ZnO Grain Boundaries Research Articles

Thermal Conductivity and Mechanical Properties of the Rubber Stopper Filled with ZnO Nanograins

ZnO nanograins (nZnO) were synthesized by a precipitation method. The nZnO exhibited the average grain size of ~30 nm and the specific surface area of 28.24 m2 g-1. Then, it was incorporated into the ethylene propylene diene monomer (EPDM) rubber and produced to be a rubber stopper. This work studied the effect of nZnO and conventional ZnO (cZnO) as a cure activator on the thermal conductivity and mechanical properties of the rubber stopper. As the content of nZnO increased, the thermal conductivity and mechanical properties are also increased. The obtained results could be reduced successfully from 5 to 2 parts per hundred parts of rubber (phr) compared with a cZnO because nZnO provides a satisfactory thermal conductivity and mechanical properties. Moreover, it also gives uniform completely vulcanized for thick rubber product due to swelling value (Q) between outer and inner portion of rubber stopper is less difference. In this work is concluded that the dosage, grain sizes and grain boundary of ZnO are effective parameter in the thermal conductivity and mechanical properties of the rubber stopper.

High-performance staggered top-gate thin-film transistors with hybrid-phase microstructural ITO-stabilized ZnO channels

In this paper, the ITO-stabilized ZnO thin films with a hybrid-phase microstructure were introduced, where a number of nanocrystals were embedded in an amorphous matrix. The microstructural and optical properties of thin films were investigated. It was found that the grain boundary and native defect issues in the pristine polycrystalline ZnO could be well suppressed. Meanwhile, such thin films also possessed relatively smooth surface and high transmittance in the visible range. Afterwards, the corresponding staggered top-gate thin-film transistors (TFTs) were fabricated at a temperature of 300 °C and exhibited fairly high electrical characteristics, especially with a field-effect mobility of nearly 20 cm2 V−1 s−1 and a subthreshold swing as low as 0.115 V/decade. In addition, the electrical uniformity and the stability of devices were also examined to be excellent. It is expected that the staggered top-gate TFTs with hybrid-phase microstructural ITO-stabilized ZnO channels are promising in the next-generation active-matrix flat panel displays.

Local probe microscopic studies on Al-doped ZnO: Pseudoferroelectricity and band bending at grain boundaries

In this paper, based on piezoforce measurements, we show the presence of opposite polarization at grains and grain boundaries of Al-doped ZnO (AZO). The polarization can be flipped by 180° in phase by switching the polarity of the applied electric field, revealing the existence of nanoscale pseudoferroelectricity in AZO grown on Pt/TiO2/SiO2/Si substrate. We also demonstrate an experimental evidence on local band bending at grain boundaries of AZO films using conductive atomic force microscopy and Kelvin probe force microscopy. The presence of an opposite polarization at grains and grain boundaries gives rise to a polarization-driven barrier formation at grain boundaries. With the help of conductive atomic force microscopy, we show that the polarization-driven barrier along with the defect-induced electrostatic potential barrier account for the measured local band bending at grain boundaries. The present study opens a new avenue to understand the charge transport in light of both polarization and electrostatic effects.

Evaluation of vacancy-type defects in ZnO by the positron annihilation lifetime spectroscopy

Thermal behavior of vacancy-type defects in polycrystalline ZnO was studied by the positron annihilation lifetime spectroscopy. Two-component analysis of the PALS spectra revealed that the defect-related longer-lifetime component decreases as the annealing temperature is raised, and almost disappears within 15 min when annealed at 1,273 K. We also found that the intensity of this component decreases with increasing density of the annealed ZnO pellets; however, little density dependence was seen in its lifetime. These observations evidently suggest that this component having long lifetime of about 400 ps corresponds to the positrons trapped in grain boundaries in the polycrystalline ZnO.

