Synthesis and electromagnetic wave absorption performances of a novel(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)high-entropy MAX phase

Herein,a novel kind of high-entropy MAX phases,(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)powders were success-fully synthesized by a newly proposed two-step solid state reaction process.The oxidation experiments demonstrate that the oxidation products of Al_(2)Mo_(3)O_(12) and rutile TiO 2 are formed at about 600 and 800℃,respectively.Besides,the dielectric and electromagnetic(EM)wave absorption properties of(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)powders and those after oxidation at different temperatures were also exam-ined.The results show that the as-synthesized(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)powders possess excellent EM wave absorption performances with the minimum reflection loss(RL)of-45.80 dB(at 1.7 mm thickness)and the maximum effective absorption bandwidth(E AB)of 3.6 GHz(at 1.5 mm thickness).After oxidation at 400-800℃,due to the coupling of conductivity loss and polarization loss,(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)AlC_(2)powders can retain good EM wave absorption properties in a certain frequency range.In this paper,the effects of oxidation on EM wave absorption properties of high-entropy MAX phases were systematically investigated for the first time.This work manifests that high-entropy MAX phases are promising EM wave absorbing candidates and can maintain good EM wave absorption performances after oxidation.

Synthesis of high-entropy MXenes with high-efficiency electromagnetic wave absorption

High-entropy MXenes,as a new emerging class of materials,possess diverse compositions,unexpected physicochemical characteristics,and great potentials for electromagnetic(EM)wave absorption.Herein,two single-to-few-layer high-entropy MXenes,(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))3 C2 T_(x)and(Mo_(0.2)Cr_(0.2)Nb_(0.2)Ti_(0.2)V_(0.2))_(4)C_(3)T_(x),were synthesized for the first time.During the exfoliation and delamination processes,the structural,morphological,and compositional evolutions were analyzed,verifying the successful formation of single-to-few-layer two-dimensional MXene nanosheets.Investigations indicate that with the filling content of only 35 wt%,MXene powder filled composites exhibit high-efficiency EM wave absorption performances.The f-(Mo_(0.25)Cr_(0.25)Ti_(0.25)V_(0.25))_(3)C_(2)T_(x)possesses the minimum reflection loss(RL min)of−45.0 dB with the matching thickness of 1.52 mm and the maximum effective absorption bandwidth(EAB)of 5.6 GHz at 1.65 mm thickness.Also,f-(Mo_(0.2)Cr_(0.2)Nb_(0.2)Ti_(0.2)V_(0.2))_(4)C_(3)T_(x)can attain an RL min of−52.8 dB with the thickness of 1.58 mm and an optimum EAB value of 3.6 GHz at 1.50 mm.The satisfactory EM wave absorption efficiency and bandwidth,thin matching thickness,and low filling content prove the lightweight advantage and great application potential of high-entropy MXenes in EM wave absorption.In this work,the high-entropy strategy is applied to tune the EM wave absorption performances for MXenes.Furthermore,high-entropy engineering is expected to provide control and tunability of many other properties,such as electrochemical,catalytic,and mechanical behaviors.

Effect of grain size on mechanical properties and tribological behavior of size-tunable high entropy diboride ceramics obtained by two-step SPS sintering

High-entropy diboride(HEB)ceramics constitute a novel class of ultrahigh-temperature ceramics that are appealing for applications in extreme environments.The relative density and grain size play important roles in tailoring the mechanical properties and wear resistance of HEBs,affecting their applications,such as high-temperature structural parts and thermal protection systems.In this study,highly dense(HfZrTaVNb)B_(2) ceramics with size-tunable microstructures were successfully synthesized by spark plasma sintering combined with an ingenious two-step strategy.The effects of grain size on the mechanical properties and wear resistance of(HfZrTaVNb)B_(2) ceramics were comprehensively investigated.The results indicated that the smaller grain size led to higher hardness and fracture toughness,and the relationship between hardness and grain size fitted the Hall–Petch equation well.In particular,the sample featuring a grain size of 1.64µm and 97.6%density had the highest hardness and fracture toughness,26.7 GPa and 4.6 MPa·m^(1/2),respectively.Notably,it also demonstrated optimal wear resistance,displaying a minimal wear rate of only 2.53×10^(−6) mm^(3)/(N·m)under a 20 N load.Microstructure analysis revealed that the primary wear mechanism observed in(HfZrTaVNb)B_(2) was oxidative wear under a 5 N load.Under a 10 N load,the wear mechanism comprised both oxidative and fracture wear.The wear mechanism became more complex and involved oxidation wear,fracture wear,abrasive wear,and fatigue wear at a 20 N load.