DSC and TEM investigations on multiple melting phenomena in isotactic polystyrene

The multiple melting behavior and morphologies of isotactic polystyrene (iPS) isothermally crystallized from the glassy state have been investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The combination of thermal analysis and morphological results indicates that two lamellar populations are responsible for the so-called double melting behavior in iPS. The low-temperature melting peak is attributed to the melting of less perfect (thinner or defect containing) subsidiary lamellae formed in the framework of the dominant (thicker or more perfect crystalline) lamellae upon isothermal crystallization. The high-temperature one is mainly due to the melting of the dominant thicker lamellae, and to some less extent, the melting of a recrystallized population coming from the melted defect lamellae during the heating process in DSC.     

Dark field imaging of semicrystalline polymers by scanning transmission electron microscopy

Scanning transmission electron microscopy (STEM) has been suggested to have advantages over conventional transmission electron microscopy (CTEM) for the observation of diffraction contrast features and diffraction patterns from radiation sensitive crystalline polymers. Because of image intensification, control of illumination location and magnification independent focus, STEM operation for focusing, area selection and set up of optics permits a high yield of systematic data. Dark field (DF) imaging is most useful when employed in conjunction with scanning microarea diffraction. For convergent beam microdiffraction and efficient DF imaging of thin crystals the beam divergence should be less than 5×10^–3 radians. For single beam DF, the reflection of interest is selected by the intermediate lens aperture. Use of a STEM annular detector to collect more than one reflection results in increased DF image intensity and resolution. Use of the entire azimuthal range of a single powder pattern reflection permits examination of crystal texture — in particular, images produced by chain axis reflections show the detailed arrangements of lamellae.     

Properties of layered structured PE/PP and PB-1/PP films having ultra-thin layers

Highly oriented polymer films containing ultra-thin layers of polyethylene/polypropylene (PE/PP) or polybutene-1/polypropylene (PB-1/PP) were investigated with transmission electron microscopy, differential scanning calorimetry, optical birefringence methods and mechanical testing. The determination of the microstructures of the above systems before and after melting and recrystallization of the low-melting component revealed the existence of epitaxially crystallized PE lamellae in the PE/PP system, whereas no epitaxy was observed in the PB-1/PP system. However, if PB-1 was sandwiched between two layers of the polypropylene, a non-isotropic arrangement of the recrystallized PB-1 crystals was detected. The mechanical properties, as well as thermal behaviour and optical birefringence of these systems, were determined and the results were related to the specific morphologies present in the different samples. Reinforcement mechanisms resulting from epitaxially crystallized PE lamellae are discussed.     

Microstructural characterization of unidirectionally frozenin situ Sn-Se eutectic composites

An array of lamellar SnSe-SnSe₂ structure is obtained by unidirectional solidification, in which SnSe and SnSe₂ are, respectively,p andn type semiconductors. Their structural morphology was examined with use of transmission and scanning electron microscopes. It was found that the ordering of the alternative layers of the phases could be accomplished by a suitable choice of freezing rates, although several kinds of structural defects such as terminations, misfit lamellae and colony structure were observed. The mechanisms of these defect formations were considered in terms of the constitutional supercooling. Furthermore, the crystallographic relationship between the two phases in the solidified state was determined.     

STEM characterization for lithium-ion battery cathode materials

This article briefly reviews the status and new progress on the characterization of popular cathode materials for lithium-ion batteries by scanning transmission electron microscopy (STEM) and presents some of our own research work in this field, especially the direct observation of light elements such as Li and H with atomic resolution using the annular bright-field imaging (ABF) technique. These results demonstrate that STEM combined with high-angle annular dark-field imaging, electron energy-loss spectroscopy (EELS) and ABF imaging is a powerful tool for investigation of the atomic level microstructure of various cathode materials and resolving many fundamental issues in the battery related research field and industries, such as the mechanism of capacity fading and diffusion behavior across the interface between electrode and electrolyte.     

Microstructural investigation of quartz submitted to ultra-short shock loading

A high-energy pulsed laser was used to induce very short (2 ns) pressure pulses in quartz single crystals. The microstructure of recovered specimens was characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy. Whatever the peak pressures (20–90 GPa), the shocked materials showed no shock defects (amorphous lamellae, Brazil twins, etc.). The microstructure was dominated by fracturing. The present study thus suggests that for very short pulse durations, quartz can be loaded at pressures well above the Hugoniot elastic limit without undergoing solid-state amorphization. The behaviour of quartz is purely elastic-brittle.     

