Crystallography of precipitates in Mg alloys

Crystallography of precipitates in Mg alloys is indispensable to explain and predict alloy microstructures and properties.In order to obtain a global understanding of diversified experimental results,a general theory of singular interface is introduced,which provides the physical base and calculation methodology for interpreting precipitate morphology and orientation relationship(OR),especially useful for understanding irrational facets and ORs.Guided by the theory,recent experimental findings are systematically summarized,including thermally stable and metastable precipitates with various crystal structures.Then,theoretical advances inspired by the findings are introduced,which deepens our understanding on OR selection and preference of irrational facets.At last,future research directions in this field are proposed.

Medium and high-entropy transition mental disilicides with improved infrared emissivity for thermal protection applications

Transition metal disilicides are widely used as heating elements and infrared emission coatings.However,the limited intrinsic infrared emissivity and high thermal conductivity are the main limitations to their applications as infrared emission coatings in the thermal protection system.To cope with these prob-lems,four medium and high-entropy transition metal disilicides,i.e.,(V_(0.25)Ta_(0.25)Mo_(0.25)W_(0.25))Si_(2)(ME-1),(Nb_(0.25)Ta_(0.25)Mo_(0.25)W_(0.25))Si_(2)(ME-2),(V_(0.2)Nb_(0.2)Ta_(0.2)Mo_(0.2)W_(0.2))Si_(2)(HE-1),and(Cr_(0.2)Nb_(0.2)Ta_(0.2)Mo_(0.2)W_(0.2))Si_(2)(HE-2),were designed and synthesized by spark plasma sintering method using transition metal binary disilicides as precursors.The introduction of multi-elements into transition metal disilicides not only im-proved the infrared emissivity but also reduced the electrical and thermal conductivity.Among them,(Cr_(0.2)Nb_(0.2)Ta_(0.2)Mo_(0.2)W_(0.2))Si_(2)(HE-2)had the lowest electrical conductivity of 3789 S cm-1,which is over one order of magnitude lower than that of MoSi_(2)(50000 S cm^(-1)),and total infrared emissivity of 0.42 at room temperature,which is nearly double of that of TaSi_(2).Benefiting from low electrical conductivity and phonon scattering due to lattice distortion,the medium and high-entropy transition metal disilicides also demonstrated a significant decline in thermal conductivity compared to their binary counterparts.Of all samples,HE-2 exhibited the lowest thermal conductivity of 6.4 W m^(−1)K^(−1).The high-entropy tran-sition metal disilicides also present excellent oxidation resistance at high temperatures.The improved infrared emissivity,reduced thermal conductivity,excellent oxidation resistance,and lower densities of these medium and high-entropy transition metal disilicides portend that they are promising as infrared emission coating materials for applications in thermal protection systems.

The highest melting point material:Searched by Bayesian global optimization with deep potential molecular dynamics

The interest in refractory materials is increasing rapidly in recent decades due to the development of hypersonic vehicles.However,the substance that has the highest melting point(Tm)keeps a secret,since precise measurements in extreme conditions are overwhelmingly difficult.In the present work,an accurate deep potential(DP)model of a Hf-Ta-C-N system was first trained,and then applied to search for the highest melting point material by molecular dynamics(MD)simulation and Bayesian global optimization(BGO).The predicted melting points agree well with the experiments and confirm that carbon site vacancies can enhance the melting point of rock-saltstructure carbides.The solid solution with N is verified as another new and more effective melting point enhancing approach for HfC,while a conventional routing of the solid solution with Ta(e.g.,HfTa_(4)C_(5))is not suggested to result in a maximum melting point.The highest melting point(~4236 K)is achieved with the composition of HfCo.638No.271,which is~80 K higher than the highest value in a Hf-C binary system.Dominating mechanism of the N addition is believed to be unstable C-N and N-N bonds in liquid phase,which reduces liquid phase entropy and renders the liquid phase less stable.The improved melting point and less gas generation during oxidation by the addition of N provide a new routing to modify thermal protection materials for the hypersonic vehicles.

