Characterization of Tannic Acid-Coated AZ31 Mg Alloy for Biomedical Application and Comparison with AZ91.

Magnesium alloys are promising materials for bioresorbable implants that will improve patient life and reduce healthcare costs. However, their clinical use is prevented by the rapid degradation and corrosion of magnesium, which leads to a fast loss of mechanical strength and the formation of by-products that can trigger tissue inflammation. Here, a tannic acid coating is proposed to control the degradation of AZ31 and AZ91 alloys, starting from a previous study by the authors on AZ91. The coatings on the two materials were characterized both by the chemical (EDS, FTIR, XPS) and the morphological (SEM, confocal profilometry) point of view. Static degradation tests in PBS and electrochemical measurements in different solutions showed that the protective performances of the tannic acid coatings are strongly affected by the presence of cracks. The presence of fractures in the protective layer generates galvanic couples between the coating scales and the metal, worsening the corrosion resistance. Although degradation control was not achieved, useful insights on the degradation mechanisms of coated Mg surfaces were obtained, as well as key points for future studies: it resulted that the absence of cracks in protective coatings is of uttermost importance for novel biodegradable implants with proper degradation kinetics.

Optimization of Hexagonal Structure for Enhancing Heat Transfer in Storage System.

Thermal performance was tested during cycling work for latent heat storage systems based on KNO3 and NaNO3 (weight ratio 54:46). For heat transfer improvement, cast aluminum honeycomb-shaped structures were produced via 3D printing of polymer model and investment casting. Different wall thicknesses were tested at 1.2 mm and 1.6 mm. The obtained results were compared to working cycles of pure PCM bed. The use of enhancers is reported to improve the rate of charging and discharging of the deposit. In the next step, the structures were examined with numerical simulation performed with ANSYS Fluent software. The wall thicknesses taken into consideration were the following: 0.8, 1.2, 1.6, and 2.0 mm. An insert with a greater wall thickness allows for smaller dT/dt and better heat distribution in the vessel. The investment casting process enables the manufacturing of complex structures of custom shapes without porosity and contamination.

Aluminum Inserts for Enhancing Heat Transfer in PCM Accumulator.

Phase change materials (PCMs) are applied in heat storage units, as they are able to accumulate the energy in the form of the latent heat of fusion. Thus, they can be used in recovering the excess of heat from various industrial processes. Their main weakness is their low thermal conductivity coefficient, which strongly limits their usage. In this paper, the benefits of the application of metallic inserts in heat storage PCM-based units were elaborated. Two kinds of Al-Si spatial elements (foams and honeycomb structures) were produced with the use of means of the investment casting method. Key factors influencing the technological process were established. The surface's roughness was measured in order to compare the obtained structures with their patterns in terms of the casting's accuracy. The compressive strength of the samples was tested, and their fatigue resistance was considered. The thermal performance of manufactured inserts in the PCM (paraffin)-based accumulator, supported by the calculation of heat fluxes, was analyzed and adjusted. Finally, further optimization was conducted in terms of the volume ratio of the metal insert to the PCM. Metallic inserts were found to significantly affect the performance of the entire energy storage system, as their use results in reduced charging time, a longer heat release time, increased maximum temperature, and a significant reduction in the temperature gradient in the heat storage unit.