Abstract:
A ceramic composition is herein disclosed, which is a lead-type perovskite compound capable of being subjected to low temperature-sintering among ceramic compositions for use in making capacitors, which has a high dielectric constant, low temperature-dependency thereof and a low decrease in capacitance upon application of a DC bias and which comprises a ternary system comprising lead magnesium niobate (Pb(Mg 1/3  Nb 2/3 )O 3 ), lead nickel niobate (Pb(Ni 1/3  Nb 2/3 )O 3 ) and lead titanate (PbTiO 3 ) z  or a ternary system comprising lead magnesium tungstate (Pb(Mg 1/2  W 1/2 )O 3 ) lead titanate (PbTiO 3 ) and lead nickel niobate (Pb(Ni 1/3  Nb 2/3 )O 3 ) and a desired amount of lanthanum manganese niobate La(Mn 2/3  Nb 1/3 )O 3 ) incorporated into the ternary system or a predetermined amount of La 3+  or Ca 2+  ions with which the Pb 2+  ions present in the ternary system are substituted.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ceramic composition and in particular to a ceramic composition which has a high dielectric constant and a low rate of variation in dielectric constant with temperature and which is capable of being sintered at a low temperature in the order of 1050° to 1100° C. or lower. 
     2. Description of the Prior Art 
     When a multilayer capacitor is formed from a ceramic composition, the ceramic composition should be selected so that it satisfies several requirements such that it should have a dielectric constant and a specific resistivity as high as possible. Furthermore the rate of variation in dielectric constant with temperature, dielectric loss and drop in dielectric constant due to DC bias application should be as low as possible. Among ceramic compositions having high dielectric constants, those mainly comprising barium titanate (BaTiO 3 ) are well known and already put into practical use. However, they require a high sintering temperature. Accordingly, an additive such as calcium titanate (CaTiO 3 ) or lead titanate (PbTiO 3 ) is incorporated to improve the temperature-dependent characteristics however, but the sintering temperature thereof is remains on in the order of not less than 1300° C. For this reason, when the ceramic composition mainly comprising barium titanate is used in making a multilayer ceramic capacitor, materials for internal electrodes thereof are limited. More particularly, the internal electrode specific expensive materials such as noble metals (e.g., platinum and palladium) which can withstand such a high sintering temperature. Further, the dielectric constant achieved by these dielectric ceramic compositions is at highest about 8000. The dielectric constant thereof may be increased, but, in this case, variation thereof with temperature is impaired. More specifically, these ceramic compositions simply satisfy Y5V characteristics (-30° to 85° C.; +22%, -82%) as defined in the EIA Standards. 
     Although a ceramic material showing a low rate of variation in dielectric constant with temperature can be produced from the conventional material, the dielectric constant of the resulting ceramic composition is too low (in the order of about 2000) to use as a material for capacitors. 
     As has been discussed above, to reduce the expenses for producing multilayer ceramic capacitors, it is necessary to develop a ceramic composition capable of being sintered at a low temperature of the order of not more than 1050° C., which makes it possible to use cheaper materials for internal electrodes of capacitors such as those mainly comprising siver or nickel. Recently, there have been proposed various lead-based composite perovskite type compounds having low sintering temperatures and high dielectric constants. For instance, a ternary composition comprising lead magnesium niobate (Pb(Mg 1/3  Nb 2/3 )O3), lead nickel niobate (Pb(Ni 1/3  Nb 2/3 )O 3 ) and lead titanate (PbTiO 3 ) can achieve a dielectric constant at room temperature of not less than 10000 (see U.S. Pat. No. 4,712,156). In addition, Japanese Unexamined Patent Publication No. Sho 58-161972 discloses that a ternary composition comprising lead magnesium tungstate (Pb(Mg 1/2  W 1/2 )O 3 ), lead titanate (PbTiO 3 ) and lead nickel niobate (Pb(Ni 1/3  Nb 2/3 )O 3 ) can likewise achieve a dielectric constant of not less than 10000. However, these three-component systems suffer from a problem of high temperature-dependency of the dielectric constant. Moreover, the dielectric constant thereof is greatly reduced upon application of a DC bias and correspondingly, the resulting capacitors are greatly limited in their applications. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a ceramic composition which comprises the foregoing three-component system and exhibits a high dielectric constant, a relatively low temperature-dependency thereof and a small drop in dielectric constant upon application of a DC bias. 
     The foregoing and other objects and features of the present invention will be apparent from the following description. 
     According to a first aspect of the present invention, there is provided a ceramic composition which comprises, as a major constituent, a ternary system essentially consisting of lead magnesium niobate (Pb(Mg 1/3  Nb 2/3 )O 3 ), lead nickel niobate (Pb(Ni 1/3  Nb 2/3 )O 3 ) and lead titanate [PbTiO 3  ] and being expressed by the following general formula (Pb(Mg 1/3  Nb 2/3 )O 3 ) 2  (Pb(Ni 1/3  Nb 2/3 )O 3 ) y  (PbTiO 3 ) z  wherein the subscripts x, y and z satisfy the following relation: x+y+z=1.0 and fall within the range defined by and be on the line segments joining the following seven points (a) to (g) which are given by the coordinates: 
     
         ______________________________________(x = 0.10,   y = 0.70,   z = 0.20)     (a)(x = 0.10,   y = 0.475,  z = 0.425)    (b)(x = 0.625,  y = 0.05,   z = 0.325)    (c)(x = 0.75,   y = 0.05,   z = 0.20)     (d)(x = 0.75,   y = 0.15,   z = 0.10)     (e)(x = 0.50,   y = 0.40,   z = 0.10)     (f)(x = 0.15,   y = 0.70,   z = 0.15)     (g)______________________________________ 
    
     on the triangular ternary-system diagram; and, as an additive, 0.01 to 10 mole% of lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 3 ). 
     According to a second aspect of the present invention, there is provided a ceramic composition which comprises, as a major constituent, a ternary system essentially consisting of lead magnesium tungstate (Pb(Mg 1/2  W 1/2 )O 3 ), lead titanate (PbTiO 3 ) and lead nickel niobate (Pb(Ni 1/3  Nb 2/3 )O 3 ) and being expressed by the following general formula (Pb(Mg 1/2  W 1/2 )O 3 ) x  [PbTiO 3  ] y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  wherein the subscripts x, y and z satisfy the following relation: x+y+z=1.0 and fall within the range defined by and be on the line segments joining the following four points (h) to (k) which are given by the coordinates: 
     
         ______________________________________(x = 0.693,  y = 0.297,  z = 0.01)     (h)(x = 0.39,   y = 0.60,   z = 0.01)     (i)(x = 0.05,   y = 0.55,   z = 0.40)     (j)(x = 0.06,   y = 0.24,   z = 0.70)     (k)______________________________________ 
    
     on the triangular ternary-system diagram; and, as an additive, 0.01 to 10 mole% of lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 3 ). 
     According to a third aspect of the present invention, there is provided a ceramic composition which comprises, as a major constitutent, a ternary system essentially consisting of lead magnesium niobate (Pb(Mg 1/3  Nb 2/3 )O 3 ), lead nickel niobate (Pb(Ni 1/3  Nb 2/3 )O 3 ) and lead titanate (PbTiO 3 ) and being expressed by the following general formula: (Pb(Mg 1/3  Nb 2/3 )O 3 ) x  (Pb(Ni 1/3  Nb 2/3 )O 3 ) y  (PbTiO 3 ) z  wherein the subscripts x, y and z satisfy the following relation: x+y+z=1.0 and fall within the range defined by and be on the line segments joining the following seven points (a) to (g) which are given by the coordinates: 
     
