Effects of ZnO nanoparticulate addition on the properties of PMNT ceramics
© Promsawat et al; licensee Springer. 2012
Received: 6 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
This research was conducted in order to study the effect of ZnO nanoparticulate addition on the properties of 0.9 Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 [PMNT] ceramics. The PMNT ceramics were prepared by a solid-state reaction. The ZnO nanoparticles were added into PMNT ceramics to form PMNT/x ZnO (x = 0, 0.05, 0.1, 0.5, and 1.0 wt.%). The PMNT/x ZnO ceramics were investigated in terms of phase, microstructure, and mechanical and electrical properties. It was found that the density and grain size of PMNT ceramics tended to increase with an increasing amount of ZnO content. Moreover, a transgranular fracture was observed for the samples containing ZnO, while pure PMNT ceramics showed only a intergranular fracture. An addition of only 0.05 wt.% of ZnO was also found to enhance the hardness and dielectric and ferroelectric properties of the PMNT ceramics.
The complex perovskite Pb(Mg1/3Nb2/3)O3 [PMN] compound has been extensively studied for uses in several applications due to its high dielectric constant and low sintering temperatures [1–5]. The maximum dielectric constant [εrmax] of PMN increased when normal ferroelectric PbTiO3 [PT] was added. The temperature related to this maximum (Tmax) also shifted upward . The εrmax of PMN reached the highest value with the addition of only 10 mol% PT [7, 8]. The 0.9PMN-0.1PT [PMNT] is thus known as one of the most popular ferroelectric compositions which show a high dielectric constant and a high electrostrictive strain for multilayer capacitor and electrostrictive actuator applications. Under an actual working environment, however, PMNT ceramics still have problems related to mechanical strength. Moreover, for applications in electronic devices, high values of strength, hardness, and fracture toughness are also required. It is well understood that improving the densification process can effectively enhance the mechanical strength of ceramics. In addition, decreasing the grain size could also enhance the hardness and fracture toughness of ceramics [9, 10]. According to previous investigations, one simple novel method to improve mechanical characteristics of oxide ceramics was based on the nanocomposite concept .
ZnO is known to have semiconductive properties and is now used in some electronic devices. It was found to improve sensitivity in materials used for sensing devices. Apart from this, the role of ZnO as a sintering aid in the sintering process was previously observed in ferroelectric ceramics such as PZT and PZT-BLT [9, 10]. Moreover, addition of a ZnO nanoparticulate into these material systems also enhanced the hardness and fracture toughness of the ceramics. In this study, the ZnO nanoparticulate was thus selected as an additive for PMNT ceramics to improve mechanical properties, while dielectric and ferroelectric properties of the ceramics were expected to be maintained. Effects of the ZnO concentration on the phase, microstructure, and mechanical and electrical properties of PMNT ceramics were investigated and discussed.
The PMNT powder was prepared by the columbite method . The columbite precursor (MgNb2O6) was prepared by mixing the stoichiometric amounts of MgO (99.9%, Fluka, Sigma-Aldrich, St. Louis, MO, USA) and Nb2O5 (99.9%, Aldrich, Sigma-Aldrich, St. Louis, MO, USA) in ethanol, followed by ball milling for 24 h using a ZrO2 grinding medium. The slurry was dried at 120°C, and the powder was calcined at 1,000°C for 4 h. The columbite precursor was then mixed and ball-milled with predetermined amounts of PbO and TiO2 (99.9%, Aldrich) powders and calcined at 850°C for 2 h. The calcined powders were added with ZnO nanoparticles (20 nm, 99.5%, Nanostructured & Amorphous Materials, Inc., Houston, TX, USA) to form PMNT/x ZnO powders where x = 0, 0.05, 0.1, 0.5, and 1 wt.%. The mixed powders were then uniaxially pressed into pellets and sintered at 1,150°C for 2 h in an atmosphere of PMN powder. Bulk density of the ceramics was determined using Archimedes' method. Phase composition of the PMNT/ZnO ceramics was characterized using an X-ray diffraction method [XRD] (X-pert, PANalytical B.V., Almelo, The Netherlands). Microstructure of the ceramics was observed via a scanning electron microscope [SEM] (JSM-6335F, JEOL Ltd., Akishima, Tokyo, Japan). Average grain size was determined using a mean linear interception method from the SEM micrographs. In this method, a number of straight lines were drawn on each micrograph, and intercepted lengths of grains were obtained and averaged. The well-polished ceramics were subjected to Vickers indentation (Galileo Microscan, LTF S.p.a., Antegnate, Italy) for hardness (HV) determination. Fracture toughness (KIC) was determined following the method described by Antis et al. . Dielectric constant and loss tangent were measured using an LCR meter (Hitester 3532-50, Hioki, Ueda, Nagano, Japan). Ferroelectric hysteresis (P-E) loops were characterized using a computer-controlled modified Sawyer-Tower circuit.
Results and discussion
Relative density, grain size, and lattice parameter of PMNT/ZnO ceramics
Lattice parameter (Å)
1.88 ± 0.05
2.15 ± 0.06
2.61 ± 0.08
2.71 ± 0.06
3.07 ± 0.07
Dielectric and ferroelectric properties of PMNT/ZnO ceramics
P r /P max
E c /E max
It can be seen that the PMNT/ZnO ceramics were successfully prepared by a solid-state mixed-oxide method. The variation of lattice parameters, microstructure, and mechanical and electrical properties of the ceramics were affected by the addition of ZnO nanoparticles. The hardness value of the pure PMNT ceramic increased from approximately 4.5 to approximately 5.3 GPa when 0.05 wt.% of ZnO was added into the ceramic. An addition of ZnO in the range of 0.5 to 1.0 wt.% tended to increase the fracture toughness value. Moreover, an addition of 0.05 wt.% ZnO enhanced the dielectric constant of the monolithic PMNT ceramic from 10,380 to 14,344. Furthermore, ferroelectric properties of the ceramic were also improved when 0.05 wt.% of ZnO was added. From this investigation, it was suggested that the optimum composition of the PMNT/ZnO system would be 0.05 wt.% ZnO due to its superior mechanical, dielectric, and ferroelectric properties.
This work is financially supported by the Thailand Research Fund (TRF) and the National Research University Project under Thailand's Office of the Higher Education Commission (OHEC). The Faculty of Science and the Graduate School of Chiang Mai University are also acknowledged. MP would also like to thank the financial support from the TRF through the Royal Golden Jubilee Ph.D. Program.
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