Failure mechanism and influencing factors of cement sheath integrity under alternating pressure

The failure of cement sheath integrity can be easily caused by alternating pressure during large-scale multistage hydraulic fracturing in shale-gas well.An elastic-plastic mechanical model of casing-cement sheath-formation(CSF)system under alternating pressure is established based on the Mohr-Coulomb criterion and thick-walled cylinder theory,and it has been solved by MATLAB programming combining global optimization algorithm with Global Search.The failure mechanism of cement sheath integrity is investigated,by which it can be seen that the formation of interface debonding is mainly related to the plastic strain accumulation,and there is a risk of interface debonding under alternating pressure,once the cement sheath enters plasticity whether in shallow or deep well sections.The matching relationship between the mechanical parameters(elastic modulus and Poisson's ratio)of cement sheath and its integrity failure under alternating pressure in whole well sections is studied,by which it has been found there is a“critical range”in the Poisson's ratio of cement sheath.When the Poisson's ratio is below the“critical range”,there is a positive correlation between the yield internal pressure of cement sheath(SYP)and its elastic modulus.However,when the Poisson's ratio is above the“critical range”,there is a negative correlation.The elastic modulus of cement sheath is closely related to its Poisson's ratio,and restricts each other.Scientific and reasonable matching between mechanical parameters of cement sheath and CSF system under different working conditions can not only reduce the cost,but also protect the cement sheath integrity.

Enhanced electrochemical performance of Si/C electrode through surface modification using SrF_(2) particle

The silicon-based material exhibits a high theoretical specific capacity and is one of the best anode for the next generation of advanced lithium-ion batteries(LIBs).However,it is difficult for the silicon-based anode to form a stable solid-state interphase(SEI)during Li alloy/de-alloy process due to the large volume change(up to 300%)between silicon and Li4.4Si,which seriously limits the cycle life of the LIBs.Herein,we use strontium fluoride(SrF_(2))particle to coat the silicon-carbon(Si/C)electrode(SrF_(2)@Si/C)to help forming a stable and high mechanical strength SEI by spontaneously embedding the SrF_(2) particle into SEI.Meanwhile the formed SEI can inhibit the volume expansion of the silicon-carbon anode during the cycle.The electrochemical test results show that the cycle performance and the ionic conductivity of the SrF_(2)@Si/C anode has been significantly improved.The X-ray photoelectron spectroscopy(XPS)analysis reveals that there are fewer electrolyte decomposition products formed on the surface of the SrF_(2)@Si/C anode.This study provides a facile approach to overcome the problems of Si/C electrode during the electrochemical cycling,which will be beneficial to the industrial application of silicon-based anode materials.

Annealing atmosphere-dependent capacitive energy storage

Electrostatic capacitors based on dielectrics with high energy density and efficiency are desired for modern electrical systems owing to their intrinsic fast charging-discharging speed and excellent reliability.The longstanding bottleneck is their relatively small energy density.Herein,we report enhanced energy density and efficiency in the Aurivillius Pb_(2)Bi_(4)Ti_(5)O_(18)films by controlling the post-annealing atmosphere.The results demonstrate that the fabrication atmosphere has significant effects on the film texture and defects.As the increase of the oxygen pressure of annealing atmosphere,the Pb_(2)Bi_(4)Ti_(5)O_(18)films show a preferred growth orientation of(00l)and fewer defects,which leads to a higher polarization and breakdown field for the film annealed in air atmosphere and thus help to achieve an ultrahigh energy density of59.4 J·cm^(-3)and an improved efficiency of 81.2%.Moreover,the film also exhibits excellent cycling reliability and good thermal stability.The Pb_(2)Bi_(4)Ti_(5)O_(18)films show a significant potential application for dielectric capacitors.

