Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy Study of Nonstoichiometric Silicon-Carbon-Oxygen Glasses

Crystallization behavior of Si-C-O glasses in the temperature range of 1000°–1400°C was investigated using transmission electron microscopy (TEM) in conjunction with electron energy-loss spectroscopy (EELS). Si-C-O glasses were prepared by pyrolysis of polysiloxane networks obtained from homogeneous mixtures of triethoxysilane, T^H, and methyldiethoxysilane, D^H. Si-C-O glass composition depended on the molar ratio of the precursors utilized. At a ratio of T^H/D^H= 1, the formation of a carbon-rich glass was observed, whereas a ratio of T^H/D^H= 9 yielded a Si-C-O glass with excess free silicon. Both materials were amorphous at 1000°C, but showed a distinct difference in crystallization behavior on annealing at high temperature. Although T^H/D^H= 1 revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400°C, T^H/D^H= 9 showed, in addition to SiC crystallites, numerous larger silicon precipitates (20–50 nm), even at 1200°C. Both materials underwent a phase separation process, SiC _x O_2(1-x)→ x SiC + (1 - x )SiO₂, when annealed at temperatures exceeding 1200°C.     

Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy Characterization of Glass Phase in Sol-Gel-Derived Mullite

Sol-gel-derived mullite ceramics were processed by pressureless sintering at 1600°, 1650°, and 1700°C for 4 h. Microstructural and microchemical characterization of the mullite materials was performed using transmission electron microscopy, in conjunction with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy (EELS). Apart from mullite grain diameter and triplepocket size, no major microstructural changes were observed with increasing sintering temperature. Residual glass was present at triple pockets and along two-grain junctions. Not all grain boundaries revealed the presence of a continuous amorphous intergranular film. Clean interfaces were observed only at boundaries with one grain parallel to the [001] orientation (low-energy configuration). Quantitative EELS analysis of mullite grains and glass pockets revealed only small changes in composition with increasing sintering temperature; i.e., the alumina:silica ratio slightly increased for mullite and glass. The analysis implied that mullite with this relatively high aluminum content would not be stable adjacent to residual glass. However, a stable glass-mullite system has been proposed, because impurity cations were detected within glass pockets, which suggested a slight shift of the subsolidus line (glass-mullite/ mullite) to a higher amount of alumina. Energy-loss nearedge structure studies of the Si- L _2,3 edge revealed a similar near-edge structure for the mullite, residual glass, and quartz. Thus, SiO₄ tetrahedra were thought to be the main building units of the glass contained in sintered mullite.     

Surface Defects in Polished Silicon Studied by Cross-Sectional Transmission Electron Microscopy

The cross-sectional TEM technique is applied to Si(100) surfaces, which are exposed to mild polishing with diamond paste in the 0.25- to 6-μm-size range as well as with the commercial polishing medium Syton HT 50. Syton polishing is also used to prepare electron-transparent foils for TEM studies, and this preparation technique shows several advantages over ion thinning. Mild diamond polishing results in small-scale abrasive grooving and production of superficial dislocation loops of 1/2〈110〉{111} type. Debris of amorphous material tends to aggregate in surface cavities. These aggregates are formed during polishing, not by ion milling. Syton polishing results in very smooth surfaces, with no signs of abrasive grooving or dislocation density. Two major mechanisms of mass removal are suggested: (1) "smearing" of very superficial material at a nearly atomic level, resulting in amorphous aggregates and (2) strongly localized plastic cutting or shearing. In mild polishing, both mechanisms are suggested to be active, localized plastic cuttinglshearing becoming progressively more dominant for increasing particle size.     

Charge-Compensating Defect in Ceria-Doped Strontium Barium Niobate by Electron Energy-Loss Spectroscopy and X-ray Photoelectron Spectroscopy

Undoped and CeO₂-doped Sr_0.5Ba_0.5Nb₂O₆ (SBN) powders were synthesized using the solid-state reaction method. The lattice parameters of undoped and CeO₂-doped SBN were evaluated using X-ray diffractometry. The valence state of the cerium ion in Ce-doped SBN was identified using X-ray photoelectron spectroscopy (XPS), and the valence of 3+ was confirmed. The charge-compensated defect of the cerium doping in SBN ceramics, i.e., excessive oxygen ions occupying the vacant O(4) or O(5) site, was further evidenced using electron energy-loss spectroscopy (EELS) and XPS. The relationships of charge-compensated defect, structure, binding energy, and temperature of the maximum of the relative permittivity ( T _m) were discussed.     

