Dispersion and Preparation of LBST

In order to avoid the agglomeration caused by van der Waals attraction resulted

from large SSA of nano-sized particles, the ceramic powders must be dispersed before sintering.¹⁸ Viscosity of 1.0LBST (the number appearing in front of LBST represents the molar ratio of La2O3 added in BST) slurry as a function of PMAA amount is presented in Fig. 4-29. It can be seen that at first the viscosity sharply decreases with the increase amount of chemical dispersant, reaches the lowest value at 3 wt.%, then increases with further increase of chemical dispersant concentration. The lowest viscosity indicates the finest dispersion of powders in slurry. Similar results were observed in other xLBST slurries and hence it is concluded that the optimum content of PMAA chemical dispersant = 3 wt.% for the dispersion the nano-scale LBST powders. By the way, in this study, the LBST materials must be prepared by premix via laboratory mill with an amphibious chemical dispersant (PDAAE). The reason is

that PDAAE molecules have more steric effects and stabilize the BST mixed slurry in a cloudy longer. In addition, a differential chemical dispersant (PMAA) has also been used to disperse and prepare LBST mixed slurries. Compared with PDAAE, PMAA molecules are anionic polyelectrolytes which would cause electrostatic repulsions to enhance dispersion and decrease viscosity of LBST mixed slurries.

Figure 4-30 shows the particle size distribution of various xLBST slurries. The d50’s of aqueous suspensions are 72, 70, 67 and 69 nm for BST, 0.5LBST, 1.0LBST and 1.5LBST slurries, respectively. Furthermore, the sharp, narrow shape of

0 1 2 3 4 5

Figure 4-29. The effect of dispersant concentration on the viscosity of 1.0LBST

suspensions subjected to various grinding/mixing processes.

0.01 0.1 1

Figure 4-30. Particle size distribution of nano-xLBST powder in the slurry subjected

distribution curves implies not only the uniform particle size in xLBST slurries, but also the success of nano-scale powders dispersion via the methodology described above.

4.3.2. Sintering Behaviors and Microstructure Evolution

Figures 4-31 presents the relative densities of BST and xLBST samples sintered at various temperatures for 1 hr. It can be seen that at the same sintering temperature the density of sample decreases with the increase of La content. When sintered at 1400°C, the samples with La2O3 content > 1.0 mol.% (e.g., 1.5 LBST sample) cannot reach the satisfactory densification (i.e., relative densities > 90%).

Figure 4-32 shows the average grain sizes of BST and xLBST samples as a function of sintering temperature deduced from SEM characterization. It can be seen that with the increase of sintering temperature, grain enlargement in the samples becomes obvious; however, the grain size increment is suppressed with the increase of La content. An illustration of above results is given in Figs. 4-33(a)-4-33(d) which depicts the SEM morphologies of BST and xLBST samples sintered at 1400°C for 1 hr. The BST and 0.5LBST samples exhibit highly dense sintered structures with few pores at grain boundaries; nevertheless, their grain sizes exceed micrometer scale. The 1.5LBST sample contains numerous pores in the sintered body and its density remains

1000 1100 1200 1300 1400

Fig. 4-31. Relative densities of BST and xLBST samples sintered at various

temperatures for 1 hr.

Fig. 4-32. Average grain sizes of the BST and the LBST samples as a function of

sintering temperature deduced from SEM observation.

low at 70% although it exhibits extremely fine grain size about 100 nm after the 1400°C-sintering. As to the 1.0LBST sample, it simultaneously accomplishes the satisfactory densification (≈ 95%) and fine grain structure (≈ 200 nm) via 1400°C/1 hr-sintering.

The study on conventional BST sample reported that it requires 1400°C sintering for 1 hr to reach densification and the grain size is about 10 µm.^19,54 As to the BST and BTO samples prepared on the base of nano-scale powders, they respectively possess the average grain sizes about 50 and 150 µm when sintered at 1400°C for 1 hr.

