Effect of sintering temperature on structural and ...

Effect of sintering temperature on structural and dielectric properties of Bi and Li cosubstituted SrTiO3 Mahmoud S. Alkathy, J. Pundareekam Goud, and K. C. James Raju Citation: AIP Conference Proceedings 1728, 020355 (2016); doi: 10.1063/1.4946406 View online: http://dx.doi.org/10.1063/1.4946406 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Dielectric investigations on co-substituted bismuth ferrite (Bi1-xLaxFe1-xMnxO3) AIP Conf. Proc. 1728, 020578 (2016); 10.1063/1.4946629 Optical properties of Y and Ti co-substituted BiFe O 3 multiferroics AIP Conf. Proc. 1591, 622 (2014); 10.1063/1.4872696 Effect of Pr substitution on structural and dielectric properties of SrTiO3 J. Appl. Phys. 112, 044106 (2012); 10.1063/1.4747937 Effects of Gd substitution on microstructures and low temperature dielectric relaxation behaviors of SrTiO3 ceramics J. Appl. Phys. 112, 034114 (2012); 10.1063/1.4745876 Structural and electro‐optic properties of laser ablated Bi4Ti3O12 thin films on SrTiO3(100) and SrTiO3(110) Appl. Phys. Lett. 61, 1516 (1992); 10.1063/1.107534

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Effect of Sintering Temperature on Structural and Dielectric Properties of Bi and Li Co-Substituted SrTiO3 1

Mahmoud. S. Alkathy,1J Pundareekam Goud, K. C. James Raju1,2,* 1

2

School of Physics, University of Hyderabad, AdvancedCentre of Research in High Energy Materials, Hyderabad-500046, India. * Corresponding author email - [email protected] Tel. No.: 040-23134305, Fax: +91 40 23010227.

Abstract. Sintering is an important factor that affects the microstructure development in ceramics. In this work, Bi and Li co-substituted Strontium Titanate (ST) ceramics samples sintered at different temperatures are prepared and analyzed by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). XRD was used to detect phase structure of the sintered samples and SEM was used for microstructure analysis. Dielectric properties of Bi and Li co-substituted SrTiO3 samples were measured using Agilent 4294A Impedance analyzer in the frequency range 40 Hz to 5 MHz at room temperature. The result reveals that the dielectric constant of Bi and Li co-substituted SrTiO3 ceramics increases with increasing sintering temperature up to 1250oC and then decreases beyond that. The highest value of dielectric constant obtained is 860 at 1KHz frequency and lowest dielectric loss observed is 0.04 at the same frequency for samples sintered at 1250oC . Also the grain size increases with increase of sintering temperature. Keywords: Strontium Titanate; co-substitution;dielectic constant; Sintering, Grain size.

INTRODUCTION Strontium Titanate is an attractive material that belongs to a quantum paraelectric and non-conducting material with cubic perovskite structure [1,2]. Strontium titanate is suitable for many applications such as tunable microwave devices due to its high DC- electric field dependence on dielectric constant [3], dynamic random access memories due to its good insulating properties and higher values of dielectric constant[4,5]. Polycrystalline SrTiO 3 (ST) is usually prepared by the conventional solid state reaction (SSR) method and sol-gel method, which have been widely used in the preparation of many materials.The earlier studies, it is noted that manganese or antimony substitionhas an effect on the dielectric properties of ST [3.6]. The dielectric properties of ST ceramics are strongly dependent on the formation of its microstructure. Generally, optimizing conditions particularly the sintering temperature, seriously influence ceramics microstructure, grain growth, densifications and resulting in better properties [7-11]. In this paper, the structural and dielectric properties of Bi and Li co-substituted ST samples sintered at different temperatures (1200oC-1300oC) are investigated.

