Morphology and Frictional Characteristics Under Electrical Currents of Al_2O_3/Cu Composites Prepared by Internal Oxidation

Two Al2O3/Cu composites containing 0.24 wt.% Al2O3 and 0.60 wt.% Al2O3 separately are prepared by internal oxidation. Effects of sliding speed and pressure on the frictional characteristics of the composites and copper against brass are investigated and compared. The changes in morphology of the sliding surface and subsurface are examined with scanning electron microscope （SEM） and energy dispersive X-ray spectrum （EDS）. The results show that the wear resistance of the Al2O3/Cu composites is superior to that of copper under the same conditions, Under a given electrical current, the wear rate of Al2O3/Cu composites decreases as the Al2O3-content increases, However, the wear rates of the Al2O3/Cu composites and copper increase as the sliding speed and pressure increase under dry sliding condition. The main wear mechanisms for Al2O3/Cu composites are of abrasion and adhesion; for copper, it is adhesion, although wear by oxidation and electrical erosion can also be observed as the speed and pressure rise.

Effect of electrical current on tribological property of Cu matrix composite reinforced by carbon nanotubes

Cu matrix composite reinforced with 10%（volume fraction） carbon nanotubes（CNTs/Cu） and pure Cu bulk were prepared by powder metallurgy techniques under the same consolidation processing condition.The effect of electrical current on tribological property of the materials was investigated by using a pin-on-disk friction and wear tester.The results show that the friction coefficient and wear rate of CNTs/Cu composite as well as those of pure Cu bulk increase with increasing the electrical current without exception,and the effect of electrical current is more obvious on tribological property of pure Cu bulk than on that of CNTs/Cu composite;the dominant wear mechanisms are arc erosion wear and plastic flow deformation,respectively;CNTs can improve tribological property of Cu matrix composites with electrical current.

Microstructure and mechanical properties of GTA weldments of titanium matrix composites prepared with or without current pulsing

The effects of current pulsing on the microstructure, hardness and tensile properties at different temperatures of gas tungsten arc （GTA） weldments of titanium matrix composites were studied. Full-penetration butt joints were made with or without current pulsing. Optical microscopy, hardness test and scanning electron microscopy were employed to evaluate the metallurgical characteristics of welded joints. Tensile properties of weldments at different temperatures were studied and correlated with the microstructure. The results exhibit that current pulsing leads to the refinement of the weld microstrucmre and TiB whisker and the redistribution of reinforcements resulting in higher hardness, tensile strength and ductility of weldments in the as-welded condition.

Effects of current density on preparation and performance of Al/conductive coating/α-PbO_2-Ce O_2-TiO_2/β-Pb O_2-MnO_2-WC-ZrO_2 composite electrode materials

Al/conductive coating/α-Pb O2-Ce O2-Ti O2/β-PbO 2-MnO 2-WC-Zr O2 composite electrode material was prepared on Al/conductive coating/α-PbO 2-Ce O2-Ti O2 substrate by electrochemical oxidation co-deposition technique. The effects of current density on the chemical composition, electrocatalytic activity, and stability of the composite anode material were investigated by energy dispersive X-ray spectroscopy（EDXS）, anode polarization curves, quasi-stationary polarization（Tafel） curves, electrochemical impedance spectroscopy（EIS）, scanning electron microscopy（SEM）, and X-ray diffraction（XRD）. Results reveal that the composite electrode obtained at 1 A/dm2 possesses the lowest overpotential（0.610 V at 500 A/m2） for oxygen evolution, the best electrocatalytic activity, the longest service life（360 h at 40 °C in 150 g/L H2SO4 solution under 2 A/cm2）, and the lowest cell voltage（2.75 V at 500 A/m2）. Furthermore, with increasing current density, the coating exhibits grain growth and the decrease of content of Mn O2. Only a slight effect on crystalline structure is observed.

Effect of Electrical Current on the Tribological Behavior of the Cu-WS_2-G Composites in Air and Vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5N/cm 2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2 , elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.

Tribological behavior of CNTs-Cu and graphite-Cu composites with electric current

CNTs-Cu and graphite-Cu composites were separately prepared by powder metallurgy technique under the same consolidation processing. Tribological behavior of the composites with electric current was investigated by using a pin-on-disk friction and wear tester. The results show that the friction coefficient and wear rate of the composites decrease with increasing the reinforcement content, and increase with increasing the electric current density; the effects of electric current are more obvious on tribological properties of graphite-Cu composites than on CNTs-Cu composites; for graphite-Cu composites the dominant wear mechanisms are electric arc erosion and adhesive wear, while for CNTs-Cu composites are adhesive wear.

