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Low-frequency dielectric measurements on Argonne Premium coals
Letter
Low-frequency dielectric measurements on Argonne Premium coals (Received
19 December
7992)
Sir Dielectric spectroscopy has been used extensively in the investigation and characterization of polymers. At frequencies below - lo* Hz, the dielectric properties often relate to charge migration and hence it is possible to deduce the morphological structure of a material. Most amorphous dipolar polymers exhibit LY and fl dielectric relaxation processes and it is possible to study these using dielectric spectroscopy. The tl process is due to micro-Brownian motions of chains and is associated with the glass transition, whereas the p process is due to side group motions. Therefore low-frequency dielectric spectroscopy is potentially a powerful technique to investigate the electronic structure of coals, and to our knowledge no such investigations have been performed. The dielectric properties of coal have received much attention with respect to monitoring coal concentration in coalwater slurries or in detecting changes within a coal seam during in situ gasification processes’-3. Previous work on coals has concentrated solely on the MHz to GHz region. This work reports measurements in the low-frequency range, lo5 to lo-’ Hz, for a series of Argonne Premium coals of different rank, of the real permittivity and real conductivity as a function of temperature. Illinois No. 6 (ILL, p,,=552 kgmm3), Pocahontas No. 3 (POC, pp = 657 kg me3), Upper Freeport (UF, pp = 497 kg me3) and North Dakota lignite (NDL, pp= 517kgme3) (all <150pm; p,=packing density), obtained from the Argonne National Laboratory Premium Coal Sample Program, were used. The coal samples were pretreated by heating in the chamber of a Mettler DSC-30 at 383K for 90 min followed by 10 min at 523 K under a nitrogen carrier gas. The samples were quenched to room temperature and used immediately. This thermal treatment was carried out to simulate the sample preparation of Hall and Mackinnon4 in which an enthalpy relaxation followed by a glass transition was observed for Illinois No. 6 coal. Dielectric measurements were performed using a Solartron 1250 frequency response analyser (FRA); the techniques for interfacing the instrument to the sample and for data analysis have been described previously5. Data were collected over the frequency range lo-* to 6.3 x lo4 Hz over
00162361/93/07/107742 0 1993 Butterworth-Heinemann
Ltd.
a period of 15min. A cell consisting of two pre-etched copper electrodes and maintained at constant separation of 1 mm with a copper spacer was used to hold the sample. This design generates a three-electrode system with an active electrode area of 1 cm’. The sample was inserted into the cell and was vigorously agitated to ensure that it settled firmly in
Permittivity
the cell. The packing density, pp, was measured for each sample. The sample arrangement was attached to a thermostatically heated copper block and the whole assembly inserted in an Oxford instruments cryostat (DN1704). Measurements were made isothermally, commencing at 450 K and decreasing in 10 K steps to 300K, with a period of 20min
at 100 kHz
3.8 3.6
3.2
1
300 Figure 1 Real No. 3; +, North
Temperature
400 (K)
500
permittivity of Argonne coals as a function of temperature: q , Pocahontas Dakota
Real Conductivity
lignite;
(l/Qm)
n , Upper Freeport;
0,
Illinois No. 6
at O.OlHz
-'l
300 Figure 2 Figure I
Real conductivity
400 Temperature of Argonne
coals as a function
Fuel 1993
500
(K) of temperature.
