High Temperature Electrical Transport Research Articles

Heat capacity and high temperature electrical transport properties of TbNiC2

The single phase polycrystalline TbNiC2 sample was obtained by annealing at 1073 K. The compound adopts structure of CeNiC2-type, confirmed by XRD Rietveld refinement: space group Amm2, a = 3.6000(2) Å, b = 4.5129(2) Å, c = 6.0563(3), RB(I) = 3.72%, Rp = 5.46%. The orthorhombic CeNiC2 crystal structure prototype is shown to be derived from the hexagonal AlB2 one (space group P6/mmm) following the Bärnighausen formalism via translationengleiche. Zero-field heat capacity measurements of TbNiC2 in the range of 1.9–300 K reveal Sommerfeld coefficient γ = 8(1) mJ mol−1 K−2, phonon coefficient β = 1.18(1)∙10−4 J mol−1 K−4 with the derived Debye temperature of ΘD ≈ 404 K and antiferromagnetic state below TN = 25 K with a spin-wave excitation gap of Δ = 26(2) K. The 4f-electron contribution to the magnetic entropy change indicates a doublet ground state for a non-Kramer's Tb3+ ion. Thermopower and electrical resistivity of TbNiC2 have been measured in the range of 310–855 K and reveal values characteristic for metallic systems. Seebeck coefficient decreases with increasing temperature approximately linearly in the all studied interval, and its sign changes from positive to negative one at ~660 K, whereby the electrical resistivity increases from 310 K reaching maximum at ~640 K and afterwards decreases slightly. Maximum power factor value of 27 μW m−1 K−2 has been observed at 310 K.

High temperature electrical transport properties of MBE-grown Mg-doped GaN and AlGaN materials

This paper discusses the results of high temperature resistivity and Hall effect studies of Mg-doped GaN and AlxGa1−xN epilayers (0.05 < x < 0.23). The studied samples were grown by molecular beam epitaxy on low temperature buffers of GaN and AlN deposited on a sapphire substrate. The experiments were carried out at temperatures ranging from 300 up to 1000 K. Up to a certain critical temperature TC (around 800 K), a typical increase of the conduction processes due to the excitation of impurity states has been observed with an activation energy of about EA = 200 meV. However, at this critical temperature TC, an annealing effect was observed in all the investigated samples. At this critical temperature, the increase in the free carrier concentration as a function of time leads to an irreversible decrease in the samples’ resistivity of more than 60%. The observed temperature dependences of the electrical transport properties are analyzed in the frame of an impurity model including shallow donors and Mg-related acceptors (with EA ∼ 200 meV). In some cases, an additional conduction channel not related to free carriers in the valence band must be taken into account. This can lead to an incorrect determination of hole concentration in the valence band, an important parameter in the process of radiative recombination.

Chromium silicide-based composites fabricated via solid-state reactions: Phase development, oxidation behavior and electrical properties at high-temperatures

The chromium silicide-based electroconductive composites were fabricated by solid-state sintering of chromium silicide and chromium oxide powders at 1370 °C in argon. The final densified composites were composed of various solid solution and silicide phases (e.g. Cr3Si, Cr5Si3) and silica, depending on the starting silicide/oxide volume ratio ranging from 0.67 to 9.0. Major phases were found to be homogeneously distributed within the metastable (cristobalite) and/or amorphous silica grain boundary phase. The controlled oxidation experiments revealed excellent oxidation resistance at 50°-870 °C, where low-level oxidation was observed. The composites all displayed a metallic-type electrical conductivity due to the presence of the Cr3Si, Cr5Si3 and CrSi as the major conductive silicide phases. Their electrical conductivities ranged from 42.1 to 213.7 S/cm at 1000 °C. High-temperature annealing of the composites resulted in phase and microstructural changes, which further improved their high-temperature oxidation resistance and electrical transport properties.

