Tiny Single Crystals Research Articles

Cationic Pb2 Dumbbells Stabilized in the Highly Covalent Lead Nitridosilicate Pb2Si5N8

AbstractDue to the weak oxidative force of N2, nitrides are only typically formed with the less electronegative metals. Meeting this challenge, we here present Pb2Si5N8, the first nitridosilicate containing highly electron‐affine cations of a metal from the right side of the Zintl border. By using advanced synchrotron X‐ray diffraction, the crystal structure was determined from a tiny single crystal of 1×3×3 μm3 in size, revealing a significantly different bonding situation compared to all other nitridosilicates known so far. Indeed, DFT calculations confirm distinct amounts of covalency not only between Pb and N but also between formal Pb2+ cations. Thus, unprecedented cationic Pb2 dumbbells with a stretching vibration at 117 cm−1 were found in Pb2Si5N8, the first representative of a crystallographically elucidated lead nitride, stabilized by high amounts of covalency.

The Analysis of Superelasticity and Microstructural Evolution in NiTi Single Crystals by Molecular Dynamics.

Superelasticity in shape memory alloys is an important feature for actuators and medical devices. However, the function of the devices is typically limited by mechanical bandwidth and fatigue, which are dominated by the microstructures. Thus, in order to correlate the mechanical response and the microstructures, the microstructural evolution in NiTi single crystals under the compression, tensile, and shearing tests is simulated by molecular dynamics (MD) in the current study. Then, the martensite variant identification method, which identifies the crystal variants/phases of each lattice based on the transformation matrix, is used to post-process the MD results. The results with the detailed information of variants and phases reveal many features that have good agreement with those reported in the literature, such as X-interfaces and the transitional orthorhombic phase between the austenite and monoclinic phases. A new twin structure consisting of diamond and wedge-shaped patterns is also discovered. The macroscopic behavior, such as stress-strain curves and the total energy profile, is linked with the distribution of dislocation and twin patterns. The results suggest that the loading cases of shear and compression allow a low critical strain for the onset of martensitic transformation and a better superelasticity behavior. Therefore, the two loading cases are suitable to apply to the NiTi actuators. The current work is expected to provide insight into the mechanical responses and design guideline for NiTi shape memory alloy actuators.

Modeling of finite deformation of pseudoelastic NiTi shape memory alloy considering various inelasticity mechanisms

A crystal plasticity finite element (CPFE) model is developed for pseudoelastic NiTi shape memory alloy (SMA). The model includes various inelastic mechanisms of deformation such as martensite transformation, dislocation glide in austenite phase and twinning in the martensite phase. This model is based on the finite deformation theory and predicts phase transformation, stress field and residual strain due to slip and twinning. Following the work of Anand and Gurtin (2003), the Helmholtz free energy is derived and further the thermodynamic driving forces for slip, twin and phase transformation are obtained by using Clausius-Duhem inequality. The developed constitutive model has been implemented within a user material subroutine interface VUMAT in ABAQUS™/Explicit. The activation of different inelasticity mechanisms at different temperatures and strains are first verified by performing various simulations of uniaxial tensile deformation. Then the model is calibrated and validated against the experimental stress-strain response of NiTi single crystal as reported in the literature. The single crystal constitutive model is extended for modeling the response of a polycrystal with both initial random and textured crystallographic orientations using Taylor scale transition technique. A series of simulations are performed on polycrystalline representative volume element (RVE) at various temperatures and strains. The effect of temperature and imposed strain on phase transformation and residual strain is investigated systematically. The residual strain increases due to slip in austenite phase and twinning in the martensite phases that are activated as temperature and imposed strain increase respectively. Further the uniaxial loading conditions are extended to multiaxial loading and the results are compared against the experimental data.

Study of phase transition and ultimate mechanical properties of orthorhombic polyoxymethylene based on the refined crystal structure

The crystal structure of the orthorhombic polyoxymethylene (POM) has been refined by analyzing the synchrotron X-ray diffraction data measured for a micrometer-size single crystal of moth-type morphology. The 3-dimensional elastic and compliance constants tensors were calculated for both the trigonal and orthorhombic phases on the basis of the thus-obtained accurate structure information. The crystal structure information allowed us also to discuss the solid-state phase transition behavior from the orthorhombic to the trigonal phase occurring at around 78 °C. For this purpose, we have performed the temperature-dependent synchrotron X-ray scattering measurement for a tiny moth-type single crystal and revealed the geometrical relation between these two crystals. The free energy was calculated as a function of temperature for these two accurately-analyzed crystal forms using a density functional theory (DFT). The result showed that the orthorhombic phase is thermodynamically more stable than the trigonal phase in a low temperature region, and the stability was reversed at about 225 K. This temperature corresponds to the phase transition point from the orthorhombic to trigonal phase, which is however considerably lower compared with the actually-observed transition point. The large discrepancy might be ascribed to the additional contribution of the entropy term due to the structural disorder and/or the thermal motion of chains with a large amplitude, which was not taken into account in the DFT calculation.

