Visualization of the electronic phase separation in superconducting KxFe2−ySe2

Type-II iron-based superconductors(Fe-SCs),the alkali-metal-intercalated iron selenide A_xFe_2?ySe₂(A=K,Tl,Rb,etc.)with a superconducting transition temperature of 32 K,exhibit unique properties such as high Néel temperature,Fe-vacancies ordering,antiferromagnetically ordered insulating state in the phase diagram,and mesoscopic phase separation in the superconducting materials.In particular,the electronic and structural phase separation in these systems has attracted intensive attention since it provides a platform to unveil the insulating parent phase of type-II Fe-SCs that mimics the Mott parent phase in cuprates.In this work,we use spatial-and angle-resolved photoemission spectroscopy to study the electronic structure of superconducting K_xFe_2?ySe₂.We observe clear electronic phase separation of K_xFe_2?ySe₂ into metallic islands and insulating matrix,showing different K and Fe concentrations.While the metallic islands show strongly dispersive bands near the Fermi level,the insulating phase shows an energy gap up to 700 meV and a nearly flat band around 700 meV below the Fermi energy,consistent with previous experimental and theoretical results on the superconducting K_1?xFe₂Se₂(122 phase)and Fe-vacancy ordered K_0.8Fe_1.6Se₂(245 phase),respectively.Our results not only provide important insights into the mysterious composition of phase-separated superconducting and insulating phases of K_xFe_2?ySe₂,but also present their intrinsic electronic structures,which will shed light on the comprehension of the unique physics in type-II Fe-SCs.     

Continuous dimethyl carbonate synthesis from CO2 and methanol over BixCe1-xOδ monoliths:Effect of bismuth doping on population of oxygen vacancies,activity,and reaction pathway

We evaluated bismuth doped cerium oxide catalysts for the continuous synthesis of dimethyl carbonate(DMC)from methanol and carbon dioxide in the absence of a dehydrating agent.Bi_xCe_1-xO_δnanocomposites of various compositions(x=0.06-0.24)were coated on a ceramic honeycomb and their structural and catalytic properties were examined.The incorporation of Bi species into the CeO₂ lattice facilitated controlling of the surface population of oxygen vacancies,which is shown to play a crucial role in the mechanism of this reaction and is an important parameter for the design of ceria-based catalysts.The DMC production rate of the Bi_xCe_1-xO_δ catalysts was found to be strongly enhanced with increasing Ov concentration.The concentration of oxygen vacancies exhibited a maximum for Bi_0.12Ce_0.88O_δ,which afforded the highest DMC production rate.Long-term tests showed stable activity and selectivity of this catalyst over 45 h on-stream at 140°C and a gas-hourly space velocity of 2,880 mL·g_cat^-1·h^-1.In-situ modulation excitation diffuse reflection Fourier transform infrared spectroscopy and first-principle calculations indicate that the DMC synthesis occurs through reaction of a bidentate carbonate intermediate with the activated methoxy(-OCH₃)species.The activation of C0₂ to form the bidentate carbonate intermediate on the oxygen vacancy sites is identified as highest energy barrier in the reaction pathway and thus is likely the rate-determining step.     

Mechanical Properties and Damage Tolerance of Bulk Yb3Al5O12 Ceramic

Yb₃Al₅O₁₂ has potential applications as thermal barrier coatings（TBCs） because it shows low thermal conductivity and close thermal expansion coefficient to nickel-based superalloys.As a prospective TBC material,besides superior thermal properties,the mechanical properties are also important.In this paper,we present the mechanical properties of Yb₃Al₅O₁₂ including elastic moduli,hardness,strength,and fracture toughness.The Young’s modulus of Yb₃Al₅O₁₂ is 282 GPa.The shear-modulus-to-bulkmodulus ratio of Yb₃Al₅O₁₂ is 0.63,which indicates relatively low shear deformation resistance.In addition,Yb₃Al₅O₁₂ exhibits high strength and fracture toughness but low hardness compared to yttria stabilized zirconia（YSZ）,the most successful TBC material.SEM observation reveals that the fracture surface of Yb₃Al₅O₁₂ displays "layered structure feature",which is caused by crack deflection.Investigation based on Hertzian contact test demonstrates that Yb₃Al₅O₁₂ is a damage-tolerant ceramic.Crack deflection and bridging can arouse shear faults,dissipate the local damage energy,and restrict the crack propagation within the material,which play an important role in enhancing the damage tolerance.The superior mechanical properties and good damage tolerance ensure Yb₃Al₅O₁₂ a promising candidate for TBC applications.     

