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Tsimpliaraki A.,Aristotle University of Thessaloniki | Tsivintzelis I.,Aristotle University of Thessaloniki | Marras S.I.,TEI of Western Macedonia | Zuburtikudis I.,TEI of Western Macedonia | Panayiotou C.,Aristotle University of Thessaloniki
Journal of Supercritical Fluids | Year: 2011

Three series of polymer nanocomposites, based on poly(d,l lactic acid) (PDLLA) and organically modified montmorillonite, were prepared by the melt and the solution intercalation technique. The first series was prepared by extrusion using different clay loadings. The second series of nanohybrids was obtained using montmorillonite modified with different types of alkylammonium surfactants in terms of carbon-chain lengths (i.e., 4, 8, 12, 16 and 18). In the third series of nanocomposites, the organic cation concentration of the surfactant was varying. Microcellular porous materials were, afterwards, fabricated from these three series of nanocomposites. The porous structures of pure and nanocomposite PDLLA were prepared by isothermal pressure quench using supercritical CO 2 as foaming agent. The morphology of the produced porous materials was investigated by scanning electron microscopy (SEM). Image processing of the samples revealed that the final cellular structure is strongly related to clay loading and, both, the type and the organic cation concentration of the alkylammonium used for the modification of the clay. The results suggest that the size of the pores decreases and the cell density and bulk foam density increase with the increase of clay loading or the surfactant's carbon chain length or the cation concentration in clay. Clay dispersion seems to be enhanced by the supercritical treatment upon foaming. © 2011 Elsevier B.V. All rights reserved.


Achilias D.S.,Aristotle University of Thessaloniki | Panayotidou E.,Aristotle University of Thessaloniki | Zuburtikudis I.,TEI of Western Macedonia
Thermochimica Acta | Year: 2011

Poly(3-hydroxybutyrate) (PHB)/organically modified clay nanocomposites were prepared by the melt mixing method and were characterized using wide-angle X-ray diffraction. Their thermal degradation kinetics was investigated using thermogravimetric analysis at various heating rates. Further kinetic analysis was performed using isoconversional methods and the invariant kinetic parameters method was used to estimate the so-called true kinetic parameters, i.e. the pre-exponential factor, A and the activation energy, E, as well as the reaction model. It was found that intercalated structures are formed and the thermal stability of the material is improved by the addition of the nano-filler. From the isoconversional analysis, it was found that the activation energy does not vary significantly with the degree of degradation denoting degradation in one step with similar values for pure PHB and for all nanocomposites. Using the invariant kinetic parameters method, it was found that the model that best describes the experimental data was that of Sestak-Berggrens with f(a) = αn(1 - α)m, where the value of n is always larger than m and is increasing with the amount of the nano-filler. The value of the true activation energy was found to be about 100 kJ mol-1 for all nanocomposites and the pre-exponential factor for PHB was estimated equal to 5.35 × 109 min-1. Finally, the values of the kinetic rate constant k were found to decrease with the amount of the nano-filler up to 3 wt%, while for amounts larger than 3 wt% k increased reaching a value greater than that of pure PHB for the 10 wt% nanocomposites. © 2010 Elsevier B.V.


Mitsoulis E.,National Technical University of Athens | Malamataris N.A.,TEI of Western Macedonia | Malamataris N.A.,George Mason University
Journal of Non-Newtonian Fluid Mechanics | Year: 2012

The free (or open) boundary condition (FBC, OBC) was proposed by Papanastasiou et al. (A new outflow boundary condition, Int. J. Numer. Methods Fluids 14 (1992) 587-608) to handle truncated domains with synthetic boundaries where the outflow conditions are unknown. In the present work, implementation of the FBC has been extended to viscoelastic fluids governed by integral constitutive equations. As such we consider here the K-BKZ/PSM model, which also reduces to the upper-convected Maxwell fluid (UCM) for a single relaxation time and an appropriate choice of material parameters. The Finite Element Method (FEM) is used to provide numerical results in simple test cases, such as planar flow at an angle and Poiseuille flow in a tube where analytical solutions exist for checking purposes. Then previous numerical results obtained with the differential UCM model are checked in highly viscoelastic flows in a 4:1 contraction. Finally, the FBC is used with the K-BKZ/PSM model with data corresponding to a benchmark polymer melt (the IUPAC-LDPE melt). Particular emphasis is based on a non-zero second normal-stress difference, which has been reported in the literature to cause problems and seems responsible for earlier loss of convergence. The results with the FBC in short domains are in excellent agreement with those obtained from long domains used until now to accommodate the highly convective nature of the stresses in viscoelastic flows, for which the FBC seems most appropriate. © 2012 Elsevier B.V.


