Helmholtz-Zentrum Hereon

Dissimilar fusion welding between NiTi shape memory alloy and AlCoCrFeNi 2.1 , eutectic high entropy alloy was achieved using a combination of Nb and Cu interlayers. Both base metals exhibit unique properties that complement each other making them attractive for integration in smart systems. However, direct joining of these materials is not feasible due to the formation of brittle intermetallic compounds that compromise the joint integrity. Thus, to avoid this metallurgical issue dissolution of the Nb and Cu interlayers promoted elemental intermixing and altered solidification pathways, leading to the formation and intensification of topologically close-packed phases. Phase evolution was assessed through thermodynamic CALPHAD simulations and validated using synchrotron X-ray diffraction, while further microstructural characterization employed backscattered electron imaging and energy dispersive spectroscopy. The weld exhibited pronounced spatial heterogeneity, with a peak hardness of 856 HV 0.3 near the NiTi-side interfacial region, consistent with enrichment of TCP phases and Ti-rich intermetallics. Tensile testing showed fracture at an ultimate tensile strength of 372 MPa and a fracture strain of 1.11%, with crack initiation in the hardened interfacial region followed by mixed-mode fracture toward the fusion zone. Although tensile ductility remains limited, the multi-interlayer strategy enabled defect-free joint formation and confined embrittlement to a narrow region compared with direct joining. • Fusion welding of NiTi shape memory alloy with AlCoCrFeNi 2.1 eutectic high entropy alloy is investigated using multiple interlayers to minimize intermetallic formation. • Interlayer dissolution produced a heterogeneous microstructure, promoting the formation of topologically close-packed phases that influenced the mechanical behaviour of the joint. • CALPHAD simulations and synchrotron X-ray diffraction revealed the impact of elemental intermixing and phase evolution across the composite weld zone.

Efficient thermal management is essential for aluminum components operating in environments with fluctuating thermal loads, especially when protective surface treatments are required, like in heat exchangers working in phase change material (PCM) environments. Plasma electrolytic oxidation (PEO) provides excellent wear and corrosion resistance but typically produces ceramic layers with low intrinsic thermal conductivity, potentially limiting heat-transfer performance. This study systematically evaluates how PEO processing mode (unipolar vs. bipolar) and nanoparticle additives influence coating microstructure, phase composition, porosity, and the effective thermal conductivity of coated AlMg3 substrates. Coatings were produced using a standard alkaline silicate-phosphate electrolyte with and without ZnO, WS 2 , TiC, TiN, and BN particles. Microstructural and compositional analyses were performed via SEM/EDS and GIXRD measurements, while thermal conductivity of the full coated system was determined using laser flash analysis (LFA). Across all conditions, PEO coatings reduced thermal conductivity relative to bare alloy (129.78 W/(m·K)) but only moderately, with most 5-min treatments remaining within ∼20% loss of the substrate conductivity (111.43 W/(m·K) for particle-free unipolar mode coatings). Particle additions resulted only in limited improvements; the best performing additive, 3 g/L ZnO in unipolar mode, yielded slight conductivity enhancement associated with more uniform particle incorporation (119. 03 W/(m·K)). Overall, results highlight that PEO coatings can deliver protective functionality while maintaining acceptable thermal performance.

This study investigates the combined effects of salinity and microplastic contamination on the evaporation process and salt crystallization in porous media through complementary column- and pore-scale experiments. Laboratory soil columns were packed with either pure sand or sand mixed with 5% (w/w) poly(vinyl chloride) (PVC) microplastics and subsequently saturated with freshwater or saline (NaCl) solution. Evaporation and crystallization dynamics were monitored by using mass loss measurements, surface optical and thermal imaging, and synchrotron X-ray tomography. Results show that salinity consistently suppressed evaporation by roughly 25–30%, whereas PVC microplastics enhanced it, generating substantial differences in cumulative water loss across treatments. Moreover, thermal imaging revealed distinct surface responses: NaCl samples developed salt crusts that progressively reduced local temperature contrasts and led to more spatially uniform surface conditions, whereas PVC-NaCl samples exhibited lower mean surface temperatures but substantially higher spatial variability, reflected in larger and more persistent temperature anomalies during drying. Pore-scale μCT imaging further confirmed that microplastics altered crystallization patterns by redistributing salt deposition over the upper part of the sand profile and modifying the nucleation behavior. Together, these findings underscore the complex interplay between microplastics and salinity, with implications for soil moisture regulation, surface energy flux, and environmental monitoring strategies.

