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  • 1
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The excess carbon of various polysilazane precursors with varying carbon contents was investigated using 13C MAS NMR, Raman and ESR spectroscopies, and microwave conductivity measurements. Microstructure characterization was investigated using TEM. The collected data were compared with those from a previous study on the same precursor. This study focused on the distribution and appearance of the free-carbon phase. Although the spectroscopic techniques showed no differences in the structure of the free-carbon phase, a clear distinction between the various precursors was found using microwave conductivity and high-resolution TEM imaging.
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  • 2
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Two different non-oxide ceramics, Si3N4 and SiC, were characterized with respect to their grain-boundary structure employing both scanning and transmission electron microscopy. The latter method, which enables one to gain direct insight of the atomistic interface structure, was utilized to verify whether grain-boundary wetting occurred. SEM imaging of plasma-etched surfaces revealed a characteristic bright contrast along interfaces for both ceramics, Si3N4 as well as SiC, suggesting the presence of an intergranular glass film. High-resolution TEM studies of the Si3N4 sample confirmed that these fine bright lines along grain boundaries represent intergranular glass films separating Si3N4 matrix grains. However, when high-resolution TEM was employed on SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual glass films was not detected. The SiC materials showed clean grain boundaries with no indication of residual glass even at triple pockets. Chemical analysis monitored yttrium and aluminum segregation at interfaces, which creates a potential barrier (space charges) and therefore affects both the inner mean potential at the interface (Fresnel fringes) and the plasma-etching response. Although SEM imaging showed a similar interface contrast for both Si3N4 and SiC ceramics, HRTEM studies clearly revealed grain-boundary wetting in the former and clean interfaces in the latter material, respectively. Hence, SEM imaging and Fresnel fringe TEM imaging alone are not conclusive when characterizing interface wetting in ceramic polycrystals.
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  • 3
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The grain-boundary structure of a model SiAlON polycrystal with nominal composition Si5AlON7 was characterized by transmission electron microscopy (TEM) both in an equilibrium (as-processed) state at room temperature and after quenching from elevated temperature. In addition, low-frequency (1–13 Hz) internal friction data were recorded as a function of temperature, showing a pronounced grain-boundary sliding peak positioned at 1030°C. High-resolution transmission electron microscopy (HRTEM) of the equilibrated low-temperature microstructure revealed residual glass only at multigrain junctions, but no amorphous intergranular films were observed. The detection of clean interfaces in the as-processed sample contradicts the internal friction data, which instead suggests the presence of a low-viscosity grain boundary phase, sliding at elevated temperatures. Therefore, a thin section of the as-sintered material was heated to 1380°C and rapidly quenched. HRTEM analysis of this sample showed, apart from residual glass pockets, wetted grain boundaries, which is in line with the internal friction experiment. This wetting-dewetting phenomenon observed in z= 1 SiAlON is expected to have a strong impact not only on high-temperature engineering ceramics but also on geological, temperature-activated processes such as volcanic eruptions.
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  • 4
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 84 (2001), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: 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, TH, and methyldiethoxysilane, DH. Si-C-O glass composition depended on the molar ratio of the precursors utilized. At a ratio of TH/DH= 1, the formation of a carbon-rich glass was observed, whereas a ratio of TH/DH= 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 TH/DH= 1 revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400°C, TH/DH= 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, SiCxO2(1-x)→xSiC + (1 - x)SiO2, when annealed at temperatures exceeding 1200°C.
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  • 5
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 84 (2001), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Internal friction experiments were conducted on a model SiC polycrystal prepared from preoxidized (high-purity) SiC powder. This material contained high-purity SiO2 glass at grain boundaries in addition to a free-carbon phase, which was completely removed upon powder preoxidation. Comparative tests were conducted on a SiC polycrystal, obtained from the as-received SiC powder with the addition of 2.5 vol% of high-purity SiO2. This latter SiC material was also investigated after annealing at 1900°C for 3 h in a nitrogen atmosphere. Electron microscopy observations revealed a glass-wetted interface structure in SiC polycrystals prepared from both as-received and preoxidized powders. However, the former material also showed a large fraction of interfaces coated by turbostratic graphite. Upon high-temperature annealing in nitrogen, partial glass dewetting occurred, and voids were systematically observed at multigrain junctions. The actual presence of nitrogen could only be detected in a limited number of wetted interfaces. A common feature in the internal friction behavior of the preoxidized, SiO2-added and nitrogen-annealed SiC was a relaxation peak that resulted from grain-boundary sliding. Frequency-shift analysis revealed markedly different characteristics for this peak: both the magnitude of the intergranular glass viscosity and the activation energy for grain-boundary viscous flow were much higher in the nitrogen-annealed material. Results of torsional creep tests were consistent with these findings, with nitrogen-annealed SiC being the most creep resistant among the tested materials.
