High Temperature Electrical Transport Properties 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.

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 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...

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 Properties of Eu and Yb-doped Skutterudites

ABSTRACTSkutterudites are a class of materials that show promise for thermoelectric applications due to their high power factor and the ability to “tune” the thermal conductivity due to the addition of “rattling” atoms into the novel structure of these materials. Thermopower and resistivity is measured and reported for a series of Eu and Yb-doped skutterudites over a temperature range of approximately 100 K – 700 K using the High Temperature Thermoelectric Probe. Sample measurement techniques are briefly discussed. Data from various dopings is presented and compared in hope of showing trends that point towards improvements in these skutterudites.

High temperature electrical transport properties of theEuFe4Sb12 and YbFe4Sb12 filledskutterudites

Polycrystalline samples of the filled skutterudites EuFe4Sb12 and YbFe4Sb12 were prepared by direct melting andhot pressing. The samples were characterized using x-ray powder diffractionand differential thermal analysis. It was found that both compounds meltincongruently at peritectic temperatures of 1023±5 K for EuFe4Sb12 and 975±5 K for YbFe4Sb12.At room temperature these materials possess a positive Seebeck coefficient of65-67 µV K-1 and high carrier concentration in the range5.6×1021-3.6×1021 cm-3. The electrical transportproperties were measured over the temperature range 100-860 K. For bothcompounds the electrical resistivity and Seebeck coefficient graduallyincrease with increasing temperature and for YbFe4Sb12exhibit a maximum at 760-800 K. The estimated ZT value of YbFe4Sb12 is 0.4 at 670 K.

The high temperature electrical properties of ZnTe: Al; self-compensation model in ZnTe

The high temperature defect equilibria and self-compensation behavior of ZnTe were investigated by measuring the high temperature electrical transport properties under equilibrium conditions of Al-doped single crystals. The conductivity type was changed from p-type to low resistivity n-type between 700 and 925°C in Zn-rich atmospheres by the addition of 6 × 10 18 cm −3 Al donors. A two-carrier treatment of the transport measurements yields nαP +0·4 Zn. It is proposed that the impurity donors are compensated by doubly ionized native acceptors.