Ultrasonic Nozzle Spray in Situ Mixing and Microwave-Assisted Preparation of Nanocrystalline Spinel Metal Oxides: Nickel Ferrite and Zinc Aluminate

Nanocrystalline spinel nickel ferrite and zinc aluminate particles (6−20 nm) can be prepared by a recently developed continuous flow method that combines microwave heating and in situ ultrasonic nozzle spray mixing. The preparations were carried out at ambient pressure (1 atm), microwave power (0−600 W), and ultrasonic nozzle with resonant frequency of 48 or 120 kHz. The products were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microcopy, X-ray electron dispersive spectroscopy, Brunauer−Emmett−Teller spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and temperature-programmed desorption mass spectrometry. The results showed that the ultrasonic nozzle and microwave irradiation complement each other, with respect to the purity of the products. The specific advantages of INM method for the preparing nickel ferrites are that pure materials with high surface areas and tunable particle sizes are produced, and this process is continuous.