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  • 2000-2004  (34)
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  • 1
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 87 (2004), 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 ultrafine, undoped ZrO2 powders with median primary particle sizes of 9 and 25 nm were used to prepare ceramic suspensions for thermoplastic extrusion. The organic vehicle consisted of an industrial-grade poly(ethylene-co-vinyl acetate) (EVA) or polyethylene (PE-HD) and decanoic acid as a dispersing agent. The powder volume loadings achieved were 44% and 52% by volume for the two powders, respectively. The amount of dispersant needed was calculated from a new model based on available chemisorption sites on the powder surface. Mixing and extrusion were conducted using a conventional modular plastic processing unit. Green bodies were dewaxed up to 450°C in an inert atmosphere and sintered to full density in air at 1060° and 1100°C, respectively. Analysis of the ceramic phase content and the microstructure of the bodies is presented.
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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: The chemical stability of an amorphous silicon carbonitride ceramic, having the composition 0.57SiC·0.43Si3N4·0.49C is studied as a function of nitrogen overpressure at 1873 K. The ceramic suffers a weight loss at pN2 〈 3.5 bar (1 bar = 100 kPa), does not show a weight change from 3.5 to 11 bar, and gains weight above 11 bar. The structure of the ceramic changes with pressure: it is crystalline from 1 to 6 bar, amorphous at ∼10 bar, and is crystalline above ∼10 bar. The weight-loss transition, at 3.5 bar, is in excellent agreement with the prediction from thermodynamic analysis when the activities of carbon, SiC, and Si3N4 are set equal to those of the crystalline forms; this implies that the material crystallizes before decomposition. The amorphous to crystalline transition that occurs at ∼10 bar, and which is accompanied by weight gain, is likely to have taken place by a different mechanism. A nucleation and growth reaction with the atmospheric nitrogen is proposed as the likely mechanism. The supersaturation required to nucleate α-Si3N4 crystals is calculated to be 30 kJ/mol.
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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 bulk modulus B0= 290(5) GPa and its first pressure derivative B′0= 4.9(6) were obtained for c-Si3N4 from volume versus pressure dependence. Measurements were performed under quasi-hydrostatic conditions in a diamond anvil cell to 53 GPa using synchrotron radiation and energy dispersive X-ray powder diffraction. This combined with nanoindentation measurements determined the shear modulus G0 of c-Si3N4 to be 148(16) GPa. The Vickers microhardness HV(0.5) for dense, oxygen-free c-Si3N4 was estimated to be between 30 and 43 GPa. Both the elastic moduli and microhardness of c-Si3N4 exceed those of the hexagonal counterparts, α- and β-phases.
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  • 4
    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: Aluminum nitride (AlN) powders were prepared from the oxide precursors aluminum nitrate, aluminum hydroxide, aluminum 2-ethyl-hexanoate, and aluminum isopropoxide (i.e., Al(NO3)3, Al(OH)3, Al(OH)(O2CCH(C2H5)(C4H9))2, and Al(OCH(CH3)2)3). Pyrolyses were performed in flowing dry NH3 and N2 at 1000°–1500°C. For comparison, the nitride precursors aluminum dimethylamide (Al(N(CH3)2)3) and aluminum trimethylamino alane (AlH3·N(CH3)3) were exposed to the same nitridation conditions. Products were investigated using XRD, TEM, EDX, SEM, and elemental analysis. The results showed that nitridation was primarily controlled by the water:ammonia ratio in the atmosphere. Single-phase AlN powders were obtained from all oxide precursors. Complete nitridation was not obtained using pure N2, even for the non-oxide precursors.
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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: Nanoindentation has revealed significant scatter of the microscale hardness and elastic modulus of an amorphous SiCN ceramic, because of structural inhomogeneities (nanopores and clusters of free carbon within the material). As a consequence of the common feature, an amorphous nature, SiCN, in regard to its mechanical properties, resembles SiO2 glass more than SiC or Si3N4. However, because of the stronger Si—C and Si—N covalent bonding, SiCN is harder and stiffer than SiO2. The mean hardness—13 ± 2 GPa, measured at a load of 250 mN for SiCN—is approximately half that of polycrystalline Si3N4 (24.9 ± 0.6 GPa) but higher than that of SiO2 glass (8.9 ± 0.04 GPa). The elastic modulus of the SiCN, measured at a load of 250 mN, is 121 ± 10 GPa.
