Dielectric and impedance properties of Li0.5Fe2.5O4 doped BaTiO3 composite ceramics

Dielectric and impedance properties of Li0.5Fe2.5O4 doped BaTiO3 composite ceramics

Results in Physics 11 (2018) 899–904 Contents lists available at ScienceDirect Results in Physics journal homepage: www.elsevier.com/locate/rinp Di...

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Results in Physics 11 (2018) 899–904

Contents lists available at ScienceDirect

Results in Physics journal homepage: www.elsevier.com/locate/rinp

Dielectric and impedance properties of Li0.5Fe2.5O4 doped BaTiO3 composite ceramics

T



Ganapathi Rao Gajulaa, , K.N. Chidambara Kumara, Lakshmi Rekha Buddigab, Gnana Praveena Nethalac a

Department of Physics, BS&H, Sree Vidyanikethan Engineering College, Tirupati 517102, A.P, India Department of Chemistry, Andhra University, Visakhapatnam 530 003, A.P, India c Department of Physics, BSH, Swarnandhra College of Engineering and Technology, Narsapur 534280, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Dielectric constant Dielectric loss Impedance Temperature Frequency XRD and FESEM

The sintered composite Li0.5Fe2.5O4 (LF) doped BaTiO3 (BT) having chemical formulae (1−x) BT+ (x) LF (where x = 0, 0.05) have been prepared using conventional solid state reaction technique. We have characterized the sintered (0.95) BT+ (0.05) LF (BTL) composite by XRD, FESEM and impedance analyzer. The XRD studies confirm the tetragonal structure and lattice parameters have calculated from the XRD peaks. The FESEM image confirms that the composite exhibits dense microstructure with an average grain size of 1.580 µm. We have studied the variation of dielectric constant and dielectric loss with frequency (in the range from 0 Hz to 10 MHz) at five different temperatures (27 °C, 123 °C, 219 °C, 323 °C and 429 °C) and the variation of dielectric constant and dielectric loss with temperature (from 20 °C to 500 °C) at four different frequencies (1 kHz, 10 kHz, 100 kHz and 1 MHz). The dielectric studies indicate that both the dielectric constant and the dielectric loss of BTL increase with increase in the temperature at different frequencies and decreases with increase in the frequency. The impedance study over a wide range of temperature and frequency indicates that these properties of all the samples are temperature dependent and frequency dependent.

Introduction The high dielectric constant ceramic materials have adequate technological application in novel electronic devices. The composite materials with combined ferroic properties can be used in specific device applications like multiple state memory elements, sensors, and spintronics [1] and potential applications like waveguides, switching [2], modulation of amplitudes, transducers [3,4]. The coexistence of multi-phase in these materials entitles their use in designing devices like transducers, actuators [5,6]. The reduction in ferroelectric distortion due to the transition ‘d’ electrons leads to the development of multiferroic [7–9]. For the miniaturization of the communication devices, we require ceramic materials having high dielectric constant. One such ceramic material is Barium titanate (BaTiO3) possessing both high dielectric constant and low dielectric loss properties [10,11] which is widely used to manufacture electronic components [12] and also used in electronic devices such as mobile phones, personal computer and electronic passive components on account of its excellent dielectric, ferroelectric and piezoelectric properties [13–15]. Also BaTiO3 is used as multilayered ⁎

ceramic capacitors which are an important passive component in electronic devices [11]. The reason for the common use of BaTiO3 in the electronic devices is that its dielectric constant can be varied by applying a d. c electric field. BT has a high bulk polarization and high dielectric constant because the Ti ions shift from random to align position when an electric field is applied [16]. LF, an inverse spinel crystal structure material, is one of a useful material for microwave devices and memory core applications. In LF, all the Li1+ ions and 3/5 of the Fe3+ ions occupy octahedral site and the remaining Fe3+ ions occupy tetrahedral sites. Iron ions in octahedral site transits from ordered phase (α) to disordered phase (β) based on the distribution of ions which also depends on sintering temperature [17]. In this paper we have mainly focused on electrical properties like the variation of the dielectric constant, dielectric loss and impedance with temperature at different frequencies (1 kHz, 10 kHz, 100 kHz and 1 MHz), the variation of the dielectric constant, dielectric loss and impedance with frequencies at different temperature (27 °C, 123 °C, 219 °C, 323 °C and 429 °C) of BT doped with LF.