High-temperature electrical and thermal transport behaviors of In2O3-based ceramics by Zn-Sn co-substitution

We report remarkably the enhanced thermoelectric performance of Sn-Zn co-doped In2O3 that were synthesized by a solid-state reaction followed by spark plasma sintering in the mid-temperature range. The X-ray diffraction study indicates that Sn and Zn were successfully co-substituted at the In site without forming any additional phase even at 8% doping concentration. The co-substitution shows a significant increase in the electrical conductivity by band structure modification resulting in a significantly enhanced power factor. The point defect engineering combined with nanostructuring using a high energy ball milling suppressed the lattice thermal conductivity, which eventually resulted in an increased ZT value of 0.32 at 973 K, that is, about 7 times higher than that of pure In2O3. Thus, this study demonstrates the important role of co-substitution in improving the thermoelectric properties of n-type oxides.

The impact of Ba substitution on high-temperature transport in lanthanum-strontium ferrite

Effect of barium content on structural, thermal, and transport properties of oxides La0.49Sr0.5–x Ba x FeO3–δ (х = 0, 0.25, 0.50) was studied with the emphasis on their potential use for electrodes and oxygen transport membranes. Barium substitution for strontium was found to enhance symmetry of crystalline lattice, facilitate the oxygen release on heating, and favor the increasing thermal expansion of La0.49Sr0.5–x Ba x FeO3–δ . Analysis of electrical conductivity depending on the partial oxygen pressure allowed to separate the contributions of oxygen ions, electrons, and holes in high-temperature electrical transport in order to trace barium effect on conductivity components. Electron conductivity of n-type and its activation energy does not reveal sensitivity to barium concentration in oxides. Oxygen ion conductivity is shown to decrease with increasing barium content in La0.49Sr0.5–x Ba x FeO3–δ while respective activation energy increases from 0.70 to 0.79 eV. About four times decrease of p-type electron conductivity at invariable activation energy was detected as a result of the substitution degree increase from x = 0 to x = 0.5. Possible mechanisms of barium influence on properties of oxides are discussed.

Synthesis, sintering, transport and thermal properties of Na2Fe2Ti6O16 freudenbergite

Na2Fe2Ti6O16 freudenbergite has been synthesized using a simple solid-state route and conventionally sintered to reach 94% density ceramics. High temperature electrical and thermal transport properties have been investigated for the first time. In the temperature range of 468 K–1000 K, the electrical conductivity of Na2Fe2Ti6O16 can be well fitted with small polaron hopping model. For a further verification of the model, we measured the room-temperature optical reflectivity and derived the optical conductivity (σ(ω)) with Kramers–Kronig analysis. A mid-infrared peak in σ(ω) confirms the presence of small polarons. From 468 K to 1000 K, Na2Fe2Ti6O16 is an n-type oxide and exhibits a temperature-independent Seebeck coefficient (∼−590 μV K−1), in agreement with the small polaron hopping mechanism. The thermal conductivity decreases with increasing temperature from 16.5 W m−1.K−1 (373 K) to 3.4 W m−1.K−1 (1000 K). Na2Fe2Ti6O16 shows a maximum figure of merit (ZT) ∼ 3 × 10−4 at 1000 K.

High temperature electrical transport study of Si-doped AlN

Electrical transport (resistivity and Hall Effect) have been studied in silicon doped aluminum nitride (AlN) thick epitaxial layers from 250 K up to 1000 K. The investigated samples, grown by molecular beam epitaxy were characterized by n-type conduction with an ambient temperature free carrier concentration of about ∼ 1 × 1015 cm−3. The donor level, situated about 250 meV below the conduction band edge, was found to be responsible for the experimentally observed increase of free carrier concentration with temperature. The temperature dependence of carrier mobility has been analyzed in the framework of a multimode scattering model. In the investigated samples the main scattering mechanism is supposed to be dislocation scattering.

