低掺杂Cr破坏La0.4Ca0.6MnO3的电荷有序相

用固相反应法制备了La0.4Ca0.6Mn1-xCroxO3(x=0.00,0.02,0.04,0.06)多晶样品.通过X射线衍射、磁化强度-温度曲线、电子自旋共振谱线研究了Cr3+替代Mn3+对La0 4Ca0.6MnO3磁性质及电荷有序相的影响.结果表明:母体La0.4Ca0.6MnO3磁结构十分复杂,在258 K...     

Atomistic origin of brittle-to-ductile transition behavior of polycrystalline 3C–SiC in diamond cutting

The machinability of hard brittle polycrystalline ceramic has a strong correlation with internal microstructures and their accommodated deformation behavior. In the present work, we investigate the mechanisms governing the brittle-to-ductile transition behavior of polycrystalline 3C–SiC in diamond cutting by means of molecular dynamics simulations. Simulation results reveal the co-existence of dislocation slip and amorphization-dominated ductile deformation and cracking along grain boundaries-mediated brittle fracture, as well as the correlation of individual deformation modes with machining force variation and machined surface morphology. In addition, inter-granular fracture, grain boundary sliding and grain pull-up are also operating brittle deformation modes of polycrystalline 3C–SiC. The strong competition between above heterogeneous deformation modes determines the brittle-to-ductile transition behavior in grooving of polycrystalline 3C–SiC. Simulation results also demonstrate that grain size has a strong impact on the brittle-to-ductile transition and material deformation behavior of polycrystalline 3C–SiC under diamond cutting.     

The role of manganese substitution on the redox behavior of La0.6Sr0.4Fe0.8Mn0.2O3-δ

Perovskite oxides such as ferrites have been widely investigated for their remarkable electrochemical activity as SOFC electrodes. However, their phase instability in reducing conditions remains an issue for anode application. The role of Mn substitution into B-site of La_0.6Sr_0.4FeO_3-δ (LSF) perovskite oxide was investigated. New insights on the structural evolution of La_0.6Sr_0.4Fe_0.8Mn_0.2O_3-δ (LSFMn) upon high temperature reduction were revealed. In oxidizing atmosphere, Mn substitution reduces the oxygen vacancy concentration while, switching to reducing conditions, it drives the transition from rhombohedral perovskite to single Ruddlesden-Popper phase, affecting the Fe⁰ exsolution. Redox-cycles of LSFMn were investigated and the properties of re-oxidized compounds were highlighted. The effect of Mn substitution on perovskite conductivity was also evaluated both in oxidizing and reducing conditions.     

固相反应法制备La0.6Sr0.25K0.15MnO3+xAg复合材料及其磁电阻效应研究

采用固相反应法使提前制备好的La0.6Sr0.25K0.15MnO3与AgNO3复合,制备了La0.6Sr025K0.15MnO3+xAg(x=0.04,0.06,0.08)系列复合样品.我们在样品的相结构、磁性质及其磁电阻效应方面进行了研究.通过相结构的研究发现:复合样品中除了有钙钛矿主相外还有少量Ag相和Mn3O4相.通过对样品的磁性测量发现:复合样品对母体样品的比饱和磁化强度和居里温度的影响很小.复合后,母体样品的磁电阻温度稳定性得到很好改善.当x=0.04时,在290 ～ 352 K的温度范围内,磁电阻值稳定在5.0％±0.2％;当x=0.06时,在260～307 K的温度范围内,磁电阻值保持在3.8％±0.2％;在282 ～339 K内x=0.08的复合样品的磁电阻值保持在5.03％±0.12％.本征磁电阻与隧穿磁电阻竞争的结果是导致磁电阻值的温度稳定性的机制.     

不同制备方法下的La0.8Ca0.2MnO3巨磁阻薄膜的形貌特征研究

采用了固相粉末烧结法、脉冲激光沉积法(Pulsed Laser Deposition)和溶胶—凝胶法(sol—gel)3种方法,制备出了La0.8Ca0.2MnO3(LCMO)巨磁阻粉末样品和在硅基底上诱导生长的薄膜样品。用原子力显微镜(AFM)、光学显微镜以及X射线衍射(XRD)对所得样品进行了结构表征。系统地研究了热处理条件对LCMO结晶形貌的影响。结果发现硅基底上的LCMO薄膜因受到诱导从而呈现出定向生长的现象,表现为类似“花”状的形态;对于固相法制备的样品,则呈现出无定型的多晶形态;对于脉冲激光沉积法制备的样品呈现出均匀致密的多晶薄膜。     

Dielectric properties and diffuse phase transition behavior of CuO-doped lead-free Ba(ZrxTi1−x)O3 ceramics

CuO-doped lead-free Ba(Zr_0.2Ti_0.8)O₃ (BZT20) ceramics were prepared through a solid state processing technique, and the effects of CuO on microstructure, dielectric properties and diffuse phase transition behavior were investigated. The average grain sizes were increased by CuO doping. The temperature and frequency dependences of the dielectric constant revealed that CuO-doped BZT20 ceramics exhibited broad diffuse phase transition behavior. The dielectric constant increased with increasing CuO concentration. The value of T_m and degree of diffusion (γ) changed regularly in the studied compositional ranges. The BZT20 samples with 1.0 mol% CuO doping, sintered at 1310 °C, showed excellent dielectric property and lower diffusivity with ε_m=21,371 and γ=1.87. These results can be explained by the disordered distribution of Cu ions in the B sites and the weakened bonding force with oxygen ions in Cu substituted BZT20 structure.     