Phase Separation in Borosilicate Glasses as Seen by Electron Microscopy and Scanning Electron Microscopy

Microstructures of two sodium borosilicate glasses were examined. Electron micrographs of replicas of water-etched fracture surfaces show that the microphases increase in size with the temperature and time of heat treatment. The relative amounts of soluble and insoluble microphases remain essentially unchanged. Scanning electron micrographs of completely leached glasses show that the insoluble phase is randomly interconnected. Scanning electron micrographs of colloidal deposits in the pores of the leached glass, which result from acid leaching of the heat-treated base glass, are shown. Electron micrographs showing re-solution of the phases are also included.     

Microstructural Phenomena Controlling Losses in NiCuZn-Ferrites as Studied by Transmission Electron Microscopy

The magnetic losses of NiCuZn-ferrites with the chemical formula (Ni_0.37−δZn_0.63Cu_δ)Fe_1.93O₄ (0<δ<0.22) are found to exhibit a minimum as a function of the copper content. This minimum corresponds to a certain sintering temperature, which is also a function of the copper content. At sintering temperatures lower than the optimum, insufficient densification is considered to be the reason for increased losses. At sintering temperatures higher than the optimum, the increased losses are caused by the presence of copper segregation to the grain boundaries and by some identified microstructural defects such as subgrain boundaries and compositional modulations. Internal stress relaxation is believed to be the reason for those chemical and morphological imperfections. In contrast to the power losses, the magnetic permeability does not show the previous dependency and increases monotonically with increased density or grain size.     

Influence of Sampling and Storage Protocol on Fractal Morphology of Soot Studied by Transmission Electron Microscopy

The aim of this work was to compare the fractal characteristics, D_f and k_f, the primary particle diameter, D_pp, the gyration diameter of aggregates, D_g, and the overlap coefficient, C_ov, of carbon nanoparticle aggregates produced by an ethylene diffusion flame and sampled by means of four commonly used techniques. The first method involves a thermophoretic piston probe (TPP) which inserts a TEM grid into the flame. Three other methods were applied at the outlet of a dilution device, also inserted in the flame. The first of these used a nuclepore filtration sampler (NFS), and is based on filtration of particles onto a polycarbonate membrane. The second, post dilution method, the insertion particle sampler (IPS), inserts a TEM grid, perpendicular to the aerosol flow. Similar to TPP, the last method is a thermophoretic particle sampler (TPS) sampling directly onto a TEM grid. After collection, the samples are stored in the dark either, (1) in a nitrogen filled cell at low humidity or, (2) in ambient air for studying atmospheric ageing. Good agreement was observed between TPP, TPS, and IPS indicating that the dilution induced for TPS and IPS does not significantly change the morphology of soot. On the other hand, the NFS protocol tended to overestimate the overlap coefficient and the size of primary particles and aggregates. Finally, with regard to the aging effect, we found that k_f and D_pp evolve slowly during storage in the atmosphere while D_f, was insensitive to the storage conditions. However, the overlap coefficient increased and the gyration diameter decreased as a function of storage duration, while storage under nitrogen tended to reduce these changes.     

Reaction-Layer Interfaces in SiC-Fiber-Reinforced Glass-Ceramics: A High-Resolution Scanning Transmission Electron Microscopy Analysis

The reaction layers between silicon carbide continuous fibers (Nicalon) and a calcium aluminosilicate glass-ceramic (anorthite composition) matrix in hot-pressed composites have been characterized both structurally and chemically using high-resolution, field-emission scanning transmission electron microscopy. Chemical compositions at 10-nm spacings, with a resolution of ∼5 nm, were collected across the fiber-matrix interface zone. The reaction sequence in the material is silicon carbide (fiber)—carbon (in this case, graphite)—silica-rich glass—anorthite. The composition of the carbon layer is constant across its width; the interfaces between the four phases are planar. This morphology and the chemical gradients observed are consistent with the simple, 'carbon-condensed' oxidation displacemetn reaction, SiC + O₂→ SiO₂+ C, being responsible for interface phase formation in the composites. The planar interfaces indicate that the rate-limiting process in the interface formation reaction is the diffusion of oxygen through the matrix and silica glass layer; a corollary of this conclusion is that the diffusion of silicon through the carbon layer is a relatively faster process.     

Nanoscale Phase Separation in Lithium Niobium Silicate Glass by Femtosecond Laser Irradiation

Understanding the phase transformation in glass and the morphology of related nanostructure after femtosecond laser irradiation is of great importance for fabricating functional optics, in which glass crystallization is involved to obtain nonlinear optical properties. We report on the crystallization inside lithium niobium silicate glass induced by fs laser irradiation. Energy‐dispersive X‐ray spectroscopy coupled to scanning transmission electron microscopy (STEM/EDS) and transmission electron microscopy confirm a nanoscale phase separation whereby LiNbO₃ crystals are embedded in lamella‐shaped frames of amorphous SiO₂. The obtained nanostructure may have applications in fabricating second‐order nonlinear optical devices.     

