P.Q. Mantas

Defect chemistry and electrical characteristics of undoped and Mn-doped ZnO

Abstract Undoped and Mn-doped ZnO samples were sintered at 1373, 1473, and 1573 K, for 2 h, in air, and then quenched to room temperature. Defect concentrations in these samples at the sintering temperatures and at room temperature and below were calculated using a refined defect model. Using the calculated electron concentrations and assuming a constant electron mobility of 100 cm2/Vs, conductivities at room temperature were calculated and compared with experimental ones. The agreement between the experimental and calculated conductivities is very good for all the samples of undoped and Mn-doped ZnO. In the Mn-doped ZnO case, the ionisation energy of the Mn defect in ZnO was estimated to be ∼2.0 eV. Using the experimental conductivities and the calculated electron concentrations, the electron mobilities were calculated between 70 and 300 K. The results show that the temperature dependence of the mobility in undoped ZnO is similar to that in ZnO single crystals observed in other works, and heavy Mn doping significantly reduces the electron mobility below room temperature probably due to impurity scattering. The role of Mn on the electrical conductivity of ZnO could be understood.

Effect of Al and Mn doping on the electrical conductivity of ZnO

Abstract Samples of Al-doped and Mn-doped ZnO with a doping level up to 1.2 mol% were sintered at temperatures from 1100 to 1400°C in air. dc Electrical conductivities of these samples at room temperature and below were measured, and the effects of the doping type, the doping level, the sintering temperature and time on the electrical conductivity of ZnO were investigated. It was found that Al increased the electrical conductivity of ZnO resulting in a manifestation of a metallic electrical conduction behaviour, and a semiconductor-metal transition occured in the Al-doped ZnO samples. For Mn-doped ZnO samples quenched from the sintering temperatures, the electrical conductivity decreased with the increase in the Mn content, but the samples still showed a semiconductor electrical conduction behaviour. In this way, one could obtain a systematic variation of the ZnO electrical conductivity from the high conductivity, Al-doped case, to the high resistivity, Mn-doped one, spanning over eight orders of magnitude, which is explained in the present communication.

Phase coexistence region and dielectric properties of PZT ceramics

In PZT ceramics it is commonly observed that the tetragonal and the rhombohedral phases may coexist around the morphotropic phase boundary (MPB). Some controversy still exists concerning the causes of the real occurrence of the phase coexistence, the distribution of the coexisting phases and their chemical and structural properties. In a previous work we found a relation between the width of the coexistence region and the grain size of the ceramic that could be explained by the statistical distribution model, as long as the elementary phase volumes were considered as the ferroelectric domains inside the grain. In the present work the structural parameters of the phases and the dielectric permittivity of PZT in a compositional range covering the phase coexistence region are determined and analysed. It is observed that in both tetragonal and rhombohedral phases the permittivity increases as the lattice distortion relative to the cubic symmetry decreases. The dielectric permittivities of PZT inside the phase coexistence region were calculated considering that the phase coexistence corresponds to a statistical distribution of phases with the same composition. This model provides dielectric results consistent with the experimental ones. It was also shown that the maximum of the dielectric properties in the MPB does not result from the phase coexistence, but it is a consequence of the approach to a minimum structure distortion.

Hopping conduction in Mn-doped ZnO

The dc and ac conductivities of Mn-doped ZnO were investigated at temperatures from 10 to 100 K. The temperature dependence of the dc conductivity from 10 to 100 K shows an abrupt change at ∼18 K, manifesting a much lower activation energy for conduction below 18 K. From 10 to 18 K, the ac conductivity, σac(ω), varies as σac(ω)=Aωs in the frequency range from 102 to 106 Hz with s in the range of 0.6–1. The dc and ac conductivity observations suggest that the dominant conduction mechanism at temperatures between 10 to 18 K in these samples is a hopping conduction.

