High Frequency Vibrational Modes Research Articles

Effect of hydrogen constraints on predicting thermal conductivity of hydrocarbons in molecular dynamics simulation

Large overestimation has been reported while predicting the thermal conductivity of hydrocarbons using an all-atom force field model in molecular dynamics (MD) simulations. Although it has been guessed as resulting from the high-frequency vibration of the hydrogen atoms and hydrogen constraints method is suggested to be employed to reduce the deviation, how hydrogen constraints affect the heat conduction and local structure of hydrocarbons in MD simulation is still not fully understood. In this work, the effect of hydrogen constraints on the prediction of thermal conductivity of n-decane in MD simulations with the all-atom force field is studied. The results show that the deviation of the thermal conductivity of n-decane can be narrowed down by 72.48 % in simulations if the hydrogen constraints with a SHAKE algorithm is employed. The analysis of heat flux decomposition indicates that employing hydrogen constraints can reduce the contribution of transport term and non-bonded interactions to the heat flux, which in turn can help improve the accuracy while predicting the thermal conductivity in MD simulations. Furthermore, results of the vibrational density of states show that hydrogen constraints can dismiss the high-frequency vibration mode of molecules, thus effectively reducing the overestimation of thermal conductivity of hydrocarbon systems in MD simulations. The findings of this work sheds light on the molecular mechanism of heat transfer in hydrocarbon systems.

3D mode shapes characterization under hammer impact using 3D-DIC and phase-based motion magnification

Modal analysis constitutes a fundamental aspect of structural investigation within diverse engineering domains, encompassing sectors such as automotive, wind energy, and aerospace. The prominence of high-frequency excitation loads, exemplified by the combustion phenomena in liquid rocket engines, necessitates an in-depth examination of the high-frequency vibrational response within structural components. However, the complexity of evaluating high-frequency vibrations arises from the negligible displacement associated with these responses. When using an optical full-field measurement system based on a high-speed camera for vibration measurement, it is usually severely affected by noise. Direct analysis of raw data using an optical measurement system (3D-DIC) is not feasible. In this paper, we combine phase-based motion magnification and digital image correlation methods to obtain the high-frequency vibration modes of the structure. 3D-DIC(3D Digital Image Correlation)analysis is performed on the magnified images to quantify the out-of-plane vibration modes of the structure. Using the cantilever beam as an example, the first five out-of-plane vibration mode shapes were separated from the response video under a single hammer excitation. Especially the 5th order natural frequency is as high as 3503 Hz, and the corresponding structural response was below the noise floor of the camera system. The vibration mode results obtained by this method are highly consistent with the vibration modes obtained by the 3D-SLDV(3D Scanning Laser Doppler Vibrometer) method. Finally, this method was applied to identify the out-of-plane vibration modes of real engine pipe. The combination of motion magnification techniques and DIC can enhance the capability of traditional 3D-DIC, which is beneficial for high-frequency structural identification. Future research could concentrate on optimizing motion amplification factors for different structures and loads, and creating automated algorithms for analyzing and visualizing amplified motion data in real time.

(Invited) Photophysical Properties and Photodynamics in CdSe Quantum Dots as Photocatalysts and Photosensitizers

Interactions between dye molecules and the semiconductor quantum dots (QDs) has a strong effect on both solar-to-electrical and solar-to-chemical energy conversion processes. In both types of applications, the charge transfer from the photoexcited QD to the molecule play a crucial role. Applying DFT-based non-adiabatic molecular dynamics, we have revealed ultrafast hole transfer (~10 fs) from the CdSe QD to the Ru(II) dye, following by a slow component (~100 fs) associated with the redistribution of population throughout QD states. The ultrafast interfacial QD-to-dye hole transfer is rationalized by strong non-adiabatic couplings between the QD surface states and the high frequency vibrational modes of isocyanide ligands in the dye. We also have studied the effect of the complex heterostructures of Janus QDs, where a half of the QD is made from CdSe and another half is from PbSe forming an interface along a specific crystalline direction. Our calculations show that the Pb-enriched (111) crystalline direction of the interface enhances optical response of the QDs. The hole relaxation is much slower than the electron relaxation, followed by the hole transfer to the dye. The hole transfer is sensitive to the polarity of the environment, rather than the ways of dye’s attachment to the surface. We also found that in nonstoichiometric QDs, a loss of a passivating ligand affects the electronic structure in the same way as injection of an electron in Cd-rich QDs or hole injection in Se-rich QDs. Moreover, electron injection to the Se-rich QDs results in highly optically active redshifted transitions associated with the introduced charge. Overall, our calculations provide atomistic insights into QD-to-molecule charge transfer, giving researchers an additional handle for enhancing QD photophysical properties by means of the QD stoichiometry, interface structure, and QD-molecule engineering.

