粒狀結構鉬-二氧化錫厚膜之熱電勢研究

Abstract

金屬－絕緣體複合物系統（MxI1-x，M是金屬、Ｉ是絕緣體，而x是金屬體積比）的物理性質是一直倍感興趣的課題。其中熱電勢是一種可觀察出物質的傳輸性質重要的物理量，此外，熱電勢和物質的傳輸性質有重要的關連性。我們測量了一系列不同鉬金屬比例Mox(SnO2)1-x厚膜的熱電勢，樣品使用濺鍍方法製備於天津大學李志青教授實驗室，其膜厚介於200-400nm，本實驗利用4He低溫量測系統量測300K至5K之熱電勢。 本實驗使用熱電勢在金屬中行為預測模型擬合量測結果，發現樣品x=1-0.637載子濃度隨著x減少而減少。亦將載子濃度與李志青教授實驗室測量之霍爾系數換算得到的載子濃度作比較。而樣品x=0.286、0.263與0.252熱電勢在低溫時有急劇減少的現象，根據此行為趨勢，我們判斷傳輸機制為nearest-neighbor hopping（NNH），著手進行擬合，發現實驗數據與擬合曲線並非完全吻合。 1991年Bergman 和Levy提出良導體與不良導體複合物，在臨界體積比xc附近熱電勢的變化，與不同的熱導率比值與電導率比值組合有關。當電導率比值遠小於熱導率比值時，熱電勢對x-xc的行為符合此式子，S正比於(x-xc)^-t 。而本實驗利用此式擬合，得到樣品臨界體積比xc[S]=0.286；而李志青教授實驗室也對電阻率的量測結果作擬合，得到臨界體積比（percolation threshold)等於0.32。熱電勢得到臨界體積比小於電阻率得到的臨界體積比，原因推測可能是由於熱電勢對於摻入少量絕緣體的樣品較不敏感所致。

The physical properties of metal-insulator composites (MxI1-x where M=metal, I=insulator, and x is the metal volume fraction) systems have attracted much interest in the past several decades. Thermoelectric power is one kind of physical properties has correlation with transport properties of materials. We have measured thermoelectric power, of a series of Mox(SnO2)1-x thick films with different Mo volume x fraction ranging from 0.252 to 1. The granular thick films were fabricated by co-sputtering method from Tianjin University professor Z. Q. Li’s lab. The thicknesses of films were between 200 to 400nm. In our measurement, thermoelectric power was measured from 300K to 5K by the 4He measuring system. We found that the magnitude of carrier concentration decreased when x was reduced from 1 to 0.637 by the fitting equation S=AT+BT^3, which was the prediction of thermoelectric power in metals. The samples x=0.286, 0.263 and 0.252, which absolute value of thermoelectric power increase sharply at low temperature. According to the tendency of thermoelectric power, we supposed the transport mechanism were nearest-neighbor hopping. But after we fitted the data line, we observed that the fitting line did not coincide with the data line. The percolation behavior of thermoelectric power was proposed by David J Bergman and Ohad Levy in 1991. Both of them claimed that thermoelectric power of metal-insulator composites near the percolation threshold xc should depend on different electrical conductivity and thermal conductivity ratios of two materials. When the electrical conductivity ratio much smaller than thermal conductivity ratios ,S=So(x-xc)^-t ,by using this equation, our fitting result of samples x=0.286-0.252, the percolation threshold was 0.286. In addition, the percolation threshold calculated from professor Z. Q. Li’s lab was 0.32±0.01. About the reason, we conjectured that thermoelectric power is insensitive to the small volume fraction of insulating inclusions.