Xi-Shu Wang

Effects of thickness on mechanical properties of conducting polythiophene films

Owing to the rapid development of conducting polymers [1, 2], it has become possible to control the electrical property of polymers over the range from insulation to high conductivity. Some polymeric materials have successively been synthesized with conductivity similar to such metals as copper [3]. Polymers and electrical conductivity are no longer mutually exclusive. In addition, conducting polymers possess some other superior properties such as low mass density, ease of fabrication, flexibility in design, and resistance to corrosion. Therefore, they can be used in various applications as substitutes of metals [4]. The promise of combining these properties with good electrical properties in polymers has prompted extensive interest in the last two decades. However, most of the previously synthesized conducting polymers are brittle, insoluble, intractable, and often decomposed before melting [5]. The preparation of conducting polymers with high strength and good chemical stability remains still a great challenge. To date, there is still a lack of investigation on the relationship between the macroscopic properties and microstructures of conducting polymer films. Recently, polythiophene thin films with high conductivity and chemical stability have been synthesized by an electrochemical method [6, 7]. The mechanical properties of such kinds of films are experimentally investigated in this paper by using an electronic speckle pattern interferometry method and the SEM technique. Significant effects of thickness on the strength and Young’s modulus of films are found. The conducting polythiophene films used in this study were synthesized in a one-compartment cell with the use of an EG&G potentiostat model 283 under computer control [6, 7]. Two AISI 304 stainless steel sheets with a spacing of 5 mm are used as the working and counter electrodes. The anodic potential is referred to the third electrode made of Ag/AgC1. Polythiophene films were yielded by electrolysis of 0.03 M thiophene in freshly distilled boron trifluoride diethyl etherate at a constant potential of 1.3 V. The film thickness was controlled by adjusting the total charge passed in the cell. Thus, polythiophene films with different thicknesses ranging from one to hundreds of microns were successively grown/deposited on the base of a stainless steel plate by controlling the strength of the electric current and the synthesis time. These films behave like metal thin films and can be cut easily into various shapes of specimens. An electronic speckle pattern interferometry (ESPI) method [8] was used to measure the displacement of a film specimen under strain. When subjected to an externally applied force, a polythiophene film may undergo a considerable shape change, especially in the longitudinal direction due to the large ratio of the length L to the width W (L/W ≥ 10). The shape change of a film can be detected by the ESPI method. Several tensile stress-strain curves of polythiophene films of different thickness are shown in Fig. 1. Following the linear elastic deformation, plastic deformation occurs at a threshold stress, referred to as the yield stress. During plastic deformation, an evident strain-hardening curve is observed, that is, the applied stress has to be increased continuously for further development of plastic strain. The maximum stress that a polythiophene film of thickness less than 10 μm can bear is similar to that of an aluminum thin film. It can be seen from Fig. 1 that the strength of a polythiophene film exhibits an evident dependence upon its thickness. As the film thickness increases, the tensile strength decreases. This size effect of tensile strength is especially evident when the film thickness is less than 10 μm. However, the film strengths approach to a constant value when the thickness is larger than 15 μm. No evident difference has been observed in the yield stress among films when their thickness is larger than about 15 μm. In our measurement, films of thickness 4 μm possess the highest strength. The physical micromechanisms of such a size effect will be analyzed below. The elastic modulus of a film can also be obtained by the same experimental system, from which both strain

Effects of dragonfly wing structure on the dynamic performances

The configurations of dragonfly wings, including the corrugations of the chordwise cross-section, the microstructure of the longitudinal veins and membrane, were comprehensively investigated using the Environmental Scanning Electron Microscopy (ESEM). Based on the experimental results reported previously, the multi-scale and multi-dimensional models with different structural features of dragonfly wing were created, and the biological dynamic behaviors of wing models were discussed through the Finite Element Method (FEM). The results demonstrate that the effects of different structural features on dynamic behaviors of dragonfly wing such as natural frequency/modal, bending/torsional deformation, reaction force/torque are very significant. The corrugations of dragonfly wing along the chordwise can observably improve the flapping frequency because of the greater structural stiffness of wings. In updated model, the novel sandwich microstructure of the longitudinal veins remarkably improves the torsional deformation of dragonfly wing while it has a little effect on the flapping frequency and bending deformation. These integrated structural features can adjust the deformation of wing oneself, therefore the flow field around the wings can be controlled adaptively. The fact is that the flights of dragonfly wing with sandwich microstructure of longitudinal veins are more efficient and intelligent.

Investigations on the Mechanical Properties of Conducting Polymer Coating-Substrate Structures and Their Influencing Factors

This review covers recent advances and work on the microstructure features, mechanical properties and cracking processes of conducting polymer film/coating- substrate structures under different testing conditions. An attempt is made to characterize and quantify the relationships between mechanical properties and microstructure features. In addition, the film cracking mechanism on the micro scale and some influencing factors that play a significant role in the service of the film-substrate structure are presented. These investigations cover the conducting polymer film/coating nucleation process, microstructure-fracture characterization, translation of brittle-ductile fractures, and cracking processes near the largest inherent macromolecule defects under thermal-mechanical loadings, and were carried out using in situ scanning electron microscopy (SEM) observations, as a novel method for evaluation of interface strength and critical failure stress.

