Sinn-wen Chen

Electromigration effect upon the Sn/Ag and Sn/Ni interfacial reactions at various temperatures

Abstract This study investigated the effects of the passage of electric currents of 500 A/cm2 density through Sn/Ag couples annealed at 120 and 160 °C. The results showed that when the direction of electron flow was from the Sn side to Ag, it enhanced the growth of the Ag3Sn phase at the interface, and it retarded the Ag3Sn phase growth when the electrons flowed from the Ag side to Sn. Similar results were found in the Sn/Ni system. The Ni3Sn4 phase formed in the Sn/Ni couples annealed at 160 and 180 °C. The growth rate of the Ni3Sn4 phase increased when the electrons flowed from the Sn side to Ni side, and decreased if the direction of electron flow was reversed. The thickness of the reaction layers was measured, and the apparent effective charges z a ∗ for Sn were determined. The values of z a ∗ decreased with increasing temperatures, which indicated that the effect of electromigration on interfacial reactions became less significant at higher temperatures.

Electromigration effect upon the Sn–0.7 wt% Cu/Ni and Sn–3.5 wt% Ag/Ni interfacial reactions

This study investigates the effect of electromigration upon the interfacial reactions between the promising lead-free solders, Sn–Cu and Sn–Ag, with Ni substrate. Sandwich-type reaction couples, Sn–0.7 wt% Cu/Ni/Sn–0.7 wt% Cu and Sn–3.5 wt% Ag/Ni/Sn–3.5 wt% Ag, were reacted at 160, 180, and 200 °C for various lengths of time with and without the passage of electric currents. Without passage of electric currents through the couples, only one intermetallic compound Ni3Sn4 with ∼7 at. % Cu solubility was found at both interfaces of the Sn–0.7 wt% Cu/Ni couples. With the passage of an electric current of 500 A/cm2 density, the Cu6Sn5 phase was formed at the solder/Ni interface besides the Ni3Sn4 phase. Similar to those without the passage of electric currents, only the Ni3Sn4 phase was found at the Ni/solder interface. Directions of movement of electrons, Sn, and Cu atoms are the same at the solder/Ni interface, and the growth rates of the intermetallic layers were enhanced. At the Ni/solder interface, the el...

Phase diagram of In–Co–Sb system and thermoelectric properties of In-containing skutterudites

In-containing skutterudites have long attracted much attention and debate partly due to the solubility limit issue of indium in CoSb3. The isothermal section of the equilibrium phase diagram for the In–Co–Sb system at 873 K is proposed using knowledge of the related binary phase diagrams and experimental data, which explains the debated indium solubility that depends on Sb content. In this paper, a series of In-containing skutterudite samples (InxCo4Sb12−x/3 with x varying from 0.075 to 0.6 and In0.3Co4−ySb11.9+y with y changing from −0.20 to 0.20) are synthesized and characterized. X-ray analysis and scanning electron microscopy images indicate that, up to x = 0.27, single-phase skutterudites are obtained with lattice constant increasing with In fraction x. A fixed-composition skutterudite In0.27±0.01Co4Sb11.9 was determined for the Co-rich side of In–CoSb3 which is in coexistence with liquid InSb and CoSb2. Indium, like Ga, is expected, from DFT calculations, to form compound defects in In-containing skutterudites. However, relatively higher carrier concentrations of In-containing skutterudites compared to Ga-containing skutterudites indicate the existence of not fully charge-compensated compound defects, which can also be explained by DFT calculations. The net n-type carrier concentration that naturally forms from the complex defects is close to the optimum for thermoelectric performance, enabling a maximum zT of 1.2 for the fixed skutterudite composition In0.27Co4Sb11.9 at 750 K.

Phase equilibria of the Cu−In system. I: Experimental investigation

Portions of the Cu-In phase diagram were redetermined experimentally, specifically in the compositional region from 32.0 to 100 at % In. The experimental techniques used were differential scanning calorimetry (DSC), differential thermal analysis (DTA), powder X-ray diffraction (XRD), and electron probe microanalysis (EPMA).The results indicate that the phase Cu11ln9 is stable to low temperatures rather than decomposing at 157 °C. The existence of the“phase bundle” at compositions ranging from 32 to 38 at.% In was not supported by our experimental data. Only two phases were found to exist: η at temperatures higher than 305 to 389 °C, and η at lower temperatures. Minor phase boundary adjustments were made in this region. The peritectic temperature for the formation of Cu11ln9 was found to be 307 ± 1 °C, and the L = Cu11ln9 + (In) eutectic temperature was found to be 154 ± 1 °C. In the study to determine the temperature stability of Cu11lns9, it was found that Culm exists at low temperatures, but the stability was not investigated.

