Multi‐layer π‐stacked emitters based on spatially confined donor/acceptor/donor (D/A/D) patterns have been developed to achieve high‐efficiency thermally activated delayed fluorescence (TADF). In ...this case, dual donor moieties and a single acceptor moiety are introduced to form two three‐dimensional (3D) emitters, DM‐BD1 and DM‐BD2, which rely on spatial charge transfer (CT). Owing to the enforced face‐to‐face D/A/D pattern, effective CT interactions are realized, which lead to high photoluminescence quantum yields (PLQYs) of 94.2 % and 92.8 % for the two molecules, respectively. The resulting emitters exhibit small singlet–triplet energy splitting (ΔEST) and fast reverse intersystem crossing (RISC) processes. Maximum external quantum efficiencies (EQEs) of 28.0 % and 26.6 % were realized for devices based on DM‐BD1 and DM‐BD2, respectively, which are higher than those of their D/A‐type analogues.
Multi‐Layer π‐stacked molecules are designed to realize efficient thermally activated delayed fluorescence. Spatially confined molecules with stereochemical structures are constructed in donor/acceptor/donor architectures with different conformations. Their organic light‐emitting diode (OLED) devices exhibit high external quantum efficiencies (EQEs) of 28.0 %/26.6 %, respectively.
Compared to efficient green and near‐infrared light‐emitting diodes (LEDs), less progress has been made on deep‐blue perovskite LEDs. They suffer from inefficient domain various number of PbX6− ...layers (n) control, resulting in a series of unfavorable issues such as unstable color, multipeak profile, and poor fluorescence yield. Here, a strategy involving a delicate spacer modulation for quasi‐2D perovskite films via an introduction of aromatic polyamine molecules into the perovskite precursor is reported. With low‐dimensional component engineering, the n1 domain, which shows nonradiative recombination and retarded exciton transfer, is significantly suppressed. Also, the n3 domain, which represents the population of emission species, is remarkably increased. The optimized quasi‐2D perovskite film presents blue emission from the n3 domain (peak at 465 nm) with a photoluminescence quantum yield (PLQY) as high as 77%. It enables the corresponding perovskite LEDs to deliver stable deep‐blue emission (CIE (0.145, 0.05)) with an external quantum efficiency (EQE) of 2.6%. The findings in this work provide further understanding on the structural and emission properties of quasi‐2D perovskites, which pave a new route to design deep‐blue‐emissive perovskite materials.
A quasi‐two‐dimensional perovskite film with stable domain distribution is prepared based on a new spacer. The film containing pure bromide perovskite exhibits enhanced deep‐blue fluorescence with quantum yield of 77% by low‐dimensional component engineering. As a result, the corresponding light‐emitting diodes deliver stable deep‐blue emission with a peak external quantum efficiency of 2.6%.
► Analyze the effect of coplanar fissure angle on mechanical parameters of sandstone. ► Characterize crack coalescence behavior of sandstone by photographic monitoring. ► Obtain the sequence and type ...of crack coalescence in sandstone samples. ► Construct the relation between coplanar fissure angle and crack coalescence stress.
This investigation presents crack coalescence behavior of brittle sandstone samples containing two coplanar fissures in the process of deformation. On basis of experimental results, the influence of coplanar fissure angle on strength and deformation behavior of sandstone samples is firstly analyzed. By adopting photographic monitoring, the crack initiation and coalescence are all observed and characterized from the inner and outer tips of pre-existing coplanar fissures in brittle sandstone sample. Moreover, the sequence and type of crack coalescence in sandstone samples containing two coplanar fissures are analyzed. Finally, the relationship between coplanar fissure angle and crack coalescence stress is constructed.
Atomically ordered intermetallic nanoparticles are promising for catalytic applications but are difficult to produce because the high-temperature annealing required for atom ordering inevitably ...accelerates metal sintering that leads to larger crystallites. We prepared platinum intermetallics with an average particle size of <5 nanometers on porous sulfur-doped carbon supports, on which the strong interaction between platinum and sulfur suppresses metal sintering up to 1000°C. We synthesized intermetallic libraries of small nanoparticles consisting of 46 combinations of platinum with 16 other metal elements and used them to study the dependence of electrocatalytic oxygen-reduction reaction activity on alloy composition and platinum skin strain. The intermetallic libraries are highly mass efficient in proton-exchange-membrane fuel cells and could achieve high activities of 1.3 to 1.8 amperes per milligram of platinum at 0.9 volts.
