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ABSTRACT: The acoustic field modelling reported in this paper finds application in the design of a scanning probe tip for measuring the near-surface elastic properties of solids and surface structures at high frequencies and with high spatial resolution. The underlying concept is for a longitudinally polarized pulse to be launched from a spherically-shaped portion of the upper surface of the pyramidal or conical shaped tip, and focused towards the narrow lower end. The change in the reflectivity when the narrow end is brought into contact with a solid will provide a measure of the local frequency dependent compliance of that solid. The calculations assume the material from which the tip is fabricated to be transversely isotropic, with symmetry axis coinciding with the axis of the tip. The main issue addressed in this paper is the role of the curvature of the radiating surface and anisotropy of the medium in determining the focal length and focal spread of the radiated field. Two complementary approaches are taken, firstly the discretization of the equations of motion on an irregular mesh of around 3×10(5) triangular elements and solution using the commercial FE package ABAQUS/Explicit, and secondly an analytical approach based on ray tracing and a Green's function method exploiting the angular spectrum method and stationary phase approximation in its evaluation. Consistency is achieved between these approaches regarding the characteristics of the focal region. With the combination of the two approaches it is thus possible to model the wave field from low frequencies, where the FE method is computationally economical and able to handle complex geometries, to high frequencies, where advantage increasingly lies with ray tracing and the Green's function method.
Ultrasonics 10/2011; 51(7):824-30. · 1.84 Impact Factor
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ABSTRACT: The ultrashort laser pulse photoacoustic method has been used to characterise the physical properties of spin-coated PMMA layers onto Si wafers with thicknesses from 586 to 13 nm, . Acoustic speeds of polymer films were derived from the measured time of flight of acoustic waves in polymers and from their calculated visco-elastic properties. A 12% increase, in comparison to the PMMA bulk value, of the acoustic speeds was measured for polymer films with thicknesses below 80 nm, which corresponds to an increase in Young's modulus of 26%. In addition, we found that adding a hexamethyldisilazane primer monolayer between the polymer film and the Si substrate lessen the increase in Young's modulus, suggesting that the nanoscale changes are due to interface effects.
Journal of Physics Conference Series 04/2010; 214(1):012049.
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ABSTRACT: Surface roughness affects the results of nanomechanical tests. The surface roughness values to be measured on a surface of a porous material are dependent on the properties of the naturally occurring pore space. In order to assess the surface roughness of hardened cement paste (HCP) without the actual influence of the usual sample preparation for nanomechanical testing (i.e. grinding and polishing), focussed ion beam nanotomography datasets were utilized for reconstruction of 3D (nanoscale resolution) surface profiles of hardened cement pastes. 'Virtual topographic experiments' were performed and root mean square surface roughness was then calculated for a large number of such 3D surface profiles. The resulting root mean square (between 115 and 494 nm) is considerably higher than some roughness values (as low as 10 nm) reported in the literature. We suggest that thus-analysed root mean square values provide an estimate of a 'hard' lower limit that can be achieved by 'artefact-free' sample preparation of realistic samples of hardened cement paste. To the best of our knowledge, this 'hard' lower limit was quantified for a porous material based on hydraulic cement for the first time. We suggest that the values of RMS below such a limit may indicate sample preparation artefacts. Consequently, for reliable nanomechanical testing of disordered porous materials, such as hardened cement paste, the preparation methods may require further improvement.
Journal of Microscopy 12/2008; 232(2):200-6. · 1.63 Impact Factor
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ABSTRACT: Results on a tunable resonant cavity enhanced detector (RCED) in the mid-infrared employing a vertically moving, comb-drive actuated micromirror are presented. A wide tuning range of 0.7 mum and a low order configuration have been achieved with a micromirror displacement range of 2.5 mum and a reduction of the optical cavity length respectively.
Optical MEMs and Nanophotonics, 2008 IEEE/LEOS Internationall Conference on; 09/2008
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ABSTRACT: Lead chalcogenide (IV–VI narrow-gap semiconductor) layers on Si or BaF2(111) substrates are employed to realize two mid-infrared optoelectronic devices for the first time. A tunable resonant cavity
enhanced detector is realized by employing a movable mirror. Tuning is across the 4μm to 5.5μm wavelength range, and linewidth is <0.1μm. Due to the thin (0.3μm) PbTe photodiode inside the cavity, a higher sensitivity at higher operating temperatures was achieved as compared to conventional
thick photodiodes. The second device is an optically pumped vertical external-cavity surface-emitting laser with PbTe-based
gain layers. It emits at ∼5μm wavelength and with output power up to 50mW pulsed, or 3mW continuous wave at 100K.
