Electrical conductivity of natural rubber cellulose II nanocomposites

[EN] Nanocomposite materials obtained from natural rubber (NR) reinforced with different amounts of cellulose II (cell) nanoparticles (in the range of 0 to 30 phr) are studied by dielectric spectroscopy (DS) in a broad temperature range (¿150 to 150 °C). For comparative purposes, the pure materials, NR and cell, are also investigated. An analysis of the cell content effect on the conductive properties of the nanocomposites was carried out. The dielectric spectra exhibit conductivity phenomena at low frequencies and high temperatures: Maxwell¿Wagner¿ Sillars (MWS) and electrode polarization (EP) conductive processes were observed in the nanocomposite samples.We thank Professor Regina Nunes of the Instituto de Macromoleculas Eloisa Mano (Universidade Federal do Rio de Janeiro) for providing us the NR and NR-cell samples. This work was financially supported by DGCYT through grant MAT2012-33483.Ortiz Serna, MP.; Carsí Rosique, M.; Redondo Foj, MB.; Sanchis Sánchez, MJ. (2014). Electrical conductivity of natural rubber cellulose II nanocomposites. Journal of Non-Crystalline Solids. 405:180-187. https://doi.org/10.1016/j.jnoncrysol.2014.09.026S18018740

Anisotropic thermoelectric power in YBa,Cu,O7-x, single crystals under pressures up to 8 GPa

The thermoelectric power (TEP) of YBa,Cu,O7-x, single crystals has been measured as a function of pressure up to 8 GPa. The ab-plane TEP decreases monotonically. The c-axis TEP shows a broad maximum around 1.5 GPa, followed by a decrease with a further increase in pressure

Phase relations and dielectric properties of BaTi03 ceramics heavily substituted with neodymium

Investigations on the phase relations and dielectric properties of (1 -x)BaTiO3 + xNd2/3TiO 3 (BNT) ceramics sintered in air below 1650 K have been carried out. X-ray powder diffraction studies indicate apparent phase singularity for compositions with x < 0.3. Nd2Ti207 is detected at higher neodymium concentrations. The unit cell parameter changes continuously with neodymium content, and BaTiO3 is completely cubic at room temperature with x -- 0.0525, whereas electron diffraction studies indicate that the air-sintered BNT ceramics with x > 0.08 contain additional phases that are partly amorphous even to an electron beam. SEM observations reveal that BaTiO3 grains are mostly covered by a molten intergranular phase, and show the presence of randomly distributed Nd2Ti207 grains. Energy dispersive X-ray analysis shows the Ba-Nd-Ti ternary composition of the intergranular phase. Differential thermal analysis studies support the formation of a partial melt involving dissolution-precipitation of boundary layers of BaTiO3 grains. These complex phase relations are accounted for in terms of the phase instability of BaTiO3 with large cation-vacancy concentration as a result of heavy Nd 3+ substitution. The absence of structural intergrowth in (1 - x)BaTiO3 + xNd2/3TiO3 under oxidative conditions leads to a separation of phases wherein the new phases undergo melting and remain X-ray amorphous. BNT ceramics with 0.1 < x < 0.3 have ~eff >~ 104 with tan 6 < 0.1 and nearly flat temperature capacitance characteristics. The grain-size dependence of ee,, variations of ~eff and tan 6 with the measuring frequency, the non-ohmic resistivities, and the non-linear leakage currents at higher field-strengths which are accompanied by the decrease in eeff and rise in tan 3, are explained on the basis of an intergranular (internal boundary layer) dielectric characteristic of these ceramics

EPR study on the role of Mn in enhancing PTC of BaTiO3

Low concentration of Mn (< 0.05 atom%) added to lanthanide-doped ceramics for enhancing the PTC effect did not show any EPR signal due to Mn in the tetragonal phase. Above Tc (400 K) it showed the six-line signal arising from Mn2+. This is explained on the basis of Mn existing as Mn3+ ion with short relaxation time at room temperature. Oxidation state changes to Mn2+ above Tc; thus Mn3+ acts as an electron trap. This augments the function of activated defect centres (VBa /ag VBa) in diminishing the charge carrier concentration across the phase transformation

Electron transport properties of irradiated polyimide thin films in single track regime.

