Shoko Yoshikawa

Resistivities of conductive composites

A steady‐state model for the resistivity of composites is presented, based on the idea that the resistance through a composite is the result of a series of a large number of resistors combined in series and parallel. There are three separate contributions to the resistance: constriction resistance at the contacts, tunneling resistance at the contacts, and the intrinsic filler resistance through each particle, with tunneling resistance generally dominating the magnitude of the overall resistance. The model predicts resistivity increases with increasing filler hardness and/or elastic modulus and insulating film thickness, while resistivity decreases with increasing particle size and intrinsic stress. The room‐temperature dc resistivity behavior of conductor‐filled silicone rubber composites was investigated to verify the model. Comparison of the model to this experimental data showed that good agreement could be obtained for filler materials in which the tarnish layer was a known quantity for a given powder...

Electric field induced phase transition of antiferroelectric lead lanthanum zirconate titanate stannate ceramics

The electric field induced phase transition behavior of lead lanthanum zirconate titanate stannate (PLZTS) ceramics was investigated. PLZTS undergoes a tetragonal antiferroelectric (AFETet) to rhombohedral ferroelectric (FERh) phase transition with the application of an electric field. The volume increase associated with this antiferroelectric (AFE)–ferroelectric (FE) phase transition plays an important role with respect to actuator applications. This volume increase involves an increase in both transverse and longitudinal strains. The E field at which the transverse strain increases is accompanied by an abrupt jump in polarization. The longitudinal strain, however, lags behind this polarization jump exhibiting a slight decrease at the onset of phase switching. This decoupling was related to the preferentially oriented AFE domain configuration, with its tetragonal c-axis perpendicular to the applied electric field. It is suggested that phase switching involves multiple steps involving both structural tran...

Transformed stress direction acoustic transducer

This invention describes an acoustic transducer assembly wherein an extremely high figure of merit (dh gh) is obtained as a result of converting incoming acoustic axial stress into radial extensional stress thereby multiplying its effect. The piezoelectric active element is encased in a metal sandwich enclosing two semilunar air spaces which allow the device to withstand extremely high hydrostatic pressure yet still respond to low level sound waves when acting as a hydrophone. The mechanical prestress induced by the differential coefficients of expansion between the metal case and the piezoelectric ceramic element also serves to prevent depolarization aging.

Piezoelectric composites with high sensitivity and high capacitance for use at high pressures

A type of piezoelectric composite has been developed for oceanographic applications. The composites have a large figure of merit (d/sub h/*g/sub h/ or d/sub h/*g/sub h//tan delta , where d/sub h/ is the hydrostatic piezoelectric voltage coefficient), a large dielectric constant (K) and low dielectric loss, and great mechanical strength. A shallow cavity between the PZT ceramics and thick metallic electrode is designed to convert a portion of the z-direction stress into a large radial and tangential stress of opposite sign. thereby causing the d/sub 33/ and d/sub 31/ contributions to d/sub h/ to add rather than subtract, and raising the figure of merit. Theoretical stress analysis was carried out using an axisymmetric finite element method. Experimental results show that the d/sub h/*g/sub h/, K, and withstandable pressure are extremely high.<<ETX>>

High Power Characterization of Piezoelectric Materials

Three techniques for measuring high voltage/power piezoelectric properties, which have been developed recently, are compared: a voltage-constant piezoelectric resonance method, a current-constant piezoelectric resonance method, and a pulse drive method. The conventional resonance method with a constant voltage circuit exhibits significant distortion (or a hysteresis) in the resonance frequency spectrum under a high vibration level due to large elastic non-linearity, which limits precise determination of the electromechanical coupling parameters. To the contrary, the resonance method with a constant current circuit (i.e., constant velocity) can determine the coupling parameters more precisely from a perfectly-symmetrical resonance spectrum. The general problem in both resonance methods is heat generation in the sample during the measurement. In order to separate the temperature characteristic from the non-linearity, it is recommended that the pulse method be used in parallel, even though the accuracy is not very high.

Metal-electroactive ceramic composite actuators

The metal-ceramic actuator includes an electroactive substrate having at least a pair of opposed planar surfaces and a determined thickness, with the ceramic substrate being poled in its thickness dimension. Conductive electrodes sandwich the ceramic substrate and are bonded to its planar surfaces. Metal caps, each having a concave cavity bounded by a rim, are bonded to both planar surfaces of the ceramic substrate. A potential is applied to the conductive electrodes to cause an expansion of the ceramic substrate in its thickness dimension and a concomitant contraction in its planar dimensions. The contraction creates a flexure of the metal caps, which flexures are used to actuate another instrumentality.

High displacement ceramic metal composite actuators (moonies)

Abstract The two most common type of piezoelectric actuators are the multilayer actuator with internal electrodes and the cantilevered bimorph actuator[1]. A new type of composite ceramic actuator is the multilayered multistacked moonie (multi-multi moonie). Normal multilayer actuators produce a large generative force, but only a small displacement. Conversely, bimorphs produce large displacements but the forces are very small. The moonie actuator combines the advantages of both, producing a large displacement as well as a reasonably large generative force.

The effect of geometry on the characteristics of the moonie transducer and reliability issue

The moonie actuator is a versatile performer that fills the gap between the multilayer actuator and the bimorph actuator. This paper describes the effect of geometrical changes in the endcap on the moonies actuator characteristics. The reliability of multilayer moonie actuators were tested for long term usage and a wide range of temperatures. The effect of cavity dimensions, which are the key parameters for transformation and amplification of lateral motion into axial motion, on the characteristics of the moonie actuator were investigated. Fatigue tests of multilayer moonie actuator were performed under a cyclic electric field of 1 kV/mm with triangular wave form at 100 Hz, up to 107 cycles, at room temperature. Deviations less than ±0.1% from the original displacement value were observed. Temperature dependence experiments were performed in the range of -20 to +70°C. Maximum ±15% nonpermanent deviations in the displacement from the room temperature value were observed

Ceramic-metal composite actuator

The main objective of this work was to develop a new type of actuator. It consists of a piezoelectric ceramic disk or multilayer stack and two metal end plates with a crescent-shaped cavity on the inner surface. The plates are used as mechanical transformers for converting and amplifying the lateral displacement of the ceramic into a large axial motion in the plates. Both d31 and d33 contribute to the axial displacement. Sizeable strains were obtained with both PZT-metal and PMN-metal actuators. Displacement amplification principle, fabrication, and measurement results are presented.

Percolation constraints in the use of conductor-filled polymers for interconnects

The properties of composite systems are understood in terms of percolation phenomena; when a sufficient amount of conductive filler is loaded into an insulating polymer matrix, the composite transforms from an insulator to a conductor, the result of continuous linkages of filler particles. The critical volume fraction at which this transformation occurs, V/sub c/, is the focus of the investigation reported. The concentration is on the properties of some silver-filled silicon rubber composites, and it is shown how the artificial percolation restrictions affect both the critical volume fraction and the resistivity of the composite. A computer simulation of the two-dimensional case shows that these restrictions affect V/sub c/ in a predictable manner. An empirical method of predicting V/sub c/ for non-restricted systems is presented.<<ETX>>