This work integrates experimental and MD simulation approaches to study the role of graphene in G-Cu-W composites. Arcing tests were conducted on G-Cu-W and Cu-W contact samples under a 5kA peak current. Experimental results show that adding graphene leads to a lower surface roughness of the sample following arcing. MD simulation results indicate that the G-Cu-W model exhibits a smoother surface and fewer lost metal atoms than the Cu-W model due to the protective effect of graphene layer

Bissett, Mark

Connolly, Andrew

Kinloch, Ian

Matharage, Shanika Yasantha

Spencer, Joseph

Wang, Xin

Wang, Zhongdong

Yan, Jiu Dun

Zhou, Yannan

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/ppt.2023.3.150Plasma Physics and Technology 10(3):150–153, 2023 © Department of Physics, FEE CTU in Prague, 2023ELECTRICAL EROSION RESISTANCE OF GRAPHENE REINFORCEDCU-W CIRCUIT BREAKER CONTACT MATERIALS UNDER 5 KA ARCX. Wanga, Y.N. Zhoub, A. Connollyc, S.Y. Matharagea, M. Bissettc,J.W. Spencerb, J.D. Yan b, I. Kinlochc, Z.D. Wang a,∗a Centre for Smart Grid, Department of Engineering, University of Exeter. EX4 4PY, UKb Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool L69 3GJ, UKc Department of Materials, University of Manchester, Manchester M13 9PL, UK∗ zhongdong.wang@exeter.ac.ukAbstract. This work integrates experimental and MD simulation approaches to study the role ofgraphene in G-Cu-W composites. Arcing tests were conducted on G-Cu-W and Cu-W contact samplesunder a 5 kA peak current. Experimental results show that adding graphene leads to a lower surfaceroughness of the sample following arcing. MD simulation results indicate that the G-Cu-W modelexhibits a smoother surface and fewer lost metal atoms than the Cu-W model due to the protectiveeffect of graphene layer.Keywords: Circuit breaker contact, Graphene-reinforced Cu-W composites, Arc erosion, Moleculardynamics simulation.1. IntroductionCopper-tungsten (Cu-W) contacts have gathered con-siderable experience over decades for their use in high-voltage circuit breakers (HVCBs). These contactscombine the high electrical conductivity of Cu withthe high mechanical strength, high melting point, andlow thermal expansion of W. During faulty conditionswhen the current exceeds tens of thousands of Amps,the opening of a circuit breaker results in a highlydestructive arc. Although Cu-W contacts are capa-ble of withstanding repeated arcing processes duringoperation, their limited operating lifetime remains achallenge due to the erosive effects of arcing conditionsthat damage the contact surface, leading to compro-mised structural integrity and electrical performance.Graphene is a 2D nanomaterial composed of amonoatomic layer of hexagonally arranged carbonatoms known for their exceptional mechanical, ther-mal, and electrical properties. Recent studies have fo-cused on the incorporation of graphene as an additiveto enhance the properties of Cu-W contact materials.Study [1] indicated that the addition of graphene canimprove the hardness and conductivity of the Cu-Wmatrix. Study [2] reported that adding graphene canenhance the breakdown strength of W70Cu30 com-posites. However, these results are only based on thesmall bulk samples, and the experimental environmentis far from the practical circuit breaker’s working con-ditions. Therefore, it is still unclear if graphene canimprove the lifetime of circuit breaker contacts andwhat the anti-erosion mechanism of graphene is.This work studies the failure mechanism of circuitbreaker contacts and the role of graphene in G-Cu-Wcomposites by implementing arcing tests on Cu-W(80 wt% W and 20 wt% Cu) and G-Cu-W (80 wt% W,20 wt% Cu and 0.02 wt% G) circuit breaker contactsunder 5 kA peak current condition. In addition, Molec-ular Dynamics (MD) simulation is used to study theelectrical erosion resistance mechanism of grapheneat the micro-level.2. Experiment SetupThe testing HVCB unit is a modification of a245 kV/40 kA live tank circuit breaker. The schematicdiagram of the HVCB unit without the nozzle is shownin Figure 1, which gives the relative positions of arc-ing contacts. In Figure 1(b), the main circuit of theexperimental system is composed of a charging circuit,a capacitor bank, a dump circuit, a test current trig-ger circuit and a test circuit breaker. The capacitorbank is