以第一原理計算探討鋰矽鍺合金之液相與非晶相之動力學與 結構性質

Abstract

本論文運用第一原理計算與分子動力學模擬探討液態及非晶相鋰矽鍺合金在不同成分下的動力學及結構等性質。其中LixSi0.5Ge0.5的成分主要介於x=0.45~4.81，藉由適當的熔化與焠火之程序產生液態及非晶相結構，並與鋰矽、鋰鍺系統進行比較。此篇論文主要分兩部分： 在第一部分的研究主要針對液態鋰矽鍺合金相關性質進行分析，而我們的結果顯示：在鋰矽鍺系統中，其擴散係數隨鋰濃度的變化與鋰矽及鋰鍺系統有相當大的不同。在鋰矽鍺(LixSi0.5Ge0.5)系統中，我們發現在鋰濃度達x=2.23時，有較強的鋰矽、鋰鍺作用力導致較低的擴散係數。另外，與LixSi及LixGe系統的比較中，我們發現，在鋰濃度0.45 < x < 2.23之間，鋰矽鍺的擴散係數較鋰矽及鋰鍺來得高，在鋰濃度x > 2.23時，三個系統間的擴散係數差異並不大。此外在結構性質的分析中，不僅驗證了動力學的結果，我們額外發現到了在鋰濃度x > 2.23時，矽、鍺原子隨著溫度上升而會有聚集的現象，而我們認為此現象主要源自於鋰矽、鋰鍺有較強的作用力。我們最後計算了液態鋰矽鍺合金的電子性質，並與鋰矽、鋰鍺系統相比，發現其導電度主要介於鋰矽、鋰鍺系統之間。 在本研究的第二個部分，則針對各濃度非晶相鋰矽鍺合金進行結構與電子性質分析，各部分皆顯示出與液態時有相似的特徵。此外我們也進行了晶體非晶相化現象的模擬，並發現期間的壓力及介面的反應速率，矽鍺合金主要介於矽、鍺之間。 總結以上的結果，雖然在電性，介面反應速率，非晶相化時的壓力，矽鍺合金皆介於純矽以及純鍺，然而在動力學上卻顯示出與鋰矽、鋰鍺相當不同的變化，我們發現在鋰濃度0.45 < x < 2.23之間，鋰矽鍺合金甚至有優於鋰矽及鋰鍺的動力學行為。

In this thesis, the first atomic calculation and molecular dynamics simulation are used to investigate the kinetics and structure of liquid and amorphous LixSi0.5Ge0.5 alloys in different compositions. The composition of LixSi0.5Ge0.5 is between x=0.45~4.81, and the liquid and amorphous phase structures are generated by appropriate melting and quenching procedures, and compared with LixSi and LixGe systems. This paper is mainly divided into two parts: The first part of the study focused on the analysis of the properties of liquid LixSi0.5Ge0.5 alloys, and our results show that the diffusivity varies with lithium concentration and is quite different from LixSi and LixGe systems. In the LixSi0.5Ge0.5 system, we found the lowest diffusivity at x = 2.23 due to stronger Li-Si and Li-Ge interaction. Comparing the three systems, it can be found that the diffusivity in LixSi0.5Ge0.5 is higher than that of LixSi and LixGe at a lithium concentration of 0.45 < x < 2.23. However, there is not much difference in diffusivity among the three systems at x > 2.23. In addition, in the analysis of structural properties, we found that at high lithium concentrations, Si and Ge atoms will aggregate as the temperature rises, and we believe that this phenomenon is mainly due to the strong interactions of Li-Si and Li-Ge. Finally, the electronic properties of the LixSi0.5Ge0.5 alloy were calculated. Compared with the LixGe and LixSi systems, the conductivity lies between the LixSi and LixGe systems. In the second part of the study, the structural and electronic properties of the amorphous phase LixSi0.5Ge0.5 alloys were analyzed. Each part shows similar characteristics to those in liquid. In addition, we perform the amorphization of crystal. We have found out that the stress during amorphization and rate of interfacial reaction is between those of Si and Ge. In summary, although the electrical, interfacial reaction rate, and the stress during amorphization in Si0.5Ge0.5 are between those of Si and Ge, LixSi0.5Ge0.5 exhibit quite different characteristics from LixSi and LixGe. We found that LixSi0.5Ge0.5 have higher Li diffusivity than that of LixSi and LixGe at 0.45 < x < 2.23.