Effect of grain size and mechanical processing on the dielectric properties of BaTiO 3
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Dielectric properties of polycrystalline BaTiO3 ceramics having grain sizes of 1 to 40 /im have been studied. Fine-grained ceramic BaTiO3 of 1 yam average grain size has 90° domains and has shown higher dielectric constant, lower ferroelectric transition temperature (Tc), and lower transition energy than coarser-grained material. 90° domain switching was preferentially produced in the fine-grained BaTiO3 as a result of abrasion. For the fine-grained BaTiO3, the dielectric constant decreased with one-dimensional pressure, whereas, for the coarse-grained material, the dielectric constant increased before decreasing with the pressure. The one-dimensional pressure resulted in increased Tc of both the fine- and coarse-grained BaTiO3, with the effect being the greatest for the coarse-grained material. The relationship between these results and internal stress, and the effect of external pressure imposed on internally stressed lattice, were discussed.
I. INTRODUCTION BaTiO3 is the most well-known ferroelectric material, but some of its properties, especially the properties as a function of grain size, remain incompletely investigated. The effects of grain size on the dielectric properties of BaTiO3 ceramics have been reported.1"8 It is known that a dense fine-grained ( = 1 /mm) ceramic BaTiO3 had a high dielectric constant around 6000, while a normal coarser-grained ceramic product was 1500-2000 at room temperature.1'2 The high dielectric constant shown in the fine-grained material far exceeds an average value calculated from the dielectric constants of single-crystal BaTiO3 of 4000 along the nonpolar a-axis and 400 along the polar c-axis.9 Several authors2^* have suggested that the fine-grained BaTiO3 has not or very few ferroelectric 90° domains, and thus the high dielectric constant results from internal stress produced in fine grains due to the absence of stress relaxation by the formation of 90° domains as normally shown in coarse grains. Buessem et al.1 have calculated the internal stress resulting from the absence of 90° domains in fine single-domain grains using Devonshire's free energy function. In an alternative approach, Arlt et al.,5 on the contrary, explained the high dielectric constant of fine-grained ceramic BaTiO3 as a 90° domain wall contribution, on the basis of electron microscopy and calculation of domain sizes. Studies of grain size effects on the ferroelectric phase transition temperature (Tc) have also shown contradictory results. Some authors have proposed that Tc increases with decreasing grain size. 67 One study6 has
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Present address: Department of Materials Engineering, Korea Maritime University, Pusan 606-791, Korea.
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J. Mater. Res., Vol. 10, No. 6, Jun 1995
http://journals.cambridge.org
Downloaded: 18 Jul 2014
suggested that a stable space charge layer is formed inside every domain. In the fine-grained ferroelectric materials, the ferroelectric polarization is locked in by the space charge layer, resulting in ferroelectric stability far beyond usual Tc. The space charge layer is
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