Semiconductor‐Insulator Transition in Undoped Rutile, TiO₂, Ceramics

Abstract

The electrical conductivity of undoped rutile ceramics is very dependent on sample processing conditions, especially the temperature and atmosphere during sintering and the subsequent cooling rate. Samples become increasingly semiconducting when quenched from temperatures above ~700°C without the need for a reducing atmosphere. Thus, samples quenched from 1400°C in air have conductivity ~1 × 10−2 Scm−1with activation energy ~0.01(1) eV over the temperature range 10–100 K, whereas similar samples that are slow cooled or annealed in air at 300°C–500°C are insulating with activation energy 1.67(2) eV and conductivity, e.g., 1 × 10−7 Scm−1 at 400°C. The very wide range of electrical properties is attributed to variations in oxygen content which are too small to be detected using thermogravimetry. Impedance analysis shows that, depending on cooling rate, partially oxidized samples may be prepared in which samples retain a semiconducting core, but have an oxidized outer layer.

The Effect of Electric Field on Sintering and Electrical Conductivity of Titania

“…4,5 In another work, the effect of rapid cooling and annealing temperature on the transition from ionic to electronic conductivity in undoped TiO 2 has been reported. 6 Quenching TiO 2 sample from a gradually increasing dwell temperature changes the nature of conductivity from insulator to semiconductor, which has been attributed to freezing of oxygen vacancies in the structure. The effect of atmosphere (with different partial pressure of oxygen) on the conductivity of sample at room temperature has also been evaluated.…”

“…The effect of atmosphere (with different partial pressure of oxygen) on the conductivity of sample at room temperature has also been evaluated. 6 Doping with pentavalent oxides, such as V 2 O 5 , Nb 2 O 5 which act as donors, raises the dielectric constant, the breakdown strength and the exponent of nonlinearity for the I-V plot in varistor applications. 7 The above properties depend on the sintering treatment, which determines the grain size and the defect concentration.…”

Electrical and Magnetic Properties of NiZn Ferrite Prepared by Conventional and Solar Sintering

“…can give rise to dramatic increases in electronic conductivity. An excellent example is rutile, TiO 2 , which changes from an insulator in slow‐cooled samples to a good semiconductor in samples quenched from e.g., 1200°C, even though the amount of oxygen loss was barely detectable by weight‐loss studies . We suggest that Ni,Zn ferrite may also lose oxygen at high temperatures; if the samples are quenched, the oxygen nonstoichiometry is retained, but if they are slow‐cooled, reoxidation occurs.…”

“…Low measuring temperatures were necessary for impedance data to be accessible over the frequency range of the instrumentation, but there was one notable difference to the solar‐sintered sample data: the C′ spectroscopic plot shows evidence of a third, poorly resolved, low‐frequency plateau at approximately 10 nFcm −1 and the impedance complex plane plots show a third, low‐frequency arc. Capacitance values of this magnitude are usually associated with sample surfaces or sample–electrode interfaces …”

“…which show that the third, lowest frequency arc in the complex plane plot decreases in size with increasing dc bias and is largely absent with 5 V bias. This indicates that the third arc is a potential barrier impedance, rather than a material impedance; it is attributed to formation of a Schottky barrier at the sample–electrode interface, associated with a mismatch in Fermi levels across the interface. In order for Schottky barriers to be detected readily in impedance data, it is usually necessary that their associated resistances are comparable to or greater than those of other resistances within the sample.…”