Chapter 3 Experiment Methods
3.4. Dispersion of CNTs
3.4.2. Experimental Flow of LIB Experiment
The experimental flow for this part of study is shown in Fig. 3-6. The LiCoO2
electrode contained 90 wt.% LiCoO2 (LICO, Taiwan), 3 wt.% polyvinylidene fluoride (PVDF, Kuraha Chemical, Japan) binder and 7 wt.% conducting additives. The conducting additives were synthetic graphite (Timrex KS6, Timcal, Switzerland) and CNTs dispersed by various amounts of PEI. The mixture containing above ingredients were mixed by ball milling for 4 hrs and then coated onto 20-µm thick Al foil, dried at 140°C and pressed to form the LiCoO2 electrode with final thickness of 120 µm. The coin cell assembly of LIB test cell is showed in Fig. 3-7.
.
Figure 3-6. Experimental flow for LIB experiment.
Resistivity measurement Electric measurement
Viscosity
Figure 3-7. The coin cell assembly of LIB test cell.
3.4.3. Optical Microscopy
The CNTs slurry was first prepared with dispersant to confirm that the CNTs was dispersed. The dispersive effect was determined to Optical Microscope (OM, OLYMPUS-Mx40, Japan).
3.4.4. Viscosity Measurement
The rheology of CNTs slurry without or with chemical dispersants subjected to ball mixing was evaluated by a viscometer (ARES-LS1, Germany).
3.4.5. Adsorption of Dispersant
The CNTs suspensions with different concentrations of PEI were prepared. These suspensions were deagglomerated by a 3D ball mixing. After ball mixing, the suspensions were centrifuged at a speed of 1.5 ×104 rpm for 2 hrs to obtain supernatants. The residual dispersant concentration in the supernatants was analyzed and determined by a UV-Visible (V-600, JASCO/Japan). The amount of dispersant adsorbed on CNTs was calculated from the difference in dispersant concentration before and after adsorption.
3.4.6. Microstructure Observation
The morphology and CNTs dispersive effect in LiCoO2 electrode was obtained by scanning electron microscopy (SEM, Hitachi 4700).
3.4.7. Cyclic Voltammograms
The test cells without and with 2 wt.% PEI in LiCoO2 electrode were evaluated
by a cyclic voltammograms (Biologic, Mac Pile II, USA) operating in between 2.9 and 4.2 V. The purpose of cyclic voltammogram test is to explore the oxidation reduction potential of the test cell and the electrochemistry change in the test cells.
3.4.8. Electrical Characterizations
Electrochemical impedance spectroscopy (EIS, Schlumberger, SI 1286 and SI
1255, England) operating at 3.5 V was adopted to characterize the impedance profile of half cell containing modified LiCoO2 electrode. The measurement was carried out at the scanning frequencies ranged from 0.01 Hz to 50 kHz and with the perturbation amplitude of 10 mV. The cycling performance and charge-discharge capability of LIB test cells were carried out with the aid of charge-discharge apparatus (Arbin, model BT2042, USA) which enables a control of 5 cells synchronously at most. A charge-discharge test consisted of five cycles. The procedure was composed of a constant-current of 2 mA followed by constant-voltage at 4.2 V until the current tapered down to 0.2 mA. Both test cells were first charged at 0.2-C current density.
CHAPTER 4
Results and Discussion
4.1. Dispersion and Preparation of Nano-scale BTO materials 4.1.1. Rheological Behaviors
Viscosity measurement is commonly adopted to evaluate the dispersed extent of concentrated suspensions. Figure 4-1 shows the effect of dispersant concentration on the viscosity of 60 wt.% BTO suspensions subjected to conventional ball milling or nano grinding/mixing via MiniZeta mill. The viscosity of suspensions without dispersion treatment is about 3200 cps. For the suspensions subjected to different mechanical milling processes, their viscosities decrease when PMAA-Na or PDAAE is incorporated. The viscosity of suspensions first decreases with the increase of dispersant concentration then reaches a plateau. At same of dispersants concentration, the viscosity of suspensions subjected to nano grinding/milling exhibits a lower value in comparison with that of suspensions subjected to conventional ball milling. For the suspensions subjected to nano grinding/mixing, the optimum chemical dispersant concentrations in BTO suspensions are about 2 wt.%
for PMAA-Na and 3 wt.% for PDAAE, which respectively result in the lowest viscosities of 15 and 76 cps.
sharpest descend of viscosity and the amount of PMAA-Na to achieve the lowest value of viscosity is less in comparison with that of PDAAE. As the PMAA-Na and PDAAE both have similar molecular weight, the difference in the rheological behaviors is attributed to the difference of chemical structures. The PMAA-Na is anionic polyelectrolyte, which causes mainly electrostatic repulsion when adsorbed on BTO particles. Though PDAAE also causes electrostatic repulsion when adsorbed on BTO surface, the effect is relatively less because PDAAE is amphibious polyelectrolyte.
