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71.
The radionuclide phosphorus-32 has been usud to measure the effects of (I) temperature, (2) time of standing, (3) stirring, (4) the molybdate-phosphate ratio, (5) hydrochloric, sulphuric, nitric and perchloric acids, and (6) ammonium nitrate on the efficiency of precipitation of ammonium l 2-molybdophosphate,The precipitate is formed quantitatively after 30 min at any temperature between 50° and 80° followed by 30 min at room temperature' provided that the liquid is stirred at 15-min intervals. Twice the stoichiometric amount of nitromolybdate reagent is sufficient for quantitative precipitation. Nitric add is essential ; perchloric acid does not interfere, but hydrochloric and sulphuric acids do. Ammonium nitrate does not affect the efficiency of the precipitation, 相似文献
72.
In start-up of steady shearing flow of two viscous unentangled liquids, namely low-molecular-weight polystyrene and -D-glucose, the shear stress catastrophically collapses if the shear rate is raised above a value corresponding to a critical initial shear stress of around 0.1–0.3 MPa. The time dependence of the shear stress during this process is similar for the two liquids, but visualization of samples in situ and after quenching reveals significant differences. For -D-glucose, the stress collapse evidently results from debonding of the sample from the rheometer tool, while in polystyrene, bubbles open up within the sample, as occurs in cavitation. Some similarities are pointed out between these phenomena and that of lubrication failure reported in the tribology literature. 相似文献
73.
M. A. Riley D. E. Archer J. Simpson T. B. Brown J. Döring J. W. Holcomb G. D. Johns T. D. Johnson T. Petters J. Pfohl S. L. Tabor O. N. Tekyi-Mensah P. C. Womble V. A. Wood 《Zeitschrift für Physik A Hadrons and Nuclei》1993,346(4):319-320
The [402]5/2 bands in177Ta which are identical to the neighboring even-even176Hf groundstate band have been extended to higher angular momentum. These bands in the two nuclei are seen to diverge from each other in the region of the first i13/2 neutron alignment. The lower observed crossing frequency for the [402]5/2 bands indicates a lower deformation for these bands compared to176Hf. Extensions to theh
9/2 [541]1/2 yrast band are also reported.Special thanks to D.C. Radford, It. MacLeod, W.T. Milner and R. Books for their invaluable software support. Thanks also to F.K. McGowan and R. Darlington for their help with the targets. This work was supported by the National Science Foundation and the State of Florida and by the UK Science and Engineering Research Council. MAR and JS acknowledge the receipt of a NATO Collaborative Research Grant. 相似文献
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76.
D. Vickers L.A. Archer T. Floyd-Smith 《Colloids and surfaces. A, Physicochemical and engineering aspects》2009,348(1-3):39-44
Narrow size distribution cubic Co3O4 nanoparticles were synthesized and rheological properties of suspensions of the cubes in oligomeric polyethylene glycol (PEG) were explored over a range of particle volume fractions and rotational shear flow conditions. At low and high particle volume fractions, the relative viscosity of the suspensions is described by a Krieger–Dougherty formula with an intrinsic viscosity consistent with expectations for suspensions of ideal cuboids. At intermediate to high particle loadings, the suspensions manifest complex rheological behavior, including shear thinning and shear-thickening features. These observations are discussed in terms of the charge carried by the cubes and the shear rate/volume fraction dependency of the transition from shear thinning to shear thickening. 相似文献
77.
Rose M. Pringle Katie Milton Brkich Thomasenia Lott Adams Cirecie West‐Olatunii Diane A. Archer‐Banks 《School science and mathematics》2012,112(4):217-229
Despite recent progress toward gender equity in science and mathematics education, the underachievement of low‐income African American girls remains a challenge when compared with their white counterparts. Furthermore, the causes of this persistent underachievement have not been explored thoroughly. We have initiated a three‐year longitudinal study of how African American girls position themselves in relation to science and mathematics learning from fifth to seventh grade, including the impact, if any, of the positioning of teachers, counselors, and parents on this process. In this article, we share findings examining science and mathematics teachers' actions and perceptions and their positioning of African American girls. This qualitative study used an interpretive design with multiple data sources including classroom observations, interviews, and field notes. Findings reveal that school‐wide policies and teachers' autonomous decisions impact the regularity of science and mathematics instruction, and that teachers do not always conceptualize the girls as science and mathematics achievers, positioning them in negative ways. 相似文献
78.
Ding S Chen JS Qi G Duan X Wang Z Giannelis EP Archer LA Lou XW 《Journal of the American Chemical Society》2011,133(1):21-23
We report an interesting approach for efficient synthesis of SnO(2) hollow spheres inside mesoporous silica "nanoreactors". The as-prepared products are shown to have a uniform size distribution and good structural stability. When evaluated for their lithium storage properties, these SnO(2) hollow spheres manifest improved capacity retention. 相似文献
79.
Landherr LJ Cohen C Archer LA 《Langmuir : the ACS journal of surfaces and colloids》2011,27(10):5944-5952
The interfacial properties of end-linked polydimethylsiloxane (PDMS) films on silicon are examined. Thin cross-linked PDMS films (~10 μm thick) were synthesized over a self-assembled monolayer supported on a silicon wafer. By systematically varying the concentration of monofunctional PDMS in a mixture with telechelic precursor molecules, structures ranging from near-ideal elastic networks to poorly cross-linked networks composed of a preponderance of dangling/pendent chains were synthesized. Lateral force microscopy (LFM) employing bead probes was used to quantify the effect of network structure on the interfacial friction coefficient and residual force. Indentation measurements employing an AFM in force mode were used to characterize the elastic modulus and the pull-off force for the films as a function of pendent chain content. These measurements were complemented with conventional mechanical rheometry measurements on similar thick network films to determine their bulk rheological properties. All networks studied manifested interfacial friction coefficients substantially lower than that of bare silicon. PDMS networks with the lowest pendent chain content displayed friction coefficients close to 1 order of magnitude lower than that of bare silicon, whereas networks with the highest pendent chain content manifested friction coefficients about 3 times lower than that of bare silicon. At intermediate sliding velocities, a crossover in the interfacial friction coefficient was observed, wherein cross-linked PDMS films with the least amount of pendent chains exhibit the highest friction coefficient. These observations are discussed in terms of the structure of the films and relaxation dynamics of elastic strands and dangling chains in tethered network films. 相似文献
80.