Relative density values of the PMNT/ZnO ceramics were measured and tabulated in Table . The results indicated that an addition of ZnO did not significantly change the relative density value of PMNT ceramics. However, the highest relative density value was obtained for the PMNT ceramic incorporated with 0.05 wt.% ZnO. This result was expected that the small amount of ZnO addition more effectively affected the densification process of the ceramic. Further addition of ZnO could more effectively influence the grain growth process according to the increase in grain size of the ceramics as shown in Table .
Relative density, grain size, and lattice parameter of PMNT/ZnO ceramics
Results of the phase characterization of PMNT/ZnO ceramics are shown in Figure . The XRD patterns were well matched with standard JCPDS file no. 27-1199 for the cubic phase in the
space group. The XRD patterns showed that an addition of ZnO did not change the crystal structure of PMNT ceramics as well as no secondary phases including the ZnO phase were observed. The result suggested that Zn2+
ions could completely enter into the lattice of the PMNT structure (within the limitations of the XRD technique). Moreover, a detailed observation of XRD peaks at 2θ
≈ 45° showed that the peaks were slightly shifted to the left with an increasing ZnO content. It was believed that the substitution of Zn2+
= 0.74 Å) for Mg2+
= 0.72 Å) or Nb5+
= 0.64 Å) [14
] in the B-site lattices of PMNT resulted in the expansion of the unit cell. This result was supported by the increasing values of the calculated lattice parameter as shown in Table .
Figure 1 XRD patterns of PMNT/ZnO ceramics sintered at 1,150°C and XRD peak at 2θ ≈ 45°. The figure on the left showed XRD patterns of PMNT/xZnO ceramics, where x = 0, 0.05, 0.1, 0.5, and 1 wt.% of CuO measured from an angle range (more ...)
SEM micrographs of the fractured surface of PMNT/ZnO ceramics are shown in Figure . Average grain sizes tabulated in Table indicated that the grain size sharply increased when 0.05 to 0.1 wt.% ZnO was added. It was believed that this behavior was due to the enhancement of mass transport caused by ZnO addition [15
] which led to more grain growth. However, the grain size was quite constant with further ZnO addition (0.5 to 1.0 wt.%). In this case, some undetected ZnO may partially distribute at grain boundary and act as a grain growth inhibitor. Microstructure of the pure PMNT as shown in Figure revealed mainly an intergranular fracture. The samples incorporated with ZnO nanoparticles show a mixed-mode of inter/transgranular fracture as shown in Figure . The degree of transgranular fracture tended to predominantly occur in large grains. This result indicated that large grains were weaker than smaller ones [16
]. Moreover, the result may also be caused by the pinning at grain boundary of added ZnO contributing to crack deflection into grain bulk which caused a higher occurrence of transgranular fracture in large grains.
SEM images of the fractured surface of PMNT/ZnO ceramics. Fractured surface of pure PMNT ceramic (a) and PMNT ceramics incorporated with 0.05 (b), 0.1 (c), 0.5 (d), and 1 (e) wt.% ZnO.
Mechanical properties of the ceramics in terms of Vickers hardness (HV
) and fracture toughness (KIC
) were investigated, and the results are shown in Figure . The hardness value of the pure PMNT ceramic was approximately 4.5 GPa, and the value increased to approximately 5.3 GPa when 0.05 wt.% of ZnO was added into the PMNT ceramic. The ZnO solute in the PMNT grain was believed to contribute to higher resistance to Vickers indentation, leading to a harder material. Among PMNT ceramics incorporated with ZnO, however, the hardness value slightly decreased with further increasing ZnO content. The decrease of hardness value was associated with an increase of grain size. It was known that grain boundaries in the ceramic having smaller grains are stress concentration sites which acted as effective obstacles to dislocation pile-up in the adjacent grains, leading to a harder material [17
]. Fracture toughness result showed that an addition of 0.05 to 0.1 wt.% ZnO decreased fracture toughness values of PMNT ceramics. Due to the increase in grain size and observation of transgranular fracture when the amount of ZnO content was increased, the crack length of PMNT/ZnO ceramics extended longer than that in the pure PMNT ceramic, leading to a decrease in the fracture toughness. Further increasing ZnO content (0.5 to 1.0 wt.%) slightly increased the fracture toughness of the ceramics. It was believed that the micropores as observed at grain boundaries in the PMNT/ZnO ceramics (in black circles in Figure ) contributed to the obstruction of crack propagation, and hence, a decrease in crack propagation led to an increase in fracture toughness values.
Vickers hardness and fracture toughness of PMNT/ZnO ceramics. The upper line indicated the relation of Vickers hardness and ZnO content. The other lower line indicated the relation of fracture toughness and ZnO content.
Dielectric constant and dielectric loss values measured at room temperature and plotted as a function of ZnO content are shown in Figure and tabulated in Table . Typical characteristics of relaxor ferroelectrics, i.e., decreasing of dielectric constant and increasing of dielectric loss values with an increasing frequency, were observed in this ceramic system. An addition of 0.05 wt.% ZnO sharply increased the dielectric constant value of the ceramics. The dielectric constant values seemed to be correlated to the density values of PMNT/ZnO ceramics. From the above relationship, dielectric constant values of ceramics with further increasing of ZnO content (0.1 to 1.0 wt.%) were believed to be due to the presence of micropores at grain boundaries of the ceramics.
Figure 4 Dielectric constant and dielectric loss of PMNT/ZnO ceramics measured at room temperature. The upper group showed the relation of dielectric constant and ZnO content. The other lower group showed the relation of dielectric loss and ZnO content. These (more ...)
Dielectric and ferroelectric properties of PMNT/ZnO ceramics
Hysteresis loops of PMNT/ZnO ceramics are shown in Figure , and the related values (i.e., Pr
) were evaluated and listed in Table . Because of the temperature and field dependence of ferroelectric properties of ceramics, these parameters were normalized in the form of Pr
]. The hysteresis loop was well developed when 0.05 wt.% ZnO was added. However, the hysteresis loop was suppressed with 0.1 to 1.0 wt.% of ZnO additions. The results were associated with dielectric characteristics of the PMNT/ZnO ceramics. As mentioned above, since dielectric properties depended on the densification behavior of the ceramics, ferroelectric property behavior was thus believed to be attributed to the densification behavior of the ceramics as well. Therefore, the important factor mainly affected by the electrical properties of PMNT/ZnO ceramics in this study seemed to be the densification behavior of the ceramics.
Figure 5 Hysteresis loops of PMNT/ZnO ceramics measured at 20 Hz. The figure showed the relation of the applied alternative current electric field at 20 Hz and measured polarization of the pure PMNT ceramic (black line and square symbol) and PMNT ceramics incorporated (more ...)