Dielectric ceramic compositions

Abstract: A dielectric ceramic composition having about 97.5 to 99.95 wt% of a main component and 0.05 to 2.5 wt% of a first subcomponent, the main component having the following general formula: {100-(a+b+c+d+e)}BaTiO 3 + aBi 2 O 3 + bNb 2 O 5 + cMaO + dMbO 2 + eMc, wherein Ma is at least one element selected from the group consisting of Mg, Ca and Zn; Mb is at least one of a metal selected from the group consisting of Ti, Sn and Zr; Mc is at least one rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Dy, Ho and Er; and a, b, c, d, and e are mole % and are in the following ranges: 1.0 ≦ a ≦ 6.0, 0.5 ≦ b ≦ 4.5, 0 ≦ c ≦ 4.0, 1.5 ≦ d ≦ 15.0, and 0.5 ≦ e ≦ 5.5; and the first subcomponent comprising a glass mainly containing SiO 2 . The composition can be subjected to firing at 1,160°C or less; satisfies the X8R characteristic of the EIA standard in spite of a high dielectric constant of 1,000 or more; and exhibits a small change rate of the electrostatic capacitance toward the temperature when a bias dc voltage is applied thereto.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a dielectric ceramic composition, and in particular, relates to a dielectric ceramic composition used as a material for a monolithic ceramic capacitor or the like. Description of the Related Art Conventionally, dielectric ceramic compositions in the field of the invention and in which the main component is BaTiO 3 and the subcomponents are a bismuth compound, e.g., Bi 2 O 3 -TiO 2 , Bi 2 O 3 -SnO 2 and Bi 2 O 3 -ZrO 2 , and a rare earth metal oxide, are widely known to exhibit a small temperature dependence of its characteristics such as dielectric constant, mechanical strength and stability of the dielectric constant over wide temperature ranges. It is also reported that another dielectric ceramic composition of which the main component is BaTiO 3 and the subcomponents are Nb 2 O 3 , a rare metal oxide, and an oxide of a transition metal, e.g., Cr, Mn, Fe, Co and Ni, has a flat temperature characteristic of the dielectric constant while having the high dielectric constant of 3,000 or more. The temperature characteristic of those dielectric ceramic compositions satisfy the X7R characteristic of the EIA standard, i.e. the percentage of capacitance change in a temperature range of -55°C to +125°C is limited within ± 15% on the basis of the reference electrostatic capacitance at 25°C. Recently, ceramic monolithic capacitors have been applied to the EEC modules which electronically control an automobile's engine. These modules are implemented in an engine room in which, when the engine operates, the temperature ranges from approximately -20°C in winter to approximately +130°C in summer. Further, the temperature is likely to rise to approximately +150°C if the engine becomes overheated. Thus, conventional dielectric ceramic compositions satisfying only the X7R characteristic cannot be employed. The dielectric ceramic composition in which the main component is BaTiO 3 and the subcomponents are Nb 2 O 3 , a rare metal oxide and an oxide of a transition metal, e.g., Cr, Mn, Fe, Co and Ni, exhibits relatively small mechanical strength. Moreover, the physical characteristics of these compositions, such as dielectric constant, largely depend on the applied voltage. Accordingly, small-sized ceramic monolithic capacitors with high capacitance cannot be attained using a thin film of the dielectric composition. Further, although the dielectric ceramic compositions having BaTiO 3 as a main component and a bismuth compound as a subcomponent exhibit low dependence of their physical characteristics on the applied electric voltage and a high mechanical strength, the dependence of the dielectric constant on temperature becomes larger when the dielectric constant of the composition is made to be high. Moreover, to use those dielectric ceramic compositions for monolithic ceramic capacitors, it is necessary to increase the amount of Pb in the material which forms internal electrodes because the required firing temperature is 1,160°C or more. Such internal electrodes are high in cost. In addition, the reaction between Pd and Bi 2 O 3 increases. An example of a dielectric ceramic composition which can be used as material of a ceramic laminated