Papers by Keyword: Polarity

Abstract: The chemical vapor deposition (CVD) growth of boron carbide (B_xC) layers on 4H-SiC, 4°off substrates was studied. Depending on the polarity of the substrate, different results were obtained. On Si face, the direct CVD growth at 1600°C under a mixture of BCl₃+C₃H₈ systematically led to polycrystalline B_xC films, whatever the C/B ratio in the gas phase. On the C face, heteroepitaxial growth was obtained for C/B ratios = 12 or higher with a step bunched morphology. If a boridation step (10 min at 1200°C under BCl₃ flow) was used before the CVD growth, then heteroepitaxy was successful on both substrate polarities. To explain these results, a mechanism is proposed which involves the nature of the chemical bonds at the early stage of nucleation. It is suggested that a full B coverage of the SiC surface should favor the nucleation of the B-rich (0001) plane of B_xC, promoting thus the heteroepitaxial growth along this direction.

Abstract: In electrical discharge machining (EDM) process, electrode polarity plays an important role during machining operation. This paper addresses the issues of EDM utilizing the positive and negative tool-electrode polarity to explore the effects on the performance criteria such as material removal rate (MRR), tool wear rate (TWR), surface roughness (SR) and micro-hardness during machining of Ti–5Al-2.5Sn (GradeVI) Ti alloy. The Ti-5Al-2.5Sn alloy was machined using copper-chromium electrode with positive and negative polarity by varying the peak current and pulse-on-time, while the pulse-off-time was kept constant. The result of study suggests that reverse polarity improves the MRR, TWR, SR while normal polarity improves the surface micro-hardness. The peak current has the maximum affect on machining performance for both types of polarities. Further the migration of different elements and formation of compounds on the machined surface was investigated using EDX and XRD analysis.

Abstract: New packings based on Silochrome C80 modified with Cu (II), Co (II) and Ni (II) 8-oxyquinolinates were studied. Their sorption characteristics and chromatographic properties were investigated and the comparative evaluation of sorbents polarity was carried out by applying Rohrschneider-McReynolds coefficients system, thermodynamic data and using the solvation parameter model of Abraham. It was found that the nature of metal and complex structure have a decisive influence on the physico-chemical properties of the packings. Modified silica gels show high selectivity for gas chromatographic separation of oxygen containing and aromatic organic compounds.

Abstract: The off-axis solution growth of 4H-SiC was studied focusing on the morphological instabilities by using conventional TSSG technique. The morphology depends strongly on the crystalline polarity, and that on Si surface can be characterized by wandering while that on C surface is characterized by strong step-bunching. By raising the temperature gradient, step bunching on Si surface is considerably suppressed which can be consistent to the constitutional super cooling scheme. However, C surface exhibits strong step bunching as the temperature gradient increase. These behaviors can be explained by the difference in Ehrlich-Schwoebel barrier and diffusion behavior of adatoms.

Abstract: Various types of shielding gases mixes are used for the industrial GMAW applications. The use of the direct current welding process with reverse or direct polarity influences the geometric characteristics of the beads as well as process dilution. In certain situations, such as weld deposits, the penetration should be as shallow as possible and the heat affected zone of the bead should be reduced. The paper presents the experimental procedure, the obtained results and the conclusions concerning the influence of polarity on the shape and geometry of the welding beam, while using various shielding gases and various welding parameters and by changing polarities.

Abstract: Electrical discharge grinding (EDG) is an advanced machining process and can be utilised to fabricate complex geometry of PCD tools. However, the PCD removal mechanism in this process is complicated. This study was carried out to understand the difference in PCD surface structure with difference EDG polarities. The study revealed that the finishing process with negative polarity is the reason for the porous structure on the surface. Further analysis on the chemical element and carbon structure were implemented as the morphological examination of the surface.

Abstract: Current-voltage (I-V) and impedance spectroscopy are employed to investigate the polarity of TiO₂ varistor. It is found that TiO₂ varistor shows typical polarity behavior, i.e., the I-V curve is asymmetric, the impedance slightly increases, and the capacitance slightly decreases when the bias voltage reverses. The effects of measuring frequency on impedance spectroscopy are systematically investigated. The results indicate that TiO₂ varistor behaves considerable polarity under low sweeping frequency. High sweeping frequency (over 10³ Hz) counteracts the polarity behavior. A model of micro-polarized zone is tentatively proposed to clarify the polarity behavior of TiO₂ varistor.

Abstract: Mechanochemical (MC) processes are often modified to a large extent with use of process control agents (PCAs). An appropriate PCA is, however, chosen semi-empirically for any MC process such as ball-milling. In this study, milling period and medium, PCA kinds and amount, and powder kinds, are altered as major parameters to systematically and mechanistically investigate the effect of PCAs on powder milling and the related phenomena. The model systems selected are TiO₂ and ZnO powders with various PCAs including alkanes, alcohols, water, and a few ionic liquids. According to X-ray line broadening analyses the viscosity and the structure of PCA molecules strongly affect the MC process. In addition, the MC process for harder TiO₂ is quite different from ZnO, and oxygen-derived polarity of PCA molecules would cause major difference on the milling behavior. Various parameters controlling the MC process will be discussed.

Abstract: Three-dimensional molecular dynamics (3D MD) simulation was carried out to investigate the deformation of single crystal gallium arsenide (GaAs) during nanoindentation. Tersoff potential was used to simulate the atomistic interaction under an extremely low load of indentation. The coordination number and atomic displacement were studied and the cross-sectional profiles of the simulated indent were examined. The simulation results revealed that the lattice deformation of GaAs was influenced by polarity, showing distinct patterns on different crystalline planes. Slip band and dislocation were found to be the dominant deformation phenomena.

Abstract: With the extensive use of electronic current transformer, check the original method cannot meet the requirements of electronic current transformer performance testing technology. In 500 kV Jincheng a variable electronic current transformer as the test object, the principle and design characteristics of electronic current transformer, proposes an effective test method, ratio and through field test of low power coils and Rogowski coils 2 coil error, phase error and polarity, verify the validity and accuracy of the method.