Advances in Natural Science

Dyson Equations for Green Functions of Electrons in Open Single-level Quantum Dot

Nguyen Van Hieu, Nguyen Bich Ha, Nguyen Van Hop — Mon, 21 Jul 2008 00:02:48 +0700

The infinite system of differential equations for the nonequilibrium Green functions of electrons in a single-level quantum dot connected with two conducting leads is truncated by applying the mean-field approximation to the mean values of the products of four operators. As the result the system of Dyson equations for the two-point real-time nonequilibrium Green functions in the Keldysh formalism as well as that of the two-point imaginary-time Green functions are derived.

Anharmonic Effective Potential, Local Force Constant and Correlation Effects

Nguyen Van Hung, Nguyen Bao Trung, Le Hai Hung — Mon, 21 Jul 2008 00:14:04 +0700

Analytical expressions for the anharmonic effective potential, local force constant, Displacement-displacement Correlation Function (DCF) CR and Debye-Waller factor described by the Mean Square Relative Displacement (MSRD) sigma^{2 and by the Mean Square Displacement (MSD) u2of bcc crystals in the X-ray Absorption Fine Structure (XAFS) have been derived. The effective interatomic potential of the system has been considered by taking into account the influences of nearest atomic neighbors, and it contains the Morse potential characterizing the interaction of each pair of atoms. Numerical results for u^{2 , sigma^{2 and CR of Fe and W are found to be in good agreement with experiment. The ratios CR/u^{2 and CR/sigma^{2 approach constant values at high temperatures showing the same properties obtained by the Debye model.}}}}}

Investigation of Thermodynamic Quantities of Cubic Zirconia by Statistical Moment Method

Vu Van Hung, Le Thi Mai Thanh, Nguyen Thanh Hai — Mon, 21 Jul 2008 00:20:28 +0700

We have investigated the thermodynamic properties of the cubic zirconia ZrO2 using the statistical moment method in the statistical physics. The free energy, thermal lattice expansion coefficient, specific heats at the constant volume and those at the constant pressure, CV and CP , are derived in closed analytic forms in terms of the power moments of the atomic displacements. The present analytical formulas including the anharmonic effects of the lattice vibrations give the accurate values of the thermodynamic quantities, which are comparable to those of the ab initio calculations and experimental values. The calculated results are in agreement with experimental findings. The thermodynamic quantities of the cubic zirconia are predicted using two different inter-atomic potential models. The influence of dipole polarization effects on the thermodynamic properties for cubic zirconia have been studied.

Influence of Substitution of the Magnetic 3d Metals for Mn in Perovskite La0.67Ca0.33Mn0.90TM0.10O3 (TM = Fe, Co, Ni) Compounds

Nguyen Huy Sinh, Vu Thanh Mai, Pham Hong Quang — Mon, 21 Jul 2008 00:27:35 +0700

Influence of substitution of 3d metals for Mn on properties of La0.67Ca0.33Mn0.9 TM0.1O3 (TM = Fe, Co, Ni) compounds was studied. Ferromagnetic – paramagnetic and metals – insulator transitions were significantly affected by Mn-site substitution. However, no observable difference was found in their crystal structure from X-ray diffraction analysis. At room temperature the structure characterization of these compounds gave the single phase and structure is the distortion orthorhombic cell with space group symmetry Pnma. The magnetoresistance measurement showed that the magnetoresistance ratio MR increases until 17% in magnetic field of 0.4 T, and in low magnetic field region (μoH < 0.05 T), MR = 7.5% at 102 K. The investigations of EPR showed that the intensity of resonance line can be well fitted by the expression: I(T) = Ioexp(Ea/kBT). The values of activation energy have been determined with Ea = 0.074 eV, 0.093 eV and 0.086 eV for substituted Fe, Co, Ni samples, respectively. These values are slightly smaller than the value of Ea = 0.12 eV for La0.67Ca0.33MnO3. We attribute the reason to the reduction of Mn3+ content caused by TM substitution for Mn. The dependence transition temperatures and transport properties of all samples are well explained by introducing the SE interaction with considering that the Fe3+, Co3+ and Ni2+ ions have high-spin configuration, the local DOS near the Fermi level at the TM site Nx(Ef ) would decrease from Ni to Co and Fe [1], thus reduces the hopping probability and increases the resistivity in order.

