Dielectric relaxation and conductivity in Lead-Free

Dielectric relaxation and conductivity in Lead-Free Sodium Bismuth Titanate Ceramics Dhananjay K Sharma*, Raju Kumar, Seema Sharma

Radheshyam Rai, Anderi L Kholkin

Ferroelectric Research Laboratory, Department of Physics, A N College, Patna 800013, India

Departmento de Engenharia Ceramica e do Vidro, Campus Universitario de Santiago, Aveiro University, 3810-193 Aveiro, Portugal

*Email:- [email protected] Abstract- Perovskite structured Na0.5Bi0.5TiO3 (NBT) ceramics were synthesized by the solid-state sintering method. The presence of constituent phases and crystalline structure of the system was confirmed by Xray diffraction technique exhibiting perovskite structure with rhombohedral symmetry. The dielectric behavior showed a broad peak at ~4000C characterized as Curie temperature. The real part of impedance (Z’) as a function of frequency has higher values at lower frequencies and decreases up to 20 kHz and attains a constant value beyond that frequency. The broadening of peaks in frequency explicit plots of imaginary part of impedance (Z”) suggests that there is a spread of relaxation times, which involves more than two equilibrium portions. The purpose of giving dielectric data obtained from the impedance studies is to give the usefulness of the impedance formalism to evaluate the dielectric behavior in the present sample. The Nyquist plot and conductivity studies showed the NTCR character of NBT ceramic samples. Keywords: NBT; Perovskite; impedance; Curie temperature

I.

dielectric

relaxation;

INTRODUCTION

Ever since the discovery of piezoelectric e ect, piezoelectric materials have been rapidly developed and widely used. At present, the most widely-used piezoelectric materials are Pb(Zr,Ti)O3(PZT)-based ceramics because of their superior piezoelectric properties. However, because the evaporation of harmful lead oxides during the preparation of Pbcontained ceramics has detrimental influence on environment, lead-free perovskite structured

piezoelectric materials such as Na0.5Bi0.5TiO3-based oxides, bismuth layer structure oxides and tungsten bronze-type oxides have been studied in order to replace PZT-based ceramics. For using as lead-free or low lead control compositions for a control-free atmosphere and to avoid pollution during the sintering process by suppressing PbO evaporation is preferable. Sodium bismuth titanate Na0.5Bi0.5TiO3, NBT) [4-6] is considered to be a good candidate to replace PZTbased ceramics. Sodium bismuth titanate (NBT) is a ferroelectric composition with an ABO3 perovskite structure of A-site complex occupation which is rich in phase transitions: cubic paraelectric (PE), antiferroelectric (AFE) and ferroelectric (FE). NBT is ferroelectric, exhibiting high coercivity (Ec~70 kV cm-1), and appreciable rema-nent polarization (Pr~38 C cm-2) The A-site cations are ordered and the ferroelectric transition is diffuse. The material is mechanically tough and less toxic being lead -free. In all ferroelectrics, in general, the study of electrical conductivity is very important since the associated physical properties like piezoelectricity, pyroelectricity and also strategy for poling are dependent on the order and nature of conductivity in these materials. In view of the increased interest in NBT ceramic, present workers have under-taken detailed studies of electrical behavior in the temperature range from room temperature (RT) to 550C. In this paper, we aim to report the dielectric relaxation and conduction mechanism of polycrystalline NBT ceramics by impedance spectroscopy using Cole-Cole plot.

The conventional mixed oxide technique was used to prepare Na0.5Bi0.5TiO3 (NBT) ceramics. The starting materials were high purity metal oxide and/or carbonate powders 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.). The carbonates were dried at 2500C for 6h prior to use, then all the powders were weighed according to the required stoichiometry and mixed for 48h using 2propanol and zirconia media. The powders were calcined at 8000C for 2 h. The powders were milled again for 12 h. After drying powders were pressed into discs with diameter of 10 mm and thickness of 2mm. The compacted discs were sintered at 10200C for 2 h. Silver paste was fired on both sides of the samples at 2000C as electrodes. The XRD X-ray diffractometer (XRD–Philips Expert System) employing CuK radiation at 50kV and pattern was obtained, to identify the crystal phase of the samples, using an 40mA. The samples were scanned at an interval of 0.020 /min for 2 in the range 10–800. The identification of the peaks was carried out using the Topas23 refinement programme. The dielectric properties of the samples were determined at RT (room temperature) to 5500C using an impedance analyzer (PSM Impedance Analyser 1735) from 100 Hz to 1MHz.

III. RESULTS AND DISCUSSION Fig 1 shows the X-ray diffraction patterns of NBT ceramics. The XRD spectra exhibit perovskite structure with rhombohedral symmetry.

(220)

(221)

(211)

(200)

(100)

(111)

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(210)

Intensity

3000

2000

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80

0

20

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2θ 50

Fig 2 shows the dielectric properties of NBT samples at different frequencies with temperature. The dielectric constant at room temperature and dielectric maxima of the ceramics decrease with increasing frequency.

6000

01 kHz 10 kHz 100kHz 1MHz

5000

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0 0

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Temp( C)

Fig 2. Variation of dielectric constant with temperature of NBT ceramic

The dielectric constant tends to peak around ~4000C at 10 kHz which the Curie temperature corresponding to the ferroelectric to paraelectric phase transition. The dielectric peak appears to broaden with respect to increase in frequency which may be attributed to the structural disorder in the system. The electrical properties of the materials have been investigated using complex impedance spectroscopy (CIS). It is an important tool to analyze the electrical properties of a polycrystalline material in view of its capability of correlating the sample electrical behaviour to its microstructure [7].

