Effect of the sintering temperature on the properties of nanocrystalline Ca1−xSmxMnO3 (0 ≤ x ≤ 0.4) powders

Nanocrystalline Ca_1−xSm_xMnO₃ (0 ≤ x ≤ 0.4) manganites were prepared by a soft chemical method (Pechini method) followed by auto-combustion and sintering in air at 1073 or 1473 K. Single-phase powders with general composition Ca_1−xSm_xMnO₃ were obtained after 18 h annealing. The particle and grain sizes of the substituted Sm-manganites did not exhibit variation with samarium content, but increase with increasing the sintering temperature. All manganites show two active IR vibrational modes near 400 and 600 cm⁻¹ characteristic of the BO₆ octahedron vibrations.For the samples sintered at T_s = 1473 K, the partial substitution of calcium by samarium in the CaMnO₃ phase induces a marked decrease in the electrical resistivity, in the temperature range of 300-900 K, and at the same time a metal-to-insulator transition occurs; for T_s = 1073 K all the samples present semiconductor behaviour. With the increase of the annealing temperature the grain size increases and a metal-semiconductor transition appears. The results can be ascribed to the Mn⁴⁺/Mn³⁺ ratio and particle grain size. The effects of particle size on the electrical properties can be attributed to the domain status, changes in the Mn-O-Mn bond angle and Mn-O bond length.     

Crystal structure and magnetic properties of DyCuxGa2−x (x = 0-2.0) antiferromagnetic compounds

DyCu_xGa_2−x (x = 0-2.0) compounds have been synthesized; meanwhile, their crystal structure and magnetic properties have been investigated by X-ray diffraction and magnetic measurements. The result shows that the continuous solid-solution series crystallize in three phases, with the structure types of AlB₂ (x = 0-0.2), DyCuGe (x = 0.3-0.6) and CeCu₂ (x = 0.7-2.0), respectively. The main reason to form the three structure types is considered to be the average atomic radius ratio of R to Cu/Ga. Magnetic-ordering transition of the compounds with x = 0.2-0.6 takes place at about 20 K and 113 K, while those of other compounds only takes place at about 20 K, which is attributed to the change of the near Dy-Dy distances and the ordered substitution of Ga by Cu.     

XPS study of Pr1−xCaxMnO3 (x = 0.2, 0.33, 0.4 and 0.84)

We have studied the Mn 2p, Ca 2p, and Pr 4d core levels of Pr_1-xCa_xMnO₃ (x = 0.2, 0.33, 0.4 and 0.84) as a function of x using X-ray photoelectron spectroscopy both at room temperature as well as 77 K. Suppression of chemical potential shifts have been observed at 77 K compared to that of room temperature spectra. We have discussed this result by considering the concept of phase separation.     

Effect of Ca-doping on the structural and electrical properties of CuY1−xCaxO2 (0 ≤x ≤0.10) ceramics

Delafossites CuY_1−xCa_xO₂ (0 ≤x ≤0.10) ceramics have been prepared by solid state reaction using Cu₂O, Y₂O₃ and CaCO₃. Liquid phase sintering, which obviously accelerates the reaction speed of Cu₂O-Y₂O₃-CaCO₃ system and promotes the formation of CuYO₂ phase is evidenced for the Ca-doped samples. During the sintering process, CuO can react with CaO to form two intermediate compounds, CaCu₂O₃ and Ca₂CuO₃, which decompose into CaO and liquid phase during 1273-1323 K. In the dopant range of 0 ≤x ≤0.10, both electrical conductivity and density of the samples are increased by Ca-doping. The room temperature conductivity of CuY_0.94Ca_0.06O₂ is more than four orders of magnitude higher than that of CuYO₂.     

Thermoelectric properties of Cu1−xPtxFeO2 (0.0 ≤ x ≤ 0.05) delafossite-type transition oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite were synthesized by solid state reaction method for studying thermoelectric properties. The properties of Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the high temperature ranging from 300 to 960 K. The results of Seebeck coefficient, electrical conductivity and power factor were increased with increasing Pt substitution and temperature. The thermal conductivity was decreased from 5.8 to 3.5 W/mK with increasing the temperature from 300 to 960 K. An important results, the highest value of power factor and ZT is 2.0 × 10⁻⁴ W/mK² and 0.05, respectively, for x = 0.05 at 960 K.     