Vacancy breathing by grain boundaries—a mechanism of memristive switching in polycrystalline oxides

Abstract

SbVO4 doped ZnO–V2O5-based varistor ceramics: microstructure, electrical properties and conductive mechanism

The effects of SbVO4 addition on the microstructure, electrical properties and characteristics of grain and grain boundary of ZnO–V2O5 based varistor ceramics were studied using SEM, E–J measurements and impedance spectroscopy. XRD analysis revealed that all the samples consist of main phase of ZnO and the second phase of BiVO4 and Zn7Sb2O12. The microstructural homogeneity of the ceramic was improved through adding SbVO4. With an increase of SbVO4, the average grain sizes decrease from 16.1 to 6.1 μm. The resistivity of grain boundary is approximately constant (~103 Ω). The ZnO–V2O5-based varistor ceramics added with 0.3 mol % SbVO4 sintered at 940 °C for 4 h exhibited good nonlinear properties of α = 51, J = 13.4 μA/mm2 and E 1mA = 416 V/mm.

Mn3O4함량에 따른 ZnO의 결함과 입계 특성

In this study, we investigated the effects of Mn dopant (0.1∼3.0 at% Mn3O4 sintered at 100 0℃ for 1 h in air) on the bulk trap (i.e. defect) and grain boundary properties of ZnO, ZM(0.1∼3.0) using admittance spectroscopy (AS), and impedance-modulus spectroscopy (IS & MS). As a result, three kinds of defect were found below the conduction band edge of ZnO as 0.09∼0.14 eV (attractive coulombic center), 0.22∼25 eV ( ·· ), and 0.32∼0.33 eV ( · ). The oxygen vacancy increased with Mn doping. In ZM, an electrically single grain boundary as double Schottky barrier was formed with 0.82∼1.0 eV of activation energies by IS & MS. We also find out that the barriers of grain boundary of Mn-doped ZnO (��-factor=0.13) were more stabilized and homogenized with temperature compared to pure ZnO.

Control of Varistor Voltage by Grain-size Control of Bi-added ZnO Varistors

Relationship between the grain size of ZnO and the amounts of Bi and Sb for Bi-Ba-Sb-Co-Mn-added ZnO was investigated. The amount of Bi at which the grain size of ZnO became the maximum shifted to the notably larger one by adding small amount of Sb such as 0.01 mol%. The maximum value of the grain size when adding Sb was approximately 1.3 times larger than that when not adding Sb. Because Ba3Mn2O8 was not formed when Sb is added and the much amount of Ba forms the coexistence phase with Bi at grain boundaries of ZnO, the grain size of ZnO becomes large. The tolerance characteristic to the electrical degradation was improved by addition of Sb, because Ba3Mn2O8 was not formed and Mn distributed more uniformly at grain boundaries.

Density functional theory study of stoichiometric and nonstoichiometric ZnO grain boundaries

We present a density functional theory analysis of stoichiometric and nonstoichiometric ZnO tilt grain boundaries (GBs) that reveals under which conditions such extrinsically undoped GBs may become electrically active. In the case of ZnO the self-interaction correction (SIC) scheme used allows a more accurate description of the formation energies as well as the electronic levels than the local density approximation (LDA). The results obtained with the SIC scheme deviate in some crucial ways from the LDA results of recent years. First, stoichiometric and nonstoichiometric ZnO GBs can show occupied deep levels whenever oxygen atoms are undercoordinated. Second, ZnO GBs with an oxygen excess at the boundary plane can exhibit unoccupieddeep levels which may account for an experimentally observed weak varistor effect found in undoped polycrystalline ZnO.

