Ceramic Composition of High Dielectric Constant

I. Field of the Invention

The present invention relates to a ceramic composition of a high dielectric constant which essentially consists of barium titanate and has a ceramic microstructure, and which is primarily used for a ceramic capacitor.

II. Description of the Prior Art

It is conventionally known that barium titanate as a major component have been used as materials for various ceramic capacitors. As a matter of fact, barium titanate has ferroelectric characteristics. More particularly, isometric perovskite barium titanate is obtained at a high temperature; tetragonal barium titanate where the crystal is slightly elongated along the C-axis is obtained at a temperature below 120.degree. C.; orthorhombic barium titanate is obtained at a temperature of about 0.degree. C.; and rhombic barium titanate is obtained at a temperature of about -80.degree. C. The phase transition point at the temperature of about 120.degree. C. is especially called the Curie point. Barium titanate shows paraelectric characteristics at a higher temperature than the Curie point, whereas it shows ferroelectric characteristics at a lower temperature than the Curie point. The dielectric constant of barium titanate is as high as 10,000 at the Curie point. It is thus noted that barium titanate by itself does not have a high dielectric constant at room temperature. Various compact capacitors are commercially available in which the high dielectric constant of barium titanate at about the Curie point is obtained at a temperature lower than this point so as to allow the capacitor to have a proper (electrostatic) capacitance at room temperature. The peak value of the dielectric constant at a high temperature can be shifted by additives so as to obtain the peak value at a lower temperature. These additives are called a shifter and are generally selected from a stannate such as BaSnO.sub.3, SrSnO.sub.3, CaSnO.sub.3, PbSnO.sub.3, CuSnO.sub.3, ZnSnO.sub.3 and CdSnO.sub.3 ; a zirconate such as BaZrO.sub.3, CaZrO.sub.3 and SrZrO.sub.3 ; and a titanate such as SrTiO.sub.3 and PbTiO.sub.3. The effect of the shifter is greatest for a stannate, followed by a zirconate and a titanate, in that order.

A barium titanate compound containing a shifter has been used for a ceramic capacitor of a single sheet lead type. Meanwhile, a laminate chip manufacturing technique has been recently developed, so that a dielectric sheet of 30 to 100 .mu.m thickness can be easily obtained. A laminate ceramic chip capacitor is then developed having a structure in which a set of thin dielectric sheets sandwiched between electrodes is laminated together. The laminate ceramic chip capacitors have been widely used in various types of electronic equipment. In this manner, the conventional dielectric ceramic composition is often utilized as a thin laminate dielectric sheet. However, the laminate ceramic chip capacitor has a dielectric of 10 to 20 .mu.m thickness, whereas the conventional single-sheet ceramic capacitor has a dielectric of 100 to 10,000 .mu.m thickness. As a result, the laminate ceramic chip capacitor is adversely affected by the intensity of the electric field about 5 to 10 times as much as is the conventional capacitor. For this reason, a strong demand has arised for a composition which is less dependent on voltage. Furthermore, when the dielectric layer is thin, crystal defects of the ceramic material tend to affect electrical characteristics of the capacitor. As a result, the ceramic material must have a uniform grain size and a small number of pores each having a small diameter.

In order to meet the above-mentioned demands, inventors of the present invention have proposed a ceramic composition which has a high dielectric constant, a small number of crystal defects, little dependency on voltage, and a high breakdown voltage. This ceramic composition of a high dielectric constant essentially consists of 100 mol parts of barium metatitanate (BaTiO.sub.3), (2/3)(7 .+-.1) mol parts of cerium dioxide (CeO.sub.2), and 7.+-.1 mol parts of titanium dioxide (TiO.sub.2).

This ceramic composition of a high dielectric constant has a ceramic microstructure in which a grain size and a pore diameter are small. At the same time, the ceramic composition has a high dielectric constant, a high breakdown voltage and little dependency on voltage. However, the above ceramic composition has a small disadvantage in that the composition has a relatively low insulation resistance, and the electrodes are visible through the thin dielectric layers, since the ceramic composition is vivid red, when the thin dielectric layer is alternately laminated with the electrode.

It is an object of the present invention to provide an improved ceramic composition of a high dielectric constant which solves the disadvantage presented by the above-mentioned ceramic composition.

In order to achieve the object of the present invention, manganese dioxide (MnO.sub.2) is added in a small amount to the ceramic composition which consists of 100 mol parts of BaTiO.sub.3, (2/3)(7 .+-.1) mol parts of CeO.sub.2, and 7 .+-.1 mol parts of TiO.sub.2.