Phase range and physical properties of the thallium tin tellurides Tl10−xSnxTe6 (x ≤ 2.2)

Crystal structures and physical property measurements were determined for Tl_10−xSn_xTe₆ with a phase range of 0 ≤ x ≤ 2.2. These tellurides are substitution variants of Tl₅Te₃. Electronic structure calculations indicate that Tl₈Sn₂Te₆ should be an intrinsic semiconductor, and the Sn-poor variants, extrinsic ones with p-type conduction. The positive Seebeck values increase with increasing Sn content, while the electrical and thermal conductivity values decrease. Low thermal conductivity values, well below 1 W m⁻¹ K⁻¹, are the best asset of these materials with respect to thermoelectric performance. At x = 2.2, the best thermoelectric properties were obtained, with a figure-of-merit ZT = 0.60 at 617 K as determined on sintered cold-pressed pellets.     

Room temperature electroresistance in Sr2−xGdxMnTiO6 perovskites (0 ≤ x ≤ 1)

We report on the room temperature strong (∼80%) electroresistance (ER) in the double perovskite with mixed Mn valence: Sr_2−xGd_xMnTiO₆, 0 ≤ x ≤ 1. Both, continuous and pulsed current-voltage curves are almost identical which indicates that the observed electroresistance is not associated with heating. This is also supported by simultaneous temperature measurements. ER is negligible (absent) in the x = 0 compound and increases with the increase of Gd content ‘x’. The amplitude of ER has a maximum for x = 0.75, suggesting that ER is determined by both the double exchange and the Mn³⁺ concentration. At the same time, magnetic interactions change from the antiferromagnetic (x = 0) to ferromagnetic ones as x → 1, thus linking the ER with ferromagnetism.     

Synthesis and thermoelectric properties of the PbSe1−xTex alloys

The PbSe_1−xTe_x alloys with x = 0.2, 0.3, 0.5, 0.85 and 1.0 were prepared by induction melting, ball milling and spark plasma sintering techniques. The thermoelectric properties of the samples were investigated. The XRD analysis indicated that all samples are NaCl-type structure solid solutions Pb(Se,Te) containing nanograins. Increasing Te content resulted in increasing the lattice parameter a. The thermoelectric measurements show that all samples are n-type semiconductors in temperature range from 300 K to 673 K. The electrical resistivity of the doped sample is much smaller than that of pure PbSe, but comparable to that of PbTe. The absolute Seebeck coefficients for the doped sample PbSe_1−xTe_x with x = 0.2, 0.3 and 0.5 range from 150 μV/K at 300 K to 250 μV/K at 673 K, which is much larger than that of pure PbSe (66-138 μV/K), but smaller than that of PbTe (230-310 μV/K) in the same experimental conditions. The thermal conductivity for the doped sample PbSe_1−xTe_x with x = 0.2, 0.3 and 0.5 range from 0.95 to 0.66 W/m K, which is much smaller than that of pure PbSe (2.1-1.3 W/m K) or PbTe (1.4-1.1 W/m K). As a result, the figure of merit for the doped sample can be enhanced. The maximum dimensionless figure of merit ZT of 1.15 was obtained in the sample PbTe_0.5Se_0.5 at 573 K, more than 50% higher than that of pure PbTe prepared in the same condition.     

Studies on the power factor of (Ba,Sr)Co2+xRu4−xO11 compounds

We have prepared polycrystalline single-phase ACo_2+xRu_4−xO₁₁ (A = Sr, Ba; 0 ≤ x ≤ 0.5) using the ceramic method and we have studied their structure, electrical resistivity and Seebeck coefficient, in order to estimate their power factor (P.F.). These layered compounds show values of electrical resistivity of the order of 10⁻⁵ Ωm and their Seebeck coefficients are positive and range from 1 μV K⁻¹ (T = 100 K) to 20 μV K⁻¹ (T = 450 K). The maximum power factor at room temperature is displayed by BaCo₂Ru₄O₁₁ (P.F.: 0.20 μW K⁻² cm⁻¹), value that is comparable to that shown by compounds such as SrRuO₃ and Sr₆Co₅O₁₅.     

