Microstructures and interfaces in directionally solidified oxide–oxide eutectics

This article reviews different studies carried out on the microstructures and oxide–oxide interfaces encountered in directionally solidified eutectics. Depending on the phase diagram and preparation conditions, lamellar, fibrous or three-dimensional network microstructures can be obtained. Interfaces between refractory oxides with various structural types (corundum, fluorite, rocksalt, perovskite and garnet) are investigated. Orientation relations and interface planes are reported and general features of the structure of interfaces are discussed from high-resolution electron microscopy (HREM) studies. The accommodation of lattice mismatch at the interface is investigated by geometric phase analysis of HREM images.     

Ti(C,N)基金属陶瓷中陶瓷相芯/壳组织的观察与分析

用ＸＲＤ，ＳＥＭ，ＴＥＭ和ＨＲＥＭ观察、分析和方法研究了Ｔｉ（Ｃ，Ｎ）基金属陶瓷中陶瓷相的芯、壳组织。结果表明，在ＳＥＭ，ＴＦＭ观察中，芯、壳之间存在相界面，而ＨＲＥＭ观察显示陶瓷相的芯／壳组织具有连续相同的点阵结构。     

Direct Observation of Channel Structures in Zeolite Y and A with a Slow-Scan, Charge-Coupled-Device Camera

Zeolite materials are very sensitive to electron beam irradiation. Therefore, there have been only limited attempts to characterize their structures using high-resolution electron microscopy (HREM). The sensitivity to the electron beam depends strongly on the Si/Al ratio in zeolites. A slow-scan, charge-coupled-device (CCD) camera provides the possibility to observe materials using HREM with very low beam doses. In this study, this method was applied to observe sensitive materials such as zeolite type A and type Y, and HREM images were recorded without any damage caused by electron irradiation under a dose density less than 0. 2 A /cm². It was found that fine channel structures, cleavage planes, and the electron irradiation process in the zeolites could be characterized using HREM with the slow-scan CCD camera.     

Microstructural and Microchemical Characterization of Silicon Carbide and Silicon Carbonitride Ceramic Fibers Produced from Polymer Precursors

Several continuous SiC and SiC/N-based ceramic fibers prepared from different polymer precursors have been characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and high-resolution electron microscopy (HREM). Methods to prepare longitudinal as well as cross-sectional thin specimens from brittle ceramic fibers were developed to facilitate HREM and EELS studies. Lattice images clearly showed nanometer-sized crystallites, as well as amorphous regions. Microchemical analysis using EELS permitted study of the form and distribution of the various chemical species within the fibers.     

The Study of Heterogeneous Catalysts by High-Resolution Transmission Electron MicroscoDV

Advances in instrumentation have made it possible in recent years to study the microstructure of inorganic materials at atomic resolution using the technique of high-resolution electron microscopy (HREM). Details of instrumentation have been described elsewhere [l], and applications and trends for HREM have recently been reviewed [2]. Although HREM is primarily a technique for studying bulk defects, it is increasingly also being applied in the profile-imaging mode to derive information about surfaces [3]. The high spatial resolution of the electron microscope makes it a valuable tool for the characterization of heterogeneous catalysts. This is evidenced by the growing number of studies wherein electron micrographs are being used to describe the morphology of a particular catalyst. Profile imaging is proving particularly useful in this regard for following changes in surface structure as a function of treatment conditions [4].     

Surface Diffusion

Advances in instrumentation have made it possible in recent years to study the microstructure of inorganic materials at atomic resolution using the technique of high-resolution electron microscopy (HREM). Details of instrumentation have been described elsewhere [l], and applications and trends for HREM have recently been reviewed [2]. Although HREM is primarily a technique for studying bulk defects, it is increasingly also being applied in the profile-imaging mode to derive information about surfaces [3]. The high spatial resolution of the electron microscope makes it a valuable tool for the characterization of heterogeneous catalysts. This is evidenced by the growing number of studies wherein electron micrographs are being used to describe the morphology of a particular catalyst. Profile imaging is proving particularly useful in this regard for following changes in surface structure as a function of treatment conditions [4].     

