Crystal structure & Microstructure analysis

4. Results & Discussion

4.1.1 Crystal structure & Microstructure analysis

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(a) (b)

Fig. 16. X-ray diffraction data of 1^st calcinated powders.

Fig. 17. X-ray Diffraction (XRD) data of all specimens after sintering.

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Fig. 18 shows the SEM images of polished and thermally etched surface of all specimens. Fig. 18(a) is the microstructure of BT pristine, consisted of many pores and large grain size. Abnormal grain growth(AGG) often occurs in BaTiO3 and (111) twin boundary is very crucial for AGG [18]. In this BT sample, (111) twin boundary is also observed, marked with red circles, and this result in large grain size. The pores were also observed by rapid grain growth leading closure of pores inside the grain [19].

In Fig. 18(b)~(e) and Fig. 19, Dy-doped sample doesn’t have any secondary phase, indicating all dy is well doped on BaTiO3 lattice. In addition, all the Dy-doped specimens shows the decreased grain size compared to BT pristine, which is because that the energy is needed for dopant to be incorporated into the lattice and so the required energy for grain growth is reduced [4]. The decreased grain size can be the evidence that Dy is incorporated on lattice.

Fig. 18. Microstructure of (a) Pristine BaTiO3, (b) Dy-A site, (c) Dy-B site, (d) Dy-A&B site, and (e) Dy excess.

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Fig. 19. Comparison of grain size of all samples.

4.1.2. Dielectric properties

Fig. 20 displays the temperature dependent dielectric permittivity and loss at 10kHz. Here, TC is Curie- temperature and TO-T is the phase transition temperature is between orthorhombic and tetragonal phase.

Fig. 21 is a summary data of Fig. 20. Temperature coefficient of capacitance (TCC) is a variation of temperature-dependent capacitance (see Eq. (17)). This coefficient indicates thermal stability and should be low for high-reliable MLCC. Here, C and RT is capacitance and room temperature, respectively.

TCC = ^{𝑪−𝑪𝑹𝑻}

𝑪_𝑹𝑻 ⅹ100 [%] ( 17 )

In Fig. 21 (a), Dy–B site shows the narrowest range of TC and- TO-T, leading to high TCC, and it has the highest dielectric loss. On Dy–A site, the widest range of TC and- TO-T, leading to lowest TCC, is observed, which means that Dy–A site is the best doping site in terms of thermal stability. Dy–A&B site shows the lowest dielectric loss. Dy–Excess has similar TC, TO-T, and loss with A&B site.

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Fig. 20. Dielectric permittivity and loss of all samples at 10kHz.

(a)

(b)

Fig. 21. (a) TC and TO-T of all samples (b) Temperature coefficient of capacitance (TCC) and maximum dielectric loss in the range of -55°C and 105°C.

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4.1.3. Electrical & Ferroelectric properties

Fig. 22 is the Nyquist plots of impedance according to frequencies at 500°C, which correlates the microstructure and electrical properties. On the impedance spectroscopy, the contribution of electrical conduction is characterized by continuous three circles that comes from bulk (grain), grain boundary and electrode. Especially, here, the degree of insulating resistance will be mainly discussed from the size of semi-circle of bulk conduction [20].

In Fig. 22, the Dy-B site has the smallest insulating resistance. This is due to leakage current from oxygen vacancies as described in Eq. (16). The Dy-A&B site shows the largest single plot, which indicates that the electrical conduction comes from the grain interior and has the best insulating resistance. In addition, while Dy-A site and Dy-B site display decreased resistance compared to pristine, Dy-excess data also shows improved resistance value like A&B site. This result indicates that Dy in Dy-excess is doped on both A site and B site, having self-compensation. The reason why there is a difference in resistance value between Dy-A&B site and Dy-Excess is because the amount of Dy occupying each A site and B site wasn’t controlled in Dy-Excess sample and so the ratio of Dy doped on the A-site and the B site would not be the same with Dy-A&B site.

Fig. 22. Nyquist plots of impedance according to frequencies in (a) 0~200𝐤𝛀, (b) 0~10𝐤𝛀, and 0~1𝐤𝛀 at 500°C.

