Patent Application
20090201628
The present invention relates to a multilayer ceramic capacitor used for various types of electronic appliances.
A description is made for the structure of a conventional multilayer ceramic capacitor using
In recent years, with thickness reduction of electronic appliances, a multilayer ceramic capacitor (multi-layer ceramic capacitor, referred to as MLCC hereinafter) as well has been demanded for thickness reduction. Means of reducing the thickness of an MLCC include lamellation of the dielectric layer. An example of such a technique is described in patent literature 1.
To lamellate a dielectric layer, ceramic particles has only to be made smaller. However, simply making ceramic particles smaller by lamellating the layer decreases the number of ceramic particles between internal electrodes, causing the insulation characteristic to deteriorate.
[Patent literature 1] Japanese Patent Unexamined Publication No. 2003-133164
The present invention is a multilayer ceramic capacitor including a laminated body layer formed by alternately laminating dielectric layers made of ceramic particles and internal electrodes; and a pair of external electrodes provided at least on both end surfaces of the laminated body layer and alternately connected to the internal electrodes electrically, where the number of boundaries between ceramic particles per unit length of the dielectric layer in the lamination direction is larger than that in the direction connecting between a pair of external electrodes. Thus increasing the number of ceramic grain boundaries in the dielectric body between internal electrodes improves the insulation characteristic. Particularly, even if the number of ceramic particles thicknesswise decreases due to lamellation, increasing the number of grain boundaries suppresses deterioration of the insulation characteristic.
Hereinafter, a description is made for an example embodiment of the present invention using the related drawings. Here, the drawings are schematic and do not represent accurate dimensions of each position. The present invention is not limited to this exemplary embodiment.
In
Dielectric layer 2 is mainly made of ceramic particles as its structure of a perovskite compound or tungsten bronze compound, for example. For a perovskite compound, the dielectric constant increases, where its principal component can be barium titanate.
For a tungsten bronze compound, meanwhile, the temperature characteristic is improved, where its representative materials include those based on Ba—Nd—Ti—O. To further improve the temperature characteristic, such as bismuth oxide can be added. Ceramic particles as a material of a tungsten bronze compound contain barium, rare earthes, and titanium.
One end surface of laminated body 3 has internal electrodes 1 exposed thereon at every other layer, and the other end surface has internal electrodes 1 exposed thereon that are not exposed on the aforementioned one end surface. Then, external electrodes 4 are respectively formed so as to electrically connect to internal electrodes 1 exposed on both end surfaces. MLCC 6 is thus structured.
A description is further made for dielectric layer 2 using
An MLCC according to an embodiment of the present invention features that the number of boundaries between ceramic particles 5 per unit length of dielectric layer 2 in the lamination direction (arrow 2A in
That is, after making the grain boundaries observable by etching the fracture surface or polished surface of an MLCC, the surface is photographed with the aid of a scanning electron microscope. Next, straight lines with a certain length are drawn on the photo in the direction connecting a pair of external electrodes 4 and the lamination direction, and the numbers of grain boundaries crossing the straight lines are counted to calculate the numbers of boundaries between ceramic particles 5 per unit length. Although increasing measurement positions increases the accuracy in measuring the number of boundaries, an average value of approximately five measurement positions generally provides a sufficient accuracy.
“Boundary” in an MLCC according to an embodiment of the present invention refers to that formed by ceramic particles as the principal component, where a grain boundary phase formed by such as additives is included in a boundary.
Ceramic particles 5 used here has shape anisotropy, concretely such as acicular, plate-like, and columnar. With either of the shapes, as long as the relationship between the major axis and the minor axis holds (major axis/minor axis≧2), the minor axis direction of each ceramic particles is likely to point in the lamination direction when producing a ceramic green sheet (referred to as CGS hereinafter), thereby increasing the number of grain boundaries in the lamination direction.
In an MLCC (the case of
A method of producing shape-anisotropic barium titanate is that owing to hydrothermal reaction using a shape-anisotropic titanium compound as crystal nuclei, for example. The hydrothermal reaction is a method of generating crystals by exerting heat and pressure on a solution produced by dispersing a titanium compound and barium compound to promote the chemical reaction.
Next, barium hydroxide octahydrate as an alkali earth metal compound is added into the above-described aqueous solution of titanium oxide to produce a mixed solution (step 3b).
At this moment, barium is desirably contained more than titanium. That is, barium titanate as a final product has a perovskite structure, and the blend ratio of titanium oxide and barium salt is adjusted so that A/B>1.0 holds assuming the perovskite-type chemical formula is ABO3 (A, B represent an element, O represents oxygen; here, element A is Ba, element B is Ti). Although barium titanate is likely to grain-grow if A/B≦1.0, arranging the blend ratio so as to hold A/B>1.0 suppresses grain growth at a hydrothermal reaction to be described later, facilitating generation of minute particles.
Here, a basic compound can be added to the aqueous solution of titanium oxide to increase the solubility of the alkali earth metal compound. Here, the basic compound is hydroxide such as sodium hydroxide and calcium hydroxide, or ammonia water, for example.
