Different processing conditions and the effect of secondary phases on ferroelectric properties of Bi0.5(Na0.8K0.2)0.5TiO3 (BNKT) are studied. Ceramic powders are prepared by solid state reaction and different sintering temperatures (temperatures between 1075 and 1150ºC) are  analyzed. Finally, samples are characterized by X-ray diffraction, Raman microspectroscopy, Scanning Electron Microscopy, impedance spectroscopy, and density measurements. Through XRD patterns, the perovskite structure is stabilized; together with small peaks corresponding to a secondary phase associated with K2-xNaxTi6O13 phase. Moreover, the content of the secondary  phase, d33 piezoelectric constant and dielectric properties increase with sintering temperature.Fil: Camargo, Javier Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Ramajo, Leandro Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Rubio Marcos, Fernando. Instituto de Ceramica y Vidrio de Madrid; EspañaFil: Castro, Miriam Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin

Camargo, Javier Eduardo

Castro, Miriam Susana

Ramajo, Leandro Alfredo

Rubio Marcos, Fernando

Advanced Materials Research

English

CONICET Digital

Ferroelectric Properties of Bi0.5(Na0.8K0.2)0.5TiO3 Ceramics  Javier Camargo1,a, Leandro Ramajo1,b, Fernando Rubio-Marcos2,c, Miriam Castro1,d *  1Institute of Research in Materials Science and Technology (INTEMA), Juan B. Justo 4302 (B7608FDQ), Mar del Plata, Argentina 2Institute of Ceramic and Glass (ICV), Campus UAM c/Kelsen 5, Madrid, Spain ajavijec@gmail.com, blramajo@fi.mdp.edu.ar, cfrmarcos@icv.csic.es, dmcastro@fi.mdp.edu.ar  Keywords: piezoelectric ceramics, lead-free compositions, ferroelectric properties.  Abstract. Different processing conditions and the effect of secondary phases on ferroelectric properties of Bi0.5(Na0.8K0.2)0.5TiO3 (BNKT) were studied. Ceramic powders were prepared by solid state reaction and different sintering conditions (1075 to 1100ºC) were analyzed. Finally, samples were characterized by X-ray diffraction, Raman microspectroscopy, Scanning Electron Microscopy, impedance spectroscopy, and density measurements. Through XRD patterns, the perovskite structure was stabilized, together small peaks corresponding to a secondary phase associated with K2-xNaxTi6O13. Moreover, the content of the secondary phase, d33 piezoelectric constant and dielectric properties increased with the sintering temperature.  Introduction Lead titanate–zirconate piezoceramics are the most important and widely used materials for piezoelectric transducers, transformers and sensors. They have played a dominant role in the piezoelectrics field for a long time because of their excellent piezoelectric properties [1]. However, the toxicity of lead is a serious threat to human health and environment [2]. Thus, considerable effort has been devoted towards the development of lead-free piezoelectric ceramics.  Numerous studies on lead-free piezoelectric ceramics, such as (K,Na)NbO3, BaTiO3-based, Bi-layered, bismuth sodium titanate and tungsten bronze-type materials, have been recently published. For this reason, K0.5Na0.5NbO3 (KNN) system attracts much attention, for its elevated Curie temperature (about 420°C) and high piezoelectric properties close to the morphotropic phase boundary (MPB) [3]. However, it is difficult to obtain pure KNN ceramics with high density and great piezoelectric performance. Sodium bismuth titanate Na0.5Bi0.5TiO3 (BNT) with a relatively large remanent polarization at room temperature and a relatively high Curie temperature, could be considered another promising candidate of lead-free piezoelectric ceramics. However, its high coercive field hinders the obtention of the desired piezoelectric properties. Therefore, a number of studies have been carried out to improve electrical properties of BNT by the formation of solid solutions with other ABO3 perovskites [4-5]. It has been reported that BNT ceramics modified with Bi0.5K0.5TiO3 (BKT) showed improved dielectric and piezoelectric properties, due to a rhombohedral–tetragonal morphotropic phase boundary (MPB) at the optimal composition of Bi0.5(Na0.85K0.15)0.5TiO3 [6]. In the current work, lead-free Bi0.5(Na0.8K0.2)0.5TiO3-based ceramics were prepared by the solid state  reaction  method using a previous mecanochemical activation step of reagents.  Results were discussed considering the effect of secondary phases on structure, microstructure, dielectric and piezoelectric properties of these ceramics. Experimental Procedure Bi0.5(K0.2Na0.8)0.5TiO3 was synthesized  through  solid state reaction, using K2CO3 and Na2CO3 (Cicarelli 99,99%; Argentina), Bi2O3 (Aldrich 99,8%; USA) and TiO2 (Aldrich 99,9%; USA). Powders were mixed and milled using zirconia balls in alcoholic medium for 5 h in a planetary mill (Fritsch, Pulverisette 7, 1450 rpm). Powders were dried and calcined at 700ºC for 2 h. The calcined powders were milled again, pressed into disks and sintered at 1075 to 1150°C for 2 h.  Crystalline phases were characterized by X-ray diffraction (XRD) (Philips PW1830), using  CuK  radiation. Raman spectra were acquired at room temperature with a Renishaw inVia microscope by means of the 514 nm Ar-ion laser line (50 mW nominal power) with a diffraction grating of 2400 lines/mm. Density values were determined using the Archimedes method. Microstructures were evaluated on polished and thermally etched samples using a Field Emission Scanning Electron Microscope, FE-SEM (Hitachi S-4700). Previous to the electrical measurements, a fired silver paste was used for the electric contacts. Dielectric properties were determined at different frequencies using impedance analyzers Hioki 3532-3550 in the frequency range 100 mHz-10 MHz at room temperature. Samples were polled in a silicon oil bath at 25ºC by applying a DC field of 30.0 kV/cm for 30 min. The piezoelectric constant d33 was measured using a piezo d33 meter (YE2730A d33 METER, APC International, Ltd., USA). Finally, the ferroelectric nature of these ceramics was determined using a hysteresis meter (RT 6000 HVS, RADIANT Technologies). Results and Discussion From XRD patterns (Fig. 1), the BNKT phase was stabilized in all sintered samples. However, samples sintered at temperatures higher than 1100°C presented secondary phases, which can be indexed to K2-xNaxTi6O13 phase (monoclinic structure, JCPDS Nos. 40-0403 and 74-0275).  20 30 40 50 60 70 80ooooo  Intensity (arbitrary units)21075°C1100°C1125°C1150°Co ox xxxx xxxx Figure 1. XRD patterns of sintered samples. (o) Peaks corresponding to BNKT phase, (x) peaks associated with a secondary phase.  