Abstract The transition from primary to secondary aluminium production offers substantial environmental benefits but introduces challenges related to impurity-induced changes in microstructure and final materials performance in mechanical integrity and corrosion resistance. This study investigates the influence of most common Fe, Mn, and Cu impurities on the formation of intermetallic phases in AlSi7Mg0.3 alloys using a combined CALPHAD and machine learning framework. High-throughput Scheil solidification simulations were conducted on 4999 alloy compositions using Thermo-Calc, and the resulting data were used to train and validate a Random Forest regression model. The model exhibited robust predictive performance (R 2 = 0.98, NRMSE = 0.07, NMAE = 0.05) and was employed to compute phase fractions for over 20 million alloy compositions within the defined impurity space. SHAP-based feature analysis revealed interactions, direct as well as indirect, between impurity elements and key phases, highlighting the opposing roles of Fe and Mn in stabilizing β -Al 5 FeSi (AL9FE2SI2) and Al15SI2M4. And the resulting impurity-phase maps provide quantitative, thermodynamics-based decision support for impurity management and manganese optimization in recycled aluminium alloys.

Wind-driven gravity–capillary waves play a key role in air–sea interactions and in small-scale energy dissipation across the surface microlayer (SML). Despite decades of studies, the transition from smooth gravity–capillary waves to intermittent microscale breaking under weak wind forcing is still not well understood. This study investigates gravity–capillary wave dynamics and micro-breaking in a 24 m long, 1 m wide wind–wave tank with a total height of 1.5 m and a mean water depth of 0.51 m. Measurements were performed using a newly developed Colour Imaging Slope Gauge (CISG), providing high-resolution spatio-temporal observations of surface slopes within a 33.2 cm × 26.8 cm field of view (FOV), at a spatial resolution of 0.024² cm² per pixel and a frame rate of 400 Hz. A total of 18 experiments were conducted over a range of low wind speeds (1.8 m s⁻¹– 4.0 m s⁻¹) with small increments. Wire-wave gauge measurements were used to support three-dimensional surface reconstructions. Spectral, wavelet, and band-pass filtering techniques were applied to isolate capillary-scale features associated with micro-breaking. Particular attention was given to surface curvature as a geometric indicator of micro-breaking. The wide FOV enables direct tracking of isolated events and reveals a clear increase in capillary activity and micro-breaking occurrence with increasing wind forcing. First results indicate a distinct transitional regime at wind speeds near 2.0 m s⁻¹, where the first clear capillary signatures associated with micro-breaking emerge in the frequency-wavenumber spectra. The CISG successfully captures the spatial onset of these micro-breaking induced capillaries with wavelengths between 0.4 cm and 3 cm. By applying wavelet and band-pass filtering, these features were isolated, allowing for the identification of the "birth" of micro-breaking induced capillaries within the FOV. This work establishes a methodological framework for detecting micro-breaking and provides new insights into the surface conditions governing small-scale dissipation processes in wind-driven wave systems.

Permafrost thaw features –such as thermokarst lakes, retrogressive thaw slumps, and thermo-erosional gullies– are becoming increasingly widespread across the Arctic as a direct result of warming global temperatures. Permafrost underlies 15% of the land surface area in the Northern Hemisphere, and globally is estimated to store substantially more carbon (1,300 Pg of C) than all the world's forests. Despite the massive amount of permafrost soil carbon, little is known about the processes that spur microbial respiration as the soils transition to a post-thaw state, resulting in increased contributions of greenhouse gas emissions. Understanding these processes is particularly important in regions underlain by late Pleistocene, ice-rich Yedoma deposits that contain large amounts of buried, poorly decomposed organic matter. To investigate the potential greenhouse gas (GHG) contribution of these thaw features in Yedoma landscapes, we sampled carbon dioxide and methane gas fluxes across eight thaw transects on the Baldwin Peninsula (Western Alaska) in the summers of 2023 and 2024. We used manual chamber measurements to measure the in-situ GHG fluxes and recorded site parameters to categorize the extent of thaw disturbance (including topography, active layer depth, soil moisture, vegetation, and more). From these measurements, we found that permafrost thaw increased CH4 fluxes exponentially relative to adjacent undisturbed tundra, with mean fluxes ranging from 0.6 to 7.0 mg CH4-C m-2 d-1 and 25.6, 67.4, 71.9 mg CH4-C m-2 d-1 in recently drained thermokarst lake basins, thaw ponds, and thermo-erosional gullies, respectively. Almost all sites were net sources of methane to the atmosphere, with the exception of two upland measurements in minimal thaw disturbance landscapes (CH4 fluxes were < -0.3 mg CH4 m-2 d-1 representing net CH4 oxidation). Across all landscapes included in the sampling, retrogressive thaw slumps, representing freshly exposed Yedoma deposits, had the highest mean CH4 flux among the other disturbed permafrost landforms (159 mg CH4-C m-2 d-1). Though undisturbed upland sites are generally thought to be net sinks of carbon, our measurements show that methane emissions were ubiquitous – even in well aerated sites, suggesting complex or inhomogeneous subsurface conditions. In these undisturbed sites, ecosystem respiration fluxes from sites that had vegetation were not significantly different than those with bare soil, suggesting that it was not the upland plant life driving net GHG patterns. Rather, we propose that the underlying carbon-rich Holocene soils and late Pleistocene Yedoma deposits provide a consistent trickle of methane and CO2 to the surface. This study provides field data on CH₄ and CO₂ fluxes across multiple thaw landforms in a Western Alaska Yedoma landscape, highlighting the potential role of deep carbon mobilization across a gradient of disturbance.

