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Optical mesophase texture and X-ray diffraction pattern of the early-stage carbonization of pitches
OPTICAL MESOPHASE TEXTURE AND X-RAY DIFFRACTION PATTERN OF THE EARLY-STAGE CARBONIZATION OF PITCHES H. HONDA,*
H. KIMURA,
Y. SANADA,
Resources Kesearch
Institute,
S. SUGAWARA
and T. FURUTA
Ka\~~agrlchi-S;~it;~r~i~~.,lap;~n
AbstractLow temper;iture carbonization of pitches was studied with polarized-light microscopic method, X-ray diffraction, solvent extraction and density measurements. The nucleation and growth of spherical bodies occur progressively with increase of residence time at a fixed temperature, until the spherical bodies eventually coalesce with each other. Fine irregular particles of insolubles in coal-tar pitch are observed in the region of boundary between spherules and matrix. They play an important role in the mesophase transformation. The typical changes of stacking height of lamellae, interlamellar spacing, weight loss, density and insolubility with residence time show nearly the same tendency at the various temperatures from 390” to 430°C. The time-temperature superposition has been successfully employed. In the master curve for coal-tar pitch reduced at 41O”C, the reduced time scale extends from lo-” to lo” hr. From the relation between the reducing factor of time translation and absolute temperature, the apparent activation energies can be estimated over the range of 35-45 kcal/mole. It seems probable that a model of the rearrangement of G-C bonding together with vaporization of low molecular substances is applicable for the growth of spherical bodies. 1. INTRODUCTION
Low temperature carbonization of pitches has been studied extensively using polarizedlight microscopic method and reviewed by Brooks et al. and several other groups[ld]. Brooks and Taylor[l] observed that the formation of graphitizing low-temperature carbons by solidification from a liquid phase proceeds through the separation of a mesophase spherical bodies. With increase of ternperature, the spheres grow up and coalesce. They have also observed that the spheres grow larger to a certain extent with increase of time at a fixed temperature. It has been clarified by Ihnatowicz et aZ.[2] that the insolubility of the spherical bodies in the solvent and in the isotropic phase accounts fol the relation between the solubility measurements and the microscopic appearance. It appears, however, that very few detailed *Present
address:
search Institute,
Government
Industrial
Kyushu, Tosu-Kyushu,.Japan.
investigations of the development of the characteristic anisotropic mosaic texture with residence time in the neighbourhood of 400”-450°C have yet been made. The present work has been done in an attempt to get the relationship between development of mesophase spherical bodies, which later become units of mosaic texture, and X-ray diffraction or some physical properties of pattern pitches. 2. EXPERIMENTAL
(:oal-tar pitch and naphtha-tar pitch were selected for these studies as samples of typical type of pitches. The elementary composition, the softening temperature and the quinoline insolubles -of the pitches were determined by routine methods. The results are shown in Table 1. ‘I‘he pitches were carbonized in a vertical silica tube within a heated cylindrical furnace Kehaving automatic temperature control. The 181
H. HONDA et al. Table 1. Characteristics of pitches used Elementary analysis (%)
Coal-tar pitch Naphtha-tar pitch
C
H
N
S
O(diff.)
sp* (“C)
91.9 95.0
4.7 5.0
I.4 -
0.3 -
1.7 -
76.5 -
YIt %
BIS
3.5 1.8
16.1 -
%
*Softening Point (ring and ball method (JIS)). IQuinoline insolubles. $Benzene insolubles.
