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A reversible dual-responsive luminescent material was introduced by our group to show the simultaneous color and lifetime switching in response to external stimuli. Pristine crystalline powder of (...

Developing organic thermally activated delayed fluorescence (TADF) emitters with high efficiency and narrowband emissions is crucial and challenging for high-quality organic light-emitting diodes (OLEDs). Here, three multiresonance TADF emitters DPACzBN1, DPACzBN2, and DPACzBN3 are designed via a peripheral decoration strategy and synthesized through a lithium intermediate cascade borylation reaction (15% yield for DPACzBN1) or a more efficient lithium-free direct borylation reaction (45% yield for DPACzBN2 and 75% yield for DPACzBN3). All the emitters exhibit a similar blue emission with small full-width at half maximum (fwhm) values as low as 20 nm in toluene solutions. The introduction of the diphenylamino moiety into the parent molecule DPACzBN1 can not only maintain the high photoluminescence quantum yields over 90% but also narrow the bandwidth and enhance the rate constant of the reverse intersystem crossing process, as well as suppress the spectral broadening in devices. Benefiting from the excellent TADF properties and good inhibition of spectral broadening, TADF OLEDs based on DPACzBN3 achieve the highest maximum external quantum efficiency of 27.7% and the smallest fwhm of 24 nm among the three emitters.

Three-dimensional asperity model of rough surfaces is proposed using valley–peak ratio (VPR) of the maximum peak. Asperities of rough surfaces are reconstructed according to paraboloid asperity function, which is derived using least square method and asperity projection plane determined via VPR. The minimum profile deviation between reconstructed asperities and rough surface is calculated using simulated annealing method to optimize VPR. Effects of simulated mutation surfaces, measured surfaces, and different sampling intervals (SIs) on profile deviation are investigated. Compared with nine–point method, the proposed model demonstrates a smaller deviation of mutational rough surface, the absence of interference among asperities, and more stable surface parameters and contact mechanics performance at different SIs.

Novel smart materials that can achieve a diverse range of practical applications with simple molecular structures and green synthesis routes are still in great demand. Herein, by a green photo-oxidation reaction, two simple aggregation-induced emission luminogens (AIEgens) CZ-PYZ and p-CZ-BYZ were developed, with the difference of linking type between the donor group (carbazole) and the acceptor group (a benzoylimino derivative). These two analogues exhibited different luminescence behaviors in the aggregation state. The molecule p-CZ-BYZ showed mechanochromic luminescence and room-temperature phosphorescence simultaneously, while none of the similar phenomena was observed in the case of CZ-PYZ. According to photophysical property tests, single crystal structural analyses and theoretical calculations, we found that this difference was induced by their different stacking manners. It was believed that the present results may provide us with a better understanding of structure–property relationship and offer an important guideline for multifunctional molecular design.

High performance solution processable thermally activated delayed fluorescence (TADF) emitters are of huge importance in the development of organic light-emitting diodes (OLEDs). In this article, two solution processable D3–A star-shaped TADF emitters, namely TATC-TRZ and TATP-TRZ, with different flexible alkyl and rigid phenyl side chains are developed. Both emitters show similar photophysical properties and obvious TADF characteristics with a sub-micro second delayed lifetime of 0.6 μs for TATC-TRZ and 0.4 μs for TATP-TRZ while the flexible alkyl chain ensures TATC-TRZ with superior film forming ability and morphological stability compared to TATP-TRZ. Owing to such advantages, solution processed non-doped OLEDs based on TATC-TRZ secure a high maximum external quantum efficiency (EQEmax) of 7.5% and current efficiency (CEmax) of 19.9 cd A−1, respectively, which is much higher than that of TATP-TRZ with EQEmax of 2.8% and CEmax of 7.4 cd A−1. This work demonstrates the effectiveness of a flexible alkyl tail on the performance of solution-processed devices.

Developing highly efficient red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge due to strict molecular design principles. In this contribution, two red emitters PXZ-AQPy and PXZ-AQPhPy with high structural rigidity and linear configurations are designed and synthesized based on phenoxazine (PXZ) as a donor and acenaphtho[1,2-b]quinoxaline (AQ) as an acceptor. Both molecules with horizontal orientation are endowed with TADF and aggregation-induced emission (AIE) properties. By elongating the acceptor segment via phenyl decoration, PXZ-AQPhPy exhibits higher horizontal ratios of the emitting dipole orientation (Θ‖ = 85%), while the emission characteristics are almost unaltered. Consequently, the PXZ-AQPhPy based device achieves a high external quantum efficiency of 19.6% with an emission peak at 610 nm. This work provides a simple and effective design strategy to selectively promote the horizontal orientation of red TADF emitters.

A tri-armed arylsulfone-based acceptor (SF) linked by triphenylbenzene (TPB) has been developed to construct solution-processable thermally activated delayed fluorescence (TADF) materials with large band gaps. The acceptor suffered limited conjugation due to the tetrahedral geometry of sulfonyl groups, as well as a meta-linkage at the central TPB moiety. By simple conjugation of the SF with appropriate donor units, luminophores SF–DMAC and SF–PXZ capable of producing efficient blue/green emissions were obtained, with peak efficiencies of 11.3% (19.0 cd A−1 and 10.0 lm W−1) and 12.4% (35.6 cd A−1 and 17.8 lm W−1) in the corresponding wet-processed OLEDs. Decent resistance to roll-off at high brightness was noticed in the device based on SF–PXZ, ascribing to reduced intermolecular interactions due to its star-shaped architecture and an efficient reverse intersystem crossing (RISC) process.

