Thermal and electronic charge transport in bulk nanostructured Zr0.25Hf0.75NiSn composites with full-Heusler inclusions

Bulk Zr_0.25Hf₀₇₅NiSn half-Heusler (HH) nanocomposites containing various mole fractions of full-Heusler (FH) inclusions were prepared by solid state reaction of pre-synthesized HH alloy with elemental Ni at 1073 K. The microstructures of spark plasma sintered specimens of the HH/FH nanocomposites were investigated using transmission electron microscopy and their thermoelectric properties were measured from 300 K to 775 K. The formation of coherent FH inclusions into the HH matrix arises from solid-state Ni diffusion into vacant sites of the HH structure. HH(1–y)/FH(y) composites with mole fraction of FH inclusions below the percolation threshold, y∼0.2, show increased electrical conductivity, reduced Seebeck coefficient and increased total thermal conductivity arising from gradual increase in the carrier concentration for composites. A drastic reduction (∼55%) in κ_l was observed for the composite with y=0.6 and is attributed to enhanced phonon scattering due to mass fluctuations between FH and HH, and high density of HH/FH interfaces.     

Electrophysical properties of GeSexTe1−x single crystals grown by physical vapor transport (PVT)

The room temperature composition dependences of the lattice parameters, mass density, thermopower, Hall constant, carrier concentration and mobility, and electrical conductivity of GeSe_xTe_1?x (x=0.1 ? 0.4) single crystals grown by the Physical Vapor Transport (PVT) method, indicate that the crystals are non-stoichiometric degenerate semiconductors having p-type conductivity. The main lattice defects in this compound are germanium vacancies. The effects of selenium content on the electrical properties of GeSe_xTe_1?x single crystals are explained qualitatively based on the character of the energy levels and on the non-stoichiometric property of this compound.     

Transport properties of undoped and Br-doped PbTe sintered at high-temperature and pressure ≥4.0 GPa

The thermoelectric properties of nominally undoped PbTe and Br doped PbTe materials sintered at high-pressure and high-temperature (HPHT) have been studied. All samples show n-type semiconducting behavior with negative thermopower. For undoped PbTe, four different HPHT treatments were performed at pressures between 4.0 and 6.5 GPa. PbTe doped with Br at 0.5, 1.0, 2.0, 3.0×10¹⁹ cm⁻³ was HPHT treated at 4.0 GPa and 1045 °C. As the dopant concentration increases, the absolute thermopower decreases, thermal conductivity increases, and electrical resistivity decreases. At a nominal dopant concentration of 1.0×10¹⁹ cm⁻³, carrier mobility of 1165 cm²/V s and dimensionless thermoelectric figure-of-merit, ZT, of around 0.27 at 300 K were obtained. These results demonstrate that HPHT post-processing is a viable and controllable way of tuning the thermoelectric properties of PbTe-based materials.     

Synthesis,characterization, dielectric and rectification properties of PANI/Nd2O3:Al2O3 nanocomposites

Polyaniline–Nd₂O₃:Al₂O₃ nanocomposites were prepared by in situ oxidative polymerization method using different weight percentages of oxide powders. The prepared nanocomposites were characterized by Fourier transform infrared spectroscopy and X‐ray diffraction for molecular and crystal structures. Scanning electron microscopy and transmission electron microscopy images show the tubular structure of polyaniline nanocomposite with embedded metal oxides. The electrical conductivity of the nanocomposites increases with increase in temperature as well as with concentration of Nd₂O₃:Al₂O₃ particles in polyaniline. This is because of the hopping of charge polarons and extended chain length of the nanocomposites as evidenced by the negative thermal coefficient (NTC) characteristic. A high NTC value of 2.67 was found in nanocomposites with 15 wt% of oxide particles. These nanocomposites show low dielectric constant and dielectric loss; the electrical conductivity is higher than 0.3 S/cm as confirmed by Cole–Cole plot that indicates a decrease in both grain resistance and bulk resistance of the nanocomposites. The current–voltage and capacitance–voltage measurements were also carried out. The carrier mobility μ values of pure polyaniline and nanocomposites were found to be 4.27 × 10^?3 and 1.45 × 10^–2 H.M^?1, respectively. A significant enhancement in carrier mobility was observed in comparison with the literature. Copyright © 2016 John Wiley & Sons, Ltd.     

