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
1 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 introduction of configuration entropy can yield a batch of unexpected properties including better resistance to environment attack, better oxidation and corrosion resistance, good high-temperature stability, and lower thermal conductivity [36–42]. Most importantly, the “cocktail” effect on electrical and magnetic properties is anticipated which will open a new door to tailor EM wave absorbing properties including absorbing capability, effective absorbing bandwidth, and impedance matching. Previous work in the authors’ lab has found that high-entropy rare-earth silicide carbide/rare-earth oxide composites show a wide efficient absorption bandwidth of 4.5 GHz with a maximum reflection loss of –50.9 dB at thickness of 2.0 mm [43]. However, by far EM wave
absorption properties of HE REB6 have not been explored.
The aims of this work are to design and synthesize HE REB6 and to explore the EM wave absorption properties of HE REB6. To achieve these goals, five high-entropy rare-earth hexaborides (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Eu0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Sm0.2Eu0.2Yb0.2)B6, and (Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6 are designed and successfully synthesized. Hereafter they are referred to as HE REB6-1, HE REB6-2, HE REB6-3, HE REB6-4, and HE REB6-5. The choice of rare-earth elements is due to the following considerations. Firstly, hexaborides containing these RE elements exhibit the same crystal structure. Secondly, as shown in Table 1 [44], the difference of selected ion radius is smaller than 15%, which makes the formation of solid solution with a single phase easy. Thirdly, the selected compounds including NdB6, SmB6, YB6, ErB6, and CeB6 are conductors with antiferromagnetism, while EuB6 is a semiconductor with two ferromagnetic transition temperature of 12.5 and 15.3 K. Fourthly, although some REB6 such as YbB6 is a semiconductor, the formation of high-entropy hexaborides could lead to the band gap narrowing which can tune the electrical conductivity [45]. The electrical conductivity and magnetic ordering in hexaborides endow these new types of HE REB6 promising EM wave absorption properties. Finally, the selected compounds also take advantages of lightweight, exceptional high-temperature stability, and favorable resistant to corrosion and oxidation. The formation of HE REB6 could lead to further improvement of the comprehensive properties including mechanical property, thermal property, and EM wave absorption property.
Table 1 Ion radius and radius difference of the selected RE ions for the design of HE REB6 (data were obtained from the revised list of effective ionic radii [44])
2 Experimental
2. 1 Synthesis of HE REB6 powders
Five HE REB6, i.e., (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Eu0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Sm0.2Eu0.2Yb0.2)B6, and (Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6 powders were synthesized by a one-step boron carbide reduction process using commercially available Y2O3, CeO2, Sm2O3, Eu2O3, Er2O3, Yb2O3 powders (99.9% purity, 1 m, HWRK Chem Co., Ltd., Beijing, China) and B4C powders (98% purity, –200 mesh, Mudanjiang Qianjin Reagent Co. Ltd., Heilongjiang, China) as starting materials. Five rare earth oxide powders in the stoichiometric ratio of required HE REB6 were mixed together with B4C powders to prepare HE REB6 powders. The initial stage of the boron carbide reduction process leads to the formation of REBO3 as an intermediate phase as described by Eqs. (1)–(4), where RE refers to Y, Sm, Eu, Er, or Yb. In particular, the molar ratio of reactants is listed in Table 2.
Table 2 Constituent of raw materials of HE REB6
Each mixture was ball-milled with ethanol and agate media in a polytetrafluoroethylene jar for 4 h. After ball milling, the homogeneously mixed slurry was fully dried in an oven at 60 ℃. After drying and being sieved through a 120-mesh screen, the mixed powders were uniaxially cold-pressed into cylindrical pellets of 30 mm diameter in a stainless-steel die. Eventually, the green bodies were heated to 1900 ℃ and held for 1 h in vacuum to synthesize pure HE REB6 powders. In order to study the influence of intermediate phases, i.e., high-entropy rare-earth borates (HE REBO3) on the EM wave absorption performance of HE REB6, the pellets were heated to 1600 ℃ and kept for 1 h in Ar atmosphere to obtain HE REB6/HE REBO3 composites. The as-prepared samples were ground into powders in an agate mortar for subsequent characterizations.
