Materials Science Forum Vols. 675-677

Abstract: The isothermal initial oxidation kinetics of Zr55Cu30Al10Ni5 bulk metallic glass in glassy state (lower than Tg = 685K) and in surpercooled liquid state (Tg = 685 K < T < Tx=774 K) is investigated under dry air by thermogravimetric method in short-term stage (for 1.5 hours). A protective parabolic law is followed in glassy state, except at 573 K where a linear law is followed. The self-limiting oxidation kinetics evolves from a short linear stage to a steady mild growth stage in supercooled liquid state at 723 K, which is induced mainly by crystallization and by the fast growth of dense scale. The growth of the scale is dominated by the formation of tetragonal-ZrO2 (t- ZrO2) in the range of 623 K - 673 K in glassy state for 1.5 hours. The activation energy of oxidation is mainly piloted by the diffusion of oxygen ions.

Abstract: The paper studied various morphologies of the quasicrystal phase in Mg-Zn-Y alloys caused by different cooling rates. The cooling rates were reached through five kinds of cooling media. The cooling curves were monitored by multichannel data acquisition cards. A further heat treatment was made to study quasicrystal growth behavior. The results showed that, with the decrease of cooling rate, the quasicrystal morphology changed and its size became larger. Moreover, a planar quasicrystal morphology evolution schematic diagram during quasicrystal growth process was observed for the first time.

Abstract: A new method, using sodium silicate and sodium aluminate synthesize 4A-molecular sieve, was developed by using microwave technology. The sodium silicate was a high modulus liquor by-product of nano-silica production from coal fly ash. Meanwhile, the sodium aluminate was a process by-product of alumina extraction from coal fly ash. Reaction mixture composition was defined as follow:SiO2/Al2O3 ratio in 2.0, Na2O/SiO2 ratio in 1.5, and H2O/Na2O ratio in 65. The gelation process was completed in 1 hr. Microwave crystallization power was (800w) 30%. Microwave crystallization period can last 25 mins. The 4A-molecular sieve was obtained by collecting crystals from the reaction mixture through filtration after washing with water to pH 11-12 and drying inside isotherm oven. The calcium exchange capacity and effective pore size of the product were 316mg/g and 0.4nm respectively. Over 90% of surface pore size reached in sizes of less than or equal to 8μm. Purity of 4A-molecular sieve up to 99%. This method significantly reduced the raw material costs for sodium silicate and sodium aluminate. In addition, the adoption of microwave technology also lowered the energy usage and shortened crystallization time. All these contributed final low costs of 4A-molecular sieve product, which made it possible for many practical applications.

Abstract: Strong alkali anion exchange resin was employed as a hydroxyl source in the synthesis. OH- releasing rates were easily controlled by varying precursor salts, salt concentrations and temperatures, etc. The ratio between the nucleation speed and the speed of oxide crystallite growth was adjusted to favor the anisotropic growth of the crystal. Zinc and aluminum nano-sized oxide were obtained in various morphologies. X-ray diffraction (XRD), scanning electron microscope (SEM) and Energy Dispersion Spectroscopy (EDS) were employed to characterize the products. It is ascertained that the surface reaction on the resin and the mechanical grinding are the key steps in controlling the product morphology. The results show the flexibility and possibility of large scale production of various nano metal oxides and as well as mixed oxides.

Abstract: Thermal behaviors, compositions and structures of silver laurate prepared in wateralcohol system with double jet method has been investigated via DSC, FTIR, XRD and SEM, respectively. Silver nitrate, sodium hydroxide and lauric acid were used as reactants and PVP as protective agent. Results show that the highly ordered layer structures of as prepared samples were weakened at 173oC and 212oC and then decomposed at 238oC. The morphology changed from fiber-like to lamellar when the silver laurate was calcined at 212oC.

