Formation of ZnO nanorod arrays on polytetraflouroethylene (PTFE) via a seeded growth low temperature hydrothermal reaction

ZnO nanorod arrays were formed by a low temperature hydrothermal process on seeded polytetraflouroethylene (PTFE) sheets. The seed layer was formed using thermal oxidation of a thin evaporated Zn film on the PTFE sheet at 300 °C in air for 10 min. The formation of ZnO nanorod arrays in the hydrothermal reactive bath consisting of hexamethylamine (HMT) and Zn ions occurred via the reaction of hydroxyl ions released during the thermal degradation of HMT with the Zn ions. The seed layer provided a template for the nucleation of the ZnO and HMT which also acted as a chelating agent that promoted growth of the ZnO along the c-axis, leading to the formation of exclusively (0 0 2) ZnO nanorods. The effect of exposure time of the seeded PTFE to the reactive solution on the formation of the nanorods was investigated. Well aligned, relatively uniform tapered 300 nm long nanorods can be formed after 8 h of exposure. Longer exposure times to 24 h resulted in the formation of more uniform nanorods with base diameter averaged of ∼100 nm and the tip diameter of ∼50 nm. XRD analysis showed that the ZnO nanorod array had a hexagonal wurtzite structure. This result is in agreement with HR-TEM observations and Raman scattering analysis. Photoluminescence study showed that a strong UV emission peak was obtained at 380 nm and a small peak at 560 nm, which is associated with green emission. The optical band gap measured from these plots was at 3.2 eV on average.     

Kinetics inversion in isothermal oxidation of uncoated WC-based carbides between 450 and 800 °C

Oxidation of uncoated WC-based carbides (hard metals) has been investigated between 450 and 800 °C. An anomalous change in the oxidation behaviour has been found in the temperature range between 528 and 630 °C. Instead of the normal increase of the oxidation rates when the temperature increases, a kinetics inversion is produced: an anomalous decrease of the oxidation rates has been observed in this temperature range. Furthermore, this anomalous behaviour produces an important change in the activation energy. At temperatures below the anomaly (below 528 °C), an activation energy of 119 ± 8 kJ/mol has been found, whereas above the anomaly (above 630 °C), an activation energy of 208 ± 8 kJ/mol has been found. This anomalous temperature dependence of the oxidation behaviour can be related to a higher amount of complex oxide CoWO₄ formed on the oxide scale, and to the effect that this oxide produces in the oxidation kinetics of the other oxides.     

High temperature scaling of Ni-xSi-10 at.% Al alloys in 1 atm of pure O2

The oxidation of Ni-xSi-10Al alloys (with x = 0, 2, 4 and 6 at.%), has been studied at 900 and 1000 °C in 1 atm of pure O₂ to examine the effect of different silicon additions on the behavior of ternary Ni-Si-10Al alloys. The kinetic curves of Ni-10Al are approximately parabolic at both 900 and 1000 °C. Conversely, the kinetics of the ternary alloys at both temperatures correspond generally to a rate decrease faster than predicted by the parabolic rate law, except for the oxidation of Ni-6Si-10Al at 1000 °C, which exhibits a single nearly-parabolic stage. Oxidation of the binary alloy formed at both temperatures an internal oxidation zone beneath a layer of NiO. Oxidation of Ni-2Si-10Al at both temperatures and of the other two alloys at 900 °C formed initially a zone of internal oxidation of Al + Si. However, a layer of alumina forming at the front of internal oxidation after some time blocked the internal oxidation and produced a gradual conversion of the metal matrix of this region into NiO, with a simultaneous decrease of the oxidation rate. Conversely, the oxidation of Ni-4Si-10Al and Ni-6Si-10Al at 1000 °C did not produce an internal oxidation, but formed an alumina layer directly on the alloy surface after an initial stage when also Ni was oxidized. Therefore, silicon exerts the third-element effect by reducing the critical Al content needed for the transition from its internal to its external oxidation with respect to the corresponding Ni-Al alloy. This result is interpreted by means of an extension to ternary alloys of Wagner’s criterion for the same transition in binary alloys based on the attainment of a critical volume fraction of internal oxide.     

