Characteristic Core Shell Structures with Composition ×= 0.01 (BaTi1-5xNb4xO3) Prepared by the Barium Titanate Route and the Solid-state Route

Most modern capacitor applications require complex perovskite compositions (ABO3) involving barium titanate (BaTiO3) and several dopant oxides; therefore, the chemical synthesis of submicron barium titanate has grown in importance since the 1980s in response to the need to increase capacitance volume efficiency in multilayer capacitors. The phenomenon of ferroelectricity was discovered by Joseph Valasek at the Rochelle Salt in 1921 [1, 2]. Since the discovery of this phenomenon until today, BaTiO3 has been the most widely studied ferroelectric material by many researchers [3, 4]. Ferroelectric materials present a permanent electric dipole moment, which can be reoriented by applying an external electric field. The properties of this electroceramic are involved in the fabrication of thermistors, dielectric ceramic capacitors, multilayer capacitors, relaxors, etc. [5, 6]. Diffuse phase transition (DPT) behavior was observed in BaTiO3 samples with additions of Zr, Cd, Bi and Nb, the microstructure of these ceramic was characterized by a core-shell structure [7–10] which featured a core and shell for ferroelectric and paraelectric phases respectively. It was suggested that the core and shell are not different phases [7]. During sintering a core-shell structure can be developed as a result of densification and grain growth which were assisted by a liquid phase [7]. It was further suggested that core-shell structure tends to disappear at high temperatures [7]. In this study, we report characteristic core-shell structure, also, chemical inhomogeneity is found as a result of (DPT). According to this in mind, in this work, X ray diffraction (see Figure 1), electrical measurements, scanning electron microscope (see Figure 2) and transmission electron microscope (see Figure 3) studies were used to investigate the BaTiO3 system with Nb5+ dopant by two different routes called mixed oxide route and barium titanate route with barium carbonate and barium titanate as initial reagents respectively, to determine effects on physical properties. In Fig. 4, fixed frequency permittivity data obtained at 10 kHz using an LCR meter for sample with x = 0.01 prepared by BT route are presented. Results are shown for three different sintering temperatures, 1400, 1500 and 1550oC; it can be seen that higher values of permittivity were obtained when the sample was heated at 1500oC, which was further characterized by TEM and compared with results from the mixed oxide route for the same composition. It is important to note that in all three cases, the values of permittivity are low and relatively independent of temperature, which makes this material interesting for possible X7R applications [11–13]. In Fig. 5, a comparison of permittivity data between composition with x = 0.01 prepared by BT-route and mixed oxide route is presented. The electrical response at two different sintering temperatures, 1500 and 1550°C is shown. From the Fig. 5, four different curves are observed, two corresponds to a samples with composition x = 0.01 prepared by mixed oxide route and are labeled as 4vta and 4maya respectively and the rest are for samples prepared by BT route. Clearly, an inflection is observed in sample 4maya prepared by mixed oxide route at 1550°C, compared with sample 4vta sintered at 1500oC, before made sample 4vta sintered at 1500°C, the powder was reheated three times at 1500°C then quenched at 1550°C. Electrical measurements revealed that, more homogeneous sample 4vta was obtained after several heating steps at 1500oC, compared with 4maya, which were prepared by mixed oxide route, Fig. 5. At 1500oC the sample 4vta prepared by the mixed oxide route has a permittivity value greater than 10000 and there is no remaining BT present in this sample, while the samples 4BTB and 4btb2 prepared by the BT-route showed an inflection which corresponds to remaining barium titanate and was further identified using TEM. Under the TEM, sample 4BTB showed the presence of typical core-shell structure inside the grains. Core-shell structures are present in the material due to inhomogeneous distribution of Nb5+ atoms in the barium titanate. By LCR measurements, we detected the presence of an inflection, which corresponds to a type I, ferroelectric phase: tetragonal barium titanate. In Figs. 4, 5 and 6, the inflection present is an indication that the ferroelectric phase barium titanate is still present. This shows that, even after sintering at very high temperatures, sometimes it is not possible to obtain fully a homogeneous sample when prepared by BT route. XRD results clearly demonstrate that when the sample prepared under BT route was heated at temperatures of 1550oC a second phase appeared which was identified, as hexagonal BaTiO3. It was clear by XRD that in samples prepared by BT route the cubic phase could be obtained at temperatures up to 1500oC, without any evidence of secondary phases. Unfortunately, when the sample prepared under BT route fired at 1550oC a mixture of phases were achieved. According to the XRD results, we cannot establish exactly at which time the mixture of phases appeared in the sample or if the sample started to melt before reaching completely the pure hexagonal phase or mixture of phases. In comparison, for the samples with x = 0.01 prepared by the solid state route and BT route respectively XRD results demonstrates that is possible to keep the cubic phase at temperatures up to 1550oC without any evidence of secondary phase (hexagonal phase) compared with sample prepared under BT route in which at the same temperature the hexagonal phase form TEM results shown that when sample with x = 0.01 prepared under BT route was sintered for extended times then dopant (Nb5+) would diffuse into the core and compositional homogenization would cause a transition from the type I (just core shell) to a type III (only high density of dislocations). TEM results showed that the core and shell structure result due to an inhomogeneous distribution of Nb5+ inside the barium titanate in sample prepared under BT route, consequently, this has an important role on the temperature dependence of the dielectric constant. For samples prepared by the BT route, electrical measurements demonstrate that the peak behavior of the dielectric constants with temperature observed is simple due to a remaining barium titanate which is related with the ferroelectric phase (core) observed by TEM and has an important effect on the permittivity value. For the same composition inhomogeneous core and shell, regions were observed only for samples prepared by the BT route. Permittivity measurement values demonstrate that samples prepared by BT route are more inhomogeneous than those prepared under solid-state route. XRD data for sample with x = 0.01 prepared by the BT route. SEM micrograph for sample prepared by BT route heated at (Figure 2). TEM bright-field image is shown for composition with x = 0.01 prepared by (Figure 3). Temperature dependence of permittivity for sample x = 0.01 (Figure 4). Temperature dependence of permittivity data against temperature. Comparison between mixed oxide route and barium titanate (Figure 6).