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 .     

Low-temperature thermoelectric characteristics of Ca3−xYbxCo3.95Ga0.05O9+δ (0 ≤ x ≤ 0.10)

Polycrystalline samples of Ca_3?xYb_xCo_3.95Ga_0.05O_9+δ (x = 0.00, 0.02, 0.05 and 0.10) have been prepared by conventional solid-state synthesis. The XRD results 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 electrical resistivity, thermopower increased and total thermal conductivity decreased with increasing Yb³⁺ content. Ca_2.95Yb_0.05Co_3.95Ga_0.05O_9+δ had the highest dimensionless figure of merit of 0.035 at 300 K, which is about 34.6 % higher than that of the undoped sample. These results suggested that the partial substitution of Yb³⁺ ion improves the thermoelectric properties of Ca₃Co₄O_9+δ.     

Low-temperature thermoelectric and magnetic characteristics of Ca2.9Bi0.1Co4−xFexO9+δ (0 ≤ x ≤ 0.10)

The effect of Fe ion doping on the low-temperature thermoelectric and magnetic properties of Ca_2.9Bi_0.1Co_4?xFe_xO_9+δ (x = 0.00, 0.025, 0.05 and 0.10) have been investigated. The samples were prepared by conventional solid-state synthesis. The X-ray diffraction patterns revealed that all the samples are single phase. The electrical resistivity results indicated that all the samples obey the variable range hopping in the low temperature regime. The T^* (transition temperature from Fermi liquid metal to incoherent metal) was increased and the slope of A value (Fermi-liquid transport coefficient) was decreased with increasing Fe content. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity, thermopower and total thermal conductivity were decreased with increasing Fe content. Among the doped samples, Ca_2.95Bi_0.10Co_3.90Fe_0.10O_9+δ had the highest dimensionless figure of merit of 0.056 at 300 K. Magnetic measurements indicated that all the samples exhibit a low-spin state of cobalt ion. The effective magnetic moments were decreased with increasing Fe content.     

Critical behavior in Fe-doped manganites La0.67Ba0.22Sr0.11Mn1−x Fe x O3 (0 ≤ x ≤ 0.2)

The critical properties of the manganese perovskite La_0.67Ba_0.22Sr_0.11Mn_1?x Fe _x O₃ (0 ≤ x ≤ 0.2) around the paramagnetic (PM)–ferromagnetic (FM) phase transition were investigated using modified Arrott plots and Kouvel–Fisher method based on the data of static magnetic measurements recorded around the curie temperature T _C. Based on the evaluated critical exponents for the compounds with different x values (0.378 ≤ β ≤ 0.411, 1.247 ≤ γ ≤ 1.393, and 4.018 ≤ δ ≤ 4.73) we concluded that the PM–FM phase transition in the compound with x = 0 belongs to the three-dimensional Heisenberg universality. However, the substitution of Fe³⁺ for Mn³⁺ resulted in shifting the value of γ toward the 3D-Ising model value and a slight increase in β. This behavior was discussed and correlated with the random distribution of Fe³⁺ at the Mn³⁺ sites and the consequent suppression the double exchange interaction.     

High-temperature thermoelectric properties of the sintered Bi2Sr2Ca1 − xYxCu2Oy (x = 0 – 1)

Electrical conductivity and Seebeck coefficient were measured in a temperature range of 320–1073 K for sintered samples of Bi₂Sr₂Ca_{1 – x} Y _x Cu₂O _y (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0). It has been found that the conduction behavior changes from n-type metallic to p-type semiconducting with increasing yttrium concentration. The power factors were in a range of 1.7–3.0 × 10^–5 Wm^–1 K^–2 for the sample with x = 0.8, being maximized by the optimization of the yttrium concentration. The thermal conductivity for the sample with x = 0.8 was 0.73 Wm^–1 K^–1 at 310 K, and decreased with increasing temperature. The values of thermoelectric figure of merit were estimated to be in a range of 3.4–4.8 × 10^–5 K^–1 at temperatures of 320–673 K for the sample with x = 0.8.     