In this composition, the insulation resistance is further improved, and the brightness of the ceramic color is decreased so as not to allow the palladium electrodes to be readily seen through the dielectric layers, thus improving the product image of the capacitors.

Other objects, features, and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.

FIG. 1 is a graph for explaining a change in capacitance as a function of the temperature of a ceramic composition which was proposed previously by the inventors of the present invention;

FIG. 2 is a sectional view of a laminated ceramic chip capacitor having the above-mentioned ceramic composition;

FIGS. 3A to 3C are graphs for explaining the dielectric constant .epsilon. of a square plate dielectric layer consisting of a ceramic composition of an embodiment of the present invention as a function of the amount of MnO.sub.2 added to the ceramic composition of this embodiment, the dissipation factor tan .delta. as a function of the amount of MnO.sub.2 added, and the insulation resistance IR as a function of the amount of MnO.sub.2 added, respectively; and

FIGS. 4A to 4C are graphs for explaining the dielectric constant .epsilon. of a laminated ceramic chip capacitor consisting of a ceramic composition of the present invention as a function of the amount of MnO.sub.2 added to the ceramic composition of this embodiment, the dissipation factor tan .delta. as a function of the amount of MnO.sub.2 added, and the insulation resistance IR as a function of the amount of MnO.sub.2 added, respectively.

A ceramic composition which was proposed previously by the inventors of the present invention will be described in detail below.

Barium metatitanate (BaTiO.sub.3) is prepared by mixing barium carbonate (BaCO.sub.3) and titanium dioxide (TiO.sub.2) such that a ratio of Ba to Ti is 1.000.+-.0.005:1. The resultant mixture is then calcined at a temperature of 1,100.degree. to 1,150.degree. C. and is pulverized. CeO.sub.2 and TiO.sub.2 are then added to the pulverized grains such that a ratio of Ce to Ti is 2:3. After BaTiO.sub.3 is mixed with CeO.sub.2 and TiO.sub.2, a binder is then added to the mixture of BaTiO.sub.3, CeO.sub.2 and TiO.sub.2. A square sheet is formed of the resultant mixture, and is then calcined at at temperature range of 1,250.degree. to 1,350.degree. C. Thereafter, silver electrodes are formed on the square sheet.

Table 1 shows the electrical characteristics of the square sheets when each sample is calcined at different temperatures.

As may be apparent from Table 1, composition Nos. 2, 3 and 4 can be used as materials which comply with EIA standards (Y 5V). More particularly, these compositions satisfy the required capacitance change (20.degree. C. reference) of +22% to -82% in a temperature range of -30.degree. C. to +85.degree. C. The capacitors made of the above compositions have the same dielectric constant as the dielectric constant (10,000) of a commercially available ceramic capacitor (Y 5V). The conventional compositions which have a zirconate additive and which are used for Y 5V capacitors are shown in the lowest row of Table 1. The grain size of the ceramic material which was proposed previously by the inventors of the present invention is 2 to 3 .mu.m; and a maximum pore size thereof is about 3 .mu.m. However, the grain size of the conventional ceramic material is as large as 10 to 20 .mu.m; and a maximum pore size thereof is as large as 20 .mu.m. Differences in the grain size and the pore size of the microstructures result in an improvement of electrical characteristics of a laminated ceramic chip capacitor using a thin dielectric film.

A laminated ceramic chip capacitor is prepared by using the material indicated by composition No. 3 at a calcination temperature of 1,300.degree. C. Comparison results between the ceramic chip capacitor described above and a zirconate type laminated ceramic chip capacitor are shown in Table 2. It is noted that in FIG. 2 reference numeral 1 denotes dielectric layers each having a thickness of 35 .mu.m; 2, palladium electrodes; and 3, silver electrodes.

As may be apparent from Table 2, the laminated ceramic chip capacitor made of the material indicated by the composition No. 3 has a force against bending twice that of the conventional zirconate type capacitor, and a breakdown voltage three times that of the conventional capacitor. Furthermore, according to Tables 1 and 2, the dissipation factor tan .delta. of the capacitor of the material indicated by composition No. 3 has little dependency on voltage.

However, the composition described above has a slightly low insulation resistance. Furthermore, the color of the composition is vivid red, so that the palladium electrodes of the laminated ceramic chip capacitor can be observed through the dielectric layers. As a result, there is room for improvement.

A preferred embodiment of a ceramic composition of the present invention, which solves the above-mentioned problems will be described in detail below.