Nano/sub-nano analysis based on high resolution transmission electron microscopy for ceramic materials

Grain boundaries, interfaces and surfaces are very attractive areas from the viewpoint of lattice discontinuity. Doped elements and point defects often segregate there, which largely changes bulk properties such as microstructure, mechanical strength, electrical conductivity. To understand the details of the dopant effects, it is necessary to use nano-scale analysis techniques based on transmission electron microscopy with energy dispersive X-ray spectroscopy and electron energy-loss spectroscopy with nano/sub-nano electron probe. This review contains some nano-scale analysis studies performed in our research group for the grain size control of BaTiO3 polycrystals, electrical properties across single grain boundaries of n-type BaTiO3, SrTiO3 and ZnO, interface structural change of WC–Co based cemented carbides and the preparation of novel dislocation nano-wires. Grain boundaries in BaTiO3 sinters were revealed to exhibit grain boundary faceting by doping a very small amount of excess TiO2. The formation of the facets due to extra Ti–O2 bonding is closely related to abnormal grain growth, which is often observed in TiO2-excess BaTiO3 sinters. A similar faceting feature is also observed in VC-doped WC–Co cemented carbides. Doped VC strongly segregates to WC/Co interfaces to form micro facets. The formation of the micro facets resulted in WC grain size reduction. In ZnO boundaries, doped Pr was revealed to segregate at the grain boundary. The segregated Pr ions are situated in specific atomic columns. Further, electron energy-loss spectroscopy has revealed that the valence of the segregated Pr ions is 3+, which means that they are not acceptors but donors to ZnO. The improvement of varistic effect observed in Pr-doped ZnO is closely related to Zn vacancies enhanced by Pr doping. Meanwhile, dislocations are a kind of one-dimensional lattice defect including extra half planes. By pipe-diffusion of dopants at dislocation cores, we can successfully develop conducting nano-wires in insulating sapphire crystals.

Cr2O3를 첨가한 ZnO의 소결과 전기적 특성

In this study, we have characterized the roles of <TEX>$Cr_2O_3$</TEX> on the sintering and electrical properties of ZnO. The densification and grain growth of Cr-doped ZnO (ZCr) system was mainly influenced by Cr contents. In the beginning of sintering, the densification of ZnO was retarded as reducing the Zni concentration in ZnO lattice with Cr doping. And the densification and grain growth of ZnO was more retarded due to a formation of spinel phase with increasing the Cr contents. ZCr system revealed varistor behavior with nonlinear coefficient <TEX>$\alpha$</TEX> of 3~23 depending on the sintering temperature, implying double Schottky barrier formation on the grain boundary of ZnO. Especially the best varistor characteristics should be developed with 0.1~0.5 at% Cr contents and under <TEX>$1100^{\circ}C$</TEX> in ZCr systems.

Ferromagnetic properties of the Mn-doped nanograined ZnO films

Dense nanograined pure and Mn-doped Zn1−xMnxO polycrystals with x ranging between 0.1–34 at. % were synthesized by the wet chemistry method from butanoate precursors. Pure and Mn-doped ZnO possesses ferromagnetic properties only if the ratio of grain boundary (GB) area to grain volume sGB exceeds a certain threshold value sth. The polycrystals in this work satisfy these conditions and, therefore, reveal ferromagnetic properties. The observed dependence of saturation magnetization on the Mn concentration shows an unexpected nonmonotonous behavior. The increase in saturation magnetization at low Mn concentration is explained by the injection of divalent Mn2+ ions and charge carriers into pure ZnO. The decrease in saturation magnetization between 0.1 and 5 at. % Mn can be explained by the increase in the portion of Mn3+ and Mn4+ ions. The second increase in saturation magnetization above 5 at. % Mn is explained by the formation of multilayer Mn segregation layer in ZnO GBs. The shape of the dependence of saturation magnetization on Mn concentration is different for the Mn-doped nanograined ZnO manufactured by different methods. It is most probably controlled by the topology of GB network (ferromagnetic GB foam) in the ZnO polycrystals.

ChemInform Abstract: Phase Identification and Electrical Properties in ZnO-Glass Varistors.

