Acoustic Impedance Mismatch Research Articles

Spinodal Decomposition and Nucleation and Growth as a Means to Bulk Nanostructured Thermoelectrics: Enhanced Performance in Pb1-xSnxTe−PbS

The solid-state transformation phenomena of spinodal decomposition and nucleation and growth are presented as tools to create nanostructured thermoelectric materials with very low thermal conductivity and greatly enhanced figure of merit. The systems (PbTe)(1-x)(PbS)(x) and (Pb(0.95)Sn(0.05)Te)(1-x)(PbS)(x) are not solid solutions but phase separate into PbTe-rich and PbS-rich regions to produce coherent nanoscale heterogeneities that severely depress the lattice thermal conductivity. For x > approximately 0.03 the materials are ordered on three submicrometer length scales. Transmission electron microscopy reveals both spinodal decomposition and nucleation and growth phenomena the relative magnitude of which varies with x. We show that the (Pb(0.95)Sn(0.05)Te)(1-x)(PbS)(x) system, despite its nanostructured nature, maintains a high electron mobility (>100 cm(2)/V x s at 700 K). At x approximately 0.08 the material achieves a very low room-temperature lattice thermal conductivity of approximately 0.4 W/m x K. This value is only 28% of the PbTe lattice thermal conductivity at room temperature. The inhibition of heat flow in this system is caused by nanostructure-induced acoustic impedance mismatch between the PbTe-rich and PbS-rich regions. As a result the thermoelectric properties of (Pb(0.95)Sn(0.05)Te)(1-x)(PbS)(x) at x = 0.04, 0.08, and 0.16 were found to be superior to those of PbTe by almost a factor of 2. The relative importance of the two observed modes of nanostructuring, spinodal decomposition and nucleation and growth, in suppressing the thermal conductivity was assessed in this work, and we can conclude that the latter mode seems more effective in doing so. The promise of such a system for high efficiency is highlighted by a ZT approximately 1.50 at 642 K for x approximately 0.08.

The detection by sonar of difficult targets (including centimetre-scale plastic objects and optical fibres) buried in saturated sediment

This paper reports on a laboratory study into the use of sonar to detect objects, two of which exhibit a poor acoustic impedance mismatch with the water-saturated sediment in which they are buried to depth of about 30 cm. The targets are solid cylinders having diameters of 20–25 mm and 50 cm length, made of polyethylene, of telecommunications optical fibre, and of steel. Steel spheres are included for comparison. A poor acoustic impedance mismatch between the target and the host sediment is one factor that can make buried targets difficult to detect with sonar, but such detection is increasingly becoming an issue in a range of applications from archaeology to defence to telecommunications. Attention is paid to those signal processing techniques which could be of potential benefit. For this range of test objects, comparisons are made between use of optimal filtering and synthetic aperture sonar. In addition, the potential of a range of acousto-optical effects (optical time domain reflectometry, Raman and Brillouin scattering, and fibre optic hydrophones) is assessed in the Appendix for the particular application of detecting non-metallised fibre optic telecommunications cables. A web page dedicated to this paper hosts movies and reports at http://www.isvr.soton.ac.uk/fdag/uaua/target_in_sand.htm.

Effect of Air-Coupled/Conventional Ultrasound Waves on Resin-Infiltrated Time of Wood Natural Materials

Air-coupled ultrasound is a non-contact technique and has obvious advantages over water-coupled experimentation. Especially, wood materials are very sensitive to water and inspection without any coupling medium of a liquid is really needed to wood materials due to the permeation of coupling medium such as water. In this work, it is desirable to perform contact-less nondestructive evaluation to assess wood material homogeneity. A wood material was nondestructively characterized with non-contact and contact modes to measure ultrasonic velocity using automated data acquisition software. We have utilized a proposed peak-delay measurement method. Also through transmission mode was performed because of the main limitation for aircoupled transducers, which is the acoustic impedance mismatch between most materials and air. The variation of ultrasonic velocity was found to be consistent to some degree with those in infiltrated area for air-coupled and conventional scan images. Through-transmission scans for aircoupled and conventional UT were used based on both amplitude and time-of-flight images. Finally, results using a peak-delay measurement method well somewhat corresponded to ultrasonic velocities of the pulse overlap method.

