【作者】 赵明；

【导师】 秦宏伟；

【作者基本信息】 山东大学 ， 材料物理与化学， 2010， 硕士

【摘要】 钙钛矿稀土氧化物ABO3可以应用在磁阻抗材料、催化材料、高温超导材料,也可以应用于气敏材料。作为气敏材料,它具有高度的稳定性、良好的选择性和优秀的灵敏度。其气敏性质可以通过对A位,B位进行部分替代或者同时进行部分替代而改变,甚至提高。近年来这种极具重要理论意义和使用价值的材料引起了广大科研工作者的强烈兴趣。人们通常会通过用低价阳离子元素比如Ca、Sr、Ba、Pb等来部分替代LaFeO3中La的位置或用Cu、Co、Ni、Mn等来替代Fe的位置来改善他们的气敏性。La1-xCaxFeO3是一种重要的催化材料,但是对于其对CO这种重要的污染性气体的催化性几乎没有被研究过,本文用溶胶凝胶法合成了La1-xCaxFeO3纳米粉体,并对其气敏特性,稳定性及其导电性质进行了研究。为了进一步深入探讨,我们基于第一性原理的计算模拟了La7CaFe8O24(0 1 0)表面的氧吸附情况,发现Fe离子对氧吸附起着主导作用。并且无论初始吸附状态如何,最终都吸附在Fe离子上,也印证了B位在化学吸附中的主导作用。同时在其基础上研究了La7CaFe7CoO24的表面O2吸附,观察到了解离吸附的过程。本文主要结论如下：1.XRD图谱表明获得的粉体均为单一的钙钛矿结构,并无杂相出现,说明Ca确实替代了La的位置而没有替代Fe的位置。随着Ca含量的提高样品La1-xCaxFeO3的晶胞参数逐渐减小,导致晶胞容量减小,晶粒大小也逐渐减小,这是因为Ca离子(100pm)比La离子(103pm)稍微小一点,当Ca替代La后晶胞参数就减小,晶胞收缩。2.通过对气敏元件的电阻测量我们发现La1-xCaxFeO3(x=0.1,0.2,0.4)在CO中的电阻在相同温度下都大于空气中的电阻,属于P型半导体,x=0.3时气敏元件在CO中导电特性出现了从p型到n型的转化,p-n转化温度为380℃。我们还发现随着x的增加,空气中的元件电阻先减小,后增大。这是由于气敏元件中电价补偿和空位补偿同时并存相互竞争的结果：电价补偿占主导时,空穴增加,电阻变小；空位补偿增大后,自由电子增加,电阻增大。3.当x=0.1,0.2,0.3时,La1-xCaxFeO3对于CO具有较好的灵敏度,其中x=0.2时,温度为100℃,灵敏度达到最高。我们又测量了x=0.2时气敏元件的稳定性及其反应时间,得到了较好的气敏元件特性参数。La0.8Ca0.2FeO3可以考虑用来做为测量CO的气敏材料。4.用基于密度泛函的第一性原理计算方法研究了La7CaFe8O24(0 10)表面和氧在其表面上的吸附。La7CaFe8O24(0 1 0)表面的表面态出现在费米能级的附近,主要由Fe 3d的轨道组成。表面Fe离子对氧吸附起到主导作用。在Fe离子上的吸附比在La和O离子上的吸附要稳定的多,吸附氧在表面Fe离子上的成键机制是O 2p和Fe 3d轨道之间的强相互作用。此外,我们的计算表面氧分子在La7CaFesO24(0 1 0)表面上的吸附属于分子化学吸附和物理吸附。具体公式如下5.LaTCaFe7CoO24(0 1 0)表面和氧气分子在其表面上的吸附用Dmol3模拟。Co原子的掺杂并没有改变钙钛矿结构。表面发生了驰豫现象而没有重构,计算预示了Fe和Co原子在吸附过程中的作用。七种吸附模型中发现了物理吸附,化学吸附,解离吸附3种吸附,在5种分子化学吸附模型中Co原子的吸附能力最小,在Fe-O-Co上初始位置的O2发生了解离吸附(O-O键长达到了4.0474A),都吸附在Fe上,这些都说明Fe原子在La7CaFe7CoO24(010)表面上比Co原子吸附能力要强,依然起主导作用。总之,我们用实验和计算两种方法研究了La1-xCaxFeO3纳米气敏材料,得到了稳定性高、响应快、灵敏度良好的气敏材料,深入了解了La1-xCaxFeO3的气敏机理,为探索新的气敏材料做了铺垫。更多还原

