排序方式： 共有37条查询结果，搜索用时 15 毫秒
1.
Katharina Marquardt Elena Petrishcheva Rainer Abart Emmanuel Gardés Richard Wirth Ralf Dohmen Hans-Werner Becker Wilhelm Heinrich 《Physics and Chemistry of Minerals》2010,37(10):751-760
In this study, we address volume diffusion of ytterbium in yttrium aluminum garnet (YAG) using thin-film single crystal diffusion
couples. We employ analytical transmission electron microscopy (ATEM) as a tool for combined microstructural and microchemical
analysis and compare the results to Rutherford backscattering (RBS) analysis. Given the high spatial resolution of the method,
we focus on microstructural changes of the thin-film diffusant source during the diffusion anneal. We evaluate the potential
influence of the associated changes in its transport properties on the evolution of concentration profiles in the single crystal
substrate. This approach allows us to test the reliability of determination of volume diffusion coefficients from thin-film
diffusion experiments. We found that for the chosen experimental setting, the influence of thin-film re-crystallization is
small when compared with the experimental uncertainty and good estimates for the volume diffusion coefficients of Yb in YAG
can be obtained using standard assumptions. Both Yb-concentration profiles analyzed with ATEM and with RBS give similar results.
At 1,450°C and 1 bar, we infer log D
Yb (m2/s) values of −19.37 ± 0.07 (TEM) and −19.84 ± 0.02 (RBS). Although the change in thin-film transport properties associated
with successive crystallization during the diffusion anneal does not play a major role for our experimental setup, this effect
cannot generally be ignored. 相似文献
2.
Thorsten?MauritsenEmail author Gunilla?Svensson Branko?Grisogono 《Boundary-Layer Meteorology》2005,117(2):259-273
We investigate the flow over Arctic leads using a mesoscale numerical model, typical of both summer and winter, under idealised
conditions. We find that Arctic leads may be the source of standing atmospheric internal gravity waves during both seasons.
The summertime wave may be compared with the wave generated by a small ridge, though with the phase reversed. The mechanism
for exciting the wave is found to be the internal boundary layer developing due to horizontal variations in surface temperature
and roughness length. During the more exploratory wintertime simulations, with substantial temperature difference between
the lead and the ice surface, we find that secondary circulations and intermittent wave-breaking may occur. The effects of
the lead appear far downstream. 相似文献
3.
The simulation of Arctic clouds and their influence on the winter surface temperature in present-day climate in the CMIP3 multi-model dataset 总被引：1，自引：1，他引：0
We investigate the influence of clouds on the surface energy budget and surface temperature in the sea-ice covered parts of the ocean north of the Arctic circle in present-day climate in nine global climate models participating in the Coupled Model Intercomparison Project phase 3, CMIP3. Monthly mean simulated surface skin temperature, radiative fluxes and cloud parameters are evaluated using retrievals from the extended AVHHR Polar Pathfinder (APP-x) product. We analyzed the annual cycle but the main focus is on the winter, in which large parts of the region experience polar night. We find a smaller across-model spread as well as better agreement with observations during summer than during winter in the simulated climatological annual cycles of total cloudiness and surface skin temperature. The across-model spread in liquid and ice water paths is substantial during the whole year. These results qualitatively agree with earlier studies on the present-day Arctic climate in GCMs. The climatological ensemble model mean annual cycle of surface cloud forcing shows good agreement with observations in summer. However, during winter the insulating effect of clouds tends to be underestimated in models. During winter, most of the models as well as the observations show higher monthly mean total cloud fractions, associated with larger positive surface cloud forcing. Most models also show good correlation between the surface cloud forcing and the vertically integrated ice and liquid cloud condensate. The wintertime ensemble model mean total cloud fraction (69%) shows excellent agreement with observations. The across-model spread in the winter mean cloudiness is substantial (36?C94%) however and several models significantly underestimate the cloud liquid water content. If the two models not showing any relationship between cloudiness and surface cloud forcing are disregarded, a tentative across-model relation exists, in such a way that models that simulate large winter mean cloudiness also show larger surface cloud forcing. Even though the across-model spread in wintertime surface cloud forcing is large, no clear relation to the surface temperature is found. This indicates that other processes, not explicitly cloud related, are important for the simulated across-model spread in surface temperature. 相似文献
4.