The results presented in Figs. 4-31 and 4-32 clearly indicate that doping of La2O3 in BST may effectively suppress the grain coarsening during sintering. However, the La-doping also raises the densification temperature of nano-BST ceramics as revealed by Fig. 4-33.

Figures 4-34(a) and 4-34(b) present the TEM micrographs of 0.5LBST and 1.0LBST, respectively. The concentrations of Ba, Sr, Ti and La elements in grain center and at grain boundary for 0.5LBST and 1.0LBST samples obtained by EDX analysis are listed in Table 4-4. The TEM images shown in Figs 4-34(a) and 4-34(b) reveal a uniform granular microstructure in both LBST samples and the grain sizes are in consistent with those obtained by SEM observations (e.g., Figs. 4 and 5). As to the element concentrations, the data listed in Table 4-4 indicate that the La element

Figure 4-33. SEM micrographs of BST samples doped with: (a) BST (b) 0.5LBST (c).

1.0LBST (d) 1.5LBST sample subjected to 1400°C/1-hr sintering.

tends to segregate at the grain boundaries. It is proposed that the donor dopants segregated at grain boundaries reduce the mobility of boundaries and effectively inhibit grain growth during sintering process.^59,60 The TEM/EDX analysis presented above clearly evidence (from Table 4-4) the segregation of La element at grain boundaries. It hence restrains the grain growth during sintering and thus the LBST samples exhibit finer grain sizes in comparison with BST sample sintered at the same

sintering conditions.

Figure 4-34. TEM micrographs of (a) 1.0LBST (b) 0.5LBST subjected to 1400°C/1-hr sintering.

Table 4-4. Element distribution in xLBST sample.

Sample Element In grain center (wt.%) At grain boundary (wt.%)

Ba 45.2 44.8

Sr 12.3 12.6

La 0.52 0.66

0.5LBST

Ti 21.3 21.9

Ba 43.9 44.3

Sr 12.1 12.4

La 0.92 1.37

1.0LBST

Ti 22.6 21.4

Figure 4-35 presents the XRD patterns of the raw La2O3/BTO/STO powder

that the raw powders of BTO, STO and La2O3 are all of cubic phases. After the 1400°C/1-hr sintering, the xLBST samples all become tetragonal BST phase. The missing of La2O3 peak indicates that La2O3 are completely soluble in BST, which is in consistent with the TEM/EDX analysis presented above as well as the results reported by previous study⁶¹. The ionic radius of La⁺³ (0.115 nm) is smaller than that of Ba⁺² (0.135 nm) and about the same as that of Sr⁺² (0.113 nm), hence the relatively small amount of La2O3 (< 3 wt.%) may completely dissolve in BTO at high sintering temperatures (> 1300°C).⁶¹

The XRD analysis for 1.0LBST samples sintered at various temperatures was also carried out so as to explore the evolution of phase constitution during the sintering of nano-scale ceramic powders. The XRD patterns presented in Fig. 4-36 indicates that sintering at temperatures greater than 1200°C is required to yield the tetragonal BST solid solution phase. It has been reported that sintering temperatures as high as 1500 and 1400°C are respectively required to form tetragonal phase in conventional BTO samples doped with La2O3.⁶¹ Hence the reduction of raw powder sizes to nanometer scale indeed provides a viable way to secure desired phase structure at lower sintering temperatures.

20 30 40 50 60 70 80

Figure 4-35. XRD patterns for raw powders and xLBST samples sintered at 1400°C for 1 hr.

Fig. 4-36. XRD patterns for 1.0LBST sample sintered at different temperatures.

TEM image and diffraction pattern of 1.0LBST sample sintered at 1400°C for 1 hr are presented in Figs. 4-37(a) and (b). The analysis of selected area electron diffraction (SAED) pattern shows that such a sample possesses the same tetragonal structure as in BST as revealed by XRD. Besides, SAED patterns does not show the existence of second phases, which is in good coincidence with the result of XRD analysis.

