2174 Letters to the Editor / Carbon 41 (2003) 2159–2179

G raphitic open-celled carbon foams: processing andcharacterization

a , b c*Rajeev Mehta , David P. Anderson , Joseph W. HageraChemical Engineering Department, Thapar Institute of Engineering and Technology, Patiala, Punjab 147004,India

bUniversity of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0168,USAcWright Research and Development Center, Wright Patterson Air Force Base, Dayton, OH, USA

Received 27 November 2002; accepted 22 May 2003

Keywords: A. Mesophase pitch, Carbon composites; B. Carbonization; C. Optical microscopy, Scanning electron microscopy

Carbon fiber-reinforced composites [1,2] have existed be achieved through high temperature treatment of thefor many years. They represent an attempt to incorporate pitch foams.the exceptional mechanical properties of fiber into a The pitch used was AR Mesophase pitch [3,4] (Mit-polymer, metal, ceramic, carbon char or pitch matrix. The subishi Corp; naphthalene precursor). The chemical blow-fibers act as the major load-bearing elements and the ing agent used was a barium salt of 5-phenyltetrazole [5]matrix serves to transfer load to the disconnected fiber which has a decomposition temperature of 3758C. Thenetwork. However, there are two important drawbacks in first foaming technique examined was the ‘micro-cellularcarbon fiber-reinforced composite technology, namely, the foaming technique’ [6–9] in which a ‘micro-cellular foam’complexity of the fabrication process and the limiting is produced by first saturating a polymer with a gas underproperties of the matrix elements. The main hypothesis of high pressure and then utilizing the thermodynamic in-this research program is that a new generation of compo- stabilities that result when the temperature is raised and thesite materials, with interconnected graphitic reinforcing pressure is reduced to produce the bubbles. The suddenligaments, offers a simplified processing alternative to the instability instantaneously nucleates a myriad of bubblesconventional carbon-fiber reinforced composite technolo- with a uniform distribution. The bubbles are then allowedgy. to grow to the desired size, at which time the process is

Reticulated foams, which display such an interconnected halted.ligament network, are readily synthesized from polymers, Solid mesophase pitch samples were obtained by com-and represent the reinforcing architecture we seek. Due to pression molding the mesophase pitch powder at antheir amorphous carbon morphology, however, these foams elevated temperature in a stainless steel mold (40310310)have relatively low moduli, much lower than we would under pressure (hydraulic press) in vacuum. The followingexpect in an aligned graphitic morphology. If the cellular conditions were used to obtain a non-porous sample:graphitic ligament microstructure is created from a ther- temperature52508C, thickness51/80 or 1/160, pressuremotropic mesophase pitch, the ligaments of these reticu- (initial)5500 p.s.i., pressure (final)520 p.s.i. and nolated foams, rather than being amorphous, should possess a applied pressure while cooling. The solid pitch samplesmorphology similar to that in advanced carbon fibers. were saturated at 1400 p.s.i. at room temperature for 2

We envision that the foaming process would impart weeks and at 1008C for 3 days. These samples werebiaxial elongation stress components to the two-dimension- foamed at 195, 200, 210, and 2208C for different lengthsal nematic planes and would align the mesogenic mole- of time. All the high pressure experiments were done in acules with the ligament axis in a manner similar to their Parr autoclave reactor. The high temperature processingalignment parallel to the fiber axis in the spinning process. was done as follows: oxidation stabilization in a FisherThe nascent mesophase pitch foam can then be oxygen Isotemp programmable furnace, carbonization in a Metlerstabilized, carbonized and graphitized, which is analogous furnace and graphitization in a hot press.to post fiber-spinning treatment for carbon fibers. Light microscopy was done on samples made by

The objectives of this paper are: (1) to describe two vacuum impregnating foams using a fluorescent dye taggeddifferent ways of foaming mesophase pitch, and (2) to epoxy resin and the foam structure was also observed in ademonstrate that a very high degree of graphitization can scanning electron microscope. X-ray diffraction measure-

ments were made after grinding the foams to a fine powderand mixing with silicon powder as an internal standard.The powder was placed in a symmetrical transmission*Corresponding author. Fax:191-175-236-4498.

E-mail address: rajeevmehta33@yahoo.com (R. Mehta). holder of a Huber four-circle diffractometer using Cu Ka

0008-6223/03/$ – see front matter 2003 Published by Elsevier Ltd.doi:10.1016/S0008-6223(03)00243-4

Letters to the Editor / Carbon 41 (2003) 2159–2179 2175

radiation (incident beam crystal monochromator). The was exposed to air during oxygen stabilization werescans were corrected for polarization, Lorentz structure carbonized without any loss in shape. On the other hand,factor, Compton scattering, and absorption before plotting the samples which were partially covered by the moldand curve fitting. (Pyrex test tubes) lost their shape during carbonization.