High-quality vertically aligned ZnO nanorods synthesized by microwave-assisted CBD with ZnO-PVA complex seed layer on Si substrates

We successfully synthesized vertically aligned zinc oxide (ZnO) nanorods on seeded silicon substrates using chemical bath deposition assisted by microwave heating. ZnO nanorods were grown on seed layers of ZnO-polyvinyl alcohol (PVA) nanocomposites spin-coated on p-type Si (1 1 1). The nanorod's diameter was found to be dependent on the annealing temperature of the ZnO-PVA seed layer. We produced ZnO nanorods with diameters in the range of 50-300 nm from five groups of seed layers annealed at 250 °C, 350 °C, 380 °C, 450 °C, and 550 °C. The nanorods were examined with X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy, which revealed hexagonal wurtzite structures perpendicular to the substrate along the z-axis in the direction of (0 0 2). Photoluminescence measurements revealed high UV emission at a high I_UV/I_vis ratio of 175. We also conducted Raman scattering studies on the ZnO nanorods to estimate the lattice vibration modes.     

Growth of ZnO nanorods by air annealing of ZnO films with an applied electric field

A novel method of ZnO nanorods growth is presented based on low temperature (300 °C) air annealing of ZnO film while applying an electric field (∼ 10 V/cm) parallel to the film. The films were deposited on glass substrates using a filtered vacuum arc deposition system equipped with a Zn cathode, at an arc current of 160 A, oxygen pressure of 3.2 mTorr, and deposition time of 30 s. Cu tape electrodes were applied on each end of the coated sample, and used to apply the electric field. The samples were annealed in a quartz furnace at 200, 300, 400 °C for 20 or 60 min. Each sample surface was examined using a Scanning Electron Microscope (SEM) and a High Resolution SEM (HRSEM) to study its micro- and nano-structure. The film crystallographic structure was studied using X-ray diffractometry (XRD). ZnO rods with lengths of ∼ 3 μm were observed on the samples annealed at 300 °C for 20 min with an electric field of ∼ 10³ V/m, while separated conical forms with lengths of ∼ 0.5 μm and base width of ∼ 150 nm were observed after annealing under the same conditions but without any electric field. The rod growth rate and area density were ∼ 2.0-2.5 nm/s, and ∼ 3 × 10⁷ cm^− 2, respectively.     

Enhanced ferroelectric, dielectric and optical behavior in Li-doped ZnO nanorods

Enhancement in the dielectric and ferroelectric properties has been observed in case of Li-doped ZnO nanorods (NR). Effect of Li-doping on ZnO structure and its optical properties has also been reported. In high resolution TEM studies, the length and diameter of as-synthesized Li-ZnO nanorods were found in the range of 100-150 nm and 20-70 nm, respectively. XRD studies, Li-doped ZnO NR exhibited wurzite structure in which lattice parameter becomes larger than the pure ZnO NR. In dielectric studies, higher dielectric constant and a ferroelectric phase transition at 72 °C were observed. In ferroelectric studies, high remnant polarization of 0.873 μC/cm² and low coercive field of 0.592 kV/cm were observed, which were better than their values in bulk Li-doped ZnO (0.044 μC/cm² and 2.0 kV/cm, respectively). In UV-Vis spectra, low absorption band edge at 352 nm was observed due to the size effect in the ZnO nanorods. In addition, PL spectra show a blue shift in both UV and visible region as a result of doping. These results are discussed in the light of the nanorods of confined geometry.     

High-temperature oxidation behavior of Nb-Si-Cr alloys with Hf additions

The oxidation behavior of two alloys from the Nb-Si-Cr system containing hafnium has been investigated under isothermal and cyclic conditions. Nb-20Si-20Cr-(5,10)Hf alloys (composition in atomic percent) were exposed to air for 24 and 168 h over a range of temperatures from 700 °C to 1400 °C. A gravimetric method was used to determine the oxidation kinetics; weight gain per unit area as a function of temperature or time. Computed isothermal sections of the quaternary Nb-Si-Cr-Hf phase diagrams were used for alloy selection. XRD, SEM and EDS were used to characterize the phases present in the oxidation products and the alloys. Oxidation experiments revealed extremely good oxidation resistance at 700 °C and 800 °C and above 1200 °C under isothermal conditions for both alloys. Partial pesting was observed when the samples were exposed to 800 °C. Complete oxide formation was observed above 1000 °C for 5Hf and above 900 °C for 10Hf up to 1200 °C. Beneficial effects have been observed with the addition of 10Hf to the alloy compared to 5Hf at 700 °C, 1200 °C and 1300 °C resulting in a reduction of weight gain per unit area.     

