Yuanchun Zhao

Microstructure evolution and thermal properties in nanocrystalline Fe during mechanical attrition

The microstructural evolution and thermal properties of nanocrystalline (nc) Fe during mechanical attrition were investigated by using quantitative X-ray diffraction and thermal analysis techniques. Upon milling of the Fe powders with coarse grains, grain refinement takes place gradually and a steady-state grain size in the nanometer regime is reached after a certain period of milling. With the further milling of the nc Fe within the stage of the steady-state grain size, we observed a grain boundary relaxation process that was manifested by evident decreases in the thermal expansion coefficient and the stored enthalpy, as well as slight decreases in the lattice strain and the Debye-Waller parameter. The grain boundary enthalpy of the nc Fe was estimated, showing a decreasing tendency with the milling time. The present work indicated with clear experimental evidence that the nc materials with the same grain size may exhibit very different properties that depend upon the microstructure of the numerous metastable grain boundaries

Microstrain effect on thermal properties of nanocrystalline Cu

The nanocrystalline (nc) Cu samples with different microstrains but the same grain size were obtained by annealing a magnetron-sputtered ne Cu specimen. Quantitative X-ray diffraction (XRD) measurements show that with an increment of the microstrain from 0.14 to 0.24% the thermal expansion coefficient (TEC) of crystalline lattice increases by about 12%, the static displacement of atom from the equilibrium position (B-S) increases from 0.47 +/- 0.09 to 1.16 +/- 0.15 Angstrom(2), and Debye characteristic temperature (Theta(D)) decreases from 307.1 +/- 3.1 to 279.2 +/- 2.8 K. The microstrain effect on thermal properties in the nc Cu might be attributed to the change in density of grain boundary defects/dislocations. The present investigation demonstrates that the thermal properties of ne materials are determined by not only the grain size but also the microstructure of grain boundaries

ZnO Tetrapods Designed as Multiterminal Sensors to Distinguish False Responses and Increase Sensitivity

Individual zinc oxide tetrapods were designed as multiterminal sensors by the e-beam lithography method. Different from double-terminal sensors, these sensors can give multiple responses to a single signal at the same time. The designed tetrapod devices were employed to detect light with different wavelength. The results indicate that they are remarkable optoelectronic devices, sensitive to ultraviolet light, and have advantages on distinguishing noises and increasing sensitivity. This should be helpful for weak signal measurements of nanodevices.

Highly Dense and Perfectly Aligned Single-Walled Carbon Nanotubes Fabricated by Diamond Wire Drawing Dies

We have developed a low-cost and effective method to align single-walled carbon nanotubes (SWNTs) using a series of diamond wire drawing dies. The obtained SWNTs are highly dense and perfectly aligned. X-ray diffraction (XRD) indicates that the highly dense and perfectly aligned SWNTs (HDPA-SWNTs) form a two-dimensional triangular lattice with a lattice constant of 19.62 A. We observe a sharp (002) reflection in the XRD pattern, which should be ascribed to an intertube spacing 3.39 A of adjacent SWNTs. Raman spectra reveal that the radical breath mode (RBM) of SWNTs with larger diameter in the HDPA-SWNTs is suppressed compared with that of as-grown SWNTs. The HDPA-SWNTs have a large density, approximately 1.09 g/cm 3, and a low resistivity, approximately 2 m Omega cm, at room temperature, as well as a large response to light illumination.

Large-scale synthesis and optical behaviors of ZnO tetrapods

Zinc oxide tetrapods were synthesized on a large scale through thermal evaporation of zinc powder with a vapor transportation deposition method. Scanning electron microscope images gave clear evidences of twin planes at the junction of the tetrapods. Photoluminescence and waveguide behaviors of a single tetrapod were demonstrated with a near-field scanning optical microscope.

Laser-induced coloration of WO3

It has long been recognized that WO3 can be colored through electro-, photo-, and thermo-chromism processes. We report here that WO3 turned to a deep blue color upon just one pulse irradiation of the XeCl excimer laser (lambda = 308 nm, pulse duration = 36 ns, output energy = 100 mJ/pulse). Scanning electron microscopy (SEM) images showed that a melted layer was formed on the surface of the sample. The color change was demonstrated by UV-visible diffuse reflectance. X-ray photoelectron spectroscopy (XPS) showed the partial reduction of WO3 and the appearance of conduction band electrons. Calorimetry measurement results indicated that reoxidation happened at 400-440 degrees C in air. From these experimental results combining with the calculation, it was concluded that the laser-induced coloration is different from ordinary photochromism induced by UV light, and may be attributed to thermochromism

Coulomb explosion: a novel approach to separate single-walled carbon nanotubes from their bundle.

A novel approach based on Coulomb explosion has been developed to separate single-walled carbon nanotubes (SWNTs) from their bundle. With this technique, we can readily separate a bundle of SWNTs into smaller bundles with uniform diameter as well as some individual SWNTs. The separated SWNTs have a typical length of several microns and form a nanotree at one end of the original bundle. More importantly, this separating procedure involves no surfactant and includes only one-step physical process. The separation method offers great conveniences for the subsequent individual SWNT or multiterminal SWNTs device fabrication and their physical properties studies.

Magnetic transitions in Dy-microalloyed Fe-based bulk metallic glasses

The magnetic properties of Fe70−xDyxZr8Mo5W2B15 (0 x 5) bulk metallic glasses (BMGs) are studied using dc and ac susceptibility measurements. A re-entrant spin glass behaviour is observed in the BMGs and the phenomenon is ascribed to Dy-microalloying-induced site frustration. The magnetic properties of the BMGs are found to be tunable by appropriate selection of the Dy content, and the schematic magnetic phase diagram as a function of Dy content is derived.

Large photocurrent generated by a camera flash in single-walled carbon nanotubes

The photocurrent generated in single-walled carbon nanotube bundles upon camera flash illumination has been studied under different ambient pressures and light intensities. The results show that the intensity of photocurrent depends closely on the ambient pressure and light intensity. With the ambient pressure reduced, the photocurrent exhibits a logarithmic growth behaviour. Meanwhile, the photocurrent increases with the increase in light intensity. In this work, a dynamic model is employed to unveil the origins of the observed photocurrent. A much smaller lifetime of photocarriers (~10?ms) is observed than that needed for gas molecular desorption or photodesorption (seconds or longer). Our results are consistent with the model of Schottky barriers being responsible for photocurrent generation.

Secondary growth of small ZnO tripodlike arms on the end of nanowires

Small zinc oxide tripodlike arms were assembled at the end of nanowires through thermal evaporation of zinc powder in a horizontal tube furnace. These arms are hundreds of nanometers in length, and they are tens of nanometers in diameter, similar to that of the based nanowires. High-resolution transmission electron microscopy investigation exhibits that they are wurtzite structure and have clear twin planes at the junction of the arms and the based nanowires. The growth mechanism, Raman, and photoluminescence properties were discussed.