Anisotropic thermal expansion in high-entropy multicomponent AlB_(2)-type diboride solid solutions

High-entropy(HE)ultra-high temperature ceramics have the chance to pave the way for future applications propelling technology advantages in the fields of energy conversion and extreme environmental shielding.Among others,HE diborides stand out owing to their intrinsic anisotropic layered structure and ability to withstand ultra-high temperatures.Herein,we employed in-situ high-resolution synchrotron diffraction over a plethora of multicomponent compositions,with four to seven transition metals,with the intent of understanding the thermal lattice expansion following different composition or synthesis process.As a result,we were able to control the average thermal expansion(TE)from 1.3×10^(−6)to 6.9×10^(−6)K^(−1)depending on the combination of metals,with a variation of in-plane to out-of-plane TE ratio ranging from 1.5 to 2.8.

Research Progress of Non-oxide and High Entropy Ceramic Coatings

Ceramic coatings play a keyrole in extending the service life of materials in aerospaceandenergy fields byprotectingmaterials from high temperature,oxidation,corrosion and thermal stress.Non-oxide and high entropy ceramics are new emerging coating materials which have been researched and developed in recent years.Compared with traditional oxide ceramics,non-oxide ceramics have better high temperature stability,oxidation resistance and erosion resistance.These characteristics make non-oxide ceramics perform well in extreme environments.It is particularly noteworthy that the non-oxide high entropy ceramic is a uniform solid solution composed of at least four or fiveatoms.Their unique structure and outstanding propertiesshow great potential application in the field of coating.In this paper,the researches aboutregulating microstructure,preparation technology and properties of nitride and its high entropy system,carbide and its high entropy system and boride and its high entropy system in coating field are summarized,and their future development and prospects are prospected.

Hierarchical porous(Ta_(0.2)Nb_(0.2)Ti_(0.2)Zr_(0.2)Hf_(0.2))C high-entropy ceramics prepared by a self-foaming method for thermal insulation

To meet the emerging demands for thermal protection materials for hypersonic aircraft,developing porous ultrahigh-temperature ceramics with both robust mechanical properties and superior thermal insulation performance is a critical challenge.Herein,we report novel porous(Ta_(0.2)Nb_(0.2)Ti_(0.2)Zr_(0.2)Hf_(0.2))C high-entropy carbide(PHEC)ceramics fabricated by a self-foaming method using commercially available metal chloride and furfuryl alcohol(FA)as precursors.The PHEC ceramics are constructed of microspheres with a size of 2µm,leading to a high porosity of 91.3%and an interconnected frame.These microspheres consist of high-entropy carbide grains(20 nm),resulting in abundant interfaces and nanosized pores in the PHEC ceramics.Due to its unique hierarchical structure,the prepared PHEC ceramics have outstanding compressive strength(28.1±2 MPa)and exceptionally low thermal conductivity(κ_(T),0.046 W·m^(-1)·K^(-1))at room temperature.This makes it a promising thermal insulation materials for ultrahigh temperature applications.This work provides a cost-effective and facile strategy for producing porous ultrahigh-temperature ceramics.

Cr-assisted low-temperature densification of(Ti,Zr,Nb,Ta,Mo)C high-entropy carbides with ultrafine grain and enhanced hardness

While the use of low-melting-point metals as sintering aids for high-entropy carbide(HEC)ceramics has been well established,their existence can compromise hardness due to residual metallic inclusions.This study demonstrates an innovative strategy to meet this challenge,where(Ti,Zr,Nb,Ta,Mo)C high-entropy carbide ceramics with ultrafine grains and enhanced hardness are obtained through chromium(Cr)-metal-assisted spark plasma sintering(SPS)at a temperature as low as 1600℃.The results show that the addition of 5 vol%Cr promotes the formation of highly densified single HEC phase ceramics with a high relative density(98.4%)and an ultrafine-grained microstructure(0.17μm).This low-temperature densification mechanism can be attributed to Cr’s solid-solution effect within the matrix and the increased carbon vacancies generated during sintering.The grain size of the(Ti,Zr,Nb,Ta,Mo)C ceramics with 5 vol%Cr metal addition is significantly smaller than that of Cr-free(Ti,Zr,Nb,Ta,Mo)C ceramics sintered at 2000℃(3.03μm)or via traditional low-temperature liquid-phase sintering(1.3–1.5μm).Importantly,the addition of 5 vol%Cr substantially increased the hardness of the ceramics,with a remarkable increase from 23.57 to 28.16 GPa compared to that of the pure(Ti,Zr,Nb,Ta,Mo)C ceramics,owing to the fine-grain strengthening and solid-solution strengthening mechanisms.This work highlights the uniqueness of Cr metal as a sintering aid in achieving densification and hardness improvements in(Ti,Zr,Nb,Ta,Mo)C ceramics,offering a promising strategy for improving the properties of HEC materials for further development in the near future.

