Now showing 1 - 10 of 14
  • Publication
    An electro-elastic theory for the mechanically-assisted photo-induced spin transition in core-shell spin-crossover nanoparticles
    (2018) ;
    Boukheddaden, Kamel
    The development of heterostructure materials may lead to new features that cannot be obtained with natural materials. Here we simulate a model structurally hybrid core-shell nanoparticle with different lattice parameters between an electronically inert shell and an active spin crossover core. The nanoparticle consists of a 2D core with 20 × 20 size with square symmetry, surrounded by a shell made of 10 atomic layers. The low temperature photoexcitation of the core shows a significant environment-dependent behavior. In particular, we demonstrate that a shell with a large lattice parameter accelerates the low-spin to high-spin photoexcitation process of the core through the single domain nucleation mechanism while a moderate shell lattice parameter leads to spatially-homogeneous growth of the high-spin fraction. We found that the mechanical retro-action of the shell may cause elastic instability of the core leading to efficient control and manipulation of its photo-conversion.
    Scopus© Citations 9  184  50
  • Publication
    Asymmetric self-organization accompanying a thermoinduced spin transition with symmetry breaking: Microscopic modeling
    (2023) ;
    Boukheddaden, Kamel
    A microscopic elastic model allowing a symmetry breaking upon a spin transition is developed based on competing interactions between the nearest and next-nearest neighbors. The model yields a structurally degenerated high-spin state with a diamond-shaped cell and a nondegenerated low-spin state with a square-shaped cell. We investigated the effect of the symmetry change of the unit cell on the thermoinduced spin transition by monitoring the ratio of the elastic energies involved in the nearest and next-nearest neighbors. The simulations are performed on a deformable two-dimensional lattice made of spins S=±1 (representing the high-spin and low-spin molecules) coupled with springs. The numerical resolution of the model is based on the Monte Carlo metropolis approach, running on spins and positions variables. The simulations of the thermoinduced spin transition disclose asymmetric thermal hysteresis loops with quite different domain distributions on the heating and cooling branches. The analysis of the magnetic and structural properties pointed out that the spin transition with symmetry breaking is dependent on the sign of the thermal gradient. We demonstrated that the nucleation and growth process of spin domains might contrast with the structural self-organization of the lattice according to the energetic contribution of the symmetry breaking. Indeed, the examination of the spatial organization aspects revealed that the structural and elastic anisotropy in the lattice hinders the long-range character of the intermolecular interactions. The lattice configurations show a labyrinthlike structure during a transition from high-symmetry to low-symmetry phases and a multidomain structure upon a transition in the other way around regardless of the spin state in both phases. Furthermore, we found that thermal fluctuations have a crucial role depending on the thermal gradient and on the direction of evolution of the symmetry (reducing or increasing). We demonstrated that stepped and even incomplete spin transitions could be obtained by adjusting the elastic contribution responsible for the symmetry breaking.
      5
  • Publication
    Comparative Study of Polyethylene Films Embedded with Oxide Nanoparticles of Granulated and Free-Standing Nature
    (2022) ; ; ;
    Le Guyon, Valerie
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    ; ;
    Angastiniotis, Nicos
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    Koutsokeras, Loukas
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    Duponchel, Benoît
    Nanocomposite polymer films are a very diverse research field due to their many applications. The search for low-cost, versatile methods, producing regulated properties of the final products, has thus become extremely relevant. We have previously reported a bulk-scale process, dispersing granulated metal oxide nanoparticles, of both unary and multi-component nature, in a low-density polyethylene (LDPE) polymer matrix, establishing a reference in the produced films’ optical properties, due to the high degree of homogeneity and preservation of the primary particle size allowed by this method. In this work, unmodified, free-standing particles, namely zinc oxide (ZnO), titanium dioxide (TiO2), aluminum oxide (Al2O3), and silicon dioxide (SiO2) are blended directly with LDPE, and the optical properties of the fabricated films are compared to those of films made using the granulation process. The direct blending process evidently allows for control of the secondary particle size and ensures a homogeneous dispersion of the particles, albeit to a lesser extent than the granulation process. Despite the secondary particle size being comparatively larger than its granulated counterpart, the process still provides a regulated degree of deagglomeration of the free-standing oxide particles, so it can be used as a low-cost alternative. The regulation of the secondary particle size tunes the transmission and reflection spectra, in both unary and mixed oxide compositions. Finally, the direct blending process exhibits a clear ability to tune the energy band gap in mixed oxides.
      76  108
  • Publication
    Control of the Speed of a Light-Induced Spin Transition through Mesoscale Core-Shell Architecture
    (American Chemical Society, 2018) ;
    Felts, A.C.
    ;
    Cain, J.M.
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    Talham, D.R.
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    Abboud, K.A.
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    Andrus, M.J.
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    Boukheddaden, K.
    ;
    Meisel, M.W.
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    Ahir, A.R.