Bonded Terfenol-D composites with low eddy current loss and high magnetostriction

Bonded Terfenol-D composites,with high electrical resistivity and low eddy current loss,can be used in an alternating magnetic field with high frequency.However,the nonmagnetic binder impairs the magnetostriction of the composites.To achieve high magnetostriction and low eddy current loss,the mixture of the alloy powder and binder was compressed at low pressure in an oriented magnetic field.After this,the aligned samples were recompressed by cold isostatic pressing（CIP）.Besides,the effect of particle size on the magnetostriction of the bonded Terfenol-D composites was also studied.The results showed that the bonded Terfenol-D composites had excellent magnetostriction when the particle size was 50-80 μm.The oriented magnetic field and CIP could improve the magnetostriction of the bonded composites,which reaches 1020×10-6.The bonded Terfenol-D composites had good compact structure and high density（7.24 g/cm3）.The magnetic loss of the bonded Terfenol-D composites was 192 mW/cm3 at a frequency of 100 kHz in a magnetic field of 960 A/m,which was about one third of that of casting Terfenol-D alloys.

Morphology and tribological properties of Ni/n-SiO_2 composite coatings by pulse-reverse current brush-plating

Ni/n-SiO2 composite coating was electrodeposited by brush-plating with pulse-reverse current（RC）. The morphology, hardness, and tribological properties of the coating were investigated and compared with those of Ni and composite coatings electrodeposited with direct current（DC）. The results indicate that Ni/n-SiO2 composite coating electrodeposited by RC, because of RC and the nano powders, has denser coating, finer crystal grains, higher hardness（HV650.0, nearly 1.5 times higher than that of Ni coating electrodeposited by DC） and lower friction coefficient（nearly 0.62）, as a result, in the wearing experiment, the Ni/n-SiO2 composite coating electrodeposited by RC has the least worn loss. So this kind of coating has better wear resistance. And RC electro brush-plating can be used as a new technology of brush-plating in the area of wear resistance.

Single-phase-to-ground fault protection based on zero-sequence current ratio coefficient for low-resistance grounding distribution network

Definite-time zero-sequence over-current protection is presently used in systems whose neutral point is grounded by a low resistance(low-resistance grounding systems).These systems frequently malfunction owing to their high settings of the action value when a high-impedance grounding fault occurs.In this study,the relationship between the zero-sequence currents of each feeder and the neutral branch was analyzed.Then,a grounding protection method was proposed on the basis of the zero-sequence current ratio coefficient.It is defined as the ratio of the zero-sequence current of the feeder to that of the neutral branch.Nonetheless,both zero-sequence voltage and zero-sequence current are affected by the transition resistance,The influence of transition resistance can be eliminated by calculating this coefficient.Therefore,a method based on the zero-sequence current ratio coefficient was proposed considering the significant difference between the faulty feeder and healthy feeder.Furthermore,unbalanced current can be prevented by setting the starting current.PSCAD simulation results reveal that the proposed method shows high reliability and sensitivity when a high-resistance grounding fault occurs.

MHz cut-off frequency and permeability mechanism of iron-based soft magnetic composites

The lack of soft magnetic composites with high power density in MHz frequency range has become an obstacle in the efficient operation of the electrical and electronic equipments.Here,a promising method to increase the cut-off frequency of iron-based soft magnetic composites to hundreds of MHz is reported.The cut-off frequency is increased from 10 MHz to 1 GHz by modulating the height of the ring,the distribution of particles,and the particle size.The mechanism of cut-off frequency and permeability is the coherent rotation of domain modulated by inhomogeneous field due to the eddy current effect.An empirical formula for the cut-off frequency in a magnetic ring composed of iron-based particles is established from experimental data.This work provides an effective approach to fabricate soft magnetic composites with a cut-off frequency in hundreds of MHz.

Current-voltage characteristics of lead zirconate titanate/nickel bilayered hollow cylindrical magnetoelectric composites

Current--voltage measurements obtained from lead zirconate titanate/nickel bilayered hollow cylindrical magnetoelectric composite showed that a sinusoidal current applied to the copper coil wrapped around the hollow cylinder circumference induces voltage across the lead zirconate titanate layer thickness. The current--voltage coefficient and the maximum induced voltage in lead zirconate titanate at 1~kHz and resonance （60.1~kHz） frequencies increased linearly with the number of the coil turns and the applied current. The resonance frequency corresponds to the electromechanical resonance frequency. The current--voltage coefficient can be significantly improved by optimizing the magnetoelectric structure geometry and/or increasing the number of coil turns. Hollow cylindrical lead zirconate titanate/nickel structures can be potentially used as current sensors.