Methane Partial Oxidation Using A (La0.6Sr0.4)(Ga0.8Mg0.05Co0.15)O3–δ Membrane

A novel sulfonated carbon composite solid acid was successfully prepared by the pyrolysis of a polymer matrix impregnated with glucose followed by sulfonation. The title catalyst has higher acid site density, better esterification activity of both small and large free fatty acids (acetic acid and palmitic acid), and better reusability than the previously reported carbon-based catalyst prepared by sulfonating pyrolyzed sugar. This catalyst also exhibited higher esterification activity than tungstated zirconia (WZ) and Silica-Supported Nafion (Nafion^®SAC-13). The higher activity of the sulfonated carbon composite solid acid catalyst was clearly due to the presence of a much higher acid site density than any of the other catalysts.     

Origin of superior dielectric and piezoelectric properties in 0.4Ba(Zr0.2Ti0.8)O3–0.6(Ba0.7Ca0.3)TiO3 at intermediate grain sizes

Grain size effect is one of the most important issues to develop next-generation multilayer microdevices. In this work, the tetragonal 0.4Ba(Zr_0.2Ti_0.8)O₃–0.6(Ba_0.7Ca_0.3)TiO₃ (BZT–60BCT) ceramics with a wide grain size from 2.1 to 24 μm were successfully prepared by using ultrafine nano powder and two-step sintering. The results demonstrate that critical/intermediate grain size of dielectric constant ε_r and piezoelectric constant d₃₃ appears at ∼12.9 μm. It was found that the presence of large lattice tetragonality, and enhanced domain wall motion induced by domain coexistence between submicron and nano size in sample with a grain size of ∼12.9 μm, resulting in the superior dielectric and piezoelectric properties. These findings and analyses of the origin of superior dielectric and piezoelectric properties at intermediate grain size have important practical implications in the design of high-performance piezoelectric ceramics.     

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.     

Fabrication of lead-free Ba(Zr0.2Ti0.8)O3–(Ba0.7Ca0.3)TiO3 nanoparticles and the application in flexible piezoelectric nanogenerator

Recently, flexible nanogenerators have attracted much attention due to the continuous demand of portable electronic devices. Thus, in our present work, a flexible piezoelectric nanogenerator (pNG) was fabricated by mixing 0.5Ba(Zr_0.2Ti_0.8)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ nanoparticles (BCTZ NPs) and a PDMS polymer matrix. The flexible device exhibited excellent performance with maximum open-circuit voltage of 0.6 V and short-circuit current of 7.5 nA. BCTZ NPs were prepared by a reaction between a metallic salt and a metallic oxide in a solution of composite-hydroxide eutectic. XRD pattern showed BCTZ NPs to have pure perovskite structure. In this paper, the structure, morphology of the BCTZ NPs, and electric property of the BCTZ-based pNG were systematically studied. With the advantages of small-size, environmentally friendly, high flexibility, and high-sensitivity to external vibration, BCTZ will open up a range of new applications.     

Creep behavior of porous La0.6Sr0.4Co0.2Fe0.8O3-δ substrate material for oxygen separation application

Advanced oxygen transport membrane designs consist of a thin functional layer supported by a porous substrate material that carries mechanical loads. Creep deformation behavior is to be assessed to warrant a long-term reliable operation at elevated temperatures. Aiming towards an asymmetric composite, the current study reports and compares the creep behavior of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) perovskite porous substrate material with different porosity and pore structures in air for a temperature range of 800–1000?°C. A porosity and pore structure independent average stress exponent and activation energy are derived from the deformation data, both being representative for the LSCF material. To investigate the structural stability of the dense layer in an asymmetric membrane, sandwich samples of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) with porous substrate and dense layers on both side were tested by three-point bending with respect to creep rupture behavior of the dense layer. Creep rupture cracks were observed in the tensile surface of BSCF, but not in the case of LSCF.     