Nanonet-/fiber-structured flexible ceramic membrane enabling dielectric energy storage

Ceramics are considered intrinsically brittle at macro scale due to the lack of slip mechanism and pre-existing defects,which greatly limits their potential applications in emerging fields including wearable electronic devices and flexible display.In this contribution,we developed BiFeO_(3)/SiO_(2) dual-networks with exceptional flexibility through a coupled electronetting/electrospun method.The hybrid nanostructured networks endow the material with high tensile strength(2.7 MPa),excellent flexibility(80%recoverable deformation),and robust fatigue resistance performance(maintain flexibility after a 1000-cyclic compress test).After in-situ compounded with dielectric polymer via a layer-by-layer solution casting method,the resultant three-dimensional(3D)composite film exhibits a twice higher dielectric constant(εr)than polyether imide(PEI)film.More importantly,the breakdown strength of the 3D composite film is almost the same as that of the PEI film,resulting in an enhanced energy density of~6.0 J/cm^(3) and a high efficiency of 80%at 4.58 MV/cm.The unique structure,combined with the excellent balance between mechanical and dielectric properties in flexible structures,is of critical significance to the design of flexible functional ceramics and broadening their applications in wearable electric devices.

Local symmetry distortion induced anomalous thermal conduction in thermoelectric layered oxyselenides

Polyhedral distortion,associated closely with the atomic arrangement and interatomic interactions,drives many unique behaviors in solids,such as phase transition and negative thermal expansion.In thermoelectric heteroanionic oxides,the anionic polyhedra are widely present,but their effect on thermal transport is rarely investigated.Here,we report an anomalous thermal conduction induced by local symmetry distortion in layered oxyselenides via solving the Boltzmann transport equation based on first-principles calculations.We found interestingly that lighter BiCuSeO exhibits lower thermal conductivity than heavier BiAgSeO.Due to the different distorted degrees of CuSe4 and AgSe4 tetrahedrons,Cu prefers the in-plane vibration,while Ag has more tendency of out-of-plane vibration.Thus,the heat-carrying phonons dominated by the rattling-like vibration of Cu are significantly suppressed,resulting in lower thermal conductivity of BiCuSeO.This study highlights the importance of polyhedral distortion in regulating thermal conduction in layered heteroanionic materials.

Colossal permittivity and ultralow dielectric loss in Nb-doped SrTiO_(3) ceramics

Defect engineering has been applied to prepare materials with modifiable dielectric properties.SrTiNbxO3(x=0,0.003,0.006,0.009,0.012)ceramics were synthesized using the traditional solid-state reaction method and sintered in a reducing atmosphere.All samples show excellent dielectric properties with giant permittivity(>3.5×10^(4))and low dielectric loss(<0.01).SrTiNb0.003O3 ceramic exhibits a colossal permittivity of 4.6×10^(4)and an ultralow dielectric loss of 0.005(1 kHz,room temperature)as well as great temperature stability in the range of(−60)–160℃.The mechanism of the presented colossal permittivity(CP)properties is investigated by conducting X-ray photoelectron spectroscopy(XPS)and analyzing activation energies.The results indicate that the introduction of Nb5+and the reducing sintering atmosphere together generated the formation of Ti^(3+)and V_(O)^(**).These defects further form Ti-V_(O)^(**)-Ti'_(Ti)defect dipoles,contributing to the coexisting giant permittivity and low dielectric loss in Nb-doped SrTiO_(3)(STN)ceramics.

Ferroelectricity survives in the sub-nanometer scale

Ultrathin materials with maintained functionalities are the prerequisites for the device miniaturization and integration.Ferroelectric materials are key ingredients in nonvolatile memories as information storage media and in field-effect transistors as gate dielectrics[1],and therefore their thickness scaling enables the high-capacity and energy-efficient electronics.However,the longstanding problem for ultrathin ferroelectrics is that the strong depolarizing field can cancel ferroelectricity[2].In traditional barium titanates,the ultrathin film turns paraelectric without spontaneous polarizations[2,3].Moreover,realizing switchable ferroelectric behaviors is even more challenging because of the unsuppressed leakage current.In recent years,although some studies have demonstrated that ferroelectricity can be stabilized in several nanometers in perovskites and fluorites[4-6],there still lacks direct evidence to confirm that ultrathin ferroelectrics have macroscopic hysteresis loops and polarization switching,which is the foundation of the practical usage of ultrathin ferroelectrics.

