Spherical powders enable new applications for metals

The Science

A newly developed process transforms large, irregular chunks of metal elements into uniform spherical particles that act like tiny ball bearings rolling past one another. This allows solid metals to be handled like liquids. In the process, vibrations eject an aerosol mist of molten metal droplets into a chamber of inert gas that surrounds and cools the droplets. Surface tension then pulls the liquid into spherical drops. The result is flowable spherical powders that can be pumped like a liquid or propelled by a gas. While this research was pursued to optimize the fabrication of targets for isotope production, the AMAZEMET rePowder® ultrasonic metal atomization process works equally well for most metallic elements and their alloys, opening possibilities for a variety of applications beyond isotope production targets.

The Impact

Additive manufacturing, or 3D printing, can fuse particles that flow like liquid into complex shapes. This includes shapes that are hard to machine, forge or assemble from sheets, rods or ingots of material. The ability to convert pieces of almost any metallic element or alloy into a spherical powder that flows and can be pumped like a liquid or propelled by a gas is an important advance. Researchers so far have used these spherical powders for a composite filter to produce the isotope californium-252 and to print silicon-germanium for nuclear batteries, in addition to targets. Traditional pressed pellets create dense structures for isotope production targets that hold materials in place during and after irradiation. In contrast, spherical powders of most elements can be welded into targets that dissolve rapidly. This could lead to partially automated production of targets and more efficient isotope production.

Summary

To create the spherical powders using ultrasonic metal atomization, an element or alloy is first placed on a water-cooled hearth that is vibrated 20,000 times per second. Next, an electrical arc rapidly melts the metal into a puddle. The vibrations create waves in the puddle that eject the small, spherical droplets of the now-liquid material. The droplets are swept away from the electrical arc with argon gas and quickly solidify in the process. A container at the bottom of the system collects the particles. A sieve can divide batches by size.

While spherical powders are not new, the ability to make small quantities across the periodic table and of special research materials is. Typically, spherical metal powders have been available only in structural-type alloys in very large batches. Most of these materials are stainless steel, titanium, and other alloys for industry. Originally, scientists developed the AMAZEMET rePowder® method so they could use spherical powders of unusual, rare materials for additive manufacturing. While that ability is still an important goal, the ability to have almost any metal or alloy flow like liquid is creating new uses. This method, while developed for making isotope production targets, opens new realms of possibilities for a variety of applications in isotopes and other fields.

Funding

This work is supported by the Department of Energy Isotope Program, managed by the Office of Science for Isotope R&D and Production.