Acoustic Mismatch Research Articles

A total-internal-reflection-based Fabry–Pérot resonator for ultra-sensitive wideband ultrasound and photoacoustic applications

In photoacoustic and ultrasound imaging, optical transducers offer a unique potential to provide higher responsivity, wider bandwidths, and greatly reduced electrical and acoustic impedance mismatch when compared with piezoelectric transducers. In this paper, we propose a total-internal-reflection-based Fabry–Pérot resonator composed of a 12-nm-thick gold layer and a dielectric resonant cavity. The resonator uses the same Kretschmann configuration as surface plasmon resonators (SPR). The resonators were analyzed both theoretically and experimentally. The experimental results were compared with those for an SPR for benchmarking. The 1.9-μm-thick-PMMA- and 3.4-μm-thick-PDMS-based resonators demonstrated responsivities of 3.6- and 30-fold improvements compared with the SPR, respectively. The measured bandwidths for the PMMA, PDMS devices are 110 MHz and 75 MHz, respectively. Single-shot sensitivity of 160 Pa is obtained for the PDMS device. The results indicate that, with the proposed resonator in imaging applications, sensitivity and the signal-to-noise ratio can be improved significantly without compromising the bandwidth.

Wrinkle Detection in Carbon Fiber-Reinforced Polymers Using Linear Phase FIR-Filtered Ultrasonic Array Data

Carbon fiber-reinforced polymers (CFRP) are extensively used in aerospace applications. Out-of-plane wrinkles frequently occur in aerospace CFRP parts that are commonly large and complex. Wrinkles acting as failure initiators severely damage the mechanical performance of CFRP parts. Wrinkles have no significant acoustic impedance mismatch, reflecting weak echoes. The total focusing method (TFM) using weak reflection signals is vulnerable to noise, so our primary work is to design discrete-time filters to relieve the noise interference. Wrinkles in CFRP composites are geometric defects, and their direct detection requires high spatial precision. The TFM method is a time-domain delay-and-sum algorithm, and it requires that the time information of filtered signals has no change or can be corrected. A linear phase filter can avoid phase distortion, and its filtered signal can be corrected by shifting a constant time. We first propose a wrinkle detection method using linear phase FIR-filtered ultrasonic array data. Linear phase filters almost do not affect the wrinkle geometry of detection results and can relieve noise-induced dislocation. Four filters with different bandwidths have been designed and applied for wrinkle detection. The 2 MHz bandwidth filter is recommended as an optimum choice.

Size and environment: The effect of phonon localization on micro-Brillouin imaging

Specifically designed samples have been analyzed to test the ability of Brillouin spectroscopy to provide reliable mechanical characterization of micro and nano-objects. The selected samples are polymeric films, whose transversal sizes from hundreds of nano- to some micro-meters cover the entire range of length-scales relevant in Brillouin scattering process. The experimental data highlight how, the size of the extended collective oscillation (acoustic phonons, in brief) is the lowest spatial resolution reachable in Brillouin mechanical characterization. Conversely, in the limit condition of phonon confinement, the technique provides the mechanical properties of nano-objects whose characteristic size is comparable with the phonon wavelength (⁓300 nm). Investigating acoustically heterogeneous materials, both size of heterogeneity and acoustic mismatch between adjacent regions are shown to be relevant in shaping the Brillouin response. In particular, a transition from a confined to a non-confined condition is obtained modulating the acoustic mismatch between the micro-objects and their local environment. The provided results and the derived analytic models for the data analysis will guide the interpretation of Brillouin spectra acquired in complex nano-structured samples such as cells, tissues or biomimetic materials. Our analysis can therefore generate new insights to tackle fundamental problems in mechanobiology or to characterize new bioengineered materials.

In Situ Design of High‐Performance Dual‐Phase GeSe Thermoelectrics by Tailoring Chemical Bonds

AbstractComposite engineering favors high thermoelectric performance by tuning the carrier and phonon transport. Herein, orthorhombic and rhombohedral dual‐phase GeSe are designed in situ by tailoring chemical bonds. Atom probe tomography verifies the coexistence of a covalently bonded orthorhombic phase and a metavalently bonded rhombohedral phase in GeSe‐InTe alloys. The production of the rhombohedral phase simultaneously increases the carrier concentration, the carrier mobility, the band degeneracy, and the density‐of‐states effective mass due to the reduced formation energy of cation vacancies and the improved crystal symmetry. These attributes are beneficial to a high‐power factor. In addition, the thermal conductivity can be significantly reduced due to the intrinsically strong lattice anharmonicity of the metavalently bonded phase, the interfacial acoustic phonon mismatch across different bonding mechanisms, and the phonon scattering at vacancy‐solute clusters. Moreover, the metavalently bonded phase embraces higher solubility of dopants that enables the further optimization of properties by Cd‐Ag doping, resulting in a zT of 0.95 at 773 K as well as enhanced strength and ductility in dual‐phase Ge0.94Cd0.03Ag0.03Se(InTe)0.15. This work indicates that in situ design of dual‐phase composites by tailoring chemical bonds is an effective method for enhancing the thermoelectric and mechanical properties of GeSe and other p‐bonded chalcogenides.

