奈米二氧化鈦之視燒結活化能與相變研究

Abstract

中文摘要 「主導燒結曲線模型」是一個可以預測陶瓷燒結過程中行為變化的模型，已經廣泛的應用在燒結領域。由於此模型是由全期燒結模型推導而來，依據推導過程中用到的許多假設，主導燒結曲線模型應該只能應用在微米級以上的粉體，但在近幾年的研究中卻發現主導燒結曲線模型也能完整描述奈米尺度下的燒結行為。而這一定與模型中的唯一關鍵參數「視燒結活化能」有關，雖然目前對其物理意義尚不清楚明白。本研究因此特別利用不同初始粒徑與初始相對密度的奈米二氧化鈦陶瓷粉末進行實驗，藉由設計不同條件的燒結實驗來探討視燒結活化能的變化，並且進對視燒結活化能的瞭解。 本研究選取粒徑35 nm與80 nm的奈米二氧化鈦陶瓷粉末，並分別壓製了初始相對密度44%、41%、36%的35 nm-TiO2與初始相對密度43%、41%、35%的80 nm-TiO2二氧化鈦，進行不同初始粒徑與初始相對密度的燒結實驗。每組樣本的升溫條件固定皆以3℃/min、5℃/min與10℃/min的升溫速率進行燒結，所得的結果代入主導燒結曲線模型分析。 在利用主導燒結曲線模型分析六組實驗數據過程中，發現主導燒結曲線具有擬合燒結與相變同時發生之能力，此擬合曲線中包含了四個反應，包括anatase與anatase的燒結、anatase與rutile的燒結、rutile與rutile的燒結還有anatase的相變，此四種不同的反應竟然可以只由一條主導燒結曲線描述其變化，是本研究重要的發現。 本研究透過主導燒結曲線模型的分析與估計，得到不同初始粒徑與初始生坯相對密度下的視燒結活化能，比較其關係結果發現，原本先前所認為的粒徑假說與相對密度假說在透過更多的資料點分析後，其證據顯示奈米尺度下所求得之視燒結活化能與初始粒徑、初始相對密度無關。本研究另外以German等人推導之三個參數的擬合方程式擬合本研究數據，結果發現其擬合結果與利用本研究之擬合方程式所得到的視燒結活化能非常相近，僅在粒徑80 nm 初始相對密度41%的燒結條件下得到較高的活化能，因此根據目前實驗結果，初始相對密度、初始粒徑與視燒結活化能應該沒有關係。

Abstract Master Sintering Curve (MSC) model, which can predict the sintering behaviors of ceramic sintering, has been widely used in sintering during the past few years. Based on the inherited assumptions from the combined stage sintering model, from which the MSC was derived, MSC should not work in the sintering of nanocrystalline ceramic, but as we have proved in our earlier work that it does work. Though we still don’t know the physical meaning of the only parameter of MSC, i.e., the apparent activation energy Qa, it must have something to do with the excellent applicability of the model. This research experimenting on the sintering of nanocrystalline titania samples with various average initial diameters and relative density, explores the variations of Qa and helps increase our understandings of the parameter. Sintering experiments were conducted using both 35 nm diameter titania samples with initial relative densities of 44%, 41% and 36%, and 80 nm diameter titania samples with initial relative densities of 43%, 41% and 35%. Each set of samples were sintered at 3oC/min, 5oC/min and 10oC/min. One of the great discoveries of the study is that the MSC can be used to analyze and describe the complex reactions that both phase transformation and sintering happened at same time. In addition, using both our five-parameter S-curve fitting and German’s three-parameter curve fitting lead to very similar results and values of Qa. Most importantly, we cannot find satisfactory data to support the hypothesis that Qa varies linearly either with particle diameter or with initial porosity of the samples.