Chung-loong Choy

High tunability in compositionally graded epitaxial barium strontium titanate thin films by pulsed-laser deposition

Compositionally graded barium strontium titanate [(BaxSr1−x)TiO3—BST, x=0.75, 0.8, 0.9, and 1.0] thin films are fabricated by pulsed-laser deposition on a LaAlO3 substrate with (La0.7Sr0.3)MnO3 as the bottom electrode. A high dielectric permittivity and temperature characteristic without Curie–Weiss law are obtained. A tunability of over 70% is obtained at frequency of 1 MHz, which is higher than that of single BST layer with the same compositions. All the results indicate that the graded thin films have better electrical properties than a single-layer film.

Evaluation of the material parameters of piezoelectric materials by various methods

The elastic, dielectric and piezoelectric constants of four piezoelectric materials, including polyvinylidene fluoride, vinylidene fluoride-trifluoroethylene copolymer, PZT/epoxy 1-3 composite, and lead metaniobate ceramic, have been evaluated from the impedance data using five different methods. A method described in ANSI/IEEE Std. 176-1987, though based on formulae derived for loss-less materials, is found to be applicable to materials with moderate loss. However, for high-loss materials such as polyvinylidene fluoride, the electromechanical coupling constant (/spl kappa//sub t/) obtained by the method of Std. 176 is substantially higher than the actual value. Calculations based on a piezoelectric resonance analysis program (PRAP) combine the best features of two earlier methods. In addition to the impedance at the parallel resonance frequency, impedances at two other frequencies are required for calculation. The PRAP method gives quite accurate material parameters regardless of the magnitude of the loss, but the parameters (including /spl kappa//sub t/) vary by as much as 15% depending on the choice of data. In the nonlinear regression method described in the present work, all the impedance data points around the resonance are least-squares fitted to the theoretical expression for the impedance. Besides the advantage of requiring no arbitrary choice of data, the nonlinear regression method can readily take account of the frequency dependence of the dielectric constant.

Pyroelectric or piezoelectric compensated ferroelectric composites

The matrix and inclusions of 0–3 composites of lead titanate (PT) in polyvinylidenefluoride trifluoroethylene (P(VDF–TrFE)) have been independently poled by a special poling method. The polarization states of both constituents are investigated by the measurement of the dynamic pyroelectric coefficients of the composites in the temperature range of 20–90 °C, within which the copolymer matrix undergoes a ferroelectric-paraelectric phase change. The pyroelectric coefficients of PT and P(VDF–TrFE) have the same sign, while their piezoelectric coefficients have opposite signs. This allows the preparation of composites with enhanced pyroelectric but reduced piezoelectric activity when the matrix and inclusions are polarized in the same direction, or vice versa if the constituents are oppositely polarized. For a PT volume fraction of 27% it was possible to prepare a pyroelectric composite with vanishing piezoelectric activity or a piezoelectric composite with vanishing pyroelectric activity by poling the matrix ...

Single crystal PMN-0.33PT/epoxy 1-3 composites for ultrasonic transducer applications

Lead magnesium niobate-lead titanate 0.67Pb (Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-0.33PbTiO/sub 3/ (PMN-0.33PT, abbreviated as PMN-PT) single crystals were used to fabricate PMN-PT/epoxy 1-3 composites with different volume fractions of PMN-PT ranging from 0.4 to 0.8. The electromechanical properties of the 1-3 composites were determined by the resonance technique. Theoretical modeling of the 1-3 composites matched quite well with the measured material properties. It was demonstrated that the thickness electromechanical coupling coefficients of the composites could reach as high as 0.8. A 2.4 MHz plane ultrasonic transducer was fabricated using a PMN-PT/epoxy 1-3 composite with 0.37 volume fraction of PMN-PT. It shows a -6 dB bandwidth of /spl sim/61% and an insertion loss of -14 dB.

Piezoelectric coefficient of aluminum nitride and gallium nitride

The piezoelectric coefficient d33 of aluminum nitride (AlN) and gallium nitride (GaN) thin films grown on silicon substrates by molecular beam epitaxy have been measured using a laser interferometer. X-ray diffraction reveals that the AlN and GaN films consist mainly of crystals with a hexagonal wurtzite structure. In order to grow epitaxial GaN films, an AlN film was first deposited on silicon as the buffer layer, so the d33 measurement for GaN was actually performed on GaN/AlN/Si multilayer systems. The relative permittivity and electrical resistivity of each constituent layer of the film and the potential drop across each layer were determined as a function of frequency. The potential drops were then used to calculate the piezoelectric coefficient d33 of GaN. After correcting for substrate clamping, d33 of AlN and GaN were found to be (5.1±0.1) and (3.1±0.1) pm V−1, respectively.