叶腊石干法研磨过程中偏叶腊石相的HRTEM研究

采用高分辨透射电镜(HRTEM)对叶腊石研磨的中间产物偏叶腊石相进行晶体结构的研究,并就研磨与热处理对叶腊石晶体结构的影响首次进行了傅里叶变换红外(FTIR)光谱的对比.结果表明:(1)干法研磨过程中,叶腊石研磨的中间相为偏叶腊石,且研磨使得叶腊石的晶体结构出现明显的晶格膨胀,其中研磨作用对叶腊石沿其c轴方向的晶体结构的影响较大.(2)研磨或者热处理条件下,叶腊石的FTIR光谱中3673 cm-1处峰形尖锐的羟基伸缩振动峰的强度随着研磨时间或煅烧温度的增大而逐渐降低,并最终消失.该结论表明热与机械力研磨处理对叶腊石晶体结构的破坏具有相似性.     

Characterization of Phase Separation and Thermal History Effects in Magnesium Silicate Glass Fibers by Nuclear Magnetic Resonance Spectroscopy

Liquid–liquid immiscibility, leading to the separation of silica-rich and silica-poor domains, is a common phenomenon in binary silicate glasses, but can be difficult to detect and characterize when rapid cooling results in nano-scale domain dimensions. ²⁹Si nuclear magnetic resonance (NMR) spectroscopy can be very useful for detecting such phase separation, because the exclusion of paramagnetic impurity ions from the silica-rich regions can greatly slow their spin-lattice relaxation rates. Properly designed experiments can therefore largely isolate the NMR signals from high-silica and low-silica domains, and thus provide information about their proportions, compositions, and short- to intermediate-range structures. We demonstrate this approach here for fiber glasses that are predominantly magnesium, or calcium-magnesium silicates, with minor contents of alumina. For bulk compositions within the known region of stable liquid immiscibility, phase separation occurs even when extremely rapid cooling yields fibers less than 1 μm in mean diameter. Slower cooling increases the extent of separation, while the addition of small amounts of alumina reduces it.     

Critical Particle Size and Phase Transformation in Zirconia: Transmission Electron Microscopy and X-ray Diffraction Studies

A study was undertaken to examine the crystallite size effect on the low-temperature transformation of tettragonal zirconia. Zirconia weas prepared by precipitation from a solution of zirconium tetrachloride by adding ammonium hydroxide to produce a pH of 2.95. Portions of the sample, after drying, were calcined at 500°C for various time intervals. Phase transformation was followed by X-ray diffraction; the data show that the tetragonal phase was initially formed and it was transformed to the monoclinic phase at longer periods of calcination. It was observed by TEM particle size and XRD crystallite size that the transformation does not appear to be due to a critical particle size effect.     

High-Fat Diet Impairs Mouse Median Eminence: A Study by Transmission and Scanning Electron Microscopy Coupled with Raman Spectroscopy

Hypothalamic dysfunction is an initial event following diet-induced obesity, primarily involving areas regulating energy balance such as arcuate nucleus (Arc) and median eminence (ME). To gain insights into the early hypothalamic diet-induced alterations, adult CD1 mice fed a high-fat diet (HFD) for 6 weeks were studied and compared with normo-fed controls. Transmission and scanning electron microscopy and histological staining were employed for morphological studies of the ME, while Raman spectroscopy was applied for the biochemical analysis of the Arc-ME complex. In HFD mice, ME β2-tanycytes, glial cells dedicated to blood-liquor crosstalk, exhibited remarkable ultrastructural anomalies, including altered alignment, reduced junctions, degenerating organelles, and higher content of lipid droplets, lysosomes, and autophagosomes. Degenerating tanycytes also displayed an electron transparent cytoplasm filled with numerous vesicles, and they were surrounded by dilated extracellular spaces extending up to the subependymal layer. Consistently, Raman spectroscopy analysis of the Arc-ME complex revealed higher glycogen, collagen, and lipid bands in HFD mice compared with controls, and there was also a higher band corresponding to the cyanide group in the former compared to the last. Collectively, these data show that ME β₂-tanycytes exhibit early structural and chemical alterations due to HFD and reveal for the first-time hypothalamic cyanide presence following high dietary lipids consumption, which is a novel aspect with potential implications in the field of obesity.     

