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ABSTRACT: We investigate the size-dependent optical extinction properties of colloidal Si NCs from the near-IR to UV. Experimental results are compared to the Mie solution to Maxwell's equations using the same refractive index as bulk Si in order to evaluate the deviation from bulk properties. We find that the energy for the lowest direct transition (E1) continuously blueshifts from near bulk-like at ~3.4 eV, in large NCs (16 nm) to ~3.6 eV for small NC (3.9 nm), contrary to the Mie solution. The extinction cross section of NCs on a per-atom basis was found to be independent of NC size, within our experimental resolution. The results suggest that quantum confinement effects strongly influence excitons associated with the E1 transition.
Langmuir 01/2013; · 4.19 Impact Factor
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ABSTRACT: Silicon nanocrystals have recently attracted significant attention for applications in electronics, optoelectronics, and biological imaging due to their size-dependent optical and electronic properties. Here a method for synthesizing luminescent silicon nanocrystals from silicon tetrachloride with a nonthermal plasma is described. Silicon nanocrystals with mean diameters of 3-15 nm are synthesized and have a narrow size distribution with the standard deviation being less than 20% of the mean size. Control over crystallinity is achieved for plasma pressures of 1-12 Torr and hydrogen gas concentrations of 5-70% through adjustment of the plasma power. The size of nanocrystals, and resulting optical properties, is mainly dependent on the gas residence time in the plasma region. Additionally the surface of the nanocrystals is covered by both hydrogen and chlorine. Oxidation of the nanocrystals, which is found to follow the Cabrera-Mott mechanism under ambient conditions, is significantly faster than hydrogen terminated silicon due to partial termination of the nanocrystal surface by chlorine.
Nanotechnology 06/2011; 22(30):305605. · 3.98 Impact Factor
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ABSTRACT: This paper presents the reaction mechanism of single-step methane partial oxidation to methanol at room temperature using non-thermal plasma microreactor. Macroscopic quantities of hydrogen peroxide (H2O2) and methyl hydroperoxide (CH3OOH) are produced when methane is partially oxidized at room temperature (about 5 °C). CH3OOH is known to be the principle intermediate of incomplete methane oxidation product such as CH3OH and HCHO, but has not been demonstrated experimentally so far. H2O2 promotes post-plasma oxidation of oxygenates in the condensed plasma-synthesized liquid. At an early stage of in-liquid oxidation, H2O2 oxidizes HCHO into HCOOH preferentially; subsequently, HCOOH is fully oxidized to CO2 and H2O. Depending upon the concentration of oxygenates and H2O2, electrical conductivity of the plasma solution dramatically increased, which detrimentally influences plasma properties. Methane partial oxidation with air was also investigated from a practical viewpoint. Generation of active nitrogen species (ANS) is the key to promoting overall methane conversion in the presence of oxygen; however, fragile oxygenates were also decomposed by ANS, thus selectivity for useful oxygenates was degraded in the presence of nitrogen. When oxygen is fully consumed, CH4 conversion is also terminated and water gas shift reaction (CO + H2O = CO2 + H2) becomes predominant.
Journal of Physics D Applied Physics 06/2011; 44(27):274010. · 2.54 Impact Factor
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ABSTRACT: Methane partial oxidation was investigated using a plasma microreactor. The experiments were performed at 5 and 300 °C. Microreactor configuration allows an efficient evacuation of the heat generated by methane partial oxidation and dielectric barrier discharges, allowing at the same time a better temperature control. At 5 °C, liquid condensation of low vapour pressure compounds, such as formaldehyde and methanol, occurs. 1H-NMR analysis allowed us to demonstrate significant CH3OOH formation during plasma-assisted partial oxidation of methane. Conversion and product selectivity were discussed for both temperatures. In the second part of this work, a numerical simulation was performed and a gas-phase chemical mechanism was proposed and discussed. From the comparison between the experimental results and the simulation it was found that CH3OO formation has a determinant role in oxygenated compound production, since its fast formation disfavoured radical recombination. At 5 °C the oxidation leads mainly towards oxygenated compound formation, and plasma dissociation was the major phenomenon responsible for CH4 conversion. At 300 °C, higher CH4 conversion resulted from oxidative reactions induced by OH radicals with a chemistry predominantly oxidative, producing CO, H2, CO2 and H2O.
