Sintering of powders of polyaromatic mesophase to high-strength isotropic carbons: III. Powders based on an iron-catalyzed mesophase synthesis

A spherulitic mesophase pitch with about 50 vol% mesophase has been synthesized by an iron-catalyzed pyrolysis of a coal tar pitch. The isotropic pitch is removed to various extents by solvent extraction. The thermoplasticity of the resulting raw mesophase powders is adjusted by treatment with oxygen with respect to optimum sintering properties. After milling of these powders to a particle size d 10 μm they were compacted with various molding pressures, sintered and graphitization-treated with various heating programs. The individual process steps and intermediate products were analyzed. Carbons sintered at 1300 °C show flexural strengths up to 225 MPa and strains-to-failure up to 1.25%. After graphitization treatment at 2400 °C, carbons with a flexural strength of about 170 MPa and a strain-to-failure of about 1% were obtained.     

Preparation and microstructure of hollow mesophase pitch-based carbon fibers

Hollow mesophase pitch fibers with rather thin diameter were successfully prepared by spinning through a C-shaped capillary. Die-swell was found to be the main factor affecting the formation of hollow fiber, and this was controlled by varying the spinning temperature. After carbonization at 1000°C, the hollow fibers possess a relatively small outer diameter of 21 μm and an inner diameter of 6 μm, and show better mechanical properties than solid fibers with similar outer diameter. The higher mechanical properties are attributed to the orientation of mesophase molecules which is related to the shape and dimension of the spinneret. The transverse microstructure of hollow carbon fibers is illustrated by scanning electron microscopy (SEM) observations on fracture sections.     

Preparation of carbon fiber from isotropic pitch containing mesophase spheres

Naphthalene based synthetic isotropic pitch was found to give spinnable precursors which carried 10–100 μm anisotropic spheres of 45 and 50 vol% in the isotropic matrix, when the pitch was heat-treated at 375 and 380 °C for 20 and 23 hours, respectively. The spun fiber was stabilized and carbonized into a carbon fiber, which showed anisotropic belts running along the fiber axis, suggesting that both anisotropic spheres and isotropic matrix are deformed to align within the nozzle. The structure and mechanical properties of the carbon fibers, thus prepared, were examined under microscopes and by mechanical testing.     

New insights on the direct activation of isotropic petroleum pitch by alkaline hydroxides

The paper provides interesting evidences that a low softening point isotropic petroleum pitch can be used as a good carbon precursor for the preparation of activated carbons. The activation is carried out by KOH and/or NaOH and the resulting activated carbons present well developed porosity. Such hydroxide activations can be done directly on the pristine petroleum pitch (P) or on the pitch that has been submitted to an air stabilisation followed by a N₂ heat treatment (TAN). In general, KOH activation produces better results than NaOH, both in terms of porosity and yield, the results obtained for the activation of TAN being impressive because of the good porosity developments and high yields reached. The different treatments carried out over the petroleum pitch precursor clearly show that they significantly influence the extent of microporosity development. This is due to different changes occurring in the porous structure of the precursor as a function of the treatment carried out. The efficiency of the activation process increases as the mesophase content of the precursor decreases, as well as the mesophase formation during the activation process is avoided.     

纺丝温度对中间相沥青炭纤维结构性能的影响

以萘催化合成中间相沥青(MP-1)和热缩聚法制备中间相沥青(MP-2)两种中间相沥青为原料,对其进行簇组成分析、偏光显微镜观察及红外光谱分析,研究其组成及结构.采用实验室气压式单孔纺丝装置在不同温度下对两种中间相沥青进行熔融纺丝,探讨纺丝温度对炭纤维结构及性能的影响.研究表明:MP-1低温获得无规结构,高温出现中心放射状边缘洋葱皮混合结构;MP-2随纺丝温度升高依次出现无规结构、准洋葱皮结构和洋葱皮结构.中间相沥青原料的性质影响着纤维截面结构随纺丝温度变化的规律.     

Influence of heat and pressure treatment on the rheological behavior of petroleum pitches

Pitch rheological properties are extremely important during the manufacturing process of carbon materials, in mesophase formation, and with regard to the final properties of the carbon products. In this work, pitch samples have been prepared from three different FCC decant oils by heat-treatment, under 0.9 MPa pressure, in a reactor at 390 °C, 410 °C, and 430 °C. These samples were analyzed in a rotational rheometer using a parallel-plate sensor. The rheometric softening points matched the results obtained using conventional equipment and exponential relationships were found to exist between these softening points and the pitch cosity when the former approached 180 °C. The quinoline-insoluble content (QI) has been shown to be more important in increasing the pitch viscosity than the toluene-insoluble content (TI). Oscillatory rheometry analysis has shown that an elastic response is not always found in creep and recovery tests, even when the elastic modulus G′ is dominant over the viscous modulus G′′. Pitch elasticity was found to be independent of the mesophase, and this pitch property was either only observed when the cross-over point occurred at very high frequencies or did not occur at all within the frequency range studied.     

