碳基材料的制备与表征及其电子结构的理论研究
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摘要
碳元素在地壳中的丰度仅为0.027%,与铁、铝、氧等富有元素比较相差甚远,但放眼地球表面,碳元素却无处不在。它几乎可涵盖地球上所有物质的性质甚至相对立的两种性质,如从最硬到极软、全吸光到全透光、绝缘体到半导体到导体、绝热到良导热等。就某种意义上讲,人类世界就是碳的世界,人类文明史就是人类利用碳的历史。碳基材料在诸多领域都体现着重要的应用价值,比如石墨材料作为中子慢化剂用于核工程,磁性碳基材料用于自旋电子学领域,新型碳同素异形体用作纳米电子器件等。
对于作为慢化剂的核石墨,它在传统反应堆型中的应用技术已经比较成熟,然而,其应用于熔盐堆的研究却鲜有报道。作为六种第四代反应堆候选堆型之一的熔盐堆,具有负反应性温度系数高、工作压力低、热力学稳定性高等优点,这些优点均建立在核石墨能适应堆芯中熔盐环境和辐照环境的前提下,石墨是多孔材料,为了避免熔盐浸渗,在石墨表面制备低孔隙率热解碳涂层并研究其辐照响应将变得十分必要。对于碳基材料的磁性,区别于含有d电子和f电子的磁性金属原子,碳原子中仅含有s电子和p电子,所以研究碳基材料自旋极化的机制势在必行。尽管人们已通过掺杂过渡金属制得碳基磁性材料,但磁性的起源及磁化强度的调控问题尚未得到根本解决,这一方面的研究也便成为自旋极化领域实验和理论的研究重点之一。对于新型碳同素异形体,碳原子最外层有四个价电子,除了可以通过sp3杂化轨道形成单键外,还能以sp2及sp杂化轨道形成稳定的双键和叁键,同时也可以和多种原子形成共价键,组成丰富的结构。1985年,日本的稻垣道夫教授出版了《炭的材料工程》一书。在该书中,他强调了纳米结构的重要性,而且开始讨论基于纳米取向排列的碳化和石墨化原理,即拥有不同面取向、轴取向、点取向和无序排列的碳／石墨构成了石墨材料家族。随后,富勒烯和碳纳米管的出现,开启了碳材料研究的新方向。然而,追逐新型碳同素异形体的脚步从未停止过,2010年,石墨炔的成功制备再一次引发了这种由sp2杂化和sp杂化碳原子构成的碳同素异形体的研究热潮。
本论文依据以上所涉及到的三个应用领域,研究的碳基材料包括：多晶热解碳涂层、多晶碳化硅(SiC)涂层、单晶SiC (3dimensional,3D)、单层SiC (2D)和石墨炔(2D&3D)。在材料合成中采用化学气相沉积方法,这是制备热解碳基涂层最有效的方法之一。理论研究则采用基于密度泛函理论的第一性原理计算方法。
本论文实验与理论相结合,系统探讨了一系列碳基材料的制备与表征及其电子结构的理论研究。第一章为有关本论文的背景；第二章讲解了本论文所采用的实验手段和理论计算方法；第三章到第五章,归纳了作者在攻读博士学位期间所做的主要科研工作。
本论文的主要科研成果如下：
1.在1300℃下,利用化学气相沉积方法在核石墨基体上制备了各向同性热解碳,针对它作为核能领域的涂层材料的应用进行了研究。用129Xe26+离子对涂层进行辐照,利用偏光金相显微镜、扫描电子显微镜、透射电子显微镜、X射线衍射、拉曼光谱、纳米压痕和X射线光电子能谱等实验手段,表征了各向同性热解碳涂层辐照前后的微结构变化。实验结果显示,Xe离子辐照会导致热解碳中多晶结构的物理结构和化学性质同时发生变化。另外还表征了辐照之后的热解碳样品受500℃和1000℃退火之后的影响。离子辐照对石墨结构的a轴和c轴都有影响,且会在辐照区域产生大量碎屑状的形态。各向同性热解碳中存在一个调节其机械性能的剂量窗口：硬度和杨氏模量随着辐照剂量增加而升高,当晶格结构达到无序化时,剂量继续升高,硬度和杨氏模量反而下降。650℃下,原始核石墨和带有各向同性热解碳涂层覆盖的核石墨样品的FLiNaK盐浸渗实验表明：没有涂层覆盖的核石墨增重明显高于带有涂层覆盖的核石墨。扫描电子显微镜和压汞实验显示,各向同性热解碳涂层的孔径和开孔率很小,因此可以有效阻挡熔盐对核石墨的浸渗。另外,在1150℃下利用化学气相沉积方法在核石墨基体上制备的SiC涂层,也在熔融状态下的FLiNaK盐中做了浸渗实验。浸渗实验表明：5个大气压下,没有涂层保护的样品增重非常明显,而有涂层覆盖的样品增重很小。同步辐射X射线荧光实验显示,连续且密实的SiC涂层在液态盐中对核石墨起到很好的保护作用。
