La1-xSrxMnO3:Ag0.2 (0.1 ≤ x ≤ 0.2) ceramics with large room-temperature TCR for uncooled infrared bolometers

In this study, high-density La_1-xSr_xMnO₃:Ag_0.2 (x = 0.1, 0.125, 0.15, 0.175, and 0.2) ceramics were prepared by the conventional sol-gel method. The peak values of temperature coefficient of resistivity (TCR) in all La_1-xSr_xMnO₃:Ag_0.2 samples were systematically controlled by changing the Sr content. A significant improvement in peak TCR have been observed through adjusting the Sr content. At doping molar ratio of x = 0.15 in La_1-xSr_xMnO₃:Ag_0.2 ceramics, the peak TCR value reached 14.7% K⁻¹, and peak TCR temperature (T_K, 288.2 K) was estimated to room-temperature (290 K). Visibly, it is encouraging to get such a high room-temperature TCR value. These findings suggested that La_0.85Sr_0.15MnO₃:Ag_0.2 ceramics could be used to prepare room-temperature uncooled infrared bolometers.     

Enhanced room-temperature TCR of La0.67Ca0.33-xSrxMnO3 (0.06 ≤ x ≤ 0.11) polycrystalline ceramics by Sr content adjustment

Herein, the influence of Sr doping on chemical composition, microstructure, and electrical transport properties of La_0.67Ca_0.33-xSr_xMnO₃ (LCSMO, 0.06 ≤ x ≤ 0.11) polycrystalline ceramics is systematically investigated. The study was performed by using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), elemental mapping, energy dispersive spectrometry (EDS), X-ray photoemission spectroscopy (XPS), and four-probe method (ρ-T). XRD results verified the synthesis of high-purity orthogonal perovskite structure with Pnma space group. EDS and XPS results confirmed presence and uniform distribution of La, Ca, Sr, Mn and O. Furthermore, ρ-T curves indicated that the resistivity decreased with increasing Sr content, whereas metal-insulator transition temperature (T_MI) gradually increased. Moreover, cell volume (V) and electron bandwidth (W) increased with the increase in Sr content, which effectively reduced the resistivity of LCSMO ceramics. One should note that double exchange (DE) enhancement between Mn³⁺ and Mn⁴⁺ is responsible for gradually increasing T_MI. Finally, maximum TCR value of 14.3% K⁻¹ was achieved at x = 0.08, which rendered TCR peak temperature of 299.0 K. High room-temperature TCR of LCSMO ceramics is promising for next-generation infrared bolometers, operating at room-temperature.     

Critical behavior and its correlation with magnetocaloric effect in La0.7Sr0.25Na0.05Mn(1−x)TixO3 (0≤x≤0.1) manganite oxide

The critical properties of the electron-doped manganite La_0.7Sr_0.25Na_0.05Mn_(1−x)Ti_xO₃ (x=0.0 and 0.1) were investigated based on the data of static magnetization measurements around Curie temperature T_C. Various techniques such as modified Arrott plot, a Kouvel–Fisher method, and critical isotherm analysis were used to determine the values of the ferromagnetic transition temperature T_C, as well as the critical exponents of β, γ and δ. The estimated results are close to those expected for three-dimensional Heisenberg class with short-range interaction. From the field dependence of RCP and ΔS_M, it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott plot (MAP).     

Influence of Ce addition on the structural,magnetic, and magnetocaloric properties in La0.7−xCexSr0.3MnO3 (0≤x≤0.3) ceramic compound

We report improvement in the magnetocaloric properties of Ce-doped lanthanum manganites. Polycrystalline La_0.7−xCe_xSr_0.30MnO₃ (0≤x≥0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transition in parent sample and as well as in doped sample with Curie temperature decreasing progressively with increasing Ce-concentration from ~370 K for x=0.0 to 310 K for x=0.30. Magnetic entropy change (ΔS_M) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. A large ΔS_M associated with the ferromagnetic–paramagnetic transition in La_0.7−xCe_xSr_0.30MnO₃ samples has been observed. The value of ΔS_M was found to increase with Ce-doping up to x=0.15 and the highest value of 2.12 J kg⁻¹ K⁻¹ (at ΔH=2 T) was observed for La_0.55Ce_0.15Sr_0.30MnO₃ sample near the Curie temperature of 356 K. Also, improved relative cooling power of ~122 J kg⁻¹ was observed for the same sample. Due to the large magnetic entropy change and high Curie temperature, the La_0.55Ce_0.15Sr_0.30MnO₃ sample is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature.     