Katharina?Marquardt Elena?Petrishcheva Emmanuel?Gardés Richard?Wirth Rainer?Abart Wilhelm?Heinrich 《Contributions to Mineralogy and Petrology》2011,162(4):739-749
Yb-Y inter-diffusion along a single grain boundary of a synthetic yttrium aluminium garnet (YAG) bicrystal has been studied
using analytical transmission electron microscopy (ATEM). To investigate the diffusion, a thin-film containing Yb as the diffusant
was deposited perpendicular to the bicrystal grain boundary by pulsed laser deposition (PLD). Structural properties and their
change with time in both the diffusant source and the grain boundary are reported. The diffusion profiles are incorporated
in a numerical diffusion model, which is applied to determine the grain boundary diffusion coefficient, D
gb
, at 1.723 K it is equal to 3 × 10−15 m2/s. We find that grain boundary diffusion is 4.85 orders of magnitude faster than volume diffusion, which was determined from
the same diffusion experiment. This result is discussed in the context of special versus general grain boundaries. Finally,
we successfully tested the capability of synchrotron-based nano-X-ray fluorescence analysis to map micro-chemical patterns
in two dimensions with sub-micrometre resolution. 相似文献
5.
Part I of this contribution (Gardés et al. in Contrib Mineral Petrol, 2010) reported time- and temperature-dependent experimental growth of polycrystalline forsterite-enstatite double layers between
single crystals of periclase and quartz, and enstatite single layers between forsterite and quartz. Both double and single
layers displayed growth rates decreasing with time and pronounced grain coarsening. Here, a model is presented for the growth
of the layers that couples grain boundary diffusion and grain coarsening to interpret the drop of the growth rates. It results
that the growth of the layers is such that (Δx)2 ∝ t
1−1/n
, where Δx is the layer thickness and n the grain coarsening exponent, as experimentally observed. It is shown that component transport occurs mainly by grain boundary
diffusion and that the contribution of volume diffusion is negligible. Assuming a value of 1 nm for the effective grain boundary
width, the following Arrhenius laws for MgO grain boundary diffusion are derived: log D
gb,0Fo (m2/s) = −2.71 ± 1.03 and E
gbFo = 329 ± 30 kJ/mol in forsterite and log D
gb,0En (m2/s) = 0.13 ± 1.31 and E
gbEn = 417 ± 38 kJ/mol in enstatite. The different activation energies are responsible for the changes in the enstatite/forsterite
thickness ratio with varying temperature. We show that significant biases are introduced if grain boundary diffusion-controlled
rim growth is modelled assuming constant bulk diffusivities so that differences in activation energies of more than 100 kJ/mol
may arise. It is thus important to consider grain coarsening when modelling layered reaction zones because they are usually
polycrystalline and controlled by grain boundary transport. 相似文献
6.
