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Crystal Structure, Microstructure, Dielectric and Piezoelectric Properties of Lead-Free KNN Ceramics Fabricated via Combustion Method a
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Chakkaphan Wattanawikkam , Suphornphun Chootin & Theerachai Bongkarn
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Department of Physics, Faculty of Science, Naresuan University, Phitsanulok, 65000, Thailand b
Research Center for Academic Excellence in Applied Physics, Faculty of Science, Naresuan University, Phitsanulok, 65000, Thailand Published online: 06 Dec 2014.
To cite this article: Chakkaphan Wattanawikkam, Suphornphun Chootin & Theerachai Bongkarn (2014) Crystal Structure, Microstructure, Dielectric and Piezoelectric Properties of Lead-Free KNN Ceramics Fabricated via Combustion Method, Ferroelectrics, 473:1, 24-33, DOI: 10.1080/00150193.2014.974438 To link to this article: http://dx.doi.org/10.1080/00150193.2014.974438
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Ferroelectrics, 473:24–33, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 0015-0193 print / 1563-5112 online DOI: 10.1080/00150193.2014.974438
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Crystal Structure, Microstructure, Dielectric and Piezoelectric Properties of Lead-Free KNN Ceramics Fabricated via Combustion Method CHAKKAPHAN WATTANAWIKKAM,1 SUPHORNPHUN CHOOTIN,1 AND THEERACHAI BONGKARN1,2,∗ 1
Department of Physics, Faculty of Science, Naresuan University, Phitsanulok, 65000, Thailand 2 Research Center for Academic Excellence in Applied Physics, Faculty of Science, Naresuan University, Phitsanulok, 65000, Thailand The combustion method has been developed to prepare lead-free K0.5 Na0.5 NbO3 : KNN ceramics. Urea and glycine were used as a fuel to reduce the reaction temperatures. The effects of fuel type, calcination and sintering conditions on phase formation, morphology evolution, density, and electrical properties were investigated. A pure orthorhombic perovskite structure was obtained from KNN powders calcined at 700◦ C using glycine as a fuel. The average particle size increased approximately from 0.54 to 0.64 μm as the sintering temperature increased from 650 to 700◦ C. For the sintered ceramics, pure perovskite was found in the KNN samples sintered below 1150◦ C. The microstructure of the KNN ceramics exhibited a square or rectangular shape and the average grain size increased with increasing sintering temperatures. The transition temperatures slightly decreased with increasing sintering temperatures. The density and dielectric constant of the ceramics tended to increase with increasing temperature up to 1050◦ C, where it reached a value of 94.6% and 5839, respectively, and then decreased with further increase of sintering temperature. The piezoelectric d33 value of the most-dense ceramics was 114 pC/N. Keywords Lead-free KNN; perovskite phase; firing conditions; combustion method
1. Introduction Piezoelectric ceramics are important and widely used for sensor, actuator, transducer, buzzer and electric devices due to their excellent electrical properties [1–3]. However, these piezoelectric ceramics are mostly PZT-base ceramics, which contain more than 60% lead [4]. The toxicity of lead is a serious threat to human health and the environment [5–6]. Thus, it is urgent to develop lead-free piezoelectric ceramics to replace PZT-lead ceramics. Recently, much attention has been paid to KNN based piezoelectric properties, because of their high Curie temperature, high piezoelectric properties and compatibility with human tissue [7–10]. Unfortunately, pure KNN-ceramics have difficulty becoming fully Received in final form August 1, 2014. This paper was originally submitted to Ferroelectrics Volume 470 to honor Professor Amar S. Bhalla. ∗ Corresponding author; E-mail:
[email protected]
