本文選題：第一性原理計(jì)算 + FeMnP_(1-x)T_x(T=Si�。� 參考：《內(nèi)蒙古師范大學(xué)》2017年碩士論文

【摘要】：本文主要研究了兩個(gè)不同的磁性體系的相結(jié)構(gòu)、力學(xué)性質(zhì)、磁有序性和熱力學(xué)性質(zhì).論文主要內(nèi)容分為兩個(gè)部分:第一部分是Fe_2P型FeMnP_(1-x)T_x(T=Si,Ga,Ge)化合物的相結(jié)構(gòu)、力學(xué)性質(zhì)和熱力學(xué)性質(zhì)的理論研究;第二部分是稀釋摻雜的Fe_(14)Tm_2(Tm=Cr,Mn,Co,Ni)合金的結(jié)構(gòu)與磁性的關(guān)聯(lián)、力學(xué)性質(zhì)和間隙(C和N)原子對(duì)合金四方相穩(wěn)定性的影響.作為磁制冷工質(zhì)材料,FeMnP_(1-x)Si_x化合物在反復(fù)磁化和熱加載下工作,該化合物在有限溫度下應(yīng)具備良好的機(jī)械性能和力學(xué)穩(wěn)定性.實(shí)驗(yàn)發(fā)現(xiàn)FeMnP_(1-x)Si_x化合物的機(jī)械性能較差,然而到目前為止,針對(duì)此類化合物的機(jī)械性能、韌脆性的研究較少.本文從理論上預(yù)測(cè)了Ga原子替代P而形成的Fe_2P型FeMnP_(1-x)Ga_x化合物可能具有良好的機(jī)械性能和較大的磁卡效應(yīng).以密度泛函理論、線彈性理論、經(jīng)典統(tǒng)計(jì)理論為基礎(chǔ),采用VASP、Phonopy、Gibbs等軟件包計(jì)算研究了Fe MnP_(1-x)T_x(T=Si,Ga,Ge)系列化合物的總能量、磁矩、形成焓、電子結(jié)構(gòu)、力學(xué)性質(zhì)和熱力學(xué)性質(zhì).首先,對(duì)比研究了Fe_2P型FeMnP_(1-x)Ga_x化合物可能形成的六方相和體心正交相的穩(wěn)定性.計(jì)算結(jié)果表明化合物的六方相和體心正交相的總能量、磁矩和形成焓非常接近且形成焓均為負(fù)值.彈性常數(shù)的計(jì)算結(jié)果表明,兩個(gè)相均為力學(xué)穩(wěn)定相,而且彈性模量的計(jì)算可知鐵磁(FM)態(tài)六方相的呈韌性,體心正交相呈脆性.計(jì)算兩相的聲子譜發(fā)現(xiàn),六方相是動(dòng)力學(xué)穩(wěn)定的,而體心正交相的聲子色散關(guān)系中存在明顯的虛頻,動(dòng)力學(xué)不穩(wěn)定.進(jìn)一步研究了六方相FeMn P1-xGax化合物的力學(xué)性能,并與FeMnP_(1-x)T_x(T=Si,Ge)化合物進(jìn)行了對(duì)比.為了模擬順磁態(tài)化合物的力學(xué)性能,我們采用反鐵磁(AFM)模型計(jì)算了化合物體系的彈性常數(shù).計(jì)算結(jié)果表明六方相FeMnP_(1-x)T_x(T=Si,Ga,Ge)化合物在力學(xué)上穩(wěn)定,FM態(tài)Fe MnP_(0.67)T_(0.33)(T=Si,Ga,Ge)化合物呈現(xiàn)韌性,FeMnP_(0.67)Ga_(0.33)的韌性最好,FeMnP0.67Si0.33的韌性相對(duì)較差.AFM態(tài)FeMnP0.33T0.67(T=Si,Ga,Ge)化合物呈現(xiàn)韌性,FeMnP0.33Ge0.67的韌性最好,FeMnP_(0.33)Ga_(0.67)的韌性相對(duì)較差.FM態(tài)FeMnP0.33T0.67(T=Si,Ga,Ge)和AFM態(tài)FeMnP_(0.67)T_(0.33)(T=Si,Ga,Ge)化合物處在韌脆性的臨界狀態(tài).原子占位的無(wú)序性可能會(huì)改善化合物的韌性.彈性常數(shù)隨不同摻雜原子的變化規(guī)律可通過電子結(jié)構(gòu)分析和Force theorem解釋.最后,通過對(duì)比鐵磁態(tài)和順磁態(tài)的Gibbs自由能G(P,T)確定了FeMnP_(1-x)Ga_x化合物的居里溫度,計(jì)算了體系經(jīng)歷鐵磁順磁相變時(shí)的熱容、熵變以及熱膨脹系數(shù),并與FeMnP_(1-x)Ge_x化合物進(jìn)行了對(duì)比.化合物的熱容在相變時(shí)產(chǎn)生躍變,體系經(jīng)歷了一級(jí)相變.FeMnP_(0.67)Ga_(0.33)化合物的居里溫度為Tc=500K,此時(shí)的熵變?yōu)?35)S=69.34 J K~(-1) kg~(-1),而FeMn P0.67Ge0.33在Tc=590K時(shí)的熵變?yōu)?35)S=66.69 J K~(-1) kg~(-1).具有高Ga組份的FeMnP_(0.33)Ga_(0.67)化合物的Tc=770K,體系的熵變?yōu)?35)S=89.26 J K~(-1) kg~(-1).總之,FeMnP_(1-x)Ga_x化合物的六方相為穩(wěn)定相,且各項(xiàng)性能都非常接近巨磁卡FeMnP_(1-x)Ge_x化合物,因此FeMnP_(1-x)Ga_x化合物是有巨磁卡效應(yīng)的室溫磁制冷材料.金屬鐵及其合金中存在顯著的磁性-結(jié)構(gòu)關(guān)聯(lián)效應(yīng),即體系的晶格結(jié)構(gòu)決定磁有序性.因此通過調(diào)控合金的結(jié)構(gòu)可達(dá)到調(diào)控磁性狀態(tài)的目的.另外,四方相稀釋Fe合金的磁晶各向異性與其結(jié)構(gòu)四方性(c/a)有關(guān).因此,關(guān)于不同結(jié)構(gòu)與磁有序性的稀釋Fe合金的結(jié)構(gòu)穩(wěn)定性和力學(xué)性質(zhì)的研究顯得尤為重要.我們研究了Fe和Fe_(14)Tm_2(Tm=Cr,Mn,Co,Ni)二元合金處于不同結(jié)構(gòu)(bcc,fcc,bct)和磁有序性(FM,AFM)時(shí)的相對(duì)穩(wěn)定性和力學(xué)性質(zhì).計(jì)算結(jié)果表明:Fe和Fe_(14)Tm_2(Tm=Cr,Mn,Co,Ni)二元合金的bcc相FM態(tài)比f(wàn)cc相FM態(tài)、AFM態(tài)更穩(wěn)定,fcc相的FM態(tài)具有高自旋(HS)和低自旋(LS)的特性,fcc相AFM態(tài)要比FM態(tài)穩(wěn)定.進(jìn)一步研究了四方結(jié)構(gòu)的Fe_(14)Tm_2合金,發(fā)現(xiàn)Fe14Cr2、Fe_(14)Mn_2的AFM態(tài)要比FM態(tài)穩(wěn)定,Fe14Co2、Fe14Ni2的FM態(tài)要比AFM態(tài)穩(wěn)定.彈性常數(shù)的計(jì)算表明,僅fcc相FM-HS態(tài)的Fe_(14)Tm_2(Tm=Cr,Mn,Co,Ni)二元合金,fcc相FM-LS態(tài)的Fe14Ni2合金,四方相FM-HS態(tài)金屬Fe不滿足力學(xué)穩(wěn)定性條件,其它相均在力學(xué)上穩(wěn)定.說明合金化使得體系的HS態(tài)在力學(xué)上穩(wěn)定.將C或N原子填充在四方相Fe14Co2二元合金的八面體間隙時(shí),其LS態(tài)是比bcc相更穩(wěn)定的結(jié)構(gòu),同時(shí)含間隙原子的Fe和Fe14Co2合金均滿足四方結(jié)構(gòu)力學(xué)穩(wěn)定性條件,即間隙原子的填充使體系四方相更加穩(wěn)定,電子結(jié)構(gòu)的定性分析表明含有間隙原子的合金可能具有更大的磁晶各向異性。