Barium carbonate (BaCO.sub.3) and titanium dioxide (TiO.sub.2) were mixed such that a ratio of Ba to Ti was 1.000.+-.0.005:1. The resultant mixture was then calcined at a temperature of 1,100.degree. to 1,150.degree. C. and was pulverized to obtain barium metatitanate (BaTiO.sub.3). A hundred mol parts of BaTiO.sub.3, (2/3)(7.+-.1) mol parts of CeO.sub.3, and 7.+-.1 mol parts of TiO.sub.2 were mixed together. Various amounts of MnO.sub.2 were added respectively, to the mixture of BaTiO.sub.3, CeO.sub.3 and TiO.sub.2. The binders were respectively added to the resultant compounds to form square plates, respectively. The square plates were then calcined at a temperature of 1,250.degree. to 1,350.degree. C. Thereafter, silver electrodes were formed on each of the calcined square dielectric plates to prepare capacitors.

FIGS. 3A to 3C are graphs for explaining the electrical characteristics of the resultant dielectric plates as functions of the amounts of MnO.sub.2 added to the respective composition described above. As may be apparent from FIG. 3A, the dielectric constant .epsilon. tends to decrease gradually as an increasing amount in excess of 0.2 mol part of MnO.sub.2 is added to the composition. The dielectric loss angle tan .delta. shown in FIG. 3B increases as an increasing amount in excess of 0.5 mol part of MnO.sub.2 is added to the composition. Furthermore, the insulation resistance IR shown in FIG. 3C increases proportionally as the amount of MnO.sub.2 added to the composition increases.

It is noted that the dielectric plates described above are made of the composition indicated by composition No. 3 at a calcination temperature of 1,300.degree. C. The dissipation factor tan .delta. and the insulation resistance IR are measured in the same manner as for Table 1.

Laminated ceramic chip capacitors as shown in FIG. 2 were then prepared by using the composition which is indicated by composition No. 3 at a calcination temperature of 1,300.degree. C. and which has various amounts of MnO.sub.2 added thereto.

FIGS. 4A to 4C are graphs for explaining the electrical characteristics of the laminated ceramic chip capacitors as functions of the amount of MnO added to the composition, respectively. The hatched portions in FIGS. 4A to 4C indicate portions at which the palladium electrodes can be seen through the dielectric layer of 35 .mu.m thickness. As may be apparent from FIGS. 4A to 4C, the electrical characteristics of the capacitors are influenced more greatly than those of the square dielectric plates. The dielectric constant .epsilon. shown in FIG. 4A is greatly decreased as an increasing amount in excess of 0.2 mol part of MnO.sub.2 is added to the composition; the dielectric constant .epsilon. is not substantially changed when the amount of MnO.sub.2 does not exceed 0.2 mol part. The insulation resistance IR shown in FIG. 4C is increased by about five times when the amount of MnO.sub.2 is 0.2 mol part, as compared with the capacitor which does not contain any MnO.sub.2. The dissipation factor tan .delta. shown in FIG. 4B increases as an increasing amount of MnO.sub.2 in excess of 0.3 mol part is added.

When compositions indicated by composition Nos. 2 and 4 in Table 1 are used and No. 3 except that calcined at a temperature of 1,300.degree. C. are used, it is found that the same effects can be obtained in the same manner as in the composition indicated by composition No. 3.

The optimal amount of MnO.sub.2 falls within a range of 0.05 mol part to 0.2 mol part. When 0.05 mol part of MnO.sub.2 is added, the palladium electrodes may not be observed through the dielectric layers. Furthermore, addition of 0.2 mol part of MnO.sub.2 allows a state immediately prior to the abrupt decrease in the dielectric constant.

The average grain size of the ceramic was 2 to 3 .mu.m, and the average pore size was not more than 2 to 3 .mu.m.

Any electrical characteristics other than the dielectric constant .epsilon., the dissipation factor tan .delta., and the insulation resistance IR remain constant, independently of the addition of MnO.sub.2.

In the above example, the barium titanate is prepared using barium carbonate and titanium dioxide. However, commercially available barium titanate may also be used.

As described above, the ceramic composition of the present invention has a ceramic microstructure in which a grain size is small, only a small number of pores is present, and a pore size is also small. Furthermore, the ceramic composition has a high dielectric constant, a high breakdown voltage, and little dependency on voltage. When a small amount of MnO.sub.2 is added, the insulation (dielectric) resistance is further improved. Furthermore, although the palladium electrodes can be seen through the dielectric layers without addition of MnO.sub.2, such a drawback can be eliminated by the addition of MnO.sub.2. Thus, the composition of the present invention has a great advantage as used in laminated ceramic chip capacitors for a variety of industrial applications.