Evangelia A. Pavlatou

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) ⇆ M_nF_5n(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) ΔH_R=−104.3±2.4 kJmol⁻¹, ΔS_R=−175±20 Jmol⁻¹K⁻¹ for 2NbF₅(g) ⇆ Nb₂F₁₀(g); (b) ΔH_R = −101.1±3.7 kJmol⁻¹, ΔS_R=−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, τ_V and vibrational frequency modulation τ_ω. 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 MX_n⁻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, τ_V, τ_ω, and τ_d, as well as the time between collisions τ_BC. 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, τ_V≥T/2. The modulation time may vary from this same half-period of vibration or the time between collisions τ_BC to very long times, τ_ω≥T/2, τ_ω≥τ_BC. For short-lived complexes, the longest of two characteristic times describing the phase decay cannot exceed possible duration of collision, τ_ω⩽τ_d, τ_V⩽τ_d. Cs-containing configurations follow this definition and therefore should be considered instantaneous short-lived collision complexes: their τ_V≈T/2∼0.1ps, and τ_ω≈τ_BC∼0.03ps. Li-containing configurations appear to be relatively long-lived: their lifetimes could be associated with τ_ω∼0.17ps, which is several times longer than any other shortest possible characteristic time in the system (τ_BC∼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

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

Electroplating and Surface Treatment (2001), IX (1), 23 (in Russian)

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

Galvanotecnica e nueve finiture AIFM (2001), XI-LII, 23 (in Italian)

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(A₁) isotropic 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 XBeCl_20.5 the liquid structure is predominated by tetrahedral and trigonal beryllium species in equilibrium, BeCl₄²⁻=BeCl₃⁻+Cl⁻; and (b) at X_BeCl2>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 T_e=642°C, interesting structural changes of borate glasses formed have been evidenced by means of IR and Raman spectroscopy. Over 80-100wt% Na₂B₄O₇ range AlF₆^2- 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 T_g, 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 A_c1 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(S_1/2)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

E.A. Pavlatou, P.A. Madden, M. Wilson

The Journal of Chemical Physics (1997), 107, 10446-10457

Computer simulations of the short-time, vibrational dynamics of the network-forming ionic melts, LaCl₃ and ZnCl₂, and of their mixtures with network-breaking alkali halides are described. In the mixtures, high frequency peaks in the vibrational density of states are shown to be describable in terms of the normal coordinates of vibration of transient molecular ion species, like LaCl₆³⁻ and ZnCl₄²⁻. Novel simulation methods are presented which allow this association to be established. In the pure melts, the vibrational motions retain a strong aspect of this local polyhedral unit vibrational character, but the effects of network-induced coupling between the vibrations of different units become pronounced, particularly in ZnCl₂. The calculated vibrational spectra are compared with extensive Raman data on these systems, and with infrared and neutron spectra in pure ZnCl₂. For the mixtures, remarkably good agreement with experiment is found, confirming the high quality of the representation of the interionic interactions obtained with the polarizable ion model potentials used. For the melts, there are discrepancies between the peak frequencies observed in the vibrational DOS and the Raman spectra. These discrepancies are likely to be due to the network-induced couplings, whose effect on the Raman (and infrared) spectra is not fully included in the calculated DOS. 

doi: 10.1063/1.474209

E.A. Pavlatou, S.N. Yannopoulos, G.N. Papatheodorou, G. Fytas

The Journal of Physical Chemistry B (1997), 101, 8748-8755

Photon correlation spectroscopy in both polarized and depolarized geometries has been utilized to study density and orientation fluctuations in ZnCl₂, ZnBr₂, and their symmetric mixture in the metastable supercooled state. The pure components display behavior intermediate between strong and fragile glasses reflected in the shape (L(logτ)) and dynamics (τ) of the relaxation function C(t), being similar for both density (C_ρ(t)) and orientation (C₂(t)) correlation functions. In contrast to polymer blends, concentration fluctuations have no measurable broadening effect on the shape of C(t). Instead, L(logτ) becomes narrower with increasing temperature and affects differently C_ρ(t) and C₂(t) for the zinc halide mixture. Despite local homogeneity in the thermodynamically ideal mixture, ZnBr₂ with the higher glass transition temperature (T_g=395K) was found to dominate the primary α-relaxation in the symmetric binary mixture. In the framework of "two-fluid" models, the distribution and the temperature dependence of τ support rigid microheterogeneities on the order of 2nm. The additional slow process, observed in fragile glasses so far, appears to affect also the anisotropic scattering from the pure components.