Atomic-scale simulations in multi-component alloys and compounds:A review on advances in interatomic potential

Multi-component alloys have demonstrated excellent performance in various applications,but the vast range of possible compositions and microstructures makes it challenging to identify optimized alloys for specific purposes.To overcome this challenge,large-scale atomic simulation techniques have been widely used for the design and optimization of multi-component alloys.The capability and reliability of large-scale atomic simulations essentially rely on the quality of interatomic potentials that describe the interactions between atoms.This work provides a comprehensive summary of the latest advances in atomic simulation techniques for multi-component alloys.The focus is on interatomic potentials,including both conventional empirical potentials and newly developed machine learning potentials(MLPs).The fitting processes for different types of interatomic potentials applied to multi-component alloys are also discussed.Finally,the challenges and future perspectives in developing MLPs are thoroughly addressed.Overall,this review provides a valuable resource for researchers interested in developing optimized multicomponent alloys using atomic simulation techniques.

High-entropy ceramics:Present status,challenges,and a look forward

High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements.Although in the infant stage,the emerging of this new family of materials has brought new opportunities for material design and property tailoring.Distinct from metals,the diversity in crystal structure and electronic structure of ceramics provides huge space for properties tuning through band structure engineering and phonon engineering.Aside from strengthening,hardening,and low thermal conductivity that have already been found in high-entropy alloys,new properties like colossal dielectric constant,super ionic conductivity,severe anisotropic thermal expansion coefficient,strong electromagnetic wave absorption,etc.,have been discovered in HECs.As a response to the rapid development in this nascent field,this article gives a comprehensive review on the structure features,theoretical methods for stability and property prediction,processing routes,novel properties,and prospective applications of HECs.The challenges on processing,characterization,and property predictions are also emphasized.Finally,future directions for new material exploration,novel processing,fundamental understanding,in-depth characterization,and database assessments are given.

High porosity and low thermal conductivity high entropy(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2C

Porous ultra-high temperature ceramics(UHTCs)are promising for ultrahigh-temperature thermal insulation applications.However,the main limitations for their applications are the high thermal conductivity and densification of porous structure at high temperatures.In order to overcome these obstacles,herein,porous high entropy(Zr(0.2)Hf(0.2)Ti(0.2)Nb(0.2)Ta(0.2))C was prepared by a simple method combing in-situ reaction and partial sintering.Porous high entropy(Zr(0.2)Hf(0.2)Ti(0.2)Nb(0.2)Ta(0.2))C possesses homogeneous microstructure with grain size in the range of 100–500 nm and pore size in the range of 0.2–1μm,which exhibits high porosity of 80.99%,high compressive strength of 3.45 MPa,low room temperature thermal conductivity of 0.39 W·m^-1K^-1,low thermal diffusivity of 0.74 mm^2·s^-1and good high temperature stability.The combination of these properties renders porous high entropy(Zr(0.2)Hf(0.2)Ti(0.2)Nb(0.2)Ta(0.2))Cpromising as light-weight ultrahigh temperature thermal insulation materials.

Porous high entropy(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2: A novel strategy towards making ultrahigh temperature ceramics thermal insulating

Transition metal diborides based ultrahigh temperature ceramics(UHTCs) are characterized by high melting point, high strength and hardness, and high electrical and thermal conductivity. The high thermal conductivity arises from both electronic and phonon contributions. Thus electronic and phonon contributions must be controlled simultaneously in reducing the thermal conductivity of transition metal diborides. In high entropy(HE) materials, both electrons and phonons are scattered such that the thermal conductivity can significantly be reduced, which opens a new window to design novel insulating materials. Inspired by the high entropy effect, porous HE(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2 is designed in this work as a new thermal insulting ultrahigh temperature material and is synthesized by an in-situ thermal borocarbon reduction/partial sintering process. The porous HE(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2 possesses high porosity of 75.67%, pore size of 0.3–1.2 μm, homogeneous microstructure with small grain size of 400–800 nm, which results in low room temperature thermal diffusivity and thermal conductivity of 0.74 mm2 s^-1 and 0.51 W m^-1K^-1, respectively. In addition, it exhibits high compressive strength of3.93 MPa. The combination of these properties indicates that exploring porous high entropy ceramics such as porous HE(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2 is a novel strategy in making UHTCs thermal insulating.

