Bent Crystals Research Articles

All-atom simulations of bent liquid crystal dimers: the twist-bend nematic phase and insights into conformational chirality.

The liquid crystal dimer 1,7-bis-4-(4'-cyanobiphenyl)heptane (CB7CB) is known to exhibit a nematic-nematic phase transition, with the lower temperature phase identified as the twist-bend nematic (NTB) phase. Despite the achiral nature of the mesogen, the NTB phase demonstrates emergent chirality through the spontaneous formation of a helical structure. We present extensive molecular dynamics simulations of CB7CB using an all-atom force field. The NTB phase is observed in this model and, upon heating, shows phase transitions into the nematic (N) and isotropic phases. The simulated NTB phase returns a pitch of 8.35 nm and a conical tilt angle of 29°. Analysis of the bend angle between the mesogenic units reveals an average angle of 127°, which is invariant to the simulated phase. We have calculated distributions of the chirality order parameter, χ, for the ensemble of conformers in the NTB and N phases. These distributions elucidate that CB7CB is statistically achiral but can adopt chiral conformers with no preference for a specific handedness. Furthermore, there is no change in the extent of conformational chirality between the NTB and N phases. Using single-molecule stochastic dynamics simulations in the gas phase, we study the dimer series CBnCB (where n = 6, 7, 8 or 9) and CBX(CH2)5YCB (where X/Y = CH2, O or S) in terms of the bend angle and conformational chirality. We confirm that the bent molecular shape determines the ability of a dimer to exhibit the NTB phase rather than its potential to assume chiral conformers; as |χ|max increases with the spacer length, but the even-membered dimers have a linear shape in contrast to the bent nature of dimers with spacers of odd parity. For CBX(CH2)5YCB, it is found that |χ|max increases as the bend angle of the dimer decreases, while the flexibility of the dimers remains unchanged through the series.

Pionic hydrogen and deuterium

The strong-interaction effects both in pionic hydrogen and deuterium atoms have been re-determined with improved precision. The hadronic shift and width in pionic hydrogen together with the hadronic shift in pionic deuterium constitute a one-fold constraint for the two independent pion-nucleon scattering lengths. Furthermore, the hadronic width in pionic deuterium measures the transition strength of s-wave pions on an isoscalar nucleon-nucleon pair which is an independent quantity not related to the pion-nucleon scattering lengths. The experiment was performed at the Paul Scherrer Institute by stopping a high-intensity low-energy pion beam in gaseous targets using the cyclotron trap. The X-rays emitted by the πH and πD atoms were analysed with a high resolution Bragg spectrometer equipped with spherically bent crystals. The pion-nucleon scattering lengths and other physical quantities extracted from the atom data are in good agreement with the results obtained from pionnucleon and nucleon-nucleon scattering experiments and confirm that a consistent picture is achieved for the low-energy pion-nucleon sector with respect to the expectations of chiral perturbation theory.

Spherically Bent Crystal Imaging System for Interface Trajectory Measurement During Ablation

Spherically Bent Crystal Imaging System for Interface Trajectory Measurement During Ablation

Reflection Efficiency and Spectra Resolutions Ray-Tracing Simulations for the VOXES HAPG Crystal Based Von Hamos Spectrometer

The VOXES collaboration at INFN National Laboratories of Frascati developed a prototype of a high resolution Von Hamos X-ray spectrometer using HAPG (Highly Annealed Pyrolytic Graphite) mosaic crystals. This technology allows the employment of extended isotropic sources and could find application in several physics fields. The capability of the spectrometer to reach energy precision and resolution below 1 and 10 eV, respectively, when used with wide sources, has been already demonstrated. Recently, the response of this device, for a ρ = 206.7 mm cylindrically bent HAPG crystal using CuKα1,2 and FeKα1,2 XRF lines, has been investigated in terms of reflection efficiency by a dedicated ray-tracing simulation. Details of the simulation procedure and the comparison with the experimental results are presented. This study is crucial in order to retrieve information on the spectrometer signal collection efficiency, especially in the energy range in which the standard calibration procedures cannot be applied.