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.     

Structural and optical properties of Cd1−xZnxS (0 ≤ x ≤ 0.3) nanoparticles

Cd_1?xZn_xS nanoparticles for Zn = 0–30 % were successfully synthesized by a conventional chemical co-precipitation method at room temperature. X-ray diffraction spectra confirmed the pure zinc blend cubic structure of undoped CdS; but Zn-doping on Cd–S matrix induced the mixed phases of cubic and hexagonal structure. The reduced crystal size, d-value, cell parameters and higher micro-strain at lower Zn concentration were due to the distortion produced by Zn²⁺ in Cd–S lattice. The enhancing diffraction intensity at lower Zn concentrations was due to the substitution of Zn²⁺ ions instead of Cd²⁺ ions whereas the reduced intensity after 20 % was due to the presence of Zn²⁺ ions both as substitutionally and interstitially in Cd–S lattice. The nominal stoichiometry and chemical purity was confirmed by energy dispersive X-ray analysis. The initial blue shift of energy gap from undoped CdS (3.75 eV) to Zn = 10 % (3.82 eV) was due to the size effect and also the incorporation of Zn²⁺ in the Cd–S lattice. The observed red shift of energy gap at higher Zn concentrations could be attributed to the improved crystallinity. The band gap tailoring was useful to design a suitable window material in fabrication for solar cells and other opto-electronic devices. The characteristic IR peaks around 617–619 cm^?1 and the reduced intensity by Zn-doping confirmed the presence of Zn in Cd–S lattice.     

Electrical transport properties of Nd0.67−x Eu x Sr0.33MnO3 (0 ≤ x ≤ 0.67) manganites

A systematic investigation of electrical transport properties viz., electrical conductivity and thermopower of Eu-doped Neodymium-based colossal magnetoresistive manganites with compositional formula, Nd_0.67−x Eu _x Sr_0.33MnO₃ (x = 0–0.67) has been undertaken. These materials were prepared by citrate gel route and characterized by X-ray diffraction, scanning electron microscopy, AC susceptibility, and electrical resistivity measurements. With a view to understand the complex conduction mechanism of these materials, electrical resistivity and thermoelectric power (TEP) data have been analyzed using various theoretical models. It has been concluded that the ferromagnetic metallic part of the conduction mechanism is explained by grain/domain boundary, electron–electron, and magnon scattering mechanisms, while that of high temperature paramagnetic insulating region might be due to small polaron hopping mechanism. The sign change of charge carriers observed in TEP measurements is attributed to the oxygen deficiency of the samples.     

Magnetocaloric Effect in Nanopowders of Pr0.67Ca0.33Fe x Mn1−x O3

Magnetocaloric effect in nanopowders of Pr_0.67Ca_0.33Fe _x Mn_1?x O₃ (x=0,0.1,0.3,0.4), in the vicinity of magnetic phase transitions, was investigated. It is shown that Pr_0.67Ca_0.33MnO₃ exhibits the largest magnetic entropy change (ΔS _M ) of 0.61 J/kg/K at 208 K upon 0.5 T magnetic field variation. Furthermore, the ΔS _M distribution of the Pr_0.67Ca_0.33Fe _x Mn_1?x O₃ is much more uniform than that of gadolinium. Because of these results, nanopowders of Pr_0.67Ca_0.33Fe _x Mn_1?x O₃ have some potential applications for magnetic refrigerants in an extended high-temperature range. Moreover, it can be used as a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.     

