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Carbon disc of high density and strength prepared from synthetic pitch-derived mesocarbon microbeads
Carbon 37 (1999) 1049–1057
Carbon disc of high density and strength prepared from synthetic pitch-derived mesocarbon microbeads Yong-Gang Wang*, Yozo Korai, Isao Mochida Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan Received 10 September 1998; accepted 1 November 1998
Abstract Binderless carbon discs were prepared from mesocarbon microbeads (MCMB) derived from the synthetic naphthalene pitch without and with carbon black. The carbonized discs were evaluated in terms of physical properties and textures. Carbon black included in the stabilized MCMB was found to enhance significantly the compressive strength of the disc by suppressing the formation and propagation of the fissures and voids in the disc. Strikingly high compressive strength of 420 MPa, bulk density of 1.71 g / cm 3 , and large volumetric shrinkage of 33% were successfully obtained by calcining the disc at 13008C, which were prepared from the stabilized MCMB with 3% BP2000 (Black Pearls 2000). The disc of no stabilization without carbon black exhibited a flow texture with large voids and deformed shape. Oxidative stabilization at 3008C for 1 h provided the fine mosaic texture in the disk with the excellent shape stability and homogeneous shrinkage. The discs from MCMB with BP2000 and KB (Ketjen Black) stabilized at 2708C for 1 h showed the intimate adhesion among MCMB particles with homogeneous texture, no large fissure being found in the calcined disc, corresponding to their higher mechanical strength. No stabilization of MCMB with carbon black provided inferior strength and shape stability to that of stabilized MCMB with carbon black, but much superior to those of MCMB without carbon black regardless of the stabilization. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon beads; B. Stabilization; D. Mechanical properties
1. Introduction High density and high strength artifacts of carbon and graphite have been widely applied in the modern advanced technologies. They are indispensable for the nuclear reactors, electric discharge, drilling bits, crucibles for chemicals and semiconductors [1]. Such carbon artifacts have been usually prepared by the following two procedures. The first one uses pulverized coke as filler and pitch as binder [2]. Natural graphite and petroleum coke have been used as such filler. The filler blended with coal tar pitch as a binder is moulded by a cold isostatic press, baking, impregnation required, and graphitization into the final product. This process often results in a rather low density of insufficient strength, hence baking and impregnation are repeated. The second one uses self-adhesive carbonaceous grains for binderless *Corresponding author.
moulding [3–6]. Mesocarbon microbeads [MCMB] have been recognized to be an excellent precursor for such high density and strength artifacts [7–10]. The cost of MCMB is a problem. Pulverized particles of green cokes [11,12], oxidized or heat treated mesophase pitch [13–15] have been also moulded without any binding substances to give high density and high strength. Recently, the present authors have reported that high yield and uniform size of MCMB can be prepared from the synthetic naphthalene isotropic pitch [16] and that carbon black enhanced further their yield and homogeneity [17]. In the present study, such MCMB prepared from the synthetic pitch were moulded into discs and their physical properties were examined. The proper stabilization conditions of MCMB were carefully selected because such MCMB suffer from the excess fusion when they are carbonized after moulding. The influence of the carbon black on the stabilization of MCMB and physical properties and textures of the resultant artifacts were also
0008-6223 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 98 )00298-X
Y. Wang et al. / Carbon 37 (1999) 1049 – 1057
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studied to find a way to facilitate the stabilization and enhance their strength.
2. Experimental
2.1. Preparation and analyses of MCMB Mesocarbon microbeads [MCMB] were prepared from a synthetic naphthalene isotropic pitch [17], which was supplied by Mitsubishi Gas Chemical Company. The isotropic pitch was heat treated in a Pyrex glass tube at 4108C by a heating rate of 28C / min for a certain time under mild agitation. The heat treated pitch was extracted first in tetrahydrofuran at 508C for 12 h, and then particles were filtered at this temperature. This process was repeated several times to isolate MCMB. The MCMB prepared without carbon black at 4108C for 1.5 h were named as MCMB-FCB. In order to increase the yield and improve homogeneity of MCMB, the pitch blended with carbon black was heat treated at 4108C for 2 h to prepare the approximately same size of MCMB to that without carbon black because carbon black tended to produce more MCMB of the smaller size. The MCMB prepared with 3% Black Pearls 2000 (BP2000) and Ketjen Black (KB) were named as MCMB-BP-3 and MCMB-KB-3, respectively, as summarized in Table 1. The average diameters of BP2000 and KB are 15 and 30 nm, respectively. More details are to be found in a previous paper [17]. Thermogravimetric analyses of MCMB were performed by a TG / DTA thermal analyzer (Seiko, TG / DTA 220) at a heating rate of 58C / min to 8008C. The size distribution of MCMB was measured by a laser diffraction particle size distribution analyzer (Nikkiso, FRA 9220).
2.2. Stabilization of MCMB
2.3. Preparation of carbon discs As-prepared and stabilized MCMB (ca. 2 g) were moulded without a binder under 40 MPa at room temperature into a hard disc of 20 mm in diameter and ca. 5 mm in thickness. The disc was carbonized at 7008C for 1 h at the heating rate of 18C / min in a horizontal furnace under nitrogen flow, and further calcined at 1000 or 13008C for 1 h at the heating rate of 58C / min in argon flow.
2.4. Property evaluation of carbon discs The bulk density of discs after moulding, carbonization, and calcination were calculated by measuring the dimensions and weight. The X-ray diffraction profiles of the artifacts were measured using Cu Ka radiation (Rigaku, RAD 2VB). The interlayer spacing, d 002 , and the crystallite height, Lc(002), were calculated from the diffraction angle and half width of C(002) according to the procedure prescribed by the Japan Society for Promotion of Science [18]. The resultant artifact was cut into a test piece (ca. 63634 mm) for measurement of the compressive strength by an autograph (Shimazu, AG-2000B). A cross-head speed was 1 mm / min. The fractured surfaces of the test pieces were observed by a high resolution scanning electron microscope (JEOL, JSM 6400F). The cross sections of the discs after the carbonization and calcination were examined with a polarized microscope (Olympus, B061) after the conventional polishing.
