Ceramic capacitor and manufacturing method therefor

A ceramic capacitor made from a ceramic mainly containing a CaZrO 3 -CaTiO 3 solid solution exhibiting a high solid solubility is provided. The powder X-ray diffraction pattern of the ceramic satisfies conditions: (X-ray intensity of valley D)/(X-ray intensity of peak B)<0.2; and (X-ray intensity of valley E)/(X-ray intensity of peak B)<0.2, wherein the peak B is assigned to the ( 121 ) plane of the CaZrO 3 -CaTiO 3 solid solution at approximately 32.0°, the valley D lies at approximately 31.8° between a peak A which is assigned to the ( 200 ) plane of the CaZrO 3 -CaTiO 3 solid solution detected at approximately 31.6° and the peak B, and the valley E lies at approximately 32.2° between the peak B and a peak C which is assigned to the ( 002 ) plane of the CaZrO 3 -CaTiO 3 solid solution detected at approximately 32.4°. A manufacturing method therefor is also provided.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiments of a ceramic capacitor and a manufacturing method therefor according to the present invention will be described below with reference to the drawings by way of Examples. 
 EXAMPLE 1 First, CaCO 3 , ZrO 2 and TiO 2 , which were the starting materials for dielectric ceramic green sheets 1 shown in FIG. 1 , were prepared and were weighed to achieve a CaZrO 3 /CaTiO 3 molar ratio of 6:4. Subsequently, the starting materials were mixed with a binder and an organic solvent for several hours and were pulverized by a wet process to prepare a slurry. The slurry was dried and then calcined for two hours at temperatures shown in Table 1 to obtain the calcined stocks for Samples 1 to 5. 1 TABLE 1 Sample 1* Sample 2 Sample 3 Sample 4 Sample 5* Calcination 1,050° C. 1,100° C. 1,150° C. 1,200° C. 1,250° C. Temperature MTTF (hr.) 46.8 216.6 253.1 262.6 128.6 Value m 0.88 4.24 4.79 5.15 2.65 Note: Asterisked samples are not within the scope of the invention. Each of the calcined stocks was mixed with a binder and an organic solvent blended for several hours, and pulverized by a wet process. A sintering aid containing MnCO 3 and SiO 2 as the primary components was added thereto to again prepare a slurry. The resulting slurry was then shaped into sheets, each approximately 7 &mgr;m in thickness, by using a shaping machine such as a doctor blade, and the resulting sheets were dried to obtain the ceramic green sheets 1 . A conductive paste containing Cu, Ag, Ag—Pd or Pd, or a base metal such as Ni, etc., was applied by screen printing onto the ceramic green sheets 1 to form internal electrodes 13 and 14 . The ceramic green sheets 1 were stacked so that the internal electrodes 13 and 14 opposed each other with the ceramic green sheet 1 therebetween and were press-bonded to form a laminate. The laminate was treated in air for three hours at a temperature of 250° C. to remove the binder therefrom and was then sintered in a reducing atmosphere at a temperature of 1,320° C. for two hours to prepare a ceramic compact 11 shown in FIG. 2 . A vertical cross-section of each of Samples 1 to 5 was examined with a microscope, and the distance between the internal electrodes 13 and 14 of the ceramic compact 11 as found to be 4.6 &mgr;m for all of the samples. Next, the ceramic compact 11 was subjected to barrel finishing. Subsequently, an electrode paste containing Cu, Ag, Ag—Pd or the like was applied to the two ends of the ceramic compact 11 by a dipping method or the like, dried and baked to form external electrodes 15 and 16 . Next, the surfaces of the external electrodes 15 and 16 were plated with Ni and Sn to prepare a monolithic ceramic capacitor 10 . An accelerated life test (the number of the test pieces n&equals;36) was then performed on the resulting monolithic capacitors 10 of Samples 1 to 5 at 150° C. and 200V. Mean time to failure (MTTF) calculated from the accelerated life test results and values m in the Weibull plot are shown in Table 1 for each of Samples 1 to 5. The value m is a parameter related to the early failure rate. Large MTTF and m values are preferable. In Table 1, the asterisked samples (Samples 1 and 5) are comparative examples which are outside the scope of the present invention. As is apparent from Table 1, the calcination temperature significantly affects the reliability of the ceramic capacitors 10 , the MTTF and the value m. In order to examine the solid solubility between CaZrO 3 and CaTiO 3 constituting the ceramic of the monolithic ceramic capacitor 10 , the ceramic portion of each of Samples 1 to 5 was pulverized and was subjected to a structural analysis by a powder X-ray diffraction. The results showed that the X-ray diffraction peaks identifying crystal phases were only the peaks of CaZrO 3 -CaTiO 3 solid solution in all Samples 