La1−xCaxMnO3:Ag0.2 (0.25 ≤ x ≤ 0.31) ceramics with high temperature coefficient of resistivity under magnetic field

The polycrystalline La_1?xCa_xMnO₃ ceramics exhibit good electromagnetic performance, i.e., high temperature coefficient of resistivity (TCR), which can be tuned flexibly with respect to structures. Unfortunately, the magnetic field applied to these materials causes a massive decrease in TCR, which hinders their practical applications. In this study, polycrystalline ceramic La_1?xCa_xMnO₃:Ag_0.2 was fabricated by sol–gel and solid–phase doping methods, and subsequently vast TCR was obtained in the magnetic field for ceramic with x = 0.2. For this polycrystalline material, high value of TCR (58.65%·K^?1, 62.00%·K^?1) could be maintained with or without magnetic field with metal–insulator (M ? I) transition temperature near room temperature range. Extremely high value of TCR in the presence of magnetic field is attributed to the spin–spin coupling effect, which is beneficial to the sensitivity of M ? I transition, generating vast TCR in the magnetic field. Overall, these findings provide new prospects for future applications in infrared bolometers.     

LaCr1-xFexO3 (0≤x≤0.7): A novel NTC ceramic with high stability

Perovskite-phase LaCr_1-xFe_xO₃ (0 ≤ x ≤ 0.7) ceramics were fabricated by two-step sintering of powders synthesized by conventional solid-state reaction. XRD patterns reveals that the ceramics are single-phased LaCr_1-xFe_xO₃ solid solution at lower Fe contents, but form a biphasic mixture of LaCrO₃-LaFeO₃ at higher Fe contents. SEM analysis indicates that Fe doping causes a reduction in porosity, and a corresponding increase in density. The effects of Fe doping on electrical properties have also been investigated. The electrical resistivity ρ_25°C decreases initially and then increases within a wide range of 634.4–83915.9 Ω·cm with increasing Fe content, whereas B varies within a relatively narrow range of 3651−4301 K. Such combination enables it to be a potential candidate for NTC thermistors. An investigation of elevated-temperature aging behavior shows that the resistance shifts at 25 °C were less than 0.65 % after annealing at 125 °C in air for 1000 h, revealing that the material exhibits excellent stability.     

Catalytic activity improvement for efficient hydrogen oxidation of infiltrated La0.3Sr0.7Ti0.3Fe0.7O3-δ anode for solid oxide fuel cell

In this paper, a ceramic perovskite anode for solid oxide fuel cells (SOFCs) is prepared by infiltrating La and Fe co-doped strontium titanium, La_0.3Sr_0.7Ti_0.3Fe_0.7O_3-δ (LSTF0.7) into porous backbone of scandia-stabilized zirconia (ScSZ) and tested in pure H₂ at 700–850 °C. LSTF0.7 crystal exhibits high reduction stability and good compatibility with ScSZ electrolyte under reducing atmosphere. In order to improve the electrocatalytic activity, 15 wt% of CeO₂ and 7 wt% of Ni are infiltrated into the backbone pores respectively, thus forming LSTF0.7-CeO₂ and Ni-CeO₂-LSTF0.7 composite anodes. The cell with LSTF0.7 single anode shows a relatively lower maximal power density (MPD) of 401 mW cm⁻² in H₂ at 800 °C. While the maximal power densities of the cells with LSTF0.7-CeO₂ and Ni-CeO₂-LSTF0.7 composite anodes are 612 mW cm⁻² and 698 mW cm⁻² operated at the same conditions, respectively. The three anode polarization resistances (Rp,a) distinguished from the corresponding full cells are 0.176, 0.086 and 0.076 Ω cm² at 800 °C, respectively. The values of the activation energy (Ea) towards H₂ oxidation for the three anodes can be calculated to be 52.2, 46.0, 43.9 kJ mol⁻¹ based on their respective Rp,a. Therefore, the LSTF0.7-based anodes are considered to be promising alternatives for solid oxide fuel cell applications.     

