Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite
Structural and phase transformations in precursors of the exfoliated graphite (EG) and an effect of various conditions of heating, extension and chemical modification on structure and physical properties of EG-based carbon materials were studied. Some aspects of the forming of the strength character...
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Інститут хімії поверхні ім. О.О. Чуйка НАН України
2002
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| Цитувати: | Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite / Yu.I. Sementsov, M.L. Pyatkovsky, G.P. Prikhod’ko, V.M. Ogenko, I.G. Sidorenko, V.V. Yanchenko // Поверхность. — 2002. — Вип. 7-8. — С. 190-214. — Бібліогр.: 33 назв. — англ. |
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| author | Sementsov, Yu.I. Pyatkovsky, M.L. Prikhod’ko, G.P. Ogenko, V.M. Sidorenko, I.G. Yanchenko, V.V. |
| author_facet | Sementsov, Yu.I. Pyatkovsky, M.L. Prikhod’ko, G.P. Ogenko, V.M. Sidorenko, I.G. Yanchenko, V.V. |
| citation_txt | Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite / Yu.I. Sementsov, M.L. Pyatkovsky, G.P. Prikhod’ko, V.M. Ogenko, I.G. Sidorenko, V.V. Yanchenko // Поверхность. — 2002. — Вип. 7-8. — С. 190-214. — Бібліогр.: 33 назв. — англ. |
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| description | Structural and phase transformations in precursors of the exfoliated graphite (EG) and an effect of various conditions of heating, extension and chemical modification on structure and physical properties of EG-based carbon materials were studied. Some aspects of the forming of the strength characteristics of low-density materials from EG at the principle stages of their preparation are considered. The preliminary results of obtaining of aggregate models of EG-carbon and EG-carbon-carbon fibre are considered.
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190
SOME ASPECTS OF FORMING OF MECHANICAL
CHARACTERISTICS OF CARBON MATERIALS BASED ON
EXFOLIATED GRAPHITE
Yu.I. Sementsov1, M.L. Pyatkovsky1, G.P. Prikhod’ko1, V.M. Ogenko1,
I.G. Sidorenko1, and V.V. Yanchenko2
1Institute of Surface Chemistry, National Academy of Sciences
Gen.Naumov Str.17, 03680 Kyiv-164, UKRAINE
2TMSpetsmash Ltd., Konstantinovskaya Str. 2a, 04071 Kyiv-71, UKRAINE
Abstract
Structural and phase transformations in precursors of the exfoliated graphite (EG) and
an effect of various conditions of heating, extension and chemical modification on structure
and physical properties of EG-based carbon materials were studied. Some aspects of the
forming of the strength characteristics of low-density materials from EG at the principle
stages of their preparation are considered. The preliminary results of obtaining of aggregate
models of EG-carbon and EG-carbon-carbon fibre are considered.
1. Introduction
Among the dispersed forms of carbon a special place is occupied by exfoliated
graphite. EG is derived as a result of structural transformations of graphite intercalation
compounds or their hydrolyzed forms in conditions of fast heating. In particular, in the
conditions of heating of residual compounds of graphite bisulfate (C24
+HSO4
-×2.5H2SO4) up
to temperatures 800-1100°С the powder with a specific surface area 50-100 m2/g can be
prepared. The ability of EG to be moulded in a continuous material without binder allows
one to use it as a basis for a wide row of carbon materials: electrotechnical, antifriction,
constructional, thickeners, fillers of polymers and rubbers [1, 2]. The consumer properties of
composite materials (CM) based on EG, including physical and mechanical characteristics,
are determined by peculiarities of its crystalline and phase structure, as well as by the
strength of bonds between the EG particles, originated during the forming of the material
[3, 4]. To adjust a structural state of the surface and the volume of EG particles within certain
limits is possible by changing conditions of oxidizing and thermal processing of natural
graphite. The chemical modification of the surface of EG particles, in particular
thermotreatment with organic compounds, allows to strengthen their interparticle interaction,
and the quasi-continuous distributed carbon structure in a graphite matrix changes
qualitatively the natural and mechanical characteristics of CM. A considerable increase of the
strength characteristics of CM is reached by an introduction of a high-modulus component -
carbon fibre into the EG-carbon system.
2. Crystalline structure
The source materials - highly oriented pyrolytic graphite of the mark UT and natural
graphite of the mark GAC-2 from Zavalyev's deposit — had the perfect crystalline structure
close to a structure of monocrystalline graphite. It allows generalize the results of
experiments conducted with each of these materials. The intercalation by a sulfuric acid was
conducted by the technique [8] in stoichiometric conditions of oxidation. Crystals of graphite
bisulfate were washed by distilled water up to pH 6-7 of flushing waters, and treated in high
temperature under atmospheric pressure and final temperatures of heating up to 100, 250,
400, and 1000°C. The endurance at the final temperature lasted till the cessation of the loss of
191
the sample weight. Thus, different states of samples were achieved, which were determined
in [9] as residual compounds of graphite bisulfate. The process of intercalation of samples of
graphite and various states of residual compounds were controlled with a X-ray
diffractometer with CoKa radiation (l = 0.17902 nm). Electronic-microscopic analysis was
carried out on a device Tesla BS-540 with maximum accelerating power of 120 kV and
resolution 0.7 nm. Thinning of samples was conducted by the technique [10]. The mode of
matching with a quasimonocrystal source allowed to exclude structural features disconnected
with processes of introduction and deintercalation from consideration [7].
Similar to [5, 7], the key feature of the X-ray pattern of residual compounds of
graphite bisulfate in the basic state (that is after its heat treatment at 100°C) is a significant
broadening of graphite diffraction reflections (002) and the presence of additional peaks.
After processing the samples at temperatures up to 400°С the intensity of additional
diffraction of reflections is redistributed. The width of the line (002) of graphite matrices
decreases too. Such change of diffractogram character testifies on restructuring of residual
compounds, which, apparently, ends at 1000°C. On the electron diffraction pattern from a
plot of a source sample (Fig. 1, a) non-periodically located reflexes, not belonging to a
graphite matrix, are visible. The analysis of the dark field images, according to [14], testifies
that the structure contains thin flap-type selections of phases, distinct from the matrices,
which however have a certain crystal structural conformity with it. With allowance for
structural state of the initial sample it is possible to characterize the detected derivations as
areas, into a certain degree enriched by the intercalant or its residues. Areas of structural
discontinuities (the light fields) have an extended and bent form, both with alternation of
dark and light bars and without it. It can be stipulated by the fact that the structure of the
indicated areas is similar, but differs by a degree of ordering, including a completely
disordered state. The structural discontinuities in the form of flap-type selections, apparently,
are the reason of elastic curving of a chip, to which testify the rows of brightly expressed
reflexes along certain directions curving of a chip, to which testify the rows of brightly
expressed reflexes along certain directions on the picture of microdiffraction (Fig. 1, а).
