本文選題：第一性原理計(jì)算 + 負(fù)熱膨脹��； 參考：《鄭州大學(xué)》2016年博士論文

【摘要】：隨著新技術(shù)、新方法、新手段的出現(xiàn),負(fù)熱膨脹材料作為材料科學(xué)領(lǐng)域的新寵,得到了越來(lái)越多的關(guān)注。由于目前已有的負(fù)熱膨脹材料大多為絕緣體,雖有大量的工作采用摻雜、與金屬?gòu)?fù)合等手段改善其導(dǎo)電性能,但金屬性的缺失仍然極大地限制了其在電、熱、機(jī)械等領(lǐng)域的應(yīng)用。本論文著眼于負(fù)熱膨脹材料的導(dǎo)電性能,采用基于密度泛函理論的第一性原理方法對(duì)幾種負(fù)熱膨脹材料的反常熱膨脹機(jī)理進(jìn)行研究,并與相關(guān)的絕緣負(fù)熱膨脹材料對(duì)比,去解釋負(fù)熱膨脹特性與導(dǎo)電性的內(nèi)在聯(lián)系。本論文的研究?jī)?nèi)容和結(jié)論如下:1.實(shí)驗(yàn)報(bào)道了立方結(jié)構(gòu)的Re O_3不僅具有負(fù)熱膨脹特性,而且其優(yōu)異的導(dǎo)電性可與Ag、Cu媲美。借助于第一性原理和準(zhǔn)諧近似,我們的計(jì)算發(fā)現(xiàn):倒空間M和R點(diǎn)處的格林艾森參數(shù)具有負(fù)向極大絕對(duì)值。與此對(duì)應(yīng)的低頻光學(xué)支的振動(dòng)引起Re O_6剛性八面體耦合轉(zhuǎn)動(dòng),導(dǎo)致體積受熱收縮。O-2p和Re-t2g電子態(tài)的雜化使其呈現(xiàn)優(yōu)良的導(dǎo)電特性。實(shí)驗(yàn)還指出與其同為立方結(jié)構(gòu)的絕緣體Sc F3,呈現(xiàn)出更強(qiáng)烈的負(fù)熱膨脹特性,負(fù)熱膨脹系數(shù)約是Re O_3的十倍。計(jì)算發(fā)現(xiàn)二者負(fù)熱膨脹成因一樣,均為M和R點(diǎn)的格林艾森參數(shù)出現(xiàn)負(fù)極值,以及低頻光學(xué)支引起的剛性八面體耦合轉(zhuǎn)動(dòng)。熱脹系數(shù)差別巨大歸因于電子結(jié)構(gòu)的不同。Re-O共價(jià)鍵強(qiáng)于Sc-F離子鍵,而且按照M和R點(diǎn)振動(dòng)模式移動(dòng)原子,發(fā)現(xiàn)O原子移位勢(shì)壘比F要陡。這均使得Re O6八面體的扭轉(zhuǎn)變得困難,導(dǎo)致熱脹系數(shù)偏小。2.立方相Re O_3可認(rèn)為是A-位缺失的ABO_3型鈣鈦礦結(jié)構(gòu)。A-位原子缺失所致的體心充足空位使得Re O_3很容易出現(xiàn)相變。隨著壓強(qiáng)增加,Re O_3呈現(xiàn)的一系列相變過(guò)程中,四方相P4/mbm是否存在,一直存在爭(zhēng)議。在本章節(jié)中我們利用第一性原理計(jì)算,借助于聲子譜振動(dòng)曲線、形成熱焓和動(dòng)力學(xué)能量勢(shì)壘,澄清了從立方Pm-3m到P4/mbm相變的可能性,并定量給出轉(zhuǎn)變壓強(qiáng)是5.0 kbar。雖然Re O_3有多種結(jié)構(gòu)相,但只有立方Pm-3m相存在負(fù)熱膨脹現(xiàn)象。通過(guò)計(jì)算不同壓強(qiáng)值下的色散關(guān)系和對(duì)晶體結(jié)構(gòu)分析,立方Pm-3m相在5.0 kbar時(shí),倒空間M點(diǎn)的M3振動(dòng)模出現(xiàn)虛頻,結(jié)構(gòu)不再穩(wěn)定。M3振動(dòng)模是立方相出現(xiàn)負(fù)熱膨脹現(xiàn)象的原因,M3模的軟化對(duì)應(yīng)于O原子從x=0.2500移位至x=0.2401,出現(xiàn)相變,負(fù)熱膨脹現(xiàn)象消失。3.過(guò)渡金屬鑭系碳化物,不僅具有良好的機(jī)械性能,還是潛在的超導(dǎo)材料。實(shí)驗(yàn)發(fā)現(xiàn)四方相La C2在低溫區(qū)呈現(xiàn)超導(dǎo)電性,還表現(xiàn)出各向異性的負(fù)熱膨脹特性。在本章節(jié)中,借助于第一性原理計(jì)算和準(zhǔn)諧近似,我們模擬再現(xiàn)了跨越超導(dǎo)溫區(qū)的負(fù)熱膨脹現(xiàn)象。計(jì)算結(jié)論顯示c-軸的負(fù)熱膨脹系數(shù)和溫區(qū)均與實(shí)驗(yàn)吻合一致。計(jì)算結(jié)果還預(yù)言了可能由于實(shí)驗(yàn)儀器精度不夠而沒(méi)能測(cè)出的a-軸的負(fù)熱膨脹現(xiàn)象。通過(guò)計(jì)算格林艾森參數(shù)和因子群分析,布里淵區(qū)中心的Eu和Eg模,邊界M和Z點(diǎn)的三支振動(dòng)模對(duì)應(yīng)負(fù)格林艾森參數(shù)值,C-C二聚體的橫向振動(dòng)引起晶格La……La有效距離縮短,導(dǎo)致負(fù)熱膨脹現(xiàn)象。4.實(shí)驗(yàn)發(fā)現(xiàn),隨著溫度升高,鈣鈦礦Bi Ni O_3從低溫三斜相轉(zhuǎn)變?yōu)楦邷卣幌嗟倪^(guò)程中,出現(xiàn)了顯著的負(fù)熱膨脹現(xiàn)象,而且還伴隨有絕緣體→導(dǎo)體和反鐵磁→鐵磁的轉(zhuǎn)變。這使得Bi Ni O_3的負(fù)熱膨脹現(xiàn)象貌似與磁、電、熱振動(dòng)均相關(guān),使其負(fù)熱膨脹機(jī)理更復(fù)雜。本章節(jié)從第一性原理出發(fā),通過(guò)計(jì)算分析電、磁和晶體結(jié)構(gòu),證實(shí):高溫正交相中,由于Bi-6s與O-2p電子態(tài)的雜化,使得費(fèi)米能級(jí)處出現(xiàn)雜化峰,表現(xiàn)為導(dǎo)電性,磁矩為1.732μB/Ni。通過(guò)分析電子態(tài)密度,發(fā)現(xiàn)了Ni與Bi金屬間電荷轉(zhuǎn)移。BVSs計(jì)算表明兩個(gè)相的氧化態(tài)分別是Bi+3Ni+3O_3和Bi+30.5 Bi+50.5Ni+2O_3.電荷態(tài)密度圖顯示低溫相為G-型反鐵磁絕緣體,而且伴隨著金屬間電荷轉(zhuǎn)移,體系從小體積正交相變成大體積三斜相,出現(xiàn)負(fù)熱膨脹行為。我們的理論計(jì)算很好的解釋和證實(shí)了金屬間電荷轉(zhuǎn)移是Bi Ni O_3呈現(xiàn)負(fù)熱膨脹的成因,為豐富負(fù)熱膨脹的機(jī)理解釋提供了新內(nèi)容。
