Doctoral dissertation
Charge compensation mechanisms and twinning in doped SnO2-based ceramics
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Abstract
Tin (IV) oxide, SnO2 (cassiterite), is an n-type semiconductor with an advantageous
combination of chemical, electric and optical properties that make it suitable for a range
of applications, from solid-state gas sensors, transparent conductive coatings, oxidation
catalysts and varistors. In varistor ceramics, grain growth studies are of particular interest,
because understanding the growth process can be beneficially used for controlling the
microstructure development and for tailoring grain size-dependent electrical properties,
such as the threshold voltage. Unlike in ZnO varistor ceramics, where the role of dopants
on microstructure evolution is well understood, these effects have not yet been fully
examined in SnO2 ceramics.
In my doctoral thesis, I focused on the synthesis of various compositions of two varistor
systems: SnO2-CoO-Nb2O5 and SnO2-CoO-Ta2O5, and suggested ionic charge compensation
mechanism for both systems. In CoO–Nb2O5–doped SnO2 ceramics, I have shown that Nb5+
ions compensate for Co2+ and Sn2+ ions according to the following charge compensation
mechanism:
6 Sn(IV)Sn (IV )
× ⇌ Sn(II)Sn (IV )
″ + Co(II)Sn (IV )
″ + 4 Nb(V)Sn (IV )
·
Important distinction between Ta2O5 and Nb2O5 addition is considerably lower solid
solubility of Ta in SnO2 and two times higher Co:Ta ratio, suggesting that Co2+ is the only
divalent cation that compensates the charge of Ta5+ on Sn–sites obeying the following
compensation mechanism:
3 Sn(IV)Sn (IV )
× ⇌ Co(II)Sn (IV )
″ + 2 Ta(V)Sn (IV )
· .
The second part of my doctoral thesis is focused on twinning of SnO2. This otherwise
quite common phenomenon in SnO2 has not yet been clarified, while in structurally related
rutile TiO2, the mechanism of twinning is well understood. I tried to determine how specific
dopants influence the formation of twin boundaries, and in turn, how they affect SnO2
grain growth, microstructure development, and the electrical properties of the material. I
have shown that twinning in SnO2 is not chemically induced, but it is probably a result of
topotaxial recrystallization process in the initial stages of SnO2 grain growth.