Rapid one-step preparation of SrZrO3 using Zr4+ gel and SrSO4 ore under alkaline hydrothermal conditions

Abstract

SrZrO₃-structured perovskite particles were prepared under hydrothermal conditions in a KOH (5 M) solution using Zr-gel and SrSO₄ mineral precursors. The treatments were conducted between 150 and 240 °C for different reaction intervals (1–96 h), and the KOH solution volume varied between 7.5 and 30 mL. To evaluate the effect of the Zr-gel precursor, the treatments were preliminarily conducted with a coprecipitated pasty Zr-gel (Zr(OH)₄·9.64H₂O) and subsequently with a lyophilized Zr-gel Zr(OH)₄ powder. Generally, SrZrO₃ particles were produced by a single-step reaction following the simultaneous bulk dissolution of the Zr⁴⁺ gel precursor and the SrSO₄ powder. However, in the preliminary experiments, a dehydration reaction of the pasty Zr-gel preceded the ultimate single-step reaction, resulting in complete SrZrO₃ particle formation taking place over a longer interval of 96 h at 240 °C. In contrast, when using the dried Zr-gel powder, complete feedstock dissolution occurred more rapidly, producing SrZrO₃ particles at 200 °C over 48 h. The SrZrO₃ particle sizes varied significantly depending on whether the pasty gel or dried powder Zr precursor was used. Particles prepared with the pasty gel exhibited a bimodal size distribution with mean particle sizes of 25 and 65 μm with pseudocubic and star-shaped cuboidal morphologies, respectively. In contrast, particle growth resulting from the rapid dissolution of solid powder feedstock produced cubic-shaped particles, monomodally distributed with an average particle size of 10 μm. Furthermore, byproduct (SrCO₃) formation occurred predominantly under earlier stages together with SrZrO₃ particle irrespective of the 5 M KOH filling volume; however, at a volume of 15 mL spontaneously achieved in situ the SrCO₃ dissolution at intermediate stages of reaction. This reaction pathway did not proceed at small (7.5 mL) and large (30 mL) volumes of the alkaline fluid. A kinetic study indicated that the activation energy required to produce the SrZrO₃ cubic-shaped particles was low in both cases, i.e., 15.05 and 22.27 kJ mol⁻¹ between the powder and pasty Zr⁴⁺ precursors, respectively.