Preparation and electrical properties of perovskite ceramics in the system BaBi1−xSbxO3 (0 ≤ x ≤ 0.5)

The effect of the composition on the electrical properties of BaBi_1−xSb_xO₃ (0 ≤ x ≤ 0.5) negative temperature coefficient (NTC) thermistors was studied. Major phases present in the sintered bodies of BaBi_1−xSb_xO₃ (0 < x < 0.5) ceramics were BaBi_0.5Sb_0.5O₃ compounds with a rhombohedral structure and BaBiO₃ compounds with a monoclinal structure. Most pores were located in the grains of BaBiO₃ and BaBi_0.5Sb_0.5O₃ ceramics. It was apparent that the ρ₂₅ and B_25/85 constant of the thermistors increased with increasing Sb content.     

Effect of Ca on the microstructure and magnetocaloric effects in the La1−xCaxFe11.5Si1.5 compounds

The effect of Ca on the microstructure and magnetocaloric effects has been investigated in the La_1−xCa_xFe_11.5Si_1.5 (x = 0, 0.1, 0.2 and 0.3) compounds. The introduction of Ca leads to the appearance of minor α-Fe and Ca-rich phases, which affects the actual compositions of the main phases for the Ca containing samples. With increasing the Ca concentration, the Curie temperature T_C increases from 183 to 208 K, and the maximum magnetic entropy changes |ΔS| at the respective T_C with a magnetic field change from 0 to 5 T are 21.3, 19.5, 16.9, and 11.2 J/kg K for x = 0, 0.1, 0.2, and 0.3, respectively. The nature of the magnetic transition changes from first-order to second-order with an increase in Ca concentration, which leads to a reduction of the hysteresis and a decrease of the magnetic entropy change. However, the relative cooling power for La_1−xCa_xFe_11.5Si_1.5 compounds remains comparable with or even larger than that of other magnetocaloric materials over a wide temperature range. The higher T_C and the smaller hysteresis in comparison with those of the parent compound suggest that the La_1−xCa_xFe_11.5Si_1.5 compounds could be suitable candidates for magnetic refrigerants in the corresponding temperature range.     

Infrared emissivities and microwave absorption properties of perovskite Sm0.5Sr0.5Co1−xFexO3 (0 ≤ x ≤ 0.5)

Sm_0.5Sr_0.5Co_1−xFe_xO₃ (0 ≤ x ≤ 0.5, SSCF) with perovskite-type structure has been successfully prepared by conventional solid-state reaction as a microwave and infrared multi-functional material. The effects of Fe incorporation on the structure, electrical conductivity, infrared emissivity and microwave-absorbing properties were investigated in detail. XRD results have shown that the perovskite structure of SSCF has an orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5, respectively. The incorporation of Fe in SSCF could contribute to the decrease of electrical conductivity, while the infrared emissivities are increased. Moreover, microwave-absorbing properties in the frequency range of 2-18 GHz at room temperature are sensitive to Fe content. The complex permittivity, complex permeability and electromagnetic loss tangent have suddenly a step change at a certain frequency and the step-change frequency position moves slightly to lower frequencies with Fe increased. The optimal reflection loss calculated from the measured permittivity and permeability is 29.33 dB at 7.97 GHz with a thickness of 2.0 mm.     

Anisotropy compensation and high low-field magnetostriction of epoxy/Tb1−xHox(Fe0.8Co0.2)2 composites (0.60 ≤ x ≤ 1.0)

The anisotropy compensation and magnetostrictive properties of Tb_1−xHo_x(Fe_0.8Co_0.2)₂ (0.60 ≤ x ≤ 1.0) alloys have been investigated. The easy magnetization direction (EMD) at room temperature rotates from the 〈1 1 1〉 axis (x ≤ 0.75) to the 〈1 0 0〉 axis (x ≥ 0.90) through an intermediate state 〈1 1 0〉, subjected to the anisotropy compensation between Tb³⁺ and Ho³⁺ ions. Composition anisotropy compensation is realized near x = 0.75. The Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ alloy has a minimum anisotropy and a large spontaneous magnetostriction coefficient λ₁₁₁ (≈740 ppm) at room temperature. The strong 〈1 1 1〉-oriented 1-3 epoxy-bonded composite has been fabricated by curing under a moderate magnetic field. A high low-field magnetostriction of about 400 ppm at 3 kOe is obtained for the 1-3 epoxy/Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ composite with 40-vol% alloy particles, which can be attributed to the low magnetic anisotropy, EMD lying along 〈1 1 1〉 direction, the strong 〈1 1 1〉-textured orientation and the chain structure.     

