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Dielectric and electrical properties of Na$a
2V
50
15ceramic
Banarji Behera1*, P Nayak1 and R N P Choi£ihary2
*P G Department of Physics, Sambalpur University, Sambalpur-768 019, Orissa, India department of Physics and Meteorology, IIT, Kharagpur-721 302, West Bengal, India
E-mail : banarjiJ>ehera(a>yahoo.co.in
Abstract : The dielectric and electrical properties of polycrystalltne sample NaBd2V5015 (NBV) prepared through solid-state reaction technique is presented here X-Ray diffraction study has confirmed the formation of single-phase compound with an orthorhombic structure at room temperature. The analysis of the dielectric constant (c) and loss tangent (tantf) as a function of frequency and temperature suggest the occurrence of phase transition above the room temperature and has a very low value of dielectric constant {e) at the transition temperature The electrical properties analysis (/ e ac conductivity) shows that the compound exhibit negative temperature coefficient of resistance (NTCR) type of behavior like that of a semiconductor Keywords : Dielectric, phase transition, electrical properties, NTCR
PACSNo*. : 72.80 -r, 77.22.Ch, 77 80.Bh
1. Introduction
Ferroelectric materials of tungsten bronze (TB) family has been great interest in recent years because of their growing use in electric, electro-optic, acoustic, microwave resonator, phase shifter, computer memory and display etc. devices [1-6]. Moreover a constant effort is being given to develop new ferroelectric oxides with high dielectric constant (2000-20000) and low dielectric loss (0.003-0.03) [7,8] to meet the requirement of these properties necessary for recent devices and advance technologies. These include piezoelectric transducers and actuators, non-volatile ferroelectric memories, dielectric for microelectronics and wireless communication, pyroelectric arrays and non- linear optical applications [9-11 J. Ferroelectric compounds with tungsten bronze structure have the general chemical formula [(A^AzMCMKB^tB^alCV where the A site usually filled by divalent or trivalent cations, and the B sites by Nb+5, Ta*6 or V*5 atoms. This structure consists of an arrangement of corner sharing B06 octahedrons forming three interstitial sites. Generally the smallest interstice C is empty, so the general formula is AeB^Ogo for filled tungsten bronze structure. There is a scope for variety of cations substitution at the many interstitial sites (i.e. A,, A2, B,, B2) that can tailor the physical 'Corresponding Author © 2007 IACS
64 Banarji Behera, P Nayak andRNP Choudhary
properties of the materials for device applications. Detailed literature survey reveals that there has not been reported on the titled compound NaBa2V5015 (NBV). The present paper summarizes the results of an extensive study made on the dielectric and electrical properties.
2. Experimental details
The ceramic composition mentioned in this paper has been prepared by solid-state reaction technique using high purity raw materials : Na2C03 (99.9%, M/S. Sarabhai M.
Chemicals, India), BaC03 (99.9%, M/S. Sarabhai M. Chemicals, India) and V205 (99%, Koch Light Ltd, England) in a suitable stoichiometry. The oxides and carbonates powders are weighted in stoichiometric ratios and mixed thoroughly; first in an air atmosphere for 1 h and then in alcohol (LB. methanol) for 1 h. Then mixed powders are calcined for 7 h at 833 K in an alumina crucible. The process of grinding and calcinations is repeated several times till the formation of the desired material. The calcined fine powders are cold pressed into cylindrical pellets of 10 mm diameter and 1-2 mm of thickness at a pressure of 4 x 106 N/M2 using a hydraulic press by adding poly vinyl alcohol (PVA). PVA is used as binder to reduce the brittleness of the pellet and burnt out during high temperature sintering process. Then the pellets are sintered at 773 K for 6 h in an air atmosphere using an alumina crucible. Then the sintered pellets are polished by fine emery paper to make both surfaces flat and parallel. The formation and quality of compound is studied by an X-ray diffraction technique at room temperature with a powder diffractometer (Rigaku Miniflex, Japan) using CuK„ radiation (A = 1.5405 A) in a wide range of Bragg's angles 20 (20° < 20 < 80°) with a scanning rate of 37minute. To measure the electrical properties of the compound, air-drying conducting silver paste is painted on both flat surfaces of the pellets to serve as electrodes. After electroding, the pellets are dried at 423 K for 4 h to remove moisture, if any, and then cooled to room temperature before taking any measurement.