2. 2 Phase composition, microstructure, and EM absorbing properties
Phase composition was analyzed by an X-ray diffractometer (XRD, D8 Advanced, Bruker, Germany) utilizing Cu Kα radiation (λ = 1.5406 Å) with a step size of 0.02° at a scanning rate of 2 (°)/min. The weight fraction of intermediate phase is quantitatively analyzed using the total pattern solution software (TOPAS, Bruker Corp., Karlsruhe, Germany). The microstructure of as-prepared HE REB6 powders was investigated by a scanning electron microscope (SEM, Apollo 300, CamScan, UK) equipped with an energy dispersive spectroscopy system (EDS Inca X-Max 80 T, Oxford, UK).
The relative complex permeability (μr = μ'+ jμ'') and permittivity (εr = ε' + jε'') were determined by a vector network analyzer (Agilent N5244A, USA) using the coaxial method in the frequency range of 1.0–18.0 GHz. High-entropy REB6 powders and paraffin wax were mixed at a mass ratio of 7:3 and compacted into a toroidal shape (Φout = 7.00 mm, Φin = 3.04 mm). The reflection loss (RL), which is one of the evaluation criteria for the EM wave absorption properties, was determined from the relative complex permeability and permittivity for a given frequency and absorber thickness based on the transmission-line theory and metal back-panel model [46], according to Eqs. (5) and (6):
where Z0 is the impedance of free space, Zin is the normalized input characteristic impedance at the interface between atmosphere and absorber, μr is the relative complex permeability, εr is the relative complex permittivity, f is the frequency, d is the thickness of samples, c is the velocity of light (3×108 m/s). The effective absorption bandwidth (EAB) is defined by the range of frequencies over which RL is smaller than –10 dB, which represents that 90% of the microwave energy is absorbed while only 10% of that is reflected [47].
3 Results and discussion
3. 1 Synthesis and microstructure of pure HE REB6powders
As shown in Fig. 1, rare earth hexaborides exhibit CsCl-type cubic structure with a space group of Pm3¯m(No. 221). The rare-earth and boron atoms are located at 1a (0, 0, 0) and 6f (0.5, 0.5, z) positions, respectively, where the position parameter z of B atom is related to the lattice parameter (a) and B–B bond lengths [48]. On the one hand, the structure of REB6 is constructed by the strong covalent σ bonds that link boron octahedrons, which limits the change of the lattice parameters but only allows the adjustments of the lengths of inter-octahedra σ bond (B–Binter) and intra-octahedron τ bond (B–Bintra) [19,23,24,33,49]. The changes in B–Binter and B–Bintra distances have great impact on the electrical properties, leading to the electric conduction transition. Each boron octahedron in REB6 is deficient of two electrons and thus the stability is ensured by the transformation of an electron from rare earth metals to boron octahedron, which give rise to good electrical conductivity in trivalent hexaborides [25,26,50]. As for the intriguing magnetism found in REB6, the magnetic ordering might be attributed to an indirect exchange of the RKKY type via conduction electrons donated from trivalent rare-earth ions or impurities [51,52].
Fig. 1 Crystal structure of REB6. Rare earth (RE) atoms (brown) and B6 octahedron (cyan) are arranged in a CsCl-like packing.
Figure 2 shows the XRD pattern of (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 (HE REB6-1) synthesized at 1900 ℃ for 1 h in vacuum. Standard ICDD/JCPDS cards of CeB6, YB6, SmB6, ErB6, and YbB6 are also presented to confirm the formation of HE REB6, which is structural ordered but compositional disordered with multi-principle RE elements.
Fig. 2 XRD pattern of (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 (HE REB6-1) together with those of CeB6, YB6, SmB6, ErB6, and YbB6 obtained from ICDD/JCPDS cards.
To characterize the particle size and morphology, the HE REB6-1 powders after grinding in an agate mortar are observed by SEM as shown in Fig. 3(a). The powders are equiaxial and the average particle size is about 1.86 μm as shown in Fig. 3(b).
Fig. 3 (a) SEM image and (b) particle size distribution of (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 powders.
