To reproduce the rolling contact fatigue (RCF) damage of rail steel used in heavy haul railways and to clarify its possible mechanism, the RCF characteristics of pearlitic steel with different carbon ...content and initial hardness values are evaluated using a two-disk-type machine. The results indicate that the number of spalls and the crack depth decrease with increasing carbon content of pearlitic steel even when the initial hardness is almost the same level. Furthermore, an increase in the carbon content increases the hardness of the rolling contact surface, resulting in the suppression of the plastic flow development. Results also indicate that the crack inclination angle varies depending on the carbon content. According to these results, a possible mechanism for the suppression of fatigue crack propagation in higher carbon pearlitic steel is proposed on the basis of the fracture mechanics approach.
•Carbon content of rail steel affects heavy loaded rolling contact fatigue resistance.•Increase in carbon content increases in hardness of the rolling surface.•Increase in carbon content suppresses development of plastic flow.•Crack inclination angle varies depending on plastic flow.•Increase in carbon content results in suppression of crack propagation.
Helically twisted π-conjugated compounds are promising candidates for chiroptical dyes with strong circular dichroism and circularly polarized luminescence. Herein we report the synthesis of ...tetrabenzof,jk,mn,r7helicene as a near-infrared (NIR) emitter with thermally stable helical chirality. Tetrabenzo7helicene (C44H24), which consists of only 44 carbon atoms, showed broad fluorescence in the NIR region up to 850 nm. On the basis of density functional theory calculations, the NIR emission was attributed to the large orbital interaction between two dibenzoa,canthracene units.
Helicenes are promising candidates for chiral optoelectronic materials because of their helically twisted π-conjugated system. However, the emission intensity of unsubstituted helicenes is very weak ...(Φ
< 0.05) due to a small oscillator strength for the S
→ S
transition. In this work, we investigated the substitution position of the 7helicene framework so that the S
→ S
transition has a large transition magnetic dipole moment (TMDM) and is partially symmetry-allowed. A 7helicene derivative thus designed showed a large fluorescence emission rate (
= 0.02 ns
) and a large TMDM for the S
→ S
transition (|
| = 2.37 × 10
erg·Gauss
), which are more than 10 times greater than those of unsubstituted 7helicene (
= 0.001 ns
, |
| = 0.045 × 10
erg·Gauss
). As a result, we achieved the 7helicene derivative whose dissymmetry factor of CPL and fluorescence quantum yield were both high (|
| = 1.3 × 10
, Φ
= 0.17) in the solution phase.
Helically twisted conductive nanocarbon materials are applicable to optoelectronic and electromagnetic molecular devices working on the nanometer scale. Herein, we report the synthesis of ...per-peri-perbenzo5- and 9helicenes in addition to previously reported π-extended 7helicene. The homogeneously π-extended helicenes can be regarded as helically fused oligo-phenanthrenes. The HOMO-LUMO gap decreased significantly from 2.14 to 1.15 eV with increasing helical length, suggesting the large effective conjugation length (ECL) of the π-extended helical framework. The large ECL of π-extended helicenes is attributed to the large orbital interactions between the phenanthrene subunits at the 9- and 10-positions, which form a polyene-like electronic structure. Based on the experimental results and DFT calculations, the ultrafast decay dynamics on the sub-picosecond timescale were attributed to the low-lying conical intersection.
An amphiphilic rectangular‐shaped photochromic diarylethene bearing two hydrophobic alkyl chains and two hydrophilic tri(ethylene glycol) chains was synthesized, and its photoinduced morphological ...transformation in water was investigated. Two unexpected phenomena were revealed in the course of the experiments: a re‐entrant photoinduced macroscopic morphological transformation and temperature‐dependent kinetic products of supramolecular assembly. When the pure closed‐ring isomer was dispersed in water, a re‐entrant photoinduced morphological transformation, that is, a photoinduced transition from the hydrated phase to the dehydrated phase and then back to the hydrated phase, was observed by optical microscopy upon irradiation with green light at 20 °C; this was interpreted by the V‐shaped phase diagram of the LCST transition. The aqueous assembly of the pure closed‐ring isomer was controlled by changing the temperature; specifically, rapid cooling to 15 and 5 °C gave J and H aggregates, respectively, as the kinetic products. The thermodynamic product at both temperatures was a mixture of mostly H aggregate with a small amount of J aggregate. This behavior was rationalized by the temperature‐dependent potential energy surface of the supramolecular assembly.
