Direct synthesis of nm-sized barium titanate crystallites using a new modified LTDS method under CO2-free atmosphere

Abstract

Ferroelectric fine particles such as barium titanate (BaTiO3) had been used as raw materials for multilayered capacitor. Recently, with miniaturization for electronic device, down-sizing of multilayered capacitor has been developed for past years and accelerated. As a result, in the multilayered capacitor, the thickness between electrodes has been reduced to 1 μm at present, and in the future can be expected to become 0.1 μm. Consequently, with down-sizing thickness in the multilayered capacitor, decrease of particles sizes in BaTiO3 raw materials is also required, and in the future, it can be expected that particle sizes will reduce from a few hundred μm to a few ten μm. However, in ferroelectric fine particles, it is well-known that ferroelectricity decreases with reducing particle and grain sizes, and disappears below certain critical sizes, this is called the “size effect” in ferroelectrics [1–9]. Therefore, size effect for ferroelectrics such as BaTiO3 is one of the most important phenomena of interest to industry and science. To date, many researchers have studied the size effect, and some have estimated a critical size around 10–20 nm in BaTiO3 fine particles [9–12]. This critical size was just an estimated value, and there is no experimentally determined critical size. Thus, it is very important to prepare BaTiO3 ferroelectric crystals with sizes below the estimated critical size, and investigate its size effect. However, it is so difficult to prepare nm-sized BaTiO3 crystallites using conventional wet processes such as sol-gel and hydrothermal methods because of their limitation. To prepare nm-sized BaTiO3 crystals with sizes below 10 nm, a new innovative preparation method was required. We proposed a new preparation method (low temperature direct synthesis, LTDS, method) to prepare nmsized BaTiO3 crystallites [13, 14]. This concept can be described as follows. Ti ions in strong acid solution (pH 13.0) with excess Ba ions. Next, near an interface between Ti droplet and Ba solution, a neutralization reaction between strong acid and base occurs, with a large heat of neutralization. As a result, it can be expected to synthesize BaTiO3 particles directly from Ti and Ba ions using the heat of neutralization as follows,