Method for manufacturing dielectric ceramic powder, and multilayer ceramic capacitor obtained by using the ceramic powder

Abstract

The invention relates to a method for manufacturing dielectric ceramic powder and a multilayer ceramic capacitor using the ceramic powder. According to the invention, BaCO.sub.3 powder is dispersed into a solution of solvent and dispersant to prepare BaCO.sub.3 slurry and then the resultant BaCO.sub.3 slurry is wet-milled. Also, TiO.sub.2 powder slurry is mixed into the wet-milled BaCO.sub.3 slurry to form mixed slurry and then the mixed slurry is dried into mixed powder. Finally, the dried mixed powder is calcined to produce BaTiO.sub.3 powder.

Description

CLAIM OF PRIORITY

[0001] This application claims the benefit of Korean Patent Application No. 2005-25891 filed on Mar. 29, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for manufacturing dielectric ceramic powder. More particularly, the present invention relates to a method for manufacturing dielectric ceramic powder by using wet-milled, BaCO.sub.3 as raw powder to prepare raw ceramic powder via solid state reaction, thereby ensuring fine particle size and uniform particle size distribution, and a multilayer ceramic capacitor made from the ceramic powder.

[0004] 2. Description of the Related Art

[0005] The information society of the 21st century has seen an increasing trend of digitalization, high-performance and high-reliability and multimedia in products such as electronic appliances, PC, HHP which chiefly utilize Multilayer Ceramic Capacitor (MLCC), one of the necessary passive devices of the electronics industry. Accordingly, MLCC parts have undergone higher-capacity and minimization fast. But this requires sheet lamination and fine BaTiO.sub.3 particles having uniform size distribution as the dielectric power used. Also, tetragonality indicated by a c/a ratio of c-axis to a-axis of BaTiO.sub.3 powder crystal needs to be higher (more than 1.008).

[0006] BaTiO.sub.3 powder has been produced by hydrothermal synthesis, alkoxide method, solid state reaction and the like.

[0007] In hydrothermal synthesis, jel-type titanium hydrate is added to a great quantity of barium hydroxide to react at a high temperature of about 150.degree. C. and under a high atmospheric pressure of 10, thereby producing crystalline BaTiO.sub.3. This method has the advantage of directly producing spherical crystal BaTiO.sub.3 sized about 100 nm but has the disadvantage of difficult design and maintenance of a reactor, and expensive manufacturing cost. Furthermore, recently, BaTiO.sub.3 powder produced by hydrothermal synthesis has revealed significant defects such as oxygen vacancy and barium vacancy, which develop into pores in the case of heat treatment, thus deteriorating properties of BaTiO.sub.3 powder.

[0008] Meanwhile in synthesizing BaTiO.sub.3 via hydrolysis of metal alkoxide, metal alkoaxide alcohol solution and Ba (OH) aqueous solution are mixed in a tube-type static mixer to react at a temperature of 80.degree. C. This method is advantageous due to following reasons. Liquid phase employed herein as starting material is more reactive than solid jel-type titanium hydrate used in hydrothermal synthesis. This allows synthesizing at a relatively low temperature, and easy adjustment of the synthesized powder particle size to about 20 to 100 nm. However this method has drawbacks in that a synthesis device is hard to configure, and alkoxide reagent used as starting material is expensive. Furthermore, material cost is expensive due to use of alcohol solvent, and complicated process conditions such as synthesis temperature hamper mass productions.

[0009] Therefore, to manufacture low-priced BaTiO.sub.3, solid state reaction is most advantageous. In solid state reaction, BaCO.sub.3 powder and TiO.sub.2 powder are used as starting powder. The BaCO.sub.3 powder and TiO.sub.2 powder are mixed, and then undergo solid phase reaction in a calcination process to be synthesized into final BaTiO.sub.3 powder. To achieve lamination of dielectric layers, dielectric material powder should have small particle size and uniform particle distribution. But BaTiO.sub.3 manufactured by solid state reaction reportedly does not exhibit uniform particle size distribution compared to BTO manufactured via other methods described above. In the end, in sold state reaction, one of essential factors for obtaining final uniform BaTiO.sub.3 powder concerns uniform dispersion of BaCO.sub.3 powder and TiO.sub.2 powder in the early stage. Such technologies have been consistently developed.