Photoluminescence quantum yield (PLQY) and the reverse intersystem crossing (RISC) process are critical for next-generation highly efficient organic light-emitting diodes (OLEDs). However, it is not easy to simultaneously obtain high PLQY and fast RISC, especially in TADF emitters with twisted donor–acceptor structures due to their conflicting requirements for wave function overlapping. Herein, based on a rigid planar dibenzo[a,c]phenazine (DPPZ) unit as an acceptor, four emitters (isomers of DMAC-11-DPPZ and DMAC-10-DPPZ, and isomers of PXZ-11-DPPZ and PXZ-10-DPPZ) with 9,9-dimethylacridine (DMAC) and 10H-phenoxazine (PXZ) as donors substituted via 11- and 10-positions were designed and synthesized to balance the wave function overlapping and explore the relationship between their structures and luminescence properties. Photophysical characterization and theoretical calculations suggest small energy gaps (ΔEST) featuring fast RISC rates in all these compounds. Meanwhile, DMAC-11-DPPZ and PXZ-11-DPPZ achieve higher PLQYs due to the largely suppressed non-radiative transition as revealed by the decreased Huang–Rhys factors. As a result, high external quantum efficiencies (EQEmax) of 23.8% for DMAC-11-DPPZ and 13.7% for PXZ-11-DPPZ are obtained. This work provides a promising strategy to improve the PLQY of TADF materials by adjusting the substituted site, and also shows the potential of phenazine derivatives for OLED applications.

Three donor–acceptor–donor type emitters of 2TPA-BT, 2MeTPA-BT and 2DMeTPA-BT were synthesized. Crystal structures for all compounds were determined by single-crystal X-ray diffraction analysis, showing that by introducing one or two methyl groups between the donors and acceptor, the spatial configuration changes greatly for 2MeTPA-BT and 2DMeTPA-BT compared to their parent compound 2TPA-BT. Density functional theory analysis reveals that the emission of 2MeTPA-BT has a more obvious hybridized local and charge-transfer (HLCT) feature based on the influence of steric hindrance of methyl substituents. Attributed to their different spatial configurations and luminescence mechanisms, different emission wavelengths with high photoluminescence quantum yields (ΦPLs) of 75.43%, 93.43% and 89.63% in toluene solvent, as well as 44.76%, 52.19 and 33.87% in neat film, were observed for 2TPA-BT, 2MeTPA-BT and 2DMeTPA-BT, respectively. Furthermore, strong electroluminescence was observed with the emission peaks at 602, 560 and 544 nm, maximum current efficiencies of 11.05 cd A−1, 30.40 cd A−1 and 25.02 cd A−1, maximum power efficiencies of 6.01 lm W−1, 23.67 lm W−1 and 20.37 lm W−1, and maximum external quantum efficiencies of 4.43%, 8.47% and 7.29% for 2TPA-BT, 2MeTPA-BT and 2DMeTPA-BT doped organic light-emitting devices (OLEDs), respectively. This work clearly illustrates the effect of spatial configuration changes on the luminescence properties of organic emitters.

Herein, we report a new pyrene (PY)-based derivative, 1-(4-(phenylsulfonyl)phenyl)pyrene (1-DPS-PY), whose two crystals (crystal-B and crystal-G) exhibit sky-blue and green excimer fluorescence, respectively. Crystal-B and crystal-G have a similar dimeric π–π PY stacking in the crystals, but with a different single-molecule conformation. The experimental and theoretical investigations demonstrate that the substituent orientation of 4-(phenylsulfonyl)phenyl (DPS) plays a crucial role in modulating the excited-state properties of the PY excimer. The small twist angle (Φ) between DPS and PY facilitates the larger overlap between two PY planes in the excited state, resulting in more red-shifted excimer emission, and vice versa. Moreover, crystal-B shows a photoluminescence (PL) efficiency (ηPL) as high as 77% due to the better discreteness of PY dimer stacking, in contrast to ηPL = 60% of crystal-G with the non-discrete PY dimer stacking. Organic light-emitting diodes (OLEDs) were fabricated with doped and non-doped device structures. The devices showed a gradual emission change from deep blue to sky blue with increasing doping concentration, corresponding to the emission species transformation from a monomer to an excimer. A non-doped device exhibited the best performance with an external quantum efficiency (EQE) of nearly 4.5% and a maximum luminance of 13 733 cd m−2. These results provide us with a new understanding of the mechanism of the PY excimer, and a new strategy to design the polymorphic PY dimers for high-efficiency excimer fluorescence with tunable emission color.