Exchanges of momentum and energy across the sea surface microlayer (SML) are controlled by turbulent dynamics within the first millimeters above/below the wavy water surface. Wind-generated waves, ubiquitous at the ocean surface, strongly influence turbulent processes in the air and water near the surface, especially as the waves grow and (microscale) break. Surface-active substances, commonly found in coastal waters, are known to dampen waves over a wide range of scales. However, the influence of these surfactants on the coupled air-water flow dynamics and associated fluxes remains unclear. Indeed, some of the phenomena involved take place at a sub-millimeter scale, which makes it challenging to investigate the complex mechanisms at stake.A combination of two experimental techniques (PIV, particle image velocimetry, and LIF, Laser Induced Fluorescence) with a high resolution (33 µm/pixel for the PIV and 55 µm/pixel for the LIF) were used to determine flow motions on both sides of the SML. The complex set-up was installed at a fetch of 15.5m at the 24-m long, 1-m wide, 1-5m high wind wave tank of the University of Hamburg (Germany) which is specially designed for studies with surfactants (Oleyl Alcohol, OLA in this work). Here, we focus on conditions with a reference windspeed of 4.5m/s measured by an ultrasonic anemometer at 64 cm above the water surface.The wide field of view (51cm) enables us to capture the evolution in time and space of turbulent shear stress above and below individual wind waves. As the waves move through the field of view, steepen and microbreak, high magnitude turbulent shear is produced in the airflow past the wave-crest and can sometimes spread over several wavelengths when intense air-flow separation events occur. A quadrant analysis shows that negative momentum flux (Q1 and Q3) events are usually encountered before wave-crests whereas positive momentum fluxes (Q2 and Q4) events are produced past them on average. In the water, positive turbulent shear stress mainly shows up below the windward side of the waves, while negative turbulent shear is present below their leeward sides. An estimation of the viscous and turbulent energy dissipation integrated over the first centimeter underneath the water surface shows that the production of bound capillary waves enhances the energy dissipation, which becomes more intense as the capillary train grows up.When surfactants are present, a reduction of sweeps and ejections (Q2 and Q4) past the dampened wave crests is notable and can be associated with the reduced occurrence and intensity of air-flow separation events. In the water, the removal of most capillary waves leads to a reduction in energy dissipation, as well as in the (phase) averaged turbulent kinetic energy below crests.

The Arctic is warming four times faster than the global average, triggering the widespread degradation of permafrost and enhancing mobilization of vast, previously frozen soil nitrogen (N) and carbon (C) stocks. While C release associated with permafrost thaw is better documented, the liberation of permafrost N, a potential precursor to the strong greenhouse gas (GHG) nitrous oxide (N2O), remains a critical knowledge gap. This is particularly relevant for ice-rich Yedoma deposits, which are highly vulnerable to abrupt thaw and the formation of disturbance features, such as retrogressive thaw slumps (RTSs), thermokarst lakes, and drained thermokarst lake basins (DTLBs). While RTSs are known hotspots for N2O, recently formed DTLBs underlain by ice-rich Yedoma deposits with a high content of buried, poorly decomposed organic matter, remain largely understudied, although they are widespread across Yedoma landscapes. Importantly, there are no studies reporting N2O fluxes from DTLBs, despite the high N mineralization expected after drainage, which might support high emissions.Here, we report N2O fluxes and N turnover processes from RTS and DLB on the Baldwin Peninsula, Western Alaska, underlain by ice-rich late Pleistocene Yedoma deposits. In situ fluxes were measured during the summer of 2024, alongside aerial surveys for high-resolution elevation and vegetation mapping with lidar and optical cameras. We conducted laboratory incubations for GHG production, including denitrification and 15N labeling to quantify gross rates of mineralization, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA) using the 15N tracer method.Our study reveals high N2O emissions at both disturbance sites, demonstrating that DTLBs are emerging as significant sources of N2O (up to 7.7 mg N m-2 d-1) emissions, comparable to known high emissions from thaw slumps. supported by high nitrate concentrations, reaching up to 205.1 µg N gDW-1. We identified the role of environmental factors in driving the spatial variability in N2O fluxes as well as N cycling. These findings suggest that as thermokarst lake drainage events increase across the Arctic, DTLBs in Yedoma uplands represent a major, expanding source of permafrost-driven N emissions that must be integrated into global climate feedback models. By tracking how N moves through the soil and how different environmental conditions, like moisture and thaw, trigger specific microbial processes, we can better understand the overall behavior of the N cycle and its growing role in these disturbance landforms.

