Lithium-ion batteries(LIBs)undergo various degradation phenomena such as material decomposition,structural change and uneven lithium ion distribution during long-term cycles,which would affect their performance and sa...Lithium-ion batteries(LIBs)undergo various degradation phenomena such as material decomposition,structural change and uneven lithium ion distribution during long-term cycles,which would affect their performance and safety.In order to improve the performance of the LIBs during their life cycle,preload force is preset when the batteries are assembled.Different preload forces will in turn affect the cycle life and heat generation of the battery.In order to address this issue,this work carries out charge/discharge cycle tests on a NCM811 battery under different preload forces.Isothermal calorimetry tests are performed to investigate the battery heat generation under different states of health(SOHs)and preload forces.Based on the test results,an empirical prediction model for heat generation power as a function of SOH is established.Results show that when the preload force is 5 N·m,the battery capacity decreases in the slowest rate and the average heat generation power is the lowest.Changes in peaks of the incremental capacity curve can be used to characterize the loss of lithium at the electrode,which in turn characterizes the change of heat generation power of the battery.The average heat generation power is mainly affected by the SOH,going through a period of trough with the decrease of the SOH and continuing to increase after crossing the critical point.In general,these findings emphasize the relationship between preload force,SOH and heat generation power,which is helpful for the judgment of optimal preload to improve the efficiency of LIBs.展开更多
Al-containing coatings were prepared on AZ31 magnesium alloy by pack-cementation technology.X-ray diffraction(XRD),backscattered electron imaging(BSEI)and energy dispersive spectroscopy(EDS)were jointly employed to ch...Al-containing coatings were prepared on AZ31 magnesium alloy by pack-cementation technology.X-ray diffraction(XRD),backscattered electron imaging(BSEI)and energy dispersive spectroscopy(EDS)were jointly employed to characterize the phases,microstructure and composition of the coated samples.The results show that the feedstock composition has a significant impact on the phases,microstructure and thickness of the coatings.For the sample with AlCl3 powder as the activator,the coating is very thick and composed of gradient phases and structures from surface to inside,including small amount ofb-Mg2Al3,coarse eutectic-like structure ofγ-Mg17Al12+δ-Mg,and fineγ-Mg17Al12 precipitations.In contrast,for the sample with AlCl3 and pure Al composite powders as the activator,the coating is relatively thin and contains a thin Al2O3 layer and a small amount of fineγ-Mg17Al12 precipitates.For the pack-cementation aluminizing that is not protected by high-vacuum or inert gas,the addition of pure Al powders can easily introduce the Al2O3 layer into the coating to prevent active Al ions further penetrating into the magnesium matrix,resulting in the thin Al-containing coating.The microhardness and corrosion behavior of the two kinds of aluminized coatings were also studied and discussed.展开更多
Automotive collision avoidance technology can effectively avoid the accidents caused by dangerous traffic conditions or driver's manipulation errors.Moreover,it can promote the development of autonomous driving fo...Automotive collision avoidance technology can effectively avoid the accidents caused by dangerous traffic conditions or driver's manipulation errors.Moreover,it can promote the development of autonomous driving for intelligent vehicle in intelligent transportation.We present a collision avoidance system,which is composed of an evasive trajectory planner and a path following controller.Considering the stability of the vehicle in the conflict-free process,the evasive trajectory planner is designed by polynomial parametric method and optimized by genetic algorithm.The path following controller is proposed to make the car drive along the designed path by controlling the vehicle's lateral movement.Simulation results show that the vehicle with the proposed controller has good stability in the collision process,and it can ensure the vehicle driving in accordance with the planned trajectory at different speeds.The research results can provide a certain basis for the research and development of automotive collision avoidance technology.展开更多
With the worsening of energy crisis and environmental pollution,electric vehicles with four in?wheel motors have been paid more and more attention. The main research subject is how to reasonably distribute the driving...With the worsening of energy crisis and environmental pollution,electric vehicles with four in?wheel motors have been paid more and more attention. The main research subject is how to reasonably distribute the driving torque of each wheel. Considering the longitudinal motion,lateral motion,yaw movement and rotation of the four wheels,the tire model and the seven DOF dynamic model of the vehicle are established in this paper. Then,the torque distribution method is proposed based on road adhesion margin,which can be divided into anti ? slip control layer and torque distribution layer. The anti?slip control layer is built based on sliding mode variable structure control,whose main function is to avoid the excessive slip of wheels caused by road conditions. The torque distribution layer is responsible for selecting the torque distribution method based on road adhesion margin. The simulation results show that the proposed torque distribution method can ensure the vehicle quickly adapt to current road adhesion conditions,and improve the handling stability and dynamic performance of the vehicle in the driving process.展开更多
