Showing papers by "Xi'an Jiaotong University published in 1995"

Detection of ECG characteristic points using wavelet transforms

Abstract: An algorithm based on wavelet transforms (WT's) has been developed for detecting ECG characteristic points. With the multiscale feature of WT's, the QRS complex can be distinguished from high P or T waves, noise, baseline drift, and artifacts. The relation between the characteristic points of ECG signal and those of modulus maximum pairs of its WT's is illustrated. By using this method, the detection rate of QRS complexes is above 99.8% for the MIT/BIH database and the P and T waves can also be detected, even with serious base line drift and noise. >

Operational principle, fabrication and displacement characteristics of a functionally gradient piezoelectric ceramic actuator

Abstract: In this paper, the theoretical relationship between the bending displacement at the free end of a functionally gradient material (FGM) actuator and the FGM coefficient of the sandwich has been derived from the structure of an FGM actuator. FGM actuators based on PNN-PZT piezoelectric ceramics have been developed by the powder mould stacking press method and their displacement characteristics have also been measured. The results show that the bending displacement at the free end of a 20 mm×8 mm×0.7 mm FGM actuator is about 20 μm when a voltage of 1000 V is applied. The morphology and the compositional distribution across the profile of the FGM actuator are examined by using SEM and EDS, respectively.

Space charge in XLPE power cable under dc electrical stress and heat treatment

Abstract: It is well known that the existence and accumulation of space charge in insulation can be harmful to polymer power cable. Although there is much research done on space charge distribution in various samples, there are still some important problems left unsolved, such as the mechanism of formation and elimination of space charge in dielectrics. Because of its superiority, the pulsed electro-acoustic (PEA) measurement method is used widely in the measurement of space charge in various geometries of samples such as the multiple layer plate and cylindrical cables. The space charge in crosslinked polyethylene (XLPE) power cable under dc voltage and heat treatment is investigated by the PEA method in this paper. After heat treatment, the space charge, which previously formed in cable insulation under the applied voltage, disappears. If electrical stress is applied to the insulation again, space charge will appear once more. However, after the cable is heated and degassed in vacuum for a long time, no matter how long the dc voltage is applied to the cable, no more space charge will be formed in the cable. The mechanism of formation and elimination of space charge under the effect of electrical and thermal stress is discussed. >

The prediction of dimensional error for sculptured surface productions using the ball-end milling process. Part 1: Chip geometry analysis and cutting force prediction

Abstract: The study of machining errors caused by tool deflection in the balkend milling process involves four issues, namely the chip geometry, the cutting force, the tool deflection and the deflection sensitivity of the surface geometry. In this paper, chip geometry and cutting force are investigated. The study on chip geometry includes the undeformed radial chip thickness, the chip engagement surface and the relationship between feed boundary and feed angle. For cutting force prediction, a rigid force model and a flexible force model are developed. Instantaneous cutting forces of a machining experiment for two 2D sculptured surfaces produced by the ball-end milling process are simulated using these force models and are verified by force measurements. This information is used in Part 2 of this paper, together with a tool deflection model and the deflection sensitivity of the surface geometry, to predict the machining errors of the machined sculptured surfaces.

Neutral‐donor‐bound exciton recombination dynamics in GaN grown by metalorganic chemical vapor deposition

Abstract: Neutral‐donor‐bound exciton recombination (I2) dynamics have been studied by photoluminescence in an unintentionally doped n‐type GaN epitaxial layer grown by metalorganic chemical vapor deposition. The luminescence emission line shape, peak position, and intensity as functions of temperature have been measured. In particular, time‐resolved emission spectroscopy has been employed to study the dynamic processes of the bound exciton recombination, from which the temperature and the emission energy dependencies of the recombination lifetime of this transition have been obtained.

Pore size and pore-size distribution control of porous silica

Abstract: Porous silica is a unique matrix material for chemical sensors. In this paper, porous silica is prepared via the sol-gel technique. The porosity, pore size, pore shape and pore-size distribution can be controlled by catalysts and preparation conditions. A porosity as high as 60−70% can be reached. The pore size can be controlled in the range 2−100 nm. A single narrow peak and double separate peak distributions can be achieved by controlling the pH values. The pore shape can be controlled from narrow-necked and large-abdomened shapes to fine cylindrical pores. Applications of porous silica for chemical sensors are also discussed in this paper.

