What's missing in materials?

Abstract

Often in materials engineering we can envisage what new materials we wish to have and so point research in directions that make our aims achievable. As time progresses, such aims evolve, become increasingly diverse, and occasionally lead to quite unexpected provisions. From other viewpoints, we can also consider what we would like to be missing and this leads in further directions, to the removal of materials, or of specific elements within them, in relation to disposal, recycling, or purification to a level where subsequent additions can have highly beneficial effects. It is less obvious that, within materials, some atoms are always missing. These comprise only a small fraction, but enough to have major effects, and they can be regarded as vacancies that exist at places where atoms are normally situated. Their influence is greatest at elevated temperatures, for then they can move most easily, and so permit diffusion of the surrounding atoms. Their movement is random but under driving forces they drift in specific directions: under stress or chemical influences this can lead to distortion through transfer of atomic positions or to fracture through the formation of holes if the vacancies agglomerate. Such effects may be disastrous but, conversely, they can also be beneficial in assisting materials to bond together, especially when use can be made of the space created within them. It is natural to think of the characteristics of materials but important also to think of the usefulness of the space they provide. Lightweight, large scale structures covering wide spans come into this category, as do vessels for the containment of gases at high pressures. Sometimes, it is constructive to regard space as a valuable component of a composite material.