used as the power source of the arcing current.There are four ignitrons to turn current on or off inthe main circuit: the dump ignitron, DC ignitron,forward and reverse AC ignitrons. An ignitron is agas-filled tube used to control the current in a circuit.The arc voltage and current are measured using ahigh-voltage probe (Tektronix P6015 A) and a Ro-gowski coil respectively, and the data is recorded by adigital oscilloscope (Tektronix DPO 2024). The typi-cal waveforms (arc voltage and current) are shown inFigure 2. Before the arcing contacts are separated, alow and slowly decaying DC current passes throughthe arcing contacts and maintains an arc for tens ofmilliseconds. At a certain point in time after contactseparation, a positive half-cycle AC is then gener-ated, and the arc is extinguished automatically at thefirst current zero crossing point. The present workconducted one time 5 kA current peak sine half-wavearcing test on the G-Cu-W and Cu-W contacts, re-spectively.150vol. 10 no. 3/2023 Graphene reinforced Cu-W HVCB materialsFigure 1. (a) Schematic diagram of the HVCB withoutnozzle; (b) schematic diagram of the main circuit.Figure 2. Current and voltage waveforms under 5 kAarcing condition.3. Material CharacterisationRaman spectroscopy and Raman mapping are em-ployed to determine the distribution of grapheneacross the surface of the G-Cu-W contact samplesusing a Renishaw inVia Qontor confocal Raman micro-scope, under illumination of a 532 nm laser. Spectralacquisition was performed on different locations onthe surface of the contact head. In Figure 3(b), aRaman intensity map can be seen, which shows theintensity of the characteristic G-peak Raman signalassociated with graphene on the G-Cu-W contact sur-face. The graphene signal is located predominantlyin the Cu-rich regions, indicating that some graphenefillers are distributed across the Cu phase of the G-Cu-W composite.The Scanning Electron Microscope (SEM) image ofthe Cu-W contact surface of the cathode subjected toa single arcing test is shown in Figure 4(a). Cu (blackphase) and W (white phase) can still be observed.However, the W grain structures are no longer visible.Instead, some melting and resolidification of the Wskeleton can be observed. Figure 4(b) shows the SEMimage of the G-Cu-W contact surface subjected toa single arcing test. W grains can still be observedon the contact surface, with interstitial Cu domainsremaining. Arc erosion has not entirely eroded thesurface Cu, indicating contacts can maintain a lowresistance state in the closed position. This representsa significant difference in performance from Cu-Wcontacts. The presence of graphene provides reinforce-ment of the Cu-W metal matrix, as well as absorbingFigure 3. (a) White-light image of G-Cu-W con-tact surface before arcing test, showing Cu-heavy re-gion. (b) Raman intensity map of graphene G-peak(1580 cm−1) of the same region. The G-peak signalis coloured blue – the brighter the blue in the imageindicates a stronger G-peak signal.Figure 4. SEM images of cathode contact surface aftera single arc shot at 5 kA of (a) Cu-W; (b) G-Cu-W.energy during the arcing process, reducing damage tothe contact surface.Laser scanning confocal microscopy is utilised toestablish roughness parameters of the contact surfaceafter the arcing test. As shown in Figure 5, the surfaceroughness, which measures the average deviation ofa surface profile from a flat plane, increases afterthe arcing test compared with the contact surfacebefore the arcing test. Across this range, the G-Cu-Wcontacts exhibit a lower surface roughness than theCu-W contacts.4. Anti-erosion Mechanism ofGraphene at Micro-level4.1. Simulation DetailsThe Cu-W model, depicted in Figure 6(a), representsa typical Cu-W composite structure where Cu fillsthe gaps between W grains. Based on the Ramanmapping results, graphene is found in the Cu-richregion of the contact surface. To incorporate this, asingle layer of graphene is added to the Cu surface ofthe Cu-W model, forming a G-Cu-W model shownin Figure 6(b). The fixed boundary condition is usedalong the z direction, while the periodical boundarycondition is applied in the x and y directions. Addi-tionally, three bottom layers of substrates are fixed.Both models are first energy minimised and then fullyrelaxed at 300 K for 30 ps before ion bombardmentsimulation. Then, three models’ atom layers at allfour vertical surfaces are forced to maintain 300 K toprevent the waves caused by the bombardment fromreturning through the periodic boundary. It is notedthat neutral sulphur (S) atoms instead of S ions are151X. Wang, Y.N. Zhou, A. Connolly et al. Plasma Physics and TechnologyFigure 5. 