Titanium-based Functional Nanomaterials:Flame Aerosol Synthesis And Reactions

Flame synthesis,or flame aerosol synthesis,has been widely adopted to make commodity nanoparticle products like TiO₂,due to its high-throughput production,fast processing time and apparent simplicity as a continues one-step process.However,it is still a great challenge in flame synthesis to manufacture nanostructured materials with complex compositions,or with special demands in crystal and geometric structures.In this thesis,advanced flame synthesis technology is developed to manufacture multi-component titanium-based functional materials.Through the theoretical modelling by time scale analysis and population balance methods,as well as the online laser diagnostics techniques,the evolution of nanoparticles from precursors is investigated in detail at multiple scales.Three important nanocomposites,i.e.,the bandgap engineered optical materials,the supported nano-catalysts and non-oxide metal propellants were successfully synthesized in this work.Further study were conducted in their characteristics and reactions.The reaction mechanisms and chemical kinetics of the precursors are studied as the initiation step in flame synthesis.The precursor reactions are classified as temperature-driven decomposition and radical–driven decomposition with striking difference in characteristic reaction times.Based on the time scale analysis,the flame synthesis process was decoupled,and the growth of nanoparticles driven by Brownian collision and coalescence was simulated by poly-dispersed population balance model.Two fundamental synthesis routes were proposed in the thesis for the flame synthesis of functional nanomaterials,one is intra-particle doping,and the other is inter-particle mixing.The synthesis diagram for nanocomposites is thus developed,providing the possible synthesis pathways for multi-component metal oxides with various crystal and geometric structures.The PS?phase selective?-LIBS was first time introduced into the flame spray pyrolysis in this work,which shows the combination effects of the evaporation of solvents,the decomposition of the precursors and the nucleation of the particles at the very early stage of flame synthesis.Through flame synthesis,a series of bandgap engineered optical materials are manufactured and the particle bandgap is measured with in situ laser diagnostics.As shown in the experiments,the emission intensity of Ti is enhanced by 23% as doped by V,and the intensity is weakened by 22% as doped by Zr,which corresponds to the tuning of the bandgap.The supported nano-catalysts with the size under 10 nm in this work is made in stagnation swirl flame equipped with a sub-micron atomizer.The Pd dispersed on TiO₂ supports shows exceptional high activity in methane catalytic combustion.The 20% conversion temperature of methane is decreased to as low as 293°C,which is lower than the reports in literature.The thermal stability of the catalysts can be further enhanced by adding Ce into supports.These distinguished properties originate from the strong metal-support interaction formed during flame synthesis process.The non-oxide metal propellants are synthesized in a special designed “flame environment”,a plume of laser induced plasma,and the online burn times of the nanoparticles are measured.The results show that once sintering is accounted for,the rate of combustion obeys a near ^diameter1 power-law dependence.The optical emission from combustion was also used to model the oxidation process,which can be reasonably described with a kinetically controlled shrinking core model.