Besides distinctly expressed structurally ordered phase derivations of two kinds, residues of
intercalant are statistically rare. In areas with obvious texture of a matrix the selections of
structural discontinuities of the round form with brightly expressed primary orientation are
fixed.
The analysis of the change of a diffractogram, and also the pictures of
microdiffraction and appropriate dark and light field of the images of samples after
processing at 250 and 400°С (Fig. 1, d-i) and during heating at 400°С (in situ) [15] detected
the following regularities in the change of a structure of residual compounds. The heat
treatment of samples at 250°С results in such structural transformations as stratification,
which deal mainly with most disordered areas of structural discontinuities, and apparently
causes, the transformation in areas with a more ordered structure. Alongside with the
deintercalation the probability of derivation of phases of intercalant grows too. The heating
up to 400°С results in the dynamic development of processes of structural and phase
transformations. Obviously, all areas of structural discontinuities detected in a source sample
undergo changes too. The phases of the intercalant residues are formed. These samples are
characterised by the greatest diversity both in amount of phases of the intercalant residues
and in an amount of areas of structural discontinuities.
The increase of the heating temperature up to 1000°С causes deep structural
transformations both of disordered phase derivations and phases of the intercalant residues. It
is reflected first of all in the change of a diffractogram. The additional reflections in a small
angular area disappear, half-line width of a graphite matrix decreases. However heating and
endurance of samples even at 1000°С do not result in full deintercalation.
192
Fig. 1. Electron diffraction patterns and appropriated dark and light fields of the images of
specimens of residual compounds of graphite bisulfate, heating under 100 (a, b, c),
250 (d, e, f), 400 (g, h, i), and 1000°C (j, k, l).
193
As a result of transformations of areas of structural discontinuities at least two phases
of the remainders of intercalant of different dispersity are formed. A higher dispersed phase
having an obviously expressed texture of a matrix, can be determined as COS from the
obtained sets of d/n. The areas with the ordered distribution of the remainders of intercalant
are also served.
Thus, the residual compounds of graphite bisulfate (a precursor of exfoliated
graphite) at all stages of heat treatment, including high-temperature fast heating with the
purpose of obtaining EG, represent structurally a heterogeneous system. It includes chips of
graphite, areas of structural discontinuities stipulated by the distribution of the remainders of
intercalant with a various degree of ordering, and also at least two phases of the remainders
of intercalant of various dispersity. The temperature of processing determines the qualitative
structure and quantitative ratio of phase derivations.
3. Structural transition
The structural transformations in samples of residual compounds of graphite bisulfate
(GBRC) under volumetric compression were studied by methods of measurement of
electroresistance and thermo-emf within the temperature range from 200 to 1200°С
according to technique described in [11]. Two variants of structural transitions were
considered: "free" extension (the sample was placed on a substrate) and extension to a
limited volume (the sample was placed in the form limiting changes of its volume).
Overpressure of argon in the high-temperature camera was created by thermocompression
[12]. Conductivity and thermo-emf were measured by a four-probe method [13].
The preliminary measurements of temperature of structural transition of GBRC under
atmospheric pressure conducted by differential thermal analysis, in accordance with [2], have
shown that the temperature range is from 180 to 200°С. The area of maximum rate of weight
loss (according to the data of the thermogravimetric analysis) is 200-250°С. The temperature
dependencies of electroconductivity (s) and thermo-emf (S) while "free" expansion were
studied under the pressure of 5, 10, and 20 MPa.
While heating the s value increases to a certain temperature, then it becomes by leaps
several tens times less and after that decreases linearly with a constant temperature
coefficient (Fig. 2). The thermo-emf decreases from 30-50 to 15-20 mV/K under heating,
then it rapidly reduces to the values close to zero or negative ones. The temperatures of the
leap of electroconductivity and thermo-emf, that is the temperatures of structural transition,
registered when s(Ts
tr.) and S(TS
tr.) are changed, differ significantly (Fig. 3). The results of
their measurement under various pressures are shown in Table 1. Thus, the increasing
pressure causes an offset of temperature of structural transition to the area of higher values
and the extension of its temperature range (DTs, DTS). The dependencies s(Т) and S(Т) have
anomalies at the temperature of about 200°C, that corresponds to the beginning of intensive
weight loss of GBRC under normal pressure.
Table 1. Effect of pressure on the temperature of structural transition of residual compounds
of graphite bisulfate.
Pressure, МPa Тs
tr.,°С DТs
tr.,°С Тs
tr.,°С DТs
tr.,°С
5 290 65 500 65
10 330 100 555 100
20 345 110 575 110
Note: DT - temperature range of structural transition
194
227 727 1227
0
10
20
S, mB/K
s
/K
, O
hm
-1
0
50
T, K
Fig. 2. Temperature dependencies of electroconductivity (s) and thermo-emf (S) of residual
compounds of graphite bisulfate while “free” expansion under pressure of 10 MPa.
1.5 2 2.5
2
4
6
8
10
(16)
(1/T)×103, K-1
ln(P)
0
o
o
o
o
o
Fig. 3. Dependence of the temperature of the structural transition of residual compounds of
graphite bisulfate registered while the leap of electroconductivity under pressure.
The dependence of structural transition temperature on pressure (Fig. 3) registered
while conductivity leap has linear character in accord with [16]. The DH value, defined
according to an inclination of straight line, is 70+2 kJ/mol that almost coincides with DH
value of graphite-bromine intercalation compound.
In this case the affinity of the DH values may testify that the activation process energy
of the thermal expansion is determined by properties of a graphite matrix instead of
individual intercalant characteristics.
The structural transition of GBRC while expansion in a limited volume was studied
under the pressure of 10 MPa. The monotonous change of s(Т) and S(T) is observed when
temperature increasing up to 1200°С (Fig. 4). Electroconductivity increases at the
temperatures up to 700°C and then slightly decreases within the whole temperature range.
