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Conduction Mechanisms of Lead Free Potassium Sodium Niobate (KNN) Single Crystals and Polycrystals
Keywords: KNN, piezoelectrics, Lead Free
Piezoelectrics are functional materials that account for a huge market of products, extending from daily use products to more sophisticated devices. Within piezoelectrics, single crystals and ceramics are options currently considered when a material of choice has to be selected. Though single crystals normally exceed their ceramic counterpart in terms of cost production, the search for optimum performance may dictate the need to choose between piezoelectric single crystals or polycrystalline ceramics. In the present work dealing with lead free K0.5Na0.5NbO3 (KNN) piezoelectrics, it is shown that the option for single crystals or ceramics must be guided by the final temperature of use. Using impedance spectroscopy, the electric charge transport in KNN (001) oriented single crystals and its polycrystalline ceramics counterpart is compared. It is shown that in a temperature range close to room temperature (Tr.t 200 C), simple crystals exhibit higher conductivity than the polycrystalline ceramics which grain boundaries behave as barriers to charge transport. This difference in charge conduction becomes enhanced at even higher temperatures due to the contribution of ionic conduction in the case of single crystals. This is confirmed by the activation energy, 1.60 and 0.97 eV, calculated for the temperature range above 300 C, which point out for an ionic conduction in the case of single crystals and ionized oxygen vacancies as charge carriers in the case of the polycrystalline KNN, respectively. These results suggest the simple crystals to be more suitable for applications at room temperature whereas for high temperatures ceramics seem to be a better choice.
Maria Elisabete Costa, Associate Professor
CICECO, University of Aveiro
Aveiro, Aveiro

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