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Tsinghua Science and Technology

Issue 4 • Date Dec. 1997

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Displaying Results 1 - 25 of 25
  • [Front cover]

    Page(s): c1
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    Freely Available from IEEE
  • Contents

    Page(s): 1 - 2
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    Freely Available from IEEE
  • Tunneling of electrons in multibarrier heterostructures under crossed electric and magnetic fields

    Page(s): 759 - 764
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    Taking Hermitian functions as envelope functions, this paper presents a calculation of the transmissi on coefficient for electrons tunneling through multibarrier heterostructures with parabolic quantum wells under crossed electric and magnetic fields by using the transfer-matrix method. Electric field effect, magnetic field effect, well width and barrier height effects on resonant-tunneling are studied in detail. It is found that all of these four factors affect the peak-value and peak position of the transmission coefficient. For symmetric double-barrier systems, the wider the well is and the lower the barrier is, the more drastically are peaks reduced by the magnetic field. But for triple-barrier systems, with increasing magnetic field strength, the variation of peak-value exhibits complicated behavior due to the coupling of the quasibound states in two quantum wells of the system. View full abstract»

    Open Access
  • Calculation of the neutron direct inelastic scattering cross section of 238U

    Page(s): 765 - 769
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    In this work, Rotation model and rotation-vibration coupled channel optical model (CCOM) were first used to carry out systematic calculation of neutron direct inelastic scattering cross section (DISCS) for the reaction 1U (n, n) with an incident energy from 0.01 MeV to 20.0 MeV. The number of coupled energy levels involved in this calculation increased to eight at the first time, namely 0+, 2+, 4+, 6+, 8+ in the ground rotation band of 1U and 1, 3, 5 in the octupole vibration band with K = 0. Some physical problems, such as how to get the optical model parameters of deformed nuclei and phonon amplitudes, are discussed. View full abstract»

    Open Access
  • Stable ferroelectric structures determinedusing screened dipole interactions

    Page(s): 770 - 773
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    This paper proposes a simple model for studying ferroelectricity by introducing a screened dipole-dipole interaction Hamiltonan. By minimizing the total energy, the body-centered tetragonal lattice structure is shown to be stable under the screened di pole-dipole interaction, while the cubic structure is not. The effectiveness of this model is estimated by simplifying the model to the nearest-neighbor Ising model for evaluating the physical quantities. The results are in qualitative agreement with experimental data. A method is also suggested to verify the results. In addition, the critical temperature of the 3 D-Ising model for body-centered tetragonal crystals is obtained using the Monte Carlo method. View full abstract»

    Open Access
  • Correcting theory of an ultrasonic stepping motor having self-correcting functionm

    Page(s): 774 - 776
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (504 KB)  

    An open-looped ultrasonic stepping motor with self-correction function was studied theoretically. In order to analyse the mechanism for the motor's function, A linear ultrasonic self-correcting stepping motor is designed, and the dynamics of the motor was studied. For clarification, a set of experiment device was also designed, the calculated results were consistent with the experimental ones. View full abstract»

    Open Access
  • Pinch analysis of heat exchange problems with uncertain parameters

    Page(s): 777 - 782
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    A system method is proposed to analyze pinch change of heat exchange problems with uncertain stream flowrates and temperatures. By an analysis of the maximum heating point and maximum cooling poi nt of uncertain parameters, existence of pinch jump can be determined. Based on the principle of reduced temperature interval analysis, a sufficient and necessary condition is derived for each stream supply temperature to determine if it is possible to become pinch within the specified range of uncertain parameters, then this range may be partitioned into different pinch regions. The study is a foundation for the flexibility analysis, synthesis, and retrofit of heat exchanger networks. View full abstract»

    Open Access
  • New process of fine grinding-activation of heavy CaCO3 and its application to PVC rigid sheet

    Page(s): 783 - 786
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (611 KB)  

