The Ames Laboratory's Materials Preparation Center is the source
for research-grade samples
Rare-earth metals play a crucial, but largely hidden, role in most of the technological breakthroughs of the past 30 years. You’ll find them in all types of electronics and virtually anything with an electric motor, including electric-powered cars and the generators in wind turbines. They are also the “metal” in nickel-metal-hydride batteries found in hybrid cars and cordless power tools.
Getting rare-earth metals requires a multiple-step process. First rare-earth oxides, like the yellow (cerium), black (praseodymium) and blue (neodymium) powders in the dishes are exposed to hydrogen fluoride gas. This turns the powder into a crystalline fluoride, such as the green praesodymium fluride crystal (far right). A reduction reaction and further processing turns the rare-earth fluorides into their final, pure metal forms, (from top center) scandium (arc cast), dysprosium (sublimated), dysprosium (arc cast) and gadolinium (single crystal, arc zone).
So just what are rare-earth metals? The name commonly applies to 17 elements on the periodic table that include scandium, yttrium and the lanthanides, as highlighted in the periodic table on Inquiry’s back cover. None of these chemically similar elements exist naturally on their own, but are mined as comingled oxides in minerals, such as bastnasite and monazite.
Rare-earth metals are typically used in combination with other metals to create alloys with enhanced properties not found in the individual components. Neodymium, for example, when combined with iron and boron, creates an alloy with very strong magnetic properties.
At the research forefront
When it comes to studying and producing ultra-high purity rare-earth materials, the Ames Laboratory is the world recognized leader. The work began in the 1940s when Frank H. Spedding and his colleagues developed the ion-exchange process, a technique that separates the “fraternal fifteen” plus yttrium and scandium.
That pioneering work and further rare-earth research efforts led to a constant demand for samples from the Ames Lab from research groups around the globe. To better serve the scientific community’s demand for research materials, the Materials Preparation Center was formed. The MPC is a DOE, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences & Engineering specialized research center that’s recognized throughout the worldwide research community for its unique capabilities in the preparation, purification, single crystal growth, and characterization of rare-earth metals, alkaline-earth metals, and refractory-metal materials. MPC operations are primarily funded by the DOE’s Materials Discovery, Design, and Synthesis team’s Synthesis and Processing Science core research activity.
Established in 1981, the MPC is a one-of-a-kind facility, which is acutely sensitive to the needs of researchers. Providing research and developmental quantities of high-purity materials and unique characterization services to scientists at university, industry and government facilities on a cost-recovery basis, the MPC allows access to novel materials as they are developed.
The MPC is renowned for its outstanding technical expertise in alloy design, creating materials that exhibit ultra-fine microstructures and high strength and high conductivity — properties of great potential value to American technology. The MPC also has established a reputation for close interaction with its clients, providing personalized service to meet each client’s individual needs. Each year, the MPC satisfies hundreds of requests for customized materials and services that are unavailable from commercial suppliers and unmatched in quality anywhere else in the world.
Purity is key
In order for researchers to get an accurate picture of the properties of a particular alloy, it’s essential to start with the best possible materials.
“Impurities can cause all kinds of problems,“ says Trevor Riedemann, an Ames Laboratory scientist and manager of the MPC’s rare-earth materials section. “We’ve seen a number of cases where competing research groups have seen very disparate properties because the experimenters used different grades of rare-earth materials.”
Producing such high-purity rare-earth metals on a research scale requires a multiple-step process that’s basically unchanged from the 1940s. Using the best rare-earth oxides available, the oxides are first converted to their respective fluorides by exposing them to hydrogen-fluoride gas. These rare-earth “salts” are packed with purified calcium in tantalum crucibles. Tantalum is used to minimize contamination that would occur if ceramic crucibles, such as alumina or zirconia, were used. All the preparation steps take place in highly controlled environments, such as an argon gas-filled glove box, to prevent oxidation.
The reduction process is driven by an induction furnace. During the reduction process, when the rare-earth fluoride salts and calcium melt, from 1500 to 1800 degrees Celsius (2732 to 3272 degrees Fahrenheit), the calcium bonds with the fluoride, leaving the rare-earth metal. Depending on the specific rare-earth metal being refined, additional steps, such as vacuum casting, sublimation or distillation, are used to purify the metal.
Ames Lab technician Ross Anderson prepares
to refine lanthanum by packing a tantalum
crucible with lanthanum fluoride crystals and
purified calcium in an argon-filled glovebox.
The tanatlum crucible (left) is placed inside
a second “heater can” crucible (right) that is
wrapped with a graphite jacket. The assembly is then placed in an induction furnace.
The crucible assembly is placed inside a
quartz tube that in turn is placed within an
induction coil. Electrical current applied to the coil then heats the lanthanum and calcium.
As the temperature rises, the material inside the crucible begins to glow. Depending
on the rare-earth metal being used, the rare-earth fluoride and calcium melt between
1500 and 1800 degrees Celsius, at which point the calcium bonds with the fluoride,
leaving the purified rare-earth metal.
Calcium-fluoride salts (black crystals) formed during the reduction process are separated from the rare-earth metal, which typically stays at the bottom of the crucible and is decanted out.
“Gadolinium, terbium and lutetium are more difficult to refine,” Riedemann says, “because they eat tantalum (the crucible material) for lunch. So we have to take additional steps to remove the tantalum that leaches from the crucible.”
Once the materials are refined, they’re stored in a controlled environment to prevent oxidation. Depending on what’s needed, the MPC can provide the raw, rare-earth material, or produce specific alloys that researchers at the Ames Laboratory and other labs around the world might require. In the 2009 calendar year, the MPC provided research materials to clients throughout the United States, as well as Australia, Austria, Canada, Demark, France, Germany, Italy, Japan, Korea, the Netherlands, Singapore, Spain, South Africa, Switzerland, and the United Kingdom.
~ by Kerry Gibson