Manuel Valente

Materials Physics, Materials Science

PhD
26.87

Publications

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    Ah. Dhahri · M. Jemmali · E. Dhahri · M.A. Valente
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    ABSTRACT: The La0.75Sr0.1Ca0.15MnO3 sample have been prepared by sol-gel method. The crystallographic study shows that the sample crystallize in the rhombohedral structure with space group. The field dependence of magnetocaloric properties of manganite La0.75Sr0.1Ca0.15MnO3 with second order phase transition material is investigated using a phenomenological model. The model parameters were determined from the magnetization data adjustment and were used to give better fits to magnetic transition and to calculate the magnetocaloric thermodynamic quantities. The magnetic entropy change reaches a peak of about 5.8 J/kg .K and the relative cooling power (RCP) value of 195 J/kg at 305 K upon 5T applied magnetic field. The critical exponents associated with ferromagnetic transition have been determined from magnetocaloric effect methods.
    Journal of Alloys and Compounds 03/2015; 638. DOI:10.1016/j.jallcom.2015.01.314 · 2.73 Impact Factor
  • F. Issaoui · Y. Baklouti · E. Dhahri · F. Zouari · M. A. Valente
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    ABSTRACT: 1-(2Aminoethyl) pyrrolidinium tetrachlorocobaltat (C6H16N2) CoCl4 has been prepared and analyzed by X-ray diffraction. The compound crystallizes in the orthorhombic system with space group P212121. The unit cell dimensions are a= 7.3279(13), b= 13.051(2) and c= 13.831(2) Å with Z= 4. The crystal structure of this salt was solved by Patterson methods and refined by full-matrix least squares on F 2 to the final values of R=0.0486 and wR =0.1347. The structure consists of diprotonated 1-(2aminoethyl) pyrrolidinium cations and monomeric CoCl \(_{4}^{2-}\) These entities are interconnected by means of hydrogen bonding contacts forming a threedimensional network. Magnetization was used to investigate the magnetic properties. This compound exhibits an antiferromagnetic (AFM) to paramagnetic (PM) phase transition at a temperature (T N) lower than 2 K. The values of paramagnetic Curie-Weiss temperature (𝜃 cw) and the exchange parameter (J/ K B) emphasize the existence of an antiferromagnetic interaction between the neighboring cobaltions.
    Journal of Superconductivity and Novel Magnetism 01/2015; DOI:10.1007/s10948-015-3057-y · 0.93 Impact Factor
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    F. I. H. Rhouma · A. Dhahri · J. Dhahri · M. A. Valente · K. Khirouni
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    ABSTRACT: Three perovskite-structured ceramic phases Ba1−3x Pr2x Ti0.95Sn0.05O3 (BPTS-x) with x=0.01, 0.03, and 0.05 have been synthesized by solid-state reaction method. Obtained BPTS-x ceramics were investigated by X-ray diffraction and dielectric properties measurements. Analysis of all the ceramic samples using X-ray diffraction method at room temperature and its Rietveld refinement inferred rhombohedric structure with the space group $R\bar{3}c$ . The temperature variation of real permittivity gives evidence of the ferroelectric phase transition and of the relaxation behavior. With increasing Pr concentration, the degree of diffuse phase transition was enhanced, and a linear reduction in the transition temperature was produced. The conductivity spectra have been investigated by the Jonscher universal power law (σ(ω)=σ dc+Aω s ) and attributed to hopping conduction mechanism.
    Applied Physics A 02/2014; 114(3). DOI:10.1007/s00339-013-7760-x · 1.69 Impact Factor
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    ABSTRACT: Polycrystalline sample of Bi0.8Dy0.1C0.1Fe0.9Ti0.1O3 (where C=Ba (DB), La (DL) and Pb (DP)) was prepared by a high-temperature solid state reaction technique. For the understanding of the multiferroic property, the relation between the crystal structures, magnetic transition and ferroelectric transitions with increasing temperature have been analyzed. X-ray diffraction analysis of the powders suggests the formation of a single-phase material with tetragonal structure. Studies of dielectric properties of the compound, studied in a frequency range 104–106 Hz at different temperatures (25–30 °C), exhibits two dielectric anomalies one at 225 °C and other around 375 °C (ferroelectric–paraelectric type transition). By using VSM technique, a significant change in the magnetic properties was observed in the Bi0.8Dy0.1C0.1Fe0.9Ti0.1O3 system with Pb doping. It was seen that coercive field HC and remnant magnetization MR increase with for Pb but for La and Ba it decreases.
