Sub-microsecond X-ray crystallography: techniques,challenges, and applications for materials science

Dynamics of crystals is a subject of recent interest in solid-state physics and a challenge for modern X-ray crystallography. Time-dependent response of solids to an external perturbation on atomic and microstructural length scales is the key to understanding many physical properties. This paper reviews the challenges and opportunities for probing of sub-micro-, micro-, and millisecond dynamics of solids using the methods of X-ray crystallography. It starts with an overview of recent time-resolved X-ray diffraction techniques. It then focuses on the processes that are important for understanding functional materials: dynamics of ferroelectric and ferroelastic domain patterns, texture in piezoelectric ceramics, mechanical resonances in solids, and dynamics of structural disorder. Knowledge available from macroscopic experiments is summarized, and opportunities for X-ray crystallography to resolve existing controversies are presented. This paper suggests the possible synergy of macroscopic and X-ray crystallographic experimental techniques.     

Crystallography and the liquid crystal phase: a new approach to structural studies on a thermo-tropic smectic Schiff base

In spite of the apparent contradiction between ‘liquid crystals’ (LC, materials exhibiting some degree of disorder) and ‘crystallography’ (a paradigmatic ordered kingdom), X-ray diffraction (XRD) studies make a substantial contribution to the field of LC. Focusing this review on smectic (Sm, lamellar) LC, we first describe how extremely careful XRD studies performed on mono-domain samples in the LC phase helped to elucidate the molecular structure of ordered Sm phases. Then, we describe selected examples in which single-crystal (SC) XRD on the solid-state phase of the mesogens provided information about their supra-molecular organization in the Sm phase. Finally, we present a different approach to this problem in the case of a thermo-tropic Schiff base (SB) which undergoes crystal???LC???isotropic liquid phase transformations. By combined SC and variable-temperature powder XRD, we show that the SB LC is a hexatic smectic B phase that derives from the crystal phase by relatively small topological changes promoted by the set-in of thermal rotational disorder around the long SB molecular axis.     

Integration of X-ray crystallography and electron cryo-microscopy in the analysis of virus structure and function

Atomic models of small icosahedral RNA viruses were first derived from X-ray crystal structures between 1978 and 1990. In that same period, electron cryo-microscopy with image reconstructions (cryoEM) was being developed, and viruses were among the early successful subjects of study. By 1990 it was clear that many important virus-related structures (e.g. antibodies and receptor molecules bound to viruses) would be difficult or impossible to crystalize but could be imaged by cryoEM at low to moderate resolution. Combining crystal structures of Fab fragments, receptor molecules and virus particles with cryoEM structures of the complexes allowed accurate ‘pseudo-atomic' models to be derived and resulted in a fundamental understanding of the chemistry for complex formation. Later, cryoEM produced structures of enveloped viruses that could not be crystallized, but the ecto domains of subunits forming the capsids were crystallized and fitted into the cryoEM density as rigid models. This created highly informative ‘pseudo-atomic' models of the virions. In this review we track the early development of these methods with specific examples described in more detail and also provide a more comprehensive table of references that illustrate the breadth and depth of combining X-ray crystallography and cryoEM.     

Adaptive inverse control of a galvanometer scanner considering the structural dynamic behavior

In industrial applications, scanner systems are utilized for several operations, but do not always meet the desired dynamics. Various controller-based approaches, which concentrate on simple trajectories or changes in operation points, have been presented within the last years; however, these are not sufficient for many production processes. In this paper, adaptive inverse control is utilized to enhance the scanner’s dynamics. The influence of structural dynamics is compensated by taking the behavior of the galvanometers’ mirrors into account. The performance of the new approach is verified by experimental results, reducing the optical error by almost 95% compared to current scanner systems.     

Crystallography using an X-ray free-electron laser

An X-ray free-electron laser (XFEL) produces short pulses (10–50 fs) of intense (mJ μm^?2 at 120 Hz) X-rays, with high transverse coherence. Such pulses open novel spectroscopic and scattering methods for static and time-resolved studies of matter, and many are based on X-ray crystallography. With serial femtosecond crystallography, the XFEL allows high-resolution structural determination on sub-micron protein crystals. Although the XFEL pulse is destructive, its short duration ensures that effectively undamaged material is probed. Coherent scattering features provide information on the physical crystal form and may assist in determining the crystallographic phase. By introducing synchronized optical laser pulses, one can perform ‘pump-probe’ measurements of dynamic properties, on the sub-picosecond timescale. These include photo-initiated structural modifications in biomolecules, photo-excited lattice vibrations and photo-driven structural phase transitions. As with synchrotron radiation, the XFEL wavelength can be tuned to atomic resonances, allowing time-resolved resonant-diffraction measurements, which are particularly sensitive to selected order parameters (lattice, charge, spin, and orbital) in magnetic or correlated electron materials. Finally, it is anticipated that the special properties of XFEL pulses will allow entirely new types of X-ray scattering measurements, such as ptychographic crystallography on 2D bio-crystals, correlation-function determination of nanoparticle geometry and nonlinear crystallographic mixing of optical and X-ray pulses.     