MBN explorer atomistic simulations of 855 MeV electron propagation and radiation emission in oriented silicon bent crystal: theory versus experiment

The method of relativistic molecular dynamics is applied for accurate computational modeling and numerical analysis of the channelling phenomena for 855 MeV electrons in bent oriented silicon (111) crystal. Special attention is devoted to the transition from the axial channelling regime to the planar one in the course of the crystal rotation with respect to the incident beam. Distribution in the deflection angle of electrons and spectral distribution of the radiation emitted are analyzed in detail. The results of calculations are compared with the experimental data collected at the MAinzer MIctrotron facility.

Halogen Bonded Network Modulating the Mechanical Property Elastic and Plastic Bending in Nonconventional Molecular Solid Solutions

Anisotropic mechanical response of a material to an applied external stress results in bending of organic crystals in particular directions. This phenomenon is essentially dictated by intermolecular interactions. In this work, we have tactically designed solid solutions of two non-isostructural molecular crystalline phases, 4-bromo-3-chlorophenol (4BR, I41/a) and 3-bromo-4-chlorophenol (3BR, P21/c)─an exception to the Kitaigorodsky rule. Single crystals of 4BR show elastic bending, whereas 3BR crystals are brittle in nature. The solid solutions of 4BR and 3BR in 1:1 and 1:2 stoichiometric ratios attain a unique solid solution crystal structure (P21/c, Z′ = 2). In response to mechanical stimuli, we observed elastic bending in 1:1 solid solution crystals and plastic bending in 1:2 solid soltion crystals. A detailed investigation combining laboratory single crystal X-ray diffraction, powder X-ray diffraction, thermal analysis, and periodic DFT calculations reveals intriguing structural features of these solid solutions and explores the role of halogen bonding in modulating the mechanical property from elastic (1:1) to plastic (1:2) with stoichiometric variation.

Channeling and radiation of electrons and positrons in the diamond heterocrystals

We analyze numerically the radiation and channeling properties of ultrarelativistic electrons and positrons propagating through a periodically bent diamond crystal grown on a straight single-crystal diamond substrate. Such systems can be called hetero-crystals and they are one of the experimentally realized samples for the implementation of crystalline undulators. We state that in such systems the channeling and radiation properties of projectiles are sensitive to the projectile particles energy as well as on the beam propagation direction, i.e. on whether the beam of particles enters the crystal from the side of substrate or from the side of periodically bent crystal. The predictions made are important for design and practical realization of new crystalline undulators.

Tronox site in Thann inaugurates sulfuric acid recycling unit

Efficient steering of GeV-energy negatively charged particle beams was demonstrated to be possible with a new generation of thin bent silicon crystals. Suitable crystals were produced at the Sensor Semiconductor Laboratory of Ferrara starting from Silicon On Insulator wafers, adopting proper revisitation of silicon micromachining techniques such as Low Pressure Chemical Vapor Deposition, photolithography and anisotropic chemical etching. Mechanical holders, which allow to properly bend the crystal and to reduce unwanted torsions, were employed. Crystallographic directions and crystal holder design were optimized in order to excite quasi-mosaic effect along (1 1 1) planes. Prior to exposing the crystal to particle beams, a full set of characterizations were performed. Infrared interferometry was used to measure crystal thickness with high accuracy. White-light interferometry was employed to characterize surface deformational state and its torsion. High-resolution X-rays diffraction was used to precisely measure crystal bending angle along the beam. Manufactured crystals were installed and tested at the MAMI MAinz MIcrotron to steer sub-GeV electrons, and at SLAC to deflect an electron beam in the 1 to 10 GeV energy range.

Spectral monitoring at SwissFEL using a high-resolution on-line hard X-ray single-shot spectrometer.

The performance and parameters of the online photon single-shot spectrometer (PSSS) at the Aramis beamline of the SwissFEL free-electron laser are presented. The device operates between the photon energies 4 and 13 keV and uses diamond transmission gratings and bent Si crystals for spectral measurements on the first diffraction order of the beam. The device has an energy window of 0.7% of the median photon energy of the free-electron laser pulses and a spectral resolution (full width at half-maximum) ΔE/E on the order of 10-5. The device was characterized by comparing its performance with reference data from synchrotron sources, and a parametric study investigated other effects that could affect the reliability of the spectral information.

