G. Korakakis, E.A. Pavlatou, J.A. Palyvos, N. Spyrellis

Computers & Education (2009), 52, 390-401

This research aims to determine whether the use of specific types of visualization (3D illustration, 3D animation, and interactive 3D animation) combined with narration and text, contributes to the learning process of 13- and 14-years-old students in science courses. The study was carried out with 212 8th grade students in Greece. This exploratory study utilizes three different versions of an interactive multimedia application called “Methods of separation of mixtures”, each one differing from the other two in a type of visuals. The results indicate that multimedia applications with interactive 3D animations as well as with 3D animations do in fact increase the interest of students and make the material more appealing to them. The findings also suggest that the most obvious and essential benefit of static visuals (3D illustrations) is that they leave the time control of learning to the students and decrease the cognitive load.

doi: 10.1016/j.compedu.2008.09.011

S. Spanou, E.A. Pavlatou, N. Spyrellis

Electrochimica Acta (2009), 54, 2547-2555

Nanocomposite coatings were obtained by electrochemical codeposition of TiO2 nano-particles (mean diameter 21nm) with nickel, from an additive-free Watts type bath. The electrodeposition of Ni–TiO2 composites was carried out on a rotating disk electrode (RDE), by applying direct current. Pure Ni deposits were also produced under the same experimental conditions for comparison. The surface morphology, the crystallographic orientation of nickel matrix and the grain size of the deposits were investigated, along with the distribution and the percentage, of the embedded nano-particles in nickel matrix, as a function of pH, current density and concentration of TiO2 nano-particles in the bath. The observed textural modifications of composite coatings are associated with specific structural modifications of Ni crystallites provoked by the adsorption–desorption phenomena occurring on the metal surface, induced by the presence of TiO2 nano-particles. It has been observed that the presence of TiO2 nano-particles favours the [100] texture of nickel matrix. Moreover, the codeposition of titania nano-particles with nickel was found to be favoured at low pH and low applied current values. As the titania incorporation percentage is increased, a considerable grain refinement in the nanometer region was revealed followed by an improvement of the quality of the nickel preferred orientation.

doi: 10.1016/j.electacta.2008.06.068

A. Zoikis Karathanasis, E.A. Pavlatou, N. Spyrellis

Electrochimica Acta (2009), 54, 2563-2570

In the present work pure nickel–phosphorous and nickel–phosphorous composite coatings with WC particles (mean diameter 200nm) have been produced under both direct and pulse current conditions, from a modified, organic-free, Watts bath. The produced coatings were thermally treated at various temperatures and the structure, morphology and the microhardness of the deposits were examined after each cycle of heat treatment. Imposition of pulse current conditions led to composite deposits with high-incorporation percentage of WC particles in the matrix. Also, the presence of WC particles resulted in coatings with enhanced microhardness values and retarded diffusion phenomena in the deposits during thermal treatment. Annealing of all coatings at 400°C revealed complete crystallization of the matrix to the phases of Ni, Ni2P and Ni3P accompanied by a significant increase of microhardness for all deposits. Further heating at higher temperatures demonstrated a decrease in microhardness of both kinds of deposits. The experimental data proved that it is possible to produce NiP–WC composite coatings under specific pulse current conditions followed by proper thermal treatment that exhibited considerable enhanced microhardness.

doi: 10.1016/j.electacta.2008.07.027

P. Gyftou, E.A. Pavlatou, N. Spyrellis

Applied Surface Science (2008), 254, 5910-5916

Pure nickel and nickel matrix composite deposits containing nano-SiC particles were produced under both direct and pulse current conditions from an additive-free nickel Watts’ type bath. It has been proved that composite electrodeposits prepared under pulse plating conditions exhibited higher incorporation percentages than those obtained under direct plating conditions, especially at low duty cycles. The study of the textural perfection of the deposits revealed that the presence of nano-particles led to the worsening of the quality of the observed [100] preferred orientation. Composites with high concentration of embedded particles exhibited a mixed crystal orientation through [100] and [211] axes. The embedding SiC nano-particles in the metallic matrix by an intra-crystalline mechanism resulted in the production of composite deposits with smaller crystallite sizes and more structural defects than those of pure Ni deposits. A dispersion-hardening effect was revealed for composite coatings independently from applied current conditions. Pulse electrodeposition significantly improved the hardness of the Ni/SiC composite deposits, mainly at low duty cycle and frequency of imposed current pulses.

