本文選題：納米氮化硅 + 第一性原理　； 參考：《西安交通大學》2017年博士論文

【摘要】：作為一種先進的結(jié)構(gòu)和功能陶瓷,Si_3N_4已經(jīng)在各個領(lǐng)域展示出了廣泛的應用,特別是隨著材料向小尺度化的發(fā)展,納米Si_3N_4陶瓷以其獨特的力、電和光學性能將會在納米器件上展現(xiàn)巨大的應用潛力,但目前由于各種原因,還很難通過實驗對這類材料性能進行測試與評價,基于此,本文利用第一性原理對摻雜和吸附的Si_3N_4模型進行電子結(jié)構(gòu)和光學性能模擬研究,利用分子動力學對Si_3N_4納米線和薄膜進行力學行為模擬研究,為其進一步的應用提供理論基礎。(1)利用第一性原理模擬了Al、Ga、As和P摻雜Si_3N_4體系的電子結(jié)構(gòu)和光學性質(zhì),結(jié)果表明:Al和Ga摻雜后體系帶隙分別為4.21和4.12e V,而As和P摻雜之后帶隙減小,甚至消失,說明后兩種元素摻雜對體系的帶隙影響較大;摻雜后四種體系的形成能按照Al、Ga、P和As的順序逐漸增加,說明Al摻雜體系比其他體系的結(jié)構(gòu)更穩(wěn)定;差分電荷密度圖表明,P摻雜后周圍的電子缺失增加,說明P與N成鍵的共價性有所提高,而As、Ga和Al摻雜后周圍的電子缺失降低,表明其與N成鍵的共價性有所降低,并且Al摻雜時,原子附近的電子缺失幾乎消失,電子云逐漸向N原子附近靠近,說明共價性向離子性轉(zhuǎn)化的程度最高,摻雜元素與N原子成鍵的共價性強弱順序為:PAsGaAl,與計算的布居值相一致;摻雜體系靜態(tài)介電常數(shù)隨共價半徑的增加先降低,后升高,其中As摻雜體系最低,且在低能區(qū)的介電損耗較小,有利于在光電材料上的應用。(2)利用第一性原理模擬了稀土元素(Yb、Gd、Sc、Sm、La和Lu)吸附氮化硅體系的電子結(jié)構(gòu)和光學性能,稀土元素吸附成鍵后周圍的電子缺失變?nèi)?與N成鍵的離子性的增強,其中Yb,Gd和Sc原子附近還出現(xiàn)了不同程度的電子富集,說明電子云分布的不均勻性增加,與N成鍵共價性強弱順序為YbGdScSm(La,Lu),和計算的布居值相一致;靜態(tài)介電常數(shù)和介電損耗都隨共價半徑的增加先增大,后降低,最后趨于平緩,其中Sc吸附體系最低;[010]極化方向?qū)偨殡姾瘮?shù)的貢獻大于其他兩個方向,說明吸附后體系都顯示了一定的各向異性;780~2500nm(0.496~1.59eV)近紅外光區(qū),La,Yb,Gd和Sc吸附體系的反射率較低,都小于6%,而Lu和Sm吸附體系可達20%,反射程度相對較高,對應折射率較低,說明光在前四種吸附體系中更容易傳播;390~780nm(1.59~3.18eV)可見光區(qū),六種吸附體系都具有較低的吸收系數(shù)和反射率,說明吸附體系具有“透明型”性質(zhì);80~390nm(3.18~15.5eV)紫外光區(qū),吸附體系對光的吸收較強,呈現(xiàn)出“阻隔型”性質(zhì)。(3)將第一性原理和分子動力學相結(jié)合,建立了[001]方向氮化硅納米線模型,利用分子動力學模擬了氮化硅納米線的壓縮和拉伸力學行為,結(jié)果表明:在拉伸應力下,材料彈性極限獨立于納米線長徑比,并大都發(fā)生在應變?yōu)?.05處,納米線的斷裂應力隨長徑比的升高而減少,同時在變形中產(chǎn)生大量的硅-硅鍵和團簇狀氮原子缺陷;在壓縮應力下,當應力達到最大值時,在納米線下表面的中心部位和上表面的兩個對稱位置產(chǎn)生了新的四重硅原子缺陷,使應力開始下降。(4)利用分子動力學模擬了氮化硅納米線在彎曲應力作用下的力學行為,結(jié)果表明:應力位移曲線由準彈性階段和非線性階段組成,隨著長徑比的增加(3:1、5:1和7:1),材料斷裂應力下降;在壓頭接觸納米線之后,彎曲應力隨壓頭位移的增加而增大,當初始斷裂出現(xiàn)時,彎曲應力急速下降;彎曲應力下,在上表面中心位置,可以觀察到硅-硅鍵和配位數(shù)為2的氮原子缺陷,隨壓頭位移的增加,氮的配位數(shù)逐漸轉(zhuǎn)變?yōu)?和1,硅的配位數(shù)轉(zhuǎn)變?yōu)?和7。(5)建立了基面氮化硅納米薄膜,原子的數(shù)量分別為5600、7406和9464個,利用最快下降法進行弛豫,得到了穩(wěn)定結(jié)構(gòu)。利用分子動力學模擬了薄膜的拉伸行為,結(jié)果表明:在拉伸加載過程中,當應變小于0.06時,薄膜展示了非線性應力應變關(guān)系;在0.06-0.09應變范圍內(nèi),薄膜展示了線性應力應變關(guān)系;超過0.09時,又變成非線性應力應變關(guān)系,直到最后的斷裂。斷裂應力、應變隨薄膜邊長的增加而增加,楊氏模量變化不大,y方向的斷裂應力大于x方向的斷裂應力,拉伸過程中的斷裂源來自N6h-Si鍵。隨著應變速率的增加,初始N6h-Si斷鍵缺陷出現(xiàn)的位置和應變點是相同的,是不依賴于應變速率的,當應變速率增加到5.3×109s-1時薄膜兩部分完全分離的應變數(shù)值降到0.113,其原因是由于在N6h-Si鍵的斷鍵缺陷擴展過程中出現(xiàn)了N2c-Si鍵的斷鍵缺陷,加速了薄膜裂紋的擴展過程。隨著拉伸溫度的升高,最大拉伸應力先升高后降低,對應的拉伸應變逐漸降低。
[Abstract]:As a kind of advanced structural and functional ceramics, Si_3N_4 has been widely used in various fields, especially with the development of material to small scale. Nano Si_3N_4 ceramics, with its unique force, electrical and optical properties, will show great potential in the application of nano devices. The performance of this kind of material is tested and evaluated. Based on this, this paper uses the first principle to simulate the electronic structure and optical properties of the Si_3N_4 model of doping and adsorption. The mechanical behavior of Si_3N_4 nanowires and films is simulated by molecular dynamics, which provides a theoretical basis for its further application. (1) use the first theory. The first principle simulates the electronic structure and optical