Journal of Electronic Materials 08/2008; 37(9):1497-1503. · 1.47 Impact Factor
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ABSTRACT: Active fiber composites (AFC) are thin and conformable transducer elements with orthotropic material properties, since they are made of one layer of piezoelectric ceramic fibers. They are suitable for applications in structural health monitoring systems (SHM) with acoustic non-destructive testing methods (NDT). In the presented work the transfer behavior of an AFC as an emitter of transient elastic waves in plate-like structures is investigated. The wave field emitted by an AFC surface bonded on an isotropic plate was simulated with the finite-difference method. The model includes the piezoelectric element and the plate and allows the simulation of the elastic wave propagation. For comparison with the model experiments using a laser interferometer for non-contact measurements of particle velocities at different points around the AFC on the surface of the plate were performed. Transfer functions defined as the ratio of the electric voltage excitation signal and the resulting surface velocity at a specific point are separately determined for the two fundamental Lamb wave modes. In order to take the orthotropic behavior of the AFC into account the transfer functions are determined for several points around the AFC. Results show that the AFC is capable to excite the fundamental symmetric and antisymmetric Lamb wave mode. The antisymmetric mode is mainly radiated in the direction of the piezoelectric fibers, while the symmetric mode is spread over a larger angle. The amplitudes of the emitted waves depend on the frequency of the excitation as well as on the geometric dimensions of the transducer.
Ultrasonics 07/2008; 49(1):73-82. · 1.84 Impact Factor
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ABSTRACT: Abstract— Fatigue crack propagation characteristics are measured by continuously determining the resonant frequency in vibrating systems, which is stabilized through a feedback control loop. The precisely controlled resonant frequency is related to the crack length by a nonlinear model based on fracture mechanics, hence crack growth can be monitored with respect to time with very high accuracy. The nonlinearity due to the opening and closing of the crack needs to be taken into account. In contrast to conventional fatigue tests, which require a long duration of time due to the high numbers of load cycles at low frequencies, the proposed technique operates at much higher frequencies, i.e. in the range of 100 Hz to 100 kHz. Thus the required time for measurements in the high cycle fatigue range is considerably reduced. The experimental setup is simple and inexpensive and does not require high energy inputs.
Fatigue & Fracture of Engineering Materials & Structures 02/2008; 20(7):1051 - 1058. · 0.85 Impact Factor
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ABSTRACT: laser acoustic setup is used to determine mechanical properties of polymer thin films which are used for nanoimprinting. Until now mechanical properties like the Young's Modulus or the Poisson Ratio are not well known for these polymers in such small dimensions (100 - 600 nm thickness). The polymer films are spincoated on a silicon wafer and covered with a thin aluminum layer for a better energy absorption of the laser pulses. The measurements are performed on a femtosecond laser pump- probe setup with a collinear beam guidance. This measurement method is contact-free and non-destructive. Mechanical waves are excited and detected thermoelastically using infrared laser pulses of approximately 80 fs duration. The entire experimental setup is simulated numerically: The heat distribution and wave excitation in the thin films caused by the laser pulse, the wave propagation, and the photoacoustic detection. Results of the simulation are shown and a short overview of the simulation procedure is given. With the simulation it is possible to interpret and assign the various measured wave pulses. The laser acoustic measurements are compared with profilometry measurements performed on the same thin film structures in order to quantify the mechanical properties of the polymer films.
Ultrasonics Symposium, 2007. IEEE; 12/2007
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ABSTRACT: Synchrotron radiation phase-contrast X-ray tomographic microscopy (srPCXTM) was applied to observation and identification of the features of spruce anatomy at the cellular lengthscale. The pilot experiments presented in the paper clearly revealed the features of the heartwood of Spruce (Picea abies [L.] Karst.), such as lumina and pits connecting the lumina, with a theoretical voxel size of 0.7 x 0.7 x 0.7 microm(3). The experiments were carried out on microspecimens of heartwood, measuring approximately 200 by 200 micrometers in cross-section. The technique for production and preparation of wood microsamples was developed within the framework of this investigation. The total porosity of the samples was derived and the values of the microstructural parameters, such as the diameters of tracheid, cell wall thicknesses and pit diameters were assessed non-invasively. Microstructural features as thin/small as approximately 1.5 microm were revealed and reconstructed in 3D. It is suggested that the position of sub-voxel-sized features (such as position of tori in the bordered pit pairs) can be determined indirectly using watershed segmentation. Moreover, the paper discusses the practical issues connected with a pipelined phase-contrast synchrotron-based microtomography experiment and the possible future potentials of this technique in the domain of wood science.
