Physical properties of a magnetically-oriented carbonaceous mesophase

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PHYSICAL PROPERTIES OF A MAGNETICALLY-ORIENTED CARBONACEOUS MESOPHASE P. DELHAES,J. C. ROUILLON and G. FUG Centre de RecherchePaul Pascal,DomaineUniversitaire,Universityof BordeauxI, 33405Talence,France

L. S. SINGER Union Carbide Corporation,

Carbon Products Division, Parma Technical Center, Parma, OH 44130, U.S.A (Received 7 June 1979)

Abstract-Diamagnetic susceptibility, electron spin resonance, and specific heat measurements have been made on a magnetically-oriented mesophase pitch. The diamagnetic anisotropy (xl - ~11)was found to be -0.69 x lo-” emu CGS g-’ and the average susceptibility i = (2x11t x1)/3 = -0.73 x lo-” emu CGSg-l. ESR measurements of the g-factor and linewidth confirmed the anistropic behavior and indicated free radical concentrations of - 1.2 x 10’9g-‘.The low-temperaturespecific heat results showed evidence of an anomalous linear term. A consideration

of the magnitude of the susceptibility and atomic C/H ratio suggests that the molecules consist of relatively small

aromaticregionsconnectedby aliphaticand aryl bridges.

1. INTRODUCTION During the pyrolysis of certain tars, pitches, and pure organic compounds, a high-melting, optically-anisotropic liquid phase appears[l]. This liquid phase, called the carbonaceous mesophase, is generally assumed to be a nematic liquid crystal composed of polynuclear aromatic molecules [ I, 21. Since the diamagnetic susceptibility of such molecules is much larger perpendicular to the aromatic layer planes than in the planes[3], the optic axes (also perpendicular to the layer planes) orient themselves perpendicular to a magnetic field. These molecular axes can thus be oriented approximately parallel to each other by rotating the liquid sample about an axis perpendicular to the magnetic tieldI41. The purpose of these studies was to investigate the thermal and magnetic properties of an oriented mesophase pitch, in particular, a mesophase pitch which is essentially a single anisotropic phase[5]. It was hoped that information concerning the molecular structure of these large complex molecules could be inferred from the results. The paper includes a description of sample preparation, microscopy, and X-ray results, diamagnetic susceptibility measurements, both anisotropy and mean-value determinations, electron spin resonance, and low-temperature specific heat experiments. Two different types of molecular structures are proposed, only one of which seems to be consistent with both chemical constitution and magnetic properties.

2. SAMPLE PREPARATION AND CHARACTERIZATION The mesophase pitch used for these magnetic orientation studies was prepared by heat treating a petroleum pitch. The mesophase pitch contained 95.O%C, 4.3%H (atomic C/H ratio 1.85), contained 50% pyridine inCAR Vol

17. No 64

435

solubles (P.I.), had a density of 1.33g/em-3, and was essentially 100% anisotropic [5]. The oriented samples were prepared in a manner similar to that described previously[4]. A few tenths of a gram of mesophase pitch were placed in standard 5 mm O.D. Pyrex NMR tubes which were evacuated, filled with approx. 1 atm of argon, and sealed off. The heat-treatment furnace was the previously described high-temperature ESR cavity[6] which had been slightly modified. A simple rotation scheme was used to rotate the sample about its axis at 1Orpm in a static magnetic field of -1OKGauss. After -3Omin of rotation at 400°C. the samples were cooled in the field and then cut into transverse slices or wafers 2 mm thick. Figure I shows polarized light photomicrographs of polished sections of the mesophase pitch before and after magnetic orienting. The photomicrographs were obtained by reflection utilizing crossed polars. Figure I(a) shows the mesophase pitch cooled in the absence of a magnetic field while (b) and (c) are edge views of a wafer sliced from a boule rotated and cooled in a magnetic field of IOKGauss. It can be seen that the entire wafer sample, except for the interesting small disinclination loop, is essentially one large oriented mesophase domain. When the wafer was viewed along the sample rotation axis, the sample appeared completely black and independent of stage rotation, even with four times the exposure time used in the photograph in Fig. l(b). The degree of preferred orientation of the wafers was measured by Dr. S. L. Strong employing X-ray diffraction. The incident and detected beams were set for the (002) reflection and the sample rocked about a direction perpendicular to the wafer axis. The orientation distribution of molecular layer normals with respect to the normal to the flat wafer surface (i.e. to the axis of sample

