The paper investigates the possibility of synthesizing the nanocrystalline phase of spinel (Al,Cu)Fe₂O₄ by mechanical grinding of quasicrystalline alloys Al–Cu–Fe and Al–Cu–Cr–Fe in an oxygen atmosphere. It is shown that during the grinding process, oxidation of metals occurs with the formation of oxides, which then react with the formation of the spinel phase.
Spinel-based materials have unique magnetic, electrical and catalytic properties, which makes them promising for use in various fields. Mechanical grinding is an effective method for synthesizing nanocrystalline materials, allowing for the production of homogeneous products with a high specific surface area.
Quasicrystalline Al–Cu–Fe and Al–Cu–Cr–Fe alloys were obtained by arc melting. Grinding was carried out in a planetary ball mill in an oxygen atmosphere. The phase composition and microstructure of the powders were studied using X-ray diffraction and scanning electron microscopy.
X-ray analysis showed that after mechanical grinding for 20 hours, a nanocrystalline phase of spinel (Al, Cu)Fe₂O₄ is formed in the powders. The size of the spinel crystallites is 10-20 nm. Adding chromium to the original alloy slows down the process of spinel formation.
Mechanical grinding of quasicrystalline Al–Cu–Fe and Al–Cu–Cr–Fe alloys in an oxygen atmosphere is an effective method for synthesizing the nanocrystalline phase of spinel (Al,Cu)Fe₂O₄. The obtained materials can be used as catalysts and magnetic materials.
Further studies were aimed at studying the influence of milling parameters, such as the rotation speed of the mill drums and the ratio of the powder mass to the mass of the grinding bodies, on the process of formation of the spinel phase. It was found that an increase in the rotation speed leads to an acceleration of the oxidation process and the formation of spinel, but at the same time an increase in the defectiveness of the crystal structure is observed. The optimal ratio of the powder mass to the mass of the grinding bodies is 1:10, at which the maximum rate of spinel formation and the minimum size of the crystallites are achieved.
A study of the morphology of the obtained powders using scanning electron microscopy showed that they are agglomerates of spinel nanoparticles. Ultrasonic treatment in various solvents was used to reduce the size of the agglomerates and improve the dispersion of the powders. It was found that the most effective is the use of ethanol, which allows achieving a uniform distribution of nanoparticles and reducing the size of the agglomerates to 50-100 nm.
To study the magnetic properties of the obtained nanocrystalline spinels, magnetization measurements were performed depending on temperature and magnetic field. It was found that the spinels have ferrimagnetic properties with a Curie temperature of about 400 °C. The saturation magnetization is about 20 emu/g, which is slightly lower than that of bulk spinels, which is due to the presence of defects in the crystal structure and the small size of the crystallites.
The obtained nanocrystalline spinels were tested as catalysts in the CO oxidation reaction. They were shown to have high catalytic activity, exceeding the activity of bulk analogs. The best results were obtained for the spinel obtained from the Al–Cu–Fe alloy, which demonstrates complete conversion of CO at a temperature of 300 °C. This is due to the high specific surface area and the presence of defects in the crystal structure, which are active centers of catalysis.
Author: Nilay K. Mukhopadhyay, Thakur Prasad Yadav, Radhi Shyam Tiwari and Onkar Nath Srivastava
Institute:
Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India
Department of Physics, Banaras Hindu University, Varanasi 221005, India