A tooth wear prediction methodology for helical gears in point contact is developed to evaluate their wear resistances using a lead crown. The load distribution coefficient is proposed in accordance with the elastic approach of each contact tooth pair being equal, and contact pressure are determined, and the sliding distance is obtained by adopting a generalized moving distance model. Then, the wear depth is computed in accordance with Archard’s wear equation, and the differences in tooth wear on standard and crowned helical gears are analyzed comparatively. The effects of crowned amount, fundamental geometry, and operating parameters on the wear resistance of the crowned helical gear pair are investigated. The results reveal that the tooth wear is lower on the gear surface with a moderate crowned amount than on the standard one, and that wear depths decrease with the increase in the helix angle, normal pressure angle, normal module, tooth number, or tooth width but increase with input torque rises. Furthermore, the rational lead crown, the geometric, and operating parameters optimization can be applied to wear resistance in the gear design.

This study proposes a tribo-dynamic model of a spur gear by coupling an elastic dynamic model and an isothermal elastohydrodynamic lubrication (EHL) model. In this model, the gear shafts are modelled as flexible bodies by the resultant elements of Timoshenko beam and bar elements, and the gyroscopic effect of gear rotors and shafts is also included. The transient and non-Newtonian effects are considered in the EHL model to predict tooth friction forces and moments. Then, an example is provided to analyze the interaction between gear tribology and dynamic behavior. Results indicate that the fluctuations of mesh force and surface tangential velocity caused by gear vibration considerably affect the pressure and thickness of the lubricant film. The gyroscopic effect exerts a significant influence on the dynamic and tribology properties of the gear drive at high speed. The proposed model has important guidance for the design of lubrication systems and dynamic properties in gear drives.

s The start-up motion is the significant process of many mechanical applications especially for high-frequency start-stop equipment; the formation of separating film and the main wear approximately occur in this process. In this paper, the improved mixed algorithm and a new methodology of describing Gaussian rough surface are developed and theoretically validated based on Stribeck curve. Furthermore, the transient film formation of smooth contact and rough contact with the start-up motion is analyzed and validated. Finally, the wear mechanism of different friction pairs is investigated, meanwhile, the applicability of Evans model and Archard model is evaluated. Results show that the contact ratio of smooth contact is considerably large but the transition time is much short in comparison with rough contact because the valleys of rough surface conserve lubricant avoiding solid contact. Combining the Evans equation and this mixed technique, the wear volume and wear rate were successfully calculated. The type of friction pair influences greatly the wear volume and wear rate of contact surfaces. The wear volume and wear rate of TiN–TiN the minimum in the start-up process, and alumina ceramics with satisfactory fracture toughness is a potential coating that can be applied to mechanical elements with a wide range of slide-roll ratio.

Adhesive wear in mixed elastohydrodynamic lubrication (EHL) has been one of the most prominent problems for heavy-loaded helical gears. However, this issue was rarely investigated in previous researches. In this work, an adhesive wear model for helical gears is established in line-contact mixed EHL. The contact parameters of the pinion and gear are derived according to the equivalent tapered roller contact model, and the load is attained in consideration of the varying contact line ratio. Then the asperity contact pressure is calculated according to Hertz's elastic contact theory and load sharing. Moreover, the sliding distances of the points on tooth surface of driving pinion and driven gear are achieved by a single point observation method, and Archard theory in dry contact is extended to the mixed lubrication to estimate the wear rate in mixed EHL by using fractional film defect. The modified Archard's wear model is then employed in formulating and accounting for the gear tooth wear. Effects of surface roughness, geometrical parameters and working parameters on wear depth of the driving pinion are furtherly investigated. The results show that the wear depth in mixed EHL is lower than that under dry contact, which indicates that tooth wear can be reduced with the reasonable lubricants. As the surface roughness becomes large, the asperity contact pressure, the tooth surface temperature and wear depth are increased. Additionally, the wear depths decrease with the increase in module, helix angle, pressure angle, tooth width or rotation speed but increase with input torque.

The thermo-mechanical characteristics of high-speed and heavy-load modified gears in non-Newtonian transient micro thermo-elasto-hydrodynamic contacts are investigated. The synthetic stiffness model is developed from the stiffness of both the oil film and gear pair. Scuffing prediction and mesh efficiency are determined by the total contact temperature and frictional loss ratio respectively. By utilizing the verified numerical procedure, effects of geometric parameters on aforementioned thermo-mechanical characteristics are discussed. The results show that crown modification and the increase of tooth number, module, and pressure angle contribute to vibration and impact reduction of gear drive. Larger tooth number and module are beneficial to scuffing resistance and efficiency improvement. It indicates that gear system can achieve high-performance transmission by rationally matching geometric parameters.

An enhanced spur gear dynamic model considering the combined stiffness and damping of both gear tooth and oil film is established. To acquire the combined stiffness and damping involved in the modified dynamics equations, Ishikawa formulas are adopted to calculate the gear mesh stiffness, and given the viscous-elastic oil film in elastohydrodynamic lubrication line contact equivalent to massless spring and damping elements, the models of oil film stiffness and damping in normal and tangential directions are then developed. The combined stiffness is deduced from the stiffness of both the gear tooth and oil film, while the combined damping is derived from the damping of these parts. Effects of oil film stiffness and damping on the gear dynamics are investigated, and the dynamic response of the developed model is in contrast to that of the conventional model. The results show that by utilizing the enhanced dynamic model, the displacement fluctuation in transient stage fast decays and displacement response reaches steady state faster. The speed and acceleration fluctuations in the period converting from transient to steady stages are obviously reduced, and the response curves of speed and acceleration in steady stage are smoother. Moreover, the oil film normal damping plays large role in the gear periodic motion. This indicates that the oil film stiffness is prone to effectively alleviate impact and the oil film damping is inclined to substantially reduce vibration and frictional heat for a gear drive.