Microscale phase field modeling of the martensitic transformation during cyclic loading of NiTi single crystal

A microscale phase field model developed in Levitas et al. (2004) and Idesman et al. (2005) is slightly advanced for different and anisotropic elastic moduli of phases and is employed for the study of the stress-induced cubic-monoclinic phase transition in NiTi single crystal involving all 12 martensitic variants. The model is scale-independent, without the gradient term, and it is applicable for any scale greater than 100 nm. This model includes strain softening and the corresponding transformation strain localization, and it reproduces a discrete martensitic microstructure. The model only tracks finite-width interfaces between austenite and the mixture of martensitic variants, and does not consider the interfaces between martensitic variants. The model is implemented as a UMAT subroutine in a commercial finite element (FE) package, ABAQUS. Multiple problems for a uniaxial cyclic loading are solved to study the effect of mesh, strain rate, crystal orientation, different numbers of pre-existing nuclei, and the magnitude of the athermal threshold on the stress-strain responses as well as the microstructure evolution. The obtained results exhibit that the microstructure and global stress-strain responses are practically independent of mesh discretization and the applied strain rate for relatively small strain rates. While the presence of the initial nuclei in the sample decreases the nucleation stress, it slightly increases the total energy dissipation. The observed microstructure, the sudden drop in the stress-strain curve after initiation of the martensitic transformation, and the absence of a similar jump for the reverse phase transformation are in qualitative agreement with known experiments. Changing the crystallographic orientation of the sample varies the entire behavior, namely, the variants which are involved in the phase transformation, the morphology of the associated microstructure, the stress-strain curve, and the total dissipation. Athermal threshold, in addition to the expected increase in the magnitude of hysteresis, leads to some strain hardening for the direct phase transformation.

Seeded growth of boron arsenide single crystals with high thermal conductivity

Materials with high thermal conductivities are crucial to effectively cooling high-power-density electronic and optoelectronic devices. Recently, zinc-blende boron arsenide (BAs) has been predicted to have a very high thermal conductivity of over 2000 W m−1 K−1 at room temperature by first-principles calculations, rendering it a close competitor for diamond which holds the highest thermal conductivity among bulk materials. Experimental demonstration, however, has proved extremely challenging, especially in the preparation of large high quality single crystals. Although BAs crystals have been previously grown by chemical vapor transport (CVT), the growth process relies on spontaneous nucleation and results in small crystals with multiple grains and various defects. Here, we report a controllable CVT synthesis of large single BAs crystals (400–600 μm) by using carefully selected tiny BAs single crystals as seeds. We have obtained BAs single crystals with a thermal conductivity of 351 ± 21 W m−1 K−1 at room temperature, which is almost twice as conductive as previously reported BAs crystals. Further improvement along this direction is very likely.

Phase field modeling for cyclic phase transition of NiTi shape memory alloy single crystal with super-elasticity

From the Ginzburg-Landau’s theory and thermodynamics, a three-dimensional phase field model was developed to simulate the cyclic phase transition of the NiTi shape memory alloy (SMA) single crystal with super-elasticity. The phase transition from the austenite phase to martensite one and its reverse were numerically simulated by the developed phase field model considering the repeated tension-unloading. The simulations indicated the nucleation and evolution of the multi-variant martensite and austenite phases in a scale of single crystal during the cyclic phase transition. It is found that the NiTi SMA single crystal with super-elasticity exhibits a remarkable localized deformation, accompanied by a local softening feature. Moreover, the microstructure evolution of the NiTi single crystal during the cyclic deformation was investigated. It is shown that the super-elasticity of the NiTi single crystal degrades gradually until it reaches to a stable state after certain load cycles, and a progressive accumulation of remnant martensite phase occurs, simultaneously; the phase transition of the NiTi single crystal gradually alters from a local mode into a uniform one, which comes from the internal defects and their evolutions; a large number of remnant martensite phase occurs because the enriched defects can greatly prevent the occurrence of reverse phase transition.

