Silicon Corrosion in Neutral Media: The Influence of Confined Geometries and Crevice Corrosion in Simulated Physiological Solutions

Silicon (Si) based implantable components are widely used to restore functionalities in the human body. However, there have been reported instances of Si corroding after only a few years of implantation. A key parameter often overlooked when assessing Si stability in-vitro, is the added constricting geometries introduced through in-vivo implantation. The influence of crevices and confined solutions on the stability of Si is presented in this study, considering two simulated physiological solutions: 0.01 M phosphate buffered saline (PBS) and HyClone Wear Test Fluid (WTF). It was found that Si is highly vulnerable to corrosion in confined/crevice conditions. High pitting corrosion susceptibility is found in a crevice, whereas a dissolution rate of ca. 3.6 nm/h at body temperature occurred due to local alkalization within a confined cathodic area. The corrosion rates could be increased by elevating the temperature and yielded linear Arrhenius relations, with activation energies of 106 KJ/mol in 0.01MPBS and 109 KJ/mol in HyClone WTF, corresponding to a phosphorous-silicon interaction mechanism. Phosphorous species favored corrosion and contributed to enhanced Si dissolution, while chlorides were not so influential, and applied anodic potential induced pseudo-passivation. These results highlight the importance geometrical configurations can have on a material's surface stability. (c) The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.

Preparation of high-conductivity, high-capacity phase change media capsules with enhanced thermos-chemical stability in thermal energy storage applications

Selmar Rudolf Binder

Thermal energy storage using the effects of the latent heat in the solid-liquid phase transformation has found its way into every day applications. Reusable, organic phase change media stabilize low temperatures of parcels, and salt/water tanks act as buffer storage in residential heating/cooling and industrial processes. Emerging, renewable technologies for energy production and conversion profit from high operation temperatures. Alloys from abundant metals such as aluminum, silicon, and copper have suitable melting characteristics to preheat chemical conversion catalysts and process gas streams for heat-to-power applications. They typically absorb and release a large heat of fusion, facilitate better heat transport compared to salts, and are inert to thermal decomposition compared to organic compounds. Fundamental challenges emerge from the high reactivity of molten metals towards the preferred metal encapsulation materials. Stable encapsulations are necessary to protect the storage medium, and to preserve a constant heat exchange surface during the phase transition. This thesis explores the fabrication of stainless steel capsules for use with pure aluminum, as well as Al-12%Si and Al-26%Cu-5%Si alloys. Experimental capsules of cylindrical shape were prepared with or without interface modification, and tested by isothermal exposure up to 800 °C and in a newly developed test-bench with 100+ melting cycles and in-situ performance measurement. Iron aluminide formation on the interior capsule surface reduced performance. Interface modifications and corrosion layers were characterized by scanning electron microscopy, energy dispersive x-ray spectrometry, differential scanning calorimetry, and Vickers hardness micro-indentation. Results were integrated into numerical diffusion and heat transfer modelling, allowing predictions about the effects of atomic diffusivity, solubility, and thermal conductivity on heat storage performance. Ceramic diffusion barrier coatings for stabilized performance were deposited via modified reverse-dip-coating on steel encapsulation and silicon substrates, and characterized using the same methodology. Ceramic coatings and guidelines for conservative operation conditions were effective in stabilizing heat storage density and power. Boron nitride coating was demonstrated to prevent 90 % of the capacity loss occurring in the control group, in a scaled up pilot experiment with 200 MJ latent heat capacity in encapsulated Al-26%Cu-5%Si. Temperature scanning calorimetric data from the cycling test-bench validated numerical heat transfer simulations and performance predictions from diffusion modelling. Results from validated heat and diffusion modelling were used in the design of a 1.4 MJ lab-scale prototype storage, with automated charging and discharging of stacked encapsulated samples, using air as heat transfer fluid. Herein presented guidelines allow design and operation of robust steel encapsulations, which conserve high performance for 10¿000s of full-load-hours. Our experimental data and interpretative modelling focuses on a relevant material class, that combines low material cost with high conductivity, large capacity, and melting temperatures relevant to advanced power cycles. Experiments on lab-, prototype- and pilot-scale demonstrate how our findings contribute to practical implementation of novel highly performant metal latent heat thermal energy storage systems.

EPFL2019

Influence of Bovine Serum Albumin (BSA) on the Tribocorrosion Behaviour of a Low Carbon CoCrMo Alloy in Simulated Body Fluids

Shoufan Cao, Stefano Mischler, Choshun Yoneyama

Tribocorrosion, as the interaction between mechanical wear and electrochemical corrosion, has been found to be the main problem causing the failure and limiting the lifetime of metal-on-metal artificial hip joints. Better understanding of the tribocorrosion mechanisms of CoCrMo alloys is needed in order to reduce the degradation of this alloy, especially in the presence of proteins as one of the organic components present in synovial fluid. In this study, tribocorrosion tests of a low carbon CoCrMo alloy in phosphate buffer solution (PBS) with and without bovine serum albumin (BSA) in two different concentrations at different applied potentials (passive and cathodic) were carried out. The results show that the effect of proteins on wear and friction was concentration and potential dependent. In the cathodic domain (absence of very thin passive film), wear was very low in all solutions and the friction was significantly reduced by the addition of BSA to PBS even at low BSA concentrations. However, in the passive domain, the friction and wear were found not to be affected when the BSA concentration was 0.5 g/L, while they were reduced when increasing the BSA concentration to 36 g/L. The tribocorrosion results were rationalized through an existing tribocorrosion model and the effect of BSA on wear and friction was explained by the consideration of physical factors such as changes in viscosity and double layer structure, because in the present results no tribofilm formation was observed.

2020

Preparation of high-conductivity, high-capacity phase change media capsules with enhanced thermos-chemical stability in thermal energy storage applications

Selmar Rudolf Binder

EPFL2019