Experimental Correlation between Particle Size and Laser-Induced Incandescence (LII) Time Decay in TiO 2 Flame Synthesis

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Biaggio Moretti
7 anni fa
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1 Consiglio Nazionale delle Ricerche Istituto per l Energetica e le Interfasi Milano ITALY Experimental Correlation between Particle Size and

1 1 Consiglio Nazionale delle Ricerche Istituto per l Energetica e le Interfasi Milano ITALY Experimental Correlation between Particle Size and Laser-Induced Incandescence (LII) Time Decay in TiO 2 Flame Synthesis C. Bellomunno, S. Maffi, F. Cignoli, G. Zizak and G. Angella bellomunno@ieni.cnr.it

2 Consiglio Nazionale delle Ricerche 2 Particles formation & growth FLAME SYNTHESIS: AN OUTSTANDING METHOD TO PRODUCE NANOPARTICLES burner Flame manufacturing of nanoparticles: After Wegner et al., Chemical Engineering Science 58 (2003) 4581

3 3 Reaction Burner and Sampling set-up 10 cm V CH4 = 0.7 Nl/min; V AIR = 8 Nl/min; Φ = 0.81; Ti(OC 3 H 7 ) 4 TiO 2 V N2 = 0.2 Nl/min

4 4 TiO 2 size evolution along the reaction flame TEM images of particles collected at: a) 5mm HAB b) 15mm HAB c) 25mm HAB d) 50mm HAB a b c d 20 nm HAB =Height Above the Burner

5 5 TiO 2 phase evolution along the reaction flame % R ANATASE (A) HAB [mm] Phase composition from XRD spectra RUTILE (R) 1 1 R = A R

6 6 It would be very useful the availability of a diagnostic technique allowing insights in the particle formation and growth process. In particular, an ON-LINE DIAGNOSTICS would be very attractive : near real-time response; spatial resolution; non-intrusive. in gas-phase reaction some chance

Consiglio Nazionale delle Ricerche 7 Laser Induced Incandescence: PRINCIPLES Absorption Qabs D 3 Incident laser Change of the internal energy Sublimation Q sub D 2 α j (P sat -P surf ) j Heat

7 Consiglio Nazionale delle Ricerche 7 Laser Induced Incandescence: PRINCIPLES Absorption Qabs D 3 Incident laser Change of the internal energy Sublimation Q sub D 2 α j (P sat -P surf ) j Heat conduction Q cond D 2 α T P(T-T a ) Q int = Q abs - Q sub -Q cond -Q rad Thermal radiation Qrad D 3 T 5 The LII signal is based on the energy and mass balance between a single particle and its surrounding

8 Consiglio Nazionale delle Ricerche 8 LII signal: A powerful tool in SOOT diagnostic 0 L II s ig nal [ a.u.] prompt LII f V Decay rate f(d p ) E E E E E E-07 time [sec] FROM SOOT TO OTHER NANOMATERIALS

9 Consiglio Nazionale delle Ricerche 9 How could react particles different from soot to strong laser irradiation? R.L. Vander Wal et al., 1999 Laser Induced Emission (LIE)

10 Consiglio Nazionale delle Ricerche 10 LII on Non-Carbonaceous particles: STATE OF THE ART Metals nanoparticles by laser ablation (R.L. Vander Wal et al., 1999) Various nanoparticles at room T in a cell (A.V. Filippov et al., 1999) Oxides np by laser evaporation reactor (A. Leipertz, S. Danckers, 2003) Manganese oxide by laser evaporation reactor (T. Lehre et al., 2005) Fe from a hot-wall reactor (B. F. Kock et al., 2005) TiO 2 flame synthesis (S. Maffi et al., 2008)

11 11 LII ON TITANIA FLAME LIE and TIRE-LII : Operative Condition Excitation Wavelength Laser Fluence Detection Wavelength 266 nm (4 harmonic of the Nd:YAG laser)

12 TIRE-LII and LIE set up 12 TEMPORAL ANALYSIS SPECTRAL ANALYSIS At different laser fluences: On time and delayed LIE spectra LII signals at different HAB Excitation wavelength: 266 nm Detection wavelength: 300nm-600 nm (20 nm step)

13 Consiglio Nazionale delle Ricerche 13 LIE spectra at High Laser Fluence Highly structured shape mm HAB 30 mm 50 mm 4000 Intensity (a.u.) at different HAB Wavelength (nm) Excitation: 266 nm Fluence: 600 mj/cm 2

