Amplitude Of Atomic Vibrations Research Articles

Assessment of the potential structure and nucleation efficacy of multi-component heterogeneous substrate

In recent years, nucleation of multi-component heterogeneous interface structures has attracted increasing attention due to their excellent grain refinement and anti-poisoning capabilities in Al-Si alloy. In this research, a general methodology was proposed to predict the surface structure and meanwhile evaluate the grain refinement efficacy of the multi-component boride nucleants based on a comprehensive experimental and theoretical study of a novel Al-Si alloy refiner, i.e., Al-3.5 Nb-1 V-1B. By using high-resolution electron microscopy, the (Nb, V)B2 particle with a characteristic “core-shell” surface structure was observed, attributed to V adsorption driven by interfacial energy minimization, validated by ab initio calculations. A general interfacial energy-based model was then proposed which predicts the multi-component boride substrate structure, well consistent with the experimental findings. The nucleation potency of different substrates was analyzed in atomic scale and only if the interfacial atomic vibration was considered together with the nucleation-influencing factors (i.e., the lattice mismatch and the anti Si-poisoning effect), the predicted nucleation potency order can agree well with the experimental results. A thermodynamics-kinetics combined analytic model was summarized by coupling the lattice mismatch parameter, the interfacial Si adsorption energy and the interfacial atomic vibration amplitude, which accurately evaluates the multi-component boride grain refiners for the Al-Si alloys.

Molecular dynamics study on dynamic interlayer friction of graphene and its strain effect

Abstract This study delves into the mechanism of dynamic sliding friction between layers of graphene and its strain effect, through numerical analysis using molecular dynamics simulations. To eliminate the influence of commensurability and edge effect, a friction pair model with annular graphene as a slider is established. The research explores the quantifying effects of temperature, normal load, sliding velocity, support stiffness, and axial strain on the friction between graphene layers. The coupling effect of temperature and other influencing factors is also clarified. The results indicate that the interlayer friction increases with normal load by decreasing the interlayer spacing and increasing the atomic vibration amplitude. The ploughing phenomenon does not appear since the edge effect is eliminated by the model. Friction is initially enhanced at higher sliding velocities, but is later reduced by severe residual deformation and lattice resonance frequency. The support stiffness regulates interlayer friction by affecting the atomic vibration amplitude of the graphene lattice. Mechanism analysis shows that the number of effective contact atoms increases under axial strain, and the lattice vibration frequency is the main way to regulate the interlayer friction by strain effect. Our findings provide a fundamental understanding of the strains engineering of nanoscale friction and reveal the influence mechanism of affecting factors on the dynamic friction of graphene.

Isobaric Thermal Expansivity and Isothermal Compressibility of Liquid Metals.

The relationship between the volumetric thermodynamic coefficients of liquid metals at the melting point and interatomic bond energy was studied. Using dimensional analysis, we obtained equations that connect cohesive energy with thermodynamic coefficients. The relationships were confirmed by experimental data for alkali, alkaline earth, rare earth, and transition metals. Cohesive energy is proportional to the square root of the ratio of melting point Tm divided by thermal expansivity αp. Thermal expansivity does not depend on the atomic size and atomic vibration amplitude. Bulk compressibility βT and internal pressure pi are related to the atomic vibration amplitude by an exponential dependence. Thermal pressure pth decreases with an increasing atomic size. Fcc and hcp metals with high packing density, as well as alkali metals, have the relationships with the highest coefficient of determination. The contribution of electrons and atomic vibrations to the Grüneisen parameter can be calculated for liquid metals at their melting point.

Analysis of melting behavior of some transition metals at high pressures

It has been reinforced that the Lindemann law does not work satisfactorily for determining melting curves of some transition metals such as Mo, Ta and W. The slopes of melting curves for these metals predicted from the Lindemann law are significantly larger than the corresponding observed values. We have used three methods for determining the volume dependence of Grüneisen parameter. The strong discrepancy between the calculated and experimental values of melting slopes reveal inadequacy of the Lindemann law. We have examine the original assumption considered in the derivation of the Lindemann law viz. the Gilvarry criterion according to which the root mean square amplitude of atomic vibrations is a definite fraction of the interatomic distance at melting. It is found in the present study that this critical fraction at melting is not same at different pressures. We have presented an analysis of melting for the transition metals by taking into account the pressure dependence of vibrational elongation. The vibrational elongation changes with the increase in pressure, and can be represented by a simple power law dependence on volume with exponent depending on the material.