Atomistic study on the super-elasticity of single crystal bulk NiTi shape memory alloy under adiabatic condition

The temperature-induced phase transition and the super-elasticity (from the stress-induced phase transition) of equiatomic single crystal bulk NiTi shape memory alloys are investigated by the molecular dynamics method. By the simulation to the thermo-mechanical response of the single crystal NiTi alloy along the 〈001〉B2 under the compression/unloading and an adiabatic condition, the temperature change and the nucleation and growth of martensite transformation during the compression/unloading are discussed. The simulated results of molecular dynamics show that the single crystal bulk NiTi shape memory alloy exhibits a significant temperature change during the martensite transformation and its reverse under an adiabatic condition; moreover, a localized instability occurs apparently in the process of martensite transformation, which is closely related to the nucleation and growth rates of martensite phase; finally the effect of model size and strain rate on the thermo-mechanical response of the single crystal bulk NiTi alloy is also discussed, and no instability is observed in the simulated stress-strain curves if the model size is relatively larger, e.g., 8V0 and 13.824V0.

Influence of Hydrogen and Number of Particle Variants on Ordinary and Two-Way Shape Memory Effects in Ti–Ni Single Crystals

The ordinary and two-way shape memory effects (SMEs) are investigated for [ $$ \overline{1} $$ 12] single crystals of Ti–51.3Ni (at.%) alloy aged at 823 K for 1.5 h in free state and under tensile stress of 150 MPa without hydrogen and after saturation by hydrogen. It is established that without hydrogen in [ $$ \overline{1} $$ 12] single crystals with one and four variants of Ti3Ni4 particles the maximum magnitude of the ordinary SME is 1.9–2.6% under the external stress σext = 250 MPa. Under σext > 250 MPa, crystals are destroyed. The magnitude of the two-way SME caused by the B2–R–B19' MT equal to 1.1% at σext = 0 is observed in [ $$ \overline{1} $$ 12] single crystals with one variant of Ti3Ni4 particles. The physical reason for the observed two-way SME is the internal compressive stresses oriented along the [ $$ \overline{1} $$ 12] directions arising from one variant of Ti3Ni4 particles as a result of aging under tensile stress of 150 MPa. It is established that hydrogen does not influence the TR temperature, reduces the plasticity, and suppresses the two-way SME. The suppression of two-way SME in the [ $$ \overline{1} $$ 12] single crystals of the Ti–51.3Ni (at.%) alloy with one variant of Ti3Ni4 particles is caused by shielding of stress fields from one variant of Ti3Ni4 particles and multiple nucleation of R- and B19' martensite variants under loading with saturation by hydrogen.

Effect of hydrogen on the two-way shape memory effect in TiNi single crystals

[1¯12]-oriented single crystals of Ti-51.3Ni (at. %) alloy, aged at 823 K for 1.5 h under a tensile stress of 150 MPa, have been investigated regarding the two-way shape memory effect (TWSME) in a hydrogen-free state and after hydrogenation. It has been established that a TWSME, with a value of -1.1 %, is observed in hydrogen-free [1¯12]-oriented single crystals with one variant of dispersed Ti3Ni4 particles. The physical reason for the TWSME occurrence is associated with the advent of orienting internal compressive stresses from one variant of Ti3Ni4 particles, which results from ageing under a tensile stress of 150 MPa. It has been shown that hydrogen suppresses the TWSME. Therefore, it has been confirmed that suppression of the TWSME occurs in [1¯12]-oriented single crystals of Ti-51.3Ni (at. %) alloy with one variant of dispersed Ti3Ni4 particles, which are associated with shielding the stress fields from one variant of Ti3Ni4 particles, and with the multiple generation of the R- and B19'-martensite variants in crystals with the presence of hydrogen.

Surface Treatment of Boron Doped Diamond Using Atmospheric-Pressure Nitrogen Plasma Jet

