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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.30 No.10 pp.495-501
DOI : https://doi.org/10.3740/MRSK.2020.30.10.495

# Effect of Sintering Temperature on Dielectric Properties of 72 wt%(Al2O3):28 wt%(SiO2) Ceramics

Manisha Sahu1, Basanta Kumar Panigrahi2, Hoe Joon Kim3, PL Deepti4, Sugato Hajra1, Kalyani Mohanta5
1Department of Electronics and Instrumentation, Siksha O Anusandhan (Deemed to be University), Bhubaneswar-751030, India
2Department of Electrical Engineering, Siksha O Anusandhan (Deemed to be University), Bhubaneswar-751030, India
3Department of Robotics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea
4Department of Physics, Veer Surendra Sai University of Technology, Burla-768018, India
5Department of Ceramic Engineering, Indian Institute of Technology-Banaras Hindu University, Varanasi, Uttar Pradesh 221005, India
Corresponding author E-Mail : manishafl@outlook.com (M. Sahu, Siksha O Anusandhan)
July 3, 2020 September 5, 2020 September 7, 2020

## Abstract

The various sintered samples comprising of 72 wt%(Al2O3) : 28 wt%(SiO2) based ceramics were fabricated using a colloidal processing route. The phase analysis of the ceramics was performed using an X-ray diffractometer (XRD) at room temperature confirming the presence of Al2O5Si and Al5.33Si0.67O9.33. The surface morphology of the fracture surface of the different sintered samples having different sizes of grain distribution. The resistive and capacitive properties of the three different sintered samples at frequency sweep (1 kHz to 1 MHz). The contribution of grain and the non-Debye relaxation process is seen for various sintered samples in the Nyquist plot. The ferroelectric loop of the various sintered sample shows a slim shape giving rise to low remnant polarization. The excitation performance of the sample at a constant electric signal has been examined utilizing a designed electrical circuit. The above result suggests that the prepared lead-free ceramic can act as a base for designing of dielectric capacitors or resonators.

## 초록

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

In recent days, mullite which is prepared from aluminosilicates becomes a potential candidate for a high-temperature application.1-3) It has superior physical characteristics like low dielectric constant, high resistance towards heat shock, larger creep resistance and its melting point is high too.4) It is reported that in any experimental condition whether it be air or oxygen Al2O3-SiC composites are metastable thermodynamically.5) The recent days are in a quest to develop lead-free ceramics for device engineering. Sakka and co-workers adopted the colloidal consolidation of Al2O3 and SiC further sintering of green pellets to form mullite nanocomposites. 6) Firstly, the SiC was oxidized to evolve SiO2 then it was reacted with alumina to produce mullite. In this work, the surface morphology depicts SiC spread over the mullite phase while mullite whiskers in the SiC phase. Wang and co-workers carried out the reaction of Al2O3 and SiC leading to the formation of mullite. The surface morphology depicts the presence of voids along with mullite, silicate and silica pockets. Hajra et al also reported the Al2O3 doped silica and studied its relaxation as well as conduction mechanism.7) The dielectric properties of the tungsten ion-doped mullite sample fabricated by the sol-gel process done by Paul et al.8) Yl2O3 mixed with cordierite-mullite ceramics carried out by Kim et al lead to less sintering temperature and enhancement of mechanical strength.9) Zhang et al. studied the microstructural and mechanical properties of Al2O3-mullite composite.10) Carvalho et al. synthesis aluminosilicate and alumina of various compositions with properties related to dielectric ceramic devices.11)

As per the literature review reports, we find that there is a need to investigate the dielectric and ferroelectric properties of such an Al2O3-SiO2 sample. The colloidal processing route was undertaken and the same sample was sintered at different temperatures. This paper highlights the effect of sintering temperature on the various property of the synthesized material further leading to a correlation between the structure and electrical properties.

