Large magnetocaloric effect of La0.67Pb0.33Mn1−xCoxO3 in small magnetic field variation

In this work, we studied in detail the magnetocaloric properties of La_0.67Pb_0.33MnO₃ according to the phenomenological model. Based on this model, a large magnetic entropy change has been discovered in La_0.67Pb_0.33MnO₃ when subjected to low magnetic field variation of 0.05 T. Furthermore, the results of Co doping clearly indicate that the magnetocaloric effect in this system is tunable. Therefore, this tuning makes this ceramic series potentially practical for the improvement of multimaterial layered magnetocaloric regenerators.     

Synthesis and electrochemical characterization of La0.75Sr0.25Mn0.5Cr0.5−xAlxO3, for IT- and HT-SOFCs

The main emphasis of this work is to create a new perovskite material with three different compositions (La_0.75Sr_0.25Mn_0.5Cr_0.5−xAlxO₃, x = 0.1, 0.2, 0.3) applied in both Intermediate- and High-temperature Solid Oxide Fuel Cells (IT- and HT-SOFCs). Perovskite-type polycrystalline La_0.75Sr_0.25Mn_0.5Cr_0.5−xAl_xO_3−δ (x = 0.1, 0.2, 0.3) powders were synthesized and formed in a single phase structure by a dry chemistry route (standard solid-state reaction method). The effect of Al doping on physicochemical and surface properties has been discovered. The compounds were crystallized in single phase rhombohedral symmetry (R-3C Space. Group). Total conductivity of Al doping in wet 5% H₂ was higher than both dry 5% H₂ and air. The obtained results enhance the electro-catalytic performance and the material conductivity as well, which will be good for anode materials in IT- and HT-SOFCs and the optimum doping is 10%.     

Experimental and theoretical study on enhanced electrical properties of nickel-substituted La0.5Sr0.5CoO3−δ ceramics

The effect of Ni substitution on the thermal behavior, crystal structure, densification, and electrical properties of La_0.5Sr_0.5Co_1–yNi_yO_3-δ (y = 0.00–0.08) (LSCN) ceramics was discussed based on experimental measurements and theoretical calculations to search for a ruthenium–free and lead–free conductive oxide for thick film resistors. Ceramics were synthesized by the solid–state reaction, and calculations were performed with first–principle density functional theory (DFT). Results showed that the replacement of Ni ion to Co ion could help decrease the densification temperature and enhance the densification level and improve the conductivity of LSCN. Theoretical calculations, including the crystal structure, bond population, total energy, and density of states (DOS), supported the experimental results well. The maximum conductivity of 3155 S/cm was achieved as y = 0.04 was sintered at 1200 °C, and the peak temperature coefficient of resistance (TCR) of 2405.7 ppm/°C occurred at y = 0.06.     

Sol–gel synthesis of La0.6Sr0.4CoO3−x and Sm0.5Sr0.5CoO3−x cathode nanopowders for solid oxide fuel cells

Nano-powders of La_0.6Sr_0.4CoO_3?x (LSC) and Sm_0.5Sr_0.5CoO_3?x (SSC) compositions, which are being investigated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) with La(Sr)Ga(Mg)O_3?x (LSGM) as the electrolyte, were synthesized by low-temperature sol–gel method using metal nitrates and citric acid. Thermal decomposition of the citrate gels was followed by simultaneous DSC/TGA methods. Development of phases in the gels, on heat treatments at various temperatures, was monitored by X-ray diffraction. Sol–gel powders calcined at 550–1000 °C consisted of a number of phases. Single perovskite phase La_0.6Sr_0.4CoO_3?x or Sm_0.5Sr_0.5CoO_3?x powders were obtained at 1200 °C and 1300 °C, respectively. Morphological analysis of the powders calcined at various temperatures was done by scanning electron microscopy. The average crystallite size of the powders was ～15 nm after 700 °C calcinations and slowly increased to 70–100 nm after heat treatments at 1300–1400 °C.     