[0010] For example, conventional technologies are disclosed in Korean Patent Application Publication Nos. 2002-0053749 and 2004-0038747. The Patent Application No. 2002-0053749 discloses barium titanate powder obtained by mixing barium compound and titanium dioxide having rutile ratio of up to 30% and BET specific surface area of at least 5 m.sup.2/g and calcining the same. Meanwhile, the Patent Application No. 2004-38747 teaches a technology of absorbing organic polymer compound into barium carbonate powder. According to inventions disclosed in the aforesaid patent application publications, advantageously, barium compound and titanium dioxide are mixed uniformly to enhance the degree of mixing. However despite dispersion of each element, the acicular shape of barium compounds remains unchanged, leading to inevitable contact among barium compounds due to their morphological properties. Consequently, there is a limit in obtaining optimal degree of mixing with titanium dioxide.

[0011] Another conventional technology is disclosed in Korean Patent Application Publication No. 2004-0020252. Herein, BaCO.sub.3 powder is dry-milled spherically, mixed with TiO.sub.2 powder, and then calcined. However according to the aforesaid technology, disadvantageously, such dry-milling does not reduce the number of BaCO.sub.3 particles, and high stress placed on BaCO.sub.3 does not disperse BaCO.sub.3 particles properly, thus leading to agglomeration. Large specific surface area of powder, or small particle size results in uniform dispersion, but BaCO.sub.3 according to the aforesaid technology does not diminish particle numbers, rendering uniform mixing with TiO.sub.2 difficult. Thus, BaTiO.sub.3 powder finally obtained agglomerates heavily among primary particles and forms secondary particles relatively bigger than primary particles, also causing uneven particle distribution of powder. BaTiO.sub.3 powder with such properties may be hardly dispersible when applied to the MLCC, and unsuitable for the dielectric ceramic use for up to 1 .mu.m lamination to ensure a high-capacity capacitor.

SUMMARY OF THE INVENTION

[0012] The present invention has been made to solve the foregoing problems of the prior art and it is therefore an object of the present invention to provide dielectric ceramic powder having fine particles and uniform particle size distribution, and high tetragonal crystalinity.

[0013] It is another object of the invention to provide a multilayer ceramic capacitor obtained by using the dielectric ceramic powder.

[0014] The invention will be explained hereunder.

[0015] As identified above, solid state reaction is the most economical method for producing BaTiO.sub.3 powder having fine particles and uniform particle size, and high tetragonality to manufacture a high-capacity MLCC.

[0016] In solid state reaction, fine BaTiO.sub.3 powder may be produced via BaCO.sub.3 powder and TiO.sub.2 powder having big specific surface area. But the acicular shape of BaCO.sub.3 powder obstructs uniform mixing with TiO.sub.2 powder even in the case of mechanical mixing via beads mill equipment, and renders it difficult to obtain uniform BaTiO.sub.3 powder after final calcination. Further, despite uniform dispersion of fine BaCO.sub.3 and TiO.sub.2 powders, BaCO.sub.3 power particles grow easily in the calcination process. Therefore the BaCO.sub.3 powder particles grow even before reacting with TiO.sub.2 and reaching a temperature at which BaTiO.sub.3 particles are formed, thus making uniform reaction with TiO.sub.2 difficult.

[0017] This increases unevenness of particles in case where fine BaTiO.sub.3 powder is produced via solid state reaction to laminate dielectric layers.

[0018] Therefore, the inventors have conducted studies and experiments to solve problems of the solid state reaction. As a result, they confirmed that fine particles of BaCO.sub.3 powder could be obtained effectively by wet-milling acicular-shaped BaCO.sub.3 raw powder into a slurry and changing the particle shape from acicular to spherical. Also, the inventors found that fine BaTiO.sub.3 powder with high tetragonality and uniform particle size distribution could be produced by mixing TiO.sub.2 powder having a big specific surface area into such fine and spherical BaCO.sub.3 slurry, drying and cacinating the mixed slurry.

[0019] According to an aspect of the invention for realizing the object, there is provided a method for manufacturing dielectric ceramic powder comprising steps of:

[0020] dispersing BaCO.sub.3 powder into a solution of solvent and dispersant to prepare a slurry and then wet-milling the slurry;

[0021] mixing TiO.sub.2 powder slurry into the wet-milled BaCO.sub.3 slurry to form mixed slurry and then drying the mixed slurry into mixed powder; and

[0022] calcining the dried mixed powder to produce BaTiO.sub.3 powder.

