Patent Application
20080068778
The present invention will be explained in detail with reference to preferred embodiments. However, the preferred embodiments are not intended to limit the present invention.
A preferred embodiment of a multi-layer ceramic capacitor according to the invention is to be described. A multi-layer ceramic capacitor 1 of this embodiment has, as shown in
The dielectric ceramics 3 have grains comprising, as a main ingredient, a perovskite dielectric substance represented by ABO3 (in which A represents one or more member selected from Ba, Ca, and Sr and B represents one or more member selected from Ti and Zr) and the grain growth is controlled such that the average value for the diameter of the grains is within a range from 40 to 150 nm.
Within the range for the average value of the diameter of the grains described above, where the thickness of the dielectric substance ceramics 3 is, for example, 0.9 μm (in other embodiments, 0.7 μm-10 μm including 1.0 μm, 5.0 μm, 8.0 μm, and values between any two numbers of the foregoing), when grains are arranged by the number of nine in the direction of the thickness, and grain boundaries are present at 8 positions. In an existent case where the grain is larger than 0.15 μm, that is, 150 nm, the positions for the grain boundaries are less than eight. Accordingly, since the effect of the barrier for the movement of oxygen defects due to the grain boundary is increased in the multi-layer ceramic capacitor of an embodiment of the invention, the life time property can be improved.
Further, the dielectric ceramics 3 contain rare earth compounds (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, and Y), Si compounds, compound of alkaline earth metals (Mg, Ca, and Sr) or compounds of transition metals (Mn, Cr, V, Zn, Fe, Co, etc.) as the extraneous material, and the grain has a structure in which the perovskite dielectric substance and the extraneous material are uniformly solid solubilized, or a core shell structure having a perovskite dielectric substance as a core and a shell containing extraneous material therearound.
Since it is important for the grains to suppress grain growth, it is preferred to restrict within a range: 1.2≦D/d≦1.5 that is, to restrict the grain growth within a range from 1.2 to 1.5 times in which D represents the average value for the diameter of grains and d represents an average value for the particle diameter of the raw material powder for the perovskite dielectric substance as a raw material of the dielectric ceramic layer.
The grain growth can be defined within the range of an embodiment of the invention by controlling the firing temperature, as well as controlling the composition for the dielectric ceramics 3. For example, the grain growth can be suppressed within a range of from 1.2 to 1.5 times by using a ceramic material formed by mixing the rare earth compounds by from 1 to 3 mol being indicated as oxide conversion, the alkaline earth metal compounds by from 1 to 2 mol being indicated as oxide conversion, and the transition metal compound by from 0.3 to 1.0 mol being indicated converted as oxide conversion based on 100 mol of the raw material powder for the perovskite dielectric substance such as BaTiO3.
Further, for the raw material powder for the perovskite dielectric substance as the raw material, it is preferred to use those having the average value for the particle diameter within a range from 30 to 100 nm. By using the raw material powder as described above, the average value for the diameter of the grains can be easily defined within a range from 40 to 150 nm by suppressing the grain growth within a range from 1.2 to 1.5 times. Further, since the raw material powder with the average value for the particle diameter is from 30 to 100 nm has high surface activity and high sinterability, grain boundaries with high resistance can be obtained with less grain growth.
The internal electrodes 4 comprise a base metal such as Ni. The base metal includes, Ni, Cu or alloys thereof. The internal electrodes 4 are formed by printing a conductive paste to a ceramic green sheet by a method, for example, of screen printing. The conductive paste contains a metal material, as well as a ceramic material substantially identical with the ceramic material constituting the dielectric ceramics 3 in order to mitigate the differential shrinkage relative to the firing shrinkage of the dielectric ceramics 3.
The end termination electrodes 5 comprise Cu, Ni, Ag, a Cu—Ni alloy, or a Cu—Ag alloy and are formed by coating and baking a conductive paste to the multi-layer ceramics 2 after firing, or coating a conductive paste on a not-fired multi-layer ceramics 2 and baked simultaneously with firing of the dielectric ceramics 3. Plating layers 6, 7 are formed on the end termination electrodes 5 by electrolytic plating or the like. The first plating layer 6 has a function of protecting the end termination electrodes 5 and comprises Ni, Cu, etc. The second plating layer 7 has a function of improving solder wettability and comprises Sn or an Sn alloy.
