Electrospinning is a process that produces continuous polymer fibers with diameters of a nanometric scale. Nylon 6 in formic acid was electrospun to obtain the nanofibers. Fibers with different diameters were obtained using flow rates of 0.1, 0.5, 1 and 1.5 mL/hr, 20 wt% solution concentration, with an applied voltage of 20 kV and 15 cm spinning distance. Flow rate influenced the fiber diameter distribution, droplet size and its initiating shape at the capillary tip, the trajectory of the jet, maintenance of Taylor cone, areal density and nanofiber morphology. The morphology of the electrospun nanofibers was analyzed by using the scanning electron microscope (SEM). The effect of flow rate on the deposition area was also investigated for better control of the process. It was observed that a stabilized Taylor cone, small average droplet size, narrowest fiber diameter distribution, more stability in the originating jet, and uniform morphology of nanofiber is obtained at a flow rate of 0.5 mL/hr.

To provide a complete picture of the energy landscape of Al 2 O 3 at the nanoscale, we directed this study toward understanding the energetics of amorphous alumina (a-Al 2 O 3 ). a-Al 2 O 3 nanoparticles were obtained by condensation from gas phase generated through laser evaporation of α-Al 2 O 3 targets in pure oxygen at25 Pa. As-deposited nanopowders were heat-treated at different temperatures up to 600 °C to provide powders with surface areas of 670−340 m 2 /g. The structure of the samples was characterized by powder X-ray diffraction, transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. The results indicate that the microstructure consists of aggregated 3−5 nm nanoparticles that remain amorphous to temperatures as high as 600 °C. The structure consists of a network of AlO 4 , AlO 5 , and AlO 6 polyhedra, with AlO 5 being the most abundant species. The presence of water molecules on the surfaces was confirmed by mass spectrometry of the gases evolved on heating the samples under vacuum. A combination of BET surface-area measurements, water adsorption calorimetry, and high-temperature oxide melt solution calorimetry was employed for thermodynamic analysis. By linear fit of the measured excess enthalpy of the nanoparticles as a function of surface area, the surface energy of a-Al 2 O 3 was determined to be 0.97 ± 0.04 J/m 2 . We conclude that the lower surface energy of a-Al 2 O 3 compared with crystalline polymorphs γand α-Al 2 O 3 makes this phase the most energetically stable phase at surface areas greater than 370 m 2 /g.

The garnet structure is a promising nuclear waste form because it can accommodate various actinide elements. Yttrium iron garnet, Y 3 Fe 5 O 12 (YIG), is a model composition for such substitutions. Since cerium (Ce) can be considered an analogue of actinide elements such as thorium (Th), plutonium (Pu), and uranium (U), studying the local structure and thermodynamic stability of Ce-substituted YIG (Ce:YIG) can provide insights into the structural and energetic aspects of large ion substitution in garnets. Single phases of YIG with Ce substitution up to 20 mol % (Y 3−x Ce x Fe 5 O 12 with 0 ≤ x ≤ 0.2) were synthesized through a citrate−nitrate combustion method. The oxidation state of Ce was examined by X-ray absorption near edge structure spectroscopy (XANES); the oxidation state and site occupancy of iron (Fe) as a function of Ce loading also was monitored by 57 Fe− Mossbauer spectroscopy. These measurements establish that Ce is predominantly in the trivalent state at low substitution levels, while a mixture of trivalent and tetravalent states is observed at higher concentrations. Fe was predominately trivalent and exists in multiple environments. High temperature oxide melt solution calorimetry was used to determine the enthalpy of formation of these Cesubstituted YIGs. The thermodynamic analysis demonstrated that, although there is an entropic driving force for the substitution of Ce for Y, the substitution reaction is enthalpically unfavorable. The experimental results are complemented by electronic structure calculations performed within the framework of density functional theory (DFT) with Hubbard-U corrections, which reproduce the observed increase in the tendency for tetravalent Ce to be present with a higher loading of Ce. The DFT+U results suggest that the energetics underlying the formation of tetravalent Ce involve a competition between an unfavorable energy to oxidize Ce and reduce Fe and a favorable contribution due to strain-energy reduction. The structural and thermodynamic findings suggest a strategy to design thermodynamically favorable substitutions of actinides in the garnet system.

Amorphous silicon oxycarbide polymer-derived ceramics (PDCs), synthesized from organometallic precursors, contain carbon-and silica-rich nanodomains, the latter with extensive substitution of carbon for oxygen, linking Si-centered SiO x C 4-x tetrahedra. Calorimetric studies demonstrated these PDCs to be thermodynamically more stable than a mixture of SiO 2 , C, and silicon carbide. Here, we show by multinuclear NMR spectroscopy that substitution of C for O is also attained in PDCs with depolymerized silica-rich domains containing lithium, associated with SiO x C 4-x tetrahedra with nonbridging oxygen. We suggest that significant (several percent) substitution of C for O could occur in more complex geological silicate melts/glasses in contact with graphite at moderate pressure and high temperature and may be thermodynamically far more accessible than C for Si substitution. Carbon incorporation will change the local structure and may affect physical properties, such as viscosity. Analogous carbon substitution at grain boundaries, at defect sites, or as equilibrium states in nominally acarbonaceous crystalline silicates, even if present at levels at 10-100 ppm, might form an extensive and hitherto hidden reservoir of carbon in the lower crust and mantle.T he carbon cycle is one of the most important components in the sustenance of life and the evolution of the environment on our planet. The exchange of carbon between the atmosphere, biosphere, and oceans primarily controls the near-surface carbon cycle in the short term (decades to millennia) (1, 2). However, a large fraction of the carbon in the planet is stored at a greater depth (i.e., in the crust, mantle, core), and it exchanges with the surficial carbon only on much longer time scales (millions of years) through processes involving subduction and volcanic activity (3, 4). The form of stored carbon in these deep reservoirs and the amounts of carbon in various reservoirs, as well as the balance between the inward and the outward fluxes of carbon from them, are poorly constrained (3, 4). In the crust and upper mantle under relatively oxidizing conditions, CO 3 2− provides the familiar trigonal (sp 2 ) coordination environment for carbon in carbonates, mixed CO 2 -H 2 O fluid phases and carbonatiteforming melts. Under more reducing conditions, graphite, diamond, and methane become important and carbonate becomes less important or absent. The substitution of tetrahedral (sp 3 ) carbon for silicon in silicate minerals and melts has often been suggested as a possibility at high pressure (5-13), but there was little experimental evidence for it until recent phase equilibrium studies indicated that CO 2 can undergo transformation from a molecular gas to an extended solid at very high pressure and temperature to form a cristobalite-like structure consisting of a network of corner-sharing CO 4 tetrahedra (13,14). Recent studies have also indicated the possibility of a chemical reaction between SiO 2 and CO 2 to form a silicon carbonate phase under pressure (12) and the...