Microscopic growth mechanism and edge states of monolayer 1T'-MoTe_(2)

Transition metal ditellurides(TMTDs)have versatile physical properties,including non-trivial topology,Weyl semimetal states and unique spin texture.Controlled growth of high-quality and large-scale monolayer TMTDs with preferred crystal phases is crucial for their applications.Here,we demonstrate the epitaxial growth of 1T'-MoTe_(2) on Au(111)and graphitized silicon carbide(Gr/SiC)by molecular beam epitaxy(MBE).We investigate the morphology of the grown1T'-MoTe_(2) at the atomic level by scanning tunnelling microscopy(STM)and reveal the corresponding microscopic growth mechanism.It is found that the unique ordered Te structures preferentially deposited on Au(111)regulate the growth of monolayer single crystal 1T'-MoTe_(2),while the Mo clusters were preferentially deposited on the Gr/SiC substrate,which impedes the ordered growth of monolayer MoTe_(2).We confirm that the size of single crystal 1T'-MoTe_(2) grown on Au(111)is nearly two orders of magnitude larger than that on Gr/SiC.By scanning tunnelling spectroscopy(STS),we observe that the STS spectrum of the monolayer 1T'-MoTe_(2) nano-island at the edge is different from that at the interior,which exhibits enhanced conductivity.

Mechanical response and microscopic damage mechanism of pre-flawed sandstone subjected to monotonic and multilevel cyclic loading:A laboratory-scale investigation

This study aims to investigate the mechanical response and acoustic emission(AE)characteristic of pre-flawed sandstone under both monotonic and multilevel constant-amplitude cyclic loads.Specifically,we explored how coplanar flaw angle and load type impact the strength and deformation behavior and microscopic damage mechanism.Results indicated that being fluctuated before rising with increasing fissure angle under monotonic loading,the peak strength of the specimen first increased slowly and then steeply under cyclic loading.The effect of multilevel cyclic loading on the mechanical parameters was more significant.For a single fatigue stage,the specimen underwent greater deformation in early cycles,which subsequently stabilized.Similar variation pattern was also reflected by AE count/energy/b-value.Crack behaviors were dominated by the fissure angle and load type and medium-scale crack accounted for 74.83%–86.44%of total crack.Compared with monotonic loading,crack distribution of specimen under cyclic loading was more complicated.Meanwhile,a simple model was proposed to describe the damage evolution of sandstone under cyclic loading.Finally,SEM images revealed that the microstructures at the fracture were mainly composed of intergranular fracture,and percentage of transgranular fracture jumped under cyclic loading due to the rapid release of elastic energy caused by high loading rate.

Rheo-optic in situ synchronous study on the gelation behaviour and mechanism of waxy crude oil emulsions

An improved rheo-optic in situ synchronous measurement system was employed to investigate the gelation behaviour and mechanism of waxy crude oil emulsions. By combining transmitted natural light and reflected polarized light microscopy, a multiangle composite light source was built to achieve the simultaneous observation of wax crystals and emulsified water droplets, as well as their dynamic aggregation process. Main outcomes on the microscopic mechanism were obtained by developed microscopic image processing method. It was found that the microstructure of W/O waxy crude oil emulsion has the evolution of “individual structure--homogeneous aggregate structure--heterogeneous coaggregate structure--floc structure” during the static cooling, which results in the four stages during gelation process. Different from previous studies, the aggregation of emulsified water droplets was found to be more significant and contributes to the formation and development of the wax crystals-emulsified water droplets coaggregate, which plays a decisive role in the further evolution of the gelled microstructure. Time series microscopic images show the dynamic aggregation of emulsified water droplets and wax crystals. Two different aggregation behaviours between wax crystals and water droplets were observed. That wax crystals can not only embed in gaps between adjacent water droplets and enhance the structure, but also surround the outside of the water droplets and continue to grow resulting in the interconnection of different coaggregates to form a larger floc structure. In addition, correlation between viscoelasticity and microstructure evolution of waxy crude oil emulsions of different water contents was discussed. With increasing water contents, the microstructure is changed from wax crystal flocculation structure as the main skeleton and the emulsified water droplets embedded in it, into the aggregation of emulsified water droplets occupying the main position. When the number of wax crystals and water droplets reaches a certain ratio, did wax crystals form coaggregates with emulsified water droplets, and the remaining wax crystals formed an overall flocculation structure, the viscoelasticity of the waxy crude oil emulsion is the highest.

