本文選題：第一性原理　切入點(diǎn)：過(guò)渡金屬硼化物　出處：《上海海洋大學(xué)》2017年碩士論文

【摘要】：超硬材料具有硬度高、熔點(diǎn)高、耐磨損和化學(xué)惰性良好等優(yōu)異性質(zhì),在眾多前沿機(jī)械領(lǐng)域廣泛用做表面涂層、金屬鉆頭和切割刀具等。由于金剛石在高溫下會(huì)與氧、鐵等發(fā)生反應(yīng),故限制了其廣泛的應(yīng)用,突破現(xiàn)有超硬材料的局限,尋找更為穩(wěn)定的新型超硬材料已成為一種必然的趨勢(shì)。目前,最主流的方法就是將硼、氧、碳、氮等輕的元素加入到過(guò)渡金屬中,形成類似于金剛石中的共價(jià)鍵。本文通過(guò)以密度泛函理論為框架的第一性原理計(jì)算,以過(guò)渡金屬硼化物(WB_x、Mo B_x和Mn B_4)為對(duì)象,對(duì)其力學(xué)特性進(jìn)行系統(tǒng)研究,并闡述了力學(xué)特性的微觀機(jī)制,為設(shè)計(jì)新型超硬材料提供有力依據(jù)。(1)根據(jù)W-B和Mo-B體系結(jié)構(gòu)的結(jié)構(gòu)特點(diǎn),提出了TMB_3多型體這一結(jié)構(gòu),即由相同的AH單元根據(jù)不同的堆垛順序構(gòu)成的。我們列舉了15種最小周期的結(jié)構(gòu)相,并給出了詳細(xì)的原子位置坐標(biāo)、空間群以及堆垛序。根據(jù)第一性原理,我們計(jì)算了這15種結(jié)構(gòu)的形成能,得出TMB_3多型體可視為2H和3R結(jié)構(gòu)的組合,并且TMB_3多型體是穩(wěn)定的。最后,我們計(jì)算了15種結(jié)構(gòu)的體變模量、剪切模量和理論硬度,發(fā)現(xiàn)其力學(xué)特性幾乎不受晶胞內(nèi)金屬原子層數(shù)的影響。此外,TMB_3多型體由于結(jié)構(gòu)無(wú)序和聲子折疊等因素顯示出了極低的晶格熱導(dǎo)率。由此,我們進(jìn)一步提出可以合成多型體共存的多相固溶物來(lái)提高外在硬度,達(dá)到設(shè)計(jì)新型超硬材料的目的。(2)通過(guò)第一性原理對(duì)W-B和Mo-B體系的結(jié)構(gòu)、能量和動(dòng)力學(xué)特性進(jìn)行了全面地計(jì)算。研究結(jié)果表明,h P3-TMB_2相在能量和動(dòng)力學(xué)上是不穩(wěn)定的,對(duì)其計(jì)算出的結(jié)構(gòu)參數(shù)、力學(xué)特性以及XRD圖譜都明顯偏離了實(shí)驗(yàn)結(jié)果。相比之下,我們提出的TMB_3多型體在能量和動(dòng)力學(xué)上更穩(wěn)定,并且多型體的力學(xué)性能都很好的與實(shí)驗(yàn)數(shù)據(jù)吻合。因此總結(jié)出長(zhǎng)期以來(lái)所認(rèn)為的h P3-TMB_2相事實(shí)上是對(duì)一類在軸向有著納尺度有序但長(zhǎng)程無(wú)序的TMB_3多型體。更重要的是,我們展示了這種結(jié)構(gòu)和組成上的改變是通過(guò)減輕理想h P3結(jié)構(gòu)中強(qiáng)反鍵態(tài)相互作用的結(jié)果。因此,本工作不僅解決了一些長(zhǎng)期以來(lái)存在的關(guān)于W-B和Mo-B體系晶體結(jié)構(gòu)和力學(xué)特性的疑惑,而且還證實(shí)了一類多型體的存在。我們的研究結(jié)論為開(kāi)發(fā)功能性的超硬納米復(fù)合材料提供了新的思路。(3)通過(guò)第一性原理計(jì)算全面地研究了Mn B_4的三種磁態(tài)(NM、FM和AFM)的三種結(jié)構(gòu)(m S10、o P10和m P20)的相穩(wěn)定性、力學(xué)特性和電子結(jié)構(gòu)。通過(guò)計(jì)算否定了m S10結(jié)構(gòu)是Mn B_4的結(jié)構(gòu)相,但m P20和o P10結(jié)構(gòu)是可行的。更重要的是,我們發(fā)現(xiàn)了Mn B_4有著伴隨磁結(jié)構(gòu)與電子的相變,在438K(824K)時(shí)經(jīng)受了從低對(duì)稱性的NM m P20絕緣體到高對(duì)稱性的FM(AFM)o P10金屬的轉(zhuǎn)變。根據(jù)Peierls扭曲,通過(guò)打破結(jié)構(gòu)簡(jiǎn)并性來(lái)使NM m P20相穩(wěn)定,而根據(jù)Stoner機(jī)制,通過(guò)解除自旋簡(jiǎn)并來(lái)使FM(AFM)o P10相穩(wěn)定。因此,這種獨(dú)特的由絕緣體向金屬的轉(zhuǎn)化,以及高剛度、高硬度等特性使得Mn B_4在技術(shù)應(yīng)用方面潛力巨大,比如在極端條件下的熱電開(kāi)關(guān)和場(chǎng)效應(yīng)晶體管。
[Abstract]:Super hard materials with high hardness, high melting point, abrasion resistance and excellent properties such as good chemical inertness, widely used as a surface coating in many advanced machinery, metal drilling and cutting tools. The diamond in high temperature and oxygen, iron reaction, which limits its application widely, break through the limitations of the existing super hard materials, looking for a more