J.Q. Wang

Oleic acid (OA)-modified LaF3 : Er,Yb nanocrystals and their polymer hybrid materials for potential optical-amplification applications

Oleic acid (OA)-modified LaF3 : Er,Yb nanocrystals (NCs) have been prepared and investigated. The OA-capped nanocrystals have a nearly spherical shape with a mean diameter of 7 nm and show excellent dispersibility in common organic solvents and a PMMA matrix. A 0.25 g ml−1 transparent colloidal solution in toluene can be obtained. The resulting nanocrystal–polymer hybrid materials exhibit strong emission around the 1550 nm telecommunication window when excited at 980 nm and are promising materials for polymer-based optical waveguide amplifiers.

One-step synthesis of highly water-soluble LaF3: Ln3+ nanocrystals in methanol without using any ligands

Water-soluble infrared-to-visible fluorescent LaF(3) nanocrystals doped with different lanthanide ions (Er(3+)/Yb(3+), Eu(3+), Nd(3+), Tb(3+)) have been synthesized in methanol without using any ligands. These nanocrystals are easily dispersed in water, producing a transparent colloidal solution. The colloids of the Er(3+)/Yb(3+), Eu(3+), Nd(3+), Tb(3+) doped nanocrystals exhibit strong luminescence in the visible and near-infrared spectral regions.

Influence of pressure on crystallization kinetics in an Al-Ni-Ce-Fe amorphous alloy

The influence of pressure on crystallization kinetics in an Al-Ni-Ce-Fe amorphous alloy was studied by means of piston-cylinder measurements over a pressure range up to 1.07 GPa. It was found that an applied pressure enhances the primary crystallization of the fcc-Al phase from the amorphous phase, i.e., the growth process of quenched-in Al nuclei in the amorphous Al-Ni-Ce-Fe alloy. The primary crystallization temperature decreases from 187 to 138 degreesC when the pressure increases from 0 to 1.07 GPa. The crystallization temperature for the remaining amorphous phase decreases from 328 to 306 degreesC when the pressure increases from 0 to 0.85 GPa. The negative volume change associated with the growth process of the fcc-Al phase has a significant role in the crystallization kinetics of Al-based amorphous alloys under pressure

Pressure-enhanced thermal stability against eutectic crystallization in Al-based metallic glasses

The pressure effect on thermal stability against eutectic crystallization in an Al84Ni10Ce6 and an Al82.5Ni5Y8Co2Zr2.5 metallic glasses was investigated in a pressure range up to 1.3 GPa. It was found that the applied pressure enhances the eutectic crystallization temperature for both glasses

Bonding characters of Al-containing bulk metallic glasses studied by 27Al NMR

We report very small (27)Al metallic shifts in a series of Cu-Zr-Al bulk metallic glasses. This observation and the Korringa type of spin-lattice relaxation behavior suggest that s-character wavefunctions weakly participate in bonding and opens the possibility of enhanced covalency (pd hybridization) with increasing Al concentration, in good agreement with elastic constants and hardness measurements. Moreover, ab initio calculations show that this bonding character originates from the strong Al 3p band and Zr 4d band hybridization since their atomic energy levels are closer to each other while the Al 3s band is localized far below the Fermi level. This study might provide a chemical view for understanding flow and fracture mechanisms of these bulk glass-forming alloys.

Distinguishing medium-range order in metallic glasses using fluctuation electron microscopy: A theoretical study using atomic models

We have simulated the variance in the diffracted electron intensity as a function of scattering vector in fluctuation electron microscopy for a range of fully amorphous and nanocrystal-containing metallic glass models. Variance peaks are obvious for glasses that contain well-developed short-to-medium range order but absent for their parent liquids. The variance peaks are very sensitive to the presence and especially the size of the nanocrystal embryos or nuclei embedded in the glass matrix but much less so to the variations in the type or degree of the cluster ordering developed in fully amorphous materials.

Fluctuation electron microscopy of Al-based metallic glasses: effects of minor alloying addition and structural relaxation on medium-range structural homogeneity

Al-based metallic glasses with relatively high glass-forming ability have been studied using fluctuation electron microscopy (FEM). These glasses exhibited well-defined glass transition and remained fully amorphous throughout the FEM experiments. Peaks were observed in the normalized intensity variance curves, evolving with sample composition and thermal history, while diffraction patterns remained identical. Specifically, minor alloying of Co into the Al85Ni5Y10 glass improved its medium-range homogeneity. This is in accord with, and partly explains, the increase in the glass-forming ability. After annealing of Al85Ni5Y8Co2 below its glass transition temperature, FEM suggests structural homogenization or intensified medium-range fluctuations, depending on the annealing temperature. Such dissolution versus growth of quenched-in heterogeneity accompanying structural relaxation was not detectable using other techniques.

Valence fluctuation and electron-phonon coupling in La68-xCexAl10Cu20Co2 (x=0, 34, and 68) metallic glasses

We study the anomalous acoustic softening behavior in La68−xCexAl10Cu20Co2 (x=0, 34, and 68) metallic glasses by magnetic susceptibility, thermal expansion, and specific heat at low temperatures. The Ce-based metallic glass exhibits valence fluctuation phenomenon, anomalous thermal expansion and acoustic Gruneisen parameter at low temperatures. And we provide experimental evidence of the existence of strong electron–phonon coupling in the Ce-based metallic glasses by specific heat. The anomalous acoustic and thermal properties are attributed to the valence instability in the metallic glasses.