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.

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.

Use of Solid State Carbothermic Reduction in Production of Transition Metals and Their Carbides

The use of solid state carbothermic reduction as a precursor to the smelting of transition metal ores was examined . The advantages of the introduction of a prereduction stage include enabling the more efficient use of fines and the achievement of higher energy efficiencies. A solid state reduction using carbon as the reductant offers a simpler alternative for their treatment. Subsequent treatment of the reduced material could include intensive bath smelting to produce ferroalloys or, in some case, solid state separation of the transition metal carbide where this has commercial significance.

Recent Progress in Emerging Two‑Dimensional Transition Metal Carbides

As a new member in two-dimensional materials family,transition metal carbides(TMCs)have many excellent properties,such as chemical stability,in-plane anisotropy,high conductivity and flexibility,and remarkable energy conversation efficiency,which predispose them for promising applications as transparent electrode,flexible electronics,broadband photodetectors and battery electrodes.However,up to now,their device applications are in the early stage,especially because their controllable synthesis is still a great challenge.This review systematically summarized the state-of-the-art research in this rapidly developing field with particular focus on structure,property,synthesis and applicability of TMCs.Finally,the current challenges and future perspectives are outlined for the application of 2D TMCs.

Two-Dimensional Transition Metal Carbides and Nitrides(MXenes): Synthesis, Properties, and Electrochemical Energy Storage Applications

A family of 2D transition metal carbides and nitrides known as MXenes has received increasing attention since the discovery of Ti3C2 in 2011. To date, about 30 different MXenes with well-defined structures and properties have been synthesized, and many more are theoretically predicted to exist. Due to the numerous assets including excellent mechanical properties, metallic conductivity,unique in-plane anisotropic structure, tunable band gap, and so on, MXenes rapidly positioned themselves at the forefront of the 2D materials world and have found numerous promising applications. Particular interest is devoted to applications in electrochemical energy storage, whereby 2D MXenes work either as electrodes,additives, separators, or hosts. This review summarizes recent advances in the synthesis, fundamental properties and composites of MXene and highlights the state-of-the-art electrochemical performance of MXene-based electrodes/devices.The progresses in the field of supercapacitors and Li-ion batteries, Li-S batteries, Naand other alkali metal ion batteries are reviewed, and current challenges and new opportunities for MXenes in this surging energy storage field are presented. In the focus of interest is the possibility to boost device-level performance, particularly that of rechargeable batteries, which are of utmost importance in future energy technologies. Very recently, the 2019 Nobel Prize in Chemistry was awarded to the inventors of the Li-ion battery. For sure, this will provide an additional stimulation to study fundamental aspects of electrochemical energy storage.

Adhesion between Nonreactive Liquid Metals and Transition Metal Carbides

On the basis of the experimental work of adhesion(W)data,the adhesion between transition metal car- bides and pure liquid metals which do not react with carbides is studied.In view of great scattering of the ex- perimental values of W,a critical analysis of these results is performed.The selected W values for 9 copper/carbide systems and 6 metal/TiC systems are used to discuss the various suggestions concerning the mechanism of adhesion and to evidence the role of the valence electrons of the both carbide and metal on the interactions between metals and carbides.The interactions between a metal and a carbide are essentially metal- lic interactions,resulting from the overlapping of the valence electrons at the metal/carbide interface.

Transition metal carbides in electrocatalytic oxygen evolution reaction

Oxygen evolution reaction(OER) is admitted to an important half reaction in water splitting for sustainable hydrogen production.The sluggish four-electron process is known to be the bottleneck for enhancing the efficiency of OER.In this regard,tremendous efforts have been devoted to developing effective catalysts for OER.In addition to Ir-or Ru-based oxides taken as the benchmark,transition metal carbides have attracted ever-increasing interest due to the high activity and stability as low-cost OER electrocatalysts.In this review,the transition metal carbides for water oxidation electrocatalysis concerning design strategies and synthesis are briefly summarized.Some typical applications for various carbides are also highlighted.Besides,the development trends and outlook are also discussed.

Carbon deficiency introduced plasticity of rock-salt-structured transition metal carbides

High-hardness rock-salt structured transitional metal carbides(TMC)are attracting substantial interest as potential next-generation thermal protection materials.However,the intrinsic brittleness of TMC ceramics impedes their performance in aerodynamically harsh environments.In this work,a promising strategy is proposed to introduce plasticity in TaC–HfC solid solutions by manipulating carbon deficiency.The approach combines density-functional theory(DFT)with experiments and takes Pugh's ratio(k)as the criteria.Depletion of carbon atoms in TaC–HfC solid solutions results in the de-localizing of valence electrons,deviation of spatial modulus along different crystal plane directions,and leading to significant elastic anisotropy.The carbon deficient Ta_(0.8)Hf_(0.2)C_(0.8) is predicted to be a‘softer phase’with reduced modulus and Pugh's ratio(k=0.58).A series of Ta1–xHfxCy(x=0.2 and 0.8,y=0.8,0.9,and 1.0)bulk ceramics are experimentally fabricated by an excessive metal alloying method.Trigonal and hexagonal close-packed structured carbides are derived when the carbon deficiency y decreased to 0.7.The indentation modulus drops from 641.8±14.8 GPa for Ta_(0.8)Hf_(0.2)C_(1.0) to 555.8±9.9 GPa for Ta0.8Hf0.2C0.8.The specific stoichiometric composition of Ta_(0.8)Hf_(0.2)C_(0.8) is experimentally verified to possess both plasticity(k=0.41)and ultra-high nanohardness(41.3±1.3 GPa).

