Future of global governance and climate change action in a changing political landscape

1.Editor's note On 30 November 2016,the Brookings-Tsinghua Center for Public Policy(BTC),Caixin Video,and Columbia Global Centers(Beijing)jointly hosted a Seminar on the Future of Global Governance and Climate Change Action in a Changing Political Landscape.Leading experts on China's environmental policy and

A review on additive manufacturing of ceramic matrix composites

Additive manufacturing(AM)of ceramic matrix composites(CMCs)has enabled the production of highly customized,geometrically complex and functionalized parts with significantly improved properties and functionality,compared to single-phase ceramic components.It also opens up a new way to shape damage-tolerant ceramic composites with co-continuous phase reinforcement inspired by natural ma-terials.Nowadays,a large variety of AM techniques has been successfully applied to fabricate CMCs,but variable properties have been obtained so far.This article provides a comprehensive review on the AM of ceramic matrix composites through a systematic evaluation of the capabilities and limitations of each AM technique,with an emphasis on reported results regarding the properties and potentials of AM man-ufactured ceramic matrix composites.

Revealing structural degradation in layered structure oxides cathode of lithium ion batteries via in-situ transmission electron microscopy

Transition metal oxides with layered structure have been widely used as cathode materials for lithium-ion batteries(LIBs)which have relatively high energy density,large capacity and long life.However,in the long-term electrochemical cycle,the inevitable degradation of performance of LIBs due to structural degradation in cathodes severely restricts their large-scale practical applications.Understanding the underlying mechanism of structural degradation is the most critical scientific problem.Recently,in situ transmission electron microscopy(TEM)has become a useful tool to study the structural and compositional evolutions at atomic scale in electrochemical reactions,which provided a unique and in-depth understanding of the structural degradation.In this review,we discuss the recent advances in the in situ TEM,focusing on its role in revealing the structural degradation mechanisms in the four key places:(1)the interface between the cathodes and electrolyte;(2)the cathode surface;(3)the particle interior and(4)those induced by thermal effect.The insight gained by the in-situ TEM which is still developing at its fast pace is unique and expected to provide guidance for designing better layered cathode materials.

在导电陶瓷基体中加入hBN薄片来创制材料性能的各向异性

导电陶瓷的物理性能由连续的导电基体所决定,调控困难.在这项研究中,通过在陶瓷基体中引入hBN薄片,成功实现了导电陶瓷力学和物理性能各向异性的调控.以包含TiN、Al、硼和hBN的混合粉末为原料,通过放电等离子烧结制备了两种hBN含量不同的导电TiB_(2)-AlNhBN陶瓷.在垂直于烧结压力的方向上,TiB_(2)-AlN-60vol%hBN陶瓷在室温下的弯曲强度、断裂韧性、电导率和热导率分别为151±6.34MPa、2.62±0.31MPam^(1/2)、5.3×10^(5)S m^(−1)和77.87Wm^(-1)K^(-1).而在平行于烧结压力的方向上,室温电导率和热导率分别降低到2.0×10^(5)S m^(−1)和32.25Wm^(-1)K^(-1).这些差异是通过在导电基体中加入少量额外排列的hBN薄片而产生的,为材料选择和设计提供了新的机会.

1800℃下具有优异高强度且含氧化物杂质的高熵二硼化物陶瓷

以硼热/碳热还原合成高熵二硼化物粉体为原料,在2000℃/单轴加压50 MPa条件下,经10分钟放电等离子烧结,成功制备了含有~2 vol%的氧化物和~1 vol%气孔的高熵(Ti_(0.2)Zr_(0.2)Nb_(0.2)Hf_(0.2)Ta_(0.2))B_(2)陶瓷(HEBs).经研究确认,其中残留氧化物是固溶少量硼和碳的m-(Hf,Zr)O_(2).室温下HEBs的弹性模量、维氏硬度和断裂韧性分别为508.5 GPa、17.7±0.4 GPa和4.2±0.2 MPa m^(1/2).烧结得到的HEBs具有优良的抗弯强度,特别是其高温强度.HEBs在室温、1600和1800℃下的四点抗折强度分别为400.4±47.0 MPa、695.9±55.9 MPa和751.6±23.2 MPa.对1800℃下断裂的HEBs样品进行了失效分析,在其拉伸和断裂面附近区域,仅在裂纹尖端和孔隙边缘发现了数量有限的位错线的存在,没有观察到位错运动的痕迹.本研究首次报道了高熵二硼化物陶瓷的高温强度,发现其强度直至1800℃并无衰减,并对其高温下的强韧化机制进行了有益的探讨.