Numerical investigation on dynamic interactions of non-contact maglev vehicle and two-span steel continuous bridge induced by high-frequency vibration modes

Numerical investigation on dynamic interactions of non-contact maglev vehicle and two-span steel continuous bridge induced by high-frequency vibration modes

Vibration mode identification method for structures using image correlation and compressed sensing

All objects are subject to deformations of their shapes that correspond to their natural vibration modes. The analysis of these modes is extremely useful for structural design and structural health monitoring. Traditionally, expensive measurement equipment and many vibration sensors have been required to analyze vibration modes. Recently, a method that measures displacement from images (digital image correlation) has enabled the identification of vibration modes with a single camera. However, the identification of high-frequency vibration modes requires a high-speed camera, which increases costs and reduces spatial resolution. In this study, a low-cost and highly accurate method for identifying vibration modes and frequencies, randomized single-exposure sampling (RSES), was developed by applying compressed sensing to images captured with a strobe flash and a low-speed camera. This method extracts vibration modes using proper orthogonal decomposition and applies compressed sensing to the time function to recover high-speed vibrations from low-speed measurement results. The performance of RSES was evaluated by conducting vibration experiments on beams and comparing the results with the theoretical values using acceleration sensor measurements as boundary conditions. The results demonstrated that images captured at a shutter speed of 10 fps could measure the vibration modes and amplitudes of beams vibrating at a single frequency (ranging 170–3210 Hz). From 40 images, the amplitude of vibration could be measured with errors of less than 1 %, 2 %, and 26 % for vibrational frequencies of 170, 1130, and 3210 Hz, respectively. The larger error at 3210 Hz was attributed to the amplitude of vibration being only a few micrometers owing to the shaker limitations, a value close to the resolution of digital image correlation. This method can be used to accurately analyze structures that vibrate at high speeds, which is useful for vibration control design and other applications.

Dynamic surface stress field of the pure liquid-vapor interface subjected to the cyclic loads.

We demonstrate a methodology for computationally investigating the mechanical response of a pure molten lead surface system to the lateral mechanical cyclic loads and try to answer the following question: how does the dynamically driven liquid surface system follow the classical physics of the elastic-driven oscillation? The steady-state oscillation of the dynamic surface tension (or excess stress) under cyclic load, including the excitation of high-frequency vibration mode at different driving frequencies and amplitudes, was compared with the classical theory of a single-body driven damped oscillator. Under the highest studied frequency (50GHz) and amplitude (5%) of the load, the increase of in (mean value) dynamic surface tension could reach ∼5%. The peak and trough values of the instantaneous dynamic surface tension could reach (up to) 40% increase and (up to) 20% decrease compared to the equilibrium surface tension, respectively. The extracted generalized natural frequencies seem to be intimately related to the intrinsic timescales of the atomic temporal-spatial correlation functions of the liquids both in the bulk region and in the outermost surface layers. These insights uncovered could be helpful for quantitative manipulation of the liquid surface using ultrafast shockwaves or laser pulses.