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Fig. 23. Ferroelectric hysteresis loops (P-E curve) of all samples.

Fig.23 is room temperature polarization versus electric field of all samples. All samples show hysteresis loop which indicates ferroelectric property but in Dy-B site distortion of hysteresis loop is observed due to leakage current. This result reveals the presence of oxygen vacancies in Dy-B site [21].

4.2. Comparison between site controlled and uncontrolled samples

As mentioned on introduction, previous research synthesized Dy doped BaTiO3 through conventional solid solution method where dopant and oxides mixed all together and conducted calcination. Therefore, here, properties between conventionally synthesized sample and 2-step calcinated samples will be compared to confirm whether the site control is possible through conventional method.

4.2.1. Crystal structure & Microstructure analysis

Fig. 24 shows XRD data between conventionally synthesized sample and 2-step calcinated samples.

There is no any secondary phase on all XRD data and also the Dy is well doped on target site showing peak shift in all conventional (marked with C.S.) and step samples. Dy-A site samples display peak shift to higher angle, which indicates lattice contraction. In Dy-B site samples, peak shift to lower angle

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is observed due to lattice expansion. In Fig.25, step and conventional samples all have no secondary phase and no difference in grain size on SEM images. Therefore, in terms of crystal structure and micro- structure, there is no noticeable difference between conventional sample and step sample in Dy-A site and Dy-B site.

(a) (b)

Fig. 24. Comparison of XRD data between conventionally synthesized sample and 2-step calcinated sample of (a) Dy-A site and (b) Dy-B site.

(a) (b)

Fig. 25. (a) Scanning electron microscopy images and (b) comparison of grain size between step and conventionally synthesized sample in Dy-A site and Dy-B site

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4.2.2.

Dielectric properties

The difference between conventionally synthesized sample and 2-step calcinated sample is noticeable in dielectric property. Fig. 26 shows the dielectric permittivity and loss of Dy-A site.

The step sample shows wider range of TC and TO-T and lower TCC than conventional sample. The reason about this phenomenon is not yet unclear, but it is clear that there is some difference in result between two methods and this must be related to whether the dopant site is really controlled or not. Fig.

27 displays the dielectric permittivity and loss in Dy-B site. Dy-B site with step has leakier property and worse thermal stability than conventional sample. This means that in conventional method Dy is also doped on not only B site but also A site due to self-compensation, and so the amount of formed oxygen vacancies is quite reduced compared to B site with step. Therefore, we can conclude that, through conventional method, sample couldn’t be completely doped on the target site.

(a) (b) (c)

Fig. 26. (a) Temperature dependent dielectric permittivity and loss, (b )TO-T and TC, and (c) TCC and maximum dielectric loss in the range of -55°C and 105°C of Dy-A site.

(a) (b) (c)

Fig. 27. (a) Temperature dependent dielectric permittivity and loss, (b )TO-T and TC, and (c) TCC and maximum dielectric loss in the range of -55°C and 105°C of Dy-B site.

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4.2.3. Electrical properties

The difference between conventionally synthesized sample and 2-step calcinated sample also appeared in impedance data.In, Fig. 28, Conventional samples (C.S.) in both Dy-A site and Dy-B site has larger semi-circles than step samples, indicating improved insulating resistance in Conventional samples. This results is because, in conventional method, charge carrier formation is suppressed because Dy is also doped on untargeted site, occuring self-compensation. In other words, it is difficult to obtain a site- controlled composition through conventional method which was used in previous research. To control the dopant site accurately, the 2-step calcination is necessary.

(a) (b)

Fig. 28. Nyquist plots of impedance according to frequencies compared between conventionally synthesized sample and 2-step calcinated sample of (a) Dy-A site and (b) Dy-B site.

