Papers by Author: Evangelos Hristoforou

Abstract: In recent decades the application of magnetic iron oxide micro-and nanoparticles has been established in various technological fields, such as magnetic separation of biomolecules and ions, biosensors, biofuel production and others [1-4]. Working with iron oxide particles is becoming main stream subject thanks to the facility that this kind of materials can be functionalized with a variety of chemical groups which confer them specific selective or catalytic properties [5]. Furthermore, iron oxide nanoparticles present magnetic properties, and in particular super-paramagnetism, which allows to remotely control them making their manipulation easy and cost-effective [6]. In addition, a new method of synthesis has been recently proposed, which can guarantee a cost-effective production of magnetic particles that may further reduce the running cost of separation methods based on magnetism [7]. Nevertheless, biotechnological applications of iron oxide particles are still confined to research level (lab scale devices) or for low throughput clinical applications [8,9]. Indeed, most systems based on the use of magnetic elements are design to work with microfluid dynamic or are able to process samples in bath-based fashion, therefore discontinuously. The need of robust and high-productive methods is demanded especially in bioscience where, independently from the reaction or process involving magnetic particles, once such composite materials are mixed or added to a given solution, inevitably at the end of workflow they must be separated/harvested from the reaction vessel. Therefore, it is vital for a good productivity and processivity of reactions involving magnetic particles to ensure that large volumes of solution can be treated, and magnetic particles withdrew in the most fast and accurate way. The purpose of this paper is to compare an open and a closed type magnetic trapping system regarding their efficiency using two different types of magnetic sources.

Abstract: Desalination can be carried out in water to remove any salt. The process takes place after tertiary treatment of municipal and industrial waste water as a final treatment before discharge to the recipient. The desalination of seawater using Reverse Osmosis (R.O.) technique is now a tried and proven technology that can provide high-quality water supply. The resulting from R.O. water is not suitable for drinking. The method proposed in this paper is selected desalination via zeolitic micro-particles. The method exhibits some significant advantages such as the easy recovery and reuse of powders (with negligible losses during the release process of trapped ions in liquid solution), the selective removal of ions and the ability of producing directly potable water without the need for further enrichment.

Abstract: The present paper investigates the utilization of the magnetic Barkhausen noise and magnetic permeability methods for the nondestructive characterization of annealed non-oriented electrical steel samples which were isothermally annealed in a wide range of temperatures (400°C – 950°C) and subsequently cooled in air. The resulting magnetic properties were compared with the microstructural changes occurring during annealing.

Abstract: In the present research work the determination of residual stress distribution in welded non-oriented electrical steel samples is discussed. Tungsten Inert Gas and Electron Beam Welding were used as the welding methods. The residual stresses induced by welding estimated by the magnetic, non-destructive method of Barkhausen noise and were compared with the values resulting from the semi-destructive method of X-ray diffraction Bragg-Brentano (XRD-BB). In order to evaluate accuracy and reliability of the magnetic methods applied, the steel samples were subjected in both compressive and tensile stress and the magnetic noise values were correlated to residual stress values through an appropriate calibration curve. Furthermore, the fluctuations of the residual stresses in the welding zones of the welded samples were discussed on the basis of the experimental evidence and the microstructural changes occurring during welding. It was found that the residual stress determined by MBN method was in good agreement with the XRD-BB results. However, the residual stress determined by magnetic permeability method was not in good agreement with the XRD-BB results. In contrast to the XRD-BB method, the magnetic techniques required a precise calibration procedure in all zones with noticeably different microstructure.

Abstract: The aim of this study is to describe the synthetic procedure of superparamagnetic nanoparticles of magnetite and maghemite and to use the magnetic merit of thesenano-sized ferrite particles coated byorganic substances having good water solubility to desalinate saline water. The idea derives from the experimental results of research groups using magnetic particles covered by polymers to increase the efficiency of membranes in forward osmosis desalination plants. The magnetic particles can beseparatedfrom water by an external magnet field easily.As magnetic particles, Fe₃O₄ can be prepared in different sizes from nanoto microscale by the help of co-precipitation or thermal decomposition techniques. These superparamagnetic nanoparticles are well-promising candidates for use in desalination purposes either from own or after their fabrication with polymer molecules, such as cyclodexrins, in their original form or in a modified one in order to enhance their water solubility, according to some preliminary experimental results found by our research team but not referred here. Herein, various inexpensive synthetic routes for superparamagnetic nanoparticles of magnetite (Fe₃O₄) and maghemite ( -Fe₂O₃) are described, as well as the characterization results of the produced nanoparticles with XRD, TEM, FT-IR, RAMAN, DFT and TGA/DTG analytical techniques are also referred.

