Properties of graphite prepared from boron-doped pitch as an anode for a rechargeable Li ion battery

Boron-doped graphites were derived from a naphthalene-based pitch mixed with para-xylene glycol (PXG) or dimethyl para-xylene glycol (DMPXG) as a cross-linking agent and three types of boron-containing compounds as a graphitization catalyst, and their anode performances were investigated. The structural analysis of the obtained graphites revealed that PXG functioned mainly as a two-dimensional cross-linking agent during the heat treatment process and DMPXG functioned partially as a three-dimensional. The average interlayer spacing decreased and lattice constant, a₀, and graphitizability increased with increasing the amount of boron atoms added. The result indicated that the carbon atoms were replaced by boron atoms. The anode performance was improved by the enhancement of graphitizability. The structural parameters and anode performance of boron-doped graphites did not depend on the kind of boron-containing compounds but the amount of boron atoms added in pitch and the kind of cross-linking agent.     

Quantum chemical study of the reactivity of boron-doped graphite layers towards water formation

Density functional calculations were used to study some fundamental features of boron-doped graphite layers (C_xB_y) and the boron influence on the mechanisms leading to the formation of water molecules on the C_xB_y graphite like layers. The Langmuir-Hinshelwood reactions leading to water formation on the graphite-like layers containing 12 at% boron take place with activation energies 3-5 times lower than on pure graphite ones. For the Eley-Rideal mechanism, the activation energies are always very low whether or not the graphite contains boron. Similar results were observed for 25 at% B doping. As a consequence, the oxygen and hydrogen can be more easily eliminated from the doped surfaces in the form of water molecules than from the corresponding pure ones. The C_xB_y layers with high boron content or having accumulations of boron, lose their planar structure. Two such parallel layers strongly interact through the boron atoms with the formation of a B-B bond and the displacement of the boron atoms into the inter-layer space. As a whole the system deviates from the graphite-like structure.     

Doped carbon black catalyst supports: I. Characterization by magnetic susceptibility and ESR spectroscopy

As-received Monarch carbon blacks (MC) from Cabot, graphitized carbons (GMC), desulfurized Carbolac-2, and CSX-203 were characterized by magnetic susceptibility and ESR spectroscopy at room temperature and those in the cryogenic range. The MC samples showed normal Langevin-type diamagnetism, whereas the GMC showed a temperature-dependent Landau diamagnetism. From this, the degeneracy temperature, the effective electron mass of the delocalized electrons, and the Fermi level (E_F) were estimated, assuming the approximations of a “two-dimensional electron gas” model for graphite. The GMC was doped with boron at the ppm level at heat treatment temperatures (HTT) of 2073 and 2773 K. Boronation at low HTT indicated that boron atoms could enter the interstitial or defect sites, in addition to the substitutional (sp²) sites, whereas boronation at the higher HTT showed that all boron atoms entered at the substitutional sites. ESR studies indicated a lowering of g-values, and half-line widths, and a ten-fold decrease in the spin concentration as a result of boron doping. A transition from Landau to almost Langevin type diamagnetic behavior was observed; a decrease in diamagnetic susceptibility corresponded to a lowering in the E_F level of the carbons. Shifts in the E_F have been estimated as a function of boron doping. In a sequel to this paper, the preparation of Fe catalysts on the above carbon supports are presented along with their magnetic, Mössbauer, and CO-hydrogenation studies. A rationale for changing the electronic properties of the substrate by boron doping is given, in relation to changing the properties of supported metal (M₁) by “alloying” it with another metal (M₂) to form the socalled “bimetallic” clusters.     

Graphitized boron-doped carbon foams: Performance as anodes in lithium-ion batteries

The electrochemical performance as potential anodes in lithium-ion batteries of several boron-doped and non-doped graphitic foams with different degree of structural order was investigated by galvanostatic cycling. The boron-doped foams were prepared by the co-pyrolysis of a coal and two boron sources (boron oxide and a borane–pyridine complex), followed by heat treatment in the 2400–2800 °C temperature interval. The extent of the graphitization process of the carbon foams depends on boron concentration and source. Because of the catalytic effect of boron, lightweight graphite-like foams were prepared. Boron in the foams was found to be present as carbide (B₄C), in substitutional positions in the carbon lattice (B–C), bonded to nitrogen (B–N) and forming clusters. Larger reversible lithium storage capacities with values up to ∼310 mA h g⁻¹ were achieved by using the boron oxide-based carbon foams. Moreover, since the electrochemical anodic performance of these boron-doped foams with different degree of structural order is similar, the beneficial effect of the presence of the B–C boron phase was inferred. However, the bonding of boron with nitrogen in the pyridine borane-based has a negative effect on lithium intercalation.     