Adding a basic compound in this way shifts the aqueous solution of titanium oxide to alkali, thereby increasing the solubility of the alkali earth metal compound. In this way, increasing the concentration of the reactant in the mixed solution increases the reactivity in a hydrothermal reaction to be described later.
Next, the above-described mixed solution is put into a container for a hydrothermal reaction to cause a hydrothermal reaction at 200° C. (step 3c), where the temperature for a hydrothermal reaction is preferably 200° C. or higher. This is because a hydrothermal reaction at 200° C. or higher produces a product with higher crystallinity.
Next, the mixed solution that has completed its hydrothermal reaction is dried to yield barium titanate (step 3d). Next, this dried barium titanate is washed in an acid solution as required to remove remaining carbonate (step 3e). The carbonate is assumed to be generated by the reaction between a carbon dioxide gas dissolved in the solution and unreacted barium ions. Weak acid such as acetic acid is used as the acid solution. Removing impure substances remaining by washing in this way improves the reliability of an MLCC with this material used.
Next, the barium titanate washed in the acid solution is dried (step 3f) to produce desired barium titanate.
Here, a compound containing at least one of Mg, rare earthes, Mn, and Si may be added to the above-described mixed solution, which improves the temperature characteristic of the dielectric constant. The compound containing Mg, rare earthes, Mn, and/or Si may be added in any step as long as it is before a hydrothermal reaction. Rare-earth elements here include Y, Dy, Ho, and Er, for example.
Next, a description is made for a method of manufacturing MLCCs with shape-anisotropic ceramic particles used, of the present invention, using
First, ceramic slurry is produced by mixing ceramic particles primarily containing shape-anisotropic barium titanate, additives (e.g. MgO, MnO2, SiO2, Y2O3) for adjusting the electrical characteristics, polyvinyl butyral resin as a binder, and butyl acetate as a solvent and by dispersing them (step 4a).
Next, the slurry is applied on a film of polyethylene terephthalate (referred to as PET hereinafter) by a method such as doctor blading and dried to produce a CGS (step 4b).
At this moment, ceramic particles 5 has shape anisotropy, and thus the minor axis direction of the particles is likely to point in the lamination direction, thereby increasing the number of grain boundaries in the lamination direction.
Subsequently, a paste for internal electrodes, primarily containing metal nickel powder, composed of a binder, plasticizer, and solvent is produced by a publicly known method; a pattern for internal electrodes is applied on the PET film by screen printing; and dried to produce internal electrodes (step 4c). The dimensions, shape, and position of the pattern for internal electrodes are set so that fragmented MLCCs are obtained when cut and separated in the subsequent fragmentation step.
Then, a laminated body is produced by alternately laminating the above-described CGS and the above-described internal electrodes (step 4d).
Subsequently, after laminated body 3 is fired by a publicly known method to produce a sintered body (step 4e) and then fragmented (step 4f), external electrodes are formed so as to electrically connect to the internal electrodes exposed on both end surfaces of the sintered body fragmented (step 4g) to produce MLCC 6.
In the above-described firing step, the following method can be employ to further increase the number of grain boundaries in the lamination direction.
That is, the method is firing laminated body 3 while pressurizing it. By this method, grain growth of ceramic particles 5 in the CGS is likely to occur in the direction orthogonal to the pressurizing direction by pressurizing the laminated body when firing, thereby further increasing the number of grain boundaries in the lamination direction.
Meanwhile, there is another method. That is, laminated body 3 is fired with constrained layer 7 shown in
Constrained layer 7 does not sinter at a temperature at which dielectric layer 2 sinters, and thus contraction is suppressed of the outermost layer of laminated body 3 due to sintering at firing in the direction orthogonal to the lamination direction. Meanwhile, nothing constrains contraction in the lamination direction, and thus contraction is likely to occur. Grain growth of ceramic particles 5 inside the CGS is likely to occur in the direction orthogonal to the lamination direction, thereby increasing the number of grain boundaries in the lamination direction.
Constrained layer 7 is removed after firing. At this moment, removing constrained layer 7 so that part of ceramic particles 5 contained in constrained layer 7 remains suppresses a solder flow.
In this case, the ceramic particles contained in constrained layer 7 preferably remain in an island-shaped manner so as to be dotted uniformly to the extent possible, where concretely such as blasting, polishing, or brushing is used. Thus, insulative particles can be formed (remain) on the surface of the outermost layer in the lamination direction on which external electrodes 4 are not formed.
There is another method in which internal electrode 1 is used instead of constrained layer 7. That is, the method uses the difference in sintering temperature between internal electrode 1 and dielectric layer 2. Particularly, making internal electrode 1 0.5 times or more thicker than dielectric layer 2 brings about the same effect as constrained layer 7 described above, where the upper limit of the thickness of internal electrode 1 is twice the thickness of dielectric layer 2 or less.
A multilayer ceramic capacitor of the present invention is particularly useful for such as an electronic appliance requiring thickness reduction.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007066119 | Mar 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/000584 | 3/14/2008 | WO | 00 | 12/10/2008 |