3. Results
3.1. Some properties of MCMB The thermogravimetric analyses of as-prepared MCMB in nitrogen flow are shown in Fig. 1. The sharp decrease in
MCMB-FCB were oxidatively stabilized in air at 270, 300 and 3308C for 1 h by a heating rate of 18C / min, while MCMB-BP-3 and MCMB-KB-3 were oxidatively stabilized at 2708C for 1 h by a heating rate of 58C / min, respectively.
Table 1 Preparation conditions of mesocarbon microbeads (MCMB) derived from a synthetic naphthalene isotropic pitch with and without carbon black Code
MCMB-FCB MCMB-BP-3 MCMB-KB-3
Heat treatment conditions
Additives
Temp. (8C)
Time (h)
Carbon black
wt. %
410 410 410
1.5 2 2
– BP2000 KB
– 3 3
Fig. 1. TG weight loss profiles of as-prepared MCMB. (a) MCMB-FCB, (b) MCMB-BP-3, (c) MCMB-KB-3.
Y. Wang et al. / Carbon 37 (1999) 1049 – 1057
weight of all samples began around 4008C, and their coking yields were over 90% at 8008C, although MCMB prepared in the presence of carbon black showed the slightly higher value. Size distributions of MCMB measured with a laser diffraction type particle analyzer are shown in Fig. 2. The size distribution of MCMB-FCB is within a narrow range from 0.3 to 10 mm. Their average size was about 5 mm. Carbon black tended to shift the size of MCMB towards a smaller range even though the heat treatment time is longer. Average sizes of MCMB-BP-3 and MCMB-KB-3 were 4.5 and 3.6 mm, respectively.
3.2. Shape stability during carbonization In Fig. 3 the appearances of the carbonized discs at 10008C prepared from as-prepared and stabilized MCMB under moulding pressure of 40 MPa are shown. A disc of
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as-prepared MCMB-FCB lost its shape, suffered from a significant expansion, and broke into pieces after it was carbonized (Fig. 3(a)). A disc of as-prepared MCMB-KB-3 slightly deformed its shape during carbonization and calcination as shown in Fig. 3(b). A disc of as-prepared MCMB-BP-3 maintained its shape even though its surface suffered from a slight expansion by the carbonization up to 10008C (Fig. 3(c)). A disc of stabilized MCMB-FCB at 2708C for 1 h at a heating rate of 18C / min maintained its shape during carbonization, however, its surface slightly swelled even though neither crack nor deformation was found in the disc (Fig. 3(d)), insufficient stabilization extent being suggested. Stabilization at 3008C for 1 h produced the disc of no deformation (Fig. 3(e)), exhibiting the sufficient stabilization. The less extent of stabilization at 2708C for 1 h with 58C / min was enough for MCMB-BP-3 to provide excellent shape stability of the disc during carbonization, showing the homogeneous shrinkage (Fig. 3(f)). MCMBKB-3 gave similar results.
3.3. Physical properties of carbon discs
Fig. 2. The size distribution of the MCMB. (a) MCMB-FCB, (b) MCMB-BP-3, (c) MCMB-KB-3.
Bulk densities of discs are shown in Fig. 4, where significant influences of starting MCMB are demonstrated. The bulk density increased gradually up to 7008C and then sharply in the temperature range of 700–13008C regardless of the types of MCMB. Addition of carbon black caused the slight but significant increase of the bulk density of the disc at each carbonization temperature after stabilization although it did not provide the highest green density. The maximum bulk densities of 1.71 and 1.70 g / cm 3 were achieved by the carbonization at 13008C when MCMBBP-3 and MCMB-KB-3 were moulded after stabilization at 2708C for 1 h, indicating favorable effects of the carbon black on the dense packing at carbonization. The remarkable expansion and deformation of the moulds during carbonization were observed without oxidative stabilization though carbon black significantly reduced their expansion extent. As-prepared MCMB-BP-3 provided somewhat lower densities of 1.32, 1.42, and 1.48 g / cm 3 at 700, 1000, 13008C, respectively. The oxidation of MCMBFCB at 2708C for 1 h failed to suppress the expansion of the disc, whereas a higher temperature of 3008C still allowed adhesion among the MCMB. MCMB-FCB stabilized at 3008C for 1 h provided the maximum bulk density of 1.66 g / cm 3 at 13008C. The weight loss of discs as a function of carbonization temperature is shown in Fig. 5. The discs after stabilization lost their weight steadily by the increased carbonization temperature to 13008C. Oxidative stabilization decreased the weight loss of the discs. The disc prepared from as-prepared MCMB-BP-3 showed a sharp weight loss, losing 8, 10, and 11% of its weight at 700, 1000 and 13008C, respectively. The MCMB-BP-3 stabilized at 2708C for 1 h lost the least weight, providing 5.7, 6.2, and
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Fig. 3. The appearances of the carbonized discs at 10008C. Prepared from a moulding pressure of 40 MPa at room temperature from: (a) MCMB-FCB, (b) MCMB-BP-3, (c) MCMB-KB-3, (d) MCMB-FCB stabilized at 2708C for 1 h with 18C / min, (e) MCMB-FCB stabilized at 3008C for 1 h with 18C / min, (f) MCMB-BP-3 stabilized at 2708C for 1 h with 58C / min.
Fig. 4. The influences of the carbonization temperature on the bulk densities of the discs.
Fig. 5. Change in weight of the discs as a function of carbonization temperatures.
