researchers aim to magnify the triboelectric charge on polymer surfaces. Polymer triboelectrification can be enhanced in several ways, including surface functionalization, [8] adjustment of the electronic and physicochemical properties between contacting materials, [9][10][11][12][13][14][15] or by increasing the specific contact area via nanostructuring. [16,17] Surface contact electrification can be observed also in nature. Spider ballooning is one of the most exciting natural phenomena. Using electrified strands of silk, [18] spiders can travel in the airstream for distances of hundreds of kilometers. [19] Spider silk is electrified due to contact and friction with airborne particles or during the spinning process. [20] To ensure the electrostatic flight, holding the weight of a spider, even in the absence of any aerodynamic lift, a strong surface charge is required.To understand how silk achieves this strong surface charge, understanding of its macromolecular structure is required. Spider silk has a hierarchical structure composed of highly ordered macromolecular inclusions which are interconnected by disordered elastomeric chains. [21] This macromolecular structure may lead to stress concentration, which we hypothesize is (partially) responsible for the strong surface charge.Recently, it has been found that the surface electrification of polymers is strongly connected to their physicochemical Triboelectrification of polymers enables mechanical energy harvesting in triboelectric generators, droplet generators, and ferroelectrets. Herein, triboelectric polymers, inspired by the ordering in spider-silk, with strongly enhanced contact electrification are presented. The ordering in polyether block amide (PEBA) is induced by the addition of inorganic goethite (α-FeOOH) nanowires that form H-bonds with the elastomeric matrix. The addition of as little as 0.1 vol% of α-FeOOH into PEBA increases the surface charge by more than order of magnitude (from 0.069 to 0.93 nC cm -2 ). The H-bonds between α-FeOOH and PEBA promote the formation of inclusions with higher degree of macromolecular ordering, analogous to the structure of spider silk. The formation of these inclusions is proven via nanoindentation hardness measurements and correlated with H-bond-induced chemical changes by Fourier transform infrared spectroscopy and direct scanning calorimetry. Theoretical studies reveal that the irregularity in hardness provides stress accumulation on the polymer surface during contactseparation. Subsequent molecular dynamic studies demonstrate that stress accumulation promotes the mass-transfer mechanism of contact electrification. The proposed macromolecular structure design provides a new paradigm for developing materials for applications in mechanical energy harvesting.

Surfaces only characterized by a roughness Ra or Sa may have a totally different surface texture and include complex patterns such as grooves, dimples or a mirror-polish. Here, the bearing ratio is proposed as an additional characterization measure to determine the sliding performance of a steel-ice friction pair. Different steel surfaces were produced by milling, shot blasting, and scratching followed by texture assessment with a stylus type 3D profilometer. The bearing ratio and other 3D roughness parameters were determined. Tribology experiments involved a 3 m long inclined plane tribometer and the speed measured at four points during the sliding experiment. Correlation between the steel sliding speed and the bearing ratio was observed under two different regimes: at warmer conditions and at colder conditions. Experiment 1 depicting warmer conditions exhibited a relative humidity of 64%, an air temperature of -2°C and an ice temperature of -9°C. Experiment 2 for colder conditions showed a relative humidity of 78%, an air temperature of 1°C and an ice temperature of -4°C. The sliding speed correlated with the bearing ratio in these two conditions showing -0.91 and -0.96, respectively. A strong correlation between the sliding speed and the bearing ratio shows the value of the bearing ratio as an additional surface characteristic for considering larger surface features.

Frictional interaction with a surface will depend on the features and topography within the contact zone. Describing this interaction is particularly complex when considering ice friction, which needs to look at both the macroscopic and microscopic levels. Since Leonardo da Vinci shared his findings that roughness increases friction, emphasis has been placed on measuring surface coarseness, neglecting the contact area. Here, a profilometer was used to measure the contact area at different slicing depths and identify contact points. Metal blocks were polished to a curved surface to reduce the contact area; further reduced by milling 400 µm grooves or laser-micromachining grooves with widths of 50 µm, 100 µm, and 150 µm. Sliding speed was measured on an inclined ice track. Asperities from pileup reduced sliding speed, but a smaller contact area from grooves and a curved sliding surface increased sliding speed. An analysis of sliding speed versus contact area from incremental slicing depths showed that a larger asperity contact surface pointed to faster sliding, but an increase in the polished surface area reduced sliding. As such, analysis of the surface at different length scales has revealed different design elements—asperities, grooves, curved zones—to alter the sliding speed on ice.

An eco-friendly method for the synthesis of granular activated carbon was developed in this study. Two types of activated carbon and three types of activated carbon granules have been obtained using different binders, and their properties have been determined. The approach requires adding other binders and waste materials to improve the granulation of activated carbon. Activated carbon was prepared from birch wood chips. Prepared carbon was granulated with a) gas generator tar, b) phenol-formaldehyde resin, and c) polyvinyl acetate to obtain granular activated carbon. This work aims to study the possibilities of using activated carbon adsorbents for CO2 adsorption. The activated carbon produced was characterized by BET, FTIR, and SEM. The adsorption behavior on CO2 was also studied. Granular activated carbons compression strength was enough to study it in an adsorption bed, and an optimal binder was to be phenol-formaldehyde resin and polyvinyl acetate. The obtained results show that activated carbon granules are suitable for CO2 adsorption and can be used, for example, for the removal of CO2 in the biogas upgrading process. As the sustainability problems are increasing, granules from waste materials could be promising materials for further studies.

The recent surge in interest in the densification of calcium phosphate powders needs consideration of all the influencing factors. Spark plasma sintering with the primary contribution from the spark plasma and cold compaction that densifies from the large compaction pressures were considered. X-ray diffraction and Fourier transform infra-red spectroscopy characterized the powder and tablet to confirm the retention of the amorphous phase. Density was measured using the Archimedes method and the microstructure was viewed by scanning electron microscopy. The densified tablets were indented by nanoindentation to determine the hardness and elastic modulus. Reports on the density showed that the smallest contribution to density arose from vacuum, a marginally higher densification from the spark plasma effect, but the largest densification arose from the use of significantly higher pressures. Nanoindentation showed a small difference in elastic modulus between tablets densified at 25 °C and 200 °C, but a larger difference in the hardness.