The Experts below are selected from a list of 273 Experts worldwide ranked by ideXlab platform
Ilona Riipinen - One of the best experts on this subject based on the ideXlab platform.
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Near‐Pure Vapor condensation in the Martian atmosphere: CO2 ice crystal growth
Journal of Geophysical Research: Planets, 2013Co-Authors: Constantino Listowski, Anni Määttänen, Ilona Riipinen, Franck Montmessin, Franck LefèvreAbstract:[1] A new approach is presented to model the condensational growth of carbon dioxide (CO2) ice crystals on Mars. These condensates form in very particular conditions. First, ∼95% of the atmosphere is composed of CO2 so that near-Pure Vapor condensation takes place. Second, the atmosphere is rarefied, having dramatic consequences on the crystal growth. Indeed, the subsequently reduced efficiency of heat transport helps maintain a high temperature difference between the crystal surface and the environment, inhibiting the growth. Besides, the Stefan flow which would have been expected to increase the growth rate of the crystal, because of the near-Pure Vapor condensation, is negligible. We show that the heritage of the convenient and explicit linearized crystal growth rate formula used for Earth clouds, initially derived for a trace gas, has to be reconsidered in the case of near-Pure Vapor condensation for high saturation ratios that appear to be common in the Martian mesosphere. Nevertheless, by comparing our approach with a more complex condensation model, valid for all atmospheric conditions and all Vapor abundances, we show that a very simple set of equations can still be used to efficiently reproduce the CO2 ice crystal growth rate. Our model, referred to as the CLASSIC model here, provides similar crystal growth rates than the traditionally used linearized growth rate models at low supersaturations but predicts lower crystal growth rates at high supersaturations. It can thus be used to model the condensational growth of CO2 ice crystals in the mesosphere where high supersaturations are observed.
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Near-Pure Vapor condensation in the Martian atmosphere: CO2 ice crystal growth
Journal of Geophysical Research. Planets, 2013Co-Authors: Constantino Listowski, Anni Määttänen, Ilona Riipinen, Franck Montmessin, Franck LefèvreAbstract:A new approach is presented to model the condensational growth of carbon dioxide (CO2) ice crystals on Mars. These condensates form in very particular conditions. First, ~95% of the atmosphere is composed of CO2 so that near-Pure Vapor condensation takes place. Second, the atmosphere is rarefied, having dramatic consequences on the crystal growth. Indeed, the subsequently reduced efficiency of heat transport helps maintain a high temperature difference between the crystal surface and the environment, inhibiting the growth. Besides, the Stefan flow which would have been expected to increase the growth rate of the crystal, because of the near-Pure Vapor condensation, is negligible. We show that the heritage of the convenient and explicit linearized crystal growth rate formula used for Earth clouds, initially derived for a trace gas, has to be reconsidered in the case of near-Pure Vapor condensation for high saturation ratios, which appear to be common in the martian mesosphere. Nevertheless, by comparing our approach with a more complex condensation model, valid for all atmospheric conditions and all Vapor abundances, we show that a very simple set of equations can still be used to efficiently reproduce the CO2 ice crystal growth rate. Our model, referred to as the CLASSIC model here, provides similar crystal growth rates than the traditionally used linearized growth rate models at low supersaturations, but predicts lower crystal growth rates at high supersaturations. It can thus be used to model the condensational growth of CO2 ice crystals in the mesosphere where high supersaturations are observed.
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Theoretical constraints on Pure Vapor‐pressure driven condensation of organics to ultrafine particles
Geophysical Research Letters, 2011Co-Authors: Neil M. Donahue, Erica R. Trump, Jeffrey R. Pierce, Ilona RiipinenAbstract:[1] Organic condensation to freshly nucleated particles contributes substantially to their growth. Here we explore a range of constraints on this process, under the assumption that gas-phase oxidation of organic Vapors by hydroxy radical is forming organics with a sufficiently low volatility to condense onto particles in the 2–20 nm size range. To condense but not homogeneously nucleate, Vapors need to have saturation concentrations (C*) in the 10−3 − 10−2μg m−3 range, and this is exactly the range that gas-phase chemistry is likely to produce. At least half of the observed growth rate of ultrafine particles can be explained by these simple considerations and constraints.
