[Show abstract][Hide abstract] ABSTRACT: In this study we analyze the metal substitution effects on the structural, morphological, charge transport, and spin transition properties of the [Fe1–xZnx(Htrz)2(trz)](BF4) (trz = triazole, x = 0, 0.26, or 0.43) compound using electron microscopy, powder X-ray diffraction, optical reflectivity, Raman, FTIR, 57Fe Mössbauer, and broadband (10–2–106 Hz) dielectric spectroscopies. The crystal structure and the morphology of the objects remain nearly unaffected, whereas the thermal spin transition shifts from 362 to 316 K and the thermal hysteresis width decreases from 45 to 8 K for increasing values of x. For each compound the electrical conductivity drops when the iron(II) electronic configuration is switched from the low-spin to the high-spin state. A strong overall decrease in conductivity with increasing Zn concentration is also observed in both spin states. These results, together with the analysis of the charge carrier dynamics, suggest that the ferrous ions participate directly in the charge transport mechanism, explaining the strong spin-state dependence of the electrical properties in this compound.
No preview · Article · Apr 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: In this study we analyze the metal substitution effects on the structural, morphological, charge transport and spin transition properties of the [Fe1-xZnx(Htrz)2(trz)](BF4) (trz = triazole, x = 0, 0.26 or 0.43) compound using electron microscopy, powder X-ray diffraction, optical reflectivity, Raman, FTIR, 57Fe Mössbauer and broadband (10-2 to 106 Hz) dielectric spectroscopies. The crystal structure and the morphology of the objects remain nearly unaffected, whereas the thermal spin transition shifts from 362 K to 316 K and the thermal hysteresis width decreases from 45 K to 8 K for increasing values of x. For each compound the electrical conductivity drops when the iron (II) electronic configuration is switched from the low spin (LS) to the high spin (HS) state. A strong overall decrease in conductivity with increasing Zn concentration is also observed in both spin states. These results together with the analysis of the charge carrier dynamics suggest that the ferrous ions participate directly to the charge transport mechanism, explaining the strong spin-state dependence of the electrical properties in this compound.
Full-text · Article · Apr 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Composites of the spin crossover complex [Fe(trz)(H-trz)2](BF4) (H-trz = 1,2,4-4H-triazole and trz = 1,2,4-triazolato) dispersed in a poly(methylmethacrylate) (PMMA) matrix were synthesized and investigated for their spin crossover properties by optical reflectivity, Raman spectroscopy and calorimetry. These composite films were used to fabricate bilayer cantilevers that can perform efficient and tuneable mechanical actuation based on the spin transition. A prototype device that uses the spin transition phenomenon to convert electrical energy into mechanical motion through Joule heating is described. This device is used to perform oscillatory actuation driven by a modulated current. The ability to tune the performance of this electromechanical system is demonstrated by varying the working temperature, the applied ac current and its frequency.
[Show abstract][Hide abstract] ABSTRACT: The anelastic properties of PMMA in the temperature range of the main retardation mode are strongly dependent upon tacticity. The thermally stimulated creep (TSCr) technique was used to analyse the distribution function of retardation times. A series of PMMA, with various tacticities, synthesized by living polymerization, were investigated.
One of the most important findings was the observation of a sub-mode, at 150°C, with a magnitude increasing with syndiotactic triads. The existence of physical interactions might explain this behaviour. On the low-temperature tail of the a retardation mode, a sub-mode was observed at 60°C: it was associated with remaining isotactic triads.
No preview · Article · Mar 2014 · Journal of Thermal Analysis and Calorimetry
[Show abstract][Hide abstract] ABSTRACT: Spin crossover compounds are considered to be a viable alternative for creating display, memory and switching devices due to the bistability of their magnetic, optical, mechanical and electrical properties. This Letter presents the study of the dielectric and transport properties of the [Fe(Htrz)(2)(trz)](BF4) (Htrz = 1H-1,2,4-triazole) complex in a wide temperature and frequency range. Our results reveal a singular behavior of the dielectric modulus upon the spin transition in conjunction with the switching of the conductivity between the high spin and low spin states. [GRAPHICS] ((c) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Full-text · Article · Feb 2014 · physica status solidi (RRL) - Rapid Research Letters
[Show abstract][Hide abstract] ABSTRACT: The nanoscale manipulation and charge transport properties of the [Fe(Htrz)(2) (trz)](BF(4) ) spin-crossover compound is demonstrated. Such 1D spin-crossover nanostructures are attractive building blocks for nanoelectronic switching and memory devices.