Bio-inspired high-efficiency photosystem by synergistic effects of core-shell structured Au@CdS nanoparticles and their engineered location on{001}facets of SrTiO_(3)nanocrystals

Natural photosynthesis,which provides a green and high-efficiency energy conversion path by spatial separation of photogenerated carriers through combined actions of molecules ingeniously arranged in an efficient solar nanospace,highlights the importance of rational nanostructure design to realize artificial high-efficiency photosystem.Inspired by these unique features,we constructed a high-efficiency ternary photosystem by selectively decorating the{001}facets of 18-facet SrTiO_(3)with Au@CdS photosensitizers via a green photo-assisted method.Benefiting from the dual-facilitated charge carriers transportation in core-shell structured Au@CdS heterojunction and well-faceted 18-facet SrTiO_(3)nanocrystal,such a photo-catalyst could realize the effective spatial separation of photogenerated electrons and holes.As expected,the 18-facet SrTiO_(3)/Au@CdS photocatalyst exhibits superior activity in visible-light-driven photocatalytic hydrogen evolution(4.61 mmol h^(−1)g^(−1)),166%improvement in comparison with randomly deposited Au@CdS(1.73 mmol h^(−1)g^(−1)).This work offers new insight into the development of green and high-efficiency photocatalytic systems based on the rational nanostructure design by crystal facet engineering.

Grain-refining fabrication of nanocrystalline(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)high-entropy ceramics by ultra-high pressure sintering

As an important A_(2)B_(2)O_(7)-type ceramic,(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)high-entropy pyrochlore pos-sesses promising properties such as high melting point,high chemical durability,and low thermal conductivity.However,the low sintering ability limits its application in thermal barrier coating and radioactive waste immobilization.It usually needs long-term high-temperature soaking to achieve full density,but with inevitable grain growth.In this work,dense and grain-refined nanocrystalline(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)ceramics were prepared with ultra-high pressure sintering(UHPS)method under 10 GPa at a low temperature of 800℃.The densification behavior,microstructure evo-lution,and properties of the UHPS-ed samples were then investigated.The grain size of as-prepared(La_(0.2)Nd_(0.2)Sm_(0.2)Gd_(0.2)Eu_(0.2))_(2)Zr_(2)O_(7)ceramic was only 151 nm,which is 40%smaller than that of raw pow-der.In addition,it exhibited advantageous properties including both high hardness and aqueous durabil-ity.Plastic deformation under ultra-high pressure was believed as the dominant densification mechanism responsible for grain refinement and property improvement.

Titanium nitrides as novel reactants for in-situ synthesis of TiB_(2)-based composites with improved properties

A novel in-situ reactive approach based on the reactions among TiN,aluminum and boron has been developed to synthesize TiB_(2)-based composites including TiB_(2)-h BN(TB)and TiB_(2)-h BN-AlN(TBA).Fully dense ceramics with fine-grained microstructure were successfully obtained via spark plasma sintering at 1850℃/60 MPa/5 min.Microstructure analysis suggests h BN flakes were homogenously distributed in the TiB_(2)matrix.For AlN,however,they were elongated into plate-like grains during sintering,in which lots of defects in terms of stacking faults and twinning structures were observed.The mechanical and thermophysical properties of as-sintered ceramics were comprehensively investigated and compared.In-corporating AlN significantly improved the flexure strength,hardness,fracture toughness and thermal conductivity of TiB_(2)-h BN ceramics.The electrical conductivity of TB(3.06×10^(6)S/m)is larger than that of TBA(2.35×10^(6)S/m)at room temperature,but the value(6×10^(5)S/m)was lower than that of TBA(6.9×10^(5)S/m)at 1173 K.Based on the measurement of electrical and thermal conductivity,electron and phonon contributions to thermal conductivities of TB and TBA were calculated and their temperature dependences were illustrated.

4D printing for future manufacturing

Additive manufacturing(AM),also called three-dimensional(3D)printing,offers disruptive design freedom and manufacturing flexibility due to the layer-wise incremental formation manner of components.Advancements in AM techniques provide unique advantages and feasibilities for overcoming challenges arising from fabricating parts with complexities in geometry,materials,and properties.

