Aromatic Zipper Topology Dictates Water-Responsive Actuation in Phenylalanine-Based Crystals

Water-responsive (WR) materials that reversibly deform in response to relative humidity (RH) changes are gaining increasing interest for their potential in energy harvesting and soft robotics applications. Despite progress, there are significant gaps in the understanding of how supramolecular structure underpins the reconfiguration and performance of WR materials. Here, three crystals are compared based on the amino acid phenylalanine (F) that contain water channels and F packing domains that are either layered (F), continuously connected (phenylalanyl-phenylalanine, FF), or isolated (histidyl-tyrosyl-phenylalanine, HYF). Hydration-induced reconfiguration is analyzed through changes in hydrogen-bond interactions and aromatic zipper topology. F crystals show the greatest WR deformation (WR energy density of 19.8 MJ m⁻³) followed by HYF (6.5 MJ m⁻³), while FF exhibits no observable response. The difference in water-responsiveness strongly correlates to the deformability of aromatic regions, with FF crystals being too stiff to deform, whereas HYF is too soft to efficiently transfer water tension to external loads.  These  findings reveal aromatic topology design rules for WR crystals and provide insight into general mechanisms of high-performance WR actuation. Moreover, the best-performing crystal, F emerges as an efficient WR material for applications at scale and low cost.     

Direct measurements of the Young's modulus of a single halloysite nanotube using a transmission electron microscope with a bending stage

Halloysite nanotubes (HNTs) are a naturally occurring nanotubular aluminosilicate mineral, which has been used to prepare nanocomposites with exceptional mechanical properties. In order to understand the roles of nanotubes during the deformation and fracture of nanocomposites, a state-of-the-art transmission electron microscope (TEM) with a bending stage was used to measure the Young's modulus of individual HNTs. TEM micrographs showed that the HNTs were surprising flexible and could be bent to almost 90 degrees without fracture. There was no observable reduction in the cross-sectional area of the bent HNTs. The findings suggest that HNTs, as a nanofiller, have a good potential to be used in high-performance structural materials, especially polymer-based nanocomposites.     

The fractal properties of a geometric model of the high-resolution X-ray diffractor

Perfect and mosaic crystals are conventionally used in X-ray monochromators operating in the energy range from several hundred to tens of thousands of electronvolts. The focusing X-ray optics for nonparallel beams employs either cylindrical bent crystals (the methods of Johann [1], Johansson [2], and Cauchois [3]) or crystals with spherical or toroidal [4] bending of the crystallographic planes. Special variants of high-resolution stepped-crystal diffractors [5–8] were developed to study the possibility of high-precision focusing of a monochromatic X-ray radiation. The fractal properties of a geometric model of such a high-resolution steppedcrystal diffractor are considered.     

Missing Link between Helical Nano‐ and Microfilaments in B4 Phase Bent‐Core Liquid Crystals,and Deciphering which Chiral Center Controls the Filament Handedness

The range of possible morphologies for bent‐core B4 phase liquid crystals has recently expanded from helical nanofilaments (HNFs) and modulated HNFs to dual modulated HNFs, helical microfilaments, and heliconical‐layered nanocylinders. These new morphologies are observed when one or both aliphatic side chains contain a chiral center. Here, the following questions are addressed: which of these two chiral centers controls the handedness (helicity) and which morphology of the nanofilaments is formed by bent‐core liquid crystals with tris‐biphenyl diester core flanked by two chiral 2‐octyloxy side chains? The combined results reveal that the longer arm of these nonsymmetric bent‐core liquid crystals controls the handedness of the resulting dual modulated HNFs. These derivatives with opposite configuration of the two chiral side chains now feature twice as large dimensions compared to the homochiral derivatives with identical configuration. These results are supported by density functional theory calculations and stochastic dynamic atomistic simulations, which reveal that the relative difference between the para‐ and meta‐sides of the described series of compounds drives the variation in morphology. Finally, X‐ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) data also uncover the new morphology for B4 phases featuring p2/m symmetry within the filaments and less pronounced crystalline character.     

