NiO Grains Research Articles

Optical and structural characterizations of NiO-doped PMMA films and fabrication of Ag/NiO-doped PMMA/n-Si/AuSb for analogue-digital switching applications

Herein, the authors report the NiO-doped PMMA nanocomposites fabrication process with different Ni concentrations of 0.001–10 wt% on pre-cleaned glass, ITO, and n-type Si via spin-coating technique. X-ray diffraction (XRD) was used to analyze the crystal structure of NiO in powder form and the NiO-doped PMMA nanocomposite thin films. The Rietveld refinement method was used to analyze the manufactured NiO's powder-form structural characteristics. The analysis data revealed a single phase of NiO nanoparticles with face-centred cubic of Fm-3m space group. The average crystallites' mean size and the microstrain value were determined based on the Williamson-Hall model and were equal to 32.47 ± 1.210 nm and 5 × 10−5, respectively. The FESEM micrographs illustrate dispersed cubic-shaped NiO grains in the PMMA matrix. The estimated energy gaps and the width of the localized energy states (Urbach tails) are plotted against the photon energy. The estimated energy gap, EgOp is increasing from 3.649 ± 0.003 eV to 3.782 ± 0.005 eV with increasing the NiO nanoparticles' doping ratio in the PMMA as a host insulating polymer. The fabricated Ag/NiO-PMMA (S4)/n-Si/AuSb heterojunction device revealed two different (HRS and LRS) resistance-switching behaviours. For second and third electrical (I–V) paths, the extracted magnitudes of the ideality factor are 2.397 ± 0.006 and 2.691 ± 0.003 and the calculated barrier height are 0.727 ± 0.004 and 0.886 ± 0.005, respectively. The interfacial electronics states NSS increasing from 6.58 × 1012 to 3.7 × 1014 cm−2.eV−1 and 3.48 × 1013 to 3.65 × 1014 cm−2 .eV−1 for the 2nd HRS path and 3rd LRS, respectively. The investigated hysteresis loop and the variation in interfacial electronics states reveal that the NiO-PMMA nanocomposite may be promising in non-volatile memory and electrical switching electronic devices.

Epitaxial growth of nano-texturized NiO films on MgO (001) substrates by a reactive magnetron sputtering method

Epitaxial films with nanostructures are of great significance for some special applications. In this work, nano-texturized NiO films are epitaxially grown on MgO (001) substrates by a reactive magnetron sputtering method. The evolution in morphology is studied as a function of deposition time, and the influences of substrate temperature, oxygen partial pressure, and the substrate treatment before deposition of NiO on the formation of nano-texturized NiO films are explored. It is found that the non-symmetrically epitaxial growth of nano-texturized NiO films can be linked with the surface damages of MgO (001) substrate, which leads to the coalescence of adjacent NiO grains preferentially occurs along the direction of 〈100〉 and 〈010〉, but they do not coalesce simultaneously following the other directions. In addition, the optical property of nano-texturized NiO films is studied using the Cauchy model.

Promoting methane combustion activity and stability by tuning multiple Ni–Si interactions in catalysts

Catalytic combustion holds great promise for eliminating unburned methane released by the natural gas industry, and the crucial factor is the development of cost-effective and efficient catalysts. Herein, the silicon component was introduced into nickel oxide by simultaneously regulating the precipitation of nickel precursors and the hydrolysis of silicon precursors. The generation of SiO4 groups in NiO inhibited the growth and sintering of NiO grains, meanwhile promoting the formation of active Ni2+ and adsorbed oxygen species for activation and oxidation of methane. With the increase of Si content, the existence of surface nickel silicate on NiO further improved the sinter resistance of NiO, but caused the diminish of Ni2+ and surface adsorbed oxygen species. The catalytic activity and stability were co-enhanced via facilely tuning the multiple Ni–Si interactions. The catalyst with abundant SiO4 groups and negligible nickel silicate exhibited considerably higher catalytic activity, sinter-resistance and water-resistance than NiO toward methane combustion.

Nickel oxide scale microstructure and accelerated growth in combustion flue gas: Effect of ash particles

Model Ni-25Cr and Ni-25Cr-2Mn-1Si (wt%) alloys coated with an industrial fly ash were exposed to a wet CO2 gas at 750 °C. It was found that the outer thermally-grown NiO scale encapsulated ash particles at an accelerated rate. TEM combined with TKD analysis revealed that nano structured NiO grains on the ash surface contributed to the observed NiO encapsulating effect.

