Structural and dielectric properties of ZrO2 added (Na1/2Bi1/2)TiO3 ceramic

Structural and dielectric properties of ZrO₂ added (Na_1/2Bi_1/2)TiO₃ ceramic

K. Kumari^I; K. Prasad^I,*; K.L. Yadav^II; S. Sen^III

^IMaterials Research Laboratory, University Department of Physics, T. M. Bhagalpur University, Bhagalpur – 812007, India

^IIDepartment of Physics, Indian Institute of Technology, Roorkee – 247667, India

^IIIMaterials Science and Technology Division, National Metallurgical Laboratory Jamshedpur – 831007, India

ABSTRACT

Polycrystalline samples of ZrO₂ added (Na_1/2Bi_1/2)TiO₃ were prepared using high-temperature solid-state reaction method at 1050ºC and subsequent sintering at 1090ºC in air atmosphere. Rietveld analyses of XRD data indicated the formation of a single-phase hexagonal structure with R3c symmetry. Dielectric studies revealed the relaxor behaviour. The addition of ZrO₂ to (Na_1/2Bi_1/2)TiO₃ shifted phase transition temperature as well as depolarization temperature to higher side which is desirable for piezoelectric applications. The frequency dependence of the temperature of the permittivity maximum was modeled using Vogel-Fulcher relation. The dielectric relaxation in the system is found to be analogous to the magnetic relaxation in spin-glass system.

Keywords: Lead free compound; Perovskite; Dielectric properties; Relaxor.

I. INTRODUCTION

Perovskite ABO₃-type oxides exhibit interesting properties providing potential applications in piezoelectric transducers, pyroelectric detectors, electrostrictive actuators, MEMs, etc. The materials used for these applications are mainly lead bearing compounds e.g. PbTiO₃, Pb(Zr,Ti)O₃, Pb(Mg_1/3Nb_2/3)O₃, etc. However, volatilization of toxic PbO during high-temperature sintering not only causes environmental pollution but also generate instability of composition and electrical properties of the products. Also, products containing Pb-based gadgets are not recyclable. Taking these aspects into consideration, search of eco-friendly lead-free compounds having either comparable or superior electrical properties for such applications are the main trends of research nowadays [1].

Sodium bismuth titanate, (Na_1/2Bi_1/2)TiO₃ (NBT) is considered to be an excellent candidate as a key material of lead-free piezoelectric ceramic, which shows strong ferroelectric properties having a phase transition temperature at 320ºC with relaxor behaviour [2-6]. Further, the electronic structure of different components of (Na_1/2Bi_1/2)TiO₃ and Pb(Zr,Ti)O₃ are: Na(Z = 11) [(Ne)3s¹]; Bi(Z = 83) [(Xe)6s²4f¹⁴5d¹⁰6p³]; Ti(Z = 22)[(Ar)4s²3d²]; O(Z = 8)[(He)2s²2p⁴]; Pb(Z = 82)[(Xe)6s²4f¹⁴5d¹⁰6p²] and Zr(Z = 40)[(Kr)5s²4d²]. The covalency between unoccupied states of the A-site ion in the perovskite structure, such as Pb 6d-states and Bi 6d-states, and O p-states favoured ferroelectric ground states [7, 10]. Also Bi³⁺ ions are isoelectronic with Pb²⁺, both showing a lone pair effect, encouraged further studies of NBT as alternative to PZT ceramics. In addition, NBT exhibits an anomaly in its dielectric properties as a result of low temperature phase transition from the ferroelectric to the anti-ferroelectric phase at about 200ºC, which is termed as depolarization temperature T_d. This T_d is an important factor for NBT and NBT-based ceramics in view of their practical uses, because the piezoelectric response disappears above T_d. It has been reported that the additives like MnCO₃ [11], La₂O₃ [12, 13], CeO₂ [13], Bi₂O₃–Sc₂O₃ [14], LiTaO₃ [15], NaNbO₃ [16], etc. showed improvement in the electrical properties of NBT while the T_d was greatly reduced. Recently, it has been observed that WO₃ added NBT system showed relaxor behaviour and both T_d (~300ºC at 1 kHz) as well as T_m ( = 438ºC at 1 kHz) shifted towards higher temperature side [1, 17]. In view of these, in the present work, structural, microstructural and dielectric studies of (1–x)NBT–xZrO₂; (0 < x < 0.1) (abbreviated hereafter NBT–ZrO₂) ceramics are reported.

