Volatilization of alkali ions and effects of molecular weight of polyvinylpyrrolidone introduced in solution-derived fer
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Yaoa) and Phoi Chin Goh Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), Singapore 117602
Wei Ren Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China (Received 18 April 2009; accepted 27 May 2009)
Ferroelectric K0.5Na0.5NbO3 (KNN) thin films were prepared by a chemical solution deposition approach with polyvinylpyrrolidone (PVP) of different molecular weights introduced in the precursor solutions. The volatilization of the alkali ions and the effects of the molecular weight of PVP were examined with x-ray diffraction (XRD), thermal analysis, mass spectrometry, and x-ray photoelectron spectroscopy (XPS). The results clearly showed that the volatilization of the alkali ions mainly happened at moderate temperatures before the crystallization of the KNN perovskite phase. Loss of Na was more significant than K ions during the heating process of KNN. The introduction of PVP with the appropriate molecular weight could effectively promote the crystallization of the KNN perovskite phase at reduced temperature and substantially suppress the loss of the alkali ions before crystallization. Therefore, a high dielectric constant, piezoelectric coefficient, and well saturated ferroelectric hysteresis loops were obtained in the KNN films in which PVP of the right molecular weight were introduced.
I. INTRODUCTION
K0.5Na0.5NbO3 (KNN)-based ferroelectric bulk ceramic materials with a large piezoelectric coefficient are attracting attention as a promising lead-free candidate to replace the current market dominant lead-based piezoelectric ceramics.1–3 However, the performance properties of KNN-based ceramic thin films are still far below the lead-based ferroelectric thin films. One of the major reasons is the high volatility of the alkali constituents, potassium and sodium. Significant K and Na losses have been reported by several groups in Na- and K-based thin films grown by physical vapor deposition, such as pulsed laser deposition (PLD) and sputtering techniques,4–5 and also in KNN films derived from chemical solution deposition (CSD).6–9 Severe K and Na losses make it difficult to control the composition of the obtained KNN films, and the vacancies of the alkali ions are a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0433
3516
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 12, Dec 2009 Downloaded: 12 Mar 2015
also thought to be a main reason for the large leakage current which substantially degrades ferroelectric performance.10 The major strategy reported in the literature to overcome the problems caused by the loss of the alkali ions is simply adding excess K and Na in the starting composition,6,7,11,12 but the resulting ferroelectric properties are far from satisfactory. How alkali compositions are lost is an important area of inquiry, including questions such as what is the loss quantity, what is the lo
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