[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.
The Journal of Physical Chemistry C 04/2015; 119(16):8522–8529. · 4.77 Impact Factor
[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.
The Journal of Physical Chemistry C 04/2015; 119(16). DOI:10.1021/acs.jpcc.5b01117 · 4.77 Impact Factor
[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: 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)
physica status solidi (RRL) - Rapid Research Letters 02/2014; 8(2). DOI:10.1002/pssr.201308256 · 2.14 Impact Factor
[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.
[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.
[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.
[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.%.
[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.
Chemical Communications 04/2012; 48(35):4163-5. DOI:10.1039/c2cc30528c · 6.83 Impact Factor
[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 (
[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.
[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.
Journal of Physics D Applied Physics 04/2011; 44(18):185401. DOI:10.1088/0022-3727/44/18/185401 · 2.72 Impact Factor
[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).
Electrets (ISE), 2011 14th International Symposium on; 01/2011
[Show abstract][Hide abstract] ABSTRACT: Combining conductive micro and nanofillers is a new way to improve electrical, thermal and mechanical properties of polymer composites for electronic packaging. Micrometric silver flakes and nanometric carbon nanotubes (CNT) exhibit high electrical and thermal conductivity. Moreover CNT improve strength, stiffness and fracture toughness of the polymer matrix. A new type of hybrid conductive adhesive filled with silver flakes and carbon nanotubes were investigated. Thermal and electrical conductivities were measured as well as improved mechanical properties were evaluated based on the reliability tests of joints made of tested material. The electrical, thermal and mechanical properties were consistent with the morphologies of the hybrid composites characterized by SEM.
[Show abstract][Hide abstract] ABSTRACT: This chapter contains sections titled: Introduction Structure and mechanical properties of nanocomposites Structure and electroactive properties of nanocomposites Summary and outlook References
[Show abstract][Hide abstract] ABSTRACT: High aspect ratio gold nanowires were uniformly dispersed into a poly(vinylidene difluoride–trifluoroethylene) [P(VDF-TrFE)] matrix. The nanowires were synthesized by electrodeposition using nanoporous anodic alumina oxide templates. The intrinsic optical conductivity of the gold nanowires was determined by valence electron energy loss spectroscopy. The effect of increasing volume fraction of Au nanowires on the morphology and crystallization of P(VDF-TrFE) matrix was investigated by differential scanning calorimetry. The crystallinity of P(VDF-TrFE) is strongly depressed by the randomly dispersed nanowires. Above 30 vol% the crystallization of P(VDF-TrFE) is suppressed. The bulk electrical conductivity of nanocomposite films, at room temperature, obeys a percolation behaviour at a low threshold of 2.2 vol% and this was confirmed using the surface resistivity value. An electrical conductivity of 100 S m−1 is achieved for a 3 vol% filler content.
Journal of Physics D Applied Physics 08/2010; 43(34):345401. DOI:10.1088/0022-3727/43/34/345401 · 2.72 Impact Factor