June-Gunn Lee

Role of Electronic Perturbation in Stability and Activity of Pt-Based Alloy Nanocatalysts for Oxygen Reduction

The design of electrocatalysts for polymer electrolyte membrane fuel cells must satsify two equally important fundamental principles: optimization of electrocatalytic activity and long-term stability in acid media (pH <1) at high potential (0.8 V). We report here a solution-based approach to the preparation of Pt-based alloy with early transition metals and realistic parameters for the stability and activity of Pt(3)M (M = Y, Zr, Ti, Ni, and Co) nanocatalysts for oxygen reduction reaction (ORR). The enhanced stability and activity of Pt-based alloy nanocatalysts in ORR and the relationship between electronic structure modification and stability were studied by experiment and DFT calculations. Stability correlates with the d-band fillings and the heat of alloy formation of Pt(3)M alloys, which in turn depends on the degree of the electronic perturbation due to alloying. This concept provides realistic parameters for rational catalyst design in Pt-based alloy systems.

High temperature strength and oxidation behaviour of hot-pressed silicon nitride-disilicate ceramics

Twelve different silicon nitride-disilicate ceramics have been fabricated by hot-pressing Si3N4 with the oxides of Y, Yb, Ho, Dy, Er, Sm, Ce, Lu, La, Pr, Gd, and Sc that are used as sintering additives. The high temperature strength and oxidation behaviour of the hot-pressed ceramics were investigated and correlated with the cationic radii of the oxide additives. The flexural strength at 1200°C increased, from 666 MPa for Si3N4-La2Si2O7 to 965 MPa for Si3N4-Sc2Si2O7 which is correlated with a decreasing cationic radius of the oxide additive. The weight gain during oxidation at 1400°C for 192 h in air decreased, from 0.8732 mg cm-2 for a Si3N4-Sm2Si2O7 ceramic to 0.1089 mg cm-2 for a Si3N4-Sc2Si2O7 ceramic, which is a function of the decreasing cationic radius of the oxide additive.

Continuous synthesis of silicon carbide whiskers

A two-step reaction scheme has been employed for the synthesis of SiC whiskers at 1450 °C under an argon or hydrogen flow. First, SiO vapour was generated via the carbothermal reduction of silica in a controlled manner. Second, the generated SiO vapour was reacted with carbon-carrying vapours such as CO and CH4, which resulted in the growth of SiC whiskers on a substrate away from the batch. A higher growth rate was observed in the hydrogen atmosphere due to the formation of CH4 which provides a more favourable reaction route. By the use of thermodynamic calculations, the preferred reaction routes have been selected for an efficient synthesis of SiC whiskers, and a continuous reactor has been designed. The system consists of a boat-train loaded with the silica-carbon mixture and iron-coated graphite substrate above it in an alumina-tube reactor. By pushing the boat-train into the hot zone at a fixed speed, SiO vapour is constantly generated. High-quality SiC whiskers have been grown on the substrate with diameters of 1–3 μm. The yield was about 30% based on the silicon input as SiO2 and silicon output as SiC whiskers. This demonstrates the feasibility of continuous production of high-quality SiC whiskers which does not require additional processes such as purification and classification.

Promoting effects of La for improved oxygen reduction activity and high stability of Pt on Pt–La alloy electrodes

The design of polymer electrolyte fuel cell electrocatalysts depends on two equally important fundamental principles: the optimization of electrocatalytic activities as well as the long-term stability under operating conditions (e.g., pH 0.8 V). Pt-based alloys with transition metals (i.e., Pt–La) address both of these key issues. The oxygen reduction kinetics depends on the alloy composition which, in turn, is related to the d-band center position. The stability of the oxygen reduction reaction is predictable by correlation of the d-band fillings and vacancies of Pt–M (M = Ti, Fe, Zr and La).

Comparison of structural and optical properties of InAs quantum dots grown by migration-enhanced molecular-beam epitaxy and conventional molecular-beam epitaxy

We strongly support Guryanov’s speculation—that a thinner wetting layer is expected with quantum dots (QDs) grown by migration-enhanced epitaxy—with structural and optical measurements. InAs QDs grown by migration-enhanced molecular-beam epitaxy showed a larger size, lower density, ∼40% enhanced uniformity, ∼2 times larger aspect ratio, and a measurement temperature insensitivity of the photoluminescence linewidth compared to QDs grown by conventional molecular-beam epitaxy. The thickness of the wetting layer for the migration-enhanced epitaxial InAs QD (2.1nm) was thinner than that of the counterpart (4.0nm).