[0023] According to another aspect of the invention for realizing the object, there is provided a method for manufacturing dielectric ceramic powder comprising steps of:

[0024] dispersing BaCO.sub.3 powder into a solution of solvent and dispersant to prepare a slurry and then wet-milling the slurry;

[0025] mixing CaCO.sub.3 powder slurry and TiO.sub.2 powder slurry into the wet-milled BaCO.sub.3 slurry to form mixed slurry, and then drying the mixed slurry; and

[0026] calcining the dried mixed powder to produce BaCaTiO.sub.3 powder.

[0027] According to further another aspect of the invention for realizing the object, there is provided a multilayer ceramic capacitor comprising:

[0028] a multilayer ceramic structure having a plurality of dielectric layers and a plurality of internal electrodes alternating with the dielectric layers; and

[0029] external electrodes provided at both ends of the multilayer ceramic, electrically connected to at least one of the internal electrodes,

[0030] wherein the dielectric layers comprise the dielectric ceramic powder manufactured according to the methods.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0032] FIG. 1 illustrates a process for producing dielectric ceramic powder of the invention;

[0033] FIG. 2a is a graph illustrating particle size change in accordance with wet-milling duration of the invention;

[0034] FIG. 2b is a graph illustrating effects of ammonia addition on viscosity of BaCO.sub.3 slurry in a wet-milling process of the invention;

[0035] FIG. 3 is a sectional view illustrating a multilayer ceramic capacitor manufactured via the dielectric ceramic powder of the invention;

[0036] FIG. 4a is SEM picture of BaCO.sub.3 powder before wet milling;

[0037] FIG. 4b is SEM picture of BaCO.sub.3 powder wet-milled according to the invention;

[0038] FIG. 5a is FE-SEM picture illustrating mixed powder of BaCO.sub.3 powder and TiO.sub.2 powder obtained without wet milling;

[0039] FIG. 5b is FE-SEM picture illustrating mixed powder of BaCO.sub.3 powder wet-milled according to the invention and TiO.sub.2 powder;

[0040] FIG. 6a is FE-SEM picture of the mixed powder of FIG. 5a which was heat-treated at a temperature of 900.degree. C.;

[0041] FIG. 6b is FE-SEM picture of the mixed powder of FIG. 5b which was heat-treated at a temperature of 900.degree. C.;

[0042] FIG. 7a is FE-SEM picture illustrating morphology of BaTiO.sub.3 powder manufactured according to a conventional solid state reaction;

[0043] FIG. 7b is a graph illustrating particle size distribution of BaTiO.sub.3 powder of FIG. 7a;

[0044] FIG. 8a is FE-SEM picture illustrating an example of morphology of BaTiO.sub.3 powder produced according to the invention;

[0045] FIG. 8b is a graph illustrating particle size distribution of BaTiO.sub.3 powder of FIG. 8a;

[0046] FIG. 9a is FE-SEM picture illustrating another example of morphology of BaTiO.sub.3 powder obtained according to the invention;

[0047] FIG. 9b is a graph illustrating particle size distribution of BaTiO.sub.3 powder of FIG. 9a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0048] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0049] FIG. 1 illustrates a process for manufacturing dielectric ceramic powder of the invention. As shown in FIG. 1, according to the invention, first, BaCO.sub.3 powder is dispersed into a solution of solvent and dispersant to prepare BaCO.sub.3 slurry. The dispersant, e.g. polyacrylic dispersant, is added to increase dispersibility of powder. Preferably, the dispersant is added to 1-5 weight parts with respect to BaCO.sub.3 raw powder. The BaCO.sub.3 raw powder is acicular, and preferably should have a specific surface area of 5 to 30 m.sup.2/g by BET measurement.

[0050] Further, according to the invention, distilled water and alcohol may be used as the solvent, but distilled water is preferable.

[0051] More preferably, the BaCO.sub.3 powder is dispersed into the solution to such an extent that that the BaCO.sub.3 slurry contains 10 to 60 wt % BaCO.sub.3. Less than 10 wt % BaCO.sub.3 out of the BaCO.sub.3 slurry adversely affects productivity (mass productivity). Also, BaCO.sub.3 in excess of 60 wt % out of the BaCO.sub.3 slurry degrades dispersibility and impairs wet milling.

[0052] Then, according to the invention, the BaCO.sub.3 slurry is wet-milled. The wet-milling is carried out at a speed of 1800 rpm via beads mill type equipment that uses zirconia beads with a diameter of 0.3 mm. Preferably, the milling duration should be limited to up to 20 hours. More preferably, the BaCO.sub.3 powder is wet-milled so as to have a specific surface area of at least 30 m.sup.2/g by BET measurement.

