Graphene's unique gapless band structure and remarkably large third-order optical susceptibility have drawn significant attention to its nonlinear optical response, particularly in the context of coherent anti-Stokes Raman scattering (CARS). Under the combined influence of phononic and electronic resonances, the CARS response of graphene has been observed to exhibit a distinctive feature of time-resolved dip-to-peak evolution. Here, we report a greatly enhanced double resonance Raman mode beyond the G mode of multi-layer graphene with broadband CARS measurements. The significant difference in the intensity ratio between CARS and SR for this mode may be attributed to the preferential activation of low-frequency phonons in the impulsive stimulated Raman scattering process (ISRS) and a lower dephasing rate. Our results build on a foundation towards a deeper exploration of the coherent Raman response of two-dimensional materials.

Monolayer molybdenum disulfide (MoS2), among other two-dimensional transition-metal dichalcogenides materials, is widely used in a broad range of industries due to its extraordinarily different material properties compared to its bulk counterpart. However, such unique behavior may be greatly affected by its capacity of energy dissipation or heat conduction, largely attributed to its inherent phonon scattering properties. In addition, the phonon properties of MoS2 may be greatly affected by parameters such as temperature, defect concentration, etc., reflected by the Raman spectra evolution of A1g or E2g peaks. In this light, we analyze the combined influences of temperature and defect concentration on phonon scattering for the first time. We specifically elaborate experiments based on the temperature-dependent Raman spectroscopy in order to characterize the effect of defects on phonon scattering properties of MoS2. On this basis, a predictive model is developed for the estimation of phonon lifetime under different defect concentrations that may be served as a brief yet accurate and efficient designer tool in the early stage of defect/phonon engineering. In addition, our study may provide more physical insights toward a comprehensive understanding of the phonon behavior of MoS2, thus paving a way for more practical application potentials enabled by low-dimensional materials.

Twisted bilayer graphene (tBLG) exhibits remarkable optical and electrical properties, rendering it a promising material for future micro/nano devices. Nonetheless, efficient heat transport poses a critical challenge, and comprehending the influence of the twist angle on phonon properties is vital. In this study, we present the first analysis of the effects of the twist angle on phonon scattering near the Van Hove singularity. Our investigation reveals an extended lifetime of the G mode phonon due to electron-hole pair excitation near the critical angle accompanied by a substantial enhancement response. This phenomenon occurs when electron-phonon scattering is suppressed, leading to the dominance of phonon-phonon scattering in the transport process. Our study unveils the predominant control of the phonon-phonon scattering process by three-phonon scattering with the proportion directly influenced by the twist angle. This effect arises from the alteration in the phonon band structure, aligning with theoretical predictions acquired via molecular dynamics simulations. These findings contribute significant scientific insights into the phonon scattering behavior of tBLG, thereby informing the development of advanced micro/nano devices with enhanced performance. Furthermore, our study may open avenues for investigating the potential practical applications of tBLG and other twisted materials.

This paper proposes an impedance-based design methodology for cylindrical sandwich shells, with the aim to improve the sound transmission loss properties near the ring and coincidence frequency regions. The approach enables to systematically address the poor acoustic performance, characteristic of these problematic frequency regions, while retaining the mechanical properties of these structures. This is done by seeking to suppress the mass-controlled region in the frequency spectrum, with properly tuned characteristic frequencies of the structure, completed by a degree of damping treatment. The impedance-based approach allowing this tuning is derived from the canonical wave equation with a view to sound transmission through cylindrical shells. In addition to offering fast, early design possibilities, the method provides physical insights into the vibroacoustic performance of the shell, for instance introduced to estimate the sound transmission loss of shallow curved sandwich panels in the low-frequency range. Oblique-and diffuse-field conditions are investigated, validating the analytical developments against finite element calculations.