Large-scale syntheses of uniform ZnO nanorods and ethanol gas sensors application

Uniform ZnO nanorods with a gram scale were prepared by a low temperature and solution-based method. The samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL). The results showed that the sample had uniform rod-like morphology with a narrow size distribution and highly crystallinity. Room-temperature PL spectra of these nanorods show an exciton emission around 382 nm and a negligible deep level emission, indicating the nanorods have high quality. The gas-sensing properties of the materials have been investigated. The results indicate that the as-prepared nanorods show much better sensitivity and stability. The n-type semiconductor gas sensor exhibited high sensitivity and fast response to ethanol gas at a work temperature of 400 °C. ZnO nanorods are excellent potential candidates for highly sensitive gas sensors and ultraviolet laser.     

Effects of microstructure on native oxide scale development and electrical characteristics of eutectic Cu-Cu6La alloys

A combination of electron microscopy, focused ion beam and conductive atomic force microscopy techniques have been used to study the microstructure, oxide scale development, and electrical behavior of a Cu-9 at.% La alloy. The as-cast alloy exhibits a eutectic microstructure comprising 30 vol.% Cu rods in a Cu₆La matrix. The eutectic colonies exhibit a singular orientation relationship with [0 1 0] Cu₆La parallel to 〈0 1 1〉 Cu along the rod axis, and it is shown that this corresponds to lattice matching of the two phases along this direction (∼0.02% misfit). Oxidation of the alloy at 100 °C to accelerate formation of a native oxide scale led to the development of a Cu₂O layer less than 25 nm thick on the Cu rods and a La-doped Cu₂O scale up to 1 μm thick on the Cu₆La matrix. The La-doped regions of the scale are more conductive despite being much thicker, which is consistent with previous contact resistance data obtained for this alloy. The mechanisms responsible for the formation of this non-equilibrium oxide scale structure and for the enhanced electrical conductivity of the La-doped regions are discussed.     

Influence of oxide grain morphology on formation of the CuO scale during oxidation of copper at 600-1000 °C

To explain the very slow growth rate of the CuO scale during copper oxidation at high temperatures above 850 °C as reported in the literature, the influence of the morphology of the CuO grains on scale growth was investigated by oxidizing copper with different purities in 0.1 MPa oxygen atmosphere at 600-1000 °C. Oxidation of 99.99% copper shows that the growth of the CuO scale depends on the morphology of the CuO grains. The very thin CuO scale observed at 1000 °C is related to flat CuO grains resulted from fast lateral growth of the CuO grains, whereas the thicker CuO scale at lower temperatures results from the fine CuO grains. This dependence of scale growth on oxide grain morphology was confirmed by oxidizing 99.9999% and floating zone refined (>99.9999%) copper at 800 °C. It reveals that grain boundary diffusion can favour the growth of the CuO scale, and its contribution is related to the morphology of the CuO grains depending on the purity of copper specimens.     

Evolution of structural and optical properties in the course of thermal evolution of sol-gel derived cobalt-doped gahnite

Thermal evolution of sol-gel derived gahnite (ZnAl₂O₄) with 4, 8 and 12 at.% of Zn replaced with Co was studied by thermal analysis techniques (DTA/TGA), X-ray diffraction (XRD) and UV-vis diffuse reflectance spectroscopy (DRS). Zinc-cobalt spinel powders were produced by gel heat treatment at temperatures as low as 400 °C. Crystal structure was characterized using Rietveld refinement of X-ray diffraction patterns for the samples annealed at 800 °C, simultaneously with the analysis of diffraction line broadening. It was found out that the distribution of Co²⁺ ions in tetrahedral and octahedral sites of zinc cobalt aluminate crystal lattice, crystallite size and lattice strain depend on Co loading. The green color of samples thermally treated at T < 800 °C has been explained as a consequence of partial oxidation of Co²⁺ ions at lower temperatures and accommodation of Co³⁺ ions in octahedral sites. Thermal treatment at higher temperatures promote gradual change of color from green to blue, characteristic for tetrahedrally coordinated Co²⁺ ions. The spectra evolution could be interpreted as a progressive reduction of Co³⁺ to Co²⁺ ions at higher temperatures.     

