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

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

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

Silicon Algae with Carbon Topping as Thin-film Anodes for Lithium-ion Microbatteries by a Two-step facile Method

Silicon-based electrodes for Li-ion batteries (LIB) attract much attention because of their high theoretical capacity. However, their large volume change during lithiation results in poor cycling due to mechanical cracking. Moreover, silicon can hardly form a stable solid electrolyte interphase (SEI) layer with common electrolytes. We present a safe, innovative strategy to prepare nanostructured silicon-carbon anodes in a two-step process. The nanoporosity of Si films accommodates the volume expansion while a disordered graphitic C layer on top promotes the formation of a stable SEI. This approach shows its promises: carbon-coated porous silicon anodes perform in a very stable way, reaching the areal capacity of ~175µAh cm-2, and showing no decay for at least 1000 cycles. With requiring only a two-step deposition process at moderate temperatures, this novel very simple cell concept introduces a promising way to possibly viable up-scaled production of next-generation nanostructured Si anodes for lithium-ion microbatteries