Showing papers by "Tsinghua University published in 1997"

Synthesis of Gallium Nitride Nanorods Through a Carbon Nanotube-Confined Reaction

Abstract: Gallium nitride nanorods were prepared through a carbon nanotube–confined reaction. Ga2O vapor was reacted with NH3 gas in the presence of carbon nanotubes to form wurtzite gallium nitride nanorods. The nanorods have a diameter of 4 to 50 nanometers and a length of up to 25 micrometers. It is proposed that the carbon nanotube acts as a template to confine the reaction, which results in the gallium nitride nanorods having a diameter similar to that of the original nanotubes. The results suggest that it might be possible to synthesize other nitride nanorods through similar carbon nanotube–confined reactions.

Incentives, Information, and Organizational Form

Abstract: We model an organization as a hierarchy of managers erected on top of a technology (here consisting of a collection of plants) In our framework, the role of a manager is to take steps to reduce the adverse consequences of shocks that affect the plants beneath him We argue that different organizational forms give rise to different information about managers'performance and therefore differ according to how effective incentives canbe in encouraging a good performance In particular, we show that, undercertain assumptions, the M-form (multi-divisional form) is likely to provide better incentives than the U-form (unitary form) because it promotes yardstick competition (ie relative performance evaluation) more effectively We conclude by presenting evidence that the assumptions on which this comparison rests are satisfied for Chinese data

Quantum interference effects in spontaneous emission from an atom embedded in a photonic band gap structure

Abstract: The spontaneous emission from a three-level atom embedded in a photonic band gap structure is studied. Interference between two transitions leads to quasiperiodic oscillations of population between the two upper levels with large amplitudes. The spontaneous emission of the atom is characterized by three components in the radiated field: a localized part, a traveling pulse, and a $(1/\sqrt{t}{)}^{3}$ decaying part. An analytical expression for the localization distance of the localized field is obtained. The energy velocity for the traveling pulse could be close to zero. By selecting an appropriate initial superposition state, a large amount of population trapping can be achieved.

Global rigidity theorems of hypersurfaces

Abstract: which is the natural generalization of one of the following two conditions, (1) tr r (2) (tr r ]r =constant_>0. We also note that the condition (0.1) comes out naturally when we study the operatot []. Let M be an n-dimensional hypersurface in an (n+ 1)-dimensional real space form Rn+l(c). Observing that the second fundamental form tensor hij is a natural Codazzi tensor on M, in Section 2, we apply the study of Section 1 to these hypersurfaces and obtain general rigidity results (see Theorem 2.1 and Theorem 2.2) which unify some existing results. Condition (0.1) becomes in this case

Dependent-Chance Programming: A Class of Stochastic Optimization

Abstract: This paper provides a theoretical framework of dependent-chance programming, as well as dependent-chance multiobjective programming and dependent-chance goal programming which are new types of stochastic optimization A stochastic simulation based genetic algorithm is also designed for solving dependent-chance programming models

Mixed mode near-tip fields for cracks in materials with strain-gradient effects

Abstract: Large strain gradients exist near the tip of a crack due to stress singularity. The strain-gradient effect becomes significant when the size of the fracture process zone around a crack tip is comparable to the intrinsic material length, l , typically on the order of microns. Fleck and Hutchinson's [(1993) A phenomenological theory for strain-gradient effects in plasticity. J. Mech. Phys. Solid 41 , 1825–1857], strain-gradient plasticity theory is applied to investigate the asymptotic field near a mixed mode crack tip in elastic as well as elastic-plastic materials with strain-gradient effects. It is established that the dominant strain field is irrotational. For an elastic material, stresses and couple stresses have the square-root singularity near the crack tip, and are governed by three variables (two for mode I and II stress fields, and the third, resulting from higher order stresses, for the couple stress field). Stresses ahead of a crack tip in elastic materials with strain-gradient effects could be more than 50% higher than their counterparts in materials without strain-gradient effects. For an elastic-power law hardening strain-gradient material, an analytical solution is obtained. The mixed mode stress field in strain-gradient plasticity is the linear superposition of their counterparts in mode I and II. The angular distribution of stresses and couple stresses for several near-tip mode mixities clearly indicate that the new near-tip field in strain-gradient plasticity differs significantly from the HRR field. Stresses ahead of a crack tip in elastic-plastic solids with strain-gradient effects could be more than 2.5 times their counterparts in the HRR field. The asymptotic analysis compares favorably with available finite element results. The relevance of this solution to materials, in particular the size of the dominant zone, is discussed.

Highly adhesive hydroxyapatite coatings on titanium alloy formed by ion beam assisted deposition

Abstract: A compact crystalline hydroxyapatite coating on Ti–6Al–4V substrate with an atomic intermixed coating/substrate interface about 27 nm in width was synthesized by ion beam assisted deposition (IBAD) and a following post-treatment. The coating after post-treatment was identified by X-ray diffraction as crystalline hydroxyapatite. The interface between coatings and substrates was studied by Auger electron spectroscopy. The adhesive strength between coatings and substrates was measured by scratch tester. The results showed that the adhesive strength of IBAD coatings is nearly twice that of ion beam sputtered coatings. The study also showed that coatings prepared by IBAD eliminated the interfacial deficiencies existing in plasma-sprayed coatings.

