A. Shaulov

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.

Investigation of harmonic generation in the alternating magnetic response of superconducting Y‐Ba‐Cu‐O

The magnetic behavior of a sintered Y‐Ba‐Cu‐O superconductor has been investigated by monitoring the harmonic components of its magnetic response to an alternating field. As the steady bias field is incrementally raised, a transition from a linear, reversible behavior of the magnetization to a nonlinear, irreversible behavior is indicated by the appearance of magnetic losses and odd harmonic components in the response. These harmonics disappear as the bias field or the temperature is increased above a certain point, indicating a linear behavior. The disappearance of the odd harmonics, while the magnetic losses persist, is interpreted as signifying a transition to a state of dissipative flux motion without pinning. These measurements demonstrate a new technique for determination of the lower critical field and the ‘‘irreversibility’’ field below which irreversibility in the magnetization sets in as a result of flux pinning.

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>>

Ultrasonic Transducer Arrays Made from Composite Piezoelectric Materials

A method for providing a curved composite ultrasound transducer of a relatively large size. A blank of piezoceramic material has its rear side reticulated with a series of relatively coarse grooves. The front side of the material is reticulated with a series of relatively deep fine grooves to provide a matrix of upstanding piezoceramic rods. The spaces between the rods on the front side of the blank are therefore filled with a curable polymeric resin (epoxy). The resin filling the blank spaces on the front side is partially cured. After partial curing, the blank is placed into a spherical compression mold and molded so that its front side is formed concave and its rear side convex. The resin is cured while holding the molded blank under pressure in the mold until the resin has cured. After curing, the blank is removed from the mold and the back side is ground away to provide a spherically curved transducer having a multiplicity of individual piezoceramic rods disposed in a matrix of cured resin.

Modified-lead-titanate/polymer composites for hydrophone applications

Abstract Composite piezoelectrics made from Calcium-modified lead titanate rods embedded in a polymer matrix have been evaluated for hydrophone applications. These composites behave quite differently from the conventional 1–3 composites made with lead zirconate titanate ceramics. Specifically, in the modified lead-titanate case the magnitude of the d 31 coefficient is enhanced in the composite structure, and consequently the hydrostatic dh coefficient is suppressed. Nevertheless, these composites exhibit a large gh coefficient and a remark able pressure stability. An analysis shows that a substantial contribution to the composites d 31 coefficient arises from internal stresses which develop along the ceramic rods and produce a piezoelectric charge through the d 33 coefficient of the ceramic. This effect is particularly pronounced in the composite structure of the modified lead-titanate ceramic since the ratio d 33/d 31 in this ceramic is exceptionally large.

Crossed linear arrays for ultrasonic medical imaging

An improved piezoelectric transducer permitting linear scanning along two intersecting planes. The transducer is fabricated from a cross shaped piezoelectric plate covered on both faces with metallic electrodes. First and second linear arrays are formed by the partial dicing of the opposite faces of the electrodes and plate. The two segments of the orthogonal arrays are integrated in the crossed area. An aperture may be disposed in the central area so as to permit a surgical implement, such as a biopsy needle, to be inserted into the tissue under ultrasonic visualization. Scanning electronics are alternately connectable to each of the arrays by means of a multiple pole switch under the control of a clock.

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.

Improved figure of merit in obliquely cut pyroelectric crystals

In pyroelectric crystals with high dielectric anisotropy, a significantly higher figure of merit can be obtained using cuts at oblique angles to the pyroelectric axis, rather than the conventional cuts lying perpendicular to it. For a particular material an appropriate cut can be selected to provide the maximum figure of merit attainable at the chosen temperature of operation. Experimental data obtained in deuterated triglycine fluoroberylate crystals show that optimum oblique cuts can yield an increase as great as a factor of 3 in the figure of merit of this material.