Abstract: A redox polymer, poly(ethylenimine)ferrocene (PEI-Fc) was synthesized by attaching ferrocene groups to the backbone of water soluble poly(ethylenimine), and multilayer film in nanoscale was assembled on gold electrode by alternate layer-by-layer adsorption (LBL) of the positively charged PEI-Fc and the negatively charged thermostable diaphorase (DI) from B.Stearothermophilus. The LBL process was monitored and analyzed by quartz crystal microbalance (QCM) technique, which confirmed the formation of the multilayer structure. The electrochemical oxidation of coenzyme (reduced nicotinamide adenine dinucleotide, NADH) was observed on the electrode fabricated with PEI-Fc/DI multilayer film, and the influence of layer number on current response was investigated. The modified electrode retained ca. 65% relative response after storage in buffer for two months and 50% relative response after incubation at 80 °C for 3 min, which inferred that the multilayer structure was unique stable. A NAD-dependent glucose-6-phosphate dehydrogenase (G6PDH) was also immobilized via the same LBL technique, and electrode modified with PEI-Fc/DI/G6PDH film exhibited current response to glucose-6- phosphate in the presence of free NAD+.

Abstract: Multi-walled carbon nanotubes (MWCNTs) were dispersed in an aqueous solution containing water-soluble poly(ethylenimine), and used to prepare MWCNTs modified glassy carbon electrode (GC/MWCNTs). A scanning electron microscopy was employed for characterizing the surface morphology of the modified electrode. The electrochemical behaviors of coenzyme (reduced nicotinamide adenine dinucleotide, NADH) on GC/MWCNTs were studied by both cyclic voltammetry and amperometry, and it is found clearly that the high overpotential for the direct oxidation of NADH on bare GC electrode was largely decreased in the presence of MWCNTs. The GC/MWCNTs electrode exhibited excellent properties as a platform for the construction of electrochemical biosensors based on NAD-dependent dehydrogenases, and an alcohol biosensor was successfully developed by immobilizing alcohol dehydrogenase on GC/MWCNTs.

Abstract: The effect of the strain rate on the surface nanocrystallization of titanium is investigated both theoretically and experimentally in this paper. The strain rate variation and stress distribution from surface to the interior of titanium during shot peening are estimated firstly using finite element method. Then shot peening experiment is carried out on a commercially pure titanium (CP-Ti) plate, and the obtained surface microstructures is characterized by transmission electron microscopy (TEM). Combining theoretical simulations and experimental observations, the effect of strain rate on the strain accommodation mechanism and plastic deformation mode are discussed. It is concluded that the strain rate and stress achieve the highest at the top surface layer of CP-Ti, and the strain rate decrease dramatically from the surface to the interior. The strain rate at the top surface layer is up to 104 s-1, which leads to superplastic deformation of Ti. There is no mechanical twin in the surface layer, instead, deformation lamella and adiabatic shear bands are the dominating microstructures. By means of rotation recrystallization, those deformation bands evolve to nanocrystallines.

Abstract: ZnO nanowires are promising for photonic devices, biosensor and cancer cell imaging. We have performed a first-principles study to evaluate the electronic and optical properties of ZnO nanowires. We have employed the Perdew–Burke–Ernzerhof form of generalized gradient approximation in the frame work of density functional theory. Calculations have been carried out at different configurations. With decreasing diameter, the band gap of ZnO nanowires is increased due to the increase of quantum confinement effects. The results of imaginary part of the dielectric function indicate that the optical transition between valence band and conduction band has shifted to the high energy range as the diameter decreases. The ZnO nanowires show size-tunable optical properties.

Abstract: Micro- and nano-structures of the Cr-Mo electroplated layers were studied mainly by Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM) and Positron Annihilation Lifetime Spectroscopy (PALS). These electroplated layers which were deposited in Cr-Mo electrolyte containing an organic sulfonic acid, showed surface structures having severe ups and downs of small crystal grains. Both selected area diffraction and dark-field image of TEM confirmed the presence of very small crystal grains of less than 50 nm. These small crystal grains exhibited textured structure when the electrolyte contained an organic sulfonic catalyst. PALS results indicated the presence of high density nano-size voids, and HRTEM analysis confirmed the presence of high density voids of 1 nm to 2 nm in diameter. Size and density of these nano-voids increased with the amount of catalyst in the electrolyte.