[Abstract]:With the emergence of new technology, new methods and new means, negative thermal expansion materials have been paid more and more attention as the new favorite in the field of material science. Because most of the existing negative thermal expansion materials are insulators, although a large number of work is used to improve their electrical properties, such as doping and metal complex, the loss of metal is still great. Its application in electrical, thermal and mechanical fields is restricted. This paper focuses on the conductivity of negative thermal expansion materials. The first principle method based on density functional theory is used to study the anomalous thermal expansion mechanism of several negative thermal expansion materials and to compare with the related negative thermal expansion materials to explain the negative thermal expansion characteristics and the characteristics of negative thermal expansion. The contents and conclusions of this paper are as follows: 1. the experiment reported that the cubic structure of Re O_3 not only has negative thermal expansion, but its excellent conductivity is comparable to that of Ag and Cu. With the help of the first principle and quasi harmonic approximation, our calculation shows that the Green Eisen parameter at the inverted space M and R points has a negative pole. Large absolute value. The vibration of the corresponding low frequency optical branch caused by the coupling rotation of the Re O_6 rigid eight body, resulting in the hybrids of the.O-2p and Re-t2g electronic states of the volume heating shrinkage to show excellent conductivity. The experiment also pointed out that the Sc F3 of the insulator with the same cubic structure showed a more intense negative thermal expansion characteristic and the negative thermal expansion coefficient. It is about ten times that of Re O_3. It is found that the negative thermal expansion of the two is the same as the Green Eisen parameter of M and R points, and the rigid eight body coupling rotation caused by the low frequency optical branch. The difference of thermal expansion coefficient is greatly attributed to the different.Re-O covalent bond of the electronic structure than the Sc-F ion bond, and the vibration mode of the M and R points is moved. It is found that the O atom shift barrier is steeper than that of F, which makes the torsion of the Re O6 eight surface difficult, resulting in a small.2. cubic phase Re O_3, which is considered to be the ABO_3 type of the ABO_3 type perovskite structure of the A- bit loss of the.A- bit atom, which makes the Re O_3 easily appear in the phase transition. In a series of phase transitions, there is always controversy whether the Quartet phase P4/mbm exists. In this chapter, we use the first principle to calculate the enthalpy and kinetic energy barrier by means of the phonon spectrum vibration curve, clarify the possibility of the phase transition from cubic Pm-3m to P4/mbm, and give a quantitative transition pressure of 5 kbar. although Re O_3 is much more. But only cubic Pm-3m phase has negative thermal expansion. By calculating the dispersion relation and crystal structure analysis under different pressure values, the M3 vibration mode of the M point in the inverted