ZrC coatings were deposited on graphite substrates by low pressure chemical vapor deposition(LPCVD) with the Br_2-Zr-C_3H_6-H_2-Ar system. The effects of deposition time on the microstructures and growth behavior of Z...ZrC coatings were deposited on graphite substrates by low pressure chemical vapor deposition(LPCVD) with the Br_2-Zr-C_3H_6-H_2-Ar system. The effects of deposition time on the microstructures and growth behavior of ZrC coatings were investigated. ZrC coating grew in an island-layer mode. The formation of coating was dominated by the nucleation of ZrC in the initial 20 minutes, and the rapid nucleation generated a fine-grained structure of ZrC coating. When the deposition time was over 30 min, the growth of coating was dominated by that of crystals, giving a column-arranged structure. Energy dispersive X-ray spectroscopy showed that the molar ratio of carbon to zirconium was near 1:1 in ZrC coating, and X-ray photoelectron spectroscopy showed that ZrC was the main phase in coatings, accompanied by about 2.5mol% ZrO_2 minor phase.展开更多
The simulation on pyrolysis products of pure PF resin with different chemical structure was investigated and validated by pyrolysis gas-chromatography mass spectrometry(Py-GC/MS).The simulation of pyrolysis products o...The simulation on pyrolysis products of pure PF resin with different chemical structure was investigated and validated by pyrolysis gas-chromatography mass spectrometry(Py-GC/MS).The simulation of pyrolysis products of phenolic resin with different chemical structure was investigated by AMBER(Assisted Model Building with Energy Refinement)force field.The content of pyrolysis products phenol and cresol decreases with the increase of F/P(formaldehyde/phenol)value.The content of pyrolysis products dimethylphenol and trimethylphenol increases with the enhancement of F/P value.The crosslink density of phenolic mixture can be measured by the content of pyrolysis products dimethylphenol and trimethylphenol.Consequently,the results of simulation were validated by the Py-GC/MS experiment.展开更多
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,...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.展开更多
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 ...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.展开更多
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 applica...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.展开更多
High-entropy carbides are a nascent group of ceramics that are promising for high-temperature applications due to the combination of good stability,high hardness(H),high strength,and superior creep resistance that the...High-entropy carbides are a nascent group of ceramics that are promising for high-temperature applications due to the combination of good stability,high hardness(H),high strength,and superior creep resistance that they display.Due to high melting points and low lattice diffusion coefficients,however,the high-entropy carbides are usually difficult to consolidate to a nearly full density.To cope with this challenge,herein,binary carbides including TiC,V_(8)C_(7),NbC,Mo_(2)C,and WC with different carbon stoichiometry were used to prepare dense high-entropy(TiVNbMoW)C_(4.375),and the influence of carbon vacancy on formation ability and mechanical properties of carbon-deficient high-entropy(TiVNbMoW)C_(4.375) were investigated.Intriguingly,although the starting binary carbides have different crystal structures and carbon stoichiometry,the as-prepared high-entropy material showed a rock-salt structure with a relatively high density(98.1%)and good mechanical properties with hardness of 19.4±0.4 GPa and fracture toughness(KIC)of 4.02 MPa·m^(1/2).More importantly,the high-entropy(TiVNbMoW)C_(4.375) exhibited low coefficient of friction(COF)at room temperature(RT)and 800℃.Wear rate(W)gradually increased with the temperature rising,which were attributed to the formation of low-hardness oxidation films at high temperatures to aggravate wear.At 800℃,lubricating films formed from sufficient oxidation products of V_(2)O_(5) and MoO_(3) effectively improved tribological behavior of the high-entropy(TiVNbMoW)C_(4.375).Wear mechanisms were mainly abrasive wear resulting from grain pullout and brittle fracture as well as oxidation wear generated from high-temperature reactions.These results are useful as valuable guidance and reference to the synthesis of high-entropy ceramics(HECs)for sliding parts under high-temperature serving conditions.展开更多
