Functional Properties of BaTiO3/Ni Composites
Date Issued
2006
Date
2006
Author(s)
Huang, Yung-Ching
DOI
en-US
Abstract
Multilayered ceramic capacitors (MLCCs) composing BaTiO3 based dielectric and metallic inner electrodes are one the most important passive components. The electrodes materials such as Ni or Cu are widely used for reducing the cost of MLCCs. These base-metal electrodes (BME) capacitors have to be fired in a reducing atmosphere to avoid the oxidation of metals. Many approaches have been reported to maintain the insulation resistance of BaTiO3 based dielectrics even when they are sintered in low oxygen partial pressure. In the present study, the Ni particles are mixed with the BaTiO3 powders. The sintering of the Ni/BaTiO3 composites is carried out in different atmosphere with various oxygen partial pressures. The microstructure, densification behaviour, electrical, ferroelectric and ferromagnetic properties of Ni-doped BaTiO3 are measured.
Depending on the partial pressure in the sintering atmosphere, a very small amount of Ni ion may dissolve into BaTiO3 to replace Ti ion. The Ni2+ ion thus act as acceptor to BaTiO3. The tetragonality and grain size are thus reduced with the increase of Ni content. The solubility of Ni in BaTiO3 as sintered in N2 at 1330 oC is about 0.075-0.1 vol.% Ni. In addition, the diffuse distance of Ni in BaTiO3 matrix also depends on the oxygen partial pressure in the sintering atmosphere. The solubility of Ni in BaTiO3 is negligible as sintering is carried out in a highly reducing atmosphere (Po2=10-7-10-8Pa). The densification behaviour is affected by the Ni2+ solutes. The grain size of the Ni-doped BaTiO3 is also affected by sintering atmosphere. The grain size of BaTiO3 is increased and then decreased with the decrease of oxygen partial pressure.
The electrical properties of Ni-doped BaTiO3 are affected by the Ni addition. The presence of pores and the Ba6Ti17O40 phase degrades the dielectric properties of Ni-doped BaTiO3. The undoped BaTiO3 becomes semiconducting as sintering in Po2=10-15 Pa and then re-oxidized in N2 (Po2=1 Pa) or Po2=10-2-10-3 Pa. However, the solution of Ni2+ into BaTiO3 can improve the reduction resistance of BaTiO3. Electrical resistivity higher than 1010 Ω•m and dissipation factor lower than 2% are thus obtained in the Ni-doped BaTiO3 samples even they are sintered in a relatively low oxygen partial pressure. The process windows for the preparation of insulating BaTiO3-Ni can be built by applying defect models. According the experimental results, the ionic conductivity constant (Ki) and reduction equilibrium constant (KR) can be estimated.
The monolithic BaTiO3 is an anti-ferromagnetic material; the addition of metallic Ni particles introduces a ferromagnetic response into BaTiO3. The specific saturation magnetization increases with the increase of Ni content. For the BaTiO3/35%Ni composite, the specific saturation magnetization reaches a value of 25 emu/g. On the other hand, by adding an amount of Ni particles slightly below the percolation limit of the Ni particles can push the dielectric constant to a value as high as 28,800. The phenomena are well fitted by a percolation equation. The value of percolation threshold (Vc) as determined from the experimental data is 0.35.
Subjects
鈦酸鋇
多層電容陶瓷
鎳
氧分壓
授體
固溶度
缺陷
微結構
晶粒尺寸
電阻係數
介電損失
介電常數
鐵磁性
滲流
BaTiO3
MLCCs
Ni
oxygen partial pressure
acceptor
solubility
defect
microstructure
grain size
electrical resistivity
dielectric loss
dielectric constant
ferromagnetic
percolation
Type
thesis
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