Journal of Physics D Applied Physics 06/2011; 44(27):274011. · 2.54 Impact Factor
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ABSTRACT: An atmospheric-pressure radio-frequency discharge (APRFD) has great advantages over vacuum-oriented plasma-enhanced chemical vapour deposition (PECVD) as well as other types of atmospheric-pressure plasma sources in terms of single-walled carbon nanotube (SWCNT) growth. We first provide an overview on the recent advances in PECVD synthesis of CNTs, ranging from low pressure to atmospheric pressure, and then we present our current work focusing on the analysis of reactive species generated in the cathodic plasma sheath for further understanding of the SWCNT growth mechanism in PECVD. It was found that the plasma-generated C2H2 is the main CNT growth precursor in PECVD. Approximately 30% of the CH4 (initial feedstock) was converted into C2H6, C2H4 and C2H2. A trace amount of C2H2 enabled the synthesis of SWCNTs in the thermal chemical vapour deposition (CVD) regime. H2 is necessary to grow SWCNTs using PECVD because H2 suppresses the formation of excess amount of C2H2; however, H2 does not eliminate amorphous carbon even at H2/C2H2 ratios of 300. PECVD using a binary mixture of C2H2 and isotope-modified 13CH4 demonstrated that CH4 does not contribute to CNT growth in C2H2-assisted thermal CVD. Atmospheric-pressure PECVD performed with a He/CH4/H2 system is equivalent to C2H2-assisted thermal CVD without an etching gas. APRFD appears to produce a hidden species, which influences the CNT growth process.
Journal of Physics D Applied Physics 04/2011; 44(17):174007. · 2.54 Impact Factor
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ABSTRACT: The reaction enhancement mechanism of methane steam reforming (MSR) in the nonthermal discharge and catalyst hybrid reaction is presented. Coke deposited on Al2O3-supported Ni catalyst was investigated using temperature-programmed oxidation (TPO) analysis and micro-Raman spectroscopy. Although methane conversion in the hybrid reaction is greater than that of normal catalytic reforming, the normal reaction deposited 5 times more coke than the hybrid reaction. Raman spectroscopy revealed that the superposition of the nonthermal discharge on catalysts produced less graphitized coke compared to the normal catalytic reaction, by which it is easily removed by H2O during steam reforming. Excited species produced by nonthermal discharge are so reactive that their reaction is completed only on the pellet surface: coke formation in the catalyst pore was only slightly detectable. In contrast, large amounts of coke were detected from the surface and catalyst pores in the normal reaction. In the hybrid reaction, CH4 and H2O are primarily excited by electron impact. Therefore, methane dehydrogenation followed by coke oxidation is promoted, resulting in a small amount of coke formation with greater methane conversion than in the normal reaction. The results are well-correlated with Arrhenius plot analysis of the overall forward rate constant for MSR in the hybrid reaction.
09/2008;
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ABSTRACT: Bioresources, such as landfill gas and agricultural residues, attract considerable attention because of growing concerns of global energy and environmental protection. However, the efficient usage of bioresource poses major challenges and generally necessitates appropriate pre-reforming processes. We propose a low-temperature (300−500 °C) upgrading method using an atmospheric pressure nonthermal discharge generated in a reforming catalyst bed reactor for profitable recovery of poor bioresources. Excited species produced by high-energy electron impact, which proceed independent of the temperature, accelerate methane steam reforming at lower temperatures than normal catalyst reactions with minimum energy required. The resultant hydrogen-enriched biogas is then available for use in conventional energy utility systems, such as internal combustion engines. This paper introduces fundamental characteristics of nonthermal discharge and the catalyst hybrid reactor. Furthermore, a detailed mechanistic study of synergistic effects between nonthermal discharge and the reforming catalyst is presented on the basis of the Arrhenius plot method.
