Synthesis of graphene on SiC substrate via Ni-silicidation reactions

In this work, the features of graphene layers are studied with the aim of preparing the thinnest layers possible. The graphene layers were prepared by the annealing of Ni/SiC structures. The main advantage of this process is a relatively low temperature compared with the method of graphene epitaxial growth on SiC and short annealing times compared with the chemical vapor deposition method. We prepared graphene layers from several Ni/SiC structures in which the Ni layer thickness ranged from 1 to 200 nm. The parameters of the annealing process (temperature, rate of temperature increase, annealing time) were modified during the experiments. The formed graphene layers were analyzed by means of Raman spectroscopy. From the spectra, the basic parameters of graphene, such as the number of carbon layers and crystallinity, were determined. The annealing of the Ni(200 nm)/SiC structure at 1080 °C for 10 s, produced graphene in the form of 3-4 carbon monolayers. The value was verified by X-ray Photoelectron Spectroscopy (XPS). Good agreement was achieved in the results obtained using Raman spectroscopy and XPS.     

Synthesis and structure of BN coatings on SiC nanofibers

The Gibbs potential has been calculated as a function of temperature for a number of BN synthesis reactions. Detailed studies of BN chemical vapor deposition on SiC nanofibers were used to develop a technique for BN coating production from nontoxic precursors. The optimal synthesis temperature was 1300°C. Transmission electron microscopy characterization of the BN/SiC nanocables thus produced showed that the coatings were 10–20 nm thick and had a hexagonal structure.     

Modification of carbon solubility in metals at preparation of graphene from the metal/SiC structure

Graphene has, due to its favourable features, a wide usage in a range of industries. A number of methods are being used for its preparation, each of them being suitable for another application of graphene. One of the methods is based on annealing of a metal/SiC structure at temperatures ranging from 800 to 1000?°C; graphene is formed here on the basis of carbon segregation from the metallization. This process is dependent on the reaction of metals with SiC and on solubility of carbon within metals. Our work was focused on modification of solubility of carbon in basic metals (Ni and Co) by adding suitable admixtures—Ge, Cu, Au. Principal influence onto parameters of graphene films had copper added into the metallization with cobalt. We succeeded to prepare graphene containing two carbon mono-layers (BLG).     

Scalable templated growth of graphene nanoribbons on SiC

In spite of its excellent electronic properties, the use of graphene in field-effect transistors is not practical at room temperature without modification of its intrinsically semimetallic nature to introduce a bandgap. Quantum confinement effects can create a bandgap in graphene nanoribbons, but existing nanoribbon fabrication methods are slow and often produce disordered edges that compromise electronic properties. Here, we demonstrate the self-organized growth of graphene nanoribbons on a templated silicon carbide substrate prepared using scalable photolithography and microelectronics processing. Direct nanoribbon growth avoids the need for damaging post-processing. Raman spectroscopy, high-resolution transmission electron microscopy and electrostatic force microscopy confirm that nanoribbons as narrow as 40 nm can be grown at specified positions on the substrate. Our prototype graphene devices exhibit quantum confinement at low temperatures (4 K), and an on-off ratio of 10 and carrier mobilities up to 2,700 cm(2) V(-1) s(-1) at room temperature. We demonstrate the scalability of this approach by fabricating 10,000 top-gated graphene transistors on a 0.24-cm(2) SiC chip, which is the largest density of graphene devices reported to date.     

C/SiC材料表面Si/SiC涂层及其对基底结构的影响

采用泥浆预涂层反应法在C/ SiC 复合材料表面制备Si/ SiC 涂层。通过理论计算和实验确定了制备致密不开裂涂层的泥浆配比; 研究了埋粉烧结和气相硅真空反应烧结2 种不同烧结气氛对Si/ SiC 涂层微观形貌和成分的影响; 比较了单涂层和双涂层2 种不同涂层制备方法对C/ SiC 复合材料基底结构的影响; 用SEM 观察涂层形貌, 用XRD 分析涂层成分与晶体结构。结果表明: 泥浆中C∶Si (质量比) 在1∶1. 25 左右制备的涂层不开裂; 埋粉烧结制备的涂层成粉, 而气相硅真空反应烧结制备的涂层致密且与基底结合好; 单涂层法制备涂层后基底材料致密度高, 而双涂层法制备涂层后基底仍然保持多孔结构。      

