Transformation of graphite structure under mechanical grinding

Using x-ray diffractometry and computer simulation, the process of graphite structure transformation under mechanical grinding is investigated. It is established that all of main structural parameters vary over grinding time and are interrelated. The major parameter, whose variation affects the changes of the other, is the average size of the regions of coherent scattering along the crystallographic axis a, L_a. Reduction in the grain size is the main change under mechanical grinding, the change in the lattice parameters being its consequence. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 29–34, August, 2006.     

Structural transformations on a platinum surface under mechanical tension

The effect of uniaxial tension on the structure of a recrystallized platinum surface is investigated using low-energy electron diffraction (LEED). The initial LEED patterns indicate that the (111) facet emerges on the surface of a platinum foil after a series of heating cycles under vacuum and in oxygen. After loading at ～80 MPa, the clean platinum surface is characterized by systems of regular and irregular atomic terraces. The regular terraces have a (9(111) × 100) structure. As the load increases to 90–100 MPa, the ordered arrangement of terraces transforms into a disordered arrangement. After the samples are held under these loads for ～2 h, the surface structure undergoes a transformation into the diffraction-disordered state. Under tensile deformation, the island structure of graphite molecules on the recrystallized platinum surface containing ～10 at. % C also undergoes a transformation.     

Transformation of diamond to graphite under heat treatment at low pressure

Diamond is a very persistent metastable form of carbon. At low pressure, graphitization of the diamond surface is observed at temperatures lower than 1700 °C. The basic cause of this is known to be interaction with oxygen. In the present paper, the microphysics of the phenomenon of diamond surface graphitization was studied. It has been shown that surface graphitization proceeds in several stages. First, a thin surface layer of graphite is developed due to interaction with oxygen (and probably other substances). As temperature is increased, nuclei of 5–10 nm in size are formed within it. The graphite nuclei are capable of migrating along the surface of diamond. They form ‘nests,’ where they are coalesced into aggregates 10–100 nm in size. These ‘nests’ give rise to diamond surface layer graphitization, which proceeds along diamond {211} crystallographic planes and forms graphitization figures. At this stage, the factor that determines the transformation is the pressure exerted on graphitizing material by the surrounding diamond.     

Electronic states on the surface of graphite

Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip-sample distance beyond which the effect of sample-tip interaction is negligible.     

Hot-electron-induced plasma formation on the rear surface of a foil

The plasma jet formed on the rear surface of a foil in laser–solid interaction is investigated by laser probing. The rear plasma jet, which is in line with the laser, formed a few picoseconds after the incidence of the focused laser, is due to a beam of fast electrons propagating through the target and is collimated by a strong magnetic field in the plasma. Received: 14 January 2003 / Revised version: 2 April 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531 E-mail: jzhang@aphy.iphy.ac.cn     

On the crystallographic structure of small particles of platinum supported on graphite

The shape and crystallography of small metallic particles of platinum on a graphite substrate are studied by high resolution weak beam dark field and microdiffraction electron microscopy techniques. It is found that they have the fcc bulk crystal structure and their shape is a cubooctahedron bounded by {111} and {100} faces. The implications for the catalytic reaction of neopentane on Pt/C catalyst are discussed.     

Computer simulation of graphite target ablation under the action of a nanosecond laser pulse

A quasi-gasdynamic approach is used for computer simulation of plasma expansion from a graphite plate subjected to a nanosecond laser pulse. A one-component plasma consisting of carbon molecules alone is considered. This simplifies the experimental conditions used previously to study the dynamics of the gas resulting from evaporation. The results of computer experiment conducted for different initial temperatures and pressures of the plasma are in good qualitative agreement with the real experimental data including in the time instant the density of the expanding gas reaches a maximum. It is shown that high-density clusters are likely to appear in front of the main plasma flux. The results of the computer simulation are compared with the Singh approximation of pressure, velocity, and density of the gas flow. It is concluded that this approximation is valid only within a short (compared with the entire expansion length) plasma expansion interval existing during the initial spread for t = 4 × 10^?9 s.     

On the cryogenic layer formation under conditions of high-frequency mechanical action

The results of a series of experiments using a piezovibration formation module for producing cryogenic targets with a given fuel layer structure are presented.     

The mechanical property of single-walled carbon nanotube under combined electromechanical loading

The mechanical properties and failure process of single-walled carbon nanotube (SWCNT) under combined electric field and tensile loading are investigated using the semi-empirical quantum mechanical method. The local and global structural deformation and variation of mechanical properties of SWCNT under different directions and intensity of external electric field are discussed systematically. It is shown that the electric field induced deformation in the radial and axial directions of the SWCNT are strongly dependent on the direction of electric field. The analysis of mechanical properties shows that the structure stiffness, tensile strength and failure strain of the SWCNT all decrease with the increase of the field intensity, which is particularly evident under the longitudinal electric field. The Young's modulus of SWCNTs vary with the tube diameter and are affected by the electric field. The increase of the length of the tubes intensifies the charge concentration at the tube ends under the electric field and lead to the decrease of mechanical properties of SWCNTs. The failure process of SWCNTs under the coupling effect of electric field and tensile loading is found to be controlled by the field strength and also affected by the electric charge accumulation.     

