Shih Kang Lin

Ab initio study of sodium intercalation into disordered carbon

Graphite, a predominantly chosen anode material for commercial lithium ion batteries (LIBs), has been reported to have negligible intercalation capacity as an anode for sodium ion batteries (NIBs). Disordered carbon exhibits high Na intercalation capacity and emerges as a leading candidate for NIB applications. However, the mechanism of Na+ ion insertion into disordered carbon is still controversial. Here, we propose an ab initio model for disordered carbon and investigate the intercalation mechanism of Na into the layered domains. Our ab initio calculations reveal that a larger interlayer distance and the presence of defects can effectively overcome the van der Waals interaction between graphene sheets and help Na intercalation to form NaC8. The calculation results clarify the mechanism of the Na intercalation and account for the presence of sloping and flat regions of charge–discharge curves in disordered carbon reported in numerous experiments. This reveals new prospects for helping Na intercalation into graphite.

Thermodynamic description of the Cu-Sn system

The Cu–Sn binary system is important for various applications, especially for recent developments in the electronics packaging industry. The ϵ-Cu 3 Sn and η-Cu 6 Sn 5 (η′ phases) phases are frequently encountered in electronics products. However, the two phases have been described as line compounds in previous thermodynamic modeling, and their compositional homogeneities were not considered. In this study, the thermodynamic properties of the Cu–Sn binary system are modeled and the phase diagram is calculated by the CALPHAD method, using experimental information reported in the literature. The ϵ and η (η′) phases are described using compound energy models with two and three sublattices, respectively, so that their compositional homogeneities could be calculated. Good agreement was observed between the calculated result and the existing experimental data.

Geometric and Electronic Properties of Edge-decorated Graphene Nanoribbons

Edge-decorated graphene nanoribbons are investigated with the density functional theory; they reveal three stable geometric structures. The first type is a tubular structure formed by the covalent bonds of decorating boron or nitrogen atoms. The second one consists of curved nanoribbons created by the dipole-dipole interactions between two edges when decorated with Be, Mg, or Al atoms. The final structure is a flat nanoribbon produced due to the repulsive force between two edges; most decorated structures belong to this type. Various decorating atoms, different curvature angles, and the zigzag edge structure are reflected in the electronic properties, magnetic properties, and bonding configurations. Most of the resulting structures are conductors with relatively high free carrier densities, whereas a few are semiconductors due to the zigzag-edge-induced anti-ferromagnetism.

Nano-volcanic eruption of silver

Silver (Ag) is one of the seven metals of antiquity and an important engineering material in the electronic, medical, and chemical industries because of its unique noble and catalytic properties. Ag thin films are extensively used in modern electronics primarily because of their oxidation-resistance. Here we report a novel phenomenon of Ag nano-volcanic eruption that is caused by interactions between Ag and oxygen (O). It involves grain boundary liquation, the ejection of transient Ag-O fluids through grain boundaries, and the decomposition of Ag-O fluids into O2 gas and suspended Ag and Ag2O clusters. Subsequent coating with re-deposited Ag-O and the de-alloying of O yield a conformal amorphous Ag coating. Patterned Ag hillock arrays and direct Ag-to-Ag bonding can be formed by the homogenous crystallization of amorphous coatings. The Ag “nano-volcanic eruption” mechanism is elaborated, shedding light on a new mechanism of hillock formation and new applications of amorphous Ag coatings.

Formation of solid-solution Cu-to-Cu joints using Ga solder and Pt under bump metallurgy for three-dimensional integrated circuits

AbstractThree-dimensional (3D) integrated circuits (ICs) are the most important packaging technology for next-generation semiconductors. Cu-to-Cu throughsilicon via interconnections with micro-bumps are key components in the fabrication of 3D ICs. However, significant reliability concerns have been raised due to the formation of brittle intermetallic compounds in the entire 3D IC joints. This study proposes a Ga-based Cu-to-Cu bonding technology with Pt under bump metallurgy (UBM). A systematic analysis of reactive wetting between Ga solders and polycrystalline, single-crystalline, and Ptcoated Cu substrates was conducted. Pt UBM as a wetting layer was identified to be a key component for Ga-based Cu-to-Cu bonding. Pt-coated Cu substrates were bonded using Ga solders with various Ga-to-Pt ratios (n) at 300℃. When n ≥ 4, the Cu/Pt/Ga/Pt/Cu interface evolves to Cu/facecentered cubic (fcc)/γ1-Cu9Ga4/fcc/Cu, Cu/fcc/γ1-Cu9Ga4 + Ga7Pt3/fcc/Cu, and finally Cu/fcc + Ga7Pt3/Cu structures. The desired ductile solid solution joint formed with discrete Ga7Pt3 precipitates. When n ≤ 1, a Cu/Ga7Pt3/Cu joint formed without Cu actively participating in the reactions. The reaction mechanism and microstructure evolution were elaborated with the aid of CALPHAD thermodynamic modeling.

