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ESR imaging of free radicals in mesophase pitch
ESR imaging of free radicals in mesophase pitch (Received
Key Words-Electron
26 Srptemher
spin resonance,
When coal liquid products are heated to 35O”C-500°C in an inert atmosphere, optically anisotropic carbonaceous rnesophase spheres appear and coalesce[ l-31. The importance of carbonaceous mesophase from carbonaceous materials such as coal tar pitch, petroleum residues and organic polymers is well recognized because the texture and microstructure found in coke, graphite and carbon material products clearly originate from their earlier mesophase state. The mesophase contains several times as many free radicals as the other part (matrix) of a carbonaceous sample does, providing singlet ESR spectra. It is important to investigate the spatial distribution of mesophase and/or the quantity of free radicals contained in it[4]. We have successfully observed the spatial distribution using ESR imaging, which has been developed over the last several years[5-121. More recently, a lowfield ESR imaging technique has been successfully devised for biological systems[ 131 and a rotary magnetic field gradient method for paramagnetic species with anisotropic parameters[ 141. ESR imaging enables us to observe the spatial distribution of paramagnetic centers and thus offers a new approach in the study of chemical reactions and diffusion. It also throws further light on data yielded by other important analyzers such as chromatography. While basically similar to NMR imaging, ESR-imaging has many additional characteristics. Firstly, ESR linewidths are so broad, perhaps from a few gauss to several ten gauss, that this necessitates field gradients two or three orders of magnitude higher than are currently used for NMR imaging. Secondly, for two-dimensional ESR imaging involving rotation of samples, anisotropic magnetic properties become a serious problem when the sample is mounted at different orientations in the applied field. Finally, there may be hyperfine structure present due to electron-nucleus interactions and this must be eliminated. Several investigators have overcome these problems and demonstrated the feasibility of ESR imagining in various fields. Two coils of copper wire in an elliptical shape, cooled with flowing transformer oil, provide a magnetic field gradient of 450 gauss/cm. Pulsed D.C. current is fed from two regulated programmable D.C. power supply units (KIKUSUI PAE 3530) with a maximum current of 30 A and a very fast transient response of 200 psec. The pulsation (its duty cycle IO%, pulse width IO msec) serves to protect the coils and the cavity from thermal disturbance, but also reduces the ratio of signal to noise one or two orders of magnitude. For the pulsation, an analog
Fig.
I. Mesophase pitch sample
1985)
mesophase.
free radicals
gate is interposed between a preamplifier and a phase-sensitive detector to pass the ESR signal during the current flows. All spectra from the spectrometer (JEOL FE3XG) are digitized into 1024 points, with the average of Y spectra yielding one projection spectrum, and these are fed into a microcomputer (NEC PC9801) for two-dimensional image reconstruction after deconvolution. A 256 x 256 grid is used for the back-projection method with 36 projection spectra oriented at 5” interval in the range O”180”. The hydrogenated coal tar pitch used as a sample had the following composition: 91.5% C, 5.5% H. 1.7% N, 0.3% S, I .O% 0 diff. The pitch sample was placed in a Pyrex tube reactor and heated to 450°C for I hr under nitrogen gas flow. Mesophase pitch thus obtained was mounted in resin, and polished surfaces were prepared for ESR imagining. Optical texture was assessed using a Nikon Apophoto optical microscope with polarized light. The separation of mesophase from matrix was achieved by quinoline extraction centrifugation. The ESR measurements indicated that the separated mesophase contains radicdls of 4.3 X 10”’ spins/g and the matrix I .2 x IO”’ spins/g. while the singlet line has a linewidth of 2.6 gauss which was used as a response function. Figure I shows the mesophase pitch sample in mounted resin. The specimen, 3 mm in diameter and 5 mm long, was inserted in an ESR sample tube. In Fig. 2 an optical micrograph of mesophase in the pitch matrix is shown. Anisotropic colored areas show mesophase spheres of various sizes and shapes, the largest ones of which are about 50 pm in diameter. The linewidth of the singlet line and the applied magnetic field gradient provide a one-dimensional spatial resolution of about 30 pm when the resolution is defined as a distance giving rise to a shift of the response function equal to the linewidth under the field gradient. It is assumed that deconvolution reduces the linewidth of the response function to a half its original value. However in order to obtain a two-dimensional spatial resolution of 30 pm approximately 200 projection spectra are required, but only 36 projection spectra were acquired in the present experiment. This means that the image is reconstructed under an assumption of the mission projection spectra being similar to the available ones. Thus the two-dimensional resolution may be several times greater than 30 p. Figure 3 shows a perspective view of the ESR intensity due
Fig. 2. Optical texture of mesophase
pitch
518
Letters to the Editor In addition, this method has great potential applicability to wider fields of scientific research, such as the geochemical study of kerogen in sedimentary rocks. Acknowledgment-One of the authors (K.O.) would like to thank the financial support provided by a Grant-in-Aid for Scientific Research of the Ministry of Education Science and Culture in Japan. Material Science Laboratory Faculty of Engineering Hokkaido University Sapporo 060 Japan
Fig 3. Perspective view of the distribution of free radicals in mesophase and matrix of coal tar-pitch. Vertical magnitude is proportional to the radical density per unit surface. The maximum peak height is normalized so that the height means the relative total radical density per unit surface.
