Internal Friction in Coal C

INTERNAL FRICTION IN COAL C. Wert, M. Weller

To cite this version:

C. Wert, M. Weller. INTERNAL FRICTION IN COAL. Journal de Physique Colloques, 1981, 42 (C5), pp.C5-581-C5-585. 10.1051/jphyscol:1981589. jpa-00221132

HAL Id: jpa-00221132 https://hal.archives-ouvertes.fr/jpa-00221132 Submitted on 1 Jan 1981

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE CoZZoque CS, suppZe'ment au nO1O, Tome 42, octobre 1981 page CS-581

INTERNAL FRICTION IN COAL

C.A. Wert and M. Weller

Mm-PZanck-Institut fiir MetaZZforschung, Institut fiir Werkstoffwissenschaften Stuttgart, F. R. G.

Abstract.- We have mured the internal friction of a rod of bituminous coal at a frequency near 1 Hz. Three dqing peaks are observed over the temperature range 20 to 400 K. They are sbilar to the dqing peaks observed in many polymers. One peak is thought to be caused by reorientation under stress of short s-ts of the -C-C-C- backbone. Another my be caused by reorientation under stress of appended radicals, perhaps @H. Another may be caused by confiqrational chanqes in the re~ionof the glass- rubber transition of the amorphous raterial.

1. Introduction.- Most scientists think of the carbonaceous constituent of coal as being akin to graphite. That picture may be approximately correct for the anthracites, but it is far from being true for bituminous coals, even less for the lignites (Braunkohle). That being true, we must picture what is the structure of the carbonaceous matter in coal and how we might study it. The composition of coal may be seen on a ternary phase diagram as depicted in Fig. 1. Since coal is derived from plant material, in its earliest stages of formation - the peats and lignites - it must have a chemical composition close to that of cellulose and lignin /l/. Chemi- cal changes occur during later stages of coalification, the hydrogen and oxygen concentrations are reduced, but the structure of the carbonaceous material is believed to retain much of the polymeric structure characteristic of the parent cellulose and lignin. Spectroscopic techniques and other chemical methods have been used in an attempt to characterize the presumed chain-like structure Of coal. In spite Of immense effort, all of these attempts have not been fully successful; however, several guesses have been made of the structure. They include the existence of -C-C-C-C- chains with hydrogen, oxygen, nitrogen and many trace elements in a great variety of radicals attached as side appendages. Perhaps benzene rings, fully or nartly saturated, also occur. C=C double bonds may also exist alonq the chain. These structural features being likely, we thought that internal friction peaks might be seen for coal similar to those seen for polymers.

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Further, we felt that such peaks, if found, might help elucidate some of the structural features of coal, as they have done for polymers. Thus we have made an internal friction study of a specimen of coal -- a coal somewhere around point C of Fig. l, in the bituminuous range. We cut a square "wire-like" specimen from a piece of cannel coal from eastern Kentucky in the USA. It was a rod about 1 mm on an edqe and about 5 cm long. We used cannel coal for this first attempt since it is less friable than several other bituminous coals from which we also attempted to make specimens.

2. Experiment.-The internal friction apparatus was an inverted torsio- nal pendulum previously developed and used in the MP1 at Stuttqart for the investigation of metals after low temperature irradiation /2/. Its features have been described previously /3/. The rigidity modulus of coal being about the same as for metals (perhaps half as large), the frequency ranye around 1 Hz is attainable with the same pendulum bobs as have been used previously for metals of the same dimensions. Starting at temperatures around 25 K, we first measured the spectrum to 380 K. That measurement is shown in Fig. 2. Three internal friction peaks are seen over this temperature range. The first occurs at about 130 K for a frequency of about 1.2 Hz; the second around 200 K, and a sharp peak around 350 K. The spectrum was rerun after it had been heated to about 380 K. That curve is shown in Fig. 3. The lowest-temperature peak is the same; the upper two are gone. Successive measurements after the specimen had been heated to about 550 K in steps of about 50 K showed no further change in the spectrum, except for a gradual lowering of the damping above 400 K.