Selective hydrogenation of oleic acid over Flower-like Ni-Fe/SiO2-ZrO2 catalyst to produce fatty alcohol: Effect of SiO2

To solve the problem of harsh reaction conditions in lipid hydrogenation into fatty alcohol over Cu based bimetallic catalysts, the cheap Ni-base catalyst with high activity was synthesized. By using the hydrothermal method, the flower-like SiO2-ZrO2 support was prepared. The Ni-Fe/SiO2-ZrO2 catalyst was further prepared by the impregnation method and utilized in the fatty acid hydrogenation to produce fatty alcohol. Under the reaction conditions (240 °C, initial H2 pressure: 2 MPa and reaction time: 3 h), the Ni-Fe/0.13SiO2-ZrO2 catalyst attained 96.0% conversion and 86.4% fatty alcohol yield, much higher than that of Ni-Fe/ZrO2 without SiO2 (conversion 86.1% and fatty alcohol yield 61.9%). The high catalytic activity of Ni-Fe/0.13SiO2-ZrO2 was ascribed to the increase of surface area, reduction of NiO grain size, and formation of flower-like morphology. The high selectivity to fatty alcohol over Ni-Fe/0.13SiO2-ZrO2 was related to the increase of Lewis acidity (extra active sites) and the decrease of oxygen vacancy which weakened the adsorption of aldehyde (enol) on the catalyst and thus facilitated the formation of fatty alcohol.

X-ray Micro-Computed Tomography: A Powerful Device to Analyze the 3D Microstructure of Anode-Electrolyte in BaZr0.8Y0.2O3 Protonic Ceramic Electrochemical Cells and the Reduction Behavior

New advanced fuel cell technologies are moving towards high-temperature proton conductors (HTPCs) to meet environmental issues. Their elaboration remains a challenge and micro-computed tomography (µCT) is an innovative way to control their quality. NiO-BZY anodic supports of a protonic ceramic electrochemical cell (PCEC), elaborated by co-tape casting and co-sintered at 1350 °C, were coated with a BZY20 electrolyte layer by DC magnetron sputtering. The µCT allowed to observe defects inside the volume of these PCEC half-cells and to show their evolution after an annealing treatment at 1000 °C and reduction under hydrogen. This technique consists in obtaining a 3D reconstruction of all the cross-sectional images of the whole sample, slice by slice. This allows seeing inside the sample at any desired depth. The resolution of 0.35 µm is perfectly adapted to this type of problem considering the thickness of the different layers of the sample and the size of the defects. Defects were detected, and their interpretation was possible thanks to the 3D view, such as the phenomenon of NiO grain enlargement explaining defects in the electrolyte, the effect of NiO reduction, and finally, some anomalies due to the shaping process. Ways to anticipate these defects were then proposed.

Formation of NiO/SnO2 p-n heterostructures grown by a single-step process based on a vapor–solid method

Fabrication of p-n heterostructures in low dimensional structures has been considered one of the most promising strategies to develop optoelectronic devices with enhanced applicability in photocatalysis or solar cells, among other fields of research, although it is still a challenging task. Different approaches have been used in the fabrication of p-n heterostructures, usually involving expensive processing and subsequent growth stages. In this work, NiO/SnO2 heterostructures formed by SnO2 micro and nanowires grown on NiO grains, have been fabricated in a single step following a vapor–solid process which avoids the use of catalyst or external substrates.

Oxidation behavior of a novel nickel-based single crystal superalloy at elevated temperature

The oxidation behavior of a novel low-cost single crystal superalloy was thoroughly studied at high temperature. Three distinctive stages were observed in the oxidation kinetics curve, which corresponded to the different oxide film structures and oxidation mechanisms. The outer NiO grains, several spinels as well as inner Al2O3 layer took shape at first stage while the oxidation dynamics obeyed cubic law. However, the NiO particles began to spall off the specimen surface and the continuity of Al2O3 layer was partly destroyed in the next several hours. The alloy went through severe oxidation weight gaining at second stage which could be rationalized by the severe oxidation of W and Mo. The non-protective NiWO4 and NiMoO4 spinels were surprisingly found and the Al2O3 particles demonstrated the rod-like morphology during this period. The re-formation of dense Al2O3 layer and the volatilization of MoO3 gave rise to the reduction of mass gaining rate in the final stage, additionally, the oxidation kinetics transformed to subparabolic characteristic. Moreover, the experimental alloy exhibited good microstructural stability at elevated temperature, however, the microstructure degradation was considerably serious. The entire thickness of γ′-free layers had exceeded 0.1 mm on the alloy surface after 200 h oxidation at 1120 °C.