II. EXPERIMENTAL PROCEDURE

A high temperature solid-state reaction method was used to prepare ZrO₂ added (Na_1/2Bi_1/2)TiO₃ ceramics using AR-grade (99.9%+ pure) chemicals Na₂CO₃, Bi₂O₃, TiO₂ and ZrO₂. The calcination temperature was kept at 1050ºC for 4h. Circular disc shaped pellets were made by applying uniaxial stress of 650 MPa. The pellets were then subsequently heated at 1090ºC for 3h in air atmosphere followed by furnace cooling. Completion of the reaction and the formation of the desired compound were checked by X-ray diffraction method. The weights of the samples were monitored before and after heat treatments. The maximum difference was about 1.12 mg for the total of 10 g for all the compounds. Therefore, the compositions of the samples were considered to be the same as the initial one. The XRD data were collected on calcined powder with a X-ray diffractometer (Rikagu miniflex, Japan) at room temperature, using CuK_α radiation (λ = 0.15418 nm), over a wide range of Bragg angles (20º < 2θ < 80º) with a scanning speed of 2º min^-1. Rietveld analyses [18] were carried on the samples to estimate the unit cell parameters, their crystal structure, profile matching, etc. The refinements were carried out using the program FULLPROF 2000 [19] under Windows XP together with WinPLOTR. The microstructure of the sintered NBT–ZrO₂ samples were taken on the fractured surface using a computer controlled scanning electron microscope (JEOL-JSM840A). The temperature dependence of dielectric constant (ε), phase angle (θ) and loss tangent (tanδ) were measured at different frequencies using a computer-interfaced LCR Hi-Tester (HIOKI 3532-50), Japan on a symmetrical cell of type Ag|ceramic|Ag, where Ag is a conductive paint coated on both the flat faces of pellets.

III. RESULTS AND DISCUSSION

Fig. 1 depicts the observed, calculated and difference profiles for each compositions of NBT–ZrO₂ after final cycle of refinement. Rietveld refinements were done on NBT–ZrO₂ system, selecting the space group R3c in Glazers notation [20, 21]. The fractional coordinates for the hexagonal setting of rhombohedral perovskites with space group R3c in terms of independent refinable parameters used for the analyses were originally developed by Megaw and Darlington [22]. It can be seen that the profiles for observed and calculated one are perfectly matching. The value of χ² comes out to be of the order of 3, which is considered to be very good for estimations. The profile fitting procedure adopted was minimizing the χ² function [18]. Also, XRD analyses indicated that ZrO₂ added NBT system do not change their basic structure i.e. rhombohedral symmetry: a = 3.890(6) Å and α = 89.71º [6]; however, some shifting in the peak positions and changes in intensities of the peaks could be observed. All the compounds were found to have single-phase hexagonal crystal structure. The refined structural parameters for the all the compositions along with their unit cell volume are illustrated in Fig. 2. It is observed that the value of a increases (from 5.4812 Å for x = 0 to 5.4897 Å for x = 0.10) while that of c decreases (from 13.4832 Å for x = 0 to 13.4774 Å for x = 0.10) with the increasing ZrO₂ content. Also an increase in the unit cell volume has been observed with the increment in additive percentage. Further, it can be seen that the changes in unit cell edges are at the third place of decimal (the overall volume increase was less than 0.3%). The average crystallite size of NBT–ZrO₂ system was estimated from some strong reflections of low 2θ values of X-ray profile using Scherrer's equation: L_hkl = 0.89λ/β_1/2 cosθ, where β_1/2 is the full width at half maximum. The average particle size was estimated to be of the order of 50 nm.

Fig. 3 shows the SEM-micrographs of NBT–ZrO₂ at 1µm magnification. Grain shapes are clearly visible, indicates the existence of polycrystalline microstructure. The grain of unequal sizes (~ 1-5µm) appears to be distributed throughout the sample. The ratio of the average crystallite size to the grain size of NBT–ZrO₂ is found to be of the order of 10^-2.

