Paulo Inácio

Recent advances in ceramic-polymer composite electrets

Recent work on ceramic-polymer composites for piezoelectric and pyroelectric applications is presented with special regard to the production and characterisation of new composite materials as well as two new applications of these composite materials. One of these composites is made using ceramic powders obtained using the sol-gel technique. This technique allows a better control of the stoichiometry as well as a lower temperature of crystallisation as compared with the conventional mixed oxides route. A better control of powder morphology also produces ceramic grains in the submicron range enabling the production of nanocomposites with electroactive properties. A second type of composite is reported using high temperature polymer PEEK, thus extending the temperature range of common electroactive composite materials. Finally the use of these materials is demonstrated in two piezoelectric applications, an angular acceleration accelerometer and an acoustic emission sensor.

Development of a Biosensor Based on a Piezoelectric Film

Most gravimetric biosensors use thin piezoelectric quartz crystals (quartz crystal microbalance, QCM), either as resonating crystals, or as bulk/surface acoustic wave (SAW) devices. This paper illustrates a related technique where a polymer film system made of two different types of the polymer polyvinylidene difluoride (PVDF) are used: one has piezoelectric properties and the other has a high protein binding capacity. Acoustic waves are launched in this polymer film to produce an oscillatory resonant device. This device is being tested in protein detection. Current work and preliminary results are presented.

Development of a biosensor based on a piezoelectric film

Most gravimetric biosensors use thin piezoelectric quartz crystals (quartz crystal microbalance, QCM), either as resonating crystals, or as bulk/surface acoustic wave (SAW) devices. A system is described in which acoustic waves are launched in very thin (25 microns) polymer films to produce an oscillatory resonant device. A theoretical equation for this system is almost equal to the Sauerbrey equation used in the QCM method. A system using a PVDF film was tested in protein detection. Results showed that the PVDF film can act has a microbalance. Currently, a new system using a PVDF/Immobilon-P/PVDF film, to be used in protein detection, is being tested. Results for the PVDF system and current work with the new system are presented.

Acoustic emission sensors based on ferroelectric composites

The authors develop an acoustic emission sensor using a Ca-modified PbTiO/sub 3/ ceramic-PVDF copolymer composite as piezoelectric element. The sensors experimental impedance and frequency bandwidth are determined and it is concluded that the device can operate over a large frequency range.

A Piezoelectric Pseudo-Composite Polymer Film for the Detection of Proteins

Most piezoelectric biosensors use thin piezoelectric crystals, either as longitudinal acoustic waves devices (e.g. quartz crystal microbalances, QCM), or as bulk/surface acoustic waves (SAW) devices. A novel system is described in which acoustic waves are launched in very thin polymer films to produce an oscillatory resonant device for the detection of proteins. The device uses a polymeric material in two distinct forms: common PVDF (which has good piezoelectric properties) and the Immobilon-P membrane (a porous type of PVDF with a high protein binding capacity). Current work with this system is presented. Introduction Thin quartz crystals for the detection of small additions of bound mass to its surface have been used over a period of more than 40 years [1]. This paper illustrates a related technique where a film system made of a piezoelectric polymer (PVDF Solef, from Solvay) and a porous membrane (Immobilon-P, a special type of porous PVDF, from Millipore Co.), with a high protein binding capacity, are used to produce an oscillatory resonant device. This device is to be used in the detection of proteins. Its response was tested with the protein bovine serum albumin (BSA). Also, we present some results for the electromechanical characterization of the film systems. Operating Principle Fig.1 shows a schematic of a generic piezoelectric film clamped between two pairs of electrodes: one set of electrodes acts as the transmitter and the other as the receiver. In the central part of this film the electrodes have been etched off. When a voltage signal is applied to the transmitter an acoustic wave is launched across the film, due to the piezoelectric effect, whose velocity is given by v=( s11) -1/2 [2], where s E 11 is the elastic compliance and is the density of the film (for PVDF: s E 11=2.7x10 -10 m 2 /N and =1,8 g/cm ). The film will have a resonance frequency, which depends on its length, L (between the clamping electrodes), and its properties (elastic compliance and density), which is given by: fo=(1/2L) ( s E 11) -1/2 (1) Fig. 1. A piezoelectric film clamped between two pairs of electrodes. The deposition of a small mass in the surface of the film has the effect of changing the film density. It can be shown [3] that there will be a change in the resonance frequency given by: * corresponding author: dias@dcm.fct.unl.pt Materials Science Forum Online: 2004-05-15 ISSN: 1662-9752, Vols. 455-456, pp 411-414 doi:10.4028/www.scientific.net/MSF.455-456.411