Dense and pure high-entropy metal diboride ceramics sintered from self-synthesized powders via boro/carbothermal reduction approach

Equimolar quinary diboride powders,with nominal composition of(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2,were synthesized by boro/carbothermal reduction(BCTR)of oxide mixtures(MOx,M=Ti,Hf,Zr,Nb and Ta)using B4 C as source of B and C in vacuum.By adjusting the B4 C/MOxratios,diboride mixtures without detectable MOxwere obtained at 1600℃,while high-entropy diboride(HEB)powders with particle size of<1μm was obtained at 1800℃.The phase,morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated.Although X-ray diffraction pattern indicated the powders synthesized at 1800℃ were in a single-phase Al B2 structure,elemental mappings showed that(Ta,Ti)-rich and(Zr,Nb)-rich solid solution coexisted in the HEB powders.The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000°C.(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2 ceramics with a relative density of 97.9%were obtained after spark plasma sintering the HEB powders at 2050℃ under 50 MPa.Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050℃,and the averaged grain size increased from 6.67 to 41.2μm.HEB ceramics sintered at 2000℃ had a Vickers hardness of 22.44±0.56 GPa(under a load of 1 kg),a Young’s modulus of^500 GPa and a fracture toughness of 2.83±0.15 MPa m1/2.This is the first report for obtaining high density HEB ceramics without residual oxide phase,benefiting from the high quality HEB powders obtained.

High-entropy carbide ceramics: a perspective review

High-entropy carbide ceramics(HECCs)exhibited a series of property superiority,such as high hardness,high oxidation resistance and adjustable range of thermal conductivity,making them great candidates for structural materials used in extreme service conditions.However,current HECC-related reports can only provide limited guidance for the design of HECC materials with promising properties and stable structure or for their high-quality fabrication.To fill this gap,we proposed in the current review that integrated efforts should be taken in the following three aspects to advance the design and fabrication of high-performance HECCs.First,the rule for the single-phase formation needs to be theoretically identified using high-throughput density functional theory calculations(HTDFT)and a highly accurate predictive model for rapid compositional design warrants establishment through the combination of HTDFT and machine-learning studies.In parallel with the compositional design,the lack of theoretical foundation and guidance for the synthesis of highly dense and highly pure HECC materials necessitates extensive studies focusing on the principle for the synthesis of HECC pre-alloy powders and on the powder densification mechanisms during high-temperature sintering.Moreover,great attentions are to be paid to the mechanistic understandings on the mechanical,oxidation and thermal conduction behaviors of HECC materials;for example,the toughening and strengthening mechanisms of HECC materials could be elucidated through discerning the"HECC composition-stacking fault energy distribution/bonding state-dislocation behavior"correlations.

Thermodynamic analysis of metal segregation in dual phase high entropy ceramics

Equilibrium Gibbs'free energy calculations were used to determine metal segregation trends between boride and carbide solid solutions containing two metals that are relevant to dual phase high entropy ceramics.The model predicted that Ti had the strongest tendency to segregate to the boride phase followed by Zr,Nb,Mo,V,Hf,and Ta,which matches experimental results of measured compositions.The ratio of a metal in the carbide phase to the content of the same metal in the corresponding metal boride had a linear trend with the change in standard Gibbs'free energy of reaction for a metal carbide reacting with B_(4)C to produce its corresponding metal boride and carbon.The proposed model was used to predict the changes in standard Gibbs'free energy for CrC→CrB_(2) to be−260 kJ and WC→WB_(2) to be 148 kJ,which indicates that Cr has the strongest segregation to the boride and W has the strongest segregation to the carbide.The proposed model can be used to estimate the segregation of metals in dual phase high entropy boride-carbide ceramics of any boride/carbide ratio or metal content.