An elastic dynamics model for a valve train including combined stiffness of oil film and dry contact between a cam and a tappet, is proposed. Multi-directional elastic deformations of the valve train are solved by discretizing slender components into Rayleigh beam and bar elements. The oil film stiffness is determined by non-Newtonian elastohydrodynamic lubrication in the line contact, whereas the dry contact stiffness is achieved by using finite element contact analysis. A numerical solution for the dynamic model is developed and verified by conducting a dynamic stress experiment. The influences of rotation speed on the oil film stiffness and contact force between the cam and the tappet are discussed, and the combined effects of rotation speed and oil film stiffness on a dynamic transmission error of the valve train are analyzed. Results show that the oil film stiffness decreases when the cam rotation speed increases within a certain range, whereas the sharp increased contact force may enlarge the oil film stiffness when the speed exceeds a critical value. The oil film stiffness variation slight influences the contact force, but such variation significantly affects the dynamic transmission error.

A new series of tetrahedral heteroleptic copper(I) complexes exhibiting efficient thermally-activated delayed fluorescence (TADF) in green to orange electromagnetic spectral regions has been developed by using D-A type N^N ligand and P^P ligands. Their structures, electrochemical, photophysical, and electroluminescence properties have been characterized. The complexes exhibit high photoluminescence quantum yields (PLQYs) of up to 0.71 at room temperature in doped film and the lifetimes are in a wide range of 4.3–24.1 μs. Density functional theory (DFT) calculations on the complexes reveal the lowest-lying intraligand charge-transfer excited states that are localized on the N^N ligands. Solution-processed organic light emitting diodes (OLEDs) based on one of the new emitters show a maximum external quantum efficiency (EQE) of 7.96%.

Highly efficient blue thermally activated delayed fluorescence (TADF) materials with high horizontal dipole orientation (Θ//) remain a big challenge due to the concentration quenching effects and serious efficiency roll-offs in their corresponding organic light-emitting diodes (OLEDs). We herein report two novel TADF compounds, namely 2TDBA-SBA and TDBA-SBA, which were facilely synthesized by utilizing a rigid oxygen-bridged cyclized triarylboron-based unit (TDBA) as the acceptor and a spiro acridine (SBA) as the donor. Benefiting from the rigid and planar skeleton of the acceptor and the linear molecular shape, high Θ//s and photoluminescence quantum yields (PLQYs) of ∼ 90% were achieved. Significantly, TDBA-SBA exhibited excellent TADF properties with a short delayed fluorescence lifetime of only 684 ns and narrow full-width at half-maximum (FWHM) of 44 nm in solution state, accompanied by the high PLQYs of over 80% at high doping concentrations, manifesting the alleviated exciton quenching effects. Consequently, OLEDs based on TDBA-SBA achieved a high maximum external quantum efficiency (EQE) of 29.3% and a maximum brightness of 27663 cd cm−2. Importantly, an EQE over 20% was realized even at the high brightness of 10000 cd cm−2, signifying a small efficiency roll-off.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) offered exceptional solution for narrowband organic light emitting diode (OLED) devices in terms of color purity and luminescence efficiency, while the development of new MR skeleton remained an exigent task, especially for long wavelength region. We hereby demonstrate that a simple modification of the B (boron)-N (nitrogen) framework by sp3 -carbon insertion would significant bathochromic shift the short-range chargetransfer emission and improve the device performances. The bis(acridan)phenylenebased skeleton developed in this contribution presented a non-planar conformation and allowed facile introduction of isolating units to prevent triplet-involved quenching, deriving two luminophores with quantum yields approaching 90% and narrow FWMHs below 30 nm in film state. Corresponding green-emissive devices realized superior performances comparing to the planar carbazolyl-based MR-TADF analogues, with a maximum external quantum efficiency (EQEmax) up to 28.2% and small efficiency rolloff without the involvement of any sensitizing host

In this study, a modified wear model considering contact temperature for spur gears in mixed elastohydrodynamic lubrication (EHL) is proposed. The contact temperature consists of bulk temperature and flash temperature. The bulk temperature is determined by the thermal network model, whereas the flash temperature is estimated through the published method. The bulk temperature, which was rarely included in the previous works, substantially has a considerable influence on the tooth wear in mixed EHL. It is also found that the lower contact temperature contributes to the reduction of gear wear depth. Furthermore, the effects of gear basic geometrical parameter and operating parameter on wear depth are investigated. The results show that the wear depth decreases with the increased tooth width, module, pressure angle and rotational velocity but increases with the surface roughness and torque. It indicates that wear resistance of tooth surfaces can be enhanced by optimising the design parameters of gear drives.

A novel electron donor, namely, TMDBP is designed by angularly fusing two acridine moieties. The DMAC subunits in TMDBP exhibit a dominant quasi-equatorial conformation due to steric H-H repulsion. The new TADF emitters render excellent OLED performances with EQE up to 24.2% and small efficiency roll-off.

