Microstructural and thermal investigations of HfO2 nanoparticles

Abstract

Monodispersed HfO₂ nanoparticles can be readily prepared at room temperature by the ammonia catalyzed hydrolysis and condensation of hafnium(IV) tert-butoxide in the presence of a surfactant. The nanoparticles are faceted with an average diameter of about 4 nm. The as-synthesized amorphous nanoparticles crystallize upon post-synthesis heat treatment. The crystallization temperature of the nanoparticles can be controlled by adjusting the annealing atmosphere. The HfO₂ nanoparticles have a narrow size distribution, large specific surface area and the thermal conductivity of pressed pellets is drastically reduced compared to the bulk counterpart. The specific surface area was about 239 m² g⁻¹ on as-prepared samples while those annealed at 500 °C have a surface area of 221 m² g⁻¹ showing that the heat treatment produced no significant increase in particle size. Transmission electron microscopy (TEM) further confirmed that the nanoparticles annealed at different temperatures while X-ray diffraction studies of the crystallized nanoparticles revealed that HfO₂ nanoparticles were monoclinic in structure. High density pellets of the as-synthesized HfO₂ nanoparticles were obtained, using both spark plasma sintering and uniaxial hot pressing, and their thermal conductivity was measured in the temperature range from 300 to 775 K. A large reduction of the thermal conductivity was observed for HfO₂ nanoparticles as compared to that of bulk HfO₂. The decrease in thermal conductivity is discussed in terms of the microstructure of the compacted samples. The synthetic procedure used in this work can be readily modified for large scale production of monodispersed HfO₂ nanoparticles.