Laser vaporization reactor

Laser vaporization reactor. At LTT-Erlangen, first investigations with other nanomaterials have been carried out in cooperation with the group of Staupendahl at the University of Jena in a laser vaporization reactor (LVR) (Staupendahl, 2003). The basic principle of this reactor is dispersing raw material by a fountain into the CO2 laser beams, in which particles are vaporized and nanoparticles are formed by the subsequent condensation. As the LII measurement volume was located slightly above the vaporization zone, it was not possible to prevent coarse structures of material to occur inside the measurement volume (Figure 20). 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Figure. 1. Schematic of essential components of the Exxon group cluster laser vaporization source and fast flow tube chemical reactor. On the far left is a 1 mm diameter pulsed nozzle that emits an -200 ysec long pulse of helium which achieves an average pressure of approximately one atmosphere above the sample rod. Immediately before the sample rod position the tube is expanded to 2 mm diameter. The length of this extender section can be varied form 6 mm to 50 mm depending upon the desired integration time for cluster growth. The reactor flow tube is 10 mm in diameter and typically 50 mm long. The reactants diluted in helium are added and mixed with the flow stream via the second pulsed valve.

The field of gas-phase transition metal cluster chemistry has expanded rapidly due to the development of the laser vaporization source and the fast flow chemical reactor. The work from the three major laboratories have been reviewed. Many additional laboratories are developing cluster chemistry programs and will soon certainly make significant contributions. 

There are several preparative methods for the production of bare metal clusters including the fast flow reactor (PER), the fast flow tube reactor (FTR), the SIDT (24), the GIB (23), and a supersonic cluster beam source (SCBS) (198). Essentially, all of these methods are similar. The first process is to vaporize the metal sample producing atoms, clusters, and ions. Laser vaporization is generally favored although FAB or FIB may be used. The sample is located in a chamber or a tube and so vaporization generally takes place in a confined environment. An inert gas such as helium may be present in the vaporization source or may be pulsed in after the ionization process. 

Many of the metal cluster anions have been produced by cathode discharge in He doped with Ar and studied in a flow tube reactor. A few clusters have been prepared by laser vaporization using a supersonic cluster beam source and the reac-... 

Figure 1. Schematic of laser vaporization source/fast-flow reactor configuration currently employed in our laboratory.

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

Leuchtner, Haims and Castleman [74] have reacted anion and cation A1 clusters with Oxygen under thermal conditions in a flow tube reactor. The clusters were produced using laser vaporization. An etching reaction was ob-... 

The laser beam was focused onto a point inside a reaction chamber, where localized heating promotes vapor deposition in the direction of the laser. The reactor could be moved parallel and orthogonal to the laser beam direction. Motion parallel to the laser beam was driven at selected rates by a computer operated by a stepper motor. Growth of the fiber at a location where the laser beam converges to its focal point becomes self-regulating [2]. 

P. Ho, M.E. Coltrin, and W.G. Breiland. Laser-Induced Fluorescence Measurements and Kinetic Analysis of Si Atom Formation in a Rotating Disk Chemical Vapor Deposition Reactor. J. Phys. Chem., 98(40) 10138—10147,1994. 

This chapter discusses four methods of gas phase ceramic powder synthesis by flames, fiunaces, lasers, and plasmas. In each case, the reaction thermodynamics and kinetics are similar, but the reactor design is different. To account for the particle size distribution produced in a gas phase synthesis reactor, the population balance must account for nudeation, atomistic growth (also called vapor condensation) and particle—particle segregation. These gas phase reactors are real life examples of idealized plug flow reactors that are modeled by the dispersion model for plve flow. To obtain narrow size distribution ceramic powders by gas phase synthesis, dispersion must be minimized because it leads to a broadening of the particle size distribution. Finally the gas must be quickly quenched or cooled to freeze the ceramic particles, which are often liquid at the reaction temperature, and thus prevent further aggregation. 

Hydrogen Pressure Ranges for Liquid-Only or Vapor-Only Tritium Recovery for IMW IGF Reactors. For a 2700-MWf IGF reac-tor with a 600-m chamber, a 30% tritium bum fraction and a 1.75 tritium breeding ratio, 19.5 mg/s of tritium are added to the reaction chamber. If tritium is to be recovered from only the liquid, a maximum loss through the vacuum system of 1% (0.2 mg/s) is assumed to be acceptable. With a laser driver (26), the vacuum system might operate at a 1.3 Pa (10 2 Torr) with about 10% of the vapor being pumped per second. Then, about 3.5 mPa (2.6 x 10 Torr) of tritium, or 10 times the pressure associated with 0.2 mg tritium, is allowable. 

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