EXPERMENTAL A conventional solid state reaction method with microwave assisted heating of the starting materials was used to prepare the Bi and Li co-substituted SrTiO3 samples. The general formula used for preparing the sample isSr (1-x) [Bi , Li] xTiO3 where (x=0.03). Highly pure powders of SrCO 3, TiO2, Bi2O3 and Li2CO3 (Sigma – Aldrich 99.99% purity) were used as starting materials, which were weighed and then milled for 2 h in acetone and zirconia media. After drying, the mixture was calcined using (Microwave Sintering System of ENERZI, India with1.45KW magnetron). The target temperature was kept at 1000oC and the dwell time was 20 minutes while heating and cooling rates are

International Conference on Condensed Matter and Applied Physics (ICC 2015) AIP Conf. Proc. 1728, 020355-1–020355-6; doi: 10.1063/1.4946406 Published by AIP Publishing. 978-0-7354-1375-7/$30.00

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50oC/min. The calcined powder was remilled again for 2 h and then dried. The obtained powders were added with 1 wt-% PVA and were pressed in to pellets of diameter 10 mm and 1.5 mm thickness. The green pellets were heat treated to 500ºC at a heating rate of 2oC/min for binder evaporation. The green pellets of Bi and Li co-substituted ST were sintered at three different temperatures (1200, 1250 and 1300 oC) respectively for 4hr with heating and cooling rates at 5ºC/min.

Ceramics Characterization The bulk density of the sintered pallets ware measured using Archimedes method. Distilled water was used as

the immersion liquid.The measured density ( U m ) of the sample was calculated using the following formula:

m1 U L ………………………………. (1) m1 m2 Where m1 is the sample weight measured in air, m2 is the sample weight measured in liquid and ρL is the density

Um 3

of pure water which is 0.99821( gm / cm ) at room temperature. Powder X-ray diffraction (Bruker D8 Advance) at 40 kV and 30 mA, with Cu Kα source radiation (ߣ ൌ ͳǤͷͶ Ao) at 36kV was used to identify the phase and structural properties of sintered pellets. The crystallite size “L” was calculated by using Scherrer,s formula.

kO ………………………….. (2) E cos T

L

where λ is the X-ray wavelength in angestrom (Å), β is the peak width of the diffraction peak profile at half maximum height resulting from small crystallite size in radians and K is a constant related to crystallite shape, normally taken as 0.9. The lattice parameter was calculated by using the formula.

1 2 d hkl

h2 k 2 l 2 ……………… … (3) a2

The surface morphology of the sintered ceramic was determined by Scanning Electron Microscope FE-SEM (Carl Zeiss, Ultra 55). For measuring the dielectric constant, silver paste was painted on both sides of the polished ceramic sample as electrodes and fired at 200oC for 30 min. The frequency dependence of the capacitance and loss tangent was measured by an Agilent 4294A at room temperature. From the capacitance, the dielectric constant was calculated using the following formula [12].

H

Cd / H 0 A ………………………. (4)

Where C is the capacitance (F), d the thickness of the pellet (m), A is the area of the pellet (m 2) and ɛ0 is the relative permittivity of free space = 8.85x10 -12 F. m-1.

RESULT AND DISCUSSION Crystal structure The XRD pattern of Bi and Li co-substituted ST ceramics sintered at different temperatures of 1200 oC, 1250oC and 1300oC respectively are shown in fig 1. The results were compared with standard JCPDS . The samples shows the presence of (100), (110), (111), (200), (210), (211) and (220) diffraction peaks in the scanning range of 2ߠ ൌ ͳͲ െ ͹Ͳo and exhibit single phase with cubic structure with pm3m space group. The crystallite size and lattice parameters of the samples were calculated using Eqs. (1) and (2). Table 1 gives the lattice parameters, unit cell volume, crystallite size and lattice strain for the samples sintered at different temperatures.


FIGURE 1: XRD patterns of SrTiO3 sintered at different temperatures for Bi (3%) +Li (3%) co-substituted SrTiO3 ceramic.

“TABLE 1,” Unit Cell Parameter, Unit Cell Volume, Crystallite Size and Lattice Strain of Bi,Li co-substituted ST Ceramic Sintered at Different Temperatures.

Sintering temperature °C 1200 1250 1300

Unit cell constant (Å) 3.904 3.915 3.927

Unit cell volume (Å)3 59.50 60.01 60.56

Crystallite size (nm) 117 127 189

Lattice strain 0.0011 0.0010 0.0007

FIGURE 2. Lattice parameter and crystallite size Vs sintering temperature for Sr (1-x) (Bi , Li)x TiO3- ceramic.

