Page 156 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 5

for significantly enhanced
dielectric properties and energy

storage capability

B.M. Pirzada, S. Sabir
Aligarh Muslim University, Aligarh, India

5.1 Introduction

5.1.1 Dielectrics

An electric insulator is called a dielectric when the dipoles, which are otherwise in
random orientations, align in an applied electric field. When the field is applied, elec-
tric polarization occurs in the dielectric as the dipoles align themselves in the field.
The mode of polarization in the dielectric is depicted in Fig. 5.1. The positive poles
of the dipole are moved in the direction of lower voltage and the negative poles move
in the reverse direction [1].
The polarization that develops in the dielectric lowers the effective electric field
between the plates and thus enhances the capacitance of the parallel plate dielectric
structure. The dielectric properties of a dielectric material is characterized by the
dielectric constant, k, or the relative permittivity, ε r , and it is an intrinsic material
property. The dielectric constant is the ratio of the permittivity of the dielectric
(ε) to the permittivity of vacuum (ε 0 ). It determines the polarization of the dielectric
that in turn determines the surface charge densities on the dielectric. Recent attrac-
tion in the microelectronics research is the fabrication of low-dimensional and high-
permittivity dielectrics for microelectronic devices, namely, memory devices [2–6],
capacitors [7,8], gate insulators [2,9–11], energy storage [12,13], and high-
frequency communication devices [14,15]. For the efficient operation of these
devices, higher permittivity (ε r ), reduced leakage current (J), and lower loss tangent
(tanδ)are essential [14]. Thus, larger dielectric constant materials [14] are seen to
replace SiO 2 at gate insulators in silicon devices for SiO 2 has a small permittivity
(ε r ¼3.9) [2,9,10].
As a step forward, two types of materials have been explored on the basis of their
mode of polarization. One type includes metal oxides such as Al 2 O 3 [9,16], TiO 2
[9,282,283],Nb 2 O 5 [16,17], HfO 2 [9], ZrO 2 [9], and Ta 2 O 5 [9,16]. These contain a
nonpolar atomic arrangement [14], and the residual polarization can persist long after
the applied field is removed [18]. This phenomenon is called dielectric relaxation [18].

Polymer-based Nanocomposites for Energy and Environmental Applications. https://doi.org/10.1016/B978-0-08-102262-7.00005-2
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