Chapter 4 Results and Discussion
4.1. Dispersion and Preparation of Nano-scale BTO materials
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 Fig. 4-16. Figure 4-16 also shows that when the grain sizes exceeds 200 nm, the dielectric constants decrease and dielectric losses increase with the increase of grain
sizes. Though further grain growth implies denser sintered body, the grain sizes become micro-scale and the samples exhibit the dielectric properties similar to those of conventional BTO samples.19,25 The results above evidence that the adoption of nano-BTO powder for device fabrication requires specific control on the sintering condition so as to obtain desired dielectric properties.
Figure 4-16. 1-kHz dielectric constants and loss versus grain sizes of nano-BTO sintered at 1100°C.
A comparison of Figs. 4-17 and 4-18 shows that the dielectric properties of 1200°C-sintered BTO are inferior to those of 1100°C-sintered BTO thought they exhibit a similar trend of property change with the grain sizes. The decrease of
dielectric constant of 1200°C-sintered BTO was attributed to the presence of monoclinic BaTi2O5 phase as indicated by XRD and EDS analyses. As stated previously, BaTi2O5 is paraelectric19 and its presence reduces the spontaneous polarization and thus deteriorates the overall dielectric performance of the capacitor sample.19
Figure 4-17. 1-kHz dielectric constants and loss versus grain sizes of BTO sintered at 1200°C.
Figure 4-18. 1-kHz dielectric constants and loss versus grain sizes of BTO sintered at 1300°C.
4.2. Preparation of Nano-BST Ceramics and Their Characteristics
Barium Strontium titanate (Ba1-xSrxTiO3, BST) is widely used in many application because of its exception high dielectric constant. The BST have been used in the multilayer ceramic capacitor (MLCC) and positive temperature coefficient resistor (PTCR).56 Many studies have been done on the dielectric properties of BST and the best dielectric properties of BST at about x = 0.3, in this work we hence chose to prepare the BST samples at such a stoichiometric ratio and investigated the dielectric properties and their relationships to microstructure.54
In this work we try to identify the dispersion method of nano-BST powder and the
sintering process of capacitor samples based on such a BST powder so that an in-depth understanding on the microstructure evolution and its effects on dielectric properties of BST could be obtained.
4.2.1. Dispersion and Preparation of Nano-BST Ceramic
Before sintering, the mixture of BTO and STO powders must be dispersed in
order to eliminate the agglomeration caused by van der Waals attraction resulted from large SSA of nano-sized particles. Figure 4-19 presents the morphology of the mixed powders subjected to chemical dispersion and physical grinding described previously.
The mean particle diameter measured from Fig. 4-19 is about 60 nm. Particle size distribution of the dispersed mixed powder is plotted in Fig. 4-20. It shows the average particle size is about 60 nm and, in conjunction with the morphology shown in Fig. 4-19, hence there is no aggregation of BTO/STO powders in slurry.
In Section 4.1 reported that PDAAE is able to provide the electrostatic repulsion and steric hindrance between the particles to inhibit the agglomeration. This study hence adopted the same dispersion formula to disperse the BTO/STO powders. The rheological measurement of aqueous suspension shown in Fig. 4-21 reveals that the viscosity of suspension is rather low (η ≈ 10 mPa⋅s) and its variation along with the shear rate is relatively small. This implies the Newtonian fluid behaviors of BTO/STO
slurry and hence, according to the studies of Cesarano et. al.,4 a good dispersion of BTO/STO powders in the aqueous suspension. The dispersion method for nano-BTO powder is hence equally applied to the dispersion of nano-scale BTO/STO powders.
Figure 4-19. SEM micrograph of nano-scale BST powders subjected to chemical dispersion and physical grinding process.
4.2.2. 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
-1)
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 related19,25, 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.
4.2.4. Characterization of TC
STO is usually added in BTO in order to shift the TC to lower temperatures and it is well known that the TC of BTO decreases linearly with amount of STO in BTO.54 Figure 4-28 shows the variation of dielectric constants of BST samples sintered at 1200 and 1300°C for 1 hr as a function of temperature during a heating cycle and a cooling cycle. The grain sizes of the BST sample sintering at 1200°C and 1300°C for 1 hr are about 200 nm and 220 nm, respectively. It is found that the TC corresponding to the heating and cooling cycle is about 36°C and 33°C for the BST sintered at 1200°C, respectively. As to the sample sintered at 1300°C, the TC’s are about the same at 25°C for the heating and cooling cycles. This indicates a negligible thermal hysteresis of TC’s in the dense, nano-BST samples studied in this work.
Stress/strain-induced phase transitions such as martensite transformation is commonly characterized by the hysteresis of transition temperature. Lack of thermal hysteresis in nano-BST sample hence implies that the minor effect of stress/strain energy term on the transition. Further, the effect of volume free energy term is ruled out since the phase transition of BST samples at TC involves no composition change. According to the theory of phase transitions in solids, interfacial energy term would dominate the transition and thus the presence of grain boundaries in fine-grained BST samples should play a important role in the phase transition of BST at TC.
Previous studies reported that the TC of micro-BTO is about 130°C.19 In addition to the suppression of TC by adding Sr into BTO, the results above also illustrate that the reduction of grain size in the nano-BST achieves lower TC’s in comparison with conventional BST whose TC is about 50°C.54 It is known that the lattice of BST switches from cubic to tetragonal structure when the paraelectric-to-ferroelectric transition occurs at TC. Internal stresses are built up in the samples when crystal structure changes and somehow they must be eliminated so that the phase transition may proceed. However, the stress relaxation via the presence of 90°-domain is no longer applicable to the nano-BST sample with grain size ≤ 1 µm. It is hence speculated that the fine grain structure in nano-BST samples provides numerous grain boundaries for stress relaxation. Besides, grain boundaries are known as the preferential sites for heterogeneous nucleation and fine grain sizes effectively shorten the distance of atom rearrangement during phase transition. From these reasons, it should be easier to initiate the phase transition in nano-BST samples and hence the lower TC’s were observed.
-50 0 50 100 150
Figure 4-28. The dielectric constants of BST sample sintered at 1200 and 1300°C for 1 hr as a function of temperature.
4.3. Preparation and Characterizations of La-doped BST Ceramics
Previous results show that the dielectric constant of nano-BST is higher than that of conventional BTO and BST systems. However, the dielectric loss of nano-BST is comparatively high. In order to improve this disadvantage, we dope the La2O3 in BST samples (called LBST samples hereafter) and investigate its dielectric properties relation to microstructure and phase transformation.
4.3.1. Dispersion and Preparation of LBST Samples
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)
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)