Visible-Light Active Sulfur-Doped Titania Nanoparticles Immobilized on a Silica Matrix: Synthesis, Characterization and Photocatalytic Degradation of Pollutants

The photocatalytic oxidation (PCO) of pollutants using TiO(2)-based materials can significantly improve indoor air quality (IAQ), which in turn, has a significant impact on human health and life expectancy. TiO(2)-based nanoparticles (NPs) are widely used as part of building materials to function as photocatalysts in PCO. In this work, a series of sulfur-doped TiO(2) NPs immobilized on a silica matrix were synthesized by combining a sol-gel process with ball milling. The samples were structurally characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), Fourier-transform infrared spectroscopy (FT-IR) and N(2) adsorption-desorption isotherms. Furthermore, the morphological characteristics were determined by dynamic light scattering (DLS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photocatalytic activity of the as prepared S-doped TiO(2)/SiO(2) NPs in the degradation of liquid and air pollutants under visible-light irradiation was investigated. Our results show that sulfur is an effective dopant for activating TiO(2)/SiO(2) photocatalysts under visible-light irradiation. Silica constitutes a “safe-by-design” approach and inhibits the aggregation of NPs during synthesis. The most efficient photocatalyst afforded 79% removal of methyl orange (5 h), 26% removal of acetaldehyde (1 h) and 12% oxidation of NO (1 h)

Evolution of structural, mechanical and tribological properties of Ni-P/MWCNT coatings as a function of annealing temperature

Collaborative research project Hardalt (Grant No. 606110) funded by the EU's seventh framework programme

Electrolytic codeposition of Ni-P matrix with SiC and WC particles utilizing pulse current regime: Structural and mechanical characterization

