Thermoelectric properties of Ge-doped Bi85Sb15 alloys at low temperatures

Bulk polycrystalline Bi₈₅Sb_15−xGe_x (x=0, 0.5, 1, 1.5, 2) composites were prepared by mechanical alloying followed by pressureless sintering. The thermoelectric properties were studied in the temperature range of 77–300 K. The results indicate that increasing the Ge concentration causes the Seebeck coefficient to change sign from negative to positive. Moreover, it is found that the maximum value of the Seebeck coefficient can be precisely controlled with the Ge concentration. The maximum dimensionless figure of merit reaches 0.07 at 140 K. These results suggest that the preparation of p-type Bi–Sb alloys is possible by using the Ge-doping approach.     

Impedance spectroscopy and conduction mechanism of multiferroic (Bi0.6K0.4)(Fe0.6Nb0.4)O3

Polycrystalline (Bi_0.6K_0.4) (Fe_0.6Nb_0.4)O₃ material has been prepared using a mixed-oxide route at 950 °C. It was shown by XRD that at room temperature structure of the compound is of single-phase with hexagonal symmetry. Some electrical characteristics (impedance, modulus, conductivity etc.) were studied over a wide frequency (1 kHz–1 MHz) and temperature (25–500 °C) ranges. The Nyquist plot (i.e., imaginary vs real component of complex impedance) of the material exhibit the existence and magnitude of grain interior and grain boundary contributions in the complex electrical parameters of the material depending on frequency, input energy and temperature. The nature of frequency dependence of ac conductivity follows Joncher׳s power law, and dc conductivity follows the Arrhenius behavior. The appearance of P–E hysteresis loop confirms the ferroelectric properties of the material with remnant polarization (2P_r) of 1.027 µC/cm² and coercive field (2E_c) of 16.633 kV/cm. The material shows very weak ferromagnetism at room temperature with remnant magnetization (2M_r) of 0.035 emu/gm and coercive field (2H_c) of 0.211 kOe.     

Thermoelectric properties of Bi-Sb semiconducting alloys prepared by quenching and annealing

Bi_100−xSb_x (x=8-17) alloys were prepared by direct melting of constituent elements, which was followed by quenching and annealing. The synthesis of high-homogeneity alloys was confirmed by X-ray diffraction, differential thermal analyses and electron microprobe analysis. The semiconducting and thermoelectric properties of the samples were investigated by measuring Hall coefficient, electrical resistivity and Seebeck coefficient in the temperature range from 20 to 300 K for both the as-quenched and annealing samples. The properties change gradually with the Sb concentration x, which is attributed to the variation of the energy gap. The Hall mobility was enhanced by annealing, which leads to a small electrical resistivity and a large Seebeck coefficient. Consequently, large values of about 8.5 mW/mK² for the power factor were obtained in the annealed alloys of x=8,12, and 14.     

Synthesis and magnetic properties of size-controlled FeNi alloy nanoparticles attached on multiwalled carbon nanotubes

FeNi alloy nanoparticles with controllable sizes were attached on the multiwalled carbon nanotubes by adjusting the atomic ratio of metal to carbon in the mixed solution of nitrate with Fe:Ni=1:1 (atomic ratio) via wet chemistry. Transmission electron microscopy (TEM) and high-resolution TEM indicated that quasi-spherical FeNi alloy nanoparticles with sizes in the range 12-25 nm are obtained. FeNi alloy composed of major face center cubic (fcc) and minor body center cubic (bcc) structures, which is proved by the X-ray powder diffraction (XRD). Magnetization measured by vibrating sample magnetometer demonstrated that both the coercive force and saturation magnetizations decrease as the size of the FeNi alloy nanoparticles decreased. The chemical method is promising for fabricating FeNi alloy nanoparticles attached on carbon nanotubes for magnetic storage and ultra high-density magnetic recording applications.     

Thermoelectric performance of ternary Bi-Sb-Ag alloys prepared by mechanical alloying and subsequent pressureless sintering

The alloys with the general formula of Bi₈₅Sb_15−xAg_x (x=0, 1, 3, 5, 7) were prepared by mechanical alloying and subsequent pressureless sintering (Bi₈₅Sb₁₅ alloy was used for comparison). Their transport properties involving electrical conductivity, Seebeck coefficient, and thermal conductivity had been investigated in the temperature range of 80-300 K. The maximum absolute value of Seebeck coefficient (120 μV/K) was found at 160 K in the alloy Bi₈₅Sb_15−xAg_x (x=3). The figure-of-merit of alloy Bi₈₅Sb_15−xAg_x (x=1) reached a maximum value of 2.16×10⁻³ K⁻¹ at 219 K, which is as large again as that of the reference sample Bi₈₅Sb₁₅.     