capacitor is shown in EP 06 37 041 A2. SUMMARY OF THE INVENTION In accordance with the present invention, it is intended to provide a dielectric ceramic composition, which can be fired at 1,160°C or less; has a high dielectric constant of 1,000 or more; satisfies the X8R characteristic of the EIA standard, i.e. the change of the electrostatic capacitance ("TC") in a temperature range of -55°C to +150°C is limited within ± 15% on the basis of the reference electrostatic capacitance at 25°C; exhibits a high mechanical strength; and shows small bias TC which is defined as the change of electrostatic capacitance when a dc voltage having a half magnitude of the rated voltage is applied to a film of the dielectric ceramic composition of 10 to 15µ thick in accordance with the Japanese Industrial Standard ("JIS") C6429 for the RB characteristic, as small as from +15% to -40% in the above-temperature range. According to the present invention, the above mentioned object is achieved by a dielectric ceramic composition as described in claim 1. According to another aspect of the invention, the above mentioned object is achieved by a dielectric ceramic composition as described in claim 2. Preferred embodiments of the invention are laid down in dependent claims. Accordingly, a dielectric ceramic composition is provided, consisting of 97.5 to 99.95 wt% of a main component and 0.05 to 2.5 wt% of a subcomponent; the main component having the following general formula: {100-(a+b+c+d+e)}BaTiO 3 + aBi 2 O 3 + bNb 2 O 5 + cMaO + dMbO 2 + eMc, wherein Ma is at least one element selected from the group consisting of Mg and Ca; Mb is at least one metal selected from the group consisting of Ti, Sn and Zr; Mc is at least one oxide of a rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Dy, Ho and Er; and a, b, c, d, and e are mole % and are in the following ranges; 1.0 ≤ a ≤ 6.0, 0.5 ≤ b ≤ 4.5, 0 ≤ c ≤ 4.0, 1.5 ≤ d ≤ 15.0, and 0.5 ≤ e ≤ 5.5; and a first subcomponent consisting of a glass mainly containing SiO 2 , i.e., the silica is present in the greatest amount of all of the glass components. Furthermore, a dielectric ceramic composition is provided, consisting of 97.0 to 99.94 wt% of a main component, 0.05 to 2.5 wt% of a first subcomponent, and 0.01 to 0.5 wt% of a second subcomponent; the main component having the following general formula: {100-(a+b+c+d+e)}BaTiO 3 + aBi 2 O 3 + bNb 2 O 5 + cMaO + dMbO 2 + eMc, wherein Ma is at least one element selected from the group consisting of Mg and Ca; Mb is at least one metal selected from the group consisting of Ti, Sn and Zr; Mc is at least one oxide of a rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Dy, Ho and Er; and a, b, c, d, and e are mole % and are in the following ranges; 1.0 ≤ a ≤ 6.0, 0.5 ≤ b ≤ 4.5, 0 ≤ c ≤ 4.0, 1.5 ≤ d ≤ 15.0, 0.5 ≤ e ≤ 5.5; and the first subcomponent consisting of a glass mainly containing SiO 2 ; and the second subcomponent consisting of at least one oxide of an element selected from the group consisting of Cr, Mn, Fe, Co and Ni. BaO-SeO-CaO-Li 2 O-SiO 2 is an example of the glass mainly containing SiO 2 , which glass is the first subcomponent and is used as a sintering aid for reducing the firing temperature to 1,160°C or less. Other kinds of oxide glass, such as a BaO-Li 2 O-B 2 O 3 -SiO 2 glass containing boron oxide, may be used instead. Further, non-oxide glass, such as an SiO 2 -B4C glass, may be employed. When the glass contains boron oxide and aqueous forming binders are employed for the ceramic material, it is preferred to use B4C, which is stable to water, as the material for the boron oxide. In accordance with the present invention, the dielectric ceramic composition can be fired at a low temperature of 1,160°C or less and the TC thereof is satisfactorily maintained within 15% over a wide temperature range of from -55°C to 150°C, indicating a flat temperature characteristic. Thus, monolithic ceramic capacitors including the ceramic compositions can be applied to electrical machinery and apparatuses employed in places where there are great temperature changes. Moreover, since the ceramic composition has excellent mechanical strength, the occurrence of cracking or chipping of a ceramic part mounted on a printed circuit board is prevented. Further, the small bias