Stress sensitivity of carbonate gas reservoirs and its microscopic mechanism

In order to evaluate the stress sensitivity of carbonate reservoirs,a series of rock stress sensitivity tests were carried out under in-situ formation temperature and stress condition.Based on the calibration of capillary pressure curve,the variable fractal dimension was introduced to establish the conversion formula between relaxation time and pore size.By using the nuclear magnetic resonance(NMR)method,the pore volume loss caused by stress sensitivity within different scales of pore throat was quantitatively analyzed,and the microscopic mechanism of stress sensitivity of carbonate gas reservoirs was clarified.The results show that fractures can significantly affect the stress sensitivity of carbonate reservoirs.With the increase of initial permeability,the stress sensitivity coefficient decreases and then increases for porous reservoirs,but increases monotonously for fractured-porous reservoirs.The pore volume loss caused by stress sensitivity mainly occurs for mesopores(0.02–0.50μm),contributing more than 50%of the total volume loss.Single high-angle fracture contributes 9.6%of the stress sensitivity and 15.7%of the irreversible damage.The microscopic mechanism of the stress sensitivity of carbonate gas reservoirs can be concluded as fracture closure,elastic contraction of pores and plastic deformation of rock skeleton.

Studies on the Microscopic Mechanism of the Thermodynamic Stability of Pesticide Microemulsion

The cryo-fracture electron microscope was used to study the micro-structure of pesticide mi-croemulsions. The hydromechanical radius (Rh) and the distribution (fRh) of pesticide microemulsions were determined by photo-correlation spectroscopy. This study showed that the Rh was significantly greater when the ratio of surfactants to water (w/w) decreased to 20/31 from 27/26, and a bicontinuous structure was formed when the ratio dropped to 15/36. These results explained the relationship between pesticide properties and the microscopic structure, and provided a good method for studying the microscopic structure of pesticide formulations.

Microscopic mechanism of periodical electroosmosis in reservoir rocks

Based on the electric double layer （EDL） theory and the momentum equation governing the electroosmosis flow, this paper presents an analytical solution to the peri- odical electroosmosis with a parallel straight capillary bundle model of reservoir rocks to reveal the microscopic mechanism of the electroosmotic flows in rocks. The theory shows that both the frequency dispersion characteristics of the ma^roscQpic electroosmotic Darcy velocity in unsealed rocks and the electroosmotic pressure coefficient in sealed rocks de- pend on the porosity and electrochemical properties of reservoir rocks. The mathematical simulation indicates that the distribution of the periodical electroosmotic velocity is wave- like in the rock pore. The greater the porosity is, the greater electroosmotic the Darcy velocity and the smaller electroosmotic pressure coefficient are generated. The module values of the electroosmotic Darcy velocity and the electroosmotic pressure coefficient increase with the decreasing solution concentration or the increasing cation exchange ca- pacity without affecting the phase of the electroosmotic Darcy velocity.

Modeling and petrophysical properties of digital rock models with various pore structure types: An improved workflow

Pore structure is a crucial factor affecting the physical properties of porous materials,and understanding the mechanisms and laws of these effects is of great significance in the fields of geosciences and petroleum engineering.However,it remains a challenge to accurately understand and quantify the relationship between pore structures and effective properties.This paper improves a workflow to focus on investigating the effect of pore structure on physical properties.First,a hybrid modeling approach combining process-based and morphology-based methods is proposed to reconstruct 3D models with diverse pore structure types.Then,the characteristics and differences in pore structure in these models are compared.Finally,the varia-tion laws and pore-scale mechanisms of the influence of pore structure on physical properties(permeability and elasticity)are discussed based on the reconstructed models.The relationship models between pore structure parameters and perme-ability/elastic parameters in the grain packing model are established.The effect of pore structure evolution on permeability/elasticity and the microscopic mechanism in three types of morphology-based reconstruction models are explored.The influence degree of pore structure on elastic parameters(bulk modulus,shear modulus,P-wave velocity,and S-wave veloc-ity)is quantified,reaching 29.54%,51.40%,18.94%,and 23.18%,respectively.This work forms a workflow for exploring the relationship between pore structures and petrophysical properties at the microscopic scale,providing more ideas and references for understanding the complex physical properties in porous media.