Iron-aluminum alloy has been widely used in aerospace, nuclear power, electronics and other fields. Taking into account the limitations of the continuum mechanics model at the microscopic scale, it is critical to use nanoscale simulation methods like molecular dynamics (MD) to have a deeper understanding of the friction and wear behavior. Through MD simulation, this thesis mainly studied the friction and wear process of iron aluminum alloy under different conditions. The main conclusions are as follows:(1) The frictional force first experiences a period of growth, and then stabilizes with some fluctuations due to the release of stress-energy caused by deformation.(2) Temperature and friction speed has no significant effect on friction force.(3) The higher the friction speed is, the more wear debris it produces, and the more severely the material is damaged.(4) The force and coefficient of friction will increase with the rise of aluminum content, and the plastic deformation of the alloy is strengthened, while the elastic deformation decreases.(5) The frictional force increases with the wear depth since the accumulation phenomenon of atoms becomes more obvious.

A design method is proposed for curved sandwich panels in order to improved their sound transmission loss in certain frequency regions. The ring frequency and coincidence effects may be suppressed simultaneously based on such method, resulting improved sound transmission loss properties of the curved sandwich panel in the low frequency range. The proposed method is based on an impedance approach that is developed for the estimation of the sound transmission loss of curved sandwich panels. Physical insights are drawn for the observed sound transmission behaviour of the panel based on the impedance approach over the frequency range of interest. The results are validated against the finite element simulations.

A locally resonant metamaterial curved double wall is proposed and studied. The aim is to improve the sound insulation by introducing a metamaterial design targeting a narrow fre-quency band region associated with characteristic frequencies of curved double walls, thus enabling an overall improvement of sound insulation properties in a broader frequency range. This metamaterial is realized by introducing periodically distributed resonators to a curved double wall. The sound transmission loss properties of such curved double walls are first investigated by using the concept of 'apparent impedance', which expresses the properties of the whole structure in terms of the impedances of the constituting panels and air cavity. The apparent impedance approach is validated against Finite Element models. It is shown that, instead of a dip in the sound transmission loss around the ring fre-quency of a single curved panel, the curved double wall may exhibit a broad 'valley' with low sound transmission loss, whose bandwidth is determined by the spacing between the two characteristic frequencies of the structure (associated with the ring frequency and mass-spring-mass resonance of the curved double wall). The curved double wall is then specifically designed by adjusting the two characteristic frequencies to be close to each other in order to narrow the region associated with a low transmission loss. This enables, subsequently, to improve the transmission loss in this region by effectively inserting tuned local resonators. The design principles are discussed, and applications of double walls consisting either of the same curved panels or different curved panels are both included.