Corresponding author. E-mail address: [email protected] (G.R. Gajula).

https://doi.org/10.1016/j.rinp.2018.10.057 Received 30 September 2018; Received in revised form 28 October 2018; Accepted 29 October 2018 Available online 02 November 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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Experimental technique

and small pores are visible in the sample. The BT and BTL composite exhibits clear grain, grain boundaries. The average grain size of the BT and BTL samples calculated using imageJ software are 1.269 µm and 1.580 µm respectively. From these values we strongly confirm that the grain size of BT increases with substitution of LF in it. The higher value of the grain size indicates the high dense nature of BTL composite which is due to the presence of stress between the LF and BT phases [20].

We have prepared the BTL composites using conventional solid state preparation method. The powders Ba2Co3, TiO2, Li2CO3, and Fe2O3 are grinded using agate motor for 10 h. This sample was calcinated at 900 °C temperature for 3 h. The calcinated powder was again grinded till fine powder is obtained, then a binder is prepared by Polyvinyl alcohol ((-C2H4O)n PVA) powder and added to the calcinated powder which is further grinded to obtain a uniform fine powder. The fine powder is drawn into pellets using a hydraulic press. These pellets were sintered at temperature 1150 °C for 3 h. We have used this sintered powder and pellets for different characterizations like XRD, FESEM, dielectric studies. The XRD Measurements were carried out using Bruker D8 Advance X-Ray Diffractometer (Cu Kα radiation of wavelength 1.5406 Å). The FESEM images of the grown samples are analyzed using Carlzeiss ultra55. The dielectric measurements have been obtained using impedance analyzer, Novo control technology.

Dielectric studies Temperature dependence of dielectric constant (ε′) The dependence of the dielectric constant of BTL with the temperature at different frequencies 1 kHz, 10 kHz, 100 kHz and 1 MHz is shown in Fig. 3. We see from Fig. 3, the dielectric constant increases slowly initially with an increase in the temperature till 300 °C and beyond 300 °C, the dielectric constant increases steeply and attain maximum value at 500 °C temperatures. As the frequency is increased, we see from Fig. 3, that the dielectric constant decreases. The high value of dielectric constant is due to the incorporation of LF in BT. At low frequency, the reason for the rapid increase in the dielectric constant when the material is at high temperature is mainly due to dipolar polarization. The accumulation of charges on the grain boundaries increases, causing an increase in the interfacial polarization with increase in the temperature. This phenomenon is dominant at lower frequencies. The relatively insignificant variation of the dielectric constant with temperature observed at higher frequencies is attributed to an electronic polarization which is independent of both frequency and temperature [21]. According to the Maxwell-Wagner theory, the dependence of the dielectric constant upon the temperature at low frequencies is ascribed to the presence of uncompensated surface charge at ferroelectric ferrite interface of the composites [22,23].

Results and discussions X-ray diffraction The powder X-ray pattern of BT and BTL composites are presented in Fig. 1. The diffraction peaks of BT and BTL, indexed using JCPDS no 79-2263, indicates that the BT and BTL composite possess the tetragonal perovskite structure. Also these indexed peaks indicate the ferroelectric phases. The incorporation of LF in BTL is confirmed by the presence of peak at 34° which has been indexed using JCPDS no. 897832 & 88-6711. Fig. 1 shows that the parent phase BT has not changed after incorporation of LF in it [18]. This clearly indicates that the BTL composite is in phase with the BT. The diffraction peaks of BTL composite exhibits ferroelectric and low ferrite phases without the presence of impurities. The lattice parameters of BTL for ferroelectric phase and ferrite phase have been calculated using following equations 1 dhkl = 2 2 2 for ferroelectric phase and dhkl

