The development of phosphorescent materials with time‐dependent phosphorescence colors (TDPCs) is of considerable interest for application in advanced dynamic information encryption. In this study, TDPC is realized in carbon dots (CDs) synthesized by the one‐pot hydrothermal treatment of levofloxacin. CD ink printed on paper (CD@paper) exhibits a change in phosphorescence color from orange to green, 1 s after irradiation with 395 nm light. However, when irradiated with wavelengths shorter or longer than 395 nm, the CD@paper exhibits only green or red phosphorescence, respectively. The red and green phosphorescence originates from the low‐energy surface oxide triplet state and high‐energy N‐related triplet state, respectively. When irradiated with a suitable light energy (around 395 nm wavelength), the two phosphorescent centers can be simultaneously activated, emitting red and green phosphorescence with different decay rates. The red and green phosphorescence merge into an orange phosphorescence initially, exhibiting the TDPC phenomenon. Based on the unusual phosphorescent properties of the CDs, a kind of multilevel, dynamic phosphorescence colored 3D code is designed for advanced dynamic information encryption.
An easy, large-scale synthesis of N-doped carbon quantum dots (CQDs) was developed by using isophorone diisocyanate (IPDI) as a single carbon source under microwave irradiation. The yield of raw N-doped CQDs was about 83%, which is suitable for industrial-scale production. A detailed formation mechanism for N-doped CQDs involving self-polymerization and condensation of IPDI was demonstrated. Moreover, the obtained N-doped CQDs can be homogeneously dispersed in various organic monomers and do not need toxic organic solvents as dispersing agents. This advantage expands the range of applications of CQDs in composites. The N-doped CQDs dispersed in polyurethane (PU) matrixes emit not only fluorescence but also phosphorescence and delayed fluorescence at room temperature upon excitation with ultraviolet (UV) light. Furthermore, the phosphorescence of CQD/PU composites is sensitive to oxygen and therefore, the obtained-CQDs could be exploited in the development of novel oxygen sensors.
Metal-free room-temperature phosphorescence (RTP) materials are of great significance for many applications; however, they usually exhibit low efficiency and weak intensity. This article reports a new strategy for the preparation of a high-efficiency and strong RTP materials from crystalline thermal-annealed carbon dots (CDs) and boric acid (BA) composite (g-t-CD@BA) through grinding-induced amorphous to crystallization transition. Amorphous thermal-annealed CDs and BA composite (t-CD@BA) is prepared following a thermal melting and super-cooling route, where the CDs are fully dispersed in molten BA liquid and uniformly frozen in an amorphous thermal annealed BA matrix after super-cooling to room temperature. Upon grinding treatment, the fracture and fragmentation caused by grinding promote the transformation of the high-energy amorphous state to the lower energy crystalline counterparts. As a result, the CDs are uniformly in situ embedded in the BA crystal matrix. This method affords maximum uniform embedding of the CDs in the BA crystals, decreases nonradiative decay, and promotes intersystem crossing by restraining the free vibration of the CDs, thus producing strong RTP materials with the highest reported phosphorescence quantum yield (48%). Remarkably, RTP from g-t-CD@BA powder is strong enough to illuminate items with a delay time exceeding 9 s.
This article surveys recent and not so recent literature in the field of rotational molding. The mechanisms of heat transfer, sintering and bubble removal are evaluated; as are degradation and dimensional stability. The parameters that affect the surface finish are highlighted and a number of the control systems available to the rotational molding process are mentioned. Improvements in molds and machinery, and the extent to which they reduce cycle times are also described. Finally, the range of materials available to the rotational molding process is examined and recent developments are highlighted. Of particular interest is the rotational molding of liquid polymer systems; which are shown to possess great potential for fulfilling many of rotational molding's surface quality requirements while simultaneously reducing cycle times.
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