3. Interpretation.-How can one interpret these results? To attempt this, we have used the analog of internal friction results for a typical hydro-carbon polymer, polyethylene, a typical spectrum of which is sketched in Fig. 4. This figure was taken from the paper of Kline, Sauer and Woodword /4/ as portrayed by McCrur.1 et a1 (p.367) /5/. Three peaks are found in roughly the same temperature region as for our coal Similar peaks are found for a great many other polymers, both for mechanical and dielectric loss /6-11/. Examination of the interpretation of such peaks shows enormous diversity of opinion in assignment of molecular models. However, following the views of McCrum et al, we will describe the =-peak as being caused in amorphous polymers by con- formational rearrangements on a large scale which accompany the glass- rubber transformation. The A-peak has often been ascribed to motion of side-groups. In cellophane, Bradley and Carr have attributed the 8-relaxation to a radical containing 0-H. The 8-peak is presumed to be caused by motion of short segments of the basic -C-C-C- chain, the exact geometry of which is not certain, and which may, in fact, differ from polymer to polymer. The disappearance of the K-peak and the /3-peak after high temperature annealing is consistent with observations made for other polymers. The CC -peak is notoriously unstable, apparently because of irreversible changes in gross character of the macro-molecular net- works occuring during annealing. The B-peak also can be made to appear and disappear in many polymers after heating. In certain polymers, it can be removed by drying in a dessicator and restored by storing the polymer in a humid atmosphere. The carbonaceous part of coal is largely an amorphous arrangement of semi-saturated carbon chains. With all the oxygen, nitrogen, sulfur, and metallic elements which are thought to be incorporated with the -C-C-C- network, comparison with a specific polymer is uncertain. Yet the following conclusions seem warranted for this coal: 1. This bituminous coal has mechanical characteristics of a polymeric character /12/. 2. The a-peak is probably caused by a large-scale cooperative change in molecular conformation. 3. The fi-peak seems likely to be caused by relaxation of some side- group, perhaps incorporating 0-H. 4. The r-peak is probably caused by relaxation of short-segments of the -C-C-C- chain. Other measurements are under way to better elucidate the character of this coal and to generalize the conclusions to other coals.

Acknowledgement.- This work was sponsored by the DOE through a contract with the University of Illinois, by the Max-Planck-Institut fiir Metall- forschung and by and award to one of us (CW) from the Humboldt-Stif- tung, Bonn. JOURNAL DE PHYSIQUE

References

/ l/ N. Berkowitz, An Introduction to Coal Technology, Academic Press, New York, 1979. / 2/ M. Weller, J. Diehl and W. Mensch, phys-stat-sol. (a) 60, 93, (1980). / 3/ M. Weller, Thesis, Universitat Stuttgart, 1972. / 4/ D.E. Kline, A.J. Sauer, and A.E. Woodward, Jour. Polymer Science, 22, 455 (1956). / 5/ N.G. f?cCrum, B.E. Read and G. Williams, Anelastic and Dielectric Effects in Polymeric Solids, John IYiley and Sons, London 1967. / 6/ S.A. Bradley and S.H. Carr, Jour. of ??olymer Science, Polymer Physics Edition, 14, 1 (1976). / 7/ R.W. Seymour, S. Weinhold and S.K. Haynes, Jour. of Yacrornolecu- lar Science, Physics, B16 (31, 337 (1979). / 8/ R.J.v. Hojfors, E. Baer and ?.H. Geil, Jour. of rlacromolecular Science B1 3 (3) , 323 (1977). / 9/ J.M. Charlesworth, Jour. of Polymer Science, Polymer Physics Edition, 17, 329 (1979). /10/ Charles R. Ashcroft and Richard H. Boyd, Jour. of Polymer Science, Polymer Physics Edition, 14, 2153 (1976). /11/ J.M. Pochan, R.J. Gruber and D.F. Pochan, Jour. of Polymer Science, Polymer Physics Edition, 19, 143 (1981) . /12/ John W. Larsen and Jeffrey Kovac, Chapter from Organic Chemistry of Coal, J.W. Larsen, Ed., ACS Syrcposium, Series 71, ACS, Wash. DC., p. 36-49 (1978) .

Lignite , Lignin e,Bituminous Anthracite 100 % 0 0 20-- L0 60 10O0/o Carbon -

Fig. 1: The region of coal conpositions in the ternary phase diagram C-0-H. Fig. 2: The internal friction spectrum of a rod of cannel coal on the first heating from low temperature.

6 I l I 1 I I I N - Y Cannel Coal - -? 5 Specimen 1 a ... . .- .. Run 2 # L - i . . fs1Hz ,." X - J - 3 - .C !; '. Q 2- ..a+ '---./8' - 1 - / 0. .--";* I I I I I 0 100 200 300 LOO T [K1

Fig. 3: The internal friction spectrum of Specimen 1 after it had been heated to 380 K at the end of Run 1. 20- l l I l I l N I Polyethylene P a 16- -

12 - -

8 - Y -

L -

0 I l I l l I l 0 100 200 300 LOO T [K] Fig. 4: The internal friction spectrum of polyethylene Sketched from Fig. 10.9 of Ref. /5/.