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Electron spin resonance in turbocrystalline carbons—II
Cwbmt 1968, Vol. 6, pp. 841-856.
ELECTRON
Pergamon Press.
SPIN
Printed in Great Britain
RESONANCE IN CARBONS-II*
TURBOCRYSTALLINE
s. MROZOWSKI Carbon Research Laboratory and Department of Physics, State University of New York at Buffalo, Buffalo, New York 14214 (Received 27 November 1967)
Abstract-In continuation of the work on ESR in turbostratic carbons, experiments on introduction of boron acceptors and sodium donors were performed using P33 carbon black samples heat treated to various temperatures mainly in the range 1600-2400°C. The experiments with boronation show a complicated behavior and lead to the conclusion that boron atoms can act not only as acceptors, but neutralize some of the existing localized spin centers and also can under special conditions become spin centers themselves. Experiments with sodium doping have shown that the stability of the sodium against oxidation increases with decrease in HTT. With increasing doping the “Curie-like” behavior (the factor R = ILN/IRT) goes through a maximum and the intensity of the ESR line through a minimum, as expected from the simple band model with the Fermi level originally located below, and moving across and then above the band overlap with increase in doping. Usingtwo spectrometers 9.2 Gc and 35 Gc and a variety of samples it was found that while for low heat treatments the widths are the same (in Gauss), for well graphitized materials the use of the higher frequency results in a considerably broader line. Considerations concerning the temperature and heat treatment dependence of the Ag,,,, and its relation to the diamagnetic susceptibilitv, are presented. The nature of the band structure in turbostratic carbons _ and of-the localized spin centers are discussed. 1. INTRODUCTION IT HAS
provided valuable
been shown by an exhaustive
in the g-value
study of the
information
spreading
due to both
Part I of this worko)
spin centers, line
being
exchange
conduction
the single g-value the
mixing
further
carriers
result
of intensity
contributions
planes
has been applied to two carbon
of an
Thermax,
It was
temperatures
(M-effect).
permits to determine
the
of both kinds of spin centers
and
caused and by
(S-effect).
In
the same kind of analysis
presence
for a wide range
blacks,
P33 and
of heat-treatment
(1400-3000°C)
neutron irradiation. While for partially graphitized
shown that a study of the temperature
dependence
of the graphitic
of the composite
of the
mechanism
and localized
carriers
by the shift of the Fermi level (F-effect)
in a P33 carbon black heat treated to 2600W1* *) that the single absorption line is
ESR
as to the changes
of the conduction
before
and
after
it was expected that materials (HTT >
2200”) such analysis will lead to results not much
thus to obtain information as to the influence of various factors, such as neutron irradiation and
differing
doping
work was to check the applicability of the analysis to carbon materials in a turbostratic polycrystalline state. The results reported in Part I@) leave no doubt as to the correctness of
each
with donors of
these
or acceptors
resonances.
Such
separately an
on
analysis
*Supported by the U.S. Office of Naval Research. Reproduction in whole or in part is permitted for any purpose of the U.S. Government. 841
HTT
from
26OO”W*
the
former
2), the
ones
main
for
purpose
the
P33
of this
this expectation. It has been found that both the contributions of conduction carriers and of
842
S. MROZOWSKI
localized centers increase greatly for lower heat treated samples, the Agcond= g-gr,,, for the conduction carriers changing at room temperature in proportion to the Landau diamagnetism XL. Neutron irradiation causes, similarly as in the case of graphitized samples, a large increase in concentration of localized spin centers, a relatively much smaller one in the density of the conduction states n(EF) with an interesting indication of a possible saturation of n(EF) however (slowing down of intensity increase with lowering of the Fermi level) for the lowest heat treated samples. The band structure of turbocrystalline carbons not being understood yet@) it is necessary to try all avenues of approach. The ESR technique as developed here being a most direct way of obtaining such vital quantity as the density of states n(EF) at the Fermi level, the experiments were continued and extended to a study of the influence of donors and acceptors on the ESR in turbostratic carbons. All this work was performed using the apparatus and techniques described in Part I.@) While this work was in progress, in addition to our regular 3 cm spectrometer, an 8 mm spectrometer became available. Thus a few supplementary comparative checks of the width of the lines were carried out. 2. DOPING
WITH BORON
The technique used for introduction of boron was the same as previously.ca) Boric acid powder was mixed with the carbon black solid chunks (use of powder results in much broader line(a)) and heated to a temperature 1650-1700°C. While heated the boric acid melts and soaks into the carbon solid forming a very hard glass; it is only above 1500°C that it decomposes and the free boron starts diffusing into the lattice. For graphitized samples even at 1600” the diffusion is not yet complete (not much shift in g-value) and the line is relatively broad. The best results were obtained for heating between 1700” and 1800”. This method of introduction is a very good one since the boron is introduced after the carbon structure has been formed. thus ner-
mitting a direct comparison between the original and doped material. However, for the same reason materials heat treated below 1700°C were not investigated-it was feared that the second heating to 1650°C might cause additional heat treatment. Only P33 carbon black was used in this part of the study. It was found in the previous work(2) that as boron is introduced into a material heat treated to 2600°C in increasing amounts and the ESR line shifts gradually towards gf, the intensity of the line after a short flat portion (Fig. 1, ref. 2) begins to climb faster and faster as g, is approached. The intensity increase is due to the dropping of the Fermi level down and away fi-om the band overlap, thus leading to an increase in the density of states n(EF) which determines the contribution of the carriers to the line intensity. The increased contribution of carriers brings about a change in the temperature dependence of the intensity from a slightly Curie type (ratio R = &/I,, of about 1.4) to a pure conduction type (R = 0.9-1.0). What was however surprising, is that this drop in ratio R occurs quickly at the beginning in the region of the plateau, thus making necessary the assumption that boron atoms not only act as acceptors for conduction carriers, but that in the they also beginning stages of boronation neutralize in some way the localized spin centers present in the material. While it was found in the present work that all samples of P33 heat treated to 2200°C or higher show a similar response to boronation as above, the ESR intensity for materials in turbocrystalline state even after a strong boronation does not increase at all. P33 carbon black heat treated to 2200°C (boronated at 1700°C) and to 1700°C (boronated at 1650°C) were particularly investigated. As the boron is introduced in increasing amounts the line broadens considerably at first (by a factor 2 to 3) but then the trend becomes reversed and at high boron contents the line reverts almost to the original width (factor 1.O to 1.5). This is a behavior which closely narallels the one found for nolvcrvstailine
ELECTRON
graphite
(Fig.
7, ref. 2),
with
temperature
to g,
(the
> 1 to
the change
reversing
factor
r =
producibility scatter,
wLN/wRTchanging
from
at a somewhat in width
of various
by
25~40%
the
tendency
or the
to the
other
up
On the other hand, additions
quickly
intensity
earlier data
the
from
in the initial the localized
almost constant Since again
stages boron atoms neutralize
that
some of
spin centers without creating holes in the spin centers are internal
lattice vacancies,
or
then this neutraliza-
with
boron
atoms
(filling
an existing
in the electronic pensated
by an increase
boron
the experiments
atoms
localized
can
not
spin centers
only
com-
absorption
become a source of local&d number
of instances
very sharp ESR a high relative showing
intensity
(3-4
up to 2.0). It is now believed
to 1.0, but the 3.8.
Thus
and neutral
boron
I
=
into acceptors
and
the
subsequent
reheat
remaining
then
to indicate
at this time.
The
results boron
obtained
seem
introduced
or graphite
several different
into
polycrystalline
structure
can
behave
ways. The interpretation
in
of the
observed effects is therefore complex and the boronation method will not become useful for the study of the band structure
until the process
of boronation
in detail
meaningful
is investigated
technique
and
of separation
of a reproducible
types of spin centers
and
a
of the various
introduction is worked
3. DOPING
of
out.
duction
and
experiments
driving
out
graphitized
depresses
existing
overlap permits
a
with
times the original) (ratio
WITH SODIUM
R
that the R anomaly
black
the Fermi region,
boronated
the
while the Fermi
above
the band
from
carbon
a
black
Since boronation
level away from the band
subsequent
observing
with intro-
of sodium
(P33 26OO’C) were described.
boronations
behavior
and
but also
1G) was obtained
Curie-like
the
gas the
the Fermi level, the action of exposure
partially
centers themselves. In a
for heavy
line (<
a partial
neutralize
to
spin centers
In an earlier pape0
seem to show that
in the material,
to air
that
increased
must have changed
depressed
of holes
caused by other boron atoms acting as acceptors. However,
groups
After reheating
in flow of Nitrogen
to 3G, R dropped
boron
gf
2.5 (relative
vacancy
about
in carrier
localized
I =
material).
intensity
definite
band !). The decrease in localized
spins at later stages has to be just
total
line (3/4G ) at g z
intensity
to 1200%*
effects
tion would be the result of filling the vacancies with boron does not increase the number
to the nondoped
carbon
the
spins in turbostratic
one has to assume
band. If the localized peripheral
to
1.7 and
line broadened
843
CARBONS-II
a very sharp
with R =
obscure
1.9 down
boronations.*
of localized
is large,
boronations.
the ratio R for small boron
decreases
heaviest
concentration carbons
not
to change one way highest
1.2, and seems to remain
to
does
produced
lumps
and poor re-
runs the intensity
show any definite
up
dependence
but it is clearly seen that after an initial
decrease
about
in width close
reversing
Due to variation
IN TURBOCRYSTALLINE
for g-values
< 1) and the temperature
of the g-value stage.