Thermoelectric properties of hot-pressed Tl9LnTe6 (Ln = La, Ce, Pr, Nd, Sm, Gd, Tb) and Tl10−xLaxTe6 (0.90 ⩽ x ⩽ 1.05)

In the first part of this work, thallium lanthanide tellurides with composition Tl9LnTe6 (Ln=La, Ce, Pr, Nd, Sm, Gd, Tb) have been prepared and their high temperature electrical and thermal transport properties investigated. Nearly single phase samples were obtained. Rietveld refinement results showed that Tl9CeTe6 had an exceptionally small unit cell volume of 1033Å3. The electrical conductivity and thermal conductivity increased across the lanthanide series from La to Tb. On the other hand, the Seebeck coefficient values decreased with Ln varying from La to Tb. Tl9SmTe6 or Tl9GdTe6 constituted an exception, as the Seebeck coefficient of Tl9SmTe6 was smaller than that of Tl9GdTe6. Tl9LaTe6 has the largest figure-of-merit of this series.Thereafter, Tl10−xLaxTe6 samples were prepared with x=0.90, 0.95, 1.00, 1.05, 1.10, aiming toward higher zT through composition optimization and expanding of the measurement temperature range. With the increasing La content, the unit cell volume increased, while the electrical conductivity, thermal conductivity and lattice thermal conductivity decreased, and the Seebeck coefficient increased. The highest zT=0.57 was observed for Tl9LaTe6 at 600K.

High temperature study of flexible silicon-on-insulator fin field-effect transistors

We report high temperature electrical transport characteristics of a flexible version of the semiconductor industry's most advanced architecture: fin field-effect transistor on silicon-on-insulator with sub-20 nm fins and high-κ/metal gate stacks. Characterization from room to high temperature (150 °C) was completed to determine temperature dependence of drain current (Ids), gate leakage current (Igs), transconductance (gm), and extracted low-field mobility (μ0). Mobility degradation with temperature is mainly caused by phonon scattering. The other device characteristics show insignificant difference at high temperature which proves the suitability of inorganic flexible electronics with advanced device architecture.

Electrical and thermal transport properties of CdO ceramics

High temperature electrical and thermal transport properties, that is, electrical conductivity, Seebeck coefficient and thermal conductivity, of CdO ceramics have been investigated. Because of the good electrical properties and low thermal conductivity, the dimensionless figure-of-merit ZT of the CdO ceramics reaches 0.34 at 1023 K. This value is comparable to the best reported ZT for the n-type oxide ceramic thermoelectric materials and remains as potential to be further improved by porosity controlling or nanostructuring.

An investigation on high-temperature electrical transport properties of graphene-oxide nano-thinfilms

High-temperature electrical transport properties are investigated for graphene-oxide nano thinfilms. The graphene-oxide nanoparticles are synthesized by modified Hummers method and characterized by UV–vis, Raman and X-ray diffraction techniques. The surface morphology of graphene-oxide film is analyzed using scanning electron and atomic force microscopy. The experimental results on high-temperature electrical studies of thinfilms exhibit metallic behavior followed by three-dimensional variable range hopping mechanism. The current–voltage characteristics at various temperatures (from 293K to 573K) were investigated. The effect of high-temperature on the functional groups of graphene-oxide film is evidently examined using X-ray photoelectron, thermal gravimetric analysis and Fourier transform infra-red spectroscopy. Transistor characteristics were performed after heat treatment resulting ambipolar behavior with holes and electron mobility of 127 and 66.9cm2V−1s−1 respectively. Our results are comparable to reduced graphene-oxide, indicating the advantage of our approach requires no further reduction to develop graphene-based transparent and conductive electrodes for dye-sensitized solar cells and ultra-capacitor applications.