         ______________________________________(x = 0.10,   y = 0.70,   z = 0.20)     (a)(x = 0.10,   y = 0.475,  z = 0.425)    (b)(x = 0.625,  y = 0.05,   z = 0.325)    (c)(x = 0.75,   y = 0.05,   z = 0.20)     (d)(x = 0.75,   y = 0.15,   z = 0.10)     (e)(x = 0.50,   y = 0.40,   z = 0.10)     (f)(x = 0.15,   y = 0.70,   z = 0.15)     (g)______________________________________ 
    
     on the triangular ternary-system diagram; and in which 0.01 to 30 mole% of lead ions (Pb 2+ ) of the major constituent are substituted with lanthanum ions (La 3+ ). 
     According to a fourth aspect of the present invention, there is provided a ceramic composition which comprises, as a major constituent, a ternary system: essentially consisting of lead magnesium tungstate (Pb(Mg 1/2  W 1/2 )O 3 ), lead titanate (PbTiO 3 ) and lead nickel niobate (Pb(Ni 1/3  Nb 2/3 )O 3 ) and beingexpressed by the following general formula: (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  wherein the subscripts x, y and z satisfy the following relation: x+y+z=1.0 and fall within the range defined by and be on the line segments joining the following four points (h) to (k) which are given by the coordinates: 
     
         ______________________________________(x = 0.693,  y = 0.297,  z = 0.01)     (h)(x = 0.39,   y = 0.60,   z = 0.01)     (i)(x = 0.05,   y = 0.55,   z = 0.40)     (j)(x = 0.06,   y = 0.24,   z = 0.70)     (k)______________________________________ 
    
     on the triangular ternary-system diagram; and in which 0.01 to 30 mole% of lead ions (Pb 2+ ) of the major constituent are substituted with lanthanum ions (La 3+ ). 
     According to a fifth aspect of the present invention, there is provided a ceramic composition which comprises, as a major constituent, a ternary system essentially consisting of lead magnesium tungstate [Pb(Mg 1/2  W 1/2 )O 3  ], lead titanate (PbTiO 3 ) and lead nickel niobate (Pb(Ni 1/3  Nb 2/3 )O 3 ) and being expressed by the following general formula (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  wherein the subscripts x, y and z satisfy the following relation: x+y+z=1.0 and fall within the range defined by and be on the line segments joining the following four points (h) to (k) which are given by the coordinates: 
     
         ______________________________________(x = 0.693,  y = 0.297,  z = 0.01)     (h)(x = 0.39,   y = 0.60,   z = 0.01)     (i)(x = 0.05,   y = 0.55,   z = 0.40)     (j)(x = 0.06,   y = 0.24,   z = 0.70)     (k)______________________________________ 
    
     on the triangular ternary-system diagram; and in which 0.01 to 30 mole% of lead ions (Pb 2+ ) are substituted with calcium ions (Ca 2+ ) 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     FIG. 1 is the ternary-system diagram for Pb(Mg 1/3  Nb 2/3 )O 3  -Pb(Ni 1/3  Nb 2/3 )O 3  -PbTiO 3  showing the acceptable compositional range of the major constituent of the ceramic composition according to the present invention; 
     FIG. 2 is the ternary-system diagram for Pb(Mg 1/2  W 1/2 )O 3  -PbTiO 3  -Pb(Ni 1/3  Nb 2/3 )O 3  showing the acceptable compositional range of the major constituent of the ceramic composition according to the present invention; 
     FIG. 3 is a graph showing the temperature-dependency of the dielectric constant of an embodiment of the ceramic composition according to the present invention in which x, y and z are 0.5, 0.3 and 0.2 respectively and the amount of lanthanum manganese niobate (La(Mn 2/3  Nb 1/3  O 3 ) incorporated is 0 or 10 mole%; 
     FIG. 4 is a graph showing the temperature-dependency of the dielectric constant of another embodiment of the ceramic composition according to the present invention in which x, y and z are 0.2, 0.4 and 0.4 respectively and the amount of lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 3 ) incorporated is 0 or 10 mole%; 
     FIG. 5 is a graph in which the rate of variation in capacitance observed when a DC bias is applied to a multilayer ceramic capacitor is plotted against the strength of DC electric field per layer of the capacitor (as determined in Example 3 and Comparative Example 1); 
     FIG. 6 is a graph is which the rate of variation in capacitance observed when a DC bias is applied to a multilayer ceramic capacitor is plotted against the strength of DC electric field per layer of the capacitor (as determine in Example 4 and Comparative Example 2); 
     FIG. 7 is a graph showing the temperature-dependency of the dielectric constant of a further embodiemnt of the ceramic composition according to the present invention in which x, y and z are 0.5, 0.3 and 0.2 respectively and the amount of La 3+  -substitution is 0, 10 or 30 mole%; 
     FIG. 8 is a graph showing the temperature-dependency of the dielectric constant, of a further embodiment of the ceramic composition according to the present invention in which x, y and z are 0.2, 0.5 and 0.3 respectively and the amount of La 3+  -substitution is 0, 10 or 30 mole%; 
     FIG. 9 is a graph in which the rate of variation in capacitance observed when a DC bias is applied to a multilayer ceramic capacitor is plotted against the strength of DC electric field per layer of the capacitor (as determined in Example 7 and Comparative Example 3); 
     FIG. 10 is a graph in which the rate of variation in capacitance observed when a DC bias is applied to a multilayer ceramic capacitor is plotted against the strength of DC electric field per layer of the capacitor (as determined in Example 8 and Comparative Example 4); 
     FIG. 11 is a graph showing the temperature-dependency of the dielectric constant of a further embodiment of the ceramic composition according to the present invention in which x, y and z are 0.2, 0.5 and 0.3 respectively and the amount of Ca 2+  -substitution is 0, 10 or 30 mole%; and 
     FIG. 12 is a graph is which the rate of variation in capacitance observed when a DC bias is applied to a multilayer ceramic capacitor is plotted against the strength of DC electric field per layer of the capacitor (as determined in Example 10 and Comparative Example 5). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The ternary-system diagram of the Pb(Mg 1/3  Nb 2/3 )O 3  -Pb(Ni.sub. 1/3 Nb 2/3 )O 3  -PbTiO 3  system, illustrating the acceptable compositional range of the major constituent of the ceramic composition according to one aspect of the present invention is shown in FIG. 1 and that of the Pb(Mg 1/2  W 1/2 )O 3  -PbTiO 3  -Pb(Ni 1/3  Nb 2/3 )O 3  system showing the acceptable compositional range of the major constituent of the ceramic composition according to another aspect of the present invention is shown in FIG. 2. In each of FIGS. 1 and 2, (a) to (g) or (h) to (k) are coordinates in the ternary-system diagram and the acceptable compositional range is shown as the shadowed portion including the boundary lines. 
     In the first and second aspects of the present invention, the amount of lanthanum manganese niobate: La (Mn 2/3  Nb 1/3 )O 3  to be incorporated into the cramic composition ranges from 0.01 to 10 mole% and preferabley 2 to 8 mole%. Further, in the third and fourth aspects of the present invention, the amount of Pb 2+  ions to be substituted with La 3+   ions ranges from 0.01 to 30 mole% and preferably 2 to 20 mole%. Moreover, the amount of Pb 2+  ions to be substituted with Ca 2+  ions ranges from 0.01 to 30 mole% and preferabley 2 to 20 mole%. 
     The present invention will hereinafter be described in more detail with reference to the following non-limiting working Examples and the effects practically achieved by the present invention will also be discussed in detail in comparison with Comparative Examples. 
     EXAMPLE 1 
     In this Example, there were used, as starting materials, lead oxide (PbO), magnesium oxide (MgO), niobium oxide (Nb 2  O 5 ), nickel oxide (NiO), titanium oxide (TiO 2 ), manganese carbonate (MnCO 3 ) and lanthanum oxide (La 2  O 3 ) and these starting materials were weighed so as to satisfy the compounding ratio as shown in Tables 1 to 3. These weighed starting materials were subjected to wet-milling and mixed in a ball mill, calcined at 750° to 800° C., followed by re-milling of the resulting powder in a ball mill, filtration, drying, addition of an organic binder, sizing and pressing to give cylindrical samples having a diameter of about 16 mm and a thickness of about 10 mm as well as disk-like samples having a diameter of about 16 mm and a thickness of about 2 mm. Then these samples having desired compositions were fired at a temperature ranging from 1000° to 1100° C. for one hour. 
     Silver electrodes were printed onto both faces of the fired disk-like samples at 600° C. and the capacitance thereof was determined at a frequency of 1 KHz, a voltage of 1 V r.m.s. and room temperature using a digital LCR meter to obtain dielectric constants and variation thereof with temperature. 
     To evaluate the mechanical strength of the samples in terms of the bending strength thereof, each fired cylinder was cut into 10 rectangular plates having a thickness (t) of 0.5 mm, a width (W) of 2 mm and a length of about 13 mm. The span (l) was set at 9 mm, the fracture loads P m  (kg) were determined in accordance with the three-point bending test and the bending strength τ (kg/cm 2 ) was obtained from the following relation: τ=3P m  l/2Wt 2  (kg/cm 2 ). Each bending strength obtained was an average of those for the 10 rectangular plates. 
     Tables 1 to 3 and 4 to 6 show the compounding ratio: x, y and z of the main constituent: [Pb(Mg 1/3  Nb 2/3 )O 3  ] x  [Pb(Ni 1/3  Nb 2/3 )O 3  ] y  [PbTiO 3  ] z  of the resulting ceramic composition, the amount of the additive added, the dielectric constant, the bending strength, and the ratio of the variations of dielectric constants determined at -30° C. and 85° C. In these Tables, asterisk (*) means that the compounding ratio of the major constituent of the corresponding sample is beyond the range defined in the present invention. Moreover, each variation in dielectric constant is a value relative to the dielectric constant determined at 20° C. 
     Moreover, to make clear the effect of the addition of lanthanum manganese niobate [La(Mn 2/3  Nb 1/3 )O 3  ], there is shown, in FIG. 3, the temperature-dependncy of the dielectric constants of ceramic compositions whose compounding ratio (x, y, z) was (0.5, 0.3, 0.2) and in which the amount of lanthanum manganese niobate [La(Mn 2/3  Nb 1/3 )O 3  ]was 0 and 10 mole%. 
     