Magic angle in thermal conductivity of twisted bilayer graphene

We report a local minimum in thermal conductivity in twisted bilayer graphene (TBG) at the angle of 1.08°, which corresponds to the ‘magic angle’ in the transition of several other reported properties. Within the supercell of a moiré lattice, different stacking modes generate phonon scattering sites which reduce the thermal conductivity of TBG. The thermal magic angle arises from the competition between the delocalization of atomic vibrational amplitudes and stresses on one hand, and the increased AA stacking density on the other hand. The former effect weakens the scattering strength of a single scatterer while the latter one increases the density of scatterers. The combination of these two effects eventually leads to the apparition of the highlighted irregularity in heat conduction. The manifestation of a magic angle, disclosing new thermal mechanisms at nanoscale, further uncovers the unique physics of two-dimensional materials.

Quantitative STEM: A method for measuring temperature and thickness effects on thermal diffuse scattering using STEM/EELS, and for testing electron scattering models

In the last two decades, advances in the dark field detectors and microscopes of scanning transmission electron microscopy (STEM) have inspired a resurgence of interest in quantitative STEM analysis. One promising avenue is the use of STEM as a nanothermometric probe. In this application, thermal diffuse scattering, captured by a CCD camera or an annular dark field detector, acts as an indirect measurement of the specimen temperature. One challenge with taking such a measurement is achieving adequate sensitivity to quantify a change in scattered electron signal on the order of 1% or less of the full electron beam. Another difficulty is decoupling the thermal effect on electron scattering from scattering changes due to differing specimen thicknesses and materials. To address these issues, we have developed a method using STEM, combined with electron energy loss spectroscopy (EELS), to produce a material-specific calibration curve. On silicon, across the range 89 K to 294 K, we measured a monotonically increasing HAADF signal ranging from 4.0% to 4.4% of the direct beam intensity at a thickness-to-mean-free-path ratio of 0.5. This yielded a calibration curve of temperature versus full-beam-normalized, thickness-normalized HAADF signal. The method enables thermal measurements on a specimen of varying local thickness at a spatial resolution of a few nanometers. We demonstrated the potential of the technique for testing electron scattering models by applying single-electron scattering theory to the data collected to extract a measurement of the mean atomic vibration amplitude in silicon at 294 K. The measured value, 0.00738 ± 0.00002 nm, agrees well with reported measurement using X-rays.

Pressure weakening unusual-caged dynamics of La80Al20 metallic glass-forming liquid

Caged dynamics is the precursor of β-relaxation and is part of the complex relaxation dynamics of metallic glasses. However, limited by the lack of suitable experimental techniques, the pressure dependence of the caged dynamics of glass-forming metallic liquids and its underlying mechanism are still an open question to date. An unusual caged dynamics manifests a sudden drop near 0.1 ps in the partial non-Gaussian parameter of Al atoms in metallic alloy La80Al20 liquid was recently found by molecular dynamics simulations. The present work investigates the caged dynamics of the La80Al20 liquid with strong atomic interactions under different pressures again by molecular dynamics simulations. It is found that the dominant polyhedra are transformed from the stable icosahedral-like polyhedra to the unstable ones that are compatible with crystal symmetry, resulting in the promotion of the rearrangement of atoms. These changes in turn cause the central Al atoms to be less hindered by the surrounding atoms, enhancing the cage-breaking motion and weakening the unusual caged dynamics. In addition, the applied pressure not only increases the diversity of vibrational frequencies but also weakens the difference in atomic vibrational amplitudes, leading to the weakening of the unusual caged dynamics. These findings provide new insights into the caged dynamics under pressure from the perspective of microstructure and vibrational properties.

