Electronic Friction Research Articles

A path integral approach to electronic friction of a nanometer-sized tip scanning a metal surface

In this work, we study the dissipation mechanism and frictional force of a nanometer-sized tip scanning a metal surface via a path integral approach. The metal, with internal degrees of freedom (c, c †) and a tip with an internal degree of freedom (d, d †) couple with one another by means of an exchanged potential, V. Having integrated out all internal degrees of freedom, we obtain the in-out amplitude. Moreover, we calculate the imaginary part of the in-out amplitude and the frictional force. We find the imaginary part of the in-out amplitude to be positive, and correlated to the sliding velocity in most cases. The frictional force is proportional to the sliding velocity for the case where v < 0.01. However, for cases where v > 0.01, the frictional force demonstrates nonlinear dependence on sliding velocity.

Determining the Effect of Hot Electron Dissipation on Molecular Scattering Experiments at Metal Surfaces.

Nonadiabatic effects that arise from the concerted motion of electrons and atoms at comparable energy and time scales are omnipresent in thermal and light-driven chemistry at metal surfaces. Excited (hot) electrons can measurably affect molecule–metal reactions by contributing to state-dependent reaction probabilities. Vibrational state-to-state scattering of NO on Au(111) has been one of the most studied examples in this regard, providing a testing ground for developing various nonadiabatic theories. This system is often cited as the prime example for the failure of electronic friction theory, a very efficient model accounting for dissipative forces on metal-adsorbed molecules due to the creation of hot electrons in the metal. However, the exact failings compared to experiment and their origin from theory are not established for any system because dynamic properties are affected by many compounding simulation errors of which the quality of nonadiabatic treatment is just one. We use a high-dimensional machine learning representation of electronic structure theory to minimize errors that arise from quantum chemistry. This allows us to perform a comprehensive quantitative analysis of the performance of nonadiabatic molecular dynamics in describing vibrational state-to-state scattering of NO on Au(111) and compare directly to adiabatic results. We find that electronic friction theory accurately predicts elastic and single-quantum energy loss but underestimates multiquantum energy loss and overestimates molecular trapping at high vibrational excitation. Our analysis reveals that multiquantum energy loss can potentially be remedied within friction theory whereas the overestimation of trapping constitutes a genuine breakdown of electronic friction theory. Addressing this overestimation for dynamic processes in catalysis and surface chemistry will likely require more sophisticated theories

Spintronics Meets Nonadiabatic Molecular Dynamics: Geometric Spin Torque and Damping on Dynamical Classical Magnetic Texture due to an Electronic Open Quantum System.

We analyze a quantum-classical hybrid system of steadily precessing around the fixed axis slow classical localized magnetic moments (LMMs), forming a head-to-head domain wall, surrounded by fast electrons driven out of equilibrium by LMMs and residing within a metallic wire whose connection to macroscopic reservoirs makes electronic quantum system an open one. The model captures the essence of dynamical noncollinear magnetic textures encountered in spintronics, while making it possible to obtain the exact time-dependent nonequilibrium density matrix of electronic systems and split it into four contributions. The Fermi surface contribution generates dissipative (or dampinglike in spintronics terminology) spin torque on LMMs, as the counterpart of electronic friction in nonadiabatic molecular dynamics (MD). Among two Fermi sea contributions, one generates geometric torque dominating in the adiabatic regime, which remains as the only nonzero contribution in a closed system with disconnected reservoirs. Locally geometric torque can have nondissipative (or fieldlike in spintronics terminology) component, acting as the counterpart of geometric magnetism force in nonadiabatic MD, as well as a much smaller dampinglike component acting as "geometric friction." Such current-independent geometric torque is absent from widely used micromagnetics or atomistic spin dynamics modeling of magnetization dynamics based on the Landau-Lifshitz-Gilbert equation, while previous analyses of how to include our Fermi-surface dampinglike torque have severely underestimated its total magnitude.