Figure 4 compares the XRD patterns of five HE REB6 synthesized at 1900 ℃ for 1 h in vacuum. The diffraction peaks of five samples show consistency in spite of different constituents, which indicate that single-phase high-entropy solid-solutions are formed among these rare-earth hexaborides. The lattice parameters of these HE REB6 are calculated according to Bragg’s law as shown in Eq. (7):
where λ = 1.5406 Å, θ is obtained from peak position of {100} by fitted Gaussian peak functions. The lattice parameter of a cubic lattice is equal to the interplanar spacing of {100} plane. Then the theoretical density is calculated by the lattice parameter. The lattice parameter (a) and theoretical density (ρth) of five HE REB6 are listed in Table 3. The variation of lattice parameter is derived from the substitutions of RE elements, which leads to different degrees of lattice distortions. By replacing rare-earth ions (Y3+, Sm3+, Er3+ with ionic radius of 0.9 Å, 0.96 Å, 0.89 Å respectively) in HE REB6-1 with Eu2+ (r = 1.17 Å), (Ce0.2Eu0.2Sm0.2Er0.2Yb0.2)B6 (HE REB6-2), (Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6 (HE REB6-3), and (Ce0.2Y0.2Sm0.2Eu0.2Yb0.2)B6 (HE REB6-4) with larger lattice parameters are obtained, which can be attributed to the substitution of Eu2+ that has larger ionic radius. Likewise, replacing Ce3+ (r = 0.87 Å) in HE REB6-4 with Nd3+ (r = 0.98 Å) will lead to (Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6 (HE REB6-5) with a larger lattice parameter.
Fig. 4 XRD patterns of HE REB6-1 (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6, HE REB6-2 (Ce0.2Eu0.2Sm0.2Er0.2Yb0.2)B6, HE REB6-3 (Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6, HE REB6-4 (Ce0.2Y0.2Sm0.2Eu0.2Yb0.2)B6, and HE REB6-5 (Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6.
Table 3 Lattice parameter (a) and theoretical density (ρth) of HE REB6
3. 2 Synthesis and microstructure of HE REB6/HE REBO3 composite powders
From Fig. 5, standard ICDD/JCPDS cards of borates show that cerium borate exhibits orthorhombic aragonitetype structure which is different from the rest borates with hexagonal vaterite-type structure. After synthesized at 1600 ℃ for 1 h in Ar atmosphere, the main phases of the product are (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 and (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)BO3, with the existence of a small amount of cerium borate and boron carbide, which is attributed to the uncompleted reduction.
Fig. 5 XRD pattern of (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 and (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)BO3 composite powders together with those of CeBO3, YBO3, SmBO3, ErBO3, and YbBO3 obtained from ICDD/JCPDS cards.
Figure 6 presents the backscattered electron image (BSEI) and EDS patterns of two selected points in (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6/(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)BO3 composites. Through semi-quantitative analysis, the atomic ratio of O/B of two selected points in the golden phase and the blue phase are 1.602 and 0.018, respectively. Combining with XRD analysis, it can be concluded that the golden phase is HE REBO3 and the blue phase is HE REB6. Therefore, it comes to the conclusion that HE REBO3 phase is distributed on the surface of HE REB6 in HE REB6/HE REBO3 composite powders, as shown in Fig. 7.
Fig. 6 (a) BESI of (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6/(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)BO3 composite powders, (b) EDS spectrum of selected point in the golden phase, and (c) EDS spectrum of the selected point in the blue phase.
Fig. 7 BESI of (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6/(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)BO3 composite powers indicating the distribution of (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)BO3 phase (golden color).
Figure 8 shows the XRD patterns of the composite powders synthesized at 1600 ℃ for 1 h in Ar atmosphere. The diffraction peaks of these five composites are similar to each other and all can be indexed as vaterite-type structure although they possess different compositions. Therefore, it can be concluded that composites with HE REB6 and HE REBO3 phases are formed. Hereafter, the five composites synthesized at 1600 ℃ in Ar atmosphere are referred to as HE REB6/HE REBO3-1, HE REB6/HE REBO3-2, HE REB6/HE REBO3-3, HE REB6/HE REBO3-4, and HE REB6/HE REBO3-5, respectively. To estimate the relative content of different phases in these composites, quantitative analysis was performed using the total pattern solution software (TOPAS). The weight fraction of HE REBO3 and reliability factors, Rp and Rwp, are listed in Table 4.
