Control of nanoparticle size in RF thermal plasma synthesis of ...

Control of nanoparticle size in RF thermal plasma synthesisof silicon oxide starting from solid and liquid precursorsM. Boselli 2 , V. Colombo 1,2 , E. Ghedini 1,2 , M. Gherardi 1 , F. Lo Iacono 1 , F. Rotundo 2 , P. Sanibondi 1 , E. Traldi 1Alma Mater Studiorum-Università di Bologna1 Department of Industrial Engineering (D.I.N.)2 Industrial Research Centre for Advanced Mechanics and Materials (C.I.R.I.-M.A.M.)Via Saragozza 8, 40123 Bologna, ItalyAbstract: An inductively coupled plasma torch (ICPT) system was used to synthesizenanosilica particles starting both from glass powders from fluorescent lamps and tetraethylorthosilicate (TEOS) as precursors. The effect of the curtain gas on nanoparticles size,morphology and composition was studied by the use of BET, SEM and EDS analysis.Keywords: RF Plasma synthesis, fluorescent lamps, TEOS, nanosilica, quenching gas.1. IntroductionNanomaterials, i.e. materials that show nanoscalemorphology, have become increasingly important inthe last decades due to their particular characteristics incomparison with bulk materials. Nanoparticles haveshowed to be the starting point of many “bottom-up”processes to realize nanomaterials for severalapplications since their addition can strongly improvethe physical properties of almost all bulk materialstypes. This improvement is given primarily by thenanometric size, which leads to a surface to mass ratio(order of several tens of m 2 /g) much higher than that ofmicro-sized materials. For example, the improvementof mechanical properties has been proved to be afunction of particle size [1]. Furthermore in thenanoscale range the physical properties ofnanoparticles are changed, with respect to the bulkmaterial properties, due to quantum effects such as theSurface Quantum Effect and Quantum confinementsize effect [2] that makes them very promising forelectronic and magnetic applications. In particular,ceramic nanopowders like nanosilica and nanoalumina(nano SiO₂, nano Al₂ ) have being studied and usedin many applications as medicine, concrete science,nano-reinforcement, fuel cells [3-6]. As a consequenceof the growing request for these materials, the interestfor large-scale industrial systems for their synthesis isstrongly increased in the last years.Between the different nanoparticles productionmethods, thermal plasma synthesis is one of the mostpromising, having potential for high production rates(more than 1 kg/h) [7]. Moreover, in a plasma systemlow valuable material can be used as precursors whileother processes, like flame pyrolysis and sol-gel, needmore expensive precursors. However plasma synthesisshows a poor control of the nanoparticles compositionand size. Among all the plasma sources, the radiofrequency inductively coupled plasma torches arecharacterized by higher operating conditions flexibility,higher discharge stability and higher precursorresidence time into the core of the plasma discharge ifcompared with the most widespread DC torches.In this work, the synthesis of nanosilica powders hasbeen carried out using an inductively coupled plasmatorch (ICPT) system, starting from two differentprecursors. In particular, micrometric glass powdersobtained from milled fluorescent lamps and a solutionof tetraethyl orthosilicate (TEOS) has been tested assolid and liquid precursors, respectively. Since glassfrom fluorescent lamps is categorized as waste, itstreatment in an ICPT would be of great economicinterest, going in the direction of producing high valuematerial from waste. On the other hand TEOS has beenchosen as precursor since it’s easier to vaporize andcan provide higher purity with respect to the previousglass precursor.The aim of this work is to identify the main processparameters that influence the synthesis and thecharacteristics of SiO 2 nanopowders.2. Experimental setupAs reported in Figure 1, the plasma system consists ofan inductively coupled plasma torch (Tekna PL-35)powered by a 35 kW generator working at 3 MHz,equipped with a reaction chamber designed for theproduction of metallic nanoparticles and metallicoxides with low deposition on chamber walls. Thechamber is composed of a conical part with two curtaingas injectors, the first at 5 mm and the second at 50mm downstream the torch nozzle and a cylindrical partwith a lateral outlet connected to a sampling filter,where nanoparticles can be collected.