doi: 10.1021/jp970343s

G.N. Papatheodorou, S.G. Kalogrianitis, T.G. Mihopoulos, E.A. Pavlatou

The Journal of Chemical Physics (1996), 7, 2660-2667

Molten mixtures of lithium chloride and cesium chloride have been studied using Raman spectroscopy at temperatures up to 850°C. Reduced isotropic and anisotropic Raman relative intensities have been measured at different compositions and temperatures. The spectra of the mixtures show two bands with ‘‘characteristic’’ frequencies ωCs and ωLi close to the frequencies of the bands of the pure components. Both ωCs and ωLi shift to higher energies with increasing mole fraction of CsCl and LiCl, respectively. The scattering intensity spectra of the mixtures are compared with the simulated additive spectra of the component salts. Increasing temperature increases drastically the isotropic intensity of the ωLi band but has minor effects on the anisotropic intensities. The data are discussed and interpreted in terms of interaction‐induced polarizability fluctuations. Short range overlap interactions, mainly between Li⁺ and Cl⁻, and the "symmetry" of the local structure around the anion determine the breathing‐like fluctuations which contribute to isotropic scattering and account for the drastic temperature and composition dependence of the Raman intensities. The main contribution to anisotropic scattering arises from near‐neighbor dipole‐induced‐dipole interactions between the highly polarized Cs⁺ and Cl⁻ ions.

doi: 10.1063/1.472129

E.A. Pavlatou

Chimica Chronica-New Series (1994), 23, 265-270

Raman spectra of ZnCl₂, ZnBr₂ and their mixtures were measured in the glassy and liquid state, at various temperatures and compositions. The overall behavior of the spectra indicates the presence of different bonding states in the "network" structure, where halogen atom sharing of the ZnX₄ (X=Cl, Br) tetrahedral groups occurs. Furthermore, at elevated temperatures the "network" structure breaks up, creating in the melt non bridging halogen atoms which are bound to a three-fold coordinated Zn atom. The spectra of the binary ZnCl₂-ZnBr₂ system show that the mixtures are formed by a random closest packing of halogen atoms. The observed similarities between depolarized reduced Raman spectra and the total density of vibrational states in the glassy and liquid state are presented and discussed.

S.N. Yannopoulos, E.A. Pavlatou

Chimica Chronica-New Series (1994), 23, 257-263

Photon correlation and Brillouin spectroscopy have been applied in the glass forming systems ZnCl₂, ZnBr₂ and the 50% ZnCl₂-ZnBr₂ mixture, in order to study the dynamics of local density fluctuations below T_g and far above T_m. The density correlation function of pure ZnCl₂ near T_g is well represend by the non-exponential form exp(-t/τ)^β with β=0.706±0.012 insensitive to temperature. The high value of β parameter categorizes ZnCl₂ in the class of relative "strong" glasses with an Arrhenius temperature dependence of relaxation time τ near T_g (T_g+80°C). The hypersonic properties of pure components and the 50% mixture were also studied in the GHz region, where a single relaxation process was revealed (β=1). The limiting sound velocities uo and u∞ were calculated, showing up a linear decrease with temperature. The longitudinal relaxation time τ_i was found to follow an Arrhenius type relation.

S.G. Kalogrianitis, T.G. Mihopoulos, E.A. Pavlatou, G.N. Papatheodorou

Reduced isotropic and anisotropic Raman spectra were measured for a series of molten alkali metal halides and their mixtures. The systems involving CsX and LiX (X=Cl, Br, I) were investigated at different temperatures and compositions. The spectra of the common anion mixtures show two bands which appear to be a superposition of the bands of the component salts. Similar is the spectra behavior of the common cation salts. The data are interpreted in terms of the fluctuations of the X^- and Cs⁺ polarizabilities induced by short range ionic interactions. The Li-X pair interactions appear to be important in determining the scattered light intensity and its temperature variation. 

doi: 10.1149/199413.0284PV