One-step synthesis and electromagnetic absorption properties of high entropy rare earth hexaborides (HE REB 6 ) and high entropy rare earth hexaborides/borates (HE REB 6 /HE REBO 3 ) composite powders

Journal of Advanced Ceramics 2020, 9(6): 0–0 https://doi.org/10.1007/s40145-020-0417-2 ISSN 2226-4108 CN 10-1154/TQ Research Article One-step synthesis and electromagnetic absorption properties of high entropy rare earth hexaborides (HE REB6) and high entropy rare earth hexaborides/borates (HE REB6/HE REBO3) composite powders Weiming ZHANGa, Biao ZHAOb, Huimin XIANGa, Fu-Zhi DAIa, Shijiang WUc, Yanchun ZHOUa,* a Science and Technology on Advanced Functional Composite Laboratory, Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, China b Henan Key Laboratory of Aeronautical Materials and Application Technology, School of Material Science and Engineering, Zhengzhou University of Aeronautics, Zhengzhou 450046, China c Zibo Firststar New Material Incorporated Co. Ltd., Zibo 255000, China Received: July 29, 2020; Revised: August 23, 2020; Accepted: August 30, 2020 © The Author(s) 2020. Abstract: Considering the emergence of severe electromagnetic interference problems, it is vital to develop electromagnetic (EM) wave absorbing materials with high dielectric, magnetic loss and optimized impedance matching. However, realizing the synergistic dielectric and magnetic losses in a single phase material is still a challenge. Herein, high entropy (HE) rare earth hexaborides (REB6) powders with coupling of dielectric and magnetic losses were designed and successfully synthesized through a facial one-step boron carbide reduction method, and the effects of high entropy borates intermedia phases on the EM wave absorption properties were investigated. Five HE REB6 ceramics including (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Eu0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Sm0.2 Eu0.2Yb0.2)B6, and (Nd0.2Y0.2Sm0.2Eu0.2 Yb0.2)B6 possess CsCl-type cubic crystal structure, and their theoretical densities range from 4.84 to 5.25 g/cm3. (Ce0.2Y0.2Sm0.2Er0.2 Yb0.2)B6 powders with the average particle size of 1.86 μm were found to possess the best EM wave absorption properties among these hexaborides. The RLmin value of (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 reaches –33.4 dB at 11.5 GHz at thickness of 2 mm; meanwhile, the optimized effective absorption bandwidth (EAB) is 3.9 GHz from 13.6 to 17.5 GHz with a thickness of 1.5 mm. The introduction of HE REBO3 (RE = Ce, Y, Sm, Eu, Er, Yb) as intermediate phase will give rise to the mismatching impedance, which will further lead to the reduction of reflection loss. Intriguingly, the HEREB6/HEREBO3 still possess wide effective absorption bandwidth of 4.1 GHz with the relative low thickness of 1.7 mm. Considering the better stability, low density, and good EM wave absorption properties, HE REB6 ceramics are promising as a new type of EM wave absorbing materials. Keywords: high entropy rare earth hexaborides (HE REB6); one-step synthesis; electromagnetic wave absorbing properties; synergistic dielectric and magnetic losses; wide effective absorption bandwidth * Corresponding author. E-mail: www.springer.com/journal/40145 2 1 J Adv Ceram 2020, 9(6): 0–0 Introduction With the electromagnetic (EM) wave as an important carrier of information exchange penetrating into all aspects in human life, it is critical to develop high performance EM wave absorbing materials to eliminate the adverse effects of EM wave, such as the electromagnetic interference and microwave pollution [1–3]. Under increasingly complex serving environment, EM wave absorbing materials are required to possess the following properties: (1) strong absorption, broad efficient absorption bandwidth, and optimized impedance matching; (2) lightweight, good mechanical property, and favorable high-temperature stability; (3) resistance to corrosion and oxidation; (4) easy fabrication and cost-efficient [3–5]. However, achieving high EM wave absorption performance with foregoing features in a single phase material is still a challenge [6]. For examples, magnetite with superior dual electromagnetic properties (ferrimagnetic and dielectric properties) has shortcomings of high density and inferior stability [7,8]; magnetic metals with high permeability have disadvantages of high density and poor resistance to environment corrosion [9,10]; lightweight carbon-based nanomaterials and conducting polymers both take the advantage of high conductivity but also suffer from dissatisfactory electromagnetic impedance matching [11–13]. Through element doping [14], structure design [15], surface treatment [10], and composite construction [16], weight reduction and impedance matching adjustment can be effectively realized. However, realizing synergistic dielectric and magnetic losses in a single phase material is still difficult. Thus, searching for materials with good high-temperature stability, corrosion and oxidation resistance is appealing. Consequently, it is of great significance to develop lightweight high temperature EM wave absorbing ceramics with exceptional corrosion and oxidation resistance. EM wave absorbing materials are capable of converting absorbed EM wave into thermal energy in terms of dielectric loss and/or magnetic loss [17]. Considering the conductivity and magnetism of rare-earth (RE) hexaborides (REB6), it is expected that lightweight REB6 ceramics possess promising EM wave absorption properties. REB6 exhibits a CsCl-type crystal structure, with an RE atom surrounded by eight boron octahedra [18]. The boron framework is deficient of 2 electrons that can be donated by RE atoms, making divalent REB6 (EuB6, YbB6) semiconductors or semimetals while other trivalent REB6 conductors [19–22]. Besides, the rigidity of octahedral boron cage endows REB6 with high melting point, high hardness in lattice a direction, and superior chemical stability [23,24]. The substitution of RE atoms leads to the adjustment of the distances of intra-octahedron bond (B–Bintra) and inter-octahedra bond (B–Binter), which will affect the electrical properties and then give rise to the transition of conductivity from semiconductors to conductors [25,26]. It is also found that with the exception of paramagnetic LaB6, trivalent REB6 are antiferromagnetic at low temperature and EuB6 exhibits unusual ferromagnetism below 15.3 K [27–31]. Surprisingly, (Ca1–xLax)B6 has a ferromagnetic Curie temperature of 600 K, which equals to that of transition-metal ferromagnets such as Fe [32]. Combining excellent high-temperature property, good electrical conductivity and magnetic property, REB6 are expected to be a new type of promising EM wave absorbing materials. The similar lattice parameters of different REB6 is beneficial to tuning the properties of REB6 through element doping [26,33]. Besides, boron carbide reduction method is found effective to synthesize REB6 without post-synthesis treatment such as acid washing [34,35]. Consequently, it is feasible to design and synthesize high-entropy rare-earth hexaborides (HE REB6) with multi-principal rare-earth elements. It is expected that the in (...truncated)