Bertram A. Auld

Modeling 1-3 composite piezoelectrics: thickness-mode oscillations

A simple physical model of 1-3 composite piezoelectrics is advanced for the material properties that are relevant to thickness-mode oscillations. This model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions for the composites material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer are derived. A number of examples illustrate the implications of using piezocomposites in medical ultrasonic imaging transducers. While most material properties of the composite roughly interpolate between their values for pure polymer and pure ceramic, the composites thickness-mode electromechanical coupling can exceed that of the component ceramic. This enhanced electromechanical coupling stems from partially freeing the lateral clamping of the ceramic in the composite structure. Their higher coupling and lower acoustic impedance recommend composites for medical ultrasonic imaging transducers. The model also reveals that the composites material properties cannot be optimized simultaneously; tradeoffs must be made. Of most significance is the tradeoff between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced.<<ETX>>

Improving the characteristics of a transducer using multiple piezoelectric layers

A type of transducer composed of several active piezoelectric layers is described. These layers are independently stimulated electrically to achieve performance, in terms of sensitivity and bandwidth, not available in conventional, single active layer designs. A straightforward modeling technique is employed for predicting the performance of these transducers and for determining the optimal excitation characteristics. The technique is initially developed for transmission, but its extension to reception and pulse-echo conditions is discussed. A selection of experimental results demonstrates the feasibility of the technique.<<ETX>>

Design of piezocomposites for ultrasonic transducers

Abstract 1–3 piezoelectric-rod/passive-matrix composites offer advantages over the conventional piezoceramics and piezopolymers for the pulse-echo transducers used in medical ultrasonic imaging. Their benefits include high electromechanical coupling, acoustic impedance close to that of tissue, a wide range of dielectric constants, low dielectric and mechanical losses, an adjustable sound speed, low coupling to spurious oscillations, ease of subdividing into acoustically isolated array elements, and formability into complex curved shapes. Not all benefits are achieved simultaneously. In designing a material for a specific application, the material engineer can choose the piezoceramic, the passive matrix, their relative proportions and the spatial scale of the composite. We delineate the trade-offs in designing piezocomposites which enhance the performance of present ultrasonic transducers as well as make new transducer designs feasible.

Improving transducer performance using multiple active layers

The performance of a transducer possessing several piezoelectric layers is discussed. Techniques are presented for determining excitation functions so that a pre-defined transmission characteristic is obtained in an optimal manner. The performance of a multiple layer transducer in the reception mode is considered in detail. It is evident that a high degree of transmission and reception efficiency is attainable continuously from below the fundamental thickness mode resonance to above its third harmonic. This contrasts with conventional designs which possess a null at the second harmonic. Issues regarding the stability of the technique are addressed.

Wideband micromachined capacitive microphones with radio frequency detection

Silicon microphones based on capacitive micromachined ultrasonic transducer membranes and radio frequency detection overcome many of the limitations in bandwidth, uniformity of response, and durability associated with micromachined condenser microphones. These membranes are vacuum-sealed to withstand submersion in water and have a flat mechanical response from dc up to ultrasonic frequencies. However, a sensitive radio frequency detection scheme is necessary to detect the small changes in membrane displacement that result from utilizing small membranes. In this paper we develop a mathematical model for calculating the expected output signal and noise level and verifies the model with measurements on a fabricated microphone. Measurements on a sensor with 1.3 mm2 area demonstrate less than 0.5 dB variation in the output response between 0.1 Hz to 100 kHz under electrostatic actuation and an A-weighted equivalent noise level of 63.6 dB(A) SPL in the audio band. Because the vacuum-sealed membrane structure ha...

Spectroscopy of resonant torsional modes in cylindrical rods using electromagnetic‐acoustic transduction

Two ultrasonic techniques employing electromagnetic‐acoustic transduction are presented for performing measurements of the resonant torsional frequencies and Q of solid cylindrical metallic rods. One of these techniques uses long radio‐frequency pulses to drive the sample into resonance and the other uses continuous‐wave excitation. Measurements are performed on an aluminum alloy. Since the transduction involves no mechanical coupling, the background damping is low; the Q is 1.2×105 at 755 kHz with the sample simply supported on its side. The shear velocity is determined with an accuracy of better than 2 parts in 104 (limited by the uncertainty in the measurement of the sample radius).

Trapped torsional modes in solid cylinders

An approximate theory is presented for torsional modes in a solid cylinder having a larger‐diameter central section. Modes in a narrow frequency range just above the cutoff of each branch are ‘‘trapped’’ such that, in long samples, the amplitudes decay exponentially with distance from the central section. To confirm the theory, measurements were performed on aluminum alloy cylinders using noncontacting electromagnetic‐acoustic transduction. Vibrational amplitudes of a trapped mode measured as a function of position along the length of a sample are in good agreement with theoretical calculations. The resonant frequencies of three samples with different dimensions also closely match the theory for trapped modes. Additional generally weak resonances that are observed may be associated with torsional modes that vary sinusoidally along the entire length of the samples.

Shear horizontal wave propagation in periodically layered composites

The transverse resonance approach to guided wave analysis is applied to shear horizontal (SH) wave propagation in periodically layered composites. It is found for SH waves that at high values of the guided wavevector /spl beta/, the wave energy is trapped in the slower of the two media and propagates accordingly at the slower wavespeed. At low values of /spl beta/, however, the modes demonstrate a clustering behavior, indicative of the underlying Floquet wave structure. The number of modes in a cluster is observed to correlate with the number of unit cells in the layered plate. New physical insights into the behavior of these systems are obtained by analyzing the partial waves of the guided SH modes in terms of Floquet waves. We show that the fast and slow shear waves in the periodically layered composite play an analogous role to the longitudinal and shear partial waves comprising Lamb waves in a homogeneous plate.

Finite element method and normal mode modeling of capacitive micromachined SAW and Lamb wave transducers

Surface wave and Lamb wave devices without piezoelectricity are the latest breakthrough applications of the capacitive micromachined ultrasonic transducers (CMUTs). CMUTs were introduced for airborne and immersion applications. However, experiments showed that those devices couple energy not only to the medium but also to the substrate they are built on. By placing the CMUTs on a substrate in an interdigitated configuration, it is possible to couple energy to Lamb wave or Rayleigh wave modes with very high efficiency without a need for any piezoelectric material. In this study, we calculate the acoustic field distribution in a silicon substrate as well as the acoustic impedance of the CMUT membrane, which includes the power coupled to the substrate. We apply the normal mode theory to find the distribution of the acoustic power among different Lamb wave modes. For low frequency (1 MHz) devices, we find that the lowest order antisymmetric (A/sub 0/) mode Lamb wave is the dominant mode in the substrate, and 95% of the power propagates through this mode. For high frequency devices (100 MHz), interdigital CMUTs excite Rayleigh waves with efficiencies comparable to piezoelectric surface acoustic wave (SAW) devices.

Design and modeling of 0:3 composite transducers

Abstract The derivation of a simple quantitative expression that assists in the design of 0:3 composite transducers is presented. This expression also yields insight into critical phenomena affecting transducer performance. Existing empirical data provides useful corroboration. It is concluded that efficient 0:3 composite transducers are only obtained when particle size distribution, ceramic volume fraction and particle packing pressure are tightly controlled.