    Solid State Communications 02/2014; 180:56–63. DOI:10.1016/j.ssc.2013.11.005 · 1.70 Impact Factor
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    ABSTRACT: journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t Polycrystalline sample of Bi 0.8 Dy 0.1 C 0.1 Fe 0.9 Ti 0.1 O 3 (where C¼ Ba (DB), La (DL) and Pb (DP)) was prepared by a high-temperature solid state reaction technique. For the understanding of the multiferroic property, the relation between the crystal structures, magnetic transition and ferroelectric transitions with increasing temperature have been analyzed. X-ray diffraction analysis of the powders suggests the formation of a single-phase material with tetragonal structure. Studies of dielectric properties of the compound, studied in a frequency range 10 4 –10 6 Hz at different temperatures (25–30 1C), exhibits two dielectric anomalies one at 225 1C and other around 375 1C (ferroelectric–paraelectric type transition). By using VSM technique, a significant change in the magnetic properties was observed in the Bi 0.8 Dy 0.1 C 0.1 Fe 0.9 Ti 0.1 O 3 system with Pb doping. It was seen that coercive field H C and remnant magnetization M R increase with for Pb but for La and Ba it decreases.
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    ABSTRACT: Dielectric and magnetic properties of the Bi1−x−yGdxLayFe1−yTiyO3 (x=0.1 and y=0.0, 0.1, 0.2, 0.3 and 0.5) solid solution ceramics have been presented this work. Effects of La substitution on the structural, electrical and ferroelectric properties of Bi1−x−yGdxLayFe1−yTiyO3 samples have been studied by performing X-ray diffraction and dielectric measurements as well as characterizing M–H hysteresis loop. The results indicate that, when La content exceeds 0.2, the crystal structure of the ceramic samples changes from rhombohedral to tetragonal phase. The maximum ferroelectric transition temperature (Tc) of this system was 190 °C with the dielectric constant peak of 2500 at 100 kHz for y=0.5. With increasing La content, the remanent magnetization increases and the coercive magnetic field decreases.
    Journal of Physics and Chemistry of Solids 07/2013; 74(7):905–912. DOI:10.1016/j.jpcs.2013.01.033 · 1.59 Impact Factor
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    ABSTRACT: Nickel ferrite nanoparticles were prepared by a modified sol-gel technique employing coconut oil, and then annealed at different temperatures in 400-1200 °C range. This route of preparation has revealed to be one efficient and cheap technique to obtain high quality nickel ferrite nanosized powder. Sample particles sizes obtained with XRD data and Scherrer's formula lie in 13 nm to 138 nm, with increased size with annealing temperature. Hysteresis loops have been obtained at room temperature with an inductive method. Magnetic field induced microwave absorption in nanoscale ferrites is a recent an active area of research, in order to characterize and explore potential novel applications. In the present work microwave magnetoabsorption data of the annealed nickel ferrite nanoparticles are presented. These data have been obtained with a system based on a network analyzer that operates in the frequency range 0 - 8.5 GHz. At fields up to 400 mT we can observe a peak according to ferromagnetic resonance theory. Sample annealed at higher temperature exhibits different absorption, coercivity and saturation magnetization figures, revealing its multidomain character.
    The European Physical Journal Conferences 01/2013; 40:17003-. DOI:10.1051/epjconf/20134017003
  • M. P. F. Graça · P. R. Prezas · M. M. Costa · M. A. Valente
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    ABSTRACT: Lithium niobate is an artificially synthesized material with wide technological applications, due to its numerous physical properties, such as: ferroelectricity, birefringence and large pyroelectric, piezoelectric, non-linear, acousto-optic, electro-optic and photo-elastic coefficients. It also exhibits very strong bulk photovoltaic and photo refractive effects. In the present work, LiNbO3 powders were prepared by the Pechini (sol–gel) method. The heat-treatment of the base amorphous powders, at 450 °C promotes the formation of LiNbO3 crystalline phase. The electrical characterization, in function of the heat-treatment time, shows that the dc conductivity depends on the quantity of crystalline phase, which increases with the increase of heat-treatment time. The sample treated for 96 h shows a dielectric constant value of ~35, at low frequencies. A dielectric relaxation phenomenon independent of the temperature of measurement was detected in the samples treated at 48 and 96 h, shifting to higher frequencies with the increase of the heat-treatment time.
