1,087 research outputs found

    Grain boundary partitioning of Ar and He

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    An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the intergranular medium (ITM) that surrounds them in synthetic melt-free polycrystalline diopside aggregates. ^(37)Ar and ^(4)He are introduced into the samples via neutron irradiation. As samples are crystallized under sub-solidus conditions from a pure diopside glass in a piston cylinder apparatus, noble gases diffusively equilibrate between the evolving crystal and intergranular reservoirs. After equilibration, ITM Ar and He is distinguished from that incorporated within the crystals by means of step heating analysis. An apparent equilibrium state (i.e., constant partitioning) is reached after about 20 h in the 1450 °C experiments. Data for longer durations show a systematic trend of decreasing ITM Ar (and He) with decreasing grain boundary (GB) interfacial area as would be predicted for partitioning controlled by the network of planar grain boundaries (as opposed to ITM gases distributed in discrete micro-bubbles or melt). These data yield values of GB-area-normalized partitioning, K¯^(Ar)_(ITM), with units of (Ar/m^3 of solid)/(Ar/m^2 of GB) of 6.8 x 10^3 – 2.4 x 104 m^(-1). Combined petrographic microscope, SEM, and limited TEM observation showed no evidence that a residual glass phase or grain boundary micro-bubbles dominated the ITM, though they may represent minor components. If a nominal GB thickness (δ) is assumed, and if the density of crystals and the grain boundaries are assumed equal, then a true grain boundary partition coefficient (K^(Ar)_(GB) = X^(Ar)_(crystals)/X^(Ar)_(GB) may be determined. For reasonable values of δ, K^(Ar)_(GB) is at least an order of magnitude lower than the Ar partition coefficient between diopside and melt. Helium partitioning data provide a less robust constraint with K¯^(He)_(ITM) between 4 x 10^3 and 4 x 10^4 cm^(-1), similar to the Ar partitioning data. These data suggest that an ITM consisting of nominally melt free, bubble free, tight grain boundaries can constitute a significant but not infinite reservoir, and therefore bulk transport pathway, for noble gases in fine grained portions of the crust and mantle where aqueous or melt fluids are non-wetting and of very low abundance (i.e., <0.1% fluid). Heterogeneities in grain size within dry equilibrated systems will correspond to significant differences in bulk rock noble gas content

    The preparation, characterization and hydrolysis of epoxidized natural rubber

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    The double bonds in NR can be partially epoxidized using peracids and mild experimental conditions. More forcing conditions result in hydrolysis and the ring-opening of epoxy groups. The epoxidation of NR latex concentrates using performic acid generated in situ from hydrogen peroxide and formic acid has been studied intensively; variability in the reaction is consistent with the presence of adventitious iron and the intervention of Fenton's Reagent. The determination of epoxy groups in ENR by a range of analytical techniques has been examined and conflicting results in the literature have been resolved. The fractionation of ENR by molecular mass and particle size in latices has yielded further evidence for epoxidation being a random chemical reaction. However, there is conflicting chemical evidence for a non-random process. The acid-catalysed hydrolysis of ENR results in ring-opening and the preferential formation of tetrahydrofuran rings and/or ether crosslinks. The products of hydrolysis are determined by the concentration of epoxy groups and acid and the activity of counter-ions. Intermolecular ether bonds in ENR have been characterized. The formation of crosslinks can be monitored by 13C-NMR spectroscopy via the concentration of pendant alcohol groups. Model studies have demonstrated that ring-expansion results in the exclusive formation of tetrahydrofuran rings. Cis- and trans-epoxy groups are formed during the uninhibited autoxidation of NR and ENR in an apparent trans:cis ratio of 1.4:1.0. This result is consistent with the exclusive formation of epoxy groups by the addition of peroxyl radicals to double bonds

    Thirteen Stories

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    The Apogee of the Commodity

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    Slavery is death. The body of this death is white-over-black, white-over-black only, and that continually. The body of this death is eternal and therefore with us still. Slavery is white-over-black, segregation is white-over-black, neosegregation is white-over-black, and all of it is death. White-over-black is the death that it is the slave’s calling to produce. The slave produces this death through its juridical prayers for equality of right. The slave perfects its own slavery in this manner. Rights cannot be equal. There are always ambiguities. The ambiguities are always available for a white-over-black reading. The fact of white-over-black, of the general habit of reading things in white-over-black ways, which is what prompts the slave to pray for equality of right in the first place, shows that whatever is granted will be read in the general way, white-over-black. The fact of white-over-black shows the way that we have been trained. We follow our training into our future and so rights are white-over-black, white-over-black only, and that continually. The dream of equal rights, so important to slaves, results only in the continued fact of white-over-black. If the dream is the disguise of the wish then the dream of equal rights is the disguise of the slave’s own desire for white-over-black, for death. The slave’s unacknowledgable death drive is the secret of the commodity and its fetish. Contra Marx, commodities do speak. Commodities speak of equality of right, equality of right only, and that continually
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