Volume
Symbols
72 Number
7
as in
1077
letter allowed between the commencement of each run. The dielectric properties of a polar organic material may be described in terms of the frequency dependence of the complex permittivity6,
E*= E’(W)- is”(w)
(1)
where E’(W)and E”(W) are the real and imaginary components of the dielectric permittivity, and i=& 1. A simple dipolar medium exhibits a frequency dependence which has the form 1
E’(W)-&I, PC && - Eb, E”(W) p= &b--E:,
1+ c.?T2
(2)
0.X 1 +w*r2
(3)
where cb and &b, are respectively the low- and high-frequency limiting values of the dielectric permittivity for a process with relaxation time t; E’ and E” are experimentally observable quantities which may be used to characterize the dielectric dispersion over a range of frequencies. The associated imaginary permittivity contains components for a dipolar process (E:), as well as a d.c. conduction process (&): El1= Ei $ Eic
(4)
and the combined components require to be separated before the dipolar relaxation can be observed. The dependence of the
1078
imaginary permittivity on the real conductivity, c’, is given by
An estimate of the d.c. conductivity, Q&, was obtained by measuring 0’ at lo- ’ Hz. Figure 1 illustrates that for all coals, the real permittivity is temperaturedependent. The low-temperature values compare well with the work of Chatterjee and Misra’ and Nelson et a1.3 There would appear to be no correlation between the rank of the coal and the real permittivity. In the case of POC, NDL and ILL, there is little difference between the values of real permittivity. The samples were predried up to 523 K and hence the water content would be minimal, resulting in similar real permittivities. This effect has been discussed by Chatterjee and Misra’. UF shows slightly greater values compared with the other coals. There is no correlation between the bulk density and real permittivity as previously observed by Nelson et a1.3. However, the range of packing densities studied by Nelson et al. was significantly greater than in this work, and this may account for the difference. Figure 2 illustrates that the real conductivity increases with temperature for all coals, with UF showing anomalous behaviour with consistently lower values. The similarity in values for POC, NDL and ILL can also be attributed to the absence of water. The
Fuel 1993 Volume 72 Number 7
increase in conductivity with increasing temperature can be attributed to an increase in charge carrier mobility. ACKNOWLEDGEMENT This work was funded by SERC grant GR/H18821. Alexander J. Mackinnon, Peter J. Hall, David Hayward and Richard A. Pethrick Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow Gl IXL, UK
REFERENCES Goldberg, I. B. and Chung, K. E. Fuel 1982,61, 735 Chatterjee, I. and Misra, M. J. Microware Power Technol. 1990, 25, 224 Nelson, S. O., Beck-Montgomery, S. R., Fanslow, G. E. and Bluhm, D. D. J. Microwave Power Technol. 1981, 16, 319 Hall, P. J. and Mackinnon, A. J. Fuel 1992, 71,974 D.. Mahoubibian-Jones, Havward. M.b. B. gnd Pithrick, R. A. J. Phys. E, Sci. Insir. 1984, 17, 683 Baird, M. E. ‘Electrical Properties of Polymeric Materials’, The Plastics Institute, London, 1973
Low-frequency dielectric measurements on Argonne Premium coals (Received
19 December
7992)
Sir Dielectric spectroscopy has been used extensively in the investigation and characterization of polymers. At frequencies below - lo* Hz, the dielectric properties often relate to charge migration and hence it is possible to deduce the morphological structure of a material. Most amorphous dipolar polymers exhibit LY and fl dielectric relaxation processes and it is possible to study these using dielectric spectroscopy. The tl process is due to micro-Brownian motions of chains and is associated with the glass transition, whereas the p process is due to side group motions. Therefore low-frequency dielectric spectroscopy is potentially a powerful technique to investigate the electronic structure of coals, and to our knowledge no such investigations have been performed. The dielectric properties of coal have received much attention with respect to monitoring coal concentration in coalwater slurries or in detecting changes within a coal seam during in situ gasification processes’-3. Previous work on coals has concentrated solely on the MHz to GHz region. This work reports measurements in the low-frequency range, lo5 to lo-’ Hz, for a series of Argonne Premium coals of different rank, of the real permittivity and real conductivity as a function of temperature. Illinois No. 6 (ILL, p,,=552 kgmm3), Pocahontas No. 3 (POC, pp = 657 kg me3), Upper Freeport (UF, pp = 497 kg me3) and North Dakota lignite (NDL, pp= 517kgme3) (all <150pm; p,=packing density), obtained from the Argonne National Laboratory Premium Coal Sample Program, were used. The coal samples were pretreated by heating in the chamber of a Mettler DSC-30 at 383K for 90 min followed by 10 min at 523 K under a nitrogen carrier gas. The samples were quenched to room temperature and used immediately. This thermal treatment was carried out to simulate the sample preparation of Hall and Mackinnon4 in which an enthalpy relaxation followed by a glass transition was observed for Illinois No. 6 coal. Dielectric measurements were performed using a Solartron 1250 frequency response analyser (FRA); the techniques for interfacing the instrument to the sample and for data analysis have been described previously5. Data were collected over the frequency range lo-* to 6.3 x lo4 Hz over
00162361/93/07/107742 0 1993 Butterworth-Heinemann
Ltd.