Temperature dependent phase transition of (Bi0.5Na0.5)1−xBaxTiO3 lead-free piezoelectric

Phase transformation and electric properties of lead-free piezoceramics (Bi_0.5Na_0.5)_1?xBa_xTiO₃ with x=0.05, 0.06, and 0.07(BNB5T, BNB6T and BNB7T) were investigated using dielectric, piezoelectric and ferroelectric measurements. Electric field induced strain measurement shows “W” shape bipolar strain characteristics for BNB5T with typical ferroelectric P–E curve, while BNB6T and BNB7T, possessing pinch-off P–E, exhibit “V” shape field-induced strain. All the BNBxT specimens exhibit relaxor characteristic, identified by the Debye Law. Dielectric properties measured at elevated temperatures with the frequency variation (10–500 kHz) reveal frequency dispersion below the T_d point, but no dispersion between T_d and T_m, which may be ascribed to an intermediate phase transition. By adding more Ba²⁺ ions, the region of intermediate phase, distinguished by frequency dependence dielectric constant, expands to lower temperature. Moreover, the ferroelectric properties measured at elevated temperature were carried out below and at the depolarization temperature to well investigate the existence of this phase. Much less ε–T profile dispersion were observed during the investigation of BNB6T and BNB7T, leading to possible existence of an intermediate phase in the investigated compositions. The results suggest that the linear field-induced-strain of (Bi_0.5Na_0.5)_1?xBa_xTiO₃ are expected to be attributed to the intermediate phase.     

High- and low-field dielectric responses and ferroelectric properties of (Bi0.5Na0.5)Zr1−xTixO3 ceramics

Bismuth sodium zirconate titanate (Bi_0.5Na_0.5)Zr_1?xTi_xO₃ with (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) ceramics was fabricated by a conventional sintering technique at 850–1000 °C for 2 h. From X-ray diffraction study, three regions of different phases were observed in the ceramic system; i.e., orthorhombic phase region (0≤x≤0.2), mixed-phase region (0.3≤x≤0.4), and rhombohedral phase region (0.5≤x≤0.6). It was observed that the phase evolution from orthorhombic to rhombohedral symmetry resulted in a noticeable increase of the dielectric properties. The results from the high- and low-field dielectric responses indicated that the dielectric properties of both BNZ and BNZT ceramics were dominantly attributed to the reversible contribution. It was also noticed that grain size showed only partial influence on the increase of low-field dielectric constant in Ti-rich BNZT ceramic.     

Percolation like transitions in phase separated manganites La0.5Ca0.5Mn1-xAlxO3-δ

We show that the replacement of Mn with Al strongly affects the magnetization and electrical transport behaviors in La_0.5Ca_0.5Mn_1-xAl_xO_3-δ (x = 0, 0.05, 0.07 and 0.09). Nonmagnetic Al³⁺ ions substitution at Mn sites dilutes Mn³⁺-O^2--Mn⁴⁺ network, thus suppresses the ferromagnetic metallic state in La_0.5Ca_0.5MnO_3-δ and causes a phase separation phenomenon, in which ferromagnetic phase coexists with antiferromagnetic charge ordered phase at low temperatures. With applying sufficiently high magnetic field, step-like metamagnetic transitions were observed in x = 0.05–0.09 systems below helium temperature, in which the antiferromagnetic charge ordered phase collapsed into ferromagnetic phase. Corresponding to the sharp step-like metamagnetic transitions, the resistivity decreases dramatically with increasing magnetic field, exhibiting a percolative insulator-metal transition. The variation of temperature and magnetic field changes the relative fractions of ferromagnetic and charge ordering phases, and percolative insulator-metal transition occurs due to the development of percolation paths between the growing FM domains.     