Tunable strength of SiCf/β-Yb2Si2O7 interface for different requirements in SiCf/SiC CMC:Inspiration from model composite investigation

Model composites consisting of Si C fiber embedded inβ-Yb₂Si₂O₇ matrix were processed by Spark Plasma Sintering method and the feasibility of tunable Si Cf/Yb₂Si₂O₇ interface in Si C-based CMCs were estimated.Weak and strengthened Si Cf/Yb₂Si₂O₇ interfaces were achieved by adjusting sintering temperatures.The indentation crack test and fiber push out experiments clearly demonstrated the different debonding mechanisms in the samples.Weak interfaces sintered at 1200 and 1250℃exhibited crack deflection at interface in indentation test.Their low debond energy at the interface,which were comparable to those of Py C or BN,satisfied the well-recognized interfacial debond and crack deflection criteria for CMCs.The interface was strengthened by atomic bonding in model composite sintered at 1450℃,leading to crack penetrating into Si C fiber and high debond energy.The strong interface may be promising in Si Cf/Si C CMC to withstand higher combustion temperature,because Yb₂Si₂O₇ will provide plastic deformation capacity,which would serve as weak interphase for crack deflection and energy dissipation.Therefore,it is possible to design the capability of Si C_f/RE₂Si₂O₇ interface for different requirements by adjusting interfacial strength or debond energy to reach optimal mechanical fuse mechanism in SiC_f/SiC CMC.     

Structural and Physical Property Analysis of ZnO-SnO2—In2O3—Ga2O3 Quaternary Transparent Conducting Oxide System

The increasing demand in the diverse device applications of transparent conducting oxides（TCOs） requires synthesis of new TCOs of n- or p-type conductivity.This article is about materials engineering of ZnO—SnO₂—ln₂O₃—Ga₂O₃ to synthesize powders of the quaternary compound Zn_2-xSn_1-xJn_xGa_xO_4-δ in the stoichiometry of x = 0.2,0.3,and 0.4 by solid state reaction at 1275℃.Lattice parameters were determined by X-ray diffraction（XRD） technique and solubility of ln³⁺ and Ga³⁺ in spinel Zn₂SnO₄ was found at 1275℃.The solubility limit of ln³⁺ and Ga³⁺ in Zn₂SnO₄ is found at below x = 0.4.The optical transmittance approximated by the UV—Vis reflectance spectra showed excellent characteristics while optical band gap was consistent across 3.2 eV with slight decrease along increasing x value.Carrier mobility of the species was considerably higher than the older versions of zinc stannate spinel co-substitutions whereas the carrier concentrations were moderate.     

Relationship between Microstructure Evolution and the Luminescent Properties of Eu3+-doped Yttrium Aluminum Garnet and Y2O3 Nano-powders

Eu³⁺-doped yttrium aluminum garnet（YAG） and Y₂O₃ composite phase nanoparticles were synthesized using a modified hydrothermal method.Sintering was performed at 800℃,which is considerably lower than the sintering temperature used in the conventional method.Y³⁺ in YAG and Y₂O₃ was partially substituted with Na⁺ by adding NaNO₃ into the solution during sample synthesis.The microstructures and phase transformation process were characterized through X-ray diffraction,scanning electron microscopy,and transmission electron microscopy.The obtained results verified that the addition of Na⁺ promoted crystallinity and grain growth of the three phases,namely,Y₂O₃,Y₄AI₂O₉,and Y₃AI₅O₁₂.Highly efficient luminescence properties excited by 254 and 365 nm ultraviolet were achieved.In conclusion,the remarkable enhancement of the luminescence intensity with the addition of Na⁺ should be attributed to grain growth and improvement of crystallinity.     

