3,614 research outputs found
Polymorphism of the glass former ethanol confined in mesoporous silicon
X-ray diffraction patterns of ethanol confined in parallel-aligned channels
of approx. 10 nm diameter and 50 micrometer length in mesoporous silicon have
been recorded as a function of filling fraction, temperature and for varying
cooling and heating rates. A sorption isotherm, recorded in the liquid state,
indicates a three monolayer thick, strongly adsorbed wall layer and a capillary
condensed fraction of molecules in the pore center. Though the strongly
adsorbed film remains in an amorphous state for the entire temperature range
investigated, the capillary condensed molecules reproduce the polymorphism of
bulk solid ethanol, that is the formation of either crystalline or glass-like
states as a function of cooling rate. The critical rate necessary to achieve a
vitrification in the mesopores is, however, at least two orders of magnitude
smaller than in the bulk state. This finding can be traced both to pure
geometrical constraints and quenched disorder effects, characteristic of
confinement in mesoporous silicon.Comment: 6 pages, 4 figure
Crystallization of medium length 1-alcohols in mesoporous silicon: An X-ray diffraction study
The linear 1-alcohols n-C16H33OH, n-C17H35OH, n-C19H37OH have been imbibed
and solidified in lined up, tubular mesopores of silicon with 10 nm and 15 nm
mean diameters, respectively. X-ray diffraction measurements reveal a set of
six discrete orientation states (''domains'') characterized by a perpendicular
alignment of the molecules with respect to the long axis of the pores and by a
four-fold symmetry about this direction, which coincides with the crystalline
symmetry of the Si host. A Bragg peak series characteristic of the formation of
bilayers indicates a lamellar structure of the spatially confined alcohol
crystals in 15 nm pores. By contrast, no layering reflections could be detected
for 10 nm pores. The growth mechanism responsible for the peculiar orientation
states is attributed to a nano-scale version of the Bridgman technique of
single-crystal growth, where the dominant growth direction is aligned
parallelly to the long pore axes. Our observations are analogous to the growth
phenomenology encountered for medium length n-alkanes confined in mesoporous
silicon (Phys. Rev. E 75, 021607 (2007)) and may further elucidate why porous
silicon matrices act as an effective nucleation-inducing material for protein
solution crystallization.Comment: 4 pages, 4 figures, to appear as a Brief Report in Physical Review
Crossover from a square to a hexagonal pattern in Faraday surface waves
We report on surface wave pattern formation in a Faraday experiment operated
at a very shallow filling level, where modes with a subharmonic and harmonic
time dependence interact. Associated with this distinct temporal behavior are
different pattern selection mechanisms, favoring squares or hexagons,
respectively. In a series of bifurcations running through a pair of
superlattices the surface wave pattern transforms between the two incompatible
symmetries. The close analogy to 2D and 3D crystallography is pointed out.Comment: 4 pages, 4 figure
Phase relaxation of Faraday surface waves
Surface waves on a liquid air interface excited by a vertical vibration of a
fluid layer (Faraday waves) are employed to investigate the phase relaxation of
ideally ordered patterns. By means of a combined frequency-amplitude modulation
of the excitation signal a periodic expansion and dilatation of a square wave
pattern is generated, the dynamics of which is well described by a Debye
relaxator. By comparison with the results of a linear theory it is shown that
this practice allows a precise measurement of the phase diffusion constant.Comment: 5 figure
Climate, COâ‚‚ and demographic impacts on global wildfire emissions
Abstract. Wildfires are by far the largest contributor to global biomass burning and constitute a large global source of atmospheric traces gases and aerosols. Such emissions have a considerable impact on air quality and constitute a major health hazard. Biomass burning also influences the radiative balance of the atmosphere and is thus not only of societal, but also of significant scientific interest. There is a common perception that climate change will lead to an increase in emissions as hot and dry weather events that promote wildfire will become more common. However, even though a few studies have found that the inclusion of CO2 fertilisation of photosynthesis and changes in human population patterns will tend to somewhat lower predictions of future wildfire emissions, no such study has included full ensemble ranges of both climate predictions and population projections, including the effect of different degrees of urbanisation.