Influence of crystal-transforming agent on the performance and mechanism ofα-high-strength gypsum prepared from FGD gypsum

This paper investigates the impact of flue gas desulfurization(FGD)gypsum's crystal modifier on the characteristics and microcosmic mechanism ofα-high strength gypsum.The results demonstrate that all three crystal modifiers can convert FGD gypsum toα-high-strength gypsum.Citric acid(CA)has the most significant influence onα-high-strength gypsum,and the preparedα-high-strength gypsum is short columnar,with an aspect ratio in the range of 1-3,and has a faster setting time,a larger specific surface area,and a smaller standard consistency,higher compressive strength,greater surface hardness,and smaller crystal particle size.The initial setting time of theα-high-strength gypsum manufactured with CA crystal modifier was decreased by 36%compared to the blank sample,the final setting time was lowered by 37.5%,and the water consumption of the standard consistency was reduced by 8%.The maximum strength is 32 MPa after 2 h,the absolute dry compressive strength is up to 38 MPa,and the surface hardness is improved by 24.43%.

Discrete element simulation of crushable rockfill materials

A discrete element method was used to study the evolution of particle crushing in a rockfill sample subjected to triaxial shear. A simple procedure was developed to generate clusters with arbitrary shapes, which resembled real rockfill particles. A theoretical method was developed to define the failure criterion for an individual particle subjected to an arbitrary set of contact forces. Then, a series of numerical tests of large-scale drained triaxial tests were conducted to simulate the behaviors of the rockfill sample. Finally, we examined the development of micro-characteristics such as particle crushing, contact characteristics, porosity, deformation, movement, and energy dissipation. The simulation results were partially compared with the laboratory experiments, and good agreement was achieved, demonstrating that the particle crushing model proposed can be used to simulate the drained triaxial test ofrockfill materials. Based on a comparison of macro behaviors of the roekfill sample and micro structures of the particles, the microscopic mechanism of the rockfill materials subjected to triaxial shear was determined qualitatively. It is shown that the crushing rate, rather than the number of crushed particles, can be used to reflect the relationship between macro- and micro-mechanical characteristics of rockfill materials. These research results further develop our understanding of the deformation mechanism of rockfill materials.

Optimizing signal collection efficiency of the VIPA-based Brillouin spectrometer

Brillouin spectroscopy is an emerging tool for microscopic optical imaging as it allows for non-invasive and direct assessment of the viscoelastic properties of materials.Recent advances of background-free confocal Brillouin spectrometer allows investigators to acquire the Brillouin spectra for turbid samples as well as transparent ones.However,due to strong signal loss induced by the imperfect optical setup,the Brillouin photons are usually immersed in background noise.In this report,we proposed and experimentally demonstrated multiple approaches to enhance the signal collction eficiency.A signal enhancement by>4 times can be observed,enabling ob-servation of ultra-weak signals.

MICRO/NANO-MACHINING ON SILICON SURFACE WITH A MODIFIED ATOMIC FORCE MICROSCOPE

To understand the deformation and removal mechanism of material on nano-scale at ultralow loads,a systemic study on AFM micro/nano-machining on single crystal ailicon is conducted. The results indicate that AFM nano- machining has a precisely dimensional controllability and a good surface quality on nanometer scale.A SEM is adopted to observe nano-machined region and chips,the results indicate that the material removal mechanisms change with the applied normal load. An XPS is used to analyze the changes of chemical composition inside and outside the nano-machined region respectively.The nano-indentation which is conducted with the same AFM diamond tip on the machined region shows a big discrepancy compared with that on the macro-scale. The calculated results show higher nano-hardness and elastic modulus than normal values .This phenomenon on be regarded as the indentation size effect(ISE).

NONEQUILIBRIUM STATISTICAL FOUNDATION OF FATIGUE FRACTURE

The physical foundation of statistical law of fatigue fracture is discussed. The universal forms of the microcrack growth rate, fluctuation growth coefficient and distribution function and fatigue life distribution function have been given.

