Conduction Band-Edge Non-Parabolicity Effects on Impurity States in （In,Ga）N/GaN Cylindrical QWWs

In this paper, the conduction band-edge non-parabolicity （NP） and the circular cross-section radius effects on hydrogenie shallow-donor impurity ground-state binding energy in zinc-blende （ZB） InGaN/GaN cylindrical QWWs are reported. The finite potential barrier between （In,Ga）N web and CaN environment is considered. Two models of the conduction band-edge non-parabolicity are taking into account. The variational approach is used within the framework of single band effective-mass approximation with one-parametric 1S-hydrogenic triaJ wave-function. It is found that NP effect is more pronounced in the wire of radius equal to effective Bohr radius than in large and narrow wires. Moreover, the binding energy peak shifts to narrow wire under NP effect. A good agreement is shown compared to the findings results.     

Synthesis and properties of novel diisocyanate based optically active polyimides

A new optically active diisocyanate was prepared from the reaction of l-leucine and pyromellitic dianhydride (PMDA) and subsequent transformation of intermediate imide-containing diacid to diisocyanate via Weinstock modification of Curtius rearrangement. Solution polycondensation reaction of the prepared diisocyanate with PMDA, 3,3,4,4-benzophenonetetracarboxylic dianhydride, and hexafluoroisopropylidene 2,2-bis(phthalic anhydride) resulted in the preparation of novel optically active polyimides. The optimal conditions for polyimidations (reaction duration and temperature programming) were obtained via study of the model compound. The monomer, model compound and polymers were characterized by FTIR, ¹HNMR, mass spectroscopy and elemental analysis and their optical and physical properties were studied as well.     

Experimental buckling of GRP cylindrical shells

During the setup of an experiment, errors may occur. Sources of such errors may be due to several factors which sometimes accumulate and then cause erroneous results. An experimental investigation on buckling of GRP (glass-reinforced-plastic) cylindrical shells, subject to axial compression and/or external pressure loading, has been carried out. At the beginning of the experiment, the initial geometrical imperfections were measured. Because of the small size of these quantities and the great effect these imperfections had on buckling loads, any small errors in the measurement procedure may lead to unreasonable results. Attempts have been made to detect these errors, and to identify and minimize their effect on experimental results. Tables are provided to show a comparison between the final experimental results and the corresponding theoretical ones.     

Optical characterization of InGaN/AlGaN/GaN diode grown on silicon carbide

Electroluminescence (EL) properties of In_xGa_1−xN/Al_yGa_1−yN/GaN/SiC diode were studied. The spectral range for which EL spectra were recorded is 1–3.5 eV. Room temperature EL was obtained for forward bias (3.18 V, 220 μA) at 446.067 nm (blue luminescence band), 606.98 nm (yellow luminescence band) and 893.84 nm (Infrared luminescence band). The EL temperature dependence shows that, BL band is mostly given by e–h recombination corresponding to indium composition equal to 0.17 ± 0.01 and 0.14 ± 0.02 obtained theoretically and experimentally, respectively. The yellow band is generally weak and absent at low temperature. The IRL band is more consistent with the DAP recombination and could be explained by the thermal activation of Mg states. The luminescence bands shift to lower energies is due probably to the larger potential fluctuations effect.     

Driving Potentials of Heat and Mass Transport in Porous Building Materials: A Comparison Between General Linear,Thermodynamic and Micromechanical Derivation Schemes

In systems of coupled transport processes the question of the appropriate driving potentials is a point of discussion. In this article, three different approaches to derive models for transport currents are systematically compared. According to a general linear approach, an arbitrary full set of independent state variables and material properties is sufficient to describe any transport current. This approach is derived here from a symmetry principle. Thermodynamic and micromechanical approaches are more complex and even less general, but they allow additional statements about the transport coefficients and they reduce the number of transport processes. In the thermodynamic approach the additional information stems from the calculation of the entropy production rate; the micromechanical approach involves a microphysical model of the considered porous system. As a practical example, the three derivation schemes are applied to the often-encountered case of non-hysteretic heat and moisture transport in homogeneous building materials. It is shown, how the general state variables of a porous system are reduced to only two. Then from the general linear approach it can be seen, that all equations for the moisture transport current using a main driving potential (e.g. moisture content, vapour pressure, chemical potential) and an independent secondary driving potential (e.g. temperature, liquid pressure) are equivalent, without recurrence to the thermodynamic or micromechanical approach. However, the transport coefficients are arbitrary phenomenological functions depending on the two state variables. Based on a literature survey it is shown, which additional statements can be made in the thermodynamic and in the micromechanical approach. The latter yields the pressure-driven model (vapour and liquid pressure as the two driving potentials). Finally it is shown, what is to be expected, if in more complex systems the number of state variables increases.     

Full-scale, low-temperature mechanical testing of prestressing systems

For a number of industrial applications, concrete prestressing elements are exposed to extremely low temperatures during their lifetime. This can cause concerns regarding their possible embrittlement. The investigation of the mechanical properties of individual materials of these systems at different temperatures is a routine task. Numerical methods are a reliable method for the calculation of stress and strain fields in complex geometry systems and load patterns, as long as a good knowledge of initial and boundary conditions is available. Temperature-dependent material properties can also be taken into consideration. The increased complexity of the calculations for a system under thermal and mechanical loads leads to a higher level of uncertainty in the results, as its cost in terms of required input and computing time increases. The obstacles in the way to a reliable numerical assessment of the safety of the operation of a prestressing system under extreme thermal conditions make the execution of full-scale system tests advantageous, in spite of the costs arising from the development of suitable technical means.     

Asymmetric hydrogenation of imines with a recyclable chiral frustrated Lewis pair catalyst

A camphor based chiral phosphonium hydrido borate zwitterion was synthesised and successfully applied in the enantioselective hydrogenation of imines with selectivities up to 76% ee. The high stability of the novel chiral FLP-system enables effective recycling of the metal-free catalyst.