The influence of aliovalent impurities on the oxidation kinetics of nickel at high temperatures

The influence of chromium and sodium on the nickel oxidation kinetics has been studied as a function of temperature (1373-1673 K) and oxygen activity (10−10⁵ Pa O₂), using microthermogravimetric techniques. It has been shown that the oxidation of Ni-Cr and Ni-Na alloys, like that of pure nickel, follows strictly the parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni_1−yO, it has been found that the oxidation rate of the Ni-Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of double ionized cation vacancies in a Ni_1−yO-Cr₂O₃ solid solution is fixed on a constant level by trivalent chromium ions, substitutionally incorporated into the cation sublattice of this oxide. In the case of the Ni-Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of the doping effect, by assuming that univalent sodium ions dissolve substitutionally in the cation sublattice of nickel oxide.     

Argon ions induced thermoluminescence properties of Ba0.12Sr0.88SO4 phosphor

Thermoluminescence properties of barium strontium mixed sulfate have been studied by irradiation with Argon ions. The sample was recrystallized by chemical co-precipitation techniques using H₂SO₄. The X-ray diffraction study of prepared sample suggests the orthorhombic structure with average grain size of 60 nm. The samples were irradiated with 1.2 MeV Argon ions at fluences varying between 10¹¹ and 10¹⁵ ions/cm². The argon ions penetrate to the depth of 1.89 μm and lose their energy mainly via electronic stopping. Due to ion irradiation, a large number of defects in the sample are formed. Thermally stimulated luminescence (TSL) glow curves of ion irradiated Ba_0.12Sr_0.88SO₄ phosphor exhibit broad peak with maximum intensity at 495 K composed of four overlapping peaks. This indicates that different sets of traps are being activated within the particular temperature range each with its own value of activation energy (E) and frequency factor (s). Thermoluminescence (TL) glow curves were recorded for each of the ion fluences. A linear increase in intensity of TL glow peaks was found with the increase in ion dose from 59 kGy to 5.9 MGy. The kinetic parameters associated with the prominent glow peaks were calculated using glow curve deconvolution (GCD), different glow curve shape and sample heating rate methods.     

Effective medium theory for the inhomogeneous PdMnxFe1−x alloys

Ternary PdMn_xFe_1−x alloys are known to form a microinhomogeneous random mixture of PdMn and PdFe phases. The unconventional ρ(x) dependence of dc resistivity and singularities in low frequency optical conductivity spectra of alloys are described footing within the effective medium approach. The essential point of the model proposed is the anomalous role of insulating interfaces, whose proliferation at intermediate x gives rise to the observed maximum of resistivity near x?0.8.     

Solid state electrical conductivity and thermal degradation studies on some phenothiazine derivatives

Electrical conductivity and thermal degradation studies of promethazine hydrochloride (PH); 2-chlorophenothiazine (CP); diethazine hydrochloride (DH) and trifluoperazine dihydrochloride (TFP) are reported. The activation energies are evaluated based on their electrical conductivity study conducted over the temperature range 30-150 °C. These energies for PH, CP, DH and TFP are found to be 0.86, 1.02, 0.68 and 1.08 eV, respectively. The materials are analyzed for the kinetic parameters like the activation energies for decomposition and the Arrhenious pre-exponential factors in their pyrolysis region using Broido's, Coats-Redfern and Horowitz-Metzger methods. Using these factors and the standard equations thermodynamic parameters such as enthalpy, entropy and free energies are calculated. Thermogravimetric study on these phenothiazine derivatives in air indicated that their stabilities are in the order CP>TFP>PH >DH.     

Preparation, characterization and ionic conductivity studies of ZrO2 dispersed mixed halide matrix (KCl)0.9-(NaCl)0.1

Composite electrolytes in the system [(KCl)_0.9:(NaCl)_0.1]_1−y:(ZrO₂)_y were prepared and their ionic conductivities were studied. In our previous study on the mixed halide system (KCl)_1−x:(NaCl)_x, maximum conductivity (∼50 times that of the base KCl matrix) was found when x=0.1. The matrix (KCl)_0.9:(NaCl)_0.1 was dispersed with different concentration of ZrO₂ (powder) for the preparation of composites and their conductivities were determined. The maximum conductivity was developed for the composite having composition y=0.5. The matrices were prepared by melt-quench technique and the dispersion of ZrO₂ was carried out in liquid medium. The conductivity measurements of the composites were carried out by impedance spectroscopy technique. The composite [(KCl)_0.9:(NaCl)_0.1]_0.5:(ZrO₂)_0.5 was characterized by X-ray diffraction (XRD) analysis, differential thermal analysis (DTA), thermogravimetric analysis (TG) and scanning electron microscopy (SEM). The conductivity of the composite [(KCl)_0.9:(NaCl)_0.1]_0.5:(ZrO₂)_0.5 as a function of temperature was also studied. The conductivity increase in the composite could be attributed to enhancement of defect concentration in the space charge region created at the interface between the host halide and the dispersoid.     