Then, the effect of an embodiment of the invention is to be described with reference the following specific experimental examples. In this case, BaTiO3 was used as the raw material powder for the perovskite dielectric substance. For the additive as the extraneous material, Ho2O3 was used as the rare earth compound, MgO was used as the alkaline earth metal compound, Mn2O3 was used as the transition metal compound, and SiO2 was used as the Si compound. The starting materials described above were provided so as to form the composition shown in Table 1. In Table 1, the addition amounts of Ho2O3, MgO, Mn2O3, and SiO2 were represented by the mol number based on 100 mol of BaTiO3. Further, for the addition amount of the extraneous material, since the grains tend to grow further as the average value for the particle diameter of the raw material powder for the perovskite dielectric substance decreases, the addition amount was increased more as the average value of the particle diameter of the raw material powder for the perovskite dielectric material was smaller in order to suppress the grain growth.
In the present disclosure where conditions and/or structures are not specified, the skilled artisan in the art can readily provide such conditions and/or structures, in view of the present disclosure, as a matter of routine experimentation.
In the present examples, the numerical numbers applied in embodiments can be modified by a range of at least +50% in other embodiments, and the ranges applied in embodiments may include or exclude the endpoints.
Provided raw material was mixed with water and wet-pulverized in a ball mill for 15 hours to obtain a mixture. The mixture was dried and, calcined in an atmospheric air at 800° C. for one hour to obtain a calcining body. Polyvinyl butyral as an organic binder and ethanol as a solvent were added to the calcining body and mixed to obtain a ceramic slurry. The ceramic slurry was molded into a sheet by a doctor blade to obtain a ceramic green sheet of 1.0 μm thickness.
A conductive paste was coated on the ceramic green sheet by a screen printing method to form an internal electrode pattern. Ten ceramic green sheets each formed with the internal electrode pattern were layered and hot press bonded to obtain a layered body. The layered body was cut and divided into a size of 4.0 mm×2.0 mm to obtain not fired chips. The not-fired chips were removed with the binder in a nitrogen atmosphere and then fired in a nitrogen-hydrogen gas mixture containing 1% hydrogen at a firing temperature shown in Table 1 to obtain sintered chips.
A conduction paste was coated on the internal electrode exposure surface of the obtained sintered chips and baked in a nitrogen atmosphere at 700° C. to form an end termination electrode. As described above, a multi-layer ceramic capacitor sized 3.2 mm×1.6 mm with the thickness of the dielectric ceramics between the inter electrodes of 0.75 μm was obtained.
For the obtained multi-layered ceramic capacitor, a dielectric breakdown voltage (BDV), an insulation resistance value (IR), an average life time, and permittivity were measured. The specimens were used each by the number of 10, the applied voltage was increased at a rate of 10 V/sec in an atmosphere at 25° C. and a voltage value causing short circuit was measured and the average value thereof was defined as the dielectric breakdown voltage.
Further, the specimens were used each by the number of 10, a DC voltage at 100 V was applied at 25° C. and the resistance after one min was measured and the average value thereof was defined as the insulation resistance value. Further, the specimens were used each by the number of 30, a high temperature accelerated life time test was conducted under the condition at 150° C. and at a DC voltage 50 kV/mm, and the average value for the time at which the insulation resistance value lowered to 105Ω or less was defined as the average life time. The specimens were used each by the number of five and an electrostatic capacitance was measured at an AC voltage of 0.5 V and at 1 kHz, at a temperature of 200° C., and the average value for the values calculated based on the intersection area of internal electrodes, the number of layers and the thickness of dielectric material was defined as the permittivity.
