Zsolt Fogarassy

Green synthesis of gold nanoparticles by thermophilic filamentous fungi

Alternative methods, including green synthetic approaches for the preparation of various types of nanoparticles are important to maintain sustainable development. Extracellular or intracellular extracts of fungi are perfect candidates for the synthesis of metal nanoparticles due to the scalability and cost efficiency of fungal growth even on industrial scale. There are several methods and techniques that use fungi-originated fractions for synthesis of gold nanoparticles. However, there is less knowledge about the drawbacks and limitations of these techniques. Additionally, identification of components that play key roles in the synthesis is challenging. Here we show and compare the results of three different approaches for the synthesis of gold nanoparticles using either the extracellular fraction, the autolysate of the fungi or the intracellular fraction of 29 thermophilic fungi. We observed the formation of nanoparticles with different sizes (ranging between 6 nm and 40 nm) and size distributions (with standard deviations ranging between 30% and 70%) depending on the fungi strain and experimental conditions. We found by using ultracentrifugal filtration technique that the size of reducing agents is less than 3 kDa and the size of molecules that can efficiently stabilize nanoparticles is greater than 3 kDa.

Highly wear-resistant and low-friction Si 3 N 4 composites by addition of graphene nanoplatelets approaching the 2D limit

Graphene nanoplatelets (GNPs) have emerged as one of the most promising filler materials for improving the tribological performance of ceramic composites due to their outstanding solid lubricant properties as well as mechanical and thermal stability. Yet, the addition of GNPs has so far enabled only a very limited improvement in the tribological properties of ceramics, particularly concerning the reduction of their friction coefficient. This is most likely due to the challenges of achieving a continuous lubricating and protecting tribo-film through a high GNP coverage of the exposed surfaces. Here we demonstrate that this can be achieved by efficiently increasing the exfoliation degree of GNPs down to the few-layer (FL) range. By employing FL-GNPs as filler material, the wear resistance of Si3N4 composites can be increased by more than twenty times, the friction coefficient reduced to nearly its half, while the other mechanical properties are also preserved or improved. Confocal Raman spectroscopy measurements revealed that at the origin of the spectacular improvement of the tribological properties is the formation of a continuous FL- GNP tribo-film, already at 5 wt% FL-GNP content.

The Influence of Impurity Content on Thermal Stability of Low Stacking Fault Energy Silver Processed by Severe Plastic Deformation

The effect of the impurity content on the evolution of the ultrafine-grained (UFG) microstructure in low stacking fault energy Ag and its stability at room and elevated temperatures were investigated. Samples of silver having high (99.995%) and somewhat lower (99.99%) purity levels were processed by equal-channel angular pressing (ECAP) at room temperature (RT) up to 16 passes. Although, the minimum grain size achieved by ECAP was ~200 nm for both series, the lattice defect structure was strongly influenced by the impurity content. In the samples processed by 4-16 passes of ECAP a self-annealing occurred during storage RT that was promoted by the higher twin boundary frequency. Both room-and high-temperature thermal stability of 99.99% purity Ag were much better due to the pinning effect of impurities. It was found that a large number of dislocation loops remained in the microstructure even after recrystallization at high temperatures.

Complex Study of H-Induced Structural Rearrangements in FeZr Glasses

Microstructural changes and relaxation processes were examined in Fe-Zr based rapidly solidified samples. These phenomena occur far below crystallization temperature in the course of heat treatment, or even at room temperature, induced by absorbed hydrogen.

Influence of 20 MeV electron irradiation on the optical properties and phase composition of SiOx thin films

Homogeneous films from SiO1.3 (250 nm thick) were deposited on crystalline Si substrates by thermal evaporation of silicon monoxide. A part of the films was further annealed at 700 °C to grow amorphous Si (a-Si) nanoclusters in an oxide matrix, thus producing composite a-Si-SiO1.8 films. Homogeneous as well as composite films were irradiated by 20-MeV electrons at fluences of 7.2 × 1014 and 1.44 × 1015 el/cm2. The film thicknesses and optical constants were explored by spectroscopic ellipsometry. The development of the phase composition of the films caused by the electron-beam irradiation was studied by transmission electron microscopy. The ellipsometric and electron microscopy results have shown that the SiOx films are optically homogeneous and the electron irradiation with a fluence of 7.2 × 1014 el/cm2 has led to small changes in the optical constants and the formation of very small a-Si nanoclusters. The irradiation of the a-Si-SiOx composite films caused a decrease in the effective refractive index a...