TEMPERATURE DEPENDENCE ON DIELECTRIC

Advanced Materials Research Vol 1107 (2015) pp 45-52 © (2015) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.1107.45

TEMPERATURE DEPENDENCE ON DIELECTRIC RESPONSE OF La0.7Ba0.3Mn0.4Ti0.6O3 CERAMIC Aaliyawani Ezzerin Sinina, Walter Charles Primusa*, Abdul Halim Shaarib, Zainal Abidin Talibb, Sinin Hamdanc a

Department of Basic Science and Engineering, Faculty of Agriculture and Food Sciences, Universiti Putra Malaysia Bintulu Campus, 97008 Bintulu, Sarawak, Malaysia. b Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia. c Mechanical Department, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Samarahan, Sarawak, Malaysia * email: [email protected] Keywords: ceramic, crystal structure, dielectric properties, circuit model.

Abstract Ceramic sample of La0.70Ba0.30Mn0.40Ti0.60O3 oxide has been prepared by the conventional solid-state reaction method. The sintered sample was characterized by using x-ray diffraction (XRD) and low frequency LCR meter. XRD result shows that the sample has a cubic structure with the existence of impurity phase. The dielectric properties of La0.70Ba0.30Mn0.40Ti0.60O3 measured from room temperature to 200ºC shows that the dielectric permittivity is temperature dependence with strong dispersion at low frequencies. A circuit model based on the universal capacitor response function is also being used to represent the dielectric properties of the sample. 1. Introduction An ideal perovskite structure has centrosymmetric with the general formula ABO3, where A is a monovalent or divalent metal and B is a tetravalent or pentavalent metal. Lanthanum Manganite, LaMnO3 is a Mott insulator and has a canted anti-ferromagnetic layered structure [1]. It exhibits a spin order of an A-type antiferromagnet. The interesting doping range of 0.2 ≤ x ≤ 0.5, the perovskite manganites La1-xMxMnO3 (M = Ca, Sr, Ba) have attracted much interest due to their characteristic magnetic and transport properties like the colossal magnetoresistance [2]. However, doping with 30% amount of calcium in La site leads to the lattice distortion and thus makes the material to be ferromagnetic and conductive. This phenomenon is known as the Jahn-Teller (JT) effect [3].Barium Titanate, BaTiO3 is a ferroelectric material with high dielectric constant. It has a perovskite structure which was published in 1945 by an Irish crystallographer H. D. Megaw (1907-2002). The crystal is cubic with Ba2+ ions at the cube corners, O2- ions at the centers of the cube faces, and Ti4+ ions at the cube centers [4]. Barium Titanate is widely utilized to manufacture electronic components such as multilayer capacitors (MLCs), PTC thermistors, piezoelectric transducers, and a variety of electro-optic devices [5]. The attempts to combine both ferromagnetic and ferroelectric properties started in 1960s, predominantly by the group of Smolenskii in St. Petersburg (then Leningrad) and by Venevtsev in Moscow. This phenomenon occurred due to the existence of both electric and magnetic ordering at the same phase. To date, few compounds such as TbMnO3, BiMnO3, BiFeO3, La0.7Sr0.3Mn0.96Co0.4O3, La0.7Sr0.3MnO3 and PbVO3 have been found to show both properties. In the last few years the interest of multiferroic materials has increased due to its wide range of applications especially in electronic devices. In this paper, the dielectric properties of La0.70Ba0.30Mn0.40Ti0.60O3 sample have been measured as a function of temperature. The microstructures of this sample were also discussed. 2. Theory In dielectric spectroscopy, frequency dependent contributes to the dielectric polarization. Dielectric polarization can be categorized as electronic polarization, atomic polarization, All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 119.40.120.193-31/03/15,05:51:46)

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SOLID STATE SCIENCE & TECHNOLOGY Towards an Immersive Breakthrough

orientational polarization, and interfacial polarization. Electronic polarization happens to be in the ultraviolet (UV) frequency range. As for atomic polarization and orientational polarization concerned, they happen to be in the infrared frequency range and microwave frequency range respectively. Interfacial polarization is caused by the dielectric material itself or defects on the crystal and free charge carriers such as electrons that happens at the low frequency range [6]. An equivalent circuit model representing the electrical characteristic of a material can be given based on the universal capacitor response function,

Cn  Bi 

n 1

*

* Cn  Co  f   fo 

n 1

 2  iC  f f 

sin n 

o

o

n 1

 2

cos n 

(1)

where B is a constant, i   1 , ω is the angular frequency, and n is the correlation coefficients for specific intracluster, a variable from 0 to 1. Hence, the universal capacitor response function is used to fit the dielectric data obtained. The universal capacitor equation, Equation 1 is substituted in Equation 2 to get the quasi d.c. response. *

