Conductivity Of ZnO Research Articles

Correlations for thermal conductivity and viscosity of water based nanofluids

The thermo-physical properties of nanofluids such as thermal conductivity, viscosity, density and specific heat of nanofluids are required for the analysis of convection heat transfer coefficients. The density and specific heat of nanofluids can be estimated with the mixture relations in literature. Information regarding the properties at different volume concentration and temperature is required for the estimation of heat transfer coefficient. The two most fundamental properties which are, experimentally, determined, are viscosity and thermal conductivity. Investigators have been determining the properties of nanofluids at different temperatures and base liquids. The present work is an attempt to analyze the available data to develop a non-linear regression equation for the estimation of thermal conductivity and viscosity of water based nanofluids. In the present study, nanofluids are considered as a homogenous medium and the parameters influencing the thermo physical properties identified. Equations are developed for the analysis of thermo-physical properties of nanofluids as a function of parameters viz., material, concentration, temperature and particle size useful for designer. The opposing nature of thermal conductivity rise and viscosity decrease with temperature; dependence of nanofluid thermal conductivity on material properties alters the range of applicability of nanofluids for heat transfer applications. The thermal conductivity and viscosity of Al2O3, ZnO and TiO2 dispersed in water are measured to validate the proposed equations. The result shows that the equations are able to predict the thermal conductivity and viscosity of different types of nanofluids of different particle diameters closely.

Preparation and corrosion resistance of nanostructured PVC/ZnO–polyaniline hybrid coating

ZnO–polyaniline nanocomposite with core–shell nanostructure was prepared by in situ polymerization of aniline monomer in the presence of ZnO nanoparticles. Fourier transform infrared spectroscopy, X-ray diffraction patterns, field emission scanning electron microscopy and transmission electron microscopy techniques were used to characterize the composition and structure of ZnO–polyaniline nanocomposite. d.c. electrical conductivity measurement showed that the electrical conductivity of ZnO–polyaniline nanocomposite pellets is higher than that of pristine polyaniline and ZnO nanoparticles pellets. The addition of ZnO nanoparticles causes to the increasing of polyaniline electrical conductivity. ZnO–polyaniline nanocomposite was mixed with polyvinyl chloride (PVC) through a solution mixing method and the three components PVC/ZnO–polyaniline hybrid material was applied as coating on iron coupon by the solution casting method. Corrosion protection efficiency of PVC/ZnO–polyaniline hybrid coating on iron coupons was studied by open circuit potential and Tafel techniques in 3.5% NaCl solution as corrosive environment. According to the results, PVC/ZnO–polyaniline hybrid nanocomposite coating showed dramatically increased corrosion protection effect on iron samples compared to that of uncoated iron coupon and pure polyaniline anticorrosive coating. It was found that ZnO nanoparticles improve the barrier and electrochemical anticorrosive properties of polyaniline and the addition of polyvinyl chloride increases the barrier effect of polyaniline coating.

Mg doping in wurtzite ZnO coupled with native point defects: A mechanism for enhanced n-type conductivity and photoluminescence

From first-principles calculations, we show that Mg energetically prefers to substitute Zn atoms rather than occupying interstitial octahedral or tetrahedral sites in ZnO structure. The Mg substitutions significantly reduce the formation energies of Zn/O vacancy and Zn interstitial defects, which consequently promotes the n-type conductivity and photoluminescence in ZnO. The defects form complexes with moderate binding energies, as supported by the shorter Mg-O bonds relative to Zn-O bonds. In agreement with available experimental data, Mg substitutions result in contraction along c-axis and extension along a-axis in wurtzite ZnO.

Comprehensive investigations on the feasibility of nitrogen as a p-type dopant in ZnO

It is controversial whether nitrogen can dope ZnO p-type or not. According to our first-principles calculations, no defects or defect complexes can result in p-type conductivity in ZnO. NO and NZn–2Oi have deep transition levels, while NZn–2VZn has a high formation energy at the valence band maximum (VBM), rendering all of them unlikely to contribute to the p-type conductivity. Besides, NZn will heavily compensate the p-type doping of ZnO in O-rich conditions, while (N2)O and (NO)O will do that in Zn-rich conditions. All these results raise serious questions about the feasibility of nitrogen as a p-type dopant.

