Phase Separation in an SiCO Glass Studied by Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy

SiCO glasses prepared from sol–gel precursors via pyrolysis in argon at temperatures ranging from 1000° to 1400°C were studied by transmission electron microscopy (TEM), in conjunction with electron energy-loss spectroscopy (EELS). EELS analysis showed that stoichiometric SiCO glass underwent phase separation, forming SiO₂- and SiC-based environments. This process started at ∼1200°C. However, at temperatures >1300°C, precipitation of nanometer-sized SiC particles embedded in vitreous SiO₂ was monitored by high-resolution TEM.     

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 of Thin Glass Samples

Mechanical thinning and fracturing techniques for preparing thin glass samples for direct-transmission electron microscopy are discussed. A modification of the Doherty and Leombruno procedure for mechanically thinning ceramic materials is described. These techniques make possible more reliable electron microscope studies of fine-scale submicrostructure in glass systems. Electron microscope observations on fused silica, an alkali borosilicate glass, and some binary silicate glasses are reported and discussed in terms of present understanding of glass structure.     

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.     

Transmission Electron Microscopy of Glass-Ceramics

A new technique has been developed by which thin sections of glass-ceramics can be prepared for direct observation by transmission electron microscopy. Techniques for identifying crystalline phases are discussed, and the degree of crystallinity in several glass-ceramics is presented.     

Application of Electron Energy-Loss Spectroscopy to the Study of Solid Catalysts

The addition of an electron spectrometer to a transmission electron microscope provides electron energy-loss spectra and energy-filtered images, from which thickness and chemical information can be extracted. This article reviews the techniques and instrumentation required, together with recent applications involving heterogeneous catalysts.     

Phase Transformation Sequence from Kaolinite to Mullite Investigated by an Energy-Filtering Transmission Electron Microscope

The phase transformation from kaolinite to mullite was examined with new electron diffraction data obtained using an energy-filtering transmission electron microscope. Kaolinite was finally transformed to mullite and cristobalite through several steps of endothermic and exothermic reactions, which resulted in metakaolinite, a microcrystalline spinel-type phase and amorphous silica. Metakaolinite maintained its short-range order even at 920°C. The spinel-type phase resulted from a topotactic transformation of metakaolinite. Mullite first appeared at around 940°C, showing no clear crystallographic relationships to the parent metakaolinite structure. It coexisted with metakaolinite and the spinel-type phase produced previously. The first strong exothermic peak on the DTA curve was mainly due to the extraction of amorphous silica from metakaolinite and the gradual nucleation of the mullite phase. The initially formed spinel-type and mullite phases were suggested to be Al-rich, but to progressively gain Si in their structures at higher temperatures. The spinel-type phase decomposed completely through a second weak exothermic reaction, promoting the crystallization of cristobalite from amorphous silica, and the growth of mullite.     

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.     

Preparation of Thin Sections of Coated Fibers for Characterization by Transmission Electron Microscopy

A method of preparing transmission electron microscopy (TEM) specimens of coated ceramic fibers has been developed, which produces large electron transparent areas due to the minimal preferential milling of the fiber, coating, or epoxy matrix. Multiple individual fibers or tows are impregnated with a high-temperature epoxy and contained to assure a high fiber-to-epoxy volume ratio. The samples are then sectioned and mechanically thinned either parallel or normal to the fiber axes using a wedge polisher on diamond lapping films to achieve a thickness of less than 5 μ.m. The thinned sample is then ion-milled to electron transparency in less than 30 min, giving representative specimens of the coating, fiber, and coating-fiber interface. This technique is also well suited to preparing extremely flat specimens for scanning electron microscopy analysis of thin coatings. Examples of TEM sections of coated fibers prepared using this technique are presented.     

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.     

Growth of Oxide Layers on Thin Aluminum Nitride Samples Measured by Electron Energy-Loss Spectroscopy

AlN ceramics with different amounts of oxygen impurities were investigated by electron energy-loss Spectroscopy (EELS). Because of the high dynamics of EEL spectra, a method was developed to record partial spectra and then to join them together to form a complete spectrum. The data obtained from EEL spectra were the nitrogen/oxygen concentration ratio, sample thickness, and energy-loss nearedge structures (ELNES). Because of spontaneous formation of an oxide layer on AlN samples immediately after ion milling, a method had to be developed which yielded the oxide layer thickness and the bulk oxygen content. The growth kinetics of the oxide layer were investigated by exposing the AIN samples at room temperature to air and to water for various times. From these measurements a logarithmic rate law for the oxidation of AlN at room temperature was obtained.     

An Introduction to Energy-Filtered Transmission Electron Microscopy

This article introduces the topic of energy-filtered transmission electron microscopy (EFTEM). It reviews the technique combining theory with a number of applications from materials science to highlight the progress made in the subject. Examples of EFTEM of catalysts are also reviewed with a discussion of how the technique could be used to study many more catalyst structures in the future.     

Structural Implications of the Electron Microscopy of Glass Surfaces

Electron microscopy of fractured and polished surfaces of commercial silicate glasses by the preshadowed carbon replica technique reveals the presence of inhomogeneities in the glass ranging in size from 30 to 150 a.u. in diameter. Experiments were carried out to prove that the observed features resulted neither from the experimental technique used nor from the reaction of the glasses with atmospheric moisture. It is probable that these inhomogeneities are related to the bulk structure of the glass.     

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.