Ceramic dielectric composition

A ceramic dielectric compositon is provided which consists essentially of a sintered metal oxide mixture expressed by the formula: EQU [(Ba.sub.1.sub.-u Me.sub.u)O].sub.x.[(Ta.sub.1.sub.-v Nb.sub.v).sub.2 O.sub.5 ].sub.y.[Bi.sub.2 O.sub.3 ].sub.z where Me is at least one divalent metal selected from lead and strontium, and x, y, z, u and v are numbers falling within the following ranges EQU 0.05 .ltoreq. x .ltoreq. 0.80, EQU 0.05 .ltoreq. y .ltoreq. 0.30, EQU 0.10 .ltoreq. z .ltoreq. 0.80, EQU 0 .ltoreq. u .ltoreq. 1.00, EQU 0 .ltoreq. v .ltoreq. 1.00 and EQU x + y + z = 1.0. This composition may contain specified metal oxide additives. The ceramic dielectric composition is sintered at a relatively low temperature and exhibits a dielectric constant varying linearly depending upon temperature variations over a wide use range, a reduced dielectric loss, enhanced resistivity and breakdown voltage, and therefore, is particularly suitable for "temperature-compensating" capacitors of a compact type with large capacity.

This invention relates to a ceramic dielectric composition and more 
particularly to an improved ceramic dielectric composition, which is 
characterized by being sintered at a relatively low temperature, i.e., 
from approximately 700.degree. C to approximately 900.degree. C or so, 
exhibiting a dielectric constant varying linearly depending upon 
temperature variations over a wide use range, a reduced dielectric loss 
and enhanced resistivity and breakdown voltage. This ceramic dielectric 
composition is particularly suitable for "temperature-compensating" 
capacitors of a compact-type with large capacity such as laminar 
capacitors. 
As ceramic dielectrics used in temperature-compensating capacitors, those 
which exhibit an enhanced dielectric constant, a reduced dielectric loss 
factor and a linear dependence of the dielectric constant upon temperature 
variations over a wide use range are required. Most ceramic dielectrics 
heretofore proposed for temperature-compensating capacitors contain as the 
basic ingredient titanium dioxide (TiO.sub.2) such as SrTiO.sub.3, 
CaTiO.sub.3, MgTiO.sub.3 or La.sub.2 O.sub.3.TiO.sub.2. Suitable sintering 
temperatures of these ceramic dielectric compositions are generally 
1,200.degree. to 1,400.degree. C. 
Recently, a ceramic capacitor which is more compact and of more improved 
capacity and possesses a high reliability has been desired in the field 
including the communication industry. Thus, a capacitor of a thin ceramic 
film having 0.1 to 0.2 mm thickness and a ceramic laminar capacitor 
composed of a plurality of superimposed laminae each having a thickness of 
approximately 50 microns or less have been put to practical use. 
However, such laminar ceramics of the known TiO.sub.2 -base type are not 
satisfactory from the following points of view. First, since suitable 
sintering temperatures are high, the dielectric bodies are liable to be 
distorted and poor in yield. Second, laminar internal electrodes used in 
the laminar capacitor must be made of metals having a high melting point 
such as platinum or a platinum-palladium alloy and are, therefore, costly. 
It is an object of the present invention to provide ceramic dielectric 
compositions having optimum sintering temperatures of approximately 
600.degree. to approximately 950.degree. C, i.e. lower to a considerable 
extent than those of the known TiO.sub.2 -base type dielectrics. 
It is another object of the present invention to provide ceramic dielectric 
compositions having a dielectric constant varying linearly depending upon 
temperature variations over a broad use range. 
It is still another object of the present invention to provide ceramic 
capacitors of enhanced reliability, i.e. improved insulation resistance 
and breakdown voltage. 
It is a further object of the present invention to provide ceramic 
dielectric compositions particularly suitable for a 
temperature-compensating capacitor of a compact and laminar type. 
Other objects and advantages will be apparent from the following 
description. 
