Ultra-light metal

Dr. Neil Canter, Contributing Editor | TLT Tech Beat March 2012

Researchers develop the world’s lightest cellular metal with improved mechanical properties.

KEY CONCEPTS
• Low-density cellular materials are very lightweight with density values below one gram per cubic centimeter and high porosity, though they suffer from inferior mechanical properties.
• A new ultra-light-density material prepared from nickel and composed of a unique microlattice cellular architecture is the world’s lightest metal with a density of 0.9 milligrams per cubic centimeter.
• Mechanical testing shows the strength of this ultra-light metal is significantly improved, as is the stiffness.

IMPROVING FUEL ECONOMY IN AUTOMOBILES remains a very important driver not just for the lubricant industry but also for development of lighter metals. Several approaches are underway to achieve this goal.

In a previous TLT feature article, an update was provided on the development of ultra high-strength steel alloys (A-UHSS) (1). Five different types of AUHSS have become available that offer different mechanical properties so that they can be used in specific applications.

While efforts have been made to reduce the weight of steel alloys to improve fuel economy, aluminum use in automobiles has been limited because this metal has not reached the mechanical strength of steel. In a previous TLT article, a new process known as high-pressure torsion has been developed to increase the mechanical strength of aluminum (2). The reason aluminum is strengthened is due to the formation of smaller grains below 100 nanometers in diameter and the preparation of hierarchical nanosize clusters of aluminum with alloy elements such as copper, silicon, chromium and titanium.

A class of even lighter materials known as low-density cellular materials has been used in some applications. Dr. Bill Carter, manager, architected materials department in the Sensors and Materials Laboratory at HRL Laboratories, LLC in Malibu, Calif., says, “Low-density cellular materials exhibit density values below one gram per cubic centimeter and are defined by having significant porosity.”

Materials that exhibit densities below 10 milligrams per cubic centimeter are defined as ultralow-density. Carter says, “Aerogels are the most widely used ultralow-density materials. Typically, they are used as insulation but have found applications as fluid absorbers or catalyst supports.”

Other examples of ultralow-density materials are aerogels, metal foams and carbon nanofoams or carbon nanotube forests. All of these materials contain structures characterized by random sizes and shapes of internal air pockets.

Ultralow-density materials display some beneficial properties such as a high-specific surface area but suffer from some detrimental properties. Their random structures lead to inferior mechanical properties such as irreversible deformation and low strength.

Carter says, “The fragile nature of aerogels makes them difficult to use without reinforcement but once you add in reinforcements, their density increases, making them heavier. Cost of manufacture also can be a problem as most ultralow-density materials are difficult to produce at scales sufficient to make them useful for commercial applications.”

The need exists for developing an ultralow-density material that has improved mechanical properties. Such a material has now been developed.

MICROLATTICE CELLULAR ARCHITECTURE
Carter and his fellow researchers at HRL Laboratories, in collaboration with researchers at the University of California-Irvine and the California Institute of Technology, have developed an ultra-light-density material that is the world’s lightest cellular metal and exhibits superior mechanical properties as compared to other materials of its class.

The material is composed of a unique microlattice cellular architecture that enables an ordered structure to be prepared at three-dimensional levels. A self-propagating photopolymer waveguide method is used initially to prepare a lattice structure through the polymerization of a thiol-ene liquid photomonomer in the presence of ultraviolet light.

In a second step, a nickel coating is applied to the polymer structure by an electroless plating process. Then the polymer is dissolved in a washing step leaving the ultra-light nickel metal structure.

Carter says, “We can control the cellular architecture on three levels of hierarchy at the nanometer, micrometer and millimeter scales, which enables us to design these materials with tailored properties for specific applications.”

The researchers were inspired by the observation that aerogels can be super-light, but at a microscopic scale they are very random, making them very fragile. Carter says, “We figured if we could order the material at the microscale, we could make a much more robust material.”

The result is the world’s lightest metal with a density of 0.9 milligrams per cubic centimeter. This makes the material approximately one hundred times lighter than Styrofoam.

Figure 1 shows an image of a structure of the ultra-light nickel metal sitting on top of a dandelion fluff. This structure consists only of 0.01% solid, which enables it to be so light. The remaining component in the ultra-light nickel metal is 99.99% air.


Figure 1. The world’s lightest metal is composed of a unique microlattice cellular architecture, and is lighter than dandelion fluff. (Courtesy of HRL Laboratories)

Mechanical properties of the ultralight nickel metal were also measured. Carter says, “During testing of the mechanical properties of these novel microlattice materials, we were very surprised to find that they recover from compression exceeding 50% strain and absorb energy similar to a viscoelastic polymer although they are metallic.”

The researchers determined that the ultra-light nickel metal tend to become weaker after repeated mechanical cycling during the compression tests. Carter adds, “They still retain significant energy absorption capability after multiple cycles.”

Carter believes that compared to aerogels, the strength of this ultralight metal is significantly improved as is the stiffness. Potential applications for this material could be in aerospace structural components. Carter adds, “Lightweight is very beneficial for anything that needs to fly. Also, the unique energy absorption properties are very interesting for acoustic, vibration and shock dampening.”

Over the long-term, the researchers are striving to revolutionize lightweight materials by establishing a cost-efficient, scalable process to design and manufacture microlattice materials. Carter says, “Much like modern buildings such as the Eiffel Tower that are incredibly light- and weight-efficient by virtue of their architectures, we plan to make new materials by bringing this concept to the materials level and designing material architecture at the nano- and microscales.”

Further information can be found in a recent article (3) or by contacting the lead author, Tobias Schaedler at taschaedler@hrl.com.

REFERENCES
1. Rensselar, J. (2011), “The Riddle of Steel: A-UHSS,” TLT, 67 (7), pp. 38-46.
2. Canter, N. (2011), “Super-Strong, Ductile Aluminum,” TLT, 67 (1), pp. 10-11.
3. Schaedler, T., Jacobsen, A., Torrents, A., Sorensen, A., Lian, J., Greer, J., Valdevit, L. and Carter, W. (2011), “Ultralight Metallic Microlattices,” Science, 334 (6058), pp. 962-965.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.