temperature of carbonization had to be maintained constant within -+l”C, since the mesophase transformation and its hardening process are quite sensitive to temperature and time. All carbonization runs were carried out in an inert atmosphere, usually pure nitrogen gas. Specimens were heated with the heating rate of S”C/min and maintained for desired residence time from 0 to 20 hr at a constant temperature in the range from 350” to 500°C. The heated material was allowed to cool to room temperature before examination. Microscopic observation was done with a Leitz Ortholux microscopic using reflected polarized light under Xenon arc illumination. The polished surface of the specimen moulded with epoxy resin was prepared by using Carborundum papers and woolen cloth with fine powders of alumina and chrome oxide. The X-ray diffraction patterns of powdered specimens were obtained by using a recording diffractometer with Ni-filtered C,K, radiation. The diffraction intensities were corrected for the Lorentz polarization and the absorption factors according to Mirberg’s equation[6]. The height of hypothetical cylinders (crystallites) of stacked lamellar molecules in a random layer structure and the inter-lamellar (interlayer) spacing were obtained from the half width of the diffraction profile and the position of reflection in the vicinity of 28 = 25”, respectively. Changes of sample weight with residence time at a constant temperature were deter-
mined from the weight difference before and after heat treatment. True densities were determined picnometrically at 25VC with distilled water containing 0.01 per cent of sodium oleate as a detergent. Solvent extractions with quinoline were followed by the instruction manual of Japanese Industrial Standards (JIS K2421-1966). Quinoline was used without further purification of guaranteed grade reagent.
3. RESULTS 3.1 General de.scr$tion of changes observed with polarized light microscope The present work shows that the appearance of spherical bodies and the development of anisotropic mosaic texture are recognized as functions of residence time and heat-treatment temperature (HTT). The nucleation and growth of spherical bodies occur progressively with increase of residence time until the spherical bodies eventually coalesce with each other, with complete disappearance of the isotropic liquid phase. The spherical bodies grow to 10-100~ in diameter with increase in residence time. It is observed that there are some differences in the growth of spherical bodies between coal-tar pitch and naphtha-tar pitch as’ shown in Figs. 1 and 2. The coal-tar pitch heat-treated at 400”430°C has spherical bodies of nearly the same size, which are covered by fine irregular par-
Fig. 1. hfesophase spherules times at 41OY; (plane-polarirecl
in coal-t:u. pitch for various residence light): (a) !2hr; (b) 1 hr; (c) 8 hr; (cl) I3 hr (370 X ).
Fig. 2. Mesophase spherules in naphtha-tar pitch for various residence times at 400°C (plane-polarized light): (a) 1 hr; (b) 1.5 hr; (c) 3 hr; (d) 5 hr (370 X ).
3.2 X-ray dajkxtion The changes of inter~dmellar spacing, d,, and stacking heigbt of lame&e, L,, with residence time at a fixed temperature are shown in Figs. 4 and 5, respectively. The fact, that only one reflection [i.e. (002) band corresponding to stacked aromatic tamellar molecules in a random layer structure of pitch] was utibzed for crystallite size evafuation, places doubt upon the absofute accuracy of the height of srdcked lamellar and the inter-lamellar (interlayer) spacing. irrespective of the absolute ln~~gnitude of the crystallite parameter, the results draw atten-
ticks of insaluble matters. The insoluble matters aggregate locally in the bulk mesophase or the mosaic struct.ure. On the other hand, the naphtha-tar pitch heat-treated at the same tem~)eratLlt~e r;inge has a clean surface of spherical bodies in various sizes, as the same with the spherical bodies in toluenesoluble extract of the pitch shown by Brooks f-&al.[l]* Figure 3 shows the relations among the nucleation, growth and coalescence of spherical bodies in naphtha-tar pitch heat-treated under various temperatures and residence times.
7
Naphtha-tar
I I
X
No
0
Appoorontr
0)
Growth
pitch
spherical
bodies af
of
spherical
sphericat conversion
bodies bodies to
mesophasr
-J--
1 / / i
to
012345 Reridrncc
I5
time ~
20
kr
Fig. 3. Relations between the nucleation, tar pitch heat-treated
growth and coalesoence uf‘spherical bodies in naphthaat variws temper~~ttlres and residence times.
It is clear frr,m the Fig. 3 that nucleation and growth of spherical bodies can be observed at a low temperature such as 350°C when residence time is long enough.
tion to the behaviour of structural change at early stage car~njzation. The d, decreases progressively with increase in the residence time and approaches to 3435 _Ain value. The
Fig. 4. Relation between interlamellar
H. HONDA
et al.
