Effect of chelating agents on crystal structure of nm-sized barium titanate crystallites prepared using a LTDS method

Abstract

With miniaturization for electronic device, down-sizing of electronic components has been developed in past years and accelerated at present. As a result, in the electric devices such as multilayered capacitor, particle sizes of barium titanate (BaTiO3) and barium strontium titanate (Ba1−x Srx TiO3) raw materials are also down-sizing, and today, ferroelectric fine particles with sizes below 100 nm are required. However, in ferroelectric fine particles, a phenomenon was well known that ferroelectricity decreased with decreasing particle and grain sizes, and disappeared below certain critical sizes, which was called “size effect” in ferroelectrics [1–9]. Therefore, size effect for ferroelectrics such as BaTiO3 and lead titanate (PbTiO3) is one of the most important phenomena for an interest of industry and science. Ishikawa et al. prepared PbTiO3 fine particles by a sol-gel method, and reported that using both Raman scattering and XRD, a critical size of PbTiO3 is estimated around 10.7 nm [8]. McCauley et al. also prepared BaTiO3-glass ceramics, i.e., BaTiO3 fine particles dispersed in glass, and reported that using a dielectric measurement, the critical size of BaTiO3 is estimated around 17± 4 nm [9]. However, these critical sizes were just estimated values, and there is no experimental critical size because of difficulty in preparation of ferroelectric particles with sizes around 10 nm. Thus, it is very important to prepare ferroelectric crystals with a size below their critical sizes. To date, using wet process such as sol-gel and hydrothermal methods, a preparation of nm-sized ferroelectric particles with sizes below 10 nm has been tried, but has never succeeded. One of reasons why it was so difficult to prepare nm-sized ferroelectric particles using the above methods may be an indirect formation of ferroelectric particles via intermediate products synthesized from each ion. Therefore, it can be expected that if ferroelectric particles can be prepared directly from each ion, not via intermediates, there will be no minimum limit in their particle sizes. Considering this point, we proposed a new innovative preparation method for direct synthesis of nm-sized ferroelectric crystallites, i.e., low temperature direct synthesis (LTDS) method [10, 11]. Using this LTDS method, it can be expected that BaTiO3 can be prepared directly from Ba and Ti ions, not via intermediates, as shown in the following Equation 1,