Spiky nanostructured metal particles as filler of polymeric composites showing tunable electrical conductivity

This work presents a comprehensive investigation of the piezoresistive response of a composite material based on conductive nickel filler in a silicone‐insulating matrix. In the absence of a deformation, the prepared composite shows no electric conductivity, even though the metal particle content is well above the expected percolation threshold. Upon samples deformation (compressive or tensile stress), the composite exploits a variation of electrical resistance up to nine orders of magnitude. This huge variation can be explained with the quantum tunnelling mechanism where the probability of an electron to tunnel from a particle to the next one is exponentially proportional to the thickness of the insulating layer between them and strongly enhanced by the morphology of the nickel particles, showing spiky nanostructured tips. Two different conduction theoretical models are proposed and compared with the experimental results. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Electrical conductivity and thermopower of Fe-phosphate compounds with the lazulite-type structure

Measurements of the electrical DC and AC conductivities were performed on three Fe-phosphate compounds in the temperature range ∼210 K?T?∼450 K. The Fe-phosphates are semiconducting with activation energy of DC conductivity σ_DC of E_A∼0.35-0.45 eV. For α-Fe₂PO₄O, the AC conductivity σ_AC in the low-temperature range is considerably enhanced relative to σ_DC. The frequency dependence of σ_AC can be described by an approximate power law. For the same compound, the thermopower Θ (Seebeck effect) was found to be positive in the range ∼330 K?T?∼700 K, i.e. seemingly p-type conduction occurs; at T>700 K, Θ appears to become negative. The results are described in terms of a small polaron hopping model between localized states with electron transfer of the type Fe²⁺→Fe³⁺ with Fe²⁺ as donors. In this model Θ can be described suggesting equal concentrations of Fe²⁺ and Fe³⁺ to take part in conduction.     

Simultaneous electrical conductivity and DTA measurements in the study of the phosphonate-phosphate rearrangement and accompanying processes

The application of DTA with the simultaneous recording of electrical conductivity variation in the study of the thermal behaviour ofα-oxyalkylphosphonates and their analogues is considered. The investigation of electrical conductivity variation during a reaction helps clarify certain problems of the phosphonate-phosphate rearrangement and the decomposition ofα-oxyalkylphosphonates to their components.     

Phase transformations in bulk nanostructured potassium niobiosilicate glasses

In potassium niobiosilicate (KNS) glasses, nanostructuring can be driven and controlled by thermal treatments at the glass transition temperature and/or by modulation of the chemical composition. The tight relationship between nanostructure and nonlinear optical properties suggests these bulk nanomaterials as an appealing route to nanophotonics. The focus of this paper is placed on assessing the phase transformations which occur in these materials upon annealing at the glass transition temperature and subsequent heating. High-temperature resolved X-ray diffraction (HTXRD) and high-resolution transmission electron microscopy (HRTEM) experiments are integrated with previously published results for in-depth insight. It will be shown that nanostructuring evolves from nucleation of niobium-rich nanocrystals, which are up to 20 nm large, uniformly distributed in the matrix bulk, and metastable. Formation kinetics as well as phase transformation of the nanocrystals are determined by the glass composition. Depending on it, nanocrystal nucleation can be preceded or not by phase separation, and the nanocrystals' phase transition can be of first or second order.     

Thermal conductivity of nanostructured boron nitride materials

We have measured the thermal conductivity of bulky pellets made of various boron nitride (BN)-based nanomaterials, including spherical nanoparticles, perfectly structured, bamboo-like nanotubes, and collapsed nanotubes. The thermal conductivity strongly depends on the morphology of the BN nanomaterials, especially on the surface structure. Spherical BN particles have the lowest thermal conductivity while the collapsed BN nanotubes possess the best thermoconductive properties. A model was proposed to explain the experimental observations based on the heat percolation passage considerations.     

Effect of ammonia on the temperature‐dependent conductivity and thermopower of polypyrrole

We report the first measurements of the effect of ammonia gas on the temperature dependence of the conductivity and thermoelectric power of polypyrrole films. Our data are for samples of very different conductivities, extending down to a temperature of 200 K for low‐conductivity polypyrrole gas sensors, and down to 4.2 K for more highly‐conducting PPy(PF₆) samples. We demonstrate that (except for the most metallic case) our polypyrrole samples show greater sensitivity to ammonia as the temperature is lowered (i.e. the fractional reduction in conductivity is greater at lower temperatures). Remanent decreases in conductivity are present after the removal of ammonia for higher pressure exposures, and remanent increases in the metal‐like thermoelectric power for the PPy(PF₆) for samples grown at higher temperatures. Our results indicate that the mechanism of this conductivity decrease in PPy(PF₆) is that ammonia causes a reduction in the size of metallic regions as disordered barrier regions are thickened. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1331–1338, 2006     