SPIN RESONANCE
doping with sodium ESR
level
overlap
of
the
carbon
moves across and
and again
also on its
way back to the depressed position as the sodium is being driven out. The Fermi level being depressed in turbostratic carbons even without boronation
it was the purpose
of this work to
observed in the previous work at Ag = 40 x IO-4 (Fig. 2, ref. 2) must have a similar origin.
carry out the same type of observations
on these
carbons
hope
The most instructive was a heavy boronation of material HTT 2900°C (R = 1 .l) which
obtaining
*Results in some ways similar to ours, but less definite and somewhat confusing were obtained by DELHAES and MARCHAND on boronation of a soft coke.
*Reheat of heavily boronated lumps does not broaden the line. Evidently the abundance of boron protects the surface from contact with air and from the ensuing Hennig-Smaller broadening.
(without
boronation)
information
of turbostratic
carbons.
in the
of
as to the band structure As shown
in Part
I
844
S. MROZOWSKI
turbostratic carbons possess large concentration oflocalized spin centers which presumably are not affected by sodium doping (Table 5, ref. 1). Should carbon in turbostratic state possess an energy gap a temperature behavior of the ESR little differing from a Curie type (R = 3.65) would be expected when the Fermi level is located exactly between the two energy bands. In any case as the Fermi level moves across from one to the other band a minimum in ESR intensity accompanied by a maximum in the ratio R would be expected.
sodium. Extremely small amounts of sodium can be introduced by this technique-amounts which form across the entrance to the capillary C a barely visible deposit (causing just a change in the tint of the glass). The reaction is performed by heating the chamber P to a temperature at which the deposit disappears. The sample at the bottom of the tube P is being shaken while heated so as to secure a uniform distribution ofsodium throughout the powder. After that the sample is returned into the tube S, new ESR measurements performed and the operation repeated. After a relatively large total amount of sodium is introduced sticking of particles causes a decrease in observedintensity by increased conduction (see pp. 235-237, ref. 2). To correct for such effect about a third or a quarter of the sample is transferred into the arm T, separated from the rest by sealing off, opened and the ESR intensity quickly measured before and after grinding in a mortar. After all the sodium needed is introduced into the main sample, air is admitted by breaking off the tip of the container M and as sodium is oxidized the gradual return of the Fermi level towards the original position is observed. The driving-out is achieved in steps by heating the sample in air to increasing temperatures (up to 500°C). Some driving-out series were performed in vacuum, without opening the sample to air but such procedure is much less effective. Figure 2 shows the results of such doping runs on P33 of HTT 2000°C. In distinction to the case of boronated P33 of HTT 26OO”C, starting from the original line (#O) introduction of sodium not only decreases the intensity at first, but also causes a slow shift of the line towards g,. After the line has shifted about one third of the way towards g, the continuity of the motion is interrupted by a sudden jump in the g-value resulting in a double line (#3). The intensity then continues to be transferred to the lower gvalue line (#4) and the total intensity continues to decrease almost close to the gf value. An intensity minimum is reached at Ag z 8 x IO-* (#5) after which the line starts quickly
_-y. :i_l ---.,’
T
GLASS
QUARTZ
\
:s FIG. 1. Apparatus for ESR study of introduction and driving out sodium under vacuum. S-carbon sample, M-sodium container, P-reaction chamber, C-capillary through which small amounts of sodium are distilled into the reaction zone, T-side tube for separating a fraction of reacted carbon.
The experiments were performed using sample tubes as shown in Fig. 1, being a somewhat improved version of the earlier 0nes.o) After the ESR of the nondoped sample was measured at room and liquid Nitrogen temperature a very small amount of sodium was distilled from the container M into the chamber P where the sample S was transferred and reacted with the
ELECTRON
SPIN RESONANCE
I I
\
IN TURBOCRYSTALLINE
P33
(2000°HTT)+
845
CARBONS-II
Na
FIG. 2. Changes in ESR line position and intensity for a P33 carbon black (HTT 2000°C) when sodium is introduced in steps (l-8) and when it is driven out by heating in air, also in steps (2’-6’).
gaining return
at further
in intensity trip (driving
minimum
is not reached
to the original
doping.
out of sodium)
position
until
(#5’)
On the
The effort was made to determine
the intensity
erature
dependence
the line returns
sodium
introduction.
and is not as deep
determine
the ratio
of
R
While it at the start
after was
at the end of the doping series (#8,
the sample
exceeding
intensity remnants
could
at the end, the original
not be restored-evidently
of sodium
are
well
trapped
the in the
1.2),
determination cause
one
of the unsymmetric
geneity)
ing-out process supports the previous contention(a)
the signal (factor
that when driven-in the sodium preferentially goes between the layers, but in driving-out the
considerably material
(factor
atoms lodged between
variation
(1.3)
and which
do not lower the g-value by the S-effect, last ones to leave.*
are the
*The so&urns go preferentially between layers of those microcrystals which are under c-directional tensile stress. After the body has sufficiently expanded, microcracks release some of these stresses and the intercalated sodiums become more easily removable. CARBON 6/6-o
get
structure
greater 1.5).
than
for
(#5),
the
doping
in the width
be-
show clearly
of the height
It decreases
at further
good
(inhomo-
2.0) at the minimum
ever the changes z
variation
much
a
stages,
on the line. Observations
that the temperature
important material
not
at the intermediate
material and cannot be easily driven out. The lack of reversibility in the driving-in and driv-
the crystallites
could
each
easy to 2.0) and
(R = R not
as on its way down to gf. No matter how strongly was heated
the temp-
intensity
of is
undoped
to a smaller (#8).
which
How-
are more
(r = wLN/wRTis for the original 1.15 and for a strongly doped
one z 0.9) could sufficient accuracy
not be pinned to be absolutely
down with sure of the
presence of a maximum in R at the intensity minimum. Similarly the results on the return
846
S. MROZOWSKI
trip did not turn out satisfactory for such a demonstration, especially since the return was never complete. Much more meaningful results were obtained using P33 heat treated to 16OO”C, because of the greatly reduced spread in g-values between the original and the doped material. In Fig. 3 the dependence of R and I on doping as obtained in one of the series of experiments is presented. In introducing sodium a definite peak in R is observed before the intensity minimum is reached. The delay in reaching the intensity minimum is understandable since in transferring the sample from the tube S to the reaction
P33
IN
INCREASING 0.
(1600’HTT)
chamber P and back more and more of the powder sticks to the walls and thus the quantity of carbon black in the tube 5’ is continuously decreasing. At later stages sticking of the particles diminishes the signal, and had to be corrected for. No matter how carefully the dopings were performed in trying to keep the distribution of sodium uniform throughout the carbon black, under no condition were R values at the maximum greater than 2.3-2.4 obtained. After opening the sample in driving-out the sodium, no such large maximum in R was found, indicating even less homogeneity in the sodium distribution throughout the material.
+
No
VACUUM
IN
AMOUNT .
OF
Na
HEATING
AIR
IN AIR
-
FIG. 3. Changes in ESR line intensity and in the ratio R for a P33 carbon black (HTT 16OO’C) as sodium is stepwise introduced and then driven out by heating in air.
