First-principles Study Of Structure And Mechanical Properties Of The CoSb₃/Ti Interface

The skutterudite?CoSb₃?have been regarded as one of the most promising materials operating at intermediate temperatures for an excellent figure of merit.The thermoelectric device needs to subject the combined effect of cyclic thermal load and mechanical load during the service process.With the increase of service time,mutual diffusion of Sb and Ti elementsthe at the interface between CoSb₃ and the electrode material will tanken placed.The thickness of this diffusion layer will continue to increase,eventually causing interface cracking under the mechanical load,which affects the reliability of the thermoelectric device significant.The mechanism of interfacial atom diffusion between thermoelectric materials and electrodes and their effects on the mechanical properties of the interface are extremely important for the application of thermoelectric materials.In this paper,the first-principles principle is employed to study the atomic structure,electronic characteristic,bonding characteristics and mechanism of atomic diffusion at the CoSb₃/Ti interface.Based on this,the mechanical properties of the interface structure and the failure mechanism are studied.A feasible scheme for decreas the atomic diffusion at the interface and improving the interface bonding performance is theoretically verified.In this paper,the first principle principle is used to study the microstructure and mechanical properties of CoSb₃/Ti interface.The main research contents are as follows:We performed a first-principles calculations to study the CoSb₃/Ti interfaces aimed at estimating adhesion energy,determining stable interface geometry,and getting further insight into interfacial bonding character.We find that the T3-terminated CoSb₃/Ti interface is the most stable interface.Therefore,the electronic characteristics of the T3 interface and the bonding characteristics of the atoms have been systematically studied.The electrons of atoms near the interface transfer towards the gap between the interfacial Ti and CoSb₃ layer.Electron transfer and orbital hybridization together promote the bonding between atoms.The interfacial bonds are mainly metallic.Few covalence is involved in interface as well.The atom diffusion phenomenon of Sb vacancies in the CoSb₃/Ti interface and Ti vacancies in the Ti layer was investigated respectively.The influence of vacancies on the atomic diffusion of the interface is studied.The microscopic mechanism of atomic diffusion and the bonding characteristics of the interface after diffusion are analyzed.It is found that when there are vacancies at the interface,the interdiffusion of atoms is occur at the interface,mainly the interdiffusion of Sb and Ti atoms.The Co atom at the interface is relatively stable.Finally,the interface diffusion model with the lowest vacancy formation energy was selected as the representative,and the atomic structure and electronic properties of the interface after diffusion were studied.It is used as an initial model for the subsequent study of the mechanical properties of the interface after atom diffusion.At the same time,it is also found that at the interface,the Sb-4 atomic ring structure is very fragile and easily to damage,which may also cause subsequent diffusion of Sb atoms.After Sb diffuses into the Ti layer,the interaction between Sb and Ti atoms is a metal bond.We find that atomic migration occurs easily across the interface.The tensile tests indicate the ideal tensile stress of the CoSb₃/Ti interface after atomic migration decreases about 8.1%than that of the perfect one.The failure of the perfect CoSb₃/Ti interface and the CoSb3/Ti interface after atomic migration both occurs in CoSb₃layer,which consistent with the experimental results.However,the failure mechanism of the perfect CoSb₃/Ti interface is different with that of the migrated CoSb₃/Ti interface.The main reason is the rearrangement of the atom near the interfacial region after atomic migration,such as the new ionic bonds form between Ti and Sb atoms.In addition,the new ionic Ti-Sb bonds make the electrons redistributed and weaken the stiffness of the Co-Sb bonds.For the perfect CoSb₃/Ti interface,the fail of the Co-Sb?Co2-Sb8?bond leads to the failure of the system.For the CoSb₃/Ti interface after atomic migration,the fail of the Sb-Sb?Sb3-Sb4?bond leads to the failure of the system.In order to decrease the atomic diffusion of the interface and improve the mechanical properties of the interface,the Al element is introduced into the Ti in the interface to study the influence of Al element on the atom diffusion of the interface.Considering that the filled CoSb₃ thermoelectric material is better performence.The effect of Yb filling on the atomic diffusion of CoSb₃/Ti interface was also studied.Finally,the influence of doping a small amount of Al in Ti and filling Yb in CoSb₃ on the mechanical properties of the interface was discussed.It is found that the effect of Yb filling in CoSb₃ on interface atom diffusion is negligible.The diffusion mechanism and tensile mechanical properties of interface atoms before and after Yb filling have no difference.The diffusion mechanism and mechanical properties of interface before and after Yb filling have not changed.The doping of Al in Ti has a great influence on the atomic diffusion of the interface,which can significantly inhibit the atomic diffusion at the interface and improve the mechanical properties of the interface.The ideal strength of the relative interface and the perfect interface is4.92 GPa.After doping Al in Ti,the ideal strength of the interface is 6.52 GPa,which is 33%higher.The main reason is that the electronegativity of the Al atom is very weak,and it exhibits a cationic state in the interface structure,and forms a strong ionic bond with the surrounding Co and Sb atoms.At the same time,Al loses electrons in the interface,which is transferred to the CoSb₃ layer near the interface,which enhances the strength of the Sb-Sb bond,and thus the mechanical properties of the interface are greatly improved.