錳與鈷摻雜物對無鉛鐵電陶瓷 (Bi0.5Na0.5)TiO3- BaTiO3-(Bi0.5K0.5)TiO3 鐵電性與顯微組織之影響

Abstract

針對無鉛鐵電陶瓷 0.854(Bi0.5Na0.5)TiO3 – 0.026BaTiO3 – 0.12(Bi0.5K0.5)TiO3，本論文以摻雜不同比例的 MnO2 與 Co3O4 二系統，來探討隨添加量的增加，BNBK 85.4/2.6/12 的鐵電性質與顯微組織的變化，選擇的摻雜劑量為 0 wt%、0.1 wt%、0.25 wt%、0.5 wt%、0.75 wt% 及 1.0 wt%。 顯微組織上，顯示隨 MnO2 與 Co3O4 的添加量增加，晶粒變大，且在0.1 wt% 添加量下可增加燒結體密度。然而，當摻雜高劑量的 MnO2 與 Co3O4 ，必須降低燒結溫度以避免針狀第二相的生成，此針狀第二相的出現會提升試片的導電率並增加高壓電實驗的失敗率。XRD 分析顯示所有組成皆為單一鈣鈦礦結構，且因菱體對稱、正方對稱兩相的共存使得 XRD peaks 高度重疊，利用 XRD 擬合軟體可定性的得知兩相的比例變化。鐵電性的表現上，摻雜 MnO2 與 Co3O4 皆使得殘留極化量及矯頑電場同時增加。電致應變方面，隨 MnO2 添加量增加，電致應變值不規則變動。當摻雜 Co3O4 之後，電致應變值小幅下降但幾乎無明顯差異，添加量造成壓電性的變化與菱體對稱、正方對稱兩相的比例有關。 去極化溫度方面，摻雜 MnO2 與 Co3O4 皆能提升去極化溫度，兩者差異點為摻雜 MnO2 可由電滯曲線觀察到明顯的中間過渡相轉換﹔然而摻雜 Co3O4 則因高溫下的離子導電行為而無法明確得知相變化。此外藉由阻抗頻譜分析可幫助吾人釐清在實驗過程中所發現之電性異常的原因。例如在相同的組成中，含針狀第二相之試片其導電率明顯高於含單一鈣鈦礦相之試片。若比較添加 0.1 wt%、0.75 wt%、1.0 wt% 之 Co3O4 其 30℃ 與 150℃ 之導電率大小，發現隨 Co3O4 添加量增加，離子在高溫的傳導變得容易且主宰此系統之傳導機制。 BNBK 85.4/2.6/12 添加 MnO2 與 Co3O4 兩系統中，最全面的性質為添加0.75 wt% MnO2，其殘留極化量為 29.74 μC/cm2，矯頑電場為 3.40 MV/m，去極化溫度約 170℃，電致應變值為 0.11 %，壓電電荷係數為 195 pC/N﹔添加0.25 wt% Co3O4，其殘留極化量為 37.44 μC/cm2，矯頑電場為3.33 MV/m，去極化溫度約 175℃，電致應變值為 0.14 %，壓電電荷係數為 224 pC/N。此兩成份具有相當的潛力來應用在無鉛的致動器。

The microstructure and ferroelectric properties of lead-free 0.854(Bi0.5Na0.5)TiO3– 0.026BaTiO3–0.12(Bi0.5K0.5)TiO3 (BNBK 85.4/2.6/12) ferroelectric ceramics doped with manganese (Mn) or cobalt (Co) are investigated in this study. The chosen doping amounts are: 0 wt%, 0.1 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt% and 1.0 wt%. The observed SEM images indicate that both the Mn and Co dopants can increase the average grain size, and small contents of the additives are beneficial to the sintering density. However, with high Mn or Co doping levels, it’s necessary to decrease the sintering temperature to prevent the formation of needle-like second phases. The appearance of the needle-like second phases promotes the conductivity of the ceramic specimens, hindering the subsequent ferroelectric characterizations under high voltage. The XRD analysis shows that all Mn or Co doped BNBK 85.42.6/12 compositions have a single perovskit structure, and display the coexistence of rhombohedral and tetragonal phases. Using a XRD peak fitting software, the contribution of different phases are seperated. On the aspect of ferroelectric properties, both Mn and Co dopings increase the value of remanent polarization and coercive field simultaneously. With increasing Mn doping level, the induced electrostrain changes randomly.In contrast, with increasing the Co doping level, the induced electrostrain stays approximately the same. Variations of the ferroelectric proterties are believed to be closely related to the ratio between the mole contents of rhombohedral and tetragonal phases. On the aspect of temperature properties, both Mn and Co dopings are able to increase the depolarization temperature. The difference is that with Mn dopants the intermediate transiton phase between the depolarization and Curie temperatures can be defined; while with Co dopants, it is more difficult. On the aspect of impedance properties, doped BNBK 85.4/2.6/12 specimens with needle-like second phases have higher conductivities. The increase in ionic conductivity for the Co-doped specimens at high temperatures is also observed. In the present study, the composition of 0.75 wt% Mn-doped BNBK 85.4/2.6/12 has a remanent polarization of 29.74 μC/cm2, a coercive field of 3.40 MV/m, depolarization temperature of about 170℃, an electrostrain of 0.11 %, and an apparent piezoelectric charge coefficient of 195 pC/N. The composition of 0.25 wt% Co-doped BNBK 85.4/2.6/12 has a remanent polarization of 37.44 μC/cm2, a coercive field of 3.33 MV/m, a depolarization temperature of about 175℃, an electrostrain of 0.14 %, and an apparent piezoelectric charge coefficient of 224 pC/N. These two compositions are candidate materials for lead-free actuator applications.