Now showing 1 - 10 of 12
PublicationComparative Study of Polyethylene Films Embedded with Oxide Nanoparticles of Granulated and Free-Standing NatureNanocomposite 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.
PublicationEffect of sintering temperature of NBT–6BT lead-free ceramics on the structural, ferroelectric and piezoelectric properties( 2022)
; ;O. Turki ;Z. Sassi ;H. Khemakhem ;N. AbdelmoulaL. LebrunThe 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. 15 6
PublicationUltrafast, Light, Soft Martensitic Materials( 2022)
; ;Ejaz Ahmed ;Durga Prasad Karothu ;Jad Mahmoud Halabi ;Ibrahim Tahir ;Kevin Quirós CanalesPanče NaumovMartensitic transformations are well documented in metals and alloys where the atoms connected via metallic bonds rearrange concertedly and rapidly; however, due to the metal atoms, these materials are inherently very dense and add significant weight and bulkiness to actuating devices. Here, remarkably rapid lattice switching of molecular martensitic materials is reported where the rate of structural transformation exceeds other phase transitions several orders of magnitude. With a determined speed in the range of 0.3–0.6 m s−1, the new phase advances throughout the crystal about ten thousand times faster relative to spin-crossover transitions, and about hundred to hundred thousand times faster than other common structural phase transitions. Macroscopic crystals of these materials respond by rapid expansion or contraction of about 0.02 m s−1 for unrestrained crystals and 0.02–0.03 m s−1 for clamped crystals. Monte–Carlo simulation of the spatiotemporal profile of the transition and of the local distribution of elastic and kinetic energies induced by domain growth reveals the critical role of the dynamic phase boundary and the lattice edges in the structure switching. Within a broader context, this study indicates that the martensitic organic crystals are prospective lightweight substitutes of metals for ultrafast and clean energy transduction. 17 2
PublicationMicroscopic electroelastic modeling of a spin transition with symmetry breakingA theoretical approach combining Monte-Carlo and molecular-dynamics techniques is developed to deal with the structural anisotropy upon the spin transition in molecular materials. The simulations were done on a 2D lattice, where the cells have two structural symmetries: square-shaped (non-degenerated) at a low spin (LS) state and a diamond-shaped (degenerated) at a high spin (HS) state. We investigated the thermally induced spin transition and the relaxation of a metastable HS state trapped at low temperatures. We demonstrated that the structural parameters have a crucial impact on the spin transition, and by adjusting the lattice and the elastic parameters, we were able to generate a two-step thermally induced spin transition. The analysis of magnetic and structural properties pointed out that the symmetry breaking reduces significantly the cooperativity between the lattice’s cells. The maps of the difference between cell diagonals reveal an auto-organized HS lattice with an alternation of different symmetries over the state, confirming the symmetry breaking when switching from an LS to HS state. The mechanical relaxation of an LS lattice containing HS defects shows an anisotropic distribution of the elastic energy, channelized over the shortest paths toward the borders of the lattice. The interaction between two HS defects placed in a LS lattice is as well investigated as a function of their separating distance for different symmetries of the HS state. We demonstrated that the HS symmetry impacts the HS/LS elastic barrier as well as the dependence of the relaxed elastic energy on the distance separating two HS defects introduced in an LS lattice
Scopus© Citations 1 37 4
PublicationLanthanides effects on the ferroelectric and energy-storage properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramic: Comparative approachIn this work, (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramic doped with three lanthanides La3+, Gd3+, Ho3+ were prepared and investigated the corresponding structural, vibrational, ferroelectric and energy storage properties. The X-ray diffraction and Raman spectra reveal a pure perovskite with the coexistence of rhombohedral and tetragonal phases at room temperature for all samples. The thermal dependence of the dielectric constant shows a ferroelectric/ antiferroelectric phase transition around Td, confirmed by the change in the shape of P-E hysteresis loops for all samples. We found that the optimal piezoelectric and ferroelectric properties were obtained in the ceramic doped with Ho3+. Furthermore, the later sample revealed relatively remarkable features of energy storage density with temperature, reaching a maximum of 0.63 J/cm3 around 100 ◦C. The results discussed in this work highlight the great potential of the doped NBT-6BT based lead-free ceramics.
PublicationMagneto-elastic properties of a spin crossover membrane deposited on a deformable substrate(Institute of Physics, 2020)
; ;Maalej, ASingh, YSpin-crossover (SCO) solids have been studied for several years due to their fascinating physical properties and their potential applications as optical switches and reversible high-density memories for information storage. Through this article, we will examine in details the effects of substrate's lattice parameters, on a deformable spin crossover membrane, simulated using an electro-elastic model taking into account the volume change at the transition. The molecules of the membrane can be either in the low spin state (LS) or the high spin state (HS), while those of the substrate are electronically neutral. Magnetic properties of the SCO membrane and the pressure distribution as a function of the lattice parameter of the substrate have been investigated. We demonstrated that the thermally induced first-order spin transition is significantly affected by the structural properties of the substrate, where a rise in the lattice parameter of the latter lowers the transition temperature and reduces the width of the thermal hysteresis loop. The investigations on the spatiotemporal aspects of the spin transition in the membrane demonstrates that the nucleation and growth processes are sensitive to the structural properties of the elastic misfit between the substrate and the SCO membrane. 372Scopus© Citations 3
PublicationEnhancement of dielectric, piezoelectric, ferroelectric, and electrocaloric properties in slightly doped (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramic by samarium( 2019)
;Seveyrat, Laurence ; ;Khemakhem, Hamadi ;Turki, Olfa ;Sassi, ZinaLebrun, LaurentWe 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 12 181 13
PublicationEffects of high-spin-low-spin lattice misfit on the nucleation and propagation velocities of elastic interfaces in cooperative spin-crossover solids( 2019)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 4 134 2
PublicationElectro-elastic modeling of thermal and mechanical properties of a spin crossover core/shell nanoparticle( 2019)
; ;Maalej, Ahmed ;Boukheddaden, KamelAffes, KarimWe 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 8 155 3
PublicationControl of the Speed of a Light-Induced Spin Transition through Mesoscale Core-Shell Architecture(American Chemical Society, 2018)
; ;Felts, A.C. ;Cain, J.M. ;Talham, D.R. ;Abboud, K.A. ;Ahir, A.R. ;Andrus, M.J. ;Boukheddaden, K.Meisel, M.W.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 41 172 3