NiTi(110)表面氧原子吸附的第一性原理研究

为了研究给定的NiTi的表面氧化过程,在保持体系中Ni和Ti原子总数相等的条件下,构建了一系列Ti原子在表面反位的c(2×2)-NiTi(110)缺陷体系,并利用第一性原理计算研究了氧原子在各种NiTi(110)反位缺陷体系的吸附行为以及表面形成能.计算结果表明:吸附氧原子的稳定性与表面Ti原子的富集程度有很大的关联性,体系表面Ti原子富集程度越高,氧原子吸附的稳定性越高;当覆盖度较高时,由于氧原子的吸附,可使Ni和Ti原子在表面出现反位.在富氧条件(μ_o≥-9.35 eV)下,氧原子在表面第1层中的全部Ni原子与第3层全部Ti换位的反位缺陷体系上的吸附最稳定,此时随着氧原子的吸附,表面上的Ti原子升高,导致向上膨胀生长形成二氧化钛层,且在其下方形成富Ni层,由此可合理地解释实验上发现NiTi合金氧化形成二氧化钛层的可能原因.

First-principles study on the adsorption of oxygen at NiTi (110) surface

NiTi alloys with equiatomic compositions have been widely used as structural materials in aerospace, aviation and other fields due to their shape memory effects and good mechanical performances. At the same time, they are considered as excellent biomedical materials for their biocompatibilities and high fatigue resistances. As structural materials, the oxidation resistance of NiTi alloy should be improved. However, as biomedical materials, the formation of dense TiO₂ layers on the surface of NiTi alloy is required to suppress the release of Ni ions in body liquid. As a result, it is of great significance to study the oxidation mechanism of NiTi alloy. In this work, while the total number of Ti is kept the same as that of Ni atoms in the whole system, a series of defected c(2×2)-NiTi (110) surfaces with antisite of Ti are constructed to further understand the oxidation mechanism of NiTi alloy. The adsorption of oxygen atom at the NiTi (110) surface is investigated by the first-principles calculations. The calculated results show that the stability of the oxygen adsorption is strongly related to the enrichment of Ti atoms on the surface. The higher the enrichment of Ti atoms on the surface, the stronger the adsorption of oxygen atoms is. When the coverage of oxygen is high enough, the adsorption of oxygen atoms on the surface could cause the antisite of Ti atoms on the surface by the exchange of Ni atoms in the first layer with Ti atoms in other layers. Under the O-rich conditions (μ_O ≥-9.35 eV), it is the most stable that the oxygen atoms adsorbed on Ti antisite surface, with the whole Ni atoms in the first surface layer exchanged with the whole Ti atoms in the third surface layer. With the increase of the adsorbed oxygen atoms on the surface, the heights of Ti atoms in the surface layers are raised by the adsorption of oxygen. The TiO₂ layer is formed by the expansive growth, while Ni atoms are enriched beneath the TiO₂. As a result, the reason why the TiO₂ layer is formed on the NiTi alloy surface in the experimental conditions is well explained.