Uniaxial compression experiments were performed for brittle sandstone samples containing a single fissure by a rock mechanics servo-controlled testing system. Based on the experimental results of ...axial stress-axial strain curves, the influence of single fissure geometry on the strength and deformation behavior of sandstone samples is analyzed in detail. Compared with the intact sandstone sample, the sandstone samples containing a single fissure show the localization deformation failure. The uniaxial compressive strength, Young’s modulus and peak axial strain of sandstone samples with pre-existing single fissure are all lower than that of intact sandstone sample, which is closely related to the fissure length and fissure angle. The crack coalescence was observed and characterized from tips of pre-existing single fissure in brittle sandstone sample. Nine different crack types are identified based on their geometry and crack propagation mechanism (tensile, shear, lateral crack, far-field crack and surface spalling) for single fissure, which can be used to analyze the failure mode and cracking process of sandstone sample containing a single fissure under uniaxial compression. To confirm the subsequence of crack coalescence in sandstone sample, the photographic monitoring and acoustic emission (AE) technique were adopted for uniaxial compression test. The real-time crack coalescence process of sandstone containing a single fissure was recorded during the whole loading. In the end, the influence of the crack coalescence on the strength and deformation failure behavior of brittle sandstone sample containing a single fissure is analyzed under uniaxial compression. The present research is helpful to understand the failure behavior and fracture mechanism of engineering rock mass (such as slope instability and underground rock burst).
Long-term experiments were performed on red sandstones after different thermal treatments (25, 300, 700 and 1000 °C) under multi-step loading and unloading cycles and a confining pressure of 25 MPa. ...Furthermore, to quantitatively analyse the temperature influence on the deformation behaviours of the specimens, the concept of the temperature–strain rate was proposed to describe the relationship between strain and temperature, and the experimental results were corrected to identical temperatures (i.e., 20 °C), to overcome the influence of periodic fluctuations in ambient temperature. The results show that the axial mean temperature–strain rate first increased as temperature increased from 25 to 300 °C and then decreased with increasing temperature, whereas the lateral mean temperature–strain rate decreased with increasing temperature. The total strain was divided into the instantaneous elastic strain, the instantaneous plastic strain, the visco-elastic strain and the visco-plastic strain. The total axial strain increased with increasing deviatoric stress, and the irrecoverable strain increased with increasing loading and unloading history. Furthermore, the total axial strain increased with increasing temperature; specifically, at 1000 °C, it was approximately two times that at 700 °C and three times those at 25 and 300 °C. The instantaneous elastic strain and the instantaneous plastic strain increased approximately linearly with increasing deviatoric stress, whereas the creep strain varied with deviatoric stress in complicated ways at different temperatures. However, under identical deviatoric stress, the instantaneous elastic strain and the instantaneous plastic strain increased slightly as temperature increased from 25 to 700 °C and then increased substantially as temperature reached 1000 °C, whereas the variations in the creep strain, the visco-elastic strain and the visco-plastic strain were dependent on temperature and stress level. Finally, the permeability first decreased slightly as temperature increased from 25 to 300 °C and then increased with increasing temperature.
The purpose of this review is to discuss the development and the contribution of the experiments and numerical simulations in compression‐induced failure characteristics of flawed rock specimens. ...Investigation is essential to understand the fundamental failures occurring in a rock bridge, for assessing anticipated and actual performances of the structures built on or in rock masses. The review begins by representing and discussing compression‐induced crack initiation and growth in flawed rocks and explaining the significance of studying these issues in Section 1. The crack initiation and growth behaviors in flawed rocks under uniaxial, biaxial, and triaxial compressions are described in depth in Section 2 where it is expected to distinguish 2‐D crack growth from 3‐D crack growth. After that, the failure characteristics of flawed rock specimens due to temperature treatments and hydraulic pressure are systematically reviewed. Numerical studies of the compression‐induced failure characteristics of flawed rock specimens based on different numerical theories and numerical models are comprehensively evaluated in Section 3. In Section 4, the new findings obtained recently from experimental and numerical studies are drawn. Finally, some aspects of prospective research and a brief summary are presented in Sections 5 and 6.
Based on a series of triaxial compression experiments using hollow sandstone specimens with various hole diameters (d=0, 11, 15 and 26mm), the deformation, peak strength and crack damage behavior of ...hollow sandstone specimens under different confining pressures are investigated. The experimental results show that the Young's modulus of hollow sandstone only depends on the confining pressure and is not affected by hole diameter in the tested range of d=0–26mm. Two types of methods used to confirm the elastic modulus and Poisson's ratio of rock material are proposed to evaluate the triaxial deformation characteristics of hollow sandstone. The effects of confining pressure and hole diameter on the Poisson's ratio and peak strain of hollow sandstone are analyzed. The peak strength and crack damage parameters of hollow sandstone depend on not only the confining pressure (σ3) but also the hole diameter. Under uniaxial compression, the peak strength and crack damage threshold (σcd) of hollow sandstone are independent of hole diameter, whereas the triaxial compressive strength of hollow sandstone decreases linearly with increasing hole diameter. The peak strength and crack damage threshold of hollow sandstone increase with increasing confining pressure, which can be better described by the nonlinear Hoek-Brown criterion than by the linear Mohr-Coulomb criterion. Furthermore, the sensitivity of the crack damage threshold of hollow sandstone to hole diameter is lower than that of peak strength on hole diameter. The concluding remarks can be used to improve the stability and safety of deep underground engineering.
