Carbon Microbeads Research Articles

Evolution of the structure and mechanical behaviour of a carbon foam at very high temperatures

Mechanical behaviour of a low density carbon/carbon composite at very high temperature is studied in relation with its microstructure. This composite is a syntactic foam made of carbon microbeads with a binder and voids. The resulting geometrical density is 0.3 g cm −3. Compressive tests from room temperature up to 3100 °C with a very high heating rate (180 °C s −1) have been conducted. Intermediate temperature tests have also been performed and show an obvious modification of mechanical behaviour from around 2000 °C. This result is related to a sudden modification of structure and texture of the carbonaceous matter during the high temperature mechanical test. A strong plastic deformation occurs when the mechanical experiment is performed at 3100 °C whereas the material elastically deforms at room temperature.

A study on the irreversible capacity of initial doping/undoping of lithium into carbon

The initial irreversible capacity, Q i, is one of the parameters to express the material balancing of the cathode to anode in lithium-ion cells. In this work, a new terms are introduced, namely, the initial intercalation Ah efficiency (IIE) and the initial irreversible specific capacity at the surface ( Q is), to express precisely the irreversibility of an electrode/electrolyte system. The terms depend on the active-materials and composition of the electrode, but do not change with charging state. Mesophase carbon microbeads (MPCF) have the highest value of IIE and the lowest value of Q is in 1 M LipF 6/ethylene carbonate (EC)+diethyl carbonate (DEC) (1:1 volume ratio) electrolyte. The IIE value of a LiCoO 2 electrode is 97–98%, although the preparation conditions of the material and the electrolyte are different. The Q is value of LiCoO 2 is 0–1 mAh g −1. The MPCF-LiCoO 2 cell system has the lowest latent capacity. The Q is value increases slightly on adding conductive material. The IIE and Q is values vary with the electrolyte. By introducing propylene carbonate to the EC+DEC mixed solvent, the IIE values are retained, but Q is is increased. With the addition of methyl propionate, the IIE value is increased, and the Q is value is also increased slightly.

Studies on Capacity Loss and Capacity Fading of Nanosized SnSb Alloy Anode for Li-Ion Batteries

Nanosized SnSb alloy exhibits much higher reversible capacity as an anode active material for Li-ion batteries. However, rather large capacity loss at the first charge and discharge cycle as well as capacity fading during cycling for pure nanosized alloy has been observed. These phenomena originate from the following factors: the decomposition reaction of surface oxide and the formation of solid electrolyte interphase on the surface of alloy, the irreversible trapping of Li ions by host atoms, serious aggregation of alloy particles during electrochemical cycling, and the existence of an impure phase. Several strategies have been proposed to overcome these drawbacks, including surface modification, addition of dispersant, and coating on stable frame cores, such as mesophase carbon microbeads, to form composite materials. © 2001 The Electrochemical Society. All rights reserved.

Static and dynamic non-linear behaviors of carbon particle filled polymers with positive thermal coefficient

Polymers filled with conductive particles like carbon black, short carbon fibers or microbeads, may exhibit a very large positive thermal coefficient of the electrical resistance when they are heated above room temperature. Such a behavior is particularly useful in industrial applications for producing electrical safety devices. We present a comprehensive study of the electrical properties of epoxies filled with carbon microbeads when heating is achieved by the Joule effect. In that case, the electrical responses up to a large enough electrical excitation must be non-linear as a result of progressive heating of the material. As expected, we have found that it depends on both the 'intrinsic' properties of the sample (essentially a function of the composition) and the extent of thermal coupling between the investigated sample and the external thermal bath (variable heat leak). We have particularly studied the stationary states under constant applied voltage and the transients following a step like time dependent voltage. Most of our findings agree with the general results of percolation theory combined with theoretical models of rupture on a random fuse network and with a simple model of heat flow through the sample and from the sample to the thermal bath.

Nano-SnSb alloy deposited on MCMB as an anode material for lithium ion batteries

Nanosized SnSb alloy has been deposited and dispersed uniformly on the surface of mesophase carbon microbeads (MCMB) by a co-precipitation method in glycerin solution. As an active anode material for lithium ion batteries these composites show improved cycling performance. This improvement is due to the dispersion of the nano-sized alloy particles on the carbon surface that can alleviate the aggregation of nano-sized alloy particles during Li insertion and extraction. Furthermore, as both SnSb alloy and MCMB are active materials for Li-storage, a higher reversible capacity of 420 mA h g−1 has been achieved for the composite containing 26 wt% of SnSb alloy deposited on MCMB.

Irreversible capacity loss of graphite electrode in lithium-ion batteries

The irreversible capacity loss of the carbon electrode in lithium-ion batteries at the first cycle is caused mostly by surface film growth. We inspected an unknown irreversible capacity loss (UICL) of the natural graphite electrodes. The charge/discharge behavior of graphite and meso-phase carbon microbeads heat-treated at 2800°C (MCMB28) as the materials of the carbon anode in the lithium-ion battery were compared. It was found that the capacity loss of the natural graphite electrode in the first cycle is caused not only by surface film growth, but also by irreversible lithium-ion intercalation on the new formed surface at the potential range of lithium intercalation, while the capacity loss of the MCMB28 electrode is mainly originated from surface film growth. The reason for the difference of their irreversible capacity losses of these two kinds of carbon material was explained in relation to their structural characteristics.

