Study of the lowest time and temperature thresholds in formation of homogeneous ferrite-spinels CuFe2O4, NiFe2O4, CoFe2O4, ZnFe2O4 for solidand liquid-phase synthesis methods

Determination of the minimum temperature and time values in formation of homogeneous samples of ferritic materials by different methods is important, because the final ferritization temperature, together with the exposure time, affects the final particle size. If the ferritization temperature and sintering time decrease, the final particle size of the resulting material also decreases. The particle size is an important parameter for heterogeneous catalysts, because the smaller the particle size of the catalyst, the higher its catalytic activity. The ceramic, sol-gel (citrate) method, coprecipitation method, and hydrothermal method have been investigated in the article, as exemplified in the production of СuFe2O4, CoFe2O4, NiFe2O4, and ZnFe2O4. The lower temperature thresholds for the formation of ferrite phases have been experimentally determined for the ceramic, sol-gel (citrate), hydrothermal and co-precipitation methods for CuFe2O4, NiFe2O4, CoFe2O4, and ZnFe2O4 samples. The temperature threshold of formation decreases in the following order: ceramic method (1000-1100 °C), coprecipitation method (400-800 °C), hydrothermal method (180 °C), sol-gel (citrate) method (150-300 °C); at the same time, for each of the methods, exposure in the furnace can be from several hours to a day. Using the data of powder diffraction patterns, the lattice parameters for the entire series of samples have been determined according to the full-profile analysis by Rietveld method. For СuFe2O4, obtained by ceramic method: a = 8.376 Å, V = 587.74 Å3; by sol-gel (citrate) method: a = 8.270, V = 56.63 Å3; by coprecipitation method: a = 8.392, V = 590.90 Å3; by hydrothermal method: a = 8.278, V = 567.24 Å3. For СoFe2O4, obtained by ceramic method: a = 8.385, V = 598.49 Å3; by sol-gel (citrate) method: a = 8.353, V = 582.82 Å3; by coprecipitation method: a = 8.378, V = 588.04 Å3; by hydrothermal method: a = 8.393, V = 591.31 Å3. For NiFe2O4, obtained by ceramic method: a = 8.338, V = 579.59 Å3; by sol-gel (citrate) method: a = 8.357, V = 583.49 Å3; by coprecipitation method: a = 8.347, V = 581.40 Å3; by hydrothermal method: a = 8.351, V = 582.38 Å3. For ZnFe2O4, obtained by ceramic method: a = 8.439, V = 600.97 Å3; by sol-gel (citrate) method: a = 8.439, V = 8.439 Å3; by coprecipitation method: a = 8.440, V = 601.10 Å3; by hydrothermal method: a = 8.453, V = 604.02 Å3.