Microwave-absorbing characteristics for the composites of thermal-plastic polyurethane (TPU)-bonded NiZn-ferrites prepared by combustion synthesis method

NiZn ferrite with the chemical formula Ni_0.5-xZn _0.5-xM_2xFe₂O₄ (M = Co, Cu, or Mg, and x = 0 or 0.05) was synthesized by a combustion synthesis method using metal nitrates and urea ((NH₂)₂CO) as reactants. The nanocrystallite of these materials were mixed with a thermal-plastic polyurethane (TPU) elastomer to be converted into a microwave-absorbing composite. The complex relative permittivity (ε_r = ε′-jε″) and permeability (μ_r = μ′-jμ″) of the absorber were measured in a frequency range of 2-12 GHz. The reflection loss (R.L.), matching frequency (f_m) and matching thickness (d_m) were calculated using the theory of the absorbing wall. Effects of both the particle size of ferrite and the dopant presented in the ferrite on the electromagnetic properties and microwave-absorbing characteristics were investigated. It was found that nanoparticles around 40 nm exhibit higher reflection loss than both those obtained from micro-sized powders and those with size less than 25 nm. Also, it was shown that the Co-doped, Mg-doped, and Cu-doped NiZn ferrite-TPU composites could be designed to be more negative than -20 dB, in a frequency range of 2-12 GHz, to become promising materials for microwave-absorbing application. These doped ferrite-containing composites have more effective microwave-absorbing characteristics compared to undoped one.