Role of iron in the ferromagnetic-antiferromagnetic boundary of La0.5Ca0.5Mn1-xFexO3+δ (0≤x≤0.5) manganites

Samples of La_0.5Ca_0.5Mn_1−xFe_xO_3+δ (0≤x≤0.5) were synthesized using a solid-state reaction method involving a milling process and thermal treatment up to 1200 °C in an air atmosphere. Samples were characterized structurally with X-ray diffraction analysis and Rietveld refinement with morphology characterization using scanning electron microscopy. Magnetic properties were investigated using a physical property measurement system to obtain zero field cooling and the associated curves to plot hysteresis loops. Our results revealed the interplay between the structural and magnetic properties as Fe ions attached to the crystalline structure. A mechanism based on the substitution of Mn³⁺ and Mn⁴⁺ by low-spin Fe³⁺ and Fe⁴⁺ ions, respectively, was hypothesized to interpret the experimental data. More specifically, the temperature at which the transition from ferromagnetic to paramagnetic occurred increased with increasing Fe content as a result of a greater density of oxygen-mediated ferromagnetic bonds. Conversely, the magnetization weakened because the t_2g electrons were distributed in the respective d orbitals by adopting a low-spin configuration. Such a configuration is preferred as a result of the unit cell distortion in the milling process where the greater ionic radius of the Fe⁴⁺ ions leads to an elongated c-axis tetragonal symmetry and a greater unit cell volume. Finally, low-temperature magnetic behavior revealed the occurrence of a reentrant spin-glass type state within the ferromagnetic matrix favored by a milling-driven structural disorder and the existence of competitive superexchange interactions.     

Structural and magnetic features of La0.7Sr0.3Mn1−xCoxO3 nano-catalysts for ethane combustion and CO oxidation

Structural, magnetic and catalytic properties of La_0.7Sr_0.3Mn_1−xCo_xO₃ (x=0.00, 0.25, 0.50, 0.75, 1.00) nano-perovskites prepared by the citrate method are investigated. The structural characterization of the compounds by X-ray powder diffraction and using X׳Pert package and Fullprof program is an evidence for a monoclinic structure (P21/n space group) with x=0.50 and a rhombohedral structure (R-3c space group) for other samples. These results have been confirmed by the FT-IR measurements. Crystallite size of the powders obtained from Halder–Wagner method has been compared with the Scherrer method. The structural and magnetic results suggest the presence of different concentrations of various cations of Co⁺², Co⁺³, Co⁺⁴, Mn⁺⁴ and Mn⁺³ in the samples. Activation energy, band gap energy, and electrical conductivity measurements have been employed to explain catalytic performance of the samples. The results of performance tests show that the sample with x=0.25 has the highest catalytic activity for CO oxidation, whereas, the sample with x=0.75 has the highest catalytic activity for C₂H₆ combustion.     

Surface Exchange of Oxygen in La1−x Sr x FeO3−δ (x = 0, 0.1)

The electronic charge carrier concentration in La_1?x Sr _x FeO_3?δ was shown to depend on the partial pressure of O₂ (pO ₂). Chemical diffusion coefficient and surface exchange coefficient, k _chem, were determined by conductivity relaxation in O₂/N₂ and CO/CO₂ mixtures. k _chem was proportional to pO ₂ ^1.06 in O₂/N₂, while in CO/CO₂ k _chem was controlled by a reaction mechanism involving both CO and CO₂.     

Variable optical properties of La0.5Sr0.5Co1-xNixO3-δ for high-temperature resistant code application

La_0.5Sr_0.5Co_1-xNi_xO_3-δ (x = 0, 0.1, 0.3, 0.5) ceramics were prepared via tape casting and solid state reaction process. The influence of Ni concentration on the optical properties of La_0.5Sr_0.5Co_1-xNi_xO_3-δ has been investigated. Results showed that the reflectance in the range of 0.3–15 μm decreased with the increment of Ni concentration, thereby causing a change in the color phase parameters and emissivity. Based on the difference in L* values and emissivity, the letters (HOT) and QR codes (NJTECH) were fabricated. The developed letters and QR codes could be identified both at room and high temperatures. Furthermore, the QR codes were read out successfully even underwent heat treatment at 1000 °C. The results in this work demonstrate a new application of La_0.5Sr_0.5Co_1-xNi_xO_3-δ ceramics.     

