Dielectric ceramic composition

A dielectric ceramic composition composed mainly of lead magnesium-niobate Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 !, strontium titanate SrTiO.sub.3 !, and lead titanate PbTiO.sub.3 !, characterized in that the molar ratio of said three major components is defined by PA1 Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 !.sub.x .multidot.SrTiO.sub.3 !.sub.y .multidot.PbTiO.sub.3 !.sub.z (where x+y+z=100 parts by mole, and each value of x, y, and z is on lines or within the area enclosed by lines passing through four points A (72, 10, 18), B (76, 5, 19), C (57, 5, 38), and D (48, 20, 32) in a triangular composition diagrams), and said three major components are supplemented by (Pb.sub.1-x Ba.sub.x) (Cu.sub.1/2 W.sub.1/2)O.sub.3 (where 0.ltoreq.x.ltoreq.1) in an amount less than about 5 parts by mole (excluding 0 part by mole) for 100 parts by mole of said major components. This dielectric ceramic composition can be sintered at low temperatures and has a high permittivity and a high mechanical strength.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a dielectric ceramic composition and, more 
particularly, to such a composition which is to be used as a raw material 
for laminated ceramic capacitors. 
2. Description of the Related Art 
A dielectric ceramic compound based on barium titanate (BaTiO.sub.3) has 
been in general practical. However, it suffers the disadvantage of 
requiring firing at high temperatures (usually 1300.degree.-1400.degree. 
C.). When used for laminated capacitors, needed are internal electrodes of 
expensive precious metals (such as platinum and palladium) which withstand 
the high firing temperature. This leads to high production costs. 
Therefore, cost reduction of laminated ceramic capacitor requires a 
dielectric ceramic composition capable of being fired at low temperatures 
so that inexpensive metals (based on silver and nickel) can be used for 
the internal electrodes. 
The basic electrical properties required of dielectric ceramic compositions 
include high permittivity, low dielectric loss and high insulation 
resistance. In addition, high mechanical strength is another requirement 
for dielectric ceramic compositions. This is true particularly in the case 
where they are used for laminated chip capacitors, which are subject to 
mechanical strain when mounted on a substrate because of difference in 
thermal expansion between the substrate and the dielectric ceramic 
composition constituting chip capacitors. Such strain causes cracking and 
damage to chip capacitors. Thus, chip capacitors depend for their 
reliability on the mechanical properties of the dielectric ceramic 
composition from which they are made. 
Such being the case, there has been a demand for development of a 
dielectric ceramic composition which is capable of sintering at low 
temperatures and is superior in mechanical strength. 
Japanese Patent Laid-open No. 21850/1980 discloses a dielectric ceramic 
composition capable of sintering at low temperatures, which is a 
two-component composition composed of Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 and 
PbZrO.sub.3. This composition, however, has a high dielectric loss and a 
low specific resistance, which need to be remedied. To overcome these 
disadvantages, there has been proposed a three-component composition 
containing Pb(Mn.sub.2/3 W.sub.1/3)O.sub.3 as a third component (Japanese 
Patent Laid-open No. 23058/1980). This three-component composition, 
however, is poor in flexural strength and hence is used only for laminated 
capacitors intended for certain limited uses. Another ternary dielectric 
ceramic composition composed of Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3, 
Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3, and PbTiO.sub.3 has been disclosed in 
Japanese Patent Laid-open No. 60670/1983. In common with the 
above-mentioned dielectric ceramic composition, it is not satisfactory in 
mechanical strength. 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a dielectric ceramic 
composition which can be sintered at low temperatures and has a high 
permittivity and sufficient mechanical strength. 