Critical properties in Dy-doped La0.7−xDyxSr0.3MnO3 (x=0.00, 0.03) manganites

The critical properties of La_0.7−xDy_xSr_0.3MnO₃ (x=0.00 and 0.03) manganites elaborated using sol-gel method have been investigated around the ferromagnetic-paramagnetic (FM-PM) phase transition. The X-ray diffraction characterizations show that the parent compound (x=0.00) crystallized in the rhombohedral structure with R-3C space group without any detectable impurity, while doped sample with (x=0.03) crystallized in the orthorhombic structure with Pnma space group. Various techniques such as modified Arrott plots, Kouvel-Fisher method and critical isotherm analysis were used to determine the values of the Curie temperature T_C, as well as the critical exponents β (corresponding to the spontaneous magnetisation), γ (corresponding to the initial susceptibility) and δ (corresponding to the critical magnetisation isotherm). The estimated results are close to those expected by the mean-field model for both samples. The obtained values from critical isotherm M(T_C, μ₀H) are close to those determined using the Widom scaling relation, and all data fall on two distinct branches, one for T<T_C and the other for T>T_C, indicating that the critical exponents obtained in this work are accurate.     

Thermal and chemical induced expansion of La0.3Sr0.7(Fe,Ga)O3−δ ceramics

The linear thermal expansion coefficients (TECs) of perovskite-type La_0.3Sr_0.7Fe_1−xGa_xO_3−δ (x=0–0.4), determined by dilatometric and high-temperature X-ray diffraction techniques, are in the range (19–41)×10⁻⁶ K⁻¹ at 770–1170 K, decreasing when the oxygen partial pressure or gallium concentration increases. At oxygen pressures from 10⁻⁴ to 1 atm, the isothermal chemically induced expansion of La_0.3Sr_0.7Fe(Ga)O_3−δ ceramics is a linear function of the oxygen nonstoichiometry. The magnitude of changes in δ and, thus, chemical expansion both are reduced by gallium doping. The ratio between isothermal chemical strain and nonstoichiometry variations, (ε_C/Δδ), follows an Arrhenius-type dependence on temperature and varies in the range (1.7–5.9)×10⁻². The drastic increase in the thermal expansion at temperatures above 700 K, typical for ferrite-based ceramics, was shown to be mainly apparent, resulting from the chemically-induced expansion of the lattice due to oxygen losses. The TEC values, corrected for the chemical strain on heating, are close to the TECs at low temperatures and increase with gallium content. The observed correlations between the thermal and chemical expansion and ionic conductivity of La_0.3Sr_0.7Fe_1−xGa_xO_3−δ are discussed in terms of their relationships with the oxygen deficiency and cation composition.     

Electrical and magnetic properties of La1−xSrxMnO3 (0.1 ≤ x≤ 0.25) ceramics prepared by sol–gel technique

La_1−xSr_xMnO₃ (0.1 ≤ x ≤ 0.25) high density ceramics were prepared by sol-gel method using methanol as solvent. X-ray diffraction analysis showed that all samples exhibited single perovskite structure and no second phase was detected. Scanning electron microscopy images exhibited good particle connectivity on the surface of sample, and grain size increased with the increase in Sr doping. Resistivity-temperature curves of samples were measured by standard four-probe method, and curves exhibited significant differences in studied range of Sr doping. Magnetic measurement results indicated that the variation of susceptibility of different samples was quite different, and the Curie temperature of samples increased with the increase in Sr content. For x = 0.2, temperature coefficient of resistance value of the sample was larger, and corresponding peak TCR temperature was 307.1 K, which is very close to room temperature. Thus, La_0.8Sr_0.2MnO₃ ceramics exhibited high TCR value close to room temperature. Combined with its excellent magnetic properties, La_0.8Sr_0.2MnO₃ ceramics may potentially act as effective candidates for uncooled radiation calorimeter and uncooled magnetic sensor. Applications of La_0.8Sr_0.2MnO₃ ceramics in uncooled infrared radiation calorimeter at room temperature will be highly beneficial.     

Structural,magnetic and magnetocaloric study of La0.7−xEuxSr0.3MnO3 (x=0.1, 0.2 and 0.3) manganites

This paper reports the structural, magnetic and magnetocaloric properties of La_0.7−xEu_xSr_0.3MnO₃ (x=0.1, 0.2 and 0.3) polycrystalline manganites elaborated using the solid-state reaction at high temperatures. The X-ray powder diffraction shows that all the prepared compounds are single phase. La_0.6Eu_0.1Sr_0.3MnO₃ is crystallized in the rhombohedral symmetry, whereas a structural transition towards orthorhombic system is observed for x≥0.2. Eu doping was found to induce a decrease of the Curie temperature T_C from 343 K (x=0.1) to 272 K (x=0.3). All compounds undergo a large magnetocaloric effect and have consequently potential applications in magnetic refrigeration domain around room temperature.     

