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L C Feldman - One of the best experts on this subject based on the ideXlab platform.
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oxidation induced stress in sio2 sic structures
Applied Physics Letters, 2017Co-Authors: Xiuyan Li, A V Ermakov, Voshadhi Amarasinghe, Eric Garfunkel, T Gustafsson, L C FeldmanAbstract:Physical stress in SiO2/SiC stacks formed by the thermal oxidation of SiC is studied experimentally through both room temperature ex-situ and variable temperature (25–1150 °C) in-situ investigations. Mechanisms giving rise to the stress are a thermal component, associated with differences in thermal expansion coefficients of the oxide and the substrate, and an intrinsic component associated with the different atomic densities and structure of the film and substrate. Ex-situ results show a ∼108 Pa compressive stress in the SiO2 film in a SiO2/SiC stack with a strong Crystal Face dependence (C Face(000ī) and Si Face (0001)) and processing (temperature, growth rate) dependence. Real-time stress determination demonstrates that at temperatures above ∼900 °C, the total intrinsic stress and a portion of the thermal stress may be relieved. On the basis of these findings, a viscous model is proposed to discuss the stress relaxation.
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chemical properties of oxidized silicon carbide surFaces upon etching in hydrofluoric acid
Journal of the American Chemical Society, 2009Co-Authors: Sarit Dhar, Oliver Seitz, Mathew D Halls, Sungho Choi, Yves J Chabal, L C FeldmanAbstract:Hydrogen termination of oxidized silicon in hydrofluoric acid results from an etching process that is now well understood and accepted. This surFace has become a standard for studies of surFace science and an important component in silicon device processing for microelectronics, energy, and sensor applications. The present work shows that HF etching of oxidized silicon carbide (SiC) leads to a very different surFace termination, whether the surFace is carbon or silicon terminated. Specifically, the silicon carbide surFaces are hydrophilic with hydroxyl termination, resulting from the inability of HF to remove the last oxygen layer at the oxide/SiC interFace. The final surFace chemistry and stability critically depend on the Crystal Face and surFace stoichiometry. These surFace properties affect the ability to chemically functionalize the surFace and therefore impact how SiC can be used for biomedical applications.
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Modified Deal Grove model for the thermal oxidation of silicon carbide
Journal of Applied Physics, 2004Co-Authors: S Dhar, L C Feldman, G Chung, J.r. WilliamsAbstract:A modified Deal Grove model for the oxidation of 4H-SiC is presented, which includes the removal of the carbon species. The model is applied to data on the oxidation rates for the ~0001! Si, (0001¯ ) C, and (112¯ 0) a Faces, which are performed in 1 atm dry oxygen and in the temperature range 950–1150 °C. Analysis within the model provides a physical explanation for the large Crystal-Face dependent oxidation rates observed.
Teruhisa Ohno - One of the best experts on this subject based on the ideXlab platform.
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photocatalytic reduction of co2 over exposed Crystal Face controlled tio2 nanorod having a brookite phase with co catalyst loading
Applied Catalysis B-environmental, 2014Co-Authors: Teruhisa Ohno, Takayoshi Higo, Naoya Murakami, Hirofumi Saito, Qitao Zhang, Yin Yang, Toshiki TsubotaAbstract:Abstract Photocatalytic reduction of carbon dioxide (CO 2 ) was carried out using exposed-Crystal- Face-controlled titanium(IV) oxide (TiO 2 ) having a brookite phase. Methanol (CH 3 OH) was detected as the main product, and trace amounts of formic acid, carbon monoxide, methane, and hydrogen were also detected in some cases. The prepared nanorod-shaped brookite TiO 2 with large {2 1 0} and small {2 1 2} exposed Crystal Faces showed larger CH 3 OH generation than that of commercial brookite TiO 2 powder (Kojundo Chemical Laboratory Co., Ltd.). The activity of a brookite TiO 2 nanorod for CO 2 reduction depended on its aspect ratio because the {2 1 0} Crystal Faced worked as a reduction site, whereas an oxidation site was assigned to {2 1 2} Crystal Faces. Photodeposition of gold (Au) or silver (Ag) nanoparticles on the nanorod-shaped brookite TiO 2 induced a dramatic increase in CH 3 OH production because the deposited metal particles work as reductive sites for multi-electron reduction of CO 2 . Among the co-catalyst-loaded brookite TiO 2 nanorods, nanorod-shaped brookite TiO 2 loaded with Ag showed higher activity. The source of carbon of CH 3 OH obtained by CO 2 reduction is discussed on the basis of results of a labeling experiment using 13 CO 2 .