Synthetic experimental and numerical investigation on the vertical seismic effect on underground structures

Seismic damage patterns of underground structures indicate unignorable effects of vertical seismic actions. To deal with this, this study distinguished the impact of the vertical seismic effect on the seismic safety of underground structures experimentally and theoretically and provides important shaking table test results for this topic. A typical shallow buried subway station-tunnel junction structure was designed by following the Buckingham similarity law, based on which the shaking table test was performed to investigate the vertical seismic effect on underground structures. The vertical wave propagation laws of the free field, the model structure, and the soil–structure interaction (SSI) system were comparatively analyzed. Numerical simulations considering the soil’s nonlinear behaviors under vertical loads were performed using the equivalent linear analysis method and were verified by the test records. By combining both the experimental and numerical results, it was found that the surrounding soils have very weak constraints on the structural vertical movements. By distinguishing lateral soil pressures from horizontal actions, the vertical seismic action is found to be mainly an inertia force that can stimulate high-frequency vibration modes of the system and cause considerable vertical relative deformations within the underground structure. Consistent with the observed structural damage at the Daikai station, the participation of high-frequency vibration modes is a rational approach that can explain the amplification effect of vertical ground motions on structural seismic responses. In addition, the equivalent linear method is generally satisfactory for simulating the soil compressional nonlinearity.

High-temperature superconductivity in two-dimensional hydrogenated titanium diboride: Ti2B2H4

In recent years, superconductivity in two-dimensional (2D) layered hexagonal metal borides has aroused much interests. Here, based on first-principles calculations, we theoretically report a new type of 2D hydrogenated transition-metal diboride: Ti2B2H4. Results reveal that the introduction of hydrogen atoms expands the frequency range of phonon spectrum of monolayer Ti2B2, and significantly increases the electron-phonon coupling (EPC). The calculated EPC constant λ of Ti2B2H4 is 1.18, and the corresponding superconducting critical temperature (Tc) is 48.6 K, exceeding the McMillan limit (39 K). The EPC mainly originates from the coupling between electrons in Ti-3d orbitals and the low-frequency vibration modes of Ti atoms as well as high-frequency vibration modes of H atoms. Additionally, the coupling strength can be further increased to 1.61 by applying appropriate biaxial compressive strain, and the corresponding Tc can be boosted to 69.4 K, approaching to liquid nitrogen (N2) temperature. The predicted Ti2B2H4 provides a new platform for 2D superconductivity and may have potential applications in 2D nanodevices.

A study into the FSI modelling of flat plate water entry and related uncertainties

This paper presents systematic comparisons of experiments against fluid structure interaction (FSI) simulations for flat water plate entry. Special focus is attributed on hydroelasticity and air trapping effects, quantification of the experimental and numerical uncertainties and the validity of modelling assumptions for the prediction of bottom slamming induced loads. Consequently, the American society of Mechanical Engineers standard for Verification and Validation is used to estimate the errors. Numerical and experimental results agree favourably. It is shown that pressures and strains may be prone to spatial effects that lead to minor deviations between experiments and simulations. High frequency vibration modes and model boundary conditions may also influence the results.

Development of Structure Monitoring Technique by Vibration Mode Identification Using Digital Image Correlation and Compressive Sensing

By combining digital image correlation method and compressed sensing, we propose a low-cost method for identifying high-frequency vibration modes without the use of a high-speed camera. Sub-Nyquist sampling was achieved by randomizing the timing of strobe flashes. The applicability of the method was verified through vibration tests on an aluminum plate using bandpass noise.

Vibration Mode Identification Method for Structures Using Image Correlation and Compressed Sensing

All objects have deformed shapes (vibration modes) that tend to vibrate, and measuring these modes is extremely useful in structural design and failure diagnosis. Traditionally, expensive measurement equipment and many vibration sensors were commonly used to measure vibration modes, but a method that measures displacement from images (Digital Image Correlation) has made it possible to identify vibration modes with a single camera. However, the identification of high-frequency vibration modes requires a high-speed camera, which increases the cost and reduces the resolution. In this study, a low-cost and highly accurate method for identifying vibration modes and frequencies (RSES: Randomized Single Exposure Sampling) was developed by applying compressed sensing to images taken with a strobe flash and a low-speed camera. This method successfully identified vibration modes and frequencies of structures vibrating at frequencies more than 300 times the shooting speed.

