Electrical properties of ZnO varistors prepared by direct mixing of constituent phases

Varistor samples containing different amounts of constituent phases were prepared by direct mixing of constituent phases. Detailed electrical characterization was performed to explain the influence of minor phases (spinel and intergranular phases) on overall properties. Characterization included investigation of the non-linear coefficients (alpha), breakdown electric field (E-B), leakage currents (J(L)), grain boundary barrier hight (Phi(B)) and constant beta from current-voltage characteristics, as well as calculation of activation energies for conduction (E-A) from ac impedance spectroscopy in the temperature interval 30-410 degrees C. Varistors sintered at 1100 degrees C for 1 h showed pronounced differences in electrical properties depending on relative molar ratios of the phases. Results were discussed in the sense of possible reduction of the content of minor phases in ZnO varistors

TEM and DFT Study of Basal-plane Inversion Boundaries in SnO2-doped ZnO

In our recent study (Ribie et al. 2020) we reported the structure of inversion boundaries (IBs) in Sb2O3 -doped ZnO. Here, we focus on IBs that form in SnO2-doped ZnO. Using atomic resolution scanning transmission electron microscopy (STEM) methods we confirm that in SnO2-doped ZnO the IBs form in head-to-head configuration, where ZnO4 tetrahedra in both ZnO domains point towards the IB plane composed of a close packed layer of octahedrally coordinated Sn and Zn atoms. The in-plane composition is driven by the local charge balance, following Pauling's principle of electroneutrality for ionic crystals, according to which the average oxidation state of cations is 3+. To satisfy this condition, the cation ratio in the IB-layer is Sn4+ : Zn2+ =1:1. This was confirmed by concentric electron probe analysis employing energy dispersive spectroscopy (EDS) showing that Sn atoms occupy 0.504 +/- 0.039 of the IB layer, while the rest of the octahedral sites are occupied by Zn. IBs in SnO2-doped ZnO occur in the lowest energy, IB3 translation state with the cation sublattice expansion of Delta IB(zn-zn) of +91 pm with corresponding O-sublattice contraction Delta IB(O-O) of -46 pm. Based on quantitative HRTEM and HAADF-STEM analysis of in-plane ordering of Sn and Zn atoms, we identified two types of short-range distributions, (i) zigzag and (ii) stripe. Our density functional theory (DFT) calculations showed that the energy difference between the two arrangements is small (similar to 6 meV) giving rise to their alternation within the octahedral IB layer. As a result, cation ordering intermittently changes its type and the direction to maximize intrinsic entropy of the IB layer driven by the in-plane electroneutrality and 6-fold symmetry restrictions. A long-range in-plane disorder, as shown by our work would enhance quantum well effect to phonon scattering, while Zn2+ located in the IB octahedral sites, would modify the the bandgap, and enhance the in-plane conductivity and concentration of carriers

Effect of parameters of pulsating current (PC) regimes on morphology of electrodeposited copper in hydrogen co-deposition range

The most often employed electrolytes for the electrodeposition of copper are those based on aqueous solutions of sulfuric acid (H2SO4) and cupric sulfate (CuSO4). There is an ionic equilibrium of a lot of species in the CuSO4-H2SO4-H2O system. Electrodeposition processes at high current densities and overpotentials are very suitable for experimental verification of this ionic equilibrium, because there is parallelism between copper electrodeposition and hydrogen evolution reaction at high current densities and overpotentials. In this study, it is shown that effects attained by the choice of appropriate parameters of square-waves pulsating current (PC) on morphology of electrodeposited copper were equivalent to those obtained by electrodepositions at the constant overpotential in the hydrogen co-deposition range from six solutions of different concentrations of CuSO4 and H2SO4. In this way, it was clear that it is possible to simulate the ionic equilibrium of the species in the CuSO4-H2SO4-H2O system using the only one electroplating solution if the appropriate parameters of PC regimes are used.Belgrade, Serbia, June 6-10, 201

Effect of parameters of pulsating current (PC) regimes on morphology of electrodeposited copper in hydrogen co-deposition range

The most often employed electrolytes for the electrodeposition of copper are those based on aqueous solutions of sulfuric acid (H2SO4) and cupric sulfate (CuSO4). There is an ionic equilibrium of a lot of species in the CuSO4-H2SO4-H2O system. Electrodeposition processes at high current densities and overpotentials are very suitable for experimental verification of this ionic equilibrium, because there is parallelism between copper electrodeposition and hydrogen evolution reaction at high current densities and overpotentials. In this study, it is shown that effects attained by the choice of appropriate parameters of square-waves pulsating current (PC) on morphology of electrodeposited copper were equivalent to those obtained by electrodepositions at the constant overpotential in the hydrogen co-deposition range from six solutions of different concentrations of CuSO4 and H2SO4. In this way, it was clear that it is possible to simulate the ionic equilibrium of the species in the CuSO4-H2SO4-H2O system using the only one electroplating solution if the appropriate parameters of PC regimes are used.Belgrade, Serbia, June 6-10, 201

Effect of the electrolysis regime on the structural characteristics of honeycomb-like electrodes

The effect of different current regimes of electrolysis on the micro- and nanostructural characteristics of open porous structures was examined by the analysis of honeycomb-like copper electrodes obtained by constant galvanostatic (DC) electrodeposition and by regimes of pulsating (PC) and reversing (RC) current. An increase in the number of holes fowled by detached hydrogen bubbles, the decrease in wall width between holes and changes in surface morphology around holes from cauliflower-like agglomerates of copper grains to dendrites were observed in the following order: the DC, PC and RC regime. The hole size formed in the RC regime was smaller than the hole size formed in the DC and PC regimes. Analysis of the obtained structural characteristics showed that the specific surface area of the honeycomb-like electrodes was increased by the application of the PC and RC regimes in relation to the DC regime