Investigation of time-dependent characteristics of materials and micromechanisms of plastic deformation on a submicron scale by a new pulse indentation technique

A new pulse technique is proposed to investigate dynamics and micromechanisms of plastic deformations of materials underneath the indenter during microindentation. It is established that the process of indenter penetration under pulse loading conditions can be described by several distinct stages (as many as four in some cases) differing in kinetics and activation parameters. In each investigated material, the first stage was characterized by a high strain rate (10³ s^–1) and the high contact stresses (dynamic hardness), which exceeded static hardness by factor of 5–10. Typical values of activation volumes during the first stage were of the order of 10^–30 m³, i.e. close to the volume occupied by an atom (ion) in a lattice. During the second and the subsequent stages, the activation volume in ionic crystals increased up to about 10^–28 m³,( i.e. 10b ³, where b is Burger's vector of glide dislocations). Dynamics of initial stages of the indenter penetration was, therefore, determined by elastic and subsequently by plastic deformation, which is carried out by non-equilibrium point defects (most likely by interstitials or crowdions). A relative role of point defects in the process of mass transfer underneath the indenter and its contribution to microhardness is estimated. In soft materials (NaCl, LiF, Pb) during long indentation times (1 s) contribution of point defects can be estimated as more than 10%, and in the case of hard materials (Si, amorphous alloys) became closer to 100%. Dynamics of the final stages of the indenter penetration in soft crystals were governed by the dislocation creep. In all investigated materials for short indentation times (1 s) plastic deformation underneath the indenter occurred predominantly via generation and motion of interstitials and their clusters containing a few atoms.     

Deformation mechanisms in polymer crystals

The deformation of polydiacetylene single crystals has been investigated by scanning electron microscopy. Three types of deformation twins have been identified. The twinning planes are (0 1 2), (¯2 1 2), and (2 1 2), types not previously reported in polymer crystals. Deformation features in polydiacetylene single crystals observed by other workers are interpreted in terms of these three twins. The possibility of similar types of twins occurring in other polymer crystals is discussed. The twins in the polydiacetylene single crystals and those found in other non-polymeric materials are compared.     

Nonlinear microstructural constitutive equation of nanocrystalline metals

Summary. Based on experimental observations, nanocrystalline materials are modeled as composite systems in which the amorphous interfacial phase is treated as the matrix, whereas the nano-scale single crystals are modeled as inclusions. Generally speaking, the elastic moduli of nanoscale crystals are higher than those of the amorphous matrix phase, and the deformation mechanism of nanocrystalline materials depends heavily on the size of the crystals. For conventional macro size crystal materials, such as coarse-grained polycrystalline materials, the deformation mechanism due to dislocation movement is dominant. When the crystal size is reduced to a certain critical value, plastic deformation is caused by shear banding in the amorphous matrix. In order to model such a deformation mechanism in nanocrystalline materials, constitutive equations are established based on internal variable theory. The proposed model reveals the relation between the yield strength and the grain size of the material.     

Evaluation of bent-crystal x-ray backlighting and microscopy techniques for the Sandia Z machine

X-ray backlighting and microscopy systems for the 1-10-keV range based on spherically or toroidally bent crystals are discussed. These systems are ideal for use on the Sandia Z machine, a megajoule-class x-ray facility. Near-normal-incidence crystal microscopy systems have been shown to be more efficient than pinhole cameras with the same spatial resolution and magnification [Appl. Opt. 37, 1784 (1998)]. We show that high-resolution (< or = 10 microm) x-ray backlighting systems using bent crystals can be more efficient than analogous point-projection imaging systems. Examples of bent-crystal-backlighting results that demonstrate 10-microm resolution over a 20-mm field of view are presented.     