                                           TABLE 1__________________________________________________________________________Compounding Ratios of Sample Nos. 1 to 20    Main Constituent Compounding                        AdditiveSample    Ratio (Mole %)           (Mole %)No. Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3         Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                   PbTiO.sub.3                        La(Mn.sub.2/3 Nb.sub.1/3)O.sub.3__________________________________________________________________________ 1* 10        70        20   02   10        70        20   0.013   10        70        20   24   10        70        20   1 5* 10        70        20   15 6* 15        60        25   07   15        60        25   0.018   15        60        25   29   15        60        25   1010* 15        60        25   1511* 15        70        15   012  15        70        15   0.0113  15        70        15   214  15        70        15   1015* 15        70        15   1516* 40        35        25   017  40        35        25   0.0118  40        35        25   219  40        35        25   1020* 40        35        25   15__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________Compounding Ratios of Sample Nos. 21 to 40    Main Constituent Compounding                        AdditiveSample    Ratio (Mole %)           (Mole %)No. Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3         Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                   PbTiO.sub.3                        La(Mn.sub.2/3 Nb.sub.1/3)O.sub.3__________________________________________________________________________ 21*    60        20        20   022  60        20        20   0.0123  60        20        20   224  60        20        20   10 25*    60        20        20   15 26*    70        20        10   027  70        20        10   0.0128  70        20        10   229  70        20        10   10 30*    70        20        10   15 31*    50        40        10   032  50        40        10   0.0133  50        40        10   234  50        40        10   10 35*    50        40        10   15 36*    60        25        15   037  60        25        15   0.0138  60        25        15   239  60        25        15   10 40*    60        25        15   15__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________Compounding Ratios of Sample Nos. 41 to 60    Main Constituent Compounding                        AdditiveSample    Ratio (Mole %)           (Mole %)No. Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3         Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                   PbTiO.sub.3                        La(Mn.sub.2/3 Nb.sub.1/3)O.sub.3__________________________________________________________________________ 41*    10        47.5      42.5 042  10        47.5      42.5 0.043  10        47.5      42.5 244  10        47.5      42.5 10 45*    10        47.5      42.5 15 46*    62.5      5         32.5 047  62.5      5         32.5 0.0148  62.5      5         32.5 249  62.5      5         32.5 10 50*    62.5      5         32.5 15 51*    75        5         20   052  75        5         20   0.0153  75        5         20   254  75        5         20   10 55*    75        5         20   15 56*    75        15        10   057  75        15        10   0.0158  75        15        10   259  75        15        10   10 60*    75        15        10   15__________________________________________________________________________ 
    
     
                       TABLE 4______________________________________Properties of Sample Nos. 1 to 20   Bending  Dielectric   Variation of Dielec-Sample  Strength Constant     tric Constant (%)No.     (kg/cm.sup.2)            (room temp.) -30° C.                                85° C.______________________________________ 1*      600     9500         -52.3  -43.22       1030     9100         -50.7  -42.13       1160     8700         -48.8  -39.54       1320     6950         -46.2  -38.1 5*      620     5100         -46.0  -37.9 6*      580     18700        -58.7  -27.17       1070     16300        -56.8  -26.28       1230     13250        -54.3  -25.19       1420     9100         -50.1  -22.710*      590     6230         -49.8  -21.911*      620     16250        -53.8  -41.212      1100     15300        -52.3  -39.913      1280     13750        -49.1  -38.514      1390     8670         -46.3  -37.215*      650     5250         -45.1  -36.616*      650     21570        -62.5  -15.817      1110     18750        -59.1  -13.918      1320     16200        -48.8  -12.119      1390     9200         -45.5  -11.320*      680     6350         -44.8  -11.1______________________________________ 
    
     
                       TABLE 5______________________________________Properties of Sample Nos. 21 to 40   Bending  Dielectric   Variation of Dielec-Sample  Strength Constant     tric Constant (%)No.     (kg/cm.sup.2)            (room temp.) -30° C.                                85° C.______________________________________ 21*     630     17500        -65.8  -43.222      1020     16300        -64.1  -42.723      1280     13150        -61.1  -40.124      1430      8800        -55.7  -35.2 25*     650      5710        -54.8  -34.5 26*     610     13750        -63.1  -48.727       980     12500        -62.7  -45.128      1190      9300        -58.5  -43.229      1350      7350        -55.2  -40.1 30*     660      5050        -55.1  -39.5 31*     590     12300        -48.7  -38.532       970     11200        -46.3  -36.133      1100      9150        -44.7  -33.634      1250      7300        -40.3  -31.1 35*     600      4800        -39.9  -30.2 36*     620     14800        -52.5  -42.337      1050     13750        -51.1  -41.638      1210     10500        -50.1  -39.739      1370     7650         -48.6  -35.5 40*     650     4700         -47.5  -34.9______________________________________ 
    
     
                       TABLE 6______________________________________Properties of Sample Nos. 41 to 60   Bending  Dielectric   Variation of Dielec-Sample  Strength Constant     tric Constant (%)No.     (kg/cm.sup.2)            (room temp.) -30° C.                                85° C.______________________________________ 41*     760     4500         -52.5  +33.542      1010     4350         -51.3  +31.043      1130     4030         -32.5  +25.344      1250     3500         -28.1   +5.3 45*     780     3200         -26.5   -5.5 46*     710     4200         -60.3  +45.247       980     4050         -59.5  +44.248      1150     3810         -55.1  +25.649      1230     3500         -48.2  -15.6 50*     810     3160         -40.2  -17.2 51*     750     6300         -55.1  +20.152      1050     6210         -54.6  +19.553      1130     6100         -51.2   -5.554      1240     5500         -48.7  -15.3 55*     780     4800         -41.2  -22.1 56*     710     18500        -35.6  -55.257       970     17800        -34.1  -52.658      1050     15200        - 31.5 -35.659      1125     13000        -30.6  -31.2 60*     820     8600         +11.2  -52.5______________________________________ 
    
     EXAMPLE 2 
     There were used, as starting materials, lead oxide (PbO), magnesium oxide (MgO), tungsten oxide (WO 3 ), niobium oxide (Nb 2  O 5 ), nickel oxide (NiO), titanium oxide (TiO 2 ), manganese carbonate (MnCO 3 ) and lanthanum oxide (La 2  O 3 ) and these starting materials were weighed so as to satisfy the compounding ratio as shown in Tables 7 to 8. Then the same procedures used in Example 1 were repeated to give cylindrical samples as well as disk-like samples and the samples were fired at a temperature rainging from 1000° to 1050° C. for one hour. 
     In the same manner used in Example 1, the dielectric constant, variation thereof with temperature and bending strength of each sample were determined. Tables 7 to 8 and 9 to 10 show the compounding ratio: x, y and z of the main constituent: (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  [Pb(Ni 1/3  Nb 2/3 )O 3 ) z  of the resulting ceramic composition, the amount of the additive added, the dielectric constant, the bending strength, and the variations of dielectric constants determined at -30° C. and 85° C. In these Tables, asterisk (*) means that the compounding ratio of the major constituent of the corresponding sample is beyond the range defined in the present invention. Moreover, each variation in dielectric constant is a value relative to the dielectric constant determined at 20° C. 
     Moreover, to make clear the effect of the addition of lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 3 ), there is shown, in FIG. 4, the temperature-dependncy of the dielectric constants of ceramic compositions whose compounding ratio (x, y, z) was (0.2, 0.4, 0.4) and in which the amount of lanthanum manganese niobate [La(Mn 2/3  Nb 1/3 )O 3  ] was 0 and 10 mole%. 
     
