Solvothermal synthesis of Co_xFe_3-xO₄ spheres and their microwave absorption properties

Co_xFe_3-xO₄ (x = 0-1) spheres are synthesized via a solvothermal reaction using ethylene glycol (EG) as a solvent. They are characterized, and the results show that the prepared spheres are mainly 300-500 nm in diameter and constituted by small grains. For the EG solution containing stoichiometric ingredients (atomic ratio of Co ²⁺:Fe³⁺ = 1:2), the obtained spheres are Co _0.9Fe_2.1O₄ at 200 °C (sphere A) and Co _0.74Fe_2.26O₄ (sphere B) at 300 °C, whose crystallites are 23 nm and 30 nm in size, respectively. VSM measurements reveal improved properties with sphere B. Variations of complex permittivity and permeability for different composite (75% mass ratio of spheres) have been studied as a function of frequency. The calculated reflectivity value indicates that the composite containing sphere A displays better microwave absorption capability. The minimum reflection loss reaches -41.089 dB at 12.08 GHz, with a matching thickness of 2 mm. The dielectric loss contributes even more than magnetic loss in the frequency range of 3-14 GHz. The synergistic effect of dual losses makes the submicrosphere a promising absorbent in X and Ku bands. The composite consisting of sphere B is inferior in dielectric properties owing to ferrous ion migration from octahedral to tetrahedral sites and due to the big crystallites lacking defects. After the calcination treatment of the spheres at 700 °C, the dielectric loss turns out to be low due to the disappearing Fe²⁺ ↔ Fe³⁺ pairs in adjacent octahedral sites and the loss of defects. Variations of the cobalt ratio in spheres can change the resonance frequency and crystallinity of the spheres and ultimately the minimum reflection loss and corresponding frequency band. The microwave absorption properties of mixed magnetite and cobalt ferrite spheres are influenced by the cationic stoichiometry and crystalline integrity.