Synthesis of ultra-fine and controllable size distribution nanocrystalline MgAl2O4 powders and ascertainment of aluminum loss by introducing inert atmosphere pre-calcination during combustion

Abstract

Nanocrystalline magnesium aluminate spinel (MgAl2O4) powders were synthesized in this study by employing a modified nitrate–citrate combustion route, in which the heat treatment was continuously performed by two separate processes. One process was in nitrogen during the carbonization of the precursors, and the other was subsequently in air. This can greatly decrease the heat release during the combustion process. X-ray diffraction and high-resolution transmission electron microscopy, differential scanning calorimetry, thermogravimetry, infrared spectroscopy, and inductively coupled plasma atomic emission spectrometer were utilized to reveal the reaction process and characterize the physical properties of precursors and powders. To achieve well-crystallized spinel powders with smallest possible and controllable crystallite sizes, the effect of carbonization in nitrogen on the calcined spinel powders was investigated in detail. The introduction of the inert atmosphere at the pre-calcination stage, with appropriate temperature and holding time, allowed ultra-fine, well-crystallized and size-controllable MgAl2O4 powders with average crystallite sizes of 5–25 nm to be obtained. Despite the benefit from the pre-calcination in nitrogen, it was found that the stoichiometry of precursors can be compromised due to the loss of aluminum during the heat treatment in N2 as the temperature was too high (>900 °C) with sufficient holding time. The mechanism of the carbothermal reduction was investigated in this study. By introducing inert atmosphere pre-calcination combustion, ultra-fine well-crystallized and crystalline size-controllable MgAl2O4 spinel powders were thus obtained.