Showing papers by "Tsinghua University published in 1990"

Confined electron and hydrogenic donor states in a spherical quantum dot of GaAs-Ga1-xAlxAs.

Abstract: According to hydrogenic-effective-mass theory, exact solutions and quantum-level structures are presented for confined electron and hydrogenic donor states in a spherical quantum dot (SQD) of GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As. Calculated results reveal that the values of the quantum levels of a confined electron in a SQD can be quite different for cases with finite and infinite barrier heights. The quantum-level sequence and degeneracy for an electron in a SQD are similar to those of a super- atom of GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As but different from those in a Coulomb field. There is stronger confinement and larger binding energy for a hydrogenic donor in a SQD of GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As than in the corresponding quantum-well wires and two-dimensional quantum-well structures. The binding energy and its maximum of the ground state of a donor at the center of a quantum well are found to be strongly dependent on the well dimensionality and barrier height.

Evaluation by indentation of fracture toughness of ceramic materials

Abstract: A transition fracture mode from Palmqvist to median has been observed in a number of ceramic materials A new expression to determine the fracture toughness (KIC) by indentation is presented The KIC values calculated by this formula are independent of the crack profile (median or Palmqvist) and of the applied load This formula has been obtained by modifying the universal curve of Evans and Charles to incorporate Palmqvist and median cracks over a wide range of loads in the case of brittle materials with different mechanical properties (elastic properties: E, v, KIC)

Texture analysis and discrimination in additive noise

Abstract: Texture feature extraction and discrimination in an additive noise environment is considered. A new set of 28 texture features derived in the spatial frequency domain is presented. A successive addition and deletion feature selection scheme based on the Wilks criterion is used to obtain a subset of features which effectively discriminate among a set of sample textures. The selected features are further evaluated according to their ability to discriminate and classify samples of natural textures corrupted by additive noise at various signal to noise ratios (SNR). Samples from five texture classes were classified using only four features with an accuracy of at least 92% for all SNR greater than or equal to one. Performance for a SNR of 0.1 dropped to 70%. The four texture features extracted measure the dominant peak energy. power spectrum shape, and entropy. These measures are insensitive to additive noise and are effective for texture discrimination.

Thermal vacancies and positron-lifetime measurements in Fe 76.3 Al 23.7

Abstract: The formation of thermal vacancies in the intermetallic compound ${\mathrm{Fe}}_{76.3}$${\mathrm{Al}}_{23.7}$ was investigated between 20 and 1060 \ifmmode^\circ\else\textdegree\fi{}C by positron-lifetime measurements. The positron lifetime ${\mathrm{\ensuremath{\tau}}}_{\mathit{f}}$=112 ps at 20 \ifmmode^\circ\else\textdegree\fi{}C indicates that no structural vacancies can be detected. The increase of the mean positron lifetime \ensuremath{\tau}\ifmmode\bar\else\textasciimacron\fi{} due to thermal vacancy formation starts at relatively low temperatures (${\mathit{T}}_{3}$=475 \ifmmode^\circ\else\textdegree\fi{}C). The fit of a simple two-state trapping model to the temperature variation of \ensuremath{\tau}\ifmmode\bar\else\textasciimacron\fi{} yields an effective vacancy formation enthalpy ${\mathit{H}}^{\mathit{F}}$=1.18\ifmmode\pm\else\textpm\fi{}0.04 eV, which suggests a thermal vacancy concentration at the melting temperature of several atomic percent which is much higher than in pure metals and similar to that found in some intermetallic compounds with a B2 structure. From a comparison with self-diffusion results, a vacancy migration enthalpy of ${\mathit{H}}^{\mathit{M}}$=1.4--1.6 eV is deduced. The phase transitions in the present alloy are discussed in terms of vacancy formation and migration.

A mechanism for the formation of lower bainite

Abstract: A diffusional mechanism for the formation of lower bainite is proposed based primarily on transmission electron microscopy (TEM) observations of isothermally reacted specimens of Fe-C-2 pct Mn alloys. The mechanism involves the initial precipitation of a nearly carbide-free ferrite“spine,” followed by sympathetic nucleation of“secondary (ferrite) plates” which lie at an angle to the initial“spine.” Carbide precipitation subsequently occurs in austenite at ferrite: austenite boundaries located in small gaps between the“secondary plates.” An“annealing” process then occurs in which the gaps are filled in by further growth of ferrite and additional carbide precipitation; the annealing out of ferrite: ferrite boundaries between impinged“secondary plates” completes this process. This annealing stage contributes to the final appearance of lower bainite sheaves as monolithic plates containing embedded carbides. The present mechanism accounts for the single variant of carbides oriented at an angle to the sheaf axis repeatedly reported in lower bainite; it is also consistent with the previous observation of one“rough” side and one“smooth” side of lower bainite“plates.”

