China Science

The article describes and analyses the conflicts over land, authority and natural resources among the non-governmental organisations (NGOs) and the ethnic groups in West Caprivi in Namibia. It also offers information on the tribalism, tradition and ethnicity in South Africa, and also presents and analysis on the multi-layered political nature of identities. The ethnic groups, namely Khwe and Mbukushu, were also discussed, as well as their contested authority in relation to the state and how the people of each group were constructed.

Taylor, Julie J.1
[1]Department of International Development, University of Oxford,

Unlike normal chemical bonds, hydrogen bonds (H-bonds) characteristically feature binding energies and contact distances that do not simply depend on the donor (D) and acceptor (:A) nature. Instead, their chemical context can lead to large variations even for a same donor?acceptor couple. As a striking example, the weak HO?H···OH2 bond in neutral water changes, in acidic or basic medium, to the 6-fold stronger and 15% shorter [H2O···H···OH2]+ or [HO···H···OH]? bonds. This surprising behavior, sometimes called the H-bond puzzle, practically prevents prediction of H-bond strengths from the properties of the interacting molecules. Explaining this puzzle has been the main research interest of our laboratory in the last 20 years. Our first contribution was the proposal of RAHB (resonance-assisted H-bond), a new type of strong H-bond where donor and acceptor are linked by a short ?-conjugated fragment. The RAHB discovery prompted new studies on strong H-bonds, finally leading to a general H-bond classification in six classes, called the six chemical leitmotifs, four of which include all known types of strong bonds. These studies attested to the covalent nature of the strong H-bond showing, by a formal valence-bond treatment, that weak H-bonds are basically electrostatic while stronger ones are mixtures of electrostatic and covalent contributions. The covalent component gradually increases as the difference of donor?acceptor proton affinities, ?PA, or acidic constants, ?pKa, approaches zero. At this limit, the strong and symmetrical D···H···A bonds formed can be viewed as true three-center-four-electron covalent bonds. These results emphasize the role PA/pKa equalization plays in strengthening the H-bond, a hypothesis often invoked in the past but never fully verified. In this Account, this hypothesis is reconsidered by using a new instrument, the pKa slide rule, a bar chart that reports in separate scales the pKa’s of the D?H proton donors and :A proton acceptors most frequently involved in D?H···:A bond formation. Allowing the two scales to shift so to bring selected donor and acceptor molecules into coincidence, the ruler permits graphical evaluation of ?pKa and then empirical appreciation of the D?H···:A bond strength according to the pKa equalization principle. Reliability of pKa slide rule predictions has been verified by extensive comparison with two classical sources of H-bond strengths: (i) the gas-phase dissociation enthalpies of charged [X···H···X]? and [X···H···X]+ bonds derived from the thermodynamic NIST Database and (ii) the geometries of more than 9500 H-bonds retrieved from the Cambridge Structural Database. The results attest that the pKa slide rule provides a reliable solution for the long-standing problem of H-bond-strength prediction and represents an efficient and practical tool for making such predictions directly accessible to all scientists.

Paola Gilli?Loretta Pretto?Valerio Bertolasi?Gastone Gilli
Dipartimento di Chimica and Centro di Strutturistica Diffrattometrica, Universitdi Ferrara, I-44100 Ferrara, Italy

The purpose of this study was to examine the suitability of polystyrene-coated (PS-coated) microcapsules of drug-resin complex for achieving prolonged release of diltiazem-HCl, a highly water-soluble drug, in simulated gastric; intestinal fluid. The drug was bound to Indion 254, a cation-exchange resin,; the resulting resinate was microencapsulated with PS using an oil-in-water emulsion-solvent evaporation method. The effect of various formulation parameters on the characteristics of the microcapsules was studied. Mean diameter; encapsulation efficiency of the microcapsules rose with an increase in the concentration of emulsion stabilizer; the coat/core ratio, while the same characteristics tended to decrease with an increase in the volume of the organic disperse phase. The desorption of drug from the uncoated resinate was quite rapid; independent of the pH of the dissolution media. On the other hand, the drug release from the microcapsules was prolonged for different periods of time depending on the formulation parameters; was also found to be independent of the pH of the dissolution media. Both the encapsulation efficiency; the retardation of drug release were found to be dependent on the uniformity of coating, which in turn was influenced by the formulation parameters. Kinetic studies revealed that the desorption of drug from the resinate obeyed the typical particle diffusion process, whereas the drug release from the microencapsulated resinate followed the diffusion-controlled model in accordance with the Higuchi equation. PS appeared to be a suitable polymer to provide prolonged release of diltiazem independent of the pH of the dissolution media.