Improved Carbon Anode Materials for Lithium-Ion Cells

ABSTRACTSeveral carbon materials have been studied for suitability as anode materials in lithium-ion cells. Carbons that have been included in this evaluation are three grades of commercially available mesophase carbon microbeads (MCMB) 6–28, 10–28 and 25–28, two specially prepared mesophase fibers (Amoco), a foreign mesophase fiber and KS-15 graphite (Lonza). Differences in cycling behavior between the three types of MCMB material are shown. Data of full lithium-ion cells demonstrate the effect that the choice of carbon material has on the cell discharge voltage and capacity. Lithium reference electrode experiments in full cells (3.0–4.0Ah capacity), elucidate the dynamics under several charge/discharge regimes and provide a comparison between the performance of carbon fiber and graphite anode materials. These test results indicate that the fibers can be charged at significantly higher rates than graphite without showing polarization at the anode. Full and half cell data also demonstrates the high coulombic efficiencies of the mesophase materials and first cycle efficiencies as compared to graphite. A comparison of two mesophase materials with different textures in full cells under strenuous cycling conditions shows significant differences in capacity retention. SEM photos of fibers showing the different textures are also presented.

Activated carbons and double layer capacitance

The porous structures and electrochemical double layer capacitance of activated carbon microbeads and carbon fibers were investigated using nitrogen gas adsorption and electrochemical constant current cycling (CCC) methods. Porous structural information on pore size distribution (PSD) and surface area were extracted through a gas adsorption analysis program based on density functional theory (DFT). The relation between the porous surface areas (ranging from 500 m 2/g to 1700 m 2/g) and specific double layer capacitance is discussed and a simple model is presented to correlate these parameters. It was found that the specific double layer capacitance on the micropore surface (about 15–20 μF/ cm 2) was determined by the property of carbon basal layer. Additionally, the specific double layer capacitance on the external surface strongly depends on the surface morphology and possibly also on the functional groups. It was found that the magnitude could change significantly from one carbon type to another.

Preparation of hollow carbon-microbeads from water-in-oil emulsion using amphiphilic carbonaceous material

Abstract Carbon microbeads were prepared from water-in-oil emulsion using amphiphilic carbonaceous material and urea. With urea as a forming agent with a concentration 1–20 wt.%, hollow carbon-microbeads were obtained, and their particle size ranged between 2 and 15 μm. The true density of the hollow carbon-microbeads was changed, by varying the concentration of urea and the heat-treatment temperature. In particular, a pore size of about 50 A was predominantly produced under a certain condition.

Carbon electrode materials for lithium-ion batteries

Five kinds of carbonaceous materials: graphite, carbon black, coke, petroleum coke, mesophase carbon microbeads in terms of the cell carbon|EC-DEC-LiPF 6|Li, were examined by X-ray diffractometry and impedance spectrum.

Surface Modification of Carbon Microbead by the Grafting of Polymers

Abstract The surface grafting of polymers onto carbon microbead, a sphere of carbonized phenol resin, was achieved by two methods: 1) the radical graft polymerization initiated by azo groups introduced onto the surface and 2) the trapping of polymer radicals by a carbon microbead surface. The introduction of azo groups onto a carbon microbead surface was achieved by the reaction of surface carboxyl groups on the bead with 2,2′-azobis[2-(hydroxymethyl)-propionitrile] using N, N′-dicyclohexyl-carbodiimide as a condensing agent. The radical polymerization of methyl methacrylate (MMA) was initiated by surface azo groups introduced onto the surface, and polyMMA was grafted onto the bead through the propagation of polymer chains from the surface: the percentage of grafting reached to 18% after 16 hours. Furthermore, it was found that by the reaction of an azo polyamide (a polyamide having an azo bond in the main chain) and peroxide polymers (polymers having peroxide groups in the main chain and/or pendant group...

Spherical Carbon Packings Prepared from Spherical Cellulose Particles for High-Performance Liquid Chromatography

Packings of spherical carbon beads were prepared by graphitizing spherical particles of cellulose. The carbon bead obtained was a carbon microbead which was physically and chemically stable and which retained the spherical shape of the original material of cellulose particles. The diameter of the microbead was found to gradually become smaller as carbonization proceeded. The diameter of the resultant graphitized carbon microbead was <5 μm. This carbon bead was used as HPLC packing. Carbon microbead packing, unlike conventional carbon packings, was found not to show excessive retention or adsorption of electron-rich or hydrophobic compounds. It was also found that the peaks obtained using the carbon microbead packing became more symmetrical along with an increase in the graphitization temperature. The capability of the obtained carbon packings in retaining hydrophobic compounds was similar to that of alkylated silica gels, rather than to that of the conventional carbon packings. In addition, the carbon microbead packing, like conventional carbon packings, could separate positional isomers by the mechanism of a charge-transfer interaction.