14 Consiglio Nazionale delle Ricerche 14 LIE spectra at different Laser Fluences: TIME EVOLUTION Intensity (a.u.) mJ/cm 2 60mJ/cm 2 prompt 20 ns 50 ns Intensity (a.u.) prompt 40 ns 75 ns Wavelength (nm) Particles destroyed Wavelength (nm) Spectrum is less structured

15 15 Prompt LIE spectra at Different Laser Fluences intensity (a.u.) mj/cm 2 60 mj/cm 2 20 mj/cm wavelength (nm) fluence values up to about 40 mj/cm 2 give satisfactory results for titania at 266 nm

16 Consiglio Nazionale delle Ricerche 16 Time decay of the emission signal at Different Laser Fluences Excitation: 266 nm Detection: 415 nm HAB: 20 mm dp= 20 nm (by TEM)

17 Consiglio Nazionale delle Ricerche 17 LII signals at Different HAB Excitation: 266 nm Detection: 415 nm Fluence: 20 mj/cm 2 LII signal is sensitive to particle size S. Maffi, F. Cignoli, C. Bellomunno, S. De Iuliis, G. Zizak, Spectrochim. Acta Part B (2008)

18 Operative procedure 18 TEM - IA particle sampling on TEM grids at different location in the flame; TEM images; image analysis; particle size statistics and lognormal distribution of particles for the whole flame LII choice of the suitable LII operative conditions (best detection wavelength, trade-off between SNR, probe volume size, spectral integration, dynamic range); TIRE-LII recording and processing for the whole flame

$19 TEM IA Particle Size Statistics a (fraction/δln d) 2.0 1.5 1.0 0.$ 5 particle diameter (nm) b) 0.0 1 10 100 particle diameter (nm) d (nm) 48.

19 19 TEM IA Particle Size Statistics a (fraction/δln d) a) d (nm) 17.5 σ 1.23 b (fraction/ Δ ln d) particle diameter (nm) b) particle diameter (nm) d (nm) 48.5 σ 1.30 TEM images and lognormal distribution of particles at 15 mm a) and 50 mm b)

20 20 Counter Median Diameter (CMD) of titania particles in the reaction flame CMD (nm) particle size INCREASES along the flame HAB (mm) counter median diameter obtained by the lognormal distribution of particles

21 21 Temporal evolution of the Emission spectrum (uncorrected for system spectral response) ns delay 500ns delay intensity (a.u.) intensity (a.u.) wavelength (nm) wavelength (nm) just after the laser pulse the maximum signal is at short wavelengths (higher temperature) but

22 LII signal at different detection wavelengths 1.0 at 20 mm above the burner 22 Norm. LII intensity nm 450nm 320nm time (ns) at longer wavelengths the signal drops more slowly allowing better SNR at longer times

23 1.0 Consiglio Nazionale delle Ricerche LII signal taken with a 5mm step along the titania flame 23 Norm. LII intensity Excitation: 266 nm Detection: 600 nm Fluence: 35mJ/cm 2 probe volume: 1x2x3 mm 3 increasing particle size; decreasing T time (ns)

24 Conclusion, Problems and Perspectives LII signal can be rather safely obtained in flame reactors; LII decay time is clearly and continuously depending on particle size; Suitable LII operative conditions should be selected: 1. Excitation wavelength in the UV region 2. Laser fluence carefully determined through spectral test 3. Detection wavelength Signal quality can be further increased; a real-time measurement is in principle feasible (as it is for soot already) at least for TiO 2. A suitable model seems to be mandatory and TiO 2 parameters must be well known. If successful, the method offers unique advantages 24

25 25 Milano Thanks for your attention!

26 26 ABSORPTION OF LASER IRRADIATION Q abs : absorption efficiency E i : laser irradiance HEAT LOSS DUE TO CONDUCTION Λ: heat transfer coeff. T: particle temperature T 0 : gas temperature VAPORIZATION ΔH v : heat of vaporization M: molar mass RADIATION ε: emission coeff. M b λ : blackbody spectral radiant exitance λ: wavelength CHANGE IN INTERNAL ENERGY ρ: density C: specific heat

27 Consiglio Nazionale delle Ricerche 27 LIE spectra at Medium Laser Fluence LOW HAB HIGH HAB Intensity (a.u.) a) 5 mm prompt delay 36ns 10 mm prompt delay 36ns Intensity (a.u.) b) 45 mm prompt delay 36ns 50 mm prompt delay 36ns Wavelength (nm) Young particles seem to be more sensitive to the fluence value Excitation: 266 nm Fluence: 60 mj/cm 2

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