High-resolution nuclear magnetic resonance spectra from samples located in inhomogeneous static and radio frequency magnetic fields can be obtained by applying a train of z-rotation radio frequency pulses to repeatedly refocus the inhomogeneous broadening during signal detection. z-rotation pulses based on an adiabatic double passage are effective over wide bandwidths using a limited amount of radio frequency power at the expense of being time consuming and, consequently, sensitive to motion of the spin bearing molecules. The signal damping resulting from molecular self-diffusion during the pulse was studied experimentally and using Brownian dynamics simulations. The results show that the analytical expression for diffusion damping during a double spin echo is a reasonable approximation for the signal decay during an adiabatic z-rotation pulse. Methods to alleviate the effects of diffusion are discussed. 相似文献
Current efficient magnetic resonance imaging (MRI) methods such as parallel-imaging and k-t methods encode MR signals using a set of effective encoding functions other than the Fourier basis. This work revisits the proposition of directly manipulating the set of effective encoding functions at the radiofrequency excitation step in order to increase MRI efficiency. This approach, often termed "broadband encoding," enables the application of algebraic matrix factorization technologies to extract efficiency by representing and encoding MR signal content in a compacted form. Broadband imaging equivalents of fast multiecho, parallel and k-t MRI are developed and analyzed. The potential of these techniques to increase the time efficiency of data acquisition is experimentally verified on a commercial MRI scanner using simple spin-echo imaging. A three-dimensional gradient-echo dynamic imaging application that demonstrates the potential benefits of this approach compared to the present state of the art for certain applications is also presented. 相似文献
Ketoreductases were utilized for the stereoselective synthesis of δ-hydroxy-β-keto esters, β-hydroxy-δ-keto esters, and β,δ-dihydroxy esters. Seven out of eight possible stereoisomers were obtained from the enzymatic reduction of the corresponding β,δ-diketo ester in high enantio- and diastereomeric excess. 相似文献
A modified spin-flip model is used to investigate the stability of optically-pumped quantum-dot spin-vertical cavity surface-emitting lasers. in terms of pump intensity and polarization. The stability maps exhibit pronounced polarization switching in the stable regions. Periodic oscillations in unstable regions are attributed to competition between spin relaxation and birefringence. 相似文献
Spatial planning is an important and complex activity. It includes land use planning and resource allocation as basic components. An abundance of papers can be found in the literature related to each one of these two aspects separately. On the contrary, a much smaller number of research reports deal with both aspects simultaneously. This paper presents an innovative evolutionary algorithm for treating combined land use planning and resource allocation problems. The new algorithm performs optimization on a cellular automaton domain, applying suitable transition rules on the individual neighbourhoods. The optimization process is multi-objective, based on non-domination criteria and self-organizing. It produces a Pareto front thus offering an advantage to the decision maker, in comparison to methods based on weighted-sum objective functions. Moreover, the present multi-objective self-organizing algorithm (MOSOA) can handle both local and global spatial constraints. A combined land use and water allocation problem is treated, in order to illustrate the cellular automaton optimization approach. Water is allocated after pumping from an aquifer, thus contributing a nonlinearity to the objective function. The problem is bi-objective aiming at (a) the minimization of soil and groundwater pollution and (b) the maximization of economic profit. An ecological and a socioeconomic constraint are imposed: (a) Groundwater levels at selected places are kept above prescribed thresholds. (b) Land use quota is predefined. MOSOA is compared to a standard multi-objective genetic algorithm and is shown to yield better results both with respect to the Pareto front and to the degree of compactness. The latter is a highly desirable feature of a land use pattern. In the land use literature, compactness is part of the objective function or of the constraints. In contrast, the present approach renders compactness as an emergent result. 相似文献