In the present thesis the electrodeposition of Ni-P coatings as well as the codeposition of SiC and WC particles (mean diameter of 1 μm and 200 nm, respectively) with Ni-P matrix from an additive-free modified Watts type bath using a rotating disc electrode and direct and pulse current regime, were studied. The aim of this work was the study of the effect of various electrolysis parameters on the structure, morphology and composition of the coatings, along with the study of the mechanical properties of the coatings focusing in their hardness and tribological performance. Furthermore, the structural properties were correlated with the examined mechanical properties. For the Ni-P coatings a dependence of phosphorous content of the alloy on the imposed conditions of the pulse current regime (i.e. the value of duty cycle and the frequency of the pulses) was revealed. The structure of the coating is characterized as amorphous, while under intense conditions of pulse current (low duty cycle values - high pulse frequencies) the production of the crystalline phase of Νi12P5 was achieved for the first time. Regarding the Ni-P/SiC and Ni-P/WC composite coatings the imposition of pulse current regime led to significant higher codeposition percentage of reinforcing particles, as well as to more homogeneous dispersion in the Ni-P matrix. It should be mentioned that the codeposition of Ni-P/WC coatings was achieved/recorded for the first time. Although SiC codeposition resulted in a decrease of phosphorous content in the matrix, this effect was observed in the case of WC particles. However, it was revealed that the Ni-P matrix of all composite coatings had also an amorphous structure. After thermal treatment at 400 oC the amorphous phase was crystallized in the steady phases of Ni, Ni3P and Ni2P. Moreover, during thermal treatment diffusion phenomena took place, as well as chemical reactions in the interfaces of substrate - coating and reinforcing particles - matrix. The hardness of the Ni-P coatings was in the range of 4,5-5,5 GPa, higher than that of pure nickel, due to the formation of the solid solution of nickel - phosphorous. After thermal treatment the hardness demonstrated a significant increase due to the formation of Ni crystals and the precipitation of Ni3P phase and reached the value of 12 GPa. The hardness values of the composite coating before thermal treatment varied between 5 and 7 GPa, while after thermal treatment were in the range of 12 and 18,5 GPa. The study of the tribological performance of the coatings revealed that the main wear mechanisms were those of abrasion and adhesion. In some cases of Ni-P coatings, fatigue was also detected. Regarding the wear volume loss, it was concluded that the imposition of pulse current, the codeposition of reinforcing particles and the thermal treatment have a beneficial effect on the wear resistance of the coatings. It should be mentioned that among all the coating that were studied in the present work the Ni-P/WC ones showed a superior wear resistance.Στην παρούσα διδακτορική διατριβή μελετήθηκε αφενός η ηλεκτρολυτική παρασκευή απλών επικαλύψεων Ni-P και αφετέρου η συναπόθεση σωματιδίων SiC και WC (μέσης διαμέτρου 1 μm και 200 nm, αντίστοιχα) σε μήτρα Ni-P, από τροποποιημένο λουτρό Watts απουσία οργανικών προσθέτων με χρήση ηλεκτροδίου περιστρεφόμενου δίσκου και την εφαρμογή τόσο συνεχούς όσο και παλμικού ρεύματος σταθερής φοράς. Σκοπός της εργασίας ήταν η μελέτη της επίδρασης διαφόρων παραμέτρων της ηλεκτρόλυσης στη δομή, στη μορφολογία και στο ποσοστό εγκλεισμού των σωματιδίων των παραγόμενων επικαλύψεων, καθώς και η μελέτη των μηχανικών ιδιοτήτων τους και κυρίως της σκληρότητας και της τριβολογικής τους συμπεριφοράς. Επιπλέον, έγινε συσχέτιση των δομικών χαρακτηριστικών με τις μηχανικές ιδιότητες των επικαλύψεων. Στις απλές επικαλύψεις Ni-P διαπιστώθηκε εξάρτηση του ποσοστού φωσφόρου στο κράμα από τις επιβαλλόμενες συνθήκες παλμικού ρεύματος (δηλαδή της τιμής του duty cycle και της συχνότητας των παλμών). Οι επικαλύψεις χαρακτηρίστηκαν ως άμορφες ενώ κάτω από ιδιαίτερα έντονες συνθήκες παλμικού ρεύματος (χαμηλές τιμές duty cycle, υψηλές συχνότητες παλμών) έγινε εφικτή η παρασκευή της κρυσταλλικής φάσης Ni12P5 για πρώτη φορά. Όσον αφορά στις σύνθετες επικαλύψεις Ni-P/SiC και Ni-P/WC η επιβολή παλμικών ρευμάτων οδήγησε σε σημαντικά υψηλότερα ποσοστά απόθεσης των ενισχυτικών σωματιδίων, καθώς και σε πιο ομοιόμορφη διασπορά τους στη μήτρα. Σημειώνεται ότι η συναπόθεση Ni-P/WC πραγματοποιήθηκε για πρώτη φορά. Διαπιστώθηκε ότι η συναπόθεση μικροσωματιδίων SiC επηρεάζει το ποσοστό φωσφόρου στη μήτρα Ni-P μειώνοντάς το, ενώ αντίθετα τα νανοσωματίδια WC δεν παρουσιάζουν αντίστοιχη επίδραση. Παρόλα αυτά, η μήτρα Ni-P στις σύνθετες επικαλύψεις χαρακτηρίστηκε επίσης ως άμορφη. Μετά τη θερμική κατεργασία των αποθεμάτων, διαπιστώθηκε κρυστάλλωση της άμορφης φάσης Ni-P στις φάσεις Ni, Ni3P και Ni2P. Επίσης κατά τη θερμική κατεργασία διαπιστώθηκαν φαινόμενα διάχυσης καθώς και χημικών αντιδράσεων στις διεπιφάνειες υποστρώματος - επικάλυψης και μήτρας - ενισχυτικών σωματιδίων. Η σκληρότητα των επικαλύψεων Ni-P κυμαίνεται στα 4,5-5.5 GPa, σημαντικά μεγαλύτερη από τις επικαλύψεις Ni, λόγω του σχηματισμού του στερεού διαλύματος νικελίου - φωσφόρου. Μετά τη θερμική κατεργασία τους η σκληρότητα αυξάνεται σημαντικά λόγω του σχηματισμού του κρυσταλλικού Ni, και κυρίως των κατακρημνισμάτων Ni3P, και φτάνει τα 12 GPa. Αντίστοιχα οι σκληρότητες των σύνθετων επικαλύψεων Ni-P/SiC και Ni-P/WC κυμαίνονται στα 5-7 GPa πριν τη θερμική τους κατεργασία, ενώ μετά από αυτή διαπιστώνεται αύξηση και είναι μεταξύ 12 και 18,5 GPa. Τέλος, η μελέτη της τριβολογική συμπεριφοράς των επικαλύψεων έδειξε ότι κυρίαρχοι μηχανισμοί φθοράς είναι η άροση και η πρόσφυση, ενώ σε ορισμένες περιπτώσεις επικαλύψεων Ni-P διαπιστώθηκε και η κόπωση. Σε γενικές γραμμές η επιβολή παλμικών ρευμάτων, η συναπόθεση σωματιδίων και η θερμική κατεργασία ενισχύουν την αντίσταση σε φθορά λόγω τριβής των επικαλύψεων. Αξίζει να αναφερθεί ότι οι σύνθετες επικαλύψεις Ni-P/WC παρουσίασαν ιδιαίτερα αυξημένη αντοχή στη φθορά λόγω τριβής