Single step fabrication of N-doped graphene/Si3N4/SiC heterostructures

In-plane heteroatom substitution of graphene is a promising strategy to modify its properties. The ability to dope graphene with electron-donor nitrogen heteroatoms is highly important for modulating electrical properties of graphene. Here we demonstrate a transfer-free method to directly grow large area quasi free-standing N-doped graphene bilayers on an insulating substrate （Si3N4）. Electron-bombardment heating under nitrogen flux results in simultaneous growth of N-doped graphene and a Si3N4 layer on the SiC surface. The decoupling of N-doped graphene from the substrate and the presence of Si3N4 are identified by X-ray photoemission spectroscopy and low-energy electron diffraction. The substitution of nitrogen atoms in the graphene planes was confirmed using high resolution X-ray photoemission spectroscopy which reveals several atomic configurations for the nitrogen atoms： Graphitic-like, pyridine-like, and pyrrolic- like. Furthermore, we demonstrated for the first time that N-doped graphene could be used to efficiently probe oxygen molecules via nitrogen atom defects.     

SiC/Co-Cr体系的润湿性研究

采用座滴法研究了反应烧结SiC/ Co-Cr 体系的润湿性。与反应烧结SiC/ 纯Co 体系进行对比, 研究了Cr 含量、实验温度和保温时间对润湿角的影响及活性元素的作用。结果表明, 加入适量的活性元素Cr 能够显著提高体系的润湿性。当体系的Cr 含量分别为5 % , 7 %和42 %时, 体系的润湿角较小, 润湿性比较好。SiC/ Co-Cr体系和SiC/ 纯Co 体系的润湿过程均属于反应性润湿, 实验温度和保温时间对体系的润湿角影响较大。微观结构研究和XRD 分析表明, 对于SiC/ 纯Co 体系, 界面区域发生了化学反应, 生成了CoSi , 减小了润湿角。加入活性元素Cr 以后, 由于Cr 元素与基体发生反应, 生成Cr₂₃C₆ , 进一步降低了体系的界面能, 提高了润湿性。      

Synthesis of WC/Co/Diamond Nanocomposites

ABSTRACT

A new method had been developed for the synthesis of a triphasic WC/Co/diamond nanocomposite, comprising a high volume fraction of diamond in a WC/Co matrix. Starting with available nanophase WC/Co powder, the new process involves: (1) partial sintering of a WC/Co powder compact to develop a porous preform, (2) chemical vapor infiltration (CVI) of the porous preform with amorphous or graphitic carbon, and (3) high pressure/high temperature (HPHT) consolidation of the carbon-infiltrated WC/Co preform to develop a fully-dense WC/Co/diamond nanocomposite. In the critical CVI step of the process, the kinetics of the carbon deposition may be controlled to develop either an uniform or graded distribution throughout the porous WC/Co preform. A functionally-graded triphasic superhard material, comprising a nanophase WC/Co core and a diamond-enriched surface, should combine high strength and toughness with superior wear resistance.     

Synthesis of Co3 O4 /graphene nanocomposite using paraffin wax for adsorption of methyl violet in water

This study discusses the use of Co₃ O₄ impregnated graphene (CoOIG) as an efficient adsorbent for the removal of methyl violet (MV) dye from wastewater. CoOIG nanocomposites have been prepared by pyrolyzing paraffin wax with cobalt acetate. The synthesised nanocomposite was characterised by X‐ray diffraction, field emission scanning electron microscope, transmission electron microscope, Fourier transform infrared spectroscope, Raman spectroscopy, and Brunauer–Emmett–Teller isotherm studies. The above studies indicate that the composites have cobalt oxide nanoparticles of size 51–58 nm embedded in the graphene nanoparticles. The adsorption studies were conducted with various parameters, pH, temperature and initial dye concentration, adsorbent dosage and contact time by the batch method. The adsorption of MV dye by the adsorbent CoOIG was about 90% initially at 15 min and 98% dye removal at pH 5. The data were fitted in Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich and Sips isotherm models. Various thermodynamic parameters like Gibbs free energy, enthalpy, and entropy of the on‐going adsorption process have also been calculated.Inspec keywords: cobalt compounds, graphene, nanoparticles, nanocomposites, nanofabrication, adsorption, dyes, scanning electron microscopy, field emission electron microscopy, transmission electron microscopy, Raman spectra, Fourier transform infrared spectra, free energy, enthalpy, entropyOther keywords: nanocomposite, paraffin wax, adsorption, methyl violet dye, water, X‐ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, Brunauer‐Emmett‐Teller isotherm, cobalt oxide nanoparticles, graphene nanoparticles, thermodynamic parameters, Gibbs free energy, enthalpy, entropy, Co₃ O₄ ‐C     