Charge carrier mobility and concentration as a function of composition in AgPO3-AgI glasses

Conductivity data of the xAgI(1 - x)AgPO(3) system (0 ≤ x ≤ 0.5) were collected in the liquid and glassy states. The difference in the dependence of ionic conductivity on temperature below and above their glass transition temperatures (T(g)) is interpreted by a discontinuity in the charge carrier's mobility mechanisms. Charge carrier displacement occurs through an activated mechanism below T(g) and through a Vogel-Fulcher-Tammann-Hesse mechanism above this temperature. Fitting conductivity data with the proposed model allows one to determine separately the enthalpies of charge carrier formation and migration. For the five investigated compositions, the enthalpy of charge carrier formation is found to decrease, with x, from 0.86 to 0.2 eV, while the migration enthalpy remains constant at ≈0.14 eV. Based on these values, the charge carrier mobility and concentration in the glassy state can then be calculated. Mobility values at room temperature (≈10(-4) cm(2) V(-1) s(-1)) do not vary significantly with the AgI content and are in good agreement with those previously measured by the Hall-effect technique. The observed increase in ionic conductivity with x would thus only be due to an increase in the effective charge carrier concentration. Considering AgI as a weak electrolyte, the change in the effective charge carrier concentration is justified and is correlated to the partial free energy of silver iodide forming a regular solution with AgPO(3).     

Electrical and Thermoelectrical Behavior of Binary Composites of Phosphate Glass Loaded with Zinc Fillers: Promising Materials for Photovoltaic’s

Homogeneous xPbO?(1?x) P₂O₅ glasses where 0 % :σ x<100 % have been successfully synthesized using a melt‐quenching method. The short range structures of the prepared samples were examined by Fourier transform infrared spectroscopy, x‐powder diffraction and scanning electron microscopy. The most stable vitreous phase is of composition 45 mol%PbO‐55 mol%P₂O₅; it was loaded with zinc volume fraction. We carried out experimental and simulative investigations of the electrical characteristics of p? n junction; the obtained results indicated that conductivity of the composites increases by increasing dopent concentration. It was also observed that the current voltage characteristics of the composite were found to be ohmic in nature, wherein drastic drop of the electrical conduction was observed at an accurate temperature of 405 K. Electrical behavior of the composites as function of filler concentration and versus temperature were explained respectively by percolation theory and positive temperature coefficient effect.     

First experimental evidence of potassium ordering in layered k4co7o14

The layered P2-K4Co7O14 oxide has been prepared and characterized by means of X-ray diffraction, electrical conductivity, thermopower, and magnetic measurements. The crystal structure of K4Co7O14 (P6(3)/m space group, Z=2, a=7.5171(1) A, and c=12.371(1) A) consists of a stacking of slabs of edge-shared CoO6 octahedra with K+ ions occupying ordered positions in the interslab space, leading to a a0 radical7xa0 radical7 supercell. Potential energy calculations at 0 K are in good agreement with the ordered distribution of potassium ions in the (ab) plane. This oxide is metallic, and the magnetic susceptibility is of Pauli-type, which contrasts with the Curie-Weiss behavior of the homologous NaxCoO2 (x approximately 0.6) oxide with close alkali content. The thermopower at room temperature is about one-third that of polycrystalline Na0.6CoO2.     

Conduction mechanism in Y3−2xCa2xFe5−xVxO12 garnet ferrites

A series of polycrystalline garnet ferrites with composition Y_3−2xCa_2xFe_5−xV_xO₁₂ (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), were prepared by the standard ceramic technique to study the effect of Ca²⁺ and V⁵⁺ ions substitution on their DC electrical conductivity, thermoelectric power, charge carrier concentration and charge carrier mobility at different temperatures. It was found that the DC electrical conductivity increases linearly with increasing temperature ensuring the semiconducting nature of samples. The lines representing the temperature dependence of σ_dc are broken at two-phase transition temperature (T_σ1, T_σ2=T_C) giving three distinct regions (I, II and III). The activation energy for electrical conduction increases going from ferrimagnetic state (regions I and II) to paramagnetic state (region III) through the transition temperature T_σ2 (Curie temperature). It also increases going from region I to region II thorough the temperature T_σ1. The dc electrical conductivity does not vary uniformly with Ca²⁺ and V⁵⁺ ion substitution. The values of the thermoelectric power were positive for samples of 0.0⩽x⩽0.6 indicating that the majority of the carrier are holes in these samples while it were negative for samples of x⩾0.8 indicating that the majority of charge carriers are electrons in this samples. Using the values of the DC electrical conductivity and thermoelectric power, the values of the charge carrier concentration and the charge carrier mobility were calculated.     

