Electrical conductivity of a silicone network upon electron irradiation: influence of formulation

In this study, the electrical conductivity of a silicone elastomer filled with inorganic fillers was investigated upon electron irradiation. Neat samples consisting of the isolated polysiloxane matrix (with no fillers) were studied in parallel to identify the filler contribution to this evolution. It was shown that exposure to 400 keV electron doses induced a decrease in electrical conductivity for both the filled and neat materials. This decrease was much more pronounced with the filled samples than with the neat ones. Moreover, the activation energy of electrical conductivity (Arrhenius behaviour) doubled in the filled case, while it varied only weakly for the neat case. In light of these results, structure–property relationships were proposed on the basis of the radiation-induced crosslink processes to which this material is subject. In the framework of electronic percolation theory, it is suggested that the radiation-induced formation of SiO3 crosslinks in the polysiloxane network and SiO4 crosslinks at filler–matrix interfaces affects the percolation path of the material, which can be simply modelled by a network of resistors in series. On one hand, their densification increases the overall resistance of the percolation path, which results in the observed decrease of effective electrical conductivity. On the other hand, the steep increase in activation energy in the filled material attributes to the SiO4 crosslinks becoming the most restrictive barrier along the percolation path. In spite of the misleading likeness of electrical conductivities in the pristine state, this study presented evidence that silicone formulation can affect the evolution of electrical properties in radiative environments. To illustrate this conclusion, the use of this material in space applications, especially when directly exposed to the radiative space environment, was discussed. The decrease in electrical conductivity was associated with a progressively increasing risk for the occurrence of electrostatic discharge and consequent spacecraft failures

Inorganic fillers influence on the radiation-induced ageing of a space-used silicone elastomer

A space-used filled silicone rubber (silica and iron oxide fillers) and its polysiloxane isolated matrix were exposed to high energy electrons in order to determine their ageing mechanisms from a structural point of view. Physicochemical analysis evidenced that both filled and unfilled materials predominantly crosslink under such irradiation. Solid-state 29Si NMR spectroscopy allowed the identification of T-type SiO3 units as the main new crosslinks formed in the polymer network. It also revealed an increase in Qtype SiO4 units in the irradiated filled sample. Thanks to the combination of NMR spectroscopy and ammonia-modified swelling tests, these Q-type units were associated with new crosslinks formed at the silica fillers-matrix interface. While the main interaction between the polysiloxane network and the fillers was shown to proceed mainly through hydrogen bonding in the pristine filled samples, it was suggested that the hydrogen bonds were progressively replaced with SiO4 chemical bonds. These additional chemical crosslinks induced evolutions of the shear modulus on the rubber plateau and crosslink density that were significantly more pronounced in the filled material than in the neat one

Electrical behaviour of a silicone elastomer under simulated space environment

The electrical behavior of a space-used silicone elastomer was characterized using surface potential decay and dynamic dielectric spectroscopy techniques. In both cases, the dielectric manifestation of the glass transition (dipole orientation) and a charge transport phenomenon were observed. An unexpected linear increase of the surface potential with temperature was observed around Tg in thermally-stimulated potential decay experiments, due to molecular mobility limiting dipolar orientation in one hand, and 3D thermal expansion reducing the materials capacitance in the other hand. At higher temperatures, the charge transport process, believed to be thermally activated electron hopping with an activation energy of about 0.4 eV, was studied with and without the silica and iron oxide fillers present in the commercial material. These fillers were found to play a preponderant role in the low-frequency electrical conductivity of this silicone elastomer, probably through a Maxwell–Wagner–Sillars relaxation phenomenon

Climatically-Active Gases in the Eastern Boundary Upwelling and Oxygen Minimum Zone (OMZ) Systems

International audienceThe EBUS (Eastern Boundary Upwelling Systems) and OMZs (Oxygen Minimum Zone) contribute very significantly to the gas exchange between the ocean and the atmosphere, notably with respect to the greenhouse gases (hereafter GHG). From in-situ ocean measurements, the uncertainty of the net global ocean-atmosphere CO2 fluxes is between 20 and 30%, and could be much higher in the EBUS-OMZ. Off Peru, very few in-situ data are available presently, which justifies alternative approaches for assessing these fluxes. GHG air-sea fluxes determination can be inferred from inverse modeling applied to Vertical Column Densities (VCDs) from GOSAT, using state of the art modeling, at low spatial resolution. For accurately linking sources of GHGs to EBUS and OMZs, the resolution of the source regions needs to be increased. This task develops on new non-linear and multiscale processing methods for complex signals to infer a higher spatial resolution mapping of the fluxes and the associated sinks and sources between the atmosphere and the ocean. The use of coupled satellite data (e.g. SST and/or Ocean colour) that carry turbulence information associated to ocean dynamics is taken into account at unprecedented detail level to incorporate turbulence effects in the evaluation of the air-sea fluxes. We will present a framework as described above for determining sources and sinks of GHG from satellite remote sensing with the Peru OMZ as a test bed