[0053] With increase in milling time for BaCO.sub.3 slurry, particle size of BaCO.sub.3 powder decreases, leading to continuous increase in the specific surface area thereof. But as shown in FIG. 2(a), the specific surface area does not increase any more after 8 hours, increasing viscosity of slurry significantly. However, increased viscosity renders continuous wet-milling process difficult and thus it is necessary to reduce viscosity.

[0054] Therefore, according to the invention, to reduce viscosity of slurry, as shown in FIG. 2(b), ammonia should be preferably added in the wet-milling process. More preferably, the ammonia can be added to at least 0.1 wt % with respect to the solvent.

[0055] Then, according to the invention, TiO.sub.2 powder slurry is mixed into the wet-milled BaCO.sub.3 slurry to form mixed slurry. TiO.sub.2 slurry can be easily manufactured by dispersing TiO.sub.2 powder into a solution of solvent and dispersant. The TiO.sub.2 raw powder preferably have a specific surface area of at least 20 m.sup.2/g, and more preferably at least 45 m.sup.2/g.

[0056] At this time, to produce BaTiO.sub.3 powder in a following process, TiO.sub.2 powder is mixed into a slurry to such an extent that a Ba/Ti mole ratio becomes 1. In this mixing process, the BaCO.sub.3 slurry and TiO.sub.2 slurry can be wet-mixed via zirconia beads with a diameter of 0.3 mm.

[0057] Meanwhile, sheet lamination of a high-capacity MLCC increases induced electric field, resultantly deteriorating IR and TCC properties. Therefore, to solve this problem, if necessary, TiO.sub.2 slurry and CaCO.sub.3 slurry as well may be mixed into the wet-milled BaCO.sub.3 slurry. With such mixing of CaCO.sub.3 slurry, Ca-doped BaTiO.sub.3, or fine BaCaTiO.sub.3 powder can be obtained in a following process.

[0058] Thereafter, the mixed slurry is dried to produce dried mixed powder, preferably, at a temperature of up to 200.degree. C. The invention is not limited to the aforesaid drying method but spray drying method may be more preferable.

[0059] Also, if necessary, the dried powder may be coarsely crushed via atomizer.

[0060] And the dried mixed powder is calcined to synthesize dielectric ceramic powder such as BaTiO.sub.3 powder or BaCaTiO.sub.3 powder. In a calcination process, BaCO.sub.3 powder and TiO.sub.2 powder may undergo solid state reaction to form BaTiO.sub.3 dielectric powder. Further, in case where CaCO.sub.3 powder is additionally mixed, Ca-doped BaCaTiO.sub.3 power can be obtained. Preferably, the calciantion temperature ranges from 900 to 1100.degree. C.

[0061] Powders synthesized in this fashion have necking among primary particles. To be used for the MLCC, typically, the mixed powders could go through a process of separation from primary particles undamaged. To this end, according to the invention, the synthesized ceramic powder can be pulverized. The pulverizing process can be carried out effectively through deagglomeration in beads mill.

[0062] Typical solid state reaction is applied to the dielectric ceramic powder manufactured according to the aforesaid process. Still the dielectric ceramic powder may have uniform particle size distribution, with mean particle size of 150 nm to 250 nm, D10/D50 of at least 0.6 and D90/D50 of up to 1.4. based on FE-SEM picture

[0063] Also, the dielectric ceramic powder may have at least 5.0 m.sub.2/g of BET specific surface area, and based on FE-SEM picture, a c/a ratio of c-axis to a-axis of the powder crystal lattice is at least 1.009.

[0064] Meanwhile, organic binder, solvent and other additives may be mixed with the dielectric ceramic powder obtained to prepare ceramic slurry, and a dielectric layer for the MLCC, or green sheets may be manufactured by using the ceramic slurry via a general tape casting method. Y.sub.2O.sub.3, Mn.sub.3O.sub.4, Cr.sub.2O.sub.3 and glass are used as additives.

[0065] FIG. 3 is a sectional view illustrating a multilayer ceramic capacitor manufactured via the dielectric ceramic powder. As shown in FIG. 3, the multilayer ceramic capacitor (MLCC) 10 includes a multilayer ceramic structure having a plurality of dielectric layers 1 and a plurality of internal electrodes 3 alternating with the dielectric layers; and external electrodes 5 provided at both ends of the multilayer ceramic structure.