Fig. 8 XRD patterns of HE REB6/HE REBO3-1, HE REB6/HE REBO3-2, HE REB6/HE REBO3-3, HE REB6/HE REBO3-4, and HE REB6/HE REBO3-5 composite powders.
Table 4 Weight fraction of HE REBO3 and reliability factors, Rp and Rwp in five HE REB6/HE REBO3 composites
3. 3 EM wave absorption properties
3.3.1 EM wave absorption properties of HE REB6
Figure 9 reveals the frequency dependences of the electromagnetic parameters for HE REB6/paraffin composites. As illustrated in Fig. 9(a) and Fig. 9(b), across the testing frequency range, the values of ε' and ε″ show a decreasing tendency from HE REB6-1 to HE REB6-4, which is attributed to the changes of electrical conductivity of HE REB6 as a result of the changes of lattice constant a due to the substitution of Eu2+. The decrease of lattice constant a will lead to the increasement of the bond length difference between B–Binter and B–Bintra, which will further give rise to the increasement of shear anisotropy. On this basis, REB6 with smaller lattice constant would possess better electrical conductivity [49]. Therefore, the conductivity of HE REB6 decreases from HE REB6-1 to HE REB6-4 as the lattice constant increases. In particularly, in exception of HE REB6-3, ε″ values of HE REB6 decrease with the increasing of frequency at low frequency, which indicates the dispersion phenomenon; while the value of ε″ exhibits growing tendency as the frequency increases. Generally, the magnitude of dielectric loss can be estimated by the dielectric loss tangent
. From Fig. 10(a), the variation of dielectric loss tangent shows a similar tendency to that of ε″ value. It is expected that HE REB6-1 and HE REB6-2 have considerable dielectric loss ability, which is derived from dipolar polarization, conduction loss, and polarization loss [53,54]. In specific, numerous dipoles in HE REB6 can provide multiple ways for the transmission of the electromagnetic wave [55]. Moreover, abundant polarization charges and a sufficient number of interfaces between the HE REB6 ceramics and paraffin wax are beneficial to spur interfacial polarization (called the Maxwell–Wagner effect) and related relaxation, which contribute to the dielectric loss [56,57]. The electron migration because of natural electrical conductivity will also lead to the reduction of electromagnetic energy. The polarization mechanism can be described in the assist of Debye theory [58] as shown in Eq. (8):
where εs , ε∞ are the static permittivity, and relative dielectric permittivity at high-frequency limit, respectively. Therefore, the relationship between ε' and ε″ is represented by a curve in the shape of semicircle (called the Cole–Cole semicircle), and each semicircle corresponds to one Debye relaxation process [59]. Figure 10(b) presents the ε'–ε″ plots of HE REB6, where clear semicircles are pictured to reflect the polarization relaxations of HE REB6 powders.
The relative complex permeability and magnetic loss tangent of HE REB6 are shown in Figs. 9(c), 9(d), and 10(c). From Fig. 9(c), the μ' values of HE REB6-2, HE REB6-4, and HE REB6-5 decrease in the range of 2–8 GHz and then remain steady because of the relaxation of magnetic moments procession, while those of HE REB6-1 and HE REB6-3 exhibit a small amount of increasement. Figure 9(d) shows frequency dependent of μ″ of HE REB6. Resonance peaks are observed, which can be described by natural resonance[60]. The corresponding values of magnetic loss tangent of HE REB6 exhibit similar fluctuation with μ″ values. Generally, the magnetic loss is derived from hysteresis loss, domain wall displacement loss, natural resonance loss, and eddy current loss [61]. Since the characterization is performed in a weak external magnetic field at the frequency range of gigahertz, hysteresis loss and domain wall displacement loss can be ruled out from the mechanism of magnetic loss in HE REB6. The contribution of Eddy current loss to the reflected loss is related to the electrical conductivity of materials (σ) and the matching thickness (d), as represented by Eq. (9)[62]:
where μ0 is the vacuum permeability. If the eddy current loss is the dominate factor for the magnetic loss, the value of C0 (μ''(μ') -2 f-1 )should be a constant [63]; otherwise, the natural resonance loss is preponderance. Figure 10(d) presents the values of C0 of HE REB6 as a function of frequency. In the frequency range of 1–12 GHz, the values of C0 decrease with the increase of frequency. With raising frequency, the values of C0 remain as a constant. Therefore, it can be concluded that both the eddy current loss and the natural resonance loss contribute to the magnetic loss. It is worth noting that the μ″ values of HE REB6 in Fig. 9(d) are close to that of Fe3O4, which is a typical magnetic EM wave absorber with exceptional magnetic features [64,65]. Drawn from the analysis of dielectric and magnetic loss tangents, it is concluded that the absorption abilities of HE REB6 are generated by the coupling of dielectric and magnetic losses.
