Thermal expansion of e-Co and v-Co nanocrystals in glass matrices

Metastable ɛ-Co nanocrystals precipitate in silicate glass matrices together with stable v-Co nanocrystals at temperatures up to 873 K, whereas rapid thermal decomposition of e-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 e-Co stabilized in glasses. The liquid character of the matrix provides free expansion of e-Co nanocrystals above the glass transition temperature (Tg), while a strain correction was performed for lattice constants below Tg using the co-precipitated v-Co as internal reference. It is found that CTE (298-873 K) of e-Co nanocrystals is 10.1 ppm K−1, which is slightly larger than CTE of v-Co nanocrystals (9.4 ppm K−1).