煤沥青基中间相沥青的制备研究

以纯化的煤焦油沥青为原料,考察了热聚合温度和恒温时间对中间相沥青的收率、光学显微形态、软化点和族组成的影响.结果表明:反应温度在420℃,恒温5 h时得到了软化点为312℃的流线体型中间相沥青,其收率为79.1%;热聚合反应在相对较低的温度400℃,反应时间为10 h时形成了软化点为305℃、收率为81.4%的优质广域型可纺性中间相沥青.对该原料煤沥青而言,通过控制热聚合反应温度和恒温时间可以达到制备优质中间相的目的.     

Improving graphitization degree of mesophase pitch-derived carbon fiber by solid-phase annealing of spun fiber

The solid-phase annealing of the mesophase pitch spun fiber was examined between the glass transition (T_g) and softening (T_s) temperatures of the pitch to improve the graphitization degree of the graphitized fiber through recovering or further improving the stacking height of the mesogen molecules in the spun fiber, since the rapid spinning reduced markedly stacking height in the as-spun fiber. A naphthalene mesophase pitch as received carried stacking height of 2.9 nm which was markedly reduced to 1.7 nm by spinning at 230 m/min, giving Lc=40 nm for its graphitized fiber. Annealing at 206 °C improved the stacking height of the spun fiber to 2.4 nm and Lc(002) of the graphitized fiber to 54 nm. Annealing of the methylnaphthalene mesophase pitch fiber at 200 °C was much more effective in improving the stacking height from 3.5 to 5.0 nm and its graphitized fiber to Lc=91 from 40 nm. Such an improved graphitization degree led to improved thermal conductivity and tensile modulus of the graphitized fiber. It must be noted that the annealing of the spun fiber reduced its stabilization rate, indicating densification of molecular stacking in the fiber. The transformation scheme of mesophase pitch into graphite fibers is discussed to clarify the roles of molecular stacking in the clusters and their arrangement in the mesophase pitch fiber during the carbon manufacturing process.     

High rate capability of carbonaceous composites as anode electrodes for lithium-ion secondary battery

Anode materials with high rate capability for Li-ion secondary batteries were investigated by using the mixture of graphite, cokes, and petroleum pitch. Since obvious potential plateaus were obtained at graphite contents above 40 wt.%, which would cause difficulties in perceiving the capacity variations as a function of electrical potential, the graphite content were determined at 20–30 wt.%. The composites with a given content of graphite and remaining content of petroleum pitch/cokes mixtures at 1:4, 1:1, and 4:1 mass ratios were heated at a temperature range of 800–1200 °C. For a given composition of carbonaceous composite, the discharge rate capability improved but the reversible capacity decreased with increasing the heat treatment temperature. Although the reversible capacity increased with increasing content of the petroleum pitch for given graphite content and heat treatment temperature, the discharge rate capability decreased. The carbonaceous composites prepared by the mixture of 30 wt.% graphite and 70 wt.% petroleum pitch/cokes mixture at 1:4 mass ratio with the heat treatment at 800 °C showed relatively high electrochemical properties, of which reversible capacity, initial efficiency, discharge rate capability (retention of discharge capacity in 5 C/0.2 C) and charge capacity at 5 C were 312 mAh/g, 79%, 89% and 78 mAh/g, respectively.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites: Part II. Stabilization process

In the fabrication of carbon-carbon composites by mesophase injection through a fiber preform, it is essential to stabilize the flow-induced microstructure in the flow channels and to prevent relaxation and exudation of the mesophase. Oxidation stabilization studies were conducted on preforms injected with the naphthalene-based AR mesophase pitch. Oxidation mass gain (OMG) curves at 170, 222, and 270 °C were generated for 60°-wedges cut from full size composite disks. The rates of OMG at 170 °C of first- and second-cycle injection wedges and full-size disks were comparable to those using as-spun filaments 30 μm in diameter, and particles sieved to 200 to 340 μm. The results suggest that oxygen is accessible deep into a mesophase matrix and the transport is facilitated by connected array of shrinkage cracks. Oxidation at 170 °C has strong advantage over higher oxidation temperatures by having a higher carbon yield and lower OMG threshold and thus oxidation time required for stabilization. The 60°-wedges could be stabilized at 170 °C after a 25 h oxidation with a 7.2% OMG and attaining a carbon yield above 85%.     

