Negative thermal expansion of nanoporous anodic aluminum oxide membranes

Abstract

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 $⟨\alpha {⟩}_{\mathrm{NiNW}}=-(1.6\pm 1.5)\times 10^{-6}$/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 $⟨\alpha {⟩}_{\mathrm{AAO}}=-(5\pm 1)\times 10^{-6}$/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.