Nanocrystalline Spinel from Freeze-Dried Nitrates:  Synthesis, Energetics of Product Formation, and Cation Distribution

• Published in: Chemistry of materials Vol. 10; no. 4; pp. 1083 - 1090
• Main Authors: McHale, J. M, Navrotsky, A, Kirkpatrick, R. J
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 20.04.1998
• Subjects: Clusters, nanoparticles, and nanocrystalline materials; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Growth from solid phases (including multiphase diffusion and recrystallization); Materials science; Methods of crystal growth; physics of crystal growth; Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals; Physics; Structure of solids and liquids; crystallography; Crystal growth; Inorganic compounds; Spinels; Ternary compounds; Magnesium oxides; Calcination; Experimental study; Nanocrystal; Aluminium oxides; Crystal structure
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm9706659

The formation of MgAl2O4 from a freeze-dried nitrate precursor was studied by thermogravimetric analysis, differential thermal analysis, powder X-ray diffraction, transmission electron microscopy, 27Al magic angle spinning NMR, and high-temperature solution calorimetry. A single phase, slightly alumina-rich spinel of composition Mg0.957Al2.028O4 was obtained from the precursor by calcination at temperatures ≥ 1073 K. Transmission electron microscopy revealed that material calcined at 1073 K was nanocrystalline, with grain sizes on the order of 20 nm. 27Al NMR revealed that this material had an unusually high degree of cation disorder, with an order parameter of 0.59 at room temperature. This degree of disorder, which has previously only been achieved in MgAl2O4 via neutron bombardment, provides strong thermodynamic evidence that the freeze-dried precursor contained a highly disordered and probably close to random mixture of cations. Significant levels of five-coordinated Al3+ were detected in amorphous samples calcined at 973 K. Increasing calcination temperatures resulted in a decrease in the percentage of tetrahedral Al3+ and a simultaneous increase in the average particle size of the material. Drop solution calorimetry in 2PbO·B2O3 at 975 K revealed an enthalpy difference of 39.9 ± 7.4 kJ mol-1 between the disordered nanophase MgAl2O4 synthesized at 1073 K and the well-crystallized material synthesized at 1773 K. Particle size, cation distribution, and adsorbed H2O affect the energetics, with the surface energy term dominant.