The gist of the present invention resides in a dielectric ceramic 
composition composed mainly of lead magnesium-niobate Pb(Mg.sub.1/3 
Nb.sub.2/3)O.sub.3 !, strontium titanate SrTiO.sub.3 !, and lead titanate 
PbTiO.sub.3 !, characterized in that the molar ratio of said three major 
components is defined by 
Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 !.sub.x .multidot.SrTiO.sub.3 !.sub.y 
.multidot.PbTiO.sub.3 !.sub.z 
(where x+y+z=100 parts by mole, and each value of x, y, and z is on lines 
or within the area enclosed by lines passing through four points A (72, 
10, 18), B (76, 5, 19), C (57, 5, 38), and D (48, 20, 32) on a triangular 
composition diagram), and said three major components are supplemented by 
(Pb.sub.1-a Ba.sub.a) (Cu.sub.1/2 W.sub.1/2)O.sub.3 (where 
0.ltoreq.a.ltoreq.1) in a positive amount of less than about 5 parts by 
mole (i.e., excluding 0 part) per 100 parts by mole of said major 
components. 
The dielectric ceramic composition of the present invention can be sintered 
at low temperatures and has good dielectric properties and high mechanical 
strength, not achieved before. It has a flexural strength higher than 120 
MPa. Because of this high strength, it finds use as a raw material for 
laminated chip capacitors. Some samples of this invention have a flexural 
strength higher than 130 MPa and a relative permittivity higher than 9000 
and meet the D characteristics prescribed by JIS. 
These and other objects and advantages of the invention may be readily 
ascertained by referring to the following description and appended 
drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Each sample was prepared from Pb.sub.3 O.sub.4, MgO, Nb.sub.2 O.sub.5, 
TiO.sub.2, SrCO.sub.3, CuO, and WO.sub.3 (technical grade) as starting 
materials. They were weighed such that the composition shown in Table 1 
was established. They were mixed by ball-milling in deionized water (as a 
solvent) for 16 hours. The resulting powder was calcined in the atmosphere 
at 750.degree. C. for 2 hours. The calcined powder was mixed with an 
organic solvent (such as toluene), a binder (such as PVB), and a 
plasticizer (such as DOP) to give a slurry. The slurry was formed into a 
green sheet (50-60 .mu.m thick) by the doctor blade method. The green 
sheet was fabricated into a discoid specimen (1.0 mm thick, 10 mm in 
diameter) and a rectangular parallelepipedic specimen (45 mm long, 5 mm 
wide, 1.5 mm thick). These specimens were freed of binder in air and then 
fired in air at 950.degree.-1100.degree. C. Thus there were obtained the 
desired ceramic samples. 
TABLE 1 
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Flexural 
Relative 
Sample 
Composition (parts by mole) 
strength 
permit- 
TCC (%) 
No. PMN 
ST PT PBCW .sigma.(m value) 
tivity 
-25.degree. C. 
85.degree. C. 
__________________________________________________________________________ 
1* 80 0 20 5 98 (8.3) 
4800 -25.7 
111 
2* 76 5 19 5 131 (4.3) 
11300 
-43.8 
-13.3 
3 72 10 18 5 135 (6.4) 
8400 -16.6 
-49.7 
4 74.4 
7 18.6 
5 132 (6.8) 
9400 -29.5 
-28.2 
5* 60 0 40 5 92 (12.6) 
2100 -14.7 
36 
6 57 5 38 5 129 (11.8) 
2800 -21 53.7 
7 54 10 36 5 128 (7.9) 
3500 -23.4 
97.8 
8 51 15 34 5 152 (6.4) 
5600 -33.6 
54.3 
9 48 20 32 5 138 (9.9) 
4700 1.3 -37.4 
10 49.2 
18 32.8 
5 145 (8.5) 
5300 -15.8 
7.5 
11 63 10 27 5 133 (6.9) 
10000 
-45 -20 
12* 50 10 40 5 90 (7.4) 
3000 -30 90 
13* 60 20 20 5 115 (9.5) 
5000 -5 -32 
14* 80 10 10 5 105 (8.3) 
11000 
-50 -10 
15* 63 10 27 0 not sinterable even at 1200.degree. C. 