Lattice crossover and mixed valency in the LaCo1−xRhxO3 solid solution

The full LaCo_1−xRh_xO₃ solid solution was investigated utilizing structural, electrical transport, magnetic, and thermal conductivity characterization. Strong evidence for at least some conversion of Rh³⁺/Co³⁺ to Rh⁴⁺/Co²⁺ is found in both structural and electrical transport data. The crystal structure is that of a rhombohedrally distorted perovskite over the range 0.0≤x≤0.1. The common orthorhombic distortion of the perovskite structure is found over the range 0.2≤x≤1.0. A crossover of all three orthorhombic cell edges occurs at x=0.5 giving the appearance of a cubic structure, which actually remains orthorhombic. The octahedra in the orthorhombic structure must be distorted for x values less than 0.5, and the observed distortion suggests orbital ordering for Co²⁺. Electrical resistivity measurements as a function of temperature show semiconducting-like regions for all compositions. There is a steady increase in electrical resistivity as the Rh content increases. Large positive thermopower values are generally obtained above 475 K. With increasing Rh substitution there is a decrease in thermal conductivity, which slowly rises with increasing temperature due to increased electrical conductivity. The electronic part of the thermal conductivity is suppressed significantly upon Rh substitution. A thermoelectric figure-of-merit (ZT) of about 0.075 has been achieved for LaCo_0.5Rh_0.5O₃ at 775 K, and is expected to reach 0.15 at 1000 K.     

Conductivity and carrier traps in La1−xSrxCo1−zMnzO3−δ (x=0.3; z=0 and 0.25)

Measurements of the equilibrium oxygen content, electrical conductivity and thermopower in the perovskite-like solid solution La_0.7Sr_0.3Co_1-zMn_zO_3−δ (z=0 and 0.25) as a function of the temperature and oxygen partial pressure are used to determine the temperature dependence of the conductivity and thermopower at different values of the oxygen deficiency. A model for a hopping conductor with screened charge disproportionation is applied for the data analysis in combination with trapping reactions of n- and p-type carriers on local oxygen vacancy clusters and manganese cations, respectively. Changes in the ratio of n-type to p-type mobility are due to variations in oxygen vacancy concentration and manganese content, while the energetic parameters governing charge disproportionation of the trivalent cobalt cations and formation of vacancy associates are shown to be essentially invariable. These calculated charge carrier site occupancies are used to model temperature variations of the electrical properties in La_0.7Sr_0.3Co_1−zMn_zO_3−δ in favorable correspondence with experimental observations.     

Thermoelectric properties of Yb(x)Eu(1-x)Cd2Sb2

The thermoelectric performance of EuCd(2)Sb(2) and YbCd(2)Sb(2) was improved by mixed cation occupation. The composition, structure, and thermoelectric properties of Yb(x)Eu(1-x)Cd(2)Sb(2) (x=0, 0.5, 0.75, and 1) have been investigated. Polycrystalline samples are prepared by direct reaction of the elements. Thermoelectric properties were investigated after densification of the materials by spark plasma sintering. Yb(x)Eu(1-x)Cd(2)Sb(2) crystallizes in the P3m1 space group. The lattice parameters increase with the europium content. These materials show low electrical resistivity, high Seebeck coefficient, and low thermal conductivity together with high carrier concentration and high carrier mobility. ZT values of 0.88 and 0.97 are obtained for Yb(0.5)Eu(0.5)Cd(2)Sb(2) and Yb(0.75)Eu(0.25)Cd(2)Sb(2) at 650 K, respectively.     

Improvement of thermoelectric properties of alkaline-earth hexaborides

Thermoelectric (TE) and transport properties of alkaline-earth hexaborides were examined to investigate the possibility of improvement in their TE performance. As carrier concentration increased, electrical conductivity increased and the absolute value of the Seebeck coefficient decreased monotonically, while carrier mobility was almost unchanged. These results suggest that the electrical properties of the hexaboride depend largely on carrier concentration. Thermal conductivity of the hexaboride was higher than 10 W/m K even at 1073 K, which is relatively high among TE materials. Alloys of CaB₆ and SrB₆ were prepared in order to reduce lattice thermal conductivity. Whereas the Seebeck coefficient and electrical conductivity of the alloys were intermediate between those of CaB₆ and SrB₆ single phases, the thermal conductivities of the alloys were lower than those of both single phases. The highest TE performance was obtained in the vicinity of Ca_0.5Sr_0.5B₆, indicating that alloying is effective in improving the performance.     

