Molecular Dynamics Simulation Of Thermal Conductivity Of Nanoscale Lanthanum Aluminate Thin Films

As an oxide material with perovskite structure,lanthanum aluminate nanofilms have high melting point and nice lattice compatibility,which have wide development prospects in semiconductor devices,high-temperature superconductivity and thermal barrier coatings.At room temperature,the structure appears as a rhombohedral phase.While when the temperature exceeds 800 K,it will be transformed into a cubic structure.The optical,electrical and thermal transport properties of lanthanum aluminate materials can be improved by forming heterojunction with other materials or doping metal ions.To study the thermal transport characteristics of materials,thermal conductivity is an indispensable parameter,which is related to the thermal design and thermal management of devices and determines the performance and reliability of devices.In this thesis,the thermal conductivity of cubic phase and rhombohedral phase lanthanum aluminate nanofilms was simulated based on the NVE ensemble and COMPASS force field by non-equilibrium molecular dynamics simulation method.The effects of film thickness,temperature and oxygen vacancy defects on the thermal conductivity of the films were also explored.The results are mainly divided into two parts:（1）At 1200 K,the thermal conductivity of cubic phase lanthanum aluminate films with film thickness of 3.05-12.21 nm was simulated,and the values ranged from 2.39 Wm^-1K^-1 to2.88 Wm^-1K^-1.With the increase of film thickness,the thermal conductivity also increases correspondingly,showing an approximate linear relationship.Compared with the thermal conductivity of bulk lanthanum aluminate,film has an obvious size effect,which is analyzed theoretically by Boltzmann transport theory.Temperature is also one of the factors affecting the thermal conductivity of thin films.In this thesis,the thermal conductivity of cubic lanthanum aluminate nanofilms in the range of 1200-2000 K was simulated.It is found that the thermal conductivity of lanthanum aluminate films with film thickness of 6.11 nm and9.16 nm is less affected by temperature,which proves that lanthanum aluminate materials have great thermal stability.The thermal conductivity of cubic phase lanthanum aluminate nanofilms with oxygen vacancy concentration between 0.03%and 0.20%was also studied.At1200 K,the higher concentration of oxygen vacancy is,the lower the corresponding thermal conductivity of film with thickness of 4.58 nm is,which value is from 2.51 Wm^-1K^-1 to 2.59Wm^-1K^-1.This result is consistent with the change trend of thermal conductivity in the literature.（2）Since the rhombohedral structure of lanthanum aluminate at room temperature is ideal,the thermal conductivity of the rhombohedral phase lanthanum aluminate films simulated in this thesis is set at room temperature.The film thickness ranges from 5.25 nm to20.99 nm,and the film thermal conductivity ranges from 1.44 Wm^-1K^-1 to 1.81 Wm^-1K^-1which increases with the increase of film thickness.The results are consistent with the trend of cubic phase thermal conductivity,which reflects the effect of film size on thermal conductivity.It can be explained by the theory of gas dynamics.In the preparation of lanthanum aluminate films,oxygen vacancy is the most common defect.In this thesis,the effect of oxygen vacancy concentration in the range of 0.03%-0.14%on the thermal conductivity of lanthanum aluminate films at room temperature was simulated.With the increase of oxygen vacancy concentration,the thermal conductivity of 10.49 nm thin films decreases,ranging from 1.60 Wm^-1K^-1 to 1.68 Wm^-1K^-1.In addition,the thermal conductivity of cubic phase and rhombohedral phase lanthanum aluminate films is compared.It is found that the thermal conductivity of rhombohedral phase lanthanum aluminate films with the same thickness is lower,which can be explained by molecular scattering theory.