Ball-milled Graphite Research Articles

Hydrogen storage properties of hydrogenated graphite destabilized by mixing with lithium hydride

Hydrogen storage properties of the ball-milled mixtures composed of hydrogenated nano-structural graphite (CnanoHx) and lithium hydride (LiH) were examined from thermodynamic and structural points of view, where the CnanoHx was synthesized by ball-milling graphite powder under hydrogen atmosphere. Since hydrogen atoms in the CnanoHx and LiH are strongly bonded with each host atom, heating up to >600°C is necessary to release hydrogen from each solid product. The CnanoHx and LiH composites desorbed hydrogen and hydrocarbon gases below 400°C with about 9.4 mass% weight loss. The 2: 1 composite of the CnanoHx and LiH reversibly stored hydrogen with an effective capacity of about 4.5 mass % at 350°C. No evident peaks were observed in XRD profiles after the dehydrogenation, indicating that the nano-structural feature remained after dehydrogenation due to formation of (CLi) nano clusters. However, only the LiH phase was crystallized after rehydrogenation at 350°C. From these results, it is concluded that hydrogen in the composites is destabilized by a novel interaction between the CnanoHx and LiH in a nanometer scale, and is desorbed at lower temperatures than each of the components. Therefore, this Li-C-H system can be recognized to be a new family of hydrogen storage materials.

Microstructure and low-temperature hydrogen storage capacity of ball-milled graphite

Hydrogen adsorption in ball-milled graphite is investigated in the low temperature range from 110 to 35 K and at pressures up to 20 MPa. The adsorption data are compared to the results of detailed quantitative microstructural analyses of the samples used for the adsorption experiments. The amount of hydrogen adsorbed at temperatures well below 77 K exceeds considerably that what is expected from adsorption on plane graphitic planes. The results can be explained assuming the following mechanisms: (i) adsorption in trapping states on plane surfaces at and below 110 K; (ii) adsorption in small micropores with diameter of less than 1 nm at 77 K and pressure of 10 MPa, and (iii) multilayer adsorption in mesopores at temperatures from 35 to 40 K and pressure of 2 MPa. The effects observed in the low temperature range are reversible and make the investigated material interesting as a supporting component for liquid hydrogen storage systems.

Highly adsorbent carbon formed by ball milling

The adsorption capacity of activated carbon formed by ball milling from alumina and synthetic graphite was investigated. It was observed that ball milling graphite with a harder phase led to a greater adsorption capacity for a shorter milling time. The mass loss of the sample was found to be a function of temperature. The mass losses during leaching were 9.75% and 25.2% for graphite milled for 50 h without and with alumina respectively. Compared with graphite milled alone, the sample milled with alumina has a capacity of 3.7 time greater.

The nucleation and growth of carbon nanotubes in a mechano-thermal process

Separate nucleation and growth processes of carbon nanotubes were found in a mechano-thermal method in which carbon nanotubes are produced by first mechanical milling of graphite powder at room temperature and subsequent thermal annealing up to 1400 °C. The ball-milled graphite contains nucleation structures (nanosized metal particles and deformed (0 0 2) layers containing pentagons), and disordered carbon as a free carbon atom source. The subsequent annealing activates the growth of two types of multi-walled nanotubes in the absence of carbon vapor. Thin nanotubes (diameter <20 nm) are formed via crystallization of the disordered carbon with the preferred formation of the (0 0 2) basal planes. Thick nanotubes (diameter >20 nm) are formed through a metal catalytic solution–precipitation process (solid–liquid–solid). In both cases, carbon nanotubes grew out from disordered carbon particles with closed tips.

Influence of Oxygen and Hydrogen Milling Atmospheres on the Electrochemical Properties of Ballmilled Graphite

The effect of the grinding oxygen or hydrogen atmosphere on the physical/electrochemical properties of ballmilled graphite was studied. These properties strongly depend on the atmosphere and pressure of the milling container of the oxygen and the hydrogen contents of the samples. For instance, grinding under a 10 bars of atmosphere leads to a highly disordered carbon, while 10 bars under gives a heterogeneous carbon having a well and poorly crystallized graphite component. In both cases, the reversible and irreversible capacities and the double-layer capacitance increase with milling time. The graphite ground 10 h under a strongly differs from the other samples. We ascribed such behavior to the presence of a large amount of oxygenated functional groups in relation with the formation of nanoporosity and/or the predominance of edge carbon upon grinding. In contrast, under or the variation of the Brunauer-Emmett-Teller (BET) surface area with milling time is correlated with the irreversible capacity. Differences in the voltage composition curves are also noticed with a low oxygen pressure strongly reducing the polarization of the charge/discharge cycling curve while leading to Li for Finally, 10 h of milling under were shown to produce a carbon with an Li and Li for while 80 h under led to a carbon having a double-layer capacitance of 57 F/g in © 2001 The Electrochemical Society. All rights reserved.

