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The present work elaborates on the correlation between the amount and ordering of the free carbon phase in silicon oxycarbides and their charge carrier transport behavior. Thus, silicon oxycarbides possessing free carbon contents from 0 to ca. 58 vol.% (SiOC/C) were synthesized and exposed to temperatures from 1100 to 1800 °C. The prepared samples were extensively analyzed concerning the thermal evolution of the sp2 carbon phase by means of Raman spectroscopy. Additionally, electrical conductivity and Hall measurements were performed and correlated with the structural information obtained from the Raman spectroscopic investigation. It is shown that the percolation threshold in SiOC/C samples depends on the temperature of their thermal treatment, varying from ca. 20 vol.% in the samples prepared at 1100 °C to ca. 6 vol.% for the samples annealed at 1600 °C. Moreover, three different conduction regimes are identified in SiOC/C, depending on its sp2 carbon content: (i) at low carbon contents (i.e., <1 vol.%), the silicon oxycarbide glassy matrix dominates the charge carrier transport, which exhibits an activation energy of ca. 1 eV and occurs within localized states, presumably dangling bonds; (ii) near the percolation threshold, tunneling or hopping of charge carriers between spatially separated sp2 carbon precipitates appear to be responsible for the electrical conductivity; (iii) whereas above the percolation threshold, the charge carrier transport is only weakly activated (Ea = 0.03 eV) and is realized through the (continuous) carbon phase. Hall measurements on SiOC/C samples above the percolation threshold indicate p-type carriers mainly contributing to conduction. Their density is shown to vary with the sp2 carbon content in the range from 1014 to 1019 cm−3; whereas their mobility (ca. 3 cm2/V) seems to not depend on the sp2 carbon content. [ABSTRACT FROM AUTHOR]

Dimension reduction, cost efficiency, and environmental sustainability are important factors in absorbent designs. Geopolymers represent an eco-friendly and cost-efficient solution for such applications, and the objective of this study is to develop new geopolymer-based composites with tailored dielectric properties. To develop such composites, different formulations based on three types of carbon and various surfactants are tested. The nonionic surfactant is preferred over the anionic surfactant. Dielectric investigations between 2 and 3.3 GHz are performed. The results reveal that the carbon content and its type (origin) have significant effects on the dielectric characteristics and less on the magnetic characteristics. Indeed, an increase in permittivity from 2 to 24 and an increase from 0.09 to 0.6 for loss tangent are shown with changes in the carbon content and type. A permittivity (ε) of 2.27 and loss (tan δ) of 0.19 are obtained for a pore size of 1.6 mm, for the carbon type with the lowest purity, and with a nonionic surfactant. Finally, it is shown that the addition of magnetite has little impact on the overall magnetic properties of the geopolymer. [ABSTRACT FROM AUTHOR]

We report preferential orientation control in photochromic gadolinium oxyhydride (GdHO) thin films deposited by a two-step process. Gadolinium hydride (GdH2-x) films were grown by reactive magnetron sputtering, followed by oxidation in air. The preferential orientation, grain size, anion concentrations and photochromic response of the films were strongly dependent on the deposition pressure. The GdHO films showed a preferential orientation along the [100] direction and exhibited photochromism when synthesized at deposition pressures of up to 5.8 Pa. The photochromic contrast was larger than 20% when the films were deposited below 2.8 Pa with a 0.22 H2/Ar flow ratio. We argue that the relation of preferential orientation and the post deposition oxidation since oxygen concentration is known to be a key parameter for photochromism in rare-earth oxyhydride thin films. The experimental observations described above were explained by the decrease of the grain size as a result of the increase of the deposition pressure of the sputtering gas, followed by a higher oxygen incorporation. [ABSTRACT FROM AUTHOR]