Calculation of DC and AC flashover voltage of polluted insulators

Abstract: The phenomena and the propagation processes of DC and AC arcs on the surface of polluted insulators were observed and investigated by means of a high-speed camera. Time-resolved photographs indicate that there is an obvious distinction between DC and AC flashover processes. Based on the alternating character of an AC arc on the polluted surface, the relationship between its reignition and recovery conditions is analyzed. The most important factor is the recovery rather than the reignition condition. A formula expressing the resistance of the pollution layer on the insulator is derived. From this, the flashover voltage of a polluted insulator of complex shape can be calculated. A computer program has been developed to calculate the flashover under both DC and AC. Some arc phenomena, such as arc bridging between the sheds or ribs of insulators, the drift of the arc from the insulator surface, and the number of arcs in series before flashover, are taken into account. Much experimentation has been done to determine the flashover voltages of polluted insulators with complex shapes under both DC and AC. Experimental data are in good agreement with the calculated results. >

Mechanism of flow regime transition from bubbling to turbulent fluidization

Abstract: The flow regimes of a dense-phase gas-solid fluidized bed have become a topic of increasing interest as a result of intensive development in fluidization research Through a plot of the bed's pressure fluctuation vs gas velocity curve, most investigators have determined the critical transition velocity from bubbling to turbulent fluidization This critical velocity, u{sub c}, was defined as the velocity corresponding to the peak point of that curve However, the mechanism of this flow regime transition has not been realized clearly The mechanism that can fairly fit the experimental phenomena is still not available, although some attempts have been proposed This paper focused on revealing that mechanism

Birefringent tuning double frequency He-Ne laser.

Abstract: Experimental details are given of a novel, promising kind of double frequency laser—with power output of 0.8–1.4 mW, large frequency difference from 37 MHz to a longitudinal mode interval, and frequency stability of 10−5, and its potential uses are discussed.

A fast transistor-chaining algorithm for CMOS cell layout

Abstract: A fast algorithm is proposed for the transistor-chaining problem in CMOS functional cell layout based on the layout style of T. Uehara and W.M. van Cleemput (1981). The algorithm takes a transistor-level circuit schematic and outputs a minimum set of transistor chains. Possible diffusion abutments between the transistor pairs are modeled as a bipartite graph. A depth-first search algorithm is used to search for the optimal chaining. Theorems on the set of branches that needs to be explored at each node of the search tree are derived. A theoretical lower bound on the size of the chain set is also derived. This bound enables one to prune the search tree efficiently. The algorithm has been implemented and tested and is able to find optimal solutions almost instantly for all the cases from the literature that were examined. >

Simulation of Vortex-Shedding Flow About a Circular Cylinder at High Reynolds Numbers

Abstract: Vortex shedding over a circular cylinder is modeled based on the weakly compressible flow equations with a simple subgrid scale turbulence model and a simple hybrid boundary condition. An explicit finite volume method is used. A subcritical and a supercritical case are computed. The large-scale vortex-shedding phenomenon, the primary vortices, and the related oscillatory lift and drag can be calculated fairly well with a grid system coarser than the boundary layer thickness

Hadronic transitions of D-wave quarkonium and psi (3770)-->J/ psi + pi pi.

Abstract: The hadronic transition $\ensuremath{\psi}(3770)\ensuremath{\rightarrow}\frac{J}{\ensuremath{\psi}}+\ensuremath{\pi}\ensuremath{\pi}$ is studied in the framework of the QCD multipole expansion. The calculated rate compares favorably with the recent result from the Mark III Collaboration. We also have compared our theoretical approach with others.

Low temperature sintering of lead magnesium nickel niobate zirconate titanate (PMN-PNN-PZT) piezoelectric ceramic, with high performances

Abstract: Low temperature sintering of quarternary system lead magnesium nickel niobate zirconate titanate (PMN-PNN-PZT) piezoelectric ceramics has been investigated. This ceramic with proper additives could be sintered at about 900°C and exhibited excellent piezoelectric properties: Kp = 0.61–0.65, Qm ≥ 1000, ϵT/33/ϵ0 = 2300–2800, d 33 = 430–465 × 10−12 C/N, tgδ ≤ 50 × 10−4, ρ = 7.9–8.01 g/cm3. The microstructure and sintering mechanism of the materials has been studied by means of SEM and TEM.