Microstructure and thermophysical properties of B4C/graphite composites containing substitutional boron

B₄C/graphite composites (BGC) containing substitutional boron were fabricated by pressureless sintering of powder mixtures of petroleum coke, coal tar pitch and B₄C. After sintering at 900 °C and graphitizing at 2200 °C, the microstructure of BGC was characterized by SEM, TEM, XRD, Raman spectroscopy and optical microscopy. XPS measurements revealed the formation of BC₃, and the matrix carbon contained around 6 wt.% substitutional boron. The thermal conductivity of the BGC at room temperature is 52.7 W/m K and the flexural strength is up to 35.1 MPa. The bulk density and electrical resistivity are 1.72 g/cm³ and 13.4 μΩ m, respectively. The correlation between microstructure and properties was investigated. The results showed that the microstructure improvement of the BGC has obvious effect on the thermal conductivity, flexural strength, and electrical resistivity.     

B/B 1D/2D solid-state high-resolution NMR studies on boron-doped diamond

In this work, three boron-doped diamond samples prepared from a high-pressure high-temperature synthesis method with the same starting materials but with different initial ratios for boron are studied. Magnetic susceptibility measurement shows that the increment of the initial amount of boron does not straightforwardly bring lower superconducting transition temperature. In accordance with our previous ¹¹B high-resolution magic-angle spinning (MAS) NMR results, we show that there are at least four boron signal components and the one at 28.5 ppm is ascribed to the substitutional boron in the diamond structure providing the carriers responsible for conductivity. From observed relative intensities of the four signal components, it is suggested that the excess boron, which does not contribute to the conductivity appears as a broad signal at around 65.5 ppm. We apply two-dimensional (2D) NMR to examine ¹H dipolar broadening and ¹¹B–¹¹B boron spin diffusion, and candidates purported so far for the excess boron, that is, a boron + hydrogen complex and –B–B– and/or –B–C–B– clusters are negated. From its chemical-shift value, it is suggested that the excess boron exists as trigonally coordinated boron. We further apply 2D nutation NMR to examine the size of its quadrupolar coupling and show that it is not very large. As for a structure that compromises the trigonal coordination invoked from the chemical-shift value and the small quadrupolar coupling, we postulate boron locally in a graphite-like structure but the symmetry of the electric field gradient around it is high. Furthermore, we show that ¹⁰B MAS NMR is useful to selectively observe the substitutional boron in the diamond structure appearing at 28.5 ppm, whose quadrupolar coupling is much smaller than that of the excess boron at 65.5 ppm.     

Synthesis of heterofullerene using a direct BN substitution reaction of fullerene

A heterofullerene in which carbon atoms of the fullerene cage are substituted by heteroatoms (boron and nitrogen) was synthesized by BN substitution reaction of fullerene C₆₀ upon irradiation with a KrF excimer laser at room temperature. The products were purified by a high performance liquid chromatography and analyzed with a double focusing mass and mass/mass spectrometer, revealing that BNC₅₈ (m/z 721) was formed. The existence of boron and nitrogen in the product was confirmed by X-ray photoelectron spectroscopy and ¹³C NMR analyses. Moreover, ¹¹B NMR spectrum of the sample indicated the existence of BN bond in the compound.     

A first-principles study of calcium-decorated, boron-doped graphene for high capacity hydrogen storage

Hydrogen adsorption and storage on calcium-decorated, boron-doped graphene was explored using density functional theory simulations based on local density approximation and generalized gradient approximation methods. The clustering problem for calcium-decorated graphene was investigated and it was shown that individual calcium atoms are not stable on pure graphene, and formation of aggregates is favorable. Substitutional boron doping can eliminate the clustering problem for Ca atoms on graphene. Up to four hydrogen molecules can stably bind to a Ca atom on a graphene plane with substitutional doping of a single boron atom. The average binding energy of ∼0.4 eV/H₂ is in the range that permits H₂ recycling at ambient conditions. Two binding mechanisms contribute to the adsorption of H₂ molecules: polarization of the H₂ molecule under the electric field produced by the Ca–graphene dipole, and hybridization of the 3d orbitals of Ca with the σ orbitals of H₂. Double-sided Ca-decorated graphene doped with individual boron atoms of 12 at.% can theoretically reach a gravimetric capacity of 8.38 wt.% hydrogen.     

