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We developed a new one-pot method for the synthesis of Cu nanoparticles capped with fatty acids and amines from an insoluble salt, such as Cu carbonate and Cu hydroxide, in ethylene glycol. This method could be completed within a short period of time and provides a high collection rate from inexpensive raw materials without impurities. The mean diameter of the Cu nanoparticles was controlled from 93 to 13 nm as the alkyl carbon number increased from C10 to C22. The same fraction of fatty acids and amines used for capping agents was suitable to obtain the minimum size of Cu nanoparticles. The thermal decomposition temperature of the capping layer was lower than 300 C even in an inert atmosphere. Higher strengths of the Cu plates bonded by the Cu nanoparticles were achieved owing to the more densely packed sintered structures by the smaller Cu nanoparticles. The shear strength of the Cu plates bonded by the Cu nanoparticles was higher than 30 MPa which was the same level as for ordinary solders even though the process temperature of 300 C was much lower than high-temperature solders. The minimum electrical resistivity of the sintered Cu nanoparticle film was 13 mU cm which was obtained after annealing at 300 C.

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Magnetic cores of passive components are required to have low hysteresis loss, which is dependent on the coercive force. Since it is well known that the coercive force becomes zero at the superparamagnetic regime below a certain critical size, we attempted to synthesize Ni nanoparticles in a size-controlled fashion and investigated the effect of particle size on the magnetic properties. Ni nanoparticles were synthesized by the reduction of Ni acetylacetonate in oleylamine at 220 °C with trioctylphosphine (TOP) as the capping agent. An increase in the TOP/Ni ratio resulted in the size decrease. We succeeded in synthesizing superparamagnetic Ni nanoparticles with almost zero coercive force at particle size below 20 nm by the TOP/Ni ratio of 0.8. However, the saturation magnetization values became smaller with decrease in the size. The saturation magnetizations of the Ni nanoparticles without capping layers were calculated based on the assumption that the interior atoms of the nanoparticles were magnetic, whereas the surface-oxidized atoms were non-magnetic. The measured and calculated saturation magnetization values decreased in approximately the same fashion as the TOP/Ni ratio increased, indicating that the decrease could be mainly attributed to increases in the amounts of capping layer and oxidized surface atoms.

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