Study of high-speed-impact-induced conoidal fracture of Ti alloy layer in composite armor plate composed of Ti-and Al-alloy layers

In order to understand the mechanism of conoidal fracture damage caused by a high-speed fragmentsimulating projectile in titanium alloy layer of a composite armor plate composed of titanium-and aluminum-alloy layers,the ballistic interaction process was successfully simulated based on the Tuler eButcher and GISSMO coupling failure model.The simulated conoidal fracture morphology was in good agreement with the three-dimensional industrial-computed-tomography image.Further,three main damage zones(zones I,II,and III)were identified besides the crater area,which are located respectively near the crater area,at the back of the target plate,and directly below the crater area.Under the high-speed-impact conditions,in zone II,cracks began to form at the end of the period of crack formation in zone I,but crack formation in zone III started before the end of crack formation in zone II.Further,the damage mechanism differed for different stress states.The microcracks in zone I were formed both by void connection and shear deformation.In the formation of zone I,the stress triaxiality ranged from2.0 to1.0,and the shear failure mechanism played a dominant role.The microcracks in zone II showed the combined features of shear deformation and void connection,and during the formation process,the stress triaxiality was between 0 and 0.5 with a mixed failure mode.Further,the microcracks in zone III showed obvious characteristics of void connection caused by local melting.During the zone III formation,the triaxiality was 1.0e1.9,and the ductile fracture mechanism was dominant,which also reflects the phenomenon of spallation.

EXPERIMENTAL RESEARCH OF NANO-CUTTING BY USING SCANNING PROBE MICROCOPE

An experimental study on cutting amorphous alloy at nanometer scale is conducted by applying the principle and technology of scanning probe microscope(SPM) It is revealed from the experiments that cutting inside SPM is an excellent and direct way to research the material removal process at small size Based on the experimental results,the chip formation mechanism for the cutting of amorphous alloy is discussed It is found that the deformation along the direction of chip flow occurs ahead of the appearance of localized shear,and a simplified geometrical model is proposed to illustrate the deformation.

Conformational isomerization:A novel mechanism to realize the AIE-TADF behaviors

Thermally activated delayed fluorescence(TADF)materials with aggregationinduced emission(AIE)features can overcome aggregation-caused quenching(ACQ)and emit intensely in aggregate states and thus have attracted enormous attention in the fields of high-efficiency organic light-emitting diodes,bioimaging,photodynamic therapy,photocatalysis,etc.However,their corresponding exact working mechanisms at the microscopic level are still far from clear.Herein,by carefully investigating the physical properties of our newly designed TADF material 6-(10H-spiro[acridine-9,9′-fluoren]-10-yl)nicotinonitrile in various states,we concluded that conformational isomerization plays an important role in realizing high photoluminescence quantum yields in its amorphous neat film state,in which the high-lying quasi-axial conformations with non-TADF features and low-lying quasiequal conformations with TADF characteristics serve as the host matrix and dopant,respectively,thus suppressing ACQ in disordered aggregate states.Our work not only offers a new possible microscopic mechanism by using conformational isomerization for the AIE-TADF phenomenon but also provides a novel method for designing high-efficiency AIE-TADF emitters.

Mechanism of magnetic field-modulated luminescence from lanthanide ions in inorganic crystal:a review

The luminescence from lanthanide ions has potential applications in light emitting diodes,biomedical,solar cells,sensors,display,etc.However,the luminescence is suffered from the various problems,such as low luminescence efficiency and inharmonious wavelength for energy transfer.Magnetic field is an efficient method to modulate the wavelength and intensity of luminescence from lanthanide ions.Magnetic field redistributes the populated electrons in the excited states to tune the wavelength of lanthanide ions by Zeeman effect,mixing effect,and quantum confinement effect.Magnetic field enhances or suppresses the luminescence intensity by the administration of cross-relaxation,energy transfer,and Boltzmann population.In this review,we first introduce the various phenomena and mechanisms of magnetic field modulated downshift luminescence from lanthanide ions,including Zeeman effect,cross-relaxation,crystal structure,absorption,quantum confinement effect,and magneticoptical hysteresis.Then,we explain the regulation of upconversion luminescence by magnetic field,containing energy transfer and mixing effect.Finally,different options regarding how to understand the mechanism of magnetic field-modulated luminescence from lanthanide ions in the future are outlined.