Preparation and electrical conductivity of organoheterobimetallic-maleonitriledithiolates

The new complexes [PhHg]₂[M(mnt)₂] [M = Ni^II, Cu^II, Zn^II or Pd^II; mnt^2– = 1,2-dicyano-1,2-ethylenedithiolate (maleonitriledithiolate)] have been characterized spectroscopically and magnetically and their solid phase conductivity measured. All compounds exhibit solid phase _rt in the 1.29 × 10^–12–5.68 × 10^–10 S cm^–1 range and semiconduct in the 313–383 K range.     

Hydrogenation of nanostructured alloys and composites based on magnesium

The capability of sorbing hydrogen was studied for the magnesium alloys and related composites. The microstructures of the Mg-Ni binary eutectic alloys and Mg-La-Ni and Mg-Mm-Ni ternary eutectic alloys were studied. Both the initial alloys and alloys modified by the method of equal channel angular pressing were used as objects of the study. Features of interaction of the alloys with hydrogen were revealed. Sorption of hydrogen by the metal hydride composites based on alloys and ??pseudoalloys,?? viz., alloys obtained by sintering of mechanochemically treated highly dispersed powders formed by the hydride dispersion of metallic phases, was studied. Metal??carbon composites based on highly dispersed magnesium alloys or pseudoalloys and carbon nanostructures were formed, and the absorption of hydrogen by these composites was examined.     

Measurements of thermal conductivity and specific heat of dilute metallic alloys in the form of small samples by electrical method

The method of determining the thermal conductivity depends upon a relation between the maximum temperature ( _m ) attained for a given current and potential difference (V) in a current carrying specimen. Heat is assumed to enter and leave specimen only through the surfaces through which electric current enters and leaves, other surfaces being insulated against flow of both heat and electricity. The plane ends of the rod were taken to be isothermal and equipotential surfaces held at a constant temperature.For the measurement of the specific heat, a homogeneous constant Joule heating is imposed. The initial slope of ( _m ) as a function of time is inversely proportional of heat capacity regardless of heat losses. Measurements on dilute Ni base Cr alloy sample up to 360 °C.

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Zusammenfassung Die Methode zur Bestimmung der Wärmeleitfähigkeit hängt vom Verhältnis zwischen der bei einer bestimmten Stromstärke erreichten Maximumtemperatur ( _m ) und dem Spannungsabfall an einem stromdurchflossenen Prüfling ab. Es wird davon ausgegangen, daß die Wärme nur an den Flächen in die Probe gelangt bzw. diese verlässt, an denen auch der elektrische Strom ein- bzw. austritt, da die übrigen Oberflächen gegen Strom- und Wärmefluss isoliert wurden. Die planen Enden des Messstabes wurden bei konstanter Temperatur gehalten und als isotherme bzw. äquipotentiale Flächen angesehen.

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( _m ), (V) . , , , . , , . . _m , , . Ni-Cr 360°.

     

Fluorination and electrical conductivity of BN nanotubes

Fluorination of BN nanotubes has been performed using a catalytic growth method, which leads to the appearance of markedly curved fluorine-doped BN sheets and converts originally insulating BN nanotubes to semiconductors, as confirmed by the comparative electron transport four-probe measurements on doped and undoped individual BN nanotubes.     

Measurements of electrical conductivity in solid bromine and iodine

Measurements of the electrical conductivity were performed with bromine and iodine in the liquid and the solid states, both containing low concentrations of the corresponding halide ions. In bromine the specific conductivity increases dramatically upon solidification and in iodine it changes only slightly. In both systems the conductivity in the solid is rather high, with remarkably low temperature coefficients, pointing to an unusual mechanism of conduction (of the Grotthuss type) requiring very little movement of the heavy nuclei while the charge is transferred. In mixtures of bromine with a small amount of nitrobenzene (NB) an equivalent conductivity as high as 12 cm² mol^?1 Ω^?1 was observed at ?25°C. In iodine the specific conductivity reached a value of about 0.01 Ω^?1 cm^?1 at 100°C. The energy of activation for conduction in bromine down to ?40°C was found to be about 23 kJ mol^?1, increasing sharply below this temperature. In iodine, values of about 21–27 kJ mol^?1 were observed over the whole temperature range measured.     