硼热/碳热还原技术制备高致密度、高纯度高熵硼化物陶瓷（英文）

以过渡金属氧化物(MOx)和碳化硼(B4C)为原料,采用硼热/碳热还原技术在1800℃下制备得到了等摩尔比的亚微米级五元高熵硼化物(Ti0.2Hf0.2Zr0.2Nb0.2Ta0.2)B2粉体,并深入探讨了硼热/碳热还原过程中产物的物相、形貌以及固溶体变化过程.X射线衍射谱表明所制备的粉体在1800℃即形成了单相结构,但直到2000℃样品中各元素才分布均匀.将得到的粉体在2050℃/50 MPa的条件下进行放电等离子烧结,获得了相对密度为97.9%的高熵硼化物陶瓷.研究发现,当烧结温度从2000℃增加到2050℃时,高熵陶瓷晶粒迅速长大,平均晶粒尺寸从6.67μm增大到41.2μm.2000℃下制备得到的高熵硼化物陶瓷具有良好的力学性能,其硬度、杨氏模量和断裂韧性分别为22.44±0.56 GPa,~500 GPa,2.83±0.15 MPa m^1/2.

Advances in ultra-high temperature ceramics,composites,and coatings

Ultra-high temperature ceramics(UHTCs)are generally referred to the carbides,nitrides,and borides of the transition metals,with the Group IVB compounds(Zr&Hf)and TaC as the main focus.The UHTCs are endowed with ultra-high melting points,excellent mechanical properties,and ablation resistance at elevated temperatures.These unique combinations of properties make them promising materials for extremely environmental structural applications in rocket and hypersonic vehicles,particularly nozzles,leading edges,and engine components,etc.In addition to bulk UHTCs,UHTC coatings and fiber reinforced UHTC composites are extensively developed and applied to avoid the intrinsic brittleness and poor thermal shock resistance of bulk ceramics.Recently,high-entropy UHTCs are developed rapidly and attract a lot of attention as an emerging direction for ultra-high temperature materials.This review presents the state of the art of processing approaches,microstructure design and properties of UHTCs from bulk materials to composites and coatings,as well as the future directions.

Oxide dispersion strengthened stainless steel 316L with superior strength and ductility by selective laser melting

Dense oxide dispersion strengthened(ODS) 316 L steels with different amount of Y2O3 additions were succe s s fully fabricated by selective laser melting(SLM) even though part of the added Y2O3 got lost during the process.The microstructure was characterized in details and the mechanical properties were tested at room temperature,250℃ and 400℃,respectively.The effect of the scanning speed on agglomeration of nanoparticles during SLM process was discussed.Superior properties,e.g.,yield strength of 574 MPa and elongation of 91%,were achieved at room temperature in SLM ODS 316 L with additional 1% of Y2 O3.At elevated temperature s,the strength kept high but the elongations dropped dra matically.It was observed that nano-voids nucleated throughout the whole gauge section at the sites where nanoinclusions located.The growth and coalescence of these voids were suppre s sed by the formation of an element segregation network before necking,which relieved local stress concentration and thus delayed necking.This unusual necking behavior was studied and compared to the previous theory.It appeared that the strong convection presented in the melt pool can evenly redistribute the short-time milled coarse Y2O3 precursor powder during SLM process.These findings can not only solve the problems encountered during the fabrication of ODS components but also replenish the strengthening mechanism of SLM 316 L thus pave a way for further improving of mechanical properties.

Nanoceramic composites with duplex microstructure break the strength-toughness tradeoff

There is a strength and fracture toughness tradeoff in nanoceramic composite. The strength varies reciprocally with the grain size whereas the toughness contributed by compressive residual stress increases with the dimension of the second phase. In this work, a novel duplex microstructure with reinforced clusters composing of nanosized grains was proposed and validated using a model system of B_(4)C-TiB_(2) ceramics densified by carbide boronizing. As-obtained ceramics exhibit excellent combined mechanical properties at room temperature, including Vickers hardness, Young's modulus, flexural strength and fracture toughness(by surface crack in flexure method) of 32.1 ± 2.7 GPa, 506.9 ± 2.0 GPa,1175 ± 71 MPa and 5.1 ± 0.4 MPa m^(0.5), respectively. Both strength and toughness are at least ~30 % higher than the counterparts with similar composition but homogenously distributed TiB_(2) grains. Graphite onion was confirmed to be an intermediate product during reactive sintering, it facilitated the grain pullout during fracture and retained the nanometric TiB_(2) grain in the cluster, both of which also contribute the toughening and strengthening mechanisms in the B_(4)C-TiB_(2) ceramics.

