Materials Chemistry and Physics xxx (2013) 1e8
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Effect of BaTiO3 additive on the electrical properties of Na0.50Bi0.50TiO3 lead free ceramics Dhananjay K. Sharma a, *, Nawnit Kumar b, Seema Sharma a, Radheshyam Rai c a
Ferroelectric Research Laboratory, Department of Physics, A N College, Patna 800013, India Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, India c Department of Ceramics and Glass Engineering and CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b
h i g h l i g h t s Detailed analysis and interpretation of impedance data of BT doped NBT reported. Complex impedance analyses of compounds reveal grain and grain boundary effect. Magnitude of Z0 decreases at lower frequencies with temperature showing NTCR effect. Decrease in relaxation time with increasing temperature represents semiconducting behaviour. Semiconducting behaviour of the compounds is believed to be due to the presence of oxygen defects.
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Article history: Received 4 December 2012 Received in revised form 28 March 2013 Accepted 26 April 2013
The near morphotropic phase boundary (MPB) compositions of lead-free piezoelectric ceramics based on sodium bismuth titanate (Na0.50Bi0.50TiO3: NBT) and barium titanate (BaTiO3: BT) were carefully investigated by conventional high temperature mixed-oxide method. All the ceramics exhibit single phase rhombohedral symmetry. The frequency (100 Hz to 1 MHz) and temperature (Room temperaturee500 C) dependence of impedance spectroscopy of (1 x)Na0.50Bi0.50TiO3exBaTiO3 (x ¼ 0.0, 0.06, 0.07 and 0.08) ceramics were investigated by impedance analyzer. The frequency explicit plots of Z00 versus frequency at various temperatures show peaks in the higher temperature range (>400 C). The compounds show dielectric relaxation, which is found to be of non-Debye type and the relaxation frequency shifted to higher side with increase in temperature. The activation energy values obtained for different BT content suggest that the electrical conduction in NBT is mainly due to the mobility of the ionized oxygen defects. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: D. Crystal structure C. Piezoelectric C. Electron microscopy (SEM) D. Electrical properties
1. Introduction Piezoelectric materials play an important role for electronic devices such as actuators, accelerators, piezoelectric motors, transducers, filters and resonators, and microelectromechanical systems (MEMS). The most widely used piezoelectric materials are PbZrO3ePbTiO3 (PZT)-based multicomponent systems [1e4] because of their excellent piezoelectric properties. However, it is recently desired to use lead-free materials for environmental protection during the waste disposal of products. Therefore, lead-free piezoelectric materials have been attracting attention worldwide
* Corresponding author. Tel.: þ91 9199017908. E-mail addresses:
[email protected] (D.K. Sharma), seema_sharma26@ yahoo.com (S. Sharma).
[5e8] as new materials in place of PZT-based piezoelectric ceramics. Lead-free piezoelectric materials, such as piezoelectric single crystals, e.g. langa-site [9], and ferroelectric ceramics with a perovskite structure [10e18], a tungsten bronze structure [19,20], and bismuth layer-structured ferroelectric [21e24], have been extensively reported. Recently, various perovskite-structured ferroelectrics such as BaTiO3 (Na0.5Bi0.5)TiO3, (Bi0.5K0.5)TiO3, KNbO3, (K,Na)NbO3, and their solid solutions have been actively studied as candidates for lead-free piezoelectric ceramics. Sodium Bismuth Titanate, (Na0.5Bi0.5)TiO3, is a perovskitestructured ferroelectric with rhombohedral symmetry (R3C) at room temperature (RT) and its phase transition is complicated. The phase transition temperatures, from rhombohedral to tetragonal and from tetragonal to cubic are approximately 340 C and 540 C on heating, respectively, for NBT single crystals. NBT ceramic shows the strong ferroelectric properties of a large remanent polarization
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.04.038
Please cite this article in press as: D.K. Sharma, et al., Effect of BaTiO3 additive on the electrical properties of Na0.50Bi0.50TiO3 lead free ceramics, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.04.038
and relatively high piezoelectric properties compared with other lead-free piezoelectric ceramics. Therefore, the NBT is considered to be an excellent candidate as a key material of lead-free piezoelectric ceramics. However, NBT ceramic is difficult to pole due to a large coercive field [25]. In the recent 2 decades, NBT-based solid solutions [17,19e32] and A-site substituted NBT [25e28] that can be poled easily were extensively studied. Among these NBT-based systems, (1 x) (Na0.5Bi0.5)TiO3exBaTiO3 is more focused. Takenaka et al. [29] pointed out that at room temperature it has a rhombohedral (Fa)etetragonal (Fb) Morphotropic Phase Boundary (MPB) at x ¼ 0.06e0.07 where the system shows outstanding piezoelectric and dielectric properties of all the lead free ceramic resources. In the present work, the 1 x(Na0.5Bi0.5TiO3)ex(BaTiO3) system with x ¼ 0.0, 0.06, 0.07, and 0.08, (NBTeBT) near MPB were prepared by the conventional solid state reaction method and their structural, microstructural and electrical properties were studied systematically. 