Effects of boron doping in low- and high-surface-area carbon powders

Two distinctive carbon materials (Saran char and SP-1 graphite) were doped with B at different loading to clarify the intrinsic effect of substitutional B on carbon reactivity. The carbon precursors would be affected in different style by substitutional B due to different important properties (crystallinity and surface area). The B retentivity depended on the nature of B dopant and carbon substrate; a less ordered carbon has higher B loading than its counterpart. Graphitization was enhanced by substitutional B, as expected. Furthermore, the B incorporation was still beneficial for SP-1 although it already had high crystallinity. An interesting behavior was noticed; the increase in L_a was greater than L_c. The intrinsic effect of substitutional B in carbon oxidation was proved to be a catalytic one. Unlike highly ordered SP-1 graphite, Saran char showed both a catalytic effect at low B loading and low conversion, and an inhibiting effect at high B loading and high conversion. The inductive effect was proposed to explain this catalytic effect on different crystallite size. Different sizes of carbon clusters were calculated by Gaussian 98W; the extent of the effect of substitutional B did get smaller to the carbon in bigger size of carbon cluster.     

Electrochemical performance of low temperature fluorinated graphites used as cathode in primary lithium batteries

The present study highlights the electrochemical performance of two series of fluorinated graphites used as the cathode in primary lithium batteries. These compounds were prepared under fluorine gas at room temperature using a catalytic atmosphere made of boron or chlorine fluoride, and then thermally treated between 100 and 600 °C. The electrochemical properties are correlated to a complete physico-chemical characterization, already performed by XRD, NMR, FT-IR and EPR. In particular, important parameters are taken into account: C-F bonding, carbon hybridization, fluorine content (i.e. F/C ratio) and amount of intercalated catalyst residues. It is shown that the average discharge potential of fluorinated graphite used in primary lithium batteries can be predicted owing to the chemical shift values (δ_C-F) obtained by solid ¹³C NMR. On the other hand, the higher capacity values are achieved for low temperature fluorinated graphite treated at the highest temperatures, i.e. for high fluorination level. The electrochemical performance study of these materials is completed by the study of the effect of simulated storage. The differences between the various samples during electrochemical tests and those observed using different electrolytes are discussed. Fluorinated graphites obtained with a chlorine catalyst or post-treated at temperatures higher than 450 °C are unaffected by ageing.     

Synchrotron radiation X-ray analysis of boron-doped diamond films grown by hot-filament assisted chemical vapor deposition

This paper presents a synchrotron X-ray radiation analysis of boron-doped diamond films grown by hot-filament assisted chemical vapor deposition (HFCVD). The diamond films were grown at different doping levels with the introduction of boron to the gas mixture by bubbling hydrogen in a B₂O₃ solution in methanol. The B/C ratio in methanol varied from 2000 to 20 000 ppm and the gas flow rates were controlled so that boron incorporation to the film varied in the range from 10¹⁸ to 10²¹ boron/cm³. All other process parameters were kept unchanged to allow comparison only of the influence of the doping level. The film analyses were performed at the X-ray diffraction beamline of the Laboratório Nacional de Luz Sincrotron – LNLS, Brazil. The Debye–Scherrer configuration was used in this study. A high intensity monochromatic beam at λ=1.46 Å was used and an excellent signal to noise ratio was obtained for 2θ varying from 20° to 150°. The difractogram for the undoped diamond film show intense peaks from the (111), (220), (311), (400) and (331) crystallographic planes. For the boron-doped films a set of new diffraction lines appear and their intensities increase considerably with the doping level. The set of diffraction peaks of similar intensities are related to a hexagonal structure and were assigned with high confidence to tungsten carbide. This reveals that the boron-doping process in HFCVD facilitates the incorporation of tungsten carbide from the filament in the diamond film. The FWHM analysis of the diamond diffraction peaks shows a dependence of film crystallinity with doping level with a definite maximum at approximately 2.3×10¹⁹ boron/cm³.     

