Sinterização por micro-ondas em uma e em duas etapas de varistores à base de ZnO
Klein-Gunnewiek, Rodolfo Foster
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This thesis is aimed at one-step microwave sintering at frequencies of 2.45 GHz and 30 GHz and the development of two-step microwave sintering (MW-TSS) at 2.45 GHz of ZnO-based nanostructured semiconductor compositions for application in varistors. The synthesis of nanostructured additives Mn, Cu, Cr, Co and Bi oxides with particles below 100 nm, based on the cationic exchange of soluble salt/polymer was developed and improved. After preparation and conformation, the samples were ultra-rapidly sintered at heating rates of 100 °C/min at 2.45 GHz and 20 °C/min at 30 GHz, both at 10 minutes of holding time, and in the two-step sintering the t2 holding time did not exceed one hour. The MW-TSS technique enabled us to reduce the pure ZnO average grain size from 4.85 (micro) to 2.85 (micro) while increasing density from 96.9% to 97.9%. For the Matsuoka nano composition, sintering at 30 GHz proved more effective, reaching a non-linear coefficient (atto) of 37 and a density of 96.5%. The addition of copper to the composition improved the value of (atto) reaching 71.4 at 2.45 GHz. However, the two-step sintering by microwave permitted a reduction in the average grain size and an increase in the electrical properties, reaching of 40 in the copper-free composition, and with the addition of copper it reached 83 with the breakdown electrical field achieving 14 kV/cm, with average grain size always below 3 μm, and, in the best condition in which the electrical properties were optimized, of merely 1.84. Although the average grain size attained is not located within the nanometric scale, sintering by microwave proved effective in obtaining varistors with excellent electrical properties, achieving finer microstructures when the two-step sintering by microwave technique is applied, with greatly reduced times for both one-step and two-step sintering techniques. The accelerated diffusional processes relative to the possible nonthermal effects caused by the microwave processing were also discussed.