TANG Yiwen, HE Xiaoying, PAN Yingtong, LIU Jixuan, ZHANG Guojun

(State Key Laboratory of Advanced Fiber Materials, Institute of Functional Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China)

Extended abstract:[Background and purposes] With the rapid development of radar and semiconductor technologies, intelligent electronic devices are widely used in autonomous vehicles, 5G base stations and unmanned aerial systems. The electromagnetic radiation pollution not only disrupts the function of nearby electronic equipment but also compromises information security. The most efficacious approach to address this issue is to develop advanced electromagnetic wave absorbers with strong absorption, thin thickness and wide absorption bandwidth. Bismuth ferrite ( BiFeO₃) exhibits magnetoelectric coupling effects at room temperature, holding promise for applications as microwave absorbing materials. However, its inherent weak magnetism and insufficient impedance matching restrict its development as an absorbing material. In this study, BiFeO₃, Bi_0.9La_0.1FeO₃, Bi_0.8La_0.2FeO₃ and Bi_0.33La_0.33Nd_0.33FeO₃ powders were synthesized by using chemical co-precipitation method. The effects of rare-earth element on phase composition, microstructure, elemental distribution, oxygen vacancy concentration, electromagnetic parameters, and microwave absorption properties of the  BiFeO₃-based solid solution materials were systematically studied. With increasing content of rare-earth element,  BiFeO₃ gradually transitioned from a rhombohedral structure to an orthorhombic structure, while the Bi₂O₃ impurity phase in the obtained materials gradually disappeared and the oxygen vacancy concentration increased. Excessively high oxygen vacancy concentration tended to weaken the microwave absorption performance of the materials. Bi_0.9La_0.1FeO₃ exhibited a moderate oxygen vacancy concentration, achieving a minimum reflection loss (RLmin) of –42.62 dB at a thickness of 2.36 mm and a maximum effective absorption bandwidth (EABmax) of 1.38 GHz, demonstrating the optimal microwave absorption performance among the synthesized BiFeO3-based solid solutions. The results provided guiding significance for the solid solution composition design and performance regulation of novel microwave absorbing materials.[Methods] A series of  BiFeO₃-based powders, including BiFeO₃, Bi_0.9La_0.1FeO₃, Bi_0.8La_0.2FeO₃ and Bi_0.33La_0.33Nd_0.33FeO₃, were synthesized by using a chemical co-precipitation method. Stoichiometric amounts of metal nitrates (Bi, La, Nd, Sm, Fe) were dissolved in deionized water, followed by a series of processes, including ammonia precipitation, centrifugation and drying. The precursors were calcined at 600–1000 ℃ for 2 h to obtain phase-pure powders. Phase composition of the as-synthesized particles was characterized by using X-ray diffraction analyzer (XRD, D8 Advance, Bruker). Particle sizes of the synthesized powders were measured by using a laser particle size analysis (Litersizer500, Anton Paar, Austria). Microstructure and element distribution were analyzed by using field emission scanning electron microscope (SEM, SU8010, Hitachi, Japan) equipped with an energy dispersive X-ray spectrometer (EDS, SU8220, Hitachi, Japan). Chemical composition and electronic states of the synthesized powders were examined by using X-ray photoelectron spectroscopy (XPS, Escalab 250Xi). Electromagnetic parameters, namely complex permittivity and permeability, were measured with a vector network analyzer in the 2–18 GHz frequency range. Microwave absorption properties, including reflection loss (RL) and effective absorption bandwidth (EAB), were calculated according to transmission line theory.[Results] XRD results revealed that 20 at.% La solid solution induced a rhombohedral-to-orthorhombic phase transition in BiFeO₃. Further multi-element solid solutions (Bi_0.33La_0.33Nd_0.33FeO₃) stabilized the single orthorhombic perovskite phase (PDF#86-1518). The average particle size of BiFeO₃ is ~713 nm. The particle size of 10 at.% La solid solution decreased to ~201 nm, whereas those of Bi_0.8La_0.2FeO₃ and Bi_0.33La_0.33Nd_0.33FeO₃ increased to ~824 nm and ~2.23 μm, respectively. EDS results confirmed uniform element distribution without segregation. XPS results showed that the multi-element solid solutions significantly had increased oxygen vacancy concentrations. Among the synthesized powders, Bi_0.9La_0.1FeO₃ exhibited optimal microwave absorption performance, achieving a minimum reflection loss (RLmin) of –42.62 dB at 12.4 GHz (2.36 mm thickness) and an EABmax of 1.38 GHz (5 mm thickness). In contrast, Bi_0.8La_0.2FeO₃ and Bi_0.33La_0.33Nd_0.33FeO₃ showed degraded microwave absorption performance, with RLmin of –16.35 dB and –24.79 dB, respectively, due to excessive oxygen vacancies, which weakened the unevenness of charge distribution, thereby leading to a reduction in polarization effects and consequently disrupted impedance matching and weakened polarization relaxation.[Conclusions] BiFeO₃ powder and Bi_0.9La_0.1FeO₃, Bi_0.8La_0.2FeO₃ and Bi_0.33La_0.33Nd_0.33FeO₃ solid solution powders were synthesized by using a chemical co-precipitation method, to study the effects of rare-earth element at the Bi-site on phase composition, microstructure, oxygen vacancy concentration, electromagnetic parameters and microwave absorption properties of  BiFeO₃. The main conclusions are as follows.(1) Rare-earth element doping at the Bi-site altered the crystal structure of  BiFeO₃ and inhibited the presence of Bi₂O₃ n the final products. The as-synthesized pure BiFeO₃ exhibited a rhombohedral structure with a certain amount of Bi₂O₃. When the La content increased from 10 at.% to 20 at.%, phase transition occurred from rhombohedral to orthorhombic structure, while the content of Bi₂O₃ impurity phase significantly reduced. Further increasing the doping content and types of rare-earth elements resulted in Bi_0.33La_0.33Nd_0.33FeO₃ with single orthorhombic perovskite structure without the presence of Bi₂O₃.(2) Rare-earth element doping at the Bi-site varied the oxygen vacancy concentration in BiFeO₃, thereby regulating the microwave absorption performance of the materials. The oxygen vacancy concentrations of pure  BiFeO₃ powder and Bi_0.9La_0.1FeO₃, Bi_0.8La_0.2FeO₃ and Bi_0.33La_0.33Nd_0.33FeO₃ solid solution powders increased with increasing content of the rare-earth element. Bi_0.9La_0.1FeO₃ exhibited a moderate oxygen vacancy concentration and the optimal microwave absorption performance, achieving a minimum reflection loss (RLmin) of −42.62 dB at a thickness of 2.36 mm at 12.4 GHz, with a maximum effective absorption bandwidth (EABmax) extended to 1.38 GHz (at 5 mm thickness). Its excellent performance originated from the significant increase in dielectric loss and ideal impedance matching, enabling efficient electromagnetic wave absorption and dissipation near 12 GHz. In contrast, Bi_0.8La_0.2FeO₃ and Bi_0.33La_0.33Nd_0.33FeO₃ had excessively high oxygen vacancy concentrations, which weakened the uneven charge distribution and thus reduced the polarization effect, leading to reduction in microwave absorption performance.

Key words: bismuth ferrite; solid solution; oxygen vacancy; reflection loss; microwave absorption