Nd0.7Sr0.3MnO3中显微结构相关电致电阻效应

用固相反应和高能球磨合成后续热处理两种方法分别得到钙钛矿结构Nd_0.7Sr_0.3MnO₃氧化物.两种不同方法得到的多晶样品,虽然晶体结构相同,化学成分和晶粒大小相近,但它们电输运性质却表现出很大差异.用固相反应法制得的样品的电阻几乎不随负载电流的变化而变化,即不表现电致电阻行为;而通过高能球磨合成后续热处理方法得到的样品电阻随外加电流增大而急剧减小,出现显著电致电阻效应.产生这种截然不同电输运特性的原因可能与样品的显微结构和界面性质有关. 关键词： 电致电阻效应 显微结构 钙钛矿结构锰氧化物 界面电阻     

钙钛矿锰氧化物(La1-xGdx)4/3Sr5/3Mn2O7 (x=0, 0.025) 磁性和输运性质研究

采用传统的高温固相烧结法制备了双层钙钛矿锰氧化物(La_1-xGd_x)_4/3Sr_5/3Mn₂ O₇ (x=0, 0.025)多晶样品. 通过X射线衍射仪研究发现样品为Sr₃Ti₂O₇型四方结构, 空间群为I4/mmm; 磁性测量表明, Gd³⁺掺杂后的样品(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂O₇的三维磁有序转变温度(T_C1^3D)、磁化强度(M)均降低, 这是由于Gd³⁺的掺杂引起晶格的畸变, 从而使得晶格常数发生改变, 减弱了铁磁耦合而导致的; 通过电子自旋共振谱测量发现, 在T_C^3D<T<300 K温度范围内, 两样品在顺磁的基体上均有短程的铁磁团簇存在, 出现了相分离现象. 电性测量表明: 两样品分别在T_C1^3D (La_4/3Sr_5/3Mn₂O₇ 样品的三维磁有序转变温度, T_C0^3D)<T<300 K温度范围内均以三维变程跳跃的方式导电, 分析得出Gd³⁺的掺杂使得载流子局域长度的减小. 这表明载流子需要吸收更多的能量才能克服晶格的束缚进行跳跃, 因此(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂ O₇ 样品的电阻较高.     

Sr1−xBaxSnO3 system applied in the photocatalytic discoloration of an azo-dye

Semiconductor materials have received substantial attention as photocatalysts for controlling water pollution. Among these materials, perovskite-structured SrSnO₃ is a promising candidate for this application, whereas BaSnO₃ exhibits very low activity. In the present work, Sr_1−xBa_xSnO₃ (x = 0, 0.25, 0.50, 0.75 and 1) was synthesized by solid-state reaction and was applied in the photocatalytic discoloration of the organic dye Remazol Golden Yellow. The perovskite structure was obtained for all compositions of the solid solutions with both Sr²⁺ and Ba²⁺ present in the lattice. A remarkable change in the short-range symmetry was observed as the amount of Ba²⁺ increased, and this change led to a decrease in the band gap of the material. Although the BaSnO₃ was not active toward water photolysis, the discoloration induced by this perovskite was twice that induced by SrSnO₃. The two materials appear to feature different mechanisms of photodegradation: the direct mechanism prevails in the case of BaSnO₃, whereas the indirect mechanism appears to play a key role in the case of SrSnO₃.     

Structural and electrical properties of lead-free perovskite ceramic: Ba(In1/2Nb1/2)O3

Lead-free perovskite Ba(In_1/2Nb_1/2)O₃ was prepared by conventional ceramic fabrication technique at 1350 °C/5 h in air atmosphere. The crystal symmetry, space group and unit cell dimensions were determined from Rietveld analysis using FullProf software whereas crystallite size and lattice strain were estimated from Williamson-Hall approach. XRD analysis of the compound indicated the formation of a single-phase cubic structure with the space group Pm3m. EDAX and SEM studies were carried out in order to evaluate the quality and purity of the compound. Complex impedance as well as electric modulus analyses suggested the dielectric relaxation to be of non-Debye type. The correlated barrier hopping (CBH) model was employed to successfully explain the mechanism of charge transport in Ba(In_1/2Nb_1/2)O₃.     

Synthesis of hybrid organic-inorganic perovskite platelets by vacuum impregnation

In perovskite solar cells and optoelectronics, perovskite film morphology controls the performance of the device. Various methods have been developed to control the morphology and coverage of the perovskite films. In this article platelet type perovskite morphlogy was synthesized using low temperature vacuum impregnation of the perovskite solution CH₃NH₃PbI₃ resulting in complete coverage on TiO₂ film. Vacuum impregnation synthesis of perovskites has the advantage of low cost and low temperature which faciliates application in flexible electronics and solar cells.     

