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, 2004, 46, . 4 Synthesis Mechanism and Technology of Ultrafine Diamond from Detonation Huang Fenglei, Tong Yi, Yun Shourong The National Key Laboratory of Explosion and Safety Science, Beijing Institute of Technology, 100081 Beijing, China Ultrafine diamond (UFD) is a hybrid product of explosion mechanics and material science. In this paper, we established the synthesis mechanism of UFD, built the direct-simulation Monte-Carlo computing method for the generating of liquid drops from free carbon through coagulating in detonation, and constructed the computing programs for explosion thermodynamics conditions and phase-transition dynamics. The synthesis and purification technology were studied systemically.

1. Introduction 2.2. The sect i on of chemi cal react i on. At the hot spots, the decomposed components of the explosive Ultrafine Diamond (UFD) is formed from the un-oxidized are reunited, and release chemical energy, which make the carbon in the negative oxygen balance explosive at high temperature rise and lead to decomposition and reaction pressure and high temperature produced in detonation. The of explosive. Consequently, the chemical energy impels analysis by SAXS and TEM has shown that the particle the leading shock wave move forward continually. As the size distribution mainly is 1-60 nm, and the median size oxygen is deficient, a part of carbon in the explosive is (D50) is 9-11 nm. It possesses some properties of natural left and become free carbon after the detonation reaction.

diamond, simultaneously possesses some properties of When the free carbon collide each other in the reaction nano-metric particle. This perfect combination constructs its zone, the liquid state carbon will coagulate to bigger drop special quality, which is different from the traditional manand the solid state carbon will depart away again after the made diamond and natural diamond. Its bright prospect colliding. In traditional carbon phase plot, the temperature has been predicted in the field of material science, material and pressure of the detonation should sit in the solid surface improving, and even in medicine [1]. A lot of zone. We found that the melt point of carbon particles has research work about it has been done in some countries relation with the particle size, for instance, the melt point include Russian [2], Japan and America. Some research of nanometer metal particles is lower than the block metal.

results have already put into commercial use. In China On the basis of former researchers work and by means of several institutes and universities are doing the related thermodynamics relations, we have a relation between the research. Beijing Institute of Technology is one of them, melt point (T ) and the ultrafine particle size (rs ) as follows:

in which a research group started the systematic research in 1990. Here is a partial research work of ours.

2 s 1 - = s - l, (1) s Hmrs l 2. Synthesis mechanism of UFD here = T /Tm, Tm the melt point of block material, In the detonation process of negative oxygen balance Hm the melt enthalpy of the solid, density, explosive, free carbon was released, and then turn into UFD, surface tension, subscribe s solid phase, subscribe to which mechanism scholars have different viewpoints.

They could not decide that UFD was formed in which section of detonation process involving leading shock wave (Von-Neumann peak), chemical reaction zone (ended in the point CJ) and Taylor zone (Fig. 1). Most of the scholars put up with their mechanism only by theory analysis or experiment and necessary calculation is neglected [3]. In this paper, we managed to find the forming mechanism of UFD by quantitative calculation.

2.1. The sect i on of l eadi ng shock wave.

The molecular calculation shows that the wave front energy can focus several electron volts of energy on the bands of the explosive molecule in 10-10 second and the energy is enough to break the molecular bonds. This splitting only occurs at the local hot spots. The explosive molecule absorbs the energy from the splitting at first, then breaks into individual atoms, free radicals and fragments of molecules, Figure 1. The relation between melt points and the size (rs ) of in which the free carbon is included.


602 Huang Fenglei, Tong Yi, Yun Shourong and passed the test of analytic solution of Smoluchowski equation.

The calculating results by DSMC program are as follows.

1) The number of atom in the original cluster has a little influence on the coagulating.

2) The numerical calculation results is shown in the solid line in the Fig. 3. It shows that when the coagulating time is 1.18 10-9 second, the particle size (rs ) is concentrated on 2 nm, and located at the lower limit comparing with the experiment data 2-4 nm. Here the distribution finction of the coagulating particles is logarithm. The distribution plot by the calculation is shown in dot line in the Fig. 3.