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shape controlled anatase titanium iv oxide particles prepared by hydrothermal treatment of peroxo titanic acid in the presence of polyvinyl alcohol
Journal of Physical Chemistry C, 2009Co-Authors: Naoya Murakami, Toshiki Tsubota, Yu Kurihara, Teruhisa OhnoAbstract:Anatase titanium(IV) oxide (TiO2) particles with specific exposed Crystal Faces were prepared by hydrothermal treatment of peroxo titanic acid (PTA) solution with polyvinyl alcohol as a shape-control reagent. Crystal phase, shape, and size of TiO2 particles were found to be greatly dependent on pH value of PTA solution and time of hydrothermal treatment. TiO2 particles prepared from PTA solution of pH 7 had {101} and {001} exposed Crystal Faces, and the shape of TiO2 particles changed with the time of hydrothermal treatment. The prepared TiO2 particles with specific exposed Crystal Faces showed higher photocatalytic activity for acetaldehyde decomposition than commercial spherical TiO2 particles. This result implies that back reaction was prevented by spatial separation of redox sites in the particles because of selective migration of electrons and positive holes to specific exposed Crystal Faces and/or different reactivity of electrons and positive holes on the specific exposed Crystal Face. Furthermore,...
D S Thomson - One of the best experts on this subject based on the ideXlab platform.
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single particle measurements of the chemical composition of cirrus ice residue during Crystal Face
Journal of Geophysical Research, 2004Co-Authors: P K Hudson, D J Cziczo, D M Murphy, D S ThomsonAbstract:[1] The first real-time, in situ, investigation of the chemical composition of the residue of cirrus ice Crystals was performed during July 2002. This study was undertaken on a NASA WB-57F high-altitude research aircraft as part of Crystal-Face, a field campaign which sought to further our understanding of the relation of clouds, water vapor, and climate by characterizing, among other parameters, anvil cirrus formed about the Florida peninsula. A counter flow virtual impactor (CVI) was used to separate cirrus ice from the unactivated interstitial aerosol particles and evaporate condensed-phase water. Residual material, on a Crystal-by-Crystal basis, was subsequently analyzed using the NOAA Aeronomy Laboratory's Particle Analysis by Laser Mass Spectrometry (PALMS) instrument. Sampling was performed from 5 to 15 km altitude and from 12° to 28° north latitude within cirrus originating over land and ocean. Chemical composition measurements provided several important results. Sea salt was often incorporated into cirrus, consistent with homogeneous ice formation by aerosol particles from the marine boundary layer. Size measurements showed that large particles preferentially froze over smaller ones. Meteoritic material was found within ice Crystals, indicative of a relation between stratospheric aerosol particles and tropospheric clouds. Mineral dust was the dominant residue observed in clouds formed during a dust transport event from the Sahara, consistent with a heterogeneous freezing mechanism. These results show that chemical composition and size are important determinants of which aerosol particles form cirrus ice Crystals.
J V Pittman - One of the best experts on this subject based on the ideXlab platform.
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validation and determination of ice water content radar reflectivity relationships during Crystal Face flight requirements for future comparisons
Journal of Geophysical Research, 2008Co-Authors: D S Sayres, Gerald M Heymsfield, E M Weinstock, J V Pittman, Ann M Fridlind, J B Smith, J G Anderson, Lihua Li, Andrew S AckermanAbstract:[1] In situ measurements of cirrus ice water content (IWC) by the Harvard water vapor and total water instruments during Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment are compared with remote sensing data made by the Cloud Radar System instrument in order to derive and validate an empirical IWC-radar relflectivity Ze relationship. The comparisons show that for measurements of in situ IWC and remotely measured radar reflectivity, collocated within 2 km of each other, a single IWC-Ze relationship can be found that fits the data with an uncertainty of ±20–30%. A cloud resolving model shows this level of uncertainty to be consistent with sampling errors associated with comparing two measurements that are not collocated. Uncertainties are quantified in the use of in situ data to validate the retrieval algorithms used to derive the IWC of clouds from remote sensing observations, such as radar reflectivity Ze. Uncertainties are classified into instrumental uncertainties, uncertainties related to sampling errors, and uncertainties in using a single IWC-Ze relationship to describe a cloud.