2.运用实验和理论相结合的手段探讨了3MeV质子辐照对6H-SiC单晶磁性的影响。研究发现：质子辐照可以在非磁性的6H-SiC中诱发产生居里温度高于300K的稳定铁磁性。样品中存在一可调节磁性大小的剂量窗口,辐照所产生的最大磁矩(0.17emu/g)比之前报道的中子辐照SiC单晶所产生的磁矩(1×10-4emu/g)高三个数量级。基于密度泛函理论的第一性原理计算显示,铁磁性主要源自于质子辐照所产生的与双空位有关的复合缺陷(VsiVc+nH,n=1-3)。利用14N+离子辐照也可以在6H-SiC单晶中产生磁矩大小可调、室温下稳定存在的铁磁性。理论研究表明,N原子的替代缺陷结合空位缺陷可以增强局域磁矩的铁磁性耦合。理论计算的结果与实验值相一致。此外,针对单层SiC的空位缺陷所引起的电子自旋极化和磁有序的理论研究表明：Si空位可以导致体系自旋极化的产生,而C空位则不会产生自旋极化,Si空位导致的电子自旋极化主要来源于缺陷周围C原子的贡献。由此产生的局域磁矩倾向于亚铁磁序排列。
3.运用基于密度泛函理论的第一性原理计算方法研究了由sp和sp2杂化的碳原子构成的石墨炔(graphyne)的电子结构和光学性质,结合紧束缚模型揭示了graphyne中带隙的起源,即：不同杂化形态的C原子的不等价π键导致graphyne具有与石墨烯完全不同的电子结构。层与层之间的相互作用将体graphyne的带隙缩减至0.16eV,与单层graphyne相比,多层graphyne的光学谱峰出现红移。我们还对graphyne纳米带的结构、能量和电子性质进行了理论研究。计算显示,结构形变主要发生在纳米带的边缘,扶手椅形的纳米带在能量上更稳定。所有的graphyne纳米带均呈现半导体特性。由于量子限制效应,纳米带的带隙随纳米带宽度增加而减小。边缘裸露的锯齿形graphyne纳米带具有自旋极化的边缘态。上述理论结果为石墨炔在纳米电子器件中的应用打下了基础。
Carbon is one of the most fundamental elements in the earth. The abundance of carbon in the earth's crust is only0.027%, which is far less than that of Fe, Al and O. However, carbon element is everywhere on earth. Carbon can possess almost all the material properties on the planet, even the antithetical properties. For example, carbon-based materials can be the hardest or softest, transparent or not, insulator, conductor or semiconductor, heat insulation or good thermal conductivity. In a sense, the human world is the world of carbon, and the history of human civilization is the history of using of carbon. So far, Carbon-based materials have been employed in many areas, such as using as a neutron moderator in nuclear reactor, fabricating spintronics and electronic devices, etc.
