47,539 research outputs found
From building blocks to 2D networks
No full textThe aim of this work is to further the understanding of the important parameters in the formation process of 2D nanostructures and therewith pioneer for novel applications. Such 2D nanostructures can be composed of specially designed organic molecules, which are adsorbed on various surfaces. In order to study true 2D structures, monolayers were deposited. Their properties have been investigated by scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions as well as under ambient conditions. The latter is a highly dynamic environment, where several parameters come into play. Complementary surface analysis techniques such as low-energy electron diffraction (LEED), X-Ray photo-emission spectroscopy (XPS), and Raman spectroscopy were used when necessary to characterize these novel molecular networks. In order to conduct this type of experiments, high technical requirements have to be fulfilled, in particular for UHV experiments. Thus, the focus is on a drift-stable STM, which lays the foundation for high resolution STM topographs. Under ambient conditions, the liquid-solid STM can be easily upgraded by an injection add-on due to the highly flexible design. This special extension allows for adding extra solvent without impairing the high resolution of the STM data. Besides the device, also the quality of the tip is of pivotal importance. In order to meet the high requirements for STM tips, an in vacuo ion-sputtering and electron-beam annealing device was realized for the post-preparation of scanning probes within one device. This two-step cleaning process consists of an ion-sputtering step and subsequent thermal annealing of the probe. One study using this STM setup concerned the incorporation dynamics of coronene (COR) guest molecules into pre-existent pores of a rigid 2D supramolecular host networks of trimesic acid (TMA) as well as the larger analogous benzenetribenzoic acid (BTB) at the liquid-solid interface. By means of the injection add-on the additional solution containing the guest molecules was applied to the surface. At the same time the incorporation process was monitored by the STM. The incorporation dynamics into geometrically perfectly matched pores of trimesic acid as well as into the substantially larger pores of benzentribenzoic acid exhibit a clearly different behavior. For the BTB network instantaneous incorporation within the temporal resolution of the experiment was observed; for the TMA network, however, intermediate adsorption states of COR could be visualized before the final adsorption state was reached. A further issue addressed in this work is the generation of metal-organic frameworks (MOFs) under ultra-high vacuum conditions. A suitable building block therefore is an aromatic trithiol, i.e. 1,3,5-tris(4-mercaptophenyl)benzene (TMB). To understand the specific role of the substrate, the surface-mediated reaction has been studied on Cu(111) as well as on Ag(111). Room temperature deposition on both substrates results in densely packed trigonal structures. Yet, heating the Cu(111) with the TMB molecules to moderate temperature (150 °C) yields two different porous metal coordinated networks, depending on the initial surface coverage. For Ag(111) the first structural change occurs after annealing the sample at 300 °C. Here, several disordered structures with partially covalent disulfur bridges were identified. Proceeding further in the scope of increasing interaction strength between the building blocks, covalent organic frameworks (COFs) were studied under ultra-high vacuum conditions as well as under ambient conditions. For this purpose, a promising strategy is covalent coupling through radical addition reactions of appropriate monomers, i.e. halogenated aromatic molecules such as 1,3,5-tris(4-bromophenyl)benzene (TBPB) and 1,3,5-tris(4- iodophenyl)benzene (TIPB). Besides the correct choice of a catalytic surface, the activation energy for the scission of the carbon-halogen bonds is an essential parameter. In the case of ultra-high vacuum experiments, the influence of substrate temperature, material, and crystallographic orientation on the coupling reaction was studied. For reactive Cu(111) and Ag(110) surfaces room temperature deposition of TBPB already leads to a homolysis of the C-Br bond and subsequent formation of proto-polymers. Applying additional heat facilitates the transformation of proto-polymers into 2D covalent networks. In contrast, for Ag(111) just a variety of self-assembled and rather poorly ordered structures composed of intact molecules has emerged. The deposition onto substrates held at 80 K has never resulted in proto-polymers. For ambient conditions, the polymerization reaction of 1,3,5-tri(4-iodophenyl)benzene (TIPB) on Au(111) was studied by STM after drop-casting the monomer onto the substrate held either at room temperature or at 100 °C. For room temperature deposition only poorly ordered non-covalent arrangements were observed. In accordance with the established UHV protocol for halogenated coupling reaction, a covalent aryl-aryl coupling was accomplished for high temperature deposition. Interestingly, these covalent aggregates were not directly adsorbed on the Au(111) surface, but attached on top of a chemisorbed monolayer comprised of iodine and partially dehalogenated TIPB molecules. For a detailed analysis of the processes, the temperature dependent dehalogenation reaction was monitored by X-ray photoelectron spectroscopy under ultra-high vacuum conditions
