Density of RDX Crystals Grown During High Acceleration in an Ultracentrifuge
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Density Of RDX Crystals Grown During High Acceleration In An Ultracentrifuge Mary Y. D. Lanzerotti, Richard Z. Squillace, Alexander Gandzelko and J. Sharma1 U. S. Army ARDEC Picatinny Arsenal, NJ 07806-5000, U. S. A. 1 Carderock Division, Naval Surface Warfare Center West Bethesda, MD 20817-5700, U. S. A. ABSTRACT RDX (cyclotrimethylene-trinitramine) crystals were grown during high acceleration (high g) in an ultracentrifuge. These crystals are found to have a density ~ 0.6% greater than 1 g crystals, and to have a greatly reduced defect content. The high g crystals should therefore have significantly reduced shock sensitivity as compared to commercial grade RDX. When a RDX saturated acetone solution is accelerated at 200,000 g, the RDX solute molecules move individually through the acetone solvent molecules to form a RDX crystal because the density of the RDX (Theoretical Maximum Density 1.806 g/cc) solute is more dense than the acetone (0.79 g/cc) solvent. Crystal growth is controlled by the g-force. Crystal defects including voids and solution inclusions caused by temperature variation or evaporation at 1 g are minimized and the RDX crystal density increases. A nitrogen pycnometer was used to measure the density of the RDX crystals grown at 1 g and at 200,000 g. The density of the RDX crystals grown at 200,000 g (1.7980 + 0.0003 g/cc)is found to be greater than the density of RDX crystals grown at 1 g (1.7881 + 0.0003 g/cc) by 0.0099 g/cc. The density of the high g crystal is 99.6% of the Theoretical Maximum Density of RDX. INTRODUCTION Crystal growth from a solution can be considered a heterogeneous chemical reaction of the type where a portion of the liquid goes into crystal form [1,2]. In the laboratory at 1 g, crystal growth methods include solvent evaporation at constant temperature and slow cooling. Crystal growth occurs when the solution becomes supersaturated. The crystal growth is controlled by simultaneous movement of solute and solvent in convection currents [3-4]. During solvent evaporation the density of the solution increases and descends to the crystal. The process of crystallization results from mass transfer from the surrounding solution to the crystal surface. At 1 g the density of the surrounding solution is reduced during crystal growth. The lower density solution then rises, creating a convection current. The convection current can create voids, defects and solution inclusions in the crystal. AFM (atomic force microscopy) has been used to study crystal growth that is caused by a change in concentration or temperature. [5] Supersaturation can also occur in an initially saturated solution during high acceleration [610]. If the solution is initially saturated, then under acceleration the solution at the outer edge of the accelerating tube becomes supersaturated. A density gradient is established. Thus at high g (above 1,000 g), the crystal growth mechanism is different. The solute molecules move individually through the solvent molecules to form a crystal if the density of the sol
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