Titanium oxides as an innovation basis for electrodes, varistors and heating conductors

Titanium oxides are highly variable in their chemical composition. This means that their electrical properties also vary widely, so that electrical components can be manufactured with a variety of electrical functions. Fraunhofer IKTS develops titanium oxides for applications in the areas of energy technology, medical technology, electronics, electrical engineering and nuclear fusion.

Titanium oxides have properties typically also found in ceramics, such as abrasion resistance, hardness, rigidity and mechanical strength, which opens up the opportunity for new multifunctional components and parts. Fully oxidized titanium dioxide (TiO₂ ) can be transformed from an electrical insulator to an electrical conductor through doping (Table 1). Titanium dioxide is chemically stable in an oxidizing atmosphere up to more than 1500 °C and can therefore be used in the high-temperature range. Under reducing conditions, titanium dioxide forms oxygen vacancies, resulting in electrical conductivity even without doping. The titanium oxide Magnéli phases of the formula TinO_2n-1 have significant oxygen deficits and very low electrical resistances (Table 1).

Of a lesser importance from a technical standpoint are metal-like oxides such as TiO and TiO_0,5, which behave electrically like metals. However, compared with TiO₂ , titanium oxides with an oxygen deficit are sensitive to oxidation at temperatures above 400 °C and are therefore only stable at moderate temperatures. For single oxides, titanium dioxide achieves an exceptionally high electrical permittivity value (e_r ) of 100. Depending on the frequency, the permittivity values can even reach almost 1000 (Fig. 1). The electrical permittivity can also be adapted. In combination with ZrO2 or CeO₂ , a TiO₂ composite can achieve an almost frequency-independent er value between 30 and 50.

A combination of doped titanium dioxide grains with grain boundary regions that create electronic barriers between the TiO₂ grains (Fig. 2) leads to low-voltage varistors (change from insulator to electrical conductor at a defined switching voltage). In contrast to commercial varistors, this switch from electrical insulation to electrical conduction starts at electric field strength in the range of 10 to 100 V/mm.