Journal of Non-Crystalline Solids 354 (2008) 5321–5322
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Dielectric properties of polystyrene–CCTO composite F. Amaral a, C.P.L. Rubinger a, F. Henry b, L.C. Costa a,*, M.A. Valente a, A. Barros-Timmons c a b c
Physics Department and I3N, University of Aveiro, 3810-193 Aveiro, Portugal Laboratoire de Recherche en Polymères, CNRS, 94320 Thiais, France Chemistry Department and CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e
i n f o
Article history: Available online 6 September 2008 PACS: 77.84.Lf 77.22.Ch Keywords: Dielectric properties Relaxation Electric modulus Polymers Organics
a b s t r a c t The control of the dielectric properties of polymer composites is a relevant tool to synthesize a material to a specific industrial application. Polystyrene (PS) is a suitable host because it is readily available, and is easy to cast into desired shapes, maintaining the mechanical integrity of the matrix. CaCu3Ti4O12 (CCTO) is a well-known high dielectric constant material, very useful for capacitors and memory devices. In this work, we studied the dielectric properties of the composite PS–CCTO, in the frequency range 10 Hz to 100 kHz, for CaCu3Ti4O12 grains concentrations up to 64% by volume. Different mixture laws were used to fit the data: Hanai, Wiener, Maxwell–Wagner, Kraszewsky, Looyenga and Generalized Looyenga. The last one presents the best results. The calculated exponent of this law was then correlated with the shape particles observed by scanning electron microscopy. Finally, using Generalized Looyenga law, we can carefully select the adequate CCTO concentration in order to tailor the desired behavior, producing interesting composites for potential applications. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The electrical properties of an insulating polymer can be modified by adding particles like carbon [1], iron [2], nickel [3] or conducting polymer [4]. The properties of such composite material can thus be controlled by properly choosing the components, their shape and their relative concentrations. CaCu3Ti4O12 (CCTO), a perovskite like compound, has been studied profoundly in the last years, because of their colossal dielectric permittivity [5–8]. CCTO ceramics when associated with other kind of materials, such as polymers [9], metals [10] or even other ceramics [11– 14], can produce composites with interesting electric and dielectric properties. Polymer composites using high dielectric ceramics, like CCTO, are particularly important, once they can be manufactured as high density energy storage flexible materials for capacitor applications [12–14]. On the other hand, polystyrene is a suitable host because it is readily available, and is easy to cast into desired forms maintaining the mechanical integrity of the matrix.
2.1. Materials
* Corresponding author. E-mail addresses:
[email protected], kady@fis.ua.pt (L.C. Costa). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.05.056
CCTO powders were prepared by the solid state traditional method [5]. At first, stoichiometric quantities of CaCO3, TiO2 and CuO were mixed in a planetary mill for 20 min, with a rotational speed of 200 rpm, and the mixture was calcined during 12 h, at 1050 °C. The single phase structure was confirmed by DRX analyses. The obtained ceramic was ball milled. Polystyrene was prepared by aqueous emulsion polymerization technique yielding monodispersed PS microparticles, used later on as host matrix. The powder samples were pressed into thin plates (2 cm 2 cm 0.2 cm) at room temperature and 4.5 MPa during 20 min for the dielectric characterization. 2.2. Characterization To make the dielectric measurements we used a SR850 DSP Lock-in Amplifier, from Stanford Research Systems. The method consists in measuring the ‘in phase’ and the ‘out of phase’ components of the output signal relatively to a reference signal. These quantities were then used to calculate the values of effective resistance and capacitance in a parallel RC (resistance–capacitor) model of the sample. The experimental parameters were the frequency
5322
F. Amaral et al. / Journal of Non-Crystalline Solids 354 (2008) 5321–5322
range from 10 Hz to 100 kHz, and sinusoidal voltage (Vrms) of 1.0 V, without dc bias. All the measurements were made at a constant temperature (23 °C). The samples microstructure was observed by Scanning electron microscopy (SEM), performed in a Hitachi-S4100 operated at 25 kV on the surface of carbon coated by sputtering samples. 3. Discussion Fig. 1 shows the real part of the complex permittivity, as a function of frequency, for samples with different concentrations of CCTO in the host matrix of polystyrene. No relaxation processes were identified in this frequency range. A mixture law permits to link the complex permittivity of the composite, e*, with that of its constituents. If filler particles with a complex permittivity, ef , and volume fraction uf, are dispersed in a matrix material with complex permittivity em , then
e ¼ f ðem ; ef ; uf ; kÞ;
ð1Þ
where k is a parameter related to the shape of the inclusion particles. In this study we tried different mixture laws to fit the experimental results. In particular, Maxwell–Wagner, Hanai and Looyenga, used for spherical particles, Wiener and Kraszewsky for
Fig. 3. SEM image of CCTO particles (sample without polymer).