doi: 10.1016/j.apsusc.2008.03.151

E.A. Pavlatou, N. Spyrellis

Russian Journal of Electrochemistry (2008), 44, 745-754

An additive-free Watts type bath containing micron- and nano-SiC particles (1μm and 20nm respectively), as well as ultrafine-WC particles (200nm), was used for the production of pure Ni and nickel matrix composite electrocoatings under both direct and pulse current conditions. Moreover, nickel nanocrystalline deposits were obtained from a Watts type bath containing small amounts of 2-butyne-1,4-diol, in order to investigate the combined advantages of additives and pulse technique on the properties of the deposits. The influence of the variable electrolysis parameters, the particle size and the organic additive concentration on the surface morphology, the structure and properties of the deposits were discussed. It has been proved that the application of pulse electrodeposition affects drastically the structural characteristics and properties of the deposits and under well-defined conditions could lead to the preparation of nanostructured materials with improved mechanical properties.

doi: 10.1134/S1023193508060165

E.A. Pavlatou, M. Raptakis, N. Spyrellis

Surface & Coatings Technology (2007), 201, 4571-4577

Nanocrystalline nickel deposits were produced by applying both direct current (DC) and pulse current electroplating (PC) from an additive-free Watts type bath and in the presence of 2-butyne-1,4-diol (BD). The surface morphology, the preferred orientation, the grain size and the microhardness of the deposits were investigated. Under DC conditions, the addition of BD enhanced the perfection of [100] preferred crystal orientation and promoted a considerable grain refinement. The application of PC technique in presence of small amount of BD (CBD=0.25 mmolL−1) accelerated the textural modification from [100] to [211] with respect to DC conditions and deposits with grain size of 20–60nm were produced. A significant enhancement of microhardness of the produced bright deposits was observed compared to those deposits prepared under DC regime, which could be correlated to the observed crystal orientation mode [211], as well as to the corresponding nanosized crystallites. Progressive addition of BD promoted a peculiar textural modification from [211] to [100] and a possible mechanism of action of BD is proposed taking into account as inhibiting species of the Ni crystal growth not only Ni(OH)2, but also cis-butene-1,4 diol.

doi: 10.1016/j.surfcoat.2006.09.113

E.A. Pavlatou, M. Stroumbouli, P. Gyftou, N. Spyrellis

Journal of Applied Electrochemistry (2006), 36, 385-394

Pure Ni and nickel matrix composite electrocoatings containing micron- and nano-SiC particles (1μm and 20nm respectively) were produced under direct and pulse current conditions from an additive-free Watts type bath. The effect of the particle size, codeposition percentage of SiC and type of imposed current on the microhardness as well as on the microstructure of the electrodeposits were investigated. Ni/SiC composite deposits prepared under either direct or pulse current conditions exhibited a considerable strengthening effect with respect to pure Ni coatings. The improved hardness of composite coatings was associated to specific structural modifications of Ni crystallites provoked by the adsorption of H+ on the surface of SiC particles, thus leading to a (211) texture mode of Ni crystal growth. Pulse electrodeposition significantly improved the hardness of the Ni/SiC composite coatings, especially at low duty cycles, in which grain refinement and higher SiC incorporation (vol.%) was achieved. The enhanced hardness of Ni/nano-SiC deposits, as compared to Ni/micron-SiC composites, was attributed to the increasing values of the number density of embedded SiC particles in the nickel matrix with decreasing particle size. In addition, the observed hardening effects of the SiC particles might be associated to the different embedding mechanisms of the particles, which could be characterized as inter-crystalline for micron-SiC and partially intra-crystalline for nano-SiC particles.

doi: 10.1007/s10800-005-9082-y

P. Gyftou, M. Stroumbouli, E.A. Pavlatou, P. Asimidis, N. Spyrellis

Electrochimica Acta (2005), 50, 4544-4550

Nickel matrix composite coatings containing micro and nano-sized SiC particles were prepared from an additive-free Watts’ type solution under direct and pulse current conditions, in order to study the correlation between SiC particles embedding and the tribological behaviour of deposits. The wear properties of Ni/SiC composite coatings were shown to depend on the type of current, the size of the embedded particles, the weight fraction of codeposited particles, the microstructural modifications induced by codepositing SiC particles and the plating conditions. It was proved that the presence of SiC particles influences the adsorption–desorption phenomena occurring at the metal-catholyte interface during electrocrystallization and, synergically with the plating conditions, modifies the deposits microstructure thus affecting wear properties.