                                           TABLE 7__________________________________________________________________________Compounding Ratios of Sample Nos. 61 to 80    Main Constituent Compounding                        AdditiveSample    Ratio (Mole %)           (Mole %)No. Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3         Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                   PbTiO.sub.3                        La(Mn.sub.2/3 Nb.sub.1/3)O.sub.3__________________________________________________________________________ 61*    69.3       1        29.7 062  69.3       1        29.7 0.0163  69.3       1        29.7 264  69.3       1         29.7                        10 65*    69.3       1        29.7 15 66*    49.5       1        49.5 067  49.5       1        49.5 0.0168  49.5       1        49.5 269  49.5       1        49.5 10 70*    49.5       1        49.5 15 71*    19.5      35        45.5 072  19.5      35        45.5 0.0173  19.5      35        45.5 274  19.5      35        45.5 10 75*    19.5      35        45.5 15 76*    10        50        40   077  10        50        40   0.0178  10        50        40   279  10        50        40   10 80*    10        50        40   15__________________________________________________________________________ 
    
     
                                           TABLE 8__________________________________________________________________________Compounding Ratios of Sample Nos. 81 to 100    Main Constituent Compounding                        AdditiveSample    Ratio (Mole %)           (Mole %)No. Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3         Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                   PbTiO.sub.3                        La(Mn.sub.2/3 Nb.sub.1/3)O.sub.3__________________________________________________________________________ 81*    6         70        24   082  6         70        24   0.0183  6         70        24   284  6         70        24   10 85*    6         70        24   15 86*    30        30        40   087  30        30        40   0.0188  30        30        40   289  30        30        40   10 90*    30        30        40   15 91*    39         1        60   092  39         1        60   0.0193  39         1        60   294  39         1        60   10 95*    39         1        60   15 96*    5         40        55   097  5         40        55   0.0198  5         40        55   299  5         40        55   10100*    5         40        55   15__________________________________________________________________________ 
    
     
                       TABLE 9______________________________________Properties of Sample Nos. 61 to 80Bending     Dielectric   Variation of Dielec-SampleStrength    Constant     tric Constant (%)No.  (kg/cm.sup.2)            (room temp.) -30° C.                                85° C.______________________________________ 61*  650        3060         +35.2  -42.862   1020        2980         +33.8  -41.563   1250        2850         +30.5  -36.264   1380        2670         +26.5  -31.7 65*  620        1910         +26.0  -30.1 66*  680        4530         -52.5  -25.367   1010        3870         -51.1  -24.868   1125        3510         -50.8  -23.669   1280        3050         -45.7  -21.8 70*  700        2100         -45.5  -20.9 71*  580        3450         -56.5  -12.872    980        2950         -52.8  -11.773   1260        2810         -49.7  -10.874   1410        2530         -46.1   -9.3 75*  620        2050         -45.8   -9.0 76*  640        5620         -46.2  -35.577   1050        5410         -45.7  -33.878   1180        4730         -41.2  -31.579   1350        3500         -35.6  -27.2 80*  660        2400         -35.1  -26.8______________________________________ 
    
     
                       TABLE 10______________________________________Properties of Sample Nos. 81 to 100   Bending  Dielectric   Variation of Dielec-Sample  Strength Constant     tric Constant (%)No.     (kg/cm.sup.2)            (room temp.) -30° C.                                85° C.______________________________________ 81*     600     9250         -45.2  -43.882       980     9120         -42.6  -41.383      1230     8350         -39.1  -37.584      1360     6800         -36.8  -35.2 85*     610     3850         -36.1  -34.3 86*     590     9800         -42.3  -38.787      1010     9370         -40.5  -36.288      1170     8910         -37.7  -34.389      1310     6250         -33.8  -29.6 90*     630     4800         -33.1  -28.2 91*     610     4800         -65.3  +25.592       850     4750         -64.1  +23.293      1050     4500         -56.7  +25.394      1230     4320         -48.7   +1.5 95*     750     4250         -43.3  -15.8 96*     600     6050         -68.3  +35.597       810     5830         -67.5  +33.198      1030     5510         -58.2  +28.399      1260     4930         -47.6   +7.8100*     820     4610         -43.3   -5.5______________________________________ 
    
     As seen from the data shown in Tables 1 to 6, the ceramic composition comprising a ternary composition: Pb(Mg 1/3  Nb 2/3 )O 3  -Pb(Ni 1/3  Nb 2/3 )O 3  -PbTiO 3  to which 0.01 to 10 mole% of (La(Mn 2/3  Nb 1/3 )O 3 ) as an additive is added has a low variation of dielectric constant with temperature and a high bending strength and thus would be useful as a material for use in making multilayer ceramic capacitors. 
     Further, as seen from the data listed in Tables 7 to 10, the ceramic composition comprising a ternary composition: Pb(Mg 1/2  W 1/2 )O 3  -PbTiO 3  -Pb(Ni 1/3  Nb 2/3 )O 3  to which 0.01 to 10 mole% of (La(Mn 2/3  Nb 1/3 )O 3 ) as an additive is added has a low variation of dielectric constant with temperature and a high bending strength and thus would be useful as a material for use in making multilayer ceramic capacitors. 
     EXAMPLE 3 
     The same procedures used in Example 1 were repeated to give a dielectric powder having a compunding ratio: (x, y, z) in the major constituent represented by [Pb(Mg 1/3  Nb 2/3 )O 3  ] x  (Pb(Ni 1/3  Nb 2/3 )O 3 ) y  (PbTiO 3 ) z  of (0.5, 0.3, 0.2) and comprising 10 mole% of lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 3 ), as an additive. The resulting dielectric powder was dispersed in an organic solvent, kneaded with an organic binder to give a slurry and the resulting slurry was formed into a film having a thickness of 40 μm according to the doctor blade technique currently used. Then a paste for internal electrode was printed on the film in accordance with the usual screen printing method, followed by stamping out, lamination, hot-pressing to give a multilayer body which was then cut into pieces having a desired shape to obtain green chips for capacitors. The resulting green chips were heated to desired temperatures to remove the binder and to fire and then silver paste was applied thereto to form external electrodes. 
     The capacitance of the resulting capacitor was determined at room temperature while a DC bias of 0 to 50V was applied to the multilayer ceramic capacitor using a digital multimeter. The results thus obtained are plotted in FIG. 5. 
     EXAMPLE 4 
     The same procedures used in Example 1 were repeated to give a dielectric powder having a compunding ratio: (x, y, z) in the major constituent represented by (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  of (0.2, 0.4, 0.4) and comprising 10 mole% of lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 3 ) as an additive. 
     Further, in the same manner used in Example 3, a multilayer ceramic capacitor was produced and the behavior thereof upon applying a DC bias was determined in the same manner described in Example 3. The results obtained are plotted in FIG. 6. 
     COMPARATIVE EXAMPLE 1 
     The same procedures used in Example 3 were repeated except for using a composition having the compounding ratio (x, y, z) in the major constituent represented by (Pb(Mg 1/3  Nb 2/3 )O 3 ) x  (Pb(Ni 1/3  Nb 2/3 )O 3 ) y  (PbTiO 3 ) z  of (0.2, 0.6, 0.2) and free of lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 3 ) to give a capacitor and the capacitance thereof upon applying a DC bias was determined in the same manner described in Example 3. The results obtained are plotted in FIG. 5 together with the results obtained in Example 3. 
     COMPARATIVE EXAMPLE 2 
     The same procedures used in Example 3 were repeated except for using a composition having the compounding ratio (x, y, z) in the major constituent represented by (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  of (0.3, 0.4, 0.3) and free of Pb 2+  -substitution to give a capacitor and the capacitance thereof upon applying a DC bias was determined in the same manner described in Example 3. The results obtained are plotted in FIG. 6 together with the results obtained in Example 4. 
     The results shown in FIGS. 5 and 6 clearly indicate that the capacitors obtained from the lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 31 )-containing ceramic composition of the present invention show the behavior upon application of a DC bias superior to those for the capacitors obtained using the compositions of Comparative Examples 1 and 2. 
     EXAMPLE 5 
     In this Example, there were used, as starting materials, lead oxide (PbO), magnesium oxide (MgO), niobium oxide (Nb 2  O 5 ), nickel oxide (NiO), titanium oxide (TiO 2 ) and lanthanum oxide (La 2  O 3 ) and these starting materials were weighed so as to satisfy the compounding ratio as shown in Tables 11 to 13. These weighed starting materials were subjected to wet-milling and mixed in a ball mill, calcined at 750° to 850° C., followed by re-milling of the resulting powder in a ball mill, filtration, drying, addition of an organic binder, sizing and pressing to give disk-like samples having a diameter of about 16 mm and a thickness of about 2 mm. Then the samples having desired compositions were fired at a temperature ranging from 1000 to 1100° C. for one hour. 
     Silver electrodes were printed onto both faces of the fired disk-like samples at 600° C. and the capacitance and dielectric loss thereof were determined at a frequency of 1 KHz, a voltage of 1 V r.m.s. and room temperature using a digital LCR meter to obtain dielectric constants and variation thereof with temperature. Then a voltage of 50V was applied to the samples for one minute using an insulation resistivity tester to determine insulation resistivities and to hence obtain specific resistivities. 
     Tables 11 to 13 and 14 to 16 show the compounding ratio: x, y and z of the major constituent: (Pb(Mg 1/3  Nb 2/3 )O 3 ) x  (Pb(Ni 1/3  Nb 2/3 )O 3 ) y  (PbTiO 3 ) z  of the resulting ceramic composition, the amount of La 3+  -substitution (mole%), the specific resistivity and the variations of dielectric constants determined at -30° C. and 85° C. In these Tables, asterisk (*) means that the compounding ratio of the major constituent of the corresponding sample is beyond the range defined in the present invention. Moreover, each variation in dielectric constant is a value relative to the dielectric constant determined at 20° C. 
     Moreover, to make clear the effect of La 3+  -substitution, there is shown, in FIG. 7, the temperature-dependency of the dielectric constant of a ceramic composition whose compounding ratio (x, y, z) was (0.5, 0.3, 0.2) and in which the amount of La 3+  -substitution was 0, 10 and 30 mole%. 
     