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dense using ordinary sintering methods [4, 10]. The reason for this problem is because Na2 O and K2 O easily evaporate at high temperatures [8]. The phase stability of pureceramics KNN is limited to 1140◦ C according to phase diagram for KNbO3 -NaNbO3 [9]. There are several methods for the fabrication of dense KNN ceramics such as; the convention method [11], the conventional method with press-less sintering [10, 12] and the polymerized complex method [13]. H. Birrol et al. [11] prepared KNN ceramics by the conventional method. The mixing powders were calcined at 825◦ C for 4 h. with a heating rate of 3◦ C/min and a cooling rate of 10◦ C/min. After being sintered at 1114◦ C for 2 h. with a heating rate of 5◦ C/min and cooling rate of 10◦ C/min in an oxygen rich atmosphere. The results showed a maximum density, a dielectric constant at Tc and a d33 of 95.9%, ∼5800 and 100 pC/N, respectively. Afterwards, H. Du et al. [10,12] fabricated KNN ceramics by a conventional method (with press-less sintered using double calcined) at 900◦ C for 5 h. and sintering temperature of 1100–1120◦ C for 2 h. The highest density, dielectric constant at Tc and d33 of 97.6%, ∼5600 and 120 pC/N were obtained from the sample sintered at 1120◦ C. H. Bo et al. [13] synthesized KNN ceramics by the polymerized complex method (PC method). K2 CO3 and Na2 CO3 were dissolved in a niobium citrate solution with an adjusted PH of 7 by using NH3 H2 O. The polymeric sol was obtained by slowly adding ethylene glycol to the solution. The high dielectric constant value of ∼1800 (at Tc ), d33 of 125 and kp of 0.40 were obtained in the sample calcined and sintered at 800 and 1100◦ C. The conventional method is relatively simple, yet it is time consuming, energy intensive, and the calcined powders are often large and inhomogeneous [14]. While the chemical route can provide ultra-fine and more homogeneous powders, these methods require a longer processing time, require expensive chemicals, special equipment and have complex procedure [15]. Recently, the combustion method has invoked interest as a way of preparing ferroelectric ceramics. It is an uncomplicated method, which produces pure ultra-fine powders, high density (resulting in excellent electrical properties) [16–18], use lower firing temperatures and a lower soak time compared to other methods [19–21]. This is because the combustion reaction provides heat that can be effectively applied to the starting materials and this energy speeds up the reaction of the materials [22]. A detailed study of KNN ceramics fabrication using the combustion method has not been reported in the literature. Therefore, in this work, the preparation of pure KNN ceramics via the combustion method using urea and glycine as a fuel was studied. The effects of fuel type, calcination and sintering conditions on phase formation, microstructure, density and electrical properties were investigated and compared to previous works.
2. Experimental KNN ceramics were fabricated by the combustion method. Reagent-grade oxide and carbonate powders of K2 CO3 , Na2 CO3 and Nb2 O5 were used as raw materials. They were milled for 24 h. using planetary milling with zirconia ball media and ethanol, dried and sieved. The mixing powders were mixed with a fuel (urea and glycine) in a ratio of 1:2 and then calcined at 650–700◦ C for 0.5–2 h. The calcined powders with PVA binder were ball-milled again for 12 h., dried and pressed into disks of 15 mm in diameter and 1.5 mm in thickness. The pellets were sintered at 1000–1150◦ C for 2 h. The crystalline phase of the sample was identified by the X-ray diffraction (XRD). The microstructure evolution was observed using a scanning electron microscope (SEM). The temperature dependence
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Figure 1. XRD pattern of KNN powders calcined at 650–700◦ C using urea and glycine as a fuel.
of the dielectric constant was determined by a LCR meter in temperatures ranging from room temperature to 500◦ C. The bulk density was measured by the Archimedes method using distilled water as a medium. The piezoelectric constant d33 was measured using a quasi-static piezoelectric d33 meter.
