W.A. Smith

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>>

Modeling 1-3 composite piezoelectrics: hydrostatic response

A simple physical model of 1-3 composite piezoelectrics that was advanced for the material properties relevant to thickness-mode oscillations is extended to address the hydrostatic response. The 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 are derived 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. The results are similar to those derived by Haun and Newnham (1983, 1986) using a parallel-series connectivity model. The model is illustrated by analyzing composites made from conventional PZT5 and anisotropic modified lead titanate piezoelectric ceramics. For PZT5, the composite structure enhances its hydrostatic charge coefficient, hydrostatic voltage coefficient, hydrophone figure of merit, and hydrostatic coupling coefficient, while three of these quantities fall short of their pure ceramic values in the modified lead titanate composites. The shortfall is due to an enhanced composite that arises from lateral stress on the polymer being transferred to a longitudinal stress along the ceramic rods by the Poisson effect in the polymer, thus producing a charge through the ceramics d/sub 33/.<<ETX>>

Composite piezoelectrics: basic research to a practical device

Abstract The adaptability of 1–3 PZT-rod/polymer-matrix piezoelectric composites commends them for the requirements of medical ultrasonic imaging transducers. We describe material fabrication techniques, material design considerations, and transducer design trade-offs involved in this application. A simple dice-and-fill fabrication technique yields material suitable for transducers with frequencies approaching 10 MHz. The piezocomposites provide high electromechanical coupling and low acoustic impedance – properties superior to those of the conventional piezoceramics and piezopolymers. Composite piezoelectric plates can be formed into complex shapes for beam focusing and steering. Moreover, arrays defined by simply patterning the electrode – not cutting the plate – exhibit no spurious modes in individual elements and low cross-talk between array elements. The resulting device performance is illustrated with a concave annular array which shows high sensitivity (8 dB insertion loss), large bandwidth (57%), ...

Biplane phased array for ultrasonic medical imaging

The authors report a transducer structure that combines two orthogonal phase arrays that enable the real-time scanning of two orthogonal sectors. This biplane phased array is formed by partially dicing the opposite faces of a composite piezoelectric plate in orthogonal directions. Individual elements of the orthogonal arrays show broad radiation patterns close to the theoretical expectations for isolated elements. Arrays formed by patterning the electrode alone, without cross dicing, show a narrower directivity pattern for a single element. This narrowing is a refractive effect due to the relatively high acoustic velocities in the composite material as compared with the sound velocity in the propagation medium. It is noted that the concept of the biplane phased array forms the basis for a new class of transducer systems that combine two transducer arrays in a single piezocomposite plate.<<ETX>>

Composite Piezoelectric Materials for Medical Ultrasonic Imaging Transducers - A Review

This paper surveys published research on the use of composite physicians. piezoelect.ric materials to make the acoustic transducers used in medical ultrasonic imaging. For medical imaging transducers, attention has focused on the 1-3, PZT-rod/polymer composite structure. The piezoelectric plates used in such ultrasonic transducers consist of a polymer matrix which holds together parallel thin rods of piezoelectric ceramic oriented perpendicular to the faces of the plate. By varying the properties, proportions and spatial scales of the polymer and piezoceramic constituents a rich variety of material properties are achieved. Of particular interest for medical imaging transducers is the ability to engineer materials whose electromechanical coupling is higher than that of conventional materials and whose acoustic impedance is close to that of tissue. Besides these basic advantages in material parameters. composites have properties that facilitate meeting other technological requirements. Flexible composite piezoelectrics can be formed into complex curved shapes for steering and focusing the acoustic waves. Transducer arrays can he made from composites by patterning the electrode alone -- cutting the piezoelectric is not required.

Pyroelectric pzt-polymer composites

Abstract PZT-Spurrs composites fabricated by the replamine technique improves the pyroelectric figure of merit p/e by a factor of 7 over that of solid PZT. The temperature dependence of the pyroelectric response exhibits an interesting compensation point where the primary and secondary effects cancel out. At room temperature the hard plastic mechanically clamps the PZT, but at higher temperatures the plastic softens. PZT-soft polymer composites showed no enhancement of the pyroelectric figure of merit. Major changes in the pyroelectric response were noted when the sample thickness was comparable to the scale of the composite heterogeneity.

Dielectric, pyroelectric, and thermal properties of LiNH4SO4 and LiND4SO4

Measurements of optical second harmonic generation, specific heat, dielectric constant, and pyroelectric coefficient of LiNH4SO4 and its deuterated analog are reported over a temperature range which includes the two phase transitions near 283 K and 459 K. The results indicate that these materials are potentially useful as pyroelectric detectors. Pyroelectric and second harmonic generation data show that the phase below ∼ 283 K is polar.

Performance of Ultrasonic Transducers Made from Composite Piezoelectric Materials

Single element transducers and linear arrays made from composite piezoelectric materials were characterized for application in medical ultrasonic imaging. The composite transducers showed a compact temporal response and high sensitivity over a broad frequency range. Prototype linear arrays with elements defined by an electrode pattern alone exhibited low acoustic cross talk between neighboring elements and overall performance equivalent to that of state of the art diced arrays.

Dielectric anomalies in boracites

Abstract Experimental results are reported on the dielectric behavior of Fe3B7O13I, Ni3B7O13Br, Cu3B7O13CI, and Mn3B7O13I, near their cubic-orthorhombic phase transition. Fe3B7O13I and Ni3B7O13Br show the typical dielectric behavior of boracites, whereas Cu3B7O13CI and Mn3B7O13I exhibit a different dielectric anomaly. Both types of behavior can be discussed on the basis of a Landau expansion of the thermodynamic potential.

Calorimetric investigation of the ferroelectric 43m-mm2 phase transition in boracite crystals

The isobaric molar heat capacity of CrCl, FeI, CuCl, NiBr and ZnBr boracite at the ferroelectric 43m-mm2 phase transition is reported. The magnitude of the rise in Cp at the transition, and the large upper bound values of ΔH and ΔS prove that the phase transition is first order. The values of ΔH follow the trend ZnBr ⪢ NiBr > CrCl ⪢ CuCl > FeI reflecting possible structural dissimilarities amongst the boracites. Thermal annealing of single crystal boracite samples of NiBr and CrCl is found to remove multiple peaking of the heat capacity at the transition resulting in single peak heat capacity curves. The multiple peaking is thought to arise from internal stresses within the crystal.