Differential scanning calorimetry（DSC） has been widely applied to study crystallization and melting of materials. However, for polymeric lamellar crystals, the melting thermogram during heating process usually exhib...Differential scanning calorimetry（DSC） has been widely applied to study crystallization and melting of materials. However, for polymeric lamellar crystals, the melting thermogram during heating process usually exhibits a broad endothermic peak or even multiple endotherms, which may result from changes of metastability via recrystallization process. Sometimes, the recrystallization exotherm cannot be observed due to its overlapping with the melting endotherm. In this work, we employed a step heating procedure consisting of successive heating and temperature holding stages to measure the metastability of isothermally crystallized poly（butylene succinate）（PBS） crystals. With this approach we could gain the fraction of crystals melted at different temperature ranges and quantitatively detect the melting-recrystallization behavior. The melting-recrystallization behavior depends on the polymer chain structure and the crystallization temperature. For instance, PBS block copolymer hardly shows recrystallization behavior while PBS oligomer and high molecular weight PBS homopolymer demonstrate remarkable melting-recrystallization phenomenon. High molecular weight PBS isothermally crystallized in the low temperature range shows multiple melting-recrystallization while those isothermally crystallized at elevated temperatures do not exhibit observable recrystallization behavior. Furthermore, the melting endotherms were fitted via the melting kinetics equations. The original isothermally crystallized lamellae demonstrate quite different melting kinetics from the recrystallized lamellar crystals that melt at the highest temperature range, which is attributed to the different degrees of stabilization. Finally, the mechanism of melting-recrystallization is briefly discussed. We propose that apparent meltrecrystallization phenomenon be observed when melting of preformed lamellar crystals and recrystallization of thicker lamellae have similar free energy barrier.展开更多
The motivation of this paper is to explore the application of Step-Heating Thermography(SHT)as a technique capable of inspecting new composite rail carbodies using demanding requirements set by the rail manufacturing ...The motivation of this paper is to explore the application of Step-Heating Thermography(SHT)as a technique capable of inspecting new composite rail carbodies using demanding requirements set by the rail manufacturing industry.A large composite sample,with Polytetrafluoroethylene(PTFE)artificial defects,replicating a side-wall section of a new rail carbody,was manufactured and inspected for surface and subsurface defects in this research.The sample,characterized by its large thickness,consists of a monolithic Carbon Fibre Reinforced Polymers(CFRP)component(20mm thickness)and a CFRP-PET foam-CFRP sandwich(40mm total thickness)component fused together.The main challenge of the inspection procedure was to apply reflection mode thermography and detect defects in the entire thickness of the sample that exhibits both low emissivity and thermal insulating properties,especially at the sandwich sections of the sample.The paper explored thermography procedures that would be able to detect large numbers of defects under one single acquisition and would be applied under an automated inspection process leading to the detection of defects only up to 5mm in the CFRP sections of the sample while no defects were able to be detected at the back skin of the sample.展开更多
Single-crystal fibers(SCFs)exhibit a significant potential for high-power laser applications owing to their superior thermal management capabilities.To enhance pump power and mitigate damage at the injection end of SC...Single-crystal fibers(SCFs)exhibit a significant potential for high-power laser applications owing to their superior thermal management capabilities.To enhance pump power and mitigate damage at the injection end of SCFs,a segmented YAG/Yb:YAG/YAG SCF with a diameter of~200μm was directly grown using a one-step laserheated pedestal growth(LHPG)method.The crystallinity and uniformity of the as-grown fiber were evaluated through a real-time back-reflection Laue camera system.Employing a low-loss segmented YAG/Yb:YAG/YAG SCF as the laser gain medium,investigations were conducted on both continuous-wave(CW)oscillators and ultrafast pulse amplifiers.The CW laser operation at 1030.9 nm achieved a maximum output power of 117.6 W with a slope efficiency of~46%,while the ultrafast amplifier system generated pulses with an average power of 101.2 W at a repetition rate of 30.8 MHz.To the best of our knowledge,this represents the highest average power achieved from a single-stage amplifier based on an Yb:YAG SCF.Preliminary pulse compression was performed at a signal power of 5o.6 W using a grating pair,resulting in a compressed pulse width of 786 fs with a compression efficiency of 58%.This study highlights the potential of crystal fiber lasers for a new generation of high-power laser systems.展开更多
MgAl-LDH（layered double hydroxides） were prepared with CO（NH2）2, NH4 Cl and NH3·H2O by the coprecipitation method, respectively. Corresponding composite membranes were prepared by the coating method. LDHs wer...MgAl-LDH（layered double hydroxides） were prepared with CO（NH2）2, NH4 Cl and NH3·H2O by the coprecipitation method, respectively. Corresponding composite membranes were prepared by the coating method. LDHs were characterized by WAXS, CO2-TPD and SEM. The morphology of the PVC/LDHs composite membranes were characterized by means of SEM. The thermal stability of the membranes was analyzed by air aging box and TGA-FTIR. The SEM results show that nano-particles can be compatible with poly（vinyl chloride）（PVC） matrix homogeneously by the stirring-ultrasound blend method with two steps. Furthermore, the air aging box results proved that MgAl-CO（NH2）2-LDH has the best effect on thermal stability of PVC. TGA-FTIR results show that MgAl-CO（NH2）2-LDH could adsorb more HCl that resulted from the degradation of PVC and improve the pyrolysis temperature of the first degradation stage by 15 K compared with PVC.展开更多