Figure 4-1. The effect of dispersant concentration on the viscosity of 60 wt.% BTO suspensions subjected to various grinding/mixing processes.
4.1.2. Particle Size Distribution
In conventional ball milling, slurry and grinding media are sealed in a bottle and
the grinding/mixing is achieved by the rotation motion of grinding balls. Such a milling process provides a relatively low shear force that it is insufficient to achieve nano-scale dispersion. In this work, the MiniZeta mill using the 2-mm-diameter ZrO2
balls as the grinding media was able to grind the BTO powder down to the sizes smaller then 100 nm. By using the principle of agitator bead mills, a special agitator shaft rotating at a speed as high as 3600 rpm accelerated the grinding media in the MiniZeta mill chamber. It not only provided superfine grinding, but also generated a rather large shear force to disperse the slurry. In addition, the multi-passage grinding/mixing path adopted by MiniZeta mill is able to reduce the sizes of BTO powders in a relatively short time span.
Figure 4-2 presents the mean particle size (d50) of powder as a function of dispersant concentration in 30 wt.% BTO suspensions subjected to nano or conventional milling processes. The initial value of d50 of powder without chemical dispersants in suspensions is 1 µm. With the increasing amount of chemical dispersant, the values of d50 of powders in all types of suspensions decrease. As shown in Figure 4-2, the variation of d50 with the dispersant concentration is very much the same as that of viscosity presented in Fig. 4-2. The smallest d50 of BTO suspensions occurs at
about 2 wt.% for PMAA-Na and 3 wt.% for PDAAE; it equals to 80 nm and 83 nm, respectively. Apparently, more PDAAE are required to achieve the smallest d50 in suspensions and the minimum particle size in suspension containing PDAAE is slightly larger than that in suspension containing PMAA-Na.
0 1 2 3 4 5 6
Figure 4-2. The effect of dispersant concentration on the mean particle size (d50) of powder in 30 wt.% BTO suspensions subjected to different grinding/mixing processes.
Figures 4-3 and 4-4 respectively show the particle size distributions of 30 wt.%
BTO slurries containing 2 wt.% PMAA-Na and 3 wt.% PDAAE subjected to conventional and nano milling processes. The d50’s of suspensions subjected to the
nano grinding/mixing are 80 nm for PMAA-Na and 83 nm for PDAAE, while that of suspensions subjected to conventional ball milling are 180 nm for PMAA-Na and 181 nm for PDAAE. Furthermore, narrower shape of distribution curve implies a more uniform size of BTO particles in the suspensions subjected to nano grinding/mixing process. It was also found that average size of BTO in suspension without any mechanical milling is in the range of 200 nm. This indicated that the high shear force provided by an appropriate mechanical milling process is also beneficial to the stability of suspensions. Owing to the larger SSA and van der Waals force, the nano-scale particles in slurry tend to agglomerate together. In addition to the chemical dispersant, a mechanical milling process that is able to provide sufficient shear force is required in order to achieve satisfactory dispersion of nano-scale BTO particles in aqueous suspension.
Experimental results of viscosity and particle size distribution also indicated that a sole mechanical milling process is insufficient to disperse nano-sized BTO particles in suspensions. Though nano grinding/mixing via the MiniZeta mill provides a large shear force to disperse the particles, it cannot stabilize the suspensions. Owing to the large SSA and van der Waals force, the particles subjected to nano grinding/mixing still tend to agglomerate together if the suspensions were free of chemical dispersant.
Hence, in addition to mechanical milling, appropriate amount of chemical dispersant
has to be added to provide sufficient electrostatic repulsions and steric effect in suspensions so that a stable, uniform dispersion in nanometer scale could be obtained.
0.0 0.4 0.8 1.2
Figure 4-3. Particle size distribution of BTO powder containing 2 wt.% PMAA-Na subjected to conventional ball milling and nano grinding/mixing processes.
4.1.3. Zeta Potentials
Electrokinetic measurements are widely used to assess the magnitude and sign of
particle surface charge as a function of pH value in suspension.21,55 Figure 4-5 presents the zeta potentials of 30 wt.% BTO powder as function of pH values of suspension. Isoelectric points for 80-nm-BTO was at pH = 3.8; for 180-nm-BTOwas at pH = 4.5; and for 1-µm-BTO was at pH = 5.2. This indicates the BTO particles tend
to stabilize in aqueous solutions of high pH values.
Figure 4-4. Particle size distribution of BTO powder containing 3 wt.% PDAAE subjected to conventional ball milling and nano grinding/mixing processes.
