Enabling highly efficient and broadband electromagnetic wave absorption by tuning impedance match in high-entropy transition metal diborides (HE TMB2)

The advance in communication technology has triggered worldwide concern on electromagnetic wave pollution. To cope with this challenge, exploring high-performance electromagnetic (EM) wave absorbing materials with dielectric and magnetic losses coupling is urgently required. Of the EM wave absorbers, transition metal diborides (TMB2) possess excellent dielectric loss capability. However, akin to other single dielectric materials, poor impedance match leads to inferior performance. High-entropy engineering is expected to be effective in tailoring the balance between dielectric and magnetic losses through compositional design. Herein, three HE TMB2 powders with nominal equimolar TM including HE TMB2-1 (TM = Zr, Hf, Nb, Ta), HE TMB2-2 (TM = Ti, Zr, Hf, Nb, Ta), and HE TMB2-3 (TM = Cr, Zr, Hf, Nb, Ta) have been designed and prepared by one-step boro/carbothermal reduction. As a result of synergistic effects of strong attenuation capability and impedance match, HE TMB2-1 shows much improved performance with the optimal minimum reflection loss (RLmin) of −59.6 dB (8.48 GHz, 2.68 mm) and effective absorption bandwidth (EAB) of 7.6 GHz (2.3 mm). Most impressively, incorporating Cr in HE TMB2-3 greatly improves the impedance match over 1–18 GHz, thus achieving the RLmin of −56.2 dB (8.48 GHz, 2.63 mm) and the EAB of 11.0 GHz (2.2 mm), which is superior to most other EM wave absorbing materials. This work reveals that constructing high-entropy compounds, especially by incorporating magnetic elements, is effectual in tailoring the impedance match for highly conductive compounds, i.e., tuning electrical conductivity and boosting magnetic loss to realize highly efficient and broadband EM wave absorption with dielectric and magnetic coupling in single-phase materials.

Spontaneous Dynamical Disordering of Borophenes in MgB2 and Related Metal Borides

Layered boron compounds have attracted significant interest in applications from energy storage to electronic materials to device applications, owing in part to a diversity of surface properties tied to specific arrangements of boron atoms. Here, first-principles calculations coupled with global optimization are performed to explore the energy land scape for surface atomic configurations of MgB2, a prototypical layered metal diboride. We demonstrate, that contrary to previous assumptions, multiple reconstructions are thermodynamically preferred and kinetically accessible within the exposed B surfaces in MgB2, and other layered metal diborides. Such a dynamic environment and intrinsic disordering of the B surface atoms in metal borides presents new opportunities to realize a diverse set of 2D boron structures. We validated the predicted dynamic surface disordering by characterizing exfoliated boron-terminated MgB2 nanosheets. Application-relevant implications are discussed, with a particular view towards understanding the impact of boron surface heterogeneity on hydrogen storage performance.