Fig. 1. Polarized light photomicrographs of unoriented and oriented samples of mesophase pitch under crossed polars. The crossed arrows indicate the polarizationdirections;the approximatedirectionsof the edge planes with respect to the magnetic field and the sample rotation axis are also shown. The two views in (b) were taken with identical exposure times.

x1 - ~11and the ,Q value respectiveiy[7]. The quantities xI and ,Q are the diamagnetic components measured with the magnetic field perpendicular and parallel, respectively, to the flat wafer surface, i.e. to the aromatic layer planes. The mean diamagnetic susceptibility value j is calculated from the following relationship: 1

x’= $(X’ -Ml) + 3Xlll. * -42-36-30-24A6-12 -6 0 6 ~~~rn~(~e~~

12 I6 24 30-3642

I

Fig. 2. Preferred orientation of magnetically-oriented mesophase wafer as determined by X-ray diff~etion. The circles were calculated for a Gaussian curve of the same height and width.

rotation) was found to be approximately Gaussian with a full width at half-maximum (FWHM) of 22”. The X-ray o~entation diagram is shown in Fig. 2. 3. RMJLTS OF MAGh’El’K AND THERMAL ~~~~

3.1 ~a~ugne~ic su~cep~jb~i~y We used Krishnan’s torque method and Faraday’s method in order to obtain the diamagnetic anistropy

The diamagnetism was found to be temperature independent between 295 and 78°K. These results are given in Table 1. The results in Table 1 constitute the first determination of the diamagnetic anisotropy of an oriented mesophase. 3.2 electron spin reso~u~ce (ESR) The ESR was measured at room temperature on wafers mounted on edge so that the wafer axis direction could be continuously varied with respect to the magnetic field direction. The measurements were made at -9.5 GHz employing a microwave power level less than - 10 pw to prevent saturation effects. The paramagnetic susceptibility (and derived spin concentrations) were

437

Physical properties of a carbonaceous mesophase Table 1. Magnetic and ESR properties of oriented-mesophase pitch waferst Diamagnetic susceptibility XL-XII XL-XII X Xl XB

(29S”K) (78°K) (295°K) (295°K) (295°K)

ESR (295°K)

-0.69 x 10e6emu/g -0.70 X W emu/g -0.73 X 10e6emu/g -0.50 X 10m6 emu/g -1.19X 10-6emu/g

% gll g, Ag = (g)~ - g,) SF Sk

2.5 x lo-* emu/g 1.2x 10’9g 2.002792 00002 2.00231200002 4.8 x 10m4 5.7 Gauss 6.3 Gauss

tThe designations (1and i denote the magnetic field parallel and perpendicular to the flat wafer surface, respectively. fThe spin or free radical concentration was calculated from Xp, the paramagnetic susceptibility, using the formula N = 3&TX/g’p*S(St I) assuming spins of l/2 and g = 2.0023.

obtained by digital integration techniques. The lineshapes

were intermediate between Gaussian and Lorentzian. The g-values, linewidths, paramagnetic susceptibility and free radical concentration are given in Table I. In order to check the nature of the electron localized centers, we have determined the thermal variation of the paramagnetism down to 4.3”K with a Faraday balance. A classical Curie’s law has been found; the magnitude of the paramagnetism exceeded the observed diamagnetism below liquid nitrogen temperature. Both the nature and the concentration of free radicals observed by ESR have, therefore. been confirmed.

3.3 Specific heat The specific heat (C,) has been measured in the liquid helium temperature range by using a standard adiabatic method[81. The sample, a pressed pellet with a weight -0.4g, possessed a poor thermal conductivity. It was impossible, therefore, to cool the compound below 2°K. The thermal variation observed between 2 and 10°K is presented in Fig. 3. We note that the following relationship is obeyed in the lowest temperature range: C, = aT t bT3.

The cubic temperature-dependent term allows one to calculate the Debye temperature (6, = lSO”K), but a

small linear term is also present. A linear term of similar magnitude has already been observed in anthracene chars [9]. 4. Dl!JCUSSIONAND COMMENTS

4.1 Electron spin resonance (ESR) The ESR results are similar to those obtained previously for a magnetically-oriented acenaphthylene mesophase pitch containing mesophase spkeres[4]. The g-anisotropy for the spheres was slightly less (3.2 x 10e4) than that observed for the single-phase pitch in this study, since presumably, the degree of preferred orientation of molecules in the spheres is somewhat poorer due to the “layer curvature” in the “Brooks and Taylor” sphere structure. The g-values and anisotropy are again relatively close to those predicted by Stone for oddalternate neutral r-radicals [ IO]. Note that the sign of the anisotropy is opposite to that observed for high-temperature carbons and graphite but is the same as that observed for anthracene chars [ 1I] and acenaphthylene coke[l2]. The spin concentration of 1.2x 10’9gm’ is similar to that observed in other mesophase pitches[l3]. If the average molecular weight of this single-phase pitch is assumed to be -1000, the spin concentration indicates that approx. 2% of the molecules are free radicals. The linewidth and lineshape of the ESR together with the small anisotropy and spin concentration, strongly suggest that the primary line broadening mechanism is proton hyperfine interaction.

