Showing papers by "Tsinghua University published in 1999"

Improving plant drought, salt, and freezing tolerance by gene transfer of a single stress-inducible transcription factor

Abstract: Plant productivity is greatly affected by environmental stresses such as drought, salt loading, and freezing. We reported previously that a cis-acting promoter element, the dehydration response element (DRE), plays an important role in regulating gene expression in response to these stresses. The transcription factor DREB1A specifically interacts with the DRE and induces expression of stress tolerance genes. We show here that overexpression of the cDNA encoding DREB1A in transgenic plants activated the expression of many of these stress tolerance genes under normal growing conditions and resulted in improved tolerance to drought, salt loading, and freezing. However, use of the strong constitutive 35S cauliflower mosaic virus (CaMV) promoter to drive expression of DREB1A also resulted in severe growth retardation under normal growing conditions. In contrast, expression of DREB1A from the stress inducible rd29A promoter gave rise to minimal effects on plant growth while providing an even greater tolerance to stress conditions than did expression of the gene from the CaMV promoter.

Federalism and the Soft Budget Constraint

Abstract: The government's incentives to bail out inefficient projects are determined by the trade-off between political benefits and economic costs, the latter depending on the decentralization of government. Two effects of federalism are derived: First, fiscal competition among local governments under factor mobility increases the opportunity costs of bailout and thus serves as a commitment device, (the "competition effect"). Second, monetary centralization, together with fiscal decentralization, induces a conflict of interests and thus may harden budget constraints and reduce inflation (the "checks and balance effect"). Our analysis is used to interpret China's recent experience of transition to a market economy.

Study on poly(methyl methacrylate)/carbon nanotube composites

Abstract: Carbon nanotubes (CNTs) can be used to compound poly (methyl methacrylate)/carbon nanotube (PMMA/CNT) composites by an in situ process. The experimental results show that CNTs can be initiated by AIBN to open their π-bonds, which imply that CNTs may participate in PMMA polymerization and form a strong combining interface between the CNTs and the PMMA matrix. Through the use of an improved in situ process, the mechanical properties and the heat deflection temperatures of composites rise with the increase of CNTs. The dispersion ratio of CNTs in the PMMA matrix is proportional to the reaction time of polymerizing MMA before CNTs are added into the PMMA mixture.

A simple method, the -history method, of determining the heat of fusion, specific heat and thermal conductivity of phase-change materials

Abstract: A simple method of determining the melting point, heat of fusion, specific heat and thermal conductivity of phase-change materials (PCMs) is presented. Compared with other methods, such as conventional calorimetric methods, differential thermal analysis and differential scanning calorimetry methods, it has the following salient features: the experimental unit is simple, able to measure the heat of fusion, specific heat and thermal conductivity of several samples of PCMs simultaneously and allows one to observe the phase-change process of each PCM sample. Using the method, the thermophysical properties of various salt hydrates, paraffin and some PCMs developed by us were measured. For the PCMs whose thermophysical properties are available in the literature, our results gave fairly good agreement. The method is especially useful for the selection of lots of candidate PCMs used for the purpose of engineering and for preparing new PCMs.

Three‐dimensional nano‐HAp/collagen matrix loading with osteogenic cells in organ culture

Abstract: Transplantation of osteogenic cells with a suitable matrix is one strategy for engineering bone tissue. Three-dimensional distribution and growth of cells within the porous scaffold are of clinical significance for the repair of large bony defects. A nano-HAp/collagen (nHAC) composite that mimics the natural bone both in composition and microstructure to some extent was employed as a matrix for the tissue engineering of bone. A porous nHAC composite was produced in sheet form and convolved to be a three-dimensional scaffold. Using organ culture techniques and the convolving method, we have developed three-dimensional osteogenic cells/nHAC constructs in vitro. Scanning electron microscopic and histological examination has demonstrated the development of the cells/material complex. Spindle-shaped cells migrating out of bone fragments continuously proliferated and migrated throughout the network of the coil. The porous nHAC scaffold provided a microenvironment resembling that seen in vivo, and cells within the composite eventually acquired a tridimensional polygonal shape. In addition, new bone matrix was synthesized at the interface of bone fragments and the composite.

