Qingqi Zhang

Nanocomposite ultrasonic hydrophones

Abstract Needle-type hydrophones for the calibration of medical ultrasonic transducers have been fabricated using lead zirconate titanate (PZT)/poly(vinylidene fluoride-trifluoroethylene) (PZT/P(VDF-TrFE)) 0–3 nanocomposite films. The 0–3 nanocomposite film has been prepared by incorporating nanosized PZT powder (derived from a sol–gel process) in a P(VDF-TrFE) matrix. Since the piezoelectric coefficients of PZT and P(VDF-TrFE) have opposite signs, the PZT and P(VDF-TrFE) phases in the nanocomposites have been poled in opposite directions in order to give enhanced piezoelectric activity. A nanocomposite hydrophone with 0.2 volume fraction of PZT has been characterized and found to be more sensitive than a P(VDF-TrFE) hydrophone of similar structure.

Dielectric permittivity of PCLT/PVDF-TRFE nanocomposites

The use of ferroelectric polymer films as pyroelectric sensors and ultrasonic transducers has attracted considerable interest. Polymer-based 0-3 nanocomposites, consisting of nanocrystalline calcium and lanthanum modified lead titanate (PCLT) powder embedded in a vinylidene fluoride-trifluoroethylene (polyvinylidenefluoride (PVDF)-trifluoroethylene (TRFE)) copolymer matrix, also have shown good potential in pyroelectric and piezoelectric applications. The dielectric permittivity and loss in these composites are important parameters characterizing their performance. In this study, the relative permittivity and loss of PCLT/PVDF-TRFE nanocomposites with various volume fractions of ceramic have been measured as function of frequency and temperature. The copolymer and nanocomposites exhibit a dielectric relaxation at the ferroelectric-to-paraelectric phase transition and another relaxation near room temperature (at /spl sim/1 MHz). The influence of the room temperature relaxation on transducer performance is discussed.

Dielectric and pyroelectric properties of P(VDF-TrFE) and PCLT–P(VDF-TrFE) 0–3 nanocomposite films

Abstract Nanocrystalline calcium and lanthanum modified lead titanate (PCLT) powder prepared by a sol–gel process was incorporated into a polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] copolymer matrix to form PCLT–P(VDF-TrFE) nanocomposite thin films with 0.11 volume fraction of ceramic. The relative permittivity and pyroelectric coefficient of the P(VDF-TrFE) copolymer and nanocomposite films were measured as functions of the poling electric field. After poling under the same conditions, the nanocomposite film was found to have a higher pyroelectric coefficient (by ∼35%) and figures of merit than those of the P(VDF-TrFE) film of a similar thickness.

Dielectric and pyroelectric properties of PCaT/P(VDF–TrFE) 0–3 composite thin films

Abstract Powder of calcium modified lead titanate [(Pb0.8Ca0.2)TiO3 or PCaT] with particle sizes nm prepared by a sol–gel process has been embedded in a vinylidene fluoride–trifluoroethylene copolymer [P(VDF–TrFE)] matrix to form 0–3 composites. PCaT/P(VDF–TrFE) films and pyroelectric sensors have been fabricated by spin-coating. The dielectric and pyroelectric properties of the composite film have been investigated. The current and voltage responsivities as well as the voltage noise have been measured, resulting in a specific detectivity D * =1.2×10 7 cm Hz 1/2 / W . The results show that this kind of composite material has a good potential for pyroelectric sensor applications.

Integrated pyroelectric array based on PCLT/P(VDF-TrFE) composite

Abstract Calcium and lanthanum modified lead titanate (PCLT) powder was prepared by a sol–gel process followed by annealing at 850°C. The PCLT powder was dispersed in vinylidene fluoride-trifluoroethylene copolymer [P(VDF-TrFE)] to form a composite containing 12 vol.% of ceramic powder. A film of thickness 12 μm was prepared by spin-coating four layers of the composite on a glass substrate. The film was poled and then removed from the substrate. It was then bonded to a silicon chip with readout electronic circuitry to form an 8×1 integrated linear pyroelectric array. The voltage responsivity, voltage noise, and specific detectivity of the sensors in the array were measured as functions of frequency and found to agree well with the values calculated from a one-dimensional heat diffusion model.