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theoretical constraints on Pure Vapor pressure driven condensation of organics to ultrafine particles
Geophysical Research Letters, 2011Co-Authors: Neil M. Donahue, Ilona Riipinen, Erica R. Trump, Jeffrey R. PierceAbstract:[1] Organic condensation to freshly nucleated particles contributes substantially to their growth. Here we explore a range of constraints on this process, under the assumption that gas-phase oxidation of organic Vapors by hydroxy radical is forming organics with a sufficiently low volatility to condense onto particles in the 2–20 nm size range. To condense but not homogeneously nucleate, Vapors need to have saturation concentrations (C*) in the 10−3 − 10−2μg m−3 range, and this is exactly the range that gas-phase chemistry is likely to produce. At least half of the observed growth rate of ultrafine particles can be explained by these simple considerations and constraints.
Zu-hong Shen - One of the best experts on this subject based on the ideXlab platform.
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The influence of preparation parameters on the mass production of Vapor-grown carbon nanofibers
Carbon, 2000Co-Authors: Yue-ying Fan, Hui-ming Cheng, Yong-liang Wei, Zu-hong ShenAbstract:An effective production of Pure Vapor-grown carbon nanofibers (VGCNFs) was obtained at the temperature of 1373-1473 K in the presence of sulfur by an improved floating catalyst method in a horizontal reactor using benzene as carbon source, ferrocene as catalyst precursor and hydrogen as carrier gas. It was Found that higher than 30 wt.% of the carbon source from benzene can be transformed to VGCNFs which are very Pure and straight, and have uniform diameters. The experimental results demonstrated that the concentration of the sulfur-containing compound (thiophene) together with the molar ratio of C6H6/H-2, the gas residence time and the reaction time strongly influenced the yield of VGCNFs. (C) 2000 Elsevier Science Ltd. All rights reserved.
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Preparation, morphology, and microstructure of diameter-controllable Vapor-grown carbon nanofibers
Journal of Materials Research, 1998Co-Authors: Yue-ying Fan, Hui-ming Cheng, Zu-hong ShenAbstract:Pure Vapor-grown carbon nanofibers (VGCNF's) with controllable diameters of 10–200 nm were prepared by an improved floating catalyst method. Through transmission electron microscopy (TEM) observation, it was found that VGCNF's have a duplex structure, a hollow and high-crystallinity graphite filament called primary carbon fiber surrounded by a pyrocarbon layer with low graphite crystallinity. It was observed using high-resolution TEM that VGCNF's have excellent graphitic crystallinity with graphite layers stacked neatly parallel to fiber axis. Moreover, x-ray diffraction results showed that the graphitic crystallinity of carbon fibers became higher with decreasing diameter of carbon fibers.
A. Guha - One of the best experts on this subject based on the ideXlab platform.
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A unified theory of aerodynamic and condensation shock waves in Vapor‐droplet flows with or without a carrier gas
Physics of Fluids, 1994Co-Authors: A. GuhaAbstract:A unified theory for aerodynamic and condensation shock waves in Vapor‐droplet flows in the presence of an inert carrier gas is presented. Same conservation equations apply across discontinuous models for both types of wave. Exact (as well as approximate), explicit analytical jump conditions across such discontinuities are derived subject to several boundary conditions. Collectively they may be called the generalized Rankine–Hugoniot equations for Vapor‐droplet mixtures. All the equations derived are general and can be applied in the case of a Pure Vapor‐droplet flow by letting the mass fraction of the carrier gas go to zero. Much physical insight may be obtained from this integral analysis. It is shown that four types of aerodynamic shock waves (viz., equilibrium partly dispersed, equilibrium fully dispersed, partly dispersed with complete eVaporation, and fully dispersed with complete eVaporation) may occur. Conditions for each type of these waves to occur are specified and the appropriate jump conditio...