Residence
time
spacing of heat-treated
higher the temperature is kept, the faster the value of d, decreases. The same behaviour of L, under various temperatures and times have been observed as shown in Fig. 5. Blayden et al.[7] showed the maxima in the L, values of the British coals and pitch plotted against heat-treatment temperature. They considered the maxima in the L, values to be intimately associated with the fluid state through which these samples pass on carbonization. However, no occurrence of the maxima in the L, plotted against residence time have been found within the experimental conditions in the present work. 3.3 Weight loss, density and solvent extraction As carbonization progresses, spherules, which are more dense
,
mesophase than the iso-
hr
coal-tar pitch and residence time.
tropic parent phase, grow together with the vaporization of low moleculk substances. Weight loss measurements reflect the fractions of the volatile low molecular substances which are resultant products due to thermal decomposition reactions, and components in original starting material. Density and insolubility measurements provide the estimation of mesophase fractions. Figures 6, 7 and 8 show the variations of weight loss, AW, density, p, and quinoline insolubles, QZ, with residence tike, respectively. No occurrence of the maxima in QZ plotted against residence time have been found within the experimental conditions. In a relationship between solubility and HT? for British bituminous coal, Wyne-*Jones et a1.[8] have found a maximum which is presumably due
OPTICAL
MESOPHASE
TEXTURE
AND
X-RAY
DIFFRACTION
PATTERN
185
1
0
Residence Fig. 5. Relation
to the more hand ment) weight
opposing
between
stacking
tendencies
15
10
5
height
of removal
time,
of lamellae time.
of the
soluble products of pyrolysis on one and polymerization (and/or rearrangeproducing material of higher molecular on the other.
4. DISCUSSION
4.1 Mesophase spherical bodies as studied by polarized light microscope For both of coal-tar and naphtha-tar pitches, general scheme of growth of mesophase spherical bodies as observed by polarized light microscope with the increase of HTT and/or residence time within the experimental conditions; (i.e. nucleation, growth, coalescence and mosaic structure; see Figs. 1 and 2) is as follows:
of’heat-treated
20
hr coal-tar
pitch and residence
As has been pointed out, it is observed that the naphtha-tar pitch heat-treated at 400”430°C has a clean surface of spherical bodies in various sizes. In a single mesophase spherule, the changes of the extinction contours (pleochroic phenomena) under crossed polarizers with gypsum plate are simple and almost the same as those presented by Brooks and Taylor[ll]. On the other hand, the coaltar pitch at the same temperature shows spherical bodies which are covered by tine irregular particles of insoluble matter. The changes of the extinction contours are more complex than those of naphtha-tar pitch. A lot of crosses and nodes, as classified by White et a1.[5], is observed even in a single spherule. When the insoluble matter is removed by solvent extraction or hot filtration, the spherical
186
H. HONDA
et al.
50
40
30
20
t
0
390 “c
@
LOO “c
@
410 “c
$
420°c
0
430%
( 10
f 0
0
5 Residence
Fig. 6. Relation
between
weight
390 410
Oc
.
430
*c
coal-tar
pitch and residence
time.
1
15
10
5
between
hr
I
Residence
Fig. 7. Relation
,
20
Oc
0
0
time
loss of heat-treated
1
0
15
10
true density
time
of heat-treated
,
20
hr
coal-tar
pitch and residence
time.