Medium-or high-entropy materials have great potential for applications due to their diverse compo-sitions and unexpected physicochemical properties.Herein,a novel medium-entropy(TiVNb)_(2)AlC was synthesized via hot p...Medium-or high-entropy materials have great potential for applications due to their diverse compo-sitions and unexpected physicochemical properties.Herein,a novel medium-entropy(TiVNb)_(2)AlC was synthesized via hot pressing at 1400℃from three individual M_(2)AlC(M=Ti,V,Nb)MAX phases.The microstructure of(TiVNb)_(2)AlC was characterized from the microscale to the atomic scale by scanning electron microscope microscopy(SEM),scanning transmission electron microscopy(STEM),and energy dispersive spectroscopy(EDS).The results showed that Ti,V,and Nb atoms were fully solid-soluble in the M-sites of the M_(2)AlC MAX phase.Compared with three individual MAX phases,the thermal conduc-tivity of(TiVNb)_(2)AlC was reduced greatly in the temperature range of 293-1473 K,and its mechanical properties(including Young’s modulus,Vickers hardness,and bending strength)were all increased due to the solid solution strengthening and electronic mechanism.展开更多
The oxidation behaviors of graphite and ZrB_2-SiC modified graphite composite were investigated at 2100℃ in1X105 Pa air and 0.2X105 Pa O2. The oxidation tests were conducted in an induction heating furnace. The oxida...The oxidation behaviors of graphite and ZrB_2-SiC modified graphite composite were investigated at 2100℃ in1X105 Pa air and 0.2X105 Pa O2. The oxidation tests were conducted in an induction heating furnace. The oxidation of these two materials followed the linear rate law. The determined radius loss rates of graphite and C-ZrB_2-Si C at 2100℃ were 2.18X10-2and 1.05X10-2%/s in 1X105 Pa air, and 3.23X10 2 and 2.21X10 2%/s in 0.2X105 Pa O2, respectively. The incorporation of ZrB2 and SiC decreased remarkably the oxidation rate of graphite because the oxide scale formed on the sample surface during oxidation helps in reducing the exposed surface area of the underneath substrate. In two different atmospheres with the same oxygen partial pressure, both graphite and ZrB_2-SiC experienced more severe oxidation at 2100℃ in0.2X105 Pa O2than in 1X105 Pa air. The oxidation rate-controlling step for graphite and ZrB_2-SiC was proposed to be the inward diffusion of oxygen through the boundary layer and through the pores in the oxide scale, respectively. A model based on diffusion theory was established to discuss the effect of the total gas pressure on their oxidation behaviors.展开更多
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 densificat...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.展开更多
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. I...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.展开更多
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,...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.展开更多
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-e...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.展开更多
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 oxi...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.展开更多
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. T...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.展开更多
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 conductivi...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.展开更多
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 permeab...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.展开更多
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 ...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.展开更多
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 applica...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.展开更多
High entropy diborides are new categories of ultra-high temperature ceramics,which are believed promising candidates for applications in hypersonic vehicles.However,knowledge on high temperature thermal and mechanical...High entropy diborides are new categories of ultra-high temperature ceramics,which are believed promising candidates for applications in hypersonic vehicles.However,knowledge on high temperature thermal and mechanical properties of high entropy diborides is still lacking unit now.In this work,variations of thermal and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) with respect to temperature were predicted by molecular dynamics simulations.Firstly,a deep learning potential for Ti-Zr-Hf-Nb-Ta-B diboride system was fitted with its prediction error in energy and force respectively being 9.2 meV/atom and 208 meV/A,in comparison with first-principles calculations.Then,temperature dependent lattice constants,anisotropic thermal expansions,anisotropic phonon thermal conductivities,and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) from 0℃to 2400℃were evaluated,where the predicted room temperature values agree well with experimental measurements.In addition,intrinsic lattice distortions of(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) were analyzed by displacements of atoms from their ideal positions,which are in an order of 10^(-3) A and one order of magnitude smaller than those in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C.It indicates that lattice distortions in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) is not so severe as expected.With the new paradigm of machine learning potential,deep insight into high entropy materials can be achieved in the future,since the chemical and structural complexly in high entropy materials can be well handled by machine learning potential.展开更多