Inspecting Carbon Matrix Composites with Airborne/Conventional Ultrasound

Carbon/phenolic composite (CPC) materials have obvious advantages over conventional materials, which are consisting of carbon fibers embedded in a carbon matrix. The CPCs have become to be utilized as parts of aerospace applications and its low density, high thermal conductivity and excellent mechanical properties at elevated temperatures make it an ideal material for aircraft brake disks. Because of permeation of coupling medium such as water, it is desirable to perform contact-less nondestructive evaluation to assess material properties and part homogeneity. In this work, a C/P(Carbon/phenolic) composite material was characterized with non-contact and contact ultrasonic methods using automated acquisition scanner. Also through transmission mode was performed because of the main limitation for air-coupled transducers, which is the acoustic impedance mismatch between most materials and air. Especially ultrasonic images and velocities for C/C(Carbon/Carbon) composite disk brake was measured and found to be consistent to some degree with the non-contact and contact ultrasonic measurement methods. Low frequency throughtransmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. Measured results were compared with those obtained by the motorized system with using dry-coupling ultrasonics and through transmission method in immersion. Finally, results using a proposed peak-delay measurement method well corresponded to ultrasonic velocities of the pulse overlap method.

Application of Nonlinear Ultrasonic Measurement for Quality Assurance of Diffusion Bonds of Gamma Titanium Aluminum Alloy and Steel

A new nonlinear C-scan acoustic microscope (NCAM) has been developed and applied to inspect diffusion bonds of gamma titanium aluminum alloy–chromium molybdenum steel (γ TiAl–steel). The large mismatch of acoustic impedance gives strong ultrasonic reflection and masks a small signal from imperfections at the interface. However, NCAM detects indications of minute cracks or gaps, which are rarely observed by a conventional pulse–echo inspection method. Application of high-power ultrasound, with amplitude ten times greater than the conventional method, results in generation of second harmonic signals. All A-scope signals were stored in a computer, and frequency components were analyzed by fast Fourier transformation (FFT). As a result, no indication of fundamental frequency component was observed on a C-scan image, whereas clear indications of the second harmonic components were observed on the C-scan image. The minute cracks or gaps were confirmed at the indicated area by micrographic analysis following the inspection. Moreover, a quality assurance process for the γ TiAl–steel bond with a combination of linear and nonlinear ultrasonic responses is proposed.

Peculiar transmission property of acoustic waves in a one-dimensional layered phononic crystal

In this article, we report both theoretical calculation and experimental observation of acoustic waves abnormally through a one-dimensional layered transmitted phononic crystal at frequencies within the band gap into a material of large acoustic impedance mismatch, with an efficiency as high as unity. The transmission peaks can be interpreted as a result of the interference of acoustic waves reflected from all periodically aligned interfaces. The condition for the appearance of peaks is analyzed in detail and the optimized layer number is given for different configurations.

Adaptive Pulse Repetition Frequency Technique for an Ultrasonic Transit-Time Gas Flowmeter for Hot Pulsating Gases