Conductive boron-doped diamond (BDD) exhibits high physical and chemical stability, a wide potential window, a low background current. Owing to the typical characters, BDD is expected for use as an electrode material in the electrochemical applications such as electrolysis, waste water treatment, pH sensing, and electroanalysis. In order to further functionalize BDD, the surface treatment technique, e.g., enlargement the surface (reaction) area, control the morphology, chemical functionalization, is required and several kinds of the treatment method have been attempted including catalytic etching, oxygen treatment and so-on. We attempted to use atmospheric-pressure nitrogen plasma jet (APN2PJ) for the surface treatment of BDD for the first time. APN2PJ is a unique technique and can generate NH and nitrogen radicals at high density even under atmospheric pressure. In this presentation, the effects of APN2PJ on the chemical as well as electrochemical properties of BDD, especially the electrochemical double layer capacitance and the behavior at high potential region up to 2.5 V vs. RHE, are reported. In addition, the effect of following ultrasonic treatment is also reported. BDD electrode (De Nora Permelec Ltd., Nb substrate, thickness of BDD layer = 2.5 mm, boron content = 9.000 ppm) was used as purchased. APN2PJ treatment was carried out original apparatus. The treatment time was restricted at 3, 5 and 7 minutes. Mixture gas of nitrogen and hydrogen was used as the atmospheric gas. The applied voltage and pulse current of APN2PJ was set at 5 kV and 1 A, respectively. Hereafter, BDD without APN2PJ treatment is denoted as BDD-0 and BDD after plasma treatment as BDD-(APN2PJ treatment time). After APN2PJ treatment, the morphology of BDD was observed by FE-SEM and the chemical analysis was carried out by XPS. Cyclic voltammetry was carried out in a three-electrode electrochemical cell under nitrogen atmosphere. A gold mesh and Ag|AgCl electrode was used as the counter electrode and the reference electrode. 0.5 mol dm-3 H2SO4 aqueous solution was used as the electrolyte. The electrochemical double layer capacitance of each BDD electrodes was estimated in the potential region between 0.5 V and 0.6 V vs. RHE. Figure 1 a) and b) shows FE-SEM images of BDD-0 and BDD-7. The surface of BDD-0 was consisted by tiny single crystals of BDD of several micrometer sizes. On the other hand, many boundary (grooves), pits and the granular precipitates were clearly observed on the surface of BDD-7, which shows APN2PJ treatment gives morphology change of the surface of BDD. From XPS analysis, the peaks were observed in the spectra of C1s, O1s and N1s for each BDD electrodes after APN2PJ treatment, although only C1s peak was observed for BDD-0. In particular, in N1s XP spectra of BDD electrodes after APN2PJ treatment, four peak components assigned to Nitride, N-C sp3 bond, N-C sp2 bond and N-O bond were observed, showing that nitrogen was introduced into BDD surface and simultaneously, the morphology change as shown in Fig 1 b) was taken place. We also investigated the effects of following ultrasonic treatment. This was carried out in pure water. Figure 1 c) shows FE-SEM image of BDD-7 following ultrasonic treatment. Here, thus BDD was named as U-BDD-(APN2PJ treatment time). The granular precipitates, which are observed on the surface of BDD-7 (Fig. 1 b)), were disappeared and the grooves and holes became to be clearly observed. The electrochemical double layer capacitance was increased with an increase in APN2PJ treatment time and was further increased by following ultrasonic treatment. If we assumed that the capacitance relates to the geometric surface area of the electrode, the surface area of BDD would be increased with appearance of the grooves and the pits by APN2PJ. Furthermore, it was reasonably considered that the ultrasonic treatment gives disappearance of the precipitates as well as cleaning of the surface, which would lead to increase in the capacitance (surface area). The behavior at high potential region will be also presented. Figure 1

The surface layers structure of differently oriented single titanium nickelide crystals subjected to ion implantation

The structure of the surface and near-surface layers of titanium nickelide (NiTi) single crystals having [111]В2 and [001]В2 orientation relative to the direction of Si-ion beam treatment was investigated. It was found that the orientation effects of selective sputtering and the channeling control the thickness of the oxide and amorphous layers, the depth of penetration of ions and impurities, the distribution of Ni with depth. Different scenarios of deformation microstructure evolution are observed in the near-surface layers of NiTi crystals having of ‘soft’ orientation [111] В2 and ‘rigid’ orientation [001] В2.

AC calorimetry system using commercially available microchip device and its application for tiny single crystals of molecule-based compounds

The constructional details and performance of a low-temperature ac calorimetric system for extremely small single crystal samples of molecule-based compounds of which sensing parts is consisting of commercially available micro-chip device of XEN-39390 are reported. We succeeded to detect the thermal anomaly related to the glass formation of the ethylene conformation in a charge transfer complex of κ-(BEDT-TTF)2Cu[N(CN)2]Br, where BEDT-TTF is bisethylenedithiotetrathiafulvalene. From the analyses of temperature and frequency dependences of heat capacity, we suggest that XEN-39390 can give similar experimental resolution as that of TCG-3880 with much smaller single crystals less than 100ng. We also observed a glass-like freezing of molecular dynamics in the large asymmetric cations in EtMe3Sb[Pd(dmit)2]2 which are known as a spin liquid compound by single piece of crystal where dmit is 1,3-dithiole-2-thione-4,5-dithiolate. The comparison with the relaxation calorimetry result with absolute precision is performed.