    Journal of Sol-Gel Science and Technology 10/2012; 64(1). DOI:10.1007/s10971-012-2829-0 · 1.55 Impact Factor
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    ABSTRACT: /p>The production of a Tantalum solid electrolytic capacitor requires the impregnation of MnO2 by pyrolysis in one of the several manufacturing steps. It has been reported that niobium oxides are a good alternative, presenting potentially better dielectric properties and a better cost effectiveness. Thus, it is important to study the conditions and the effect of the MnO2 impregnation on niobium oxide in order to understand and optimize the parameters of this process. The morphology and microstructure of the anode is one of the most important aspects that interfere with the dielectric properties of the capacitor. In this work, it is presented a study of the morphology and microstructure of different niobium oxide anodes after electrochemical oxidation (NbO/Nb2O5 core-shell grain structure), and after MnO2 impregnation with different pyrolysis temperatures. This impregnation is made by dipping the anodes, with the NbO/Nb2O5 core-shell structure, in a slurry of Mn(NO3)2. Heating this slurry while the anode is dipped, will lead to a pyrolysis reaction where the liberation of NO2 occurs as a gas, and where the product MnO2 solidifies around the grains.
    Microscopy and Microanalysis 09/2012; DOI:10.1017/S1431927612013153 · 1.76 Impact Factor
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    T Satyanarayana · M A Valente · G Nagarjuna · N Veeraiah
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    ABSTRACT: Glass system of composition (50 À x)TeO 2 –5Sb 2 O 3 –45B 2 O 3 : xMnO with 'x' ranging from 0.3 to 1.5 mol% has been prepared via melt quenching followed by controlled crystallization. Further, the samples were characterized by X-ray diffraction, scanning electron microscopy and DSC techniques. A number of studies viz., optical absorption, EPR, and luminescence of these glass ceramics have been carried out. The spectroscopic studies have confirmed the existence of manganese ions in trivalent state in addition to divalent. EPR spectra of all investigated glass ceramics exhibit resonance signals that are characteristic for Mn 2 þ ions. The changes found in all the properties as function of the crystallizing agent (MnO) have been discussed in detail.
    Journal of Physics and Chemistry of Solids 09/2012; 74(2):229-235. DOI:10.1016/j.jpcs.2012.09.011 · 1.59 Impact Factor
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    F. I. H. Rhouma · A. Dhahri · J. Dhahri · M. A. Valente
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    ABSTRACT: Titanate barium (BaTiO3)-type oxide ceramics Ba0.8La0.133Ti1−x Snx O3 (BLTS) (here x=0.15 and 0.2) have been synthesized by the standard solid-state reaction method. Preliminary room temperature X-ray study confirms the formation of single-phase compounds in a rhombohedral crystal system. The electrical properties of BLTS were studied using the ac impedance spectroscopy technique over a wide range of temperature (120–320 K) in the frequency range of 40 Hz to 10 MHz. The presence of a single arc in the complex modulus spectrum at different temperatures confirms the single-phase character of the BLTS compounds.
    Applied Physics A 09/2012; 108(3). DOI:10.1007/s00339-012-6935-1 · 1.69 Impact Factor
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    ABSTRACT: Bi(1-x-y)GdxBayFe(1-y)TiyO3 (x = 0.1 and y = 0.1, 0.2, 0.3) solid solutions have been prepared via solid state reaction method with the aim to obtaining magnetoelectric coupling (i.e., linear relation between magnetization and electric field) at room temperature. Optimum calcination and sintering strategies for obtaining pure perovskite phase, high density ceramics and homogeneous microstructures have been determined. The maximum ferroelectric transition temperature (Tc) of this system was 150-170 degrees C with the dielectric constant peak of 2300 at 100 kHz for y = 0.1. Well saturated piezoelectric loops were observed for all composition indicating room temperature ferroelectricity. Hardness and Young's modulus decrease with depth and with increasing concentration y.
