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  • 1
    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 characterization of Si-B-C-N amorphous ceramics using isothermal compression creep testing in the temperature range of 1200°–1500°C is reported. The deformation rate contains a stress-dependent component that is proportional to the applied stress, which indicates that this portion of the deformation mechanism is based on viscous flow. An increase in the creep resistance is observed, following either preliminary annealing or hot isostatic pressing, which may be explained by a reduction of free volume in the amorphous material. The application of two deformation models that are used to predict similar deformation behavior in metallic glasses also is discussed. Although both models accurately predict the time dependence of the deformation rate of precursor-derived amorphous ceramics, the free-volume model fits the observed temperature dependence better than the “two-step” rearrangement model.
    Type of Medium: Electronic Resource
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  • 2
    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 toughness of amorphous precursor-derived ceramics in the Si-C-N system is investigated. Crack-growth data are obtained from DCB specimens, whereas the crack-tip toughness is determined from the crack-tip profile of indentation cracks. For amorphous Si-C-N ceramics that have been fabricated via the powder route, an R-curve effect is observed. The initial values of the rising R-curve are consistent with the estimated crack-tip toughness.
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  • 3
    ISSN: 1573-515X
    Keywords: biogeochemistry of sulfur ; ecosystem analysis ; nutrient cycling ; stable sulfur isotopes ; δ34S ; isotopic fractionation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract In natural ecosystems, differences often exist in the relative abundanceof stable S isotopes (°34S) that can provide clues as tothe source, nature, and cycling of S. Values of °34S inprecipitation, throughfall, soils, soil solution, and stream waters weremeasured at the Hubbard Brook Experimental Forest (HBEF), New Hampshire.Values of °34S in precipitation and throughfall weresimilar to each other but differed seasonally. Precipitation°34S values were higher in the dormant season[°34S = 5.9±0.6‰ (17)][Mean + SE(N)]than in the growing season [°34S = 5.0±0.6‰(40)] but throughfall growing-season values were higher[°34S = 5.6±0.6‰(68)] than for the dormantseason [°34S = 4.9±0.7‰ (9)]. Different treespecies did not affect throughfall °34S values. In soilsolution, °34S values were higher in the growing season(°34S = 8.9±2.8‰; 8.8±1.7‰;and 4.0±0.6‰ for Oa, Bh, and Bs horizons, respectively) thanin the dormant season (°34S = 5.6±1.5‰;3.7±2.4‰; and 3.4±1.2‰ for Oa, Bh, and Bshorizons, respectively). These seasonal differences in°34S were probably caused by biological isotopicfractionation. The °34S values in streams were generally2‰ lower and more variable than those in precipitation andthroughfall, suggesting fractionation and/or different isotopic sources inthe soil.
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  • 4
    ISSN: 1573-515X
    Keywords: base cations ; calcium ; forest ecosystem ; mobile anions ; soil acidification ; surface-water acidification
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract Anion fluxes from a forest soil are usually correlated with those of base cations (BC). Declines in base cation deposition or long-term depletion from the soil may change these relationships. We used multiple regression to identify biogeochemical variables predicting annual volume-weighted concentrations of BC in streamwater draining a forested watershed, and analysis of variance to compare the effects of Ca and Cl inputs on BC fluxes out of soil horizons in irrigated plots. For the watershed, anion concentrations in streamwater predicted BC export most precisely (R2 = 0.84). The best two-variable model (adjusted R2 = 0.91) also included BC concentration in bulk deposition. Consistent with predictions from equations governing exchange chemistry, the proportion of charge contributed by Ca2+ increased with increasing total anion concentration, while that of Na+ decreased. At the plot scale, Cl- concentrations in treatment solutions had a stronger effect (p = 0.06) on BC concentration in Oa-horizon solutions than did Ca2+ concentrations (p = 0.33). In individual horizons of individual plots, BC and total ion concentrations were correlated, but cation composition was not consistent within horizons from different plots. This study detected no evidence of long-term cation depletion in the soils controlling streamwater, but did detect extremely base-poor plots. Because acid deposition affects surface horizons first, streamwater chemistry may not be an adequate way to assess nutrient supply of forest soils.
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  • 5
    ISSN: 1573-515X
    Keywords: base cations ; calcium ; forest ecosystem ; mobile anions ; soil acidification ; surfacewater acidification
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract Anion fluxes from a forest soil are usually correlated with those of base cations (BC). Declines in base cation deposition or long-term depletion from the soil may change these relationships. We used multiple regression to identify biogeochemical variables predicting annual volume-weighted concentrations of BC in streamwater draining a forested watershed, and analysis of variance to compare the effects of Ca and Cl inputs on BC fluxes out of soil horizons in irrigated plots. For the watershed, anion concentrations in streamwater predicted BC export most precisely (R 2=0.84). The best two-variable model (adjustedR 2=0.91) also included BC concentration in bulk deposition. Consistent with predictions from equations governing exchange chemistry, the proportion of charge contributed by Ca2+ increased with increasing total anion concentration, while that of Na+ decreased. At the plot scale, Cl− concentrations in treatment solutions had a stronger effect (p=0.06) on BC concentration in Oa-horizon solutions than did Ca2+ concentrations (p=0.33). In individual horizons of individual plots, BC and total ion concentrations were correlated, but cation composition was not consistent within horizons from different plots. This study detected no evidence of longterm cation depletion in the soils controlling streamwater, but did detect extremely base-poor plots. Because acid deposition affects surface horizons first, streamwater chemistry may not be an adequate way to assess nutrient supply of forest soils.
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