Uncertainty in the physicochemical and optical properties of volcanic ash particles creates errors in the detection and modeling of volcanic ash clouds and in quantification of their potential impacts. In this study, we provide a data set that describes the physicochemical and optical properties of a representative selection of volcanic ash samples from nine different volcanic eruptions covering a wide range of silica contents (50–80 wt % SiO2). We measured and calculated parameters describing the physical (size distribution, complex shape, and dense‐rock equivalent mass density), chemical (bulk and surface composition), and optical (complex refractive index from ultraviolet to near‐infrared wavelengths) properties of the volcanic ash and classified the samples according to their SiO2 and total alkali contents into the common igneous rock types basalt to rhyolite. We found that the mass density ranges between ρ = 2.49 and 2.98 g/cm3 for rhyolitic to basaltic ash types and that the particle shape varies with changing particle size (d < 100 μm). The complex refractive indices in the wavelength range between λ = 300 nm and 1500 nm depend systematically on the composition of the samples. The real part values vary from n = 1.38 to 1.66 depending on ash type and wavelength and the imaginary part values from k = 0.00027 to 0.00268. We place our results into the context of existing data and thus provide a comprehensive data set that can be used for future and historic eruptions, when only basic information about the magma type producing the ash is known.
We report on properties of ZnO nanoparticles synthesized via non-aqueous sol–gel routes. The role of the hydrates in the zinc precursor (Zn(acac)2, xH2O) on the structure and surface termination during the synthesis is studied for the first time. The structural and chemical properties of the ZnO nanoparticles were studied by standard structural and optical characterization methods. A broad luminescence was observed from the nanoparticles stretching throughout the visible region of the spectrum and comprising characteristic blue and green emission bands commonly associated with intrinsic defects in ZnO. A tentative model is proposed to explain differences in the luminescence of nanoparticles synthesized using different routes by taking into account the role of oxygen vacancies and other native defects: most likely being zinc vacancies and interstitials, located near the surface of the nanoparticles.
Hafnium dioxide is a wide band-gap, high-κ material, and Hafnium based compounds have already been integrated into micro-electronic devices. The pure cubic HfO2 phase is promising as it presents a higher permittivity (κ > 25), but needs to be stabilized by addition of divalent or trivalent dopants, which in turn modify the electronic properties of HfO2. Here, we employ a one-pot synthesis approach to produce undoped cubic and monoclinic HfO2 nanoparticles by choice of solvent alone. The average size of these nanoparticles from transmission electron microscopy studies was estimated to be around 2.6 nm. We present a study of the morphology and microstructure and also demonstrate the presence of a strong visible photoluminescence linked to the nanosize of the particles. Furthermore, the synthesis in equivalent conditions of these two phases of HfO2 provides means for direct comparison of the chemical composition and electronic structures of the two polymorphs. This has therefore allowed us to experimentally elucidate similarities and differences in the valence band, band gap states, and conduction band of these pure phases seconded by first principles calculations within the density functional theory.
Oxygen permeation measurements are performed on dense samples of CaTi0.85Fe0.15O3-δ, CaTi0.75Fe0.15Mg0.05O3-δ and CaTi0.75Fe0.15Mn0.10O3-δ in combination with density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) in order to assess Mg and Mn as dopants for improving the O2 permeability of CaTi1-xFexO3-δ based oxygen separation membranes. The oxygen permeation measurements were carried out at temperatures ranging between 700-1000 °C with feed side oxygen partial pressures between 0.01-1 bar. The O2 permeability was experimentally found to be highest for the Mn doped sample over the whole temperature range, reaching 4.2×10 -3 ml min -1 cm -1 at 900 °C and 0.21 bar O2 in the feed which corresponds to a 40% increase over the Fe-doped sample and similar to reported values for x=0.2. While the O2 permeability of the Mg doped sample was also higher than the Fe-doped sample, it approached that of the Fe-doped sample above 900 °C. According to the DFT calculations, Mn introduces electronic states within the band gap and will predominately exist in the effectively negative charge state, as indicated by XPS measurements. Mn may therefore improve the ionic and electronic conductivity of CTF based membranes. The results are discussed in terms of the limiting species for ambipolar transport and O2 permeability, i.e., oxygen vacancies and electronic charge carriers.
The long-term stability over a period of up to 50 days has been reported for various designs of microstructured Pd 77 Ag 23 membrane modules for H 2 production and purification. Even though microchannels provide sufficient mechanical support for moderate trans-membrane pressure difference and temperatures, i.e., 4-5 bars and 400-450 °C, long-term operation under these operating conditions results in a large deformative settling of the Pd 77 Ag 23 film into the microchannel support. This settling leads to microstructural changes and pore formation on the feed surface of the membrane film that ultimately results in membrane failure. For pressures above approximately 5 bars, the application of microchannel-supported modules is thus not feasible, and for that purpose a continuous porous stainless steel support is introduced that allows for a sufficient stabilisation of the thin Pd 77 Ag 23 films. For such a porous stainless steel supported microchannel module, a hydrogen flux of 195.3 mL•min -1 •cm -2 is obtained at 450 C and 5 bars feed pressure, corresponding to a permeability of 3.4•10 -8 mol•m -1 •s -1 •Pa -0.5 . During the complete operation of 1100h at 450 C, the module shows a very good stability up to the highest feed pressure applied of 15 bars.The N 2 leakage flux has remained below the detection limit of the equipment, 5 µL•cm -2 •min -1 , resulting in a minimum value for the H 2 /N 2 permselectivity of 39.000 applying the pure H 2 flux value obtained at 5 bars.
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