[0066] To manufacture the capacitor 10, first, ceramic slurry including the dielectric ceramic powder prepared as described above is used to form the dielectric layers 1 through the typical tape casting method. Then the internal electrodes 3 are formed on the dielectric layers 1 via screen printing. Subsequently firing is carried our for the multilayer ceramic structure including the unfired dielectric layers 1, and then applying a conductive paste on both ends of the multilayer ceramic structure and finally firing it, thereby producing the multilayer ceramic capacitor 10 having the external electrodes 5.

[0067] As described above, according to the invention, to uniformly disperse and mix BaCO.sub.3 powder and TiO.sub.2 powder, before mixing with TiO.sub.2 powder, only acicular-shaped BaCO.sub.3 powder is wet-milled to be made spherical. Spherical particle shape or significant reduction in particle size allows uniform mixing with TiO.sub.2. Also, dielectric ceramic powder having fine particle size of 150 to 250 nm and high tetragonality can be manufactured by reacting BaCO.sub.3 powder with TiO.sub.2 powder before BaCO.sub.3 powder particles grow in a calcination process.

[0068] Further, in case where the multilayer ceramic capacitor is manufactured via the dielectric ceramic powder produced by the aforesaid process, sheet lamination is ensured to effectively realize higher-capacity of the MLCC while reducing the size thereof.

[0069] The invention will be explained in detail with reference to the unlimited examples which follow.

EXAMPLE 1

[0070] BaCO.sub.3 raw powder having a specific surface area of 20 m.sup.2/g was prepared. Some of BaCO.sub.3 raw powder was dispersed into a mixed solution of distilled water and polyacrylic dispersant to manufacture BaCO.sub.3 slurry. BaCO.sub.3 raw powder was dispersed into the solution to such an extent that the BaCO.sub.3 slurry would contain 10 to 60 wt % BaCO.sub.3. The slurry was wet-milled for 18 hours via beads mill type equipment that uses zirconia beads with a diameter of 0.3 mm as milling media. During wet-milling, considering a sudden increase in viscosity in accordance with decrease in BaCO.sub.3 particle numbers, ammonia was added after 8 hour milling to reduce viscosity. A specific surface area of the wet-milled BaCO.sub.3 powder was 31 m.sup.2/g, a significant increase from the initial one, and the particles had almost a spherical shape.

[0071] Field Emission (FE-SEM) picture before and after wet-milling BaCO.sub.3 is shown in FIG. 4(a-b). As shown in the aforesaid FIG. 4(a-b), wet-milling changed BaCO.sub.3 from acicular powder into finer spherical powder.

[0072] Meanwhile, slurried TiO.sub.2 powder having a specific surface area of 45 m.sup.2/g was mixed into the wet-milled BaCO.sub.3 slurry, and then the mixed slurry was mixed via beads mill. At this time, mixed powder was slurried so that BaTiO.sub.3 powder would have a Ba/Ti ratio of 1. Then for comparison, BaCO.sub.3 raw powder, which was not wet-milled, was mixed with TiO.sub.2 powder to produce mixed powder.

[0073] FIG. 5(a-b) shows FE-SEM picture of the final mixed powder. FIG. 5(a) is FE-SEM picture illustrating BaCO.sub.3 powder mixed with TiO.sub.2 powder without wet-milling, while FIG. 5(b) is FE-SEM picture of wet-milled BaCO.sub.3 powder mixed with TiO.sub.2 power. As shown in FIG. 5 (a-b), when BaCO.sub.3 powder without wet-milling was mixed with TiO.sub.2 powder, it leads to uneven mixing but use of the wet-crushed BaCO.sub.3 powder led to uniform mixing among each component.

[0074] Also, to confirm whether BaCO.sub.3 powder particles grow in case of rising temperature during a calcination process, mixed powders prepared as above were calcined and heat-treated at a temperature ranging from 600.degree. C. to 1000.degree. C. Consequently, as in FIG. 6(a), in case of using BaCO.sub.3 powder without wet-milling, BaCO.sub.3 particles grew considerably at a temperature of 900.degree. C., while as in FIG. 6(b), in case where wet-milled BaCO.sub.3 powder was used, particle growth was not observed, indicating that BaTiO.sub.3 powder can be synthesized.