doi: 10.1016/j.electacta.2004.10.090

M. Stroumbouli, P. Gyftou, E.A. Pavlatou, N. Spyrellis

Surface and Coatings Technology (2005), 195 (2-3), 325-332

The electrolytic codeposition of ultrafine WC particles (mean diameter of 0.2μm) from an additive-free nickel Watts' solution by applying both direct (DC) and pulse (PC) electroplating, has been investigated. Electrodeposition of Ni/WC composites was carried out on a rotating disk electrode (RDE) at various rotation velocities. The effect of type current and hydrodynamic conditions of the plating bath on the codeposition of WC particles with Ni matrix has been reported. The crystallographic orientation of nickel matrix, the distribution and the percentage of the embedded particles, were examined as well as the structure and the surface morphology of the produced composite coatings. It was found that electrodeposits prepared at DC conditions and low rotation velocities are highly porous. On the contrary, when applying PC conditions and high rotation velocities, compact deposits with high concentration of embedded WC particles and uniform distribution were produced. It has been observed that the presence of WC particles in the metal matrix imposes an almost random orientation of Ni crystallites along with a reinforcement of [210] orientation. Taking into consideration a surface complexation model and the experimental data, a mechanism of nickel electrocrystallization in the presence of WC particles has been proposed.

doi: 10.1016/j.surfcoat.2004.06.034

S. Boghosian, E.A. Pavlatou, G.N. Papatheodorou

Vibrational Spectroscopy (2005), 37, 133-139

The vapors of NbF5 and TaF5 have been investigated by Raman spectroscopy in the temperature range 475–675K and at total pressures from ∼0.1 to ∼4.0atm. The temperature and pressure dependence of the spectra is indicative of equilibrium shifts between monomer and associated (polymer) gaseous species. The intensities of Raman bands, which were characteristic of the various species present, were exploited in order to establish the stoichiometry and thermodynamic functions of the reaction nMF5(g) ⇆ MnF5n(g) (M=Nb, Ta). It was found that unambiguously n=2 in the studied temperature range, which implied that the predominant vapor species are monomers MF5(g) and dimers M2F10(g). The thermodynamic functions according to the studied dimerization reactions were measured from the Raman data as: (a) ΔHR=−104.3±2.4 kJmol−1, ΔSR=−175±20 Jmol−1K−1 for 2NbF5(g) ⇆ Nb2F10(g); (b) ΔHR = −101.1±3.7 kJmol−1, ΔSR=−150±30 Jmol−1K−1 for 2TaF5(g) ⇆ Ta2F10(g).

doi: 10.1016/j.vibspec.2004.08.004

S.A. Kirillov, E.A. Pavlatou, G.N. Papatheodorou

The Journal of Chemical Physics (2002), 116 (21), 9341-9351

The picosecond dynamics of molten alkali halides is discussed, and the low-frequency Raman spectra of molten LiCl, CsCl, and the LiCl–CsCl eutectic are fitted to the model enabling to obtain the times of vibrational dephasing, τV and vibrational frequency modulation τω. In terms of the Wilmshurst criterion [J. Chem. Phys. 39, 1779 (1963)] and using the data of NMR studies and molecular dynamics simulations, a conclusion is drawn that molten alkali halides cannot contain long-lived stable complexes with lifetimes greater than 10−8 s. The low-frequency Raman spectra of molten alkali halides and their mixtures probe the presence of instantaneous spatial configurations of MXn−n+1 type, where M+ is the alkali metal cation and X is the halide anion existing in melts during the time intervals equal to the time of duration of collision of oppositely charged ions τd, which is less than 0.5ps. This time is sufficient to a collision complex to execute several (at least one) vibrations. Vibrational dephasing and modulation processes elapse during this same time, thereby indicating the instantaneous nature of configurations in question. To discern between short-lived and long-lived complexes, we propose relations between the minimal damping time of the probe oscillator set equal to the half-period of vibration T/2, τV, τω, and τd, as well as the time between collisions τBC. The duration of an act resulting in the vibrational phase shift (or energy transfer) must be equal to (or longer than) the half-period of vibration of the probe oscillator, τV≥T/2. The modulation time may vary from this same half-period of vibration or the time between collisions τBC to very long times, τω≥T/2, τω≥τBC. For short-lived complexes, the longest of two characteristic times describing the phase decay cannot exceed possible duration of collision, τω⩽τd, τV⩽τd. Cs-containing configurations follow this definition and therefore should be considered instantaneous short-lived collision complexes: their τV≈T/2∼0.1ps, and τω≈τBC∼0.03ps. Li-containing configurations appear to be relatively long-lived: their lifetimes could be associated with τω∼0.17ps, which is several times longer than any other shortest possible characteristic time in the system (τBC∼0.026ps or T/2∼0.05ps). In light of these conclusions, an a priori assumption of autocomplex MX4n−4 anions and Mn+ cations as being structural elements of molten halides made in the so-called autocomplex model by Smirnov, Shabanov, and Khaimenov [Elektrohim. 2, 1240 (1966)] is discussed, and the autocomplexes are identified as instantaneous short-lived configurations detectable by the Raman method.

