|Ph.D Student||Baram Mor|
|Subject||Morphology and Thermodynamics of Thin|
Equilibrium Films at Metal-Ceramic Interfaces
|Department||Department of Materials Science and Engineering||Supervisor||Professor Wayne D. Kaplan|
|Full Thesis text|
In recent years there are an increasing number of applications which contain metal-ceramic interfaces (e.g. metal-ceramic composites, electronic packaging systems, etc.). In addition, the decrease in device dimensions increased the influence of interfaces on materials’ properties, which initiated extensive studies on interfaces. As a result, ~1 nm thick intergranular films (IGFs) have been found to exist at grain boundaries and interfaces in different systems, and recently at metal-ceramic interfaces. Although there is a considerable amount of data on this phenomenon, still a significant number of fundamental questions need to be answered. For example, it is a common opinion that the films existence reduces the interface energy but this has never been experimentally proven. In addition, there is great deal of uncertainty regarding the structure of the films.
In this study a novel experimental approach was developed to form equilibrated Au particles on sapphire substrates, in the presence of anorthite glass (CaO-2SiO2-Al2O3). This configuration allowed for analysis of the interface structure and the interface energy by preparing site-specific cross-section transmission electron microscope (TEM) specimens using a modified in-situ lift-out technique in a dual-beam focused ion beam system.
First, a ~1.2nm thick film was indeed observed at the Au-sapphire interfaces. Quantitative high resolution TEM (aberration corrected) was conducted to study the structure of the films, and order was observed in the film adjacent to the sapphire crystal. This periodicity in the film was correlated with “Ca cages” by comparing the experimentally determined atomistic structure with molecular dynamics studies. Additionally, the interface energy was measured and it was found the IGF indeed reduces the interface energy by ~190 mJ/m2 compared to the dry interface (which may be used to improve the poor adhesion of metal-oxide interfaces by introducing IGFs).
This phenomenon was discussed using different models such as: Gibbs isotherm, wetting transitions, and force balance models. The diffuse interface models were found to be the most adequate to explain the IGFs observed in this system (correlating the order and the reduction of interface energy), which can be defined as a ‘complexion’, with a distinct structure and chemistry which corresponds to minima in free energy at the interface. Furthermore, this may assist correlating specific complexions with macroscopic properties (e.g. grain boundary mobility and electrical properties), providing engineering criteria for interface properties. It was also suggested that conventional thin films (e.g. high-k films used in microelectronics) may be considered as complexions as well.