Femtosecond laser processing of nitride-based thin films to improve their tribological performance

The ability of femtosecond laser pulses to pattern coated tribological surfaces in order to improve their wear behavior was investigated. Experiments were performed with a Ti:sapphire laser (wavelength: 800nm, energy density: 2J/cm2, pulse duration: 100fs) on TiN- and on TiCN-coated surfaces. Morphological analyses of the laser-treated surfaces were carried out and did not reveal any film delamination or other coating damage after laser processing. Tribological tests simulating rapidly increasing contact pressures under boundary friction were performed on both unpatterned and laser-patterned coated surfaces using a steel counter body. The patterned surfaces showed significantly better tribological performance with respect to stability and the value of the friction coefficient during testing. EDX analyses of the tested unpatterned samples revealed complete coating removal and material transfer from the counter body to the sample surface. In the case of the laser-patterned surfaces, only slight coating damage and an accumulation of debris from the steel counter body in the laser-induced pores were observe

Femtosecond laser ablation of cemented carbides: properties and tribological applications

Laser ablation with fs laser pulses was performed in air on cobalt cemented tungsten carbide by means of a Ti : sapphire laser (800nm, 100fs). Small and moderate fluences (2, 5, 10J/cm2) and up to 5×104 pulses per irradiated spot were used to drill holes with aspect ratios up to 10. Cross-section cuts from laser-irradiated samples were produced and they were analysed with optical microscopy and SEM. EDX analyses were carried out on selected zones. Quasi-cylindrical holes were found for 2J/cm2, whereas for 5 and 10J/cm2 irregular shapes (lobes, bottoms wider than hole entrances) were found to occur after a given number of incident pulses. Layers with modified structure were evidenced at pore walls. SEM revealed a denser structure, while EDX analyses showed uniform and almost similar contents of W, C, and Co in these layers. As a direct application, patterning of coated WC-Co was carried out with 2J/cm2 and 100 pulses per pore. The resulted surfaces were tribologically tested and these tests revealed an improved friction and wear behaviou

Power-Based Droop Control in DC Microgrids Enabling Seamless Disconnection From Upstream Grids

This paper proposes a local power-based droop controller for distributed energy resource converters in dc microgrids that are connected to upstream grids by grid-interface converters. During normal operation, the grid-interface converter imposes the microgrid bus voltage, and the proposed controller allows power flow regulation at distributed energy resource converters\u2019 output. On the other hand, during abnormal operation of the grid-interface converter (e.g., due to faults in the upstream grid), the proposed controller allows bus voltage regulation by droop control. Notably, the controller can autonomously convert from power flow control to droop control, without any need of bus voltage variation detection schemes or communication with other microgrid components, which enables seamless transitions between these two modes of operation. Considering distributed energy resource converters employing the power-based droop control, the operation modes of a single converter and of the whole microgrid are defined and investigated herein. The controller design is also introduced. Furthermore, the power sharing performance of this control approach is analyzed and compared with that of classical droop control. The experimental results from a laboratory-scale dc microgrid prototype are reported to show the final performances of the proposed power-based droop control

Bridging the gap between atmospheric concentrations and local ecosystem measurements

This paper demonstrates that atmospheric inversions of CO₂ are a reliable tool for estimating regional fluxes. We compare results of an inversion over 18 days and a 300 x 300 km 2 domain in southwest France against independent measurements of fluxes from aircraft and towers. The inversion used concentration measurements from 2 towers while the independent data included 27 aircraft transects and 5 flux towers. The inversion reduces the mismatch between prior and independent fluxes, improving both spatial and temporal structures. The present mesoscale atmospheric inversion improves by 30% the CO₂ fluxes over distances of few hundreds of km around the atmospheric measurement locations. Citation: Lauvaux, T., et al. (2009), Bridging the gap between atmospheric concentrations and local ecosystem measurements, Geophys. Res. Lett., 36, L19809, doi: 10.1029/2009GL039574

CEFLES2: the remote sensing component to quantify photosynthetic efficiency from the leaf to the region by measuring sun-induced fluorescence in the oxygen absorption bands