Retraction Note: Mechanically interlocked architecture aids an ultra-stiff and ultra-hard elastically bendable cocrystal

Molecular crystals are not known to be as stiff as metals, composites and ceramics. Here we report an exceptional mechanical stiffness and high hardness in a known elastically bendable organic cocrystal [caffeine (CAF), 4-chloro-3-nitrobenzoic acid (CNB) and methanol (1:1:1)] which is comparable to certain low-density metals. Spatially resolved atomic level studies reveal that the mechanically interlocked weak hydrogen bond networks which are separated by dispersive interactions give rise to these mechanical properties. Upon bending, the crystals significantly conserve the overall energy by efficient redistribution of stress while perturbations in hydrogen bonds are compensated by strengthened π-stacking. Furthermore we report a remarkable stiffening and hardening in the elastically bent crystal. Hence, mechanically interlocked architectures provide an unexplored route to reach new mechanical limits and adaptability in organic crystals. This proof of concept inspires the design of light-weight, stiff crystalline organics with potential to rival certain inorganics, which currently seem inconceivable. Molecular crystals are typically less stiff than metals or ceramics. Here the authors report an organic elastically bendable co-crystal with stiffness comparable to low-density metals, hardness similar to stainless steel and reveal the molecular mechanism which lead to these mechanical properties.

High Resolution Residual Strain/Stress Measurements on Three Axis Neutron Diffractometer

Focusing 3-axis diffractometer set-up equipped with bent perfect crystal (BPC) monochromator and analyzer offers the sensitivity in determination of strains Dd/d < 10-4 in polycrystalline materials which is about one order of magnitude higher with respect to that of conventional 2-axis neutron scanners. It also offers possibility of line profile analysis for reasonable sample volumes and counting times. In this paper, the feasibility of using the 3-axis set-up even for measurements of rather large bulk polycrystalline samples with an acceptable resolution is presented. As the 3-axis set-up exploits focusing in real and momentum space, by a proper adjustment of the curvature of the analyzer, a high-resolution determination of the lattice changes can also be achieved even on large irradiated gauge volumes, though with a slightly relaxed resolution. It can be successfully exploited namely, in the strain/stress measurements on samples exposed to an external load, e.g. in tension/compression rig, in aging machine etc. In addition to the original performance where the analysis is carried out by rocking the BPC analyzer and the neutron signal registered by a point detector, a new alternative is offered which uses a fixed rocking angle position of the analyzer and the detector signal is registered by a one-dimensional position sensitive detector (PSD). This trick makes possible in some cases the elastic strain/stress measurements considerably faster and thus reduces the drawback of the time consuming step-by-step analysis.

Mechanically Responsive Organic Crystals by Light

AbstractMechanically responsive molecular crystals that exhibit macroscopic motions such as bending, twisting, and locomotion by light and heat have been studied intensively over the past decade. Photoisomerization has been applied to induce various motions, especially the bending of typical photochromic crystals, e. g., diarylethene and azobenzene. Phase transition is another mechanism underlying crystal actuation. Moreover, photothermal effect is a promising mechanism that has the potential to actuate any crystals that absorb light, including those for which actuation cannot be achieved by photoisomerization or phase transitions. Molecular crystals have an advantage over polymers and gels in terms of having a higher elastic modulus and stronger output force. However, previous studies of mechanical crystals have been limited mostly to basic research. There is a need to address the practical application of such mechanically responsive crystals in sensors, switches, actuators, and soft robots.

Interface modification in solid-state lithium batteries based on garnet-type electrolytes with high ionic conductivity

