J.K.L. Lai

Synthesis and structural characterization of rutile SnO 2 nanocrystals

Nanocrystalline tin dioxide (SnO 2 ) thin films were prepared on glass substrate by pulse laser deposition for the first time. The thin films were characterized for their composition, morphology, and crystalline structure by x-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. It was found that the thin films consisted only of the tetragonal phase SnO 2 with no structural change, and they were well crystallized during deposition. In most cases, SnO 2 particles were overlapped, predominantly grown on preferred (101) plane, and connected with two or three neighbors through necks. The average grain size of the as-prepared thin films was about 12 nm. These facts are of great importance for sensor characteristics, since smaller grains and preferred orientation properties provide higher gas sensitivity to the whole thin films. Our findings indicate that the n-type wide-band-gas semiconductor nanocrystalline thin films can be manipulated by using pulse laser deposition techniques, offering new opportunities to control material fabrication.

Grain growth kinetics of nanocrystalline SnO2 for long-term isothermal annealing

Long-term isothermal grain growth kinetics of nanocrystalline SnO2 was investigated. A limiting grain size for each annealing temperature was observed and the grain growth behaviour can be described with a non-linear relaxation equation. Structural relaxation of the interfaces in nanostructure materials plays an important role in the grain growth process.

Effects of Cu Contents in Sn–Cu Solder on the Composition and Morphology of Intermetallic Compounds at a Solder/Ni Interface

The reaction between Sn– x Cu ( x = 0.1, 0.3, 0.7, 0.9, and 1.5 wt%) solder alloys and Ni at 260, 280, and 290 °C for 60 s was studied to reveal the effect of Cu content on the composition and morphology of intermetallic compounds (IMCs) formed at the interface between solder and substrate. The results indicated that Cu concentration greatly affects both the composition and morphology of the IMC between the solder and Ni substrate. In particular, when the Cu concentration was less than or equal to0.3 wt%, (Cu x Ni 1− x ) 3 Sn 4 IMCs were formed at the interface. When the Cu concentration was 0.7 wt%, large facets type of (Cu x Ni 1− x ) 6 Sn 5 were mixed with (Cu x Ni 1− x ) 3 Sn 4 in the IMC layer. At Cu concentrations higher than the eutectic one, e.g., 0.9 and 1.5 wt%, stick-shaped (Cu x Ni 1− x ) 6 Sn 5 compounds were detected, but the (Cu x Ni 1− x ) 3 Sn 4 IMCs disappeared. The formation and growth mechanism of the (Cu x Ni 1− x ) 6 Sn 5 compound were analyzed. The evolution tendency of the composition and morphology of the IMCs at the three testing temperatures was found to be the same.

Nucleation site and mechanism leading to growth of bulk-quantity Mn3O4 nanorods

We report a simple and effective method for the generation of bulk-quantity nanorods of manganese oxide, Mn3O4, under surroundings of a suitable surfactant and alkaline solution. It is found that the Mn3O4 nanorod is smooth, straight, and that the geometrical shape is structurally perfect, which is produced with lengths from several hundreds nanometers to a few micrometers, and diameters range from 10nmto30nm. We amazedly found that the dripping speed of the NaOH solution plays an important role in formation of bulk-quantity Mn3O4 nanorods. The difference of dripping speed of the NaOH solution leads to a large difference of Mn3O4 morphologies, which is observed in the transmission electron microscopy images. The growth of the Mn3O4 nanorods is suggested first to follow a self-catalyzed solution-liquid-solid mechanism.

Quantum dot formation and dynamic scaling behavior of SnO2 nanocrystals induced by pulsed delivery

Quantum dot formation and dynamic scaling behavior of SnO2 nanocrystals in coalescence regime for growth by pulsed-laser deposition is explored experimentally and theoretically, and the same is compared with that for continuous vapor deposition such as molecular-beam epitaxy. Using high-resolution transmission electron microscopy, unusual quantum dots of SnO2 nanocrystals are studied. We present kinetic Monte-Carlo simulations for pulsed-laser deposition in the submonolayer regime and give a description of the island distance versus pulse intensity. We found that the scaling exponent for pulsed-laser deposition is 1.28±0.03, which is significantly lower as compared to that for molecular-beam epitaxy (1.62±0.03). Theoretical simulations reveal that this attractive difference can be pursued to the large fraction of multiple droplet coalescence under pulsed vapor delivery.