Graphite–Tin composites as anode materials for lithium-ion batteries

Graphite–tin composites were produced by high-energy bail-milling. X-ray diffraction and HREM observation showed that graphite became amorphous and tin became nanocrystalline after the intensive ball milling. The element Sn was encapsulated in the ductile graphite matrix on a nanometer scale. Electrochemical tests show that the lithium storage capacity increases with the addition of Sn, which could be attributed to the reaction of Sn with Li to form Li x Sn alloys. The volume expansion due to the alloying process may be buffered by the amorphous graphite matrix. The C 0.9Sn 0.1 electrode can deliver a discharge capacity of 1250 mAh/g in the initial cycle. Generally, the capacity of the ball-milled C, C 0.9Sn 0.1 and C 0.8Sn 0.2 electrodes decrease with cycling quite quickly, but the C 0.9Sn 0.1 and C 0.8Sn 0.2 electrodes have better cyclability than that of the ball-milled graphite electrode. The combination of C and Sn could be an anode material with high capacity for lithium-ion batteries.

Large-scale synthesis of ultrafine Si nanoparticles by ball milling

Si nanoparticles have been synthesized by ball-milling of graphite and SiO 2 powders. The solid-phase reaction, C(graphite)+SiO 2→Si+CO 2, was found to be a novel technique for fabrication of ultrafine Si particles. The reduced Si in the powder assembled as nanoparticles with a wide range of diameters. Multiple-peak structures were observed in the photoluminescence spectra of the Si nanoparticles at room temperature. The peak energies were found to coincide with the PL peak pinning energies previously reported in porous Si. Thus, the fine structures of luminescence spectra could be attributed to the size quantization of the Si nanoparticles formed via solid-phase reaction during ball-milling.

Generation of curved or closed-shell carbon nanostructures by ball-milling of graphite

Curved or closed-shell carbon nanostructures were produced by ball-milling of graphite. A high resolution indicates that the ball-milling not only produces bend of graphite sheets, forming carbon nanoarches, but also produces closed-shell carbon nanostructures, nearly carbon ‘onions’. The possible formation mechanism is proposed.

Short-range order in disordered carbons: where does the Li go?

Disordered carbons offer promise of performance gains relative to graphite as the anode host material in Li ion batteries. They also present major challenges in determining structure-property relationships, in particular the location, density and properties of the site (or sites) at which reversible Li uptake occurs. Our understanding of graphite anodes is built on 35 years of research on the crystal compound LiC 6. Here we review our efforts to obtain similar understanding of disordered carbons, using local structure determination via radial distribution function analysis, quantum chemical simulations and neutron vibrational spectroscopy. The ultimate goals are to understand the origin of the very high capacity for Li uptake exhibited by some of these materials, and to establish a scientific basis for optimizing their performance in real batteries. Two illustrative examples will be discussed in detail: the effect of residual hydrogen and ‘edge sites’ on Li binding, and the detailed evolution from long-range to short-range order in ball-milled graphite.

Molecular orbital calculation of graphite K- V X-ray emission spectra

The DV-Xα molecular orbital (MO) calculation method has been applied to the carbon K- V X-ray emission spectra of graphite. The calculated 2 p density of states (DOS) of a larger model cluster, which avoided the influence of dangling bonds of the cluster edge, was in excellent agreement with the X-ray emission spectra. The calculated π and σ subbands of 2 p DOS explained the polarized behavior of the spectra. The orientation of graphite was quantitatively discussed by means of polarization of the spectra. These results were applied to the analysis of the structural change of mechanical milled graphite; the structure change of the ball-milled graphite was studied by the X-ray spectroscopy and MO calculation. It was shown that the refinement of graphite powder occurred for up to 1000 h of milling, keeping the graphite structure, and that a further milling changed the C-C bonding within the graphite layer network.

Lithium insertion in ball-milled graphite

The effects of mechanical milling on the microstructure, morphology and electrochemical performance of graphite powders with respect to lithium insertion are studied. After 150 h of ball-milling, the well-graphitized graphite has been pulverized into small particles with a size of about 50 nm, in which there are a lot of excess vacancies, microcavities and metastable carbon interstitial phases with sizes around 13 Å. Due to the large surface energy, the merging of single particles is favoured and results in the formation of agglomerates with average size about 1 μm. Voids are formed among the agglomerated particles. The ball-milled graphite shows reversible specific capacity for lithium of 700 mA h g −1 (L 1.88 C 6 ) with large hysteresis. The large reversible capacity is due mainly to Li doping at vacancies, microcavities (or at the edges of the metastable carbon interstitial phase) and voids. The bonding change between the interstitial carbon and the carbon in the aromatic plane that is induced by insertion of Li atoms leads to hysteresis. During charge–discharge cycles, the reversible capacity above 1 V decreases rapidly, which may be due to some vacancies and microcavities being annihilated by moveable and some bound interstitial carbon and to electrolyte penetrating gradually into voids formed by agglomerated particles during the Li insertion and desertion process.