基金financially supported by the National Natural Science Foundation of China(No.21374054)the SinoGerman Center for Research Promotion and the National Basic Research Program of China(No.2014CB932202)
文摘Differential scanning calorimetry（DSC） has been widely applied to study crystallization and melting of materials. However, for polymeric lamellar crystals, the melting thermogram during heating process usually exhibits a broad endothermic peak or even multiple endotherms, which may result from changes of metastability via recrystallization process. Sometimes, the recrystallization exotherm cannot be observed due to its overlapping with the melting endotherm. In this work, we employed a step heating procedure consisting of successive heating and temperature holding stages to measure the metastability of isothermally crystallized poly（butylene succinate）（PBS） crystals. With this approach we could gain the fraction of crystals melted at different temperature ranges and quantitatively detect the melting-recrystallization behavior. The melting-recrystallization behavior depends on the polymer chain structure and the crystallization temperature. For instance, PBS block copolymer hardly shows recrystallization behavior while PBS oligomer and high molecular weight PBS homopolymer demonstrate remarkable melting-recrystallization phenomenon. High molecular weight PBS isothermally crystallized in the low temperature range shows multiple melting-recrystallization while those isothermally crystallized at elevated temperatures do not exhibit observable recrystallization behavior. Furthermore, the melting endotherms were fitted via the melting kinetics equations. The original isothermally crystallized lamellae demonstrate quite different melting kinetics from the recrystallized lamellar crystals that melt at the highest temperature range, which is attributed to the different degrees of stabilization. Finally, the mechanism of melting-recrystallization is briefly discussed. We propose that apparent meltrecrystallization phenomenon be observed when melting of preformed lamellar crystals and recrystallization of thicker lamellae have similar free energy barrier.
文摘The motivation of this paper is to explore the application of Step-Heating Thermography(SHT)as a technique capable of inspecting new composite rail carbodies using demanding requirements set by the rail manufacturing industry.A large composite sample,with Polytetrafluoroethylene(PTFE)artificial defects,replicating a side-wall section of a new rail carbody,was manufactured and inspected for surface and subsurface defects in this research.The sample,characterized by its large thickness,consists of a monolithic Carbon Fibre Reinforced Polymers(CFRP)component(20mm thickness)and a CFRP-PET foam-CFRP sandwich(40mm total thickness)component fused together.The main challenge of the inspection procedure was to apply reflection mode thermography and detect defects in the entire thickness of the sample that exhibits both low emissivity and thermal insulating properties,especially at the sandwich sections of the sample.The paper explored thermography procedures that would be able to detect large numbers of defects under one single acquisition and would be applied under an automated inspection process leading to the detection of defects only up to 5mm in the CFRP sections of the sample while no defects were able to be detected at the back skin of the sample.
基金National Key Research and Development Program of China(2023YFB406900)National Natural Science Foundation of China(62435012,U24A20312,52202008,62175132)+1 种基金Natural Science Foundation of Shandong Province(ZR2022QE013)Outstanding Young and Middle-aged Scholars of Shandong University(Tao Li)。
文摘Single-crystal fibers(SCFs)exhibit a significant potential for high-power laser applications owing to their superior thermal management capabilities.To enhance pump power and mitigate damage at the injection end of SCFs,a segmented YAG/Yb:YAG/YAG SCF with a diameter of~200μm was directly grown using a one-step laserheated pedestal growth(LHPG)method.The crystallinity and uniformity of the as-grown fiber were evaluated through a real-time back-reflection Laue camera system.Employing a low-loss segmented YAG/Yb:YAG/YAG SCF as the laser gain medium,investigations were conducted on both continuous-wave(CW)oscillators and ultrafast pulse amplifiers.The CW laser operation at 1030.9 nm achieved a maximum output power of 117.6 W with a slope efficiency of~46%,while the ultrafast amplifier system generated pulses with an average power of 101.2 W at a repetition rate of 30.8 MHz.To the best of our knowledge,this represents the highest average power achieved from a single-stage amplifier based on an Yb:YAG SCF.Preliminary pulse compression was performed at a signal power of 5o.6 W using a grating pair,resulting in a compressed pulse width of 786 fs with a compression efficiency of 58%.This study highlights the potential of crystal fiber lasers for a new generation of high-power laser systems.
基金supported by the Shanghai Municipal Education Commission with“Twelfth Five”scientific connotation construction project(No.nhky-2012-05)foreign visiting scholar fellowship program(No.B-8938-12-0406)Opening Project of Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering
文摘MgAl-LDH（layered double hydroxides） were prepared with CO（NH2）2, NH4 Cl and NH3·H2O by the coprecipitation method, respectively. Corresponding composite membranes were prepared by the coating method. LDHs were characterized by WAXS, CO2-TPD and SEM. The morphology of the PVC/LDHs composite membranes were characterized by means of SEM. The thermal stability of the membranes was analyzed by air aging box and TGA-FTIR. The SEM results show that nano-particles can be compatible with poly（vinyl chloride）（PVC） matrix homogeneously by the stirring-ultrasound blend method with two steps. Furthermore, the air aging box results proved that MgAl-CO（NH2）2-LDH has the best effect on thermal stability of PVC. TGA-FTIR results show that MgAl-CO（NH2）2-LDH could adsorb more HCl that resulted from the degradation of PVC and improve the pyrolysis temperature of the first degradation stage by 15 K compared with PVC.