No preview · Article · Mar 2013 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: Polymer composites filled with silver nanowires enable the highest value of electrical conductivity known up to now in the case of conductive nanoparticle dispersion with a percolation threshold less than 1 vol%. Silver nanowires with high aspect ratio were elaborated by two types of synthesis: electrochemical deposition in a template and polyol synthesis. For the first time the influence of each kind of nanowires in composites was studied and compared to spherical nanoparticles as reference. The value of percolation threshold and conductivity level above the percolation threshold were measured and compared. These silver nanowires were introduced into poly(vinylidenedifluoride-trifluoroethylene) in comparison to spherical silver nanoparticles. The preparation method modified the effective aspect ratio of nanowires. The low percolation threshold and the microscopy observations confirmed the good dispersion of nanowires in composites. The lowest percolation threshold was determined in the case of the polyol synthesis nanowires (0.63 vol%) in comparison with electrochemical deposited nanowires (2.2 vol%). The level of conductivity above the percolation threshold obtained with each kind of particles is in the same range near 100 S.m− 1. The value of electrical conductivity obtained above the percolation threshold is unusual at this low content of conductive filler and is observed for the first time in a conductive polymer composite.
Full-text · Article · Dec 2012 · Journal of Non-Crystalline Solids
[Show abstract][Hide abstract] ABSTRACT: Carbon nanotubes represent new emergent multifunctional materials that have potential applications for structural and electrically conductive composites. In the current paper we present a suitable technique for the integration of Double Walled Carbon Nanotubes (DWCNTs) in a unidirectional Carbon Fiber Reinforced Polymer (CFRP) with high volume content of carbon fiber. We showed that the electrical conductivity of the laminates versus temperature follows a non-linear variation which can be well described by the Fluctuation-Induced Tunneling Conduction (FITC) model. The parameters of this model for CFRP/DWCNTs and CFRP without DWCNTs were determined using best fit curves of the experimental data. This study has shown that DWCNTs have strong influence in the conductivity through laminate thickness. However, there are no significant effects on the electrical conductivity measured in the other two principle directions of the composite laminate. Furthermore, it was found that electron conduction mechanism of carbon fibers is dominated by the FITC.
Full-text · Article · Nov 2012 · Composites Science and Technology
[Show abstract][Hide abstract] ABSTRACT: An increase and homogenization of electrical conductivity is essential in epoxy carbon fiber laminar aeronautical composites. Dynamic conductivity measurements have shown a very poor transversal conductivity. Double wall carbon nanotubes have been introduced into the epoxy matrix to increase the electrical conductivity. The conductivity and the degree of dispersion of carbon nanotubes in epoxy matrix were evaluated. The epoxy matrix was filled with 0.4 wt.% of CNTs to establish the percolation threshold. A very low value of carbon nanotubes is crucial to maintain the mechanical properties and avoid an overload of the composite weight. The final carbon fiber aeronautical composite realized with the carbon nanotubes epoxy filled was studied. The conductivity measurements have shown a large increase of the transversal electrical conductivity. The percolative network has been established and scanning electron microscopy images confirm the presence of the carbon nanotube conductive pathway in the carbon fiber ply. The transversal bulk conductivity has been homogenized and improved to 10− 1 S·m− 1 for a carbon nanotubes loading near 0.12 wt.%.
Full-text · Article · Aug 2012 · Journal of Non-Crystalline Solids
[Show abstract][Hide abstract] ABSTRACT: We studied the spin state dependence of the electrical conductivity of the spin crossover compound [Fe(Htrz)(2)(trz)](BF(4)) (Htrz = 1H-1,2,4-triazole) by means of dc electrical measurements. The low spin state is characterized by higher conductance and lower thermal activation energy of the conductivity, when compared to the high spin state.
Full-text · Article · Apr 2012 · Chemical Communications
[Show abstract][Hide abstract] ABSTRACT: Nickel nanowires with high aspect ratio (250) were elaborated and incorporated into poly(vinylidene difluoride-trifluoroethylene) up to 30vol% via solvent mixing way. These nanocomposites are characterized by a conductive behavior with a high electrical conductivity value (102 S m−1) above a very low percolation threshold (0.75vol% of metallic nanowires). The introduction of nanowires strongly depressed the matrix crystallinity. Static and dynamic mechanical analysis have been realized at low nanowire volume fraction (
Full-text · Article · Jan 2012 · Journal of Non-Crystalline Solids
[Show abstract][Hide abstract] ABSTRACT: The determination of intrinsic conductivity of nanowires (NWs) is essential to understand the charge transport behaviour involved in hybrid nanocomposites. These high conductive metallic fillers are good candidate to improve electrical properties of composites in aeronautic industry. The main difficulty is often to achieve the combination of both high spatial resolution and information on the physical properties as electrical conductivity. One of the suitable methods to give the desired information is electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) mode, especially in the low-loss region. This is demonstrated by studying the nickel and gold nanowire.
