The grain growth mechanisms for 0.8(Bi,Na)TiO₃-0.2(Sr,Ti)O₃ ceramics prepared using a two-step sintering process

ABSTRACT

In this study, lead-free 0.8(Bi,Na)TiO₃-0.2(Sr,Ti)O₃ piezoelectric ceramics were prepared using a two-step sintering process to analyze their sintering mechanisms. Two-step sintering process has benefits of being able to be conducted at lower temperatures than conventional sintering process, but the complicated sintering mechanisms involved in this process have not been yet fully investigated. Therefore, in the present study, two-step sintering mechanism for lead-free piezoelectric ceramics was analyzed by estimating the activation energy required depending on the sintering conditions. Using the two-step sintering process, the piezoelectric charge and electromechanical coupling coefficients improved from 146 pC/N and 0.347 to 163 pC/N and 0.377, respectively. By comparing the grain-growth mechanisms for the conventional and two-step sintering processes, it appeared that the sintering mechanisms differed. By introducing the two-step sintering process, piezoelectric ceramics with improved piezoelectric charge and electromechanical coupling coefficients were produced.

KEYWORDS:

• Piezoelectric properties
• piezoelectric materials
• sintering mechanism

1. Introduction

Lead-based piezoelectric ceramics have advantages of higher piezoelectric properties compared with those of lead free piezoelectric ceramics. The sintering temperature of PZT or PMN-PT ceramics is around 1200°C with excellent piezoelectric properties which is a relatively lower sintering temperature compared with that of lead-free BZT-BCT ceramics which is around 1500°C Also, lead-free piezoelectric ceramics have advantages of environmental aspects unlike lead-based ceramics [1,2]. However, they have lower piezoelectric properties as well as higher sintering temperatures [3]. High sintering temperatures induce volatilization problems and complicate processing. Therefore, to lower the sintering temperature, many attempts have been made including two-step sintering process. Generally, ceramics fabricated by two-step sintering process have a lower temperature than conventional sintering process. Still, due to the long sintering time in the second step sintering process, grains can grow with uniform grain size, and pores are reduced by large margin, resulting in dense ceramics. The two-step sintering process is divided into two stages: coarsening and densification [4]. The two-step sintering process produces ceramics with a uniform grain size and high density compared with the existing process in which coarsening and densification co-occur. Another advantage of the two-step sintering process is that elemental losses can be avoided because sintering is performed at a relatively low temperature [5,6]. For example, alkali niobate-based ceramics prepared by two-step sintering have been reported to have a piezoelectric charge coefficient of 510 pC/N, resulting in improved piezoelectric properties compared with ceramics made with conventional processes (~250 pC/N) [7,8]. In addition, it is reported that barium titanate produced by the two-step sintering process showed a higher piezoelectric charge coefficient of 460 pC/N than the ceramic produced by the conventional sintering process (200 pC/N) [9,10].

Bi_0.5Na_0.5TiO₃ (BNT)-based ceramics is another lead-free piezoelectric material that has a relatively moderate d₃₃ (~180 pC/N), a high T_c (~300°C), and a large residual polarization of 38 pC/cm² [11–13]. However, its high conductivity makes the polarization process difficult. Doping has been suggested as a solution to increase the resistivity of piezoelectric ceramics. In particular, SrTiO₃ was added to BNT ceramics in this study to increase their resistivity since SrTiO₃ is an incipient paraelectric material with high resistivity. Volatile Na and Bi components can easily evaporate during the sintering process, leaving vacancies in the BNT ceramic structure. These vacancies induce leakage current [14,15]. To increase densities and resistivity in the piezoelectric ceramics, high pressure was applied to the specimens.

