Investigation of srilankite phase formation in low-pressure premixed flame synthesis of titania nanoparticles
Creators: name: Halim, Hadi Dharma
Publisher: name: No Publisher Supplied
Year: 2021

description: The effects of experimental and simulation parameters on phase formation of TiO2 nanoparticles in metalorganic-precursor-seeded ethylene/hydrogen/oxygen burner-stabilized premixed flames impinging on a chilled substrate are investigated. Using titanium tetra-iso-propoxide (TTIP) as precursor, anatase, rutile, and srilankite phases of titania nanoparticles are produced. Experimental conditions vary temperature from 1590K to 2650K, pressure from 20 to 40 torr, equivalence ratio from 0.37 to 1.08, and burner-exit gas flow velocities from 150 to 300 cm/s. Final nanoparticle sizes range from 3-10 nm ± 3nm with mixed phases. As-produced samples are stable without any sign of subsequent phase change, as characterized using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), up to 500°C. X-ray photoelectron spectroscopy (XPS) indicates Ti4+ valence and presence of carbon (13-30 atomic %), with absence of Ti3+, Ti-C, and nitrogen. Scanning Electron Microscopy (SEM) and transmission electron microscopy (TEM) confirm mixed micro- and nano-meter features. X-ray diffraction (XRD) and selected area electron diffraction (SAED) divulge srilankite, rutile, and anatase phases produce under different experimental conditions. Qualitatively, lower equivalence ratio seems to favor anatase formation, and higher equivalence ratio seems to favor srilankite formation, with other parameters fixed.The rare srilankite phase of TiO2 is created in this work using ethylene as both fuel and precursor dopant in the burning premixture. TiO2 nanoparticles (i.e., 5–10 nm) of polymorphs of srilankite TiO2-II, anatase, and rutile are produced by changing residence time and carbon content of the synthesis flow field. By tuning the residence time and fraction of included carbon, different polymorphs are generated. Anatase is formed at 20 torr by a H2/O2/N2 premixed flame, while rutile and srilankite are formed at 20 torr in a H2/C2H4/O2/N2 flame. Longer particle residence time facilitates srilankite formation over rutile. Additional carbon facilitates the formation of srilankite over that of anatase, as evidenced by the greater weight percentage of carbon in the srilankite samples than that in the anatase samples. To rule out the temperature effect on phase formation, srilankite is also synthesized in an ethylene-based flame at a lower characteristic temperature that matches that of an anatase-synthesizing ethylene-free flame. Decomposition of feed ethylene allows a higher percentage of carbon to be doped into the TiO2 lattice, which is the key (along with small nanoparticle size with high internal pressures) in stabilizing the high-pressure metastable srilankite phase in our low-pressure system. Computational simulation, using the Sandia SPIN code for the flow field and flame structure, a code for particle trajectory, and monodisperse and sectional models for particle growth dynamics, determines final properties, including primary and aggregate particle size, to compare with experiments. Characteristic particle residence times are estimated to be from 4.42 to 15.7 ms. Using pseudo-Voigt fitting on XRD and SAED spectra, nanopowders produced are determined to have anatase percentages varying from 0 to 90%, rutile percentages from 7 to 100%, and srilankite percentages from 0 to 83%. Pseudo-Monte-Carlo method is employed to account for change of direction between “rate of change” of input to output parameters.Using statistical analysis, the significant contributors to phase changes are narrowed down to equivalence ratio, pressure, residence time, and temperature. By establishing case to case Δ as 0.3 equivalence ratio, 20 torr pressure, 1 ms residence time, and 75K temperature, the pseudo-Monte-Carlo output is analyzed to yield the following conclusions:-Equivalence ratio exerts large influence on specific phase formation with -37.92 anatase, 16.69 srilankite, and 12.27 %rutile/ Φ case Δ. -Pressure exerts medium influence on specific phase formation with 19.83 rutile, 15.06 srilankite, and 7.37% %anatase/P case Δ. -Residence time exerts small influence on specific phase formation with 5.85 srilankite, -5.19 rutile, and -1.45 anatase%/t case Δ. -Temperature exerts small influence on specific phase formation with 5.49 srilankite, -3.18 rutile, and -2.04 anatase%/T case Δ.These results are consistent to that observed in the experiments, along with the reasons for the formation of specific phases.