3D confocal microscopy profile scans of (a)contact surface before arcing; (b) Cu-W and (c) G-Cu-W composite contact surfaces after arcing.Figure 6. (a) Cu-W model, (b) the space independentlyoccupied by each incident ion, (c) G-Cu-W model withthe same dimensions of Cu-W model, (d) Top viewof G-Cu-W. Red circle shows the position of incidentarea. Black, yellow and blue particles represent W, Cuand C atomsused to bombard the models in the simulation, as ionsare neutralised before bombarding the surface in thepractical working environment [3].Due to the complexity of arc erosion process, onlythe impact of ion bombardment is studied as one ofthe critical erosion processes in this work. In the sim-ulation, the model surface is one-by-one bombardedby 1500 S ions from random sites within a circularregion with a diameter of 12 nm above the model sur-face. The energy of each incident ion is 50 eV, whichcombines thermal energy from the arc, kinetic energyfrom the voltage drop and recombination energy [3].The time interval between continuous incident atomsis set as 0.1 ps. Figure 6(b) exhibits the space indepen-dently occupied by each incident ion. Therefore, theparticle density (average number of particles in unitvolume) of S ions in the simulation is 5.10×1018 cm−3,which is consistent with the particle density (1017to 1018 cm−3) in [4]. An NVE (constant Number ofparticles, Volume, and Energy) ensemble is used dur-ing the ion bombardment process, with a variabletimestep depending on the incident energy. After thebombardment of 1500 ions, models are cooled downto 300 K.Figure 7. Surface morphologies of models after ionbombardment and cooling down to 300 K: (a) and (b):CuW; (c) and (d): G-Cu-W.Classic MD simulations are performed using theLarge-scale Atomic/Molecular Massively Parallel Sim-ulator (LAMMPS). The adaptive intermolecular re-active empirical bond order (AIREBO) potential de-scribes the interactions between C atoms in graphene.The embedded-atom method (EAM) is used to de-scribe the interactions between Cu atoms and Watoms. The interactions of the Cu-C atom pairs andW-C atom pairs are described by 12–6 Lennard-Jones(LJ) potentials [5, 6]. The interactions between theincident S atom with C, Cu, W and other S atoms arecalculated with the Ziegler-Biersack-Littmark (ZBL)repulsive potential [7].4.2. Simulation ResultsSurface morphologies of Cu-W and G-Cu-W modelsafter ion bombardment and cooling down to 300 Kare shown in Figure 7. In the Cu-W system, a largeerosion crater is formed on the surface, as shownin Figure 7(a) and the corresponding surface meshFigure 7(b). This phenomenon is attributed to themovement of metal atoms away from the incidentregion under ion bombardment, leading to some atomsbeing sputtered from the surface. Simultaneously,metal atoms gain energy through collisions, causingCu with a low melting point to rapidly melt andexpand, eventually overflowing from the gaps betweenW grains and forming an erosion crater on the surface.However, no distinct erosion crater is formed on theG-Cu-W surface, as shown in Figure 7(c) and (d). Incomparison to the Cu-W system, the G-Cu-W systemdemonstrates a noticeable reduction in the number ofCu atoms overflowing from the gaps between W grains.This reduction contributes to a relatively smoothersurface in the G-Cu-W system, consistent with theresults obtained from confocal microscopy.Table 1 compares the number of lost metal atomsbetween the Cu-W and G-Cu-W systems, revealing asignificantly lower number of lost metal atoms in theG-Cu-W system. This reduction can be attributed tographene with exceptional strength and a high melt-ing point. The graphene layer on the surface acts asa barrier, preventing direct ion bombardment on the152vol. 10 no. 3/2023 Graphene reinforced Cu-W HVCB materialsContact material Number of lost atomsCu-W 785 Cu atomsG-Cu-W 136 Cu atoms, 99 C atomsTable 1. Number of lost metal atomsmodel surface. The majority of the incident