mainly to the mesophase spheres and partially to the matrix. Considering the size of the mesophase spheres in the pitch matrix and the two-dimensional resolution, the vertical magnitude in this image means relative total average radical density per unit surface both of the mesophase and the matrix contained in the unit surface. Thus, the more mass of mesophase spheres existing there, the more intense the magnitude obtained, without regard to the size of the mesophase sphere. An intense peak suggests an aggregation of a number of mesophase spheres or the presence of large mesophase spheres at that position in the sample. Some other weak peaks can be recognized surrounding the main peak, in particular a moderate aggregation on the right which forms the second most intense peak. The perspective view does not necessarily reveal all the details of the distribution and sometimes hides many peaks. A contour line map is often instructive. Figure 4 shows the contour line map of ESR intensity in the specimen. From it one can get a more detailed picture of the spatial distribution of the mesophase. The resolution of this image is several tenth of the scale in the figure and is approximately equivalent to one pixel. In conclusion, ESR imaging will serve well in the investigation of the correlation between the properties of coal products and those of their mesophases.
Coal Research Institute Faculty of Engineering Hokkaido University Sapporo 060 Japan
K. OHNO
T. YOKONO J. YAMADA Y. SANADA
REFERENCES I. J. D. Brooks and G. H. Taylor, Carbon 3, 185 (1965). 2. H. Marsh, J. M. Forster, G. Herman, M. Iley and J. N. Melnin, Fuel 52, 243 (1973). 3. H. Honda, H. Kimura, Y. Sanada, S. Sugawara and T. Furuta, Carbon 8, I81 (1970). 4. L. S. Singer, I. C. Lewis and R. A. Greinke, Proc. Inr. Carbon Conf 352 (1984) Bordeaux, France. 5. M. J. R. Hoch and A. R. Day, Solid State Commun. 30, 211 (1979). 6. M. J. R. Hoch, J. Phys. C, Solid Stare Phys. 14, 5659 (1981). 7. W. Karthe and E. J. Wehrsdorfer, J. Magn. Reson. 33, 107 (1979). 8. T. Herding, N. Klimes, W. Karthe, U. Ewert and B. Ebert, J. Magn. Reson. 49, 203 (1982). 9. T. Herrling and U. Ewert, J. Magn. Reson. 61, I I (1985). IO. K. Ohno, Japan. J. Appl. Phys. 20, L179 (1981); 23, L224 (1984). Il. K. Ohno, J. Magn. Reson. 49, 56 (1982); 50, I45 (1982). 12. S. S. Eaton and G. R. Eaton, J. Magn. Reson. 59, 474 (1985). 13. L. J. Berliner and H. Fujii, Science 277, 517 (1985). 14. K. Ohno, J. Magn. Reson. 64, 109 (1985).
Fig. 4. Contour representation of free radicals in the same sample in Fig. 3. The tone bar indicates higher radical density from the bottom to the top. The orientation of this map corresponds to the clockwise 90”-rotated one of the perspective plane in Fig. 3 so that the two arrows orient in the same direction. The full horizontal range of Fig. 2 corresponds to about 7 pixels in Fig. 4.
