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.

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).

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.

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.

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。

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.

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。

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.

过渡金属碳化物Ti_(3)C_(2)T_(x)的制备及其电磁屏蔽性能研究

二维过渡金属碳化物(MXene)具有优异的导电性、良好的柔韧性和亲水性等特点,其中Ti_(3)C_(2)T_(x)在电磁波屏蔽领域研究广泛。Ti_(3)C_(2)T_(x)是通过选择性刻蚀MAX相Ti 3AlC 2中Al元素后进一步剥离得到,因此刻蚀和剥离工艺直接决定了Ti_(3)C_(2)T_(x)的物化性质。采用LiF/HCl选择性刻蚀前驱体Ti 3AlC 2,优化刻蚀时间和剥离时间,制备了单/少层Ti_(3)C_(2)T_(x)。通过X射线衍射(XRD)、拉曼光谱(Raman)、X射线光电子能谱(XPS)和扫描电镜(SEM)等对Ti_(3)C_(2)T_(x)表面形貌、化学组成进行表征,并通过矢量网络分析仪(VNA)和四探针测试仪等研究了Ti_(3)C_(2)T_(x)的电磁屏蔽效能及导电性能。结果表明,采用LiF/HCl选择性刻蚀法能够有效制备单/少层Ti_(3)C_(2)T_(x),在刻蚀时间为48 h,超声剥离时间为50 min时,获得的Ti_(3)C_(2)T_(x)薄膜电磁屏蔽性能最高达到52.5 dB,电导率可达7342.1 S/m。同时,Ti_(3)C_(2)T_(x)自支撑薄膜在低厚度下绝对屏蔽效能达18347.65 dB cm^(2)g^(-1),表现出优异的电磁屏蔽性能。

新型磁性异质结材料的制备及其光降解染料废水的性能

为了解决光催化剂在光生电子与空穴中分离效率低且难以回收利用的问题,以钛碳化铝(Ti_(3)AlC_(2))为原料,通过LiF+HCl刻蚀法、共沉淀法、溶剂热法和异种电荷静电自组装,获得具有磁分离性能的新型异质结光催化材料AMHNTs/TiO_(2)-Ti_(3)C_(2)T_(x)(H&M),对其进行结构表征,并选取偶氮类染料甲基橙(MO)为污染物模型,考察模拟太阳光下H&M的降污性能及降解机理。结果表明:制备的异质结H&M融合了二维层状与一维管状的结构特征,提高电子的传导输送,具有良好的光催化活性和磁回收性,且对MO有较好的降解性能,为构筑新型磁性无机异质结光催化材料和染料的光降解提供有价值的参考。

过渡金属碳化物纳米材料的制备方法及在催化析氢中的应用

过渡金属碳化物(TMCs)因高导电、高硬度、耐酸碱、热稳定性良好等优势,被广泛应用于化学化工、电子器件等领域。研究发现,TMCs纳米材料因特殊的d带电子排布和暴露的活性位点,能促进氢的吸附与脱附,进而表现出较高的催化析氢活性。但是,TMCs在合成中易团聚,活性位点易被覆盖会严重阻碍其性能的发挥。通过对纳米结构的合理调控,在维持TMCs自身活性的同时可进一步提高材料自身的催化活性。不同维度TMCs纳米材料的结构优势集中体现在材料表面的有效利用与活性位点的暴露上。介绍了TMCs纳米材料的制备方法及其催化析氢性能,指出了TMCs纳米材料面临的挑战及存在的问题,以期为不同维度TMCs纳米材料的研究提供参考。

Rational design of 3D porous niobium carbide MXene/rGO hybrid aerogels as promising anode for potassium-ion batteries with ultrahigh rate capability

An effective method is designed to construct three-dimensional(3D)Nb_(2)C/reduced graphene oxide(rGO)hybrid aerogels through a low-temperature graphene oxide(GO)-assisted hydrothermal self-assembly followed by freeze-drying and annealing.The intimately coupled Nb_(2)C/rGO hybrid aerogel combines the advantages of large specific surface area and rich 3D interconnected porous structure of aerogel as well as high conductivity and low potassium diffusion energy barrier of Nb_(2)C,which not only effectively prevents the self-restacking of Nb2C nanosheets to allow more active sites exposed and accommodate the volume change during the charge/discharge process,but also increases the accessibility of electrolyte and promotes the rapid transfer of ions/electrons.As a result,Nb_(2)C/rGO-2 as the anode of potassium ion batteries(KIBs)delivers a large reversible specific capacity(301.7 mAh·g^(−1)after 500 cycles at 2.0 A·g^(−1)),an ultrahigh rate capability(155.5 mAh·g^(−1)at 20 A·g^(−1)),and an excellent long-term large-current cycle stability(198.8 mAh·g^(−1)after 1,000 cycles at 10 A·g^(−1),with a retention of 83.3%).Such a high-level electrochemical performance,especially the ultrahigh rate capability,is the best among transition metal carbides and nitride(MXene)-based materials reported so far for KIBs.The diffusion kinetics of K+is investigated thoroughly,and the synergetic charge–discharge mechanism and the structure–performance relationship of Nb_(2)C/rGO are revealed explicitly.The present work provides a good strategy to solve the self-restacking problem of two-dimensional materials and also enlarges the potential applications of MXenes.

CNTs/MXene对复合涂层导电性能的影响

导电涂料是一种具有传导电子、排除累积电荷能力的功能涂料,在航天航空、电子、石油化工等领域广泛应用。本研究将CNTs/MXene作为功能填料分散到水性丙烯酸中,制备了导电复合涂层。结果表明,涂层中的CNTs/MXene互相搭接形成了导电通路,当CNTs/MXene添加量为0.20%(质量分数)时,电导率达到了1.54×10^(-2)S/cm。管状CNTs与片层状MXene结合,使CNTs更易分散搭接形成网络结构,低添加量的导电填料即可提升复合涂层的导电性能,又能大大降低导电涂料的成本,进一步促进低成本导电功能涂料的产业化。