ELECTRON
SPIN
RESONANCE
IN TURBOCRYSTALLINE
On the way back the intensity goes through a minimum, and again at the end is not returning up to the original intensity. In general the stability of the sodium increases with decrease in HTT: while it can be driven out by oxidation from graphitized samples relatively easily the remnants refuse stubbornly to be removed from their hiding in the turbostratic carbons and increasingly so as the crystal size diminishes. 4. STUDlES
OF THE WID’PH AND OF THE g-FACTOR
When a 35 Gc spectrometer of Varian (Model V4502-13) b ecame available a large number of samples was checked for the line width on both spectrometers in the hope of obtaining some information as to the origin of the width under various conditions. It was found that while for the lower heat treatment temperatures the width of the line in Gauss is exactly the same*, for higher heat treatments as the HTT increases the width recorded using the Q band spectrometer increases in relation to the width observed on the 9.2 Gc spectrometer. On the 35 Gc spectrometer, frequently a complicated structure in the line can be observed for samples of HTT ZOOO-2400°C which originates from small inhomogeneities in the heat-treatment temperature. This is the region of the steepest variation of the g-value with HTT. The total change in Ag covering a 3.8 times wider range in Gauss than for the X-band spectrometer results in a much greater resolution in view of almost the same line widths. Heat treatment of powders in Iarger containers always *In the course of comparisons of the g-values on both spectrometers, it was discovered that our homemade X-band spectrometer gives consistently larger Ag-values, probably due to a faulty calibration of the sweep amplitude. Thus all the X-band widths have been multiplied by the proper factor of 0.88. In view of this discrepancy we do believe that all our previous data for Ag and width should be also corrected by the same factor. (To avoid confusion however this correction was not introduced in Figs. 2, 5-8 of this paper.)
CARBONS-II
847
leads to such structure, good results (no structure) being obtained only when a small grain (lump) of the solid carbon black is picked out from the container. While the width of the lines is not well reproducible, varying from one to another heat treatment run (see section 4, Part I, ref. 3), the ratio of the widths as obtained on both spectrometers (p = wQ/wx) does not depend on the absolute value of the width and seems to be a characteristic parameter varying with the heat-treatment temperature. The results are presented in Fig. 4, where the inhomogeneity region is indicated by the dotted curve. BeIow a curve of minimum widths as obtained on the & band spectrometer is given. On Fig. 5 the same data are plotted in relation to the corresponding Ag values with results added for irradiated and doped materials (mostly P33). As noted in Part I neutron irradiation decreases the width when HTT> 2000°C and increases it when HTT <2OOO”C. The comparison of widths shows that samples irradiated retain (at least partly) the p value of the origina material-this seems to indicate that p > 1 is caused by increasingly poor directional averaging as the crystallite size increases, the slight decrease in p occurring as a result of higher admixture of localized spins (&f-effect). On the other hand in case of dopings which bring the line close to gf through the F-and S-effects, sometimes a single line is obtained with p = 1 as expected, but frequently structures are observed on the 35 Gc spectrometer showing a nonuniform distribution of the doping material. The results also show that broadening caused by heat treatment in powder form is definiteIy not a g-inhomogeneity (Fig. 4). Using both spectrometers the line shape and width were measured for the French GFEC coke heat treated to 2000°C for 5 min, 20 min and 5 hr. The strongly asymmetric Iine showing correspondingly a width of 7.5,8.5 and 15 G on 9.2 Gc, shows the same asymmetric shape on the 35 Cc spectrometer, but much greater widths of 22, 25 and 40 G. The ratio p of the
I
I
1400
I I
1600 HEAT
I
I
2200 TREATMENT
I
2600 TEMPERATURE (“Cl
I
I
3000
I
A
FIG. 4. Plot of the ratio of widths as obtained on two spectrometers vs. heat treatment temperature for varions samples of P33(0) and Thermax( A). P means powder, L-heat treated in Inmp form. The continuous curve is drawn through the minimum p-values obtained. In the lower part of the figure the lowest values of the width as observed on the 8 mm spectrometer are plotted.
I
,o
/
A
ELECTRON SPIN RESONANCE IN TURBOCRYSTALLINE
CARBONS-II
849
3
l6OOfNa
I
0
I
I
I
40
80 Pg = g -gf
I
I
120
ix to41
Fm. 5. Plot of the ratio of widths as obtained on two spectrometers vs. the observed position of the line for variously heat treated, irradiated and doped samples of P33 and Thermax. widths being about 2.9, not far from the ratio of the magnetic field strengths (3.81, most of the broadening is caused by a g-inhomogeneity. Introduction of sodium makes the line much sharper (factor < 4) and the shape symmetric; even after most of the sodium is driven out and the line reverts to its original g-value, it still retains its symmetrical shape, the intensity continuing to decrease as the line gradually begins to show some hornets (no intensity minimum observed). This seems to support our views expressed in Part I that in the ESR of soft type carbon materials, part of the intensity is lost in the wings as a result of g-inhomogene&y. 4.2 Ballmilling effect Having found that the 35 Gc spectrometer is
such an excellent and sensitive tool for checking the uniformity of the material, carbon blacks were compared after prolonged ballmilling in air (2 weeks). The ESR of the raw Thermax black did not show any change caused by such treatment (no change in g-value nor width) but Thermax graphitized at 2500°C did; the width increased by a factor of 2.6 and the gvalue decreased slightly from 2.0135 to 2.0126, thus indicating some destructive influence of grinding, which could not be detected by direct electron microscopic observation.(e) 4.3 Anneal of irradiated samples It has been demonstrated time and again that even quite heavily irradiated polycrystalline graphites regain the original structure after anneal to temperature above 1500°C. Since it
850
S. MROZOWSKI
was found in Part I that the stability of damage against anneal increases as lower heat treated materials are irradiated, irradiated P33 samples of HTT ranging from 1600°C to 2600°C were annealed together with nonirradiated samples to temperatures from 2000” to 2600°C. Although the g-values change strongly in this range with HTT no differences in g-value (nor width) greater than f 0.000015 were found on the 35 Gc spectrometer, thus showing that irradiation has been completely annealed before further heat treatment has resumed. The negative result might be due to the relatively low neutron dose in our experiments. There is no doubt that strong damage at a sufficiently early stage of heat treatment should effect the subsequent graphitization process (making soft carbons less graphitizable and the glassy carbons softer). The question of interest is, how large neutron dose at each given stage is necessary to destroy the rudimentary structural skeleton which permits the returning displaced atoms to reestablish the original structure. 4.4
Temperature
dependence of Ag
In a previous publication the temperature dependence of the Agcondwas determined for the P33 2600°C by correcting the actually measured Ag-values for the M-effect (Fig. 12, ref. 1). The experimentally found linear variation of Ag changes into an inverse relation for Agcond(intermediate between l/d/T and l/T) which closely parallels the directionally averaged curve for a graphite crystal. Y. Yokozawa* has found that for all samples of P33 heat treated from 1600-3000°C the g-values vary linearly with temperature. Using this information and the data from Part I, one can obtain the temperature dependence of Ag,.,, for variously heat treated samples. The results are presented in Fig. 6. One can see that for HTT 2200°C and above, all the curves reduce roughly to a common Agc,..d-curve, which is located about 8 per
cent
below
the
directionally
averaged
*Private information on unpublished data.
WAOONER’Sdata for a graphite crystal.(r) Above HTT 2700°C the corrected values more and more deviate from this common curve, the correction becoming insufficient to raise the values to the common curve due to an overestimation of the carrier contribution. That this is so can be seen clearly from Fig. 7 where the carrier contribution XRT is plotted vs. the measured Ag: all the experimental points for highly graphitized P33 and Thermax samples are located above the indicated line Ag = 145 x 10-4. XRT, the point for P33 3000°C being as it looks greatly in error. It is believed that such experimental overestimate of the carrier contribution is caused by the incipient incompleteness in the directional averaging of the g-value for highly graphitized samples, as evidenced by the fast increase in the p value. This causes part of the intensity to become lost in the wings, increasingly so as the temperature is lowered (see also the end of section 4.1). It was shown in Part I that the room temperature Ag,,.,, value changes with heat treatment parallel with the Landau diamagnetism and that it reaches the graphitization plateau at about HTT of 2200°C. Having obtained the Ago.,,,*for a range of temperatures the existence of a simple relationship between Agco.,,and the Landau diamagnetisms xr. can now be subjected to a test on a broader basis. In Fig. 8 the AgcDnd is replotted as a function of the inverse temperature (thick lines). On the same plot a system of diamagnetic susceptibility curves X, = X-X,, as taken from the publications of PINNICKand KI~vE(~) and from Les Carbones are drawn (thin lines) on such scale as to have the points Agpaphand X,,,,, about coincide at room temperature. It is clearly seen that although the two systems of curves look quite different, for each definite temperature the change in Agcondand in X, with temperature of heat treatment (rather with degree of graphitization) occur in parallel (if the scales are adjusted for the g graph to coincide at this temperature). For instance in Fig. 8 the point Agcotldfor P33 2000°C is located between
ELECTRON
SPIN RESONANCE
IN TURBOCRYSTALLINE
CARBONS-II
P33
“X,,,/ ~ggroph.ref* 7 f
‘\\
9%
%
.,,p,_6~ifference
\\
60
_ ,*~~_--_---_--
I
100
I
-
-
_
__
2ooc
----__-,600
-___
R 0
_
I
I
TEMPERATURE
I
300
200 (OKI
FIG. 6. Temperature dependence of the g-factor for variously heat treated P33 carbon black samples as observed (broken lines) and as corrected for the mixing effect (continuous lines).
