Yanqing Yang

Robust self-cleaning and micromanipulation capabilities of gecko spatulae and their bio-mimics

Geckos have the extraordinary ability to prevent their sticky feet from fouling while running on dusty walls and ceilings. Understanding gecko adhesion and self-cleaning mechanisms is essential for elucidating animal behaviours and rationally designing gecko-inspired devices. Here we report a unique self-cleaning mechanism possessed by the nano-pads of gecko spatulae. The difference between the velocity-dependent particle-wall adhesion and the velocity-independent spatula-particle dynamic response leads to a robust self-cleaning capability, allowing geckos to efficiently dislodge dirt during their locomotion. Emulating this natural design, we fabricate artificial spatulae and micromanipulators that show similar effects, and that provide a new way to manipulate micro-objects. By simply tuning the pull-off velocity, our gecko-inspired micromanipulators, made of synthetic microfibers with graphene-decorated micro-pads, can easily pick up, transport, and drop-off microparticles for precise assembling. This work should open the door to the development of novel self-cleaning adhesives, smart surfaces, microelectromechanical systems, biomedical devices, and more.

First-principles study of stability and properties on β-SiC/TiC(111) interface

The interfacial properties of β-SiC/TiC(111), such as work of adhesion, interface energy, fracture toughness, bonding nature, were investigated using first-principles calculations. Twenty four interface models with different terminations, carbon sublattice, and stacking sites were investigated. The thermodynamic stability of SiC/TiC(111) decreases as the order of C/C, Si/Ti, C/Ti, and Si/C terminations. The C/C-terminated top-site-stacked models (CCU3, CCT3) are most stable with the largest work of adhesion, smallest interface energy, and largest interfacial fracture toughness. The interfacial fracture toughness is predicted as 3.6 ∼ 4.3 MPa·m1/2. The valence electron density and partial density of states indicate that the interfacial bonding is mainly contributed from covalent C-C interactions caused by the hybridization of C-2p. The interfacial Si-C and Ti-C bonds are less covalent and much weaker than the interior ones, and the interfacial bonds are more inclined to decompose. The carbon layer is likel...

Interfacial properties and electronic structure of β-SiC(111)/α-Ti(0001): A first principle study

First-principles calculations of β-SiC(111)/α-Ti(0001) interface have been performed and the adhesion strength, interface energy, interfacial fracture toughness, and electronic structure are obtained. Six C-terminated β-SiC(111)/α-Ti(0001) interface models are investigated to clarify the influence of stacking sites and Ti atoms tilt direction on the interface bonding and fracture toughness. The hollow-site-stacked interfaces, in which Ti atoms locate on the hollow site of interfacial C atoms (cases III and IV), are more thermodynamically stable with larger work of adhesion, and interfacial fracture toughness. The center-site-stacked (cases I and II) and top-site-stacked (cases V and VI) interfaces have a decreasing interface adhesion as the order. The electronic structure of hollow-site-stacked interface (case IV) gives the evidence that atomic bonding exists between interfacial C, Si, and Ti atoms, and the C-Ti bonds exhibit more covalent features than Si-Ti. The tilt direction of Ti atoms, namely the stacking style of Ti, has a subtle and secondary effect on the interface stability.

First-principles calculation on β-SiC(111)/α-WC(0001) interface

The α-WC(0001) surface and β-SiC(111)/α-WC(0001) interface were studied by first-principles calculation based on density functional theory. It is demonstrated that the α-WC(0001) surface models with more than nine atom-layers exhibit bulk-like interior, wherein the surface relaxations localized within the top three layers are well converged. Twenty-four specific geometry models of SiC/WC interface structures with different terminations and stacking sites were chosen. The calculated work of adhesion and interface energy suggest that the most stable interface structure has the C-C bonding across the interface, yielding the largest work of adhesion and the lowest interface energy. Moreover, the top-site stacking sequence is preferable for the C/C-terminated interface. The effects of the interface on the electronic structures of the C/C-terminated interfaces are mainly localized within the first and second layers of the interface. Calculations of the work of adhesion and interface energy provide theoretical evidence that the mechanical failure may initiate at the interface or in SiC but not in WC.

Deformed microstructure and texture of Ti6Al4V alloy

Abstract Microstructure and texture evolution during hot compression of Ti6Al4V alloy with an initial equiaxed microstructure were studied in the temperature range of 850–930 °C, strain rate range of 0.01–1 s −1 and engineering compressive strain of 70%. The results indicate that when temperature is below 900 °C and strain rate is higher than 0.1 s −1 , the microstructure is mainly composed of elongated a grains. While deforming at higher temperatures and lower strain rates, dynamic recrystallization takes place. Electron back scattered diffraction (EBSD) result shows that during dynamic recrystallization, subgrain boundaries absorb dislocations and the recrystallized grains with high angle grain boundary form. At 930 °C dynamic recrystallization has basically completed, and needlelike α phase forms after water quenching. Pole figure analysis indicates that compared with the initial specimen, textures below 930 °C are weaker, while at 930 °C they are stronger.