Hydrothermal synthesis of superconductors Ba1−xKxBiO3 and double perovskites Ba1−xKxBi1−yNayO3

Superconductors Ba_1−xK_xBiO₃ and body-centered double perovskites Ba_1−xK_xBi_1−yNa_yO₃ have been selectively synthesized by a facile hydrothermal route. The appropriate ratio and adding sequence of initial reagents, alkalinity, reaction temperature and time are the critical factors that influence the crystal growth of the compounds. The purity and homogeneity of the crystals were detected by the ICP, SEM, EDX and TEM studies. Magnetic measurements show that the superconducting transition temperatures T_C of Ba_1−xK_xBiO₃ decrease from 22 K (for x = 0.35) to 8 K (for x = 0.55) with increasing the K doping level.     

Near room temperature magnetic entropy changes in as-cast Gd100−xMnx (x = 0, 5, 10, 15, and 20 at.%) alloys

A series of Gd_100−xMn_x (x = 0, 5, 10, 15, and 20 at.%) alloys were prepared by arc-melting. The Curie temperature (T_C) associated with the ferromagnetic-paramagnetic transitions, derived from M-T curves, show decrease in T_C for as-cast alloys (∼279 K) as compared to as-cast Gd (∼292 K). No appreciable decrease in the |ΔS_M|_max values ∼4.6 J/kg K (0-2 T) and ∼8.6 J/kg K (0-5 T) were observed upon alloying Gd with Mn up to x ≤ 15 at.%. Refrigerant capacity (q) showed negligible variation ∼195 J/kg (0-2 T) and ∼450 J/kg (0-5 T) with increasing Mn (up to x ≤ 15 at.%) content. Similar values of |ΔS_M|_max and q coupled with ∼13 K decrease in T_C for as-cast Gd_100−xMn_x (0 ≤ x ≤ 15) alloys as compared to Gd, suggests expansion of working temperature region of Gd upon alloying with Mn up to 15 at.%. Low cost, adjustable T_C, favorable magnetocaloric properties make Gd_100−xMn_x alloys potential candidates as second-order transition based magnetic refrigerants for near room temperature air-conditioning and magnetic refrigeration.     

Structural, magnetic and dielectric properties of La2−xCaxNiO4+δ (x = 0, 0.1, 0.2, 0.3)

The synthesis, structural, magnetic and dielectric properties of a new type of high permittivity materials La_2−xCa_xNiO_4+δ (x = 0, 0.1, 0.2, 0.3) (abbreviated as LCNs) were reported. The samples were prepared through conventional solid state reaction route. Detailed structural information was retrieved by Rietveld refinement; normalized bond length and bond valence was calculated to investigate the compression/dilation effect of bonds and atoms in unit cell. It can be found all samples belong to K₂NiF₄ structure with space group I4/mmm. Doping of Ca in La₂NiO_4+δ shrinks the unit cell and makes the structure tend to become instable. Three types of (La, Ca)-O bonds, and two kinds of Ni-O bonds exist in LCNs. Along c axis there are alternately compressed (La,Ca)O₉ dodecahedra and lengthened NiO₆ octahedra. Room temperature magnetic measurements show that the materials are paramagnetic and Ca doping can improve the spontaneous magnetization. Furthermore, all samples have colossal values of the dielectric constant (?) at frequencies lower than 1 kHz. Interestingly, La_1.8Ca_0.2NiO_4+δ maintains its high permittivity at frequencies up to 1 MHz while La_1.7Ca_0.3NiO_4+δ has the lowest dielectric loss (tan δ). Calcium doping can effectively enhance ? and inhibit tan δ. The distortion of (La,Ca)O₉ dodecahedra can well explain their dielectric properties.     