Theoretical investigations of structural, electronic, and physical properties of Rb2BX6 (B = Ti, Se, Pd; X = F, Cl, Br, I) double perovskites

Using the density functional theory prediction, we investigate the structural, elastic, electronic, optical, and vibrational properties of B- and X-dependent double perovskites Rb2BCl6 (B = Ti, Se, Pd) and Rb2PdX6 (X = F, Cl, Br, I) in this work. They are indirect bandgap semiconductors. For Rb2BCl6, the top valence bands are dominated by Cl-3p states and the bottom conduction bands by Ti-4d/Se-5p/Pd-5d states. For Rb2PdX6, the top valance bands are formed by X-p (F-2p, Cl-3p, Br-4p, and I-5p) states, and the bottom conduction bands are mainly composed of Pd-5d states. The high values of imaginary dielectric functions, high absorption coefficients, and low reflection coefficients indicate that Rb2BCl6 (B = Ti, Se, Pd) and Rb2PdX6 (X = F, Cl, Br, I) double perovskites are potential lead-free photoelectric materials. In addition, Rb2PdX6 (X = Br, I) double perovskites have the potential to be used in photovoltaic applications as they have broad optical absorption in the visible light range and dynamic stability under 400 K. The lattice vibrations of Rb2PdF6, examined as an example, show that the high-frequency vibration modes (T1u, T2g, Eg, and A1g) are derived from PdF62− clusters, while the low-frequency vibration modes (T2g and T1u) are contributed by Rb atoms.

Ab initio investigation of hydrogen-based high T c superconductor that is stable under ambient environment

We report an ab initio investigation of a hydrogen-based high-T c superconductor candidate—crystalized C4H4 in the cubic-gauche (cg-C4H4) structure, with the symmetry of space group I213. We find the cg-C4H4 structure to be stable under ambient environment; and the evaluation of the electron-phonon coupling strength indicates that the heavily doped cg-C4H4 can generate superconductivity with a T c ∼ 72 K. The high frequency vibrational modes of hydrogen atoms are found to play an important role in the total electron-phonon interaction strength, and the reduction of structural symmetry compared with graphene further enhances the electron–phonon coupling of the carbon framework. Our investigation illustrates a BCS route to realizing the hydrogen-based high T c superconductivity.

Brightness attenuation mechanisms of Er3+ self-sensitized upconversion nanocrystals under 1.5 μm pumping

The Er3+ self-sensitized upconversion nanoparticles (UCNPs) under 1.5 μm pumping presents great promises due to their high luminescence efficiency and safe excitation wavelength. However, its practical application ability are limited due to unsatisfactory brightness induced by organic compounds. We have even observed completely quench when it dispersed in polymer. Herein, the effects of high-frequency vibration modes from the organics on luminescence properties of β-NaYF4:Er3+ excited by 1.5 μm were studied in detail. The results showed that decreased emission of β-NaYF4:Er3+ in organic compounds containing CH bonds is mainly attributed to the excitation energy attenuation induced by molecules. The CH bonds in aliphatic molecules possessing strong absorption in near infrared region will dissipate the 1.5 μm photons energy. Equally importantly, an ignored quenching pathway was considered that the Er3+ 4I13/2 state is directly quenched by the near infrared vibrations of CH at ~7000 cm−1. The de-excitation of the Er3+ first-excited state eventually affects the population of upper electronic levels. After considering to avoid the above quenching pathways, the ligand-free NaYF4:Er@NaYF4 UCNPs exhibited satisfactory brightness upon 1.5 μm excitation and could be used for anti-counterfeiting labels. Our findings will promote the developments of many advanced applications based on Er3+ doped UCNPs.

Double parabolic reflectors wave-guided high-power ultrasonic transducer (DPLUS) based ultrasonic tweezers for micro/nano manipulation

Micro manipulation by ultrasonic technology has attracted wide attention recently due to its efficient manipulation process. In this work, Double Parabolic refLectors wave-guided high-power Ultrasonic tranSducer (DPLUS) which realized a low-loss and high-power ultrasound generation device was used as ultrasonic tweezers. The DPLUS can excite low (154.4 kHz) and high frequency (1.49 MHz) vibration modes, which can achieve two kinds of micro/nano multi manipulation modes in water. In this study, the axially vibrating needle of DPLUS was vertically immersed into the droplet to manipulate micro/nano samples including copper wire, TiO2 powders and yeast cells.

Development of a transient thrust stand with sub-millisecond resolution.

