發(fā)布時(shí)間：2018-03-26 08:07

  本文選題：熱電材料　切入點(diǎn)：MgAgSb　出處：《浙江大學(xué)》2017年博士論文

【摘要】：MgAgSb合金是近年來新開發(fā)的近室溫高性能熱電材料,具有組成元素儲(chǔ)量豐富的優(yōu)點(diǎn)。但是仍存在許多問題,譬如由于存在多晶型轉(zhuǎn)變以及Mg的高活性,高性能純相α-MgAgSb難以獲得;電學(xué)性能尚未優(yōu)化,MgAgSb基熱電材料具有較低熱導(dǎo)率然而內(nèi)在物理機(jī)制尚不明確;高溫相理論預(yù)測(cè)與實(shí)驗(yàn)結(jié)果不相符等。本文重點(diǎn)研究純相α-MgAgSb的制備,電學(xué)性能優(yōu)化,本征低熱導(dǎo)率的內(nèi)在機(jī)理及不同晶型MgAgSb的晶體結(jié)構(gòu)與熱電性能,取得如下成果:(1)通過控制制備工藝,成功獲得純相α-MgAgSb,并通過In摻雜調(diào)節(jié)體系的載流子濃度并研究了該體系基本物理特性。利用第一性原理計(jì)算了 α-MgAgSb的能帶結(jié)構(gòu),其為窄禁帶間接帶隙半導(dǎo)體,禁帶寬度Eg～0.26eV,價(jià)帶頂具有高能谷簡(jiǎn)并度NV=8。采用SPB模型計(jì)算得到該體系的態(tài)密度有效質(zhì)量m*= 2.0me,單帶有效質(zhì)量mb*=0.5me。利用變形勢(shì)理論結(jié)合實(shí)驗(yàn)結(jié)果得到該體系的變形勢(shì)E=20.0eV。In摻雜的α-MgAgSb在525K獲得最高zT～1.1,分析得到該體系的最優(yōu)載流子濃度為8～9×1019cm-3。(2)基于晶格動(dòng)力學(xué)和聲子傳輸理論,從化學(xué)鍵角度解釋了 α-MgAgSb本征低晶格熱導(dǎo)率的起因,即全局和局域弱化學(xué)鍵共同存在是非籠狀結(jié)構(gòu)Nowotny-Juza化合物α-MgAgSb本征低晶格熱導(dǎo)的物理機(jī)制。低價(jià)電子數(shù)(VEC=8)引起α-MgAgSb全局弱鍵,表現(xiàn)為低聲速;α-MgAgSb扭曲立方結(jié)構(gòu)中獨(dú)特的Mg-Ag-Sb三中心局域弱鍵引起低頻光學(xué)支的共振散射。低溫?zé)嵝阅軠y(cè)量和模型分析也證明愛因斯坦低頻振動(dòng)模的存在。這類分級(jí)化學(xué)鍵特征可以推廣到其他Nowotny-Juza化合物,晶格動(dòng)力學(xué)計(jì)算表明Nowotny-Juza化合物CdCuSb和CaAgSb也具有低聲速和低頻光學(xué)聲子的特點(diǎn)。(3)通過變溫高分辨同步輻射粉末X射線衍射(SR-PXRD)分析了 MgAgSb的多晶型晶體結(jié)構(gòu)特征與相變。Sb和Ag3Sb雜質(zhì)出現(xiàn)在400-600 K的高溫下,含量隨溫度升高而升高。MgAgSb結(jié)構(gòu)對(duì)稱性隨溫度升高。兩種MgAgSb多晶型物(β-MgAgSb和γ-MgAgSb)在700K下共存,但在高溫(800-1000K)下僅發(fā)現(xiàn)立方結(jié)構(gòu)γ-MgAgSb相。y-MgAgSb相的結(jié)晶度隨溫度增加而降低,樣品在1000 K時(shí)晶體僅為43.8%。雜質(zhì)液化很大程度上影響了 y-MgAgSb的穩(wěn)定性。γ-MgAgSb的高分辨率粉末數(shù)據(jù)可以使用Ⅰ型(Mg,Ag和Sb分別占在4b,4c和4a位置)和Ⅱ型(Mg,Ag和Sb分別占在4a,4b和4c位置)的half-Heusler晶體結(jié)構(gòu)模型。對(duì)精修的結(jié)構(gòu)因子進(jìn)行的最大熵法表明,Ⅱ型給出了更多的物理聲學(xué)電子密度。熱電性能測(cè)量γ-MgAgSb具有半導(dǎo)體特征,與Ⅱ型結(jié)構(gòu)模型結(jié)果更加相符合。
[Abstract]:MgAgSb alloy is a newly developed thermoelectric material with high performance near room temperature in recent years, which has the advantage of abundant element reserves. However, there are still many problems, such as the existence of polycrystalline transformation and the high activity of mg, which makes it difficult to obtain high performance pure phase 偽 -MgAgSb. The electrical properties of MgAgSb based thermoelectric materials have not been optimized but the intrinsic physical mechanism is not clear. The theoretical prediction of high temperature phase does not accord with the experimental results. In this paper, the preparation of pure phase 偽 -MgAgSb, the optimization of electrical properties, etc. The intrinsic mechanism of intrinsic low thermal conductivity and the crystal structure and thermoelectric properties of different crystalline MgAgSb