2. Experimental procedure Polycrystalline samples of (1 x)Na0.50Bi0.50TiO3exBaTiO3 (NBTeBT), where (x ¼ 0.0, 0.06, 0.07, and 0.08), were synthesized from high purity oxides of Bi2O3 (99.9%, Aldrich Chem. Co), Na2CO3 (99.5%, Aldrich Chem. Co), BaCO3 (99.9%, Aldrich Chem. Co.) and TiO2 (99.9%, Aldrich Chem. Co.) using high temperature solid state reaction technique in an ambient atmosphere. The constituent compounds in suitable stoichiometry were thoroughly mixed in a ball milling unit for 24 h. The calcined fine powder was cold pressed into cylindrical pellets of 10 mm in diameter and 1e2 mm in thickness using a hydraulic press at a pressure of 6 107 kg m2. These pellets were sintered between 1000 and 1050 C for 2 h in air. The formation and quality of compounds were verified by X-ray diffraction (XRD) technique. The XRD pattern of the compounds was recorded at room temperature using X-ray powder diffractometer (Rigaku Minifiex Japan) with Cu Ka radiation (¼1.5418 A) in a wide range of Bragg angles 2q (20 2q 80 ) at a scanning rate of 2 min1. The microstructural examination was carried out by scanning electron microscopy (SEM) using Jeol 6300 and Philips XL30, equipped with energy dispersive spectrometer (EDS) for chemical analysis. For electrical measurements, the polished surfaces of the sintered pellets were electroded with air drying silver paste. Impedance spectroscopy of the compounds was investigated using an Impedance Analyzer (Newtons 4th Limited) as a function of frequency at room temperature (RT) and temperature (RT to 500 C) at different frequencies (100 Hz to 1 MHz). 3. Result and discussion Fig. 1 shows the X-ray diffraction patterns of NBTeBT ceramics. The spectra examined are single phase with perovskite type structure without any secondary phases. At room temperature, the symmetry of NBT is rhombohedral and BT is tetragonal. Our solid solutions have rhombohedral morphotropic phase boundary. The narrow and symmetric X-ray diffraction peaks of the NBTeBT compounds indicate homogeneity and good crystallization of the samples. All the reflection peaks were indexed using observed interplanar spacing d, and lattice parameters of compound were determined using a least-square refinement method. A good agreement between calculated and observed d values of all diffraction lines (reflections) of NBTeBT system with different x content suggests that the basic crystal structure is rhombohedral. It is also noted that the diffraction peaks shift slightly towards low diffraction angles as x increases means lattice parameter increases with increase in x. This may be attributed to the larger ionic radii of Ba2þ than those of (Na0.50B0.50)þ and Ti4þ. In addition, it was seen
Intensity (arb. unit)
D.K. Sharma et al. / Materials Chemistry and Physics xxx (2013) 1e8
Intensity (arb. unit)
2
(110)
(111)
(d)(100)
(200)
(211) (210)
2θ (degrees) (220) (221) (310)
(c) (b) (a) 20
30
40
50
60
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2 (degrees) Fig. 1. Room temperature XRD pattern of (1 x)NBTexBT, (a) x ¼ 0.00, (b) x ¼ 0.06, (c) x ¼ 0.07 and (d) x ¼ 0.08.
that the cell volumes of the ceramics increased gradually with increasing x, which can explain why the diffraction peaks shifted slightly towards low diffraction angles as x increased. Speculation regarding the A-site substitution is made based on the random solid solution model, ionic radii, and coordination number of the introduced guest ion in respect to the A- and B-sites. Since the perovskite is a closed-packed structure, the possible interstitial sites are bounded by charged ions (negative or positive ions). This implies that there is a smaller chance of having a high concentration of interstitial sites. Therefore the cation lattice sites (A or B) are locations with the highest possibility of occupation by the guest ion. The ionic radii of B-site ion Ti4þ with a coordination number (CN of 6) is 0.60 A respectively. The ionic radii of A-site ions such as Naþ and Biþ (CN of 6) are 1.02 A and 1.03 A respectively. Thus due to the close proximity of the ionic radii of Naþ and Biþ3, it is postulated that the Ba2þ (CN: 6 and rþ Ba: 1.35 A) substitutes for the potassium in the A-site of perovskite lattice. Fig. 2 exhibits the microstructure (SEM) of the compounds with different x values. The fracture surfaces show homogeneous grains and dense structure which is in good agreement with the density (5.65 g cm3), determined experimentally by Archimedes method. The introduction of BT in NBTeBT ceramics results in the reduction of grain size of the parent compound. It is evident from the figure that with BaTiO3 content above 6%, the grain size is well smaller than 1 mm. For PZT- and PLZT-based ceramics, the substitution of Ba2þ for Pb2þ generally leads to a great inhibition of grain growth. Similar to Ba-modified lead-based ceramics, in the NBTeBT ceramics, grain growth is inhibited after the introduction of BaTiO3 into (Na0.5Bi0.5)TiO3. Fig. 3 represents the dielectric spectrum 3 0 (u) of (1 x)NBTexBT (x ¼ 0, 0.06, 0.07 and 0.08) at different temperatures. The observed variation of 3 0 with frequency can be attributed to frequency relaxation in the material. A high degree of dispersion of the permittivity is identified at high temperature (from 300 C) and at low frequencies (