Dynamic glass transition of filled polysiloxane upon electron irradiation

The influence of radiation-induced crosslinking on the molecular mobility of a filled silicone elastomer near the glass transition (α-relaxation) was analyzed using broadband dielectric spectroscopy. Samples of the isolated polysiloxane matrix (neat) were also studied so as to assess the filler influence on the evolution of the α-relaxation. A slowing-down of the segmental dynamics was observed with increasing ionizing dose. It was ascribed to the relaxing dipoles losing degrees of freedom as a result of network stiffening. An enhancement of intermolecular coupling, associatedwith the cooperativity of the α-relaxation,was deduced fromthe dielectric analysis. Similar observations were made in the past with chemically crosslinked polysiloxanes. This study evidenced that even though the crosslinks formed upon chemical crosslinking (mainly Si\\CH2\\CH2\\Si) differ in nature from those formed upon irradiation (mainly SiO3 and SiO4), they affect the dynamic glass transition in a very similar way. The filler influence on the dynamic glass transition was also studied upon irradiation. One of the main outcomes of this study is the fading of the filler-related effect in themost irradiated samples: both the shape and dynamics of the α-relaxation were identical in the most highly irradiated neat and filled samples

Giant Hydrogen Sulfide Plume in the Oxygen Minimum Zone off Peru Supports Chemolithoautotrophy

In Eastern Boundary Upwelling Systems nutrient-rich waters are transported to the ocean surface, fuelling high photoautotrophic primary production. Subsequent heterotrophic decomposition of the produced biomass increases the oxygen-depletion at intermediate water depths, which can result in the formation of oxygen minimum zones (OMZ). OMZs can sporadically accumulate hydrogen sulfide (H2S), which is toxic to most multicellular organisms and has been implicated in massive fish kills. During a cruise to the OMZ off Peru in January 2009 we found a sulfidic plume in continental shelf waters, covering an area >5500 km2, which contained ~2.2×104 tons of H2S. This was the first time that H2S was measured in the Peruvian OMZ and with ~440 km3 the largest plume ever reported for oceanic waters. We assessed the phylogenetic and functional diversity of the inhabiting microbial community by high-throughput sequencing of DNA and RNA, while its metabolic activity was determined with rate measurements of carbon fixation and nitrogen transformation processes. The waters were dominated by several distinct γ-, δ- and ε-proteobacterial taxa associated with either sulfur oxidation or sulfate reduction. Our results suggest that these chemolithoautotrophic bacteria utilized several oxidants (oxygen, nitrate, nitrite, nitric oxide and nitrous oxide) to detoxify the sulfidic waters well below the oxic surface. The chemolithoautotrophic activity at our sampling site led to high rates of dark carbon fixation. Assuming that these chemolithoautotrophic rates were maintained throughout the sulfidic waters, they could be representing as much as ~30% of the photoautotrophic carbon fixation. Postulated changes such as eutrophication and global warming, which lead to an expansion and intensification of OMZs, might also increase the frequency of sulfidic waters. We suggest that the chemolithoautotrophically fixed carbon may be involved in a negative feedback loop that could fuel further sulfate reduction and potentially stabilize the sulfidic OMZ water

Evaluating future climate change exposure of marine habitat in the South East Pacific based on metabolic constraints

