臺灣博碩士論文加值系統

摘要
本論文之主要研究目的是在探討Pb0.6Sr0.4TiO3正溫度係數電阻材料之電阻-溫度特性與其微觀結構之間的關係，實驗中分別利用各種不同的燒結方法來進行試片的製作，在經過電阻-溫度曲線的量測之後發現：不同的燒結方法會導致有極為不同的電阻-溫度曲線產生，其中利用微波來進行燒結的試片呈現了單一且Tc約為380℃的曲線，而一般傳統燒結的試片則形成了特殊的雙Tc曲線，此外微波燒結的方式也會使得試片整體的電阻率降低許多。
為了能夠正確的找出造成電性差別的原因所在，我們有系統的使用電子顯微鏡及能譜分析儀來進行微觀結構的觀察與成份分析，結果顯示，試片中具有一因成份不均勻所產生的核-殼結構存在，即在晶粒的外層會形成一Pb含量較高的殼層。經詳細分析後顯示，此一殼層主要是由於高溫燒結期間的Pb擴散層及冷卻過程的析出層所控制，燒結溫度越高、時間越長則殼層中的擴散層越厚，冷卻速率越慢則析出層會較厚。在不同的燒結方法下，此一核-殼結構亦會有所不同，其中，在傳統燒結並以緩慢冷卻的試片中形成了最厚但Pb含量卻最低的殼層，而以微波燒結的試片則相對的形成了厚度較薄但Pb含量卻最高的殼層。
根據核-殼結構的不同以及Heywang和Jonker對正溫度係數電阻特性的解釋，本文結合了理論及微觀的分析，對文獻上Pb0.6Sr0.4TiO3正溫度係數電阻材料雙Tc電阻-溫度特性的成因加以修正，來解釋不同燒方法及條件下，電阻-溫度特性所產生的差異。在微波燒結的試片中，由於殼層的Pb含量高、且厚度佔晶粒尺寸的百分比最大，因此當試片溫度介於殼層(Tcs)及核層(Tcc)之鐵電相變化溫度(Tc)時，殼層內鐵電電域的自發極化電荷補償了晶界及核-殼邊界的蕭特基能障，使得殼層形成一低電阻的電子通道，導致單一高溫Tc的產生；而傳統燒結的試片就是由於晶界及核-殼邊界的蕭特基能障無法充份補償，使得殼層形成一較高電阻的電子通道(當Tcc< T < Tcs時)，並導致雙Tc現象的產生。
除了核-殼結構的差異外，以微波燒結的試片在液相燒結特性以及鐵電電域結構上也呈現了較為不一樣現象。由SiO2-PbTiO3或SiO2-PbO的共晶作用所產生的液相燒結區域具有非晶質的結晶結構，但在這些非晶質的燒結區中卻具有一些結晶的小顆粒存在；另外，在某些晶粒的外圍則發現了不一樣的結晶質TiO2燒結區。在電域結構上，微波燒結的試片展現了極為複雜的電域安排情形，綜合分析顯示，這些特性的產生應是由於微波對試片內不同的元素產生不同的反應所導致。
最後，我們以SrTiO3披覆PbTiO3的原始核-殼結構粉末來模擬Pb0.6Sr0.4TiO3中的核-殼結構，在經過不同的燒結方法後，經由微觀結構的觀察與成份分析發現：以微波燒結的方法可以使得原始的核-殼結構保持得較為完整，即高Pb以及較厚的殼層；而傳統燒結與快速燒結的方法則會使得核層與殼層之間嚴重的相互擴散，造成殼層的Pb含量以及厚度大量的減小。根據前面的說法，微波燒結的試片將是最有可能形成單一Tc的電阻-溫度曲線，而傳統燒結以及快速燒結的試片則是可能會形成雙Tc的電阻-溫度曲線。在經過實際的電阻-溫度特性量測後，證實了我們的說法，也幾乎可以確定：試片中核-殼結構的差異的確是造成電性不同的主因。