Dithiocarbamate-protected ruthenium nanoparticles: Synthesis, spectroscopy, electrochemistry and STM studies

Stable ruthenium nanoparticles were synthesized in a biphasic system with a protecting monolayer of dithiocarbamate derivatives. The core size of the resulting Ru particles was found to vary with the initial ligand-metal feed ratio. UV-vis spectroscopic measurements showed a Mie scattering profile, with no obvious surface-plasmon resonance. The size and crystal structures of the particles were characterized by transmission electron microscopic (TEM) measurements. A significant fraction of the nanoparticles was found within the size range of 2-4 nm in diameter and of spherical shape from the TEM measurements. Clear lattice fringes could be observed in high-resolution TEM images with the fringe spacing consistent with the Ru(1 0 1) lattice planes. Electrochemical studies of Ru particles with different core size exhibited the solution-phase quantized charging of the particle double layers, analogous to those reported for gold and other transition-metal particles. The potential spacing between adjacent quantized charging peaks was found to vary with the particle core size, corresponding to the variation of the particle molecular capacitance. These charge-transfer properties were very consistent with the STM measurements of isolated nanoparticles which exhibit clear Coulomb blockade and staircase features.     

碳包覆铁纳米金属颗粒的制备

以酚醛树脂为碳源,硝酸铁为催化剂大量制备碳包覆铁纳米金属颗粒。利用扫描电子显微镜（SEM）、透射电子显微镜（TEM）、高分辨透射电子显微镜（HRTEM）等方法,分析和表征了产物的微观形貌和结构。结果表明：产物为纳米级的准球形碳一铁核壳结构,由同心碳纳米石墨壳层包覆纳米铁组成。产物外径30～100nm,碳层间距为0．34mm,与石墨层间距非常接近。     

Variation of Width and Composition of Grain-Boundary Film in a High-Purity Silicon Nitride with Minimal Silica

Contrary to the widely accepted observation that grain-boundary amorphous films for a given Si₃N₄ composition have common (equilibrium) widths and compositions, a significant variation for both parameters from film to film was observed in an undoped high-purity Si₃N₄ prepared using a hot isostatic pressing method. This material previously has been reported to have an equilibrium film width of 0.6 nm, as measured using a high-resolution electron microscopy (HREM) method; this value is significantly different from that which is typical for other high-purity Si₃N₄ ceramics (1.0 nm). A total of four boundaries were analyzed, using spatially resolved electron energy-loss spectroscopy methods, which can give the chemical width and composition for the film. Widths of these grain-boundary films were substantially different from each other; only the thinnest matches the previous HREM observations. The nitrogen content in the film decreased concurrently as the film thickened. This material had many cavities and complicated configurations at triple pockets, because of the very low total-SiO₂ content (0.55 vol%). They created locally different equilibrium conditions for grain-boundary films, in comparison with other fully densified Si₃N₄, causing such strong variation in both film structure and chemistry. This observation reveals the importance of triple pockets in equilibrium film structures, providing new insight in evaluating the absorption and wetting models. The thinnest film may correspond to the amorphous structure that is required to bind two randomly oriented Si₃N₄ grains under greater local stress.     

high-resolution electron microscopy of aSr-containing sialon polytypoid phase

A new type of Sr-containing sialon polytypoid phase with the structural formula SrSi_10-xAl_18+xN_32-xO_x (x≈1) has been found in the Sr–Si–Al–O–N system. The phase was characterised by X-ray powder diffraction (XRPD), and its structure was investigated by electron diffraction (ED) and high resolution electron microscopy (HREM). It is considerably disordered, but the average structure has a rhombohedral unit cell with a=5·335(5)≈ ·a_AlN and c=79·1(1)Å≈30·c_AlN. The Sr atoms are located in layers M–Sr–M, M=(Si/Al), at the origin of the unit cell with 12 X=(O,N) atoms around it, at distances of 3 Å, forming a cubo-octahedron. The X atoms that form a hexagon around the Sr atom in the ab plane are corner shared by M=(Si/Al) tetrahedra with opposite polarity in adjacent layers in which 2/3 of the tetrahedra are occupied. The M–Sr–M layers alternate with normally eight-layer-thick AlN type blocks, although the thickness of these blocks frequently varies. The structural model obtained from the HREM images includes a polarity reversal of the tetrahedra in the AlN blocks, similar to that proposed to occur in Si–Al–O–N polytypoid phases. The model with one Sr layer and 10 M=(Si,Al) layers per 1/3 of the repeat unit agrees with the composition of the phase and experimental HREM images.     

Formation of polymer/carbon nanotubes nano-hybrid shish-kebab via non-isothermal crystallization

Multi-walled carbon nanotubes (MWNTs) periodically decorated with polyethylene (PE) lamellar crystals had been prepared using the non-isothermal crystallization method. The morphology and structure of polyethylene attached to MWNTs were investigated by means of transmission electron microscopy (TEM). A nano-hybrid shish-kebab (NHSK) structure was observed wherein the average diameter of PE lamellar crystals varies from 30 to 150 nm with average periodicity of 35-80 nm. The TEM images of samples obtained at 125 °C showed that MWNTs were first wrapped by a homogeneous coating of PE with few subglobules, then PE chains epitaxially grew from the subglobule and formed lamellar crystals perpendicular to the carbon nanotube axis. It is suggested that the homogeneous coating plays a key role in the formation of NHSK structures. And the formation process was discussed based on the intermediate state images of samples obtained at 95 °C. While NHSK structures cannot be formed by using polypropylene (PP). This may attribute to the zigzagged conformation of PP chains on the surface of MWNTs, which hinders the formation of homogeneous coating of PP on it.     

