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- ItemCleaning of tungsten tips for subsequent use as cold field emitters or STM probes(FEI STU, 2024-01-31) Košelová, Zuzana; Horáková, Lenka; Burda, Daniel; Allaham, Mohammad Mahmoud; Knápek, Alexandr; Fohlerová, ZdenkaThis study investigates the crucial process of cleaning cold field emission electron emitters and scanning tunnel microscopy (STM) probes, particularly focusing on tungsten tips. The cleanliness of these tips is essential for maintaining optimal cathode properties, preventing impurities that can significantly affect the emission process. Various cleaning methods, including macroetching, ammonia cleaning, and hydrofluoric acid (HF) cleaning were explored and compared by scanning electron microscopy. The macroetching method, involving a mixture of hydrochloric acid, nitric acid, and hydrogen fluoride, proved to be too reactive, causing significant material removal and altering the tip's structure. Ammonia cleaning did not significantly improve or harm the samples. However, oxide islands appeared in some areas, suggesting the potential formation of ammonium tungsten oxide. HF cleaning, specifically at 20% and 50% concentrations, demonstrated effectiveness in removing tungsten oxides without damaging the tip. Pre-cleaning with water and ethanol proved beneficial for subsequent HF refinement. Results suggest that HF is the most suitable method for oxide removal but a rinse with water is essential for removing residual sodium hydroxide. To maintain optimal properties, it is crucial to apply a less reactive layer quickly or transfer the tips to a water/ethanol bath to prevent oxidation.
- ItemMONITORING THE INTERNAL STRUCTURE BEHAVIOUR OF ALKALI-ACTIVATED SLAG PASTE: EFFECT OF THE CURING MODE(CZECH TECHNICAL UNIV PRAGUE, 2023-12-31) Nápravník, Petr; Kocáb, Dalibor; Bílek, Vlastimil; Lisztwan, Dominik; Kucharczyková, BarbaraThis paper deals with the monitoring of the internal structure behaviour of an alkali -activated slag (AAS) paste. The slag was activated with a 4M solution of sodium hydroxide. The behaviour of the internal structure of the paste was regularly monitored through the changes in the resonant frequency and the mechanical properties, until the paste reached the age of 90 days. The main aim of the article is to show the long-term maturation and degradation process of an AAS paste under different curing modes. The results obtained suggest that the curing mode of the specimens has a significant effect on the behaviour of the internal structure of the paste based on the AAS. The development of both the dynamic properties and the flexural strength indicates the occurrence of a higher porosity in the internal structure of the paste, especially when the free drying process is started earlier. Insufficient hydration of the binder system is also a likely cause of cracks. The reduction in the relative dynamic moduli values ranging from 50 % to 80 % was observed for drying specimens at the age of 90 days. What is very interesting is that the occurrence of cracks was not prevented even by intensive moist curing of the paste as, between the 21st and the 28th day of maturing, there was a significant decrease of about 20 % in the relative dynamic modulus of elasticity and also a 50 % reduction in the flexural strength.
- ItemCarboxymethyl starch as a reducing and capping agent in the hydrothermal synthesis of selenium nanostructures for use with three-dimensional-printed hydrogel carriers(ROYAL SOC, 2023-10-11) Vishakha, Vishakha; Abdellatif, Abdelmohsen Moustafa; Michalička, Jan; White, Paul B.; Lepcio, Petr; Tinoco Navarro, Lizeth Katherine; Jančář, JosefThe hydrothermal method is a cost-effective and eco-friendly route for preparing various nanomaterials. It can use a capping agent, such as a polysaccharide, to govern and define the nanoparticle morphology. Elemental selenium nanostructures (spheres and rods) were synthesized and stabilized using a tailor-made carboxymethyl starch (CMS, degree of substitution = 0.3) under hydrothermal conditions. CMS is particularly convenient because it acts simultaneously as the capping and reducing agent, as verified by several analytical techniques, while the reaction relies entirely on green solvents. Furthermore, the effect of sodium selenite concentration, reaction time and temperature on the nanoparticle size, morphology, microstructure and chemical composition was investigated to identify the ideal synthesis conditions. A pilot experiment demonstrated the feasibility of implementing the synthesized nanoparticles into vat photopolymerization three-dimensional-printed hydrogel carriers based on 2-hydroxyethyl methacrylate (HEMA). When submersed into the water, the subsequent particle release was confirmed by dynamic light scattering (DLS), promising great potential for use in bio-three-dimensional printing and other biomedical applications.
- ItemMaximizing the electrochemical performance of supercapacitor electrodes from plastic waste(Elsevier, 2023-08-07) Dědek, Ivan; Bartusek, Stanislav; Dvořáček, Josef Jan; Nečas, Jan; Petruš, Josef; Jakubec, Petr; Kupka, Vojtěch; Otyepka, MichalThe management of the increasing volume of plastic waste has become a key challenge for society. A promising strategy now consists in the transformation of plastic waste into high-value materials that can be utilized in energy storage devices such as batteries and supercapacitors. In this study, we demonstrate a two-step procedure, involving pyrolysis, followed by chemical activation that will convert common plastic waste into activated carbons (ACs). This technique makes ACs suitable for supercapacitor electrode materials. Further, the electrochemical performance of ACs is outstanding in terms of capacitance, energy density, and cycling stability. Besides the well-established parameters, including a specific surface area and micropore volume, we found that other critical factors such as polymer glass transition temperature, polymer-activating agent miscibility, activating agent (K2CO3):AC ratio, and AC water dispersion stability also play a crucial role in determining the supercapacitors performance. Controlling these parameters, we obtained ACs as supercapacitor electrodes from a range of plastic waste materials with a competitive electrochemical performance. Specifically, the ACs exhibited a specific capacitance of 220 F g1 (at a current density of 1 A g1), energy and power densities of 61.1 Wh kg1 and 36.9 kW kg1, respectively, and excellent cycling stability (95 % retention after 30,000 cycles). Our findings provide a pathway towards transforming plastic waste into valuable electrode materials for supercapacitors.
- ItemAccelerated carbonation of oil-well cement blended with pozzolans and latent hydraulic materials(SPRINGER, 2023-08-12) Kuzielova, Eva; Slaný, Michal; Žemlička, Matúš; Másilko, JiříAccelerated carbonation of cement mixtures consisting of class G cement, silica fume, metakaolin, or blast furnace slag was studied by thermogravimetric, X-ray diffraction and Fourier transform infrared analyses for 1 year. Surface parts were fully carbonated during the first 7 days. Polymerization of amorphous hydrates due to their decalcification was observed together with the rising amount of calcium carbonates and reformation of gypsum from ettringite and monosulfate. Decalcification of clinker phases took place fast after the depletion of portlandite. Although portlandite was still present in unblended pastes, because of slower carbonation rate in the surface parts and higher C/S ratio of amorphous hydrates, the carbonation front moved inward during 3 months. Pozzolanic reactions in the samples with 30 mass% of additives depleted portlandite, however, higher amounts of C-(A)-S-H phases with lower C/S ratio, denser microstructure and faster carbonation of surface parts ensured their resistance against carbonation throughout the monitored period.