851
852
S. MROZOWSKI
0
P33
A THERMAX x
1
irr, HTT>PlOO%
+ irr. t annralrd, HTT~2100*C I
Ag= g-g,%04)
120
FIG.7. Plot of the fractional contribution X,r of conduction carriers to the ESR intensity at room temperature for variously heat treated P33 and Thermax samples in relation to the observed position of the line.
Xr for Texas coke 2000” and 1800” and the XL for point Agcontifor P33 1800”C-between Texas 1800” and 1600” as they should. The discrepancies such as for XL P33 1500” and XL Spheron 2000” are probably due to the longer heat treatment residence time. However, ;f a relation between Agcondand XL existi, it cannot be very simgile, because all Agcondcurves can be brought almost to overlap by proper change of the Ag scale, while the XLcurves cannot, becoming more and more flat as the heat treatment decreases. The two curves Ag,raph and XI,,Dh cannot be brought into coincidence by a proper multiplication of their scale and by introducing a relative shift in their origins (see the broken curve), as PACAULT et al.(lO) believed it to be possible, but can be brought to overlap without shifting the origin by multiplication of the vertical XLscale by a factor 2.56 (dotted curve) and change in the temperature scale by a factor To/To’ = 3.4. Unfortunately the temperature factor increases
greatly as one tries to bring into overlap curves for the lower heat treatments, and it is questionable if such a large change in temperature scale possibly can have any physical meaning. 5. DISCUSSION
AND CONCLUSIONS
The experiments with sodium doping provide a direct proof of the contention that for turbostratic carbons the Fermi level is depressed below the band overlap and more so, as the heat-treatment temperature is lower. While an addition of very small amounts of sodium causes an immediate increase in intensity of the ESR of a P33 sample heat treated to 26OO”C, a few small fractions of sodium are necessary to reach the intensity minimum for HTT 2OOO”C, the fractions becoming greater in case of HTT 1600°C. Thus the Fermi level at HTT 1600” must be located somewhat lower than for HTT 2000” as required by the band model proposed many years ago.(n) Unfortunately, the
ELECTRON
SPIN RESONANCE
IN TURBOCRYSTALLINE
CARBONS-II
853
36C
3oc
e
J
b
T
120
-2
x
60
I
293
I
I
I
200 160 I20 TEMPERATURE (OK 1
I
100
1
80
FIO. 8. Comparison between the temperature dependence of the A.gf,,,, value for various samples of P33 and the temperature variation of the Landau diamagnetism xt for various materials (for details see text).
S. MROZOWSKI
854
CUMULATIVE
FERMI LEVEL
E
FIG. 9. Density of states vs. energy relation resulting from an unequal distribution of traps among crystallites. doping
experiments
success:
the intensity
in R observed
ended
in
partial
and maximum
are not as great as expected
only for a band model bands,
only
minimum
not
with a gap or touching
but even for a graphitic
type band over-
lap. For this last model a value of 2.7 for R at the overlap would be expected when
for a model
dependence
with
(for HTT
a gap
1600”),
a pure
R = 3.65 should be obtained.
Curie The
highest R value observed however was 2.4! It is the contention of the author that this discrepancy inability formly
is not only due to the experimental in doped
preparing
a
microcrystals
sample all
with
uni-
through
the
material but that there might be a basic reason why obtaining the limiting value will not be possible. The statistical fluctuations in distribu-
and less by the band shape crystallites
of the individual
(Fig. 9). Thus unless crystallites
more traps should happen
to be accepting
with more
donors we will have to resign ourselves to much more
roundabout
structure Figure
10 tries
knowledge
of tests
to summarize
concerning
tensity in carbons. sharp
types
for fine polycrystalline
peak
the ESR
of the
band
materials.* our
present
absorption
in-
The curve on the left with a
at HTT
700”
represents
the
be-
havior of chars, the curve being representative for almost all carbonaceous substances with little deviation
in shape
or absolute
value.
On
the
right the curve is drawn for the two carbon blacks Thermax and P33 as obtained in Part I. It is believed that for other soft carbons in which the growth of crystals is not arrested by the
tion of traps between microcrystals in conjunction with the requirement of matching the
particle size, the corresponding decay faster, reach the intensity
curve might minimum and
Fermi level of all crystallites should result even in case of a model with a gap in an average band structure with an apparent overlap largely controlled by the statistics of the trap distribution
*Smearing out of the overlap region will occur even in a single crystal due to local deformations of band structure by ionic fields of the donors or acceptors.
ELECTRON SPIN RESONANCE IN TURBOCRYSTALLINE
CARBONS-II
855
CARBON
SPlN
CENTERS
POLYCRYSTALLlNE
HEAT
TREATMENT
TEMPERATURE
PC)
FIG.10.Schematic presentation of the ESR intensity in dependence on heat treatment temperature as believed to apply essentially to most carbons, with smail deviations depending on graphitizability and possibly other factors. level off at the graphitic value earlier than for Thermax or P33 while for smaller carbon blacks even at HTT 2900°C the minimum might not yet be reached, thus leading to a scatter in intensity values observed by ARNOLD."~) The shaded area on the graph Fig. 10 gives the contribution of conduction carriers to the ESR intensity (that is the dependence of a(&) on heat treatment) as obtained from the curve for the P33 carbon black and extrapoIated to the low heat treatments on the well founded assumption that conduction holes start appearing in chars somewhere above HTT of 600°C.(r3’ The presence of carrier contribution to ESR is expected to show in form of an increasing deviation from the Curie law temperature dependence at HTT above 700°C and there are good indications that this is what in fact is observed.(r4) In this context the term “delo-
calized spins” was carefully avoided, because it possesses also another meaning-of a spin which is able to travel to and fro about an unsaturated molecular chain or ring system, but which as the temperature dependence is concerned will conform to the Curie law as long as it cannot leave the molecule. Using this latter terminology the localized spins in chars indicated in Fig. 10 might well be “delocalized.” Leaving aside the interesting question of why is the transition through the intensity minimum (HTT 26OO’C) up to the final plateau in Fig. 10 not reflected in variations of Ag and of diamagnetic susceptibility, the key question concerns the nature of the localized spin centers. Are the spin centers in chars different from localized spin centers in carbons ? Aren’t they all manifestations of the presence of broken (freely dangling) carbon bonds ? If so, why is the resonance suddenly
856
S. MROZOWSKI
changing from air sensitive to air insensitive above HTT of I 100°C and what is the reason for the Hennig-Smaller broadening in the range lOOO”-15OO”C? Finally, what is the connection if any between the decrease of the ESR intensity above HTT 700°C and the incipient conductivity? These are the questions towards which our future efforts shall be directed. REXERENCES 1. MROZOWWS., Carbon3, 305 (1965). 2. MROZO~~KIS., Carbon 4,227 (1966). 3. A~NOLI)G. and MROZOW~XI S., Carbon 6, 243 (1968). 4. See the Discussion at the 8th Carbon Conference: &huXiAND A., Carbon6, 193 (1968).
P. and MARCHANDA., Curbon 3, 115 5. DEL-S (19653. 6. ZANCHETTA J., Carbon 5,411 (1967). 7. WAGONER G., Proceedings of the Fourth Carbon Conference,p. 197. Pergamon Press, Oxford (1960). 8. PINNICKH. T. and KIWE P., Phys. Rev. 102, 58 (1956). A. (editor) Les Curbones, Vol. I, p. 412. 9. PACAULT Masson et Co., Paris (1965). J., THEOBALD J. G. 10. PACAULTA., UEBERWELD and ~ERUTTI M., C. R. Acad. Sci. Paris 251, 3589 (1965). S., Phys. Rev. 85, 609 (1952); and 11. MROZOW~KI Errata, Phys. Rev. 86, 1056 (1952). 12. ARNOLD G., Curbon 5, 33 (1967). 13. KMETKO E, A., Phys. Rev. 82,456 (1951). A., DELHAESP. and ZANCHRTTA J., 14. MARCHAND J. Chim. Phys. 60,668 (1963).