Pliable solid medium as a plasma confinement layer for laser peening

Laser peening is a promising surface treatment technique for enhancing the mechanical performance of metals. In laser peening, the plasma confinement layer contributes to the generation of high-pressure shockwaves by suppressing the expansion of laser-induced plasma. Therefore, the choice of a plasma confinement layer is important for improving laser peening effectiveness. For laser peening in environments where liquid materials cannot be used as a plasma confinement layer, alginate gel, which is a pliable material with shape-following capabilities, has been proposed to reduce the acoustic impedance mismatch caused by surface roughness when using solid materials. In this study, the feasibility of alginate gel as a plasma confinement layer and an appropriate process window for laser irradiation were investigated. The results of this study are then presented.

The effect of alloying element on the thermophysical properties of carbon fiber-graphite flakes/copper composites

In this study, carbon fiber-graphite flake/copper composites were prepared by powder metallurgy. The impact of the alloying elements Ti and Zr on the composites’ properties was investigated, and the influence of varying Zr contents on the phase structure and thermal conductivity of the composites was also explored. The results indicate that composites containing Zr exhibit better flexural strength and thermal conductivity compared to those containing Ti. When the volume fraction of carbon fiber is 2% and 2 wt% Zr is added to copper matrix, the carbide layer at the composite interface is uniform and has a thickness of 0.36 μm. The thermal conductivity of the composite material is up to 597.5 W/(m∙K). The mechanical properties of the composites are enhanced by the synergistic effect of adding carbon fiber and alloying matrix and the composite shows a remarkable flexural strength of 150.5 MPa, which is 58.3% higher compared to composites without carbon fiber. Furthermore, the strengthening mechanism of carbon fiber on the mechanical properties of the composites was examined. The thermal conductivity of the multiphase composites was effectively predicted using the Acoustic Mismatch Model (AMM) combined with the MF module of Digimat software, and the impact of Ti or Zr elements on the composites’ thermal conductivity was analyzed.

Mixed mismatch model predicted interfacial thermal conductance of metal/semiconductor interface

The acoustic mismatch model and diffuse mismatch model are widely used to calculate interfacial thermal conductance. These two models are respectively based on the assumption of extremely smooth and rough interfaces. Owing to the great difference between the actual interface structure and the two hypotheses, the predictions of these two models deviate greatly from the actual interfacial thermal conductance. The recently proposed mixed mismatch model considers the effect of interface structure on the ratio of phonon specular transmission to diffuse scattering transmission, and the prediction accuracy is improved. However, this model requires molecular dynamics simulation to obtain phonon information at the interface. In this work, the mixed mismatch model is simplified by introducing the measured roughness value, and the influence of interface structure on the contact area is taken into account to achieve a simple, fast and accurate prediction of interface thermal conductance. Based on this model, the interfacial thermal conductances of metals (aluminum, copper, gold) and semiconductors (silicon, silicon carbide, gallium arsenide, gallium nitride) are calculated and predicted. The results of Al/Si interface are in good agreement with the experimental results. This model is helpful not only in understanding the mechanism of interface heat conduction, but also in comparing with the measurement results.

Theoretical analysis of interfacial design and thermal conductivity in graphite flakes/Al composites with various interfacial coatings

Abstract Graphite flakes/Al composites are promising thermal management materials due to high thermal conductivity (TC) in basal plane orientation, matched coefficient of thermal expansion, and good machinability. In this article, the acoustic mismatch model and the effective medium approach are applied to predict the influence of different interfacial coatings on the interfacial thermal conductance (ITC) and the TC of graphite flakes/Al composites, respectively. With the increase in the thickness of interfacial coatings, the ITC and the TC of graphite flakes/Al composites decrease. For the composites with Ni, Cr/Cr7C3/Cr3C2, Si/SiC, Ti/TiC, WC, and Mo/Mo2C coatings, the ITC is sensitive to coating thickness. In order to obtain ideal TC of graphite flakes/Al composites, the thickness of the coatings should be controlled below 1 μm. It is reasonable that the TC of the graphite flakes/Al composites increases as the volume fraction of graphite flakes increases. The TC of the graphite flakes/Al composites increases with the ITC and changes slowly when the ITC increases to a certain extent. Si/SiC and WC coatings are proposed to be the most promising candidates to improve the thermal performance of graphite flakes/Al composites.