Effect of A-Site Ionic Radius on the Structure and Microwave Dielectric Characteristics of Sr1+xSm1−xAl1−xTixO4 Ceramics

SrSmAlO₄ microwave dielectric ceramics were modified by Sr/Ti cosubstitution for Sm/Al. The effects of radius difference of A-site ions on the microwave dielectric characteristics were investigated together with the structure. Sr_1+xSm_1−xAl_1−xTi_xO₄ (x=0, 0.05, 0.10, and 0.15) ceramics were prepared by a solid-state reaction approach. X-ray diffraction studies revealed a single-phase K₂NiF₄-type solid solution with corresponding peaks shifting to lower 2θ as x increased. Minor inhomogeneous grain morphology for x=0.05 and a trace amount of second phases for x=0.10, 0.15 were detected by backscattered-electron microscopy and energy-dispersive X-ray analysis. With increasing Sr/Ti cosubstitution, the dielectric constant ɛ_r increased from 18.4 to 20.4, and the temperature coefficient of resonant frequency τ_f was adjusted from −1.8 to 7.4 ppm/°C almost linearly. However, the Q×f value decreased from 74,500 GHz at x=0–53,000 GHz at x=0.15. The internal stresses caused by the decreased tolerance factor and the large ionic radii difference between Sr²⁺ and Sm³⁺ should be the predominant reasons for such a decrease in the Q×f value. The high-resolution transmission electron microscopic results revealed an increase in the lattice distortion with increasing Sr/Ti cosubstitution, and subsequently supported the above conclusion upon the increased internal stresses.     

Diffuse phase transition and high electric field properties of BaCeyTi1−yO3 relaxor ferroelectric ceramics

Pure perovskite BaCe_yTi_1−yO₃ ceramics with compositions y=0.10, 0.20 and 0.30 have been prepared by solid state reaction. The temperature and frequency dependence of dielectric properties (permittivity, dielectric loss and dielectric modulus) of the ceramics has been investigated. A diffuse phase transition is typical for all the compositions, with a reduction of the Curie temperature with increasing Ce addition. If in the case of the sample with 0.10 Ce content, no frequency shift of the phase transition temperature (T_m) is noticed, a relaxor-like ferroelectric character become predominant for concentrations 0.20 and 0.30. The ceramic with y=0.20 presents higher tunability, a reduced hysteretic behavior and reasonable low dielectric losses at room temperature, which makes this composition a very good candidate for tunable capacitors applications.     

Effects of Cr substitution and oxygen reduction on elastic anomaly and ultrasonic velocity in charge-ordered Nd0.5Ca0.5Mn1−xCrxO3−δ ceramics

Charge-ordered Nd_0.5Ca_0.5Mn_1?xCr_xO_3?δ ceramics have been investigated by electrical resistivity, AC susceptibility and ultrasonic velocity measurements to elucidate the effects of Cr substitution and oxygen reduction on charge ordering (CO). Resistivity and susceptibility measurements showed that the x=0 sample exhibits insulating behavior and an anti-ferromagnetic transition at 230 K as well as a CO transition at 280 K. The substitution of Cr induces a ferromagnetic–paramagnetic and metal–insulator (MI) transition as well as gradually suppressing the CO state due to weakening of the Jahn–Teller (JT) effect. Quenching to reduce the oxygen content of the x=0.05 sample caused the MI transition temperature to shift to lower temperatures most likely due to oxygen reduction. On the other hand, both longitudinal and shear velocities at 100 K increased significantly with Cr substitution indicating improvement in elastic properties. However, quenching the x=0.05 sample slightly deceased both velocities and related elastic moduli. A step-like longitudinal velocity anomaly characterized by a slope change suggests the existence of CO state for x=0, 0.02 and 0.05 samples. The step-like anomaly shifts to lower temperatures from 266 K (x=0) to 222 K (x=0.05) with Cr substitution indicating a weakening of the CO state. Absence of the step-like anomaly for the quenched x=0.05 sample suggests suppression of the CO state due to oxygen reduction. Analysis of the step-like anomaly using the mean-field theory suggests involvement of the JT effect which transforms from dynamic to static type with decreasing temperature. Cr substitution weakened the CO state as a result of weakening the JT effect while reducing the oxygen content suppresses the CO state as a result of oxygen reduction.     