Standardless atom counting in scanning transmission electron microscopy

We demonstrate that high-angle annular dark-field imaging in scanning transmission electron microscopy allows for quantification of the number and location of all atoms in a three-dimensional, crystalline, arbitrarily shaped specimen without the need for a calibration standard. We show that the method also provides for an approach to directly measure the finite effective source size of a scanning transmission electron microscope.     

Direct electron microscopic observation of transcrystalline layers in microfibrillar reinforced polymer-polymer composites

Microfibrillar reinforced composites (MFC) comprising an isotropic matrix from a lower melting polymer, i.e., low density polyethylene (LDPE), reinforced by microfibrils of a higher melting polymer, recycled from bottles, i.e., poly(ethylene terephthalate) (PET), were processed under industrially relevant conditions via injection molding in a weight ratio of PET/LDPE = 50/50. Dog bone samples with MFC structure were characterized by means of scanning (SEM) and transmission (TEM) electron microscopy. SEM observations on cryogenic fracture surfaces show an isotropic LDPE matrix reinforced by more or less randomly distributed PET microfibrils. By means of TEM on stained ultrathin slices one observes the formation of transcrystalline layers of LDPE matrix on the surface of the PET microfibrils. In these layers the crystalline lamellae are aligned parallel to each other and are placed perpendicularly to the fibril surfaces. This is in contrast to the bulk matrix where the lamellae are quasi-randomly arranged.     

Reaction and diffusion phenomena upon oxidation of a (γ+α2) TiAlNb alloy in air

Abstract

A two-phase fully lamellar titanium aluminide alloy, Ti46Al8Nb, was exposed to air at temperatures ranging from 700 to 900°C. Isothermal and cyclic oxidation tests were conducted for up to 1000 h. Oxidation kinetics was followed by gravimetric measurements. Post-test examination of specimens comprised light microscopy, scanning electron microscopy/energy dispersive X-ray spectrometry (EDS), transmission electron miscoscopy/EDS, X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, glow discharge spectrometry, nanoindentation and scratch test. Very thin and perfectly adherent scales were composed of nanometric grains of aluminium oxide with a corundum structure on the surface, followed by a compact layer of titanium dioxide with a rutile structure and a fine-grained porous mixed-oxide layer. Chains of voids appeared locally between the compact and the porous part of the scale after extended exposure. The alloy–scale interface was complex, and contained coarse grains of alumina and Nb-rich precipitates embedded in a Ti-rich matrix. Niobium, as the least mobile element, marked the diffusion front. Major transport processes in the near-surface zone were nitrogen and oxygen inward diffusion and aluminium outward diffusion. The scale was not uniform in thickness, which might be related to grain-boundary diffusion, different orientations of the colonies of lamellae and phase composition of the lamellae on the specimen surface.     

Imaging domains in BaTiO<Subscript>3</Subscript> single crystal nanostructures: comparing information from transmission electron microscopy and piezo-force microscopy

This article compares and contrasts information obtained, using transmission electron microscopy (TEM) and piezo-force microscopy (PFM), on domain configurations adopted in single crystal lamellae of BaTiO₃, that had been cut directly from bulk using a focused ion beam microscope with top and bottom surfaces parallel to {100}_pseudocubic. Both forms of imaging reveal domain walls parallel to {110}_pseudocubic, consistent with sets of 90° domains with dipoles oriented parallel to the two <001>_pseudocubic directions in the plane of the lamellae. However, the domain width was observed to be dramatically larger using PFM than it was using TEM. This suggests significant differences in the surface energy densities that drive the domain formation in the first place, that could relate to differences in the boundary conditions in the two modes of imaging (TEM samples are imaged under high vacuum, whereas PFM imaging was performed in air). Attempts were made to map local dipole orientations directly, using a form of ‘vector’ PFM. However, information inferred was largely inconsistent with the known crystallography of the samples, raising concern about the levels of care needed for accurate interpretation of PFM images.     