When cooling the s(Т) dependence is similar in general to the dependence under heating,
however the electroconductivity has smaller values and the inflection point corresponds to
900°С.
195
227 727 1227
1000
2000
S,
m
V
/K
+10
0
-10
s,
O
hm
-1
×c
m
-1
T, K
-1
-2
Fig. 4. Temperature dependencies of electroconductivity (s) and thermo-emf (S) of graphite
bisulfate residual compounds while expansion in limited volume under pressure of
10 MPa: 1 - heating; 2 - cooling.
Thus, the leap of electronic properties, which accompanies the structural transition of
graphite bisulfate residual compounds under heating, is stipulated by the change of specimen
geometry. When the sizes of specimen are fixed by extension to a limited volume the
properties are monotonously changed. The hysteresis of temperature dependencies s(Т) and
S(T), obviously, testifies to the quasi-continuous "phase" transformations of microareas of
the heterogeneous system (residual compounds of graphite bisulfate) almost in the whole
studied range of temperatures. The decrease of conductivity and thermo-emf transition to the
area of positive values testify that such transformations (intercalant loss, formation of new
phases etc.) result in a decrease of total concentration of current charges and a growth of
relative concentration of positive charges.
4. Exfoliated graphite moulding
The pressing is the main method of materials moulding from EG. The method of
direct pressing (one and double-side) in a mould or rolling of the EG powder is most
widespread. The characteristics of EG powder as a moulding material significantly differ
from that of powders of other materials. Its particles have a complex structure, advanced
surface area, specific “worm-shaped” form, high volatility and propensity to self-compacting.
The pressing of graphite without using a binder is accompanied by formation of
bonds of two types: Van-der-Vaals bonds - between graphite planes, and chemical bonds -
between active sites, which are available on graphite planes and on the sides of crystallites
[18]. The process of EG pressing without using a binder is stipulated by the availability of
thermal destruction products of graphite intercalation compounds (GIC) on the surface of EG
particles [17]. These products play a plasticizer role, providing sliding of EG layers and its
joining during pressing. At the same time the concentration of paramagnetic centres in a
briquette is reduced approximately by five orders while EG moulding [18]. It is explained by
the fact that a lot of carbon radicals with free bond are formed as a result of thermal splitting
196
of graphite intercalation compounds, which recombine and, apparently, influence the strength
of the final product while moulding.
The exfoliated graphite moulding has several stages, and morphology and presence of
defects of EG particles structure render essential influence on the kinetics of pressing
process, i.e. they change dependence of pressed material density (r) on pressure Р [20]. It is
possible to obtain materials with density from 0.1 to 2.2 g/cm3 by a method of direct pressing.
Other perspective method is the thermal pressing of exfoliated graphite in the closed
gas-permeable mould [21]. In this case at first graphite intercalation compounds should be
obtained by any known method, then it should be placed in the closed gas-permeable mould
and heated up. While heating the GIC powder swells out and renders pressure on the EG
powder formed. As the walls of the mould limit the expansion of EG it is packed. The
processes of the expansion and packing occur practically simultaneously. By this method it is
possible to obtain highly porous graphite materials with apparent density from 0.05 to
0.40 g/cm3. In order to obtain material with density of more than 0.2 g/cm3, it is necessary to
pack primarily the GIC powder in a mould. The achievement of density of more than
0.4 g/cm3 is impeded by necessity to apply heat and corrosion resistant moulds and by
tendency of swelled graphite to penetrate through the gas went holes.
Pressings of specimens by a method of unilateral moulding with record of the
pressing diagram as well as their compression test were conducted on 2167-Р50 test unit
[22]. To exclude influence of the dynamic factor on process of structural formation, pressing
was executed with continuous loading and off-loading and at low speed (10 mm/min).
According to the EG pressing diagrams (Fig. 5) the process has complex character; at the
same time the elastic after-effect in a material after removal of pressure (curve 2) plays a
significant role in creation of a pressed material structure. The dependence of pressed
material density on pressure while pressing (curve 1) considerably differs from similar
dependence obtained after removal of load and ejecting of compact according to the results
of measurement of final density (curve 4). It is explained by elastic dimensional change of
pressed materials after removal of pressure. The elastic after-effect is a result of elastic
deformation of the mould and clearing of elastic forces in a powder. The crystallographic
factor of an anisotropy of particles influences significantly the elastic extension [18]. During
EG pressing the stresses in the mould and punches are rather small therefore their elastic
deformation may be neglected. At the same the density of pressed materials approaches
(Fig. 5, curve 1) to X-ray density of graphite equal to 2.26 g/cm3 while pressing of high
density materials from exfoliated graphite [23]. Apparently, in this case the forces of elastic
interaction between graphite planes influence considerably the elastic after-effect. The
products of thermal destruction of graphite bisulfate located on planes of EG particles, which
play a role of a binder in the process of pressing, become a peculiar damper between layers
of graphite. As a result of all this the density of the material is changed by 20-30 % after
removal of pressing pressure.
The coefficient of anisotropy Кa of pressed exfoliated graphite was determined as the
ratio of intensity of X-ray reflections from the (002) plane measured in parallel and
perpendicularly to the direction of application of pressing pressure. The character of change
of a given factor depending on pressed material density (Fig. 6) corresponds to the character
r=f(P) shown on Fig. 5. The pressing process has three stages: I - packing and partial
destruction of particles (up to the density of 0.5 g/cm3); II - mutual moving, splitting,
destruction of particles and their fragments (up to the density of 1.3 g/cm3); III - plastic
deformation of fragments of particles in the whole volume [18, 20, 24].
197
0 20 40 60
0.6
1.2
1.8
2.4
2
1
4
3
P, MPa
III II I
r ,
g/
cm
3
Fig. 5. Dependence of apparent density of pressed exfoliated graphite on pressing pressure
while pressing (1), unloading (2), after removal of pressure (3) and after specimen
pressing off (4).
At the stage I, described by linear change of density depending on pressure of
pressing, Ка increases that is apparently connected with deformation of contact surfaces of EG
particles and partial orientation of their basic planes perpendicularly to axes of pressing. At
the stage II the dependency r=f(P) has curvilinear character, and coefficient of anisotropy
practically is not changed. It can be explained by crushing and mutual moving of already
oriented particles. As a result of such process the increase of anisotropy is stopped, the
porosity is decreased and the boundaries of contacts of initial particles are blurred. At the end
of this stage the structure of a surface layer acquires homogeneous small fragment character,
and the visible boundaries between particles disappear.
