Scholarly article on topic 'Structural and electrical properties of (1-x)(Na0.465K0.465 Li0.07)NbO3 – x CaTiO3 lead-free piezoelectric ceramics with high Curie temperature'

Structural and electrical properties of (1-x)(Na0.465K0.465 Li0.07)NbO3 – x CaTiO3 lead-free piezoelectric ceramics with high Curie temperature Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — P. Bomlai, N. Muensit, S.J. Milne

Abstract Accompanied by the calling for the suitable development, lead-based piezoelectric ceramics will be gradually substituted by an environment-friendly of lead-free piezoelectric ceramics. Among them, alkaline niobate (Na, K)NbO3 -based ceramics are more promising because of their good piezoelectric properties and high Curie temperatures. In this study, lead-free piezoelectric ceramics compositions: (1-x)(Na0.465K0.465Li0.07)NbO3 – x CaTiO3 (x = 0.005, 0.01, 0.015, 0.02) were prepared by a conventional mixed oxide route, and the effects of CaTiO3 on structural and electrical properties were investigated. X-ray diffraction analysis results indicated that CaTiO3 diffuses into the (Na0.465K0.465Li0.07)NbO3 lattice to form a solid solution during sintering. The samples with x ≤ 0.01 showed the coexistence of orthorhombic and tetragonal phases. The tetragonality increased with further increasing x. Grain growth during secondary recrystallization was also affected. The dielectric and piezoelectric properties are enhanced for the composition near the orthorhombic-tetragonal polymorphotropic phase boundary. The 0.995(Na0.465K0.465Li0.07)NbO3 – 0.005 CaTiO3 ceramics exhibit optimum electrical properties (d33 = 258 pC/N, ɛr = 1014, tan δ = 0.038, and TC = 464°C). The results reveal that (1-x)(Na0.465K0.465Li0.07)NbO3 – x CaTiO3 ceramics are promising candidate materials for lead-free piezoelectric application.

Academic research paper on topic "Structural and electrical properties of (1-x)(Na0.465K0.465 Li0.07)NbO3 – x CaTiO3 lead-free piezoelectric ceramics with high Curie temperature"

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W70 ELSEVIER

Procedía Engineering

Procediíi Engineering 32 (2012) 814 - 820

www.elsevier.com/Iocate/procedia

I-SEEC2011

Structural and electrical properties of (1-x)(Na0465K0465 Li0.07)NbO3 - x CaTiO3 lead-free piezoelectric ceramics with

high Curie temperature

aDepartment of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Songkhla, 90112, Thailand bDepartment of Physics, Faculty of Science, Prince of Songkla University, Songkhla, 90112, Thailand cInstitute for Materials Research, University of Leeds, Leeds LS2 9JT, United Kingdom

Elsevier use only: Received 30 September 2011; Revised 10 November 2011; Accepted 25 November 2011.

Accompanied by the calling for the suitable development, lead-based piezoelectric ceramics will be gradually substituted by an environment-friendly of lead-free piezoelectric ceramics. Among them, alkaline niobate (Na, K)NbO3 -based ceramics are more promising because of their good piezoelectric properties and high Curie temperatures. In this study, lead-free piezoelectric ceramics compositions: (1-x)(Na0 465K0 465Li0 07)NbO3 - x CaTiO3 (x = 0.005, 0.01, 0.015, 0.02) were prepared by a conventional mixed oxide route, and the effects of CaTiO3 on structural and electrical properties were investigated. X-ray diffraction analysis results indicated that CaTiO3 diffuses into the (Na0 465K0.465Li0.07)NbO3 lattice to form a solid solution during sintering. The samples with x < 0.01 showed the coexistence of orthorhombic and tetragonal phases. The tetragonality increased with further increasing x. Grain growth during secondary recrystallization was also affected. The dielectric and piezoelectric properties are enhanced for the composition near the orthorhombic-tetragonal polymorphotropic phase boundary. The 0.995(Na0 465K0.465Li0.07)Nb03 - 0.005 CaTiO3 ceramics exhibit optimum electrical properties (d33 = 258 pC/N, E = 1014, tan S= 0.038, and TC = 464°C). The results reveal that (1-x)CNa0.465K0.465Li0.07)NbO3 - x CaTiO3 ceramics are promising candidate materials for lead-free piezoelectric application.

© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of I-SEEC2011

Keywords: Lead-free materials; Phase transition; Electrical properties

* Corresponding author. Tel.: +66-74-288-250; fax: +66-74-288-395. E-mail address: ppornsuda@yahoo.com.

P. Bomlaia*, N. Muensitb, S. J. Milne'

Abstract

1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.02.017

1. Introduction

Lead oxide based ferroelectrics, represented by lead zirconate titanate, Pb(Zr, Ti)O3 (PZT), are widely used for piezoelectric actuators, sensors and transducers due to their excellent piezoelectric properties. However, the toxicity of lead oxide and its high vapor pressure during processing demand alternative environmentally friendly materials to replace PZT based ceramics.

A solid solution of ferroelectric KNbO3 and anti-ferroelectric NaNbO3, such as (Na0.5K0.5)NbO3 (NKN), has attracted much attention as an alternative, lead-free piezoelectric materials because of their relatively high piezoelectric and ferroelectric properties, and high Curie temperature [1-2]. However, it is well known that dense and well sintered NKN ceramics are very difficult to obtain by ordinary sintering processes because of the high volatility of alkali metal oxides at high temperatures. To improve densification and electrical properties of NKN ceramics, many modifiers are added into NKN to form new solid solution, such as NKN-BaTiO3 [3], NKN-LiNbO3 [4], NKN-LiSbO3 [5] and NKN-LiTaO3 [6]. The effects of sintering aids such as CuO [7], ZnO [8], and Bi2O3 [9] have also been studied. The recent research results also found that the piezoelectric properties of KNN-based lead-free ceramics could be improved, but the Curie temperature (Tc) of the ceramics usually decreases [10-11]. Therefore, it is important to prepare the lead-free piezoelectric materials with high Curie temperature and good piezoelectric properties. As one important member of alkaline niobate systems, Li doped NKN ceramics show relatively high Tc and good electrical properties [4]. In this work, (1-x)(Na0 465K0 465Li0 07) NbO3 - x CaTiO3 ceramics with high Curie temperature were obtained by the conventional sintering method. The effects of CaTiO3 content on densification, microstructure and electrical properties of the NKN - LiNbO3 composition were studied.

2. Experimental procedure

The lead-free ceramic compositions: (1-x)(Na0.465K0.465Li0.07)NbO3 - x CaTiO3 (x = 0.005, 0.01, 0.015 and 0.02 mol.) were prepared via conventional mixed - oxide method. Reagent - grade oxide and carbonate powders of K2CO3 (99.5%), Na2CO3 (99.5%), Li2CO3 (99.5%) and Nb2O5 (99.9%) were used as starting materials. A (Na0.465K0.465Li0.07)NbO3 (NKLN) powder was prepared before reacting with CaTiO3 (CT) reagent. The starting powders were weighed according to the stoichiometric formula and ball milled with zirconia grinding ball and ethanol for 24 h. The dried powder was then calcined at 800 oC for 2 h. The NKLN powders were then ground, weighed, and ball milled again for 24 h with CaCO3, and TiO2 to obtain compositions (1-x) NKLN - x CT. A reaction-sintering approach was used to produce the CT-modified NKLN ceramics, in that no second powder calcination step was employed. The combined powders were dried, ground, and pressed into 1.6 cm diameter disks, placed in alumina crucibles, and sintered at low temperatures ranging from 980 to 1020 oC, for 2 h.

The density of the sintered samples was determined by the Archimedes method. The phase structures were analyzed by X-ray diffraction (XRD) analysis obtained by using CuKa radiation (Philips X' Pert MPD). The as-sintered surface microstructures of the samples were observed using scanning electron microscopy (SEM; Jeol: JSM-5800LV). After polishing, silver paste was fired on both sides of the samples at 600 °C for 10 min. as electrodes for the electrical properties measurement. The temperature dependence of the dielectric properties of ceramics was measured using a high precision LCR meter (GW Instek; LCR 821) by measuring the capacitance (C) and dissipation factor (D) from 30 to 500 oC, using a heating rate of 3 oC /min. The piezoelectric constant of the ceramics was measured using a piezo-d^ meter (APC International, Ltd.; YE 2730A) after poling in silicone oil at 160 oC under 3-4 kV/mm for 25 min.

