Vietnam Academy of Science and Technology: Journals Online
Vietnam Academy of Science and Technology: Journals OnlineNot a member yet
7064 research outputs found
Sort by
Synthesis of ZnSe Nanocrystals for Solid-state Lighting Applications
No full textWe report the large-scale synthesis of highly luminescent ZnSe nanocrystals (NCs) by a simple and low-cost hydrothermal method. XRD (X-ray Diffraction) and HR-TEM (High Resolution Transmission Microscopy) characterization studies confirmed the formation of as-synthesized ZnSe NCs in cubic structure. The optical property of ZnSe NCs were tunable via controlling the Zn:Se molar precursor ratio (0.5:1–1.5:1), reaction temperature (150–200 0C), and reaction time (5–30 h). The resulting ZnSe NCs with the Zn:Se precursor ratio of 1:1, hydrothermally treated at 190 0C for 20 h exhibited the highest photoluminescence quantum yield obtained by PL spectra with the 355 nm excitation. The current–voltage (I–V) characteristics of the ZnSe NCs show its promising application in the solid-state lighting
An improved meshless method for finite deformation problem in compressible hyperelastic media
No full textHyperelastic materials are considered as special category of elastic solid materials because of their nonlinear complicated constitutive laws. Due to large strain state, the behaviour of such materials is often considered in finite deformation analysis. The nonlinear large deformation behavior of such materials is important. In this study, a novel meshless radial point interpolation method (RPIM) enhanced by Cartesian transformation method (CTM), an effective numerical integration, is presented for nonlinear behavior of hyperelastic media under finite deformation state with total Lagrange formulation. Unlike the mesh-based approaches, the meshless methods have shown their advantages in analysis of large deformation problems. The developed CTM-based RPIM is thus free from the need for background cells, which are often used for numerical integration in many conventional meshfree approaches. The developed meshfree method owns some desirable features of an effective technique in solving large deformation, which will be illustrated through the numerical experiments in which our computed results are validated against reference solutions derived from other approaches.
Simulation Study on Supercontinuum Generation at Normal Dispersion Regime of a Carbon Disulfide-core Photonic Crystal Fiber
No full textA photonic crystal fiber with a hollow core filled with carbon disulfide (CS2) is proposed as a new source of supercontinuum light. We numerically study guiding properties of modeled fibers including the dispersion and the effective mode area of the fundamental mode. As a result, octave spanning of the SC spectrum was achieved in the wavelength range of near-IR from 1.25 μm to 2.3 μm with 90 fs pulse and energy of 1.5 nJ at a pump wavelength of 1.55 μm. The proposed fibers are fully compatible with all-silica fiber systems, in particular, could be used for all-fiber SC sources and new low-cost all-fiber optical systems
Morpho-kinematics of the Molecular Gas in a Quasar Host Galaxy at Redshift Z=0.654
No full texte present a new study of archival ALMA observations of the CO(2-1) line emission ofthe host galaxy of quasar RX J1131. The quasar, at redshift z S ∼0.654, is lensed by a foreground galaxy at z L ∼0.30. Particular attention is paid to the mechanism of gravitational lensing. A simple lens model, shown to well reproduce the optical images obtained by the Hubble Space Telescope, is applied to the ALMA CO(2-1) images, providing a tool to understand the uncertainties attached to the evaluation of the source brightness and kinematics. Uncertainties attached to the process of data reduction are carefully evaluated. Evidence for the robustness of previously published results is obtained. A system of polar coordinates is introduced to better match the specificity ofthe imaging process. It provides particularly clear evidence for rotation of the gas contained in the galaxy. A simple rotating disc model is shown to give an excellent overall description of the morpho-kinematics of the source. It gives evidence for a hot spot of emission located near the quasar, overlapping the caustic and corresponding to an enhancement of emission by a factor ∼2.5. The possible presence of a companion galaxy suggested by some previous authors is not confirmed. The rotation curve is studied with reference to the predictions of the disc model. De-tailed comparison between model and observations gives evidence for a more complex dynamics than implied by the model. Doppler velocity dispersion within the beam size in the image plane is found to contribute 60±10 km s −1 to the line width. It accounts for the observed line width when a2- σ cut is applied to the data. However, when using a less severe cut, a significant amount of turbulence may be accommodated, preventing a reliable evaluation of the contribution of turbulence to the line width
A UNIFIED PORT-HAMILTONIAN APPROACH FOR MODELLING AND STABILIZING CONTROL OF ENGINEERING SYSTEMS
No full textThis work deals with systems whose dynamics are affine in the control input. Such dynamics are considered to be significantly differentially expressed in a canonical form, namely the quadratic (pseudo) port-Hamiltonian representation, in order to explore further some structural properties usable for the tracking-error passivity-based control design. Different kinds of linear and nonlinear engineering systems including an open isothermal homogeneous system and a continuous biochemical fermenter are used to illustrate the approach
Population fluctuation of Paederus fuscipes Curtis (causes blistering of the skin) in the field on the Red River alluvial plain (Baigiua), Hanoi, Vietnam