Enabling highly efficient and broadband electromagnetic wave absorption by tuning impedance match in high-entropy transition metal diborides (HETMB_(2))

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

Interfacial electronic coupling of V-doped Co_(2)P with high-entropy MXene reduces kinetic energy barrier for efficient overall water splitting

Developing efficient,low-cost non-noble metal-based bifunctional catalysts to achieve excellent hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)kinetics in alkaline media is challenging but very meaningful.However,improving the electronic structure of the catalyst to optimize the adsorption of intermediates and reduce the reaction energy barrier is the key to improve the reaction efficiency.Herein,a V-doped Co_(2)P coupled with high-entropy MXene heterostructure catalyst(V-Co_(2)P@HE)was prepared by a two-step electrodeposition and controlled phosphorization process.The analyses of X-ray absorption spectroscopy,X-ray photoelectron spectroscopy and theoretical calculations jointly show that the introduction of V and the strong electron coupling between the two components optimize the adsorption energy of water molecules and reaction intermediates.Benefiting from the abundant active sites and optimizing intermediate adsorption energy and heterogeneous interface electronic structure,V-Co_(2)P@HE has excellent HER and OER activity and long-term stability under alkaline condition.In particular,when assembled as cathode and anode,the bifunctional V-Co_(2)P@HE catalyst can drive a current density of 10 mA cm^(-2)with only 1.53 V.This work provides new strategies for the application of highentropy MXene and the design of novel non-noble metal-based bifunctional electrolytic water catalysts.

Single-source precursor derived high-entropy metal–carbide nanowires:Microstructure and growth evolution

In recent years,high-entropy metal carbides(HECs)have attracted significant attention due to their exceptional physical and chemical properties.The combination of excellent performance exhibited by bulk HEC ceramics and distinctive geometric characteristics has paved the way for the emergence of one-dimensional(1D)HECs as novel materials with unique development potential.Herein,we successfully fabricated novel(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C nanowires derived via Fe-assisted single-sourced precursor pyrolysis.Prior to the synthesis of the nanowires,the composition and microstructure of(Ti,Zr,Hf,Nb,Ta)-containing precursor(PHECs)were analyzed,and divinylbenzene(DVB)was used to accelerate the conversion process of the precursor and contribute to the formation of HECs,which also provided a partial carbon source for the nanowire growth.Additionally,multi-branched,single-branched,and single-branched bending nanowires were synthesized by adjusting the ratio of PHECs to DVB.The obtained single-branched(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C nanowires possessed smooth surfaces with an average diameter of 130–150 nm and a length of several tens of micrometers,which were a single-crystal structure and typically grew along the[11¯1]direction.Also,the growth of the(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C nanowires was in agreement with top-type vapor–liquid–solid mechanism.This work not only successfully achieved the fabrication of HEC nanowires by a catalyst-assisted polymer pyrolysis,but also provided a comprehensive analysis of the factors affecting their yield and morphology,highlighting the potential application of these attractive nano-materials.

三维一体针织结构超级电容器的储能性能

为提高能源储存器件的柔韧性及安全性,开发柔性可穿戴三维一体储能设备。制备二维过渡金属碳化物Ti_(3)C_(2)T_(x)(MXene)/锌(Zn)三维一体针织结构超级电容器(ZSC)。通过透射电子显微镜和X射线衍射仪对MXene纳米片微观形貌和涂层情况进行表征,利用电化学工作站探究其储能性能。结果表明,ZSC具有优异的倍率性能,在电流密度为1 mA/cm^(2)时,ZSC面积电容为345.56 mF/cm^(2)。分析超级电容器的储能机制为负极侧的可逆Zn沉积/剥离和正极离子吸附/解吸。经过10000次充放电循环,ZSC仍具有93.51%的电容保持率和92.43%的库伦效率,且能量密度为25.05μW·h/cm^(2)时,功率密度为10 mW/cm^(2)。ZSC在空气中放置30 d,其储能性能保持稳定,表现出良好的电化学耐久性与稳定性。