The goal of the work is to find a way to improve the sound insulation properties of different types of panels in order to meet different requirements. Inspired by the nontrivial behavior of the locally resonant acoustic metamaterials, this concept is introduced into the design of structures in order to explore the potential ways to improve the sound insulation behavior in the relevant specific frequency regions. At relatively low frequency region when the bending wavelength is much longer than the distance between isolated resonators, which is also the interesting frequency range in the most part of the work, it may be assumed that the effects of the resonators are uniformly distributed over the entire surface. An impedance approach is hence proposed to estimate the sound transmission loss of the metamaterial panels in order to get more insights from physics. This is realized, in general, by integrating the equivalent impedance of the resonators together with the corresponding impedance of the host panel. Valuable theories are derived based on that, laying a solid foundation for effective/efficient design of metamaterial panels. This approach also provides a fast and reliable tool for the designs prior to a time-consuming and computationally expensive numerical simulation. Based on that, a new design for locally resonant metamaterial sandwich plates is proposed to improve the sound transmission loss performance in the coincidence frequency region. A systematic method to tune the resonance frequency of local resonators is developed. This approach also supplies a method to remove the possible side-dips associated with the resonance of the resonators. The influence of the sound radiation from the resonators is further investigated with the Finite Element models. It is proposed to embed the resonators inside the core material in order to eliminate the possible influence, and also to make a smooth surface. The metamaterial sandwich panel designed in this way combines improved acoustic insulation properties with the lightweight nature of the sandwich panel. Besides the coincidence frequency region, the ring frequency area of a cylindrical shell is another important frequency region for bad sound transmission loss. The effectiveness of locally resonant metamaterial is also investigated. Similar to the case of the flat panel, both impedance model and Finite Element model are developed for the problem of the sound transmission loss properties. The influence of the resonators is presented, and compared with the case of the flat panel. Unlike the case of the metamaterial flat panel, two side-dips around the sharp improvement cannot be avoided when applying the resonators near the ring frequency of the curved panel. The reason for that is explored by using the impedance approach. It is noticed that, while the impedance of a flat panel near the critical frequency is shifted from a masstype impedance to stiffness-type impedance, the impedance of a cylindrical shell is shifted from a stiffness-type (tension-type) impedance to mass-type iv impedance. For a traditional mass-spring type resonator, however, the equivalent impedance is always shifted from a mass-type impedance to stiffness-type impedance when the frequency crosses the resonance frequency. Therefore, when the traditional resonators are applied near the ring frequency, there are always frequencies at which the impedances cancel each other, resulting in the worsened sound transmission loss. In order to have better improvement of the sound transmission loss in this frequency region, new types of resonators have to be developed. A locally resonant metamaterial curved double wall is proposed and studied, with the aim of addressing the mass-spring-mass resonance and ring frequency effects of the wall. The sound transmission loss properties of a curved double wall are first investigated by introducing the concept of ‘apparent impedance’, which expresses the properties of the entire structure in terms of the impedances of the constituting panels and air cavity. The apparent impedance derivation is validated against Finite Element models. The curved double wall is then specifically designed by adjusting the two characteristic frequencies to be close to each other in order to narrow the region associated with a poor transmission loss. This enables, subsequently, to improve the transmission loss in this region by effectively inserting tuned local resonators. The design principles are discussed, and applications for double walls consisting the same curved panels or different curved panels are both included.

Locally resonant metamaterial flat panels have proved to potentially exhibit extraordinary sound transmission loss properties when the resonance frequency of the resonators is tuned to the coincidence frequency region. Whether this technique is also effective to address the ring frequency effect for curved panels is investigated in this paper. For this purpose, a cylindrical shell, as a representation of curved panels, is studied from a theoretical and numerical point of view, with a specific focus on the transmission loss behaviour around the ring frequency region when the shell is mounted with local resonators. The influence from the resonators is presented and compared with that for a flat panel. An inverse effect of the resonators is observed on the sound transmission loss between the metamaterial cylindrical shell and the metamaterial flat panel when the resonance frequency of the resonators is tuned to be below or above the ring or coincidence frequency, respectively. Rather than the extraordinary improvement observed for the metamaterial flat panel, tuning such conventional resonators to the ring frequency of curved panels generates two side dips despite a sharp improvement at the ring frequency itself. This phenomenon is explained from an effective impedance point of view developed in this paper. The approach proposed and the conclusions provided may subsequently allow for the design of suitable resonators in order to resolve the ring frequency effect for curved panels.

This paper investigates the suppression of vibration and sound radiation of a sandwich plate through the use of periodic design. A periodic sandwich plate is constructed and its dispersion relation is calculated. The vibration and sound radiation properties of the periodic sandwich plate are studied. Via the comparison of the periodic and bare sandwich plate, the effects of the periodic design on the vibration and sound radiation are analysed. Further, to know the sound radiation properties better, sound radiation efficiency of the periodic and bare sandwich plates is compared. In addition, the effects of the boundary conditions on the properties of the periodic sandwich plate are analysed. The numerical results demonstrate that the vibration and sound radiation are greatly suppressed over the stop band of the periodic sandwich plate. The suppression can also be obtained in part of pass bands. It is also shown that the periodic design can be an effective method for the reduction of the sound radiation efficiency. The suppression for the vibration and sound is greater than that caused by only increasing the mass of the plate in the designing frequency range.