Negative thermal expansion of nanoporous anodic aluminum oxide membranes.

We have measured the thermal expansion of Ni nanowires electrodeposited into self-organized nanoporous amorphous aluminum oxide (AAO) membranes without Al substrate using X-ray diffraction between 110K and 350K. The results indicate an average thermal expansion of the Ni nanowires -- along the wire axis -- of $\langle\alpha\rangle_{\rm NiNW}=-(1.6 \pm 1.5)\times 10^{-6}$/K. Assuming a bulk-like thermal expansion of the isolated Ni nanowires, this result indicates that AAO has also a negative thermal expansion. We estimate the thermal expansion of nanoporous AAO to be $\langle\alpha\rangle_{\rm AAO}=-(5 \pm 1) \times 10^{-6}$/K. We show that data obtained previously on the thermal expansion of metallic nanowires grown in the nanoporous AAO may be interpreted as originated in a negative thermal expansion of the matrix.