Papers by Keyword: Oxidation

Abstract: Ni-Fe-P coatings were deposited on ferritic stainless steel by means of electroless method. The oxidation behavior of the steel with these coatings were investigated at 800°C in air. The mass gain of the steel with Ni-Fe-P coatings deposited from solutions with different mass ratios of FeSO4 to NiSO4 decreased with increasing mass ratio. For the steel with Ni-Fe-P coating deposited in the solution with 0.25 of the mass ratio, the mass gain increased with the increase in oxidation time. While the mass gain of the steel with coatings deposited in solution with 0.5 and 0.75 of the mass ratio, respectively, became stable after 10-h oxidation. Compared with Ni-P, The Ni-Fe-P coatings improved the spallation resistance of the surface oxide scale. The oxide scale formed the bare steel consisted of Cr2O3 with TiO2 and (Mn,Cr)3O4 spinel. However, the oxide scale thermally grown on the steel with Ni-Fe-P coatings was composed of NiO, (Ni,Fe)3O4 spinel and Cr2O3.

Abstract: A multi-layer coating SiC/ Si - MoSi2 /MoSi2 with good oxidation resistance has been developed by using the slurry-sintering method on the surface of graphite to improve the oxidation resistance performance at 1700 °C for more than 200 h. Microstructure and anti-oxidation property of the as-received coating were investigated. It was found that functionally gradient SiC/ Si - MoSi2 /MoSi2 coating was formed on the surface of the graphite substrate. The coating characterized by excellent thermal shock resistance can effectively protect the graphite from oxidation at 1700 °C for 200 h. The mechanism of the oxidation resistance of the coated graphite is also dicussed, and a dense SiO2 layer was formed on the surface of coating at 1700°C during the isotheral oxidation, which can protect graphite from oxidation at longer time.

Abstract: This study demonstrates nano-scale lithography process on localized (100) silicon (p-type) surface using modified AFM apparatuses and controlling methods. AFM-based experimental apparatuses are connected the customized pulse generator that supplies electricity between conductive tip and silicon surface maintaining constant humidity during processes. Then pulse durations are controlled according to various experimental conditions. The pulsed electrochemical reaction within the gap between conductive tip and silicon surface induces the formation of oxide with nano-scale topographies. Various heights and widths of oxides can be created by AFM surface modification according to various pulse durations and applied electrical conditions under humidity environment. In addition, it can be known that oxides are completely removed after sample surface is etched in diluted HF solution, which shows micro/nano-scale grooves can be fabricated after predefined chemical treatment. They are wider than oxides widths and have several nanometer depths. Nano patterning technique from this experiment suggests that pulse electrochemical machining process has bright potential for advancing nano machining technologies.

Abstract: Glass/yttrium silicates/SiC multi-layer coating for carbon/carbon (C/C) composite was prepared and the flexural strength and weight changes of the coated samples after oxidation in air and combustion atmosphere at 1500oC were investigated. Results show that when oxidation in air and combustion atmosphere, the C/C composites lost weight by a linear and a parabolic law respectively. The weight loss reaches its maximum value at about 800oC in air but the strength of the specimen arrives at the lowest value at about 850oC. There are three oxidation zones along the specimens. A non-oxidation zone is below 400oC. Close to it is the cracking-oxidation zone (400-1100oC). At the temperature range from 1100oC to 1500oC is the coating-oxidation zone. Fracture of the coated sample at the temperature spot of 850oC is observed after oxidation in combustion atmosphere at 1500oC for 70 hours, which are considered to be responsible for the cracking and decreasing in strength during the test in combustion atmosphere.

Abstract: Titania nanotube arrays were synthesized via anodic oxidization of titanium foil in dimethyl sulfoxide (DMSO) solution containing 2 wt% HF and 3 wt% H2O at 40 V. The microstructure of the arrays was characterized with scanning electron microscopy (SEM). The results show that morphology of titania nanotube arrays is evidently influenced by the anodization time, and with the extension of oxidation time, the better morphology could be obtained. The possible formation mechanism of titania nanotube arrays has been discussed.