A technique of using an adaptive pulse repetition frequency (PRF) to operate an ultrasonic contrapropagation transit-time gas flowmeter (UFM) is introduced. This adaptive PRF technique allows transient measurements of hot (up to 450 degC) and pulsating (up to 1.5 kHz) gas flows. Such conditions occur in the exhaust gas of a combustion engine. Here, a UFM with the widely used fixed PRF technique is not applicable, because the large gas temperature variations would prevent a reliable detection of ultrasonic pulse arrival times. Coherently reflected waves are generated within the gas because of the unavoidable acoustic impedance mismatch between the gas and the transducers, and, depending on the gas temperatures, these echoes overlap with the main signal. The adaptive PRF technique overcomes this problem and allows correct pulse detection over the whole temperature range required. The UFM utilizes special high-temperature-resistant capacitance ultrasonic transducers (CUTs) to meet the requirements in terms of operating temperature range and dynamic response. Results, which are obtained with a preliminary laboratory prototype, are presented for the exhaust gas mass flow rate in a otimes 50-mm pipe measured at gas temperatures of up to 450 degC and at PRFs of up to 5.5 kHz, which is an increase in frequency response of one order of magnitude in comparison to existing measurement systems

Phonon scattering cross section of polydispersed spherical nanoparticles

An approximate analytical solution is proposed to estimate the phonon scattering cross section of polydispersed spherical nanoparticles. Using perturbation of the Hamiltonian due to differences in mass and bond stiffness between a host medium and a spherical nanoparticle, an analytical solution is obtained for the scattering cross section in the Rayleigh limit when the size parameter approaches zero. In the geometrical scattering limit, when the size parameter approaches infinity, the van de Hulst approximation for anomalous diffraction is used to estimate the scattering cross section as a function of acoustic impedance mismatch between the host medium and the spherical nanoparticle. Finally, these two limiting cases are bridged by a simple expression to estimate the scattering cross section for intermediate values of the size parameter. Using this, the scattering cross section for a polydispersed distribution of spherical nanoparticles was also estimated as a function of the parameters defining the statistical size distribution.

Amplitude non-linearity of the electrically evoked compound action potential (eCAP) of the auditory nerve

The function of the middle ear is to resolve the acoustic impedance mismatch between the air in the ear canal and the fluid of the inner ear. Without this impedance matching, very little acoustic energy would be absorbed into the cochlea. The first step in this process is the tympanic membrane (TM) converting sound in the ear canal into vibrations of the middle ear bones. Understanding how the TM manages its task so successfully over such a broad frequency range should lead to more satisfactory and less variable TM repairs (myringoplasty). In addition, understanding the mechanics of the TM is necessary to improve the coupling between ossicular prostheses and the TM. Mathematical models have played a central role in helping the research community understand the mechanics of the eardrum. However, all models require parameters as inputs. Unfortunately, most of the parameters needed for modeling the TM are not well known. In this work, several approaches for inferring the material properties of the TM are explored. First, constitutive modeling is used to estimate an elastic modulus based on the elastic modulus of collagen and experimentally observed fiber densities. Second, experimental tension and bending test results from the literature are re-interpreted using composite laminate theory. Lastly, dynamic measurements of the cat TM are used in conjunction with a composite shell model to bound the material parameters. Values from the literature, both measurement and modeling efforts, and from the present analysis are brought together to form a coherent picture of the TM's material properties. In the human, the data bound the elastic modulus between 0.1 and 0.3 GPa. In the cat, the data suggest a range of 0.1–0.4 GPa. These values are significantly higher than previous estimates.

Adaptive characteristics-based matching for compressible multifluid dynamics

This paper presents an evolutionary step in sharp capturing of shocked, high acoustic impedance mismatch (AIM) interfaces in an adaptive mesh refinement (AMR) environment. The central theme which guides the present development addresses the need to optimize between the algorithmic complexities in advanced front capturing and front tracking methods developed recently for high AIM interfaces with the simplicity requirements imposed by the AMR multi-level dynamic solutions implementation. The paper shows that we have achieved this objective by means of relaxing the strict conservative treatment of AMR prolongation/restriction operators in the interfacial region and by using a natural-neighbor-interpolation (NNI) algorithm to eliminate the need for ghost cell extrapolation into the other fluid in a characteristics-based matching (CBM) scheme. The later is based on a two-fluid Riemann solver, which brings the accuracy and robustness of front-tracking approach into the fast local level set front-capturing implementation of the CBM method. A broad set of test problems (including shocked multi-gaseous media, bubble collapse, underwater explosion and shock passing over a liquid drop suspended in a gaseous medium) was performed and the results demonstrate that the fundamental assumptions/approximations made in modifying the AMR prolongation/restriction operators and in using the NNI algorithm for interfacial treatment are acceptable from the accuracy point of view, while they enable an effective implementation and utility of the structured AMR technology for solving complex multiphase problems in a highly compressible setting.