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The disc prepared from MCMB-BP-3 stabilized at 2708C for 1 h showed 33% volumetric shrinkage at 13008C, which is larger than that without oxidative stabilization (21%). The carbon black enhanced volumetric shrinkage at any carbonization temperature. The disc of stabilized MCMB-KB-3 provided 17, 27 and 32% of shrinkage at 700, 1000, and 13008C, respectively. However, the values of stabilized MCMB-FCB were 15, 25 and 29%, respectively, at respective temperatures. The compressive strength of calcined discs at 13008C are summarized in Table 2. The disc of stabilized MCMBFCB exhibited the high compressive strength of 320 MPa, while the excessive oxidation at 3308C for 1 h decreased slightly its value to 300 MPa. Strikingly high compressive strengths of 420 and 400 MPa were obtained from the discs prepared from stabilized MCMB-BP-3 and MCMBKB-3, respectively, indicating the favorable effects of carbon black on the compressive strength of the disc. An excessive oxidation of MCMB-KB-3 at 3008C for 1 h markedly decreased its compressive strength to the lowest value of 250 MPa. The values of interlayer spacing, d 002 , and crystallite height, Lc, of as-prepared and calcined discs are summarized in Table 3. The Lc value of MCMB-FCB increased gradually to 1.8 from 1.6 nm by 10008C, and then sharply to 5.6 nm at 13008C. The MCMB-BP-3 and MCMB-KB-3 showed the slightly low Lc of 1.4 nm which increased to 4.7 and 4.5 nm at 13008C, respectively. The d 002 value of MCMB-FCB decreased from 0.3541 at 10008C to 0.3456 at 13008C. The calcined discs of MCMB-BP-3 and MCMB-KB-3 at 13008C gave slightly larger d 002 values of 0.3464 and 0.3475 nm, respectively.
Fig. 6. Change in voluminal shrinkage of the discs as a function of carbonization temperature.
9.0% loss at 700, 1000 and 13008C, respectively. Carbon black reduced slightly the weight loss of the disc, as indicated by 9.2% of the stabilized MCMB-KB-3 and 12% of the stabilized MCMB-FCB at 13008C. The volumetric shrinkage of the discs prepared from as-prepared and stabilized MCMB are shown in Fig. 6.
Table 2 Compressive strengths of carbonized discs at 13008C for 1 h Starting MCMB
Stabilization conditions Temp. (8C)
Time (h)
Heating rate (8C / min)
MCMB-FCB
300 330 270 270 300
1 1 1 1 1
1 1 5 5 5
MCMB-BP-3 MCMB-KB-3
Bulk density (g / cm 3 )
Compressive strength (MPa)
1.66 1.64 1.71 1.70 1.69
320 300 420 400 250
Table 3 Crystallographic properties of as-prepared and carbonized discs Starting MCMB
d 002 (nm)
Lc (nm)
As-prepared
10008C
13008C
As-prepared
10008C
13008C
MCMB-FCB MCMB-BP-3 MCMB-KB-3
0.3589 0.3577 0.3601
0.3541 0.3547 0.3542
0.3456 0.3464 0.3475
1.6 1.4 1.4
1.8 1.6 1.5
5.6 4.7 4.5
Stabilization conditions: MCMB-FCB: 3008C, 1 h, 18C / min, MCMB-BP-3 and MCMB-KB-3: 2708C, 1 h, 58C / min.
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3.4. Textures of carbon discs The polarized optical microphotographs of carbonized discs at 13008C, prepared from as-prepared and stabilized MCMB, are shown in Fig. 7. The disc of as-prepared MCMB-FCB exhibited flow texture with large voids and lost the shape of MCMB (Fig. 7(a)), indicating excess fusibility of MCMB. Oxidative stabilization at 3008C for 1 h removed pore and cracks in the carbonized disc, providing the fine mosaic texture of MCMB with the excellent shape and homogeneous shrinkage (Fig. 7(b)). Oxidative stabilization at a higher temperature of 3308C provided also a fine mosaic texture of MCMB for maintaining its shape, then a lot of fine voids can be induced in the carbonized disc (Fig. 7(c)), suggesting the insufficient adhesion among the grains. MCMB-BP-3 stabilized at 2708C for 1 h provided a
homogeneous distribution of MCMB in the carbonized disc (Fig. 7(d)). No pores and cracks were observable, showing the uniform shrinkage during the carbonization. MCMB-KB-3 stabilized at 2708C for 1 h gave a similar texture with MCMB-BP-3, then very few fine crevices can be found in the carbonized disc (Fig. 7(e)). Oxidative stabilization of MCMB-KB-3 stabilized at 3008C for 1 h increased the crevices of the carbonized disc (Fig. 7(f)), reflecting the lower density and strength after the carbonization. SEM photographs of the fractured surfaces of the carbonized discs at 13008C after measurements of compressive strength, prepared without and with carbon black, are shown in Figs. 8 and 9, respectively. The carbonized disc from MCMB-FCB stabilized at 3008C for 1 h showed an adhesion among MCMB with homogeneous and densified appearance as observed in Fig. 8(a). In Fig. 8(b) it
Fig. 7. Optical textures of the carbonized discs at 13008C for 1 h. Prepared from (a) as-prepared MCMB-FCB, (b) MCMB-FCB stabilized at 3008C for 1 h, (c) MCMB-FCB stabilized at 3308C for 1 h, (d) MCMB-BP-3 stabilized at 2708C for 1 h, (e) MCMB-KB-3 stabilized at 2708C for 1 h, (f) MCMB-KB-3 stabilized at 3008C for 1 h.