Lithium Insertion in Carbon‐Silicon Composite Materials Produced by Mechanical Milling

Carbons containing nanosize silicon particles were prepared by mechanical milling of graphite and silicon mixtures with different atomic ratios. The microstructure, morphology, and electrochemical performance of ballmilled (x = 0, 0.1, 0.2, 0.25) were analyzed by X‐ray diffraction, Raman, high‐resolution and transmission electron microscopy, and electrochemical methods. After ballmilling, the crystal size of graphite increased but the size of silicon decreased with increasing content of silicon. Ballmilled materials reacted reversibly with lithium, and the reversible specific capacity increased from 437 mAh/g in the ballmilled pure graphite to 1039 mAh/g in ballmilled materials. The excess capacity due to the Li extraction from silicon appeared at a potential about 0.4 V vs. Li metal. After 20 charge/discharge cycles the reversible capacity of was 794 mAh/g. This behavior is a result of nanosize silicon particles decreasing the crumbling rate during Li insertion and extraction. These materials appear to be promising candidates for negative electrodes in lithium‐ion rechargeable batteries.

Structure and chemical reactivity of ball-milled graphite

Nanostructured carbon powders are prepared from graphite powder in a planetary mill using WC, stainless steel, and agate milling tools. The evolution of microstructure and chemical reactivity is studied by X-ray diffraction and combustion methods, respectively. The specific surface area of milled samples is determined by BET measurement and the impurity content is estimated by spark source mass spectroscopy. There is no monotonous relationship between reactivity and specific surface area of particles and agglomerations of particles. In contrast to impurity contents, distortion of atomic short- and medium-range order is shown to be essential for the observed enhancement of chemical reactivity. A statistical model for the milling process is formulated and used to discuss the experimental data. As a result, an expression for the dependence of combustion temperature on milling time is proposed.

Raman Spectra of Graphite and Diamond Mechanically Milled with Agate or Stainless Steel Ball-Mill

The reduction of crystalline size and amorphization of graphite and diamond during agate and stainless ball-milling are investigated by Raman spectroscopy. The ultimate crystalline size of graphite, estimated by the Raman intensity ratio, of 2.5 nm for the agate ball-mill is smaller than that of 3.5 nm for the stainless ball-mill, while the milling time to reach the ultimate size for the former is about 10 times larger than for the latter, indicating more stability of the nanocrystalline graphite. After reaching the ultimate crystalline size, a significant broadening of the Raman spectra, which indicates the completion of amorphization, is detected only for the stainless ball-milled graphite at ∼ 500 h of milling. Also the increase rate of the Raman peakwidth for the stainless ball-milled graphite before amorphization is higher than that for the agate ball-milled graphite, indicating a larger introduction of disorder from the start of milling. Amorphization of diamond is also observed for the stainless ball-mill. The difference in the results between the agate and the stainless ball-mill is discussed in terms of the effect of impurity mixed from the milling apparatus on the stability of nanocrystalline carbon materials.

The friction and wear properties, during abrasion, of compressed graphite-powder compacts and commercial graphitised carbons

The friction and wear properties during abrasion of compressed compacts made from ball-milled graphite powder are shown to vary systematically with the pressure of compaction. For abrasion on emery papers of particle diameter D > 70 rmmicrons the friction coefficient μ. is practically constant up to D = 150 microns or more, but the wear rate M still increases gradually with D. From the high wear volume rate in comparison with that for metals under similar conditions, and from the particulate nature of the wear debris, it is concluded that much of the wear occurs by brittle fracture; and this also accounts for the observed non-linear relation between M and μ. Some plastic flow by slip on (0001) planes also occurs. Both M and μ (at D > 70 microns) decrease with increase in the pressure of compaction of the graphite, and with the corresponding increase in the hardness of the compact. Some commercial samples which were well graphitised showed similar friction and wear properties, but in others a lower degree of graphitisation was associated with a lower particle size, higher hardness, and particularly low friction and wear. Theoretical considerations indicate that the adhesion component μa, of the total μ, is ~ 0.1, the remainder (0.2-0.5) being that due to the ploughing component μp.