Investigation of electrical and dielectricstudies of calcium doped magnesiumtitanate[Mg(1-x)CaXTiO3 (x=0, 0.9)] ceramics V. S. Samyukthaa,*, A. G. Sampath Kumarb , T. S.Raoc, R. P. Suvarnad
aDepartment of Physics, Maris Stella College (Autonomous), Vijayawada, India
bDepartment of Physics, MallaReddy Engineering College(A), Hyderabad- 500100, T.S., India
cMaterial Science Lab, Department of Physics, Sri Krishnadevaraya University, Anantapuramu, India
dDepartment of Physics, Jawaharlal Nehru Technological University, Anantapuramu, India
The pure and calcium doped magnesium titanateceramics were synthesised by conventional solid-state diffusion method. The intension of the present work was to investigate the influence of Ca2+ on electrical and dielectric properties of the above ceramics. The dielectric constant from room temperature to 3500C was calculated using HIOKI 3532-50 LCR HiTESTER in the frequency range of 1KHz-1MHz. The dielectric constant at room temperature at a frequency of 1MHz for Calcium doped Magnesium Titanate was found to be and as the concentration of Calcium increases dielectric constant increases. The AC conductivity, Dielectric loss of the samples were measured and thermoelectric properties were studied. XRD studies revealed that the compound exhibits single phase with Orthorhombic structure and space group 62 :Pbnm.
(Received March 28, 2021; Accepted July 13, 2021)
Keywords: Solid state diffusion, Dielectric constant, AC conductivity
1. Introduction
Magnesium titanate ceramics play an important role in microwave technologies such as global positioning system operating at microwave frequencies, resonators, filters, antennas for communication system and multilayer capacitors [1-4]. It is a versatile material of low dielectric loss with high quality factor (Q above 20000 at 8GHz) and intermediate dielectric constant (εr=17) [5]. An MgTiO3 ceramic exhibits ilmenite structure with rhombohedral crystal system and belongs to R-3 group [6, 7]. MgTiO3 are being prepared by various techniques such as thermal decomposition of peroxide precursors, hydrothermal mechano-chemical complexation routes, Sol- gel method, MOSD and auto- igniting combustion technique [8-13], Chemical co-precipitation [14], metalorganic chemical vapour deposition [15]. In this work, the effect of calcium dopant study is studied.
2. Experimental procedure
Reagent grade raw materials of MgO, TiO2 and CaO were used as starting materials.
These materials were taken according to the composition and then milled for 12h in a Bill Miller (PM 200). These mixtures were dried and calcined at 11000C for 36h in air in a programmable furnace with intermediate grindings. These calcined powders were again milled for 10h. Then the powders were uniaxially pressed into pellets 0.589cm in diameter and 0.262 cm in thickness. The readily formed pellets were sintered for 4h with a heating rate of 100C/min and then cooled to room temperature.
The crystalline phases and lattice parameters of the sintered samples were identified by X- Ray diffractometer (Rigaku) using CuK α radiation. Microstructural analysis of the sintered
*Corresponding author: [email protected]
samples was performed by SEM and Energy Dispersive X-Ray Spectroscopy (EDS). The dielectric constant, dielectric loss and AC conductivity were measured by LCR meter HIOKI 3252-50 in the frequency range 0.1KHz to 5MHz. The dielectric constant [16] is measured by
A d C
r *
*
0
(1)
where C is capacitance of the pellet, d is thickness of the pellet; A is the area of the cross section of the pellet and 0 is the permittivity of free space. The AC conductivity of the samples was estimated from dielectric parameters. The AC conductivity (σAC) was calculated using the relation [17,18]:
σAC = ωεε0tanδ (2)
where ε0 is the permittivity of the free space, ω is the angular frequency and tanδ is the loss tangent.
3. Results and discussions
3.1. XRD analysis
The crystalline structure of Mg0.1Ca0.9TiO3 ceramic sample was evaluated by XRD analysis at room temperature using Rigaku X-Ray Powder Diffract Meter (Japan) are shown in Fig. 1 and Table 1.
