Carnegie’s high-pressure history
Until the 20th century, we could only imagine how materials would act under the extreme pressures and temperatures found in the deep interior of our planet.
The seminal work of Nobel Laureate Percy Bridgman, who pioneered high-pressure research in the early decades of the 20th century, broke this barrier with the invention of a piston-cylinder press. This invention allowed scientists to create and maintain pressures in the lab at around 2-3 kilobars—enough to study minerals under pressure up to about 80 kilometers into Earth’s crust and shallowest mantle. However, only a very few labs had access to such equipment.
In the early 1960s, the late Joe Boyd, a staff scientist at the Geophysical Laboratory (GL)* teamed up with Joseph England, the GL engineer, to create the Boyd-England Piston Cylinder. This press could reach and maintain pressures around 40 kilobars or more at temperatures exceeding 1600 degrees Celsius—finally allowing scientists to study the uppermost mantle. The machine was easy to build and use and remains a workhouse in high-pressure studies to this day. Still, the pressures and temperatures that would unlock the secrets of the rest of the mantle and core—most of the planet—were inaccessible.
*The Geophysical Lab merged with the Department of Terrestrial Magnetism in 2020 to form the Earth and Planets Laboratory (EPL).
In 1975, GL's Peter M. Bell and Ho-kwang Mao took the materials science world by storm. Following the development of the first diamond anvil cell by scientists at the National Bureau of Standards, which could apply around 500 kilobars between the tips of diamonds, Bell and Mao published a paper describing a new diamond anvil cell design that could apply pressures of 1 Megabar—double the previous record. About a decade later the same team was able to achieve pressures of 2.8 Megabars—again blowing the old record out of the water.
Reaching the 2.8 Megabar threshold kicked off a new era of ultra-high-pressure science and opened the door to exploring material properties in the untapped pressure region above 1 Megabar. This innovation allowed access to pressures equivalent to Earth’s outer core for the first time.
Studies exploring these extreme conditions provide a rare window into planetary interiors. At the same time, these pressures allow scientists to create entirely new technologically relevant materials with a whole host of novel physical phenomena. We do both at the Earth and Planets Laboratory.
With each new pressure frontier, we unveil new details of our planet and of materials themselves. Today, the Earth and Planets Laboratory continues to be one of the premier institutions in the world engaged in high-pressure science. Our high-pressure work falls into three main categories:
- What is the fundamental behavior of materials under extreme conditions?
- How do extreme materials affect planetary processes?
- How can we improve technological materials using extreme conditions?
What is the fundamental behavior of materials under extreme conditions?
Generally speaking, materials under high pressures shrink in volume because the average space between atoms gets smaller. This new close-knit configuration allows different types of bonds to form between atoms, which can fundamentally change the properties of a material—like its electrical conductivity, thermal conductivity, viscosity, melting, and reaction kinetics.
What is a phase change?
Pressure generally changes a material’s properties on a smooth continuum—until it doesn’t!
At specific pressures, some materials’ properties change dramatically. These reproducible material boundaries designate phase changes in which the entire structure of the material suddenly and reliably rearranges. For example, molecular solids, like water ice, go through several phases of structural transition as pressure increases. Carnegie’s Timothy Strobel showed some of those phases during a live demonstration in a recent Neighborhood Lecture event.
In compounds that form rock (e.g., oxides and silicates), a central positively charged atom of metal like magnesium or silicon is surrounded by a bunch of negatively charged oxygens. As pressure increases, the number of oxygens that can fit around the central metallic cation increases, leading to a more tightly packed material. For example, in common quartz that we find at the surface (SiO2), the silicon is bound to four oxygens.
However, under 80 kilobars of pressure, it transforms to a dense phase, called stishovite—a high-pressure version of SiO2 found as microscopic grains in impact craters and high-pressure rock. In stishovite, each silicon atom bonds to the nearest six oxygens surrounding it instead of four, but both quartz and stishovite are different phases of silicon dioxide.
Scientists like those at EPL investigate the changing properties of materials under pressure. They are constantly seeking to expand their understanding of materials to apply that knowledge to models of planetary interiors and seek out new materials with properties that can help us expand our current technologies.Read more
We cannot take direct samples from the deepest interior of our planet. The Earth’s core is about 2,900 km below the surface. Meanwhile, the furthest we’ve managed to drill is only about 12 km, which doesn’t even get us past the crust and extend into the mantle. Sometimes we can find natural samples of high-pressure materials at meteorite impact sites or trapped inside of a diamond, but normally the best we can do is take samples of the materials we think are inside our planet and test them in the lab.
We perform this foundational research using high-pressure devices such as diamond anvil cells and large-volume presses such as the piston cylinder. We load precursor materials into these devices and then subject them to high pressures and temperatures to push the material to the limit. Scientists create detailed logs of how its mineralogy and properties change.
We can then use what we find to inform the physics and chemistry in computational models of planetary interiors—enhancing our understanding of how our planet developed and changed through time.