Key Words-Electron
26 Srptemher
spin resonance,
When coal liquid products are heated to 35O”C-500°C in an inert atmosphere, optically anisotropic carbonaceous rnesophase spheres appear and coalesce[ l-31. The importance of carbonaceous mesophase from carbonaceous materials such as coal tar pitch, petroleum residues and organic polymers is well recognized because the texture and microstructure found in coke, graphite and carbon material products clearly originate from their earlier mesophase state. The mesophase contains several times as many free radicals as the other part (matrix) of a carbonaceous sample does, providing singlet ESR spectra. It is important to investigate the spatial distribution of mesophase and/or the quantity of free radicals contained in it[4]. We have successfully observed the spatial distribution using ESR imaging, which has been developed over the last several years[5-121. More recently, a lowfield ESR imaging technique has been successfully devised for biological systems[ 131 and a rotary magnetic field gradient method for paramagnetic species with anisotropic parameters[ 141. ESR imaging enables us to observe the spatial distribution of paramagnetic centers and thus offers a new approach in the study of chemical reactions and diffusion. It also throws further light on data yielded by other important analyzers such as chromatography. While basically similar to NMR imaging, ESR-imaging has many additional characteristics. Firstly, ESR linewidths are so broad, perhaps from a few gauss to several ten gauss, that this necessitates field gradients two or three orders of magnitude higher than are currently used for NMR imaging. Secondly, for two-dimensional ESR imaging involving rotation of samples, anisotropic magnetic properties become a serious problem when the sample is mounted at different orientations in the applied field. Finally, there may be hyperfine structure present due to electron-nucleus interactions and this must be eliminated. Several investigators have overcome these problems and demonstrated the feasibility of ESR imagining in various fields. Two coils of copper wire in an elliptical shape, cooled with flowing transformer oil, provide a magnetic field gradient of 450 gauss/cm. Pulsed D.C. current is fed from two regulated programmable D.C. power supply units (KIKUSUI PAE 3530) with a maximum current of 30 A and a very fast transient response of 200 psec. The pulsation (its duty cycle IO%, pulse width IO msec) serves to protect the coils and the cavity from thermal disturbance, but also reduces the ratio of signal to noise one or two orders of magnitude. For the pulsation, an analog
Fig.
I. Mesophase pitch sample
1985)
mesophase.
free radicals
gate is interposed between a preamplifier and a phase-sensitive detector to pass the ESR signal during the current flows. All spectra from the spectrometer (JEOL FE3XG) are digitized into 1024 points, with the average of Y spectra yielding one projection spectrum, and these are fed into a microcomputer (NEC PC9801) for two-dimensional image reconstruction after deconvolution. A 256 x 256 grid is used for the back-projection method with 36 projection spectra oriented at 5” interval in the range O”180”. The hydrogenated coal tar pitch used as a sample had the following composition: 91.5% C, 5.5% H. 1.7% N, 0.3% S, I .O% 0 diff. The pitch sample was placed in a Pyrex tube reactor and heated to 450°C for I hr under nitrogen gas flow. Mesophase pitch thus obtained was mounted in resin, and polished surfaces were prepared for ESR imagining. Optical texture was assessed using a Nikon Apophoto optical microscope with polarized light. The separation of mesophase from matrix was achieved by quinoline extraction centrifugation. The ESR measurements indicated that the separated mesophase contains radicdls of 4.3 X 10”’ spins/g and the matrix I .2 x IO”’ spins/g. while the singlet line has a linewidth of 2.6 gauss which was used as a response function. Figure I shows the mesophase pitch sample in mounted resin. The specimen, 3 mm in diameter and 5 mm long, was inserted in an ESR sample tube. In Fig. 2 an optical micrograph of mesophase in the pitch matrix is shown. Anisotropic colored areas show mesophase spheres of various sizes and shapes, the largest ones of which are about 50 pm in diameter. The linewidth of the singlet line and the applied magnetic field gradient provide a one-dimensional spatial resolution of about 30 pm when the resolution is defined as a distance giving rise to a shift of the response function equal to the linewidth under the field gradient. It is assumed that deconvolution reduces the linewidth of the response function to a half its original value. However in order to obtain a two-dimensional spatial resolution of 30 pm approximately 200 projection spectra are required, but only 36 projection spectra were acquired in the present experiment. This means that the image is reconstructed under an assumption of the mission projection spectra being similar to the available ones. Thus the two-dimensional resolution may be several times greater than 30 p. Figure 3 shows a perspective view of the ESR intensity due
Fig. 2. Optical texture of mesophase
pitch
518
Letters to the Editor In addition, this method has great potential applicability to wider fields of scientific research, such as the geochemical study of kerogen in sedimentary rocks. Acknowledgment-One of the authors (K.O.) would like to thank the financial support provided by a Grant-in-Aid for Scientific Research of the Ministry of Education Science and Culture in Japan. Material Science Laboratory Faculty of Engineering Hokkaido University Sapporo 060 Japan
Fig 3. Perspective view of the distribution of free radicals in mesophase and matrix of coal tar-pitch. Vertical magnitude is proportional to the radical density per unit surface. The maximum peak height is normalized so that the height means the relative total radical density per unit surface.
