Effect of copper substitution on the microstructure and ferroelectric properties of barium titanate

Abstract

Barium titanate (BaTiO3) is one of the most extensively studied ferroelectric materials particularly because of its high dielectric constant, which is useful for ceramic capacitors. Besides this BaTiO3 has also generated much interest as a piezoelectric transducer [1], a chemical sensor [2] and a thermistor [3]. Owing to its diverse applications, synthesis of BaTiO3 has become the subject of several recent papers that include the coprecipitation [4–6], sol–gel [7, 8] hydrothermal [9, 10] and combustion techniques [11]. It has also been well established that the ferroelectric properties of BaTiO3 are markedly altered by substituents as well as grain size in polycrystalline ceramics [12–14]. Isovalent substituents such as Zr and Sn for Ti were reported to lower the ferroelectric to paraelectric transition temperature, i.e. Curie temperature (Tc), while enhancing the dielectric constant with increasing concentration of the substituent [15, 16]. Heterovalent substituents like Fe, Cu, Ni and Nb were also reported to shift the Tc of BaTiO3 ceramics [17]. Attainment of lower firing temperatures without sacrificing high densities and dielectric constants is an important factor in the use of BaTiO3 ceramics for multilayer capacitors. Hence, in order to lower the sintering temperature of BaTiO3 ceramics, many oxides, such as B2O3, Bi2O3, CuO and SiO2, are added as flux, which has a tendency to lower the dielectric constant either by dilution effect or by formation of compounds with low dielectric constant. In view of the observation that Cu can act as a Curie point shifter and the oxide CuO can act as a flux agent, the study reported here was undertaken to investigate the effect of copper ion substitution to yield Ba(Ti1yxCu2x)O3 compositions with x ˆ 0.025, 0.05 and 0.1. The variation of dielectric properties and corresponding microstructure as a function of Cu concentration in BaTiO3 are discussed in comparison with those of isovalent Sn modified BaTiO3 ceramics. Powders of Ba(Ti1yxCu2x)O3 with x ˆ 0.025, 0.05 and 0.1, and Ba(Ti1yxSnx)O3 with x ˆ 0.025, 0.05, 0.075 and 0.1 were prepared by conventional powder mixing method using BaCO3, TiO2, CuO and SnO2 of analytical grade chemicals as starting materials. The weighed amounts of raw materials were mixed thoroughly by grinding with addition of alcohol and calcined at 900 8C for 4 h. The calcined powder after grinding was mixed with 5 wt % PVA as binder, pelletized and sintered at 1200 8C for 4 h. Phase identification in the sintered pellets was performed using an X-ray diffractometer (Scintag model DMC 105) with Ni filtered CuKAE radiation. Variation of dielectric constant as a function of temperature from room temperature at 150 8C was measured at 1 kHz using an LCR meter. Prior to dielectric measurements the pellets were coated with a silver paint and cured. Microstructural studies on the fractured surfaces of sintered pellets were performed with a scanning electron microscope (SEM; Jeol model T330A). The X-ray diffraction (XRD) pattern of pure BaTiO3, typical of all other samples, is shown in Fig. 1 along with that of 0.1 Sn modified BaTiO3. The XRD patterns of pure and Cu-modified BaTiO3 samples were in complete agreement with the XRD pattern of tetragonal BaTiO3 of the Joint Committee on Powder Diffraction Standards (JCPDS) file number 5-626. However, in Sn-modified samples, some peaks of low intensity characteristic of BaSnO3 were noticed for x ˆ 0.1 composition. Variation of dielectric constant as a function of temperature for pure as well as Cuand Sn-modified samples is depicted in Figs 2 and 3. As can be seen from Fig. 2, progressive addition of Cu (except for the x ˆ 0.025 sample) resulted in an increase of peak dielectric constant (aTc ) from 6135 to 16 349 with a simultaneous decrease in Tc from 120 to 75 8C. The observed ferroelectric to paraelectric phase transition for pure BaTiO3 coincides well with the literature value, suggesting that a grain size higher than 1 im and addition of Cu seem to render the transition diffuse. Also, there was an increase in room temperature dielectric constant (aRT) from 1285 for pure BaTiO3 to 7598 for 0.1Cu modified BaTiO3,