Journal of the Korean Physical Society, Vol. 68, No. 12, June 2016, pp. 1439∼1444 Giant Strain in Lead-free Relaxor/Ferroelectric Piezocomposite Ceramics Thi Hinh Dinh, Jin-Kyu Kang, Hoang Thien Khoi Nguyen, Trang An Duong and Jae-Shin Lee ∗ School of Materials Science and Engineering, University of Ulsan, Ulsan 44610, Korea Vu Diem Ngoc Tran School of Materials Science and Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam Ky Nam Pham School of Geology and Mineral Resources, Ho Chi Minh University of Natural Resources and Environment, Ho Chi Minh, Vietnam (Received 2 October 2015) The crystal structural, ferroelectric, and electric-field-induced-strain (EFIS) properties of lead- free relaxor/ferroelectric piezocomposites were investigated. The relaxor-matrix phases were mixed with the ferroelectric-seed phases by using a conventional ceramic processing route. The addition of the ferroelectric seed phase dramatically enhanced the EFIS of the relaxor matrix phase at low electric fields. Giant strains of 745 pm/V at 4 kV/mm and 466 pm/V at 3 kV/mm were obtained when the seed contents were 30 wt% and 50 wt%, respectively, which are much higher than those of the relaxor matrix phase without ferroelectric seeds (575 pm/V at 4 kV/mm and 327 pm/V at 3 kV/mm). PACS numbers: 77.65.-j, 77.22.-d, 81.05.Je, 85.50.-n Keywords: Lead-free piezoelectric ceramics, Relaxor, Strain, Ferroelectric, Composites DOI: 10.3938/jkps.68.1439 I. INTRODUCTION Piezoelectric ceramics play an important role in electronic device applications, especially lead-oxide- based piezoelectric ceramics such as Pb(Zr,Ti)O 3 , (Pb,La)(Zr,Ti)O 3 , and Pb(Mg,Nb)O 3 [1, 2]. However, these materials contain more than 60 wt.% of lead [3], which is a very toxic substance as it can cause damage to the kidneys, the brain, the nervous system, and espe- cially the intelligence of the children. The volatilization of lead oxide (PbO) during the high-temperature sinter- ing process not only causes environmental pollution but also generates instability in the composition and the elec- trical properties of the final products. In recent years, many countries (Europe, Japan, US, Korea etc.) have required all of their new electronic products to be lead- free for environmental protection and human health [4, 5]. Therefore, a great need exists to develop lead-free piezoelectric ceramics that are environment-friendly and possess excellent electrical properties for replacement of existing lead-based materials. Among various lead-free materials, binary (Bi,Na) TiO 3 -(Bi,K)TiO 3 (BNKT) solid solutions are consid- ∗ E-mail: jslee@ulsan.ac.kr; Fax: +82-52-259-1688 ered as potential candidates due to their excellent elec- tromechanical properties near the morphotropic phase boundary (MPB) [6–8]. A number of previous stud- ies have reported that the electromechanical properties can be improved by modification of the MPB in BNKT with various dopants or modifiers [9]: BNKT-BiAlO 3 [10], BNKT-LiSbO 3 [11], and BNKT-BaTiO 3 [12] for their excellent ferroelectric and piezoelectric properties; Nb-doped BNKT [13,14], BNKT-Bi(Mg,Sn)O 3 [15], Li- and Ta-codoped BNKT [16], BNKT-LaFeO 3 [17], Sn- doped BNKT [18,19], BNKT-Ba 1/2 Ca 1/2 ZrO 3 [20], La- doped BNKT [21–23], BNKT-(K,Na)NbO 3 [24], Nb- doped BNKT-SrTiO 3 [25], and BNKT-SrTiO 3 [26] for their giant electric-field-induced strain (EFIS) proper- ties; BNKT-(K,Na)NbO 3 [27] and BNKT-BaZrO 3 [28] for their temperature-insensitive electrostrictive coeffi- cient. Bismuth-based materials demonstrating giant EFIS have been considered as incipient piezoceramics [29], in which the materials become macroscopically piezo- electric under an applied electric field. Incipient piezoceramics, however, required high electric fields to induce a phase transformation to obtain the giant strain. This can be ameliorated by using composites consisting of a relaxor (matrix) and a ferroelectric phase (seed). Through this approach, a maximum strain of 0.29% -1439-