1 to 5 and no different phase was identified. However, when the diffraction patterns were examined in detail, peak resolutions among the three peaks assigned to the ( 200 ) plane, the ( 121 ) plane and the ( 002 ) plane of the CaZrO 3 -CaTiO 3 solid solution observed at around 2&thgr;&equals;32°, where &thgr; represents the Bragg angle, were different among the samples. To be more specific, as shown in FIG. 3 , when the X-ray diffraction peak assigned to the ( 200 ) plane detected at approximately 2&thgr;&equals;31.6° was represented by A, the X-ray diffraction peak assigned to the ( 121 ) plane detected at approximately 2&thgr;&equals;32.0° was represented by B, the X-ray diffraction peak assigned to the ( 002 ) plane detected at approximately 2&thgr;&equals;32.4° was represented by C, the valley lying at approximately 2&thgr;&equals;31.8° between the X-ray diffraction peaks A and B was represented by D, and the valley lying at approximately 2&thgr;&equals;32.2° between the X-ray diffraction peaks B and C was represented by E, the ratios of X-ray intensities d and e of the valleys D and E, respectively, to an X-ray intensity b of the main peak B were calculated. The results are shown in Table 2. 2 TABLE 2 Sample 1* Sample 2 Sample 3 Sample 4 Sample 5* Calcination 1,050° C. 1,100° C. 1,150° C. 1,200° C. 1,250° C. Temperature d/b ratio 0.221 0.187 0.166 0.158 0.184 e/b ratio 0.228 0.196 0.182 0.177 0.203 Note: Asterisked samples are not within the scope of the invention. As is apparent from Table 2, the peak resolutions among the diffraction patterns of the ( 200 ), ( 121 ) and ( 002 ) planes correlate with the reliability results shown in Table 1. High reliability is achieved when the ceramic portion of the monolithic ceramic capacitor 10 satisfies relationships (1) and (2) below: (X-ray intensity at the valley D )/(X-ray intensity at the peak B )<0.2  (1) (X-ray intensity at the valley E )/(X-ray intensity at the peak B )<0.2  (2) In other words, high peak resolutions and high reliability are achieved when the calcination temperature is between about 1,100° C. and 1,200° C. Generally, the solid-solubility of CaZrO 3 -CaTiO 3 can be improved by employing higher calcination temperatures; however, the subsequent pulverization by a wet process cannot be performed efficiently in such a case, resulting in degraded sinterability and failing to improve solid solubility of the resulting compact. Thus, it is preferable that the calcination temperature of the starting materials be set at a temperature between about 1,100° C. and 1,200° C. 
 EXAMPLE 2 First, CaCO 3 , ZrO 2 and TiO 2 , which were starting materials for dielectric ceramic green sheets 1 shown in FIG. 1 , were prepared and were weighed to achieve a CaZrO 3 /CaTiO 3 molar ratio of 6:4. Subsequently, the starting materials were blended with a binder and an organic solvent, for several hours and pulverized by a wet process to prepare a slurry. The slurry was dried and then calcined at 1,150° C. for two hours to obtain a calcined stock. The calcined stock was blended with a binder and an organic solvent for several hours and pulverized by a wet process. A sintering aid containing MnCO 3 and SiO 2 as the primary components was added thereto to again prepare a slurry. The slurry was then shaped into sheets each approximately 7 &mgr;m in thickness by using a shaping machine such as a doctor blade, and the resulting sheets were dried to prepare the ceramic green sheets 1 . A conductive paste was applied by screen-printing or the like on the ceramic green sheets 1 to form internal electrodes 13 and 14 . The ceramic green sheets 1 were stacked so that the internal electrodes 13 and 14 opposed each other with the ceramic green sheet 1 therebetween and were press-bonded to form a laminate. The laminate was treated in air for three hours at a temperature of 250° C. to remove the binder therefrom and was then sintered in a reducing atmosphere at temperatures shown in Table 3 for two hours to obtain a ceramic compact 11 shown in FIG. 2 . A vertical cross-section of each of Samples 6 to 10 was examined with a microscope, and the distance between the internal electrodes 13 and 14 of the ceramic compact 11 was found to be 4.6 &mgr;m for all of the samples. 