The second foaming technique used was to employ a The carbonized samples were then heat treated to aboutchemical blowing agent, barium salt of 5-phenyltetrazole, 24008C in a hot press.as the source of gas. The foam processing steps are The first foaming technique, the so-called ‘micro-cel-illustrated in Fig. 1. Briefly, mesophase pitch was ground lular foam’ technique, was used in an attempt to obtaininto a powder and dried to drive off moisture (about 10% micro-cellular mesophase foams, i.e. with cell-sizes lessby weight). The powdered pitch was mixed with the than 10mm. Even though there was some difference withchemical blowing agent in a Pyrex test-tube and melted in respect to the cell-sizes in the foams with different time–a nitrogen environment above the decomposition tempera- temperature foaming histories, none of the foams wereture of the blowing agent (about 3758C). The foaming micro-cellular (i.e. cell sizes were much larger than 10temperature was kept below 4008C at which temperature mm). Also, unlike a typical micro-cellular foam, the cell-volatiles are observed to come off. The mold was size distribution was extremely broad. It is suspected thatquenched with water to stabilize the nascent foam. Before failure to obtain micro-cellular foams might be due to thethe nascent mesophase foam was subjected to high tem- low solubility of nitrogen in the crystalline morphology ofperature it was oxygen stabilized. The stabilization process the mesophase pitch. A pitch sample which had not been[10,11] occasionally called the infusibilization or ther- supersaturated (under pressure) was foamed at 2108C andmosetting step, is essential to ensure that the carbon very similar foam was obtained. This suggested that all thestructure does not change in shape or lose layer alignment samples were foaming simply because of homogeneousimposed during bubble growth. The foam was stabilized in nucleation from nitrogen or water vapor in equilibriumair at about 2508C (heating from room temperature at with the samples at room temperature or by heterogeneous

211 8C min ). The stabilized foam was carbonized using the nucleation from the impurities in the mesophase pitch. Afollowing thermal profile: room temperature to 3758C at further solubility study confirmed that the solubility of

21 212 8C min , 375 to 4508C at 0.58C min and from 450 to nitrogen in pitch at room temperature is very low (less21 21 218508C at 58C min . The samples whose entire surface than 0.0001 cc (atm) (g) . This is consistent with the

reports that the solubility of non-polar gases is negligiblein the crystalline regions of semi-crystalline polymers like

polyethylene. It can be concluded that the ‘micro-cellular’approach as discussed above cannot be used to obtainmicro-cellular mesophase pitch foams.

The foams obtained by the second foaming techniquewere open-celled foams and the cell-sizes were fairlyuniform (Fig. 2). X-ray diffraction (Fig. 3) analysis of theheat treated foams indicated a very high degree of graphiti-

˚zation with interlayer spacing53.361 nm andL 5200 A.a

Fig. 2. SEM of foam produced from the mesophase pitch usingFig. 1. Schematic of the foam processing of AR Mesophase pitch. the second technique.

2176 Letters to the Editor / Carbon 41 (2003) 2159–2179

Fig. 3. X-ray diffraction patterns of graphitized and carbonized foams with Si internal standard.

In comparison, the corresponding values for fibers spun R eferencesfrom AR mesophase pitch and heat treated at 1500, 2000and 25008C are 0.3457, 0.3428 and 0.3389 nm, and 44, [1] F itzer E. Carbon fibers and their composites. Berlin: Spring-

˚ er; 1983.140, and 210 A, respectively [12]. It is believed that these[2] W hite JL, Sheaffer PM. Carbon 1989;27(5):697–707.foams are not produced as a result of porosity formation[3] M ochida I, Shimizu K, Koral Y, Otsuka H, Sakai Y,during heat treatment of the carbonaceous material [13].

Fujiyama S. Carbon 1990;28(2/3):311–7.The objective of processing graphitized foam from the[4] M ochida I, Yooh SH, Takano N, Ferlin F, Korai Y,mesophase pitch used as a precursor for carbon fibers has

Yokogawa K. Carbon 1996;34(8):941–56.been achieved. However, the graphitized cellular structure[5] E xpandex 5—PT high temperature blowing agent, Bulletinappeared to be quite fragile. It is postulated that this could

00R-03-0675, Stephen Chemical Company, Polychem De-be due to two reasons: (1) the graphitic planes in the partment (now part of Olin Corporation), Wilmington, MA.ligaments are not aligned along the length of the ligaments, [6] R amesh NS, Rasmussen DH, Campbell GA. Polym Eng Sciand (2) the presence of defects (either at the point of 1991;31(23):1657–65.contact of individual ligaments or within the individual [7] Y oung JR, Suh NM. Polym Composites 1985;6(3):175–80.ligaments). The first hypothesis will be tested by examin- [8] C olton JS, Suh NP. Polym Eng Sci 1987;27(7):493–8.ing the ligament structure in transmission electron micro- [9] M artini J, Waldman F, Suh NM. US patent 4,473,665, 1984.

[10] L in SS. SAMPE J 1991;27:1.scopy. The second hypothesis, namely, the presence of[11] S inger LS. US patent 4,605,183.defects, is confirmed from the SEM micrographs of the[12] S inger LS. Carbon 1978;16:409–18.foams. It is worth noting that there is no significant[13] E hrburger P, Sansisgne E, Tahon B. Carbon 1996;34:1493–difference between the crack morphology of the nascent

9.foam and graphitized foam. This would imply that thedefects are introduced in the process of foaming ratherthan in the subsequent heat treatment.