Structure and Electromagnetic Properties of Single-Crystalline Fe<SUB>3</SUB>O<SUB>4</SUB> Hollow Nanospheres

Abstract

Magnetite (Fe3O4) hollow nanospheres with an average diameter of 300 nm and an average shell thickness of 40 nm were synthesized by a surfactant-free solvothermal reduction method, and their structure and electromagnetic (EM) properties were investigated. The Fe3O4 hollow nanospheres showed single-crystalline features along the [111] crystal growth direction and a ferrimagnetic behavior at room temperature. The Fe3O4 hollow nanosphere/paraffin composites exhibited a flatter response in the real complex relative permittivity (epsilon') and a lower value of -0.5 in the imaginary complex relative permittivity (epsilon") in comparison with other Fe3O4-based nanomaterials because of the enhanced electrical resistivity. Their imaginary complex relative permeability (mu") displayed a resonance peak at -4 GHz and a negative value up to -0.03 in the 17.2-18 GHz range due to the dissipation of EM energy in the cavity of the hollow nanospheres. Their reflection loss (RL) exceeded -10 dB from 3.1 to 10.1 GHz at a thickness of 2.6-5 mm and attended an optimal value of -43.5 dB at 4 GHz at 5 mm thickness as a result of an effective complementation between the dielectric and magnetic losses.

Porous Fe3O4 Nanospheres with Controlled Porosity for Enhanced Electromagnetic Wave Absorption

“…For a sample thickness of thickness of 2.7 mm, the following maximum RL values were observed: −14.9 dB for U0 at 12.5 GHz, −49.2 dB for U1 at 11.9 GHz, and −35.1 dB, and −29.1 dB for U2 and U4, respectively, at 12.8, and 12.1 GHz (Figure (a–d), respectively). Previous studies have noted a link between the excellent EMW absorption performance of porous samples and their unique morphologies (e.g., flower‐like), in terms of effective permittivity and impedance matching . The present porous Fe 3 O 4 nanospheres showed strong absorption and a thin coating thickness compared with previous reports (Table ).…”

Fe3O4Nanoparticles Embedded Hollow Mesoporous Carbon Nanofibers and Polydimethylsiloxane-Based Nanocomposites as Efficient Microwave Absorber

“…In this study, μ r ″ showed a resonance peak at ∼4 GHz and assumed a negative value up to −0.03 in the frequency range of 17.2–18 GHz owing to the dissipation of electromagnetic wave energy in the cavity of the hollow nanospheres. Similarly, a μ r ″ value of −0.41 has been reported in the frequency range of 12.96–16.56 GHz for porous Fe 3 O 4 /SnO 2 core/shell nanorod based wax composites . A negative value of ε r ″ in some frequency ranges may be because of the formation of continuous conductive network of CNFs inside the polydimethylsiloxane polymer matrix.…”

Solvothermal Synthesis of Single-Crystal Magnetite Hollow Sub-Microspheres: A Novel Formation Mechanism and Magnetic Properties

“…Hence, the in situ emerged CO 2 bubbles played a key role in the formation of the hollow structures, which was essential for the inside-out Ostwald ripening and accelerated the hollowing process distinctly. Without the assistance of gas bubbles, only solid microspheres were obtained even under prolonged reaction times of 30 h. [8,12] This confirmed the determining role of the bubbles in the formation of the hollow structures.…”

“…[5][6][7][8][9][10][11][12] In those previous studies, though magnetite hollow spheres with a narrow diameter distribution have been successfully synthesized, the diameter and shell thickness of the spheres could not be tuned conveniently. In the solvothermal process, an alkaline substance, such as NaOH, was added to the system to supply OH À [6][7][8][9] because Fe 3þ could not be reduced by ethylene glycol in the absence of OH À .…”

“…[10][11][12] In those solvothermal processes, it was observed that the small amount of water played an important role for the formation of magnetite hollow spheres, [11,12] and the freshly produced gas bubbles were regarded as a soft template for the formation of hollow spheres. [12] According to the formation mechanism proposed in previous reports, the final hollow spheres obtained should be polydisperse because it is impossible to form monodisperse gas bubbles in the reaction solution. Hence, to date, no reports have established the detailed role of water in the formation of magnetite hollow spheres with a narrow diameter distribution-the formation mechanism of magnetite hollow spheres using the solvothermal process is still elusive.…”