                                           TABLE 11__________________________________________________________________________Compounding Ratios of Sample Nos. 1 to 20                           Amount ofMain Constituent Compounding                           SubstitutionSampleRatio (Mole %)             (Mole %)No.  Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3           Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                     PbTiO.sub.3                           La__________________________________________________________________________ 1*  10         70        20    02    10         70        20    0.013    10         70        20    104    10         70        20    30 5*  10         70        20    32.5 6*  15         60        25    07    15         60        25    0.018    15         60        25    109    15         60        25    3010*  15         60        25    32.511*  15         70        15    012   15         70        15    0.0113   15         70        15    1014   15         70        15    3015*  15         70        15    32.516*  40         35        25    017   40         35        25    0.0118   40         35        25    1019   40         35        25    3020*  40         35        25    32.5__________________________________________________________________________ 
    
     
                                           TABLE 12__________________________________________________________________________Compounding Ratios of Sample Nos. 21 to 40                           Amount ofMain Constituent Compounding                           SubstitutionSampleRatio (Mole %)             (Mole %)No.  Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3           Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                     PbTiO.sub.3                           La__________________________________________________________________________ 21* 60         20        20    022   60         20        20    0.0123   60         20        20    1024   60         20        20    30 25* 60         20        20    32.5 26* 70         20        10    027   70         20        10    0.0128   70         20        10    1029   70         20        10    30 30* 70         20        10    32.5 31* 50         40        10    032   50         40        10    0.0133   50         40        10    1034   50         40        10    30 35* 50         40        10    32.5 36* 60         25        15    037   60         25        15    0.0138   60         25        15    1039   60         25        15    30 40* 60         25        15    32.5__________________________________________________________________________ 
    
     
                                           TABLE 13__________________________________________________________________________Compounding Ratios of Sample Nos. 41 to 60                           Amount ofMain Constituent Compounding                           SubstitutionSampleRatio (Mole %)             (Mole %)No.  Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3           Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                     PbTiO.sub.3                           La__________________________________________________________________________ 41* 10         47.5      42.5  042   10         47.5      42.5  0.0143   10         47.5      42.5  1044   10         47.5      42.5  30 45* 10         47.5      42.5  32.5 46* 62.5       5         32.5  047   62.5       5         32.5  0.0148   62.5       5         32.5  1049   62.5       5         32.5  30 50* 62.5       5         32.5  32.5 51* 75         5         20    052   75         5         20    0.0153   75         5         20    1054   75         5         20    30 55* 75         5         20    32.5 56* 75         15        10    057   75         15        10    0.0158   75         15        10    1059   75         15        10    30 60* 75         15        10    32.5__________________________________________________________________________ 
    
     
                       TABLE 14______________________________________Properties of Sample Nos. 1 to 20  Specific  Dielectric   Variation of Dielec-Sample Resistivity            Constant     tric Constant (%)No.    (Ω · cm)            (room temp.) -30° C.                                85° C.______________________________________ 1*    6.5 × 10.sup.12            9500         -52.3  -43.22      1.3 × 10.sup.13            8800         -48.7  -41.53      1.5 × 10.sup.13            7650         -47.5  -38.84      2.5 × 10.sup.13            6820         -45.2  -37.5 5*    8.7 × 10.sup.12            4200         -45.1  -37.1 6*    5.3 × 10.sup.12            18700        -58.7  -27.17      1.8 × 10.sup.13            15450        -55.3  -25.98      1.5 × 10.sup.13            12300        -53.8  -24.89      3.1 × 10.sup.13            8750         -48.7  -22.510*    6.8 × 10.sup.12            3200         -48.5  -22.311*    4.3 × 10.sup.12            16250        -53.8  -41.212     1.5 × 10.sup.13            13700        -51.5  -38.213     1.8 × 10.sup.13            11400        -48.3  -37.114     1.3 × 10.sup.13            7650         -45.1  -35.515*    5.8 × 10.sup.12            4500         -44.9  -35.316*    3.8 × 10.sup.12            21570        -62.5  -15.817     1.6 × 10.sup.13            17550        -58.7  -13.218     2.3 × 10.sup.13            15300        -47.5  -11.719     3.1 × 10.sup.13            8700         -43.2  -10.520*    5.5 × 10.sup.12            4850         -42.8  -10.3______________________________________ 
    
     
                       TABLE 15______________________________________Properties of Sample Nos. 21 to 40  Specific  Dielectric   Variation of Dielec-Sample Resistivity            Constant     tric Constant (%)No.    (Ω · cm)            (room temp.) -30° C.                                85° C.______________________________________ 21*   6.8 × 10.sup.12            17500        -65.8  -43.222     1.1 × 10.sup.13            14780        -63.8  -41.223     1.9 × 10.sup.13            11500        -60.5  -38.724     2.3 × 10.sup.13             7600        -52.1  -33.1 25*   8.7 × 10.sup.12             4150        -51.9  -33.0 26*   7.3 × 10.sup.12            13750        -63.1  -48.727     1.8 × 10.sup.13            11700        -61.5  -44.528     1.7 × 10.sup.13             8630        -57.2  -42 129     3.0 × 10.sup.13             5250        -54.6  -38.8 30*   9.5 × 10.sup.12             4050        -54.4  -38.7 31*   7.7 × 10.sup.12            12300        -48.7  -38.532     1.5 × 10.sup.13            10500        -45.3  -35.233     1.3 × 10.sup.13             8750        -43.3  -32.134     2.7 × 10.sup.13             6200        -38.8  -30.5 35*   8.8 × 10.sup.12             3950        -38.6  -30.1 36*   5.8 × 10.sup.12            14800        -52.5  -42.337     1.2 × 10.sup.13            12080        -50.8  -40.738     1.8 × 10.sup.13             9700        -48.7  -38.539     2.0 × 10.sup.13             6260        -45.2  -33.1 40*   6.2 × 10.sup.12             4020        -45.0  -33.0______________________________________ 
    