Shock wave refraction patterns at interfaces

Interactions of shock waves with gas–gas and gas–liquid interfaces (under both slow–fast and fast–slow configurations) are studied using the recently developed Adaptive Characteristics-based Matching (aCBM) method for capturing interfaces in compressible multi-fluid media. First, we verify our approach for the gas–gas case; a class of problems for which a substantial body of knowledge already exists. Then, we consider slow–fast, gas–liquid interfaces under weak shocks, and fast–slow, liquid–gas interfaces under strong shocks. The very high acoustic impedance mismatch situation here creates significant numerical (simulation) and experimental (visualization) difficulties, and the literature for it is meager and sporadic. Compared to gas–gas interfaces we note both similarities and differences. We discuss the sources for these differences, as well as potential implications of generalizing and embedding such results in multi-dimensional simulation schemes towards improving their front-capturing performance.

Non-destructive high-resolution characterization of buried interfaces for advanced interconnect and packaging architectures: Experiments and modeling

Characterization of buried interfaces in advanced interconnect and packaging structures is a critical challenge as complementary metal-oxide-semiconductor (CMOS) devices are scaled and as novel packaging and interconnect concepts and materials such as three-dimensional (3-D) circuits, and ultra-low dielectric constant insulators are developed. Critical information that needs to be obtained from such inspection includes detection of unbonded bumps in flip-chip packages; missing interconnections in bonded wafers and presence of delaminations in interconnect layers. In this paper, we discuss in detail our experimental and theoretical investigation of different non-destructive characterization techniques namely acoustic imaging, infra-red imaging and transient thermal microscopy to analyze and quantify the quality of buried interfaces. The imaging resolution is shown to critically depend on the material thickness through which the incident probe beam has to propagate to form the images. Defects in buried interfaces such as voids in bonded wafers appear as regions of bright contrast in acoustic imaging due to large acoustic impedance mismatch at the voids. In the case of transient thermal microscopy, unbonded areas will lead to non-uniform temperature distribution at the surface of the substrate when the sample is rapidly heated with a flash lamp. The localized temperature rise at the defect and its temporal evolution can be related in turn to the defect size. We discuss capabilities and limitations of the different techniques for inspection of buried interfaces with an emphasis on characterizing bonded interfaces for 3-D integrated circuits. It is shown that acoustic imaging with single spherical lens is better suited to study bond quality compared to thermal microscopy due to its better detection capabilities. Infra-red imaging was found to be unsuitable for investigation of bonded interfaces in wafers containing multiple layers of metallization.

Acoustic phonon engineering in coated cylindrical nanowires

We have theoretically investigated the effect of a coating made of the elastically dissimilar material on the acoustic phonon properties of semiconductor nanowires. It is shown that the acoustic impedance mismatch at the interface between the nanowire and the barrier coating affects dramatically the phonon spectra and group velocities in the nanowires. Coatings made of materials with a small sound velocity lead to compression of the phonon energy spectrum and strong reduction of the phonon group velocities. The coatings made of materials with a high sound velocity have opposite effect. Our calculations reveal substantial re-distribution of the elastic deformations in coated nanowires, which results in modification of the phonon transport properties, and corresponding changes in thermal and electrical conduction. We argue that tuning of the coated nanowire material parameters and the barrier layer thickness can be used for engineering the transport properties in such nanostructures.

