RESUMEN
We present a two-step surface modification process to tailor the micro and nano morphology of niobium oxide layers. Niobium was firstly anodized in spark regime in a Ca- and P-containing solution and subsequently treated by acid etching. The effects of anodizing time and applied potential on the surface morphology is investigated with SEM and AFM, complemented by XPS compositional analysis. Anodizing with a limiting potential of 250 V results in the fast growth of oxide layers with a homogeneous distribution of micro-sized pores. Cracks are, however, observed on 250 V grown layers. Limiting the anodizing potential to 200 V slows down the oxide growth, increasing the anodizing time needed to achieve a uniform pore coverage but produces fracture-free oxide layers. The surface nano morphology is further tuned by a subsequent acid etching process that leads to the formation of nano-sized pits on the anodically grown oxide surface. In vitro tests show that the etching-induced nanostructure effectively promotes cell adhesion and spreading onto the niobium oxide surface.
RESUMEN
Metal-metal composites are a class of composite materials studied for their high ductility and strength, but their potential applications are currently limited by the complex manufacturing processes involved. Electro-sinter-forging (ESF) is a single-pulse electro discharge sintering technique that proved its effectiveness in the rapid sintering of several metals, alloys, and composites. Previous studies proved the processability of Ti and AlSi10Mg by ESF to produce metal-metal composites and defined a correlation between microstructure and processing parameters. This paper presents the wear and corrosion characterizations of two metal-metal composites obtained via ESF with the following compositions: 20% Ti/80% AlSi10Mg and 20% AlSi10Mg/80% Ti. The two materials showed complementary resistance to wear and corrosion. A higher fraction of AlSi10Mg is responsible for forming a protective tribolayer in dry-sliding conditions, while a higher fraction of Titanium confers improved corrosion resistance due to its higher corrosion potential.
RESUMEN
A commercially available glass-ceramic composition is applied on a ferritic stainless steel (FSS) substrate reproducing a type of interface present in solid oxide fuel cells (SOFCs) stacks. Electrochemical impedance spectroscopy (EIS) is used to study the electrical response of the assembly in the temperature range of 380-780 °C and during aging for 250 h at 780 °C. Post-experiment analyses, performed by means of X-ray diffraction (XRD), and along cross-sections by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis, highlight the microstructural changes promoted by aging conditions over time. In particular, progressive crystallization of the glass-ceramic, high temperature corrosion of the substrate and diffusion of Fe and Cr ions from the FSS substrate into the sealant influence the electrical response of the system under investigation. The electrical measurements show an increase in conductivity to 5 × 10-6 Sâcm-1, more than one order of magnitude below the maximum recommended value.
RESUMEN
International newspapers and experts have called 3D printing the industrial revolution of this century. Among all its available variants, the fused deposition modeling (FDM) technique is of greater interest since its application is possible using simple desktop printers. FDM is a complex process, characterized by a large number of parameters that influence the quality and final properties of the product. In particular, in the case of semicrystalline polymers, which afford better mechanical properties than amorphous ones, it is necessary to understand the crystallization kinetics as the processing conditions vary, in order to be able to develop models that allow having a better control over the process and consequently on the final properties of the material. In this work it was proposed to study the crystallization kinetics of two different polyamides used for FDM 3D printing and to link it to the microstructure and properties obtained during FDM. The kinetics are studied both in isothermal and fast cooling conditions, thanks to a home-built device which allows mimicking the quenching experienced during filament deposition. The temperature history of a single filament is then determined by mean of a micro-thermocouple and the final crystallinity of the sample printed in a variety of conditions is assessed by differential scanning calorimetry. It is found that the applied processing conditions always allowed for the achievement of the maximum crystallinity, although in one condition the polyamide mesomorphic phase possibly develops. Despite the degree of crystallinity is not a strong function of printing variables, the weld strength of adjacent layers shows remarkable variations. In particular, a decrease of its value with printing speed is observed, linked to the probable development of molecular anisotropy under the more extreme printing conditions.
RESUMEN
The structural and thermoelectric properties of the filled skutterudite (Sm,Gd)y(FexNi1-x)4Sb12 were investigated and critically compared to the ones in the Sm-containing system with the aim of unravelling the effect of double filling on filling fraction and thermal conductivity. Several samples (x = 0.50-0.90 and y = 0.15-0.48) were prepared by melting-sintering, and two of them were densified by spark plasma sintering in order to study their thermoelectric features. The crystallographic study enables the recognition of the role of the filler size in ruling the filling fraction and the compositional location of the p/n crossover: It has been found that the former lowers and the latter moves toward lower x values with the reduction of the filler ionic size, as a consequence of the progressively weaker interaction of the filler with the Sb12 cavity. The analysis of thermoelectric properties indicates that, despite the Sm3+/Gd3+ small mass difference, the contemporary presence of these ions in the 2a site significantly affects the thermal conductivity of both p- and n-compositions. This occurs by reducing its value with respect to the Sm-filled compound at each temperature considered, and making the overall thermoelectric performance of the system comparable to several multi-filled (Fe, Ni)-based skutterudites described in the literature.