Atmospheric methanol measurement using selective catalytic methanol to formaldehyde conversion

International audienceA novel atmospheric methanol measurement technique, employing selective gas-phase catalytic conversion of methanol to formaldehyde followed by detection of the formaldehyde product, has been developed and tested. The effects of temperature, gas flow rate, gas composition, reactor-bed length, and reactor-bed composition on the methanol conversion efficiency of a molybdenum-rich, iron-molybdate catalyst [Mo-Fe-O] were studied. Best results were achieved using a 1:4 mixture (w/w) of the catalyst in quartz sand. Optimal methanol to formaldehyde conversion (>95% efficiency) occurred at a catalyst housing temperature of 345°C and an estimated sample-air/catalyst contact time of <0.2 seconds. Potential interferences arising from conversion of methane and a number of common volatile organic compounds (VOC) to formaldehyde were found to be negligible under most atmospheric conditions and catalyst housing temperatures. Using the new technique, atmospheric measurements of methanol were made at the University of Bremen campus from 1 to 15 July 2004. Methanol mixing ratios ranged from 1 to 5 ppb with distinct maxima at night. Formaldehyde mixing ratios, obtained in conjunction with methanol by periodically bypassing the catalytic converter, ranged from 0.2 to 1.6 ppb with maxima during midday. These results suggest that selective, catalytic methanol to formaldehyde conversion, coupled with existing formaldehyde measurement instrumentation, is an inexpensive and effective means for monitoring atmospheric methanol

Characteristics of CVD ternary refractory nitride diffusion barriers

A range of different ternary refractory nitride compositions have been deposited by CVD (chemical vapor deposition) for the systems TiSiN, WBN, and WSiN. The precursors used are readily available. The structure, electrical, and barrier properties of the films produced by CVD are similar to those observed for films with similar compositions deposited by PVD (physical vapor deposition). The step coverage of the CVD processes developed is good and in some cases, exceptional. A combination of desirable resistivity, step coverage, and barrier properties exists simultaneously over a reasonable range of compositions for each system. Initial attempts to integrate WSiN films into a standard 0.5 micrometer CMOS process flow in place of a sputtered Ti/TiN stack were successful

Direct simulation of ion beam induced stressing and amorphization of silicon

Using molecular dynamics (MD) simulation, we investigate the mechanical response of silicon to high dose ion-irradiation. We employ a realistic and efficient model to directly simulate ion beam induced amorphization. Structural properties of the amorphized sample are compared with experimental data and results of other simulation studies. We find the behavior of the irradiated material is related to the rate at which it can relax. Depending upon the ability to deform, we observe either the generation of a high compressive stress and subsequent expansion of the material, or generation of tensile stress and densification. We note that statistical material properties, such as radial distribution functions are not sufficient to differentiate between different densities of amorphous samples. For any reasonable deformation rate, we observe an expansion of the target upon amorphization in agreement with experimental observations. This is in contrast to simulations of quenching which usually result in denser structures relative to crystalline Si. We conclude that although there is substantial agreement between experimental measurements and most simulation results, the amorphous structures being investigated may have fundamental differences; the difference in density can be attributed to local defects within the amorphous network. Finally we show that annealing simulations of our amorphized samples can lead to a reduction of high energy local defects without a large scale rearrangement of the amorphous network. This supports the proposal that defects in amorphous silicon are analogous to those in crystalline silicon.Comment: 13 pages, 12 figure

Erbium in crystal silicon: Optical activation, excitation, and concentration limits

7 pags.; 7 figs.The optical activation, excitation, and concentration limits of erbium in crystal Si are studied. Preamorphized surface layers of Czochralski-grown (Cz) Si(100), containing 1.7×1018 O/cm3, were implanted with 250 keV Er at fluences in the range 8×1011-8×10 14 cm-2. After thermal solid-phase epitaxy of the Er-doped amorphous layers at 600°C, Er is trapped in the crystal at concentrations ranging from 3×1016 to 7×1019 Er/cm 3, as measured by secondary-ion-mass spectrometry. Photoluminescence spectra taken at 77 K show the characteristic Er3+ intra-4f luminescence at 1.54 ¿m. Photoluminescence excitation spectroscopy shows that Er is excited through a photocarrier-mediated process. Rapid thermal annealing at 1000°C for 15 s increases the luminescence intensity, mainly due to an increase in minority-carrier lifetime, which enhances the excitation efficiency. Luminescent Er forms clusters with oxygen: the maximum Er concentration that can be optically activated is determined by the O content, and is (3±1)×1017 Er/cm3 in Cz-Si. The internal quantum efficiency for electrical excitation of Er in Cz-Si is larger than 3×10-6. © 1995 American Institute of Physics.This work is part of the research program of the Foundation for Fundamental Research on Matter (FOM) and was made possible by financial support from the Dutch Organization for the Advancement of Pure Research @IWO), the Netherlands Technology Foundation (STW), and the IC Technology Program (IOP Electra-Optics) of the Ministry of Economic Affairs. R.S. acknowIedges financial support from CSIC, Spain.Peer Reviewe