Electronic friction coefficients from the atom-in-jellium model for Z=1–92

The break-down of the Born-Oppenheimer approximation is an important topic in chemical dynamics on metal surfaces. In this context, the most frequently used "work-horse" is electronic friction theory, commonly relying on friction coefficients obtained from density functional theory (DFT) calculations from the early 80s based on the atom-in-jellium model. However, results are only available for a limited set of jellium densities and elements ($Z=1-18$). In this work, these calculations are revisited by investigating the corresponding friction coefficients for the entire periodic table ($Z=1-92$). Furthermore, friction coefficients obtained by including the electron density gradient on the Generalized Gradient Approximation (GGA) level are presented. Finally, we show that spin polarization and relativistic effects can have sizeable effects on these friction coefficients for some elements.

Development of Sn-added in situ autoreactivity Zr–Si biomedical alloys

In this study, the impact of the amount of Sn on microstructure, mechanical properties and wear resistance of Zr–8.8Si–xSn (x=0, 5, 10, 20at.%) alloys was investigated by X-ray diffraction, scanning electron microscopy, compression experiments and friction and wear experiments. It is found that the added Sn reacts with Zr to form Zr4Sn and Zr5Sn3 mesophases to improve the properties of the alloys. The formation of Zr4Sn in the alloys not only greatly increases the strength of the alloys, but also affects the distribution of the Zr–Si strengthening phase and can be used to coordinate the plasticity of the alloys. When the content of Sn reaches 20at.%, Zr5Sn3 begins to appear and Zr2Si disappears. The wear resistance of the alloys becomes worse due to the formation of Zr4Sn and the aggregation of Zr2Si. In this paper, the second phase strengthening by Zr2Si, the solid solution strengthening after the addition of Sn and the second phase strengthening by Zr4Sn are used to govern the mechanical properties and wear resistance of the alloys. It is promising biomedical alloys with higher strength and wear resistance.

Effect of PZT doping amount on microstructure and properties of Fe-based memory alloy composite coating by laser cladding

ABSTRACT To enhance the hardness and wear resistance of 304 stainless steel, Fe-based memory alloy/PZT piezoelectric ceramic (SMA/PZT) composite coatings with different PZT doping amounts (0.5–2 wt.%) were fabricated on 304 stainless steel by laser cladding. Coatings show good metallurgical bonding with the substrate. The microstructure, phase composition, microhardness, wear and corrosion resistance of the composite coating were investigated by scanning electron microscopy, X-ray diffractometer, microhardness tester, friction and wear tester and electrochemical workstation respectively. The results show that the microhardness, wear resistance and corrosion resistance of the composite coating are improved compared to the SMA coating. When the PZT doping amount is 1 wt.%, the SMA/PZT composite coating has the highest microhardness, and its wear and corrosion resistance are optimal compared to other coatings. It proved that 1 wt.% was optimal doping amount of PZT in the composite coating.

Effect of Cr3C2 content on microstructure and properties of 310 stainless steel coating by PTA welding

ABSTRACT In this work, the effect of Cr3C2 on the microstructure and properties of 310 coatings by plasma transfer arc welding was studied. The microstructure, composition, hardness and wear resistance of the coatings were analyzed by scanning electron microscope, EDS, X-ray diffractometer, microhardness tester and friction wear tester. The results show that when Cr3C2 is less, the coating is composed of columnar crystal and cellular crystal, mainly including (Fe, Ni) Fe3Ni2FeCr0.29Ni0.16C0.06Cr2Fe14C and CrFe7C0.45. With the increase of Cr3C2, the structure becomes dendrites and new phases Cr7C3 and Cr23C6 appeared. However, the crystallinity of 5% is poor and there is no obvious grain structure. The hardness of the coatings increases first and then decreases with the content of Cr3C2. The hardness with the content of 3% is the highest. The friction coefficient corresponds to the hardness and the main wear mechanism is abrasive wear and oxidation wear.

