Search

The temperature dependences of the electrical conductivity σ(T), Seebeck coefficient σ(T), and heat capacity Cp(T) of polycrystalline samples of Bi2Te3, Bi2Te3+1%CuI, and Bi2Te3+1%(CuI+1/2Pb) are investigated in the temperature range below room temperature. Based on the temperature dependences of all investigated physical properties, it is discovered that phase transition occurs at 120–200 K. Investigation of single crystals shows that anomalies in the electrical resistivity (ρ(T)=1/σ(T)) occur only across the crystal growth axis (across the well‐conducting Bi–Te plane). Investigation of the low‐temperature dependence of electrical conductivity shows that all polycrystalline samples exhibit quasi‐two‐dimensional electron transport. Additionally, quasi‐two‐dimensional transport is detected in single crystals based on anisotropy analysis ρ⊥(T)/ρ‖(T):10 (where ρ‖(T) is the resistivity along the crystal growth axis, and ρ⊥(T) is resistivity across the crystal growth axis) and temperature dependence ρ(T):T2 below 50 K. The Fermi energy EF is estimated using the temperature dependence of S(T). It is discovered that an increase in EF at T > 200 K is associated with the phase transition. For single‐crystal samples, the maximum thermoelectric figure of merit ZT, as observed along the crystal growth axis, increases with doping. A maximum ZT value of ∼1.1 is observed for the Bi2Te3+1%(CuI+1/2Pb) sample at room temperature (T=300K). [ABSTRACT FROM AUTHOR]

The temperature dependences of the electrical conductivity σ(T), Seebeck coefficient σ(T), and heat capacity Cp(T) of polycrystalline samples of Bi2Te3, Bi2Te3+1%CuI, and Bi2Te3+1%(CuI+1/2Pb) are investigated in the temperature range below room temperature. Based on the temperature dependences of all investigated physical properties, it is discovered that phase transition occurs at 120–200 K. Investigation of single crystals shows that anomalies in the electrical resistivity (ρ(T)=1/σ(T)) occur only across the crystal growth axis (across the well‐conducting Bi–Te plane). Investigation of the low‐temperature dependence of electrical conductivity shows that all polycrystalline samples exhibit quasi‐two‐dimensional electron transport. Additionally, quasi‐two‐dimensional transport is detected in single crystals based on anisotropy analysis ρ⊥(T)/ρ‖(T):10 (where ρ‖(T) is the resistivity along the crystal growth axis, and ρ⊥(T) is resistivity across the crystal growth axis) and temperature dependence ρ(T):T2 below 50 K. The Fermi energy EF is estimated using the temperature dependence of S(T). It is discovered that an increase in EF at T > 200 K is associated with the phase transition. For single‐crystal samples, the maximum thermoelectric figure of merit ZT, as observed along the crystal growth axis, increases with doping. A maximum ZT value of ∼1.1 is observed for the Bi2Te3+1%(CuI+1/2Pb) sample at room temperature (T=300K). [ABSTRACT FROM AUTHOR]

The thermoelectric properties of a series of the polycrystalline samples of titanium dichalcogenides with partial substitution of Ti for Nb and S for Se were investigated. It was found that sintering of the samples improved the thermoelectric efficiency (ZT), and the maximum ZT was achieved at sintering temperature of 600°C. A further increase in the sintering temperature (850°C and 950°C) led to the recrystallization of the samples, as a result, the Seebeck coefficient sharply decreased and electrical conductivity dramatically increased. The temperature dependences of electrical conductivity σ(T) in the temperature range from 4.2 to 300 K and Seebeck coefficient S(T) in the temperature range from 77 to 300 K were investigated in order to determine the nature of the observed improvements in thermoelectric properties due to double substitutions and sintering. Two‐dimensionalization of electron transport properties of Ti1−xNbxS2−ySey solid solutions was found. The Fermi energy EF2D was estimated using the temperature dependences of Seebeck coefficient. The relationship between the Fermi energy EF2D and figure of merit ZT was established. The effect of sintering on parameters σ(T), S(T), charge carrier concentration (n2D), mobility (µ), and thermal conductivity (k) was found. The optimal value of Fermi energy EF2D in terms of figure of merit ZT = 0.31 at room temperature (T = 300 K) was found for Ti0.98Nb0.02S1.3Se0.7 sample sintered at 600°C. [ABSTRACT FROM AUTHOR]