0 0.4 0.8 1.2 1.6
4
8
12
16
20
r , g/cm 3
K a
III II I
Fig. 6. Change of coefficient of anisotropy depending on apparent density of the pressed
material.
198
The intensive increase of the coefficient of anisotropy at the stage III (Fig. 6) is
stipulated by formation of a laminated structure with primary orientation of graphite planes
perpendicularly to the direction of effect of the external load. At this stage the reconstruction
of macrolayers of materials, their linking and mutual penetration take place that may cause
formation of similar to lens microdelamination owing to elastic after-effect after removal of
pressure.
5. Mechanical characteristics
According to the sections 3-4 the exfoliated graphite is a complex heterogeneous
system, structural state of which is determined by conditions of EG synthesis and defines
physical and mechanical properties of obtained materials. Some aspects of the formation of
strength characteristics of low-density materials from EG are presented below at main stages
of their production: preliminary stage, stage of intercalation and heat treatment and stage of
moulding.
The specimens 20 mm in height and diameter made of EG obtained from natural
graphite of Zavalyev’s deposit (Ukraine) were used for the researches. A powder of natural
graphite of GAK-2 mark was processed by the concentrated sulfuric acid in the presence of
an oxidizer - ammonium peroxosulfate. After washing and drying the obtained product –
compound of intercalant graphite – was subjected to heat treatment at various temperatures,
therefore the exfoliated graphite with various bulk densities was obtained. The specimens
were produced by the method of unilateral moulding of EG powder in a cylindrical mould as
well as by the method of thermochemical pressing of GIC powder in the closed cylindrical
gas-permeable steel mould at the temperature of 500°C.
The compression tests - both by method of continuous deformation of a specimen and
method of repeated deformation after unload with further increase of load – were conducted
until full destruction (for materials with high density) or achievement of 20 % deformation of
a specimen. The speed of deformation was 2 mm/min. The loading was executed along the
direction of applied pressure while specimens moulding. The value of compression stress at
10% deformation (s10) was used to compare the characteristics of resistance to compression.
5.1. Influence of granulometric composition. Granulometric composition of the
natural flake graphite and the graphite intercalation compounds obtained under standard
conditions of compound oxidation are shown in Table 2.
Table 2. Granulometric composition and fraction parameters of natural graphite of GAK-2
mark and GIC powder obtained on its basis.*
Fraction, Mass content, % Bulk density, g/dm3 Loss of GIC
weight while heat
treatment, %
mm GАК-2 GІC GАК-2 GІC EG
<100 27 9 446 189 22.5 15
100-200 57 51 469 214 7.0 20
>200 16 40 530 193 4.0 26
Total 100 100 476 200 6.0 22
*- mox../mgr.= 0.7; the concentration of H2SO4 is 93%; the temperature of heat treatment
is 800°С.
More than half of particles have dimensions lying within the limits of 100-
200 microns. Intercalation results in growth of average dimensions of particles. Bulk density
of graphite decreases more than twice. A further heat treatment of each separate GIC fraction
has shown that the bulk density of obtained EG decreases when dimensions of particles
increase. Larger particles have bigger loss of weight that is higher intercalant content. The
intercalation of separate fractions of initial graphite particles in identical conditions and their
199
consequent heat treatment have shown, according to [25], the similar tendency of increase of
intercalant content and decrease of bulk density when particles dimensions increase (Table
3). The strength of the pressed EG specimens, obtained from different fractions, also
increases when particles dimensions of initial graphite increase in the studied area.
Table 3. Dependence of loss of GIC weight, EG bulk density and strength of specimens
on initial graphite dispersity.*
Initial graphite
fraction, mkm
Loss of weight
of GIC, %
EG bulk density,
g/dm3
Specimen
density, g/cm3
s10, MPa
> 100 13 26 0.6 1.1
100-200 19 6.5 0.6 1.35
>200 28 3 0.6 1.55
* mox../mgr.=0.7; concentration of H2SO4 is 93.64%; Тh.tr.=800°С
5.2. Influence of acid concentration and heat treatment. Fig. 7 shows the results of
researches of dependencies of EG bulk density on conditions of chemical and consequent
thermal treatment of initial graphite. When one of process parameters (amount of an oxidizer
or concentration of sulfuric acid under intercalation, temperature of heat treatment) was
changed, other parameters remained constant and corresponded to standard conditions
determined in [26]. As Fig. 7 shows, EG bulk density is monotonously reduced when the
amount of oxidizer or concentration of acid or temperature of GIC treatment are increased.
The value of EG bulk density is the most sensitive to concentration of sulfuric acid: decrease
of concentration by 10% results in 10-times increase of bulk density. The increase of the
amount of an oxidizer in the process of intercalation or temperature of heat treatment in the
studied interval results in almost identical decrease of bulk density. More intensively the bulk
density is changed when the amount of an oxidizer increase to approximately 0.7 kg of an
oxidizer per 1 kg of graphite. The further increasing of the ratio mox./mgr. changes
insignificantly the value of bulk density. In Tables 4, 5 the results of compression tests of
specimens made of EG, obtained with different concentration of sulfuric acid (Table 4) and
temperatures of heat treatment (Table 5) are shown. When the concentration of sulfuric acid
increases the strength of specimens monotonously increases. In the dependence of strength
on heat treatment temperature the maximum is observed at the temperature of about 600°С. It
would be expected that EG specimens that have equal density obtained of EG powder with
equal bulk density should have identical strength.
Table 4. Dependence of compression stress of specimens (s10) on concentration of sulfuric
acid (mox../mgr.=0.7; Тhtr=1000°С).*
Concentration
of H2SO4, %
85.3 86.4 88.7 93.64
s10, MPa 1.15 1.7 2.1 2.9
*Specimens density – 1.0g/cm3
Table 5. Dependence of compression stress of specimens (s10) on temperature of GIC
heat treatment (mox../mgr.= 0.7; concentration of H2SO4 is 93%)*.