    To make heavy calcium carbonate filled UHMW-PVC rigid sheet, the calcium carbonate powder is activated in a specially designed fine grinding-activation equipment (SM-10) by adding rare-earth aluminate coupling agent. The calcium carbonate powder is ground to micron order. The activated calcium carbonate filler shows excellent properties wheni t is added to UHMW-PVC, since the activated powder surface has strong compatibili ty with polymer. At a filling rate of 70 phr(per hundred resin), the impact strength with U notch is above 7 kJ/m2 and the tensile strength is above 20 MPa. The fine grinding-activation and PVC sheet pressing process are given. Both the influence of the amount of plasticizer and coupl ing agent on the mechanical properties and the influence of fill ing rate on PVC rigid sheet are discussed. View full abstract»

    Open Access
  • Fractal medium model for diffusion in dense polymer membranes

    Page(s): 787 - 792
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    The mechanism of penetrant's diffusion in polymer membrane is described on the basis of free volume theory and the concept of diffusion path is presented. The statistical character of diffusion path is then related with the size of penetrant molecules, the temperature, and the freevolume of merrbranes based on percolation theory. The membrane structure can be divided into two regimes, i. e., the fractal regime and the homogeneous regi me. The geometrical and the topological structures of diffusion path in the fractal regime can be described by the fractal dimension df and the graph dimension d1. On these basis, the transport behavior of penetrant in diffusion path is studied. In the fractal regime, the diffusion is nomalous and can not be described by the Fick' s law. With the change of the size of penetrant molecules, the temperature and the thickness of membrane, the transport behavior of penetrant can transform from normal diffusion to anomalous diffusion. View full abstract»

    Open Access
  • Ion chromatographic determination of organic acids and cations via exchange columns connected in parallel

    Page(s): 793 - 797
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    A column switching technique has been developed for the sequential determination of organic acids and cations in alcoholic drinks. An anion-exchange column and a cation-exchange column were connected in parallel for the measurement. Oxalic acid was employed as a common eluent for both the anion- and cation-exchange columns. Under the same chromatographic conditions, the organic acids and cations can be separated sequentially by a switching operation selecting the path way through the anion-and cation-exchange columns. This method is simple to operate and is very sensitive with detection limits of 0. 1 1. 0 mg/L for organic acids and 10 ~ 20 μg/L for cations. View full abstract»

    Open Access
  • Alignment mechanism of liquid crystal molecules on the fluorated polyimide surface

    Page(s): 798 - 800
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    The generation of pretilt angles for nematic liquid crystal aligned rubbed polyimide on the rubbed polyimide surface containing tri fluoromethyl moieties has been investigated. The characters of the rubbed polyimide films were studied by using X-ray photoelectron spectroscopy(XPS) and atomic force microscopy(AFM). The uneven distribution extent of fluorine content along the depth of the polyimide films could be reduced by rubbing, thus the interaction between liquid crystal molecules and polyimide molecules decreased. It is also showed that the aggregation state of polyimide molecules was changed and polyimide chains aligned along the rubbing direction, so the orientation direction of the liquid crystal molecules was the same as the rubbing direction. The orientation effect of the liquid crystal molecules on the rubbed polyimide surface can be attributed to the combined effects of the Van der Waals dispersion interaction between and polyimide and liquid crystal molecules, and the alignment of the orientation direction of polyimide chains with the rubbing direction. View full abstract»

    Open Access
  • Stereoselectivity of acetalization and ketalization reactions of giycerol and its modified derivatives

    Page(s): 801 - 805
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    Twenty-one 1, 3-dioxdanes with varieties of substituents at 2, 4 positions were synthesized by acid-catalyzed acetalization and ketalization reactions of glycerol and its modified derivatives. The assignment and quantitative analyses of thermodynamically equilibrated cis- and trans-1, 3-dioxolanes were completed by 1H-NMR. The stereoselective regularity was found in acetalization and ketalization reactions of glycerol and its modified derivatives. The stereoselective regularity of the condensation reactions was reasonablyexplained by anomeric effect and 2, 4-nonbonded interaction. View full abstract»

    Open Access
  • Copper (II) 15-crown-5 coordination compound with 3dz2

    Page(s): 806 - 808
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (633 KB)  

