Ferroelectric Nanocrystal Dispersed Oxide Glasses

Recently it has been observed that some multicomponent oxide glasses [29-39] dispersed with ferroelectric nanocrystalline phases are formed for suitable choice of the glass compositions with TiO2, BaTiO3, PbTiO3, SrTiO3, PbZrO3, KTiOPO4, Bi2VO55, Li2Ge7O15, PbS, etc. Some of these nanocrystal-dispersed glasses (referred to as FGNC) are found to show ferroelectric or relaxor type ferroelectric behavior [29-30, 40-43]. Figure 1 schematically represents the nanocrystals dispersed in the glassy matrix. Some important parameters of studied FGNC materials (both opaque and transparent) are shown in Table 1 for comparison.

Many low melting TMO glasses like vanadium phosphate glasses (V2O5-P2O5, for example), containing different concentrations of TiO2, BaTiO3, PbTiO3 etc., have been prepared in recent years, which contain nanocrystalline ferroelectric phases embedded in the glass matrix [29, 30]. All these FGNCs are, however, opaque. The alkali metal doped

Figure 1. Schematic diagram of a ferroelectric nanocomposite glass. The dispersed nanocrystals are shown by dark black spots in the glass matrix.

Bi-Sr-Ca-Cu-A-O (A = Li, K, Na) glasses are also found to be ferroelectric for typical concentrations [42, 44]. As mentioned, the dielectric constants of these opaque TMO glasses are found to be extremely large (more than BaTiO3) [30, 44]. Such a giant dielectric constant has also been obtained very recently in crystalline CaCu3Ti4O12 oxide [45, 46] and La containing PbTiO3 [47]. These materials of high dielectric constant are very important for their applications in devices. However, the origin of such a high dielectric constant is not yet clearly elucidated. In the nanoparticle dispersed glassy systems, the presence of nanocrystalline TiO2, BaTiO3, PbTiO3, SrTiO3, PbZrO3, KTiOPO4, Bi2VO55, Li2Ge7O15, PbS, etc. phases are considered to be responsible for giant dielectric constant. As stated, this is a new class of material of immense technological as well as fundamental importance. By varying the glass compositions or the quenching rate, the size as well as the concentration of the FE nano-particles/clusters can be varied.

The amorphous characters in the aforementioned oxide glasses and FGNCs are, in general, examined by an X-ray powder diffraction (XRD) technique. Nevertheless, depending on the smaller size and concentrations of the embedded nanocrystals, XRD is not sufficient to detect the presence of the nanocrystals in these glasses and FGNCs. XRD simply indicates amorphous character of these systems. Transmission electron microscopic (TEM) study can clearly reveal the presence of nanocrystalline phases in these FGNC materials. Some physical properties (like high dielectric constant) also indicate the presence of such nanocrystalline phase in some typical TMO glasses. It is also noticed from the TEM studies that many of the semiconducting TMO glasses, earlier reported to be pure homogeneous glasses, are found to contain nanocrystalline particles or clusters of different sizes (5-100 nm) and concentrations embedded in the corresponding glass matrices. Using the TEM technique, very little microstructural study of the semiconducting glassy system was, however, accomplished earlier [6, 11]. It is found that the size and concentration of the nanocrystals affect the transport, optical, and other properties of these ferroelectric nanocrystal-dispersed glasses. This behavior is similar to that observed in ferroelectric materials, for example, PbTiO3 where dielectric permittivity and other properties strongly depend on the grain sizes [22].

Table 1. Some important physical parameters of different ferroelectric glass nanocrystal composites.

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