SHOCK-WAVE SYNTHESIS OF METAL NANOPARTICLES FROM CARBOXYLATES B.P. Tolochko1, V.M. Titov2, A.P. Chernyshev1,3*, K.A. Ten2, E.P. Pruuel2, I.L. Zhogin1, P.I. Zubkov2, N.Z. Lyakhov1, L.A. Lukiyanchikov2, M.A. Sheromov4 1Institute of SolidState Chemistry and Mechanochemistry, Novosibirsk, 630128, Russia 2Lavrentiev Institute of Hydrodynamics, Novosibirsk, 630090 Russia 3Novosibirsk State Technical University, Novosibirsk, 630092 Russia 4Budker Institute of Nuclear Physics, Novosibirsk, 630090 Russia * firstname.lastname@example.org
5 4 3 2 1 0 0 1 2 Dependence of TNT conductivity vs. time (schematic) b Conductivity, a.u. d The sections ab and bc correspond to growing hot spots and ceasing chemical reaction, respectively. c a Time, µs
Here is a typical distance between “hot points”, χ is a mean temperature coefficient of conductivity.Fortrinitrotolueneχ is about 10–7m2/s.Atτ~100 nsequation we obtain using Michelson that approximately equals 10–7m. 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 calculationsgivesτ =10–3s, that is ~103times as large asthetypical timeof the reaction mixture formation ~ 10–6s. Thereforeat firstthe particles of precursorare heatedunder shockcompression (at first 0.2–0.5 mcs).
The temperature of precursor Hot spot The temperature T2 was found by the equation: Where is the Hugoniot adiabat. Pressure T2 is approximately equal to 2300 K for AgSt and 1800 K for ZnSt2 at pressure 34 GPa. It is supposed that /V=const.
Calculations T, K P, GPa AgSt U, m/s U, m/s ZnSt2 T, K P, GPa U, m/s U, m/s
Thermodynamics under HP Hence the precursor heating caused bywork of compressionis not enoughforpyrolysis of carboxylatesunder super high pressure. More ofheat is appearedwhen the matter of precursorfills poresand other defectsunder external pressure. Asa result of that hot points are formed. • Kinetics restrictions on the rate of transformationunder high pressure • The constant of the chemical reaction rate, r, depends onthe pressure: • lnr = const – ΔV#·p/(RT), (3) • hereΔV#= V#–Vi > 0, ΔV# - the molar volume of the activated complex, Vi–the sum of molar volumes of initial species. The temperature is much higher into hot spots.
Silver stearate shock wave compression a) b) c) a) AgSt; b)T<2300 K, P<340 kbar; c) diamond block structure. Diamond 111 Ag Diamond 200 Ag 50 Å Ag nanoparticles capsulated in amorphous carbon X-ray diffraction patterns of Ag and diamond
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 ~ < x2>/t ~10-10 m²/s. The value of D is near to the value of diffusivity of liquid Dliq ~ 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.
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 CyHx→Cy(алмаз)+0.5xH2, 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 sp3 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 shockactiononzincstearateproducedthe formation of ultra dispersed ZnO. The boiling temperatures ofsilver, bismuth and zinc are equal to 2485 К, 1837 К and 1180 К respectively.Thereforeit is not possible to obtain zincnanoparticlesfrom ZnSt2because of theirvaporization. The interaction between vapours of water and zinc leads to formation of ZnO. Fig. 2. Zn(II) n-octadecanoate (C36H70O4Zn)
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 Solid solution of M1St+M2St Precursors M1St M2St Metal nanoparticles M2 M1 HE explosion Alloy nanoparticles HE explosion 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 ~103 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.