Semi-annual carbon and nitrogen isotope variations in the water column of Lake Bled, NW Slovenia

The variability in the stable isotope signature of carbon and nitrogen in particulate organic matter and dissolved species in the water column of the mesotrophic subalpine Lake Bled in NW Slovenia has been determined. After the algae bloom from August to December in 2008, samples were taken from the deepest part of the lake which develops an anoxic hypolimnion for most of the year. C/N molar ratios and δ¹³C_POC and δ¹⁵N_PN values suggest an autochthonous source for particulate organic matter (POM). According to the isotope model, autochthonous carbon accounted for a major part of the particulate organic carbon (POC), ranging from 86% to 96% in September and October, while in December the proportion of allochthonous carbon was more pronounced, ranging from 57% to 59%. Low δ¹³C_POC and δ¹⁵N_PN values (from −36 to −33‰ and from 0.8 to 1.8‰), observed below 24 m in August and September, indicate the bacterial origin of POM, mainly from methanotrophic bacteria. δ¹⁵N_NO₃ and δ¹⁵N_PN values decreased with depth. The relations between δ¹⁵N_PN and NO₃^- and NH₄⁺ concentrations suggest that NH₄⁺ is the main assimilation species for nitrogen in POM. Nitrification was active between 12 and 18 m deep in September and October, indicated by increased NO₃^- concentrations and decreased δ¹⁵N_NO₃ values. The correlation between nitrate concentrations and δ¹⁵N_NO₃ values suggests active water column denitrification in October 2008. The decrease in δ¹⁵N_NO₃ values observed in December could be explained by degradation of organic matter, followed by nitrification of the degradation products. During our sampling period, there was no evident influence of sewage, agriculture, or atmospheric deposition on the nitrogen balance in the lake

NbS3_{3}: A unique quasi one-dimensional conductor with three charge density wave transitions

Through transport, compositional and structural studies, we review the features of the charge-density wave (CDW) conductor of NbS$_{3}$ (phase II). We highlight three central results: 1) In addition to the previously reported CDW transitions at $T_{P1}$ = 360\,K and $T_{P2}$ = 150\,K, another CDW transition occurs at a much higher temperature $T_{P0}$ = 620-650\,K; evidence for the non-linear conductivity of this CDW is presented. 2) We show that CDW associated with the $T_{P2}$ - transition arises from S vacancies acting as donors. Such a CDW transition has not been observed before. 3) We show exceptional coherence of the $T_{P1}$-CDW at room-temperature. Additionally, we report on the effects of uniaxial strain on the CDW transition temperatures and transport.Comment: 16 pages, 18 figure

Nb S3: A unique quasi-one-dimensional conductor with three charge density wave transitions

© 2017 American Physical Society.We review the features of the charge density wave (CDW) conductor NbS3 (phase II) and include several additional results from transport, compositional, and structural studies. Particularly, we highlight three central results: (1) In addition to the previously reported CDW transitions at TP1=360K and TP2=150K, a third CDW transition occurs at a much higher temperature TP0≈620-650K; evidence for the nonlinear conductivity of this CDW is presented. (2) We show that the CDW associated with the TP2 transition arises from S vacancies acting as donors. Such a CDW transition has not been observed before. (3) We demonstrate the exceptional coherence of the TP1 CDW at room temperature. The effects of uniaxial strain on the CDW transition temperature and transport are reported

Stable Isotope Biogeochemistry of Seabird Guano Fertilization: Results from Growth Chamber Studies with Maize (Zea Mays)

Stable isotope analysis is being utilized with increasing regularity to examine a wide range of issues (diet, habitat use, migration) in ecology, geology, archaeology, and related disciplines. A crucial component to these studies is a thorough understanding of the range and causes of baseline isotopic variation, which is relatively poorly understood for nitrogen (δ(15)N). Animal excrement is known to impact plant δ(15)N values, but the effects of seabird guano have not been systematically studied from an agricultural or horticultural standpoint.This paper presents isotopic (δ(13)C and δ(15)N) and vital data for maize (Zea mays) fertilized with Peruvian seabird guano under controlled conditions. The level of (15)N enrichment in fertilized plants is very large, with δ(15)N values ranging between 25.5 and 44.7‰ depending on the tissue and amount of fertilizer applied; comparatively, control plant δ(15)N values ranged between -0.3 and 5.7‰. Intraplant and temporal variability in δ(15)N values were large, particularly for the guano-fertilized plants, which can be attributed to changes in the availability of guano-derived N over time, and the reliance of stored vs. absorbed N. Plant δ(13)C values were not significantly impacted by guano fertilization. High concentrations of seabird guano inhibited maize germination and maize growth. Moreover, high levels of seabird guano greatly impacted the N metabolism of the plants, resulting in significantly higher tissue N content, particularly in the stalk.The results presented in this study demonstrate the very large impact of seabird guano on maize δ(15)N values. The use of seabird guano as a fertilizer can thus be traced using stable isotope analysis in food chemistry applications (certification of organic inputs). Furthermore, the fertilization of maize with seabird guano creates an isotopic signature very similar to a high-trophic level marine resource, which must be considered when interpreting isotopic data from archaeological material

