1. Field of the Invention
The present invention relates to dielectric ceramic compositions and laminated ceramic capacitors formed by using the same, and more particularly, relates to a dielectric ceramic composition for forming thin dielectric ceramic layers having a small temperature coefficient of relative dielectric constant and to a highly reliable laminated ceramic capacitor composed of the thin dielectric ceramic layers.
2. Description of the Related Art
In general, laminated ceramic capacitors have been manufactured by the method described below.
First, ceramic green sheets, which are composed of materials for forming a dielectric ceramic and are provided with a conductive material, are prepared. The conductive material is provided on surfaces of the green sheets and is formed into internal electrodes having predetermined patterns. As the materials for forming the dielectric ceramic, a material primarily composed of, for example, barium titanate is used.
Next, a plurality of ceramic green sheets including the ceramic green sheets provided with the conductive material described above are laminated to each other and is bonded together by applying heat and pressure, thereby forming an integral green laminate.
Next, this green laminate is fired, thereby forming a sintered laminate. The internal electrodes composed of the conductive material described above are formed inside this laminate.
External electrodes are formed on external surfaces of the laminate while being electrically connected to predetermined internal electrodes. The external electrodes are formed by, for example, applying a conductive paste containing powdered conductive metal and a glass frit to the external surfaces of the laminate followed by firing. A laminated ceramic capacitor is thus formed.
As the conductive material for forming the internal electrodes described above, a relatively inexpensive base metal, such as nickel or copper, has been frequently used in recent years in place of an expensive noble metal, such as platinum, palladium, or a silver-palladium alloy, in order to decrease the production cost of laminated ceramic capacitors as low as possible. However, when a laminated ceramic capacitor having internal electrodes which contain a base metal is manufactured, firing must be performed in a neutral or a reducing atmosphere in order to prevent the base metal from being oxidized during firing, and hence, a dielectric ceramic composition for forming the laminated ceramic capacitor must have reduction resistance.
As the dielectric ceramic composition having reduction resistance described above, for example, there may be mentioned a dielectric ceramic composition disclosed in Japanese Examined Patent Application Publication No. 57-42588, which is a solid solution composed of a barium titanate in which the ratio of barium to titanium is not less than 1, that is, is not less than the stoichiometric ratio; a dielectric ceramic composition disclosed in Japanese Unexamined Patent Application Publication No. 61-101459, which is a solid solution composed of barium titanate containing a rare earth oxide composed of, for example, lanthanum (La), neodymium (Nd), samarium (Sm), dysprosium (Dy), or yttrium (Y); and a BaTiO3-CaZrO3-MnO-MgO-based dielectric ceramic composition which has been disclosed in, for example, Japanese Unexamined Patent Application Publication No. 62-256422 as a composition having a smaller temperature coefficient of relative dielectric constant.
When the dielectric ceramic composition described above is fired in a reducing atmosphere, a sintered ceramic material having no semiconductive properties, that is, a dielectric ceramic layer for forming a laminated ceramic capacitor, can be obtained, whereby an inexpensive laminated ceramic capacitor having internal electrodes composed of a base metal such as nickel can be formed.
Concomitant with the recent advance of electronic technology, there has been an increasing trend toward miniaturization of electronic elements, and the miniaturization and larger capacitance of laminated ceramic capacitors have also been actively pursued. Accordingly, dielectric ceramic compositions which are formed into thin layers having a small temperature coefficient of relative dielectric constant, superior insulating properties and superior reliability have been increasingly in demand.
Although the dielectric ceramic compositions disclosed in Japanese Examined Patent Application Publication No. 57-42588 and Japanese Unexamined Patent Application Publication No. 61-101459 can form a material having a high relative dielectric constant, the sintered materials thereof have larger crystal particles, and when the thickness of dielectric ceramic layers forming a laminated ceramic capacitor is 10 xcexcm or less, the number of crystal particles existing in each dielectric ceramic layer is decreased, whereby a problem may arise in that the reliability of the laminated ceramic capacitor is decreased. In addition, since there is still another problem in that the temperature coefficient of relative dielectric constant is large, the dielectric ceramic composition described above has not satisfactory met the requirements from the market.