Fig. 4 shows the variation of dielectric constant (ε) and dielectric loss (tanδ) with temperature at different frequencies for 0.025 < x < 0.1. It can be seen that the temperature of maximum relative permittivity (T_m) shifted to higher temperature and dielectric maximum (ε_m) decreases with the increase in frequency for all the compositions. Also, the plots show the diffuse phase transition (DPT) with strong frequency dispersion, which clearly indicates the relaxor behavior in NBT–ZrO₂. Besides, it is important to note that the addition of ZrO₂ to NBT shifts T_m as well as T_d to higher temperature side which is desirable in case of NBT and NBT-based solid solutions for piezoelectric applications. Further, the value of tanδ in the working temperature region (from room temperature to 150ºC) was found to be of the order of 10^-2 for all the compounds. The low tanδ of this kind can be advantageous when improved detectivity is required. The variation of tanδ with temperature follows similar as ε-T behaviour for all the compounds. Also, a decrease in the value tanδ at room temperature (from 0.097 for x = 0.025 to 0.077 for x = 0.10) has been observed with the increase in ZrO₂ content (x).

Here the region around the dielectric peak is broadened, which is one of the important characteristics of disordered perovskite type structure with DPT and does not follow the Curie-Weiss law, exhibiting the following type of temperature dependence:

where γ is a critical exponent which lies in the range 1 < γ < 2. γ = 1 represents ideal Curie-Weiss behaviour while between 1 and 2 indicate diffuse behaviour [23,24]. The value of exponent γ was estimated using linear least square fitting of experimental data to expression (1) [25]. We find γ > 1 for all the cases. The value of γ > 1 imply diffuse phase transition which may be due to compositional fluctuations where the local curie points of different microregions are statistically distributed around the mean Curie temperature.

It is known that the frequency dependence of T_m in relaxor ferroelectrics cannot be described by simple Arrhenius law, which is to be expected for Debye-type relaxation process, but instead this dependence obeys the Vogel-Fulcher law [1]. In order to analyse relaxation features of NBT–ZrO₂ ceramics, the experimental data of lnf vs. 1/T_m were modeled using the Vogel-Fulcher relationship:

where f is the operating frequency, f₀ is the attempt frequency, E_a is the activation energy, T_f is the static freezing temperature and k_B is Boltzmann constant. Insets of Fig.4 shows the variation of lnf with inverse of temperature T_m where the solid circles represent the experimental data. It can be observed from this figure that the frequency derivative of 1/T_m is smaller at lower frequencies. This illustrates that as f → 0, a static freezing temperature is approached. Further, the relaxation time (τ = 1/f) is distributed over a certain temperature region. As the temperature is lowered, τ increases and at a critical value T_m = T_f, τ becomes extremely large and consequently the stable polarization is frozen to the glassy state [26]. This phenomenon has been observed in spin-glass system [27]. Excellent fitting of Vogel-Fulcher relation with experimental data suggests that this mechanism can also be employed to explain relaxor behavior in such ceramics. The fitting parameters: E_a, f_o and T_f are depicted in Table 1, which are consistent with the earlier reports on similar system [1, 26, 28]. The value of f_o is found to be in the optical frequency range of lattice vibrations in all the cases. We speculate that the addition of oxides have caused nanoscale heterogeneity within the material leading to relaxor behaviour.

IV. SUMMARY

Polycrystalline samples of NBT–ZrO₂, prepared through a high-temperature solid-state reaction technique, were found to have single-phase perovskite type hexagonal structure showed the relaxor behavior. The addition of ZrO₂ to NBT shifts T_m as well as T_d to higher temperature side and decreases the dielectric loss which is desirable for piezoelectric applications. Modeling of frequency dependent dielectric data using Vogel-Fulcher relationship gives a strong evidence for the static freezing temperature of thermally activated polarization fluctuations in the system. Therefore, the dielectric relaxation in the system may be considered analogous to the magnetic relaxation in spin-glass system.

(Received on 26 January, 2009)

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Publication Dates

• Publication in this collection
  17 July 2009
• Date of issue
  June 2009

History

• Received
  26 Jan 2009