ELECTRON
Pergamon Press.
SPIN
Printed in Great Britain
RESONANCE IN CARBONS-II*
TURBOCRYSTALLINE
s. MROZOWSKI Carbon Research Laboratory and Department of Physics, State University of New York at Buffalo, Buffalo, New York 14214 (Received 27 November 1967)
Abstract-In continuation of the work on ESR in turbostratic carbons, experiments on introduction of boron acceptors and sodium donors were performed using P33 carbon black samples heat treated to various temperatures mainly in the range 1600-2400°C. The experiments with boronation show a complicated behavior and lead to the conclusion that boron atoms can act not only as acceptors, but neutralize some of the existing localized spin centers and also can under special conditions become spin centers themselves. Experiments with sodium doping have shown that the stability of the sodium against oxidation increases with decrease in HTT. With increasing doping the “Curie-like” behavior (the factor R = ILN/IRT) goes through a maximum and the intensity of the ESR line through a minimum, as expected from the simple band model with the Fermi level originally located below, and moving across and then above the band overlap with increase in doping. Usingtwo spectrometers 9.2 Gc and 35 Gc and a variety of samples it was found that while for low heat treatments the widths are the same (in Gauss), for well graphitized materials the use of the higher frequency results in a considerably broader line. Considerations concerning the temperature and heat treatment dependence of the Ag,,,, and its relation to the diamagnetic susceptibilitv, are presented. The nature of the band structure in turbostratic carbons _ and of-the localized spin centers are discussed. 1. INTRODUCTION IT HAS
provided valuable
been shown by an exhaustive
in the g-value
study of the
information
spreading
due to both
Part I of this worko)
spin centers, line
being
exchange
conduction
the single g-value the
mixing
further
carriers
result
of intensity
contributions
planes
has been applied to two carbon
of an
Thermax,
It was
temperatures
(M-effect).
permits to determine
the
of both kinds of spin centers
and
caused and by
(S-effect).
In
the same kind of analysis
presence
for a wide range
blacks,
P33 and
of heat-treatment
(1400-3000°C)
neutron irradiation. While for partially graphitized
shown that a study of the temperature
dependence
of the graphitic
of the composite
of the
mechanism
and localized
carriers
by the shift of the Fermi level (F-effect)
in a P33 carbon black heat treated to 2600W1* *) that the single absorption line is
ESR
as to the changes
of the conduction
before
and
after
it was expected that materials (HTT >
2200”) such analysis will lead to results not much
thus to obtain information as to the influence of various factors, such as neutron irradiation and
differing
doping
work was to check the applicability of the analysis to carbon materials in a turbostratic polycrystalline state. The results reported in Part I@) leave no doubt as to the correctness of
each
with donors of
these
or acceptors
resonances.
Such
separately an
on
analysis
*Supported by the U.S. Office of Naval Research. Reproduction in whole or in part is permitted for any purpose of the U.S. Government. 841
HTT
from
26OO”W*
the
former
2), the
ones
main
for
purpose
the
P33
of this
this expectation. It has been found that both the contributions of conduction carriers and of
842
S. MROZOWSKI
localized centers increase greatly for lower heat treated samples, the Agcond= g-gr,,, for the conduction carriers changing at room temperature in proportion to the Landau diamagnetism XL. Neutron irradiation causes, similarly as in the case of graphitized samples, a large increase in concentration of localized spin centers, a relatively much smaller one in the density of the conduction states n(EF) with an interesting indication of a possible saturation of n(EF) however (slowing down of intensity increase with lowering of the Fermi level) for the lowest heat treated samples. The band structure of turbocrystalline carbons not being understood yet@) it is necessary to try all avenues of approach. The ESR technique as developed here being a most direct way of obtaining such vital quantity as the density of states n(EF) at the Fermi level, the experiments were continued and extended to a study of the influence of donors and acceptors on the ESR in turbostratic carbons. All this work was performed using the apparatus and techniques described in Part I.@) While this work was in progress, in addition to our regular 3 cm spectrometer, an 8 mm spectrometer became available. Thus a few supplementary comparative checks of the width of the lines were carried out. 2. DOPING
WITH BORON
The technique used for introduction of boron was the same as previously.ca) Boric acid powder was mixed with the carbon black solid chunks (use of powder results in much broader line(a)) and heated to a temperature 1650-1700°C. While heated the boric acid melts and soaks into the carbon solid forming a very hard glass; it is only above 1500°C that it decomposes and the free boron starts diffusing into the lattice. For graphitized samples even at 1600” the diffusion is not yet complete (not much shift in g-value) and the line is relatively broad. The best results were obtained for heating between 1700” and 1800”. This method of introduction is a very good one since the boron is introduced after the carbon structure has been formed. thus ner-
mitting a direct comparison between the original and doped material. However, for the same reason materials heat treated below 1700°C were not investigated-it was feared that the second heating to 1650°C might cause additional heat treatment. Only P33 carbon black was used in this part of the study. It was found in the previous work(2) that as boron is introduced into a material heat treated to 2600°C in increasing amounts and the ESR line shifts gradually towards gf, the intensity of the line after a short flat portion (Fig. 1, ref. 2) begins to climb faster and faster as g, is approached. The intensity increase is due to the dropping of the Fermi level down and away fi-om the band overlap, thus leading to an increase in the density of states n(EF) which determines the contribution of the carriers to the line intensity. The increased contribution of carriers brings about a change in the temperature dependence of the intensity from a slightly Curie type (ratio R = &/I,, of about 1.4) to a pure conduction type (R = 0.9-1.0). What was however surprising, is that this drop in ratio R occurs quickly at the beginning in the region of the plateau, thus making necessary the assumption that boron atoms not only act as acceptors for conduction carriers, but that in the they also beginning stages of boronation neutralize in some way the localized spin centers present in the material. While it was found in the present work that all samples of P33 heat treated to 2200°C or higher show a similar response to boronation as above, the ESR intensity for materials in turbocrystalline state even after a strong boronation does not increase at all. P33 carbon black heat treated to 2200°C (boronated at 1700°C) and to 1700°C (boronated at 1650°C) were particularly investigated. As the boron is introduced in increasing amounts the line broadens considerably at first (by a factor 2 to 3) but then the trend becomes reversed and at high boron contents the line reverts almost to the original width (factor 1.O to 1.5). This is a behavior which closely narallels the one found for nolvcrvstailine
ELECTRON
graphite
(Fig.
7, ref. 2),
with
temperature
to g,
(the
> 1 to
the change
reversing
factor
r =
producibility scatter,
wLN/wRTchanging
from
at a somewhat in width
of various
by
25~40%
the
tendency
or the
to the
other
up
On the other hand, additions
quickly
intensity
earlier data
the
from
in the initial the localized
almost constant Since again
stages boron atoms neutralize
that
some of
spin centers without creating holes in the spin centers are internal
lattice vacancies,
or
then this neutraliza-
with
boron
atoms
(filling
an existing
in the electronic pensated
by an increase
boron
the experiments
atoms
localized
can
not
spin centers
only
com-
absorption
become a source of local&d number
of instances
very sharp ESR a high relative showing
intensity
(3-4
up to 2.0). It is now believed
to 1.0, but the 3.8.
Thus
and neutral
boron
I
=
into acceptors
and
the
subsequent
reheat
remaining
then
to indicate
at this time.
The
results boron
obtained
seem
introduced
or graphite
several different
into
polycrystalline
structure
can
behave
ways. The interpretation
in
of the
observed effects is therefore complex and the boronation method will not become useful for the study of the band structure
until the process
of boronation
in detail
meaningful
is investigated
technique
and
of separation
of a reproducible
types of spin centers
and
a
of the various
introduction is worked
3. DOPING
of
out.
duction
and
experiments
driving
out
graphitized
depresses
existing
overlap permits
a
with
times the original) (ratio
WITH SODIUM
R
that the R anomaly
black
the Fermi region,
boronated
the
while the Fermi
above
the band
from
carbon
a
black
Since boronation
level away from the band
subsequent
observing
with intro-
of sodium
(P33 26OO’C) were described.
boronations
behavior
and
but also
1G) was obtained
Curie-like
the
gas the
the Fermi level, the action of exposure
partially
centers themselves. In a
for heavy
line (<
a partial
neutralize
to
spin centers
In an earlier pape0
seem to show that
in the material,
to air
that
increased
must have changed
depressed
of holes
caused by other boron atoms acting as acceptors. However,
groups
After reheating
in flow of Nitrogen
to 3G, R dropped
boron
gf
2.5 (relative
vacancy
about
in carrier
localized
I =
material).
intensity
definite
band !). The decrease in localized
spins at later stages has to be just
total
line (3/4G ) at g z
intensity
to 1200%*
effects
tion would be the result of filling the vacancies with boron does not increase the number
to the nondoped
carbon
the
spins in turbostratic
one has to assume
band. If the localized peripheral
to
1.7 and
line broadened
843
CARBONS-II
a very sharp
with R =
obscure
1.9 down
boronations.*
of localized
is large,
boronations.
the ratio R for small boron
decreases
heaviest
concentration carbons
not
to change one way highest
1.2, and seems to remain
to
does
produced
lumps
and poor re-
runs the intensity
show any definite
up
dependence
but it is clearly seen that after an initial
decrease
about
in width close
reversing
Due to variation
IN TURBOCRYSTALLINE
for g-values
< 1) and the temperature
of the g-value stage.