stratified geometries. For unknown morphologies, Generalised Looyenga can be used, allowing the estimation of the shape of the inclusions. Fig. 2 shows the fits using these laws, at 1 kHz. The estimated minimum mean square errors permit to conclude that Generalised Looyenga law [15], 1
1
1
t e t ¼ uf eft þ ð1 uf Þum em
160
120
ε'
CCTO
80
64%
ð2Þ
fits correctly the data. The obtained parameter t, in expression (2) was 1.76, which indicates that the shape of the particles is far from spherical for which t is equals to 3 [16]. In Fig. 3 a SEM image shows the observed CCTO particles, in a sample without polymer. Density measurements confirm that we have very low porosity, even for the composites with high concentration of filler. 4. Conclusion
47%
40
20%
0
6% 3% PS
2
3
4
5
References
log (frequency) / Hz Fig. 1. Real part of the complex permittivity as a function of frequency, for different samples.
160
Looyenga
Kr
140 Wiener direct Hanaï
120
ε'
100 80 60
Generalised Looyenga
Maxwell-Wagner direct
40 20
Wiener inverse
0 0.0
0.2
0.4
0.6
0.8
Dielectric properties of the composite PS–CCTO can be accurately fitted by the Generalised Looyenga law. The obtained values for the exponent parameter of this law allow to infer the particles shape, which is confirmed by SEM images.
1.0
1.2
Volume fraction of CCTO Fig. 2. Mixture laws fit of the real part of the complex permittivity.
[1] C. Brosseau, F. Boulic, P. Queffelec, C. Bourbigot, Y. Le Meste, J. Loaec, A. Beroual, J. Appl. Phys. 81 (2) (1997) 882. [2] M.A. Valente, L.C. Costa, S. Mendiratta, F. Henry, L. Ramanitra, Solid State Commun. 112 (1999) 67. [3] A.A. Khoroshilov, I. Koroleva, Y. Bolodin, Russ. J. Appl. Chem. 73 (11) (2000) 1918. [4] Y. Roichman, M. Silverstein, A. Siegmann, M. Narkis, J. Macromol. Sci. Phys. 38 (1–2) (1999) 145. [5] M.A. Subramanian, D. Li, N. Duan, B.A. Reisner, A.W. Sleight, Solid State Chem. 151 (2000) 323. [6] A.F.L. Almeida, R.S. de Oliveira, J.C. Goês, J.M. Sasaki, A.G. Souza Filho, J. Mendes Filho, A.S.B. Sombra, Mater. Sci. Eng. B 96 (2002) 275. [7] B.A. Bender, M.-J. Pan, Mater. Sci. Eng. B 117 (2005) 339. [8] A.F.L. Almeida, R.R. Silva, H.H.B. Rocha, P.B.A. Fechine, F.S.A. Cavalcanti, M.A. Valente, F.N.A. Freire, R.S.T.M. Sohn, A.S.B. Sombra, Phys. B: Condens. Matter 403 (2008) 586. [9] B. Shri Prakash, K.B.R. Varma, Compos. Sci. Technol. 67 (2007) 2363. [10] C.C. Wang, Y.J. Yan, L.W. Zhang, M.Y. Cui, G.L. Xie, B.S. Cao, Scripta Mater. 54 (2006) 1501. [11] A.F.L. Almeida, P.B.A. Fechine, J.C. Góes, M.A. Valente, M.A.R. Miranda, A.S.B. Sombra, Mater. Sci. Eng. B 111 (2004) 113. [12] Y. Yan, L. Jin, L. Feng, G. Cao, Mater. Sci. Eng. B 130 (2006) 146. [13] J. Kułek, I. Szafraniak, B. Hilczer, M. Połomska, J. Non-Cryst. Solids 353 (2007) 4448. [14] Y. Bai, Z.Y. Cheng, V. Bharti, H.S. Xu, Q.M. Zhang, Appl. Phys. Lett. 76 (2000) 3804. [15] H. Looyenga, Physica 31 (1965) 401. [16] V. Beek, Dielectric behaviour of heterogeneous systems, in: J.B. Birks (Ed.), Progress in Dielectrics, 1967.