doi: 10.1063/1.1473810

P. Gyftou, M. Stroumbouli, E.A. Pavlatou, N. Spyrellis

Transactions of the Institute of Metal Finishing (2002), 80 (3), 88-91

Nickel matrix composite coatings containing micron- and nano-sized SiC particles were prepared in order to study the interdependence of the SiC particles embedding and the deposits 'mechanical behaviour. SiC particles of two different sizes, namely 1μm and 20nm, were codeposited with nickel from Watts solutions under pulse current conditions. It has been observed that the embedding of SiC particles in the nickel matrix and the pulse current application result in deposits with more uniform particle distribution and better surface morphology than those obtained under direct current conditions. The study of the composite deposits revealed that the microhardness is not only increased by the presence and the reduced size of the particles, but also influenced by the current conditions, i.e. duty cycle and pulse frequency. Moreover, microhardness of the deposits can be further ameliorated by specific thermal treatment.

doi: 10.1080/00202967.2002.11871440

S.A. Kirillov, G.A. Voyiatzis, I.S. Musiyenko, G. M. Photiadis, E.A. Pavlatou

The Journal of Chemical Physics (2001), 114 (8), 3683-3691

In this article we present the first quantitative estimates of the spectroscopically active part of the interaction potential in molten complex chlorides based on dephasing studies. We have selected the molten salt systems containing quasispherical complex MCl4−2 anions (M=Mn+2 and Zn+2) and performed the study of their ν1(A1isotropic Raman line profiles as a function of the temperature and concentration. We have analyzed the form of the time correlation function of vibrational dephasing and determined the type of modulation events, which cause the line broadening processes in these systems; these are found to be purely discrete Markovian. Within the formalism of the purely discrete Markovian modulation, we have made a judgement about spectroscopically active interactions in these systems. Interionic potential in complex chlorides is dominated by the attraction forces, which depend on the interparticle distance r as r−4, and by repulsion of the r−7-type.

doi: 10.1063/1.1340031

E. M. Anghel, E.Α. Pavlatou, M. Balasoiu, S. Zuca

High Temperature Material Processes (2000), 4 (3), 431-440

As part of the ternary system Na3AlF6-Na2B4O7-TiO2, the binary subsystem: Na2B4O7TiO2 and the pseudobinary subsystem Na2B4O7-[Na3AlF6-TiO2](e) were investigated. The phase diagrams established by DTA measurements evidenced a simple eutectic point for both systems except for the concentration range >85wt.% Na2B4O7 of the Na2B4O7-[Na3AlF6TiO2](e), system where the binary paragenesis rules are no longer obeyed. Additional information regarding the structure of the mixtures were obtained by X-ray diffraction and Raman spectroscopy. Both vitreous and crystalline state were evidenced, without any new compound formation. The presence of titanium, mainly as TiO4 tetrahedra, was also proved.