Study on the Structure and Properties of Polypropylene/Clay Nanocomposites

Nguyen Thac Kim, Thai Hoang, Phan Quang Thai, Nguyen The Anh — Mon, 21 Jul 2008 00:33:51 +0700

Polypropylene/ maleic anhydride /organic montmorillonite nanocomposites (PP/ MA/ Org-MMT) have been prepared via direct melt intercalation in an internal mixer. Maleic anhydride (MA) was used as a compatibilizer to improve the dispersability of the clay. The structures of nanocomposites have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Mechanical properties and thermal stability were determined by tensile analysis and thermogravimetric analysis (TGA), respectively. The XRD patterns, TEM and SEM image showed exfoliation of Org-MMT layers in PP matrix and existence of both the exfoliated, intercalated structures of the formed nanocomposites in presence of 1% MA. The tensile strength, elongation at break, and thermal stability of PP/MA/Org-MMT nanocomposites was higher than those of neat PP. MA played a very important role in reduction of size of Org-MMT and improved its dispersion in PP matrix.

Final Breakdown on Water Tree Degraded Polymer Insulation

Vu Thanh Hai, Nguyen Dinh Thang — Mon, 21 Jul 2008 00:38:21 +0700

This report deals with the mechanism of the final breakdown of polymeric power cables which have been degraded by water trees. Water treeing is one of the major degradation mechanisms in cable insulation. It has been believed that the final breakdown takes place when a water tree penetrates the insulation. In order to reproduce penetrating water trees in a film specimen, a double layer specimen with low-density polyethylene films was subjected to an aging test to generate water trees. After confirming that penetrating water trees had bridged across one layer of the ”double-layer” specimen, the breakdown test was performed with several kinds of combinations of degraded and non-degraded films by applying an AC voltage. It was concluded from the result that the penetrated water tree region would withstand relatively high voltage stresses if the ambient relative humidity was as low as 59 %, however, the breakdown voltage rapidly drops at higher humidities. Finally, it was suggested that, under less humid conditions, a water tree would extend until it bridges across the insulation without causing breakdown, however, the breakdown would likely take place when the ambient humidity turns to be higher.

Preparation, Optical Properties and Energy Transfer of Y2O3: Rare Earth (re: Tb,Eu, Er, Yb) Nanophosphors

Tran Kim Anh, Nguyen Vu, Man Hoai Nam, Le Quoc Minh — Mon, 21 Jul 2008 00:44:54 +0700

This paper describes the preparation, optical properties and the role of the concentration of rare earth ions (RE: Eu, Er, Tb, Yb) as active centers in the energy transfer of Y2O3: RE nanophosphors. The materials are prepared by the combustion method using urea or glycine. Nanoparticles can be prepared with sizes ranging from 5 to 100 nm and can be estimated by high resolution TEM. Optimization of Eu and Er content in these nanophosphors was reported. The effect of energy transfer between Tb and Eu and the up-conversion between Er and Yb were studied. We will survey recent advances in nanophosphor development and discuss their future application potential.

The Phase Transformation in the Crystallization Process of Fe-Mordenite Zeolite

Do Xuan Dong, Dang Tuyet Phuong, Nguyen Huu Phu, Dinh Quang Khieu — Mon, 21 Jul 2008 00:49:50 +0700

In the present paper, we reported the influence of the crystallization time on the crystallinity of the Fe-mordenite zeolite (Fe- MOR). The obtained results showed that, in order to reach the high crystallinity of zeolite the optimal duration of crystallization was about 36 h. When crystallization time increased from 48h to 72h, there was phase transformation from MOR zeolite to ZSM- 5 zeolite. However, if crystallization time decreased (< 24 h) the obtained samples had low crystallinity.

