Combustion, Explosion, and Shock Waves, Vol. 39, No. 4, pp. 464–469, 2003
IR Spectroscopic Study of the Organic Component of Ultrafine Diamond Produced by Detonation Synthesis A. Ya. Korets,1 E. V. Mironov,1 and E. A. Petrov2
UDC 666.233:534.222
Translated from Fizika Goreniya i Vzryva, Vol. 39, No. 4, pp. 113–119, July–August, 2003. Original article submitted June 9, 2001; revision submitted April 4, 2002.
IR spectra of ultrafine diamonds produced by different teams of researchers are studied. The effects of heating and radiation on the properties of ultrafine diamonds are studied. Quantitative assumptions on the kinetics of formation of ultrafine diamonds are made from analysis of IR spectra. Key words: ultrafine diamonds, detonation synthesis, IR spectrum.
INTRODUCTION Ultrafine diamond (UFD) produced by detonation [1–4] of a mixture of TNT and RDX is an agglomerate of particles with an average grain size of 3–9 nm and a specific surface area of at least 250 m2 /g. The physicochemical properties of ultrafine diamond are analyzed in terms of different concepts; this ambiguity complicates the understanding of nanostructures. From general thermodynamic considerations, the large ratio of the surface energy to the volume energy for these particles should lead to distortion of the energy band and crystal structures, and, as a consequence, to changes in the electron orbital hybridization and spontaneous graphitization. As is shown in a theoretical study [5], a diamond particle of size less than 5 nm is thermodynamically more stable than a similar graphite particle. Experimental results show that UFD can be treated as a system with a dual level of aggregation: a primary explosive cluster of size 20–60 nm and a secondary aggregation up to 1000 nm and larger [6]. Finally, most chemical experimental studies revealed a constant contribution of organic functional groups to the UFD composition [7–10]. From a chemical viewpoint, this assumption is quite logical. During explosive decomposition of high explosives (HE), the carbon content is 20–30 atoms per 100 atoms, depending on the type of HE. Therefore, one can naturally expect that organic molecules and functional groups formed during 1
Krasnoyarsk State Technical University, Krasnoyarsk 660074;
[email protected]. 2 Federal Scientific and Production Center “Altai,” Biisk 659322.
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HE decomposition, as well as chemical reactions involving these organic components, must affect the processes of UFD formation. The goal of the present work was to study the organic constituents of this process and their effect on the UFD synthesis. Because aggregation has a strong effect on the physicochemical properties of UFD [6], we determined aggregate sizes in the present work using the experimental method proposed in [11] and adjusted to UFD systems in [12]. This method allowed us to avoid difficulties connected with analysis of an X-ray amorphous phase of detonation diamond [13]. To identify and analyze the molecular groups of the UFD organic component, we used infrared (IR) spectroscopy.
1. EXPERIMENT The following samples produced at the scientific centers of Biisk, Krasnoyarsk, and Chelyabinsk were examined: • UFD-1 sample produced by detonation of a mixture of TNT and RDX (40 : 60 mass ratio) and isolated from detonation products by oxidation in the presence of boric anhydride following the procedure of [8]; • UFD-2 sample produced by detonation of a mixture of TNT and RDX (60 : 40 mass ratio) and isolated by treatment with a chrome mixture; • GS sample produced by a shock-wave method from a mixture of RDX and carbon black following the method of [14] and extracted with a sulfur–nitrogen mixture;
c 2003 Plenum Publishing Corporation 0010-5082/03/3904-0464 $25.00