Relationship Between Electric Properties and Temperature of ZrB_2-SiC Composite Ceramic Heating Element

ZrB2 -SiC composite ceramic has been successfully introduced as heating element in super high temperature .field. This paper further investigated the microstructure of ZrB2 - SiC composite ceramic heating element and the relationship between electric properties and temperature. SEM photos show that the heating element consists of SiC grains and ZrBz grains smaller than 10 μm. The voltage and current gradually increase and the furnace tempera- ture rises lineally with heating time prolonging. The electric resistance increases linearly with the temperature rising. The service temperatltre of the heating element can reach 1 800 ℃ and 2 150 ℃ in air and argon at- mosphere, respectively.

热障涂层潜在高熵陶瓷材料的研究进展

提升高温腐蚀部件的隔热耐蚀性对提高石油化工设备的使用寿命有重要意义。热障涂层是提升高温部件隔热耐蚀性能的关键技术。现役热障涂层陶瓷层材料已达到高温服役极限,急需能够耐极限高温的陶瓷材料。作为近些年开发的高稳定材料,高熵陶瓷具有高温稳定性、低热导率等优点,是替换现有陶瓷涂层材料的潜在材料之一。综述了高熵碳化物陶瓷、高熵硼化物陶瓷、高熵氧化物陶瓷、高熵氮化物陶瓷以及高熵稀土系列陶瓷的研究现状。分别从制备方法以及相应的物理化学性质和晶体结构等方面介绍了各种高熵陶瓷的力学性能和物理化学性能。同时指出,对新型的高熵陶瓷材料,通过寻求更加合理的摩尔配比和精准的元素搭配来优化高熵系统对提升高熵陶瓷材料的耐高温性、高温稳定性和抗氧化性具有指导意义,有望作为未来新型热障涂层材料。最后展望了高熵陶瓷的发展方向。

超高温碳化物和硼化物陶瓷及碳纤维增强超高温陶瓷的研究进展

随着现代军事发展对空天飞行器和高超声速飞行器的迫切需求,急需发展抗热冲击性能和力学性能等良好的超高温陶瓷材料。然而,超高温陶瓷材料存在易脆、抗热冲击性能差等问题。针对于此,近年来通过将碳纤维引入到超高温陶瓷基体中,极大地提升了超高温陶瓷本身的断裂韧性和抗热冲击性能。本文首先介绍了超高温碳化物和硼化物陶瓷的性能,并讨论了引入第二相对其抗氧化性能和力学性能等的增强效果;其次介绍了碳纤维对超高温陶瓷材料的增强机制,并且在碳纤维引入后对超高温陶瓷的抗热冲击性能、抗氧化烧蚀性能、力学性能等方面的增强进行了细致讨论;最后,对未来碳纤维增强超高温陶瓷基复合材料的发展方向进行了展望。

非氧化物陶瓷纤维的研究进展

简述了非氧化物陶瓷纤维的分类及特点。综述了各类非氧化物陶瓷纤维的国内外研究现状及技术难点。其中,碳化物陶瓷纤维具有产业化优势,今后需大力发展低氧含量、近化学计量比、高结晶结构类陶瓷纤维;硼化物陶瓷纤维具有耐超高温、抗氧化性优异等独特优势,但存在烧结性差,断裂韧性偏低的不足,而采用先进的烧结技术(热压、闪电烧结和火花等离子体烧结)是一条可行的途径;氮化物陶瓷纤维具有结构功能一体化的优势,目前的研究仍处于实验室研究水平,规模化应用比较困难。展望了非氧化物陶瓷纤维未来的重点发展方向,认为低氧含量、近化学计量比、高结晶结构、良好的机械性能、结构功能一体化将是非氧化物陶瓷纤维未来的发展重点。

Frist-principles prediction of elastic, electronic,and thermodynamic properties of high entropy carbide ceramic (TiZrNbTa)C

High-entropy carbide ceramics (HECs) have drawn increasing attention as their excellent mechanical and thermal properties. In this work, the crystal stability,mechanical behavior, electronic and thermodynamic properties of (TiZrNbTa)C HEC are investigated by the first-principles calculations. Obtained results reveal that the disordered transition-metal (TM) atoms result in serious local lattice distortion within the crystal. The lattice distortion plays a key role for the structural stabilization,mechanical anisotropy and thermodynamic behaviors of(TiZrNbTa)C. Increasing pressure leads to decrease the lattice parameter, volume and brittleness, meanwhile increase the elastic constants, elastic moduli, mechanical anisotropy, sound velocity, and Debye temperature. It is also discovered that charge delocalization occurs with the increase in pressure. The mechanical stability and anisotropy of (TiZrNbTa)C are attributed primarily to TM-C bonding.

