Dielectric properties of BaTiO3 - CaBi4Ti4O15 systems for automotive MLCC

다국어 초록 (Multilingual Abstract)

Popular BaTiO3 has been still one of the most selected base materials for ceramic-based capacitors. The Curie temperature of BaTiO3 is about 120 oC, which makes it unsuitable for use at high temperatures, which has been recently required for automotive applications. In this work, we introduce an unprecedented choice of CaBi4Ti4O15 as a major constituent to control the Curie temperature toward the higher temperature for less sensitive change of capacitance with temperature. First, for the stabilization of temperature dependence of capacitance, CaBi4Ti4O15 having a high Curie temperature of ~790 oC was gradually added into BaTiO3 by following conventional solid-state reactions. As a highlight, 0.8 BaTiO3 – 0.2 CaBi4Ti4O15 showed a temperature coefficient of capacitance (TCC) of within 15 % in the temperature range from room temperature to 200 oC, with the reduction of dielectric constant, all compared to the pure BaTiO3 samples. After the optimization process of composition ratio, each additive (ZnO, ZnF2, BaF2) is introduced to 0.8 BaTiO3 – 0.2 CaBi4Ti4O15 at a constant ratio. All samples showed improved relative density and the linear shrinkage rate was significantly increased when ZnF2 was added, which is due to the liquid phase at the sintering temperature. For the dielectric properties, 0.8 BaTiO3 – 0.2 CaBi4Ti4O15 with 3 mol% BaF2 showed the slightly higher dielectric constant and lower dielectric loss than without additives in all frequency ranges (100 Hz ~ 1 MHz). Only 8.2% TCC was obtained from room temperature to 200 oC. Based on the effect of CaBi4Ti4O15 on temperature stability, the base material BaTiO3 was replaced with 0.9BaTiO3-0.1Bi0.5Na0.5TiO3, which has better temperature stability than pure BaTiO3. After the replacement, all samples showed improved relative density and linear shrinkage rate than before. For the dielectric properties, samples other than x = 0.0 in the 0.9BaTiO3-0.1Bi0.5Na0.5TiO3+xCaBi4Ti4O15 systems showed lower dielectric loss in low frequency range. The addition of CaBi4Ti4O15 resulted lower TCC values, and especially when x = 0.20, TCC values of -1.3 ~ +7.9 % TCC was obtained from room temperature to 200 oC. Based on the above experimental results, an anion additive (BaF2) was added and the optimal composition was investigated according to the content of the additive. When 5 mol% BaF2 was added, TCC value was -1.6 ~ 0.3 in the range of room temperature ~ 200oC. In the range of -55 ~ 200oC corresponding to X9R, TCC value -34.3 ~ +0.4 with rapidly decreases in the low temperature. TCC improvement effect was confirmed through CaBi4Ti4O15 and BaF2, and additional research is needed to be suitable for X9R.

국문 초록 (Abstract)

