| The friction causes energy loss accounting for approximately 1/3～1/2 of the global primary energy and causes mechanical parts to wear and eventually fail.With the advance of nanomaterials and nanotechnology,it is found that macroscopic friction is essentially determined by microscopic structural details at the atomic or molecular scale.The study of friction at the microscale is known as nanofriction,aiming to establish the structure-property relationship between microscopic structures and macroscopic frictional property,thereby maximizing the potential of nanomaterials in practical applications.In nanomaterials with strong size effects and large specific surface areas,frictional behaviors no longer follow traditional macroscopic laws.Therefore,the study of nanofriction shifted to a new research paradigm,where theoretical methods,such as density functional theory(DFT)and molecular dynamics(MD),hold significant advantages in unraveling the mechanisms of nanofriction.Carbon nanotubes(CNTs),typical of one-dimensional nanomaterials,serve as an ideal model system for nanofriction.On the one hand,due to their excellent electronic and mechanical properties,CNTs have been successfully commercialized in fields,such as biotechnology,energy,and microelectronics.On the other hand,their unique hollow cylindrical structure enables CNTs in contact with other materials both inside and outside,leading to relative motion and frictional losses.The friction inside reflects the confinement effect,such as the confined water in CNTs;the friction outside is reflected in the motion of CNTs on planes;the friction both inside and outside is reflected in concentric tubes.Based on the DFT and MD methods,we employ three typical systems of friction inside and outside CNTs and unravel their underlying mechanism for further regulation:1.The regulation of friction coefficient for water inside CNTs by size and temperatureWater exhibits ultrahigh flow rates through CNTs,far exceeding the predictions of classic theories,which makes CNTs promising for artificial aquaporin.This is attributed to the ultra-low friction at the water-CNTs interface,or rather the exceptionally long slip length.We first obtain the accurate potential corrugation based on the DFT calculations and then extend the phononic friction and Einstein diffusion models to the water-CNTs systems for predicting slip lengths.Our scheme effectively captures the size dependence of slip lengths for water in CNTs,which agrees well with previous experimental and simulational observations.Notably,we also identify the continuum-subcontinuum transition that remains debated in previous studies and further reveals that this transition is likely to be determined by the hydrogen bond,instead of structural transition.In addition,the size dependence of slip lengths can be significantly regulated by the temperature.Our methods reshape the understanding of water slippage through CNTs and are expected to be extended to other confined systems.2.The regulation of frictional behavior for CNTs on graphite planes by doping and chargingThere is more motion freedom for CNTs on graphite(GRA)substrate,facilitating the realization of complex functionality in the nano(electro)mechanical systems(NEMS).Regulating the motion of CNTs on GRA(e.g.,rolling and sliding)is to regulate the friction force in between,which can be traced back to the regulation of potential energy surfaces.The regulation from a geometric perspective in previous studies has proven to be less effective and far from feasible in experiments.Based on the potential energy profiles obtained from DFT,we explore how doping and charging affect the competition between sliding and rolling for CNT on GRA.When charging,the N-doped CNTs realize the transition from rolling to sliding.No matter with or without charging,however,the B-,Al-,and P-doped CNTs maintain the sliding preference.Doping and charging synergize to regulate motion: firstly,doping is an essential prerequisite for charging to work;doping introduces local polarization,while based on that,charging reshapes the overall charge density,inducing electrostatic repulsion between CNT and GRA.Our results are expected to provide some insights into the regulation of relative motion between nanomaterials from the electronic perspective.3.The regulation of superlubricity for double-walled CNTs by heterostructure and chargingDue to the superlubricity(i.e.,ultra-low friction)between shells,multi-walled CNTs show the potential for a new generation of nano-oscillators,where the inner tubes oscillate within the outer tubes in a telescoping way at the frequency of GHz.Since close mechanical control lacks precision when the oscillator is encapsulated into nanodevices,this requires remote driving(such as electrical or magnetic field),while maintaining the ultra-low friction between shells.Due to the intrinsic lattice mismatch,the CNTs and boron nitride nanotubes(BNNT)heterostructure show smaller intershell friction than the homogenous double-walled CNTs.We propose a conceptual design—a combination of the single electron transistor(SET)and CNT@BNNT oscillator—to remotely drive the CNT by the electric field.The SET enables CNT to accept electrons and thus charge them,making them respond to the electric field exerted by the electrodes,while BNNT accepts no electron and shows no response to external fields,which is because the BNNT feels net zero electric force due to the same charge but with opposite sign of N and B.Based on the MD calculations,we found that driving by external force and electric field produces similarly long-term sustainable oscillation,which means that the trapped electrons in CNTs have a negligible effect on the friction force between inner CNT and outer BNNT(i.e.,the superlubricity maintains).Our design is expected to provide a remedy for the practical application of CNT-based oscillators. |
