Effect of detonation synthesis conditions on the phase composition of diamond

Article abstract of DOI:10.1023/A:1012318824667

The phase composition of carbon produced by detonation synthesis was studied as a function of detonation conditions. The results demonstrate that, at an insufficient detonation power, the formation of diamonds may depend on the nature of the electron hybridization in the carbon atoms of the explosive. If the cooling rate of the detonation products is not high enough, the diamond yield may be notably reduced by amorphization.

Full text of DOI:10.1023/A:1012318824667

Inorganic Materials, Vol. 37, No. 10, 2001, pp. 1006–1010. Translated from Neorganicheskie Materialy, Vol. 37, No. 10, 2001, pp. 1185–1189.
Original Russian Text Copyright © 2001 by Chiganova.
Effect of Detonation Synthesis Conditions
on the Phase Composition of Diamond
G. A. Chiganova
Department of the Physics of Nanophase Materials, Krasnoyarsk Scientific Center, Siberian Division,
Russian Academy of Sciences, Krasnoyarsk, Russia
Received August 2, 2000
Abstract—The phase composition of carbon produced by detonation synthesis was studied as a function of det-
onation conditions. The results demonstrate that, at an insufficient detonation power, the formation of diamonds
may depend on the nature of the electron hybridization in the carbon atoms of the explosive. If the cooling rate
of the detonation products is not high enough, the diamond yield may be notably reduced by amorphization.
INTRODUCTION
other reason, supposed to be more likely, is the larger
oxygen balance of hexogen and insufficient mixing of
the detonation products. The higher conversion of hex-
ogen to UFD at higher hexogen contents of the starting
mixture was attributed to the enlargement of the diffu-
sion-mixing zone [2]. According to Anisichkin [3], the
low conversion of hexogen to diamond is associated
with the higher hydrogen content in comparison with
TNT. In the initial stage of transformations in the shock
wave, diamond and methane are formed. Next, methane
oxidizes yielding nondiamond carbon. Accordingly, the
carbon of TNT passes mainly into diamond, and that of
hexogen passes into nondiamond carbon.
Shock and detonation waves are widely used to syn-
thesize new materials. Negative-oxygen-balance deto-
nation of carbon-containing explosives yields highly
dispersed carbon in an amount governed by the equilib-
rium in the disproportionation of CO into C and CO2.
Under certain detonation conditions, the reaction prod-
ucts contain ultrafine diamond (UFD). The known pro-
cedures for extracting UFD from detonation products
rely on the selective oxidation of more reactive, nondi-
amond carbon. Detonation-produced UFD finds wide
practical application in the fabrication of polishing
pastes, composite electroplates, and diamond-contain-
ing composites and films. Although considerable theo-
It was found that, as the fraction of hexogen in the
retical and experimental effort has been concentrated starting mixture increases, the UFD content of the det-
on detonation synthesis, additional work is still neces- onation products first rises rapidly and then (at
sary to understand the detailed mechanisms of detona- 40−50 wt % hexogen) remains virtually constant [4, 5].
tion and raise the efficiency of the process.
Kozyrev and Golubeva [4] explained this finding by the
fact that, at an insufficient power of the detonation
wave, either only partial destruction of TNT molecules
occurs and the resulting fragments are unfavorable for
diamond formation or the rates of particle formation
and growth are too slow, and a significant fraction of the
carbon passes into nondiamond phases. After the
threshold power of the detonation wave is attained, mol-
ecules break down into carbon-containing fragments that
convert to diamond at a high rate. A further increase in
hexogen content does not change the picture.
The products of trinitrotoluene (C H N O ) + hexo-
7
5
3
6
gen (C H N O ) detonation are the best studied, since
3
6
6
6
trinitrotoluene (TNT) is a carbon-rich explosive, and
hexogen decomposition ensures a high pressure in the
reaction system [1]. The composition of such explo-
sives has a significant effect on the yield of condensed
carbon, diamond content, and particle size of UFD. The
results obtained in different studies are typically in
good agreement, whereas their interpretations may dif-
fer drastically. For example, it was found by the
radiotracer technique that diamond is formed mainly
The effect of the retention medium is also inter-
from carbon atoms of TNT, whereas the carbon of hex- preted in different ways. According to Savvakin and
ogen passes, for the most part, into nondiamond carbon Trefilov [6], liquid media enable full retention of deto-
[1–3]. Titov et al. [1] assumed that TNT decomposition nation products, whereas gaseous media lead to graph-
is accompanied by the formation of diamond nuclei itization of the finest diamond particles, accompanied
from bonded carbon groups just behind the front of the by a reduction in diamond yield and specific surface.
shock wave, that is, in the earliest stage of detonation. Petrov et al. [7] found that, as the heat capacity of the
Kozyrev et al. [2] considered two possible reasons for detonation medium decreases, the specific surface of
the predominant formation of diamond from the carbon the resultant diamond powder increases, and the parti-
of TNT. One of them is that diamond particles are cle size decreases. According to Aleksenskii et al. [8],
formed from TNT and hexogen at different rates. The after the removal of nondiamond carbon under identical
0
020-1685/01/3710-1006$25.00 © 2001 MAIK “Nauka/Interperiodica”

