Geeta Ray

Performance of crystal violet doped triglycine sulfate single crystals for optical and communication applications

Single crystals of 0.01 and 0.02 mol% crystal violet dye doped triglycine sulfate (NH2CH2COOH)3·H2SO4 (TGS) have been grown by a slow evaporation technique. Enhanced dielectric, optical, thermal and mechanical properties have been achieved by dye doping. Monoclinic structures showing particular coloring patterns and morphology change with dye concentration were obtained. Various functional groups and dye incorporation in the grown crystals were confirmed qualitatively by FTIR and Raman analysis. A significant increase in Curie temperature from 50 to 55 °C with a decrease in maximum permittivity has been observed. The UV-vis spectra showed an increased transmittance window and an increased optical band gap from 5.61 to 6.11 eV as a result of doping. An increase in the sharpness and intensity of the photoluminescence peak with a blue shift has been observed in doped crystals. The thermal stability and the decomposition temperature were found to increase by about 10 °C in 0.01 mol% dye doped TGS. The mechanical strength of the grown crystals was estimated by the Vickers microhardness test and was found to be high for dye doped TGS. A high piezoelectric charge coefficient d33 of 16 pC N−1 was observed for pure TGS which decreased as a result of the dye effect. The fabrication of a patch antenna was carried out after simulating its resonant frequency, making it suitable for telecommunication applications.

Combined structural, electrical, magnetic and optical characterization of bismuth ferrite nanoparticles synthesized by auto-combustion route

Abstract Phase-pure multiferroic bismuth ferrite (BFO) nanoparticles were synthesized by energy efficient, simple and low temperature sol–gel followed by auto-combustion route. Highly crystalline and well-shaped BFO nanoparticles of size about 50 nm were observed in TEM. Thermal analysis was used to optimize the calcination temperature as 500 °C. An endothermic peak at 834 °C has been detected in the DTA curve, representing the Curie temperature. The dielectric anomaly around Neel temperature (TN) was observed signifying the magnetoelectric coupling. The BFO nanoparticles were found to be highly resistive (ρ ∼ 3 × 109 Ω-cm) and had very low leakage current of the order of μA/cm2, which resulted from phase purity. A significantly enhanced weak ferromagnetism was observed due to smaller particles size and remnant magnetization and coercive field were 0.067 emu/g and 185 Oe, respectively. P–E loop confirmed the ferroelectric behavior of BFO nanoparticles. The direct band gap energy was calculated to be 2.2 eV from UV–vis studies.

Flux growth of lead free (Na0.5Bi0.5)TiO3–(K0.5Bi0.5)TiO3 ferroelectric single crystals and their characterization

We have grown single crystals of lead-free (Na0.5Bi0.5)TiO3–(K0.5Bi0.5)TiO3 (abbreviated as BNKT) as promising materials for ferroelectric applications. The single crystals were grown by a high temperature self-flux solution method in which A-site compounds were used as complex flux. The crystal structure was analyzed by means of an X-ray diffraction technique showing a perovskite phase with tetragonal symmetry at room temperature. The surface morphology of the BNKT crystals was examined by scanning electron microscopy (SEM) and was found depicting a layered like hopper structure. The temperature and frequency dependence of the dielectric constant and loss for the as-grown single crystals was investigated, showing a decrease in Tmax from 316 °C at 50 kHz to 310 °C at 1 MHz. To study the degree of dielectric relaxation, modified power law fitting and Lorentz type quadratic fitting were performed. The relative composition of the studied sample and its phase transition behavior were confirmed by Raman spectroscopy. The piezoelectric coefficient d33 with optimized poling field was found to be 216 pC N−1. The ferroelectric response of the BNKT crystals with temperature at an applied field of 37 kV cm−1 was studied. The results suggest a promising approach for growing high quality alkali based single crystals with a relatively large size for piezoelectric and ferroelectric applications.

Flux growth of 0.94[Na0.5K0.5NbO3]–0.06LiNbO3 piezo-/ferroelectric crystals for long duration and high temperature applications

High quality lead free ferroelectric single crystals of 0.94[Na0.5K0.5NbO3]–0.06LiNbO3 (NKLN) were grown by the self-flux method. The crystals were characterized for structural and dielectric properties, piezo/ferro response (both on the macro and microlevel) and electrical behavior. The characterization confirmed a perfectly homogeneous stoichiometry having negligible defects in the sample. Further, the crystals were found to exhibit high Curie phase transition (~420 °C) with a high value of d33 coefficient (=115 pC N−1) and remnant polarization (=4.01 μC cm−2 at 30 °C). Its pyroelectric coefficient was found to be ~309.8 μC m−2 °C−1. In addition, the crystals did not show any fatigue. Further, ferroelectric domains were analyzed using piezoresponse force microscopy and the effective d33 coefficient on the microlevel was calculated to be ~80 pm V−1 after a 20 min relaxation of 75 V induced bias. The results suggest NKLN crystals to be a very promising material in the family of available lead free materials for various ferroelectric applications.