Bastian Joachim Emmanuel Gardés Rainer Abart Wilhelm Heinrich 《Contributions to Mineralogy and Petrology》2011,161(3):389-399
Growth rates of monomineralic, polycrystalline åkermanite (Ca2MgSi2O7) rims produced by solid-state reactions between monticellite (CaMgSiO4) and wollastonite (CaSiO3) single crystals were determined at 0.5 GPa dry argon pressure, 1,000–1,200°C and 5 min to 60 h, using an internally heated pressure vessel. Inert Pt-markers, initially placed at the monticellite–wollastonite interface, indicate symmetrical growth into both directions. This and mass balance considerations demonstrate that rim growth is controlled by transport of MgO. At 1,200°C and run durations between 5 min and 60 h, rim growth follows a parabolic rate law with rim widths ranging from 0.4 to 16.3 μm indicating diffusion-controlled rim growth. The effective bulk diffusion coefficient \( D_{\text{eff,MgO}}^{\text{Ak}} \) is calculated to 10?15.8±0.1 m2 s?1. Between 1,000°C and 1,200°C, the effective bulk diffusion coefficient follows an Arrhenius law with E a = 204 ± 18 kJ/mol and D 0 = 10?8.6±1.6 m2 s?1. Åkermanite grains display a palisade texture with elongation perpendicular to the reaction interface. At 1,200°C, average grain widths measured normal to elongation, increase with the square root of time and range from 0.4 to 5.4 μm leading to a successive decrease in the grain boundary area fraction, which, however, does not affect \( D_{\text{eff,MgO}}^{\text{Ak}} \) to a detectible extent. This implies that grain boundary diffusion only accounts for a minor fraction of the overall chemical mass transfer, and rim growth is essentially controlled by volume diffusion. This is corroborated by the agreement between our estimates of the effective MgO bulk diffusion coefficient and experimentally determined volume diffusion data for Mg and O in åkermanite from the literature. There is sharp contrast to the MgO–SiO2 binary system, where grain boundary diffusion controls rim growth. 相似文献
7.
Perch (Perca fluviatilis), were sampled from unpolluted and polluted areas in Swedish coastal waters. The level of aromatic/hydrophobic DNA adducts in liver tissue was analyzed using the nucleas P1 version of the32P-post-labeling assay. The level of total adducts measured in the individual fish from polluted areas was between 6 and 22 nmol of adducts/mol of nucleotides, and in the fish from the reference area between 0.2 and 0.6 nmol of adducts/mol of nucleotides. 相似文献
8.
9.
DNA adducts in gills and digestive gland, as well as polycyclic aromatic hydrocarbon (PAH) tissue levels were analysed in blue mussels (Mytilus spp.) from Nordic coastal areas (Iceland, Norway and Sweden) with diffuse or point sources of PAHs of various origins. Both DNA adduct and PAH tissue levels were generally low, indicating low PAH exposure to the mussels in the areas studied. DNA adducts were found to be higher in gills than in digestive gland of the mussels at all sites studied. Elevated DNA adduct levels in gills were found at 6 sites out of 18 compared to reference sites in respective coastal zones. Adduct levels ranged from 0.5 to 10 nmol adducts/mol normal nucleotides, being highest in mussels from Reykjavík harbour, Iceland (intertidal mussels), and from Fiskaatangen, Norway (subtidal mussels). Total PAH tissue levels in the mussels ranged between 40 and 11,670 ng/g dry wt., and were significantly correlated with DNA adduct levels (r(2)=0.73, p<0.001). PAH ratio values indicated that the PAHs were in most cases of pyrolytic origin, but with petrogenic input near harbours and an oil refinery. 相似文献
10.
Simulations of subtropical marine low clouds and their radiative properties by nine coupled ocean-atmosphere climate models
participating in the fourth assesment report (AR4) of the intergovernmental panel on climate change (IPCC) are analyzed. Satellite
observations of cloudiness and radiative fluxes at the top of the atmosphere (TOA) are utilized for comparison. The analysis
is confined to the marine subtropics in an attempt to isolate low cloudiness from tropical convective systems. All analyzed
models have a negative bias in the low cloud fraction (model mean bias of −15%). On the other hand, the models show an excess
of cloud radiative cooling in the region (model mean excess of 13 W m−2). The latter bias is shown to mainly originate from too much shortwave reflection by the models clouds rather than biases
in the clear-sky fluxes. These results confirm earlier studies, thus no major progress in simulating the marine subtropical
clouds is noted. As a consequence of the combination of these two biases, this study suggests that all investigated models
are likely to overestimate the radiative response to changes in low level subtropical cloudiness. 相似文献