Ultrafine MnFe 2 O 4 powder preparation by combusting the coprecipitate with and without Mg 2+ or Zn 2+ additives
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Ultrafine MnFe2O4 powder preparation by combusting the coprecipitate with and without Mg2+ or Zn2+ additives Hsuan-Fu Yua) and Wen-Bing Zhong Department of Chemical Engineering, Tamkang University, Taipei Hsien, Taiwan 25137, Republic of China (Received 14 June 1999; accepted 11 October 1999)
Ultrafine MnFe2O4 powder with its crystallites less than 100 nm was prepared using a combustion process. The coprecipitates containing the stoichiometric amount of Mn2+ and Fe3+ to form MnFe2O4 were prepared by dissolving the required metallic nitrates in de-ionized water and adding NH4OH to adjust the pH of the solutions to 9. The collected dried precipitates were then heated up to predetermined temperatures and then quickly contacted with the acetone spray. Upon contacting with the heated precipitates, the acetone spray was ignited. The combustion of acetone caused the precipitates to form crystalline MnFe2O4 without chemical segregation. The crystallinity of MnFe2O4 powder so obtained depended on the ignition temperature of acetone spray. MnFe2O4 powder obtained at acetone ignition temperature of 773 K had higher crystallinity than that obtained at acetone ignition temperature of 523 K. The presence of a small amount of Mg2+ or Zn2+ in the composition of the coprecipitates promoted the mobility of constituent ions of the combusted powder and resulted in bigger MnFe2O4 crystallites at a lower acetone ignition temperature.
I. INTRODUCTION
Ferrites with an inverse spinel molecular structure have soft magnetic properties and are important components in the electronic industries. The quality and performance of ferrite products are greatly influenced by their microstructures. To have a better-controlled microstructure, the ferrite powder used in production should have the characteristics of high chemical homogeneity, ultrafine sizes with narrow size distribution, spherical-like shapes, and high sinterability. Several nonconventional routes for high-quality ferrite powder preparation have been used or are under development, such as the sol-gel method, 1 the hydrothermal method, 2 the aerosol method,3–5 and the coprecipitation method.6–11 Among them, the coprecipitation technique, because of its easy operation and because it is ready for mass production, is the most common one adopted in the production of ferrite powder. The coprecipitation technique used in preparing ferrite powder involves preparation of homogeneous solution that contains the required ratio of cations, precipitation of insoluble precursors of the ferrites by adding precipitating agents, separation of the solid precipitates from the liquid solution, removal of the soluble residuals by washing and drying, and formation of the ferrite powder by pyrolysis. In preparing ferrite
a)
FIG. 1. XRD patterns for the combusted specimens (without any additives) obtained by igniting the acetone spray at (a) 523 K and (b) 773 K.
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