EFFECT OF DETONATION SYNTHESIS CONDITIONS ON THE PHASE COMPOSITION
1007
conditions, UFD obtained in a gaseous atmosphere
The specific surface was determined by nitrogen
2
contains a larger amount of an amorphous sp phase on BET measurement. The equilibrium water content was
diamond cores. The amorphous phase originates from determined gravimetrically at 290 K and 70% relative
the reverse transformation diamond
graphite dur- humidity. The surface properties of diamond particles
ing cooling of the detonation products. High-resolution in hydrosols were studied by selective neutralization,
electron-microscopic studies demonstrate that the pH measurements, and macroelectrophoresis (the
retention conditions and the initial density of the explo- experimental conditions and calculation procedure
sive charge have a significant effect on the structure of were reported in [12]).
condensed carbon, consisting of extended and small
graphite-like particles, amorphous carbon, and dia-
mond particles [9].
All of the reagents used were of pure grade. The data
presented for the detonation product and UFD are aver-
ages over 4–6 samples. The confidence interval was
It was also reported that the detonation product is calculated for a 95% confidence level using the Student
nonuniform in its reactivity with nitric acid: the amor- distribution.
phous component oxidizes first; next, oxidation of
graphite microparticles occurs. UFD particles are very
nonreactive with nitric acid and oxidize with a very
RESULTS AND DISCUSSION
slow rate [10]. The separation of nondiamond carbon
phases is impeded by the small particle size of these
phases and the large exothermic effect of oxidation. By
introducing special additions (boric anhydride) and
adjusting the temperature of holding in air, we were
able to develop a procedure for chemical phase analysis
which makes it possible to quantitatively determine
amorphous carbon, graphitic carbon, and diamond [11].
The contents of nondiamond phases in the detona-
tion product were found to depend on the TNT : hexo-
gen ratio (Table 1). The mass yield of condensed carbon
increases with TNT content, in agreement with earlier
data [1, 4, 5]. The UFD content varies very little up to
6
0% TNT and decreases at higher TNT contents, simi-
lar to what was reported in [4, 5].
According to Kozyrev and Golubeva [4], the
increase in the yield of condensed carbon with increas-
ing TNT content is due to the smaller amount of oxygen
in TNT compared to hexogen. The data in Table 1 can
be used to calculate the yield of each carbon species as
a function of the carbon content in the explosive charge.
The results are shown in the figure.
It follows from these data that, under the conditions
of this study, increasing the hexogen (and, hence, oxy-
gen) content of the charge does not reduce the net yield
of condensed carbon. Probably, the higher degree of oxi-
The purpose of this work was to study the effect of
synthesis conditions on the structure of the detonation
product and properties of UFD.
EXPERIMENTAL
We studied carbon powders produced by detonation
of TNT + hexogen charges of the same weight and den-
3
sity in a 2-m chamber and the UFD isolated from the
synthesis products.
The relative contents of different carbon species dation is compensated by a shift of the reaction toward
were determined by chemical phase analysis [11]. liberation of free carbon with increasing pressure.
X-ray diffraction (XRD) analysis was carried out with
The diamond yield varies nonmonotonically with
hexogen content: at 40% hexogen, there is a plateau
a DRON-3 diffractometer (CuK radiation, Ni filter).
α
UFD was isolated by oxidation of nondiamond car- (curve 1). The yield of the amorphous phase increases
bon with atmospheric oxygen at 720 K in the presence steadily (curve 2). The yield of graphite-like carbon
of boric anhydride, followed by boiling in dilute (1 : 5) decreases with increasing hexogen content (curve 3).
HCl. The treatment duration was adjusted based on the These findings correlate with XRD data: with increas-
residual nondiamond-carbon content, which was no ing hexogen content, the intensity of the 002 peak from
greater than 0.5%. The extraction product was washed graphite (2θ = 26°) decreases, and its asymmetry on the
with water to constant pH; the precipitate was separated low-angle side becomes stronger, testifying to an
and dried to constant weight at 380 K.
increase in the content of disordered carbon. Since the
Table 1. Weight yield and phase composition of condensed carbon as functions of charge composition
Weight percent
wt % TNT
Carbon yield, %
UFD
amorphous carbon
graphite-like carbon
4
5
6
7
0
0
0
0
3.2
3.4
3.6
3.8
37
36
36
32
50
48
44
42
12
16
20
26
INORGANIC MATERIALS Vol. 37 No. 10 2001

