Size effect of the thermal expansion of nanostructural copper oxide

The temperature dependence of the thermal expansion $\Delta L/L$ of samples of high-density nanostructural CuO ceramics with a crystallite size of 20, 70, and 90 nm was measured. The nanoceramics were obtained from coarse-grained CuO powders under converging spherical shock waves. It is found that, at temperatures $T >$ 50 K, the thermal expansion coefficient $\alpha(T)$ of the nanoceramic samples increases with decreasing crystallite sizes and exceeds the value of $\alpha(T)$ of the CuO single crystal by a factor of 3.5 to 4.5. At temperatures $T <$ 50 K, regions with zero and negative values of $\alpha(T)$ were revealed. The possible reasons for the increase in the thermal expansion coefficient of nanoceramics based on 3$d$-metal oxides are discussed.