High-temperature thermoelectric properties of Ca0.9Y0.1Mn1−x Fe x O3 (0 ≤ x ≤ 0.25)

Polycrystalline compounds of Ca_0.9Y_0.1Mn_{1? x} Fe _x O₃ for 0 ≤ x ≤ 0.25 were prepared by solid-state reaction, followed by spark plasma sintering process, and their thermoelectric properties from 300 to 1200 K were systematically investigated in terms of Y and Fe co-doping at the Ca- and Mn-sites, respectively. Crystal structure refinement revealed that all the investigated samples have the O′-type orthorhombic structure, and the lattice parameters slightly increased with increasing Fe concentration, causing a crystal distortion. It was found that with increasing the content of Fe doping, the Seebeck coefficient of Ca_0.9Y_0.1Mn_{1? x} Fe _x O₃ tended to increase, while the tendency toward the electrical conductivity was more complicated. The highest power factor was found to be 2.1 × 10^?4 W/mK² at 1150 K for the sample with x = 0.05 after annealing at 1523 K for 24 h in air. Thermal conductivity of the Fe-doped samples showed a lower value than that of the x = 0 sample, and the highest dimensionless figure of merit, ZT was found to be improved about 20 % for the sample with x = 0.05 as compared to that of the x = 0 sample at 1150 K.     

Low-temperature thermoelectric and magnetic properties of Ca3−x Bi x Co4O9+δ (0 ≤ x ≤ 0.30)

Polycrystalline samples of Ca_3?x Bi _x Co₄O_9+δ (x = 0.00, 0.05, 0.10, 0.15, 0.20 and 0.30) have been prepared by conventional solid-state synthesis. Thermopower of all the samples is positive, indicating that the predominant carriers are holes over the entire temperature range. The resistivity of all the samples, except the one with x = 0.30, exhibits nonmetal to metal transition (T _MI) in the low temperature regime. The resistivity results indicate that all the doped samples obey the variable range hopping in the low temperature regime. The T _MI and T ^* (transition temperature from Fermi liquid metal to incoherent metal) increase, and the slope of A value (Fermi-liquid transport coefficient) decreases with the increasing Bi content due to an increase in chemical pressure in the lattice. Among the samples, Ca_2.7Bi_0.3Co₄O_9+δ has the highest dimensionless figure of merit of 0.091 at 300 K. This value represents an improvement of about 135 % compared to the undoped Ca₃Co₄O_9+δ . Magnetic measurements indicate that all the samples exhibit a low-spin state of cobalt ion. The ferrimagnetic transition temperature is suppressed by the Bi dopant. These results suggest that Bi is an effective doping element for improving the thermoelectric properties of Ca₃Co₄O_9+δ .     

Synthesis and thermoelectric characterization of polycrystalline Ni1−x Ca x Co2O4 (x=0–0.05) spinel materials

Ca doped NiCo₂O₄ spinel materials were synthesized by conventional solid state reactions at 900 °C. Thermoelectric properties of polycrystalline products were characterized at high temperature range of 800 °C in air. d.c. conductivity of the prepared polycrystalline 5 mol % Ca doped NiCo₂O₄ was about 60 S m^–1 at 300 °C. The value of d.c. conductivity was increased with the temperature increasing. Thermoelectric voltage of polycrystalline Ni_1–x Ca _x Co₂O₄ (x=0–0.05) was positive at 300–800 °C, this showed p-type thermoelectric properties. The Seebeck coefficient of 5 mol % Ca doped NiCo₂O₄ was ca. 300 V/K at 600 °C. The value of the Seebeck coefficient of Ni_1–x Ca _x Co₂O₄ polycrystalline products decreased with the increasing temperature. Thermal conductivity of 5 mol % Ca doped NiCo₂O₄ was ca. 2.2 W m^–1 K^–1 at 600 °C. The estimated thermoelectric figure-of-merit, Z, of 5 mol % Ca doped NiCo₂O₄ spinel polycrystalline product was about 3.5×10^–5 K^–1 at 600 °C.     