High Performance Accelerated Tests to Evaluate Hard Cr Replacements for Hydraulic Cylinders

To prolong the lifetime of hydraulic cylinders, a wear-resistant low-friction surface is required. Until now, hard Cr coatings were the best materials for this. However, in recent years, there has been an increasing pressure on the manufacturing of hard Cr plating and plated products, because of environmental and health hazards. The replacement of these coatings by alternatives has not been highly successful yet, because it requires extensive component testing, which is costly and time-consuming and thus not appropriate for material development. For this reason, there is a high need to develop tribological methods that simulate hydraulic cylinders’ component-testing closely. In addition, these new methods should also provide additional information (e.g., friction evolution) that can assist in the further development and optimization of alternative coatings. Having the above in mind and building on an existing method from the American Society for Testing and Materials (ASTM G133), a new test method that allows users to test directly on hydraulic cylinders was developed. This method can provide a relative ranking of both the wear resistance and frictional performance of alternative coatings in direct comparison to state-of-the-art hard Cr. Importantly, the method is repeatable and has a much shorter test duration than full-scale component tests, thereby accelerating material development significantly

Effect of annealing temperature on microstructure, mechanical and tribological properties of nano-SiC reinforced Ni-P coatings

The tribological properties of Ni-P/SiC nanocomposite coatings annealed at different temperatures (350–500 °C) were investigated in order to determine the optimal temperature needed to enhance their wear resistance as well as to reveal the underlying wear mechanisms. With increasing annealing temperature, the hardness of the annealed coatings gradually decreased from 8.2±0.5 to 7.1±0.6 GPa as a result of the Hall-Petch effect, nevertheless these values obtained were constantly higher than that of the as-plated coating (6.3±0.3 GPa) due to the formation of a hard Ni3P phase. Regarding to tribological properties, the Ni-P/SiC coating annealed at 350 °C presented a poorer wear resistance (6.1×10-5 mm3/Nm) compared to the as-plated coating (3.9×10-5 mm3/Nm) owing to a rougher original contact surface and the subsequent generation of nickel and iron oxides on the wear track. In contrast, coatings annealed at temperatures ranging between 400–500 °C exhibited the improved wear resistance (4.3×10-5 – 7.8×10-6 mm3/Nm) attributable to their smoother surfaces and to the lubrication effect of H3PO4 arising from the tribochemical reaction between Ni3P and the environment. Overall, the Ni-P/SiC coating annealed at 500 °C containing the largest amount of Ni3P exhibited the lowest friction coefficient (0.51) and wear rate (7.8×10-6 mm3/Nm)

Evolution of structural, mechanical and tribological properties of Ni–P/MWCNT coatings as a function of annealing temperature

The structural, mechanical and tribological properties of Ni–P/MWCNT coatings annealed at various temperatures (350–500 °C) were investigated using XRD, SEM, nanoindentation and tribometer to determine the optimal annealing temperature for their enhanced tribological properties. The results showed that the annealed coatings comprised a hard Ni3P phase, and consequently presented a higher hardness (from 7.0 ± 0.3 to 8.2 ± 1.4 GPa) than the as-plated sample (6.0 ± 0.9 GPa). With the annealing temperature increasing from 350 °C to 500 °C, the crystallinity of coating was enhanced with larger crystal grains of Ni and Ni3P, which led to a decline in hardness (from 8.2 to 7.0 GPa) due to the Hall-Petch effect. Owing to the lubrication effect of H3PO4 arising from the tribochemical reaction of Ni3P with ambient environment, the annealed samples exhibited lower friction coefficients (0.71 ~ 0.86) compared to the as-plated coating (0.87). A combination of low surface roughness and the reduction of oxides on wear track contributed to the lowest friction coefficient of Ni–P/MWCNT annealed at 400 °C. However, the decomposition of amorphous carbon in MWCNT over 380 °C produced less dense coatings (for annealing temperatures 400–500 °C), and their incompact structure led to a higher wear rate (2.9–3.0 × 10? 5 mm3/Nm) compared to the as-plated sample (2.4 × 10? 5 mm3/Nm). In contrast, Ni–P/MWCNT coating annealed at 350 °C (< 380 °C) exhibited a better wear resistance (4.3 × 10? 6 mm3/Nm). Thus, 350 °C was found to be the optimal annealing temperature to lower the friction coefficient and enhance the wear resistance of Ni–P/MWCNT coating