Enhancement of immune function in mice fed high doses of soy daidzein.

Abstract: High soy consumption leading to high exposures of soy isoflavones has been associated with a reduced risk of cancers at many sites. As part of a study focusing on the chemopreventive mechanisms, we have investigated the modulating effects of daidzein, a prominent and more bioavailable isoflavone in soy foods, on murine immune function. Swiss mice were fed daidzein at various doses daily for seven consecutive days. At high doses (20 and 40 mg/kg), daidzein exerted a stimulatory effect on nonspecific immunity, as shown by increases in the phagocytic response of peritoneal macrophages and thymus weight, in a dose-dependent manner. Augmentation of spleen immunoglobulin M-producing cells against sheep red blood cells demonstrated an-activation of humoral immunity. Enhanced cell-mediated immunity was also observed as increases in lymphocyte proportion of peripheral blood. However, no significant immunoregulatory effect was found when mice were fed 10 mg/kg daidzein. These results demonstrate for the first time that daidzein at high doses enhances several immunologic functions and suggest a novel approach to understanding the mechanism(s) by which soy foods may contribute to observed cancer prevention.

Size and shape effects of quantum dots on two-electron spectra

Abstract: The exact spectra of two electrons confined by two-dimensional and three-dimensional quantum dots (2D and 3D QD's) with parabolic potentials are obtained. Using the present results, the size and shape effects of QD's on the spectra are revealed. It is found that the spectra are dramatically changed with the variation of the dot size, and then the crossover of two levels can appear. The variation of spectra with size and the positions of crossover points are quite different between 2D and 3D QD's. It is well explained based on the study of electron-electron interaction energies in 2D and 3D QD's. The size and shape effects predict a possibility to observe phenomena related to electron-electron interactions in QD's.

Production of poly-b-hydroxybutyrate by Azotobacter vinelandii in a two-stage fermentation process

Abstract: Poly-b-hydroxybutyrate (PHB) production in Azotobacter vinelandii UWD, a mutant that produces PHB constitutively, was suppressed by high aeration of beet molasses medium. Thus a two-stage process was designed using aeration to promote growth and suppress PHB production in the first phase, while lower aeration of raw sugar medium containing fish peptone was used to promote PHB formation in the second phase. A PHB yield of 36 g/l and productivity of > 1 g polymer l -1 .h was obtained by this approach.

Raman Phonon Modes and Ferroelectric Phase Transitions in Nanocrystalline Lead Zirconate Titanate

Abstract: Micro-Raman spectra for PbZr x Ti 1-x O 3 (PZT) with grain sizes of 60 and 40 nm have been obtained. The results show that the E(TO 1 ) phonon mode of PbTiO 3 ('soft' mode) displays a decrease in frequency and an increase in linewidth with increasing Zr concentration. A discontinuous behavior in the phonon energy for the soft mode occurs at a morphotropic phase boundary (NPB) of x 0.4 and 0.2 for grain sizes of 60 and 40 nm, respectively, and it can be attributed to a phase transition from ferroelectric tetragonal to ferroelectric rhombohedral phase. The nonzero soft mode frequency near the MPB results from a level repulsion between an additional phonon mode at 10 cm -1 and the soft mode. Raman enhanced behavior has been found for the lowest phonon mode with Zr contents in the range of 0.3 to 0.6. The dependence of Raman phonon modes for PbZr 0.3 Ti 0.7 O 3 upon grain size has been analyzed and the results indicate a grain-size-induce(I phase transition at 13 nm.

Analysis of an optimal control model of multi-joint arm movements

Abstract: In this paper, we propose a model of biological motor control for generation of goal-directed multi-joint arm movements, and study the formation of muscle control inputs and invariant kinematic features of movements. The model has a hierarchical structure that can determine the control inputs for a set of redundant muscles without any inverse computation. Calculation of motor commands is divided into two stages, each of which performs a transformation of motor commands from one coordinate system to another. At the first level, a central controller in the brain accepts instructions from higher centers, which represent the motor goal in the Cartesian space. The controller computes joint equilibrium trajectories and excitation signals according to a minimum effort criterion. At the second level, a neural network in the spinal cord translates the excitation signals and equilibrium trajectories into control commands to three pairs of antagonist muscles which are redundant for a two-joint arm. No inverse computation is required in the determination of individual muscle commands. The minimum effort controller can produce arm movements whose dynamic and kinematic features are similar to those of voluntary arm movements. For fast movements, the hand approaches a target position along a near-straight path with a smooth bell-shaped velocity. The equilibrium trajectories in X and Y show an ‘N’ shape, but the end-point equilibrium path zigzags around the hand path. Joint movements are not always smooth. Joint reversal is found in movements in some directions. The excitation signals have a triphasic (or biphasic) pulse pattern, which leads to stereotyped triphasic (or biphasic) bursts in muscle control inputs, and a dynamically modulated joint stiffness. There is a fixed sequence of muscle activation from proximal muscles to distal muscles. The order is preserved in all movements. For slow movements, it is shown that a constant joint stiffness is necessary to produce a smooth movement with a bell-shaped velocity. Scaled movements can be reproduced by varying the constraints on the maximal level of excitation signals according to the speed of movement.