Polarization and hysteresis in low resistivity ferroelectric composites

Abstract A compensating technique suitable for measuring the polarization and hysteresis in ferroelectric materials with low resistivity is reported. In this method, three different poling voltages waveforms are successively applied to a sample while the current i, is measured. These are: a sinusoidal, a positively rectified and a negatively rectified sinusoidal voltages. The polarization hysteresis loop is then computed through the integration of the difference between the current obtained during the full sinusoidal voltage loop (i.e. it ) and the sum of the other two currents (i + + i −). In this paper we use this method to determine the remanent polarization and coercive field in a ferroelectric composite of calcium modified lead titanate (PTCa), of a low resistivity formulation, and a copolymer of polyvinylidene fluoride (i.e. P(VDF/TrFE)) as a function of the temperature and field.

Electromechanical Characterization of a Biosensitive and Electroactive Polymer Membrane

Most piezoelectric biosensors normally use crystals, as in the quartz crystal microbalances or surface acoustic wave devices. A new system is described in which piezoelectric polymer films (made of polyvinylidene fluoride, PVDF, and Immobilon, a porous type of PVDF) are used to produce a mass sensitive oscillatory resonant device. Recent results demonstrated that this system successfully detects the binding between bovine IgG (immunoglobulin G) and anti bovine IgG. In order to improve the performance of the biosensor the electromechanical behaviour of such film-membrane is being studied. Miniaturization is also a desirable trend which will be pursued. Thus reports on the dependence on the electrical, mechanical and biological properties with the size of the film will be presented.

Discrimination between space charge and dipolar contributions in ferroelectric polymers

The final thermally stimulated discharge current method allows a better selection of the experimental conditions for sample polarization. By decreasing the ratio between the charging time and the discharging time, the apparent peak is of the same order of magnitude as the genuine peaks and there is only a partial overlap between then. Two peaks have been identified for polyamide 11, one associated with the glass transition around 60 °C and the second associated with the Curie transition around 96 °C.

The study of the molecular movements in the range of glass transition by the final thermally stimulated discharge current technique

The electrical methods used to study the molecular movements are based on the movement of the dipoles under DC or AC electric field. We have proposed recently a combined measuring protocol to analyze charge injection/extraction, transport, trapping and de-trapping in polar or non-polar dielectric materials. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 572 K. A strong relaxation was observed around 402 K and a very weak relaxation around 345 K. This is the β relaxation which is quite complex. As concern the behavior at high temperatures, above the β relaxation, a high peak was observed that shifts continuously to higher temperatures as the charging temperature and/or the charging field increase. The maximum current of the peak increases and the temperature corresponding to the maximum current increases as the charging temperature and/or the charging field increase, given a direct observation of the so called cross-over effect related to current decay for sample charged at high fields and/or high temperatures.

On the width of the thermally stimulated discharge current peak

The Thermally Stimulated Discharge Current (TSDC) method is a very sensitive technique to analyze the movement of dipoles and of space charge (SC). To increase the selectivity of the method we have proposed a variant of the TSDC method, namely the final thermally stimulated discharge current (FTSDC) technique. The experimental conditions can be selected so that the FTSDC is mainly determined by SC de-trapping. The aim of this paper is to analyze if the elementary peaks obtained by using the two methods can be assumed as elementary Debye peaks and to determine the best experimental conditions to obtain a narrow experimental peak which means to increase the selectivity of the method.