TC allows the dielectric ceramic layer to be as thin as from 10 to 15 mm. Therefore, the ceramic monolithic capacitors can be prepared smaller in size and larger in capacitance. The above and other objects, features and advantages of the present invention will be better understood from the following detailed description of the preferred embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The main component of the dielectric ceramic composition was prepared as follows. As the starting materials, the following industrial materials were employed: BaTiO 3 ; Bi 2 O 3 ; Nb 2 O 5 ; MaO, wherein Ma is Mg, Ca or Zn; MbO 2 , wherein Mb is Ti, Sn or Zr; and Mc which is an oxide of Y, La, Ce, Pr, Nd, Sm, Dy, Ho or Er. Those starting materials were combined in amounts to satisfy the composition ratio shown in Table 1, subjected to wet grinding in a ball mill for 16 hours while blending, and dried by evaporation to give a powder mixture. The obtained powder mixture was calcinated in a zirconium container in air at 1,000°C for 2 hours, and then was ground to pass through a 200 mesh sieve. In this way, a raw powder of the main component of the ceramic composition was obtained. The first subcomponent of the dielectric ceramic composition was prepared as follows. In the present embodiment, an oxide glass having the composition of 8BaO-6SrO-6CaO-30Li 2 O-50SiO 2 (mol %) was used as the first subcomponent to control the firing temperature to 1,160°C or less. As the starting materials, the following industrial materials were employed: BaCO 3 , SrCO 3 , CaCO 3 , Li 2 O and SiO 2 . Those starting materials were combined in amounts to satisfy the foregoing composition ratio, subjected to wet grinding in a ball mill for 16 hours while blending, and dried by evaporation to obtain a powder mixture. The obtained powder mixture was placed in an aluminum melting pot at 1,300°C for 1 hour and abruptly quenched to vitrify the glass and then ground to pass through a 200 mesh sieve. In this way, a raw powder of the first subcomponent of the ceramic composition was obtained. It was then added to the raw powder of the main component in the weight ratio shown in Table 2. The second subcomponent of the dielectric ceramic composition was prepared as follows. As the starting materials, the following industrial materials were employed: Cr 2 O 3 , MnO 2 , Fe 2 O 3 , Co 2 O 3 , and NiO. To satisfy the composition ratio shown in Table 3, the second subcomponent was added to the main component having the composition of 83.5 BaTiO 3 -4.5Bi 2 O 3 -1.0Nb 2 O 5 -1.0CaO-1.0ZnO-6.0TiO 2 -0.5SnO 2 -0.5ZrO 2 -2.0Nd 2 O 3 -1.0 Dy 2 O 3 (mole %) and to which 1.0 wt% of the first subcomponent had been added. The ceramic composition shown in Table 3 indicates the combined amount of the main component and the first subcomponent. Polyvinylbutyral binders and an organic solvent, such as toluene and ethyl alcohol, were added to the resulting material, including the main, the first subcomponents, and the second subcomponents. The mixture was subjected to wet grinding in a ball mill for 16 hours while blending and was shaped into sheets by the doctor blade process to give green sheets 19 mm thick. After internal electrode patterns were printed using a Ag/Pd (70/30 wt%) paste on the green sheets, six green sheets and a summy sheet were laminated and thermally-compressed to give a laminated product. A compact of 5.5 mm by 4.5 mm by 1 mm was cut from the laminated product. The compact was then fired at a temperature shown in Tables 4 and 5 for 2 hours to obtain a 13 mm thick sintered body. Finally, a monolithic capacitor was prepared by baking a silver electrode onto the end surface of the sintered body, and the dielectric constant (ε) at room temperature, dielectric loss (tan δ), TC, and the bias TC thereof were measured. The dielectric constant (ε) and the dielectric loss (tan δ) were measured at 25°C, 1 KHz and 1 Vrms. The maximum TC (ΔC max ) which was the maximum change of the capacitance between -55°C and 150°C was measured on the basis of the reference electrostatic capacitance at 25°C. The maximum bias TC (ΔC maxB ) was measured in such a way that the electrostatic capacitance thereof was measured in the above temperature range on the basis of a reference electrostatic capacitance at 