For the application of using as a neutron moderator, graphite has been successfully used in conventional nuclear reactor. However, the characterization of nuclear graphite in molten salt reactor is scarcely studied. For all the six promising future reactors proposed by the Generation IV International Forum, molten salt reactor has incomparable advantages:a high negative temperature coefficient of reactivity, a very low vapor pressure, thermodynamic stability, and so on. Notably, the main advantages of molten salt reactor are as a result of the prerequisite that the graphite can adapt the molten-salt and irradiation environment. In order to improve the barrier property of the porous graphite to molten salt, it is necessary to coat the graphite with pyrolytic carbon and investigate the irradiation response of the coating. Subsequently, the mechanism of ferromagnetism in carbon-based materials, which contain only s and p electrons in contrast to traditional ferromagnets based on3d or4f electrons, is important but unclear. Although the magnetic carbon-based materials have been fabricated by doping transition metal, the origin of magnetism and the control of the intensity of magnetization have not been solved. Therefore, it has been one of the important task in the area of spin polarization. Additionally, carbon has4valence electrons and can bond together via single bonds, double bonds or triple bonds in the form of sp3-, sp2-and sp-hybridized carbon atoms, respectively. Moreover, carbon atoms can form covalent bonds with some other atoms to compose materials with diverse structures and properties. In1985, Inagaki published a book entitled Materials Engineering of Carbons in Japanese. In the book, he began to emphasize the importance of nanotexture in carbon materials belonged to the graphite family and discussed carbonization and graphitization of carbon materials on the basis of their nanotextures, planar, axial, point and random orientation schema. The discovery of various fullerenes and later carbon nanotubes opened a new era for carbon materials. Nonetheless, many science researchers have been engaged in seeking for the novel carbon allotropes. Due to the fabrication of graphdiyne in2010, great interest has been aroused in the research on the layer composed of sp2-and sp-hybridized carbon atoms.
Based on the above application areas, the carbon-based materials involved in the present work include polycrystalline pyrolytic carbon coating, polycrystalline SiC coating, SiC crystal, SiC monolayer and graphyne (2D or3D). Chemical vapor deposition, i.e., one of the effective methods to obtain the pyrolytic carbon-based coating, was used to prepare the samples. First-principles calculations based on density functional theory was employed to investigate the geometry, energetic stability, electronic structure, etc.
With the combination of experimental and the theoretical analyses, in this paper we systematically introduce the preparation, characterization and theoretical studies of electronic structures of carbon-based materials. Chapter I introduces a brief introduction of research background. Chapter II describes the experimental and computational details used in our paper. Chaper III to VI introduce in detail and summarize the work completed during the period of study for a doctorate.
The main results are summarized as follows:
1. IPyC prepared at1300℃by chemical vapor deposition was implanted with129Xe26+ions to obtain a wide range of information as well as to understand the coating materials in nuclear energy field. Microstructure of the pristine and ion-implanted IPyC on nuclear graphite substrate was investigated using polarized light microscopy, scanning and transmission electron microscopy, X-ray diffraction, Raman spectroscopy, nanoindentation, and X-ray photoemission spectroscopy. It was demonstrated that the Xe ion irradiation resulted in concurrent changes in both physical and chemical structures of our standard polycrystalline sample. Influences of the thermal annealing temperature on the properties of the implanted IPyC at500and1000℃were also studied. Ion-irradiation gave rise to the formation of structural deterioration along a and c axis, accompanying with the appearance of widespread clastic morphology among the irradiated zone of IPyC. There was a dose window that could be used to tune the mechanical properties of IPyC:the nanohardness and Young's modulus increased after an irradiation, but decreased as the amorphization was reached. Infiltration studies were performed on uncoated nuclear graphite and PyC coated graphite in molten FLiNaK salt at650℃under argon atmosphere at1,3and5atm. Uncoated graphite shows weight gain more obviously than that of PyC coated graphite. Nuclear graphite with PyC coating exhibits excellent infiltration resistance in molten salt due to the small open porosity as conformed from scanning electron microscopy and mercury injection experiments. SiC coating is produced on a nuclear graphite substrate using chemical vapor deposition at1150℃to protect it from molten salt diffusion. Infiltration studies, performed in molten FLiNaK salt under an argon atmosphere at5atm, show that uncoated nuclear graphite exhibits significantly higher weight gain than SiC-coated nuclear graphite. The continuous and compact SiC coating exhibits excellent infiltration resistance in liquid fluoride salt as confirmed by synchrotron radiation X-ray microbeam fluorescence.
