Configurational-Force-Based Adaptive FE Solver for a Phase Field Model of Fracture

The computational modeling of failure mechanisms in solids due to fracture based on sharp crack discontinuities suffers in situations with complex crack topologies. This can be overcome by diffusive crack modeling, based on the introduction of a crack phase field as outlined in [1, 2]. Following these formulations, we outline a thermodynamically consistent framework for phase field models of crack propagation in elastic solids, develop incremental variational principles and, as an extension to [1, 2], consider their numerical implementations by an efficient h-adaptive finite element method. A key problem of the phase field formulation is the mesh density, which is required for the resolution of the diffusive crack patterns. To this end, we embed the computational framework into an adaptive mesh refinement strategy that resolves the fracture process zones. We construct a configurational-force-based framework for h-adaptive finite element discretizations of the gradient-type diffusive fracture model. We develop a staggered computational scheme for the solution of the coupled balances in physical and material space. The balance in the material space is then used to set up indicators for the quality of the finite element mesh and accounts for a subsequent h-type mesh refinement. The capability of the proposed method is demonstrated by means of a numerical example. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The h-SbxWO3+2x Oxygen Excess Antimony Tungsten Bronze

We describe the previously unreported oxygen excess hexagonal antimony tungsten bronze with composition Sb_0.5W₃O₁₀, in the following denoted as h-Sb_xWO_3+2x with x=0.167, to demonstrate its analogy to classical A_xWO₃ tungsten bronzes. This compound forms in a relatively narrow temperature range between 580 °C<T<620 °C. It was obtained as a dark-blue polycrystalline powder, and as thin, needle-shaped, blue single crystals. h-Sb_xWO_3+2x crystallizes in the hexagonal space group P6/mmm with the cell parameters a=7.4369(4) Å and c=3.7800(2) Å. The antimony and excess oxygen occupy the hexagonal channels within the network of corner-sharing WO₆ octahedra. h-Sb_xWO_3+2x has a resistivity of ρ_{300 K}≈1.28 mΩ cm at room temperature, with little if any temperature-dependence on cooling. DFT calculations on a simplified model for this compound find a metallic-like electronic structure with the Fermi level falling within rather flat bands, especially around the Γ point.     

Persistent Room Temperature Phosphorescence from Triarylboranes: A Combined Experimental and Theoretical Study

Achieving highly efficient phosphorescence in purely organic luminophors at room temperature remains a major challenge due to slow intersystem crossing (ISC) rates in combination with effective non‐radiative processes in those systems. Most room temperature phosphorescent (RTP) organic materials have O‐ or N‐lone pairs leading to low lying (n, π*) and (π, π*) excited states which accelerate k_isc through El‐Sayed's rule. Herein, we report the first persistent RTP with lifetimes up to 0.5 s from simple triarylboranes which have no lone pairs. RTP is only observed in the crystalline state and in highly doped PMMA films which are indicative of aggregation induced emission (AIE). Detailed crystal structure analysis suggested that intermolecular interactions are important for efficient RTP. Furthermore, photophysical studies of the isolated molecules in a frozen glass, in combination with DFT/MRCI calculations, show that (σ, B p)→(π, B p) transitions accelerate the ISC process. This work provides a new approach for the design of RTP materials without (n, π*) transitions.     

Functional Additive Mixed Models

We propose an extensive framework for additive regression models for correlated functional responses, allowing for multiple partially nested or crossed functional random effects with flexible correlation structures for, for example, spatial, temporal, or longitudinal functional data. Additionally, our framework includes linear and nonlinear effects of functional and scalar covariates that may vary smoothly over the index of the functional response. It accommodates densely or sparsely observed functional responses and predictors which may be observed with additional error and includes both spline-based and functional principal component-based terms. Estimation and inference in this framework is based on standard additive mixed models, allowing us to take advantage of established methods and robust, flexible algorithms. We provide easy-to-use open source software in the pffr() function for the R package refund. Simulations show that the proposed method recovers relevant effects reliably, handles small sample sizes well, and also scales to larger datasets. Applications with spatially and longitudinally observed functional data demonstrate the flexibility in modeling and interpretability of results of our approach.     

Regularity of Local Times Associated with Volterra–Lévy Processes and Path-Wise Regularization of Stochastic Differential Equations

Journal of Theoretical Probability - We investigate the space-time regularity of the local time associated with Volterra–Lévy processes, including Volterra processes driven by $$\alpha...     

Precision measurements with LPCTrap at GANIL

The experimental achievements and the results obtained so far with the LPCTrap device installed at GANIL are presented. The apparatus is dedicated to the study of the weak interaction at low energy by means of precise measurements of the β ? ν angular correlation parameter in nuclear β decays. So far, the data collected with three isotopes have enabled to determine, for the first time, the charge state distributions of the recoiling ions, induced by shakeoff process. The analysis is presently refined to deduce the correlation parameters, with the potential of improving both the constraint deduced at low energy on exotic tensor currents (⁶He¹⁺) and the precision on the V _{u d} element of the quark-mixing matrix (³⁵Ar¹⁺ and ¹⁹Ne¹⁺) deduced from the mirror transitions dataset.     

Light‐induced degradation of PECVD aluminium oxide passivated silicon solar cells

Light‐induced degradation (LID) has been identified to be a critical issue for solar cells processed on boron‐doped silicon substrates. Typically, Czochralski‐grown silicon (Cz‐Si) has been reported to suffer from stronger LID than block‐cast multicrystalline silicon (mc‐Si) due to higher oxygen concentrations. This work investigates LID under conditions practically relevant under module operation on different cell types. It is shown that aluminium oxide (AlOx) passivated mc‐Si solar cells degrade more than a reference aluminium back surface field mc‐Si cell and, remarkably, an AlOx passivated Cz‐Si solar cell. The defect which is activated by illumination is shown to be doubtful a sole bulk effect while the AlOx passivation might play a certain role. This work may contribute to a re‐evaluation of the suitability of boron‐doped Cz‐ and mc‐Si for solar cells with very high efficiencies. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Convergence theory for nonconvex stochastic programming with an application to mixed logit

Monte Carlo methods have extensively been used and studied in the area of stochastic programming. Their convergence properties typically consider global minimizers or first-order critical points of the sample average approximation (SAA) problems and minimizers of the true problem, and show that the former converge to the latter for increasing sample size. However, the assumption of global minimization essentially restricts the scope of these results to convex problems. We review and extend these results in two directions: we allow for local SAA minimizers of possibly nonconvex problems and prove, under suitable conditions, almost sure convergence of local second-order solutions of the SAA problem to second-order critical points of the true problem. We also apply this new theory to the estimation of mixed logit models for discrete choice analysis. New useful convergence properties are derived in this context, both for the constrained and unconstrained cases, and associated estimates of the simulation bias and variance are proposed. Research Fellow of the Belgian National Fund for Scientific Research