PAS烧结SiC-石墨复相陶瓷的阻力曲线行为和抗热震性能

采用PAS烧结工艺,在1 680℃成功制备出Si C-石墨复相陶瓷,并研究了石墨含量对复相陶瓷物理力学性能、韧性、R曲线(阻力曲线)行为和抗热震性能的影响。结果表明,随石墨含量的增加,复相陶瓷的致密度、抗弯强度、弹性模量和维氏硬度均有所降低;以韧化比(TI)作为复相陶瓷的韧性指标,表明石墨含量越多,复相陶瓷的韧性越好,存在陡峭的上升R曲线行为,该指标更能反映复相陶瓷的韧性特征;显微形貌分析发现,复相陶瓷中存在的层状石墨增加了能量耗损,裂纹在扩展过程中发生偏转、分叉和桥联等现象,使得Si C复相陶瓷的韧性和抗热震性能得以改善。

Effect of wear conditions on tribological properties of electrolessly-deposited Ni-P-Gr-SiC hybrid composite coating

The friction and wear properties of the electrolessly-deposited Ni-P-Gr-SiC composites were investigated. The effects of graphite content, load and rotation speed on the friction coefficient and wear resistance of the composite coatings were mainly investigated. The worn surface and cross section of the coatings were characterized by scanning electron microscopy and energy-dispersive X-ray analysis. The results show that the composite coatings reveal good antifriction and wear resistance due to the synergic effect of graphite and SiC particles. The formation of graphite-rich mechanically mixed layer （GRMML） on the surface of Ni-P-Gr-SiC coating contributes to the good tribological behavior of the wear counterparts and SiC particles play a load bearing role in protecting GRMML from shearing easily.

Wetting of pure aluminium on graphite,SiC and Al_2O_3 in aluminium filtration

The wettability of pure aluminium on filter materials and on inclusions is believed to be an important factor affecting the filtration of aluminium. The contact angles of molten aluminium on alumina, SiC and graphite were measured under 10-8 bar high vacuum in the temperature range of 1000-1300 °C. To describe the wetting behaviour of the Al on ceramic at lower temperatures used in filtration and casting aluminium, a semi-empirical calculation was employed. The calculated contact angles at 700 °C were around 97° for alumina, 92° for vitreous graphite, 126° for single- and poly-crystal graphite, and 79° for single crystal SiC, respectively. This indicates that aluminium does not wet alumina or graphite （or Al4C3） around the casting temperature, but wets SiC at this temperature. Thus a priming height is required for aluminium to infiltrate an alumina filter. Increasing temperature can also improve the wettability of Al on ceramic.

Fabrication, microstructure, friction and wear properties of SiC3D/Al brake disc-graphite/Si C pad tribo-couple for high-speed train

The friction and wear properties of interpenetrating phase composites(IPC) SiC3D/Al sliding against graphite/SiC(G/SiC) composites were investigated using a sub-scale brake dynamometer. The testing conditions included a braking pressure of 1.25 MPa and an initial braking speed(IBS) of 200-350 km/h in a braking process of high-speed train according to the scale-conversion rules. The tribo-couple materials were characterized using scanning electron microscopy(SEM), X-ray diffractometry(XRD), and energy-dispersive X-ray spectrometry(EDS). It is found that the matching tribo-couple features low friction surface temperature, reliable friction factor, and high durability. The continuous lubricating mechanically-mixed layer(MML) forms gradually on the worn surfaces of ring in the friction process. The MML is heterogeneous, which greatly controls wear rate and coefficient of friction(COF) of the composites. The wear mechanism of SiC3D/Al is typically abrasive wear at an IBS of 200-300 km/h. When the IBS increases to 350 km/h, oxidation wear and delamination are observed. The friction behavior of the tribo-couple predicted using Solidwork simulation software agrees well with the experimental results. The tribo-couple meets the requirement of emergency braking of high-speed train.