11B and 15N solid state NMR investigation of a boron nitride preceramic polymer prepared by ammonolysis of borazine

A poly(aminoborazine), precursor for hexagonal boron nitride (h-BN) obtained by reaction of borazine B₃N₃H₆ with ammonia, and its pyrolysis derivatives have been extensively characterised by ¹⁵N and ¹¹B MAS NMR. The various B and N sites have been identified according to their first neighbouring atoms, as well as to the second ones in the case of ¹⁵N, and have also been quantified. This study demonstrates that a suitable choice of NMR techniques together with the use of isotopic enrichment can lead to a large improvement in spectral resolution, which allows a better understanding of such complex BN preceramic polymer structures and permits to follow the polymer-to-ceramic transformation.     

Superconductivity in diamond

We survey methods of synthesis of boron doped diamond with high pressure-high temperature techniques. New route is proposed for synthesis of relatively large heavily boron doped diamond single crystals, which exhibit superconductivity. Superconducting boron-doped diamond samples were synthesized with isotopes of ¹⁰B, ¹¹B, ¹³C and ¹²C. We claim the presence of a carbon isotope effect on the superconducting transition temperature, which supports the “diamond–carbon”-related nature of superconductivity and the importance of the electron–phonon interaction as the mechanism of superconductivity in diamond. Isotope substitution permits us to relate almost all bands in the Raman spectra of heavily boron-doped diamond to the vibrations of carbon atoms. The 500 cm^? 1 Raman band shifts with either carbon or boron isotope substitution and may be associated with vibrations of paired or clustered boron.     

Fabrication and microstructure of boron-doped isotropic pyrolytic carbon

Boron-doped isotropic pyrolytic carbon (pyrocarbon) was prepared by CVD using CH₄ + BCl₃ + H₂ as gaseous precursors. Microstructure of the deposited pyrocarbon on a graphite substrate was investigated using X-ray diffraction, X-ray photoelectron spectroscopy, polarized light microscopy, and scanning and transmission electron microscopy. It was found that the boron-containing pyrocarbon prepared exhibits a multi-scale structure, different from the pure isotropic pyrocarbon deposited under nearly the same experimental conditions. The multi-scale structure consists of carbon agglomerates of fine wrinkled graphitic sheets which contain substitutional boron and some micrometer-sized boron carbide particles uniformly distributed in the carbon agglomerates. Influence and mechanism of boron co-deposition during CVD process are also discussed.     

The effect of B4C addition to MnO2 in a cathode material for battery applications

Boron carbide (B₄C) added manganese dioxide (MnO₂) used as a cathode material for a Zn-MnO₂ battery using aqueous lithium hydroxide (LiOH) as the electrolyte is known to have higher discharge capacity but with a lower average discharge voltage than pure MnO₂ (additive free). The performance is reversed when using potassium hydroxide (KOH) as the electrolyte. Herein, the MnO₂ was mixed with 0, 5, 7 and 10 wt.% of boron carbide during the electrode preparation. The discharge performance of the Zn|LiOH|MnO₂ battery was improved by the addition of 5-7 wt.% boron carbide in MnO₂ cathode as compared with the pure MnO₂. However, increasing the additive to 10 wt.% causes a decrease in the discharge capacity. The performance of the Zn|KOH|MnO₂ battery was retarded by the boron carbide additive. Transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy analysis (EDS) results show evidence of crystalline MnO₂ particles during discharging in LiOH electrolyte, whereas, manganese oxide particles with different oxygen and manganese counts leading to mixture of phases is observed for KOH electrolyte which is in agreement with X-ray diffraction (XRD) data. The enhanced discharge capacity indicates that boron atoms promote lithium intercalation during the electrochemical process and improved the performance of the Zn|LiOH|MnO₂ battery. This observed improvement may be a consequence of B₄C suppressing the formation of undesirable Mn(III) phases, which in turn leads to enhanced lithium intercalation. Too much boron carbide hinders the charge carrier which inhibits the discharge capacity.     

EPR studies of a nickel–boron centre in synthetic diamond

Possible models are discussed for a new EPR centre (NOL1) observed in a diamond grown from a nickel-containing solvent catalyst with a titanium nitrogen getter. Nickel and boron are the most prevalent impurities in the diamond. The centre has trigonal (C_3v) symmetry about 〈111〉, S=1, g_∣∣=2.0235(5), g_⊥=2.0020(10) and D=−171(1) GHz. The EPR lineshape is unusual and we tentatively attribute this to partially resolved ¹¹B hyperfine structure. This centre may be the same as NIRIM-5, observed in boron doped synthetic diamond. The most likely structure for this centre is an interstitial Ni²⁺ with a substitutional B⁻ ion at the nearest neighbour site along 〈111〉.     