P-Type Conductivity in N-Doped ZnO: The Role of theNZn−VOComplex

Although nitrogen-doped zinc oxide has been fabricated as a light-emitting diode, the origin of its p-type conductivity remains mysterious. Here, by analyzing the surface reaction pathway of N in ZnO with first-principles density functional theory calculations, we demonstrate that the origin of p-type conductivity of N-doped ZnO can originate from the defect complexes of N(Zn)-V(O) and N(O)-V(Zn). Favored by the Zn-polar growth, the shallow acceptor of N(O)-V(Zn) actually evolves from the double-donor state of N(Zn)-V(O). While N(Zn)-V(O) is metastable, the p-doping mechanism of N(Zn)-V(O)→N(O)-V(Zn) in ZnO will be free from the spontaneous compensation from the intrinsic donors. The results may offer clearer strategies for doping ZnO p-type more efficiently with N.

Transport and surface conductivity in ZnO

Abstract

Stable interstitial dopant–vacancy complexes in ZnO

Using ab initio calculations, we have identified stable group-V dopant-vacancy complexes complexes in ZnO consisting of interstitial dopants surrounded by three V${}_{\mathrm{Zn}}$ (${D}_{I}$-3V${}_{\mathrm{Zn}}$ with $D=\mathrm{P}$, As, or Sb). In contradiction with previous reports, our calculations show that the acceptor level of group-V dopant--vacancy complexes is too deep to be the shallow acceptor level identified experimentally as contributing to $p$-type conductivity in ZnO. The interstitial-vacancy complexes we have identified can be generalized to other compositions, dopants, and structures.

Investigation of Source of N-Type Conductivity in Bulk ZnO

In this study, we have investigated the source of the n-type conductivity in bulk ZnO. Four samples with Zn concentrations 52%, 53%, 54% and 55 % were prepared by molecular beam epitaxy (MBE). ZnO layers exhibited reamarkable increase in carrier concentration (5.0×1016 cm-3 to 2.2×1019 cm-3) and decrease in resistivity (14.4 Ω cm to 0.009 Ω cm) as a function of Zn contents (52% to 55%) in the layers, respectively. We observed that with increasing thickness, the microstructure of films was improved. The thick films have less Zn-interstitials defects and consequently have lower carrier concentration and higher resistivity. The shifting of Raman E2high mode of ZnO towards higher frequency with increasing Zn contents in the samples also confirmed the presence of excessive Zn-interstitials in the layer.

Influence of ZnO on the Electrical Properties of YSZ

Impedance spectroscopy was performed on dense YSZ/ZnO composites with a ZnO content varying from 3 to 25 vol% and under different oxygen pressures. Raman spectroscopy showed that the cubic zirconia phase is not fully stable in contact with ZnO, under the selected sintering conditions (1300°C, 2h). The impedance diagrams reveal a peculiar blocking effect associated with the ZnO grains. Its magnitude is independent of the large variations in conductivity of ZnO, associated with changes in the equilibrium oxygen pressure. Its relaxation frequency markedly depends on this parameter. These peculiar properties are interpreted in terms of a pseudo-capacitive behavior of the electronically conducting ZnO in contact with the YSZ ionic conductor. This contact forms an electrode, the potential of which being determined by the predominant point defect concentration in ZnO.

Sol-gel derived transparent conducting ZnO:Al thin films: Effect of crystallite orientation on conductivity and self-assembled network texture

The effect of crystallite orientation on the conductivity of ZnO:Al (AZO) thin films has been investigated by means of sol-gel technique. The sol composition has a pronounced influence on the crystallite orientation and size of the AZO thin films. A lower sheet resistance of the films with high c-axis orientation is attributed to the improved packing density caused by perfect crystallite alignment rather than an anisotropic effect. Network texture with uniformly protruding “walls” arranged randomly on the film surface is formed at a higher Al content of 5.0at.%. A further optimization of film resistivity and texture may find an application of sol-gel AZO thin films in thin film solar cells.

Improved conductivity of aluminum-doped ZnO: The effect of hydrogen diffusion from a hydrogenated amorphous silicon capping layer

Plasma-deposited aluminum-doped ZnO (ZnO:Al) demonstrated a resistivity gradient as function of the film thickness, extending up to about 600 nm. This gradient decreased sharply when the ZnO:Al was capped by a hydrogenated amorphous silicon layer (a-Si:H) and subsequently treated according to the solid phase crystallization (SPC) procedure at 600 °C. The resistivity reduced from 1.2 · 10−1 to 2.6 · 10−3 Ω · cm for a film thickness of 130 nm, while for thicker films the decrease in resistivity was less pronounced, i.e., a factor of 2 for a film thickness of 810 nm. While the carrier concentration was not affected, the mobility significantly increased from 7 to 30 cm2/V · s for the thick ZnO:Al layers. This increase was ascribed to the passivation of grain boundary defects by hydrogen, which diffused from the a-Si:H toward the ZnO:Al during the SPC procedure. The passivation effect was more pronounced in thinner ZnO:Al layers, characterized by a smaller grain size, due to the presence of large grain boundaries. For thicker films with grain sizes up to 200–300 nm the mobility became progressively less affected by the presence of grain boundaries. Therefore, the hydrogen-induced improvement in conductivity was less significant for the thick ZnO:Al films.

