Reaction mechanisms for the hydrothermal synthesis of barium titanate are evaluated. Feedstocks of barium hydroxide octahydrate and anatase titania are reacted for varying durations (1–72 h) to provide intermediate‐stage samples for characterization by transmission electron microscopy/ energy‐dispersive spectroscopy (TEM/EDS), X‐ray diffractometry (XRD), and inductively coupled plasma spectroscopy (ICP). Quantitative evaluation of the extent of reaction by ICP and XRD methods permits the analysis of data with the Johnson‐Mehl‐Avrami equation. This analysis reveals two reaction‐rate regimes. Kinetic analysis, based on reaction progress, yields insight into the first reaction‐rate regime but is inconclusive in the analysis of the second reaction‐rate regime. In the first regime, at the early stage of barium titanate formation, a dissolution‐precipitation mechanism dominates. In contrast, in the second regime, at longer reaction times, an in‐situ transformation mechanism is probably dominant. However, multiple reaction mechanisms (e.g., in‐situ transformation and dissolution‐precipitation) may be competing for rate control. Alternatively, dissolution‐precipitation may be the dominant mechanism throughout the barium titanate synthesis, with nucleation and growth controlling the first regime and dissolution rate controlling the second regime.
A comprehensive model for calculating the electrical conductivity of multicomponent aqueous systems has been developed. In the infinite-dilution limit, the temperature dependence of ionic conductivities is calculated on the basis of the concept of structure-breaking and structuremaking ions. At finite concentrations, the concentration dependence of conductivity is calculated from the dielectric continuum-based mean-spherical-approximation (MSA) theory for the unrestricted primitive model. The MSA theory has been extended to concentrated solutions by using effective ionic radii. A mixing rule has been developed to predict the conductivity of multicomponent systems from those of constituent binary cation-anion subsystems. The effects of complexation are taken into account through a comprehensive speciation model coupled with a technique for predicting the limiting conductivities of complex species from those of simple ions. The model reproduces the conductivity of aqueous systems ranging from dilute to concentrated solutions (up to 30 mol/kg) at temperatures up to 573 K with an accuracy that is sufficient for modeling industrially important systems. In particular, the conductivity of multicomponent systems can be accurately predicted using data for single-solute systems.
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