Fabrication And Properties Of Ni-based Porous Alloys

This paper systematically present Ni-based porous alloys can be prepared by reactive synthesis method. The fabricated porous material is a novel substitute for current inorganic porous materials. The mentioned fabricated method has the special characteristics of simplicity, controllability and low cost, which show great significance of the development of other systems of porous materials and the corresponding industry applications.The preparation process of porous Ni3Al intermetallics is firstly investigated. The sintering process is composed of three steps. At low temperature, the gas of the samples is wiped off; at middle temperature, the solid reaction between Ni and Al is occurred and the formation of the aluminium rich intermetalllics have the effect of reducing the enthalpy of formation of the mixture; at high temperature, the distributing of the phase of the porous Ni3Al become uniformity. The key process of the preparation is the thickness of the diffusion layer is enough.This affects the controllability of the pore structures and the shape of the materials. After sintering at1250℃, the open porosity of porous Ni3Al is45%and the maximum pore size is14μm.The corrosion resistance of porous Ni3Al intermetallics is investigated in6mol/L KOH solution. Porous Ni3Al intermetallics show good corrosion resistance in the solution. Materials exhibit passive characterization and the corrosion products are the oxides of the nickel. During the corrosion, diffusion process is occurred. Porous Ni3Al intermetallics with different porosity exhibit excellent corrosion resistance in strong alkali solution. The samples with higher porosity suffer more corrosion than the one with lower porosity which due to the real surface area of porous Ni3Al interemtallics generally increases with increasing porosity. The corrosion rates are not proportional to the true surface area of porous Ni3Al interemtallics because the surface area is not the only factor influencing the corrosion rate. The pore size, pore size distribution and pore shape should also be taken into account. Porous Ni3Al electrodes exhibit porous morphology and excellent activity toward the HER. The main contribution towards the apparent activity of porous Ni3Al is a consequence of the increase of the intrinsic activity and the real surface. It is also shown that during the anodic polarization of porous Ni3Al electrode in6mol/L KOH at298K the oxidation of the electrode surface occurs, changing polarization characteristic (increasing overpotential for the hydrogen evolution) of this material. If after such a treatment, electrodes are exposed to the hydrogen evolution for a certain time, almost identical polarization diagrams for the HER are obtained as the ones before any anodic treatment (the oxide layer is completely reduced). If such an experiment is performed under the industrial application (6mol/L KOH at358K) the electrode can also retain their original performance. Porous Ni3Al electrodes show good stability in the long-run experiment.Porous Ni3Al-Mo electrodes show good activity towards HER and when the Mo content is10%the activity is the best. Results suggest the significant catalytic performance is not only from the increase of the real surface area of electrodes, but also from the true catalytic effect. The stability of hydrogen evolution activity and corrosion behavior of porous Ni3Al-Mo electrode with long-term electrolysis is investigated in6mol/L KOH solution. Their activity towards HER is assessed by recording cathodic current-potential curves and electrochemical impedance spectroscopy (EIS) techniques after different electrolysis time. It is found the porous Ni3Al-Mo electrode has a high surface roughness. The long-term operation at-100mA cm-2cathodic current density shows that the electrochemical activity of the porous Ni3Al-Mo electrode for the HER increases slightly with increasing electrolysis time. The activation of electrode is related to the removal of any existing oxidation products during sintering process. The corrosion tests show that the corrosion resistance of the porous Ni3Al-Mo electrode changes when a cathodic current is applied to electrolysis system. The impedance, LPR and current-potential measurements suggest that the formation of a passive film on the surface of electrode increases its corrosion resistance.Porous Ni-Al-Si alloys are fabricated by elemental powder reactive synthesis method. The pore size of porous Ni-Al-Si alloys is depended on the Si content. The increasing of the Si content can elevate the diffusion rate of Al element and Si atoms also diffuse into Ni Jatoms which produce large pore size. The oxygen resistance of porous Ni-Al-Si alloys is increased with the increasing of Si content indicating the Si element can prevent the Ni-Al alloys to be oxidation.Porous Ni-Cr-Fe alloys are fabricated by elemental powder reactive synthesis method. After sintering at1350℃, the permeability and maximum pore size are453m3KPa-1h-1m2and23.5μm respectively, and the open porosity is38%. The pores are Kirkendall pores due to the diffusion of Cr and Fe into Ni elemtent. The tortuosity factor of porous Ni-Cr-Fe alloys is2.11, which is a small value indicate the pores are interconnected. Porous Ni-Cr-Fe alloys show good corrosion resistance in pure Cl2environment at400℃after60days. The weight and pore structures of the alloys do not change a lot. Porous Ni-Cr-Fe alloys also shows good activity towards HER and corrosion resistance in6mol/L KOH solution.