Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Reexamination Certificate
2001-11-09
2004-02-10
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
C419S036000, C419S037000
Reexamination Certificate
active
06689311
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for manufacturing a sinter by sintering a laminate including a layer containing a ceramic powder or a metal powder and an organic layer such as a binder or a plasticizer, and other techniques related thereto.
In recent years, a multilayer ceramic component, including ceramic layers and conductive layers layered on one another, has been downsized and the performance thereof has been improved. A multilayer ceramic component will now be described with respect to a multilayer ceramic capacitor, which is one type of a multilayer ceramic component. For a multilayer ceramic capacitor, which has been used as a main component of a mobile station (hand held terminal) in a mobile communication system, such as a portable telephone, there is a demand for reducing the size thereof while increasing the capacity thereof, i.e., improving two contradicting characteristics thereof. To meet such a demand, attempts have been made in the art to reduce the thickness of a dielectric layer, increasing the number of layers, and increasing the dielectric constant thereof by developing a new material therefor. However, since there is a limit on the reduction of the thickness of a dielectric layer or the increase in the dielectric constant thereof by developing a new material therefor, the area of a multilayer ceramic capacitor has been increasing each year in order to increase the capacity thereof.
A multilayer ceramic capacitor is typically manufactured as follows. First, a dielectric material powder, a binder, a plasticizer, a dispersant, a solvent, etc., are mixed together and stirred to produce a dielectric slurry having an appropriate viscosity. Typically, barium titanate is used for the dielectric material, an acrylic resin for the binder, an ester-based plasticizer such as dibutyl phthalate for the plasticizer, an anionic surfactant such as carboxylate for the dispersant, and an ester such as butyl acetate, an alcohol, an ether or a hydrocarbon for the solvent. On the other hand, an internal electrode paste is produced by mixing a metal powder such as nickel and an organic substance together into a paste.
Then, a dielectric film is produced by directly screen-printing the dielectric slurry using a print laminator, or the like. Then, an internal electrode pattern is produced similarly by directly screen-printing the internal electrode paste using a different screen. The screen printing process for a dielectric film and an internal electrode pattern as described above is repeated for a desired number of times so as to produce a green sheet including electrodes and dielectric films alternating with each other. After the lamination step, the green sheet is cut into pieces according to the size of each printed chip, thereby obtaining a green laminate. Then, the green laminate is placed into a case in preparation for baking, and the process proceeds to a degreasing step and a baking step. The degreasing step refers to a step of removing organic substances contained in the green laminate such as a binder, a plasticizer, a solvent, etc. The baking step refers to a step of sintering through a reaction between ceramic grains. After the completion of the baking step, an external electrode, etc., are formed, after optional steps such as polishing the side surface, etc., in order to connect the internal electrodes to each other and at the same time to extract the terminals to the outside. Depending on the structure of the ceramic laminate component, a base electrode, an intermediate electrode, etc., may be formed.
The degreasing step takes a large portion of the total amount of time for the manufacture of the multilayer ceramic capacitor. As described above, in the degreasing step, organic substances such as a binder and a plasticizer, which are used for maintaining the shape of the green sheet, are decomposed and removed. Normally, the degreasing step is performed in the air. In order to increase the manufacturing efficiency, it is preferred to perform the degreasing step while rapidly increasing and decreasing the temperature of the furnace. However, if the furnace is rapidly heated, the organic substances such as a binder and a plasticizer are rapidly evaporated and decomposed, which may induce structural defects such as delamination or a crack. Such defects significantly influence the quality of the product. In view of this, the green laminate is presently degreased by slowly increasing and decreasing the temperature through a temperature range of 150 to 300° C. over a time period of 20 hours to several days. In recent years, the amount of time for the degreasing step has been increasing due to an increase in the size (particularly, the area) of a chip along with an increase in the capacitance of a capacitor. Moreover, while degreasing is done in order to remove organic substances from a green laminate, as described above, the green laminate contains a mixture of various organic substances, including a binder, a plasticizer, a dispersant, a solvent, etc., and these organic substances have different evaporation temperatures and different decomposition temperatures. Therefore, the degreasing step requires a very precise temperature control and process control.
As described above, the degreasing step requires a very precise temperature control and process control, and the amount of time for the degreasing step has been increasing due to an increase in the size and capacitance of a capacitor. With the conventional thermal decomposition/removal method performed in the air, it is very difficult to shorten the process time by speeding the degreasing step. On the other hand, many problems occur if the baking step is performed without sufficiently removing the organic substances such as a binder. Particularly, if there remains an amount of plasticizer as a result of failing to sufficiently remove it during the degreasing step, benzene rings in the plasticizer react in the baking step to produce a graphite-like substance, and the graphite-like substance causes various defects in the multilayer ceramic component. First, the graphite-like to substance has an expansion coefficient different from those of ceramics, an internal electrode material, etc., whereby it is likely to induce structural defects such as delamination or a crack as described above. Moreover, the graphite-like substance has a &pgr; electron and thus a high conductivity, whereby it may cause a leak current between the internal electrodes. As described above, if the plasticizer is not removed in the degreasing step, the manufacturing yield decreases, and the performance of the multilayer ceramic component also decreases. Therefore, it has been unavoidable to perform a time-consuming degreasing step in the prior art.
Moreover, with the conventional thermal decomposition/removal method performed in the air, an organic substance may be present in a gas that is discharged during the de-binder step, thereby also presenting an environmental problem. Therefore, it has been necessary to completely decompose and incinerate the organic substance or remove the organic substance by adsorption, etc., thus presenting a cost problem.
Methods for degreasing ceramics by using a supercritical fluid have been proposed in the art (e.g., Chemical Engineering, 1986 May issue, pp. 46-49, Shozaburo Saito, “Chourinkairyutai No Kagaku to Gijutsu (Science And Technology Of Supercritical Fluid)”, Sankyo Business). Generally, a supercritical fluid has a high dissolving ability. The degreasing of a ceramics compact can be done in a very short time by removing a binder using a supercritical fluid. With this method, however, the function of maintaining the shape of a green laminate deteriorates when removing a plasticizer and a binder from the green laminate, thereby deteriorating the precision in the shape of the product.
SUMMARY OF THE INVENTION
The present invention is based on a discovery that the total amount of time for the degreasing step can be reduced by quickly removing only a plasticizer a
Itakura Gen
Morita Kiyoyuki
Okinaka Hideyuki
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