SiC/C nanocomposites with inverse opal structure

The synthesis, morphology, structural and optical characteristics of SiC/C nanocomposites with an inverse opal lattice have been investigated. The samples were prepared by thermochemical treatment of opal matrices filled with carbon compounds which was followed by silicon dioxide dissolution. The samples were studied by electron microscopy, x-ray diffraction, photoluminescence, IR and Raman scattering spectroscopy. The electron microscopy data revealed a highly porous periodic structure which was a three-dimensional replica of the voids of the initial opal lattice. The hexagonal silicon carbide was found to be non-uniformly distributed throughout the volume, its greater part located in the surface layer up to 50?μm deep. The data of x-ray diffraction, IR and Raman scattering spectroscopy enabled us to assume that the composite had hexagonal diamond fragments. The photoluminescence and optical reflection spectra of the composites have been measured.     

In-situ high temperature X-ray diffraction study of Co/SiC interface reactions

In situ experiments on the Co/SiC interface reaction were carried out with a high temperature X-ray diffractometer capable of measuring the X-ray diffraction pattern in 1–4s using an imaging plate. The kinetic formation processes of the interface reaction layer were measured in short-period exposure experiments with the apparatus. The time-temperature phase diagram of Co/SiC in N₂was determined. Co₂Si and CoSi were formed at the Co/SiC interface between 921 and 1573 K in N₂. The formation of CoSi obeyed the parabolic rate law. The value of the activation energy was 95 kJ/mol. The results of thermal expansion coefficient measurements suggest that when a sample is cooled to room temperature, compressive strain caused by CoSi occurs on SiC.     

Enchanced high-temperature performances of SiC/SiC composites by high densification and crystalline structure

We report the enchanced in situ performances of tensile strength and thermal conductivity at elevated temperatures of the PCS-free SiC/SiC composite with a high fiber volume fraction above 50% fabricated by NITE process for nuclear applications. The composite was fabricated by the optimized combination of the fiber coating, the matrix slurry and the pressure-sintering conditions, based on our previous composites’ study history. The composite showed the excellent tensile strength up to 1500 °C, that it retained approximately 88% of the room-temperature strength. Also, the thermal conductivity of the composites represented over 20 W/m K up to 1500 °C, which was enough high to take the advantage of the assumed design value for nuclear applications. Microstructural observation indicated that the excellent high-temperature performances regarding tensile strength and thermal conductivity up to 1500 °C were the contribution to the high densification and crystalline structure in matrix.     

Synthesis and high temperature mechanical properties of Ti3SiC2/SiC composite

A high density Ti₃SiC₂/20 vol % SiC composite was hot pressed under a uniaxial pressure of 45 MPa for 30 min in an Ar atmosphere at 1600 °C. The grain size of the Ti₃SiC₂/SiC composite was finer than that of monolithic Ti₃SiC₂, though the composite was hot pressed at a higher temperature, due to the dispersion of SiC particles in the Ti₃SiC₂ matrix. Room temperature fracture toughness of the composite and Vickers hardness were measured as 5.4 MPa m^1/2 and 1080 kg mm^–2, respectively. A higher flexure strength of the composite compared to that of monolithic Ti₃SiC₂ was measured both at room temperature and up to 1200 °C. At 1000 °C, the composite showed a lower oxidation rate than that of monolithic Ti₃SiC₂.     

A Coprecipitation Coating Synthesis of SiC/YAG Composites

The α-SiC in 0.5μm size powders were coated with Al_2O_3 and Y_2O_3 by a coprecipitation coating (CPC) method forfabrication of SiC/YAG composites. The same powder preparation was carried out by conventional mechanical mixing(MM) method for comparison. Two kinds of SiC/YAG composites were manufactured by pressureless sintering usingthe different powders, named CPC composite and MM composite thereafter respectively. It is shown that the CPCcomposite has the advantages of homogeneous distribution of YAG phase and of being sintered to high density ata low temperature, 100℃ lower than that of MM composite. The strength (573 MPa) and hardness (23.3 GPa) ofthe CPC composite are significantly higher than those (323 MPa and 13.5 GPa) of the MM composite, respectively.     

