Tiny Single Crystals Research Articles

Formability of thermally cured and of nanoclay-reinforced polyelectrolyte films on NiTi substrates

The present study characterizes the formability of thin polyelectrolyte films which were modified by curing and alternatively reinforced with nano-sized clay-platelets. To investigate the suitability of modified polyelectrolyte coatings for biomedical applications, films were applied on both the polycrystalline and single crystal NiTi 50.9 at.% substrates and mechanically deformed in simulated body fluid (Hank’s solution) under cyclic tensile loading. In situ electron backscatter diffraction, in situ confocal laser scanning microscopy, photoelastic-modulated infrared reflection absorption spectroscopy, and ellipsometry measurements were employed to study defect formation. Based on observations from the substrates and films, conducted before and after to mechanical testing, defect formation during tensile cycling was related to substrate and film characteristics. Defects emerged particularly in unmodified and cured polyelectrolyte films on both polycrystalline and single crystalline substrates within areas of pronounced topographic changes. In contrast, the nanoclay-modified coatings remained defect free, indicating a remarkable improvement of formability, which can be related to the reinforcing clay-platelets within the polyelectrolyte matrix.

Determining orientation relationships for the B2 → B19′ transformation in single crystal titanium nickelide according to the texture of B19′ martensite

A precision method for determining orientation relationships upon the B2 → B19′ transformation in titanium nickelide has been developed. The method is based on analyzing the martensite texture formed in the initial high-temperature B2 phase of a single crystal having a high degree of perfection. The orientation relationships between the B19′ lattice and the initial B2 lattice of the TiNi single crystal were established from the B19′ martensite texture formed in single crystals of titanium nickelide upon the B2 → B19′ transformation and from measurements of the lattice parameters. Crystal mechanism of B19′ martensite was proposed which included the shear in the (21-1) B2 plane in the direction [−11-1] B2 by 10°. Such a shear system is typical of the bcc crystals at deformation by twinning. Absolute shear values are in a ratio of 1:4 for B2 → B19′ transformation and for twinning, respectively. Martensite deformation at an invariant lattice is accompanied by small rotations of martensite crystals (±1.6°), that increases the quantity of martensite orientations from 12 to 24.

Multiple-step martensitic transformations in the Ni51Ti49 single crystal

Multiple-step martensitic transformations of an aged Ni51Ti49 single crystal using calorimetric method were investigated. Results show that for short aging times (10–45 min) multiple-step martensitic transformations on cooling occur in two steps. Applying intermediate aging times (1.25–4 h) results in three steps and long aging times (more than 8 h) lead to two-step martensitic transformations again. This behavior has not been recognized in NiTi single crystals in literatures. It can be related to the heterogeneity of composition and stress fields around Ni4Ti3 precipitates.

Strain localization and temperature dependence of yield stress in TiNi-based single crystals

In the work, we study the temperature dependence of yield stress and/or martensite shear stress, compression curves δ(∈) and mechanisms of plastic deformation in [001]-oriented TiNi(Fe, Mo) single crystals. It is shown that stress-induced B2 → B19’ martensite transformation and mechanical twinning in the B2 phase are the basic deformation micromechanisms over the entire temperature range studied (300–773 K). The transition of these mechanisms to mesoscale localization at T > Md changes the slope of the σ0.1(T) curves.

Focused ion beam/scanning electron microscopy tomography and conventional transmission electron microscopy assessment of Ni 4Ti 3 morphology in compression-aged Ni-rich Ni–Ti single crystals

The size, morphology and configuration of Ni 4 Ti 3 precipitates in a single-crystal Ni–Ti alloy have been investigated by two-dimensional transmission electron microscopy-based image analysis and three-dimensional reconstruction from slice-and-view images obtained in a focused ion beam/scanning electron microscopy (FIB/SEM) dual-beam system. Average distances between the precipitates measured along the compression direction correlate well between both techniques, while particle shape and configuration data is best obtained from FIB/SEM. Precipitates form pockets of B2 of 0.54 μm in the compression direction and 1 μm perpendicular to the compression direction.