Measurement of Molecular Orientation in Retrieved Ultra-high-molecular-weight Polyethylene (UHMWPE) Hip Sockets using Fourier-transform Infrared Spectroscopy

Abstract:  This paper describes the use of Fourier-transform infrared spectroscopy (FTIR) to measure the degree of orientation in retrieved ultra-high-molecular-weight polyethylene (UHMWPE) acetabular components of prosthetic hip joints quantitatively. Multidirectional shearing has been found to result in a wear rate that is several orders of magnitude higher than that for linear shear. This is because linear wear of UHMWPE is believed to induce orientation of the polymer lamellae. The FTIR technique described in this paper enables a direct comparison to be made between patient biomechanics and the molecular orientation of UHMWPE hip sockets. Patients were identified prior to revision surgery of the hip for loosening of components. Individual patient's hip-joint kinematics were quantified by the aspect ratio of movement loci developed from clinical gait analysis. It was found that patients with high aspect ratios and therefore more linear wear paths, exhibited measurable orientation of the UHMWPE polymer lamellae in their retrieved hip sockets. An aspect ratio of 5.36 resulted in 67% of the polymer lamellae being oriented in the a- axis direction. The technique was qualitatively validated by the use of transmission electron microscopy and it was found that an aspect ratio of 4.46 or greater resulted in the observed orientation of the polymer lamellae.     

Volume holographic hyperspectral imaging

A volume hologram has two degenerate Bragg-phase-matching dimensions and provides the capability of volume holographic imaging. We demonstrate two volume holographic imaging architectures and investigate their imaging resolution, aberration, and sensitivity. The first architecture uses the hologram directly as an objective imaging element where strong aberration is observed and confirmed by simulation. The second architecture uses an imaging lens and a transmission geometry hologram to achieve linear two-dimensional optical sectioning and imaging of a four-dimensional (spatial plus spectral dimensions) object hyperspace. Multiplexed holograms can achieve simultaneously three-dimensional imaging of an object without a scanning mechanism.     

From tissue retrieval to electron tomography: nanoscale characterization of the interface between bone and bioactive glass

The success of biomaterials for bone regeneration relies on many factors, among which osseointegration plays a key role. Biogran (BG) is a bioactive glass commonly employed as a bone graft in dental procedures. Despite its use in clinical practice, the capability of BG to promote osseointegration has never been resolved at the nanoscale. In this paper, we present the workflow for characterizing the interface between newly formed bone and BG in a preclinical rat model. Areas of bone–BG contact were first identified by backscattered electron imaging in a scanning electron microscope. A focused ion beam in situ lift-out protocol was employed to prepare ultrathin samples for transmission electron microscopy analysis. The bone–BG gradual interface, i.e. the biointerphase, was visualized at the nanoscale with unprecedented resolution thanks to scanning transmission electron microscopy. Finally, we present a method to view the bone–BG interface in three dimensions using electron tomography.     

Scanning transmission electron microscopy analysis of grain structure in perpendicular magnetic recording media

The key component of a hard disk medium is a Co-based magnetic layer (ML) grown on a Ru seed layer. The ML nanostructure, composed of less than 10 nm grains, is believed to be controlled by this seed layer. We successfully used scanning transmission electron microscopy energy dispersive spectrometry simultaneous composition-based imaging and Moire? pattern analysis for determining the mutual structural and orientation relationship between the two layers revealing a grain-to-grain agreement. The method presented here can be utilized for observing structural correlations between consecutive polycrystalline thin film layers in general.     

The influence of sterilization technique and ageing on the structure and morphology of medical-grade ultrahigh molecular weight polyethylene

The effects of four sterilization treatments (gamma radiation in nitrogen, electron-beam radiation, ethylene oxide gas, and no sterilization) on the structure and morphology of ultrahigh molecular weight polyethylene (UHMWPE) were monitored as a function of ageing time in air for a period of 1.5 y. Characterization techniques employed include differential scanning calorimetry, density gradient column, transmission electron microscopy, and small-angle X-ray scattering. Ethylene oxide gas does not affect the structure of the polymer. Both forms of radiation lead to measurable alterations of the material's structure, including an increase in crystallinity, an increase in density, and the enhancement of lamellae crystalline stacking. Most changes in structure occur in the first few months with little differences observed upon subsequent ageing in air. The sharpness of the crystalline–amorphous boundaries decreases with time for irradiated UHMWPE and is believed to be linked to the oxidation of the polymer. © 1998 Chapman & Hall     

Experimental determination of refractive index of condensed reflectin in squid iridocytes