The influence of carbon content on the microstructure, mechanical and frictional property of chromium carbon nitride composite films

In this paper, a series of chromium carbon nitride (CrCN) composite films with various carbon content were deposited by changing the power of carbon target using RF-reactive magnetron sputtering system in the argon and nitrogen mixture gases. The microstructure, mechanical and frictional properties of CrCN films were investigated by X-ray diffraction (XRD), Raman spectrometer, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), nanoindentation and friction tester. According to the experimental results, the deposited binary CrN film exhibited a single face-centered cubic (fcc) structure, whereas the addition of carbon into the CrN matrix induced the formation of other phases of carbon and CNx. Both the solid solution strengthening and the increase of compressive residual stress improved the hardness from ~17 GPa at 0 at.% carbon to ~19 GPa at 6.3 at.% carbon. With the increase of C content, the friction curve of the CrCN film stabilized gradually and the average friction coefficient decreased gradually from 0.62 at 0 at.% carbon to 0.40 at 23.7 at.% carbon. The formation of lubricating amorphous carbon and CNx phase in the films mainly attributed to the improvement of friction property.

Single-asperity sliding friction across the superconducting phase transition.

In sliding friction, different energy dissipation channels have been proposed, including phonon and electron systems, plastic deformation, and crack formation. However, how energy is coupled into these channels is debated, and especially, the relevance of electronic dissipation remains elusive. Here, we present friction experiments of a single-asperity sliding on a high-T c superconductor from 40 to 300 kelvin. Overall, friction decreases with temperature as generally expected for nanoscale energy dissipation. However, we also find a large peak around T c. We model these results by a superposition of phononic and electronic friction, where the electronic energy dissipation vanishes below T c. In particular, we find that the electronic friction constitutes a constant offset above T c, which vanishes below T c with a power law in agreement with Bardeen-Cooper-Schrieffer theory. While current point contact friction models usually neglect such friction contributions, our study shows that electronic and phononic friction contributions can be of equal size.

Effect of Substrate Bias Voltage on the Structure and Properties of Mos2-Ti Composite Films on Titanium Alloy Surface

In order to discuss the effect of negative bias on the structure and properties of MoS2-Ti composite films on titanium alloy surface. In this paper, MoS2-Ti solid lubrication films were deposited on titanium alloy surface by unbalanced magnetron sputter. The effects of negative bias on the structure and properties of MoS2-Ti solid lubrication films were discussed. The morphology, phase, hardness, friction and wear resistance and wear mark morphology of solid lubricating film were analyzed and tested by scanning electron microscope, XRD, microhardness tester, friction and wear tester and surface profilometer. The results shows that wwhen the negative bias voltage was -150V and -200V, the structure of MoS2-Ti films is compact and the bonding with the substrate is compact. With the increase of negative bias voltage, the hardness of the film increases gradually. The wear rate of the films prepared at -150V bias voltage is the lowest, which is 2.2×10−17mm3/(N×m). The MoS2-Ti composite films deposited on the surface of titanium alloy exhibit excellent tribological properties with low friction and wear resistance, which effectively improves the surface friction and wear properties of titanium alloy.

Influence of Y2O3 on the microstructure and tribological properties of Ti-based wear-resistant laser-clad layers on TC4 alloy

