Carlo Badiola

On Weak Effect of Particle Size on Its Burn Time for Micron-Sized Aluminum Powders

This article aims to verify and validate recent experiments with single Al particles demonstrating a weak effect of particle size on its burn time. Individual particles were fed through two laser beams: a low-power laser to in-situ measure particle size using light scattering, and a CO2 laser for ignition. The sensitivity and dynamic ranges of photo-detectors were selected to capture emission from particles in the size range of 1–10 µm. The data processing method was modified. Measured color temperatures for individual particles were supported by spectroscopic measurements from multiple burning particles. The results confirmed that the burn time, t, as a function of the particle diameter, D, can be approximately described as t ∼ D n with n < 1, which is a much weaker function than that expected based on classic droplet combustion models. Experimental data further confirmed that Al particles finer than 10 µm burn in room temperature air, achieving high combustion temperatures and producing significant molecular AlO emission. Aluminum consumption rates were estimated analyzing the particle temperature histories. Estimates showed that thermal conductivity of gas surrounding burning Al particles is much less than that of the heated air, and that vapor phase reactions become non-negligible for particles greater than ∼3 µm.

Aluminum Rich Al-CuO Nanocomposite Materials Prepared by Arrested Reactive Milling at Cryogenic and Room Temperatures

Nanocomposite materials with reactive components are of interest for many applications in pyrotechnics, explosives, and propellants. Several such materials have been recently prepared by Arrested Reactive Milling (ARM), a method based on mechanical milling of µm-sized component powders to form µm-sized composite particles in which the components are mixed on a scale of 100 nm or finer. The temperature at which the milling is performed affects significantly both the rate at which the material is refined and the final properties of the product. In the present paper we report on an effort to prepare Al-CuO reactive nanocomposites at cryogenic temperatures. A SPEX Certiprep 6815 Freezer/Mill was used to prepare the nanocomposites with aluminum-rich compositions from µm-sized component powders. The material was processed in steel vials using steel balls of different sizes as milling medium. The number and dimensions of the milling balls as well as the milling time were systematically varied. The prepared powders were characterized by X-ray diffraction, particle size analysis, and scanning electron microscopy. Thermal characteristics were studied using a custom wire-ignition setup and differential scanning calorimetry. Results show that the uniformity of mixing and reactivity of the nanocomposite powders can be improved using milling at cryogenic temperatures.

Mechanically Alloyed Al-Ti Powders Prepared by Mechanical Milling at Cryogenic Temperatures

�ew metalbased reactive materials are being develo ped aimed to replace aluminum as a fuel additive in propellants, explosives, and pyrotechnics. Recently, potential benefits of mechanically alloyed Alrich AlTi powders were dis cussed in the literature; however, preparation of fine powders of such alloys could not be achieved by mechanical milling at room temperature because of ductility of the material. This paper investigates the feasibility of preparation of fine mechanically alloyed Alrich AlTi powders using mechanical milling at cryogenic temperatures. Attrition milling, a readily scalable technique is used in this study. Powders initially alloyed at room temperatures are postprocessed at the liquid nitrogen temperature to reduce the particle sizes. In addition, mechanically alloyed powders are prepared directly by the cryogenic milling from elemental starting materials. Particle sizes of the prepared powders are measured using lowangle laser light scattering. Morphology is studied using scanning electron microscopy. The structures and compositions of the prepared materials are examined using xray diffraction. Finally, combustion performance of the prepared powders is investigated and compared to that of pure Al powders with different sizes using constant volume explosion experiments. It is shown that cryogenic milling enables one to reduce the particle sizes of the Alrich AlTi alloys. It is further shown that the ignition temperatures of the mechanically alloyed powders are substantially reduced compared to pure Al.