Relaxor-based piezoelectric single crystals such as Pb($Zn_{1/3}Nb_{2/3}$)$TiO_3$-$PbTiO_3$ (PZN-PT) and Pb($Mg_{1/3}Nb_{2/3})$TiO_3$-$PbTiO_3$ (PMN-PT) are considered as promising for potential applications in medical ultrasonic, sonar transducers and solid state actuators owing to their ultrahigh coupling coefficient as high as 94\% and piezoelectric coefficient ($d_{33}$) of 2500 pC/N in contrast to conventional Pb(Zr,Ti)$O_3$ (PZT) ceramics with $k_{33}$ ~ 75\% and $d_{33 }$~ 500 pC/N. However, the toxicity of PbO and its high vapor pressure during sintering has led to a demand for alternative eco-friendly lead-free materials as several countries have already restricted the utility of hazardous substances in electrical and electronic equipments. Therefore, attention has recently been focused on the fabrication of lead-free piezoelectric ceramics and a parallel search for high performance relaxor-based lead-free piezoelectric single crystals.
The solid solution 100-$\textit{x}(Na_{0.5}Bi_{0.5}$)$TiO_{3}$-$\textit{x}BaTiO_{3}$ (NBT-$\textit{x}$BT) emerged as one the lead-free systems that can replace lead-based pizoelectrics. Over last decade, tremendous efforts have been made to fabricate high quality ceramics and single crystals of NBT-$\textit{x}$BT, particularly the compositions close to morphotropic phase boundary. In producing NBT-\textit{x}BT single crystals by melt or solution related techniques such as Bridgeman, Czochralski, top-seeded-solution growth (TSSG), and flux techniques, however, the inhomogeneity problem could not be overcome due to incongruent solidification of the compound and volatilization of low melting-point elements, bismuth and sodium, during crystal growth. To overcome this inherent and critical problem, the solid-state crystal growth (SSCG) technique, which utilizes the frequently observed phenomenon of abnormal grain growth in polycrystals, has been developed. In addition, this technique is much simpler and more cost effective than the conventional techniques.
To be successful with the SSCG technique, however, coarsening of fine matrix grains needs to be suppressed during the growth of a seed crystal embedded into the powder compact. Therefore, it is very essential to study the interface morphology and grain growth behaviour of any system before growing the crystal via SSCG. For this present study, a composition ($Na_{0.5}Bi_{0.5}$)$TiO_{3}$-$30BaTiO_{3}$ (NBT-30BT) that exhibited relatively strong relaxor behaviour with adequate dielectric permittivity and piezoelectric properties has been considered for the single crystal growth by SSCG technique. Prior to crystal growth, the grain coarsening behaviour of NBT-30BT system has been investigated with varying mol\% of excess $Na_{2}CO_{3}$. The powders of NBT-30BT, NBT-30BT with 0.2 and 0.5 mol\% $Na_{2}CO_{3}$ (0.2N 30BT, 0.5N 30BT) were prepared by conventional solid state reaction and sintered at different temperatures (1100-1200℃) and durations (10 min-10 h). When the mol\% of $Na_{2}CO_{3}$ increased, the 3-dimensional grain structure changed from rounded to cube shape with more faceted interface and the grain growth behaviour changes toward the abnormal grain growth behaviour. Grain growth behaviour with sintering temperature is explained in terms of the effect of excess $Na_{2}CO_{3}$ on interface-reaction controlled grain growth and the critical driving force. The optimal conditions have been figured out and single crystals of 0.2N and 0.5N 30BT with dimension 4 mm x 4 mm x 0.5 mm have been grown using (110) $SrTiO_{3}$ seed crystal by SSCG.
Pb($Zn_{1/3}Nb_{2/3}$)$TiO_{3}$-$PbTiO_{3}$(PZN-PT), Pb($Mg_{1/3}Nb_{2/3}$)$TiO_{3}$-$PbTiO_{3}$ (PMN-PT) 압전단결정은 94\%의 매우 높은 coupling coefficient와 2500 pC/N에 달하는 압전상수를 갖기 때문에 medical ultrasonic, sonar transducers 그리고 solid state actuators와 같은 소자에 응용이 된다. 하지만, PbO의 독성과 소결시 이 물질의 높은 증기압 때문에 전세계적으로 전자, 전기부품소자를 친환경적인 물질로 대체하고자 하는 추세이다. 그러므로, 최근의 연구는 relaxor-based 비연계압전세라믹스의 제조 및 비연계압전단결정에 집중되고 있다. 고용체인 100-$\textit{x}(Na_{0.5}Bi_{0.5}$)$TiO_{3}$-$\textit{x}BaTiO_{3}$ (NBT-$\textit{x}$BT) 연계압전세라믹스를 대체할 물질 중 하나로 떠오르고 있다. 최근 10년동안, MPB조성근처 NBT-BT계의 다결정체 및 단결정에 대한 연구가 이루어졌다.
하지만 Bridgeman, Czochralski, top-seeded-solution growth (TSSG), 그리고 flux techniques을 이용한 NBT-BT계의 단결정성장실험에서의 조성불균일문제가 아직도 해결되지 못했다. 이러한 문제를 해결하기 위해서 다결정체에서의 비정상입자성장을 이용한 고상단결정성장법(SSCG)을 통한 단결정의 성장에 대한 연구가 이루어졌다. SSCG법을 통한 단결정성장을 위해서는 종자단결정이 성장하는 동안 matrix 입자에서의 입자성장이 억제되어야 한다.
따라서, SSCG법을 통해 단결정을 성장시키기에 앞서 다결정체 내에서 입계의 구조 및 입자성장거동을 관찰하는 과정이 필요하다. 본 연구에서는, 상대적으로 강한 relaxor거동을 보이는 ($Na_{0.5}Bi_{0.5}$)$TiO_{3}$-$30BaTiO_{3}$ (NBT-30BT)계의 입자성장거동 및 SSCG법을 통한 단결정성장가능성에 대해 연구하였다. 고상단결정성장에 유리한 조건을 찾기 위하여 NBT-30BT 에 $Na_{2}CO_{3}$의 양을 0.2~0.5 mol\%로 변화시켜가며 1100-1200℃ 에서 다양한 시간 동안 입자성장거동을 관찰하였다. $Na_{2}CO_{3}$의 mol\% 가 증가함에 따라, 3차원 입자모양은 round shape에서 faceted shape으로 변화하였으며, 점점 비정상입자성장거동을 보였다. 이러한 입자성장거동은 첨가된 $Na_{2}CO_{3}$의 양의 변화에 따른 입자성장의 임계구동력과 최대입자성장구동력과의 관계로 설명할 수 있다.