A solution-processable small-organic molecules containing carbazole or phenoxazine structure as hole-transport materials for perovskite solar cells

Three low molecular weight compounds bearing carbazole units (1,6-di{3-[2-(4-methylphenyl)vinyl]carbazol-9-yl}hexane and 9,9'-di{6-[3-(2-(4-methylphenyl)vinyl)-9-carbazol-9-yl]hexyl}-[3,3']bicarbazole) and phenoxazine structure (10-butyl-3,7-diphenylphenoxazine) were tested as hole-transporting materials in perovskite solar cells. Two of them were successfully applied as hole transporting layers in electroluminescent light emitted diodes. The examined compounds were high-thermally stable with decomposition temperature found at the range of 280–419 °C. Additionally, DSC measurement revealed that they can be converted into amorphous materials. The compounds possess adequate ionization potentials, to perovskite energy levels, being in the range of 5.15–5.36 eV. The significant increase in power conversion efficiency from 1.60% in the case of a device without hole-transporting layer, to 5.31% for device with 1,6-di{3-[2-(4-methylphenyl)vinyl]carbazol-9- yl}hexane was observed.     

Influence of CO2 on Oxygen Surface Exchange Kinetics of Mixed-Conducting Ba0.5Sr0.5Co0.8Fe0.2O3?δ Oxide

The poisoning effect of CO₂ on the oxygen surface exchange kinetics of BSCF (Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ) is investigated with a novel pulse isotopic exchange technique. The surface exchange rate of BSCF severely decreases after in situ exposure to CO₂, which is ascribed to carbonate formation on the material surface. The detrimental effect of CO₂ starts at a low temperature of 375 ^oC and concentration as low as 1%, and becomes more pronounced at higher temperatures. Degradation of the surface exchange kinetics is associated with a rapid loss of oxygen permeation performance of BSCF in CO₂.     

Li mobility in Li0.5 − xNaxLa0.5TiO3 perovskites (0 ≤ x ≤ 0.5): Influence of structural and compositional parameters

The dependence of Li mobility on structure and composition of Li_0.5 − xNa_xLa_0.5TiO₃ perovskites (0 ≤ x ≤  0.5) has been investigated by means of neutron diffraction, nuclear magnetic resonance and impedance spectroscopy. At 300 K, all samples display a rhombohedral superstructure (R-3c S.G.), where octahedra are out of phase tilted along [111] direction of the ideal cubic cell. The elimination of the octahedral tilting is responsible for the rhombohedral–cubic transformation, detected near 1000 K. In these perovskites, La and Na cations are randomly distributed in A sites, but Li ions are fourfold coordinated at unit cell faces of the cubic perovskite. Lithium conductivity, σ_300 K, decreases with the sodium content, decreasing from values typical of fast ionic conductors, 10^− 3 S/cm, to those of good insulators, 10^− 10 S/cm, when the interconnectivity between vacant A sites is lost (x > 0.3). In samples with x < 0.3, dc conductivity displays a non-Arrhenius behaviour, decreasing activation energy from ~ 0.37 to 0.25 eV when the sample is heated between 77 and 500 K. The temperature dependence of B_Li factors shows the existence of two regimes for Li motion. Below 373 K, Li ions remain partially located near square oxygen windows that connect contiguous A sites, but above 400 K, extended Li motions become dominant. The additional decrease of activation energy from 0.25 to 0.16 eV (low-temperature ⁷Li NMR value), should require the full elimination of octahedral tilting which is only produced above 1000 °C.     

一锅法合成四芳基吡咯并[3,2-b]吡咯空穴传输材料及其在无掺杂倒置钙钛矿太阳电池的应用

本文采用一锅法合成了四芳基吡咯并[3,2-b]吡咯有机空穴输运材料(D41、D42、D43和D44),制备出无掺杂的倒置型平面钙钛矿太阳电池. 材料D41的芳环上含有甲基,具有供体-π-给体-π-供体结构;而D42、D43和D44具有受体-π-给体-π-受体结构,其中,芳环上分别含有氰基、氟和三氟甲基. 研究表明,芳环上取代基对其分子表面电荷分布和空穴输运层薄膜形貌有显著影响,钙钛矿晶体颗粒的大小与空穴输运材料分子结构有关,含有氰基的材料D42最有利于形成较大的钙钛矿晶粒,这主要是由于吡咯并[3,2-b]吡咯结构具有丰富的电子性质的缘故. D42制备的倒置型平面钙钛矿太阳电池光电转换效率为17.3%,在黑暗条件下22天后,仍保留了初始效率的55%. 吡咯并[3,2-b]吡咯结构具有良好的给电子特性,可作为高效钙钛矿薄膜的空穴传输材料.     

Anomalies of ultrasonic velocities, attenuation and elastic moduli in Nd1−xSrxMnO3 perovskite manganite materials

Nd_1−xSr_xMnO₃ perovskite manganite material with different compositions (x=0.31, 0.35, 0.37, 0.39 and 0.41) have been prepared employing solid-state reaction technique. The ultrasonic velocities and attenuation of the above samples have been measured employing through transmission method operated at a fundamental frequency of 5 MHz over wide range of temperatures. The temperature dependence of ultrasonic velocities, attenuation, relative percentage variation in velocities and elastic constants show an interesting anomaly in all compositions. The observed anomalies in ultrasonic parameters at T_c in all compositions have been revealed in terms of existence of ferromagnetic (FM) state. Similarly, the anomalies at T_co show the transition from FM to charge-ordered antiferromagnetic (AFM) state. The observed results have been used to explore the competitions between FM and AFM.