Categoría publications

Viscoelastic characterization of electrochemically prepared conducting polymer films by impedance analysis at Quartz Crystal. Study of the surface roughness effect on the effective values of the thickness and viscoelastic properties of the coating

Authors: A. Arnau, Y. Jiménez, R. Torres

Event: 5th edition of Acoustic Wave Based Sensors: Fundamentals, Concepts, New Applications. Physikzentrum Bad Honnef, Germany (2005)

 

An Electrochemical Quartz Crystal Microbalance is used for a continuous monitoring of the growth of the polymer poly(3,4-ethylenedioxy) thiopene tetrabutyammonium perchlorate (PEDT-TBAP), which is electro-polymerized in acetonitrile of a gold electrode of a 10 MHz quartz crystal resonator. The surface acoustic impedance of the resonator is analyzed starting from the electrical admittance continuously measured by means of a network analyzer. Changes in the acoustic impedance suggest that a mechanical resonance phenomenon occurs during the electro-deposition of the coating. In order to determine the origin of this effect, the evolution of the physical properties of the coating is analyzed. The extraction of the physical properties of the coating is carried out by means of an algorithm recently developed by the authors. Four sets of coating properties are obtained with the algorithm in four different conditions. In one of them an additional technique, ellipsometry, is used to have an alternative measurement of the coating thickness, no additional techniques are necessary in the rest of the cases. All the cases show a significant change in the viscoelastic properties of the coating during the time-interval of the suspected mechanical resonance. Changes in the surface roughness of the coating during the experiment are confirmed by scanning electron microscopy (SEM). The analysis of the surface roughness of the coating and its effect on the effective values of the coating properties, seem to indicate that the changes in the viscoelastic properties are due to the changes in the surface roughness of the coating, and that these changes are responsible for the mechanical resonance effect. This analysis is carried out by means of the same algorithm which is used, in this case, as a simulation tool.

 

 

A new roughness physical model for describing the mechanical impedance of a coated shear resonator immersed in liquids

Authors: A. Arnau, Y. Jiménez, R. Fernández

Event: 5th edition of Acoustic Wave Based Sensors: Fundamentals, Concepts, New Applications. Physikzentrum Bad Honnef, Germany (2005)

 

The models that describe the surface acoustic impedance at the surface of a coated AT quartz sensor should include the effects of the roughness, both the roughness of the crystal surface and the roughness of the coating. When these effects  are not kept in mind, the values of the physical magnitudes extracted starting from the measurable characteristic parameters of the sensor, usually the complete admittance spectrum around resonance or the frequency and motional resistance shifts, can be erroneous. In this work a model that facilitates the understanding of the physical phenomena that happen when a rough surface oscillates in contact with a fluid is presented. This model characterizes the roughness like a surface of spheroids of variable magnitudes. The solution of the Navier-Stokes equation applied to this allows the obtaining of the acoustic impedance of the rough surface in contact with the liquid, and even to extend the pattern to those cases in which the rough layer has viscoelastic properties.

Systematic error analysis in the determination of physical parameters of the coating in Quartz Crystal Resonator sensors

Authors: Y. Jiménez, A. Arnau, T. Sogorb, M. Otero, E. Calvo

Event: 4th Acoustic Wave Sensor Workshop. Salbris, France (2003)

 

A new algorithmic strategy which permits an unambiguous extraction of the effective surface mass density, loss tangent and characteristic impedance of the first coating layer in AT cut quartz crystal resonators (QCR) is introduced. A preliminary analysis of the propagating error in the determination of these effective physical parameters, due to practical errors in the admittance measuring system or due to a lack of precision in the admittance model for describing the real admittance of the sensor, is presented as well. This analysis makes clear the necessity of a standard indicating the specifications of the measuring system including the cell interface for an appropriate sensor characterization.