In accordance with the present invention, there is provided a ceramic 
dielectric composition consisting essentially of sintered metal oxide 
mixture expressed by the formula: 
EQU [(Ba.sub.1.sub.-u Me.sub.u)O].sub.x.[(Ta.sub.1.sub.-v Nb.sub.v).sub.2 
O.sub.5 ].sub.y.[Bi.sub.2 O.sub.3 ].sub.z 
where Me is at least one divalent metal selected from the group consisting 
of lead and strontium, and x, y, z, u and v are numbers falling within the 
following ranges 
EQU 0.05 .ltoreq. x .ltoreq. 0.80, 
EQU 0.05 .ltoreq. y .ltoreq. 0.30, 
EQU 0.10 .ltoreq. z .ltoreq. 0.80, 
EQU 0 .ltoreq. u .ltoreq. 1.00, 
EQU 0 .ltoreq. v .ltoreq. 1.00 and 
EQU x + y + z = 1.0. 
If the ratio of x:y:z is 1:1:1 in the abovementioned formula, the ceramic 
dielectric composition is a ferroelectric substance which is popularly 
known as one of the Bi-Layer type dielectrics represented by the formula 
A.sup.2.sup.+ Bi.sub.2 R.sup.5.sup.+ O.sub.9, where A.sup.2.sup.+ and 
R.sup.5.sup.+ are divalent and pentavalent metals, respectively. It has 
been found that, when y of the above-mentioned formula falls within the 
range of approximately 0.30 to approximately 0.33, the ceramic dielectric 
composition is comprised of mixed crystals of a ferroelectric substance 
and a paraelectric substance. This ceramic dielectric composition is of 
little or no practical use because they exhibit large dielectric loss. 
Further, it has been found that, when y of the above-mentioned formula is 
not more than approximately 0.30, the ceramic dielectric composition is 
comprised of a single crystal and paraelectric, and the optimum sintering 
temperature is low, i.e. approximately 600.degree. to approximately 
950.degree. C. From this finding, the ceramic dielectric composition of 
the invention has been completed. 
When x of the above-mentioned formula is less than 0.05, variations in 
dielectric constant of the dielectrics depending upon temperature 
variations are non-linear. In contrast, when x exceeds 0.80, the 
dielectrics are poor in resistivity. Therefore, x should be within the 
range of 0.05 to 0.80, preferably 0.20 to 0.60. 
The optimum sintering temperature of the dielectrics of the above-mentioned 
formula becomes low with a decrease of y. However, when y is too small, 
the dielectrics are poor in resistivity. In contrast, when y is too large, 
the dielectrics exhibit undesirably large dielectric loss. The range of y 
is from 0.05 to 0.30, preferably from 0.10 to 0.25. In such range, the 
dielectrics exhibit a resistivity of the order of 10.sup.13 to 10.sup.15 
ohm-cm, which is far larger than those of the known TiO.sub.2 -base type. 
This high resistivity results in enhancement in breakdown voltage. The 
dielectrics with y in the range of 0.05 to 0.30 possess a low sintering 
temperature of approximately 600.degree. to approximately 950.degree. C, 
and the permissible sintering temperature range is wide, i.e. 
approximately 50.degree. to 100.degree. C. Further, the dielectrics with y 
in the range of 0.05 to 0.30 exhibit a low dielectic loss factor of 
approximately 0.5 .times. 10.sup..sup.-4 to approximately 12 .times. 
10.sup..sup.-4. 
When z of the above-mentioned formula is less than 0.10, variations in 
dielectric constant depending upon temperature variations are non-linear. 
In contrast, when z is more than 0.80, the dielectrics are poor in 
sintering characteristics. The preferable range of z is from 0.20 to 0.65. 
Both u and v of the above-mentioned formula may be varied within the range 
of 0 to 1.0. In other words, the ratio of BaO to PbO and/or SrO and the 
ratio of Ta.sub.2 O.sub.5 to Nb.sub.2 O.sub.5 may be optionally varied. 
