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Ultrafine Oxide Powders Produced by Rapid Thermal Decomposition of Precursors in Solution

Matson, D. W., J. C. Linehan and M. E. Geusic

ABSTRACT

The Rapid Thermal Decomposition of precursors in Solution (RTDS) process is described as an approach to nanoscale particle production. The RTDS method involves a brief ( < 2 sec) exposure of an aqueous solution containing dissolved metal oxide precursors to hydrothermal conditions amenable to particle nucleation and growth. Particle growth is terminated by passing the solution through a pressure restrictor and into a cooled collection region. Initial RTDS results involving the production of iron, titanium, and zirconium oxide powders from aqueous solutions are described. Powders generated were characterized using transmission electron microscopy, powder X-ray diffraction, BET surface area analysis, and Mossbauer spectroscopy. Iron oxide (hematite phase) having crystallite sizes ranging from a few nanometers to tens-of-nanometers were produced from 0.1 M Fe(NO3)3 and Fe(NH)4(SO4) 2 solutions. Crystallite size in the hematite powders were found to be dependent on the temperature to which the solutions were exposed during RTDS processing. Anatase phase TiO2 powders having crystallite sizes of 3 to 5 nm were generated from TiO(C2O4)2 solutions, and cubic phase ZrO2 powders with 3.5 nm crystallites were produced using a solution containing a zirconium (IV) citrate ammonium complex.

 

 

1. INTRODUCTION

Ultrafine powders having individual particles in the nanometer to tens-of-nanometer size range have the potential to play important roles in advanced technology processes. Such "nanoscale" powders can have specific surface areas far in excess of 100 m2/g, making them highly desirable for applications where particle surface or interface characteristics are important. The use of nanoscale powders as heterogeneous catalysts is attractive because of the large number of catalytically active surface sites which are available per gram of material and because of the high mobilities of particles in this size range within reaction media (Derbyshire, 1988; Pradhan et al., 1991). Development of ion exchange materials from ultrafine powders offers the potential for high loading capacities, again because of the large percentage of active surface sites which are available for exchange reactions.

Consolidation of ultrafine ceramic and metal powders into bulk products by high- temperature sintering has been found to yield materials having grain structures on the nanometer size scale. These "nanocrystalline" solids have been found to exhibit a number of unusual bulk properties thought to result from the high percentage of atoms associated with grain boundaries (30-60% for 5 nm grains). Reduced sintering temperatures required for full densification (Siegel et al., 1988; Hahn et al., 1990), low- temperature plastic deformation (Karch et al., 1987), and high solute and solvent diffusion coefficients (Horvath et al., 1987; Mutschele and Krichheim, 1987) are among the novel properties attributed to these materials.

Despite the range of interesting and unusual properties exhibited by ultrafine powders and their consolidated products, a full evaluation of the potential of these materials has been limited by the small quantities of nanometer scale starting powders available for testing. At Pacific Northwest Laboratory (PNL) we are currently investigating a solution-based powder formation method which we refer to as the Rapid Thermal Decomposition of precursors in Solution (RTDS) process. This is a continuous flow process rather than a batch method and has the potential for generating large quantities of ultrafine powders. Tens-of-grams of oxide powders per hour can be produced using an existing bench-scale RTDS apparatus.

The RTDS process is amenable to a number of solvent/solute combinations. Aqueous solutions containing dissolved salts or other soluble precursor species have been found to be suitable for the production of several different ultrafine oxide powders. The results of preliminary efforts aimed at generating nanometer scale Fe203, TiO2, and ZrO2 powders from precursors dissolved in aqueous solvents are contained in this report.

2. EXPERIMENTAL

2.1 Powder Formation by RTDS

 

The RTDS process is a method for producing ultrafine solid particles by inducing abrupt changes in the temperatures and pressures of rapidly flowing solutions containing dissolved precursors. Because RTDS is a fiow-through process, the volumes of fluid being heated at a given moment are small, allowing for temperature changes of several hundred degrees C in less than 2 sec. The process is operated at elevated pressures so that the precursor solutions can be maintained in the liquid or highly compressed gas (supercritical fluid) phase at temperatures considerably above the solvent's ambient boiling temperature. Elevating the system pressure also provides a mechanism for rapid movement of the solutions through the RTDS apparatus.by means of a pressure drop at the downstream end of the apparatus. In aqueous systems consisting of water and dissolved metal containing precursors, the rapid heating from room temperature to between 150°C and 400°C results in the forced hydrolysis and/or condensation of hydrated metal species into insoluble oxide particles. The rapid heating is followed by an abrupt pressure drop and thermal quench to inhibit further particle growth.

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