07/2007;
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ABSTRACT: An atmospheric-pressure microplasma reactor was developed for the fabrication of tunable photoluminescent silicon nanocrystals. A mixture of argon, hydrogen, and silicon tetrachloride was activated by a capacitively coupled non-equilibrium plasma generated in a capillary glass tube with a volume less than 1 µl. The microplasma efficiently decomposes silicon tetrachloride into atomic silicon even though the residence time is approximately 100 µs. Supersaturated silicon vapour then leads to gas phase crystal nucleation via three-body collision, followed by rapid termination of crystal growth due to the short reactor residence time. Silicon nanocrystals are continuously synthesized in gas phase at room temperature. The room-temperature photoluminescence (PL) of as-synthesized material with hydrogen concentration around 0.7–0.8% exhibited intense visible light emission with peak intensity centred around 670 nm. The PL spectrum was blue-shifted to 520 nm with increasing hydrogen content, implying that partially oxidized nanocrystals of diameter less than 3 nm were synthesized.
Nanotechnology 05/2007; 18(23):235603. · 3.98 Impact Factor
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ABSTRACT: Nonthermal plasma-assisted methane conversion has been widely investigated as a potential low-temperature process. The desired end objective is synthesis gas production, one-step production of liquid oxygenates, or coupling products. Either oxygen or steam or both is used as a second reactant or reactants, to provide additional O atoms and initiate plasma-assisted radical processing. The present paper intends to investigate the various possible products/reactive species that are formed during plasma processing. In situ Fourier transform infrared (FTIR) absorption spectrometry is used to monitor the products/reactive groups. Experiments are performed at room temperature and at low energy inputs (50 kJ/(mol CH4)) in a gas mixture of CH4/O2/N2/H2O (atmospheric humidity). In the absence of oxygen, alkane, alkene, and alkyne groups are formed as the products, which indicates termination reactions. With increasing oxygen concentration (11, 15, and 33%) a gradual shift from alkane to a −CHO group is observed. In addition, alcohol group formation is detected with oxygen input, which indicates coupling between the CH4 dissociation products and the O radicals as a primary step. The effect of higher energy input and the presence of catalytic surfaces such as platinum also are investigated.
04/2007;
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ABSTRACT: This study demonstrates that aequorin, a luminescent natural dye, is useful for vascular cell intracellular Ca2+ concentration ([Ca2+]i) determination. A new single-photon counting technique was developed to resolve the effects of fluid flow shear stress on [Ca2+]i in human aortic smooth muscle cells (HASMCs). Confluent HASMCs were grown on petri dishes loaded with aequorin. Then the dishes were placed in a luminometer chamber after the physiological level of shear stress was applied to the HASMC surfaces. The chamber was housed inside a highly sensitive photomultiplier tube. It detected ultraweak photon emission in response to the [Ca2+]i transient. In the presence of 2.0 mM extracellular Ca2+, a shear stress of 12 dyn cm2, applied for 60 s to the top surface of the HASMC monolayer, elicited a sharp increase in [Ca2+]i.
Journal of Biomechanical Engineering 11/2006; 128(5):777-81. · 1.90 Impact Factor
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ABSTRACT: Deposition of vertically oriented carbon nanofibers (CNFs) has been studied in an atmospheric pressure radio frequency discharge without dielectric barrier covering the metallic electrodes. When the frequency is sufficiently high so that ions reside in the gap for more than one rf cycle (“trapped ions”), the operating voltage decreases remarkably and the transition from a uniform glow discharge to an arc discharge is suppressed even without dielectric barriers. More importantly, the trapped ions are able to build up a cathodic ion sheath. A large potential drop is created in the sheath between the bulk plasma and the electrode, which is essential for aligning growing CNFs. At the same time, the damage to CNFs due to ion bombardment can be minimized at atmospheric pressure. The primary interest of the present work is in identifying the cathodic ion sheath and investigating how it influences the alignment of growing CNFs in atmospheric pressure plasma-enhanced chemical-vapor deposition. Spectral emission profiles of He (706 nm), Hα (656 nm), and CH (432 nm) clearly showed that a dark space is formed between the cathode layer and the heated bottom electrode. However, increasing the rf power induced the transition to a nonuniform γ-mode discharge which creates intense plasma spots in the dark space. Aligned CNFs can be grown at moderate input power during the initial stage of the deposition process. Catalyst particles were heavily contaminated by precipitated carbon in less than 5 min. Alignment deteriorates as CNFs grow and deposition was virtually terminated by the deactivation of catalyst particles.