Sensorimotor coordination and metastability in a situated HKB model

Oscillatory phenomena are ubiquitous in nature and have become particularly relevant for the study of brain and behaviour. One of the simplest, yet explanatorily powerful, models of oscillatory Coordination Dynamics is the Haken–Kelso–Bunz (HKB) model. The metastable regime described by the HKB equation has been hypothesised to be the signature of brain oscillatory dynamics underlying sensorimotor coordination. Despite evidence supporting such a hypothesis, to our knowledge, there are still very few models (if any) where the HKB equation generates spatially situated behaviour and, at the same time, has its dynamics modulated by the behaviour it generates (by means of the sensory feedback resulting from body movement). This work presents a computational model where the HKB equation controls an agent performing a simple gradient climbing task and shows (i) how different metastable dynamical patterns in the HKB equation are generated and sustained by the continuous interaction between the agent and its environment; and (ii) how the emergence of functional metastable patterns in the HKB equation – i.e. patterns that generate gradient climbing behaviour – depends not only on the structure of the agent's sensory input but also on the coordinated coupling of the agent's motor–sensory dynamics. This work contributes to Kelso's theoretical framework and also to the understanding of neural oscillations and sensorimotor coordination.     

‘Whin Brow’: the house at which the new science of X-ray crystallography began

A long handwritten letter by Professor W.H. Bragg, dated 16 December 1912 and addressed to the Vice-Chancellor of the University of Leeds (an extract of which is presented in this article), provides an authoritative record of the progress in X-ray crystallography from the beginning of the Bragg's father and son collaboration in August 1912–December 1914. The collaboration began at the village of Cloughton on the North Yorkshire coast but hitherto the house itself had not been identified. As a result of an historical investigation initiated by the writer and with the help of members of the Bragg and Barran families and the Cloughton Local History Group, it is now identified as ‘Whin Brow’, the summer home of Henry and Rosalie Barran, close friends of William and Gwendoline Bragg. A plaque at ‘Whin Brow’, commemorating the ‘new science of X-ray crystallography’, was unveiled by Mr Charles Bragg (great-grandson of William Bragg) on 5 July 2013.     

Evaluation of the quality of the components of the process of manual arc welding using entropy methods

This paper examines important technical issues pertaining to the methodological limitations of the welding techniques currently in use, with particular consideration given to welding mathematical modelling procedures. The recent progress achieved with high nano-technologies suggests the application of similar approaches to conventional mechanics and materials science, with particular reference to welding processes. Recent studies on welded joints have highlighted the benefits to be derived from ‘Industrial Applications of Neutron Techniques’ giving access to fundamental information – not obtainable using other methods – for improving quality and durability. ‘Small angle neutron scattering’ and ‘determination of residual stress by neutron diffraction’ make it possible to characterize materials at the atomic and nanoscale level, offering existing technologies the essential contribution of precise structural methods. This paper describes the fundamental theoretical aspects of both techniques, along with several welding studies conducted using ‘small angle neutron scatter’. An experimental programme has recently been devised by Rogante Engineering, in collaboration with the Budapest Research Reactor, to develop welding processes aimed at improving safety, quality control, and durability control for the joints under consideration.     

Real-space imaging of nucleation and size induced amorphization in PdSi nanoparticles

Nucleation has been studied for hundreds of years and can be described by classical nucleation theory (CNT) in many cases. However, the understanding of nucleation has been limited by its short time and small size scale, which causes substantial experimental challenges in direct visualization of the nucleation process. Here, using double-aberration-corrected scanning transmission electron microscope, we show the real space imaging of the atomic level nucleation process and the evolution of nucleus in PdSi nanoparticles produced by pulsed laser deposition. Critical nucleus size, indicated by a structural transition from amorphous to crystal, is clearly observed and found to be in consistent with CNT prediction. We also show that the nano-scale nucleation process is analogous with size induced amorphization process which is confirmed by the molecular dynamics simulation.     

分子动力学在材料科学中的应用

分子动力学方法是进行物质原子或分子层次计算机模拟时所采用的一种基本方法。通过分子动力学模拟，可以给出原子尺度上材料及其演化过程细节的可能性，具有无先例的准确性，使材料设计和性能预测成为可能。本文分析了分子动力学模拟的基本原理和算法；综述了分子动力学在材料科学中的应用，介绍了最近发展的第一原理分子动力学模拟，指出材料科学中第一原理分子动力学模拟的应用还有待进一步发展。     

Real-time capable Finite Element Models with closed-loop control: a method for Hardware-in-the-Loop simulation of flexible systems

Modern simulation methods are increasingly applied in all areas of research and development in production engineering. Moreover, the application area can be extended by the combination of different simulation tools e.g. Finite Element Method (FEM) with Computer Aided Control Engineering (CACE). The dynamic behaviour of machine tools depends not only on their distribution of mass, damping and stiffness, but also on their applied control mechanisms. The following article presents a method for simulating the influence of the actual control system on the structural dynamics in real-time by using Hardware-in-the-Loop Simulation.     