Loliginid squid dynamically tune the structural iridescence of cells in their skin for active camouflage and communication. Bragg reflectors in these cells consist of membrane-bound lamellae periodically alternating with low refractive index extracellular spaces; neuronal signalling induces condensation of the reflectin proteins that fill the lamellae, consequently triggering the expulsion of water. This causes an increase in refractive index within the lamellae, activating reflectance, with the change in lamellar thickness and spacing progressively shifting the wavelength of reflected light. We used micro-spectrophotometry to measure the functionally relevant refractive index of the high-index lamellae of the Bragg reflectors containing the condensed reflectins in chemically fixed dermal iridocytes of the squid, Doryteuthis opalescens. Our high-magnification imaging spectrometer allowed us to obtain normalized spectra of optically distinct sections of the individual, subcellular, multi-layer Bragg stacks. Replacement of the extracellular fluid with liquids of increasing refractive index allowed us to measure the reflectivity of the Bragg stacks as it decreased progressively to 0 when the refractive index of the extracellular medium exactly matched that of the reflectin-filled lamellae, thus allowing us to directly measure the refractive index of the reflectin-filled lamellae as n_{condensed lamellae} ≈ 1.44. The measured value of the physiologically relevant n_{condensed lamellae} from these bright iridocytes falls within the range of values that we recently determined by an independent optical method and is significantly lower than values previously reported for dehydrated and air-dried reflectin films. We propose that this directly measured value for the refractive index of the squid''s Bragg lamellae containing the condensed reflectins is most appropriate for calculations of reflectivity in similar reflectin-based high-index layers in other molluscs.     

Differential interference contrast x-ray microscopy with twin zone plates

X-ray imaging in differential interference contrast (DIC) with submicrometer optical resolution was performed by using a twin zone plate (TZP) setup generating focal spots closely spaced within the TZP spatial resolution of 160 nm. Optical path differences introduced by the sample are recorded by a CCD camera in a standard full-field imaging and by an aperture photodiode in a standard scanning transmission x-ray microscope. Applying this x-ray DIC technique, we demonstrate for both the full-field imaging and scanning x-ray microscope methods a drastic increase in image contrast (approximately 20x) for a low-absorbing specimen, similar to the Nomarski DIC method for visible-light microscopy.     

Extraordinary optical transmission brightens near-field fiber probe

Near-field scanning optical microscopy (NSOM) offers high optical resolution beyond the diffraction limit for various applications in imaging, sensing, and lithography; however, for many applications the very low brightness of NSOM aperture probes is a major constraint. Here, we report a novel NSOM aperture probe that gives a 100× higher throughput and 40× increased damage threshold than conventional near-field aperture probes. These brighter probes facilitate near-field imaging of single molecules with apertures as small as 45 nm in diameter. We achieve this improvement by nanostructuring the probe and by employing a novel variant of extraordinary optical transmission, relying solely on a single aperture and a coupled waveguide. Comprehensive electromagnetic simulations show good agreement with the measured transmission spectra. Due to their significantly increased throughput and damage threshold, these resonant configuration probes provide an important step forward for near-field applications.     

AFM轻敲模式下扫描参数对成像质量影响的研究

针对原子力显微镜(AFM)扫描参数对成像质量的影响规律,对现有的扫描参数影响成像质量的研究进行了分析。提出了以幅值误差作为评价AFM成像质量的指标,通过实验研究了扫描参数对AFM成像质量的耦合影响。发现了扫描频率fs、积分增益I和幅值设定点S这3个扫描参数间相互影响的现象,在S低较低情况下可设置更大的fs和I,在获取高质量成像的同时,提升了测量效率,对扫描参数的合理设置提供了可靠的操作准则。     

Direct imaging of hydrogen-atom columns in a crystal by annular bright-field electron microscopy

Enhancing the imaging power of microscopy to identify all chemical types of atom, from low- to high-atomic-number elements,would significantly contribute for a direct determination of material structures. Electron microscopes have successfully provided images of heavy-atom positions, particularly by the annular dark-field method, but detection of light atoms was difficult owing to their weak scattering power. Recent developments of aberration-correction electron optics have significantly advanced the microscope performance, enabling identification of individual light atoms such as oxygen, nitrogen, carbon, boron and lithium. However, the lightest hydrogen atom has not yet been observed directly, except in the specific condition of hydrogen adatoms on a graphene membrane. Here we show the first direct imaging of the hydrogen atom in a crystalline solid YH(2), based on a classic 'hollow-cone' illumination theory combined with state-of-the-art scanning transmission electronmicroscopy. The optimized hollow-cone condition derived from the aberration-corrected microscope parameters confirms that the information transfer can be extended to 22.5 nm(-1), which corresponds to a spatial resolution of about 44.4 pm. These experimental conditions can be readily realized with the annular bright-field imaging in scanning transmission electron microscopy according to reciprocity, revealing successfully the hydrogen-atom columns as dark dots, as anticipated from phase contrast of a weak-phase object.