The above results illustrate that it is enough for the UFD to be coagulated in reaction zone (less than 10-9 second).

3) The melt pointsize effect. The calculation about the coagulating is without any condition. Actually, when the Figure 2. The phase graph of block diamond and UFD particles become big enough, the melt point will go up.

(4nm, 10nm). Explosives: 1 TNT, 2 TNT / RDX 60 / 40, In the calculated pressure and temperature, the situation 3 RDX, 4 PETN, 5 BTF.

is already in solid state. Therefore, the particles can not be coagulated after colliding. When this limit condition is added to the process, the time that particles size is coagulated to 5 nm increased to 10-7 second range which is almost the same range as the detonation reaction time.

It is deduced that the limitation of the particle size would come from the limitation of the coagulating process.

In reaction zone, the explosive decomposes continuously and the free carbon is produced corespondingly. Along with the growth, the carbon drop began to crystallize to solid.

The following calculation will prove that the crystallizing process in this zone is very little.

2.3. T h e s e c ton of Tai l or expansi on. In i this section, the decomposing of the explosive is completed.

Although there are chemical reaction, the heat effective is no longer to support the leading shock wave. The Tailor wave will make the temperature and the pressure go down steeply.

In this thermodynamic situation, the crystallizing process Figure 3. The distribution of the particle size at starts. If the state is located in the diamond stabilization 1.18 10-9 second.

zone the diamond will be turn out, and if the state is located in the graphite zone the crystallized diamond would be turned into graphite (called graphitization). At the same time, another kind solid carbon would be formed. As l liquid phase. The above formula is that the one order the temperature dropping, the graphitization process of the Tailor Formula. Put the related parameters of diamond diamond will stop, and the diamond exists in metastable into formula (1), we find that the pressure only has a little state this is the UFD we want to get.

influence on the melt point, but the influence of the particle As the heat effective of the phase transition and the size is bigger (shown in Fig. 1). When the size is less than volume effect are very small relatively, the process can be 10 nm, the melt point decreased rapidly. It can be explained divided into two steps. In the first step, the mechanics as that when the free carbon collide and coagulate to make process and the chemical reaction process are considered the drop bigger the melt point is higher until it arrives the and we can get rules of temperature, pressure, specific detonation temperature (TCJ). In the Fig. 2, the CJ locations volume and the free carbon quantity along with time. In of five explosives are given. It is clear that the CJ location the second step, the crystal dynamics process is considered of the TNT60 / RDX40 explosive is sited in the liquid phase on the basis of conditions gained in the first step.

zone, which indicate that the free carbon from this explosive 2.4. The cal cul at i on on t he cryst a d y n a l could coagulate to bigger drops.

m i c s o f UFD f o r m i n g. In the detonation, the The colliding and coagulating of the free carbon can be crystallizing process of the liquid carbon is carried out described by the Smoluchowski equation [4], which can be under the supercooling condition, and it can be described solved by Direct Simulation Monte-Carlo. For this purpose, by using the spontaneous crystallization theory according to we worked out a computing program by the C language the process of crystal nucleuss nucleation and growth.

, 2004, 46, . Synthesis Mechanism and Technology of Ultrafine Diamond from Detonation vessel as the explosion tank was used in the experiment. It is clear that the partial of the explosives, the kind of the protection media and the charge structure are key factors to determine the synthesis rate of UFD.

3.2. Chemi cal puri f i cat i on. The purification technology plays a very important role in the production of UFD, which contributes the main part to the production cost and affects the products quality in a certain extent.

By means of XRD, TES and laser Raman spectroscopy, it was found that the main impurity of the detonation soot is nanometric graphite and amorphous carbon, which can be cleaned away by the oxidation technology. The normal pressure perchloric acid technology (as perchloric acid Figure 4. The nucleation rate and the growing velocity of UFD. technology), the high-pressure dilute nitric acid technology (as nitric acid technology) and the using permanganate kalium and sulfuric acid technology (as permanganate kalium technology) are three of the practical methods in our study.

