| 研究生: |
林容成 Lin, Jung-Cheng |
|---|---|
| 論文名稱: |
巨磁阻Sr2Fe2-xMoxO6 之反應動力學、合成機構及其性質之探討 Reaction Kinetics, Formation Mechanism and Properties of Sr2Fe2-xMoxO6 |
| 指導教授: |
方滄澤
Fang, Tsang-Tse |
| 學位類別: |
碩士 Master |
| 系所名稱: |
工學院 - 材料科學及工程學系 Department of Materials Science and Engineering |
| 論文出版年: | 2003 |
| 畢業學年度: | 91 |
| 語文別: | 中文 |
| 論文頁數: | 109 |
| 中文關鍵詞: | 反應動力學 、巨磁阻 |
| 外文關鍵詞: | Kinetics, Sr2Fe2-xMoxO6 |
| 相關次數: | 點閱:118 下載:4 |
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雙鈣鈦礦之Sr2FeMoO6氧化物屬於半金屬性之鐵磁體,由於在室溫下具有顯著的磁阻效應,因此以物理及工程的觀點來看,此特性是相當吸引人的。而在本研究中以鍶鐵氧氧化物與鍶鉬氧氧化物為固態反應的前導粉末,並針對其反應動力學、殘留SrMoO4相及合成機構等方面作探討。
對於反應動力學部分,以固態反應之SrFeO3-x和SrMoO4在不同的升溫速率及溫度下來探討Sr2FeMoO6反應動力學。以非恆溫的反應動力學模式來計算其反應活化能。以固態反應之SrFeO3-x和SrMoO4所合成Sr2FeMoO6,可有效降低合成溫度約100℃。而對於其電性及磁阻性質並不會有太的影響。
在探討殘留SrMoO4相對Sr2FeMoO6之性質的影響上,一般認為殘留的SrMoO4相會被侷限於晶界中,且由於SrMoO4相本身為非磁及絕緣性,所以在試片中的殘留SrMoO4會導致整體的電阻率及磁阻率提高。然而,在我們實驗中發現殘留的SrMoO4相以類似非結晶型態的nanosized cluster存在於晶粒中並非是殘留在晶界中,反而是Sr離子在晶界的偏析。
然而,非計量比組成之Sr2Fe2-xMoxO6固溶體中,所添加的Mo元素可以穩定非計量比Sr2Fe2-xMoxO6固溶體的結構,使其維持在一單相結構。而其電荷平衡方程式:Sr2+2(Fe4+2-2xFe3+xMo5+x)O2-6,完全符合非記量比組成x=0.2~1.0時的電荷平衡。此外,發現在組成x=0.6時,於某些晶粒發現具有類似Moiré fringe之條紋狀顯微結構;同時也發現到差排的存在於其他晶粒中。在組成x=0.6時為相結構轉變的臨界點,再加上試片中局部成份擾動的因素,因此可以解釋為何在組成x=0.6之固溶體中可以觀察到奇異的缺陷結構。
Double Perovskite-type oxides Sr2FeMoO6 are related to half-metallic Ferromagnets (or ferrimagnets), which shows remarkable magnetoresistance at room temperature. Therefore, this property is attractive from the standpoint of both physics and engineering. The solid-state reaction of those precursors of Strontium iron oxide and Strontium molybdenum oxide were used to this study. In the purposes of this study are to evaluate the reaction kinetics and polycrystalline sample with residual SrMoO4 and the formation mechanism of SrFeMoO6.
The solid-state reaction of SrFeO3-x and SrMoO4 to form the Sr2FeMoO6 at different temperature and heating rate was to investigate the formation kinetics. The non-isothermal energy kinetic empirical model was proposed to evaluate the activation energy. The reaction temperature of Sr2FeMoO6 was performed by the solid-state reaction of SrFeO3-x and SrMoO4 had been reduced about 100℃. Nevertheless, there have no much influence in electric and magnetic properties.
In the study of the sample with residual SrMoO4 phase, the sample would have the high resistivity and low field magnetoresistance (LFMR). It was found that the non-magnetic and insulating SrMoO4 phase dose not reside at the boundary of granular sample but some boundaries are rich in the Sr ion. It is suggested that SrMoO4 might not play a role in enhancing LFMR. The possible mechanism of the increase of LFMR is discussed.
However, in aspect of formation mechanism of non-stoichiometric Sr2Fe2-xMoxO6, the additional element of Mo could be help to stabilize the solid solution of Sr2Fe2-xMoxO6, the equation of electrical neutrality could be written as Sr2+2(Fe4+2-2xFe3+xMo5+x)O2-6, any composition will be obeyed the equation. Then, we found that the TEM micrographs of samples with composition of x=0.6 whose to appear the striped structure such as Moiré figure or stacking fault in the grain, and some dislocations could be existed in other grain. However, the composition of x=0.6 is a critical point for the phase transition. Therefore, both factors of local compositional variations and phase transition could be interrupted the existence strange defect in the solid solution at x=0.6.
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