As hybrid vehicles introduced the motor, the vehicle structure has a significant change in the power matching. A driver-vehicle-road closed-loop semi-physical simulation system, which makes real driving parts together...As hybrid vehicles introduced the motor, the vehicle structure has a significant change in the power matching. A driver-vehicle-road closed-loop semi-physical simulation system, which makes real driving parts together with the simulation car, will bring convenience to the new car design. We used the computer software to simulate the road with a slope, curve and some other features based on the actual road condition, and analyzed the whole road scene in addition to geometry and physical characteristics. Analyzing and constructing the vehicle dynamics basic template, appropriate changes to the template can obtain the desired vehicle dynamics model with an external device to control the model vehicle. It combined the physical operation system with visual display, which gave us real driving feelings and increased the vehicle design predictive accuracy.展开更多
Catalyzed diesel particulate filter(CDPF)combines the functions of the oxidization catalyst and the diesel particulate filter.Due to good redox capacity and oxygen storage capacity,CeO2 is used as the catalyst of CDPF...Catalyzed diesel particulate filter(CDPF)combines the functions of the oxidization catalyst and the diesel particulate filter.Due to good redox capacity and oxygen storage capacity,CeO2 is used as the catalyst of CDPF.Since the effects of key factors on the performance of Ce02-based CDPF were rarely reported,it was performed in this study based on a zero-dimensional numerical model using plug flow reactor in which a reaction mechanism was established and validated by the experiment of the thermal gravimetric analyzer.The effects of exhaust gas temperature and three defined parameters including the ratios of NO2 in NO,(α),NOx to soot(β),and catalyst coated amount to carbon loading amount(y)on catalyst poisoning temperature,N20 concentration,NOx reduction rate and soot regeneration rate were investigated.The results show that the rising exhaust gas temperature causes the reduction of NOx concentration,and the NOx reduction rate comes to 66%when the catalyst poisoning temperature is reached.The soot regeneration rate and the N2O concentration first increase and then decrease as the exhaust gas temperature increases.Meanwhile,the higher exhaust gas temperature suppresses the production of N2O,but raises the possibility of catalyst poisoning.The increasing a and p result in the increase of soot regeneration rate and the decrease of NOx reduction rate.The catalyst poisoning temperature is improved at higher a and lower p.The soot regeneration rate has a fast increase with y at first and then stabilizes rapidly.The results of this study are valuable to optimize the operation of CDPF.展开更多
Because of the complex constraint effects among layers in multi-layered metallic bellows hydroforming,the stress concentration and defects such as wrinkling and fracture may easily occur.It is a key to reveal the defo...Because of the complex constraint effects among layers in multi-layered metallic bellows hydroforming,the stress concentration and defects such as wrinkling and fracture may easily occur.It is a key to reveal the deformation behaviors in order to obtain a sound product.Based on the ABAQUS platform,a 3 D-FE model of the four-layered U-shaped metallic bellow hydroforming process is established and validated by experiment.The stress and strain distributions,wall thickness variations and bellow profiles of each layer in the whole process,including bulging,folding and springback stages,are studied.Then deformation behaviors of bellows under different forming conditions are discussed.It is found that the wall thinning degrees of different layer vary after hydroforming,and is the largest for the inner layer and smallest for the outer layer.At folding stage,the wall thinning degree of the crown point increases lineally,and the difference among layers increases as the process going.The displacements of the crown point decrease from the inner layer to the outer layer.After springback,the U-shaped cross section changes to a tongue shape,the change of convolution pitch is much larger than the change of convolution height,and the springback values of the inner layer are smaller than the outer layer.An increase in the internal pressure and die spacing cause the maximum wall thinning degree and springback increase.With changing of process parameters,bellows with deep convolution are easily encountered wall thinning during hydroforming and convolution distortion after springback.This research is helpful for precision forming of multi-layered bellows.展开更多
