Papers by Alexandros Vradis
Journal Of The Chinese Advanced Materials Society, Jul 3, 2014
Epoxy composites reinforced with (1) micro-size barium titanate (BaTiO 3), (2) nanosize BaTiO 3 ,... more Epoxy composites reinforced with (1) micro-size barium titanate (BaTiO 3), (2) nanosize BaTiO 3 , (3) graphite nanoplatelets (GNP) , (4) combination of micro and nano BaTiO 3 and (5) combination of BaTiO 3 and GNP were fabricated and studied, in terms of the type, size and amount of the filler content. Scanning electron microscopy and X-ray diffraction spectra were employed for the investigation of the systems' morphology. Dynamic mechanical analysis, static mechanical tests and broadband dielectric spectroscopy were used in order to study the properties of the produced systems. Reinforced systems exhibit higher values of storage modulus and dielectric permittivity compared to neat epoxy. Dielectric spectra reveal the presence of two relaxation processes arising from the glass to rubber transition of the polymer matrix (a-mode), and the reorientation of polar side groups of the polymer chains (b-mode). Finally, the energy storage efficiency of the prepared composite systems was examined in terms of the energy density as a function of the temperature, frequency, type, size and amount of the employed filler.
Dielectric response and molecular dynamics in epoxy-BaSrTiO3 nanocomposites: Effect of nanofiller loading
Polymer, Jul 1, 2016
Abstract BaSrTiO 3 /epoxy resin nanocomposites were prepared in a wide range of filler concentrat... more Abstract BaSrTiO 3 /epoxy resin nanocomposites were prepared in a wide range of filler concentrations. Electron microscopy images demonstrated fine dispersion of the nanoinclusions. Variation of glass transition temperature with particles concentration was determined via differential scanning calorimetry. Three relaxation processes were detected in the dielectric spectra, namely interfacial polarization, glass to rubber transition of the matrix (α-relaxation), and re-arrangement of polar side groups (β-relaxation). Nanofiller content affect the dielectric and thermal response of the composites via the particle-polymer and particle–particle interactions. At low filler content, reduction of the macromolecular mobility causes a decrease of permittivity and an enhancement of glass transition temperature. At intermediate filler content, interparticle interactions appear to dominate, leading to altered polymer flexibility, higher permittivity and lower glass transition temperature. At high filler loading, glass transition temperature increases in tandem with permittivity because of the limited macromolecular mobility, and the high permittivity of the particles. Systems’ performance can be interpreted by a three “filler load zones” model.
Maǧalaẗ ǧameʼaẗ al-malīk Abdul Aziz. Al-uʼlum, 1993
Niobium thin films have been prepared by thermal evaporation techniques in a UHV system using an ... more Niobium thin films have been prepared by thermal evaporation techniques in a UHV system using an electrostatically focussed electron beam evaporator. The starting material was MARZ grade niobium, with 99.999% purity, and in discs 12 mm diameter by 3 mm. Depositioñ as, in general, on to thermally oxidised silicon slices with deposition rates of the order of 2 AS-I. Film properties were examined as a function of film thickness, deposition rate and substrate temperature during deposition. The critical temperature (Tc) was assessed by four-probe assessment of the sheet resistivity ratio at room temperature and in liquid nitrogen. These indicated a critical temperature close to the bulk value of 9.2 K appropriate to niobium and, .in all cases, the films were superconducting in liquid helium (4.2 K). Films of thicknesses < 500 Ao revealed Tc values < 9.2 K and the film resistivity was significantly greater than should have been expected. This was thought to be due to incomplete coalescence of the nucleation processes and a contribution to surface scattering il1 electron transport.
Journal of Applied Physics, 2015
The aim of the present work is to optimize a well-known plasmon-based aluminum/porous anodic alum... more The aim of the present work is to optimize a well-known plasmon-based aluminum/porous anodic alumina sensor. This kind of sensors is produced by partial electrochemical anodization of an aluminum film, while the remaining, non-anodized metallic film is used for supporting the propagation of surface plasmons. The anodized porous alumina is preferable against a flat solid surface as it presents much larger sensing area and thus enhanced detecting efficiency. In this work, a novel method for controlling the remaining aluminum film thickness is reported, based on a simple optical reflectance measurement during the electrochemical anodization of the initial metallic film.
Bulletin of the American Physical Society, Mar 15, 2006
The investigation of the internal structure of thin polymer films on conventional substrates by X... more The investigation of the internal structure of thin polymer films on conventional substrates by X-Ray reflectometry is restrained due to the high electron densities of these substrates compared to those of most polymers. A methodology is proposed and applied to circumvent this problem by utilizing polymeric substrates of appropriate thickness and roughness, which can be reproducibly deposited on the standard substrates in order to render them invisible to the X-rays. The resolution of the X-ray reflectometry is significantly enhanced as illustrated utilizing thin films of PS-b-PMMA diblock copolymers of various thicknesses. The proposed methodology will allow the study of internal polymer interfaces using in-house diffractometers/reflectometers.