Table 1. Process parameters and filter yield for differenttests with solid precursor.Name ofthe test1A 2B 3A 4A 5A 6APrecursortypeA B A A A AUppercurtain gas 0 3 3 10 0 0(m³/h)Lowercurtain gas 0 0 0 0 0 12(m³/h)Feed rate(g/min)6 6.5 2.3 1 N.A. N.A.Filter yield(g/h)N.A. 21 25 25 5 21N.A. = Not AvailableFigure 1. Schematic of the plasma nanoparticles synthesissystem.A Tekna Plasma Systems powder feeder PF400 hasbeen used for the synthesis of silica nanopowdersstarting from a solid glass precursor, injecting powdersin the plasma torch trough an injection probe. Anelectrical oven has been used to pre-heat precursorsbefore injection.A Tekna Plasma Systems suspension feeder SF-300has been used for the liquid precursor injection usingan atomization probe sending atomized precursordroplets in the core of the plasma discharge by meansof a carrier gas (Ar).3. Nanoparticle and precursor characterizationSize, morphology and composition of synthetizedpowders have been analysed. Specific surface area(SSA) analysis was carried out using a NOVA 2200eanalyser (Quantachrome Instruments), based on BETtheory [8]. Before nitrogen adsorption, samples weredried at 300°C and degassed. A mean diameter ofnanoparticles can be evaluated specific surface areaassuming spherical and dense particles and using thefollowing relation:where D is the mean diameter and ρ is the density(≈2300 kg/m³ for the glass of fluorescent lamps, ≈2650kg/m³ for pure silica). A scanning electron microscope(EVO 50 from ZEISS) was used to study the size andmorphology of precursor particles and of the producednanoparticles; analysis of the chemical composition ofthe particles was carried out using Energy-DispersiveX-ray Spectroscopy (EDS).(1)4. Nanosilica synthesis from solid precursorsMicrometric glass powders used as precursor wereobtained from grinded fluorescent lamps with differentmeshes (average diameter: precursor A = 38-75 μmprecursor B = 75-125 μm). The mercury content hasbeen removed from the glass powders before theplasma treatment, for safety reasons. EDS analysis ofpowders showed that they are mainly composed bySiO₂ and Na₂O with small fraction of other elements asmetallic oxides (O 52.40%, Na 10.87%, Mg 1.50%, Al1.14%, Si 30.19%, K 0.90%, Ca 2.99% by weight, withonly slight variations in different samples). Thesamples were heated at a temperature of 180°C for onehour in an oven to increase the powder flowabilityduring injection. Different tests for the production ofsilica nanoparticles have been performed keeping fixedplasma operating conditions and changing the curtaingas flow conditions, precursor feed rate and precursortype, as reported in Table 1. As a comparison betweendifferent tests, the amount of nanoparticles collected inthe sampling filter has been measured and normalizedwith respect to the duration of the test, thus obtaining amean collection rate in the sampling filter that will becalled “filter yield” in the next sections. Argon hasbeen used as carrier gas and plasma gas with a flowrate of 6 slpm and 13 slpm, respectively. Air was usedfor sheath gas, injected with a flow rate of 60 slpm. Airhas also been used as curtain gas in the reactionchamber, with conditions reported in Table 1. Totalpower of the RF system was set to 30 kW and thereaction chamber operating pressure was set to 70 kPa.Only one test has been done with precursor B since itslow vaporization efficiency induced the formation ofmillimetric SiO₂ crystals in the chamber and near thetorch outlet.Tests with precursor A have been carried out fordifferent conditions of the curtain gas. As can be seenin Table 1 comparing test 5A with other ones, an

Table 2. Results of BET analysisName ofthe testSpecific SurfaceArea (m 2 /g)Mean Diameter(nm)1A 9.4 2782B 17.8 1463A 18.6 140Table 3. Composition (w%) of samples collected in the filterTest O Na Mg Al Si K Ca2B 51 22 0 0.6 24.2 1.4 0.63A 51 18 0.6 0 27.9 1.2 0.7Table 4. Operating parameters for different tests with liquidprecursorTestSheath Gas(slpm)Curtain Gas(m 3 /h)Feed rate(g/min)T1 60 air 6 U/D 2.5T2 60 air 3 U/D 2.5T3 60 air 6 U/D 5Figure 2. SEM image of nanosilica powders synthesized intest 3AFigure 3. Crystalline particle in the sample of test 3Aincrease in curtain gas flow rate results in a higheramount of powders collected in the filter per unit time.High precursor feed rates don’t ensure greatimprovements on filter yield (test 2B and 3A) Nosignificant variation of filter yield has been obtainedincreasing the upper curtain gas flow rate from 3 to 10m 3 /h while reducing the precursor feed rate from 2.3 to1 g/min (tests 3A and 4A). The use of the lower curtaingas inlet is less effective than the upper one, as shownfrom filter yield obtained in tests 4A and 6A. Resultsfrom specific surface area (SSA) analysis and thecorresponding mean diameter have been reported inTable 2. The use of curtain gas induces an increase ofSSA and a reduction of the mean diameter of particlescollected in the sampling filter (see Table 2).SEM images of the test 3A are reported in Figure 2 and3, respectively. Though most of the particles werespheroidized and nanometric, even with diametersmaller than 40 nm (Figure 2), some crystallineparticles reached the filter (Figure 3). These crystallineparticles are micrometric and they reduce the value ofthe specific surface area. Their presence could be theeffect of a not completed vaporization of the precursorin the plasma torch. An excess or an inhomogeneousfeed rate could be the reasons leading to unevaporatedprecursor. The EDS analysis of samples collected inthe filter from tests 2B and 3A (see Table 3) shows thedecrease of Si weight fraction and the increase of Naand K after plasma treatment. The different boilingpoints of Na and Si oxides (1950°C and 2230°Crespectively) lead to an easier nucleation of Na₂Onanoparticles, penalizing the formation of SiO₂ andthen the quality of the final product. Also the presenceof N₂ may lead to this effect, reducing the partialpressure of gaseous SiO and the nucleation of nanoSiO 2 [9].5. Nanosilica synthesis from liquid precursorSynthesis tests have been carried out with liquidprecursor. The influence on the process of the liquidfeeding rate (regulated directly on the suspensionfeeder) and of the curtain gas (injected from twopositions in the reaction chamber) has been evaluated.Process parameters for different tests have beenreported in Table 4, whereas results for filter yield andspecific surface area are shown in Table 5. An increaseof the process yield can be observed for the cases withhigher injection of curtain gas (cases T1 and T3). CaseT1 reaches 130% of improvement of filter yield withrespect to case T2. As can be seen in Table 5,synthetized powders collected in the filter have a lowerdiameter with respect to that of powders obtained withsolid precursors, with BET values of 70-90 m 2 /g.

