Doctoral defence: Helle-Mai Piirsoo "Mechanical Properties of Nanocomposites with Artificial Periodic Structure"

On 29 August at 14:00, Helle-Mai Piirsoo will defend her doctoral thesis "Mechanical Properties of Nanocomposites with Artificial Periodic Structure" for obtaining the degree of Doctor of Philosophy (in Material Sciences).

Supervisors: 
Aile Tamm, University of Tartu
Taivo Jõgiaas, University of Tartu
Professor Kaupo Kukli, University of Tartu

Oponents:
Ivo Utke (Swiss Federal Laboratories for Materials Science and Technology)
Nicolae Spalatu (Tallinn University of Technology)

Summary: 
Microelectromechanical systems (MEMS) are important components in electronic devices like smart phones and watches. MEMS sensors and actuators consist of microstructures and nanomaterials, that must resist deformation and wear. In the present work, the possibility of modifying the mechanical properties of nanomaterials by fabricating nanocomposites with layered structures was investigated.
Double and triple layered Al₂O₃/Ta₂O₅ thin films were fabricated by atomic layer deposition, accurately controlling the thickness of Al₂O₃ and Ta₂O₅ components in 70 nm thick composite films. The thickness of the oxide layers and their deposition order influenced the mechanical hardness, i.e., material’s resistance to wear. The Al₂O₃/Ta₂O₅ composite films were hardened after annealing, that caused the crystallization of Ta₂O₅ layers. The mechanical properties were influenced by crystallization temperature and orientation of crystallites that depended on the thickness and order of the Al₂O₃ and Ta₂O₅ layers.
In regard with alternatives to the abovementioned compounds, the hardness and elastic modulus, describing material’s resistance to deformation, of 20 nm thick double layered SnO₂/ZrO₂ composite films depended significantly on the deposition order of the component layers.
Further, regarding metal oxides with mixed structure as alternative to multilayered films, the phase composition of ZrO₂ films was dependent on the content of Al₂O₃, used as dopant. The addition of Al₂O₃ densified the crystal lattice and hardened the host material. In addition, the continuity of polycrystalline graphene, and consequently the elasticity of carbon-based materials could be improved, growing Al₂O₃ on graphene.
The research indicated, that it is possible to modify the mechanical properties of thin metal oxide films with an artificial periodic layered structure. Development of mechanically resilient composite films could promote the production of more reliable, powerful and smaller MEMS devices.