    0.10       1040 4280                232   32  -64 7.4 × 10.sup.10__________________________________________________________________________ Compositions of the Nos. with an asterisk are outside the scope of the present invention. Compositions: Pb(Ni.sub.1/3 Nb.sub.2/3).sub.x (Fe.sub.1/2 Nb.sub.1/2).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z O.sub.3. 
    
     From Table 1 it is obvious that ceramic compositions Nos. 2, 4, and 6 to 12 within the scope of the present invention provide high dielectric constant (ε r  =8230--23410) and low dielectric loss (D&lt;500×10 -4 ), and can be sintered at a temperature below 1000° C., and further provide high specific resistivity and low temperature coefficient of dielectric constant. 
     In the ceramic compositions represented by the formula Pb(Ni 1/3  Nb 2/3 ) x  (Fe 1/2  Nb 1/2 ) y  (Fe 2/3  W 1/3 ) z  O 3  wherein x&lt;0.01, the specific resistivity of the ceramics is low. In the compositions of x&gt;0.40 , the ceramics cannot be sintered at a temperature below 1000° C. And the ceramic compositions of y&lt;0.45, y&gt;0.80, z&lt;0.05 and/or z&gt;0.50 provide relatively small dielectric constant. Therefore such compositions are not suited for use as a capacitors. 
     EXAMPLE 2 
     The starting materials, viz., lead oxide (PbO), nickel oxide (NiO), ferric oxide (Fe 2  O 3 ), niobium oxide (Nb 2  O 5 ), tungsten oxide (WO 3 ), manganese dioxide (MnO 2 ), cromium oxide (Cr 2  O 3 ), cobalt oxide (CoO) and lithium carbonate (Li 2  CO 3 ), all of relatively pure grade, were intimately mixed in a ball mill with distilled water. Thereafter the mixtures were dried and then calcined at 750° C. for 2 hours. Thus obtained materials were wet-ground in a ball mill, dried, mixed with polyvinyl alcohol as a binder solution, and then pressed into columns of about 13 mm in diameter and about 8 mm in length at a pressure of 700 kg/cm 2 . After burning out binder at about 700° C., the pressed columns which were put into magnesia crucible were sintered at a temperature of 840° to 980° C. for 2 hours. The sintered bodies were cut into discs of about 1 mm in thickness, and then were attached Cr--Au electrodes on the both surfaces of the discs by a method of vacuum evaporation. ε r , D and specific electrical resistivity of the ceramic discs are shown in Table 2. ε r  and D were measured at a frequency of 1 kHz and a voltage of 1 V under room temperature. The specific resistivity was measured at room temperature by applying a D.C. voltage of 1 kV. 
     
