本文選題：第一性原理 + 超硬材料��； 參考：《西南大學(xué)》2015年碩士論文

【摘要】：超硬材料以其優(yōu)異的高硬度、高彈性模量、良好的耐磨性和不可壓縮性,在工程機(jī)械、切削加工、航空航天、礦物開采、耐磨涂層以及其它工業(yè)領(lǐng)域有著廣泛的應(yīng)用。眾所周知此前已知的應(yīng)用最廣泛的最硬和次硬材料分別是金剛石(維氏硬度Hv=96GPa,下同)和立方氮化硼(c-BN,Hv=63 GPa)。然而金剛石較低的熱穩(wěn)定性和較差的化學(xué)穩(wěn)定性很大程度上限制了它的應(yīng)用空間,而熱穩(wěn)定性相對(duì)較高的立方氮化硼(c-BN),雖然是硬度僅次于金剛石的物質(zhì),卻遠(yuǎn)小于金剛石。因此工業(yè)和技術(shù)的需要促使人們繼續(xù)尋找硬度高而且穩(wěn)定性也很好的新型超硬材料,B-C-N系化合物結(jié)合金剛石的高硬度和c-BN的熱穩(wěn)定性有望成為新型超硬材料而受到廣泛關(guān)注。本文基于Luo等研究的bc-結(jié)構(gòu)的超硬BC2N材料的基礎(chǔ)上,設(shè)計(jì)和構(gòu)建了bc-BCN和bc-BC4N,并利用基于密度泛函理論的第一性原理的方法,研究了這幾種超硬材料(包括bc-BCN,bc-BC2N,和bc-BC4N,文中統(tǒng)稱為BCxN,或者BCN,BC2N,和BC4N)的性質(zhì)(彈性常數(shù),彈性模量,彈性各向異性,硬度,理想強(qiáng)度和高溫下的最小熱導(dǎo)率,電子結(jié)構(gòu)和光學(xué)性質(zhì)等),同時(shí)還計(jì)算了金剛石和c-BN的結(jié)果并進(jìn)行了對(duì)比。系統(tǒng)的分析了三種正交系bc-結(jié)構(gòu)的BCxN的物理性能,希望能夠?yàn)锽CxN在實(shí)驗(yàn)上的研究和工業(yè)上的應(yīng)用提供些理論參考。本文第三章給出了BCN、BC2N和BC4N的晶體結(jié)構(gòu),并繪制和分析電荷密度圖和電荷差分密度圖。發(fā)現(xiàn)圍繞著N原子的電荷密度最高,B原子周圍的電荷密度則最低,電荷差分密度圖顯示電荷從N原子轉(zhuǎn)移到鄰近的C-C鍵,而B-N鍵周圍的電荷則傾向于轉(zhuǎn)向N原子。對(duì)BCxN的力學(xué)性質(zhì)研究表明,BCN、BC2N和BC4N都是偏脆性的材料,其中BCN有相對(duì)較好的延展性,屈服強(qiáng)度也較好,其次是BC2N不過BC4N的彈性模量最高,且硬度也是最高的,達(dá)到68GPa (LDA方法計(jì)算,下同)。BC2N次之,硬度為65.2 GPa,BCN的硬度值最小。此外對(duì)BCxN的理想強(qiáng)度進(jìn)行了分析,發(fā)現(xiàn)BC4N剪切應(yīng)力值最小的方向是(110)[001],應(yīng)力值為63GPa,與c-BN的理想強(qiáng)度值相差無幾。力學(xué)各向異性研究表明,BCN、BC2N和BC4N都是彈性各向異性的,三者的各向異性程度隨著BC4N→BC2N→BCN的順序增加,因此綜合分析認(rèn)為BC4N力學(xué)性質(zhì)最佳。高溫下最小熱導(dǎo)率的研究表明,硬度值較高的晶體結(jié)構(gòu),其熱導(dǎo)率也相對(duì)較高。數(shù)據(jù)顯示BCN的熱導(dǎo)率最低,而BC4N的熱導(dǎo)率值最大。且沿著BCN、BC2N和BC4N晶體材料的不同晶向,它們的熱導(dǎo)率都是各向異性的。在第六章我們研究了BCxN的電子結(jié)構(gòu)和光學(xué)性質(zhì),發(fā)現(xiàn)BCN、BC2N和BC2N的能隙都非常小,分別是0.819eV,0.595eV和0.340eV,且導(dǎo)帶的最低點(diǎn)和價(jià)帶的最高點(diǎn)都不在同一布里淵區(qū)對(duì)稱點(diǎn),屬于間接帶隙半導(dǎo)體。BCxN的光學(xué)性質(zhì)非常相近,在可見光區(qū),三種晶體的吸收系數(shù)非常小,反射率也很低,折射率在2.0～2.5之間,且BC4N的折射率要相對(duì)較高一點(diǎn),其次是BC2N。
[Abstract]:Super hard materials have been widely used in engineering machinery, cutting, aerospace, mineral mining, wear-resistant coatings and other industrial fields due to their excellent high hardness, high elastic modulus, good wear resistance and incompressibility. It is well known that the most widely used hard and subhard materials are diamond (Vickers hardness Hvn 96GPA) and cubic boron nitride (CBN) c-BN Hvn 63 GPA. However, the low thermal stability and poor chemical stability of diamond limit its application space to a great extent, and the cubic boron nitride (CBN), which has relatively high thermal stability, is much less than diamond in hardness, although it is second only to diamond in hardness. Therefore, the need of industry and technology urges people to continue to look for new superhard materials with high hardness and good stability. The high hardness of B-C-N system compounds and the thermal stability of c-BN are expected to become new superhard materials and attract wide attention. In this paper, we design and construct bc-BCN and bc-BC4N based on bc- structure superhard BC2N materials studied by Luo et al, and use the first principle method based on density functional