Fast amperometric response of ambient temperature oxygen sensor based on PbSnF 4; iron (II) phthalcyanine-based sensing electrodes containing carbon microbeads

Several kinds of carbon microbeads and noble metal powders have been tested as an additive to the iron (II) phthalocyanine-based sensing electrode (SE) for the amperometric oxygen sensors; Ag|Ag 6I 4WO 4|PbSnF 4|SE, O 2. The use of the SE containing surface-modified carbon microbeads enables the sensors not only to respond fast at room temperature but to exhibit the highest sensitivity. Furthermore, it is capable of determining oxygen partial pressures of 2–750 kPa in a wide operating pressure range 10 kPa–3.5MPa.

High voltage, rechargeable lithium batteries using newly-developed carbon for negative electrode material

Carbon is a good candidate for negative electrodes because it can take the form of lithium intercalation compounds. We discussed the characteristics of typical carbon materials which have been studied as negative electrode materials. We have found that the mesophase pitch-based carbon microbead (MCMB) of high graphitization stage which have been graphitized at a high temperature such as 2800 °C gives good characteristics as a negative electrode for rechargeable lithium batteries. The cylindrical ‘AA-size’ batteries of our trial products using LiCoO 2 as the positive electrode and the MCMB graphitized at 2800 °C as the negative electrode have been found to provide large capacities of 500 mA h and high voltages of 3.7 V with high energy densities of 240 W h/l, 100 W h/kg.

Superhigh surface area determination of microporous solids

A new method is proposed for the measurement of specific surface area of microporous carbons of superhigh BET surface area. The method (designated the subtracting pore effects (SPE) method) is based on the analysis of two upward swings from linearity, due to the pore effects, in the high resolution α s plot. The measurement of slope of the linear α s plot after subtraction of the swings gives a definite surface area, even in a microporous system. The surface area of activated mesophase carbon microbeads with a BET surface area of 3000–4000 m 2 g −1 was determined by N 2 adsorption using the SPE method. The BET surface area obtained in the usual manner was an overestimate by about 40%. The overestimation was not attributed to enhanced micropore filling but to capillary condensation.

Influence of Organometal Compounds on The Formation of Carbon Microbeads from n-Paraffin under Pressurized Condition

Influence of Organometal Compounds on The Formation of Carbon Microbeads from n-Paraffin under Pressurized Condition

Application of carbon microbeads for column packing for high-performance liquid chromatography

Carbon microbeads have been recognized as an advanced carbonaceous material with exciting possibilities just like carbon fibre. Among their potential applications are high-density, high-strength, isotropic carbon solids [1] and bulking agents for liquid chromatography [2]. The microbeads have usually been obtained by carbonizing pitch, separating them from each other with solvent or in a high-temperature centrifuge. To name a few advantages of carbon microbeads, they offer (a) high resistance to very acidic and very basic reagents, (b) low expansion and shrinkage coefficients and (c) the ability to withstand an operating temperature of 250 °C. Recently we have developed a method for obtaining carbon microbeads from n-paraffin or a mixture of n-paraffin and coal tar pitch (CTP) under mild conditions at 873 K under a nitrogen pressure of 2 MPa [3, 4]. In this letter a study is made on the application of carbon microbeads for column packing for highperformance liquid chromatography (HPLC) for the separation of aromatic and polar compounds. n-Paraffin (melting point 341 to 343 K) was used as the starting material for production of carbon microbeads. Carbonization of paraffin was performed in a 15 ml autoclave. The autoclave charged with an 0.8 g sample was pressurized with nitrogen at an initial pressure of 5 MPa and put into an image furnace for carbonization at 873 K (the yield of carbon microbeads was 18 wt %). The rate of heating was 5 K min-~. The carbon microbeads formed in the autoclave were taken out and were heat-treated further at 973, 1073 and 1173 K for I h under nitrogen gas flow. Heat-treated carbon microbeads thus obtained were tested as a Figure 1 SEM micrographs of carbon microbeads from n-paraffin used for HPLC column packing.

Column Packing of High Performance Liquid Chromatography by use of Carbon Microbeads from N-Paraffin and Powder Coke

Carbon microbeads obtained from n-paraffin and petroleum powder coke were found to have potential to use of column packing for the separation of some useful compounds. Separation capability of carbon for hplc column is closely related to its optically anisotropic texture.

Mechanism of carbon microbead formation from paraffin-pitch mixtures

We have proposed a model of carbon microbead formation from the mixture of coal-tar pitch and n-paraffin under mild conditions, supported by results obtained by high-temperature and high-pressure1H nuclear magnetic resonance and electron spin resonance in conjunction with thermogravimetric and differential thermal analysis. The data obtained by different techniques are in good agreement with each other.

The Manufacture of High Frequency Vibration Autoclave and Its Application for The Preparation of Carbon Microbeads

An unique high frequency vibration autoclave has been constructed and applied inthe preparation of carbon microbeads from coal tar pitch/n-paraffin system.The average particle size of the carbon microbeads obtained under high frequencyvibration has been 5.2μm. The carbon microbeads obtained without vibration have been relatively larger, with the average size of 8.2μm.