IR Spectroscopic Study of the Organic Component of Ultrafine Diamond • SAA sample produced by shock-wave synthesis from a mixture of RDX and a ASM artificial diamond powder; • samples produced by detonation of a mixture of TNT and RDX (60 : 40 mass ratio): UFD-G is a mixture of diamond and non-diamond carbon (charge) purified from technological additives after explosion and UFD-O was additionally purified by air oxygen. 1.1. Determination of Average Integral Particle Sizes of UFD UFD-G consists of detonation products (carbon black and UFD), and its X-ray spectra show bands typical of graphite and diamond. X-ray patterns of the remaining samples only have bands typical of diamond. Detonation diamond samples produced from the same high explosive composition by different teams of researchers have similar aggregate sizes ranging from 100–200 nm. Samples produced by a shock-wave method have a larger grain size and a slightly larger integral size of aggregates. In both cases, however, the aggregate sizes are within the size range for secondary aggregation. Therefore, it is unnecessary to know the exact size. 1.2. IR Spectrum Structure The real IR absorption of KBr pellets with UFD includes: • absorption of the grain; in our case, this is pure diamond with an ideal crystal lattice (the selection rules exclude this absorption in the single-phonon region of lattice vibrations [15]); • absorption due to crystal lattice defects that violate the rule of prohibition of single-phonon absorption; the most common defects of the diamond crystal lattice are IR active, and their concentration is proportional to the optical density of this absorption; • absorption of the UFD organic component consisting mainly of different functional groups; • weakening related to Rayleigh scattering on heterogeneities. The detonation synthesis samples contain 10–20% (by weight) organic functional groups, which contribute to absorption. The carbon content in the elemental composition is higher in the shock-wave synthesis products than in the detonation synthesis products, and this enhances absorption, i.e., weakening due to scattering. This fact explains some qualitative differences in the IR spectra of samples produced by shock-wave and detonation syntheses, which are generally of similar physicochemical origin [16].
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The elemental composition of detonation UFD is as follows: 84–90% C, 0.8–1.2% H, 1.5–2.5% N, and 8.0–10.0% O [4, 7, 8, 17]. This is due to the fact that a certain portion of carbon and most of oxygen and hydrogen are on the surface. An analysis of the IR spectra revealed the following spectral features of the samples produced by detonation synthesis. 1. The spectra exhibit a characteristic stepwise absorption with the main peak at 1260 cm−1 for UFD-1 (Fig. 1) and 1250 cm−1 for UFD-G with side shoulders at 1205 and 1110 cm−1 , respectively. This absorption is due to the inclusion of two neighboring nitrogen atoms into the diamond crystal lattice. This distortion is called an A-defect [15]. In natural diamonds, the main peak of the nitrogen A-defect is at 1280 cm−1 . The band shift is due to the effect of the finite particle size of UFD on the absorption structure [18]. 2. Absorption at 1110–1140 cm−1 is most typical of UFD-2 (see Fig. 1) and UFD-O. This absorption is obviously due to C O stretching vibration [8] because, in general, oxygen is 8–10% of the mass of UFD particles. For the GS and SAA samples produced by shockwave synthesis, the IR spectra are similar in the structure and positions of absorption bands, indicating that these samples were formed as a result of similar physicochemical processes. The spectra show bands at 3440–3450 and 1620–1640 cm−1 due to the presence of hydroxyl groups. The absorption bands in the spectral regions 2920–2945 and 2830–2850 cm−1 correspond to C H stretching vibrations (asymmetric and symmetric, respectively) within paraffin hydrocarbon groups. The nature of the absorption band at 1100 cm−1 is less clear, and the absorption band at 450–600 cm−1 is also due to the presence of nitro and nitroso groups. In general, the spectral features are typical of the IR spectra of samples produced by detonation. We note that in addition to oxygen-containing functional groups, there are many paraffin hydrocarbon radicals on the surface of diamond particles of both samples (Fig. 2). This leads to the assumption that paraffin groups also contribute significantly to the stability of the diamond particle surfaces.
1.3. Effect of Thermal Treatment on the UFD Properties The UFD-1 and UFD-2 samples were heated in air to 373, 673, and 973 K for 1 h. Below 373 K, the aggregate size did not change. Above 673 K, oxidation began and the aggregates disintegrated, which led to a ≈30% decrease in the integral size. A further increase in temperature enhanced oxidation, and the smallest
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Fig. 1. IR spectra of UFD-1 (curve 1) and UFD-2 (curve 2): Abs is absorption and ν is the wavenumber.
