Materials Science

Thermally Stable Reflections

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Science  14 Mar 2014:
Vol. 343, Issue 6176, pp. 1179
DOI: 10.1126/science.343.6176.1179-c

A simple thermostat exploits the difference in the thermal expansion of two metal films to sense temperature changes and trigger the heating or cooling system on or off as needed. In some cases, though, the change in dimension with temperature is a problem; for example, a satellite built on Earth has to be engineered to allow for the shrinking of the parts that will occur when it reaches the low temperatures found in outer space. A few materials exist that show either limited or negative thermal expansion, including complex metal oxides, silica glasses, and an iron-nickel alloy, but these materials may be brittle or operate only over a narrow temperature range. One route to creating a material with a tailorable coefficient of thermal expansion (CTE) is to create an engineered structure. Yamamoto et al. created a periodic lattice composed of hexagonal plates of aluminum combined with a frame of titanium, which has a lower CTE, to make a thin-film material. When heated, the expansion of the aluminum is accommodated by stretching and bending of the titanium, in such a way that the connection points stay stationary. The samples were tested from room temperature to 185°C and showed a very low and slightly negative CTE. The authors envision using this architecture for making an array of mirrors, and demonstrate this capability by showing that the reflected image quality remained constant over the test temperature range.

Adv. Mater. 10.1002/adma.201304997 (2014).

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