The price-performance ratio of PHEV determines its market penetration.Besides engine and battery,the power coupling system including traction motor and automatic transmission is a key influence factor of system perfor...The price-performance ratio of PHEV determines its market penetration.Besides engine and battery,the power coupling system including traction motor and automatic transmission is a key influence factor of system performance and costs.This article introduces an electro-mechanical power coupling system for PHEV with high price-performance ratio,which integrates an electro-mechanical CVT and a flat traction motor.As an example,a PHEV system is configured to conform the vehicle dynamic specifications.展开更多
Vehicle collision avoidance system is a kind of auxiliary driving system based on vehicle active safety,which can assist the driver to take the initiative to avoid obstacles under certain conditions,so as to effective...Vehicle collision avoidance system is a kind of auxiliary driving system based on vehicle active safety,which can assist the driver to take the initiative to avoid obstacles under certain conditions,so as to effectively improve the driving safety of vehicle.This paper presents a collision avoidance system for an autonomous vehicle based on an active front steering,which mainly consists of a path planner and a robust tracking controller.A path planner is designed based on polynomial parameterization optimized by simulated annealing algorithm,which plans an evasive trajectory to bypass the obstacle and avoid crashes.The dynamic models of the AFS system,vehicle as well as the driver model are established,and based on these,a robust tracking controller is proposed,which controls the system to resist external disturbances and work in accordance with the planning trajectory.The proposed collision avoidance system is testified through CarSim and Simulink combined simulation platform.The simulation results show that it can effectively track the planning trajectory,and improve the steering stability and anti-interference performance of the vehicle.展开更多
Methane activation is a pivotal step in the application of natural gas converting into high-value added chemicals via methane steam/dry reforming reactions.Ni element was found to be the most widely used catalyst.In p...Methane activation is a pivotal step in the application of natural gas converting into high-value added chemicals via methane steam/dry reforming reactions.Ni element was found to be the most widely used catalyst.In present work,methane activation on MgO supported Ni–M(M=Fe,Co,Cu,Pd,Pt)cluster was explored through detailed density functional theory calculations,compared to pure Ni cluster.CH_(4)adsorption on Cu promoted Ni cluster requires overcoming an energy of 0.07 eV,indicating that it is slightly endothermic and unfavored to occur,while the adsorption energies of other promoters M(M=Fe,Co,Pd and Pt)are all higher than that of pure Ni cluster.The role of M on the first C–H bond cleavage of CH_(4)was investigated.Doping elements of the same period in Ni cluster,such as Fe,Co and Cu,for C–H bond activation follows the trend of the decrease of metal atom radius.As a result,Ni–Fe shows the best ability for C–H bond cleavage.In addition,doping the elements of the same family,like Pd and Pt,for CH_(4)activation is according to the increase of metal atom radius.Consequently,C–H bond activation demands a lower energy barrier on Ni–Pt cluster.To illustrate the adsorptive dissociation behaviors of CH_(4)at different Ni–M clusters,the Mulliken atomic charge was analyzed.In general,the electron gain of CH_(4)binding at different Ni–M clusters follows the sequence of Ni–Cu(–0.02 e)<Ni(–0.04 e)<Ni–Pd(–0.08 e)<Ni–Pt(–0.09 e)<Ni–Co(–0.10 e)<Ni–Fe(–0.12 e),and the binding strength between catalysts and CH_(4)raises with the CH_(4)electron gain increasing.This work provides insights into understanding the role of promoter metal M on thermal-catalytic activation of CH_(4)over Ni/MgO catalysts,and is useful to interpret the reaction at an atomic scale.展开更多
基金the financial supports from the National Key R&D Plan of China(No.2021YFB2402001)the Chongqing Municipal Innovation Project(No.CYS23657)。
文摘Lithium-ion batteries(LIBs)undergo various degradation phenomena such as material decomposition,structural change and uneven lithium ion distribution during long-term cycles,which would affect their performance and safety.In order to improve the performance of the LIBs during their life cycle,preload force is preset when the batteries are assembled.Different preload forces will in turn affect the cycle life and heat generation of the battery.In order to address this issue,this work carries out charge/discharge cycle tests on a NCM811 battery under different preload forces.Isothermal calorimetry tests are performed to investigate the battery heat generation under different states of health(SOHs)and preload forces.Based on the test results,an empirical prediction model for heat generation power as a function of SOH is established.Results show that when the preload force is 5 N·m,the battery capacity decreases in the slowest rate and the average heat generation power is the lowest.Changes in peaks of the incremental capacity curve can be used to characterize the loss of lithium at the electrode,which in turn characterizes the change of heat generation power of the battery.The average heat generation power is mainly affected by the SOH,going through a period of trough with the decrease of the SOH and continuing to increase after crossing the critical point.In general,these findings emphasize the relationship between preload force,SOH and heat generation power,which is helpful for the judgment of optimal preload to improve the efficiency of LIBs.