    This paper illustrates the origination, preparation, electron configuration, crystal structure, and properties of copper (II) 15-crown-5 coordination compound with 3dz2 ground state. The characteristics of electron paramagnetic resonance(EPR) spectrum, d-d transition in electron spectrum and differential thermal analysis (DTA) of the compound are shown. The future development in biochemistry and material science is discussed. View full abstract»

    Open Access
  • Synthesis of amino alkyl substituted phosphonates by kabachnik-fields reaction

    Page(s): 809 - 813
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    In order to find more efficient extracting agents with di functional group, seven amino alkyl substituted phosphonates were synthesized by Kabachnik-fields reaction. Their structures were elucidated on the basis of IR, MS, 1HNMR, 13CNMR, and 3lPNMR spectral methods. Four of them are new compounds. Meanwhile, the reaction was reformed by using imine for the synthesis of N-monoalkyl substituted phosphonates. View full abstract»

    Open Access
  • Adaptive lattice filter based approach to fault detection in the absence of a system model

    Page(s): 814 - 818
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    This paper proposes a novel approach to the incipient and abrupt fault detection for a class of unknown linear dynamic systems. The proposed method generates a set of residual sequences from the system input and measured output using an adaptive bi-channel lattice filter. The fault detection is carried out by performing statistical tests on the residuals. The distinctive advantage of this method is that a model of the unknown system is not required a priori for the purpose of fault detection. An application of this approach in a practical control system has been implemented, and satisfactory results have been obtained. View full abstract»

    Open Access
  • System architecture for CIMOSA model execution

    Page(s): 819 - 823
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    A CIMOSA integrating infrastructure (IIS) provides only a set of general services and a group of execution guidelines. Therefore, in order to execute a CIMOSA model, it is necessary to develop the architecture for its execution system. An execution kernel needs to be developed. The kernel makes use of the services of CIMOSA IIS to control and coordinate the occurrences of CIMOSA model constructs. An extended color Petri net, the control and integration net (CIN), is proposed. This CIN is used to build the execution kernel for the CIMOSA model. The system architecture is given. The execution system's computational view is also presented. The CIMOSA model execution system presented here is under implementation in a production system at the Berlin Fraunhofer Institute of Production Systems and Design Technology. The system's layout and relevant components are introduced. The proposed CIN method provides a new approach for CIM system integration. View full abstract»

    Open Access
  • Lifetime evaluation of medium and small size power transformers part 1: Theory

    Page(s): 824 - 827
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    A new methodology that combines analytical and experimental methods is proposed for evaluating transformer lifetime. This first part of a two part series focuses on aging and lifetime evaluation of oil sealed transformers. The second part focuses on an experimental study of the various elements of transformers. A new computational formula based on the degree of polymerization of (DP) of the insulation paper is proposed for lifetime evaluation of transformers. View full abstract»

    Open Access
  • Lifetime evaluation of medium and small size power transformers part 2: Experimental study

    Page(s): 828 - 832
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    This paper is the experimental verification of the theoretical analysis in Part 1 of a new method to evaluate the lifetime of power transformers. A new experimental method is proposed to detect the degree of polymerization (Dp) values during the paper aging process. Experiments show that the aging process of insulating paper is related to the temperature, the oxygen and the water content. If the oxygen and the water content are low, the Dp value decreases logarithmically with time. At the same temperature level, Dp values are submitted to normal distribution. Applying the presented methodology, expected lifetime of new type of power transformer SBS9 is evaluated. View full abstract»

    Open Access
  • Automatic fixture design for non-rotational parts using fuzzy comprehensive judgment method

    Page(s): 833 - 837
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (908 KB)  

    Automatic fixture design is a key link in computer aided process planning. The urcertainty in the selection of fixture design schemes necessitates the use of the fuzzy comprehensive judgment method to automatically select an optimal fixture design. This paper reports on the development of a computer aided fixture design system for non-rotational parts using the fuzzy comprehensive judgment method running in an integrated process planning environment. Several typical non-rotational parts with complicated configurations are used to demonstrate the automatic fixture design system. View full abstract»

    Open Access
  • Calculation of transient voltage distribution in transformers using bergeron's method