Structure, morphology and magnetic properties of flowerlike γ-Fe2O3@NiO core/shell nanocomposites synthesized from different precursor concentrations

The flowerlike γ-Fe2O3@NiO core/shell nanocomposites are synthesized by the two-step method. Their structure and morphology can be controlled by tuning the precursor concentration. Microstructural analysis reveals that all the samples have distinct core/shell structure without impurities, and the NiO shells are built of many irregular nanosheets which enclose the surface of γ-Fe2O3 core. As the precursor concentration decreases (i.e., more NiO content), the NiO grain grows significantly, and the thickness of NiO shells increases. Magnetic experiments are performed to analyze the influences of different microstructures on magnetic properties of samples and we have the following two results. First, at 5 K, along with increasing thickness of NiO shell, the saturation magnetization increases, while the residual magnetization decreases slightly. Second, the hysteresis loops under cooling field demonstrate that the value of exchange bias effect fluctuates between 13 Oe and 17 Oe. This is mainly because of the NiO shell that (i) is composed of irregular nanosheets with disordered orientations, and (ii) does not form a complete coating around γ-Fe2O3 core.

Stability of complex catalyst with NiO@TiO2 core-shell structure for hydrogen production

Core-shell NiO@TiO2 and CaO complex catalyst was prepared to improve stability used in the reactive sorption-enhanced reforming (ReSER) process for hydrogen production. NiO@TiO2 as catalyst component was prepared by synthesizing Ni(OH)2 as core via urea hydrolysis, by coating TiO2 shell via adsorption phase reaction technique, and was mixed with nano CaO as CO2 adsorbent to form a complex catalyst. The morphology of NiO@TiO2 and the complex catalyst were characterized by means of TEM, EDS, XRD and N2 physisorption measurements. Hydrogen production performance was investigated in a fixed-bed stainless steel reactor. Results showed that the complex catalyst with core-shell NiO@TiO2 structure has 36 cyclic runs of the ReSER process for hydrogen production, while only nine cycle runs obtained without TiO2 coated as shell. It was revealed that core-shell NiO@TiO2 structure could increase the surface area of complex catalyst and promote the separation of NiO core particles. It was also found that the optimum TiO2 coating time could obtain relative smaller NiO grain. And smaller initial NiO grain in core-shell structure would improve the stability of the complex catalyst.

CHARGE TRANSPORT FEATURES IN THE COMPOSITE ANODES OF SOLID OXIDE FUEL CELLS: MICROSTRUCTURAL AND IN-SITU RAMAN SPECTROSCOPY ANALYSES

In this work, we carry out the high-resolution electron microscopy of microstructure of grains boundaries of anion and electronic conductors in composite Ni/YSZ anodes before and after study of the current–voltage characteristics of model SOFCs. We propose a mechanism of 2-stage reaction of hydrogen oxidation occurring in the vicinity of triplephase boundary of Ni/YSZ SOFC anodes. On the first stage, metallic nickel is oxidized to nickel oxide by oxygen anion coming from the solid electrolyte membrane. On the second one, hydrogen reduces nickel oxide to metallic nickel, and water is formed. Decrease of the Ni grains size in the vicinity of contact with anion conductor grains is shown to be the result of NiO nano-grains appearance and their consequent reduction to metallic Ni during SOFC operation. High-resolution electron microscopy analysis demonstrates the significant changes in microstructure of grains boundaries of anion and electronic conductors in composite Ni/YSZ anodes after application of load current to SOFC. Nano-sized NiO grains appear in near-boundary regions of Ni grains after current tests. Orientation alignment between YSZ and nano-sized NiO lattices is unambiguous evidence of epitaxial growth of nickel oxide at YSZ surface as on a substrate that is possible only as a result of oxygen anion transport from anion conductor YSZ to the metal surface during current passage through the solid oxide fuel cell. We study the chemical transformations in the electrochemical reaction zone in SOFC composite electrodes depending on the current density passing the SOFC by new “in-situ” Raman spectroscopy technique. Increase of the current passing is shown to lead to growth in the intensity of Raman peak connected with symmetric oscillations of CeO 2 group. We connect this result with the change of the cerium cations charge state from Ce 3+ to Ce 4+ and consider this to be direct proof of the charge transfer in composite anode via oxygen anion transfer.