Effect of polycarbosilane addition on mechanical properties of hot-pressed silicon carbide

Silicon carbide ceramics containing 20 wt% polycarbosilane was fabricated by hot-pressing with various additives (B, Al, AlN). The addition of polycarbosilane resulted in a considerable increase in flexural strength up to 1050 MPa for the 1 wt% AlN and 0.5 wt% B-doped specimens and fracture toughness up to 4.0 MPa m1/2 for the 1 wt% Al-doped specimen. The improved fracture strength was a result of liquid-phase sintering and the improved toughness was a result of crack deflection along the grain boundaries. Crack branching was also observed in 1 wt% AlN and 0.5 wt% B-doped specimens.

In situ enhancement of toughness of SiC—TiB2 composites

A process based on liquid phase sintering and subsequent annealing for grain growth is presented to obtain the in situ enhancement of toughness of SiC–30 wt%, 50 wt%, and 70 wt% TiB2 composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, relatively equiaxed TiB2 grains, and yttrium aluminium garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiB2 powders with the liquid forming additives of Al2O3 and Y2O3 by hot-pressing at 1850°C and subsequent annealing at 1950°C. The annealing led to the in situ growth of elongated α-SiC grains, due to the β→α phase transformation of SiC, and the coarsening of TiB2 grains. The fracture toughness of the SiC–50 wt% TiB2 composites after 6 h annealing was 7.3 MPa m1/2, approximately 60% higher than that of as-hot-pressed composites (4.5 MPa m1/2). Bridging and crack deflection by the elongated α-SiC grains and coarse TiB2 grains appear to account for the increased toughness of the composites.

Influence of alloy buffer and capping layers on InAs/GaAs quantum dot formation

We have investigated the influence of alloy buffer and capping layers on the shape, size, and density of self-assembled InAs/GaAs quantum dots. Cross-sectional scanning tunneling microscopy (XSTM) images reveal ellipse-shaped dots with highest (lowest) diameter, height, and density, for dots with (without) surrounding alloy layers. Furthermore, the wetting layer is thicker in the presence of the alloy layers. We propose a strain-based mechanism for dot formation and collapse in the absence and presence of alloy buffer and capping layers. This mechanism is likely to be applicable to a wide range of lattice-mismatched thin-film systems.

Effect of initial α-phase content of SiC on microstructure and mechanical properties of SiC-TiC composites

Abstract By using α- and β-SiC starting powders, the effects of initial α-phase content of SiC on microstructure and mechanical properties of the hot-pressed and subsequently annealed SiC–30 wt.% TiC composites were investigated. The microstructures developed were analyzed by image analysis. Their microstructures consist of uniformly distributed elongated α-SiC grains, equiaxed TiC grains and an amorphous grain boundary phase. During annealing, the β→α phase transformation of SiC leads to the in-situ growth of elongated α-SiC grains. The average diameter of SiC increases with increasing α-SiC content in the starting powder and the aspect ratio shows a maximum at 1% α-SiC and decreases with increasing α-SiC content in the starting powder. Such results suggest that microstructure of SiC–TiC composites can be controlled by changing α-SiC content in the starting powder. The strength increased with increasing α-SiC content when α-SiC content is higher than 10% while the fracture toughness decreased with increasing α-SiC content, i.e. the same trend with the variation of aspect ratio of SiC in the composites.

Effect of annealing on mechanical properties of self-reinforced alpha-silicon carbide

Alpha-SIC powder containing 7.2 wt % Y3Al5O12 (YAG, yttrium aluminum garnet) and 4.8 wt % SiO2 as sintering aids were hot-pressed (SC0) at 1820°C for 1 h and subsequently annealed at 1920°C for 2 h (SC2), 4 h (SC4) and 8 h (SC8). When the annealing time was increased, the microstructure changed from equiaxed to elongated grains and resulted in self-reinforced microstructure consisted of large elongated grains and small equiaxed grains. Development of self-reinforced microstructure, consisted of mostly 6H phase, resulted in significant improvements in toughness. However, the improved toughness was offset by a significant reduction in strength as in the materials consisted of 4H originated from β-SiC. The fracture toughness and strength of the 8-h annealed materials were 5.5MPa · m1/2 and 490 MPa, respectively.