A.M.R. Senos

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.

WET SYNTHESIS AND CHARACTERIZATION OF MODIFIED HYDROXYAPATITE POWDERS

Hydroxyapatite powders with reproducibly different Ca/P ratios and powders with varying amounts of co-precipitated magnesium, sodium and potassium were synthesized by a wet method. Solids composition, particle size and morphology, crystalline structure, sintering behaviour and microstructure were investigated in order to understand the effect of composition in the properties of the powders. Under the present conditions of synthesis, it was concluded that magnesium, sodium and potassium will enter the hydroxyapatite lattice in vestigial amounts. Magnesium gives rise to a co-precipitated amorphous phase which crystallizes as β-whitlockite on calcining. A relationship was found between the Ca/P ratio in hydroxyapatite and its sinterability. The sintering process of pure and alkali-containing hydroxyapatites is completed at 1300 °C and leads to dense ceramics in the case of pure and sodium-containing hydroxyapatites but not in potassium-containing material.

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.

Synthesis and dielectric properties of tungsten-based complex perovskites

Ba 2 MeWO 6 (Me = Mg, Ni, Zn) double perovskites were prepared by the conventional solid-state reaction in a wide temperature range. Single-phase ceramics were obtained only at low temperatures approximately 1200 °C, whereas a small amount of second phases existed in the samples sintered at higher temperatures. All the compounds are characterized by the cubic perovskite structure (space group Fm3m) with a complete NaCl type ordering between B-site ions. Anomalous temperature variation of the dielectric loss tangent found in the Ba 2 NiWO 6 perovskite is supposed to be connected with a dielectric relaxation due to electronic hopping within thermally activated Ni 3 + -6W ( 6 - 1 / 6 ) + /W 5 + -6Ni ( 2 + 1 / 6 ) + clusters. Dielectric measurements showed that the other two perovskites-Ba 2 ZnWO 6 and Ba 2 MgWO 6 -exhibit a positive value of the temperature coefficient of permittivity. Such temperature variation is assumed to be caused by a considerable influence of the second polar mode involving B-site ion vibrations on the low-frequency dielectric properties.

Thermal and mechanical properties of aluminum titanate–mullite composites

The thermal and mechanical properties of aluminum titanate–mullite composites prepared by a gel-coated powder processing method, namely a mullite precursor gel coating aluminum titanate particles (containing 2.5 wt% MgO), were investigated. A microstructure with both elongated and equiaxed mullite grains was observed in the composite samples. Both the mechanical strength and the thermal expansion coefficient increased with the mullite amount. The mechanical strength showed a strong dependence on the grain size of aluminum titanate with an exponent of −3. Inhibition of aluminum titanate grain growth due to the presence of the mullite phase enhanced the mechanical strength of the composites. The thermal shock resistance tended to decrease as the mullite content increased. The thermal stability of aluminum titanate was improved in the present composite system.

Stainless steel coatings sputter-deposited on tungsten carbide powder particles

Abstract The aim of this work was to study the feasibility of a sputtering technique to coat WC powder particles, regarding it as an alternative to the conventional mixture of powders. For such purpose, a stainless steel 304 (AISI) coating was sputter deposited on WC powder particles using a magnetron sputtering equipment specially developed to coat powder particles. The morphology of the coated powder was characterized by scanning electron microscopy observations, Brunauer–Emmett–Teller and laser diffraction measurements. The crystallographic structure was determined by X-ray diffraction. Inductively coupled plasma–atomic emission spectrometer and electron microprobe analysis were used to characterize the amount, chemical composition and distribution of the sputtered coating. The characterization results indicated that all WC particles were coated and that all the steel constituent elements were deposited in the same original proportion. The coating had a ferrite b.c.c. structure and presented a columnar growth with some porosity. The compaction behavior of the coated powders was characterized by unidirectional pressing using pressures between 60 and 250 MPa. The maximum of relative density was attained for P ≥190 MPa, with values of 57–58% of relative density, comparable to that of non-coated powders, and without the need of any pressing binder to obtain green compacts resistant to handling. High sintered densities, of approximately 95%, were obtained at a relatively low temperature of 1325 °C with only ∼6 wt.% of binder phase in the coated powders.