Study of the Ion‐Irradiation Behavior of Advanced SiC Fibers by Raman Spectroscopy and Transmission Electron Microscopy

6H–SiC single crystals and two types of SiC fibers, Hi‐Nicalon type S and Tyranno SA3, have been irradiated with 4‐MeV Au³⁺ up to 2 × 10¹⁵ cm^?2 (4 dpa) at room temperature, 100°C and 200°C. These fibers are composed of highly faulted 3C–SiC grains and free intergranular C. Stacking fault linear density and grain size estimations yield, respectively, 0.29 nm^?1 and 26–36 nm for the Hi‐Nicalon type S fibers and 0.18 nm^?1 and 141–210 nm for the Tyranno SA3 fibers. Both transmission electron microscopy and surface micro‐Raman spectroscopy reveal the complete amorphization of all the samples when irradiated at room temperature and 100°C and a remaining crystallinity when irradiated at 200°C. The latter observations reveal a multi‐band irradiated layer consisting in a partially amorphized band near the surface and an in‐depth amorphous band. Also, nanocrystalline SiC grains with high stacking fault densities can be found embedded in amorphous SiC at the maximum damage zone of the Hi‐Nicalon type S fibers irradiated at 200°C.     

Influence of Barium Dissolution on the Electrokinetic Properties of Colloidal BaTiO3 in an Aqueous Medium

Hysteresis in the electrokinetic behavior of colloidal hydrothermal BaTiO₃ occurs during sequential acid and base titrations. Ba dissolution during acid titration results in an oxide-rich surface. When the acid-treated BaTiO₃ is titrated back to pH 10, dissolved Ba is specifically adsorbed and/or precipitated onto the particle surface. The combined effects of dissolution and subsequent adsorption–precipitation results in titration hysteresis. Most of the labile Ba can be removed by multiple acid treatments, which result in a TiO₂-like surface layer composition. Barium dissolution increases with decreasing pH but levels off below pH 4 due to diffusion through the surface oxide layer as predicted previously. A phenomenological model is offered to explain the electrokinetic behavior as a function of pH. It is suggested that inherent BaCO₃ contamination is not the primary source of dissolved Ba from hydrothermal BaTiO₃ in acidic solution.     

Chemical and Structural Changes in Manganese-Doped Yttria-Stabilized Zirconia Studied by Electron Energy Loss Spectroscopy Combined with Electron Diffraction

Solid solution of manganese in yttria-stabilized zirconia (YSZ) may occur in the electrolyte of solid oxide fuel cells. Possible changes in valence, coordination, and site occupancy of Mn in YSZ are of interest. Also, subsequent structural modification of the cubic YSZ, as well as the possible ordering of vacancies, has important consequences for the ionic conductivity. Electron energy loss spectroscopy was used to measure the O K and the Mn L edge of Mn in solid solution in a zirconia host lattice. The ratio Mn L ₃/ L ₂was determined for some manganese oxides and for Mn in solid solution. It is shown that the L ₃/ L ₂ratio does not simply reflect the oxidation state of the Mn ions in solid solution. Selected area diffraction experiments were also made in the TEM. This showed ordering of the cations and the anion vacancies at high doping levels. It is concluded that the number of O ions surrounding each Mn ion may be very important in interpreting the obtained L ₃/ L ₂ratio.     

Preparation of Strontium- and Zinc-Doped LaGaO3 Powders via Precipitation in the Presence of Urea and/or Enzyme Urease

Solid oxide fuel cell powders having a composition of La_0.8Sr_0.2Ga_0.8Zn_0.2O_2.8 (LSGZ) were prepared by aqueous chemical precipitation in the presence of decomposing urea, followed by single-step calcination in air. In some synthesis experiments the decomposition of urea was catalyzed by the enzyme urease. The calcination behavior of the precursor powders was studied over the temperature range of 90–1300°C, in an air atmosphere. Characterization of the samples was performed by XRD, TG/DTA, FTIR, FESEM, EDS, and carbon analyses. Two tentative XRD patterns have been created for the hydroxycarbonate precursors and the product LSGZ ceramics, respectively.     

Characterization of β-Silicon Carbide by Silicon-29 Solid-State NMR, Transmission Electron Microscopy, and Powder X-ray Diffraction

Increased interest in ceramic materials, particularly for high-temperature, high-stress applications, has created the need for rapid and reliable analytical techniques to monitor microcrystalline structure of commercial ceramic powders. A comparative evaluation of commercially available β-SiC powders is undertaken to analyze the potential of nuclear magnetic resonance (NMR) in the characterization of β-SiC powder. NMR provides an acceptable, rapid method for characterization of powders both during powder manufacturing as well as for powder analyses priror to sintering studies. The results of transmission electron microscopy and X-ray diffraction are correlated with the NMR spectra to explain some newly observed features in the NMR spectra of β-SiC powders and to illustrate the sensitivity of NMR to microcrystalline disorder.