Influence of ECAP process on mechanical and corrosion properties of pure Mg and ZK60 magnesium alloy for biodegradable stent applications

Equal channel angular pressing (ECAP) was performed on ZK60 alloy and pure Mg in the temperature range 150-250 °C. A significant grain refinement was detected after ECAP, leading to an ultrafine grain size (UFG) and enhanced formability during extrusion process. Comparing to conventional coarse grained samples, fracture elongation of pure Mg and ZK60 alloy were significantly improved by 130% and 100%, respectively, while the tensile strength remained at high level. Extrusion was performed on ECAP processed billets to produce small tubes (with outer/inner diameter of 4/2.5 mm) as precursors for biodegradable stents. Studies on extruded tubes revealed that even after extrusion the microstructure and microhardness of the UFG ZK60 alloy were almost stable. Furthermore, pure Mg tubes showed an additional improvement in terms of grain refining and mechanical properties after extrusion. Electrochemical analyses and microstructural assessments after corrosion tests demonstrated two major influential factors in corrosion behavior of the investigated materials. The presence of Zn and Zr as alloying elements simultaneously increases the nobility by formation of a protective film and increase the local corrosion damage by amplifying the pitting development. ECAP treatment decreases the size of the second phase particles thus improving microstructure homogeneity, thereby decreasing the localized corrosion effects

Performance of the Uncoated Titanium Anode in the Chemical Oxygen Demand Removal of Industrial Wastewaters; (a Case Study on the Electrochemical Treatment of the Textile Effluent at the Pre-Pilot Scale)

In the current work, the capability of uncoated titanium anode in the electrochemical treatment of textile wastewater has been investigated with the aim of simultaneously benefiting from electrooxidation and electrocoagulation treatment processes. In the present work, the feasibility of using uncoated titanium anodes for wastewater treatment is studied in an electrochemical pre-pilot set-up with polymeric casing and an electrical supply power of 150 W, operated under galvanostatic regime in batch mode, focusing on the current density as the main subject of assessment, and its performance is evaluated using metrics such as chemical oxygen demand removal and specific energy consumption. A noticeable finding of this work, is the flexibility of the set-up to combine the electrocoagulation and electro-oxidation mechanisms with the current density as the controlling parameter, leading to a remarkable decontamination capability, so that reductions in the total chemical oxygen demand as large as 75–80% in the neutral and 90–95% in the acidic environments were achieved. At low current densities ( 100 μA/cm2), the anodic corrosion was accelerated and the dominant wastewater treatment mechanism was switched to electrocoagulation. Along with the chemical oxygen demand removal capability, the energetic cost-effectiveness of the set-up was a major concern, particularly from the industrial point of view, which was assessed in both neutral and acidic environments, and it was realized optimization occurred at 600 μA/cm2, so that the specific energy consumption and the rate specific energy consumption, were both minimized at this current density, in respective order, 8.9 kWh/kgCOD and 3.52 kWh/kgCOD/h in neutral, and 10 kWh/kgCOD and 2.34 kWh/kgCOD/h in acidic environments

Evaluation of the operating potential window of electrochemical capacitors

Widening of the operating potential window is a straightforward route towards increasing the specific energy of electrochemical capacitors. Usually, the assessment of the viable potential range is committed to thermodynamic considerations over the electrolyte potential window and experimental probing of the electrochemical stability of both electrolyte and electrode materials through cyclic voltammetry. However, while the former approach is too conservative, the latter is prone to failure. In the present work, we consider quantitative approaches for monitoring the influence of the potential window on the dissipative behaviour of aqueous electrochemical capacitors. For proving the concept underlying this work, we analyse as a case study an asymmetric cell with a reduced graphene oxide / MnO2 positive electrode and a carbon nanotube based negative electrode, as well as nominally symmetric cells assembled with either electrode. We apply and compare different procedures to define the safe potential window of these systems: the usual potential window opening technique, applied either to single electrodes or packed cells; and not conventional methods, based on the analysis of either the energy efficiency or the cell impedance as a function of the cell potential. Precisely, we analyse the trend of the energy efficiency, derived from galvanostatic charge / discharge experiments, and that of the real component of the cell impedance vs. the potential window, to discriminate the onset of irreversible processes leading to dissipative losses. The viability of the proposed methods and the reliability of the attendant criteria are finally checked in the light of the results of cycling performance of the asymmetric cell

Comparative Study of the Growth of CNTs on Stainless Steel with and without the External Catalyst

Multi-walled carbon nanotubes were synthesized on 316 stainless steel substrate by chemical vapor deposition through two different methods: 1- without use of any external catalyst and using ethylene as the carbon precursor and 2- using ferrocene as an external source of catalyst particles, dissolved in toluene, as the carbon precursor. Carbon nanotubes grown by the two methods were characterized by scanning and transmission electron microscopy and X-ray diffraction methods and were compared subsequently to determine certain characteristics of each method. Good coverage and homogeneity was observed in both cases. However, the carbon layer was thicker and denser in externally catalyzed samples. Two different mechanisms, namely, base and tip modes, were observed for the nanotubes growth, each with particular characteristics stemming from the synthesis methods. Surface nano-features and external catalyst behavior were found to have the dominant role in determining the morphology of carbon filaments in intrinsically and externally catalyzed samples, respectively