Insights into the stable and fast lithium storage performance of oxygen-deficient LiV3O8 nanosheets

Oxygen-deficient LiV₃O₈ is considered as one of the promising cathode materials for lithium ion batteries(LIBs)because of its high cycling stability and rate capability.However,it is very difficult to control and study the content and position of V⁴⁺and oxygen vacancies in LiV₃O₈,and therefore the mechanism of improving electrochemical performance of LiV₃O₈ is still unclear.Herein,we developed four LiV₃O₈ nanosheets with different V⁴⁺and oxygen vacancy contents and positions.The physicochemical and lithium storage properties indicate that the V⁴⁺and oxygen vacancies in the surface layer increase the contribution of pseudocapacitive lithium storage on the nanosheet surface.The V⁴⁺and oxygen vacancies in the lattice improve the electrical conductivity of LiV₃O₈,and enhance the phase transformation and lithium ion diffusion rates.By adjusting the content of V⁴⁺and oxygen vacancies,we obtained an oxygen-deficient LiV₃O₈ nanosheet which maintained more than 93%of the initial reversible capacity after 300 cycles at 5,000 mA·g^?1.The V⁴⁺and oxygen vacancies play an important role in improving the stability and rapidity of lithium storage.This work is helpful to understand the stable and fast lithium storage mechanism of oxygen-deficient LiV₃O₈,and might lay a foundation for further studies of other oxygen-deficient metal oxide electrodes for long-life and high-power LIBs.     

Nonlinear Electrical Properties of SnO2—LiO—Nb2O5 Varistor System

The electrical properties of (Nb,Li)-doped SnO2 ceramics as a new varistor material were investigated.The sample 97.95% SnO20.50%Li2O0.05%Nb2O5( mol fraction)sintered at 1450℃ possess the highest density(p=6.77g/cm^3) and nonlinear electrical coefficient(α=11.6).The substitution of Sn^4 with Li^ increases the concentration of oxygen vacancies,together with the formation of solid solution ,which will increase the sintering rate greatly and decrease the optimized sintering temperature.The substitution of Sn^4 with Li^ and the variation of temperature play very important effects on the densities,dielectric constant,nonlinear electrical properties and other characteristics of the samples.The properties of the grain boundary barrier and the microstructural characteristics were investigated to ensure the effect of the dopants and the temperature.A grain boundary defect barrier model was used to illustrate the grain boundary barriers formation in SnO2-Li2O-Nb2O5 varistors.     

Strain-regulated sensing properties of α-Fe2O3 nano-cylinders with atomic carbon layers for ethanol detection

The sensitivity of ethanol sensor is of paramount importance in a variety of areas,including chemical production with ethanol,alcohol testing for driving safety,etc.Herein,α-Fe₂O₃ nano-cylinders with atomic carbon layers are synthesized,for the first time,through in-situ catalytic chemical vapor deposition combined with hydrothermal techniques for the detection of ethanol.The reported α-Fe₂O₃@C nano-cylinders with double surficial strain effects deliver an ethanol detection sensitivity of 8 times as compared with α-Fe₂O₃ nano-cylinders,10 times higher as compared with its detection sensitivity to ammonia,para-xylene,methanol and benzene.The sensor also exhibits over-14-day operation stability and the minimum detection limit of 10 ppm.To our best knowledge,the performances surpass those of previously reported α-Fe₂O₃.Such attractive performances are attributed to the enhanced charge transfer in α-Fe₂O₃ owing to the double surficial strain effects of α-Fe₂O₃@C nano-cylinders and the efficient adsorption of ethanol with atomic carbon layers.     

