Time-resolved two-color LII: size distributions of nano-particles from gas-to-particle synthesis

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Abstract

A two-color LII technique for in situ measurements of particle size distributions is described. The technique is based on the simultaneous detection of time-resolved LII signals at two different wavelengths with one-dimensional spatial resolution using a newly developed experimental setup. The ratio of both LII signals yields particle temperatures as a function of time and location. Measured particle temperature decays are numerically simulated based on a detailed cooling model for particle ensembles. Particle size distributions are obtained by fitting simulated particle temperature decays to measured ones using multi-dimensional non-linear regression. The two-color LII technique for particle sizing can be applied to a wide range of materials because it is independent of the optical properties of the particle material. Exemplarily, the measuring technique is applied to investigate the synthesis of nanoscaled metal oxide particle in a laser vaporization reactor.

Introduction

Nano-particles are found in, e.g., high performance ceramics, catalysts, pigments, coatings, pharmaceuticals, car tires or optical fibers. The notable relevance of nanoscaled particles is due to a physical peculiarity of nano-particles: whilst physical properties of bulk materials are mainly determined by their chemical composition, many properties of nanoscaled particles and powders or aerosols additionally depend on the particle size as well as the particle size distribution. The ductility of ceramics made of nanoscaled titanium oxide particles is, e.g., twice as high as of ceramics fabricated of particles with micrometer length scale [1]. Light absorption of nano-particles is proportional to the particle volume, instead of to the particle surface area [2]. Ferrofluids are stable suspensions of ferromagnetic nano-particles with superparamagnetic properties [3].

Gas-to-particle synthesis of nano-particles is one major technical synthesis route. Precursors are either chemically reacting gases or condensing vapor. In technical systems, e.g., the flame synthesis of titanium dioxide or soot, product properties are determined by a complex interaction of gas phase chemistry, mixing effects, particle nucleation, surface growth, coagulation, and oxidation. The dynamics of these processes depend on global process parameters like pressure, temperature, precursor concentration, and turbulence intensity. For customized synthesis of products with well-defined properties, knowledge and understanding of processes involved in particle formation are indispensable. The validation of particle formation models and the ability to control technical synthesis reactors fundamentally require in situ measuring techniques for determination of local particle size distributions.

Laser-induced incandescence (LII) allows non-intrusive temporally and spatially resolved in situ measurements of particle volume fraction and primary particle size. The technique is based on the detection of the enhanced thermal radiation of particles heated by absorbing an intense nanosecond laser pulse. The subsequent decrease in particle temperature is mainly due to heat transfer and evaporation. The heat and mass transfer rates depend on the particle surface area, while the internal energy of a particle is proportional to its volume. Consequently, the temperature, and thus, the LII signal of small particles decreases faster than that of large particles. Analysis of the time evolution of LII signals to obtain information about the particle size distribution in the measurement volume requires a detailed model describing these cooling processes. In the past, several authors introduced and applied heat and mass transfer models of soot [4], [5], [6], [7], [8], [9], [10] and other particles [11], [12] in LII, and the feasibility of particle sizing has been demonstrated in several studies. However, some discrepancies between model predictions and experimental findings introducing errors to size measurements still exist [13], [14], [15]. It can be shown that most significant are errors related to modeling laser absorption [16]. If LII signals are measured at two wavelengths, the particle temperature evolution during LII experiments can be calculated based on Planck’s law. The major advantage of two-color LII experiments is that laser absorption is not to be modeled since the particle temperature after the laser pulse is experimentally determined. Additionally, the obtained particle size distributions are independent of the optical particle properties, which are often inaccurately known and may vary with experimental conditions [17] or change during LII experiments [18]. Two-color LII methods can effectively eliminate these significant error sources of particle sizing in LII and can be applied to a wide range of particle materials.

In this study, the evaluation of particle size distributions from time-resolved two-color LII experiments is discussed in detail. The presented method is applied to investigate the synthesis of nanoscaled metal oxide particles in a laser vaporization reactor.

Section snippets

Experimental

The evaluation of particle size distributions from measured LII signals comprehends three steps. First, particle temperature evolutions of laser heated particles are to be determined from LII signals measured simultaneously at two different wavelengths. Second, measured particle temperature decays are numerically simulated based on a detailed cooling model for particle ensembles. Third, particle size distributions are obtained by fitting simulated particle temperature decays to measured ones

Applications and results

The principal feasibility of assessing p (r) from two-color LII experiments has been demonstrated in sooting flames [16], [23], [25]. In this study, the technique has been used for the first time to investigate size distributions of metal oxide particles from a laser vaporization reactor.

In a laser vaporization reactor, coarse powders are vaporized in the focus of an intense laser beam. The vapor condenses above the feed material, and nanosized primary particles are formed. A continuous gas flow

Conclusions

A new modification of the two-color LII technique for in situ measurements of particle size distributions is described. The technique is based on the simultaneous detection of time-resolved LII signals at two different wavelengths with one-dimensional spatial resolution. The ratio of both LII signals yields particle temperatures as a function of time and location. Measured particle temperature decays are numerically simulated based on a detailed cooling model for particle ensembles. Particle

Acknowledgments

This work has been supported by the Deutsche Forschungsgemeinschaft and Forschungsvereinigung Verbrennungskraftmaschinen e.V., which is gratefully acknowledged. The cooperation with “Projekthaus Nanomaterialien, degussa.” has been extremely generative. We specially thank Dr. M. Pridöhl, Dr. H. Mühlenweg, and Dr. A. Gutsch. The measurements in the laser evaporation reactor are performed in cooperation with the Friedrich-Schiller-Universität Jena. We specially thank J. Grabow, Dr. H.-D. Kurland,

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