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The hysteresis loops of magnetic samples can be measured by the magneto‐optical Kerr effect (MOKE), while magnetic force microscopy (MFM) is used to depict domain walls or spatially resolved magnetization orientation in magnetic nanostructures or thin films. Both methods are commonly applied on thin films with a perfectly flat surface or on nanostructures. Here, we report MOKE and MFM measurements on a new class of magnetic materials, namely electrospun mats of randomly oriented nanofibers which influence the 30 nm Co coating layer, proving especially the MOKE method utilizable for rough surfaces. Possible interpretations of these first MOKE measurements on highly randomly structured magnetic layers are given. [ABSTRACT FROM AUTHOR]

Polyacrylonitrile can be used as a base material for thermochemical conversion into carbon. Especially nanofiber mats, produced by electrospinning, are of interest to create carbon nanofibers. Optimal stabilization and carbonization parameters, however, strongly depend on the spatial features of the original material. While differences between nano- and microfibers are well known, this paper shows that depending on the electrospinning method and the solvent used, considerable differences between various nanofiber mats have to be taken into account for the optimization of the stabilization conditions. Here, we examine for the first time polyacrylonitrile nanofiber mats, electrospun with wire electrospinning from the low-toxic dimethyl sulfoxide as a solvent, instead of the typically used needle electrospinning from the toxic dimethylformamide. Additionally, we used inexpensive polyacrylonitrile from knitting yarn instead of highly specialized material, tailored for carbonization. Our results show that by carefully controlling the maximum stabilization temperature and especially the heating rate, fully stabilized polyacrylonitrile fibers without undesired interconnections can be created as precursors for carbonization. [ABSTRACT FROM AUTHOR]

Carbon nanofibers are used for a broad range of applications, from nano-composites to energy storage devices. They are typically produced from electrospun poly(acrylonitrile) nanofibers by thermal stabilization and carbonization. The nanofiber mats are usually placed freely movable in an oven, which leads to relaxation of internal stress within the nanofibers, making them thicker and shorter. To preserve their pristine morphology they can be mechanically fixated, which may cause the nanofibers to break. In a previous study, we demonstrated that sandwiching the nanofiber mats between metal sheets retained their morphology during stabilization and incipient carbonization at 500 °C. Here, we present a comparative study of stainless steel, titanium, copper and silicon substrate sandwiches at carbonization temperatures of 500 °C, 800 °C and 1200 °C. Helium ion microscopy revealed that all metals mostly eliminated nanofiber deformation, whereas silicone achieved the best results in this regard. The highest temperatures for which the metals were shown to be applicable were 500 °C for silicon, 800 °C for stainless steel and copper, and 1200 °C for titanium. Fourier transform infrared and Raman spectroscopy revealed a higher degree of carbonization and increased crystallinity for higher temperatures, which was shown to depend on the substrate material. [ABSTRACT FROM AUTHOR]

Electrospun poly(acrylonitrile) (PAN) nanofibers are typical precursors of carbon nanofibers. During stabilization and carbonization, however, the morphology of pristine PAN nanofibers is not retained if the as-spun nanofiber mats are treated without an external mechanical force, since internal stress tends to relax, causing the whole mats to shrink significantly, while the individual fibers thicken and curl. Stretching the nanofiber mats during thermal treatment, in contrast, can result in fractures due to inhomogeneous stress. Previous studies have shown that stabilization and carbonization of PAN nanofibers electrospun on an aluminum substrate are efficient methods to retain the fiber mat dimensions without macroscopic cracks during heat treatment. In this work, we studied different procedures of mechanical fixation via metallic substrates during thermal treatment. The influence of the metallic substrate material as well as different methods of double-sided covering of the fibers, i.e., sandwiching, were investigated. The results revealed that sandwich configurations with double-sided metallic supports not only facilitate optimal preservation of the original fiber morphology but also significantly accelerate the carbonization process. It was found that unlike regularly carbonized nanofibers, the metal supports allow complete deoxygenation at low treatment temperature and that the obtained carbon nanofibers exhibit increased crystallinity. [ABSTRACT FROM AUTHOR]