SPIN RESONANCE
doping with sodium ESR
level
overlap
of
the
carbon
moves across and
and again
also on its
way back to the depressed position as the sodium is being driven out. The Fermi level being depressed in turbostratic carbons even without boronation
it was the purpose
of this work to
observed in the previous work at Ag = 40 x IO-4 (Fig. 2, ref. 2) must have a similar origin.
carry out the same type of observations
on these
carbons
hope
The most instructive was a heavy boronation of material HTT 2900°C (R = 1 .l) which
obtaining
*Results in some ways similar to ours, but less definite and somewhat confusing were obtained by DELHAES and MARCHAND on boronation of a soft coke.
*Reheat of heavily boronated lumps does not broaden the line. Evidently the abundance of boron protects the surface from contact with air and from the ensuing Hennig-Smaller broadening.
(without
boronation)
information
of turbostratic
carbons.
in the
of
as to the band structure As shown
in Part
I
844
S. MROZOWSKI
turbostratic carbons possess large concentration oflocalized spin centers which presumably are not affected by sodium doping (Table 5, ref. 1). Should carbon in turbostratic state possess an energy gap a temperature behavior of the ESR little differing from a Curie type (R = 3.65) would be expected when the Fermi level is located exactly between the two energy bands. In any case as the Fermi level moves across from one to the other band a minimum in ESR intensity accompanied by a maximum in the ratio R would be expected.
sodium. Extremely small amounts of sodium can be introduced by this technique-amounts which form across the entrance to the capillary C a barely visible deposit (causing just a change in the tint of the glass). The reaction is performed by heating the chamber P to a temperature at which the deposit disappears. The sample at the bottom of the tube P is being shaken while heated so as to secure a uniform distribution ofsodium throughout the powder. After that the sample is returned into the tube S, new ESR measurements performed and the operation repeated. After a relatively large total amount of sodium is introduced sticking of particles causes a decrease in observedintensity by increased conduction (see pp. 235-237, ref. 2). To correct for such effect about a third or a quarter of the sample is transferred into the arm T, separated from the rest by sealing off, opened and the ESR intensity quickly measured before and after grinding in a mortar. After all the sodium needed is introduced into the main sample, air is admitted by breaking off the tip of the container M and as sodium is oxidized the gradual return of the Fermi level towards the original position is observed. The driving-out is achieved in steps by heating the sample in air to increasing temperatures (up to 500°C). Some driving-out series were performed in vacuum, without opening the sample to air but such procedure is much less effective. Figure 2 shows the results of such doping runs on P33 of HTT 2000°C. In distinction to the case of boronated P33 of HTT 26OO”C, starting from the original line (#O) introduction of sodium not only decreases the intensity at first, but also causes a slow shift of the line towards g,. After the line has shifted about one third of the way towards g, the continuity of the motion is interrupted by a sudden jump in the g-value resulting in a double line (#3). The intensity then continues to be transferred to the lower gvalue line (#4) and the total intensity continues to decrease almost close to the gf value. An intensity minimum is reached at Ag z 8 x IO-* (#5) after which the line starts quickly
_-y. :i_l ---.,’
T
GLASS
QUARTZ
\
:s FIG. 1. Apparatus for ESR study of introduction and driving out sodium under vacuum. S-carbon sample, M-sodium container, P-reaction chamber, C-capillary through which small amounts of sodium are distilled into the reaction zone, T-side tube for separating a fraction of reacted carbon.
The experiments were performed using sample tubes as shown in Fig. 1, being a somewhat improved version of the earlier 0nes.o) After the ESR of the nondoped sample was measured at room and liquid Nitrogen temperature a very small amount of sodium was distilled from the container M into the chamber P where the sample S was transferred and reacted with the
ELECTRON
SPIN RESONANCE
I I
\
IN TURBOCRYSTALLINE
P33
(2000°HTT)+
845
CARBONS-II
Na
FIG. 2. Changes in ESR line position and intensity for a P33 carbon black (HTT 2000°C) when sodium is introduced in steps (l-8) and when it is driven out by heating in air, also in steps (2’-6’).
gaining return
at further
in intensity trip (driving
minimum
is not reached
to the original
doping.
out of sodium)
position
until
(#5’)
On the
The effort was made to determine
the intensity
erature
dependence
the line returns
sodium
introduction.
and is not as deep
determine
the ratio
of
R
While it at the start
after was
at the end of the doping series (#8,
the sample
exceeding
intensity remnants
could
at the end, the original
not be restored-evidently
of sodium
are
well
trapped
the in the
1.2),
determination cause
one
of the unsymmetric
geneity)
ing-out process supports the previous contention(a)
the signal (factor
that when driven-in the sodium preferentially goes between the layers, but in driving-out the
considerably material
(factor
atoms lodged between
variation
(1.3)
and which
do not lower the g-value by the S-effect, last ones to leave.*
are the
*The so&urns go preferentially between layers of those microcrystals which are under c-directional tensile stress. After the body has sufficiently expanded, microcracks release some of these stresses and the intercalated sodiums become more easily removable. CARBON 6/6-o
get
structure
greater 1.5).
than
for
(#5),
the
doping
in the width
be-
show clearly
of the height
It decreases
at further
good
(inhomo-
2.0) at the minimum
ever the changes z
variation
much
a
stages,
on the line. Observations
that the temperature
important material
not
at the intermediate
material and cannot be easily driven out. The lack of reversibility in the driving-in and driv-
the crystallites
could
each
easy to 2.0) and
(R = R not
as on its way down to gf. No matter how strongly was heated
the temp-
intensity
of is
undoped
to a smaller (#8).
which
How-
are more
(r = wLN/wRTis for the original 1.15 and for a strongly doped
one z 0.9) could sufficient accuracy
not be pinned to be absolutely
down with sure of the
presence of a maximum in R at the intensity minimum. Similarly the results on the return
846
S. MROZOWSKI
trip did not turn out satisfactory for such a demonstration, especially since the return was never complete. Much more meaningful results were obtained using P33 heat treated to 16OO”C, because of the greatly reduced spread in g-values between the original and the doped material. In Fig. 3 the dependence of R and I on doping as obtained in one of the series of experiments is presented. In introducing sodium a definite peak in R is observed before the intensity minimum is reached. The delay in reaching the intensity minimum is understandable since in transferring the sample from the tube S to the reaction
P33
IN
INCREASING 0.
(1600’HTT)
chamber P and back more and more of the powder sticks to the walls and thus the quantity of carbon black in the tube 5’ is continuously decreasing. At later stages sticking of the particles diminishes the signal, and had to be corrected for. No matter how carefully the dopings were performed in trying to keep the distribution of sodium uniform throughout the carbon black, under no condition were R values at the maximum greater than 2.3-2.4 obtained. After opening the sample in driving-out the sodium, no such large maximum in R was found, indicating even less homogeneity in the sodium distribution throughout the material.
+
No
VACUUM
IN
AMOUNT .
OF
Na
HEATING
AIR
IN AIR
-
FIG. 3. Changes in ESR line intensity and in the ratio R for a P33 carbon black (HTT 16OO’C) as sodium is stepwise introduced and then driven out by heating in air.