본 논문에서는 몰 비율에 따라 혼합된 BaTiO3 - CaBi4Ti4O15 시스템에 고상반응법을 이용하여 다양한 화합물을 첨가하였으며 CaBi4Ti4O15 함량과 첨가된 화합물에 따른 유전특성을 연구하였다. 혼합된 CaBi4Ti4O15의 양과 화합물에 따른 결정구조, 격자 상수, 미세구조, 유전특성의 변화가 어떠한 상관관계를 가지고 있는지 고찰 하였다.
(1-x) BaTiO3 - x CaBi4Ti4O15 시스템에서 최적의 비율을 찾기 위해 x = 0.0, 0.1, 0.2, 1.0 의 4가지 조성의 유전체 샘플을 제작하였으며, 제작된 샘플들을 XRD를 통해 확인해본 결과, 2차상은 확인되지 않았다. 샘플의 소결온도는 1100 oC로 BaTiO3의 소결온도 보다 낮은 온도로, 순수한 BaTiO3(x = 0.0)는 65.59% 의 낮은 상대 밀도를 가지는 반면, x = 0.1, 0.2의 경우에서는 95% 이상의 높은 상대 밀도를 획득하였다.
유전특성의 경우, x = 0.2의 경우 상온에서 200 oC까지의 온도범위에서 15% 이내의 TCC를 보이며 가장 안정적인 특성을 나타내었다. 가장 우수한 조성인 0.8 BaTiO3 – 0.2 CaBi4Ti4O15에 ZnO, ZnF2, BaF2, ZnF2/BaF2를 3mol% excess로 첨가하여, 이에 대해 미세구조 분석과 유전특성 변화에 대해 연구하였다. 이를 통해, 다양한 첨가제에 따른 결정구조의 변화와 유전특성 변화에 대해서 연구하였으며, BaF2를 첨가를 통해 유전특성의 개선과 200 oC까지의 온도범위에서 TCC의 최대값이 8.2%에 달하는 안정적인 유전특성을 가지는 유전체를 개발하였다.
위의 실험결과를 통해 확인한, CaBi4Ti4O15와 음이온 첨가제의 첨가효과를 기반으로 순수한 BaTiO3보다 온도안정성이 뛰어난 BaTiO3-Bi0.5Na0.5TiO3에 CaBi4Ti4O15를 첨가하였다. 0.9BaTiO3-0.1Bi0.5Na0.5TiO3 + x CaBi4Ti4O15 를 기준으로 x = 0.00, 0.05, 0.10, 0.20을 첨가하였으며, x = 0.20의 경우 가장 우수한 특징을 가지는 것을 확인하였다. 이를 토대로 온도안정성 개선에 효과를 보인 BaF2를 1, 3, 5 mol%를 첨가하였으며 5 mol%를 첨가한 경우 상온에서 200oC까지의 온도범위에서 -1.6 ~ 0.3 의 TCC를 가지며 가장 안정적인 특성을 가지는 것을 확인하였다.
X9R에 해당하는 -55 ~ 200oC 범위를 측정한 결과, -55oC의 저온에서 허용범위인 15%를 초과하는 값을 가지는 것을 확인하였지만, CaBi4Ti4O15 및 BaF2의 온도안정성 향상 효과를 확인할 수 있었다.

목차 (Table of Contents)

• Chapter 1. Introduction
• Chapter 2. Backgrounds
• 2.1 Multilayer Ceramic Capacitor
• 2.1.1 Ceramic Capacitor
• 2.1.2 Classification and Status of MLCC
• 2.2 Curie temperature, TC
• 2.3 Dielectric Ceramics
• 2.3.1 Perovskite
• 2.3.2 Barium Titanate (BaTiO3)
• 2.3.3 Calcium Bismuth Titanate (CaBi4Ti4O15)
• 2.3.4 Bismuth Sodium Titanate (Bi0.5Na0.5TiO3)
• 2.3.5 Dielectric Properties
• Chapter 3. Experimental Procedure
• 3.1 Sample Preparation
• 3.2 Measurements and Characterization
• 3.2.1 Density
• 3.2.2 X-ray Diffraction Analysis
• 3.2.3 Scanning Electron Microscope (SEM)
• 3.2.4 Measurement Frequency Dependence of Dielectric Properties
• 3.2.5 Measurement Temperature Dependence of Dielectric Properties
• Chapter 4. Results and Discussion
• 4.1 Synthesis of CaBi4Ti4O15
• 4.2 BaTiO3-CaBi4Ti4O15 Systems
• 4.3 Dielectric Properties of BaTiO3-CaBi4Ti4O15 Systems with Contents of CaBi4Ti4O15
• 4.4 Dielectric Properties of 0.8BaTiO3–0.2CaBi4Ti4O15 Systems with Various Additives
• 4.5 Dielectric Properties of (0.9BaTiO3-0.1Bi0.5Na0.5TiO3)–CaBi4Ti4O15 Systems with Contents of CaBi4Ti4O15
• 4.6 Dielectric Properties of (0.9BaTiO3-0.1Bi0.5Na0.5TiO3)+0.20CaBi4Ti4O15 Systems with Contents of Additive (BaF2)
• Chapter 5. Conclusions
• References
• Abstract in Korean