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co signatures in subtropical convective clouds and anvils during Crystal Face an analysis of convective transport and entrainment using observations and a cloud resolving model
Journal of Geophysical Research, 2006Co-Authors: Jimena P Lopez, M Loewenstein, E M Weinstock, Ann M Fridlind, H Jost, Andrew S Ackerman, T Campos, D S Sayres, J B Smith, J V PittmanAbstract:[1] Convective systems are an important mechanism in the transport of boundary layer air into the upper troposphere. The Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (Crystal-Face) campaign, in July 2002, was developed as a comprehensive atmospheric mission to improve knowledge of subtropical cirrus systems and their roles in regional and global climate. In situ measurements of carbon monoxide (CO), water vapor (H2Ov), and total water (H2Ot) aboard NASA's WB-57F aircraft and CO aboard the U.S. Navy's Twin Otter aircraft were obtained to study the role of convective transport. Three flights sampled convective outflow on 11, 16 and 29 July found varying degrees of CO enhancement relative to the free troposphere. A cloud-resolving model used the in situ observations and meteorological fields to study these three systems. Several methods of filtering the observations were devised here using ice water content, relative humidity with respect to ice, and particle number concentration as a means to statistically sample the model results to represent the flight tracks. A weighted histogram based on ice water content observations was then used to sample the simulations for the three flights. In addition, because the observations occurred in the convective outflow cirrus and not in the storm cores, the model was used to estimate the maximum CO within the convective systems. In general, anvil-level air parcels contained an estimated 20–40% boundary layer air in the analyzed storms.
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formation of a tropopause cirrus layer observed over florida during Crystal Face
Journal of Geophysical Research, 2005Co-Authors: E J Jensen, P Lawson, Leonhard Pfister, Thaopaul V Bui, A J Weinheimer, E M Weinstock, Jessica Smith, J V Pittman, D Baumgardner, Matthew J McgillAbstract:[1] On 13 July 2002 a widespread, subvisible tropopause cirrus layer occurred over the Florida region. This cloud was observed in great detail with the NASA Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (Crystal-Face) instrumentation, including in situ measurements with the WB-57 aircraft. In this paper, we use the 13 July cloud as a case study to evaluate the physical processes controlling the formation and evolution of tropopause cirrus layers. Microphysics measurements indicate that ice Crystal diameters in the cloud layer ranged from about 7 to 50 μm, and the peak number mode was about 10–25 μm. In situ water vapor and temperature measurements in the cloud indicated supersaturation with respect to ice throughout, with ice saturation ratios as large as 1.8. Even when the ice surFace area density was as high as about 500 μm2 cm−3, ice supersaturations of 20–30% were observed. Trajectory analysis shows that the air sampled near the tropopause on this day generally came from the north and cooled considerably during the previous few days. Examination of infrared satellite imagery along air parcel back trajectories from the WB-57 flight track indicates that the tropopause cloud layer formation was, in general, not simply left over ice from recently generated anvil cirrus. Simulations of cloud formation using time-height curtains of temperature along the trajectory paths show that the cloud could have formed in situ near the tropopause as the air was advected into the south Florida region and cooled to unusually low temperatures. If we assume a high threshold for ice nucleation via homogeneous freezing of aqueous sulfate aerosols, the model reproduces the observed cloud structure, ice Crystal size distributions, and ice supersaturation statistics. Inclusion of observed gravity wave temperature perturbations in the simulations is essential to reproduce the observed cloud properties. Without waves, Crystal number densities are too low, Crystal sizes are too large, and the Crystals fall out too fast, leaving very little cloud persisting at the end of the simulations. In the cloud simulations, coincidence of high supersaturations and high surFace areas can be produced by either recent nucleation or sedimentation of Crystals into supersaturated layers. The agreement between model results and observed supersaturations is improved somewhat if we assume that the steady state relative humidity within cirrus at T < 200 K is enhanced by about 30%. The WB-57 measurements and the model results suggest that the cloud layer irreversibly dehydrated air near the tropopause.