                                           TABLE 2__________________________________________________________________________                    SinteringCompositions             temper-       Specific       Additives    ature    D    resistivityNo.   x  y  z  (wt %)       (°C.)                         ε.sub.r                             (× 10.sup.-4)cm)                                    (Ω__________________________________________________________________________15 0.01 0.59    0.40       --           920  18690                             245  6.1 × 10.sup.816 &#34;  &#34;  &#34;  0.2 MnO.sub.2                    920  17920                             153  7.5 × 10.sup.1017 &#34;  &#34;  &#34;  0.2 Cr.sub.2 O.sub.3                    920  16870                             107  4.1 × 10.sup.1018 &#34;  &#34;  &#34;  0.2 CoO      920  18150                             124  3.9 × 10.sup.1019 &#34;  &#34;  &#34;  0.2 Li.sub.2 O                    920  19430                              82  5.4 × 10.sup.1020 0.05 0.45    0.50       --           900   8570                              80  2.3 × 10.sup.921 &#34;  &#34;  &#34;  0.01 MnO.sub.2                    900   8810                              72  4.3 × 10.sup.922 &#34;  &#34;  &#34;  0.01 Cr.sub.2 O.sub.3                    900   8490                              65  4.1 × 10.sup.923 &#34;  &#34;  &#34;  0.01 CoO     900   8530                              68  5.7 × 10.sup.924 &#34;  &#34;  &#34;  0.01 Li.sub.2 O                    900   9070                              52  3.3 × 10.sup.925 0.10 0.80    0.10       --           900   8260                             483  3.2 × 10.sup.1126 &#34;  &#34;  &#34;  0.1 MnO.sub.2                    900   8200                             195  6.1 × 10.sup.1127 &#34;  &#34;  &#34;  0.1 Cr.sub.2 O.sub.3                    900   8440                             241  8.3 × 10.sup.1128 &#34;  &#34;  &#34;  0.1 CoO      900   8610                             307  5.0 × 10.sup.1129 &#34;  &#34;  &#34;  0.1 Li.sub.2 O                    900   8150                             239  9.4 × 10.sup.1130 0.10 0.60    0.30       --           880  22620                             260  7.1 × 10.sup.1131 &#34;  &#34;  &#34;  0.5 MnO.sub.2                    880  20590                             207  2.5 × 10.sup.1232 &#34;  &#34;  &#34;  1.5 MnO.sub.2                    840  16200                              98  2.1 ×  10.sup.1233 &#34;  &#34;  &#34;  0.2 MnO.sub.2 + 0.2 CoO                    900  21730                             134  4.8 × 10.sup.1234 &#34;  &#34;  &#34;  0.5 Li.sub.2 O                    900  24100                             123  9.8 × 10.sup.1235 &#34;  &#34;  &#34;  1.5 Li.sub.2 O                    900  17350                             214  1.3 × 10.sup.1236 &#34;  &#34;  &#34;  0.2 MnO.sub.2 + Li.sub.2 O                    880  23470                              72  8.3 × 10.sup.1237 0.20 0.60    0.20       --           940  23410                             136  5.0 × 10.sup.1138 &#34;  &#34;  &#34;  0.05 CoO     940  22160                             108  7.2 × 10.sup.1139 &#34;  &#34;  &#34;  0.2 CoO      920  21500                              96  4.3 × 10.sup.1240 &#34;  &#34;  &#34;  1.5 CoO      920  14020                             115  8.1 × 10.sup.11 41*   &#34;  &#34;  &#34;  2.0 CoO      920   9380                             324  2.3 × 10.sup.1142 &#34;  &#34;  &#34;  0.2 Cr.sub.2 O.sub.3                    940  24340                             123  7.5 × 10.sup.1243 &#34;  &#34;  &#34;  1.5 Cr.sub.2 O.sub.2                    940  16600                             101  5.9 × 10.sup.11 44*   &#34;  &#34;  &#34;  2.0 Cr.sub.2 O.sub.3                    940  10130                             157  9.4 × 10.sup.1045 &#34;  &#34;  &#34;  0.2 CoO + 0.2 Cr.sub.2 O.sub.3                    940  21940                              89  5.3 × 10.sup.1246 &#34;  &#34;  &#34;  0.2 CoO + 0.1 Li.sub.2 O                    900  18350                             108  7.4 × 10.sup.1247 &#34;  &#34;  &#34;  0.2 Cr.sub.2 O.sub.3 + 0.1 Li.sub.2 O                    900  19610                              84  6.4 × 10.sup.1248 &#34;  &#34;  &#34;  0.1 MnO.sub.2 + 0.1 CoO +                    900  21090                              98  5.7 × 10.sup.12       0.1 Cr.sub.2 O.sub.349 &#34;  &#34;  &#34;  0.1 MnO.sub.2 +0.1 CoO +                    880  18720                             110  2.0 × 10.sup.12       0.1 Cr.sub.2 O.sub.3 + 0.1 Li.sub.2 O50 0.30 0.60    0.10       --           960  15950                             293  4.3 × 10.sup. 1151 &#34;  &#34;  &#34;  0.2 MnO.sub.2                    960  13490                             218  8.0 × 10.sup.12 52*   &#34;  &#34;  &#34;  2.0 MnO.sub.2                    920   7410                             472  2.1 × 10.sup.1153 &#34;  &#34;  &#34;  0.2 Li.sub.2 O                    940  17020                             240  5.2 × 10.sup.12 54*   &#34;  &#34;  &#34;  2.0 Li.sub.2 O                    880   6740                             325  6.4 × 10.sup.1055 0.30 0.50    0.20       --           960  11290                             442  8.9 × 10.sup.1056 &#34;  &#34;  &#34;  0.1 MnO.sub.2                    960  12030                             279  2.7 × 10.sup.1157 &#34;  &#34;  &#34;  0.1 MnO.sub.2 + 0.1 Cr.sub.2 O.sub.3                    960  10170                             194  1.3 × 10.sup.1258 &#34;  &#34;  &#34;  0.1 MnO.sub.2 + 0.1 Cr.sub.2 O.sub.3 +                    920  11400                             213  2.8 × 10.sup.12       0.1 Li.sub.2 O59 0.40 0.55    0.05       --           980   8230                             124  1.7 × 10.sup.1160 &#34;  &#34;  &#34;  0.1 MnO.sub.2                    980   8610                              62  2.9 × 10.sup.1261 &#34;  &#34;  &#34;  0.1 Cr.sub.2 O.sub.3                    980   8500                              49  5.6 × 10.sup.1162 &#34;  &#34;  &#34;  0.1 CoO      980   8140                              70  3.1 × 10.sup.1263 &#34;  &#34;  &#34;  0.1 Li.sub.2 O                    980   8160                              72  4.5 × 10.sup.11__________________________________________________________________________ Basic compositions: Pb(Ni.sub.1/3 Nb.sub.2/3).sub.x (Fe.sub.1/2 Nb.sub.1/2).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z O.sub.3. Compositions of the Nos. with an asterisk are outside the scope of the present invention. 
    
