Temperature effects in ion-electron emission of graphite quasicrystal

Analysis of the comparative influence of complete amorphization of a polycrystalline graphite surface layer and radiation-induced disordering of highly oriented pyrolytic graphite UPV-1T on the character of temperature dependences of the ion-electron emission yield under high-dose (10¹⁸–10¹⁹ ion/cm²) 30 keV Ar⁺ ion irradiation has been carried out.     

Kossel lines in angular distribution of X-rays excited by protons in pyrolytic graphite quasi-crystal

Kossel line profiles were experimentally studied upon 1.5-MeV proton excitation of characteristic K _α radiation of argon atoms implanted into an UPV-1T graphite quasi-crystal. It was found that Kossel line profiles depend on argon ion implantation conditions, i.e., the lines generated in crystals implanted at temperatures of 150 and 200°C exhibit different widths and contrasts. In addition to the main Kossel cone line, a second cone line was detected caused by an additional texture component in the UPV-1T pyrolytic graphite quasi-crystal.     

Sputtering of highly oriented pyrolytic graphite with 30 keV Argon ions

The influence of the incidence angle of 30 keV Ar⁺ ions, ion fluence and target temperature on the sputtering yield and surface microgeometry of highly oriented pyrolytic graphite (UPV-1T) samples was experimentally studied. It was found that at fluences more than 5 × 10¹⁹ ion cm^?2 the sputtering yield at room temperature in the range of the ion incidence angle from 0° to 80° is twice as small as the corresponding experimental data for both polycrystalline graphite and glassy carbon. The analysis of ion-induced relief permits us to suppose the topographical suppression mechanism of highly oriented pyrolytic graphite sputtering.     

Effect of the ion current density on the temperature dependences of ion-induced electron emission from carbon-based materials

The effect of ion current density j on the temperature dependences of the ion-induced electron emission yield γ(T) of carbon-based materials (polycrystalline graphite MPG-8, highly oriented pyrolytic graphite UPV-1T, and some glassy carbons) under irradiation by atomic and molecular nitrogen ions with energies of several tens of keV has been experimentally studied. The most significant effect of the density j on the dependences γ(T) is observed for low-temperature glassy carbons.     

Study of carbon material surface layers modified by nitrogen and argon ion irradiation

We present the results of the study of the elemental composition and defects of the electronic structure of the surface layer modified by high-dose irradiation (10¹⁸–10¹⁹ ion/cm²) of highly oriented pyrolytic graphite (UPV-1T) by 30-keV N ₂ ⁺ and Ar⁺ ions in the temperature range from 180 to 400°C. The EPR spectra observed during irradiation with argon ions at high temperatures and with nitrogen ions at temperatures near the liquid-nitrogen temperature T = 77 K exhibit anomalously narrow lines which probably result from the exchange interaction inside paramagnetic clusters of displaced carbon atoms. During nitrogen ion irradiation at room and higher temperatures, paramagnetic defects typical of many carbon materials (single EPR lines with g = 2.0027–2.0029) and belonging to carbon atoms bound to one or three nitrogen atoms were detected.     

Electrical conductivity and chemical bond of graphite intercalation compound with fluorine and metal fluoride

Graphite intercalation compounds with fluorine and metal fluoride (MgF₂ or CuF₂) were prepared from petroleum coke and pyrolytic graphite. With progress in the intercalation reaction, the first stage compound with identity period 9.4 Å changed to another structure of identity period 10.7 Å. It was found from ESCA measurements that the chemical interaction between intercalated fluorine and carbon was similar to the covalent bond around the surface and slightly ionic in the bulk. The maximum electrical conductivities in the direction of the ab-axis were (1.9–2.0) × 10⁵ (ωcm)^-1, which were 10–13 times that of the original pyrolytic graphite.     

Surface structure changes in carbon-based materials and polymers under an oxygen-plasma beam

The surface topography of samples of graphite USB-15, pyrolytic carbon PGI, pyrolitic graphite UPV-1, and polyimide PM-1E exposed to accelerated oxygen-plasma beams has been studied by scanning electron microscopy. Surface etching by fast oxygen particles results in the formation of a system of cone/column-like protrusions. As the material mass loss increases, the protrusions increase in size, and the surface topography transforms. Etching of carbon-based materials leads to enrichment of the surface layer with carbon, which is most pronounced in the boron-doped graphite.     

Formation of functional groups on graphite during oxygen plasma treatment

Improved sample wettability was obtained by oxygen plasma functionalization of pyrolytic graphite. The samples were exposed to highly dissociated oxygen plasma with the density of 1 × 10¹⁶ m⁻³, the electron temperature of about 5.5 eV and the density of neutral oxygen atoms of 8 × 10²¹ m⁻³ for 20 s. The surface wettability was measured by a contact angle of water drop. The contact angle dropped from original 112° down to about 1°. The functional groups were detected by XPS analyses. The survey spectrum showed a substantial increase of oxygen concentration on the surface, while high-resolution analyses showed additional oxygen was bonded onto the graphite surface in the form of C-O polar functional group responsible for the increase of the surface energy.     