A theoretical study of the feasibility of acoustical tweezers: Ray acoustics approach

The optical tweezer has been found to have many biomedical applications in trapping macromolecules and cells. For the trapping mechanism, there has to be a sharp spatial change in axial optical intensity and the particle size must be much greater than the wavelength. Similar phenomenon may exist in acoustics. This work was undertaken to demonstrate theoretically that it is possible to acoustically trap particles near the focal point where most of the acoustic energy is concentrated if certain conditions are met. Acoustic force exerted on a fluid particle in ultrasonic fields is analyzed in a ray acoustics regime where the wavelength of acoustic beam is much smaller than the size of the particle. In order to apply the acoustical tweezer to manipulating macromolecules and cells whose size is in the order of a few microns or less, a prerequisite is that the ultrasound wavelength has to be much smaller than a few microns. In this paper, the analysis is therefore based on the field pattern produced by a strongly focused 100 MHz ultrasonic transducer with Gaussian intensity distribution. For the realization of acoustic trapping, negative axial radiation force has to be generated to pull a particle towards a focus. The fat particle considered for acoustic trapping in this paper has an acoustic impedance of 1.4 MRayls. The magnitude of the acoustic axial radiation force that has been calculated as the size of the fat particle is varied from 8lambda to 14lambda. In addition, both Fresnel coefficients at various positions are also calculated to assess the interaction of reflection and refraction and their relative contribution to the effect of the acoustical tweezer. The simulation results show that the feasibility of the acoustical tweezer depends on both the degree of acoustic impedance mismatch and the degree of focusing relative to the particle size.

Progress towards an electro-acoustic resonance technique for determining quantitative material and geometrical properties in high contrast multi-layer elastic structures.

High contrast multilayered elastic structures continue to be problematic for ultrasonic inspection. Large acoustic material impedance mismatches, refraction, reverberation, multiple echoes, and high elastic attenuations are just some of the issues one is faced with standard high frequency (1–20 MHz) pulse echo detection and imaging methods. In this presentation, we will present progress towards developing a low frequency resonance technique that operates in the 20 to 70 kHz regime. The technique is based on a direct correlation between the electrical impedance of a standard electro-acoustic transducer and the mechanical loading it experiences when placed in contact with a layered elastic structure. Preliminary experimental and theoretical results are in good agreement.

A theoretical study of the feasibility of acoustical tweezer: Ray acoustics approach

Optical tweezer has been found to have many biomedical applications in trapping macromolecules and cells. For the trapping mechanism, there has to be a sharp spatial change in axial optical intensity and the particle size must be much greater than the wavelength. Similar phenomenon may exist in acoustics. This work was undertaken to demonstrate theoretically that it is possible to acoustically trap particles near the focal point if certain conditions are met. Acoustic force exerted on fat tissue in ultrasonic fields is analyzed in ray acoustics regime where the wavelength of acoustic beam is much smaller than the size of the particle. In this paper, the analysis is therefore based on the field pattern produced by a strongly focused 100 MHz ultrasonic transducer with Gaussian intensity distribution. The magnitude of force and Fresnel coefficients at various positions are calculated. According to the simulation results, acoustical tweezer works particularly when the beam width at focus is one wavelength and the tolerance of acoustic impedance mismatch between two media lies within 6.7%. [Work supported by NIH Grant P41-EB2182.]