ELECTRON
SPIN
RESONANCE
IN TURBOCRYSTALLINE
On the way back the intensity goes through a minimum, and again at the end is not returning up to the original intensity. In general the stability of the sodium increases with decrease in HTT: while it can be driven out by oxidation from graphitized samples relatively easily the remnants refuse stubbornly to be removed from their hiding in the turbostratic carbons and increasingly so as the crystal size diminishes. 4. STUDlES
OF THE WID’PH AND OF THE g-FACTOR
When a 35 Gc spectrometer of Varian (Model V4502-13) b ecame available a large number of samples was checked for the line width on both spectrometers in the hope of obtaining some information as to the origin of the width under various conditions. It was found that while for the lower heat treatment temperatures the width of the line in Gauss is exactly the same*, for higher heat treatments as the HTT increases the width recorded using the Q band spectrometer increases in relation to the width observed on the 9.2 Gc spectrometer. On the 35 Gc spectrometer, frequently a complicated structure in the line can be observed for samples of HTT ZOOO-2400°C which originates from small inhomogeneities in the heat-treatment temperature. This is the region of the steepest variation of the g-value with HTT. The total change in Ag covering a 3.8 times wider range in Gauss than for the X-band spectrometer results in a much greater resolution in view of almost the same line widths. Heat treatment of powders in Iarger containers always *In the course of comparisons of the g-values on both spectrometers, it was discovered that our homemade X-band spectrometer gives consistently larger Ag-values, probably due to a faulty calibration of the sweep amplitude. Thus all the X-band widths have been multiplied by the proper factor of 0.88. In view of this discrepancy we do believe that all our previous data for Ag and width should be also corrected by the same factor. (To avoid confusion however this correction was not introduced in Figs. 2, 5-8 of this paper.)
CARBONS-II
847
leads to such structure, good results (no structure) being obtained only when a small grain (lump) of the solid carbon black is picked out from the container. While the width of the lines is not well reproducible, varying from one to another heat treatment run (see section 4, Part I, ref. 3), the ratio of the widths as obtained on both spectrometers (p = wQ/wx) does not depend on the absolute value of the width and seems to be a characteristic parameter varying with the heat-treatment temperature. The results are presented in Fig. 4, where the inhomogeneity region is indicated by the dotted curve. BeIow a curve of minimum widths as obtained on the & band spectrometer is given. On Fig. 5 the same data are plotted in relation to the corresponding Ag values with results added for irradiated and doped materials (mostly P33). As noted in Part I neutron irradiation decreases the width when HTT> 2000°C and increases it when HTT <2OOO”C. The comparison of widths shows that samples irradiated retain (at least partly) the p value of the origina material-this seems to indicate that p > 1 is caused by increasingly poor directional averaging as the crystallite size increases, the slight decrease in p occurring as a result of higher admixture of localized spins (&f-effect). On the other hand in case of dopings which bring the line close to gf through the F-and S-effects, sometimes a single line is obtained with p = 1 as expected, but frequently structures are observed on the 35 Gc spectrometer showing a nonuniform distribution of the doping material. The results also show that broadening caused by heat treatment in powder form is definiteIy not a g-inhomogeneity (Fig. 4). Using both spectrometers the line shape and width were measured for the French GFEC coke heat treated to 2000°C for 5 min, 20 min and 5 hr. The strongly asymmetric Iine showing correspondingly a width of 7.5,8.5 and 15 G on 9.2 Gc, shows the same asymmetric shape on the 35 Cc spectrometer, but much greater widths of 22, 25 and 40 G. The ratio p of the
I
I
1400
I I
1600 HEAT
I
I
2200 TREATMENT
I
2600 TEMPERATURE (“Cl
I
I
3000
I
A
FIG. 4. Plot of the ratio of widths as obtained on two spectrometers vs. heat treatment temperature for varions samples of P33(0) and Thermax( A). P means powder, L-heat treated in Inmp form. The continuous curve is drawn through the minimum p-values obtained. In the lower part of the figure the lowest values of the width as observed on the 8 mm spectrometer are plotted.
I
,o
/
A
ELECTRON SPIN RESONANCE IN TURBOCRYSTALLINE
CARBONS-II
849
3
l6OOfNa
I
0
I
I
I
40
80 Pg = g -gf
I
I
120
ix to41
Fm. 5. Plot of the ratio of widths as obtained on two spectrometers vs. the observed position of the line for variously heat treated, irradiated and doped samples of P33 and Thermax. widths being about 2.9, not far from the ratio of the magnetic field strengths (3.81, most of the broadening is caused by a g-inhomogeneity. Introduction of sodium makes the line much sharper (factor < 4) and the shape symmetric; even after most of the sodium is driven out and the line reverts to its original g-value, it still retains its symmetrical shape, the intensity continuing to decrease as the line gradually begins to show some hornets (no intensity minimum observed). This seems to support our views expressed in Part I that in the ESR of soft type carbon materials, part of the intensity is lost in the wings as a result of g-inhomogene&y. 4.2 Ballmilling effect Having found that the 35 Gc spectrometer is
such an excellent and sensitive tool for checking the uniformity of the material, carbon blacks were compared after prolonged ballmilling in air (2 weeks). The ESR of the raw Thermax black did not show any change caused by such treatment (no change in g-value nor width) but Thermax graphitized at 2500°C did; the width increased by a factor of 2.6 and the gvalue decreased slightly from 2.0135 to 2.0126, thus indicating some destructive influence of grinding, which could not be detected by direct electron microscopic observation.(e) 4.3 Anneal of irradiated samples It has been demonstrated time and again that even quite heavily irradiated polycrystalline graphites regain the original structure after anneal to temperature above 1500°C. Since it
850
S. MROZOWSKI
was found in Part I that the stability of damage against anneal increases as lower heat treated materials are irradiated, irradiated P33 samples of HTT ranging from 1600°C to 2600°C were annealed together with nonirradiated samples to temperatures from 2000” to 2600°C. Although the g-values change strongly in this range with HTT no differences in g-value (nor width) greater than f 0.000015 were found on the 35 Gc spectrometer, thus showing that irradiation has been completely annealed before further heat treatment has resumed. The negative result might be due to the relatively low neutron dose in our experiments. There is no doubt that strong damage at a sufficiently early stage of heat treatment should effect the subsequent graphitization process (making soft carbons less graphitizable and the glassy carbons softer). The question of interest is, how large neutron dose at each given stage is necessary to destroy the rudimentary structural skeleton which permits the returning displaced atoms to reestablish the original structure. 4.4
Temperature
dependence of Ag
In a previous publication the temperature dependence of the Agcondwas determined for the P33 2600°C by correcting the actually measured Ag-values for the M-effect (Fig. 12, ref. 1). The experimentally found linear variation of Ag changes into an inverse relation for Agcond(intermediate between l/d/T and l/T) which closely parallels the directionally averaged curve for a graphite crystal. Y. Yokozawa* has found that for all samples of P33 heat treated from 1600-3000°C the g-values vary linearly with temperature. Using this information and the data from Part I, one can obtain the temperature dependence of Ag,.,, for variously heat treated samples. The results are presented in Fig. 6. One can see that for HTT 2200°C and above, all the curves reduce roughly to a common Agc,..d-curve, which is located about 8 per
cent
below
the
directionally
averaged
*Private information on unpublished data.
WAOONER’Sdata for a graphite crystal.(r) Above HTT 2700°C the corrected values more and more deviate from this common curve, the correction becoming insufficient to raise the values to the common curve due to an overestimation of the carrier contribution. That this is so can be seen clearly from Fig. 7 where the carrier contribution XRT is plotted vs. the measured Ag: all the experimental points for highly graphitized P33 and Thermax samples are located above the indicated line Ag = 145 x 10-4. XRT, the point for P33 3000°C being as it looks greatly in error. It is believed that such experimental overestimate of the carrier contribution is caused by the incipient incompleteness in the directional averaging of the g-value for highly graphitized samples, as evidenced by the fast increase in the p value. This causes part of the intensity to become lost in the wings, increasingly so as the temperature is lowered (see also the end of section 4.1). It was shown in Part I that the room temperature Ag,,.,, value changes with heat treatment parallel with the Landau diamagnetism and that it reaches the graphitization plateau at about HTT of 2200°C. Having obtained the Ago.,,,*for a range of temperatures the existence of a simple relationship between Agco.,,and the Landau diamagnetisms xr. can now be subjected to a test on a broader basis. In Fig. 8 the AgcDnd is replotted as a function of the inverse temperature (thick lines). On the same plot a system of diamagnetic susceptibility curves X, = X-X,, as taken from the publications of PINNICKand KI~vE(~) and from Les Carbones are drawn (thin lines) on such scale as to have the points Agpaphand X,,,,, about coincide at room temperature. It is clearly seen that although the two systems of curves look quite different, for each definite temperature the change in Agcondand in X, with temperature of heat treatment (rather with degree of graphitization) occur in parallel (if the scales are adjusted for the g graph to coincide at this temperature). For instance in Fig. 8 the point Agcotldfor P33 2000°C is located between
ELECTRON
SPIN RESONANCE
IN TURBOCRYSTALLINE
CARBONS-II
P33
“X,,,/ ~ggroph.ref* 7 f
‘\\
9%
%
.,,p,_6~ifference
\\
60
_ ,*~~_--_---_--
I
100
I
-
-
_
__
2ooc
----__-,600
-___
R 0
_
I
I
TEMPERATURE
I
300
200 (OKI
FIG. 6. Temperature dependence of the g-factor for variously heat treated P33 carbon black samples as observed (broken lines) and as corrected for the mixing effect (continuous lines).