Fig. 2. General structure of IR spectra of samples produced by detonation and shock-wave syntheses: D is the optical density and ν is the wavenumber; for both types of samples, the characteristic absorption of paraffin hydrocarbon groups in the spectral region is at 2800–3000 cm−1 ; 1) UFD-1; 2) GS; 3) UFD-G.
aggregates were first to oxidize. Integral information yields an approximately 38–40% increase in the aggregate size. We note that in general, interaction with air oxygen decreases the aggregate size, and, hence, the stability of UFD particles. It is quite possible that the same trend occurs during formation of the UFD system.
1.4. Effect of Radiation on the UFD Properties Preliminary studies were performed of UFD-G and UFD-1 samples irradiated in a high-energy reactor(fastneutron and gamma radiation). No changes in the IR spectra of UFD-G due to radiation were observed.
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Neither did radiation of the second sample with fast neutrons (E > 1 MeV) at a flow rate Φ = (1.30 ± 0.19) · 1017 neutron/cm2 led to significant changes in the IR spectra. In a sample exposed to γ-radiation of (0.5 ± 0.17) Mrad, absorption changed after the sample was subjected to radiation related to vibrations of hydrocarbon groups (1439, 2859, 2930, and 2970 cm−1 ) [19]. In the present work, we used only high-energy γ-radiation [(183 ± 24) Mrad with energy up to 2.4 MeV). After irradiation of the detonation synthesis products, their IR spectra did not change, in contrast to the UFD-O sample, whose spectrum is shown in Fig. 3. In the UFD-O sample, the absorption band shifted from 1315 to 1275 cm−1 , which indicates the presence of Atype centers. After irradiation, the average aggregate size also decreased from 191 to 130 nm, which also supports this hypothesis. The samples produced by shock-wave synthesis had similar IR spectra. In the SAA sample, the absorption band shifted from 2945 to 2920 cm−1 (see Fig. 3). This shift is due to the effect of radiation on hydrocarbon groups, which leads to changes in the characteristic frequencies of C H stretching vibrations. A similar shift was observed for the GS sample. Generally, the exposure to radiation had a weak effect on the structure of the samples studied. We note only a 30% decrease in the aggregate size for UFD-O, a 15% decrease for GS, and a 3% decrease for SAA; for UFD-G, no changes were observed. 2. RELATIONSHIP OF IR SPECTRA WITH FORMATION OF ULTRAFINE DIAMOND The stability of the IR absorption spectra of some functional groups (hydroxyl, methyl, etc.) in samples subjected to various physicochemical actions (oxidation, radiation) agrees with the assumption that the formation of these organic groups and UFD synthesis are closely related to each other. It is obvious that the molecular groups and defects of the ultrafine diamond phase formed under nonequilibrium conditions should be connected with synthesis either directly or indirectly and contain information on the processes that affect the synthesis. 2.1. Formation of Nitrogen Oxide Most IR spectra of UFD show absorption at 450–600 cm−1 (see Fig. 3). This absorption is contributed mainly by nitrogen– oxygen vibrations in nitro
Fig. 3. IR spectra of radiated and unradiated UFD samples: (a) absorption spectra; (b) optical density; spectra 1 and 4 refer to unradiated UFD-O and SAA samples, respectively, and spectra 2 and 3 refer to radiated UFD-O and SAA samples, respectively.
(R–NO2 ) and nitroso (R–NO) groups. For UFD-1 synthesized in carbon dioxide (and, probably, for all other samples), the source of this absorption band is NO. This oxide can be produced from elementary substances at 3000–4000◦ C [20]. Formation of NO and NO2 oxides was supported experimentally in [21].
468 Formation of this oxide can affect nucleation of the diamond phase, thus increasing the carbon content in the region of formation of the UFD system. The temperature dependence of the UFD yield obtained in [22] for a HE with high oxygen and nitrogen contents can be explained by a temperature acceleration of this reaction.
