Soft particle model for block copolymers

A soft particle model for diblock (AB) copolymer melts is proposed. Each molecule is mapped onto two soft spheres built by Gaussian A- and B-monomer distributions. An approximate analytical expression for the joint distribution function for the distance between both spheres and their radii of gyration is derived, which determines the entropic contribution to the intramolecular free energy. Adding a mean-field expression for the intermolecular interactions, we obtain the total free energy of the system. Based on this free energy, Monte Carlo simulations are carried out to study the kinetics of microphase ordering in the bulk and its effect on molecular diffusion. This is followed by an analysis of thin films, with emphasis on pattern transfer from walls with a periodic structure. It is shown that the level of coarse graining in the soft particle model is suitable to describe structural and kinetic properties of copolymers on mesoscopic scales.     

Constant dielectric loss in disordered ionic conductors: Theoretical aspects

Cooperative charge relaxation within a random system of electrostatically interacting defect centers provides a mechanism for a “nearly constant dielectric loss” (NCL) response in structurally disordered ionic conductors. Pertinent models based on statistical mechanics are reviewed briefly. In addition, we present a theoretical frame for the problem of how two kinds of ionic motion, hopping migration and NCL-type local charge relaxation, are superimposed in the total ac-response. Using renewal theory, the modification of NCL-spectra due to hopping is calculated in terms of the waiting time distribution for ionic hops. In the special case of Poissonian hops with average rate λ the modified complex dielectric susceptibility in the NCL-regime is obtained by analytic continuation from the corresponding susceptibility in the absence of hops. Implications with respect to the crossover between hopping transport and NCL-behavior are discussed.     

Sparsity reconstruction in electrical impedance tomography: An experimental evaluation

We investigate the potential of sparsity constraints in the electrical impedance tomography (EIT) inverse problem of inferring the distributed conductivity based on boundary potential measurements. In sparsity reconstruction, inhomogeneities of the conductivity are a priori assumed to be sparse with respect to a certain basis. This prior information is incorporated into a Tikhonov-type functional by including a sparsity-promoting ?¹-penalty term. The functional is minimized with an iterative soft shrinkage-type algorithm. In this paper, the feasibility of the sparsity reconstruction approach is evaluated by experimental data from water tank measurements. The reconstructions are computed both with sparsity constraints and with a more conventional smoothness regularization approach. The results verify that the adoption of ?¹-type constraints can enhance the quality of EIT reconstructions: in most of the test cases the reconstructions with sparsity constraints are both qualitatively and quantitatively more feasible than that with the smoothness constraint.     

Static and Time Dependent Density Functional Theory with Internal Degrees of Freedom: Merits and Limitations Demonstrated for the Potts Model

We present an extension of the density-functional theory (DFT) formalism for lattice gases to systems with internal degrees of freedom. In order to test approximations commonly used in DFT approaches, we investigate the statics and dynamics of occupation (density) profiles in the one-dimensional Potts model. In particular, by taking the exact functional for this model we can directly evaluate the quality of the local equilibrium approximation used in time-dependent density-functional theory (TDFT). Excellent agreement is found in comparison with Monte Carlo simulations. Finally, principal limitations of TDFT are demonstrated.     

Modelling of the Positive Column in an Argon-Hexamethyl-disiloxane dc Glow Discharge

A model is presented of the positive column of a dc glow discharge in argon with small admixtures of hexamethyldisiloxane (HMDS). The axial electric field, the ion production rates for direct-, stepwise-, pair-, and Penning ionization, the densities of metastable Ar atoms and of electrons, and the wall current of HMDS ions are calculated in dependence on HMDS admixture and discharge current density. For the calculations particle balance equations were used for a diffusion determined plasma in a mixture of two gaseous components. The reaction rates for the electron collision processes were determined applying the electron distribution function calculated for pure argon. Taking into account PENNING ionization of HMDS molecules by metastable argon atoms the decrease of electric field for increasing HMDS admixtures is according to the experimen-tally measured values. Also ion wall currents and electron densities are compared with experimen-tal values for thin film formation rate and results of probe measurements.     

Struktur und Bindungscharakter von Ba3P2

The structure of the compound Ba₃P₂ was elucidated. It cristallizes with the inverse Th₃P₄ structure, or strictly, with the inverse Ce₂S₃ structure with randomly distributed phosphorus vacancies. The lattice parameter a₀ is 9.775 Å, the x-ray density dx is 4.52 g/cm³. The coordinate parameter x of the barium sites is 0.0665. Barium is surrounded by 6 phosphorus and 5 barium atoms; the distance of 3,81 Å between 3 of the barium atoms is extremely short. Phosphorus is surrounded by 8 barium atoms. The phosphorus atoms are isolated from one another. The structure is best described as a filling of the octahedral spaces in the bee lattice of the barium metal by phosphorus atoms. The partially negative charge of the phosphorus atoms (P^? + 2e ? P^?? + e) causes a strong distortion of the interstitial lattice. The bonding character of Ba₃P₂ has to be interpreted as mixed metallic-ionic.     

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