Researching for interphase materials that can protect SiC fibers from oxygen and water vapor attacks has become one of the most important issues for the applications of SiC_f/SiC composites in high-temperature combust...Researching for interphase materials that can protect SiC fibers from oxygen and water vapor attacks has become one of the most important issues for the applications of SiC_f/SiC composites in high-temperature combustion environment. However, such kinds of interphase materials are not available yet. Herein,we report theoretically predicted properties of two promising interphase materials Y_5Si_3 C and Y_3Si_2C_2.Although crystallizing in different structures, they share the common features of layered structure,anisotropic chemical bonding, anisotropic electrical and mechanical properties, and low shear deformation resistance. The bulk moduli for Y_5Si_3C and Y_3Si_2C_2 are 78 and 93 GPa, respectively; while their shear moduli are 52 and 50GPa, respectively. The maximum to minimum Young's modulus ratios are1.44 for Y_5Si_3C and 3.27 for Y_3Si_2C_2. Based on the low shear deformation resistance and low Pugh's ratios(G/B = 0.666 forY_5Si_3C and 0.537 for Y_3Si_2C_2; G: shear modulus; B: bulk modulus), they are predicted as damage tolerant and soft ceramics with predicted Vickers hardness of 9.6 and 6.9 GPa, respectively.The cleavage plane and possible slip systems are(000 l) and(0001)[1120] and(1010)[0001] forY_5Si_3C,and those for Y_3Si_2C_2 are {h00} and(010)[101]. Since the oxidation products are water-vapor resistant Y2 Si2 O7, Y2 SiO5 and/or Y_2 O_3 upon oxidation, and the volume expansions are ca 140% and ca 26% for Y_5Si_3C and Y_3Si_2C_2, they are expected to seal the interfacial cracks in SiC_f/SiC composites. The unique combination of easy cleavage, low shear deformation resistance, volume expansions upon oxidation, and the resistance of the oxidation products to water vapor attack warrant them promising as interphase materials of SiC_f/SiC composites for water-vapor laden environment applications.展开更多
基金Founded by the National Natural Science Foundation of China(No.91216302)the National Program on Key Basic Research Project of the People's Republic of China(No.2015CB655200)
文摘ZrC coatings were deposited on graphite substrates by low pressure chemical vapor deposition(LPCVD) with the Br_2-Zr-C_3H_6-H_2-Ar system. The effects of deposition time on the microstructures and growth behavior of ZrC coatings were investigated. ZrC coating grew in an island-layer mode. The formation of coating was dominated by the nucleation of ZrC in the initial 20 minutes, and the rapid nucleation generated a fine-grained structure of ZrC coating. When the deposition time was over 30 min, the growth of coating was dominated by that of crystals, giving a column-arranged structure. Energy dispersive X-ray spectroscopy showed that the molar ratio of carbon to zirconium was near 1:1 in ZrC coating, and X-ray photoelectron spectroscopy showed that ZrC was the main phase in coatings, accompanied by about 2.5mol% ZrO_2 minor phase.
文摘The simulation on pyrolysis products of pure PF resin with different chemical structure was investigated and validated by pyrolysis gas-chromatography mass spectrometry(Py-GC/MS).The simulation of pyrolysis products of phenolic resin with different chemical structure was investigated by AMBER(Assisted Model Building with Energy Refinement)force field.The content of pyrolysis products phenol and cresol decreases with the increase of F/P(formaldehyde/phenol)value.The content of pyrolysis products dimethylphenol and trimethylphenol increases with the enhancement of F/P value.The crosslink density of phenolic mixture can be measured by the content of pyrolysis products dimethylphenol and trimethylphenol.Consequently,the results of simulation were validated by the Py-GC/MS experiment.
基金the National Natural Science Foundation of China under Grant Nos.51972089,51672064,U1435206.
文摘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.
基金This work was financially supported by the National Natural Science Foundation of China under Grant Nos.51972089,51672064,and U1637210.
文摘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.
基金This work was financially supported by the National Natu-ral Science Foundation of China(Nos.51972082,51972089,and 52172041)the Key Program of National Natural Science Foun-dation of China(No.52032003).
文摘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.
基金supported by the National Natural Science Foundation of China (51902317)the Key Research Program of the Chinese Academy of Sciences (XDPB24)+1 种基金the State Key Laboratory of New Ceramic and Fine Processing Tsinghua University (KF201911)the Key Research and Development Project of Gansu Province (20YF8GA001).
