Ali Abdali

Gas-Phase Synthesis of Non-Agglomerated Nanoparticles by Fast Gasdynamic Heating and Cooling

A novel method for the production of oxide nanoparticles from gas-phase organic precursors in a high-velocity flow apparatus is described. In contrast to conventional methods of reaction initiation by flames, plasmas or heat conduction, the gas mixture is instantaneously heated by a stationary shock wave of an overexpanded supersonic nozzle flow above a critical initiation temperature. Following the reaction initiation, different molecular and intermolecular processes lead to the generation and growth of nanoparticles. After an adjustable reaction time that depends on the gas velocity and the reactor length the particle growth by surface growth and coagulation is interrupted by fast expansion and, therefore, cooling of the gas in a convergent-divergent nozzle flow. The total enthalpy of the flow is finally reduced by injecting water into the flow behind the nozzle exit. The aim of this gas-dynamic nanoparticle synthesis strategy is the generation of non-agglomerated single particles with a narrow size distribution. In this paper, a process is described which aims at establishing flow conditions for the heating and quenching process that are as uniform as possible in order to generate material with well-defined properties. This synthesis path is in contrast to today’s industrial production facilities where the heating of the precursor gas is typically achieved by a low velocity flow, e.g. through a flame in connection with low cooling rates at the end of the particle growth zone. This results in inhomogeneous particle formation conditions along the individual flow paths leading to agglomerated nanoparticles with strongly varying primary particle sizes and agglomerate sizes and structures. This paper focuses on the design strategy of the planned facility that is based on numerical and experimental investigations of the underlying fluid-dynamic and chemical-kinetic requirements. This joint project of five German universities and the German Aerospace Center is funded by DFG (Deutsche Forschungsgemeinschaft) and is performed in close cooperation with Degussa AG, Germany. First experiments with this facility are planned for mid of 2007.

Gas-Temperature Imaging in a Microwave-Plasma Nanoparticle-Synthesis Reactor Using Multi-Line NO-LIF Thermometry

Abstract Multi-line NO-LIF thermometry is used to determine the two-dimensional temperature distribution inside a low-pressure plasma reactor. The applicability of multi-line temperature measurements to non-equilibrium plasma environments was evaluated. Temperatures between 300 and 3000 K have been observed, while microwave power, and pressure show a strong effect on the temperature distribution. Metal-organic precursors added for particle synthesis additionally influence the temperature through the heat release during particle formation and the oxidation of organic ligands.

Surface functionalization of microwave plasma-synthesized silica nanoparticles for enhancing the stability of dispersions

Abstract Gas phase-synthesized silica nanoparticles were functionalized with three different silane coupling agents (SCAs) including amine, amine/phosphonate and octyltriethoxy functional groups and the stability of dispersions in polar and non-polar dispersing media such as water, ethanol, methanol, chloroform, benzene, and toluene was studied. Fourier transform infrared spectroscopy showed that all three SCAs are chemically attached to the surface of silica nanoparticles. Amine-functionalized particles using steric dispersion stabilization alone showed limited stability. Thus, an additional SCA with sufficiently long hydrocarbon chains and strong positively charged phosphonate groups was introduced in order to achieve electrosteric stabilization. Steric stabilization was successful with hydrophobic octyltriethoxy-functionalized silica nanoparticles in non-polar solvents. The results from dynamic light scattering measurements showed that in dispersions of amine/phosphonate- and octyltriethoxy-functionalized silica particles are dispersed on a primary particle level. Stable dispersions were successfully prepared from initially agglomerated nanoparticles synthesized in a microwave plasma reactor by designing the surface functionalization.

A Shock-Tube with High-Repetition-Rate Time-of-Flight Mass Spectrometry for the Study of Complex Reaction Systems

The process of soot particle formation has been extensively studied. The thermal decomposition of acetylene as major decomposition product of some soot-producing hydrocarbons in different temperature ranges has been previously studied by several authors [1–3]. Shock tubes with different modern diagnostics are versatile tools that are widely used for gas-phase reaction kinetics studies [4–7]. In shock tubes, high-temperature reactions can be investigated on short time scales. The high temperature and pressure conditions behind the reflected shock wave are stable up to a fewmilliseconds. Considering these conditions, fast detectionmethods are required, that provide time-resolved information about the change of composition of the investigated mixture. A high-repetition-rate (HRR) time-of-flight mass spectrometer (TOF-MS) can simultaneously measure the concentration-time profiles for numerous species with approximately one measurement every 10 μs. Combinations of shock tubeswith TOF-MSare established for the study of complex reaction systems. The most extensive set of TOF-MS shock-tube measurements was published by Kern et al. [2, 8–10]. There are some difficulties in coupling a shock tube with a TOF-MS. The most important problems are extracting the sample from the shock tube in a reproducible way and also acquiring the resulting large amounts of data in a very short time. In the present study we used a shock tube coupled to a HRRTOF-MS to investigate the thermal decomposition of acetylene as a test system to validate the new setup. After validation, this setup has been used to investigate the thermal decomposition of (CH3)6Si2O (HMDSO).

Functionalization of SiO2 nanoparticles and their superhydrophobic surface coating

Silica (SiO2) nanoparticles synthesized from the gas phase in a microwave plasma reactor were dispersed in organic solvents and subsequently chemically modified with silane coupling agents (SCAs) to reduce agglomeration. A transparent and stable dispersion of silica nanoparticles in toluene could be achieved after functionalization with octyltriethoxysilane (OTES). FTIR measurements indicated that the concentration of free hydroxyl groups on the nanoparticle surface was notably decreased. Nanodispersions of functionalized silica particles with several SCAs were prepared and coated onto glass substrates. The resulting highly hydrophilic/hydrophobic glass substrates were investigated by water contact angle measurements. As a result, the hydrophobically OTES-functionalized silica nanoparticles in toluene showed an average diameter of 28 nm and remained for more than eight weeks.