(Y0.25Yb0.25Er0.25Lu0.25)2(Zr0.5Hf0.5)2O7: A defective fluorite structured high entropy ceramic with low thermal conductivity and close thermal expansion coefficient to Al2O3

Al2O3f/Al2O3 ceramic matrix composites(CMC)are promising candidate materials of blades and combustor liners of future gas turbines in light of their higher temperature capability,higher environmental stability and oxidizing-free capacity[1–3].Nevertheless,grain growth,sintering and creep deformation at high operation temperatures are still serious problems for Al2O3f/Al2O3 ceramic matrix composites,which can lead to a reduction in the strength and damage tolerance[2].Moreover,Al2O3 can be corroded by the high temperature water vapor in combustion environments and yields volatile products,such as Al(OH)3[4].Consequently,environmental barrier coatings(EBCs)are necessary for Al2O3f/Al2O3 ceramic matrix composites,which can protect Al2O3f/Al2O3 CMC from high temperature and flowing combustion gas corrosion and thus increase the high temperature capability and the service life of components.

(La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4: A high-entropy rare-earth phosphate monazite ceramic with low thermal conductivity and good compatibility with Al2O3

Low thermal conductivity, matched thermal expansion coefficient and good compatibility are general requirements for the environmental/thermal barrier coatings(EBCs/TBCs) and interphases for Al2O3 f/Al2O3 composites. In this work, a novel high-entropy(HE) rare-earth phosphate monazite ceramic (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 is designed and successfully synthesized. This new type of HE rare-earth phosphate monazite exhibits good chemical compatibility with Al2O3, without reaction with Al2O3 as high as 1600℃ in air. Moreover, the thermal expansion coefficient(TEC) of HE (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4(8.9 × 10^-6/℃ at 300–1000℃) is close to that of Al2O3. The thermal conductivity of HE (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 at room temperature is as low as 2.08 W·m^-1·K^-1, which is about 42% lower than that of La PO4. Good chemical compatibility, close TEC to that of Al2O3, and low thermal conductivity indicate that HE (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 is suitable as a candidate EBC/TBC material and an interphase for Al2O3 f/Al2O3 composites.

High entropy(Yb0.25Y0.25Lu0.25Er0.25)2SiO5 with strong anisotropy in thermal expansion

A novel high entropy(HE) rare earth monosilicate(Yb0.25Y0.25Lu0.25Er0.252 SiO5 was synthesized by solid-state reaction method.X-ray diffraction and scanning electron microscopy analysis indicate that a single solid solution is formed with homogeneous distribution of rare-earth elements.HE(Yb0.25Y0.25Lu0.255 Er0.252 SiO5 exhibits excellent phase stability and anisotropy in thermal expansion.The coefficients of thermal expansion(CTEs) in three crystallographic directions are:αa=(2.57±0.07)×10^-6 K^-1,αb=(8.07±0.13)×10^-6 K^-1,αc=(9.98±0.10)×10^-6 K^-1.The strong anisotropy in thermal expansion is favorable in minimizing the coating/substrate mismatch if preferred orientation of HE(Yb0.25Y0.25Lu0.25Er0.252 SiO5 is controlled on either metal or ceramic substrate.

High entropy defective fluorite structured rare-earth niobates and tantalates for thermal barrier applications

Rare-earth tantalates and niobates(REjTaO7 and REjNbO7)have been considered as promising candidate thermal barrier coating(TBC)materials in next generation gas-turbine engines due to their ultra-low thermal conductivity and better thermal stability than yttria-stabilized zirconia(YSZ).However,the low Vickers hardness and toughness are the main shortcomings of RE;TaO-and REjNbOr that limit their applications as TBC materials.To increase the hardness,high entropy(Yu3Ybu3Er/3)sTaOr,(Y13YbnErns)NbO,and(Sm1/6Eu1/6Y 1/6Yb1/6Lu1/6Er1/6)3(Nb1/2Ta1/2)O7 are designed and synthesized in this study.These high entropy ceramics exhibit high Vickers hardness(10.912.0 GPa),close thermal expansion coefficients to that of single-principal-component RE3TaO,and RE;NbO,(7.9×10^-6-10.8×10-6 C-1 at room temperature),good phase stability,and good chemical compatibility with thermally grown Al2O3,which make them promising for applications as candidate TBC materials.