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5. Conclusion

In a lot of previous research, the analysis of properties by the role of Dy as donor and acceptor was lack. Dy is an amphoteric dopant in BaTiO3 system, and so it is important to control dopant site to dope on target site. Here, by definitely controlling the doping site through 2- step calcination, we systematically analyze properties according to the role of Dy. Dy doping on A site has advantage of thermal stability and in case of A&B site reliability. In addition, it would be better to avoid Dy doping on B-site which result in leakage current. In addition, Dy- excess displayed different aspects with Dy- A site and Dy-B site but similar with A&B site in terms of impedance and dielectric property. That is, self-compensation occurs when Dy was added excessively on BaTiO3. These results could be a guild- line to select appropriate Dy³⁺ doping site following purpose.

To confirm whether the dopant site is well controlled or not, we compared the properties between 2- step calcination and conventional solid solution methods. As a result, in Dy-A site and Dy-B site, samples synthesized from conventional solid solution method showed improved property. This indicates that it is inevitable to occur the self-compensation in conventional solid solution method and also it is hard to control the doping site from conventional solid solution method. Therefore, site control through 2-step calcination is crucial for dopants with amphoteric behavior.

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6. Reference

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[14] Zhang, J., Hou, Y., Zheng, M., Jia, W., Zhu, M., & Yan, H. (2016). The occupation behavior of Y2O3 and its effect on the microstructure and electric properties in X7R dielectrics. Journal of the American Ceramic Society, 99(4), 1375-1382.

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Acknowledgment

지난 2 년의 학위과정 동안 깊고 높으신 학식을 바탕으로 헌신적인 지도를 아끼지 않으셨던 조욱 교수님, 존경합니다. 연구 진행이 원활하지 않을 때 늘 격려해주시고 지지해주신 자상한 모습, 그리고 가끔은 애정 어린 조언으로 부족한 저를 일깨워 주고 이끌어 주셔서 정말 감사드립니다. 또한 바쁘신 와중에도 논문심사를 맡아주신 손재성 교수님 그리고 차채녕 교수님께 감사의 말씀 드립니다. 논문에 대한 유익한 말씀과 조언이 많은 도움이 되었습니다.

늘 옆에서 많은 도움을 줬던 강호 오빠, 건주 오빠, 영진 오빠, 원희 선생님 감사합니다.

마르지 않는 간 zi 를 가진 뇌섹남 황필 오빠. 아무것도 모르던 저를 도 닦는 마음으로 돌봐준 츤데레 우석 오빠. 사회력, 인성, 지성 모든 걸 다 가진 재현 오빠. 따뜻하고 포근한 마음을 가진 혜림 언니. 연애를 위해 항상 노력하지만 성과가 없는 옆자리 신사 준용 오빠, 이번에는 꼭 성공하길 바래. 중요한 건 꺾이지 않는 마음! 삼대 오백 1 등을 향해 달려가봐요, 우진 오빠. 부처 같은 멘탈을 가진 지훈 오빠. 고맙고 미안한 우리 귀여운 누리. 대학원 오고 더 멋있어진 정우 오빠, 박사 따면 연예인 하겠어요. 더 친해지지 못해서 아쉬운 민서 언니. 다들 감사합니다. 그리고 많이 부족한 저를 이끌어주고 때로는 친한 언니로서 인생의 조언도 건내 준 사수 주현 언니, 언니 덕분에 성장할 수 있었고, 또 행복한 대학원 생활 할 수 있었어요! 교수님 다음으로 존경하고 너무 감사해요.

나의 20 대 반 이상을 함께 보내준 은영이, 즐겁고 힘들고 행복한 시간들 함께 보내줘서 고마워. 돈 없는 대학원생에게 망설임 없이 베풀어준 김가연 정말 고맙다!

해운대 암소 갈비 flex 해줄게. 나의 정신적 지주이자 반쪽 상현이, 툴툴대는 나의 모습을

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불평불만 없이 받아줘서 고마워. 마지막으로 멀리서도 항상 내 편이 되어준 우리 가족들.

감사드리고 사랑합니다.

앞으로 사회에 나가서, 현재에 안주하지 않고 꾸준히 성장해 나가는 자랑스러운 SFC 졸업생이 되겠습니다.

2022.12.05 -김보경 올림-