851
852
S. MROZOWSKI
0
P33
A THERMAX x
1
irr, HTT>PlOO%
+ irr. t annralrd, HTT~2100*C I
Ag= g-g,%04)
120
FIG.7. Plot of the fractional contribution X,r of conduction carriers to the ESR intensity at room temperature for variously heat treated P33 and Thermax samples in relation to the observed position of the line.
Xr for Texas coke 2000” and 1800” and the XL for point Agcontifor P33 1800”C-between Texas 1800” and 1600” as they should. The discrepancies such as for XL P33 1500” and XL Spheron 2000” are probably due to the longer heat treatment residence time. However, ;f a relation between Agcondand XL existi, it cannot be very simgile, because all Agcondcurves can be brought almost to overlap by proper change of the Ag scale, while the XLcurves cannot, becoming more and more flat as the heat treatment decreases. The two curves Ag,raph and XI,,Dh cannot be brought into coincidence by a proper multiplication of their scale and by introducing a relative shift in their origins (see the broken curve), as PACAULT et al.(lO) believed it to be possible, but can be brought to overlap without shifting the origin by multiplication of the vertical XLscale by a factor 2.56 (dotted curve) and change in the temperature scale by a factor To/To’ = 3.4. Unfortunately the temperature factor increases
greatly as one tries to bring into overlap curves for the lower heat treatments, and it is questionable if such a large change in temperature scale possibly can have any physical meaning. 5. DISCUSSION
AND CONCLUSIONS
The experiments with sodium doping provide a direct proof of the contention that for turbostratic carbons the Fermi level is depressed below the band overlap and more so, as the heat-treatment temperature is lower. While an addition of very small amounts of sodium causes an immediate increase in intensity of the ESR of a P33 sample heat treated to 26OO”C, a few small fractions of sodium are necessary to reach the intensity minimum for HTT 2OOO”C, the fractions becoming greater in case of HTT 1600°C. Thus the Fermi level at HTT 1600” must be located somewhat lower than for HTT 2000” as required by the band model proposed many years ago.(n) Unfortunately, the
ELECTRON
SPIN RESONANCE
IN TURBOCRYSTALLINE
CARBONS-II
853
36C
3oc
e
J
b
T
120
-2
x
60
I
293
I
I
I
200 160 I20 TEMPERATURE (OK 1
I
100
1
80
FIO. 8. Comparison between the temperature dependence of the A.gf,,,, value for various samples of P33 and the temperature variation of the Landau diamagnetism xt for various materials (for details see text).
S. MROZOWSKI
854
CUMULATIVE
FERMI LEVEL
E
FIG. 9. Density of states vs. energy relation resulting from an unequal distribution of traps among crystallites. doping
experiments
success:
the intensity
in R observed
ended
in
partial
and maximum
are not as great as expected
only for a band model bands,
only
minimum
not
with a gap or touching
but even for a graphitic
type band over-
lap. For this last model a value of 2.7 for R at the overlap would be expected when
for a model
dependence
with
(for HTT
a gap
1600”),
a pure
R = 3.65 should be obtained.
Curie The
highest R value observed however was 2.4! It is the contention of the author that this discrepancy inability formly
is not only due to the experimental in doped
preparing
a
microcrystals
sample all
with
uni-
through
the
material but that there might be a basic reason why obtaining the limiting value will not be possible. The statistical fluctuations in distribu-
and less by the band shape crystallites
of the individual
(Fig. 9). Thus unless crystallites
more traps should happen
to be accepting
with more
donors we will have to resign ourselves to much more
roundabout
structure Figure
10 tries
knowledge
of tests
to summarize
concerning
tensity in carbons. sharp
types
for fine polycrystalline
peak
the ESR
of the
band
materials.* our
present
absorption
in-
The curve on the left with a
at HTT
700”
represents
the
be-
havior of chars, the curve being representative for almost all carbonaceous substances with little deviation
in shape
or absolute
value.
On
the
right the curve is drawn for the two carbon blacks Thermax and P33 as obtained in Part I. It is believed that for other soft carbons in which the growth of crystals is not arrested by the
tion of traps between microcrystals in conjunction with the requirement of matching the
particle size, the corresponding decay faster, reach the intensity
curve might minimum and
Fermi level of all crystallites should result even in case of a model with a gap in an average band structure with an apparent overlap largely controlled by the statistics of the trap distribution
*Smearing out of the overlap region will occur even in a single crystal due to local deformations of band structure by ionic fields of the donors or acceptors.
ELECTRON SPIN RESONANCE IN TURBOCRYSTALLINE
CARBONS-II
855
CARBON
SPlN
CENTERS
POLYCRYSTALLlNE
HEAT
TREATMENT
TEMPERATURE
PC)
FIG.10.Schematic presentation of the ESR intensity in dependence on heat treatment temperature as believed to apply essentially to most carbons, with smail deviations depending on graphitizability and possibly other factors. level off at the graphitic value earlier than for Thermax or P33 while for smaller carbon blacks even at HTT 2900°C the minimum might not yet be reached, thus leading to a scatter in intensity values observed by ARNOLD."~) The shaded area on the graph Fig. 10 gives the contribution of conduction carriers to the ESR intensity (that is the dependence of a(&) on heat treatment) as obtained from the curve for the P33 carbon black and extrapoIated to the low heat treatments on the well founded assumption that conduction holes start appearing in chars somewhere above HTT of 600°C.(r3’ The presence of carrier contribution to ESR is expected to show in form of an increasing deviation from the Curie law temperature dependence at HTT above 700°C and there are good indications that this is what in fact is observed.(r4) In this context the term “delo-
calized spins” was carefully avoided, because it possesses also another meaning-of a spin which is able to travel to and fro about an unsaturated molecular chain or ring system, but which as the temperature dependence is concerned will conform to the Curie law as long as it cannot leave the molecule. Using this latter terminology the localized spins in chars indicated in Fig. 10 might well be “delocalized.” Leaving aside the interesting question of why is the transition through the intensity minimum (HTT 26OO’C) up to the final plateau in Fig. 10 not reflected in variations of Ag and of diamagnetic susceptibility, the key question concerns the nature of the localized spin centers. Are the spin centers in chars different from localized spin centers in carbons ? Aren’t they all manifestations of the presence of broken (freely dangling) carbon bonds ? If so, why is the resonance suddenly
856
S. MROZOWSKI
changing from air sensitive to air insensitive above HTT of I 100°C and what is the reason for the Hennig-Smaller broadening in the range lOOO”-15OO”C? Finally, what is the connection if any between the decrease of the ESR intensity above HTT 700°C and the incipient conductivity? These are the questions towards which our future efforts shall be directed. REXERENCES 1. MROZOWWS., Carbon3, 305 (1965). 2. MROZO~~KIS., Carbon 4,227 (1966). 3. A~NOLI)G. and MROZOW~XI S., Carbon 6, 243 (1968). 4. See the Discussion at the 8th Carbon Conference: &huXiAND A., Carbon6, 193 (1968).
P. and MARCHANDA., Curbon 3, 115 5. DEL-S (19653. 6. ZANCHETTA J., Carbon 5,411 (1967). 7. WAGONER G., Proceedings of the Fourth Carbon Conference,p. 197. Pergamon Press, Oxford (1960). 8. PINNICKH. T. and KIWE P., Phys. Rev. 102, 58 (1956). A. (editor) Les Curbones, Vol. I, p. 412. 9. PACAULT Masson et Co., Paris (1965). J., THEOBALD J. G. 10. PACAULTA., UEBERWELD and ~ERUTTI M., C. R. Acad. Sci. Paris 251, 3589 (1965). S., Phys. Rev. 85, 609 (1952); and 11. MROZOW~KI Errata, Phys. Rev. 86, 1056 (1952). 12. ARNOLD G., Curbon 5, 33 (1967). 13. KMETKO E, A., Phys. Rev. 82,456 (1951). A., DELHAESP. and ZANCHRTTA J., 14. MARCHAND J. Chim. Phys. 60,668 (1963).