Caracterização e modelagem das propriedades ópticas de sensores de dióxido de vanádio.

Abstract

I n this work the characterization and modelling of the optical properties of a vanadium dioxide sensor ( V 0 2 ) are studied. These sensors present a crystalographic transformation induced by temperature and exhibit hysteresis. For the modelling of the optical properties in the phase plan i t was necessary to adapt the classic Preisach model and the adiabatic limit approximation model. In the Preisach model it was necessary to redefine the elementary hysteresis operator, to choose a distribution function and to adopt a new interpretation of the Preisach triangle. In the adiabatic limit approximation model the expression that represents the adiabatic equilibrium limits was also modified. An experimental platform was developed to submit the film of V02 to a controlled temperature variation in order to allow the characterization of its optical properties. The wiping-out and congruency properties and the accommodation phenomenon were observed experimentally. The temperature was controlled by using a Peltier module. The linearization around an operation point made possible the Peltier module to be represented by a second order linear system with temperature dependent parameters. The model of the Peltier module was identified for nine operating points and a scheduling PID temperature controller was designed by the zero-pole cancellation technique. The distribution function used in the modified Preisach model was determined from the first-order descending transition curves by using a genetic algorithm. The parameters of the modified adiabatic limit approximation model were obtained directly from the experimental data of the major hysteresis loop. The models obtained experimentally were used to generate hysteresis loops that were compared with the experimental data. The obtained results are considered satisfactory. However, the developed models reproduce approximately the hysteresis phenomenon of the film of VQ2. The discrepancies between the calculated and the experimental data, mainly for the congruency property, can be attributed to limitations in the numeric implementation of the model, to the film temperature control and the experimental platform limitations.