Jeffrey F. Rhoads
Professor of Mechanical Engineering
Westphal, Eric R. ; Murray, Allison K. ; Mcconnell, Miranda P. ; Fleck, Trevor J. ; Chiu, George T. ‐C. ; Rhoads, Jeffrey F. ; Gunduz, I. Emre ; Son, Steven F.
Propellants, Explosives, Pyrotechnics, January 2019, Vol.44(1), pp.47-54
Nanothermites have shown the potential to controllably fracture substrates in applications such as electromechanical systems security. In prior work, both equivalence ratio and material formulation have been varied to tailor fracturing performance. In this paper, material confinement was utilized to further tailor the fracturing performance of aluminum bismuth (III) oxide (Al/BiO) and aluminum copper (II) oxide (Al/CuO) nanothermites. These nanothermites were selectively deposited onto representative substrates through inkjet printing. Al/BiO nanothermites were prepared over a range of equivalence ratios and showed a range of resulting fragmentation, with a maximum near the equivalence ratio of ϕ=2. Burning rate measurements correlated with the trends seen in these experiments. All of the previous attempts at fragmenting a substrate using unconfined Al/CuO were unsuccessful. The prepared Al/CuO nanothermites at stoichiometric conditions resulted in fractured silicon substrates when confined. These results demonstrate the ability of confinement to further tailor the fracturing performance of nanothermites.
Mesoscale observations of the thermal decomposition of energetic composites under ultrasonic excitation
Roberts, Z. A. ; Wickham, J. A. ; Sorensen, C. J. ; Manship, T. D. ; Gunduz, I. E. ; Son, S. F. ; Rhoads, J. F.
Journal of Applied Physics, 07 June 2019, Vol.125(21)
Polymer bonded explosives (PBXs) have exhibited localized heating and, in some cases, subsequent reactions in response to ultrasonic excitation. The objectives of this work are to investigate the conditions for, and locations of, hot spot initiation of energetic crystals embedded within a polymer binder subjected to periodic mechanical excitation from a contacting transducer operating at 210.5 kHz. Crystal and binder interactions and events such as delamination, solid-solid phase change, and gas production were observed in real time via optical microscopy. We conclude that there are two main pathways of heat generation which are capable of driving an explosive to decomposition in the systems of interest: frictional heating from a delaminated and moving binder interface and viscoelastic heating in the binder near an embedded crystal. Formulations that address the vibration initiation sensitivity of PBX composites require knowledge of the key internal heat generation mechanisms. The results included here indicate that improving binder adhesion to energetic crystals or improving crystal morphology to reduce heating during cyclic loading may only address one of the available pathways of energy dissipation and that binder and crystal selection should be done concurrently. Furthermore, the results presented herein appear to indicate that rounded particles, in contrast to faceted crystals, with strong adhesion to the binder are expected to result in decreased heating rates under ultrasonic excitation.
Mares, Jo ; Roberts, Za ; Gunduz, Ie ; Parab, Nd ; Sun, T ; Fezzaa, K ; Chen, Ww ; Son, Sf ; Rhoads, Jf
Applied Materials Today, 2019 Jun, Vol.15, pp.286-294
Ultrasound is used to study “hot spot” formation and explosive initiation. This work details observations of the heating and decomposition of an explosive. Interfacial friction is shown to be a dominant heating mechanism. High-strain mechanical loading of polymer-bonded explosives can produce significant stress concentrations due to microstructural heterogeneities, resulting in localized thermal “hot spots”. Ultrasound produces similar effects and has been proposed as a tool to study the thermomechanical interactions related to explosive initiation. Detailed observations of the processes governing the generation of heat in these materials are severely lacking, yet they are vital for identifying salient physics, improving the modeling tools used to predict mechanical response, improving explosives safety, and providing insight into the initiation mechanisms of explosion. Here we report on high-speed, high-resolution in-situ observations, obtained via synchrotron X-ray phase contrast imaging and diffraction, of the heating and decomposition of an explosive material under ultrasonic excitation. We demonstrate that interfacial friction is a dominant heating mechanism and can lead to a violent reaction in the explosive particles.
Furthermore, sub-surface particle temperatures are estimated via diffraction.
Murray, Allison K ; Meseke, Joseph R ; Bajaj, Nikhil ; Rhoads, Jeffrey F
2019 IEEE SENSORS, October 2019, pp.1-4
This work highlights the development of an array of oscillator-based sensors suitable for the detection of volatile organic compounds (VOCs). A field-programmable gate array (FPGA) was leveraged to monitor the oscillation frequency of 16 sensing elements in parallel. It was shown that three functional chemistries can be deposited via inkjet printing and subsequently used to identify various analytes of interest. This response is valuable for identifying sample air makeup using techniques such as principle component analysis. Additionally, this system utilized an array of sufficient scale to incorporate several functional chemistries with intentional redundancies. Ultimately, this work demonstrates a low-cost oscillator-based sensing array capable of simultaneously monitoring chemical processes with single second resolution using 16 independent channels.
Pyles, Conor S ; Bajaj, Nikhil ; Rhoads, Jeffrey F ; Weinstein, Dana ; Quinn, D. Dane
2019 IEEE SENSORS, October 2019, pp.1-4
In this work, we examine the dynamics of a network of Colpitts oscillators which are coupled through a series of color- filtered CdSe photocells. We establish that with the analysis of particular emergent behaviors (most notably, frequency-locking within the network), this type of system has promise as a threshold color sensor. By exploiting such emergent behaviors, this type of system may find utility in optical pattern recognition.
Habermehl, Scott ; Bajaj, Nikhil ; Shah, Shreyas ; Weinstein, Dana ; Rhoads, Jeffrey
Nonlinear Dynamics, Dec 2019, Vol.98(4), pp.3037-3050
The experimental characterization of a network of three resistively coupled microelectromechanical systems (MEMS)-based Colpitts oscillators is presented. The oscillators were fabricated on a printed circuit board and comprised of a 16 MHz quartz resonator in a positive feedback loop, which generated a sustained limit cycle oscillation when powered by a DC voltage supply. It was demonstrated that by varying the coupling strength and uncoupled frequency distribution between the oscillators, frequency synchronization and phase relationships could be controlled. This was examined in isolated pairs of oscillators and in cases where the entire network was interacting. Coupling was achieved with discrete resistors that could be autonomously interchanged with digitally controlled switches. These results build upon a small body of experimental studies by examining a coupling architecture wherein impedance interactions between the oscillators made amplitude dynamics relevant. This is a fundamental step toward fully understanding patterns and behavior in practical networks of coupled MEMS oscillators. The potential for the network to be used in neuromorphic computing and pattern recognition applications is also discussed.