摘要
從近年的研究中可以發現在非鉛壓電材料中，主要以鈦酸鉍鈉系與鈮酸鉀鈉為主的固溶系統最具有發展的潛力。其中鈮酸鉀鈉的固溶系統被用以特殊的製程合成出較佳的機電耦合常數(kp~0.36)與高的相變化溫度(420℃)的壓電特性，但是由一般的製程中無法得到緻密性高的陶瓷體，所以普通的燒結方式一直無法獲得電性較佳的陶瓷體，最近文獻發現鈮酸鉀鈉若添加助燒劑則可以在一般空氣中燒結就可得到緻密性高的陶瓷體。
本研究利用固態氧化物法燒結製做鈮酸鉀鈉陶瓷體，以摻雜鋰做為助燒劑，研究鋰的摻雜對於煆燒溫度及微觀組織所產生的影響，之後再摻雜銀做為調整其電性之研究，探討摻雜鋰及銀後對鈮酸鉀鈉系統相結構、缺陷及基本電性之間的關係進行分析。
本研究的成果，可以歸納如下：
(Na0.5K0.5)(1-x) LixNbO3陶瓷體的成份為x=6 mol%時以650℃煆燒，1060℃燒結，可獲得相對密度為96%之陶瓷體，其X光圖譜分析為鈮酸鉀鈉的O相與T相之共存，電滯曲線的量測得到的矯頑電場為 12kV/cm，殘留極化量為30μC/cm2。
從相對密度與X光圖譜交叉分析中可以發現，以固態氧化物法燒結 (Na0.5K0.5)(1-x) LixNbO3陶瓷體時，粉末的預成相愈接近鈮酸鉀的O相時，愈容易燒結出不含鎢青銅相的鈣鈦礦結構，此時陶瓷體的相對密度較高，顯示其緻密性較高，粉末在超過650℃的煆燒溫度煆燒時，容易產生鎢青銅的相結構，則會降低陶瓷體燒結後的緻密性；粉末在低於650℃時由於不純物量仍多且鈮酸鉀的預成相不完整，經燒結時還是會產生鎢青銅相的結構。
顯微組織的觀察鋰的摻雜有促進(Na0.5K0.5)(1-x) LixNbO3陶瓷體的異常晶粒成長的現象，異常晶粒成長的現象也可以視為液相燒結的一種，有提昇陶瓷體緻密的效果，與相對密度比對x=8 mol%時，相對密度已沒有提昇的作用，但在含鋰量在x=6 mol%及x=4 mol%時不同的煆燒溫度會使鋰的摻雜有提昇陶瓷體緻密的現象。
(Na0.5K0.5)(1-x) LixNbO3陶瓷體由於鋰的摻雜使燒結溫度的工作範圍縮小，並且使雜質與鎢青銅相容易形成，燒結溫度太低無法使鈉順利固溶進入鈣鈦礦結構中，溫度太高又已形成明顯的異常晶粒成長。陶瓷體產生異常晶粒成長時產生了二個機制：一方面促進了陶瓷體的相對密度的提昇，降低了燒結溫度，避免鉀、鈉在高溫時的揮發，並抑制了潮解的現象；另一方面則妨礙了固相燒結作用的進行，陶瓷體如是以固相燒結作用進行燒結相信可以形成較純的鈣鈦礦結構，以及較為均勻的晶粒大小而獲得較佳的電性。從鋰的摻雜為量4 mol%可以發現使鈮酸鉀的預成相在550℃開始形成，鋰的摻雜為量6 mol%可以發現在650℃有最大的固溶量，但鋰的摻雜量亦不能超過8 mol%否則不僅不能達到降低煆燒溫度的效果，反而容易形成鎢青銅相造成妨礙燒結的效果。
(Na0.5K0.5)(1-x)(1-y) LixAgyNbO3陶瓷體，煆燒溫度的範圍會從750℃下降。經750℃煆燒的陶瓷體，其二次相大幅出現及相對密度下降都顯示出陶瓷體的電性會下降的理由，顯示銀的摻雜應使煆燒溫度下降，推論其燒結溫度範圍亦會隨之縮小。
(Na0.5K0.5)(1-x)(1-y) LixAgyNbO3陶瓷體，煆燒溫度在650℃時有最大的固溶量， 在此條件下燒結之陶瓷體， 觀察其X光圖譜還可以在 44.5°~ 47°發現有O相與T相轉換(MPB)的特徵，並由電滯曲線的觀察只要再改變燒結條件，如燒結溫度、持溫時間或昇溫速率必定可以得到電性較佳的陶瓷體。從壓電特性的量測也可以發現雖然未能達到最佳的極化條件，但摻銀之後可以降低極化的溫度及持溫時間，如能找出最佳極化條件也是這一系統可以發展成非鉛壓電材料的最大優勢。

Abstract
Among the development of lead-free piezoelectric ceramics, bismuth sodium titanate and potassium sodium niobate based ceramics are the most potential systems. Solid solution of potassium sodium niobate materials can be made to possess relative high coupling coefficient of Kp=0.36 and high curie temperature of 420℃ using special processing procedures. However, ordinary processing methods appear to be difficult to derive dense ceramics. Appropriate sintering aids were reported to be helpful to obtain denser ceramics with better properties, but how the added elements influence the microstructural arrangements in materials are mostly unclear. In this study, oxide mixing method was employed to fabricate potassium sodium niobate ceramics. Lithium oxide was adopted as the sintering aids, and its effect on the calcination/sintering temperatures. Then silver was chosen to adjust the electrical properties and phase transition, microstructural arrangements, as well as electrical variations of the system doped with Li and Ag were investigated.
The results were concluded as following:
