A multilayer ceramic capacitor, one example of an electronic device, is comprised of a multilayer structure device body including pluralities of dielectric layers and internal electrode layers alternately stacked and a pair of external terminal electrodes formed on the two ends of the device body.
This multilayer ceramic capacitor is produced by first alternately stacking exactly the required number of pre-firing dielectric layers and pre-firing internal electrode layers to produce a pre-firing device body, then firing this, then forming a pair of external terminal electrodes on the two ends of the fired device body.
As the pre-firing dielectric layers, ceramic green sheets are used, while as the pre-firing internal electrode layers, internal electrode paste layers of predetermined patterns, metal thin films, etc. are used.
Ceramic green sheets can be produced by the sheet method, the drawing method, etc. The “sheet method” is the method of coating a dielectric coating containing a dielectric powder, a binder, a plasticizer, an organic solvent, etc. by the doctor blade method etc. on a PET or other carrier sheet then drying it by heating. The “drawing method” is the method of biaxially drawing a film shaped body obtained by extruding a dielectric suspension comprised of a dielectric powder and a binder mixed in a solvent.
Internal electrode paste layers of predetermined patterns are produced by printing method. The “printing method” is the method of coating a conductive coating containing a conductive material including Pd, Ag—Pd, Ni, or another metal, a binder, an organic solvent, etc. on a ceramic green sheet in a predetermined pattern. Metal thin films of predetermined patterns are produced by sputtering or another thin film method.
In this way, when producing a multilayer ceramic capacitor, the pre-firing dielectric layers and pre-firing internal electrode layers are simultaneously fired. For this reason, it is required that the conductive material contained in the pre-firing internal electrode layers have a higher melting point than the sintering temperature of the dielectric powder contained in the pre-firing dielectric layers, not react with the dielectric powder, and not disperse in the dielectric layers after firing.
In the past, to satisfy these demands, Pt, Pd, and other precious metals have been used for the conductive material contained in the pre-firing internal electrode layers. However, precious metals are themselves expensive. As a result, there was the defect that the finally obtained multilayer ceramic capacitors became high in cost. Therefore, in the past, it has been widely known to lower the sintering temperature of the dielectric powder to 900 to 1100° C. and use an Ag—Pd alloy for the conductive material contained in the pre-firing internal electrode layers or to use Ni or another inexpensive base metal.
However, in recent years, the reduction in size of various types of electronic equipment has led to demand for smaller in size and larger capacity multilayer ceramic capacitors for assembly into electronic equipment. To further reduce the size and increase the capacity of multilayer ceramic capacitors, it is required to stack not only dielectric layers, but also internal electrode layers thinner and with fewer defects.
However, illustrating the case of use of Ni for the conductive material included in the pre-firing internal electrode layers, this Ni has a lower melting point than the dielectric powder included in the pre-firing-dielectric layers. For this reason, when simultaneously firing these, a large difference occurred between the sintering temperatures of the two. If sintering at the higher temperature when there is a large difference in the sintering temperatures, the internal electrode layers will crack or peel off, while if sintering at the lower temperature, firing defects of the dielectric powder will sometimes occur.
Further, if reducing the thickness of the pre-firing internal electrode layers, during the firing in a reducing atmosphere, the Ni particles included in the conductive material grow and become spheroidal. The adjoining Ni particles which had been connected before firing become separated resulting in voids at all different locations. As a result, it becomes difficult to continuously form fired internal electrode layers. When the fired internal electrode layers are not continuous, there is the problem that the multilayer ceramic capacitor falls in electrostatic capacity.
However, the following Patent Document 1 shows the method of alloying the internal electrode layers so as to prevent disconnection of the internal electrodes. However, this Patent Document 1 considers control of the alloy difficult with the thin film formation method, prepares internal electrode layers made of multilayer metal films before firing, and alloys them in the firing stage.
However, this Patent Document 1 does not disclose anything regarding what kind of metal may be used to alloy with so as to suppress growth of nickel particles in the firing stage and prevent spheroidization so as to prevent electrode disconnection when using internal electrodes having nickel as their main ingredients. Depending on the composition forming the multilayer metal films, conversely the sintering temperature ends up being lowered and growth of the nickel particles in the firing stage cannot be suppressed.
Further, when the metal film contacting a ceramic when forming a multilayer metal film is poor in wettability and bondability with the ceramic, conversely spheroidization and disconnection end up occurring and the capacitor falls in electrostatic capacity.
Further, the following Patent Document 2 proposes a conductive paste having nickel as a main ingredient and containing a metallocene expressed by a general formula M[(C5H5)2] (M is at least one kind of Ru, Os, Pd, Cr, and Co).
However, in this Patent Document 2, the conductive paste only contains the nickel particles and metallocene separately dispersed and does not cover the nickel particles with precious metal particles.
For this reason, the effect of suppression of grain growth of the nickel particles at the firing stage was weak, the effect of preventing spheroidization was weak, and a method able to effectively prevent electrode disconnection has been sought.
Therefore, the applicant filed a patent application shown in the following Patent Document 3 to solve these problems. This patent application discloses conductive particles obtained by sputtering to form a thin film of an alloy of Ni and a precious metal and pulverizing this thin film to obtain an alloy powder for use for forming internal electrode layers and conductive particles comprised of nickel particles coated with a platinum layer. By using such conductive particles, the applicant discovered that it is possible to provide a multilayer ceramic capacitor or other electronic device able to suppress growth of the Ni particles in the firing stage, effectively prevent spheroidization and electrode disconnection, and effectively suppress a drop in electrostatic capacity even when the internal electrode layers are reduced in thickness.
However, to produce the above conductive particles, in the case of alloy particles, there was the problem that the steps were increased such as by the sputtering and the production process became longer. Further, in the case of coated particles, when depositing platinum around the nickel particles, segregated particles of platinum of several μm or more were produced and the nickel particles were sometimes not coated by platinum. Therefore, there was the problem of a poor efficiency of production of platinum-coated nickel particles.
Note that as shown in Patent Document 4, the method of production of conductive particles of forming a nickel or other base metal coating layers on the surfaces of the particles of palladium or other precious metal powder is known. However, with the art disclosed in this publication, the precious metal particles have a distribution of size of 0.3 to 0.5 μm. The particle size is too large for internal electrode layers reduced in thickness, so finer sized particles have been sought. Further, in the invention shown in this Patent Document 4, since Ni or another base metal is used for the outside coating layers, when making Ni deposite on the surfaces of the cores of Pt particles during the reduction deposition step, the Ni coatings sometimes are oxidized to form Ni oxides when the coatings are too thin. Therefore, there were limits with the art described in Patent Document 4 when trying to use an easily oxidizable substance such as Ni as the main metal material of the internal electrodes.
Patent Document 1: Japanese Patent Publication (A) No. 3-126206
Patent Document 2: Japanese Patent Publication (A) No. 10-214520
Patent Document 3: International Publication No. WO04/070748
Patent Document 4: Japanese Patent Publication (A) No. 8-212827