Nan Dong

Synthesis and Thermal Stability of Nanocrystalline Nickel Coatings by Direct Current Electrodeposition

Abstract The influences of current density and saccharin concentration on nanocrystalline nickel coatings synthesized by direct current electrodeposition were analyzed. Results show that in the current density range from 0.5 A/dm2 to 1.5 A/dm2, nanocrystalline nickel coatings with broad microhardness HV distribution from 4150 MPa to 6030 MPa can be synthesized by adjusting saccharin concentration. The preferred orientation in the deposits exists at 0.5 A/dm2, (111), (200) double orientation texture changes to a (200) orientation texture as saccharin concentration and (200) intensity increase. The internal stresses of the samples prepared at 0.5 A/dm2 decrease to nearly 0 MPa as saccharin concentration increases to 1.2 g/L. The results of DSC curves show that the grains start to grow until 600 °C and keep the thermal stability near to bulk nickel when grain sizes are in the range from 28 nm to 98 nm. However, when the grain size is 10 nm, the grains grow abnormally at 317 °C, and their thermal stability decrease sharply.

Combined experiment and first-principles study of the formation of the Al2O3 layer in alumina-forming austenitic stainless steel

A combination of experiment and first-principles research on the formation of the Al2O3 layer in alumina-forming austenitic stainless steel is presented. The results show that the oxide layer has a multilayer structure with an outer oxidized Cr2O3 and an inner Al2O3. Further, theoretical simulation using the first principles method is applied to research the formation process of the Al2O3 layer in the Fe/Cr2O3 interface as well as the impact of four alloying elements (Cr, Ni, Mn, and Si) on the formation of Al2O3. Results indicate that the Al atom originating from the Fe-based matrix prefers to diffuse into the Cr2O3 slab, thereby resulting in the formation of the Fe/Al2O3/Cr2O3 construction, which agrees with the experimental behaviour. Moreover, the introduction of Cr, Ni, Mn, and Si can slow down the diffusion of Al and result in a slower growth rate of Al2O3. The effects of Cr–Y (Ni, Mn, and Si) co-doping are more significant than those of X (Cr, Ni, Mn, and Si) single doping. Furthermore, Si can improve the adhesion of the Fe/Al2O3/Cr2O3 interface, thus can improve the adhesion of the oxide scales.

Acceleration of oxidation process of iron in supercritical water containing dissolved oxygen by the formation of H2O2

To improve fuel use and energy-conversion efficiency and reduce the emission of pollutants, oxygenation is now deemed an effective chemical treatment of water in supercritical and ultra-supercritical power plants. Supercritical water with dissolved oxygen significantly enhances the oxidation rate of steels in the main steam pipeline and super-heater header. However, at the atomic scale, the mechanism of metal oxidation in supercritical water containing dissolved oxygen is unknown and has not been investigated by simulation. In this work, the oxidation of iron in supercritical water containing dissolved oxygen is studied by ab initio molecular dynamics and first principles calculations. The results indicate that dissolved oxygen in supercritical water dramatically accelerates the oxidation of iron. With the help of oxygen, the decomposition of water occurs on the iron surface, thereby producing more iron oxides and iron hydroxides. Additionally, hydrogen peroxide (H2O2) forms as an intermediate product, instantaneously decomposing to form iron hydroxides, and this is another reason for the enhancement in the oxidation of steel by supercritical water containing dissolved oxygen. Based on the results from ab initio molecular dynamics, we develop herein typical models of water molecules and oxygen molecules reacting directly on the iron surface and then carry out first-principles calculations. The results show that water decomposes on the iron surface only with the assistance of adjacent oxygen molecules and in the absence of surrounding water molecules. This investigation deepens our understanding of the oxidation mechanism of metal in supercritical water containing dissolved oxygen. The ideas and methods implemented in this work can also be used to study other materials exposed to supercritical water involving oxygen.

A combined experimental and first-principle study on the oxidation mechanism of super austenitic stainless steel S32654 at 900 °C

A combined experimental and first-principle study on the oxidation mechanism of super austenitic stainless steel S32654 at 900 °C for a short time period (1, 3, and 5 h) in air is presented. The samples exhibit excellent oxidation resistance because of the initial and gradual formation of the denser Fe- and Cr-rich layer with increasing oxidation time. Meanwhile, the Mo-rich layer gradually forms because of the Mo diffusion, which results in the formation of the oxide layer with two distinct regions: an inner Fe- and Cr-rich layer and an outer Mo-rich layer. Density functional theory is applied to investigate the diffusion behaviour of Mo atom in the Fe-Cr-Ni/Cr2O3 interface and the effects of alloying elements (Fe, Ni, and Mn) on the Mo diffusion. The Mo originating from the alloy matrix tends to diffuse into the Cr2O3 part, thereby resulting in the formation of the continuous Mo-rich layer, which is consistent with the experimental behaviour. Moreover, the introduction of Ni to the Cr2O3 part can promote the Mo diffusion and the formation of the Mo-rich oxide layer, whereas Fe and Mn can hinder the Mo diffusion. The calculated results provide a microcosmic explanation of the experimental results.

Design of three-channel filters with broad nontransmission bandgap by the use of dual-periodic heterostructures

The bandgap characteristics of 1D dual-periodic photonic crystal (PC) heterostructures composed of two different 1D PCs containing TiO2/MgF2 multilayer films were theoretically studied through a transfer matrix method. With broad nontransmission bandgap and high-transmission peaks for electromagnetic and magnetic electric modes, the optimization design provides a promising method to fabricate the dual-periodic PC three-channel filter with a wide nontransmission range in the visible range. At the incident angle of less than 28°, the nontransmission range of the dual-periodic PC three-channel filters can be substantially enlarged over the entire visible range, and the phenomenon of three-channel filters in blue, green, and red light can be realized by adjusting the repeat cycle counts of two 1D PCs. Thus the proposed approach can be utilized for display applications.