Ferroelectric-to-non-ergodic relaxor phase transition of (Bi0.5Na0.3K0.2) TiO3– (Ba0.8Ca0.2) TiO3 lead-free ceramics by SrTiO3 effect

In this work, a series compositions of [(0.95?x)(Bi_0.5Na_0.3K_0.2)TiO₃–xSrTiO₃–0.05(Ba_0.8Ca_0.2) TiO₃] Pb-free ceramics (0.0?≤?x?≤?0.2) abbreviation (BKNT–ST–BCT) were synthesized by conventional sintering method. Effect of ST addition on the crystal structure, domain structure, dielectric and ferroelectric properties were investigated. The crystal structure was monitored by XRD and the patterns shown phase crossover from tetragonal to pseudo-cubic caused by lower crystal symmetry of lattice effect at high content of ST. Morphology of sintered ceramics were characterized by SEM, while the domain structure of ST?=?0.0 at different temperatures was examined by In situ TEM. Diffused phase transition corresponding to ferroelectric to ergodic relaxor at lower T has been observed at depolarization temperature (T_d) at (ST < 0.15), while the permittivity peak which detected at ST?=?0.2 in whole range of temperature denote non-ergodic relaxor to paraelectric phase transition. All compositions show normal ferroelectric (P–E) loop at room temperature even ST?=?0.2 with low coercive field (E_c?~?15 kV/cm), while slim relaxor (P–E) loop was observed at (T?=?200 °C). Present normal ferroelectric properties of ST?=?0.2 are attributed to the domain growth and domain wall displacement above the domain switching electric field. All samples belonging to (x?≤?0.1) shown current peak corresponding to ferroelectric phase transition at domain switching field. Samples belonging to (x?≥?0.15) shown two nominal peaks where the second peak is corresponding to ferroelectric-to-another ferroelectric phase transition with different crystal symmetry.

     

A Langrangian Study of Solids Suspension in a Stirred Vessel by Positron Emission Particle Tracking (PEPT)

A technique of Positron Emission Particle Tracking (PEPT) was used to obtain information on the flow behavior of coarse particles suspended in pseudoplastic liquids agitated by axial‐hydrofoil Lightnin impellers A320 and A410. PEPT enables the position of a 600 μm radioactive particle tracer inserted inside one of the suspended particles to be detected many times per second and its full trajectory followed inside the vessel. Particle trajectory analysis yielded information on particle circulation, velocity distribution, and spatial occupancy. The minimum speed for complete particle suspension, N_js, was also determined. The well‐known Zwietering correlation failed to predict the measurements by a substantial margin, suggesting that it is inadequate for viscous non‐Newtonian liquids.     

Determination of minimum speed required for solids suspension in stirred vessels using pressure measurements

The minimum speed required for complete suspension (N_js) is a major parameter for solids suspension in stirred tanks. Micale et al. 7th UK Conference on Mixing (2002) determined N_js by using a pressure gauge technique in a model vessel. In the present work N_js was measured by the same later technique in a more practical vessel with varying C/T ratio and the impeller direction of rotation. The results were compared to those obtained by Zwietering Chem. Eng. Sci. 8 , 244–253, (1958) correlation. Agreement was found between results from present work and predictions by Zwietering's correlation with maximum difference not exceeding 17%.