Columnar Structures from Asymmetrically Tapered Biphenylamide

An asymmetrically tapered N,N'-tris[[(2-dodecylaminocarbonyl)ethyl]methyl]-4-biphenylamide (asym-C(12)PhA, where n is the number of carbon atoms in the alkyl chains, n = 12) was newly designed and synthesized. In this asymmetrically tapered asym-C(12)PhA biphenylamide, H-bondable hydrophilic amide moieties are located at between a rigid hydrophobic biphenyl rod and three flexible hydrophobic alkyl chains. Computer energy minimization indicated that three-dimensional (3D) geometry of asym-C(12)PhA biphenylamide looks like a cone with dimensions of 3.01 nm in height and 1.44 nm in bottom radius. Phase transitions and supra-molecular structures were identified utilizing the combined techniques of differential scanning calorimetry, 1D wide-angle X-ray diffraction (1D WAXD), Fourier-transform infrared spectroscopy, and solid-state (13)C nuclear magnetic resonance analyses. The asym-C(12)PhA self-assembled into a highly ordered columnar mesophase just below the isotropization temperature and then transformed to 3D columnar crystalline phase (Phi(Cr)) on further cooling. Selected area electron diffractions in transmission electron microscopy (TEM) along with 1D WAXD and cross-polarized optical microscopy suggested that discotic building blocks were constructed by rotating 120 degrees of three asym-C(12)PhA with respect to neighboring ones and the tmb (top-middle-bottom) stacked discotic building blocks further self-organized into columns. These columns are laterally intercalated to form the Phi(Cr) phase. On the basis of the TEM image and polyethylene surface decoration technology, it was identified that the self-assembled asym-C(12)PhA fibers with approximately 1 mum in diameter and several millimeters in length were braids of tiny single crystals.

D-61 Distribution of Residual Microstresses in Rolled Ti-Ni Single Crystals

D-61 Distribution of Residual Microstresses in Rolled Ti-Ni Single Crystals

Solvothermal Synthesis, Cathodoluminescence, and Field‐Emission Properties of Pure and N‐Doped ZnO Nanobullets

AbstractHomogenous crystallization in solution, in the absence of external influences, is expected to lead to growth that is symmetric at least in two opposite facets. Such was not the case when we attempted to synthesize ZnO nanostructures by employing a solvothermal technique. The reaction product, instead, consisted of bullet‐shaped tiny single crystals with an abrupt hexagonal base and a sharp tip. A careful analysis of the product and the intermediate states of the synthesis reveals that one of the reaction intermediates with sheet‐like morphology acts as a self‐sacrificing template and induces such unexpected and novel growth. The synthesis was further extended to dope the nanobullets with nitrogen as previous studies showed this can induce p‐type behavior in ZnO, which is technologically complementary to the naturally occurring n‐type ZnO. Herein, a soft‐chemical approach is used for the first time for this purpose, which is otherwise accomplished with high‐temperature techniques. Cathodoluminesce (CL) investigations reveal stable optical behavior within a pure nanobullet. On the other hand, the CL spectra derived from the surfaces and the cores of the doped samples are different, pointing at a N‐rich core. Finally, even though N‐doped ZnO is known to have high electrical conductivity, the study now demonstrates that the field‐emission properties of ZnO can also be greatly enhanced by means of N doping.

Magnetic detection of high-resolution electron spin resonance using a microcantilever in the millimeter-wave region up to 240 GHz

Highly sensitive magnetic detection of electron spin resonance (ESR) using a microcantilever is presented. By combining a modulation technique with the use of a piezoresistive cantilever, we successfully observed ESR signals of a tiny single crystal (mass<1 microg) of Co Tutton salt, Co(NH(4))(2)(SO(4))(2) x 6 H(2)O, in the frequency region of 80-240 GHz. The achieved spin sensitivity was approximately 10(9) spins/G at 4.5 K, providing promising applications to high-resolution and high-sensitivity terahertz ESR.