Influence of oxidative stabilization on the electrochemical behaviour of coal tar pitch derived carbons in lithium batteries

A commercial coal tar pitch was thermally treated at 430 °C for 4 h and then submitted to hot filtration in order to separate the isotropic phase from the mesophase developed during the treatment. Each phase was then oxidatively stabilized in order to preserve its structure during carbonization and then carbonized at temperatures ranging from 700 to 1000 °C. The effect of the microstructure, particle morphology and chemical composition of the carbons and also the influence of their carbonization temperature on the electrochemical behaviour as electrode materials in lithium cells were studied.Galvanostatic cycling of lithium test cells using the carbon materials as positive electrodes showed the improvement of the electrochemical performance in both isotropic and anisotropic phases by stabilization with air previous to carbonization. More subtle differences between isotropic and anisotropic samples were evidenced and interpreted in terms of their textural properties. Moreover, the electrochemical impedance spectroscopy (EIS) has been demonstrated to be an interesting technique to elucidate the changes occurred in the electrode interfaces when these coal tar pitch based carbons are cycled.     

Preparation of mesophase-pitch-based carbon foams at low pressures

A simple method for preparing the mesophase-pitch-based carbon foams at low pressures through prolonging the soaking time in the preparation process of the mesophase pitch was disclosed. The physical properties, morphologies and the crystal structure of the as-obtained foams were investigated. Bulk density of the resultant carbon foams cover a range 0.514-0.624 g/cm³, under the preparation pressure range 0.5-2 MPa. The SEM micrographs revealed that the thermal shrinkage of the graphitized foams derived from the higher softening point mesophase pitch was less than that of the foam from the lower softening point; Optical micrographs showed that higher softening point mesophase pitch derived carbon foams exhibited better orientation and less microcracks in both junctions and ligaments; The XRD results revealed that higher softening point pitch derived graphitized foams exhibited smaller interlayer spacing and larger crystallite size. The properties of the carbon foam can be severely affected by the properties of the precursor, so it is critical to tailor the properties of the pitch precursor so as to obtain high performance and low cost products.     

中介相沥青及其碳纤维的应用

介绍了中介相沥青及其碳纤维作为复合材料、导热和绝热防辐射材料、高级电池阳极、高温润滑剂、碳泡沫等的各种用途,重点讨论了含碳复合材料的各种制备方法和应用。指出中介相沥青已成为制作碳材料的主要原料,它比碳纤维的另一种原料聚丙烯腈（ＰＡＮ）纤维具有更广阔的应用领域。     

Thermal stability and mechanical properties of SiC particulate-reinforced Si–C–N composites after heating at 2000 °C

A SiC particulate-reinforced Si–C–N ceramic composite was fabricated using the precursor impregnation and pyrolysis method, and its thermal and mechanical properties were analyzed. The weight loss of the composite was 5% after a heating at 2100 °C in Ar. The pores of the composite enlarged at and above 1700 °C in Ar due to the decomposition of the Si–C–N matrix. However, the composite retained mechanical properties such as strength and hardness after heating at 1700 °C. 88% of the original strength was remained after heating at 2000 °C for 10 h although the fabrication temperature was 1350 °C. The weight gain of the composite was 3.2% after an oxidation at 1450 °C for 30 min in air. The inner oxidation of the particulate-reinforced composites (PRC) was suppressed above 1400 °C due to the closure of the open pores by SiO₂. Consequently, the composite possessed excellent creep resistance at 1400 °C in air. The SiC/Si–C–N composite is a challenging candidate for the application at high temperature.     

Thixotropic changes occurring on reheating a coal tar pitch containing mesophase

A coal tar impregnation pitch has been heated to 420°C under shear conditions and cooled. Microstructural investigation of the material showed ellipsoidal mesophase particles produced by shear deformation. On reheating in quiescent conditions, these, particles were observed to revert to the spherical shape after a 3.5 hr treatment at 300°C. Material which experienced the same initial heating cycle but no shear was also reheated, with isothermal intervals, up to 300°C. Flow curves measured isothermally at temperatures between 250 and 300°C showed marked thixotropic behaviour. The thixotropic loop widened with increasing ageing time at each temperature, but the effects were reversible indicating a physical phenomenon. It is suggested that this behaviour is probably the result of a slow coagulation of the “solid-like” mesophase particles to form a weak structure which can be broken down at low rates of shear.     

