Papers by Author: C.J. Dias

Abstract: Chemocapacitors are polymer coated Interdigital electrodes (IDE) where the transducer mechanism relies on the permittivity changes and swelling of the coating polymer (sensitive layer), usually in a form of a thin film, when exposed to an volatile organic compound (VOC). Despite several synthetic and natural polymers have already been produced by electrospinning, there have been fewer studies on rubbery polymers with low glass transition temperature (e.g. Poly(dimethyl siloxane) – PDMS). In this work we produce PDMS:PMMA 3:1 nanofiber (NF) layers by electrospinnig to be used as chemical sensitive layers on IDE chemocapacitors. The results show an enhanced response from the sensors with NFs with respect with sensors prepared with the same sensitive layers in the form of a homogeneous film.

Abstract: The influence of humidity content on the electrical and dielectric properties of a composite made from recycled TetraPak® containers and granulated cork was studied. The material components have been dried before preparation and after the composite was conditioned by keeping the samples in a dry environment (desiccator) or in an oven at high temperature (70°C in air). The differences observed in electrical properties (investigated by isothermal charge and discharge current measurement) and dielectric properties (measured using dielectric relaxation spectroscopy) show that the thermal treatment at high temperature is more efficient on removing water and slows down the re-absorption rate.

Abstract: Information about the spatial distribution of the electric field can be obtained by measuring the electrode impedance as a function of the diameter of the electrolyte surrounding the electrode. The non-uniform distribution of the electric field around the electrode is supported by the variation of the geometry factor (GF) with the electrical conductivity and geometry of the volume conductor. A comparison of the values obtained for the GF from experimental data, from model calculations and simulations help to understand the non-uniform distribution of the electric field. The GF calculated from four-electrode-measurements is significantly higher. GF should be used with caution in calculations of the deep brain stimulation (DBS) electrode impedance.

Abstract: Most piezoelectric gravimetric biosensors are based on quartz crystal microbalances or surface acoustic wave devices. In this paper we describe a polymer film system, made of piezoelectric polyvinylidene difluoride (PVDF) and the Immobilon-P membrane (porous PVDF), for biosensing applications. In operation a film is accommodated in a flow cell and connected to an electronic circuit, constituting an oscillatory resonant device; the output signal is its resonance frequency. This device successfully detected the binding between bovine IgG and bovine anti-IgG. Work on the modelling of this film system is presented. Two different approaches are being considered: the finite element method (FEM) and the ABCD matrices. Results comparing theoretical and experimental data are presented.

Abstract: The isothermal charging current and the isothermal discharging current in low mobility materials are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms in dielectric materials. We demonstrate that besides a polarization current there is a current related to charge injection or extraction at the metal-dielectric interface and a reverse current related to the charge trapped into the shallow superficial or near superficial states of the dielectric and which can move at the interface in the opposite way that occurring during injection. Two important parameters can be determined (i) the highest value of the relaxation time for the polarization mechanisms which are involved into the transient current and (ii) the height of the potential barrier W0 at the metal-dielectric interface. The experimental data demonstrate that there is no threshold field for electron injection/extraction at a metal-dielectric interface.

Abstract: For the characterization of the new materials and for a better understanding of the connection between structure and properties it is necessary to use more and more sensible methods to study molecular movement at nanometric scale. This paper presents the experimental basis for a new electrical method to study the fine molecular movements at nanometric scale in dielectric materials. The method will be applied for polar and non-polar materials characterization. Traditionally, the electrical methods used to study the molecular movements are based on the movements of the dipoles that are parts of the molecules. We have proposed recently a combined protocol to analyze charge injection/extraction, transport, trapping and detrapping in low mobility materials. The experimental results demonstrate that the method can be used to obtain a complex thermogram which contains information about all molecular movements, even at nanoscopic level. Actually during the charging process we are decorating the structure with space charge and during the subsequent heating we are observing an apparent peak and the genuine peaks that are related to charge de-trapping determined by the molecular movement. The method is very sensitive, very selective and allows to determinate the parameters for local and collective molecular movements, including the temperature dependence of the activation energy and the relaxation time.

Abstract: An interdigital chemocapacitor (IDC) sensor array has been developed to discriminate organic vapour samples with pure organics and mixtures. Sensors were coated with diverse polymers (PEUT, PDMS and modified PDMS). Dedicated signal conditioning circuits were developed with a reduced S/N ratio enabling measurement of capacitance changes as low as 0.07 fF. A new sampling technique was also developed to improve isobaric sampling conditions on the sensors chamber. Linear techniques such as Principal Component Analysis (PCA) for discrimination between the different samples were sucessfully applied. As far as we know, this is the first time that an array of interdigital chemocapacitors is used for organic vapour discrimination.

Abstract: 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.

Abstract: Ultrasonic motors are of great interest since their operation is not affected by a magnetic field. On this work the development of a prototype of a smart ultrasonic motor and its characteristics will be presented. The stator of the motor is formed by a cantilever made of PZT ceramic plate sandwiched between two unidirectional CFRP composite sheets. The composite sheets are oriented to 45º and -45º in the longitudinal direction. By application of a sinusoidal electric field to the piezoelectric ceramic plate, it vibrates. The anisotropy of the CFRP composite sheets induces an expansion/torsion movement. This effect is transformed into rotation by friction. The study of adequate voltages and frequencies of the applied electric field allows the optimization of the rotating motor performance.