     
                       TABLE 16______________________________________Properties of Sample Nos. 21 to 40  Specific  Dielectric   Variation of Dielec-Sample Resistivity            Constant     tric Constant (%)No.    (Ω · cm)            (room temp.) -30° C.                                85° C.______________________________________ 41*   5.5 × 10.sup.12            4500         -52.5  +32.542     1.8 × 10.sup.13            4800         -50.8  +31.143     2.3 × 10.sup.13            5500         -55.3  -20.144     2.5 × 10.sup.13            5200         -45.1  -50.3 45*   7.5 × 10.sup.12            5100         -43.2  -51.3 46*   7.0 × 10.sup.12            4200         -60.3  +45.247     1.1 × 10.sup.13            4400         -58.8  +43.148     1.8 × 10.sup.13            6200         -55.6  +18.149     2.5 × 10.sup.13            5200         -54.8  -11.6 50*   8.8 × 10.sup.12            4950         -50.5  -11.0 51*   3.8 × 10.sup.12            6300         -55.1  +20.152     1.5 × 10.sup.13            6400         -53.2  +16.553     1.8 × 10.sup.13            7500         -12.6  -50.554     2.3 × 10.sup.13            6800         +20.5  -56.5 55*   7.6 × 10.sup.12            6200         +22.1  -58.7 56*   5.1 × 10.sup.12            18500        -35.6  -55.257     8.9 × 10.sup.12            17500        -32.4  -56.558     2.0 × 10.sup.13            10000        -10.5  -53.859     2.5 × 10.sup.13            8500         +11.3  -50.2 60*   5.0 × 10.sup.12            8150         +12.5  -50.5______________________________________ 
    
     EXAMPLE 6 
     There were used, as starting materials, lead oxide (PbO), magnesium oxide (MgO), tungsten oxide (WO 3 ), niobium oxide (Nb 2  O 5 ), nickel oxide (NiO), titanium oxide (TiO 2 ) and lanthanum oxide (La 2  O 3 ) and these starting materials were weighed so as to satisfy the compounding ratio as shown in Tables 17 to 18. Then the same procedures used in Example 1 were repeated to give disk-like samples and the samples were fired at a temperature ranging from 1000° to 1050° C. for one hour. 
     In the same manner used in Example 1, the dielectric constant, variation thereof with temperature, dielectric loss and specific resistivity of each sample were determined. Tables 17 to 18 and Tables 19 to 20 show the compounding ratio: x, y and z of the major constituent: (Pb(Mg 1/2  W 1/2 )O 3  .sub.(PbTiO 3 ) y  (Pb(Ni 1/3  Nb2/3)O 3 ) z  of the resulting ceramic composition, the amount of La 3+  -substitution, the specific resistivity and the variations of dielectric constants determined at -30° C. and 85° C. 
     Moreover, to make clear the effect of La 3+  -substitution, there is shown, in FIG. 8, the temperature-dependncy of the dielectric constant of a ceramic composition whose compounding ratio (x, y, z) was (0.2, 0.5, 0.3) and in which the amount of La 3+  -substitution was 0, 10 and 30 mole%. 
     
                                           TABLE 17__________________________________________________________________________Compounding Ratios of Sample Nos. 61 to 80                           Amount ofMain Constituent Compounding                           SubstitutionSampleRatio (Mole %)             (Mole %)No.  Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3           Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                     PbTiO.sub.3                           La__________________________________________________________________________ 61* 69.3        1        29.7  062   69.3        1        29.7  0.0163   69.3        1        29.7  1064   69.3        1        29.7  30 65* 69.3        1        29.7  32.5 66* 49.5        1        49.5  067   49.5        1        49.5  0.0168   49.5        1        49.5  1069   49.5        1        49.5  30 70* 49.5        1        49.5  32.5 71* 19.5       35        45.5  072   19.5       35        45.5  0.0173   19.5       35        45.5  1074   19.5       35        45.5  30 75* 19.5       35        45.5  32.5 76* 10         50        40    077   10         50        40    0.0178   10         50        40    1079   10         50        40    30 80* 10         50        40    32.5__________________________________________________________________________ 
    
     
                                           TABLE 18__________________________________________________________________________Compounding Ratios of Sample Nos. 81 to 100                           Amount ofMain Constituent Compounding                           SubstitutionSampleRatio (Mole %)             (Mole %)No.  Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3           Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                     PbTiO.sub.3                           La__________________________________________________________________________ 81*  6         70        24    082    6         70        24    0.0183    6         70        24    1084    6         70        24    30 85*  6         70        24    32.5 86* 30         30        40    087   30         30        40    0.0188   30         30        40    1089   30         30        40    30 90* 30         30        40    32.5 91* 39          1        60    092   39          1        60    0.0193   39          1        60    1094   39          1        60    30 95* 39          1        60    32.5 96*  5         40        55    097    5         40        55    0.0198    5         40        55    1099    5         40        55    30100*  5         40        55    32.5__________________________________________________________________________ 
    
     
                       TABLE 19______________________________________Properties of Sample Nos. 61 to 80  Specific  Dielectric   Variation of Dielec-Sample Resistivity            Constant     tric Constant (%)No.    (Ω · cm)            (room temp.) -30° C.                                85° C.______________________________________ 61*   9.5 × 10.sup.12            3060         +35.2  -42.862     1.1 × 10.sup.13            2950         +33.0  -40.563     1.3 × 10.sup.13            2710         +28.5  -35.164     2.5 × 10.sup.13            2520         +23.1  -30.5 65*   9.8 × 10.sup.12            1800         +23.2  -29.8 66*   2.5 × 10.sup.12            4530         -52.5  -25.367     1.3 × 10.sup.13            3520         -50.5  -24.568     1.8 × 10.sup.13            3230         -48.3  -23.269     2.0 × 10.sup.13            3010         -42.1  -20.5 70*   3.5 × 10.sup.12            2070         -42.0  -20.1 71*   4.2 × 10.sup.12            3450         -56.5  -12.872     1.5 × 10.sup.13            2870         -53.2  -11.573     2.3 × 10.sup.13            2650         - 48.5 -10.874     2.6 × 10.sup.13            2430         -45.3   -9.5 75*   5.7 × 10.sup.12            1950         -44.5   -8.7 76*   4.5 × 10.sup.12            5620         -46.2  -35.577     1.7 × 10.sup.13            5320         -43.1  -32.178     2.3 × 10.sup.13            4500         -40.5  -28.879     2.8 × 10.sup.13            3200         -33.7  -24.1 80*   6.3 × 10.sup.12            2200         -33.1  -24.0______________________________________ 
    
     
                       TABLE 20______________________________________Properties of Sample Nos. 81 to 100  Specific  Dielectric   Variation of Dielec-Sample Resistivity            Constant     tric Constant (%)No.    (Ω · cm)            (room temp.) -30° C.                                85° C.______________________________________ 81*   2.8 × 10.sup.12            9250         +45.2  -43.882     2.5 × 10.sup.13            9050         -41.7  -40.283     2.8 × 10.sup.13            8200         -37.8  -36.584     3.0 × 10.sup.13            6570         -35.5  -34.8 85*   3.6 × 10.sup.12            3200         -34.8  -34.2 86*   3.5 × 10.sup.12            9800         -42.3  -38.787     2.1 × 10.sup.13            9260         -39.7  -35.588     2.5 × 10.sup.13            8770         -35.2  -32.889     2.8 × 10.sup.13            6070         -33.6  -28.8 90*   5.6 × 10.sup.12            4050         -32.8  -27.9 91*   3.5 × 10.sup.12            4800         -65.3  +25.592     2.0 × 10.sup.13            5000         -63.3  +32.593     2.5 × 10.sup.13            5250         - 53.2 +10.594     2.8 × 10.sup.13            5100         -49.8  -23.8 95*   8.2 × 10.sup.12            4700         -48.5  -22.5 96*   1.2 × 10.sup.12            6050         -68.3  +35.597     5.3 × 10.sup.12            6200         -55.3  +27.698     7.5 × 10.sup.12            7500         -50.5  +15.899     7.8 × 10.sup.12            7300         -49.8  -13.2100*   3.3 × 10.sup.12            7050         -45.5  -14.5______________________________________ 
    