Mitsoulis E.,National Technical University of Athens | Malamataris N.A.,TEI of Western Macedonia | Malamataris N.A.,George Mason University
International Journal for Numerical Methods in Fluids | Year: 2012

The free (or open) boundary condition (FBC, OBC) was proposed by Papanastasiou et al. (A new outflow boundary condition, International Journal for Numerical Methods in Fluids, 1992; 14:587-608) to handle truncated domains with synthetic boundaries where the outflow conditions are unknown. In the present work, implementation of the FBC has been tested in several benchmark problems of viscous flow in fluid mechanics. The FEM is used to provide numerical results for both cases of planar and axisymmetric domains under laminar, isothermal or non-isothermal, steady-state conditions, for Newtonian fluids. The effects of inertia, gravity, compressibility, pressure dependence of the viscosity, slip at the wall, and surface tension are all considered individually in the extrudate-swell benchmark problem for a wide range of the relevant parameters. The present results extend previous ones regarding the applicability of the FBC and show cases where the FBC is inappropriate, namely in the extrudate-swell problem with gravity or surface-tension effects. Particular emphasis has been given to the pressure at the outflow, which is the most sensitive quantity of the computations. In all cases where FBC is appropriate, excellent agreement has been found in comparisons with results from very long domains. The formulation for Picard-type iterations is given in some detail, and the differences with the Newton-Raphson formulation are highlighted regarding some computational aspects. © 2011 John Wiley & Sons, Ltd.


Danis T.G.,DELCOF SA | Karagiozoglou D.T.,DELCOF SA | Tsakiris I.N.,TEI of Western Macedonia | Alegakis A.K.,University of Crete | Tsatsakis A.M.,University of Crete
Food Chemistry | Year: 2011

The incidence and levels of pesticide residues in peaches grown using Integrated Crop Management (ICM) methods in Pella and Imathia, districts of Macedonia, Northern Greece, are hereby presented. A total of 1150 peach samples were collected pre-harvest (June-September) and were analysed during the period of 2002-2007. Residual levels of selected insecticides, fungicides and acaricides were determined by Gas Chromatography-Mass Spectrometry following solid phase extraction. The monitoring program is extended up to 31 pesticides. Twenty-two of them were measured above detection limit, and 8 of them were found present every year. Insecticides represent the highest incidence amongst pesticide categories (654 positive samples, 56.9%) while chlorpyrifos is the most frequent within the category (491 positive samples). The incidence of pyrethrins was the highest in 2007 (26.8%) while bifenthrin was detected most frequently within this category (67 positive samples in 2007). Traceable levels of pesticides were lower than the Maximum Residues Limits (MRLs) in all peach samples. Variances in incidence and levels of pesticides are attributed to some extent to the weather conditions as well as due to abidance by the guidelines of agronomists during blowing and harvest period. Monitoring in agricultural products could be employed as the first step in a chain of biomonitoring studies in humans, and may also be proven to be a good and ample indicator of exposure. © 2010 Elsevier Ltd. All rights reserved.


Vergados J.D.,TEI of Western Macedonia | Vergados J.D.,University of Ioannina
Advances in High Energy Physics | Year: 2015

We show how to obtain the energy distribution f(E) in our vicinity starting from WIMP density profiles in a self-consistent way by employing the Eddington approach and adding reasonable angular momentum dependent terms in the expression of the energy. We then show how we can obtain the velocity dispersions and the asymmetry parameter β in terms of the parameters describing the angular momentum dependence. From this expression, for f(E), we proceed to construct an axially symmetric WIMP a velocity distribution, which, for a gravitationally bound system, automatically has a velocity upper bound and is characterized by the same asymmetriy β. This approach is tested and clarified by constructing analytic expressions in a simple model, with adequate structure. We then show how such velocity distributions can be used in determining the event rates, including modulation, in both the standard and the directional WIMP searches. © 2015 J. D. Vergados.


