Effect of temperature, time and particle size of Ti precursor on hydrothermal synthesis of barium titanate

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Abstract

Barium titanate (BaTiO3) nanopowder is synthesized using two TiO2 powder precursors with different particle sizes and barium hydroxide via hydrothermal route. Effect of temperature, time and particle size is studied using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. Crystallite size of barium titanate is observed to decrease with increasing temperature, while morphology of obtained BaTiO3 changed from porous irregular shape at lower reaction temperature (90 °C) to compact facetted shape at higher reaction temperature (150 °C). TEM observation of low reaction temperature samples (60 °C) supports in situ transformation or short range dissolution–precipitation reaction mechanism. The fine grained TiO2 (∼25 nm) precursor reacted faster than coarse grained TiO2 (∼110–125 nm) precursor. Reaction rate may depend on size of TiO2 precursor particles. The phase of obtained BaTiO3 in all samples is found to be cubic.

Introduction

Barium titanate (BaTiO3) perovskite is famous for its high dielectric constant. BaTiO3 powder is often used in ceramic capacitors, multilayer capacitors (MLCC), positive temperature coefficient of resistivity (PTCR) devices [1]. BaTiO3 nanocrystals have been synthesized by hydrothermal method, sol–gel processing, microwave heated micro-emulsion method and oxalate route [2]. The synthesis by hydrothermal route is interesting as the sub-micrometer BaTiO3 particles obtained via this route under moderate conditions have exact stoichiometry, homogeneity, and uniform size distribution.

Thermodynamics and kinetics of hydrothermal synthesis of BaTiO3 is investigated by many researchers [3], [4], [5]. Hertl [4] reported the kinetics of BaTiO3 proposing the in situ transformation mechanism for hydrothermal conversion of TiO2 to BaTiO3. While using Ba(OH)2 and particulate TiO2 as precursors he found that the rate determining step at high concentration of Ba(OH)2 is the topochemical reaction of Ba2+ with TiO2 at the interface, whereas at low concentration of Ba(OH)2 the diffusion of Ba2+ through the BaTiO3 product layer limits the reaction rate. He found that particle size of BaTiO3 is inversely proportional to initial concentration of Ba(OH)2. Eckert et al. [5] evaluated reaction mechanism of hydrothermal formation of BaTiO3 using same precursors for various reaction times. Their analysis revealed two reaction regimes. In first regime, at early stage of BaTiO3 formation dissolution–precipitation was observed to be the transformation mechanism. While in the second regime, an in situ transformation mechanism dominated at longer reaction times. According to them no trends of crystallite sizes relative to reaction time at a particular temperature were evident.

Hu et al. [6] supported the in situ transformation as reaction mechanism as they observed the size and morphology of BaTiO3 obtained by hydrothermal conversion of TiO2 remained the same as that of the titanium precursor. Qi et al. [7] while investigating the crystallization mechanism of BaTiO3 during the hydrothermal reaction of Ba(OH)2 and TiO2 in a modified autoclave, observed that crystallization mechanism is short range dissolution–precipitation.

In a recent work Wang et al. [8] proposed a formation process of porous spheres of TiO2, SrTiO3 and BaTiO3, a mechanism analogous to the Kirkendall effect [9].

The present study aims at finding the influence of temperature, time and particle size of titania precursor on the hydrothermal crystallization of BaTiO3 and the morphological changes occurring during BaTiO3 formation.

Section snippets

Experimental

Ba(OH)2·8H2O is used as barium precursor, while TiO2 anatase (∼110–125 nm) type-A and TiO2 P25 Degussa (∼25 nm) type-B are used as Ti precursors (Table 1). As the formation of BaTiO3 consumes equimolar amounts of Ba and Ti, the barium to titanium ratio is kept at one (Ba:Ti = 1). Both precursors are added into a Teflon vessel along with double distilled water. No mineraliser such as KOH or NaOH is used for pH adjustment. The concentration of Ba is used where the alkalinity of Ba(OH)2 aqueous

XRD

The reaction at 60 °C with type-A precursor yields an incomplete reaction. Even after 48 h un-reacted anatase is observed by XRD analysis, yet some BaTiO3 perovskite cubic structure can be seen at this temperature after 16 h (Fig. 1). The reactions at 90, 120 and 150 °C for type-A precursor yielded BaTiO3, with some TiO2 as anatase present in all samples, whose amount decreases as the reaction is given longer time. Type-B precursor reacted faster than type-A precursor. As we can observe the

Discussion

A systematic study of hydrothermal synthesis of BaTiO3 was carried out to understand the effects of parameters such as temperature and time on the formation of BaTiO3 under alkaline conditions. Two titania precursors were used to investigate the effect of particle size on crystallization of BaTiO3. Following observation gave an insight into BaTiO3 nanoparticles formation under different temperature, time and Ti precursor particle size states.

Conclusions

For hydrothermal synthesis of BaTiO3 using particulate TiO2 and barium hydroxide it can be summarised that fine grained TiO2 precursor reacted faster due to large surface area for reaction of Ba with TiO2. Higher reaction temperature and longer reaction time helped to complete crystallization of BaTiO3 by hydrothermal route. The crystallite size of BaTiO3 calculated using Scherrer's formula is observed to decrease at higher temperature, while no trends regarding the dependence of crystallite

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