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Fig. 8. SEM photographs of the fractured surfaces of the discs prepared from MCMB-FCB. Carbonization conditions; 13008C for 1h, stabilization conditions; (a) and (b) 3008C for 1 h, (c) and (d) 3308C for 1 h.
is exhibited that the fissure was produced in the boundaries among MCMB, indicating that the binding part among MCMB easily caused fragmentation of the discs. Excessive oxidation at 3308C for 1 h provided the clear boundaries of MCMB with a coarse appearance as shown in Fig. 8(c), a lot of large fissures and voids in the carbonized disc being observable. In Fig. 8(d) an individual MCMB without any adhesion is shown, and the fissure around the sphere was observed, corresponding to insufficient fusion among the grains. The disc prepared from MCMB-BP-3 after stabilization showed the adhesion among MCMB particles with homogeneous texture as shown in Fig. 9(a). No larger fissure can be found after the measurement of the compressive strength although very fine fissures and few voids
were observed (Fig. 9(b)), exhibiting the homogeneous fragmentation of the disc. The carbonized disc from MCMB-KB-3 after stabilization showed the homogeneous appearance (Fig. 9(c)), and again no bigger voids, but more numbers of fine fissures and voids (Fig. 9(d)) than that from MCMB-BP-3. Increased fine fissures and voids of MCMB-KB-3 provided a slightly low compressive strength of 400 MPa than that of MCMB-BP-3 (420 MPa). Thus, the disc prepared from MCMB with carbon black after proper stabilization produced the homogeneous fragmentation of the disc after the measurement of the compressive strength, corresponding to higher mechanical strength, while the disc prepared from MCMB without carbon black induced the larger fissure during carbonization, leading to lower compressive strength.
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Fig. 9. SEM photographs of the fractured surfaces of the discs prepared from MCMB-BP-3 ((a) and (b)) and MCMB-KB-3 ((c) and (d)). Carbonization conditions; 13008C for 1 h, stabilization conditions; 2708C for 1 h.
4. Discussion The present study reports that a carbonized artifact of excellent properties with strikingly high compressive strength of 420 MPa, bulk density of 1.71 g / cm 3 , and large volumetric shrinkage of 33% was successfully prepared from the stabilized MCMB with 3% BP2000 by the calcination at 13008C. Such high mechanical performance is expected to be applied widely for the commercial areas of advanced technologies. MCMB prepared from the synthetic pitch are still fusible although carbon black significantly reduced the fusibility even if they are isolated by extraction. The mesogen molecules of the sphere from the synthetic pitch are considered to be a series of naphthalene-oligomers. The
lighter components are more or less included in the heavier components in the sphere. Oxidative stabilization in air effectively reduces their lighter components or naphthenic structure through the condensation and dehydrogenation, leaving the adequate adhesion among oxidized MCMB. Thus, adequate oxidation can remove the excessive fusion, maintaining their self-adhesive ability. Excessive oxidation at a high temperature or a longer period removed too much the adhesive ability of MCMB, leaving and inducing a lot of fine crevice or voids among the MCMB to lower the mechanical strength of the artifact after the carbonization. Insufficient oxidation of MCMB leaves too much fusibility and volatile matters, leading to expansion because of the evolution of volatile matters and fusible deformation of MCMB during carbonization, and too many voids are
Y. Wang et al. / Carbon 37 (1999) 1049 – 1057
induced in the carbonized disc. Thus, it is important to achieve the balance of their thermosetting properties to maintain the shape stability and sufficient adhesive ability. It is worthwhile to discuss the roles of the carbon black in the stabilization and carbonization of the discs. Carbon black, which are mainly adsorbed on the periphery of MCMB as observed in a previous study [17], enhances the adhesion force among MCMB as filler to control the carbonization of a lighter fraction at their surface and effectively inhibits the crack propagation at the interface during carbonization of the discs. Carbon black among the MCMB provides the gas evolution pathways, controlling the expansion of the artifacts at less stabilization which allows dense packing and uniform contraction of MCMB in the discs. Thus, carbon black improves significantly the mechanical performance of the artifact. Uniform composition of MCMB inherited from the parent synthetic pitch and uniform distribution of their size are considered to favour the high performance of the carbon artifact through the controlled oxidation and closer packing in the disc. The strikingly high compressive strength of the carbon discs obtained by the present study, confirms the excellent properties of the present MCMB as the precursor of advanced carbon materials. Further control of the synthetic pitch increases its potential for a carbon source of high performances.
5. Conclusions MCMB prepared from the synthetic naphthalene pitch, exhibited the excellent performance as the precursor materials of high density and high strength artifacts. Particularly, MCMB prepared in the presence of carbon black enhanced their mechanical performance because carbon black effectively inhibits the crack formation and propagation at the interface during carbonization of the discs. The compressive strengths of 420 and 400 MPa were successfully obtained by the carbonized discs of MCMB prepared with 3 wt. % BP2000 and KB, respectively. These compressive strength values are certainly higher than those found in the literature, which are derived from the artifacts of coal tar pitch based MCMB, such as ca. 200
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MPa by Honda [3], ca. 300 MPa by Yamada [4], and 380 MPa by Nakagawa [10].
Acknowledgements The authors are very grateful to Mr. R. Fujiura from Mitsubishi Gas Chemical Company Inc. for the measurement of the compressive strength.