Fig. 9 Electromagnetic parameters of HE REB6: (a) real permittivity (ε'), (b) imaginary permittivity (ε″), (c) real permeability (μ'), (d) imaginary permeability (μ″).
Fig. 10 Frequency dependences of (a) the dielectric loss tangent (ε″/ε') and (c) the magnetic loss tangent (μ″/μ'); (b) Cole–Cole semicircles and (d) C0–f curves of HE REB6.
Based on above discussions, high electric and magnetic loss tangents will lead to high reflection loss (RL), which is one of the vital evaluation indexes of the EM wave absorption abilities. With relatively higher dielectric and magnetic loss tangents, HE REB6-2 might possess the highest RL value. However, from Fig. 11(a), it can be found that HE REB6-1 rather than HE REB6-2 exhibits the highest RL value, which is due to the better impedance match. It has come to light that the closer the impedance match values to 1, the better the impedance match [66]. From this angle, HE REB6-1 possesses the best impedance matching, and HE REB6-2 ranks the second, as shown in Fig. 11(b). As shown in Figs. 11(a) and 11(c), the RLmin value of HE REB6-1 reaches –33.4 dB at 11.5 GHz with a thickness of 2 mm, and the optimized effective absorption bandwidth (EAB) is 3.9 GHz from 13.6 to 17.5 GHz with a thickness of 1.5 mm. As shown in Fig. 11(d), HE REB6-2 reaches the largest EAB of 4.3 GHz from 12.6 to 16.9 GHz at thickness of 1.7 mm. Therefore, the superior EM absorption properties require not only strong absorption, but also good impedance matching. In addition, the variation of impedance match values of HE REB6 indicates the impedance matching can be tuned via compositional design. The addition of multi-principle elements with different atomic sizes in rare-earth hexaborides brings about severe lattice distortion effect, which has an impact on the changes of B–B bonding lengths and electronic structure [25,26]. Thus, different electronic conductivity and magnetic performance in HE REB6 can be achieved through composition design, which will lead to the difference of EM wave absorption properties in HE REB6 with different constituent.
Fig. 11 Comparison of (a) reflection loss (RL) values and (b) impedance match (Zin/Z0) for HE REB6. (c) and (d) Frequency dependence of RL values at various thickness for HE REB6-1 and HE REB6-2, respectively.