LaCo(1-x)Ni(x)O(3) with Improved Electrical Conductivity

To determine the applicability of LaCo(1-x)Ni(x)O(3) in a conductive material for electrical wiring, the dependence of the electronic transport property on the Ni content is investigated via Hall effect measurements, Rietveld analyses, and band-structure calculations. Ni doping (50 mol %) into the Co sites realizes a high electrical conductivity of 1.9 × 10(3) S/cm, which is an unexpectedly high value for a LaCo(1-x)Ni(x)O(3) system, at room temperature due to the high carrier concentration of 2.2 × 10(22) cm(-3) and the small effective mass of 0.1 m(e). In addition, the high electrical conductivity is maintained from room temperature to 900 °C; that is, the temperature coefficient of the conductivity is smaller than that of standard metals. Thus, the results indicate that LaCo(0.5)Ni(0.5)O(3) is suitable as a conductive material for electrical wiring at high temperatures in air.     

Thermal and electrical behaviors of acid functionalized MWCNT/ABC-type triblock copolymer grafted-MWCNT nanocomposites

In this work, ABC-type triblock copolymer grafted onto the surface of the MWCNT/acid functionalized MWCNT (MWCNT-COOH) composites were prepared and the properties of nanocomposites were characterized extensively using differential scanning calorimetry (DSC), scanning electronic microscopy (SEM), thermogravimetric analysis (TGA), ac electrical conductivity and dielectrical measurements.

DSC study showed that the glass transition temperatures of the nanocomposites are a some higher than that of the matrix polymer. The increase in oxidized MWCNT in the nanocomposite improved the thermal stability of the composite, according to initial decomposition temperatures. The ac electrical conductivity has increased moderately with increasing frequency, but has increased slowly with increase in the oxidized MWCNT content in the nanocomposites. The electrical conductivity increases slowly with increasing temperature to about the glass transition temperature, then it increases faster. The dielectric constants for the matrix polymer and all the composites decreases slightly with increasing frequency from 0.1 kHz to 2.0 kHz. The dielectric constant increases slightly with increasing temperature up to about the glass transition temperature region and then the increase in temperature is accelerated the increase in the dielectric constant.     

Transport properties, specific heat and thermal conductivity of GaN nanocrystalline ceramic

The structural and transport properties (resistivity, thermopower and Hall effect), specific heat and thermal conductivity have been measured for GaN nanocrystalline ceramic prepared by hot pressing. It was found that the temperature dependence of resistivity in temperature range 10-300 K shows the very low activation energy, which is ascribed to the shallow donor doping originating in amorphous phase of sample. The major charge carriers are electrons, what is indicated by negative sign of Hall constant and Seebeck coefficient. The thermopower attains large values (−58 μV/K at 300 K) and was characterized by linear temperature dependence which suggests the diffusion as a major contribution to Seebeck effect. The high electron concentration of 1.3×10¹⁹ cm⁻³ and high electronic specific heat coefficient determined to be 2.4 mJ/molK² allow to conclude that GaN ceramic demonstrates the semimetallic-like behavior accompanied by very small mobility of electrons (∼0.1 cm²/V s) which is responsible for its high resistivity. A low heat conductivity of GaN ceramics is associated with partial amorphous phase of GaN grains due to high pressure sintering.     

Facile one‐pot synthesis and characterization of novel nanostructured organic dispersible polyaniline

Novel polyaniline (PANI) with nanotube, nanosheet, and nanofiber shapes was facilely synthesized by a self‐templating one‐pot process. Anilinium‐dodecylsulfate (DS) complex, obtained by mixing equivalent amounts of aniline and sodium dodecylsulfate (SDS), played a template‐like role in forming PANI nanostructures. It was found that the morphology and electrical conductivity of the PANI nanostructures changed with the amount of SDS and the reaction temperature. Nanotube‐shaped PANI synthesized at the temperature of anilinium‐DS complex formation in presence of SDS (concentration <0.12 M) showed high conductivity (12.9 S/cm). At higher temperature, the morphology changed to shape of a rose flower and electrical conductivity decreased to 3.95 S/cm. This suggested that both temperature and SDS concentration were key parameters for controlling the formation of the anilinium‐DS complex that acted as a template. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1024–1029, 2009     

High-temperature thermopower and conductivity of La1−xBaxMnO3 (0.02?x?0.35)

Thermopower and conductivity are measured for the perovskite-like manganite La_1−xBa_xMnO₃ (0.02?x?0.35) in the temperature range 200-950°C and oxygen pressure varying between 10⁻¹¹ and 0.5 atm. The data for thermopower and electrical conductivity indicate a transport mechanism dominated by adiabatic hopes of small polarons. The high-temperature limit for the thermopower is mainly defined by the spin contribution while configurational contribution does not depend essentially on the doping level. Changes in the conductivity with doping are shown to reflect variations in the mobility of charge carriers.     

