Piezoelectric properties and surface potential of green abalone shell studied by scanning probe microscopy techniques

Seashells have prominent mechanical properties, such as the coexistence of high strength and toughness in contrast with the properties of their constituent components. The hierarchical structure of seashells is commonly believed to be the key factor sustaining their extraordinary mechanical properties from a biomechanics viewpoint. However, the electromechanical coupling behavior in seashells may also have significant effects on its outstanding mechanical performance. This paper describes a study of the electromechanical coupling behavior in green abalone shell, whose structure has been abundantly studied previously. The objective of this study is to qualify the electromechanical coupling phenomena, which are mainly from piezoelectricity and possibly limited ionic motion, and their relations to the microhardness and surface potential in the abalone shell. It is found that the stronger piezoelectric response is associated with the higher surface microhardness in the abalone shell. Furthermore, both organic layers and some mineral grains exhibit the piezoelectric response. The shell deformation at nanoscale is found to be a linear function of the external electric field. The inherent polarization switching associated with piezoelectricity can be one of the mechanisms triggering energy dissipation when external mechanical energy is applied. Furthermore, a ferroelectric hysteresis loop can also be observed in abalone shell. The shape of the loop suggests that abalone shell has the capability to store and release the external energy by its internal mechanisms. Thus, it is presumed that the abalone shell is a type of piezoelectric material and is inborn with a mechanism to absorb external mechanical energy internally, and effectively to convert it to electrical energy. In such a way, the effective external stresses acting on the structure would be reduced to a certain extent, and thus electromechanical coupling may be another important source of the extraordinary mechanical property of the abalone shell.