Тhtr.,°С 300 400 600 800 1000
s10, MPa 1.8 2.76 3.14 3.04 2.9
* Specimens density – 1.0 g/cm3
200
0 0.4 0.8 1.2 1.6 mox./mgr., kg/kg
0
10
20
30
a
2
1
T, °C
g,
g
/d
m
3
200 400 600 800 1000
84 88 92
0
20
40
60
80
b
g,
g
/d
m
3
Ksulfuric acid, %
Fig. 7. Dependencies of EG bulk density on: (a) 1 – oxidizer/graphite ratio (concentration
H2SO4 is 93%, Th,tr=850°C); 2 – temperature of heat treatment of GIC (concentration
H2SO4 is 93%, mox./mgr.=0.7); (b) H2SO4 concentration (mox./mgr.=0.7, Th,tr=1000°C).
However, the analysis of results of strength tests has shown (Table 6) that as well as
in case of different bulk density at various temperatures (Table 5) there is a certain maximum
of strength of specimens made of EG, which was obtained at 600°С.
Table 6. Dependence of compression stress of specimens (s10) on heat treatment temperature
while identical EG bulk density (U=11 g/dm3; specimens density 1.0 g/cm3).
Т,°С 400 600 800
s10, МПа 2.80 3.15 2.85
*(mox../mgr.= 0.7; concentration of H2SO4 is 93%).
201
Such effect, obviously, is stipulated by features of thermal decomposition of
compounds of graphite intercalated with sulfuric acid. The temperature dependence of rate of
GIC-H2SO4 weight loss has two maximums: first at the temperature of about 250°С and
second - in the range from 600 to 850°С [27]. Therefore on a surface of EG particles
obtained at 400°С there is a considerable amount of products of graphite bisulfate
decomposition (sulfate sulfur and sulfur chemically bound with carbon of graphite) [28]. At
temperatures of about 800°С and higher the amount of residual compounds of sulfur
decreases considerably (second peak of thermogravimetric curve, [27]), that, apparently,
worsens the pressing ability of EG and, accordingly, decreases the strength of specimens.
Furthermore, EG obtained at 600°С is characterised by high degree of discontinuity of
particles structure [20] which requires more significant costs of energy for its modification
while deformation. Therefore specimens of low density (up to 1.3 g/cm3) made of EG
obtained at 600°С, have higher resistance to deformation [29].
Fig. 8 shows general dependencies of compression strength on bulk density of EG,
obtained under various conditions of intercalation and heat treatment. For the analysis the
following specimens made of exfoliated graphite were used: specimens obtained under
standard intercalation conditions of oxidation and at variable temperatures of heat treatment;
under condition of variable sulfuric acid concentration and constant content of an oxidizer and
at constant temperature; obtained under standard intercalation conditions at step rise of heat
treatment temperature from 200 to 600°C with interval of 30 minutes after each 100°C as
well as specimens obtained by crushing particles of EG, produced under standard conditions,
in distilled water. As Fig. 8 demonstrates, the most significant decrease of specimens strength
is observed when the bulk density increases at the account of decrease of temperature of GIC
heat treatment (curve 1) and decrease of concentration of sulfuric acid in the process of
graphite intercalation (curve 2). Despite of considerable increase of EG bulk density
(5-8 times) at the account of gradual GIC heating, obtained under standard conditions of
intercalation (curve 3), or at the account of liquid-phase crushing of EG (curve 4), significant
decrease of compression strength is not observed. Therefore, the strength of pressed
exfoliated graphite depends not only on the degree of structure loosening (value of a specific
surface area or bulk density value) but also on a physical and chemical state of the surface of
EG particles that is determined as by properties of initial graphite so by conditions of
chemical and consequent thermal treatments of graphite.
5.3. Influence of graphite over oxidation on the mechanical characteristics. The
graphite intercalation with sulfuric acid in the presence of strong oxidizers (KМnO4,
K2Cr2O7, (NH4)2S2O8, SO3 etc.) is rather complex process, which determines to the large
extend properties of obtained materials. The authors of works [30, 31] have shown that the
GIC phase composition depends on oxidising mixture redox-potential, which in its turn
depends on concentration of a sulfuric acid and amount of an oxidizer in mixture. Authors
affirm that the bulk density of obtained EG is the function of GIC phase composition and
practically does not depend on the nature of an oxidizer. The authors of work [32] assert that
while chemical oxidation of graphite in the presence of strong oxidizers GIC intercalated
layer composition includes peroxodisulfuric HS2O8-anions.
The similar GIC hydrolytic deintercalation may cause formation of surface layer of
graphite oxide in amount up to 6-10 wt. % [33] and increase the strength of final materials.
In this work the influence of concentration of an oxidizer in the system "graphite -
(NH4)2S2O8 - Н2SO4" and sulfuric acid concentration on the strength of pressed EG (with
conditions being equal) was studied. For tensile tests the specimens like strips of variable
cross-section 100 mm in length, minimum width of 5 mm and thickness of 0.3 mm were
used. Specimens were moulded by a method of unilateral pressing of EG powder obtained at
the temperature of 800°С. The density of specimens was 0.9 g/sm3. For compression tests
specimens of 20 mm diameter and height were produced by a method of thermochemical
moulding at the temperature of 500°С in the closed gas-permeable mould; density of
202
specimens was 0.07 g/sm3. Fig. 9 shows dependency of EG bulk density and specimens
tensile strength on amount of an oxidizer per unit of graphite mass with constant content of
sulfuric acid in reactionary mixture.
0 20 40 60
1
2
3
2
3
4
1 s 1
0,
M
P
a
80
g, g/cm3
Fig. 8. Dependence of compression strength (s10) of specimens on change of EG bulk
density (U) while:
1 - change of temperature of GIC treatment (concentration of H2SO4 is 93%,
mox./mgr.=0.7); 2 - change of concentration of a sulfuric acid under intercalation
(mox./mgr.=0.7; HTT=1000°С); 3 - step GIC heating from 200 to 600°С (mox./mgr=0.7;
concentration of H2SO4 is 93%); 4 - liquid-phase crushing of EG particles
(mox./mgr.=0.7; concentration of H2SO4 is 93%, НТТ=800°С). The density of the
specimens is r=1g/cm3.
The sharp decrease of bulk density, when the amount of an oxidizer increases, is
accompanied by considerable increase of tensile strength, which is obviously caused by
increase of EG specific surface area, and accordingly by increase of amount of active sites of
its surface. In the section of bulk density stabilisation the further strength growth is
apparently caused by increase of amount of free radicals of EG surface owing to introduction
of peroxodisulfuric acid anions in graphite and its further decomposition [33]. The further
increase of oxidizer amount results in a sharp strength decrease owing to as GIC particles
sticking together in drying process so EG particles in process of heat treatment. In this case
active sites recombine that results in EG compactibility deterioration and, accordingly, in
material strength decreasing. The similar picture is observed for dependencies of
thermochemical moulded GIC compression strength on amount of oxidizer in reactionary
mixture (Figs. 10, 11).