1
008
CHIGANOVA
η, %
from hexogen. During TNT decomposition in a detona-
2
1
tion wave with a relatively low power, clusters are
formed from already existing carbon groups, but the
6
5
4
3
2
sp -hybridized state is partially retained, which inhibits
diamond formation. Raising the pressure accelerates
restructuring, and the diamond yield increases with
hexogen content. A further increase in pressure may
lead to more active dissociation of TNT molecules into
constituent atoms, and the process will again be limited
by the first step—clustering. In such a case, the break in
the plot of the diamond yield vs. hexogen content (fig-
ure, curve 1) may be associated with a change from one
rate-limiting process to another.
2
1
3
2
2
5
35
45
55
The observed reduction in the yield of sp -hybrid-
wt % hexogen
ized (more highly ordered) carbon with increasing hex-
ogen content (curve 3) is attributable not only to the
corresponding decrease in TNT content; calculations
indicate that the ratio of this form of carbon to the car-
bon of TNT decreases with increasing hexogen content.
It is therefore reasonable to assume that, with increas-
Yield of (1) UFD, (2) amorphous carbon, and (3) graphite-
like carbon in terms of carbon vs. hexogen content of the
explosive charge.
retention conditions (and, hence, the degree of graphi- ing pressure, the breakup of the hybridization configu-
tization) were kept unchanged, it is reasonable to ration in TNT becomes predominant. The increase in
2
expect that the yield of graphite-like, sp -hybridized the yield of the most disordered, amorphous carbon
carbon is influenced by the nature of the molecules of (curve 2) can be explained in an analogous way: first,
the explosive. Indeed, the formation of simple sub- the contribution from the carbon of hexogen increases
stances from atoms can be generally thought of as a and, second, the graphite-like carbon becomes more
two-step process: atom
cluster
crystal. The disordered. Thus, the phase composition of the detona-
crystal structure is controlled by external influences— tion product is governed by the nature of the explosive
pressure and temperature. At the same time, the forma- and the detonation parameters.
tion of diamond and graphite is, in large measure, influ-
The formation of a large amount of disordered car-
enced by hybridization (energetics of carbon atoms).
bon at a low hexogen content may be associated with
For example, in the kinetic stability region, diamond is
the poor retention of diamond in a CO atmosphere. If
the cooling rate is not high enough, the diamond
formed is exposed to high temperature for a short time.
2
extremely difficult to deposit on a substrate if use is
made of uni-, di-, and trivalent carbon atoms [13].
Depending on carbon hybridization in the starting
carbon-containing compounds, two types of graphitiza-
tion are distinguished: homogeneous (fast, without
phase transformations) and heterogeneous [14]. Under
dynamic compression to pressures above 20 GPa, the
crystal lattice of graphite begins to break down, which
is accompanied by changes in bonding configuration
and nucleation of carbynes [15]. It would be expected
that, below a certain power of the detonation wave, the
forming carbon clusters retain the original type of
hybridization. Therefore, the formation of diamond in
Considering that the thermally induced conversion of
diamond to graphite involves the formation of a disor-
dered structure [16] and that the detonation time is very
small, it is reasonable to assume that the dominant pro-
cess is the breakdown of the diamond structure, without
subsequent graphitization. Therefore, more effective
cooling of the detonation products should reduce the
content of amorphous carbon and raise the diamond
yield.
The data in Table 2 illustrates the effect of the reten-
the detonation zone may involve three steps: (1) carbon tion medium on the yield and composition of the deto-
clustering, (2) changes in bonding configuration, and nation product obtained from a 60 : 40 mixture of TNT
(
3) formation of diamond particles. Which step will and hexogen. Improving the retention conditions
limit the rate of diamond formation depends on pres- increases the intensity of the 111 XRD peak from dia-
sure, temperature, and the nature of the molecules of mond (2θ Ӎ 44°) and reduces the intensity of the amor-
3
phous halo. The observed changes in phase composi-
tion lend support to the assumption that UFD is amor-
the explosive. During decomposition of hexogen (sp
hybridization of separated carbon atoms), the rate of the
first step is slow, and diamond particles have no time to
phized in CO . Moreover, the notably reduced yield of
2
be formed. Increasing the power of the detonation wave condensed carbon in CO testifies to partial oxidation.
2
increases the residence time of particles in the high- Taking into account the data in Table 2 and the differ-
pressure zone; in this regime, the conversion of hexo- ence in oxidability between amorphous carbon (highly
gen to diamond increases with hexogen content [2], but reactive) and UFD (inert), we conclude that the reduc-
the first step still limits the rate of diamond formation tion in diamond yield is mainly due to amorphization.
INORGANIC MATERIALS Vol. 37 No. 10 2001

