Lead-based piezoelectric ceramics such as $Pb(Zr_xTi_{1-x})O_3$ (PZT) show excellent piezoelectric properties and are widely used in electric, and electronic devices. However, these ceramics are associated with serious environmental problems as they contain more than 60 wt% of lead. Thus, in recent years, many studies have concentrated on developing environmentally friendly lead-free compounds to replace lead-based piezoelectric ceramics in piezoelectric applications. Among the candidates for lead-free materials, alkaline niobate-based materials such as $K_{0.5}Na_{0.5}NbO_3$ are considered to be good alternatives to PZT due to the good piezoelectric properties of these materials. Recently, mechanochemical synthesis has been considered as a nano-size powder preparation method because the major advantages of mechanochemical synthesis lie in the formation of reaction products at a low temperature and the refinement of produced powders to a nanometer size range. Several attempts have been made to synthesize various lead-free piezoelectric ceramic powders using mechanochemical synthesis. Attempts have also been made to synthesize $K_{0.5}Na_{0.5}NbO_3$ by mechanochemical synthesis, but these did not succeed; it was found that only amorphization occurs - even after prolonged milling. Moreover, no research results have demonstrated success in synthesizing nano-sized $K_{0.5}Na_{0.5}NbO_3$ powders via mechanochemical synthesis. In the present study, the mechanochemical synthesis was studied using a high-energy milling apparatus in an effort to make nano-size powders. Mechanochemical synthesis of $NaNbO_3$ and $K_{0.5}Na_{0.5}NbO_3$ from a powder mixture of $K_2CO_3, Na_2CO_3$ and $Nb_2O_5$ has been investigated. The densification behavior of $K_{0.5}Na_{0.5}NbO_3$ by pressureless sintering using mechanochemically synthesized powders and its electric properties are also investigated.
In chapter III, mechanochemical synthesis of $NaNbO_3$ was studied using a high-energy milling apparatus (SPEX mill) in an effort to decrease the milling time required for synthesis. The effects of the milling ball density and Ball-to-Powder ratio (BPR) on the reaction time were also investigated. Nano-size (10―20 nm) $NaNbO_3$ powder was synthesized when the powder mixture was milled in a vibratory mill using a $ZrO_2$ vial and WC or $ZrO_2$ balls. The time required for powder synthesis decreased as the density of balls increased. It was possible to synthesized powder within 120 min when using WC balls with a BPR of 30. The synthesized $NaNbO_3$ powder also had good thermal stability. $NaNbO_3$ powder can thus be easily produced via high energy milling of low cost $Na_2CO_3$ and $Nb_2O_5$ powders.
In chapter IV, mechanochemical synthesis of $K_{0.5}Na_{0.5}NbO_3$ was studied using a high-energy milling apparatus (SPEX mill). The effects of the milling vial density, milling ball density and Ball-to-Powder ratio (BPR) on the reaction time were also investigated. Nano-size (10―20 nm) $K_{0.5}Na_{0.5}NbO_3$ powder was synthesized when the powder mixture was milled in a vibratory mill using a $ZrO_2$ or WC vial and $ZrO_2$ or WC balls. The time required for powder synthesis decreased as the density of vial and balls increased. It was possible to synthesize powder within 420 min when using a WC vial and balls with a BPR of 30. The synthesized $K_{0.5}Na_{0.5}NbO_3$ powders form agglomerates during mechanochemical processing. The XRD patterns shows $K_{0.5}Na_{0.5}NbO_3$ powder has been almost fully synthesized by mechanochemical processing. However, we can see that there is a small amount of unreacted powders by themogravimetric analysis, evolved-gas analysis and residual carbon analysis. These unreacted powders can be eliminated by annealing above 500℃.
In chapter V, the densification of the mechanochemically synthesized $K_{0.5}Na_{0.5}NbO_3$ powders was investigated by pressureless sintering. As mechanochemically synthesized powders consisted of agglomerates of nano-sized particles of 10-20 nm, the compacts of agglomerated $K_{0.5}Na_{0.5}NbO_3$ powders were difficult to sinter due to the coexistence of intra- and inter-agglomerate pores. In order to overcome these drawbacks, additional wet milling was introduced. The sintered density of mechanochemically synthesized $K_{0.5}Na_{0.5}NbO_3$ powders increased as the wet-milling time increased. It was possible to sinter about 99% TD after 30h wet milling. Moreover, mechanochemically synthesized powders contained a small amount of unreacted powders, which can lead $CO_2$ gas release during sintering. This phenomenon caused crack formation in the specimen. With an additional annealing treatment, it was possible to sinter about 99% TD without cracking in the specimen. In the present study, the sintered specimens of mechanochemically synthesized $K_{0.5}Na_{0.5}NbO_3$ show excellent piezoelectric and electromechanical responses, $d_{33}$~119 pC/N, $k_p$ ~0.41. These results show that the piezoelectric properties of $K_{0.5}Na_{0.5}NbO_3$ pellets can be improved by using mechanochemically synthesized powders in pressureless sintering.
본 연구에서는 기계화학법으로 제조된 KNN 분말을 이용하여 상압소결로 고밀도 KNN 비연계 압전 세라믹스를 제조하였다. 기계화학법으로 합성된 분말은 응집체 형태를 가지고 있기 때문에 상압소결 공정으로 치밀한 소결체를 제조할 수 없었지만, 응집체를 제거하기 위한 습식밀링 공정을 도입한 결과 습식밀링 시간이 길어질수록 소결밀도가 증가하는 경향을 보였으며, 30시간의 습식밀링을 한 후에는 약 99%이상의 높은 소결밀도를 갖는 소결체를 제조할 수 있었다.
기계화학법으로 제조된 KNN 분말에는 합성되지 못한 미반응 상태의 잔류 carbonate가 소결 중 $CO_2$가 발생을 유도하기 때문에 소결 중 $CO_2$ 발생구간에서 시편에 crack을 유도하여 건전한 소결체를 제조하기 어렵지만, 열처리를 통해 미반응 잔류 carbonate를 제거한 후 소결을 실시하면 crack이 발생하지 않고 높은 소결밀도를 갖는 KNN 비연계 압전 세라믹스를 제조할 수 있었다.
본 연구에서 제조한 기계화학법으로 제조된 KNN 분말을 이용한 고밀도 KNN 비연계 압전 세라믹스는 압전상수 119 pC/N, 전기기계결합계수 0.41를 가지며, 이 결과는 상압소결 공정으로 KNN 비연계 압전 세라믹스를 제조한 이전의 어떠한 결과보다도 우수한 전기적 특성의 향상을 나타내었다.
