Oxidation of Thin Sheet Reaction-Sintered Silicon Nitride

Oxidation of reaction-sintered silicon nitride was studied in damp air. The formation of "passive" silica films was investigated at 1 atm and 700 to 1100°C and some limited work on weight loss behavior was performed in vacuo of 10⁻⁸ to 10⁻⁵ atm at 1050 to 1200°C. Passive behavior was dominated by reaction in the pore network. Oxidation was extensive at 900 to 1000° but slight at 700 to 800°C. At 1100°C a protective skin limited reaction. Weight loss in vacuo was slight at 1050°C. The vacuum pressure required to suppress the weight loss increased from 4 to 5 × 10⁻⁷ atm at 1050° to 1.5 to 2.5 × 10⁻⁵ atm at 1200°C.     

Computer Simulation of the Microstructure Developed in Reaction-Sintered Silicon Nitride Ceramics

A simulation model of the microstructure developed during the reaction sintering of ultrafine silicon powders is presented. The model employs interactive subroutines that describe particle compaction, sintering, and nitridation. The particle compaction model is based on a random particle packing model, whereas the sintering and reaction sintering models are based on the modified grain model and the sharp interface model (SIM). Microstructural changes due to the competition between chemical reaction and sintering are taken into account in the model. The results predicated by this model show good agreement with experimental data from previous studies.     

High-Resolution Electron Microscopy Investigation of Viscous Flow Creep in a High-Purity Silicon Nitride

The redistribution of intergranular amorphous films during creep deformation of Si₃N₄ has been studied by high-resolution transmission electron microscopy. The film thickness distribution of a high-purity Si₃N₄ material before and after creep was measured. A narrow range of film widths in the uncrept material and a bimodal distribution after creep were observed. These observations provided convincing evidence of the occurrence of viscous flow of intergranular amorphous films during creep deformation of Si₃N₄. Moreover, the creep response predicted by a viscous flow model was in good agreement with experimental data.     

Transmission Electron Microscopy Investigation of the Oxidation of Hot Isostatically Pressed Silicon Nitride with and without Sintering Aids

Analytical transmission electron microscopy of thin-foil cross sections has been used to examine the oxidation behavior of hot isostatically pressed silicon nitride (Si₃N₄) materials. The transmission electron microscopy (TEM) cross sections are prepared by a special technique that provides electron transparency through the entire oxide, interfacial, subscalar, and matrix regions simultaneously. The materials are oxidized in an alumina furnace at 1250°C for 100 h. TEM investigation indicates that oxidation of Si₃N₄ occurs in an oxidation reaction zone that is comprised of the scale, oxide/matrix interface, and subscalar regions; therefore, the silica (SiO₂)/Si₃N₄ interfacial surface area that is available for oxidation is very large. The oxidative attack on the Si₃N₄ grains is not uniform or sequential, and oxygen diffuses into the matrix before the surface grains are consumed. Gas bubbles, probably nitrogen gas, accumulate at all levels of the scale, and no evidence is found for the existence of an "oxynitride" layer. Disintegration of the secondary phase, Y₂Si₂O₇, in the subscalar region is observed to occur, indicating that secondary, oxidation-related phenomena are occurring.     

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.     

Electron Microscopy of Ferrites

This paper describes some electron microscopy studies of magnetic ferrites with special emphasis on LiFe₄O₈ and including some results on NiFe₂O₄ and SrFe₁₂O₁₈, The paper is concerned with analysis of the crystallography, defect structure, and phase transformations and their relations to magnetic properties, viz. coercivity and hysteresis characteristics.     

Scanning Electron Microscopy and Transmission Electron Microscopy Study of Ferroelectric Domains in Doped BaTiO3

This paper describes the results of detailed studies carried out on Ca-doped and Nb-doped BaTiO₃ using scanning electron microscopy and transmission electron microscopy. The techniques used were topographical contrast, selected-area diffraction, and microdiffraction Kikuchi pattern analysis. By these methods it was possible to unambiguously identify the different types of domain boundaries. Also, evidence was obtained for the existence of an unconventional 90% domain boundary which has not previously been reported.     

Thin Films in Electron Microscopy

Thin films play an important role in electron microscopy as they can be used to improve the contrast and stability of specimens, as well as to make specimens electrically conductive. In order to avoid overlapping of specimen and coating structures, it is necessary to understand how thin films are formed in the various coating technologies and how to create them reproducibly as part of the different preparation techniques for electron microscopy. In contrast, electron microscopy can be applied to learn more about the structural details of thin films used, for instance, in the optical coating industry. Heat shock fracturing and Pt-C surface replication of the cross sections resulted in reliable transmission electron micrographs (TEM) of the coating microstructure. These studies demonstrate that, under optimal conditions, it is possible to find a correlation between the measured optical properties and the microstructure of the coatings. TEM replica investigations reveal single events, so they can be useful if discrepancies in the (statistical) physical data have to be investigated.     

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.     