properties of the Al, Ga, As and P doped Si_3N_4 systems. The results show that the band gaps of Al and Ga doped systems are 4.21 and 4.12e V respectively, while the band gap decreases and even disappeared after As and P doping, indicating that the doping of the two elements has great influence on the band gap of the system, and the formation of the four systems after doping can be based on Al. The order of As is increasing gradually, indicating that the Al doping system is more stable than the structure of other systems. The differential charge density diagram shows that the electron deletion around P increases after doping, indicating that the covalence of P and N is improved, while the electron deletion around As, Ga and Al decreases, which indicates that the covalence with N is reduced, and Al doping is reduced. The electron loss near the atom almost disappeared, and the electron cloud gradually approached the N atom, indicating that the covalently to the ionic conversion was the highest. The covalence order of the doped elements and N atoms was PAsGaAl, which was the same as the calculated distribution value; the static dielectric constant of the doping system decreased first and then increased with the increase of covalent radius. The As doping system is the lowest, and the dielectric loss in the low energy region is small, which is beneficial to the application on the photoelectric materials. (2) the electronic structure and optical properties of the adsorption of rare earth elements (Yb, Gd, Sc, Sm, La and Lu) are simulated by the first principle. The electron loss around the rare earth element is weaker and the ionic property of the bond with N There are different degrees of electron enrichment in the vicinity of Yb, Gd and Sc atoms, indicating that the distribution of the electron cloud increases. The order of the bond covalence with N is YbGdScSm (La, Lu), which is the same as that of the calculated values. The static dielectric constant and dielectric loss increase first, then decrease, and finally tend to be flat. Slowly, the Sc adsorption system is the lowest; the contribution of the [010] polarization direction to the total dielectric function is greater than the other two directions, indicating that the adsorption system shows a certain anisotropy; the reflectance of the 780~2500nm (0.496~1.59eV) near infrared light region, La, Yb, Gd and Sc adsorption system is lower than 6%, and Lu and Sm adsorption system can reach 20%, reflection degree phase 390~780nm (1.59~3.18eV) visible light region, the six adsorption systems have low absorption coefficient and reflectivity, indicating that the adsorption system has a "transparent" property, and 80~390nm (3.18~15.5eV) ultraviolet light region, the absorption system has a stronger absorption of light. "Barrier type" is presented. (3) a [001] orientation silicon nitride nanowire model is established by combining the first principle with molecular dynamics, and the compressive and tensile mechanical behavior of the nanowires of silicon nitride are simulated by molecular dynamics. The results