The graphite crystals swell much more in the intercalation process than while
intercalation under stoichiometric conditions. This process is accompanied by significant
decrease of bulk density of dried GIC (Fig. 10). The swelling of graphite crystals is
apparently caused by not only interlayer intercalant introduction, but also by intensive
extraction of oxygen between carbon layers owing to partial decomposition of introduced
HS2O8-peroxosulfate ions. When graphite-acid mass ratio is equal to 2 (Fig. 10, curve 1) and
at certain amount of an oxidizer the GIC bulk density begins to increase owing to the
particles sticking together in drying process. The particles sticking together is apparently
caused by formation under these conditions of considerable amount of graphite oxide on the
surface of particles in GIC hydrolysis process [33]. The character of change of compression
203
strength of specimens (Fig. 11) and the reasons of this change are similar to ones described
above for tensile strength. In this case with greater amount of sulfuric acid in reaction
mixture the specimens strength becomes slightly higher (Fig. 11, curve 2) than the similar
one with smaller amount of acid (Fig. 11, curve 1).
-0.8 -0.4 0 0.4 0.8
1
2
3
4
10
20
40
g, g/cm3
2
1
s, MPa
lg(mox./mgr.)
Fig. 9. Dependency of EG bulk density (U) (1) and specimens tensile strength (st) (2) on
oxidizer-graphite mass ratio (rsp.=0.9 g/cm3, mac./mgr=2).
0 2 4 6 8
50
100
150
200
250
2
1
1 - m ac. /m gr. = 2
2 - m ac. /m gr. = 4
go
x,
g
/d
m
3
m ox. /m gr.
Fig. 10. Dependency of GIC bulk density on the oxidizer-graphite mass ratio.
204
0 2 4 6 8 10
0.04
0.08
0.12
2
1 1 – mac./mgr. = 2
2 – mac./mgr. = 4
s 1
0,
M
P
a
mox./mgr.
Fig. 11. Dependency of specimens compression strength (s10) on the oxidizer-graphite mass
ratio.
5.4. Influence of material density. The experimental researches have shown that a
loading mode (continuous or repeatedly-static with increasing load) does not influence on
value of stress while given deformation. Fig. 12 shows the data of strength of single-axis
compression of specimens with different density made of EG powders obtained at various
temperatures of the GIC heat treatment. As it is shown, the results of tests are well
approximated by piece-wise linear dependence, which has some characteristic section.
Temperature of the GIC heat treatment changes the compression resistance of specimens.
The analysis of the deformation diagrams has allowed to determine the reasons of this change
(Fig. 13). So, when specimens have low density, irrespective of the EG powders obtaining
temperature, the diagram of specimens deformation is similar to the deformation diagram of
cellular polymeric materials with a small initial linear section stipulated by packing of weak
structure elements, section of elastic deformation of "frame work" of structure and section of
plastic deformation (Fig. 13, а).
The visual-optical experiments have shown [20] that it was stipulated by the above-
mentioned description. The axis of a hysteresis loop is placed inside it that testifies to energy
dissipation in material due to significant structure reconstruction under deformation. In the
density range approximately from 800 to 1300 kg/m3 the structure of the material has finely
grained character, when the EG particles are already destroyed [24]. In the process of
repeatedly-static compression of such structure the line of repeated loading begins to acquire
positive curvature similar to the shape of the off-loading curve and the hysteresis loop axis
begins to exceed its limits (Fig. 13, b). This testifies that most part of energy dissipating in the
material is used for deformation of structure formed during moulding, and to a less degree is
stipulated by the structure reconstruction. Mostly laminated structure with interlayer defects
of continuity forms in EG specimens with density of more than 1300 kg/m3 according to the
visual-optical microscopy. While continuous or repeatedly static single-axis compression of
such structure the deformation curve has S-shaped form. The hysteresis loop axis while
repeatedly-static compression considerably exceeds the limits of a hysteresis loop, and the
curve of repeated deformation is similar to the shape of the curve of the initial section
(Fig. 13, c). Such character of deformation of specimens is apparently stipulated by effect
connected with the presence of the lens-visible pores and microseparations in laminated
205
structure. While deformation of these structures, their elastic resistance similar to lens-visible
springs and consequent manifestation of elastic loss of stability are realized; in this case the
initial section of deformation curve has positive curvature. As a result of collapse of pores
and microseparations the section of the consequent increase of stress and compression
deformation has linear character before the moment of a plastic deformation inside the
material layers. Collapse of microseparations does not provide their disappearance, therefore
the initial section of the compression diagram after relief and repeated application of load is
non-linear till specimen destruction. The analysis of the deformation diagrams has shown
that irrespective of the EG obtaining temperature the inclination corner of hysteresis loop
axis differs from the inclination corner of the initial section tangent line ao, which reflects an
initial rigidity of a structure “frame” (Fig. 13). Subsequently the "frame " rigidity can be
estimated by tangent of the inclination corner of the hysteresis loop axis tga, changing
character of which reflects ability of a material to cyclic hardening or loss of strength during
the process of deformation. It has been determined that during process of repeatedly-static
compression the tangent of inclination corner of hysteresis loop axis ambiguously depends on
exfoliated graphite obtaining temperature (Fig. 14). So, tga of hysteresis loop axis of low-
density specimens has a downward tendency when deformation increases (line I), and its
value does not depend on EG obtaining temperature. The material is cyclically losing
strength up to density approximately equal to 800 kg/m3. For the specimens with density of
800-1300 kg/m3 tga is changed insignificantly when deformation increases (lines 2-5), and in
this case materials are cyclically stable, but they have different rigidity of structure “frame"
depending on EG obtaining temperature. While the density of more than 1300 kg/m3 tga
increases when deformation increases (lines 6-9), in this case materials are cyclically loss
strength, and specimens of exfoliated graphite obtained at 600°С have higher ability to cyclic
hardening.
0 400 800 1200 1600
2
4
6
8
3
4
2
1
s 1
0 , M
Pa
r,kg/m3
Fig. 12. Dependences of compression strength (s10) on the bulk density of the EG specimens
obtained at treatment temperature: 1 – 400, 2 – 600, 3 – 800, and 4 - 1000°C.
