Key Engineering Materials Vols. 336-338

Abstract: Vanadium dioxide nanocrystalline of various morphology have been synthesized from V2O5 sol and organic molecules as the structure-directing templates under hydrothermal conditions. XRD, TEM and SEM are applied to study the characteristics of samples. Cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS) are demonstrated to be appropriate templates for the formation of vanadium dioxide nanocrystalline. The probable formation mechanisms of the vanadium dioxide are also discussed.

Abstract: LaMnO3-based perovskite oxides are typical functional materials applicable to various devices for its chemistry and physics properties. The conventional method solid-state reaction method has some drawbacks of high reaction temperature, large particle size and limited degree of chemical homogeneity. So, an easy, low cost and environmental method of synthesizing fine and homogenous powders of LaMnO3 is required. This paper discusses the synthesis of LaMnO3 via a mechanochemical activated process. The mixtures of La2O3 and Mn2O3 were used as starting powders. The specific surface of starting materials was measured by BET method. A planetary ball mill was used to grind the starting mixtures. During the grinding process, XRD analysis was employed to make clear the phase changes, and BET analysis was used to test the specific surface changes. After being grinded for different time, the mixtures were annealed at different temperatures. After this, the phase identification of mixtures was conducted by XRD. At the same time, a pre-mixing of starting materials with ethanol as solvent by normal ball-milling for 8 hrs was proceeded to obtain homogeneous mixture and improve the efficiency of grinding. All obtained LaMnO3 powders via the mechanochemical activated process were characterized by SEM imaging. Finally, an optimized powder processing was proposed.

Abstract: In this article, the spinel NiCo2O4 nanosheets with diameters of 100-200 nm were prepared by calcinations of co-precipitates, which were obtained by co-precipitation method. Different kinds of precipitants, including NaOH, KOH, Na2CO3 and ethanedioic acid aqueous solutions were investigated, and regular hexagonal nanosheets are synthesized only when the precipitant is strong KOH or NaOH solution. The morphology and structure of the hexagonal precursor particles and nanosheets were investigated, and the mechanism of the formation of hexagonal nanosheets is discussed.

Abstract: Cadmium selenide (CdSe) have been prepared by the reduction of mixed solution of cadmium chloride (CdCl2), sodium selenosulfate (Na2SeSO3) and diethanolamine ((CH2CH2OH)2NH) at temperatures of 80-200°C under Ar atmosphere. Characteristics of CdSe nanoparticles were analyzed using transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) and UV-Vis absorption spectra. CdSe samples prepared at relatively low temperature of below 100°C showed cubic structure while samples synthesized at temperature above 100°C showed hexagonal phase. The increased reaction temperature caused the particle size increase as well as the agglomeration of particles to form a hollow sphere. UV-Vis absorption spectra also showed a red-shift as increasing reaction temperatures due to the gradual growth of CdSe nanoparticles in the solution. The relatively sharp absorption band from samples synthesized at 80 and 100°C can be attributed to homogeneous particle size distribution compared with those of prepared at elevated temperatures. This paper will mainly discuss the phase stability of CdSe nanoparticles as well as the hollow sphere formation.

Abstract: CdSe nanoparticles with average particles size of ~ 3.4 nm were successfully prepared by the sonochemical reduction of a mixed solution of CdCl2, Na2SeSO3 and CH2CH2OH)2NH at room temperature. CdSe nanoparticles characterized by TEM, XPS and XRD analysis revealed the formation of high purity cubic phase CdSe nanoparticles with lattice constant of a=6.415Å. CdSe nanoparticles showed relatively large range of particle size distribution from 20 to 50 nm as the sonication time increased. UV-Vis absorption spectra showed a red-shift in the wavelength region of 545-590 nm as increasing sonication time due to the gradual growth of CdSe nanoparticles in the solution. This paper will mainly present the effects of ultrasonic irradiation on the formation and properties of CdSe nanoparticles.

Abstract: Cadmium selenide (CdSe) of hollow sphere with 30-50nm in diameter have been prepared form the mixed solution of cadmium chloride (CdCl2), sodium selenosulfate(Na2SeSO3) and ethylenediamine (H2NCH2CH2NH2) at room temperature. The hollow sphere of CdSe was characterized by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The molar ratio of EDA to Cd2+ ions in this system was important factor on controlled synthesis in the self-encapsulated morphology. Based on the observation on morphological difference with molar ratio of EDA to Cd2+ ion, the possible mechanism for the formation of CdSe hollow sphere was discussed.

Abstract: Gallium-doped zinc oxide nanopowders with high electrical conductivity were prepared by calcining the precursor synthesized from ZnSO4·7H2O, GaCl3 and NH4HCO3 by co-precipitation method. The obtained samples were characterized by FE-SEM, TEM, XRD, XPS and BET techniques. The precursor particles were in the shape of sheet-like with size about several hundred nanometers. The electrically conductive zinc oxide particles, in the shape of sphere-like, were about 10~40 nm with low agglomeration, and the specific surface area was 28.7 m2/g. XRD and XPS results showed that all the doped Ga exists as the substitution form of Ga• Zn in the ZnO system. The volume resistivity of the product was as lower as 4.5 :·cm.

Abstract: PCS (particle composite system) is a kind of dry coating equipment. The PCS process involves encapsulating UHMWPE (ultra-high molecular polyethylene) particles with a layer of SCB (super conductive carbon black) or ACB (acetylene black), and subsequently compacting these CB-encapsulated UHMWPE powders by compression molding to manufacture conductive polymer composites respectively. Morphologies of these composite particles were investigated by SEM. By SEM, we can see the conductive networks of CB in polymer composites. Coating-molding process with PCS can be used to form network structure in ceramics and metal as well.

Abstract: Non-aggregated nanoscale α-Al2O3 powders with average size ranging from 7 to 20 nm were prepared successfully at various conditions with the use of the hydrated aluminum sulfate extracted from kaolin by polyacrylamine (PAA) gel method. The calcination temperature was relatively lower about 100°C, compared to conventional solid-state reaction. It revealed that PAA worked as a chelating agent to disperse the Al3+ ions homogeneously, and the size of alumina nanoparticles could be controlled by varying the concentration of crosslinking agent.

Abstract: High-purity nanoscale α-alumina powders with high specific surface area were prepared from boehmite powders synthesized by the method of aluminium isopropoxide hydrolysis. The physiochemical transformation of boehmite powders under heat-treatment was investigated. The results showed that the reaction progress of boehmite under heat-treatment is as follows: AlO(OH) → Al2O3 (amorphous) → γ-Al2O3 → δ-Al2O3 → α-Al2O3. The phase transformation from δ-Al2O3 to α-Al2O3 begins at 1050oC, and the α-Al2O3 grains growth accelerates as temperature increases. High-purity α-Al2O3 nanometer powders with higher specific surface area can be obtained after incineration at 1100oC for 4 h.