本文關(guān)鍵詞：觸媒微結(jié)構(gòu)與立方氮化硼單晶催化機理的相關(guān)性研究　出處：《山東大學(xué)》2015年博士論文　論文類型：學(xué)位論文

  更多相關(guān)文章： cBN單晶 觸媒微結(jié)構(gòu) 高溫高壓 催化機理 晶體生長 熱力學(xué)

【摘要】：立方氮化硼(cBN)具有類金剛石結(jié)構(gòu),作為一種硬度高、穩(wěn)定性好的新型晶體材料,其高溫半導(dǎo)體特性、高頻特性和壓電特性不斷表現(xiàn)出在現(xiàn)代科學(xué)技術(shù)發(fā)展中的重要作用。目前,采用靜態(tài)高溫高壓觸媒法合成cBN仍然是工業(yè)合成單晶的重要方法,而研究高溫高壓cBN單晶的催化機理對于指導(dǎo)工業(yè)生產(chǎn)優(yōu)質(zhì)大單晶具有重要意義。實驗發(fā)現(xiàn),在合成后的cBN單晶表面總是覆蓋著一層類似于熔融狀物質(zhì),此物質(zhì)應(yīng)為觸媒和六方氮化硼(hBN)融合所形成的觸媒層。高溫高壓條件下cBN單晶正是通過觸媒層的催化和擴散作用進行形核和長大。研究合成后的cBN單晶觸媒層中物相的相互作用對于揭示cBN高溫高壓催化機理提供了重要的參考依據(jù)。本文以hBN為原料,以Li3N為觸媒原料進行了高溫高壓條件下cBN單晶的合成實驗。利用掃描電子顯微鏡(SEM)、原子力顯微鏡(AFM)、透射電子顯微鏡(TEM)、高分辨透射電子顯微鏡(HRTEM)等表征手段確定了cBN單晶觸媒層的表面形貌、物相結(jié)構(gòu)等,并在此基礎(chǔ)上利用拉曼光譜儀(Raman)、俄歇電子能譜儀(AES)、X射線光電子能譜儀(XPS)和電子能量損失譜儀(EELS)等對觸媒層中B、N原子的電子結(jié)構(gòu)變化規(guī)律進行了系統(tǒng)分析,探討高溫高壓cBN單晶的催化機理。同時,結(jié)合熱力學(xué)理論計算了cBN合成過程中各反應(yīng)的自由能變化,從而進一步驗證了表征實驗的結(jié)果,為cBN高溫高壓催化機理的研究提供了理論依據(jù)。此外,從生長動力學(xué)角度對cBN單晶的形核及生長與合成條件之間的關(guān)系進行了探討。通過對cBN觸媒層的分層XRD實驗結(jié)果可知,在觸媒層中主要物相結(jié)構(gòu)為hBN、cBN、Li3BN2,而在各層中均未發(fā)現(xiàn)Li3N的存在。利用HRTEM實驗在cBN單晶／觸媒層界面中發(fā)現(xiàn)了大量的cBN納米級顆粒。同時,利用TEM在cBN單晶／觸媒界面中也發(fā)現(xiàn)了cBN聚晶顆粒,并且在界面上發(fā)現(xiàn)了hBN的存在,從而提出了cBN單晶應(yīng)由hBN直接發(fā)生結(jié)構(gòu)轉(zhuǎn)變而形成。在高溫高壓條件下,hBN在觸媒層中通過觸媒的催化作用完成形核過程,并依靠觸媒層中B、N原子的濃度差異進行生長。Raman譜和AES譜的精細(xì)結(jié)構(gòu)表明,B原子和N原子的電子結(jié)構(gòu)在觸媒層的不同區(qū)域是不同的,在B、N原子由擴散方式通過觸媒層達到cBN單晶的過程中,B、N原子的性質(zhì)發(fā)生了變化。AES譜顯示觸媒外層B、N原子的精細(xì)結(jié)構(gòu)與hBN相似,而在觸媒內(nèi)層,即靠近cBN單晶處,AES譜圖更加接近于cBN單晶,在觸媒層中由外到內(nèi),具有sp2π雜化特征的俄歇峰強度逐漸減弱。利用XPS和EELS對cBN單晶觸媒層中B、N原子的電子結(jié)構(gòu)變化進行定量分析。采用深度刻蝕的方法對觸媒層不同深度的B、N電子結(jié)構(gòu)進行分析,發(fā)現(xiàn)隨著濺射時間的延長,即越來越接近cBN單晶,在主結(jié)合能高能端sp2π雜化所呈現(xiàn)的攜上伴峰的強度越來越低。利用Gauss/Lorenz混合型函數(shù)對深度刻蝕過程中B1s峰進行曲線擬合分峰處理,從而得到在觸媒層由外到內(nèi),sp2π的含量由61.18%降低到28.24%,而sp3的含量由38.82%增加到71.76%。EELS分層實驗結(jié)果表明,在cBN單晶觸媒層由外到內(nèi),sp3-B的相對含量分別是63.47%、67.24%和79.53%。這些結(jié)果表明在cBN單晶的生長中,觸媒的催化作用逐漸增大,B、N原子的電子構(gòu)型由hBN的sp2π雜化態(tài)逐漸向cBN的sp3雜化態(tài)轉(zhuǎn)化。對cBN單晶表面的AFM研究表明在(111)晶面和(100)晶面均存在大量的cBN亞顆粒,并且(100)晶面的顆粒要明顯大于(111)晶面。cBN單晶在高溫高壓下的生長在一定程度上可以看作是這些cBN亞顆�；蛟蛹瘓F在生長的cBN界面上組合與重新排列的過程。結(jié)合SEM結(jié)果可知,cBN單晶(111)奇異面存在連續(xù)生長的臺階,表明cBN在高溫高壓條件下以片層機制長大。同時,單晶中位錯的存在可以使cBN單晶的生長界面形成連續(xù)的螺形生長臺階,這為單晶長大過程提供了大量的臺階源,從而可在較低的合成條件下完成cBN單晶的生長。利用晶體生長動力學(xué)理論討論了臨界晶核半徑、臨界形核功和晶體生長速度隨合成壓力、溫度的變化關(guān)系。結(jié)果表明：在非均勻形核時,一定溫度下,臨界晶核半徑r*和臨界形核功△G*隨壓力的降低而增大；壓力一定時,兩者隨溫度的增加而增加。晶體生長速度與溫度、壓力呈拋物線規(guī)律,其中當(dāng)合成壓力為5.5GPa時,cBN單晶具有最快的生長速度。以相變熱力學(xué)為理論依據(jù),綜合考慮溫度、壓力對物相體積的影響,計算了Li3N及hBN在高溫高壓條件下向cBN結(jié)構(gòu)轉(zhuǎn)變的反應(yīng)自由能變化情況。結(jié)果表明,在合成cBN單晶的溫度、壓力范圍內(nèi),hBN→cBN反應(yīng)的Gibbs自由能變化均為負(fù)值。Li3BN2→cBN+Li3N反應(yīng)的△G0區(qū)域呈現(xiàn)“V”形區(qū),這一區(qū)域與優(yōu)質(zhì)cBN單晶生長的溫度、壓力區(qū)域近似重合,這一結(jié)果說明cBN不應(yīng)由Li3BN2分解產(chǎn)生。從熱力學(xué)角度來看,Li3BN2的形成降低了hBN向cBN轉(zhuǎn)變所需要越過的勢壘,cBN單晶來源應(yīng)為hBN的直接結(jié)構(gòu)轉(zhuǎn)變,Li3BN2在cBN合成中起到了結(jié)構(gòu)催化作用。結(jié)合表征實驗結(jié)果和理論計算,對cBN在高溫高壓下的催化機理可作如下分析：hBN是高溫高壓條件下cBN生長的直接B、N來源,Li3BN2作為觸媒可以加速這一過程的實現(xiàn),為單晶生長的催化相。高溫高壓條件下,Li3BN2與hBN形成共熔體,Li3BN2中的BN23-離子侵入六方相中,使hBN層間的范德華力受到影響,從而發(fā)生滑移或斷裂,此時hBN的遠程有序結(jié)構(gòu)消失,逐漸降低為低聚合度的BN團簇。與此同時,Li+能夠吸引處于高溫活躍狀態(tài)的hBN中N原子中的一個電子,并將其傳送給B原子,促使B、N原子的電子結(jié)構(gòu)均變?yōu)锽、N原子的s軌道上的電子被激發(fā)到空的p軌道,形成具有類似sp3雜化狀態(tài)的BN原子集團。由于熔體內(nèi)存在能量起伏和結(jié)構(gòu)起伏,合成腔體內(nèi)溫度、壓力的微小波動,就會促使sp3-BN原子團聚集、碰撞從而形成cBN單晶結(jié)構(gòu)。同時,由于觸媒層內(nèi)外存在濃度梯度,六方相不斷向觸媒內(nèi)層擴散,并不斷轉(zhuǎn)化為立方相。隨著合成的不斷進行,熔體中具有sp3雜化態(tài)的BN原子集團數(shù)目不斷增加,并在已形成的cBN晶面處含量達到最高。在隨后的生長過程中,cBN單晶將以片層生長的方式不斷長大。