Thermal Transport in Nanostructured Solid-State Cooling Devices

Low-dimensional nanostructured materials are promising candidates for high efficiency solid-state cooling devices based on the Peltier effect. Thermal transport in these low-dimensional materials is a key factor for device performance since the thermoelectric figure of merit is inversely proportional to thermal conductivity. Therefore, understanding thermal transport in nanostructured materials is crucial for engineering high performance devices. Thermal transport in semiconductors is dominated by lattice vibrations called phonons, and phonon transport is often markedly different in nanostructures than it is in bulk materials for a number of reasons. First, as the size of a structure decreases, its surface area to volume ratio increases, thereby increasing the importance of boundaries and interfaces. Additionally, at the nanoscale the characteristic length of the structure approaches the phonon wavelength, and other interesting phenomena such as dispersion relation modification and quantum confinement may arise and further alter the thermal transport. In this paper we discuss phonon transport in semiconductor superlattices and nanowires with regards to applications in solid-state cooling devices. Systematic studies on periodic multilayers called superlattices disclose the relative importance of acoustic impedance mismatch, alloy scattering, and crystalline imperfections at the interfaces. Thermal conductivity measurements of mono-crystalline silicon nanowires of different diameters reveal the strong effects of phonon-boundary scattering. Experimental results for Si/SiGe superlattice nanowires indicate that different phonon scattering mechanisms may disrupt phonon transport at different frequencies. These experimental studies provide insight regarding the dominant mechanisms for phonon transport in nanostructures. Finally, we also briefly discuss Peltier coolers made from nanostructured materials that have shown promising cooling performance.

Silicon oxide colloidal∕polymer nanocomposite films

The quarter-wavelength (λ∕4) acoustic matching layer, a vital component in medical ultrasonic transducer, can bridge the large acoustic impedance mismatch between the piezoelectric material and the human body. Composite materials are widely used as matching materials in order to cover the wide acoustic impedance range that cannot be accomplished by using a single-phase material. At high frequencies (>50MHz), the λ∕4 matching layers become extremely thin so that the fabrication of homogeneous composite material matching layers becomes very challenging. A method is reported in this letter to fabricate sol-gel silicon oxide colloidal∕polymer composite film on silicon substrate, in which the particle size of silicon oxide colloidal is between 10 and 40 nm. The acoustic impedance of the nanocomposite films versus aging temperature has been measured at the desired operating frequency.

Three approaches for estimating the elastic modulus of the tympanic membrane

The function of the middle ear is to resolve the acoustic impedance mismatch between the air in the ear canal and the fluid of the inner ear. Without this impedance matching, very little acoustic energy would be absorbed into the cochlea. The first step in this process is the tympanic membrane (TM) converting sound in the ear canal into vibrations of the middle ear bones. Understanding how the TM manages its task so successfully over such a broad frequency range should lead to more satisfactory and less variable TM repairs (myringoplasty). In addition, understanding the mechanics of the TM is necessary to improve the coupling between ossicular prostheses and the TM. Mathematical models have played a central role in helping the research community understand the mechanics of the eardrum. However, all models require parameters as inputs. Unfortunately, most of the parameters needed for modeling the TM are not well known. In this work, several approaches for inferring the material properties of the TM are explored. First, constitutive modeling is used to estimate an elastic modulus based on the elastic modulus of collagen and experimentally observed fiber densities. Second, experimental tension and bending test results from the literature are re-interpreted using composite laminate theory. Lastly, dynamic measurements of the cat TM are used in conjunction with a composite shell model to bound the material parameters. Values from the literature, both measurement and modeling efforts, and from the present analysis are brought together to form a coherent picture of the TM's material properties. In the human, the data bound the elastic modulus between 0.1 and 0.3 GPa. In the cat, the data suggest a range of 0.1–0.4 GPa. These values are significantly higher than previous estimates.

A phonon depletion effect in ultrathin heterostructures with acoustically mismatched layers

We demonstrate theoretically that modification of the acoustic phonon spectrum in semiconductor heterostructures with large acoustic impedance mismatch between the core and cladding layers may lead to strong phonon depletion in the core layer. The latter is achieved if the heterostructure parameters are properly tuned, i.e., the structure thickness is in nanometer scale to ensure phonon quantization and the cladding layers are acoustically “softer” than the core layer. Using a numerical solution of the elasticity equation, we show that one can achieve conditions when almost all acoustic phonon modes are squeezed in the cladding layers with the exception of a small fraction of phonons with very small wave vectors (q⩽0.3nm−1). The predicted phonon depletion effect in the core layer of the acoustically mismatched heterostructures may lead to increased carrier mobility in certain regions of the heterostructure as well as improved thermal management of heterostructure-based devices.