Microstructure and thermal conductivity of surfactant-free NiO nanostructures

► Fast synthesis of surfactant-free NiO nanoparticles with controllable size. ► High specific surface area for NiO nanoparticles with size range from 3 to 7 nm. ► Strong reduction of the thermal conductivity with decreasing particle size. ► NiO as nanoinclusions in high performance materials for energy conversion.