【作者】 于帅芹；

【导师】 尹衍升；

【作者基本信息】 中国海洋大学 ， 海洋化学工程与技术， 2008， 博士

【摘要】 随着凝聚态物理学、量子化学等相关基础学科的深入发展,以及计算机计算能力的空前提高,使得从原子、分子水平上进行材料设计,成为现代先进材料合成技术的重要发展方向。而在新材料的微观结构设计的研究中,团簇的计算设计扮演了十分重要的角色。本文针对金属电化学腐蚀的本质,应用密度泛函理论方法,对一系列铁铝、铁硅和铁镍的铁基合金团簇,按照“由小到大逐级生长”的方法,从理论上设计和预测了它们的几何和电子结构,寻找了这些团簇的基本结构单元、成键规则和生长机制。主要目的是可望通过理论计算获取一组量化参数并结合实验数据,从原子、分子水平上探讨铁基合金耐腐蚀性能,为今后设计最优耐腐蚀材料提供理论指导。主要研究内容及成果如下:1.应用密度泛函的BPW91方法结合Lanl2dz基组研究了Fen(n≤13)团簇的结构和电子性质,该方法被证实能正确反映团簇的基本信息,节约了计算时间,而且获得的平均结合能较其他密度泛函方法更为接近实验值。团簇向着高对称性的方向生长。二阶能量差分表明了n=6和n=8为铁团簇的稳定幻数,且Fen(n≥8)团簇倾向包含Fe6和Fe8簇的方向解离,进一步暗示Fe6和Fe8簇具有相对高的稳定性。Fe8在238 cm-1处有红外强峰出现,恰好对应Fe6团簇的八面体伸缩振动峰值,暗示八面体结构的特殊稳定性。且八面体作为BCC晶格结构的一部分,包含了晶胞的6个面原子,充分说明Fe6簇能很好的体现金属铁的特征,为Al、Si、Ni原子掺杂铁团簇提供了理论模型。2.应用BPW91/Lanl2dz/6-31G(d)方法深入研究了Al原子掺杂Fen(n≤8)团簇的基态结构和电子性质,并获得了较为准确的几何信息。为消除基组误差,在Lanl2dz/6-31G(d)混合基组优化结构的基础上,应用全电子基组6-311++G(2d, 2p)进行单点计算,获得的能量与该基组全优化结果一致。计算结果发现一个Al原子很难掺杂到铁团簇结构的中间,倾向于分布在铁簇的表面,并且它的掺杂也难以改变铁簇的主体结构。进一步,本文对Fen-1Al团簇进行了稳定性分析,其中,二阶能量差分、HOMO-LUMO能隙以及垂直电离势的分析结果都一致表明Fe3Al具有特殊的稳定性。另一方面,本文以Fe6簇为基体,逐个加入Al原子来模拟不同Al含量掺杂的铁簇的几何和电子结构,计算结果发现随杂质Al原子数目的增多,FemAln(m+n=6)团簇的最稳定构型由八面体结构变为双三角锥戴帽,向着对称性低的方向生长。本文还发现Fe5Al,Fe4Al2以及Fe3Al3簇具有相对高的HOMO-LUMO能隙和较低的HOMO能,揭示这几种团簇具有较高的稳定性,相应的Al的原子百分含量恰好处于Fe3Al和FeAl金属间化合物的Al含量范围内,说明应用密度泛函理论方法获得的量化参数如HOMO能以及HOMO-LUMO能隙可以从原子、分子角度反映Fe-Al合金的耐腐蚀性能。此外,为了验证我们的理论结果,本文从实验角度上探讨了Fe3Al金属间化合物的耐海水腐蚀性能。应用失重法测定了Fe3Al在海水环境下的年腐蚀速率,并与碳钢做了比较,发现Fe3Al较碳钢更耐海水腐蚀。通过对二者腐蚀后表面形貌分析以及开路电位的测定结果也都显示出Fe3Al具有优异的抗海水腐蚀性能,很好地支持了我们的理论结果。3.应用与铁铝合金簇相同的计算步骤,详细研究了Si掺杂铁团簇的几何和电子结构。与纯铁团簇相比,掺杂Si可以增强团簇的稳定性。此外,计算结果发现Fe3Si为铁硅合金团簇的幻数簇,具有相当高的稳定性。且Si和Fe原子组成比例为1:3时的原子百分含量恰好对应质量百分比14.36%Si,此含量的Fe-Si合金具有最优耐腐蚀性能,可见Fe3Si簇很好的体现出类块体结构特征,进一步说明所有表征Fe-Si团簇稳定性的量化参数都可以用来表征相应块体结构的耐腐蚀性能。4. Ni作为不锈钢的主要合金成分,极大的提高了其抗腐蚀性能。基于此,本文应用密度泛函方法研究了铁镍合金簇的基态结构和成键机制。经过稳定性分析发现,Fe3Ni、Fe4Ni、Fe5Ni和Fe6Ni这几种小团簇均具有较高的稳定性,且Fe与Ni的原子组成比例与具有优异抗腐蚀性能的FeNi合金中的Ni的掺杂含量一致。在这几种团簇中,Fe5Ni稳定性最高,对应于Ni的质量百分含量为16.7%,在此,我们大胆预言16.7%Ni为Fe-Ni合金抗腐蚀性能的最佳掺杂含量,期待实验的进一步验证。更多还原