OPTICAL
MESOPHASE
TEXTURE
AND X-RAY
DIFFRACTION
PATTERN
187
60
Residence
t imc
,
hr
Fig. 8. Relation between quinoline insolubles of heat-treated pitch and residence time.
bodies turn into a type of naphtha-tar
pitch.*
Fine particles of insoluble matter of irregular shape, therefore, may play an important role in the mesophase transformation during the early-stages of carbonization. 4.2 Changes of physical properties A single smooth curve is obtained for various HTT’s and residence times when quinoline insoluble fraction is plotted against weight loss as shown in Fig. 9. This result shows that the time-temperature superposition principle operates over the restricted rangt of 390”-430°C shown in Figs. 4-8 respectively overlap quite well when *Experimental results and discussion in detail will be presented elsewhere.
coal-tar
they are shifted to appropriate residence time. For instance, in the master curve of L, for coal-tar pitch reduced at 410°C the reduced time scale extends from IO-* to IO” hr as shown in Fig. 10. Since all points lie on one common curve, it may be said that such a superposition method is adequate for pitches. In the process of deducing the master curves, the shift factors, a,, for various properties are evaluated from the amounts by which the individual curves at temperature T are shifted with respect to the residence time so as to fit the curve at a reference temperature T,,. In the range from 0.01 to 1 hr of reduced residence time, the changes of L,, d,, QI, AW and p are relatively small. This corresponds to the nucleation range of spherical
H. HONDA
188 100
et al.
-
0 4’
60
-
T 0
60
0
390
*c
Qp
400
‘c
0
410
“c
@
LZO “c
0
430
0 I @ 1
Y
op I
9”
-
,:
2
0’ 40
-
20
-
0-
0
t
0
I
10
20 Weight
loss
30 ,
40
%
Fig. 9. Relation between quinoline insolubles of coal-tar pitch and weight loss under the various conditions of heat treatment.
D-01
10
0.I
1,
18,
,ilr,
Fig. 10. Master curve for stacking height of lamellae of coal-tar pitch reduced to 410°C. bodies. On the other hand, drastic changes of L,, d,, QI, AW and p are observed in the
range from 1 to 40 hr of reduced residence time. This corresponds to the region of spherical body growth. At times longer than 40 hr, all of the quantities appear to level off This corresponds to the region of again. complete conversion to mesophase. From the relation between the reducing factor of time translation, uT, and the absolute temperature, T, the apparent activation energy, AE, can be derived by using the relation of ar = A exp(AE/RT), where UT is the conversion factor in residence time so as to obtain the same characteristics of the sample when the temperature changes from T to To, A is a constant, R is the gas constant and T is the absolute temperature (Fig. 11). The calculated values of apparent activation energy for various physical properties
OPTICAL 0.6
a,
0
TEXTURE
,
o-4
0.2
MESOPHASE
I
I
0
Lc
l
Weight
AND X-RAY
J; 6
loss
4
i
/
-
0
-
- 0.4
;/,
seems C-C of low for the
PATTERN
189
then that the rearrangement in bondings together with vaporizamolecular substances are responsgrowth of the spherical bodies.
/
J
-0-Z
It the tion ible
DIFFRACTION
,:
REFERENCES 1. Brooks J. D. and Taylor G. I-f., Carbon 3, 185 (1965); also C~e~~.~t~ and Physics of Carbon (Edited by P. L. Walker, jr.), Vol. 3, p. 243. Dekker, New York (1968). Ihnatowicz M., Chiche P., Deduit J., Pregermain S. and Tournant R., Carbon 4,41 (1966). Kipling _I. J. and Shooter P. V., Carbon 4, 1
(1966). 1.42
14.4
146
‘/J Fig. 11. Relation
between
148
1.50
x lo3 shift factor and l/T.
are 35-45 kcal/mole. It is interesting to note, on the other hand, that the value of AI2 estimated from the change of quinoline-insolis 39 kcal/mole assuming uble fraction carbonization reaction is of the first order.
Marsh H. and Stadler H. P., F&46,351 (1967). White J. I., Guthrie G. L. and Gardner 1. O., Carbon 5,517 (1967). White j. I., Duboirs j. and Souillart (:., European Atomic Energy Community-Euratom Report No. EUR 4094e (1969). 6. Mirberg M. E.,J. A#. Phys. 29,64 (1958). 7. Blayden H. E., Gibson j. and Riley H. L., Proc. Conj U~tra~ne structures of Coals and Cokes, p. 176. BCURA, London (1943). 8. Wynne-Jones W. F. K., Blayden H. E. and Shaw F. H ., Brennsto&%vz. 33,201 (1952).