An investigation on oxidation wear mechanisms of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni intermetallic-based alloys

The results of investigation on oxidation wear mechanism of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni-based intermetallic alloy are presented. Oxidation was carried out in air at temperatures: 900 °C, 925 °C and 975 °C taking into account the micro-geometry of surfaces being heated and oxidized. It was determined that the rise of surface roughness to Ra = 5.8 μm definitely reduces scale of chipping in higher temperature (975 °C). Investigation of the material structure of the specimen and chemical composition of oxidation products was performed. Possibilities of an increase in the heat resistance of the tested alloy by means of application of high roughness surfaces were shown.     

Oxidation of iron at 400-600 °C in dry and wet O2

The oxidation of iron in dry and wet O₂ at 400-600 °C has been re-investigated using gravimetry, SEM/EDX, XRD and FIB. In the presence of O₂, water vapour accelerates iron oxidation at 500 and 600 °C. At 400 and 500 °C the magnetite layer is duplex and exposure to water vapour results in the formation of blades on top of a fine-grained hematite layer. At 600 °C it results in a surface without needles and blades. The increased oxidation rate at 500 and 600 °C is attributed to a smaller grain size in the hematite layer resulting in faster ion transport.     

Oxidation behavior of Fe-20Cr-5Al rare earth alloys in air and synthetic exhaust gas

Fe-20Cr-5Al alloy foils are used in automotive catalytic converters. This work examines oxidation behavior of four production-processed alloy foils in both air and synthetic exhaust gas environments. Oxidation tests were performed between 750° C and 1150° C for times to 96 hrs. Weight gain results in both atmospheres were similar, an indication that the same mechanism controls oxidation in both environments. At high temperatures (>-950° C) both atmospheres produce weight gains consistent with -alumina growth. Activation energies of 323 kJ/gmole and 271 kJ/gmole were calculated for oxidation in air and synthetic exhaust gas, respectively. At lower temperatures (<-850° C), accelerated weight gains can occur from growth of transition alumina. Despite similar weight gain results, the two atmospheres produce different oxide morphologies: at 950° C and above, air produces a rounded, porous oxide while synthetic exhaust produces a more compact, angular oxide. Unexpectedly, oxide spalling occurred on foils oxidized in synthetic exhaust at 1050° C and above.     

Oxidation and electrical behavior of AISI 430 coated with cobalt spinels for SOFC interconnect applications

Stainless steel can be used as interconnect plates in solid oxide fuel cells (SOFCs) below operating temperature of 800 °C. Unwanted reactions between the alloy and other SOFC components decrease the efficiency of these energy convertors. One approach to improving interconnect properties is to apply a surface coating to them. In this study, AISI 430 ferritic stainless steel interconnect is coated in a cobalt-base pack mixture using the pack cementation method. Isothermal oxidation, cyclic oxidation and oxidation at different temperatures (400-900 °C) are applied to evaluate the role of the coating layer during oxidation. Area-specific resistance (ASR) of the coated substrates has also been tested as a function of temperature and time. The surface morphology was examined by SEM, the chemical composition and structure of oxide formed were analysed by EDS and XRD. Results showed that the coating layer transforms into MnCo₂O₄, CoCr₂O₄ and CoFe₂O₄ spinels during isothermal oxidation. This scale is protective and acts as an effective barrier against chromium migration into the outer oxide layer and prevents weight gain. The mass gain and spallation indicated that the formation of spinel significantly improved the high temperature oxidation. These spinels also cause a reduction in ASR for coated substrates (9.7 mΩcm²) as compared to uncoated substrates (36.1 mΩcm²) after 200 h of isothermal oxidation at 800 °C.     

Formation of zinc aluminum mixed metal oxide nanostructures

We report a method for synthesizing zinc aluminum layered double hydroxide (ZnAl:LDH) nanostructures at room temperature. The ZnAl:LDH nanoplates could be converted into zinc aluminum mixed metal oxide (MMO) nanostructures by calcination in air. The crystalline nature and morphology of the MMO nanostructures could be tuned by varying the calcination temperature. At low calcination temperatures (450-650 °C), nanostructures were composed of crystalline ZnO regions and amorphous regions. The crystalline orientations of the ZnO crystal grains were almost identical throughout the nanostructure. At calcination temperatures above 750 °C, ZnAl₂O₄ crystal grains appeared and amorphous regions could not be found in MMO nanostructures. As the calcination temperature increased, the crystal grain size and surface roughness of MMO nanostructures increased. Calcination at 950 °C resulted in the formation of MMO nanoparticles. The optical properties of the MMO nanostructures were probed by UV-vis diffuse reflectance spectroscopy. The spectra varied depending on their dimensions and crystalline natures.     