Arindam Halder1?Biswanath Sa1Email:biswanathsa2003@yahoo.com
[1] Centre for Advanced Research in Pharmaceutical Sciences, Department of Pharmaceutical Technology, Jadavpur University, 700 032 Kolkata, India

The transformation of Al3Zr (L12) and Al3(Zr1?xTix) (L12) precipitates to their respective equilibrium D023 structures is investigated in conventionally solidified Al–0.1Zr and Al–0.1Zr–0.1Ti (at.%) alloys aged isothermally at 500 °C or aged isochronally in the range 300–600 °C. Titanium additions delay neither coarsening of the metastable L12 precipitates nor their transformation to the D023 structure. Both alloys overage at the same rate at or above 500 °C, during which spheroidal L12 precipitates transform to disk-shaped D023 precipitates at ca. 200 nm in diameter and 50 nm in thickness, exhibiting a cube-on-cube orientation relationship with the ?-Al matrix. The transformation occurs heterogeneously on dislocations because of a large lattice parameter mismatch of the D023 phase with ?-Al. The transformation is very sluggish and even at 575 °C coherent L12 precipitates can remain untransformed. Mechanisms of microstructural coarsening and strengthening are discussed with respect to the micrometer-scale dendritic distribution of precipitates.

Keith E. KniplingaEmail:knipling@anvil.nrl.navy.mil?David C. Dunandb?David N. Seidmanb
[a]Naval Research Laboratory, Code 6356, 4555 Overlook Avenue, SW, Washington, DC 20375-5320, USA;[b]Department of Materials Science; Engineering, Northwestern University, 2220 Campus Dr., Evanston, IL 60208-3108, USA;[c]Northwestern University Center for Atom-Probe, Tomography (NUCAPT), Northwestern University, 2220 Campus Dr., Evanston, IL 60208-3108, USA

The fabrication of C/Si–B–C–N fiber–reinforced ceramic matrix composites (FRCs) is reported. The processing basically involves three individual steps: (i) vacuum-assisted infiltration of a liquid two-component Si–B–C–N precursor into stacked woven carbon fiber fabrics, (ii) stepwise thermal cross-linking and (iii) thermal transformation of the polymer matrix composite into a ceramic matrix composite. Repeated polymer infiltration/pyrolysis cycles resulted in FRCs with 89% relative density. The average bending strength of the FRCs was 255 MPa and considerable fiber pull-out was observed throughout the fractured surfaces. The composite retained its strengths upon bending even at 1500 °C in argon and there was no evidence for brittle fracture. The flexural creep strain of the composite at 1400 °C was 0.55%.

See Hoon Lee aEmail:LEE.Seahoon@nims.go.jp?Markus Weinmann1?Fritz Aldingera
[a]Max-Planck-Institut für Metallforschung, Institut für Nichtmetallische Anorganische Materialien, Universitt Stuttgart Pulvermetallurgisches Laboratorium, Heisenbergstr. 3, 70569 Stuttgart, Germany

Analytical transmission electron microscopy was used to characterize precipitation processes in a model Al–4.0Cu–0.3Mg (wt.%) alloy aged at 200 °C. The evolution of microstructure was more complex than previously reported and involved processes common both to the binary Al–Cu and ternary Al–Cu–Mg systems. We report precipitation of a novel orientation of the common intermediate phase ??, which we have designated . Isomorphous with the well-known body-centred tetragonal Al2Cu phase that occurs in Al–Cu-based alloys, we observed the following novel orientation: , . A trace of Mg was associated with the orientation and we suggest that this is a consequence of the role that Mg atom clusters play during nucleation. For the first time, we also report the occurrence of the ? phase (Al5Cu6Mg2) in a new orientation analogous to that of , designated as and . These results are discussed in terms of microstructural design for age hardening in Al–Cu–Mg base alloys, much of which involves the ?? and ? precipitate phases.