3. Results and Discussion Figure 1 shows the XRD pattern of KNN powders using urea and glycine as a fuel with different calcination conditions. It was found that a trace amount of the secondary phase of Nb2 O3 could be detected in the powders calcined at 650 and 700◦ C using urea as a fuel. With glycine as a fuel, the pure orthorhombic perovskite phase was detected when powders were calcined at 650◦ C for 2 h. At a calcination temperature of 700◦ C for 2 h, the pure perovskite phase was detected and an obvious separation of XRD peaks such as 200 and 002 was attended which indicated a higher crystallinity. To save energy, a calcination
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Figure 2. SEM micrograph of the KNN powders with glycine calcined at; (a) 650◦ C for 2 h, (b) 700◦ C for 2 h, (c) 700◦ C for 1 h and (d) 700◦ C for 0.5 h.
temperature of 700◦ C for 0.5 and 1 h. of soak time, using glycine as a fuel, was attempted. The result revealed that although it takes only 0.5 h. of soak time, the pure perovskite phase and high crystallinity could still be observed, as shown in Fig. 1. Therefore, the appropriate preparation condition of KNN powder was to use glycine as a fuel and then calcineat 700◦ C for 0.5 h. At the same calcination temperature, the pure perovskite phase was obtained when using glycine as a fuel. This result could not be obtained when using urea as a fuel. This may be because the combustion heat from the decomposition of glycine (13.0 kJ/g) was higher than the decomposition of urea (10.5 kJ/g) [23]. In addition, the calcination temperature and soak time of the KNN powders observed in this study were lower than those using other methods i.e. ∼200◦ C compared to the conventional method [11–12] and ∼100◦ C when compared to the PC method [13]. This may cause by the occurrence of combustion energy and the liquid medium phase which are produced by the melting and decomposing of glycine fuel during the combustion process. Figure 2 shows a SEM micrograph of KNN powders calcined in different conditions. A characteristic spherical morphology of the particle with an agglomerated formed was observed. The average particle size slightly increased with increasing calcined temperatures and soak time. The average particle size was in the range of 0.54–0.64 μm. The XRD pattern of the KNN ceramics sintered between 1000 and 1150◦ C for 2 h. is shown in Figure 3. It is evident that the sintered pellets exhibited the pure orthorhombic structure and no secondary phase can be certified when the KNN was sintered ≤1125◦ C.
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Figure 3. XRD pattern of KNN ceramics sintered at 1000–1150◦ C for 2 h.
At the sintering temperature of 1150◦ C, the trace amount of a second phase of Nb2 O3 was detected. This may be caused by the evaporation of Na2 O and K2 O at high temperature (1140◦ C) [9]. The natural surface SEM micrograph of the KNN ceramics sintered between 1000 and 1150◦ C for 2 h. is displayed in Fig. 4. A square or rectangular morphology was observed in all samples. This result was similar to previous reports [11–13]. At a low sintering temperature, many distinct pores occurred in the KNN samples (Fig. 4(a) and (b)). When the sintering temperature was increased to 1050◦ C (Fig. 4(c)), the porosity decreased significantly and the average grain size slightly increased. At 1075◦ C of sintering temperature (Fig. 4(d)), the small grains began to melt and a grain boundary began to emerge. In Fig. 4(e), at 1100◦ C of sintering temperature, the small grains melted together and a large amount of the liquid phase produced a larger grain. In Fig. 4(f), the melting of grain boundaries was found and grain sizes were increased. Moreover, the inhomogeneous between grain boundary and inner grain was observed. This suggested a different composition existed which corresponded to the phase stability of pure KNN ceramics limited to 1140◦ C [8–9].
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Figure 4. Surface SEM micrograph of the KNN ceramics sintered at; (a) 1000◦ C, (b) 1025◦ C, (c) 1050◦ C, (d) 1075◦ C, (e) 1100◦ C and (f) 1150◦ C for 2 h.