The land subsurface stored around 6% of the Earth’s energy imbalance in the last five decades (of around 0.5 Wm-2, equivalent to 380 ZJ), being the second contributor to the energy partitioning after the ocean (90%). Previous studies have shown that state-of-the-art Earth System Models (ESMs) remarkably underestimate the observational land heat uptake values. This underestimation stems from Land Surface Models (LSMs) within ESMs imposing too shallow zero-flux bottom boundary conditions to correctly represent the conductive propagation and land heat uptake with depth. When realistically deep boundary conditions are prescribed, land heat uptake increases by a factor of five. However, changes in ground surface temperature are negligible. The reasons for this lack of impact of the LSM depth on surface temperatures are assessed herein.An ensemble of eight historical and RCP8.5 land-only simulations with different subsurface depths was conducted with the LSM of the Max Planck Institute for Meteorology ESM (MPI-ESM), JSBACH. Simulation-derived latent (LHF), sensible (SHF), and ground heat fluxes (GHF) were compared across simulations, and GHF was additionally evaluated against estimates from a one-dimensional heat conduction forward model. Results show that, for a global warming of 1.5 ºC with respect to 1850-1900, GHF increases from 0.04 to 0.07 Wm-2 when deepening the LSM from 10 to 22 m, saturating at around 0.12 Wm-2 when the boundary condition is placed at approximately 100 m. The increase in the incoming GHF is mainly compensated by a global decrease in the outgoing SHF, a small decrease of the LHF in wet regions, and a decrease in the surface net radiation in arid and semi-arid regions. These quantities, yet small, evidence that an insufficient LSM depth induces to an inaccurate resolution of the long-term surface energy balance, which may have implications for land-atmosphere interaction. Their accumulation over time also produces biases in the terrestrial energy partitioning.

This research examines the multifaceted experiences of Early Career Researchers (ECRs) navigating the increasingly globalized landscape of atmospheric chemistry. Drawing upon collective insights from the ECR Scientific Steering Committee and quantitative data from a survey of 180 ECRs across 40 countries, we investigate their primary motivations, challenges, and opportunities. Key obstacles identified include systemic difficulties in securing funding and resources, achieving sustainable work–life balance, and uncertainty around long-term career prospects, often compounded by precarious employment conditions. While globalization offers significant avenues for international collaboration, data sharing, and knowledge exchange, it concurrently presents challenges such as heightened competition, visa restrictions, regional disparities, and the risk of inequitable research practices. Despite these hurdles, ECRs are driven by a strong interest in their field, a desire to make a tangible impact on societal concerns, and the appeal of a supportive community. This research offers actionable insights focused on 4 key pillars: (i) strengthening mentoring programs; (ii) reforming funding mechanisms for improved accessibility and equity; (iii) providing targeted skill development workshops; and (iv) promoting equitable collaborations and advancing the decolonization of research practices. To foster a supportive, inclusive, and sustainable environment for the next generation of atmospheric scientists, this work underscores the urgent need for systemic change and sustained collaborative action by networks such as International Global Atmospheric Chemistry (IGAC), as well as by senior researchers, academic institutions, and funding agencies.The research presented is published as Maximilien Desservettaz, Martin Otto Paul Ramacher, Simone Thirstrup Andersen, Cybelli Barbosa, Sebastian Diez, Hannah Bryant, Tamryn Hamilton, Stephanie Schneider, Karn Vohra, Yuanzhe Li, Sachin Mishra, Nor Diana Abdul Halim, Shahid Uz Zaman, Flossie Brown, Shyno Susan John, Pravash Tiwari, William Apondo, Emily Matthews; A new generation of Early Career Researchers in atmospheric chemistry: Navigating a globalized scientific landscape. Elementa: Science of the Anthropocene 3 January 2025; 13 (1): 00064. doi: https://doi.org/10.1525/elementa.2025.00064.