Circuit for continuous motional series resonant frequency and motional resistance monitoring of quartz crystal resonators by parallel capacitance compensation

Authors: A. Arnau, T. Sogorb, Y. Jiménez

Event: Acusticum, Sevilla, Spain (2002)

 

A deep analysis of the problem associated to interface circuits for quartz-crystal-microbalance sensors reveals that the so-called static capacitance of the sensor is one of the elements which make the use of oscillators more critical for sensor applications. A phase-locked-loop based circuit designed for compensating the parallel capacitance in quartz crystal resonator sensors is presented. This circuit permits the calibration of the external circuitry to the sensor and provides a continuous measurement of the motional series resonant frequency and motional resistance. An extension and automation of the proposed system for multiple sensor characterization is introduced. Experimental results are also shown.

Interface circuit for characterization Quartz Crystal Resonators based on parallel capacitance compensation

Authors: A. Arnau, T. Sogorb, Y. Jiménez

Event: QCM 2002: Chemical, Biological and Pharmaceutical applications of acoustic sensor technologies. Brighton, UK (2002)

 

A sensor-controlled oscillator tracks the frequency for a certain sensor phase, therefore, changes in the motional resistance or in the parallel capacitance produce erroneous changes in the oscillating frequency. These erroneous changes would not take place if the parallel capacitance were precisely compensated and the phase of the sensor for oscillating condition were 0º. This reveals that the so-called static capacitance of the sensor is one of the elements which make the use of oscillators more critical for sensors applications. A phase-locked-loop based circuit designed for compensating the parallel capacitance in quartz crystal resonator sensors is presented. The results reported prove the reliability of the proposed system for quartz sensors. This circuit compensates the parallel capacitance and permits the calibration of the external circuitry to the sensor, in addition provides a continuous measurement of the motional series resonant frequency and motional resistance. Preliminary experimental results using this circuit as a biosensor are also shown.

 

Circuit for continuous motional series resonant frequency and motional resistance monitoring of Quartz Crystal Resonator Sensors in Fluid Media by parallel capacitance compensation

Authors: A. Arnau, T. Sogorb, P. Ventura, Y. Jiménez

Event: 3th Acoustic Wave Sensor Workshop, Taos, New Mexico,USA (2001)

 

For a complete characterization of quartz crystal resonator sensors, it is necessary the measurement of their electrical conductance (G) and susceptance (B) in a certain bandwidth around resonance. This is usually done with impedance analyzers and is particularly useful in liquid environment, where the resonator loss is high and the capacitances in parallel with the motional impedance of the sensor have a great effect on its electrical admittance. This complete characterization permits a reasonably coherent fit of a BVD model to the measured admittance, since specific points of the measured G and B can be associated with BVD parameters without incurring significant error in most cases. However, impedance analyzers are very expensive and not always suitable for in situ measurements, principally for a simultaneous multi-sensor characterization. Alternative characterization of the resonator around resonance is also used by measuring the voltage transfer function of a simple circuit in which the resonator is included. In these cases, a mathematical fit of a BVD model, in which the motional capacitance is normally considered constant and equal to that of the unperturbed sensor, is made for determining the remaining parameters. This mathematical fit normally produces, in these cases, a high correlation between parallel capacitance and motional inductance. Actually, the effect of parallel capacitances is negligible near the series resonant frequency. Thus, special oscillators have been designed to operate near the true motional series resonant frequency and, additionally, to provide the measurement of the motional resistance. However, the higher the resonator loss is, the higher the frequency error between the oscillation frequency and the true motional series resonant frequency is, and complicated adjustments have to be made to minimize this error.

A phase locked loop based circuit has been specifically designed for compensating parallel capacitance effects in quartz resonator sensors. This circuit provides a continuous measurement of the true motional series resonant frequency and motional resistance. The system permits the calibration of the effective capacitive compensation and gives an indirect measurement of this capacitance. Additionally, the circuit maintains one side of the resonator grounded, so that the parasitic capacitances due to the fluid can be more easily controlled.