References ´ [1] Oya A. In: Marsh H, Heintz EA, Rodrıguez-Reinoso F, editors. Introduction to Carbon Technologies. Spain: Universidad de Alicante, 1997:561. [2] Miyazaki K, Hagio T, Kobayashi K. J Mater Sci 1981;16:752. [3] Honda H, Yamada Y. J Jap Petrol Inst 1973;16:392. [4] Yamada Y, Shibata K, Honda H, Oi S. Tanso 1977;88:2. ¨ [5] Hoffmann WR, Huttinger KJ. Carbon 1994;32:1087. ¨ [6] Gschwindt A, Huttinger KJ. Carbon 1994;32:1105. ´ ´ [7] Martınez M, Rodrıguez-Reinoso F, Torregrosa P, Marsh H. In: Extended Abstracts, Carbon’96. Newcastle, UK, 1996:423. [8] Rand B, Stirling C. In: Extended Abstracts, 20th Biennial Conference on Carbon. Santa Barbara, CA, 1991:204. [9] Bhatia G, Aggarwal RK, Punjabi N, Bahl OP. J Mater Sci 1994;29:4757. [10] Nakagawa Y, Fujita K, Mori M. In: Extended abstracts, 17th biennial Conference on Carbon. Kentucky, 1985:409. [11] Mochida I, Korai Y, Fujitsu T, Takeshita K, Mukai K, Nagino H. J Mater Sci 1982;17:525. ´ ´ [12] Martınez M, Rodrıguez-Reinoso F, Torregrosa P, Romero D, Santamaria R. Carbon 1995;33:1182. [13] Mochida I, Fujiura R, Kojima T, Sakamoto H, Kanno K. Carbon 1994;32:961. [14] Mochida I, Fujiura R, Kojima T, Sakamoto H, Yoshimura T. Carbon 1995;33:265. ´ [15] Fernandez JJ, Figueiras A, Granda M, Bermejo J, Parra JB, ´ Menendez R. Carbon 1995;33:1235. [16] Korai Y, Ishida S, Yoon S-H, Wang Y-G, Mochida I, Nakagawa Y, Matsumura Y. Carbon 1996;34:1569. [17] Korai Y, Wang Y-G, Yoon S-H, Ishida S, Mochida I, Nakagawa Y, Matsumura Y. Carbon 1997;35:875. [18] Japan Society For Promotion of Science. Tanso 1963;36:25.
Carbon disc of high density and strength prepared from synthetic pitch-derived mesocarbon microbeads Yong-Gang Wang*, Yozo Korai, Isao Mochida Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan Received 10 September 1998; accepted 1 November 1998
Abstract Binderless carbon discs were prepared from mesocarbon microbeads (MCMB) derived from the synthetic naphthalene pitch without and with carbon black. The carbonized discs were evaluated in terms of physical properties and textures. Carbon black included in the stabilized MCMB was found to enhance significantly the compressive strength of the disc by suppressing the formation and propagation of the fissures and voids in the disc. Strikingly high compressive strength of 420 MPa, bulk density of 1.71 g / cm 3 , and large volumetric shrinkage of 33% were successfully obtained by calcining the disc at 13008C, which were prepared from the stabilized MCMB with 3% BP2000 (Black Pearls 2000). The disc of no stabilization without carbon black exhibited a flow texture with large voids and deformed shape. Oxidative stabilization at 3008C for 1 h provided the fine mosaic texture in the disk with the excellent shape stability and homogeneous shrinkage. The discs from MCMB with BP2000 and KB (Ketjen Black) stabilized at 2708C for 1 h showed the intimate adhesion among MCMB particles with homogeneous texture, no large fissure being found in the calcined disc, corresponding to their higher mechanical strength. No stabilization of MCMB with carbon black provided inferior strength and shape stability to that of stabilized MCMB with carbon black, but much superior to those of MCMB without carbon black regardless of the stabilization. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon beads; B. Stabilization; D. Mechanical properties
1. Introduction High density and high strength artifacts of carbon and graphite have been widely applied in the modern advanced technologies. They are indispensable for the nuclear reactors, electric discharge, drilling bits, crucibles for chemicals and semiconductors [1]. Such carbon artifacts have been usually prepared by the following two procedures. The first one uses pulverized coke as filler and pitch as binder [2]. Natural graphite and petroleum coke have been used as such filler. The filler blended with coal tar pitch as a binder is moulded by a cold isostatic press, baking, impregnation required, and graphitization into the final product. This process often results in a rather low density of insufficient strength, hence baking and impregnation are repeated. The second one uses self-adhesive carbonaceous grains for binderless *Corresponding author.
moulding [3–6]. Mesocarbon microbeads [MCMB] have been recognized to be an excellent precursor for such high density and strength artifacts [7–10]. The cost of MCMB is a problem. Pulverized particles of green cokes [11,12], oxidized or heat treated mesophase pitch [13–15] have been also moulded without any binding substances to give high density and high strength. Recently, the present authors have reported that high yield and uniform size of MCMB can be prepared from the synthetic naphthalene isotropic pitch [16] and that carbon black enhanced further their yield and homogeneity [17]. In the present study, such MCMB prepared from the synthetic pitch were moulded into discs and their physical properties were examined. The proper stabilization conditions of MCMB were carefully selected because such MCMB suffer from the excess fusion when they are carbonized after moulding. The influence of the carbon black on the stabilization of MCMB and physical properties and textures of the resultant artifacts were also
0008-6223 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 98 )00298-X
Y. Wang et al. / Carbon 37 (1999) 1049 – 1057
1050
studied to find a way to facilitate the stabilization and enhance their strength.
2. Experimental
2.1. Preparation and analyses of MCMB Mesocarbon microbeads [MCMB] were prepared from a synthetic naphthalene isotropic pitch [17], which was supplied by Mitsubishi Gas Chemical Company. The isotropic pitch was heat treated in a Pyrex glass tube at 4108C by a heating rate of 28C / min for a certain time under mild agitation. The heat treated pitch was extracted first in tetrahydrofuran at 508C for 12 h, and then particles were filtered at this temperature. This process was repeated several times to isolate MCMB. The MCMB prepared without carbon black at 4108C for 1.5 h were named as MCMB-FCB. In order to increase the yield and improve homogeneity of MCMB, the pitch blended with carbon black was heat treated at 4108C for 2 h to prepare the approximately same size of MCMB to that without carbon black because carbon black tended to produce more MCMB of the smaller size. The MCMB prepared with 3% Black Pearls 2000 (BP2000) and Ketjen Black (KB) were named as MCMB-BP-3 and MCMB-KB-3, respectively, as summarized in Table 1. The average diameters of BP2000 and KB are 15 and 30 nm, respectively. More details are to be found in a previous paper [17]. Thermogravimetric analyses of MCMB were performed by a TG / DTA thermal analyzer (Seiko, TG / DTA 220) at a heating rate of 58C / min to 8008C. The size distribution of MCMB was measured by a laser diffraction particle size distribution analyzer (Nikkiso, FRA 9220).