Molecular Dynamics Simulation of Nanoindentation of Nb-Zr Alloys with Different Zr Content

To understand the nanomechanical behaviors of the Nb-based alloys with Zr addition at room/high temperature, the molecular dynamics simulations of nanoindentation are conducted. In this work, the load-unload displacement curve, hardness, and dislocation characteristics of Nb-Zr alloys with varying Zr content ranging from 0 to 5 wt.% are studied. The simulation results are found to closely agree with the experimental one at 1 wt.%, therefore showing the reliability of the simulation. Moreover, considering distinct responses of alloys to different service temperature, the high-temperature nanoindentation are performed. The effects of Zr addition on the mechanical deformation under both temperatures are compared. The same phenomenon is found such that the optimum concentration range yielding the greatest hardness is 1–3 wt.%. The elastic modulus of NbZr alloy improves with elevated concentration at room temperature, while the hardness at higher temperature exhibits the opposite trend. This is attributed to the higher amplitude of atomic vibrations at high temperatures, which is more likely to deviate atoms from their equilibrium positions and weaken the pinning effect under external loading. Therefore, we believe that our studies on the nanomechanical mechanisms of materials at room/high temperature will provide an effective way for the alloying optimization design.

Dispersion effects and real atomic vibrations of impure crystals studied based on interatomic effective potential

The dispersion effects and real atomic vibrations of crystals containing doping atoms have been studied using the interatomic effective potential which includes the many-body effects based on the contributions of nearest neighbors of the host and doping atoms. The analytical expressions are derived for the dispersion relations describing the dispersion effects consisting of the acoustic and optic branches, forbidden zone, as well as for the amplitude and phase shift of the real atomic vibrations of impure crystals. The Morse potential is assumed to describe the single-pair atomic interaction. The interrupts of wave propagation caused by doping atoms and the changes of thermodynamic properties due to the real atomic vibrations, as well as the areas of localization of forbidden zones and doping atoms have been specified. The numerical results for Fe doped by atoms of W calculated by the present theory using the interatomic effective potential are found to be in good agreement with experiment obtained from the measured Morse potential parameters and quite different from those of its inverse doping process (W doped by atoms of Fe), while they are the same if the single-pair potential is used.

Supramolecular Interactions Lead to Remarkably High Thermal Conductivities in Interpenetrated Two-Dimensional Porous Crystals.

The design of innovative porous crystals with high porosities and large surface areas has garnered a great deal of attention over the past few decades due to their remarkable potential for a variety of applications. However, heat dissipation is key to realizing their potential. We use systematic atomistic simulations to reveal that interpenetrated porous crystals formed from two-dimensional (2D) frameworks possess remarkable thermal conductivities at high porosities in comparison to their three-dimensional (3D) single framework and interpenetrated 3D framework counterparts. In contrast to conventional understanding, higher thermal conductivities are associated with lower atomic densities and higher porosities for porous crystals formed from interpenetrating 2D frameworks. We attribute this to lower phonon-phonon scattering and vibrational hardening from the supramolecular interactions that restrict atomic vibrational amplitudes, facilitating heat conduction. This marks a new regime of materials design combining ultralow mass densities and ultrahigh thermal conductivities in 2D interpenetrated porous crystals.

Diffusion of water nanodroplets on graphene with double-vacancy: The constraining effects of defect

Diffusion of water across surfaces generally involves motion on an uneven or defective but otherwise stationary substrate. Studying the diffusion of water nanodroplets on the surface of such a substrate is useful for the development and progress of nano-fluidic systems. In this study, the diffusion of water nanodroplets on double-vacancy graphene is investigated based on molecular dynamics simulations. We find that defects have a constraining effect on water nanodroplets. According to the defect distribution in conjunction with the range of water nanodroplet diffusion, the constraining effects can be differentiated as fully constrained or partially constrained. Subsequently, the impact on the constraining effects from water nanodroplet size and defect distance is studied. Tensile deformation and temperature are considered as two possible ways to break the constraining effects. The results show that the constraining effects from defects on water nanodroplets can be broken when biaxial tensile strain reaches 0.6% for both cases, but temperature has no obvious effect for partially constrained conditions. The differently constrained behaviors of water nanodroplets are found to be related to the vibration amplitude of the carbon atoms. Our results reveal that tensile deformation of graphene is more helpful for controlling the diffusion of water nanodroplets on double-vacancy graphene. The phenomena reported here can enrich the knowledge of molecular mechanisms for nanofluidic systems, and may inspire more applications with structural design for graphene and other 2D materials.