Local transformation strain measurements in precipitated NiTi single crystals

The objective of this work was to determine the effect of composition on the local transformation strain in NiTi single crystals. Using in situ digital image correlation to obtain full-field strain measurements, we revealed that increasing Ni concentration and precipitate volume fraction results in smaller local transformation strains. Comparison of the local strain measurements to theoretical calculations indicated that detwinning strains are limited in Ni-rich compositions. It was also found that the Ni-rich compositions displayed a more homogeneous transformation.

Solvothermal synthesis and characterization of ZnSe nanoplates

ZnSe nanoplates were synthesized by a solvothermal method using ethylenediamine (EN) as the liganding solvent. The crystal structures, morphologies and optical properties of the precursor and ZnSe products were systematically characterized. Results reveal that precursor ZnSe(en) 1/2 with a layered structure was initially obtained through solvothermal synthesis, which can be converted into hexagonal wurtzite structured ZnSe by heat-treatment in Ar atmosphere. The as-prepared ZnSe was composed of stacked nanoplates. The field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) results reveal that every nanoplate was made up of plenty of tiny single crystals. The room temperature ultraviolet–visible (UV/vis) measurements indicate that the bandgap of the obtained ZnSe is 2.7 eV and a large blue shift about 1.3 eV was observed in precursor ZnSe(en) 1/2.

Construction of a low-temperature thermodynamic measurement system for single crystal of molecular compounds under pressures

An apparatus to obtain low-temperature thermodynamic information under high pressures for a tiny single crystal of molecular compounds was developed based on the ac technique. To detect small temperature oscillation of a sample inside the cramp-type pressure cell, we have used a small ruthenium oxide chip sensor as a thermometer. The adoption of the four-terminal method by the ac resistance bridge has made high-resolution detection of thermal anomaly possible in the low-temperature region. The constructed high-pressure thermodynamic system was mounted on a 3He refrigerator and we have succeeded to detect the thermal anomaly in relevant to magnetic order of single crystal sample of Mn4-cluster complex up to 1.05 GPa. A distinct peak of the heat capacity and its upward shift with increasing pressures was observed using a tiny crystal of about 100 microg. The high-pressure behavior of the discontinuity of heat capacity at the superconductive transition of 6 mg of metal indium has also been detected by this apparatus. The details and performance of the technique are reported.

Fracture in single crystal NiTi

This paper presents an investigation of the pseudoelastic transformation of two crystallographic orientations of a single crystal NiTi shape memory alloy (SMA). Both uniaxial tension and notched tension samples are considered with the tensile axes along the [1 0 0] and [1 1 1] directions. The phase transformation is observed using optical techniques in situ. For the uniaxial tension samples, martensite plates and Lüders-band structures are observed. For the notched samples, transformation structures in the [1 0 0] sample appeared predominantly on the sides of the notch and crack tip with stable crack propagation. In the [1 1 1] notched samples transformation occurs directly ahead of the notch and unstable crack propagation is observed. An available work criterion is used to predict the location of the transformation, with good agreement to the experimental observations. The different fracture behavior of the two notched sample orientations is explained utilizing the available work calculations.

ZnO Twin-Cones: Synthesis, Photoluminescence, and Catalytic Decomposition of Ammonium Perchlorate

ZnO twin-cones, a new member to the ZnO family, were prepared directly by a solvothermal method using a mixed solution of zinc nitrate and ethanol. The reaction and growth mechanisms of ZnO twin-cones were investigated by X-ray diffraction, UV-visible spectra, infrared and ion trap mass spectra, and transmission electron microscopy. All as-prepared ZnO cones consisted of tiny single crystals with lengths of several micrometers. With prolonging of the reaction time from 1.5 h to 7 days, the twin-cone shape did not change at all, while the lattice parameters increased slightly and the emission peak of photoluminescence shifted from the green region to the near orange region. ZnO twin-cones are also explored as an additive to promote the thermal decomposition of ammonium perchlorate. The variations of photoluminescence spectra and catalytic roles in ammonium perchlorate decomposition were discussed in terms of the defect structure of ZnO twin-cones.

