添加二價鉛離子對La4Ti9O24微波介電陶瓷特性影響之研究

Abstract

本研究重點主要針對Pb2+離子與La4Ti9O24陶瓷反應生成La2/3TiO3-type鈣鈦礦斜方晶相之機制與特性作一深入的分析與探討. 整個架構區分為三個主要部分: 首先, 以巨觀的方式觀察PbO與La4Ti9O24陶瓷體介面的反應現象;利用X-ray繞射分析以及掃描式電子顯微鏡來觀察不同的熱處理溫度以及不同的持溫時間所製作的樣品, 研究結果發現, Pb2+離子會滲入到La4Ti9O24陶瓷內, 同時在Pb2+離子所滲入的區域內會產生新的La2/3TiO3-type鈣鈦礦斜方晶相; 藉由量測不同溫度與時間之樣品, 其因Pb2+離子滲入而產生之La2/3TiO3-type相層厚度之關係, 符合反應式擴散(Reactive diffusion)機制中 x2 = kt 之關係式 (其中 x: 擴散層厚度; k: 反應速率常數; t: 反應時間); 進一步將不同實驗條件所得到之反應速率常數值代入Arrhenius方程式(ln (k) = -Ea/RT + A ,其中Ea: 活化能; T: 絕對溫度; R: 氣體反應常數; A: 常數), 可以求得Pb2+離子與La4Ti9O24陶瓷發生反應式擴散所需之活化能為607 ± 60 kJ/mol. 第二部分則是以微觀的方式來解析Pb2+離子與La4Ti9O24陶瓷以及La2/3TiO3-type相晶格之間的關係; 利用化學共沉法來合成不同Pb2+離子添加量的La4Ti9O24陶瓷起始粉體, 再以900 oC /1h的條件熱處理; 藉由選區電子繞射圖譜以及X-ray繞射分析所生成之La2/3TiO3-type相結構; 結果顯示La2/3TiO3-type相之結晶結構為斜方晶之鈣鈦礦結構, 符合空間群Ibmm(No. 74)的對稱性. 進一步利用The Rietveld method分析XRD的資料, 可以解析所生成之La2/3TiO3-type相晶格參數為a = 0.55371 nm, b = 0.55064 nm 以及 c = 0.77825 nm. 最後則是藉由添加含Pb2+離子與不含Pb2+離子之低熔點玻璃粉, 與La4Ti9O24陶瓷粉體合成低溫共燒陶瓷材料的特性分析; 由於含Pb2+離子玻璃粉體當中的Pb2+離子會在燒結過程中擴散進入到La4Ti9O24陶瓷體內, 進而發生相轉換產生La2/3TiO3-type相, 也因此造成低溫共燒陶瓷體本身之微波介電特性產生變化; 因此, 藉由調整玻璃粉的添加種類及比例, 可以在950 - 1000oC/4h的燒結條件下可以得到理論密度95%以上的緻密性, 來合成出微波介電特性範圍介於(3 GHz): εr值約20—80, Q值約1600—1000以及TCF值從0—190的低溫共燒微波介電陶瓷材料.

In this study, it was performed a systematic investigation of La4Ti9O24 ceramics with various Pb2+ doping and further discussed the correlation with the La2/3TiO3-type phase. The main structure of this study includes three parts: In the first part, it was investigated the interaction between Pb2+ and La4Ti9O24 ceramic bulk. The result shows that a significant Pb2+ diffusion into La4Ti9O24 ceramic bulk and a significant crystallization of the La2/3TiO3-type phase is clearly observed in the Pb2+ diffusion layer. The reacted zone consists of the La2/3TiO3-type phase exhibits the linear dependence of the square of the thickness, x, on the heat-treated time, t, in excellent agreement with the parabolic law x2 = kt, where k is the growth rate coefficient. The kinetic study thus indicates that the Pb2+/La4Ti9O24 interaction strictly obeys the theory of the reactive diffusion. Furthermore, the experimental k values were used to determine the associated activation energy, Ea, for the formation of La2/3TiO3-type phase using the Arrhenius plot and the following least-square equation, ln (k) = -Ea/RT + A, where T is the annealing temperature, R is the universal gas constant, and A is a constant, resulting in Ea ~ 607 □ 60 kJ/mol. The second part reveals the crystalline structure of La2/3TiO3-type phase with Pb2+ doped. From the results of electron diffraction and Rietveld analysis of the X-ray powder diffraction patterns, we find that the La2/3TiO3-type phase crystallizes in the orthorhombic space group Ibmm (No. 74) with a = 0.55371 nm, b = 0.55064 nm, and c = 0.77825 nm In the third part, we investigated the influence of lead and/or zinc containing borosilicate glass addition on the sintering temperature and the microwave dielectric property of La4Ti9O24 ceramics. With the glass addition above 20 vol%, La4Ti9O24 ceramics with >95% of the theoretical density can be achieved at □1000 oC. The crystalline intensity of the La2/3TiO3-type phase increases significantly with the increase of lead containing borosilicate glass addition due to the Pb2+-induced phase transition. The change in the crystalline phase has appreciable effects on the microwave dielectric property (at 3 GHz), εr ~ 20—80, Q ~ 1600—1000, and TCF ~ 0—190.