High-temperature electrical and thermal transport properties of fully filled skutterudites RFe4Sb12 (R = Ca, Sr, Ba, La, Ce, Pr, Nd, Eu, and Yb)

Fully filled skutterudites RFe4Sb12 (R = Ca, Sr, Ba, La, Ce, Pr, Nd, Eu, and Yb) have been prepared and the high-temperature electrical and thermal transport properties are investigated systematically. Lattice constants of RFe4Sb12 increase almost linearly with increasing the ionic radii of the fillers, while the lattice expansion in filled structure is weakly influenced by the filler valence charge states. Using simple charge counting, the hole concentration in RFe4Sb12 with divalent fillers (R = Ca, Sr, Ba, Eu, and Yb) is much higher than that in RFe4Sb12 with trivalent fillers (R = La, Ce, Pr, and Nd), resulting in relatively high electrical conductivity and low Seebeck coefficient. It is also found that RFe4Sb12 filled skutterudites having similar filler valence charge states exhibit comparable electrical conductivity and Seebeck coefficient, and the behavior of the temperature dependence, thereby leading to comparable power factor values in the temperature range from 300 to 800 K. All RFe4Sb12 samples possess low lattice thermal conductivity. The correlation between the lattice thermal resistivity WL and ionic radii of the fillers is discussed and a good relationship of WL ∼ (rcage−rion)3 is observed in lanthanide metal filled skutterudites. CeFe4Sb12, PrFe4Sb12, and NdFe4Sb12 show the highest thermoelectric figure of merit around 0.87 at 750 K among all the filled skutterudites studied in this work.

High-temperature electrical transport behaviors of the layered Ca2Co2O5-based ceramics

Textured Bi-substituted Ca2Co2O5 ceramics have been prepared using a coprecipitation method combined with spark plasma sintering. The Bi substitution is effective in increasing the grain orientation of the Ca2Co2O5-based ceramics (from 0.46 to 0.85). Electrical conductivity increases obviously with partial substitution of Bi3+ for Ca2+ (from 113.9 to 142.4 S/cm at 973 K). The Ca1.92Bi0.08Co2O5 sample exhibits higher power factor (4.4×10−4 W m−1 K−2) than that of pure Ca2Co2O5 (3.2×10−4 W m−1 K−2) at 973 K, indicating that the Ca2Co2O5 system is another promising p-type material for high-temperature thermoelectric applications.

High Temperature Thermoelectric Response of Electron-Doped CaMnO3

We report a systematical investigation on the high temperature thermoelectric response of Ca1−xRxMnO3 (R = rare-earth) perovskites in the electron-doped range. The results reveal that electron concentration is the dominant factor for the high temperature electrical transport properties whereas the weight and size of R ions dominate the thermal transport properties. As the doping level varies, the best thermoelectric performance is observed at the relative electron concentration around 0.1. However, in the case of a fixed electron concentration, structural distortions become important since bandwidth has an observable influence on resistivity. By combining the three factors, electron concentration, crystal structure, and the weight/size of R ions, the largest thermoelectric figure of merit ZT for Ca1−xRxMnO3 reaches 0.2 at 1000 K. But this ZT value is still too far from the application criterion (ZT > 1). Using the dynamical mean field theory, we demonstrate that a ZT value larger than one in electron-dope...

Electronic transport properties of La2/3Ca1/3Mn0.98Cu0.02O3 sintered at different temperature

The sol-gel method was adopted to prepare series of La2/3Ca1/3Mn0.98Cu0.02O3 samples sintered at different temperatures, and the structure of samples, high-temperature electrical conductive mechanism and electronic transport properties of double-peak resistance were investigated. X-ray diffraction pattern indicates that the samples are in perovskite structures and obey the Variable-Rang Hopping Mechanism of ρ=ρ0exp[(T0/T)1/4] under high temperatures during the electrical conduction. In addition, based on the polycrystalline granular system surface and image, the reasonable explanation was given for the double-peak resistance phemomenon.

High-temperature electrical transport behaviors in textured Ca3Co4O9-based polycrystalline ceramics

Highly (00l) oriented Ca3Co4O9-based ceramics were fabricated by spark plasma sintering combined with a dynamic forging process. The grain orientation is effective in lowering the electrical resistivity and enhancing the thermoelectric power factor but with little influence on the Seebeck coefficient. A metallic-to-semiconducting transition temperature can be observed and the activation energy is almost independent of the La-doping. All of the Ca3Co4O9-based ceramic samples exhibit large thermoelectric power, and the figure of merit for La-doped Ca3Co4O9 sample can reach 0.26 at 975 K, which can be a promising candidate of p-type material for high-temperature thermoelectric application.