     As seen from the data shown in Tables 11 to 16, the ceramic composition comprising a ternary composition: Pb(Mg 1/3  Nb 2/3 )O 3  -Pb(Ni 1/3  Nb 2/3 )O 3  -PbTiO 3  in which 0.01 to 30 mole% of lead ions (Pb 2+ ) were substituted with lanthanum ions (La 3+ ) is an excellent material for use in making multilayer ceramic capacitors having a high specific resistivity and a low variation of dielectric constant with temperature. 
     Further, as seen from the data listed in Tables 17 to 20, the ceramic composition comprising a ternary composition: Pb(Mg 1/2  W 1/2 )O 3  -PbTiO 3  -Pb(Ni 1/3  Nb 2/3 )O 3  in which 0.01 to 30 mole% of lead ions (Pb 2+ ) were substituted with lanthanum ions (La 3+ ) is likewise an excellent material for use in making multilayer ceramic capacitors having a high specific resistivity and a low variation of dielectric constant with temperature. 
     EXAMPLE 7 
     The same procedures used in Example 5 were repeated to give a dielectric powder which had a compounding ratio: (x, y, z) in the major constituent represented by (Pb(Mg 1/3  Nb 2/3 )O 3 ) x  (Pb(Ni 1/3  Nb 2/3 )O 3 ) y  (PbTiO 3 ) z  of (0.5, 0.3, 0.2) and in which 10 mole% of Pb 2+   ions were substituted with La 3+   ions. The resulting dielectric powder was dispersed in an organic solvent, kneaded with an organic binder to give a slurry and the resulting slurry was formed into a film having a thickness of 40 μm according to the doctor blade technique currently used. Then a paste for internal electrode was printed on the film in accordance with the usual screen printing method, followed by stamping out, lamination, hot-pressing to give a multilayer body which was then cut into pieces having a desired shape to obtain green chips for capacitors. The resulting green chips were heated to desired temperatures to remove the binder and to fire and then silver paste was applied thereto to form external electrodes. 
     The capacitance of the multilayer ceramic capacitor thus obtained was determined at room temperature while a DC bias of 0 to 50V was applied to the multilayer ceramic capacitor using a digital multimeter. The results thus obtained are plotted in FIG. 9. 
     EXAMPLE 8 
     The same procedures used in Example 5 were repeated to give a dielectric powder which had a compounding ratio: (x, y, z) in the major constituent represented by (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  of (0.2, 0.5, 0.3) and in which 10 mole% of Pb 2+   ions were substituted with La 3+   ions. 
     Further, in the same manner used in Example 7, a multilayer ceramic capacitor was produced and the behavior thereof upon applying a DC bias was determined in the same manner described in Example 7. The results obtained are plotted in FIG. 10. 
     COMPARATIVE EXAMPLE 3 
     The same procedures used in Example 7 were repeated except for using a composition which had a compounding ratio: (x, y, z) in the major constituent represented by (Pb(Ni 1/3  Nb 2/3 )O 3 ) x  (Pb(Ni 1/3  Nb 2/3 )O 3 ) y  (PbTiO 3 ) z  of (0.2, 0.6, 0.2) and free of La 3+  -substitution to give a capacitor and the capacitance thereof upon applying a DC bias was determined in the same manner described in Example 7. The results obtained are plotted in FIG. 9 together with the results obtained in Example 7. 
     COMPARATIVE EXAMPLE 4 
     The same procedures used in Example 7 were repeated except for using a composition which had a compunding ratio: (x, y, z) in the major constituent represented by (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  of (0.3, 0.4, 0.3) and free of substitution of Pb 2+   to give a capacitor and the capacitance thereof upon applying a DC bias was determined in the same manner described in Example 7. The results obtained are plotted in FIG. 10 together with the results obtained in Example 8. 
     The results shown in FIGS. 9 and 10 clearly indicate that the capacitors obtained from the ceramic composition of the present invention in which lead ions (Pb 2+ ) had been substituted with lanthanum ions (La 3+ ) show the behavior upon application of a DC bias superior to those for the capacitors obtained using the compositions of Comparative Examples 3 and 4. 
     EXAMPLE 9 
     There were used, as starting materials, lead oxide (PbO), tungsten oxide (WO 3 ), magnesium oxide (MgO), niobium oxide (Nb 2  O 5 ), nickel oxide (NiO), titanium oxide (TiO 2 ) and calcium carbonate (CaCO 3 ) and these starting materials were weighed so as to satisfy the compounding ratio as shown in Tables 21 to 22. These weighed starting materials were subjected to wet-milling and mixed in a ball mill, calcined at 800° to 850° C., followed by re-milling of the resulting powder in a ball mill, filtration, drying, addition of an organic binder, sizing and pressing to give cylindrical samples having a diameter of about 16 mm and a thickness of about 10 mm as well as two disk-like samples having a diameter of about 16 mm and a thickness of about 2 mm. Then these samples having desired compositions were fired at a temperature ranging from 950° to 1050° C. for one hour. Silver electrodes were print=d onto both faces of the fired disk-like samples at 600° C. and the capacitance and dielectric loss thereof were determined at a frequency of 1 KHz, a voltage of 1 V r.m.s. and room temperature using a digital LCR meter to obtain dielectric constants and the variation thereof with temperature. 
     Tables 21 to 24 show the compounding ratio: x, y and z of the major constituent: (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  of the resulting ceramic composition, the amount of Ca 2+  -substitution, dielectric constant at room temperature, dielectric loss and variations of dielectric constants determined at -30° C. and 85° C. (expressed in terms of a value relative to a dielectric constant observed at 20° C.). In these Tables, asterisk (*) means that the amount of Ca 2+  -substitution is beyond the range defined in the present invention and double asterisk (**) means that the major constituent of the corresponding sample is beyond the range defined in the present invention. 
     Moreover, to make clear the effect of the Ca 2+  -substitution, there is shown, in FIG. 11, the temperature-dependency of the dielectric constant of a ceramic composition whose compounding ratio (x, y, z) was (0.2, 0.5, 0.3) and in which the amount of Ca 2+  -substitution was 0, 10 and 30 mole%. 
     As seen from the data shown in Tables 21 to 24, the ceramic composition of the present invention comprising a ternary composition: Pb(Mg 1/2  W 1/2 )O 3  -PbTiO 3  -Pb(Ni 1/3  Nb 2/3 )O 3  in which 0.01 to 30 mole% of lead ions (Pb 2+ ) were substituted with calcium ions (Ca2+) has a high dielectric constant and a low variation of dielectric constant with temperature and can satisfy the Y5U characteristics (-30 to 85° C.; +22%, -56%) or the Y5T characteristics (-30° to 85° C.; +22%, -33%) as defined in the EIA Standards. Furthermore, the ceramic composition of the present invention can be sintered at a low temperature of not more than 1050° C. and, therefore, when it is used to form a multilayer ceramic capacitor, a cheap silver . palladium, alloy can be used for forming internal electrodes thereof. 
     