Temperature dependence of dielectric loss (tan δ) The dependence of the dielectric loss (tan δ) of BTL with temperature at four different frequencies 1 kHz, 10 kHz, 100 kHz and 1 MHz is shown in Fig. 4. We see from Fig. 4 that the dielectric loss of BTL increases with increase in the temperature and the nature of variation of dielectric loss with temperature are same as that of the dielectric constant vs temperature curve (shown in Fig. 3). Beyond 300 °C, the dielectric loss increases abruptly with an increase in the temperature. At 500 °C, the dielectric loss of BTL reaches high value for 1 kHz, but as the frequency is increased, we see from Fig. 4, the dielectric loss decreases. The hopping of electrons between Fe+3 and Fe+2 may lead to the increase in the dielectric loss of BTL as the temperature is increased. The maximum value of the dielectric loss at high temperature can be described by the Koop’s model [24], according to which a solid is made of grains and grain boundaries. The grains have low resistivity and large thickness whereas the grain boundaries have high resistivity and small thickness. Thus the high value of dielectric loss at higher temperature may be due to the effect of grains because the grains play a vital role at higher temperature.

h +k l + 2 a2 c

=

a h2 + k2 + l2

for ferrite phase

The lattice parameters of BTL are a = 4.004 Å, c = 4.009 Å and c/ a = 1.001 for ferroelectric phase and a = 8.500 Å for ferrite phase. The lattice parameters of BTL for ferroelectric phase are similar to the previous reported value of BT [19]. Morphological studies Fig. 2 shows the FESEM micrographs of BT and composite BTL. We see from Fig. 2, the micrographs of BT and BTL confirm the roughness

Frequency dependence of dielectric constant (ε′) The dependence of the dielectric constant of BTL with the frequency at five different temperatures 27 °C, 123 °C, 219 °C, 323 °C and 427 °C is shown in Fig. 5. From Fig. 5, we clearly see that the dielectric constant decreases with increase in the frequency at all temperatures. Among all the above said five different temperatures, the dielectric constant is more at 427 °C in the low frequency region and decreases with increase in frequency upto 100 Hz. Beyond 100 Hz frequency, the dielectric constant of BTL reaches a constant minimum value which confirms that at higher frequencies, the dielectric constant is independent of frequency. As and when the temperature is decreased, the dielectric constant decreases abruptly at low frequency region. Also we see that the

Fig. 1. X-ray diffraction pattern of BT and BTL composite. 900

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Fig. 2. FESEM micrographs of a) BT and b) BTL composite ceramic.

the dielectric constant of the material decreases and this can be described on the basis of the dipole orientation in composites, in which the dipoles cannot respond to the applied electric field [25,26]. At low frequency, the occurrence of electronic polarization due to the polaron hopping mechanism contributes to the dispersion and hence the dielectric constant reaches high value. Frequency dependence of dielectric loss (tan δ) The dependence of dielectric loss of BTL with the frequency at five different temperatures (27 °C, 123 °C, 219 °C, 323 °C and 427 °C) is shown in Fig. 6. From Fig. 6, we clearly see that at 427 °C, the dielectric loss of BTL is high at low frequency and decreases steeply with increase in the frequency upto 1 kHz, beyond this frequency, the dielectric loss attains a constant value. This may be due to the presence of a macroscopic distortion and the space charge polarization [27]. Similar nature of the dielectric loss is observed at the remaining temperatures 27 °C, 123 °C, 219 °C, 323 °C. We see from Fig. 6, that the dielectric loss decreases with temperature at low frequency region. Fig. 3. Variation of dielectric constant (ε′) with temperature at different frequencies of BTL composite.

Impedance studies

Fig. 4. Variation of dielectric loss (tan δ) with temperature at different frequencies of BTL composite.

