Effects of Ba/Ti ratio on tetragonality, Curie temperature, and dielectric properties of solid-state-reacted BaTiO 3 pow
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The effects of the barium/titanium (Ba/Ti) ratio on the crystalline phase, Curie temperature, and dielectric properties of solid-state-reacted BaTiO3 powder were investigated. The experimental results showed that tetragonality decreased and the Curie temperature shifted to lower temperature when the Ba/Ti ratio strayed from 1.0. The BaTiO3 powder had the maximum dielectric constant when the Ba/Ti approaching 1.0.
I. INTRODUCTION
Barium titanate (BaTiO3) is widely used in the electronics industry. With its high dielectric constant and low losses, it is used as a basic ferroelectric material in many electronic components, such as multilayer ceramic capacitors, as well as positive temperature coefficient thermistors and sensors. Previous studies on the effects of the Ba/Ti ratio on BaTiO3 have focused on doping elements and dielectric properties. However, doping elements may cause complications in studies of the Ba/Ti ratio. Therefore, to realize the effects of the Ba/Ti on tetragonality, Curie temperature, and the dielectric constant of BaTiO3 powderbecomes very important. Hu et al.1 pointed out that the solubility limit of BaO in BaTiO3 is lower than 100 ppm. Lee et al.2 revised the BaO–TiO2 phase diagram and the solubility limit of BaO in BaTiO3 and pointed out that the Ba/Ti ratio may change when the sintering temperature does. For example, the Ba/Ti ratio ranges between 0.9834–1.0051 and 0.9665–1.0097 when sintered at 1200 and 1320 °C, respectively. Lewis and Catlow found that Schottky defects occur more easily than Frenkel defects in nonstoichiometric BaTiO3, and these defects in Ba-excess and Ti-excess are shown in the following equations, respectively2:
II. EXPERIMENTAL PROCEDURE
BaTiO3
X 00 BaO ! BaX Ba þOO þVTi þ2VO BaTiO3
TiO2 !
X 0 TiX Ti þ2OO þVBa þVO
:
On the other hand, the formation energies of barium vacancies (5.5 eV) and titanium vacancies (24 eV) are lower than the diffusion energies of titanium ions (83 eV) and barium ions (20 eV) in BaTiO3, and thus, the solubility regime on the Ti-excess side should be wider than that on the Ba-excess side. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.318 J. Mater. Res., Vol. 27, No. 23, Dec 14, 2012
http://journals.cambridge.org
Abelard3 proposed that tetragonality decreases as the grain size of BaTiO3 decreases, leading to the so-called “size effect.” Previous studies4–12 had obtained results that indicate that tetragonal and cubic phases coexist in BaTiO3 in a core– shell structure (core: tetragonal, shell: cubic) when the grain size decreases. A cubic phase appearing on the BaTiO3 surface leads to a decrease in the transformation enthalpy of BaTiO3 powder and also decreases the Curie temperature. Arlt et al.13 also pointed out that permittivity shows a maximum ;5000 at grain sizes of 0.8–1 lm and strongly decreases at grain sizes ,0.7 lm. The decrease in permittivity observed at smaller grain sizes is obviously correlated to structural changes in the BaTiO3. Therefore, to determine
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