2.2. Stabilization of MCMB
2.3. Preparation of carbon discs As-prepared and stabilized MCMB (ca. 2 g) were moulded without a binder under 40 MPa at room temperature into a hard disc of 20 mm in diameter and ca. 5 mm in thickness. The disc was carbonized at 7008C for 1 h at the heating rate of 18C / min in a horizontal furnace under nitrogen flow, and further calcined at 1000 or 13008C for 1 h at the heating rate of 58C / min in argon flow.
2.4. Property evaluation of carbon discs The bulk density of discs after moulding, carbonization, and calcination were calculated by measuring the dimensions and weight. The X-ray diffraction profiles of the artifacts were measured using Cu Ka radiation (Rigaku, RAD 2VB). The interlayer spacing, d 002 , and the crystallite height, Lc(002), were calculated from the diffraction angle and half width of C(002) according to the procedure prescribed by the Japan Society for Promotion of Science [18]. The resultant artifact was cut into a test piece (ca. 63634 mm) for measurement of the compressive strength by an autograph (Shimazu, AG-2000B). A cross-head speed was 1 mm / min. The fractured surfaces of the test pieces were observed by a high resolution scanning electron microscope (JEOL, JSM 6400F). The cross sections of the discs after the carbonization and calcination were examined with a polarized microscope (Olympus, B061) after the conventional polishing.
3. Results
3.1. Some properties of MCMB The thermogravimetric analyses of as-prepared MCMB in nitrogen flow are shown in Fig. 1. The sharp decrease in
MCMB-FCB were oxidatively stabilized in air at 270, 300 and 3308C for 1 h by a heating rate of 18C / min, while MCMB-BP-3 and MCMB-KB-3 were oxidatively stabilized at 2708C for 1 h by a heating rate of 58C / min, respectively.
Table 1 Preparation conditions of mesocarbon microbeads (MCMB) derived from a synthetic naphthalene isotropic pitch with and without carbon black Code
MCMB-FCB MCMB-BP-3 MCMB-KB-3
Heat treatment conditions
Additives
Temp. (8C)
Time (h)
Carbon black
wt. %
410 410 410
1.5 2 2
– BP2000 KB
– 3 3
Fig. 1. TG weight loss profiles of as-prepared MCMB. (a) MCMB-FCB, (b) MCMB-BP-3, (c) MCMB-KB-3.
Y. Wang et al. / Carbon 37 (1999) 1049 – 1057
weight of all samples began around 4008C, and their coking yields were over 90% at 8008C, although MCMB prepared in the presence of carbon black showed the slightly higher value. Size distributions of MCMB measured with a laser diffraction type particle analyzer are shown in Fig. 2. The size distribution of MCMB-FCB is within a narrow range from 0.3 to 10 mm. Their average size was about 5 mm. Carbon black tended to shift the size of MCMB towards a smaller range even though the heat treatment time is longer. Average sizes of MCMB-BP-3 and MCMB-KB-3 were 4.5 and 3.6 mm, respectively.
3.2. Shape stability during carbonization In Fig. 3 the appearances of the carbonized discs at 10008C prepared from as-prepared and stabilized MCMB under moulding pressure of 40 MPa are shown. A disc of
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as-prepared MCMB-FCB lost its shape, suffered from a significant expansion, and broke into pieces after it was carbonized (Fig. 3(a)). A disc of as-prepared MCMB-KB-3 slightly deformed its shape during carbonization and calcination as shown in Fig. 3(b). A disc of as-prepared MCMB-BP-3 maintained its shape even though its surface suffered from a slight expansion by the carbonization up to 10008C (Fig. 3(c)). A disc of stabilized MCMB-FCB at 2708C for 1 h at a heating rate of 18C / min maintained its shape during carbonization, however, its surface slightly swelled even though neither crack nor deformation was found in the disc (Fig. 3(d)), insufficient stabilization extent being suggested. Stabilization at 3008C for 1 h produced the disc of no deformation (Fig. 3(e)), exhibiting the sufficient stabilization. The less extent of stabilization at 2708C for 1 h with 58C / min was enough for MCMB-BP-3 to provide excellent shape stability of the disc during carbonization, showing the homogeneous shrinkage (Fig. 3(f)). MCMBKB-3 gave similar results.
3.3. Physical properties of carbon discs
Fig. 2. The size distribution of the MCMB. (a) MCMB-FCB, (b) MCMB-BP-3, (c) MCMB-KB-3.
Bulk densities of discs are shown in Fig. 4, where significant influences of starting MCMB are demonstrated. The bulk density increased gradually up to 7008C and then sharply in the temperature range of 700–13008C regardless of the types of MCMB. Addition of carbon black caused the slight but significant increase of the bulk density of the disc at each carbonization temperature after stabilization although it did not provide the highest green density. The maximum bulk densities of 1.71 and 1.70 g / cm 3 were achieved by the carbonization at 13008C when MCMBBP-3 and MCMB-KB-3 were moulded after stabilization at 2708C for 1 h, indicating favorable effects of the carbon black on the dense packing at carbonization. The remarkable expansion and deformation of the moulds during carbonization were observed without oxidative stabilization though carbon black significantly reduced their expansion extent. As-prepared MCMB-BP-3 provided somewhat lower densities of 1.32, 1.42, and 1.48 g / cm 3 at 700, 1000, 13008C, respectively. The oxidation of MCMBFCB at 2708C for 1 h failed to suppress the expansion of the disc, whereas a higher temperature of 3008C still allowed adhesion among the MCMB. MCMB-FCB stabilized at 3008C for 1 h provided the maximum bulk density of 1.66 g / cm 3 at 13008C. The weight loss of discs as a function of carbonization temperature is shown in Fig. 5. The discs after stabilization lost their weight steadily by the increased carbonization temperature to 13008C. Oxidative stabilization decreased the weight loss of the discs. The disc prepared from as-prepared MCMB-BP-3 showed a sharp weight loss, losing 8, 10, and 11% of its weight at 700, 1000 and 13008C, respectively. The MCMB-BP-3 stabilized at 2708C for 1 h lost the least weight, providing 5.7, 6.2, and
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Fig. 3. The appearances of the carbonized discs at 10008C. Prepared from a moulding pressure of 40 MPa at room temperature from: (a) MCMB-FCB, (b) MCMB-BP-3, (c) MCMB-KB-3, (d) MCMB-FCB stabilized at 2708C for 1 h with 18C / min, (e) MCMB-FCB stabilized at 3008C for 1 h with 18C / min, (f) MCMB-BP-3 stabilized at 2708C for 1 h with 58C / min.