In contrast, the dielectric ceramic composition disclosed in Japanese Unexamined Patent Application Publication No. 62-256422 can form a material having a relatively high dielectric constant, and the sintered material thereof has small crystal particles and has also small temperature coefficient of relative dielectric constant. However, CaZrO3 and CaTiO3 formed during a firing step are likely to produce a secondary phase together with MnO and the like, and hence, there may be a problem of inferior reliability at a high temperature.
In order to solve the problems described above, a BaTiO3-rare earth oxide-Co2O3-based composition has been proposed in Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9067, and 5-9068.
However, the reliability required by the market has recently become increasingly demanding, and dielectric ceramic compositions which are formed into dielectric ceramic layers having further improved reliability have been strongly desired. In addition, thinner dielectric ceramic layers have also been increasingly required in order to obtain a larger capacitance of the laminated dielectric ceramic capacitors. Furthermore, dielectric ceramic capacitors have been increasingly used under the conditions wherein a high DC voltage is applied for a long period of time, and hence, dielectric ceramic compositions which are formed into dielectric ceramic layers having a smaller change in capacitance with time have also been desired.
In order to satisfy the requirements described above, it becomes necessary to realize highly reliable laminated ceramic capacitors having small changes in capacitance and insulating resistance with time under a high electric field even when the laminated capacitor is formed of thinner dielectric ceramic layers.
Accordingly, an object of the present invention is to provide a dielectric ceramic composition for forming a material which satisfies the B characteristic specified by Japanese Industrial Standard (JIS) and/or the X7R characteristic specified by Electronic Industries Association (EIA), which has a CR product of 5,000 xcexa9xe2x80xa2F or more, the CR product being the product of the insulating resistance and the electrostatic capacitance at an electric field of 3 kV/mm, which has stable insulating properties under conditions of high temperature and high voltage application, and which has superior reliability and a significantly small change in capacitance with time even when the dielectric ceramic composition is formed into a thin-film material. In addition, the present invention provides a laminated ceramic capacitor formed by using the dielectric ceramic composition described above.
A dielectric ceramic composition according to the present invention comprises a primary component having a perovskite structure represented by the formula ABO3; and an additive component represented by the formula a{(1xe2x88x92b)R+bV}+cM. In the formula ABO3, A is barium (Ba) which may be partly replaced with strontium (Sr) or calcium (Ca), and B is titanium (Ti) which may be partly replaced with zirconium (Zr) or hafnium (Hf). In the formula a{(1xe2x88x92b)R+bV}+cM, R is a compound containing at least one element selected from the group consisting of lanthanum (La), cesium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Th), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu); V is a compound containing vanadium (V); M is a compound containing at least one selected from the group consisting of manganese (Mn), nickel (Ni), magnesium (Mg), iron (Fe) and zinc (Zn); and a, b and c indicate the number of moles of the compounds R, V and M.
In addition, when the primary component and the additive component are represented by the formula 100ABO3+a{(1xe2x88x92b)R+bV}+cM, then 1.25xe2x89xa6axe2x89xa68.0, 0 less than bxe2x89xa60.2, 1.0 less than cxe2x89xa66.0, and a/c greater than 1.1 are satisfied.
In the above formula ABO3, the ratio of A to B is not limited to 1; however, the ratio of A to B is preferably in the range of from about 1.00 to 1.02.
Furthermore, the dielectric ceramic composition of the present invention contains an auxiliary sintering agent.
In the dielectric ceramic composition of the present invention, it is preferable that the auxiliary sintering agent mentioned above be an oxide containing at least one of boron (B) and silicon (Si), and that the content be in the range of from about 0.2 to 2.0 parts by weight with respect to 100 parts by weight of the primary component.
The present invention also provides a laminated ceramic capacitor formed by using the dielectric ceramic composition described above.
The laminated ceramic capacitor according to the present invention comprises a laminate having a plurality of dielectric ceramic layers laminated to each other and internal electrodes formed along specific interfaces between the dielectric ceramic layers; and external electrodes formed on external surfaces of the laminate while being electrically connected to specific electrodes among the internal electrodes; wherein the dielectric ceramic layers are each formed of a sintered body of the dielectric ceramic composition described above, and the internal electrodes contain a base metal as a conductive component.