Ceramic varistors based on ZnO with lead zinc borosilicate glass instead of Bi2O3 were prepared. The effect of sintering conditions on the electrical properties was studied by sintering samples at various temperatures and cooling them at different rates. The sample sintered at 1250°C for 1 h, then furnace cooled, possessed the best electrical properties, as characterized by the highest nonlinear coefficient, lowest leakage current, and lowest degradation. The microstructure and crystal structure of the glass phase of ZnO–glass varistors were examined by means of scanning electron microscopy, transmission electron microscopy, and powder X-ray diffractometry. The glass phase was originally amorphous, but crystallized as an intergranular layer in the sintered and furnace-cooled samples. This crystallized phase was a zinc borate phase (5ZnO·2B2O3), which was identified by X-ray diffractometry, transmission electron microscopy, and Auger electron spectroscopy. The zinc borate phase at the grain boundary of ZnO–glass samples enhanced the nonohmic characteristics of the ceramic varistors.

First-principles density functional study of dopant elements at grain boundaries in ZnO

We present a first-principles density functional theory study of doped ZnO with focus on its application as a transparent conducting oxide, having both high optical transparency and high electrical conductivity. Investigated is the impact of grain boundaries on the physics of atomic defects, and especially the formation energies of oxygen vacancies, cation dopants Al and Ga, and anion dopants N and P are determined. The main goal is to obtain information about the positions of the defect levels generated by the different dopants in the electronic band gap. Because of the known deficiency of the local density approximation (LDA) to yield accurate values for band gap energies for insulators such as ZnO a self-interaction correction (SIC) to the LDA is employed. As atomistic supercell models which contain grain boundaries and dopants are quite large in size we implemented the SIC by means of SIC pseudopotentials which merely increase the computational costs, as compared to the LDA. The main result of our study is that grain boundaries do affect the formation energies for substitutional dopants significantly. Furthermore the position and shape of dopant-induced electronic energy levels at the grain boundaries are changed considerably with respect to the single crystal. This may help us to explain, for example, why N doping can lead to $p$ conductivity at room temperature or why Al or Ga doping can increase the transparency.

Photoluminescence and Structural Properties of ZnO Films Deposited on Si Substrates by Chemical Spray Deposition

The structure and photoluminescence (PL) of ZnO films deposited on Si (111) substrates by chemical spray deposition using aqueous solution of zinc acetate [Zn (CH3 CO2)2 , 2H2O] was investigated. It was found that these properties depend strongly on growth temperature and grain size. The samples grown at the optimum deposition conditions can generate stronger luminescence. It was revealed by X-ray diffraction that the preferred orientation of poly-crystals is along C-axis, with hexagonal wurtzite structure. PL spectroscopy results shows an evident ultra-violet (UV) emission observed near 380 nm with a broad low intensity band centred at 430 nm. The band was reported as being caused by the defects in the grain boundaries of ZnO. Under optimum deposition conditions a low resistivity and a high optical transmittance of the order of 2.8×10−4 Ω cm and 90%, respectively, were obtained.

Formation of a low energy grain boundary in ZnO: The structural unit concept in hexagonal symmetry materials

ZnO thin films prepared by magnetron sputtering on c-plane sapphire may exhibit a columnar growth with an average column diameter depending on the deposition temperature. In this case two epitaxial relationships coexist and adjacent columns are rotated, one from the other, by 90° around the [0001] direction. The long range rotation between domains is in agreement with the theoretical epitaxial relationships, but the local angles oscillate between 27° and 32° due to the formation of interfaces, which settle into low energy configurations with grain boundary dislocations of a Burgers vectors and 5/7 aligned structural units for the 32.2° tilt angle.

