A Comparative Study on the Dielectric Properties of Lanthanum Copper Titanium Dioxide Ceramic

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

A Comparative Study on the Dielectric Properties of Lanthanum Copper Titanium Dioxide (La2/3Cu3Ti4O12) Ceramic with Conventional and Microwave Sintering Routes 1

Surya Mallick1, Pawan Kumar2, M. Malathi1, a 1 – Condensed Matter Research Laboratory, Department of Physics, School of Advance Sciences, VIT University, Vellore, Tamilnadu, India 2 – National Institute of Technology, Rourkela, Odisha, India a –mmalathi@vit.ac.in DOI 10.2412/mmse.6.46.507 provided by Seo4U.link

Keywords: microwave, conventional, dielectric, ceramics.

ABSTRACT. Lanthanum Copper Titanium Dioxide (La2/3Cu3Ti4O12, LCTO) precursor powders were synthesized by a cost effective solid-state reaction. The material is sintered at two different techniques one is conventional and other one is microwave. The microstructure and impedance characteristics were found to be strongly dependent on the sintering conditions. The sintering has been done at 1, 000oC for 4 thin conventional method and for 20, 40and 60 min in microwave method to compare the effects of two different sintering processes. X-ray powder diffraction study (XRD) analysis, dielectric constant, dielectric loss and Scanning Electron Microscopy (SEM) results are observed. Structural properties and phase formation was confirmed through XRD, this confirms Perovskite cubic structure of LCTO ceramics. Density of the samples determined using Archimedes principle with water as liquid medium. SEM micrographs are taken and results are being compared. Dielectric constant was investigated for different frequency values (1 kHz, 10 kHz, 100 kHz, 1 MHz) with temperature and the effective dielectric constant and loss as a function of frequency has been studied at room temperature. Dielectric constant of microwave-sintered sample was found to be higher compared to the conventional sintered sample at room temperature.

Introduction. Giant dielectric materials have become increasingly important due to the strong technological needs for the further reduction of dimensional size and the enhancement of performance in capacitance-based components like capacitors. In recent years a series of Perovskyte- related structure material, ACu3Ti4O12 (A= Ca1, La2/32, Y2/33, Na1/2Bi1/24, Na1/2La1/25) has been extremely investigated because of its giant dielectric constant accompanied by low dielectric loss at room temperature.La2/3Cu3Ti4O12 (LCTO)is a member of the ACu3Ti4O12 family but so far there are limited literatures reporting LCTO ceramics out of them most studies are focused on preparation, microstructure and dielectric properties of LCTO ceramics[1-3].LCTO ceramics can be fabricated by a conventional solid state reaction [4]. However, the solid-state reaction has some disadvantages such as long processing time, low purity and inhomogeneous grain size, which results in poor dielectric properties. In general, the improvement of the fabrication methods is an effective way to improve electrical characteristics of the ceramics. There are many alternative methods have been used to prepare electronic ceramics which includes Sol-gel method, hydrothermal synthesis, combustion route, spark plasma sintering , hot pressing, out of which Sol-gel method have been attempted to prepare LCTO ceramics[1, 4]. However, these methods are complex and expensive which makes it difficult in industrial application. Microwave sintering for electronic ceramics is superior to conventional sintering owing to its unique characteristics, such as rapid heating, enhanced densification rate and improved microstructure. Microwave heating differs significantly from 1

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

conventional heating. In the microwave, sintering process the heat is generated internally within the material instead of originating from external sources and hence there is an inverse heating profile.

The heating is very rapid as the material is heated by energy conversion rather than by energy transfer, which occurs in conventional techniques. Microwave sintering ensures considerable time and energy saving, and therefore considered as one of the most prospective sintering techniques in material processing. The method has been widely applied in the fabrication of electronic ceramics [5].

In this work, the LCTO ceramics were fabricated by conventional and microwave sintering. The influence of sintering methods, sintering time on the microstructure and dielectric properties of LCTO ceramics investigated systematically. The origin of high dielectric constant of LCTO was studied by impedance analysis. From the XRD peaks, it has been shown that LCTO has a perovskite cubic structure. LCTO ceramics produced from microwave sintering method giving uniform and dense grain morphology. Several reports have shown that many factors, such as electrodes, grain boundaries or domain boundaries are responsible for the high dielectric constant and further improvements in dielectric properties have been studied [6, 7]. We believe that our present studies would help in providing more insight and rationalizing the dielectric behaviour of the ceramics at different sintering routes.

Experimental Methods. Polycrystalline ceramic powders of LCTO were prepared via the conventional solid-state reaction route using stoichiometric amounts of La2O3, CuO and TiO2. The raw materials were measured using the high precision balance machine. These were thoroughly mixed in an acetone medium using a ball mill. Then the mixture was thoroughly grinded for one hour. This was followed by the calcination of the powder in alumina crucible at 1, 000oC for 4, 6, 8and 12h, at a heating rate of 5 oC per minute in conventional furnace. During the calcination process, ferroelectric phase is obtained because of solid phase reaction between the constituents. Single Phase formation was confirmed through XRD. The XRD patterns of the samples were taken at an angle2θ(20≤2θ≤70)o with a scanning rate of 2o per minute. The polycrystalline powder was then cold pressed into the pallets using PVA as a binder. LCTO pallets were sintered in two different methods one is conventional sintering and other one is microwave sintering. In conventional sintering process, pallets were fired at one, 000oC for 4h whereas in microwave sintering process pallets were fired at 1, 000oCfor 20, 40 and 60 min, respectively. Pallet densities were measured using Archimedes principle using water as the liquid medium. The microstructural features and grain size distribution in sintered pallets were studied by SEM. The grain sizes were found using Average Grain Intercept Method (AGI) [7].