EXAMPLE 2

[0075] TABLE-US-00001 TABLE 1 Specific surface area Wet-milling of (m.sup.2/g) Calcination No. BaCO.sub.3 BaCO.sub.3 TiO.sub.2 CaCO.sub.3 temp. (.degree. C.) 1 Not wet-milled 20 20 1020 2 Not wet-milled 20 20 1040 3 Wet-milled 31 20 1020 4 Wet-milled 31 20 1040 5 Wet-milled 31 45 960 6 Wet-milled 31 45 990 7 Wet-milled 31 45 1020 8 Wet-milled 31 45 30 960 9 Wet-milled 31 45 30 990

[0076] BaCO.sub.3 raw powder having a specific surface area of 20 m.sup.2/g was prepared. Some of BaCO.sub.3 raw powder was dispersed into a mixed solution of distilled water and polyarcrylic dispersant to produce BaCO.sub.3 slurry. BaCO.sub.3 powder was dispersed into the solution to such an extent that BaCO.sub.3 slurry would contain 10 to 60 wt % BaCO.sub.3. The resultant slurry was wet milled for 18 hours via beads mill type equipment using zirconia beads with a diameter of 0.3 mm as milling media. Considering a sudden increase in viscosity in accordance with decrease in BaCO.sub.3 particle numbers during a wet-milling process, ammonia was added to reduce viscosity after 8-hour milling. A specific surface area of the wet-milled BaCO.sub.3 powder is shown in Table 1 above.

[0077] Slurried TiO.sub.2 raw powder having different specific surface area was mixed into the wet-milled BaCO.sub.3 slurry via beads mill. The mixed powder was slurried so that BaTiO.sub.3 powder would have a Ba/Ti ratio of 1, and then the mixed powder was obtained by spray drying.

[0078] Meanwhile, in manufacturing Ca-doped BaCaTiO.sub.3 dielectric ceramic powder, as shown in Table 1, slurried TiO.sub.2 powder and slurried CaCO.sub.3 powder having a specific surface area of 30 m.sup.2/g were mixed into the wet-milled BaCO.sub.3. At this time, to obtain (Ba.sub.0.98Ca.sub.0.02).sub.1.000TiO.sub.3 powder, each of TiO.sub.2 powder and CaCO.sub.3 powder were mixed into a slurry form, and then dried by spraying dying to produce mixed powder.

[0079] For comparison, as shown in Table 1, some of BaCO.sub.3 raw powder having a specific surface area of 20 m.sup.2/g was wet-mixed with TiO.sub.2 powder having a specific surface area of 20 m.sup.2/g without undergoing wet-milling. The powders were measured and mixed so that resultant BaTiO.sub.3 powder would have a Ba/Ti ratio of 1.

[0080] The resultant mixed powders were dried and calcined under the conditions set forth in Table 1 to manufacture BaTiO.sub.3 or BaCaTiO.sub.3 dielectric ceramic powder. Thereafter, the ceramic powder was deagglomerated via beads mill to produce final powder.

[0081] To examine properties of powders manufactured as above, BET specific surface area was measured. Also, through XRD analysis, a c/a ratio of c-axis to a-axis of the powder crystal lattice was calculated to measure tetragonality, and the results are shown in Table 2 below. Mean particle size (D.sub.mean) of powder was measured via image analyzer based on FE-SEM picture. Further, to investigate uniformity of particle size distribution, measurement was conducted on 10% cumulative distribution D10, 50% cumulative distribution D50, and 90% cumulative distribution D90, respectively from small size distribution. The calculated results of D10/D50, D90/D50 are shown in Table 2. TABLE-US-00002 TABLE 2 Particle size Ceramic SSA* MPS* distribution Tetrago- No. powder (m.sup.2/g) (nm) D10/D50 D90/D50 nality 1 BaTiO.sub.3 5.66 176 0.41 1.57 1.0070 2 BaTiO.sub.3 3.97 212 0.40 1.54 1.0097 3 BaTiO.sub.3 4.58 199 0.62 1.38 1.0097 4 BaTiO.sub.3 4.01 230 0.65 1.36 1.0103 5 BaTiO.sub.3 5.68 150 0.70 1.26 1.0093 6 BaTiO.sub.3 4.53 202 0.69 1.24 1.0105 7 BaTiO.sub.3 4.08 218 0.72 1.24 1.0105 8 BaCaTiO.sub.3 5.62 155 0.71 1.27 1.0091 9 BaCaTiO.sub.3 4.57 198 0.70 1.24 1.0103 *SSA: Specific Surface Area *MPS: Mean Particle Size

[0082] As shown in Tables 1 and 2, for sample 1 in which BaCO.sub.3 was calcined at a temperature of 1020.degree. C. without wet-milling, the particles were finely-sized with 176 nm but tetragonality thereof was 1.007, which is lower than 1.008 or a requirement for high-capacity dielectric powder. For sample 2 in which BaCO.sub.3 was calcined at a temperature of 1040.degree. C., BaTiO.sub.3 having tetragonality of 1.0097 and size of about 212 nm was synthesized.

