粒狀結構金屬-絕緣體複合物之熱電效應研究

Abstract

在金屬-絕緣體複合材料MxI1-x (M：金屬, I：絕緣體)中，隨著金屬體積比例x的減少，電阻率亦隨之漸漸地增加。當x小於臨界體積比xc(又稱為電阻率的percolation threshold)時，電阻率 將開始有一明顯大幅上升(發散)的趨勢。換句話說，當 x≧ xc時，電導率會開始有明顯的增加，這個現象就是percolation。另一方面，目前的理論尚未確定其熱電效應(Seebeck效應)的percolation threshold xc'是否會等於電阻率的percolation threshold xc。 本篇論文中，我們研究二批不同成份的三維粒狀結構金屬-絕緣體複合物：(PrBa2Cu3O7)1-xAgx和Znx(SiO2)1-x，藉由測量這些樣品的Seebeck係數及電阻率，我們發現Seebeck係數的確也會在x小於某值時有了大幅改變的趨勢。而且由本篇論文的結果，我們發現電阻率和Seebeck係數兩者的percolation threshold並不相同。這是目前的理論所尚未瞭解的結果。

In metal-insulator composites MxI1-x (M : metal, I : insulator), with the gradual reduction of the metal volume fraction, x, the resistivity increases gradually thereupon. When x is smaller than the resistivity critical volume fraction, xc (called the resistivity percolation threshold), the resistivity begin to rise abruptly (i.e., diverge). In other words, once x≧xc, the conductivity begins to rise rapidly. This phenomenon is known as percolation. On the other hand, according to current theories, it is not known, with the decrease of x, whether the thermopower (Seebeck coefficient) percolation threshold, xc', would be the same with the resistivity percolation threshold . In this thesis, we study two kind of three-dimensional granular metal-insulator composites: (PrBa2Cu3O7)1-xAgx and Znx(SiO2)1-x. By measuring Seebeck coefficients and resistivities of these samples, we found that the magnitude of Seebeck coefficient increases abruptly once x is smaller than some value xc'. In particular, we found that the percolation threshold of the resistivity and Seebeck coefficient are not the same. This result cannot be understood in terms of current theories.