New thermal wave aspects on burn evaluation of skin subjected to instantaneous heating

Abstract: Comparative studies on the well-known Pennes' equation and the newly developed thermal wave model of bioheat transfer (TWMBT) were performed to investigate the wave like behaviors of bioheat transfer occurred in thermal injury of biological bodies. The one-dimensional TWMBT in a finite medium was solved using separation of variables and the analytical solution showed distinctive wave behaviors of bioheat transfer in skin subjected to instantaneous heating. The finite difference method was used to simulate and study practical problems involved in burn injuries in which skin was stratified as three layers with various thermal physical properties. Deviations between the TWMBT and the traditional Pennes' equation imply that, for high flux heating with extremely short duration (i.e., flash fire), the TWMBT which accounts for finite thermal wave propagation may provide realistic predictions on burn evaluation. A general heat flux criterion has been established to determine when the thermal wave propagation dominates the principal heat transfer process and the TWMBT can be used for tissue temperature prediction and burn evaluation. A preliminary interpretation on the mechanisms of the wave like behaviors of heat transfer in living tissues was conducted. The application of thermal wave theory can also be possibly extended to other medical problems which involve instantaneous heating or cooling.

Examination of the indentation size effect in low-load vickers hardness testing of ceramics

Abstract: The indentation size effect (ISE) in Vickers hardness for several ceramic materials was observed in a relatively wider range of applied test load. It was shown that the proportional specimen resistance (PSR) model proposed by Li and Bradt is insufficient for describing the experimental data. By considering the effect of the machining-induced plastically deformed surface on the hardness measurements, the PSR model was modified and the empirical equation proposed originally by Buckle was proven to be more suitable for correlating the measured indentation dimension to the applied test load. ©

Adaptive Sliding-Mode Guidance of a Homing Missile

Abstract: Sliding-mode control is applied to design robust homing missile guidance law. The sufe cient and necessary condition for the sliding-mode motion of a linear time-varying system not to be affected by disturbances and the sufe cient condition for that motion not to be affected by parameter perturbations are given. An adaptive reaching law of sliding mode for a linear time-varying system is then presented and used to derive an adaptive sliding-mode guidance law. Theoretical analysis and simulation results show that the adaptive sliding-mode guidance law is robust against disturbances and parameter perturbations. Furthermore, the presented guidance law is simple to implement in practice.

A novel multiscale nonlinear thresholding method for ultrasonic speckle suppressing

Abstract: This paper presents a novel speckle suppression method for medical B-scan ultrasonic images. An original image is first separated into two parts with an adaptive filter. These two parts are then transformed into a multiscale wavelet domain and the wavelet coefficients are processed by a soft thresholding method, which is a variation of Donoho's (1995) soft thresholding method. The processed coefficients for each part are then transformed back into the space domain. Finally, the denoised image is obtained as the sum of the two processed parts. A computer-simulated image and an in vitro B-scan image of a pig heart have been used to test the performance of this new method. This technique effectively reduces the speckle noise, while preserving the resolvable details. It performs well in comparison to the multiscale thresholding technique without adaptive preprocessing and two other speckle-suppression methods.

A rapid method for detecting bacterial polyhydroxyalkanoates in intact cells by Fourier transform infrared spectroscopy

Abstract: Polyhydroxyalkanoates (PHA) are synthesized by many bacteria as inclusion bodies, and their biodegradability and structural diversity have been studied with a view to their potential application as biodegradable materials. In this paper, Fourier-transform infrared spectroscopy (FT-IR) was used to carry out rapid qualitative analysis of PHA in intact bacterial cells. The FT-IR spectra of pure PHA containing short-chain-length monomers, such as hydroxybutyrate (HB), medium-chain-length hydroxyalkanoate (mclHA) monomers including hydroxyoctanoate (HO) and hydroxydecanoate (HD), or both HB and mclHA monomers, showed their strong characteristic band at 1728 cm−1, 1740 cm−1 or 1732 cm−1 respectively. Other accompanying bands near 1280 cm−1 and 1165 cm−1 helped identify the types of PHA. The intensity of the methylene band near 2925 cm−1 provided additional information for PHA characterization. In comparison, bacterial cells accumulating the above PHA also showed strong marker bands at 1732 cm−1, 1744 cm−1 or 1739 cm−1, corresponding to intracellular PHB, mclPHA and P(HB + mclHA) respectively. The accompanying bands visible in pure PHA were also observable in the intact cells. The FT-IR results were further confirmed by gas chromatography analysis.

A CBIR method based on color-spatial feature

Abstract: Along with the analysis of color features in the hue, saturation and value (HSV) space, a new dividing method to quantize the color space into 36 non-uniform bins is introduced in this paper. Based on this quantization method we propose a color-spatial method to include several spatial features of the colors in an image for retrieval. These features are area and position, which mean the zero-order and the first-order moments, respectively. Experiments on an image database of 838 images show that the algorithm performs well in precision and adaptability.