The CEFLES2 campaign during the Carbo Europe Regional Experiment Strategy was designed to provide simultaneous airborne measurements of solar induced fluorescence and CO2 fluxes. It was combined with extensive ground-based quantification of leaf- and canopy-level processes in support of ESA's Candidate Earth Explorer Mission of the "Fluorescence Explorer" (FLEX). The aim of this campaign was to test if fluorescence signal detected from an airborne platform can be used to improve estimates of plant mediated exchange on the mesoscale. Canopy fluorescence was quantified from four airborne platforms using a combination of novel sensors: (i) the prototype airborne sensor AirFLEX quantified fluorescence in the oxygen A and B bands, (ii) a hyperspectral spectrometer (ASD) measured reflectance along transects during 12 day courses, (iii) spatially high resolution georeferenced hyperspectral data cubes containing the whole optical spectrum and the thermal region were gathered with an AHS sensor, and (iv) the first employment of the high performance imaging spectrometer HYPER delivered spatially explicit and multi-temporal transects across the whole region. During three measurement periods in April, June and September 2007 structural, functional and radiometric characteristics of more than 20 different vegetation types in the Les Landes region, Southwest France, were extensively characterized on the ground. The campaign concept focussed especially on quantifying plant mediated exchange processes (photosynthetic electron transport, CO2 uptake, evapotranspiration) and fluorescence emission. The comparison between passive sun-induced fluorescence and active laser-induced fluorescence was performed on a corn canopy in the daily cycle and under desiccation stress. Both techniques show good agreement in detecting stress induced fluorescence change at the 760 nm band. On the large scale, airborne and ground-level measurements of fluorescence were compared on several vegetation types supporting the scaling of this novel remote sensing signal. The multi-scale design of the four airborne radiometric measurements along with extensive ground activities fosters a nested approach to quantify photosynthetic efficiency and gross primary productivity (GPP) from passive fluorescence

Satellite-inferred European carbon sink larger than expected

Current knowledge about the European terrestrial biospheric carbon sink, from the Atlantic to the Urals, relies upon bottom-up inventory and surface flux inverse model estimates (e.g. 0.27±0.16 GtC a−1 for 2000–2005 (Schulze et al., 2009), 0.17±0.44 GtC a−1 for 2001–2007 (Peters et al., 2010), 0.45±0.40 GtC a−1 for 2010 (Chevallier et al., 2014), 0.40±0.42 GtC a−1 for 2001–2004 (Peylin et al., 2013)). Inverse models assimilate in situ CO2 atmospheric concentrations measured by surface-based air sampling networks. The intrinsic sparseness of these networks is one reason for the relatively large flux uncertainties (Peters et al., 2010; Bruhwiler et al., 2011). Satellite-based CO2 measurements have the potential to reduce these uncertainties (Miller et al., 2007; Chevallier et al., 2007). Global inversion experiments using independent models and independent GOSAT satellite data products consistently derived a considerably larger European sink (1.0–1.3 GtC a−1 for 09/2009–08/2010 (Basu et al., 2013), 1.2–1.8 GtC a−1 in 2010 (Chevallier et al., 2014)). However, these results have been considered unrealistic due to potential retrieval biases and/or transport errors (Chevallier et al., 2014) or have not been discussed at all (Basu et al., 2013; Takagi et al., 2014). Our analysis comprises a regional inversion approach using STILT (Gerbig et al., 2003; Lin et al., 2003) short-range (days) particle dispersion modelling, rendering it insensitive to large-scale retrieval biases and less sensitive to long-range transport errors. We show that the satellite-derived European terrestrial carbon sink is indeed much larger (1.02±0.30 GtC a−1 in 2010) than previously expected. This is qualitatively consistent among an ensemble of five different inversion set-ups and five independent satellite retrievals (BESD (Reuter et al., 2011) 2003–2010, ACOS (O’Dell et al., 2012) 2010, UoL-FP (Cogan et al., 2012) 2010, RemoTeC (Butz et al., 2011) 2010, and NIES (Yoshida et al., 2013) 2010) using data from two different instruments (SCIAMACHY (Bovensmann et al., 1999) and GOSAT (Kuze et al., 2009)). The difference to in situ based inversions (Peylin et al., 2013), whilst large with respect to the mean reported European carbon sink (0.4 GtC a−1 for 2001–2004), is similar in magnitude to the reported uncertainty (0.42 GtC a−1). The highest gain in information is obtained during the growing season when satellite observation conditions are advantageous, a priori uncertainties are largest, and the surface sink maximises; during the dormant season, the results are dominated by the a priori. Our results provide evidence that the current understanding of the European carbon sink has to be revisited

Precision requirements for space-based XCO2 data

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): D10314, doi:10.1029/2006JD007659.Precision requirements are determined for space-based column-averaged CO2 dry air mole fraction (XCO2) data. These requirements result from an assessment of spatial and temporal gradients in XCO2, the relationship between XCO2 precision and surface CO2 flux uncertainties inferred from inversions of the XCO2 data, and the effects of XCO2 biases on the fidelity of CO2 flux inversions. Observational system simulation experiments and synthesis inversion modeling demonstrate that the Orbiting Carbon Observatory mission design and sampling strategy provide the means to achieve these XCO2 data precision requirements.This work was supported by the Orbiting Carbon Observatory (OCO) project through NASA’s Earth System Science Pathfinder (ESSP) program. SCO and JTR were supported by a NASA IDS grant (NAG5-9462) to JTR