Morphology and electrical response of hydrogenated acrylonitrile butadiene rubber (HNBR) and its ... more Morphology and electrical response of hydrogenated acrylonitrile butadiene rubber (HNBR) and its multiwall carbon nanotube (MWCNT) reinforced nanocomposites were studied by means of x-ray diffraction and broadband dielectric spectroscopy. HNBR systems were found to be semi-crystalline, with their crystallinity to increase with the addition of MWCNTs. In their dielectric spectra, four relaxation processes were detected. Ascending in relaxation time, these were attributed to: (i) interfacial polarization at the interface of crystalline and amorphous regions of HNBR and at the interface between HNBR and MWCNTs, (ii) glass to rubber transition of the amorphous part of HNBR, (iii) rearrangement of polar side groups, such as-CN, and (iv) local motions of small segments of the main elastomer chain. Electrical conductivity increases with MWCNT content and frequency increasing. The effect of temperature, on the electrical response, is more pronounced at low frequencies. The temperature dependence of the electrical conductivity strongly deviates from a pure Arrhenius behavior, signifying that the occurring conductance mechanisms do not correspond to a single thermally activated process. Relaxation dynamics imply that crystalline regions exert motion restrictions to large segments of the macromolecules in the amorphous phase and to polar parts of the systems.
Flow through Alumina Nanopores Bearing Responsive Polymer Brushes
Macromolecules, Jul 15, 2022
Contemporary materials, Oct 10, 2014
Renewable energy sources have an important role in global energy demand in the 21 st century. New... more Renewable energy sources have an important role in global energy demand in the 21 st century. New investigations in solar energy materials and solar energy systems offer new opportunities for a sustainable future, adapting local energy needs, environmental protection and aesthetics. Solar energy can be used almost everywhere, with visible materials and systems. In this paper, an overview of new solar energy materials and systems is presented, focused on the work that is performed in the Laboratories of the University of Patras and of the Technical Educational Institute. The research activities of these Laboratories are on nanotechnology, biotechnology, microalgae hydrogen production, photovoltaics, smart glazings and materials for solar energy systems. A brief description of the achieved improvements on these solar energy materials is included, where energy performance and system cost play an important role together with aesthetics, considering their wide application in the future.
Shear effects on the geometry of polystyrene-polyethylene oxide copolymers at the solid-solvent interface
Physica B-condensed Matter, Dec 1, 1997
The geometry of polystyrene (PS)-polyethylene oxide (PEO) diblock copolymers at the quartz-solven... more The geometry of polystyrene (PS)-polyethylene oxide (PEO) diblock copolymers at the quartz-solvent interface has been examined as a function of solvent quality and shear by neutron reflectometry. Under conditions where a brush geometry is observed for the static PS free block, extremely large shear rates (∼ 10 000/s) produce little or no effect on the structure, in agreement with the
Macromolecules, Dec 1, 1998
European Polymer Journal, May 1, 2014
Morphology and electrical response of hydrogenated acrylonitrile butadiene rubber (HNBR) and its ... more Morphology and electrical response of hydrogenated acrylonitrile butadiene rubber (HNBR) and its multiwall carbon nanotube (MWCNT) reinforced nanocomposites were studied by means of x-ray diffraction and broadband dielectric spectroscopy. HNBR systems were found to be semi-crystalline, with their crystallinity to increase with the addition of MWCNTs. In their dielectric spectra, four relaxation processes were detected. Ascending in relaxation time, these were attributed to: (i) interfacial polarization at the interface of crystalline and amorphous regions of HNBR and at the interface between HNBR and MWCNTs, (ii) glass to rubber transition of the amorphous part of HNBR, (iii) rearrangement of polar side groups, such as-CN, and (iv) local motions of small segments of the main elastomer chain. Electrical conductivity increases with MWCNT content and frequency increasing. The effect of temperature, on the electrical response, is more pronounced at low frequencies. The temperature dependence of the electrical conductivity strongly deviates from a pure Arrhenius behavior, signifying that the occurring conductance mechanisms do not correspond to a single thermally activated process. Relaxation dynamics imply that crystalline regions exert motion restrictions to large segments of the macromolecules in the amorphous phase and to polar parts of the systems.
