铂铱合金与316L不锈钢微激光点焊气孔形成机理

采用第一性原理方法研究了H2分子在Li2NH (110) 晶面的表面吸附。通过研究Li2NH (110)/H2体系的吸附位置、吸附能和电子结构，发现H2分子吸附在Li长桥位时会发生解离，并在Li2NH (110) 面形成NH2基，其吸附能为1.178 eV，属于强化学吸附，吸附最稳定。此时，NH2基中的H原子与Li2NH表面的相互作用主要源于H 1s轨道与Li2NH表层N原子的2s，2p轨道重叠杂化的贡献，且N–H键为共价键；另一个H原子与Li2NH表面的相互作用主要是与Li之间的离子键作用；H2分子的解离能垒为1.31 eV，表明在一定热激活条件下H2分子在Li2NH (110)表面发生解离吸附。N顶位吸附时，优化结束后形成NH3，但该吸附方式不稳定，可见Li2NH (110)面与H2反应不易直接生成NH3。

Porosity Formation Mechanism in Laser Spot Microwelding of Pt-Ir Alloy and 316L Stainless Steel Wires

The adsorption of H2 on Li2NH(110) crystal surfaces was studied by first principles. Preferred adsorption sites, adsorption energy, dissociation energy and electronic structure of the Li2NH(110)/H2 systems were calculated separately. Results show that H2 adsorbed on the Li long bridge site is more favorable than other sites, while NH2 are formed on the Li2NH(110) crystal surfaces. The calculated adsorption energy on the Li long bridge site is 1.178 eV belonging to a strong chemical adsorption, which is the most stable; meanwhile the interaction between one H atom in the NH2 and the Li2NH(110) surface is mainly due to the overlapping between the 1s states of the H atom and the 2s, 2p states of the N atom on the Li2NH surface, through which covalent bonds are formed between N and H atoms; the interaction between the other H atom and Li2NH(110) surface is mainly the ionic bond between Li and H atoms. A activation barrier of 1.31 eV is found for the dissociation of H2 molecule in N bridge configuration, which indicates the dissociative adsorption of H2 on Li2NH(110) surface is favorable under the certain heat activation condition; NH3 is formed after the optimization of H2 adsorbed on the N top site. The adsorption energy on the N top site is minus, and this adsorption is unstable. So it is concluded that it is not easy to produce the NH3 between Li2NH(110) surface and H2 directly.