Tribological and electric-arc behaviors of carbon/copper pair during sliding friction process with electric current applied

The tribological behaviors of carbon block sliding against copper ring with and without electric current applied were investigated by using an advanced multifunctional friction and wear tester, and the electric-arc behaviors were analyzed in detail. The results show that the normal load is one of the main controlling factors for generation of electric arc during friction process with electric current applied. The strength of electric arc is enhanced with the decrease of normal loads and the increase of electric currents. The unstable friction process and the fluctuated dynamic friction coefficients are strongly dependent upon the electric arc. The wear volumes and the wear mechanism of carbon brush were affected by the electric arc obviously. As no electric arc occurs, no clear discrepancy of the wear volumes of the carbon samples with and without electric current applied could be detected. While the wear mechanisms are mainly mechanical wear. However, under the condition of the electric arc appearance, the wear volume of carbon with electric current applied increases much more rapidly than that without electric current applied and also increases obviously with the increase of electric current strengths and the decrease of normal loads. The wear mechanisms of carbon block are mainly electric arc ablation accompanying with adhesive wear and material transferring.

Effect of electrodes and thermal insulators on grain refinement by electric current pulse

The application of electric current pulse（ECP） to a solidification process refers to the immersion of electrodes into the liquid metal and the employment of thermal insulators on the upper surface of metal.In order to ascertain the effects of these two factors on the structure refinement by the ECP technique,three groups of experiments were performed with different types of electrodes or various thermal insulators.By the comparison between solidification structures under different conditions,it is followed that the electrode and the thermal insulator have an obvious influence on the grain refinement under an applied ECP,and further analysis demonstrates that the thermal conditions of the liquid surface play a vital role in the modification of solidification structure.Also,the results support the viewpoint that most of the equiaxed grains originate from the liquid surface subjected to an ECP.

Migration behavior of solidification nuclei in pure Al melt under effect of electric current pulse

A mathematical model considering free nuclei was developed to reveal the migration behavior of the free nuclei. Numerical simulation results show that most of the nuclei on the top surface of the melt move downwards and distribute randomly inside the Al melt, which induces more nucleation sites resulting in grain refinement. At the same time, the effect of nuclei size on the nuclei distribution and refinement employing electric current pulse （ECP） was also investigated. The smaller nuclei migrate a short distance with the Al melt at lower speed. But for the larger nuclei, the migration downwards with higher speed benefits the refinement of interior grains of the melt. The research results help to better understand the refinement process and provide a more reasonable explanation of the grain refinement mechanism using ECP.

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.

Microstructure and properties of ultra-fine tungsten heavy alloys prepared by mechanical alloying and electric current activated sintering

The microstructure and properties of the 93W-4Ni-2Co-1Fe(mass fraction,%) tungsten heavy alloys prepared by mechanical alloying and electric current activated sintering from mixed elemental powders were investigated.After 15 h milling,the average W grain size in the powders is decreased to 120 nm.For the powders milled for 15 h,the density,hardness and transverse rupture strength of the alloys sintered only by an intensive pulse electric current are the maximum.When the total sintering time keeps constant,the properties of the sintered alloys can be obviously improved by optimizing the sintering time of pulse-and constant-currents.A bulk ultrafine alloy with an average W grain size of about 340 nm can be obtained by sintering 15 h-milled powders in a total sintering time of 6 min.The corresponding sintered density,hardness and transverse rupture strength reach 16.78 g /cm3,HRA84.3 and 968 MPa,respectively.

Influences of pulse electric current treatment on solidification microstructures and mechanical properties of Al-Si piston alloys

Three kinds of AI-Si piston alloys were prepared and subjected to pulse electric current treatment （PECT） at different pouring temperatures. Some aspects of the solidification microstructures were examined including the morphology and the distribution of the matrix and the secondary phases by using of optical microscopy （OM）, SEM and EDS methods. Results indicate that PECT can refine the grains of α-AI in the alloys as effectively as chemical modification by sodium salt. The processing parameters of PECT on the multi-component AI-Si alloys were then optimized through the testing of tensile strength, elongation and microhardness of the prepared alloys. A new theory was put forward to explain the mechanism of PECT.

Microstructure and Temperature Distribution in ZnAl_2O_4 Sintered Body by Pulse Electric Current

Microstructure of reaction sintering of ZnAl2O4 at 1500℃ by hot-pressing(HP) and pulse electric current was investigated. The results indicated that the existed cracks in sintered body were caused by structure mismatch. It is the evidence that periodical temperature field existed during pulse electric current sintering of nonconductive materials. The distance between high temperature areas was related to die diameter.