206
0 4 8 12
0.5
1.0
1.5
s ,
M
P
a
e , %
a
0 4 8 12
2
4
6
s ,
M
P
a
e, %
b
0 4 8 12
4
8
12
s
, M
P
a
e, %
c
Fig. 13. Diagrams of repeatedly-static single-axis compression deformation of EG specimens
with density: a – 500; b – 1300; and c – 1700 kg/m3.
207
0 4 8 12
20
40
80
100
120
o
o o o o
e, %
1
2
5
3
4
6
7 8 9
I
II
III
IV
tg
a
, M
P
a
Fig. 14. Dependence of tangent of inclination corner of hysteresis loop axis on general
compression deformation of specimens with density of 500 (1), 900 (2-4), and
1700 kg/m3 (5-9) of EG obtained at the temperatures of 400 (/), 600 (//), 800 (///),
and 1000°C (IV).
5.5. Influence of moulding method. The comparative X-ray phase analysis of natural
graphite and EG specimens obtained by the free expansion and by expansion in limited
volume of graphite intercalation compounds has been conducted. The general shape of
diffractograms has appeared to be similar. However, noticeable difference was observed in
profiles of reflections (002), half-width of which was 18.5, 28.0 and 21.0 minutes
accordingly. Physical broadening of X-ray reflection is determined, as it is known, by limits
of coherent dispersion areas and value of microstresses, that is characteristic for the structural
state of a crystal (degree of disordering). Thus, the indicated values of half-width of
reflections (002) testify that the expansion in limited volume results in considerably smaller
disordering of EG structure in comparison with a material obtained by free expansion. The
single-axis compression tests were conducted. Specimens for mechanical tests have been
produced by two methods: unilateral pressing of EG powder in a cylindrical mould and
expansion in limited volume of residual compounds of graphite bisulfate (thermochemical
moulding). Diameter and height of compression specimens were 20 mm. The tests were
conducted using 2167-Р50 test unit under condition of repeated-static loading with an
increasing load and record of the deformation diagram. The vector of loading was directed in
parallel to axis of pressing and deformation speed was 2 mm/min.
The method of repeated-static compression was used to study how the indicated
differences in a structural state influence mechanical properties of materials. To exclude the
influence of specimens macrostructure features, in particular their anisotropy, specimens had
identical low density (~0.1 g/cm3). Having such density they are practically isotropic. The
analysis of dependencies sc on e (Fig. 15) shows that the character of deformation of
specimens, obtained by the free expansion of graphite bisulfate residual compounds, has
mainly elastic character.
208
0 4 8 12
0.06
0.12
0.18
a
s, MPa
e, % 0 4 8 12 e, %
0.08
0.16
0.24
b
s, MPa
0 4 8 12
8
16
24
Kel, %
d c
2
1
0 4 8 12
20
40
60
80
e, % e, %
Ad/Ag, %
2
1
Fig. 15. Dependencies of compression stress (a, b), dissipated deformation energy share (c),
and elasticity coefficient (d) on compression deformation of specimens obtained by
methods of EG unilateral pressing (a, 1) and GBRC expansion in limited volume (b,
2).
It is explained by the fact that while expansion in free volume the particles acquire
properties of elastic elements with high residual stresses owing to high structure disordering
caused by deformation and curving of plane carbon layers and creation of specific defects of
a graphite crystalline lattice and honeycomb structure [5]. It determines the specificity of
deformation process of specimens consisting of such particles (Fig. 15, а). At the first stage
of compression after deformation of weak elements of "frame" formed owing to elastic after-
effect while specimens moulding, the process enters a stage of elastic deformation of
"frame". After that, the compression passes to stage of elastic-plastic deformation of the
structure elements. The curve of unloading-repeated loading form a wide hysteresis loop, and
at the same time dissipated deformation energy Ad (the square of a hysteresis loop) constitute
a significant part of general energy Аg., used for deformation of the material (square under
the curve of deformation). When deformation increases the share of dissipated energy
initially increases (Fig. 15, c, curve 1) and the internal stress and defectiveness of structure
209
are increased. When the critical level of internal stresses achieved the elements of the
structure stop to act as elastic elements and the processes of local and general destruction of
particles begin. As a result the share of dissipated energy considerably decreases, and the
deformation acquires mainly plastic character (Fig. 15, d, curve I).
While expansion of GBRC in limited volume the process of appearance of residual
stresses connected with disordering of the structure, is apparently accompanied by their
simultaneous relaxation owing to interaction of particles while stacking in the mould. Due to
this the plasticity of the structure elements of the specimens is increased, and the
compression diagram is essentially changed (Fig. 15, b). The elastic-plastic deformation
begins practically at the same time when compression stress begins to increase, the share of
dissipated deformation energy is insignificant (Fig. 15, c, curve 2) and continuously reduces
when deformation increasing. The deformation has mainly plastic character (Fig. 15, d, curve
2).
Thus, the difference in a structural state of the graphite particles obtained by the
extension of graphite bisulfate residual compounds in free and limited volumes, results in
change of character of resistance to compression of materials made of these particles.
6. Effect of chemical modification of EG surface by thermosetting polymers
on mechanical characteristics of composite materials
The distribution of a thin layer of a polymeric material on the EG advanced surface and
consequent its carbonization in a compact form allows to create a composite material
including two mutually penetrating structures of crystalline graphite and amorphous carbon
material. Thus, at the account of combination of EG plasticity and amorphous carbon
hardness, taking into account their high adhesive interaction, it is possible to regulate
mechanical characteristics of such composite material and to create EG-carbon-carbon fibre
systems. The average characteristics of the studied specimens are shown in Table 7.
The comparative analysis of the compression diagrams of specimens of “pure" EG
and EG-carbon compositions (Figs. 16, 17) have shown that the carbon component of
specimens causes considerable change in behaviour character of a material during
compression process.
During the process of repeated-static compression of EG specimens the lentil-shaped
hysteresis loop of large area is observed that testifies to significant dissipation of deformation
energy in the material connected with restructuring during deformation. In this case the share
of elastic component of deformation (Кel.), which can be determined as the ratio of elastic
component of deformation after unloading (eel.) to general deformation achieved on the
moment of unloading (eg.), continuously decreases when deformation increases. While large
deformations (more than 10 %) plastic component prevails in deformation process.
