Ni-NiO Porous Preform with Controlled Porosity Using Al₂O₃ Powder

Abstract

The Ni-NiO skeleton seems to be a good candidate for various applications in industry such as corrosion-proof filters or components in refrigerating systems and as preforms for reactive infiltration with molten metals.The present work was focused on preparation of Ni-NiO composite with higher, controlled porosity. Sintering of pure Ni powder always leads to a substantial closed porosity in almost whole sample volume [1,2]. To eliminate this, we added Al2O3 particles with diameter of-32 +20 μm into the Ni powder (-75 +45 μm diameters) and sintered this loose powder mixture (Ni + 25 vol. % Al2O3) in air by progressive heating up to 800 °C followed by 2 hours isothermal exposure. As a control, pure Ni powder was sintered under the same conditions. Thermal oxidation of loose powder samples performed in alumina crucible indicates that the strongest oxidation occurred in the top part of sample, while the bottom part was the least oxidized. Therefore, it was necessary to run the thermal oxidation once more, but out of the crucible, to ensure the sufficient diffusion of oxygen to the whole volume of sample.

Impact of Al2O3 Particle Size on the Open Porosity of Ni/Al2O3 Composites Prepared by the Thermal Oxidation at Moderate Temperatures

“…It has been evidenced in our previous work [34], and in agreement with others [35,36], that the weight gain was caused solely by the formation of NiO layers on the surface of Ni particles as a result of thermal oxidation. Moreover, it has been reported that the NiO growth gave rise to the sample open porosity [34].…”

“…It has been evidenced in our previous work [34], and in agreement with others [35,36], that the weight gain was caused solely by the formation of NiO layers on the surface of Ni particles as a result of thermal oxidation. Moreover, it has been reported that the NiO growth gave rise to the sample open porosity [34]. In an attempt to understand the role of Al 2 O 3 particle size in pore formation and distribution, the weight gain was associated with the sample porosity determined by the Archimedes' principal-based method.…”

“…Although the 20-32 μm Al2O3 fraction was examined in previous work [34], these data cannot be simply combined with those demonstrated in the present work. The main reason likely lies with the different volumes of samples, which could affect the open porosity.…”

“…Moreover, it is evident that the Al2O3 particle size did not play any significant role in the total weight gain, albeit a slightly stronger impact of the larger particles could be seen (Figure 4b). [34]. We found that ceramic particles impeded the formation of a robust NiO network as a result of Ni oxidation.…”

“…In 2017, we reported that Al2O3 particles increased the open porosity of the Ni/Al2O3 composite (Ni + 12.87 wt.% Al2O3-corresponding to 25 vol.% Al2O3) fabricated by two cycles of thermal oxidation in air and at temperatures ≤ 800 °C [34]. We found that ceramic particles impeded the formation of a robust NiO network as a result of Ni oxidation.…”