Study on reparation of Blends Based on Poly(vinyl Chloride), Nitrile-butadiene Rubber, Natural Rubberand Polyethylene

Do Quang Khang, Nguyen Phi Trung — Mon, 21 Jul 2008 00:53:21 +0700

The blends based on polyvinyl chloride (PVC), nitrile-butadiene rubber (NBR), natural rubber (NR) and polyethylene (PE) were made by three different methods. The mechanical properties of these blends in dependence of mixing time and temperature were characterized. The result showed that the two-step blending method, where PVC and NBR were first mixed together and then blended with NR and PE, is the best one to prepare blends with high mechanical properties.

Preparation of Superabsorbent Polymers by the Inverse Suspension Method

Nguyen Van Khoi, Nguyen Thanh Tung, Pham Thi Thu Ha, Trinh Duc Cong — Mon, 21 Jul 2008 00:57:30 +0700

The superabsorbent polymers based on acrylic acid have been prepared in the presence of ammonium persulfate (APS) as initiator and N,N-methylene- bis- acrylamide (MBA) as crosslinking agent. The suspension polymerisation occurred in aqueous drops dispersed in toluene and stabilised by sorbitan monooleate. Some effects on the swelling degree and size particles such as concentrations of MBA and initiator, neutralization degree (Dn) were studied. Swelling degree 405g/g, reach maximum at 85C and 0.09% crosslinking agent, 1.0% initiator, 75% degree of neutralization, 350 rpm agitation speed was observed.

A Study on Methanation of Carbon Monoxide over Catalysts NiO/TiO2 and NiO/-Al2O3

Luu Cam Loc, Nguyen Manh Tuan, Nguyen Kim Dung, Nguye Huu Huy Phuc, Ho Si Thoang — Mon, 21 Jul 2008 01:02:39 +0700

The catalyst 7.5% NiO/TiO2 and seven samples of NiO/ -Al2O3 with NiO content from 7.5% to 60% have been obtained and studied. Physico-chemical characteristics of the catalysts were determined by methods of Adsorption (BET), X-ray Diffraction (XRD), Temperature- Programmed Reduction (TPR) and Hydrogen Pulse Titration. The catalytic properties of the obtained samples were investigated in the reaction of CO methanation at temperatures 180 – 220C and mole ratios hydrogen/carbon monoxide 25-100. It has been found that the optimal regime for catalyst treatment is calcination at 600C and reduction at 400C. The catalyst 7.5% NiO/TiO2 expressed very high activity but in case of NiO/Al2O3 for having high activity it was necessary to raise the content of NiO up to 37.7%. This catalyst has been indicated to be the most active in all the studied conditions. The determining factors for expressing high activity in the catalyst 7.5% NiO/TiO2 are the presence of spinel NiTiO3 and the optimal average size of nickel assemblies but in the catalyst 37.7% NiO/Al2O3 – the optimal size of active centers, big values of specific surface area and content of NiO and the high thermal stability.

New Monomers for Chemical Vapor Deposition Polymerization of Poly (p-Xylylene)

Nguyen Duc Nghia, Ngo Trinh Tung, Jung Il Jin — Mon, 21 Jul 2008 01:06:35 +0700

It was demonstrated that Poly(p-Xylylene) (PPX) could be prepared from , ’-Bis(Alkoxy or Aryloxy) –p-Xylenes via chemical vapor decomposition (CVD)- method. This is one-step process and there are side products by the CVD-process. This effect depends both on the CVD- condition and the properties of the starting monomer. The structure and thermal behavior of the material were characterized by FTIR, UVvis spectroscopy, elemental analysis, wide-angle X-ray diffraction, DTA and DSC. It could be showed that the deposited PPX is semicrystalline and the melting process of PPX is characterized by the two crystalline phase transition and accompanied by decomposition. This research will open a new way to synthesize of PPX.

