Technically Speaking

An atomic engine that has all of the power but none of the moving parts

by Dr. Brendan Hanrahan

What if atomic crystals could send pulse power to light up a room?

Back in 314 B.C., a student of Socrates described bits of sawdust that gravitated to a stone thrown into a camp fire. What was an oddity then might be a solution as technology comes of age.

In 1946, more than 17,000 vacuum tubes clicked away in a crowded room and 20 seconds later, ENIAC—the Electronic Numerical Integrator And Computer—had calculated the trajectory of an artillery shell for the Ballistics Research Laboratory. Attendees got to keep a printout as a keepsake. Exciting! The following year, John Bardeen, Walter Brattain and William Shockley would invent the semiconductor transistor. The properties of the semiconductor material accomplished much of the same tasks of the vacuum tube machine, which marked the beginning of the end for vacuum tube-based systems. A material replaced a machine.

THE ATOMIC ENGINE

THE ATOMIC ENGINE
Heat causes motion of the asymmetric atom (blue) in the pyroelectric crystal, which is converted to electrical power in a repeating process. (Illustration by Eric Proctor, U.S. Army Research Laboratory (ARL)

In my research, what I’d like to know is which pyroelectric material is the one that will have the best chance of success in practical use for the Army. In general terms, an engine’s job is to convert one form of energy into another. This process is described in thermodynamics (the study of heat, energy and work). Why is this important? Because more than 75 percent of the electricity production around the world starts with heat. For example, a coal-fired power plant burns coal to create steam, which in turn drives a turbine.

We’re all familiar with the internal combustion engines that power our cars. Energy conversion begins with the piston in your car quickly compressing the air in the cylinder. Adding gasoline and a spark creates combustion, causing a quick pressure rise. Pressure pushes on the piston, which spins the crankshaft, eventually transferring energy to the wheels. The piston comes back up in the cylinder and we’re ready to start all over again. That process doesn’t just convert energy into motion, it also converts energy into heat, nearly all of which is wasted.

What if these same processes could be accomplished with less waste on the atomic scale, mimicking pistons with atoms? How could we exploit the technology?

We explore the once mystic crystals because we know that the material has polarization, which can be altered by an electric field; and we see a potential pathway between thermal and electrical energy conversion, which is the ultimate goal.

CRYSTAL CLEAR
These crystalline materials are made up of an ordered arrangement of atoms. Some atoms have a positive charge and some a negative charge. The pyroelectric materials look like a box of atoms with a single atom that is almost, but not quite, in the center. That means that the charge is more positive on one side or another. However, when you heat the material, the atom that was slightly offset centers itself to form an evenly charged surface. The asymmetry caused by the material’s polarization, or internal electric field, causes the electric charge on the surface to change when the polarization changes as the material is heated or cooled.

In the 1700s and 1800s, a number of today’s legendary scientists had explored pyroelectric properties: Carl Linnaeus, who created the two-name system we use to classify animals, plants and minerals; Joseph Priestly, who discovered oxygen; and Pierre and Marie Curie, who were credited with advances in radiation, magnetism and crystallography.

It was not until later that pyroelectrics were considered for everyday use. Nowadays, pyroelectrics are primarily used in home security systems, where infrared radiation is absorbed by the pyroelectric material, which enables motion detection.

ATOM-LEVEL POWER

ATOM-LEVEL POWER
Pyroelectric materials produce energy at the atomic level when they are heated or cooled. (Image by zoom-zoom/iStock)

CHARGE IT
We explore the once mystic crystals because we know that the material has polarization, which can be altered by an electric field; and we see a potential pathway between thermal and electrical energy conversion, which is the ultimate goal.

Let’s figure out how a pyroelectric engine would work. First, it would look a lot like a sandwich, with a pyroelectric material between metal electrodes. Let’s go back to internal combustion, where the first process in energy conversion is compressing air and fuel. In the same way, the pyroelectric engine has an electric field with polarization that pushes a charge in one direction or the other until heat is applied.

The extremely thin pyroelectric engine heats up quickly, loses polarization, and electricity gets pushed evenly onto its surface. This is analogous to the power stroke of an internal combustion engine’s piston, but you’re pushing charge, not wheels.
So, in the same way that to keep the cycle goingan engine keeps cycling, the piston has to rise and compress fuel and air again, we have to cool and remove the charge to keep the cycle going over and over again.

The voltage created through the electric field of atoms adds massive “pressures” with the ease of flipping a switch. Pyroelectric materials can also be made into sheets of thin film. Whether this material could ever replace a generator for modern uses like lighting a tent city, will be determined as the science advances.

The temporary voltage that occurs when pyroelectric materials are heated and cooled is one of the least written-about in materials science literature. Historically there have been concerns such as the efficiency of the heat transfer. Recent advances in pyroelectric materials science have suggested that a pyroelectric engine eventually could reach the potential to make it a transformative technology.

PYROELECTRIC HEAT ENGINE

PYROELECTRIC HEAT ENGINE
Pyroelectric material can be made into a thin film so these “engines” can be extreme small, scalable and made to coat uneven surfaces. (Illustration by Eric Proctor, U.S. Army Research Laboratory)

CONCLUSION
Getting the pyro-material, the cycle and the measurement right requires a diverse team of scientists and engineers working together. U.S. Army Research Laboratory scientists are confident, though, that exploring this unique connection between the thermal and electrical realms will lead to new technologies that could leapfrog the ones we are looking at today, enabling new power sources for the future.

Electrical power will continue to be both a necessity and a challenge for our armed forces and the civilian world. Most of the power we use comes from some kind of heat source and goes through a similar energy conversion process in machines. A material that produces electricity that could replace machines could, as silicon did with vacuum tubes, make processes vastly more efficient, potentially much less costly and add yet another exciting technology that lead to innovations we can’t even begin to imagine. The good news is that there are myriad new energy generation and storage technologies being researched inside and out of DOD.

For more information, contact the author at (301) 394-1960 or at brendan.m.hanrahan.civ@mail.mil.

For information about the U.S. Army Research Laboratory’s collaboration opportunities in materials science, go to http://www.arl.army.mil/opencampus/.

DR. BRENDAN HANRAHAN works in the Energy and Power Division at the U.S. Army Research Laboratory in Adelphi, Maryland, and leads a pyroelectric energy conversion project. He is also co-founder of a race series originating in Washington that has raised $11 million for research into neurofibromatosis. He holds doctorate and master’s degrees in materials science and engineering from the University of Maryland and a B.S. in ceramic and materials engineering from Clemson University.

This article will be printed in the October – December issue of Army AL&T magazine.

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