Oxygen Relief/Absorption and Superconductivity of Bi-Pb-Sr-Ca-Cu-O Compounds

The oxygen relief/absorption,superconduc—tivity and structure of(Bi_(1-x)Pb_x)_2Sr_2Ca_2—Cu_3O_y compounds have been studied by TGA,electrical resistance measurement and X-raydiffraction.The amount of oxygen which can be re-leased and superconductivity of the compounds arecorrelative to Pb content.The release of oxygen in-creases when x less than 0.2 and decreases when xexceeds 0.2 with increasing x.The appropriate con-tent of Pb for superconductivity is in the range of0.1相似文献   

Influential Factors on Electromagnetic Properties of Selected 3D Reticulated Ceramics

3D reticulated ceramics (3DRCs) with the composition containing SrFe12O19-SiC-TiO2 were prepared by a replication process with polyurethane sponges as the template in ceramic slurry. The electrical conductivity, dielectric and magnetic parameters of 3D reticulated ceramics (3DRCs) were measured with changes in cell size of the sponges, contents in the slurry and sintering temperature in this paper. Discussions about the influential factors of those parameters were focused on their electrical conductivity. The experimental results indicated that the electrical conductivity of 3DRCs raised with the increase of cell size, SiC/SrO.6Fe2O3 with weight ratio and sintering temperature. X-ray diffractions and SEM were used to investigate the relationship between electrical conductivity and sintering temperature. Deoxidizing reactions of SrO.6Fe2O3 caused the increasing electrical conductivity. The real part of permittivity (ε′) and imaginary part of permeability (μ") raised with the increase of electrical conductivity (σ). The imaginary part of permittivity (ε") has a maximum at 10° S/cm with the increase of σ, and the real part of permeability (μ′)changes slightly with the increase of σ. When σ is at the range of 10-4 S/cm to 100 S/cm (a semi conductive state),both the imagine part of permittivity and permeability raises with increasing σ, therefore, the 3DRCs present their high electromagnetic loss properties.     

Nonlinear Electrical Properties of SnO_2-Li_2O-Nb_2O_5 Varistor System

The electrical properties of (Nb, Li)-doped SnO2 ceramics as a new varistor material were investigated. The sample 97.95%SnO2·0.50%Li2O·0.05%Nb2O5 (mol fraction) sintered at 1450= possess the highest density (ρ=6.77 g/cm3) and nonlinear electrical coefficient (α=11.6). The substitution of Sn4+ with Li+ increases the concentration of oxygen vacancies, together with the formation of solid solution, which will increase the sintering rate greatly and decrease the optimized sintering temperature. The substitution of Sn4+ with Li+ and the variation of temperature play very important effects on the densities, dielectric constant, nonlinear electrical properties and other characteristics of the samples. The properties of the grain boundary barrier and the microstructural characteristics were investigated to ensure the effect of the dopants and the temperature. A grain boundary defect barrier model was used to illustrate the grain boundary barriers formation in SnO2-Li2O-Nb2O5 varistors.     

Strain-sensitive ferromagnetic two-dimensional Cr2Te3

Searching for room temperature magnetic two-dimensional(2D)materials is a charming goal,but the number of satisfied materials is tiny.Strain can introduce considerable deformation into the lattice structure of 2D materials,and thus significantly modulate their intrinsic properties.In this work,we demonstrated a remarkable strain-modulated magnetic properties in the chemical vapor deposited Cr₂Te₃ nanoflakes grown on mica substrate.We found the Curie temperature of Cr₂Te₃ nanoflakes can be positively and negatively modulated under tensile and compressive strain respectively,with a maximum varied value of -40 and-90 K,dependent on the thickness of samples.Besides,the coercive field of Cr₂Te₃ nanoflakes also showed a significant decrease under the applied strain,suggesting the decrease of exchange interaction or the change of the magnetization direction.This work suggests a promise to employ interfacial strain to accelerate the practical application of room temperature 2D magnetics.     