Conservation of Genetic Diversity of Threatened Cycads (Cycadaceae) in Vietnam

Nguyen Minh Tam — Mon, 21 Jul 2008 01:08:59 +0700

Cycads occur in lowland forests and are threatened by their degradation, fragmentation and over-exploitation. Our results indicate that cycad population sizes were very small (less 100 individuals) with high propagules. These increased mating between closely related individuals growing in clumps. The allozyme data show although all studied cycads maintained high levels of genetic diversity, an average of 0.556 (0.491 in C. hoabinhensis to 0.598 in C. chevalieri), the degree of inbreeding was high. Factors that contribute to the high levels of genetic variability include dioecy, perennial and complex habitat structure. The priorities for cycad conservation is discussed as well.

Zinc Effects on Oxidative Physiology of Oral Bacteria

Nguyen Thi Mai Phuong, Phan Tuan Nghia, Robert E. Marquis — Mon, 21 Jul 2008 01:13:18 +0700

Zinc is used as an anti-gingivitis agent in oral care products and affects multiple targets in oral bacteria. We found that it acts also to suppress respiration of oral streptococci and of Fusobacterium nucleatum, an organism associated with development of gingivitis. Zinc as either the sulfate or citrate chelate was a potent inhibitor of respiration of the oral streptococci Streptococus mutans GS-5 or UA 159 and S. sanguis NCTC 10904. 50% inhibitory concentrations for intact cells in suspensions were below 0.3 mM with nearly complete inhibition of O2 metabolism at higher zinc levels. 1.0 mM Zn2+ inhibited O2 utilization by F. nucleatum ATCC 25586 by some 50%, but almost 40% of O2 metabolism by the anaerobe was not Zn sensitive. NADH oxidase plays the major role on O2 utilization by oral streptococci and is also important for F. nucleatum. In cell extracts of streptococci, the 50% inhibitory concentration (IC50) of Zn2+ for NADH oxidase activity ranged from ca. 2.1 mM for S. sanguis NCTC 10904 to 3.0 mM for the GS-5 strains of S. mutans. The enzyme in extracts of F. nucleatum was somewhat more Zn sensitive with an IC50 value of 1.4 mM. S. sanguis is a major generator of H2O2 in dental plaque. Zn2+acted to reduce peroxide production assessed with use of horseradish-peroxidase and leuco-crystal violet. 0.1 mM ZnSO4 was more than 50% inhibitory for cells in suspensions or mono-organism biofilms. Zinc is primarily bacteriostatic rather than bactericidal, and inhibition was at least partially reversible after zinc removal. Zinc was inhibitory also for the protective enzymes thiolperoxidase (IC50= 0.1 mM), hypothiocyanite reductase (IC50 = 0.1 mM) and glutathione reductase (IC50 = 0.4 mM). Zinc acted mainly as a pro-oxidant for oral bacteria inhibiting NADH oxidase, considered to be protective against oxidative stress, and also other protective enzymes. This view is supported by findings of zinc enhanced peroxide killing of the organisms. Abbreviations. Reactive oxygen species (ROS), NADH oxidase (NOX), NADH hypothiocyanite reductase (NHOR), glutathione reductase (GR), thiolperoxidase (TSA), catalase (CAT), pseudocatalase (PCAT), superoxide dismutase (SOD), 50% inhibitory concentration (IC50).

Study on Treatment of Alkaline Black Liquor Using Sulphate Reducing Bacteria

Doan Thai Hao, Tran Dinh Man — Mon, 21 Jul 2008 01:18:22 +0700

Alkaline pulping liquors are the problematic for environment due to their toxicity and poor biodegradability. During last decade, anaerobic treatment technology has been successfully applied to various types of pulp and paper industry effluents. In the present study, the treatment of highly toxic pulping liquors in lab-scale has been done using sulfate reducing bacteria (SRB) isolated from sediment samples of pulp and paper mills in Vietnam. This treatment could reduce 70 - 75% of COD after three weeks. The COD removal could achieve 82 - 88% by subsequent aerobic treatment in 48 hours. COD of treated water was 80 - 90 mg/l.