ICMCTF2016 Session C6: Energetic Materials and Micro-Structures for Nanomanufacturing

The last decade has seen the interest in additive manufacturing of items increase drastically. Several processes including fused deposition modeling, stereo lithography, laser sintering, laser melting, and even various vapor deposition methods are all currently used for additive manufacturing of various materials. This study examines the use of an extrusion process, blade casting, to enable additive manufacture of an energetic film that is free standing once cured but sustains an exothermic reaction once ignition occurs. Three polymer binder solvent systems were chosen for use in this study, these were a tin cured polydimethylsiloxane silicone and xylene, epoxy resin and xylene, and acrylonitrile butadiene styrene, ABS, and acetone. The base energetic used in all synthesized films was aluminum and molybdenum trioxide with potassium perchlorate used as an oxidizing additive. The mass of the energetic materials was held constant for all films tested. The mass percent binder was increased from ten percent to fifty percent in order to establish the effect the binder has on the flame speed of the cured films. All films were blade cast at 1.0 mm thickness with constant volume percent solids. The films were then burned in an unconfined test apparatus and the flame propagation was recorded with a high speed camera. The results from the flame speed testing showed that as the mass percent binder is decreased the flame speed of the films increased with twenty mass percent ABS having the highest flame speed of 1.24 cm/s. Since the binders are participating in the reaction this increase in flame speed is attributed to the fuel-oxidizer ratio approaching an optimum ratio as the binder, a fuel, is removed. In addition, at the higher mass percents binder there is not enough oxidizer to completely react with the binder, thus the non participating binder acts as a heat sink drawing heat from the reaction. These extrusion synthesized films are attracting attention due to their ability to be used in fused deposition methods of additive manufacturing allowing on demand energetic manufacturing.

Energetic materials provide unique capabilities for on-chip applications due to the high amount of stored energy and breadth of potential fabrication techniques. The stored energy is ideal for a number of actuation applications including heating, fuzing, or micro-propulsion. We have previously demonstrated significant potential for on-chip porous silicon combustion, whether through a high degree of reaction tuning, or through self-propagating combustion events at the microscale. However, one of the typical limiting factors of on-chip porous silicon energetic materials is the use of hygroscopic oxidizers. In order to penetrate the nanoscale porous silicon matrix, oxidizers that dissolve in solvents such as methanol are desirable, but solubility correlates with hygroscopicity. Water intake is highly detrimental to the performance of such energetic materials, and therefore the use of these energetic systems typically requires a hermetic seal. To avoid this issue, alternative oxidizers or energetics are desirable for small-scale on-chip applications.

The present study investigates a number of different techniques for implementing energetic materials on-chip, and also investigates the use of porous silicon as a substrate for energetic materials. For small scale on-chip applications heat loss to the surrounding substrate can be critical. The thermal conductivity of porous silicon can be several orders of magnitude lower than crystalline silicon, and therefore may be a functional surface for patterning of energetic materials. Investigated materials include aluminum based nanothermites, and porous silicon with unique or novelly placed oxidizers. Application of the materials was done in a number of ways including drop-casting, deposition, or nanowicking, with an effort to have feature sizes at the microscale. Initial experiments have yielded a hierarchical porous silicon with larger, 2 micrometer pores allowing infiltration of nanoscale metal oxides, combined with a nanoscale porous silicon layer on the surface of the 2 micrometer pores. Characterization of resulting materials was primarily performed via, high-speed imaging, SEM, and DSC.

