Page 159 - Polymer-based Nanocomposites for Energy and Environmental Applications

134 Polymer-based Nanocomposites for Energy and Environmental Applications

Pb(Mg 1/3 Nb 2/3 )O 3 –PbTiO 3 (PMN-PT) [51], or Pb(Zr x Ti 1 x )O 3 [52] into the polymer
matrix. In these ceramic polymer composites, a very large concentration of ceramic
particles (e.g., >50 vol%) is needed, resulting in a significant improvement in
Young’s modulus of the polymer composites. Other structural imperfections like
pores and voids are also introduced, which causes a decreased breakdown strength
of the polymer composites. Polymer composites show dielectric breakdown at electric
fields even lower than the breakdown strength of the ferroelectric ceramic fillers, even
when polymer matrix of high breakdown strength is used. Polymer composites can
achieve polarization at very low electric fields where saturated polarization of the
ferroelectric ceramic fillers may yet be achieved. Thus, the ferroelectric properties
of polymer composites are limited by the large volume fraction of ceramic fillers
and the subsequent microstructural imperfections. To overcome the issue of the high
volume fraction of ceramic particles in the polymer composites, one effective remedy
lies in the employment of ceramic fillers with a large aspect ratio rather than the com-
monly used spherical particles. The advantages of using large-aspect-ratio fillers are
twofold: (i) They can enhance the dielectric constant of the composites at much lower
volume fractions due to their large dipole moments [53], and (ii) they possess reduced
surface energy and hence prevent agglomeration [54]. The microstructural imperfec-
tions can be reduced by better compatibility between the ceramic and polymer matrix,
which is usually obtained by surface modification of the ceramic particles [40,55,56].
Surface engineering causes stronger interfaces between the ceramic fillers and the
polymer matrices by producing interfacial bonds through active surface groups
added by surface treatments. Recently, a common biomaterial, dopamine, has been
considered as a robust and generic surface building block due to its efficient interfa-
cial adhesion strength to a variety of surfaces including polymers and metal
oxides [57,58].
To further overcome the limitations of the inorganic-polymer composites in a con-
trollable way, numerous efforts were made with chemical modification being in focus,
namely, (i) use of some specific molecules for surface modification, such as silanes
[59], phosphonic acid [55], and ethylene diamine [45]; (ii) encapsulation by smaller
metal oxide [49,60]; and (iii) in situ polymerization on the surfaces of the
nanoparticles [56,61-65]. Silane coupling agent was extensively used to modify nano-
particle surfaces to improve the dispersion of the fillers in polymer matrix. However,
the unabsorbed residual components would result in high leakage current and dielec-
tric loss [59,66].
To overcome the limitations of either polymers or ceramics, organic polymers
along with inorganic ceramics have been developed recently [32,67-73].Perry
et al. fabricated barium titanate (BT)-based polymer nanocomposites with a good
energy storage density by modifying the BT nanoparticles with coupling agents
[40,55]. Marks and coworkers synthesized a series of polypropylene (PP)
nanocomposites by in situ olefin polymerization. Efficient nanoparticle dispersion,
large dielectric constant, low dielectric loss, large breakdown strength, and
improved energy storage density were obtained in these PP nanocomposites
[53,60,62,74,75].