Dispersion as the link between basic research and commercial applications of conductive polymers (polyaniline)

A critical review is given about the status and potentials of commercial applications of conductive polymers, mainly polyaniline. It is shown that ‘dispersion’ has evolved to be the necessary key for making conductive polymers suitable for industrial applications, at least if ‘conductive’ (i.e. ‘doped’ conjugated polymers) and not neutral, ‘undoped’ conjugated polymers are under review.     

水汽在涂层中的扩散传输行为

利用石英晶体微天平(QCM)为主要手段研究了水汽在醇酸清漆和聚氨酯清漆涂层中的扩散传输行为.水汽在上述涂层中的扩散符合Fick第二扩散定律.计算出了扩散系数D.用傅利叶变换红外光谱仪(FTIR)测定表明吸水前后涂层分子结构发生了变化.证明水分子和涂层分子发生了化学反应.     

Impact specimen geometry on T23 and TP347HFG steels behaviour during steam oxidation at harsh conditions

Ferritic T23 steel and austenitic TP347HFG steel have been studied with an emphasis on understanding the impact of specimen geometry on their steam oxidation behaviour. The selected materials were tested over a wide range of temperatures from 600 to 750°C. The tests were carried out in 100% steam conditions for 1000 hours. The tests indicated that the ‘curved-shaped’ specimens show slower mass gain, scale ticking and void nucleation rates than ‘bridge-shaped’ specimens (with flat and convex surfaces combined). Furthermore, a bridge TP347HFG sample showed the formation of lower amount of flaky oxide at 750°C.     

Studies on the micro-laser spot welding of an NdFeB permanent magnet with a low carbon steel

In search of high speed and miniaturization, the welding of NdFeB permanent magnet material is currently of increasing interest. Previous studies have shown that dissimilar material joining between magnet and steel sheets can be realized by laser irradiation, but it is still not clear how the welding parameters affect the weld quality. In this paper, the results are reported of experiments studying effects of laser pulse power, pulse duration, and defocusing distance on joint dimensions and mechanical behavior in laser spot welding of an NdFeB magnet (N48) with a low carbon steel (AISI 1006). Both conduction mode and keyhole mode welding were performed in the present study. The welding modes can be altered by changing peak power or defocusing distance, but not by changing pulse duration. Three fracture modes were found in shear tests, i.e., ‘nugget pullout’, ‘through nugget’ and ‘magnet crush’, while in peel tests, only ‘nugget pullout’ fracture mode was observed. The different fracture modes under different loading conditions were attributed to the mechanical locking effects present under shear testing but not under peel testing. For ‘nugget pullout’ mode fracture, the metallurgical quality of joints is the controlling factor of fracture forces; for ‘nugget through’ and ‘magnet crush’ mode fractures, the controlling factors are the size and strength of the nugget and base magnet. Certain nugget penetration should be achieved to avoid ‘nugget pullout’ fracture, while excessive growth of nugget should be limited to avoid ‘magnet crush’ fracture by appropriately adjusting process parameters.     

Physical Characteristics of Friction Processes in a Columnar Approach-Separation Model with Tangential Force

Protection of Metals and Physical Chemistry of Surfaces - Three-dimensional molecular dynamics (MD) simulation is conducted to understanding the dynamics processes of atomic-scale friction, which...     

Spreading and wetting at the microscopic scale: recent developments and perspectives

In both the experimental and numerical approaches of wettability, the spreading phenomenon is now understood as the consequence of the competition between a driving term (i.e. the affinity of the liquid for the solid) and the friction of the liquid molecules on the substrate. The aim of this article is to review how the combination of molecular dynamics (MD) simulations and of microscopic scale experiments leads to a better understanding of the physico-chemical properties of the solid/liquid interface and to present the most recent developments achieved in the complete wetting regime.     

Review of chatter studies in cold rolling

In the past, many different modes of vibration and causes of rolling chatter have been presented, yet no complete and clear picture of the basic mechanics emerged. In order to control chatter in cold rolling operations, a much better understanding of the basic mechanics of the problem is required. To provide, therefore, a ground work for developing such a basic understanding, in this paper, existing models of a mill stand as well as of the rolling process in the open literature are presented. In addition, existing chatter models, which are formulated to investigate chatter as the consequence of the interaction between the structural dynamics of the mills and the dynamics of the rolling process, are also reviewed.