Abstract: Discovery of new high temperature alloys is a multidimensional problem which encompasses the intrinsic thermodynamic stability and their thermo-chemical and thermo-mechanical response to the combustion environment. Even when considering only the transition metals in combination with stable oxide formers, the number of ternary combinations exceeds 104. Hence, the traditional Edisonian process is not a practical approach to develop new alloy systems. Using formation enthalpy as a guide to compound stability, we propose a hierarchical scheme for identifying potential alloy systems which involves sifting through large regions of phase space with increasingly more accurate analysis. The coarsest sieve is a semi-empirical method based on the Miedema model extended to ternary systems. The next stage is ab initio simulations for a more accurate assessment and the basis for selecting system to investigate experimentally. We describe the implementation of this approach through the discovering of ternary additions that improve the oxidation stability -NiAl alloy.

Abstract: For energy production and also for the glass industry, finding new refractory alloys which could permit to increase the process temperatures to 1200°C or more is a permanent challenge. Chromium base alloys can be good candidates, considering the melting point of Cr itself, and also its low corrosion rate in molten glass. Two families of alloys have been studied for this purpose, Cr-Mo-W and Cr-Ta-X alloys (X= Mo, Si..). A finer selection of compositions has been done, to optimize their chemical and mechanical properties. Kinetics of HT oxidation by air, of corrosion by molten glass and also creep properties of several alloys have been measured up to 1250°C. The results obtained with the best alloys (Cr-Ta base) give positive indications as regards the possibility of their industrial use.

Abstract: Good dispersion of active catalytic elements, their uniform distribution on the carrier’s surface and availability for neutralised combustion gases are key factors that determine the usability of catalyst cartridges for automotive catalytic reactors. By examining the morphology of oxide films, with SEM/STEM method, obtained by oxidation of foil coated with Pt/Al two-layered system (like laminate), where an Al nanofilm is situated between steel foil and a Pt nanofilm, one can observe forked and plate whiskers growing directly of substrate. Oxide films obtained from Pt+Al composite nanofilms were formed differently (where components are intermixed). These films are marked by chaotic, mutually penetrating fine equiaxed solids and whiskers. In case of composite nanofilms the grain system obtained in the course of deposition contributed to good Pt grain refining, and due to that mostly particles from 3 to 9 nm in conventional diameter were formed. Direct contact between Pt particles and Al particles caused the partial occlusion of Pt particles with oxide in the process of Al oxidation to be effected.

Abstract: The solar furnace is a heating system based on concentrated sunrays on the material surface. It is an original method for testing ultra-high-temperature ceramics (UHTC) at very high temperature (above 2200°C) in air with an exposure time of several minutes. In this study, the solar flux is 15.5 MW.m-2 with a homogeneous exposed surface of 10 mm2. A large temperature-time composition parameters space is covered producing a large set of oxidized samples. Massive cylindrical specimens of UHTC materials are prepared by spark plasma sintering at 1900°C under a pressure of 100 MPa for 5 minutes. Then, samples are tested in air from 1750°C up to 2400°C with dwell times varied from 1 to 5 min. During oxidation of ZrB2-SiC (20%vol) material, the formed and known complex oxide scale identified from literature is easily reproduced using this method. It consists of a thin outer silica layer and zirconia columnar layer with a region of SiC depleted zone in ZrB2 phase. The impact of the reduction of Si content is quantified and the coating ZrB2-20%vol SiC is tested as protection on C-C composite.

Abstract: Microstructural evolution of Ti2AlN ceramics during high-temperature oxidation in air has been revealed by X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEGSEM), and energy-dispersive spectroscopy (EDS). After oxidation below 1200 °C, layered microstructures formed on Ti2AlN surfaces containing anatase, rutile, and α-Al2O3. Above 1200 °C, more complex layered microstructures formed containing Al2TiO5, rutile, α-Al2O3, and continuous void layers. With increasing temperature, anatase gradually transformed to rutile, and TiO2 reacted with α-Al2O3 to form Al2TiO5. Based on these microstructural observations, an oxidation mechanism for Ti2AlN ceramics is proposed.