Haimin Yao

Protection mechanisms of the iron-plated armor of a deep sea hydrothermal vent gastropod

Biological exoskeletons, in particular those with unusually robust and multifunctional properties, hold enormous potential for the development of improved load-bearing and protective engineering materials. Here, we report new materials and mechanical design principles of the iron-plated multilayered structure of the natural armor of Crysomallon squamiferum, a recently discovered gastropod mollusc from the Kairei Indian hydrothermal vent field, which is unlike any other known natural or synthetic engineered armor. We have determined through nanoscale experiments and computational simulations of a predatory attack that the specific combination of different materials, microstructures, interfacial geometries, gradation, and layering are advantageous for penetration resistance, energy dissipation, mitigation of fracture and crack arrest, reduction of back deflections, and resistance to bending and tensile loads. The structure-property-performance relationships described are expected to be of technological interest for a variety of civilian and defense applications.

Hollow-tunneled graphitic carbon nanofibers through Ni-diffusion-induced graphitization as high-performance anode materials

N-doped nanoporous graphitic carbon has attracted great interest because of its distinctive structure and physical properties. In this paper, we have proposed a novel method to control Ni-induced graphitization by diffusing Ni nanoparticles from graphitic carbon spheres into N-doped amorphous carbon nanofibers, which turns amorphous carbon into graphitic carbon and produces a hollow-tunnel structure in electrospun carbon/Ni nanofibers. The resultant materials were further treated by chemical activation and acid treatment to develop activated N-doped hollow-tunneled graphitic carbon nanofibers (ANHTGCNs). In a typical application, we demonstrate that ANHTGCNs are excellent anode materials for lithium ion batteries (LIBs), displaying a superhigh reversible specific capacity of ∼1560 mA h g−1 and a remarkable volumetric capacity of ∼1.8 A h cm−3 at a current density of 0.1 A g−1 with outstanding rate capability and good cycling stability.

Adhesion and sliding response of a biologically inspired fibrillar surface: experimental observations.

Inspired by the adhesion mechanisms of several animal species such as geckos, beetles and flies, several efforts in designing and fabricating surface engineering strategies have been made recently to mimic the adhesive and frictional behaviour of biological foot pads. An important feature of such biological adhesion systems is the ability to switch between strong attachment and easy detachment, which is crucial for animal locomotion. Recent investigations have suggested that such a ‘switching’ mechanism can be achieved by the elastic anisotropy of the attachment pad, which renders the magnitude of the detachment force to be direction dependent. This suggestion is supported by the observations that the fibres of the foot pads in geckos and insects are oriented at an angle to the base and that geckos curl their toes backwards (digital hyperextension) while detaching from a surface. One of the promising bio-inspired architectures developed recently is a film-terminated fibrillar PDMS surface; this structure was demonstrated to result in superior detachment force and energy dissipation compared with a bulk PDMS surface. In this investigation, the film-terminated fibrillar architecture is modified by tilting the fibres to make the surface vertically more compliant and elastically anisotropic. The directional detachment and the sliding resistance between the tilted fibrillar surfaces and a spherical glass lens are measured: both show significant directional anisotropy. It is argued that the anisotropy introduced by the tilted fibres and the deformation-induced change in the compliance of the fibre layer are responsible for the observed anisotropy in the detachment force.

Cracking and adhesion at small scales: atomistic and continuum studies of flaw tolerant nanostructures