Abstract: In this paper we present a new method for the monitoring of super-paramagnetic nanoparticles (SPANs) in the body. Nowadays, reliable and inexpensive device and method for monitoring the spatial distribution of SPANs in the body are not present in the market of clinical imaging equipments. Importantly, since SPANs can be conjugated to a huge variety of organic (antibodies, proteins, synthetic polymers) and inorganic molecules they can be used to selectively detect targets (e.g. cancer cells) with striking specificity. The existing imaging methods used for clinical diagnostic purposes are the nuclear magnetic resonance (NMR) and computerized axial tomography scan (CAT or CT scan). Detection of SPANs with these methods is still controversial and most import they used strong magnetic field and harmful X-ray radiation, respectively, and the cost for a single analysis is high as well. Herein we describe an innovative magnetic method promises the measurement of the distribution of SPANs with sensitivity quite better than 1 μm³. The method (patented device by our group) is based on magnetic excitation and consequent detection of nanoparticles using super-conducting or magnetic sensors (magnetometers). The device is innovative and novel, and could be considered as a universal breakthrough in tumor diagnosis. Possible other applications could be simultaneous killing of the cancer cells applying inductive heating techniques.

Abstract: The influence of Electron Beam welding technique on the microstructure, mechanical and magnetic properties of low carbon steels was presented and evaluated. Samples with dimensions of 12 cm x 6 cm x 1.5 mm were prepared from stock plate, installed on the welding table and welded at welding speeds and pulsed currents following a predesigned protocol. In order to examine the influence of welding on the magnetizing behavior of low carbon steel, magnetic Barkhausen noise were conducted on the surface of the welded samples. The results were further evaluated by examining the microstructure and mechanical properties of the welded samples by using scanning electron microscopy and micro-hardness measurements, respectively.

Abstract: The high pressure (50 MPa - 2 GPa) – high temperature synthesized MgB2 bulk materials are characterized by nearly theoretical density (1-2% porosity), 80-98% connectivity, extremely high critical current densities (e.g. at 20 K, in 0–1 T jc=1.3–1.0 MA·cm-2 (with 10% SiC) and jc= 0.92 – 0.73 MA·cm-2 (without doping)), large irreversibility fields (Birr(18.4 K) =15 T and Birr (0 K) = 32.5 T) and high upper critical fields (Bc2 (22 K) =15 T and Bc2(0 K) ~ 42.1 T). The transformation of grain boundary pinning to point pinning in MgB2-based materials with increasing manufacturing temperature (from 800 to 1050 oC) under pressures from 0.1 to 2 GPa correlates well with an increase in critical current density in low external magnetic fields caused by the redistribution of boron and the oxygen impurities in the material structure. As the manufacturing temperature increases, the discontinuous oxygen enriched layers transform into distinct Mg-B-O inclusions and the size and amount of inclusions of higher magnesium borides MgBX (X>4) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC additions.

Abstract: In the present work the determination of intrinsic properties in ferromagnetic materials by using magnetic non-destructive techniques is discussed. Barkhausen noise and bulk permeability measurements were used in various steels. The monotonic dependence of the magnetic response on microstructural features was verified.

Abstract: The Preisach formalism is used as a basis for a vector model of magnetic hysteresis in soft magnetic materials subject to tensile stress. The model uses as vector elementary hysteresis operator the Stoner-Wohlfarth mechanism of coherent rotation while the Preisach density is constructed as the weighed sum of probability density functions corresponding to the high and low induction regions. The model reproduces the basic phenomenology of stress-dependent hysteresis: the double peak in differential permeability modeled as the effect of internal demagnetizing fields emerging from residual stresses; the increase in coercivity due to increased pinning; the decrease in magnetic induction as the result of non-180^o domain rotation. The role of the negative differential permeability near remanence and its derivative is discussed with respect to residual stresses and magnetic NDT.