High-Temperature Electrical Transport and Thermoelectric Properties of La0.75Ba0.25MnO3 Ceramics

In this study, the thermoelectric power and resistivity of the perovskite manganite <TEX>$La_{0.75}Ba_{0.25}MnO_3$</TEX> were investigated in the temperature range 300K-1200K. The electrical resistivity and thermoelectric power indicate a transport mechanism dominated by adiabatic small-polaron hopping. The power factor increases from <TEX>$2{\times}10^{-6}W/mK^2$</TEX> to <TEX>$1{\times}10^{-5}W/mK^2$</TEX> as to the temperature increases from 400K to 1200K, which indicates that the compound is highly feasible as a thermoelectric material at high temperatures.

High-temperature electrical transport properties of buckypapers composed of doped single-walled carbon nanotubes

Data on temperature-dependent electrical resistance of buckypaper flakes are presented in this paper. The buckypapers are composed of ropes of aligned single-walled carbon nanotubes doped with HNO 3, which are treated as mixed systems with their properties being dependent on the treatment performed. The measurements cover rather wide temperature range from 300 up to 900 K. In case of untreated samples, curves with two well-defined activation energies are seen, which are discussed in terms of different DC conductivity mechanisms, with a great attention paid to the parallel metal-semiconductor system. In turn, in heat-treated samples the resistance is found nearly temperature-independent except for the significant peak centered at about 600–650 K. Observed characteristics are also fitted using the parallel model, although with a less accuracy suggesting influence of another conductivity mechanisms. At any rate, the resistance peak is possibly related to the metal/non-metal transition observed in disordered solids.

High-temperature electrical transport in Al-doped calcium and strontium titanates

The electrical conductivity of perovskite-related oxides CaTi1−xAlxO3−δ and SrTi1−xAlxO3−δ (x=0−0.4) were investigated within the temperature range 900 to 1000 °C and the oxygen partial pressure range between 10−20 and 0.21 atm using a dc four-point technique. The materials investigated show predominantly p-type electronic conductivity at high, n-type electronic conductivity at low, and ionic conductivity at intermediate oxygen partial pressures. The values of ionic conductivity in CaTi1−xAlxO3−δ were found to be lower than those in CaTi1−xFexO3−δ. The effect of aluminium concentration on the high-temperature transport properties was examined.

High-temperature electrical transport in La0.3Sr0.7Fe1 − xGaxO3 − δ (x = 0–0.5)

The electrical properties of perovskite-related oxide La0.3Sr0.7Fe1 - xGaxO3 − δ (x = 0–0.5) are reported within the temperature range 750 to 950 °C and the oxygen partial pressure range between 10−19 and 0.5 atm. The maximum solid solubility of gallium in the iron sublattice of La0.3Sr0.7FeO3 − δ is close to 30%. At oxygen pressures about 10−4 atm, the materials undergo a transition from perovskite to oxygen vacancy ordered structures. The type of vacancy ordering depends on gallium content. Both p- and n-type electronic conductivities decrease when insulating gallium cations having stable oxidation state are incorporated into the iron sublattice of the ferrite. The observed temperature and oxygen pressure dependencies of the p-type electronic conductivity in the ordered phases suggest a transition from the intrinsic regime, when the electron–hole conduction is governed by the band gap, to the extrinsic regime, controlled by the amount of the oxygen excess in the vacancy ordered structure. The ordered strontium–lanthanum ferrite was shown to be a mixed conductor with oxygen ionic conductivity of about 0.1 S cm−1 at 900 °C. Ionic conduction in the vacancy ordered phases decreases with increasing gallium concentration.