(TiZrHf)P2O7: An equimolar multicomponent or high entropy ceramic with good thermal stability and low thermal conductivity

Zr P2O7 is a promising material for high temperature insulating applications. However, decomposition above 1400℃ is the bottleneck that limiting its application at high temperatures. To improve the thermal stability, a novel multicomponent equimolar solid solution(Ti Zr Hf)P2O7 was designed and successfully synthesized in this work inspired by high-entropy ceramic(HEC) concept. The as-synthesized(Ti Zr Hf)P2O7 exhibits good thermal stability, which is not decomposed after heating at 1550℃ for 3 h. It also shows lower thermal conductivity(0.78 W m^-1 K^-1) compared to the constituting metal pyrophosphates Ti P2O7, Zr P2O7 and Hf P2O7. The combination of high thermal stability and low thermal conductivity renders(Ti Zr Hf)P2O7 promising for high temperature thermal insulating applications.

(La0.2Ce0.2Nd0.2Sm0.2Eu0.2)2Zr2O7:A novel high-entropy ceramic with low thermal conductivity and sluggish grain growth rate

Fine grains and slow grain growth rate are beneficial to preventing the thermal stress-induced cracking and thermal conductivity increase of thermal barrier coatings.Inspired by the sluggish diffusion effect of high-entropy materials,a novel high-entropy(HE)rare-earth zirconate solid solution(La0.2Ce0.2Nd0.2Sm0.2Eu0.2)2 Zr2 O7 was designed and successfully synthesized in this work.The as-synthesized(La0.2Ce0.2Nd0.2Sm0.2Eu0.2)2 Zr2 O7 is phase-pure with homogeneous rare-earth element distribution.The thermal conductivity of as-synthesized(La0.2Ce0.2Nd0.2Sm0.2Eu0.2)2 Zr2 O7 at room temperature is as low as 0.76 W m^-1 K^-1.Moreover,after being heated at 1500℃for 1-18 h,the average grain size of(La0.2Ce0.2Nd0.2Sm0.2Eu0.2)2 Zr2 O7 only increases from 1.69μm to 3.92μm,while the average grain size of La2Zr2O7 increases from 1.96μm to 8.89μm.Low thermal conductivity and sluggish grain growth rate indicate that high-entropy(La0.2Ce0.2Nd0.2Sm0.2Eu0.2)2Zr2O7 is suitable for application as a thermal barrier coating material and it may possess good thermal stress-induced cracking resistance.

High entropy(Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12:A novel high temperature stable thermal barrier material

Ytterbium aluminum garnet(Yb3Al5O12)is considered as a promising thermal barrier material.However,the main limitations of Yb3Al5O12 for thermal barrier applications are relative low thermal expansion coefficient and high thermal conductivity.In order to overcome these obstacles,herein,a new high entropy(Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12 ceramic was designed,and then powders and bulk were prepared through solid-state reaction method and spark plasma sintering(SPS),respectively.The thermal expansion coefficient of HE(Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12 is(8.54±0.29)×10^-6 K^-1 at 673 K–1273 K,which is about 9%higher than that of Yb3Al5O12.The thermal conductivity of HE(Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12 ceramic is 3.81 W·m^-1 K^-1 at 300 K,which is about 18%lower than that of Yb3Al5O12.Moreover,there is no reaction between HE(Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12 and thermally grown(TG)Al2O3 even at 1600℃.After annealing at 1590℃for 18 h,the average grain size of HE(Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12 increases only from 1.56μm to 2.27μm.Close thermal expansion coefficient to TG Al2O3,low thermal conductivity,good phase stability,excellent chemical compatibility with TG Al2O3 and slow grain growth rate make HE(Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12 promising for thermal barrier applications.

