Kelvin T. Higa

Thermal Initiation of Al-MoO3 Nanocomposite Materials Prepared by Different Methods

Two types of nanocomposite reactive materials with the same bulk compositions 8Al MoO3 were prepared and compared with each other. One of the materials was manufactured by arrested reactive milling and the other so-called metastable interstitial composite was manufactured by mixing of nanoscaled individual powders. The materials were characterized using differential scanning calorimetry, thermogravimetric, as well as ignition, experiments using an electrically heated filament and laser as heat sources. The experiments were interpreted using simplified models describing heat transfer in the heated material samples in different experimental configurations. Clear differences in the low-temperature redox reactions, well detectable by differential scanning calorimetry, were established between metastable interstitial composite and arrested-reactive-milling-prepared materials. However, the materials did not differ significantly from each other in the ignition experiments. In the heated filament ignition tests, their ignition temperatures were nearly identical to each other and were in the range of 750–800 K. These ignition temperatures coincided with the temperatures at which main exothermic processes were detected in differential scanning calorimetry experiments. In laser ignition experiments performed with consolidated pellets of bothmaterials, metastable interstitial composite pellets produced consistently stronger pressure pulses. The ignition delays were similar for the pellets of both materials prepared with the same porosity. Analysis of the heat transfer in the pellets heated by the laser suggested that the laser-exposed pellet surfaces were heated to approximately the same temperature before ignition for both materials. This temperature was estimated to be close to 500 K, neglecting the exothermic reactions preceding ignition and possible fragmentation of the heated pellets. Taking into account both phenomena is expected to result in a higher surface temperature, which would better represent the experimental situation. It is proposed that the ignition of both metastable interstitial composite and arrested-reactive-millingprepared materials at the same temperature can be explained by a thermodynamically driven transformation of a protective amorphous alumina into a crystalline polymorph.

Characterization of Nanometer- to Micron-Sized Aluminum Powders: Size Distribution from Thermogravimetric Analysis

Thermogravimetric analysis was used to study the reactivity of aluminum powders in air, oxygen, and nitrogen. In addition, the data were used to characterize active Al content, Al oxide content, volatile impurity content, particle size, and particle size distribution. Weight gains from complete oxidation of the Al were used to calculate average particle sizes in the range of 30 to 500 nm. These particle sizes correlated well with particle sizes derived from surface area measurement. Particle size was also examined by scanning electron microscopy, and compared with crystallite size determined by x-ray diffraction. Particle size distributions were derived from thermogravimetric analysis data based on a model of uniform oxidation of Al from the exterior to the interior of the particle. The method is well suited for analyzing samples with broad particle size distributions, and in particular, for monitoring the presence of 500-5000 nm particles within nominally nanosized samples. Quantitative information was not obtained for particles around 100 nm or smaller, due to large variations in oxidation behavior below 700° C. Nitridation of Al powders was studied for extended times at 600° C. Surprisingly, 2 μm powder was nearly completely nitrided in 1 h, indicating that the nitride product has little inhibiting effect on the reaction.

Metalorganic molecular beam epitaxy doping of II-VI compound semiconductors

Abstract Site-selective metalorganic molecular beam epitaxy (MOMBE) is a new technique that shows much promise for the controlled substitutional doping of II–VI compound semiconductors. In the present work, initial results on the use of the site-selective doping are reported. The metalorganic compound [ t -BuZnAs( t -Bu) 2 ] 2 which is employed in the present work is only one of a whole class of potential compounds in which a group II element is pre-bonded to a group V element. ZnSe epitaxial layers doped with this compound exhibit bright near edge luminescence that is dominated by acceptor-bound-exciton transitions. This indicates that the As contained in this designer dopant is incorporated on its proper subsitution site.