Raman analyses were performed on different regions of all sintered samples between 200 and 1000cm-1 (Fig. 2). From Raman spectra, six vibration bands corresponding to BNKT, in all sintered samples were observed. The amplitude and overlapping Raman bands reflected the strong anharmonicity and disorder inherent to A-sites. Moreover, new peaks related to a secondary phase assigned to K2-xNaxTi6O13 were observed. Peaks below 500 cm-1 could be attributed to the K–O–Ti stretching vibration. Peaks at about 655 cm-1 have been assigned to the Ti–O–Ti stretch in edge-shared TiO6. Peaks near 870 cm-1 were reported for a short Ti–O stretching vibration in distorted TiO6. Weak peaks around 240 and 400 cm-1 characteristic of the K–O–Ti containing short Ti–O bonds were also observed. Although secondary phases were not detected through XRD patterns at the lowest sintering temperature, Raman spectra confirmed the formation of a secondary phase with composition close to K2-xNaxTi6O13 in all samples [7].  1100°C  I (A.U.)1075°CSecondary phaseSecondary phase1150°C1125°C  Secondary phase250 500 750 1000BNKT phase  250 500 750 1000Secondary phase (cm-1)BNKT phaseBNKT phaseBNKT phase   Figure 2. Raman spectra of sintered samples.  Microstructural characteristics of sintered samples were observed through Field Emission Scanning Electron Microscopy (FE-SEM) (Fig. 3). The FE-SEM micrographs showed the typical BNKT morphology consisting of faceted grains with a very small grain size. Furthermore, it was determined that sintering temperature affected the grain size and the amount of secondary phases. A secondary phase was also detected in the form of rods, probably due to the formation of K2-xNaxTi6O13, as detected by XRD and Raman microspectroscopy. All systems show small grains (≤ 1 μm) that become finer at low sintering temperatures.    Figure 3. FE-SEM images of pure BNKT sintered at (A) 1075ºC, (B) 1100ºC, (C) 1125ºC and (D) 1150ºC.  Relative permittivity and dielectric loss values as a function of frequency for samples sintered at different temperatures were measured at room temperature (Fig. 4). In all cases, it was determined that at low frequencies, permittivity decreases drastically due to a space charge relaxation process characteristic of the polycrystalline material. Additionally, a relaxation process at high frequency (8~MHz) which was associated with a dipolar relaxation phenomena was observed. In these samples, the improvement in the real permittivity value with the sintering temperature could be related to the secondary phase formation and the densification degree.   101102103104105106107250500750100012501500A)Relative Permittivity (´) Frequency (Hz)  1075°C  1100°C  1125°C  1150°C 101102103104105106107255075100125150175200225250   1075°C  1100°C  1125°C  1150°C Dielectric Loss (´´)Frequency(Hz)B) Figure 4. Curves of relative permittivity (A) and dielectric loss (B) as a function of frequency at room temperature.  From density measurements, samples sintered at 1100°C presented the maximum value of the complete set of samples (Table 1). Interestingly, the higher the sintering temperature, the higher the piezoelectric constant (d33) and real permittivity values (Table 1). Although this secondary phase was not previously reported in BNKT systems, according to these observations the presence of this secondary phase contributes to the improvement of the ferroelectric and piezoelectric properties of BNKT-based ceramics. The apparition of this phase could be related with the mechanochemical activation of the powders in the solid state reaction method.  Table 1. Density, real permittivity ( ´), dielectric loss ( ”), and piezoelectric constant (d33) values of the sintered samples.          Finally, hysteresis loops of sintered samples were collected at room temperature (Fig. 5). BNKT sintered at 1075ºC showed a remnant polarization (Pr) 1.31 μC/cm2, which increased to 2.13, 3.09 and 9.54 C/cm2 with the sintering temperature rising. The coercive electric field (Ec), and the saturation polarization values (Ps) were 9.63, 16.59, 15.58 and 25.1 kV/cm and for samples sintered at 1075, 1100, 1125 and 1150ºC, respectively. Taking into account the increase in the coercive electric field values, a hardness effect on the ferroelectric properties of BNKT with the increase of sintering temperature was detected. This result could be associated with the secondary phase amount, the densification degree, and the anchoring effect of grain boundaries which limit the polarization of material and increase the coercive field of the samples and the energy involved in changing the polarization of the material. Sintering  temperature (ºC) Density (g/cm3) ´ ” d33 (pC/N) 1075 5.64 674 33.8 30 1100 5.79 760 29.5 50 1125 5.65 879 36.8 90 1150 5.59 1014 43.9 100 -40 -20 0 20 40-20-15-10-505101520  1075 ºC 1100 ºC 1125 ºC 1150 ºCP (µC/cm2)E (kV/cm) Figure 5. Hysteresis loops at room temperature of sintered ceramics. Summary The effect of sintering temperature on the microstructure, secondary phase formation and electric properties of Bi0.5(Na0.8K0.2)0.5TiO3 (BNKT)-based ceramics obtained by the conventional mixed oxide method has been reported. The formation of main perovskite type structure and a secondary phase whith composition close to K2-xNaxTi6O13 were detected.  The amount of this secondary phase was increased with the sintering temperature. Interestingly, this secondary phase improved the piezoelectric properties and the remnant polarization of these ceramics.   References [1] B. Jaffe, W. R. Cook, and H. Jaffe, Piezoelectric ceramics, Academic Press, London, 1971. [2] J. Rodel, W. Jo, K. T. P. Seifert, E. M Anton, T. Granzow, D. Damjanovic, Perspective on the development of lead-free piezoceramics, J. Am. Ceram. Soc. 92 (2009) 1153-1177. [3] R. Wang, R. Xie, T. Sekiya, Y. Shimojo, Fabrication and characterization of potassium-sodium niobate piezoelectric ceramics by spark-plasma-sintering method, Mater. Res. Bull. 39 (2004) 1709-1715. [4] D. Lin, K.W. Kwok, H.L.W. Chan, Structure and electrical properties of Bi0.5Na0.5TiO3–BaTiO3–Bi0.5Li0.5TiO3 lead-free piezoelectric ceramics, Solid State Ionics, 178 (2008) 1930-1937. [5] M. Jiang, X. Liu, C. Liu, Effect of BiFeO3 additions on the dielectric and piezoelectric properties of (K0.44Na0.52Li0.04)(Nb0.84Ta0.1Sb0.06)O3 ceramics, Mater. Res. Bull, 45 (2010) 220-223. [6] B. Wang, L. Luo, F. Ni, P. Du, W. Li, H. Chen, Piezoelectric and ferroelectric properties of (Bi1−xNa0.8K0.2Lax)0.5TiO3 lead-free ceramics, J. Alloys Compd, 526 (2012) 79-84. [7] X. Meng, D. Wang, J. Liu, B. Lin, Z. Fu, Effects of titania different phases on the microstructure and properties of K2Ti6O13 nanowires, Solid State Commun., 137 (2006) 146–149.  