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  • 6
    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: Three novel Si-C-B-N ceramic compositions, namely Si2.9B1.0C14N2.9, Si3.9B1.0C11N3.2 and Si5.3B1.0C19N3.4, were synthesized using the polymer-to-ceramic transformation of the polyorganoborosilazanes [B(C2H4Si(Ph)NH)3]n, [B(C2H4Si(CH3)NH)2–(C2H4Si(CH3)N(SiH2Ph))]n, and [B(C2H4Si(CH3)–N(SiH2Ph))3]n, where Ph is phenyl (C6H5), at 1050°C in argon. The Si-B-C-N ceramics exhibited significant stability with respect to composition and mass change in the temperature range between 1000° and 2200°C, including isothermal annealing of the samples at the final temperature for 30 min in argon. The mass loss rate at 2200°C was as low as 1.4 wt%·h−1 for Si5.3B1.0C19N3.4, 1.7 wt%·h−1 for Si2.9B1.0C14N2.9, and 2.4 wt%·h−1 for Si3.9B1.0C11N3.2. The measured amount of mass loss rate was comparable to that of pure SiC materials. As crystalline phases, β-Si3N4 and β-SiC were found exclusively in the samples annealed at 2200°C at 0.1 MPa in argon. For thermodynamic reasons, β-Si3N4 should have decomposed into the elements silicon and nitrogen at that particular temperature and gas pressure. However, the presence of β-Si3N4 in our materials indicated that carbon and boron kinetically stabilized the Si3N4-based composition.
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  • 7
    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: In this article we present the synthesis of three-dimensionally ordered ternary BC2N crystals produced under high-temperature and high-pressure conditions. Unit cell dimensions are a= 2.4820 ± 0.0007 Å and c= 6.620 ± 0.003 Å. The composition BC2N is derived from Vegard's law and quantitative electron energy loss spectroscopy. The ordered arrangement of atoms in the unit cell is discussed.
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  • 8
    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
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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: The crystallization behavior of organometallic-precursor-derived amorphous Si-C-N ceramics was investigated under N2 atmosphere using X-ray diffractometry (XRD), transmission electron microscopy (TEM), and solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy. Amorphous Si-C-N ceramics with a C/Si atomic ratio in the range of 0.34–1.13 were prepared using polycarbosilane-polysilazane blends, single-source polysilazanes, and single-source polysilylcarbodiimides. The XRD study indicated that the crystallization temperature of Si3N4 increased consistently with the C/Si atomic ratio and reached 1500°C at C/Si atomic ratios ranging from 0.53 to 1.13. This temperature was 300°C higher than that of the carbon-free amorphous Si-N material. In contrast, the SiC crystallization temperature showed no clear relation with the C/Si atomic ratio. The TEM and NMR analyses revealed that the crystallization of amorphous Si-C-N was governed by carbon content, chemical homogeneity, and molecular structure of the amorphous Si-C-N network.
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  • 10
    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: The structure and electronic transport properties of polymer-derived pristine and boron-doped silicon carbonitride ceramics have been studied, with particular emphasis on understanding the effect of annealing treatments. Structural analysis using the radial distribution function formalism showed that the local structure is comprised of basic building blocks of Si tetrahedra with B, C, and N at the corners. Comparison of the electrical properties of pristine and boron-doped ceramics shows that boron doping leads to enhanced p-type conductivity, with a small positive thermopower. The postpyrolysis annealing treatments at elevated temperatures have a significant effect on the conductivity. The conductivity variation with temperature for these ceramics shows Mott's variable range hopping (VRH) behavior, characteristic of a highly defective semiconductor.
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