Once the characteristic size of materials reaches nanoscale, the mechanical properties may change drastically and classical mechanisms of materials failure may cease to hold. In this paper, we focus on joint atomistic-continuum studies of failure and deformation of nanoscale materials. In the first part of the paper, we discuss the size dependence of brittle fracture. We illustrate that if the characteristic dimension of a material is below a critical length scale that can be on the order of several nanometres, the classical Griffith theory of fracture no longer holds. An important consequence of this finding is that materials with nano-substructures may become flaw-tolerant, as the stress concentration at crack tips disappears and failure always occurs at the theoretical strength of materials, regardless of defects. Our atomistic simulations complement recent continuum analysis (Gao et al 2003 Proc. Natl Acad. Sci. USA 100 5597–600) and reveal a smooth transition between Griffith modes of failure via crack propagation to uniform bond rupture at theoretical strength below a nanometre critical length. Our results may have consequences for understanding failure of many small-scale materials. In the second part of this paper, we focus on the size dependence of adhesion systems. We demonstrate that optimal adhesion can be achieved by either length scale reduction, or by optimization of the shape of the surface of the adhesion element. We find that whereas change in shape can lead to optimal adhesion strength, those systems are not robust against small deviations from the optimal shape. In contrast, reducing the dimensions of the adhesion system results in robust adhesion devices that fail at their theoretical strength, regardless of the presence of flaws. An important consequence of this finding is that even under the presence of surface roughness, optimal adhesion is possible provided the size of contact elements is sufficiently small. Our atomistic results corroborate earlier theoretical modelling at the continuum scale (Gao and Yao 2004 Proc. Natl Acad. Sci. USA 101 7851–6). We discuss the relevance of our studies with respect to natures design of bone nanostructures and nanoscale adhesion elements in geckos.

Bio-inspired interfacial strengthening strategy through geometrically interlocking designs

Many biological materials, such as nacre and bone, are hybrid materials composed of stiff brittle ceramics and compliant organic materials. These natural organic/inorganic composites exhibit much enhanced strength and toughness in comparison to their constituents and inspires enormous biomimetic endeavors aiming to synthesize materials with superior mechanical properties. However, most current synthetic composites have not exhibited their full potential of property enhancement compared to the natural prototypes they are mimicking. One of the key issues is the weak junctions between stiff and compliant phases, which need to be optimized according to the intended functions of the composite material. Motivated by the geometrically interlocking designs of natural biomaterials, here we propose an interfacial strengthening strategy by introducing geometrical interlockers on the interfaces between compliant and stiff phases. Finite element analysis (FEA) shows that the strength of the composite depends strongly on the geometrical features of interlockers including shape, size, and structural hierarchy. Even for the most unfavorable scenario when neither adhesion nor friction is present between stiff and compliant phases, the tensile strength of the composites with proper interlocker design can reach up to 70% of the ideal value. The findings in this paper would provide guidelines to the improvement of the mechanical properties of current biomimetic composites.

Mechanical principles of robust and releasable adhesion of gecko

Robust attachment to rough surfaces and controllable detachment are two features that must coexist for gecko to locomote on walls and ceilings. While robust adhesion ensures that the animal can stick reliably in the presence of random surface roughness, the same adhesion must be readily releasable upon its movement. What are the mechanical principles of such robust and releasable adhesion in nature? In this paper, we review some of the most essential results obtained in our previous studies on this question and also add a number of new results aimed to facilitate a more complete understanding. We begin with addressing the shape and size effects in adhesion between single asperities, and then extend the discussion to contact between rough surfaces, focusing on strategies to achieve robust (flaw-tolerant) and releasable adhesion across multiple length scales. Based on the fundamental principles in fracture and adhesion mechanics, we show that flaw tolerant adhesion is controlled by a dimensionless parameter which suggests a combination of strategies including size confinement, hierarchical energy dissipation, graded elasticity and strength degradation over multiple size scales for robust attachment to rough surfaces. At the same time, we show that strong elastic anisotropy allows adhesion to be controllable depending on the direction of pulling. These results and their future extensions may lead to a theoretical basis to understand robust and releasable adhesion in a variety of biological systems.