Preparation and properties of CMAS resistant bixbyite structured high-entropy oxides RE_(2)O_(3)(RE=Sm,Eu,Er,Lu,Y,and Yb):Promising environmental barrier coating materials for Al_(2)O_(3)f/Al_(2)O_(3) composites

Y_(2)O_(3) is regarded as one of the potential environmental barrier coating(EBC)materials for Al_(2)O_(3)f/Al_(2)O_(3)ceramic matrix composites owing to its high melting point and close thermal expansion coefficient to Al_(2)O_(3).However,the relatively high thermal conductivity and unsatisfactory calcium-magnesium-aluminosilicate(CMAS)resistance are the main obstacles for the practical application of Y_(2)O_(3).In order to reduce the thermal conductivity and increase the CMAS resistance,four cubic bixbyite structured high-entropy oxides RE_(2)O_(3),including(Eu_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3),(Sm_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3),(Sm_(0.2)Eu_(0.2)Er_(0.2)Y_(0.2)Yb_(0.2))2O_(3),and(Sm_(0.2)Eu_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3)were designed and synthesized,among which(Eu_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3)and(Sm_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3)bulks were prepared by spark plasma sintering(SPS)to investigate their mechanical and thermal properties as well as CMAS resistance.The mechanical properties of(Eu_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3)and(Sm_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3) are close to those of Y_(2)O_(3) but become more brittle than Y_(2)O_(3).The thermal conductivities of(Eu_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb_(0.2))2O_(3) and(Sm_(0.2)Er_(0.2)Lu_(0.2)Y_(0.2)Yb02)2O_(3)(5.1 and 4.6 W·m^(-1)·K^(-1))are only 23.8%and 21.5%respectively of that of Y_(2)O_(3)(21.4 W·m^(-1)·K^(-1)),while their thermal expansion coefficients are close to those of Y_(2)O_(3) and A12O_(3).Most importantly,HE RE_(2)O_(3) ceramics exhibit good CMAS resistance.After being attacked by CMAS at 1350℃for 4 h,the HE RE_(2)O_(3) ceramics maintain their original morphologies without forming pores or cracks,making them promising as EBC materials for Al_(2)O_(3)f/Al_(2)O_(3) composites.

Effect of reaction routes on the porosity and permeability of porous high entropy(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6 for transpiration cooling

Transpiration cooling technique is a reusable and high-efficiency thermal protection system(TPS),which is potential to improve the reusability and security of re-entry space vehicle.Relatively low density,high permeability and high porosity are general requirements for porous media of transpiration cooling systems.In this work,a new porous high entropy metal hexaboride(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6 is designed and prepared by the in-situ reaction/partial sintering method.Two reaction routes are designed to synthesize(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6,including boron thermal reduction and borocarbon thermal reduction.The as-prepared porous HE(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6 ceramics possess homogeneous microstructure and exhibit low density,high porosity,high compressive strength and high permeability.The combination of these properties makes porous HE(Y0.2Yb0.2Sm0.2Nd0.2Eu0.2)B6 promising as a candidate porous media for various transpiration cooling applications.

High-entropy(Nd_(0.2)Sm_(0.2)Eu_(0.2)Y_(0.2)Yb_(0.2))_(4)Al_(2)O_(9) with good high temperature stability,low thermal conductivity,and anisotropic thermal expansivity

The critical requirements for the environmental barrier coating(EBC)materials of silicon-based ceramic matrix composites(CMCs)include good tolerance to harsh environments,thermal expansion matches with the interlayer mullite,good high-temperature phase stability,and low thermal conductivity.Cuspidine-structured rare-earth aluminates RE_(4)Al_(2)O_(9) have been considered as candidates of EBCs for their superior mechanical and thermal properties,but the phase transition at high temperatures is a notable drawback of these materials.To suppress the phase transition and improve the phase stability,a novel cuspidine-structured rare-earth aluminate solid solution(Nd_(0.2)Sm_(0.2)Eu_(0.2)Y_(0.2)Yb_(0.2))_(4)Al_(2)O_(9) was designed and successfully synthesized inspired by entropy stabilization effect of high-entropy ceramics(HECs).The as-synthesized HE(Nd_(0.2)Sm_(0.2)Eu_(0.2)Y_(0.2)Yb_(0.2))_(4)Al_(2)O_(9) exhibits a close thermal expansion coefficient(6.96×10^(-6) K^(-1) at 300-1473 K)to that of mullite,good phase stability from 300 to 1473 K,and low thermal conductivity(1.50 W·m^(-1)·K^(-1) at room temperature).In addition,strong anisotropic thermal expansion has been observed compared to Y_(4)Al_(2)O_(9) and Yb_(4)Al_(2)O_(9).The mechanism for low thermal conductivity is attributed to the lattice distortion and mass difference of the constituent atoms,and the anisotropic thermal expansion is due to the anisotropic chemical bonding enhanced by the large size rare-earth cations.

