Materials Science Forum Vols. 654-656

Abstract: With recent research, the author intends to outline the framework of the field emission of ZnO nanostructures. However, many groups’ reports ignored the thermionic emission process in the low electric field. A recently published field emission cathode parameter extraction method [X. He et al., J. Appl. Phys. 102, 056107(2007)] provided unambiguous and reliable cathode. The method utilized Richardson-Laue-Dushman law in low electric field and Fowler-Norheim equation in high electric field to solve a one-dimensional model including both thermionic and field emission. The model gave a much better agreement with the experimental data of ZnO cathode under the applied field and acquired a revised surface field enhancement factors and work function of ZnO cathode in the electron emission process.

Abstract: CdSe particles with the wurtzite structure have been synthesized via solvothermal method using a mixed solution of triethylenetetramine (TETA) and de-ionized water (DIW) without adding a reducing agent. It was found that the ball-like CdSe precursor with the zinc-blende phase could be transformed to the wurtzite structure after heat-treating at 580 °C in Ar atmosphere and the obtained microspheres were composed of small CdSe particles. The experimental results were compared with that obtained without TETA and it was speculated that TETA in the mixed solution played a role of reducing agent and solvent. Both the as-prepared products and the annealed powders were systematically characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared absorbance spectroscopy (FTIR).

Abstract: BNC nanotubes were prepared by a high-energy ball milling and annealing method. Graphite carbon (C) and hexagonal boron nitride (h-BN) powder was first milled at room temperature to generate highly disordered nanoporous particles with a metastable structure. Upon elevated temperature annealing up to 1300 °C, these nanoporous particles were chemically and thermally activated to grow BNC nanotubes with both cylindrical and bamboo-like morphology. The cylindrical tubes have smaller diameters from 40 to 60 nm and lengths up to 2 μm. The bamboo tubes have larger diameters from 80 to 150 nm and lengths up to 10 μm. The formation mechanisms of different nanostructures are discussed.

Abstract: The plasma-induced emission properties of ZnO nanorod and carbon nanotube (CNT) arrays were investigated under the pulse electric field. The formation of plasma on the array surface was found and high intensity electron beams were obtained from the two kinds of arrays. The plasma-induced emission properties of the ZnO nanorod and CNT arrays have big differences. Under the same electric field, the CNT arrays have higher emission current than the ZnO nanorod arrays. With the emission currents changing, the electron emissions of the ZnO nanorod arrays always are very uniform; but that of the CNT arrays are non-uniform. The plasma expansion velocity of the ZnO nanorod arrays is lower than that of the CNT arrays. Accordingly, the emission stability of the ZnO nanorod arrays is better than that of the CNT arrays.

Abstract: Polythiophene (PTP) coated V2O5 nanotubes were prepared by an in-situ polymerization of thiophene monomers in the presence of prepared V2O5 nanotubes. The nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which proved the polymerization of thiophene monomer and the strong interaction between polythiophene and V2O5 nanotubes (VONTs). The gas sensing properties of PTP coated V2O5 nanotubes were studied at room temperature, which was found that PTP coated V2O5 nanotubes could detect ethanol with much higher sensitivity than pure VONTs. The sensing mechanism of PTP coated V2O5 nanotubes to ethanol is presumed to be the synergetic interaction between polythiophene (PTP) and V2O5 nanotubes.

Abstract: We report the poly(3iophene) (P3HT)/ZnO nanobelt hybrid p-n junction diodes characterized by using a conductive atomic force microscope (C-AFM). The diodes exhibited a turn-on voltage of about 2.5 V and ideality factor of about 11.6. The obvious current steps in the I-V characteristics under the reverse bias were clearly observed at room temperature. The origin of these steps is suggested to be attributed to the charge injection-trapping induced by nanoparticles on the surface of the ZnO nanobelt.

Abstract: Nanocomposite materials consisting of a nanocrystalline Ni matrix with grain size ranging from 23 to 40 nm, and nano-size SiO2 particles with average particle size of 7 nm, have been produced by pulse electrodeposition. Grain size was controlled by peak current density. It was observed that SiO2 particles precipitated within grain interior by the transmission electron microscopy (TEM). Hardness and tensile strength of nanocomposites with a grain size larger than 35 nm was higher than that of nanocrystalline pure Ni possibly as a consequence of Orowan precipitation hardening. However, hardness of Ni-SiO2 nanocomposites with a grain size smaller than 30 nm was the same as that of nanocrystalline pure Ni. It was considered that the SiO2 particles do not contribute to hardening because deformation occurs by a grain-boundary-mediated mechanism such as grain-boundary sliding.

Abstract: A physically-based model is proposed for the competitive precipitation of multiple phases (bcc-Fe, Fe3B, Nd2Fe14B, Nd2Fe23B3, NdFe4B4 and Fe2B) from an amorphous Fe-B-Nd matrix. These materials form the basis of a class of nanocomposite hard magnets. The nucleation and growth of the different phases are calculated using computational thermodynamics and kinetics tools with input from a thermodynamic assessment of this system. In some alloy compositions, the phase formation sequence during crystallization shows significant sensitivity to the heating-rate. Model calculations illustrate that this effect cannot be explained by homogeneous nucleation and growth of the phases. The possible role of heterogeneous nucleation is briefly discussed.

Abstract: In Nd9.5Fe84B6.5 melt-spun ribbon, the quenching temperature is found to be effective for the texture development of Nd2Fe14B nanocrystals. For a relatively low quenching temperature of 1250°C a (00l) texture of Nd2Fe14B crystals was found on the free-side surface of the ribbons. At a higher quenching temperature of 1350°C, the microstructure of the free-side surface of the ribbons switches into (320) and (517) texture. It is believed that the transformation of the melt at higher temperature triggers the switch of the texture.

Abstract: The magnetic and optical properties of regioregular poly(3-hexylthiophene)-capped Au nanoparticles (NPs) have been investigated in order to clarify the effectiveness of polythiophene capping on the induction of ferromagnetism in Au. The room-temperature magnetization curve of the polythiophene-capped Au NPs exhibits a clear hysteresis behavior with a spontaneous magnetization of 8.5 x 10-3 emu/g. The average magnetic moment of the surface Au atoms is estimated to be 2.6 x 10-3 B. The ultraviolet-visible spectrum shows a clear sign of the surface plasmon absorbance of metallic Au which reflects the week character of the chemical bond between the Au and S atoms on capping surface. Our results show clearly that Au NPs capped with polythiophene can be ferromagnetic.