Dielectric response of materials: extension to the Debye model

Abstract The Debye model is frequently used to explain the dielectric behaviour of materials. A good description of the experimental results is often obtained by using a distribution of relaxation times. In this work, it is proposed to extend the Debye model to the depolarisation of two or more types of dipoles occurring simultaneously. With this procedure, it is observed that permittivity Cole–Cole plots appear with two or more semicircles overlapping each other, and can give origin to one flattened semicircle. The same type of behaviour is observed for the impedance Cole–Cole plots, and both type of description of experimental data usually give very similar results. Assuming that relaxation times are thermally activated parameters, with positive or negative activation energies depending on the type of the dipoles, it is possible to obtain dielectric responses similar to those of the paraelectric regions of ferroelectric materials, and dielectric peaks on the permittivity-versus-temperature graphs similar to the relaxor behaviour found in some ferroelectric materials. ©

Dielectric properties of Bi doped SrTiO3 ceramics in the temperature range 500–800 K

The (Sr1−1.5xBix)TiO3 (0.0133⩽x⩽0.133) ceramic system reveals several sets of dielectric permittivity peaks in different temperature ranges. Dielectric permittivity and dielectric loss peaks were detected in the temperature range 500–800 K and in the present article the dielectric polarization behavior is presented and discussed. The activation energy of the dielectric relaxation is in the range of 0.99–1.12 eV. It is suggested that the permittivity peaks are related to the movement of oxygen ions or oxygen vacancies.

Deep donors in polycrystalline Mn-doped ZnO

In a previous work, significant varistor behaviour was found in polycrystalline ZnO with Mn as the only additive. In the present work, the admittance spectra of these Mn-doped ZnO samples were investigated in the temperature range from 70 to 300 K. From this study, the microscopic parameters of the deep bulk donors such as their energy levels and capture cross-sections were obtained in the samples with different Mn contents. The origin of the donors in ZnO is discussed by comparing their characteristics found in Mn-doped ZnO with those observed in other ZnO varistor systems.

Varistor behaviour of Mn-doped ZnO ceramics

Abstract Polycrystalline ZnO doped with MnO, from 0.1 to 0.6 mol%, was prepared by conventional ceramic processing. The effect of Mn doping on the electrical properties of ZnO was investigated. Samples quenched from the sintering temperature show ohmic behaviour, while pronounced varistor behaviour is found in Mn-doped ZnO obtained by slow cooling from the sintering temperature or by annealing at a lower temperature. The origin of the varistor effect in the samples of polycrystalline ZnO with MnO as the only additive is discussed. The defect equilibrium analysis suggests that the varistor behaviour in these samples is due to the oxidation of the double ionised zinc interstitial defects present at grain boundaries by ambient oxygen during cooling or annealing, and the presence of Mn in the ZnO grains induces this process.

The barrier height formation in ZnO varistors

Abstract The electrical characteristics of ZnO ceramic varistors are interpreted as being due to the existence of potential barriers at the grain boundaries. The two main models describing the origin of these barriers, surface states at the interface, and surface oxidation, are discussed in this work, where the microstructure and the electrical properties at various temperatures (up to 160 °C) of a standard varistor composition, are used in an attempt to separate both contributions. The temperature dependence of both the distribution of surface states and the Fermi level, when extrapolated to high temperatures, indicates that the surface states control mainly the shape of the I–V curves, and that the surface oxidation contribution to the barrier, although as high as 0.25 eV, is not sufficient to account for the total energy of the barrier. Values for the density of carriers calculated using the dependence of the Fermi level on temperature, were in good agreement with reported values determined by other techniques.

Dielectric relaxation behaviour of Bi:SrTiO3: I. The low temperature permittivity peak

The dielectric properties of (Sr1 − 1.5xBix)TiO3 (0.002≤x≤0.167) ceramics were systematically studied. The temperature dependence of the complexpermittivity was measured in the temperature range of 10–300 K. Permittivity peaks with obvious frequency dispersion were observed and with the increase of the Bi content, the temperature of the permittivity maximum of each one is shifted to higher temperatures. In the high temperature side of the permittivity peaks, notable departure from the Curie-Weiss law was observed. The relaxation behaviour is characterised in the present paper. The effect of Bi on the dielectric relaxation behaviour and the physical mechanism giving polarisation are briefly discussed.