基金the National Natural Science Foundation of China(No.51575073)International Cooperation Special Project in Science and Technology of China(No.2015DFR70480)Scientific and Technological Research Program of Chongqing,China(Nos.cstc2017jcyjBX0031,cstc2018jszx-cyzdX0126).
文摘Al-containing coatings were prepared on AZ31 magnesium alloy by pack-cementation technology.X-ray diffraction(XRD),backscattered electron imaging(BSEI)and energy dispersive spectroscopy(EDS)were jointly employed to characterize the phases,microstructure and composition of the coated samples.The results show that the feedstock composition has a significant impact on the phases,microstructure and thickness of the coatings.For the sample with AlCl3 powder as the activator,the coating is very thick and composed of gradient phases and structures from surface to inside,including small amount ofb-Mg2Al3,coarse eutectic-like structure ofγ-Mg17Al12+δ-Mg,and fineγ-Mg17Al12 precipitations.In contrast,for the sample with AlCl3 and pure Al composite powders as the activator,the coating is relatively thin and contains a thin Al2O3 layer and a small amount of fineγ-Mg17Al12 precipitates.For the pack-cementation aluminizing that is not protected by high-vacuum or inert gas,the addition of pure Al powders can easily introduce the Al2O3 layer into the coating to prevent active Al ions further penetrating into the magnesium matrix,resulting in the thin Al-containing coating.The microhardness and corrosion behavior of the two kinds of aluminized coatings were also studied and discussed.
基金supported by the National Key Research and Development Plan of China (No.2016YFB0101102 )the Suzhou Tsinghua Innovation Initiative(No. 2016SZ0207)+2 种基金the National Natural Science Foundation of China(No.51375007)the Research Project of Key Laboratory of Advanced Manufacture Technology for Automobile Parts(Chongqing University of Technology),Ministry of Education (No.2015KLMT04)the Fundamental Research Funds for the Central Universities (No. NE2016002)
文摘Automotive collision avoidance technology can effectively avoid the accidents caused by dangerous traffic conditions or driver's manipulation errors.Moreover,it can promote the development of autonomous driving for intelligent vehicle in intelligent transportation.We present a collision avoidance system,which is composed of an evasive trajectory planner and a path following controller.Considering the stability of the vehicle in the conflict-free process,the evasive trajectory planner is designed by polynomial parametric method and optimized by genetic algorithm.The path following controller is proposed to make the car drive along the designed path by controlling the vehicle's lateral movement.Simulation results show that the vehicle with the proposed controller has good stability in the collision process,and it can ensure the vehicle driving in accordance with the planned trajectory at different speeds.The research results can provide a certain basis for the research and development of automotive collision avoidance technology.
基金supported by the Natural Science Foundation of Jiangsu Province(No. BK20151472)the Research Project of Key Laboratory of Advanced Manufacture Technology for Automobile Parts(Chongqing University of Technology) , Ministry of Education (No. 2015KLMT04)
文摘With the worsening of energy crisis and environmental pollution,electric vehicles with four in?wheel motors have been paid more and more attention. The main research subject is how to reasonably distribute the driving torque of each wheel. Considering the longitudinal motion,lateral motion,yaw movement and rotation of the four wheels,the tire model and the seven DOF dynamic model of the vehicle are established in this paper. Then,the torque distribution method is proposed based on road adhesion margin,which can be divided into anti ? slip control layer and torque distribution layer. The anti?slip control layer is built based on sliding mode variable structure control,whose main function is to avoid the excessive slip of wheels caused by road conditions. The torque distribution layer is responsible for selecting the torque distribution method based on road adhesion margin. The simulation results show that the proposed torque distribution method can ensure the vehicle quickly adapt to current road adhesion conditions,and improve the handling stability and dynamic performance of the vehicle in the driving process.