    Page(s): 838 - 841
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    This paper presents a new method for the calculation of the transient voltage distribution over a transformer winding-the Bergeron's method. Using the nodal admittance matrix, formulae are developed for the resistances, and self and mutual inductances of the equivalent circuit. Calculated results are given. The method reduces internal memory requirement, allows easy formation of the nodal admittance matrix, reduces computational time, etc. View full abstract»

    Open Access
  • Virtual-actual space alignment based on salient color features in VR aided tele-operation system

    Page(s): 842 - 845
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (826 KB)  

    A dynamic calibration method is developed for virtual-actual space alignment in VR aided tele-operation systems. During model building natural or man-made salient color features in the actual environment are picked out and marked on the environment model. During tele-operation, if the co or camera mounted on the tele-operation platform sees some of the modeled color features, the platform orientation can be determined from a single camera view. Correspondence between detected features and model ed features is based on the color attributes and the geometr ic relationships between them. Finally, a simple closed form location calculation for a 4DOF moving camera is given based on the locations of the three feature points. View full abstract»

    Open Access
  • Pseudo-colorimetric method for detection of penetration and seam position in welding process

    Page(s): 846 - 849
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (747 KB)  

    Pseudo-color measuring of the infrared radiation from molten pool is used to gather information the seam position and on the welding penetration. Characteristics are detected using color-sensing devices. Test results from both thermal simulation and robotic arc welding are regular and reproducible. The pseudo-color values were found to be directly relative to the seam position and welding penetration, therefore, they could be used for monitoring welding penetration and for seam tracking. View full abstract»

    Open Access
  • Automatic structural synthesis for planar kinematic chains containing higher pairs

    Page(s): 850 - 852
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (607 KB)  

    This paper presents a mechanism theory principle for equivalent substitution in kinematic chain of a higher pair for lower pairs. This is the opposite of the principle of the equivalent substitution of lower pairs for a higher pair. Also presented here is a new approach for isomorphism identification in kinematic chains containing higher pairs(KCCHs). Based on this approach, an algorithm for generating nonisomorphic KCCHs has been developed. This algorithm is not restricted by the number of links, the degree of freedom or the number of higher pairs. Using this algorithm, nonisomorphic KCCHs can be generated automatically from the kinematic chains that contain only lower pairs. As an application of this algorithm, all the nonisomorphic KCCHs with N < 10(N, the number of links), F ≤3 (F, the degree of KCCHs), and H ≤ 2(H, the number of higher pairs) are computer generated. View full abstract»

    Open Access
  • Classification of cancer cells using neural neural networks in combination with expert experience

    Page(s): 853 - 855
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (536 KB)  

    This paper briefly reviews sorre neural networks and discusses their drawbacks, the main defaults is that these neural networks do not use expert experience (or knowledge) and have the human flexibility, therefore, a new better method for the combination of neural networks with expert experience (or knowledge) was proposed. Probabilistic neural networks (PNNs) classification of cancer cell image is described. This networks is simpler and faster than back-propagating neural networks (BPNNs) during training and learning. Neural networks combined with expert experience is presented in order to improve the classification accuracy of the networks and the simulation experiments were performed and the results have shown that the method presented is very efficient and feasible. View full abstract»

    Open Access
  • Extensions of Farkas-Minkowski's Lemma and Lemma and Stiemke's Lemma in the space l1

    Page(s): 856 - 860
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1122 KB)  

    This paper extends Farkas-Mnkowski's Lemma and Stiemke's Lemma from the Euclidean space to (l1, l). The extensions of Farkas-Minkowski's Lemma and Stiemke's Lemma are the Basic Valuation Theorem in the case (l1, l). The security price is weakly arbitrage-free if and only if there exists a positive state vector; the security price is strictly arbitrage-free if and only if there exists a strictly positive state vector. The present value of the securities prices at date 0 is the value of their returns over all countably infinite possible states of nature at date 1. View full abstract»

    Open Access

Aims & Scope

Tsinghua Science and Technology (Tsinghua Sci Technol) aims to highlight scientific achievements in computer science, electronic engineering, and other IT fields. Contributions all over the world are welcome.

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