Multi-track Ti-based wear-resistant composite coatings were fabricated on TC4 alloy surfaces using laser-clad TC4 + Ni45 + Co–WC mixed powders with different Y2O3 contents (0, 1, and 3 wt%). The microstructure, microhardness, and tribological properties of the coatings were characterised using X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry, electron probe X-ray micro analyser, microhardness tester, and friction and wear testing apparatus. The results showed that the number of cracks on the coating surfaces gradually decreased with the addition of Y2O3 and that residual Co–WC powders existed in the coating subsurfaces. The phase composition of the coatings with different Y2O3 contents remained unchanged and was mainly composed of reinforcing phases of TiC, TiB2, Ti2Ni, and matrix α-Ti. With the addition of Y2O3, the coating microstructure was remarkably refined, the direction characteristic of the TiC dendrites obviously weakened, and the nucleation rate significantly increased. When the added Y2O3 was 3 wt%, a large amount of TiB2–TiC-dependent growth composite phases precipitated in the coating. The two-dimensional lattice misfit between (0001)TiB2 and (111)TiC was 0.912%, which indicated that TiB2 and TiC formed a coherent interface. When the amount of Y2O3 was increased, the microhardness of the coatings gradually decreased, and the wear volume of the coatings first increased and then decreased. Under the effect of the TiB2–TiC composite phases, the wear resistance of the 3 wt% Y2O3 coating was optimal. The 3 wt% Y2O3 coating friction coefficient was the lowest, and the wear mechanism was abrasive wear.

Scattering of NO(ν = 3) from Au(111): a stochastic dissipative quantum dynamical perspective.

In this work, we present a theoretical study of the scattering dynamics of NO(ν = 3) from an ideal unreconstructed Au(111) surface. The simulations are performed in reduced dimensions at the three high-symmetry sites employing our recent modification to the stochastic wave packet approach for diatomic-metal scattering [J. Chem. Phys., 2019, 150, 184105]. Energy exchange between molecular vibrational degrees of freedom and the electron-hole pairs (EHP) of the metal is accounted for by quantized stochastic jump operators, with associated rates obtained from a microscopic model based on Fermi's golden rule. The simulations are found to reproduce the experimentally observed trend of enhanced vibrational relaxation probabilities with increasing initial translational energy. Molecular dynamics simulations with electronic friction (MDEF) in the independent atom approximation were performed to compare classical and quantum dynamical descriptions of that system. Significant differences between these two descriptions were found indicating that intermode coupling must be described accurately by using a good potential energy surface, and pointing out at the potentially important influence of a quantized description of energy relaxation in describing the scattering of NO from Au(111).

Preparation and Properties of Flame Retardant ABS/POK Alloy

<i>Background</i> ABS material has excellent mechanical properties, but its solvent resistance is poor. Acid solvents are prone to cracking, which will affect the normal use of the product. Technical research is needed to improve this performance defect and solve the technical bottleneck of poor solvent resistance of ABS material, so as to meet the technical demand of energy and other fields and improve the market application field of ABS material. <i>Subject</i> Using ABS and POK resin alloy as matrix resin, a high performance flame retardant ABS alloy material was prepared by developing a high efficiency flame retardant system of brominated antimony. <i>Methods</i> The mechanical properties, crystallization behavior, wear resistance, solvent resistance and heat resistance of the alloy materials were characterized by Fourier infrared spectroscopy, differential scanning calorimeter, universal testing machine, scanning electron microscope and friction tester. <i>Result</i> ABS has a good compatibility with POK, and the alloy material containing 20% POK has excellent mechanical properties and heat resistance, good ice acetic acid resistance and excellent solvent resistance. Through the DSC curve of the alloy material, the relationship between the melting peak area and the content of POK resin was obtained. POK resin can significantly improve the wear resistance of ABS and has a good glossiness retention rate.

Vibrational response and motion of carbon monoxide on Cu(100) driven by femtosecond laser pulses: Molecular dynamics with electronic friction