Study of pyrolytic carbon films for non-emitting grid applications

Pyrolytic carbon films were prepared on a hot substrate inside UHV system back-filled with 6 × 10⁻⁵ Torr of CH₄. The resulting films showed two different chemical states of carbon; carbides formed on W and Ta substrates and graphitic carbon layers were yound on ?. Ir substrate. Alter BaO deposition, the carbides showed a work function too low to be useful as grids. On the other hand, the work function of pyrolytic carbon films were high both before and after Ba or BaO coverage. Study of the degradation mechanisms of carbon films, formed by e-bean techniques, however, showed that they also reacted rapidly with W and T₂ resulting in a carbide formation. Thus it suggested that in the application of pyrolytic graphite grids, contact with carbide-forming metals should be avoided. Interaction of the pyrolytic carbon with BaO also resulted in the removal of carbon. However, this “leeching” phenomenon projects a life for pyrolytic graphite grids much longer than actually observed in the tube tests. The diffusion of Ba and BaO through pyrolytic carbon layer was evident in this workm but its precise relation to life failure rate is not yet understood.     

Kinetics of the formation of gas bubbles during polarization of copper and graphite electrodes in electrolytic aqueous solutions

The adsorption and kinetic processes of the formation of gas bubbles passivating the surface during polarization of copper and graphite electrodes in 1% aqueous solution of sulfuric acid have been investigated. Three stages of the process related to the recharging of the double electric layer—adsorption accumulation of the gas escaping from the surface, the critical nucleation of the gas bubbles, and their subsequent growth—have been revealed, distinguished, and quantitatively estimated. It has been shown that potential leveling at the steady-state value specified by the Tafel equation is unambiguously associated with achievement of the limiting surface area screened by the gas bubbles for each particular current density. The surface diffusion constants D _H = (1.5–4.4) × 10⁻⁴ and (0.1–3.8) × 10⁻⁵ cm²/s of hydrogen on copper and graphite, respectively, and D _O = (1.8–4.5) × 10⁻⁷ cm²/s of oxygen on graphite during the motion toward the drain (the gas bubbles) have been calculated.     

Effect of pyrolytic polyacrylonitrile on electrochemical performance of Si/graphite composite anode for lithium-ion batteries

The silicon/graphite (Si/G) composite was prepared using pyrolytic polyacrylonitrile (PAN) as carbon precursor, which is a nitrogen-doped carbon that provides efficient pathway for electron transfer. The combination of flake graphite and pyrolytic carbon layer accommodates the large volume expansion of Si during discharge-charge process. The Si/G composite was synthesized via cost-effective liquid solidification followed by carbonization process. The effect of PAN content on electrochemical performance of composites was investigated. The composite containing 40 wt% PAN exhibits a relatively better rate capability and cycle performance than others. It exhibits initial reversible specific capacity of 793.6 mAh g^?1 at a current density of 100 mA g^?1. High capacity of 661 mAh g^?1 can be reached after 50 cycles at current density of 500 mA g^?1.     

Ion-induced self-organized ripple patterns on graphite and diamond surfaces

In order to investigate the allotropic effect on ripple pattern formation, highly oriented pyrolytic graphite (HOPG) and single crystalline diamond were irradiated with 10-200 keV Xe⁺ at an incident angle of 60° with respective to the surface normal. The irradiation fluence was 2 × 10¹⁷ cm⁻² for all irradiations. Ripple patterns were observed on both HOPG and diamond surfaces. However, large differences in ripple wavelengths, amplitudes and surface roughnesses between HOPG and diamond were recognized. The reason for these differences is discussed.     

Formation of Gallium-induced nanostructures on single crystal HOPG surface

The room temperature growth of gallium atoms on the highly oriented pyrolytic graphite (HOPG) surface has been performed. The gallium atoms were deposited by thermal evaporation method in an ultra high vacuum system at a base pressure 5 × 10⁻¹⁰ torr. The X-ray photo electron spectroscopy (XPS) studies had been performed to confirm the presence of gallium atoms on HOPG surface. Scanning tunneling spectroscopy (STM) technique was employed to study the surface morphology of the clean HOPG surface and gallium covered HOPG surfaces which recognize the formation of gallium induced nanostructures. The deconvoluted XPS core level spectra of C (1s) and Ga (3d) demonstrate the possible interaction between substrate and the adsorbate atoms. The STM analysis revealed that the gallium deposition on HOPG led to significant change in the surface morphology. It was observed that the Ga atoms adsorbed as layer structure on HOPG surface for low coverage while quasi one-dimensional chain like nanostructure (1 ± 0.2 nm) has been formed for higher Ga coverage. The nanostructured surfaces induced by Ga deposition are found to be stable and could be used as a template for the growth of metallic nanostructures.     