Figure 4-37. (a) TEM image and (b) SAED pattern of 1.0LBST sample sintered at 1400°C for 1 hr

4.3.3. Dielectric Properties

In below, only the dielectric properties of 1.0LBST sample are presented since it is the sample that may achieve the satisfied densification with fine grain size. The

100 nm

to 1400°C are presented in Fig. 4-38. It can be seen that with the increase of sintering temperature, the dielectric constant increases from 1730 to 13800 while the dielectric loss decreases from 0.32 to 2.8×10⁻⁴. As shown in Figs. 4-31 and 4-32, relative density and grain size increase with sintering temperature and, therefore, to achieve high densification density without substantial grain enlargement is crucial to preserve the dielectric properties of capacitor samples prepared based on nano-ceramic powders.

Since the ionic radius of La⁺³ is comparatively similar to those of Ba⁺² and Sr⁺² and larger than that of Ti⁺⁴ (0.068 nm), the La⁺² would thus occupy Ba⁺²/Sr⁺² sites in BST. The associated defect reaction can be expressed as:

O

Above reaction depicts the formation of Ba⁺²/Sr⁺² site vacancies, V ′′ , induced _Ba/Sr

by the doping of La2O3. Previous TEM/EDX analysis revealed the segregation of La at grain boundaries, it is hence speculated that more V ′′ would residue in the _Ba/Sr vicinity of grain boundaries which, in turn, benefits the mobility and induced dipoles and thus amplify the dielectric constant property. Meanwhile, the distortion of BST lattice caused by the difference of ionic radii and doping of aliovalent impurity (i.e.,

La⁺³) somehow may contribute to the increase of polarization. Previous studies further demonstrated that the capacitor samples with nano-scale granular structures possess higher dielectric constants in comparison with samples with micro-scale granular structures due to the existence of numerous interfaces that effectively promote the mobility and induced dipoles when external bias is applied.^18,19 The reasons cited above explain the high dielectric constant property observed in 1.0LBST capacitor sample.

Dielectric property measurement also revealed that dielectric loss of 1.0LBST (loss = 2.8×10⁻⁴) is lower than that of nano-BST sample (loss = 10⁻²). In addition, the resistance of 1.0LBST is 4×10⁶ Ω which is lower than nano BST sample (3×10⁸ Ω).

This is good sufficient evidence to prove that La2O3 dopant can induce dipole; and further, decrease resistance of BST sample. Besides, the dielectric loss is a lose energy and come from dipole motion. So, the lose energy must be small when dipole move easily. From this point of view, we can understand that dielectric loss of 1.0LBST is lower than nano-BST sample.

Figure 4-38. 1-kHz dielectric constant and dielectric loss of 1.0 LBST samples sintered at versus sintering temperature.

4.3.4. Characterization of T_C

Figure 4-39 plots the 1 kHz dielectric constant versus temperature for xLBST samples. The TC decreases from 50°C for the sample free of La2O3 to 25°C for the BST samples doped with 1.0 mol.% La2O3. The result illustrates that the 1.0LBST exhibits lower TC’s in comparison with conventional BST whose TC is about 50°C.⁵⁴ It is well known that TC is a critical temperature which the lattice of BST switches from cubic to tetragonal structure when the paraelectric-to-ferroelectric transition. In this study, the nano 1.0LBST sample has many grain boundaries and the grain boundaries

effectively shorten the distance of atom rearrangement during phase transition. In addition, grain boundary can relax inter stress and shift TC to lower temperature. For reasons mentioned above, nano LBST sample has a lower TC than micro BST sample.

Furthermore, TC of nano 1.0LBST and nano BST in our previous study were similar (section 4-2-4). Nano LBST and BST have lower TC than that of micro BST, conclusively nano grain does reduce TC in BST sample.

-50 0 50 100 150

Figure 4-39. The dielectric constants of LBST sample sintered at 1400°C for 1 hr as a function of temperature.