preparation-reactivity-gasless-nanostructured-energetic

Preparation and Reactivity of Gasless Nanostructured Energetic Materials

High-Energy Ball Milling (HEBM) is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collisions from the ...

University of Notre Dame

The oxide-based supported catalysts with high specific surface area (>200 m(2)/g) were produced in one step through combination of the impregnation and solution combustion synthesis approaches. As a model system, iron oxide was selected, which was loaded on different porous supports including alpha-Al(2)O(3), gamma-Al(2)O(3), and ZrO(2), as well as activated alumina. It was shown that for the former three cases the specific surface areas of the supported catalysts are about or below the surface areas of the support. However, for the activated Al(2)O(3) this characteristic significantly increases compared to that of the support. It was demonstrated that the developed approach may be used to produce different types of oxide-based supported catalysts, including perovskites.

In situ preparation of oxide-based supported catalysts by solution combustion synthesis.

A novel apparatus for joining carbon-carbon (C-C) composites is presented. This device was designed and built based on the concept of self-sustained oxygen-free high-temperature reactions. A layer of reactive mixture is contained between two disks of C-C composite that are to be joined. The stack is held in place between two electrodes, which are connected to a dc power supply. dc current is used to uniformly initiate the reaction in the reactive layer. The electrodes are also part of the pneumatic system, which applies a load to the stack. The designed hydraulic system is effective, lending to low cost and simplified, rapid, accurate operation. It provides a very short response time ( approximately 10 ms), which is important for the considered applications. All operational parameters such as initial and final loads, applied current, delay time between ignition and final load application, duration of Joule heating, and safety interlocks are controlled by a programable logic controller system. These features make it an efficient, user-friendly and safe machine to join refractory materials. The entire joining process takes place on the order of seconds, rather than hours as required for solid-state joining methods. The mechanical properties of the obtained joints are higher than those for the C-C composites.

Novel apparatus for joining of carbon-carbon composites.

The influence of short-term (5-15 min) highly energetic ball milling on the ignition characteristics of a gasless heterogeneous Ni-Al reactive system has been investigated. By using Al-Ni clad particles (30-40 microm diameter Al spheres coated by a 3-3.5 microm layer of Ni, that corresponds to a 1:1 Ni/Al atomic ratio), it was shown that such mechanical treatment leads to a significant decrease in the self-ignition temperature of the system. For example, after 15 min of ball milling, the ignition temperature appears to be approximately 600 K, well below the eutectic (913 K) in the considered binary system, which is the ignition temperature for the initial clad particles. Thus, it was demonstrated that the thermal explosion process for mechanically treated reactive media can be solely defined by solid-state reactions. Additionally, thermal analysis measurements revealed that mechanical activation results in a substantial decrease in the effective activation energy (from 84 to 28 kcal/mol) of interaction between Al and Ni. This effect, that is, mechanical activation of chemical reaction, is connected to a substantial increase of contact area between reactive particles and fresh interphase boundaries formed in an inert atmosphere during ball milling. It is also important that by varying the time of mechanical activation one can precisely control the ignition temperature in high-density energetic heterogeneous systems.

Thermal explosion in Al-Ni system: influence of mechanical activation.

High temperature (>1000 K) reaction kinetics in the stoichiometric (1:1 by molar ratio) Al-Ni system was investigated by using the, so-called, electrothermal analysis (ETA) method. ETA is the only technique that allows studying kinetics of a heterogeneous gasless reaction at temperatures above the melting points of the precursors. Special attention was focused on methodological aspects of the ETA method. Two different reaction systems were studied: (i) initial Al/Ni clad particles; (ii) the same powders but after 15 min of high energy ball milling. Analysis of the obtained results leads to the conclusion that such mechanical treatment decreases the apparent activation energies of the reaction in the Ni-Al system, from 47 +/- 7 kcal/mol for the initial powder to 25 +/- 3 kcal/mol after ball milling. Comparison of these data with those reported previously was also made.

Kinetics of high temperature reaction in Ni-Al system: influence of mechanical activation.

A novel approach for depositing hydroxyapatite (HA) films on titanium substrates by using mechanical alloying (MA) technique has been developed. However, it was shown that one-hour heat treatment at 800°C of such mechanically coated HA layer leads to partial transformation of desired HA phase to beta-tri-calcium phosphate (β-TCP) phase. It appears that the grain boundary and interface defects formed during MA promote this transformation. It was discovered that doping HA by silicon results in hindering this phase transformation process. The Si-doped HA does not show phase transition to β-TCP or decomposition after heat treatment even at 900°C.

Nanocrystalline hydroxyapatite/si coating by mechanical alloying technique.

Bulk processing of porous silicon nanoparticles (nSi) of 50-300 nm size and surface area of 25-230 m(2)/g has been developed using a combustion synthesis method. nSi exhibits consistent photoresponse to AM 1.5 simulated solar excitation. In confirmation of photoactivity, the films of nSi exhibit prompt bleaching following femtosecond laser pulse excitation resulting from the photoinduced charge separation. Photocurrent generation observed upon AM 1.5 excitation of these films in a photoelectrochemical cell shows strong dependence on the thickness of the intrinsic silica shell that encompasses the nanoparticles and hinders interparticle electron transfer.

Article	Year
In situ preparation of oxide-based supported catalysts by solution combustion synthesis. The journal of physical chemistry. B\| PubMed ID: 16853808	2005
Novel apparatus for joining of carbon-carbon composites. The Review of scientific instruments\| PubMed ID: 17503944	2007
Thermal explosion in Al-Ni system: influence of mechanical activation. The journal of physical chemistry. A\| PubMed ID: 19831374	2009
Kinetics of high temperature reaction in Ni-Al system: influence of mechanical activation. The journal of physical chemistry. A\| PubMed ID: 20433182	2010
Nanocrystalline hydroxyapatite/si coating by mechanical alloying technique. Bioinorganic chemistry and applications\| PubMed ID: 22312324	2012
Photoactive porous silicon nanopowder. ACS applied materials & interfaces\| PubMed ID: 23510512	2013

Alexander S. Mukasyan

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