The role of crystallographic anisotropy in formation of the structure of silicon-implanted layers of NiTi single crystals

The Auger electron spectroscopy and transmission electron microscopy techniques have been used to study the composition and structure of the surface layers of NiTi single crystals implanted with silicon ions at various directions of the incident ion beam. The role of “soft” [111]B2 and “hard” [001]B2 orientations of the NiTi single crystal in formation of the ion-beam-modified surface layer and the defect structure of Siion-implanted near-surface layers is revealed.

Role of flux on synthesis of poly single crystals of Ce3+ doped yttrium aluminum garnet

Yttrium aluminum garnet powders of good morphology are composed of tiny single crystals of regular faces. However, the growth mechanism of those ploy single crystals remains unknown. This paper provides morphology images of particles uncrushed, and it is found that those tiny crystals grow from large clusters in a pomegranate‐like manner. The morphology change occurs after phase transition, and is driven by flux, which provides a semi‐liquid environment around the crystals. This paper is beneficial for understanding growth mechanism of poly tiny single crystals of yttrium aluminum garnet powders.

Shape Memory Behavior of [111]-Oriented NiTi Single Crystals After Stress-Assisted Aging

The shape memory behavior of [111]-oriented Ni51Ti49 (at.%) single crystals was investigated after stress-assisted aging at 500 °C for 1.5 h under a compressive stress of −150 MPa. It was found that a single family of Ni4Ti3 precipitates with two crystallographically equivalent variants was formed after aging under compressive stress. Stress-assisted aging resulted in tensile two-way shape memory effect strain of 1.56% under −5 MPa. Thermal cycling under −600 MPa resulted in a transformation strain of −2.15%, while the subsequent thermal cycling under −5 MPa resulted in a tensile two-way shape memory effect strain of 2.2%.

Microphase separation upon crystallization of small amphiphilic molecules: ‘low’ temperature form II of sodium benzoate (E 211)

A so far unknown low temperature polymorph (form II crystallized at 350 °C) of the common food preservative sodium benzoate (NaBz; E 211) could be crystallized by annealing (7 days) of the poorly crystalline technical material. Its crystal structure was solved by indexing a tiny single crystal by electron diffraction followed by structure solution and refinement applying powder X-ray diffraction (PXRD). The new polymorph shows many structural similarities to form I (crystallized at 420 °C). Both form I and II are the result of a microphase separation: the structures consist of pseudo-hexagonal packings of rod-shaped micelles where the core consists of sodium cations being coordinated by the carboxylic groups, while the phenyl moieties of benzoate point outwards rendering the micelle surface hydrophobic. The structures differ by the degree of elliptical distortion of the rods which is more pronounced for the new form II due to incorporation of an additional Na-benzoate moiety into the circumference of the rods.

Effect of sodium doping on the microstructure, lattice distortion and magnetic properties of GdMnO3 tiny single crystals

10% monovalent sodium doping reduces the octahedral and lattice distortion in GdMnO3 tiny single crystals and introduces ferromagnetic ordering in the compound.

NiTi superelasticity via atomistic simulations

The NiTi shape memory alloys (SMAs) are promising candidates for the next-generation multifunctional materials. These materials are superelastic i.e. they can fully recover their original shape even after fairly large inelastic deformations once the mechanical forces are removed. The superelasticity reportedly stems from atomic scale crystal transformations. However, very few computer simulations have emerged, elucidating the transformation mechanisms at the discrete lattice level, which underlie the extraordinary strain recoverability. Here, we conduct breakthrough molecular dynamics modelling on the superelastic behaviour of the NiTi single crystals, and unravel the atomistic genesis thereof. The deformation recovery is clearly traced to the reversible transformation between austenite and martensite crystals through simulations. We examine the mechanistic origin of the tension–compression asymmetries and the effects of pressure/temperature/strain rate variation isolatedly. Hence, this work essentially brings a new dimension to probing the NiTi performance based on the mesoscale physics under more complicated thermo-mechanical loading scenarios.

Mechanical detection of electron spin resonance beyond 1 THz

We report the cantilever detection of electron spin resonance (ESR) in the terahertz (THz) region. This technique mechanically detects ESR as a change in magnetic torque that acts on the cantilever. The ESR absorption of a tiny single crystal of Co Tutton salt, Co(NH4)2(SO4)2⋅6H2O, was observed in frequencies of up to 1.1 THz using a backward travelling wave oscillator as a THz-wave source. This is the highest frequency of mechanical detection of ESR till date. The spectral resolution was evaluated with the ratio of the peak separation to the sum of the half-width at half maximum of two absorption peaks. The highest resolution value of 8.59 ± 0.53 was achieved at 685 GHz, while 2.47 ± 0.01 at 80 GHz. This technique will not only broaden the scope of ESR spectroscopy application but also lead to high-spectral-resolution ESR imaging.