POSSIBLE WAY TO PREPARE NANOPARTICLES FROM AEROSOLS RELEASED AT PLASMA DEPOSITION

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1 POSSIBLE WAY TO PREPARE NANOPARTICLES FROM AEROSOLS RELEASED AT PLASMA DEPOSITION Vlastimil BROŽEK 1, Libor MASTNÝ 2, Pavel MORAVEC 3, Vladimír ŽDÍMAL 3 1 Institute of Plasma Physics AS CR, v.v.i Prague, Za Slovankou 3, Czech Republic brozek@ipp.cas.cz 2 Institute of Chemical Technology, Prague, Czech Republic, mastny@vscht.cz 3 Institute of Chemical Process Fundamentals AS CR, v.v.i., Prague, Rozvojová 135, Czech Republic Abstract zdimal@icpf.cas.cz During plasma deposition of powdered metals or inorganic compounds using WSP generator, they interact with the plasma having temperature between and K. As a result, they melt, evaporate, thermally decompose, get ionized and/or react chemically both with plasma-forming medium and with the surrounding atmosphere or carrier gas. An experimental apparatus was constructed enabling us to capture aerosol particles emitted from the surface of the free flying particles (FFP) heated in a stream of plasma. In this work we studied content, composition and size distribution of nanosized particles of Ag and oxides of Al, Ti, Cr and W, released during plasma deposition process. These particles were analyzed by dynamic light scattering - photon correlation spectroscopy using instrument MALVERN, and with aerosol spectrometers SMPS 3936 and APS 3321.Concentration of aerosols containing nanoparticles of silver or metal oxides with diameters below 100 nm was found to be in the range 85 to 140 μg/m 3, when we captured particles into an aqueous colloidal solution we obtained concentrations up to 150 mg/l. Key words: plasma spraying; aerosol; nanoparticles production; measurement of particle size distribution; 1. INTRODUCTION Plasma spraying as the most effective way of thermal spraying uses extremely high plasma temperature (e.g generator WSP to K) to melt the powdered metals or their compounds and to deposit them on temporary or permanent substrates. Powder precursors can be deposited in inert, oxidizing or reducing atmospheres and of course in vacuum. In addition, the precursors also interact with plasma or plasmaforming medium. When we select an inappropriate powder size in relation to feeding distance or to the distance from the FD and SD substrate, the powder may evaporate from the large part, and vapor is transported by the highly turbulent plasma towards the substrate. At deposition in the air atmosphere, it is mainly this evaporated material that reacts with oxygen or nitrogen to form oxides or nitrides of nanometric dimensions. Examples of spheroidal freely flying particles FFP of titanium and tungsten covered with oxide powders are shown in Fig.1 and Fig. 2. The amount of nanometric particles as an unwanted product of the plasma deposition can be eliminated to some extent by changing the aerodynamic conditions and geometry of the deposition process. On the other hand it is possible to enhance the nanoparticle production during plasma deposition in order to obtain high nanoparticle yield. Formation of aerosol nanoparticles at industrial plasma deposition can pose a serious health risk for the plant workers, who then need to use specialized and expensive protective equipment. The results of one selected measurement of aerosol nanoparticles produced during plasma deposition process were published previously [1,2].

2 100 nm 1 µm Fig.1 Spheroidized FFP of titanium metal with TiO 2 nanoparticles Fig.2 Spheroidized tungsten metal particles with detail of WO 3 nanoparticles For example, we have found that plasma sprayed chromium oxide produced a bimodal particle number size distribution with mode positions at 30 nm and between 100 and 200 nm. The primary particles with diameters around 30 nm were probably formed by condensation from the gas phase. The interaction of plasma with metallic titanium resulted in several Ti oxides (e.g. TiO, Ti 3 O and Magnéli phases) with rutile form of TiO 2 as the main product. In this case, the particle size distribution was again bimodal with the first main mode between nm and a second flat mode between nm. Plasma spraying can also be carried out by LPPS (liquid precursor plasma spraying) or SPS (suspension plasma spraying) [3,4]. AgNO 3 solution was decomposed in the plasma reactor to metal Ag and oxides of nitrogen. The solution of hexachloroplatinic acid H 2 PtCl 6 yielded the elemental Pt, the content of chlorides and hypochlorites captured in the water separator proved acid decomposition to elemental chlorine. Product parameters depend not only on concentration of the solution but also on the location where the sample is introduced into the plasma. Particle number size distribution of the nano-silver aerosol was monomodal with the mode positioned at 25 nm and a substantial fraction of the nucleation mode below 20 nm. When produced from more concentrated AgNO 3 solutions, the voluminous and deformed splats or coarse silver coatings were formed on the substrates. In this paper, we turn attention to the possibility of effective use of otherwise rather unpleasant phenomenon associated with plasma deposition, namely the formation of aerosol nanoparticles as a process side product. 2. EXPERIMENTAL Metallic and ceramic coatings and self-supporting shell parts (see for example Fig.3) are prepared with production processing capacity from 40 to 100 kg/h of powder precursor. This capacity depends on its melting point, specific heat, and other thermochemical parameters. In our plasma torch of the WSP type with power input of 160 kw (see Fig. 4) we can handle a maximum of 40 kg/h of corundum, but up to 80 kg/h of tungsten, due to its lower melting enthalpy and higher thermal conductivity, even though their melting points differ by more than a thousand degrees. Molten precursors move to the place of deposition inside a conical space with spatial angle of the neutral zone from to 0.02 sr and with angle of the catastrophic oxidation zone from 0.02 to 0.05 sr. Their speed measured by the apparatus DVP 2000 [5] was found to be m/s (see Fig.5 for illustration). The aerosol is formed in the outer zone of the plasma cone containing

