Investigations into the sythesis, structural, dielectric and optical properties of multifunctional M2NaNb5O15 (M=Ba,Sr) nanocrystal composites

Abstract

Transparent 2BaO–0.5 Na₂O–2.5 Nb₂O₅–4.5 B₂O₃ (BNNB) glass system was fabricated and characterized for their structural and various physical properties. These glasses on controlled heat treatment at appropriate temperatures yielded Ba₂NaNb₅O₁₅ nanocrystalline phase. Transmission Electron Microscopy (TEM) carried out on these samples corroborated the presence of Ba₂NaNb₅O₁₅ nanocrystals dispersed in a continuous glass matrix. The optical band gap and refractive index were found to have crystallite size (at nano scale) dependence. The refractive index dispersion with wavelength of light was analyzed on the basis of the Sellmeier relations. At room temperature under UV excitation (355 nm) these glass nanocrystal composites exhibited violet-blue emission which was ascribed to the defect states. The dielectric constant of the BNNB glass was found to be almost independent of frequency (100 Hz–10 MHz) and temperature (300–600 K). The temperature coefficient of dielectric constant was 8 ± 3 ppm/K in the 300–600 K temperature range. The relaxation and conduction phenomena were rationalized using modulus formalism and universal AC conductivity exponential power law respectively. The observed relaxation behavior was found to be thermally activated. The complex impedance data were fitted using the least square method (Nyquist plot). The relaxation times computed from Z"(ω) and M"(ω) peak maxima followed Arrhenius behavior. The fabricated glass nanocrystal composites exhibited P vs. E hysteresis loops at room temperature and the remnant polarization (Pr) increased with the increase in crystallite size. Synthesis and characterization of fine powders comprising nano crystallites of barium sodium niobate, Ba₂NaNb₅O₁₅ (BNN) via citrate assisted sol-gel route at much lower temperature than that of conventional solid-state reaction route. The phase evolution of BNN as a function of temperature was investigated by thermo gravimetric analysis (TGA), differential thermal analysis (DTA), Fourier transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD). XRD studies confirmed the BNN formation temperature to be around 923 K. Transmission electron microscopy revealed that the nanocrystallites are associated with dislocations. The optical band gap was calculated using Kubelka–Munk function. These nanocrystallites exhibited strong visible photoluminescence (PL) at room temperature. The PL mechanism was explained by invoking the dielectric confinement effect, defect states and generation of self-trapped excitons. Synthesis of Erbium (Er³⁺) doped nanocrystalline barium sodium niobate (Ba₂Na₁₋₃ₓErₓNb₅O₁₅ where x = 0, 0.02, 0.04 and 0.06) via citrate based sol-gel route. The desired phase formation was confirmed by X-ray powder diffraction followed by FTIR studies. The high resolution transmission electron microscopy facilitated the establishment of the structure of nanocrystalline phase and its morphology. Kubelka– Munk function based on diffused reflectance studies carried out on nano sized crystallites was employed to obtain the optical band-gap. The synthesized samples (x = 0.02) heat-treated at 1023 K / 2 h exhibited room temperature white light (blue, red and green) emission at a CIE coordinate (0.34, 0.40) and a color temperature of ~5280 K, (cool white) under the excitation radiation of 355 nm. Lead-free ferroelectric ceramics (Sr₂NaNb₅O₁₅ + x wt% MnO₂ (SNN–x Mn)) were fabricated from powders obtained using conventional solid-state reaction route. Effects of MnO₂ addition on the microstructural and electrical properties of Sr₂NaNb₅O₁₅ ceramics were investigated for different x values (0 < x < 0.5). The MnO₂-added SNN based ceramics were found to have tetragonal tungsten bronze structure at room temperature. MnO₂ addition in SNN ceramics were well dense and exhibited enhanced piezoelectric properties. Improved ferroelectric properties (2Pr = 10.78 μC/cm² and 2Ec = 28.06 kV/cm) was obtained for the SNN–0.25 wt% MnO₂ added ceramics. These results indicate that the lead-free SNN–x Mn ceramics are promising for piezoelectric based device applications. The effect of Ta-substitution on Nb sites (associated with the formula Sr₂NaNb₅₋ᵧTaᵧO₁₅ (0 < y < 1.5)) structural and electrical properties of Sr₂NaNb₅O₁₅ ceramics was also investigated. The Curie temperature (Tc) was found to decrease with the increase in Ta substitution on Nb site. The diffuseness of the dielectric peak was found to increase with the increment in Ta⁵⁺ content, implying that its substitution enhances the structural disorder. Vogel–Fulcher modeling of dielectric relaxation confirmed the strengthening of relaxor behavior. There is an ample scope to carry out further interesting studies based on the research outcomes reported in this thesis. In this thesis the refractive index and band gap tunability was investigated for 2BaO–0.5 Na₂O–2.5 Nb₂O₅–4.5 B₂O₃ (BNNB) glass system. The similar study can be done on some more systems and a generalized relation can be derived to predict the physical properties (refractive index, optical band gap etc.) of these materials for different size and volume fraction of crystallized phase. As Ba₂NaNb₅O₁₅ (BNN) single crystals are imperative considering their nonlinear applications (the Second Harmonic Generation (SHG) coefficient of BNN is twice than that of well known LiNbO₃ single crystals), it would be worthy investigating the non-linear optical performance of the transparent glass ceramics with BNN crystallites embedded in the system. Dielectric breakdown studies on BNNB glass and glass nanocrystal composites can be done to estimate the energy storage capacity of these materials, since their dielectric constant was found to be almost independent of frequency (100 Hz–10 MHz) and temperature (300–600 K). This study will be useful in perspective of energy storage capacitor applications. Ferroelectric BNN nano powders synthesized via sol-gel route can be studied further to establish crystallite/grain size dependency on their physical properties (ferroelectric and dielectric properties) by optimizing their sintering conditions. By grain boundary engineering it may be possible to obtain transparent BNN ceramics which can be largely exploited for non-linear optical applications. The Erbium (Er³⁺) doped BNN nano ceramics exhibited white light emission. It would be beneficial to study the optical properties doped with other rare earth ions e.g. Sm³⁺, Eu³⁺, Yb³⁺ etc. Ta substituted Sr₂NaNb₅₋ᵧTaᵧO₁₅ (0 < y < 1.5) ceramics revealed a composition dependent phase transitions. One can study the detailed structural properties with the help of Rietveld refinement and unravel the mechanism.