Diapycnal motion, diffusion, and stretching of tracers in the ocean

Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Measurements of in-situ turbulence reveal that mixing is bottom-enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. However, in-situ mixing estimates are indirect and the inferred vertical velocities have not yet been confirmed. Purposeful releases of inert tracers, and their subsequent spreading, have been used to independently infer turbulent diffusivities; however, these Tracer Release Experiments (TREs) provide estimates in excess of in-situ ones. In an attempt to reconcile these differences, Ruan and Ferrari (2021) derived exact buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. We show in a numerical simulation that tracer-averaged diapycnal motion is directly driven by the tracer-averaged buoyancy velocity, a convolution of the asymmetric upwelling/downwelling dipole. Diapycnal spreading, however, involves both the expected contribution from the tracer-averaged in-situ diffusion and an additional non-linear diapycnal stretching term. These diapycnal stretching effects, caused by correlations between buoyancy and the buoyancy velocity, can either enhance or reduce tracer spreading. Diapycnal stretching in the stratified interior is compensated by diapycnal contraction near the bottom; for simulations of the Brazil Basin Tracer Release Experiment these nearly cancel by coincidence. By contrast, a numerical tracer released near the bottom experiences leading-order stretching that varies in time. These results suggest mixing estimates from TREs are not unambiguous, especially near topography, and that more attention should be paid towards the evolution of tracers' first moments.

Kinetics and energetic analysis of the slow dispersive electron transfer from nano-TiO2 to O2 by in situ diffusion reflectance and Laplace transform

In situ diffusion reflectances reveal the trapping-filling effect in the electron transfer from TiO2 to O2 and Laplace transform was developed to derive the broadened apparent barrier energy distribution.

Destructive Effect of Water Vapour on an In Situ Diffusion Barrier Layer within an Aluminide Coating on IN738 Alloy

High-temperature interdiffusion within a hot-dipped aluminide (Al-10 wt.% Si) coating on an IN738 superalloy was investigated at 1050 °C in air and in air plus water vapour. The resulting morphology of in situ diffusion barrier layer (DBL) within the aluminide coating is affected by oxidizing atmospheres; DBL can effectively retard the interdiffusion of aluminium within the coating. The location of the in situ DBL is governed by the partial pressure of oxygen at different depths from the oxide scales in both atmospheres. Meanwhile, the diffusion fluxes of different elements led to DBLs with different morphologies in the aluminide coating on the Ni-based alloy.

Digital autoradiography on C-14-labelled PMMA impregnated rock samples using the BeaverTM

In Finland and Sweden the KBS-3 concept has been chosen for the disposal of spent nuclear fuel in crystalline rock. Recent experiments have shown that heterogeneity of rock may play a major role in the transport of radionuclides. Autoradiographic methods have been proven to be able to assist the characterization of heterogeneous structures. In this study we tested a novel filmless autoradiographic device called BeaverTM which applies a micro patterned gaseous detector in order to quantitatively map beta emissions from C-14 atoms. The studied samples were impregnated with C-14-labelled methylmethacrylate (C-14-MMA) and polymerized to C-14-PMMA with thermal initiator. The BeaverTM was then used to determine the spatial distribution of the C-14-PMMA by measuring the C-14 emissions. The porosity is determined from the amount of C-14-PMMA in the rock sample and results were compared to ones from phosphor imaging plate autoradiography. The resulting images show a heterogeneous distribution of porosity which arises from the different minerals. The samples were chosen from three sites that have been used recently for in situ diffusion experiments: Olkiluoto (Finland), Äspö (Sweden) and Grimsel (Switzerland).

A novel in situ diffusion strategy to fabricate high performance cathodes for low temperature proton-conducting solid oxide fuel cells

A novel in situ Pr diffusion strategy based on a Sm0.2Ce0.8O2−δ–Pr(Pr0.5Ba1.5)Cu3O7−δ (3 : 7 wt%) compound is developed to achieve a perovskite-related proton-blocking composite cathode Ce1−xPrxO2−δ–Ba2CeCu3O7.4–Sm2Ba1.33Ce0.67Cu3O9–CuO for BaZr0.1Ce0.7Y0.2O3−δ-based proton-conducting SOFCs.