Interfacial Reactions in the Sn-Bi/Te Couples

Sn-Bi alloys are promising Pb-free solders and Te is the primary constituent element of the bismuth telluride thermoelectric materials. Interfacial reactions in the Sn-Bi/Te couples are examined herein. Very unusual cruciform patterns have been observed in the Sn-Bi/Te couples reacted at 250°C. The reaction product is the SnTe phase. The reaction layer thickness is very uniform along the Te substrate edges, and there are no reaction products at the substrate corners. The cruciform patterns of the reaction layers gradually disappear in the Sn-Bi/Te reaction couples with higher Bi contents, and the critical composition is about Sn-36.25at.%Bi (Sn-50wt.%Bi). The reaction product is also the SnTe phase in the Sn/Te couple reacted at 210°C but without the cruciform pattern. At 210°C, the SnTe phase layer in the Sn/Te couple has a duplex structure, and its growth rate follows a parabolic law.

The relationship between the peak shape of a DTA curve and the shape of a phase diagram

Abstract DTA curves are simulated based on the heat transfer modeling of the DTA cells. The calculated results are compared with the experimental determinations. The rates of heat evolution or absorption of the specimens under the DTA scan are correlated with the shapes of phase diagrams. The relationships between the shapes of DTA curves and the shapes of phase diagrams are discussed.

Solubility design leading to high figure of merit in low-cost Ce-CoSb3 skutterudites.

CoSb3-based filled skutterudite has emerged as one of the most viable candidates for thermoelectric applications in automotive industry. However, the scale-up commercialization of such materials is still a challenge due to the scarcity and cost of constituent elements. Here we study Ce, the most earth abundant and low-cost rare earth element as a single-filling element and demonstrate that, by solubility design using a phase diagram approach, the filling fraction limit (FFL) x in CexCo4Sb12 can be increased more than twice the amount reported previously (x=0.09). This ultra-high FFL (x=0.20) enables the optimization of carrier concentration such that no additional filling elements are needed to produce a state of the art n-type skutterudite material with a zT value of 1.3 at 850 K before nano-structuring. The earth abundance and low cost of Ce would potentially facilitate a widespread application of skutterudites.

Solidification curves of AlCu, AlMg and AlCuMg alloys

Abstract Three different methods have been utilized to determine the solidification curves of the Al-rich Al Cu, Al Mg and Al Cu Mg alloys. These three methods, i.e. a conventional quenching and image analysis method, calculations using two simplistic solidification models, and a proposed DTA coupled with mathematical modeling method, have been illustrated. Obtained by the three different methods, the solid fractions versus solidification temperatures of the solidifying samples are compared with one another, and the three different methods are discussed. Advantages of using the proposed DTA coupled with mathematical modeling method to determine the solidification curves are demonstrated. A ∼20°C undercooling of these Al-rich alloys during solidification has been observed by using DTA. The primary solidification phases are α-Al phase for all seven alloys. The secondary solidification phases have also been identified by using metallography, SEM (Scanning Electron Microscopy) and EPMA (Electron Probe Microanalysis).

Calculation of phase diagrams and solidification paths of Al-rich Al−Li−Cu alloys

Thermodynamic models for the various phases in the Al-rich corner of the Al−Li−Cu system were developed on the basis of the phase equilibrium and limited thermodynamic data available in the literature and the thermodynamic descriptions of the three consitituent binary systems, Al−Li, Al−Cu, and Cu−Li. The calculated isothermal section at several temperatures and the liquidus projection are in ageement with the experimental determinations. Combining the thermodynamic models and the Scheil model or the modified Scheil model to incllude solidstate back-diffusion, quantitative solidification paths were predited. The calculated amount of the primary phase formed during solidification was compared with those obtained experimentally.

Phase equilibria of the Al-Li binary system

The solid + liquid phase equilibria between α-Al and β-AlLi were determined using differential thermal analysis (DTA), metallography, and chemical analysis. Boron nitride (BN), which was found to be inert to these alloys, was used as the container. These measurements were carried out in order to resolve the discrepancies reported in the literature. The α-Al+β-AlLi eutectic temperature and composition were determined to be 600 °C±1 °C and 25.8±0.5 at. pct Li. Using these data and data reported in the literature concerning the phase equilibria and thermodynamic properties, thermodynamic models for all the phases were obtained by optimization. The thermodynamic values obtained for the β-AlLi phase describe not only the phase equilibria, but also yield structural defect data in agreement with measured values. The assessed enthalpies of formation, excess entropies of formation, and entropies of melting for all the intermetallic phases obtained are compared with empirical correlations when experimental data are not available. In addition to the stable diagram, a metastable diagram involving the δ′-Al3Li is also calculated from the thermodynamic models. The calculated diagram is in good agreement with the experimental data.