Spark plasma sintering of microwave processed nanocrystalline barium titanate and their characterisations

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Abstract

Nanocrystalline barium titanate (BT) was synthesized by the high-energy planetary mill assisted novel hybrid processing methodology. As-milled precursors were microwave calcined and subsequently sintered by spark plasma sintering (SPS). Powders were characterised by XRD, laser Raman spectroscopy, BET and TEM techniques. Microstructure, purity, thermal stability and dielectric characteristics of the sintered BT were analyzed using FE-SEM, EDX, TGA and LCZ meter. Nanocrystalline nature of BT was evident from the enhanced peak broadening noticed in the XRD pattern and from the TEM image. Raman spectra corroborated well with the XRD results. FE-SEM image revealed a fairly dense and uniform microstructure, which was attributed to the high reactivity and needle shaped morphology of the mechanically activated microwave calcined powders, which showed a surface area of about 20 m2 g−1. Microwave calcined SPS processed BT showed a thermal stability of 99.5% as evident from the thermo-gravimetric analysis. Superior dielectric response was shown by the hybrid processed BT which is highly suitable for capacitor applications.

Highlights

► Nanostructured barium titanate synthesized by a novel 3-step hybrid method. ► The powders were densified by spark plasma sintering. ► Average grain size was found to be 150 nm. ► Agglomerated, needle shaped grains were noticed in the microwave calcined as-milled precursors. ► Plasma sintered barium titanate showed a thermal stability of 99.5% and superior dielectric response.

Introduction

Barium titanate (BT) based ceramics showing colossal dielectric behaviour are widely used in ceramic capacitors for varied applications [1], [2]. Extensive research and development works are being carried out at a rapid rate to enhance the performance of BT based capacitors. Nano-BT has been eagerly desired for the miniaturization of capacitors [3], [4]. Mechanochemical processing technique like high-energy planetary milling is considered to be one of the most efficient methods for the production of nano-BT [5]. High-energy planetary milling greatly improves the reactivity of precursors by increasing their effective surface areas and also induces in-situ phase formation during milling; thereby subsequent processing temperatures can be reduced to a greater extent [6], [7], [8], [9]. Schaffer et al. [10] reported the kinetics involved during the mechanochemical processing. Boldyrev [11] reported the processes involved in mechanical activation and mechanochemistry. Detailed study on the high-energy planetary milling process, with regard to the synthesis of ferroelectric ceramics like BT has been reported by Kong et al. [12]. Upon synthesis of mechanically activated mixture, the subsequent steps are: calcination for the complete phase formation; sintering of the calcined powders to achieve high density compacts for device applications. Heat treatment of ceramics by the application of microwave energy is gaining prominence due to its multiple benefits [13], [14], [15]. BT based dielectric ceramic materials can easily be heated by the application of microwave radiation because of their inherent ability to absorb the microwaves, and subsequently gets volumetrically heated up in a shorter period of time [16]. Microwave heating of the base-metal-electrode capacitors was reported by Fang et al. [17]. Microwave processing of lead-free BT piezoelectric ceramics was investigated by Takahashi et al. [18]. Fang et al. [19] performed a one-step microwave synthesis of nano-sized BT using an oxalate precursor. Recently, Sundararajan et al. [20] studied the combined effect of milling and calcination methods on the characteristics of nano-BT. After the calcination step, one of the most challenging tasks is to sinter the powders to high densities, and at the same time prevent the exaggerated grain growth by retaining the nanocrystalline nature. As far as sintering of ceramics is concerned, microwave assisted sintering route seems to cause a major problem of thermal runaway effect due to the uncontrolled and non-uniform exposure to microwaves leading to differential densification [21]. Selective local overheating of the specimen occurs, which leads to warpage, lower densities and poor dielectric performance. Spark plasma sintering (SPS) is considered to be an excellent methodology for the sintering of piezo-ferroelectric ceramic powders like BT [22]. SPS is a pressure assisted sintering process carried out mostly in a vacuum environment, where the powder is uniaxially pressed in a graphite die and pulsed direct current is applied simultaneously to the die through the graphite punches. It is believed that a weak plasma is generated around the powders during the spark discharge process, which is followed by other processes leading to rapid densification of the powders [23]. It is relatively energy efficient and less time consuming in nature than the conventional and microwave sintering techniques [24]. Thermal runaway problem leading to the differential densification in the microwave sintering process is completely avoided in SPS. Rapid sintering by SPS retains the nanostructure of the powders by inhibiting the exaggerated grain growth during sintering. This leads to highly dense and uniform nanostructure in the sintered compacts which greatly influences the dielectric behaviour of ferroelectric ceramics like BT. It has been reported that the room-temperature relative dielectric constant increased to a greater extent, as the grain size of the BT ceramic is decreased [25], [26]. There are many reports on the SPS of nano-BT [27], [28], [29] synthesized by the various routes like sol–gel [30], hydrothermal processing [31], self-propagating high-temperature synthesis [32], etc. But each of these methods have their own inherent demerits. Barium carbonate and barium oxide were used as the primary precursors in most of the previously reported works on the mechanochemical synthesis of BT [33], [34], [35]. It is well known fact that hydroxide materials decomposes at a relatively lower temperature than their carbonates counterpart and are also cost wise economical. By taking into account the advantages of mechanochemical processing, microwave processing and SPS, it is worthwhile to study the effect of their combined effect on the characteristics of BT by using cost effective barium hydroxide as the novel barium precursor. According to the author(s) knowledge, there is no literature available on such hybrid processing of BT ceramics. In this article, we report for the first time our preliminary studies on the SPS of nano-BT synthesized by microwave calcination of the as-milled precursors.

Section snippets

Experimental methods and characterisation

Barium hydroxide (99% purity, Loba chemicals ltd.) and anatase grade titania (99% pure, Merck laboratories) were used as the primary precursors. These precursors of stoichiometric amounts were taken and subsequently subjected to high-energy planetary milling in an indigenously designed planetary-mill. Interior of the stainless steel milling jar was lined with a synthetic rubber. Tungsten carbide (WC) balls of 12 mm diameter were used as the milling media. Powder to ball ratio used in the

X-ray diffraction analysis

XRD pattern of the hybrid processed BT is shown in Fig. 2. The JCPDS file number: 05-0626 was taken as a standard for indexing the peaks. The crystal system was tetragonal. Highly intense peaks with apparent broadening were clearly noticed in the pattern. The c/a ratio calculated from the XRD pattern was found to be 1.021. Rapid and volumetric heating during the microwave and SPS processing has prevented exaggerated grain growth during the phase formation of BT. The c/a ratio obtained was very

Conclusions

Nano-BT was synthesized by mechanical activation followed by microwave calcination at 800 °C and Spark plasma sintering (SPS) at 1000 °C. Barium hydroxide was used as the novel barium precursor. Information from the XRD pattern and Raman spectrum of the hybrid processed BT corroborated well with each other. TEM image showed needle-like agglomerated grains, which had an average BET surface area of ∼20 m2 g−1. FE-SEM image revealed that the nanostructure with average grain size of 150 nm was

Acknowledgements

Authors thank the Centre for Technology Development and Transfer (CTDT), Anna University, Chennai, India for funding this project (File No: 7672/CTDT-1/2010) under the “Students Innovative Research Projects Support Scheme” of Anna University. The support of S. L. Gupta, IIT-K; Mr. S. Balaji of CSIR-CLRI, Chennai, India; Mr. Biswajit Mishra of TIFR, Mumbai, India and Mr. V. S. Balaji of NIT, Trichy, India are gratefully acknowledged.

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