Browsing by Author "Torres-Mendieta, Rafael"
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- ItemChemical oxidation and reduction of hexachlorocyclohexanes: A review(2019-01-01) Waclawek, Stanislaw; Silvestri, Daniele; Hrabák, Pavel; Padil, Vinod V. T.; Torres-Mendieta, Rafael; Waclawek, Maria; Černík, Miroslav; Dionysiou, Dionysios D.Lindane (gamma-hexachlorocyclohexane) and its isomers (HCH) are some of the most common and most easily detected organochlorine pesticides in the environment. The widespread distribution of lindane is due to its use as an insecticide, accompanied by its persistence and bioaccumulation, whereas HCH were disposed of as waste in unmanaged landfills. Unfortunately, certain HCH (especially the most reactive ones: gamma- and alpha-HCH) are harmful to the central nervous system and to reproductive and endocrine systems, therefore development of suitable remediation methods is needed to remove them from contaminated soil and water. This paper provides a short history of the use of lindane and a description of the properties of HCH, as well as their determination methods. The main focus of the paper, however, is a review of oxidative and reductive treatment methods. Although these methods of HCH remediation are popular, there are no review papers summarising their principles, history, advantages and disadvantages. Furthermore, recent advances in the chemical treatment of HCH are discussed and risks concerning these processes are given.
- ItemFemtosecond laser settings for optimal bracket bonding to zirconia(2019-03-01) García-Sanz, Verónica; Paredes-Gallardo, Vanessa; Bellot-Arcís, Carlos; Martínez-León, Lluís; Torres-Mendieta, Rafael; Montero, Javier; Albaladejo, AlbertoBonding orthodontic brackets to ceramic materials is a challenging procedure; femtosecond (FS) laser conditioning could provide improved results, but the ideal settings for effective bracket-zirconia bonding have never been established. This study aimed to analyze the differences in surface roughness and shear bond strength (SBS) produced by different femtosecond laser settings and establish a protocol to prepare zirconia surfaces for optimal adhesion to metal orthodontic brackets. One hundred eighty zirconia samples were assigned to six groups according to surface treatment: (1) control; (2) air-particle abrasion (APA); (3) FS laser irradiation (300 mW output power, 60 mu m inter-groove distance); (4) FS laser irradiation (200 mW, 100 mu m); (5) FS laser irradiation (40 mW, 60 mu m); and (6) FS laser irradiation (200 mW, 60 mu m). Surface roughness was measured. Orthodontic brackets were bonded to the zirconia specimens, and SBS was measured. SBS in groups 3 and 6 was significantly higher than the other groups (5.92 +/- 1.12 MPa and 5.68 +/- 0.94 MPa). No significant differences were found between groups 1, 2, 4, and 5 (3.87 +/- 0.77 MPa, 4.25 +/- 0.51 MPa, 3.74 +/- 0.10 MPa, and 3.91 +/- 0.53 MPa). Surface roughness was significantly greater for FS laser than for control and APA groups (p = 1.28 x 10(-8)). FS laser at 200 mW, 60 mu m can be recommended as the ideal settings for treating zirconia surfaces, producing good SBS and more economical energy use.
- ItemLaser-induced fragmentation of carbonyl iron as a clean method to enhance magnetorheological effect(Elsevier Ltd, 2020-05-01) Cvek, Martin; Torres-Mendieta, Rafael; Havelka, Ondřej; Urbánek, Michal; Plachý, Tomáš; Černík, MiroslavMagnetic nanoparticles (NPs) are widely used as additives in magnetorheological (MR) suspensions to enhance their magnetic performance and kinetic stability. The synthesis of NPs is often resolved via chemical routes or complex manufacturing procedures, which require hazardous chemicals and generate waste products. In this study, a clean, laser-mediated strategy known as laser fragmentation in liquids (LFL) is proposed that enables the synthesis of NP-based additives directly from the given MR suspension, with the added advantage of limiting the production of waste during the fabrication process. The carbonyl iron (CI) microparticles dispersed in ethylene glycol were used as both the MR suspension and precursor agent in the production of the nanoscale additive. The size and crystalline structure of the NPs were investigated via TEM and XRD, respectively. The mixture of the MR suspension and laser-synthesized additive notably facilitated the magnetisation of the CI particles, incrementing the MR characteristics and dynamic yield stress values by up to 31% in a low-to-moderate field region, which are important aspects in various field-controlled robotic, damping and torque systems. The concept presented appears to be an effective and clean alternative for fabricating bidispersed MR suspensions through the nanoscale additive approach.
- ItemProduction of electrospun nanofibers based on graphene oxide/gum Arabic(ELSEVIER SCIENCE BV, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS, 2019-03-01) Silvestri, Daniele; Mikšíček, Jiří; Waclawek, Stanislaw; Torres-Mendieta, Rafael; Padil, Vinod V. T.; Černík, MiroslavOver the last few years, the electrospinning technique has attracted significant attention for the production of novel nanofibrous materials. At the same time, the use of graphene oxide and the natural products extracted from plants and/or trees have become very popular in various fields of science. In this work, a new method for the production of nanofibers based on a combination of Gum Arabic (GA), as a natural tree gum exudate, PVA, as an environmentally-friendly stabilizer, and graphene oxide (GO) has been developed and characterized. SEM analysis showed fundamental differences on the surface of bare nanofibers with and without GO, and also significantly smaller fiber diameters in the case of the presence of GO (fibers <100 nm present). Raman spectroscopy confirmed and TGA analysis approximated the content of GO in the nanofibers. Adsorption of methylene blue on the produced nanofibrous membrane was about 50% higher in the presence of GO, which opens the possibility to use GO/GA/PVA fibers in several applications, for example for the removal of dyes.