[0083] In contrast, for sample 3, in which BaCO.sub.3 was wet-milled and then mixed with 20 m.sup.2/g of TiO.sub.2, BaCO.sub.3 particles were finely-sized with 199 nm and tetragonality thereof was 1.0097, a high figure even at a temperature of 1020.degree. C., which is lower than when BaCO.sub.3 was not wet-milled. Also, for sample 6 in which BaCO.sub.3 was wet-milled and then mixed with TiO.sub.2 powder having a specific area of 45 m.sup.2/g, BaTiO.sub.3 powder particles were sized 202 nm, with tetragonality of at least 1.010 at a temperature of 990C. Further, for sample 5 in which BaCO.sub.3 was calcined at a temperature of 960.degree. C., BaCO.sub.3 powder was obtained with fine particle size of 150 nm and big specific surface area of 5.68 m.sup.2/g. Still, BaTiO.sub.3 powder obtained had high tetragonality of 1.0093.

[0084] In addition, to compare particle uniformity based on cumulative particle size distribution, the calculated values of D10/D50, D90/D50 were considered. Herein, bigger D10/D50 value and smaller D90/D50 value mean more uniform distribution. When the calculated values are compared, wet-milled BaCO.sub.3 indicates bigger D10/D50 and smaller D90/D50, and thus more uniform particle size distribution than that without wet-milling. For mixed powders (samples 5 to 7) in which wet-milled BaCO.sub.3 was mixed with TiO.sub.2 having a specific surface area of 45 m.sup.2/g, the particle size distributions were most uniform.

[0085] Further, Ca-added BaCaTiO.sub.3 powder (samples 8 to 9) exhibited behavior similar to BaTiO.sub.3 powder. By calcining at a temperature of 990.degree. C. and 960.degree. C., BaCaTiO.sub.3 powders having mean particle size of 198 nm and 155 nm, respectively, could be produced with tetragonalitiy of at least 1.0091 overall.

[0086] FIGS. 7a, 8a and 9a are FE-SEM pictures of dielectric ceramic powder corresponding to samples 2, 3 and 6. FIGS. 7b, 8b and 9b are graphs illustrating particle size distribution measured via image analyzer. As shown in the above FIGS. 7a, 7b, 8b and 9b, compared to sample 2 which used BaCO.sub.3 without wet-milling, sample 3 which used wet-milled BaCO.sub.3 powder indicated more uniform particle size distribution. Further, the narrowest particle size distribution was found in sample 6 which used wet-milled BaCO.sub.3 powder and TiO.sub.2 powder having big specific surface area.

[0087] As set forth above, according to the invention, BaTiO.sub.3 or BaCaTiO.sub.3 is manufactured via wet-milled BaCO.sub.3 powder to produce uniform dielectric ceramic powder having fine particles sized 150 to 250 nm and high tetragonality.

[0088] Also, the multilayer ceramic capacitor manufactured via dielectric ceramic powder allows sheet lamination and enables higher-capacity and minimization of the MLCC.

[0089] While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A method for manufacturing dielectric ceramic powder comprising steps of: dispersing BaCO.sub.3 powder into a solution of solvent and dispersant to prepare a slurry and then wet-milling the slurry; mixing TiO.sub.2 powder slurry into the wet-milled BaCO.sub.3 slurry to form mixed slurry and then drying the mixed slurry into mixed powder; and calcining the dried mixed powder to produce BaTiO.sub.3 powder.

2. The method according to claim 1, wherein the solvent comprises distilled water or alcohol.

3. The method according to claim 1, wherein the dispersant is polyacrylic, and added to 1.about.5 weight parts with respect to the BaCO.sub.3 powder.

4. The method according to claim 1, wherein the BaCO.sub.3 powder has a specific surface area ranging from 5 to 30 m.sup.2/g by BET measurement.

5. The method according to claim 1, wherein BaCO.sub.3 powder is dispersed into the solution to such an extent that the BaCO.sub.3 slurry contains 10 to 60 wt % BaCO.sub.3.