Oxygen vacancy doping effect on the electrical and magnetic behavior of Ba5−xLaxNb4−xTixO15

We report electric and magnetic properties of oxygen deficient Ba_5−xLa_xNb_4−xTi_xO_15−δ phases, which have been prepared by solid-state reaction method followed by a controlled reduction process under hydrogen atmosphere. The extra electrons added by the formation of the oxygen vacancies (δ) introduce localized spins and the magnetic susceptibility can be described by a temperature-independent contribution and a Curie-Weiss term associated to the Ti³⁺ ion formation. Besides, the experimental resistivity (ρ) data of these four reduced compounds are well described in a wide temperature range with the equation , which suggests the presence of small polarons in the system. Although, all samples present electrical insulating behavior, the electrical resistivity decreases four orders of magnitude for intermediate x values. We interpreted this fact as a consequence of the mix between the localized bands of the Nb and Ti ions, which favors the promotion of carriers due to reduction of the band gap.     

Investigations on the electrical and structural properties of polyaniline doped with camphor sulphonic acid

Polyaniline is chemically synthesised and doped with camphor sulphonic acid. FTIR studies carried out on these samples indicate that the aromatic rings are retained after polymerisation. The percentage of crystallinity for polyaniline doped with camphor sulphonic acid has been estimated from the X-ray diffraction studies and is around 56% with respect to polyaniline emeraldine base. The change in dielectric permittivity with respect to temperature and frequency is explained on the basis of interfacial polarisation. AC conductivity is evaluated from the observed dielectric permittivity. The values of AC and DC conductivity and activation energy are calculated. The activation energy values suggested that the hopping conduction is the prominent conduction mechanism in this system.     

Low temperature thermoelectric properties of Pb- or Sn-doped Bi-Sb alloys

Pb- or Sn-doped Bi₈₈Sb₁₂ alloys were prepared by direct melting, quenching, and annealing. The Bi-Sb alloy phase was predominant in all samples. Pb or Sn atoms were distributed almost uniformly in Bi₈₈Sb_12, while some segregation was confirmed at the grain boundaries when Pb or Sn was involved heavily. The thermoelectric properties of these doped materials were investigated by measuring the Hall coefficient, electrical resistivity, and Seebeck coefficient between 20 K and 300 K. The Hall and Seebeck coefficients of Pb- or Sn-doped samples were positive at low temperatures, indicating that the doping element acted as an acceptor. Temperatures resulting in positive Hall and Seebeck coefficients further increased with increasing doping amount and with respect to the annealing process. As a result, a large power factor of 1.2 W/mK² could be obtained in the 3-at% Sn-doped sample at 220 K, with a large positive Seebeck coefficient.     

Complex impedance spectroscopic analysis of Mn-modified Pb(Zr0.65Ti0.35)O3 electroceramics

The polycrystalline samples of Pb(Zr_0.65−xMn_xTi_0.35)O₃ (PZMT) (x=0, 0.05, 0.10, 0.15) were prepared by a high-temperature solid-state reaction technique. Detailed studies on the effect of compositional variation of manganese (Mn) on the electrical behavior (complex impedance Z*, complex modulus M*, electrical conductivity and relaxation mechanisms) of the PZMT systems have been carried out by a nondestructive complex impedance spectroscopy (CIS) technique at 400 °C. The Nyquist plots suggest that the grains only are responsible in the conduction mechanism of the materials. The occurrence of single arc in the complex modulus spectrum of all the compositions of Mn confirms the single-phase characteristics of the PZMT compounds, and also confirms the presence of non-Debye type of multiple relaxation in the material.     

Studies of dielectric and impedance properties of KCa2V5O15 ceramics

A polycrystalline sample, KCa₂V₅O₁₅, with tungsten bronze structure was prepared by a mixed-oxide method at low temperature (i.e., at 630 °C). A preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound was studied by scanning electron microscopy (SEM). Two dielectric anomalies at 131 and 275 °C were observed in the temperature dependency of dielectric response at various frequencies, which may be attributed to the ferroelastic-ferroelectric and ferroelectric-paraelectric transitions, respectively. The nature of variation of the electrical conductivity, and value of activation energy of different temperature regions, suggest that the conduction process is of mixed-type (i.e., ionic-polaronic and space charge generated from the oxygen ion vacancies). The impedance plots showed only bulk contributions, and non-Debye type of relaxation process occurs in the material. A hopping mechanism of electrical transport processes in the system is evident from the modulus analysis. The activation energy of the compound (calculated both from loss and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers.     