Electrical properties of La2-xPrxTi2O7 (x = 0–0.3) ceramics

The electrical properties of La₂Ti₂O₇ (LTO) ceramics have been enhanced through the substitution of La³⁺ ions by Pr³⁺ ions. Almost all doped Pr³⁺ ions will get at A - site without causing a change on monoclinic phase of LTO. The average grain size is 17.8 μm for La_1.9Pr_0.1Ti₂O₇ ceramics. The relaxation activation energy which is contributed by defect dipoles that are formed from TiO₆ oxygen octahedrons’ distortions in grains is 1.6 eV for La_1.8Pr_0.2Ti₂O₇ ceramics. This kind of defects will be activated from 520 °C and completely be activated until 650 °C. The piezoelectric coefficient d₃₃ = 3.0 pC/N of La_2-xPr_xTi₂O₇ ceramics maintains stable when the Pr³⁺ doping content x ranging from 0.1 to 0.3.     

Magnetoresistance properties of 0.99La0.7(CaxSr1−x)0.3MnO3/0.01CuZnFe4O4 composites

The composites of 0.99La_0.7(Ca_xSr_1?x)_0.3MnO₃/0.01CuZnFe₄O₄ (LCSMO/CZF) (x = 0, 0.1, 0.2, 0.3, 0.5, 0.7 and 1) were fabricated by conventional solid state reaction method, and their electrical transport and magnetoresistance (MR) properties were investigated by physical property measurement system (PPMS). The result of X-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicated that no new phase appeared in the composites except LCSMO and CZF phases. CZF is mainly distributed at the grain boundaries and surfaces of the matrix. The resistivity of all the composites was measured in the range 100–350 K at 0 T, 0.5 T and 1 T magnetic field. Room temperature MR peak appears for the composites x = 0.03. The observed variation of MR with varying Ca and Sr concentration has been qualitatively.     

Crystal structure and microwave dielectric properties of La2(Zr1−xTix)3(MoO4)9 (0 ≤ x ≤ 0.1) ceramics

La₂(Zr_1−xTi_x)₃(MoO₄)₉ (0 ≤ x ≤ 0.1) ceramics were prepared by the traditional solid-state reaction method. XRD analysis showed that La₂(Zr_1−xTi_x)₃(MoO₄)₉ (0 ≤ x ≤ 0.1) ceramics belonged to a trigonal system. Based on the chemical bond theory, the consequences of bond energy, bond ionicity, lattice energy, and thermal expansion coefficient of ceramics on microwave dielectric properties were discussed. As Ti⁴⁺ addition was increased, the reduction in dielectric constant was ascribed to the fact that the polarizability of Ti⁴⁺ is smaller than Zr⁴⁺, and the downward trend was related to the bond ionicity. Besides, the tendency of Q·f value depended on the packing fraction and the lattice energy. The improvement in τ_f value, the increase in bond energy, and the decrease in the coefficient of thermal expansion were all correlated. The far-infrared spectra implied that the absorptions of structural phonon oscillation were the main reason for the maximum polarization contribution. La₂(Zr_0.92Ti_0.08)₃(MoO₄)₉ ceramics sintered at 750°C for 4 hours exhibited the best dielectric properties (ε_r = 10.33, Q·f = 80 658 GHz, and τ_f = 3.48 ppm/°C).     

掺杂少量Sm时La0.67-xSmxSr0.33MnO3(0.00≤x≤0.30)的磁电性质

通过测量样品的磁化强度-温度(M-T)曲线、电阻率-温度(ρ-T)曲线及磁电阻(magnetoresistance)-温度(MR-T)曲线,研究了Sm掺杂(x=0.00,0.10,0.20,0.30)对La0.67-xSrmxSr0.33MnO3磁电性质的影响.发现在铁磁相主要是单磁子散射起作用,表现为金属型导电,可用公式ρ=ρ0+AT2拟合,其中ρ为温度T时的电阻率;ρ0为0 K时的电阻率;A为常数.顺磁相的输运机制主要是小极化子跃迁起作用,可以用公式ρ=BTexp·(Ea/kBT)拟合,其中Ea为激活能;kB为Bolzman常数;B为常数.在相变温区是顺磁相的小极化子与铁磁相的单磁子2种输运性质共存.     