The perchloric acid technology is used earlier. It has advantages of simple equipment and good purification efficiency, but has disadvantages that chlorine gas and other harmful gases as well as waste acid are produced in the reaction process, which can result in serious environment pollution. It is limited in doing experiment. The nitric acid technology from Russia is a relative advanced one with high automatic control level. However, at high pressure and high temperature situation, the dilute nitric acid needs expensive titanium as the reaction vessel to keep out of the corrosion, thus a big investment is inevitable for the use of this technology. Even though a medium production line with yield of UFD 107 carat / year has been set up already in the Figure 5. The yield of UFD. CJ the end of chemical reaction zone and the beginning of Taylor zone; G the graphitization experiment area of our university. Comparing the above two point.

technologies the permanganate kalium technology can make up some shortages of them. It has properties of efficient, economical, safe and less environment pollution, and is easy to be put into commercial use. The most important is that From the Von Neumann peak to the Tailor expansion the method can get better purification result than others zone, a special computing programm was worked out. The yield (out from per explosive mass) of UFD was calculated.

The relations of Id(t), Vc(t) and Yie(t) are shown in Fig. 4.

Table 1. Purifying results of UFD by different technologies (from and 5 (where Id nucleation rate of diamond, Vc XPS) growing velocity of UFD, and Yie yield of UFD). It can be seen from the Fig. 5 that the yield of UFD is only Purification Permanganate Perchloric acid Nitric acid 1.1 10-6 till the CJ point. This means that the UFD was technology kalium formed mainly in the Tailor expansion zone.

C (diamond) % 89-92 86-92 90-3. The synthesis and purification Table 2. Further purifying results of UFD (from XRF) technology of UFD Element C Fe Cr Ti Ba 3.1. Det onat i on synt hesi s. The raw materials of Content, % 98.591 0.303 0.219 0.200 0.the synthesis of UFD in our research is a kind of mixed explosive TNT / RDX. The detonation of the explosive Element Cl S P Si Al provide not only the carbon resource, but also the high presContent, % 0.025 0.025 0.001 0.021 0.sure and high temperature conditions needed for forming UFD, which makes the synthesizing method relative simple.

In order to gather the detonation products detonation Element Cu Ca K Mg Na Content, % 0.316 0.025 0.030 0.042 0.soot, and to fill the protection media, a pressurized metal , 2004, 46, . 604 Huang Fenglei, Tong Yi, Yun Shourong Table 3. Particle size distribution of UFD purified by different technologies (from SAXS) Technology Perchloric Acid Nitric Acid Permanganate Kalium Size f (D), Mass Cumula- f (D), Mass Cumula- f (D), Mass CumulaInterval, nm %nm Fraction, % tive, % %nm Fraction, % tive, % %nm Fraction, % tive, % 1-5 100 34.9 34.9 66.2 23.9 23.9 79.2 27.1 27.5-10 95.1 41.4 76.3 100 45.2 69.1 100 42.8 69.10-18 23.1 16.1 92.4 28.6 20.6 89.8 30.3 20.8 90.18-36 2.2 3.5 95.9 4.3 6.9 96.7 1.8 2.8 93.36-60 2.0 4.1 100 1.5 3.3 100 3.1 6.4 Median Size 9.3 10.5 10.D50, nm discussed above. At present, this technology is suitably applied in 105 carat / year scale production.

The purifying results of UFD tested by means of XPS are listed in the table 1, from which the purity of UFD that got from the permanganate kalium technology is superior to that from the perchloric acid technology and the nitric acid technology. If make further treatment, the purity of the products can be improved even better. Here is an example in table 2, which data comes from XRF.

Furthermore, some differences are in existence among the particle sizes of UFD purified through different technologies.

Table 3 shows the test results by SAXS. It can be found, of the three sets test of data, that the particle size distribution from nitric acid technology is closer to which from permanganate kalium technology.

4. Conclusion and Prospect 1) The synthesis mechanism of UFD was established by qualitative analysis and quantitative calculations including explosion thermodynamics, coagulating dynamics of carbon drop and phase transition. The results show that the calculation can be used to forecast the forming process and the yield of UFD.

2) The synthesis and purification technology of UFD were studied systematically. The characteristics of three purification technologies were discussed, which are suitable for different scale production.

3) It is believed that UFD that possesses some of the properties both diamond and nanometer particles will has a bright future.

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