In general, hot pressing, quenching, or two-step sintering processes are preferably used to prepare dense ceramics for such volatile materials [7,16,17]. Among these processes, two-step sintering is known to prevent element loss since it involves a long-term second sintering process that requires a lower temperature than conventional sintering processes [6,18]. In addition, two-step sintering can improve the piezoelectric properties by improving the microstructure, grain size, and homogeneity of the specimen. Nevertheless, studies on the mechanism and required energy for the two-step sintering process of BNT-ST ceramics have not been yet performed by employing numerical model between sintering temperature and grain size. In this study, a two-step sintering mechanism of lead-free piezoelectric ceramics of BNT-ST was investigated. Previously, studies on the two-step sintering of BNT-ST ceramics have been reported [19]. The sintering energy was calculated from the BNT20ST composition, and the sintering mechanism was analyzed in depth through the grain growth kinetic exponent for the first time. Thus, the sintering mechanism for two-step sintering process was studied to analyze the grain-growth process by considering the sintering temperature and holding time in order to predict and control the piezoelectric properties.

2. Experimental procedure

In this study, 0.8(Ba_0.5Na_0.5)TiO₃–0.2SrTiO₃ (BNT–ST) powder was prepared through a traditional oxide solid-phase reaction using Bi₂O₃ (purity 99.9%, Sigma-Aldrich Co., Ltd., Germany), Na₂CO₃ (purity 99.0%, Sigma-Aldrich Co., Ltd., Germany), TiO₂ (purity 99.9%, Kojundo Chemical Lab. Co., Ltd., Japan), and SrCO₃ (purity 99.9%, Sigma-Aldrich Co., Ltd., Italy). First, the stoichiometrically weighed powder was ball milled with ethanol and yttria-stabilized zirconia ball for 24 h. Thereafter, the obtained slurry was dried in an oven and calcined at 850°C for 2 h. After the calcination process, the dried slurry was filtered through a 100-μm mesh to obtain a powder. For ease of molding, PVA (Polyvinyl Alcohol) was used as a binder in an amount of 5 wt%. Uniaxial pressure molding was performed at about 200 MPa to produce the specimens as disks with a diameter of 12 mm. Uniaxial pressure molding was performed at about 200 MPa to produce the specimens as disks with a diameter of 12 mm.

To prepare the sample using the conventional sintering process, the temperature was raised to 5°C/min and maintained at 600°C for 1 h to remove the binder. Thereafter, the sample was sintered at elevated temperatures of 1125°C, 1150°C, 1175°C, 1200°C, and 1225°C for 2 h.

The two-step sintering process involved a pre-sintering step and a secondary sintering step at an adjusted temperature. The pre-sintering temperature for the two-step sintering process was selected by considering the sintering time and temperature of the conventional sintering process. The secondary-step sintering temperature was set to be 100°C lower than the pre-sintering temperature (cooldown rate, 50°C/min). The secondary-step sintering time for the two-step sintering process was adjusted to 0, 6, 12, 18, 24, 30, and 36 h. The prepared ceramics were coated with silver paste on both surfaces and fired at 700°C for 10 min. The polling process was performed in a silicone oil bath at room temperature for 30 min using a DC electric field of 4 kV/mm.

Field-emission scanning electron microscopy (FE-SEM; Carl Zeiss, SIGMA 300) was performed to study the grain size. Pt ion was coated on the unpolished sample after washing ethanol to analyze FE-SEM. We measured about 120–140 grains in the conventional sintering process sample and about 230–260 grains in the two-step sintering process sample. The grain size was measured by the Feret diameter method [20]. Average grain size was calculated as the geometric mean of the short and long edges for rectangular-shaped grains. If it was difficult to determine the grain’s longest and shortest edge, the grain size was measured at an angle of about every 30° and averaged. The density of the BNT–ST ceramics was measured using the Archimedes method. The value of d₃₃ was measured using a Berlincourt-type d₃₃ meter (YE2730A d₃₃ meter). The electromechanical coupling coefficient (k_p) was calculated and estimated using the following formula [21–23]:

(1) $k_p=2.51\frac{\frac{f_a-f_c}{f_a}}{-}{(\frac{f_a-f_r}{f_a})}^2,$(1)

where f_a and f_r are the antiresonant and resonant frequencies measured with an impedance analyzer (Agilent 4294A), respectively.

