【作者】 王优；

【导师】 邢献然；

【作者基本信息】 北京科技大学 ， 冶金工程， 2016， 博士

【摘要】 NdFeO3做为一种重要的钙钛矿型稀土铁酸盐,具有优良的化学、热力学以及机械稳定性,使其在气敏和中温-固体燃料电池阴极材料上都有潜在的应用。由于材料的形貌及化学组分都会影响其最终性能,如何控制材料的生长,实现对其形貌、组成乃至物性的调控,对于深入研究结构与性能的关联,以及最终设计并合成出具有理想性能的材料具有重要的意义。本论文首先简要介绍了钙钛矿型稀土铁酸盐的研究现状,总结和评述了这类材料的结构与性能以及形貌与制备目前的研究进展及面临的主要问题。在此基础上,选择以水热法为主要制备手段合成出形貌可控的NdFeO3单晶粉体,通过对其生长机理的研究优化了合成条件。采用固相模板法制备出具有核-壳结构的Nd1-xCaxFe03 (x=0、0.3)空心球,并研究了材料的气敏性。最后通过第一性原理计算讨论了掺入Ca2+后材料电阻减小的原因。论文的主要内容如下：(1)通过水热法合成出NdFeO3,探讨了水热温度、时间、KOH浓度、矿化剂等对产物组成及形貌的影响。研究表明,形成NdFeO3的最佳条件为：水热260℃下反应5天,溶液中KOH浓度大于10M。并发现KOH浓度对于材料形貌的影响起到至关重要的作用。当KOH的浓度在10-16M之间,产物为方块状的颗粒；当KOH浓度为17M,产物为立体的十字交叉状；当KOH浓度为19M,产物为沿着[010]方向生长的长方形颗粒。对不同形貌大小的产物的磁性进行表征,发现所有材料均表现出弱铁磁性,较小的NdFeO3棒状颗粒表现出较大的矫顽力。对水热过程中物相及形貌随时间变化的研究表明,NdFeO3是由中间产物Fe203微米片与Nd(OH)3纳米棒的溶解-结晶形成的。尝试采用蒸汽水热法以及两步水热法来调控溶液中间产物Fe203溶解速度,显著缩短了水热反应所需时间,从而证明Fe203微米片的溶解是反应过程中的控制步骤。(2)以碳球为模板制备出具有核-壳结构的Nd1-xCaxFeO3(x=0、0.3)空心球,并系统地研究了电阻值较小且具有核-壳结构的Ndo.7Cao.3Fe03空心球的气敏性。在140℃时,其对500ppm的C2H5OH的响应值为17,高于Nd0.7Ca0.3FeO3纳米颗粒。并且其表现出较快的响应与恢复速度。经过10天50次循环测试后材料仍表现出很好的稳定性。这些特性都使其有望在气敏传感器上得到应用。(3)通过第一性原理计算了(x=0.00、0.25、0.50、0.75、 1.00;δ=0.00、0.25)的态密度,发现掺钙后引入的空穴态出现在费米能级之上,表明电子/空穴沿着Fe-O-Fe能较快的移动。因为材料中氧离子的扩散是遵循空穴跃迁机制,随着Ca2+的掺入氧空位形成能降低,产生更多氧空位,材料的离子导电性也随之提高了。很好的解释了掺钙后材料电阻减小的原因,为调控材料的电阻使其达到作为气敏材料与中温-燃料电池电极材料的需求提供了理论依据。更多还原

【Abstract】 NdFeO3, which has good chemical, thermal and mechanical stability, is an important candidate material for gas sensors and intermediate-temperature solid oxide fuel cells (IT-SOFC). Since the morphology and composition have an effect on the properties of materials, the controllable preparation help us to investigate the relationship between them and design new materials with the properties we need. In this dissertation, the current status of studies on typical rare earth orthoferrities was reviewed briefly at first. The research progress and main problems about structure and properties of this kind of material have been summarized, and the various morphology and synthesize method have also been summarized and analyzed. Based on this analysis, the controllable preparation of NdFeOh was prepared by hydrothermal method, and then the synthesis conditions was modulated by analyzing their grow mechanism. The Nd1-xCaxFeO3 (x=0,0.3) core-shell hollow spheres were prepared by solid template method and their sensing properties were investigated. Finally, the first-principles calculation could help us to improve our understanding of the colossally increased conductivity of Ca doped NdFeO3. The work includes the following.(1) NdFeO3 was successfully synthesized by hydrothermal method, and the influence of the temperature, time, the concentration of KOH solution and mineralizer on products were discussed. It was found the morphologies of the final products strongly depend on the concentration of KOH solution. When the concentration of the KOH solution was adjusted from 16 M to 19 M, cuboid, cross-shaped and bar-shaped NdFeO3 crystals were obtained respectively. The bar-shaped NdFeO3 crystals are preferentially oriented along the direction of [010]. The magnetic properties of NdFeO3 crystals with different shapes were also explored, weak ferromagnetism was observed in all the samples, and NdFeO3 crystals with small sizes have the largest coercive field Hc. The studies for time-dependent phase evolution showed that the NdFeO3 was crystalized by the dissolution and precipitation of Fe2O3 microplates and Nd(OH)3 nanorods, and the control step is the dissolution of Fe2O3 microplates. Based on this result, the synthesis condition was modulated, and the hydrothermal time could be optimized.(2) Nd1-xCaxFeO3(x=0,0.3) core-shell hollow spheres were prepared by using carbonaceous microspheres as templates. The gas sensing properties of Nd0.7Ca0.3FeO3 core-shell hollow spheres were systematically investigated. The sensitivity to 500 ppm ethanol was 17 at 140 ℃, which is much higher than that of Nd0.7Ca0.3FeO3 nanoparticles. Also, the rapid response and recovery characteristics were achieved. Meanwhile, after 10 testing days and 50 cyclic tests, the gas response of the Ndo.7Cao.3Fe03 core-shell hollow spheres to 500ppm ethanol at 140℃ was maintained well and nearly constant. The long-term stability, good sensitivity and rapid response and recovery times demonstrate its potential application as the sensing material.(3) Through the electronic structure analysis of NdxCa1-xFeO3-δ (x=0.00,0.25,0.50, 0.75,1.00; δ=0.00,0.25) based on the first-principles calculations, it is found that the hole states introduced by Ca substitution appear just above the Fermi level, which implies a high mobility of electrons/holes along the Fe-O-Fe bonding network. Specifically, it becomes easier to form O vacancies after Ca doping. Since the diffusion of O2- anions occurs through a vacancy hopping mechanism, the ion conductivity is also improved. These findings help us to improve our understanding of the colossally increased conductivity of Ca doped NdFe O3 and turn the electronic conduction for its practical application in gas sensors and IT-SOFC.更多还原