Microstructural Evolution of Epitaxial MgAl 2 O 4 Thin Films Derived From Metal Nitrates and From Metal Alkoxides
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A metal alkoxide is a compound where the metal M with valence n is bonded to an alkyl group R with oxygen. The general formula is M(OR),. Metal alkoxide is first hydrolyzed into metal hydroxide and then is converted to metal oxide. A double alkoxide is formed when an oxygen atom which is bonded to one metal donates a lone pair of electrons to the empty orbital of a neighboring different metal cation. The advantages of using double alkoxide as the precursor is the homogeneity on a molecular level. Magnesium aluminum isopropoxide (MgA12(OC 3H7 )8 ) has been used in making spinel powder 4. It is used in this study as precursor for making thin films.
EXPERIMENTAL PROCEDURE Stoichiometric amount of magnesium nitrate (Mg(N0 3 )2.6H 20) and aluminum nitrate (Al(NO 3)3 .9H 20) were dissolved in methanol to form the precursor solution. The solutions were spin-coated on planar (100) LaAlO 3, (100) SrTiO 3, and (100) MgA120 4 substrates. The calcination was done in an ultra-high purity oxygen atmosphere. The thicknesses of the resultant films were controlled by the concentration of the solution and spinning rate. Multiple coatings were done to produce thicker films. Magnesium aluminum isopropoxide was made by a method similar to that used by McMahon 5. A round bottom flask was loaded with magnesium metal turnings and excess amount of aluminum isopropoxide, with small amount of mercuric chloride used as catalyst. The reactants and excess amount of isopropanol were loaded in a dry box. The isopropanol was dried by heating it with Call2 and was distilled before use. The flask was refluxed for two days under a nitrogen atmosphere. The reaction flask was exposed to a mechanical pump for 24 h after refluxing to remove the solvent. The product was vacuum distilled at approximately 120'C. The resulting magnesium aluminum isopropoxide was a clear, viscous liquid. Extreme care was taken to avoid any introduction of water during each step of processing. The magnesium aluminum isopropoxide was diluted in dried isopropanol and spin-coated on (100) LaA10 3 and (100) SrTiO 3 substrates. The precursor films were first heated in moist atmosphere to hydrolyze and then calcined in dry atmosphere. The mixed metal nitrate solution was dried to powder form for decomposition studies. The decomposition processes of the two precursor systems were analyzed by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). Additional melting experiments were done by placing dried nitrate precursors powders inside capillary tubes and observed the melting while heating them. The intermediate products of pyrolysis of these precursor systems were studied by powder X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The surface morphologies of the spinel films were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The crystallinity and in-plane orientation were studied by powder diffraction X-ray diffraction and pole figure X-ray diffraction. Plan-view transmission electron microscopy (TEM)
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