Joining of Zirconia Ceramic to Stainless Steel and to Itself Using Ag57Cu38Ti5 Filler Metal

Abstract: The brazing of ZrO2 ceramic to 1Cr18Ni9Ti stainless steel and to itself was performed using Ag57Cu38Ti5 filler metal under a vacuum of 7 × 10-3 Pa. The effects of interlayer copper on the ceramic to stainless steel joint strength, and the brazing temperature (1073 to 1323 K) and holding time (0 to 60 min) on ceramic to ceramic joint strength were investigated. The joint strength was evaluated by shear testing. An interfacial reaction layer between the ceramic and the filler metal was found, and the reaction products were δ-TiO and γ-AgTi3. The joint strength of ZrO2 ceramic to stainless steel was improved by using a layer of copper of a suitable thickness. The brazing temperature and holding time had a strong influence on the joint strength of ceramic to ceramic, and the joint strength was mainly controlled by reaction layer thickness and the properties of the reaction products. The maximum shear strength was obtained for brazing at 1123 K for 30 min and an interfacial reaction layer thickness of ∼4.4 μm.

The use of shape correction factors for elastic indentation measurements

Abstract: Elastic properties of small volumes of materials can be measured from the unloading of small indentations using the so-called nanoindenters The analytic elastic solutions for axisymmetric indenters are currently used to calculate modulus from the unloading curve, sometimes using corrections derived for flat, rigid punches. It is shown that these corrections represent an upper limit for the correction. More realistic corrections are derived for the Vickers, Berkovich, and Knoop indenter shapes, using the assumption of a uniformly loaded area. Results show that the axisymmetric solution overestimates the elastic compliance of the Vickers indenter by a factor of 1.0055, of the Berkovich indenter by a factor of 1.0226, and of the Knoop indenter by a factor of 2.682.

A more efficient Lagrangian relaxation approach to job-shop scheduling problems

Abstract: Lagrangian relaxation consists of relaxing capacity constraints using Lagrangian multipliers and of decomposing the problem into job level subproblems. In the literature, when job shop scheduling problems are considered, these subproblems are further decomposed into operation level subproblems by relaxing precedence constraints. Unfortunately, this results in solution oscillation and often prevents convergence of the algorithm. Although several methods have been proposed to avoid solution oscillation, none of them is really satisfactory. In this paper, we propose an efficient pseudopolynomial time dynamic programming algorithm to solve relaxed job level subproblems. This makes the relaxation of precedence constraints unnecessary. The solution oscillation can then be avoided. This algorithm also results in a much more efficient Lagrangian relaxation approach to job-shop scheduling problems. Computational results on randomly generated problems are given to demonstrate the efficiency of the algorithm.

The effect of interlayer metals on the strength of alumina ceramic and 1Cr18Ni9Ti stainless steel bonding

Abstract: The effects of interlayers of molybdenum and copper on the strength of alumina ceramic and 1Cr18Ni9Ti stainless steel bonding with Ag57Cu38Ti5 filler metal were investigated. The interfacial morphologies were observed and analysed by scanning electron microscopy and energy dispersive X-ray (EDX) analysis, respectively. The joint strength was examined by shear tests. When using a molybdenum interlayer, the joint strength could be greatly improved because molybdenum not only reduced the interfacial residual stress, but also did not affect the interfacial reaction between the ceramic and the filler metal, and the maximum value was obtained when it was about 0.1 mm thick. When using copper as an interlayer, the joint strength was not increased but decreased, because copper reduced the activity of titanium in the filler metal, resulting in an insufficient interfacial reaction between the ceramic and the filler metal and the formation of poor interfacial adhesion. Therefore, in selecting an interlayer metal to reduce or avoid interfacial residual stress in joining ceramics to metals, in which the interfacial reaction of ceramic and filler metal is important to the joints, the interaction of interlayer metal and filler metal must be considered.

Interfacial reaction of alumina with Ag-Cu-Ti alloy

Abstract: The interfacial reaction of Al2O3 and Ag-Cu-Ti alloy was investigated by scanning electron microscope (SEM) and X-ray diffraction (XRD), respectively. It was shown that Al2O3 ceramic reacted strongly with Ag-Cu-Ti alloy. With the increasing heating temperature and holding time, the reaction layer thickness increased and its growth was mainly controlled by the diffusion of titanium through the reaction layer. The reaction products were Cu2Ti4O and AlTi at or below 1123 K. However, there were two distinct layers at the interface at or above 1173 K, one layer in the vicinity of ceramic consisting mainly of Ti2O and TiO and the other layer near the alloy was CuTi2, a layer transition structures with a Al2O3/Ti2O+TiO/Ti2O +TiO+CuTi2/CuTi2/Ag-Cu formed at the interface according to the SEM and XRD analyses results. A lower or a higher joining temperature and a shorter or a longer holding time were disadvantageous for a stable and high reliable joined interface from the point of view of interfacial microstructures and morphologies.