Fig. 4. The influences of the carbonization temperature on the bulk densities of the discs.
Fig. 5. Change in weight of the discs as a function of carbonization temperatures.
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The disc prepared from MCMB-BP-3 stabilized at 2708C for 1 h showed 33% volumetric shrinkage at 13008C, which is larger than that without oxidative stabilization (21%). The carbon black enhanced volumetric shrinkage at any carbonization temperature. The disc of stabilized MCMB-KB-3 provided 17, 27 and 32% of shrinkage at 700, 1000, and 13008C, respectively. However, the values of stabilized MCMB-FCB were 15, 25 and 29%, respectively, at respective temperatures. The compressive strength of calcined discs at 13008C are summarized in Table 2. The disc of stabilized MCMBFCB exhibited the high compressive strength of 320 MPa, while the excessive oxidation at 3308C for 1 h decreased slightly its value to 300 MPa. Strikingly high compressive strengths of 420 and 400 MPa were obtained from the discs prepared from stabilized MCMB-BP-3 and MCMBKB-3, respectively, indicating the favorable effects of carbon black on the compressive strength of the disc. An excessive oxidation of MCMB-KB-3 at 3008C for 1 h markedly decreased its compressive strength to the lowest value of 250 MPa. The values of interlayer spacing, d 002 , and crystallite height, Lc, of as-prepared and calcined discs are summarized in Table 3. The Lc value of MCMB-FCB increased gradually to 1.8 from 1.6 nm by 10008C, and then sharply to 5.6 nm at 13008C. The MCMB-BP-3 and MCMB-KB-3 showed the slightly low Lc of 1.4 nm which increased to 4.7 and 4.5 nm at 13008C, respectively. The d 002 value of MCMB-FCB decreased from 0.3541 at 10008C to 0.3456 at 13008C. The calcined discs of MCMB-BP-3 and MCMB-KB-3 at 13008C gave slightly larger d 002 values of 0.3464 and 0.3475 nm, respectively.
Fig. 6. Change in voluminal shrinkage of the discs as a function of carbonization temperature.
9.0% loss at 700, 1000 and 13008C, respectively. Carbon black reduced slightly the weight loss of the disc, as indicated by 9.2% of the stabilized MCMB-KB-3 and 12% of the stabilized MCMB-FCB at 13008C. The volumetric shrinkage of the discs prepared from as-prepared and stabilized MCMB are shown in Fig. 6.
Table 2 Compressive strengths of carbonized discs at 13008C for 1 h Starting MCMB
Stabilization conditions Temp. (8C)
Time (h)
Heating rate (8C / min)
MCMB-FCB
300 330 270 270 300
1 1 1 1 1
1 1 5 5 5
MCMB-BP-3 MCMB-KB-3
Bulk density (g / cm 3 )
Compressive strength (MPa)
1.66 1.64 1.71 1.70 1.69
320 300 420 400 250
Table 3 Crystallographic properties of as-prepared and carbonized discs Starting MCMB
d 002 (nm)
Lc (nm)
As-prepared
10008C
13008C
As-prepared
10008C
13008C
MCMB-FCB MCMB-BP-3 MCMB-KB-3
0.3589 0.3577 0.3601
0.3541 0.3547 0.3542
0.3456 0.3464 0.3475
1.6 1.4 1.4
1.8 1.6 1.5
5.6 4.7 4.5
Stabilization conditions: MCMB-FCB: 3008C, 1 h, 18C / min, MCMB-BP-3 and MCMB-KB-3: 2708C, 1 h, 58C / min.
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3.4. Textures of carbon discs The polarized optical microphotographs of carbonized discs at 13008C, prepared from as-prepared and stabilized MCMB, are shown in Fig. 7. The disc of as-prepared MCMB-FCB exhibited flow texture with large voids and lost the shape of MCMB (Fig. 7(a)), indicating excess fusibility of MCMB. Oxidative stabilization at 3008C for 1 h removed pore and cracks in the carbonized disc, providing the fine mosaic texture of MCMB with the excellent shape and homogeneous shrinkage (Fig. 7(b)). Oxidative stabilization at a higher temperature of 3308C provided also a fine mosaic texture of MCMB for maintaining its shape, then a lot of fine voids can be induced in the carbonized disc (Fig. 7(c)), suggesting the insufficient adhesion among the grains. MCMB-BP-3 stabilized at 2708C for 1 h provided a
homogeneous distribution of MCMB in the carbonized disc (Fig. 7(d)). No pores and cracks were observable, showing the uniform shrinkage during the carbonization. MCMB-KB-3 stabilized at 2708C for 1 h gave a similar texture with MCMB-BP-3, then very few fine crevices can be found in the carbonized disc (Fig. 7(e)). Oxidative stabilization of MCMB-KB-3 stabilized at 3008C for 1 h increased the crevices of the carbonized disc (Fig. 7(f)), reflecting the lower density and strength after the carbonization. SEM photographs of the fractured surfaces of the carbonized discs at 13008C after measurements of compressive strength, prepared without and with carbon black, are shown in Figs. 8 and 9, respectively. The carbonized disc from MCMB-FCB stabilized at 3008C for 1 h showed an adhesion among MCMB with homogeneous and densified appearance as observed in Fig. 8(a). In Fig. 8(b) it
Fig. 7. Optical textures of the carbonized discs at 13008C for 1 h. Prepared from (a) as-prepared MCMB-FCB, (b) MCMB-FCB stabilized at 3008C for 1 h, (c) MCMB-FCB stabilized at 3308C for 1 h, (d) MCMB-BP-3 stabilized at 2708C for 1 h, (e) MCMB-KB-3 stabilized at 2708C for 1 h, (f) MCMB-KB-3 stabilized at 3008C for 1 h.