H. KIMURA,
Y. SANADA,
Resources Kesearch
Institute,
S. SUGAWARA
and T. FURUTA
Ka\~~agrlchi-S;~it;~r~i~~.,lap;~n
AbstractLow temper;iture carbonization of pitches was studied with polarized-light microscopic method, X-ray diffraction, solvent extraction and density measurements. The nucleation and growth of spherical bodies occur progressively with increase of residence time at a fixed temperature, until the spherical bodies eventually coalesce with each other. Fine irregular particles of insolubles in coal-tar pitch are observed in the region of boundary between spherules and matrix. They play an important role in the mesophase transformation. The typical changes of stacking height of lamellae, interlamellar spacing, weight loss, density and insolubility with residence time show nearly the same tendency at the various temperatures from 390” to 430°C. The time-temperature superposition has been successfully employed. In the master curve for coal-tar pitch reduced at 41O”C, the reduced time scale extends from lo-” to lo” hr. From the relation between the reducing factor of time translation and absolute temperature, the apparent activation energies can be estimated over the range of 35-45 kcal/mole. It seems probable that a model of the rearrangement of G-C bonding together with vaporization of low molecular substances is applicable for the growth of spherical bodies. 1. INTRODUCTION
Low temperature carbonization of pitches has been studied extensively using polarizedlight microscopic method and reviewed by Brooks et al. and several other groups[ld]. Brooks and Taylor[l] observed that the formation of graphitizing low-temperature carbons by solidification from a liquid phase proceeds through the separation of a mesophase spherical bodies. With increase of ternperature, the spheres grow up and coalesce. They have also observed that the spheres grow larger to a certain extent with increase of time at a fixed temperature. It has been clarified by Ihnatowicz et aZ.[2] that the insolubility of the spherical bodies in the solvent and in the isotropic phase accounts fol the relation between the solubility measurements and the microscopic appearance. It appears, however, that very few detailed *Present
address:
search Institute,
Government
Industrial
Kyushu, Tosu-Kyushu,.Japan.
investigations of the development of the characteristic anisotropic mosaic texture with residence time in the neighbourhood of 400”-450°C have yet been made. The present work has been done in an attempt to get the relationship between development of mesophase spherical bodies, which later become units of mosaic texture, and X-ray diffraction or some physical properties of pattern pitches. 2. EXPERIMENTAL
(:oal-tar pitch and naphtha-tar pitch were selected for these studies as samples of typical type of pitches. The elementary composition, the softening temperature and the quinoline insolubles -of the pitches were determined by routine methods. The results are shown in Table 1. ‘I‘he pitches were carbonized in a vertical silica tube within a heated cylindrical furnace Kehaving automatic temperature control. The 181
H. HONDA et al. Table 1. Characteristics of pitches used Elementary analysis (%)
Coal-tar pitch Naphtha-tar pitch
C
H
N
S
O(diff.)
sp* (“C)
91.9 95.0
4.7 5.0
I.4 -
0.3 -
1.7 -
76.5 -
YIt %
BIS
3.5 1.8
16.1 -
%
*Softening Point (ring and ball method (JIS)). IQuinoline insolubles. $Benzene insolubles.