These results agree with XRD studies. The average value of the grain size, as measured by the linear intercept method, increased from 0.45 to 6.12 μm with increasing sintering temperature from 1000 to 1150◦ C, as seen in Table 1. The linear shrinkage, bulk density and theoretical density (TD) of KNN ceramics obtained at 1000–1150◦ C are listed in Table 1. The linear shrinkage of KNN ceramics increased from 5.9 to 22.4% when sintered temperature increased. The density of sintered pellets increased with increasing of sintering temperatures up to 1050◦ C and then decreased
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1000 1025 1050 1075 1100 1150
Sintered temperature (◦ C)
0.45 0.56 0.57 0.87 1.26 6.12
Average grain size (μm) 5.9 8.6 12.9 15.5 20.3 22.4
Linear shrinkage (%) 3.89 4.04 4.34 4.21 4.10 4.00
Bulk density (g/cm3) 86.2 89.4 96.4 93.4 90.7 88.7
Theoretical density (TD) (%) 231 210 202 201 198 —
To-t
411 404 403 400 399 —
Tc
Transition temperatures (◦ C)
574 720 1319 1112 847 —
εr
To-t
0.03 0.03 0.02 0.06 0.13 —
tanδ
1401 2087 5839 3941 3089 —
εr
Tc
0.19 0.16 0.07 0.13 0.38 —
tanδ
Dielectric properties
Table 1 Average grain size, linear shrinkage, bulk density, theoretical density, transition temperatures, dielectric properties of KNN ceramics sintered at 1000–1150◦ C
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Figure 5. Temperature dependence of dielectric constant of KNN ceramics sintered at 1000–1100◦ C.
as sintered temperature increased further. The highest bulk density and theoretical density of 4.34 g/cm3 and 96.4% were observed from the sample sintered at 1050◦ C, which corresponded to the microstructure investigation. The dielectric constant as a function of temperature for KNN ceramics sintered at varies temperatures is shown in Fig. 5. The dielectric curve of the sample exhibited two peaks at low and high temperatures, which represent the transformation from the ferroelectric orthorhombic to the ferroelectric tetragonal phase (To-t ) and ferroelectric tetragonal to paraelectric cubic phase (Tc ), respectively [11–12]. The transition temperatures of sintered pellets slightly decreased with increasing sintering temperatures, as can be seen in Table 1. The dielectric constant and dielectric loss at To-t and Tc tend to increase and decrease when increasing sintering temperatures to 1050◦ C and then decreased and increased as sintered temperature increased. The highest maximum dielectric constant of 5839 and the lowest dielectric loss of 0.07 were obtained from the sample sintered at 1050◦ C for 2 h (Table 1). The dielectric properties of the pellet sintered at 1150◦ C could not be measured since the sample was distorted in shape as a result of the melting of Na2 O and K2 O [9]. The dielectric properties corresponded with the density and microstructure results. The piezoelectric constant d33 of the most-dense specimen was measured, after poling under 15 kVDC in silicone oil at 110◦ C for 15 min, as a value of 114 pC/N. The density, dielectric and piezoelectric properties of the KNN ceramics in this study were higher than those produced by the conventional method. This indicated that high quality KNN ceramics could be prepared via the combustion method.
4. Conclusion Lead-free KNN ceramics were successfully prepared by the combustion method using glycine as a fuel. The calcined and sintering conditions influenced phase formation, morphology and the electrical properties of the KNN sample. A pure orthorhombic perovskite was detected in the powders calcined at 700◦ C for 0.5 h. and the dense sample was obtained in the pellet sintered at 1050◦ C for 2 h, which is lower than other methods. The average
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grain size increased with increasing sintering temperatures. The density and dielectric constant were enhanced when sintering temperatures were increased from 1000 to 1050◦ C and droped when the sintered temperature was more than 1050◦ C. Excellent density and electrical properties of TD = 96.4%, εr = 5839, tan δ = 0.07 and d33 = 114 pC/N were observed in ceramics sintered at 1050◦ C, with superior density and better electrical properties than those obtained by other methods reported previously.
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Acknowledgments Mr. Don Hindle is acknowledged for helpful comments and corrections of the manuscript. Thanks also to Department of Physics, Faculty of Science, Naresuan University for supporting facilities.
Funding This work was financially supported by the Thailand Research Fund (TRF) and Commission on Higher Education (CHE).
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