2.2. A-Type Nitrogen Inclusion An A-type nitrogen inclusion is a defect that occurs in natural diamond. However, the role of this defect is not defined in geological studies [15]. The outer valence shell of nitrogen has five electrons, and nitrogen can undergo eight different degrees of oxidation. In the case of a nitro group, typical of many HE, all five nitrogen electrons are involved into the formation of chemical bonds. In A-defect nitrogen, as well as in the inner-cycle nitrogen of RDX, three p-electrons are involved in the formation of chemical bonds. The remaining two electrons form a lone pair of s-electrons. Indeed, experimental data show that the A-defect nitrogen is not located on the UFD particle surface, the time of UFD formation is short, and the time of existence of the high-temperature zone [23] is relatively short for the dramatic physicochemical changes related to nitrogen. Therefore, it can be reasonably assumed that two nitrogen fragments of RDX do not have enough time to change their configuration, thus forming an A-defect. The result obtained is explained by different rates of formation of diamond particles from fragments of TNT and RDX decomposition in a detonation wave. This conclusion is confirmed by experiments in [24]. This fragment can contribute to the enlargement of carbon nuclei. Therefore, the physicochemical stability of the UFD system should increase. According to [9], diamond-like carbon from detonation synthesis can be characterized as a solid solution of nitrogen in carbon of cubic structure. The stability of this phase is contributed by interstitial nitrogen atoms [9]. However, the chemical functioning of A-defects is more probable. These conditions should lead to formation of molecular nitrogen, apart from formation of nitrogen oxide. For molecular nitrogen, the synthesis temperature is quite low. The structure of molecular nitrogen is such that the lone pair of s-electrons is used. The appearance of chemically active carbon in the reaction zone leads to the formation of an A-defect in diamond. Although molecular nitrogen does not directly affect the formation of the diamond phase, this molecule consists of triple bonds, whose formation
Korets, Mironov, and Petrov requires considerable energy (≈945 kJ/mole). The concentration of nitrogen A-defects is approximately 1016 –1019 centers/cm3 for natural diamond [15] and 1018 centers/cm3 for UFD [25]. A local decrease in temperature can be critical for the stability of UFD particles. Formation of molecular nitrogen during detonation synthesis of UFD in the TNT–RDX system was supported in the experiments of [21]. 2.3. Chemical Stabilization The last important stage in the formation of an UFD particle is the stabilization of the surface. Using a mixture of propane–butane and ethylene as a cooling medium, Petrov [26] reported an anomalously high yield of UFD and, particularly, condensed carbon. Addition of paraffin hydrocarbons to HE does not increase the UFD yield [27, 28]. Thus, the hydrocarbon medium contributes to the formation of the condensed phase during detonation synthesis of UFD in the TNT–RDX system. A comparison of the shock-wave energy with the total bond energy in a TNT molecule shows that the shock-wave energy energy is insufficient to break all bonds in the molecule [23]. The most stable functional group in a TNT molecule is a methyl group. Therefore, incomplete destruction of the methyl group is likely. The formation of an A-defect requires a certain time, during which paraffin groups will most likely be mixed. Under thermodynamic conditions corresponding to the chemical peak of a detonation wave, formation of diamond nuclei is most likely but their conservation requires chemical stabilization. As follows from the aforesaid, it is quite probable that methyl groups contribute to chemical stabilization in the TNT–RDX system [23, 29]. CONCLUSIONS In this paper, we considered some chemical reactions (formation of nitrogen oxide and molecular nitrogen) that occur at high temperatures and affect the formation of the UFD system. It is shown that RDX or HMX can be the main sources of molecular nitrogen. Ultrafine diamond is formed under conditions of high reactivity. The methyl groups situated on the UFD surface decrease the effect of oxidation or other aggressive chemical processes. Since the time of UFD formation is small, incompletely destroyed methyl groups of TNT molecules are more likely to be a source of surface methyl groups of ultrafine diamond. This simple assumption explains the reason for an increased content
IR Spectroscopic Study of the Organic Component of Ultrafine Diamond of methyl labels in experimental studies using the tracer method [23, 24, 29]. The authors thank G. A. Chiganova and V. P. Maloi for assistance and helpful discussions.
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