Showing papers by "Xi'an Jiaotong University published in 1996"

Effect of powder structure on the structure of thermally sprayed WC-Co coatings

Abstract: Five representative types of WC-Co powders were selected to clarify the dependence of the structure of sprayed coatings on the structure of powders themselves. The WC-Co coatings were sprayed with the Jet-Kote process and plasma spraying as well. The structure of WC-Co coatings was primarily characterized by X-ray diffraction. The X-ray diffraction patterns of the sprayed coatings were illustrated compared with those of powders, which aims at a better understanding of the structure of thermally sprayed WC-Co coatings. The selected coating was also analysed by differential scanning calorimetry (DSC). The decarburizing process, and the effects of powder structure and spray conditions on the crystal structure of sprayed WC-Co coatings are discussed in detail.

Gas-sensing property and mechanism of CaxLa1−xFeO3 ceramics

Abstract: LaFEO3 and CaxLa1−xFeO3 ceramic powders have been prepared by the coprecipitation method from La(NO3)3, Fe(NO3)3 and Ca(NO3)2 aqueous solutions. The orthorhombic perovskite phases of LaFeO3 and CaxLa1−xFeO3 are characterized by X-ray diffraction patterns. The sensors fabricated with those powders have high sensitivity to alcohol. Partial substitution of La3+ in LaFeO3 with Ca2+ can enhance the sensitivity of the materials to reducing gases. The resistance of an LaFeO3 sensor in air, vacuum and alcohol-containing air has been measured. Complex impedance spectroscopy has been used to try and analyse the gas-sensing mechanism. According to the experimental results, it can be deduced that the surface adsorptive and lattice oxygen govern the sensing properties of LaFeO3 and CaxLa1−xFeO3 ceramics.

Formation of an amorphous phase in thermally sprayed WC-Co

Abstract: A WC-Co coating was sprayed by the high-velocity oxyfuel process using a feedstock of tungsten carbide clad with cobalt. The structure of the sprayed coating was characterized by x-ray diffraction (XRD), differential scanning calorimetry (DSC), and differential thermal analysis (DTA). It was found that an amorphous phase of Co-W-C ternary alloy observed as a large, broad peak in the XRD pattern can be formed in the as-sprayed WC-Co coating. The DSC, DTA, and XRD analyses revealed that the amorphous phase crystallized at a temperature of around 873 K to metallic cobalt, Co6W6C, and tungsten with appreciable precipitation of free carbon. The heat treatment of as-sprayed WC-Co coating at a high temperature of 1173 K suggests that annealing at a temperature higher than about 1104 K will promote the reaction of tungsten and cobalt with carbon to form the complex carbide C06W6C.

Feasibility study of fractional frequency transmission system

Abstract: In their previous paper (1994), the authors proposed a new transmission system-the fractional frequency transmission system (FFTS) which uses lower frequency to reduce the reactances of AC transmission systems. This approach can multiply increase transmission capacity and remarkably improve operating performance. The feasibility and efficiency of the FFTS is investigated in this paper by computer simulation. A mathematical model and an efficient algorithm are developed for this purpose. Computer simulation results illustrate that the FFTS is a very promising new transmission system.

Direct measurement of transverse residual strains in aorta.

Abstract: Residual strains were measured in the porcine aorta. Segments were cut from the aorta perpendicular to its longitudinal axis. Microdots of water-insoluble black ink were sprinkled onto the transverse sectional surface of the segments in the no-load state. The segments were then cut radially, and sectional zero-stress states were approached. The coordinates of selected microdots (2-20 microns) were digitized from photographs taken in the no-load state and the zero-stress state. Residual strains in the transverse section were calculated from the displacement of the microdots. The circumferential residual strains on the inner wall and outer wall were calculated from the circumferential lengths in the no-load state and the zero-stress state. Results show that the circumferential residual strain is negative (compressive) in the inner layer of the aortic wall and positive (tensile) in the outer layer, whereas the radial residual strain is tensile in the inner layer and compressive in the outer layer. This residual strain distribution reduces the stress concentration in the aorta under physiological load. The experimental results compared well with theoretical estimations of a cylindrical model. Regional difference of the residual strain exists and is significant (P < 0.01), e.g., the circumferential residual strains on the inner wall of the ascending, descending thoracic, and abdominal regions of the aorta are -0.133 +/- 0.019, -0.074 +/- 0.020, and -0.046 +/- 0.017 (mean +/- SD), respectively. More radial cuts of a segment produced no significant additional strains. This means that an aortic segment after one radial cut can be considered as the zero-stress state.