The dielectric (dielectric constant e and dielectric loss tantf) and electrical properties are studied on the sintered pellet of the compound using a computer controlled HIOKI LCR Hi TESTER, Model : 3532. The dielectric spectrum is recorded using an ac signal of amplitude 1.5 V up to a temperature of 773 K starting from room temperature. The ac conductivity and activation energy of the compound is calculated from the dielectric data.
3. Results and discussion
The room temperature XRD pattern (Figure 1) of the calcined powders of the compound NaBa2V5015 shows the formation of a single-phase new compound. An orthorhombic unit cell was selected on the basis of the good agreement between observed and calculated d spacing (2ad= c U - d^ = minimum). The lattice parameters of the selected unit cell were refined using the least-squares substraction of a standard computer program package "POWDH [12]. These are : a = 10.4605 (18)A, b = 7.2661 (18)A, and c = 14.4543 (18)A. (The number in parenthesis is estimated standard
deviation). However, with limited data, it was not possible to determine crystal structure and the space group of the compound. The average crystallite size (D) of NBV was determined from the broadening of a few XRD peaks using tie Scherrer's equation [13], p s KAI{fiyf2cosehk), where K = constant = 0.89, A = 1.5405A and /3m - peak width of the reflection at half intensity. The average crystallite sizei of the compound is found to be 43 nm.
Dielectric study : i Here we present three figures, where the first two relating dielectric properties and the
third relating to conductivity property. ] In Figure 2 the variation of dielectric constant (e) and loss (tan S) is shown as a function of frequency at room temperature. From the analysis of the plot it is observed that both dielectric constant (e) and loss (tan S) decrease with increasing frequency, which are the characteristic features seen in polar dielectric materials.
30 40 50 60 70 80 Bragg Angle(28)
Figure 1. XRD pattern of NaBa2V5015 ceramic
Frequency (kHz)
Figure 2. Variation of dielectric constant (e) and dielectric loss (tan S) as a function of frequency.
At low frequencies, the high value of e occurs owing to the presence of different types of polarization (i.e. dipolar, ionic and electronic) and at higher frequencies the main contribution to e comes from atomic and electronic polarization.
Figure 3 shows the variation of dielectric constant (e) and dielectric loss (tan S) as a function of temperature (302-773 K) at two different frequencies 50 kHz and 100 kHz respectively. From the figure it is evident that the value of dielectric constant increases gradually with increasing temperature to a certain value of temperature showing a phase transition between ferroelectric and paraelectric phase and thereafter it decreases. The transition temperature for the two frequencies is found to exist at 611 K with a maximum value of e at about 55 and 50, respectively. Further increase of temperature beyond 660 K, e increases. This is due to the presence of space charge polarization in the bulk material. The behaviour of dielectric loss increases with increase in temperature due to the same cause as e.
The temperature dependence of ac electrical conductivity &*> is calculated from the dielectric data collected with the LCR meter and using the relation a = &i%tan S%
66 Banarji Behera, P Nayak andRNP Choudhary
where w = angular frequency, 4, = dielectric permittivity in free space. Figure 4 shows the variation of crac against the 103/7 at a frequency of 50 kHz.
1E4-J
E 1E5-^
300 350 400 450 500 550 600 650 700 Temperature (K)
Figure 3. Variation of dielectric constant (t) and dielectric loss as a function of temperature
14 16 18 20 22 24 26 28 30 32 34 1CP/T (K1)
Figure 4. Temperature dependence of ac conductivity (crac) of NBV compound with inverse of absolute temperature at 50 kHz
The conductivity versus temperature response is more or less a straight line and can be explained by a thermally activated transport of Arrhenius type crac = aQ exp (~Ea/KgT)t where o0> Ea and KB represent the pre-exponential term, the activation energy of the mobile charge carriers and Boltzmann's constant respectively The activation energy (Ea) of the material at 50 kHz is found to be 0 147 eV The activation energy is found to be low. Therefore, to activate the material a small amount of energy is required
4. Conclusion
The NBV ceramic has been prepared by high temperature solid-state reaction technique The compound undergoes ferroelectric and paraelectnc phase transition well above the room temperature and hence, the material can be used in many ferroelectric devices.
We also observe the low activation energy and decreasing resistance with rise in temperature, which indicates the NTCR behaviour of the sample like that of a semiconductor.
Acknowledgments
One of the authors (BB) acknowledges the financial support extended by PG Research fellowship of Sambalpur University, Orissa. The authors also acknowledge the use of DST-FIST sponsored central computer facility of the Department of Physics, Sambalpur University.
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