3 melted splats and vaporized products. The chemical composition of aerosol particles can be derived from expected chemical reactions between precursor and plasma, or with the surrounding gaseous atmosphere. The apparatus, constructed on the principle of water skimming separator (foam separator, Fig.6), served to capture the aerosol particles semi-quantitatively in order to determine mass, size and morphology of resulting nanoparticles. Fig.3 Different free standing parts (FSP) prepared by plasma spraying on the stabile or temporary substrates Fig.4 Aerosol formation during plasma spraying Fig.5 Temperature and velocity of WO 3 particles [5] aerosol inlet, 2- high-pressure blower, 3- collection fluid, 4-sieve bulkhead, cooling, 6-outlet of the foam separator; 7- inlet of the SMPS 3936 Analyzer, SMPS APS 8- APS 3321 Analyzer, Fig.6 Scheme of the aerosol sampling and water skimming separator (horizontal sampling) 2.1 Parameters of aerosol sampling Sampling of aerosol formed during plasma deposition process at horizontal arrangement was performed as follows (Fig. 6-1.): A vertical sampling tube (40 mm in diameter) was located 50 cm in front of the nozzle of the plasmatron WSP and 30 cm below the axis of the plasma flow. A high pressure blower was used for sucking of the aerosol at the flow rate of 100 m 3 /h. The aerosol was fed into the water skimming separator horizontally through the inlet pipe of 4 cm in diameter. Main exhaust of aerosol in the axis of plasma flow was provided by a large area blower Aretbw N (cca 0.8 m 2 ) with pumping capacity of 7560 m 3 /h.

4 2.2 Analytical methods Two methods were used to measure size distribution of aerosol nanoparticles. Before entering the separator aerosol particles were analyzed by aerosol spectrometers SMPS and APS, the colloidal solution from the foam separator was analyzed by photon correlation spectroscopy. In this work, we used an aerodynamic particle sizer APS 3321 (TSI, USA), used in the Aerosol laboratory of the ICPF. This spectrometer classifies and measures concentration of particles in the size range 0,5-20 micrometers at a sample flow rate of 5 L/min. Operating range declared by the manufacturer is: pressure 0.06 to MPa, temperature 10-40º C, relative humidity 10-90%, maximum particle concentration 1000 particles/cm 3. We also used a Scanning mobility particle sizer SMPS 3936, consisting of an Electrostatic classifier EC 3080 and a Condensation particle counter CPC 3775 [2]. The SMPS covered a particle size range from 15 to 640 nm at a sample flow rate 0.3 l/min. Sampling time of both SMPS and APS was 3 minutes. The dynamic light scattering method called photon correlation spectroscopy was used to measure size distribution of nanoparticles in colloidal solution. The measurement principle is based on the fluctuation of the scattered light signal due to the Brownian motion of nanoparticles. The measured signal was evaluated using the Stokes-Einstein equation and correlation analysis. A He-Ne laser with a wavelength of nm and power of 4 mw was used for the measurements. The detector was directed at an angle of 173 relative to the incident laser radiation. This configuration enabled us to minimize diffraction signal originating from large particles (dust impurities) and perform measurements at higher concentrations. Measurements were performed using a Zetasizer (Nano ZS, Malvern). Before measurements, samples of colloidal solutions were filtered on a Teflon filter with a porosity of 450 nm. 3. RESULTS AND DISCUSSION For plasma projection, powders of Ti, Cr 2 O 3 and W, particle size μm, and 20 wt% solutions of AgNO 3, and (NH 4 ) 2 Cr 2 O 7 were used. Aerosol samples were collected at a distance of 100 cm from the mouth of a plasma torch WSP, type PAL 160. Examples of particle size distribution of nanoscale particles are shown in Fig.7. After entering and absorption of nanoparticles produced in a foam separator, colloidal solutions were separated from the sedimented microparticles, and nanoparticles were then examined by SEM and by qualitative and quantitative XRD analysis. A summary of results is shown in Table 1. SEM images of samples of particles from foam separator are shown in Fig. 8, and examples of particle size distribution are shown in Fig.9. Table 1. Results of product analysis precursor Ti W Cr 2O 3 AgNO 3 (NH 4) 2Cr 2O 7 product TiO 2 ; Ti 3O WO 3 Cr 2O 3 Ag Cr 2O 3 Particle size mean diameter (nm) Content of nanoparticles after 30s of reaction time (mg/l)