MARCH WET AVALANCHE PREDICTION AT BRIDGER BOWL SKI AREA, MONTANA

ABSTRACT: Few avalanche forecast models are tailored specifically for wet avalanche forecasting. Bridger Bowl (intermountain climate) is a good area to develop a wet avalanche probability model. The primary archived data consists of eight variables. The archived data for March from 1968 to 2001 (1996 data unavailable) were used to develop 68 predictor variables related to temperature, snowpack settlement, and precipitation. The original dataset was divided into days with snowfall in the past 48 hours (new snow) and days without (old snow). There were 33 significant old snow variables and 22 significant new snow variables. Six variables are common to both old and new snow. The best predictor variables for old and new snow are different. The variables were analyzed with binomial logistic regression to produce probability models for old snow and for new snow wet avalanche conditions. The old snow model uses the prediction day minimum temperature and the two-day change in total snow depth as predictor variables and has a 89% overall success rate. However, the majority of this success is due to correct prediction of days without wet avalanches (96% of all correct predictions). The new snow model uses the prediction day minimum temperature and three-day cumulative new snow water equivalent as predictor variables, but is less useful. The models are applicable only to Bridger Bowl. The numerical forecast models can be used as one of the tools in the forecasting toolbox but limited data and complexity of process require that the decisions about closure remain in the hands of the ski patrol

MARCH WET AVALANCHE PREDICTION AT BRIDGER BOWL SKI AREA, MONTANA

ABSTRACT: Few avalanche forecast models are tailored specifically for wet avalanche forecasting. Bridger Bowl (intermountain climate) is a good area to develop a wet avalanche probability model. The primary archived data consists of eight variables. The archived data for March from 1968 to 2001 (1996 data unavailable) were used to develop 68 predictor variables related to temperature, snowpack settlement, and precipitation. The original dataset was divided into days with snowfall in the past 48 hours (new snow) and days without (old snow). There were 33 significant old snow variables and 22 significant new snow variables. Six variables are common to both old and new snow. The best predictor variables for old and new snow are different. The variables were analyzed with binomial logistic regression to produce probability models for old snow and for new snow wet avalanche conditions. The old snow model uses the prediction day minimum temperature and the two-day change in total snow depth as predictor variables and has a 89% overall success rate. However, the majority of this success is due to correct prediction of days without wet avalanches (96% of all correct predictions). The new snow model uses the prediction day minimum temperature and three-day cumulative new snow water equivalent as predictor variables, but is less useful. The models are applicable only to Bridger Bowl. The numerical forecast models can be used as one of the tools in the forecasting toolbox but limited data and complexity of process require that the decisions about closure remain in the hands of the ski patrol

Priority sites for wildfowl conservation in Mexico

A set of priority sites for wildfowl conservation in Mexico was determined using contemporary count data (1991–2000) from the U.S. Fish & Wildlife Service mid-winter surveys. We used a complementarity approach implemented through linear integer programming that addresses particular conservation concerns for every species included in the analysis and large fluctuations in numbers through time. A set of 31 priority sites was identified, which held more than 69% of the mid-winter count total in Mexico during all surveyed years. Six sites were in the northern highlands, 12 in the central highlands, six on the Gulf of Mexico coast and seven on the upper Pacific coast. Twenty-two sites from the priority set have previously been identified as qualifying for designation as wetlands of international importance under the Ramsar Convention and 20 sites are classified as Important Areas for Bird Conservation in Mexico. The information presented here provides an accountable, spatially-explicit, numerical basis for ongoing conservation planning efforts in Mexico, which can be used to improve existing wildfowl conservation networks in the country and can also be useful for conservation planning exercises elsewhere