Interactions between cross-links and dislocations in polyethylene crystals: a model of irradiation hardening

The irradiation hardening of polyethylene (PE) crystals is explained in terms of the intersection and interaction between cross-links and dislocations. The elastic energies and forces of interaction between cross-links and the dislocations responsible for the various plastic deformation modes are calculated using a force dipole model of a cross-link and the strain field of the dislocation. The elastic energies of interaction are in all cases less than 0.7 eV (1.12×10^–19 J) and they are greater for edge than for screw dislocations. The hardening which arises from the direct intersections is calculated using a Morse potential model for the cross-link strength, and it is found that these interactions involve energies of the order of 3.6 eV (5.76×10^–19 J). From these results it is concluded that at 0 K both types of interaction produce similar hardening. However, since the elastic interaction energies are small, the hardness of cross-linked PE crystals at moderate temperatures is due solely to direct intersections of cross-links and dislocations. The strongest interactions take place between cross-links and those dislocations which produce chain-axis slip and this explains why this mode of deformation is readily suppressed by irradiation. The forces of interaction between cross-links and twin dislocations are not negligible, but since their interaction energies are of the order of 0.1 eV (0.16×10^–19 J), twinning deformation, at moderate temperatures, should not be affected by irradiation. By combining all the possible deformation modes, the relationship _c=4^1/2, is derived for the increase in yield stress, _c (GPa), in terms of the atomic concentration of cross-links,, provided that these are uniformly distributed in the crystal.     

Multi‐scale modeling of plastic deformations in nano‐scale materials; Transition to plastic limit

A large amount of research in computational mechanics has biased toward atomistic simulations. This trend, on one hand, is due to the increased demand to perform computations in nanoscale and, on the other hand, is due to the rather simple applications of pairwise potentials in modeling the interactions between atoms of a given crystal. The Cauchy–Born (CB) hypothesis has been used effectively to model the behavior of crystals under different loading conditions, in which the comparison with molecular dynamics simulations presents desirable coincidence between the results. A number of research works have been devoted to the validity of CB hypothesis and its application in post‐elastic limit. However, the range of application of CB hypothesis is limited, and it remains questionable whether it is still applicable beyond the validity limit. In this paper, a multi‐scale technique is developed for modeling of plastic deformations in nanoscale materials. The deformation gradient is decomposed into the plastic and elastic parts, i.e., F  =  F ^p F ^e. This decomposition is in contrast to the conventional decomposition, F  =  F ^e F ^p, generally encountered in continuum and crystal plasticity. It is shown that the former decomposition is more appropriate for the problem dealt within this work. Inspired by crystal plasticity, the plastic part is determined from the slip on potential slip systems. Based on the assumption that the CB hypothesis remains valid in the homogeneous deformation, the elastic deformation gradient resulting from the aforementioned decomposition is employed in conjunction with the CB hypothesis to update the state variables for face‐centered cubic crystals. The assumption of homogeneity of elastic deformation gradient is justified by the fact that elastic deformations are considerably smaller than the plastic deformations. The computational algorithms are derived in details, and numerical simulations are presented through several examples to demonstrate the capability of the proposed computational algorithm in the modeling of golden crystals under different loading conditions. Copyright © 2016 John Wiley & Sons, Ltd.     

Suitable materials for elastico mechanoluminescence-based stress sensors

Whereas the elastico mechanoluminescence (EML) of certain crystals increases linearly with the stress, nonlinearity occurs in the EML intensity versus stress plot of several crystals. The EML of crystals can be understood on the basis of piezoelectrically-induced detrapping model, whereby the localized piezoelectric field causes detrapping of electrons or holes and subsequently the capture of electrons in the excited states of activator ions, recombination of electrons in hole captured centres, recombination of holes in electron-captured centres or simply the electron-hole recombination gives rise to the light emission. Considering the piezoelectrically-induced detrapping model of EML expression is derived for the stress dependence of the EML intensity. It is shown that the crystals having uniform distribution of traps show linear relationship between the EML intensity and stress and the crystals having exponential distribution of traps show nonlinear relationship between the EML intensity and stress. The crystals having linear dependence of EML intensity on stress are suitable for the fabrication of EML-based stress sensors. The values of coefficient of deformation detrapping, relaxation time of the crosshead of the machine used to deform the samples and lifetime of the charge carriers in the shallow traps lying in the normal piezoelectric region of the crystals can be determined from the EML measurements. The values of the coefficient of deformation detrapping are 0.310, 0.018 and 0.021 MPa⁻¹ for SrMgAl₆O₁₁:Eu, Sr₂MgSi₂O₇:Eu and SrCaMgSi₂O₇:Eu crystals, respectively. The coefficient of deformation detrapping is low for SrAl₂O₄:Eu, SrAl₂O₄:Eu, Dy, SrBaMgSi₂O₇:Eu and ZnS:Mn crystals and such crystals are suitable for EML-based stress sensors. A good agreement is found between the theoretical and experimental results.     

