Improvement in dielectric and mechanical performance of CaCu3.1Ti4O12.1 by addition of Al2O3 nanoparticles

Chompoonuch Puchmark 1 Gobwute Rujijanagul 0 0 Department of Physics and Materials Science, Faculty of Science, Chiang Mai University , Chiang Mai, 50200, Thailand 1 Department of Physics, Faculty of Science, Naresuan University , Phitsanulok, 65000, Thailand The properties of CaCu3.1Ti4O12.1 [CC3.1TO] ceramics with the addition of Al2O3 nanoparticles, prepared via a solidstate reaction technique, were investigated. The nanoparticle additive was found to inhibit grain growth with the average grain size decreasing from approximately 7.5 m for CC3.1TO to approximately 2.0 m 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. - Background 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. [1], 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 [4]. The CCTO ceramics exhibit an electrically heterogeneous structure involving mobile-charged species in terms of the Maxwell-Wagner relaxation [5]. 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, [13] while Kwon et al. reported that both Cu- and Ti-deficient CCTO presented a higher dielectric constant than undoped CCTO [14]. 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. Experimental procedure It has been proposed that Cu stoichiometry is related to the dielectric response [13,14]. Fang et al. [13] 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 900C for 8 h with a heating rate of 5C/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 550C with a heating rate of 1C/min. Finally, the pellets were sintered at 1,025C for 6 h with a heating rate of 5C/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 600C 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 Phase formation The XRD results for the sintered ceramics containing up to 2 vol.% Al2O3 are illustrated in Figure 1. All of the patterns were similar to the unmodified CCTO diffraction peaks and were consistent with the results reported previously [18]. The peaks of the second phases such as Cu2O and CuO could not be observed in the XRD patterns [14]. Further, no peak was observed for the Al2O3 phase in any of the XRD patterns. This may be due to the amount of Al2O3 additive which was too little to be detected at the sensitivity level of the XRD instrument. Densification, microstructure, and hardness behavior The plot of density as a function of Al2O3 volume fraction is shown in Figure 2. The density slightly increased with the increasing amounts of Al2O3 up to 0.5 vol.% and then decreased for the 2 vol.% sample. The reduction in density for the higher Al2O3 samples suggests that the sintering mechanism of the samples was not complete. To obtain the best densification for compositions > 0.5 vol.% Al2O3, higher sintering temperatures or longer soaking times would be required. Figure 3 displays the SEM micrographs of the as-sintered surfaces of CC3.1TO-Al2O3 nanocomposites. An Figure 1 XRD patterns of the surfaces of the CC3.1TO and CC3.1TO-Al2O3 pellets. Figure 2 Density and average grain size as Al2O3 volume fraction function for CC3.1TO and CC3.1TO-Al2O3 nanocomposites. Inset shows Knoop hardness value as a function of Al2O3 content of the samples. agglomeration of Al2O3 nanoparticles was not explicitly observed, implying that the processing method (...truncated)