Hafnia and Hafnia-Zirconia Mixed Oxide Ultra Spherical Microspheres Made by a SOL-GEL-Vibrational Dropping Process
Thorsten Brandau and Egbert Brandau
Keywords: Hafnia, Zirconia, Mixed Oxides, Sol-Gel-Process, Microspheres, Catalyst Carrier, Homogeneous Catalysts, Filtering Materials, Grinding Media.
Introduction
Trends in ceramics development have recently been moving toward increasingly high quality ceramic materials such as partly stabilized Zirconia. Metal oxides such as CaO, MgO, Y2O3, CeO4, etc. are used as stabilizing additives.
The object of the recent development was to find a method for producing stabilized Hafnia, Hafnia containing or Zirconia high density spheres or spheres with tailored pore size and surface area with a uniform spherical geometry and a narrow grain size distribution. Aqueous solutions or sols of Hf or Zr preneutralized with ammonia are the precursors to get microspheres. The liquid is gently pumped through a vibrating nozzle system where upon exiting the fluid stream breaks up into uniform droplets. The surface tension of these droplets molds them into perfect spheres in which gelation is induced during a short period of free fall. Solidification can be induced in an ammonia gaseous and liquid medium through chemical reaction.
Sol-Gel Production Process
A number of metals can form hydroxide sols and can be used as starting material for the production of microspheres. Sols with low viscosity, such as Zirconium or Hafnium hydroxide stabilized with organic compounds like polyalcohols or with pore formers like urea, can easily be pressed through a nozzle system. Gelation in the gaseous phase is obtained by the reaction with ammonia. Solidification is continued by dropping the spheres in an aqueous solution of ammonia. This solidification can be tailored to the wanted properties of the microspheres by varying the reaction time and the concentration of ammonia in the precipitation bath. Not only sols can be used to obtain a fast and sufficient gelation during the short time of sphericity. Aqueous solutions of nitrates or carbonates are also excellent starting materials for the production of microspheres. The preneutralized solutions formed to spheres are also gelated in a gaseous reaction with ammonia. They are ultimately precipitated in an aqueous solution of ammonia. Even without chemical presolidification in the gaseous phase, the solidification is possible provided that the precipitation solution shows a low surface tension to avoid a deformation of the spheres.
The microspheres obtain a different composition and structure according to the subsequent treatment. The spheres may be porous, with high density, soft, hard, coated, embedded, with outstanding exactness or just free flowing, activated or doped with additives.
Precursors of Zirconia and Hafnia
Aqueous solutions of Zr, Hf in nitric acid preneutralized with ammonia are the precursors to get high purity of the produced microspheres. Precipitation as a gel is obtained even when the molar nitrate/zirconium ratio exceeds 1.1 : 1. Hafnium and/or Zirconium salts such as carbonates, nitrates, or chlorides with stoichiometric composition, such as for example HfOCl2 or ZrO(NO3)2 can also be used. Additives like Magnesia or Yttria to crystal stabilization must be used if sintered products are wanted.
Organic-derived precursors can facilitate the preparation. The use of partially alkoxide-derived precursors and completely organic-derived precursors can be used. The preparation of mixed oxides, e.g. ZrHfO4, is easily to realize with mixtures of alkoxides:
Zr(OR)4 + Hf(OR)4 -> ZrHfO4
Zr(OR)3NO3 + Hf(OR)3NO3 -> ZrHfO4.
Subsequent Treatment of Gel-Particles
The chemical reaction between metal salt solutions and ammonia produces ammonia salts (NH4NO3, NH4Cl, (NH4)2CO3, ...). These salts must be washed out before the particles are processed further, since otherwise the particles would be destroyed during heat treatment as these salts decompose. Washing of the spherical gel particles produced by the vibrational dropping process presents no difficulties, since the exchange rate (diffusion) between particles and surrounding liquid is extremely fast. It takes only a few minutes.
After washing, the particles are dried at 100° - 180°C in air, evaporating the washing fluid. The dried gel particles are then calcined in air (600° to 900°C) in order to decompose the organic polymer. To improve the reproducibility of calcining products, it is useful to work under flowing air at a constant humidity in the range between 10 to 30 g of water per cubic meter of air. Depending on the composition of the precursor solution, residual carbon contents ranging from 10 to 50 ppm, BET surface areas ranging from 10 to 200 m²/g, and crush strengths from >0.2 to 2 Newton/particle can be specifically achieved, with particle sizes between 0.2 mm and 0.6 mm final diameter for particles containing at least 80 wt% ZrO2 or HfO2.
If water is used as the washing liquid, this produces a cubic crystal structure in the powder, as demonstrated by X-ray diffraction studies of Y2O3-stabilized HfO2 and ZrO2 particles. Removal of the water from the gel particles using an alcohol that is miscible with water, e.g. isopropanol, followed by drying and calcining, leads to a Y2O3-stabilized HfO2 and ZrO2 powder that contains three crystal structures, monoclinic, tetragonal, and cubic. These particles are well suited for the production of sintered partially stabilized HfO2 and ZrO2 shaped elements. Sintering under hydrogen at 1700°C can produce microspheres of very high density (95 % to 98 % of theoretical density) without any cracking. Sintering under air at 1500°C leads also to high density spheres.
Range and Application of Produced Microspheres
The range of produced microspheres is not subject to any limitation. Nearly all compounds of Zirconium and Hafnium that can be liquiefied or peptized to a solution or sol of low viscosity and that can react to a solid compound through chemical or physical treatment are usable as precursor to produce microspheres. The microspheres attain diameters from 5 mm down to 50 µm. For alkoxy compounds of Hafnium or Zirconium it has proved useful to implement chemical hydrolysis by means of steam before introduction into the aqueous solution. Amplitude and frequency of the nozzle oscillation or the liquid oscillation are held constant to attain a monodisperse grain size distribution.
The finished microspheres can be modified by subsequent washing, further chemical reaction, drying, calcining, sintering, impregnation, coating, coloring ... As calcined spheres make excellent catalyst carriers, homogeneous catalysts or filtering materials. Unusually effective and abrasion resistant microspheres for grinding other materials are made from sintered HfO2 and ZrO2 or mixed (Hf, Zr)O2.