    Journal of Nanoscience and Nanotechnology 08/2012; 12(8):6639-44. DOI:10.1166/jnn.2012.4559 · 1.34 Impact Factor
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    ABSTRACT: Samples of ferrofluids containing chemically stabilized nanoparticles of magnetite (Fe3O4) with tetramethylammonium hydroxide (TMAOH) were prepared by a direct reduction–precipitation method. The influences of aging time and temperature on the size and monodispersion characteristics of the produced nanoparticles were investigated. Transmission electron microscopy, powder X-ray diffraction, Fourier-transform infrared, and magnetization measurements with applied magnetic field up to 2 T were used to characterize the synthesized iron oxides. Raising the temperature of the synthesized material in autoclave affects positively the monodispersion of the nanoparticles, but it was not found to significantly influence the size itself of individual particles.
    Journal of Magnetism and Magnetic Materials 05/2012; 324(10):1753–1757. DOI:10.1016/j.jmmm.2011.12.033 · 2.00 Impact Factor
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    ABSTRACT: Nanoparticles of magnetite (Fe 3 O 4 ) were obtained by reacting ferric chloride with sodium sulphite, through the reduction-precipitation method. The effects of adding tetramethylammonium hydroxide (TMAOH) during or after the precipitation of the iron oxide were studied in an attempt to obtain well-dispersed magnetite nanoparticles. Accordingly, the following experimental conditions were tested: (i) precipitation in absence of TMAOH (sample Mt), (ii) the same as (i) after peptizing with TMAOH (Mt1), (iii) TMAOH added to the reaction mixture during the precipitation of magnetite (Mt2). Analyses with transmission electron microscopy (TEM), X-ray diffraction, Mössbauer spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), zeta potential, and magnetization measurements up to 2.5 T revealed that magnetite was normally formed also in the medium containing TMAOH. The degree of particles agglomeration was monitored with laser diffraction and technique and inspection of TEM images. The relative contributions of Néel and Brownian relaxations on the magnetic heat dissipation were studied by investigating the ability of suspensions of these magnetite nanoparticles to release heat in aqueous and in hydrogel media. Based on ATR-FTIR and zeta potential data, it is suggested that the surfaces of the synthesized magnetite particles treated with TMAOH become coated with (CH 3 ) 4 N + cations.
    Journal of Nanomaterials 01/2012; 2012(1–3). DOI:10.1155/2012/454759 · 1.64 Impact Factor
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    ABSTRACT: Glasses doped with rare earth (RE) ions are widely studied due to the numerous applications of these systems (active media for solid state lasers, optical telecommunication, non-linear optical materials, electro-optic devices, etc.). Boron trioxide, B2O3, is a known glass forming oxide with a relative low melting temperature. The addition of a transition metal oxide, such as V2O5, promotes the exhibition of semiconducting properties making these promising systems for several technological applications, such that ones involved in solar energy conversion devices. It is known that alkali borovanadate glasses, like alkali borate glasses themselves, are ionically conducting materials. Despite their importance there are only few studies on these glasses reported on literature. Thus, the alkali-borovanadate glass system constitutes a family with high interest from the electrical and dielectric point of view. The effect of the increment of alkali quantity in the electrical and dielectric response of these glasses and the physical/structural explanation are questions which will be addressed. In this work, the transparent glass samples with molar composition 0.01Sm(2)O(3)-0.99[0.85B(2)O(3)-(0.15-x)Li2O-xV(2)O(5)] with x = 0, 0.1, 0.2, 0.35, 0.5 and 2 (mol%) were prepared by conventional melting technique. The prepared samples were fully characterized using different experimental techniques such as, differential thermal analysis (DTA), X-ray diffraction (XRD), electrical and dielectric measurements. The samples structure, electrical and dielectric properties as a function of vanadium ions content was explored and discussed. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    physica status solidi (c) 11/2011; 8(11-12):3107-3110. DOI:10.1002/pssc.201000762
  • M. A. Valente · L. Bih · H. Bih · M. P. F. Graça
