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

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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.
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