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Esr absorption study of oxidized pan fibres
Carbon, 1975.
Vol. 13, p. 147. Pergamon Press. Printed in Great Britain
LETTER
TO THE EDITOR
ESR ABSORPTION STUDY OF OXIDIZED PAN FIBRES (Received 20 May 1974)
Several authors have studied the electron spin resonance (ESR) absorption spectra of carbon fibres derived from a polyacrylonitrile (PAN) based precursor and heat treated at different temperatures in the temperature range of 500°C to 2800°C.The work has been critically reviewed by Singer [I] and also by Gill[2]. However the ESR absorption of raw PAN fibres heated in air, oxygen or nitrogen atmosphere at or near 200°C has not been investigated. We have studied the ESR absorption spectra in PAN fibres heated in oxygen-nitrogen at 200-230°Cfor different length of time and the results are discussed in this paper. The ESR absorption measurements were made with Varian X-band ESR spectrometer at room temperature (RT). The method of heating the PAN fibres in oxygen-nitrogen atmosphere has already been described by Bahl and Manocha[3]. For taking ESR absorption measurements, equal amounts of fibres were packed into a thin quartz tube by the method described by Robson et al [4]. The PAN fibres do not show any ESR absorption prior to heating in oxygen. The results obtained with fibres heated in oxygen (oxidized) at 205+ 2°C for different intervals of time are shown in Fig. I. It can be seen from the Fig. that the intensity of the ESR line increases for about 5 hr of heating time, remains almost constant from 5 to about 10hr of heating time and then starts decreasing. The half-width 7.5 G of the ESR line however, does not change. The line disappeared in about 40 hr of heating. The ESR absorption measurements made after keeping the oxidized fibres in air at RT for about 6 months did not show any change in the intensity, in half-width and in the g-value (2~001-+0~00005) indicating that the radicals generated during oxidation of the PAN fibres do not change at RT. Similar ESR measurements made with PAN fibres heated in nitrogen atmosphere at 205°Cfor different intervals of time did not
8
Half-width -.-.-
.-.-.
Acknowledgements-We wish to thank Dr. A. R. Verma, Director, National Physical Laboratory and Dr. V. Cl. Bhide for constant encouragement and Dr. S. S. Chari for many helpful discussions. Thanks are also due to Dr. 0. P. Bahl for providing the oxidized PAN fibres.
8
I
show any ESR absorption at RT. These fibres were brown in colour while those heated in oxygen were black. The ESR measurements were also made at RT with PAN fibres heated in oxygen atmosphere at 215°Cfor different length of time. The results were in general similar to those obtained with PAN fibres heated in oxygen at 205°C.The intensity of the absorption line increased for a heating time of about one hour and then started decreasing. In these fibres, the intensity of ESR absorption was higher than that for the fibres heated at 205°Cfor the same length of time. The ESR results described in this paper show that the unheated PAN fibres do not contain any paramagnetic radicals. The radicals are generated only when these fibres are heated in oxygen atmosphere and their concentration is dependent on the heating time and the temperature. Presumably two reactions, namely, creation and destruction of free radicals during the oxidation process are taking place simultaneously. The free radicals may be generated during the oxidation of hydroxyl groups to resonancestabilized semi-quinone structure which leads to aromatization. The destruction of free radicals may be due to the chemisorption of oxygen. Bahl and Manocha[3] have studied the variation of strength, infrared absorption, X-ray diffraction etc. of preoxidized (at 205°C) carbon fibres derived from PAN. These authors have observed that the strength of the carbon fibres is highest when the PAN fibres are oxidized at 205°C for about 8-9 hr prior to carbonization at IOOO”C or above. A comparison of the ESR results with those of Bahl and Manocha clearly indicates that the presence of a large number of free radicals during the carbonisation process increases the strength of the carbon fibres. The work is in progress and the detailed results will be published in the near future.
Nation Physical Laboratory New Delhi-12
G. D. SOOTHA T. C. TRIPATHI
India -2
2
‘I, I
REFERENCES
Singer L. S., Proc. 5th Confi on Carbon, Pergamon, New York 2, 37 (1962). 2. Gill R. M., Carbon Fibres in Composite Materials, London Iliffe (1972). 3. Bahl 0. P. and Manocha L. M., Carbon (In Press). 4. Robson D., Assabghy F. Y. I. and Ingram D. J. E., J. Phys. D. Appf. Phys. 4, 1426(1971). 1.
0
10
5
Time
15
of oxldatlon.
20 20
30
40
hr
Fig. I. Variation of intensity and half-width of the ESR absorption line with time of oxidation at 205°C of PAN fibres at room temperature.