PCLT/P(VDF-TrFE) nanocomposite pyroelectric sensors

Thin composite films consisting of 12 vol% of nanosized lanthanum and calcium-modified lead titanate (PCLT) powder embedded in a vinylidene fluoride-trifluoroethylene copolymer [P(VDF-TrFE)] matrix were deposited on silicon (Si) substrates to form pyroelectric sensors with three different configurations. The influences of a thermal buffer layer and back etching of the silicon substrate on the current and voltage responsivities of the sensors were investigated theoretically and experimentally. The specific detectivity of the sensors was also calculated from the measured voltage responsivity and noise.

Effects of dispersing agent on the microstructure and properties of PCLT/P(VDF-TrFE) 0–3 nanocomposites

Abstract Powder agglomeration is a major problem in the fabrication of calcium and lanthanum modified lead titanate/vinylidene fluoride-trifluoroethylene [PCLT/P(VDF-TrFE)] 0–3 nanocomposites. To ensure uniform dispersion of the nanocrystalline PCLT powder in the P(VDF-TrFE) matrix, the powder is often dispersed in a liquid (e.g. methyl-ethyl ketone) placed in a high power ultrasonic bath. In this study, PCLT nanocrystalline powder derived from a sol-gel process was dispersed in various liquids placed in an ultrasonic bath for different periods of time tu. The size and specific surface area of the particles were measured as functions of tu. By choosing a good dispersing agent and an appropriate t, PCLT/P(VDF-TrFE) nanocomposites with a uniform dispersion of nanosized particles can be prepared. The dielectric, piezoelectric and pyroelectric properties of these composites have been investigated.

PROPERTIES OF PCLT/P(VDF-TRFE) 0-3 NANOCOMPOSITES

Abstract Calcium and lanthanum modified lead titanate (PCLT) powder with size in the nanometer range was prepared using a sol-gel technique. PCLT powder annealed at 850°C was incorporated into a polyvinylidene fluoride/trifluoroethylene P(VDF-TrFE) copolymer matrix to form 0.15 mm thick films of 0–3 nanocomposites with ceramic volume fraction (φ) ranging from 0.05 to 0.33. The dielectric and pyroelectric properties of the composites were measured as functions of temperature. The observed permittivity and dielectric loss were found to agree with the predictions of the Bruggeman model. When the ceramic and copolymer phases in these nanocomposites are poled in the same direction, the pyroelectric activities of the two phases reinforce while the piezoelectric activities partially cancel, thereby minimizing the vibration-induced electrical noise in pyroelectric current measurements.

Structure of nanocrystalline powder and thin films of lead lanthanum titanate prepared by the sol-gel process

Ultrafine powders and thin films of lead lanthanum titanate (PLT) were prepared by the sol-gel process. The structural variation of the powder with increasing annealing temperature was monitored by differential thermal analysis, thermogravimetric analysis and X-ray diffraction. The lowest temperature for the formation of nanocrystals in the powder was found to be about 500/spl deg/C. Crystallites of various sizes were obtained by controlling the annealing temperature and time. Thin films of PLT were fabricated by spin casting the sol-gel solution onto Si(111) substrates and the microstructure was analyzed using a scanning electron microscope. By controlling the concentration of the precursor and the processing temperature, PLT thin films with uniform crystallite size could be prepared.

Structural characteristics of 0–3 ceramic/polymer composites

Abstract Calcium and lanthanum modified lead titanate (PCLT) powder sintered at 850°C has been prepared by using a sol-gel method and ferroelectric vinylidene fluoride trifluoroethylene [P(VDF-TRFE)] copolymer film has been prepared by the hot-press method. 0–3 PCLT/P(VDF-TrFE) composite films with fine PCLT powder dispersed uniformly in P(VDF-TrFE) matrix have been fabricated by using solvent-casting method. XRD and IR analysis have been used to determine and compare the structures of the single-phase materials with the composite. The XRD peaks and characteristic vibration bands of both single-phase ferroelectric PCLT and P(VDF-TrFE) can still be found which shows that the composite is composed of two ferroelectric phases.