3.3.2 EM wave absorption properties of HE REB6/HE REBO3
Combining the microstructure shown in Fig. 7 and the XRD patterns of HE REB6/HE REBO3 powders shown in Fig. 8, it comes to the conclusion that composites of HE REB6 and HE REBO3 are formed. From Fig. 12(a), except for HE REB6/HE REBO3-4, ε' values of HE composites are smaller than those of HE REB6 in the whole frequency range, indicating that the conductivity is reduced because of the instinct insulating feature of HE REBO3, which will further decrease ε″ value according to free electron theory ε''≈σ/ (2πfε0 ), where σ is the conductivity and ε0 is the permittivity of vacuum [67]. However, as shown in Fig. 12(b), the considerable improvement of ε'' value of HE REB6/HE
REBO3 compared with those of HE REB6 are observed, which indicates that there is another dominant mechanism contributing to electric loss. In specific, the coverage of HE REBO3 intermediate phase will spur more interfacial polarizations and related relaxations among HE REB6, HE REBO3 and paraffin wax phases, which will lead to more severe interface polarization under the action of electromagnetic waves. Therefore, compared to HE REB6 samples, the values of electric dissipation factor (ε″/ε') of HE REB6/HE REBO3 samples are higher (Fig. 13(a)). As for the permeability shown in Figs. 12(c) and 12(d), the μ' values of HE REB6/HE REBO3 are close to those of HE REB6, while the μ″ values are lower than those of HE REB6. According to Eq. (9), the decrease of electrical conductivity due to the formation of instinct insulating HE REBO3 phase will
give rise to reduction of μ″ value. Then, from Fig. 13(c), the magnetic loss tangent (μ″/μ') of HE REB6/HE REBO3 samples is reduced, which indicates relatively lower magnetic loss. These foregoing results indicate that both dielectric loss and magnetic loss of HE REB6/HE REBO3 contribute to dissipating the electromagnetic wave energy, but the dominant effect is due to the dielectric loss.
Fig. 12 Electromagnetic parameters of HE REB6/HE REBO3: (a) real permittivity (ε'), (b) imaginary permittivity (ε″), (c) real permeability (μ'), (d) imaginary permeability (μ″).
The dielectric loss is caused by Debye dipolar relaxation [59], which is proven by the presence of semicircle curves as shown in Fig. 13(b). The C0 values in Fig. 13(d) decrease dramatically as frequency increase in the frequency range of 1–6 GHz; then, they keep unchanged as the frequency increases. Therefore, the magnetic loss is derived from eddy current loss and natural resonance loss [60,63]. As has been discussed in the previous section, the EM wave absorption properties are strongly affected by the impedance match. Otherwise, electromagnetic wave will be reflected on the surface of the EM wave absorbing materials, and the reflect loss ability is in vain when it comes to the fact that the entering of EM wave into the absorber is difficult to realize [68]. Therefore, the reduction of reflection losses of HE REB6/HE REBO3 composite powders shown in Fig. 14(a) is due to the unfitting impedance match compared with those of HE REB6 samples. For example, from Fig. 14(a), the value of impedance match (Zin/Z0) of HE REB6-1 and HE REB6/HE REBO3-1 increases from 1.06 to 1.19, which gives rise to reduction of the RLmin value from –33.4 to –24.1 dB with the same thickness of 2 mm.
Fig. 13 Frequency dependences of (a) the dielectric loss tangent (ε″/ε') and (c) the magnetic loss tangent (μ″/μ'); (b) Cole–Cole semicircles and (d) C0–f curves of HE REB6/HE REBO3.
Fig. 14 Comparison of (a) reflection loss (RL) values and (b) impedance match (Zin/Z0) for HE REB6/HE REBO3. (c, d) Frequency dependence of RL values at various thickness for HE REB6/HE REBO3-1 and HE REB6/HE REBO3-2, respectively.
Although the reflection loss abilities of HE REB6/HE REBO3 samples become weaker, HE REB6/HE REBO3 composite powders still exhibit considerable effective absorption bandwidth with low thickness. As shown in Figs. 14(c) and 14(d), both HE REB6/HE REBO3-1 and HE REB6/HE REBO3-2 exhibit wide EAB of 4.1 GHz with the same width of 1.7 mm. Thus, the introduction of HE REBO3 as an intermediate phase into HE REB6 will lead to greater dielectric loss, relatively poorer magnetic loss, and worsen impedance matching. For example, in the frequency range of 10–14 GHz, the value of dielectric tangent loss of HE REB6/HE REBO3-1 reaches 0.35 which is 1.75 times of that of HE REB6-1, while the value of magnetic tangent loss decreases from 0.12 to 0.05, and the value of impedance match (Zin/Z0) gets further away from 1. As a result, the value of RLmin of HE REB6/HE REBO3-1 decreases, which indicates that the EM wave absorption abilities become weaker. The above results infer that although the coverage of insulator HE REBO3 phase is beneficial to spur interfacial polarization and related relaxation and give rise to greater dielectric loss, worsen of the magnetic loss and impedance matching, which will finally lead to the weaken of overall reflection loss ability.