Influence of carbon concentration and reaction temperature upon bainite morphology in Fe-C-2 Pct Mn alloys

Abstract: Isothermal transformation of austenite to bainite was studied by optical, replication, and thin foil transmission electron microscopy (TEM) in both hypo- and hypereutectoid Fe-C-2 wt pct Mn alloys (and a single 3 pct Mn alloy) containing from 0.1 to 1.37 wt pct C in order to characterize and explain the transitions which occur in bainite morphology as a function of carbon content and reaction temperature. A “morphology map” was constructed showing the temperaturecarbon composition(T-x) regions in which four different bainite morphologies predominate: upper bainite, lower bainite, nodular bainite, and inverse bainite. Calculations of the volume free energy changes associated with the nucleation of ferrite, †G v α , and cementite, †G v c , and the parabolic rate constant for growth of ferrite, αα, and cementite, αc, were performed in order to explain the observed morphological transitions. In hypoeutectoid alloys, where |†G v α | ≫ |ΔG v c | and αα ≫ α c , the Widmanstatten ferrite-dominated morphologies of upper and lower bainite predominate. The upper-to-lower bainite transition appears to be associated with the emergence of edge-to-face sympathetic nucleation at high values of |ΔG v α |. In hypereutectoid alloys, the ratios ΔG v α /ΔG v c and αα/αc are considerably smaller; hence, cementite can compete much more readily with ferrite during both nucleation and growth, resulting in the formation of nodular bainite. With decreasing temperature in the hypereutectoid regime,ΔG v α /ΔG v c , and especially αga/αc, increase, resulting in the replacement of nodular bainite by lower bainite at temperatures below about 250 °C to 275°C.

A recursive algorithm for simultaneous identification of model order and parameters

Abstract: A recursive identification algorithm for SISO CARMA systems is presented based on an augmented information matrix (AIM). Decomposition of the AIM using UDU factorization provides simultaneous, recursive estimates of both the system parameters and the loss functions from order 0 to n, where n is the maximum possible order of the real process and U and D are upper and diagonal matrices, respectively. The most appropriate model order is then determined by examination of the loss functions. This approach results in a computationally efficient and numerically robust algorithm for systems of unknown or variable model order. >

Progress in outdoor insulation research in China

Abstract: A summary is presented of studies on outdoor insulation in China. Past investigations of the characteristics of outdoor insulation are described. Due to the special features of Chinese topography and the increasing severity of pollution as a result of the development of industry in China, a series of studies was made to understand the influence of atmospheric parameters-air density, humidity, and temperature-on the dielectric strength of outdoor insulation. The influence of a few factors on flashover, such as the flashover criteria for contaminated insulators, the role of local arc with respect to the pollution performance of insulators, and the pollution discharge at low pressure, are discussed. >

Quantum levels and Zeeman splitting for two-dimensional hydrogenic donor states in a magnetic field

Abstract: We have presented exact solutions for two-dimensional (2D) hydrogenic donor states in a magnetic field and a study of quantum levels and Zeeman bifurcation of 2D donor states in different strength of magnetic field. The quadratic effect of the magnetic field partly lifts the degeneracy of 2D donor states in the pure Coulomb potential and results in an increase of all quantum levels. Then, the first-order term of a magnetic field, i.e., the Zeeman term, completely lifts the degeneracy. At strong magnetic field, the degeneracy of Landau levels is lifted by the Coulomb potential. As the magnetic field decreases, all of the values of the energies decrease linearly until the hybridization between Landau and Coulomb levels occurs. In addition, based on the comparison between energies of 2D and 3D states, we conclude that there are smaller diamagnetic shifts for 2D donor states than those for the corresponding 3D donor states.

Formation and Morphology of 2H‐SiC Whiskers by the Decomposition of Silicon Nitride

Abstract: α-SiC whiskers were synthesized by decomposition of Si 3 N 4 at 1900°C in a graphite vessel under a pressure of 10 atm of N 2 . The fibrous crystal produced was identified as 2H-SiC by X-ray diffraction. The whiskers take the shape of straight needles colored white, gray or green

Simultaneous Observation of Disclination and Texture by Optical Microscopy of a Thermotropic Aromatic Polyester

Abstract: An aromatic copolyester (m.p.148°C) film quenched from 180°C melt showed a schlieren texture. The birefringent domains increased in size with increasing temperature of the melt before it was quenched. The polymer film quenched from the melt after being heating above 240°C for 10 min showed both disclinations and a banded texture of random orientation on the same picture, although the banded texture can not be directly observed in the heated melt. However, the banded texture of the quenched film persisted on heating to a melt up to 200°C and began to vanish above this temperature, but the banded texture reappeared on being requenched into a solid film. The banded texture could also be seen in the localities displaying brushes. Consequently schlieren brushes are optical effects but not isotropic material regions.

Hydrogen molecular ions in two dimensions.

Abstract: The exact solutions of the Schr\"odinger equation of a two-dimensional hydrogen molecular ion (2D HMI), expressed in confocal elliptic coordinates, are obtained. Calculated results show that the binding energy and equilibrium distance of the ground state of a 2D HMI are, respectively, much larger and shorter than those of a 3D HMI, and that the behavior of a low-dimensional electron is similar to that of a more massive charged particle. This may be useful for understanding new physical phenomena.