Natural convections in conjugated single and double enclosures

Abstract: The natural convection in single and double conjugated enclosures are numerically investigated. The single and double enclosures are formed by low conductance walls with finite thickness. The outside vertical surfaces of the conducting walls are of the third kind of boundary condition while the top and bottom outside surfaces are adiabatic. The problem studied is characterized by a dominant horizontal temperature gradient and the thermal boundary conditions at the cavity surfaces can not be specified in priori. Numerical results reveal the characteristics in such kind of enclosures and show the importance of the thermal boundary conditions on the natural convection in enclosures. It is also found that the natural convections in the conjugated double enclosures are basically the same, with a major difference in their fluid temperature levels.

Acceptor-bound exciton recombination dynamics in p-type GaN

Abstract: Dynamics of the neutral‐acceptor‐bound exciton transition (the I1 line) in a Mg doped p‐type GaN epitaxial layer grown by metalorganic chemical vapor deposition (MOCVD) have been studied by time‐resolved photoluminescence emission spectroscopy. Two emission lines in the I1 transition region have been resolved in the time‐resolved spectra, possibly due to the existence of two energy states of the Mg impurities after postgrowth thermal annealing. The recombination lifetimes of the acceptor‐bound exciton transition have been measured under different conditions including temperature, excitation intensity, and emission energy. From these measurements, a value of about 450 ps for the radiative recombination lifetime has been obtained, which is an important physical quantity for optoelectronic device applications based on GaN.

Evaluation of the thermodynamic process of indirect injection diesel engines by the First and Second Law

Abstract: Heat transfer losses in the swirl chamber, throttling losses at the connecting passage and combustion delay in the main chamber are considered as the three factors influencing the thermal efficiency of IDI diesel engines. This paper suggests a thermodynamic model, in which three idealized diesel engines including no passage throttling engine, adiabatic diesel engine for swirl chamber and DI diesel engine are assumed to isolate heat transfer losses, throttling losses and combustion delay in IDI diesel engines. The Second Law analysis is carried out by the thermodynamic state parameters calculated by the cycle simulation of engines based on the First Law. The effects of heat transfer losses in the swirl chamber, throttling losses at the connecting passage and combustion delay in the main chamber on the irreversibilities and availability losses during the engine cycle are analyzed in detail. The relative influences among the three losses are also investigated. The results of First Law analysis indicate that heat transfer losses in the swirl chamber at low load conditions and combustion delay in the main chamber at full load conditions are the main factors impairing the fuel economy of IDI diesel engines. However, the results of further analysis of the Second Lawmore » indicate that passage throttling is a key factor affecting the fuel economy of IDI diesel engines at full load conditions. On the basis of thermodynamic analysis, a modified design of connecting passage is made on a single cylinder IDI diesel engine.The modified connecting passage has different inclination angles at both sides of the passage, and reduces throttling losses at the connecting passage, shortens combustion delay and combustion period in the main chamber, and hence reduces the engine fuel consumption and smoke emission.« less

SiGe/Si bifurcation optical active switch based on plasma dispersion effect

Abstract: Based on the plasma dispersion effect of Si1-xGex, 2 × 2 bifurcation optical active switches have been fabricated, in which the Si1-xGex was grown by molecular beam epitaxy. At a 36 mA injection current, the device reaches maximum optical switching. The crosstalk is less than –11 dB and the insertion loss is 3.8 dB. The measured response time is <100 ns.

Radiation from a dipole antenna on two-layer grounded Faraday chiral substrates

Abstract: The generalized spectral domain-exponential matrix technique (GSD-EMT) is used to investigate the radiation characteristics of a dipole antenna placed on various grounded Faraday chiral substrates, and such Faraday chiral materials include chiroferrite and semiconductor chiroplasma. Numerical results are presented to demonstrate the combined effects of gyrotropy and chirality on the pattern sharp as well as the main beam direction of dipole antenna, and the influences of different constitutive parameters are taken into consideration carefully. It is believed that the present study provide us much insight into the physical properties of Faraday chiral substrates.

Three-Dimensional Natural Convection in an Enclosure With an Internal Isolated Vertical Plate

Abstract: Natural convection in a cubic enclosure with an internal isolated heated vertical plate was investigated both experimentally and numerically. The internal plate was suspended under the lower surface of the enclosure top wall and electrically heated. The six enclosure walls were at a lower constant temperature. The plate average Nusselt number and the air temperature in the enclosure symmetry plane were experimentally determined in the range of Ra = 8 X 10{sup 4} to 5 X 10{sup 7}. Numerical simulations of laminar natural convection in the same configuration were performed. The agreement between the test data and the numerically predicted values is reasonably good, with a maximum deviation in plate average Nusselt number of 12.3 percent and that in air temperature of 9.4 percent. Detailed temperature and velocity distributions in four cross sections were presented for the case of Ra = 6.57 X 10{sup 5}. 23 refs., 7 figs., 1 tab.