•Conduct a series of triaxial experiments for hollow sandstone with various hole diameters•Analyze the effect of hole diameter and σ3 on deformation parameters of hollow sandstone•Compare two methods to confirm the elastic modulus and Poisson's ratio of hollow sandstone•Investigate the effect of hole diameter and σ3 on peak strength of hollow sandstone•Discuss the effect of hole diameter and σ3 on crack damage threshold of hollow sandstone
Fissures in natural rocks play an important role in determining the strength, deformability and failure behavior of rock mass. However in the past, triaxial compression experiments have rarely been ...conducted for rock materials containing three-dimensional (3-D) fissures and the failure mechanical behavior of fissured rocks is not well known due to the difficulty of conducting triaxial experiments on fissured rocks. Therefore in this research, conventional triaxial compression experiments were performed to study the strength, deformability and failure behavior of granite specimens with one preexisting open fissure. Thirty-one specimens were prepared to perform conventional triaxial compression tests for intact and fissured granite. First, based on the experimental results, the effects of the confining pressure and the fissure angle on the elastic modulus and the peak axial strain of granite specimens are analyzed. Second, the influence of the confining pressure on the crack damage threshold and the peak strength of granite with respect to various fissure angles are evaluated. For the same fissure angle, the crack damage threshold and the peak strength of granite both increase with the confining pressure, which is in good agreement with the linear Mohr–Coulomb criterion. With increasing fissure angle, the cohesion of granite first increases and later decreases, but the internal friction angle is not obviously dependent on the fissure angle. Third, nine crack types are identified to analyze the failure characteristics of granite specimens containing a single fissure under conventional triaxial compression. Finally, a series of X-ray microcomputed tomography (CT) observations were conducted to analyze the internal damage mechanism of granite specimens with respect to various fissure angles. Reconstructed 3-D CT images indicate obvious effects of confining pressure and fissure angle on the crack system of granite specimens. The study helps to elucidate the fundamental nature of rock failure under conventional triaxial compression.
•Analyze the thermal damage behavior of granite by optical microscopic observation.•Investigate the effect of temperature on the strength and deformation parameters of granite.•Explore the acoustic ...emission behavior of granite subjected to various high temperature treatment.•Discuss the internal crack mechanism of deformed granite specimens by X-ray micro CT system.
A detailed understanding of the thermal damage and failure mechanical behavior of granite at elevated temperatures is a key concern in nuclear waste disposal engineering, underground coal gasification, and heat mining in enhanced geothermal energy. In this research, uniaxial compression tests were first carried out to evaluate the effect of high temperature treatments (200, 300, 400, 500, 600, 700 and 800°C) on the crack damage, strength and deformation failure behavior of a granite. The results demonstrated that, in all cases, the crack damage threshold, the strength and static elastic modulus of granite were increased at 300°C, before decreasing up to our maximum temperature of 800°C. However, the static Poisson’s ratio of granite first decreased at 600°C, and then increased rapidly with the temperature. The crack damage and peak axial strain always showed an increase when the temperature was increased. However, the dynamic elastic modulus decreased with the temperature, whereas the dynamic Poisson’s ratio did not depend on the temperature. The gradual increase of temperature results in a more ductile failure of granite. Next, the thermal damage mechanism of uncompressed granite was analyzed by optical microscopic observation. At T=25–300°C, the mechanisms were favored by the thermal expansion of mineral grains but no microcracks were observed; at T=400–600°C, the mechanisms were contributed by boundary cracks and transgranular cracks in feldspar and quartz grains; and at T=700–800°C, the mechanisms were associated with the coalescence of boundary cracks and transgranular cracks. The internal crack evolution process was then monitored during deformation using acoustic emission (AE) monitoring. The results showed that the cracking process of granite depended on the heat treatment temperature. Finally, the deformation mechanism of failed granite at various temperatures was analyzed using X-ray micro CT. During loading, the uniaxial compression stress direction dominated the more brittle fracture process of granite at T=25–600°C, which led to splitting tensile main cracks induced along the axial stress, and thermal damage determined the larger ductile fracture process of granite at T=700–800°C, which resulted in a more ductile deformation after the peak strength.