Influence of current density on nano-Al_2O_3/Ni+Co bionic gradient composite coatings by electrodeposition

Metal and nano-ceramic nanocomposite coatings were prepared on the gray cast iron surface by the electrodeposition method. The Ni-Co was used as the metal matrix,and nano-Al2O3 was chosen as the second-phase particulates. To avoid poor inter-face bonding and stress distribution,the gradient structure of biology materials was found as the model and therefore the gradient composite coating was prepared. The morphology of the composite coatings was flatter and the microstructure was denser than that of pure Ni-Co coatings. The composite coatings were prepared by different current densities,and the 2-D and 3-D morphologies of the surface coatings were observed. The result indicated that the 2-D structure became rougher and the 3-D surface density of apices became less when the current density was increased. The content of nanoparticulates reached a maximum value at the current density of 40mA·cm^-2,at the same time the properties including microhardness and wear-resistance were analyzed. The microhardness reached a maximum value and the wear volume was also less at the current density of 40mA·cm^-2. The reason was that nano-Al2O3 particles caused dispersive strengthening and grain refining.

Zero-sequence Current Suppressing Strategy for Dual Three-phase Permanent Magnet Synchronous Machines Connected with Single Neutral Point

Dual three-phase permanent-magnet synchronous machines(DTP-PMSM)connected with a single neutral point provide a loop for zero-sequence current(ZSC).This paper proposes a novel space vector pulse width modulation(SVPWM)strategy to suppress the ZSC.Five vectors are selected as basic voltage vectors in one switching period.The fundamental and harmonic planes and the zero-sequence plane are taken into consideration to synthesis the reference voltage vector.To suppress the ZSC,a non-zero zero-sequence voltage(ZSV)is generated to compensate the third harmonic back-EMF.Rather than triangular carrier modulation,the sawtooth carrier modulation strategy is used to generate asymmetric PWM signals.The modulation range is investigated to explore the variation of modulation range caused by considering the zero-sequence plane.With the proposed method,the ZSC can be considerably reduced.The simulated and experimental results are presented to validate the effectiveness of the proposed modulation strategy.

Effects of pulse current on properties of electrodeposited RE-Ni-W-P-SiC composite coating

Effects of pulse current on properties of electrodeposited RE-Ni-W-P-SiC composite coating were studied. The results show that hardness of the pulse current electrodeposited composite coatings is higher than that of the direct current electrodeposited composite coatings while other parameters are the same. Otherwise, the deposited velocity is higher, and wear resistance and corrosion resistance of the pulse current electrodeposited composite coatings are also improved when the average current density is 10 A/dm2 , pulse frequency is 800 Hz and duty ratio is 1: 5. The hardness of the coating as-deposited is HV500 - 700, and it reaches HV1300 after heat treatment at 400 ℃ for 1 h.

Hydrodynamic Characteristics of Submarine Composite Pipeline in a Wave-Current Coexisting Field

-A composite pipeline is defined as a pipeline system composed of one big pipe and one or several small pipes. Based on the theory of wave- current interaction and physical model test, the hydrodynamic characteristics of the submarine composite pipeline in wave-current coexisting field (both regular and irregular waves) are investigated. The so-called 'modified diameter method' is used for analyzing the in-fine hydrodynamic coefficients of the composite pipeline, which are well related to KC number. The comparison of test data for regular and irregular waves shows that in the region of 90 > KC> 20, the results in these two cases can be unified. The effect of water depth is analyzed in details. The relationships between CD, CM and KC , which are based on the results of present research, may be used as a reference in engineering design.

