  Using Landau-Ginzburg-Devonshire theory and effective medium approximation,
we analytically calculate typical dependences of the pyroelectric and
electrocaloric coefficients on external electric field, temperature and radius
for spherical single-domain ferroelectric nanoparticles. The considered
physical model corresponds to the nanocomposite with small fraction of
ferroelectric nanoparticles. Within the framework of the analytical model we
establish how the size changes determine the temperature and field behavior
pyroelectric and electrocaloric coefficients on example of BaTiO3 nanoparticles
covered by a semiconducting shell and placed in a dielectric polymer. We show
that by changing the particle size one can induce maxima of the pyroelectric
coefficient and electrocaloric temperature variation, control their width,
height and sign. Obtained analytical expressions allow selecting the interval
of particle sizes, voltage, and/or temperature for which pyroelectric energy
conversion and electrocaloric coefficient are optimal for applications. The
observed size effect opens the possibility to control pyroelectric and
electrocaloric properties of ferroelectric nanocomposites that can be important
for their advanced applications in energy convertors and cooling systems.