Electrospinning can be used to prepare nanofibers from various polymers and polymer blends. The adhesion of nanofibers to the substrates on which they are electrospun varies greatly with the substrate material and structure. In some cases, good adhesion is desired to produce sandwich structures by electrospinning one material directly onto another. This is the case, e.g., with dye-sensitized solar cells (DSSCs). While both pure foil DSSCs and pure electrospun DSSCs have been examined, a combination of both technologies can be used to combine their advantages, e.g., the lateral strength of foils with the large surface-to-volume ratio of electrospun nanofibers. Here, we investigate the morphology and adhesion of electrospun nanofibers on different foil substrates containing materials commonly used in DSSCs, such as graphite, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) or TiO2. The results show that the foil material strongly influences the adhesion, while a plasma pretreatment of the foils showed no significant effect. Moreover, it is well known that conductive substrates can alter the morphology of nanofiber mats, both at microscopic and macroscopic levels. However, these effects could not be observed in the current study. [ABSTRACT FROM AUTHOR]

Polyacrylonitrile (PAN) nanofibers, prepared by electrospinning, are often used as a precursor for carbon nanofibers. The thermal carbonization process necessitates a preceding oxidative stabilization, which is usually performed thermally, i.e., by carefully heating the electrospun nanofibers in an oven. One of the typical problems occurring during this process is a strong deformation of the fiber morphologies—the fibers become thicker and shorter, and show partly undesired conglutinations. This problem can be solved by stretching the nanofiber mat during thermal treatment, which, on the other hand, can lead to breakage of the nanofiber mat. In a previous study, we have shown that the electrospinning of PAN on aluminum foils and the subsequent stabilization of this substrate is a simple method for retaining the fiber morphology without breaking the nanofiber mat. Here, we report on the impact of different aluminum foils on the physical and chemical properties of stabilized PAN nanofibers mats, and on the following incipient carbonization process at a temperature of max. 600 °C, i.e., below the melting temperature of aluminum. [ABSTRACT FROM AUTHOR]

Electrospinning is a frequently used method to prepare air and water filters. Electrospun nanofiber mats can have very small pores, allowing for filtering of even the smallest particles or molecules. In addition, their high surface-to-volume ratio allows for the integration of materials which may additionally treat the filtered material through photo-degradation, possess antimicrobial properties, etc., thus enhancing their applicability. However, the fine nanofiber mats are prone to mechanical damage. Possible solutions include reinforcement by embedding them in composites or gluing them onto layers that are more mechanically stable. In a previous study, we showed that it is generally possible to stabilize electrospun nanofiber mats by 3D printing rigid polymer layers onto them. Since this procedure is not technically easy and needs some experience to avoid delamination as well as damaging the nanofiber mat by the hot nozzle, here we report on the reversed technique (i.e., first 3D printing a rigid scaffold and subsequently electrospinning the nanofiber mat on top of it). We show that, although the adhesion between both materials is insufficient in the case of a common rigid printing polymer, nanofiber mats show strong adhesion to 3D printed scaffolds from thermoplastic polyurethane (TPU). This paves the way to a second approach of combining 3D printing and electrospinning in order to prepare mechanically stable filters with a nanofibrous surface. [ABSTRACT FROM AUTHOR]

Electrospinning is a new technology whose scope is gradually being developed. For this reason, the number of known polymer–solvent combinations for electrospinning is still very low despite the enormous variety of substances that are potentially available. In particular, electrospinning from low-toxic solvents, such as the use of dimethyl sulfoxide (DMSO) in medical technology, is rare in the relevant scientific literature. Therefore, we present in this work a series of new polymers that are applicable for electrospinning from DMSO. From a wide range of synthetic polymers tested, poly(vinyl alcohol) (PVOH), poly(2ethyl2oxazolene) (PEOZ), and poly(vinylpyrrolidone) (PVP) as water-soluble polymers and poly(styrene-co-acrylonitrile) (SAN), poly(vinyl alcohol-co-ethylene) (EVOH), and acrylonitrile butadiene styrene (ABS) as water-insoluble polymers were found to be suitable for the production of nanofibers. Furthermore, the influence of acetone as a volatile solvent additive in DMSO on the fiber morphology of these polymers was investigated. Analyses of the fiber morphology by helium ion microscopy (HIM) showed significantly different fiber diameters for different polymers and a reduction in beads and branches with increasing acetone content. [ABSTRACT FROM AUTHOR]