With the rapid development of wireless communication technology, electromagnetic waves are regarded as a main medium for energy and information transmission. Absorbing materials, as a focus of scientific research in recent years, play a key role in effectively reducing electromagnetic radiation pollution in the environment, thereby reducing the negative effect of electromagnetic waves. The design of absorbing materials follows the principles of "thin, light, wide, and strong" to meet practical applications. At present, there are various types of absorbing materials, mainly including magnetic metals, polymer based composite materials, ferrites, and carbon-based materials. However, all the materials have their limitations. For instance, magnetic materials have a superior performance, but they have a poor oxidation resistance, which are prone to eddy current losses, thus affecting the absorption efficiency. The preparation cost of polymer-based composite materials is high, and their application fields are limited. Ferrite materials have an insufficient temperature stability and a high surface density. In addition, carbon-based materials suffer from poor impedance matching and narrow absorption frequency bands. These inherent limitations hinder a widespread applicability of absorbing materials in diverse application scenarios. Therefore, developing novel absorbing materials with superior oxidation resistance and high-temperature stability has an important practical significance.

High-entropy multiphase oxide ceramics was prepared by a solid-state reaction method. In the preparation process, FeO, CoO, NiO, CuO, ZnO and Co₂O₃ as raw materials in a molar ratio of 1∶1∶1∶1∶1∶5 were ground with anhydrous ethanol in a mill at 200 r/min for 8 h, and then dried at 80 ℃ to obtain a mixed powder. Subsequently, 8 g of the mixed powder was pressed into a round billet with a diameter of 30 mm at 10 MPa. The obtained round billet was then heated and reacted in a model BR-12N muffle furnace at different temperatures (i.e., 800, 900, 1000 ℃ and 1100 ℃) for 6 h to obtain the samples of (Fe_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)Co_xO_y high-entropy ceramics (HEO), which were named samples HEO-800, HEO-900, HEO-1000, and HEO-1100, respectively.

The results show that the obtained samples HEO-800, HEO-900, HEO-1000, and HEO-1100 all exhibit spinel and rock salt phases, as well as pore structures and irregular blocks. The degree of crystallization gradually increases with increasing temperature, which can be attributed to the accelerated diffusion of atoms at the interface and the gradual increase in grain size due to the presence of oxygen vacancies. The XPS analysis indicates that the high-entropy multiphase oxide ceramics sintered at different temperatures all have oxygen vacancies, and the maximum oxygen vacancies occur at 900 ℃. The conductivity test shows that the sample HEO-900 has the maximum conductivity. The electromagnetic wave absorption test results reveal that the RL_min values of the samples HEO-800, HEO-900, HEO-1000, and HEO-1100 are -9.85, -36.14, -13.73 dB, and -4.61 dB, respectively. The sample HEO-900 obtained via sintering at 900 ℃ has the optimum absorbing property. Except for the sample HEO-900, the reflection loss of other samples is not high mainly due to the weak dielectric loss caused by low oxygen vacancy, while the magnetic loss is opposite to the dielectric loss. The magnetic loss and dielectric loss synergistically promote the improvement of absorption performance of the material. In addition, the impedance matching area also firstly increases and then decreases as the temperature increases. The impedance area of the sample HEO-900 is relatively large, indicating that there are more electromagnetic waves incident on the interior of the material and the absorption efficiency increases.

The results indicated that oxygen vacancies were easily generated in (Fe_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)Co_xO_y high-entropy ceramics due to the multivalent state and multiple transformations of element Co, which was conducive to electron transfer. The presence of multiphase structure and oxygen vacancies increased the dielectric loss of the material and the interface polarization and defect polarization of the system, thus significantly improving the absorption capacity of electromagnetic waves. This study found that the sample HEO-900 sintered at 900 ℃ had the optimum absorption performance (i.e., the RL_min value of -36.14 dB and an optimal bandwidth of 2.86 GHz at a matching thickness of 3.0 mm and a frequency of 9.37 GHz).