Fabrication and oxidation behavior of Cr2AlC coating on Ti6242 alloy

An ∼ 5 µm Cr₂AlC coating was synthesized on near-α titanium alloy Ti6242 using an industrially sized magnetron sputtering coater. Isothermal oxidation at 700 °C and 800 °C, and cyclic oxidation at 700 °C of the bare alloys and coated specimens were investigated in air. The results indicated that the Ti6242 alloy faced serious oxidation problems at 700 °C and 800 °C. Repeated formation and spallation of the multilayered oxide scale on the Ti6242 alloy occurred during oxidation testing. The coated specimens exhibited much better oxidation behaviour as compared to the bare alloy. A continuous Al-rich oxide scale formed on the coating surface during the initial oxidation stages. The oxide scale and coating itself acted as diffusion barriers blocking the further ingress of oxygen and protected the substrate alloy from oxidation. The oxidation mechanisms of the bare alloy and the coated specimens were investigated based on the experimental results.     

Preparation of porous flower-like ZnO nanostructures and their gas-sensing property

Porous flower-like ZnO nanostructures have been synthesized by a template-free, economical hydrothermal method combined with subsequent calcination. Calcination of the precursors produced flower-like ZnO nanostructures, composed of interconnected porous ZnO nanosheets with high porosity resulting from the thermal decomposition of the as-prepared precursors, i.e., flower-like basic zinc carbonate (BZC). Moreover, the nanostructures have been characterized through X-ray diffraction, thermogravimetric-differential thermalgravimetric analysis, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller N₂ adsorption-desorption analyses. Compared with ZnO nanorods, the as-prepared porous flower-like ZnO nanostructures exhibit a good response and reversibility to some organic gas, such as ethanol and acetone. The sensor responses to 100 ppm ethanol and acetone are 21.8 and 16.8, respectively, at a working temperature of 320 °C. In addition, the sensors also exhibited a good response to 2-propanol and methanol, which indicate that these porous flower-like ZnO nanostructures are highly promising for applications of gas sensors.     

Isothermal oxidation behaviour of Nb-W-Cr Alloys

A study of the effect of Cr content on the microstructure and isothermal oxidation behaviour of four alloys from the Nb-Cr-W system has been performed. Selection of specific alloy compositions has been based on the ternary isothermal sections. Oxidation experiments were conducted in air at 900 and 1300 °C for 24 h under isothermal conditions. Weight gain per unit area as function of the temperature has been used to evaluate the oxidation resistance. The phases present in the alloys and the oxide scales were characterized by XRD, SEM, and EDS. Microstructure consists of Nb solid solution and NbCr₂, Laves phase. The oxidation kinetics follows a parabolic behaviour at 1300 °C; the addition of 30% Cr resulted in the significant reduction of the parabolic oxidation rate. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates in comparison to alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄ and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content and the oxidation temperature. The characterization results delineate the effect of the Cr content on the oxidation mechanisms of these alloys that represent a promising base for high-temperature alloy development.     

Microstructure and oxidation resistance behavior of lanthanum oxide-doped Mo-12Si-8.5B Alloys

Mo-12Si-8.5B (at.%) alloys doped with different mass fraction of lanthanum oxide (La₂O₃) were fabricated by using arc-melting and spark plasma sintering techniques. Composition and microstructures of the samples were examined with XRD and optical microscope, respectively. Oxidation resistance behavior of the alloys at 800 °C and 1000 °C was investigated using TGA, XRD and SEM-EDS. Experimental results show that the Mo-12Si-8.5B alloys are mainly composed of α-Mo, Mo₅SiB₂ (T₂) and Mo₃Si phases and the grain sizes of alloys doped with La₂O₃ were finer than the undoped ones. At 800 °C and 1000 °C, the alloys doped with La₂O₃ have good oxidation resistances because the protective borosilicate scale is quickly formed and adhered with the substrate. The oxide scales consist of outer boronsilicate glass scale and inner reaction zone consists of molybdenum dioxide. The oxidation mechanisms of the La₂O₃-doped Mo-12Si-8.5B alloys were also discussed in light of the experiments results.