Improvement in dielectric and mechanical performance of CaCu3.1Ti4O12.1 by addition of Al2O3 nanoparticles
© Warangkanagool and Rujijanagul; licensee Springer. 2012
Received: 19 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
The properties of CaCu3.1Ti4O12.1 [CC3.1TO] ceramics with the addition of Al2O3 nanoparticles, prepared via a solid-state reaction technique, were investigated. The nanoparticle additive was found to inhibit grain growth with the average grain size decreasing from approximately 7.5 μm for CC3.1TO to approximately 2.0 μm for the unmodified samples, while the Knoop hardness value was found to improve with a maximum value of 9.8 GPa for the 1 vol.% Al2O3 sample. A very high dielectric constant > 60,000 with a low loss tangent (approximately 0.09) was observed for the 0.5 vol.% Al2O3 sample at 1 kHz and at room temperature. These data suggest that nanocomposites have a great potential for dielectric applications.
Keywordsnanocomposites dielectric properties microstructure mechanical property
CaCu3Ti4O12 [CCTO] is an interesting dielectric material which exhibits a high dielectric constant over 10,000 at room temperature and shows temperature independence over the temperature range from approximately 100 to 400 K [1–3]. Since the discovery of this material by Subramanian et al. , CCTO has been widely studied to further understand and improve its properties. The CCTO crystal has a cubic symmetry with an Im3 space group. In the CCTO lattice, the TiO6 octahedra are tilted which results in a doubling of the perovskite-like structure, involved in the planar square arrangement of the oxygen around the copper ions . The CCTO ceramics exhibit an electrically heterogeneous structure involving mobile-charged species in terms of the Maxwell-Wagner relaxation . Internal interfaces in the polycrystalline CCTO give rise to the polarization in the insulating grain boundary and at the semiconducting grains which is well explained by the internal barrier layer capacitor [IBLC] model [6, 7]. To improve the dielectric properties further, many cations have been introduced into CCTO, including Co, Zr, Fe, Sc, and Nb on the B site and substitution of La and Eu at the A site [4, 8–12]. Although some of these additives resulted in a reduction of the loss tangent, most additives also reduced the dielectric constant. Fang et al. proposed that Cu stoichiometry can affect the electrical properties of the CCTO ceramics,  while Kwon et al. reported that both Cu- and Ti-deficient CCTO presented a higher dielectric constant than undoped CCTO . Recently, many authors have reported on the properties of composites between CCTO and other materials such as BaTiO3, SrTiO3, ZnNb2O6, and polystyrene [15–17]. However, the properties of composites formed by adding nanocomposites to CCTO have still not been widely investigated. In the present work, a new nanocomposite system between CCTO (with non-stoichiometric composition) and Al2O3 nanoparticles was fabricated. We demonstrate that the dielectric behavior of the composites can be significantly improved by the addition of these nanoparticles. Some other properties of the nanocomposites were also investigated and reported.
It has been proposed that Cu stoichiometry is related to the dielectric response [13, 14]. Fang et al.  reported that Cu-excessive CCTO samples showed improved densification and dielectric behaviors. In the present work, Cu-excessive CCTO ceramics in a composition of CaCu3.1Ti4O12.1 [CC3.1TO] were fabricated. Our studies indicate that this composition exhibited a good densification and dielectric response (data not shown). The samples were fabricated using the solid-state mixed oxide method. Reagent grade CaCO3, CuO, and TiO2 powders were used as starting materials. The mixture of these powders was ground for 24 h in ethanol using zirconia grinding media. The suspension was then dried and subsequently calcined at 900°C for 8 h with a heating rate of 5°C/min. The calcined CC3.1TO powders were mixed with (0.5, 1, and 2 vol.%) Al2O3 nanoparticles (40 nm average particle size) and 1% polyvinyl alcohol [PVA] binder and were ball-milled in ethanol for 12 h using the same method as mentioned earlier. The slurry was then dried and sieved to a fine powder. The mixed powders were uniaxially pressed into pellets at a pressure of 60 MPa. The PVA binder was burnt out at 550°C with a heating rate of 1°C/min. Finally, the pellets were sintered at 1,025°C for 6 h with a heating rate of 5°C/min. The sintered pellets were investigated for phase formation by X-ray diffraction [XRD]. Density of the sintered samples was measured using the Archimedes method with distilled water as the fluid medium. The microstructures of the sintered samples were characterized using a scanning electron microscope [SEM], and the average grain size was determined using the linear intercept method. For the electrical measurement, silver paste was applied to both sides of the circular faces of the ceramics, then dried at 600°C for 15 min, and cooled naturally to room temperature. The dielectric constant and dielectric loss were then measured using a LCZ meter. The mechanical properties (hardness) of various sintered samples were studied using a Knoop microhardness tester. Indentations were applied to the polished surfaces with 0.3- and 0.5-kg loads and with an indentation period of 15 s.
Results and discussion
Densification, microstructure, and hardness behavior
The Knoop hardness values of the samples as a function of Al2O3 content are illustrated in the inset of Figure 2. The Knoop hardness data reveal that the additive improved the hardness values. The maximum hardness value in this work was 9.8 GPa (for the 1 vol.% sample) which is comparable to the value reported by Puchmark et al. for the PZT-Al2O3 nanocomposites. The improvement in the mechanical properties is most likely due to the nanoparticles reinforcing the grain boundaries and acting as effective pins against microcrack propagation . Moreover, the enhancement of hardness can be related to the reduction in grain size, i.e., small grain size samples gave a higher measured hardness.
where d is the grain size, t is the thickness of the grain boundary (barrier width), and εgb is the internal dielectric constant of the barrier material. Since the grain size of the present samples decreased with the increasing additive, Equation 1 predicts that the higher dielectric constant for the 0.5 vol.% sample is not related to the grain size, but it may be connected to a change in the grain boundary characteristics such as εgb and t after adding the additive. The reason for the change of grain boundary characteristic is still unclear, but it is possible that the Al2O3 nanoparticles had a reaction with the matrix of the CC3.1TO, and as a result, the formation of Al-metal oxide phases at the grain boundary produced other products in small amounts which could not be detect by XRD . However, the higher density for the 0.5 vol.% sample can be explained by the observed higher dielectric constant in the present work.
CC3.1TO-Al2O3 nanocomposites were fabricated for the first time. The samples were prepared using a solid-state reaction. The CC3.1TO ceramics showed a duplex microstructure, consisting of coarse and fine grains, while the nanocomposites showed mainly fine grains in their microstructure due to the fact that the additive inhibited grain growth. The additive also enhanced the hardness value especially for the 1 vol.% sample. However, the CC3.1TO and 0.5 vol.% Al2O3 showed a high dielectric constant with a strong dielectric frequency dispersion especially at low temperatures and also had a lowered loss tangent value, as compared with other samples. These results indicate that the addition of nanoparticles may be an alternative method to improve the dielectric behavior in some other giant dielectric materials.
This work was supported by The Thailand Research Fund (TRF), Thailand's Office of the Higher Education Commission (OHEC), and Faculty of Science and Graduate School, Chiang Mai University. The authors would like to thank Prof. Dr. Tawee Tunkasiri for his help in many facilities.
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