6. The method according to claim 1, wherein the BaCO.sub.3 slurry is wet-milled to such an extent that BET specific surface area of BaCO.sub.3 powder is at least 30 m.sup.2/g.

7. The method according to claim 1, wherein in the wet-milling step, ammonia is added to reduce viscosity of the slurry.

8. The method according to claim 7, wherein the ammonia is added to at least 0.1 weight parts with respect to the solvent.

9. The method according to claim 1, wherein the TiO.sub.2 powder has a specific surface area of at least 20 m.sup.2/g.

10. The method according to claim 1, wherein the calcination temperature ranges from 900 to 1100.degree. C.

11. The method according to claim 1, further comprising pulverizing the produced BaTiO.sub.3 powder.

12. The method according to claim 11, wherein the pulverized BaTiO.sub.3 powder has uniform particle size distribution, with mean particle size of 150 nm to 250 nm, D10/D50 of at least 0.6 and D90/D50 of up to 1.4 based on FE-SEM picture.

13. The method according to claim 11, wherein the pulverized BaTiO.sub.3 powder has at least 5.0 m.sub.2/g of BET specific surface area, and based on FE-SEM picture, a c/a ratio of C-axis to a-axis of the powder crystal lattice is at least 1.009.

14. A method for manufacturing dielectric ceramic powder comprising steps of: dispersing BaCO.sub.3 powder into a solution of solvent and dispersant to prepare a slurry and then wet-milling the slurry; mixing CaCO.sub.3 powder slurry and TiO.sub.2 powder slurry into the wet-milled BaCO.sub.3 slurry to form mixed slurry, and then drying the mixed slurry; and calcining the dried mixed powder to produce BaCaTiO.sub.3 powder.

15. The method according to claim 14, wherein the solvent comprises distilled water or alcohol.

16. The method according to claim 14, wherein the dispersant is polyacrylic, and added to 1-5 weight parts with respect to the BaCO.sub.3 powder.

17. The method according to claim 14, wherein the BaCO.sub.3 powder has a specific surface area of 5 to 30 m.sup.2/g by BET measurement.

18. The method according to claim 14, wherein BaCO.sub.3 powder is dispersed into the solution so that the BaCO.sub.3 slurry contains 10 to 60 wt % BaCO.sub.3.

19. The method according to claim 14, wherein the BaCO.sub.3 slurry is wet-milled to such an extent that the BaCO.sub.3 powder has a specific surface area of at least 30 m.sup.2/g.

20. The method according to claim 14, wherein in the wet-milling step, ammonia is added to reduce viscosity of the slurry.

21. The method according to claim 20, wherein the ammonia is added to at least 0.1 weight parts with respect to the solvent.

22. The method according to claim 14, wherein the TiO.sub.2 powder has a specific surface area of at least 20 m.sup.2/g.

23. The method according to claim 14, wherein the calcination temperature ranges from 900 to 1100.degree. C.

24. The method according to claim 14, further comprising pulverizing the produced BaCaTiO.sub.3 powder.

25. The method according to claim 24, wherein the pulverized BaCaTiO.sub.3 powder has uniform particle size distribution, with mean particle size of 150 nm to 250 nm, D10/D50 of at least 0.6 and D90/D50 of up to 1.4 based on FE-SEM picture.

26. The method according to claim 24, wherein the pulverized BaCaTiO.sub.3 powder has at least 5.0 m.sub.2/g of BET specific surface area, and based on FE-SEM picture, a c/a ratio of C-axis to a-axis of the powder crystal lattice is at least 1.009.

27. A multilayer ceramic capacitor comprising: a multilayer ceramic structure having a plurality of dielectric layers and a plurality of internal electrodes alternating with the dielectric layers; and external electrodes provided at both ends of the multilayer ceramic, electrically connected to at least one of the internal electrodes, wherein the dielectric layers comprise the dielectric ceramic powder manufactured according to claim 1.

28. The multilayer ceramic capacitor according to claim 27, wherein the ceramic power has uniform particle size distribution, with mean particle size of 150 nm to 250 nm, D10/D50 of at least 0.6 and D90/D50 of up to 1.4 based on FE-SEM picture.

29. The multilayer capacitor according to claim 27, wherein the ceramic powder has at least 5.0 m.sub.2/g of BET specific surface area, and based on FE-SEM picture, a c/a ratio of c-axis to a-axis of the powder crystal lattice is at least 1.009.