2. Magnetism of6H-SiC single crystals implanted with3MeV protons is studied both experimentally and theoretically. We found that proton irradiation can induce stable ferromagnetism in6H-SiC with a Curie temperature above300K. There is a dose window available for tuning the magnetization of the samples. The maximum saturation magnetizations (0.17emu/g) are three orders of magnitude larger than that reported in neutron-irradiated SiC crystals (1×10-4emu/g). First-principles calculations indicate that the ferromagnetism is related to the divacancy-related defects (VsiVc+nH,(n=1-3)) generated under proton irradiation. Room temperature, macroscopic magnetization was also induced and could be tuned in6H-SiC using 14N+ion implantation. First-principles density functional theory computation results confirmed that14N+ion implantation can enhance the ferromagnetic ordering of the local magnetic moments caused by vacancy and substitution defects. The calculated magnetization values in the energetically favored ferromagnetic ordering are qualitatively in agreement with the experimental data. We also performed first-principles calculations to investigate the spin-polarization of vacancy defects in SiC monolayer. We show that Si and C vacancy defects play different roles in the magnetism of SiC monolayer. Local magnetic moments can be induced by the presence of Si vacancy (Vsi) whereas no spin-polarization occurs in C vacancy (Vc) defects. The induced states are due to the unpaired electrons on carbon atoms surrounding the silicon vacancy. The spatial distribution of spin density displays the features of ferrimagnetic alignments for the most stable configuration.
3. We explored the electronic structures and optical properties of graphyne consisting of sp-and sp2-hybridized carbon atoms using first-principles calculations and tight-binding method. In contrast to zero-band gap graphene, the small band gap in graphyne is related to the inhomogeneous π-π bindings between differently-hybridized carbon atoms. The optical properties of single-layered and bulk graphyne are also calculated and analyzed on the basis of electron density of states. The interlayer interactions of bulk graphyne narrow the band gap to0.16eV and result in redshift of the optical spectral peaks as compared to single-layered graphyne. We also performed first-principles calculations to explore the structural, energetic, and electronic properties of graphyne nanoribbons (GYNRs), the graphyne with a nanometer-size width. We found that structural relaxation mainly takes place at the edges of GYNRs and the armchair-shaped edge is energetically preferable. All the GYNRs are semiconducting independent of edge structures. Due to the lateral quantum-confinement effects, the band gap decreases gradually with the increase of ribbon width. For the GYNRs with bared zigzag edges, the edge states are spin-polarized and coupled in an antiferromagnetic-like way along each side. With the semiconducting properties and tunable band gaps, these novel carbon nanoribbons may find applications in building nanoscaled devices.