Settling of finite-size particles in isotropically forced, homogeneous turbulence: interface-resolved simulations
No full textWe have simulated the gravity-induced settling of finite-size particles in a turbulent background flow which is forced in a statistically-stationary fashion. The simulations are accurately resolving the solid-fluid interface with the aid of an immersed boundary technique [1]. The parameters of the simulation are (apart from background turbulence) identical to those of reference [2], where particle clustering was observed at a Galileo number of 178 and a solid volume fraction of 0.005. In the present case, it is found that a relative turbulence intensity of 0.24 leads to the disappearance of the clusters; as a consequence, the increase in average particle settling velocity found in [2] also vanishes. [1] M. Uhlmann. An immersed boundary method with direct forcing for the simulation of particulate flows. J. Comput. Phys., 209(2):448–476, 2005. [2] M. Uhlmann and T. Doychev. Sedimentation of a dilute suspension of rigid spheres at intermediate Galileo numbers: the effect of clustering upon the particle motion. J. Fluid Mech., 752:310–348, 2014
Mesophilic-hydrothermal-thermophilic (M-H-T) digestion of green corn straw
No full textMesophilic-hydrothermal (80-160 degrees C, 30 min)-thermophilic (M-H-T) digestion and control tests of mesophilic (M), thermophilic (T), hydrothermal-mesophilic (H-M), and mesophilic-thermophilic digestion (M-T) of green corn straw were conducted for a 20-day fermentation period. The results indicate that M-H-T is an efficient method to improve methane production. A maximum methane yield of 371.74 mL/g volatile solid was obtained by the M (3 days)-H (140 degrees C)-T (17 days) process, which was 20.44%, 16.55%, 31.44%, and 14.31% higher than the yields of the M, T, 140-M, and M-T processes. The enhanced methane production was attributed to (1) the improved hemicellulose degradation and lignin disorganization; (2) prevention of the degradation of soluble sugar, easily hydrolyzed hemicellulose and cellulose into furfural and methylfurfural; and (3) lack of formation of Maillard reaction products during initial hydrothermal treatment. (C) 2015 Elsevier Ltd. All rights reserved
Male occurrence in Austrian Triops cancriformis (Branchiopoda, Notostraca) populations and ultrastructural observations of the male gonad
No full textRecent reports of occasional males in Austrian populations of Triops cancriformis have promoted interest in an analysis of the ultrastructure of the male gonad. It appears as a double thick and well‐defined tubular structure running along the midgut, inside the hemocoel. It is composed of two portions: the wall and the lumen. The former is made up of germ cells that are widespread and intermixed with somatic cells. The lumen is ∼200 μm wide and acts as the collecting site of spermatids and sperm. The germ cells are recognizable by their rounded appearance and by the occurrence of exceptionally long synaptonemal complexes in their nuclei. Their maturation implies a volume elongation and an aggregation in a characteristic “cyst” arrangement. The cysts migrate towards the lumen, where they break open, releasing spermatids and sperm. At the end of this process the lumen is filled with maturing gametes. The sperm have very condensed chromatin nuclei and partially reduced cytoplasm where the most striking features are the axoneme and strewn microtubules together with evident pseudopodia. Our morphological data support the full functionality of males in T. cancriformis. However, further work on gametogenesis, distribution of sexes, and genetic analysis of breeding experiments are needed to reach a fuller understanding of the role of males in middle and northern European populations of this species
Dr. Glendon Swarthout
No full textHosted by Roger M. Busfield, MSU Assistant Professor of Speech and Theater, Meet the Author is designed to introduce a general audience to a contemporary author and their work through in-depth interviews. This episode features a conversation between Dr. Glendon Swarthout, prolific author and English professor at MSU, and assistant professors Sam S. Baskett and Theodore B. Strandness
Brightness and virtual source size of a supersonic deuterium beam
No full textSupersonic beams have numerous applications in research fields ranging from spectroscopy with nanodroplets
to surface science and matter-wave microscopy. Thus, measurement and prediction of their properties is of
considerable interest. In this paper we present measurements of the virtual-source size and its brightness, as well as the terminal speed and terminal speed ratio of a supersonic deuterium (D2) beam. The speed distribution
data were measured with time-of-flight experiments and Fresnel zone-plate imaging was used to measure virtual
source size. The point-spread function of the zone plate was simulated based on the measured wavelength
distribution and used to extract the width of the virtual source and its brightness from the focus measurement.