Growth and spectroscopic characteristics of Yb3+-Doped NaBi(MoO4)2 crystals

We have determined the growth charge composition and low thermal gradient (< 1°C/cm) Czochralski pulling parameters for the growth of bulk homogeneous Yb³⁺-doped NaBi(MoO₄)₂ crystals and assessed the main spectroscopic characteristics of the Yb³⁺:NaBi(MoO₄)₂ crystals as laser materials.     

Polycrystal deformation analysis by high energy synchrotron X-ray diffraction on the I12 JEEP beamline at Diamond Light Source

Better understanding of the distribution of elastic and plastic strains in deformed polycrystalline, multiphase materials is important for structural engineering. The deformation response depends upon the interaction of grains of different orientations, and the anisotropy associated with each phase. Strain partitioning and tensile-compressive hardening asymmetry arises due to mismatches in modulus and ductility between grains and phases in alloys such as Ti-6Al-4V that displays both strong anisotropy within one phase and significant differences of properties between phases. Simple four-point bent beam samples were studied using the newly available energy-dispersive X-ray diffraction setup at the high energy white-beam synchrotron beamline I12 (JEEP) at Diamond Light Source. Diffraction patterns from the bent polycrystalline Ti6Al4V samples were collected using the new 23-cell “horseshoe” detector and interpreted using Pawley refinement to extract unit cell parameters, thus allowing elastic strain to be determined. The tensile-compressive hardening asymmetry was quantified for the grains oriented with the basal plane perpendicular to the loading direction. Initial evaluation of the performance of the new instrument is reported.     

Some aspects of the static and dynamic compressive behaviour of β-brass single crystals

The compressive behaviour of β-brass single crystals has been investigated in uninterrupted static and dynamic tests and in interrupted tests using static-static, static-dynamic, dynamic-dynamic and dynamic-static loading sequences. Static and dynamic strain-rates were 1.5×10^?4 and 3.2×10³ sec^?1 respectively. Slip traces on the statically deformed crystals were wavy and deformation occurred by single slip on either (ī01) [111] or (¯211) [111] or by a transition mode involving both (ī01) [111] and (¯211 [111]. Except for anomalous behaviour in the dynamic reload following static preload the dynamic slip traces were straight with deformation occurring by multiple slip on four {110} planes involving two 〈111〉 directions. It is shown that there is no direct causal relationship between the lower work-hardening rate and level of flow stress and the crystallography of slip in dynamic deformation. The work-hardening rate and flow stress in static and dynamic loading are rather determined by the dynamics of the deformation. The differences in the substructural features as observed by transmission electron microscopy arise principally from the differences in the slip modes and cannot be interpreted as controlling the stress-strain behaviour. The low work-hardening rate and flow stress in dynamic deformation is believed to be due to the production of short-lived disorder. The absence of a/2〈111〉 dislocations in thin foils is explained in terms of the fast reordering reaction in β-brass.     

Fracture and fracture toughness of stoichiometric MgAl2O4 crystals at room temperature

The fracture toughness and path of stoichiometric spinel (MgAl₂O₄) crystals were determined at 22 °C for key low-index planes by double cantilever beam, as well as fractography of flexure specimens failing from either machining or indentation flaws. These results are compared with other single and polycrystalline MgAl₂O₄ fracture toughness values measured by various techniques, as well as single crystal versus polycrystal results for other materials. Evaluation of experimental and theoretical results shows (1) the fracture toughness of the spinel {110} plane is only a limited amount (e.g. 6%) higher than for the {100} plane (1.2 MPa m^1/2), (2) fractography of machining flaw fracture origins was the most effective source of K _IC results, and (3) caution must be used in applying fracture toughness techniques to single crystals. Cautions include accounting for possible effects of elastic anisotropy (especially for double cantilever beam and probably double torsion tests), the nature of failure-initiating flaws (especially for notch-beam tests), and the frequent lack of symmetric plastic deformation and fracture (especially for indentation techniques).Retired.     