Mechanical behavior of ferroelastic porous La0.6Sr0.4Co0.2Fe0.8O3−δ prepared with different pore formers

The present work investigated the mechanical behavior of porous La_0.6Sr_0.4Co_0.2 Fe_0.8O_3−δ LSCF under uniaxial compression. The porous (LSCF) samples with the same grain size but different porous structures with 1.5–41% of porosity were prepared using three different pore formers. All the samples had ferroelastic domains and exhibited ferroelastic mechanical behaviors under uniaxial compression. Initial and loading moduli as well as critical stress monotonically decreased and remnant strain increased with increasing the porosity. The initial modulus can be determined by the actual porosity regardless of porous structure or grain size, whereas the other properties were more sensitive to experimental condition such as loading rate and maximum applied stress. Compressive fracture strength could be significantly influenced by porous structure.     

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.     

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.     

Ba0.95La0.05Fe0.8Ni0.2O3−δ perovskite as efficient cathode electrocatalysts for proton-conducting solid oxide fuel cells

The key issue that limits the electrochemical performance of proton-conducting solid oxide fuel cells (H⁺-SOFCs) is the sluggish kinetics of the oxygen reduction reaction (ORR) of cathode at intermediate and low temperatures. Herein, oxygen vacancy engineering is conducted on cobalt-free Ba_0.95La_0.05FeO_3?δ (BLF) by nickel substitution, which is confirmed by density functional theory computations. Nickel-substituted BLF material (Ba_0.95La_0.05Fe_1?xNi_xO_3?δ (x = 0, 0.1, 0.2, 0.3)) can promote the generation of oxygen vacancies and improve catalytic activity, which is found to be in line with the experimental results of XPS. The phase structure, microstructure, and electrochemical performance of Ba_0.95La_0.05Fe_0.8Ni_0.2O_3?δ (BLFNi0.2) are well-investigated. The single cells with the BLFNi0.2-BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3?δ (BCZYYb) composite cathode achieve low polarization resistance (R_p) of 0.099 Ω cm² and a peak power density of 631 mW cm^?2 at 700 °C while maintaining good durability for 120 h with no observable degradation. The results demonstrate that Ni-doped BLF is a promising cobalt-free cathode material for H⁺-SOFCs.     

Enhanced piezoelectric strain of BiFeO3–Ba(Zr0.02Ti0.98)O3 lead-free ceramics near the phase boundary

The xBiFeO₃-(1-x)Ba(Zr_0.02Ti_0.98)O₃ + 1.0 mol% MnO₂ (xBF-BZT) lead-free piezoelectric ceramics were prepared by conventional solid-state reaction method. The structure, dielectric, and piezoelectric properties were studied. X-ray diffraction (XRD) analysis showed that xBF-BZT ceramics exhibited pure perovskite structure with the coexistence of tetragonal and rhombohedral phases (0.66 ≤ x ≤ 0.74). The Curie temperature T_c, the dielectric constant ε_r (1 kHz), dielectric loss tanδ (1 kHz), piezoelectric constant d₃₃, coercive field E_c (80 kV/cm), and remnant polarization P_r (80 kV/cm) of 0.7BF-0.3BZT-Mn ceramics were 491°C, 633, 0.044, 165 pC/N, 35.6 kV/cm, and 22.6 μC/cm², respectively. The unipolar strain of 0.7BF-0.3BZT reached up to 0.20% under the electric field of 60 kV/cm, which is larger than that (0.15%) of BiFeO₃–BaTiO₃ ceramics. These results indicated that the xBF-BZT ceramics were promising candidates for high-temperature piezoelectric materials.     

Aging effect in Er3+-doped 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 ceramics

Usually, aging in poled ferroelectrics leads to degradation of certain physical properties. In this study, we found a remarkable aging effect in tetragonal Er³⁺-doped 0.5Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-BCT) ceramics after poling. It is observed that the domains can spontaneously rotate to keep their spontaneous polarization direction similar to that of the poling electric field during aging for the poled ceramics. Furthermore, compared with freshly poled ceramics, the thermally stimulated current (TSC) peak of the aged ones shifts toward a higher temperature (10°C). And the temperature of the TSC peak in the aged ceramics is exactly equal to their Curie temperature. Such features indicate that aging for the poled ceramics could stabilize the alignment of ordered ferroelectric domains. Additionally, a downward TSC peak above Curie temperature is obtained in both poled and aged ceramics, demonstrating that poling and aging can lead to ordered alignment of defect dipoles. The aging mechanism of poled Er³⁺-doped BZT-BCT ceramics has proposed and discussed in this article.     

Control of oxygen deficiency in Ca1−x La x MnO3−δ and its cathodic properties in alkaline solution

Ceramic samples of composition Ca_0.9La_0.1MnO_3– having various oxygen contents were prepared by a quenching method under nitrogen atmosphere. As the 3 – value decreased from 2.97 to 2.79, the sample conductivity decreased from 10² to 10^–1 S cm^–1. The porous ceramic samples showed good properties as cathode materials in alkaline solution without using conductive material such as graphite, but the discharge capacity decreased with decreasing sample conductivity. The discharge termination is explained by a simple model considering dissipation of the conductive path (high conductivity core) present in the porous sintered ceramic.