SHOCK-WAVE SYNTHESIS OF METAL NANOPARTICLES FROM CARBOXYLATES B.P. Tolochko 1, V.M. Titov 2, A.P. Chernyshev 1,3 *, K.A. Ten 2, E.P. Pruuel 2, I.L. Zhogin 1, P.I. Zubkov 2, N.Z. Lyakhov 1, L.A. Lukiyanchikov 2, M.A. Sheromov 4 I.L. Zhogin 1, P.I. Zubkov 2, N.Z. Lyakhov 1, L.A. Lukiyanchikov 2, M.A. Sheromov 4 1 Institute of Solid State Chemistry and Mechanochemistry, Novosibirsk, , Russia 2 Lavrentiev Institute of Hydrodynamics, Novosibirsk, Russia 3 Novosibirsk State Technical University, Novosibirsk, Russia 4 Budker Institute of Nuclear Physics, Novosibirsk, Russia *

Dependence of TNT conductivity vs. time (schematic) The sections ab and bc correspond to growing hot spots and ceasing chemical reaction, respectively Conductivity, a.u. a b c d Time, µs

Here  is a typical distance between “hot points”, χ is a mean temperature coefficient of conductivity. For trinitrotoluene χ is about 10 –7 m 2 /s. At τ~100 ns equation we obtain using Michelson that  approximately equals 10 –7 m. The particles as part of explosive The particles as inclusions

Heat exchange It is necessary to emphasize that the energy producing under detonation conditions into precursor particles less than in other region occupied by high explosives. The characteristic temperatures of such a particle less than one of environment due to the thermodynamic processes occurring are adiabatic. Really the time of temperature relaxation is estimated as The calculations gives τ =10 –3 s, that is ~10 3 times as large as the typical time of the reaction mixture formation ~ 10 –6 s. Therefore at first the particles of precursor are heated under shock compression (at first 0.2–0.5 mcs).

The temperature of precursor The temperature T2 was found by the equation : Whereis the Hugoniot adiabat. T2 is approximately equal to 2300 K for AgSt and 1800 K for ZnSt 2 at pressure 34 GPa. It is supposed that  /V=const. Pressure Hot spot

Calculations AgSt ZnSt 2 U, m/s T, K U, m/s T, K U, m/s P, GPa

Thermodynamics under HP Kinetics restrictions on the rate of transformation under high pressure The constant of the chemical reaction rate, r, depends on the pressure: lnr = const – ΔV # ·p/(RT), (3) here ΔV # = V # – V i > 0, ΔV # - the molar volume of the activated complex, V i –the sum of molar volumes of initial species. Hence the precursor heating caused by work of compression is not enough for pyrolysis of carboxylates under super high pressure. More of heat is appeared when the matter of precursor fills pores and other defects under external pressure. As a result of that hot points are formed. The temperature is much higher into hot spots.

Silver stearate shock wave compression a) AgSt; b) T<2300 K, P<340 kbar; c) diamond block structure. a) b)c) Ag nanoparticles capsulated in amorphous carbon X-ray diffraction patterns of Ag and diamond 111 Ag Diamond 200 Ag Diamond 50 Å

SAXS signal behavior from AgSt during detonation

Diffusivity at the initial stage of decomposition Earlier it was shown by us that the typical size of silver particles near 70 Å. Log-normal size distribution takes place. The time of nanoparticles formation is equal to or more than 0.5 mcs. Let us evaluate the order of magnitude of diffusivity employing Einstein formulae. We have D ~ /t ~ m²/s. The value of D is near to the value of diffusivity of liquid D liq ~ 10 –9 m²/s, while in solid state diffusivity equals ~ 10 –12 m²/s at the temperature close to melting point. Thus diffusion properties of medium in which metal particles are formed is close to ones of the liquid state.

The initial stage of pyrolysis occurring through the anhydride

Decomposition of metal carboxylates Decomposition of any metal carboxylate is difinded by strength of bond in molecular. The strengh of bond increase in the following row: М–О < R–COO < С–Н < С–Х (heteroatoms) < C–С. The main type of these reactions is a primary decarboxylation with rupture of R – COO-bond and separation of carbon dioxide:. or. Molecular of carboxylate can decay via formation corresponding carboxylic acid:

Formation of nanodiamonds C y H x →C y (алмаз)+0.5xH 2, Diamonds are formed in the unloading wave by the free-radical mechanism in a media with diffusion properties close to those of a liquid state substance. Nanodiamonds begin to appear from free radicals (CH3, CH2, CH etc.) after 0.5  s of explosion. The catalytic role in the detonation synthesis is performed by atomic hydrogen. Shock-wave impact on metal carboxylates leads to formation of the reaction mixture Metal – C – H – O. It was shown that in the course of these physicohemical processes, metal clusters undergo coalescence and diamond microparticles are formed (if the cathion has catalytic properties). The role of catalysts at detonation synthesis differs from their role in the HPHT process. At detonation synthesis, they are expected either to support sp 3 hybridization or to accelerate formation of compounds to do that.

