Novel Materials Proper to Liquid Process

Abstract

In this chapter, novel oxide materials, which are special for liquid processes, are introduced. In the liquid process, a final solid oxide is formed by the decomposition of a metal–organic compound and by the subsequent growth of metal oxide, which proceed simultaneously in a pyrolysis reaction. Because of their nature, the organic elements remain until a certain stage during the conversion process from liquid to solid. Carbon atoms thus remaining in the system strongly affect the properties of the solid oxide formed and also affect the gel-to-solid conversion. The complete elimination of carbon from the system generally requires a temperature greater than 1000 °C. Accordingly, carbon should remain in the system when annealing is conducted at 500 °C or 600 °C. In most cases, the remaining carbon tends to degrade the properties of the formed solid. In the case of an ITO film, for example, the resistivity of an ITO film prepared by the vacuum process is typically 1 × 10⁻⁴ Ωcm, whereas it increases to 1 × 10⁻³ Ωcm in the case of a film prepared using the liquid process. This degradation is regrettably common in liquid-processed oxide materials and has been a large drawback of the liquid process compared to the vacuum process.

However, exceptions always exist. That is also true in oxide materials prepared by the liquid process. In some cases, the liquid process gives thin films superior to those prepared by the vacuum process. One example is described in Sects. 12.1 and 12.2, “Low-temperature formation of perovskite PZT.” By controlling the decomposition of carbon, we demonstrated the direct formation of perovskite PZT bulk and films at low temperatures, which has not been accomplished by the vacuum process. Moreover, the effect of carbon decomposition on materials led to novel materials. One example is introduced in Sects. 15.1 and 15.2. A material with a high relative dielectric constant ε_r was obtained by the liquid process. The material is BiNbO, which has a pyrochlore crystal structure as a main phase and has never been previously reported in this ternary system.

Because oxide materials prepared by the liquid process always contain some amount of carbon, they can reasonably be called metal-oxide carbides. Therefore, they are essentially a novel substance from a compositional viewpoint. Sometimes novel materials are created because of remaining carbon. The example is introduced in Sect. 15.3, where p-type semiconductors were created, most likely by the effects of remaining carbon.