With the recent trend toward size reduction in electronic appliances, the electronic components for use therein are more and more required to be reduced in size. In functional components employing ceramics, such as inductors and capacitors, among those electronic components, size reductions and characteristics improvements have come to be attained by a multilayer structure. Such a multilayer component is produced by dispersing a metal powder, e.g., a noble-metal powder, in an organic-binder-containing organic solvent to obtain an electroconductive paste, printing the paste on a ceramic green sheet, subjecting the green sheet to the steps of stacking, press bonding, and cutting, thereafter burning the green sheet, and forming external electrodes.
Noble-metal powders for use in such electroconductive pastes are required to have a narrow particle-size distribution range and to have high purity and high crystallinity.
Patent Document 1, for example, describes a feature wherein a platinum powder having a narrow particle-size distribution range and high purity is obtained by a process for obtaining a highly crystalline platinum powder, in which platinum black is mixed with calcium carbonate by a wet process, the mixture is dried, subsequently pulverized, and then burned to remove carbon dioxide, the remaining calcium oxide is dissolved away with a dilute acid and removed by washing with water, and the residue is dried to obtain a platinum powder.
However, in the case where the process described in Patent Document 1 is used, there has been a problem in that the highly crystalline platinum powder to be obtained considerably varies in property in cases when either the platinum black, which is a platinum powder, or the calcium carbonate has abnormal properties. Furthermore, since platinum black, which is a platinum powder, is produced first and this platinum black is subjected to the treatments, including wet-process mixing with calcium carbonate, to produce a highly crystalline platinum powder, it is necessary to produce a platinum powder twice in total. The process described in Patent Document 1 hence has had a problem in that the number of production steps is large, resulting in an increase in cost.
Patent Document 2 describes, as a process for overcoming such problems, a process for producing a fine powder of one or more metals selected from among platinum, gold, rhodium, palladium, silver, copper, and nickel, the process being characterized by including: an aqueous-metal-compound-solution preparation step in which one or more water-soluble compounds of the metal(s) are dissolved in water to obtain an aqueous solution thereof having a pH of 4 or less; a reaction step in which a powder of the hydroxide(s) of one or more Group 2A element metals of the periodic table selected from among calcium hydroxide, magnesium hydroxide, and barium hydroxide and/or an aqueous slurry of the hydroxide(s) is mixed with the aqueous solution having a pH of 4 or less and the pH of the resultant mixture is adjusted to 10 or higher; a first separation step in which the insoluble solid obtained through the reaction step is taken out and dried; a heat treatment step in which the separated insoluble solid is heated in an inert gas or hydrogen gas atmosphere at a temperature which is 800° C. or higher but is not higher by 100° C. or more than the lowest of the melting points of the selected metals; an acid treatment step in which the solid that has undergone the heat treatment is immersed in an aqueous acid solution and the pH is kept at 4 or less; and a second separation step in which the metal particles present in the aqueous acid solution used in the acid treatment are taken out, washed, and dried.