When both u and v are zero, i.e. the dielectrics are of the formula; 
[BaO].sub.x . [Ta.sub.2 O.sub.5 ].sub.y . [Bi.sub.2 O.sub.3 ].sub.z, and 
x, y and z are within the hereinbefore-mentioned ranges, the temperature 
coefficient of dielectric constant (.epsilon..TC) is positive. However, 
the temperature coefficient of dielectric constant can be approximately 
zero or negative by partially substituting PbO and/or SrO for BaO and 
Nb.sub.2 O.sub.5 for Ta.sub.2 O.sub.5, respectively. Particularly, partial 
substitution of Nb.sub.2 O.sub.5 for Ta.sub.2 O.sub.5 reduces the 
dependence of the temperature coefficient .epsilon..TC upon the variation 
in the ratio of BaO to PbO and/or SrO in the ingredient (Ba.sub.1.sub.-u 
Me.sub.u)O, and therefore, results in dielectrics exhibiting a temperature 
coefficient .epsilon..TC of approximately zero, even if the ratio of BaO 
to PbO and/or SrO is varied over a relatively broad range. For this 
advantage and the desired dielectric loss, preferable ranges of u and v 
are from 0.2 to 0.8 and from 0 to 0.6, respectively. 
The ceramic dielectric composition of the invention may contain a minor 
amount of an additive metal oxide selected from titanium oxide, chromium 
oxide, molybdenum oxide and tungsten oxide. The ceramic dielectric 
composition having such an additive is advantageous over the fundamental 
dielectric composition of the hereinbefore-mentioned formula in that 
capacitors made of the former composition possess an improved 
load-durability of insulation resistance at a high temperature. 
Laminar capacitors made of the fundamental dielectric composition having no 
additive exhibit an insulation resistance reduction to an extent such 
that, when they are continuously used at a temperature of 125.degree. C 
and a voltage of 100V over a period of 1,000 hours, the initial insulation 
resistance on the order of 10.sup.13 ohm decreases to on the order of 
10.sup.12 ohm. In contrast, capacitors made of the dielectrics having the 
additive exhibit little or no reduction in the insulation resistance even 
after continuous use of 10,000 hours. 
The amount of the additive is preferably not more than 4% by weight, more 
preferably within the range of 0.05% to 1% by weight, based on the weight 
of the fundamental dielectric composition of the hereinbefore-mentioned 
formula and expressed in terms of the amount of titanium dioxide 
(TiO.sub.2), chromic oxide (Cr.sub.2 O.sub.3), molybdenum trioxide 
(MoO.sub.3) and tungsten trioxide (WO.sub.3), respectively. The load 
durability of insulation resistance is improved by the addition even in a 
trace amount, but becomes satisfactory by the addition of at least 0.05% 
by weight. When the amount of the additive is larger than 1% by weight, 
said load durability is approximately the same level, and when it exceeds 
4% by weight, the optimum sintering temperature becomes undesirably high. 
The above-mentioned additives may be used either alone or in combination. 
Of the additives, titanium oxide is most preferable because the resulting 
ceramic dielectrics consist of micro crystal grains. 
The ceramic dielectric compositions of the present invention may be 
prepared as follows. Finely divided particles or powders of the respective 
metal oxides are blended with each other and shaped into a suitable form 
such as tablets. The shaped product is pre-sintered at a temperature of 
approximately 500.degree. C to 800.degree. C for 0.5 to 5 hours. The 
pre-sintered product is pulverized into fine particles or powders and 
shaped into a suitable form using or not using a binder. Then, the shaped 
product is sintered at a temperature of 600.degree. C to 950.degree. C for 
0.5 to 8 hours. If desired, the aforesaid pre-sintering and the subsequent 
pulverization may be repeated prior to the sintering. 
Instead of metal oxides, the respective metal compounds may be used which 
are in the form of carbonate salts, organic acid salts and hydroxides.