Fabrication and characterization of graphene derived from SiC

Using novel ideas for the fabrication of epitaxial graphene （EG） on SiC, two forms of graphene termed as vertical aligned gra- phene sheets （VAGS） and graphene covered SiC powder （GCSP） were derived, respectively, from SiC slices and SiC powder, aimed for applications in energy storage and photocatalysis. Herein, the fabrication procedures, morphology characteristics, some intrinsic physical properties and performances for applications in field effect transistor （FET） and cold cathode field emission source are revealed and analyzed based on the graphene materials. The EG on a 2-inch SiC （0001） showed an average sheet resistance about 720 D,/~5 with a non-uniformity 7.2%. The FETs fabricated on the EG possessed a cutoff frequency 80 GHz. Based on the VAGS derived from a completely carbonized SiC slice, a magnetic phase diagram of graphene with irregu- lar zigzag edges is also reported.

Simultaneous N-intercalation and N-doping of epitaxial graphene on 6H-SiC（0001） through thermal reactions with ammonia

Surface functionalization of epitaxial graphene overlayers on 6H-SiC（0001） has been attempted through thermal reactions in NH3. X-ray photoelectron spectroscopy and micro-region low energy electron diffraction results show that a significant amount of N is present at the NHB-treated graphene surface, which results in strong band bending at the SiC surface as well as decoupling of the graphene overlayers from the substrate. The majority of the surface N species can be removed by annealing in vacuum up to 850 ~C, weakening the surface band bending and resuming the strong coupling of graphene with the SiC surface. The desorbed N atoms can be attributed to the intercalated species between graphene and SiC. Low temperature scanning tunneling spectroscopy and density functional theory simulations confirm the presence of N dopants in the graphene lattice, which are in the form of graphitic substitution and can be stable above 850 ~C. This is the first report of simultaneous N intercalation and N doping of epitaxial graphene overlayers on SiC, and it may be employed to alter the surface physical and chemical properties of epitaxial graphene overlayers.

Hydrogenation of the buffer-layer graphene on 6H-SiC （0001）： A possible route for the engineering of graphene- based devices

The hydrogenation at various temperatures of the （6√3 ×6√3）R30° reconstruction of SIC（0001）, the so-called buffer layer graphene （BLG）, is investigated. For the BLG, a significant concentration of remaining dangling bonds related to unsaturated Si atoms of the outermost SiC bilayer is evidenced in the inverse photoemission spectra. These dangling bonds give rise to a peak around 1 eV above the Fermi level, associated with the upper single-electron states of a Mott-Hubbard insulator, which vanishes upon hydrogenation. Hydrogen atoms adsorbed at ambient temperature remain covalently bound to BLG （H-BLG） up to temperatures of -500℃. They induce additional C-Si bonds at the BLG/SiC interface that saturate the remaining Si dangling bonds, as evidenced in both IPES and Auger electron spectra. The H-BLG further shows a large energy gap and an excess n-type doping in comparison to the pristine BLG. Upon hydrogen exposure at higher temperature （〉 700 ℃）, hydrogen atoms intercalate at the BLG/SiC interface, inducing the formation of a single layer of quasi-free- standing graphene （QFSG） lying on top of a hydrogenated （√3×√3）R30° reconstruction as supported by IPES. We suggest that the high-stability and the distinct electronic structure of both BLG-derived structures, H-BLG and QFSG, may open a route for the engineering of graphene-based devices.

Self-organized metal-semiconductor epitaxial graphene layer on off-axis 4H-SiC（0001）

The remarkable properties of graphene have shown promise for new perspectives in future electronics, notably for nanometer scale devices. Here we grow graphene epitaxially on an off-axis 4H-SiC（0001） substrate and demonstrate the formation of periodic arrangement of monolayer graphene on planar （0001） terraces and Bernal bilayer graphene on （1120） nanofacets of SiC. We investigate these lateral superlattices using Raman spectroscopy, atomic force microscopy/ electrostatic force microscopy （AFM/EFM） and X-ray and angle resolved photoemission spectroscopy （XPS/ARPES）. The correlation of EFM and ARPES reveals the appearance of permanent electronic band gaps in AB-stacked bilayer graphene on （1120） SiC nanofacets of 150 meV. This feature is confirmed by density functional theory （DFT） calculations. The charge transfer between the substrate and graphene bilayer results in an asymmetric charge distribution between the top and the bottom graphene layers opening an energy gap. This surface organization can be thus defined as self-organized metal-semiconductor graphene.