Mode disorder in elliptical hole PCFs

In the elliptical-hole photonic crystal fibers, when the central defect air hole is large enough, the modes will be disordered. The order of the HE₁₁-like mode is investigated in detail based on the full vector supercell overlapping method. The degeneracy will be enhanced at the frequency where the modes begin to be disordered.     

Synthesis and characterization of tungsten trioxide powders prepared from tungstic acids

WO₃ powders were prepared by the thermal decomposition of tungstic acids (WO₃·nH₂O, n=1/3, 1, 2). The tungstic acids were synthesized from WO₄²⁻ aqueous solutions under a variety of conditions of pH, temperature and W(VI) concentrations. The thermal decomposition of the tungstic acids into WO₃ was analysed by TG and DSC methods. Nano-sized WO₃ powders with different morphological characteristics were obtained by thermal treatment of the tungstic acids at 500 °C in air atmosphere. The morphologies of WO₃ powders were characterised by scanning electron microscopy and infrared absorption spectroscopy. Patterns of infrared spectra were related with distinct powder morphologies.     

Nanoscale azo pigment immobilized on carbon nanotubes via liquid phase reprecipitation approach

Nanoscale azo pigment immobilized on the outer shell of multiwalled carbon nanotubes (MWCNT–AZO) were prepared by modified liquid phase reprecipitation method, and the MWCNT–AZO hybrid was characterized by means of TEM, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–VIS absorption. The photoconductivity of single-layered photoreceptors, where MWCNT–AZO served as the charge generation material (CGM), was studied by the xerographic photoinduced discharge method. The results indicated that the MWCNT–AZO nano hybrid showed broader and enhanced photosensitivity than MWCNT/bulk azo pigments (AZO) blend or the bulk AZO did, which was interpreted in terms of nanometer size effect of MWCNT–AZO hybrid and charge transfer from AZO nanoparticles to MWCNT.     

The effect of sintering on the properties of Ba0.7Sr0.3TiO3 ferroelectric films produced by electrophoretic deposition

Ba_0.7Sr_0.3TiO₃ (BST) films were produced on metal substrate using electrophoretic deposition (EPD) followed by high temperature sintering. The composition, crystal structure and microstructure of the deposited films were characterised using scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The sintered BST films showed a sharp perovskite structure with uniform microstructure. The results indicated that a typical BST film of 20 μm thick after sintering for 2–10 h exhibited the relative dielectric constant ranging from 3100 to 5100 at 20 °C and dissipation factor of 0.061–0.118 at 1 kHz. The density, crystallinity, microstructural uniformity and dielectric properties of the BST films reached their best values after 8 h annealing at 1350 °C. These films also possess excellent adhesion to the substrate.     

Evolution of phases during mechanochemical activation in magnetite-containing systems

In a first experiment, we subjected a mixture of Fe₃O₄ and Fe (80–20 wt.%) to mechanochemical activation by high-energy ball milling, for time periods ranging from 0.5 to 14 h. Complementary X-ray diffraction (XRD) and Mössbauer spectroscopy data demonstrated a phase transformation of magnetite to hematite, accompanied by a partial oxidation of iron to hematite. The reaction can be used to obtain nanometer-size magnetite by ball milling, due to the inhibition of its transformation to hematite, caused by the presence of iron atoms. In a second experiment, we ball-milled Fe₃O₄:Co²⁺ (Fe_3−xCo_xO₄ with x=0.1) for time intervals between 2.5 and 17.5 h. Our XRD and Mössbauer measurements showed that the cobalt-doped magnetite undergoes a phase transformation to hematite, which is actually cobalt-doped hematite. We were able to show herewith that the Co ion is not kicked out of the lattice during the milling process, but undergoes the phase transformation inside the hematite lattice. Finally, we exposed a mixture of Fe₃O₄ and Co (80–20 wt.%) to mechanochemical activation for time periods ranging from 0.5 to 10 h. The XRD and Mössbauer results are consistent with the formation of cobalt ferrite (a strongly Co-substituted magnetite), with the occurrence of hematite as an intermediate product. In all three cases, the milling-induced phase transformations started with a considerable disorder of the octahedral sublattice of magnetite.     

温度调谐掺Yb3+ 双包层光纤激光器研究

实验研究了掺Yb^3＋双包层光纤激光器的温度调谐特性。当光纤的温度从13℃变化到95℃时,输出激光的中心波长从1084．9nm增加到1096．3nm,输出激光功率从0．95W减小到0．58W。波长的调谐范围为11．4nm。理论分析表明,当光纤的温度升高时,高于零点的增益范围向长波方向移动,同时增益呈下降趋势。理论分析可以很好地解释实验现象。     

Electronic and thermal transport in hot carrier solar cells with low-dimensional contacts

Hot carrier solar cells are a third generation solar cell device where electrons and holes, heated by solar radiation, are removed from the absorber via low-dimensional energy selective contacts before they can thermalise to the band edge. Here, a new model is presented for calculating the performance of these devices, which takes into account the energy spectrum of the contacts. It is shown that efficiency is maximised with a certain ideal number of contacts and that the energy spectra of these should be narrow.     

Negative bias temperature instability in n-channel power VDMOSFETs

Negative gate bias is used in some applications for faster switching off the n-channel MOS devices. It is shown in this study that NBT stress-related instability in commercial n-channel power VDMOSFETs could be actually more serious than in corresponding p-channel devices. NBT stress is found to create equal V_T shifts in both device types, whereas the subsequent positive bias annealing results in more serious overall V_T instability in n-channel devices. The changes in the densities of stress-induced interface traps in two device types are equal as well, but significant amounts of NBT stress-induced border traps are only found in n-channel devices. All the results are discussed in terms of hydrogen reaction and diffusion model.     

A formal framework for business process modelling and design

We present a formal framework for enterprise and business process modelling. The concepts of our framework (objectives and goals, roles and actors, actions and processes, responsibilities and constraints) allow business analysts to capture enterprise knowledge in a way that is both intuitive and mathematically formal. We also outline the basic steps of a methodology that allows business analysts to produce detailed, formal specifications of business processes from high-level enterprise objectives. The use of a formal language permits us to verify that the specifications possess certain correctness properties, namely that the responsibilities assigned to roles are fulfilled, and that constraints are maintained as a result of process execution.     

Stochastic finite element analysis of shells

In the present paper the stochastic formulation of the triangular composite (TRIC) facet shell element is presented using the weighted integral and local average methods. The elastic modulus of the structure is considered to be a two-dimensional homogeneous stochastic field which is represented via the spectral representation method. As a result of the proposed derivation and the special features of the element, the stochastic stiffness matrix is calculated in terms of a minimum number of random variables of the stochastic field giving a cost-effective stochastic matrix. Under the assumption of a pre-specified power spectral density function of the stochastic field, it is possible to compute the response variability of the shell structure. Numerical tests are provided to demonstrate the applicability of the proposed methodologies.