Journal of Applied Physics 01/2006; 99(2):024310-024310-7. · 2.17 Impact Factor
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ABSTRACT: We have developed a dielectric barrier discharge (DBD) and catalyst hybrid reactor for reforming low-calorific fuels such as biogas at low temperature (300−500 °C). This technique allows the use of low-temperature thermal energy wasted from various industries, which ultimately provides a variety of energy utility options. The idea behind the project is that radicals produced by DBD can be decomposed at much lower temperatures than in a normal reforming condition. However, the situation becomes even more complicated because DBD enhances chemical reactions in different ways: (1) Excited species, radicals, and ions decompose on the catalyst at a lower temperature than the stable molecule. (2) Byproducts such as acetylene and ethane decompose at lower temperatures than methane. (3) Heat that is generated by DBD also enhances regular catalytic reforming. A mechanistic study of steam reforming in a plasma hybrid reactor was performed to distinguish their respective contributions to the synergistic effect. Results are discussed on the basis of the catalyst bed temperature, which was measured accurately with an infrared camera with and without DBD. Thermal and nonthermal effects of DBD on the catalytic reforming of methane are discussed extensively.
10/2005;
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10/2005: pages 477 - 487; , ISBN: 9783527605583
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ABSTRACT: This study discusses the development of an atmospheric pressure glow discharge enhanced CVD system for the catalytic growth of carbon nanotubes (CNTs). He/H 2 /CH 4 (900 : 100 : 0–20 scc min −1) gas mixture was processed in the barrier discharge reactor operated at 760 Torr. Ni-coated (20 nm) quartz substrate was used up to 600˚C to achieve low temperature catalytic growth of CNTs. Special pretreatment of substrate using metal plating technique was employed for uniform growth; minimum requirements for CNTs growth were specified in terms of substrate temperature, H 2 /CH 4 ratio and deposition time. SEM and TEM micrographs confirmed multi-wall CNTs with outside diameter and number density of 40–50 nm and 10 9 –10 10 cm −2 , respectively. On the other hand, some of those CNTs included considerable wall defects associated with Ni particle aggregation. We also applied DBD enhanced catalytic CVD, but CNTs could not synthesized.
J. Phys. D: Appl. Phys. 01/2002; 35:2779-2784.
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ABSTRACT: The thermal structure of a methane-fed dielectric barrier discharge (DBD) and a atmospheric pressure glow-discharge (APG) has been extensively investigated in terms of time-averaged gas temperature profile between two parallel-plate electrodes separated by 1.0 mm. Emission spectroscopy of the rotational band of CH ((0, 0) A 2 → X 2 : 431 nm) was performed for this purpose. In order to minimize average temperature increase in the reaction field, DBD and APG were activated by 10 kHz with 2% duty cycle pulsed voltage (2 µs pulse width/100 µs interval). In DBD, temperature increase of a single microdischarge, on a time average, reached 200 K. It suddenly decreased below 100 K associated with the dark space formation near the dielectric barrier. Also, gas temperature in the surface discharge was fairly low because emission in these regions was limited within the initial stages of propagation (∼5 ns), whereas energy deposition would continue until microdischarge extinction; these facts implied that rotational temperature seemed to be far below the actual gas temperature in these regions. In APG, gas temperature was uniformly increased by positive column formation. In addition, a remarkable temperature increase due to negative glow formation was obtained only near the metallic electrode. For practical interest, we also investigated the net temperature increase with high frequency operations (AC-80 kHz), which depends not only on plasma properties, but also various engineering factors such as flow field, external cooling conditions, and total input power. In DBD, gas temperature in the middle of gas gap was significantly increased with increasing input power because of poor cooling conditions. In APG, in contrast, gas temperature near the electrodes was significantly increased associated with negative glow formation.
Plasma Sources Sci. Technol. 01/2002; 11:431-438.