Nanoporous alumina enhanced surface plasmon resonance sensors
Journal of Applied Physics, May 1, 2008
Journal of Cellular Biochemistry, 2011
Pleiotrophin (PTN) is a heparin-binding growth factor that plays a significant role in tumor grow... more Pleiotrophin (PTN) is a heparin-binding growth factor that plays a significant role in tumor growth and angiogenesis. We have previously shown that in order for PTN to induce migration of endothelial cells, binding to both a n b 3 integrin and its receptor protein tyrosine phosphatase beta/zeta (RPTPb/z) is required. In the present study we show that a synthetic peptide corresponding to the last 25 amino acids of the C-terminal region of PTN (PTN 112-136) inhibited angiogenesis in the in vivo chicken embryo chorioallantoic membrane (CAM) assay and PTN-induced migration and tube formation of human endothelial cells in vitro. PTN 112-136 inhibited binding of PTN to a n b 3 integrin, and as shown by surface plasmon resonance (SPR) measurements, specifically interacted with the specificity loop of the extracellular domain of b 3. Moreover, it abolished PTN-induced FAK Y397 phosphorylation, similarly to the effect of a neutralizing a n b 3-selective antibody. PTN 112-136 did not affect binding of PTN to RPTPb/z in endothelial cells and induced b 3 Y773 phosphorylation and ERK1/2 activation to a similar extent with PTN. This effect was inhibited by down-regulation of RPTPb/z by siRNA or by c-src inhibition, suggesting that PTN 112-136 may interact with RPTPb/z. NMR spectroscopy studies showed that PTN 112-136 was characterized by conformational flexibility and absence of any element of secondary structure at room temperature, although the biologically active peptide segment 123-132 may adopt a defined structure at lower temperature. Collectively, our data suggest that although PTN 112-136 induces some of the signaling pathways triggered by PTN, it inhibits PTN-induced angiogenic activities through inhibition of PTN binding to a n b 3 integrin.
Testing and simulation of a thermoacoustic transducer prototype
ABSTRACT Thermoacoustic transduction is the transformation of thermal energy fluctuations into so... more ABSTRACT Thermoacoustic transduction is the transformation of thermal energy fluctuations into sound. Devices fabricated by appropriate materials utilise such a mechanism in order to achieve acoustic wave generation by direct application of an electrical audio signal and without the use of any moving components. A thermoacoustic transducer causes local vibration of air molecules resulting in a proportional pressure change. The present work studies an implementation of this alternative audio transduction technique for a prototype developed on silicon wafer. Measurements of the performance of this hybrid solid state device are presented and compared to the theoretical principles of its operation which are evaluated via simulations.
Macromolecules, Feb 1, 2000
Flow through Alumina Nanopores Bearing Responsive Polymer Brushes
Macromolecules
The conformational behavior of end-grafted diblock copolymer brushes inside a nanoporous alumina ... more The conformational behavior of end-grafted diblock copolymer brushes inside a nanoporous alumina membrane in poor and good solvent is experimentally studied. For this purpose an apparatus has been specially designed which enables the monitoring of the volumetric flow rate vs. differential pressure generated by fluid flowing through the membrane. INTRODUCTION: Polymer brushes are formed when polymer chains are tethered at one end to a surface at sufficient density to overlap and stretch away from the surface. 1 Stimulusresponsive polymer brushes are a category of polymer brushes that exhibit a change in their conformation, surface energy, or charge state, triggered by an external stimulus such as a change of solvent, temperature, pH, ionic strength, light, mechanical stress, electric or magnetic field, e.t.c. 2 In many processes, particles or macroscopic surfaces bearing adsorbed polymer molecules pass from a good solvent for the polymer to a poor solvent. This external stimulus, by ...
Contemporary Materials, 2014
Renewable energy sources have an important role in global energy demand in the 21st century. New ... more Renewable energy sources have an important role in global energy demand in the 21st century. New investigations in solar energy materials and solar energy systems offer new opportunities for a sustainable future, adapting local energy needs, environmental protection and aesthetics. Solar energy can be used almost everywhere, with visible materials and systems. In this paper, an overview of new solar energy materials and systems is presented, focused on the work that is performed in the Laboratories of the University of Patras and of the Technical Educational Institute. The research activities of these Laboratories are on nanotechnology, biotechnology, microalgae hydrogen production, photovoltaics, smart glazings and materials for solar energy systems. A brief description of the achieved improvements on these solar energy materials is included, where energy performance and system cost play an important role together with aesthetics, considering their wide application in the future.
Journal of Applied Physics, 2018
Border trap evaluation for SiO 2 /GeO 2 /Ge gate stacks using deep-level transient spectroscopy
Neutron Reflectivity Study of End-Adsorbed Polymer Brushes: Effect of Solvent Quality and Shear Flow
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Papers by Alexandros Vradis