Strengthening of RC Beams and Frames by External Prestressing

Ph.DDOCTOR OF PHILOSOPH

Edge-pinning effect of graphene nanoflakes sliding atop graphene

Edge effect is one of the detrimental factors preventing superlubricity in laminar solid lubricants. Separating the friction contribution from the edge atom and inner atom is of paramount importance for rational design of ultralow friction across scales in van der Waals heterostructures. To decouple these contributions and provide the underlying microscopic origin at the atomistic level, we considered two contrast models, namely, graphene nanoflakes with dimerized and pristine edges sliding on graphene monolayer based on extensive ab initio calculations. We found the edge contribution to friction is lattice orientation dependence. In particular, edge pinning effect by dimerization is obvious for misaligned contact but suppressed in aligned lattice orientation. The former case providing local commensuration along edges is reminiscent of Aubry's pinned phase and the contribution of per edge carbon atom to the sliding potential energy corrugation is even 1.5 times more than that of an atom in bilayer graphene under commensurate contact. Furthermore, we demonstrated that the dimerized edges as high frictional pinning sites are robust to strain engineering and even enhanced by fluorination. Both structural and chemical modification in the tribological system constructed here offers the atomic details to dissect the undesirable edge pinning effect in layered materials which may give rise to the marked discrepancies in measured friction parameters from the same superlubric sample or different samples with the same size and identical preparation.Comment: 18 pages,6 figure

Ultra-high strength metal matrix composites (MMCs) with extended ductility manufactured by size-controlled powder and spherical cast tungsten carbide

The main challenge of particle reinforced metal matrix composites (MMCs) is balancing strength and ductility. This research uses type 420 stainless steel and spherical cast tungsten carbide (WC/W2C) with a similar powder size and range as raw powders to manufacture laser powder bed fusion (LPBF) 420 + 5 wt% WC/W2C MMCs. LPBF 420 + 5 wt% WC/W2C MMCs contain austenite, martensite, and W-rich carbides (WC/W2C, FeW3C, M6C, and M7C3) from nanometre to micrometre scale. The well-balanced composition creates a crack-free reaction layer between the reinforced particles and matrix. This reaction layer consists of two distinct layers, depending on the element concentration. The LPBF 420 + 5 wt% WC/W2C MMCs achieved an excellent compressive strength of ∼5.5 GPa and a considerable fracture strain exceeding 50 %. The underlying mechanisms for the improved mechanical properties are discussed, providing further insight to advance the application of MMCs via additive manufacturing

Synergistic improvement of pitting and wear resistance of laser powder bed fusion 420 stainless steel reinforced by size-controlled spherical cast tungsten carbides

The spherical cast WC/W2C is selected to produce a martensitic stainless steel-based composite using the laser powder bed fusion technique. W and C reacted with Fe and Cr, creating a strong bond between the particles and the matrix, reducing the wear rate by over 98%. W and C diffuse to the matrix, increasing the hardness over 100 HV0.5. The interface between cast WC/W2C and matrix is sensitive to pitting corrosion through galvanic effects. However, the formation of austenite and WO3 from spherical cast WC/W2C decomposition improves the critical pitting potential and passive film stability

Unveiling the additive-assisted oriented growth of perovskite crystallite for high performance light-emitting diodes.

Solution-processed metal halide perovskites have been recognized as one of the most promising semiconductors, with applications in light-emitting diodes (LEDs), solar cells and lasers. Various additives have been widely used in perovskite precursor solutions, aiming to improve the formed perovskite film quality through passivating defects and controlling the crystallinity. The additive's role of defect passivation has been intensively investigated, while a deep understanding of how additives influence the crystallization process of perovskites is lacking. Here, we reveal a general additive-assisted crystal formation pathway for FAPbI3 perovskite with vertical orientation, by tracking the chemical interaction in the precursor solution and crystallographic evolution during the film formation process. The resulting understanding motivates us to use a new additive with multi-functional groups, 2-(2-(2-Aminoethoxy)ethoxy)acetic acid, which can facilitate the orientated growth of perovskite and passivate defects, leading to perovskite layer with high crystallinity and low defect density and thereby record-high performance NIR perovskite LEDs (~800 nm emission peak, a peak external quantum efficiency of 22.2% with enhanced stability)

A Driving Behavior Planning and Trajectory Generation Method for Autonomous Electric Bus

A framework of path planning for autonomous electric bus is presented. ArcGIS platform is utilized for map-building and global path planning. Firstly, a high-precision map is built based on GPS in ArcGIS for global planning. Then the global optimal path is obtained by network analysis tool in ArcGIS. To facilitate local planning, WGS-84 coordinates in the map are converted to local coordinates. Secondly, a double-layer finite state machine (FSM) is devised to plan driving behavior under different driving scenarios, such as structured driving, lane changing, turning, and so on. Besides, local optimal trajectory is generated by cubic polynomial, which takes full account of the safety and kinetics of the electric bus. Finally, the simulation results show that the framework is reliable and feasible for driving behavior planning and trajectory generation. Furthermore, its validity is proven with an autonomous bus platform 12 m in length

Passivity breakdown on copper: Influence of borate anion

Passivity breakdown on copper: Influence of borate anio

Elemental decoration design with metastable cellular substructures for additively manufactured high-strength and high-corrosion resistant austenitic stainless steel

Multi-level chemical-structural heterogeneities extensively exist in additively manufactured (AM) metals due to the intrinsic layer-by-layer non-equilibrium solidification process. Strategies designed with particular metastable substructures aiming at advanced performances are significant for AM counterparts. In this work, Si and Mo additions are conducted based on the regulations of dislocation cell substructures and stacking fault energies for stainless steel (SS) 316L fabricated by laser powder bed fusion (PBF-LB). Their load-bearing performance and corrosion behavior are characterized. Results show that additional Mo segregation at cellular boundaries contributes a stronger strengthening effect than Si, which periodically hinders dislocation slip during deformation. Addition of Si triggers deformation twinning at an early stage due to decreased stacking fault energy, and subsequent dynamic Hall-Petch effects improve strain-hardening capability and plasticity for PBF-LB SS 316L+Si. Meanwhile, addition of Mo enhances pitting corrosion resistance of PBF-LB 316L+Mo SS in chloride-containing solutions, especially the pitting re-passivation, which is opposite in the Si addition case due to the increased quantiy of undesired Si/Mn-rich oxides. Underlying deformation and corrosion mechanisms for Mo/Si-added PBF-LB SSs are discussed. Our work is anticipated to motivate the alloy design concept based on particular metastable substructures for advanced AM alloys