Thermal expansion of ε-Co and β-Co nanocrystals in glass matrices

Abstract

Metastable ɛ-Co nanocrystals precipitate in silicate glass matrices together with stable β-Co nanocrystals at temperatures up to 873 K, whereas rapid thermal decomposition of ε-Co is reported if formed by wet-chemical routes. Hence, high-temperature X-ray diffraction was used to determine the unknown coefficient of thermal expansion (CTE) of ε-Co stabilized in glasses. The liquid character of the matrix provides free expansion of ε-Co nanocrystals above the glass transition temperature (Tg), while a strain correction was performed for lattice constants below Tg using the co-precipitated β-Co as internal reference. It is found that CTE (298–873 K) of ε-Co nanocrystals is 10.1 ppm K−1, which is slightly larger than CTE of β-Co nanocrystals (9.4 ppm K−1).