Lowering the thermal conductivity of Sr(Ti0.8Nb0.2)O3 by SrO and CaO doping: microstructure and thermoelectric properties

Excess SrO and CaO were added to the Sr(Ti0.8Nb0.2)O3 thermoelectric material, which was structurally compensated by the formation of Ruddlesden–Popper-type planar faults with the compositions SrO and/or (Sr, Ca)O. Both types of doping significantly changed the original isotropic Sr(Ti0.8Nb0.2)O3 microstructure and resulted in the formation of lamellar Ruddlesden–Popper-type phases within the Sr(Ti0.8Nb0.2)O3 grains. Three-dimensional networks of single Ruddlesden–Popper-type faults were also observed in the Sr(Ti0.8Nb0.2)O3 for both types of doping. The combination of both structural features significantly lowered the thermal conductivity in comparison with Sr(Ti0.8Nb0.2)O3 due to the enhanced phonon scattering observed at the planar faults, which proves that introducing such defects is a promising method for lowering the thermal conductivity of the Sr(Ti0.8Nb0.2)O3 thermoelectric material. The highest figure of merit (ZT = 0.08) was achieved with CaO doping, since the significantly reduced thermal conductivity was accompanied by an increased power factor

The (110) and (320) surfaces of a Cantor alloy

The (110) and (320) surfaces of the single-phase FeCrMnNiCo solid solution have been studied on two adjacent millimeter size grains using surface science and transmission electron microscopy (TEM) techniques. The structural and chemical evolutions of the high entropy alloy (HEA) surfaces have been determined for various sputtering conditions, annealing temperatures and durations. Up to 873 K, angle-resolved X-ray photoelectron spectroscopy measurements indicate a clear Mn and Ni surface co-segregation. We propose that the surface segregation of Mn is driven by its low surface energy. The attractive interaction between Mn and Ni promotes Ni segregation which accompanied the Mn diffusion to the surface. Regarding the structures investigated by low energy electron diffraction and scanning tunneling microscopy, the (320) surface presents a terraced morphology with an ordered structure consistent with a () termination. On the contrary, the (110) surface reveals an important degree of structural disorder and local reconstructions. Its highly anisotropic morphology resembles rows propagating along the [001] direction. Above 873 K, Mn desorption occurs while the Ni content keeps increasing linearly with the temperature. TEM analysis show no evidence for HEA decomposition into metallic or intermetallic phases even after repeated annealing and sputtering cycles. The above results set the upper temperature limit above which the surface stoichiometry departs from the quinary HEA concept. It also defines the temperature range for the use of FeCrMnNiCo based coating under high vacuum conditions and for aerospace applications

A new complex ternary phase in the Al-Cr-Sc push-pull alloy

International audienceThe purpose of this study is to find new ternary intermetallic compounds within the Al-Cr-Sc system, which is typical of a push-pull system in which two constituents (Cr and Sc) are immiscible, whilst they form compounds, respectively, with the third constituent (Al). By arc-melting of the three components aluminium (Al), chromium (Cr) and scandium (Sc) under inert atmosphere, a range of different alloys was produced. The microstructure was observed using scanning electron microscopy (SEM) and the phase compositions were analysed by electron dispersive X-ray spectroscopy (EDS). The samples were measured with X-ray diffraction (XRD) and transmission electron microscopy. The results were confirmed by Rietveld refinement. Three different phases were found in the microstructure of the samples, the two known binary phases Al 16 Cr 10 and Al 3 Sc and a new ternary phase coined f-Al 8 Cr 4 Sc. f-Al 8 Cr 4 Sc showed great similarities to other Al 8 Cr 4 RE compounds with tetragonal I4/mmm crystal structure. Starting from the structure of already known Al 8 Cr 4 RE crystals, ab initio calculations were performed to determine the crystallographic parameters of f-Al 8 Cr 4 Sc and also to investigate its electronic structure, which identified the energy band factor as the key factor that determines the stability of this compound. The lattice parameters and atomic positions were found in good agreement with the ones obtained by Rietveld refinement. Experimental atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image analysis of the f-Al 8 Cr 4 Sc phase confirmed the predicted atomic model