Yuen-wah Wong

Reactive pulsed laser deposition of CNx films

Carbon nitride (CNx) films were prepared by reactive pulsed laser deposition at nitrogen partial pressure PN2varying from 0 to 300 mTorr. It is found that the atomic fraction of nitrogen f in the films first increases with increasing PN2, reaches a maximum of 0.32 at PN2=100 mTorr, and then decreases to a saturated value of 0.26 at PN2≳200 mTorr. Because of the absence of energetic particles in reactive pulsed laser deposition, the limited nitrogen content cannot be attributed to preferential sputtering of nitrogen that is generally observed in particle‐assisted deposition of CNx films. Infrared absorption experiments show the existence of C≡N bonds and graphitic sp2 bonds. The sp2 bonds become IR active because of symmetry breaking of graphitic rings as a consequence of nitrogen incorporation. CNx films deposited at low PN2 (e.g., 5 mTorr) are more graphitic than the diamondlike pure carbon sample deposited at PN2=0, so have a slightly narrower electron band gap Eopt and a significantly higher room‐tempe...

Dielectric and pyroelectric properties of lead zirconate titanate/polyurethane composites

0-3 composite ranging between 0 and 3, of ferroelectric ceramic lead zirconate titanate (PZT) and thermoplastic elastomer polyurethane (PU) were fabricated. The pyroelectric and dielectric properties of the hot-pressed thin film samples of various PZT volume fractions were measured. The experimental dielectric permittivities and losses agreed reasonably well with the Bruggeman model. The room temperature pyroelectric coefficients of the composites were found to increase linearly with PZT volume fraction and substantially larger than expected. For example, for a composite with 30% PZT, its pyroelectric coefficient is about 90μC∕m2K at room temperature, which is more than tenfold of a PZT∕PVDF composite of the same ceramic volume fraction. We propose a model in which the electrical conductivity of the composite system is taken into consideration to explain the linear relationship and the extraordinarily large pyroelectric coefficients obtained.

Formation of core/shell structured cobalt/carbon nanoparticles by pulsed laser ablation in toluene

Magnetic cobalt nanoparticles encapsulated in shells of layered structure have been produced by the technique of pulsed laser ablation in toluene. The morphology, microstructure, and magnetic properties of the prepared nanoparticles were characterized by electron microscopy, micro-Raman spectroscopy, and superconducting quantum interference device magnetometry, respectively. The results indicated that the cobalt nanoparticles fabricated are noncrystalline but coated with the graphitic carbon layers. It is believed that the formation of these carbon layers well-protect the cobalt nanoparticles to be oxidized thus maintaining the superparamagnetic property. This is an important feature that makes the cobalt nanoparticles a useful material for medical and many other magnetic based applications.

Effect of interfacial charge on polarization switching of lead zirconate titanate particles in lead zirconate titanate/polyurethane composites

A simple model has been developed to include the effect of accumulated charge at the matrix–inclusion interface in ferroelectric 0–3 composites on the polarization reversal characteristics. This is used to illustrate a recent set of experimental results on initially polarized lead zirconate titanate (PZT)/polyurethane (PU) composites subjected to an increasing electric field in the reversed direction. Estimations based on previous models [e.g., T. Furukawa, K. Fujino, and E. Fukada, Jpn. J. Appl. Phys. 15, 2119 (1976)] suggest that the local electric field in the PZT particles at switching was an order smaller than the coercive field of PZT. By introducing the effect of interfacial charge and assuming that initially the charge has a magnitude which balances the depolarization field in the polarized PZT inclusions, theoretical calculation based on this image shows that the phenomenon of PZT switching at low fields can be explained as a manifestation of the depletion of interfacial charge which can no longe...

Enhancement of piezoelectric and pyroelectric properties of composite films using polymer electrolyte matrix

Composite films consisting of lead zirconate titanate (PZT) inclusions dispersed in a matrix of polymer electrolyte the polyethylene oxide were fabricated. Their piezoelectric coefficients d33 and pyroelectric coefficients p are increased almost proportional to the volume fraction of the ferroelectric ceramic phase. For 34% PZT composite, d33 and p are 170pC∕N and 120μC∕m2K, respectively, they are much larger than PZT/polyvinylidene fluoride-trifluoroethylene composite with similar ceramics content. It is believed that the enhancement of the measured coefficients is due to the relatively high electrical conductivity of polymer electrolyte matrix. This is confirmed by theoretical models which consider the conductivity of matrix.Composite films consisting of lead zirconate titanate (PZT) inclusions dispersed in a matrix of polymer electrolyte the polyethylene oxide were fabricated. Their piezoelectric coefficients d33 and pyroelectric coefficients p are increased almost proportional to the volume fraction of the ferroelectric ceramic phase. For 34% PZT composite, d33 and p are 170pC∕N and 120μC∕m2K, respectively, they are much larger than PZT/polyvinylidene fluoride-trifluoroethylene composite with similar ceramics content. It is believed that the enhancement of the measured coefficients is due to the relatively high electrical conductivity of polymer electrolyte matrix. This is confirmed by theoretical models which consider the conductivity of matrix.