Journal of Structural Biology 07/2007; 159(1):46-55. · 3.41 Impact Factor
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ABSTRACT: This paper presents the design, fabrication and measurement results of a vertically moving, electrostatically actuated micromirror. The single crystalline silicon substrate allows the design of a symmetrical and mechanically stable mirror suspension while keeping a geometry with high fill factors and maintaining elasticity and thus keeping the actuation voltage below 25 V. The device is being developed for the use in a tunable resonant cavity enhanced detector (RCED) for the mid-infrared (Arnold, 2005). RCED's make use of a standing wave formed in an optical cavity and are only sensitive at the resonances. The wavelengths of the resonances are hereby depending on the distance of the two cavity mirrors (Unlu, 1995). Such narrowband detector systems are sought- after in multispectral infrared (IR) thermography or infrared spectroscopy (Musca, 2005).
Micro Electro Mechanical Systems, 2007. MEMS. IEEE 20th International Conference on; 02/2007
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ABSTRACT: The wave propagation in anisotropic 3D structures with arbitrary geometries is numerically calculated in order to interpret laser acoustic measurements in microstructures. The laser acoustic Pump-Probe technique generates bulk waves in structures in a thermo elastic way. Here, the wave propagation for various geometries and materials is investigated. In the first part, the wave propagation in isotropic, axisymmetric structures is simulated with a 2D finite difference formulation. The numerical results are verified with measurements of macroscopic specimens. In a second step, the simulations are extended to 3D structures with anisotropic material properties. The implemented code allows the calculation of the wave propagation for different orientations of the material axes (orientation of the orthotropic axes relative to the geometry of the structure). Limits of the presented algorithms are discussed and future directions of the on-going research project are presented
Ultrasonics Symposium, 2006. IEEE; 11/2006
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Ultrasonics Symposium, 2005 IEEE; 10/2005
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ABSTRACT: Functionally graded materials (FGMs) are defined as materials featuring engineered gradual spatial transitions in microstructure and/or composition thus having gradually varying mechanical properties. A rarely treated topic is the elastodynamic wave propagation in FGMs. It is of particular interest since the reflection, refraction, and transmission of mechanical waves is frequency dependent provided that the spatial area in which the material properties vary is in the order of the mechanical wave lengths to be distinguished. This opens a wide field of potential engineering applications like micromechanical frequency filters, spectrum analyzers, or acoustic isolation layers. Frequency sensitive elastodynamic wave propagation phenomena are demonstrated, based on 2D numerical simulations and on a series of short-pulse-laser-acoustic experiments. There, the frequency dependent reflection and transmission behavior caused by intermetallic interface layers of 10 to 20 nm thickness has been demonstrated, which corresponds to a micromechanical filter operating in the frequency range of 0.5 THz.
Ultrasonics Symposium, 2004 IEEE; 09/2004
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ABSTRACT: In most applications of MEMS the mechanical properties of the used materials are key parameters for the perfect working of the microsystems. Measuring bulk acoustic waves excited in MEMS structures with ultra-short laser pulses is a powerful method for the accurate and non-destructive evaluation as well as for the characterization of material properties. The pump-probe laser-based acoustic method generates bulk acoustic waves in a thermo-elastic way by absorbing the pump laser pulses. The acoustic waves are partly reflected at any discontinuity of the acoustic impedance. At the surface of the specimen the reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. Thin membranes are part of numerous microelectromechanical systems (MEMS) like sensors, activators and bulk acoustic wave (BAW) filters for example. The described non-destructive and non-contact method is the right approach for testing such thin and brittle structures like membranes. Results of measurements on freestanding aluminium-silicon nitride multi-layer membranes with total thicknesses in the order of several hundred nanometers are presented and compared with thermo-elastic models and with measurements of the supported case. The measured results are used for the determination of the moduli of the membranes.
Ultrasonics 05/2004; 42(1-9):641-6. · 1.84 Impact Factor
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ABSTRACT: A method for the controlled positioning of small particles in one or two dimensions by an ultrasound field excited by a surface wave is presented. Particles of a diameter between 10 and 100 microm placed on a surface can be concentrated at certain locations and moved over the surface. In other approaches it is possible to let the particle levitate freely in the fluid. However for the use of ultrasonic positioning in for example microassembling it is necessary to move particles over a surface as well as to let them levitate over the surface. Physical principle: A two- or three-dimensional ultrasound field is excited in a fluid filled gap between a rigid surface at the bottom and a vibrating surface of a solid at the top. The height of the gap varies between 0.1 and 2 mm. A one-dimensional sinusoidal vibration of the upper surface excites a two-dimensional ultrasound field in the fluid. Particles that are arbitrarily distributed on the lower surface will be concentrated in lines by the ultrasound field. First the calculation of the field of forces on particles in the fluid layer is presented. Then the dispersion relation of a vibrating plate which is in contact with a fluid on one side is derived. The technical setup will be introduced. Finally the experiments are shown and compared to the theoretical results.