A comparative study of interfacial thermal conductance between metal and semiconductor

To understand and control thermal conductance of interface between metal and semiconductor has now become a crucial task for the thermal design and management of nano-electronic and micro-electronic devices. The interfacial alignments and electronic characteristics of the interfaces between metal and semiconductor are studied using a first-principles calculation based on hybrid density functional theory. The thermal conductance of interfaces between metal and semiconductor were calculated and analyzed using diffuse mismatch model, acoustic mismatch model and nonequilibrium molecular dynamics methods. Especially, according to nonequilibrium molecular dynamics, the values of thermal conductance were obtained to be 32.55 MW m−2 K−1 and 341.87 MW m−2 K−1 at C–Cu and Si–Cu interfaces, respectively. These results of theoretical simulation calculations are basically consistent with the current experimental data, which indicates that phonon–phonon interaction play a more important role than electron–phonon interaction during heat transport. It may be effective way to improve the interfacial thermal conductance through enhancing the interface coupling strength at the metal–semiconductor interface because the strong interfacial scattering plays a role in suppressing in the weaker interface coupling heterostructure, leading to the lower thermal conductance of interfaces. This could provide a beneficial reference for the design of the Schottky diode and thermal management at the interfaces between metal and semiconductor.

Surface-Acoustic-Wave Devices Based on Lithium Niobate and Amorphous Silicon Thin Films on a Silicon Substrate

This work presents an acoustic platform using solidly mounted thin-oriented lithium niobate (LiNbO3) film on silicon (Si). A thin layer of amorphous Si eliminates a conductive layer generated in the thin-film bonding processes and contributes to acoustic energy confinement and thermal frequency stability. The gigahertz operating frequency is achieved by resorting to the guided acoustic wave shear-horizontal wave (SH0) in a 400-nm-thick X-cut LiNbO3 thin film. Due to the elimination of the parasitic surface conduction (PSC) effect, the electromechanical coupling of the guide acoustic wave is maximized in the Si-based heterogeneous wafer. Due to the minimized acoustic impedance mismatch between surface material and substrate, longitudinal and higher order spurious modes are suppressed. Due to the stiff substrate with a small temperature expansion coefficient (TEC), the solidly mounted structure features a reduced temperature coefficient of frequency (TCF) and improved power handling. The fabricated resonator shows an extracted electromechanical coupling coefficient of 22.8%, a high loaded <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> of 1208 at 1.6 GHz, and a TCF of −36 p/min/K. The compressed filter is demonstrated with a minimum insertion loss (IL) of 0.6 dB, a fractional bandwidth (FBW) of 8%, an out-of-band rejection of 30 dB, a TCF of −38.5 p/min/K at roll-off, and a miniaturized footprint of 0.4 mm2. The performance has shown the strong potential of the LiNbO3–Si platform for front-end applications in 5G.

Predicted interfacial thermal conductance and thermal conductivity of graphite flakes/Al composites with different alloy composition

Abstract Graphite flake/Al composites are promising thermal management materials due to their light weight and excellent thermal properties. The interface structure is a key factor that impacts the thermophysical properties. In this work, prediction models based on the acoustic mismatch model (AMM) and the effective medium approach (EMA) were developed to evaluate the effect of the alloying elements on the interfacial thermal conductance (ITC) and thermal conductivity (TC) of graphite flake/Al composites. With the increase in Si content, the ITC for the graphite flakes/Al composites increases clearly. In contrast, the ITC for the graphite flakes/Al composites decreases with the increase in Mg content. The variation range of ITC of graphite flakes/Al–Mg–Si composites is (2.3 × 107–3.1 × 107) W m−1 K−1. The thermal conductivity of the graphite flakes/Al–Mg–Si composites is in the range of (412–476) W m−1 K−1. With the increase in the volume fraction of graphite flake, the thermal conductivity of the graphite flakes/Al composites is significantly improved.