Here, we present a series of 124 simulations with the LPJ–GUESS–SIMFIRE global dynamic vegetation–wildfire model, including a semi-empirical formulation for the prediction of burned area based on fire weather, fuel continuity and human population density. The simulations use Climate Model Intercomparison Project 5 (CMIP5) climate predictions from eight Earth system models. These were combined with two Representative Concentration Pathways (RCPs) and five scenarios of future human population density based on the series of Shared Socioeconomic Pathways (SSPs) to assess the sensitivity of emissions to the effect of climate, CO2 and humans. In addition, two alternative parameterisations of the semi-empirical burned-area model were applied. Contrary to previous work, we find no clear future trend of global wildfire emissions for the moderate emissions and climate change scenario based on the RCP 4.5. Only historical population change introduces a decline by around 15 % since 1900. Future emissions could either increase for low population growth and fast urbanisation, or continue to decline for high population growth and slow urbanisation. Only for high future climate change (RCP8.5), wildfire emissions start to rise again after ca. 2020 but are unlikely to reach the levels of 1900 by the end of the 21st century. We find that climate warming will generally increase the risk of fire, but that this is only one of several equally important factors driving future levels of wildfire emissions, which include population change, CO2 fertilisation causing woody thickening, increased productivity and fuel load and faster litter turnover in a warmer climate
Evidence for Kosterlitz-Thouless type orientational ordering of CFBr monolayers physisorbed on graphite
Monolayers of the halomethane CFBr adsorbed on graphite have been
investigated by x-ray diffraction. The layers crystallize in a commensurate
triangular lattice. On cooling they approach a three-sublattice
antiferroelectric pattern of the in-plane components of the dipole moments. The
ordering is not consistent with a conventional phase transition, but points to
Kosterlitz-Thouless behavior. It is argued that the transition is described by
a 6-state clock model on a triangular lattice with antiferromagnetic nearest
neighbor interactions which is studied with Monte-Carlo simulations. A
finite-size scaling analysis shows that the ordering transition is indeed in
the KT universality class.Comment: 4 pages, 5 figure
Peat decomposition records in three pristine ombrotrophic bogs in southern Patagonia
Ombrotrophic bogs in southern Patagonia have been examined with regard to paleoclimatic and geochemical research questions but knowledge about organic matter decomposition in these bogs is limited. Therefore, we examined peat humification with depth by Fourier Transformed Infrared (FTIR) measurements of solid peat, C/N ratio, and &delta;<sup>13</sup>C and &delta;<sup>15</sup>N isotope measurements in three bog sites. Peat decomposition generally increased with depth but distinct small scale variation occurred, reflecting fluctuations in factors controlling decomposition. C/N ratios varied mostly between 40 and 120 and were significantly correlated (<i>R</i><sup>2</sup> > 0.55, <i>p</i> < 0.01) with FTIR-derived humification indices. The degree of decomposition was lowest at a site presently dominated by <i>Sphagnum</i> mosses. The peat was most strongly decomposed at the driest site, where currently peat-forming vegetation produced less refractory organic material, possibly due to fertilizing effects of high sea spray deposition. Decomposition of peat was also advanced near ash layers, suggesting a stimulation of decomposition by ash deposition. Values of &delta;<sup>13</sup>C were 26.5 &plusmn; 2&permil; in the peat and partly related to decomposition indices, while &delta;<sup>15</sup>N in the peat varied around zero and did not consistently relate to any decomposition index. Concentrations of DOM partly related to C/N ratios, partly to FTIR derived indices. They were not conclusively linked to the decomposition degree of the peat. DOM was enriched in <sup>13</sup>C and in <sup>15</sup>N relative to the solid phase probably due to multiple microbial modifications and recycling of N in these N-poor environments. In summary, the depth profiles of C/N ratios, &delta;<sup>13</sup>C values, and FTIR spectra seemed to reflect changes in environmental conditions affecting decomposition, such as bog wetness, but were dominated by site specific factors, and are further influenced by ash deposition and possibly by sea spray input
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