Generalized Butler-Volmer relation on a curved electrode surface under the action of stress

According to the principle of thermal activation process, the energy state of a material under the action of stress is a function of local stress. A generalized Butler-Volmer relationship for the electrode reaction on the surface of a curved electrode is derived, which takes account of the effects of local stress and the radius of mean curvature. From this relationship, the overpotential is found to be proportional to hydrostatic stress and the activation volume under the condition of open circuit. The conditions for the deposition of the material made solely from solute atoms and the formation of surface pits and porous structures are obtained, using the generalized Butler-Volmer relationship.     

Direct observation of dynamic shape transformation and coalescence in platinum nanosheets on graphite surface at room temperature by time-resolved AFM

The shape transformation of platinum (Pt) nanosheets with a uniform thickness of as thin as 3.5 ± 1 nm supported on graphite was investigated by in situ atomic force microscopy (AFM). The AFM observations revealed the shape transformation and the coalescence in preferred directions for the Pt nanosheets at room temperature (25 °C), which is much lower than the melting point of bulk metallic platinum (1769 °C). The behavior may be attributed to the high surface energy for the edge parts of Pt nanosheets with the small curvature of the nanometer scale.     

Consideration of mechanical properties of single-walled carbon nanotubes under various loading conditions

In this article, mechanical properties of single-walled carbon nanotubes (SWCNTs) with various radiuses under tensile, compressive and lateral loads are considered. Stress–strain curve, elastic modulus, tensile, compressive and rotational stiffness, buckling behaviour, and critical axial compressive load and pressure of eight different zigzag and armchair SWCNTs are investigated to figure out the effect of radius and chirality on mechanical properties of nanotubes. Using molecular dynamic simulation (MDS) method, it can be explained that SWCNTs have higher Young’s modulus and tensile stiffness than compressive elastic modulus and compressive stiffness. Critical axial force of zigzag SWCNT is independent from the radius, but that of armchair type rises by increasing of radius, also these two types show different buckling modes.     

Formation of relief on a metallic target surface under the action of compression-plasma flows

The results of experimental and theoretical investigations of relief formation on the surface of a steel target (grade St 3 steel, GOST (State Standard) 380) during treatment by compression-plasma flows are represented. The density of energy absorbed by the target varied in the range from 15 to 25 J/cm², the pulse duration was 100 μs, and the pulse number was N = 1, 3, 5, 7. The experiment revealed the expansion of boundaries of the central area (the area on which the plasma flow is incident normally to the surface) with increasing pulse number. This is explained by the more uniform surface treatment at a greater pulse number. It is shown that to describe relief formation in the central area there is a need to take into account the pressure of the plasma flow on the target surface, apart from surface tension forces and energy dissipation due to viscosity.     

强激光辐照下纯铝的力学响应和层裂的实验测量与分析

 采用速度干涉（VISAR）测试技术,对强激光辐照下纯铝的动态力学响应和层裂特性进行了实验测量和分析。样品厚度分别为200 μm 和485 μm,激光脉冲的半高宽约为10 ns,功率密度变化范围为10¹⁰~10¹¹ W·cm^-2。实测了样品自由面速度波形,反映了强激光加载作用下材料损伤演化过程以及损伤对材料动态响应的影响。计算得到了冲击波强度(2.0~13.4 GPa) 和不同拉伸应变率下铝的层裂强度(1.6~2.3 GPa)。在所采用的实验条件和1维近似下,激光辐照产生的冲击波强度与激光功率密度之间成线性关系。最后讨论了层裂强度与拉伸应变率之间的关系,显示层裂强度随着拉伸应变率的增加而增大。     

强激光辐照下纯铝的力学响应和层裂的实验测量与分析

采用速度干涉（VISAR）测试技术,对强激光辐照下纯铝的动态力学响应和层裂特性进行了实验测量和分析。样品厚度分别为200 μm 和485 μm,激光脉冲的半高宽约为10 ns,功率密度变化范围为1010~1011 W·cm-2。实测了样品自由面速度波形,反映了强激光加载作用下材料损伤演化过程以及损伤对材料动态响应的影响。计算得到了冲击波强度(2.0~13.4 GPa) 和不同拉伸应变率下铝的层裂强度(1.6~2.3 GPa)。在所采用的实验条件和1维近似下,激光辐照产生的冲击波强度与激光功率密度之间成线性关系。最后讨论了层裂强度与拉伸应变率之间的关系,显示层裂强度随着拉伸应变率的增加而增大。     

Thermomechanical response of aluminum alloys under the combined action of tensile loading and laser irradiations

This study reports the investigation of the thermomechanical behavior of aluminum alloys(Al-1060, Al-6061, and Al-7075) under the combined action of tensile loading and laser irradiations. The continuous wave ytterbium fiber laser(wavelength 1080 nm) was employed as the irradiation source, while tensile loading was provided by the tensile testing machine. The effects of various pre-loading and laser power densities on the failure time, temperature distribution, and the deformation behavior of aluminum alloys are analyzed. The experimental results represent the significant reduction in failure time for higher laser power densities and for high preloading values, which implies that preloading may contribute a significant role in the failure of the material at elevated temperature. Fracture on a microscopic scale was predominantly ductile comprising micro-void nucleation, growth, and coalescence. The Al-1060 specimens behaved plastically to some extent, while Al-6061 and Al-7075 specimens experienced catastrophic failure. The reason and characterization of material failure by tensile and laser loading are explored in detail. A comparative behavior of under-tested materials is also investigated. This work suggests that studies considering only combined loading are not enough to fully understand the mechanical behavior of under-tested materials. For complete characterization, one should consider the effect of heating as well as loading rate and the corresponding involved processes with the help of thermomechanical coupling and the thermal elastic-plastic theory.