A transient thrust stand has been developed to offer 0.1 ms time-resolved force measurements up to 22 N. The system uses a predictor-based subspace system identification (PBSID) algorithm to obtain a high order state space model of the thrust stand. The state space model defines high-frequency vibration modes within the thrust stand. The high-frequency vibration modes are necessary to provide the time response of 0.1 ms. Thruster forces are then estimated using an augmented Kalman filter (AKF) to combine sensor traces from four accelerometers, a velocity sensor, and a displacement transducer. Fusing low-frequency displacement data with high-frequency acceleration measurements provides accurate force data from 0 kHz to 10 kHz. Combining the AKF with the PBSID state space model inherently attenuates external noise sources such as pumps. The transient thrust stand uses a torsional configuration to minimize influence from external vibrations and achieve high force resolution independent of thruster weight. Results demonstrated that the system was capable of obtaining dynamic thrust profiles with less than 5% error and a time resolution of 0.1 ms. To date, no thrust stand is capable of measuring up to 22 N forces with a time response of 10 kHz.

Singlet fission in spiroconjugated dimers.

Spiroconjugation results in a unique arrangement of conjugated fragments providing a novel way to chemically connect chromophoric units and control their electronic interaction, which is a key factor for the viability of the singlet fission photophysical reaction. In this study, we computationally explore the possibility of intramolecular singlet fission in spiroconjugated dimers by characterizing the nature of the low-lying excited electronic states, evaluating the magnitude of interstate couplings, describing possible singlet fission mechanisms, and investigating the potential role of low and high frequency vibrational modes in the exciton fission process. The spiro linkage of organic chromophores with the proper excited singlet and triplet energies favors the presence of low-lying charge resonance states, which play a major role in the formation of the triplet pair state. Overall, our results suggest that spiroconjugated dimers are potentially good candidates to efficiently generate independent triplet states through singlet fission.

Terahertz-driven phonon upconversion in SrTiO3

Direct manipulation of the atomic lattice using intense long-wavelength laser pulses has become a viable approach to create new states of matter in complex materials. Conventionally, a high frequency vibrational mode is driven resonantly by a mid-infrared laser pulse and the lattice structure is modified through indirect coupling of this infrared-active phonon to other, lower frequency lattice modulations. Here, we drive the lowest frequency optical phonon in the prototypical transition metal oxide SrTiO3 well into the anharmonic regime with an intense terahertz field. We show that it is possible to transfer energy to higher frequency phonon modes through nonlinear coupling. Our observations are carried out by directly mapping the lattice response to the coherent drive field with femtosecond x-ray pulses, enabling direct visualization of the atomic displacements.

Modulating Band Gap Structure by Parametric Excitations

Artificial periodic structures are used to control spatial and spectral properties of acoustic or elastic waves. The ability to exploit band gap structure creatively develops a new route to achieve excellently manipulated wave properties. In this study, we introduce a paradigm for a type of real-time band gap modulation technique based on parametric excitations. The longitudinal wave of one-dimensional (1D) spring-mass systems that undergo transverse periodic vibrations is investigated, in which the high-frequency vibration modes are considered as parametric excitation to provide pseudo-stiffness to the longitudinal elastic wave in the propagating direction. Both analytical and numerical methods are used to elucidate the versatility and efficiency of the proposed real-time dynamic modulating technique.

High Shock-Resistant Design for Wafer-Level-Packaged Three-Axis Accelerometer With Ring-Shaped Beam

A novel high shock-resistant beam design for a piezoresistive three-axis accelerometer with a distinctive ring-shaped beam structure introduced for relaxing assembling stresses is discussed. Through simulations modeling shock tests, the transient responses of a sensor device having a mass suspended with four beams and encapsulated using a wafer-level-packaging process were analyzed in detail. The beams are negatively affected by large axial tensile stress originating from a high-frequency vibration mode, and rapid stress increases just after the crashing of the mass into the encapsulation caps. Thus, the ring beam should be designed to simultaneously reduce the maximum stress for shock resistance and increase flexibility for stress relaxation against axial tensile load. A noticeable shock resistance against a peak acceleration of 20 000 G was achieved without sacrificing the stress-relaxation effect of the ring by applying an optimized ring design and minimizing the gap between the mass and cap to soften the crash. [2017-0237]