Research Papers

Micro-Optical Initiation of Nanoenergetic Materials Using a Temporally Tailored Variable-Pulse-Width Laser

[+] Author and Article Information
Mikhail N. Slipchenko

Weldon School of Biomedical Engineering,
Purdue University,
West Lafayette, IN 47907;
Spectral Energies, LLC,
Dayton, OH 45431

Joseph D. Miller

Department of Mechanical Engineering,
Iowa State University, Ames, IA 50011

Sukesh Roy

Spectral Energies, LLC,
Dayton, OH 45431

James R. Gord

Air Force Research Laboratory,
Wright-Patterson Air Force Base,
OH 45433

Terrence R. Meyer

Department of Mechanical Engineering,
Iowa State University,
Ames, IA 50011
e-mail: trm@iastate.edu

1Corresponding author.

Manuscript received April 17, 2012; final manuscript received September 11, 2012; published online January 18, 2013. Assoc. Editor: Debjyoti Banerjee.

J. Nanotechnol. Eng. Med 3(3), 031007 (Jan 18, 2013) (6 pages) doi:10.1115/1.4007887 History: Received April 17, 2012; Revised September 11, 2012

Nanoenergetic materials can provide a significant enhancement in the rate of energy release as compared with microscale materials. The energy-release rate is strongly dependent not only on the primary particle size but also on the level of agglomeration, which is of particular interest for the inclusion of nanoenergetics in practical systems where agglomeration is desired or difficult to avoid. Unlike studies of nanoparticles or nanometer-size aggregates, which can be conducted with ultrafast or nanosecond lasers assuming uniform heating, microscale aggregates of nanoparticles are more sensitive to the thermophysical time scale of the heating process. To allow control over the rate of energy deposition during laser initiation studies, a custom, temporally tailored, continuously variable-pulse-width (VPW) laser was employed for radiative heating of nanoenergetic materials. The laser consisted of a continuous-wave master oscillator, which could be sliced into desired pulses, and a chain of amplifiers to reach high peak power. The slicer allowed control over the time profile of the pulses via the combination of an arbitrary waveform generator and acousto-optic modulator (AOM). The effects of utilizing flat-top or ramped laser pulses with durations from 100 ns to 150 μs and energies up to 20 mJ at 1064 nm were investigated, along with a broad range of heating rates for single particles or nanoparticle aggregates up to 100-μm diameter. In combination with an optical microscope, laser heating of aggregates consisting of 70-nm diameter Al nanoparticles in a Teflon matrix showed significant dependence on the heating profile due to the sensitivity of nanoenergetic materials to heating rate. The ability to control the temporal pulse-intensity profile leads to greater control over the effects of ablative heating and the resulting shockwave propagation. Hence, flexible laser-pulse profiles allow the investigation of energetic properties for a wide size range of metal/metal-oxide nanoparticles, aggregates, and composites.

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Fig. 1

Optimal laser-pulse duration and energy dependence on particle size. (a) Thermal-diffusion length for uniform heating of solid Al with given pulse time duration. Shaded areas roughly correspond to nanoparticles or aggregates of sizes less than 300 nm (left) and greater than 300 nm (right). (b) Illustration of how laser heating of large particle with fixed duration (10-ns) laser can be limited by damage threshold of dielectric substrate. (c) Optical image of 30 -μm size Al aggregate placed on the surface of the cover slip. (d) Glass substrate damage as a result of initiation of aggregate in (c) by 1-mJ, 10-ns pulse.

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Fig. 2

Temporally tailored, variable-pulse-width laser layout: MO—master oscillator, AOM—fiber-pigtailed acousto-optic modulator, TPF—thin-film polarizer. Amp #1, Amp #2, and Amp #3 are Nd:YAG flashlamp amplifiers with 101-mm rod length and 4-mm, 5-mm, and 6.3-mm rod diameters, respectively.

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Fig. 3

Experimental setup for micro-optical initiation and optical and SEM images of aggregates. (a) Coupling of VPW laser to micro-optical test bench. PD—photodiode, HeNe—632-nm helium–neon laser, LS—light source with a condenser, TS—motorized XY translational stage, PMT—photomultiplier tube, M1—10% beam splitter, M2—632-nm dielectric mirror, M3—10% beam splitter, M4—1064-nm dielectric mirror, M5—50% beam splitter or silver mirror. (b) Optical images of aggregates. (c) and (d) SEM images of individual aggregates indicated in (b).

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Fig. 4

Performance of VPW laser. (a) Output of AOM driven by rectangular pulse of 10 -μs length. (b) Same pulses amplified through amplifier chain. Temporal profiles are measured using photodiode, and peak power is calculated from measured pulse energy.

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Fig. 5

Dependence of particle initiation on pulse duration and temporal profile. (a) Photodiode traces of 100-ns, 10 -μs near-flat-top pulses, and 50 -μs (FWHM) slow-rising pulse. Transmission images of aggregates before and after initiation by 100-ns, 10 -μs, and 50 -μs pulses are shown in (b) and (c), (d) and (e), and (f) and (g), respectively. Arrows in (e) point to shockwave ring. Scale bar is 10 μm.

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Fig. 6

SEM images of the aftermath of nanoparticle initiation for (a) 10 -μs flat-top laser profile and (b) 50 -μs slowly ramped pulse. The lighter outer region is the undisturbed gold coating used for enhancing contrast in the SEM images.

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Fig. 7

Analysis of emission from laser-initiated particles. (a) PMT traces of emission from aggregate initiated with 10 -μs flat-top laser pulse and from aggregate initiated with slow-rising 50 -μs (FWHM) pulse. (b) 300-kHz-rate image sequence showing spatial distribution of emission from large aggregate by slow-rising 50 -μs pulse. Each frame size is 15 × 30 μm2. Time position of images is shown in insert above PMT trace in (a). (c) Emission spectra from flat-top (10-μs) and slow-rising pulses (50-μs).

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Fig. 8

Time-gated emission spectroscopy. (a) and (c) are time traces during laser–matter interaction (PMT—particle emission, PD—laser intensity). (b) and (d) are spectra collected during time gates indicated in (a) and (c), respectively. Time gates in (a) and (c) are 200 ns in duration and delayed by 0 and 3.5 μs, respectively, from the arrival of the 10 -μs, 111 -μJ pulse. Data are for separate realizations but show typical spectra at each time gate.




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