Influence of La addition on the structural,magnetic and magnetocaloric properties in Sr2−xLaxFeMoO6 (0≤x≤0.3) double perovskite

We report the effect of carrier doping via partial substitution of La³⁺ for Sr²⁺ on the structural, magnetic and magnetocaloric properties of Sr₂FeMoO₆ double perovskite. Polycrystalline Sr_2−xLa_xFeMoO₆ (x=0.0, 0.1, 0.2, 0.3) samples were prepared using the conventional solid state reaction method. Using the X-ray diffraction (XRD) analysis it was established that all the samples crystallized in a tetragonal structure with I4/mmm space group. An increase in the La doping lead to an increase in the lattice parameter ‘a’ and the volume of the unit cell. The lattice parameter ‘c’ however remained unchanged. The temperature variation in magnetization and Arrott analysis suggested a second order of ferromagnetic phase transition in all samples with Curie temperature, T_C increasing from 358 K for x=0.0–365 K for x=0.3. A gradual increase in magnetization was also observed with the increasing La content up to x=0.2. The magnetic entropy change was calculated from the measurement of isothermal magnetization versus magnetic field at different temperatures. The tunability of magnetization and T_C simply by adjusting the concentration of La and synthesis conditions makes Sr₂FeMoO₆ an attractive material for magnetic refrigeration at desired temperature.     

Electrical transport properties driven by magnetic competition in hole-doped perovskite Pr1-xBaxMnO3 (0.25 ≤ x ≤ 0.36)

In this work, polycrystalline Pr_1-xBa_xMnO₃ (0.25 ≤ x ≤ 0.36) ceramics were synthesized, and their magnetic and electrical transport properties were systematically studied. All samples show two metal-insulator transitions (MITs) corresponding to the high temperature T_MI1 and low temperature T_MI2, respectively, besides the non-Griffith phase above the ferromagnetic (FM) transition temperature T_C. Combining the results of the transport and magnetic properties, it is found that the FM transition temperature T_C coincides with the temperature T_MI1, which is linearly related to the A-site ionic radius mismatch variance σ², indicating the enhancement of FM interactions due to the increase of the degree of B-site ordering of Mn³⁺/Mn⁴⁺ ions. The positive correlation between ferromagnetic insulators (FMI) and magnetic interactions, including the FM and short-range antiferromagnetic (AFM) interactions, is confirmed. It is suggested that the first MIT at T_MI1 is attributed to the Mn³⁺/Mn⁴⁺ double exchange interactions and the second MIT at T_MI2 is closely related to the suppression of the AFM interactions under the internal FM field induced by the Mn³⁺/Mn⁴⁺ DE interactions. This work provides not only a theoretical understanding on the origin of MIT at low temperature, but also a new way for adjusting the FMI in perovskite manganese oxide Pr_1-xBa_xMnO₃ for application.     

Utilizing the localized surface piezoelectricity of centrosymmetric Sr1-xFexTiO3 (x≤0.2) ceramics for piezocatalytic dye degradation

Centrosymmetric Sr_1-xFe_xTiO₃ (x = 0, 0.01, 0.05, 0.1, 0.2) ceramics were synthesized using solid-state reaction method. XRD confirmed the perovskite SrTiO₃ phase in all samples and indicated the conversion of cubic to pseudocubic crystal symmetry with increase in Fe substitution. Raman spectroscopy showed first and second order Raman scattering modes in all samples and a new Raman mode in 691–701 cm^-1 in Fe substituted samples. The irregular morphology of grains in Sr_1-xFe_xTiO₃ (x≤0.1) was interestingly transformed into rhombicuboctahedron morphology in x = 0.2 sample. All samples showed significant piezocatalytic degradation of methylene blue (MB) dye. The piezocatalytic dye degradation was increased with increase in Fe substitution. The best piezocatalytic dye degradation rate of 0.00448 min^-1 was achieved in x = 0.2 sample. The localized surface piezoelectricity along with some other sono-induced phenomena might be responsible for piezocatalytic activity. The sample with x = 0.2 also showed promising MB dye degradation under visible light (k = 0.00364 min^-1).