25°C at 0 V. The anti-flexure strength of the ceramic was measured by the three-point flexure test, as follows: Each of the materials corresponding to the compositions shown in Tables 1, 2 and 3 was formed into sheets and pressed to form a laminated product. A compact of 35 mm by 7 mm by 1.2 mm was cut from the laminated product. After that, the resulting compact was baked for 2 hours at the corresponding baking temperature shown in Tables 4 and 5 to provide a ceramic strip. The anti-flexure strength of each composition was evaluated as the mean value of twenty samples. Table 4 shows the result of the measurements of the compositions according to Tables 1 and 2, and Table 5 shows that of the compositions according to Table 3. In the Tables, samples marked with an asterisk (*) are outside the scope of the invention. Sample No. Main Component Wt % First Subcomponent Wt % 1 99 1 * 2 9 2 * 3 99.5 0.5 * 4 99 1 5 98.5 1.5 * 6 99.5 0.5 7 98 2 * 8 98 2 * 9 99 1 * 10 99.5 0.5 * 11 98 2 * 12 99.5 0.5 * 13 99 1 * 14 99 1 * 15 99 1 * 16 99.5 0.5 * 17 100 0 * 18 99.97 0.03 * 19 97.3 2.7 Sample No. Main Component Second Subcomponent Wt % Cr Mn Fe Co Ni 21 99.99 0 0.01 0 0 0 22 99.85 0.05 0.05 0.5 0 0 23 99.95 0 0 0 0.05 0 24 99.9 0.05 0 0 0 0.05 25 99.85 0 0.1 0 0 0.05 26 99.65 0.05 0.2 0.05 0.05 0 27 99.6 0 0.3 0 0 0.1 28 99.55 0 0.3 0 0.1 0.05 29 99.5 0.1 0.3 0 0.1 0 *30 99.45 0 0.35 0.05 0.1 0.05 Sample No. Firing Temp. °C Electric Characteristics Mechanical Characteristics ε tan δ % T C |ΔC/C| max % T C |ΔC/C| maxB % kg/cm 2 1 1140 1910 1.3 11.5 33.5 2060 * 2 1100 1800 1.4 12.1 30.0 2400 * 3 1100 1680 1.4 9.5 31.0 2180 * 4 1160 1900 1.2 12.0 31.5 1900 5 1120 1650 1.4 10.5 33.0 2200 * 6 1080 1250 1.2 6.5 28.5 2310 7 1100 1480 1.2 14.5 33.0 2100 * 8 1140 1630 1.5 13.5 31.5 2130 * 9 1160 1580 2.1 17.2 33.8 1470 * 10 1120 980 1.3 21.5 28.0 2050 * 11 1140 1620 1.7 23.0 41.0 2210 * 12 1100 2220 1.1 18.2 42.5 2300 * 13 1100 1710 1.8 22.5 31.0 2260 * 14 1140 1320 1.9 16.2 34.0 2000 * 15 1150 1450 1.3 29.3 35.5 1970 * 16 1120 1300 1.8 24.1 43.0 1800 * 17 1100 1590 1.1 22.1 35.2 2350 * 18 1160 Not Sintered at 1160°C * 19 1080 990 1.0 11.2 29.4 2150 20 1120 2000 1.3 12.1 33.0 2060 21 1120 1920 1.4 12.0 33.3 2020 22 1120 2010 1.4 13.0 34.2 1980 23 1120 1980 1.3 12.0 33.0 2050 24 1120 2050 1.4 13.0 34.0 2100 25 1120 1890 1.4 12.2 33.0 2120 26 1120 1920 1.7 12.5 32.8 2110 27 1120 1950 1.8 11.9 34.5 2130 28 1120 1890 2.0 13.2 34.8 1970 29 1120 1920 2.3 12.6 35.0 1980 * 30 1120 1800 2.6 12.2 35.0 1980 The preferable range of the amounts of the main component, the first subcomponent, and the second subcomponent are explained below. With respect to the main component, the a value which represents Bi 2 O 3 mole % is preferably in a range of from 1.0 to 6.0 mole %, and most preferably 2.5 to 5.5%. The absolute value of the maximum TC (ΔC max ) exceeds 15% and the anti-flexure strength is as small as 1,500 kg/cm 2 or less when the a value is less than 1.0 mole %, as shown in sample 9, while the maximum TC (ΔC max ) exceeds 15% and the dielectric constant (ε) is less than 1,000 when the a value is more than 6.0 mole %, as shown in sample 10. In addition, the b value which represents Nb 2 O 5 mole % is preferably in a range of from 0.5 to 4.5 mole %, and most preferably 1.5 to 4%, because the maximum TC (ΔC max ) exceeds -15% and the maximum bias TC (ΔC maxB ) exceeds -40% when the b value is less than 0.5 mole % or more than 4.5 mole %, as respectively shown in samples 11 and 12. Further, the c value which represents MaO mole % is preferably in a range of from 0 to 4.0 mole %, and most preferably 1 to 3.5%, because the maximum TC (ΔC max ) exceeds -15% when the c value is more than 4.0 mole %, as shown in sample 13, while a high dielectric constant (ε) can be obtained even if c value is 0 mole %, as shown in sample 5. The d value which represents MbO 2 mole % is preferably in a range of from 1.5 to 15.0 mole %, and most preferably 3.5 to 14%, because the maximum TC (ΔC max ) exceeds -15% when the d value is less than 1.5 mole % or more than 15.0 mole %, as respectively shown in samples 14 and 15. The e value which represents Mc mole % is preferably in a range of from 0.5 to 5.5 mole %, and most preferably 1 to 5%, because the maximum TC (ΔC max ) exceeds 15% and