The dynamic measurement of surface charge distribution deposited from partial discharge in air by Pockels effect technique

Abstract: A recently developed electro-optical Pockels effect technique, which allows the dynamic measurement of surface charge distributions, is employed to observe the dynamic behaviour of a partial discharge in a point - dielectric gap. One period of an 8 kV ac voltage is applied to a needle electrode with gap spacings ranging from 0 to 7 mm. Different discharge behaviour and different deposited surface charge distribution patterns are observed for different point - dielectric gap spacings. The electrode gap spacings can be roughly divided into three regions, namely the surface discharge region, the transition region and the space discharge region. In the transition region, both surface discharges and space discharges which consist of a spark discharge and corona discharge are observed. The memory effect associated with the accumulated surface charges plays an important role in controlling the discharge behaviour. The surface charge field component may either suppress or enhance the local field near the point electrode depending on the phase of the applied voltage. `Back-discharges' are the typical result of the memory effect. Negative surface charge deposited from previous discharges can have a considerable influence on the propagation of the subsequent positive surface streamer channels depending on whether they develop in the surface discharge region or in the transition region.

The lamellar structure of a detonation gun sprayed Al2O3 coating

Abstract: The structural features of a detonation gun sprayed Al 2 O 3 coating were examined using a copper electroplating technique. It is revealed that the detonation gun sprayed Al 2 O 3 coating has a typical layer structure similar to that of coatings deposited using other thermal spraying processes. Despite having the highest particle velocity of the thermal spraying processes, lamellar bonding at the interfaces between flattened particles in detonation gun sprayed Al 2 O 3 coatings is very poor. The mean bonding ratio of bonded interface area to apparent bonding interface for a typical detonation gun Al 2 O 3 coating is about 10%, which is less than one-third the value for a typical plasma-sprayed Al 2 O 3 coating. However, the high particle velocity results in the formation of a rough surface on the spray splat, which may be effective in enhancing interlocking between flattened particles.

Theory of including degrees and its applications to uncertainty inferences

Abstract: Uncertainty inference is a key problem in artificial intelligence. The concept of including degree is introduced and some methods of generating including degrees are given. Some applications of including degrees to the retrieval and inference in expert systems are demonstrated.

A Modified Entropy Generation Number for Heat Exchangers

Abstract: This paper demonstrates the difference between the entropy generation number method proposed by Bejan and the method of entropy generation per unit amount of heat transferred in analyzing the thermodynamic performance of heat exchangers, points out the reason for leading to the above difference. A modified entropy generation number for evaluating the irreversibility of heat exchangers is proposed which is in consistent with the entropy generation per unit amount of heat transferred in entropy generation analysis. The entropy generated by friction is also investigated. Results show that when the entropy generated by friction in heat exchangers is taken into account, there is a minimum total entropy generation number while the NTU and the ratio of heat capacity rates vary. The existence of this minimum is the prerequisite of heat exchanger optimization.

Triangulations intersect nicely

Abstract: We show that there is a matching between the edges of any two triangulations of a planar point set such that an edge of one triangulation is matched either to the identical edge in the other triangulation or to an edge that crosses it. This theorem also holds for the triangles of the triangulations and in general independence systems. As an application, we give some lower bounds for the minimum-weight triangulation which can be computed in polynomial time by matching and network-flow techniques. We exhibit an easy-to-recognize class of point sets for which the minimum-weight triangulation coincides with the greedy triangulation.

Zero-gap directional coupler switch integrated into a silicon-on insulator for 1.3-microm operation.

Abstract: A silicon-on insulator (SOI) zero-gap directional coupler switch is studied based on the large-cross-section singlemode rib waveguide condition, the dual-mode interference principle, and the free-carrier plasma dispersion effect, in which the SOI technique uses silicon and silicon dioxide thermal bonding and backpolishing. The SOI is fabricated by potassium hydroxide anisotropic etching. Its insertion loss and cross talk are measured to be less than 4.81 dB and 218.6 dB, respectively, at a wavelength of 1.3 microm and a switching voltage of 0.91 V. Response time is ~210 ns.