show that the elastic limit of the material is independent of the nanoscale length to diameter ratio under tensile stress and mostly occurs in the nanowire ratio of nanoscale. When the strain is 0.05, the fracture stress of nanowires decreases with the increase of the ratio of length to diameter. At the same time, a large number of silicon silicon bonds and cluster like nitrogen atoms are produced in the deformation. Under the compressive stress, when the stress reaches the maximum, a new four silicon atom defect is produced at the center of the surface of the nanowire and the two symmetrical positions of the upper surface. The stress begins to decline. (4) the mechanical behavior of silicon nitride nanowires under the action of bending stress is simulated by molecular dynamics. The results show that the stress displacement curve is composed of quasi elastic phase and nonlinear stage. With the increase of the ratio of length to diameter (3:1,5:1 and 7:1), the fracture stress of the material decreases, and the bending stress follows the pressure head contact nanowires. The increase of the head displacement increases, when the initial fracture occurs, the bending stress rapidly decreases. Under the bending stress, the silicon silicon bond and the coordination number of 2 nitrogen atom defects can be observed. With the increase of the pressure head displacement, the coordination number of nitrogen is gradually changed to 0 and 1, the coordination number of silicon to 6 and 7. (5) is to establish the base silicon nitride. The number of nanometers, the number of atoms is 56007406 and 9464 respectively, is relaxed by the fastest descent method and the stable structure is obtained. The tensile behavior of the film is simulated by molecular dynamics. The results show that the film shows the nonlinear stress-strain relationship when the strain is less than 0.06 during the tensile loading process, and it is within the strain range of 0.06-0.09. The film shows the linear stress-strain relationship; more than 0.09, it becomes nonlinear stress strain relationship until the final fracture. The fracture stress and strain increase with the increase of the length of the film, the young's modulus changes little, the fracture stress in the direction of Y is larger than the X direction, and the fracture source in the tensile process comes from the N6h-Si bond. Along with the strain. As the rate increases, the position of the initial N6h-Si breaking defect is the same and the strain point is the same. It is not dependent on the strain rate. When the strain rate increases to 5.3 * 109s-1, the total separation strain value of the two part of the film is reduced to 0.113. The reason is that the broken bond defect of the N2c-Si bond appears during the break down extension of the N6h-Si key. With the increase of tensile temperature, the maximum tensile stress increases first and then decreases, and the corresponding tensile strain decreases.
【學位授予單位】：西安交通大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TQ174.1

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