The dielectric constant measurement was carried out as a function of temperature for different frequency values (1 kHz, 10 kHz, 100kHz, 1MHz), along with that dielectric constant and loss as a function of frequency were measured at room temperature using independence gain phase analyzer.

For these purpose surfaces of sintered pallets sputtered with silver. Ideally silver should adhere strongly to the ceramics, it should be very thin, practically zero resistance and with a good chemical and physical durability.

Results and Discussions. Fig. 1 shows the XRD patterns of LCTO powders calcined for different times (4, 6, 8 and 12 h). The XRD patterns are virtually the same and show only single phaseperovskite (ABX3) structure, without the evidence of the second phase. XRD patterns of LCTO ceramics are in agreement with the respective joint committee on powder diffraction standards (cubic, space group- Im3, space group number- 204, JCPDS file no. 75-2188). As the calcination time increases, the substance begins to melt, these results secondary peaks to the XRD pattern. From the pattern, it has cleared that cupper and calcium has diffused completely into the LCTO ceramic lattice to form a solid.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 1. XRD pattern of LCTO calcined at 1, 000°C in conventional way for (a) 4h, (b) 6 h, (c) 8 h and (d) 12h, ♣ -Unidentified Phase.

Fig. 2. SEM images of sample Sintered for (a) 4h, (b) 20 min, (c) 40 min and (d) 60 min.

Fig. 2 (a-d) show the surface morphologies of LCTO ceramics sintered at 1, 000oC for 4h in conventional furnace and for 20, 40and 60minin microwave furnace. We termed conventionally sintered sample for 4 hours as C-4 and microwave sintered sample for 20, 40, and 60 min as M-20, M-40, and M-60, respectively. It is clear from the micrographs that the grains have smooth faces associated with cubic appearance.

From the table 1 it can be shown that the gran size of the conventionally sintered sample (C-4) is less compared to microwave sintered samples (M-20, M-40 and M-60). Ceramics with larger grain size have a small volume fraction taken up by Schottky barriers at the grain boundary, which will lead to the decrease of the effective thickness of charge storage regions. This may corresponds to the thinner barrier width and consequently leads to larger dielectric constant [8], [9]. As the microwave sintering time extends LCTO ceramics become uniform, denser and grain size increases. The sample M-20, M-40 and M-60exhibits relatively homogeneous grain sizes and low porosity. Density of M-40 found to be 5.06 g /cm3, which is comparatively higher than C-4.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Table 1. Calculated grain sizes are listed below.

*

Sample code

Grain size (µm)

M-60

2.4

M-40

2.2

M-20

1.4

C-4

1.3

M- Microwave, C- conventional

Based on practical application, ceramics with high dielectric constant and low dielectric loss must be selected firstly. Fig. 3 (a), shows the variation of dielectric constant with temperature at different frequency values (1 kHz, 10 kHz, 100 kHz, 1 MHz), for M-40. It can be seen that there is a phase transition from ferroelectric to paraelectric at Curie temperature (Tc) (250 oC, 1 kHz), large value of Tc can be found at higher frequency range (106 Hz). The Dielectric constants values showing weak temperature and frequency dependence up to Tc, after that it increases sharply with increase in temperature which is due to increase in polarization at higher temperature. It is found that there is weak temperature dependence of dielectric constant at higher frequency range, (106 Hz). Fig. 3 (b), shows the variation of dielectric constant and dielectric loss of M-40as a function of frequency at room temperature. Same analysis has been done for M-60, M-20 and C-4, which are not being shown here. The results indicate that dielectric constant of M-40 is around 1.097, which is comparatively higher than C-4at room temperature. All the samples reasonably exhibited high dielectric constants at low frequencies. Furthermore, the dielectric losses of the microwave sintered samples for 20, 40 and 60 min are lower than that of the conventional sintered sample for 4 h at room temperature. The dielectric loss of M-40 found out to be 1.04 at room temperature.

Fig. 3. (a)Variation of dielectric constant (εr) at different frequency values as a function of temperature, (b) Variation of dielectric constant (εr) and dielectric loss (tan δ) with frequency at room temperature forM-40.

Summary. The LCTO ceramics have been successfully prepared by microwave and conventional sintering route. The effect of sintering process on microstructure and dielectric properties of LCTO ceramics has been investigated systematically. The sample calcined for 4, 6, 8 and 12 h in MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

conventional heating are found out to be single phase perovskite cubic structure. SEM analysis showed dense microstructure in the sample with grain size of 1-2μm. As the microwave sintering time extends the grain size and density of LCTO ceramics increases and the sample gradually becomes denser. The microwave sintered pallets exhibited more homogeneous microstructure, less porosity as well as comparatively higher value of dielectric constant (M-40, 1, 097) and lower value of dielectric loss (M-40, 1.04) compared to conventional sintered sample (C-4) at room temperature.

M-40 showed much higher value of dielectric constant (40 x104, at 103Hz), which displays weak temperature and frequency dependence over a certain temperature range. The Curie temperature (Tc) found out to be225 oC at 1 kHz, the value increases at higher frequency range.

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