3. Results and discussion

To determine the optimum two-step sintering temperature, the BNT–ST ceramics were sintered at 1125°C, 1150°C, 1175°C, 1200°C, and 1225°C using the conventional sintering process. Figure 1 shows the BNT-ST ceramic fabricated by the conventional sintering process. The grain size increased in the sintering temperature increased from 1125°C to 1175°C. However, the grain size became non-uniform when the sintering temperature was between 1200°C and 1225°C. Numerous pores were observed as the temperature approached 1225°C. In general, it is a reported that the electrical properties were improved in a ceramic having a dense microstructure [24]. As a result, it can be predicted that the electrical properties can be improved near the sintering temperature of 1175°C.

Figure 1. FE-SEM micrographs and distribution of the BNT–ST ceramics after the conventional sintering at sintering temperatures of (a) 1125°C, (b) 1150°C, (c) 1175°C, (d) 1200°C, and (e) 1225°C and (f) FE-SEM micrographs of the BNT–ST powder before the sintering process.

Figure 2 shows the relative density and average grain size of the BNT–ST ceramics sintered at 1125°C–1225°C. The relative density and average grain size of the BNT–ST ceramics increased as the sintering temperature increased from 1125°C to 1175°C. It can be observed that the relative density slightly increased at 1200°C and decreased at 1225°C. As shown in Figure 1(e), the density decreased as the pores increased. The grain size was estimated based on the sintering temperature and sintering time using the following formula [25–27]:

(2) $\text{ln}(G^n-G_0^n)=lnt+ln{k}_0-\frac{\frac{Q}{RT}}{,}$(2)

Figure 2. Relative density and average grain size of the BNT–ST ceramics as a function of the sintering temperature after the conventional sintering process.

where G and G₀ are the average grain sizes after and before the sintering process, n is the grain growth kinetic exponent, t is the sintering time, k₀ is the proportional constant based on the sintering mechanism, Q is the activation energy of the grain growth, R is the gas constant, and T is the sintering temperature. As described later, G_0c is G₀ in the conventional sintering process, and G_0t is G₀ in the two-step sintering process. If G is considerably larger than G₀, then G₀ can be omitted as Equation (3)(3) $\text{ln}{G}^n=lnt+ln{k}_0-\frac{\frac{Q}{RT}}{.}$(3) :

(3) $\text{ln}{G}^n=lnt+ln{k}_0-\frac{\frac{Q}{RT}}{.}$(3)

As shown in Figure 1(f), since the grain size of ceramics sintered through the conventional sintering process is much larger than G_0c, the activation energy can be calculated using Equation (3)(3) $\text{ln}{G}^n=lnt+ln{k}_0-\frac{\frac{Q}{RT}}{.}$(3) . The only factors that affect the slope of the sintering temperature and average grain size are Q and n. Therefore, the Q of the sintering process can be calculated from the average grain size of the log scale in Figure 2 and the slope (Q/R) of the equation for the reciprocal of temperature. Therefore, Q and k₀ were calculated over the sintering temperature range of 1125°C–1175°C for different values of n. The slope was 2 in this range.

Figure 3 shows the calculated Q and k₀. At n = 1, 2, 3, and 4, Q was calculated as 681, 511, 341, and 170 kJ/mol, respectively, and k₀ was calculated 5.90 × 10⁹ nm/h, 7.00 × 10¹⁹ nm²/h, 8.29 × 10²⁹ nm³/h, and 9.68 × 10³⁹ nm⁴/h, respectively. It can be seen that Q increases linearly, and k₀ increases exponentially according to the value of n. When n was the same in the sintering temperature range of 1125°C–1175°C, Q was also constant. As shown in Figure 2, the grain growth was completed at 1175°C because the trend changed at 1175°C. Gao et al. confirmed that n was 1.7 and Q was 393.4 kJ/mol when 0.92Bi_0.5Na_0.5TiO₃–0.08BaTiO₃ ceramics were made using the conventional sintering process [28]. From Gao’s report and our result in Figure 3, we can argue that n = 2 and Q = 341 kJ/mol can be the optimized parameters for the sintering conditions in our experiment, since BNT-BT has similar components and structure with those of BNT-ST.