Ferroelectric properties of Bi0.5(Na0.8K0.2)0.5TiO3 ceramics

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas

Different processing conditions and the effect of secondary phases on ferroelectric properties of Bi0.5(Na0.8K0.2)0.5TiO3(BNKT) are studied. Ceramic powders are prepared by solid state reaction and different sintering temperatures (temperatures between 1075 and 1150°C) are analyzed. Finally, samples are characterized by X-ray diffraction, Raman microspectroscopy, Scanning Electron Microscopy, impedance spectroscopy, and density measurements. Through XRD patterns, the perovskite structure is stabilized; together with small peaks corresponding to a secondary phase associated with K2-xNaxTi6O13phase. Moreover, the content of the secondary phase, d33piezoelectric constant and dielectric properties increase with sintering temperature.</jats:p

Javier Camargo

Leandro Ramajo

Fernando Rubio-Marcos

Miriam Castro

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Ferroelectric Properties of Bi&lt;sub&gt;0.5&lt;/sub&gt;(Na&lt;sub&gt;0.8&lt;/sub&gt;K&lt;sub&gt;0.2&lt;/sub&gt;)&lt;sub&gt;0.5&lt;/sub&gt;TiO&lt;sub&gt;3&lt;/sub&gt; Ceramics

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Ferroelectric properties of Bi0.5(Na0.8K0.2)0.5TiO3 ceramics

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