A catalyst-free growth of aluminum-doped ZnO nanorods by thermal evaporation

The growth of Al:ZnO nanorods on a silicon substrate using a low-temperature thermal evaporation method is reported. The samples were fabricated within a horizontal quartz tube under controlled supply of O₂ gas where Zn and Al powders were previously mixed and heated at 700°C. This allows the reactant vapors to deposit onto the substrate placed vertically above the source materials. Both the undoped and doped samples were characterized using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) measurements. It was observed that randomly oriented nanowires were formed with varying nanostructures as the dopant concentrations were increased from 0.6 at.% to 11.3 at.% with the appearance of ‘pencil-like’ shape at 2.4 at.%, measuring between 260 to 350 nm and 720 nm in diameter and length, respectively. The HRTEM images revealed nanorods fringes of 0.46 nm wide, an equivalent to the lattice constant of ZnO and correspond to the (0001) fringes with regard to the growth direction. The as-prepared Al:ZnO samples exhibited a strong UV emission band located at approximately 389 nm (E _g  = 3.19 eV) with multiple other low intensity peaks appeared at wavelengths greater than 400 nm contributed by oxygen vacancies. The results showed the importance of Al doping that played an important role on the morphology and optical properties of ZnO nanostructures. This may led to potential nanodevices in sensor and biological applications.     

PLZT陶瓷的晶界结构

根据PLZT透明铁电陶瓷晶界的HREM象的实验结果,提出了可能存在的晶界原子结构模型。这些晶界模型不仅清晰地层现了PLZT晶界的结构特点,而且有助于工艺改进,进一步提高材料的性能。同时,对PLZT陶瓷晶界的研究结果也可供类似结构的陶瓷材料参考。     

Direct Observation of Multilayer Adsorption on Alumina Grain Boundaries

Grain-boundary films 0.6 nm in size have been observed on the grain boundaries of neodymia (Nd₂O₃)-doped alumina (α-Al₂O₃) sintered at 1800°C. Direct observation by high-angle annular dark-field imaging in the aberration-corrected scanning transmission electron microscope shows that this type of grain-boundary structure is the result of multilayer adsorption. Neodymium cations adsorb onto the faces of each of the two grains that comprise the grain boundary by substituting for aluminum cations. The positions of these cations are slightly distorted relative to the perfect lattice, and a third atomic layer in the core of the grain-boundary resides between these two layers. The measurements also confirm that the thickness deduced from high-resolution transmission electron microscopy lattice images are accurate.     

The effect of temperature on nascent morphology of polyethylene polymerized over solution-phase flat model catalysts

The structure and morphology of polyethylene (PE) produced during solution polymerization using bis(imino)pyridyl metal catalysts supported by flat SiO₂/Si(100) wafers were investigated by atomic force microscopy (AFM) and electron diffraction. Depending on the polymerization temperature, ranging from RT to 85 °C, different morphologies of the nascent PE have been observed. “Sea weed” like supermolecular structures are the predominant nascent morphologies of the PE polymerized at low temperatures. This should be associated with the high PE yield and high nucleation rate at low temperature; the catalyst is highly active and the PE macromolecules have low solubility in toluene and nucleate immediately after formation. With increasing polymerization temperature, e.g. at 60 or 70 °C, larger single crystals with roughly a lozenge shape but saw-tooth-like facets have been created. The multilayer overgrowth of the PE crystals demonstrates that the generated PE materials exceed what is required for single layer crystal growth. At 85 °C, decreasing crystal growth rate results in the formation of small PE single crystals. At the same time, the high solubility of the PE in toluene results in continuous diffusion of the macromolecules to the existing PE crystals and therefore single crystals in regular truncated lozenge shape have been formed. Electron diffraction indicates that in the whole temperature range, PE crystallizes in flat-on crystals in chain-folded structure with different chain folding stem length.     