Application of Transmission Electron Microscopy to the Study of Deformation in Ceramic Oxides

Slip systems, dislocation reactions, and dislocation dissociations are reviewed in relation to the crystal structures of ceramic oxides. Techniques of deformation and transmission electron microscopy (TEM) are described. The impact of TEM on the understanding of deformation in oxides is discussed by relating the observed dislocation substructures to the mechanical properties in several oxide systems. Important information can be obtained on dislocation glide and climb processes in relation to work hardening, recovery, and creep. Yielding and the brittle-to-ductile transition temperature are discussed in terms of the magnitude of the Burgers vector in various crystal structures and the influence of dissociation reactions and diffusion. Examples include oxides with close-packed anions such as MgO, MgAl₂O₄ BeO, A1₂O₃, TiO₂, and Mg₂SiO₄, and other oxides such as Cu₂O, UO₂, and SiO₂.     

Application of Electron Microscopy to the Study of Photochemical Reactions in Organic Solids

Currently available electron microscopic techniques are discussed and illustrated with specific reference to some of the unsolved problems in the photochemistry of organic solids. New techniques, such as dynamical electron diffraction, are outlined and some future prospects are assessed.     

Promotional Effect of Iron for the Nitridation of Niobium Oxide to Niobium Nitride

Supported NbN catalysts were prepared under a N₂–H₂ gas stream by the addition of Fe species to Nb/SiO₂. The nitriding process was monitored by in situ XAFS measurements. Fe K-edge XAFS and Nb K-edge in situ XAFS measurements revealed that Fe species in close proximity to niobium oxide can effectively induce nitridation of the niobium oxide with the nitriding rate being dependent upon the dispersion of Fe species. It was concluded that the dispersed Fe species close to niobium effectively assisted the nitridation of niobium oxide into NbN. The nitriding degree can be expressed by the coordination number of Nb–N.     

Electron Microscopy of Sintered Beryllia

Porosity, etching characteristics, and general microstructural features of extruded and sintered beryllium oxide were studied by electron microscope observation of surface replicas. Both pore structures and etching characteristics were shown to be related to the hemimorphic hexagonal crystal structure of BeO. A form of twinning possible only in hemimorphic systems was also observed.     

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.     

照相明胶后处理用硅藻土的电子显微术的研究

硅藻土是当代照相明胶大生产后处理工序中愈来愈广泛使用的一种过滤剂,它的功能是除去明胶中的非胶原蛋白和其它有色有机质。明胶工业生产的实践表明,有的硅藻土过滤性能优良,而有的硅藻土则效果很差。本工作收集了广泛使用的几个典型的硅藻土样品,对比了它们的过滤性能,并用电镜和X射线特征能谱(EDAX)对这些样品的形貌、晶态及组成进行了初步研究,指出了硅藻土的过滤性能的好坏与其形貌、晶态密切相关。     

Study of Crystallization of Glass Coatings on Metal by Electron Microscopy

Controlled growth of crystals and immiscible phases in glass coatings on steel increased their resistance to mechanical and thermal shock. The electron microscope was used to follow crystal and immiscible phase development during heat treatment and to study the effect of crystals on fracture propagation in several complex glass compositions applied to metals by standard enameling techniques. Initial firing temperature, heating rates, heat treatment, and other enamel processes were found to affect the nature of the phases developed in the coatings. Crystals and immiscible phases caused fracture paths to be disrupted, resulting in nonconchoidal types of failure. A correlation is made between resistance to thermal shock and fracture properties.     

直接法测定氮化钒铁中的铁含量

利用先低温预熔,再高温熔融的熔样方法,采用一次沉淀过滤分离法直接检测氮化钒铁中铁含量。结果表明,该方法避免了传统间接差减法操作繁琐、费时费工和结果准确度低的缺点,大幅度提高了检测速度,且测定数据重现性好,准确度高。     

Analytical Electron Microscopy of Heterogeneous Catalysts

Abstract

Conventional transmission electron microscopy (CTEM) is by now a quite well-established technique for the study of heterogeneous catalysts. It is commonly used for the determination of metallic particles shapes and size distribution on various types of carriers [1, 2]. In addition, dark field imaging and diffraction measurements (in selected area or microdiffraction mode) allows advantage to be taken of the diffraction of the electrons in order to identify the active phase on the basis of its crystal structure [3, 4].     

Scanning Electron Microscopy of Fracture Surfaces

Scanning electron microscopy has supplied information concerning the mechanism of failure of aluminium/epoxide joints: (1) evidence for a critical concentration of A-1100 silane that inhibits stress corrosion cracking; (2) evidence of plastic deformation at a crack tip; (3) observation of aluminum corrosion products.     

Electron Microscopy of Etched Plagiodase Feldspar

Single crystals of plagiodase feldspar were selectively etched by immersion in aqueous solutions of Na₂CO₃ and K₂CO₃ for 48 hr and in 48% HF for 1 to 3 sec. The etched surfaces are replicated using a two-stage Formvar-carbon replica shadowed with palladium. These replicas reveal that etching varies in different twin lamellas and that etch pits align along twin boundaries.