Control of an external-cavity laser diode operating in the regular pulse package regime with a frequency shift of the optical feedback

I demonstrate numerically that the regular pulse package regime observed in an external-cavity laser diode can be controlled by means of an adequate shift of the optical feedback frequency. This control leads to a stable pulsed behavior.     

Current Status and New Perspectives on Chitin and Chitosan as Functional Biopolymers

The natural biopolymer chitin and its deacetylated product chitosan are found abundantly in nature as structural building blocks and are used in all sectors of human activities like materials science, nutrition, health care, and energy. Far from being fully recognized, these polymers are able to open opportunities for completely novel applications due to their exceptional properties which an economic value is intrinsically entrapped. On a commercial scale, chitosan is mainly obtained from crustacean shells rather than from the fungal and insect sources. Significant efforts have been devoted to commercialize chitosan extracted from fungal and insect sources to completely replace crustacean-derived chitosan. However, the traditional chitin extraction processes are laden with many disadvantages. The present review discusses the potential bioextraction of chitosan from fungal, insect, and crustacean as well as its superior physico-chemical properties. The different aspects of fungal, insects, and crustacean chitosan extraction methods and various parameters having an effect on the yield of chitin and chitosan are discussed in detail. In addition, this review also deals with essential attributes of chitosan for high value-added applications in different fields and highlighted new perspectives on the production of chitin and deacetylated chitosan from different sources with the concomitant reduction of the environmental impact.     

Predicting ductile fracture of low carbon steel sheets: Stress-based versus mixed stress/strain-based Mohr–Coulomb model

Two distinct implementations of the Mohr–Coulomb failure model are used in conjunction with a non-associated quadratic plasticity model to describe the onset of fracture in low carbon steel sheets. The stress-based version corresponds to the original Mohr–Coulomb model in stress space. For the mixed stress/strain-based version, the Mohr–Coulomb failure criterion is first transformed into the space of stress triaxiality, Lode angle parameter and equivalent plastic strain and then used as stress-state dependent weighting function in a damage indicator model. Basic fracture experiments including tensile specimens of different notch radii and a punch test are performed to calibrate the material parameters of the respective models. Subsequently, the models are used to predict the crack initiation in a Hasek test and during the stamping of an anticlastic structure. Unlike for the calibration experiments, the loading history during stamping is highly non-linear. Both models can be calibrated with similar accuracy, but the strain-based model predicts the instant of onset of fracture with greater accuracy in the stamping experiment which is an advantage of the empirical damage accumulation rule.     

Numerical modeling of guided wave interaction with non-axisymmetric cracks in elastic cylinders

A three dimensional (3D) hybrid method combining the classical finite element (FE) method with the semi-analytical finite element (SAFE) technique is developed. This hybrid method is employed to study the interaction of guided waves with non-axisymmetric damages in cylinders. The near field surrounding the damage is analysed with the 3D FE method. The solution is expanded into sums of guided modes on both inlet and outlet cross-sections. Such eigenmode expansions enable separation into ingoing and outgoing waves, i.e., incident, reflected and transmitted waves. Using the SAFE method, elastic guided modes are then computed at the aforementioned cross-sections thus reducing the analysis to two dimensions (2D). The amplitudes of the incident modes are imposed, whereas those of the scattered modes are determined by solving the global system of the 3D hybrid FE-SAFE model. In this paper, a formula is proposed for the calculation of eigenforces and modal power flows from eigendisplacements and SAFE matrices. This has the advantage of simplifying the post-process of load eigenvectors in hybrid FE-SAFE methods. Results obtained for a vertical free-end cylinder are in good agreement with those published in the literature. Moreover, first results of the interaction of the fundamental compressional, flexural and torsional Pochhammer–Chree modes with non-axisymmetric vertical cracks are obtained and discussed. Then, the interactions of the fundamental compressional mode with oblique free-ends and cracks are briefly addressed. The power balance is shown to be satisfied with a good accuracy.     

Design and demonstration of a high-energy booster amplifier for a high-repetition rate petawatt class laser system

We have successfully developed a high-energy, high-repetition rate Ti:sapphire laser system that delivers 33 J before compression at 0.1 Hz. The final booster amplifier is based on a 100 mm diameter Ti:sapphire crystal pumped with 72 J of energy in six beams delivered by three frequency-doubled high-repetition rate Nd:glass lasers. This system is, to the best of our knowledge, the first demonstrated petawatt class laser system running at a high repetition rate.     

Theoretical studies of the tunneling splitting of malonaldehyde using the multiconfiguration time-dependent Hartree approach

Full dimensional multi-configuration time-dependent Hartree calculations of the zero point energy and the tunneling splitting of malonaldehyde using a recently published potential energy surface [Y. Wang, B. J. Braams, J. M. Bowman, S. Carter, and D. P. Tew, J. Chem. Phys. 128, 224314 (2008)] are reported. The potential energy surface has been approximated by a modified version of the n-mode representation and careful convergence check has been performed to ensure accurate results. The obtained value for the splitting (23.4 cm(-1)) is in acceptable agreement with the experimental value of 21.583 cm(-1). The computed zero-point-energy is 14,670 cm(-1) which is lower than previous results of Wang et al., but likely to be about 4 cm(-1) too low because of shortcomings of the n-mode representation of the potential. The energies reported in this abstract contain a correction to account for neglected vibrational angular momentum terms.     

Debundling, selection and release of SWNTs using fluorene-based photocleavable polymers

Photocleavable polymers based on 9,9-dialkylfluorene backbone and o-nitrobenzylether were designed and synthesized to obtain stable (n,m) enriched suspensions of semiconducting SWNTs in toluene. Photoirradiation of the suspensions triggered the precipitation of the SWNTs and TEM images indicate close packing of SWNTs pointing at partial removal of the coating polymer.     

Kinetics of iron–copper sulphides oxidation in relation to protohistoric copper smelting

This article deals with one specific step of the copper extractive metallurgy process: the roasting of iron–copper sulphides. It aims at shedding light on an archaeological issue: the reconstruction of the copper extractive metallurgy processes during protohistory (IV^e–II^e millennium BC). Experimental simulations are performed at laboratory scale by modelizing the conditions of protohistoric furnaces. Kinetic of roasting is studied by thermogravimetry combined with the physico-chemical analysis of synthetic products. The influence of two parameters is studied: (i) the temperature (773, 973 and 1173 K) and (ii) the granularity of the roasted ores (1 mm and 100 μm). In each case, the chemical mechanism governing the oxidation of iron copper sulphide is proposed. Apart from one extreme case (∅ = 1 mm; T = 773 K), it is showed that kinetic is controlled by the transport of molecular oxygen (O₂) from the gas to the grain surface. Moreover, we prove that, in some cases where the diffusivity of gaseous oxygen is low, roasting can be accelerated by the presence of an oxide, which constitute an in-situ source of oxygen. Theses experiments support the hypothesis that such a technique could have allowed a roasting process where iron and sulfur were removed by the solid oxygen instead of the gaseous oxygen. These results allow to validate a one-step copper smelting process starting from sulphidic ores, and to identify the experimental parameters of this process.     

Die Analyse von Wolframbronce

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