Yield stress asymmetry in [001] TiNi single crystals

Yield stress asymmetry in [001] TiNi single crystals

Direct transmission electron microscopy observations of martensitic transformations in Ni-rich NiTi single crystals during in situ cooling and straining

We investigate martensitic transformations using transmission electron microscopy (TEM) in compression aged Ni-rich NiTi single crystals with one family of Ni 4Ti 3 precipitates. Small cylinders from a Ni-rich NiTi single crystal with a Ni content of 51.0 at.% were compression aged at 550 °C in the [1 1 1] B2 direction for different aging times. Differential scanning calorimetry (DSC) investigations show that a three-step martensitic transformation (three DSC peaks on cooling from the high temperature regime) can be observed for aging times of 4 ks. In situ cooling TEM investigations reveal that the first peak on cooling is associated with a transformation from B2 to R-phase, starting from all precipitate/matrix interfaces. On further cooling, the B19′-phase appears and grows along precipitate/matrix interfaces (second step). With further decreasing temperature, the remaining R-phase between the precipitates transforms to B19′ (third peak). In situ TEM straining experiments of B2 above the martensitic start temperature reveal that first some microstructural regions directly transform in microscopic burst like events from B2 to B19′. On further straining, the B19′-phase grows along precipitate/matrix interfaces. However, no formation of R-phase precedes the formation of stress-induced B19′.

Root-mean-square displacements of atoms in the B2- and R-phases of TiNi single crystal

Using Mo Kα radiation on a single crystal of a titanium nickelide, intensities of 15 structural and 11 superstructural reflections of the B2-phase are measured. Structure factors are calculated for these reflections and root-mean-square displacements of nickel and titanium atoms are determined. The mean square displacement of Ni is equal to 〈 u 2〉 Ni = (8.7 ± 0.6) × 10 −4 nm 2 and is 〈 u 2〉 Ti = (3.9 ± 0.3) × 10 −4 nm 2 for Ti at 363 K. The temperature dependence of the mean square displacements of Ni and Ti upon cooling is determined in the interval 375–283 K.

Conventional and in-situ transmission electron microscopy investigations into multistage martensitic transformations in Ni-rich NiTi shape memory alloys

Investigations of multi-step martensitic transformations in Ni-rich NiTi single crystals and polycrystals are reported. Attention is mainly given to processes that involve a two-step R → B19′ transition. Single crystals and polycrystals with a Ni content close to 51 at.% were subjected to standard heat-treatments during which microstructures with a homogeneous distribution of Ni 4Ti 3 precipitates form. Results obtained using differential scanning calorimetry and conventional and in-situ transmission electron microscopy clearly show that these microstructures transform from R phase into B19′ martensite in two steps. We suggest that the nucleation conditions for the B19′ phase must be taken into account to rationalize the obtained results.

Loss of pseudoelasticity in nickel–titanium sub-micron compression pillars

Nickel–titanium (NiTi) is capable of undergoing pseudoelastic deformation wherein relatively large amounts of inelastic deformation are recovered upon load removal due to a martensitic phase transformation. This study investigates pseudoelastic as well as plastic deformation in sub-micron diameter NiTi compression pillars. Pillars ranging in diameter from approximately 2 μm to 200 nm were prepared using focused ion beam micro-machining of aged [1 1 1] single crystal NiTi. Results reveal pseudoelasticity in all samples tested with diameters between 2 μm and 400 nm, although permanent strain was introduced at relatively low strains compared to bulk. Decreased sample size generally showed a smaller stress–strain hysteresis, with a full loss of recoverable pseudoelastic strain for samples with a diameter smaller than 200 nm. In addition, plastic flow stress of the martensite was shown to be independent of sample diameter for the aged NiTi material. Lastly, it is observed that crystallographic orientation has a stronger influence on martensite plastic flow strength than pillar size.

Observation of Antiferromagnetic Spin-Flop Transition in λ-type BETS Salts Using AFM Microcantilever

Antiferromagnetic spin-flop transitions in a series of λ-(BETS)2FeBrxCl4-x are observed using piezoresistive AFM microcantilever. The measurements are carried out on a tiny single crystal using a sample rotator and the easy axis changes with increasing the bromine content x. The Neel temperature and spin-flop transition field also rise with increasing the bromine content x, suggesting an enhancement of π-d interactions by replacement of chlorine with bromine atoms in this mixed-halide anion system.