Probing the Earth’s core
For instance, this type of research can help us understand what is driving Earth’s geodynamo—the planetary engine of convecting metals that drives our planet’s protective magnetic field. We know that our planet came from the disk of dust and gas that surrounded our Sun in its youth. Eventually, the densest material sank inward in the forming planet, creating the layers that exist today—core, mantle, and crust. Although the core is predominantly iron, seismic data indicates that some lighter elements like oxygen, silicon, sulfur, carbon, and hydrogen, were dissolved into it as it developed.
Over time, the inner core crystallized and has been continuously cooling since then. Scientists want to know if heat flowing out of the core and into the mantle could drive the geodynamo on its own. Or does this thermal convection need an extra boost from the buoyancy of light elements, not just heat, moving out of a condensing inner core?
A team that included Carnegie’s Alexander Goncharov recently addressed this question by directly measuring the thermal conductivity of solid iron and iron-silicon alloys up to 1.14 megabars and 3300K—mimicking conditions found in Earth’s outer core.
The team found that with a concentration of about eight weight percent silicon in their simulated inner core, the geodynamo could have functioned on heat transmission alone for the planet’s entire history.
A third extreme condition: time!
Extreme heat and pressure aren’t only found in the center of the planet! Sometimes they occur during sudden events—like when a meteor collides with a planet. When large impacts occur, time is also at an extreme. A material might go through extreme heat and pressure changes in less time than it takes to snap your fingers. Carnegie’s Sally June Tracy is an expert at studying materials under these conditions.
She uses gas guns or high-powered pulsed lasers to generate shock waves that travel through a sample producing a fleeting high pressure-temperature state. During the experiment, she collects data using X-ray or laser probes to learn how the material changes on a structural and chemical level.
Says Tracy, “This work provides experimental constraints on subjects ranging from planetary formation, high-pressure phases of deep planetary interiors, and materials for extreme applications.”
In a recent Neighborhood Lecture, someone in the audience asked Carnegie’s Tim Strobel what the top global challenges materials science can help solve.
“Clean water, renewable energy, world peace—you name it, and materials science can help solve it,” said Strobel. “Just look around where you’re sitting right now; maybe you’re at your desk looking at your computer screen. These are all materials with particular properties. The world we live in is possible because of materials science and novel materials research”.
In addition to allowing scientists to directly observe minerals behaving as they would in the center of the planet, EPL researchers like Strobel harness these high-pressure tools to create brand new materials that usually* only form under very specific high-temperature and pressure conditions. We now use computational chemistry to predict materials and then work in the lab to synthesize them and study their properties.
Things like next-generation superconductors, photovoltaics, and insulators are all on the table when pressure is involved.
*While many of these exotic materials are discovered under extreme conditions, scientists can develop novel techniques to recreate them under more normal conditions. For example, diamond, the high-pressure form of carbon, can also be synthesized under low-pressure conditions using vapor deposition techniques. Nature figured out how to do this already as nanometer diamonds are a common component of interstellar dust.
Searching for superconductors
Of specific interest to our campus (and the world at large) is the search for new superconducting materials. A superconductor has no electrical resistance; it does not restrict electron movement or lose any energy as electrons travel through it. A stable room temperature superconductor—the holy grail of materials science—would revolutionize technology by substantially reducing energy use and massively increasing the speed of computing.
Typically these materials have to be cooled below a very low temperature, which often makes them impractical for widespread use.
Discovering a superconductor that exists at ambient room temperatures and pressures is likely more than a decade off, but we are building the path that will get us there.
At Carnegie, the search for the superconductor goes back to the 1980s. In 1987, Carnegie scientists Ho-kwang Mao and Robert Hazen, identified the composition and structure of the first high-temperature superconductor. (If you want to learn more about that, Hazen wrote a popular book The Breakthrough: The Race for the Superconductor.) We've been searching for superconducting materials ever since!
More recently, Strobel and his team predicted and synthesized the first carbon-based clathrate. Clathrates are cage-like structures that can trap other elements inside. Scientists more than 50 years ago predicted that a carbon-based clathrate would have useful properties, like the potential for superconductivity.
Former Carnegie postdoc Li Zhu said in an interview, “We used a structure prediction method, which we developed, to predict the first stable carbon clathrate. Then, my colleagues used that prediction to synthesize the clathrate in the lab under very high pressures. So, this is the first carbon clathrate in the world.”
He continued, “This material is as hard as diamond and has the potential to act as a superconductor or an insulator depending on which atoms we put in the cage.”
The team has since confirmed superconductivity in these carbon-boron clathrates, a paper laying out their discovery is in the works.
Every day, the Earth and Planets Laboratory builds on the Geophysical Lab’s legacy to push the boundaries of high-pressure science. We develop techniques, probe new experimental questions, expand our computation capabilities, collaborate with colleagues, and fill our labs with top-tier instrumentation (and scientists).
All of this allows us to make new observations about our planet’s interior, better understand planetary development, and ultimately develop new materials that could make our world a better place.