(110)

4000

A secondary phase of Bi2O3 was also identified as at 2 ~ 290. The intensity of this diffraction peak is very small compared to the indexed diffraction lines and suggests that the secondary phase is very small (less than 2%).

ε

II. EXPERIMENTAL PROCEDURE

60

Fig 1: Room Temperature XRD pattern of NBT ceramic

Fig 3 presents the real part of impedance (Z´) as a function of frequency. Z´ has higher values at lower frequencies and decreases up to 100 kHz and attains a constant value beyond that. The pattern shows the sigmoidal variation as a function of frequency in the low frequency region followed by a saturation region in the high frequency region. This suggests

the presence of mixed nature of polarization behaviour in the material.

3.0

35

2.5

Z'(KΩ)

30

Z(K Ω)

40

200C, 240C, 280C, 320C, 360C, 400C, 440C, 480C,

420C 440C 460C 480C 500C

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25

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1.0 10000

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220 C 260 C 300 C 340 C 380 C 420 C 460 C 500 C

1000000

Freq

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towards higher frequency side with increasing temperature showing that the resistance of the bulk material is decreasing. Also, the magnitude of Z” decreases with increasing frequency. This would imply that dielectric relaxation is temperature dependent, and there is apparently not a single relaxation time. It is evident that with increasing temperature, there is broadening of the peaks and at higher temperatures, the curves appear almost flat. Fig 5 shows temperature dependent spectra (Nyquist plot) of NBT material. The impedance spectrum is featured by semicircular arcs. The nature of variation of the arcs with temperature and frequency provides various clues of the materials.

10 5 0 10000

100000

1000000

Freq(Hz)

Fig 3. Variation of real part (Z’) of impedance with frequency of NBT ceramic

Fig 4 presents the variation of imaginary part of impedance (Z´´) as a function of frequency at different set of temperatures. With the increase of frequency, the real part of impedance (Z´) and imaginary part of impedance (Z´´) decreases with increase of frequency.

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200 C, 240 C, 280 C, 320 C, 360 C, 400 C, 440 C, 480 C,

-45 -40 -35

Z''(KΩ)

-30 -25

-0.7

420 C 440 C 460 C 480 C 500 C

-0.6

-20

220 C 260 C 300 C 340 C 380 C 420 C 460 C 500 C

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Z ''(KΩ)

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0.0 100

1000

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Fig 4. Variation of imaginary part (Z ) of impedance with frequency of NBT ceramic

At higher frequency side all the curves merge. As the temperature increases, the peak in Z" vs. frequency curve appears [8]. The peak shifts

Fig 5. Variation of real and imaginary part of impedance with temperature of NBT ceramic

The impedance spectra are characterized by the appearance of a two semicircular arcs. The presence of single semi circular arc indicates that the electrical process contribution is form a bilk material (grain interior), which can be modeled as an equivalent circuit comprising of a parallel combination of bulk resistance (Rb) bulk capacitance (Cb) and leaky capacitance (Qb). Appearance of another semi circular arc near 4200 C indicates the beginning of the intergranular activities (grain boundary effect) within the sample with definite contribution from both bulk and grain boundary effects (Fig 5). The intercept of the semicircular arc with the real axis (Z’) gives us an estimate of the bulk resistance (Rb) and grain

boundary resistance (Rgb) of the material. It has been observed that the bulk resistance of the material decreases with increase in temperature showing a typical semiconducting property, i.e. negative temperature coefficient of resistance (NTCR) behavior [9]. It can be noticed that the complex impedance plots are not represented by full semicircle, rather the semicircular arc is depressed and the centre of the arc lies below the real (Z ) axis suggesting the relaxation to be of poly dispersive Non-Debye type in samples. This may be due to the presence of distributed elements in the material electrode system.

CONCLUSIONS The solid solution of pure NBT was prepared using high temperature solid state reaction technique. XRD analysis showed perovskite structure with rhombohedral symmetry. The cubic-rhombohedral transition is indicated by a broad maximum in the dielectric spectrum. The grain and grain boundary contribution have been separated using impedance spectroscopy analysis equivalent circuit to explain electrical phenomena occuring inside the material which revealed the Non-Debye type relaxation present in the material. REFERENCES [1] D.A.Hall, A. Steuwer, B. Cherdhirunkorn, P.J. Withers, T. Mori “Micromechanics of residual stress and texture development due to poling in polycrystalline ferroelectric ceramics” Journal of the Mechanics and Physics of Solids, 53, pp-249-260 (2005) [2] Rajiv Ranjan, Rajiv Kumar, Banarji Behera, R.N.P.Choudhary ” Effect of Sm on structural, dielectric and conductivity properties of PZT ceramics” Materials Chemistry and Physics, 115, pp-3-477 (2009) [3] Radheshyam Rai, Seema Sharma, R.N.P. Choudhary ” Dielectric and piezoelectric studies of Fe doped PLZT ceramics” Materials Letters, 59, pp-3921-3925 (2005) [4] B.V. B. Saradhi, K. Srinivas, G. Prasad, S.V. Suryanarayana,T. Bhimasankaram,” Impedance spectroscopic studies in ferroelectric (Na1/2Bi1/2)TiO3” Materials Science and Engineering B 98, pp-10-16, (2003) [5] J-R Gomah-Pettry, S Saı¨ d, P Marchet, J-P Mercurio,” Sodium-bismuth titanate based leadfree ferroelectric materials” Journal of the European Ceramic Society 24, pp-1165–1169, (2004)

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