Behavior of BaCe0.9−x Zr x Y0.1O3−δ in water and ethanol suspensions

The stability of perovskite proton conductors BaCe_0.2Zr_0.7Y_0.1O_2.95 and BaCe_0.5Zr_0.4Y_0.1O_2.95 (referred to as BCZY) has been tested by exposure of powders to water and ethanol at room temperature. The liquid phase was monitored as a function of time with inductively coupled plasma-optical emission spectrometry. With water, leaching of barium continued for over 200 days, the duration of the test. There was no systematic difference in leaching with solutions of native pH or alkalified with NH₄OH. The powders, before and after treatment, were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM confirmed the degradation of the perovskite phase and revealed barium carbonate nanobranches and nanoparticles from the agglomeration of the fragmented perovskite. The effect of ethanol was much less severe; leaching of elements after immersion was negligible for up to 17 days, and XRD patterns before and after treatment did not show any structural degradation. It is concluded that ethanol is an appropriate choice as a processing solvent for tapecasting BCZY powders.     

Normal and Anomalous Pseudogap of Superconducting Y0.9−x Pr x Ca0.1Ba2[Cu1−y Zn y ]3O7−δ

Electrical resistivity measurements on the superconducting oxides of the compositions Y_0.9−x Pr _x Ca_0.1Ba₂[Cu_1−y Zn _y ]₃O_7−δ (0≤x≤0.20 and 0.0≤y≤0.10) sintered in oxygen atmosphere were carried out to obtain the normal and anomalous pseudogaps in underdoped and overdoped samples. It is observed that pseudogap temperature T ^* decreases with increasing doping level p in the underdoped case. For the overdoped sample with y=0.06, T ^* shows no p dependence.      

Composition range and physicochemical properties of La1 − x Ba x Mn1 − y Fe y O3 solid solutions

We have determined the extent of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ solid solutions with orthorhombically and rhombohedrally distorted perovskite structures. A partial phase diagram of the LaMnO_{3 + δ}-BaMnO₃-BaFeO_2.5-LaFeO₃ system in air at a temperature of 1373 K has been proposed for the first time. We have measured the relative length change of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ samples and calculated their thermal expansion coefficients.     

Multiferroic Properties of (Bi0.9Gd0.1FeO)1−x (BaTiO3) x Ceramics

Conventional solid-state reaction method has been employed for the synthesis of polycrystalline (Bi_0.9Gd_0.1FeO)_1?x (BaTiO₃) _x for x=0.1, 0.2 and 0.3, ceramics samples. The effect of BaTiO₃ content on the multiferroic properties of Gd-doped BiFeO₃ ceramics has been presented. Pure perovskite phase with high density has been obtained by optimizing the synthesis approach, calcination and sintering strategies. Structural analysis carried out using X-ray diffraction confirms the formation of desired morphotropic phase. The dielectric properties have been investigated at different concentration of BaTiO₃ as function of temperature, revealing that by increasing the BaTiO₃ content dielectric constant increases while dielectric losses decrease. Magnetic study shows that initially saturation magnetization increases with increase in BaTiO₃ content up to x=0.1; however, afterwards it decreases for higher concentration of BaTiO₃. According to ferroelectric measurements, PE loops (with low coercive field) are observed at room temperature. The remnant polarization (P _r ) has been found to be 0.169, 0.619 and 0.760 μC/cm², respectively, for samples with x=0.1, 0.2 and 0.3. Magnetoelectric coupling in as-synthesized samples has been indirectly deduced by an anomaly observed at magnetic transition temperature.     

Investigation of wide range magnetoresistance in La0.7Ca0.3−xAgxMnO3 (0 ≤ x ≤ 0.2) system

We synthesized magnetoresistive compounds with nominal compositions La_0.7Ca_0.3?xAg_xMnO₃ (0 ≤ x ≤ 0.2) by solid state reaction method with different sintering temperatures. All the samples exhibited orthorhombic structure with variable lattice parameters. The metal-insulator transitions were observed below room temperature in most of the samples. Depending upon the composition and sintering temperature, the samples showed very high magnetoresistance in narrow range or significant magnetoresistance over wide range of temperature. The ac susceptibility measurement revealed compositional dependence of Curie temperature.