Degradation in titanium-containing oxides

The degradation of capacitor dielectrics has been discussed in general terms in Section 5.2.2 and 5.4.1. Here the topic is amplified with regard to titanium-containing oxides. Deterioration can occur under two different sets of conditions and most probably with differing mechanisms. 

Another circumstance is when deterioration becomes apparent under fields in excess of 0.5 MVm-1 at temperatures above 85 °C, and occurs more rapidly the higher the field or the temperature. The fall in resistance has been observed in single crystals of rutile and barium titanate and so must be assumed to be a bulk rather than a grain boundary effect, although there is evidence that grain boundaries play a part in degradation processes in ceramics. 

Degradation can be slowed down by suitable substituents. The presence of donor ions at levels exceeding 2mol.%, e.g. substituting Nb5+ for Ti4+, La3+ for Ba2+ or F for O2, prolongs useful life. Donor ions reduce the concentration of oxygen vacancies, which are relatively mobile, and increase the concentration of cation vacancies. The latter have low mobilities at room temperature and, when combined with holes in the valence band, behave as acceptors (see Section 2.6.2, and Section 5.7.3). 

Manganese at the 1 % level in air-fired dielectrics acts as a palliative. It is present as both Mn4+ and Mn3 +. The former ion must be expected to act as an effective electron trap since it is readily converted into Mn3 +. However, the presence of Mn3+ will result in a corresponding concentration of oxygen vacancies. 

Humidity is known to accelerate degradation and it is argued that the mechanism involves protons incorporated into the structure, as discussed in the context of hydrogen sensors (see Section 4.6.1). Because an hydroxyl group can occupy a vacant oxygen ion site without distorting the lattice, water may be incorporated into the structure according to the reaction  

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