Barium titanate is widely used as various electronic components because of its excellent dielectric properties and chemical stability. Since the electrical properties of $BaTiO_3$ vary considerably with microstructure, the grain growth and densification behaviors have been much studied to control the resultant microstructure. The structural transition of grain boundaries in $BaTiO_3$ and related grain growth behavior has been investigated for a wide range of oxygen partial pressure $(P_{O2})$.
In the present investigation, the effect of sintering atmosphere on the densification and grain growth in 0.1 mol% Ti-excess $BaTiO_3$ has been studied. The microstructures obtained after sintering at 1250°C below the eutectic temperature were very different depending on the oxygen partial pressure, $P_{O2}$. At a high $P_{O2}$ of $1.54^{\ast}10^{-8}$ atm, most grain boundaries were faceted and the grain growth hardly occurred. The relative density of the sample increased slowly as sintering time increased. At a $P_{O2}$ of $7.85^{\ast}10^{-12}$ atm, as fraction of the faceted grain boundaries decreased with the reduction of $P_{O2}$, the grain growth and densification enhanced in the initial stage, but finally only relative density increased without coarsening as sintering time increased. As the fraction of faceted grain boundaries decreased gradually, abnormally large grains appeared and the distribution became bimodal. Further sintered at this $P_{O2}$, the abnormal grains have impinged on each other and the relative density of the sample increased rapidly as sintering time increased. However, with a further reduction of $P_{O2}$, the fraction of faceted segments became very small. Therefore, grain growth behavior became normal and densification behavior increased slowly with increasing of sintering time.
This investigation also included the effect of two step sintering. Same samples pre-fired at a high temperature 1300℃ for a very short time and fulfilled 1250℃ as conventional isothermal sintering at $P_{O2}$ of $1.0^{\ast}10^{-9}$ atm. Samples prepared by two-step sintering had high relative density and fine grains with uniform microstructure as compared normal isothermal sintering.
From the observations, densification without coarsening was effectively controlled via the transition of grain boundary structure through the atmosphere changing with oxygen partial pressure control and two step sintering.
$BaTiO_3$ 계에서 산소분압과 two-step sintering이 미세구조에 미치는 영향에 대해서 연구하였다.
$BaTiO_3$ 의 입계구조는 산소분압에 따라 다르게 나타나고 또한 입계구조의 변화는 입자성장과 치밀화에 영향을 미치게 된다. $BaTiO_3$ 계에서 산소분압이 높을수록 즉 산화분위기일수록 입계구조는 faceted되며 step mechanism에 따르고 산소분압이 낮을수록 즉 환원분위기일수록 입계구조는 rough해지며 cubic law를 따르게 된다. 산소분압이 입자성장과 치밀화에 대한 영향을 보면 산소분압이 높을 때 $(P_{O2} \sim 1.54^{\ast}10^{-8} atm)$ 입자성장은 억제되고 치밀화 율은 낮으며 산소분압이 중간 정도의 값을 $(P_{O2} \sim 7.85^{\ast}10^{-12} atm)$ 가질 때에는 소결 후기 첫 단계에서 어느 정도 입자성장이 일어나지만 오랜 시간을 유지하면 입자성장이 일어나지 않고 치밀화만 진행된다. 그리고 그 보다 더 낮은 산소분압에서는 $(P_{O2} \sim 8^{\ast}10^{-18} atm)$ stagnant한 입자성장으로부터 twin이 없는 비정상이 발생하였고 나중에 이런 비정상 입자들이 서로 충돌되는 양상을 보였으며 소결 시간에 따라 빠른 치밀화 율을 보여주었다. 산소분압이 낮을 때는 $(P_{O2} \sim 4^{\ast}10^{-19} atm)$ 입자성장이 촉진되고 밀도도 시간에 따라 증가되었지만 소결 후기에 빠른 입자성장으로 인한 기공 포획 때문에 밀도가 95%정도에 머물렀다.
$BaTiO_3$ 계를 normal sintering, two-step sintering 방법으로 열처리 해 보았다. Normal sintering을 한 경우 상대밀도가 입자성장과 함께 증가되었고 시간에 따라 평균입자크기가 증가되었다. 반면에 Two-step sintering을 한 경우 미세조직이 균일하면서 치밀화도 잘 되었으며 평균 입자 크기도 normal sintering에 비하여 작았다. 따라서 two-step sintering은 고상에서 일반 소결보다 입자성장도 억제되고 미세구조 균일화에도 효과가 있음이 증명되었다.