Fig.1 XRD pattern of Mg0.1Ca0.9TiO3 ceramics
Table 1. XRD profile data of Mg0.1Ca0.9TiO3 sample.
2θ d-space A0 FWHM (h k l) Dp (nm) 23.26958 3.819539 0.102895 (1 0 0) 82.23 25.71824 3.461158 0.208826 (1 1 1) 40.75 33.09667 2.704464 0.196316 (1 1 2) 44.09 40.70905 2.21459 0.147334 (2 0 2) 60.6 49.01521 1.856968 0.441709 (2 2 1) 20.64 54.70815 1.676423 0.304735 (1 3 1) 30.65 58.9192 1.566249 0.239672 (1 3 2) 39.76 59.29216 1.557284 0.191372 (2 1 1) 49.88 60.57413 1.527358 0.815422 (2 2 3) 11.78
Orthorhombic crystalline structure having single reflection peaks with the exception of few additional phases corresponding to the presence of MgTiO3 (MT) and MgTi2O5 exhibited by the sample is observed. The MgTi2O5 requires a high sintering temperature of 14500C. So second phase MgTi2O5 was formed as intermediate phase and difficult to eliminate from sample prepared by mixed oxide route [17]. The intensity of the diffraction pattern depends on structure factor shows the XRD data of present sample and the average crystallite size (Dp) of 50.52nm was obtained using Scherer formula [19]
Dp = kλ/βCosθ (3)
where k is a constant and is equal to 0.9, θ is diffraction angle, λ=0.154056 nm (CuKα) and β is full width half maxima. The increase in the ionic radius from Mg2+ is 0.65A0 [20] to Ca2+ is 0.99A0 leads to small shift towards lower angles. The dislocation density is calculated by the formula ρ=D-2 where its value is found to be 3.918x1014 m-2. The lattice constants are found to be a=5.484465 A0, b=5.443375 A0 and c=7.529209 A0 and Unit cell volume is found to be 224.777026 (A0)3 are calculated for Mg0.1Ca0.9TiO3 are in good agreement with the crystallographic parameters.
3.2. SEM and EDS results
The surface morphology was studied by Scanning Electron Microscopy. Fig.2 shows SEM images of Mg0.1Ca0.9TiO3 which have been made at two different spots having 5000 X and 10,000X magnifications and in 5μm, 10μm range respectively. In the images the grains are almost spherical in shape containing homogeneous distribution containing with distinguished boundaries with less porosity have been observed. The average particle size was calculated as 175nm, using the following ImageJ software. The average grain size was measured to be 5.31μm using the formula given in the equation (4). The increase in grain size is due to sintering which enhances the grain size. This high grain growth when compared with crystalline size computed from XRD may be due to low elastic strain and thus high grain boundary creeping occurs. The concentrations of elements such as Mg, Ca,Ti and O present in ceramic sample were determined by EDS analysis which were shown in the Fig.3 Table 2 illustrates the At%, and Wt% of various elements.
Fig.2. SEM images of Mg0.1Ca0.9TiO3 sample.
Average grain size=
MN L 5 .
1 (4)
where L is L=the total test line length, M=the magnification, N=the total number of intercepts which the grain boundary makes with the line.
Table 2. Shows EDS profile data of thMg0.1Ca0.9TiO3 sample.
Fig.3. EDS Spectra recorded from Mg0.1Ca0.9TiO3 Sample.
3.3. Dielectric Studies
The dielectric properties were performed on disks for pure and doped MgTiO3 in the temperature range from room temperature to 3500C. The variation of dielectric constant with temperature at 1kHzfrequency is shown in the Fig. 4. The dielectric constant found was 14.5 for MgTiO3 and for Mg0.1Ca0.9TiO3 was 90.26 at 1kHz at room temperature. The dielectric constant at different frequencies for the sample Mg0.1Ca0.9TiO3 were plotted in the figure 5.The dielectric constant is high at low frequency due to the accumulation of charge at the grain boundary and at the interface of electrode sample and electrode which is called space –charge polarization [21, 22].