Electrical and thermal transport behaviours of high-entropy perovskite thermoelectric oxides

Oxide-based ceramics could be promising thermoelectric materials because of their thermal and chemical stability at high temperature.However,their mediocre electrical conductivity or high thermal conductivity is still a challenge for the use in commercial devices.Here,we report significantly suppressed thermal conductivity in SrTiO_(3)-based thermoelectric ceramics via high-entropy strategy for the first time,and optimized electrical conductivity by defect engineering.In high-entropy(Ca_(0.2)Sr_(0.2)Ba_(0.2)Pb_(0.2)La_(0.2))TiO_(3)bulks,the minimum thermal conductivity can be 1.17 W/(m·K)at 923 K,which should be ascribed to the large lattice distortion and the huge mass fluctuation effect.The power factor can reach about 295μW/(m·K^(2))by inducing oxygen vacancies.Finally,the ZT value of 0.2 can be realized at 873 K in this bulk sample.This approach proposed a new concept of high entropy into thermoelectric oxides,which could be generalized for designing high-performance thermoelectric oxides with low thermal conductivity.

High energy storage capability of perovskite relaxor ferroelectrics via hierarchical optimization

Ultrafast charge/discharge process and ultrahigh power density enable dielectrics essential components in modern electrical and electronic devices, especially in pulse power systems. However, in recent years, the energy storage performances of present dielectrics are increasingly unable to satisfy the growing demand for miniaturization and integration, which stimulates further researches on dielectrics with higher energy density and efficiency.Among various inorganic dielectrics, perovskite relaxor ferroelectrics are recognized as promising candidates for energy storage applications, with high permittivity and relatively high efficiency. Here, we focus on recent progress and achievements on optimizing perovskite relaxor ferroelectrics toward better energy storage capability through hierarchical design. The principles and key parameters of dielectric energy storage, together with the definition of majority types of dielectrics, are introduced at first. Strategies within various scales include domain, grain size, orientation, and composite engineering are summarized. The existing challenges are presented and future prospects are proposed in the end, with the background of both academic explorations and industrial applications.

BiCuSeO as state-of-the-art thermoelectric materials for energy conversion:from thin films to bulks

BiCuSeO-based thermoelectric material has attracted great attention as state-of-the-art thermoelectric materials since it was first reported in 2010. In this review, we update the studies on the BiCuSeO thin films first. Then, we focus on the most recent progress of multiple approaches that enhance the thermoelectric performance including advanced synthesized technologies, notable mechanisms for higher power factor （optimizing carrier concentration, carrier mobility, Seebeck coefficient） and doping effects predicted by calculation. And finally, aiming at further enhancing the performance of these materials and ultimately commercial application, we give a brief discussion on the urgent issues to which should be paid close attention.

Self-etching Ni-Co hydroxides@Ni-Cu nanowire arrays with enhancing ultrahigh areal capacitance for flexible thin-film supercapacitors

Flexible thin-film supercapacitors with high specific capacitance are highly desirable for modern wearable or micro-sized electrical and electronic applications. In this contribution, Ni-Co hydroxides（NCH）nanosheets were deposited on top of Ni-Cu alloy（NCA）nanowire arrays forming a freestanding thin-film composite electrode with hierarchical structure for supercapacitors.During electrochemical cycling, the dissolution of Cu into Cu ions will create more active sites on NCA, and the redeposited copper oxide can be coated onto NCH, giving rise to substantial increase in specific capacitance with cycling. Meanwhile, NCA and NCH have excellent conductivity, thus leading to excellent rate performance. This flexible thin-film electrode delivers an ultrahigh initial specific capacitance of 0.63 F·cm^（-2）（or 781.3 F·cm^（-3））.During charge-discharge cycles, the specific capacitance can increase up to 1.18 F·cm^（-2）（or 1475 F·cm^（-3）） along with the“self-etching”process. The electrode presents a better specific capacitance and rate capability compared with previously reported flexible thin-film electrode, and this novel design of etching technique may expand to other binary or ternary materials.

