High speed propulsion: Performance advantage of advanced materials

High-speed air breathing propulsion systems have many attractive military and civil applications. The high propulsive efficiency of these systems allows the exploitation of speed, distance, and bigger payloads, or any combination of the three. The severe operating conditions of these systems require particular attention to overall thermal management of the engine/air-frame. Fuel-cooling the engine structure is a viable way of maintaining thermal balance over a range of flight conditions. Air Force applications have focused on using endothermic hydrocarbon fuels to address this issue because of their compatibility with the military operations. Recent ground tests of scramjet engines have demonstrated adequate performance utilizing state-of-the-art technology in materials. This progress has paved the way for an expendable flight test vehicle in the near future. In order to take full advantage of the capabilities of this propulsion system, advances in fuel-cooled structures, high temperature un-cooled materials, and increased heat capacity of hydrocarbon fuels will be needed to enable expendable systems to reach higher Mach numbers. An additional benefit would be realized in future reusable systems.     

An in situ TEM study of phase formation in gold-aluminum couples

In the gold wire bonding of aluminum in microelectronic devices the presence of aluminum oxide on the metallization surface may be expected. Electron transparent couples containing an oxide layer at the interface were heated in a TEM to determine the effects of a passivation layer on intermetallic formation. Intermetallic phases were evidenced by changes in sample appearance and their structure was determined by electron diffraction. The presence of an oxide at the interface hindered second phase formation at temperatures at which they were usually expected to form. In aluminum rich couples, the formation of the AuAl₂ intermetallic was not observed to form until about 350°C with the oxide present. In a reverse configuration involving a gold rich couple, an amorphous oxide phase was observed between the Al and the advancing Au₂Al front. The movement of the reaction front appeared to be controlled by surface diffusion across this phase.     

The mechanochemical processing of aerospace metals

The status of mechanochemical processing of aerospace metals (aluminum and titanium) is reviewed. It is demonstrated that the activation of chemical reactions by mechanical energy can lead to many interesting applications including production of advanced materials with novel constitutional and microstructural effects leading to enhanced mechanical properties.     

Use of polymers for cement-based structural materials

Cement-based materials are widely used in the civil infrastructure. Polymers as admixtures can improve the properties, particularly in relation to water absorption reduction, toughness enhancement, vibration damping and increase of the bond strength of cement to reinforcements. Polymeric admixtures include particles, short fibers and organic liquids. Latex in the form of an aqueous particle dispersion is most common. Other than being used as admixtures, polymers are used as partial replacement of fine aggregate, for coating, sealing and repairing concrete and for coating steel reinforcing bars for corrosion protection.