Cq.d .c *  C1  C2

*

(2)

where C1  Bi  and C2  Bi  respectively. p is the fractional correlation index for the transport of the quasi-free charge between clusters. For diffusion phenomena, the diffusion function is given by * s Cdiffusion  Bi  (3) where s is close to 0.5. *

p

*

n 1

3. Experimental Sample of La0.70Ba0.30Mn0.40Ti0.60O3 was prepared by the conventional solid-state reaction method. The powders for each element were firstly weighed according to the stoichiometric ratios required before mixing thoroughly for 24 hours in order to obtain a homogeneous mixture. The mixture was then ground for 30 minutes using a mortar. Then the mixed powder was calcined for two times; first at 800 °C for 12 hours, and second at 1000 °C for 12 hours with intermediate grinding before pressed into pellet form for sintering process at 1200 °C for 12 hours. After sample preparation, their microstructure and dielectric properties were characterized using XRD method and low frequency LCR meter respectively. To measure the dielectric properties, the pellet has been polished on both sides of the surface before coated with silver paint for better contact. The dimension of pellet was measured for further analysis on dielectric properties. The sample was measured at different temperature starting at room temperature to 200ºC. For each temperature, frequency ranging from 0.01 Hz to 100 kHz was applied. To find the crystal structure of the sample, the X-ray diffraction method was used and scanned from 20º to 80º. 4. Results 4.1 Microstructure In Figure 1, it can be seen that the sample's grain has merged due to the high sintering temperature. The sample also shows to be dense and less void. The shape of the grains observed is like a rectangular shape. The consolidation of the grains might reduce the grain boundary area. Figure 2 shows the X-ray diffraction pattern of La0.70Ba0.30Mn0.40Ti0.60O3. The diffraction pattern that was observed is attributed to cubic perovskite structure with lattice parameter a = b = c =3.944 Å. This result is in agreement with Ming Li et al [7].

Advanced Materials Research Vol. 1107

47

Figure 1: SEM micrograph of La0.70Ba0.30Mn0.40Ti0.60O3. (110)

(211)

(200) (100)

*

(111) *

(220)

(310)

Figure 2: XRD pattern of La0.70Ba0.30Mn0.40Ti0.60O3. 4.2 Dielectric properties Temperature dependence of the dielectric permittivity of La0.70Ba0.30Mn0.40Ti0.60O3 is shown in Figure 3 and Figure 4. In Figure 3, It is seen that the dielectric constant decreases towards higher frequencies and at low frequencies,  ' increases at higher temperature. In Figure 4,  " decreases towards higher frequencies. At lower temperature (room to 80º), at lower frequencies, strong dispersion is observed, while weak dispersion at higher frequencies. Both figure shows that the sample is affected by temperature change where each real,  ' and imaginary,  " parts of the permittivity increases as the temperature increased. Since electron hoping is thermally activated, an increase in temperature result in increases dielectric polarization. Thus in turn causes an increase in dielectric constant [8].

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Figure 3: Real component of permittivity of La0.70Ba0.30Mn0.40Ti0.60O3 at different temperatures

Figure 4: Imaginary component of permittivity of La0.70Ba0.30Mn0.40Ti0.60O3 at different temperatures The measured complex capacitance for La0.70Ba0.30Mn0.40Ti0.60O3 at several different temperatures is shown in Figure 6. By using the universal capacitor response function, the circuit model for each temperature was obtained and the parameters for fitting are tabulated in Table 1. There is a change in circuit arrangement as the sample is heated compared to the sample measured at room temperature. At room temperature, the dielectric properties can be represented by two parallel circuits connected in series as shown in Figure 5. As temperature increase, the dielectric response can be represented by universal capacitors arranged in parallel with high frequency capacitance.


(a)

Cquasi d.c.

49

(b) Cdiffusion

C∞1

Cquasi d.c.

C∞2

C∞2

Figure 5: Circuit arrangement for La0.70Ba0.30Mn0.40Ti0.60O3 at (a) higher temperatures, (b) room temperature.

(a)

(b)

C E' C E" CF' CF"

C E' C E" CF' CF"

50


C E' C E" CF' CF"

(c)

C E' C E" CF' CF"

(d)

(e)

(f)

C E' C E" CF' CF"

C E' C E" CF' CF"

Figure 6: Complex capacitance of La0.70Ba0.30Mn0.40Ti0.60O3 at (a) room temperature, (b) 40º, (c) 80º, (d) 120º, (e) 160º, (f) 200º.