基于家蚕平板丝结构的柔性压力传感器制备及其传感性能

为获得基于天然平板丝结构的柔性压力传感器,以家蚕平板丝为柔性基底,在家蚕吐丝过程中循环喷洒过渡金属碳化物/氮化物和银纳米线作为导电材料,制备出具有天然多层结构的导电传感层复合材料。借助扫描电子显微镜和傅里叶红外光谱仪对制备的平板丝传感层进行形貌观察和结构分析,并通过优化传感层的层数开发出性能更优异的传感器。结果表明:具有2层传感层的传感器的灵敏度最高,在42.03~60.00 kPa的高压范围内灵敏度可达0.20 kPa^(-1),比仅有1层传感层的传感器灵敏度提高了20倍;该传感器响应/恢复时间均在1 s以内,且在施加不同压缩速率时仍能保持电流值稳定,可准确响应动态变化的压力,在超过1500次压力加载/卸载循环后传感性能依旧保持稳定;该传感器可很好地贴附到人体关节表面用于监测人体运动状态,通过按压时间长短来模拟摩斯密码,可用于信息加密传输和困境救援;将传感器制成传感阵列,可应用于压力位置轨迹追踪。

A first-principles investigation on mechanical and metallic properties of titanium carbides under pressure

The titanium carbides are potential candidates to achieve both high hardness and refractory property. We carried out a structural search for titanium carbides at three pressures of 0 GPa, 30 GPa and 50 GPa. A phase diagram of the Ti-C system at 0 K was obtained by elucidating formation enthalpies as a function of compositions, and their mechanical and metallic properties of titanium carbides were investigated sys- tematically. We also discussed the relation of titanium concentration to the both mechanical and metallic properties of titanium carbides. It has been found that the average valence electron density and tractil-ity improved at higher concentrations of titanium, while the degree of covalent bonding directionality decreased. To this effect, the hardness of titanium carbide decreases as the content of titanium increases. Our results indicated that the titanium content significantly affected the metallic properties of the Ti-C system.

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.

全固态MnO_(2)@CNT/MXene纤维超级电容器的制备与性能研究

利用氯磺酸共混碳纳米管与过渡金属碳化物MXene形成液晶态纺丝液,通过湿法纺丝技术制备CNT/MXene复合纤维,分别使用电化学沉积法及水热法在复合纤维表面沉积二氧化锰(MnO_(2)),制备MnO_(2)@CNT/MXene复合纤维,纤维用固态电解质封装成为柔性超级电容器。结果表明,电化学沉积较水热法沉积的纤维超级电容器具有更高的体积比电容,在1 A/cm^(3)的电流密度下的比电容约为97.5 F/cm^(3),在循环充放电2500次后,最终电容保持率约为原有的74.5%。

过渡金属碳化物超级电容器电极材料研究进展

在能源问题日益突出的背景下,超级电容器作为一种高效的电化学储能装置备受关注,其快速充放电、循环稳定性等特点使其成为当前研究的热点。过渡金属碳化物因其具有良好的导电性和化学稳定性而被用作新型电极材料。首先介绍了过渡金属碳化物的晶体结构与基本物理化学性质,随后探讨了过渡金属碳化物及其作为电极材料在超级电容器中的应用。此外详细介绍了双电层电极材料和赝电容电极材料的特性,结合材料特性探讨了过渡金属碳化物电极材料的制备方法,包括碳化钼、碳化钛、碳化铌、碳化钒以及其他的碳化物。依据材料结构与性能的密切关系,阐述了过渡金属碳化物作为电极材料对提高超级电容器电化学性能的重要意义。最后总结了过渡金属碳化物在超级电容器领域的重要性,展望了未来可能的研究方向。

基于MXene-MB-cDNA材料构建新型传感器方法用于镉离子检测

本研究中,开发了一种新颖的基于过渡金属碳化物-亚甲基蓝-互补DNA(MXene-MB-cDNA)探针构建的竞争型电化学传感器,可用于重金属镉离子(Cd^(2+))的检测。该传感器一部分是基于MXene片层结构扩大MB负载率,用以放大电化学检测信号;另一部分以电极上修饰的Cd^(2+)Apt做为纽带连接MXene-MB-cDNA。当Cd^(2+)存在时,与cDNA竞争性结合Apt,引起双链结构变化,产生电化学信号,其最低检测限达到0.0034 mg·L^(-1),并在0.01至1.6 mg·L^(-1)范围内呈现良好的线性关系(R2=0.991)。相较传统方法,该方法更简单灵敏,为环境保护、食品安全、生物医药等研究方向中镉离子的检测提供了新的思路。

MXenes基材料热电性能的研究进展

MXenes(过渡金属碳/氮化物)作为一种新型二维层状材料,有着独特的层状结构,即层内是由共价键连接,层外由较弱的范德华力作用连接。这种结构和键合特性使得MXenes材料备受关注。因MXenes有着金属导电性以及可调节的电子带隙,故其在热电材料领域有着较大的潜在应用价值。简单综述了MXenes材料在热电领域的发展,归纳了经理论计算预测及实验证实热电性能的MXenes材料,最后展望了MXenes作为热电材料的发展前景。

过渡金属碳化物的研究进展

过渡金属碳化物作为一种催化新材料显示了其独特的性能 ,在石油馏份的HDS、HDN、加氢和脱氢反应、烃的异构化和芳构化、甲烷转化、F T合成和电催化等反应中都表现出了优良的催化性能。本文综述了国内外关于钼、钨碳化物的制备方法以及在上述反应中的应用研究进展。