СРЕДНЕКВАДРАТИЧНЫЕ ОТКЛОНЕНИЯ АТОМОВ – НАНОМАСШТАБНЫЙ ИНДИКАТОР СОСТОЯНИЯ ПОВЕРХНОСТИ

Актуальность исследования связана с повышением надежности сварных соединений паросиловых установок и тем самым с рациональным использованием материалов изделий за счет совершенствования и развития методов изучения их ресурсных физических характеристик. Цель: тестирование новых наномасштабных структурных характеристик III рода для оценок состояния поверхности. Объект: образцы жаропрочной стали трубных поверхностей нагрева паровых котлов. Методы: физическое моделирование условий эксплуатации путем термоциклирования в электропечи МИМП-10УЭ, холодного циклического деформирования с применением гидравлического пресса, рентгенометрия образцов с оценкой среднеквадратичных отклонений атомов и внутренних структурных напряжений на рентгеновских дифрактометрах типа ДРОН, анализ морфологии поверхности с помощью портативного электронного микроскопа типа PENSCKOPE с увеличением ´20. Результаты. Проведены экспериментальные исследования, связанные с количественной оценкой колебательной амплитуды атомов и ее эволюцией при циклических термических и механических нагрузках. В результате исследований предложен новый диагностический параметр в виде колебательной амплитуды атомов , отражающий фундаментальную природу сил межатомных связей. Продемонстрирована корреляция между среднеквадратичными отклонениями атомов (микромасштабный уровень свойств), представляющими микроструктурные характеристики III рода, с макромеханическими характеристиками и (макромасштабный уровень свойств). Так как условный предел текучести и временное сопротивление разрушению связаны с прочностью и разрушением, то и среднеквадратичные отклонения атомов могут быть использованы в качестве индикатора поврежденности и признака достижения предельных состояний. Параметр может служить также характеристикой текущего индивидуального физического состояния металла. Предложена новая методология оценки текущего и наступления предельных состояний. Показано, что восстановление структуры контролируется на атомном микромасштабном уровне по колебательной амплитуде и напряжениям I рода.

ОСОБЕННОСТИ ЭВОЛЮЦИИ НАНОМАСШТАБНЫХ ХАРАКТЕРИСТИК МЕТАЛЛА ПРИ НАГРУЖЕНИИ ТЕРМИЧЕСКИМИ ЦИКЛАМИ

Актуальность исследования обусловлена отсутствием экспериментальных данных, устанавливающих связь между субмикроскопическими свойствами (свойствами III рода) и макроскопическими свойствами I рода. Такая взаимосвязанность, являющаяся фактором влиянияна отношение «микроструктура–прочность», может способствовать дальнейшим успехам в решении вопросов ресурса. Цель исследования заключается в установлении влияния циклических термических нагрузок на среднеквадратичные смещения атомов как показатель прочности межатомных связей и микроповрежденности металла пароперегревательных труб. Объектом исследования являются образцы из околошовных зон однородного сварного шва, выполненного из жаропрочной перлитной стали 12Х1МФ. Методы: физическое моделирование условий эксплуатации путем воздействия на образцы циклами термической нагрузки в электропечи МИМП-10УЭ, рентгенометрия образцов, оценка внутренних структурных напряжений на рентгеновских дифрактометрах ДРОН, морфология поверхности с помощью портативного электронного микроскопа типа PENSCKOPE. Результаты.Установлено влияние циклических термических нагрузок на среднеквадратичные смещения атомов. Показано, что в результате термического влияния прочность межатомных связей увеличивается. Проиллюстрирована корреляция между субмикро- и макроскопическими характеристиками прочности, и показано, что амплитуда тепловых колебаний атомов, являясь характеристикой прочности межатомных связей, может быть диагностическим признаком накопления и развития повреждаемости металла. Уменьшение амплитуды среднеквадратичных смещений атомов при термоциклировании объяснено на основе гипотезы о возникновении твердых растворов замещения в результате процесса азотирования поверхности молекулярным азотом атмосферного воздуха. В рамках этой гипотезы показано, что одним из механизмов осцилляции внутренних напряжений I рода при термоциклировании может быть процесс азотирования поверхности, приводящий к организации в многокомпонентных легированных сталях сложных связей «металл–неметалл», направленных на поддержание устойчивости системы твердых растворов.

