Synthesis of Ba-ferrite microspheres doped with Sr for thermoseeds in hyperthermia

M-type hexagonal ferrite, MFe12O19 (M = Ba, Sr, etc.) has been intensively investigated as a material for permanent magnets, high-density recording media, and microwave devices [1]. The sol-gel technique has emerged in recent years as a versatile method for synthesizing barium and strontium ferrite. The sol-gel technique offers the following advantages: the temperature needed for the crystallization process is lower, the particles produced are ultrafine, and they have a narrow size distribution [1]. Spray pyrolysis is a valuable technique for forming a non-agglomerated complex mixed-metal oxide powders with high purity, controlled stoichiometry and crystallinity, and a relatively narrow particle-size distribution [2]. Therefore, the solgel method prior to spray pyrolysis was used to prepare the samples for obtaining powders from a sol in much shorter periods compared to those needed using the traditional evaporation-drying procedure [3]. Appropriate portions of Ba(NO3)2, (CH3CO2)2Sr, and Fe(NO3)2·9H2O were dissolved in ethylene glycol, methyl alcohol and distilled water in the stoichiometric ratio, namely (Ba, Sr)/Fe = 1/10. The solution was refluxed at 80 ◦C for 12 hrs. The spray pyrolysis equipment used in this study consisted mainly of an ultrasonic nebulizer (H556, LG Electronics, Korea), a furnace, a glass fiber filter (GF/F, Whatman, England) and a rotary pump [4]. The aerosol droplets were produced by ultrasonic frequency (1.67 MHz) and were nebulized at flow rates of 1–5 /min. The droplets were then passed through a low temperature first heating zone (300 ◦C) where the solvent was evaporated. The resulting dried particles were subsequently passed through a second heating zone (400–1000 ◦C) where the particles decomposed [5]. The powders were finally recovered by a glass fiber filter of 0.7 μm of pore size and were dried at 100 ◦C for 24 hrs. In order to study the changes in a crystalline