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    ABSTRACT: Glasses with transition metal ions have interest because of their potential use in electrochemical, electronic and electro-optical devices. Molybdenum oxide is not a glass-forming, but it is able to enter the glass structure in the form of MoO4 tetrahedral or MoO6 octahedral with some glass-forming, like P2O5. The binary MoO3–P2O5 glasses were stable over a wide range of compositions. The ternary glass P2O5–MoO3–Li2O presents high potentiality as fast ion-conductors. Ferroelectric crystalline particles (LiNbO3) can be obtained by heat-treatments of the glass P2O5-Li2O-Nb2O5. The heat-treatment does not promote glass-ceramics with a single crystalline phase. The quaternary system P2O5-MoO3-Nb2O5-X2O, (X=Li, Na), prepared by melt quenching, allowed the control of the crystalline phases obtained by heat-treatments. In this work, the transparent as-prepared glasses were heat-treated at 550 ºC and 650 ºC for 4 hours. The structure of the samples was analyzed by differential thermal analyses (DTA), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The change of the MoO3/P2O5 ratio is the main chemical parameter that governs the nucleation and growth of different crystallites. The decrease of this ratio induces the formation of non-linear optical NbOPO4 crystallites in the glass. The increase of the ratio leads to the formation of other phases, such as LiPO3 (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 11/2011; 8(11‐12):3091 - 3094. DOI:10.1002/pssc.201000765
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    ABSTRACT: A family of antimony tellurite glasses of the composition (55-x)TeO2-10Sb2O3-35B2O3 mixed with different concentrations of TiO2 (0 to 2 mol%) were prepared and crystalized. The samples were characterized by XRD and SEM techniques; spectroscopic (optical absorption and Raman) and PIB properties have been investigated. Randomly distributed crystals were identified by SEM. The XRD studies have revealed the presence of Sb2Te2O7, Sb2Te2O9, TiTe3O8,TiBO3 crystalline phases in these samples. The diffraction data also indicated that, the antimony ions coexist in Sb5+ state with Sb3+ state in these samples; however, the concentration of Sb3+ ions seems to be dominant over Sb5+ ions in the samples containing lower concentrations of TiO2. Optical absorption spectral studies have indicated that a considerable proportion of Ti ions do exist in Ti3+ state in addition to Ti4+ state and the redox ratio seems to be increasing with increase in the concentration of TiO2. The Raman spectral studies have revealed the presence of several conventional structural groups viz., TeO4, SbO3, BO3, BO4, TiO4,TiO6 in the glass ceramic network; these studies have also indicated a growing degree of disorder in the glass ceramic with increase in the concentration of TiO2. The experiments on PIB with 7 ns 1064 nm Nd:YAG laser beam indicated the maximum intensity of birefringence for the sample crystallized with 2.0 mol% of TiO2. This result combined with the optical absorption studies allowed us to conclusion that Ti ion surrounding ligands play principal role in the observed PIB effects (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 11/2011; 8(11‐12):3082 - 3086. DOI:10.1002/pssc.201000750
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    Filipe Amaral · Manuel Valente · Luís Cadillon Costa
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    ABSTRACT: We report the synthesis and characterization of CaCu3Ti4O12 (CCTO) powders prepared by two different wet-chemical methods: ethylenediaminetetraacetic acid-gel method (EDTA) and modified Pechini (MP) process. In both cases, almost single-phase CCTO fine powders were obtained from the calcination of the gel precursors at lower temperatures and shorter times than those reported for the conventional solid-state reaction process. The structure of the calcinated powders and sintered pellets were characterized by X-ray diffraction and scanning electron microscopy. Impedance spectroscopy measurements were performed on the sintered pellets obtained from the different powders, in the frequency range from 40 Hz to 110 MHz, at room temperature. The results are discussed and compared. High frequency relaxation data was fitted with Cole–Cole model and the calculated relaxation parameters were correlated with morphological and structural properties.
    Physica Status Solidi (A) Applications and Materials 10/2011; 208(10):2284 - 2287. DOI:10.1002/pssa.201000760 · 1.62 Impact Factor
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    Manuel Pedro Fernandes Grac?a · Manuel Almeida Valente
    Advances in Ceramics - Electric and Magnetic Ceramics, Bioceramics, Ceramics and Environment, 09/2011; , ISBN: 978-953-307-350-7

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