A new class of high-entropy M_(3)B_(4) borides

A new class of high-entropy M3B4 borides of the Ta_(3)B_(4)-prototyped orthorhombic structure has been synthesized in the bulk form for the first time.Specimens with compositions of(V0.2Cr0.2Nb0.2Mo0.2Ta0.2)3B4 and(V0.2Cr0.2Nb0.2Ta0.2W0.2)_(3)B_(4) were fabricated via reactive spark plasma sintering of high-energy-ball-milled elemental boron and metal precursors.The sintered specimens were〜98.7%in relative densities with virtually no oxide contamination,albeit the presence of minor(4-5 vol%)secondary high-entropy M5B6 phases.Despite that Mo_(3)B_(4) or W_(3)B_(4) are not stable phase,20%of M03B4 and W3B4 can be stabilized into the high-entropy M3B4 borides.Vickers hardness was measured to be 18.6 and 19.8 GPa at a standard load of 9.8 N.This work has further expanded the family of different structures of high-entropy ceramics reported to date.

Achieving ultra-broadband electromagnetic wave absorption in high-entropy transition metal carbides (HE TMCs)

Electronic devices pervade everyday life,which has triggered severe electromagnetic(EM)wave pollution.To face this challenge,developing EM wave absorbers with ultra-broadband absorption capacity is critically required.Currently,nano-composite construction has been widely utilized to realize impedance match and broadband absorption.However,complex experimental procedures,limited thermal stability,and interior oxidation resistance are still unneglectable issues.Therefore,it is appealing to realize ultra-broadband EM wave absorption in single-phase materials with good stability.Aiming at this target,two high-entropy transition metal carbides(HE TMCs)including(Zr,Hf,Nb,Ta)C(HE TMC-2)and(Cr,Zr,Hf,Nb,Ta)C(HE TMC-3)are designed and synthesized,of which the microwave absorption performance is investigated in comparison with previously reported(Ti,Zr,Hf,Nb,Ta)C(HE TMC-1).Due to the synergistic effects of dielectric and magnetic losses,HE TMC-2 and HE TMC-3 exhibit better impedance match and wider effective absorption bandwidth(EAB).In specific,the exclusion of Ti element in HE TMC-2 endows it optimal minimum reflection loss(RL_(min))and EAB of−41.7 dB(2.11 mm,10.52 GHz)and 3.5 GHz(at 3.0 mm),respectively.Remarkably,the incorporation of Cr element in HE TMC-3 significantly improves the impedance match,thus realizing EAB of 10.5,9.2,and 13.9 GHz at 2,3,and 4 mm,respectively.The significance of this study lays on realizing ultra-broadband capacity in HE TMC-3(Cr,Zr,Hf,Nb,Ta),demonstrating the effectiveness of high-entropy component design in tailoring the impedance match.

Textured and toughened high-entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C-SiCw ceramics

High-entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C ceramics,with different contents(0,5,10,and 20 vol.%)of Si C whiskers(SiCw),were fabricated by spark plasma sintering using raw powders synthesized via carbothermal reduction.The application of a uniaxial compaction force led to texture development of the SiCw within the(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C matrix.Fracture toughness increased with the increase in SiCw content,while Vickers hardness remains almost unchanged.The toughness of(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C-20 vol.%SiCw ceramics reached 4.3±0.3 MPa m^(1/2),which was approximately 43%higher than that of the monolithic(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C ceramic(3.0±0.2 MPa m1/2).The main toughening mechanisms were attributed to crack deflection,whisker debonding,and whisker pullout.