Moire-pattern-modulated electronic structures in Sb2Te3/graphene heterostructure

Moire superlattice has recently been found in topological insulators,which can lead to periodic modulation on the electronic structure.In this work,we report the low-temperature scanning tunneling microscopy study of Sb₂Te₃ films grown on graphitized 4H-SiC.We find that substrate temperature can strongly influence the rotation angles between Sb₂Te₃ film and graphene substrate.Three kinds of moire patterns are observed at the first quintuple layer Sb₂Te₃ film under different substrate temperatures.One shows complicated patterns with a rotation angle of nearly 0°relative to the substrate,another just exhibits simple 1 ×1 structure with a rotation angle of 30°.Other rotation angle like 8.2°is observed at higher substrate temperature as well,which is relatively rare.Comparison of the d//dV curves from Sb₂Te₃ films with different moire patterns indicates that the superstructure can offer degrees of freedom in tailoring electronic structure.This work may stimulate the further study on the moire modulation to the electronic properties of topological insulators.     

Electrical and Optical Properties of GaSe Thin Films

The structure, electrical transport, and optical properties of GaSe films fabricated by means of radio-frequency (RF) magnetron sputtering in Ar were investigated. The as-sputtered GaSe films were amorphous, and their optical energy gap Eg are 1.9～2.6 eV. The effect of the synthesis conditions on the optical and electrical properties of the GaSe films has also been studied     

Ambient flash sintering of reduced graphene oxide/zirconia composites:Role of reduced graphene oxide

Fabrication of graphene/ceramic composites commonly requires a high-temperature sintering step with long times as well as a vacuum or inert atmosphere,which not only results in property degradation but also significant equipment complexity and manufacturing costs.In this work,the ambient flash sintering behavior of reduced graphene oxide/3 mol% yttria-stabilized ZrO₂(rGO/3 YSZ) composites utilizing rGO as both a composite component and a conductive additive is reported.When the sintering condition is carefully optimized,a dense and conductive composite can be achieved at room temperature and in the air within 20 s.The role of the rGO in the FS of the rGO/3 YSZ composites is elucidated,especially with the assistance of a separate investigation on the thermal runaway behavior of the rGO.The work suggests a promising fabrication route for rGO/ceramic composites where the vacuum and furnace are not needed,which is of interest in terms of simplifying the fabrication equipment for energy and cost savings.     

Phase growth patterns for Al_2O_3/GdAlO_3 eutectics over wide ranges of compositions and solidification rates

Phase selection and growth characteristics of directionally solidified Al₂O₃/GdAlO_3(GAP)faceted eutectic ce ramics are investigated over wide ranges of compositions and solidification rates to explore the eutectic coupled zone.Through the obse rvation of the quenched solid-liquid interface,the competitive growth of primary faceted Al₂O₃phase,prima ry non-faceted GAP phase and Al₂O₃/GAP eutectic with diffe rent morphologies is detected.Microstructure transitions from wholly eutectic to primary Al₂O₃(GAP)dendrite plus eutectic and then to wholly eutectic are found in Al₂O₃-2 O mol%Gd₂O₃hypoeutectic(Al₂O₃-26 mol%Gd₂O₃hypereutectic)ceramics with the increase of solidification rate.The dendrite growth of faceted Al₂O₃and non-faceted GAP phases are well predicted by KGT model,which have introduced appro p riate dimensionless supersaturationΩto characterize the anisotropic growth of dendrites.Based on the maximum interface temperature criterion,the competitive growth of primary phase and eutectic is analyzed theoretically and the predicted coupled zone of Al₂O₃/GAP eutectic ceramics is in good agreement with the experimental results.Besides,the influence of microstructure with these different morphologies on the flexural strength of Al₂O₃/GAP eutectic ceramics is studied.     

