Date of Award

Spring 2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)

Department

Committee for the Interdisciplinary Program in Materials Science and Engineering

First Advisor

Lev N. Krasnoperov

Second Advisor

Haim Grebel

Third Advisor

Zafar Iqbal

Fourth Advisor

Frank J. Owens

Fifth Advisor

Leonid Tsybeskov

Abstract

Cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), a well-known energetic material, is highly explosive. Reduction of the sensitivity of RDX is desired for safe handling and

storage in military applications. Data on the sensitivity of RDX in the 10-1000 µm crystal size range suggested that the impact sensitivity could be further reduced by reduction of the crystal size to the sub-micron or nano-scale (Armstrong, 1990). Recent research with nano-RDX obtained using Rapid Expansion of Supercritical Solutions (RESS) confirmed these expectations (Stepanov, 2005).

RESS process appeared to be one out of a few techniques of production of nanoscale energetic materials free of the risk of explosion. Current research was aimed at the understanding of the fundamentals of the RESS process and the mechanism of nanoparticle formation via an in situ particle monitoring and characterization in the RESS jet combined with the characterization of the final product.

In this research, nanoparticles of RDX generated by RESS using supercritical CO2 were characterized in situ by a pulse laser light scattering imaging technique using gated ICCD camera. The sensitivity was determined using Rayleigh scattering from air as well as light scattering from standard polystyrene spheres. The size distribution functions of the particles formed in the RESS jet were determined using the calibrated sensitivity. The final diameter of RDX particles at the pre-expansion pressure of 180 bar was 73 nm at the maximum of the size distribution function. Assuming that the particles near the nozzle consisted mainly of CO2 and the log-normal size distribution, the diameter of the particles near the nozzle (7.5 mm from the nozzle) at the distribution maximum was 3.3 µm at the pre-expansion pressure of 180 bar. The number densities of the particles in the RESS jet were determined by counting individual particles in the light scattering images. Based on the measured particle size distributions and the number density of particles along the RESS jet, the mechanism of particle formation in the RESS is discussed. The homogeneous nucleation mechanism is rejected as it fails to explain the large particle size experimentally observed. Instead, a modified "spray-drying" mechanism is suggested.

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