The dielectric constant decreases as frequency increases due to the gradual deteriorating of the space - charge polarization, indicating the domination of the electronic and atomic contribution.
Figure 6 depicts the variation of tangent loss with temperature for Mg0.1Ca0.9TiO3 ceramic sampleat different frequencies ranging from 1kHz, to 1MHz. At low frequency there is increase in dielectric loss with temperature nearly at 2000C and then decreases. At higher frequencies the dielectric loss is very low and almost stable. This may be an extrinsic loss dominated by secondary phases, oxygen vacancies, grain sizes and densification. The variation ac conductivity as a function of frequency at different temperatures is plotted in the figure 7. As the temperature goes on increasing, the dielectric relaxation gets thermally activated and hence this can be governed by the Arrhenius equation [23, 24].
0
K TEa e B
(5)
where K=8.6X10-5eV, σo= pre exponential factor, and T= absolute temperature. In general σac can be calculated by the following equation.
ac o r
tan
(6)
Element Wt% At%
OK 15.14 32.36
MgK 2.76 3.88
CaK 37.2 31.73
TiK 44.9 32.04
where εr =dielectric constant, εo=8.9X10-12 F/m, ω=2πf and tanδ= loss tangent.The slopes of the curves attribute activation energies and were attained as 0.643 eV and 0.105 eV at 1 kHz and 1MHz frequencies respectively. This definitely establishes a fact that conductivity increases with increasing frequency and becomes independent of temperature at high frequencies. At lower frequencies the conductivity increases with increase of temperature due to thermal activation process and this may be due to hopping of charge carriers which are bound in the localized states while conductivity decreases with increasing frequencies because of hopping of free charge carriers from one lattice to the other due to electronic and ionic conduction [16]. Another reason could be attributed to the fact that the dopants can introduce defects which may lead to segregate at the grain boundaries due to diffusion process resulting from sintering and cooling processes.
Fig. 4. Variation of Dielectric Constant wit Temperature at 1KHz frequency for Mg(1-x) CaxTiO3 samples.
Fig. 5. Variation of Dielectric Constant of Mg0.1Ca0.9TiO3 with Temperature at Different frequencies.
Fig. 6. Variation of Dielectric loss (tanδ) with Temperature at 1KHz frequency for Mg0.1Ca0.9TiO3 sample.
Fig. 7. Variation of ac conductivity of Mg0.1Ca0.9TiO3 with Temperature at Different frequencies.
3.4. Electrical Studies
Figure 8 shows the variation of Thermoemf (E) with temperature. The Seebeck coefficient (thermoelectric power) as a function of temperature of Mg0.1Ca0.9TiO3 ceramics sintered at 12000C is shown in fig.9. The Seebeck coefficient is positive and increases with increase of temperature revealing that holes are being introduced by Ca addition. At high temperatures the Seebeck coefficient remains constant and later decreased with an overall slope of 248.78μV/K.. The carrier concentration (n) was calculated for the Mg0.1Ca0.9TiO3 pellet of thickness 0.262cm and radius 0.589cm as 0.197X1022 cm-3 using the following equation [25].
exp( )
N Se
n V K
(7)
where N=1022 cm-3 (Density of states), V=0.15 cm3 (Volume of the sample),
k
e = 86.4 μV/K S=Seebeck Coefficient
Fig. 8. Variation of Thermoemf with Temperature for Mg0.1Ca0.9TiO3 sample.
Fig. 9. Variation of Seebeck Coefficient with Temperature for Mg0.1Ca0.9TiO3 sample.
4.Conclusions
This study reveals that the Mg0.1Ca0.9TiO3 sample is of Orthorhombic structure except that of few additional phases of MgTi2O5 with good homogenity and crystalline nature by XRD analysis. From the SEM analysis it is revealed that grains are almost spherical in shape and are distributed with less porosity, homogenously. In the low frequency region the dielectric constant decreases with the frequency where as it shows frequency-independent behaviour in the high frequency region. It is also a high dielectric constant material with low dielectric loss which means it is suitable for microwave applications.
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