Assessment of NiO-CGO composites as cermet precursors

NiO-Ce0.9Gd0.1O1.95 (NiO-CGO) composite powders produced by a one-step chemical route were assessed as precursors of fuel cell anode cermets. Porous NiO-CGO composites were prepared by uniaxial pressing followed by firing at 1400 °C for 4 h. Ni-CGO cermets were subsequently obtained by reduction in 10 vol% H2 + 90 vol% N2 at 750 °C. Similar materials obtained by conventional mechanical mixing of commercial powders were also prepared and tested. The structure and particle size of the calcined material were investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The electrical properties of all materials were duly studied by impedance spectroscopy (NiO-CGO) (in air and several N2 + O2 gas mixtures) or dc conductivity (Ni-CGO). The total and relative magnitude of impedance arcs observed in the low temperature impedance spectra can be used to obtain significant insight on microstructural characteristics, due to a variable role of NiO when shifting from a well percolated electronic conducting pathway to a dispersed ion-blocking phase. The confirmed superior conductivity of one-step materials was interpreted as a consequence of the presence of a network of well-connected and homogeneously distributed NiO(/Ni) grains, as shown by SEM.

Ion transfer in Ni-containing composite anodes of solid oxide fuel cells: A microstructural study

High-resolution transmission and scanning electron microscopy of the solid oxide fuel cell (SOFC) anodes made of Ni and yttria-stabilized zirconia (YSZ), revealed a substantial microstructural reconstruction induced by the anodic current. These microstructural alterations, primarily the growth of NiO inclusions at the Ni| YSZ interface, the resultant orientation relationship between the NiO and YSZ grains, and Ni grain fragmentation on subsequent reduction, provide a straightforward evidence for the so-called oxygen spillover mechanism of the anodic reaction.

Effect of anode calcination on the performance and redox stability of low-temperature solid oxide fuel cells prepared via impregnation

Development of low-temperature solid oxide fuel cells (SOFCs) is crucially important for their stable and continuous operation as a reliable power source. With Ni nanoparticles impregnated into the thick anode support, Sr and Mg doped LaGaO3 (LSGM) based electrolyte has been demonstrated to be adept at reducing the ohmic resistance for high performance at low temperatures (450–650 °C). In this study, 55-μm LSGM-electrolyte-based SOFCs are prepared via the impregnation for both cathode and anode, and high performance is obtained at low temperatures. The durability and reduction-oxidation (redox) stability are examined in terms of the calcination temperature of LSGM/Ni(O) anode that determines the thermal growth of NiO crystals and interfacial reaction between NiO and LSGM. We find that the cell with anode calcined at 1100 °C does contain larger NiO grains and more secondary phases, but it shows similar power density comparing to the one calcined at 800 °C. Both cells are stable at 650 °C and no damage on the electrolyte is found during the redox cycle, but the performance of the former is more sensitive to the redox cycle due to the high ohmic resistance arisen from the possible discontinuous connection between the large Ni grains.

Percolative NiO decorated reduced-graphene oxide with a giant dielectric permittivity

NiO decorated reduced graphene oxide (r-GO) composites with varying r-GO weight percentages are synthesized using molecular level mixing technique. These composites are found to follow percolation theory with respect to dielectric behavior and conductivity values. A giant permittivity (ε′) value of 3688 was measured at 1kHz when 2.4wt% of r-GO was used in preparing NiO/r-GO composite. In comparison to NiO, enhanced permittivity values are recorded in the case of NiO/r-GO composites due to Maxwell-Wagner interfacial polarization between functionalized graphene sheets and NiO particles. It was also observed that ε′ values increased with decreasing NiO grain size up to the percolation threshold value. Moreover, ε′ values are found to be consistent with universal threshold values. Cole–Cole plots’ analysis revealed that the interfaces in the composites are more active than NiO grains.

