Basics of qualitative research: Grounded theory procedures and techniques

The aim of this review article on recent developments of mechanochemistry (nowadays established as a part of chemistry) is to provide a comprehensive overview of advances achieved in the field of atomistic processes, phase transformations, simple and multicomponent nanosystems and peculiarities of mechanochemical reactions. Industrial aspects with successful penetration into fields like materials engineering, heterogeneous catalysis and extractive metallurgy are also reviewed. The hallmarks of mechanochemistry include influencing reactivity of solids by the presence of solid-state defects, interphases and relaxation phenomena, enabling processes to take place under non-equilibrium conditions, creating a well-crystallized core of nanoparticles with disordered near-surface shell regions and performing simple dry time-convenient one-step syntheses. Underlying these hallmarks are technological consequences like preparing new nanomaterials with the desired properties or producing these materials in a reproducible way with high yield and under simple and easy operating conditions. The last but not least hallmark is enabling work under environmentally friendly and essentially waste-free conditions (822 references).

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Hallmarks of mechanochemistry: From nanoparticles to technology

Metal-based reactive nanomaterials

Metal combustion has received renewed interest largely as a result of the ability to produce and characterize metallic nanoparticles. Much of the highly desirable traits of nanosized metal powders in combustion systems have been attributed to their high specific surface area (high reactivity) and potential ability to store energy in surfaces. In addition, nanosized powders are known to display increased catalytic activity, superparamagnetic behavior, superplasticity, lower melting temperatures, lower sintering temperatures, and higher theoretical densities compared to micron and larger sized materials. The lower melting temperatures can result in lower ignition temperatures of metals. The combustion rates of materials with nanopowders have been observed to increase significantly over similar materials with micron sized particles. A lower limit in size of nanoenergetic metallic powders in some cases may result from the presence of their passivating oxide coating. Consequently, coatings, self-assembled monolayers (SAMs), and the development of composite materials that limit the volume of non-energetic material in the powders have been under development in recent years. After a brief review of the classifications of metal combustion based on thermodynamic considerations and the different types of combustion regimes of metal particles (diffusion vs. kinetic control), an overview of the combustion of aluminum nanoparticles, their applications, and their synthesis and assembly is presented.

Metal particle combustion and nanotechnology

Tansy fruit mediated greener synthesis of silver and gold nanoparticles

Laser ignition of nanocomposite thermites

Combustion behavior of energetic composite materials was experimentally examined for the purpose of evaluating the unique properties of nano-scale compared with traditional micron-scale particulate media. Behavior of composite systems composed of aluminum (Al) and molybdenum trioxide (MoO3) were studied as a function of Al particle size, equivalence ratio and bulk density. Samples were prepared by mechanically mixing individual fuel and oxidizer particles and combustion experiments included measurements of ignition and flame propagation behavior. Ignition was achieved using a 50-W CO2 laser and combustion velocities were measured from photographic data. Reaction kinetics were studied with differential scanning calorimetry (DSC). Results indicate that the incorporation of nano-Al particles (1) significantly reduces ignition temperatures and (2) produces unique reaction behavior that can be attributed to a different chemical kinetic mechanism than observed with micron-Al particles.

Combustion Behavior of Highly Energetic Thermites: Nano versus Micron Composites

Combustion velocities were experimentally determined for nanocomposite thermite powders composed of aluminum (Al) fuel and molybdenum trioxide (MoO3) oxidizer under well-confined conditions Pressures were also measured to provide detailed information about the reaction mechanism Samples of three different fuel particle sizes (44, 80, and 121nm) were analyzed to determine the influence of particle size on combustion velocity Bulk powder density was varied from approximately 5% to 10% of the theoretical maximum density (TMD) The combustion velocities ranged from approximately 600 to 1000m∕s Results indicate that combustion velocities increase with decreasing particle size Pressure measurements indicate that strong convective mechanisms are integral in flame propagation

Combustion velocities and propagation mechanisms of metastable interstitial composites

Combustion wave speeds of nanocomposite Al/Fe2O3: the effects of Fe2O3 particle synthesis technique

Reactive mixtures of aluminum (Al) and polytetrafluoroethylene (PTFE or Teflon) have applications in propellants, explosives, and pyrotechnics. This study examines the thermal degradation behavior of Teflon and nanometer scale Al particles compared with micron-scale Al particles. Differential scanning calorimetry and thermo-gravimetric analyses were performed in an argon environment on both nanometer and micron scale particulate mixtures revealing lower ignition temperatures and larger exothermic activity for the nanometer Al/Teflon mixture. This increased ignition sensitivity and exothermicity is caused by a pre-ignition reaction unique to the nano-Al mixture. Experiments suggest that the pre-ignition reaction mechanism is controlled by the fluorination of the Al particle passivation shell. Micron-scale Al particles have a lower specific surface area and therefore the influence of the passivation shell promoting a pre-ignition reaction is reduced. Chemical kinetics are discussed along with parti...

Michelle L. Pantoya

Papers

Laser ignition of nanocomposite thermites

Combustion Behavior of Highly Energetic Thermites: Nano versus Micron Composites

Combustion velocities and propagation mechanisms of metastable interstitial composites

Combustion wave speeds of nanocomposite Al/Fe2O3: the effects of Fe2O3 particle synthesis technique

Effect of al particle size on the thermal degradation of al/teflon mixtures