Effects of boron content on the microstructures and mechanical properties of reactive hot-pressed BxC-TiB2-SiC composites

B_xC-TiB₂-SiC ceramic composites were fabricated via reactive hot pressing using TiC, B, and Si as the raw materials. The phase transition process was studied by heating powder mixtures to different temperatures in combination with X-ray diffraction analysis. The stoichiometric ratio between B and C in boron carbide is variable. A series of powder mixtures containing excess boron (0 wt%, 10 wt%, 20 wt%, or 30 wt% B) were sintered, and the microstructures and mechanical properties of the composites were investigated. The results showed that the B_6.1C-TiB₂-SiC composite prepared from the starting powders with 30 wt% excess boron had the best comprehensive mechanical properties, with a relative density, hardness, bending strength, and fracture toughness of 98.32%, 33.2 GPa, 840 MPa, and 5.22 MPa m^1/2, respectively. Excessive boron substitution may cause lattice distortion in boron carbide, and the boron carbide grains in this state may form a large number of twins under the compressive stress generated by the TiB₂ grains, which will affect the properties of the composites.     

Effect of zirconia addition on pressureless sintering of boron carbide

This paper presents the results of experiments on pressureless sintering of boron carbide with varying addition of zirconia (ZrO₂: 0–30 wt.%). Green pellets were densified by sintering at 2275 °C in vacuum for 60 min and characterized by measurement of density, hardness, thermal conductivity and microstructure. Samples prepared with the addition of ≥5 wt.% ZrO₂ showed higher densities in the range of 93–96% ρth, compared to 86.63% ρth for boron carbide only. Addition of ZrO₂ was found to increase the hardness of sintered samples and regardless of ZrO₂ content, the hardness values ranged between 30 and 31.5 GPa. XRD of the sintered pellets showed the presence of ZrB₂. Optical microscope as well as electron probe microanalysis (EPMA) showed the presence of two phases, grey matrix with white precipitates. EPMA analysis of second phase revealed the presence of Zirconium in this phase. Fractography of boron carbide with 25% ZrO₂ showed the failure to be by mixed fracture (transgranular and intergranular). Thermal conductivity values of the samples measured in the temperature range of 400–1000 °C were marginally higher with the addition of ZrO₂.     

Isotopic substitution of boron and carbon in superconducting diamond epilayers grown by MPCVD

Sets of heavily boron-doped {100}-oriented diamond epilayers have been grown by MPCVD from isotope-enriched gases such as ¹³CH₄ and ¹⁰B₂H₆ or ¹³CH₄ and naturally abundant ¹¹B₂H₆, and the resulting concentration profiles of the various boron isotopes were measured by SIMS. In the particular case of the ¹³C¹⁰B set, the high absorption cross section of ¹⁰B made neutron depth profiling possible for this element.The vibrational and superconducting properties of these epilayers were compared to those of films grown from gases containing the isotopes in their natural abundance. The critical temperature T_C values were found to be lower both in the ¹³C¹¹B and ¹³C¹⁰B sets, as expected for a conventional pairing mechanism mediated by phonons. In good agreement with recent studies, the isotope-effect coefficient for T_C was found to be much higher than the classical 0.5 value. Various origins for this behaviour are suggested.     

Characterization of aluminum and boron containing beta zeolites prepared by solid-state recrystallization of magadiite

A method for the preparation of phase-pure [Al]-BEA and [Al,B]-BEA zeolites based on solid-state recrystallization of synthetic aluminum-containing magadiites is presented. For comparison [B]-BEA was prepared by conventional hydrothermal synthesis. From X-ray diffraction (XRD) measurements conclusions could be drawn in respect to kinetics and mechanism of the recrystallization process. Coordination state and resistance to hydrolysis of framework boron species in as-synthesized and ion-exchanged BEA varieties were investigated by ¹¹B MAS NMR spectroscopy. The acidity of the samples was studied by temperature-programmed ammonia elimination (TPAE) from the NH₄-forms and by IR spectroscopy using pyridine as probe molecule. An effect of framework boron on the acid strength of bridged hydroxyls associated with framework aluminum could not be evidenced. Nevertheless, incorporated boron gives rise to weak acid sites which are involved in ion-exchange processes. The number of strong Brønsted sites can be controlled by the aluminum content of the layer silicate.