Fabrication of a Ti/TiO2/NiO electrode for electrocatalytic nitrite removal

Ti/TiO2/NiO (Ti/TNO) electrode used for electrocatalytic oxidation of nitrite was fabricated by a simple thermal decomposition method in this work. The influences of calcination temperature on crystal phase and morphology of Ti/TNO composite were explored by XRD and TEM. After being calcined at 700°C, NiO grains are closely surrounded by TiO2 grains, in which TiO2 acts as an adhesion agent to fix the NiO grains on the Ti plate. The relative contents of Ni(III) and Ti(III) species were found increase due to the close heterojunction between NiO and TiO2, which reduces the impedance and improves the electrocatalytic activity of Ti/TNO. Ti/TNO-350-700 not only exhibits as high activity as those of Ti/TNO-350 and Ti/TNO-700 for electrocatalytic removal of nitrite contaminant, but also owns a superior electrocatalytic stability, which is more than 30 times and 2.5 times longer than those of Ti/TNO-350 and Ti/TNO-700, respectively.

Breakdown of antiferromagnet order in polycrystalline NiFe/NiO bilayers probed with acoustic emission

Magnetization reversal of polycrystalline NiFe/NiO bilayers was investigated using magneto-optical indicator film imaging and acoustic emission techniques. Sporadic acoustic signals were detected in a constant magnetic field after the magnetization reversal. It is suggested that they are related to elastic waves excited by sharp shocks in the NiO layer with strong magnetostriction. Their probability depends on the history and number of repetitions of the field cycling, thus testifying the thermal-activation nature of the long-time relaxation of an antiferromagnetic order. These results provide evidence of spontaneous thermally activated switching of the antiferromagnetic order in NiO grains during magnetization reversal in ferromagnet/antiferromagnet (FM/AFM) heterostructures. The respective deformation modes are discussed in terms of the thermal fluctuation aftereffect in the Fulcomer and Charap model which predicts that irreversible breakdown of the original spin orientation can take place in some antiferromagnetic grains with disordered anisotropy axes during magnetization reversal of exchange-coupled FM/AFM structures. The spin reorientation in the saturated state may induce abrupt distortion of isolated metastable grains because of the NiO magnetostriction, leading to excitation of shock waves and formation of plate (or Lamb) waves.

Effect of Sputtering Temperature on Structure and Optical Properties of NiO Films Fabricated by Magnetron Sputtering

NiO thin films were deposited on sapphire and Corning 1737 glass substrates using the radio frequency magnetron sputtering system. The structures, optical and electrical properties of NiO films grown under different sputtering temperatures were thoroughly studied. The NiO films were composed of different-size NiO nano-grains with a strong (111) preferred orientation. The NiO grain size increased with the sputtering temperature increase. The band gap values decreased from 3.69 eV to 3.38 eV on the sputtering temperature increase from 30°C to 450°C. Moreover, the electrical property variations of the NiO samples were studied by the Hall effect in detail.

High-Temperature Decomposition of Fe- and Cr-Doped NiO Produced by a Novel Flame Spray Technique

A novel thick film technology has been developed for the manufacture of electric heating elements from partially oxidised Ni–Cr–Fe alloy powder. The sprayed films were a mixture of metallic alloys and Ni-based oxides. Electron microscopy and analysis were used to monitor the transformations in the films during subsequent heat treatment. In particular, it was found that homogeneous grains of Cr- and Fe-doped Ni-based oxides transformed into NiO grains containing numerous Ni-, Cr- and Fe-rich spinel particles with a typical size of 40 nm.

Effect of lithium-ion doping concentration on structural and optical properties of NiO films fabricated by magnetron sputtering

In the present work, we report the dependence of lithium-ion (Li+) doping concentration on structure, optical and electrical properties of NiO films fabricated by radio frequency (rf) magnetron sputtering. It was found that the NiO grains were increased with increase in lithium-ion concentration from an anomalous state to a cubic shape following the NaCl-type structure. The NiO films presented highly c-axis preferred orientation with significant (111) diffraction. The samples marked A-C under the lithium-ion concentration of 1 at.%, 3 at.%, and 5 at.% respectively have optical band gap values of 3.72 eV, 3.61 eV and 3.47 eV, respectively. Comparatively, the controllable electrical properties of NiO films can be achieved by the variations of crystal quality arise from the lithium-ion doping concentration.