P. Gyftou, E.A. Pavlatou, N. Spyrellis, K.S. Hatzilyberis

Transactions of the Institute of Metal Finishing (2000), 78 (6), 223-226

Coatings applied to several mechanical parts involved in the cotton textile industry are expected to improve production efficiency and to decrease the relevant costs. The quality of cotton products is one of the most crucial parameters in the development of textile industry and is significantly associated with the mechanical and tribological properties of the coated machinery parts. For this purpose, nickel matrix coatings containing silicon carbide (SiC) micro-particles, prepared under pulse plating conditions were applied on several parts. Surface morphology, preferred orientation of the crystals, incorporated SiC percentage and mechanical properties of these coatings were investigated. Moreover, the influence of the surface coating on the cotton products quality was studied by the means of a prototypal method, which determines the extent of wear on a typical cotton fabric, under strictly controlled conditions. It has been proved that Ni/SiC coatings, prepared under very well defined conditions (pulse current density, frequency, duty cycle, etc), present relatively low roughness, high microhardness, low sliding friction coefficient between the coating and typical cotton products and reduced wear against cotton fabrics or yarns.

doi: 10.1080/00202967.2000.11871345

E.A. Pavlatou, G.N. Papatheodorou

Physical Chemistry Chemical Physics (2000), 2, 1035-1043

Raman spectra of solid and liquid beryllium chloride and of binary liquid BeCl2–CsCl mixtures have been measured. Systematic investigation of the Raman spectra upon heating the crystalline forms of BeCl2 revealed two different solid phase transitions at 350 and 405°C. A glass phase was also obtained by vapour transport with a crystallisation temperature at ∽250°C. The Raman spectra of the molten and glassy BeCl2 show bands at common frequencies that are correlated to certain vibrational modes of two solid allotropic phases. Temperature dependent measurements of the Raman spectra of a series of BeCl2–CsCl melt compositions suggest that: (a)at XBeCl20.5 the liquid structure is predominated by tetrahedral and trigonal beryllium species in equilibrium, BeCl42−=BeCl3+Cl; and (b) at XBeCl2>0.5 two types of polynuclear beryllium ionic species are formed, one consisting of a "chain" of edge-bridged BeCl4 tetrahedra and the other having a "cluster" like structure of vertex-bridged BeCl4 tetrahedra. The spectra of pure BeCl2 liquid and glass are interpreted in terms of similar neutral "chain" and "cluster" structures having as ending units trigonally coordinated beryllium atoms. On going from the glass to the melt and/or with increasing temperature the "cluster" structures unfold to form "chains". Similarities also exist between the structures of molten BeCl2 and of glassy SiSe2.

doi: 10.1039/A909120C

E.M. Anghel, M. Zaharescu, S. Zuca, E.Α. Pavlatou

Journal of Materials Science (1999), 34, 3923-3929

A complex study was carried out on Na2B4O7-Na3AlF6 binary system by means of differential thermal analyses (DTA), X-ray diffraction as well as infrared (IR), and Raman spectroscopy. Although the system exhibits a simple eutectic at 83wt% Na2B4O7 and Te=642°C, interesting structural changes of borate glasses formed have been evidenced by means of IR and Raman spectroscopy. Over 80-100wt% Na2B4O7 range AlF62- anions are diluted in the vitreous matrix without significant structural changes while the further increase of Na3AlF6 content diminishes the concentration of BO4 groups and promotes the appearance of complex anions AlF(x−3)−x (x = 6, 5, 4), as well as the possible new BF4- and/or BO3F4− species.

doi: 10.1023/A:1004635109184

A. Perakis, I.P. Kotsalas, E.A. Pavlatou, C. Raptis

Physica Status Solidi (B) (1999), 211, 421-427

The evolution of the Raman spectrum of amorphous GeS2 is reported over wide temperature (300 to 1075K) and pressure (0 to 10.8GPa) ranges. The temperature dependence shows that a-GeS2 turns gradually crystalline above Tg, first to the 3D-phase and then to the layered (2D)-phase, with the latter one being retained up to the melting point and upon normal cooling to 300K. The evolution of the Ac1 companion band of a-GeS2 into the crystalline phases indicates that this band is due to the symmetric stretch of S atoms in bridges of edge-sharing Ge(S1/2)4 tetrahedra. The Raman bands of a-GeS2 harden with increasing pressure, indicating a continuous decrease of the Ge–S bond length, while the material remains amorphous throughout the pressure range. Above 10 GPa, the material becomes almost opaque, thus making difficult the observation of Raman scattering. All these pressure induced effects are reversible after bringing the sample to ambient pressure.

doi: 10.1002/(SICI)1521-3951(199901)211:1<421::AID-PSSB421>3.0.CO;2-F

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