Gas-induced electrical conductivity changes in metal phthalocyanines

The effect of NO₂, NO, F₂, BCl₃, BF₃, NH₃, H₂S, SO₂ and HCl at low concentrations (< 1 ppm) in air on the eletrical conductivity of a number of metal phthalocyanines has been investigated. Vacuum sublimed films of Pb, Zn, Cu, Ni, Co and Fe phthalocyanine have been shown to be sensitive to NO₂ and F₂ but insensitive to more weakly electrophilic and electrophobic gases. Increasing sensitivity to NO₂ was observed as phthalocyanines containing progressively heavier metals were used. Magnesium phthalocyanine was affected only by HCl. The potential of these materials as gas sensors is discussed.     

Half-metallicity,magnetism, mechanical,thermal, and phonon properties of FeCrTe and FeCrSe half-Heusler alloys under pressure

The half-metallicity of Heusler alloys is quite sensitive to high pressure and disorder. To understand this phenomenon better, we systematically studied the half-metallic nature, magnetism, phonon, and thermomechanical properties of FeCrTe and FeCrSe Heusler alloys under high pressure using ab initio calculations based on density functional theory. The ground-state lattice constants for FeCrTe and FeCrSe alloys are 5.93 and 5.57 Å, respectively, consistent with available theoretical results. Formation energy, cohesive energy, elastic constants, and phonon dispersion confirmed that both compounds are thermodynamically and mechanically stable. The FeCrTe and FeCrSe alloys showed a half-metallic character with a band gap of 0.68 and 0.58 eV at 0 GPa pressure, respectively, and magnetic moments of 2.01 μ_B for both alloys, using generalized gradient approximation (GGA) approximation. FeCrTe alloy changes from metallic to half-metallic above 30 GPa pressure using GGA + U. The elastic properties were scrutinized, and it was found that, at 0 GPa pressure, FeCrTe is ductile, and FeCrSe is brittle. Under pressure, FeCrSe becomes brittle above 10 GPa pressure. Average sound velocity V_m, Debye temperature Ɵ_D, and heat capacity C_V were predicted under pressure. These outcomes will improve the integration of Fe-based half-Heusler alloys in spintronic devices.     

Secondary relaxations and electrical conductivity in hyperbranched polyester amides

Broadband dielectric spectroscopy is used to investigate molecular dynamics and charge transport in three hyperbranched polyester amides with hydroxyl, phenyl, and stearate terminal groups. At higher temperatures, the dielectric spectra are interpreted in terms of hopping conduction in a spatially randomly varying energy landscape, whereas two secondary dipolar relaxations attributed to librations of the terminal and amide groups dominate the low temperature regime. Despite a shift of more than 3 decades in the dc conductivity upon variation of the end groups, the Barton–Nakajima–Namikawa relation is shown to hold. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1651–1657, 2010     

Analysis of surface and bulk composition of amorphous metallic alloys

Summary Surface and bulk composition of amorphous metallic alloys (AMA) has been analyzed by AES and SIMS. Original surface segregation and its variation has been studied in detail under the effect of different treatment techniques. Secondary ion energy distributions were measured for different structural states and primary ions. Data obtained can be used in practical SIMS analysis. Kinetical characteristics of secondary ion emission (SIE) are shown to be useful for surface characterization. Structure-sensitive SIE is changed greatly and irreversibly during transition from the amorphous to the crystalline state.

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Analyse der Oberflächen und Bulk-Zusammensetzung von amorphen Metall-Legierungen

     

Simultaneous enhancement of electrical conductivity and impact strength via formation of carbon black‐filler network in PP/EPDM Blends

The electrical conductivity and impact strength of polypropylene(PP)/EPDM/carbon black ternary composites were investigated in this paper. Two processing methods were employed to prepare these ternary composites. One was called one‐step processing method, in which the elastomer and the filler directly melt blended with PP matrix. Another one was called two‐step processing method, in which the elastomer and the filler were mixed first, and then melt blended with pure PP. To get an optimal phase morphology that favors the electrical conductivity and impact strength, controlling the distribution of CB in PP/EPDM blend was a crucial factor. Thus the interfacial tension and the work of adhesion were first calculated based on the measurement of contact angle, and the results showed that CB tended to be accumulated around EPDM phases to form filler‐network structure. Expectably, the filler‐network structure was observed in PP/EPDM/CB(80/20/3) composite prepared by two‐step processing method. The formation of this filler‐network structure decreased the percolation threshold of CB particles in polymer matrix, and the electrical conductivity as well as Izod impact strength of the composite increased dramatically. This work provided a new way to prepare polymer composites with both improved conductivity and impact strength. Copyright © 2009 John Wiley & Sons, Ltd.