Table 5. Results of tests with liquid precursorTest Filteryield (g/h)SSA(m 2 /g)Meandiameter(nm)T1 3.8 90 29T2 1.44 70 37T3 3.89 60 43Figure 4. SEM image of nanosilica powders from filterTable 6. Composition (w%) of samples collected in the filterTest C O Si AuT2 5.35 50.96 35.32 8.36T3 3.74 48.95 37.78 9.52In Figure 4, a SEM picture shows that all particlescollected are nanometric and that they show a tendencyto form agglomerates. It can be noticed from EDS thata higher purity of product can be obtained from TEOSwith respect to that of powders obtained from solidprecursors (see Table 6). Comparison of results of testT1 and test T3 (see Table 5) show that increasing thefeed rate the filter yield slightly changes and samplednanoparticles are characterized by slightly lower SSA.6. ConclusionResults show that the ratio between the filter yield andthe precursor feed rate is higher for solid than for liquidprecursors. However, non-vaporized precursor powdershave been collected in the filter in case of solidprecursor, whereas with liquid precursor only nanosizedparticles have been obtained. Furthersimulative/experimental studies will be conducted todetermine operating conditions for the silica synthesisprocess that guarantee optimal evaporation rate of solidprecursors as dependent from their dimension and massflow rate, also taking into account loading effects.Moreover, it has been shown that the use of curtaingases generally gives the possibility to collect a higheramount of powders in the filter and at the same timeleads to a reduction of their mean diameters.To better understand the effects of the curtain gasescomposition on the chemical-physical process ofnanosilica synthesis further studies on the use of aquenching gas need to be conducted.References[1] S. Y. Fu, X. Q. Feng, B. Lauke, Y. W. Mai, Effectsof particle size, particle/matrix interface adhesion andparticle loading on mechanical properties ofparticulate–polymer composites, Composites: Part B39 (2008) 933-961.[2] J. A. Smyder, T. D. Krauss, Coming attractions forsemiconductor quantum dots, Materials Today 14(2011) 382-396.[3] T. K. Barik, B. Sahu, V. Swain, Nanosilica frommedicine to pest control, Parasitology Research 103(2008) 253-258.[4] B. W. Jo, C. H. Kim, G. H. Tae, J. B. Park,Characteristics of cement mortar with nano-SiO2particles, Construction and Building Materials 21(2007) 1351-1355.[5] C. Borgonovo, D. Apelian, Manufacture ofAluminum Nanocomposites: A Critical Review,Advances in Metal Matrix Composites - MaterialsScience Forum 678 (2011) 1-22.[6] D. Kim, M. A. Scibioh, S. Kwak, I. H. Oh, H. Y.Ha, Nano-silica layered composite membranesprepared by PECVD for direct methanol fuel cells,Electrochemistry Communications 6 (2004) 1069-1074.[7] D. Vollath, Plasma synthesis of nanopowders,Journal of Nanoparticle Research 10 (2008) 39-57.[8] S. Brunauer, P. H. Emmett, E. Teller, Adsorptionof Gases in Multimolecular Layers, Journal of theAmerican Chemical Society 60 (1938) 309-319.[9] D. Harbec, F. Gitzhofer, A. Tagnit-Hamou,Induction plasma synthesis of nanometric spheroidizedglass powder for use in cementitious materials, PowderTechnology 214 (2011) 356-364.