Structure and electrical properties of MnO2-doped Sr2−xCaxNaNb5O15 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics Sr_2−xCa_xNaNb₅O₁₅ + y wt% MnO₂have been prepared by the conventional solid state reaction method. Our results reveal that Ca²⁺and Mn ions have entered into the Sr₂NaNb₅O₁₅ lattices to form a solid solution with tungsten-bronze structure. The substitution of Ca²⁺ induces a decrease in piezoelectric coefficient d₃₃, electromechanical coupling factors k_p and k_t, while, the addition of Mn ions decreases the sintering temperature and effectively promotes the densification of the ceramics. The effect of substitution of Ca²⁺and Mn ions on the structure, electrical properties and diffused phase changing were investigated systematically. For the ceramic with x = 0.05 and y = 0.5, the piezoelectric, dielectric and ferroelectric properties become optimum, giving a piezoelectric coefficient d₃₃ = 190 pC/N, electromechanical coupling factors k_p = 13.4% and k_t = 36.5%, ?_r = 2123, loss tangent tan δ = 0.038, remanent polarization P_r = 4.76 μC/cm², coercive field E_c = 12.68 kV/cm, and Curie temperature T_c = 260 °C.     

Electrochemical hydrogen storage properties of non-stoichiometric La0.7Mg0.3−xCaxNi2.8Co0.5 (x = 0-0.10) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of La_0.7Mg_0.3−xCa_xNi_2.8Co_0.5 (x = 0, 0.05 and 0.10) alloys prepared by arc-melting and subsequent powder sintering method are investigated. The electrochemical measurement results show that the cycle stability after 100 charge/discharge cycles first increases from 46.4% (x = 0) to 54.3% (x = 0.05), then decreases to 43.2% (x = 0.10), and the high rate dischargeability increases from 64.5% (x = 0) to 68.5% (x = 0.10) at the discharge current density of 1200 mA/g. The electrochemical impedance spectroscopy analysis indicates that the electrochemical kinetics of the alloy electrodes is improved by increasing Ca. The entire results exhibit that a suitable content of Ca (x = 0.05) can improve the overall electrochemical hydrogen storage characteristics of the alloys.     

Large magnetic entropy change near room temperature in La0.7(Ca0.27Ag0.03)MnO3 perovskite

In this paper, the magnetic properties and magnetocaloric effect (MCE) of La_0.7(Ca_1−xAg_x)_0.3MnO₃ (x = 0, 0.1, 0.2, 0.7, and 1) powder samples are reported. Our polycrystalline compounds were synthesized using the solid state reaction method at high temperature. Magnetization measurements versus temperature showed that all our samples exhibited a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature, T_C, has been found to increase from ∼250 K for x = 0-270 K for x = 1. Ag doping weakens the first order phase transition, and at higher Ag doping, the phase transition is of second order. For the La_0.7(Ca_0.27Ag_0.03)MnO₃ composition, the maxima of the magnetic entropy changes from the applied magnetic field (ΔS_M) at 2 and 5 T are about 4.5 and 7.75 J/kg K, respectively, at the Curie temperature of ∼263 K. The relative cooling power (RCP) values without hysteresis loss are about 102 and 271 J/kg for the applied fields of 2 and 5 T, respectively. Due to the large ΔS_M, large RCP, and high Curie temperature, La_0.7(Ca_0.27Ag_0.03)MnO₃ is promising for application in potential magnetic refrigeration near room temperature.     

Correlating structure with ferromagnetism in melt-spun Gd100−xFex

X-ray diffraction and transmission electron microscopy measurements of melt-spun Gd_100−xFe_x (0 ≤ x ≤ 40) and inert-gas condensed/compacted samples (3.8 ≤ x ≤ 12.7) reveal a structure of crystalline hcp-Gd grains surrounded by a non-crystalline Gd1−xeffFexeff phase, where x_eff > x is the effective iron concentration within the amorphous region. The two-phase structure is responsible for an unusual dependence of the coercivity on temperature in which non-zero coercivity is observed above the hcp-Gd T_c with a peak near 320 K. The coercivity decreases as the hcp-Gd grains order, then increases with decreasing temperature. This behavior is explained by the presence of magnetically correlated Fe-rich regions.