Vol. 13, p. 147. Pergamon Press. Printed in Great Britain
LETTER
TO THE EDITOR
ESR ABSORPTION STUDY OF OXIDIZED PAN FIBRES (Received 20 May 1974)
Several authors have studied the electron spin resonance (ESR) absorption spectra of carbon fibres derived from a polyacrylonitrile (PAN) based precursor and heat treated at different temperatures in the temperature range of 500°C to 2800°C.The work has been critically reviewed by Singer [I] and also by Gill[2]. However the ESR absorption of raw PAN fibres heated in air, oxygen or nitrogen atmosphere at or near 200°C has not been investigated. We have studied the ESR absorption spectra in PAN fibres heated in oxygen-nitrogen at 200-230°Cfor different length of time and the results are discussed in this paper. The ESR absorption measurements were made with Varian X-band ESR spectrometer at room temperature (RT). The method of heating the PAN fibres in oxygen-nitrogen atmosphere has already been described by Bahl and Manocha[3]. For taking ESR absorption measurements, equal amounts of fibres were packed into a thin quartz tube by the method described by Robson et al [4]. The PAN fibres do not show any ESR absorption prior to heating in oxygen. The results obtained with fibres heated in oxygen (oxidized) at 205+ 2°C for different intervals of time are shown in Fig. I. It can be seen from the Fig. that the intensity of the ESR line increases for about 5 hr of heating time, remains almost constant from 5 to about 10hr of heating time and then starts decreasing. The half-width 7.5 G of the ESR line however, does not change. The line disappeared in about 40 hr of heating. The ESR absorption measurements made after keeping the oxidized fibres in air at RT for about 6 months did not show any change in the intensity, in half-width and in the g-value (2~001-+0~00005) indicating that the radicals generated during oxidation of the PAN fibres do not change at RT. Similar ESR measurements made with PAN fibres heated in nitrogen atmosphere at 205°Cfor different intervals of time did not
8
Half-width -.-.-
.-.-.
Acknowledgements-We wish to thank Dr. A. R. Verma, Director, National Physical Laboratory and Dr. V. Cl. Bhide for constant encouragement and Dr. S. S. Chari for many helpful discussions. Thanks are also due to Dr. 0. P. Bahl for providing the oxidized PAN fibres.
8
I
show any ESR absorption at RT. These fibres were brown in colour while those heated in oxygen were black. The ESR measurements were also made at RT with PAN fibres heated in oxygen atmosphere at 215°Cfor different length of time. The results were in general similar to those obtained with PAN fibres heated in oxygen at 205°C.The intensity of the absorption line increased for a heating time of about one hour and then started decreasing. In these fibres, the intensity of ESR absorption was higher than that for the fibres heated at 205°Cfor the same length of time. The ESR results described in this paper show that the unheated PAN fibres do not contain any paramagnetic radicals. The radicals are generated only when these fibres are heated in oxygen atmosphere and their concentration is dependent on the heating time and the temperature. Presumably two reactions, namely, creation and destruction of free radicals during the oxidation process are taking place simultaneously. The free radicals may be generated during the oxidation of hydroxyl groups to resonancestabilized semi-quinone structure which leads to aromatization. The destruction of free radicals may be due to the chemisorption of oxygen. Bahl and Manocha[3] have studied the variation of strength, infrared absorption, X-ray diffraction etc. of preoxidized (at 205°C) carbon fibres derived from PAN. These authors have observed that the strength of the carbon fibres is highest when the PAN fibres are oxidized at 205°C for about 8-9 hr prior to carbonization at IOOO”C or above. A comparison of the ESR results with those of Bahl and Manocha clearly indicates that the presence of a large number of free radicals during the carbonisation process increases the strength of the carbon fibres. The work is in progress and the detailed results will be published in the near future.
Nation Physical Laboratory New Delhi-12
G. D. SOOTHA T. C. TRIPATHI
India -2
2
‘I, I
REFERENCES
Singer L. S., Proc. 5th Confi on Carbon, Pergamon, New York 2, 37 (1962). 2. Gill R. M., Carbon Fibres in Composite Materials, London Iliffe (1972). 3. Bahl 0. P. and Manocha L. M., Carbon (In Press). 4. Robson D., Assabghy F. Y. I. and Ingram D. J. E., J. Phys. D. Appf. Phys. 4, 1426(1971). 1.
0
10
5
Time
15
of oxldatlon.
20 20
30
40
hr
Fig. I. Variation of intensity and half-width of the ESR absorption line with time of oxidation at 205°C of PAN fibres at room temperature.