Full-text · Article · Dec 2011 · Materials Letters
[Show abstract][Hide abstract] ABSTRACT: We report on the deposition of thin films of the [Fe(HB(pz)3)2] (pz 1⁄4 pyrazolyl) molecular spin crossover complex by thermal evaporation. By means of impedance measurements and Raman microspectroscopy, we show that the films maintain the structure and properties of the bulk material. The conductivity of the films decreases by ca. 2 orders of magnitude when the freshly deposited compound goes through a first (irreversible) thermal phase change above ca. 380 K. This property can be exploited as a non-volatile (read-only) memory effect.
Full-text · Article · Aug 2011 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: ABSTRACT
Combining micro and nanofillers is a new way to improve thermal and electrical conductivities. Micrometric silver flakes and nanometric carbon nanotubes exhibit very high thermal and electrical conductivities. This work investigates a new type of hybrid conductive adhesives filled with carbon nanotubes (DWCNTs or MWCNTs) and silver flakes. Very high electrical conductivities are measured as well as improved mechanical properties. Electron microscopy has been used to characterize the microstructure of the hybrid composites.
Key words: hybrid composite, carbon nanotubes, silver flakes
[Show abstract][Hide abstract] ABSTRACT: Raman spectroscopy is used to access the dispersion state of DWNTs in a PEEK polymer matrix. The interaction of the outer tube with the matrix can be detd. from the line shape of the Raman G band. This allows us to distinguish regions where the nanotubes are well dispersed and regions where the nanotubes are agglomerated. The percolation threshold of the elec. cond. of the double wall carbon nanotubes (DWNTs)/PEEK nanocomposites is found to be at 0.2-0.3 wt%. We find a max. elec. cond. of 3 Ã 10-2 S cm-1 at 2 wt% loading. We detect nanotube wt. concns. as low as 0.16 wt% by Raman spectroscopy using a yellow excitation wavelength. We compare the Raman images with transmission electron microscopy images and elec. cond. measurements. A statistical method is used to find a quant. measure of the DWNTs dispersion in the polymer matrix from the Raman images. [on SciFinder(R)]
[Show abstract][Hide abstract] ABSTRACT: The electrical and dielectric properties of CuFe1−xCrxO2 (0 ≤ x ≤ 1) powders, doped with 3% of Mg and prepared by solid-state reaction, were studied by broadband dielectric spectroscopy in the temperature range from −100 to 150 °C. The frequency-dependent electrical and dielectric data have been discussed in the framework of a power law conductivity and complex impedance and dielectric modulus. At room temperature, the ac conductivity behaviour is characteristic of the charge transport in CuFe1−xCrxO2 powders. The substitution of Fe3+ by Cr3+ results in an increase in dc conductivity and a decrease in the Cu+–Cu+ distance. Dc conductivity, characteristic onset frequency and Havriliak–Negami characteristics relaxation times are thermally activated above −40 °C for x = 0.835. The associated activation energies obtained from dc and ac conductivity and from impedance and modulus losses are similar and show that CuFe1−xCrxO2 delafossite powders satisfy the BNN relation. Dc and ac conductivities have the same transport mechanism, namely thermally activated nearest neighbour hopping and tunnelling hopping above and below −40 °C, respectively.
Preview · Article · Apr 2011 · Journal of Physics D Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Much attention has been focused on the identification of promising valence tautomeric complexes as switchable molecular materials for potential applications, such as memories, switching devices and sensors. Here we report a dielectric investigation of the charge transfer transition (CT) in three cyanometalate complexes: Rb0.8Mn[Fe(CN)6]0.93 1.62H2O, Na2.2Co4[Fe(CN)6]3.3 15H2O and CO3[W(CN)8]2(pyrimidine)4 6H2O, involving change in their dielectric permittivity and electrical conductivity. The CT phase transition is founded to be coupled to a paraferroelectric transition between the high temperature (HT) and the low temperature phase (LT).