Defect Induced Polarization Loss in Multi-Shelled Spinel Hollow Spheres for Electromagnetic Wave Absorption Application

The development of efficient electromagnetic wave absorbers needed to reduce interference has gained significant research attention in recent years. Cobalt-based spinel structure metal oxides have been identified as favorite candidates for this application. Co-based spinel MCo₂O₄ electromagnetic wave absorbers have been prepared recently and show potential electromagnetic wave absorption properties. Despite these promising results, the high density of MCo₂O₄ based absorbers still hamper their application. However, more research is required on these absorbers and their electromagnetic wave attenuation.

In light of these insufficiencies, a design of specific morphology to lower the density of MCo₂O₄ based EM wave absorbers is highly desired. Developing hollow-structured materials is seen as a sure path to solving this important research issue. This is why multi-shelled hollow spheres have attracted much attention due to their lightweight structure, shell porosity, intershell spacing, and controllable shell numbers and thickness. The multi-shelled hollow spheres micro/nanostructure promotes EM wave absorption by: the multiple shells lead to numerous reflections when the incident electromagnetic wave propagates through two adjacent shells, secondly, the generated shell-air-shell interfaces boost interfacial polarization, thirdly, multiple cavities in this structure help circumvent the MCo₂O₄ high-density issue.

Unfortunately, only a handful of studies have focused on multi-shelled hollow spheres-based EM absorbers. Besides morphology regulation, it would also be essential to investigate the quantitative relationship and desirable dissipation mechanism in dielectric loss materials. Previous studies have paid much attention to the design of unique structures to distinguish the dominant loss mechanism. Still, an investigation on defect engineering and its induced polarization loss dominant electromagnetic wave absorbers hasn’t received much attention. And this is why it would be necessary first to clarify the EM wave absorbing mechanism primarily caused by defect-induced polarization loss.

Inspired by this thought, researchers Dr. Ming Qin, Dr. Limin Zhang, Dr. Xiaoru Zhao, and Professor Hongjing Wu from the Northwestern Polytechnical University in China proposed a versatile route for the preparation of multi-shelled MCo₂O₄ based hollow spheres structured materials. The authors demonstrated for the first time the defect-induced dielectric loss dominant mechanism as a result of multi-shelled spinel hollow spheres. Their research work is published in the journal Advanced Science.

The authors rapidly attained samples with the same tripled-shelled structure through identical synthetic routes. They first selected NiCo₂O₄-based materials as representatives and modified their defects in crystal and oxygen vacancy. The authors then extended this principle to serial Co-based multi-shelled hollow sphere materials under ideal defect site engineering temperature.

The authors observed that various bivalent metals ions had a different affinity for carbon spheres than Co ions, which led to the formation of multi-shelled hollow spheres with varied crystalline phases. By investigating the composition, conductivity, and defect sites in ferrites, the authors observed that electromagnetic dissipation from conduction loss, interfacial polarization, and dipolar polarization didn’t play a significant role compared to defect-induced polarization loss. Using NiO/NiCo₂O₄ composites as representatives, the researchers observed two defects: oxygen vacancy and crystal structure in the spinel structure. Crystal defect contribution was more profound; therefore, electromagnetic wave dissipation characteristics followed its variance tendency.

In a bid to validate their discovery, the authors analyzed the electromagnetic wave absorption performance of serial Co-based multi-shelled hollow sphere materials. By analyzing the characterization results, they found that NiO/NiCo₂O₄ composites had the highest defect sites in the spinel structure, therefore reporting the best electromagnetic wave absorption performance.

When it came to the peculiar multi-shelled hollow sphere’s structure, the researchers demonstrated the defect-induced dielectric loss dominant mechanism for the first time. They also observed that electron spin resonance signal measurement proportionally matched the EM wave absorption capacities of the measured materials. This discovery could go a long way in opening an avenue to select superior EM wave absorbers rapidly. Ultimately, the findings of this study will inspire the fabrication of high-performance and lightweight ferrite EM wave absorbers by defect engineering.