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ABSTRACT: Energy distribution and heat transfer mechanisms in atmospheric pressure non-equilibrium plasmas were investigated extensively through energy balance analysis, emission spectroscopy of the rotational band of CH (A 2 → X 2), and gas chromatographic analysis. Two plasma sources were examined: methane-fed dielectric barrier discharge (DBD) and atmospheric pressure glow-discharge (APG). The DBD features filamentary microdischarges accompanied by surface discharge along a dielectric barrier. As a result, 60% of the input power was measured as heat transfer to the dielectric electrode, whereas 20% was to the metallic electrode. Consequently, feed gas average temperature was increased only by 20–40 K. On the other hand, rotational temperature of the corresponding emission region exceeded average gas temperature by 100 K. In APG, heat transfer to electrodes was dominated by formation of negative glow regardless of whether the electrode was covered by a dielectric. However, negative glow tended to be thinner and more intense when it formed on a metallic electrode, leading to slightly higher metallic heating. Rotational temperature in APG was close to average gas temperature since APG does not show radial localization of plasma. Energy efficiency for methane decomposition process to produce ethane, ethylene, and hydrogen was about 1% regardless of the plasma source. Energy distribution and heat transfer mechanisms depend strongly on the plasma spatial structure rather than flow fields or feed gas physical properties.
J. Phys. D: Appl. Phys. 01/2001; 34:3383-3390.
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ABSTRACT: The gas temperature of reactive microdischarges has been extensively investigated in a methane-fed dielectric barrier discharge (DBD) configured by a two parallel plate reactor with 0.5 mm gap spacing. Emission spectroscopy of the rotational bands of CH(2 → 2) coupled with heat transfer experiments have been employed for this purpose. Stationary and space-averaged gas temperature between the discharge gap was estimated from the heat transfer experiment; thus heat capacity and enthalpy gained by the feed gas stream resulted in a ≈20 K temperature increase. The rotational temperature showed fairly good sensitivity to the inlet gas temperature variation in the range 370–670 K. However, the local gas temperature increase inside the microdischarges represented an additional 100 K above the average gas temperature, indicating one order of magnitude higher value than the theoretically expected gas temperature increase (5–10 K) for a single microdischarge. High-frequency operation (80 kHz) is responsible for memory effect, thus such a high gas temperature increase was the result of multiple microdischarges rather than a single one.
J. Phys. D: Appl. Phys. 01/2001; 34:2504-2511.
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ABSTRACT: A flow-type, microscale, non-equilibrium plasma reactor was developed for partial oxidation of methane without a catalyst. A wide range of oxygen and methane mixtures was directly processed without dilution or explosion at ambient temperature because the microscale plasma reactor removes excess heat generated by partial oxidation, thereby maintaining a reaction field at temperatures near room temperature. Consequently, the least reactive methane was excited by high-energy electrons, whereas successive destruction of reactive oxygenates was minimized simultaneously within the extremely confined environment. A highly reactive and quenching environment is thereby obtained within a single reactor: these are paradoxical conditions in conventional thermochemical processes. A major product among liquid oxygenates was methanol, whose selectivity reached 34% at 30% of methane conversion. Selectivity of oxygenates such as methanol and formaldehyde depends strongly on the fragmentation pattern of methane dissociation by electron impact. Maximum selectivity of oxygenates, which is estimated from numerical simulation of a filamentary microdischarge, reaches 60% when the applied electric field corresponds to the breakdown field of methane (80 Td, 1 Td = 10−17 V cm2). The discharge current increases markedly with an applied electric field, but the selectivity of oxygenates decreases as the field strength increases.
Catalysis Today.
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ABSTRACT: Direct dehydrogenation of methane to produce more useful chemicals was examined using low temperature plasmas such as DBD, corona and spark discharge under the conditions of room temperature and atmospheric pressure. In spark discharge, acetylene was produced with the selectivity higher than 85% and small amount of deposited carbon. The energy efficiency in spark discharge was much higher than that in DBD and corona discharge. By the emission spectroscopy, it was found that methane was highly dissociated to atomic carbon and hydrogen in spark discharge. The gas temperature in spark discharge channel remained as low as 420–460 K determined by Boltzmann plot method of CH rotational band (431 nm). The specific energy requirement for acetylene was improved by the optimization of reactor size and residence time and reached 12.1 kW h/kg-C2H2, which was as same as Huels process with DC arc plasma.
Catalysis Today.
