The effects of diameter on detonation velocity of packed granular beds of HNS (2,2',4,4',6,6'-hexanitrostilbene) and CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, HNIW) will be discussed. Due to the novel nature of the diagnostic technique utilized here, a thorough discussion of the experimental method is provided. The dimension at which finite diameter effects occur was characterized by conducting simultaneous streak camera and framing camera measurements on miniature rate sticks similar in concept to traditional rate sticks. A significant difference between historical rate sticks and those discussed here comes in the form of how they were produced. A femtosecond laser was used to generate precision miniature rate sticks down to diameters of 187 μm. Finally, we will discuss the somewhat unexpected result of nano particulate generation of energetic materials due to the laser machining process.
A new approach to explosive sample preparation is described in which microelectronics-related processing techniques are utilized. Fused silica and alumina substrates were prepared utilizing laser machining. Films of PETN were deposited into channels within the substrates by physical vapor deposition. Four distinct explosive behaviors were observed with high-speed framing photography by driving the films with a donor explosive. Initiation at hot spots was directly observed, followed by either energy dissipation leading to failure, or growth to a detonation. Unsteady behavior in velocity and structure was observed as reactive waves failed due to decreasing channel width. Mesoscale simulations were performed to assist in experiment development and understanding. We have demonstrated the ability to pattern these films of explosives and preliminary mesoscale simulations of arrays of voids showed effects dependent on void size and that detonation would not develop with voids below a certain size. Future work involves experimentation on deposited films with regular patterned porosity to elucidate mesoscale explosive behavior.
It has been shown that thermal energy imparted to a metallic substrate by laser heating induces a transient temperature gradient through the thickness of the sample. In favorable conditions of laser fluence and absorptivity, the resulting inhomogeneous thermal strain leads to a measurable permanent deflection. This project established parameters for laser micro forming of thin materials that are relevant to MESA generation weapon system components and confirmed methods for producing micrometer displacements with repeatable bend direction and magnitude. Precise micro forming vectors were realized through computational finite element analysis (FEA) of laser-induced transient heating that indicated the optimal combination of laser heat input relative to the material being heated and its thermal mass. Precise laser micro forming was demonstrated in two practical manufacturing operations of importance to the DOE complex: micrometer gap adjustments of precious metal alloy contacts and forming of meso scale cones.
Femtosecond pulse laser drilling has evolved to become a preferred process for selective (maskless) micromachining in a variety of materials, including metals, polymers, semiconductors, ceramics, and living tissue. Manufacturers of state-of-the-art femtosecond laser systems advertise the inherent advantage of micromachining with ultra short pulses: the absence of a heat affected zone. In the ideal case, this leads to micro and nano scale features without distortion due to melt or recast. However, recent studies have shown that this is limited to the low fluence regime in many cases. High dynamic range autocorrelation studies were performed on two commercial Ti:sapphire femtosecond laser systems to investigate the possible presence of a nanosecond pedestal in the femtosecond pulse produced by chirped pulse amplification. If confirmed, nanosecond temporal phenomena may explain many of the thermal effects witnessed in high fluence micromachining. The material removal rate was measured in addition to feature morphology observations for percussion micro drilling of metal substrates in vacuum and ambient environments. Trials were repeated with proposed corrective optics installed, including a variable aperture and a nonlinear frequency doubling crystal. Results were compared. Although the investigation of nanosecond temporal phenomena is ongoing, early results have confirmed published accounts of higher removal rates in a vacuum environment.
Proceedings of SPIE - The International Society for Optical Engineering
Palmer, Jeremy A.; Hsieh, Wen T.; Quijada, Manuel; Mott, Brent; Akpan, Eddie; Brown, Gary L.; Jacobson, Mindy B.; Greenhouse, Matthew A.
A miniature Fabry-Perot tunable infrared filter under development at the NASA Goddard Space Flight Center is fabricated using micro opto electromechanical systems (MOEMS) technology. Intended for wide-field imaging spectroscopy in space flight, it features a large 10-mm diameter aperture structure that consists of a set of opposing suspended thin films 500 nanometers in thickness, supported by annular silicon disks. Achieving the desired effective finesse in the MOEMS instrument requires maximizing the RMS flatness in the film. This paper presents surface characterization data for the suspended aperture film prior to, and following application of a multi-layer dielectric mirror. A maximum RMS flatness of 38 nanometers was measured prior to coating, leading to an estimate of the maximum effective finesse of 14. Results show evidence of initial deformation of the silicon support structure due to internal stress in the substrate and thin film layers. Film stress gradients in the dielectric coating on either side of the aperture add convexity and other localized deflections. The design of a tuning system based upon electrostatic positioning with feedback control is presented.
A commercial stereolithography (SL) machine was modified to integrate fluid dispensing or direct-write (DW) technology with SL in an integrated manufacturing environment for automated and efficient hybrid manufacturing of complex electrical devices, combining three-dimensional (3D) electrical circuitry with SL-manufactured parts. The modified SL system operates similarly to a commercially available machine, although build interrupts were used to stop and start the SL build while depositing fluid using the DW system. An additional linear encoder was attached to the SL platform z-stage and used to maintain accurate part registration during the SL and DW build processes. Individual STL files were required as part of the manufacturing process plan. The DW system employed a three-axis translation mechanism that was integrated with the commercial SL machine. Registration between the SL part, SL laser and the DW nozzle was maintained through the use of 0.025-inch diameter cylindrical reference holes manufactured in the part during SL. After depositing conductive ink using DW, the SL laser was commanded to trace the profile until the ink was cured. The current system allows for easy exchange between SL and DW in order to manufacture fully functional 3D electrical circuits and structures in a semi-automated environment. To demonstrate the manufacturing capabilities, the hybrid SL/DW setup was used to make a simple multi-layer SL part with embedded circuitry. This hybrid system is not intended to function as a commercial system, it is intended for experimental demonstration only. This hybrid SL/DW system has the potential for manufacturing fully functional electromechanical devices that are more compact, less expensive, and more reliable than their conventional predecessors, and work is ongoing in order to fully automate the current system.