Ni-doping effect on the anisotropic resistivity in Bi2Sr2Ca(Cu1−xNix)2Oy single crystals

When Ni is substituted for Cu in Bi-2212 single crystals, the in-plane resistivity is increased with maintaining the linear temperature dependence while the out-of-plane resistivity is decreased. The anisotropy parameter estimated from the angular dependence of the flux flow resistivity is also decreased from 250 for undoped sample to 80 for a sample doped with 2.4% of Ni. The doping dependence of _ab and _c is discussed on the basis of the t-J model and the Fermi liquid theory.     

Crystalline structure and dielectric properties of (Sr1−1.5x Bi x ) TiO3 ceramics

The crystalline structure, microstructure and dielectric properties of the (Sr_1–1.5x Bi _x )TiO₃ (0 x 0.267) ceramics were studied. Cubic solid solutions were determined for x 0.2 at room temperature. However, lattice distortion was detected by Raman spectra. A dense microstructure with the grain sizes of 2–4 m was obtained for (Sr_1–1.5x Bi _x )TiO₃ (0 x 0.2) ceramics. The Bi concentration was examined and found to be in agreement with the nominal composition and overall uniformly distributed in the sample. Different from the observations in the earlier literature for other doped quantum paraelectrics, where only an induced dielectric anomaly was reported, there are three Bi induced dielectric modes A, B, and C in the Bi doped SrTiO₃ samples. The occurrence of the impurity modes and the ferroelectric relaxor mode and their evolution are demonstrated as a function of Bi concentration.     

Preparation and thermoelectric properties of β-Fe1−x Ru x Si2

-Fe_1–x Ru _x Si₂ solid solution was synthesized by solid state reaction at 1100 °C for 48 h and subsequent annealing at 850 °C for 168 h in an evacuated silica tube. Single phase solid solution was obtained in the composition range 0 x 0.1. The thermal stability range of the -phase is extended to higher temperature region by partial substitution of Ru atom. The thermoelectric properties of Cr or Co-doped -Fe_1–x Ru _x Si₂ strongly depend on the sintering conditions. The samples with optimum thermoelectric properties are obtained by high-pressure sintering at 3 GPa and 800 °C for 1 h. The optimum compositions are found to be Fe_0.92Ru_0.05Cr_0.03Si₂ and Fe_0.92Ru_0.05C_0.03Si₂ for p-type and n-type materials, respectively. The power factors (²) of these materials are higher than that of -FeSi₂ based materials.     

Dielectric studies of Ca x Sr1−x F2 mixed crystals

The dielectric constant (K) and loss (K) of CaF₂, SrF₂ and their mixed crystals as a function of frequency in the range 10² to 10⁵ Hz at room temperature were measured. Except at 10² Hz, the value of the dielectric constant was found to be independent of frequency. At any frequency the value ofK for mixed crystals was found to vary non-linearly as a function of composition, showing a maximum at about equimolar composition. The value ofK was less in mixed crystals compared to the end members. An attempt is made to explain the results.     

Dielectric properties of the Sr1−x La x Sn1−x Co x O3 system

The possibility of the formation of a solid solution in the Sr_1–x La _x Sn_1–x Co _x O₃ system has been explored. Single-phase solid solution forms in the compositions for x0.10. All single-phase solid solution compositions have a cubic structure similar to SrSnO₃. The dielectric behaviour of these solid solution compositions has been studied as a function of temperature and frequency. The frequency dependence of dielectric constant and dielectric loss in these materials indicates that space charge polarization contributes significantly to their observed dielectric parameters. Microstructural studies show the presence of well-faceted grains. The average grain size in these samples is small.     

High-temperature thermoelectric properties of Ca3−xPrxCo3.95Ga0.05O9+δ

Ca₃Co₄O_9+δ and Ca_3?xPr_xCo_3.95Ga_0.05O_9+δ (x = 0.02, 0.05 and 0.10) samples were prepared by conventional solid-state synthesis. The thermoelectric (TE) properties were measured at 300–750 K. The XRD patterns revealed that all the samples are single phase. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The power factor of Ca_2.9Pr_0.10Co_3.95Ga_0.05O_9+δ was improved about 52.6 % compared to Ca₃Co₄O_9+δ at 300 K. Ca_2.9Pr_0.10Co_3.95Ga_0.05O_9+δ had the highest dimensionless figure of merit of 0.030 at 300 K, representing an improvement of about 50 % compared to Ca₃Co₄O_9+δ. These results indicate that partial substitution of Pr³⁺ ion improves the TE properties of Ca₃Co₄O_9+δ.     