Synthesis, Crystal Structural and Electrical Conductivity Properties of Fe-Doped Zinc Oxide Powders at High Temperatures

The synthesis, crystal structure and electrical conductivity properties of Fe-doped ZnO powders (in the range of 0.25{15 mol%) were reported in this paper. I-phase samples, which were indexed as single phase with a hexagonal (wurtzite) structure in the Fe-doped ZnO binary system, were determined by X-ray difiraction (XRD). The solubility limit of Fe in the ZnO lattice is 3 mol% at 950 ‐ C. The above mixed phase was observed. And the impurity phase was determined as the cubic-ZnFe2O4 phase when compared with standard XRD data using the PDF program. This study focused on single I-phase ZnO samples which were synthesized at 950 ‐ C because the limit of the solubility range is the widest at this temperature. The lattice parameters a and c of the I-phase decreased with Fe-doping concentration. The morphology of the I-phase samples was analyzed with a scanning electron microscope. The grain size of the I-phase samples increased with heat treatment and doping concentration. The electrical conductivity of the pure ZnO and single I-phase samples was investigated using the four-probe dc method at 100{950 ‐ C in air atmosphere. The electrical conductivity values of pure ZnO, 0.25 and 3 mol% Fe-doped ZnO samples at 100 ‐ C were 2£10 i6 , 1.7£10 i3 and 6.3£10 i4 S¢cm i1 , and at 950 ‐ C they were 3.4, 8.5 and 4 S¢cm i1 , respectively.

Texture‐etched ZnO as a versatile base for optical back reflectors with well‐designed surface morphologies for application in thin film solar cells

AbstractZnO layers with a wide range of surface morphologies were studied for applications in thin film silicon solar cells. The texture of ZnO layers, achieved by chemical etching, was measured by atomic force microscopy (AFM) and statistically analyzed in terms of the roughness, the crater depth, the crater diameter, and the crater opening angle. We present a method to independently tune the lateral and vertical dimensions of the etched crater structures by combined variations of ZnO layer thickness and etching time. The optical scattering properties of the textured ZnO layers are investigated. The effects of these morphologies on the performance of the solar cells, prepared in both p–i–n and n–i–p configurations are analyzed. For the latter cell type the roughness is introduced at the back side, thus the cell does not sense ZnO absorption and conductivity of the different morphologies. The link between the morphology, the optical properties of ZnO layers, and the solar cell performance is discussed.

P-Type behaviour of electrodeposited ZnO:Cu films

Cu-doped ZnO (ZnO:Cu) thin films and ZnO/ZnO:Cu homojunction devices were electrodeposited on conductive glass substrates in a non-aqueous electrolyte containing Cu and Zn salts. The Cu content of the films is proportional to the Cu/Zn precursor ratio in the deposition electrolyte. ZnO:Cu was found to be of a hexagonal wurtzite structure with (002) preferred orientation. A transition from n-type to p-type was observed for ZnO:Cu films with a Cu/Zn ratio higher than 2% as inferred from the change in the direction of the photocurrent. The rectifying characteristics shown by homojunction devices further confirm the p-type conductivity of ZnO:Cu layers.

Stable p-Type Conduction from Sb-Decorated Head-to-Head Basal Plane Inversion Domain Boundaries in ZnO Nanowires

We report that Sb-decorated head-to-head (H-H) basal plane inversion domain boundaries (b-IDBs) lead to stable p-type conduction in Sb-doped ZnO nanowires (NWs) due to Sb and O codoping. Aberration-corrected Z-contrast scanning transmission electron microscopy shows that all of the Sb in the NWs is incorporated into H-H b-IDBs just under the (0001) NW growth surfaces and the (0001) bottom facets of interior voids. Density functional theory calculations show that the extra basal plane of O per H-H b-IDB makes them electron acceptors. NWs containing these defects exhibited stable p-type behavior in a single NW FET over 18 months. This new mechanism for p-type conduction in ZnO offers the potential of ZnO NW based p-n homojunction devices.

