Toshitaka Ishizaki

A new one-pot method for the synthesis of Cu nanoparticles for low temperature bonding

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 μΩ cm which was obtained after annealing at 300 °C.

Effect of Particle Size on the Magnetic Properties of Ni Nanoparticles Synthesized with Trioctylphosphine as the Capping Agent

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.

Synthesis of Au@Ag@Cu trimetallic nanocrystals using three-step reduction

Au@Ag@Cu trimetallic nanocrystals were prepared using a three-step reduction method. In the first step, decahedral Au core seeds were prepared by reducing HAuCl4·4H2O in diethylene glycol (DEG) under oil-bath heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. In the second step, Ag shells were overgrown on these Au seeds in N,N-dimethylformamide (DMF) in the presence of PVP under oil-bath heating to prepare decahedral Au@Ag nanocrystals. In the third step, Cu shells were overgrown further on Au@Ag core–shell nanocrystals in ethylene glycol (EG) in the presence of PVP under oil-bath heating. The resultant crystal shapes were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and X-ray diffraction (XRD) measurements. Results show that Cu shells of two kinds are grown over Au@Ag core seeds: a phase-separated major Cu component attached to one or two side edges of decahedral Au@Ag cores, and a minor Cu component that appears as thin Cu shells over decahedral Au@Ag cores. Partial reservation of pentagonal shape and appearance of Moire patterns in Au@Ag@Cu particles suggest that epitaxial growth occurs on some parts of the Au@Ag cores despite a large lattice mismatch between Ag and Cu (11.5%). The growth mechanism of Au@Ag@Cu nanocrystals was discussed in terms of lattice mismatch, decahedral particle defects, and the favorable shape of metallic shells. Optical properties of Au@Ag@Cu nanocrystals were determined by measuring extinction spectra.

Syntheses of Au–Cu-rich AuAg(AgCl)Cu alloy and Ag–Cu-rich AuAgCu@Cu core–shell and AuAgCu alloy nanoparticles using a polyol method

Core–shell and alloy types of nanoparticles including Au, Ag, and Cu components were prepared by reducing mixtures of HAuCl4·4H2O, AgNO3, and Cu(OAc)2·H2O in ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP) at 175 °C. At a HAuCl4·4H2O : AgNO3 : Cu(OAc)2·H2O molar ratio of 1 : 2 : 1, mixtures of Au–Cu-rich AuAg(AgCl)Cu alloy nanoparticles and AgCl precipitates were formed after 2.5–35 min heating. On the other hand, at a HAuCl4·4H2O : AgNO3 : Cu(OAc)2·H2O molar ratio of 0.0065 : 2 : 1, at which the formation of AgCl precipitate was suppressed, Ag–Cu-rich AuAgCu alloy particles were prepared via AuAgCu@Cu core–shell particles after 2.5–34 min heating. The growth mechanisms of AuAg(AgCl)Cu, AuAgCu@Cu, and AuAgCu particles were examined using TEM-energy dispersed X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectral data. The time dependence of UV-Vis-NIR spectral data indicated that the Cu components of AuAg(AgCl)Cu and AuAgCu alloy particles retained good anti-oxidation properties about 1 month after preparation.

Thermal simulation of joints with high thermal conductivities for power electronic devices

Abstract The thermal properties of new power modules joined by materials with high thermal conductivities, such as Ag or Cu nanoparticle joints, can differ from those of current modules joined by ordinary solders with low thermal conductivities. However, these properties have not been thoroughly investigated thus far. The overall thermal resistance of a simple simulation module was calculated by the 3-dimensional finite element method to study the correlation between the thermal conductivity of the joint layer and the thermal properties. The calculation results identified an optimal thickness to achieve the minimum thermal resistance when the thermal conductivity of the joint layer is much higher than that of the heatsink. This is presumed to occur because the thermal resistance decreases in the heatsink much more than it increases in the joint layer, owing to the increased uniformity of thermal spreading as the joint-layer thickness increases to the optimal value. This effect of thermal resistance reduction with thickening of the joint layer is seen when the thermal conductivity of the joint layer is sufficiently higher than that of the heatsink and the area of the joint layer is sufficiently smaller than that of the heatsink. The same effect is also expected in an actual module with a joint between a silicon carbide chip and a direct bonded copper substrate. This study reveals that the design concept for power modules should change to preliminarily estimate the optimal thickness to achieve the minimum thermal resistance when the thermal conductivity of the joint layer is much higher than that of the heatsink.

Enhancement of pressure-free bonding with Cu particles by the addition of Cu–Ni alloy nanoparticles

Cu–Ni alloy nanoparticles (∼20 nm) were synthesized with precise control over the resulting Cu and Ni proportions. Nanoparticles composed of 29 wt% Cu and 71 wt% Ni exhibited significant resistance to surface oxidation and were readily sintered, suggesting that they could serve as bonding materials. The adhesive strength of Cu plates bonded by Cu particles of 150–200 nm in size was enhanced by adding these Cu–Ni alloy nanoparticles to the larger Cu particles. This is the first example of the use of base metal nanoparticles at a low-temperature (250 °C) under pressure-free conditions to achieve bonding strengths equivalent to those obtained when applying the standard Pb-free solder (Sn–Ag–Cu). Furthermore, a sintered Cu film with good electric conductivity was obtained using the mixture of Cu–Ni nanoparticles and Cu particles. The enhanced properties of these bonded layers were due to the formation of a densely sintered layer resulting from the addition of the Cu–Ni alloy nanoparticles to the Cu paste. This method could potentially have applications in the electrical packaging industry, since it represents a simple, economical process that results in highly conductive bonds.

Effects of Au fraction on the morphology and stability of Au–Ag–Cu trimetallic particles prepared using a polyol method

AuAgCu alloy and phase-separated AuAgCu/Cu nanoparticles were prepared by reducing mixtures of Au(OAc)3, AgNO3, and Cu(OAc)2·H2O in ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP) at 175 °C. At Au(OAc)3:AgNO3:Cu(OAc)2·H2O molar ratios of 1:1:1, short string and peanut types of AuAgCu alloy nanoparticles were formed after 2.5–40 min heating. For Au(OAc)3:AgNO3:Cu(OAc)2·H2O molar ratios of 0.5:1:1 and 0.1:1:1, peanut-type or spherical AuAgCu alloy core Cu shell nanoparticles denoted as AuAgCu@Cu were produced after 5 min heating. They changed to a mixture of AuAgCu alloy and phase-separated AuAgCu/Cu nanoparticles after 17.5–37 min heating. The growth mechanisms of AuAgCu, AuAgCu@Cu, and AuAgCu/Cu particles were examined using TEM energy dispersed X-ray spectroscopic (EDS), XRD, and ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectral data. The time dependence of UV-Vis-NIR spectral data indicated that AuAgCu and AuAgCu/Cu nanoparticles retain high stability including anti-oxidation properties for a long time after preparation.