对于作为慢化剂的核石墨,它在传统反应堆型中的应用技术已经比较成熟,然而,其应用于熔盐堆的研究却鲜有报道。作为六种第四代反应堆候选堆型之一的熔盐堆,具有负反应性温度系数高、工作压力低、热力学稳定性高等优点,这些优点均建立在核石墨能适应堆芯中熔盐环境和辐照环境的前提下,石墨是多孔材料,为了避免熔盐浸渗,在石墨表面制备低孔隙率热解碳涂层并研究其辐照响应将变得十分必要。对于碳基材料的磁性,区别于含有d电子和f电子的磁性金属原子,碳原子中仅含有s电子和p电子,所以研究碳基材料自旋极化的机制势在必行。尽管人们已通过掺杂过渡金属制得碳基磁性材料,但磁性的起源及磁化强度的调控问题尚未得到根本解决,这一方面的研究也便成为自旋极化领域实验和理论的研究重点之一。对于新型碳同素异形体,碳原子最外层有四个价电子,除了可以通过sp3杂化轨道形成单键外,还能以sp2及sp杂化轨道形成稳定的双键和叁键,同时也可以和多种原子形成共价键,组成丰富的结构。1985年,日本的稻垣道夫教授出版了《炭的材料工程》一书。在该书中,他强调了纳米结构的重要性,而且开始讨论基于纳米取向排列的碳化和石墨化原理,即拥有不同面取向、轴取向、点取向和无序排列的碳／石墨构成了石墨材料家族。随后,富勒烯和碳纳米管的出现,开启了碳材料研究的新方向。然而,追逐新型碳同素异形体的脚步从未停止过,2010年,石墨炔的成功制备再一次引发了这种由sp2杂化和sp杂化碳原子构成的碳同素异形体的研究热潮。
本论文依据以上所涉及到的三个应用领域,研究的碳基材料包括：多晶热解碳涂层、多晶碳化硅(SiC)涂层、单晶SiC (3dimensional,3D)、单层SiC (2D)和石墨炔(2D&3D)。在材料合成中采用化学气相沉积方法,这是制备热解碳基涂层最有效的方法之一。理论研究则采用基于密度泛函理论的第一性原理计算方法。
本论文实验与理论相结合,系统探讨了一系列碳基材料的制备与表征及其电子结构的理论研究。第一章为有关本论文的背景；第二章讲解了本论文所采用的实验手段和理论计算方法；第三章到第五章,归纳了作者在攻读博士学位期间所做的主要科研工作。
本论文的主要科研成果如下：
1.在1300℃下,利用化学气相沉积方法在核石墨基体上制备了各向同性热解碳,针对它作为核能领域的涂层材料的应用进行了研究。用129Xe26+离子对涂层进行辐照,利用偏光金相显微镜、扫描电子显微镜、透射电子显微镜、X射线衍射、拉曼光谱、纳米压痕和X射线光电子能谱等实验手段,表征了各向同性热解碳涂层辐照前后的微结构变化。实验结果显示,Xe离子辐照会导致热解碳中多晶结构的物理结构和化学性质同时发生变化。另外还表征了辐照之后的热解碳样品受500℃和1000℃退火之后的影响。离子辐照对石墨结构的a轴和c轴都有影响,且会在辐照区域产生大量碎屑状的形态。各向同性热解碳中存在一个调节其机械性能的剂量窗口：硬度和杨氏模量随着辐照剂量增加而升高,当晶格结构达到无序化时,剂量继续升高,硬度和杨氏模量反而下降。650℃下,原始核石墨和带有各向同性热解碳涂层覆盖的核石墨样品的FLiNaK盐浸渗实验表明：没有涂层覆盖的核石墨增重明显高于带有涂层覆盖的核石墨。扫描电子显微镜和压汞实验显示,各向同性热解碳涂层的孔径和开孔率很小,因此可以有效阻挡熔盐对核石墨的浸渗。另外,在1150℃下利用化学气相沉积方法在核石墨基体上制备的SiC涂层,也在熔融状态下的FLiNaK盐中做了浸渗实验。浸渗实验表明：5个大气压下,没有涂层保护的样品增重非常明显,而有涂层覆盖的样品增重很小。同步辐射X射线荧光实验显示,连续且密实的SiC涂层在液态盐中对核石墨起到很好的保护作用。
2.运用实验和理论相结合的手段探讨了3MeV质子辐照对6H-SiC单晶磁性的影响。研究发现：质子辐照可以在非磁性的6H-SiC中诱发产生居里温度高于300K的稳定铁磁性。样品中存在一可调节磁性大小的剂量窗口,辐照所产生的最大磁矩(0.17emu/g)比之前报道的中子辐照SiC单晶所产生的磁矩(1×10-4emu/g)高三个数量级。基于密度泛函理论的第一性原理计算显示,铁磁性主要源自于质子辐照所产生的与双空位有关的复合缺陷(VsiVc+nH,n=1-3)。利用14N+离子辐照也可以在6H-SiC单晶中产生磁矩大小可调、室温下稳定存在的铁磁性。理论研究表明,N原子的替代缺陷结合空位缺陷可以增强局域磁矩的铁磁性耦合。理论计算的结果与实验值相一致。此外,针对单层SiC的空位缺陷所引起的电子自旋极化和磁有序的理论研究表明：Si空位可以导致体系自旋极化的产生,而C空位则不会产生自旋极化,Si空位导致的电子自旋极化主要来源于缺陷周围C原子的贡献。由此产生的局域磁矩倾向于亚铁磁序排列。
3.运用基于密度泛函理论的第一性原理计算方法研究了由sp和sp2杂化的碳原子构成的石墨炔(graphyne)的电子结构和光学性质,结合紧束缚模型揭示了graphyne中带隙的起源,即：不同杂化形态的C原子的不等价π键导致graphyne具有与石墨烯完全不同的电子结构。层与层之间的相互作用将体graphyne的带隙缩减至0.16eV,与单层graphyne相比,多层graphyne的光学谱峰出现红移。我们还对graphyne纳米带的结构、能量和电子性质进行了理论研究。计算显示,结构形变主要发生在纳米带的边缘,扶手椅形的纳米带在能量上更稳定。所有的graphyne纳米带均呈现半导体特性。由于量子限制效应,纳米带的带隙随纳米带宽度增加而减小。边缘裸露的锯齿形graphyne纳米带具有自旋极化的边缘态。上述理论结果为石墨炔在纳米电子器件中的应用打下了基础。
Carbon is one of the most fundamental elements in the earth. The abundance of carbon in the earth's crust is only0.027%, which is far less than that of Fe, Al and O. However, carbon element is everywhere on earth. Carbon can possess almost all the material properties on the planet, even the antithetical properties. For example, carbon-based materials can be the hardest or softest, transparent or not, insulator, conductor or semiconductor, heat insulation or good thermal conductivity. In a sense, the human world is the world of carbon, and the history of human civilization is the history of using of carbon. So far, Carbon-based materials have been employed in many areas, such as using as a neutron moderator in nuclear reactor, fabricating spintronics and electronic devices, etc.