Ultrasonics 05/2004; 42(1-9):75-80. · 1.84 Impact Factor
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ABSTRACT: The measurement of bulk acoustic waves (BAW) excited in thin films or microstructures with ultrashort laser pulses is a powerful method for accurate and nondestructive evaluation of material or geometrical properties. Optical techniques like the pump-probe laser-based acoustic method generate BAW in a thermoelastic way by absorbing the pump laser pulses at the surface of the specimen. The acoustic waves are partly reflected at any discontinuity of the acoustic impedance. Back at the surface the reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. The measurement technique is explained for the case of an aluminium thin film on sapphire. The influence of the film thickness and the deposition method of the thin films on the bulk wave speed is shown. In the second part of the paper this technique is used for measuring the bulk wave propagation in very thin membranes. The BAW propagation in freestanding silicon-nitride aluminium multilayer membranes with total thickness in the order of several hundred nanometers is measured. The measurements of the freestanding membranes are compared with measurements of the supported case. The technique presented in this paper can also be applied for the characterization of material or geometrical properties of thin film BAW resonators. The advantage of the method lies in its nondestructive and noncontact approach, which is necessary for ultrathin and brittle structures.
Ultrasonics Symposium, 2002. Proceedings. 2002 IEEE; 11/2002
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ABSTRACT: In this work a tissue aspiration method for the in vivo determination of biological soft tissue material parameters is presented. An explicit axisymmetric finite element simulation of the aspiration experiment is used together with a Levenberg-Marquardt algorithm to estimate the material model parameters in an inverse parameter determination process. An optimal fit of the simulated experiment and the real experiment is sought with the parameter estimation algorithm. Soft biological tissue is modelled as a viscoelastic, non-linear, nearly incompressible, isotropic continuum. Viscoelasticity is accounted for by a quasi-linear formulation. The aspiration method is validated experimentally with a synthetic material. In vivo (intra-operatively during surgical interventions) and ex vivo experiments were performed on human uteri.
Medical Image Analysis 10/2002; 6(3):275-87. · 4.42 Impact Factor
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ABSTRACT: For the controlled positioning of small particles with ultrasound a standing wave in a fluid is used. The standing wave is implemented in a resonator, that consists of a fluid filled tube and two piezoelectric transducers on each end. A one-dimensional model of a piezo-device including the fluid-loading on one side and a backside support is introduced. This model allows the calculation of the transmitted wave as a function of the applied electric voltage and the incident wave. In addition, when an electrical impedance is connected to the piezo-device, the reflection coefficient can be varied in amplitude and phase, so that the parameters of the reflected wave can be controlled completely. The resonator itself, consisting of a piezo-device on each end and the fluid between, is included in the model. Several methods to shift the nodes of the standing wave in the resonator are investigated and the ability to position particles is discussed.
Ultrasonics 06/2002; 40(1-8):317-22. · 1.84 Impact Factor
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ABSTRACT: Pulsed laser acoustic experiments have the advantage of very high temporal resolution. However, the lateral resolution amounts to several wavelengths of light. To improve the lateral resolution a focussing tip in which the mechanical waves are focussed is introduced. The combination of high resolution in time and space domain leads to a new potential time resolved scanning probe method. Therefore several axi-symmetric structures are investigated numerically using a finite difference method. The ultrasonic wave propagation in different tips is discussed. By varying the geometry of the tip, the displacement at the sharp end is maximized. The numerically calculated results are verified experimentally on structures having macroscopic dimensions. Scaling effects are considered in order to translate the results into the microscopic scale where arbitrary geometries are much more challenging to implement.
Ultrasonics 06/2002; 40(1-8):747-52. · 1.84 Impact Factor
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ABSTRACT: The development and optimization of non-destructive testing procedures usually needs experimental data. As experiments are time-consuming and expensive to conduct, we would like to use numerical data instead. This is admissible, if the simulation describes the physical experiments accurately. A three-dimensional displacement-stress finite-difference model is presented for a piezoelectric transducer coupled to an anisotropic tube. The allocation of the displacement and stress components on a staggered grid leads to a stable scheme. A full piezoelectric model of the transducer is used, including transverse isotropy in the elastic, dielectric, and piezoelectric constants. Similar to an experiment, elastic waves are excited in the corresponding simulation by applying a voltage signal to the electrodes of the piezoelectric transducer. Predictions of the simulation model for a piezoelectric ring transducer coupled to a carbon-fibre-reinforced shell are compared to experimental results to test the validity of the numerical data.
Ultrasonics 06/2002; 40(1-8):181-6. · 1.84 Impact Factor