[Abstract]:Cubic boron nitride (cBN) with diamond-like structure, as a new type of crystal materials, high hardness, good stability, the high temperature characteristics of semiconductor, high frequency characteristics and piezoelectric properties continue to play an important role in the development of modern science and technology. At present, the static high temperature high-pressure catalytic synthesis of cBN method is still an important method of industrial synthesis single crystal, catalytic mechanism and research of high temperature and high pressure cBN crystal has an important significance for guiding the industrial production of high quality single crystal. The experimental results showed that, in cBN single crystal surface after synthesis is always covered with a layer similar to the molten material, this material should be as catalyst and six boron nitride (hBN) catalyst layer formed by fusion. Under high temperature and high pressure cBN crystal is through catalytic and diffusion of the catalyst layer of nucleation and growth of cBN crystal. The catalyst layer after synthesis in the phase interaction to reveal cBN at high temperature Provides important reference for the high pressure catalytic mechanism. This paper takes hBN as the raw material, the synthesis experiments under high temperature and high pressure cBN single crystal with Li3N as catalyst materials. By using scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) etc. the characterization of the surface morphology of cBN single crystal catalyst layer, phase structure, and on this basis by Raman spectroscopy (Raman), Auger electron spectroscopy (AES), X ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) on the catalyst layer in B, electronic structure changes of N atoms were analyzed, to investigate the catalytic mechanism of cBN crystal under high temperature and high pressure. At the same time, the reaction in the synthesis of cBN free energy change was calculated with the theory of thermodynamics, which verifies the characterization of experimental results for cBN high temperature and high pressure. The study provides the theoretical basis for the mechanism. In addition, the relationship between nucleation and growth and synthesis conditions of cBN crystal growth from the perspective of dynamics are discussed. Through the experimental results of hierarchical XRD cBN catalyst layer shows that in the main catalyst layer in phase structure of hBN, cBN, Li3BN2, and in each layer were not found in the presence of Li3N. The HRTEM assay showed that cBN nanoparticles in a large number of single crystal cBN / catalyst layer interface. At the same time, the use of TEM cBN polycrystalline particles are also found in the cBN crystal / catalyst