We have prepared a suite of polyimide thin films containing spatially separated one-dimensional conductive-nanowires by ion-beam irradiation exhibiting temperature dependent electrical resistance consistent with thermally activated electron hopping with activation energies about 1 eV arising from localized states spatially distributed along the ion tracks. Dielectric measurements showed the formation of high dielectric constant interphase regions surrounding each ion track generated during the irradiation process, responsible for space-charge accumulation which influences electron transport within the ion tracks. This behavior suggests a role for space-charge effects and dielectric properties in this interphase region in the control of electron transport within single track nanowires. © 2009, American Institute of Physic

EPR Study on the Role of Manganese in Enhancing PTC of BaTiO3BaTiO_3

Low concentration of Mn (< 0.05 atom%) added to lanthanide-doped ceramics for enhancing the PTC effect did not show any EPR signal due to Mn in the tetragonal phase. Above $T_c$ (400 K) it showed the six-line signal arising from Mn^2^+. This is explained on the basis of Mn existing as Mn^3^+ ion with short relaxation time at room temperature. Oxidation state changes to Mn^2^+ above $T_c$; thus Mn^3^+ acts as an electron trap. This augments the function of activated defect centres (V_{Ba} \rightleftharpoons V^{\prime}_B_a) in diminishing the charge carrier concentration across the phase transformation

EPR studies on donor doped BaTiO3 grain boundary layer ceramic dielectrics

Donor doped BaTiO3 ceramics become insulating5 under controlled conditions with effective dielectric constants >10. The changes in EPR signals indicate that a certain fraction of the donor doped BaTiO3 is cubic even at room temperature and that the cubic fraction increases with the donor content. X-ray powder diffraction data support the EPR results. The coexistence of both the phases over a range of temperature is characteristic of diffused phase transition. The effect of grain size variation on EPR signal intensities indicate that the boundary layers surrounding the grains may constitute the cubic phase as a result of higher Ba-vacancies and donor contents at the grain boundary layer than in the bulk. Since the acceptor states arising from the Ba-vacancies and the impurities are activated in the cubic phase, they capture electrons from the conduction band, rendering the cubic phase electrically more insulating than the semiconductive tetragonal grain interiors. Thus, the cubic grain boundary layers act as effective dielectric media where the field tends to concentrate

Dielectric enhancement in polymer-nanoparticle composites through interphase polarizability

Dielectric measurements on polyimide-oxide nanoparticle composite thin films show a composite dielectric constant (composite) that increased monotonically with increasing oxide content well above the value predicted by Maxwell&#039;s rule for dielectric mixtures below the percolation threshold. Above certain volume fractions, the measured epsilon composite values were found to exceed the corresponding nanoparticle epsilon such that epsilon polymer&lt; epsilon particle&lt; epsilon composite contrasted to conventional composites where epsilon polymer&lt; epsilon composite&lt; epsilon particle. The epsilon composite was independent of frequency to 10 MHz with dielectric loss of &lt;0.005 throughout this range, indicating that the observed enhancement in epsilon does not originate from space-charge related contributions and hence should be due to dipolar contributions. The observed epsilon enhancement (epsilon composite- epsilon Maxwell) showed a correlation with the total surface area of the nanoparticles. The dielectric model of Vo and Shi [Microelectron. J. 33, 409 (2002), and references therein] showed that the enhanced dielectric behavior originates from significant interfacial nanoparticle-polymer interactions and the critical role of additional contributions to polarizability through specific physicochemical interactions within the interphase region. An interphase epsilon int considerably higher than that of the nanoparticle and a high interface interaction constant of 3.24 for the nanocomposite suggest a strong interaction between the functional groups of the polymer and the nanoparticle surface. Although modeling suggests a maximum of epsilon~65 vol %, loss in micromechanical stability occurred above 20% due to incomplete polymer wetting films arising from the high nanoparticle surface areas