[Abstract]:This paper mainly studies the phase structure, mechanical properties, magnetic ordering and thermodynamic properties of two different magnetic systems. The main contents of this paper are divided into two parts: the first part is the phase structure of Fe_2P type FeMnP_ (1-x) T_x (T=Si, Ga, Ge) compounds, the theoretical study of the mechanical properties and thermodynamic properties; the second part is the dilution doped Fe_ (14) Tm_. The relationship between the structure and magnetic properties of 2 (Tm=Cr, Mn, Co, Ni) alloys, the mechanical properties and the effects of the interstitial (C and N) atoms on the stability of the tetragonal phase of the alloy. As a magnetic refrigerant material, FeMnP_ (1-x) Si_x compound works under repeated magnetization and thermal loading. The compound should have good mechanical and mechanical stability at a limited temperature. Experimental hair The mechanical properties of the present FeMnP_ (1-x) Si_x compounds are poor. However, to date, there are few studies on the mechanical properties of such compounds. This paper predicts that Fe_2P type FeMnP_ (1-x) Ga_x compounds formed by Ga atoms instead of P may have good mechanical properties and larger magnetic card effects. On the basis of linear elasticity theory and classical statistical theory, the total energy, magnetic moment, formation enthalpy, electronic structure, mechanical properties and thermodynamic properties of Fe MnP_ (1-x) T_x (T=Si, Ga, Ge) series compounds are calculated by using VASP, Phonopy, Gibbs and other software packages. First, the possible six square and body centers of Fe_2P type FeMnP_ are compared and studied. The calculation results show that the total energy of the six square phase and the orthogonal phase of the body center is very close and the formation enthalpy is negative. The calculation results of the elastic constants show that the two phases are all mechanical stable phase, and the calculation of the modulus of elasticity shows the toughness of the ferromagnetic (FM) state six square phase and the orthogonal phase of the body center. Brittleness. The calculation of the phonon spectra of the two phase shows that the six phase phase is dynamic and stable, while the phonon dispersion relation of the body center quadrature phase has obvious virtual frequency and dynamic instability. The mechanical properties of the six square phase FeMn P1-xGax compound are further studied and compared with the FeMnP_ (1-x) T_x (T=Si, Ge) compound. The elastic constants of the compound system are calculated by the anti ferromagnetic (AFM) model. The results show that the six square phase FeMnP_ (1-x) T_x (T=Si, Ga, Ge) compounds are stable in mechanics, and the FM state Fe MnP_ (0.67) T_ (0.33) compounds exhibit toughness, and (0.67) the toughness is the best. The poor.AFM state FeMnP0.33T0.67 (T=Si, Ga, Ge) compounds exhibit toughness, and the toughness of FeMnP0.33Ge0.67 is the best. The toughness of FeMnP_ (0.33) Ga_ (0.67) is relatively poor.FM state FeMnP0.33T0.67 (T=Si, Ga, 0.67) is at