For the application of using as a neutron moderator, graphite has been successfully used in conventional nuclear reactor. However, the characterization of nuclear graphite in molten salt reactor is scarcely studied. For all the six promising future reactors proposed by the Generation IV International Forum, molten salt reactor has incomparable advantages:a high negative temperature coefficient of reactivity, a very low vapor pressure, thermodynamic stability, and so on. Notably, the main advantages of molten salt reactor are as a result of the prerequisite that the graphite can adapt the molten-salt and irradiation environment. In order to improve the barrier property of the porous graphite to molten salt, it is necessary to coat the graphite with pyrolytic carbon and investigate the irradiation response of the coating. Subsequently, the mechanism of ferromagnetism in carbon-based materials, which contain only s and p electrons in contrast to traditional ferromagnets based on3d or4f electrons, is important but unclear. Although the magnetic carbon-based materials have been fabricated by doping transition metal, the origin of magnetism and the control of the intensity of magnetization have not been solved. Therefore, it has been one of the important task in the area of spin polarization. Additionally, carbon has4valence electrons and can bond together via single bonds, double bonds or triple bonds in the form of sp3-, sp2-and sp-hybridized carbon atoms, respectively. Moreover, carbon atoms can form covalent bonds with some other atoms to compose materials with diverse structures and properties. In1985, Inagaki published a book entitled Materials Engineering of Carbons in Japanese. In the book, he began to emphasize the importance of nanotexture in carbon materials belonged to the graphite family and discussed carbonization and graphitization of carbon materials on the basis of their nanotextures, planar, axial, point and random orientation schema. The discovery of various fullerenes and later carbon nanotubes opened a new era for carbon materials. Nonetheless, many science researchers have been engaged in seeking for the novel carbon allotropes. Due to the fabrication of graphdiyne in2010, great interest has been aroused in the research on the layer composed of sp2-and sp-hybridized carbon atoms.
Based on the above application areas, the carbon-based materials involved in the present work include polycrystalline pyrolytic carbon coating, polycrystalline SiC coating, SiC crystal, SiC monolayer and graphyne (2D or3D). Chemical vapor deposition, i.e., one of the effective methods to obtain the pyrolytic carbon-based coating, was used to prepare the samples. First-principles calculations based on density functional theory was employed to investigate the geometry, energetic stability, electronic structure, etc.
With the combination of experimental and the theoretical analyses, in this paper we systematically introduce the preparation, characterization and theoretical studies of electronic structures of carbon-based materials. Chapter I introduces a brief introduction of research background. Chapter II describes the experimental and computational details used in our paper. Chaper III to VI introduce in detail and summarize the work completed during the period of study for a doctorate.