In-situ ZrB2-hBN ceramics with high strength and low elasticity

ZrB2 ceramics with various hexagonal BN(hBN)additions up to 37 vol%were reactively densified by spark plasma sintering using powder mixtures containing ZrB2,ZrN and boron.ZrN-boron based additives effectively promoted the densification process,ZrB2 ceramics reached>99%relative density at 2000℃and an applied pressure of 60 MPa with only 5 vol%in-situ formed hBN,whereas the relative density of pure ZrB2 was only 91.2%at the same conditions.Increasing thehBN contents,the morphology of hBN grains gradually changed from quasi-spherical to flake dominated,which has substantial influence on their mechanical properties.In-situ ZrB2-10 vol%hBN ceramics demonstrated high flexural strength of 597±22 MPa,relatively low Young’s modulus of 406 GPa and good machinability,especially for the impressively large strain to failure(1.47×10^-3)which is superior to most of their counterparts in the ZrB2 based particulate reinforced ceramics.

Core-rim structure,bi-solubility and a hierarchical phase relationship in hot-pressed ZrB2-SiC-MC ceramics(M=Nb,Hf,Ta,W)

Core-rim structures were identified as a common feature in hot-pressed ZrB2-SiC-MC ceramics(M=Nb,Hf,Ta and W)by a combination of X-ray diffraction,scanning and transmission electron microscopies.Quantitative analyses associate them with the bi-solubility of M in ZrB2 phase,in which transition of solubility across the core/rim boundary is abrupted,signifying their creation via dissolution-reprecipitation process facilitated by transient liquid-phase.The cores were retained from starting powder after surface melting and the rims were grown from the liquid-phase to incorporate more solutes,leaving the residual liquid to turn into ZrC phase with higher solubility of M.We propose g-point scheme in the ZrB2-MB2 diagrams to combine the bi-solubility and the core-rim structures into an intra-phase relationship created by sintering,leading further to a hierarchical phase relationship.The temperature dependence of flexural strength in the ZrB2-SiC-MC ceramics varies with MC additions,which can be respectively strengthened by the strain energy created in the core-rim structures and metal segregation to grain boundaries.

Reactive sintering of B4C-TaB2 ceramics via carbide boronizing:Reaction process, microstructure and mechanical properties

Carbide boronizing is a promising approach to obtain fine grained boron carbide based ceramics with improved mechanical properties. In this work, reaction process, microstructural characteristics and mechanical properties of BxC-TaB2(x = 3.7, 4.9, 7.1) ceramics were comprehensively investigated via this method. Dense BxC-TaB2 ceramics with refined microstructure were obtained from submicro tantalum carbide and boron powder mixtures at 1800℃/50 MPa/5 min by spark plasma sintering. The stoichiometry of boron carbide was determined from lattice parameters and Raman shift. It was found that uniformly distributed TaB2 grains in the BxC matrix is favor of the densification process and restricting grain growth.Besides, planar defects with high density were observed from the as-formed B7.1 C grains and transient stress was considered to contribute to the densification involved with plastic deformation. Microstructural observations indicate the dissolution of oxygen in the TaB2 lattice and most of the B7.1 C/TaB2 phase boundaries were clean. Owing to the highly faulted structure and finer grain size, as-obtained BxC-TaB2 ceramics exhibit high Vickers hardness(33.3–34.4 GPa at 9.8 N) and relatively high flexural strength ranging from 440 to 502 MPa.

Self-supported yttria-stabilized zirconia mesocrystals with tunable mesopores prepared by a chemi-thermal process

Mesoporous mesocrystals are highly desired in catalysis,energy storage,medical and many other applications,but most of synthesis strategies involve the usage of costly chemicals and complicated synthesis routes,which impede the commercialization of such materials.During the sintering of dense ceramics,coarsening is an undesirable phenomenon which causes the growth of the grains as well as the pores hence hinders the densification,however,coarsening is desired in the sintering of porous ceramics to expand the pore sizes while retaining the total pore volume.Here we report a chemi-thermal process,during which nanocrystallite aggregates were synthesized by hydrothermal process and then converted to the product by sintering.Through this strategy,we synthesized mesoporous self-supported mesocrystals of yttria-stabilized zirconia with tunable pore size and the process was then scaled-up to industrial production.The thermal conductivity measurement shows that the mesoporous powder is a good thermal isolator.The monolith pellets can be obtained by SPS sintering under high pressure and the mesoporosity is retained in the monolith pellets.This work features facile and scalable process as well as low cost raw chemicals making it highly desirable in industrial applications.

Integrating thin wall into block:A new scanning strategy for laser powder bed fusion of dense tungsten

Due to a combination of outstanding properties including high melting point,high density,low thermal expansion coefficient,low saturated vapor pressure and excellent thermal conductivity,tungsten(W)parts have been widely used as electrodes,heating elements,anti-scatter grids for Computed Tomography(CT)equipment,rocket nozzles and divertor materials for nuclear fusion[1].