Auto ignition synthesis of nanocrystalline MgAl 2 O 4 and related nanocomposites
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Auto ignition synthesis of nanocrystalline MgAl2O4 and related nanocomposites S. Bhaduri, S.B. Bhaduri, and K.A. Prisbrey Department of Metallurgical Engineering, University of Idaho, Moscow, Idaho 83844-3024 (Received 9 November 1998, accepted 25 May 1999)
Nanocrystalline powders of various compositions in the Al2O3–MgO binary system were synthesized using a novel “auto ignition” process. The respective nitrates were used as starting materials and urea as fuel. Thermodynamic calculations of the adiabatic temperatures were performed for various compositions from Al2O3-rich to the MgO-rich side of the phase diagram. The combustion temperatures of the different compositions were also determined experimentally. The as-synthesized powders were characterized by x-ray diffraction (XRD) and transmission electron microscopy (TEM). As a result of processing, spinel, alumina, magnesia, and solid solutions/ nanocomposites thereof formed. Grain sizes and the lattice parameter were calculated based on XRD results. Where appropriate, the lattice parameter versus the composition of these solid solutions satisfied Vegard’s law. Spinel grains were in the 13–20 nm range, alumina grains were 30–40 nm, and MgO grains were 2–28 nm. The grain sizes calculated from XRD results were in good agreement with the TEM results.
I. INTRODUCTION
Nanocomposites consist of a material in which at least one phase is in the nanometer ( 100 nm), commercially available, high-purity spinel powder (Baikowski International Corporation, Charlotte, NC) and Magnesium Oxide powder (Allied Chemical, Morristown, NJ). For an ␣-alumina standard, our laboratory-produced powder was used after a heat treatment at 1400 °C for 10 h. Lattice parameters were determined using an x-ray diffraction (XRD) pattern following Cohen’s method. For lattice parameter determination, at least 5 peaks were under consideration, especially those corresponding to higher angles. In order to prepare TEM samples, the powders were crushed and picked off by 200 mesh Cu grids coated with carbon. However, in some compositions, powders were ultrasonicated in high purity ethanol for 10 min to disperse uniformly. The as-dispersed powders were examined using a transmission electron microscope JEOL 2010 (Peabody, MA), and a Phillips CM 200 (Mahwah, NJ), both operating at 200 kV. In some cases, local compositions were determined by energy dispersive analysis. Lattice parameter and grain sizes were calculated based on XRD
The equilibrium phase diagram (Fig. 2) shows the various phases that form in this binary system. Table I depicts the theoretical calculation of adiabatic temperatures corresponding to different kinds of chemical reactions. Reactions 1 and 5 depict the chemical reactions of both pure aluminum nitrate, pure magnesium nitrate with urea. Reaction 2 is an alumina-rich composition that results in mostly alumina with a trace of MgAl2O4. Reaction 3 is where a pure spinel (MgAl2O4) formation is expected. Reaction 4 is almost 50–50 mol% aluminum
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