Enhancement of thermoelectric power in layered Bi2Sr2Co2−x Ir x O y single crystals

We report on the structural, electrical, and thermal transport properties of the single crystals of Bi₂Sr₂Co_2?x Ir _x O _y (0 ≤ x ≤ 0.2). Large-sized (centimeter-level) and good-quality single crystals were grown by a modified flux method. The substitution of Ir ions for Co ones makes the in-plane resistivity ρ _ab increase monotonically, whereas, the in-plane thermopower S _ab initially increases and then decreases as x is larger than 0.1. Therefore, among all samples, the in-plane power factor P _ab (=S _ab ² /ρ _ab) of Bi₂Sr₂Co_1.9Ir_0.1O _y would reach a maximum value of 116.4 μWm^?1K^?2 at 300 K and shows a significant improvement of about 20 % compared to that of the parent sample. Our results indicate that an appropriate doping of Ir ions at Co-sites may provide an effective way to enhance the thermoelectric performance of Bi₂Sr₂Co₂O _y system.     

Microwave dielectric properties of (Ca0.8Sr0.2)(SnxTi1−x)O3 ceramics

In this paper, we study the behavior of the B-site behavior with the incorporation of Sn⁴⁺ ion in (Ca_0.8Sr_0.2)TiO₃ ceramics. An excess of Sn⁴⁺ resulted in the formation of a secondary phase of CaSnO₃ and SrSnO₃ affecting the microwave dielectric properties of the (Ca_0.8Sr_0.2)(Sn_xTi_1?x)O₃ ceramics. The dielectric properties of the (Ca_0.8Sr_0.2)(Sn_xTi_1?x)O₃ ceramics were improved because of the solid solution of Sn⁴⁺ substitution in the B-site. The temperature coefficient of resonant frequency (τ_f) of the (Ca_0.8Sr_0.2)(Sn_xTi_1?x)O₃ ceramics also improved with increasing Sn content.     

Crystal structure and anisotropy of iodine-intercalated Bi2Sr2Ca n−1Cu n O x

The electronic state and structural configuration of the intercalated iodine species in stage-1, I-Bi₂Sr₂Ca _n?1Cu _n O _x (n = l, 2), have been studied through polarization-resolved Raman and¹²⁹I Mössbauer spectroscopy. The polarization dependence of the Raman spectra and the Mössbauer measurement confirmed the dominant species to be triiodide ions, I ₃ ^? , with alignment of these linear molecules either along thea- orb-axis in the host crystals. Transport measurements such as thermoelectric power and Hall coefficient clearly indicated that hole carriers are doped into the CuO₂ planes upon intercalation, by whichT _c of the host superconductor is changed. Furthermore, based on resistivity measurements in a magnetic field, we suggest that the iodine intercalation leads to a decrease of the anisotropy both in normal and superconducting states, suppressing the extremely two-dimensional character of the Bi₂Sr₂Ca _n?1Cu _n O _x systems.     

Thermodynamic properties of Ca1 − x Er x F2 + x and Ca1 − x Yb x F2 + x heterovalent solid solutions

The heat capacity of single crystals of the Ca_{1 ? x} Er _x F_{2 + x} (x = 0.05, 0.10) and Ca_0.95Yb_0.05F_2.05 fluorite solid solutions was determined by adiabatic calorimetry in the temperature range 55–300 K. The results were used to obtain temperature dependences of the Debye characteristic temperature, entropy, and enthalpy for the solid solutions.     

Dielectric properties of the system Ca1−xYxTi1−xCoxO3(0.00 ⩽ x ⩽ 0.15)

The dielectric behaviour of the compositions withx0.15 in the system Ca_1–x Y _x Ti_1–x Co _x O₃ sintered and cooled in air has been studied. Space-charge polarization which arises due to the presence of chemical inhomogeneities at the micro-level contributes significantly to their dielectric constant. The composition with x=0.05 exhibits temperature and frequency-independent dielectric constant and very small dielectric loss.     