Electric Current-induced Failure of 200-nm-thick Gold Interconnects

200-nm-thick Au interconnects on a quartz substrate were tested in-situ inside a dual-beam microscope by applying direct current, alternating current and alternating current with a small direct current component. The failure behavior of the Au interconnects under three kinds of electric currents were characterized in-situ by scanning electron microscopy. It is found that the formation of voids and subsequent growth perpendicular to the interconnect direction is the fatal failure mode for all the Au interconnects under three kinds of electric currents. The failure mechanism of the ultrathin metal lines induced by the electric currents was analyzed.

Photon-Assisted Heat Generation by Electric Current in a Quantum Dot Attached to Ferromagnetic Leads

Heat generated by electric current in a quantum dot device contacting a phonon bath is studied using the non- equilibrium Green function technique. Spin-polarized current is generated owing to the Zeeman splitting of the dot level. The current＇s strength and the spin polarization are further manipulated by changing the frequency of an applied photon field and the ferromagnetism on the leads. We find that the associated heat by this spin- polarized current emerges even if the bias voltage is smaller than the phonon energy quanta and obvious negative differential of the heat generation develops when the photon frequency exceeds that of the phonon. It is also found that both the strength and the resonant peaks＇ position of the heat generation can be tuned by changing the value and the arrangement configurations of the magnetic moments of the two leads, and then provides an effective method to generate large spin-polarized current with weak heat. Such a result may be useful in designing low energy consumption spintronic devices.

Influence of Electric Current on Kirkendall Diffusion of Zn/Cu Couples

The influence of electric current on Kirkendall diffusion in Zn/Cu couples was investigated. Under the action of different electric currents, the Zn/Cu diffusion couples were annealed at 785℃ for different holding time. The experimental results show that the displacement of the Kirkendall plane increases with increasing holding time. However, the displacement of the Kirkendall plane with electric current is larger than that without electric current. The relationship between the displacement of the Kirkendall plane and the holding time is changed under the action of electric current. The likely reason for the electric current enhancing effect is the energy transfer from electron to jumping atom, increasing the integrated diffusion coefficient, which leads to the increase in the velocity of Kirkendall plane.

Evolution of melt convection in a liquid metal driven by a pulsed electric current

Gain refinement in metal alloy can be achieved by applying an electric current pulse(ECP)in solidification process.Forced flow inside the melt has been proved to be a key role in grain refinement.In this paper,the fluid flow inside Ga 20 wt%-In 12 wt%-Sn alloy induced by a damping sinusoidal ECP flowing through two parallel electrodes into the cylindrical melt was investigated by both experimental measurements and numerical simulations.Experimental results showed that a strong descending jet was induced beneath the bottom of electrodes under the application of ECP.Besides,it was found that flow intensity increases with the increase of amplitude,frequency,and pulse width,respectively.In order to unlock the formation mechanism of flow pattern and the relevance of flow intensity varied with electrical parameters,a three-dimensional numerical model under the application of ECP was established.Meanwhile,a comparative study was conducted by numerical simulations to reveal the distributions of electromagnetic fields and forced flow.Numerical results showed that the downward Lorentz force induced by ECP was concentrated beneath the bottom of electrodes.This downward Lorentz force induces a descending jet and provokes a global forced flow.According to numerical simulations,the evolution of flow intensity with electrical parameters under the application of ECP can be understood by the time averaged impulse of Lorentz force.

NONLINEAR STABILITY ANALYSIS OF A CLAMPED ROD CARRYING ELECTRIC CURRENT

This paper is devoted to the analysis of the nonlinear Stability of a clamped rodcarrying electric current in the magnetic field which is produced by the current frowingin a pair of inifinitely long parallel rigid wires. The natural State of the rod is in theplane of the wires and is equidistant from them.Firstly under the assumption of apatial deformation, the governing equations of the problem are derived, and the linearizedproblem and critical currents are discussed. Secondly, it ls proved that the buckledstates of the rod are always in planes. Finally. the global responses of the bifurcationproblem of the rod are compuled numerically and the distributions of the deflections.axial forces and bending monents are obtained. The results show that the buckledslates of the rod may be either supercritical or Subcritical. depending on the distancebetween the rod and the wires. Furthermore, it is found that -there exists a limit pointon the branch solution of the supercritical buckled State. This is distinctively differentfrom the buckled slate of the elastic compressive rods.

Synthesis of Ni/Al System Intermetallic Compound Under the Influence of Pulsating Electric Current

Synthesis of Ni/Al system intermetallic compound under the influence of pulsating electric current is researched.Reactions of Ni/Al system intermetallic compound are analyzed.It is found that solid-state reactions occur at Ni/Al interface and the main way of reactions is atoms diffusing each other under the influence of a high-density pulsating electric current.