a period of 15min. A cell consisting of two pre-etched copper electrodes and maintained at constant separation of 1 mm with a copper spacer was used to hold the sample. This design generates a three-electrode system with an active electrode area of 1 cm’. The sample was inserted into the cell and was vigorously agitated to ensure that it settled firmly in
Permittivity
the cell. The packing density, pp, was measured for each sample. The sample arrangement was attached to a thermostatically heated copper block and the whole assembly inserted in an Oxford instruments cryostat (DN1704). Measurements were made isothermally, commencing at 450 K and decreasing in 10 K steps to 300K, with a period of 20min
at 100 kHz
3.8 3.6
3.2
1
300 Figure 1 Real No. 3; +, North
Temperature
400 (K)
500
permittivity of Argonne coals as a function of temperature: q , Pocahontas Dakota
Real Conductivity
lignite;
(l/Qm)
n , Upper Freeport;
0,
Illinois No. 6
at O.OlHz
-'l
300 Figure 2 Figure I
Real conductivity
400 Temperature of Argonne
coals as a function
Fuel 1993
500
(K) of temperature.
Volume
Symbols
72 Number
7
as in
1077
letter allowed between the commencement of each run. The dielectric properties of a polar organic material may be described in terms of the frequency dependence of the complex permittivity6,
E*= E’(W)- is”(w)
(1)
where E’(W)and E”(W) are the real and imaginary components of the dielectric permittivity, and i=& 1. A simple dipolar medium exhibits a frequency dependence which has the form 1
E’(W)-&I, PC && - Eb, E”(W) p= &b--E:,
1+ c.?T2
(2)
0.X 1 +w*r2
(3)
where cb and &b, are respectively the low- and high-frequency limiting values of the dielectric permittivity for a process with relaxation time t; E’ and E” are experimentally observable quantities which may be used to characterize the dielectric dispersion over a range of frequencies. The associated imaginary permittivity contains components for a dipolar process (E:), as well as a d.c. conduction process (&): El1= Ei $ Eic
(4)
and the combined components require to be separated before the dipolar relaxation can be observed. The dependence of the
1078
imaginary permittivity on the real conductivity, c’, is given by
An estimate of the d.c. conductivity, Q&, was obtained by measuring 0’ at lo- ’ Hz. Figure 1 illustrates that for all coals, the real permittivity is temperaturedependent. The low-temperature values compare well with the work of Chatterjee and Misra’ and Nelson et a1.3 There would appear to be no correlation between the rank of the coal and the real permittivity. In the case of POC, NDL and ILL, there is little difference between the values of real permittivity. The samples were predried up to 523 K and hence the water content would be minimal, resulting in similar real permittivities. This effect has been discussed by Chatterjee and Misra’. UF shows slightly greater values compared with the other coals. There is no correlation between the bulk density and real permittivity as previously observed by Nelson et a1.3. However, the range of packing densities studied by Nelson et al. was significantly greater than in this work, and this may account for the difference. Figure 2 illustrates that the real conductivity increases with temperature for all coals, with UF showing anomalous behaviour with consistently lower values. The similarity in values for POC, NDL and ILL can also be attributed to the absence of water. The
Fuel 1993 Volume 72 Number 7
increase in conductivity with increasing temperature can be attributed to an increase in charge carrier mobility. ACKNOWLEDGEMENT This work was funded by SERC grant GR/H18821. Alexander J. Mackinnon, Peter J. Hall, David Hayward and Richard A. Pethrick Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow Gl IXL, UK
REFERENCES Goldberg, I. B. and Chung, K. E. Fuel 1982,61, 735 Chatterjee, I. and Misra, M. J. Microware Power Technol. 1990, 25, 224 Nelson, S. O., Beck-Montgomery, S. R., Fanslow, G. E. and Bluhm, D. D. J. Microwave Power Technol. 1981, 16, 319 Hall, P. J. and Mackinnon, A. J. Fuel 1992, 71,974 D.. Mahoubibian-Jones, Havward. M.b. B. gnd Pithrick, R. A. J. Phys. E, Sci. Insir. 1984, 17, 683 Baird, M. E. ‘Electrical Properties of Polymeric Materials’, The Plastics Institute, London, 1973