基金Funded by the National Natural Science Foundation of China(No.51305475)Chongqing Research Program of Basic Research and Frontier Technology(No.cstc2013jcyj A60004)the Scientific and Technological Research Program of Chongqing Municipal Education Commission(No.KJ1500927)
文摘As hybrid vehicles introduced the motor, the vehicle structure has a significant change in the power matching. A driver-vehicle-road closed-loop semi-physical simulation system, which makes real driving parts together with the simulation car, will bring convenience to the new car design. We used the computer software to simulate the road with a slope, curve and some other features based on the actual road condition, and analyzed the whole road scene in addition to geometry and physical characteristics. Analyzing and constructing the vehicle dynamics basic template, appropriate changes to the template can obtain the desired vehicle dynamics model with an external device to control the model vehicle. It combined the physical operation system with visual display, which gave us real driving feelings and increased the vehicle design predictive accuracy.
基金This work is supported by National Natural Science Foundation of China(Grant No.51976016 and 51806250)We also thank to the support by Open Research Subject of State Key Laboratory of Engines,China(Grant No.K2018-07)Open Research Subject of Key Laboratory of Advanced Manufacture Technology for Automobile Parts(Grant No.2017KLMT02,Chongqing University of Technology).
文摘Catalyzed diesel particulate filter(CDPF)combines the functions of the oxidization catalyst and the diesel particulate filter.Due to good redox capacity and oxygen storage capacity,CeO2 is used as the catalyst of CDPF.Since the effects of key factors on the performance of Ce02-based CDPF were rarely reported,it was performed in this study based on a zero-dimensional numerical model using plug flow reactor in which a reaction mechanism was established and validated by the experiment of the thermal gravimetric analyzer.The effects of exhaust gas temperature and three defined parameters including the ratios of NO2 in NO,(α),NOx to soot(β),and catalyst coated amount to carbon loading amount(y)on catalyst poisoning temperature,N20 concentration,NOx reduction rate and soot regeneration rate were investigated.The results show that the rising exhaust gas temperature causes the reduction of NOx concentration,and the NOx reduction rate comes to 66%when the catalyst poisoning temperature is reached.The soot regeneration rate and the N2O concentration first increase and then decrease as the exhaust gas temperature increases.Meanwhile,the higher exhaust gas temperature suppresses the production of N2O,but raises the possibility of catalyst poisoning.The increasing a and p result in the increase of soot regeneration rate and the decrease of NOx reduction rate.The catalyst poisoning temperature is improved at higher a and lower p.The soot regeneration rate has a fast increase with y at first and then stabilizes rapidly.The results of this study are valuable to optimize the operation of CDPF.
基金the funds of the National Natural Science Foundation of China(No.51875456)the Natural Science Basic Research Plan in Shaanxi Province of China(No.2019JM-450)+3 种基金the Young Talent fund of University Association for Science and Technology in Shaanxi,China(No.20170518)the Key Laboratory of Advanced Manufacture Technology for Automobile Parts(Chongqing University of Technology),Ministry of Education(No.2018KLMT03)Materials Science and Engineering provincial-level superior discipline funding project of Xi'an Shiyou Universitythe Program for Young Innovative Research Team in Xi'an Shiyou University(No.2015QNKYCXTD02)。
文摘Because of the complex constraint effects among layers in multi-layered metallic bellows hydroforming,the stress concentration and defects such as wrinkling and fracture may easily occur.It is a key to reveal the deformation behaviors in order to obtain a sound product.Based on the ABAQUS platform,a 3 D-FE model of the four-layered U-shaped metallic bellow hydroforming process is established and validated by experiment.The stress and strain distributions,wall thickness variations and bellow profiles of each layer in the whole process,including bulging,folding and springback stages,are studied.Then deformation behaviors of bellows under different forming conditions are discussed.It is found that the wall thinning degrees of different layer vary after hydroforming,and is the largest for the inner layer and smallest for the outer layer.At folding stage,the wall thinning degree of the crown point increases lineally,and the difference among layers increases as the process going.The displacements of the crown point decrease from the inner layer to the outer layer.After springback,the U-shaped cross section changes to a tongue shape,the change of convolution pitch is much larger than the change of convolution height,and the springback values of the inner layer are smaller than the outer layer.An increase in the internal pressure and die spacing cause the maximum wall thinning degree and springback increase.With changing of process parameters,bellows with deep convolution are easily encountered wall thinning during hydroforming and convolution distortion after springback.This research is helpful for precision forming of multi-layered bellows.
