Laser Raman spectroscopy for determination of the C-C bonding length in carbon

594

Letters to the Editor

Laser

Raman

spectroscopy

for determination carbon

of the C-C bonding

length

in

(Received 6 October 1987; accepted in revised form 8 March 1988) Key Words - Raman spectra, G-line, D-line, blue shift

The first order Raman spectra of the two polymorphic modifications of solid carbon are characterized by the socalled G line with wavenumber 1575 cm-1 for graphite and the D line with wavenumber 1332 cm-t for diamond [l]. In the case of structurally disordered carbon materials both of these lines are measurable, but the D line wavenumber is always increased (blue shifted) relative to that of diamond. Both red and blue shifts have been observed for the Cl line [2-51. A decreased wavenumber (red shift) of the G line, indicating increased bonding length, was reported in the case of vapor grown carbon fibers measured under tensile stress [6]. The fiber elongation under stress was found to correspond with the fiber modulus. A mathematical treatment of this finding is given elsewhere [7]. Conversely, the normally measured blue shift of the G line relative to the ideal graphite position would indicate internal compressive stresses and decreased basal C-C distance. The existence of carbon atom bond lengths smaller than 1.415 A was theoretically predicted some 30 years ago by Coulson [8] and is explained by the approach of two carbon atoms in the edee of a laver ‘2 while-still remaining part of the layer structure (sp electron orbital configuration). Carbon atoms in residual “tails“ extending from the layer edge can be arranged according to the sp3 configuration and are registered by the D line in Raman spectroscopy as randomly placed abnormal boundary oscillating groups. The bonding length of such sp3 bonded edge atoms should be smaller than in diamond. This explains why the D line in disorded carbons is always blue shifted as compared with the wavenumber of pure sp3 bonds in diamond. Restriction of sp3 hybridization to edge atoms excludes explanation of the D line appearance in-disordered carbon in terms of a partial diamond structure. This finding promises to offer a new method to characterize quantitatively the atomic arrangements in carbon materials and opens a way to bridge the gap between amorphous carbon and disordered graphite.

As an example, the Raman spectra of various types of carbon fibers are presented in Fig 1. T 300, AS4, Celion G30-500 and G40-700 are PAN-based HT type fibers with final heat treatment temperatures (I-ITT) around 1300-1400°C and average c/2 distances of 3.5 A. M40 is a PAN-based HM fiber with final HIT around 27OO’C and c/2 of 3.45 A. T50 and T75 are rayonbased, stretchaphitized high modulus fibers with c/2 around 3.42 R P55 and PlOO are mesophase pitch based fibers with final HlT of 18OO’C and about 2500°C respectively and cl2 distances of 3.438 A for P55 and 3.392 A for PlOO. One can recognize a rough correlation between G and D line appearance and the interlayer distance c/2 for these fibers. T800 and TlOOO belong to the new generation of PAN-based intermediate modulus fibers and do not exactlv fit into this correlation. An additional influence of &ace treatment cannot be excluded in the case. Recently, laser Raman spectra have even been proposed for characterization of the carbon fiber surface [9]. Further data on various graphites, including HOPG and polygranular carbons, are discussed elsewhere [lo]. Institutfur Chemische Technik UniversitatKarlsruhe

E. FITZER

Kaiserstrab 12 D-7500 Karlsruhe 1, FGR Institute of Physics Nicholas Copernicus University Grudziadzka Street 517 P-87100 Torun, POLAND

F. ROZPLOCH

Letters to the Editor

595 REFERENCES

1.

T I 000

2.

ASI

3. 4.

GL01700

5.

6.

7.

T 800

8.

T 300

9.

T 75 10.

T 50

F. Tuistra and J.L. Koenig, Journal of Chem. Phys. 1126 (1970). C.H. Chang, R.A Beyerlein, and S.A. Chan, Carbon 22,393 (1984). R.P. Vidano, D.B. Fischbach, L.J. Willis, and T.M. Loehr, Solid Stare Corn, 39, 341 (1981). P. Lespade, R. Al-Jishi and M.S. Dresselhaus, Carbon ,20, 427 (1982). A. Richter, H.-J. Scheibe, W. Pompe, K.-W. Brzezinka and I. Muhling, .I. Non-Ctyst. Sol., 88, 131 (1986). H. Sakata, G. Dresselhaus, and M. Endo, in Proceedings of the 18th Carb. Conf. (Worcester Polytechnic Institute, Worcester, MA USA) (1987), p. 18. E. Fitzer and F. Rozploch, High TemperaturesHigh Pressures (to be published). C.A. Coulson, in Proceedings of the 4th Carbon Conf. Pergamon Press, New York (1960). p. 215. K. Morita, Y. Murata, A. Ishitani, K. Marayama and T. Ono, A. Nakajiama, Pure and Appl. Chem., 58, 455 (1986). E. Fitzer, E. Gantner, F. Rozploch and D. Steinert, High Temperature-High Pressures (to be published).

ML0

P 55 s P 55 PI00 1700

1610

1520

1430

1340

1250

Wavenumber (cm-l)

Fig. 1 Raman spectra of lateral measured carbon fibres

Preparation

of meso-carbon

microbeads

with a narrow

size distribution

(Received 28 January 1988; accepted in revised form 30 March 1988) Key Words - Meso-carbon

microbeads,

bulk mesphase pitch, particle size distribution

It is widely known that the formation of mesophase takes place in organic materials, such as coal-tar pitch, coalliquefaction products, petroleum heavy residual oil, etc. When these pitch materials are heat treated at a temperature between 350 and 450°C, the carbonaceous mesophase spherules formed in the pitches are generally called meso-carbon microbeads [ 1,2].

Although there have been many studies on the preparation of meso-carbon microbeads [3-61, mesocarbon microbeads with a narrow size distribution have not been obtained so far because of difficulties in controlling the reaction process and in separating the meso-carbon microbeads from the matrix. If mesocarbon microbeads with a narrow size distribution were