【Abstract】 ABO3 perovskite rare-earth oxides can be used in magneto-impedance materials, catalytic materials, high-temperature superconducting materials, but also can be served as gas-sensing meterials. As a gas sensor, it has sound stability, good selectivity and excellent sensitivity. Another excellence is that its characteristic can be controlled to get even better by electing suitable A and B atoms or chemical doping in ABO3. ABO3 material which has great theorical significance and utility value has aroused interest among the researchers.Generally, gas-sensing properties of LaFeO3 can be improved by substituting lower valance-cations, such as Ca, Sr, Ba, Pb for La, or substituting Fe by Cu, Co, Ni, Mn and so on. La1-xCaxFeO3 is a kind of very important photo catalysis material. Its gas sensing property of CO has not being studied yet. Since CO is a kind of very important toxic gas, in this article La1-xCaxFeO3 powders were prepared by the sol-gel method, and were characterized for the gas sensing property, stability and conductivity.For the further understanding of the ABO3 materials, we calculated the oxygen adsorption on LaFeO3 surface based on first-principle calculation. We found that Fe ions dominated the oxygen adsorption. Wherever intial position was placed, the optimized position all focused on the surface Fe ion, which illustrated B ion dominated chemisorption.The abstract of our results as follows:1. X-ray diffraction patterns (XRD) of La1-xCaxFeO3 show that La1-xCaxFeO3 nanocrystalline is perovskite phases with the orthorhombic structure, no obvious other phases are observed. It also illustrates that La ion is replaced by Ca ion. With an increase of x, the mean crystallite size decreases, cell volume decreases and also the lattice parameter. That is because calcium ion (100pm) is slightly smaller than lanthanum ion (103pm). When lanthanum ion is substitutd by calcium ion, the lattice parameter decreaces leading to a shrink of unit cell. 2. After measuring the conductivity of the gas-sensing device, we find that resistance of La1-xCaxFeO3 (x=0.1,0.2,0.4) in CO is bigger than resistance in air under the same temperature, which illustrates that La1-xCaxFeO3(x=0.1,0.2,0.4) is a kind of P-type semiconductor. La1-xCaxFeO3 (x=0.3) is found a transformation through p-type semiconductor to n-type semiconductor. The turnover temperature is 380℃. It is found that with an increase of x, the device resistance decreases at first then increases. That is because electrical valence compensation and vacancy compensation. Two kinds of mechanism both exist in La1-xCaxFeO3, and combat with each other. When the electrical valence compensation dominates, holes increase and resistance increases. On the other hand, the domination of oxygen vacancy compensation leads just opposite consequence.3. La1-xCaxFeO3 (x=0.1,0.2,0.3) have good sensitivity. La1-xCaxFeO3 (x=0.2) reachs the peak of sensitivity at 100 degrees Centigrade. The stability and respond time are detectived. The gas sensing device property parameters are pretty good. Lao.8Cao.2Fe03 could be used as a CO gas sensor in the future.4. Clean La7CaFegO24 (010) surface and adorption on it have been investigated at the level of density functional theory based on first-principle calculation. The surface states of LaFeO3 (010) surface appear near Fermi energy level mainly caused by Fe 3d orbital. The surface Fe ions dominate the oxygen adsorption process. The adsorbed O2 on Fe ion is much more stable than that on La and O ions, and the bonding mechanism of adsorbed O2 on surface Fe ions is the strong interaction between O 2p and Fe 3d orbital. In addition, our calculations indicate that O2 dissociation on LaFeO3 surface belongs to chemisorbed-precursor mechanism. The following equilibria show the adsorption process 5. La7CaFe7CoO24(010)surface and O2 adsorption on La7CaFe7CoO24(010)surface have been studied at the level of density functional theory based on first principles calculation using Dmol3. After doping Co into La7CaFegO24, La7CaFe7CoO24 maintained perovskite phase. In the La7CaFe7CoO24(01 0)surface, surface relaxation was found and surface reconstruction was not. Surface partial density of states suggested Fe and Co atom play an important role in O2 adsorption process.7 modes was built to simulate the different adsorption site of O2. Physisorption, molecular chemisorption and dissociative chemisorption were found in 7 modes. Co adsorption energy was smaller than other 4 kinds of molecular chempisorption. Dissociative chemisorption was found in O-site (O atom in Fe-O-Co) mode (O-O bonds length was 4.0474A.) and dissociative O atoms were both adsorbed on Fe atom. The result shows that Fe atom adsorption ability is stronger than Co in La7CaFe7CoO24(010)surface.Above all, perovskite nano-crystalline La1-xCaxFeO3 is studied using calculation and experimental method. The gas-sensing material has good stability and sensitivity. We gain further insight into the gas-sensing mechanism of La1-xCaxFeO3 material. The study foreshadows the seeking for new gas-sensing materials.更多还原