Regulating the formation ability and mechanical properties of high-entropy transition metal carbides by carbon stoichiometry

Tremendous efforts have been dedicated to promote the formation ability of high-entropy transition metal carbides.However,the majority of methods for the synthesis of high-entropy transition metal carbides still face the challenges of high temperature,low efficiency,additional longtime post-treatment and uncontrollable properties.To cope with these challenges,high-entropy transition metal carbides with regulatable carbon stoichiometry(HE TMC)were designed and synthesized,achieving improved ability for single phase solid solutions formation,promoting of sintering and controllable mechanical properties.Two typical composition series,i.e.,easily synthesized(ZrHfTaNb)C(ZHTNC)and difficultly synthesized(Zr_(0.25)Hf_(0.25)Ta_(0.25)Ti_(0.25))C(ZHTTC)are selected to demonstrate the promoting formation ability of single phase solid solutions from carbon stoichiometry deviations.Single phase high-entropy ZHTTC,which has been proven difficult in forming a single phase solid solution,can be prepared with the decrease of C/TM ratio under 2000℃;while the high-entropy ZHTNC,which has been proven easy in forming a single phase solid solution,can be synthesized at lower temperatures with the decrease of C/TM ratio.The synergistic effect of entropy stabilization and reduced chemical bond strength gaining from carbon stoichiometry deviations is responsible for the formation of single phase solid solutions and the promoted sintering of HE TMC.For example,the relative density of bulk(ZrHfTaNb)C(SPS-ZHTNC)increases from 90.98%to 94.25%with decreasing the C/TM atomic ratio from 0.9 to 0.74.More importantly,the room temperature flexural strength,fracture toughness and brittleness index of SPS-ZHTNCcan be tuned in the range of 384 MPa–419 MPa,4.41 MPam–4.73 MPamand 3.679μm–4.083μm,respectively.Thus,the HE TMCprepared by adjusting the ratio of carbon to refractory transition metal oxides have great potential for achieving low temperature synthesis,promoted sintering and tunable properties.

超高温材料的研究现状与展望

超高温材料是航天飞行器长时飞行、大气层再入飞行和跨大气层飞行不可或缺的关键材料,其直接影响了航天飞行器的研制进程并对飞行试验的成败起到了至关重要的作用。综述了C/C复合材料、陶瓷基复合材料、碳化物超高温陶瓷、硼化物超高温陶瓷及氮化物超高温陶瓷等超高温材料近年来的最新研究成果,重点评述了C/C复合材料的组织形成机理、疲劳特性、基体改性及抗氧化行为,Cf/Si C及Si Cf/Si C陶瓷基复合材料和超高温陶瓷在制备工艺、力学性能、抗氧化和抗烧蚀等方面的国内外研究现状,探讨了常用几种超高温材料的优缺点并分析了其研究重点,提出了超高温材料当前研究中存在的主要问题,指出了超高温材料未来的研究目标和发展方向。

ZrB_2-SiC复相陶瓷制备方法研究现状

综述了ZrB2-SiC复相陶瓷近年来的研究现状。总结了无压烧结、反应热压烧结、热压烧结、放电等离子烧结和一些其他方法制备ZrB2-SiC复相陶瓷的研究成果,并提出了ZrB2-SiC复相陶瓷目前研究中存在的问题和今后潜在的发展方向。

超高温陶瓷材料的抗氧化性

氧化失效制约着在航空发动机等领域应用的超高温陶瓷的发展。本文对硼化物、碳化物超高温陶瓷材料在氧化机理和抗氧化研究方面作了综合评述。硼化物主要依靠表面非晶膜来阻止氧的扩散,温度过高,非晶膜失效;而碳化物则是靠金属碳氧化物层(MeCxOy)来阻止材料的进一步氧化。但大多数研究并未表明这些材料在2000℃以上仍有较好的抗氧化性。最后提出超高温陶瓷抗氧化的研究方向。

超高温陶瓷材料的研究

由于在极端环境中具有优异的物理化学性能,超高温陶瓷成为未来高超声速飞行和可重复使用运载飞船领域最具前途的候选材料之一。本文评述了超高温陶瓷材料包括材料体系、粉体合成、烧结和涂层的研究现状,提炼出尚未解决的部分问题,提出超高温陶瓷基涂层材料是其发展趋势之一。