mainly to the mesophase spheres and partially to the matrix. Considering the size of the mesophase spheres in the pitch matrix and the two-dimensional resolution, the vertical magnitude in this image means relative total average radical density per unit surface both of the mesophase and the matrix contained in the unit surface. Thus, the more mass of mesophase spheres existing there, the more intense the magnitude obtained, without regard to the size of the mesophase sphere. An intense peak suggests an aggregation of a number of mesophase spheres or the presence of large mesophase spheres at that position in the sample. Some other weak peaks can be recognized surrounding the main peak, in particular a moderate aggregation on the right which forms the second most intense peak. The perspective view does not necessarily reveal all the details of the distribution and sometimes hides many peaks. A contour line map is often instructive. Figure 4 shows the contour line map of ESR intensity in the specimen. From it one can get a more detailed picture of the spatial distribution of the mesophase. The resolution of this image is several tenth of the scale in the figure and is approximately equivalent to one pixel. In conclusion, ESR imaging will serve well in the investigation of the correlation between the properties of coal products and those of their mesophases.
Coal Research Institute Faculty of Engineering Hokkaido University Sapporo 060 Japan
K. OHNO
T. YOKONO J. YAMADA Y. SANADA
REFERENCES I. J. D. Brooks and G. H. Taylor, Carbon 3, 185 (1965). 2. H. Marsh, J. M. Forster, G. Herman, M. Iley and J. N. Melnin, Fuel 52, 243 (1973). 3. H. Honda, H. Kimura, Y. Sanada, S. Sugawara and T. Furuta, Carbon 8, I81 (1970). 4. L. S. Singer, I. C. Lewis and R. A. Greinke, Proc. Inr. Carbon Conf 352 (1984) Bordeaux, France. 5. M. J. R. Hoch and A. R. Day, Solid State Commun. 30, 211 (1979). 6. M. J. R. Hoch, J. Phys. C, Solid Stare Phys. 14, 5659 (1981). 7. W. Karthe and E. J. Wehrsdorfer, J. Magn. Reson. 33, 107 (1979). 8. T. Herding, N. Klimes, W. Karthe, U. Ewert and B. Ebert, J. Magn. Reson. 49, 203 (1982). 9. T. Herrling and U. Ewert, J. Magn. Reson. 61, I I (1985). IO. K. Ohno, Japan. J. Appl. Phys. 20, L179 (1981); 23, L224 (1984). Il. K. Ohno, J. Magn. Reson. 49, 56 (1982); 50, I45 (1982). 12. S. S. Eaton and G. R. Eaton, J. Magn. Reson. 59, 474 (1985). 13. L. J. Berliner and H. Fujii, Science 277, 517 (1985). 14. K. Ohno, J. Magn. Reson. 64, 109 (1985).
Fig. 4. Contour representation of free radicals in the same sample in Fig. 3. The tone bar indicates higher radical density from the bottom to the top. The orientation of this map corresponds to the clockwise 90”-rotated one of the perspective plane in Fig. 3 so that the two arrows orient in the same direction. The full horizontal range of Fig. 2 corresponds to about 7 pixels in Fig. 4.