Lori J. Groven

Magnetic field enhanced thermal conductivity in heat transfer nanofluids containing Ni coated single wall carbon nanotubes

In this paper, we report that the thermal conductivity (TC) of heat transfer nanofluids containing Ni coated single wall carbon nanotube can be enhanced by applied magnetic field. A reasonable explanation for these interesting results is that Ni coated nanotubes form aligned chains under applied magnetic field, which improves thermal conductivity via increased contacts. On longer holding in magnetic field, the nanotubes gradually move and form large clumps of nanotubes, which eventually decreases the TC. When we reduce the magnetic field strength and maintain a smaller field right after TC reaches the maximum, the TC value can be kept longer compared to without magnetic field. We attribute gradual magnetic clumping to the gradual cause of the TC decrease in the magnetic field. We also found that the time to reach the maximum peak value of TC is increased as the applied magnetic field is reduced. Scanning electron microscopy images show that the Ni coated nantubes are aligned well under the influence of a ...

Non-aqueous synthesis of silver nanoparticles using tin acetate as a reducing agent for the conductive ink formulation in printed electronics

We have developed a process for the synthesis of silver nanoparticles protected with a passivating shell of dodecylamine in toluene media using tin(II) acetate as a reducing agent. Based on the electrochemical series, during the reduction process Sn(II) oxidizes into Sn(IV) which reduces Ag(I) into Ag(0). The nucleation and growth processes result in particles with diameters in the range 5–20 nm. This simple non-aqueous one pot synthesis can be easily scaled up to produce grams of nanoparticles in a matter of hours. The particles can also be dispersed in many non-aqueous solvents which make them a suitable candidate for many applications. Characterization of the end product using TEM, UV-Vis spectroscopy, and powder X-ray diffraction verified the presence of a silver metallic core whereas TGA confirmed the presence of a dodecylamine shell. The resulting particles were used in non-aqueous conductive ink formulation. The ink was used to print conductive tracks on flexible substrates like Epson photo paper and polyimide (Kapton) using an Aerosol Jet based printing technique.

The role of microstructure refinement on the impact ignition and combustion behavior of mechanically activated Ni/Al reactive composites

Metal-based reactive composites have great potential as energetic materials due to their high energy densities and potential uses as structural energetic materials and enhanced blast materials however these materials can be difficult to ignite with typical particle size ranges. Recent work has shown that mechanical activation of reactive powders increases their ignition sensitivity, yet it is not fully understood how the role of microstructure refinement due to the duration of mechanical activation will influence the impact ignition and combustion behavior of these materials. In this work, impact ignition and combustion behavior of compacted mechanically activated Ni/Al reactive powder were studied using a modified Asay shear impact experiment where properties such as the impact ignition threshold, ignition delay time, and combustion velocity were identified as a function of milling time. It was found that the mechanical impact ignition threshold decreases from an impact energy of greater than 500 J to an...

Thermal and mechanical response of PBX 9501 under contact excitation

The thermal and mechanical responses of a cyclotetramethylene-tetranitramine-based explosive (PBX 9501) and two non-energetic mock materials (900-21 and PBS 9501) under high-frequency mechanical excitation are presented. Direct contact ultrasound transducers were used to excite samples through a frequency range of 50 kHz to 40 MHz. The mechanical response of each sample was approximated from a contact receiving transducer and trends were confirmed via laser Doppler vibrometry. The steady-state thermal response of the samples was measured at discrete excitation frequencies via infrared thermography. A maximum temperature rise of approximately 15 K was observed in PBX 9501, and the mock materials exhibited similar thermal characteristics. Temperature gradients were calculated to estimate the total heat generated within the samples due to the mechanical excitation. The active heating mechanisms were found to be highly dependent on the frequency of excitation. Possible mechanisms of heating at frequencies bel...

Dependence of Nano-Aluminum and Water Propellant Combustion on pH and Rheology

Over the past few years, the combustion of nano-aluminum/water (nAl/H2O) propellants has been widely reported, but further progress has been slowed for the following reason: the loosely correlated trends in combustion data are insufficient in guiding further research efforts, and they cannot be used to significantly improve the Isp observed in static rocket motor tests. It was previously found that different mixing techniques (hand, planetary and resonant mixers, duration and temperature), or equivalence ratio gave rise to different burning rates, but the influence of pH and rheology on nAl/H2O propellants was not considered. We find that the effects of pH on nAl/H2O propellants are profound, and correlate well with viscosity, low pressure deflagration limits, burning rate exponents, and rocket motor performance. Our findings suggest that coagulation can influence the pressure exponent over a wide range of values (0.34–0.68). For particle diameters <1 µm, dispersion during mixing is affected more by electrostatic repulsion from charged ions than from mechanical agitation, and this is reflected through zeta potential and viscosity measurements at different pH levels. Additionally, we observe that pH has an influence on nAl/H2O reaction kinetics during ignition, as the propellant transitions from low temperature oxidation to high temperature combustion.

