Synchrotron‐Based Micro‐CT and Refraction‐Enhanced Micro‐CT for Non‐Destructive Materials Characterisation

X‐ray computed tomography is an important tool for non‐destructively evaluating the 3‐D microstructure of modern materials. To resolve material structures in the micrometer range and below, high brilliance synchrotron radiation has to be used. The Federal Institute for Materials Research and Testing (BAM) has built up an imaging setup for micro‐tomography and ‐radiography (BAMline) at the Berliner storage ring for synchrotron radiation (BESSY). In computed tomography, the contrast at interfaces within heterogeneous materials can be strongly amplified by effects related to X‐ray refraction. Such effects are especially useful for materials of low absorption or mixed phases showing similar X‐ray absorption properties that produce low contrast. The technique is based on ultra‐small‐angle scattering by microstructural elements causing phase‐related effects, such as refraction and total reflection. The extraordinary contrast of inner surfaces is far beyond absorption effects. Crack orientation and fibre/matrix debonding in plastics, polymers, ceramics and metal‐matrix‐composites after cyclic loading and hydro‐thermal aging can be visualized. In most cases, the investigated inner surface and interface structures correlate to mechanical properties. The technique is an alternative to other attempts on raising the spatial resolution of CT machines.     

特种功能材料中的固态相变及应用

固态相变已经从传统结构材料的增强增韧延伸到新型功能材料研究领域,引发多种奇特的物理效应。目前已经形成一批基于固态相变的新型功能材料。这些新型功能材料蕴含着丰富的固态相变理论。固态相变现已成为新型功能材料设计与功能特性实现的重要手段之一。文章结合作者在固态相变及特种功能材料方面的研究工作,重点介绍高温形状记忆合金、高阻尼形状记忆合金、磁致伸缩材料和热障涂层材料中的固态相变特征及其在这些特种功能材料设计与功能实现和调控中应用的研究进展。     

Thermodynamic Analysis on Relation Between T0 and σM in a Ti44Ni47Nb9 Shape Memory Alloy

The effect of heat treatment on martensitic transformation behavior has been investigated in Ti44Ni47Nb9 alloy.The relation between transformation temperatures and critical stress of stress induced martensitic transformation is interpreted in terms of thermodynamic theory.It is shown that the decrease in transformation temperature in specimens of slow cooling rate or low temperature aging after solution heat treatment results from the changes of Ni/Ti ratio in the matrix.The increase of critical stress of stress induced martensitic transformation is a consequence of the decrease of transformation temperatures.     

Fractal‐Theory‐Based Control of the Shape and Quality of CVD‐Grown 2D Materials

The precise control of the shape and quality of 2D materials during chemical vapor deposition (CVD) processes remains a challenging task, due to a lack of understanding of their underlying growth mechanisms. The existence of a fractal‐growth‐based mechanism in the CVD synthesis of several 2D materials is revealed, to which a modified traditional fractal theory is applied in order to build a 2D diffusion‐limited aggregation (2D‐DLA) model based on an atomic‐scale growth mechanism. The strength of this model is validated by the perfect correlation between theoretically simulated data, predicted by 2D‐DLA, and experimental results obtained from the CVD synthesis of graphene, hexagonal boron nitride, and transition metal dichalcogenides. By applying the 2D‐DLA model, it is also discovered that the single‐domain net growth rate (SD‐NGR) plays a crucial factor in governing the shape and quality of 2D‐material crystals. By carefully tuning SD‐NGR, various fractal‐morphology high‐quality single‐crystal 2D materials are synthesized, achieving, for the first time, the precise control of 2D‐material CVD growth. This work lays the theoretical foundation for the precise adjustment of the morphologies and physical properties of 2D materials, which is essential to the use of fractal‐shaped nanomaterials for the fabrication of new‐generation neural‐network nanodevices.     

Effect of Training on Two-way Shape Memory Effect and Its Stability in a Ti-Ni-Hf High Temperature Shape Memory Alloy

The Effect of the thermal cycling training under constant strain on the two-way shape memory effect (TWSME) in a Ti36Ni49Hfl5 high temperature shape memory alloy (SMA) has been investigated by bending tests. The results indicated that the training procedure is beneficial to get the better TWSME. The two-way shape memory strain increases with increasing the training strain. And it decreases with increasing the training temperature. The TWSME obtained in the present alloy shows poorer stability compared with that obtained in the TiNi alloys.     

Polymer‐Based Resistive Memory Materials and Devices

Due to the advantages of good scalability, flexibility, low cost, ease of processing, 3D‐stacking capability, and large capacity for data storage, polymer‐based resistive memories have been a promising alternative or supplementary devices to conventional inorganic semiconductor‐based memory technology, and attracted significant scientific interest as a new and promising research field. In this review, we first introduced the general characteristics of the device structures and fabrication, memory effects, switching mechanisms, and effects of electrodes on memory properties associated with polymer‐based resistive memory devices. Subsequently, the research progress concerning the use of single polymers or polymer composites as active materials for resistive memory devices has been summarized and discussed. In particular, we consider a rational approach to their design and discuss how to realize the excellent memory devices and understand the memory mechanisms. Finally, the current challenges and several possible future research directions in this field have also been discussed.     

Phosphorene and Phosphorene‐Based Materials – Prospects for Future Applications

Phosphorene, a single‐ or few‐layered semiconductor material obtained from black phosphorus, has recently been introduced as a new member of the family of two‐dimensional (2D) layered materials. Since its discovery, phosphorene has attracted significant attention, and due to its unique properties, is a promising material for many applications including transistors, batteries and photovoltaics (PV). However, based on the current progress in phosphorene production, it is clear that a lot remains to be explored before this material can be used for these applications. After providing a comprehensive overview of recent advancements in phosphorene synthesis, advantages and challenges of the currently available methods for phosphorene production are discussed. An overview of the research progress in the use of phosphorene for a wide range of applications is presented, with a focus on enabling important roles that phosphorene would play in next‐generation PV cells. Roadmaps that have the potential to address some of the challenges in phosphorene research are examined because it is clear that the unprecedented chemical, physical and electronic properties of phosphorene and phosphorene‐based materials are suitable for various applications, including photovoltaics.     

Freestanding 3D Mesostructures,Functional Devices,and Shape‐Programmable Systems Based on Mechanically Induced Assembly with Shape Memory Polymers

Capabilities for controlled formation of sophisticated 3D micro/nanostructures in advanced materials have foundational implications across a broad range of fields. Recently developed methods use stress release in prestrained elastomeric substrates as a driving force for assembling 3D structures and functional microdevices from 2D precursors. A limitation of this approach is that releasing these structures from their substrate returns them to their original 2D layouts due to the elastic recovery of the constituent materials. Here, a concept in which shape memory polymers serve as a means to achieve freestanding 3D architectures from the same basic approach is introduced, with demonstrated ability to realize lateral dimensions, characteristic feature sizes, and thicknesses as small as ≈500, 10, and 5 µm simultaneously, and the potential to scale to much larger or smaller dimensions. Wireless electronic devices illustrate the capacity to integrate other materials and functional components into these 3D frameworks. Quantitative mechanics modeling and experimental measurements illustrate not only shape fixation but also capabilities that allow for structure recovery and shape programmability, as a form of 4D structural control. These ideas provide opportunities in fields ranging from micro‐electromechanical systems and microrobotics, to smart intravascular stents, tissue scaffolds, and many others.