Pyrochlore-based high-entropy ceramics for capacitive energy storage

High-performance dielectrics are widely used in high-power systems,electric vehicles,and aerospace,as key materials for capacitor devices.Such application scenarios under these extreme conditions require ultra-high stability and reliability of the dielectrics.Herein,a novel pyrochlore component with high-entropy design of Bi1.5Zn_(0.75)Mg_(0.25)Nb_(0.75)Ta_(0.75)O_(7)(BZMNT)bulk endows an excellent energy storage performance of Wrec≈2.72 J/cm3 together with an ultra-high energy efficiency of 91%at a significant enhanced electric field Eb of 650 kV/cm.Meanwhile,the temperature coefficient(TCC)of BZMNT(~−220 ppm/℃)is also found to be greatly improved compared with that of the pure Bi1.5ZnNb1.5O7(BZN)(~−300 ppm/℃),demonstrating its potential application in temperature-reliable conditions.The high-entropy design results in lattice distortion that contributes to the polarization,while the retardation effect results in a reduction of grain size to submicron scale which enhances the Eb.The high-entropy design provides a new strategy for improving the high energy storage performance of ceramic materials.

Thermoelectric thin films:Promising strategies and related mechanism on boosting energy conversion performance

Thermoelectrics can be capable of direct and reversible conversion between heat and electricity.Low dimensional thermoelectric materials,especially two dimensional(2D)thin films,have been considered as a breakthrough to decouple the correlations between electronic and thermal transport,contributing to the optimization of thermoelectric performance.During the past few decades,some effective strategies combined with physical concepts like quantum confinement effect,energy filtering effect,band structure tuning and interface engineering are introduced to design high performance thermoelectrics.Having a thorough understanding the underlyingmechanisms is essential to develop thermoelectric materials.Here,our review summarizes the major strategies that can be utilized in thermoelectric thin films,including fabrication of superlattice structure,formation of two-dimensional electron gas(2DEG)system,orientation regulation,strain engineering and magnetic manipulation,from the aspects of deep mechanisms analyses,recent progress and prospects,inspiring significant improvement of thermoelectric properties.

Polymer-infiltrated layered silicates for dental restorative materials

Layered porous ceramic used for polymer-infiltrated-ceramic-network materials(PICNs) may be a promising candidate for dental restoration.The effect of sintering temperature of ceramic green bodies on mechanical and optical properties of PICNs is unclear.The purpose was to fabricate PICNs and evaluate their mechanical and optical properties.Polymer-infiltrated layered silicates for dental restorative materials were prepared via infiltrating polymerizable monomers into partially sintered porous silicates and thermo-curing.Bending samples for flexural strength and fracture toughness were fabricated(sample numbers of n=15).Vickers hardness and elastic modulus were measured via nano-indentation(n=10).One-way ANOVA and Weibull statistics were used for statistical analysis.Optical property was characterized by spectral reflectance.Brittleness index was used to characterize the machinability of the materials.Microstructures and phase structures were investigated using scanning electron microscopy(SEM) and X-ray diffractometer(XRD),respectively.Flexural strength of polymer-infiltrated layered silicates varied from 91.29 to 155.19 MPa,fracture toughness ranged from 1.186 to 1.782 MPa·m^1/2,Vickers hardness ranged from 1.165 to 9.596 GPa,and elastic modulus ranged from 25.35 to 100.50 GPa.The formed glass phases at 1200 and 1300℃ showed influences on corresponding optical property,which could be observed from spectral reflectance.A kind of PICNs was fabricated by infiltrating polymerizable monomers into layered porous ceramic networks.Sintering temperature could have dramatic effects on the mechanical and optical properties of porous ceramics and PICNs.These kinds of materials possess similar properties to that of natural tooth and could be used for dental restoration.

Low-dimensional nanostructured photocatalysts

Low-dimensional nanostructures are a promising class of ideal high-performance candidates for energy storage and conversion owing to their unique structural,optical,and chemical properties.Low-dimensional nanostructured photocatalysts have attracted ever-growing research attention.In this review,we mainly emphasize on summarizing the 0-,1-,and 2-dimensional nanostructured photocatalysts systematically,including their photocatalytic performance,synthesis methods,and theoretical analysis.From the viewpoint of dimension,we try to figure out the way to design more high-efficiency photocatalysts towards numerous applications in the field of solar energy conversion,hoping to promote efficient control and rational development of photocatalysts.