文摘High-entropy carbides are a nascent group of ceramics that are promising for high-temperature applications due to the combination of good stability,high hardness(H),high strength,and superior creep resistance that they display.Due to high melting points and low lattice diffusion coefficients,however,the high-entropy carbides are usually difficult to consolidate to a nearly full density.To cope with this challenge,herein,binary carbides including TiC,V_(8)C_(7),NbC,Mo_(2)C,and WC with different carbon stoichiometry were used to prepare dense high-entropy(TiVNbMoW)C_(4.375),and the influence of carbon vacancy on formation ability and mechanical properties of carbon-deficient high-entropy(TiVNbMoW)C_(4.375) were investigated.Intriguingly,although the starting binary carbides have different crystal structures and carbon stoichiometry,the as-prepared high-entropy material showed a rock-salt structure with a relatively high density(98.1%)and good mechanical properties with hardness of 19.4±0.4 GPa and fracture toughness(KIC)of 4.02 MPa·m^(1/2).More importantly,the high-entropy(TiVNbMoW)C_(4.375) exhibited low coefficient of friction(COF)at room temperature(RT)and 800℃.Wear rate(W)gradually increased with the temperature rising,which were attributed to the formation of low-hardness oxidation films at high temperatures to aggravate wear.At 800℃,lubricating films formed from sufficient oxidation products of V_(2)O_(5) and MoO_(3) effectively improved tribological behavior of the high-entropy(TiVNbMoW)C_(4.375).Wear mechanisms were mainly abrasive wear resulting from grain pullout and brittle fracture as well as oxidation wear generated from high-temperature reactions.These results are useful as valuable guidance and reference to the synthesis of high-entropy ceramics(HECs)for sliding parts under high-temperature serving conditions.
基金This work was supported by the National Natural Science Foun-dation of China(Grant No.52071318)and the Fund of Science and Technology on Advanced Functional Composites Laboratory(Grant No.6142906210305).
文摘Medium-or high-entropy materials have great potential for applications due to their diverse compo-sitions and unexpected physicochemical properties.Herein,a novel medium-entropy(TiVNb)_(2)AlC was synthesized via hot pressing at 1400℃from three individual M_(2)AlC(M=Ti,V,Nb)MAX phases.The microstructure of(TiVNb)_(2)AlC was characterized from the microscale to the atomic scale by scanning electron microscope microscopy(SEM),scanning transmission electron microscopy(STEM),and energy dispersive spectroscopy(EDS).The results showed that Ti,V,and Nb atoms were fully solid-soluble in the M-sites of the M_(2)AlC MAX phase.Compared with three individual MAX phases,the thermal conduc-tivity of(TiVNb)_(2)AlC was reduced greatly in the temperature range of 293-1473 K,and its mechanical properties(including Young’s modulus,Vickers hardness,and bending strength)were all increased due to the solid solution strengthening and electronic mechanism.
基金supported by the National Scientific Instrument and Equipment Development Project (No. 2011YQ14014504)
文摘The oxidation behaviors of graphite and ZrB_2-SiC modified graphite composite were investigated at 2100℃ in1X105 Pa air and 0.2X105 Pa O2. The oxidation tests were conducted in an induction heating furnace. The oxidation of these two materials followed the linear rate law. The determined radius loss rates of graphite and C-ZrB_2-Si C at 2100℃ were 2.18X10-2and 1.05X10-2%/s in 1X105 Pa air, and 3.23X10 2 and 2.21X10 2%/s in 0.2X105 Pa O2, respectively. The incorporation of ZrB2 and SiC decreased remarkably the oxidation rate of graphite because the oxide scale formed on the sample surface during oxidation helps in reducing the exposed surface area of the underneath substrate. In two different atmospheres with the same oxygen partial pressure, both graphite and ZrB_2-SiC experienced more severe oxidation at 2100℃ in0.2X105 Pa O2than in 1X105 Pa air. The oxidation rate-controlling step for graphite and ZrB_2-SiC was proposed to be the inward diffusion of oxygen through the boundary layer and through the pores in the oxide scale, respectively. A model based on diffusion theory was established to discuss the effect of the total gas pressure on their oxidation behaviors.
基金supported by the National Natural Science Foundation of China under Grant Nos. U1435206 and 51672064Beijing Municipal Science & Technology Commission under Grant No. D161100002416001
文摘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.