The experiments were carried out with a 10-μm-diameter nozzle and a source temperature of T0 = 310 K in the
pressure range p0 = 3–171 bars and for T0 = 106 K in the pressure range p0 = 3–131 bars.We found that using
deuterium as opposed to helium results in a virtual source that is about a factor 2 brighter under similar stagnation
conditions. A comparison between the measured data and the predictions from a theoretical model based on
the Boltzmann equation, which explicitly include the coupling between translational and rotational degrees of
freedom as well as the real-gas properties of D2, resulted in good correspondence for the two different interaction
potentials we tried. A careful comparison with the experimental results shows that the potential by Buck et al.
[J. Chem. Phys. 78, 4439 (1983)] is moderately better than the Lennard-Jones potential at describing the expansion
dynamics
Büchertode
No full textKörte M. Büchertode. In: Eder T, Kobenter S, Plener P, eds. Seitenweise : was das Buch ist. Wien : Ed. Atelier ; 2011: 204-218
The maternal immune system during pregnancy and its influence on fetal development
No full textThe maternal immune system plays a critical role in the establishment, maintenance, and completion of a healthy pregnancy. However, the specific mechanisms utilized to achieve these goals are not well understood. Various cells and molecules of the immune system are key players in the development and function of the placenta and the fetus. Effector cells of the immune system act to promote and yet limit placental development. The T helper 1 (Th1)/T helper 2 (Th2) immune shift during pregnancy is well established. A fine balance between proinflammatory and anti-inflammatory influences is required. We herein review the evidence regarding maternal tolerance of fetal tissues and the underlying cell-mediated immune and humoral (hormones and cytokines) mechanisms. We also note the many unanswered questions in our understanding of these mechanisms. In addition, we summarize the clinical manifestations of an altered maternal immune system during pregnancy related to susceptibility to common viral, bacterial, and parasitic infections, as well as to autoimmune diseases.Peer reviewe
Chromosomes in sexual populations of notostracan and con-chostracan taxa (crustacea, branchiopoda)
No full textBranchiopods reproductive mechanisms range from gonochorism to unisexuality, passing through an-drodioecy. In order to contribute to still lacking or controversial basic knowledges, we analyzed the karyotype of the main Italian taxa: the Notostracan living fossils Triops cancriformis (parthenogenetic) and Lepidurus apus lubbocki (bisexual), and the Conchostracan Eoleptestheria ticinensis (bisexual). Also one male obtained from a supposedly hermaphroditic Austrian population of T. cancriformis was checked. In T. cancriformis a diploid number of 12 chromosomes is observed in both females and male; this is in line with previous results on other Italian populations, but contrasts with observations on European samples. The richness of normal meiotic pictures indicates that the male specimen is able to produce sperms. L. apus lubbocki shows a diploid number of 10 chromosomes; in male specimens the mispairings during meiosis I could explain the haploid number of 6 chromosomes obtained from diakinetic plates in Palestinian samples of the same subspecies found in literature. Irregular meiotic divisions also highlight the ultrastructurally observed abortive spermatogenesis. Finally, in the presently analyzed population of E. ticinensis, the adult females show 10 chromosomes as diploid number, while males present 10 or 11 elements; in the nauplii, chromosome numbers range from 8 to 12, differing among unrelated individuals, between nauplii produced by the same female and even within the same nauplius. B elements are taken into account. © 2005 Taylor & Francis Group, LLC
Erosie door open taludbekledingen. Samenvattend verslag + Bijlage A t/m D
No full textOpen taludbekledingen die bestaan uit in verband geplaatste betonblokken met gaten, bieden de mogelijkheid vegetatie te doen groeien, waardoor mogelijk een milieuvriendelijke oever kan worden verkregen. In het pioniersstadium van de vegetatie is het evenwel ongewenst dat de gatvulling uitspoelt. Teneinde de relatie tussen waterbeweging en erosie van de gatvulling vast te stellen, is door de Dienst Weg- en Waterbouwkunde van Rijkswaterstaat per brief d.d. 16 maart 1987 (kenmerk WB 570), opdracht verleend aan het Waterloopkundig Laboratorium tot het uitvoeren van onderzoek naar de erosie door open taludbekledingen. Het doel van het onderzoek is het ontwikkelen van ontwerprichtlijnen voor taludbekledingen met gaten die groter zijn dan de zand- of filterkorrels eronder. Hiertoe dient de kritieke waterbeweging bij een oever- of dijkbekleding te worden vastgesteld, waarbij nog toelaatbare erosie is te verwachten. De toelaatbare erosie mag daarbij maximaal gelijk zijn aan de hoeveelheid sediment in de gaten. Filter- of basismateriaal gelegen onder de elementen mag dus niet uitspoelen. Bij oeverbekledingen waar vegetatie een rol moet gaan spelen, is de toelaatbare erosie kleiner, dat wil zeggen in de gaten dient sediment achter te blijven.Steenzettingen - TAW/EN
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