【Abstract】 With the deep development of the condensed matter physics and quantum chemistry as well as the improvement of the level of the computer, the material design based on the atomic and molecular becomes an important direction of development for the synthesis technology of advanced materials. Moreover, the theoretical calculation of clusters plays a very important role in the study of the new material microstructure design.According to the nature of electrochemical corrosion of metal and guided by the DFT, a series of iron-aluminium, iron-silicon and iron-nickel alloy clusters are studied in order to discover their ground state structures and the growth mechanisms as well as the building blocks, to design and predict of their geometric and electronic structure in this paper. Our main purpose is to explore the corrosion-resistant property of iron-based alloy clusters by theoretical calculations in order to obtain a group of quantitative parameters combined with experimental data at the atomic and molecular level, and provide theoretical guidance for designing the most optimal corrosion-resistant materials. The main study contents and results of this paper are described in detail as follows:1. Using the BPW91 method combined with Lanl2dz basis sets, we study on the geometric, electronic properties of Fen(n≤13) clusters in this chapter. The BPW91 method is proved to be able to obtain correct information and save the computing time. Moreover, the average bonding energy per atom obtained is closer to the experimental results than that of other DFT methods. The small iron clusters favors highest-dimension. The results of the second finite difference of the total energy and the HOMO-LUMO energy gap show that n=6 and 8 are magic number which are more stable than neighboring ones. And the Fen(n≥8) cluster are easy to be dissociated to Fe6 and Fe8, which further indicate Fe6 and Fe8 clusters have special stablility. A strong peak at 238 cm-1 is found for Fe8 cluster agreed with the stretching vibration frequency of octahedron for Fe6, which indicates the octahedron structure has special stability. Moreover, the octahedral structure is part of BCC lattice structure, including atoms of six cell surface. That fully prove Fe6 cluster can very well reflect the properties of iron metal, and it can provide a theoretical model for Al, Si, and Ni doping iron clusters.2. After study the ground state and electronic properties of Al-doped Fen(n≤8) clusters by using the BPW91/Lanl2dz/6-31G(d) method, We obtain accurate geometric information in a kind of way. In order to eliminate the error of basis sets, single-point BPW91/6-311++G(d, p) calculations are performed for all the structures by using the geometries obtained from the BPW91/lanl2dz//6-31g(d) optimizations. Furthermore, the single-point BPW91 calculations for Al-doped Fen(n=2~4) are very close to those obtained by the fully optimized BPW91 results.Calculated results show an doped Al atom prefers to the surface of the iron structures and do not change the frame structures of iron atoms. Further, we analyze the stability of Fen-1Al clusters, and combined with the results of the second finite difference of the total energy as well as the HOMO-LUMO energy, the vertical ionization potential (VIP) shows that the Fe3Al has special stability. On the other hand, the Fe6 cluster is selected to model the geometric and electronic structures of FemAln with different Al content by adopting Al atom one by one. The most stable structures of FemAln(m+n=6) clusters transfer from octahedron to the capped trigonal bipyramid, growing towards low- dimension. Our calculations show Fe5Al,Fe4Al2 and Fe3Al3 have large HOMO-LUMO gap and low HOMO value, indicating these clusters have high stablility, of which the Al content of atom percentage for these cluster methioned above are within Al content of Fe3Al and FeAl intermetallic compounds. Therefore, the quantitative parameters such as HOMO-LUMO gap and HOMO can characterize Fe-Al alloy corrosion resistance at atom and molecular level.Experimentally, we present the electrochemical corrosion behaviors of Fe3Al in seawater due to vertify our theoretical results. the annual corrosion rate of the Fe3Al is compound by using weight loss method, the corresponding results indicate Fe3Al have more excellent corrosion resistance than carbon steel. Subsequently, by analyzing the surface morphology of corrosion of Fe3Al and carbon steel as well as the measurement of the open circuit potential, Fe3Al is found to have excellent corrosion-resistance which indicates that the properties of bulk materials can be obtained from a finite size cluster model.3. The geometric and electronic structures of Si-doped iron clusters are investigated by using the same method as that of Fe-Al alloy clusters. Compared with pure iron clusters, the doping of Si atom can improve the stability of iron clusters. By analyzing the stability of Fe-Si alloy clusters, the Fe3Si and Fe5Si are found to be magic clusters. The Si content of Fe5Si is 14%Si which locates in the best Si-doped scope of corrosion resistance of Fe-Si alloy. Clearly, the study on small clusters can make a better understanding of the corresponding materials.4. As a major component of stainless steel, Ni-doping steel improved its corrosion resistance greatly. Theoretical, we study the ground-state and the bonding mechanism of Fe-Ni alloy clusters with the density functional theory method. By analyzing the stability of the ground state structures, Fe3Ni、Fe4Ni、Fe5Ni和Fe6Ni are found have high stability, and the corresponding atomic ratio of Fe and Ni atoms for those clusters is consisted with the doped Ni content of Fe-Ni alloy possessing excellent corrosion resistance. Our theoretical results show Fe5Ni is most stable in those clusters, the corresponding content of Ni is 16.7%Ni. In light of the results, we predict 16.7%Ni is the optimized doped contend for the corrosion resistance of Fe-Ni alloy.更多还原