the critical state of the toughness. The disorder of atomic occupying potential may improve the compound The variation of elastic constants with different doping atoms can be explained by electronic structural analysis and Force theorem. Finally, the Curie temperature of FeMnP_ (1-x) Ga_x compounds is determined by comparing the Gibbs free energy G (P, T) of the ferromagnetic and paramagnetic states. The heat capacity, entropy change and thermal expansion coefficient of the system have been calculated when the system has undergone ferromagnetic paramagnetic phase transition. The heat capacity of the compound is compared with the FeMnP_ (1-x) Ge_x compound. The heat capacity of the compound produces a jump in the phase transition. The system experiences the first order phase transition of the.FeMnP_ (0.67) Ga_ (0.33) compound and the Curie temperature is Tc=500K, and the entropy becomes (35) S=69.34 J K~ (-1) kg~ (-1), and the entropy becomes (35). The entropy of the FeMnP_ (0.33) Ga_ (0.67) compound of the high Ga component changes to (35) S=89.26 J K~ (-1) kg~ (-1). In a word, the six square phase of the FeMnP_ (1-x) compound is a stable phase, and the properties are very close to the giant magnetocaloric compound. Therefore, the compound is a room temperature magnetic refrigeration material with giant magnetic card effect. There is a significant magnetic structure association effect in metal iron and its alloys, that is, the lattice structure of the system determines the magnetic ordering. Therefore, the purpose of regulating magnetic state can be achieved by regulating the structure of the alloy. In addition, the magnetocrystalline anisotropy of the tetragonal Fe alloy is related to the structure of the Quartet (c/a). Therefore, about the different structures and magnetic order The study of structural stability and mechanical properties of Fe alloys with sexual dilution is particularly important. We have studied the relative stability and mechanical properties of Fe and Fe_ (14) Tm_2 (Tm=Cr, Mn, Co, Ni) alloy in different structures (BCC, FCC, BCT) and magnetic ordering. The results show that the two yuan alloy of 14 The FM state of C phase is more stable than FCC phase FM state, AFM state is more stable, FM state of FCC phase has the characteristics of high spin (HS) and low spin (LS), FCC phase AFM state is more stable than FM state. It is clear that the Fe_ (14) Tm_2 (14) Tm_2 (Tm=Cr, Mn, Co, Ni) alloy, FCC phase FM-LS state Fe14Ni2 alloy, and the Quartet phase FM-HS state metal does not satisfy the mechanical stability conditions, and the other phases are stable in mechanics. In the gap, the LS state is a more stable structure than the BCC phase. Both Fe and Fe14Co2 alloy with interstitial atoms meet the mechanical stability conditions of the tetragonal structure. That is, the filling of the interstitial atoms makes the tetragonal phase more stable. The qualitative analysis of the electronic structure shows that the gold inclusion containing the interstitial atoms may have a larger magnetocrystalline anisotropy.
【學(xué)位授予單位】：內(nèi)蒙古師范大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TG132.2;O614.811

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