Intercalation of lithium ions into bulk and powder highly oriented pyrolytic graphite

The electrochemical reactions of highly oriented pyrolytic graphite (HOPG) bulk and powder electrodes in 1 M LiPF₆ 1:1 EC/DMC solution were investigated and the results show that the intercalation reaction of lithium ion into HOPG electrode occurs only at the edge plane and SEI formation reaction on the basal plane is negligible in comparison with that on the edge plane. The active surface area of HOPG powder electrode could be deduced by comparing the peak area (consumed charge for SEI formation) at potential of 0.5 V on voltammograms with that of bulk HOPG edge electrode. The diffusion coefficients of lithium ion in HOPG bulk layers and in HOPG powder was for the first time measured by use of electrochemical impedance spectra and potential step chronamperameter methods. It was found that the diffusion coefficients of lithium in HOPG were in the range of 10⁻¹¹-10⁻¹² cm² s⁻¹ for the lithium-HOPG intercalation compounds at potentials from 0.2 (vs. Li/Li⁺) to 0.02 V, decreasing with the increase of lithium intercalation degree. A good agreement was obtained between the results from bulk and powder HOPG electrodes by electrochemical impedance spectra method.     

A new approach for the determination of the C ls binding energy

The binding energy of the C 1s-level of clean pyrolytic graphite and amorphous carbon has been measured by means of the excitation-curve technique. Values ofE _{C 1s} ^B =284.31±0.2 eV for graphite and amorphous carbon have been found.     

Low-density carbon material modified with pyrolytic carbon

Here we report a physicochemical investigation of low-density carbon materials modified with pyrolytic carbon (PC). Exfoliated graphite (EG) obtained by nitrate expandable graphite thermal destruction was pressed into low-density graphite materials (LDGMs) with densities of 0.045-0.50 g/cm³ and saturated with PC by impact CVI technique. LDGM infiltration with PC leads to sample weight and density growth. The amount of deposited PC strictly depends on synthesis conditions. The maximum surface and volume deposition of PC occurred for samples with density of 0.05 g/cm³. XRD, Raman spectroscopy and scanning electron microscopy revealed that the deposited PC is of smooth laminar (SL) type. Composite thermal conductivity is about 2-3.5 Wt/m K.     

Nuclear electric quadrupole interaction of implanted ions in graphite

A number of isotopes have been implanted into thin foils of highly oriented pyrolytic graphite in order to investigate the possibility of using this material as a catcher for on-line LT-NO experiments to measure the quadrupole interaction of short-lived nuclei. Pure nuclear electric quadrupole interaction of nuclei of^188,189Ir,²⁰³Hg,^69mZn,^182m,183Re and¹¹¹In has been observed. Values for the electric field gradient along theC-axis are: +10.0(3) +8.2(9), +5.4(2.6) and +1.5(1)·10²² V/m², respectively, for Ir, Hg, Re and In in graphite. A value ofQ=+0.79(6) b is deduced for the quadrupole moment of¹⁸⁹Ir. Two theoretical models provide a better understanding of the origin of the electric field gradient in graphite. The first is based on induced electric polarization of graphite atoms, while in the second one hybridization of impurity and graphite electronic wave functions is calculated.     

Observation of Raman band shifting with excitation wavelength for carbons and graphites

The Raman spectra of crystalline graphite, graphite damaged by ion-etching, and structurally disordered pyrolytic and glassy carbons were examined as a function of excitation wavelength λ over the range 488.0 to 647.1 nm. The bands located at 1360, 2720 and 2950 cm ¹ with λ = 488.0 nm undergo a progressive red-shift as λ increases, while the positions of the other major bands (1580 and 1620 cm ¹) are invariant. The significance of these observations is briefly discussed.     

TDPAD measurements with highly oriented pyrolytic graphite using the19F(p,p′)19F* and19F(α, n)22Na* reactions

The TDPAD technique has been used to investigate quadrupole interactions of¹⁹F*(E _x =197 keV,J ^π=5/2⁺,T _1/2=88.5 ns) and²²Na*(E _x =583 keV,J ^π=1⁺,T _1/2=243 ns) in highly oriented pyrolytic graphite (HOPG). For¹⁹F* a single static quadrupole interaction was found with field gradientV _ZZ =3.24(19) × 10¹⁸ V/cm². For the case of²²Na* no quadrupole interaction was detected, leading to the conclusion that |QV _ZZ |<5.2(5) × 10¹⁵ b V/cm². TakingQ=0.06 b for the 583 keV state on the basis ofB(E2) measurements and theoretical estimates, we find that |V _ZZ | <8.7(8) × 10¹⁶ V/cm². The data demonstrate the advantages of using HOPG as a host material for the study of quadrupole interactions. Former Central Bureau for Nuclear Measurements (CBNM).