     It is obvious from Table 2 that Pb(Ni 1/3  Nb 2/3 ) x  (Fe 1/2  Nb 1/2 ) y  (Fe 2/3  W 1/3 ) z  O 3  ceramics containing at least one element selected from the group consisting of Mn, Cr, Co and Li in a total quantity equivalent to from 0.01 to 1.5 weight percent of respective oxides (MnO 2 , Cr 2  O 3 , CoO and Li 2  O) exhibit high specific electrical resistivity as compared with that of compositions with no addition, exhibit low D as compared with that of the ceramics with no addition, and exhibit still high ε r  (8140-24100) along with low sintering temperature (840°-980° C.). In the compositions containing over 1.5 wt. % of MnO 2 , Cr 2  O 3 , CoO or Li 2  O as additive the specific electrical resistivity of the ceramics is low. 
     EXAMPLE 3 
     The starting materials, viz., lead oxide (PbO), magnesium oxide (MgO), zinc oxide (ZnO), ferric oxide (Fe 2  O 3 ), niobium oxide (Nb 2  O 5 ) and tungsten oxide (WO 3 ), all of relatively pure grade, were intimately mixed in a ball mill with distrilled water. Thereafter the mixture were dried and then calcined at about 750° C. for 2 hours. Thus obtained materials were wet-ground in a ball mill, dried, mixed with polyvinyl alcohol as a binder solution, and then pressed into columns of about 13 mm in diameter and about 8 mm in length at a pressure of 700 kg/cm 2 . After burning out binder at about 700° C., the pressed columns which were put into magnesia crucible were sintered at a temperature of 780° to 1040° C. for 2 hours. The sintered bodies were cut into discs of about 1 mm in thickness, and then were attached Cr--Au electrodes on the both surfaces of the discs by a method of vacuum evaporation. ε r , D and change of ε r  with temperature of the ceramic discs are shown in Table 3. ε r  and D were measured at a frequency of 1 kHz and a voltage of 1 V under room temperature. The change of ε r  with temperature were measured at a temperature range of -25 to 85° C. and calculated with reference to the ε r  at 20° C. 
     
                                           TABLE 3__________________________________________________________________________        Sintering        temper-         Change of ε.sub.rCompositions ature     D     (%)No.   x  y   z  (°C.)              ε.sub.r                  (× 10.sup.-4)                        -25° C.                             85° C.__________________________________________________________________________ 64*   -- 0.40     0.60        920   5830                   95    28  -4365 0.01 0.40     0.59        900   6650                  108    26  -3966*   0.10 0.50     0.40        860   5240                  281   -33  -2567 0.10 0.45     0.45        820   6490                  235    -8  -2868 0.10 0.40     0.50        780   8120                  223    16  -3569 0.10 0.30     0.60        820   6380                   67    27  -38 70*   0.10 0.25     0.65        840   4900                   85    36  -5271 0.20 0.40     0.40        820   8210                   98    -9  -3472 0.20 0.30     0.50        800   7300                   57    22  -3773 0.30 0.40     0.30        840   9670                  112   -31  -3474 0.40 0.40     0.20        820   11820                  203   -33  -3575 0.40 0.30     0.30        820   10530                   64   -21  -4076 0.40 0.15     0.45        860   6440                   71    28  -3577 0.50 0.30     0.20        900   8790                  135   -16  -3078 0.60 0.30     0.10        940   8980                  183   -29  -3179 0.70 0.25     0.05        960   8210                  236   -34   1280 0.70 0.15     0.15        940   6550                  124   -15  -32 81*   0.70 0.10     0.20        1000  5670                   98     9  -46 82*   0.73 0.25     0.02        1040  8420                  269   -47    7__________________________________________________________________________ Compositions of the Nos. with an asterisk are outside the scope of the present invention. Compositions: Pb(Mg.sub.1/3 Nb.sub.2/3).sub.x (Zn.sub.1/3 Nb.sub.2/3).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z O.sub.3. 
    
     It is clear from Table 3 that ceramic compositions Nos. 65, 67, 68 and 71 to 80 within the scope of present invention provide high ε r  (6380-11820), low D (≦236×10 -4 ) and low temperature coefficient of ε r  along with low sintering temperature (780°-960° C.). 
     The ceramic compositions represented by the formula Pb(Mg 1/3  Nb 2/3 ) x  (Zn 1/3  Nb 2/3 ) y  (Fe 2/3  W 1/3 ) z  O 3  wherein x&lt;0.01, y&lt;0.15, y&gt;0.45 and/or z&gt;0.60, provide relatively small ε r . In the compositions of x&gt;0.70 and/or z&lt;0.05, the ceramics cannot be sintered at a temperature below 1000° C. 
     As apparent from these Examples 1 to 3, the ternary ceramic compositions of the present invention have low sintering temperature below 1000° C., so that relatively cheap metal such as silver can be employed as internal electrodes of multilayer ceramic capacitors, and the durability of furnace materials for sintering use may be extended and electric power for sintering may be lowered. Moreover, the ceramic compositions according to the present invention exhibit high dielectric constant along with high specific electrical resistivity, low temperature coefficient of the dielectric constant and/or low dielectric loss. Therefore, the ceramic compositions are suitable for use of ceramic capacitors. 
     It will be evident that the starting materials to be used in the present invention are not limited to those used in the above examples. Other oxide or compounds which are easily decomposed at elevated temperature may be used in place of the starting materials of the above examples.