SiC/AlN复合球体的制备与表征

以粉煤灰和碳黑为原料n(SiO2)/n(C)=4.2,采用微波加热碳热还原法在1300℃下制备了SiC/AlN复合球体。利用X射线衍射仪(XRD)、拉曼光谱(RS)和扫描电子显微镜(SEM)对SiC/AlN复合球体的形貌和结构进行了表征,并分析了其形成机理。结果表明,以炭黑球为模板,粉煤灰提供Si源和Al源,通过微波加热碳热还原氮化反应可以制得具有梯度结构的SiC/AlN复合球体。所制备的SiC/AlN复合球体具有AlN-多型体的外壳、SiC纳米线过渡层和SiC晶须与花朵状SiC晶体构成的核心。     

Field emission properties of pointed cathodes based on graphene films on SiC

Electrical properties of low-threshold field emission cathodes produced by growth nanocluster graphene films on the pointed surface of heavily doped n ⁺SiC by sublimation epitaxy have been considered. The quality of the graphene coating has been assessed based on the morphological studies and Raman spectroscopy. Using the volt–ampere characteristics the work function from a pointed cathode with graphene coating was calculated (～ 0.76 eV). Such a low value of the work function is explained on the assumptions that the graphene film has the nanocluster nature and the sources of the field emission are graphene nanoclusters.     

三维聚苯乙烯/聚苯胺/石墨烯复合微粒的制备及电化学性能

为扩展石墨烯的宏观应用,制备性能优异的三维聚苯乙烯/聚苯胺/石墨烯(PS/PANI/graphene)复合微粒具有重要意义.以聚苯乙烯微粒为模板,通过2种浓度苯胺单体的原位生长得到2种聚苯乙烯/聚苯胺复合微粒,再利用氧化石墨烯与苯乙烯/聚苯胺微粒间的静电、共轭相互作用制备三维PS/PANI/graphene复合微粒.利用红外光谱(FTIR)、扫描电镜(SEM)、X射线衍射(XRD)、热重分析(TG)对其微观形貌、结构进行表征,利用电化学测试对三维复合微粒电化学性能进行测试.结果表明,复合材料保持了聚合物微粒的基本形貌,具有三维结构,并有优异的比电容(578 F/g)和循环稳定性(循环900次,容量保持81.5%),其电性能远优于单纯石墨烯和聚苯胺.     

Co_3O_4/介孔SiO_2复合体的制备及其介观结构的调控

以合成的螯合型表面活性剂N-月桂酰基乙二胺三乙酸(LED3A)为模板剂,利用其与Co2+的配位作用,一步法制备了Co3O4/介孔SiO2复合体。借助紫外光谱验证了LED3A与Co2+间的配位作用,热重分析仪测定样品合适的煅烧温度,通过傅里叶变换红外光谱、X射线衍射、高分辨透射电子显微镜以及N2吸附-脱附等方法对制备的Co3O4/介孔SiO2复合体进行了组分分析和结构表征。发现所形成的Co3O4纳米颗粒分布于SiO2孔道中,并可通过调节Co2+的加入量来控制LED3A极性头电负性,分别得到Pm3n立方相、p6mm二维六方相和Ia3d立方相等不同的介观结构。     

Atomic structure of graphene on SiO2

We employ scanning probe microscopy to reveal atomic structures and nanoscale morphology of graphene-based electronic devices (i.e., a graphene sheet supported by an insulating silicon dioxide substrate) for the first time. Atomic resolution scanning tunneling microscopy images reveal the presence of a strong spatially dependent perturbation, which breaks the hexagonal lattice symmetry of the graphitic lattice. Structural corrugations of the graphene sheet partially conform to the underlying silicon oxide substrate. These effects are obscured or modified on graphene devices processed with normal lithographic methods, as they are covered with a layer of photoresist residue. We enable our experiments by a novel cleaning process to produce atomically clean graphene sheets.