Theoretical prediction on thermal and mechanical properties of high entropy(Zr(0.2)Hf(0.2)Ti(0.2)Nb(0.2)Ta(0.2))C by deep learning potential

High entropy materials(HEMs, e.g. high entropy alloys, high entropy ceramics) have gained increasing interests due to the possibility that they can provide challenge properties unattainable by traditional materials. Though a large number of HEMs have emerged, there is still in lack of theoretical predictions and simulations on HEMs, which is probably caused by the chemical complexity of HEMs. In this work,we demonstrate that the machine learning potentials developed in recent years can overcome the complexity of HEMs, and serve as powerful theoretical tools to simulate HEMs. A deep learning potential(DLP) for high entropy(Zr(0.2) Hf(0.2) Ti(0.2) Nb(0.2) Ta(0.2))C is fitted with the prediction error in energy and force being 9.4 me V/atom and 217 me V/?, respectively. The reliability and generality of the DLP are affirmed,since it can accurately predict lattice parameters and elastic constants of mono-phase carbides TMC(TM = Ti, Zr, Hf, Nb and Ta). Lattice constants(increase from 4.5707 ? to 4.6727 ?), thermal expansion coefficients(increase from 7.85×10-6 K^(-1) to 10.58×10-6 K^(-1)), phonon thermal conductivities(decrease from 2.02 W·m-1·K^(-1) to 0.95 W·m-1·K^(-1)), and elastic properties of high entropy(Zr(0.2) Hf(0.2) Ti(0.2) Nb(0.2) Ta(0.2))C in temperature ranging from 0°C to 2400°C are predicted by molecular dynamics simulations. The predicted room temperature properties agree well with experimental measurements, indicating the high accuracy of the DLP. With introducing of machine learning potentials, many problems that are intractable by traditional methods can be handled now. It is hopeful that deep insight into HEMs can be obtained in the future by such powerful methods.

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 REB03)composite powders

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,(Ce0.2Hu0.2Sm0.2Er0.2Yb0.2)B6,(Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6,(Ce0.2Ya2Sm0.2Eu0.2Yb0.2)B6,and(Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6 possess CsCl-type cubic crystal structure,and their theoretical densities range from 4.84 to 5.25 g/cm^(3).(Ce02Y0.2Sm0.2Er0.2Yb02)B6 powders with the average particle size of 1.86 jim 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 REB03(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/HEREB03 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.

Achieving strong microwave absorption capability and wide absorption bandwidth through a combination of high entropy rare earth silicide carbides/rare earth oxides

Developing electromagnetic(EM) wave absorbing materials with low reflection coefficient and optimal operating frequency band is urgently needed on account of the increasingly serious EM pollution. However, the applications of common EM absorbing materials are encumbered by poor high-temperature stability, poor oxidation resistance, narrow absorption bandwidth or high density. Herein, the strong EM absorption capability and wide efficient absorption bandwidth of high entropy ceramics are reported for the first time, which are designed by a combination of the novel high entropy(HE) rare earth silicide carbides/rare earth oxides(RE3 Si2 C2/RE2 O3). Three HE powders, i.e., HERSC-1(HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2),HERSC-2 HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2/HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)2 O3) and HERSC-3(HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)3 Si2 C2/HE(Tm0.2 Y0.2 Dy0.2 Gd0.2 Tb0.2)2 O3), are synthesized. Although HERSC-1 exhibits a limited absorption effect(the minimum reflection loss(RLmin) is-11.6 d B at 3.4 mm) and a relatively narrow effective absorption bandwidth(EAB) of 1.7 GHz, the optimal absorption RLminvalue and EAB of HERSC-2 and HERSC-3 are-40.7 d B(at 2.9 mm), 3.4 GHz and-50.9 d B(at 2.0 mm), 4.5 GHz,respectively, demonstrating strong microwave absorption capability and wide absorption bandwidth.Considering the better stability, low density and strong EM absorption effect, HE ceramics are promising as a new type of EM absorbing materials.