3. Results and discussion:

3.1 The structural properties of (l-x)NKLN - x CT ceramics

Fig. 1 shows the XRD patterns of (l-x)NKLN - x CT samples, which had been sintered at 1000 °C for 2 h. The intensity ratio of the pair of peaks at 28 = 45 - 46.5° in each pattern was used as an indication of the tetragonal/ orthorhombic phase content. A change in the relative intensities of certain main-phase peaks, for example the 002 and 200 peaks at 45-46.5 °20, was apparent with changing CT content. It was found that addition of CT to NKLN had obvious influence on the crystal structure of the ceramics. A mixture of orthorhombic and tetragonal phases is expected for samples with 0.05 < x < 0.01 from a measured intensity ratio (Ioo/hoo) of ~1.0, Table 1. This suggests that substitution of Ca2+ and Ti4+ ions on the perovskite lattice occurs and affects phase stability. The dopant may promote the stability of the tetragonal phase in the NKLN parent composition through a slight change in the position of the tetragonal-orthorhombic phase boundary on the NKN-LN phase diagram. In addition, faint extra peaks were present which were of similar J-spacings to a tungsten bronze phase (e.g. K6Li4Nb10O30). It was also found that peaks of secondary phase became lower with increasing CT content, indicating that the CT addition can reduce the formation of unstable secondary phases enhancing the stability. Moreover, it was also found the positions of the diffraction peak of the (1-x)NKLN - x CT ceramics shifted to higher angles with the increase of x. This is due the host Na+ (ionic radius 1.39 A) and K+ (1.64 A) are replaced by doped Ca2+ (0.99 A) and Nb5+ (0.64 A) is replaced by Ti4+ (0.61 A), respectively [12]. As a result, the geometrical distortion of the ceramics was induced by the substitution of CT for NKLN lattice.

o 0 1 o o o o O T-

Ü Üx = 0 02wr 2

»Ix = 0.015 I IUI w M

JUx = 001 A/\ M

J r ~ 1 1 1 • • • • 1 1 I, . Mx=00M M 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

20 25 30 35 40 45 50 55 60 26 (degree)

Fig. 1. XRD patterns of the (1-x)NKLN - x CT (* = K6Li4Nb10O30)

Figure 2 shows the bulk density of the ceramics as a function of x and sintering temperature. The relative density of the x = 0.005 ceramics prepared by reaction-sintering method at 980 °C was very low, which is about 85% of the theoretical density. However, the relative density of the ceramics increased quickly with the increase of CT content, and reached a maximum value of 93% of the theoretical density at x = 0.02. Liquid phase sintering should be responsible for the improvement of relative density. The highest density samples were produced at a temperature of 1000 oC. For x = 0.005 CT modified NKLN the relative density was 94%, increased significantly to 96% for x = 0.01 samples and then decreased to 90% for x = 0.02 . Increasing the temperature from 1000 oC to 1020 oC led to a significant decrease in the

densities of x < 0.02 compositions. This result indicated that the optimum sintering temperature for (1-x)NKLN - x CT samples with x < 0.01 was 1000 oC, and there was only a slight enhancement in density for CT additions at this temperature. The decrease in sintered density between 1000 oC and 1020 oC, is most probably due to the effects of loss of volatile oxides (K2O and Na2O).

CaTiO3 content (mol.)

Fig. 2. Density of the (1-x)NKLN - x CT samples sintered at different temperatures

The microstructures of samples sintered at 1000 oC, was also sensitive to CT content and typical of secondary recrystallization (secondary grain growth), with a bimodal grain size composed of large grains up to, ~ 5-7 ^m in size, co-existing with ~ 1 ^m grains, Fig. 3. Moreover, it was also found that the grain size of the samples increased to 8-12 ^m with increasing CT content, up to x = 0.015. This increase probably can be attributed to segregation of the Ca2+ and Ti4+ ions at the grain boundaries and increases the mobility substantially as densification occurs. The increase in the mobility of the grain boundary increases the mass transport. As a result, grain growth is promoted and bigger grains are formed in the (1-x)NKLN - x CT ceramics at higher concentration x of CT. In other perovskites such as BaTiO3, secondary recrystallization is often thought to be associated with liquid phase formation. A related mechanism leading to the distinctive bimodal grain size distributions may be occurring in the NKLN-CT systems. Increasing of CT content to x = 0.02, led to more advanced secondary grain growth and resulting in a greater proportion of the large (secondary) grain fraction, and a narrower range of grain sizes.