3 TABLE 3 Sample 6* Sample 7* Sample 8 Sample 9 Sample 10 Sintering 1,240° C. 1,270° C. 1,300° C. 1,330° C. 1,360° C. Temperature MTTF (hr.) 174.1 218.8 264.5 271.2 288.6 Value m 1.22 1.87 4.57 4.73 5.11 Note: Asterisked samples are not within the scope of the invention. Next, the ceramic compact 11 was subjected to barrel finishing. Subsequently, an electrode paste was applied to the two ends of the ceramic compact 11 by a dipping method or the like, dried and baked to form external electrodes 15 and 16 . Next, the surfaces of the external electrodes 15 and 16 were plated with Ni and Sn to complete a monolithic ceramic capacitor 10 . An accelerated life test (the number of test pieces n&equals;36) was then performed on the resulting monolithic capacitors 10 of Samples 6 to 10 at 150° C. and 200V. Mean time to failure (MTTF) calculated from the accelerated life test results and values m in the Weibull plot are shown in Table 3 for each of Samples 6 to 10. The value m is a parameter related to the early failure rate. Large MTTF and m values are preferable. In Table 3, the asterisked samples, i.e., Samples 6 and 7, are comparative examples which are outside the scope of the present invention. As is apparent from Table 3, the sintering temperature significantly affects even the reliability of the ceramic capacitor 10 using the material calcined at a temperature in the suitable range demonstrated in Example 1. In order to examine the solid solubility in the CaZrO 3 -CaTiO 3 contained in the ceramic of the monolithic ceramic capacitor 10 , the ceramic portion of each of Samples 6 to 10 was pulverized and was subjected to a structural analysis by a powder X-ray diffraction. The results showed that the X-ray diffraction peaks identifying crystal phases were only the peaks of CaZrO 3 -CaTiO 3 solid solution in all Samples 6 to 10 and no different phase was identified. The peak resolutions among the three peaks assigned to the ( 200 ), ( 121 ) and ( 002 ) planes of the CaZrO 3 -CaTiO 3 solid solution observed at around 2&thgr;&equals;32° were then examined for each of Samples 6 to 10. That is, as shown in FIG. 3 , the ratios of X-ray intensities d and e of the valley D and E to an X-ray intensity b of the main peak B were calculated. The results are shown in Table 4. 4 TABLE 4 Sample 6* Sample 7* Sample 8 Sample 9 Sample 10 Sintering 1,240° C. 1,270° C. 1,300° C. 1,330° C. 1,360° C. Temperature d/b ratio 0.206 0.187 0.169 0.161 0.159 e/b ratio 0.211 0.201 0.184 0.178 0.176 Note: Asterisked samples are not within the scope of the invention. The results show that while solid-solution formation is mostly achieved by calcination at a temperature in the range determined in Example 1, the solid-solution formation also progresses during sintering. Although a low sintering temperature does not lead to a significant degradation in reliability such as that experienced when the calcination temperature is changed, it leads to a decrease in the value m which is a parameter related to the early failure rate. Thus, the sintering temperature is one of the important control items and is preferably set at a temperature not less than about 1,300° C. As described above, in the monolithic ceramic capacitor 10 composed of a dielectric material containing CaZrO 3 and CaTiO 3 as the primary components, and MnCO 3 and SiO 2 , the solid solubility in CaZrO 3 -CaTiO 3 can be improved by optimizing the temperature for calcining the dielectric starting materials and the temperature for sintering the laminate to make the monolithic capacitor 10 . Thus, a highly reliable monolithic ceramic capacitor can be manufactured even when the thickness of the ceramic green sheet is reduced to 5 &mgr;m or less. Other Embodiments The ceramic capacitors and the manufacturing method therefor of the present invention are not limited to the above preferred embodiments and are subject to various changes and modifications within the scope of the invention. For example, the molar ratio of CaZrO 3 to CaTiO 3 is not limited to the 6:4 ratio described in the above embodiments. Since the peak resolution is not affected by the ratio of CaZrO 3 to CaTiO 3 , no limit is imposed as to the ratio and any desired ratio may be employed. Moreover, the present invention can be applied not only to a monolithic ceramic capacitor but also to a single-layer ceramic capacitor. Furthermore, the ceramic green sheets having the internal electrodes thereon are stacked and then sintered in the preferred embodiment. The method for making the ceramic capacitor is not limited to this and other methods may be employed. For example, the method comprising the steps of forming a ceramic insulating layer by printing or the like using a ceramic material paste, applying a conductive material paste onto the surface of the ceramic insulating layer to form an internal electrode, and applying the ceramic material paste thereon to form another ceramic insulating layer may be used. By repeating the above steps, a ceramic capacitor having a multilayer structure can be obtained. As is apparent from the above, a CaZrO 3 -CaTiO 3 -based ceramic exhibiting a high solid solubility can be obtained according to the present invention, and a ceramic capacitor having a high reliability at high temperatures can be manufactured.