Electrostriction of lead zirconate titanate/polyurethane composites

Electrostriction of a ferroelectric inclusion/nonferroelectric matrix composite system was studied. The samples were prepared by blending the lead zirconate titanate (PZT) particles with the thermoplastic polyurethane through extrusion and subsequently by hot pressing. Quasistatic cyclic electric fields were applied across the samples while strains and currents were monitored simultaneously. It was found that the electrostriction of the composites depended on the applied electric field in a hysteretic manner. In particular at the high-field regime, the samples exhibited a reversal in the electrostrictive strain. This switching effect occurred at a critical field which was inversely proportional to the PZT content. An associated increase in the displacement current with the critical field was also observed. It indicates that the switching in strain of the composites was mainly due to the flipping of the PZT dipoles in the nonferroelectric polymer matrix. A model was developed for describing the electrostri...

Magnetoelectric effect of polymer electrolyte composites with Terfenol-D and lead zirconate titanate inclusions

The magnetoelectric effects of three-phase composites in 0-0-3 connectivity were investigated. The composites consist of particulate Terfenol-D and lead zirconate titanate blended in different polymer matrices. The magnetoelectric coefficient αq, which is the charge density change in response to a change in the applied magnetic field, of the samples was measured under short circuit condition. The results show that the αq of the samples with an electrolytic polymer matrix is larger than that of the samples with an insulating matrix, while samples with an ion-doped electrolytic polymer matrix exhibit the largest αq. These results conform with the expectation that higher matrix conductivity has an effect of enhancing the magnetoelectric signals.

Temperature dependence of magnetoresistivity of cobalt-polytetrafluoroethylene granular composite films

Heterogeneous granular films consisting of ferromagnetic cobalt particles embedded in a polytetrafluoroethylene polymer matrix were fabricated by the pulsed laser deposition technique carried out with a Nd-doped yttrium aluminum garnet laser at 355nm. The samples exhibit a magnetoresistance (MR) change of 4% (at 8kOe) at room temperature. An enhancement of MR to a value of 6% was observed at 20K which is attributed to the increasing order of magnetic moments in the ferromagnetic state at low temperature. Significant loop opening of the MR profile at low temperature and the temperature dependence of MR reveal the interplay of superparamagnetic relaxation behavior and spin-dependent electron tunneling.

MODELING OF THE MAGNETOELECTRIC EFFECT OF THREE-PHASE MULTIFERROIC PARTICULATE COMPOSITES

ABSTRACT Using the effective medium approximation, we set up a model for the investigation of 3-phase magnetoelectric composites of 0-0-3 connectivity. The two particulate phases are magnetostrictive particles and piezoelectric particles, which are considered to be spherical in shape and dispersed in a continuous polymer matrix of non-negligible electrical conductivity. The electric, magnetic and elastic properties of the constituents are assumed to be isotropic. We calculate the magnetoelectric voltage/charge coefficient of the composites as a function of bias magnetic field and compare the theoretical results with measurements on Terfenol-D/PZT/LiClO4-PEO and Terfenol-D/PZT/PEO 0-0-3 composite systems. Reasonably good agreement between model predictions and experimental results is obtained.

Mechanism of bending electrostriction in thermoplastic polyurethane

The mechanism of bending electrostriction in polyurethane films is discussed and elucidated through a numerical calculation. The simulations are carried out on a model in which charge carriers are assumed to be electrons injected from the cathode by the Schottky effect, and the positive charges are immobile. Under a dc field, our simulation results show that the electrons go out of the anode, leaving behind a large quantity of positive charge around the anode. As a result, the electric field near the anode eventually becomes much larger than that near the cathode. The asymmetrical electric field distribution leads to an asymmetrical stress distribution through the electrostriction effect and thus to bending of the polyurethane film under the application of a dc electric field. The results can also explain the gradual change in bending direction after reversing the polarity of the electric field.