Liquid-Mix Synthesis of Oxide Powders and Thin Films Using a Starch-Based Polymer
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LONE-WEN F. TAI AND HARLAN U. ANDERSON University of Missouri-Rolla, Ceramic Engineering Department Rolla, Missouri 65401
ABSTRACT A commercially available water-soluble starch derivative was used as the sole organic precursor in the Liquid-Mix synthesis of mixed-cation oxide powders and thin films. The acidified polymer (by nitric acid) was able to complex metal ions through the carboxylate ligands. Loosely agglomerated fine powders as well as dense thin films of complex oxides have been prepared using the same type of polymer. Oxide powders of Cr-doped lanthanum aluminate and yttrium aluminum garnet both crystallized in a single step, without forming any intermediate or second phases, when the amorphous resin intermediates were calcined at 650'C and 750'C for 2 hours, respectively. It was demonstrated that nitric acid could effectively reduce the viscosity of the polymer-nitrate solution to make it suitable for spin coating process. Dense thin films of Y(8 mol%)-doped ZrO2 were formed on Si and A12 0 3 substrates by spin coating the polymeric solution and heating at temperatures below 1000°C. INTRODUCTION The Liquid-Mix (LM) process' of homogeneous and fine particulate oxide powders was originally developed by Pechini 2 in the late 1960's. This liquid mix (LM) process has been extensively used to produce mixed-cation 3oxide powders with controlled particle sizes and morphologies for different applications."1 The LM process starts from the formation of a homogeneous aqueous solution containing citric acid (CA), ethylene glycol (EG), and the nitrate
(or citrate complex) of all ingredient cations. Upon removal of water, a viscous polymeric solution forms in which the cations are uniformly mixed at an atomic level. Upon heating to temperatures in the 250-300'C range an amorphous resin intermediate forms. Calcination of these intermediate products in the 500 to 900'C temperature range yields loosely agglomerated small (generally < 500 nm) crystallite powders. Since the organics used in the LM synthesis of oxides eventually is combusted, it is desirable to minimize their cost. The purpose of this investigation was to replace the current CA-EG polymeric precursor by less expensive organics. In our early studies, 4 it was found that a starch derivative of hydrolyzed polysaccharide (C3446 water-soluble polymer, A. E. Staley Co) has a moderate gelling capability and a high stability over wide temperature ranges and pH values. In addition, it is compatible with metal nitrates and being capable of forming a foam upon heating. The microstructure of the porous foam is shown in figure 1. The formation of metal complexes in the polymer-nitrate mixture and the thermal decomposition of a resin intermediate into an oxide product has been studied by means of FTIR spectroscopy, X-ray diffraction, and thermal analyses.5 It was found that the end group of -CH 2 OH in a glucose unit (as shown in figure 2) can be oxidized by nitric acid to form the -COOH ligand which can complex metal ions. It appeared that a homogeneous mixture of all
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