Fabrication and Characterization of Dielectric ZnCr2O4 Nanopowders and Thin Films for Parallel-Plate Capacitor Applications.

We report here the successful shape-controlled synthesis of dielectric spinel-type ZnCr2O4 nanoparticles by using a simple sol-gel auto-combustion method followed by successive heat treatment steps of the resulting powders at temperatures from 500 to 900 °C and from 5 to 11 h, in air. A systematic study of the dependence of the morphology of the nanoparticles on the annealing time and temperature was performed by using field effect scanning electron microscopy (FE-SEM), powder X-ray diffraction (PXRD) and structure refinement by the Rietveld method, dynamic lattice analysis and broadband dielectric spectrometry, respectively. It was observed for the first time that when the aerobic post-synthesis heat treatment temperature increases progressively from 500 to 900 °C, the ZnCr2O4 nanoparticles: (i) increase in size from 10 to 350 nm and (ii) develop well-defined facets, changing their shape from shapeless to truncated octahedrons and eventually pseudo-octahedra. The samples were found to exhibit high dielectric constant values and low dielectric losses with the best dielectric performance characteristics displayed by the 350 nm pseudo-octahedral nanoparticles whose permittivity reaches a value of ε = 1500 and a dielectric loss tan δ = 5 × 10-4 at a frequency of 1 Hz. Nanoparticulate ZnCr2O4-based thin films with a thickness varying from 0.5 to 2 µm were fabricated by the drop-casting method and subsequently incorporated into planar capacitors whose dielectric performance was characterized. This study undoubtedly shows that the dielectric properties of nanostructured zinc chromite powders can be engineered by the rational control of their morphology upon the variation of the post-synthesis heat treatment process.

Superelastic Multimaterial Electronic and Photonic Fibers and Devices via Thermal Drawing.

Electronic and photonic fiber devices that can sustain large elastic deformation are becoming key components in a variety of fields ranging from healthcare to robotics and wearable devices. The fabrication of highly elastic and functional fibers remains however challenging, which is limiting their technological developments. Simple and scalable fiber-processing techniques to continuously codraw different materials within a polymeric structure constitute an ideal platform to realize functional fibers and devices. Despite decades of research however, elastomeric materials with the proper rheological attributes for multimaterial fiber processing cannot be identified. Here, the thermal drawing of hundreds-of-meters long multimaterial optical and electronic fibers and devices that can sustain up to 500% elastic deformation is demonstrated. From a rheological and microstructure analysis, thermoplastic elastomers that can be thermally drawn at high viscosities (above 103 Pa s), allowing the encapsulation of a variety of microstructured, soft, and rigid materials are identified. Using this scalable approach, fiber devices combining high performance, extreme elasticity, and unprecedented functionalities, allowing novel applications in smart textiles, robotics, or medical implants, are demonstrated.

Steering surface topographies of electrospun fibers: understanding the mechanisms.

A profound understanding of how to tailor surface topographies of electrospun fibers is of great importance for surface sensitive applications including optical sensing, catalysis, drug delivery and tissue engineering. Hereby, a novel approach to comprehend the driving forces for fiber surface topography formation is introduced through inclusion of the dynamic solvent-polymer interaction during fiber formation. Thus, the interplay between polymer solubility as well as computed fiber jet surface temperature changes in function of time during solvent evaporation and the resultant phase separation behavior are studied. The correlation of experimental and theoretical results shows that the temperature difference between the polymer solution jet surface temperature and the dew point of the controlled electrospinning environment are the main influencing factors with respect to water condensation and thus phase separation leading to the final fiber surface topography. As polymer matrices with enhanced surface area are particularly appealing for sensing applications, we further functionalized our nanoporous fibrous membranes with a phosphorescent oxygen-sensitive dye. The hybrid membranes possess high brightness, stability in aqueous medium, linear response to oxygen and hence represent a promising scaffold for cell growth, contactless monitoring of oxygen and live fluorescence imaging in 3-D cell models.

In vivo confirmation of hydration-induced changes in human-skin thickness, roughness and interaction with the environment.

The skin properties, structure, and performance can be influenced by many internal and external factors, such as age, gender, lifestyle, skin diseases, and a hydration level that can vary in relation to the environment. The aim of this work was to demonstrate the multifaceted influence of water on human skin through a combination of in vivo confocal Raman spectroscopy and images of volar-forearm skin captured with the laser scanning confocal microscopy. By means of this pilot study, the authors have both qualitatively and quantitatively studied the influence of changing the depth-dependent hydration level of the stratum corneum (SC) on the real contact area, surface roughness, and the dimensions of the primary lines and presented a new method for characterizing the contact area for different states of the skin. The hydration level of the skin and the thickness of the SC increased significantly due to uptake of moisture derived from liquid water or, to a much lesser extent, from humidity present in the environment. Hydrated skin was smoother and exhibited higher real contact area values. The highest rates of water uptake were observed for the upper few micrometers of skin and for short exposure times.