Vapor deposition of aluminium oxide into N-rich mesoporous carbon framework as a reversible sulfur host for lithium-sulfur battery cathode

Restraining the shuttle effects of lithium polysulfides is the key to improve the cycling reversibility and stability of lithium-sulfur(Li-S)batteries for which design of robust sulfur hosts has been regarded as the most effective strategy.In this work,we report a new type of hybrid sulfur host which is composed of Al₂O₃ homogenously decorated in nitrogen-rich mesoporous carbon framework(NMC-Al₂O₃).The NMC-Al₂O₃ hybrid host features a poly-dispersed spherical morphology and a mesoporous configuration with high surface area and large pore volume that can accommodate a high sulfur content up to 73.5 wt.%.As a result,the fabricated NMC-Al₂O₃-S cathode exhibits all-round improvements in electrochemical properties in term of capacities(1,212 mAh·g^-1at 0.2 C;755 mAh·g^-1at 2 C),cycling charge-discharge reversibility(sustainably 100%efficiencies)and stability(1,000 cycles with only 0.023%capacity decay per cycle at 0.5 C).By contrast,the Al₂O₃-free NMC-S cathode shows both decreased capacities and rapidly descending Coulombic efficiencies during cycling.Density functional theory(DFH")calculations further reveal that the implanted Al₂O₃ can greatly enhance the chemical adsorption and catalytic conversion for various lithium polysulfides and thereby effectively prevent the polysulfide shuttling and significantly improve the utilizability,reversibility and stability of sulfur cathode.     

Electrical Conductivity of Sintered Chromia Mixed with TiO2, CuO and Mn-Oxides

The electrical conductivity of sintered Cr2O3 mixed with 2% and 5% （in molar fraction） TiO2 or CuO was investigated in the temperature range 500-900℃ in air and in At/4 vol. pct H2 atmospheres. The effect of different Mn-oxides on the electrical conductivity of Cr2O3 was also studied under the same conditions. From the conductivity measurements it is established that additions of TiO2 change the defect structure of Cr2O3 and the effect of TiO2 on the electrical conductivity is controlled by TiO2 concentration as well as temperature and O2 partial pressure of the surrounding atmosphere. The conductivity of Cr2O3 increased in air and decreased in the At/H2 atmosphere by CuO additions. The effect of CuO was discussed with possible changes in the defect concentration in Cr2O3. Mixing of Cr2O3 with different Mn-oxides at the same Mn to metal atom fraction decreased the conductivity in air and in Ar/H2 atmospheres. No clear correlation between the spinel fraction and the changes in conductivity could be found.     

Growth mechanism of primary Ti5Si3 phases in special brasses and their effect on wear resistance

In the present work, in-situ Ti₅Si₃ reinforced special brasses were prepared by melt reaction method. The synthesized Ti₅Si₃ phase shows various morphologies in brasses with different Ti₅Si₃ content, and the3 D morphological evolution of primary Ti₅Si₃and its growth mechanism were investigated. The Ti₅Si₃ crystal, which bears D8₈ hexagonal crystal structure, grows along <0001> direction and is revealed by{1010} faces during growth. With the increase of Ti₅Si₃ content in the brasses, the morphology of primary Ti₅Si₃significantly changes from fibers to hexagonal prisms to short-rods with hollow. In addition,the influence of Ti₅Si₃ volume fraction and morphology on the wear behavior of special brass was also revealed. It was substantiated that the wear resistance increases with the increasing volume fraction of Ti₅Si₃, and the corresponding wear mechanism changes from delamination to slight adhesive wear and abrasive wear. However, the friction coefficient shows an abnormal increase when most of the Ti₅Si₃ containing hollows appears in the brass. That is mainly due to the fact that the Ti₅Si₃ is easier to break and fall off resulted by the hollow as a crack source, which makes it unable to resist the plastic deformation of the contact surface during the sliding.