Manufacturing Bits: May 1

Adaptive materials; stronger armor; AIN wafers.


Adaptive materials
The U.S. Army Research Laboratory (ARL) and the University of Maryland have developed a technique to make adaptive materials.

Using ultraviolet light, researchers have devised a way that causes a composite material to become stiffer and stronger on-demand. This in turn could enable a variety of new capabilities for the U.S. military, such as rotorcraft design.

In this technique, the idea is to attach ultraviolet light reactive molecules to agents like carbon nanotubes. Carbon nanotubes act as reactive reinforcing agents, as they are embedded in a polymer material. Then, a chemical reaction occurs. The interaction between the reinforcing agents and the polymer increases. As a result, the resulting composite materials could become 93% stiffer and 35% stronger after a five- minute exposure to ultraviolet light.

Army researchers imagine a rotorcraft concept, which represents reactive reinforcements that when exposed to ultraviolet light will increase the mechanical behavior on-demand. The engineers said control of mechanical behavior could potentially lead to increased aerodynamic stability in rotorcraft structures. (U.S. Army illustration)

“This research shows that it is possible to control the overall material property of these nanocomposites through molecular engineering at the interface between the composite components. This is not only important for fundamental science but also for the optimization of structural component response,” said Zhongjie Huang, a postdoctoral research fellow at the University of Maryland, on ARL’s Web site.

“An important motivation for this work is the desire to engineer new structures, starting from the nanoscale, to enable advanced rotorcraft concepts that have been proposed in the past, but were infeasible due to limitations in current composites. One of the most important capabilities envisioned by these concepts is a significantly reduced maintenance burden due to compromises we make to fly at high speeds,” said Bryan Glaz, chief scientist of ARL’s Vehicle Technology Directorate, on the site. “The enhanced mechanical properties with potentially low weight penalties, enabled by the new technique, could lead to nanocomposite based structures that would enable rotorcraft concepts that we cannot build today.”

Stronger armor
The U.S. Naval Research Laboratory (NRL) has made a breakthrough in the field of nanocrystalline ceramics for use in developing stronger armor in the battlefield.

Researchers demonstrated that it may be possible to one day to design a lightweight, nanocrystalline ceramic material, which in turn could dissipate more mechanical energy. For example, the material could absorb more damage from a sharp projectile, and maintain its high hardness. This discovery could pave the way for more efficient armor for U.S. military forces.

In the lab, NRL advanced its nanosintering technology to form large-scale nanostructured solids. This technology is called Environmentally Controlled Pressure Assisted Sintering (EC-PAS). Sintering is the process of forming a solid mass of material. This is done by heat or pressure.

At the NRL, the goal is to reduce the grain size of metals and ceramics. This, in turn, can increase strength and hardness of a given material. With EC-PAS, NRL broke the world’s record for the smallest grain size in dense ceramics at 3.6nm. This s about 30,000 times smaller than the width of a human hair.

“A few years ago, NRL was the first to show that if you decrease the grain size of ceramics to tens of nanometers, the hardness and strength increase,” said James Wollmershauser, a materials research engineer in NRL’s Materials Science and Technology Division, on the agency’s Web site. “Our current work takes this much further. We decreased the grain size of fully dense ceramics to record breaking single digits, and analyzed the elasticity, hardness, energy dissipation and fracture behavior in ceramics with a wide range of nanosize grains.

“In general, the Navy wants to lighten the load of the warfighter,” said Wollmershauser. “If you can make harder armor, or better performing armor, then you can put less armor on a person or vehicle, in turn increasing capacity for other things like munitions and electronics.”

AIN wafers
HexaTech has rolled out a 2-inch aluminum nitride (AlN) substrate line for use in laser diodes, power semiconductors, ultraviolet LEDs and other applications.

AlN is a compound semiconductor technology with a wide bandgap of 6.2eV, which is higher than silicon, silicon carbide (SiC) and gallium nitride (GaN).

To develop its wafers, HexaTech uses a proprietary technology, dubbed physical vapor transport (PVT). First, the company starts with AlN source materials. Then, it grows single crystalline AlN boules in furnaces with temperatures exceeding 2000°C. The boules are then sliced into wafers and polished.

AlN growth process (Source: HexaTech)

“By challenging perceived constraints and aggressively pursuing solutions at each step of the crystal growth process, we have developed a significant shift in capability which breaks previously observed limitations,” said Raoul Schlesser, HexaTech co‐founder and vice president of crystal and wafer Development. “(This) sets the stage for both continued diameter expansion and increased process yields, ultimately rivaling the price:performance ratio of other mature compound semiconductor technologies, such as SiC and GaAs.”

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