Numerical Modelling of a Valveless Impedance Pump with Various Pinch Locations

Microfluidics devices offers reliable transport mechanism to drive fluid within a microscale flow. This research work aims to identify the characteristic of flow in valveless impedance pump which uses acoustic impedance mismatch to drive flow using numerical simulations. The focus of the studies is designing a well performed pump where the required displacement of elastic wall can still be achieved even when the force applied in different locations and also a mechanism that requires low input current. Shear rate resulting from the pumping mechanism were measured at different input boundary conditions. Important parameters such as pinch location can affect the direction of the velocity magnitude and shear rate. It is found that the maximum shear rate occurred at 2 mm pinch location at 400 s-1. Hence, the pump design from this study can be used as the best application for microfluidic systems where a high pumping amount is desired.

Synergistic Effect of Work Function and Acoustic Impedance Mismatch for Improved Thermoelectric Performance in GeTe-WC Composite.

The preparation of composite materials is a promising methodology for concurrent optimization of electrical and thermal transport properties for improved thermoelectric (TE) performance. This study demonstrates how the acoustic impedance mismatch (AIM) and the work function of components decouple the TE parameters to achieve enhanced TE performance of the (1-z)Ge0.87Mn0.05Sb0.08Te-(z)WC composite. The simultaneous increase in the electrical conductivity (σ) and Seebeck coefficient (α) with WC (tungsten carbide) volume fraction (z) results in an enhanced power factor (α2σ) in the composite. The rise in σ is attributed to the creation of favorable current paths through the WC phase located between grains of Ge0.87Mn0.05Sb0.08Te, which leads to increased carrier mobility in the composite. Detailed analysis of the obtained electrical properties was performed via Kelvin probe force microscopy (work function measurement) and atomic force microscopy techniques (spatial current distribution map and current–voltage (I–V) characteristics), which are further supported by density functional theory (DFT) calculations. Furthermore, the difference in elastic properties (i.e., sound velocity) between Ge0.87Mn0.05Sb0.08Te and WC results in a high AIM, and hence, a large interface thermal resistance (Rint) between the phases is achieved. The correlation between Rint and the Kapitza radius depicts a reduced phonon thermal conductivity (κph) of the composite, which is explained using the Bruggeman asymmetrical model. Moreover, the decrease in κph is further validated by phonon dispersion calculations that indicate the decrease in phonon group velocity in the composite. The simultaneous effect of enhanced α2σ and reduced κph results in a maximum figure of merit (zT) of 1.93 at 773 K for (1-z)Ge0.87Mn0.05Sb0.08Te-(z)WC composite for z = 0.010. It results in an average thermoelectric figure of merit (zTav) of 1.02 for a temperature difference (ΔT) of 473 K. This study shows promise to achieve higher zTav across a wide range of composite materials.

Self-heating hotspots in superconducting nanowires cooled by phonon black-body radiation

Controlling thermal transport is important for a range of devices and technologies, from phase change memories to next-generation electronics. This is especially true in nano-scale devices where thermal transport is altered by the influence of surfaces and changes in dimensionality. In superconducting nanowire single-photon detectors, the thermal boundary conductance between the nanowire and the substrate it is fabricated on influences all of the performance metrics that make these detectors attractive for applications. This includes the maximum count rate, latency, jitter, and quantum efficiency. Despite its importance, the study of thermal boundary conductance in superconducting nanowire devices has not been done systematically, primarily due to the lack of a straightforward characterization method. Here, we show that simple electrical measurements can be used to estimate the thermal boundary conductance between nanowires and substrates and that these measurements agree with acoustic mismatch theory across a variety of substrates. Numerical simulations allow us to refine our understanding, however, open questions remain. This work should enable thermal engineering in superconducting nanowire electronics and cryogenic detectors for improved device performance.

Ultrafast formation of quantized interlayer vibrations in Bi2Se3 by photoinduced strain waves.

This study comprehensively investigated the coherent lattice dynamics in Bi2Se3 by ultrafast optical pump-probe spectroscopy with tunable near-infrared probe pulses. Sample-thickness- and probe-wavelength-dependent experiments revealed the key role of Bi2Se3 optical property in the generation and detection of photoinduced strain waves, whose confinement initiated coherent interlayer vibrations. Furthermore, the frequency and lifetime of the interlayer vibrations could be quantitatively explained with a modified linear chain and an acoustic mismatch model considering elastic coupling at sample-substrate interfaces. The results of this work provide insights for analyzing and interpreting, through ultrafast optical spectroscopy, nanomechanical interactions in layered materials.