16* 63 10 27 10 90 (7.9) 
9500 -41 -26 
17 63 10 27 5 138 (7.1) 
9800 -42 -23 
18 63 10 27 5 131 (7.6) 
9900 -39 -21 
__________________________________________________________________________ 
*Samples not conforming to the invention. 
PMN: Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 
ST: SrTiO.sub.3 
PT: PbTiO.sub.3 
PBCW: (Pb.sub.1-a Ba.sub.a)(Cu.sub.1/2 W.sub.1/2)O.sub.3 
After coating with silver paste on both sides and baking at 800.degree. C. 
in air, the discoid specimen was tested for dielectric properties. The 
relative permittivity (.epsilon..sub.r) was measured at 25.degree. C. for 
a frequency of 1 kHz and voltage of 1 V (rms). The temperature 
characteristics of electrostatic capacity (TCC) was expressed in terms of 
change (%) in the capacity at -25.degree. C. and 85.degree. C. relative to 
the capacity at 20.degree. C. The capacity was measured for a frequency of 
1 kHz and voltage of 1 V (rms). 
The rectangular specimen was used to measure flexural strength by 
three-point bending test in the following manner. The specimen is placed 
on two supports, 30 mm apart, and bent by advancing the crosshead at a 
rate of 0.5 mm/min. The flexural strength (.sigma.) is calculated from the 
formula below. 
EQU .sigma.=3.times.P.times.L/2.times.W.times.t.sup.2 
where 
P: breaking load 
L: distance between supports 
W: width of the specimen 
t: thickness of the specimen 
The value of flexural strength is an average of 15-20 measurements 
calculated from Weibull distribution. 
The flexural strength and dielectric properties of the specimens are 
summarized in Table 1. 
Samples Nos. 1 to 16 contain PBCW in which a=0. Sample No. 17 contains PBCW 
in which a=0.5. Sample No. 18 contains PBCW in which a=1. 
The flexural strength (.sigma.) is expressed in terms of MPa, and "m value" 
is obtained from the Weibull distribution. 
The composition of each sample is plotted in the triangular composition 
diagram in mole percent (FIG. 1). Numerals in the triangular coordinates 
correspond to the sample numbers. 
According to the present invention, the composition is specified as 
mentioned above. The reason for this is explained with reference to Table 
1 and FIG. 1. Compositions which are plotted under the line B-C give rise 
to a flexural strength lower than 100 MPa, as in samples Nos. 1 and 5. 
Compositions which are plotted outside the lines C-D and A-B give rise to 
flexural strengths of 90 MPa and 105 MPa as in samples Nos. 12 and 14. 
Such values are similar to those of conventional lead-based dielectric 
ceramic compositions. Compositions which are plotted above the line D-A 
give rise to a flexural strength of 115 MPa, as in sample No. 13. This 
value is not high enough for the present invention. In the case where the 
auxiliary component is not added, as in sample No. 15, the sample cannot 
be sintered even at 1200.degree. C. In other words, the sample does not 
take on the property that the lead-based dielectric ceramic composition 
can be sintered at low temperatures. In the case where the auxiliary 
component is added in an excess amount (more than about 5 parts by mole), 
as in sample No. 16, the sample is extremely poor in flexural strength (90 
MPa). 
As Table 1 shows, sample No. 4 has a relative permittivity of 9400 and a 
flexural strength of 132 MPa and sample No. 10 has a relative permittivity 
of 5300 and a flexural strength of 145 MPa. Moreover, the values of TCC 
suggest that both samples satisfy the D characteristics prescribed in JIS. 
This is noteworthy in view of the fact that there have never been 
conventional lead-based dielectric ceramic compositions which exhibit such 
high performance. 
In preferred embodiments, x is about 48-74.2, y is about 5-20, more 
preferably about 7-18, and z is about 18-38, more preferably about 
18.6-36.