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Advanced Science Research Center, Japan Atomic Energy Agency
High Energy Accelerator Research Organization, KEK
A joint research group comprising researchers from the Advanced Science Research Center at the Japan Atomic Energy Agency, Institute of Materials Structure Science at KEK, and J-PARC Center has elucidated the mechanism of insulation degradation due to trace amounts of hydrogen impurities in barium titanate (BaTiO3), a representative ferroelectric material used in ceramic capacitors.
Multilayer ceramic capacitors are fundamental components of electronic devices such as computers and smartphones, and are indispensable to the modern electronics industry. BaTiO3, a ferroelectric material, is one of the main components of multilayer ceramic capacitors. The electronic properties of BaTiO3 change greatly upon the introduction of trace amounts of impurities and defects; therefore, it is important to elucidate the role of the impurities in BaTiO3 to control its electronic properties.
This research focused on hydrogen since there is a risk for hydrogen incorporation into multilayer ceramic capacitors in the annealing process during their fabrication. We used positive muons in place of hydrogen to investigate the role of hydrogen impurities in BaTiO3. The positive muon implanted into the BaTiO3 crystal behaves as a light isotope of hydrogen and its radioactive decay enables sensitive detection of the local electronic state around the muon.
The experimental results showed that a significant fraction of implanted muons bound electrons below -190°C and the electron orbital spreads over several tens of unit cells (Fig. (a)). The spreading of the electron orbital was several tens of times that of an isolated case, suggesting that the electron was very weakly bound to the muon. The weakly bound electrons were gradually released with increasing temperature due to the increased thermal energy (Fig. (b)). The released electrons can move freely around the crystal and lead to electric conductivity, thus decreasing the insulating performance of BaTiO3. Hydrogen impurities in BaTiO3 are also considered to release electrons according to a similar mechanism, resulting in insulation degradation.
The insights from this work can be applied to improve the performance of BaTiO3-based ceramic capacitors by removing the possibility of hydrogen incorporation during their fabrication process.
(a) Schematic showing the positive muon-electron bound state in a BaTiO3 crystal and that in vacuum (an isolated case). The electron orbital size for the former is several tens of times that of the latter.
(b) Temperature dependence of the ratio of positive muons that bound an electron (black squares) and positive muons that released an electron (red circles). Solid lines show the theoretical curves.
When the temperature is increased, electrons weakly bound to positive muons are released because of the increased thermal energy. As a result, the number of positive muons that bound an electron decreases and the number of positive muons that released an electron increases. The released electrons can move freely around the crystal and decrease the insulating performance of the crystal.
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