3.1 Densification mechanisms
Fig.2 (a) and (b) show the densification tendency in x and y directions of SP1 and SP2 when sintering. It can be seen from the results shown in Fig. 2(a), with increasing of temperature, there shows a linear densification in x direction before 1200°C and a sharp increasing of densification rate after 1200°C. A platform is occurring around 1500°C, which indicated the pausing of densification. With the temperature increasing to 1625°C, the sintering end-point, the shrinkage value declines to zero again. There also illustrates linear densification in y direction before 1200°C and shows a sharp increasing of densification rate after 1200°C. However, the densification rate is higher than that in x direction before 1400°C, and the untimely densification in y direction is occurring comparison with that in x direction. From Fig. 2b, it shows that the densification of unprocessed SP2 is very different with that of SP1. The densification rate in y direction is smaller than that of x direction before 1250°C. The platform, indicated the stopping of densification, is occurring
at 1500°C later than that of SP1; and the final point of sintering in x direction is not observed before 1650°C. According to sintering theory, a densification platform indicates a neck forming or mass transport at grain boundary, which needs more Gibbs energy as sintering force.
From results of the main sintering curve (shown in Fig. 2), the densification rate change associated with unequal distance among particles will be described as three stages-initial, intermediate and final. The porous phase is removed slowly with the increasing of temperature before 1200°C; the densification or the arrangement of particles is occurring in the range of 1200-1500°C. In this stage, the solid particles are connected and the pores are filled through mass transport. After 1500°C, the densification rate falls off gradually until stopping again. Volume diffusion accompanied with an elastic flow but plastic deformation (called high temperature creep deformation) in the final stage of sintering.
We suggested the densification mechanisms to be divided into two types: 1) the chemical potential induced; 2) the spatial force induced. But it can be divided to six sub-stages, to characterize the sintering period. The six sub-stages, to describe the densification, are presented in Fig.3. In the first initial sub-stage (I-1), with the PVA coating in SP1, the elastic flow of PVA increases the densification rate in y direction until the volatile component burnt. After the organic combusted, the second initial sub-stage (I-2), the slower densification rate in x direction, as a gas emission passage, is beneficial to further densification in y direction. With the gas discharging, the pore adjacent to grains slowly closed. In first intermediate sub-stage (II-1), because of the high activation energy of processed powders by mechanical method, the surface energy decreases with the particle rearrangement or particle restoration through bonding the tiny powder to the small one. The pores located near the tiny powders closed while the bonding continues in the second intermediate sub-stage (II-2), and the further particle rearrangement was continued. The most significant change in intermediate stage is the secondary powder combined with others. The densification rate in this stage is great higher than that in initial stage, but the total shrinkages are similar to that in initial stage. The occurring of first shrinkage platform was the start of final stage. During the final sub-stage (III-1), the grain boundary migration and coalescence continues when the separate pores consumed. The bonded grains were becoming one grain with the disappearance of grain boundary which causes the densification rate near to zero. This phenomenon indicated that the densification is nearly completed according to conventional sintering theory. Differently, in this study, the further densification following the sintering platform takes place in the final sub-stage (III-2) and the mass transport in particle is considered as a global concept. According to traditional sintering theory, the grain boundary will be straight in the final stage of sintering, which cannot give a clear interpretation for partially stability zirconia polycrystalline ceramics. In this study, the spatial force balance is a key factor to affect the final densification of ceramics, especially for the polycrystalline material.
3.2 Elements distribution, microstructure and phase assemblage
Fig.4 implicates the Y and Mg element distribution in sintered S1 sample. The Y ion is clearly visible in the area of grains (shown in Fig.4c), and it can be seen that the Mg ion is mostly distributed along the grain boundaries (shown in Fig.4a). Furthermore, the accumulative Mg ion is obviously seen in Fig.4a owing to the forming of Mg-Al spinel. With the aim of studying the influence of additive ion ratio on phase assemblage of Zr-Y-Mg-Al-Ca-O ceramic, XRD is conducted on the samples with different Y/Mg ion rate. The XRD patterns and microstructure are illustrated in Fig. 5, respectively. As shown in Figure 5d, it can be seen that the fractions of m phase in the three samples, which are mainly similar, do not correlate with the increasing of Y/Mg ion ratio. Moreover, the number of coarse grains shown in SEM pictures is varied with the content of addition. With the increase of Y ion, the number of coarse grains drops gradually. But there are some sub-coarse grains with the increasing of Y/Mg ion ratio. This could be associated with the block effect of spinel [12]. With the increasing of Mg ion content, the black spinel grains are conspicuous. According to our previous studying [13], the coarsening grains were related to heat treat process. But in this study, a same sintering process has been conducted on all samples; therefore, from Fig.5, it can be concluded that there has a more important influence on grain size rather than the m phase content with the increasing of Y/Mg ion ratio if sintered by a similar sintering process.
3.3 Ionic conductivity and conduction mechanism
The ionic conductivities of the sintered samples were studied by Frequency Response Analyzer in temperature range from 723K to 1273K. As for the Zr-Y-Mg-Al-Ca-O ceramic, the typical impedance spectra is consisted with a high-frequency impedance arc and a low-frequency impedance arc related to the grain and grain boundary resistor-capacitor (RC) effect, respectively (shown in Fig.6). From the impedance spectra shown in Fig.6, it can be seen that with increase of Y/Mg ion ratio, there are all gradually declining in the total electrical resistance, grain electrical resistance and grain boundary electrical resistance tested at 1223K. The declined rate in grain electrical resistance is diminishing. But it is proportionate drop in grain boundary electrical resistance at that temperature with the increasing of Y/Mg ion ratio. Based on the element distribution, as shown in Fig.4, the decreasing grain boundary electrical resistance is related to the decreasing of Mg ion content along grain boundary. On the other hand, the enrichment of Mg ion in grain boundary acts as a disincentive to grain boundary ionic conductivity. Furthermore, the grain ionic conductivity increasing effect by the increasing of Y ion content is weakened slowly as the saturated substitutional effect of Y ion for Zr ion lattice site.
As the result of ionic conductivity shown in Fig.7, calculated by Arrhenius function, it reveals that the ionic conductivity of S3 is the highest at high temperature which is in accordance with impedance spectra. However, a random arrangement of that over lower temperature windows is found, which may be related to the influence extent of grain boundary electrical resistance or grain electrical resistance on the total electrical resistance. From Fig.6 and Fig.7, it can be concluded that the influence of grain
boundary conductivity on the total conductivity is more extraordinary than that of grain conductivity. This also infers that the ionic transmitted route is changed from grain conduction model into multi-channel grain boundary conduction model, which is preferred transport route. By contrasting with typical microstructure graph, the number of coarse grains and the distribution of that could control the grain impedance. The disadvantageous factor of more coarse grains on ionic conductivity at high temperature is involved in the limitation of grain electrical resistance. The effect of Mg ion on the grain boundary conductivity reveals that high-pure grain boundary is beneficial to total conductivity at high temperature. Although the grain boundary of Zr-Y-Mg-Al-Ca-O ceramic in this study is not high-pure, there is as a second-phase among grains, called partial clogged grain boundary, which will be transforming to good conductor with increase of temperature.
From the figure 7, the maximum ionic conductivity obtained in Zr-Y-Mg-Al-Ca-O ceramics is 0.143 Scm-1 at 1223K. The highest ionic conductivity obtained in this work is better than that of Sm2-xLaxZr2O7 [14] and Sc-Yb-Zr-Ca-O at 1223K [15]. The complex doped electrolyte in this work showed an increased ionic conductivity which is also better than that of 8YSZ reported in literature [16] at the same temperature.