EFFECT OF DETONATION SYNTHESIS CONDITIONS ON THE PHASE COMPOSITION
1009
Table 2. Weight yield and phase composition of condensed carbon as functions of the retention medium
Weight percent
Retention medium
Carbon yield, %
UFD
amorphous carbon
graphite-like carbon
CO2
3.6
6.0
7.5
36
44
56
44
33
18
20
23
26
CO + H O
2
2
H O
2
Table 3. Effect of the retention medium on the properties of UFD
2
Medium
S, m /g
N, mg-equiv/g
pH
–ζ, mV
[H O], %
2
H O
270
280
300
0.60 ± 0.03
0.70 ± 0.04
0.90 ± 0.05
5.8 ± 0.3
5.2 ± 0.2
4.6 ± 0.3
43.0 ± 0.6
41.0 ± 0.5
36.0 ± 0.5
2.5 ± 0.3
2.9 ± 0.2
4.0 ± 0.3
2
CO + H O
2
2
CO2
The retention conditions also influence the proper- carbon species present. As a result, the diamond surface
ties of the UFD isolated from the detonation products becomes more hydrophilic.
(
the necessary heat treatment time was shorter at lower
contents of graphite-like carbon, which is difficult to
oxidize). Upon deterioration of the retention condi-
tions, the specific surface S and equilibrium water con-
ACKNOWLEDGMENTS
I am grateful toA.I. Lyamkin andA.I. Babushkin for
supplying the detonation-produced carbon powders.
tent [H O] increase slightly. This is accompanied by
2
changes in the surface properties of the UFD parti-
cles—number of protogenic groups, N; electrokinetic
potential, ζ; and pH of the hydrosol (Table 3). Clearly,
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1
2
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CONCLUSION
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of diamonds may depend on the nature of the electron
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