                                           TABLE 21__________________________________________________________________________Compounding Ratios of Sample Nos. 1 to 20                           Amount ofMain Constituent Compounding                           SubstitutionSampleRatio (Mole %)             (Mole %)No.  Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3           Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                     PbTiO.sub.3                           Ca__________________________________________________________________________ 1*  69.3       1.0       29.7  02    69.3       1.0       29.7  0.013    69.3       1.0       29.7  2 4** 40.0       35.0      25.0  0.01 5** 40.0       35.0      25.0  2 6*  6.0        70.0      24.0  07    6.0        70.0      24.0  28    6.0        70.0      24.0  5 9*  5.0        40.0      55.0  010   5.0        40.0      55.0  3011*  5.0        40.0      55.0  32.512*  20.0       30.0      50.0  013   20.0       30.0      50.0  1014   20.0       30.0      50.0  30 15**15.0       25.0      60.0  30 16**15.0       25.0      60.0  32.517*  39.0       1.0       60.0  018   39.0       1.0       60.0  1019   39.0       1.0       60.0  2020*  30.0       30.0      40.0  0__________________________________________________________________________ 
    
     
                                           TABLE 22__________________________________________________________________________Compounding Ratios of Sample Nos. 21 to 25                           Amount ofMain Constituent Compounding                           SubstitutionSampleRatio (Mole %)             (Mole %)No.  Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3           Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                     PbTiO.sub.3                           Ca__________________________________________________________________________21   30.0       30.0      40.0  0.0122   30.0       30.0      40.0  5 23* 10.0       50.0      40.0  024   10.0       50.0      40.0  225   10.0       50.0      40.0  10__________________________________________________________________________ 
    
     
                       TABLE 23______________________________________Properties of Sample Nos. 1 to 20  Specific  Dielectric   Variation of Dielec-Sample Resistivity            Constant     tric Constant (%)No.    (Ω · cm)            (room temp.) -30° C.                                85° C.______________________________________ 1*    2.8 × 10.sup.12            9250         +45.2  -43.82      1.2 × 10.sup.13            3240         +29.6  -41.73      2.3 × 10.sup.13            3410         +22.3  -39.6 4**   4.7 × 10.sup.12            3720         +58.6  -47.5 5**   6.5 × 10.sup.12            3140         +52.4  -41.0 6*    2.8 × 10.sup.12            9250         -45.2  -43.87      3.4 × 10.sup.12            8580         -29.0  -41.18      5.1 × 10.sup.12            6740         +15.3  -31.4 9*    1.2 × 10.sup.12            6050         -68.3  +35.510     4.7 × 10.sup.12            6230         -41.9   -4.611*    1.6 × 10.sup.12            4860         -48.6   +8.312*    2.9 × 10.sup.12            3950         -34.6  +11.513     4.6 ×  10.sup.12            5740         -30.2  -15.114     6.2 × 10.sup.12            5090         -3.8   -33.2 15**  6.8 × 10.sup.12            3740         -28.6  +45.5 16**  3.3 × 10.sup.12            3500         -25.6  +54.617*    3.5 × 10.sup.12            4800         -65.3  +25.518     5.1 × 10.sup.12            6930         -39.4  +26.419     6.9 × 10.sup.12            7160         -29.5  -17.820*    2.3 × 10.sup.12            11200        -40.8  -38.5______________________________________ 
    
     
                       TABLE 24______________________________________Properties of Sample Nos. 21 to 25  Specific  Dielectric   Variation of Dielec-Sample Resistivity            Constant     tric Constant (%)No.    (Ω · cm)            (room temp.) -30° C.                                85° C.______________________________________21     2.6 × 10.sup.12            11850        +40.2  -41.322     4.5 × 10.sup.12             7380        +12.4  -29.5 23*   1.6 × 10.sup.12             5350        -29.6  +78.824     2.5 × 10.sup.12             6200        -24.5  +47.525     4.3 × 10.sup.12            10060        -35.4  -44.6______________________________________ 
    
     EXAMPLE 10 
     The same procedures used in Example 9 were repeated to give a dielectric powder which had a compounding ratio: (x, y, z) in the major constituent represented by (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  of (0.3, 0.5, 0.2) and in which 10 mole% of Pb 2+  ions were substituted with Ca 2+  ions. The resulting dielectric powder was dispersed in an organic solvent, kneaded with an organic binder to give a slurry and the resulting slurry was formed into a film having a thickness of 40 μm according to the doctor blade technique currently used. Then a paste for internal electrode was printed on the film in accordance with the usual screen printing method, followed by stamping out, lamination, hot-pressing to give a multilayer body which was then cut into pieces having a desired shape to obtain green chips for capacitors. The resulting green chips were heated to desired temperatures to remove the binder and to fire and then silver paste was applied thereto to form external electrodes. 
     The capacitance of the multilayer ceramic capacitor thus obtained was determined at room temperature while a DC bias of 0 to 50V was applied to the multilayer ceramic capacitor using a digital multimeter. The results thus obtained are plotted in FIG. 12. 
     COMPARATIVE EXAMPLE 5 
     The same procedures used in Example 10 were repeated except for using a composition which had a compounding ratio (x, y, z) in the major constituent represented by (Pb(Mg 1/2  W 1/2 )O 3 ) x  (PbTiO 3 ) y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  of (0.3, 0.4, 0.3) and free of substitution of Pb 2+  and Ca 2+  to give a capacitor and the capacitance thereof upon applying a DC bias was determined in the same manner described in Example 10. The results obtained are plotted in FIG. 12 together with the those obtained in Example 10. 
     The results shown in FIG. 12 clearly indicate that the capacitors obtained from the ceramic composition of the present invention in which lead ions (Pb 2+ ) are substituted with calcium ions (Ca 2+ ) show the behavior upon application of a DC bias superior to that for the capacitor obtained using the composition of Comparative Example 5. 
     Incidentally, the Curie points of ceramic compositions whose compounding ratios of the major constituents are beyond the range defined in the present invention are deviated from room temperature to the temperature side extremely higher or lower than the room temperature and, therefore, such compositions suffer from such problems that the dielectric constants thereof at room temperature are very low and that the temperature-dependency of the dielectric constant is high within the practical temperature range. Moreover, if the amount of lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 3 ) to be incorporated into the ceramic composition is beyond the range defined in the present invention, the resulting composition is not applicable as a material for use in making capacitors since it suffers from such a problem that the depressing effect is too high and the capacitance is low or that the bending strength thereof is lowered. In addition, if the amount of the La 3+  -substitution is beyond the range defined in the present invention, the resulting composition is not applicable as a material for use in making capacitors since it suffers from such a problem that the depressing effect is too high and the capacitance is low, that the Curie point of the resulting ceramic composition is greatly deviated from room temperature and accordingly the dielectric constant thereof at the room temperature is very low or that the firing temperature must be raised to a high level (if the composition is fired at a temperature range of from 1050° to 1100° C., it is insufficiently fired and this leads to a decrease in the specific resistance). Further, if the amount of the Ca 2+  -substitution is beyond the range defined in the present invention, the resulting composition is not applicable as a material for use in making capacitors since it suffers from such a problem that the depressing effect is too high and the capacitance is lowered, that the Curie point of the resulting ceramic composition is greatly deviated from room temperature and accordingly the dielectric constant thereof at the room temperature is very low or that the firing temperature must be raised (if the composition is fired at 1050° C., it is insufficiently fired and this leads to a decrease in the specific resistivity). 
     The ceramic composition of the present invention has a low temperature-dependency of the dielectric constant, which can be achieved by incorporating lanthanum manganese niobate (La(Mn 2/3  Nb 1/3 )O 3 ) into the major constituent thereof: (Pb(Mg 1/2  W 1/2 )O 3 ) x  [PbTiO 3  ] y  (Pb(Ni 1/3  Nb 2/3 )O 3 ) z  or (Pb(Mg 1/3  Nb 2/3 )O 3 ) x  (Pb(Ni 1/3  Nb 2/3 )O 3 ) y  (PbTiO 3 ) z  or through the substitution of Pb 2+   ions in the major constituent with a predetermined amount of La 3+   or Ca 2+   ions. Further, the ceramic composition of the present invention shows a low decrease in the capacitance upon application of a DC bias. Thus, the ceramic composition makes it possible to provide a multilayer ceramic capacitor having excellent temperature-dependency of the dielectric constant and high reliability. Further, the firing temperature thereof is not more than 1050° to 1100° C. and this allows the use of silver-palladium alloys abundant in silver as materials for internal electrodes of capacitors and, in turn, allows the reduction in expeses for the production of internal electrodes. The composition is likewise applicable to such a multilayer ceramic capacitor as a power source-smoothing capacitor which is used while applying a DC bias since the decrease in the capacitance upon application of a DC bias is relatively small a has already been discussed above. Furthermore, the composition can provide a product having a high bending strength when lanthanum manganese niobate is incorporated, while if Pb 2+   ions are substituted with La 3+   or Ca 2+   ions, the composition can provide a product having a high specific resistivity.