Precipitation sequence of η phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy during artificial aging

The precipitation sequence of η(MgZn2) phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy was investigated by examining samples aged at 135 °C for various times from 5 min to 6 h. High resolution transmission electron microscopy (HRTEM) observations and energy dispersive X-ray spectroscopy (EDX) analysis indicate that the precipitation sequence of η phase along low-angle grain boundaries should be supersaturated solid solution (SSS)→vacancy-rich clusters (VRC)→GP II zones→η′→η. Based on the theory of non-equilibrium grain boundary segregation (NGS) and non-equilibrium grain boundary co-segregation (NGCS), the excessive solute elements gradually segregate to the grain boundaries by the diffusion of the solute-vacancy complex during aging treatment. The grain boundary segregation plays an important role in the nucleation and growth of VRC, GP II zones, η′ phase as well as η phase.

Investigation of interfacial reaction product of SiCf/C/Mo/Ti6Al4V composite through Raman spectroscopy

Interfacial reaction products of heat-treated SiCf/C/Mo/Ti6Al4V are studied. Energy dispersive spectrometer (EDS) mapping indicates two sublayers existence in the interface with Si-rich layer 1 next to SiC fiber and C-rich layer 2 next to matrix. Raman line scanning along interface also shows two sublayers with different reaction products, which is consistent with EDS mapping. Si-rich layer 1 is proved mainly Ti5Si3(Cx). C-rich layer 2 is proved mainly TiCx comparing the spectrum with that of TiCx particles in matrix. Peak shifts are also detected, which is possibly due to residual stress or/and microstructural evolution. Existence of SiC and excess carbon throughout the interface is also confirmed by Raman spectroscopy.

Effect of amorphous carbon coatings on the mechanical behavior of silicon carbide nanowire

Silicon carbide nanowires (NWs) are promising candidates for structural applications owing to their excellent mechanical, thermal, and electronic properties. The effect of amorphous carbon coatings on the mechanical behavior of the nanowires was studied via molecular dynamics methods at room temperature. The results show that the amorphous carbon coatings can shield opening cracks on silicon carbide nanowires, making them damage-tolerant. With increasing the defect size, the tensile strength and fracture energy of uncoated silicon carbide nanowires rapidly decrease; however, the properties of coated nanowires maintain nearly constant. Increasing the coating thickness leads to a brittle-to-ductile transition for the nanowires. Careful tailoring of the coatings permits engineering of these nanostructures for higher strength and damage tolerance at submicron scales.

Development of advanced electron tomography in materials science based on TEM and STEM

The recent developments of electron tomography (ET) based on transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) in the field of materials science were introduced. The various types of ET based on TEM as well as STEM were described in detail, which included bright-field (BF)-TEM tomography, dark-field (DF)-TEM tomography, weak-beam dark-field (WBDF)-TEM tomography, annular dark-field (ADF)-TEM tomography, energy-filtered transmission electron microscopy (EFTEM) tomography, high-angle annular dark-field (HAADF)-STEM tomography, ADF-STEM tomography, incoherent bright field (IBF)-STEM tomography, electron energy loss spectroscopy (EELS)-STEM tomography and X-ray energy dispersive spectrometry (XEDS)-STEM tomography, and so on. The optimized tilt series such as dual-axis tilt tomography, on-axis tilt tomography, conical tilt tomography and equally-sloped tomography (EST) were reported. The advanced reconstruction algorithms, such as discrete algebraic reconstruction technique (DART), compressed sensing (CS) algorithm and EST were overviewed. At last, the development tendency of ET in materials science was presented.

Modeling of push-out test for interfacial fracture toughness of fiber-reinforced composites

An analytical model of the push-out test was developed to predict the interfacial fracture toughness of fiber-reinforced matrix composites. Elastic deformation, thermal residual stress, Poisson effect, and energy dissipation due to interfacial friction are all accounted for in the model based on a fracture mechanics approach and a general solution was derived from the analysis. In addition, finite element analysis was performed to validate the analytical model. The results show that the predictions from the analytical model are in good agreement with the finite element results and experimental data on SiC-reinforced Timetal834 composites. The effects of residual stress, the friction stress ,and the debonding crack length on the interfacial fracture toughness were discussed for a better understanding of the interfacial phenomena in push-out test. The analytical model could be applied to other composite systems as well.