Equivalent Resistance in Pulse Electric Current Sintering

The sintering resistance for conductive TiB2 and non-conductive A12O3 as well as empty die during pulse current sintering were investigated in this paper. Equivalent resistances were measured by current and valtage during sintering the conductive and non-conductive materials in the same conditions. It is found that the current paths for conductive are different from those for non-conductive materials. For non-conductive materials, sintering resistances are influenced by powder sizes and heating rates, which indicates that pulse current has some interaction with non-conductive powders. For conductive TiB2 , sintering resistances are influenced by heating rates and ball-milling time, which indicates the effect of powders activated by spark.

Evolution of Vertical Electric Currents and Flares in An Active Region

In this paper, solar active region (AR) 6891 is studied. The long term temporal (over a 8 day period) relationahip between vertical current evolution and flare are examined. We found that the day to day variation of the flare number in the active region is more closly associated with the variation of net current intensity than that of total current intensity. The increase in negative current in the weak following polarity area is due to the reduction in negative current in the strong preceding area.

Effect of Electric Current on Diffusion of Aluminum in Ti3AlC2 into Zirconium Alloy

The spark plasma sintering（SPS） method was used to study the mechanism of reaction interface between Zr and Ti3AlC2 with electric current going through it. It was found that electric current greatly reduced the bonding temperature of Zr and Ti3AlC2. By the micro-structure analysis of the interface through SEM/EDS, it was found that Al atoms diffused from the Ti3AlC2 substrate into the Zr side and reacted with Zr to form the Zr-Al compounds at the interface, which is the strengthening mechanism of Ti3AlC2-Zr bonding. The thickness of reaction layers（Zr-Al alloy） was from 0.879 to 13.945 mm depending on different sintering condition. Current direction, heating rate, soaking time, pulse patterns all influenced the diffusion of Al atoms which affected the joining quality of Zr and Ti3AlC2.

Migration Behavior of Impurity Iron in Silicon Melt Under Pulsed Electric Current

The impurity iron in silicon material will seriously affect the photoelectric conversion efficiency of silicon solar cells.However,the traditional silicon purification method has the disadvantages of long cycle,high energy consumption and serious pollution.In this study,an efficient and green pulsed electric current purification technology is proposed.The electromigration effect of iron elements,the current density gradient driving of iron phase,and the gravity of iron phase all affect the migration behavior of iron phase in silicon melt under pulsed electric current.Regardless of the depth of electrode insertion into the silicon melt,the solubility of iron in silicon decreases under the pulsed electric current,which helps to form the iron phase.At the same time,the iron phase tends to sink toward the bottom under the influence of gravity.When the electrode is shallowly inserted,a non-uniform electric field is formed in the silicon melt,and the iron phase is mainly driven by the current density gradient to accelerate sink toward the bottom.When the electrode is fully inserted,an approximately uniform electric field is formed in the silicon melt,and iron elements are preferentially migrated to the cathode by electromigration,forming iron phase sinking at the cathode.The study of impurity iron migration behavior in silicon melt under pulsed electric current provides a new approach for the purification of polycrystalline silicon.

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.

Effects of direct current electric field on corrosion behaviour of copper, Cl- ion migration behaviour and dendrites growth under thin electrolyte layer

Effect of direct current electric field （DCEF） on corrosion behaviour of copper printed circuit board （PCB-Cu）, Cl-ion migration behaviour, dendrites growth under thin electrolyte layer was investigated using potentiodynamic polarization and scanning electron microscopy （SEM） with energy dispersive spectrometer （EDS）. Results indicate that DCEF decreases the corrosion of PCB-Cu;Cl-ions directionally migrate from the negative pole to the positive pole, and enrich on the surface of the positive pole, which causes serious localized corrosion; dendrites grow on the surface of the negative pole, and the rate and scale of dendrite growth become faster and greater with the increase of external voltage and exposure time, respectively.

Columnar to Equiaxed Transition During Solidification of Small Ingot by Using Electric Current Pulse

A new approach to applying the electric current pulse （ECP） with parallel electrodes to the promotion of the transition from columnar crystal to equiaxed crystal and the improvement of macrosegregation was introduced. The ECP was applied to different stages of the solidification. The results showed that the application of the ECP in both the initial stage （the thickness of solidified shell reached 2 mm approximately） and the late stage （the thickness of solidified shell reached 14 mm approximately） of solidification can promote the columnar to equiaxed transition （CET）. The analysis showed that during solidification, a large number of nuclei around the upper surface fell off due to ECP, which subsequently showered on the melt and impinged the growth front of the columnar crystal. Therefore, the CEToccurred. In addition, this method was also employed to influence the solidification process of bearing steel, and the results showed that the structure was changed from columnar crystal to equiaxed crystal, indicating that ECP can enhance the homogeneity of structure and composition of bearing steel.