    The rate of the light-induced spin transition in a coordination polymer network solid dramatically increases when included as the core in mesoscale core-shell particles. A series of photomagnetic coordination polymer core-shell heterostructures, based on the light-switchable RbaCob[Fe(CN)6]c·mH2O (RbCoFe-PBA) as core with the isostructural KjNik[Cr(CN)6]l·nH2O (KNiCr-PBA) as shell, are studied using temperature-dependent powder X-ray diffraction and SQUID magnetometry. The core RbCoFe-PBA exhibits a charge transfer-induced spin transition (CTIST), which can be thermally and optically induced. When coupled to the shell, the rate of the optically induced transition from low spin to high spin increases. Isothermal relaxation from the optically induced high spin state of the core back to the low spin state and activation energies associated with the transition between these states were measured. The presence of a shell decreases the activation energy, which is associated with the elastic properties of the core. Numerical simulations using an electro-elastic model for the spin transition in core-shell particles supports the findings, demonstrating how coupling of the core to the shell changes the elastic properties of the system. The ability to tune the rate of optically induced magnetic and structural phase transitions through control of mesoscale architecture presents a new approach to the development of photoswitchable materials with tailored properties. © 2018 American Chemical Society.
    Scopus© Citations 41  235  71
  • Publication
    Effect of sintering temperature of NBT–6BT lead-free ceramics on the structural, ferroelectric and piezoelectric properties
    (2022) ;
    O. Turki
    ;
    Z. Sassi
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    H. Khemakhem
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    N. Abdelmoula
    ;
    L. Lebrun
    The lead-free Na 0.5 Bi 0.5 0.94 Ba 0.06 TiO 3 (NBT-6BT) ceramics was fabricated at various sintering temperatures using a conventional solid-state reaction method. The effect of calcination temperature was systematically investigated on the structural properties. The XRD results show that the 850 °C is the best calcination temperature, where the NBT-6BT ceramic had the largest crystallite size with a dense sample, 96% of the theoretical value. However, the density decreased significantly with increasing the calcination temperature above 850 °C due to the secondary phase formation. On the other hand, the effect of sintering temperature was studied on the piezoelectric and ferroelectric properties evolutions. It is found that the optimal ferroelectric and piezoelectric values were obtained at 1150 °C (P r = 27 µC/cm 2 , E c = 3.89 kV/mm and d 33 = 110 pC/N). According to these results, we suggest that 850 °C and 1150 °C can be considered as optimal calcination and sintering temperature in NBT-6BT ceramic, respectively.
    Scopus© Citations 6  29  6
  • Publication
    Effects of high-spin-low-spin lattice misfit on the nucleation and propagation velocities of elastic interfaces in cooperative spin-crossover solids
    Switchable Spin crossover (SCO) materials have been studied for many years for their promising applications as sensors of pressure, memories, and molecular switches. Recent optical microscopy studies demonstrated that, in cooperative SCO single crystals, the first-order spin transition proceeds through nucleation of a macroscopic single domain with a clear elastic high-spin=low-spin interface that propagates over the whole crystal, with very small velocities, measured in the range: 2–10 µm·s−1. The present theoretical work is devoted to investigate the nucleation and propagation of high-spin (HS) low-spin (LS) domains in spin crossover materials during the thermally-induced relaxation of a metastable HS state towards the stable LS state. The analysis is performed on a 2D rectangular lattice using an electro elastic model, which takes into account for the change of spin states and the volume along the transition process. We found that the increase of lattice parameters misfits affects the nucleation dynamics of the spin-crossover transformations, successively from homogenous domain nucleation to two sites nucleation leading to two coexisting domains and ending up with a macroscopic single domain features beyond a threshold value of lattice misfit. The evolution of the HS=LS interface has been monitored during the relaxation and a universal law of the HS–LS interface velocity has been derived and discussed.
    Scopus© Citations 4  158  256
  • Publication
    Electro-elastic modeling of thermal and mechanical properties of a spin crossover core/shell nanoparticle
    (2019) ;
    Maalej, Ahmed
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    Boukheddaden, Kamel
    ;
    Affes, Karim
    We investigated theoretically the thermo-induced spin transition of a nanostructure made of an active spin crossover core surrounded by an inert shell with a misfit of lattice parameters between the two constituents. We demonstrated that (i) the structural and magnetic features of the SCO core are very sensitive to the structural properties of the surrounding environment. (ii) The misfit of lattice parameters influences the nature of the spin transition from gradual to abrupt one with an important shift of the transition temperature. (iii) The structural heterogeneity of the nanoparticle affects as well the spatiotemporal kinetics of the thermo-induced spin transition. The mechanical properties of the nanoparticle were as well studied and correlated with the magnetic behaviour of the nanoparticle.