Figure 3. Plots of grain size versus sintering temperature for estimating Q after the conventional sintering process.

Figure 4 depicts the d₃₃ and k_p at the measured sintering temperature. At sintering temperatures of 1125°C, 1150°C, and 1175°C, d₃₃ was 96, 123, and 146 pC/N, respectively, while k_p increased to 0.259, 0.301, and 0.348, respectively. However, at sintering temperatures of 1200°C and 1225°C, d₃₃ decreased to 145 and 142 pC/N, respectively, while k_p decreased to 0.344 and 0.332, respectively. When the ceramics were sintered at 1175°C, the most uniform and dense microstructure appeared, and the highest piezoelectric properties were obtained. Therefore, the sintering temperature of 1175°C was set as the pre-sintering temperature of the two-step sintering.

Figure 4. Piezoelectric charge coefficient, d₃₃ and electromechanical coupling coefficients, k_p of the BNT–ST ceramics after conventional sintering at sintering temperatures of 1125°C, 1150°C, 1175°C, 1200°C, and 1225°C.

To fabricate the BNT–ST ceramics with the two-step sintering process, pre-sintering was performed for 10 min to achieve pre-coarsening at 1175°C, which is the optimum sintering temperature for the existing sintering process. Thereafter, the secondary sintering holding time (t₂) was set to 0, 6, 12, 18, 24, 30, and 36 h, and the BNT–ST ceramics were fabricated by performing secondary sintering at 1075°C.

Figure 5 shows the average grain size and relative density of the BNT–ST ceramics as a function of t₂. As shown in Figure 5, relative density was increased up to 98% with increasing holding time t₂. When holding time t₂ approached 24 h and then relative density was saturated. Also, the average grain size was continuously increased with holing time t₂; however, the increasing ratio was decreased after 24 h holding time t₂.

Figure 5. Average grain size and relative density of the BNT–ST ceramics as a function of t₂ of the secondary sintering process.

Figure 6 shows the grain size as a function of t₂. The plots for estimating in Figure 6 refer to k₀ and Q calculated in Figure 3. As shown in Figure 5, G_0t is small enough to be compared with the grain size after the two-step sintering process, so Equation (2)(2) $\text{ln}(G^n-G_0^n)=lnt+ln{k}_0-\frac{\frac{Q}{RT}}{,}$(2) was used. The portion indicated by the dotted line represents the theoretical value of the grain size according to n. When t₂ was less than 24 h, the exponent n of grain size G was found between 2 and 3. Exponent n is more converged to 2 rather than 3, while n converged to 4 when the sintering time was more than 24 h. From the result of the calculated values of n, we can argue that 24 h of sintering time t₂ located in the time boundary for the different sintering mechanism for the two-step sintering process. Inset in Figure 6 explains grains size variation dependent on the holding time with different slope k from Equation (2)(2) $\text{ln}(G^n-G_0^n)=lnt+ln{k}_0-\frac{\frac{Q}{RT}}{,}$(2) . As we have discussed before, holding time 24 h has important meaning of boundary time for different sintering mechanisms. As shown in the inset, slope has different values of 0.46 and 0.76. It means that two different sintering mechanisms exist in the BNT-ST two-step sintering process.

Figure 6. Ln G versus t₂ plots for estimating the grain growth kinetic exponent (n) after two-step sintering.