The garnet-structure lithium-stuffed solid electrolyte Li7La3Zr2O12 is a promising candidate as lithium-ion conductors for next-generation lithium batteries. We present a comprehensive investigation on the effect of alkaline-earth-metal elements (Ca, Sr, Ba) doping on the structure, mechanical and electrochemical properties in the garnet-type solution Li6.6La3Zr1.6Sb0.4O12. The crystal structure, micromorphology, bending strength, fracture toughness and ionic conductivity of Li6.65La2.95A0.05Zr1.6Sb0.4O12 (A = Ca, Sr, Ba) samples are analyzed systematically by X-ray diffraction (XRD), scanning electron microscopy (SEM), three-point bending method, single edge notch beam and impedance analysis. The results show that alkaline-earth-metal ion substitution may increase the total Li-ion conductivity by promoting ceramic densification in combination with enhancing Li-ion concentration. Among the investigated compounds, Li6.65La2.95Ba0.05Zr1.6Sb0.4O12 samples show excellent performances with a relative density of 96.6%, bending strength of 148.2±2.1 MPa, fracture toughness of 0.94 ± 0.055 MPa•m1/2, Li-ion conductivity of 1.14 × 10-3 S•cm-1 as well as activation energy of 0.361 eV. Additionally, Li/garnet interfacial problems in the solid-state lithium batteries need to be addressed because the poor contact between the garnet electrolyte and Li electrode will cause large interfacial impedance and uneven Li-ion flux during cycling. A methodology is proposed to negate the Li/garnet interfacial impedance by screen-printing the Ag layer on the garnet. A decrease in the interfacial resistance from 986 to 136.7 Ω•cm2 and a stable lithium stripping/platting profile at different current densities are observed, indicating a homogeneous Li-ion flux and appreciable electrochemical performance at Li/garnet interface. The fabricated full batteries with LiFePO4 cathode, modified garnet electrolyte and lithium metal anode show excellent cycling performances under high current density at room temperature.

Channeling efficiency reduction in high dose neutron irradiated silicon crystals for high energy and high intensity beam collimation and extraction

The channeling process in bent silicon crystals are used since '70s to manipulate beams of high energy particles. During the last decade, several studies and experiments carried out by the UA9 Collaboration at CERN demonstrated the possibility to use bent crystals for beam collimation, extraction, focusing and splitting in particle accelerators. These crystals are subject to deterioration due to the interaction of the particles with the crystal lattice, degrading the beam steering performance. For this reason, robustness tests are crucial to estimate their reliability and operational lifetime. A ∼8% of reduction in channeling efficiency on crystals irradiated with 2.5·1021/cm2 thermal neutrons was measured and reported in this manuscript. Extrapolations to possible operational scenarios in high energy accelerators are also discussed.

Surface generation of tungsten carbide in laser-assisted diamond turning

The removal mechanism of conventional cutting and in-process-heat laser-assisted cutting (In-LAC) of binderless polycrystalline tungsten carbide (WC) material is studied through systematic numerical analysis and experimental investigation. The proposed In-LAC model considers the accumulated thermal effect and the laser in-process-heat thermal boundary condition simultaneously. Results of molecular dynamics (MD) analysis are remarkably consistent with the experimental results qualitatively. The high-temperature nanoindentation test reveals the improved machinability of WC at an elevated temperature and provides a theoretical basis for cutting force reduction. A small Young's modulus measured at an elevated temperature presents a large elastic recovery value for the In-LAC model. The critical depth of no observed surface cracks of binderless WC increases from 26.6 nm to 106.3 nm, which can be attributed to in-process laser assistance. Furthermore, the In-LAC method is beneficial to avoid subsurface crystal bending and reduce subsurface damage in the MD model and the taper cutting sample subsurface. The existence of the laser annealing effect during the In-LAC process is directly proven by binderless WC cross-section transmission electron microscopy (TEM). According to the simulation analysis results and diamond turning chip morphology, the optimal laser power for polycrystalline WC ranges from 10 W to 15 W, which facilitates obtaining the surface finish of 4.66 nm in Sa and significantly improving the tool life.

X-ray rocking curve imaging on large arrays of extremely tall SiGe microcrystals epitaxial on Si

This work investigates layers of densely spaced SiGe microcrystals epitaxially formed on patterned Si and grown up to extreme heights of 40 and 100 µm using the rocking curve imaging technique with standard laboratory equipment and a 2D X-ray pixel detector. As the crystalline tilt varied both within the epitaxial SiGe layers and inside the individual microcrystals, it was possible to obtain real-space 2D maps of the local lattice bending and distortion across the complete SiGe surface. These X-ray maps, showing the variation of crystalline quality along the sample surface, were compared with optical and scanning electron microscopy images. Knowing the distribution of the X-ray diffraction peak intensity, peak position and peak width immediately yields the crystal lattice bending locally present in the samples as a result of the thermal processes arising during the growth. The results found here by a macroscopic-scale imaging technique reveal that the array of large microcrystals, which tend to fuse at a certain height, forms domains limited by cracks during cooling after the growth. The domains are characterized by uniform lattice bending and their boundaries are observed as higher distortion of the crystal structure. The effect of concave thermal lattice bending inside the microcrystal array is in excellent agreement with the results previously presented on a microscopic scale using scanning nanodiffraction.