The development of the thermally activated delayed fluorescence (TADF) emitters with circularly polarized luminescence (CPL), particularly those exhibiting deep-blue emission is still a formidable challenge. In this work, we reported a simple and easily accessed molecular design strategy for deep-blue circularly polarized TADF (CP-TADF) enantiomers. Two chiral compounds, namely (S)-NPE-AcDPS and (R)-NPE-AcDPS, were successfully designed and synthesized, which featured concurrently TADF, CPL, and aggregation-induced enhanced emission (AIEE) properties. The CPL exhibited a maximum gPL value of 3.0 × 10−4. These emitters exhibited deep blue emission peaking at 451 nm in doped film with a high photoluminescence of 86% and a small singlet-triplet splitting of 0.05 eV. Furthermore, the deep blue OLED based on (S)-NPE-AcDPS demonstrated a high external quantum efficiency (EQE) up to 18.5%.

An adhesive wear prediction method for double helical gears is proposed according to enhanced coordinate transformation and generalized sliding distance model in conjunction with Archard's wear equation. To describe transient contact ellipse and identify the contact point pairs conveniently, a transform coordinate plane is set in coincidence with the plane of action and a coordinate axe parallels to the contact line. The contact pressure distribution is determined by contact line length, contact width and normal force, and a modified sliding distance model is proposed by generalized moving distance replacement of Hertz contact width. As the wear coefficient, contact pressure and sliding distance are given, the tooth wear depths are predicted by a developed numerical procedure. Effects of major geometrical and working parameters on the wear depth are investigated. The results show that the wear depth becomes smaller, which is mainly determined by the contact force per unit length, equivalent curvature radius and sliding distance as normal module, normal pressure angle, helix angle, tooth width or transmission ratio increases. However, the wear depth becomes larger when input torque is improved. It is indicated that rational parameters match in gear design and uniform wear distribution are beneficial for wear resistance.

Innovative stiffness and damping models for oil films are developed to account for the impacts in both normal and tangential directions. Given that these models are applied to a gear drive in line contact elastohydrodynamic lubrication (EHL), the combined stiffness is derived from the stiffness of both the oil film and gear tooth while the combined damping is established from the damping of these parts. The effects of three fundamental parameters (contact force, rotation speed, and tooth numbers) of the gear drive in line contact EHL on the combined stiffness and damping are then investigated. The results reveal that the small normal and tangential stiffness of the lubricant can alleviate meshing impact and shear vibration, while the impact and friction heat can be reduced by using an oil film with either a large normal damping or small tangential damping. Given that its amplitude and fluctuation are closely related to shear rate, effective viscosity, entrainment velocity, and curvature radii, the improved combined stiffness and damping can be obtained by rationally matching the geometric and operating parameters.

Excited-state properties (characteristics and composition) play a crucial role in luminescence properties of the new-generation organic light-emitting materials. As a special excited species, the hybridized local and charge-transfer (HLCT) state is composed of locally excited (LE) state and charge-transfer (CT) state, which can harvest simultaneous high photoluminescence (PL) efficiency and high exciton utilization in organic light-emitting diodes. In this work, we designed and synthesized three donor (D)–acceptor (A) compounds with different donor units and a typical electron-deficient unit acridine as the acceptor unit to investigate the different hybridization statuses among them. As is revealed by the molecular design, the three compounds precisely show the different hybridization statuses: LE-dominated hybridization (CZP-1AC), quasi-equivalent HLCT (qHLCT, TPA-1AC), and CT-dominated hybridization (PTZ-1AC). As a result, TPA-1AC exhibits the highest PL efficiency and multifold device performance, meri...

To develop colorless and soluble polyimide films, a novel dianhydride containing 1,4:3,6-dianhydro-D-mannitol unit, 2,5-bis(3,4-dicarboxyphenoxy)-1,4:3,6-dianhydromannitol dianhydride (IMDA) was synthesized. And two series of polyimides were prepared via a two-step thermal imidization, PI−(1–4) were obtained from IMDA and four kinds of aromatic diamines while PI−(5–7) from IMDA and three kinds of semiaromatic diamines. All the polyimides were readily soluble in common polar solvents and could afford flexible, tough and colorless films with transparency up to 89% at 450 nm. Especially, polyimides simultaneously containing 1,4:3,6-dianhydrohexitol units in diamine and dianhydride exhibited comparable optical and soluble performance with the alicyclic fluorinated ones. Meanwhile, it was certified that 1,4:3,6-dianhydrohexitol fragment in dianhydride was more determinant in solubility and transmittance of polyimides than that in diamine. An overall investigation of these polyimides on thermal, mechanical, morphological, soluble, optical and dielectric properties was presented, and their structure-property relationships were discussed in detail.

This paper provides a method coupling the 2D microtopography filtered by ISO Gaussian filter of spur gears, which is typically used in power transmission devices, and Transmission error (TE) together. A gearbox with certain transmission ratio is selected as a research object for analysis. This gearbox comprises a mathematical model of TE calculation based on the Monte Carlo method and the coordinate transformation method, which couples the 2D microtopography of spur gears and TE. Two TE models are established to study their effects on the coupling results: a) The single TE model, which means only one gear owns TE, and b) the synthetic TE model, which means both gears possess TE. The new model of the 2D microtopography of tooth surfaces can be utilized further to investigate the mechanism of engagement and the approximate contact position of asperities on the tooth surface. The new model is also significant in studying the tooth root bending stress.