energy isabsorbed by the graphene layer and dissipated in theform of waves to the surroundings, thereby reducingthe energy exerted on the metal atoms. Additionally,the graphene layer prevents the underlying Cu atomsfrom leaving the surface, thereby enhancing the re-sistance to arc erosion in the Cu-W composites. Itis worth noting that in both systems, the lost metalatoms are exclusively Cu atoms, indicating that Wexhibits better resistance to arc erosion than Cu dueto its higher melting point and strength. In additionsimulation results revealed that the edges of grapheneare more susceptible to damage because the carbonatoms at the edges indeed lack C-C bonds comparedto the internal C atoms, resulting in reduced strength.Figures 8(a) and (b) show cross-sectional imagesof the Cu-W and G-Cu-W systems after ion bom-bardment, respectively. The surface expansion ob-served in the Cu-W model is higher than that in theG-Cu-W model. Figures 8(c) and (d) display the cor-responding atomic temperature distributions, wherethe temperature of part atoms exceeds the meltingpoint of Cu (1357 K) and W (3695 K). It can be seenthat the hottest region (3700 K) in Cu-W model issignificantly larger than that in the G-Cu-W model.This is attributed to the much lower thermal conduc-tivity across the G/Cu interface than the Cu matrix.Consequently, the heat in graphene cannot rapidlyconduct to the underlying Cu matrix. Figures 8(e)and (f) show the morphologies of W grains on theright side of both models, indicating greater damageto W grains in the Cu-W system than in the G-Cu-Wsystem, which agrees with SEM results.5. ConclusionsIn this work, experimental and MD simulation wereintegrated to study the role of graphene in G-Cu-Wcontact materials. Arcing tests were conducted onG-Cu-W and Cu-W contact samples under a 5 kApeak current. Experimental findings revealed that theaddition of graphene reduced the damage to contactsurface and surface roughness of the Cu-W matrixfollowing arcing tests. MD simulations demonstratedthat the graphene-covered Cu-W model exhibited asmoother surface and fewer lost metal atoms comparedto the Cu-W model, which agrees with the materialcharacterisation results. This can be attributed tographene acting as a protective layer, effectively reduc-ing the incident energy on the substrate. In addition,graphene prevents heat transfer to the substrate, sig-nificantly reducing the molten pool’s size.Figure 8. Cross section of (a) Cu-W and (b) G-Cu-Wafter ions bombardment; (c) and (d) are correspondingtemperature distribution profiles. (e) and (f) are Wbulk on right side after cooling to 300 K.AcknowledgementsThis work was supported by the State Grid Corpora-tion of China Science and Technology Foundation (5500-201958505A-0-0-00).References[1] L. Dong, W. Chen, C. Zheng, and N. Deng.Microstructure and properties characterization oftungsten–copper composite materials doped withgraphene. Journal of Alloys and Compounds, 695:1637–1646, 2017. doi:10.1016/j.jallcom.2016.10.310.[2] L. Dong, W. Chen, N. Deng, et al. Investigation onarc erosion behaviors and mechanism of W70Cu30electrical contact materials adding graphene. Journal ofAlloys and Compounds, 696:923–930, 2017.doi:10.1016/j.jallcom.2016.12.044.[3] R. Xu, M. Zhou, X. Wang, et al. A moleculardynamics simulation study on the role of graphene inenhancing the arc erosion resistance of Cu metal matrix.Computational Materials Science, 212:111549, 2022.doi:10.1016/J.COMMATSCI.2022.111549.[4] H. Ito et al. Switching Equipment. Springer, 2019.doi:10.1007/978-3-319-72538-3.[5] W. Peng and K. Sun. Effects of Cu/graphene interfaceon the mechanical properties of multilayer Cu/graphenecomposites. Mechanics of Materials, 141:103270, 2020.doi:10.1016/J.MECHMAT.2019.103270.[6] A. Dinsdale. Sgte data for pure substances. Calphad,15(4):317–425, 1991.doi:10.1016/0364-5916(91)90030-N.[7] J. F. Ziegler and J. P. Biersack. The stopping andrange of ions in matter. Springer, 1985.doi:10.1007/978-1-4615-8103-1_3.153

Electrical Erosion Resistance of Graphene Reinforced Cu-W Circuit Breaker Contact Materials under 5 kA Arc

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Electrical Erosion Resistance of Graphene Reinforced Cu-W Circuit Breaker Contact Materials under 5 kA Arc

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