By University of Waterloo - Waterloo Institute for Nanotechnology (WIN)
TiRe-LII uses a laser pulse to heat aerosolized nanoparticles within a small probe volume to incandescent temperatures. A common setup involves a laser beam oriented perpendicular to a detector assembly, which most often consists of photomultiplier tubes (PMTs) equipped with bandpass filters aligned to wavelengths in the visible and near-infrared spectrum. The intersection between the laser beam and detection optics forms a probe volume. The spectral incandescence of the nanoparticle ensemble is measured as the nanoparticles return to the ambient gas temperature. Since larger nanoparticles cool more slowly than small particles, the particle size distribution can be determined from the observed spectral incandescence decay rate. The TiRe-LII model consists of two sub-models: a spectroscopic sub-model, which relates the measured incandescence to the nanoparticle temperatures; and a heat transfer model, which predicts the nanoparticle cooling rate. TiRe-LII: Spectroscopic sub-model The spectral incandescence emitted by an ensemble of laser-heated nanoparticles is given by [equation] where d p is the nanoparticle diameter, Q abs,λ(d p) is the spectral absorption cross-section, I λ,b[T p(d p)] is the blackbody spectral intensity at a temperature T p, p(d p) is a probability density function for the nanoparticle diameters, and coefficient C accounts for the particle number density and collector solid angles. TiRe-LII: Heat transfer sub-model Nanoparticle temperatures are found from an energy balance that relates the sensible energy of the nanoparticle to laser heating and conductive and evaporative cooling [equation] Other cooling terms, including radiative and thermionic heat transfer, are usually at least one order of magnitude lower than conduction and evaporation, and can be neglected from the model. Conductive and evaporative cooling usually occurs in the free molecular regime (FMR), in which molecules travel between the nanoparticle and the equilibrium state of the gas without colliding with other gas molecules. FMR conduction heat transfer is given by [equation] where N g ² is the incident gas molecular number flux, <E g,o-E g,i>max is the maximum average energy transferred when a gas molecule scatters from the nanoparticle allowed by the 2 nd Law of Thermodynamics, and α is the thermal accommodation coefficient (TAC), which defines the energy transfer efficiency between the gas molecule and the nanoparticle surface. Evaporative heat transfer is given by [equation] where N v ² is the number flux of evaporated molecules (calculated from the Clausius-Clapeyron equation) and Δ H v is the latent heat of vaporization, which is the energy required to liberate one molecule from the nanoparticle. TiRe-LII: Applications TiRe-LII is most commonly applied to soot, and much of the literature devoted to the in situ investigation of soot in flames, and black carbon in the atmosphere. There is growing interest in measuring the properties of synthetic nanoparticles, including those made of iron, nickel, molybdenum, silver, gold, silicon, and germanium. While TiRe-LII is mainly used to infer particle size, the diagnostic is increasingly used as a tool for scientific inquiry, e.g. to characterize the TAC, Δ H v, and the electromagnetic properties of liquid metals and molten semiconductors at high temperatures. Recent work has focused on characterizing few layer graphene (FLG), used to enhance the performance of batteries and supercapacitors.