Table 7. Initial and final parameters of specimens of EG-carbon and EG-carbon-carbon fibre
(CF) compositions.
Composition Concentration of
polymer in initial
mixture, wt. %
Initial density of
specimens, g/cm3
Final density of
specimens, g/cm3
Carbon
concentration
in specimens,
%
EG-carbon 30 1.20 1.10 22
EG-carbon 50 1.20 1.07 35
EG-carbon 70 1.20 1.00 47
EG-carbon-CF 30 1.20 1.06 21
EG-carbon-CF 50 1.20 0.97 47
EG-carbon-CF 70 1.20 0.99 61
210
4 8 12 16 20
0
1
2
3
eS
eel. eg
s,
M
P
a
e, %
Fig. 16. Repeatedly-static single-axis compression deformation diagrams of EG.
The introduction of carbon component within the limits of the studied concentration
changes behaviour character of materials while compression (Fig. 17). The diagrams of
deformation become similar to the diagrams of plastic materials compression, which has
ability of deformation without destruction [29]. Such materials are characterized by the
positive curvature of the diagram in the section of elastic-plastic deformation and fore-
stalling stress growth while increase of deformation. As it is shown in Fig. 17, for the studied
compositions of the unloading-repeated loading diagram the hysteresis loop also forms. This
loop has also lentil-shaped form under condition of small deformations. However, when
deformation increases the loop begins to curve and hysteresis loop axis exceeds its limits.
This, apparently, testifies to formation of the closed hollow defects of a lens-visible shape in
the studied materials while large deformations. During repeated loading such defects are
deformed as the lens-visible springs and cause such shape of the hysteresis loop. Under
condition of small deformations the stress values in the EG-carbon compositions differ
insignificantly from the same values for initial EG specimens. However, while large
deformations the compositions EG-carbon have considerably large strength.
0 4 8 12 16
1
2
3
4
e, %
s,
M
P
a
Fig. 17. Diagrams of repeatedly-static single-axis compression deformation of EG-carbon
composite materials.
211
As it is shown in Fig. 18, the change of carbon component concentration within
studied limits has little influence on compression resistance, and the studied compositions
have approximately equal strength.
0 4 8 12 16
1
2
3
4
o
o
o
o
o
o
o
o
o
- 22
- 35
o- 47
s,
M
P
a
e , %
Fig. 18. Compression deformation diagrams of EG-carbon composite materials with carbon
concentration 22, 35 and 47 wt. %.
The share of elastic component of deformation for the studied compositions, as well as for
"pure" EG, continuously decreases when deformation increases, although while large
deformations some stabilisation of elastic properties of materials (Fig. 19, curves 1-3) is
observed.
0 4 8 12 16
0.4
0.6
Kel., %
6
5
4
7
3
2
1
0.2
e, %
Fig. 19. Dependencies of elastic coefficient on specimens deformation for EG-carbon (1-3);
EG-carbon-CF (4-6); and EG (7).
212
As it is shown in the compression diagram of EG-carbon-carbon fibre compositions,
the introduction of small amount of carbon fibre into material influences considerably
behaviour character and strength of composition during compression (Fig. 20).
0 4 8 12 16
2
4
6
8
10
12
3
2
1 s ,
M
P
a
e, %
Fig. 20. Compression deformation diagrams of EG-carbon-carbon fiber composite materials
with carbon concentration 21 (1); 47 (2) and 61 wt. % (3).
If at small carbon concentration the compression diagrams differ insignificantly from
the diagrams for EG-carbon compositions as to the character and stress level, the
compression diagrams acquire practically linear character as far as the specimens destruction,
and the destruction has a brittle character while increase of the carbon component
concentration. The strength of specimens is increased by a factor of 3-5, and the share of
elastic component prevails in deformation process (Fig. 19, curves 5,6). An elasticity
coefficient Кel. increases even before specimen destruction in case of EG-carbon-CF
compositions with concentration of carbon of 61 wt. %.
Thus, the introduction of carbon component in EG changes the behaviour character of
materials during compression and increases their strength while large deformations. Within
the studied interval of carbon concentration (20-45%) it has little influence on compression
resistance, therefore specimens have approximately identical strength. The deformation of
EG-carbon-carbon fibre composition with large carbon concentration mostly has elastic
character. The strength of such compositions exceeds the strength of EG-carbon materials by
a factor of 3-5, and the destruction has a brittle character.
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| id | nasplib_isofts_kiev_ua-123456789-126366 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | XXXX-0106 |
| language | English |
| last_indexed | 2025-12-07T13:39:24Z |
| publishDate | 2002 |
| publisher | Інститут хімії поверхні ім. О.О. Чуйка НАН України |
| record_format | dspace |
| spelling | Sementsov, Yu.I. Pyatkovsky, M.L. Prikhod’ko, G.P. Ogenko, V.M. Sidorenko, I.G. Yanchenko, V.V. 2017-11-20T18:54:55Z 2017-11-20T18:54:55Z 2002 Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite / Yu.I. Sementsov, M.L. Pyatkovsky, G.P. Prikhod’ko, V.M. Ogenko, I.G. Sidorenko, V.V. Yanchenko // Поверхность. — 2002. — Вип. 7-8. — С. 190-214. — Бібліогр.: 33 назв. — англ. XXXX-0106 https://nasplib.isofts.kiev.ua/handle/123456789/126366 Structural and phase transformations in precursors of the exfoliated graphite (EG) and an effect of various conditions of heating, extension and chemical modification on structure and physical properties of EG-based carbon materials were studied. Some aspects of the forming of the strength characteristics of low-density materials from EG at the principle stages of their preparation are considered. The preliminary results of obtaining of aggregate models of EG-carbon and EG-carbon-carbon fibre are considered. en Інститут хімії поверхні ім. О.О. Чуйка НАН України Поверхность Surface properties of inorganic materials Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite Article published earlier |
| spellingShingle | Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite Sementsov, Yu.I. Pyatkovsky, M.L. Prikhod’ko, G.P. Ogenko, V.M. Sidorenko, I.G. Yanchenko, V.V. Surface properties of inorganic materials |
| title | Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite |
| title_full | Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite |
| title_fullStr | Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite |
| title_full_unstemmed | Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite |
| title_short | Some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite |
| title_sort | some aspects of mechanical characteristics forming in carbon materials based on exfoliated graphite |
| topic | Surface properties of inorganic materials |
| topic_facet | Surface properties of inorganic materials |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/126366 |
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