Fabrication of (Na0.5K0.5)(1-x)LixNbO3 ceramics with density higher than 96% can be achieved by calcinations of ceramics at 650℃ and sintered at 1060℃. Analysis of X-ray diffraction pattern implies that the orthorhombic phase and the tetragonal phase co-exist. Ferroelectric properties of the material exhibit a coercive field of 12kV/cm and a remenant polarization of 30μC/cm2.
To prepare (Na0.5K0.5)(1-x)LixNbO3 ceramics, if the structure of the calcined powder is closer to the orthorhombic potassium niobate, it would be easier to obtain perovskite structure and the tungsten bronze phase can be suppressed. If employing the powders prepared at a temperature higher than 650℃, tungsten bronze phase appear in the ceramics and the relative density of ceramics decreases. On the other hand, impurities and fewer well-crystallized phase in the powders calcined at a temperature lower than 600℃, tungsten bronze phase appears in the sintered specimens.
Microstructural investigations indicates that Li2O addition enhance the abnormal grain growth in ceramics with an addition of up to 6mol%, due to liquid sintering. If addition of Li2O higher than 8mol%, the density of specimens decreases again and the amount of tungsten bronze phase increases quickly.
The Li2O addition produces liquid phase sintering, and it is quite often abnormal grain growth happens if the sintering temperature is high, and it may also cause difficulty of solid solution of Na+ into the perovskite substrate if the sintering temperature is relatively low. Formation of liquid phase sintering reduces the sintering temperature, and enhances the density of specimens and therefore avoids the evaporation of sodium and potassium as well as deliquesces of the specimen; on the other hand, tungsten bronze phase formation occurs, which retards the densification of the specimens.
It is found that formation of perovskite ferroelectric phase starts from 550℃ when adding simply Li2O. Maximum solid solution of Li2O into the NKN substrate is 6 mol% at 650℃. If the amount of Li2O addition is higher than 8 mol%, tungsten bronze phase forms. Calcination process determines the formation of the second phases as well as atomic arrangements in powders. Simultaneous addition of Li2O and Ag may produce large amount of second phases and reduces specimen density, indicating that the range of calcination temperature was narrowed down when Ag was added. Material electrical properties seriously deteriorate, when second phases form.
(Na0.5K0.5)(1-x)(1-y)LixAgyNbO3 ceramics exhibit the best calcination solubility of the dopants at 650℃. X-ray diffraction patterns show co-existence of the orthorhombic and the tetragonal phases. Although the sintering conditions of the specimens have not been optimized, Ag-addition reduces the poling temperature and holding time. Optimization of the processing and poling conditions of the present material system may enhance the electrical properties of the specimens.

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