temperature of carbonization had to be maintained constant within -+l”C, since the mesophase transformation and its hardening process are quite sensitive to temperature and time. All carbonization runs were carried out in an inert atmosphere, usually pure nitrogen gas. Specimens were heated with the heating rate of S”C/min and maintained for desired residence time from 0 to 20 hr at a constant temperature in the range from 350” to 500°C. The heated material was allowed to cool to room temperature before examination. Microscopic observation was done with a Leitz Ortholux microscopic using reflected polarized light under Xenon arc illumination. The polished surface of the specimen moulded with epoxy resin was prepared by using Carborundum papers and woolen cloth with fine powders of alumina and chrome oxide. The X-ray diffraction patterns of powdered specimens were obtained by using a recording diffractometer with Ni-filtered C,K, radiation. The diffraction intensities were corrected for the Lorentz polarization and the absorption factors according to Mirberg’s equation[6]. The height of hypothetical cylinders (crystallites) of stacked lamellar molecules in a random layer structure and the inter-lamellar (interlayer) spacing were obtained from the half width of the diffraction profile and the position of reflection in the vicinity of 28 = 25”, respectively. Changes of sample weight with residence time at a constant temperature were deter-
mined from the weight difference before and after heat treatment. True densities were determined picnometrically at 25VC with distilled water containing 0.01 per cent of sodium oleate as a detergent. Solvent extractions with quinoline were followed by the instruction manual of Japanese Industrial Standards (JIS K2421-1966). Quinoline was used without further purification of guaranteed grade reagent.
3. RESULTS 3.1 General de.scr$tion of changes observed with polarized light microscope The present work shows that the appearance of spherical bodies and the development of anisotropic mosaic texture are recognized as functions of residence time and heat-treatment temperature (HTT). The nucleation and growth of spherical bodies occur progressively with increase of residence time until the spherical bodies eventually coalesce with each other, with complete disappearance of the isotropic liquid phase. The spherical bodies grow to 10-100~ in diameter with increase in residence time. It is observed that there are some differences in the growth of spherical bodies between coal-tar pitch and naphtha-tar pitch as’ shown in Figs. 1 and 2. The coal-tar pitch heat-treated at 400”430°C has spherical bodies of nearly the same size, which are covered by fine irregular par-
Fig. 1. hfesophase spherules times at 41OY; (plane-polarirecl
in coal-t:u. pitch for various residence light): (a) !2hr; (b) 1 hr; (c) 8 hr; (cl) I3 hr (370 X ).
Fig. 2. Mesophase spherules in naphtha-tar pitch for various residence times at 400°C (plane-polarized light): (a) 1 hr; (b) 1.5 hr; (c) 3 hr; (d) 5 hr (370 X ).
3.2 X-ray dajkxtion The changes of inter~dmellar spacing, d,, and stacking heigbt of lame&e, L,, with residence time at a fixed temperature are shown in Figs. 4 and 5, respectively. The fact, that only one reflection [i.e. (002) band corresponding to stacked aromatic tamellar molecules in a random layer structure of pitch] was utibzed for crystallite size evafuation, places doubt upon the absofute accuracy of the height of srdcked lamellar and the inter-lamellar (interlayer) spacing. irrespective of the absolute ln~~gnitude of the crystallite parameter, the results draw atten-
ticks of insaluble matters. The insoluble matters aggregate locally in the bulk mesophase or the mosaic struct.ure. On the other hand, the naphtha-tar pitch heat-treated at the same tem~)eratLlt~e r;inge has a clean surface of spherical bodies in various sizes, as the same with the spherical bodies in toluenesoluble extract of the pitch shown by Brooks f-&al.[l]* Figure 3 shows the relations among the nucleation, growth and coalescence of spherical bodies in naphtha-tar pitch heat-treated under various temperatures and residence times.
7
Naphtha-tar
I I
X
No
0
Appoorontr
0)
Growth
pitch
spherical
bodies af
of
spherical
sphericat conversion
bodies bodies to
mesophasr
-J--
1 / / i
to
012345 Reridrncc
I5
time ~
20
kr
Fig. 3. Relations between the nucleation, tar pitch heat-treated
growth and coalesoence uf‘spherical bodies in naphthaat variws temper~~ttlres and residence times.
It is clear frr,m the Fig. 3 that nucleation and growth of spherical bodies can be observed at a low temperature such as 350°C when residence time is long enough.
tion to the behaviour of structural change at early stage car~njzation. The d, decreases progressively with increase in the residence time and approaches to 3435 _Ain value. The
Fig. 4. Relation between interlamellar
H. HONDA
et al.