Dossis M.F.,TEI of Western Macedonia
ACM International Conference Proceeding Series | Year: 2014

The current difficulty to deliver complex digital integrated circuits (ICs) on time in the market has created the need for formal, fully automated and rapid methods for designing and developing such products. These methods exploit approaches such as compiler technology, High-level Synthesis (HLS) and Electronic System Level (ESL) flows to handle such complexity. Designing custom VLSI hardware units motivate for developing large arithmetic blocks such as floating-point units, using HLS tools. Here, the formal and automated CCC high-level synthesis tools are used in the floating-point paradigm. The CCC tools generate provably-correct floating-point blocks due to the formality of the method, in order to avoid time-consuming RTL and gate-level simulations. The floating-point operations are coded in the ADA programming language and competitive hardware performance is delivered from the rapid CCC compiler experiment, therefore the presented approach is proven useable. © 2014 ACM.


Dossis M.F.,TEI of Western Macedonia
Proceedings - IEEE Annual Symposium on VLSI, ISVLSI 2010 | Year: 2010

High-level synthesis has been envisaged as a suitable methodology to design and deliver on time, at least large parts of today's complex IC systems. This paper describes a unified and integrated HLS framework, to automatically produce custom and massively-parallel hardware, including its memory and system interfaces from high-level sequential program code. Using compiler-generators and logic programming techniques, provably-correct hardware compilation flow is achieved. The utilized hardware optimization inference engine is driven by a set of resource constraints, which limit to a certain boundary the number of available hardware operators to function in parallel during each control step. This optimization reduces drastically the number of different control steps (states) of the implemented application. The hardware compilation runs are completed in orders-of-magnltude less time than that which would be needed by even very experienced HDL designers to implement the same applications in RTL code. Implementation results from synthesis of a number of control-dominated, linear and repetitive, applications including a MPEG video compression engine with up to a few hundred states, are presented. In all cases the HLS framework delivers quickly provably-correct, implementable RTL code and the optimized schedule is reduced at up to 30% in comparison with the initial schedule. © 2010 IEEE.


Marras S.I.,TEI of Western Macedonia | Zuburtikudis I.,TEI of Western Macedonia
Journal of Applied Polymer Science | Year: 2012

The structural and thermal characteristics of poly(L-lactic acid)/layered-silicate hybrid materials that were produced via two different routes, namely by solvent casting and by melt mixing, were compared in association with the degree of clay modification. Investigation of the produced materials' structure revealed that, at low modification levels, melt blending is necessary in dispersing the amine-treated clay into the polymer matrix. At intermediate degrees of modification, both techniques are capable of swelling the silicate clay with the solution casting to be a more effective method. Thermal measurements showed that the clay modification level influences significantly the thermal stability of both solution and melt processed hybrids. Moreover, the material derived from melt mixing displayed a higher onset decomposition temperature. The glass transition temperature of the polymer was not significantly affected by the preparation method followed. However, the crystallization process was found to be strongly dependent on both the preparation method and the degree of clay modification. © 2011 Wiley Periodicals, Inc.


Dossis M.,TEI of Western Macedonia
ACM International Conference Proceeding Series | Year: 2013

In this paper, a formal design framework is described that it allows for correct-by-construction complex circuit generation from high-level program models with arbitrary data and control flow. The formal nature of the program compilation allows for design flow iteration - free design, rapid development times and bug-free product implementation. The discussed formal framework, translates program subroutines into hardware coprocessors. The translation process is implemented with formal compiler-compiler and logic programming techniques. Due to this the functionality of the generated finite-state-machines and datapaths is formally equivalent to that of the source programs. This is practically proven in this paper with verification of the designed systems at both the program code and at the produced circuit RTL simulations level. The presented design flow enables rapid prototyping and it allows for compression of the development time from months down to a few hours. Copyright 2013 ACM.

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