Interfacial thermal resistance between nanoconfined water and silicon: Impact of temperature and silicon phase

Molecular dynamics simulations are used to investigate the interfacial thermal resistance (Kapitza resistance) between crystalline or amorphous silicon and nanoconfined water at nanoscale. The simulations are performed under various conditions such as: different silicon phases (crystalline or amorphous), various water slab thicknesses, average system temperature and temperature difference between the thermostats. The results indicate that the Kapitza resistance is larger between crystalline silicon slabs and water (≈1.2 10−8 K m2 W−1) than between amorphous silicon slabs and water (≈0.7 10−8 K m2 W−1), which can be interpreted as a density effect using the acoustic mismatch model. We have not observed significant size effects related to the water slab thickness on the Kapitza resistance nor on the thermal conductivity of the nanoconfined water. Furthermore, the interfacial thermal resistance is linearly impacted by temperature unless the temperature difference between the thermostats is larger than 50 K. The presented results provide new insights in nano heat transfer in presence of a solid/liquid interface.

Role of magnons and the size effect in heat transport through an insulating ferromagnet taking into account the magnetic field

We solve a kinetic, space-dependent problem in an insulator/ferromagnet/insulator heterostructure in the presence of a magnetic field. At the same time, to interpret experiments on thermal transfer across ferromagnet / insulator boundaries, the Little acoustic mismatch theory is usually used, in which the temperature jump at the interface is determined only by the acoustic characteristics of the medium. We show that the theory of acoustic mismatch is convenient only for sufficiently thick insulating ferromagnets. Of the greatest interest to us is the opposite limiting case of thin ferrodielectric films, where the role of magnons in the formation of a temperature jump at the ferromagnet/insulator interface becomes decisive. We show that for T ≪ ΘD a nonlinear heat flux Q crossing the ferrodielectric/insulator interface, both the size effect and its decreasing dependence on the external magnetic field increase. Namely, Q depends on the magnetic properties of thin heated ferromagnetic films, and the role of magnons is essential in the heat transfer regime, which we call magnon overheating. On the contrary, for thick heated layers of a ferromagnet, Q can be described by the Little formula, which does not take into account the magnetic properties of the ferromagnet layer.

Substrate-supported nano-objects with high vibrational quality factors

Recent optical time-resolved experiments on single supported nano-objects (gold nanodisks with various diameter over thickness ratios) have demonstrated a marked enhancement of their vibrational quality factors for specific nano-object morphologies, resulting from the near-suppression of radiative vibrational damping associated with the emission of acoustic waves in the nano-object environment. This paper clarifies the origin of this phenomenon, which is ascribed to the creation of a “quasi-bound state in the continuum” vibrational mode by radiative coupling between two nano-object modes whose frequencies become close for specific nano-object shapes. The symmetry breaking induced by the presence of a substrate, which limits nanodisk acoustic emission to a half-space, is shown to play an essential role in enabling such radiative coupling. The impact of the acoustic mismatch between the nano-object and the substrate is explored, and it is shown that a moderate acoustic mismatch can still enable the creation of near-localized vibrational modes with high radiative quality factors, while allowing radiative coupling effects to occur over a broad range of nano-object geometries. Although this paper focuses on the situation of a substrate-supported gold nanodisk, which has already been the object of experimental investigations, the effects that it describes are general and constitute a promising approach to enhance the vibrational quality factors of nano-objects.

Planes approximation method for investigating the physical origins of deep, wide phononic bandgaps

The objective of this paper is to formulate a simple analytical model that describes Bragg and Mie scattering leading to the creation of bandgaps in phononic crystals (PnCs). The model is a homogenized, mean-field 1D model, dubbed the Planes Approximation Method (PAM). Bragg and Mie scattering are decoupled into independent 1D models and then recombined graphically to understand better and visualize bandgap creation. Due to the model's simplicity, it allows for rapid solutions compared to other computational methods. This paper argues that acoustic impedance mismatches yield bandgaps based on the formulation. PAM is demonstrated using a square lattice of microscale circular inclusions of tungsten with a lattice constant of 45 μm in a SiO2 matrix and then compared to results from finite difference time domain and plane wave expansion simulations.

Couplants in Acoustic Biosensing Systems.

Acoustic biosensors are widely used in physical, chemical, and biosensing applications. One of the major concerns in acoustic biosensing is the delicacy of the medium through which acoustic waves propagate and reach acoustic sensors. Even a small airgap diminishes acoustic signal strengths due to high acoustic impedance mismatch. Therefore, the presence of a coupling medium to create a pathway for an efficient propagation of acoustic waves is essential. Here, we have reviewed the chemical, physical, and acoustic characteristics of various coupling material (liquid, gel-based, semi-dry, and dry) and present a guide to determine a suitable application-specific coupling medium.