In Silico Determination of Gas Permeabilities by Non-Equilibrium Molecular Dynamics: CO2 and He through PIM-1.

© 2015 by the authors; licensee MDPI, Basel, Switzerland.We study the permeation dynamics of helium and carbon dioxide through an atomistically detailed model of a polymer of intrinsic microporosity, PIM-1, via non-equilibrium molecular dynamics (NEMD) simulations. This work presents the first explicit molecular modeling of gas permeation through a high free-volume polymer sample, and it demonstrates how permeability and solubility can be obtained coherently from a single simulation. Solubilities in particular can be obtained to a very high degree of confidence and within experimental inaccuracies. Furthermore, the simulations make it possible to obtain very specific information on the diffusion dynamics of penetrant molecules and yield detailed maps of gas occupancy, which are akin to a digital tomographic scan of the polymer network. In addition to determining permeability and solubility directly from NEMD simulations, the results shed light on the permeation mechanism of the penetrant gases, suggesting that the relative openness of the microporous topology promotes the anomalous diffusion of penetrant gases, which entails a deviation from the pore hopping mechanism usually observed in gas diffusion in polymers

Systematic X-ray absorption study of hole doping in BSCCO - phases

X-ray absorption spectroscopy (XAS) on the O 1s threshold was applied to Bi-based, single crystalline high temperature superconductors (HTc's), whose hole densities in the CuO2 planes was varied by different methods. XAS gives the intensity of the so-called pre-peak of the O 1s line due to the unoccupied part of the Zhang-Rice (ZR) singlet state. The effects of variation of the number n of CuO2 - planes per unit cell (n = 1,2,3) and the effect of La-substitution for Sr for the n = 1 and n = 2 phase were studied systematically. Furthermore the symmetry of the states could be probed by the polarization of the impinging radiation.Comment: 4 pages, 2 figures, to appear in the proceedings of SCES2001, Ann Arbor, August 6-10, 200

A Canted Double Undulator System with a Wide Energy Range for EMIL

At BESSY II a canted double undulator system for the Energy Materials In situ Laboratory EMIL is under construction. The energy regime is covered with two undulators, an APPLE II undulator for the soft and a cryogenic permanent magnet undulator CPMU 17 for the hard photons. The layout and the performance of the undulators are presented in detail. The minimum of the vertical betatron function is shifted to the center of the CPMU 17. The neighboring quadrupoles and an additional quadrupole between the undulators control the vertical betatron function. Prior to the undulator installation a testing chamber with four movable vertical scrapers has been implemented at the CPMU 17 location. Utilizing the scrapers the new asymmetric lattice optics will be tested and optimize

Cryogenic Design of a PrFeB Based Undulator

A PrFeB based cryogenic undulator has been built at Helmholtz Zentrum Berlin HZB in collaboration with the Ludwig Maximilian University München LMU . LMU will operate the undulator at a laser plasma accelerator at the Max Planck Institut für Quantenoptik in Garching. The 20 period device has a period length of 9mm and a fixed gap of 2.5mm. The operation of a small gap device at a high emittance electron beam requires stable magnetic material. A high coercivity is achieved with PrFeB material cooled down to 20 30K. In this paper we present the mechanic, magnetic and cryogenic design and compare predictions with measured dat

Nanoscale structural and chemical analysis of F-implanted enhancement-mode InAlN/GaN heterostructure field effect transistors

We investigate the impact of a fluorine plasma treatment used to obtain enhancement-mode operation on the structure and chemistry at the nanometer and atomic scales of an InAlN/GaN field effect transistor. The fluorine plasma treatment is successful in that enhancement mode operation is achieved with a +2.8 V threshold voltage. However, the InAlN barrier layers are observed to have been damaged by the fluorine treatment with their thickness being reduced by up to 50%. The treatment also led to oxygen incorporation within the InAlN barrier layers. Furthermore, even in the as-grown structure, Ga was unintentionally incorporated during the growth of the InAlN barrier. The impact of both the reduced barrier thickness and the incorporated Ga within the barrier on the transistor properties has been evaluated theoretically and compared to the experimentally determined two-dimensional electron gas density and threshold voltage of the transistor. For devices without fluorine treatment, the two-dimensional electron gas density is better predicted if the quaternary nature of the barrier is taken into account. For the fluorine treated device, not only the changes to the barrier layer thickness and composition, but also the fluorine doping needs to be considered to predict device performance. These studies reveal the factors influencing the performance of these specific transistor structures and highlight the strengths of the applied nanoscale characterisation techniques in revealing information relevant to device performance.</jats:p

Stress relaxation of AlGaN on nonpolar m-plane GaN substrate

The stress relaxation with increasing thickness of metal-organic vapor phase epitaxy grown Al0.19Ga0.81N on quasi-bulk (101¯0) m-plane GaN substrates was investigated by x-ray diffraction. The anisotropic in-plane stress leads to an orthorhombic distortion of the lattice, which requires special mathematical treatment. Extending earlier works, we developed a method to calculate the distortion along [12¯10], [0001], and [101¯0] and obtained the lattice parameters, Al content, and strain values. The stress relaxation along the two in-plane directions involves two different mechanisms. First, the stress along [12¯10] relaxes by the onset of misfit dislocations through the {101¯0}⟨12¯10⟩ slip system while for thicker layers the stress along [0001] relaxes by crack formation. Comparing the cathodoluminescence emission at room temperature with the expected bandgap showed that both tensile in-plane strains along [12¯10] and [0001] decrease the bandgap.</jats:p

Defect characterization of { 10 1 ¯ 3 } GaN by electron microscopy

Advances in obtaining untwinned (101¯3)-oriented semi-polar GaN enable a new crystal orientation for the growth of green and red LED structures. We present a scanning electron microscopy study that combines the structural characterization of electron channeling contrast imaging with the optical characterization of cathodoluminescence hyperspectral imaging on a (101¯3) GaN layer. An extensive defect analysis revealed that the dominant defects consist of basal plane stacking faults (BSFs), prismatic stacking faults, partial dislocations, and threading dislocations. With a defect density of about an order of magnitude lower than in comparable. The optical properties of the defects have been characterized from 10 to 320 K, showing BSF luminescence at room temperature indicating a reduced density of non-radiative recombination centers in the as-grown samples compared to established semi- and non-polar orientations. Our findings suggest that growth along (101¯3) has the potential for higher radiative efficiency than established semi-polar orientations.</jats:p