Effects and mechanisms of surfactants on physical properties and microstructures of metakaolin-based geopolymer

In this study,the effects of five different ionic and nonionic surfactants on the physical properties and microstructures of a metakaolin-based geopolymer(MKG)were investigated.It is the first comprehensive comparative study of the effects of sodium lauryl sulfonate(SLS),alkyl polyglycoside(APG),benzalkonium chloride(BAC),sucrose fatty acid esters(SE),and stearic acid(STA)on MKG.Viscosity,densities,apparent water absorption,and compressive strength were measured,and pore structures,micro-defects,and gels observed through scanning electron microscopy(SEM)and mercury intrusion porosimetry(MIP).In the MKG slurry,a high affinity of surfactants to liquid-air interfaces increased viscosity and promoted the generation of bubbles.Based on both the ionic types and molecular configurations of the surfactants,stronger adsorption of a surfactant on the surface of the metakaolin resulted in better dispersion of metakaolin particles and a denser microstructure of the MKG.The surfactants with weaker adsorption(SLS and APG)caused higher porosity,a larger pore size,and more micro-defects,while those with stronger adsorption(BAC,SE,and STA)led to relatively lower porosity and denser microstructures.Density,water absorption,and compressive strength were closely related to the total intrusion porosity of the MKG.The mechanisms underlying surfactant adsorption to the surface of metakaolin are proposed.

THE MICROSCOPIC MECHANISM OF THE(p,π^+)REACTION

The microscopic desecription of the π nucleus scattering is generalized to study the （p,π）reactions.The differential eross section of the 3He（p, π）4He at Tplab,=415 MeV iscalculated by this method. The shape of the theoretical angular distribution coincideswith the experimental data,but the absolute value is larger than the experimental value by afactor of 2.

Effect of tempering temperature on strain hardening exponent and flow stress curve of 1000MPa grade steel for construction machinery

To study the effect of tempering temperature on strain hardening exponent and flow stress curve,one kind of 1000 MPa grade low carbon bainitic steel for construction machinery was designed,and the standard uniaxial tensile tests were conducted at room temperature.A new flow stress model,which could predict the flow behavior of the tested steels at different tempering temperatures more efficiently,was established.The relationship between mobile dislocation density and strain hardening exponent was discussed based on the dislocation-stress relation.Arrhenius equation and an inverse proportional function were adopted to describe the mobile dislocation,and two mathematical models were established to describe the relationship between tempering temperature and strain hardening exponent.Nonlinear regression analysis was applied to the Arrhenius type model,hence,the activation energy was determined to be 37.6kJ/mol.Moreover,the square of correlation coefficient was 0.985,which indicated a high reliability between the fitted curve and experimental data.By comparison with the Arrhenius type curve,the general trend of the inverse proportional fitting curve was coincided with the experimental data points except of some fitting errors.Thus,the Arrhenius type model can be adopted to predict the strain hardening exponent at different tempering temperatures.

Analysis of Sulfate Corrosion Resistance and Reliability of Recycled Concrete

In order to study the effect of different recycled fine aggregate and recycled coarse aggregate substitution rate on the sulfate resistance of recycled concrete under dry-wet cycle conditions,the substitution rate of recycled fine aggregate and recycled coarse aggregate was designed to be 0%/0%,10%/0%,0%/25%,10%/25%,0%/50%and 10%/50%of the six groups of mixture ratio.The mass loss rate and relative dynamic modulus of each group of test blocks were tested respectively and the optimal ratio was selected.The Wiener distribution probability method was used to establish the reliability function to optimize the remaining life of the ratio.The results show that the dynamic elastic modulus and mass show a trend of increasing first and then decreasing during the dry-wet cycles of sulfate.The recycled concrete has the strongest resistance to sulfate attack under the substitution rate of recycled fine aggregate and recycled coarse aggregate is 10%and 0%,and the test blocks will be damaged after being subjected to 182 times of dry-wet cycles of sulfate.The effect of recycled coarse aggregate on the sulfate attack resistance of the test block is more obvious than that of the recycled fine aggregate,which leads to the durability of the concrete being greatly reduced.