Synthesis And Properties Of Nano-Fe/Al₂O₃ Composite Particles Prepared By Mechanochemical Method

Metal nanoparticles exhibit unique properties and hereby draw much attention. However, because of their high surface energy and activity, they are easily suffered from agglomeration and oxidation, which have limited their wide applications. In this paper, the nano-Fe/Al₂O₃ composite particles with core-shell structure were prepared from the powder mixture of Al and Fe₃O₄ by mechanochemical method. In the composite particles, Fe nanoparticle is embedded into Al₂O₃ matrix, which prevents the agglomeration and oxidation of Fe nanoparticles.The charge ratio and the powder mass were determined firstly according to relative references. Suitable milling rotation rate was chosen by X-ray diffraction(XRD). Then, based on the phase analysis for the XRD patterns of samples milled for different time, it was studied that the physicochemical changes of Al-Fe₃O₄ system in the whole mechanochemical process. And suitable milling time was fixed up by it.Phase compositions of the sample milled for 6h were identified by X-ray diffractometer. The microstructure of the composite particles in that sample was observed by TEM. Phases of the core and of the shell in the microstructures were determined by selected electron diffraction patterns (SEDP). Size distribution of the composite particles was characterized by using laser diffraction particle size analyzer.Differential scanning calorimeter (DSC) was used to analyze the thermal stability of the core-shell structured nano-Fe/Al₂O₃ composite particles in the sample milled for 6h under Ar atmosphere. The oxidation resistance of the composite particles under air atmosphere at room temperature was analyzed by comparing the XRD patterns of the samples before and after room-temperature-oxidation experiment. Thermogravity (TG) was employed to charaterize the oxidation resistance of the composite particles under air atmosphere at a certain heating rate. Based on the TG result and the XRD patterns of the sample after TG, we calculated the content ofα-Fe phase involved in the core-shell structured composite particles.Alternating gradient magnetometer (AGM) was used to measure magnetic parameters of the samples milled for different time such as coercivity H_c, saturated magnetization M_s, squareness M_r/M_s, etc. Also the variation regularities of the magnetic parameters during the mechaochemical process were studied. With the diffusion-reaction-assembly (abbr. for DRA) model set for analyzing the mechanochemical process, the micromechanism for the formation of core-shell structured nano-Fe/Al₂O₃ composite particles was investigated and applied to the explanation of phase variation and performance of samples. At last, with the help of the DSC curves obtained at different heating rates, it was analyzed by Starink model-free method that the chemical reactions happening to the samples milled for a certain time and that the affection of solid-state diffusion on the chemical reactions. The existence of the solid-state diffusion in the milling process was also confirmed.