Historically, high-strength materials have been heavy and dense. The need for high-strength but lightweight materials has become more widespread when designing everything from vehicles and aircraft, to buildings and wind turbines. These advanced materials are enabling engines to operate efficiently at higher temperatures, use less fuel, and emit fewer pollutants, as well as finding uses in many other applications.

The LEAP engine features ceramic matrix composites developed by GE, with help from DOE and ORNL. (General Electric)

Massive Magnetic Materials

NASA researchers are working to improve and advance the technology electric aircraft use to make them energy efficient and worthwhile.

“As the country, and the world, moves away from fossil fuels and toward renewable sources like solar cells and electric cars, the efficiency of power conversion will become a major issue,” said Randy Bowman, head of a new Magnetic Material Fabrication and Characterization Lab at NASA Glenn Research Center in Cleveland, OH.

With a newly acquired magnetic material caster, the lab allows researchers to create custom alloy magnetic ribbons up to one mile long and 50 mm wide. This ribbon can then be used to manufacture parts that generate or transform electrical power more efficiently and at a larger scale than ever before

This large-scale caster is the largest in the nation for conducting magnetic material research — providing NASA and its partners with the capability to produce materials large enough to move out of the lab and into commercial use.

This new casting capability allows researchers to customize the metallic makeup of a magnetic ribbon, produce a large-scale version of it, and use it exactly as it applies to their subjects of research, projects, or full-scale tests.

“We have the ability to alter the magnetic properties to tailor their properties to match the unique requirements of each specific application,” said Bowman. “For example, some applications want the material to be easily magnetized while others want it to be difficult.”

According to Bowman, a group of researchers is testing these materials to convert the power generated by solar cells into a form that can be smoothly integrated into the national power grid system and easily managed with little to no electrical variations or disruptions.

Lightweight Material is Ten Times Stronger than Steel

MIT researchers designed one of the strongest lightweight materials known by compressing and fusing flakes of graphene, a two-dimensional form of carbon. The new material — a sponge-like material with a density of 5 percent — can have a strength 10 times that of steel. In its 2D form, graphene is one of the strongest materials, but translating that 2D strength into useful 3D materials has been difficult.

The crucial aspect of the new 3D forms has more to do with their unusual geometrical configuration than with the material itself, which suggests that similar strong, lightweight materials could be made from a variety of materials by creating similar geometric features.

The MIT team analyzed the material's behavior down to the level of individual atoms within the structure. They produced a mathematical framework that very closely matches experimental observations. Two-dimensional materials — basically, flat sheets that are just one atom in thickness, but can be indefinitely large in the other dimensions — have exceptional strength as well as unique electrical properties. But because of their extraordinary thinness, they are not very useful for making 3D materials that could be used in vehicles, buildings, or devices.

The team compressed small flakes of graphene using a combination of heat and pressure. This process produced a strong, stable structure that resembles some corals and microscopic creatures called diatoms. These shapes, which have an enormous surface area in proportion to their volume, proved to be remarkably strong. The team created a variety of 3D models and then subjected them to various tests. In computational simulations that mimic the loading conditions in the tensile and compression tests performed in a tensile loading machine, one of the samples had 5 percent the density of steel, but 10 times the strength.

The new materials were made using a high-resolution, multimaterial 3D printer. The unusual geometric shapes that graphene naturally forms under heat and pressure look something like a Nerf ball — round, but full of holes. These shapes, known as gyroids, are so complex that making them using conventional manufacturing methods may be impossible. The team used the 3D-printed models of the structure, enlarged to thousands of times their natural size, for testing purposes.

Because the shape contains very tiny pore spaces, the material might find application in water or chemical filtration systems.

Ceramic Matrix Composites for Jet Engines

This coil of soft magnetic ribbon was produced on NASA Glenn's large-scale caster. The ribbon measures 20 microns thick and approximately 30 mm wide. (NASA)

Ceramic matrix composite (CMC) materials are made of coated ceramic fibers surrounded by a ceramic matrix. They are tough, lightweight, and capable of withstanding temperatures 300 to 400 °F hotter than metal alloys can endure. If certain components were made with CMCs instead of metal alloys, the turbine engines in aircraft and power plants could operate more efficiently at higher temperatures, combusting fuel more completely, and emitting fewer pollutants.

A quarter-century ago, the U.S. Department of Energy (DOE) began a program, led by DOE's Oak Ridge National Laboratory, to support U.S. development of CMC materials. Last year, LEAP, a new aircraft engine, became the first widely deployed CMC-containing product. CFM International, a 50/50 joint venture of Safran and GE, manufactures LEAP.

The engine has one CMC component, a turbine shroud lining its hottest zone, so it can operate at up to 2400 °F. The CMC needs less cooling air than nickel-based superalloys, and is part of a suite of technologies that contributes to 15 percent fuel savings for LEAP over its predecessor. GE's CMC is made of silicon carbide (SiC) ceramic fibers (containing silicon and carbon in equal amounts) coated with a proprietary material containing boron nitride. The coated fibers are shaped into a preform that is embedded in SiC containing 10-15 percent silicon.

Typically, combining two brittle materials yields a brittle material, but altering the bond between fiber and matrix allows the material to act more like a piece of wood. Cracks don't propagate into the fibers from the matrix around them. The fibers hold the material together and carry the load while slowly pulling from the matrix, adding toughness.

DOE's Continuous Fiber Ceramic Composite (CFCC) program ran from 1992 to 2002, with the goal of developing CMCs for industrial gas turbine engines that produce electricity. (GE manufactures both power and propulsion turbines.) A follow-on DOE program ran through 2005, and funded the most promising CFCC companies to further develop materials and components.

A land-based gas turbine to generate electricity can be more demanding than an aircraft engine application because it spends much more time operating at high temperature. Advances in the next generation of materials would enable breakthrough improvements in efficiency and emissions that could lower the cost of electricity.

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