Synthesis and structure-property characterization of BaTiO3/xNio nanocomposite ceramics

Abstract

Microelectronics industry is confronted with a challenging issue of fluctuating dielectric constant with temperature and frequency in device applications. Thus, there is a great demand to find ceramic capacitors with diffuse and stable dielectric properties. This research thus aimed at fabricating new BaTiO3/xNiO doped ceramics having diffuse and stable dielectric properties where x = 0, 0.5, 1.5 and 2.5 % of BaTiO3 (by weight) added NiO doped ceramic samples. In this context, NiO nano-power was added in the range of 0.5 to 2.5 % of BaTiO3 (by weight) added NiO doped ceramic samples. BaTiO3/xNiO doped ceramics were fabricated from BaTiO3 powder and NiO nanoparticles employing a conventional mixed oxide method. At first, nano sized pure BaTiO3 powders (100nm) and NiO (20nm) were properly milled, dried and pressed into pellets to prepare green samples. Mixed powders were pressed into pellets at a pressure of around 40 kN. Then, for densification the green samples were sintered in a high temperature furnace at 12800C, 13000C, and 13200C. Single stage sintering was used for densification of the samples. To ascertain crystallinity of fabricated samples, an XRD analysis was conducted. XRD analysis confirmed well-defined diffraction peaks from (100), (101), (111), (002), (200), (201), (210), (211), (202), (212), (103), (301), (113), (311) and (003) plane of BaTiO3. Interestingly, the addition of NiO in BaTiO3 imparted significant strain in crystals evident from the merging of twin peaks {for 0.5-2.5 % of BaTiO3 (by weight) added NiO doped ceramic samples} which is characteristics of tetragonal BaTiO3 phase. Thus, i.e. only doping effects have been observed in fabricated ceramics. Grain size, its distribution and morphology were investigated using Field- Emission Secondary Electron Microscopy (FESEM). NiO additions are present in the position of octahedral site located in the BaTiO3 peroveskite crystal structure. Abnormal grain growth was found in pure BaTiO3 ceramic with a 2.74 μm average grain size. After adding NiO nanoparticles into the system, the grain size significantly decreased to 1.15 μm. The melting temperature difference of NiO and BaTiO3 suppresses the grain growth during the sintering process. The addition of NiO nanoparticles served two purposes: 1) A part of it modifies the dielectric properties by substitution of Ti4+ sites thus creating a vacancy at an anion site for required charge neutrality, 2) The rest of NiO may present in the form excessive inclusions in the BaTiO3 and located in the BaTiO3 grain boundaries. These NiO may hinder domain wall motion sufficiently to reduce the dielectric constant. Moreover, NiO at grain boundaries develops conducting phases and can affect the capacitance and dielectric loss values of the materials. With increasing NiO content the dielectric constant of BaTiO3 decreased due to the development of a conducting phase in the microstructure. It was found that sintering in the range of 12800C to 13200C for 2 hours showed controlled grain size near to 1 m required for obtaining optimal properties. Energy Dispersive X-ray (EDX) analysis was conducted for elemental analysis and to understand the role of doped Ni2+ in the BaTiO3/xNiO doped ceramics. EDX analysis confirmed the Ni2+ presence at phase boundary. Dielectric properties of the samples were measured using an impedance analyzer. The change of dielectric properties was investigated varying modulating frequency (100 Hz to 10 MHz) and temperature (300C-1400C). Curie point of 0.5, 1.5 and 2.5 % of BaTiO3 (by weight) added NiO doped ceramic samples were found respectively at 1030C, 960C and 980C by LCR meter whereas the Curie temperature (Tc) for pure BaTiO3 is 1200C. The best room temperature dielectric constant of 1950 was obtained for BaTiO3 (BTO) + 0.5 % of BaTiO3 (by weight) added NiO doped ceramic sample sintered at 12800C for 2 hours. Temperature dependence was investigated at 10 kHz and a stable dielectric constant of 700 was obtained as a function of temperature. For BaTiO3 (BTO) + 0.5 % of BaTiO3 (by weight) added NiO doped ceramic sample, addition of NiO contributed to enhancement of dielectric properties primarily due to doping effects. In contrast, BaTiO3 (BTO) + 1.5 to 2.5 % of BaTiO3 (by weight) added NiO doped ceramic samples exhibited doping effects resulting stable dielectric behavior.