stable new superhard materials has become an inevitable trend. At present, most mainstream is boron, oxygen, carbon, nitrogen and other light elements added to the transition metal, the formation of a covalent bond is similar to that of diamond in. Through the calculation based on density functional theory as the first principle frame, with transition metal boride (WB_x, Mo B_x and Mn B_4) as the object, the systematic study on the mechanical properties, and discusses the microscopic mechanism of the mechanical properties, provide a strong basis for the design of new superhard materials. (1) according to W-B and M The structure characteristics of o-B system structure, puts forward the structure of TMB_3 POLYTYPOIDS, AH from the same unit according to the different composition of the stacking sequence. We list structure of 15 minimum period, and gives the detailed atomic coordinates, space group and stacking sequence. According to the principle, we calculate the formation of these 15 kinds of structure can be obtained, TMB_3 polytypoid can be considered as a combination of 2H and 3R structure, and the TMB_3 polytype is stable. Finally, we calculate the structure of the 15 kinds of bulk modulus, shear modulus and hardness theory, found that the mechanical properties of almost not affected by the metal atoms in the cell the number of TMB_3 POLYTYPOIDS. In addition, due to the disordered structure and phonon folding factors such as showing the lattice extremely low thermal conductivity. Therefore, we further propose a synthesis polytype coexistence of multiphase solids to improve external hardness, achieve the design of a new type of super Hard material. (2) the structure of the first principle of the W-B and Mo-B system, energy and kinetic characteristics of the calculation. The results show that the H phase in P3-TMB_2 energy and dynamics is unstable, the calculated structural parameters on its mechanical properties, and deviated from the XRD map the results of the experiment. In contrast, we propose TMB_3 polytypoid more stable in energy and dynamics, and the mechanical properties of polytypes are good agreement with the experimental data. It summarizes that the long h P3-TMB_2 phase is in fact of a class in the axial direction with TMB_3 nano scale ordered but long rangedisorder polytypes. More importantly, we show that changes in structure and composition is through reducing the ideal h P3 structure of strong anti bonding state interaction results. Therefore, this work not only solves some long existing in the 鍏充簬W-B鍜孧o-B浣撶郴鏅朵綋緇撴瀯鍜屽姏瀛︾壒鎬х殑鐤戞儜,鑰屼笖榪樿瘉瀹炰簡(jiǎn)涓，

本文編號(hào)：1700596