The hydrolysis of BTO in acidic environment is as follows18:
BaTiO3(s) + H2O(l) = Ba2+(aq) + TiO2(rutile)+ 2OH−(aq) (4-1) BaTiO3(s) + 2H+(aq) = Ba2+(aq) + TiO2(rutile)+ H2O(l) (4-2)
Nano-scale BTO particles possess large SSA. When HNO3(aq) was added into the
surface of BTO. The Ba2+ ions could nullify some negative charges on the surface of the BTO, so the zeta potential shows a more positive value as revealed by Eq.(4-2).
On the other hand, when NH4OH was added into the suspension, the pH increased and the resulted OH− ions could neutralize the positive charges on the surface of BTO so that the negative trend of the zeta potential increased.
2 4 6 8 10 12
Figure 4-5. Zeta potentials of BTO suspensions at different particle sizes.
Figure 4-6 shows that the zeta potentials for BTO suspensions containing PMAA-Na or PDAAE at pH = 9.5. It can be seen that at the same particle size, the zeta potential for the suspension containing PMAA-Na is more negative than that
negative charged so that the adsorption of dispersant on particle surface is more difficult in comparison with the micro-scale particles. This suggests that the rejection caused by the net negative electric charge of the particles and, for PMAA molecules, rejection becomes more severe when the particle size is down to nanometer scale.
However, the PDAAE molecules become amphibious in a basic solution (i.e., at the condition of pH = 9.5). The molecules contain cationic functional groups (−N+R3) would favor themselves to be adsorbed on the surface of BTO particles.
0 200 400 600 800 1000
Figure 4-6. Zeta potentials of BTO suspensions containing different chemical dispersants.
4.1.4 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.
4.1.6. Phase Constitution
Figure 4-15 presents the XRD patterns for the samples sintered at temperatures ranging from 1000 to 1300°C for 6 hrs. At the temperatures below 1000°C, tetragonal BTO phase remains insignificant. However, the tetragonal BTO becomes the dominant phase at 1100°C and, when the sintering temperature is raised to 1200°C, most of tetragonal phase evolves into the monoclinic phase. After the 1300°C-sintering, the BTO is overwhelmed by the cubic phase. The XRD pattern for the sample sintered at 1200°C indicates that diffraction peaks corresponding to tetragonal phase are relatively weak in comparison with those corresponding to monoclinic phase. Table 4-3 shows the Ba:Ti ratio obtained by using the EDS attached to SEM. The higher Ba:Ti ratio ≈ 1:1.76 indicates that the stoichiometry of monoclinic phase is likely BaTi2O5. It was hence concluded that the phase constitution for the sample sintered at 1200°C is monoclinic BaTi2O5 mixed with a relatively small amount of tetragonal BTO.
Previous studies reported that the BaTi2O5 may form in the surface reaction layer and also in the bulk of BaTiO3 pellets when the BaO-TiO2 samples were sintered in oxygen-rich ambient at high temperatures.19 It was also found that the BaTi2O5 is of paraelectric19 and, when monoclinic BaTi2O5 is present in BTO, it would shift the capacitor sample from ferroelectric to paraelectric and hence decrease its dielectric
constant.19
Figure 4-15. The XRD patterns for the BTO sintered at different temperatures.
Table 4-3. The EDS analysis of Ba:Ti ratio of samples sintered at different
temperatures for 6 hrs.
Sintering temperatures Ba:Ti ratio
1100°C 1:1.12
1200°C 1:1.76
1300°C 1:1.14
4.1.7. Dielectric Properties
Figures 4-16, 4-17 and 4-18 presents the 1-kHz dielectric constants and dielectric
losses versus the grain sizes of BTO sintered at 1100, 1200 and 1300°C, respectively.
It can be seen that for the samples sintered at 1100 and 1200°C, the dielectric constant first increases with the increase of grain sizes then decreases with the further increase of grain sizes and vice versa for the dielectric loss. The highest dielectric constant about 8000 and lowest dielectric loss about 5×10−3 were achieved in the sample sintered at 1100°C for 6 hrs with grain size about 140 nm. As to the samples sintered at 1300°C, the dielectric constant decreases and dielectric loss increases monotonically with the increase of grain sizes, which is in the same trend as reported by previous studies relating to micro-scale BTO.25
The sintering temperatures for conventional BTO process usually exceed 1300°C.
The results above clearly indicate that, in addition to the upgrading of dielectric properties, the sintering temperature for nano-BTO is 200°C lower than that for conventional process. The reduction of sintering would be a merit of thermal budget for the BTO device fabrication.
During the sintering of nano-BTO, the voids might be rapidly eliminated at the early stage of sintering due to the mass transport processes driven by the large SSA of nano-particles. A relatively dense sintered body thus might achieve prior to the apparent coarsening of grains, resulting in distinct dielectric properties as shown in
During the sintering of nano-BTO, the voids might be rapidly eliminated at the early stage of sintering due to the mass transport processes driven by the large SSA of nano-particles. A relatively dense sintered body thus might achieve prior to the apparent coarsening of grains, resulting in distinct dielectric properties as shown in