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Structure of partly dispersed normal shock waves in Vapor‐droplet flows
Physics of Fluids A: Fluid Dynamics, 1992Co-Authors: A. GuhaAbstract:A numerical and analytical study has been made to reveal the internal structures of partly dispersed shock waves in Pure Vapor‐droplet media. The results clearly demonstrate the effects of different relaxation phenomena present and their associated time scales on the structure. The study systematically analyzes the relative effects of different flow parameters on the structure and thickness of the shock wave. Wherever possible, the structure in a Vapor‐droplet medium is contrasted against similar structures in more familiar solid particle‐laden gas flow. The case of complete eVaporation through which a two‐phase Vapor‐droplet medium reverts to a single‐phase nonrelaxing one is discussed. Although linearized analyses are often presented for relaxing flows downstream of frozen shocks, they are of limited applicability to Vapor‐droplet flows. The paper gives many examples of why a linearized analysis is unlikely to be successful in such cases.
Yue-ying Fan - One of the best experts on this subject based on the ideXlab platform.
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The influence of preparation parameters on the mass production of Vapor-grown carbon nanofibers
Carbon, 2000Co-Authors: Yue-ying Fan, Hui-ming Cheng, Yong-liang Wei, Zu-hong ShenAbstract:An effective production of Pure Vapor-grown carbon nanofibers (VGCNFs) was obtained at the temperature of 1373-1473 K in the presence of sulfur by an improved floating catalyst method in a horizontal reactor using benzene as carbon source, ferrocene as catalyst precursor and hydrogen as carrier gas. It was Found that higher than 30 wt.% of the carbon source from benzene can be transformed to VGCNFs which are very Pure and straight, and have uniform diameters. The experimental results demonstrated that the concentration of the sulfur-containing compound (thiophene) together with the molar ratio of C6H6/H-2, the gas residence time and the reaction time strongly influenced the yield of VGCNFs. (C) 2000 Elsevier Science Ltd. All rights reserved.
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Preparation, morphology, and microstructure of diameter-controllable Vapor-grown carbon nanofibers
Journal of Materials Research, 1998Co-Authors: Yue-ying Fan, Hui-ming Cheng, Zu-hong ShenAbstract:Pure Vapor-grown carbon nanofibers (VGCNF's) with controllable diameters of 10–200 nm were prepared by an improved floating catalyst method. Through transmission electron microscopy (TEM) observation, it was found that VGCNF's have a duplex structure, a hollow and high-crystallinity graphite filament called primary carbon fiber surrounded by a pyrocarbon layer with low graphite crystallinity. It was observed using high-resolution TEM that VGCNF's have excellent graphitic crystallinity with graphite layers stacked neatly parallel to fiber axis. Moreover, x-ray diffraction results showed that the graphitic crystallinity of carbon fibers became higher with decreasing diameter of carbon fibers.
Hui-ming Cheng - One of the best experts on this subject based on the ideXlab platform.
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The influence of preparation parameters on the mass production of Vapor-grown carbon nanofibers
Carbon, 2000Co-Authors: Yue-ying Fan, Hui-ming Cheng, Yong-liang Wei, Zu-hong ShenAbstract:An effective production of Pure Vapor-grown carbon nanofibers (VGCNFs) was obtained at the temperature of 1373-1473 K in the presence of sulfur by an improved floating catalyst method in a horizontal reactor using benzene as carbon source, ferrocene as catalyst precursor and hydrogen as carrier gas. It was Found that higher than 30 wt.% of the carbon source from benzene can be transformed to VGCNFs which are very Pure and straight, and have uniform diameters. The experimental results demonstrated that the concentration of the sulfur-containing compound (thiophene) together with the molar ratio of C6H6/H-2, the gas residence time and the reaction time strongly influenced the yield of VGCNFs. (C) 2000 Elsevier Science Ltd. All rights reserved.
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Preparation, morphology, and microstructure of diameter-controllable Vapor-grown carbon nanofibers
Journal of Materials Research, 1998Co-Authors: Yue-ying Fan, Hui-ming Cheng, Zu-hong ShenAbstract:Pure Vapor-grown carbon nanofibers (VGCNF's) with controllable diameters of 10–200 nm were prepared by an improved floating catalyst method. Through transmission electron microscopy (TEM) observation, it was found that VGCNF's have a duplex structure, a hollow and high-crystallinity graphite filament called primary carbon fiber surrounded by a pyrocarbon layer with low graphite crystallinity. It was observed using high-resolution TEM that VGCNF's have excellent graphitic crystallinity with graphite layers stacked neatly parallel to fiber axis. Moreover, x-ray diffraction results showed that the graphitic crystallinity of carbon fibers became higher with decreasing diameter of carbon fibers.