Phase-microstructure evolution and microwave dielectric characteristic of (1−x)(Sr0.5Ce0.5)TiO3+δ−xNdAlO3 solid solution

A perovskite solid solution (1−x)(Sr_0.5Ce_0.5)TiO_3+δ−xNdAlO₃, x = 0.1 to 0.4 was prepared by conventional solid state method. X-ray diffraction spectra revealed a single phase with tetragonal structure, indicating that doping of NdAlO₃ significantly stabilized the perovskite-like structure. The addition of NdAlO₃ facilitated the formation of large plate-like grains with porous microstructure. The dielectric constant (ε_r) decreased with increasing x because of the small ionic polarizability of NdAlO₃. The Q × f value was strongly dependent on the microstructure of these ceramics. The temperature coefficient of resonant frequency (τ_f) gradually shifted to near zero with a rise of x, which resulted from the decrease in tolerance factor (t). The solid solution with x = 0.4 sintered at 1550 °C for 4 h showed a good combination of dielectric properties: ε_r = 72, Q × f = 12052 GHz and τ_f = +5 pmm/°C.     

Mn掺杂对La0.5Ca0.5MnxTi1-xO3介电行为的影响

利用传统的固相烧结方法,成功制备了La和ca,Mn和Ti相互替代的正交钙钛矿相La0.5Ca0.5MnxTi1-xO3(0≤x≤0.6)陶瓷。通过X射线衍射,阻抗分析仪对体系物相结构和介电性能进行了测试分析。结果表明：由于La,Ca和Mn,Ti的共存以及保持电中性的要求,该体系极易受从Mn^4 转变为Mn^3 的作用,在Mn含量不同时引入不同的空位缺陷,当Mn^3 含量与La^3 含量相当时体系中产生的缺陷相对最少,其介电常数最大,反之随两者含量偏差的增大而下降;La0.5Ca0.5MnxTi1-xO3分别在400～450℃和150～300℃之间出现2个介电峰。体系中La^3 和氧空位等产生的p型载流子和Mn离子变价引入的n型载流子在外电场下发生局域重排,产生了载流子极化,并在150～300℃范围被活化。同时由于这两种载流子对温度影响的响应不同,产生了与Mn离子的掺入相关的介电峰。在400～450℃范围,体系出现了相应为正交与立方结构转变的介电峰,也即Curie点。     

Ammonia oxidation in Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor

Selective oxidation of ammonia to NO was studied in a dense mixed ion electron conducting Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ membrane reactor, which integrates the separation and catalytic reaction process in a single reactive separation unit. The influence of the temperature and feed concentration on the membrane reaction performance were investigated in detail. Under reaction conditions, the oxygen permeation flux through the dense membrane increases with increasing temperature and ammonia flow rate. The lower temperature and ammonia concentration can favor the formation of NO, in which higher catalytic performance is obtained, suggesting that the membrane reactor operation is much beneficial for selective oxidation of ammonia.     

Irreversible metamagnetic behaviors and H-T phase diagram in phase separated La0.5Ca0.5Mn1-xAlxO3-δ

Magnetization relaxation, ac susceptibility, dc magnetization and magnetoelectrical transport measurements have been carried out on Al³⁺-doped manganites La_0.5Ca_0.5Mn_1-xAl_xO_3-δ (x = 0.05, 0.07, 0.09, 0.10, 0.15). Experimental results suggest that nonmagnetic Al³⁺ substituting Mn suppresses the ferromagnetic phase and induces a charge ordered antiferromagnetic phase. The coexistence and competition of these two kinds of interactions result in a spin-glass like state in x = 0.1 and 0.15 system. Obvious hysteresis was observed in both magnetization and resistivity versus temperature and magnetic field curves, suggesting an inhomogeneous metastable phase transition. Upon applying a magnetic field of H_C^A-F, the Al³⁺-doped samples undergo an irreversible metamagnetic transition from the charge ordering antiferromagnetic insulating to ferromagnetic metallic phase in the low temperature region. Phase diagram in the H_C-T plane has been determined according to the magnetization and magnetoelectric transport measurements. The magnetic disorders and antiferromagnetic matrix produced by Al³⁺ dopants and spontaneous oxygen vacancies play an important role in the pinning of the FM phase.