Intermetallic Compounds as Fuels for Composite Rocket Propellants

Aluminized composite propellants have long suffered from efficiency and thermal challenges related to production of condensed phase slag droplets during operation. In an effort to mitigate the production of large droplets, a mechanically activated intermetallic forming nickel-aluminum compound was substituted for a portion of a propellant’s aluminum fuel. The resulting agglomerate size and burning rate of this propellant was compared to a standard aluminized AP/HTPB propellant. Addition of mechanically activated fuel particles increased the burning rate exponent of the propellant, while simultaneously decreasing condensed phase agglomerate size from 235 µm (for the control propellant) to 90 µ m( for the propellant containing 75 wt.% Ni-Al fuel). As such, intermetallic forming fuels may provide a route for increasing efficiency in solid rocket motors by simultaneously reducing the need for burning rate catalysts and minimizing two-phase nozzle flow losses.

Further Development of an Aluminum and Water Solid Rocket Propellant

Nanoscale aluminum and water has been used as a stepping stone towards in-situ rocket propellants and as a testbed for nanoenergetic composite propellants. A baseline formulation of nanoscale aluminum and water was developed and demonstrated with a sounding rocket flight in 2009. Performance of the propellant was not optimized, hence a reformulation was sought with an emphasis on improved safety and more efficient combustion. The chosen reformulation is a bimodal powder distribution of 70 wt.% Novacentrix 80 nm Al and 30 wt.% Valimet 2 µm Al at an equivalence ratio of 0.813 (optimized for sea level Isp). The mixture also includes 3 wt.% ammonium dihydrogen phosphate, to inhibit the slow reaction of nanoaluminum with water, and 1 wt.% polyacrylamide to improve material suspension. Ammonium dihydrogen phosphate can protect nanoaluminum in solution for several hours, but degradation can occur while mixing, and pH increases from slightly acidic to basic with increased mixing time and temperature. The stress of mixing might be removing the coating and exposing nanoaluminum to water. It is also shown that nanoaluminum reacts faster in basic aqueous solutions than in solutions with neutral pH. Static motor tests reveal that propellant formulations with neutral pH provide better performance. Implementations of shorter mixing times and reduced temperatures are used to control the pH of the propellant, resulting in increased Isp values of as much as 30%.

Effect of carbon nanotube addition on morphology of SHS synthesized materials

During the past few years, a considerable amount of research has been focused on the addition of carbon nanotubes (CNTs) to various matrix materials including polymers, ceramics, metals, and intermetallics. However, the processing methodologies for ceramics, metals, and intermetallics have been primarily those used in traditional power technology. In this paper combustion synthesis (CS), also known as self-propagating high temperature synthesis (SHS) or volume combustion synthesis (VCS) was explored as a route for the formation of carbon nanotube reinforced composites. The effect of single-walled carbon nanotube (SWCNT) addition on the product morphology is presented for the following reactive systems: (i) Ni-Al and (ii) 3Ti-B4C. It was determined that the SWCNTs survive a short exposure to the high temperatures associated with the combustion synthesis of NiAl (1911 K) and TiB2-TiC (3200 K) and effectively act as a diluent which results in grain refinement of the resulting product. The results also show that effective dispersion of the SWCNTs is achieved; however, the SWCNTs are not well embedded in the matrix. Based on the survival of the SWCNTs in the above reactive systems it was shown that metal coated nanotubes may be a very promising reinforcement material for combustion synthesized materials and could effectively solve the issue of CNT to matrix adhesion.

Low loss, high index of refraction metamaterials based on multiple enhancement mechanisms

A new metallo-dielectric metamaterial is described in this paper that is comprised of a lattice of so-called dumbbell particles. Low loss with potentially large index of refraction is possible with these artificial electromagnetic materials using multiple enhancement effects including thin layers of high dielectric constant, low loss composite films printed between layers of dumbbell particles. Spray deposition of these composite dielectric layers is described.

Nanoscale Characterization of Mock Explosive Materials Using Advanced Atomic Force Microscopy Methods

Most explosives are micro- and nanoscale composite material systems consisting of energetic crystals, amorphous particles, binders, and additives whose response to mechanical, thermal, or electromagnetic insults is often controlled by submicrometer-scale heterogeneities and interfaces. Several advanced dynamic atomic force microscopy (AFM) techniques, including phase imaging, force volume mode, and Kelvin probe force microscopy with resonance enhancement for dielectric property mapping, have been used to map the local physical properties of mock explosive materials systems, allowing the identification of submicrometer heterogeneities in electrical and mechanical properties that could lead to the formation of hotspots under electromagnetic or mechanical stimuli. The physical interpretation of the property maps and the methods of image formation are presented. Possible interpretations of the results and future applications to energetic material systems are also discussed.