A constitutive theory is developed for shape memory polymers. It is to describe the thermomechanical properties of such materials under large deformations. The theory is based on the idea, which is developed in the work of Liu et al. [2006. Thermomechanics of shape memory polymers: uniaxial experiments and constitutive modelling. Int. J. Plasticity 22, 279-313], that the coexisting active and frozen phases of the polymer and the transitions between them provide the underlying mechanisms for strain storage and recovery during a shape memory cycle. General constitutive functions for nonlinear thermoelastic materials are used for the active and frozen phases. Also used is an internal state variable which describes the volume fraction of the frozen phase. The material behavior of history dependence in the frozen phase is captured by using the concept of frozen reference configuration. The relation between the overall deformation and the stress is derived by integration of the constitutive equations of the coexisting phases. As a special case of the nonlinear constitutive model, a neo-Hookean type constitutive function for each phase is considered. The material behaviors in a shape memory cycle under uniaxial loading are examined. A linear constitutive model is derived from the nonlinear theory by considering small deformations. The predictions of this model are compared with experimental measurements. 相似文献
A constitutive theory is developed for shape memory polymers. It is to describe the thermomechanical properties of such materials under large deformations. The theory is based on the idea, which is developed in the work of Liu et al. [2006. Thermomechanics of shape memory polymers: uniaxial experiments and constitutive modeling. Int. J. Plasticity 22, 279-313], that the coexisting active and frozen phases of the polymer and the transitions between them provide the underlying mechanisms for strain storage and recovery during a shape memory cycle. General constitutive functions for nonlinear thermoelastic materials are used for the active and frozen phases. Also used is an internal state variable which describes the volume fraction of the frozen phase. The material behavior of history dependence in the frozen phase is captured by using the concept of frozen reference configuration. The relation between the overall deformation and the stress is derived by integration of the constitutive equations of the coexisting phases. As a special case of the nonlinear constitutive model, a neo-Hookean type constitutive function for each phase is considered. The material behaviors in a shape memory cycle under uniaxial loading are examined. A linear constitutive model is derived from the nonlinear theory by considering small deformations. The predictions of this model are compared with experimental measurements. 相似文献
Investigations of thermotropic phase transitions performed on organocyclosiloxanes [PhSi(O)OSiR]6, where R is Me3, Me2(CH2Cl) or Me2(CH≃CH2), have revealed that all these hexamers are mesomorphic compounds. The hexamers exhibit uncommon polymesomorphic behaviour forming two quite different mesomorphic structures. The molecular arrangement in the low temperature (LT) modification is characterized by two-dimensional (2D) long-range order with hexagonal packing. The X-ray diffraction pattern and peculiarities of molecular packing in the crystal lead us to suggest that the LT-mesophase is columnar, presumably of the Colhd type. The LT-mesophase is formed by dimeric moieties, which associate with each other in column-like substructures, the ring planes not orthogonal to the stack axis. The high temperature (HT) mesophase is a plastic crystal (3D-order), where molecules take up positions in a face-centred cubic lattice. This is a very uncommon example of thermal behaviour for plastic crystals that provides a unique opportunity to bridge the gap between plastic crystalline and liquid crystalline mesomorphic behaviour. The thermal and structural properties of the mesophases depend upon the type of side groups of the hexamers. The size of the ring also affects the phase behaviour and the mesomorphic structure. This conclusion is consistent with data obtained by us earlier for cyclotetrasiloxanes. 相似文献
Mesoporous solids which possess average pore diameters between 7 and 20 nm, depending on the composition, have been prepared. The solids have the general formula Al100PχM20 where M = Al or Fe, and χ = 0, 4.5, 9, 18, 36, 72 or 144. The initial addition of phosphorus as phosphate transforms the originally crystalline oxide/oxides into amorphous solids. These amorphous materials possess a narrow pore size distribution: 80–90% of the pores lie within 1–2 nm of the average pore diameter. Subsequent incremental amounts of phosphorus transform the material into a crystalline solid whilst the pore size distribution becomes much wider and the maximum moves towards larger pore diameters. Substitution of 20% of the aluminium by iron results, at a low phosphorus content, in pores with smaller pore volumes and smaller surface areas.
The data in the dVp/dDp = > Dp) graphs, where Vp is the incremental pore volume and Dp is the average pore diameter, can be approximated using an admixture of Gaussian and Lorentzian curves. For low phosphorus contents the dVp/dDp = (Dp) curves have a mainly Gaussian profile but the gradual addition of phosphorus transforms them to Lorentzian-type curves. An attempt to approximate the histograms dVp =(Dp) with the minimum number of distribution curves made up of the corresponding Gaussian and Lorentzian components indicates that each successive addition of phosphorus creates a dominant new pore component at a larger pore diameter. At the same time, the components at smaller pore diameters are diminished and eventually disappear as more phosphorus is added. 相似文献