NIOBIUM

PAMELA S. ZURER, C&EN WASHINGTON

I
never handled any niobium until my 50th birthday. As a science journalist, I'd heard of superconducting alloys of niobium, of sophisticated porous materials made from niobium oxide, and of the interesting chemistry of some niobium complexes. But it was my husband's birthday gift of a niobium necklace that put the metal into my hands and made me want to learn more about it.

My necklace is a simple choker made of small metal rectangles linked together. Yet it never fails to draw compliments when I wear it. A design on the surface of each link shimmers in soft luminous blues, greens, and purples--reflecting light more like the wings of a butterfly than the cold luster of gold or silver.

The captivating colors, I figured, must be due to a variety of enamels applied to the metal's surface. Wrong. The rainbow of colors is generated by thin films of niobium oxide, with the thickness of the oxide layer governing the color perceived. Jewelry artists produce brilliant colors on niobium by anodizing the metal.

"Thin-film interference is responsible for the color," says Bill Seeley, founder of Reactive Metals Studio in Clarksdale, Ariz., which sells niobium, titanium, and anodizing equipment to jewelry makers. "The oxide is transparent and has a high refractive index. Light waves bounce off the oxide, but some go through and reflect off the metal below, reappearing at the surface after a time delay that depends on the thickness of the oxide layer. Those two sets of waves either interfere with or reinforce each other, creating the color you see."

Surface oxide layers can be produced by heating niobium in air. Jewelry artists, however, prefer the control over the oxide thickness that they can achieve with electrochemistry.

NIOBIUM AT A GLANCE
Name: Named for the Greek mythological figure Niobe, daughter of Tantalus, because of its position above tantalum.
Atomic mass: 92.91.
History: Discovered in 1801 by Charles Hatchett while working with a sample of columbite.
Appearance: Shiny gray metal that takes on a bluish tinge when exposed to air at room temperatures for a long time. Soft and ductile.
Behavior: Oxidizes in air at high temperatures.
Uses: Used in various superconductor applications and in welding rods, cutting tools, and pipelines. Niobium-stabilized steel is very heat resistant.
8136element.niobium1
COURTESY OF BILL SEELEY
VIBRANT The colors in Seeley's niobium and silver broach (top) and deBeixedon's "Night Color Squid" (below) spring from thin-film interference.
8136element.niobium2

COURTESY OF DIANNE DEBEIXEDON

"The thickness of the oxide is controlled by the voltage in the anodizing bath," Seeley explains. "An extremely thin layer--600 to 1,000 Å--grows at the interface. The oxide itself is resistant to the passage of current. If you set the voltage at 30 V, for example, the oxide film grows to a certain thickness and stops. Artists make multicolored pieces by using masks to temporarily protect parts of their pieces from the electrolyte."

In addition to niobium, interference colors can be created by anodizing titanium, zirconium, molybdenum, and tantalum, says Seeley, whose master of fine arts thesis was on studio preparation and coloring of titanium. Niobium, however, is particularly attractive to work.

"It's a beautiful, ductile metal," Seeley notes. "You can form it, re-form it, chase it, repousse it, spin it, shape it any number of ways. And it needs no special cleaning to create the beautiful anodized colors."

Dianne deBeixedon, professor of metalworking at Old Dominion University, Norfolk, Va., agrees. "Niobium is very malleable, very cooperative, a wonderful metal to shape. Plus there are the vibrant colors.

"For example, at 60 V, niobium produces a beautiful deep yellow," deBeixedon says. "But at about 65 V, it starts turning a pinky peach. At various voltages there are purples, fuchsia, a gorgeous turquoise, greens, a beautiful cobalt blue. The only color you can't get is red."

In her own niobium work, deBeixedon uses a technique called anodic painting that allows exquisite control of color. She dips a paintbrush to which a wire is attached into the electrolyte, sets her apparatus to the voltage corresponding to the color she wants, and "paints" on the surface of the jewelry with the paintbrush electrode.

Like many other jewelry artists working with niobium today, deBeixedon first learned of its potential from Seeley. Recently, Seeley shared his expertise with a different group of professionals. At a workshop at the North Carolina School of Science & Mathematics in Durham, he taught high school chemistry teachers to anodize niobium and titanium.

The project was the idea of Myra J. Halpin, a chemistry teacher at the school, a statewide magnet for students with high aptitudes for science and math. She received a Toyota Tapestry grant to purchase anodizers and supplies and brought in Seeley to work with teachers from the local area. She's already used what they learned with her own students.

"I've always been fascinated by chemistry and colors," Halpin says. She introduced the unit after her class had tackled electrochemistry. The students designed, shaped, and anodized earrings and other small pieces. In a more advanced research class, Halpin's students investigated the effect of different variables on the colors produced.

"The kids get something real and tangible from chemistry to take home," Seeley notes. "Electrochemistry makes beautiful things."

Beautiful, indeed. For my next birthday, I'm hoping for more niobium jewelry.


Pamela S. Zurer, C&EN's managing editor, has been with the magazine for 22 years. She's still amazed she gets paid to talk with people about the interesting chemistry they do.




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