interface, and found the existence of hBN in the interface, and then put forward the cBN single crystal by hBN the structural change happens. Formed under high temperature, hBN in the catalyst layer by the catalytic action of the catalyst to complete the nucleation process, and rely on the catalyst layer in B, shows that the fine structure of the growth of.Raman and AES spectra of N atom concentration difference, B Different areas of electronic structure and N atoms in the catalyst layer is different, in the process of B, N atoms by diffusion through the catalyst layer to cBN crystal, B, properties of N atoms changed.AES spectrum shows that the catalyst layer B, hBN and N atomic fine structure is similar, and in the media contact the inner layer, which is near to cBN crystal, the AES spectrum is more close to the cBN crystal in a catalyst layer from outside to inside, the auger peak intensity with SP2 PI hybrid characteristics gradually weakened. For single crystal cBN catalyst layer in B by XPS and EELS, the change of electronic structure of N atoms were quantitatively analyzed. The method adopts deep etching the catalyst layer in different depth of B, analyze the electronic structure of N, with the increase of the sputtering time, which is more and more close to the cBN crystal, the main binding energy of hybrid PI SP2 end with more and more low intensity peaks. Using Gauss/Lorenz mixed function of the depth of moment The B1s peak erosion process curve fitting peak processing, resulting in a catalyst layer from outside to inside, the content of SP2 PI decreased from 61.18% to 28.24%, while the SP3 content increased from 38.82% to 71.76%.EELS layered experimental results show that cBN single crystal in the catalyst layer from outside to inside, the relative content of sp3-B was 63.47% 67.24% and 79.53%., these results indicated that the growth of cBN crystal, catalytic effect of catalyst increases, B electron configuration N atoms by SP2 PI hybrid state hBN to SP3 hybrid state cBN transformation. The research on AFM cBN single crystal surface indicated that in (111) plane and (100) crystal there were cBN large number of sub grains, and (100) crystal plane of the particles to be significantly greater than the (111) crystal face of.CBN crystal under high temperature and high pressure growth can to a certain extent as these cBN particles or sub atomic group combination in the growth of the cBN interface and the combination of re arranged. SEM results show that cBN single crystal (111) is the continuous growth of the steps of the singular surface, showed that cBN grew up with lamellar mechanism at high temperature and high pressure. At the same time, the existence of dislocations in single crystal growth interface can make the cBN crystal formed spiral continuous growth step, this is a single crystal grow up process provides a great source of steps thus, the completion of cBN crystal growth in the condition of low synthesis. Using the crystal growth kinetics theory to discuss the critical nucleus radius, the critical nucleation energy and crystal growth velocity with