Fig 2 shows that as the sintering temperature increases, the position of all peaks shifted to lower angles, indicating an increase in the lattice parameter as shown in table 1. This was probably because more oxygen defects were introduced with increasing of sintering temperature. Also it is observed that the crystallite size increases with increasing the sintering temperature. The size is corresponding to the observed results from FE-SEM.


Morphological Analysis Fig. 3 shows the micro structure of the sintered pellets. The grain sizes were measured by using the linear intercept method with 30 kX magnifications. In high temperature sintered samples, the melting of the sample was observed. The observed grains are of varying size. The average grain size for pure ST and Bi, Li co-substituted ST samples sintered at 1200oC, 1250 oC and 1300oC are 0.3 μm, 1.8 μm, 2.5 μm and 2.7 μm respectively. The increase of grain size may be due to the mass transport mechanism in samples during the sintering process [13].

FIGURE 3:SEM Images of SrTiO3Sintered at Different Temperatures: (a) 1200°C; (b) 1250°C; and (c) 1300°C

Dielectric Properties The frequency dependences of the dielectric constant and dielectric loss measured at room temperature shows that, initially the dielectric constant of Bi and Li co-substituted ST ceramics sintered at three different temperatures decreases with increasing the frequency. Second, it is also found that the dielectric constant for the sample sintered at 1250oC shows higher values comparing with others. The dielectric relaxation peaks in the dissipation factor were appeared at frequency >1 kHz and shifted towards higher frequency with increase of sintering temperature as shown in fig 4b. It is suggested that the appearing of relaxation peaks is due to the movement of oxygen ions or oxygen vacancies [14]. The samples sintered at 1250 oC gave higher values of dielectric constant with lower values of loss tangent along with higher density. This means, at this condition the samples are in the most optimized state and that should be suitable for applications. “TABLE 2,” Change of density, porosity, dielectic constant and dielectric loss with sintering temperature for Sr (1-x)

[Bi , Li]x TiO3 ceramics. Sintering temperature °C 1200

Um

(gm/cm3) 4.6

Relative density %

Porosity %

90

10

1250

4.9 96 6 1300 4.9 96 6 Where H exp : is the dielectric constant measured before porosity correction

H exp @

H corr @1

tan G @

1kHz 330

kHz 412

0.056

860

923

0.04

710

788

0.08

1kHz

H corr : is the dielectric constant after porosity correction.


FIGURE 4. Frequency dependence of dielectric constant and dielectric loss for Bi and Li co-substituted strontium titanate ceramics sintered at three different temperatures.

The porosity corrected dielectric constant for all samples were calculated using the following formula [15]. 3 p(H r ,exp 1) …………(5) H r ,exp H r ,corr (1 ) 2H r ,exp 1 Where

H r ,exp

is the measured permittivity,

H r ,corr

is the permittivity after porosity correction and

p is the

fraction of porosity, which is defined by p 1 D , where D is the ratio of measured density to the theoretical density of the material. It is observed that the value of the dielectric constant for all samples were increased after the porosity correction as expected.

FIGURE 5. Frequency dependence of dielectric constant for Bi and Li co-substituted strontium titanate ceramics sintered at three different temperatures before and after porosity corrections.


CONCLUSION Sr (1-x) [Bi , Li]x TiO3 ( x=0,03) ceramics sintered at different temperatures were fabricated. With the increase of sintering temperature, the peaks are shifted towards lower diffraction angles which shows the increasing of lattice parameter. The SEM result showed that the grain size increases with increase of sintering temperature, which is the result in the mass transport mechanism in samples during the sintering process. The dielectric properties shows that, the maximum value of dielectric constant is 570 at 1kHz which is obtained for the samples sintered at 1250oC sintering temperature, where the lowest dielectric loss of 0.04 is also found showing that this is the optimum temperature for sintering of these dielectric samples. The dielectric constant was enhanced with porosity correction as expected.

ACKNOWLEDGMENTS Authors thank Mr. Binoy of School of the Physics, University of Hyderabad in helping the dielectric measurements. MSA also acknowledge the financial support from (Al Bayda University) Government of Yemen.

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