CI T (mJ/g’K’) 1 15

4.2 Diamagnetic susceptibility It is possible to calculate the true molecular diamagnetic anisotropy value x3-x1 of this oriented mesophase pitch. We have the following relationships [ 141: x~=xI+(x~-xI)cos2 at T = constant

B

XII= XI + $0 - XI) -1 sin* 0

05 t

006

YY 0

IO

I 20

1 30

I 40

1 T21’K2)

Fig. 3. Temperature dependence of the specific heat C, plotted as CJT vs T2, in order to make evident the linear and cubic specific heat terms.

from which one obtains:

(a-XI)T =

+4w

Xllh

P.

438

DELHAESet al.

A structural and constitutional analysis of molar volume of the type employed by Van Krevelen[l6] for coals was not pa~icularly conclusive. For exampfe, using the measured room-temperature density of 1.33g/cm3, the Van Krevelen empirical relationship can be evaluated.

with

Using the X-ray result that the orientation half-width at half-height is 6 = 1I”, a (x3-x,) value of -0,80 10e6is calculated. This can be compared with the standard values given in the following table for small aromatic molecules.

R -= 0c

3.1(W)- (K/d)+ 9.1 - 3.q~~~

9.9

where (R/C) is the number of total rings, both aromatic

Table 2. Diamagnetismof pure aromaticcompoundst Number of fused rings, n Benzene Naphthalene Anthracene Pyrene Chrysene Anthanthrene Ovalene

: 3 4 ;t IO

Magneticsusceptibility Molecular weight, M 78 128 178 202 228 178 398

t

(emu CGS g-‘) x 106 AX=XL-XII

-0.71 -0.718 -0.727 -0.732 -0.731 -0.734 -0.88

-0.692 -1.00 -1.275 -1.10 -

tValues quoted from Ubbelhode and Lewis, “Graphite and its Crystal Compounds” (Clarendon Press 1960).

It is clear that the diamagnetic susceptibility and anisotropy are comparable to values found for relatively small aromatic molecules. A consideration of the molecular weight, constitution, and density of this mesophase pitch leads to a similar conclusion, viz. that the molecules in the mesophase pitch cannot be totally aromatic. For example, if we assume that the average molecular weight of the mesophase pitch in these studies is similar to that reported for other mesophase pitches[l, 151,viz. -1000, two possible structures might look like those shown in Fig. 4. The structure (A) is totally aromatic but has an atomic C/H ratio of 3.16 (compared to 1.85) and would be expected to have a large diamagnetism. On the other hand, the structure in Fig. 4(B) has a more comparable C/H ratio of 1.57, and because of the small size of the aromatic ring systems, would be expected to have a relatively small diamagnetism, close to that observed for the pitch in this study.

n

s

Fig. 4. Two hypothetical structures for the “average” molecules in mesophase pitch.

and naphthenic per carbon atom, i& is the moIe~ular weight/mole of carbon, d is the density and (HlC) is the atomic ratio of hydrogen and carbon. The calculation yields an (R/C) value of 0.27 for the experimental mesophase pitch which is intermediate between the actual values 0.35 and 0.20 for molecules (A) and (B), respective y. The structure (B), although perhaps not quanti~tively correct, does have approximately the correct C/H ratio and possesses the small aromatic regions which are consistent with the observed magnetic susceptibility and ansiotropy. Similar structures have also been proposed by Mochida et af.[17]. 4.3 Specific heat From the specific heat measurements shown in Fig. 3, we are able to present further analysis of each component and the departure from the standard T3 dependence. A detailed discussion of these points follows: 4.3.1 The linear co~p~~e~f. As proved for anthracene chars, the linear term is not an electronic term but is due to density and composition fluctuations [ 181. 4.3.2 Anisotropic lattice vibrations. The departure from the standard Debye law at very low temperatures is evidence for an ansiotropic vibrational system with low frequency phonons. Such behavior is detected for this mesophase. Therefore, to compare with graphite, we have plotted in Fig. 5, in reduced coordinates, the intrinsic lattice component after subtracting the linear term. For graphite, we have used the experimental results obtained by Keesom and Pearlman[l9] on a ~ly~rystalline sample. Here the same kind of behavior is observed, allowing us to use a Debye-Tarasov model as for polymers[8]. In this case, we assume that the