The Ghadames Basin contains important oil- and gas-producing reservoirs distributed across Algeria, Tunisia, and Libya. Regional two-dimensional (2-D) modeling, using data from more than 30 wells, has been undertaken to assess the timing and distribution of hydrocarbon generation in the basin. Four potential petroleum systems have been identified: (1) a Middle-Upper Devonian (Frasnian) and Triassic (Triassic Argilo Greseux Inferieur [TAG-I]) system in the central-western basin; (2) a Lower Silurian (Tannezuft) and Triassic (TAG-I) system to the far west; (3) a Lower Silurian (Tannezuft) and Upper Silurian (Acacus) system in the eastern and northeastern margins; and (4) a Lower Silurian (Tanezzuft) and Middle-Upper Devonian (Frasnian) system to the east-southeast. The Lower Silurian Tanezzuft source rock underwent two main phases of hydrocarbon generation. The first phase occurred during the Carboniferous, and the second started during the Cretaceous, generating most hydrocarbons in the eastern (Libyan) basin. The Frasnian shales underwent an initial, minor generative phase in the central depression during the Carboniferous. However, the main generation occurred during the Late Jurassic-Cenozoic in the western and central depression. The Frasnian shales are currently only marginally mature in the eastern part of the basin. Modeling indicates that the Alpine (Eocene) exhumation of the eastern (Libyan) basin margin had a significant control on the timing of hydrocarbon generation from the Lower Silurian source rock. The preferred burial-history model calibrates source rock maturity data by incorporating late exhumation and reduced subsidence prior to the Hercynian (Carboniferous) orogeny. As a result, the Tannezuft shales preserve their generative potential into the Mesozoic-Cenozoic, with renewed hydrocarbon generation during subsequent reburial, which can migrate to post-Hercynian (Carboniferous] traps, hence favoring the preservation of hydrocarbon accumulations.

Ruth Underdown?Jonathan Redfern
North Africa Research Group, School of Earth, Atmospheric and environmentalSciences, University of Manchester, Oxford Road, Manchester M13 9PL, United kingdom

In addition to seismically mapped fault structures, a large number of faults below the limit of seismic resolution contribute to subsurface deformation. However, a correlation between large- and small-scale faults is difficult because of their strong variation in orientation. A workflow to analyze deformation over different scales is described here. Based on the combination of seismic interpretation, coherency analysis, geostatistical analysis, kinematic modeling, and well data analysis, we constrained the density and orientation of sub-seismic faults and made predictions about reactivation and opening of fractures. We interpreted faults in seismic and coherency volumes at scales between several kilometers and a few tens of meters. Three-dimensional (3-D) retrodeformation was performed on a detailed interpreted 3-D structural model to simulate strain in the hanging wall at the time of faulting, at a scale below seismic resolution. The modeling results show that (1) considerable strain is observed more than 1 km (0.62 mi) away from the fault trace and (2) deformation around the fault causes strain variations, depending on the fault morphology. This strain variation is responsible tor tne heteroge-neous subseismic fracture distribution observed in wells. We linked the fracture density from the well data with the modeled strain magnitude and used the strain magnitude as a proxy for fracture density. With this method, we can predict the relative density of small-scale fractures in areas without well data. Furthermore, knowing the orientation of the local strain axis, we predict a fault strike and opening or reactivation of fractures during a particular deformation event.

Tina Lohr?Charlotte M. Krawczyk?David C. Tanner
GeoForschungsZentrum Potsdam, Sektion 3.1, Telegrafenberg, 14473 Potsdam, Germany; Leibniz Institute for Applied Geosciences, Stilleweg 2, 30655 Hannover, Germany; Geoscience Center of Goettingen University, Goldschmidtstr. 3, 37077 Goettingen, Germany