The effective modulus of super carbon nanotubes predicted by molecular structure mechanics

A super carbon nanotube (ST) is a kind of hierarchical structure constructed from carbon nanotubes (named as CNT arm tubes). With the detailed construction of a Y-junction considered, the effective mechanical properties of ST structures are studied by the molecular structure mechanics (MSM) method. The Youngs modulus and shear modulus of STs are found to depend mainly on the aspect ratio of CNT arm tubes instead of the chirality of the ST. A scale law is adopted to express the relation between the effective modulus (Youngs modulus or shear modulus) and the aspect ratio of the CNT arm tubes. The Poissons ratio of the ST is affected by both the aspect ratio of the CNT arm tubes and the chirality of the ST. The deformation of the ST comes from both the bending and the stretching of the CNT arm tubes. The Y-junction acts as an reinforcement phase to make the bending and stretching couple together and induce large linearity in ST structures.

Buckling behavior of metal film/substrate structure under pure bending

Many studies on the thin film/substrate structure and its failure mechanism were reported in recent years. The direct experimental results of thin film/substrate structure by scanning electron microscopy presents an intriguing problem: there exists a buckling failure mechanism at the lateral edge of metal film under pure bending. The qualitative theoretical analysis has been done on such buckling failure of thin film/substrate structure. The experimental results and theoretical analysis are helpful to understand the extrinsic stresses or deformations that are induced by external physical effects.

Effects of Ca combined with Sr additions on microstructure and mechanical properties of AZ91D magnesium alloy

Abstract Weight reduction to improve automobile fuel economy has triggered renewed interest in magnesium. The effects of Ca/Sr separate and composite additions to AZ91D magnesium alloy on its microstructure and mechanical properties have been investigated. The results indicate Ca can refine both the grain and eutectic phase of AZ91D magnesium alloy. Sr hampers microstructure refinement when composite Ca/Sr additions are made. In addition, separate Ca additions to AZ91D magnesium alloy increase yield strength but decrease elongation of this alloy. By adjusting the Ca/Sr composite proportions, additions to AZ91D magnesium alloy are able to improve both microstructure and mechanical properties of the alloy.

Hierarchical Dragonfly Wing: Microstructure-Biomechanical Behavior Relations

The dragonfly wing, which consists of veins and membrane, is of biological hierarchical material. We observed the cross-sections of longitudinal veins and membrane using Environmental Scanning Electron Microscopy (ESEM). Based on the experiments and previous studies, we described the longitudinal vein and the membrane in terms of two hierarchical levels of organization of composite materials at the micro- and nano-scales. The longitudinal vein of dragonfly wing has a complex sandwich structure with two chitinous shells and a protein layer, and it is considered as the first hierarchical level of the vein. Moreover, the chitinous shells are concentric multilayered structures. Clusters of nano-fibrils grow along the circumferential orientation embedded into the protein layer. It is considered as the second level of the hierarchy. Similarly, the upper and lower epidermises of membrane constitute the first hierarchical level of organization in micro scale. Similar to the vein shell, the membrane epidermises were found to be a paralleled multilayered structure, defined as the second hierarchical level of the membrane. Combining with the mechanical behavior analysis of the dragonfly wing, we concluded that the growth orientation of the hierarchical structure of the longitudinal vein and membrane is relevant to its biomechanical behavior.

A comprehensive study on the mechanical properties of super carbon nanotubes

Super carbon nanotubes (STs) are a kind of hierarchical structure built by carbon nanotubes (called CNT arm tubes). Due to their complicated structure, the mechanical properties of STs are difficult to predict. In this work, an invariability phenomenon is uncovered in the mechanical properties of STs: both the in-plane stiffness and the shear stiffness of STs are independent of the ST chirality or CNT arm tube chirality. The bending rigidities of STs are also studied. From the in-plane stiffness and bending rigidity, the equivalent thickness of STs is calculated and found to be approximately a linear function of the CNT arm tube aspect ratio.

Microstructure-mechanical properties relationship in conducting polypyrrole films

In this paper, the microstructures of electrochemically synthesized conducting polypyrrole (PPy) films were studied by scanning electron microscopy (SEM). It was found that the polymer film growth condition has strong effects on mechanical properties of the polymer film. The relationship between mechanical properties (such as tensile strength and brittle—tough properties) and the microstructures of PPy films was described for the first time. The effect of electrochemical polymerization condition including temperature and electrolyte composition on the strength and brittle—tough properties was also studied. Films deposited both on the surface of the anode facing the counter electrode and on the back surface were characterized. In order to improve the mechanical properties of PPy films, an optimal condition of electrochemical synthesis of conducting PPy films has been recommended.

Improvement on the Fatigue Performance of 2024-T4 Alloy by Synergistic Coating Technology

In this paper, rotating bending fatigue tests of 2024-T4 Al alloy with different oxide coatings were carried out. Compared to the uncoated and previously reported oxide coatings of aluminum alloys, the fatigue strength is able to be enhanced by using a novel oxide coating with sealing pore technology. These results indicate that the better the coating surface quality is, the more excellent the fatigue performance under rotating bending fatigue loading is. The improvement on the fatigue performance is mainly because the fatigue crack initiation and the early stage of fatigue crack growth at the coating layer can be delayed after PEO coating with pore sealing. Therefore, it is a so-called synergistic coating technology for various uses, including welding thermal cracks and filling micro-pores. The effects of different oxide coatings on surface hardness, compressive residual stress, morphology and fatigue fracture morphology are discussed. A critical compressive residual stress of about 95–100 MPa is proposed.