Temperature dependence of impedance (Z′) The variation of impedance of BTL with the temperature at four different frequencies 1 kHz, 10 kHz, 100 kHz and 1 MHz is shown in Fig. 7. From Fig. 7 we see that the impedance of BTL at 1 kHz increases with increase in the temperature upto a certain temperature, then decreases with further increase in temperature and beyond 170 °C, the impedance attains a constant minimum value. The temperature at which the impedance reaches maximum value is called transition temperature Tc. The transition temperature of the BTL composite at 1 kHz is 170 °C. A similar nature of the variation of the impedance with temperature is also seen for 10 kHz, 100 kHz and 1 MHz frequency. The transition temperatures of BTL composite are 222 °C, 278 °C and 412 °C at frequencies 10 kHz, 100 kHz and 1 MHz respectively. We see from Fig. 6 that as and when the frequency is increased, the transition temperature Tc gets shifted towards high temperature regions. We also see from Fig. 6 that at the curie temperature, the impedance of the BTL composite decreases with increases in the frequency. This results in the enhancement of resistivity of material with temperatures at lower frequencies [28]. Frequency dependence of impedance (Z′) The variation of impedance with frequency of BTL at five different temperatures 27 °C, 123 °C, 219 °C, 323 °C and 427 °C is shown in Fig. 8. We see from Fig. 8, the impedance is more at low frequencies and

dielectric constant reaches minimum value at all five different temperatures which signifies that the dielectric constant is independent of frequencies beyond 100 Hz. On account of the interfacial polarization, 901

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Fig. 5. The dielectric constant (ε′) varies with frequency at different temperatures of BTL composite.

323 °C and 427 °C respectively attaining constant may be due to the fact that space charge effect is negligible at high frequency region [32,33].

Conclusions The BTL composite have successfully synthesized using solid state technique and discussed the structural, dielectric and impedance properties of BTL using XRD, FESEM, impedance analyzer. The XRD pattern depicted the diphasic nature with tetragonal structure and no structural change has been observed in BT after doping LF in it. The FESEM image revealed that the composite exhibits dense microstructure with small pores and clear grain boundaries. The dielectric constant of composite BTL is higher at 1 kHz, the dielectric constant increased with increase in temperature and the high value of dielectric constant is attributed to the presence of LF in BT. The variation of dielectric constant with temperature observed at higher frequencies is attributed to electronic polarizations which are independent of both frequency and temperature. The increase of the dielectric loss of BTL with temperature may be due to the increase in the hopping rates of electrons between Fe+3-Fe+2. At higher frequencies, the dielectric constant is independent of frequency over all temperatures. The impedance is more at low temperatures in the low frequency region and the low value of impedance at high temperatures might be due to an increase in conductivity with the frequency because of the liberation of space charge as a result of reduction in its barrier height, which might be used in electronic components and capacitor applications.

Fig. 6. The dielectric loss (tan δ) of BTL varies with frequency at different temperatures.

steeply decreases with increase in frequency upto 1 kHz, 3.2 kHz, 32 kHz, 316 kHz and 1 MHz at 27 °C, 123 °C, 219 °C, 323 °C and 427 °C respectively and beyond those frequencies, the impedance becomes constant. At low frequency, the impedance is high and the impedance value decreases gradually as and when the applied frequency is increased. Also, as and when the temperature is varied, the impedance value decreases. The reason for the material to exhibit constant impedance value beyond frequencies 1 kHz, 3.2 kHz, 32 kHz, 316 kHz and 1 MHz for 27 °C, 123 °C, 219 °C, 323 °C and 427 °C respectively may be due to increase in the conductivity with frequency and the liberation of space charge as a result of reduction in its barrier height [29–31]. The reason for the impedance of BTL at frequencies beyond 1 kHz, 3.2 kHz, 32 kHz, 316 kHz and 1 MHz at temperatures 27 °C, 123 °C, 219 °C,

Acknowledgements We thankful to Indian Nano User Program (INUP), IIT Bombay, India is gratefully acknowledged for extending, FESEM, dielectric and Impedance properties. 902

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Fig. 7. The Impedance (Z′) varies with temperature of BTL ceramics at different frequencies.

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