Molecular dynamics simulation on deformation mechanisms in body-centered-cubic molybdenum nanowires

A systematic study on the deformation mechanisms of molybdenum (Mo) nanowires (NWs) was conducted using molecular dynamics simulations. Both axial orientation and wire thickness were found to play important roles in determining the deformation pathways. In the NWs with orientation 〈110〉/{111}, full dislocation plasticity is referentially activated on {110} planes. For both 〈100〉/{110} and 〈100〉/{100} NWs, twinning is the dominant mechanism with {112} being the coherent twin boundaries. A progressive slip process leads to a uniform elongation of 41% and the 〈100〉 wire axis reorients to 〈110〉. For 〈100〉/{100} NWs, the reorientation mechanism ceases to operate when the diameter d   8 nm. The atomic chains are energetically preferred for ultrathin NWs after yielding due to the resemblance of the surface to the close-packed bcc planes, while multiple slip systems tend to be activated for larger NWs. Finally, a theoretical model is proposed to explain the underlying mechanism of size dependence of t...

A porous graphene/carbon nanowire hybrid with embedded SnO2 nanocrystals for high performance lithium ion storage

A high-yield porous graphene/carbon nanowire (PG/CNW) hybrid is synthesized via in situ chemical oxidative polymerization followed by carbonization and KOH activation. An easy and efficient vacuum-assisted impregnation followed by thermal treatment at a temperature of 350 °C is adopted to crystallize the SnO2 nanocrystals, realizing their embedment into the PG/CNW matrix. When evaluated for the electrochemical properties in lithium ion batteries, the obtained SnO2–PG/CNW composite exhibits a high lithium storage capacity of up to 1200 mA h g−1, good cycling performance and outstanding rate capability. The preparation strategy is easy and efficient, providing a novel methodology for various types of heterogeneous nanocrystals embedded into porous matrix configuration for energy storage applications.

Core/shell TiO2–MnO2/MnO2 heterostructure anodes for high-performance lithium-ion batteries

Core/shell TiO2–MnO2/MnO2 heterostructures were synthesized by combining an electrospinning technique with a hydrothermal reaction. To create the starting materials, porous TiO2–carbon nanofibers were first prepared using a simple electrospinning technique followed by calcination. The porous structure in TiO2–carbon nanofibers caused by the partial decomposition of polystyrene is beneficial to the diffusion of KMnO4 from the outer surface into inner fibers to completely react with carbon and produce MnO2 nanosheets. Some MnO2 nanosheets in the TiO2 core connect with other MnO2 nanosheets surrounding the TiO2 core to form core/shell TiO2–MnO2/MnO2, which can enhance the stability of the structure. The large surface area of the resulting materials offers a sufficient electrode–electrolyte interface to promote the charge-transfer reactions, which yields a better rate capability. The porous structure of TiO2–MnO2/MnO2 nanofibers not only facilitates Li-ion access, but also accommodates large volumetric expansion during the charging–discharging processes, resulting in an excellent cycle performance. As an anode, this material delivered a high reversible capacity of 891 mA h g−1 at the first cycle and maintained the capacity of 888 mA h g−1 after 50 cycles at the current density of 0.1 A g−1; it also showed a remarkable rate capability of 2 A g−1 while retaining a capacity of 185 mA h g−1 after 500 cycles. Given their enhanced electrochemical performance, core/shell TiO2–MnO2/MnO2 heterostructure nanofibers are promising anode candidates for lithium-ion batteries.

Acoustic band gaps of three-dimensional periodic polymer cellular solids with cubic symmetry

The band structure and sound attenuation of the triply periodic co-continuous composite materials with simple cubic lattice, body-centered cubic lattice, and face-centered cubic lattice consisting of PMMA and air are investigated using finite element method. Complete band gaps are found in these structures and the width of band gaps is depending on volume fraction. It is shown that the width of band gaps along different directions in the first irreducible Brillouin zone enlarges as the volume fraction increases from 0.2 to 0.7. The largest complete band gap widths of the three types of co-continuous structures are 0.29, 0.54, and 0.55, respectively. As the complete band gaps appear in audible range of frequencies, these triply periodic co-continuous composite materials can be utilized to control noise.