Calculation of Uniform Corrosion Current Density of Iron in Hydrochloric Acid Solutions based on the Principle of Maximum Entropy Production Rate Applied to Literature Data

In this paper a new procedure for the calculation of uniform corrosion current density is presented. The procedure is based on the following criterion: corrosion current and extent of anodic and cathodic areas are such that the entropy production rate is maximum. Experimental data for the reaction Fe(s) + HCl(aq) = FeCl2(aq) + H2(g), taken from literature, are used to test the new model. A comparison between corrosion current density calculated from weight loss measurements and current density calculated from polarization curves by means of the proposed model is carried out. The proposed model provides a better agreement, between Tafel line extrapolation and gravimetric measurement, in comparison to the mixed potential theory usually adopted

Supercapacitor electrodes by direct growth of multi-walled carbon nanotubes on Al: a study of performance versus layer growth evolution

Supercapacitor electrodes were fabricated by direct growth of multi-walled carbon nanotubes (CNTs) on Al current collectors via a chemical vapor deposition process in the presence of a spin-coated Co-Mo catalyst. A detailed study of the dependence of the CNT layer structure and thickness on growth time set the basis for the assessment of supercapacitors assembled with the CNTs/Al electrodes. As the main features of the layer growth evolution, an increase in the population of finer CNTs and a shift from a random entanglement to a rough vertical alignment of nanotubes were noted with proceeding growth. The growth time influence on the performance of supercapacitors was in fact apparent. Particularly, the specific capacitance of CNTs/Al electrodes in 0.5 M K2SO4 aqueous electrolyte increased from 35 to 80 F g-1 as the CNT layer thickness varied from 20 to 60 mm, with a concurrent loss in rate capability (knee frequency from 1 kHz to 60 Hz). The latter was excellent in general, arguably due to both a fast ion transport through the interconnected CNT network and a negligible contribution of the active layer/current collector contact to the equivalent series resistance (0.15–0.22 mV g), a distinct advantage of the direct growth fabrication method. Overall, a relatively simple process of direct growth of CNTs on Al foils is shown to be an effective method to fabricate supercapacitor electrodes, notably in the absence of special measures and processing steps finalized to a tight control of nanotubes growth and organization

Effects of CVD direct growth of carbon nanotubes and nanofibers on microstructure and electrochemical corrosion behavior of 316 stainless steel

In this work, the corrosion behavior of three differently treated AISI 316 stainless steel plates is investigated, viz. (1) pristine, (2) coated with a carbon nanotube (CNT) layer and (3) coated with a carbon nanofiber (CNF) layer.CNTs andCNFswere directly grown on stainless steel via a CVD method, using ethylene as the carbon source and without the addition of an external catalyst. The corrosion behavior of these materials was investigated by a combination of microstructural and electrochemical methods. Electrochemical tests included potentiodynamic and potentiostatic techniques in hot sulfuric acid solutions. A strong deterioration in corrosion resistance was revealed by the electrochemical tests and confirmed by microstructural examination of the samples. It was found that carbon diffusion into the substrate material during the CNT/CNF growth process results in chromium depletion of the near-surface region of 316 SS and chromiumcarbide precipitation at grain boundaries causing accelerated intergranular corrosion. Accordingly, notwithstanding the obvious inability of CNT/CNF layers to provide protection to the substrate due to their porous nature, a real corrosion damage arises from the high temperature exposure of stainless steel to the CVD atmosphere, suggesting that a similar risk may be present even for compact carbon coating deposited by CVD process, in the event of local damage of the coating

Manganese Oxide Particles as Cytoprotective, Oxygen Generating Agents

Cell culture and cellular transplant therapies are adversely affected by oxidative species and radicals. Herein, we present the production of bioactive manganese oxide nanoparticles for the purpose of radical scavenging and cytoprotection. Manganese comprises the core active structure of somatic enzymes that perform the same function, in vivo. Formulated nanoparticles were characterized structurally and surveyed for maximal activity (superoxide scavenging, hydrogen peroxide scavenging with resultant oxygen generation) and minimal cytotoxicity (48-h direct exposure to titrated manganese oxide concentrations). Cytoprotective capacity was tested using cell exposure to hydrogen peroxide in the presence or absence of the nanoparticles. Several ideal compounds were manufactured and utilized that showed complete disproportionation of superoxide produced by the xanthine/xanthine oxidase reaction. Further, the nanoparticles showed catalase–like activity by completely converting hydrogen peroxide into the corresponding concentration of oxygen. Finally, the particles protected cells (murine β-cell insulinoma) against insult from hydrogen peroxide exposure. Based on these observed properties, these particles could be utilized to combat oxidative stress and inflammatory response in a variety of cell therapy applications