基金financially supported by the National Natural Science Foundation of China (Nos. 51672064 and U1435206)
文摘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.
基金financially supported by the National Natural Science Foundation of China (Nos. 51672064 and U1435206)
文摘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.
基金financially supported by the National Natural Science Foundation of China (Nos.51672064 and U1435206)
文摘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.
基金financial supported by the National Natural Science Foundation of China(Nos.51672064 and U1435206).
文摘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.
基金supported financially by the National Natural Science Foundation of China(Nos.51672064 and No.U1435206)。
文摘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.
基金This study was financially supported by the National Natural Science Foundation of China(Nos.51672064 and 51972089).
文摘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.
基金financial supported by the National Natural Science Foundation of China(Nos.51672064 and U1435206).
文摘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.
基金financial support from the National Natural Science Foundation of China(Grant Nos.51672064 and 51972089).
文摘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.
基金financially supported by the National Natural Science Foundation of China(Nos.51672064 and 51972089)。
文摘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.
基金supported by Natural Sciences Foundation of China under Grant No.51972089 and No.51672064。
文摘High entropy diborides are new categories of ultra-high temperature ceramics,which are believed promising candidates for applications in hypersonic vehicles.However,knowledge on high temperature thermal and mechanical properties of high entropy diborides is still lacking unit now.In this work,variations of thermal and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) with respect to temperature were predicted by molecular dynamics simulations.Firstly,a deep learning potential for Ti-Zr-Hf-Nb-Ta-B diboride system was fitted with its prediction error in energy and force respectively being 9.2 meV/atom and 208 meV/A,in comparison with first-principles calculations.Then,temperature dependent lattice constants,anisotropic thermal expansions,anisotropic phonon thermal conductivities,and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) from 0℃to 2400℃were evaluated,where the predicted room temperature values agree well with experimental measurements.In addition,intrinsic lattice distortions of(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) were analyzed by displacements of atoms from their ideal positions,which are in an order of 10^(-3) A and one order of magnitude smaller than those in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C.It indicates that lattice distortions in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) is not so severe as expected.With the new paradigm of machine learning potential,deep insight into high entropy materials can be achieved in the future,since the chemical and structural complexly in high entropy materials can be well handled by machine learning potential.
基金supported financially by the National Natural Science Foundation of China (Nos. U1435206 and 51672064)the Beijing Municipal Science & Technology Commission (No. D161100002416001)
文摘Researching for interphase materials that can protect SiC fibers from oxygen and water vapor attacks has become one of the most important issues for the applications of SiC_f/SiC composites in high-temperature combustion environment. However, such kinds of interphase materials are not available yet. Herein,we report theoretically predicted properties of two promising interphase materials Y_5Si_3 C and Y_3Si_2C_2.Although crystallizing in different structures, they share the common features of layered structure,anisotropic chemical bonding, anisotropic electrical and mechanical properties, and low shear deformation resistance. The bulk moduli for Y_5Si_3C and Y_3Si_2C_2 are 78 and 93 GPa, respectively; while their shear moduli are 52 and 50GPa, respectively. The maximum to minimum Young's modulus ratios are1.44 for Y_5Si_3C and 3.27 for Y_3Si_2C_2. Based on the low shear deformation resistance and low Pugh's ratios(G/B = 0.666 forY_5Si_3C and 0.537 for Y_3Si_2C_2; G: shear modulus; B: bulk modulus), they are predicted as damage tolerant and soft ceramics with predicted Vickers hardness of 9.6 and 6.9 GPa, respectively.The cleavage plane and possible slip systems are(000 l) and(0001)[1120] and(1010)[0001] forY_5Si_3C,and those for Y_3Si_2C_2 are {h00} and(010)[101]. Since the oxidation products are water-vapor resistant Y2 Si2 O7, Y2 SiO5 and/or Y_2 O_3 upon oxidation, and the volume expansions are ca 140% and ca 26% for Y_5Si_3C and Y_3Si_2C_2, they are expected to seal the interfacial cracks in SiC_f/SiC composites. The unique combination of easy cleavage, low shear deformation resistance, volume expansions upon oxidation, and the resistance of the oxidation products to water vapor attack warrant them promising as interphase materials of SiC_f/SiC composites for water-vapor laden environment applications.