    Scopus© Citations 8  196  74
  • Publication
    Enhancement of dielectric, piezoelectric, ferroelectric, and electrocaloric properties in slightly doped (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramic by samarium
    (2019)
    Seveyrat, Laurence
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    ;
    Khemakhem, Hamadi
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    Turki, Olfa
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    Sassi, Zina
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    Lebrun, Laurent
    We investigated the structural, dielectric, piezoelectric, ferroelectric, and electrocaloric properties of the samarium doped (Na0.5Bi0.5)0.94Ba0.06TiO3 (NBT-6BT) ceramic. X-ray diffraction and Raman spectra confirm the coexistence of the rhombohedral and tetragonal structures for a low amount of Sm (≤8 mol. %), while the compositions with a higher amount of Sm2O3 (11 mol. %) have a pseudocubic structure. The thermal dependency of the dielectric permittivity revealed two phase transitions from ferroelectric to antiferroelectric at low temperatures, then to paraelectric phase at higher temperatures. The substitution of NBT-6BT with 2 mol. % of Sm2O3 remarkably enhances the ferroelectric and the piezoelectric properties of the (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramic. Furthermore, a large electrocaloric effect (ΔT=1.4K) (ΔT=1.4K) was directly measured on the ceramic doped with 2 mol. % of Sm2O3 under an applied electric field of 50 kV/cm.
    Scopus© Citations 13  264  108
  • Publication
    Ferroelectric Properties and Electrocaloric Effect in Dy2O3 Substitution on Lead-Free (Na0.5 Bi0.5)0.94 Ba0.06TiO3 Ceramic
    (2023)
    Turki, O.
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    Zouari, I.
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    Sassi, Z.
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    Seveyrat, L.
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    Khemakhem, H.
    We report the impact of dysprosium (Dy, with y = 0.01, 0.02, 0.05 and 0.08) substitution in (Na0.5 Bi0.5)0.94 Ba0.06TiO3 (NBT-6BT) lead-free ceramics. The structural, vibrational dielectric, ferroelectric and electrocaloric (EC) properties of all samples were systematically investigated. X-ray diffraction analysis revealed the coexistence of tetragonal (P4mm) and rhombohedral (R3c) structural phases at low content of Dy (y = 0.01, 0.02 and 0.05). The structural changes with the introduction of Dy were confirmed by Raman spectroscopy at room temperature. The evolution of the Raman spectra with temperature was found to be strongly correlated with the dielectric measurements. Higher stability of the ferroelectric (FE) phase was obtained at room temperature for the compositions y = 0.01, 0.02 and 0.05, with optimum values for y = 0.02 as remanent polarization Pr = 32µC/cm2, as well as piezoelectric coefficients d33 = 137 pC/N, kp = 0.27 and kt = 0.16, whereas a higher content of Dy (y = 0.08) induced a remarkable decrease of the ferroelectric and piezoelectric properties. Using the direct EC measurement, the ceramic corresponding to y = 0.02 exhibited a significant EC response, where ΔT = 1.2 K under 5 kV/mm. The incorporation of Dy was found to enhance the EC responsivity coefficient ζ = iT/ΔE), with a best value of ζ = 0.24 K.mm/kV for y = 0.02.
      8
  • Publication
    Interplay between a crystal's shape and spatiotemporal dynamics in a spin transition material
    (Royal Society of Chemistry, 2018) ;
    Fourati, H.
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    Milin, E.
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    Boukheddaden, K.
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    Abid, Y.
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    Triki, S.
    ;
    Chastanet, G.
    We investigated by means of optical microscopy (OM) the spatiotemporal features of the thermo-induced spin transition of [Fe(2-pytrz)2{Pd(CN)4}]·3H2O (1) (2-pytrz = 4-(2-pyridyl)-1,2,4,4H-triazole) single crystals having two different shapes (triangle and rectangle). While magnetic and calorimetric measurements, performed on a polycrystalline material, showed the respective average heating and cooling transition temperatures of (Tdown1/2 ∼ 152 K, Tup1/2 ∼ 154 K) and (Tdown1/2 ∼ 160.0 K, Tup1/2 ∼ 163.5 K), OM studies performed on a unique single crystal revealed significantly different switching temperatures (Tdown1/2 ∼ 152 K and Tup1/2 ∼ 162 K). OM investigations showed an interface spreading over all crystals during the spin transition. Thanks to the color contrast between the low-spin (LS) and the high-spin (HS) states, we have been able to follow the real time dynamics of the spin transition between these two spin states, as well as access the thermal hysteresis loop of each single crystal. After image processing, the HS-LS interface's velocity was carefully estimated in the ranges [4.4-8.5] μm s-1 and [2.5-5.5] μm s-1 on cooling and heating, respectively. In addition, we found that the velocity of the interface is shape-dependent, and accelerates nearby the crystal's borders. Interestingly, we observed that during the propagation process, the interface optimizes its shape so as to minimize the excess of elastic energy arising from the lattice parameter misfit between the LS and HS phases. All of these original experimental results are well reproduced using a spatiotemporal model based on the description of the spin-crossover problem as a reaction diffusion phenomenon. © the Owner Societies 2018.
    Scopus© Citations 23  173  72