Formation of carbonaceous mesophase at lower temperature

The effects of heat-treatment at lower temperature on the formation of carbonaceous mesophase were studied. Heat-treatments of QS pitch below 340°C and of KF asphalt below 370°C lead to the formation of mesophase spherules having a different structure from those found previously. For QS pitch, the lowest temperature of mesophase formation was 290°C but more than one month was required for heat-treatment. The structure of the fourth-type spherule is proposed from the microscopic observations and its orientational behavior in a magnetic field. The structure of this type is very similar to that of the Brooks-Taylor-type one, and it is believed that the fourth-type spherule is a meta-stable phase of the Brooks-Taylor-type one.     

Short residence time graphitization of mesophase pitch-based carbon fibers

The effects of graphitization time and temperature on the properties of three mesophase pitch-based carbon fibers have been characterized. Graphitization temperatures studied were 2400, 2700, and 3000 °C and residence times ranged from 0.7 to 3600 s. Helium pycnometry, measurements of fiber tow resistance, and X-ray diffraction were employed to study fiber properties. As anticipated, substantial variations in fiber properties were noted for the range of graphitization conditions studied and among the three fiber types. Significant structural evolution and property development occurred even at the shortest furnace residence times. For example, for one of the fibers, a furnace residence time of 0.7 s at 3000 °C resulted in a degree of graphitization value of ∼50%, a density of 1.98 g/cm³, and an electrical resistivity of 6.3 μΩ m (corresponding thermal conductivity ∼200 W m⁻¹ K⁻¹). A simple energy consumption analysis suggests that short residence time graphitization at high temperature may result in both lower costs and substantially higher production rates for fibers prepared from mesophase pitch.     

Kinetics of petroleum pitch polymerization by gel permeation chromatography

The thermal polymerization kinetics of petroleum pitch molecules with molecular weights (MW) between 400–2000 were studied utilizing gel permeation chromatography (GPC). Two distinctly different reaction mechanisms of the petroleum pitch were observed from the GPC curves during transformation of the pitch to mesophase and semi-coke. When mesophase is forming in the pitch, molecules in the MW range of 400–700 were the most reactive, while molecules with MW greater than 1200 were essentially unreactivc. This lack of reactivity of the larger molecules ensures that the mesophase remains highly fluid during its formation. A change in reaction mechanism occurs after the mesophase has solidified as a result of the thermal polymerization reactions. At this stage, there is an abrupt decrease in the reactivity of molecules with MW less than 1100 and the reactivity of all measured molecules is the same. This significant kinetic change is due to a physical phenomenon in which solid state reactions occur in the infusible (vitrified) polymerized pitch. The significantly reduced reaction rates occurring in petroleum pitch after vitrification are similar to the results observed for other thermosetting materials.     

The correspondence between loss of H-NMR signal intensity and mesophase growth during the thermal maturation of a petroleum pitch

Samples of petroleum pitch, Ashland 240, were heated isothermally at 380, 400, 430 and 450°C and the hydrocarbon evolution and mesophase growth monitored simultaneously by the use of high temperature ¹H-nuclear magnetic resonance (NMR) analysis. For all four temperatures the NMR signal indicated no mesophase formation until the signal intensity had dropped by 30%, which corresponded to 20% weight loss from the pitch. The correlation between the onset and increase in mesophase formation with signal intensity loss was found to be independent of temperature. This temperature independence for the petroleum pitch agrees with the work presented previously by Honda et al. (Carbon 1970, 8, 181). who compared weight loss with the growth of quinoline-insoluble material for a coal-derived pitch. Given the correlation between signal loss and mesophase formation, it is suggested that a relative measure of NMR signal intensity could be used to estimate mesophase content of petroleum pitch, during maturation.     

Primary carbonization of an anisotropic ‘mesophase’ pitch compared to conventional isotropic pitch

The evolution during carbonization treatments of a 100% anisotropic pitch (pitch C) was compared to that of Ashland 240 (100% γ resins). The anisotropic pitch C results from a gas-sparge preparation leading to a composition of 93% β resins (QS-TI) and 7% γ resins (QS). It is made of a major component (QS-TI), in which droplets (100-300 nm in size) partially toluene soluble are distributed. The physicochemical, textural and microtextural evolutions of the two pitches were studied. During pitch C primary carbonization, anisotropic droplets grow by coalescence, then decompose into Brooks and Taylor mesophase spheres suspended in isotropic drops. These drops develop at the expense of the anisotropic matrix by a continuous regeneration of the small anisotropic droplets which feed the isotropic drops by diffusion process. Then inside these drops, mesophase spheres grow then coalesce and the behaviour of a conventional pitch is restored. These various molecular associations are due to absence of chemical events below 450 °C, leading to the global mass spectrum being constant. At 500 °C the material is homogeneously anisotropic though plastic, the metastable system is destroyed and the evolution of conventional pitches is recovered, i.e. above 550 °C macropores develop up to solidification at 600 °C (semi-coke stage).