Electrical properties of mechanically activated zinc oxide

Microstructural properties of a commercial zinc oxide powder were modified by mechanical activation in a high-energy vibro-mill. The obtained powders were dry pressed and sintered at 1100 degrees C for 2 h. The electrical properties of grain boundaries of obtained ZnO ceramics were studied using an ac impedance analyzer. For that purpose, the ac electrical response was measured in the temperature range from 23 to 240 degrees C in order to determine the resistance and capacitance of grain boundaries. The activation energies of conduction were obtained using an Arrhenius equation. Donor densities were calculated from Mott-Schottky measurements. The influence of microstructure, types and concentrations of defects on electrical properties was discussed

New inversion boundary structure in Sb-doped ZnO predicted by DFT calculations and confirmed by experimental HRTEM

Today, ab-initio calculations are becoming a powerful tool to perform virtual experiments that have the capacity to predict and to reproduce experimentally observed non-periodic features, such as interfaces, that are responsible for quantum properties of materials. In our paper we investigate 2D quantum-well structures, known as inversion boundaries OM. Combining atomistic modeling, DFT calculations and HRTEM analysis we provide a new fundamental insight into the structure and stability of Sb-rich basal-plane IBs in ZnO. DFT screening for potential IB model was based on the known stacking deviations in originating wurtzite structure. The results show that the model with A beta-B alpha-A beta C-gamma B-beta C sequence (IB3) is the most stable translation for Sb-doping, as opposed to previously accepted A beta-B alpha-A beta C-gamma A-alpha C (IB2) model. The key to the stability of IB structures has been found to lie in their cationic stacking. We show that the energies of constituting stacking segments can be used to predict the stability of new IB structures without the need of further ab-initio calculations. DFT optimized models of IBs accurately predict the experimentally observed IB structures with lateral relaxations down to a precision of similar to 1 pm. The newly determined cation sublattice expansions for experimentally confirmed IB2 and IB3 models, Delta(IB(zn-zn)) are +81 pm and +77 pm, whereas the corresponding O-sublattice contractions Delta(IB(0-0)) are -53 pm and -57 pm, respectively. The refined structures will help to solve open questions related to their role in electron transport, phonon scattering, p-type conductivity, affinity of dopants to generate IBs and the underlying formation mechanisms, whereas the excellent match between the calculations and experiment demonstrated in our study opens new perspectives for prediction of such properties from first principles

Synthesis and characterization of anodic substrates for IT-SOFCs based on proton conductors

Cermet anode substrates based on high temperature proton conductors Ni-BaCe0.9Y0.1O3- δ (Ni-BCY10) were prepared using wet chemical method. The microstructural properties of sintered pellets were investigated using X-ray diffraction analysis and field emission scanning electron microscopy. Impedance spectroscopy measurements were used for evaluation of electrical properties of the anode pellets. The high conductivity values of reduced anodes confirmed the percolation through Ni particles even for anode with reduced amount of nickel. Fuel cell tests were carried out and fuel cells with the cermet anode showed higher power output comparing to the fuel cells with platinum electrode

Bonding nature in Mg doped ZnO system - charge density topology approach

Doping of semiconductors is commonly used for tailoring electrical, magnetic, optical and structural properties of ceramic materials. Understanding the effect of doping on stability of the material is important in order to improve the process and obtain targeted properties. Commonly used approach for determining energy contribution of doping is based on comparing the heat of formation of doped and undoped crystals. Our idea is to go beyond this thermodynamic approach and determine energy contribution of each particle in bulk structure. Doped zinc oxide has been a subject of many studies concerning doping, and this makes it a good choice to compare our results to pervious findings. Magnesium as an isovalent dopant makes the model simpler for computation, and Mg-doped zinc oxide has been previously studied in both theory and experiment. Our assumption was that periodic placement of dopants would lower the energy of the system compared to a random placement of dopants. Therefore, a supercell model replicated in 3D space has been used. The model is made of eight unit cells (2×2×2) with a single zinc replaced with magnesium (6.25 at%). Our aim is to determine energy contribution of both, a zinc in pure zinc oxide structure and of magnesium in doped supercell. We combined linearized augmented planewave (LAPW) method with quantum theory of atoms in molecules (QTAIM). This approach enables us to determine the effect of doping on stability of the structure. Various methods that extend beyond DFT, like LDA+U Hubbard approach and hybrid functionals for exact description of exchange term in the Hamiltonian of the system are used to determine precise electronic structure, while QTAIM-based partition of supercell space allows us to calculate various integral contributions (charge density, energy) of each particle taking any site in the supercell

Mechanochemically assisted solid-state synthesis of Cu substituted thermoelectric sodium cobaltite

Polycrystalline samples of Cu substituted NaCo2-xCuxO4 (x=0, 0.01, 0.03, 0.05, 0.1) were prepared using mechanochemically assisted solid-state reaction method starting from the Na2CO3, Co3O4 and CuO powders. Each powder mixture was mechanically activated by grinding for 3 h in a planetary ball mill with ball to powder mixture ratio 20:1, at the basic disc rotation speed of 360 rpm. The asprepared powders were pressed into disc-shaped pellets and subsequently subjected to a heat treatment at 880 °C for 24 h in inert argon atmosphere. Changes in structural characteristics of the samples and particle morphology, caused by Co substitution by Cu, were characterized using X-ray diffraction and scanning electron microscopy, respectively. It should be emphasized that milling process reduced the time necessary for obtaining pure sodium cobaltite. From the results obtained in this study, observed changes in microstructure were correlated with changes in the lattice parameters, indicating the influence of Cu ion incorporation in NaCo2O4 crystal lattice