3.2 The electrical properties of (l-x)NKLN - x CT ceramics

Measurements of dielectric constant as a function of temperature provided information on the phase transitions in NKLN. The values of dielectric constant (at 1 kHz) as a function of CT content for the highest density samples (produced at 1000 °C) are shown in Fig. 4. In Fig. 4(a), two-phase transitions are observed obviously above the room temperature. At the room temperature, the good dielectric constant of ~ 1014 and lowest dissipation factor of ~ 3% were found in x = 0.005 samples. This is considered to relate principally to the effects of Ca2+ and Ti4+ ions substitution on the NKLN crystal lattice, and to resultant changes in phase content. After that, the x = 0.005 modified NKLN sample showed a low-temperature transition due to an orthorhombic-tetragonal polymorphic phase transitions (TT-O) at ~ 96 °C. A transition at higher temperatures corresponded to the tetragonal-cubic ferroelectric phase transition (Curie temperature, TC) at ~ 464 °C. Increasing of CT content, TC value significantly decreased to ~ 421 °C for x = 0.02. It was clear that the TC in the (1-x)NKLN - x CT system is higher than the reported values of NKN-based ceramics [13]. The low temperature transition, TO-T shifted to below 50 °C on doping,

Table 1. A decrease in the temperatures of the O-T dielectric discontinuity in the CT- modified samples would increase the amount of tetragonal phase in the sample at room-temperature. The dissipation factor was lowered by the incorporation of CT dopant. The value fell to ~0.01 at temperatures below 200 °C for x > 0.01, from a value of 0.04 for the x = 0.005 sample. At temperatures above the Curie temperature the dissipation factors increased rapidly, owing to conductive losses (Fig. 4(b)).

The room temperature piezoelectric constant C33 of (1-x)KNLN - x CT ceramics was found as a function of x. The C33 significantly decreased with the addition of CT, the maximum value (258 pC/N) was obtained at x = 0.005, Table 1. This value is higher than previous reported for 0.93(Nao.5Ka5NbO3) -0.07LiNb03 ceramics [4]. Addition of small amounts of CT yields to larger piezoelectric constant C33 than those of pure NKLN samples (200 pC/N) [4]. The promotion may be attributed to the increased density, lowering the leakage current and enhancing the poling process.

Fig. 3. SEM images of (1-x)NKLN - x CT samples sintered at 1000 °C

Table 1. The Intensity ratios (I002/200), orthorhombic-tetragonal polymorphic phase transition temperature (TO-T), Curie temperature (TC) and piezoelectric (C33) constant of (1- x)NKLNT-xCT samples

CaTi03 content (mol.) I002/200 T0-t (oC) Tc (oC) C33 (pC/N)

0.005 0.99 96 464 258

0.01 0.97 94 440 234

0.015 1.26 38 426 160

0.02 1.40 38 421 186

Temperature (oC) Temperature (oC)

Fig. 4. Dielectric properties of (1-x)NKLN - x CT samples sintered at 1000 °C; a) dielectric constant, b) dissipation factor 4. Conclusion

CT-doped NKLN lead-free piezoelectric ceramics were prepared by a reaction-sintering method. The proper amount of CT was effective in promoting the phase evolution, densification, microstructure and electrical insulation of ceramics. The 0.995NKLN - 0.005 CT ceramic possess high Curie temperature > 460 °C. The 0.995(Na0.465K0.465Li0.07)Nb03 - 0.005 CaTi03 ceramics exhibit optimum electrical properties (d33 = 258 pC/N, sr = 1014, tan S = 0.038, and Tc = 464°C). The results reveal that (1-x)(Na0.465K0465Li0.07)Nb03 - x CaTi03 ceramics are promising candidate materials for lead-free piezoelectric application.

Acknowledgements

This research is financially supported by the Thailand Research Fund (TRF) and Commission on Higher Education (CHE), and Prince of Songkla University.

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