ASU Research E-Magazine: Pressure Tactics

Designing and developing devices that exert immense amounts of pressure is a major preoccupation of scientists that make up ASU’s Materials Research Group. As is true in all experimental work, the key to scientific credibility is that each device they develop must be able to exert pressure in a reproducible manner.

Pressure is the result of force applied to a given area. The same force applied to a smaller area geometrically increases pressure.

“We’ve had to build our laboratories from scratch,” says project leader Paul McMillan, an ASU professor of chemistry. Regardless, the group has enjoyed considerable success in the two years since initial grant support was received from the National Science Foundation.

In that time, the ASU researchers have built a pair of multi-anvil devices. Each device is capable of exerting more than 200 kilobars of pressure at temperatures up to almost 2,000 degrees Centigrade. The 200 kilobar total is equal to 200,000 times the normal atmospheric pressure found at sea level. A third multi-anvil device is nearing completion.

Each multi-anvil device consists of a series of containers within containers. The outer containers are made of machined steel or carbide wedges. When assembled, the wedges form an octahedral space in the center.

John Holloway (left) and Paul McMillan display the innards of a multi-anvil press.

Researchers place samples inside a drilled ceramic holder which fits inside the octahedral hole. Force exerted on the outer container moves the wedges to constrict the volume of the hole. The reduction in surface area between each successive layer serves to multiply the pressure applied to the sample. Some multi-anvil devices can generate more than 300 kilobars of pressure.

McMillan credits group member John Holloway, a professor of chemistry and geology, with the new design. “John is a master at designing compact high-pressure equipment,” he says.

ASU researchers use the multi-anvil press to produce sample materials in amounts of only tens of milligrams, which is much to little for industrial purposes. But the experiment can be scaled to larger size. McMillan says that Japanese laboratories have huge devices capable of making a liter of material at 100 kilobars of pressure.

Holloway also designed and built a piston-cylinder machine that exerts up to 25 kilobars of pressure. The machine is used to produce larger samples for study.

The new devices are used to complement research conducted by George Wolf at ASU’s first high pressure laboratory. Wolf, an associate professor of chemistry, uses diamond anvil cell technology in his work.

With less torque than is needed to tighten a spark plug, a scientist using diamond anvil cell technology can create pressures that exceed that found at the center of the Earth. “In fact, you can exert pressure that is roughly equal to that found a third of the way through Jupiter,” McMillan says.

The diamond anvil fits easily within the palm of one hand. The device is just what its name implies. At the center of the cell are two flawless, gem-cut diamonds, each about a third of a carat in weight. The points of the diamonds are ground off to small, flat surfaces set in opposition to each other.

Tiny slivers of sample material are placed between the diamond faces. The thrust generated by hand-cranking a pressure bolt is transmitted to the sample. The diamond anvil cell multiplies that pressure by 500 to 1,000 times.

Wolf uses the diamond anvil to test materials under pressures in excess of one megabar. A megabar is one million times the atmospheric pressure at sea level. Think of it as a pressure roughly equivalent to the weight of six full-grown elephants concentrated on an area of four square millimeters, an area the size of a matchstick’s base. One megabar also represents the pressure found at about 2,000 kilometers beneath the Earth’s crust.

An added advantage to using the diamond anvil cell is that Wolf and other scientists can directly observe samples through a microscope while they are being squashed. Scientists also can use spectroscopy to study samples by beaming lasers or x-rays through the diamond anvils. They can even measure electrical conductivity or magnetic properties of the sample.

The major disadvantage of diamond anvil research is that only a minuscule amount of material can be studied at any one time—less than a microgram.

Other group members use powerful computer work stations to do theoretical calculations of materials under pressure. The combination of experimental and theoretical techniques sets ASU apart from other research groups.

“Other laboratories have decided to follow one approach or another,” McMillan explains. “We have married both experimental technologies with theoretical methods.”

In the past, most high pressure research was conducted by geochemists or geophyscists. Intense interest in new materials that might be technologically important is changing the scene.

“There is increased interest in new materials that might be produced using high pressure techniques,” McMillan says. “We need to work out ways of producing these materials commercially in large batches.”

Some might say that the pressure is on.—Grant E. Smith