the maximum bias TC (ΔC maxB ) exceeds -40% when the e value is less than 0.5 mole %, as shown in sample 16, and the maximum TC (ΔC max ) exceeds -15% when the e value is more than 5.5 mole %, as shown in sample 17. With respect to the first subcomponent, the content thereof is preferably in a range of from 0.05 to 2.5 wt%, and most preferably 0.5 to 2%. This is because the firing temperature exceeds 1,160°C when the content is less than 0.05 wt%, as shown in sample 18, while the dielectric constant (ε) was less than 1,000 when the content was more than 2.5 wt%, as shown in sample 19. With respect to the second subcomponent, it was used to prevent the dielectric ceramic from being reduced. The content of the second subcomponent is preferably in the range of from 0.01 to 0.5 wt%, and most preferably 0.1 to 0.4%. This is because reduction cannot be prevented effectively if the content is less than 0.01 wt%, while the dielectric loss (tan δ) exceeds 2.5% when the content is more than 2.5 wt%, as shown in sample 30. In the foregoing examples, the first subcomponent premixed according to a prescribed composition ratio was subjected to a high temperature in order to be melted and then ground to give a glass powder. The resulting glass powder was added to the main component of the ceramic composition. The first subcomponent may also be added to the main component as a modified material obtained by heating, but not melting, the starting materials premixed according to a prescribed composition ratio. Moreover, each constitutional element of the first subcomponent may be separately added to the main component in an appropriate state, such as a metal alkoxide, and may be baked to become melted and vitrified. In the foregoing examples, although the second subcomponent was added in an oxide form from the beginning, a material which changes to an oxide in the calcination or firing process, for example, a carbonate of each element, may be used as a starting material.

Claims: A dielectric ceramic composition consisting of 97.5 to 99.95 wt% of a main component and 0.05 to 2.5 wt% of a first subcomponent consisting of a glass mainly containing SiO 2 , said main component having the following general formula {100-(a+b+c+d+e)}BaTiO 3 + aBi 2 O 3 + bNb 2 O 5 + cMaO + dMbO 2 + eMc, wherein Ma is at least one element selected from the group consisting of Mg and Ca; Mb is at least one of a metal selected from the group consisting of Ti, Sn and Zr; Mc is at least one rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Dy, Ho and Er; and a, b, c, d, and e are mole % and are in the following ranges: 1.0 ≤ a ≤ 6.0 0.5 ≤ b ≤ 4.5 0 ≤ c ≤ 4.0 1.5 ≤ d ≤ 15.0 0.5 ≤ e ≤ 5.5. A dielectric ceramic composition consisting of 97.0 to 99.94 wt% of a main component, 0.05 to 2.5 wt% of a first subcomponent and 0.01 to 0.5 wt% of a second subcomponent, said main component having the following general formula {100-(a+b+c+d+e)}BaTiO 3 + aBi 2 O 3 + bNb 2 O 5 + cMaO + dMbO 2 + eMc, wherein Ma is at least one element selected from the group consisting of Mg and Ca; Mb is at least one of a metal selected from the group consisting of Ti, Sn and Zr; Mc is at least one rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Dy, Ho and Er; and a, b, c, d, and e are mole % and are in the following ranges: 1.0 ≤ a ≤ 6.0 0.5 ≤ b ≤ 4.5 0 ≤ c ≤ 4.0 1.5 ≤ d ≤ 15.0 0.5 ≤ e ≤ 5.5, and said first subcomponent consisting of a glass mainly containing SiO 2 , and said second subcomponent consisting of at least one oxide of an element selected from the group consisting of Cr, Mn, Fe, Co and Ni. A dielectric ceramic composition according to claim 2 containing 0.1 to 0.4 wt% of said second subcomponent. A dielectric ceramic composition according to one of the preceding claims containing 2.5 ≤ a ≤ 5.5 1.5 ≤ b ≤ 4 1 ≤ c ≤ 3.5 3.5 ≤ d ≤ 14 1 ≤ e ≤ 5. A dielectric ceramic composition according to one of the preceding claims containing 0.5 to 2 wt% of said first subcomponent. A dielectric ceramic composition according to claim 5 wherein at least one of Ma, Mb and Mc is a mixture of at least two members of the group which constitutes it.

Inventors: SHIBATA MASAMITSU, SHIBATA, MASAMITSU
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1996
Main classification: H01B3
IPC codes: H01G4, H01B3, C04B35