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Fig. 8. SEM photographs of the fractured surfaces of the discs prepared from MCMB-FCB. Carbonization conditions; 13008C for 1h, stabilization conditions; (a) and (b) 3008C for 1 h, (c) and (d) 3308C for 1 h.
is exhibited that the fissure was produced in the boundaries among MCMB, indicating that the binding part among MCMB easily caused fragmentation of the discs. Excessive oxidation at 3308C for 1 h provided the clear boundaries of MCMB with a coarse appearance as shown in Fig. 8(c), a lot of large fissures and voids in the carbonized disc being observable. In Fig. 8(d) an individual MCMB without any adhesion is shown, and the fissure around the sphere was observed, corresponding to insufficient fusion among the grains. The disc prepared from MCMB-BP-3 after stabilization showed the adhesion among MCMB particles with homogeneous texture as shown in Fig. 9(a). No larger fissure can be found after the measurement of the compressive strength although very fine fissures and few voids
were observed (Fig. 9(b)), exhibiting the homogeneous fragmentation of the disc. The carbonized disc from MCMB-KB-3 after stabilization showed the homogeneous appearance (Fig. 9(c)), and again no bigger voids, but more numbers of fine fissures and voids (Fig. 9(d)) than that from MCMB-BP-3. Increased fine fissures and voids of MCMB-KB-3 provided a slightly low compressive strength of 400 MPa than that of MCMB-BP-3 (420 MPa). Thus, the disc prepared from MCMB with carbon black after proper stabilization produced the homogeneous fragmentation of the disc after the measurement of the compressive strength, corresponding to higher mechanical strength, while the disc prepared from MCMB without carbon black induced the larger fissure during carbonization, leading to lower compressive strength.
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Fig. 9. SEM photographs of the fractured surfaces of the discs prepared from MCMB-BP-3 ((a) and (b)) and MCMB-KB-3 ((c) and (d)). Carbonization conditions; 13008C for 1 h, stabilization conditions; 2708C for 1 h.
4. Discussion The present study reports that a carbonized artifact of excellent properties with strikingly high compressive strength of 420 MPa, bulk density of 1.71 g / cm 3 , and large volumetric shrinkage of 33% was successfully prepared from the stabilized MCMB with 3% BP2000 by the calcination at 13008C. Such high mechanical performance is expected to be applied widely for the commercial areas of advanced technologies. MCMB prepared from the synthetic pitch are still fusible although carbon black significantly reduced the fusibility even if they are isolated by extraction. The mesogen molecules of the sphere from the synthetic pitch are considered to be a series of naphthalene-oligomers. The
lighter components are more or less included in the heavier components in the sphere. Oxidative stabilization in air effectively reduces their lighter components or naphthenic structure through the condensation and dehydrogenation, leaving the adequate adhesion among oxidized MCMB. Thus, adequate oxidation can remove the excessive fusion, maintaining their self-adhesive ability. Excessive oxidation at a high temperature or a longer period removed too much the adhesive ability of MCMB, leaving and inducing a lot of fine crevice or voids among the MCMB to lower the mechanical strength of the artifact after the carbonization. Insufficient oxidation of MCMB leaves too much fusibility and volatile matters, leading to expansion because of the evolution of volatile matters and fusible deformation of MCMB during carbonization, and too many voids are
Y. Wang et al. / Carbon 37 (1999) 1049 – 1057
induced in the carbonized disc. Thus, it is important to achieve the balance of their thermosetting properties to maintain the shape stability and sufficient adhesive ability. It is worthwhile to discuss the roles of the carbon black in the stabilization and carbonization of the discs. Carbon black, which are mainly adsorbed on the periphery of MCMB as observed in a previous study [17], enhances the adhesion force among MCMB as filler to control the carbonization of a lighter fraction at their surface and effectively inhibits the crack propagation at the interface during carbonization of the discs. Carbon black among the MCMB provides the gas evolution pathways, controlling the expansion of the artifacts at less stabilization which allows dense packing and uniform contraction of MCMB in the discs. Thus, carbon black improves significantly the mechanical performance of the artifact. Uniform composition of MCMB inherited from the parent synthetic pitch and uniform distribution of their size are considered to favour the high performance of the carbon artifact through the controlled oxidation and closer packing in the disc. The strikingly high compressive strength of the carbon discs obtained by the present study, confirms the excellent properties of the present MCMB as the precursor of advanced carbon materials. Further control of the synthetic pitch increases its potential for a carbon source of high performances.
5. Conclusions MCMB prepared from the synthetic naphthalene pitch, exhibited the excellent performance as the precursor materials of high density and high strength artifacts. Particularly, MCMB prepared in the presence of carbon black enhanced their mechanical performance because carbon black effectively inhibits the crack formation and propagation at the interface during carbonization of the discs. The compressive strengths of 420 and 400 MPa were successfully obtained by the carbonized discs of MCMB prepared with 3 wt. % BP2000 and KB, respectively. These compressive strength values are certainly higher than those found in the literature, which are derived from the artifacts of coal tar pitch based MCMB, such as ca. 200
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MPa by Honda [3], ca. 300 MPa by Yamada [4], and 380 MPa by Nakagawa [10].
Acknowledgements The authors are very grateful to Mr. R. Fujiura from Mitsubishi Gas Chemical Company Inc. for the measurement of the compressive strength.
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