Physico-chemical nanomaterials science

On the nanostructural peculiarities of nucleating the microcracks in a nickel oxide containing material
Kharchenko Ye.V.1, Blikharskyy Z.Ya.1, Vira V.V.1, Vasyliv B.D.2
1 Institute of Building and Environmental Engineering, Lviv Polytechnic National University, 12 S.Bandera str., Lviv 79000, Ukraine.
2 Department of Structural Fracture Mechanics of Materials, Karpenko Physico-Mechanical Institute of the NAS of Ukraine. 5 Naukova str., Lviv 79060, Ukraine.
E-mail: mechengin1111@gmail.com

The aim of this work is to study the substructure changes in a nickel oxide containing ceramic material during its reduction and oxidation at 600°C, basing on the structural analysis. A series of the YSZ-NiO specimens were undergo three treatment modes at 600°C, namely: 1) one-time reduction in a hydrogenous atmosphere; 2) redox cycling (5 cycles), each redox cycle comprises the stages of isothermal dwell in a hydrogenous atmosphere and in air; 3) redox cycling (5 cycles), with extra stages of degassing. Modelling the substructure changes during the reduction and oxidation half-cycles (at 600°C) has been performed basing on the nano- and microstructural analysis. Corresponding conclusions have been drawn.
Increased porosity, along with increased amount of reduced Ni, has been revealed in specimens after mode 2 test (redox cycling without degassing). It was established that in case of such treatment, a reaction of oxygen with remaining hydrogen on the stage of isothermal dwell in air at 600°C takes place followed by substantial increase of water vapor local pressure. Such high pressure conditions occur in small pores causing nucleation of nanocracks on three-phase (“nickel phase-zirconium phase-pore”) boundaries. Such effect of water vapor is probably the main reason of structural degradation of the cermet.
After mode 3 test of specimens, it was revealed that the stage of degassing between half-cycles of reduction and oxidation plays a substantial role in formation of Ni-network. Contrary to mode 2, the following structural peculiarities were detected: 1) formation of a network of nanopores in the particle outer layer; 2) reduction of Ni-phase particle size by separating Ni clusters of the particle outer layer; 3) redistribution of fine Ni particles that allows the porosity to be partially decreased; 4) formation of a network of reduced Ni particles that improves electric conductivity and structural strength of the cermet.