Numerical simulation of unsteady cavitating flows using a homogenous equilibrium model

 Numerical simulations of two-dimensional cavity flows around a flat plate normal to flow and flows through a 90^∘ bent duct are performed to clarify unsteady behavior under various cavitation conditions. A numerical method applying a TVD-MacCormack scheme with a cavitation model based on a homogenous equilibrium model of compressible gas-liquid two-phase media proposed by the present authors, is applied to solve the cavitating flow. This method permits the simple treatment of the whole gas-liquid two-phase flow field including wave propagation and large interface deformation. Numerical results including detailed observations of unsteady cavity flows and comparisons of predicted results with experimental data are provided. Received: 5 August 2002 / Accepted: 6 January 2003     

Stress corrosion cracking mechanisms in ductile FCC materials

Recent results on the SCC behaviour of ductile fcc materials are reviewed. Critical experiments are presented to test the corrosion enhanced plasticity model proposed some years ago by one of the present authors to describe the SCC of austenitic stainless steels in Cl^- solutions. Slow strain rate tests on 110 and 100 316L alloy single crystals clearly confirm that the macroscopically brittle fracture is in fact achieved by microcracking on {111} microfacets in zig-zag. Moreover the corrosion deformation interactions on which the model is based are quantitatively analysed through softening effects observed in cyclic plastic deformation in the corrosive solution. The conditions for hydrogen entry in the material are described, which leads to the notion of critical surface defects for hydrogen effects. New developments of the model are then discussed and a numerical simulation of the corrosion deformation interactions is presented.Presented at Fourth Greek National Congress on Mechanics, 26–29 June 1995, held at Xanthi, Greece.     

The orientation dependence of the deformation modes of crystalline mercury at 77°K

Single crystals of mercury have been deformed either uniaxially or in four-point bending at 77‡K and the orientation dependence of the operative deformation modes investigated. The experimental techniques adopted are briefly described and results presented for thirty-five specimens. The predominant deformation mode was crystallographic {11ˉ1} 〈1ˉ10^* slip but ‘{ˉ1ˉ35’ twinning and {1ˉ10} kinking were frequently observed. Slip in the close-packed 〈011〉 direction was wavy and restricted to three specimens. The orientation dependence of these deformation modes is interpreted using Schmid-factor contour-plots of the first- and second-most highly stressed variants. The form and nature of the boundaries between different regions of these plots are discussed in detail. The operative deformation modes in the bent specimens can be satisfactorily interpreted by making use of a new analysis of plane plastic strain developed as a result of this work. The results on deformation kinking have also led to a new general theory of the crystallography of this phenomenon. A critical resolved shear stress of 18±2 g mm⁻² was found for {11ˉ1} 〈1ˉ10〉 slip for specimens with Schmid-factors greater than about 0.3. Much larger values were measured or deduced for other specimens and possible explanations of this surprising result are advanced. Several of the observations are considered to be of general importance to the understanding of the mechanical properties of all crystalline materials but they can only be thoroughly investigated on a material, like mercury, of comparatively low crystal symmetry.     

Mechanical properties of low-carbon steels over a wide range of temperatures and strain rates applied to processes of thin sheet rolling

Results are given for mechanical tests on a number of low-carbon steels in uniaxial tension and compression in the strain rate range 10^–3–4·10³ sec^–1 and at temperatures of 293–573 K. An increase is confirmed for strength and ductility characteristics with an increase in strain rate. It is shown that the sensitivity of metals to strain rate depends on the specific temperature and rate conditions of loading. On the basis of experimental data obtained analytical dependences are suggested for the resistance of materials to deformation on the degree and rate of deformation, and also on temperature.Translated from Problemy Prochnosti, No. 8, pp. 76–84, August, 1990.     

Scintillation properties of SrF2 and SrF2-Ce3+ crystals

This Letter presents the results of measuring scintillation properties of pure SrF₂ crystals and crystals activated by various concentrations of Ce³⁺ ions. The light yield of these materials is compared to that of the known scintillators NaI-Tl and CaF₂-Eu²⁺. Strontium fluoride crystals activated with Ce³⁺ ions are found to be characterized by high light yield and to be promising materials for use in scintillation detectors employed for γ-ray well logging.