Study of nanostructural changes in a Ni-containing cermet material during reduction and oxidation at 600 °C

Abstract

In this work, the nanostructural changes in a Ni/NiO cermet material, during its reduction and oxidation at 600 °C have been studied. A series of the NiO ceramic specimens (mode 1) were undergone to the next three treatment modes at 600 °C, namely: (2) one-time reduction in hydrogen of 99.99 vol% H2 purity; (3) one-time reduction in Ar–5 vol% H2 mixture; (4) redox cycling (5 cycles), each redox cycle comprises the stages of isothermal holding in Ar–5 vol% H2 mixture and in air, with intermediate stages of degassing. Increased porosity, along with an increased amount of reduced Ni, has been revealed in specimens after mode 2 test. In contrast to treatment in pure hydrogen, no substantial volume changes in the material after mode 3 test were found, and single micropores were detected only at the sites of contact of nickel phase particles. In contrast to modes 2 and 3, the following structural peculiarities in the course of redox cycling of as-sintered NiO ceramics (mode 4) were detected: (1) formation of a network of nanopores in the outer layer of a Ni-phase particle; (2) reduction of Ni-phase particle size by separating thin pieces of reduced Ni subgrains; (3) coagulation of fine Ni pieces that allows the porosity to be partially decreased and “bridges” to be formed; (4) clustering of the initial Ni-phase particles with the formation of nanopores at the sites of former boundaries; (5) formation of a network of reduced Ni that consists of Ni fringes and “bridges” and improves electrical conductivity and structural strength of the cermet.