We investigate the thermoelectric properties of bulk polycrystalline samples of WSe2‐based compounds with partial substitutions in the cationic (W) and the anionic (Se) sublattices in the temperature range from 4.2 to 650 K. The substitution of W for Nb leads to a significant increase in the charge carrier concentration, however, deteriorates the charge carrier mobility. In contrast, the substitution of selenium for sulfur increases the charge carrier mobility, the thermal conductivity, and the Seebeck coefficient but conductivity changes non‐monotonical. We show that the addition of sulfur in anionic sublattice affects the grain sizes in the polycrystalline material. Using substitutions in the anionic and cationic sublattices, we find the optimal ratio of the elements for better thermoelectric efficiency. The W0.98Nb0.02Se1.7S0.3 sample showed the best value of the figure of merit ZT = 0.26 (T = 650 K). [ABSTRACT FROM AUTHOR]

The present paper is devoted to the influence of the growth temperature of multi‐walled carbon nanotubes (MWCNTs) on their defective structure. The MWCNTs obtained within the range of 610–750 °C have been studied using two methods: Raman spectroscopy and the analysis of the temperature dependence of conductivity. This approach allows us to obtain independent data on the concentration of the defects within the MWCNTs via the intensity ratio of 2D and D bands, on one hand, and charge carrier concentration, on the other hand. The results obtained using Raman spectroscopy and the analysis of the conductivity have provided asymptotic and volcano‐like curves, respectively, of defect concentration within the temperature range studied. This can be attributed to the difference in the probing depth of each method (∼50 nm for Raman spectroscopy, and ∼2–3 nm for conductivity measurements) providing different sensitivity to surface impurities or defects. The secondary factors (the size of the active component, the amount of the lateral carbon deposits) have been shown to mask the influence of the growth temperature. Nevertheless, as these secondary factors and the synthesis temperature independently affect the effective nanotube defectiveness, we have shown the defect concentration to fall with increasing MWCNT growth temperature. [ABSTRACT FROM AUTHOR]

The structure and electronic properties of the N‐doped bamboo‐like carbon nanotubes with different nitrogen content (N‐CNTs) are studied by the use of transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy, and X‐ray photoelectron spectroscopy supplemented by measurements of the temperature dependence of conductivity. XPS data are used to estimate the content of nitrogen represented by pyridinic, pyrrolic, graphitic, and oxidized N species and encapsulated molecular nitrogen. The graphene blocks surrounded by borders containing hole defects and N‐species are considered as building blocks of N‐CNT walls. The intensity ratio of D, G, and 2D bands in Raman spectra and XPS data on the concentration of N‐species are used to analyze the size of defect‐free graphene blocks in N‐CNTs. The size of such blocks depends on the N‐defect content and is estimated to be 1–2 nm. The increase of N‐species content resulting in a decrease of graphene block size correlates with the decrease of current carrier concentration (nES) estimated within the Efros–Shklovskii model. [ABSTRACT FROM AUTHOR]

A novel lightweight onion-like carbon (OLC)-based polymer composite with high electromagnetic (EM) shielding properties is presented. OLC have been produced via the large-scale production technology based on the annealing of detonation nanodiamond under vacuum conditions (or in inert atmosphere). EM shielding effectiveness has been tested in the frequency range of 26–37 GHz. The highest EM attenuation at 36.6 GHz reaching −34 dB was observed for polymethylmethacrylate films comprising 20 wt.% of OLC. The shielding effectiveness data collected for microwave frequencies were found to correlate well with the electrical resistivity measurements by four-probe method as well as conductivity measurements provided by the broadband dielectric spectroscopy (20 Hz–3 GHz). It was proved experimentally that OLC EM shielding capacity can be optimized by varying the nanoonion cluster size and nanodiamond annealing temperature so that effective EM coatings can be produced. Both the experimental observations and theoretical simulations demonstrate that even small (smaller than percolation threshold) additions of OLC particles to a polymer host can noticeably modify the composite response to EM radiation. [ABSTRACT FROM AUTHOR]

The temperature dependence of heat capacity of superconducting YBa2Cu3O6+x system (with x=0.85, 0.90, 0.95) was investigated above Tc. For all the samples the “small” anomalies were discovered in ranges 110–180 K (Tlow), 205–235 K (Tm) and 250–290 K (Th). The analysis with involving data for TmBa2Cu3O6.95 and GdBa2Cu3O6.95 reveals that one may consider some of these anomalies as a characteristic property of the 90 K samples. [Copyright &y& Elsevier]