In this study, we investigated the effect of microwave irradiation on the synthesis of ZnFe2O4 (ZFO) nanopowders. The structural, chemical, and physical features of the powders prepared via microwave-assisted heating (MWH) were analyzed with variation in the initial
precursors and synthesis temperatures. Three different types of source batches, namely Zn(CH3CO2)2 · 2H2O and FeC2O4 · 2H2O (ZAHFOH), Zn(NO3)3 · 6H2O and Fe(NO3)2
· 9H2O (ZNHFNH), and ZnO and Fe2O3 (ZOFO), were prepared. The formation of ZFO compounds was achieved at 100 °C for ZAHFOH using MWH, which is much lower than the synthesis temperature of 700 °C using conventional heating (CH). The value of the
activation energy (Q) for the ZAHFOH source in synthesis using MWH was approximately 7.9 kJ/mol, which is approximately one-tenth of the Q value (100.7 kJ/mol) for ZOFO using CH. It was determined that the inversion factors were approximately 5.4, 4.3, and 4.0, and the crystallite sizes
were 57.7, 72.5, and 78.9 nm for the ZAHFOH, ZNHFNH, and ZOFO, respectively. The ZFO powders exhibited superparamagnetism for ZAHFOH and paramagnetism for ZNHFNH and ZOFO. Based on the crystallite size-related surface effects, redistribution of the Zn and Fe ions occurred. This phenomenon
is attributed to the magnetic transformation of ZFO. Based on these results, it is expected that a sensitively tunable size and magnetic properties can be achieved using MWH.
The crystallization kinetics in BaTiO3 synthesis from hydrate precursors via microwave-assisted heating (MWH) were investigated. The structural and chemical features of powders synthesized via MWH and conventional heating (CH) were compared. The charged radicals generated under microwave irradiation were identified by chemical analysis and real-time charge flux measurements. Using Ba(OH)2∙H2O (BH1), Ba(OH)2 (BH0), and BaCO3 (BC) as the precursors for a Ba source, and TiO2∙4H2O (TH) for a Ti source, three different mixture samples, BH1TH (BH1 + TH), BH0TH (BH0 + TH), and BCTH (BC + TH), were heat-treated in the temperature range of 100–900 °C. BaTiO3 powders were synthesized at temperatures as low as 100 °C when sample BH1TH was subjected to MWH. Based on the growth exponent (n), the synthesis reactions were inferred to be diffusion-controlled processes (3 ≤ n ≤ 4) for MWH and interface-controlled processes (2 ≤ n ≤ 3) for CH. Current densities of approximately 0.073 and 0.022 mA/m2 were measured for samples BH1TH and BH0TH, respectively, indicating the generation of charged radicals by the interaction between the precursors and injected microwaves. The radicals were determined as OH− groups by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.
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