Diamond formation It was found that after the shock-wave impact on AgSt induced the formation of the particles of diamond. The supposing reaction is following: Alkyls+other radicals → diamond+hydrogen The diamond formation occurs beyond the Chapman- Jouguet plane.

Shock action on zinc stearate A shock action on zinc stearate produced the formation of ultra dispersed ZnO. Fig. 2. Zn(II) n-octadecanoate (C 36 H 70 O 4 Zn) The boiling temperatures of silver, bismuth and zinc are equal to 2485 К, 1837 К and 1180 К respectively. Therefore it is not possible to obtain zinc nanoparticles from ZnSt 2 because of their vaporization. The interaction between vapours of water and zinc leads to formation of ZnO.

The normalized distribution of metal particles by their sizes that sets in during the coalescence process is log-normal. The presence of CO and olefins leads to the growth of the amorphous carbon layer on the surface of metal clusters, which gradually lowers their catalytic activity down to zero and hinders further coalescence of metal nanoparticles.

Nanoparticles of alloys explosion Solid solution of M 1 St+M 2 St M 1 St M 2 St Metal nanoparticles M1M1 M2M2 Alloy nanoparticles Precursors HE Bi+Pb

Conclusion It has been shown that the precursor “particles” under consideration differ from the rest explosive due to the higher content of carbon and presence of the metal catalyst in their chemical composition. It should be noted that energy released at detonation inside the precursor is lower than in the area free of stearates. Since the running processes are adiabatic, the typical temperature of a “particle” will be lower than the surrounding temperature. The temperature equalization time scale is estimated to be ~10 –3 s, which is ~10 3 times as large as the reaction time scale experimentally obtained. Formation of metal nanoparticles in the reaction time scale requires high density of the substance at high mobility of the metal-containing compounds from which the nanoparticles form. High mobility in the dense substance (density of the explosive is about 1,6 kg/m³) is provided by high temperature of the reaction mixture (T~1800÷2300 K). The model suggested implies the metal clusters to grow by the diffusion mechanism, i.e. the “building material” is delivered via diffusion. According to computations, in this case diffusion properties of the medium where metal particles form are close to those of a liquid state. The most important type of reactions at disintegration of carboxylates is the transfer of free radicals. Under detonation, temperature and pressure significantly exceed the analogous parameters in experiments on thermal destruction of metal carboxylates. The short time of the reaction mixture life is compensated by high mobility and concentration of the reagents. The model implies that in the course of the physical-chemical processes metal clusters undergo coalescence and, if the cathion has catalytic properties, diamond micro-particles form. Presence of CO and olefins leads to growth of the amorphous carbon layer on the surface of metal clusters, which gradually lowers their catalytic activity down to zero and impedes further coalescence of metal nanoparticles. It has been shown that the precursor “particles” under consideration differ from the rest explosive due to the higher content of carbon and presence of the metal catalyst in their chemical composition. It should be noted that energy released at detonation inside the precursor is lower than in the area free of stearates. Since the running processes are adiabatic, the typical temperature of a “particle” will be lower than the surrounding temperature. The temperature equalization time scale is estimated to be ~10 –3 s, which is ~10 3 times as large as the reaction time scale experimentally obtained. Formation of metal nanoparticles in the reaction time scale requires high density of the substance at high mobility of the metal-containing compounds from which the nanoparticles form. High mobility in the dense substance (density of the explosive is about 1,6 kg/m³) is provided by high temperature of the reaction mixture (T~1800÷2300 K). The model suggested implies the metal clusters to grow by the diffusion mechanism, i.e. the “building material” is delivered via diffusion. According to computations, in this case diffusion properties of the medium where metal particles form are close to those of a liquid state. The most important type of reactions at disintegration of carboxylates is the transfer of free radicals. Under detonation, temperature and pressure significantly exceed the analogous parameters in experiments on thermal destruction of metal carboxylates. The short time of the reaction mixture life is compensated by high mobility and concentration of the reagents. The model implies that in the course of the physical-chemical processes metal clusters undergo coalescence and, if the cathion has catalytic properties, diamond micro-particles form. Presence of CO and olefins leads to growth of the amorphous carbon layer on the surface of metal clusters, which gradually lowers their catalytic activity down to zero and impedes further coalescence of metal nanoparticles.

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