Field-responsive materials external stimuli

Insulating perovskites are generally described as dielectrics (a term used to characterise a polarisable insulator), piezoelectrics, pyroelectrics and ferroelec-trics. These names (which are general and not confined to perovskites) describe the response of the material, which is always an electric polarisation, to an applied external stimulus. In a dielectric, for example, the stimulus is an applied electric field, and the response is an electric polarisation of the material. Both the response and the stimulus must be described with respect to the crystal structure of the material, as vectors. Thus, for a dielectric, the response of the material, the electric polarisation, needs to be characterised by a vector P, and the stimulus, the applied electric field, needs to be specified as a vector E, both described with respect to the crystal structure of the perovskite. In a cubic crystal P is proportional and parallel to E, but for most crystals this is not true and the relationship between these two vectors needs to be described in tensor notation. 

In both cases, the best functional properties are found in those compositions on the MPB. Due to the energetic degeneracy of the two structural phases on the MPB, the system cannot decide which structure to take so that both phases may co-exist. Any external stimulus could tip the balance of the situation. Therefore, this uncertainty effectively creates high responsiveness to external fields, leading to enhanced functional properties of the material. These kind of methods to enhance functional properties of materials are guided by the following design philosophy ... 

Stimuli-responsive polymers consist of a class of smart materials that exhibit a physical response to changes in external conditions. Such stimuli include changes in pH, ionic strength, solvent polarity, and temperature, as well as mechanical force or electric fields. On the basis of their ability to switch conformations, stimuli-responsive polymers are being applied as sensors, actuators, and transducers (e.g., mechano-electrical or mechano-optical). Nanoporous membranes can be functionalized with stimuli-responsive polymers to modify their permeability, that is, to reversibly open and close the pores upon a given stimulus. ... 

Stimuli-Responsive (SR) materials, also called smart materials have been attracting great interest within scientific community in the last few decades [1-4], They possess uitique properties that have made this class of materials very promising for several applications in the field of nanoscience. In particular, the smart materials undergo changes in response to small external variations in enviroiunen-tal conditions or to physical or biochenfical stimuli. In addition, there are dual SR materials that simultaneously respond to more than one stimulus [5-7]. For instance, temperature-sensitive polymers may also respond to pH changes [8-11]. 

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