CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International ] (]]]]) ]]]–]]] High densification and dielectric properties of lead-free (K 0.5 Na 0.5 )NbO 3 piezoelectric ceramics with optimum excess Na 2 O and K 2 O contents Rattiphorn Sumang a , Chamaiporn Wicheanrat b , Theerachai Bongkarn b,c,n , Santi Maensiri d a Department of Physics, Faculty of Science and Technology, Pibulsongkram Rajabhat University, Phitsanulok 65000, Thailand b Department of Physics, Faculty of Science, Naresuan University, Phisanulok 65000, Thailand c Research Center for Academic Excellence in Applied Physics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand d School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand Received 26 October 2014; accepted 14 March 2015 Abstract Lead-free piezoelectric ceramics based on the (K 0.5 Na 0.5 )NbO 3 (KNN) system were prepared by the solid-state reaction method. The effects of firing temperatures and excess Na 2 O and K 2 O on KNN ceramics were investigated. The samples were calcined between 750 and 950 1C for 4 h and the sintering range was between 1060 and 1140 1C. Impurities phases of Nb 2 O 5 and K 2 CO 3 appeared in all sintered samples, due to evaporation of some elements during in the sintering process. The average particle size and grain size increased with increased firing temperatures. After addition of excess Na 2 O and K 2 O with 0, 1, 2, 3, 4, 5 and 10 wt% in the system, the impurities phase did not appear in all samples. The porous microstructure gradually decreased with increased of excess Na 2 O and K 2 O up to 1 wt% and then slightly increased with higher than 1 wt% of excess Na 2 O and K 2 O. The density and dielectric constant can be improved by adding 1 wt% of excess Na 2 O and K 2 O. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Excess Na 2 O and K 2 O; Lead-free; Dielectric; Firing temperature 1. Introduction Lead-free piezoelectric ceramics have attracted attention as new piezoelectric materials to replace PZT based ceramics because of environmental problem due to its high vapor pressure during sintering process. Recently, (K 0.5 Na 0.5 )NbO 3 (KNN)- based lead-free piezoelectric ceramics are attracting attention which are characterized by high piezoelectric properties and high temperature of phase transition (  410 1C) [1–7]. These materials are widely used to develop sensors and actuators [8]. However, it was difficult to achieve the densification in pure KNN ceramic due to high reactivity with the volatilization of K 2 O and Na 2 O during sintering process and difficulty in sintering an elaborate body [9–13]. Various fabricating techniques, such as hydrothermal method [14–16], hot-pressed sintering, spark-plasma sintering (SPS) have been utilized to improve the densification and properties of KNN lead-free materials. Hydrothermal synthesis and microwave sinter- ing showed the maximum relative density 88% at the sintering temperature of 1040 1C [14]. Hot-pressed sintering method can improve the densification and presented maximum relative density of 98.9% at the sintering temperature of 1100 1C [15]. At low temperature of 920 1C, the theoretical density of 99% with enhanced properties was observed by SPS method [16]. However, all of these techniques are costly for production and unsuitable for practical application in industry [17,18]. The conventional solid- state reaction method is more economical for large batch processing of KNN ceramics which have been reported by several research groups [10,19,20]. Chen et al. [19] prepared KNN ceramic by solid-state reaction using a calcination temperature of 865 1C and sintering temperature of 1100 1C for 3 h. The www.elsevier.com/locate/ceramint http://dx.doi.org/10.1016/j.ceramint.2015.03.228 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. n Corresponding author at: Department of Physics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand, Tel.: þ 66 55963528; fax: þ66 55963501. E-mail addresses: rattiphorn_11@hotmail.com (R. Sumang), chamaiphornw@hotmail.com (C. Wicheanrat), researchcmu@yahoo.com (T. Bongkarn), santimaensiri@g.sut.ac.th (S. Maensiri). Please cite this article as: R. Sumang, et al., High densification and dielectric properties of lead-free (K 0.5 Na 0.5 )NbO 3 piezoelectric ceramics with optimum excess Na 2 O and K 2 O contents, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.228