Dielectric and pyroelectric properties of P(VDF-TrFE) and
PCLT–P(VDF-TrFE) 0–3 nanocomposite films
QQ Zhang, HLW Chan
*
, CL Choy
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, ROC
Abstract
Nanocrystalline calcium and lanthanum modified lead titanate (PCLT) powder prepared by a sol–gel process was incorporated into a
polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] copolymer matrix to form PCLT–P(VDF-TrFE) nanocomposite thin films with
0.11 volume fraction of ceramic. The relative permittivity and pyroelectric coefficient of the P(VDF-TrFE) copolymer and nanocomposite
films were measured as functions of the poling electric field. After poling under the same conditions, the nanocomposite film was found to
have a higher pyroelectric coefficient (by ϳ35%) and figures of merit than those of the P(VDF-TrFE) film of a similar thickness. ᭧1998
Elsevier Science B.V. All rights reserved.
Keywords: Poling electric field; A. Nano-structures; A. thin films
1. Introduction
Polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)]
copolymers with TrFE content higher than 20% crystallizes
from the melt or from solution in a polar b phase [1], and
can be poled to give pyroelectric activity without prior
mechanical stretching. Integrated pyroelectric sensor arrays
have been fabricated by spin-coating P(VDF-TrFE) copoly-
mer films onto silicon wafers that contain readout electro-
nics [2–5]. However, the dielectric and pyroelectric
properties of the copolymer vary appreciably depending
on the supplier, composition, heat treatment and poling
electric field. In order to prepare high-performance nano-
composites, it is necessary to characterize the copolymer
matrix first.
Lead titanate (PT) is a ferroelectric ceramic having a
good pyroelectric property. PT doped with calcium and
lanthanum (PCLT) was reported to have pyroelectric coeffi-
cient ϳ3 times higher than that of PT [6]. As the ceramic in
the composite contributes substantially to the overall pyro-
electric property of the composite, it is imperative to use a
ceramic with high pyroelectric activity to give the compo-
site a high pyroelectric coefficient. In the preparation of
homogeneous films for integrated pyroelectric sensor appli-
cations, it is necessary to use ceramic powder with sizes
significantly smaller than the thickness of the film (several
mm). Hence PCLT powder with sizes in the nanometer
range was prepared by the sol–gel method [7] and homo-
geneous PCLT–P(VDF-TrFE) composite films were
prepared and characterized.
2. Experiments and results
2.1. Fabrication of copolymer and composite films
The P(VDF-TrFE) 70–30 mol% copolymer pellets
supplied by Piezotech Co. has a Curie temperature of
102ЊC upon heating and a melting temperature of 150ЊC
as determined by differential scanning calorimetry (DSC).
The procedure for preparing PCLT powder by the sol–gel
method has been described in our previous report [7]. The
powder used in this study was annealed at 850ЊC. It has an
average crystallite diameter of 50 nm (determined by X-ray
diffraction) and an average particle diameter of 200 nm
(determined using a particle analyzer). 1.5 g of copolymer
pellets were dissolved in 10 ml. of methyl-ethyl-ketone
(MEK) to form a solution. To prepare 0–3 nanocomposites,
a suitable amount of PCLT nanocrystalline powder was
blended into the copolymer solution to form a mixture.
The powder in the mixture was dispersed in an ultrasonic
bath for 4 h to produce a composite suspension. P(VDF-
TrFE) and nanocomposite films were prepared by spin-
coating (2000 rpm for 1 min) the copolymer solution and
the composite suspension, respectively, onto glass
substrates previously coated with aluminum electrodes
(400 nm thick). After the spin-coating procedure, the film
Composites: Part A 30 (1999) 163–167
1359-835X/99/$ - see front matter ᭧ 1998 Elsevier Science B.V. All rights reserved.
PII: S1359-835X(98)00113-4
* Corresponding author. Fax: 00 852 2333 7629, e-mail: apahlcha
@polyu.edu.hk