Different temperature, different assembly: An amphiphilic rectangular‐shaped photochromic diarylethene showed photoinduced morphological transformation upon irradiation with UV and visible light in water. A re‐entrant transformation was observed, which was interpreted by the V‐shaped phase diagram of the LCST transition. Moreover, by changing the cooling temperature, different kinetic products of supramolecular assembly were obtained.
Designed porosity in coordination materials often relies on highly ordered crystalline networks, which provide stability upon solvent removal. However, the requirement for crystallinity often impedes ...control of higher degrees of morphological versatility, or materials processing. Herein, we describe a supramolecular approach to the synthesis of amorphous polymer materials with controlled microporosity. The strategy entails the use of robust metal-organic polyhedra (MOPs) as porous monomers in the supramolecular polymerization reaction. Detailed analysis of the reaction mechanism of the MOPs with imidazole-based linkers revealed the polymerization to consist of three separate stages: nucleation, elongation, and cross-linking. By controlling the self-assembly pathways, we successfully tuned the resulting macroscopic form of the polymers, from spherical colloidal particles to colloidal gels with hierarchical porosity. The resulting materials display distinct microporous properties arising from the internal cavity of the MOPs. This synthetic approach could lead to the fabrication of soft, flexible materials with permanent porosity.
Considering the application of cryogenic treatment in aerospace manufacturing field, the hot rolled 7055 Al–Zn–Mg–Cu alloy thick plate was selected in order to solve its mechanical anisotropy. ...Several cryogenic-aging processes were applied by introducing deep cryogenic treatment, pre-deformation and natural aging as pre-treatments and two-stage artificial aging as final treatment. An optimized artificial aging parameter in this paper is 110 °C/6 h + 130 °C/16 h, for a best preformance (Rm-max = 668 Mpa, Rp0.2-max = 635 Mpa, A25mm-max = 8.52%) in transverse direction. Nearly all the samples pre-treated by various cryogenic processes obtain stable plastic deformation capacity. Generally, there exist geometric recrystallization, continuous recrystallizaion and discontinuous recrystallization, to some extent in different period of cryogenic-aging process. The final morphology is the result of a mutual compromise between pinning effect originated from cryogenic or natural aging precipitates and recrystallization driving force originating from the energy stored in cryogenic or pre-tensile deformed microstructure. For the thick plate of hot-rolled AA7055, the pair of original rolling Brass components can be enhanced by cryogenic treatment or compressive deformation along transverse direction. But the final texture and the morphology of Σ3n GBs are affected mainly in artificial stage due to various abilities of recovery and discontinuous recrystallization in different directions. Tensile plastic deformation and cycled cryogenic treatment can destroy the Brass components, together with continuous straight geometrically necessary boundaries, more effectively. Since precipitation is probablely accelerated through pre-treatments, a modification of artificial aging time has promising prospect to achieve more excellent mechanical performance.
Self-assembled microstructures of an amphiphilic diarylethene featuring an alkyl chain and triethylene glycol groups showed a photoinduced reversible morphological change in water. Reversible ...photoisomerization of the core diarylethene gave rise to a reversible morphological transformation between colorless microspheres and colored fibers. When colorless microspheres were irradiated with UV light, colored fibers were formed, and when the colored fibers were irradiated with visible light, the spheres were restored to their original positions where the spheres originally existed. This system showed reversible morphological change through not only photoirradiation but also temperature change. These behaviors can be interpreted as a phase transition between the sphere and fiber states. The dynamic process of the phase transition was monitored by polarized optical microscopy (POM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). It was revealed that the formation of fibers upon UV irradiation occurred radially at the surface of the sphere and the formation of the spheres upon visible-light irradiation occurred at the middle of the fiber. The unique photoinduced mechanical motion provides useful information for the design of sophisticated photoactuators.
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