Isotopic effects in structural properties of graphene

Isotopic effects are relevant to understand several properties of solids, and have been thoroughly analyzed along the years. These effects may depend on the dimensionality of the considered solid. Here we assess their magnitude for structural properties of graphene, a paradigmatic two-dimensional material. We use path-integral molecular dynamics simulations, a well-suited technique to quantify the influence of nuclear quantum effects on equilibrium variables, especially in cases where anharmonic effects are important. Emphasis is put on interatomic distances and mean-square displacements, as well as on the in-plane area of the graphene layer. At low temperature, the relative difference in C–C distance for 13C and 14C, with respect to 12C, is found to be − 2.5 and − 4.6 × 10−4, respectively, larger than in three-dimensional carbon-based materials such as diamond. For the in-plane area, the relative changes amount to − 3.9 and − 6.9 × 10−4. The magnitude of anharmonicity in the lattice vibrations is estimated by comparing the internal energy and atomic vibrational amplitudes with those derived from a harmonic approximation.

The generalized Lindemann melting coefficient

Lindemann developed the melting temperature theory over 100 years ago, known as the Lindemann criterion. Its main assumption is that melting occurs when the root-mean-square vibration amplitude of ions and atoms in crystals exceeds a critical fraction, η of the inter-atomic spacing in crystals. The Lindemann coefficient η is undefined and scientific papers report different η values for different elements. Here we present previously unobserved data trends pointing to the fact that the Lindemann coefficient could be linked to the periodic groups of the periodic table, having an exact value for each element belonging to a given periodic group. We report 12 distinctive Lindemann coefficient values corresponding to 12 groups of the periodic table containing solid elements with identifiable melting temperature. Using these vales, the recalculation of the melting temperatures indicates a good match to the experimental values for 39 elements, corresponding to 12 out of 15 periodic groups. This newly observed result opens up the possibility of further refining the Lindemann melting criterion by stimulating analytical studies of the Lindemann coefficient in the light of this newly discovered result.

Comparative optical, FTIR and photoluminescence spectral analysis of copper ions in BaO–B2O3, SrO– B2O3 or Bi2O3– B2O3 glasses and impact of gamma irradiation

Three basic borate glasses were prepared from the systems (BaO–B2O3), (SrO–B2O3) and (Bi2O3–B2O3) together with samples doped with 0.25% CuO from the three systems. Collective characterizations of the optical, FTIR, photoluminescence spectra of the prepared glasses were carried out before and after impinging to gamma irradiation. The results were analysed in relation with the structural building groups and their housing in the glass network. Optical spectra of the alkaline earth borates reveal distinct UV absorption bands due to unavoidable trace iron impurities (Fe3+ ions). While the Bi2O3–B2O3 glass shows an additional near visible band related to absorption of Bi3+ ions. The CuO doped glasses exhibit distinct greenish blue colors imparting an additional very broad visible near IR band in all samples due to octahedrally coordinated Cu2+ ions. FTIR spectra of the two glass systems of (SrO–B2O3) and (BaO–B2O3) glasses display vibrational bands due to both BO4 and BO3 groups, while Bi2O3–B2O3 glass shows the interference of additional vibrational bands due to BiO6 and BiO3 groups. Photoluminescence spectra of the samples exhibited similar excitation and emission peaks for the three CuO-doped glasses. They are assumed to originate from the excitation of (3 d9 4s3d10) transitions in Cu2+ ions. Gamma irradiation caused some changes in all spectral measurements, and upon inclusion CuO doping no noticeable changes have been identified. Thermal expansion data were analysed and correlated with increasing the amplitude of atomic vibrations, field strength and bond strength of the cations with oxygen.

