F. Montero de Espinosa

Design of piezocomposite materials and piezoelectric transducers using topology optimization— Part III

SummaryCurrently developments of piezocomposite materials and piczoelectric actuators have been based on the use of simple analytical models, test of prototypes, and analysis using the finite element method (FEM), usually limiting the problem to a parametric optimization. By changing the topology of these devices or their components, we may obtain an improvement in their performance characteristics. Based on this idea, this paper discusses the application of topology optimization combined with the homogenization method and FEM for designing piezocomposite materials. The homogenization method allows us to calculate the effective properties of a composite material knowing its unit cell topology. New effective properties that improves the electromechanical efficiency of the piezocomposite material are obtained by designing the piezocomposite unit cell. This method consists of finding the distribution of the material and void phases in a periodic unit cell that optimizes the performance characteristics of the piezocomposite. The optimized solution is obtained using Sequential Linear Programming (SLP). A general homogenization method applied to piczoelectricity was implemented using the finite element method (FEM). This homogenization method has no limitations regarding volume fraction or shape of the composite constituents. The main assumptions are that the unit cell is periodic and that the scale of the composite part is much larger than the microstructure dimensions. Prototypes of the optimized piezocomposites were manufactured and experimental results confirmed the large improvement.

Viscoelasticity of silica aerogels at ultrasonic frequencies

In this letter, the possibility to use a technique based on the analysis of thickness resonances of air-surrounded aerogel plates at ultrasonic frequencies to obtain viscoelastic properties is investigated. These resonances were excited and sensed by airborne ultrasonic waves. Toward this purpose, specially designed air-coupled, high-sensitivity, and broadband piezoelectric transducers were used. Precise and simultaneous measurements of the velocity and attenuation of longitudinal and shear waves at different frequencies, as well as aerogel density, were obtained. It allowed us to afford a full characterization of the viscoelastic properties of these materials at ultrasonic frequencies.

Ultrasonic measurement of milk coagulation time

Using a pulse reflection technique an ultrasonic system has been developed to monitor in situ the coagulation process of rennetted milk. The velocity and attenuation of ultrasonic waves through coagulating milk were continuously monitored. The observed changes in ultrasonic velocity during coagulation were used to predict the coagulation time. The coagulation time is indicative of the transition from the enzymatic phase to the physicochemical phase. The determination of coagulation time has a decisive role in determining the qualities of the end product in cheesemaking.

On the resonance frequencies of water-loaded circular plates

The studies on the increase of inertia, and the corresponding reduction in frequency, of plates when vibrating in water have been confined mainly to the lower modes of vibration and for plate radii much less than a wavelength of sound in the fluid. In this paper, a method is presented which, subject to certain approximations, permits the evaluation of the influence of the fluid on the frequency of any vibration mode whose deflection curve in a vacuum is known. The method has been applied to water-loaded circular plates vibrating in their axisymmetric modes.

Broad-band driving of echographic arrays using 10 ns-500 V efficient pulse generators

Abstract To obtain a good dynamic range on echographic signals produced by broad-band piezoelectric arrays with frequencies > 4 MHz, it is necessary to excite each of the array elements with very fast electric spikes. Therefore, for most depths used in medical echographic visualization, or NDT applications, electric pulses of some hundreds of volts with a rise-time of

Highly coupled dielectric behavior of porous ceramics embedding a polymer

We present a theoretical approach to study the dielectric properties of porous ceramics and composite materials and explain the experimentally observed notable influence of the coupling between the components of a saturated porous ceramic on the final behavior of the sample. This model is based on the assumption of a dielectric coupling between the components that modifies the expected averaged properties of the material.

64 Elements two-dimensional piezoelectric array for 3D imaging

Ultrasound has a large potential on non-invasive inspection with main applications in medical imaging and non-destructive testing (NDT). The increasing interest in 3D imaging applications leads to investigate new solutions for two-dimensional (2D) ultrasonic arrays with an affordable number of electronic channels without resolution degradation. 2D segmented annular arrays (SAAs) are a good compromise between resolution--image quality--and number of electronically active channels. A 1-3 piezoelectric composites are used as basis material to manufacture the array transducers due to their low planar coupling and high electromechanical coupling coefficients. A 1.5 MHz SAA of 64 elements and 20 mm of diameter was designed, manufactured and tested. The design key point is the use of a flexible circuit with electrodes and tracks that define the array geometry. The piezocomposite was used as a monolithic support. Soft backing and one matching layer were used. The array elements have been tested electrically and acoustically showing good agreement with a KLM-based simulation model. Acoustical field measurements in water at different steering angles were made and compared with simulations performed with a model that uses an exact solution of the impulse response approach. Side lobes are important because the array geometry used was designed to work in metals for NDT purposes. Smaller array elements should be made for medical applications.

A small 2D ultrasonic array for NDT applications

A prototype of two-dimensional transducer array with reduced number of elements, based on segmented annular distribution is presented. The capability of this array to produce volumetric imaging is compared to the equivalent conventional 2D squared matrix array. The comparison between both apertures is made for the cases of the full-array emission/reception mode and SAFT mode. From the analysis it is deduced that the segmented annular arrays produce lower grating lobes than squared arrays, improving the image contrast. The fabrication process of a segmented annular array of 64 elements and the experimental work made with this array transducer is also presented in this paper.

Ceramic powder-polymer piezocomposites for electroacoustic transduction : modeling and design

Abstract A measurable piezoelectric response in loaded ceramic powder–polymer piezocomposites requires a certain degree of connectivity among the individual ceramic grains, so that the spatial connectivity of the composite can be located somewhere between the ideal (0–3) and (3–3) connectivity limits. This non-zero spatial connectivity of the ceramic grains in the polymer matrix makes the use of classical theoretical methods to assess dielectric and mechanical properties difficult, since these are mainly based on scattering or averaging approaches. Here, a new theoretical approach, recently developed to study porous piezoelectric ceramics, is applied to assess the properties of such composite materials. In addition, previous approaches are reviewed. Two different ceramic materials and polymers are used to produce composite materials following different fabrication routes. The samples are characterized following the new approach, paying special attention to the determination of the real spatial connectivity of the composites, the bounding between ceramic grains and polymer, and the degree of poling. As a result, a comprehensive characterization route for these materials is proposed.

Ultrasonic measurement device for the characterization of microbiological and biochemical processes in liquid media

A measuring device for the characterization of liquid media based on the propagation of ultrasonic waves is presented. It is a four-channel system especially designed for monitoring microbiological and biochemical processes. The liquid samples are placed in commercial glass bottles which can be sterilized. The bottles have inlet and outlet tubes, which can be used for adding substances or extracting samples during the measuring process without interruption. Magnetic stirring can be used to keep the liquid agitated for homogenization purposes. Thermal control elements assure the temperature stability during the measurement. The liquid characterization is based on the detection of amplitude and time-of-flight changes in the sample under study. The main features, operation and performance of this ultrasonic device are analysed in this work, and some measurements and preliminary results are shown.