Thermoresponsive Inorganic Materials

Abstract

AbstractThe effect of temperature on the vast majority of materials is well known—as materials are heated, they initially expand in volume before eventually melting, subliming, or decomposing. Thermal expansion is often viewed as a deleterious property. Thermal expansion can, however, also be put to good use. It has long been known that the walls of bowed buildings can be pulled back into shape by a cooling iron bar; steel tires can be shrink‐fitted onto the wheels of railway carriages. Although these examples represent a technologically useful response to the stimulus of increased (or decreased) temperature, they do not quite fall into the category of “smart.” However, the very simple concept of coupling together two materials, one that has a large and one a smaller coefficient of thermal expansion to produce a bimetallic strip can certainly be considered to create a smart composite body. Here, the strains induced by the higher expansion of one material cause the strip to bend as temperature is increased, leading to a simple temperature‐sensing/responsive device.The vast majority of materials known and used in technological applications have a positive coefficient of thermal expansion; the reasons for this behavior are discussed. Certain materials, however, display the opposite property and contract in volume when heated. These materials thus have a negative coefficient of thermal expansion, which has led to their somewhat confusing description as “negative thermal expansion” (NTE) materials. The properties of these materials and the origin of these effects form the main topic of this article.Materials that have this unusual thermoresponse have a number of important technological applications. Applications related to specific materials are discussed. The first major area of application is in producing composite bodies. By mixing normal materials with a negative thermal expansion phase, one can achieve a composite that has a precisely controlled positive, negative, or even zero coefficient of expansion. Second, certain of the materials discussed can be chemically doped to control their expansion properties. Thus, one can envisage a single material that could be adjusted to have zero overall expansion. Such materials are of use where repeated thermal shock might lead to mechanical failure (an everyday example is oven‐to‐table cookware) or in optical devices such as high precision mirrors where any thermal expansion might distort optical properties. Materials that have strong intrinsic thermal contraction are most likely to be used as compensators for the positive expansion of other phases. In “negative thermal expansion” materials considered in the remainder of this article, there will always be an underlying expansive component caused by vibrational modes that tend to increase bond distances. In certain circumstances, however, these modes may be dominated by other more exotic effects.