As one type of latest emitters with simultaneous high efficiency and color-purity, the development of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials represents an important advancement for organic light-emitting diodes (OLEDs). We herein present a new strategy to improve the performance of MR-TADF emitters by fusing sulfur element into the B-N based framework, aiming to utilize the non-metal heavy-atom effect in accelerating the reverse intersystem crossing (RISC) process of the emitter. Two compounds, namely 2PTZBN and 2PXZBN, were developed in this work through rigidifying the DABNA-1 skeleton by sulfur or oxygen atoms. The theoretical calculations and photoluminescence studies revealed that the sulfur-incorporated 2PTZBN enabled considerable rate constant of RISC (kRISC) up to 2.8 × 105 s-1 in toluene due to larger spin-orbital coupling (SOC) values and smaller singlet-triplet energy splitting (ΔEST) compared with 2PXZBN. Consequently, organic light-emitting diodes based on 2PTZBN exhibited highly efficient green emission with maximum external quantum efficiency (EQE) of 25.5%.

Double helical gears are widely applied in the aviation, marine and energy equipment, and generally required to be modified as the crowned since the machining error and assembling error of the gear drive are inevitable. The crowned gears usually operate in point contact mixed elastohydrodynamic lubrication (EHL), and adhesive wear has been one of the most prominent problems, but few previous researches were focused on this problem. In this work, a wear life prediction model of crowned double helical gears in point contact mixed EHL is proposed. The tooth profiles of the crowned gears are obtained according to the generating principle, and the load is attained in consideration of multi-tooth pairs contact. The contact pressure is derived from Hertzian elastic contact theory and load sharing concept, and the sliding distance is determined by a generalized model. Then, the wear rate in point contact mixed EHL is evaluated by extending the Archard theory to lubricated case, and the extended Archard's wear model is used to formulate the gear tooth wear. Moreover, the wear life prediction model which takes account of rotation speed and input torque is established, and the influences of surface roughness and crowned amount on the wear life of the crowned gears are further studied. Results show that surface roughness has a great influence on the wear life at different rotational speeds, while input torque has a great influence on the wear life at different crowned amount. It indicates that the wear life of the crowned double helical gears can be improved by reasonable matching lubrication, modification and working parameters.

Wear is directly related to rough surface contact, and the accuracy of a wear prediction model depends on the accurate determination of contact pressure. In this work, a numerical model with high computational accuracy for rough surface contact is developed and applied to the wear prediction of spur gears. To acquire contact pressure, the integral equation that contact pressure should satisfy is discretized into a set of linear equations by piecewise linear interpolating contact pressure. Then, a gear wear model including rough surface contact is established. The proposed contact model is verified by comparing with smooth and rough surface contact, and the wear depths of a spur gear drive with geometry and pressure updates are analyzed. Result shows that a small roughness value among contact surfaces may cause a large change in contact pressure. The effects of main design parameters on gear wear are subsequently investigated. Gear wear is found to decrease with increased module and pressure angle but increases with increased transmission ratio and input torque. These findings indicate that reasonable parameter matching is beneficial to improving gear wear resistance.

Non-Newtonian thermal elastohydrodynamic lubrication (EHL) theory in point contact which deals with the lubrication for a crowned herringbone gear drive is developed, combined with the verified numerical procedure. Effects of the major geometry and working parameters on the lubrication performance (the pressure, film thickness, temperature rise and shear stress), are investigated. The results show that the enlargement of normal module, normal pressure angle, helix angel, and transmission ratio is beneficial for pitting resistance, and that the excessive crowned can debase the contact fatigue strength. It is indicated that severe thermal deformation and scuffing are prone to generation under large load and speed.

A new normal stiffness model of the oil film is established by the viscous-elastic fluid between a spur gear drive being equivalent to a massless spring element, and the tangential stiffness model is proposed according to the hypothesis of equal shear stress on laminar element surfaces. Effects of operating parameters (contact force and rotation speed) and geometry parameters (module, tooth number, pressure angle and modification) on the oil film stiffness from the root to the tip are investigated. The results show that the amplitude and fluctuation of the stiffness are closely related to the shear rate, effective viscosity and curvature radii. It is indicated that the rational parameter match is valid in mesh impact reduction and stationarity enhancement.

Throwing light upon the structure-property relationship of the excited state properties for next-generation fluorescent materials is crucial for the organic light emitting diode (OLED) field. Herein, we designed and synthesized three donor-acceptor (D-A) structure compounds based on a strong spin orbit coupling (SOC) acceptor benzo[a, c]phenazine (DPPZ) to research on the three typical types of excited states, namely, the locally-excited (LE) dominated excited state (CZP-DPPZ), the hybridized local and charge-transfer (HLCT) state (TPA-DPPZ), and the charge-transfer (CT) dominated state with TADF characteristics (PXZ-DPPZ). A theoretical combined experimental research was adopted for the excited state properties and their regulation methods of the three compounds. Benefiting from the HLCT character, TPA-DPPZ achieves the best non-doped device performance with maximum brightness of 61,951 cd m-2 and maximum external quantum efficiency of 3.42%, with both high photoluminescence quantum efficiency of 40.2% and high exciton utilization of 42.8%. Additionally, for the doped OLED, PXZ-DPPZ can achieve a max EQE of 9.35%, due to a suppressed triplet quenching and an enhanced SOC.