Contribution of point defects and nano-grains to thermal transport behaviours of oxide-based thermoelectrics

Point defects and nano-grains are very effective ways to control the thermal conductivity in oxide-based thermoelectrics.Here we use the optimised Debye–Callaway model to understand how the effect of point defects and nano-grains to reduce the thermal conductivity by inducing normal process and oxygen vacancy in oxide-based thermoelectrics.Our results reveal that this model can be effective to fit the experimental data of thermal conductivity in ZnO-,CaMnO_(3)-,BiCuSeO-,SrTiO_(3)^(-)and In_(2)O_(3)-based systems,which indicate that the normal scattering process and the oxygen vacancy will make obvious contribution to the thermal conductivity as compared with alloy compounds system.These calculations also propose that it could be desirable to obtain higher ZT by controlling the concentration of oxygen vacancy in the nano-grained thermoelectric oxides.

Microstructure,wear and corrosion performance of plasma electrolytic oxidation coatings formed on D16T Al alloy

The plasma electrolytic oxidation(PEO)coatings were produced on D16 T Al alloy in the aluminate and silicate electrolyte with and without graphene.The phase composition,microstructure and elemental distribution of the coatings were tested by X-ray diffraction(XRD),scanning electron microscope(SEM)and energy dispersive X-ray spectroscopy(EDX).The wear and corrosion resistance of PEO coatings were evaluated by dry sliding wear tests and electrochemical impedance spectroscopy(EIS).The morphology feature of the wear tracks was compared and analyzed by SEM and three-dimensional microscope.The results demonstrate that the structure,wear and corrosion resistance of PEO coatings with graphene are better than that of PEO coatings without graphene.The coating fabricated in the aluminate electrolyte with graphene exhibited the lowest roughness.The coated samples formed in silicate electrolyte with graphene displayed the thickest,densest and the most compact coating.It exhibited the best wear and corrosion resistance due to the incorporation mode of graphene in the coatings.The mechanism of graphene improving the wear and corrosion resistance of PEO coating was further discussed.In summary,the comprehensive performances of PEO coatings formed in silicate electrolyte on D16 T Al alloy are superior to that produced in aluminate electrolyte.

MAGNETOELECTRIC RESPONSES IN MULTIFERROIC COMPOSITE THIN FILMS

Multiferroic composite thinfilms of ferroelectrics and magnets have attracted ever-increasing interest in most recent years.In this review,magnetoelectric(ME)responses as well as their underlying ME coupling mechanisms in such multiferroic composite thinfilms are discussed,oriented by their potential applications in novel ME devices.Among them,the direct ME response,i.e.,magnetic-field control of polarization,can be exploited for micro-sensor applications(sensing magneticfield,electric current,light,etc.),mainly determined by a strain-mediated coupling interaction.The converse ME response,i.e.,electric-field modulation of magnetism,offers great opportunities for new potential devices for spintronics and in data storage applications.A series of prototype ME devices based on both direct and converse ME responses have been presented.The review concludes with a remark on the future possibilities and scientific challenges in thisfield.

HIGH DIELECTRIC PROPERTIES OF LOW-TEMPERATURE SINTERED(Ca_(0.5)Cu_(0.5))TiO_(3)CERAMICS

High dielectric-permittivity(Ca_(0.5)Cu_(0.5))TiO_(3)-based ceramics have been prepared by a sol-gel method combined with a solid state sintering process.The results indicate that the additives of H_(3)BO_(3)have a remarkable effect on the sintering temperature,microstructure and dielectric properties.High density(Ca_(0.5)Cu_(0.5))TiO_(3).bulk ceramics can be obtained after sintering at 930℃.The as-sintered ceramics show high dielectric constants(~1000),and low losses(~0.05).The dielectric properties are nearly independent of frequency and temperature in a wide range.The activation energy is calculated as about_(0.5)0 eV by impedance spectrum method.