Sistema de aquecimento controlado para caracterização elétrica de sólidos

Abstract

Impedance Spectroscopy, IE, is an important tool for characterizing electrical properties and it is used in the research and development of new materials, as well as in various sectors of the industry related to manufacturing and quality control. An important factor in the application of the technique is the temperature control in the analysed samples, as well as measuring the linearity in the heating rate inside the samples, since the thermal sensitivity of the materials generally significantly influences the results of the electrical properties. The present work presents the construction of a vertical heating system controlled and monitored by computer, aiming at stability and thermal control, for electrical evaluation by IE of different solid materials. The system is basically composed of three sections: data acquisition and control software, temperature and power control system, and the oven itself. An important factor in the construction of the system was the effort to reduce the costs involved, in order to allow access to a technology considered costly within the national scenario. The built heating system allows the analysis of ambient temperature up to 500 °C, with these limits being defined by the specific needs of the materials to be studied by the research group. A total of 6 electrodes installed on the top of the oven allow the electrical evaluation of up to 5 samples. To obtain the linearity of the heating rate, ensuring control and efficiency of the measurements, a 4 to 20 mA signal from a temperature controller was used, which controls the rectified voltage of the electrical network via pulse width modulation (PWM). A control software uses the Modbus protocol (RTU), with communication through the emulated serial (USB) port, searching the controller data synchronously with the computer's real time clock. This software also allows configuring the internal program of the controller and graphically displaying in real time the parameters of temperature, control ramp and percentage power consumed by the oven. The temperature was measured with a high-performance digital multimeter and and the use of a multiplexer for 6 sensors. The system was validated by measurements on two different ferroelectric samples, BaTiO3 (polycrystalline barium titanate) and TGS (monocrystalline triglycine sulfate). It was verified, as already well established in the literature, the dependence between the peak of the dielectric constant of the materials as a function of temperature, allowing to consider as a standard validation of the constructed system.