Photothermally Driven High-Speed Crystal Actuation and Its Simulation.

Mechanically responsive crystals have been increasingly explored, mainly based on photoisomerization. However, photoisomerization has some disadvantages for crystal actuation, such as a slow actuation speed, no actuation of thick crystals, and a narrow wavelength range. Here we report photothermally driven fast-bending actuation and simulation of a salicylideneaniline derivative crystal with an o-amino substituent in enol form. Under ultraviolet (UV) light irradiation, these thin (<20 μm) crystals bent but the thick (>40 μm) crystals did not due to photoisomerization; in contrast, thick crystals bent very quickly (in several milliseconds) due to the photothermal effect, even by visible light. Finally, 500 Hz high-frequency bending was achieved by pulsed UV laser irradiation. The generated photothermal energy was estimated based on the photodynamics using femtosecond transient absorption. Photothermal bending is caused by a nonsteady temperature gradient in the thickness direction due to the heat conduction of photothermal energy generated near the crystal surface. The temperature gradient was calculated based on the one-dimensional nonsteady heat conduction equation to simulate photothermally driven crystal bending successfully. Most crystals that absorb light have their own photothermal effects. It is expected that the creation and design of actuation of almost all crystals will be possible via the photothermal effect, which cannot be realized by photoisomerization, and the potential and versatility of crystals as actuation materials will expand in the near future.

Electromagnetic characterization of the crystal primary collimators for the HL-LHC

The crystal-based primary collimators installed in the Large Hadron Collider (LHC) at CERN use the channelling process in bent crystals to steer halo particles efficiently onto downstream collimators. This scheme, called crystal collimation, is also considered for applications of fixed-target implementations in the context of the Physics Beyond Collider at the LHC. Crystal collimation uses 4 mm-long silicon crystals that need to be approached very close to the high-intensity circulating beams, posing obvious concerns for machine impedance. A complex mechanical assembly was developed for this purpose. The setup includes also a system to control with sub-μrad accuracy the angular orientation of the crystal, which is done with a high-precision interferometric system. In order to prevent possible beam-induced instabilities and/or damage of the device components from excessive RF-heating, the electromagnetic (EM) characterization of this device is essential prior to its usage with high-intensity beams. In this article, the longitudinal impedance of the crystal primary collimator is studied extensively and estimations of power loss inside the device are provided for the different beam types planned at the LHC and at its High-Luminosity upgrade (HL-LHC). Electromagnetic simulations are performed on a realistic model that includes all the relevant components. The model is described in detail and computational challenges coming from its complexity are discussed. Care is taken to characterize the materials of each relevant sub-component. In particular, the lossy properties of silicon, whose complex permittivity is also evaluated through RF rectangular-cavity perturbation measurements, are taken into account. Numerical results are then compared with dedicated RF measurements performed on a prototype built for the LHC.

Detailed diffraction imaging of x-ray optics crystals with synchrotron radiation

Rocking curve topography at the Advanced Photon Source's beamline 1-BM measures the x-ray reflection from large (many cm2) flat crystals on a sub-mm scale with microradian angular resolution. The (011̄1) reflection at 8 keV is uniform across the crystal and close to theory for three thick quartz wafers well-polished with increasingly finer grit. However, the reflection is non-uniform for some ∼0.1 mm thin, bendable crystals that are made flat by optical contact with a flat substrate. These thin crystals are bent to serve in certain x-ray diagnostics of plasmas, and similar non-uniformities could then occur in bent crystals as well. The same detail in x-ray reflection in bent crystals is unachievable with the existing topography setup: One way to get the desired resolution is with a standard microfocusing approach.

Resolution of a bent-crystal spectrometer for X-ray free-electron laser pulses: diamond versus silicon.

The resolution function of a spectrometer based on a strongly bent single crystal (bending radius of 10 cm or less) is evaluated. It is shown that the resolution is controlled by two parameters: (i) the ratio of the lattice spacing of the chosen reflection to the crystal thickness and (ii) a single parameter comprising crystal thickness, its bending radius, distance to a detector, and anisotropic elastic constants of the chosen crystal. The results allow the optimization of the parameters of bent-crystal spectrometers for the hard X-ray free-electron laser sources.