## 2. Experimental Procedure

The pure oxides of Al2O3 (particle size 60 nm) and SiO2 (particle size 20 nm) (M/S Sigma Aldrich) powders were commercially purchased and used for further processing of the samples. The processing technique was followed as per our previous reported reports.12) Egg white (derived from fresh eggs) and sucrose (Loba Chemie, India) acted as a binder for suspension preparation. The homogenization egg white was achieved by employing stirring at a magnetic stirrer for 2 h. Further various ratios of distilled water were mixed along with egg white and/or sucrose to prepare the premix. This premix was used to prepare the Al2O3-SiO2 slurries. Then the solution was transferred into polypropylene bottles and further 3 mm zirconia as grinding media for 24 h. The bubble formation from egg white during the suspension preparation was controlled by antifoaming agent 1- octanol (5 mL/100 mL of egg white). The zirconia media was separated from the slurries after 24 hrs of milling, and different measurements were performed. The obtained slurries were cast into petroleum jelly-coated rectangular aluminum molds and kept in a hot oven at 70 °C for 1 day for drying purposes. The molds were cooled to room temperature; green parts were separated from the mold, and further dried in an oven. All samples were kept at 900 °C for 2 h for burnout of the organics and gaining sufficient for handling strength. Then sintering of samples was performed at 1,400, 1,500, 1,600 °C in a hightemperature tube furnace with a soaking period of 3 h. Fig. 1 shows the systematic representation the sintering of samples was carried out at three different temperatures like 1,400, 1,500, 1,600 °C. The structural analysis of the specimens was done by X-Ray Powder Diffraction (XRD) (Rigaku Smartlab) using Cu-Kα radiation (λ=1.54056 Å) and operating at 40 kV and 40 mA with a step size of 2 degrees/minute. Scanning electron microscopy (SEM) sample was subjected to surface gold spray treatment to make the surface conducting. The model of the SEM used was Carl Zeiss-EVO10. The hysteresis loop of synthesized samples performed by Marine India at room temperature. The electrical measurements were taken at room temperature for a wide sweep of frequency (1 kHz-1 MHz) by employing the Hioki LCR meter.

## 3. Results and Discussion

Fig. 2(a, b, c) shows the XRD pattern of the samples. It is observed a combined phase like Al2O5Si and Al5.33O9.33Si0.67 is matched by employing the Xpert high score software package. The relativity density of the sample at 1,400, 1,500, 1,600 °C are 93 %, 91.3 %, 93.5 % respectively obtained from Archimedes principal. Table 1 shows the crystallographic information of Al2O5Si and Al5.33Si0.67O9.33. Fig. 3(a, b, c) demonstrates the microstructure of the sample. For all the sintered samples surface various size of grains are witnessed with non-uniform distribution and porosity.

The relative permittivity is similar to the dielectric permittivity of the sample expressed as a ratio relative to the permittivity of vacuum. The real and imaginary component of relative permittivity is denoted as: $ε r ( ω ) = ε r ′ ( ω ) + i ε r ′ ​ ′ ( ω )$. The various kind of polarization responsible for relaxation, defects, and dielectric loss can be studied with the help of dielectric measurements. The dielectric constant and loss factor of the sintered samples are presented over a frequency sweep of 1 kHz to 1 MHz in Fig. 4(a, b) at room temperature. The samples at various sintering temperatures display similar low dielectric constant at a lower frequency. The higher value of the dielectric constant in the low-frequency range (i.e. dispersive part up to 10 kHz) indicates the contribution from all the four types of polarization (space-charge, electrode, dipolar, electronic), whereas the lower value at higher frequency region (above 10 kHz, where the entire plot merges) attributed to the absence of space charge polarization because of fast variation of the field. The trend denotes the dipolar relaxation phenomenon where at lower frequencies the dipoles can follow the applied field frequency while at high frequencies dipoles unable to track the applied field frequency.13-15) The complex dielectric parameters of the various sintered samples prepared in the colloidal route may suffer from voids, vacancy defects, dislocations, dipoles. The plot in Fig. 4(b) demonstrates the dielectric loss of the sample. The high electric conductivity may cause higher dissipation generating large leakage current for sintered samples. The resonance of the charge defect dipoles may be attributed to a high loss factor in the low-frequency area.16,17)

Impedance spectroscopy differentiates among the intrinsic (grain), extrinsic (grain boundary) and sample electrode interface contributions in the ceramics.18) Fig. 4(c) shows the real of impedance versus frequency and (inset) represents the imaginary part of impedance versus frequency. In Fig. 4(c) all the plots at different sintering temperatures show low-frequency dispersion which coalesces at the region of higher frequency and forms a plateau region. The low-frequency dispersion occurs due to the slow dynamics of the relaxation process generating from space charges.19-21) In the inset figure, it shows an imaginary part of impedance the dispersion curves appear to merge at higher frequency occurring due to the presence of space charge polarization at lower frequencies which fades at higher frequencies. Nyquist plot (Z′ vs. Z″) represents relaxation (Debye or Non-Debye) mechanisms resembled the semi-circular arc shape. A full or partial semicircle is depicted upon the relaxation strength and experimentally available range of frequency. The bulk resistance can be determined from the intercept of the semi-circular arc along the real Z-axis. The transport properties of the synthesized samples can be determined with an electric circuit made by resistance (R), capacitance (C) and constant phase elements (Q) linked in a parallel arrangement. The experimental data of the prepared sample at different sintering temperatures are fitted with a single RQC circuit, as illustrated in Fig. 4(d) (using Zsimpwin software), denoting that grain effect is present in the above-mentioned temperature range. The depressed semicircles for the composition at different sintering temperatures suggest the Non-Debye type behavior.22) From fitting the curves, the value of the grain capacitance (Cg), bulk resistance (Rg) at room temperature for different sintered samples are written in Table 2.