IntroductionOn-going climate change is now recognized to yield physiological stresses on marine species, with potentially detrimental effects on ecosystems. Here, we evaluate the prospect of using climate velocities (CV) of the metabolic index (Φ) for assessing changes in habitat in the South East Pacific.MethodsOur approach is based on a species with mean ecophysiotype (i.e. model species) and the use of a global Earth System Model simulation (CESM-LE) under RCP 8.5 scenario. The SEP is chosen as a case study as it hosts an Oxygen Minimum Zone and seamounts systems sustaining local communities through artisanal fisheries.Results and DiscussionOur results indicate that CVΦ pattern is mainly constrained by the oxygen distribution and that its sign is affected by contrasting oxygen trends (including a re-oxygenation in the upper OMZ) and warming. We further show that CVΦ is weakly dependent on physiological traits composing Φ, which conveys to this metrics some value for inferring the projected mean displacement and potential changes in viability of metabolic habitat in a region where physiological data are scarce. Based on sensitivity experiments to physiological traits and natural variability, we propose a general method for inferring broad areas of climate change exposure regardless of species-specific Φ. We show in particular that for the model used here, the upper OMZ region can be considered a “safe” area for the species with ecophysiotype close to that of 71 species used to derive the model species. Limitations of the approach and perspectives of this work are also discussed

Modulation of the vertical particle transfer efficiency in the oxygen minimum zone off Peru

The fate of the organic matter (OM) produced by marine life controls the major biogeochemical cycles of the Earth's system. The OM produced through photosynthesis is either preserved, exported towards sediments or degraded through remineralisation in the water column. The productive eastern boundary upwelling systems (EBUSs) associated with oxygen minimum zones (OMZs) would be expected to foster OM preservation due to low O2 conditions. But their intense and diverse microbial activity should enhance OM degradation. To investigate this contradiction, sediment traps were deployed near the oxycline and in the OMZ core on an instrumented moored line off Peru. Data provided high-temporal-resolution O2 series characterising two seasonal steady states at the upper trap: suboxic ([O2]  50%) and remineralisation (intermediate Teff 20  50%) has been reported in summer and winter associated with extreme limitation in O2 concentrations or OM quantity for OM degradation. However, higher levels of O2 or OM, or less refractory OM, at the oxycline, even in a co-limitation context, can decrease the OMZ transfer efficiency to below 50%. This is especially true in summer during intraseasonal wind-driven oxygenation events. In late winter and early spring, high oxygenation conditions together with high fluxes of sinking particles trigger a shutdown of the OMZ transfer (Teff < 6%). Transfer efficiency of chemical elements composing the majority of the flux (nitrogen, phosphorus, silica, calcium carbonate) follows the same trend as for carbon, with the lowest transfer level being in late winter and early spring. Regarding particulate isotopes, vertical transfer of δ15N suggests a complex pattern of 15N impoverishment or enrichment according to Teff modulation. This sensitivity of OM to O2 fluctuations and particle concentration calls for further investigation into OM and O2-driven remineralisation processes. This should include consideration of the intermittent behaviour of OMZ towards OM demonstrated in past studies and climate projections

On the interpretation of changes in the subtropical oxygen minimum zone volume off Chile during two La Niña events (2001 and 2007)

Oxygen minimum zones (OMZs) are extended oceanic regions for which dissolved oxygen concentration is extremely low. They are suspected to be expanding in response to global warming. However, currently, the mechanisms by which OMZ varies in response to climate variability are still uncertain. Here, the variability of the subtropical OMZ off central Chile of a regional coupled physical–biogeochemical regional model simulation was analyzed for the period 2000–2008, noting that its fluctuations were significant despite the relatively weak amplitude of the El Niño/Southern Oscillation (ENSO). In particular, the interannual variability in the OMZ volume (OMZVOL, defined as the volume with dissolved oxygen concentration (DO) ≤ 45μM) was approximately 38% larger than that of the seasonal cycle, with maximum and minimum anomalies of OMZVOL taking place during two cold La Niña (LN) years (2001 and 2007). The model analyses further reveal that these anomalies resulted from a combined effect of changes in (1) the oxygen-poor waters poleward transport by the Peru–Chile undercurrent (PCUC), (2) the intensity of quasi-zonal jets influencing the transport of water to and from the OMZ, and (3) the zonal DO transport related to mesoscale eddy activity. Specifically, the interannual variability of the PCUC modulated primarily the DO contents of the OMZ core [(DO) ≤ 20μM] and secondarily the OMZVOL, while cross-shore DO transport by the zonal jets and the eddy fluxes played a major role in ventilating and shaping the offshore extent of the OMZ. When the OMZVOL was maximum (minimum), the PCUC transport was slightly increased (reduced), which was associated with a reduction (increase) in the ventilation of the OMZ through negative (positive) anomalies of zonal advection and DO eddy fluxes. Our results demonstrate that significant natural interannual variability in the subtropical OMZ off Chile originates from the interplay between oceanic equatorial teleconnection (PCUC transport) and local non-linear dynamics (the zonal jets and mesoscale eddies)