Further, the raw material powder was observed by SEM (Scanning Electron Microscope) under magnification by 50,000×, and the diameter was measured for the particles by the number of 300 and the average value thereof was defined as the average value for the particle diameter of the raw material powder. Further, the lateral cross section of the multi-ceramic capacitor was exposed by polishing and observed by SEM under magnification by 50,000×, the diameters for the grains by the number of 300 were measured for dielectric ceramics between the internal electrodes and the average value thereof was defined as the average value for the diameter of the grains. Further, D/d was calculated by using the average value for each of them. Table 2 shows the result of the measurements described above.
It was evaluated as satisfactory in a case where the dielectric breakdown voltage was 75 V/μm or higher, the insulation resistance value was 5.0×1010Ω or higher, the average life time was 7500 sec or more and the permittivity was 500 or more. As a result, satisfactory properties were obtained for No. 3, Nos. 5 and 6, No. 8, Nos. 11 and 12, No. 14, Nos. 17 and 18, and Nos. 20 and 21 in which the average value for the diameter of the grains was within a range from 40 nm to 150 nm and D/d was within a range form 1.2 to 1.5.
For No. 2, No. 7, No. 9, No. 10, No. 13, No. 15 and No. 16, while the average value for the diameter of the grains was within a range from 40 nm to 150 nm, D/d was out of the range of 1.2 to 1.5. Among them, for No. 7, No. 10, No. 13 and No. 16, D/d was less than 1.2 and the insulation resistance value was 5.0×1010Ω or lower. This is because sintering was not sufficient since the extent of the grain growth was small and the resistance value of the grain boundary tends to decrease. Further, for No. 2, No. 9, and No. 15, D/d shows a value larger than 1.5 and the average life time was 7500 sec or less. This is because the coarse grains tended to increase since the extent of the grain growth was large and, therefore, the number of portions where the strength of the electric field increase locally is increased. Accordingly, in a case where the average value for the diameter of the grains was from 40 to 150 nm and the value for D/d was 1.2 or more and 1.5 or less, the dielectric breakdown voltage is 75 V/μm or higher, the insulation resistance value is 5.0×1010Ω or higher and the average life time is 7500 sec or more and more preferred characteristics are obtained for the multi-layered ceramic capacitor.
Further, among the specimens in which the average value for the diameter of the grains is from 40 to 150 nm and the value for D/d is within a range from 1.2 to 1.5, No. 3 in which the average value for the grain diameter of the raw material powder is less than 30 nm, the average life time is 7500 sec or more but less than 10,000 sec and the dielectric break voltage value is 75 V/μm or more but lower than 100 V/μm. This is because the additive tends to be less dispersed in a case where the average value for the particle diameter of the raw material powder is small tending to cause variation in the solid solution of the additive or the formation of the core shell structure. Further, also for Nos. 20, 21 in which the average value for the grain diameter of the starting powder, the dielectric breakdown voltage value was 75 V/μm or higher but lower than 100 V/μm. This is because the sintering reactivity is lowered along with increase in the average value for the grain diameter of the raw material powder and the insulation resistance of the grain boundary tends to lower relatively, and dielectric break tends to be caused at a relatively low electric field strength.
For other specimens, since the average value for the diameter of the grains is from 40 to 150 nm, the value for D/d is within a range from 1.2 to 1.5, and the average value for the grain diameter of the main material powder is 30 nm or more and 100 nm or less, the dielectric breakdown voltage is 100 V/μm or higher, the insulation substance value is 5.0×1010Ω or higher, and the average life time is 10,000 sec or more, to obtain more properties.
As described above, a multi-layer ceramic capacitor having the average grain size, D/d, and the average value for the particle diameter of the raw material powder are within the range of an embodiment of the invention can provide a multi-layer ceramic capacitor of high reliability.
The present application claims priority to Japanese Patent Application No. 2006-284356, filed Sep. 20, 2006, the disclosure of which is incorporated herein by reference in its entirety.
It will be understood by those of skill in the art that numerous and various modifications can be made without departing from the spirit of the present invention. Therefore, it should be clearly understood that the forms of the present invention are illustrative only and are not intended to limit the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-284356 | Sep 2006 | JP | national |