It is a well-known fact that ferroelectric material bears a unique hysteresis loop, like a fingerprint. The ferroelectricity could be identified through the typical hysteresis loops. It shed light upon the application of ferroelectric materials in non-volatile memories like representing storage as a positive or negative remnant polarization state. The value of the switchable polarization (the difference between the positive and negative remnant polarization, coercive field on sample thickness, a decrease of switchable polarization with several switching cycles, endurance, and retention can be easily studied. P. E. loop Tracer (Marine India) comprising of modified Sawyer Tower circuit was used to capture the P-E loop at room temperature and operating at 50 Hz. The P-E loop Tracer comprises of laptop, software, programmable voltage source (up to 5 kV), silicon oil bath and oven. The electric field is obtained by dividing the voltage by the thickness (E = V/d) across the sample by measured thickness. The horizontal x-axis shows the magnitude of the field applied. As the ferroelectric material is subjected to an electric field, current signals of two nature are basically observed, leakage current and current due to domain switching phenomenon.23) From the plot we can clearly determine that the sample is purely dielectric. The Sawyer–Tower circuit is useful for experimentally derive the ferroelectric loop. Fig. 5 shows the ferroelectric hysteresis loop of the different sintered samples. The remnant polarization of 1,400, 1,500, 1,600 °C sintered sample are 0.115, 0.023, 0.027 μC/cm2 respectively. The coercive field noted for 1,400, 1,500, 1,600 °C sintered sample are 5.332, 2.078, 5.082 V/cm respectively. The slant and slim loop are observed for all samples as it is expected a non-ferroelectric sample. The P-E hysteresis loop is far from saturation under 20 kV/ cm, although a larger electric field was not commenced due to the equipment limitation. A detailed structural work needs to be done on this sample which is beyond the scope of this study.

Fig. 6 presents the excitation performance of the sample while an electric signal (sin wave) of 10 Vp-p is applied. For this one of the pellets of various sintered temperature was made smooth both sides and silver paste was painted which behaves as an electrode making it a parallel plate capacitor arrangement. The experimental setup has certain components such as digital oscilloscope, function generator, resistor (1,000 Ω), and sample acting as a capacitor. It is observed that as the frequency rises the output voltage across the sample rises and reaches maximum voltage due to resonance and thereafter decreases. It is noted for 1,400, 1,500, 1,600 °C sintered sample the maximum voltage is obtained at 5005 kHz, 3544 kHz, 4438 kHz and the value of voltage is 0.183 V, 0.132 V, 0.181 V respectively. As per the Ohms law, as the voltage is directly proportional to current so the maximum current at 5005 kHz, 3544 kHz, 4438 kHz for 1,400, 1,500, 1,600 °C sintered sample is 1.83×10−4 A, 1.83×10−4 A, 1.83×10−4 A respectively.

## 4. Conclusions

The present work shows the effect of sintering temperature on structural, dielectric and ferroelectric properties. It is seen that the sample comprises of the mixed phases of Al2O5Si and Al5.33Si0.67O9.33. The dielectric properties were measured at room temperature and different range of frequency (1 kHz- 1 MHz). The Nyquist plots exhibit a single semicircle representing the grain contribution that denotes the mobile ions that are blocked in the grains. The impedance characterization correlates the electrical properties with the micro-structural property of the materials. The ferroelectric loop suggests nonferroelectric behaviour in the sample that is characteristics of a typical dielectric material. The 10 Vpeak-peak sin wave input signal is applied to the ceramics from function generator in an experimental setup to give an insight on its applications in electronic circuits.

## Acknowledgment

The authors would like to thank Dr. PK Parida, CET Bhubaneswar to carry out the SEM experiment.

## Figure

Systematic procedure of the synthesis of the sample.

(a, b, c) X-ray diffraction of the samples at 1,400, 1,500, 1,600 °C.

(a, b, c) surface morphology of the facture surface of sintered pellets.

(a) dielectric constant, (b) dielectric loss, (c) real/ (inset) imaginary part of impedance with frequency variation at room temperature and (d) shows Nyquist plot as well as equivalent circuit model used for fitting.

Polarization versus applied voltage (PE loop).

Excitation performance of the samples at constant electric signal produced from function generator

## Table

Crystallographic information of the Al2O5Si and Al5.33O9.33Si0.67.

Values of bulk grain resistance and grain capacitance for synthesized samples.

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