Adsorption of Dispersants on BTO

Figures 4-7 and 4-8 show the specific adsorptions of PMAA-Na and PDAAE on the BTO particles in suspensions at pH = 9.5, respectively. For both types of suspensions, the specific adsorption behaviors suggest a monolayer adsorption concentration onto the particle surface; however, the critical dispersant concentrations to reach the adsorption plateau are different. For PMAA-Na, the critical value decreases with the decrease of particle size and vice versa for PDAAE. The critical dispersant concentrations to reach the adsorption plateau for these two types of suspensions are listed in Table 4-1.

Table 4-1 clear shows that the critical value and amount of absorbent are greater for PDAAE. The differences between these values are attributed to the difference of chemical structures of these two dispersants. At pH = 9.5, the surface charge of BTO particles are negative. For PMAA-Na, its molecules are completely dissociated and become negatively charged. Therefore, it is difficult for PMAA-Na molecules to be adsorbed on the BTO surface. On the contrary, PDAAE molecules become amphibious in a basic solution and the existence of cationic functional groups (−N⁺R3) favors the absorption of PDAAE on BTO surface. Accordingly, the amount of adsorbed PDAAE molecules is more than that of PMAA-Na molecules.

Furthermore, an interesting behavior of the adsorption isotherms was observed in

this work. For the BTO particles of different sizes, the initial adsorption behaviors were distinctly different when different dispersant was added. This clearly suggested that nano-scale particles possess highly negative charge on surface that inhibits the adsorption of negatively charged dispersant or increases the adsorption of amphibious charged dispersant. Another possible explanation was that the dispersants might adsorb on the large-sized particles in an orientation that permits higher extent of adsorption on particle surface.¹²

Table 4-1. Critical dispersant concentrations to reach the adsorption plateau.

80 nm 180 nm 1 µm

0 1 2 3 4 5 6

Figure 4-8. Specific adsorption of PDAAE on BTO particles of different particle sizes.

To determine monolayer PMAA-Na and PDAAEE adsorption quantitatively, the data in shown in Figs. 4-7 and 4-8 were analyzed by using the Langmuir monolayer adsorption equation:

where Ce is the equilibrium concentration of dispersants in solution, As is the adsorbent of the dispersant, and Cm is the monolayer adsorbent of the dispersant, and k is a constant. The plots of Ce/As versus Ce for these two types of suspensions are

shown in Figs. 4-9 and 4-10, respectively. The slopes of these straight lines represent the reciprocal of the monolayer adsorption of dispersant (1/Cm) and the results are summarized in Table 4-2. It can be seen that the amount of monolayer PMAA-Na adsorption decreases with the decrease of particle size and vice versa for the case of PDAAE. However, the results of adsorption and zeta potential indicated that both PDAAE and PMAA-Na could successfully stabilize the nano BTO slurry. The dispersive capability of PDAAE is steric effect and electrostatic repulsion, but the PMAA-Na is solely electrostatic repulsion.

0 1 2 3 4

Figure 4-9. Langmuir analysis for the suspension containing PMAA-Na.

0 1 2 3 4

Figure 4-10. Langmuir analysis for the suspension containing PDAAE.

Table 4-2. Monolayer absorption of dispersants on BTO surface.

80 nm 180 nm 1 µm

PMAA-Na 0.04 mg 0.12 mg 0.32 mg

PDAAE 3.2 mg 1.2 mg 0.4 mg

(Unit: per gram of BTO)

4.1.5. Microstructures

Figure 4-11 shows the morphology of the BTO particles subjected to the

chemical dispersion and physical grinding/mixing process. The average size of BTO particles was about 80 nm and no aggregation was observed. This evidenced that the PMMA-Na salt is an appropriate chemical dispersant for the preparation of aqueous nano-BTO solution.

Dispersant Particle size

Figure 4-11. SEM Micrograph of nano-sized BTO powders subjected to chemical dispersion and physical grinding/mixing process.

The microstructures of BTO sintered in the temperatures ranging from 1100 to 1300°C for 6 hrs are depicted in Figs. 4-12(a) to 12(c). The grain sizes of BTO evolve as follows: at 1100°C, 140 nm; at 1200°C, 900 nm; and at 1300°C, 70 µm. The SEM morphology observation in conjunction with the results of density measurement shown in Fig. 4-12 indicates that both the densities of sintered body and grain sizes of BTO increase with the sintering temperatures. Further, the results presented in Figs.

4-12 and 4-13 revealed three distinct densification processes for nano-BTO particles at different sintering conditions. When sintering temperatures were below 1100°C, densification and grain growth are both insignificant that, in essential, no sintering occurred. For the samples sintered in between 1100 to 1200°C for less than 6 hrs, a

rapid densification takes place with a relatively slow grain growth. When sintering times exceeded for 5 hrs at 1300°C, grain growth dominated and a slight desintering phenomenon occurred when sintering times were further increased. As suggested by previous study,⁶ the desintering phenomenon might be caused by gas releasing and/or abnormal grain growth during sintering.

Figure 4-14 plots the relative density and grain size versus sintering time of nano-BTO sintered at 1100°C. After 6-hr sintering, the sample exhibits a relatively high densification (about 95% T.D.) with a moderate grain growth that the average grain size of BTO sample increases from about 80 to 140 nm. This indicates that for the sintering of nano-BTO powders, grain growth occurs earlier in comparison with conventional process that the apparent grain growth usually occurs at the end of sintering.⁴⁶ Occurrence of grain growth at the early stage of sintering was attributed to large SSA of nano-BTO powders.The nano-particles tended to reduce the surface energy of sample by eliminating the total surface area. Interface-driven processes such as grain boundary diffusion and surface diffusion dominated the mass transport at the early stage of sintering and hence a premature grain growth was observed.

Figure 4-12. SEM Micrographs of nano-BTO sintered at (a) 1100°C (b) 1200°C (c) 1300°C for 6 hrs.

0 2 4 6 8 10 60

70 80 90 100

R elative densi ty (% )

Time (hrs)

1300℃

1200℃

1100℃

1000℃

Figure 4-13. Relative density change versus sintering times at different temperatures.

Figure 4-14. The relative density and grain size versus sintering time of sample

sintered at 1100°C.