Roles of Dopants in Sintering Behavior of ZnO-Based Varistor

Three series of ZnO-based materials with different doping levels were prepared. The correlation between the composition and microstructure, and the roles of main dopants, Bi2O3 and Sb2O3, in the sintering behaviors were proposed. Both Bi2O3 and Sb2O3 evaporated at 1115°C, but the amount of them, in which bismuth is the majority, is not significant. Bi2O3 functioned mainly as liquid during sintering to promote the sintering of ZnO, but it doesn’t mean the materials will be denser. The bismuth-rich phase retracted into small pores during cooling, leaving the big pores as voids at room temperature. More Bi2O3 added would result in less increase in material densities and dramatic decrease in relative densities, and a little bit increase in grain sizes of matrix ZnO. Sb2O3 would react with ZnO matrix into spinel, Zn7Sb2O12, which will pin at the grain boundary of ZnO to control the ZnO grain growth. The more Sb2O3 added, the smaller the grain sizes of ZnO. Appropriate amount of Sb2O3 added will yield denser materials.

Effect of boundary plane on the atomic structure of [0001] Σ 7 tilt grain boundaries in ZnO

The atomic structure of [0001] Σ 7 tilt grain boundaries with { $$12\bar{3}0$$ }‖ { $$12\bar{3}0$$ }, { $$14\bar{5}0$$ }‖ { $$14\bar{5}0$$ }, and { $$10\bar{1}0$$ }‖ { $$35\bar{8}0$$ } boundary planes in ZnO was investigated through high-resolution transmission electron microscopy observation of fiber-textured thin films and atomistic calculations. These boundaries were found to comprise three kinds of common structural units that are characterized by fourfold- to eightfold-coordinated channels along the [0001] direction in contrast to sixfold-coordinated channels in wurtzite structure. The boundary structural units are very similar to the multiple core structures of edge dislocations with Burgers vectors of 1/3 < $$11\bar{2}0$$ > . Transformation between two of the three configurations can easily occur through an atom flipping corresponding to dislocation glide. Depending on the orientation of boundary planes with respect to the Burgers vectors, the dislocation-like units exhibit straight or zigzag arrangements with periodicities corresponding to the Σ 7 misorientation.

Electron transport behaviors across single grain boundaries in n-type BaTiO3, SrTiO3 and ZnO

In some electroceramic materials, their unique electrical properties are due to potential barriers, i.e., double Schottky barriers (DSBs), formed at grain boundaries. So far, some researchers have revealed that the electrical properties of DSB are closely related to grain boundary characters, especially grain boundary coherency. For example, highly coherent boundary does not give PTCR or varistic property, while random types exhibit clear resistivity jump or abrupt current increment. Therefore, a concept of grain boundary design will be required for future device manufacturing, even in bulk materials. But it has not been clarified yet why the electron transport behaviors depend on them. In order to address this question, it is necessary to carry out a systematic experiment focusing on single grain boundaries using well-defined bicrystals.

Atomic structure of [0001]-tilt grain boundaries in ZnO: A high-resolution TEM study of fiber-textured thin films

The atomic structure of [0001]-tilt grain boundaries in ZnO was investigated using high-resolution transmission electron microscopy (HRTEM) and atomistic calculations. HRTEM observation was conducted for [0001] fiber-textured ZnO thin films grown on quartz-glass substrates by the pulsed-laser deposition. The [0001]-tilt boundaries observed in the films can be classified into three types: low-angle boundaries composed of irregular dislocation arrays, boundaries with ${10\overline{1}0}$ facet structures, and near-low $\ensuremath{\Sigma}$ boundaries represented by symmetric periodicity units. The atomic structure of the boundaries is discussed with a focus on a $\ensuremath{\Sigma}=7$ boundary in conjunction with atomistic calculations and HRTEM image simulations. The $\ensuremath{\Sigma}=7$ boundary consists of multiple structural units that are very similar to the core structures of edge dislocations. Straight or zigzag arrangements of the dislocationlike structural units constitute other high-angle boundaries with symmetric and ${10\overline{1}0}$ facet structures as well. It is suggested that [0001]-tilt boundaries in ZnO are generally described as an array of the dislocationlike units.