PhD Thesis Arne Lüker final version V4 - Cranfield University

58
Theoretical Considerations and Literature Review
While hydrolysis and condensation reactions again play a role in chelate routes, the key
reaction is chelation of the metal alkoxides, which is illustrated here for acetic acid:
M(OR)n + xCH3COOH → M(OR)n-x(OOCCH3)x + xROH [Eq. 2.30]
The primary reason for using chelating agents is to reduce the hydrolysis sensitivity of
the alkoxide compounds, resulting in solutions that are more easily handled in air.
Chelating agents thus serve a similar function to methoxyethanol. Chelation also results
in molecular modification of the alkoxide compounds, and chelating reagents thus also
dictate the structure and properties of the resulting species. Since ligands such as acetate
and acac have different decomposition pathways than do alkoxy ligands, other important
changes in precursor properties, such as pyrolysis behavior, also result.
Compared to the 2-methoxyethanol process, chelate processes offer the advantages of
relatively simple solution synthesis; involved distillation and refluxing strategies are not
required. The addition of the chelating agent to the alkoxides is typically carried out
during the initial process stages. Although the process eventually produces solutions that
are water-insensitive, the initial phase of the process is usually still carried out under inert
atmosphere conditions. As for 2-methoxyethanol processing, in the production of PZT
films, lead acetate is used in conjunction with zirconium and titanium alkoxides. Either
lead acetate is added to acetic acid for dissolution, followed by addition of the titanium
and zirconium alkoxides [51], or the alkoxides are first chelated by acetic acid, followed
by addition of lead acetate [56]. Alcohol and water are then typically added for control of
solution viscosity and stability [51].
While chelate processes are simple and rapid, the chemistry involved in solution
preparation is quite complex due to the number of reactions that occur. Key reactions
were found to be chelation, esterification, and hydrolysis and condensation [69]. The
complexity of the reactions results in a diminished ability to control precursor structure
compared to true sol−gel approaches. The gain in process simplicity thus comes at a cost.
However, precursor properties are still typically acceptable for film formation. While
characterization of the precursor species formed in this process is not as complete as for
the methoxyethanol route, studies of crystals generated from the reactions of acetic acid