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.

Lightweight Wind Turbine Rotor Blades

As offshore wind turbines get larger, they become more difficult to transport and install. Wind turbines can have rotor blades measuring up to 80 meters in length, with a rotor diameter of more than 160 meters. Since the length of the blades is limited by their weight, it is essential to develop lightweight systems with high material strength. The lower weight makes the wind turbines easier to assemble and disassemble, and improves their stability at sea.

3D-printed gyroid models, such as this one, were used to test the strength and mechanical properties of a new lightweight material. (Melanie Gonick/MIT)

As part of the European Union's WALiD (Wind Blade Using Cost-Effective Advanced Lightweight Design) project, scientists at the Fraunhofer Institute for Chemical Technology in Germany are working with industry partners to design lightweight rotor blades.

Currently, the rotor blades are made of thermosetting resin systems. These materials, however, don't permit melting, and they aren't suitable for recycling. “In the WALiD project, we're pursuing a completely new blade design. We're switching the material class and using thermoplastics in rotor blades for the first time. These are meltable plastics that we can process efficiently in automated production facilities,” said Florian Rapp, project coordinator at Fraunhofer ICT. The goal is to separate the glass and carbon fibers, and reuse the thermoplastic matrix material.

For the outer shell of the rotor blade, as well as for segments of the inner supporting structure, the team used sandwich materials made from thermoplastic foams and fiber-reinforced plastics. In general, carbon-fiber-reinforced thermoplastics are used for areas of the rotor blade that bear the greatest load, while glass fibers reinforce the less stressed areas. For the sandwich core, thermoplastic foams are bonded with cover layers made of fiber-reinforced thermoplastics in sandwich design. This combination improves the mechanical strength, efficiency, durability, and longevity of the rotor blade.

The foams also can enable completely new applications; for instance, in the automotive, aviation, and shipping industries. In vehicles, manufacturers have been using foam materials in visors and seating, but not for load-bearing structures. The current foams have some limitations with regard to temperature stability, so they can't be installed as insulation near the engine. Meltable plastic foams, by contrast, are temperature-stable and therefore suitable as insulation material in areas close to the engine.

Magnesium Manipulation

Wind turbine rotor blades made of thermoplastic sandwich materials. (© Photo Fraunhofer ICT)

Magnesium is the lightest construction metal, but also the most reactive. This means it is very sensitive to corrosion — it easily reacts with its surroundings and rusts. This makes the use of magnesium in corrosive environments very difficult, so the potential to use magnesium in cars to make them lighter is limited. At Chalmers University of Technology in Sweden, researcher Mohsen Esmaily changed the microstructure in magnesium alloys, enabling their use in decreasing vehicle weight.

For more than a hundred years, magnesium producers have worked hard to improve corrosion characteristics by developing new, more corrosion-resistant alloys, and by developing various coatings. Esmaily's research shows a completely new way to improve the corrosion resistance of the alloys by manipulating the microstructure of the material, thereby increasing the ways in which vehicle weight can be lowered.

“In cars, where every kilo of reduced weight is important, a transition to magnesium, which is 30 percent lighter than the most common lightweight metal today — aluminum — would mean a great step forward to reduce fuel consumption,” said Esmaily.

When studying a casting method called rheocasting, Esmaily discovered that the corrosion resistance of magnesium alloys produced this way was up to four times better than the same material produced by conventional high-pressure die casting. Rheocasting also gives the alloys surprisingly good ability to withstand corrosion.

“We will be able to create cast magnesium alloys that corrode much slower and that are stronger than ever before by controlling the microstructure of the alloy,” he said.

Resources

www.nasa.gov/centers/glenn/home/index.html 

https://www.ornl.gov/news 

http://news.mit.edu 

http://www.eu-walid.com 

http://www.chalmers.se/en 

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MIT’s strong, lightweight material


NASA Tech Briefs Magazine

This article first appeared in the March, 2017 issue of NASA Tech Briefs Magazine.

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