have been obtained as follows: 1) by controlling the preparation process, The pure phase 偽 -MgAgSb was successfully obtained, and the carrier concentration of the system was adjusted by in doping and the basic physical properties of the system were studied. The band structure of 偽 -MgAgSb was calculated by first principle, which is a narrow gap band gap semiconductor. The band gap is 0.26eV, and the valence band top has high energy valley degeneracy (NVN 8). The effective mass of the system is calculated by SPB model, and the effective mass of the system is calculated by SPB model, and the effective mass of the system is 0.5me. by using the theory of variable situation and the experimental results, the variable state E=20.0eV.In of the system is obtained. The highest concentration of 偽 -MgAgSb was obtained at 525K, and the optimum carrier concentration of the system was 89 脳 1019cm-3.2) based on lattice dynamics and phonon transport theory. The cause of the intrinsic low lattice thermal conductivity of 偽 -MgAgSb is explained from the point of view of chemical bond, that is, the global and local weak chemical bonds exist together, the physical mechanism of 偽 -MgAgSb intrinsic low lattice thermal conductivity of non-cage structure Nowotny-Juza compound 偽 -MgAgSb is explained. The global weak bond of 偽 -MgAgSb is induced by low valence electron number (VEC8). The resonance scattering of the low frequency optical branch caused by the unique local weak bond of Mg-Ag-Sb in the twisted cubic structure of 偽 -MgAgSb. The low temperature thermal performance measurement and model analysis also proved the existence of Einstein's low frequency vibration mode. Chemical bond characteristics can be extended to other Nowotny-Juza compounds, Lattice dynamics calculations show that Nowotny-Juza compounds CdCuSb and CaAgSb also have the characteristics of low sound velocity and low frequency optical phonons. The crystal structure and phase transition of MgAgSb have been analyzed by means of high resolution synchrotron radiation powder X-ray diffraction (SR-PXRD). The impurities of Sb and Ag3Sb appear at 400-600K high temperature. The content of MgAgSb polycrystalline compounds (尾 -MgAgSb and 緯 -MgAgSb) coexisted at 700K, but the crystallinity of cubic structure 緯 -MgAgSb phase 路y-MgAgSb phase decreased with the increase of temperature. The crystal size of the sample at 1000 K is only 43.8%. The liquefaction of impurity greatly affects the stability of y-MgAgSb. The high resolution powder data of 緯 -MgAgSb can be used in the positions of 4b 4c and 4a for 緯 -MgAgSb) and 4a4b and 4a for type 鈪，

本文編號(hào)：1667088