theory. The properties (elastic constant, modulus of elasticity, elastic anisotropy, hardness, ideal strength and minimum thermal conductivity at high temperature) of these superhard materials (including bc-BCN-bc-BC2N and bc-BC4N, collectively known as BCxN, or BCN-BC2N, and BC4N) have been studied. The electronic structure and optical properties of diamond are also calculated and compared with those of c-BN. The physical properties of BCxN with three orthogonal systems are systematically analyzed. It is hoped that this paper can provide some theoretical references for the experimental research and industrial application of BCxN. In the third chapter, the crystal structures of BCN-BC2N and BC4N are given, and the charge-density diagram and charge-differential density diagram are drawn and analyzed. It is found that the charge density around the N atom is the highest and the charge density around the B atom is the lowest. The charge differential density diagram shows that the charge transfers from the N atom to the adjacent C-C bond, while the charge around the B-N bond tends to shift to the N atom. The mechanical properties of BCxN show that both BCN-BC2N and BC4N are brittle materials, among which BCN has relatively better ductility and better yield strength, and then BC2N has the highest modulus of elasticity and the highest hardness, which is in accordance with the 68GPa method. The hardness of 65.2 GPA BCN was the lowest. In addition, the ideal strength of BCxN is analyzed. It is found that the direction of minimum shear stress of BC4N is 110) [001], and the stress value is 63 GPA, which is similar to the ideal strength of c-BN. The study of mechanical anisotropy shows that both BCN-BC2N and BC4N are elastic anisotropy, and their anisotropy increases with the order of BC4N BC2N BCN. Therefore, the comprehensive analysis shows that the mechanical properties of BC4N are the best. The study of the minimum thermal conductivity at high temperature shows that the thermal conductivity of the crystal structure with higher hardness is also relatively high. The data show that BCN has the lowest thermal conductivity, while BC4N has the highest thermal conductivity. The thermal conductivities of BCN-BC2N and BC4N crystal materials are anisotropic. In chapter 6, we study the electronic structure and optical properties of BCxN. It is found that the band gaps of BCN-BC2N and BC2N are very small, which are 0.819 EV ~ 0.595eV and 0.340 EV, respectively, and the lowest point of conduction band and the highest point of valence band are not symmetrical in the same Brillouin zone. The optical properties of the indirect band-gap semiconductor. BCxN are very similar. In the visible region, the absorption coefficient of the three crystals is very small, the reflectivity is very low, the refractive index is between 2.0 and 2.5, and the refractive index of BC4N is relatively high, followed by BC2N.
【學(xué)位授予單位】：西南大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2015
【分類號(hào)】：TQ163

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