Influence of Co, Ni and Zn substitution for Cu on the structural and superconducting properties of (Hg0.7Cr0.3)Sr2CuO4+δ

The effect of Co, Ni and Zn substitutions for Cu on the phase stability and superconducting properties of (Hg_0.7Cr_0.3)Sr₂CuO_4+δ was investigated. X-ray diffraction (XRD) revealed that both Co and Zn are soluble in the (Hg_0.7Cr_0.3)Sr₂CuO_4+δ material up to about 5% of the Cu content, whereas the solubility of Ni is extended up to 10%. Electrical resistivity and magnetic susceptibility measurements show that the value of the superconducting critical temperature T_c decreases linearly with the impurity content. The depression of T_c indicates that the suppression of the superconductivity in Co- and Ni-substituted samples is much stronger than that in Zn-substituted ones. The residual resistivity scales linearly with the doping level as expected from the impurity scattering due to disorder. Some possible explanations for the stronger suppression of T_c by the Co and Ni substitution than by Zn substitution are provided.     

Screening effect of Ti ions on the electrical conductivity and thermoelectric power of Mg ferrite

The samples Mg_1+xTi_xFe_2−2xO₄ were prepared in a single phase spinel structure as indicated from X-ray analysis. The preference of Mg²⁺ ions to the octahedral site decreases the ratio of the normal spinel in the investigated ferrite where the Mg²⁺ increases on the expense of the Fe³⁺ ions on the same site. The increase in the conductivity was found to be due to thermally activated mobility of charge carriers. The mobility data enhances the use of Verway model of conductivity which depends on the electron exchange between iron ions of different valences located on the same crystallographic sites. The existence of Ti⁴⁺ ions on the octahedral site screens the polarization and decreases the conductivity of the samples. Peculiar behavior was obtained for Ti content of 0.7 and 0.8 due to the presence of secondary phases.     

The impact of Y substitution on the 110 K high Tc phase in a Bi (Pb):2223 superconductor

The structural and superconducting properties of Bi_1.7Pb_0.3Sr₂Ca_2−xY_xCu₃O_y superconducting samples are investigated by X-ray diffraction (XRD), resistivity and thermoelectric power (TEP) measurements. XRD results reveal that the volume percentage of the 2223 high T_c phase decreases with an increase in Y content. The replacement of the Ca²⁺ ion by the Y³⁺ ion does not influence the tetragonal structure of the pure Bi (Pb): 2223 system and the lattice parameters vary with Y content. The results of resistivity indicate that the critical temperatures T_c of the samples decrease monotonically with an increase in Y content. Further, the critical concentration of Y to completely suppress superconductivity in the Y-doped Bi (Pb):2223 system is higher (0.60) than that reported (0.20) for the other rare-earth elements. On the other hand, the values of TEP at room temperature are found to be negative for Y=0.00 and 0.10 samples, and it changed to positive with further increase in Y content. The hole-carrier concentration per Cu ion (P) is deduced by using two different ways: the first in terms of T_c values in the superconducting state and the other in terms of TEP values in the normal state. Interestingly, it is found that the values of P deduced from the formal way are not consistent with the reported parabolic behavior for superconducting systems in the under-doped region, and consequently disagree with the general roles of substitution. However, the vice versa is recorded for the values of P deduced from the latter way. The results are discussed in terms of the possible reasons for the suppression of superconductivity in the considered system.     

Selected properties of EuCo2(Si1−xGex)2 alloys

EuCo₂(Si_1−xGe_x)₂, x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 samples were synthesised by induction melting followed by annealing at 900 °C and rapid quenching. X-ray powder diffraction and Auger electron spectroscopy studies revealed that solid solutions are formed only for x?0.2 and x?0.7. Magnetic susceptibility investigations for the solid solutions revealed a dominant divalent europium valence state in the germanium-rich samples and a dominant trivalent europium component in the silicon-rich samples. In the germanium-rich samples, a long-range antiferromagnetic ordering was observed. In all samples studied, additional magnetic transitions at various temperatures were detected, which could be attributed to small clusters containing different europium chemical surrounding from that in the predominant phase.     

The effect of sulfide substitution in the mixed conductor Cu7PSe6

Cu₇PSe₆ is a mixed conductor exhibiting structural phase transitions above and below room temperature that are accompanied by step-like changes in electrical conductivity. The substitution of S²⁻ for Se²⁻ in Cu₇PSe₆ significantly enhances electrical conductivity at room temperature compared to that observed for the pure compound. In the case of Cu₇P(Se_0.80S_0.20)₆, a nearly temperature-independent electrical conductivity exceeds 1 S/cm with no evidence of any phase transitions throughout the temperature interval 200-400 K. However, the ionic contribution accounts for just 2% of the total electrical conductivity in this solid solution at room temperature.