Ga Ordering and Electrical Conductivity in Nanotwin and Superlattice-Structured Ga-Doped ZnO

We have investigated the Ga-ordering controlled by structural changes from nanotwin to superlattice in Ga-doped ZnO (GZO) targets for transparent conductive oxides (TCOs) and discussed the distribution effect of Ga atoms on electrical conductivities of GZOs. The nanotwin and superlattice structures were preferentially formed by Ga-doping and sintering at high temperature. The relative fraction of nanotwin increased above transition concentration (TC ≈ 5.6 wt % Ga). Here, we found that Ga atoms at nanotwin are distributed as clustered and disordered states, while they are completely ordered in superlattice. Ultimately, the superlattice leads to high electrical conductivity in GZOs rather than the nanotwin.

Role of deep and shallow donor levels on n-type conductivity of hydrothermal ZnO

The residual n-type conductivity of O-polar hydrothermally grown ZnO single crystals and the role of annealing on the transport properties are assessed by temperature dependent Hall measurements on a wide 20–800 K temperature range. A deep level lying 250 meV below the conduction band is responsible for the residual n-type conductivity of unannealed samples. After annealing, a shallow donor level with 25 meV ionization energy becomes responsible for the sample conductivity in the room temperature range. Thanks to high temperature Hall measurement, the coexistence of the deep and the shallow level has been demonstrated in the case of annealed sample.

Proton and oxide-ion conduction in ZnO doped SnP2O7 ceramics

A series of Sn1−xZnxP2O7 (x=0, 0.03, 0.06, 0.09, 0.12) ceramic samples have been prepared by a conventional solid state reaction method. The ceramic samples of x=0.00–0.09 exhibited a single phase structure, and the doping limit of Zn2+ in SnP2O7 was at least x=0.09. The conduction behaviors of the ceramic samples were investigated by means of electrochemical methods including AC impedance spectroscopy, isotope effect, gas concentration cells etc. in the temperature range of 300–650°C. Among the ceramic samples studied, the highest conductivities were observed for the Sn0.91Zn0.09P2O7 sample to be 1.69×10−4Scm−1 in wet air and 1.91×10−4Scm−1 in wet hydrogen at 600°C. Isotope effect confirmed the ceramic samples possessed proton conduction under water vapor-containing atmosphere. It was found that the sintering temperature of 1200°C in present investigation could result in not only appropriate densities, but also high conductivities of ceramic samples. It was also found that in wet hydrogen atmosphere Sn0.91Zn0.09P2O7 was almost a pure ionic conductor, the ionic conduction was contributed mainly to proton and partially to oxide-ion, whereas in wet air atmosphere, the ceramic sample exhibited a mixed ion and electron hole conduction.

Characterization of ZnO and ZnO: TiO2 Thin Films Prepared by Sol-Gel Spray-Spin Coating Technique

In this work, the preparation of ZnO and ZnO: TiO2 thin films were conducted on glass prepared by sol-gel spray-spin coating technique. The performance and properties of nanostructured ZnO and ZnO: TiO2 thin films at different thickness were investigated. The effect of thickness on the electrical properties and optical properties have been characterized by using 2-point probe KEITHLEY 2400 source meter and UV-Vis spectrophotometer respectively. From the current-voltage measurement it shows the conductivity of nanocomposite ZnO: TiO2 was higher compare to nanostructure ZnO. The structural properties reveal the surface morphology of nanocomposite ZnO: TiO2 exhibit high surface area as the thickness of thin films increase. The optical properties show the band gap for nanocomposite ZnO: TiO2 decreases as the thickness increase.

Blocking Effect of ZnO in YSZ/ZnO Composites

Impedance spectroscopy analysis was performed under different oxygen pressures, on dense YSZ/ZnO composites with a ZnO content varying from 3 to 25 vol% and two concentrations, 8 and 10 mol% of Y2O3 in YSZ. Raman spectroscopy showed that the cubic zirconia phase containing 8 mol% Y2O3 is not stable in contact with ZnO, under the selected sintering conditions (1300°C, 2 h), in contrast to that containing 10%. The impedance diagrams reveal a peculiar blocking effect associated with the ZnO grains. Its magnitude is quasi independent of the large variations of the ZnO conductivity, associated with changes in the equilibrium oxygen pressure. On the other hand, its relaxation frequency markedly depends on the non-stoichiometry of this compound. These peculiar properties are interpreted in terms of a pseudo-capacitive behavior of the electronically conducting ZnO in contact with the YSZ ionic conductor. This contact forms an electrode, the potential of which being determined by the non-stoichiometry of ZnO.