Residence
time
spacing of heat-treated
higher the temperature is kept, the faster the value of d, decreases. The same behaviour of L, under various temperatures and times have been observed as shown in Fig. 5. Blayden et al.[7] showed the maxima in the L, values of the British coals and pitch plotted against heat-treatment temperature. They considered the maxima in the L, values to be intimately associated with the fluid state through which these samples pass on carbonization. However, no occurrence of the maxima in the L, plotted against residence time have been found within the experimental conditions in the present work. 3.3 Weight loss, density and solvent extraction As carbonization progresses, spherules, which are more dense
,
mesophase than the iso-
hr
coal-tar pitch and residence time.
tropic parent phase, grow together with the vaporization of low moleculk substances. Weight loss measurements reflect the fractions of the volatile low molecular substances which are resultant products due to thermal decomposition reactions, and components in original starting material. Density and insolubility measurements provide the estimation of mesophase fractions. Figures 6, 7 and 8 show the variations of weight loss, AW, density, p, and quinoline insolubles, QZ, with residence tike, respectively. No occurrence of the maxima in QZ plotted against residence time have been found within the experimental conditions. In a relationship between solubility and HT? for British bituminous coal, Wyne-*Jones et a1.[8] have found a maximum which is presumably due
OPTICAL
MESOPHASE
TEXTURE
AND
X-RAY
DIFFRACTION
PATTERN
185
1
0
Residence Fig. 5. Relation
to the more hand ment) weight
opposing
between
stacking
tendencies
15
10
5
height
of removal
time,
of lamellae time.
of the
soluble products of pyrolysis on one and polymerization (and/or rearrangeproducing material of higher molecular on the other.
4. DISCUSSION
4.1 Mesophase spherical bodies as studied by polarized light microscope For both of coal-tar and naphtha-tar pitches, general scheme of growth of mesophase spherical bodies as observed by polarized light microscope with the increase of HTT and/or residence time within the experimental conditions; (i.e. nucleation, growth, coalescence and mosaic structure; see Figs. 1 and 2) is as follows:
of’heat-treated
20
hr coal-tar
pitch and residence
As has been pointed out, it is observed that the naphtha-tar pitch heat-treated at 400”430°C has a clean surface of spherical bodies in various sizes. In a single mesophase spherule, the changes of the extinction contours (pleochroic phenomena) under crossed polarizers with gypsum plate are simple and almost the same as those presented by Brooks and Taylor[ll]. On the other hand, the coaltar pitch at the same temperature shows spherical bodies which are covered by tine irregular particles of insoluble matter. The changes of the extinction contours are more complex than those of naphtha-tar pitch. A lot of crosses and nodes, as classified by White et a1.[5], is observed even in a single spherule. When the insoluble matter is removed by solvent extraction or hot filtration, the spherical
186
H. HONDA
et al.
50
40
30
20
t
0
390 “c
@
LOO “c
@
410 “c
$
420°c
0
430%
( 10
f 0
0
5 Residence
Fig. 6. Relation
between
weight
390 410
Oc
.
430
*c
coal-tar
pitch and residence
time.
1
15
10
5
between
hr
I
Residence
Fig. 7. Relation
,
20
Oc
0
0
time
loss of heat-treated
1
0
15
10
true density
time
of heat-treated
,
20
hr
coal-tar
pitch and residence
time.
OPTICAL
MESOPHASE
TEXTURE
AND X-RAY
DIFFRACTION
PATTERN
187
60
Residence
t imc
,
hr
Fig. 8. Relation between quinoline insolubles of heat-treated pitch and residence time.
bodies turn into a type of naphtha-tar
pitch.*
Fine particles of insoluble matter of irregular shape, therefore, may play an important role in the mesophase transformation during the early-stages of carbonization. 4.2 Changes of physical properties A single smooth curve is obtained for various HTT’s and residence times when quinoline insoluble fraction is plotted against weight loss as shown in Fig. 9. This result shows that the time-temperature superposition principle operates over the restricted rangt of 390”-430°C shown in Figs. 4-8 respectively overlap quite well when *Experimental results and discussion in detail will be presented elsewhere.