The conventional route for controlling reactivity in materials has focused primarily on the formulation, or parameters such as particle size, within the formulation. Here we look at using the architectural design of materials as an alternate method to control material reactivity. Electrophoretic deposition is used to probe the effects of film thickness on flame propagation velocity in Al/CuO thermite composites, between 10 and 150 microns. We find that thick films can yield ~10x faster reaction velocities than thin films, due to their ability to trap gaseous intermediate species which evolve during the reaction. 3D printing is used to generate conductive electrodes as substrates for deposition, to examine the interactive effects between neighboring reactive materials. We find that the orientation and spacing of the architecture can yield very different behavior. In these cases, the particulate film undergoes a multiphase expansion to liberate both gases and particles. With proper design, these expansion processes can be used to direct the convection of gases and/or advection of particles to modulate the forward energy transport. The results find that the material architecture, specifically the feature size and design, gives us alternate design parameters for tailoring reactivity to deliver a desired energy release rate. This is a major benefit, considering the cost, sensitivity, or environmental concerns of many existing reactive formulations, along with the persistent need for new materials with precise energy release profiles.

The possible application of luminescent Si-based materials for solid-state lighting (SSL) has emerged as an interesting area of research as it would offer substantial advantages in terms of cost and manufacturability. In order for Si-based materials to be used in SSL schemes it is necessary to have precise control of the emission from these materials. This can be accomplished through the use of rare earth dopants such as Ce, Tb, and Eu (or Pr) to obtain precise blue, green, and red emissions, respectively. Details of the luminescence mechanisms in these materials, however, remain a matter of debate, particularly in cases where the composition of the host matrix is varied and/or where nanoclusters/nanocrystals form during the anneal process. Nano-structured silicon shows quantum confinement effects that contribute strongly to the luminescence.

After a brief review of the latest developments in the field, this talk will focus on the luminescence of rare earth (Ce, Tb, Eu) doped silicon oxides, nitrides, and carbides. We have demonstrated very high, optically active concentrations of the rare earths by using in-situ doping processes, using electron cyclotron resonance chemical vapour deposition (ECR-CVD) or inductively coupled plasma (ICP) CVD as low thermal budget processes for film deposition. I will describe the salient features of the deposition systems and relate important process parameters to the observed luminescence.

I will also discuss the application of synchrotron-based techniques to the investigation of the luminescence mechanisms in such structures. Our studies at the Canadian Light Source synchrotron facility include X-ray excited optical luminescence (XEOL) and the analysis of X-ray absorption near edge structure (XANES) at the Si and O K-edges and the Si L3,2-edge. Through the analysis of the XANES and XEOL, details of the microscopic materials structure and its relation to the luminescence mechanisms can be determined. In particular, the chemical sensitivity of the XEOL process provides a site-specific method for the analysis of luminescence excitation processes.

Finally, I will discuss some of the challenges in developing electrically driven lighting cells suitable for SSL and in particular, the development of white light emitters from rare earth doped Si-based materials.

The application of ‘plasmonics’ is of interest in fields as diverse as biosensing, spectrally-selective coatings, medical treatment, photonics, nano-optics, and photo-voltaic energy. In general, nanoscale films or particles of Ag or Au are used in plasmonic devices. Here, however, we investigate whether Cu-Al alloys can serve as an alternate material. Pure Cu oxidises too readily to be of use, but alloying with Al is expected to impart oxidation resistance and alter the optical properties. The latter follows because Al increases the electron-to-atom ratio, which influences electronic structure and hence the dielectric function. Here we investigate their optical properties using the complex dielectric function ε=ε₁+i.ε₂.

Dielectric function is a sensitive way to probe the structural state of some metallic thin films. Thin films were deposited at room temperature by magnetron sputtering onto a glass substrate then annealed at 500°C for 20 minutes and characterized by ellipsometry, spectroscopy and X-ray diffraction. The dielectric function for each film was determined by fitting the ellipsometric data with a multiple Lorentz oscillator model . The Al content and hence dielectric function was varied sensitively by adjusting the relative sputtering condition, in particular deposition rates of the two metals. As-deposited thin films are poorly crystalline, but were fully crystallized by annealing in inert gas at 500°C for 20 minutes. Finally, we explore the plasmonics properties of the series of alloys and show that it is closely related to their composition and microstructure.