Figure 7 shows the FE-SEM image of the BNT–ST ceramics prepared using the two-step sintering process. As shown in the FE-SEM images, as t₂ increased up to 24 h, the pores decreased. Therefore, the density was increased as discussed in Figure 6. However, as hold time t₂ increased further than 24 h, the average grain size increased. Therefore, two different sintering mechanisms can exist in the BNT-ST two-step sintering mechanism as we have discussed in Figure 7. As shown in Figure 7(f) and (g), the grain size was increased without pore decreasing. This phenomenon can be explained by considering Figure 6, which displays two-step sintering hold time t₂ versus exponent n. As the hold time t₂ increases, n changes from 2 to 4, which explains the reason that main grain growth mechanism has changed from decreasing of pore state to grain growth state. According to Brook, sintering mechanisms are largely divided into two categories: pore control and boundary control [29]. After 24 h, the density converged as shown in Figure 5, and the decrease in pores was not clear as shown in the FE-SEM image of Figure 7. From this, it can be inferred that pore control mainly occurred, and then the sintering time exceeded 24 h and boundary control became the main mechanism. In the pore control step, n ranges from 2 to 3, and in this process, movement and volatilization of internal pores mainly occur. On the other hand, in the boundary control step, n was confirmed to be 4, and since grain boundary diffusion mainly occurs, it seems that grains of irregular size increased as a result. The simulated values of n and Q based on t₂ are listed in Table 1.

Table 1. Average grain size, n, and Q values based on the two-step sintering process.

Holding time, t2 (h)	Average grain size (nm)	n	Q (kJ/mol)
6	2284	2.27	422.00
12	2709	2.29	425.56
18	3070	2.41	444.66
24	3376	4.00	712.16
30	3557	4.01	714.12
36	3716	3.99	710.57

Figure 7. FE-SEM micrographs and grain size distribution of the BNT–ST ceramics after two-step sintering at various t₂: (a) 0 h, (b) 6 h, (c) 12 h, (d) 18 h, (e) 24 h, (f) 30 h, and (g) 36 h.

Figure 8 shows the d₃₃ and k_p of the BNT–ST ceramics fabricated through two-step sintering. The d₃₃ and k_p had maximum values of 163 pC/N and 0.377, respectively, when the sintering time was 24 h. The d₃₃ increased by 10.9%, while the k_p increased by 8.6%. This is because the density of the fabricated ceramics was increased by controlling the internal pores of the ceramics through the two-step sintering process. Therefore, it is evident that the two-step sintering process was effective in fabricating high-density ceramics and improving their electrical properties.

Figure 8. Piezoelectric charge coefficient, d₃₃ and electromechanical coupling coefficients, k_p of the BNT–ST ceramics after two-step sintering for various t₂ of 0 h, 6 h, 12 h, 18 h, 24 h, 30 h, and 36 h.

4. Conclusions

In this study, two-step sintering process was investigated to analyze the sintering mechanism of 0.8(Bi,Na)TiO₃-0.2(Sr,Ti)O₃ ceramics by considering grain size G and sintering holding time t₂. Exponent n for grain size and activation energy Q for the sintering process was simulated by varying the sintering time t₂. Two different grain growth mechanisms were observed as varying holding time t₂, n = 2–3 for 0–24 h of holding time and n = 4 for 24–36 h of holding time, respectively. By varying the holding time, at first, the porosity was decreased for n = 2–3, and second, the grain size was increased for n = 4. Different sintering mechanism was observed and explained by observing the ceramic porosity and grain size. The lead-free piezoelectric BNT–ST ceramics fabricated by the two-step sintering process under optimized conditions had a d₃₃ and k_p of 163 pC/N and 0.377, respectively. By employing the two-step sintering process, the piezoelectric properties were improved around 10%. Therefore, the two-step sintering process is effective in improving the electrical properties of high-density lead-free piezoelectric BNT–ST ceramics.

Acknowledgement

We thank Professor Masaya Ichimura for his assistance during the research.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was supported by the MOTIE (Ministry of Trade, Industry, and Energy) in Korea, under the Fostering Global Talents for Innovative Growth Program (P0017308) supervised by the Korea Institute for Advancement of Technology (KIAT) and the Human Resources Development (No. 20214000000280) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry and Energy.