Sintering Behaviors and Microstructure Evolution

Figure 4-22 depicts the relative density of capacitor samples (called nano-BST samples hereafter) prepared via the solid-state reaction of BTO/STO powders as a function of sintering temperatures. For all samples, sintering duration was fixed at 1 hr. It shows that the increase of density is relatively slow below 1000°C; in between 1000 to 1200°C, densities increases rapidly; beyond 1200°C, the densities become

0.01 0.1 1

Figure 4-20. Particle size distribution of nano-BST powder in the slurry subjected to chemical dispersion and physical grinding process.

1 10 100

1 10

V iscosit y (m Pa s)

Shear rate (S

)

Figure 4-21. Viscosity of aqueous suspensions containing nano-BST powder and 5

800 900 1000 1100 1200 1300 1400 60

70 80 90 100

R ela tiv e d en sitiy (% )

Sintering temperature ( ) ℃

Figure 4-22. Relative density change of nano-BST samples versus sintering

temperature.

nano-BST samples occurs at around 1200°C. Previous our study showed that the densification temperature of nano-BTO is about 1100°C (section 4-1-5) and, hence, the addition of Sr did not suppress the densification temperature of nano-BTO

ceramics. This is similar to the micro-scale BST which requires a higher sintering

temperature than that of conventional BTO.^19,25 However, the densification

temperatures for micro-BST system could be as high as 1400°C.^19,56 Therefore, the reduction of raw powder size to nanometer scale did suppress the sintering

temperatures of capacitor samples. The ignition of densification at lower sintering

sintering, the sample tended to reduce its surface energy by eliminating the total surface area of nano-particles. The interface-driven processes such as grain boundary diffusion and surface diffusion must dominate the mass transport at the early stage of sintering and result in the densification of nano-BST samples.

The SEM morphologies of nano-BST samples sintered at 1200 and 1300°C for 1, 3, 6 and 8 hrs are presented in Figs. 4-23 and 4-24, respectively. Figures 4-23(a) and 23(b) shows a rather mild grain growth in samples subjected to 1200°C-sintering for the sintering times up to 6 hrs. The grain sizes remain uniform in the range around 200 nm. However, the grain sizes rapidly enlarge to about 5.5 µm when the sintering time is raised to 8 hrs as illustrated in Fig. 23(d). As to the samples subjected to 1300°C-sintering (see Fig. 4-24), monotonous grain growth occurs as the average grain sizes increase from 220 nm for 1-hr sintering to about 45 µm for 8 hrs-sintering.

Previous study reported that the average grain sizes are about 1.5 µm and 100 µm for the nano-BTO sintered at 1200 and 1300°C for 6 hrs, respectively (Section 4.1.5).

This implies that the addition of Sr is able to suppress the grain growth in BTO, a result similar to that reported in micro-BST system.57 The suppression of grain growth becomes much obvious in the specimens sintered at high temperatures, e.g., 1300°C.

Ionic radius of Sr⁺² (0.113 nm) is smaller than that of Ba⁺² (0.135 nm). As we known from the knowledge of dislocation interactions, lattice irregularity induced by the

difference of ionic radii would impinges the dislocation motions in BST lattice and hence retards the grain growth. In addition, before the grain boundaries being eliminated by substantial grain growth, numerous grain boundaries presents in the samples with nano-scale grain sizes. Grain boundary is known as the structural discontinuity, which may serves as the obstacle of dislocation motions and thus suppresses the grain growth.

Figure 4-23. SEM micrographs of nano-BST samples sintered at 1200°C for (a) 1 hr,

Figure 4-24. SEM micrographs of nano-BST samples sintered at 1300°C for (a) 1 hr, (b) 3 hrs, (c) 6 hrs and (d) 8 hrs.

Figure 4-25 presents the XRD patterns of the raw BTO/STO powders and the samples sintered at 1100, 1200, 1300°C for 6 hrs, respectively. It shows that the raw sample is a mixture of cubic BTO and STO powders; the sample sintering at 1100°C becomes tetragonal BST mixed with some cubic BTO/STO phases; the samples sintered at temperatures above 1200°C are all tetragonal BST. It was reported that the tetragonal phase forms in conventional BST (average grain size ≈ 10 µm) at the

temperatures as high as 1400°C.⁵⁷ Above XRD analysis indicates that in samples fabricated by using the nano-scale powders, the tetragonal phase appears at lower sintering temperatures (about 1100°C) in comparison with the conventional BST system.⁵⁸ The results above illustrate that, with specific control on sintering process, employment of nano-scale raw powders is able to secure the desired microstructure and phase constitution in the samples at lower sintering temperatures. This would be a great merit of thermal budget for device fabrication.

20 30 40 50 60 70 80

Figure 4-25. The XRD patterns for the BST sample sintered at different temperatures.

4.2.3. Dielectric Properties

Since the dielectric properties and density of ceramics are strongly related^19,25, in below only the dielectric properties of high-density (i.e., greater than 95%) nano-BST samples sintered at 1200 and 1300°C are presented.

The 1-kHz dielectric constant and dielectric lose of nano-BST samples sintered at 1200 and 1300°C for different time spans are shown in Figs 4-26 and 4-27, respectively. For 1200°C-sintering, the dielectric constant increases from 8300 to 9700 with the increase of sintering times from 1 to 6 hrs, and then decreases to 6300 after 8 hr-sintering. As to the 1300°C-sintering samples, the dielectric constant monotonously decrease from 9800 to 2200 when sintering time increase from 1 to 6 hrs. The result above indicates that, via appropriate control of sintering conditions, high dielectric constant property could be achieved in nano-BST samples in comparison with nano-BTO (dielectric constant ≈ 8000) (Section 4.1.7) and conventional BST (dielectric constant ≈ 7200) systems¹⁹.

Addition of Sr is known to rise the dielectric constant of BTO,^19,54 however, reduction of grain size should also play an important role on the improvement of dielectric constant of samples based on nano-scale raw powders. In conventional coarse-grained samples, each of grains is divided by several electric domains and the dipole moments corresponding to neighboring domains tend to arrange in an

anti-parallel manner (called 180°-domains).⁴² It has been reported that the domain size of BTO is known to be about 1 µm.⁵⁹ Therefore, in BST samples with grain sizes smaller than 1 µm (e.g., the samples subjected to 1200°C-sintering), each of grains could treated as a single electric domain. Since grain boundary is the structural discontinuity, the coupling between adjacent electric dipoles in the samples with fine grain structure becomes weaker and, hence it is comparatively easy for the electric dipoles to align in parallel manner when external bias is applied. This implies a higher net electric polarization and thus the high dielectric constants in the samples with fine grain structure.⁵⁴

In conjunction with the SEM observations shown in Figs. 4-23 and 4-24, the results above clearly illustrate that in addition to the fine grain sizes, a high-density/low-void content structure is essential to accomplish satisfactory dielectric properties in BTO and related ceramics.^19,25 This iterates the importance on the control of sintering process to achieve desired phase and microstructure in the samples based on nano-ceramic powders.

Figure 4-26. 1-kHz dielectric constant and dielectric loss of nano-BST samples sintered at 1200°C versus sintering time.

Figure 4-27. 1-kHz dielectric constant and dielectric loss of nano-BST samples sintered at 1300°C versus sintering time.