基金Project supported by National Natural Science Fund Program"Research of Control for Electro-mechanical CVT PHEV System"(Fund Number:51275549)Natural Science Fund Program of Chongqing Science and Technology Committee(FundNumber:cstc2012jjA60003)Chongqing Scientific Research Program(Fund Number:cstc2012gg-yyjsB60002)
文摘The price-performance ratio of PHEV determines its market penetration.Besides engine and battery,the power coupling system including traction motor and automatic transmission is a key influence factor of system performance and costs.This article introduces an electro-mechanical power coupling system for PHEV with high price-performance ratio,which integrates an electro-mechanical CVT and a flat traction motor.As an example,a PHEV system is configured to conform the vehicle dynamic specifications.
基金supported by the Research Project of Advanced Manufacture Technology for Automobile Parts(Chongqing University of Technology)Ministry of Education(Grant No.2015KLMT04)the National Natural Science Foundation of China(Grant No.51375007 and 51605219)
文摘Vehicle collision avoidance system is a kind of auxiliary driving system based on vehicle active safety,which can assist the driver to take the initiative to avoid obstacles under certain conditions,so as to effectively improve the driving safety of vehicle.This paper presents a collision avoidance system for an autonomous vehicle based on an active front steering,which mainly consists of a path planner and a robust tracking controller.A path planner is designed based on polynomial parameterization optimized by simulated annealing algorithm,which plans an evasive trajectory to bypass the obstacle and avoid crashes.The dynamic models of the AFS system,vehicle as well as the driver model are established,and based on these,a robust tracking controller is proposed,which controls the system to resist external disturbances and work in accordance with the planning trajectory.The proposed collision avoidance system is testified through CarSim and Simulink combined simulation platform.The simulation results show that it can effectively track the planning trajectory,and improve the steering stability and anti-interference performance of the vehicle.
基金supported by the National Natural Science Foundation of China(Grant No.52106179)the Fundamental Research Program of Shanxi Province,China(Grant No.20210302124017)+1 种基金the Natural Science Foundation of Chongqing,China(Grant No.cstc2020jcyj-msxmX0454)the Scientific and Technological Activities for Overseas Students of Shanxi Province,China(Grant No.20200016).
文摘Methane activation is a pivotal step in the application of natural gas converting into high-value added chemicals via methane steam/dry reforming reactions.Ni element was found to be the most widely used catalyst.In present work,methane activation on MgO supported Ni–M(M=Fe,Co,Cu,Pd,Pt)cluster was explored through detailed density functional theory calculations,compared to pure Ni cluster.CH_(4)adsorption on Cu promoted Ni cluster requires overcoming an energy of 0.07 eV,indicating that it is slightly endothermic and unfavored to occur,while the adsorption energies of other promoters M(M=Fe,Co,Pd and Pt)are all higher than that of pure Ni cluster.The role of M on the first C–H bond cleavage of CH_(4)was investigated.Doping elements of the same period in Ni cluster,such as Fe,Co and Cu,for C–H bond activation follows the trend of the decrease of metal atom radius.As a result,Ni–Fe shows the best ability for C–H bond cleavage.In addition,doping the elements of the same family,like Pd and Pt,for CH_(4)activation is according to the increase of metal atom radius.Consequently,C–H bond activation demands a lower energy barrier on Ni–Pt cluster.To illustrate the adsorptive dissociation behaviors of CH_(4)at different Ni–M clusters,the Mulliken atomic charge was analyzed.In general,the electron gain of CH_(4)binding at different Ni–M clusters follows the sequence of Ni–Cu(–0.02 e)<Ni(–0.04 e)<Ni–Pd(–0.08 e)<Ni–Pt(–0.09 e)<Ni–Co(–0.10 e)<Ni–Fe(–0.12 e),and the binding strength between catalysts and CH_(4)raises with the CH_(4)electron gain increasing.This work provides insights into understanding the role of promoter metal M on thermal-catalytic activation of CH_(4)over Ni/MgO catalysts,and is useful to interpret the reaction at an atomic scale.