Enhanced electronic correlation and thermoelectric response by Cu-doping in Ca(3)Co(4)O(9) single crystals

The structure, anisotropic magnetic, electrical and thermal transport properties for single crystals of Ca(3)Co(4-x)Cu(x)O(9) (x = 0, 0.2, 0.4, 0.6 and 0.8) have been investigated systematically. The Cu-doping with x = 0.2 at Co-site is sufficient to drive the low-temperature spin-glass state in the Ca(3)Co(4)O(9) system. The value of resistivity along ab-plane decreases monotonously with increasing x in the whole temperature range studied, and around room temperature, the in-plane resistivity of Ca(3)Co(3.2)Cu(0.8)O(9) is about 71% smaller than that of the undoped sample. The temperature region where the Fermi-liquid transport mechanism dominates becomes remarkably narrowed due to the Cu-doping while the electronic correlation in the system is enhanced. With further addition of Cu in the Ca(3)Co(4)O(9) system, the in-plane thermopower (S(ab)) increases slowly and the room-temperature S(ab) for Ca(3)Co(3.2)Cu(0.8)O(9) is about 17% larger than that of the undoped sample. As a result, the power factor along the ab-plane is enhanced by about 3.8 times compared to the undoped sample. The results are suggested to originate from the variations of carrier concentration and electronic correlation in this system via the different Cu-doping states: Cu(3+)/Cu(2+) (Cu(3+) major) into the CoO(2) layer for x ≤ 0.4, while Cu(2+)/Cu(3+) (Cu(2+) major) into the Ca(2)CoO(3) layers for x > 0.4.     

Synthesis and evaluation of lead telluride/bismuth antimony telluride nanocomposites for thermoelectric applications

Heterogeneous nanocomposites of p-type bismuth antimony telluride (Bi_2−xSb_xTe₃) with lead telluride (PbTe) nanoinclusions have been prepared by an incipient wetness impregnation approach. The Seebeck coefficient, electrical resistivity, thermal conductivity and Hall coefficient were measured from 80 to 380 K in order to investigate the influence of PbTe nanoparticles on the thermoelectric performance of nanocomposites. The Seebeck coefficients and electrical resistivities of nanocomposites decrease with increasing PbTe nanoparticle concentration due to an increased hole concentration. The lattice thermal conductivity decreases with the addition of PbTe nanoparticles but the total thermal conductivity increases due to the increased electronic thermal conductivity. We conclude that the presence of nanosized PbTe in the bulk Bi_2−xSb_xTe₃ matrix results in a collateral doping effect, which dominates transport properties. This study underscores the need for immiscible systems to achieve the decreased thermal transport properties possible from nanostructuring without compromising the electronic properties.     

Enhancement of tensile,electrical and thermal properties of epoxy nanocomposites through chemical hybridization of polypyrrole and graphene oxide

This paper presents the properties of epoxy nanocomposites, prepared using a synthesized hybrid Polypyrrole-Graphene Oxide (PPy-GO) filler, via in-situ chemical polymerization, at various filler loadings (i.e., 0.5–2 w. t %). The microstructures and properties of the PPy-GO hybrids and epoxy nanocomposites were studied via Fourier transform infrared (FTIR), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), mechanical (Tensile Properties), electrical, Dynamic mechanical thermal analysis (DMTA) and thermogravimetric analyses (TGA). Morphological study demonstrated that varying the nanofiller nature (PPy-GOs, PPy or GO) lead to different states of dispersion. Mechanical, electrical and thermal analysis demonstrated that the hybrid concentration and its architecture (PPy:GO ratio) are interesting factors significantly affected the properties of the epoxy based nanocomposites. On the other hand, the mechanical performance of the cured nanocomposites outperformed the PPy-GO, with enhancements of 78% and 51% of Young's modulus and strength, respectively. Here it has been established that the embedding of PPy-GO hybrids into pristine epoxy endows optimum dispersion of PPy and GO as well as better interfacial adhesion between the fillers and matrix, which results in a significant improvement in load transfer effectiveness. Electrical conductivity measurements showed that conductivity of epoxy filled nanocomposites increased up 10⁻⁴ S/cm for Epoxy/PPy-GO nanocomposites. DMTA test indicated that incorporation of PPy-GO resulted in a significantly increase in Tg of the resultant nanocomposites, which is attributed to the highly exfoliation structure and the stronger interfacial interaction. The PPy-GO particles enhanced electrical, thermal and mechanical properties of nanocomposites, confirming the synergistic effect of PPy-GO as multifunctional filler.