5 dn/dlogdp [cm -3 ] , Brno, Czech Republic, EU Cr 2O 3 1,0E+7 1,0E+6 SMPS TiO2 SMPS Cr2O3 SMPS WO3 1,0E+5 1,0E+4 1,0E+3 1,0E+2 SMPS Ag 1,0E d p [nm] Fig. 7. Particle number size distribution of Ag, Cr 2O 3,TiO 2 and WO 3 detected by SMPS spectrometer during plasma deposition. Fig.8. SEM images of the samples of Cr 2O 3, TiO 2, WO 3 and Ag particles separated from the aerosol to carbon foil. Size (d. nm) Ag Size (r. nm) WO nm Size (d. nm) Cr2O3 Cr2O3 Size (d. nm) Cr2O3 Size (d. nm) TiO2 Fig.9 Particle size distributions of nanoparticles Ag, WO 3, Cr 2O 3, and TiO 2 in the colloidal solution from foam separator 3. CONCLUSION Aerosols formed by chemical reaction in plasma from metal or metal oxide precursors have very wide size distribution from 10 to hundreds of nm. In the region of aerosol formation there are also present free flying

6 particles of microscopic sizes; their kinetic energy is sufficiently high that they deposit onto farther substrates, where they form permanent or temporary coating. While micrometric particles arising from primary precursor melt due to high temperatures and spheroidise due to high surface tension, products of their interaction with plasma or with surrounding gas atmosphere (in our case with oxygen from the air) do not reach so high temperature and recrystalize. Nanoparticles of the secondary product are either facetted or, if they have lower melting point, they can be spherical (e.g. TiO 2 nanoparticles). On the other hand, nanoparticles Al 2 O 3 or WO 3 have microcrystalline habit. In our case, the high pressure blower is sampling only part of the aerosol product. Aerodynamic conditions of suction, given by the blower capacity and experimental geometry, control the size of suction product. As a result, partial aerodynamic separation occurs. Therefore, granulometry of nanoparticles collected in the foam separator has very narrow range. When choosing an appropriate starting material for plasma deposition, powder, solution, or suspension, it is possible to intentionally increase the proportion of otherwise undesirable by-products - aerosol nanoparticles. In this work, typical particle size distribution of the studied materials are shown. If we choose properly the precursor feeding, process residence time, and dilution of the output stream, it is possible to vary these size distributions in a relatively wide range. It was also proved that particle size distributions measured by aerosol spectrometers in the gas phase were generally different than particle size distributions of aerosols collected into colloidal solution and measured by photon correlation spectroscopy. This is not surprising, since the aging processes proceed differently in the gas phase than in the liquid. For example, if coagulation takes place in the gas phase leading to formation of larger aggregates of primary particles, after collection of these aggregates into the liquid phase and the associated change in the charging state of clusters, disintegration of the clusters to the original primary particles can occur. ACKNOWLEDGMENT Support of this work by the project of Czech Science Foundation No. CSF P503/12/G147 is gratefully acknowledged. REFERENCES [1] ZDIMAL, V., MORAVEC, P., BROZEK, V. Aerosol nanoparticles formed during plasma deposition. In Proc. Aprochem 2011, s , ed.petrochemeng, ISBN [2] BROZEK, V., MASTNY, L., ZDIMAL, V. et al. Nanoparticles forming in low-temperature plasma. In Proc. CHISA 2011, s , ed. CSCHI Praha, ISBN [3] BROZEK, V., KUTILEK, Z., BERTOLISSI, G. Ceramic coatings prepared by liquid precursor plasma spraying, In APROCHEM 2012, s , ed.petrochemeng, ISBN [4] BROZEK, V., DOLEZAL. B., MASTNY, L. Silver nanoparticles preparation in the thermal plasma, In APROCHEM 2012, s , ed. PetroChemEng Praha, ISBN [5] MATEJICEK, J., et al. Development and Properties of Tungsten-Based Coatings Sprayed by WSP. In Proc. Intl. Thermal Spray Conf., Basel, 2005, s