439

Physical properties of a carbonaceous mesophase

of this noncrystalline molecular solid is weaker. To achieve a somewhat deeper insight into these molecular interactions, we consider the relation between the Debye temperature and the elastic constants. By assuming axial symmetry for the mesophase, as for a single crystal of graphite, we can write from the elastic model proposed by Bowman and Krumhansl[20] and by Komatsu[21], _I

x* i

9

‘0

$

ed =4M

\

\

0 MESOPHASE 13~ = 150°K

GRAPHITE

f5, = 340-K

5C’

t

N uiqjj

c44d/(c331

0

!

‘(KEES~M e.

h3 0k

F PEARLMAN) = 378 OK

83 = 29 2°K e3/e2=o.oE%

C12= l80C”K e3 : I6 7 ‘K e3 /02

0 01

i 0.093

0.05

T/8,

Fig. 5. Temperature dependence of the intrinsic lattice specific heat CC;,,, =~ Cexp - UT) multiplied by (fJJTQ, vs the reduced coordinate (fl&). The dotted line shows the standard 3DDeybe model, whereas the full lines and the best fits obtained with the characteristic vibrational temperatures are given in the figure.

acoustic

phonons are tridimensional in the lowest temperature range and two-dimensional in a higher range when their energy is about of the same order of magnitude as the energy of interaction between graphitic planes. To fit the experimental results, we can use, therefore, the following relation:

with

where R is the perfect gas constant. D3(0/ T) and 4(0/T) are the standard Debye functions for 30 and 20 phonons characterized, respectively, by the temperatures e3 and f12.The ratio (&/19,)is related to the anisotropy of the lattice vibrations. The best fits are given in Fig. 5. In spite of a large difference in the bulk Debye temperature 8, discussed below, almost the same ratio (0,/f&) is found for the two materials. The physical meaning that can be deduced is the following. Only the long-wavelength acoustic phonons are excited at very low temperatures. They are, therefore, sensitive only to the long-range order and not to the details of the atomic structure. The similar vibrational anisotropy indicates that in this mesophase, a lamellar structure, in some ways similar to that in graphite, has already developed. 4.3.3 Debye temperatures and cohesive energy. The observed Debye temperature for the mesophase is smaller than that in graphite because the cohesive energy

where h and k are the Planck and Boltzmann constants, N is the Avogadro number, and M is the atomic mass. The ratio (6, g/& meso.) = 12 is thus determined by the changes in the two elastic constants C3z and CM. C33, the compressibility coefficient perpendicular to the aromatic plane must be larger in a molecular solid such as the mesophase than in graphite in which Van der Waals forces are important. However, no fundamental differences between these two materials are expected. CM, the shear coefficient is very sensitive to defects and dislocations and must decrease very sharply when the atomic order is essentially absent as in the mesophase. Therefore, the observed decrease of Debye temperature must be associated with CU. The mesophase is probably a viscoelastic medium, the hydrodynamic characteristics of which are related to this shear elastic constant. Further studies of these characteristics will be necessary to prove this assumption.

5. CONCLUSIONS

This study has shown that it is possible to obtain a highly-oriented -100% mesophase pitch by rotating the sample in its molten state in a magnetic field of -10 KG. The diamagnetic susceptibility and ansiotropy results suggest the presence of either small aromatic molecules, or larger molecules with small aromatic regions, presumably connected by aliphatic and aryl bridges. The ESR results confirm the anisotropic behavior and indicate the presence of oriented r-radicals which are presumably of the neutral odd-alternate type. Concerning the specific heat measurements, it is confirmed that although short-range order is not well established, some long-range lamellar organization is already present. Because of this fundamental characteristic, this particular mesophase can be magnetically oriented and presumably fully graphitized. In conclusion, this study suggests that the ultimate characteristics of graphite, viz. its electronic, thermal, and elastic behavior, are already determined at the mesophase stage of carbonization. Acknowledgements-Theauthors acknowledgethe assistanceof D. T. Orient for ESR measurementsand Dr. S. L. Strong for X-ray measurements. REFERENCES

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