Abstract
In this thesis, we study the microstructures, especially the core-shell structures and the corresponding ρ-T properties of Pb0.6Sr0.4TiO3 positive temperature coefficient of resistivity (PTCR) materials. Distinct ρ-T curves of materials were obtained employing different sintering processes, in which microwave sintered specimens exhibit a single Tc at about 380℃, but the specimen sintered by conventional method, on the other hand, shows an intriguing double Tc behavior. Beside the ρ-T properties, conventional sintered specimens possessed higher resistivity than microwave sintered specimens do.
In order to find out the reasons of the differences in ρ-T properties, microstructures of specimens were investigated using X-ray diffractometer (XRD), electron microscope (EM) and energy dispersive spectroscopy (EDS). During investigations, a core-shell structure caused by composition inhomogeneity was observed and formation of this core-shell structure was also discussed. Results reveal that a high Pb-content shell composed of a Pb-diffusion band and a reprecipitation-band forms within the sintering period and during cooling, respectively. When the sintering temperature is high and the sintering time is long, the thickness of diffusion-band is larger than that of reprecipitation-band. On the other hand, when the cooling is very slow, the thickness of reprecipitation-band is thicker than that of diffusion-band.
The core-shell structures are very different in various sintering methods. In microwave sintered specimens, the shell has the highest Pb-content and the thinnest thickness. The conventional sintered and slow cooled specimens, on the other hand, possess the lowest Pb-content and the thickest thickness. According to the differences of the core-shell structures and the PTCR model proposed by Heywang and Jonker, the double Tc behavior of Pb0.6Sr0.4TiO3 PTCR materials is interpreted by modified core-shell structure model and the ρ-T properties of specimens which were sintered by different sintering processes were also accounted for.
In addition to above microstructural features, sintering characteristics of sintering aids, SiO2 and TiO2, and domain arrangements of microwave sintered specimens showed intriguing characteristics too. A high Si-content regions with amorphous structure and nanocrystalline particles are frequently observed at a triple junction among grain. On the other hand, regions show entire TiO2-concentration and crystalline structure are also observed around the grains. Microwave sintered specimens show much more complicated domain arrangements than conventional sintered specimens do. Results indicate that these characteristics may be attributed to that the atomic species reacted differently with microwave.
Finally, a PbTiO3-coated SrTiO3 powders were prepared for simulating the core-shell structure in Pb0.6Sr0.4TiO3 ceramics and then sintered by different sintering processes. After TEM investigations and EDS analyses, the microwave sintered specimens preserved the starting core-shell structure more effectively than the conventional and rapid thermal sintered specimens did. According to the above-mentioned model, the microwave sintered specimens will show single Tc and conventional, rapid thermal sintered specimens will show double Tc behavior. Measuring the ρ-T properties, our interpretation was verified and confirm that the core-shell structure is the primary factor of affecting ρ-T property.

Table of contents
Abstract in Chinese………………………………………………………………..I
Abstract..…………………...………………………………………………...……III
Acknowledgments..………………………………………………………………….V
Table of contents….……………………………………...………………………….VI
List of figures…………………………….……………………...…………….…..VIII
Chapter 1 Introduction……………………………………………….…..…………...1
Chapter 2 Paper review……………………………………………..….……………..5
2.1 Theoretical models of PTCR effect….…..….…..…………….…………….5
2.2 Parameters affecting the PTCR effect……..…….……..…………….….…11
2.3 Microwave sintering process…………….…..……………………..…...…13
2.4 Core-shell structures caused by composition inhomogeneity……..….…....18
Chapter 3 Experimental procedures………………………………………….……...20
Chapter 4 Sintering and microstructures of Pb0.6Sr0.4TiO3 ceramics
prepared by mixed-oxide method…………….……….………..……...…24
4.1 Structures and ρ-T property………………………..…………………...24
4.2 Microstructural characteristics of microwave
sintered Pb0.6Sr0.4TiO3 ceramics……….……………..………..…….……28
4.3 Comparison of microstructural characteristics of conventional-
furnace- sintered and microwave sintered Pb0.6Sr0.4TiO3 ceramics………42
4.4 Formation of core-shell structure…..………………………….…..……...48
4.5 Relationship of ρ-T property and core-shell structure………..……53
Chapter 5 Sintering and microstructures of PbTiO3-coated SrTiO3 ceramics…...….59
5.1 Microstructural characteristics……………..………...……….…………....59
5.2 Core-shell structure and ρ-T property.…………..……………………..67
Chapter 6 Conclusions…………………...…………………………………….…....74
References…………………………………………………………………………...76
Vita…………...…………………………………………………………………...…80

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