Experimental and modeling study of the growth and the durability of multilayers with ZTAO as a protective layer deposited by magnetron sputtering for low emissivity applications

Abstract

This thesis project aims to understand the growth mechanisms of Ag on Ti, Al co-doped ZnO layer as well as the degradation mechanisms of a silver layer encapsulated by this ZTAO for so-called “low emissivity” (Low-E) applications. Therefore, a bottom-up approach was developed separating the project into three stages, the study of the ZTAO material, the growth of silver on it, and finally the degradation of multilayers of Low-E coatings. The study develops a modeling and experimental approach that makes it possible to highlight the physical mechanisms involved in the various properties studied.
The study of the properties of the ZTAO as a function of the titanium composition shows three main zones. The mechanism of doping occurs by the substitution of Zn into the ZnO wurtzite network by TiO2 units. The first zone (Ti 7.0 at%). In this case, the film becomes amorphous and the morphology is completely disordered.
The growth of silver on ZTAO is the second stage of the project. For the sake of simplification, this study has been limited to the growth of silver on ZnO. An original method to take into account the density of damage induced by the energy species appearing during magnetron sputtering was developed. This model requires a database of the different possible events during the interaction of silver atoms with the substrate. In addition, the simulation at a macroscopic level (with kinetic Monte Carlo) of the silver growth on ZnO requires “ab-initio” and molecular models to compute the barrier energies corresponding to the atomic motions. The conductivity of the simulated films was compared with the experimental measurements showing that the films grown by evaporation fit well to the simulations compared to those grown by magnetron sputtering. This difference is explained by the size of the grains due a to higher impurities, a faster deposition rate, and the generation of defects by sputtered atoms. The influence of a substrate with a periodic pattern as well as the density of defects has been investigated to propose an alternative method allowing 2D growth without modifying the performances of the low emissivities layers. A periodic substrate enables the control of 3D growth. However, 3% of defects promote 2D growth regardless of whether the substrate has a periodic pattern.
The degradation of Low-E coatings was finally studied in a neutral salt spray using either AZO or ZTAO as seed and barrier layers. The model proposed by Ross for Low-E coatings degraded in a wet environment has been extended to neutral salt spray. The corrosive agents diffuse and are preferentially localized at metallic interfaces. ZTAO seems to interact more with Na into the barrier layer compared to AZO. The second stage of silver restructuring does not appear in this case and this is particularly the barrier layer that is delaminated. Elements of the barrier layer and silver are lost by interacting with the environment. Finally, aluminium probably plays a major role in degradation through reactions forming AlClO and aluminium hydrates. The titanium seems to favour the reactions of the aluminium while showing better protection supporting the hypothesis of the key role of aluminium.
Finally, this thesis enables to understand the growth mechanisms of ZTAO as well as the mechanisms limiting the performances of the Low-E coatings during the growth of silver on ZnO. It has been shown that the ZTAO with 4 at% in Ti shows properties similar to the ZnO while allowing a better reactivity of aluminium with the environment and thus better durability. Moreover, the low emissivity performance of silver grown on ZnO is limited by the defect density which lowers the thickness required to get a conductor film. It is necessary to find a balance between a very thin film and desired low emissivity performance. This thesis proposes several new ways to improve and understand the sustainability of Low-E coatings.

Date of Award	22 Mar 2023
Original language	English
Awarding Institution	University of Namur
Sponsors	FSR-FNRS
Supervisor	Stephane Lucas (Supervisor), Robert Sporken (President), Jérôme Cornil (Jury), Roger Smith (Jury) & Nicolas Rivolta (Jury)