Hydrothermal epitaxy of lead free (Na,K)NbO 3 -based piezoelectric films
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Hydrothermal epitaxy of lead free (Na,K)NbO3-based piezoelectric films Albertus D. Handoko1 and Gregory K. L. Goh1,* 1 Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore. ABSTRACT Lead free niobate solid solutions can exhibit piezoelectric properties comparable to that of lead zirconate titanate piezoelectrics in the vicinity of its morphotropic phase boundary (MPB). Here we describe how (Na,K)NbO3 and (Na,K)NbO3-LiTaO3 solid solution thin films can be grown epitaxially by the hydrothermal method at temperatures of 200 °C or below in water and be made ferro- and piezoelectrically active by a simple 2 step post growth treatment. INTRODUCTION It has been shown1, 2 that lead free (Na,K)NbO3 based (NKN) solid solutions can exhibit piezoelectric properties comparable to that of lead zirconate titanate (PZT) piezoelectrics in the vicinity of its morphotropic phase boundary (MPB). Unfortunately current techniques available to grow NKN based solid solution films require high temperatures, sometimes in excess of 1000 °C, which are not only detrimental to the stoichiometry due to volatility of alkali components3 but also cause unwanted thermal-related defects such as twins and cracking upon cooling through the Curie temperature. Hydrothermal synthesis is a novel and unique materials synthesis route that allows the formation of various compounds including epitaxial films at mild conditions, using water as the main solvent.4 Advantageously, this method is not only simple but also self contained and easily scalable. Waste recovery and solution reutilisation is also possible via simple centrifugation process.5 Additionally, films grown hydrothermally can virtually be free of thermal defects, because the growth temperature is typically below most materials Curie transition temperature,4 although proton/water incorporation remains a great challenge for the electrical properties which needs to be subsequently removed by post-growth treatments at modest conditions.6, 7 In this study, hydrothermal methods are explored to grow single-phase, complex composition (Na,K)NbO3-LiTaO3 (NKN-LT) solid solution thin films. Ultimately we are seeking compositions as close to the MPB as possible to obtain reasonably high polarisation and piezoelectric constant (d33) values in order to be considered as viable lead-free alternatives to Pb(Zr,Ti)O3 based systems. Complications and challenges to grow NKN-LT films in a singlestep, single-pot synthesis is also described. EXPERIMENTAL NKN powder was synthesized by reacting 3.76 mmol of Nb2O5 (99.99%, Aldrich, St. Louis, MO) in 25 ml aqueous mixtures of KOH (>88%, J.T. Baker, Phillipsburg, NJ) and NaOH (>98%, GCE) at 200 °C for 24 hours in a PTFE lined stainless steel autoclave (Parr Co., Moline, IL). The Nb2O5 precursor (99.99%, Aldrich, St. Louis, USA) was identified to be a mixture of
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91% H-Nb2O5 8 and 9% T-Nb2O5 9. In all cases, the total [OH-] was fixed at 6 M because it was found to be the optimum concentration for the reaction to complete in 24 hours
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