Microstructure and Mechanical Properties of Submicron-grained NiAl-Al_2O_3 Composite Prepared by Pulse Current Auxiliary Sintering

Dense and submicron-grained NiAl-Al2O3 composite was fabricated by pulse current auxiliary sintering（PCAS）.Its microstructure was analyzed by XRD,SEM and TEM,and its mechanical behavior was evaluated through compression test and fracture toughness test.The average grain sizes of NiAl and Al2O3 are about 200 nm and 100 nm respectively.The Al2O3 particles dispersed in NiAl matrix,forming intergranular structure and intragranular structure.During sintering,Al2O3 particles were remarkably spherized due to the unique sintering mechanism of PCAS,which is beneficial to the improvement of toughness.The NiAl-Al2O3 composite exhibits high compressive yield strength,whether at room temperature or elevated temperature.Its room-temperature（23 ℃） and elevated-temperature（1 200 ℃） compressive yield strength are up to 2 050 MPa and 140 MPa,respectively.Meanwhile,its fracture toughness is significantly enhanced,which is up to 8.2 MPa？m1/2.It is suggested that the main strengthening-toughening mechanisms are grain refinement strengthening and Al2O3 dispersion strengthening.The fracture of larger NiAl grain is the transgranular cleavage and this is induced by crack tip deflection and grain boundary weakening which are caused by intergranular and intragranular Al2O3 particles,respectively.

Zero-Sequence Current Suppression Strategy for Open-End Winding Permanent Magnet Synchronous Motor Based on Model Predictive Control

Compared with the traditional three-phase star connection winding,the open-end winding permanent magnet synchronous motor(OW-PMSM)system with a common direct current(DC)bus has a zero-sequence circuit,which makes the common-mode voltage and the back electromotive force(EMF)harmonic generated by the inverters produce the zero-sequence current in the zero-sequence circuit,and the zero-sequence current has great influence on the operation efficiency and stability of the motor control system.A zero-sequence current suppression strategy is presented based on model predictive current control for OW-PMSM.Through the mathematical model of OW-PMSM to establish the predictive model and the zero-sequence circuit model,the common-mode voltage under different voltage vector combinations is fully considered during vector selection and action time calculation.Then zero-sequence loop constraints are established,so as to suppress the zero-sequence current.In the end,the control strategy proposed in this paper is verified by simulation experiments.

Single-phase earth fault current distribution between optical fiber composite overhead ground wire and ordinary ground wire in transmission system

It is important for the safety of transmission system to accurately calculate single-phase earth fault current distribution.Features of double sided elimination method were illustrated.Quantitative calculation of single-phase earth fault current distribution and case verification were accomplished by using the loop method.Influences of some factors,such as single-phase earth fault location and ground resistance of poles,on short-circuit current distribution were discussed.Results show that:1) results of the loop method conform to those of double sided elimination method;2) the fault location hardly influences macro-distribution of short-circuit current.However,current near fault location is evidently influenced;and 3) the short-circuit current distribution is not so sensitive to the ground resistance of poles.

The Lift Forces Acting on a Submarine Composite Pipeline in a Wave-Current Coexisting Field

A composite pipeline is defined as a main big pipe composed of one or several small pipes. The flow behaviour around a submarine composite pipeline is more complicated than that around a single submarine pipeline. A series model test of composite pipelines in a wave-current coexisting field was conducted by the authors. Both in-line and lift forces were measured, and the resultant forces were also analyzed. The results of lift forces and resultant forces are reported in this paper. It is found that the lift force coefficients for composite pipelines are well related to the KC number. The lift force coefficients for an irregular wave-current coexisting field are smaller than those for a regular wave-current coexisting field. The frequency of lift force is usually twice the wave frequency or higher. The authors test indicates that the resultant forces are about 10 to 20 percent larger than in-line forces (horizontal forces). The effect of water depth is analyzed. Finally, the relationship between lift force coefficient CL and KC number, the statistical characteristics of lift and resultant forces, are given in this paper, which may be useful for engineering practice.

Hashin Failure Theory Based Damage Assessment Methodology of Composite Tidal Turbine Blades and Implications for the Blade Design

A damage assessment methodology based on the Hashin failure theory for glass fiber reinforced polymer(GFRP)composite blade is proposed. The typical failure mechanisms including the fiber tension/compression and matrix tension/compression are considered to describe the damage behaviors. To give the flapwise and edgewise loading along the blade span, the Blade Element Momentum Theory(BEMT) is adopted. In conjunction with the hydrodynamic analysis, the structural analysis of the composite blade is cooperatively performed with the Hashin damage model. The damage characteristics of the composite blade, under normal and extreme operational conditions,are comparatively analyzed. Numerical results demonstrate that the matrix tension damage is the most significant failure mode which occurs in the mid-span of the blade. The blade internal configurations including the box-beam, Ibeam, left-C beam and right-C beam are compared and analyzed. The GFRP and carbon fiber reinforced polymer(CFRP) are considered and combined. Numerical results show that the I-beam is the best structural type. The structural performance of composite tidal turbine blades could be improved by combining the GFRP and CFRP structure considering the damage and cost-effectiveness synthetically.