Gearbox is required to serve for ≥ 30 min in helicopter design to improve survivability, when the gear drive loses lubrication due to battle damage. Under such loss-of-lubrication condition, tooth friction, wear and temperature pose serious problems to dynamic performance of gears. Therefore, a nonlinear tribo-dynamic model of spur gear drives is established and experimentally validated in this work. The time-varying friction coefficient is predicted under loss-of-lubrication condition on the basis of computational inverse technique. The temperature rise and tooth wear caused by friction were included by using thermal network model and dynamic wear model. The flexibility of gear shafts and the gyroscopic effect of gear rotors are also considered. The results indicate the gyroscopic effect apparently affects the natural frequency of the gear drive, while friction has negligible effect on the natural frequency but has considerable effect on the nonlinear behavior. The influence of temperature and gyroscopic effect on nonlinear behavior are reflected in the region of middle and high speed, while tooth wear mainly affects the bifurcation at the middle speed. In addition, tooth wear not only aggravates gear vibration but also can change the phase of dynamic transmission error. The analysis and experiment results can be an important reference in reducing friction and wear, restraining nonlinear behavior, and reducing vibration and noise.

This paper proposes a thermal network model to predict the contact temperature of spur gears. A crucial parameter in gear scuffing evaluation is the contact temperature, which comprises the bulk and flash temperatures. Herein, thermal network models of gear bulk temperature and flash temperature are established, by dispersing tooth into multiple temperature elements connected by the thermal resistances of heat conduction and heat convection. The thermal network model of gear bulk temperature is verified by the finite element model and previous work, and the presented model for gear flash temperature is validated by Blok flash temperature formula. Following ISO standard, gear scuffing is evaluated by comparing the contact temperature with scuffing critical temperature. The effects of geometric and operating parameters on the gear temperature are subsequently investigated. Contact temperature is found to decrease with increased tooth width, pressure angle and module, but increases with increased rotational velocity and input torque. These findings indicate that reasonable geometric and operating parameter matching are beneficial for increasing gear-scuffing resistance.

The effects of centring error and angular misalignment on crack initiation life in herringbone gears are investigated in this work. Herringbone gears with centring error or angular misalignment are modelled for contact analysis. A crack initiation life model of standard and error gears is proposed in accordance with tooth stress history and a critical plane method for nonproportional loading. Then, the effects of centring error and angular misalignment on crack initiation life in herringbone gears are investigated. Results show that centring error and angular misalignment sharply reduce crack initiation life, but a moderate tip relief can eliminate stress concentration and extend crack initiation life. Furthermore, angular adjustment balances the load distribution of the left and right teeth in herringbone gears with centring error, improving tooth crack initiation life.

This paper presents a continuous elastic–plastic asperity contact model with or without the consideration of friction to investigate the micro-contact properties of gear tooth profiles. The model for normal or side contact analysis is established according to Hertz contact theory and the asperity morphology feature, which yields to similar results as obtained from the model proposed by Chang W.R., Etsion I., and Bogy D.B. (CEB model) and the model proposed by Kogut L. and Etsion I. (KE model). More importantly, this model avoids the constant average contact stress as predicted by the CEB model, and the noncontinuous contact stress and deformation within the ultimate strength as given by the KE model. As a application of the present theoretical model in micro-contact analysis of rough tooth profiles, a finite element model (FE model) for elastic–plastic asperity in normal or side contact is established according to the measured surface parameters of a spur gear pair. It is shown that the extreme point of Von Mise stress of the asperities along the normal vector is ascertained by FE model, and that the extreme point is relative to the initial occurrence of the asperities plastic deformation. Compared with the present theoretical model, the similar normal contact stress along the contact radius is attained by FE model. Though the contact stress isogram in the specific plane in normal or side contact of the asperities is a circle or ellipse respectively when the plastic deformation is expanded from the inside of the asperities to their surfaces, it is in line with the distribution of elastic and plastic region of the theoretical model. Compared with CEB model, KE model, and FE model, the consistent results are attained by the present theoretical model in elastic–plastic asperity contact analysis. The results indicate that the theoretical model is applicable to the elastic–plastic asperity contact analysis on the rough surface of a spur gear drive.

This contribution describes sulfoximine (SFI) derivatives as a novel type of acceptor for efficient thermally activated delayed fluorescence (TADF) emitters. SFI presents a unique advantage by facile introduction of versatile functionalities on its secondary amine, which allows delicate manipulation of TADF properties. In a proof of principle study, three green/yellowish green TADF emitters, namely DMAC-SFIP, DMAC-SFIC and DMAC-SFIS were developed with different electron-withdrawing N-substituents based on simple donor–acceptor (D-A) typed structure. A look into the molecular conformations indicates the existence of different types of weak intramolecular interactions to suppress nonradiative decay. More importantly, effective mixing between charge transfer (CT) and locally excited (LE) triplet states is observed for DMAC-SFIP, DMAC-SFIC due to suitable electron-deficiency in the SFI segment, resulting in acceleration of the reverse intersystem crossing (RISC) rate. Consequently, these new TADF emitters render excellent organic light-emitting diode (OLED) performances with external quantum efficiencies (EQEs) up to 20.4% and small efficiency roll-off.