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Table 1: Simulated parameters obtained from the universal capacitor response function of La0.70Ba0.30Mn0.40Ti0.60O3. Temperature, T (ºC)

Cs (x10-5F)

Fs (Hz)

s

Co (x10-9F)

Fc (Hz)

C∞1 (x10-6F)

C∞2 (x10-10F)

27

3.50

0.01

0.53

95.00

1300

0.8800 0.90

4.50

8.00

40

-

-

-

3.00

230

0.9950 0.80

-

2.00

80

-

-

-

2.80

1200

0.9970 0.85

-

0.06

120

-

-

-

2.50

5000

0.9980 0.70

-

6.00

160

-

-

-

80.00

100000 0.9993 0.80

-

6.00

200

-

-

-

1.80

150000 0.9990 0.85

-

1.00

p

n

It is clear that when the sample is heated, the interception of the real and imaginary part of the complex capacitance shifts to a higher frequency. In fitting, the crossing frequency, is given by the parameter, fc. It can be seen that the fc is temperature dependence. This may be due to electron hopping results in local displacements of electrons in the direction of external filed applied. Since electron hopping is thermally activated, an increase in temperature increases electrical conductivity due to the increase in the drift mobility [9]. From the fc in Table 1, the activation energy, Ea of La0.70Ba0.30Mn0.40Ti0.60O3 is calculated using Equation 4 below.

f c  f 0e

E

(4)

kT

where fc is the crossing frequency, k is the Stefan Boltzmann constant, and T is the temperature. By using the gradient of graph ln fc versus 1/T as in Figure 13, the activation energy calculated for La0.70Ba0.30Mn0.40Ti0.60O3 is 0.55 eV.

Ea = 0.55 eV

Figure 7: Graph of ln fc versus 1/T 5. Conclusion It can be concluded that the ceramic sample of La0.70Ba0.30Mn0.40Ti0.60O3 oxide has a cubic perovskite structure with lattice parameter of 3.944 Å with high density. The permittivity increases

52


as temperature increased because electron hoping is thermally activated, an increase in temperature result in increases dielectric polarization. The interception between the real and imaginary part of the complex capacitance shifts to a higher frequency as the temperature increases. From all the data obtained, the activation energy of the sample is calculated to be 0.55 eV. As for the circuit modeling, the La0.70Ba0.30Mn0.40Ti0.60O3 sample show a change in circuit arrangement as the sample is heated from two parallel circuits connected in series to only one circuit with two capacitors arranged in parallel. Acknowledgement We would like to acknowledge the UPM under RUGS grant (Vote no.: 919987) for the research funding. References [1] M. P. Sharma Jain, S. K. Anjali Krishnamurthy, Bipin K Srivastava, Synthesis and characterization of sodium substituted lanthanum manganite, Indian Journal of Pure & Applied Physics. 46 (2008) 325-329. [2] N. Abdelmoula, E. Dhahri, N. Fourati, L. Reversat, Monovalent effect n structural, magnetic and magnetoresistance properties of doped manganite oxides, Journal of Alloys and Compounds. 365 (2004) 25-30. [3] R. S. Claudy, R. S. Jyoti, G. Anirba, Charge-order Driven Multiferroic and magneto-dielectric Properties of rare earth manganates, Bull. Mater.Sci. 33(2) (2010) 169-178. [4] W. N. Ashcroft, N. D. Mermin, Solid State Physics, CBS Publishing Asia LTD, Philadelphia, 1987. [5] Hsiao-Lin, Wang, Structure and Dielectric Properties of Perovskite-Barium Titanate (BaTiO3), 2002, San Jose State University, United States of America. [6] M. H. Abdullah, Komposit Ferit sebagai Penyerap Mikrogelombang Memanfaatkan Bahan Tempatan, Penerbit Universiti Kebangsaan Malaysia, Malaysia, 2002. [7] L. Ming, F. Antioni, D. C. Sinclair, Origin of high permittivity in (La0.4Ba0.4Ca0.2)(Mn0.4Ti0.6)O3 ceramics, Journal of Applied Physics. 98 (2005) 0841011. [8] R. S. Devan, B. K. Chougule, Magnetic properties and dielectric behavior in ferrite/ferroelectric particulate composites, Physica B. 393 (2007) 161-166. [9] S. S. Chougule, B. K. Chougule, Response of dielectric behavior and magnetoelectric effect in ferroelectric rich (x)Ni0.9Zn0.1Fe2O4+(1-x)PZT ME composites, Journal of Alloys and Compounds. 456 (2007) 441-446.