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.     

Comparison of non-stationary time series in the frequency domain

In this paper we compare two non-stationary time series using non-parametric procedures. Evolutionary spectra are estimated for the two series. Randomization tests are performed on groups of spectral estimates for both related and independent time series. Simulation studies show that in certain cases the tests perform reasonably well. The tests are applied to observed geological and financial time series.     

Global information technology: a meta analysis of key issues

Recently, there has been an increasing number of studies and articles that have identified and ranked global information technology (IT) issues in various parts of the world. The objective of this article is to aggregate the results of these studies and explore the possibility of linkage of these key IT issues to environmental and organizational factors. An organizing framework for global IT issues is developed and one of the factors in the framework, the level of economic development of the country, is analyzed to observe its impact on the ranking of key IT issues. Cluster analysis on a sample of ranked IT issues from 16 different regions indicates a linkage between the level of economic development of a region and the ranking of various types of IT issues. The analysis provides stronger support for the three-way classification of regions into developed, developing, and under-developed rather than the more recent four-way classification of developed, newly industrialized, developing, and under-developed.     

A general numerical model for grounding analysis in layered soils

Safety of persons, protection of equipment and continuity of power supply are the main objectives of the grounding system of a large electrical installation. For its accurate design, it is essential to determine the potential distribution on the earth surface and the equivalent resistance of the system. In this paper, we present a numerical approach based on the boundary element method for grounding analysis in layered soils. The feasibility of this formulation is discussed by means of its application to a real grounding system with different kinds of layered soil models.     

Digital rebalance loop design for a dynamically tuned gyroscope using H2 methodology

Presented in this paper is a wide-bandwidth digital torquing rebalance loop for a dynamically tuned gyroscope (DTG) based on H₂ methodology. The role and the importance of a rebalance loop are explained first. The augmented plant model including gyroscope and integrator is composed. Next, an H₂-based controller is designed for the augmented plant model. Frequency analyses show that the proposed controller maintains a linear relationship between applied input rate and the output of the controller over a wide operating range and keeps the tilt angle small thereby reducing dynamic errors. To verify the performance of the controller in a real environment, a digital rebalance loop for a DTG is designed, fabricated, and examined. From simulation and experimental results, it is confirmed that the controller is robustly stable and has a wide bandwidth, significantly improving the performance of a DTG.