P. J. Shang

Micro-sized and Nano-sized Fe3O4 Particles as Anode Materials for Lithium-ion Batteries

Micro-sized (1030.3 +/- 178.4 nm) and nano-sized (50.4 +/- 8.0 nm) Fe(3)O(4) particles have been fabricated through hydrogen thermal reduction of alpha-Fe(2)O(3) particles synthesized by means of a hydrothermal process. The morphology and microstructure of the micro-sized and the nano-sized Fe(3)O(4) particles were characterized by X-ray diffraction, field-emission gun scanning electron microscopy, transmission electron microscopy and high-resolution electron microscopy. The micro-sized Fe(3)O(4) particles exhibit porous structure, while the nano-sized Fe(3)O(4) particles are solid structure. Their electrochemical performance was also evaluated. The nano-sized solid Fe(3)O(4) particles exhibit gradual capacity fading with initial discharge capacity of 1083.1 mAhg(-1) and reversible capacity retention of 32.6% over 50 cycles. Interestingly, the micro-sized porous Fe(3)O(4) particles display very stable capacity-cycling behavior, with initial discharge capacity of 887.5 mAhg(-1) and charge capacity of 684.4 mAhg(-1) at the 50th cycle. Therefore, 77.1% of the reversible capacity can be maintained over 50 cycles. The micro-sized porous Fe(3)O(4) particles with facile synthesis, good cycling performance and high capacity retention are promising candidate as anode materials for high energy-density lithium-ion batteries.

Synthesis of Single-Crystal TiO2 Nanowire Using Titanium Monoxide Powder by Thermal Evaporation

TiO2 nanowires were synthesized successfully in a large quantity by thermal evaporation using titanium monoxide powder as precursor. X-ray diffraction results showed that all the products were pure rutile phase of TiO2. According to microstructural observations, the nanowires have two typical morphologies, a long straight type and a short tortuous type. The straight nanowires were obtained at a wide temperature range of 900-1050 degrees C, while the tortuous ones were formed below 900 degrees C. Transmission electron microscopy characterization revealed that both the straight and the tortuous nanowires are single-crystal rutile TiO2. The preferential growth direction of the nanowires was determined as [110] orientation according to electron diffraction and high-resolution image analyses. The morphological change of TiO2 nanowires was discussed by considering the different atomic diffusion rates of Ti atoms caused by the phase transformation in Ti substrate at around 900 degrees C.

Intermetallic compound identification and Kirkendall void formation in eutectic SnIn/Cu solder joint during solid-state aging

Microstructural investigations were performed on the interfacial reactions between eutectic SnIn solder and Cu substrate during reflowing at 433 K and solid-state aging at 373 K. Cu2(In,Sn) was identified as the only intermetallic compound (IMC) at the interface, which consists of two sublayers with different morphology, a fine-grained sublayer at the Cu side and a coarse-grained sublayer at the solder side. During solid-state aging, voids were found between these two Cu2(In,Sn) sublayers but not at the substrate interface, which is also attributed to the Kirkendall effect considering the different diffusion fluxes of Sn or In and Cu atoms in different sublayers.

Ex situ observations of fast intermetallic growth on the surface of interfacial region between eutectic SnBi solder and Cu substrate during solid-state aging process

We focused on the surface microstructure and morphology changes of Cu/eutectic SnBi/Cu joint during solid-state aging process through ex situ scanning electron microscopy (SEM) observations and energy dispersive X-ray spectroscopy (EDXS) analysis. Different intermetallic compound (IMC) growth behaviors on the surface from the bulk of solder joints were observed and investigated. The results indicated that at the initial stage of solid-state aging IMCs protruded from the interfacial region with two different microstructures, the small particles on the Cu surface and chunk-type IMC at the solder side. With the increment of solid-state aging time, although the total thickness of IMCs increased very slowly, Cu3Sn phases grew fast by the consumption of Cu6Sn5 phase. Growth kinetic analyses for IMC on the surface and in the bulk of solder joints revealed that the IMC growth on the surface was faster than that in the bulk at the initial stage. For the long-term aging, although the total IMC thickness was still thicker than that in the bulk, the rate of the IMC growth on the surface was slower than that in the bulk. The growth of IMC on the surface of interfacial region was divided into two distinct stages, which corresponds to different IMC growth behaviors.

Growth kinetics and microstructural evolution of Cu-Sn intermetallic compounds on different Cu substrates during thermal aging

The microstructure of the eutectic SnBi/Cu interface was investigated by transmission electron microscopy (TEM). In-situ and ex-situ thermal aging were conducted to study the nucleation and growth of Cu<inf>3</inf>Sn and Cu<inf>6</inf>Sn<inf>5</inf> on both polycrystalline and single crystalline Cu substrates. The growth kinetic analysis showed that although the growth of total IMCs (Cu<inf>3</inf>Sn + Cu<inf>6</inf>Sn<inf>5</inf>) was similar on single and polycrystalline Cu substrates, the Cu<inf>3</inf>Sn grew faster on polycrystalline Cu substrate than that on single crystal Cu substrate, while Cu<inf>6</inf>Sn<inf>5</inf> grew slowly on polycrystalline Cu during thermal aging.

Crack propagation of single crystal beta-Sn during in situ TEM straining

In situ tensile process of single-crystal Sn was investigated by transmission electron microscopy (TEM). Despite the traditional wedge microcrack, a new tetragonal microcrack was observed during crack propagation in the single-crystal Sn. During in situ tensile straining, the dislocation dipoles formed at the front of the wedge microcrack tip, the coalescence of which is the source of microvoids at the crack tip, and then the wedge microcrack propagated deeply by aggregation of discontinuous microvoids. The tetragonal microcrack propagated by the intersection along two vertical slip planes. Moreover, the series of high-resolution images showed that Sn islands formed at the center of the frontier crack plane due to the anisotropic self-diffusion of Sn atoms along different crystallographic planes.

Phase identification of intermetallic compounds formed during In-48Sn/Cu soldering reactions

Transmission electron microscope observations and precise diffraction analyses were performed on the interfacial reaction between In-48Sn and Cu at the temperature range from 160°C to 250°C for up to 90 minutes. The results indicated that two different morphologies formed between In-48Sn and Cu at the temperature below 200°C: small-grain Cu<inf>2</inf>(In,Sn) at the Cu side and large-grain Cu<inf>2</inf>(In,Sn) at the solder side. If the soldering temperature is above 200°C, the Cu<inf>6</inf>(In,Sn)<inf>5</inf> phase is the first phase formed at the solder/Cu interface. However, the subsequent solid-state diffusion of In and Sn atoms through the initial Cu<inf>6</inf>(In,Sn)<inf>5</inf> layer drove the formation of Cu<inf>9</inf>(In,Sn)<inf>4</inf> phase between Cu<inf>6</inf>(In,Sn)<inf>5</inf> and Cu substrate.

TEM Study of Bi Segregation in the Interconnect of Eutectic Tin-Bismuth Solder and Copper

The interfacial reaction between eutectic SnBi and Cu was studied by TEM after the sample was reflowed and aged in solid state, respectively. The microstructural evolution at the SnBi/Cu interface during reflowed and solid-state aged process was analyzed. The results show that there are two layers of intermetallic compounds (IMCs), Cu3Sn and Cu6Sn5, located at the interface between solder and Cu after the sample was reflowed. The segregation of Bi at the interface between Cu3Sn and Cu was observed. Furthermore, the segregation of Bi induced the formation of voids at Cu3Sn/Cu interface during solid-state aging process.