Preparation and characterization of NiCuZn ferrite nanocrystalline powders by auto-combustion of nitrate–citrate gels

Abstract: A novel nitrate–citrate gel process was used to synthesize NiCuZn ferrite powders. The thermal decomposition and combustion process of nitrate–citrate gels were investigated by using DTA-TG, IR spectra and XRD techniques. The results revealed that the nitrate–citrate gels exhibit self-propagating behavior after ignition in air at room temperature. NiCuZn ferrite powders with a particle size of 20–50 nm were directly formed after combustion. The ratio of metal nitrates to citric acid in the starting solution affects the combustion process, and then determines the particle size of the as-synthesized powder. The magnetic structure of powders was characterized using Mossbauer spectroscopy.

Biomaterials modification by ion-beam processing

Abstract: Biomaterials modification by ion-beam processing is becoming popular for improving medical device function, biocompatibility and as a new mutation breeding method. Ion-beam-based processes, such as ion implantation and ion-beam assisted deposition (IBAD) can provide beneficial surface layers with desirable properties without detrimentally affecting the bulk properties. Ion implantation has been successful in biomaterials modification, such as in improving the wear resistance of artificial joint components, in improving wettability, anticoagulability, anticalcific behavior of biomedical polymers, and in minimizing biofouling of medical devices, etc. IBAD has been used to prepare hydroxyapatite coatings with high adhesive strength on substrates of several implant materials. Biocompatible diamond-like carbon coatings and C–N films, antibacterial coatings and sealant coatings have also been produced by this technique. This paper reviews the present status of applications of the ion-beam process in modifying the surface of biomaterials, as well as the effects induced by ion-beam irradiation to crop seeds, cells and microbes.

Frobenius manifolds and Virasoro constraints

Abstract: For an arbitrary Frobenius manifold, a system of Virasoro constraints is constructed. In the semisimple case these constraints are proved to hold true in the genus one approximation. In particular, the genus ≤ 1 Virasoro conjecture of T. Eguchi, K. Hori, M. Jinzenji, and C.-S. Xiong and of S. Katz is proved for smooth projective varieties having semisimple quantum cohomology.

Using class-center vectors to build support vector machines

Abstract: A support vector machine builds the final classification function on only a small part of the training samples (the support vectors). It is believed that all the information about classification in the training set can be represented by these samples. However, this is actually not always true when the training set is polluted by noises (training data are not i.i.d.). We present a different method for the problem, which applies the idea of capacity control in SVM but tries to make the machine less sensitive to noises and outliers. The new method can be called a central support vector machine or CSVM, for it uses the class centers in building the support vector machine.

A geometrical representation of McCulloch-Pitts neural model and its applications

Abstract: In this paper, a geometrical representation of McCulloch-Pitts neural model (1943) is presented, From the representation, a clear visual picture and interpretation of the model can be seen. Two interesting applications based on the interpretation are discussed. They are 1) a new design principle of feedforward neural networks and 2) a new proof of mapping abilities of three-layer feedforward neural networks.

Characterisation of ZnO-based varistors prepared from nanometre Precursor powders

Abstract: Nanometre precursor powders of ZnO and various additives are synthesised by a new technology, namely chemical coprecipitation and plasma pyrolysis Transmission electron microscopy (TEM) and the Brunauer-Emmett-Teller (BET) method are employed to illustrate the precursor powders' properties, The ZnO and additive composite powders are composed of spherical particles whose size is about 10-50 nm, Varistors are prepared by sintering the precursor powders at 1050 degrees C, Scanning electron microscopy (SEM) results show that the average grain size of the varistors is about 10 mu m The prepared ZnO-based varistors have excellent electronic properties Analytical results reveal that the breakdown voltage is 5000 V mm(-1) and the non-linear coefficient is about 540 Copyright (C) 2000 John Wiley a Sons, Ltd

Power loss and junction temperature analysis of power semiconductor devices

Abstract: A newly developed electrothermal calculation method is implemented to estimate the power loss and working temperature of insulated gate bipolar transistor (IGBT) devices. Based on the measurement of the IGBT's characteristics, the exact estimation of power loss considering the junction temperature is introduced. Then, the thermal network is used to calculate the working temperature. The comparison between experimental and calculation results shows that this method is effective as a designing step with only the time-domain voltage and current data obtained from simulation results.