No full textABSTRACT-HCTN07Paederus fuscipes Curtis, a rove beetle, belongs to the coleopteran large family Staphylinidae known with 46 275 species in 3 200 genera, 31 subfamilies. This species and other Paederus species somewhat confused with ants, but taxonomically much different from the latter. P. fuscipes is very common in agrosystems in Vietnam, foraging insect pests. Besides beneficial role, Paederus species are harmful to human health, causing skin irritation called dermatitis linearis. The P. fuscipes species contain Pederin (C24H43O9N), it’s 12 to 15 times more toxic than a snake's venom but due to the small amount of exposure and only on the skin, it is not as deadly as snake venom. This substance mainly causes skin burns that can be mistaken for nerve shingles. At present, knowledge of life cycle and population dynamics of P. fuscipes in Vietnam is little. Therefore, our study aimed at providing some data on its life cycle and occurrence in the field in relation to its prey, contributing to conservation of this species for controlling insect pests and issuing warnings during its peak period of populatio
SCREENING AND PRILIMINARY STUDY ON MICROBIAL STRAINS WITH INHIBITORY ACTIVITY AGAINST ENZYMES RELATING TO DIABETES IN VIETNAM
No full textABSTRACT – HCTN7Microorganisms are considered a rich and potential source for the bioactive guided exploitation of natural products. Commonly used enzyme inhibitors for diabetes treatment hitherto such as α-amylase and α-glucosidase inhibitors are mostly microbial sources derived. The present study was carried out with the aim of screening and questing microbial inhibitors against the most popular diabetes-related enzymes (α-amylase, α-glucosidase and PTP1B) in Vietnam. The investigation results revealed 19 of all 100 tested microbial crude extracts exhibiting inhibitory activity against at least one enzyme. Basing on morphological, physiological, biochemical, and rRNA 16/18S gene sequencing, the phylogeny of the most potent microbial strains was indentfied. The research results contributed confirming the potential of the research for natural compounds from microorganisms, and supported new materials for further studies to exploit medicinal active ingredients from microbial sources
Levels of heavy metals in seawater, sediment and in the tissue of Crassostrea belcheri in the western estuary of Ganh Rai bay
No full textRecent studies have shown that the seawater, sediment and commercial bivalve molluscs in the studied area have been contaminated by some heavy metals Zn, Cu, Pb and Cr. Highly toxic heavy metals like As, Cd, Pb, Hg have tended to accumulate in the tissue of clams. This paper presents the levels of some heavy metals in the seawater, sediment and soft part of oyster (Crassostrea belcheri) samples collected from the western estuaries of Ganh Rai bay in 2015 and 2017. The results also showed that Ha Thanh and Rach Lo sites recorded the highest contents of most studied metals in oyster samples. Levels of metals in oyster were in the order of Zn Cu As Pb Cr Cd Hg and the contents of metals in oyster did not reflect a correlation with those in surrounding environment. Concerning food safety criteria, Pb, Cd and Hg contents were lower than acceptable limit given by the compilation of FAO (1983), whereas As, Cu and Zn contents exceeded the legal limit, especially Zn contents. Cu and Cr contents in sediment samples of Nga Bay and Dong Hoa estuaries were between LEL-SEL values (≥ LEL and SEL), which may cause biological impacts at moderate level
Dinoflagellate cysts in surface sediments at Van Phong Bay, Khanh Hoa Province, Vietnam: distribution, abundance and potentially harmful algal blooms
No full textStudies on dinoflagellate cysts provide important knowledge on the ecology of dinoflagellates and harmful algal blooms (HABs). In this study, distribution and abundance of dinoflagellate cysts in surface sediments from 17 stations at Van Phong Bay were analyzed. There were 55 different types of cysts representing 3 orders and 18 genera, and 8 unidentified cyst types recorded. Peridiniales was the most diverse order with 29 cyst types, including 20 Protoperidinium cyst types. There were 10 cyst types of 7 potential toxic dinoflagellate species and 4 of bloom forming species found, indicating a potential risk of harmful algal blooms in Van Phong Bay. Number of cyst types and density ranged from 12 to 31 types and from 115 to 3,760 per gram of dry weight sediment, respectively. Cysts of Leonella granifera were dominant at stations in the mouth of the Bay, while Scrippsiella trochoidea cysts dominated at all stations. Shannon diversity index (H') was low, varying from 1.192.72. There were two distinct cyst assemblages identified with 40% Bray-Curtis similarity, assemblage I with 2 stations (VP09 and VP10), and assemblage II with the other stations.
USING RADIAL BASIS FUNCTION NEURAL NETWORK FOR PMSM TO OVERCOME THE CHANGING LOAD
No full textThis paper presents a using Radial Basis Function Neural Network (RBFNN) for PMSM to overcome the changing load. Firstly, a mathematic model of the PMSM drive is derived; then, to increase the performance of the PMSM drive system, a Fuzzy PI controller in which an RBFNN adjusts its parameters is applied to the speed controller for coping with the effect of the system dynamic uncertainty and the external load. Secondly, the Very high-speed integrated circuit Hardware Description Language (VHDL) is adopted to describe the behaviour of the speed control IC which includes the circuits of space vector pulse width modulation (SVPWM), coordinate transformation, RBFNN, and Fuzzy PI. Thirdly, the simulation work is performed by MATLAB/Simulink and ModelSim co-simulation mode, provided by Electronic Design Automation (EDA) Simulator Link. The PMSM, inverter, and speed command are performed in Simulink, and the speed controller of the PMSM drive is executed in ModelSim. Finally, the co-simulation results validate the effectiveness of the proposed algorithm based speed control syste