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Synthesis and Characterization of Mixed‐Metal Oxide Nanopowders Along the CoO x –Al 2 O 3 Tie Line Using Liquid‐Feed Flame Spray Pyrolysis
Author(s) -
Azurdia Jose,
Marchal Julien,
Laine Richard M.
Publication year - 2006
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2006.01155.x
Subject(s) - materials science , thermogravimetric analysis , spinel , stoichiometry , oxide , cobalt oxide , analytical chemistry (journal) , scanning electron microscope , thermal spraying , cobalt , high resolution transmission electron microscopy , transmission electron microscopy , chemical engineering , mineralogy , nanotechnology , metallurgy , chemistry , composite material , organic chemistry , coating , engineering
We report here the use of liquid‐feed flame spray pyrolysis (LF‐FSP) to produce a series of nanopowders along the CoO x –Al 2 O 3 tie line. The process is a general aerosol combustion synthesis route to a wide range of lightly agglomerated oxide nanopowders. The materials reported here were produced by aerosolizing ethanol solutions of alumatrane [Al(OCH 2 CH 2 ) 3 N] and a cobalt precursor, made by reacting Co(NO 3 ) 2 ·6H 2 O crystals with propionic acid. The compositions of the as‐produced nanopowders were controlled by selecting the appropriate ratios of the precursors. Nine samples with compositions (CoO) y (Al 2 O 3 ) 1− y , y =0−1 along the CoO x –Al 2 O 3 tie line were prepared and studied. The resulting nanopowders were characterized by X‐ray fluorescence, BET, scanning electron microscopy, high‐resolution transmission electron micrographs, X‐ray diffraction (XRD), thermogravimetric analysis (TGA), and FTIR. The powders typically consist of single‐crystal particles <40 nm diameter and specific surface areas (SSAs) of 20–60 m 2 /g. XRD studies show a gradual change in powder patterns from δ‐Al 2 O 3 to Co 3 O 4 . The cobalt aluminate spinel phase is observed at stoichiometries (21 and 37 mol%) not seen in published phase diagrams, likely because LF‐FSP processing involves a quench of >1000°C in microseconds frequently leading to kinetic rather than thermodynamic products. Likewise, the appearance of Co 3 O 4 rather than CoO as the end member in the tie line is thought to be a consequence of the process conditions. TGA studies combined with diffuse reflectance FTIR spectroscopic studies indicate that both physi‐ and chemi‐sorbed H 2 O are the principal surface species present in the as‐processed nanopowders. The only sample that differs is Co 3 O 4 , which has some carbonate species present that are detected and confirmed by a sharp mass loss event at ∼250°C. The thermal behavior of the high cobalt content samples differs greatly from the low cobalt content samples. The latter behave like most LF‐FSP‐derived nanopowders exhibiting typical 1%–4% mass losses over the 1400°C range due mostly to loss of water and some CO 2 . The high cobalt content samples exhibit a sharp mass loss event that can be attributed to the decomposition of Co 3 O 4 to CoO.