分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: Positron irradiation effects on positronium formation in low-density polyethylene (LDPE), gamma-irradiated LDPE and unplasticized PVC (UPVC) are studied. At least in one of the three different measurements, i.e., prolonged positron annihilation measurement at room temperature, low temperature in darkness and subsequent measurement under light, changes in o-Ps intensity are observed in nonirradiated LDPE and gamma-irradiated LDPE. While in UPVC, change in o-Ps intensity is hardly observable in all the above-mentioned three measurements. Reduction of o-Ps intensity by light indicates that positronium formation via the recombination of a positron and a trapped electron exists in LDPE and gamma-irradiated LDPE. The absence of light bleaching effect, together with the fact that the value of o-Ps intensity in heating and cooling process of a thermal circle is nearly the same, indicates that in UPVC, positronium can not be formed through trapped electron mechanism. This study highlights the speciality of positronium formation in UPVC, positronium is formed exclusively by the recombination of electron-positron pairs with short separations.
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: MXenes, a novel family of two-dimensional metal carbides, are receiving intense attention for lithium-ion batteries (LIBs) and super- capacitors because they have high volumetric capacitance exceeding all carbon materials. However, serious interlayer stacking exists in MXene particles, which greatly decreases the electrical conductivity in the bulk and hinders the accessibility of interlayers to electrolyte ions. Thus, multi-stacked MXene particles exhibit low capacitance and poor rate capability. Herein, we report an effective strategy to directly improve the electrochemical performance of multi-stacked MXene (Ti3C2Tx) particles as LIB anode materials. It was successfully realized by growing conductive “carbon nanofiber (CNF) bridges” within the gaps of each Ti3C2Tx particle as well as the outside. With the help of these CNFs, the as-prepared Ti3C2/CNF particles exhibited signifi- cantly improved reversible capacity compared with pure Ti3C2Tx particles. More remarkably, even at an ultrahigh rate of 100 C, the capacity of Ti3C2/CNF hybrid particles was just slightly lower than that of pure Ti3C2Tx particles at 1 C, and there was no capacity decay after 2900 cycles at 100 C, demonstrating excellent rate capability and superior long-term stability at the ultrahigh rate.
分类: 材料科学 >> 材料科学(综合) 提交时间: 2017-05-09
摘要: Perylene tetracarboxylic anhydride (PTCDA) was reacted with 6-aminocaproic acid to form the corresponding perylene bisimide (PBI). PBI was used as the foundation for oligomerisation of glycidol in a ring-opening reaction of glycidol leading to a hyper branched, water-soluble glycidol derivative of perylene (PBI-HPG). PBI-HPG was bound to the reduced graphene oxide via p–p stacking resulting in a compound termed PBI-HPG/RGO. The structure and morphology of PBI-HPG/RGO were investigated by infrared spectroscopy (FT-IR), wide angle X-ray diffractometry (WAXD), transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). PBI- HPG/RGO was blended at different loadings in order to improve the thermal and mechanical properties of epoxy composites. The maximum Tg of the epoxy composites was about 20 C and the decomposition temperature (Td) was 26 C higher than that of neat epoxy. The incorporation of PBI- HPG/RGO yields a material with an impact strength of 39.6 kJ m 2 and a tensile strength at 0.7 wt%. It increased by 50.8% and 62.3%, respectively, compared to the neat epoxy.