Carbon monoxide on copper surfaces continues to be a fascinating, rich microlab for many questions evolving in surface science. Recently, hot-electron mediated, femtosecond-laser pulse induced dynamics of CO molecules on Cu(100) were the focus of experiments [Inoue et al., Phys. Rev. Lett. 117, 186101 (2016)] and theory [Novko et al., Phys. Rev. Lett. 122, 016806 (2019)], unraveling details of the vibrational nonequilibrium dynamics on ultrashort (subpicoseconds) timescales. In the present work, full-dimensional time-resolved hot-electron driven dynamics are studied by molecular dynamics with electronic friction (MDEF). Dissipation is included by a friction term in a Langevin equation which describes the coupling of molecular degrees of freedom to electron-hole pairs in the copper surface, calculated from gradient-corrected density functional theory (DFT) via a local density friction approximation (LDFA). Relaxation due to surface phonons is included by a generalized Langevin oscillator model. The hot-electron induced excitation is described via a time-dependent electronic temperature, the latter derived from an improved two-temperature model. Our parameter-free simulations on a precomputed potential energy surface allow for excellent statistics, and the observed trends are confirmed by on-the-fly ab initio molecular dynamics with electronic friction (AIMDEF) calculations. By computing time-resolved frequency maps for selected molecular vibrations, instantaneous frequencies, probability distributions, and correlation functions, we gain microscopic insight into hot-electron driven dynamics and we can relate the time evolution of vibrational internal CO stretch-mode frequencies to measured data, notably an observed redshift. Quantitatively, the latter is found to be larger in MDEF than in experiment and possible reasons are discussed for this observation. In our model, in addition we observe the excitation and time evolution of large-amplitude low-frequency modes, lateral CO surface diffusion, and molecular desorption. Effects of surface atom motion and of the laser fluence are also discussed.

Electrons and Phonons Cooperate in the Laser-Induced Desorption of CO from Pd(111)

Femtosecond laser induced desorption of CO from a CO-covered Pd(111) surface is investigated with abinitio molecular dynamics with electronic friction that incorporates effects due to the excited electronic and phononic systems, as well as out-of-phase coadsorbate interactions. Our simulations show evidence of an important electron-phonon synergy in promoting CO desorption that has largely been neglected in other similar systems. At the saturated coverage of 0.75ML, effects due to CO-CO interadsorbate energy exchange are also important. Our dynamics simulations, in concert with site-specific desorption energy calculations, allow us to understand the large coverage dependence of the desorption yields observed in experiments.

Application of MIG and TIG Welding in Automobile Industry

Emission and weight reduction have been identified over the years by the automobile industries as the most efficient ways to maintain fuel economy and to meet the demand of the government agencies on global warming. These challenges of reducing emission and weight are even being compounded with consumers taste for luxury features which adds to the weight of the traditional vehicle designs. To meet these demands, alloys such are Aluminium Alloys, Magnesium Alloys and Titanium Alloys have been identified as the suitable materials to replace conventional steel structures due to their superior properties such as high strength-to-weight ratio, high tensile strength and high-temperature performance. With the identification of suitable materials to replace the traditional materials, the welding of alloy materials remains a challenge faced by vehicle manufacturers. Although electron beam welding, ultrasonic welding and friction stir welding have proven to give quality weld joints of alloys used in automobile fabrication, their application is limited by the high cost of equipment, need for vacuum environment in electron beam welding, size and shape of base metals. Laser welding with its reduced heat affected zone (HAZ), good seam appearance and deep penetration has widely been applied in an automobile. However, it is not with its own shortcomings such as poor gap bridging capability, difficulty in welding reflective materials and high cost. Arc welding with its low cost compared to other welding techniques and high energy efficiency, therefore, remains a useful welding process in an automobile. In this paper, a review of the various investigations by researchers on MIG and TIG welding of alloys used in the automobile have been documented.

Arc deposition of wear resistant layer TiN on Ti6Al4V using simultaneous feeding of nitrogen and wire

As an admirable surface improvement method, arc surface cladding has been widely used in various fields. The surface properties of titanium alloy were improved by gas tungsten arc cladding. In the process of arc claddings, a certain amount of nitrogen was added in the shielding gas to form the hard phase of TiN in the cladding layer. Titanium alloy surface was cladded with the same process parameters and different nitrogen flow rate. The effects of nitrogen flow rate on the microstructure and mechanical performance of the cladding layer were studied through an optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), Vickers hardness and friction wear testing machine. The results show that the increase of the nitrogen flow rate not only changes the shape of the arc but also forms a large number of TiN hard phases in the cladding layer. With the increase of the nitrogen flow rate, the number of TiN hard phases in the cladding layer increases gradually, which improves the mechanical property of the cladding layer. The Vickers hardness of the cladding layer is much larger than 600 HV and the friction coefficient is 0.419 when the nitrogen rate is 2 L/min.