Study on the properties and application of epoxy resin/polyurethane semi‐interpenetrating polymer networks

Abstract: Damping properties of epoxy resin/polyurethane (EP/PU) semi-interpenetrating polymer networks (IPNs) were studied by the dynamic mechanical analysis (DMA) method. It shows that the semi-IPNs have excellent damping properties at ordinary temperature. The maximum value of tan δ is about 1 when the weight composition of EP/PU is 70/30. Tensile tests also indicate that the system has good tensile strength and elongation at break at this ratio. The effects of structure on the properties of the semi-IPNs are discussed. Applied to the cavitation corrosion resistant coating, the semi-IPNs show good cavitation corrosion resistance. © 1996 John Wiley & Sons, Inc.

Temperature measurement and metallurgical study of cathodes in DC arcs

Abstract: Arc-cathode erosion has been studied fur pure tungsten and 2% thoriated tungsten cathodes. Measurements of cathode temperature distributions with high spatial resolution have been carried out using an optical multichannel analyzer (OMA)/spectrometer system. The cathode spot behavior has been observed with a tele-microscope and a high speed vision system simultaneously with the temperature measurement. Cathodes have been examined by scanning electron microscope (SEM) and energy dispersive x-ray spectrometer (EDX) after arcing. The results show that the initial cathode shape has no effect on cathode erosion for tungsten cathodes because the major erosion mechanism of tungsten cathodes is the ejection of molten cathode material. For 2% thoriated tungsten cathodes, the cathode geometry has little effect on the cathode tip temperature but plays an important role in the erosion. The thorium is depleted from the cathode spot due to fast evaporation, which increases the work function of the cathode. Deposition of thorium and tungsten crystals at the cathode surface outside of the cathode spot occurs. The conclusions with respect to cathode design are that steep temperature gradients resulting from overcooling should be avoided, and that an optimization of the thermionic emitter needs to take into account both a low work function and an appropriate diffusion rate in the metal matrix.

Wavelet transform and multiresolution signal decomposition for machinery monitoring and diagnosis

Abstract: Multiresolution signal decomposition based on wavelet transform or wavelet packet provides a set of decomposed signals in independent frequency bands, which contain most independent dynamic information due to the orthogonality of wavelet functions. Wavelet transform and wavelet packet in tandem with some signal processing methods, such as autoregressive spectrum, energy monitoring, fractal dimension, etc., can produce many desirable results for condition monitoring and fault diagnosis of machinery. Nonstationary fluctuation was extracted, weak defect of ball bearings was detected from the vibrations and latent fault diagnosis was realized at the early stage. Energy condition monitoring and fractal dimension analysis for nonlinear looseness fault were introduced.

The global bifurcation characteristics of the forced van der Pol oscillator

Abstract: In this paper, the bifurcation characteristics of the forced van der Pol oscillator on a specific parameter plane, including intermediate parameter regions, are investigated. The successive arrangement of the dominant mode-locking regions, where a single subharmonic solution with the rotation number, 1 (2k + 1) , exists, and the transitional zones between them are depicted. The transitional zones are explicitly proposed to be classified into two groups according to the different global characters: (1) the simple transitional zones, in which coexistence of two mode-locked solutions with rotation numbers 1 (2k ± 1) appear; (2) the complex transitional zones, in which the sub-zones with the mode-locked solutions, whose rotation numbers are rational fractions between 1 (2k + 1) and 1 (2k − 1) , and the quasi-periodic solutions exist. The emphasis of this paper is to study the evolution of the global structures in the transitional zones. A complex transitional zone generally evolves from a Farey tree, when the forcing amplitude is small, to a chaotic regime, when forcing amplitude is sufficiently large. It is of great interest that the sub-zone with a rotation number, 1 2k , which has the largest width within a complex transitional zone, can usually intrude into the dominant regions of 1 (2k − 1) before it completely vanishes. Moreover, the features of overlaps of mode-locking sub-zones and the number of coexistence of different attractors are also discussed.