Activation Energy of Bi2Sr2Ca1−x Er x Cu2O8+d Epitaxial Thin Films

The scaling behavior of the effective activation energy U(T, H) of epitaxial oriented Bi₂Sr₂Ca_1–x Er _x Cu₂O_8+d thin films with 0x0.10 has been studied as function of temperature and magnetic field. It has been found that the activation energy scales as U(H, T)=U ₀(1–t)^m H ⁿ with exponent m=1.20±0.03 and n=–1/2. The field and temperature dependence of the activation energy, as well as its overall magnitude, is consistent with a model in which U(T, H) arises from plastic deformations of viscous flux liquid above the vortex-glass transition temperature.     

Comment on the magnetic properties of the system Sr2−xCaxFeMoO6, 0≤x≤2

The structural and magnetic properties of the n-type-metallic double-perovskite system Sr_2−xCa_xFeMoO₆ are reported. At 5 K, the saturation magnetization remains M_s(5 K)=3.5 μ_B/formula unit compared to a theoretical spin-only moment of 4.0 μ_B/formula unit for a perfectly ordered compound. From the shape of the M–H hysteresis loops below T_c, it is suggested that ferromagnetic long-range ordered domains are coupled antiferromagnetically across antiphase boundaries; random disorder within domains may be small. An unprecedented dependence of the paramagnetic susceptibility on the applied magnetic field is reported and attributed to an overlapping Fe³⁺/Fe²⁺ and Mo⁶⁺/Mo⁵⁺ redox couple, first suggested by Sleight and Weiher [Sleight AW, Weiher JF. J. Phys Chem Solids 1972; 33: 679.], which results in localized S=5/2 spins on the iron atoms coexisting with itinerant minority-spin electrons in a narrow π* band.     

Influence of KClO3 Addition on the Magnetic Properties and Microstructural Morphology of Y(1−0.2x)Ba(2−0.2x)KxCu3Oy (x = 0 ÷ 0.40) Polycrystalline HTSC

The influence of KclO ₃ addition on the AC complex susceptibility (ac) and microstructural morphology of YBCO HTSC with nominal composition Y _(1–0.2x) Ba _(2–0.2x) K _x Cu ₃ O _y (x = 0 ÷ 0.40) were investigated. Xac was recorded as a function of temperature and of the concentration of KClO ₃ in various AC magnetic fields up to 19 Oe. The data were analyzed using the relation h = a(1 – T _m /T _c ) ⁿ . The results obtained showed that the presence of K- and Cl-containing impurities on the grain boundaries change the intergranular coupling and, within certain concentration limits, improves the superconducting parameters.     

Thermoelectric properties of Bi1−xSbx films with 0 < x ⩽ 0.3

The electrical conductivity, its temperature coefficient and the thermoelectric power of Bi_1−xSb_x films with 0 < x ⩽ 0.3 and thicknesses from 20 to 400 nm were measured in the temperature range 80–400 K. The results are discussed in the framework of a previously proposed anisotropic non-degenerate two-band model.     

Structural and optical properties of Bi x Se1−x films

Bi _x Se_1–x thin films have been studied because of their structural and optical properties with a view to judging their suitability as the recording medium in phase-change type optical recording. Amorphous films deposited at room temperature were crystallized by thermal annealing. X-ray diffraction analysis and surface morphological studies are reported. A maximum reflectivity difference of 25% at =830 nm was obtained upon amorphous-to-crystalline transition. The optical constants calculated by the Newton-Raphson method using the experimental transmittance, reflectance and thickness data are reported.     

Temperature dependence of elastic and dielectric properties of (Bi2O3)1 − x(CuO)x oxide glasses

Oxide glasses with the general formula (Bi₂O₃)_{1 - x}(CuO)_x have been prepared by quick quenching technique. Their longitudinal and shear elastic moduli have been determined by measuring the corresponding ultrasonic wave velocities between 300 and 470 K, which are well below the glass transition temperature of this system. Temperature variation of ultrasonic velocity and attenuation exhibit anomalies around 435 K in glasses with x 0.3. A nonlinear behaviour is also reflected in the CuO concentration dependent dielectric constant curve around x = 0.3. These anomalies are interpreted in terms of a structural softening (or transformation) taking place in samples having CuO concentration above the critical value. The high dielectric constant of these glasses show very little increase with increase of temperature. Anomalies are also found in the temperature dependence of dielectric constant around 435 K. This behaviour is again considered to be associated with the softening of the glass network.