In recent years, microwave-absorbing composites that are both cost-effective and high-performance are developed to address the electromagnetic radiation issues like interference, pollution, and health risks. Spinel ferrite glass-ceramics as all-inorganic composites have attracted wide attention. They combine the magnetic properties of ferrite crystals with the advantages of glass, i.e., high temperature resistance, oxidation resistance, ease of formability, cost-effectiveness, and adjustable dielectric constant. Fe₃O₄ is widely used as an electromagnetic wave absorber due to its high saturation magnetization, strong spin-polarization, half-metallic properties, and low cost. Fe₃O₄ can also utilize industrial solid wastes like iron tailings as raw materials to achieve resource recycling. In this paper, silicate glass-ceramics containing flower-like Fe₃O₄ nanocrystals were prepared based on SiO₂-Al₂O₃-K₂O-Na₂O-P₂O₅-B₂O₃-Fe₂O₃ system via appropriate heat treatment. The special structure could enable the glass-ceramics to exhibit a higher reflection loss. A thorough examination of the mechanism of reflection loss was conducted, elucidating the key factors affecting dielectric loss and impedance matching on the microwave-absorbing performance of Fe₃O₄ glass-ceramics.

Raw materials (Sinopharm Chemical Reagent Co., Ltd. China) were weighed in a nominal composition of the glasses with approximately 30 g of each. Then, they were thoroughly mixed to ensure uniformity, and melted in a corundum crucible in an electric furnace at 1 580 ℃ for 2 h. Afterwards, the melt was poured into a mold for shaping. The as-quenched glasses were annealed at 500 ℃ for 1 h. Finally, the glasses were heat-treated at 620-740 ℃ with a heating rate of 10 ℃/min for 2 h. The samples were groundinto powders or cut and polished into small pieces for subsequent characterizations.

The crystalline phases of the samples were determined by X-ray diffraction (XRD). The crystallinity was determined via calculating the I_a/I_b value, where I_a is the crystalline phase intensity and I_b is the total intensity of the sample. The microstructures of the glass-ceramics were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) with energy dispersive spectroscopy (EDS). The coercive field (H_c), initial magnetic permeability (µ_i), and magnetic saturation (M_s) of the samples at room temperature were determined by vibrating sample magnetometry (VSM). The glass powder was heated and thoroughly mixed with paraffin at a ratio of 4∶1, then pressed into a ring-shape with an outer diameter of 7 mm, an inner diameter of 3.04 mm, and a thickness of 2 mm in a mold. In the frequency range of 2-18 GHz, the electromagnetic parameters were measured by a vector network analyzer (VNA), and the reflection loss values of the samples were obtained based on the transmission line theory.

The results of scanning electron microscopy (SEM), vibrating sample magnetometry (VSM), and vector network analysis (VNA) reveal that an increase in temperature results in a gradual increase in the number of Fe₃O₄ grains, while the size of the flower-like structures formed by their aggregation initially increases and subsequently decreases. Also, the saturation magnetization (M_s) increases from 5.2 to 9.5 emu·g^-1 although the reflection loss exhibits an initial increase and a subsequent decline. When heat-treated at 700 ℃, the samples exhibit the most significant enhancement in Fe₃O₄ flower-like structures and the most optimal microwave absorption performance. The interfaces of the polyphase structure in the glass-ceramics generate polarization and relaxation, which enhance the dielectric loss. The mutual migration of electrons between Fe²⁺ and Fe³⁺ ions at octahedral sites induces a loss of conductivity. Meanwhile, electron hopping triggers a resonance, leading to an enhanced dielectric loss. The enhancement of crystallinity in the glass matrix promotes the magnetism, thereby leading to an improvement in magnetic loss. As the value of dielectric loss is close to that of the magnetic loss, the glass attains an effective impedance matching, thereby enhancing its reflection loss. This allows for enhanced penetration of electromagnetic waves into the material, leading to optimal reflection loss and effective absorption bandwidth at a thickness of 7 mm.

Silicate glass-ceramics containing flower-like Fe₃O₄ were prepared. The size of the flower-like structure was adjusted via heat-treatment temperature. At 700 ℃, the size of the flower-like structure was the largest. For a thickness of 7 mm, the reflectivity reached -18 dB, and the effective absorption bandwidth was approximately 2 GHz. Based on the analysis of the Debye equation and Cole-Cole plots, the larger flower-like nanostructures enhanced the reflection and scattering of electromagnetic waves during their transmission in the glass-ceramics. This led to an increase in dielectric loss and the formation of better impedance matching, revealing a multiple conduction-reflection-electromagnetic wave absorption mechanism.