Formation of alternating interfacial layers in Au-12Ge/Ni joints

Au-Ge alloys are promising materials for high-power and high-frequency packaging, and Ni is frequently used as diffusion barriers. This study investigates interfacial reactions in Au-12Ge/Ni joints at 300°C and 400°C. For the reactions at 300°C, typical interfacial morphology was observed and the diffusion path was (Au) + (Ge)/NiGe/Ni5Ge3/Ni. However, an interesting phenomenon – the formation of (Au,Ni,Ge)/NiGe alternating layers – was observed for the reactions at 400°C. The diffusion path across the interface was liquid/(Au,Ni,Ge)/NiGe/···/(Au,Ni,Ge)/NiGe/Ni2Ge/Ni. The periodic thermodynamic instability at the NiGe/Ni2Ge interface caused the subsequent nucleation of new (Au,Ni,Ge)/NiGe pairs. The thermodynamic foundation and mechanism of formation of the alternating layers are elaborated in this paper.

Ab initio-aided CALPHAD thermodynamic modeling of the Sn-Pb binary system under current stressing

Soldering is an ancient process, having been developed 5000 years ago. It remains a crucial process with many modern applications. In electronic devices, electric currents pass through solder joints. A new physical phenomenon – the supersaturation of solders under high electric currents – has recently been observed. It involves (1) un-expected supersaturation of the solder matrix phase, and (2) the formation of unusual “ring-shaped” grains. However, the origin of these phenomena is not yet understood. Here we provide a plausible explanation of these phenomena based on the changes in the phase stability of Pb-Sn solders. Ab initio-aided CALPHAD modeling is utilized to translate the electric current-induced effect into the excess Gibbs free energies of the phases. Hence, the phase equilibrium can be shifted by current stressing. The Pb-Sn phase diagrams with and without current stressing clearly demonstrate the change in the phase stabilities of Pb-Sn solders under current stressing.

The electromigration effect revisited: Non-uniform local tensile stress-driven diffusion

The electromigration (EM) effect involves atomic diffusion of metals under current stressing. Recent theories of EM are based on the unbalanced electrostatic and electron-wind forces exerted on metal ions. However, none of these models have coupled the EM effect and lattice stability. Here, we performed in situ current-stressing experiments for pure Cu strips using synchrotron X-ray diffractometry and scanning electron microscopy and ab initio calculations based on density functional theory. An intrinsic and non-uniform lattice expansion – larger at the cathode and smaller at the anode, is identified induced by the flow of electrons. If this electron flow-induced strain is small, it causes an elastic deformation; while if it is larger than the yield point, diffusion as local stress relaxation will cause the formation of hillocks and voids as well as EM-induced failure. The fundamental driving force for the electromigration effect is elucidated and validated with experiments.

A novel mechanism of silver microflakes sinter joining

In this paper, we describe a novel mechanism of Ag microflakes sintering which is not yet reported. It involves the generation of Ag amorphous layer, the nucleation and recrystallization of Ag nano particles and dynamic replication of the above-mentioned steps. Transmission electron microscopy (TEM) is utilized to characterize the Ag microflakes sinter joining, which reveals sintering is under the combination of this novel mechanism and low-temperature diffusion bonding.

Thin-Film Photoluminescent Properties and the Atomistic Model of Mg2TiO4 as a Non-rare Earth Matrix Material for Red-Emitting Phosphor

AbstractThin-film electroluminescent devices are promising solid-state lighting devices. Red light-emitting phosphor is the key component to be integrated with the well-established blue light-emitting diode chips for stimulating natural sunlight. However, environmentally hazardous rare-earth (RE) dopants, e.g. Eu2+ and Ce2+, are commonly used for red-emitting phosphors. Mg2TiO4 inverse spinel has been reported as a promising matrix material for “RE-free” red light luminescent material. In this paper, Mg2TiO4 inverse spinel is investigated using both experimental and theoretical approaches. The Mg2TiO4 thin films were deposited on Si (100) substrates using either spin-coating with the sol–gel process, or radio frequency sputtering, and annealed at various temperatures ranging from 600°C to 900°C. The crystallinity, microstructures, and photoluminescent properties of the Mg2TiO4 thin films were characterized. In addition, the atomistic model of the Mg2TiO4 inverse spinel was constructed, and the electronic band structure of Mg2TiO4 was calculated based on density functional theory. Essential physical and optoelectronic properties of the Mg2TiO4 luminance material as well as its optimal thin-film processing conditions were comprehensively reported.