Nanoparticle Formation by Laser Ablation

The properties of nanoparticle aerosols of size ranging from 4.9nm to 13nm, generated by laser ablation of solid surfaces are described. The experimental system consisted of a pulsed excimer laser, which irradiated a rotating target mounted in a cylindrical chamber 4cm in diameter and 18-cm long. Aerosols of oxides of aluminum, titanium, iron, niobium, tungsten and silicon were generated in an oxygen carrier gas as a result of a reactive laser ablation process. Gold and carbon aerosols were generated in nitrogen by non-reactive laser ablation. The aerosols were produced in the form of aggregates of primary particles in the nanometer size range. The aggregates were characterized using a differential mobility analyzer and electron microscopy. Aggregate mass and number concentration, electrical mobility size distribution, primary particle size distribution and fractal dimension were measured. System operating parameters including laser power (100mJ/pulse) and frequency (2Hz), and carrier gas flow rate (1l/min) were held constant.A striking result was the similarity in the properties of the aerosols. Primary particle size ranged between 4.9 and 13nm for the eight substances studied. The previous studies with flame reactors produced a wider spread in primary particle size, but the order of increasing primary particle size follows the same trend. While the solid-state diffusion coefficient probably influences the size of the aerosol in flame reactors, its effect is reduced for aerosols generated by laser ablation. It is hypothesized that the reduced effect can be explained by the collision-coalescence mechanism and the very fast quenching of the laser generated aerosol.     

Characters andK-theory of discrete groups

Erratum

Characters andK-theory of discrete groups     

Regularized determinants for pseudodifferential operators in vector bundles overS 1

We express the -regularized determinant of an elliptic pseudodifferential operatorA overS ¹ with strongly invertible principal symbol in terms of the Fredholm determinant of an operator of determinant class, canonically associated toA, and local invariants. These invariants are given by explicit formulae involving the principal and subprincipal symbol of the operator. We remark that,generically, elliptic pseudodifferential operators have a strongly invertible principal symbol.Research supported in part by NSF Grants.     

Dynamo theory,vorticity generation,and exponential stretching

A discussion is given of the analogy between the dynamo equation for the generation of a magnetic field by the motion of an electrically conducting fluid and the equation for the evolution of vorticity of a viscous fluid. In both cases exponential stretching is an important feature of the underlying instability problem. For the "fast" dynamo problem, the existence of exponential stretching (i.e., the positivity of the Lyapunov exponent) somewhere in the flow is a necessary condition when the flow is smooth. An example is presented of a flow with exponential stretching (an Anosov flow) that supports fast dynamo action. A parallel treatment is described for the linearized Navier-Stokes equations for the motion of a viscous fluid. In this problem the analogous necessary condition for "fast vorticity generation" is the existence of some instability in the corresponding Euler (i.e., inviscid) equation. Dynamo theory methods give a second related result, namely a universal geometric estimate from below on the growth rate of a small perturbation in an inviscid fluid. This bound gives an effective sufficient condition for local instability for Eulers equations. In particular, it is proved that a steady flow with a hyperbolic stagnation point is unstable. The growth rate of an infinitesimal perturbation in a metric with derivatives depends on this metric. This dependence is completely described.     

Surface antigens of the embryonic chick myoblast: expression on freshly trypsinized cells.

Using an antiserum raised in rabbits against embryonic chick skeletal myoblasts (Anti-M-24), we have examined the trypsin and neuraminidase sensitivity and physiological expression of myoenic cell surface antigens. It was found that trypsin-released muscle cells more effectively inhibited, on a cell to cell basis, the cytotoxicity of Anti-M-24 for 24-h-old myoblast monolayers than did identical cells that had received a 3-4 h suspension culture recovery period from trypsinization. There was no such difference in absorptive capacities observed for any other embryonic chick tissue tested (e.g. brain, retina, liver, heart, and red blood cells) when freshly trypsinized cells were compared to ones which were given a 3-4 h culture period. If freshly trypsinized muscle cells were treated with high concentrations (30,000 international units (IU)/0.1 ml packed cells) of trypsin or with neuraminidase (30,000 IU/ml packed cells), there was a selective loss of myoblast-specific surface antigens. When single cells that had been in suspension culture for 3.5 h were reexposed to low concentrations (10,000 IU/0.1 ml packed cells) of trypsin, more antigenic sites were revealed on their surfaces as detected by an increased absorptive capacity in removing myoblast-binding antibodies from Anti-M-24. This increase in antigenic expression was time-dependent and inversely related to the length of culture time after trypsinization. Immunofluorescence studies revealed that tissue specific myoblast cell surface antigens are present on both muscle cells that were freshly dissociated and those that had been in suspension culture for 3-4 h. Furthermore, freshly trypsinized myoblasts possessed cell surface components that were highly antigenic; antiserum to such cells reacted extensively with both trypsinized and recovered muscle cells as detected by complement-dependent 51Cr release cytotoxicity assays and immunofluorescence. We conclude that embryonic chick myoblasts possess surface antigens that may be selectively removed by neuraminidase or high concentrations of trypsin. These antigens may be progressively masked, with increasing time of culture after protease-dissociation, by molecules that are sensitive to low concentrations of trypsin. Such masking of tissue-specific cell surface antigens could result in the display of molecular mosaics which may play a role in facilitating intercellular recognition and subsequent differentiation and histogenesis.     

The self-preserving size distribution theory. II. Comparison with experimental results for Si and Si3N4 aerosols

The gas to particle synthesis route is a relatively clean and efficient manner for the production of high-quality ceramic powders. These powders can be subsequently sintered in any wanted shape. The modeling of these production systems is difficult because several mechanisms occur in parallel. From theoretical considerations it can be determined, however, that coagulation and sintering are dominant mechanisms as far as shape and size of the particles are considered. In part I of this article an extensive theoretical analysis was given on the self-preserving size distribution theory for power law particles. In this second part, cumulative particle size distributions of silicon and silicon nitride agglomerates, produced in a laser reactor, were determined from TEM pictures and compared to the distributions calculated from this self-preserving theory for power law particles. The calculated distributions were in fair agreement with the measured results, especially at the high end of the distributions. Calculated and measured particle growth rates were also in fair agreement. Using the self-preserving theory an analysis was made on the distribution of annealed silicon agglomerates, of interest in applications to nanoparticle technology.