Hot tensile properties of 25Cr-8Ni duplex stainless steel containing cellular (σ+γ2) structure after various thermal treatments

Abstract The influence of cellular ( σ + γ 2 ) structure on the hot tensile properties of duplex stainless steel aged at 750°C was studied. Various solution treatments were employed and subsequent ageing of the specimens at 750°C resulted in ( σ + γ 2 ) structure with different morphology and distribution within the austenite matrix. The amounts of σ precipitated at 750°C in different samples did not depend on the prior solution treatment temperature. These samples were tensile tested at 550 and 750°C, respectively. The yield strengths and ultimate tensile strengths increased while the ductility dropped in all aged samples compared with the unaged ones. The increase in strength is due to the replacement of the softer α phase with the harder but brittle ( σ + γ 2 ) cellular structure. Fracture surface analysis showed that cracks started predominately at σ / γ 2 interfaces. The cracks then merged and propagated in a direction perpendicular to the tensile direction within the cellular structure. With appropriate solution treatment to increase the spacing between the ( σ + γ 2 ) structure, the presence of cellular structure in the steel can increase the strength of the material and retains sufficient ductility at high temperatures.

Microstructural changes and fractal Ge nanocrystallites in polycrystalline Au/amorphous Ge thin bilayer films upon annealing

Microstructural changes and fractal Ge nanocrystallites in polycrystalline Au/amorphous Ge thin bilayer films upon annealing have been investigated by scanning electron microscopy, transmission electron microscopy observations and x-ray energy-dispersive spectroscopy (EDS). Experimental results indicated that the microstructure of the metal Au film plays an important role in metal-induced crystallization for Au/Ge thin bilayer films upon annealing. Interestingly, we found the position exchange of Au and Ge films and the formation of the fractal Ge nanocrystallites induced by annealing. EDS microanalysis indicated that although there is lateral interdiffusion of Au and Ge atoms, the thickness of the fractal region and the matrix remain nearly the same. At the same time, EDS shows that there are also Au aggregates extending out of the films. It is suggested that, besides the preferred nucleation at the Au/Ge interface, the breaking of Ge?Ge bonds may stimulate the crystallization of amorphous Ge, so that the crystallization temperature of Au/Ge system is much lower than that of the isolated amorphous Ge system.

Spatial fractal characteristic of spinodal decomposition in Fe-Cr-Ni duplex stainless steel

Abstract The α and α’ domains evolved from the primary ferrite phase during spinodal decomposition have different morphologies. The α (Fe-rich) forms the matrix while the α’ (Cr-rich) domains are developed in the form of discrete regions embedded within the α matrix. The distribution of these domains follows the fractal-growth characteristics with fractal dimensions ranging from 0.1 to 0.2. In addition, there exists a lower critical dimension beyond which the self-similarity of the domains breaks down.

Creep properties of aged duplex stainless steels containing σ phase

Abstract The creep properties of a cast of duplex stainless steel were characterized at temperatures 550–800°C under different loading conditions. For fully aged specimens containing σ, the stress exponent for creep was close to 3 and the activation energy was 281±9 kJ mol −1 . The results suggested that the creep mechanism in the samples in this investigation was controlled by dislocation movement. Extensive σ/γ 2 interfaces introduced during ageing improved the creep resistance of the material and related to a reduction of the creep rate in Stage II creep and an increase in the creep rupture strength of the material. Microstructural studies revealed the dependence of the creep properties on the morphology of the microstructure. Among the aged specimens containing σ, the creep strength and ductility were higher for specimens having larger γ grain thickness measured on the longitudinal plane. This characteristic was related to the crack propagation and interconnection of voids within γ matrix during tertiary creep. With appropriate solution treatment, the creep strength of σ-containing steels can be improved to a value exceeding that of type 316 steels.

Study of the thermal stress in a Pb-free half-bump solder joint under current stressing

The thermal stress in a Sn3.5Ag1Cu half-bump solder joint under a 3.82×108A∕m2 current stressing was analyzed using a coupled-field simulation. Substantial thermal stress accumulated around the Al-to-solder interface, especially in the Ni+(Ni,Cu)3Sn4 layer, where a maximal stress of 138MPa was identified. The stress gradient in the Ni layer was about 1.67×1013Pa∕m, resulting in a stress migration force of 1.82×10−16N, which is comparable to the electromigration force, 2.82×10−16N. Dissolution of the Ni+(Ni,Cu)3Sn4 layer, void formation with cracks at the anode side, and extrusions at the cathode side were observed.