Figure 15 summarizes the optimized EAB value and the sample thickness of some reported EM absorbing materials of various kinds (SiC based [69–73], carbons based[74–78], ferrite metal-organic framework (Fe-MOF) [47], phosphides based [79], sulfides based [68,80], polymers based [81–83], metals/alloys based [62,63,67, 84,85], and oxides based materials[64,66,86–89]). As we have mentioned in previous sections, seeking for materials with thin thickness and wide effective bandwidth is the trend for the development of EM wave absorbing materials. From this aspect, searching for materials in region II are the purposes of many researches. As shown in Fig. 15, although materials in region I possess wide EAB, they are still restricted by thicker thickness. In contract, while materials in region III exhibit small thickness, their narrow effective bandwidth is the hindrances. As is self-evident, materials in region IV have disadvantages of both heavy thickness and narrow EAB. It is worthy of noting that HE REB6-1, HE REB6-2, HE REB6/HE REBO3-1, and HE REB6/HE REBO3-2 are located in region II, which exhibit wide EAB with considerable thin thickness, indicating their superior EM wave absorption properties among these materials.
Fig. 15 Effective bandwidth and corresponding sample thickness of various kinds of reported EM absorbing materials.
4 Conclusions
In conclusion, five HE REB6 powders including (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Eu0.2Sm0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6, (Ce0.2Y0.2Sm0.2Eu0.2Yb0.2)B6, and (Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6 as well as five HE REB6/HE REBO3 composite powders including (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6/(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)BO3, (Ce0.2Eu0.2Sm0.2Er0.2Yb0.2)B6/(Ce0.2Eu0.2Sm0.2Er0.2Yb0.2)BO3, (Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6/(Ce0.2Y0.2Eu0.2Er0.2Yb0.2)BO3, (Ce0.2Y0.2Sm0.2Eu0.2Yb0.2)B6/(Ce0.2Y0.2Sm0.2Eu0.2Yb0.2)BO3, and (Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6/(Nd0.2Y0.2Sm0.2Eu0.2Yb0.2) BO3 were designed and successfully synthesized through a facile one-step boron carbide reduction method. XRD analysis reveals that the five as-synthesized HE REB6 are phase-pure solid solutions with CsCl-type cubic structure, and the five as-synthesized HE REB6/HE REBO3 contain two main phases of HE REB6 and HE REBO3. The theoretical density of HE REB6 ranges from 4.84 to 5.25 g/cm3. The weight fraction of HE REBO3 in HE REB6/HE REBO3 composite powders was estimated through quantitative phase analysis to be ranging from 38.67% to 64.78%. SEM analysis indicates that the (Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 powders possess equiaxial morphology with the mean particle size of 1.86 μm and HE REBO3 is distributed on the surface of HE REB6 phases in HE REB6/HE REBO3 composite powders.
The synergetic effects of dielectric and magnetic losses and tunable impedance matching are attributed to favorable EM wave absorption properties. The RLmin value of HE REB6-1 reaches –33.4 dB at 11.5 GHz at thickness of 2 mm, and the optimized effective absorption bandwidth (EAB) is 3.9 GHz from 13.6–17.5 GHz at thickness of 1.5 mm. The introduction of HE REBO3 as an intermediate phase into HE REB6 leads to greater dielectric loss, relatively poor magnetic loss, and worse impedance matching, which further leads to the reduction of EM wave reflection loss ability in HE REB6/HE REBO3 composite powders. Although the RLmin value of HE REB6/HE REBO3-1 decreases to –24.1 dB, it still reaches wide EAB of 4.1 GHz from 13.4–17.5 GHz at thin thickness of 1.7 mm. Single-phase HE REB6 ceramics with coupling of dielectric and magnetic losses possess thin thickness, wide effective absorption bandwidth, and considerable EM absorption capability. Combining good high-temperature stability, oxidation and corrosion resistance, lightweight HE REB6 ceramics are promising as a new type of EM absorbing materials that can meet the requirements of high temperature or corrosive serving environments.
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