热分解法制备WS2纳米纤维

以硫钨酸铵为原料,用热分解法制备出了WS2纳米纤维,采用TEM, HRTEM, XRD等分析手段对产物的形貌、结构、组成进行了检测,表明制备出的灰黑色WS2纳米纤维平均直径为几十纳米,最小只有几纳米,长度为几十微米;产物属六方晶系,a=0.3151 nm, c=1.271 nm,基本无杂质,较纯净. ED分析表明纤维为单晶,晶化程度好,(100)和(002)等衍射斑点清晰,与XRD分析结果一致. 对WS2纳米纤维生长的影响因素如温度、升温速率、冷却速率和反应气氛、反应物的粒度等进行了讨论.     

A near-infrared study of thermally induced structural changes in polyethylene crystal

Thermally induced structural changes of polyethylene (PE) have been studied by means of near-infrared (NIR) spectroscopy in the course of heating up to the melting temperature. NIR bands characteristic of the regular orthorhombic phase, the conformationally disordered hexagonal phase, and the amorphous phase have been successfully identified. It has been found that for the unoriented PE sample, the disordering process of orthorhombic lattice starts above room temperature and that it mostly occurs above 100 °C for the uniaxially oriented PE sample. In the latter case, the enhancement of crystallinity has clearly been detected just below T_m due to the reorganization of crystalline lattice. For the geometrically constrained ultradrawn PE sample, the phase transition from orthorhombic to hexagonal phase has been detected immediately below the melting point. The NIR bands characteristic of the hexagonal phase have been confirmed definitely. Usefulness of NIR spectroscopy has been demonstrated successfully in such a study of thermally induced phase transition behavior of PE samples with appreciable thickness, for which mid-IR spectroscopy is difficult to apply because of the intensity saturation of various key bands.     

New evidence for an oxycarbonate phase as an intermediate step in BaTiO3 preparation

A newly developed method of spray hydrolysis of a barium-titanium double alkoxide will be described as an efficient synthetic route for the preparation of a stoichiometric BaTiO₃ powder. During the thermal treatment of the corresponding precursor a barium-titanium oxycarbonate appears as an intermediate. High resolution electron microscopy (HREM) and electron energy loss spectroscopy (EELS) at ionization edges (ELNES), along with X-ray powder diffractometry (XRD), Fourier-transformed infrared spectroscopy (FTIR), and thermoanalytical measurements provide evidence of the existence of such an oxycarbonate phase. The comparison of the measured EEL spectra with quantum-mechanical calculations using density functional theory (DFT) reveals that this intermediate phase is characterized by an electronic C-Ti interaction in the crystal lattice and a specific modification of the carbonate bond.     

Structural Studies of Ordering in the (Pb1-xBax)(Mg1/3Nb2/3)O3 Crystalline Solution Series

The ordered structures of the (Pb_{1- x} Ba _x )(Mg_1/3Nb_2/3)O₃crystalline solution series were investigated by selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). At low Ba contents (e.g., x < 0.40), the ordered structure was found to be isostructural with Pb(Mg_1/3Nb_2/3)O₃, with a doubled unit cell characterized by 1/3{111} superlattice reflections. At higher Ba contents (e.g., x > 0.60), the ordered structure was characterized by 1/3{111} superlattice reflections. For intermediate Ba contents (e.g., x - 0.60), diffuse scattering along the {111} between diffuse 1/2{111} and 1/3{111} reflections was observed. The ordering is attributed to the distribution of the B-site cations between multiple sublattices. Strong fluctuations in the B-site cation ratio between ordered and disordered regions are believed not to exist; however, the possibility of weak fluctuations is consistent with the observed lattice images.     

An electron microscopic study on the turbostratic carbon formed in phenolic resin carbon by catalytic action of finely dispersed nickel

Very fine particles of certain substances are known to promote the catalytic graphitization of carbonaceous material, resulting in a unique carbon whose X-ray parameters, lying in the range of turbostratic carbon, remain almost unchanged even after more severe heat-treatments. To elucidate the actual structure of this type of carbon, its formation mechanism, and the reason for its high thermal-stability, a high resolution electron microscopic investigation was carried out on carbons derived from phenolic resin lightly and heavily doped with an organo-nickel compound. The electron microscopic texture of this carbon was found to consist of entangled lattice fringes of a stack number comparable with its X-ray L_c-value (110 Å). This texture was analogous to that of some types of the hard carbon heated to very high temperatures, but the former was less sinuous in its lattice fringe and more open in its structure. The formation mechanism appeared to differ from that accepted for conventional nickel-catalyzed graphitization, and to be particularly related with nickel particles in a certain size range. The evidence obtained so far was not enough to suggest a new mechanism, but only to illustrate part of some of the possible, hitherto little known mechanisms which might proceed along with it. Thermal stability of the carbon was understood to be due to its structure being similar to the hard carbon, but the ultimate reason why this type of structure was thermally stable could be made clear only to a limited extent.