space appears imaginary frequency at 5 kbar, and the structure is no longer stable the.M3 vibration mode is the cause of the negative thermal expansion of the cubic phase, and the softening of the M3 mode corresponds to the.M3. O atoms shift from x=0.2500 to x=0.2401 and appear phase transition. Negative thermal expansion disappears.3. transition metal lanthanide carbides, not only with good mechanical properties, but also potential superconducting materials. The experiment found that the tetragonal phase La C2 presents superconductivity at low temperature, and shows the negative thermal expansion of anisotropy. In this chapter, with the help of the first part We simulate and reproduce the negative thermal expansion of the superconducting zone. The calculation results show that the negative thermal expansion coefficient and the temperature zone of the c- axis are in agreement with the experiment. The results also predict the negative thermal expansion of the a- axis, which may not be measured by the precision of the experimental instrument. The Eu and Eg modes of the Brillouin center, the Eu and Eg modes of the Brillouin center and the three vibrational modes at the boundary M and Z point to the negative Green Eisen parameter value. The transverse vibration of the C-C two polymer causes the effective distance of the lattice La.La to be shortened, and the negative thermal expansion is caused by the.4. experiment. The perovskite Bi Ni O_3 changes from the low temperature three oblique phase to the high temperature as the temperature rises. In the process of intersection, there is a significant negative thermal expansion, and also with the transformation of insulators, conductors and antiferromagnetism to ferromagnetic. This makes the negative thermal expansion of Bi Ni O_3 appear to be related to magnetic, electric and thermal vibration, and the mechanism of its negative thermal expansion is more complex. From the first principle, the analysis of electricity, Ci Hejing The body structure shows that, in the orthogonal phase of high temperature, the hybrid peak of the Fermi energy is caused by the hybridization of the electronic state of Bi-6s and O-2p. The magnetic moment is 1.732 u B/Ni. by analyzing the electron state density. The calculation of the charge transfer.BVSs between the Ni and Bi metals shows that the oxidation states of the two phases are Bi+3Ni+3O_3 and Bi+30.5 Bi+50.5Ni+2O_3., respectively. The charge density map shows that the low temperature phase is G- antiferromagnetic insulator, and with the charge transfer between metals, the system becomes a large volume three diagonal phase from small volume quadrature phase to negative thermal expansion. Our theoretical calculation explains and confirms that the charge transfer between metals is the cause of negative thermal expansion of Bi Ni O_3, which is rich in negative heat. The explanation of the mechanism of expansion provides new content.
【學(xué)位授予單位】：鄭州大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類(lèi)號(hào)】：O469;O551.3

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