Simultaneously achieving both effective reverse intersystem crossing (RISC) process and high values of radiative decay rate (kr)/photoluminescent quantum yield (PLQY) are critical for efficient thermally activated delayed fluorescent (TADF) emitters. The separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) through typical molecular geometry tuning method guarantees a small ΔEST but results in disfavored oscillator strength for fluorescent decay. Herein, a simple method is provided to facilitate the reverse intersystem crossing (RISC) of TADF emitters via enhancing electron donating property of donors rather than twisting the donor-acceptor configuration. Three molecules, namely Me-Cz, Me-MOC and Me-DOC, are designed and synthesized. The introduction of methoxy group enhances the electron-donating ability of carbazole. The stabilized charge transfer state features a reduced splitting energy between the first singlet and triplet excited state, thus activating TADF process. Consequently, the molecules Me-DOC and Me-MOC achieve good external quantum efficiency of 18.6% and 12.4% in electroluminescent devices.

High-performance solution-processable host materials for thermally activated delayed fluorescence (TADF) emitters are of great significance to the development of organic light-emitting diodes (OLEDs). In this study, three solution-processable hosts, namely mCP-Ph, mCP-Py, and mCP-BmPy featuring a basic structure paradigm of donor-σ-acceptor (D-σ-A) are reported for green TADF OLEDs. Attributing to the well-designed structure, the three host materials exhibited high triplet energy levels and excellent solution processability. It was found that the host materials had a significant impact on the excited state characteristics of the TADF emitter, and the mCP-Ph host with the lowest dipole moment showed the highest intersystem crossing rate constants (kISC and kRISC). In consequence, solution-processed TADF OLEDs based on the emitter 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (DMAC-PCN) and the host 9-(3-(9H-carbazol-9-yl)phenyl)-3-(2,7-di-tert-butyl-9-phenyl-9H-fluoren-9-yl)-9H-carbazole (mCP-Ph) secured a maximum external quantum efficiency of 21.6%, demonstrating the effectiveness of the host engineering by σ-spacer. This work reveals the important principle for designing bespoke host for TADF emitters.

Based on novel phenanthro[9,10-d]imidazole (PI) chromophore, three deep-blue luminescent donor-acceptor molecules with different molecular configurations at the triphenylamine (TPA) core were designed and synthesized, the linear TPA-1PI, the V-shaped TPA-2PI and the star-shaped TPA-3PI. All three molecules displayed the hybridized local and charge-transfer (HLCT) excited state character, and highly efficient deep blue emissions were observed in their solution state, but only those of linear TPA-1PI and V-shaped TPA-2PI molecule were maintained in the solid state. Meanwhile, the V-shaped TPA-2PI and star-shaped TPA-3PI showed the much better thermal properties. Thus the molecular configurations of HLCT materials significantly influence their solid-state, and the V-shaped TPA-2PI molecule was highlighted because of its both deep-blue emission and good thermal stability. A double-layer device by using TPA-2PI as the emitter showed a highly efficient deep-blue electroluminescence with a very good CIE coordinate of (0.14, 0.08) and high current efficiency of 2.7 cd A−1, which suggest that it might be a promising candidate for emission-layer materials for high-performance deep-blue OLEDs.

The models of normal and tangential oil film damping are established by modeling the viscous-elastic fluid as massless damping elements. The central pressure and film thickness distributions, friction coefficient, and maximum temperature rise with or without considering thermal effect indicate the proposed damping models and the solutions to the damping are valid. Thereafter, the thermal effect on oil film damping is discussed and the effects of contact force, rotation speed, and tooth number of spur gears in line contact non-Newtonian transient thermal elastohydrodynamic lubrication (EHL) on the oil film damping are investigated. The results imply that the larger damping in the normal direction is beneficial to meshing impact resistance and vibration reduction, whereas the smaller damping in the tangential direction is very helpful for fluidity enhancement and friction heat inhibition.

This article presents a method for localizing near-field acoustic emission sources in isotropic and homogeneous thick plate using coupled piezoelectric film strain sensors. This acoustic emission source localization method is based on the phase difference of the acoustic emission signals measured by two acoustic emission sensors spaced at a predetermined distance. This phase difference cancels out certain frequency contents of the measured acoustic emission signals, which can be related to the acoustic emission source direction. A theoretical formulation which predicts these trough frequency values and acoustic emission source direction angles for the coupled acoustic emission sensors is first presented. Rayleigh wave is employed since its wave speed does not change in homogeneous half space, and its amplitude is greater than other elastic wave modes. Both elastodynamic solution and experimental data are used to validate the acoustic emission source localization technique. It is seen that the theoretical predictions, elastodynamic solution, and the experimental results are in good agreement with each other. Experimental data from acoustic emission monitoring of a stiffened steel plate specimen under fatigue loading are also used to test this method. Results show that the coupled piezoelectric film strain sensors are effective in determining the acoustic emission source direction in thick steel plates.