synthetic pressure and temperature dependence. The results show that in heterogeneous nucleation, under certain temperature, the critical nucleus radius r* and the critical nucleation energy G* decreases with increasing pressure; pressure, which increases with the increase of the temperature. The crystal growth rate and temperature, pressure of a parabolic law and the synthesis when the pressure is 5.5GPa, cBN single crystal has the most Fast growth. In thermodynamics theory, considering the effects of temperature, pressure on the phase volume, reaction of free Li3N and hBN transition at high temperature and high pressure to cBN structure can change the calculation. The results show that the synthesis of cBN crystal temperature, pressure range, hBN, cBN the Gibbs free energy delta G0 regional changes are negative for.Li3BN2 and cBN+Li3N reaction showed "V" shape, the area with high quality cBN crystal growth temperature, pressure area is approximately coincide, this result suggests that cBN should not Li3BN2 decomposition. From the view of thermodynamics, the formation of Li3BN2 hBN decreased to cBN to cross the barrier, cBN crystal source should direct the structure of the hBN transformation, Li3BN2 has played a catalytic role in the synthesis of cBN structure. According to the experimental results and theoretical calculation of characterization, the catalytic mechanism of cBN under high temperature and high pressure can be made Analysis of the following: hBN is directly B, the growth of cBN under high temperature and high pressure N source, Li3BN2 as catalyst can accelerate the realization of this process is catalyzed by crystal growth phase. Under high temperature, Li3BN2 and the formation of hBN eutectic, BN23- ions in Li3BN2 intrusion of six party phase, hBN layer between the fan Edward affected to slip or fracture, disappear when hBN long-range order, gradually reduced to BN clusters with low degree of polymerization. At the same time, Li+ is able to attract a N atom in the active state of the electronic temperature hBN, and its transfer to the B atom, to B, the electronic structure of N atoms both are B, s orbit electrons of N atoms are excited to an empty P orbit, the formation of BN atomic group with similar SP3 hybridization state. Due to the existence of energy fluctuation and fluctuation of the melt structure, synthesis temperature cavity, small fluctuations in pressure, will promote The sp3-BN cluster aggregation, collision to form the cBN crystal structure. At the same time, the catalyst layer exists inside and outside the concentration gradient, the six phase to the catalyst layer and diffusion, and continue into the cubic phase. As the synthesis continues, the number of BN group with SP3 atomic hybridization state in the melt is increasing, and reached the highest at cBN the content of crystal surface has been formed. In the subsequent growth process, the growth of cBN single crystal by a layer the way to grow up.

【學(xué)位授予單位】：山東大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TQ128.1

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