Production of Nano-Crystalline Zirconia-Based Powders Using a Flow-Through Hydrothermal Process

Abstract

ABSTRACTNano-crystalline, zirconium oxide and oxyhydroxide particulates were produced using a novel, continuous, flow-through hydrothermal technology which was previously developed at the Pacific Northwest National Laboratory. Homogeneous aqueous solutions containing 0. LM ZrO(NO3)2, 0.2-0.5M urea, and in some cases 0.025-0.10M NaF were pumped at pressures of 6000-8000 psi through a heated stainless steel reaction tube at 340-350°C. Flow at pressure was maintained using a nozzle at the down-stream end of the reaction tube. In this process, termed Rapid Thermal Decomposition of precursors in Solution (RTDS), the formation of zirconium oxide and oxyhydroxide nano-crystals (diameters of generally less than 15-nm) occurred during the solution's brief residence time (<15 seconds) in the reaction tube, yielding aqueous slurries of agglomerated particulates upon exiting the nozzle. Powders were separated from these slurries, washed, then dried. RTDS powder prepared from 0.1M ZrO(NO3)2and 0.5M urea had a surface area of 210 m2/g and was determined to be a mixture of predominantly nano-crystalline (<10 nm crystallite diameter) cubic ZrO2 with the balance being monoclinic ZrO2. This powder was subsequently calcined to 500°C, pressed into a pellet at 20,000 psi and sintered at 500°C for one hour to yield a mechanically stable porous support with a measured surface area of 125 m2/g. In contrast, RTDS powder prepared from 0.1M ZrO(NO3)2, 0.5M urea, and 0.1M NaF was determined to be pure monoclinic ZrO2with a surface area of 58 m2/g.