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  • 6
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Three high-purity SiAlON materials (Si6−zAlzOzN8−z, z= 1, 2, 3) were characterized with respect to both structure and viscous behavior of internal grain boundaries. Internal friction experiments provided a direct measure of the intrinsic viscosity of grain boundaries and concurrently revealed the occurrence of a grain-boundary interlocking mechanism that suppressed sliding. A residual glass phase (consisting of aluminum-rich SiO2) and nanometer-sized mullite residues were found at glassy triple-grain junctions of the z= 1 SiAlON. A low-melting intergranular phase dominated the high-temperature behavior of this material and caused grain-boundary sliding at temperatures as low as 1100°C. A quantitative analysis of the grain-boundary internal friction peak as a function of oscillation frequency indicated an intergranular film viscosity of log η∼ 7.5 Pa · s at 1100°C. Glass-free grain boundaries were a characteristic of SiAlON materials with z≥ 2, which yielded a significant improvement in refractoriness as compared to the z= 1 SiAlON material. In these materials, relaxation resulting from grain-boundary sliding was suppressed, and the internal friction curve simply experienced an exponential-like increase.
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  • 7
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 82 (1999), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: During tribological testing of hydrogenated amorphous carbon coatings (a-C:H) on silicon carbide ceramics, a friction coefficient of 0.06 was observed under dry sliding conditions, which is thought to represent a lower boundary value for the given experimental setups. Based on chemical and thermal analyses of the coating material, a structural model of the as-deposited amorphous coating was modified taking simplified quantum chemical and statistical arguments into consideration. The tribological behavior of the coating could be rationalized by this modified structure model: Tribological loading leads to a partial release of internal stresses stored in the coating. This process results in an increase of the local mobility of atoms, which can be seen as the driving force for initial structural rearrangements within the coating. Proof for the occurrence of such rearrangements has been provided by electron energy-loss spectroscopy (EELS) measurements, monitoring the local sp3/sp2 hybridization ratio, of the coating before and after tribological tests.
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  • 8
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 80 (1997), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Monolithic mullite with low sintering shrinkage was synthesized from polymer/filler blendsMthat is, siloxane/alumina (siloxane/Al2O3) (and siloxane/aluminum (siloxane/Al)) mixtures. The synthesis was based on a reaction-bonding process of amorphous silica, which formed when the siloxane was oxidized, with Al2O3 (or oxidized aluminum filler) at temperatures 〉1250°C. Thermodynamic calculations were used to calculate the phase composition at equilibrium. Thermoanalytical, infrared-spectroscopic, and microscopic techniques were applied to reveal the microstructural evolution. The corresponding volume changes were used to evaluate the linear shrinkage, based on the quantitative phase assemblage.
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  • 9
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 84 (2001), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: 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.
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  • 10
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Internal friction characterization of the viscosity of a residual SiO2/BaO glass, segregated to grain boundaries of polycrystalline SiC, is presented. The anelastic relaxation peak of internal friction, arising from viscous slip along grain boundaries wetted by a glass phase, is analyzed. Two SiC polycrystals, containing SiO2/BaO glasses with different compositions, are studied and compared with a SiC polycrystal containing only pure SiO2. The internal friction peak is first analyzed with respect to its shift upon frequency change. This analysis allows quantitative assessment of both the intrinsic viscosity and the activation energy for viscous flow of the grain-boundary phase. Both parameters markedly decrease with increasing amounts of BaO dopant, which is consistent with data reported in the literature on SiO2 and SiO2/BaO bulk glasses with the same nominal composition. Analysis of the peak morphology is also attempted, considering the evolution of peak width while varying the grain-boundary glass composition. Moreover, the role of microstructural parameters, such as the distributions of grain size and grain-boundary angles, on the broadening of the internal friction peak is addressed, and a procedure is proposed that allows quantitative evaluation of the activation energy for viscous flow of intergranular glass merely from the width of the internal friction peak.
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