The main results are summarized as follows:
1. IPyC prepared at1300℃by chemical vapor deposition was implanted with129Xe26+ions to obtain a wide range of information as well as to understand the coating materials in nuclear energy field. Microstructure of the pristine and ion-implanted IPyC on nuclear graphite substrate was investigated using polarized light microscopy, scanning and transmission electron microscopy, X-ray diffraction, Raman spectroscopy, nanoindentation, and X-ray photoemission spectroscopy. It was demonstrated that the Xe ion irradiation resulted in concurrent changes in both physical and chemical structures of our standard polycrystalline sample. Influences of the thermal annealing temperature on the properties of the implanted IPyC at500and1000℃were also studied. Ion-irradiation gave rise to the formation of structural deterioration along a and c axis, accompanying with the appearance of widespread clastic morphology among the irradiated zone of IPyC. There was a dose window that could be used to tune the mechanical properties of IPyC:the nanohardness and Young's modulus increased after an irradiation, but decreased as the amorphization was reached. Infiltration studies were performed on uncoated nuclear graphite and PyC coated graphite in molten FLiNaK salt at650℃under argon atmosphere at1,3and5atm. Uncoated graphite shows weight gain more obviously than that of PyC coated graphite. Nuclear graphite with PyC coating exhibits excellent infiltration resistance in molten salt due to the small open porosity as conformed from scanning electron microscopy and mercury injection experiments. SiC coating is produced on a nuclear graphite substrate using chemical vapor deposition at1150℃to protect it from molten salt diffusion. Infiltration studies, performed in molten FLiNaK salt under an argon atmosphere at5atm, show that uncoated nuclear graphite exhibits significantly higher weight gain than SiC-coated nuclear graphite. The continuous and compact SiC coating exhibits excellent infiltration resistance in liquid fluoride salt as confirmed by synchrotron radiation X-ray microbeam fluorescence.
2. Magnetism of6H-SiC single crystals implanted with3MeV protons is studied both experimentally and theoretically. We found that proton irradiation can induce stable ferromagnetism in6H-SiC with a Curie temperature above300K. There is a dose window available for tuning the magnetization of the samples. The maximum saturation magnetizations (0.17emu/g) are three orders of magnitude larger than that reported in neutron-irradiated SiC crystals (1×10-4emu/g). First-principles calculations indicate that the ferromagnetism is related to the divacancy-related defects (VsiVc+nH,(n=1-3)) generated under proton irradiation. Room temperature, macroscopic magnetization was also induced and could be tuned in6H-SiC using 14N+ion implantation. First-principles density functional theory computation results confirmed that14N+ion implantation can enhance the ferromagnetic ordering of the local magnetic moments caused by vacancy and substitution defects. The calculated magnetization values in the energetically favored ferromagnetic ordering are qualitatively in agreement with the experimental data. We also performed first-principles calculations to investigate the spin-polarization of vacancy defects in SiC monolayer. We show that Si and C vacancy defects play different roles in the magnetism of SiC monolayer. Local magnetic moments can be induced by the presence of Si vacancy (Vsi) whereas no spin-polarization occurs in C vacancy (Vc) defects. The induced states are due to the unpaired electrons on carbon atoms surrounding the silicon vacancy. The spatial distribution of spin density displays the features of ferrimagnetic alignments for the most stable configuration.
3. We explored the electronic structures and optical properties of graphyne consisting of sp-and sp2-hybridized carbon atoms using first-principles calculations and tight-binding method. In contrast to zero-band gap graphene, the small band gap in graphyne is related to the inhomogeneous π-π bindings between differently-hybridized carbon atoms. The optical properties of single-layered and bulk graphyne are also calculated and analyzed on the basis of electron density of states. The interlayer interactions of bulk graphyne narrow the band gap to0.16eV and result in redshift of the optical spectral peaks as compared to single-layered graphyne. We also performed first-principles calculations to explore the structural, energetic, and electronic properties of graphyne nanoribbons (GYNRs), the graphyne with a nanometer-size width. We found that structural relaxation mainly takes place at the edges of GYNRs and the armchair-shaped edge is energetically preferable. All the GYNRs are semiconducting independent of edge structures. Due to the lateral quantum-confinement effects, the band gap decreases gradually with the increase of ribbon width. For the GYNRs with bared zigzag edges, the edge states are spin-polarized and coupled in an antiferromagnetic-like way along each side. With the semiconducting properties and tunable band gaps, these novel carbon nanoribbons may find applications in building nanoscaled devices.
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