coal-tar
they are shifted to appropriate residence time. For instance, in the master curve of L, for coal-tar pitch reduced at 410°C the reduced time scale extends from IO-* to IO” hr as shown in Fig. 10. Since all points lie on one common curve, it may be said that such a superposition method is adequate for pitches. In the process of deducing the master curves, the shift factors, a,, for various properties are evaluated from the amounts by which the individual curves at temperature T are shifted with respect to the residence time so as to fit the curve at a reference temperature T,,. In the range from 0.01 to 1 hr of reduced residence time, the changes of L,, d,, QI, AW and p are relatively small. This corresponds to the nucleation range of spherical
H. HONDA
188 100
et al.
-
0 4’
60
-
T 0
60
0
390
*c
Qp
400
‘c
0
410
“c
@
LZO “c
0
430
0 I @ 1
Y
op I
9”
-
,:
2
0’ 40
-
20
-
0-
0
t
0
I
10
20 Weight
loss
30 ,
40
%
Fig. 9. Relation between quinoline insolubles of coal-tar pitch and weight loss under the various conditions of heat treatment.
D-01
10
0.I
1,
18,
,ilr,
Fig. 10. Master curve for stacking height of lamellae of coal-tar pitch reduced to 410°C. bodies. On the other hand, drastic changes of L,, d,, QI, AW and p are observed in the
range from 1 to 40 hr of reduced residence time. This corresponds to the region of spherical body growth. At times longer than 40 hr, all of the quantities appear to level off This corresponds to the region of again. complete conversion to mesophase. From the relation between the reducing factor of time translation, uT, and the absolute temperature, T, the apparent activation energy, AE, can be derived by using the relation of ar = A exp(AE/RT), where UT is the conversion factor in residence time so as to obtain the same characteristics of the sample when the temperature changes from T to To, A is a constant, R is the gas constant and T is the absolute temperature (Fig. 11). The calculated values of apparent activation energy for various physical properties
OPTICAL 0.6
a,
0
TEXTURE
,
o-4
0.2
MESOPHASE
I
I
0
Lc
l
Weight
AND X-RAY
J; 6
loss
4
i
/
-
0
-
- 0.4
;/,
seems C-C of low for the
PATTERN
189
then that the rearrangement in bondings together with vaporizamolecular substances are responsgrowth of the spherical bodies.
/
J
-0-Z
It the tion ible
DIFFRACTION
,:
REFERENCES 1. Brooks J. D. and Taylor G. I-f., Carbon 3, 185 (1965); also C~e~~.~t~ and Physics of Carbon (Edited by P. L. Walker, jr.), Vol. 3, p. 243. Dekker, New York (1968). Ihnatowicz M., Chiche P., Deduit J., Pregermain S. and Tournant R., Carbon 4,41 (1966). Kipling _I. J. and Shooter P. V., Carbon 4, 1
(1966). 1.42
14.4
146
‘/J Fig. 11. Relation
between
148
1.50
x lo3 shift factor and l/T.
are 35-45 kcal/mole. It is interesting to note, on the other hand, that the value of AI2 estimated from the change of quinoline-insolis 39 kcal/mole assuming uble fraction carbonization reaction is of the first order.
Marsh H. and Stadler H. P., F&46,351 (1967). White J. I., Guthrie G. L. and Gardner 1. O., Carbon 5,517 (1967). White j. I., Duboirs j. and Souillart (:., European Atomic Energy Community-Euratom Report No. EUR 4094e (1969). 6. Mirberg M. E.,J. A#. Phys. 29,64 (1958). 7. Blayden H. E., Gibson j. and Riley H. L., Proc. Conj U~tra~ne structures of Coals and Cokes, p. 176. BCURA, London (1943). 8. Wynne-Jones W. F. K., Blayden H. E. and Shaw F. H ., Brennsto&%vz. 33,201 (1952).