Symmetry-Adapted High Dimensional Neural Network Representation of Electronic Friction Tensor of Adsorbates on Metals

Nonadiabatic effects in chemical reaction at metal surfaces, due to excitation of electron-hole pairs, stand at the frontier of the studies of gas-surface reaction dynamics. However, the first principles description of electronic excitation remains challenging. In an efficient molecular dynamics with electronic friction (MDEF) method, the nonadiabatic couplings are effectively included in a so-called electronic friction tensor (EFT), which can be computed from first-order time-dependent perturbation theory (TDPT) in terms of density functional theory (DFT) orbitals. This second-rank tensor depends on adsorbate position and features a complicated transformation with regard to the intrinsic symmetry operations of the system. In this work, we develop a new symmetry-adapted neural network representation of EFT, based on our recently proposed embedded atom neural network (EANN) framework. Inspired by the derivation of the nonadiabatic coupling matrix, we represent the tensorial friction by the first and second derivatives of multiple outputs of NNs with respect to atomic Cartesian coordinates. This rigorously preserves the positive semidefiniteness, directional property, and correct symmetry-equivariance of EFT. Unlike previous methods, our new approach can readily include both molecular and surface degrees of freedom, regardless of the type of surface. Tests on the H2+Ag(111) system show that this approach yields an accurate, efficient, and continuous representation of EFT, making it possible to perform large scale TDPT-based MDEF simulations to study both adiabatic and nonadiabatic energy dissipation in a unified framework.

Hierarchical quantum master equation approach to vibronic reaction dynamics at metal surfaces.

A novel quantum dynamical method to simulate vibronic reaction dynamics in molecules at metal surfaces is proposed. The method is based on the hierarchical quantum master equation approach and uses a discrete variable representation of the nuclear degrees of freedom in combination with complex absorbing potentials and an auxiliary source term. It provides numerically exact results for a range of models. By taking the coupling to the continuum of electronic states of the surface properly into account, nonadiabatic processes can be described and the effect of electronic friction is included in a nonperturbative and non-Markovian way. Illustrative applications to models for desorption of a molecule at a surface and the current-induced bond rupture in single-molecule junctions demonstrate the performance and versatility of the method.

Effects of electronic friction from the walls on water flow in carbon nanotubes and on water desalination.

A mechanism for removal of salt from salt water is discussed, which results from friction due to Ohm's law heating, resulting from motion of an electron charge induced in the tube walls by the water molecules' dipoles and the ions' charges. The desalination occurs because this friction is larger for salt ions than for water molecules. Friction due to Ohm's law heating might also provide an explanation for the observation by Secchi etal. [Nature 537, 210 (2016)10.1038/nature19315] that the flow velocity of water in carbon nanotubes for a given pressure gradient increases rapidly as the tube radius decreases from 50 to 15 nm, which does not occur for boron nitride nanotubes, which are insulators. This friction can have the right magnitude to produce the slip lengths reported by Secchi etal. One possibility is that the nanotubes in this experiment were metallic, and their conductivity becomes large as their radius decreases, due to ballistic conduction. Another possibility is that when the tube circumference drops below the electron mean-free path, the wall switches from behaving as a two-dimensional conductor to behaving as a one-dimensional conductor for which the electrons are more strongly localized. When the conductivity is sufficiently small, small displacements of the localized electron states can provide the dominant contribution to the motion of the induced charge, rather than current flow, thus reducing the friction due to Ohm's law heating.