On the role of intermolecular vibrational motions for ice polymorphs. II. Atomic vibrational amplitudes and localization of phonons in ordered and disordered ices.

We investigate the vibrational amplitudes and the degree of the phonon localization in 19 ice forms, both crystalline and amorphous, by a quasi-harmonic approximation with a reliable classical intermolecular interaction model for water. The amplitude in the low pressure ices increases with compression, while the opposite trend is observed in the medium and high pressure ices. The amplitude of the oxygen atom does not differ from that of hydrogen in low pressure ices apart from the contribution from the zero-point vibrations. This is accounted for by the coherent but opposite phase motions in the mixed translational and rotational vibrations. A decoupling of translation-dominant and rotation-dominant motions significantly reduces the vibrational amplitudes in any ice form. The amplitudes in ice III are found to be much larger than any other crystalline ice form. In order to investigate the vibrational mode characteristics, the moment ratio of the atomic displacements for individual phonon modes, called the inverse participation ratio, is calculated and the degree of the phonon localization in crystalline and amorphous ices is discussed. It is found that the phonon modes in the hydrogen-ordered ice forms are remarkably spread over the entire crystal having propagative or diffusive characteristic, while many localized modes appear at the edges of the vibrational bands, called dissipative modes, in the hydrogen-disordered counterparts. The degree of localization is little pronounced in low density amorphous and high density amorphous due to disordering of oxygen atoms.

Substrate Temperature Influence on Optical Properties of C60 Thin Films Within the Visible Range

Fullerene thin films of about 200 nm thicknesses have been deposited by thermal evaporation method on soda lime glass at substrate temperature 303 and 403K under pressure about 10-5 mbar. This study concentrated on the influence of substrate temperature on the optical properties of C60 thin films within the visible range. Optical characterization has been carried out at room temperature using the absorption spectra, at normal incidence, in range (200-900) nm.
 The absorption and extinction coefficients of the samples have been evaluated according to the variation in the UV- Visible spectrum. Increasing substrate temperature causes decreasing in optical band gap energy, for direct allowed transitions, and slightly changing in refractive index. This incident was due to the reducing of interatomic intervals, which may be correlating a decrease in the amplitude of atomic vibrations around their equilibrium sites.

Rainbow Scattering of Inert Gas Atoms on Aluminum and Silver Crystal Surfaces

Particle trajectories are calculated to study the rainbow scattering and the focusing of Ar atoms on the Al(111) and Ag(111) crystal surfaces and Ne, Ar, and Kr atoms on the Al(001) crystal surface. The rainbow-scattering-induced features in spectra are used to determine the thermal vibration amplitudes of atoms for the Al(111) and Ag(111) targets. These data agree with the data of independent measurements. The dependence of the position of a rainbow peak on the initial energy of atoms is used to find the atom–surface interaction potentials in the energy range of 0.3–70 eV for the case Ar–Ag and 3–45 eV for other cases. The values and the functional dependences of the potentials on the internuclear distance differ strongly from the well-known pair potential models. This difference is associated with the interaction of a projectile atom with the conduction electrons in a metal.

Tunable thermal transport and reversible thermal conductivity switching in topologically networked bio-inspired materials.

The dynamic control of thermal transport properties in solids must contend with the fact that phonons are inherently broadband. Thus, efforts to create reversible thermal conductivity switches have resulted in only modest on/off ratios, since only a relatively narrow portion of the phononic spectrum is impacted. Here, we report on the ability to modulate the thermal conductivity of topologically networked materials by nearly a factor of four following hydration, through manipulation of the displacement amplitude of atomic vibrations. By varying the network topology, or crosslinked structure, of squid ring teeth-based bio-polymers through tandem-repetition of DNA sequences, we show that this thermal switching ratio can be directly programmed. This on/off ratio in thermal conductivity switching is over a factor of three larger than the current state-of-the-art thermal switch, offering the possibility of engineering thermally conductive biological materials with dynamic responsivity to heat.