Mario Villalobos - One of the best experts on this subject based on the ideXlab platform.
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Crystal Face distributions and surFace site densities of two synthetic goethites implications for adsorption capacities as a function of particle size
Langmuir, 2017Co-Authors: Kenneth J T Livi, Mario Villalobos, Rowan K Leary, M Varela, J S Barnard, Milton Villacisgarcia, Rodolfo Zanella, Anna Goodridge, Paul A MidgleyAbstract:Two synthetic goethites of varying Crystal sizes distributions were analyzed by BET, conventional TEM, cryo-TEM, atomic resolution STEM and HRTEM, and electron tomography in order to determine the effects of Crystal size, shape, and atomic scale surFace roughness on their adsorption capacities. The two samples were determined by BET to have very different site densities based on CrVI adsorption experiments. Model specific surFaces areas generated from TEM observations showed that, based on size and shape, there should be little difference in their adsorption capacities. Electron tomography revealed that both samples Crystallized with an asymmetric {101} tablet habit. STEM and HRTEM images showed a significant increase in atomic-scale surFace roughness of the larger goethite. This difference in roughness was quantified based on measurements of relative abundances of Crystal Faces {101} and {201}for the two goethites, and a reactive surFace site density was calculated for each goethite. Singly-coordinated s...
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goethite surFace reactivity iii unifying arsenate adsorption behavior through a variable Crystal Face site density model
Geochimica et Cosmochimica Acta, 2010Co-Authors: Carlos Salazarcamacho, Mario VillalobosAbstract:Abstract We developed a model that describes quantitatively the arsenate adsorption behavior for any goethite preparation as a function of pH and ionic strength, by using one basic surFace arsenate stoichiometry, with two affinity constants. The model combines a Face distribution-Crystallographic site density model for goethite with tenets of the Triple Layer and CD-MUSIC surFace complexation models, and is self-consistent with its adsorption behavior towards protons, electrolytes, and other ions investigated previously. Five different systems of published arsenate adsorption data were used to calibrate the model spanning a wide range of chemical conditions, which included adsorption isotherms at different pH values, and adsorption pH-edges at different As(V) loadings, both at different ionic strengths and background electrolytes. Four additional goethite–arsenate systems reported with limited characterization and adsorption data were accurately described by the model developed. The adsorption reaction proposed is: FeOH + SOH + AsO 4 3 - + H + → FeOAsO 3 [ 2 - ] … SOH + H 2 O where SOH is an adjacent surFace site to FeOH; with log K = 21.6 ± 0.7 when SOH is another FeOH, and log K = 18.75 ± 0.9, when SOH is Fe2OH. An additional small contribution of a protonated complex was required to describe data at low pH and very high arsenate loadings. The model considered goethites above 80 m2/g as ideally composed of 70% Face (1 0 1) and 30% Face (0 0 1), resulting in a site density for FeOH and for Fe3OH of 3.125/nm2 each. Below 80 m2/g surFace capacity increases progressively with decreasing area, which was modeled by considering a progressively increasing proportion of Faces (0 1 0)/(1 0 1), because Face (0 1 0) shows a much higher site density of FeOH groups. Computation of the specific proportion of Faces, and thus of the site densities for the three types of Crystallographic surFace groups present in goethite, may be performed for each preparation either by experimental determination of site saturation by an index ion (e.g., chromate), or by achieving congruency of proton adsorption data with those of ideal goethites when plotted as percentage of proton-reactive ( FeOH + Fe3OH) sites occupied. The surFace arsenate complexes proposed additionally explained: (1) the higher affinity of goethite for As(V) than for Cr(VI) at high pH, and thus the gentle slope of the arsenate pH adsorption edges; and (2) the lower adsorption capacity for As(V) than for Cr(VI) at low pH on low-surFace area goethites, through incomplete FeOH site occupancy of As(V). The model is very promising as a practical means of predicting the adsorption behavior of arsenate on any goethite preparation, and may extend to predictive capabilities for adsorption behavior of many other relevant oxyanions, as well as for explaining differences in ligand-promoted surFace transformation processes on goethite as a function of particle size.