Internal Strain Measurement by Fiber Bragg Grating Sensors in Textile Composites

This paper is concerned with a study of internal strain measurement by fiber Bragg grating sensors (FBGSs) embedded inside textile composites. First, on the basis of the measurement principle of FBGSs, the effects of transverse strain and temperature on the measurement results are discussed. A composite model comprising an optical fiber, coating and resin was developed to determine the measurement effectiveness of an embedded optical fiber sensor by analyzing the strain field of the system under a uniform thermal load. Factors influencing the measurement effectiveness were considered including the elastic modulus and Poissons ratio of coating material, tension stiffness ratio and the length of the host. Secondly, an experimental investigation was carried out to determine the reliability of FBGSs embedded in textile composites with different interfaces of fiber/coating and coating/resin and with two configurations, that is, single-ended and dual-ended. The measurement errors induced by the deviation of position and direction of the sensors were estimated. Finally, Volanthens low-coherence technique was applied to measure the internal strain distribution along the length of FBGSs which were embedded into textile composite.

Fundamentals and applications of optical fiber Bragg grating sensors to textile structural composites

In this paper, a review is given on the principles of optical fiber Bragg grating (FBG) sensors and their applications in textile structural composites (TSCs). As a class of novel all-fiber based components, FBG consists of a spatially periodic modulation in the refractive index along a short length of an optical fiber. FBG sensors are immune to electromagnetic interference, small in size, and easily embedded in a variety of composite materials without compromising the host structures. They can be mass-produced in low cost and are unique in their sensing strategies for absolute wavelength coding and multiplexing. The intrinsic wavelength division multiplexing and localized sensing abilities of FBGs are their most important advantages as they provide an effective means for monitoring physical parameters along a single fiber path. This ability to determine quantitatively internal distributions of physical parameters within a TSC integrated with such sensors has been demonstrated by a number of workers and will lead to a better understanding of the relationship between the structure and physical properties.

Pyroelectric and piezoelectric properties of lead titanate/polyvinylidene fluoride-trifluoroethylene 0-3 composites

Composites of calcium modified lead titanate powder dispersed in a polyvinylidene fluoride/trifluoroethylene copolymer matrix have been prepared using solvent casting followed by compression molding. Two groups of poled samples have been prepared: one with only the ceramic phase polarized and the other with both phases polarized. The observed permittivities of the unpoled composites are consistent with the predictions of the Bruggeman model. The changes in the pyroelectric and piezoelectric coefficients of the poled composites with increasing ceramic volume fraction can be described by modified linear mixture rules. When the ceramic and copolymer phases are poled in the same direction, the piezoelectric activities of the two phases partially cancel each other while the pyroelectric activities reinforce. Hence, at a certain ceramic volume fraction, the composite is pyroelectric but not piezoelectric.

Epitaxial growth and planar dielectric properties of compositionally graded (Ba1−xSrx)TiO3 thin films prepared by pulsed-laser deposition

We have heteroepitaxially deposited compositionally graded (Ba1−xSrx)TiO3 (BST) thin films with increasing x from 0.0 to 0.25 on (100)-oriented MgO substrates using pulsed-laser deposition. The compositional gradients along the depth in the graded films were characterized by Rutherford backscattering spectroscopy. By using surface interdigital electrodes, the planar dielectric response of epitaxial graded BST films was measured as a function of frequency, temperature, and dc applied voltage. At room temperature, the dielectric constant of the graded BST film was about 450 with a dielectric loss, tan δ of 0.007 at 100 kHz. Measurements varying the dc bias voltage showed hysteresis of the dielectric response and a tunability of 25% at an applied electric field of 80 kV/cm. The graded BST films undergo a diffuse phase transition with a broad and flat profile of the capacitance versus temperature. Such behavior of the dielectric response in graded BST films is attributed to the presence of the compositional a...

EFFECT OF POLING PROCEDURE ON THE PROPERTIES OF LEAD ZIRCONATE TITANATE/ VINYLIDENE FLUORIDE-TRIFLUOROETHYLENE COMPOSITES

Lead zirconate titanate/vinylidene fluoride-trifluoroethylene [PZT/P(VDF-TrFE)] 0–3 composites for pyroelectric sensor and piezoelectric transducer applications have been fabricated by incorporating PZT powder into a P(VDF-TrFE) copolymer matrix. The properties of these composites can be tailored to suit designated applications by varying the ceramic volume fraction and by using different poling procedures. As both phases in the composite are ferroelectric, and the piezoelectric coefficients of the ceramic and copolymer phases have opposite signs while the pyroelectric coefficients have like signs, special ways can be used to produce three groups of samples with (1) only the ceramic phase poled, (2) two phases poled in the same direction to achieve reinforced pyroelectric activity and reduced piezoelectric activity, and (3) two phases poled in opposite directions to obtain reinforced piezoelectric activity and reduced pyroelectric activity. In this work, original experimental results on the properties of ...